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Wavelength-dependent chlorine photolysis and subsequent radical production using UV-LEDs as light sources
Abstract
UV-LEDs are considered as the most promising UV light sources, because it has the potential to replace conventional UV lamps in some water treatment applications in the foreseeable future. In this study, UV-LEDs at four wavelengths in the UV-C or near UV-C range (i.e., 257.7, 268, 282.3, and 301.2 nm) were used to investigate the wavelength-dependency on chlorine photolysis and its subsequent radical formation. The fluence-based photodecay rates of hypochlorous acid (HOCl) and hypochlorite (OCl-) were monotonically correlated to their molar absorption coefficients and quantum yields, and the chlorine photodecay rates were much more significantly affected by molar absorption coefficients (β = 0.949) than quantum yields (β = 0.055). An empirical model that incorporated the chlorine photodecay rate constants, quantum yields, and molar absorption coefficients of HOCl and OCl- was established, validated and then used to predict the chlorine photodecay rate at any wavelength (257.7-301.2 nm) and pH (5-10). The modelling results suggested that the maximum fluence-based rate constant (1.46 × 10-4 m2 J-1) was obtained at 289.7 nm and pH 9.95. The wavelength dependency was larger at alkaline pH than at acidic pH, and the pH dependency was the largest at the longest wavelength. The formation of hydroxyl radicals (HO·) and reactive chlorine species (RCS) decreased with increasing wavelength at pH 6, and increased with increasing wavelength at pH 7. More HO· was formed at pH 6 than pH 7, but RCS showed the opposite pH-dependency. The findings in this study provide the fundamental information in selecting UV-LEDs with specific wavelength for enhancing/optimizing chlorine photodecay and/or its radical generation at different pHs in real-world applications.
... It is worth mentioning that the UV alone performed generally in the removal of 2,4,6-TCA, but the combination of UV and chlorine showed superiority at the wavelength of 275 nm. To further investigate the effect of UV wavelength on 2,4,6-TCA degradation, trials were conducted at different pH values of 5.0-9.0, and the UV fluence-based rate constant (k obs-UV ) was calculated with Eq. (1) (Yin et al. 2018). ...
... In acidic condition, HOCl is the dominant chlorine species, while in basic condition, OCl − is the dominant one. The molar absorption and photolysis quantum yield of HOCl and OCl − are significantly different at different wavelengths (Yin et al. 2018). In acidic conditions, HOCl is the dominant chlorine species, and the absorbance coefficient of HOCl at 254 nm is greater than that at 285 and 275 nm (Fig. S5), while in alkaline conditions, OCl − is the dominant chlorine species. ...
... In acidic conditions, HOCl is the dominant chlorine species, and the absorbance coefficient of HOCl at 254 nm is greater than that at 285 and 275 nm (Fig. S5), while in alkaline conditions, OCl − is the dominant chlorine species. Therefore, the distribution of chlorine species resulted in a higher absorbance coefficient of 254 nm under acidic conditions and of 275 and 285 nm under alkaline conditions, which was consistent with previous research (Yin et al. 2018). ...
2,4,6-Trichloroanisole (2,4,6-TCA) has aroused a special concern for their odor problem and potential threats. In this study, the degradation of 2,4,6-TCA by UV/chlorination with different UV sources was compared, including low-pressure mercury lamp (LPUV, 254 nm) and ultraviolet light-emitting diode (UV-LED, 275 and 285 nm). The maximum removal of 2,4,6-TCA can be achieved by 275-nm UV-LED/chlorination in neutral and alkaline conditions which was 80.0%. The reaction, kinetics, and water matrix parameters on 2,4,6-TCA degradation were also evaluated. During UV-LED (275 nm)/chlorination, 2,4,6-TCA degradation was mainly caused by direct UV photolysis and indirect hydroxyl radical (HO·) oxidation, while reactive chlorine radicals (RCSs) had a negligible contribution. The second-order rate constant between HO· and 2,4,6-TCA was determined as 3.1 × 109 M−1 s−1. Increasing initial chlorine dosage and decreasing 2,4,6-TCA concentration or pH value significantly promoted 2,4,6-TCA degradation during UV/chlorination process. The presence of natural organic matter (NOM) and bicarbonate (HCO3−) can inhibit 2,4,6-TCA degradation, while chloride ion (Cl−) had a negligible effect. The kinetic model for 2,4,6-TCA degradation was established and validated, and the degradation pathways were proposed based on the identified intermediates. Furthermore, UV-LED (275 nm)/chlorination also exhibited a promising effect on 2,4,6-TCA removal in real water, which can be used to control 2,4,6-TCA pollution and odor problems.
... At pH > pH PZC , the surface of UV-treated TiO 2 -SiO 2 is negatively charged and at pH < pH PZC , the surface of UV-treated TiO 2 -SiO 2 is positively charged. Therefore, the adsorption of the MB + cationic dye is favored at pH > pH PZC (alkaline condition) due to the presence of a large number of active sites such as TiO − , HO − and SiO − [34,35]. ...
... This is because the HOCl species, which exists predominantly at pH 4, is a more potent oxidant than the OCl − species, which exists predominantly at pH 10. Furthermore, Yin et al. (2018) [36] documented that HOCl has a larger quantum yield of about 1.0-1.5 compared to OCl − , which has a quantum yield of 0.87-1.3. ...
... Moreover, Yin et al. (2018) [35] documented that fewer radicals (i.e., HO • and Cl • ) are produced by increasing the pH of the solution during the UV/Cl 2 process. Thus, the neutral and alkaline conditions had only 50% and 52% degradation efficiency of MB compared to the acidic condition (92%). ...
The titanium dioxide-silicon dioxide (TiO2-SiO2) nanocomposite used for the study was synthesized using a sol-gel method followed by UV-treatment. The physicochemical properties of the synthesized catalyst, TiO2-SiO2 were characterized by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), ultraviolet-visible diffuse reflectance spectroscopy (UV-vis DRS) and photoluminescence (PL). The photocatalytic degradation of methylene blue (MB) dye was evaluated in the presence of TiO2-SiO2 and reactive chlorine species (RCS) under experimental conditions. By comparing the important reaction processes in the study, including photocatalysis, chlorination and photocatalytic chlorination, it was found out that the process of photocatalytic chlorination had the highest photodegradation efficiency (95% at 60 min) of the MB under optimum reaction conditions (MB = 6 mg L−1, catalyst = 0.1 g and pH = 4). The enhanced removal of MB from the aqueous medium was identified because of the synergy between chlorination and photocatalysis activated in the presence of TiO2-SiO2. The mechanism of the photocatalytic chlorination process was scrutinized in the presence of various RCS and reactive oxygen species (ROS) scavengers. Based on the experimental data attained, Na2S2O3 exhibited the highest inhibitory effect on the degradation efficiency of MB, indicating that the RCS is the main contributor to visible light-induced photodegradation of MB.
... Fluence-based pseudo first-order rate constant k f ′ were used instead of time-based rate constant k′ because the four UV devices have different UV fluence rates identified earlier in Table S3 and the corresponding k f ′ are listed in Table 2. Fig. 7A shows that k f ′ had an upward trend as pH was increased -deviating from the trend observed at 254 nm. Yin et al. (2018) propounded that the wavelength dependency of chlorine photolysis on its molar absorption coefficients were higher at alkaline pHs -indicating that (free) chlorine photo decay rates were more dependent on their molar absorption coefficients (ε) than their quantum yields (Φ). Since OClhas a higher molar absorption coefficient compared to HOCl at Table 5 The performance of the generated models with different hidden layer sizes and learning rates through 30 iterations. ...
... Collectively, the UV-LED 275 systems triumphed all other UV systems in the application of chlorine photolysis of TMP at alkaline pHs. The k f ′ trend obtained is not congruent with Yin et al.'s hypothesis (Yin et al., 2018) If chlorine photo decay rates were extremely dependent on their molar absorption coefficients (ε), the performance of chlorine photolysis would have been: UV(310)/Cl 2 > UV(275)/Cl 2 > UV(265)/Cl 2 > UV (254)/Cl 2 because ε OCl, 310 > ε OCl, 275 > ε OCl, 265 > ε OCl, 254 (Fig. S3). This implies that chlorine photolysis could have a higher dependency on not only molar absorptivity, but also its quantum yields at higher pH. ...
... This implies that chlorine photolysis could have a higher dependency on not only molar absorptivity, but also its quantum yields at higher pH. The trend in quantum yields of OClacross different wavelengths (Φ OCl, λ ) is the polar opposite to its molar absorption coefficient: Φ OCl, 254 = 1.058 > Φ OCl, 265 = 0.995 > Φ OCl, 275 = 0.937 > Φ OCl, 310 = 0.737 (Yin et al., 2018). Although OCl − at 275 nm does not have the highest molar absorption coefficient (as compared to 310 nm) nor does it have the highest quantum yield to decompose the most OClto form radical chlorine species upon UV irradiation (as compared to 254 nm), the UV(275)/Cl 2 systems provide a sweet spot for OClto have the best of both worlds. ...
The UV/Cl2 process (also known as chlorine photolysis, which is the combination of chlorine and simultaneous irradiation of UV light) is conventionally applied at acidic mediums for drinking water treatment and further treatment of wastewater effluents for secondary reuse. This is because the quantum yield of HO• from HOCl (ϕHO•, 254 = 1.4) is greater than the one from OCl⁻ (ϕHO•, 254 = 0.278) by approximately 5 times. Moreover, chlorine photolysis in acidic mediums also tends to have lower radical quenching rates than that of their alkaline counterparts by up to 1000 times. The aim of this research is to investigate the applicability of the UV/Cl2 process by assessing its efficacy on the removal of trimethoprim (TMP) at not only acidic to neutral conditions (pH 6-7), but also alkaline mediums (pH 8-9). At alkaline pH, free chlorine exists as OCl⁻ and since OCl⁻ has a higher molar absorption coefficient as compared to HOCl at higher wavelengths, there would be higher reactive chlorine species (RCS) formation and contribution. TMP removal followed pseudo-first order kinetics and depicted that a maximum fluence based constant (kf′ = 0.275 cm²/mJ) was obtained using 42.25 μM (3 mg/L) of chlorine at pH 9, with an irradiation of 275 nm. At alkaline conditions, chlorine photolysis performance followed the trend of UV (275)/Cl2 > UV (265)/Cl2 > UV (310)/Cl2 > UV (254)/Cl2. RCS like Cl•, Cl2−• and ClO• contributed to the degradation of TMP. When the pH was increased from 6 to 8, contribution from hydroxyl radicals (HO• ) was decreased whilst that of RCS was increased. Application of UV (310)/Cl2 had the highest HO• generation, contributing to TMP removals up to 13% to 48% as compared to 5% to 27% in UV (254, 265, 275)/Cl2 systems at pH 6-9. Artificial neural networks modelling was found to be able to verify and predict the contribution of HO• and RCS conventionally calculated via the general kinetic equations in the UV/Cl2 system at 254, 265, 275 and 310 nm.
... Moreover, UV-LEDs are expected to be used as alternative light sources in UV disinfection areas [28,30], due to their additional advantages, such as long lifespan, compact size, lack of need for warm-up time and absence of mercury. Previous research had reported that longer UV wavelengths were more suitable for chlorine photolysis [18,31] in water disinfection at neutral and alkaline pH, but the relationship between wavelength dependency and both chlorine photolysis rate and its disinfection efficiency has not been established. There has been no systematic study on chlorine photolysis rate at different UV wavelengths and its application to seawater disinfection. ...
... UV fluences, respectively. This result may be attributable to the ability of UV-LEDs to achieve UV light of a specific wavelength, which could be selected to match a higher absorption spectra of free chlorine (near 292 nm), to achieve a faster rate of chlorine photolysis [18,31]. Various microorganisms have different susceptibilities, which may be attributable to different modes of action of the individual components of the combined UV/chlorine treatment. ...
... The ability of chlorine photolysis to generate power-reactive oxidants depends on the irradiation conditions, such as UV wavelength or pH [14,31]. In the current study, the wavelength-dependence of Chlorine dosage (mg L − 1 ) ; UV × Cl 2 : interaction term of UV wavelength and chlorine dosage; confidence interval: 95%. ...
Unwanted proliferation of and adhesion by microalgae, as involved in fouling and species invasion, are important problems across many maritime operations and coastal industrial activities. The algicidal mechanisms of ultraviolet light-emitting diodes (UV-LEDs) and the synergistic algicidal effects between UV-LEDs and chlorine treatments in seawater were investigated in this study. Tetraselmis sp. was employed as the challenge organism. Five UV wavelengths (265, 280, 285, 300 and 315 nm) and three chlorine dosages (0.6, 0.8 and 1 mg L⁻¹) were examined in a factorial experiment to optimize the disinfection performance achieved by combined UV-LED/chlorine treatments in seawater. Overall, the combined treatments showed a more effective inactivation of the microalgae than did the single treatments. Treatment with UV285 combined with 0.8 mg L⁻¹ chlorine resulted in 3.12-log inactivation at 100 mJ cm⁻² (i.e. 2.34 Einsteins L⁻¹, 67 s). This optimal combination showed a higher inactivation rate constant and lower energy consumption (EE,2) than did the other treatments. The contribution of UV-LEDs, chlorine and hydroxyl radical (·OH) in the inactivation of Tetraselmis sp. by the combined UV-LED/chlorine disinfection process was investigated to illustrate the mechanism of this synergistic process, the results indicated that ·OH generated by photolysis were not the only contributors to the inactivation of the microalgae. Although a UV300/chlorine treatment demonstrated the highest chlorine photolysis rate, the inactivation constant of this treatment was lower than that of UV285/chlorine. These results demonstrated a synergistic algicidal effect of the combined disinfection system caused by exposure to the 285 nm UV-LED and low dosages of chlorine, which could provide an important basis for further developments and possibly industrial applications in the future.
