Article

# Production of Hydrated Electrons from Photoionization of Dissolved Organic Matter in Natural Waters

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## Abstract

Under UV irradiation, an important primary photochemical reaction of colored dissolved organic matter (CDOM) is electron ejection to produce hydrated electrons (e-aq). The efficiency of this process has been studied in both fresh water and seawater samples with both steady-state scavenger (S-SS) and time-resolved laser flash photolysis (LFP) methods. However, the apparent quantum yields (AQYs) of e-aq for the same samples using the two methods differ by as much as a factor of 100, necessitating a closer re-examination of how the process is measured. We developed a highly sensitive multipass LFP apparatus that allows detection of transient species at very low and variable UV irradiation intensities. Under single-photon conditions, we measured the AQY of e-aq from Laurentian fulvic acid as 1.3 x 10(-4), and set the upper limit for other CDOM samples at 6 x 10(-5), bringing the LFP results into agreement with those from S-SS methods. We also examined the ionization at elevated irradiation intensities and clearly demonstrated that multiphoton ionization occurs at intensities well below those usually used in LFP experiments, but well above those likely to occur at the earth's surface. This multiphoton ionization is probably the cause of the high AQYs reported by earlier LFP work. In addition, we also observed in real time other photochemical reactions, such as triplet quenching and bleaching, in the single photon regime.

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... The good agreement between the triplet energy content calculated here for the Buffle's model and reported photoacoustic data [21,22] Table 1 are obtained. These photoionization quantum yields are on the order of those obtained for FA and CDOM by LFP [21,52]. However, it is now accepted from steady-state measurements that the photoionization quantum yields of humic substances vary from approximately 10 -5 to 10 -4 over the wavelength range 300 to 400 nm [53]. ...
... However, it is now accepted from steady-state measurements that the photoionization quantum yields of humic substances vary from approximately 10 -5 to 10 -4 over the wavelength range 300 to 400 nm [53]. To account for this discrepancy, it was argued that the higher values obtained in LFP experiments are due to multiphoton ionization [52]. ...
... Given the low efficiencies of formation in natural waters and the fact that other components such as tryptophan (free or in combined form) generate solvated electrons upon irradiation, it is unlikely that CDOM-derived eaq has a significant environmental impact [52]. However, phenoxyl radicals can be formed not only upon photoionization, but also as a result of charge-transfer processes (see reactions (4) and (3)). ...
Article
Laser flash-photolysis experiments (λexc = 355 nm) performed with aqueous solutions of Pony Lake and Waskish Peat fulvic acids at pH = 2 showed formation of transient species with absorption maxima at 470–480 nm with O2-independent lifetimes of 600–700 μs, which were assigned to a phenoxyl radicals. Formation of these radicals occurs after photoionization of the fulvic acids followed by fast deprotonation process. Photoacoustic experiments at the same excitation wavelength yielded the O2-independent fraction of energy stored at times longer than 135 ns. DFT calculations were performed to estimate the absorption spectra and energy content of the phenoxyl radicals and triplet excited state of the Buffle’s model of the fulvic acid. Combination of the DFT and photoacoustic data yielded the photoionization quantum yields of both FA, which were compared to reported values obtained by steady-state and time-resolved techniques.
... [30][31][32][33] The optical and photochemical properties of CDOM vary with season, CDOM source, and water chemistry. 2, [34][35][36][37][38][39][40][41] An outstanding challenge for environmental scientists is to understand these variations and predict their impact on elemental cycling and the fate of anthropogenic compounds. ...
... Photoionization of CDOM to produce e À (aq.) has been demonstrated at excitation wavelengths from 250 to 400 nm using both steady-state and laser ash photolysis techniques. 37,111,140,[161][162][163] The source of e À (aq.) is ...
... 161 However, recent work has shown that this value is much too large due to multi-photon excitation. 37,111 It is now established that F e À varies from approximately 10 À4 to 10 À5 over the wavelength range 300 to 400 nm. 37,111 Given such low efficiencies and the fact that other components of natural waters such as tryptophan (free or in combined form) generate e À (aq.), 111 it is unlikely that CDOMderived e À (aq.) has a signicant environmental impact. ...
Article
Absorption of sunlight by chromophoric dissolved natural organic matter (CDOM) is environmentally significant because it controls photic zone depth and causes photochemistry that affects elemental cycling and contaminant fate. Both the optics (absorbance and fluorescence) and photochemistry of CDOM display unusual properties that cannot easily be ascribed to a superposition of individual chromophores. These include (i) broad, unstructured absorbance that decreases monotonically well into the visible and near IR, (ii) fluorescence emission spectra that all fall into a single envelope regardless of the excitation wavelength, and (iii) photobleaching and photochemical quantum yields that decrease monotonically with increasing wavelength. In contrast to a simple superposition model, these phenomena and others can be reasonably well explained by a physical model in which charge-transfer interactions between electron donating and accepting chromophores within the CDOM control the optical and photophysical properties. This review summarizes current understanding of the processes underlying CDOM photophysics and photochemistry as well as their physical basis.
... The deactivation of an electronically excited state of CDOM then occurs via, e.g., ionization of CDOM to produce hydrated electrons (e À aq ) (Blough and Zepp, 1995 and refs. cited therein; Bruccoleri et al., 1993; Wang et al., 2007b; Zepp et al., 1987): ...
... 4). To assess quantum yields of primary photoproduct formation and to elucidate the mechanisms involved in light absorption and in the deactivation of excited states of CDOM, time-resolved spectroscopic methods are required, which so far have been applied in only a few environmental photochemical studies (Bruccoleri et al., 1993; Del Vecchio and Blough, 2004b; Wang et al., 2007b). Many studies have, however, determined apparent quantum yields of photochemical decomposition of CDOM (Vähätalo and Wetzel, 2004) and stable photoproduct formation like dissolved inorganic carbon (DIC) (Gao and Zepp, 1998; Johannessen and Miller, 2001; Miller and Zepp, 1995), CO (BØlanger et al., 2006; Gao and Zepp, 1998; Stubbins et al., 2008; Valentine and Zepp, 1993; Zhang et al., 2006; Ziolkowski and Miller, 2007 ), and carbonyl compounds (Kieber et al., 1990 ). Regarding stable photoproduct formation, one has to bear in mind that these can be formed via direct and indirect photochemical transformations of CDOM, e.g., attack of CDOM by reactive intermediates that are produced in primary photochemical processes (Del Vecchio and Blough, 2002) or via iron-catalyzed CDOM transformation (Gao and Zepp, 1998). ...
... For example, the study by Hertkorn et al. (2006), involving a combination of NMR, capillary electrophoresis (Schmitt-Kopplin et al., 1998b), and FT-ICR-MS, is one of the promising analytical approaches to decipher what is referred to as molecularly uncharacterized carbon (Hatcher, 2004; Hedges et al., 2000). Furthermore, for the elucidation of light-induced processes on a molecular basis, time-resolved spectroscopic methods are needed, which so far have been applied in only a few environmental-photochemical studies (Bruccoleri et al., 1993; Del Vecchio and Blough, 2004b; Wang et al., 2007b). In assessing UV-induced changes of the chemical composition of DOM and thus of carbon cycling, the role of climate change also has to be taken into account (Zepp et al., 2007) (Fig. 7). ...
Article
UV-induced transformations of colored dissolved organic matter (CDOM, which is part of dissolved organic matter, DOM) affect CDOM absorption properties resulting in the loss of color (referred to as photobleaching). CDOM photobleaching increases the penetration depths of the damaging UV-B radiation into water bodies and strongly depends on the wavelength of solar radiation and on the pH of aquatic systems. UV-induced transformations also affect DOM availability to bacterioplankton, often enhancing the bioavailability of terrigenous DOM and in turn microbial respiration. The combination of UV-induced enhancement of DOM bioavailability and increased export of terrigeneous DOM into estuaries and coastal waters due to climate-related changes in continental hydrology could result in a UV-mediated positive feedback of CO2 accumulation in the atmosphere. The extent and type of CDOM photobleaching and of UV-induced changes in DOM bioavailability depend on (C)DOM chemical composition, which in turn undergoes drastic changes upon UV-induced transformations. Therefore, the chemical characterization of (C)DOM is key for rationalizing UV-induced transformations. In the second section (after the “Introduction”), we review important methods for the elucidation of the chemical composition of (C)DOM. However, this article is not intended to be comprehensive regarding (C)DOM chemical characterization. An important purpose is to provide photochemical bases for the understanding of UV-induced changes of (C)DOM absorption properties and bioavailability (mainly discussed in the sections “Pathways of DOM phototransformations” and ”UV-induced changes of the absorption properties of CDOM”).
... However, the most apparent contradiction lies within the fact that the scientific community has accepted it, the World Health organization (WHO) endorses it as a prevention method, but it is still rather poorly elucidated. Still the main factors which are agreed to be driving the inactivation process are the direct action of UVB (when transmitted through the reactor walls) [6], the indirect actions of UVA [7], temperature [8], and their synergies [9], but the natural presence of constituents such as metals (iron, and copper) or organic matter, can blur the understanding of the process [10,11]. ...
... 2 ) of the reaction were calculated by the following equations (Eqs. (11) and (12)): ...