... while in basic condition, OClis the dominant one. The molar absorption and photolysis quantum yield of HOCl and OClare signi cantly different at different wavelengths(Yin et al. 2018). In acidic conditions, HOCl is the dominant chlorine species, and the absorbance coe cient of HOCl at 254 nm is greater than those at 285 and 275 nm(Fig. ...
... While in alkaline conditions, OClis the dominant chlorine species. Therefore, the distribution of chlorine species resulted in a higher absorbance coe cient of 254 nm under acidic conditions, and of 275 and 285 nm under alkaline conditions, which was consistent with previous research(Yin et al. 2018). ...
2,4,6-Trichloroanisole (2,4,6-TCA) has aroused a special concern for their odor problem and potential threats. In this study, the degradation of 2,4,6-TCA by UV/chlorination with different UV sources was compared, including low-pressure mercury lamp (LPUV, 254 nm) and ultraviolet light-emitting diode (UV-LED, 275 and 285 nm). The maximum removal of 2,4,6-TCA can be achieved by 275 nm UV-LED/chlorination in neutral and alkaline conditions was 80.0%. The reaction, kinetics and water matrix parameters on 2,4,6-TCA degradation were also evaluated. During UV-LED (275 nm)/chlorination, 2,4,6-TCA degradation was mainly caused by direct UV photolysis and indirect hydroxyl radical (HO•) oxidation, while reactive chlorine radicals (RCSs) had a negligible contribution. The second-order rate constant between HO• and 2,4,6-TCA was determined as 3.1×10 ⁹ M ⁻¹ s ⁻¹ . Increasing initial chlorine dosage and decreasing 2,4,6-TCA concentration or pH value significantly promoted 2,4,6-TCA degradation during UV/chlorination process. The presence of natural organic matter (NOM) and bicarbonate (HCO 3 ⁻ ) can inhibit 2,4,6-TCA degradation, while chloride ion (Cl ⁻ ) had a negligible effect. The kinetic model for 2,4,6-TCA degradation was established and validated, and the degradation pathways were proposed based on the identified intermediates. Furthermore, UV-LED (275 nm)/chlorination also exhibited a promising effect on 2,4,6-TCA removal in real water, which can be used to control 2,4,6-TCA pollution and odor problems.
... HOCl reacts with OH • and Cl • slowly compared to OCl − (Eqs. 13 and 14), which means more OH • and Cl • are present in solution to oxidize the RB19 (Yin et al. 2018c;Lee et al. 2018). ...
... Sulfite traps OH • and Cl • (Eqs. 24 and 25) and reduces their oxidizing power by converting them to other radicals (Nikravesh et al. 2020;Yin et al. 2018c) (20) Sulfite anion ( SO 2− 3 ) acts as a strong radical scavenger which prohibited the degradation of RB19 strongly. The k app reduced by 99% for 5 and 10 mM of sulfite anion, approximately. ...
In recent years, advanced oxidation processes (AOPs) have indicated the greatest potential in the removal of stable organic compounds, including dyes. In this study, the ultraviolet light-emitting diodes (UV-LEDs) combined with chlorine was evaluated to remove Reactive Blue 19 (RB19) dye from aqueous solution. The effect of key experimental parameters including pH, initial chlorine concentration, initial dye concentration, and reaction time on the performance of UV-LED irradiation, UV-LED/chlorine, and the chlorination method for the removal of RB19 was studied in this research. Results showed that, more than 99% of RB19 was removed after 30 min of reaction time under optimized conditions (pH = 5, [chlorine] = 300 μM, and [RB19] = 20 mg L−1) with apparent kinetic rate constant (kapp) of 17.1 × 10−2 min−1 in UV-LED/chlorine process. However, for the chlorination method, removal efficiency was 64.7% (kapp = 3.41 × 10−2 min−1) with an apparent kinetic rate constant of 0.0341 min−1. Results also showed that UV-LED irradiation is not effective at all in removing RB19. The scavenging assay showed that OH• radicals (67.23%) had the highest contribution in RB19 removal in UV-LED/chlorine process while Cl• (17.82%) and Cl−∙2 (8.56%) had a minor role in the degradation of the dye. The RB19 degradation kinetics analysis revealed that the processes of UV-LED/chlorine and chlorination degradation followed the pseudo-first-order kinetic model. In this study, the impact of chloride, nitrate, bicarbonate, carbonate, sulfate, and sulfite anions on the performance of the process was investigated. It indicated that sulfite anion has the most negative impact on the RB19 removal process. By evaluating the synergistic effect between UV-LED lamp and chlorine, a synergy index of 5.0 was obtained for the UV-LED/chlorine process. The results presented that the UV-LED/chlorine process has a better performance than each of them alone and has the necessary efficiency for RB19 removal. Measuring COD reported its removal efficiency of 98% during the UV-LED/chlorine process under optimized conditions. Experiments continued with textile factory wastewater and indicated 30.9% of its COD removed after treatment when 1.0 μM chlorine was used.
... Recently, UVC light-emitting diode (UVC LED) has emerged as a novel and reliable technology to inactivate microorganisms due to several benefits such as mercury-free production, flexible wavelengths, long life, small and compact size, unlimited on/off cycling, energy efficiency [20]. On the other hand, UV-based various advanced oxidation processes (AOPs) have been widely investigated in various studies to degrade various micro-pollutants [21][22][23][24][25][26]. ...
... The presence of different radicals may boost the degradation of micro-pollutants. The fluence-based photo decay of HOCl and OCl − varies with wavelength and pH ( Figure S3) [23]. ...
Extensive application of antibiotics in the last few decades has promoted selective pressure to disseminate antibiotic resistance in the aquatic environment. Antibiotic-resistant genes (ARGs) and bacteria (ARB) were detected in wastewater, effluent from wastewater treatment plants (WWTPs), surface water, and finished water from drinking water treatment plants (DWTPs), and even in tap-water. Hence, the spread of antibiotic resistance has become a threat to public health. Traditional low-pressure UV lamps (LP UV) removed ARGs, but the fluences required to deactivate ARGs were very high. The efficiency of LP UV to diminish ARGs was more strongly correlated with adjacent thymine number rather than amplicon length since cyclobutane pyrimidine dimers (CPDs) are the significant lesions generated due to UVC-caused damage to DNA. Co-exposure of LP UV with other radicals in the UV based advanced oxidation processes (AOPs) exhibited some improvement in reducing concentrations of ARGs, specifically extracellular ARGs (eARGs). However, the exhaustion of radicals by various cellular components, such as cell membrane, cell wall, and cytoplasmic protein, has limited the widespread application of UV based AOPs. In LP UV-based AOPs, DNA damage induced by LP UV was identified as a major mechanism for eliminating ARG transformation. UVC light-emitting diodes (UVC LEDs) are increasingly being investigated for disinfection performances. 265 nm LED has shown better performance in controlling antibiotic resistance than 285 nm. Further researches are required to assess the performances of UVC LEDs and UVC LEDs based AOPs to deactivate ARGs.
... Additionally, there was a higher concentration of free chlorine in the E system under alkaline conditions than in the UV-E system, which was attributed to the higher molar absorption coefficient of OClcompared to HOCl. The proportion of OClincreased as the pH increased, consequently leading to a higher photodecay (UV/chlorine conversion) rate and reducing the concentration of free chlorine compared with the E system [39]. ...
... Additionally, there was a higher concentration of free chlorine in the E system under alkaline conditions than in the UV-E system, which was attributed to the higher molar absorption coefficient of OClcompared to HOCl. The proportion of OClincreased as the pH increased, consequently leading to a higher photodecay (UV/chlorine conversion) rate and reducing the concentration of free chlorine compared with the E system [39]. In the E system, the k value of the three sulfonamides ( Figure 5) could be further explained by considering the ratio of HOCl and OClunder different pH conditions (Figure 6d). ...
Ultraviolet photolysis (UVC, 254 nm) was coupled with an electrochemical oxidation process to degrade three kinds of veterinary sulfonamide (sulfamethazine [SMZ] tablets, sulfamonomethoxine [SMM] tablets, and compound sulfamethoxazole [SMX] tablets). The treatment was applied using a flat ceramic microfiltration membrane to study the effects of photocatalysts. The effectiveness of degradation of the three sulfonamides was evaluated under different conditions. Dissolved oxygen was provided via aeration, but this resulted in a large decrease in the degradation effectiveness due to the inhibition of free chlorine electrogeneration. The photocatalysts had no promotional effect on sulfonamide removal from wastewater due to reduced UV penetration. Because of the different distribution coefficients of sulfonamides, UV irradiation had different effects on different sulfonamide species. For SMZ and SMM, anionic species exhibited a higher degradation rate, whereas for SMX, degradation was most effective for neutral species. In addition, the free chlorine yield increased as the pH increased. Free chlorine conversion reactions occurred under UV irradiation, with the reactions possibly restrained by sulfonamides. Reactive chlorine species promoted SMM degradation. Compared to UV irradiation or electrochemical oxidation alone, the UV/in-situ electrochemical oxidation process was more effective and is suitable for treating real wastewater under various environmental pH levels.
... Combination of UV-LEDs and chlorine has been studied by several studies, due to the higher molar absorbance of OCl − at wavelengths of 260-300 nm. Yin et al. (2018) reported the promoted chlorine photodecay and radical production with increased wavelengths at pH 7 by using four UV-LEDs as lights sources. Several studies observed higher synergistic efficiency on micropollutants and NOM degradation in the LED UV/chlorine process than in the LP UV/chlorine process (Kwon et al., 2018;Li et al., 2018;Yin and Shang, 2020;Zou et al., 2019). ...
... Recently, several studies revealed the major contribution of ClO • in degradation some micropollutants by the UV/chlorine process Wu et al., 2017;Yin et al., 2018). The modeled results of ClO • were shown in Table S4 and Table S5. ...
Recently, ultraviolet light-emitting diodes (UV-LEDs) and chlorine combined system has been employed as an emerging advanced oxidation process. However, UV-LEDs were commonly considered as monochromatic UV sources. In this study, the obvious quantum yields of chlorine photolysis under 265 nm and 280 nm LEDs irradiations were investigated with treating LEDs as polychromatic UV sources. Particularly, Φobs-poly of HOCl and OCl⁻ for 265 nm LED were found to be 1.50 and 0.70 mol E⁻¹, respectively, whereas Φobs-poly of HOCl and OCl⁻ for 280 nm LED were 1.28 and 0.64 mol E⁻¹, respectively. It was identified that Φobs-poly were 5.66–14.63 % lower than Φobs-mono. This suggests that obvious quantum yield using peak emission wavelength would overestimate the true quantum yield. The production of radical species in LED UV/chlorine systems were determined by the degradation of BA, and illustrated by a mathematical model. Different trends were observed for 265 nm and 280 nm LED UV/chlorine systems as pH increased from 5.0 to 10.0. As pH increased, the formation of OH continuously decreased in both 265 nm and 280 nm LED systems. The formation of Cl increased at neutral pH and more Cl and OH were formed due to the higher molar absorbance coefficient at 280 nm. The chlorine dose-dependent effects on radical productions at pH of 5.0, 7.5 and 10.0 were also assessed. At pH of 5.0, OH was the main radical product and had linear correlation with chlorine dose. At pH of 7.5, the productions of OH and Cl showed similar profiles that increased rapidly at low chlorine dosage and then slowed down.
... Current researches commonly used low-and medium-pressure mercury lamps as the main UV sources, which exist some drawbacks such as relatively short life-time, toxic mercury containing, fragility and oversized [15][16][17]. To date, the Minamata Convention on Mercury has been signed by 128 countries to gradually abandon the use of mercury [18]. ...
... Furthermore, UV-LED can be turned on and off at high and adjustable frequencies, which may improve disinfection efficiency [15]. In the future, the wall plug efficiency and service life of UV-LED are expected to greatly heighten, and prices will also drop [17]. These advantages imply that UV-LED have the potential to be widely applied in disinfection and AOPs [20,21]. ...
In this study, the degradation of carbamazepine (CBZ) was comparatively investigated by UV-LED/persulfate (PS) and UV-LED/H2O2 processes in the presence of trace Cu²⁺ at neutral pH. Results showed that the introduction of trace Cu²⁺ accelerated CBZ degradation and more favorable performance was achieved in UV-LED/PS process. As oxidants dosage raised from 0.2 to 1.0 mM, the pseudo-first-order rate constant (kobs) value of CBZ both linearly increased in UV-LED/PS/Cu²⁺ and UV-LED/H2O2/Cu²⁺ processes. The promotion effect of Cu²⁺ at low level was more significant than that at high level in both processes at neutral pH. With reducing solution pH from 7.2 to 3.1, the kobs value dramatically increased from 0.136 to 0.399 min⁻¹ in UV-LED/PS/Cu²⁺ process, while its value exhibited the imperceptible change in UV-LED/H2O2/Cu²⁺ process. OH functioned as the major contributor to CBZ degradation in two processes in pH range of 3.1–7.2. The addition of humic acid (HA), HCO3⁻ and Cl⁻ impeded CBZ degradation in UV-LED/H2O2/Cu²⁺ process. As for UV-LED/PS/Cu²⁺ process, the kobs value significantly increased in the presence of Cl⁻, while the introduction of HA and HCO3⁻ restrained the removal. Total eleven degradation intermediates of CBZ were identified and a reasonable degradation pathway was proposed based on HPLC-MS/MS analysis and the calculation of Gaussian program. The acute toxicities of reacted solution enhanced in both processes within 15 min, especially in UV-LED/PS/Cu²⁺ process. In terms of electrical energy per order value, UV-LED/PS/Cu²⁺ process was found to be more cost-effective than UV-LED/H2O2/Cu²⁺ process. It was also noticed that UV-LED/PS process was more effective for CBZ removal in natural water in the presence of trace Cu²⁺.
... It is well known that HClO predominates at pH < 7.5, while ClO − predominates at pH > 7.5 (Hua and Reckhow, 2008). Hence, the solution pH affects the chlorine photolysis and results in varied yield and distribution profiles of the DBPs, where the dominant species is •OH at acidic pH and ClO• at alkaline pH (Li et al., 2016a;Yin et al., 2018). Lowering pH from 8.0 to 6.5 facilitated the formation of adsorbable organohalide in a full-scale UV/Cl 2 , due to more efficient degradation of organics by •OH at pH 6.5 (Wang et al., 2019). ...
Combined processes of light-emitting diodes ultraviolet (LED UV) and chlorination (Cl2) are alternative disinfection technologies in drinking water, while the formation of disinfection by-products (DBPs) needs to be evaluated. This study investigated the impacts of critical water matrix factors on the DBP formation in the combined processes. Moreover, the correlation between the degraded natural organic matter (NOM) and the formed DBP was studied. Simultaneous UV/Cl2 outperformed single Cl2 and sequential combined processes in degrading humic acids (HA) and resulted in the highest DBP yield. Iodide at 5–20 μg/L and bromide at 0.05–0.2 mg/L slightly affected the degradation of organics, while increased the formation of brominated DBPs up to 36.6 μg/L. pH 6 was regarded as the optimum pH, achieving high efficiency of HA degradation and a lower level of total DBP formation than pH 7 and 8 by 11 % and 24 %, respectively. Compared to HA samples (46.8–103.9 μg/L per mg/L DOC), NOM in canal water were less aromatic and yielded fewer DBPs (19.6 and 21.2 μg/L per mg/L DOC). However, the extremely high bromide in site 1 samples (18.6 mg/L) shifted the chlorinated DBPs to their brominated analogues, posting around 1 order of magnitude higher levels of toxicities than HA samples. The reduction of absorbance at 254 nm (UV254) correlated with all DBP categories in HA samples, while the correlation coefficients were compromised when included in the canal samples. For the first time, this study found that parallel factor analysis (PARAFAC) would neglect the fluorescence change caused by iodide/bromide in UV/Cl2, while the changes could be captured by self-organising map (SOM) trained with full fluorescence spectra. Fluorescence Ex/Em pairs were proposed to predict DBP formation, suggesting a potential method to develop an online monitoring system for DBPs.
... It is well known that HClO predominates at pH < 7.5, while ClO − predominates at pH > 7.5 (Hua and Reckhow, 2008). Hence, the solution pH affects the chlorine photolysis and results in varied yield and distribution profiles of the DBPs, where the dominant species is •OH at acidic pH and ClO• at alkaline pH (Li et al., 2016a;Yin et al., 2018). Lowering pH from 8.0 to 6.5 facilitated the formation of adsorbable organohalide in a full-scale UV/Cl 2 , due to more efficient degradation of organics by •OH at pH 6.5 (Wang et al., 2019). ...
... Medium-pressure mercury vapor UV (MPUV) lamps with intense UV light can achieve the AOP dose within a shorter time (e.g., 2 min in this study) and better simulate the practical situation (e.g., a few seconds to a minute or two). Considering the common use of MPUV lamps in plants, 26 and the wavelength-dependent radical generation during chlorine photolysis, 27 it is of significance to evaluate DBP alteration related to MPUV/chlorine. This study was designed to evaluate changes in the formation of AOX and individual DBPs (including regulated and several emerging DBPs) during UV/chlorine treatment using MPUV lamps/ chlorine and subsequent postchlorination. ...
... 13,38 Thus, the contributions for • OH in competitive kinetic experiments calculated by NB are probably overestimated because NB is deemed to only react with • OH. 7,18,35 Different pH, wavelength, and coexisting components in the UV/ chlorine process will lead to the variation of these reactive radicals, which can affect the reaction rates and transformation products of selected contaminants. 17,39 Further studies should investigate the interaction mechanism using theoretical calculations and experimental verification. ...
The UV/chlorine process, by combining chlorination with UV irradiation, has been recently considered as a highly efficient advanced oxidation process (AOP) technology in water treatment. Nitrobenzene (NB), benzoic acid (BA), and p-chlorobenzoic acid (pCBA) are widely used as model probe compounds in the UV/chlorine system to calculate the second-order rate constants of the specific radical reaction with target contaminates by a competitive kinetics method. A comprehensive understanding of probe compounds' reaction mechanism with reactive radicals is critical for investigation of the UV/chlorine reaction system. Here, we evaluated the radical-mediated reaction kinetics and mechanism of NB, BA, and pCBA in the UV/chlorine process using theoretical calculations and experimental studies. The main reactive radicals •OH, •ClO, and •Cl in the UV/chlorine process for the initial reaction with NB, BA, and pCBA can be explained by H-abstraction and addition pathways. The ΔE 0,≠ values for the •OH reaction with NB, BA, and pCBA were in the range of 5.0-8.0, 3.7-8.2, and 3.4-8.2 kcal mol-1, respectively. The ΔE 0,≠ values for •ClO and •Cl reactions with these three probe compounds were higher than those of •OH, indicating slower •ClO- and •Cl-initiated reactions than that of the •OH-initiated reaction. The theoretically calculated radical-mediated reaction kinetic rate constants (k CP C) for NB, BA, and pCBA were 4.58 × 10-3, 1.28 × 10-2, and 1.6 × 10-2 s-1, respectively, which was consistent with the experimentally determined pseudo-first-order rate constant (k CP RR) in the UV/chlorine process. Interestingly, theoretical calculations showed that •ClO and •Cl played an important role in subsequent reactions of NB-OH radicals, converting to hydroxylated and chlorinated products, which were further confirmed by experimental products' identification. The findings from this study indicated that quantum chemistry calculations provide an effective means to investigate the reaction kinetics and mechanism of chemicals in the UV/chlorine process.
... However, it is known that HO • is >5-fold the concentration of Cl • and thus contributes more to the disinfection efficiency [41]. The photodecay rates during the UV/chlorine AOP are related to the wavelength-dependent molar absorption coefficient [42]. The photodecay rate of chlorine has also been shown to increase with increasing pH at any wavelength. ...
Extensive use of antibiotics for humans and livestock has led to an enhanced level of antibiotic resistance in the environment. Municipal wastewater treatment plants are regarded as one of the main sources of antibiotic-resistant bacteria (ARB) and antibiotic resistance genes (ARGs) in the aquatic environment. A significant amount of research has been carried out to understand the microbiological quality of wastewater with respect to its antibiotic resistance potential over the past several years. UV disinfection has primarily been used to achieve disinfection, including damaging DNA, but there has been an increasing use of chlorine and H2O2-based AOPs for targeting genes, including ARGs, considering the higher energy demands related to the greater UV fluences needed to achieve efficient DNA damage. This review focuses on some of the most investigated processes, including UV photolysis and chlorine in both individual and combined approaches and UV advanced oxidation processes (AOPs) using H2O2. Since these approaches have practical disinfection and wastewater treatment applications globally, the processes are reviewed from the perspective of extending their scope to DNA damage/ARG inactivation in full-scale wastewater treatment. The fate of ARGs during existing wastewater treatment processes and how it changes with existing treatment processes is reviewed with a view to highlighting the research needs in relation to selected processes for addressing future disinfection challenges.
... Although some other reactive oxygen species like singlet oxygen ( 1 O 2 ) also have slow generation from NOM photolysis , compared with the large number of radicals generated by H 2 O and oxidants photolysis, there is no need for the contribution of 1 O 2 to be concerned. Some previous investigations reported the quantum yields of oxidant photolysis would be UV-wavelength dependent (Luo et al., 2016;Wang et al., 2017;Yin et al., 2018). However, the quantum yields under VUV irradiation are speculated to be the same as those under UV irradiation with limited experimental evidence or maintained as unknown. ...
The use of vacuum-UV/UV (185/254 nm) for trace organic contaminants (TOrCs) elimination during wastewater treatments has attracted much attention. Advanced oxidation processes which combine VUV/UV and additional oxidants (vacuum-UV/UV-based advanced oxidation processes, VUV/UV-AOPs) provide a promising method for eliminating recalcitrant and toxic TOrCs for wastewater reclamation. Researches in this area are increasing but the promoting effects, mechanisms, and influencing factors have not been well summarized. A comprehensive discussion of the limitations of this technique and future research directions is needed. VUV/UV-AOPs have considerable synergistic effects by increasing usage of VUV/UV photons and the oxidant, which increases radical generation. In terms of elimination kinetics, VUV/UV-AOPs outperform conventional UV-AOPs and VUV/UV processes in most cases; a 1.2–87.7-fold increase of the fluence-based kinetic constant is achieved. In terms of energy efficiency per order (EE/O) of TOrCs elimination, the EE/O of VUV/UV-AOPs only accounts for 4% of UV-AOPs and 63% of VUV/UV. However, VUV/UV-AOPs still need to be further investigated. Firstly, although VUV and UV processes have similar radical formation pathways, limited information is available on the quantum yields of photolysis and radical formation of oxidants under VUV irradiation. Secondly, optimization of VUV/UV-AOPs operating conditions, especially oxidant dosage and water-flow patterns, is needed. Thirdly, VUV/UV-AOPs are significantly inhibited by organic and inorganic matters, but the mechanisms of inhibition on VUV/UV scattering, radical quenching, and radical conversion are not well understood. Such inhibition suggests that the use of VUV/UV-AOPs would be limited to relatively clear water treatment, e.g., reverse osmosis effluent for potable water reuse and ultrapure water production. Related research is needed to establish a clearer scheme for VUV/UV-AOPs in terms of the spatial distribution of radical species in the VUV/UV irradiation system and the relevant optimization method for promoting oxidation performance.
... As water molecules dissociate to hydroxyl radicals when the wavelength of the irradiated UV is less than 190 nm [35], the CTD in our direct photolysis experiment was removed by absorbing photons. The molar absorptivity (ε) of CTD was determined as 15400 M − 1 cm − 1 (Fig. S3), which was significantly higher than that of HOCl (101 M − 1 cm − 1 ) and OCl − (365 M − 1 cm − 1 ) [36]. And the quantum yield (Φ CTD ) value of the CTD was 0.061 ± 0.002 (Text S3). ...
Combining UV and free chlorine (UV/chlorine) is an efficient advanced oxidation process for the abatement of recalcitrant organic compounds in drinking water and wastewater. This study investigated the degradation of a neonicotinoid insecticide, clothianidin (CTD), by UV/chlorine treatment. The free chlorine concentration was optimized at 160 μM, and 90.1 ± 0.4% of 40 μM CTD was degraded after 300 s of treatment. Radical quenching tests using tert-butyl alcohol, Cl⁻, HCO3⁻, and N3⁻ indicated that chlorine monoxide (ClO•) was the main radical species for CTD degradation. The second-order rate constants of CTD reacting with ClO• (kClO•,CTD = 7.3 ± 0.1 × 10⁹ M⁻¹s⁻¹) was 4.3 times higher than that for •OH (k•OH,CTD = 1.7 ± 0.2 × 10⁹ M⁻¹s⁻¹). The presence of humic acid inhibited CTD degradation by filtering UV and scavenging ClO•. The pH was optimized at 7, and the overall reaction rate constant (k′) was 2.35 ± 0.02 × 10⁻² s⁻¹ (half-life = 0.49 min). Degradation products identified during the UV/chlorine treatment were 1-methyl-3-nitroguanidine ([M + H]⁺ = 118.9), nitroguanidine ([M + H]⁺ = 105.1), methylguanidine ([M + H]⁺ = 74.3), and clothianidin urea ([M + H]⁺ = 206). The detailed time-dependent concentrations of the generated products under different pH conditions were also provided. The results suggest that the UV/chlorine treatment can be an efficient strategy for CTD degradation.
... Recently, it has been reported that the removal of perfluorooctanoic acid by UV/Fe(NO 3 ) 3 (10 mM) process, achieved a 60 % removal within 30 min (Yuan et al., 2020). However, the present research on UV/Fe 3+ -based processes usually use UV 254 as the main light source, but with defects such as toxic mercury presence, relatively short life time, fragility, and oversize (Yin et al., 2018;Cai et al., 2021). As the promising UV light sources, UV light-emitting diodes (UV-LED) have been extensively studied recently and are regarded as alternatives to conventional UV mercury lamps because of their longer life-time, more flexible configuration, absence of harmful mercury, and less energy requirement (Zou et al., 2019;Li et al., 2020). ...
N,N-Diethyl-3-methyl benzoyl amide (DEET) has been detected as an emerging pollutant in various water bodies because of its widespread use as an insect repellent. In this study, the combination of UV-LED275 and iron-containing coagulant (FeCl3) was used for the elimination of DEET in water. It was found that UV-LED275/FeCl3 (98 %) system presented a favorable removal of DEET compared with UV254/FeCl3 (59 %) and UV-LED275/Fe2(SO4)3 (81 %) processes at initial pH 3.5. DEET degradation by both UV-LED275/FeCl3 and UV-LED275/Fe2(SO4)3 processes followed pseudo-first-order kinetics with the calculated pseudo-first-order rate constants (kobs) of 0.0105 and 0.0046 cm² mJ⁻¹, respectively. The results of ESR analysis and radicals quenching experiments indicated that hydroxyl radicals (OH) and superoxide radicals (O2⁻) were responsible for DEET degradation in UV-LED275/FeCl3 process, and the former played the major role. An increase in FeCl3 dosage was beneficial to the degradation. In the UV-LED275/FeCl3 process, DEET degradation increased with a decrease in pH from 3.5 to 3.0, whereas it was almost completely suppressed with an increase in pH from 4.3 to 6.3. DEET degradation was almost unchanged after the introduction of NO3⁻, and it impeded after the addition of humic acid (HA), HCO3⁻, and SO4²⁻. The plausible degradation pathway mainly involved hydroxylation, cleavage of the C–N bond, acetylation, and dealkylation. Among the disinfection by-products (DBPs) evaluated, UV-LED275/FeCl3 pretreatment generally increased the generation of trichloromethane, chloral hydrate, dichloroacetic acid, and trichloroacetic acid, which implied that further assessment of environmental risk was needed during its practical applications.