Article
In this work, the role of dissolved oxygen in the solar and the photo-Fenton-mediated E. coli inactivation process was put under scrutiny. The effect of transient species that were produced in the presence of various natural organic matter isolates (NOM), namely Suwannee River (SR) NOM, Nordic Reservoir (NR) NOM, SR Humic acid (SRHA), and SR Fulvic acid (SRFA) was studied in detail. The role of 1 in this reaction was systematically evaluated by modifying the O2 concentration (N2/O2 purging) and the matrix composition (10, 50, and 100% deuterium oxide (D2O) v/v). In the presence of NOM, 1O2 was generated and the enhancement of E. coli inactivation rate due to charge transfer from triplet state to molecular oxygen. The comparison between SR and NR NOM showed that for these compounds, triplet state of NOM (3NOM*), and 1O2 were the more favorable active species in E. coli inactivation, respectively. Also, the second order rate constants (k2nd) of E. coli with 3NOM*, and 1 were calculated by using the steady state approximation. The obtained results showed that the rate values of 1 related to NR NOM was ~ 5.6 times higher than SR NOM, while the rate values of 3NOM* for SR NOM was ~ 8.7 times higher than NR NOM. We also determined the effect of these organic matter isolates in the photo-Fenton process and its constituents (solar/Fe2+, solar/H2O2, and solar/Fe2+/H2O2). In presence of NOM, the photo-Fenton process inactivation rates increased which confirmed that the combined processes has additional pathways generated with disinfecting effect during solar exposure of bacteria.
... The ground state chromophores contained within HA are promoted to their excited states upon the absorption of a photon. The initial excited state can undergo a number of processes, including charge separation with the formation of hydrated electron (e − aq) and an HA radical cation (HA •+ ) [28] or generation of a singlet excited state ( 1 HA*). The molecular oxygen from the surrounding environment can scavenge the hydrated electron to produce superoxide anion radical (O2 •− ) [24]. 1 HA* is relatively short-lived, can undergo fluorescence or internal conversion (IC), leading back to the ground state (HA) [27]. 1 HA* can produce the lowest excited triplet state of HA ( 3 HA*) through intersystem crossing [27]. ...
... The ground state chromophores contained within HA are promoted to their excited states upon the absorption of a photon. The initial excited state can undergo a number of processes, including charge separation with the formation of hydrated electron (e − aq ) and an HA radical cation (HA •+ ) [28] or generation of a singlet excited state ( 1 HA*). The molecular oxygen from the surrounding environment can scavenge the hydrated electron to produce superoxide anion radical (O 2 •− ) [24]. 1 HA* is relatively short-lived, can undergo fluorescence or internal conversion (IC), leading back to the ground state (HA) [27]. 1 HA* can produce the lowest excited triplet state of HA ( 3 HA*) through intersystem crossing [27]. ...
Article
Full-text available
The oxidation of highly toxic arsenite (As(III)) was studied using humic acid-coated magnetite nanoparticles (HA-MNP) as a photosensitizer. Detailed characterization of the HA-MNP was carried out before and after the photoinduced treatment of As(III) species. Upon irradiation of HA-MNP with 350 nm light, a portion of the As(III) species was oxidized to arsenate (As(V)) and was nearly quantitatively removed from the aqueous solution. The separation of As(III) from the aqueous solution is primarily driven by the strong adsorption of As(III) onto the HA-MNP. As(III) removals of 40–90% were achieved within 60 min depending on the amount of HA-MNP. The generation of reactive oxygen species (•OH and 1O2) and the triplet excited state of HA-MNP (3HA-MNP*) was monitored and quantified during HA-MNP photolysis. The results indicate 3HA-MNP* and/or singlet oxygen (1O2) depending on the reaction conditions are responsible for converting As(III) to less toxic As(V). The formation of 3HA-MNP* was quantified using the electron transfer probe 2,4,6-trimethylphenol (TMP). The formation rate of 3HA-MNP* was 8.0 ± 0.6 × 10−9 M s−1 at the TMP concentration of 50 µM and HA-MNP concentration of 1.0 g L−1. The easy preparation, capacity for triplet excited state and singlet oxygen production, and magnetic separation suggest HA-MNP has potential to be a photosensitizer for the remediation of arsenic (As) and other pollutants susceptible to advanced oxidation.
... It has been known that polyhydroxy aromatic compounds and aromatic carboxylic acids, primary components of humic acid in soils or natural waters, produce hydrated electrons when they absorb UV or near-UV light. [33][34][35] As compared to the technologies of subcritical Fe 0 , 21 vitamin B 12 /Ti(III)-citrate 36 and UV/KI, 19 the photosensitized humic acid is more environmentally oriented and applicable since humic acid, a natural organic matter (NOM), is found in natural environments in the range of 10 ppm. ...
... For instance, Zepp et al. 35 determined φ e in a 355 nm-laser flash photolysis for various natural organic matters, ranging from 1.7×10 −3 to 7.6×10 −3 . Wang et al. 33 compared the low and high intensity irradiations of the same wavelength and found that φ e in natural sunlight decreased by a factor of 100, as compared to that of a highly intensive laser flash photolysis. Furthermore, Thomas-Smith and Blough 34 observed that φ e of colored dissolved organic matters decreased with increasing wavelength (e.g., 1.2×10 −4 ≤ φ e,296 nm ≤ 2.7×10 −4 ; 2.1×10 −5 ≤ φ e,355 nm ≤ 9.0×10 −5 ). ...
Article
Perfluorinated chemicals (PFCs), as represented by perfluorooctanoate (PFOA: C 7F 15CO 2-) and perfluorooctane sulfonate (PFOS: C 8F 17SO 3-) have been widely used and detected over the globe. Their chemical inertness makes them highly resistant toward not only conventional treatments but also advanced oxidation processes. In this study, the hydrated electron-mediated reductive degradation of PFOX (X = A for PFOA and S for PFOS) was explored in an aqueous humic acid solution in which UV light is irradiated to excite humic acid, thereby ejecting electrons to the aqueous PFOX. The PFOX degradation was monitored with MS/MS and was further confirmed by quantifying fluorides. The quantum yield for the PFOX degradation was estimated to be as low as 10 -4. The F-index, the degree of defluorination ([F -] produced / [PFOX] degraded), was 0.7 and 2.3 for PFOA and PFOS, respectively, indicating they were partially degraded. In addition, it was determined that an electron-donating ethylenediaminetetraacetic acid (EDTA) is essentially required to inhibit the recombination between the oxidized humic acid and the hydrated electron; yet it appears to interfere with the one-electron-reduced PFOX behavior at a different degree depending on the varying ionic headgroups (carboxylate vs. sulfonate).
... However, the most apparent contradiction lies within the fact that the scientific community has accepted it, the World Health organization (WHO) endorses it as a prevention method, but it is still rather poorly elucidated. Still the main factors which are agreed to be driving the inactivation process are the direct action of UVB (when transmitted through the reactor walls) [6], the indirect actions of UVA [7], temperature [8], and their synergies [9], but the natural presence of constituents such as metals (iron, and copper) or organic matter, can blur the understanding of the process [10,11]. ...
... 2 ) of the reaction were calculated by the following equations (Eqs. (11) and (12)): ...
Article
In this work, the role of dissolved oxygen in the solar and the photo-Fenton-mediated E. coli inactivation process was put under scrutiny. The effect of transient species that were produced in the presence of various natural organic matter isolates (NOM), namely Suwannee River (SR) NOM, Nordic Reservoir (NR) NOM, SR Humic acid (SRHA), and SR Fulvic acid (SRFA) was studied in detail. The role of 1O2 in this reaction was systematically evaluated by modifying the O2 concentration (N2 /O2 purging) and the matrix composition (10, 50, and 100% deuterium oxide (D2O) v/v). In the presence of NOM, 1O2 was generated and the enhancement of E. coli in-activation rate due to charge transfer from triplet state to molecular oxygen. The comparison between SR and NR NOM showed that for these compounds, triplet state of NOM (3NOM *), and 1O2 were the more favorable active species in E. coli inactivation, respectively. Also, the second order rate constants (k E coli nd. 2) of E. coli with 3NOM * , and 1O2 were calculated by using the steady state approximation. The obtained results showed that the rate values of 1O2 related to NR NOM was ∼ 5.6 times higher than SR NOM, while the rate values of 3NOM * for SR NOM was ∼ 8.7 times higher than NR NOM. We also determined the effect of these organic matter isolates in the photo-Fenton process and its constituents (solar/Fe2+ , solar/H2O2, and solar/Fe2+ /H2O2). In presence of NOM, the photo-Fenton process inactivation rates increased which confirmed that the combined processes has additional pathways generated with disinfecting effect during solar exposure of bacteria.
... 1. Free radicals and hydrated electrons [89]-products of metabolism [45,[53][54][55][56][57][58][59][60], products of action of radiation (including UV) [63] on other "-omes", and products of secondary reactions of the primary products with other "-omes" [ [45,115,116] 5. Some defects in crystals of biominerals [66,117] 6. Donor-acceptor complexes with strong charge transfer, e.g. in photosynthetic centres [118,119]. ...
... It can be accelerated by artificial irradiation of generative forms of life, and this is the way to obtain new mutations in classical genetics [180]. UV irradiation may lead to the electron-excited states in DNA of highly reactive though not necessary paramagnetic character; however, it is an important inducer of the oxidative stress and free radicals in a similar way generating mutations [79,80,89,91,174,181]. But the development and expansion of genetic information, thanks to mutagenesis, are easily imaginable without recourse to the radiation or UV-induced mutagenesis [182]. ...