... The UV/chlorine AOPs are being considered as alternatives to the UV/H 2 O 2 AOP for micropollutant abatement in water, wastewater, and reclaimed water. 3,6,9 The UV/chlorine AOPs have demonstrated their better performance, compared to the UV/H 2 O 2 AOP, in pilot-scale and large-scale water treatment operation. 2 Ammonia is commonly present in water sources, wastewater effluents, and reclaimed water. ...
The wavelength dependence of photoproduct formation and quantum yields was evaluated for fluorinated pesticides and pharmaceuticals using UV-light emitting diodes (LEDs) with 255, 275, 308, 365, and 405 nm peak wavelengths. The fluorinated compounds chosen were saflufenacil, penoxsulam, sulfoxaflor, fluoxetine, 4-nitro-3-trifluoromethylphenol (TFM), florasulam, voriconazole, and favipiravir, covering key fluorine motifs (benzylic-CF3, heteroaromatic-CF3, aryl-F, and heteroaromatic-F). Quantum yields for the compounds were consistently higher for UV-C as compared to UV-A wavelengths and did not show the same trend as molar absorptivity. For all compounds except favipiravir and TFM, the fastest degradation was observed using 255 or 275 nm light, despite the low power of the LEDs. Using quantitative 19F NMR, fluoride, trifluoroacetate, and additional fluorinated byproducts were tracked and quantified. Trifluoroacetate was observed for both Ar-CF3 and Het-CF3 motifs and increased at longer wavelengths for Het-CF3. Fluoride formation from Het-CF3 was significantly lower as compared to other motifs. Ar-F and Het-F motifs readily formed fluoride at all wavelengths. For Het-CF3 and some Ar-CF3 motifs, 365 nm light produced either a greater number of or different major products. Aliphatic-CF2/CF3 products were stable under all wavelengths. These results assist in selecting the most efficient wavelengths for UV-LED degradation and informing future design of fluorinated compounds.
Chlorite (ClO2-) is an undesirable toxic byproduct commonly produced in the chlorine dioxide and ultraviolet/chlorine dioxide oxidation processes. Various methods have been developed to remove ClO2- but require additional chemicals or energy input. In this study, an overlooked mitigation pathway of ClO2- by solar light photolysis with a bonus for simultaneous removal of micropollutant co-present was reported. ClO2- could be efficiently decomposed to chloride (Cl-) and chlorate by simulated solar light (SSL) at water-relevant pHs with Cl- yield up to 65% at neutral pH. Multiple reactive species including hydroxyl radical (•OH), ozone (O3), chloride radical (Cl•), and chlorine oxide radical (ClO•) were generated in the SSL/ClO2- system with the steady-state concentrations following the order of O3 (≈ 0.8 μΜ) > ClO• (≈ 4.4 × 10-6 μΜ)> •OH (≈ 1.1 × 10-7 μΜ)> Cl• (≈ 6.8 × 10-8 μΜ) at neutral pH under investigated condition. Bezafibrate (BZF) as well as the selected six other micropollutants was efficiently degraded by the SSL/ClO2- system with pseudofirst-order rate constants ranging from 0.057 to 0.21 min-1 at pH 7.0, while most of them were negligibly degraded by SSL or ClO2- treatment alone. Kinetic modeling of BZF degradation by SSL/ClO2- at pHs 6.0 - 8.0 suggested that •OH contributed the most, followed by Cl•, O3, and ClO•. The presence of water background components (i.e., humic acid, bicarbonate, and chloride) exhibited negative effects on BZF degradation by the SSL/ClO2- system, mainly due to their competitive scavenging of reactive species therein. The mitigation of ClO2- and BZF under photolysis by natural solar light or in realistic waters was also confirmed. This study discovered an overlooked natural mitigation pathway for ClO2- and micropollutants, which has significant implications for understanding their fate in natural environments.
Graphitic carbon nitride (g-C3N4) nanomaterials hold great promise in diverse applications; however, their stability in engineering systems and transformation in nature are largely underexplored. We evaluated the stability, aging, and environmental impact of g-C3N4 nanosheets under the attack of free chlorine and reactive chlorine species (RCS), a widely used oxidant/disinfectant and a class of ubiquitous radical species, respectively. g-C3N4 nanosheets were slowly oxidized by free chlorine even at a high concentration of 200-1200 mg L-1, but they decomposed rapidly when ClO· and/or Cl2•- were the key oxidants. Though Cl2•- and ClO· are considered weaker oxidants in previous studies due to their lower reduction potentials and slower reaction kinetics than ·OH and Cl·, our study highlighted that their electrophilic attack efficacy on g-C3N4 nanosheets was on par with ·OH and much higher than Cl·. A trace level of covalently bonded Cl (0.28-0.55 at%) was introduced to g-C3N4 nanosheets after free chlorine and RCS oxidation. Our study elucidates the environmental fate and transformation of g-C3N4 nanosheets, particularly under the oxidation of chlorine-containing species, and it also provides guidelines for designing reactive, robust, and safe nanomaterials for engineering applications.
In this study, novel light emitting diode (LED)-activated periodate (PI) advanced oxidation process (AOP) at an irradiation wavelength in the ultraviolet A range (UVA, UVA-LED/PI AOP) was developed and investigated using naproxen (NPX) as a model micropollutant. The UVA-LED/PI AOP remarkably enhanced the degradation of NPX and seven other selected micropollutants with the observed pseudo-first-order rate constants ranging from 0.069 ± 0.001 to 4.50 ± 0.145 min-1 at pH 7.0, demonstrating a broad-spectrum micropollutant degradation ability. Lines of evidence from experimental analysis and kinetic modeling confirmed that hydroxyl radical (•OH) and ozone (O3) were the dominant species generated in UVA-LED/PI AOP, and they contributed evenly to NPX degradation. Increasing the pH and irradiation wavelength negatively affected NPX degradation, and this could be well explained by the decreased quantum yield (ΦPI) of PI. The degradation kinetics of NPX by the UVA-LED/PI AOP in the presence of water matrices (i.e., chloride, bicarbonate, and humic acid) and in real waters were examined, and the underlying mechanisms were illustrated. A total of nine transformation products were identified from NPX oxidation by the UVA-LED/PI AOP, mainly via hydroxylation, dealkylation, and oxidation pathways. The UVA-LED/PI AOP proposed might be a promising technology for the treatment of micropollutants in aqueous solutions. The pivotal role of ΦPI during light photolysis of PI may guide the future design of light-assisted PI AOPs.
Efficient removal of 2-Methylisoborneol (2-MIB), a typical odor component, in water treatment plants (WTPs) poses a great challenge to conventional water treatment technology due to its chemical stability. In this study, the combination of ultraviolet light-emitting diode (UV-LED) and chlorine (UV-LED/chlorine) was exploited for 2-MIB removal, and the role of ultraviolet (UV) wavelength was investigated systematically. Results showed that UV or chlorination alone did not degrade 2-MIB effectively, but UV/chlorine process could degrade 2-MIB efficiently, followed pseudo-first-order kinetic model. The 275 nm UV exhibited higher 2-MIB degradation efficiency in this UV-LED/chlorine system than 254 nm UV, 265 nm UV and 285 nm UV due to the highest mole adsorption coefficient and quantum yield of chlorine in 275nm UV. ·OH and ·Cl produced in the 275 nm UV/chlorine system played major roles in 2-MIB degradation. HCO3⁻ and Natural organic matter (NOM), which are prevalent in water, consumed ·OH and ·Cl, thus inhibiting the 2-MIB degradation by UV-LED/chlorine. In addition, NOM and 2-MIB could form photonic competition effect. The degradation of 2-MIB by UV-LED/chlorine was mainly through dehydration and demethylation, and odorous intermediates such as camphor are produced. 2-MIB was degraded through α bond fracture and six-membered ring opening to form saturated or unsaturated hydrocarbons and aldehydes. Four DBPs, chloroform (CF), trichloroacetaldehyde (TCE), trichloroacetone (TCP) and dichloroacetone (DCP), were mainly generated, and CF was the most significant byproducts.
UV-assisted advanced oxidation processes (AOPs) are widely used and studied in degradation of bisphenol A (BPA). However, detailed information on their radical chemistry and degradation mechanisms is still lacking. In this study, degradation of BPA was comparatively evaluated to investigate the radical mechanisms, products and the toxicity variation in UV/chlorine and UV/H2O2 processes. In comparison with UV/H2O2, UV/chlorine had a higher BPA degradation efficiency and higher pH-dependency due to chlorination and the synergy of •OH and RCS. The •OH and Cl• played a pivotal role as the primary radicals in BPA degradation by UV/chlorine process at all pH investigated (6–8). The relative contributions of the secondary radicals ClO• gradually decreased with a variation of pH from 6 to 8 in this process. Presence of HCO3─ and HA inhibited BPA degradation to different extents in UV/chlorine process, while the effect of Cl─ could be neglected. According to the identified transformation products, chlorination (major), hydroxylation and breakage of the isopropylidene chain were BPA decomposition pathways in the UV/chlorine system. In the UV/H2O2 system, only hydroxylation (major) and breakage of the isopropylidene chain occurred. The toxicity analysis, based on the proposed degradation pathways, indicated that the generation of chlorinated products in the UV/chlorine system led to a higher toxicity of the resulting mixture than in the UV/H2O2 system. Although UV/chlorine has an excellent BPA degradation effect and it is cost-effective, the possible environmental risk should be carefully considered when UV/chlorine system is used to remove BPA in real waters.
Photochemical reactions generally result in cleavage of a chemical bond. As such, primary photochemical reactions can lead to degradation of compounds in irradiated media. Direct photolysis tends to be a selective process that is limited to compounds that are able to absorb incident radiation and use the energy of the absorbed photon to promote a chemical reaction. Direct photolysis is an efficient process for relatively few molecules, but for some of these compounds, photolysis is the process of choice for promoting degradation. A more common strategy involves the use of UV irradiation to promote the formation of reactive intermediates, often in the form of free radicals. Reactive intermediates are often formed in hybrid processes in which a promoter is added immediately before UV exposure to allow the formation of the reactive intermediate. Examples of these hybrid processes include: H 2 O 2 /UV, Chlorine/UV, Chloramine/UV, and Persulfate/UV, all of which result in the formation of free radicals that function as strong oxidizing agents. The principles of physics and chemistry that govern UV Advanced Oxidation Processes (UV‐AOPs) are presented along with descriptions of experimental methods that are used to estimate fundamental reaction parameters. Also included is information to describe the effects of solution composition on direct photolysis and UV‐AOP dynamics. Vacuum UV (VUV) can also be applied to aqueous solutions to promote photolysis of the solvent (water), resulting in the formation of reactive intermediates. In a similar manner, UV radiation can be used to promote the formation of reactive intermediates that function as powerful reducing agents. The most common UV Advanced Reduction Process (UV‐ARP) is the sulfite/UV process, which promotes the formation of the hydrated electron, an extremely powerful reducing agent.
The ambitious “zero pollution” goal entails the upgrade of conventional (waste)water treatment processes. Among various options, photo-based advanced oxidation processes (photo-AOPs) have shown promising capabilities in terms of eliminating contaminants of emerging concern. While photo-AOP applications exist worldwide, some key issues remain that hamper their wider practice and achieving the potential zero pollution goal. In this perspective, we briefly summarize the state of photo-AOPs, including photo-hydrogen peroxide (H2O2), -Fenton, -persulfate (S2O82- ), -ozone (O3 ), and -catalysis systems. We also discuss the major theoretical and technical knowledge gaps of photo-AOPs, which require further research to advance these treatments and to progress towards zero-toxic discharge for safe water reuse.
DOPO (9,10-Dihydro-9-oxa-10-phosphaphenanthrene-10-oxide) is emerging organic phosphorus flame retardant, which has gained greatly attention because of its adverse health effects on aquatic organisms. In this study, UV light-emitting diodes lamp driven sodium sulfite activation (UV254/Sulfite) process was employed as an advanced reduction process to remove DOPO, and its mechanisms were revealed. The pseudo first order rate constant of DOPO (kobs-DOPO) in UV254/Sulfite process initially decreased from 6.95 × 10-5 s⁻¹ to 5.11 × 10-5 s⁻¹, and subsequently increased to 1.61 × 10-4 s⁻¹ as the sulfite concentration increased from 0 to 10 mM, indicating that the negative effect of HSO3⁻ on DOPO degradation was the dominate factor in UV254/Sulfite process at low sulfite concentration (< 2 mM), while the yields of hydrated electron (eaq⁻) increased as the sulfite concentration gradually reaching to a high value (10 mM), resulting in enhancing DOPO degradation significantly. In addition, the coexisted water matrix in deionized water and natural surface water weakened DOPO degradation in UV254/Sulfite process, and this inhibition strengthened as the concentrations of water matrix raised. The DOPO degradation pathway indicated that eaq⁻ attacked phosphaphenanthrene structure of DOPO and contributed to its detoxification which was confirmed by Quantitative Structure-Activity Relationship (QSAR) assessment.
Three different UV-LED wavelengths (265, 310, and 365 nm) were used in the UV-LED/chlorine reaction to investigate the degradation mechanism of iopromide (IPM) at different wavelengths, a representative iodinated contrast media compound. The degradation rate (k'IPM) increased from pH 6-8 at 265 nm, but, decreased as the pH increased up to 9 at 310 nm and 365 nm. Radical scavenging experiments showed that reactive chlorine species (RCS) are the dominant radical species at all wavelengths, but a higher contribution of OH• was observed at lower pH and longer wavelengths. The contribution of RCS decreased but the contribution of OH• increased as the wavelength increased. Among RCS, the largest contribution was found to be ClO•. Total nine transformation products (TPs) were identified by LC-QTOF-MS during the UV-LED/chlorine reaction at 265 nm. Based on the identified TPs and their time profiles, we proposed a degradation pathway of IPM during UV-LED/chlorine reaction. The Microtox test using V. fischeri showed that no significant increase in toxicity was observed at all wavelengths. The synergistic effect of UV-LED and chlorine was greater at a higher wavelength by the electrical efficiency per order (EEO) calculation.
Ozone is formed at submicromolar concentrations from photolysis of many oxyanions and oxidants in water and contributes importantly to the degradation of emerging contaminants and inactivation of pathogenic microorganisms in the natural and engineered aquatic systems. In this study, we identified and discussed the critical limitations of the commonly-used protocols using cinnamic acid (CNA) as a probe compound to determine the submicromolar-level ozone and proposed a modified protocol that overcomes those limitations. Our experimental investigation demonstrated that the radicals (e.g., HO•) formed from photolysis of oxyanions and oxidants, other than ozone, could also oxidize CNA and form benzaldehyde, resulting in the overestimation of ozone concentrations by using the commonly-used protocols. Moreover, the benzaldehyde formed from ozone-CNA reactions could be degraded by the radicals, leading to the underestimation of ozone concentrations by using the commonly-used protocols. A new protocol with high accuracy and precision was proposed and the rationales for each operational step of the new protocol were explained in detail and supported with justifications. The new protocol was compared with two commonly-used protocols in determining the concentration of ozone in the same water sample treated by the UV/chlorine process at three different UV wavelengths. The wavelength-dependent overestimation/underestimation of the ozone concentrations by using the two commonly-used protocols was well demonstrated and explained by the overlooked interferences of radicals in the protocols.
In this study, the UVA (Ultraviolet A) drinking water disinfection was promoted by electrolysis. The influences of the UVA, electrolysis current, bubbling and temperature were investigated. The disinfection mechanisms and bacterial reactivation had been studied. The results revealed that the treatment time needed to reach the DL (detection limit, about 5.4 log removal) was shortened from 180 to 80 min by the electrolysis. The total electricity consumption decreased from about 126 to 57.0 kJ/L. Compared with increasing the UVA irradiation, increasing the electrolysis current in a certain range was more preferred to improve the disinfection rate. Oxygen bubbling or higher temperature could enhance the E. coli inactivation. The quenching experiment and EPR (Electron paramagnetic resonance) detection confirmed that ROSs (¹O2, ·O2⁻ and ·OH) played important roles for the disinfection. Compared with the treatment with UVA alone, the cell membrane damage was more severe by the promoting method. In addition to the dramatically reduced enzyme activity, the synergistic process degraded most of the bacterial genomic DNA, and the bacteria were completely killed. Therefore, hybrid with electrolysis is a better way for the application of the UVA-LED disinfection.
Advanced oxidation processes (AOPs) have been increasingly studied and practiced for micropollutant abatement in drinking water treatment and potable water reuse. This study conducted the multi-angle comparison of the UV/chlorine, UV/monochloramine (UV/NH2Cl), and UV/chlorine dioxide (UV/ClO2) AOPs with respect to reactive species generation, micropollutant degradation, byproduct formation, and toxicity change. The concentrations of radicals (HO•, Cl•, and ClO•) generated in the three AOPs followed the order of UV/chlorine > UV/NH2Cl > UV/ClO2 at an oxidant dose of 70 μM, an irradiation wavelength of 254 nm, and a pH of 7.5. The concentration of ozone generated in the UV/ClO2 AOP was higher than that in the UV/chlorine AOP, while ozone was not generated in the UV/NH2Cl AOP. The effects of pH (pH 6.0, 7.5, and 9.0) and UV wavelength (254 nm, 285 nm, and 300 nm) on the three AOPs were evaluated and compared. Using the radical and ozone concentrations determined in this study, the pseudo-first-order degradation rate constants of 24 micropollutants by the three AOPs were predicted and compared. When the three AOPs were used to treat the water containing the same concentration of natural organic matter, the formation of total organic chlorine (TOCl) and the organic byproduct-associated toxicity followed the same order of UV/chlorine > UV/NH2Cl > UV/ClO2. On the contrary, the inorganic byproduct-associated toxicity followed the order of UV/ClO2 > UV/chlorine > UV/NH2Cl, due to the high concentrations of chlorite and chlorate formed in the UV/ClO2 AOP. Findings in this study offer fundamental information useful for the selection and operation of AOPs for micropollutant abatement in drinking water treatment and potable water reuse.
Advanced oxidation processes (AOPs) use chemical treatment to remove contaminants from water by oxidation with hydroxyl radicals. These hydroxyl radicals can be produced using UV light, ozone or hydrogen peroxide, but recently reactions have been developed that use persulfates as the radical source. Persulfates are strong oxidants with flexible in situ activation characteristics, including activation with heat, alkali conditions, electricity, ultrasonic treatment, transition metals, carbon and even organics. Persulfate activation can generate sulfate radicals as well as other reactive species. These reactive species, especially the sulfate radical, can degrade most organic pollutants making them valuable in the fields of water purification, soil remediation, disinfection, sludge dewatering, and other important applications in environmental systems.
Describing recent developments in persulfate-based AOPs, this book aims to provide a summary of environmental applications for persulfate-based AOPs and to guide the reader, in a comprehensive way, through various advanced oxidation processes in environmental applications. Topics include new activation methods, activation mechanisms, and advanced materials for use in activating persulfate-based AOPs for different environmental applications.
Advanced oxidation processes (AOPs) have recently gained increasing attention as promising alternatives to conventional water and wastewater treatment technologies. As an AOP, there has been a growing interest in UV/chlorine due to its multiple disinfection modes and ability to maintain chlorine residuals in drinking water distribution systems. Compared to the popular UV/H2O2 AOP treatment process, UV/chlorine AOP possesses several advantages including a higher radical production efficiency and a lower power demand. The process has recently been applied for the degradation of a wide range of natural and synthetic organic as well as inorganic compounds including pharmaceutical and personal care products, natural organic matter, herbicides and insecticides, taste and odor, and different types of water and wastewaters. The focus of the present work is to review and evaluate the efficiency of the UV/chlorine process on the oxidation of these pollutants. A critical overview is provided of the fundamentals of UV/chlorine, controlling factors of the process performance, and a comparison with other UV-based AOPs. The formation of disinfection by-products (DBPs), one of the concerns associated with UV/chlorine treatment, is discussed and strategies to tackle this obstacle are provided for future studies and applications.
Disinfection byproduct (DBP) formation is a potential concern with regard to MPUV/Cl2 application in water treatment. In this study, five typical amino acids (AAs) were selected to investigate their DBP alteration during short-term medium pressure (MP) UV/chlorine oxidation following post-chlorination relative to parallel dark controls. The five selected AAs include two potent DBP precursors (aspartic acid and tryptophan), one modest precursor (asparagine) and two poor precursors (phenylalanine and proline). MPUV/chlorine increased the total DBP formation and DBP-associated cytotoxicity of the two poor precursors phenylalanine (Phe) and proline (Pro) as well as their chlorine demands. Conversely, DBP formation and DBP-associated cytotoxicity of the three modest-to-potent DBP precursors showed the opposite changing trends due to MPUV/Cl2 oxidation. The two aromatic AAs (tryptophan and phenylalanine) were more readily to be affected by MPUV/Cl2 oxidation especially at acidic pH condition. Conversely, DBP formation and DBP-associated cytotoxicity of the three modest-to-potent precursors showed the opposite changing trends due to MPUV/Cl2 oxidation. Among the measured DBPs, the absolute formation potential changes of haloacetic acids and haloacetonitriles were the most prominent. Presence of bromide increased the trihalomethane formation potential of five AAs. Ammonia-spiked samples resulted in notably higher chlorine demands but slightly reduced DBPFP. Photonitration caused increased haloacetonitrile and trichloronitromethane formation but lower overall DBP formation potential and DBP-associated cytotoxicity. Results indicated that increased DBP formation of unreactive aromatic AAs may be problematic with respect to MPUV/Cl2 application, while the presence of inorganic ions may not contribute to further increase in calculated cytotoxicity of measured DBPs.
Eutrophication is a globally concerned issue, which brings algal cells and algal organic matter (AOM) into drinking water treatment plants. AOM is an important branch of nitrogenous disinfection byproduct (N-DBP) precursors. The variation of AOM composition in UV-LEDs/chlorine process, and its relationship with N-DBP formation still remain much uncertainty. Herein, we used fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) to investigate AOM transformation in UV-LEDs/chlorine process, with UV285 and UV365 as light source, and screen for typical precursors of N-DBPs. We found that more nitrogen-containing compounds were generated after UV-LEDs/chlorine process, leading to the larger formation of N-DBPs in postchlorination. Compounds such as lignin, proteins, and amino sugars tends to be oxidized by reactive species in UV-LEDs/chlorine process. Further, compounds with higher O/C and higher weighted average double bond equivalence (DBEw) are easier to form N-DBPs, including dichloroacetonitrile and trichloronitromethane. Also, influence factors including pH, UV fluence, post-chlorination time and bromide concentration on N-DBP formation were evaluated. The results show that N-DBP formation generally followed the order of UV285/chlorine-postchlorination, UV365/chlorine-postchlorination, and direct chlorination. Our study provides comprehensive information on N-DBP formation from AOM in UV-LEDs/chlorine-postchlorination from molecular levels.
The presence of serotonin-norepinephrine reuptake inhibitors (SNRIs) such as venlafaxine (VEN) in surface waters has caused some concerns due to their harmful impacts on the human health and environment security. In the study, ultraviolet (UV) coupled with chlorine was developed for degrading VEN. Among the experiments, the UV/chlorine process demonstrated the highest performance in eliminating VEN. UV irradiation alone played a negligible role in VEN degradation. 39.56% of VEN was degraded by dark chlorination in 30 min, while 76.02% of VEN was decayed by UV/chlorine treatment within 30 min. Adding chlorine dosage and raising solution pH both facilitated the VEN removal. HCO3⁻, Cl⁻ and HA inhibited VEN degradation during UV/chlorine treatment in accordance with predicted data. Furthermore, Kintecus software was applied to simulate the process of VEN degradation. Under UV/chlorine co-exposure, hydroxyl radical (HO·), chlorine and reactive chlorine species (RCS) were all proved to provide significant contribution to VEN oxidation. Note that both experimental and predicted contributions of HO· decreased as solution pH increased from 5.0 to 8.0. Four transformation pathways of VEN during UV/chlorine process were elucidated on basis of the DFT calculation and LC/MS analysis. Moreover, ECOSAR model program showed that UV/chlorine process reduced ecological toxicity of VEN obviously. Considering the influence of various factors, the most economical experimental conditions consist of chlorine (2.0-4.0 mg L⁻¹) and pH (7.0-8.0).
A UV-LED at a fixed wavelength is increasingly employed in the UV/chlorine process to degrade contaminants of emerging concern. However, degradation of contaminants and formation of disinfection by-products (DBPs) in post-chlorination would be significantly altered in the presence of nitrite because of the generation of reactive nitrogen species (RNS). As nitrite has a maximum absorption at ∼350 nm, its impact on the performance of the UV-LED365/chlorine process and subsequent formation of DBPs was investigated. The presence of nitrite in the UV365 irradiation process accelerated the degradation of bisphenol A (BPA), while slightly inhibiting the degradation of BPA in the UV365/chlorine process; nitrite in UV365 and UV365/chlorine processes resulted in the non-negligible formation of trichloronitromethane (TCNM). The concentration of generated DBPs and the corresponding cytotoxicity and genotoxicity all changed because of the presence of nitrite; the concentrations of nitrite, pH, and UV fluence were important factors for the formation of TCNM during the UV365/chlorine/nitrite process. Therefore, the impact of nitrite on the TCNM formation in the UV-LED365/chlorine process is worth noting.
The ozone/peroxymonosulfate (O3/PMS) process, generating hydroxyl radicals (HO•) and sulfate radicals (SO4•-), can effectively remove organic contaminants in water, while forming a remarkable amount of bromate (BrO3⁻) in the presence of bromide (Br⁻). This paper firstly investigated the ammonia (NH3), chlorine-ammonia (Cl2-NH3) and ammonia-chlorine (NH3-Cl2) pretreatment in inhibiting BrO3⁻ generation in O3/PMS process at different conditions, and the inhibition degree was compared with O3 alone. All the three kinds of pretreatment at Cl2 and NH3 dosages of 50 and 100 mM reduced 90 % or more of the overall BrO3⁻ formation, while the NH3-Cl2 pretreatment strategy is prior to that of the NH3 and Cl2-NH3. The inhibition degree of the pretreatment strategies under different pH were: 5.0>9.0>7.0>3.0. Meanwhile, the inhibition degree increased as the initial Br⁻ concentration increased from 15 to 50 µM. After each pretreatment strategy, there was a higher BrO3⁻ inhibition efficiency accompanied with a lower ozone utilization efficiency in O3 alone than that of O3/PMS process. Furthermore, the mechanism of BrO3⁻ inhibition by the pretreatment strategies was also clarified. The main products produced by masking Br⁻ in the NH3-Cl2 pretreatment were more difficult to regenerate Br⁻, as a result, the NH3-Cl2 pretreatment strategy produced the least BrO3⁻ among the three kinds of pretreatment strategies. Finally, compared with the deionized water, the pretreatment strategies shown better inhibition performance in actual water because the water matrices would trap free radicals and HOBr to reduce BrO3⁻ formation. At the same time, the NH3-Cl2 pretreatment strategy generated fewer disinfection by-products than that of Cl2-NH3 pretreatment in actual water. Therefore, the NH3-Cl2 pretreatment strategy is a practical way to control the BrO3⁻ formation in the O3/PMS process.