Chapter
EPR spectroscopy and imaging remain still a branch of physical sciences, while its application to biology and medicine is wide and valuable. The EPR-measureable species create in biological systems important and well-defined pool, unique on the background of metabolomes and metallomes. At the same time, they seem to be these constituents of the system, which make it deserving to be called “alive”. I propose to coin the phrase “the paramagnetome” to name this pool and to replace the common, descriptive name of “biologically and medically-oriented EPR/ESR spectroscopy” with “paramagnetomics”, per analogiam to other “-omes” and “-omics”. A short characteristic of these two newly defined terms is proposed, which makes the paramagnetomics closely related to other branches of the systems biology. Relations to the problems of genomics and the central problems of molecular genetics, genetic information, as well as biological evolution are also discussed. The position of EPR spectroscopy and a special role that it plays in defining and understanding the phenomenon of life seem to accomplish the long expected establishing the paramagnetomics and research on paramagnetomes as a branch of biology.
... Photo-degradation of marine SPE-DOM could have been initiated by energy transfer from the CDOM/FDOM (see Fig. S1) to conjugated π-electrons and carbonyl derivatives (i.e. ketones, CONH, COOH and COOR derivatives), followed by less selective "general attack" from reactive oxygen species such as hydroxyl radicals, singlet oxygen and hydrogen peroxide (Schmitt-Kopplin et al., 1998;Shank et al., 2010;Wang et al., 2007). All photochemical experiments were undertaken in pure water and as a result the photo-production of hydroxyl radicals from nitrate (Calza et al., 2012;Zepp et al., 1987) could not occur. ...
... Studies show that humic substances can generate hydrated electrons under UV irradiation (Wang et al., 2007). However, in solution phase, most of the photo-ejected electrons would be quenched by recombination with positively charged chromophores of humic substances (Zepp et al., 1987). ...
Article
Perfluorinated compounds (PFCs) are considered as the most recalcitrant organic contaminants. Our previous research has shown that PFCs can be completely defluorinated in the UV/organoclay/3-indole acetic acid system, however, the factors that could affect the degradation of PFCs, are still not clear. In this study, we further investigated the effect of different indole derivatives and organo-modified montmorillonite on the degradation of perfluooctanoic acid (PFOA). Based on multiple linear regression analysis, our results clearly indicate that hydrated electron yields of indole derivatives, adsorption of PFOA and indole derivatives on organo-montmorillonite contributed independently to the degradation of PFOA. In addition, the results also show that the presence of humic substance (even at 10 mg C L(-1)) would not significantly suppress the degradation process due to the strong adsorption of humic substance on the organo-montmorillonite surface. This study would provide more information to design an efficient and environment-friendly system for degradation of PFCs, and this technique will have great potential for treatment of persistent contaminants under mild reaction conditions.
... The reason may be that under UV light illumination, some active species with strong reducing ability formed, e.g., hydrated electrons and reducing radicals. Many previous works have reported that the hydrated electrons can be photoproduced from the ionization of aromatic compounds in natural organic matter under UV irradiation (Zepp et al., 1987;Wang et al., 2007;Kumamoto et al., 1994;Thomas-Smith and Blough, 2001). The components of EPS were similar with that of natural organic matter, and the aromatic compounds were also found in our extracted EPS (Fig. S1, Table S2). ...
Article
Microbial extracellular polymeric substances (EPS) widely exist in natural environments and affect the migration and transformation of pollutants in aquatic environments. Previous works report that EPS have some reducing functional groups and can reduce heavy metals. However, because of the weak reducing capability of EPS, the reduction of heavy metals by EPS without cells is extremely slow, and its effect on heavy metals species is insignificant. In this work, the accelerated reduction of silver ions (Ag⁺) by EPS from Shewanella oneidensis MR-1 under illumination was investigated. UV–visible spectroscopy, transmission electron microscopy (TEM) coupled with an energy dispersive spectrometer (EDS) and X-ray photoelectron spectroscopy (XPS) were used to confirm the formation of silver nanoparticles (AgNPs) via the reduction of Ag⁺ by EPS under light illumination. The Ag⁺ reduction by EPS follows pseudo-first-order kinetics under both visible and UV light, and the light irradiation can significantly accelerate AgNPs formation. On the one hand, visible light can excite AgNPs for their surface plasma resonance (SPR) and accelerate the electrons from the EPS to adjacent Ag⁺. On the other hand, EPS molecules may be excited by UV light to produce strong reducing species, which enhance Ag⁺ reduction. Moreover, pH, dissolved oxygen were found to affect the formation of AgNPs by EPS. This work proves the reducing capability of EPS on the reduction of Ag⁺, and this process can be accelerated under light illumination, which may affect the speciation and transformation of heavy metals in natural waters.
... 环境水体中DOM光解可以产生水合电子(e  aq ) [19,22,23] , 研究中也有所发现 [15] . 甲基汞浓度在1~100 ng L 1 的 浓度范围内光降解速率常数基本维持不变, 然后迅 速升高, MeHg浓度超过1000 ng L 1 后, 光降解速率 常数迅速降低. ...
... And the fourth mechanism is the direct formation of extra e aq − from photoionization of HA in water. 122 However, contribution of this pathway is minor due to its low quantum yield of e aq − . 53 Meanwhile, three plausible mechanisms can inhibit the reductive degradation of PFAS in the presence of HA. ...
Article
Advanced reduction processes (ARPs) have emerged as a promising method for destruction of persistent per-and poly-fluoroalkyl substances (PFAS) in water due to the generation of short-lived and highly reductive hydrated electrons (eaq-). This study provides a critical review on the mechanisms and performance of reductive destruction of PFAS with eaq-. Unique properties of eaq- and its generation in different ARP systems, particularly UV/sulfite and UV/iodide, are overviewed. Different degradation mechanisms of PFAS chemicals, such as perfluorooctanoic acid (PFOA), perfluorooctane sulfonate (PFOS), and others (e.g. short chain perfluorocarboxylic acids (PFCAs) and perfluorosulfonic acids (PFSAs), per- and polyfluoro dicarboxylic acids, and fluorotelomer carboxylic acids), are reviewed, discussed, and compared. The degradation pathways of these PFAS chemicals rely heavily upon their head groups. For specific PFAS types, fluoroalkyl chain lengths may also affect their reductive degradation patterns. Degradation and defluorination efficiencies of PFAS are considerably influenced by solution chemistry parameters and operating factors, such as pH, dose of chemical solute (i.e. sulfite or iodide) for eaq- photo-production, dissolved oxygen, humic acid, nitrate, and temperature. Furthermore, implications of the state-of-the-art knowledge on practical PFAS control actions in water industries are discussed and the priority research needs are identified.
... DOM中的芳香基团, 如酚及其衍生物(芳香羧酸 [19] 和芳香氨基酸 [20] )等, 吸收光后跃迁至激发单线态, 易 发射出水合电子e aq (R1). 由于e aq 的量子产率(Φ e )低, 在 300~400 nm光照下, Φ e =10 −4~1 0 −5 [21,22] , 且多产生于 DOM的内部微环境中, 绝大多数e aq 与微环境中的正离 子快速结合, 因此难以扩散至溶液中与OMPs发生 反应 [23] . [23] . ...
... NOM is an omnipresent natural substance with strong redox activity and photosensitivity, which could generate multiple reactive species including . O 2 -, singlet oxygen, triplet DOM and hydrated electrons under UV/visible light irradiation [43][44][45][46]. Herein, the effect of the co-existing NOM on the photodegradation and defluorination of HFPO-TA was studied in the 2-HPA/CTAB/HFPO-TA self-assembled micelle system, given its stable micelle structure and great pH resistance. ...
Article
Herein, we developed a self-assembled micelle system to decompose/defluorinate HFPO-TA (an alternative perfluoroalkyl substances (PFASs)) under ambient conditions by using three hydroxyphenylacetic acids (2-HPA, 3-HPA, 4-HPA) as hydrated electron precursors under UV irradiation, and using cationic surfactant cetyl trimethyl ammonium bromide (CTAB) to construct the micelle structure. The formed HPAs/CTAB/HFPO-TA micelles were well characterized. All the three micelle systems could efficient destruct HFPO-TA, while 2-HPA/CTAB/HFPO-TA self-assembled micelle showed the best stability under a wide pH range from 4 to 10 attributing to its tighter micelle structure and lower surface charge density. Moreover, natural organic matter (< 10 mg L⁻¹) plays little inhibition for the 2-HPA/CTAB/HFPO-TA system. This novel ternary micelle system not only maintains the high efficiency under the conditions of oxygen enrichment, but also keeps well performance in varied pH conditions. The high decomposition/defluorination efficiency and anti-interference capability of the 2-HPAs/CTAB/HFPO-TA micelle system make it a promising technology for treating the PFASs contaminating wastewater.
... Both fulvic and humic acids in the reaction mixtures may behave like photosensitizers to result in [53]. The mechanism may involve the reduction of Ag + adsorbed onto Ag 2 O by the photochemically generated reactive species such as hydrated electrons and O 2 −Á in solution to yield AgNPs [48,54]. The O 2 −Á species can react with Ag + ions to form AgNPs [55]. ...
Chapter
In recent years, there has been growing interest in the existence of natural nanoparticles in the environment and their subsequent influence to the ecological health. This chapter presents the current status on thermally- and light-induced formation of silver nanoparticles (AgNPs) under environmentally relevant conditions. Influenced environmental parameters include temperature, pH, oxic/anoxic environment, and concentrations of precursors Ag⁺ ions and natural organic matter (NOM). Surface-catalyzed reduction of Ag⁺ could describe the formation of AgNPs under various conditions. The redox species of iron (Fe(II)/Fe(III)) in the thermally induced processes enhanced the formation of AgNPs. Moieties of NOM, Ag–NOM complexes, and reactive oxygen species, ROS (e.g., $${{\text{O}}_{2}}^{\cdot - }$$) were provoked to explain the formation of AgNPs. Stability studies on formed AgNPs from Ag(I)–NOM reaction mixtures have shown their stability for days to several months. However, cations of the natural waters such as Na⁺, K⁺, Mg²⁺, and Ca²⁺ can destabilize the AgNPs. A preliminary investigation on the toxicity of AgNPs, formed in the mixture of Ag⁺-humic acid, suggests that lower minimum inhibition concentration against Gram-negative bacteria and Gram-positive bacteria compared to engineered AgNPs.
Article
The formation of reactive oxygen species (ROS) from effluent organic matter (EfOM) was investigated under simulated solar irradiation. In this study, EfOM was isolated into three different fractions based on hydrophobicity. The productivity of ROS in EfOM was measured and compared with that of natural organic matter (NOM) isolates, including Suwannee River humic acid/fulvic acid (SRHA/FA) and Pony Lake fulvic acid (PLFA). The hydrophilic (HPI) component had a greater quantum yield of (1)O2 than those of the hydrophobic (HPO) and transphilic (TPI) fractions because the HPI contained peptides and proteins. Regarding O2(•-), the phenolic moieties acted as electron donating species after photochemical excitation and therefore electron transfer to oxygen. A positive correlation was found between the phenolic concentrations and the steady state O2(•-)concentrations. H2O2 accumulated during the irradiation process from superoxide as precursor. Potentially, due to the presence of proteins or other organic species in the HPI fraction, the decay rates of H2O2 in the dark for both the effluent wastewater and the HPI fraction were significantly faster than the rates observed in the standard NOM isolates, the HPO and TPI fractions. Autochthonous NOM showed a higher •OH productivity than terrestrial NOM. The [•OH]ss was lowest in the HPI fraction due to the lack of humic fraction and existence of soluble microbial products (SMPs), which easily reacted with •OH. Overall, the HPO and TPI fractions were the major sources of superoxide, H2O2 and •OH under simulated solar irradiation. The HPI fraction dominated the production of (1)O2 and acted as a sink for H2O2 and •OH.
Article
Addition of a series of phenol electron donors to solutions of humic substances (HS) Addition of a series of phenol electron donors to solutions of humic substances (HS) enhanced substantially the initial rates of hydrogen peroxide (H2O2) photoproduction (RH2O2), with enhancement factors (EF) ranging from a low of ~3 for 2,4,6-trimethylphenol (TMP) to a high of ~15 for 3,4-dimethoxyphenol (DMOP). The substantial inhibition of the enhanced RH2O2 following borohydride reduction of the HS, as well as the dependence of RH2O2 on phenol and dioxygen concentrations are consistent with a mechanism in which the phenols react with the triplet excited states of (aromatic) ketones within the HS to form initially a phenoxy and ketyl radical. The ketyl radical then reacts rapidly with dioxygen to regenerate the ketone and form superoxide (O2-), which subsequently dismutates to H2O2. However, as was previously noted for the photosensitized loss of TMP, the incomplete inhibition of the enhanced RH2O2 following borohydride reduction suggests that there may remain another pool of oxidizing triplets. The results demonstrate that H2O2 can be generated through an additional pathway in presence of sufficiently high concentrations of appropriate electron donors through reaction with the excited triplet states of aromatic ketones and possibly of other species such as quinones. However, in some cases, the much lower ratio of H2O2 produced to phenol consumed suggests that secondary reactions could alter this ratio significantly.
Article
The reactions between chlorinated benzenes (CBzs) and hydrated electron (e aq−) were investigated by the electron beam (EB) and laser flash photolysis (LFP) experiments. Under the EB irradiation, the effects of irradiation dose, initial concentration and the number of Cl atoms on the removal efficiencies were further examined. At 10 kGy, the removal efficiencies of mono-CB, 1,3-diCB, 1,2-diCB and 1,4-diCB were 41.2%, 87.2%, 84.0%, and 84.1%, respectively. While irradiation dose was 50 kGy, the removal efficiencies increased to 47.4%, 95.8%, 95.0%, and 95.2%, respectively. Irradiation of CBzs solutions has shown that the higher the initial concentration, the lower the percentage of CBzs removal. In addition to this, the dechlorination efficiencies of 1,2-dichlorobenzene (1,2-diCB), 1,3-dichlorobenzene (1,3-diCB) and 1,4-dichlorobenzene (1,4-diCB) were much higher than that of chlorobenzene (mono-CB). The kinetics of the reactions was achieved with nanosecond LFP. The rate constants of second-order reaction between e aq− with mono-CB, 1,2-diCB, 1,3-diCB and 1,4-diCB were (5.3±0.4) × 108, (4.76±0.1) × 109, (1.01±0.1) × 1010 and (3.29±0.2) × 109 L·mol−1·s−1, respectively. Density functional theory (DFT) calculations were performed to determine the optical properties of unstable CBzs anion radicals, and the main absorption peaks lied in the range of 300–550 nm. The primary reaction pathway of CBzs with e aq− was gradual dechlorination, and the major products were Cl− and benzene (CBzs(-Cl−)). Furthermore, biphenyl (or chlorobiphenyl) was observed during the LFP, which was probably formed by recombination of benzene radicals.
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Graphene oxide (GO) is promising in scalable production and has useful properties that include semiconducting behavior, catalytic reactivity, and aqueous dispersibility. In this study, we investigated the photochemical fate of GO under environmentally relevant sunlight conditions. The results indicate that GO readily photo-reacts under simulated sunlight with the potential involvement of electron-hole pair creation. GO was shown to photo-disproportionate to CO2, reduced materials similar to reduced GO (rGO) which are fragmented compared to the starting material, and low molecular-weight (LMW) species. Kinetic studies show that the rate of the initially rapid photo-reaction of GO is insensitive to the dissolved oxygen content. In contrast, at longer time points (>10 h) the presence of dissolved oxygen led to a greater production of CO2 than the same GO material under N2-saturated conditions. Regardless, the rGO species themselves persist after extended irradiation equivalent to two months in natural sunlight, even in the presence of dissolved oxygen. Overall, our findings indicate that GO photo-transforms rapidly under sunlight exposure, resulting in chemically reduced and persistent photoproducts that are likely to exhibit transport and toxic properties unique from parent GO.
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The APEX software predicts the photochemical transformation kinetics of xenobiotics in surface waters as a function of: photoreactivity parameters (direct photolysis quantum yield and second-order reaction rate constants with transient species, namely ˙OH, CO3(-)˙, (1)O2 and the triplet states of chromophoric dissolved organic matter, (3)CDOM*), water chemistry (nitrate, nitrite, bicarbonate, carbonate, bromide and dissolved organic carbon, DOC), and water depth (more specifically, the optical path length of sunlight in water). It applies to well-mixed surface water layers, including the epilimnion of stratified lakes, and the output data are average values over the considered water column. Based on intermediate formation yields from the parent compound via the different photochemical pathways, the software can also predict intermediate formation kinetics and overall yield. APEX is based on a photochemical model that has been validated against available field data of pollutant phototransformation, with good agreement between model predictions and field results. The APEX software makes allowance for different levels of knowledge of a photochemical system. For instance, the absorption spectrum of surface water can be used if known, or otherwise it can be modelled from the values of DOC. Also the direct photolysis quantum yield can be entered as a detailed wavelength trend, as a single value (constant or average), or it can be defined as a variable if unknown. APEX is based on the free software Octave. Additional applications are provided within APEX to assess the σ-level uncertainty of the results and the seasonal trend of photochemical processes.
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Article
The photodegradation of marine dissolved organic matter (DOM) plays a critical role in the global carbon cycle. The photodegradation of both DOM and particulate organic matter (POM) leads to the formation of carbon monoxide and/or the production of labile organic matter that can be rapidly biodegraded. DOM also regulates metal speciation, particularly important for the bioavailability of nutrient metals required by phytoplankton. Copper is one of these essential nutrients but is also toxic in elevated concentrations. Its speciation is regulated by DOM photodegradation however, DOM-copper ligands are not well characterized because of their low concentrations in natural waters. Copper immobilized metal affinity chromatography (IMAC) was used to isolate high and low affinity DOM ligands from both Pacific near-shore seawater and freshwater from the Black River (NC). Laser flash photolysis (LFP) was used to characterize excited state species from these fractions and to identify excited state species in the open ocean through a depth profile from 5 to 4532 m (North Atlantic, Bermuda Atlantic Time Series Station (BATS)). LFP generated transients characteristic of the solvated electron (T = 3.5 +/- 0.1 mu s), a microbial-sourced triplet excited state (T = 1.1 mu s +/- 0.1 mu s), and a long-lived transient, DOM*, with a millisecond lifetime. Unlike the seawater triplet excited state, a copper ligand from Black River water generated a triplet excited state with a lifetime of 3.4 +/- 0.1 mu s. Fractions were also characterized by excitation-emission matrix fluorescence spectroscopy (EEMs), UV-visible, and H-1 NMR spectroscopy. EEMs spectra correlated the marine excited state species with the protein-like, microbial fraction while the freshwater transients were associated with chromophoric DOM (CDOM). H-1 NMR with spectral database matching identified ninety-seven compounds as potential sources of these excited state species in the Pacific seawater IMAC fractions. Aqueous extracts of marine macro-algae (Sargassum natans (Atlantic Ocean), Macrocystis pyrifera), and surf grass (Phyllospadix torreyi) were analyzed to investigate potential additional sources of these excited state transients. To our knowledge, this is the first study to characterize DOM-copper ligands in seawater using a combination of EEMs, NMR and laser flash photolysis.