Photolysis of ClO2 by UVC radiation occurs in several drinking water treatment scenarios (e.g., pre-oxidation by ClO2 with post-UVC disinfection or a multi-barrier disinfection system comprising ClO2 and UVC disinfection in sequence). However, whether micropollutants are degraded and undesired byproducts are formed during the co-exposure of ClO2 and UVC radiation remain unclear. This study demonstrated that four micropollutants (trimethoprim, iopromide, caffeine, and ciprofloxacin) were degraded by 14.4–100.0% during the co-exposure of ClO2 and UVC radiation in the synthetic drinking water under the environmentally relevant conditions (UV dose of 207 mJ cm⁻², ClO2 dose of 1.35 mg L⁻¹, and pH of 7.0). Trimethoprim and iopromide were predominantly degraded by ClO2 oxidation and direct UVC photolysis, respectively. Caffeine and ciprofloxacin were predominantly degraded by the radicals (HO• and Cl•) and the in-situ formed free chlorine from ClO2 photolysis, respectively. The yields of total organic chlorine (12.5 µg L⁻¹ from 1.0 mg C L⁻¹ of NOM) and chlorate (0.14 mg L⁻¹ From 1.35 mg L⁻¹ of ClO2) during the co-exposure were low. However, the yield of chlorite was high (0.76 mg L⁻¹ from 1.35 mg L⁻¹ of ClO2), which requires attention and control.
The development of ultraviolet light-emitting diode (UV-LED) technology provides more potential for the application of photocatalytic membranes. In this study, graphene oxide-titanium dioxide (GO-TiO2) photocatalytic ceramic membranes were prepared and activated by UV-LED. We systematically evaluated the removal of naproxen (NAP), carbamazepine (CBZ) and diclofenac (DCF) under 365 nm, 300 nm and 275 nm UV-LED irradiation in photocatalytic ceramic membrane reactors (PCMRs). Our results showed that this GO-TiO2 modified ceramic membrane achieved simultaneous separation and photocatalytic degradation for tested pharmaceuticals. Through fluence-based and energy-based efficiencies analysis, the 300 nm UV-LED is a good option for achieving a high removal efficiency with low energy consumption. Membrane filtration enhanced the degradation efficiency by four times through enhancing the mass transfer effect in membrane pores, and reduced the electrical energy per order (EEO) by a factor of ten. In addition, high membrane flux, high degradation rate and strong catalytic stability could be simultaneously achieved in closed-loop dead-end filtration. The addition of H2O2 could further increased the removal efficiency. PCMRs equipped with UV-LEDs are promising for pharmaceuticals removal as well as solving the issue of membrane concentrated water.
The UV photolysis of chlorine (UV/chlorine) produces a suite of radical species and has been proposed as a novel water treatment process for advanced oxidation and enhanced disinfection purposes. In this study, the kinetics and efficacy of membrane/DNA damage to B. subtilis and autochthonous bacteria during UV/chlorine treatment were investigated under different pH and irradiation wavelengths (254 nm Hg lamp vs 275 nm LEDs) with flow cytometer analysis. Compared to UV irradiation or chlorination alone, the UV/chlorine showed the enhanced kinetics and efficacy of cell membrane or DNA damage to B. subtilis. These enhancements by UV/chlorine might be primarily attributed to the in-situ generation of radical species HO• and Cl• around intracellular DNA. With pH increasing from acidic to alkaline, the UV254/chlorine showed the decreased kinetics of cell membrane or DNA damage to B. subtilis while the UV275/chlorine exhibited the increased cell membrane and DNA damage kinetics due to the higher chlorine photolysis rates. When the chlorine dose was not excess, the DNA damage of B. subtilis were highly depended on the cumulative exposures of free available chlorine (CTFAC). During UV275/chlorine in the presence of natural organic matter, the cell membrane damage also showed linear correlations with the decreases of UV absorbance at 254 nm (UVA254), and about 20–25% decrease of UVA254 indicates a sufficient cell membrane damage. Using the CTFAC values and decrease of UVA254, this study has provided the surrogate indicators for fast determining the minimal chlorine dose for sufficient inactivation of bacteria by UV/chlorine treatment.
Water contamination by contaminants of emerging concern is one of the main challenges to be solved by our desired sustainable society. In the same time, different technologies for water treatment are becoming enough mature to be implemented. In this work, two different advanced oxidation processes (AOP) were investigated: i) electrochemical processes (electrochemical, photoassisted electrochemical, electro Fered-Fenton, and photo-electro Fered-Fenton – PEF-Fered) using a BDD and DSA® electrodes under UVA and UVC irradiation (9 W) and ii) solar-based AOP using four distinct oxidants (HOCl, H2O2, S2O82–, HSO5–) in the presence or absence of Fe²⁺ ions to oxidize and mineralize imidacloprid (IMD: 50 mg L–1) containing solutions. The PEF-Fered (1.0 mM Fe²⁺ and 50 mg L–1 h–1 H2O2) under UVA or UVC irradiation and HOCl/UVC (NaCl 17 mM) processes using a BDD and DSA® electrodes (10 mA cm –2), respectively, performed equally well to completely oxidize and mineralize (∼90%) IMD at the expense of only ∼0.3 kWh g–1. Low amounts and highly oxidized byproducts identified through liquid chromatography coupled to mass spectrometry were observed for the HOCl/UVC process using a DSA® electrode. Concerning the solar-based AOP, all assessed oxidants (4 mM h–1) successfully oxidized IMD within 3 h of treatment, whereas only H2O2 and HOCl led to significant (∼60%) TOC abatement after 6 h treatment. The use of Fe²⁺ (0.5 or 1.0 mM) had no significant improvement in the oxidation and mineralization of IMD.
Chlorine dioxide (ClO2) has emerged as a promising alternative to free chlorine for water disinfection and/or pre-oxidation due to its reduced yields of chlorinated disinfection byproducts. ClO2 decomposes to form chlorite (ClO2-), which influences the following advanced oxidation processes (AOPs) for micropollutant abatement in drinking water. This study aims at investigating the effects of ClO2- on the concentrations of reactive species (e.g., radicals and ozone) and on the formation of chlorate in the UV/chlorine AOP. Results showed that the concentration of ClO
·
in the UV/chlorine process remarkably decreased by 98.20-100.00% in the presence of ClO2- at concentration of 0.1-1.0 mg·L-1 as NaClO2. The concentrations of HO
·
and ozone decreased by 42.71-65.42% and by 22.02-64.31%, respectively, while the concentration of Cl
·
was less affected (i.e., 31.00-36.21% reduction). The overall concentrations of the reactive species were differentially impacted by ClO2-'s multiple roles in the process. UV photolysis of ClO2- generated HO
·
but not Cl
·
, ClO
·
or ozone under the drinking water relevant conditions. ClO2- also competed with chlorine for UV photons but this effect was minor (< 1.0%). The radicals/ozone scavenging by ClO2- outcompeted the above two to lead to the overall decreasing concentrations of the reactive species, in consistency with the kinetic model predicted trends. ClO2- reacted with radicals and ozone to form chlorate (ClO3-) but not perchlorate (ClO4-). HO
·
played a dominant role in ClO3- formation. The findings improved the fundamental understanding on micropollutant abatement and inorganic byproduct formation by the UV/chlorine process and other AOPs in ClO2--containing water.
Photolysis of free chlorine (HOCl/ClO-) is an advanced oxidation process (AOP) to produce hydroxyl (HO•) and other radicals for refractory micropollutant degradation. However, HOCl/ClO- is only conducive to activation and production of radicals by Ultraviolet (UV) light. For the first time we show the use of visible light (>400 nm) to produce HO• and ClO•, through use of graphitic carbon nitride (g-C3N4) and photogenerated h_vb^+, e_cb^- and O2•− in the presence of HOCl/ClO-, which termed visible light g-C3N4-enabled chlorine AOP (VgC-AOP). The VgC-AOP increased the pseudo-first order degradation rate constant of a model micropollutant, carbamazepine (CBZ), by 16 and 7 times higher than those without g-C3N4 and HOCl/ClO-, respectively, and remained active over multiple use cycles. Effects of water quality (pH, alkalinity, Cu(II) and natural organic matter (NOM)) and operational conditions (g-C3N4 and HOCl/ClO- concentrations, irradiation wavelength and dose) were investigated. Of particular significance is its superior performance in the presence of NOM, which absorbs less light at visible light wavelengths and scavenges less surface-bonded reactive species, compared against UV/TiO2 or UV/chlorine AOPs. The VgC-AOP is practically relevant, feasible and easily implementable, and it expands the potential types of light sources (e.g., LEDs, solar light).
This study investigated the fate of DCAN in the UV/Cl2 and UV/H2O2 processes under the conditions relevant to potable water reuse (e.g., two pHs and three oxidant dosages). At pH 6 and an oxidant dosage of 500 μM, the degradation of DCAN in the UV/Cl2 process was attributed to UV photolysis (4.5%), HO⁻-assisted hydrolysis (10.5%), nucleophilic attack by ClO⁻ (32.2%), and oxidation by radicals (i.e., HO● and Cl●) (52.8%), while that in the UV/H2O2 process was mainly attributed to HO2⁻ (32%) and HO● (48%). In both processes, the DCAN degradation rates were higher with increasing solution pH from 5 to 6, because the increased HO⁻-assisted hydrolysis and nucleophilic attack of DCAN surpassed the decreased radical oxidation of DCAN. The DCAN degradation was enhanced with increasing chlorine or H2O2 dosages from 100 to 1000 μM, mainly due to the increased contribution from nucleophilic attack. DCAN was mostly transformed to dichloroacetic acid (DCAA) through different pathways in the two processes. At the same pH and oxidant dosage conditions, the degradation rates of DCAN in the UV/Cl2 process were higher than those in the UV/H2O2 process, due to the higher nucleophilic attack rates and radical oxidation rates in the former process. The cost of degrading 90% of DCAN using the UV/Cl2 process is about 1/8 of that using the UV/H2O2 process, making the UV/Cl2 process a more cost-effective UV-AOP in the DCAN abatement for potable water reuse.
Prechlorination of water upstream of an ultraviolet (UV) disinfection process may possibly influence the effectiveness of UV inactivation through the absorbance of UV radiation. Alternatively, UV may act to destroy chlorine, reducing the residual in water. Monochromatic UV light was more effective in degrading monochloramine whereas polychromatic UV light was more effective in degrading free chlorine. The extent of degradation varied with the type of chlorine species, water quality and type of UV irradiation (mono- or polychromatic). However, decay of chlorine and monochloramine at typical UV disinfection doses (< 100 mJ/cm2) was negligible. The transmission of UV irradiation was affected slightly by the presence of chlorine and monochloramine, and these effects on inactivation of MS2 coliphage were modeled for LP - and MP-UV reactors. The presence of 1 mg/L free chlorine or monochloramine decreased the inactivation of MS2 coliphage by less than 0.1 log for a LP-UV reactor. For the MP-UV reactor, MS2 inactivation decreased by between 0.1 and 0.3 log.
Ultraviolet light-emitting diodes (UV-LEDs) have started replacing UV lamps. The power per LED of high-power LED products has reached 12 W (14 A), which is 100 times the values observed ten years ago. In addition, the cost of these high-power LEDs has been decreasing. In this study, we attempt to understand the technologies and potential of UV-LEDs.
Utilities incorporating the potable reuse of municipal wastewater are interested in converting from the UV/H2O2 to the UV/free chlorine advanced oxidation process (AOP). The AOP treatment of reverse osmosis (RO) permeate often includes the de facto UV/chloramine AOP because chloramines applied upstream permeate RO membranes. Models are needed that accurately predict oxidant photolysis and subsequent radical reactions. By combining radical scavengers and kinetic modeling, we have derived quantum yields for radical generation by the UV photolysis of HOCl, OCl-, and NH2Cl of 0.62, 0.55, and 0.20, respectively, far below previous estimates that incorporated subsequent free chlorine or chloramine scavenging by the •Cl and •OH daughter radicals. The observed quantum yield for free chlorine loss actually decreased with increasing free chlorine concentration, suggesting scavenging of radicals participating in free chlorine chain decomposition and even free chlorine reformation. Consideration of reactions of •ClO and its daughter products (e.g., ClO2-), not included in previous models, were critical for modeling free chlorine loss. Radical reactions (indirect photolysis) accounted for ~50% of chloramine decay and ~80% of free chlorine loss or reformation. The performance of the UV/chloramine AOP was comparable to the UV/H2O2 AOP for degradation of 1,4-dioxane, benzoate and carbamazepine across pH 5.5-8.3. The UV/free chlorine AOP was more efficient at pH 5.5, but only by 30% for 1,4-dioxane. At pH 7.0-8.3, the UV/free chlorine AOP was less efficient. •Cl converts to •OH. The modeled •Cl:•OH ratio was ~20% for the UV/free chlorine AOP and ~35% for the UV/chloramine AOP such that •OH was generally more important for contaminant degradation.
Light source is a crucial factor in the application of a photochemical process, which determines the energy efficiency. The performances of acetylacetone (AA) in conversion of aqueous contaminants under irradiation with a low-pressure mercury lamp, a medium-pressure mercury lamp, a xenon lamp, and natural sunlight were investigated and compared with those of H2O2 as reference. In all cases, AA was superior to H2O2 in the degradation of Acid Orange 7. Using combinations of the different light sources with various cut-off and band-pass filters, the spectra responses of the absorbed photons in the UV/AA and UV/H2O2 processes were determined for two colored and two colorless compounds. The photonic efficiency (φ) of the two photochemical processes was found to be target-dependent. A calculation approach for the inner filter effect was developed by taking the obtained φ into account, which provides a more accurate indication of the reaction mechanisms.