The goal of our research is to better understand the structure and reactivity of natural dissolved organic matter (DOM) in aquatic environments. A more detailed knowledge of these DOM characteristics would lead to a better understanding of carbon cycling in natural waters and processes associated with water treatment using free radical chemistry. Our specific interest in DOM in natural waters is several-fold: 1) the photochemical formation of reactive oxygen species, 2) photobleaching of the DOM in coastal oceans, and 3) using chromophoric DOM (CDOM) as a tracer of water masses and in carbon cycling. Our interest in water treatment is that DOM is the major sink of hydroxyl radicals employed in advanced oxidation processes for the destruction of pollutants and thus adversely affects the efficiency of the process. We are using the techniques of radiation chemistry to explore the fundamental free radical and redox chemistry of DOM. We have initiated a study of the free radical reactions of DOM using isolated fractions of Suwannee River fulvic and humic acids and isolates from various anthropogenic sources. We are also investigating the use of model compounds in an attempt to understand the free radical transients formed from DOM either as a result of free radical reactions or photochemical reactions.
Article
This study investigated the possible natural formation of silver nanoparticles (AgNPs) in Ag+-fulvic acid (FA) solutions under various environmentally relevant conditions (temperature, pH, and UV light). Increase in temperature (24-90 oC) and pH (6.1 - 9.0) of Ag+-Suwannee River fulvic acid (SRFA) solutions accelerated the appearance of the characteristic surface plasmon resonance (SPR) of AgNPs. The rate of AgNP formation via reduction of Ag+ in the presence of different FAs (SRFA, Pahokee Peat fulvic acid, PPFA, Nordic lake fulvic acid, NLFA) and Suwannee River humic acid (SRHA) followed the order NLFA > SRHA > PPFA > SRFA. This order was found to be related to the free radical content of the acids, which was consistent with the proposed mechanism. The same order of AgNP growth was seen upon UV light illumination of Ag+-FA and Ag+-HA mixtures in moderately hard reconstituted water (MHRW). Stability studies of AgNPs, formed from the interactions of Ag+-SRFA, over a period of several months showed that these AgNPs were highly stable with SPR peak reductions of only ~ 15 %. Transmission electron microscopy (TEM) and dynamic light scattering (DLS) measurements revealed bimodal particle size distributions of aged AgNPs. The stable AgNPs formed through the reduction of Ag+ by fulvic and humic acid fractions of natural organic matter in the environment may be transported over significant distances and might also influence the overall bioavailability and ecotoxicity of AgNPs.
Article
Prairie pothole lakes (PPLs) are glacially-derived, ecologically-important water bodies found in central North America and represent a unique setting in which extensive agriculture occurs within wetland ecosystems. In the Prairie Pothole Region (PPR), elevated pesticide use and increasing hydrologic connectivity have raised concerns about the impact of nonpoint source agricultural pollution on the water quality of PPLs and downstream aquatic systems. Despite containing high dissolved organic matter (DOM) levels, the photoreactivity of the PPL water and the photochemical fate of pesticides entering PPLs are largely unknown. In this study, the photodegradation of sixteen pesticides was investigated in PPL waters sampled from North Dakota, USA, under simulated and natural sunlight. Enhanced pesticide removal rates in the irradiated PPL water relative to the control buffer pointed to the importance of indirect photolysis pathways involving photochemically produced reactive intermediates (PPRIs). The steady-state concentrations of carbonate radical, hydroxyl radical, singlet oxygen, and triplet-excited state DOM were measured and second-order rate constants for reactions of pesticides with these PPRIs were calculated. Results from this study underscore the role of DOM as photosensitizer in limiting the persistence of pesticides in prairie wetlands through photochemical reactions.
Article
Photoionization, fluorescence time-dependent Stokes shift (TDSS), and rotational dynamics of coumarin 307 (C307) have been investigated in soft matter system such as micelles using time-resolved transient absorption and fluorescence spectroscopy. Photoionization of C307 leads to the formation of coumarin radical cation and hydrated electron, which were characterized by their respective transient absorption. The photoionization yields are significantly higher in anionic sodium dodecyl sulfate (SDS) micelle than in cationic cetyltrimethylammonium bromide (CTAB) and neutral Triton X-100 (TX-100) micelles, indicating the influence of micellar surface charge on the efficient separation of radical cation-hydrated electron pair. The CTAB micelle favors the recombination of radical cation and hydrated electron leading to the formation of triplet state of C307, which causes a decrease in the photoionization yield. C307 exhibits TDSS in all micelles; the time evolution and the magnitude of the TDSS depend on nature of the micelle. In TX-100 micelles, the decay of the TDSS exhibits ultraslow component (165 ns) and is affected by the presence of electron scavengers. The ultraslow component in TX-100 micelle originates from the recombination of radical cation-hydrated electron, which results in the formation of twisted intramolecular charge transfer (TICT) state; such formation of TICT state was not observed in SDS and CTAB micelles. To the best of our knowledge, this is the first report where the radical-ion pair recombination dynamics is probed using TDSS in combination with time-resolved transient absorption studies. The activation energy for the solvent relaxation and radical-ion pair (solvent separated) recombination process was found to be 6.1 and 3.0 kcal mol(-1), respectively. Temperature effect on TDSS in TX-100 micelles confirmed the increase in the water hydration, and size of the micelle influences the relative contribution of the solvation and radical-ion pair recombination dynamics toward the total TDSS. We propose that TDSS observed in neutral micelles and reported in other biomolecules such as proteins by the 7-amino coumarin probe is not only due to the solvation dynamics alone but also due to the radical-ion pair recombination dynamics.
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The sea-surface layer is the very upper part of the sea surface where reduced mixing leads to strong gradients in physical, chemical, and biological properties. This surface layer is naturally reactive, containing a complex chemistry of inorganic components and dissolved organic matter (DOM), the latter including amino acids, proteins, fatty acids, carbohydrates, and humic-type components,2 with a high proportion of functional groups such as carbonyls, carboxylic acids, and aromatic moieties. The di!erent physical and chemical properties of the surface of the ocean compared with bulk seawater, and its function as a gateway for molecules to enter the atmosphere or ocean phase, make this an interesting and important region for study. A number of chemical reactions are believed to occur on and in the surface ocean; these may be important or even dominant sources or sinks of climatically active marine trace gases. However, the sea surface, especially the top 1 um to 1 mm known as the sea-surface microlayer (ssm), is critically undersampled, so to date much of the evidence for such chemistry comes from laboratory and/or modeling studies. This review discusses the chemical and physical structure of the sea surface and mechanisms for gas transfer across it, and explains the current understanding of trace gas formation at this critical interface between the ocean and atmosphere.
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Titanium xerogel coagulants (TXC) is a novel Ti-based coagulants developed in recent years.
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Dissolved organic matter (DOM) exists widely in the natural water. It is one of the largest active pools of organic carbon on earth and hence plays an important role in the global carbon cycling. As it contains a large number of chromophores, such as benzene ring, carboxyl, hydroxy, carbonyl, etc., it can adsorb sunlight at certain wavelength regions of the spectrum. A variety of reactive free radicals, such as hydroxyl radicals, singlet oxygen, and hydrated electron, could be generated during this process. These reactive free radicals could play an important role in the photo-transformation and photo-degradation of contaminants in the natural water. In different natural water, the generation of reactive free radicals is very different, resulting the mechanism and pathway of photo-transformation of various contaminats in natural water. This paper reviews the DOM-mediated generating pathways of major reactive radicals, including hydroxyl radicals, singlet oxygen, hydrated electron and excited triplet states of the CDOM. Then influences of them on the photo-transformation and photo-degradation of various contaminants, inculding inorganic ion, methylmercury, poly aromatic hydrocarbon, phenols, and pesticides, etc., in the natural water are discussed. The necessary works in the future research are also briefly outlined.
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Triplet-state chromophoric dissolved organic matter (³CDOM*) plays an important role in aquatic photochemistry, yet much remains unknown about reactivity of these intermediates. To better understand the kinetic behavior and reactivity of ³CDOM*, we have developed an indirect observation method based on monitoring time-resolved singlet oxygen (¹O2) phosphorescence kinetics. The underpinning principle of our approach relies on the fact that O2 quenches almost all triplets with near diffusion-limited rate constants, resulting in the formation of ¹O2, which is kinetically linked to the precursors. A kinetic model relating ¹O2 phosphorescence kinetics to triplet excited states produced from isolated humic substances and in whole natural water samples (hereafter referred to as ³CDOM*) was developed and used to determine rate constants governing ³CDOM* natural lifetimes and quenching by oxygen and 2,4,6-trimethylphenol (TMP), a common triplet probe molecule. ³CDOM* was found to exhibit smaller O2 and TMP quenching rate constants, ~9 ×10⁸ and ~8 ×10⁸ M⁻¹ s⁻¹ respectively, compared to model sensitizers, such as aromatic ketones. Findings from this report shed light on the fundamental photochemical properties of CDOM in organic matter isolates and whole waters and will help refine photochemical models to more accurately predict pollutant fate in the environment.
Article
Perfluoroalkyl substances (PFASs) are highly toxic synthetic chemicals, which are considered the most persistent organic contaminants in the environment. Previous studies have demonstrated that hydrated electron based techniques could completely destruct these compounds. However, in the reactions, alkaline and anaerobic conditions are generally required or surfactants are involved. Herein, we developed a simple binary composite, only including PFAS and hydrated electron source chemical. The system exhibited high efficiency for the utilization of hydrated electrons to decompose PFASs. By comparing the degradation processes of perfluorooctanoic acid (PFOA) in the presence of seven indole derivatives with different chemical properties, we could conclude that the reaction efficiency was dependent on not only the yield of hydrated electrons but also the interaction between PFOA and indole derivative. Among these derivatives, indole showed the highest degradation performance due to its relatively high ability to generate hydrated electrons, and more importantly, indole could form a hydrogen bonding with PFOA to accelerate the electron transfer. Moreover, the novel composite demonstrated high reaction efficiency even with coexisting humic substance and in a wide pH range (4-10). This study would deepen our understanding of the design of hydrated electron based techniques to treat PFAS-containing wastewater.
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Pharmaceutical and personal care products (PPCPs) are a group of emerging contaminants that have frequently been detected in aqueous environments. Phototransformation driven by solar irradiation is one of the most important natural processes for the elimination of PPCPs. In this study, primidone (PMD) was chosen as a model "photorefractory" compound. A series of experiments were conducted to assess if reactive intermediates (RIs), such as hydroxyl radical (HO), singlet oxygen (1O2), and triplet states of dissolved organic matter (3DOM⁎), inhibited or enhanced the photochemical transformation of PMD under simulated solar irradiation. The results indicate that HO plays a key role in the photodegradation of PMD and that dissolved oxygen can affect the degradation rate of PMD by promoting HO formation. Our results demonstrated that PMD can not only react with free HO (HO-free) but also react with lower-energy hydroxylation agents (HO-like). The contributions of HO-free and HO-like to PMD degradation in various dissolved organic matter (DOM) solutions were estimated by a methane-quenching experiment. The results indicated that the HO-like species were important in the photodegradation of "photorefractory" compounds. The bimolecular reaction rate constant of the reaction of free HO with PMD was measured as (5.21 ± 0.02) × 109 M-1 s-1 by using electron pulse radiolysis. Furthermore, PMD was used as a probe to estimate the steady-state concentration of HO-free in various DOM solutions. Using the multivariate statistical strategies of orthogonal projection to latent structures discriminant analysis (OPLS-DA) and hierarchical clustering, 28 photochemical transformation products (TPs) of PMD were successfully identified from the DOM matrix.
Article
Due to the strong electron-withdrawing nature of Cl atom in CCl4, CCl4 could not readily be degraded by oxidation process. In present study, aqueous electron (eaq⁻), a powerful reducing agent generated in UV-Na2SO3 system, was applied to reductively degradation of CCl4. The effects of several crucial factors (e.g. Na2SO3 concentration, solution pH, inorganic ions and NOM) on CCl4 degradation as well as degradation mechanism and pathway were systematically investigated. Results indicated that CCl4 was efficiently degraded in UV-Na2SO3 system and the process could be well described by pseudo-first order kinetic model. The degradation rate increased with the elevated Na2SO3 concentration (0-10 mmol/L) and solution pH (6.0-8.0), while remained approximately constant in alkaline conditions (pH=8.0, 9.0 and 10.0). Nevertheless, O2, inorganic ions and NOM exerted negative effect on CCl4 degradation and the removal efficiency of CCl4 in groundwater was only 31.7%. Mechanistic study implied that degradation of CCl4 was primarily induced by eaq⁻. CCl4 (10 mg/L) was almost completely dechlorinated within 60 minutes and the predominant intermediate products were CHCl3, C2Cl4 and C2HCl3. CHCl3 and CH2Cl2 were also rapidly degraded in the UV-Na2SO3 system.
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A composite is generally defined as a material composed of two or more distinct phases. Dental composites consist of a polymerizable resin base containing a ceramic filler that does not interfere on the polymerization reaction. Most dental polymers are based on acrylic resins made up of monomethacrylate, dimethacrylate, or trimethacrylate monomers. The most common restorative materials are based on the bisphenol A-bis(glycidyl methacrylate) (Bis-GMA), which copolymerizes with triethylene glycol dimethacrylate (TEGDMA). Also the dimethacrylate monomers bisphenol A-dimethacrylate (Bis-EMA) and a urethane dimethacrylate (UDMA) are commonly used [1].
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Novel brominated flame retardants (NBFRs) have become ubiquitous emerging organic pollutants. However, little is known about their transformation in natural waters. In this× study, aquatic photochemical behavior of a representative NBFR, 2,3-dibromopropyl-2,4,6-tribromophenyl ether (DPTE), was investigated by simulated sunlight irradiation experiment. Results show that DPTE can undergo direct photolysis (apparent quantum yield 0.008 ± 0.001) and hydroxyl radical (∙OH) initiated oxidation (second order reaction rate constant 2.4 × 10⁹ M⁻¹•s⁻¹). Dissolved organic matter (DOM) promotes the photodegradation due to generation of excited triplet DOM and ∙OH. Two chlorinated intermediates were identified in the photodegradation of DPTE in seawaters. Density functional theory calculation showed that ∙Cl or Cl2∙- addition reactions on C-Br sites of the phenyl group and H-abstraction reactions from the propyl group are main reaction pathways of DPTE with the chlorine radicals. The ∙Cl or Cl2∙- addition proceeds via a replacement mechanism to form chlorinated intermediates. Environmental half-lives of DPTE relevant with photodegradation are estimated to be 6.5 ~ 1153.9 d in waters of the Yellow River estuarine region. This study provides valuable insights into the phototransformation behavior of DPTE in natural waters, which is helpful for persistence assessment of the NBFRs.
Article
Excited triplet state chromophoric dissolved organic matter (3CDOM*) is a short-lived mixture of excited state species that plays important roles in aquatic photochemical processes. Unlike the study of triplet states of well-defined molecules, which are amenable to transient absorbance spectroscopy, the study of 3CDOM* is hampered by it being a complex mixture and its low average intersystem crossing quantum yield (ΦISC). This study is an alternative approach to investigating 3CDOM* using transient absorption laser spectroscopy. The radical cation of N,N,N′,N′-tetramethyl-p-phenylenediamine (TMPD), formed through oxidation by 3CDOM*, was directly observable by transient absorption spectroscopy and was used to probe basic photophysical properties of 3CDOM*. Quenching and control experiments verified that TMPD•+ was formed from 3CDOM* under anoxic conditions. Model triplet sensitizers with a wide range of excited triplet state reduction potentials and CDOM oxidized TMPD at near diffusion-controlled rates. This gives support to the idea that a large cross-section of 3CDOM* moieties are able to oxidize TMPD and that the complex mixture of 3CDOM* can be simplified to a single signal. Using the TMPD•+ transient, the natural triplet lifetime and ΦISC for different DOM isolates and natural waters were quantified; values ranged from 12 to 26 µs and 4.1-7.8%, respectively.
Chapter
Soluble bio-based substances (SBO) extracted from urban bio-wastes have similar chemical properties to humic substances (HS) present in natural waters and soils. Therefore, SBO are also expected to have photochemical properties similar to HS. In this chapter, a summary of the photochemistry of HS is presented along with the recent advances related to the photogeneration of reactive species upon irradiation of aqueous solutions of SBO and some examples of pollutant degradation photo-induced by SBO.
Article
Iodide photolysis under UV illumination affords an effective method to produce hydrated electrons (eaq–) in aqueous solution. Therefore, UV/Iodide photolysis can be utilized for the reductive degradation of many recalcitrant pollutants. However, the effect of naturally occurring organic matter (NOM) such as humic and fulvic acids (HA/FA), which may impact the efficiency of UV/Iodide photoreduction, is poorly understood. In this study, the UV photoreductive degradation of perfluorooctane sulfonate (PFOS) in the presence of I⁻ and HA is studied. PFOS undergoes a relatively slow direct photoreduction in pure water, a moderate level of degradation via UV/Iodide, but a rapid degradation via UV/Iodide/HA photolysis. After 1.5 h of photolysis, 86.0% of the initial [PFOS] was degraded in the presence of both I⁻ and HA with a corresponding defluorination ratio of 55.6%, whereas only 51.7% of PFOS was degraded with a defluorination ratio of 4.4% via UV/Iodide illumination in the absence of HA. The relative enhancement in the presence of HA in the photodegradation of PFOS can be attributed to several factors: a) HA enhances the effective generation of eaq– due to the reduction of I2, HOI, IO3⁻ and I3⁻ back to I⁻; b) certain functional groups of HA (i.e., quinones) enhance the electron transfer efficiency as electron shuttles; c) a weakly-bonded association of I⁻ and PFOS with HA increases the reaction probability; and d) absorption of UV photons by HA itself produces eaq–. The degradation and defluorination efficiency of PFOS by UV/Iodide/HA process is dependent on pH and HA concentration. As pH increases from 7.0 to 10.0, the enhancement effect of HA improves significantly. The optimal HA concentration for the degradation of 0.03 mM PFOS is 1.0 mg L⁻¹.