The UV-light emitting diode (LED) has been attracting significant attention as a new UV source that can replace conventional mercury gas-filled lamps in water disinfection applications. However, the UV-LED remains a relatively new addition to the water treatment toolbox. The current lack of fundamental understanding risks underutilizing uniquely advantageous features of the UV-LED due to unguided design and non-optimized disinfection practices. Our review presents the necessary fundamental knowledge required for the successful implementation of UV-LEDs, including the mechanism of light generation, LED chip fabrication, package design, and essential properties of UV-LEDs. We introduce distinct advantages, such as wavelength tuning, control of radiation patterns, and array design, while emphasizing the significant differences between LED and mercury lamp technologies required to achieve successful technology transfer. Previous studies investigated the design of UV-LED disinfection systems; however, little consensus has yet emerged regarding the integration of LEDs into flow-through reactors. While UV-LED disinfection systems will undisputedly mature in the near future, environmental engineers face a number of urgent research needs in this area including heat sink design, radiation pattern and array design optimization for uniform UV dose delivery, targeted pathogen-wavelength considerations, improved light extraction, and component monitoring systems.
Photo-activation of aqueous chlorine could promote degradation of chlorine-resistant and photochemically stable chemicals accumulated in swimming pools. This study investigated the degradation of two such chemicals, N,N-diethyl-3-methylbenzamide (DEET) and caffeine, by low pressure ultraviolet (UV) light and simulated sunlight (SS) activated free chlorine (FC) in different water matrices. Both DEET and caffeine were rapidly degraded by UV/FC and SS/FC, but exhibited different kinetic behaviors. The degradation of DEET followed pseudo-first-order kinetics, whereas the degradation of caffeine accelerated with reaction. Mechanistic study revealed that, under UV/FC, ∙OH and Cl∙ were responsible for degradation of DEET, whereas ClO∙ related reactive species (ClOrrs), generated by the reaction between FC and ∙OH/Cl∙, played a major role in addition to ∙OH and Cl∙ in degrading caffeine. Reaction rate constants of DEET and caffeine with the respective radical species were estimated. The imidazole moiety of caffeine was critical for the special reactivity with ClOrrs. Water matrix such as pH had stronger impact on the UV/FC process than the SS/FC process. In saltwater matrix under UV/FC and SS/FC, the degradation of DEET was significantly inhibited, but the degradation of caffeine was much faster than that in non-salty solutions. The interaction between Br- and Cl- may play an important role in the degradation of caffeine by UV/FC in saltwater. Reaction product analysis showed similar product patterns by UV/FC and SS/FC, and minimal formation of chlorinated intermediates and disinfection byproducts.
The photolysis of hypochlorous acid (HOCl) and hypochlorite (OCl⁻) produces a suite of reactive oxidants, including hydroxyl radical (OH), chlorine radical (Cl), and ozone (O3). Therefore, the addition of light to chlorine disinfection units could effectively convert existing drinking water treatment systems into advanced oxidation processes. This review critically examines existing studies on chlorine photolysis as a water treatment process. After describing the fundamental chemistry of chlorine photolysis, we evaluate the ability of chlorine photolysis to transform model probe compounds, target organic contaminants, and chlorine-resistant microorganisms. The efficacy of chlorine photolysis to produce reactive oxidants is dependent on solution and irradiation conditions (e.g., pH and irradiation wavelengths). For example, lower pH values result in higher steady-state concentrations of OH and Cl, resulting in enhanced contaminant removal. We also present the current state of knowledge on the alteration of dissolved organic matter and subsequent formation of disinfection by-products (DBPs) during chlorine photolysis. Although the relative yields of DBPs during chlorine photolysis are also dependent on solution conditions (e.g., higher organic DBP yields at low pH values), there is conflicting evidence on whether chlorine photolysis increases or decreases DBP production compared to thermal reactions between chlorine and dissolved organic matter in the dark. We conclude the review by identifying knowledge gaps in the current body of literature.
The ultraviolet/chlorine (UV/chlorine) water purification process was evaluated for its ability to degrade the residues of pharmaceuticals and personal care products (PPCPs) commonly found in drinking water sources. The disinfection byproducts (DBPs) formed after post-chlorination were documented. The performance of the UV/chlorine process was compared with that of the UV/hydrogen peroxide (UV/H2O2) process in treating three types of sand-filtered natural water. Except caffeine and carbamazepine residues, the UV/chlorine process was found to be 59-99% effective for feed water with a high level of dissolved organic carbon and alkalinity, and 27-92% effective for water with a high ammonia content. Both chlorine radicals and hydroxyl radicals were found to contribute to the observed PPCP degradation. The removal efficiencies of chlorine- and UV-resistant PPCPs such as carbamazepine and caffeine were 2-3 times greater than in the UV/H2O2 process in waters not enriched with ammonia. UV/chlorine treatment slightly enhanced the formation chloral hydrate (CH), haloketone (HK) and trichloronitromethane (TCNM). It reduced haloacetonitrile (HAN) formation during the post-chlorination in comparison with the UV/H2O2 process. In waters with high concentrations of ammonia, the UV/chlorine process was only 5-7% more effective than the UV/H2O2 process, and it formed slightly more THMs, HKs and TCNM along with reduced formation of CH and HAN. The UV/chlorine process is thus recommended as a good alternative to UV/H2O2 treatment for its superior PPCP removal without significantly enhancing DBP formation.
For successful wastewater reclamation, advanced oxidation processes have attracted attention for elimination of emerging contaminants. In this study, the synergistic treatment with UV irradiation and chlorine (UV/chlorine) was used to degrade carbamazepine (CBZ). Neither UV irradiation alone nor chlorination alone could efficiently degraded CBZ. UV/chlorine oxidation showed a significant synergistic effect on CBZ degradation through generation of radical species (OH and Cl), and this process could be well depicted by pseudo first order kinetic. The degradation rate constants (kobs,CBZ) of CBZ increased linearly with increasing UV irradiance and chlorine dosage. The degradation of CBZ by UV/chlorine in acidic solutions was more efficient than that in basic solutions mainly due to the effect of pH on the dissociation of HOCl and OCl(-) and then on the quantum yields and radical species quenching of UV/chlorine. When pH was increased from 5.5 to 9.5, the rate constants of degradation of CBZ by OH decreased from 0.65 to 0.14 min(-1) and that by Cl decreased from 0.40 to 0.11 min(-1). The rate constant for the reaction between Cl and CBZ was 5.6 ± 1.6 × 10(10) M(-1) s(-1). Anions of HCO3(-) (1-50 mM) showed moderate inhibition of CBZ degradation by UV/chlorine, while Cl(-) did not. UV/chlorine could efficiently degrade CBZ in wastewater treatment plant effluent, although the degradation was inhibited by about 30% compared with that in ultrapure water with chlorine dosage of 0.14-0.56 mM. Nine main oxidation products of the CBZ degradation by UV/chlorine were identified using the HPLC-QToF MS/MS. Initial oxidation products arose from hydroxylation, carboxylation and hydrogen atom abstraction of CBZ by OH and Cl, and were then further oxidized to generate acylamino cleavage and decarboxylation products of acridine and acridione.
Ultraviolet (UV) disinfection is an effective technology for the inactivation of pathogens in water and is of growing interest for industrial application. A new UV source — ultraviolet light-emitting diode (UV-LED) — has emerged in the past decade with a number of advantages compared to traditional UV mercury lamps. This promising alternative raises great interest in the research on application of UV-LEDs for water treatment. Studies on UV-LED water disinfection have increased during the past few years. This article presents a comprehensive review of recent studies on UV-LEDs with various wavelengths for the inactivation of different microorganisms. Many inconsistent and incomparable data were found from published studies, which underscores the importance of establishing a standard protocol for studying UV-LED inactivation of microorganisms. Different UV sensitivities to UV-LEDs and traditional UV lamps were observed in the literature for some microorganisms, which requires further investigation for a better understanding of microorganism response to UV-LEDs. The unique aspects of UV-LEDs improve inactivation effectiveness by applying LED special features, such as multiple wavelengths and pulsed illumination; however, more studies are needed to investigate the influencing factors and mechanisms. The special features of UV-LEDs offer the flexibility of novel reactor designs for a broad application of UV-LED reactors.
Solar irradiation of aqueous solutions containing free available chlorine (FAC) dramatically enhances inactivation of Cryptosporidium parvum oocysts compared to FAC or sunlight alone. In pH 8, 10 mM phosphate buffer at 25 °C, exposure to FAC alone yields no oocyst inactivation at CTFAC ≤ 832 (mg min) L-1, while exposure to simulated sunlight alone for 60 min yields <0.5 log inactivation. In contrast, exposure to simulated sunlight for 60 min in the presence of [FAC]0 = 8 mg L-1 as Cl2 results in photolytic decomposition of FAC to ∼1 mg L-1 as Cl2 [yielding CTFAC ∼ 200 (mg min) L-1] accompanied by >2 log oocyst inactivation. Similar enhancement effects are observed in natural water under natural sunlight. Experiments undertaken in the presence of the reactive oxygen species (ROS) scavenger tert-butanol or in the absence of oxygen indicate that these enhancements are due to in situ ROS and ozone production via FAC photolysis.
The implementation of ultraviolet light (UV) disinfection in drinking water applications in the United States has gained momentum in recent years. An online survey and field sampling of full-scale drinking water treatment plants (WTPs) were performed with a focus on evaluation of sequenced disinfection (UV and chlorine/chloramines). Twenty-seven WTPs responded to the survey. Nearly all of these WTPs were treating an impaired water source (subject to upstream agricultural or wastewater discharges) and using UV with a design dose of 40 mJ/cm2 installed as part of a plant retrofit. The majority (n = 16) of the disinfection systems used polychromatic mediumpressure UV. Twelve WTPs applied all or part of their secondary disinfectant (free chlorine or chloramines) upstream of the UV reactor. The majority of the WTPs reported no effect on chlorine residual or formation of disinfection by-products. Full-scale sampling indicated minimal (±3 μg/L) change in trihalomethane formation resulting from UV disinfection during multiple sampling events.
Introduction and Overview Electronic, nuclear and spin state of electronically excited states Transitions between states Radiative transitions between states Radiationless transitions between states Theoretical organic photochemistry Energy and electron transfer processes Mechanistic organic photochemistry Photochemistry of carbonyl compounds Photochemistry of olefins Photochemistry of enones and dienones Photochemistry of aromatic molecules Supramolecular organic photochemistry Molecular oxygen and organic photochemistry
The objective of this study was to investigate the effects of combined low-pressure ultraviolet (LPUV) irradiation and free chlorination on the formation of trichloronitromethane (TCNM) by-product from amine precursors, including a commonly used polyamine coagulant aid (poly(epichlorohydrin dimethylamine)) and simple alkylamines dimethylamine (DMA) and methylamine (MA). Results showed that TCNM formation can increase by up to 15 folds by combined UV/chlorine under disinfection to advanced oxidation conditions. The enhancement effect is influenced by UV irradiance, chlorine dose and water pH. Extended reaction time leads to the decay of TCNM by direct photolysis. The combined UV/chlorine conditions significantly promoted degradation of polyamine to generate intermediates, including DMA and MA, which are better TCNM precursors than polyamine, and also facilitated transformation of these amine precursors to TCNM. Under combined UV/chlorine, polyamine degradation was likely promoted by radical oxidation, photodecay of chlorinated polyamine and chlorine oxidation/substitution. Promoted TCNM formation from primary amine MA was primarily due to radicals' involvement. Promoted TCNM formation from secondary amine DMA likely involved a combination of radical oxidation, photo-enhanced chlorination reactions and other unknown mechanisms. Insights obtained in this study are useful for reducing TCNM formation during water treatment when both UV and chlorine will be encountered.
The UV/free chlorine process forms reactive species such as hydroxyl radicals (HO•), chlorine atoms (Cl•), Cl2•- and O•-. The specific roles of these reactive species in aqueous micropollutant degradation in the UV/chlorine process under different conditions were investigated using a steady-state kinetic model. Benzoic acid (BA) was chosen as the model micropollutant. The steady-state kinetic model developed fitted the experimental data well. The results showed that HO• and Cl• contributed substantially to BA degradation, while the roles of the other reactive species such as Cl2•- and O•- were negligible. The overall degradation rate of BA decreased as the pH increased from 6 to 9. In particular, the relative contributions of HO• and Cl• to the degradation changed from 34.7% and 65.3% respectively at pH 6 to 37.9% and 62% respectively at pH 9 under the conditions evaluated. Their relative contributions also changed slightly with variations in chlorine dosage, BA concentration and chloride concentration. The scavenging effect of natural organic matter (NOM) on Cl• was relatively small compared to that on HO•, while bicarbonate preferentially reduced the contribution of Cl•. This study is the first to demonstrate the contributions of different reactive species to the micropollutant degradation in the UV/chlorine system under environmentally relevant conditions.
Aqueous free available chlorine (FAC) can be photolyzed by sunlight and/or artificial UV light to generate various reactive oxygen species, including HO(•) and O((3)P). The influence of this chemistry on inactivation of chlorine-resistant microorganisms was investigated using Bacillus subtilis endospores as model microbial agents and simulated and natural solar radiation as light sources. Irradiation of FAC solutions markedly enhanced inactivation of B. subtilis spores in 10 mM phosphate buffer; increasing inactivation rate constants by as much as 600%, shortening inactivation curve lag phase by up to 73% and lowering CTs required for 2 log10 inactivation by as much as 71% at pH 8.0 and 10 °C. Similar results were observed at pH 7.4 and 10 °C in two drinking water samples with respective DOC concentrations and alkalinities of 0.6 and 1.2 mg C/L and 81.8 and 17.1 mg/L as CaCO3. Solar radiation alone did not inactivate B. subtilis spores under the conditions investigated. A variety of experimental data indicate that the observed enhancements in spore inactivation can be attributed to the concomitant attack of spores by HO(•) and O3, the latter of which was found to accumulate to micromolar concentrations during simulated solar irradiation of 10 mM phosphate buffer (pH 8, 10 °C) containing [FAC]0 = 8 mg/L as Cl2.
Kinetic data for the radicals H⋅ and ⋅OH in aqueous solution,and the corresponding radical anions, ⋅O− and eaq−, have been critically pulse radiolysis, flash photolysis and other methods. Rate constants for over 3500 reaction are tabulated, including reaction with molecules, ions and other radicals derived from inorganic and organic solutes.