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Natural environments on Earth are amenable to a diverse array of chemical reactions that can convert one form of nitrogen into another, often with the participation of additional substances such as minerals, dissolved metals, and organic compounds. These processes collectively define a natural chemical nitrogen cycle, analogous to the familiar biologically driven cycle but even more intricate with respect to the number of pathways by which nitrogen can be transformed and transported across land, air, and water. The fully assembled abiotic nitrogen cycle manifests a landscape rich in opportunities for investigation either with or without parallel attention to biological processes.
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The photolysis of iodide anions promotes the reaction of carbon dioxide with hydrogen sulfide or thiols to quantitatively yield formic acid and sulfur or disulfides. The reaction proceeds in acetonitrile and aqueous solutions, at atmospheric pressure and room temperature by irradiation using a low-pressure mercury lamp. This transition-metal-free photocatalytic process for CO2 capture coupled with H2S removal may have been relevant as a prebiotic carbon dioxide fixation.
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The photochemical properties of dissolved organic matter (DOM) have been of interest to scientists and engineers since the 1970s. Upon light absorption, chromophoric DOM (CDOM) can sensitize the formation of different short-lived reactive intermediates (RIs), including hydroxyl radical (•OH), singlet oxygen (¹O2) and superoxide radical anion (O2-•). In addition, a fraction of the excited singlet states in CDOM decays into excited triplet states (³CDOM*), which are also important photochemical transients in environmental systems. These RIs have a significant impact on different processes in sunlit waters, including degradation of organic contaminants and the inactivation of pathogens. Due to their transient nature and low steady-state concentrations, the use of common analytical techniques for the direct measurement of these species is impractical. Therefore specific probe compounds (PCs) are used. PCs include furfuryl alcohol for ¹O2, and terephthalic acid for •OH. In this publication, we present a critical review of the use of PCs for the assessment of the formation of photochemically generated RIs. We first introduce the concept of a PC, including the kinetic treatment and necessary assumptions needed to conduct a specific measurement. Afterwards, we present short overviews of the most studied RIs and review relevant issues regarding the use of specific PCs for their measurement. We finalize by offering recommendations regarding the use of PCs in environmental photochemistry.
Article
Buckminsterfullerenes (C60) are widely used nanomaterials that are present in surface water. The combination of C60 and humic acid (HA) generates reactive oxygen species (ROS) under solar irradiation, but this process is not well understood. Thus, the present study focused on the photochemical formation of singlet oxygen (1O2), hydroxyl radical (HO•)-like species, superoxide radicals (O2•−), hydrogen peroxide (H2O2), and triplet excited states (3C60*/3HA*) in solutions containing both C60 and HA. The quantum yield coefficients of excited triplet states (ƒTMP) and apparent quantum yields of ROS were measured and compared to the calculated values, which were based on the conservative mixing model. Although C60 proved to have only a slight impact on the 1O2 formation from HA, C60 played a key role in the inhibition of O2•−. The photochemical formation of H2O2 followed the conservative mixing model due to the reaction of C60•− with HO2•/O2•−, and the biomolecular reaction rate constant has been be measured as (7.4 ± 0.6) × 10⁶ M⁻¹ s⁻¹. The apparent ƒTMP was significantly lower than the calculated value, indicating that the steric effect of HA was significant in the reaction of 3C60* with the TMP probe. In contrast, C60 did not have an effect on the photochemical formation of HO• from HA, suggesting that HO• is elevated from the hydrophilic surface of HA. The aforementioned results may be useful for predicting the photochemical influence of C60 on aqueous environments.
Article
Photochemical transformation of hydroxylated polyhalodiphenyl ethers (HO-PXDEs) has attracted much attention for their ubiquitous presence and the photochemical formation of highly toxic dioxins. Dissolved organic matter (DOM) plays an important role in the environmental photochemical transformation of organic pollutants. However, the effects of DOM on the photolysis kinetics and dioxin formation of HO-PXDEs are still not fully understood. Herein, the effects of Suwannee River natural organic matter (SRNOM) on the phototransformation of 2'-HO-2,4,4'-trichlorodiphenyl ether (triclosan) and 2'-HO-2,4,4'-tribromodiphenyl ether (2'-HO-BDE-28) were investigated in artificial estuarine water (AEW). The results showed that although SRNOM induced indirect photolysis of triclosan and 2'-HO-BDE-28, it decreased the observed photolytic rate constants due to light screening, static quenching and dynamic quenching effects. The above effects were quantified firstly. Direct photolysis is more important than indirect photolysis in the transformation of the target compounds and the production of dioxins. SRNOM increased the dioxin yields of the two HO-PXDEs. It was also found that SRNOM decreased the formation rate constant (kp) of 2,8-dichlorodibenzo-p-dioxin (2,8-DCDD) from triclosan and showed no obvious influence on the kp of 2,8-dibromodibenzo-p-dioxin (2,8-DBDD) from 2'-HO-BDE-28. SRNOM showed no obvious influence on the degradation of 2,8-DCDD, while it increased the degradation rate constant of 2,8-DBDD. The promoting effect on the degradation of 2,8-DBDD was attributed to the formation of chloride radicals with the concurrence of SRNOM and Cl(-) in AEW. This study revealed the roles of SRNOM in the photochemical transformation of HO-PXDEs and the photochemical formation and degradation of dioxins, which is important for elucidating the transformation fate of HO-PXDEs in aquatic environments.
Article
The kinetics of simulated low-energy daylight (UVA–vis) and powerful combined ultraviolet B and A (UVB–UVA) induced direct and indirect phototransformations of four pharmaceuticals, i.e., ibuprofen, metoprolol, carbamazepine, and warfarin, which were investigated in dilute solutions of pure laboratory and natural humic waters. The results strengthen the essential function of natural chromophores in dissolved organic material (CDOM) as principal photosensitizer toward indirect phototransformations of pharmaceuticals in natural conditions under available low-energy UVA–vis and slight UVB radiations. The results confirmed that organic micropollutants are able to undergo a direct photolysis if their absorbance spectra overlap the spectral range of the available radiation but only if the radiation is strong enough, e.g., ibuprofen is able to undergo only indirect photolysis via different pathways in all realistic conditions. The action of nitrate anions as photosensitizers in the applied conditions proved to be of little importance. High-performance size-exclusion chromatographic experiments verified that the rate constants obtained under the low-energy UVA–vis and powerful UVB–UVA irradiations for the decreased amounts of the two largest molecular size fractions of CDOM were quite close to the rate constants detected for the increased amounts of the next five molecular size fractions with smaller molecular sizes. The decreased contents of the two largest molecular size fractions correlated quite well with the decreased contents of the studied pharmaceuticals under the low-energy UVA–vis irradiation process but somewhat less under the powerful UVB–UVA irradiation. The photochemically induced decomposition of the CDOM aggregates appears to increase the amounts of smaller molecular size fractions and simultaneously produce via CDOM-stimulated radical reactions indirect structural transformations of pharmaceuticals. Apparent quantum yields were estimated for the transformation–degradation of the two largest molecular-size CDOM aggregates under low-energy UVA–vis and powerful UVB–UVA irradiations. Figure Structural difference between CDOM and pharmaceuticals studies
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DISSOLVED organic material in marine and freshwater ecosystems constitutes one of the Earth's largest actively cycled reservoirs for organic matter1. The bacterially mediated turnover of chemically identifiable, low-molecular-mass components of this pool has been studied in detail for nearly three decades, but these compounds constitute less than 20% of the total reservoir2. In contrast, little is known about the fate of the larger, biologically more refractory molecules-including humic substances-which make up the bulk of dissolved organic matter. Here we report results from bacterial bioassays and photochemical studies indicating that exposure to sunlight causes dissolved organic matter to release nitrogen-rich compounds that are biologically available, thus enhancing the bacterial degradation of humic substances. We demonstrate that ammonium is among the nitrogenous compounds released and is produced most efficiently by ultraviolet wavelengths. Photochemical release of ammonium from dissolved organic matter has important implications for nitrogen availability in many aquatic ecosystems, including nitrogen-limited high-latitude environments and coastal oceans, where inputs of terrestrial humic substances are high.
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The light intensity dependence in monophotonic and consecutive biphotonic processes under laser flash photolysis conditions is derived. In a monophotonic process the relation between photoproduct yield and exciting light intensity is linear but there is saturation at high intensity. In a biphotonic process there are three different regions. The relation is quadratic at low intensity, linear at medium intensity, and there is saturation at high intensity. The difference between the two linear relations is discussed. These results are applied for analyzing the process of hydrated electron formation from excited states of various substances. The process is found to be monophotonic for ferrocyanide. For tyrosine and tryptophan in aqueous neutral solutions, under the conditions of the laser experiments, a biphotonic process is observed. For the tyrosinate ion (alkaline solution pH 12) the findings are not conclusive. Other work on this topic is discussed with reference to the analysis.