The Greater Cincinnati (Ohio) Watsr Works strives to continually improve the effectiveness of its water treatment processes. Questions regarding the vulnerability of the Ohio River watershed to microbial contamination and concerns about compliance with the Long Term 2 Enhanced Surface Water Treatment Rule led the utility to initiate an in-depth study of ultraviolet (UV) disinfection. The study was designed to determine how exposing treated Ohio River water to UV affected (1) the inactivation of Cryptosporidium parvum, bacteria, and viruses, (2) the susceptibility of these microorganisms to free chlorine after partial inactivation with UV, (3) the formation of oxidation and chlorinated disinfection by-products (DBPs), and (4) microbial regrowth (measured by assimilated organic carbon) following UV disinfection. Results indicated that UV alone or UV followed by free chlorine effectively controlled all microorganisms tested. Additionally, no evidence of increased DBP formation or regrowth was observed at UV doses typically used for disinfection.
The radiolysis of aqueous solutions of ClO– and ClO–2 has been investigated using pulse and steady state methods. The absorption spectrum of ClO has been identified (λmax= 280 nm) and the complete reaction mechanism has been established in the case of ClO– and proposed in the case of ClO–2. The following rate constants have been evaluated (units of M–1 s–1): e–aq+ ClO–→ Cl–+ O–(5.3 ± 1.0)× 1010, e–aq+ ClO–2→ ClO–+ O–(4.5 ± 0.5)× 1010, OH + ClO–→ ClO + OH–(9.0 ± 0.5)× 109, OH + ClO–2→ ClO2+ OH–(6.3 ± 0.5)× 109, O–+ ClO–→ ClO + O2–(2.4 ± 0.1)× 108, O–+ ClO–2→ ClO2+ O2–(1.9 ± 0.1)× 108, 2ClO → Cl2O2 1.5 × 1010(2k)
Acid dissociation constants for hypochlorous acid, HOCl, relative to the second ionization constant for H3PO4 have been determined by a spectrophotometric technique with a precision of 0.005 unit in pKa at temperatures from 4 to 34°. Values of pKa have been evaluated from the data. The results, considered accurate to 0.01 unit, are: 7.825 (0°), 7.754 (5°), 7.690 (10°), 7.633 (15°), 7.582 (20°), 7.537 (25°), 7.497 (30°), and 7.463 (35°).
A batch reactor was used to evaluate the advanced oxidation process of the UV/H2O2 system for control of natural organic matter (NOM) in drinking water. The light sources used include a 450 W high-pressure mercury vapor lamp and sunlight. Both quartz and Pyrex filters were used to control the wavelength and energy of UV light applied to the aqueous systems. The results showed that NOM oxidation and H2O2 decomposition followed first-order and zero-order reaction kinetics, respectively. The optimum H2O2 dose was found to be 0.01% for the oxidation of humic acids in this study. Carbonate and bicarbonate ions inhibited the degradation of humic acids.
Ultraviolet (UV) irradiation is commonly applied as a secondary disinfection process in chlorinated pools. UV-based systems have been reported to yield improvements in swimming pool water and air chemistry, but to date these observations have been largely anecdotal. The objectives of this investigation were to evaluate the effects of UV irradiation on chlorination of important organic-N precursors in swimming pools. Creatinine, L-arginine, L-histidine, glycine, and urea, which comprise the majority of the organic-N in human sweat and urine, were selected as precursors for use in conducting batch experiments to examine the time-course behavior of several DBPs and residual chlorine, with and without UV(254) irradiation. In addition, water samples from two natatoria were subjected to monochromatic UV irradiation at wavelengths of 222 nm and 254 nm to evaluate changes of liquid-phase chemistry. UV(254) irradiation promoted formation and/or decay of several chlorinated N-DBPs and also increased the rate of free chlorine consumption. UV exposure resulted in loss of inorganic chloramines (e.g., NCl(3)) from solution. Dichloromethylamine (CH(3)NCl(2)) formation from creatinine was promoted by UV exposure, when free chlorine was present in solution; however, when free chlorine was depleted, CH(3)NCl(2) photodecay was observed. Dichloroacetonitrile (CNCHCl(2)) formation (from L-histidine and L-arginine) was promoted by UV(254) irradiation, as long as free chlorine was present in solution. Likewise, UV exposure was observed to amplify cyanogen chloride (CNCl) formation from chlorination of L-histidine, L-arginine, and glycine, up to the point of free chlorine depletion. The results from experiments involving UV irradiation of chlorinated swimming pool water were qualitatively consistent with the results of model experiments involving UV/chlorination of precursors in terms of the behavior of residual chlorine and DBPs measured in this study. The results indicate that UV(254) irradiation promotes several reactions that are involved in the formation and/or destruction of chlorinated N-DBPs in pool settings. Enhancement of DBP formation was consistent with a mechanism whereby a rate-limiting step in DBP formation was promoted by UV exposure. Promotion of these reactions also resulted in increases of free chlorine consumption rates.
The destruction of the commonly found cyanobacterial toxin, microcystin-LR (MC-LR), in surface waters by UV-C/H(2)O(2) advanced oxidation process (AOP) was studied. Experiments were carried out in a bench scale photochemical apparatus with low pressure mercury vapor germicidal lamps emitting at 253.7 nm. The degradation of MC-LR was a function of UV fluence. A 93.9% removal with an initial MC-LR concentration of 1 μM was achieved with a UV fluence of 80 mJ/cm(2) and an initial H(2)O(2) concentration of 882 μM. When increasing the concentration of MC-LR only, the UV fluence-based pseudo-first order reaction rate constant generally decreased, which was probably due to the competition between by-products and MC-LR for hydroxyl radicals. An increase in H(2)O(2) concentration led to higher removal efficiency; however, the effect of HO scavenging by H(2)O(2) became significant for high H(2)O(2) concentrations. The impact of water quality parameters, such as pH, alkalinity and the presence of natural organic matter (NOM), was also studied. Field water samples from Lake Erie, Michigan and St. Johns River, Florida were employed to evaluate the potential application of this process for the degradation of MC-LR. Results showed that the presence of both alkalinity (as 89.6-117.8 mg CaCO(3)/L) and NOM (as ∼2 to ∼9.5 mg/L TOC) contributed to a significant decrease in the destruction rate of MC-LR. However, a final concentration of MC-LR bellow the guideline value of 1 μg/L was still achievable under current experimental conditions when an initial MC-LR concentration of 2.5 μg/L was spiked into those real water samples.
Several organic compounds were used as radical scavengers/reagents to investigate the possibility of the UV/chlorine process being used as an advanced oxidation process (AOP) in the treatment of drinking water and wastewater. The UV/H(2)O(2) process was selected as a reference, so that the results from the UV/chlorine process could be compared with those of the UV/H(2)O(2) process. Methanol was added to active chlorine solutions at both pH 5 and 10 and into hydrogen peroxide samples. The photodegradation quantum yields and the OH radical production yield factors, which are significant in evaluating AOPs, were calculated for both the UV/chlorine and the UV/H(2)O(2) processes. The yield factor for the UV/chlorine process at pH 5 was 0.46 ± 0.09, which is much lower than that of the UV/H(2)O(2) process, which reached 0.85 ± 0.04. In addition to methanol, para-chlorobenzoic acid (pCBA) and cyclohexanoic acid (CHA) were added to active chlorine solutions and to H(2)O(2) solutions, to evaluate the efficiencies of oxidizing these organic compounds. The specific first-order reaction rate constants for the oxidation of pCBA and CHA, using the UV/chlorine process, were lower than those found using the UV/H(2)O(2) process.
GaN-based ultraviolet-C (UV-C) light emitting diodes (LEDs) are of great interest for water disinfection. They offer significant advantages compared to conventional mercury lamps due to their compact form factor, low power requirements, high efficiency, non-toxicity, and overall robustness. However, despite the significant progress in the performance of semiconductor based UV LEDs that has been achieved in recent years, these devices still suffer from low emission power and relatively short lifetimes. Even the best UV LEDs exhibit external quantum efficiencies of only 1-2%. The objective of this study was to investigate the suitability of GaN-based UV LEDs for water disinfection. The investigation included the evaluation of the performance characteristics of UV LEDs at different operating conditions as well as the design of a UV LED module in view of the requirements for water treatment applications. Bioanalytical testing was conducted using Bacillus subtilis spores as test organism and UV LED modules with emission wavelengths of 269 nm and 282 nm. The results demonstrate the functionality of the developed UV LED disinfection modules. GaN-based UV LEDs effectively inactivated B. subtilis spores during static and flow-through tests applying varying water qualities. The 269 nm LEDs reached a higher level of inactivation than the 282 nm LEDs for the same applied fluence. The lower inactivation achieved by the 282 nm LEDs was compensated by their higher photon flux. First flow-through tests indicate a linear correlation between inactivation and fluence, demonstrating a well designed flow-through reactor. With improved light output and reduced costs, GaN-based UV LEDs can provide a promising alternative for decentralised and mobile water disinfection systems.
The aim of our study was to determine the impact of medium-pressure UV lamps radiation on water quality in a chlorinated indoor swimming pool. An indoor swimming pool was equipped with two medium-pressure UV lamps. We collected eight samples of water daily over a four-weeks period and measured total and free chlorine, pH, water temperature, bacteriological parameters, total organic carbon and trihalomethanes. During the first week, which served as control, medium-pressure UV lamps were turned off. During the next three weeks, medium-pressure UV lamps were kept on 24 h per day. The third week, we reduced the level of the injected chlorine into water, and the last week we also reduced the water renewal volume by 27%. Our results showed that bacteriological parameters remained within allowable french limits. When medium-pressure UV lamps were kept on, total, free and active chlorine levels were significantly increased (P<0.001), whereas combined chlorine level were significantly decreased (P<0.001 and P<0.05, respectively). The levels of chloroform and bromodichloromethane were significantly increased when medium-pressure UV lamps were kept on (P<0.001), whereas chlorodibromomethane and bromoform levels significantly decreased (P<0.05 and P<0.001, respectively). The additional formation of chloroform and bromodichloromethane may be explained by the increase in active chlorine and by radicalizing mechanisms initiated by UV radiation.
A new concept is introduced to characterize and model the UV/H2O2 advanced oxidation process (AOP) in water. Similarto the Rct concept used to describe OH radical exposure per ozone dose, the ROH,UV concept is defined as the experimentally determined *OH radical exposure per UV fluence. ROH,UV was determined by examining the destruction of a probe compound, para-chlorobenzoic acid in four different waters: DI water and three natural waters. ROH,UV was found to be affected greatly by water quality, specifically background *OH radical scavenging, which competed forthe formed *OH radical with the probe compound, and background UV absorbance, which screened UV irradiation from the hydrogen peroxide. The ROH,UV values determined in the experiments using low-pressure Hg lamp were greater than those for the medium-pressure Hg lamp . Finally, the ROH,UV concept was utilized to calculate an overall scavenging factor for each water matrix, and this was successfully utilized in conjunction with the steady-state *OH radical model to improve the prediction of the oxidation of endocrine-disrupting compounds 17-alpha-ethinyl estradiol and 17-beta-estradiol in the natural waters.
The photodegradation of chlorine-based disinfectants NH(2)Cl, HOCl, and OCl(-) under UV irradiation from low- (LP) and medium-pressure (MP) Hg lamps was studied. The quantum yields of aqueous chlorine and chloramine under 254 nm (LP UV) irradiation were greater than 1.2 mol Es(-1) for free chlorine in the pH range of 4-10 and 0.4 mol Es(-1) for monochloramine at pH 9. Quantum yields for MP (200-350 nm) ranged from 1.2 to 1.7 mol Es(-1) at neutral and basic pH to 3.7 mol Es(-1) at pH 4 for free chlorine, and 0.8 mol Es(-1) for monochloramine. Degradation of free chlorine was enhanced under acidic water conditions, but water quality negatively impacted the MP Hg lamp degradation of free chlorine, compared to the LP UV source. The production of hydroxyl radical via chlorine photolysis was assessed along with the rate of reaction between (*)OH and HOCl using radical scavengers (parachlorobenzoic acid and nitrobenzene) in chlorinated solutions at pH 4. The quantum yield of OH radical production from HOCl at 254 nm was found to be 1.4 mol Es(-1), while the reaction of HOCl with OH radical was measured as 8.5 x 10(4)M(-1)s(-1). NH(2)Cl was relatively stable in all irradiated solutions, with <0.3 mg L(-1) increase in nitrate following a UV dose of 1000 mJ cm(-2). For water treatment plants, no significant changes in chlorine concentration would be expected under typical pH levels and UV doses; however, the formation of (*)OH could have implications for chlorinated byproducts or decay of unwanted chemical contaminants.
The transport and discharge of ship ballast water has been recognized as a major vector for the introduction of invasive species. Chemical oxidants, long used in drinking water and wastewater treatment, are alternative treatment methods for the control of invasive species currently being tested for use on ships. One concern when a ballasted vessel arrives in port is the adverse effects of residual oxidant in the treated water. The most common oxidants include chlorine (HOCl/OCl-), bromine (HOBr/OBr-), ozone (03), hydrogen peroxide (H2O2), chlorine dioxide (ClO2), and monochloramine (NH2Cl). The present study was undertaken to evaluate the sunlight-mediated photochemical decomposition of these oxidants. Sunlight photodecomposition was measured at various pH using either distilled water or oligotrophic Gulf Stream water for specific oxidants. For selected oxidants, quantum yields at specific wavelengths were obtained. An environmental photochemical model, GCSOLAR, also provided predictions of the fate (sunlight photolysis half-lives) of HOCI/OCl-, HOBr/OBr-, ClO2, and NH2Cl for two different seasons at latitude 40 degrees and in water with two different concentrations of chromophoric dissolved organic matter. These data are useful in assessing the environmental fate of ballast water treatment oxidants if they were to be discharged in port.
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