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Dissolved organic carbon (DOC) in sea water represents one of the largest reservoirs of carbon on the earth1. The main fraction of this DOC is generally believed to be composed of old2, biologically refractory material3 such as humic substances, for which the removal mechanisms remain largely unknown. One potentially important removal process in the ocean that has not been investigated is the photochemical breakdown of this DOC in the photic zone to form biologically labile organic products. Here we show that biological uptake of pyruvate is highly correlated to its rate of photochemical production in sea water (r = 0.964), and that the photochemical precursor(s) of pyruvate is from the fraction of DOC having a nominal molecular weight of 500. This is the first evidence that photochemical breakdown of high-molecular-weight marine DOC, which is presumably biologically refractory, results in the production of a compound that is used by plankton as a substrate. Our results have important implications for the oceanic carbon cycle, particularly with respect to planktonic-food-web dynamics and the global carbon budget.
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Studies to characterize sources and sinks of carbon monoxide (CO) in the mixed layer were carried out at sites covering large regions of the north and south Pacific. Apparent quantum yield spectra for the photochemical production of CO from colored dissolve organic matter were measured, as were first-order net microbial CO consumption rate constants. Contrary to initial expectations, neither photoproduction nor biooxidation parameters exhibited strong regional variations, except that in the Southern Ocean CO biooxidation rate coefficients were very low. Global "blue-water" CO flux terms derived from the data (in Tg carbon 'from CO per year, CO-C a-1) are: photochemical source, 50 (estimated range, 30-70), microbial sink, 32 (estimated range, 10-60) and total CO sink (microbial plus gas exchange), 38 (estimated range, 13-60). Considering uncertainties and extrapolation biases, these independently estimated source and sink terms are thus in, or close to, balance at ∼40 (range of overlap, 30-60) Tg CO-C a-1. The simplest interpretation of this balance is that no major net sources or sinks (i.e., light-independent production, photoproduction at >450 nm) remain undiscovered, though considerable uncertainty in actual process rates remains. These CO fluxes are, however, very much smaller than some recently estimated values. The origins and implications of these discrepancies are discussed, and the coastal budget term is approximated.
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A new approach to define the photoreaction quantum yield in case of two-quantum excitation of molecules in solution is proposed.
Chapter
The mechanism for photoejection of electrons from simple inorganic anions in aqueous solution is being explored using ultrafast UV pump - visible/IR probe spectroscopy in close connection with quantum simulations. The pathway for detachment in the prototype aqueous iodide system, excited with a single photon into the quasi-bound charge-transfer-to-solvent (CTTS) electronic state, is probed in detail. Our experiments observe, for the first time, the timescale of ejection of the electron into the solvent from the lowest CTTS state, and the subsequent relaxation of the solvent to accommodate and solvate the electron. The ejection process is compared with resonant detachment of the molecular anion [Fe(CN)6]4-. These low energy ejection pathways are contrasted with multi-photon ionization of solutes, or the solvent itself, into the conduction band.
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The effects of the excitation wavelength on the quantum yields Q(eaq−) and QF of electron ejection and fluorescence respectively were measured for 4-cyano-N,N-dimethylaniline (CDMA) and related compounds in aqueous solutions. In the case of CDMA no electron ejection was found for excitation of the long-wavelength (1La, 1Lb) band system. At shorter wavelengths (λexc < 270 nm) Q(eaq−) increases gradually whereas QF decreases. The quantum yields level off at excitation within the third absorption band. This shows that internal charge transfer (twisted intramolecular charge transfer formation) is not accompanied by charge transfer to the solvent. At higher excitation energies electron ejection competes with internal conversion.
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Hydrogen peroxide was determined in seawater at the Hiroshima Bay, the Seto Inland Sea under sunny conditions. Concentration of hydrogen peroxide was in accordance with the intensity of the solar radiation. Photochemical production under strong biological activity might be responsible for the profiles of the diurnal and seasonal variation of hydrogen peroxide concentration. The spatial distribution of hydrogen peroxide in the Seto Inland Sea gave a good correlation with salinity, suggesting that the photosensitizer to produce hydrogen peroxide was substances brought from river waters. Hydrated electron as a possible precursor of hydrogen peroxide in the photochemical reaction was also investigated. -from Authors
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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.
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Transient absorption spectra are obtained after photoexcitation of the reduced form of nicotinamide adenine dinucleotide (NADH) and are identified as due to a solvated electron (eaq−) based on comparison with known spectra and the observation of the same transient upon photolysis of N-propyl-1,4-dihydronicotinamide. The risetime of the photogenerated transient is 40 ps, with no perceptible decay within 2 ns following excitation. The quantum yield of solvated electron formation is estimated as 0.46 ± 0.20.
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Quantification of photochemical O 2 uptake provides a measure of total chemical photooxidation of dissolved organic matter (DOM). Here we study this process and present estimates suggesting that photooxidation has the potential to significantly modify marine DOM pools, complementing or exceeding oxidation via coupled chemical- biological pathways. We measured apparent quantum yields (AQYs) of photobleaching, O2 uptake, and H2O2 pro- duction in several coastal marine samples and in dilutions of a tropical estuarine water with oligotrophic seawater. O2-loss AQYs varied little among samples or with dilution but decreased linearly from 1.2 3 10 23 at 300 nm to 0.3 3 10 23 at 400 nm and dropped about threefold to near-constant values with increasing absorbed light dose. H2O2 production, about 45% of O2 uptake, showed similar dependencies, whereas singlet oxygen (O2( 1 Dg)) reactions contributed less than 1% of O2 uptake for typical coastal water. Implications of these findings for photochemical O2 ,H 2O2, and DOM cycling are discussed. Modeling the dose-dependence of O2 loss and photobleaching at 310 nm required three DOM pools. In the simplest case, about 90% is a weakly absorbing, low-AQY pool of DOM admixed with two similar-sized pools of more photochemically reactive DOM. This result suggests that rigorously extrapolating laboratory data to the en- vironment requires detailed mapping of dose-wavelength-photobleaching AQY surfaces. Action spectra and DOM flux estimates for coastal photooxidative chemistry were derived. Site-specific potential rates are comparable to available in situ data. Globally, the DOM photolysis capacity appears to be larger than estimated coastal DOM inputs, especially in tropical and temperate areas, including areas with maximal DOM inputs.
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Dissolved organic matter (DOM) can be degraded by sunlight into a variety of photoproducts that stimulate the growth and activity of microorganisms in aquatic environments. All biologically labile photoproducts identified to date fall into one of four categories: (1) low-molecular-weight (MW) organic compounds (carbonyl compounds with MW of 20% of the bacterial carbon demand. Likewise, 30% of the bacterial nitrogen demand can be met by photodegradation of the nitrogen components of DOM, a process likely to be of particular importance in nitrogen-limited systems. When considered on a depth-integrated basis around the globe, at least 1.0
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In a humic-rich, Shallow lake (Lake Neusicdl), the seasonal dynamics of the humic and the nonhumic dissolved organic carbon (DOC) were investigated and the photochemical oxygen consumption rates of these DOC fractions exposed to surface solar radiation levels were compared with that of the bulk DOC and bacterial respiration. Furthermore, bacterial utilization of the humic, nonhumic, and bulk DOC pm-exposed to solar radiation was com- pared with utilization of the different fractions of DOC held in the dark prior to inoculating natural bacterial assemblages. The concentration of the unfractionated DOC pool ranged from -3 mmol C liter-' during summer to 1.3 mmol C liter-' in late spring. The mean contribution of humic DOC was 35.2% of bulk DOC. Under the full spectrum of solar radiation, photochemical oxygen consumption of the unfractionated DOC was 3.3 pmol O2 liter I h-l, 1.8 pmol 0, liter-' h I of the humic DOC, and 1.7 pmol 0, liter I h-l of the nonhumic DOC. In the absence of UVB, photochemical oxygen consumption was reduced by 35% in the unfractionated DOC, 38% in the humic, and 27.5% in the nonhumic DOC. Under the full spectrum of solar radiation, the photochemical oxygen consumption normalized to DOC was more than twice as high (2.83 pmol 0, mmol -I C h I) for humic than for nonhumic DOC. The bacterial oxygen consumption rate was -30% of the photochcmical oxygen consumption of the unfractionated DOC. In batch culture experiments with natural bacterial assemblages as inocula, the bacterial yield was generally higher with substrate exposed to the full spectrum of solar radiation than with substrate held in the dark prior to inoculation. Exposure of O.&pm filtered water to the full spectrum of surface solar radiation for 2-3 h resulted in a decline in activity (measured by thymidine incorporation) to 47% of the activity measured in the dark. If UVB was excluded, bacterial activity was 62% of that in the dark. Subsequent incubation at 5-20-cm depth under in situ radiation for another 2-3 h resulted in bacterial activity similar to that detected in the dark incubations at the surface. Bacteria exposed to the full range of solar radiation at the surface and incubated subsequently in the dark exhibited significantly lower activity than bacteria exposed to in situ solar radiation in distinct depth layers. This result indicates that bacteria rapidly recover from previous UV stress in the absence of UVB. Based on our results, we estimate that the photooxidation-mediated residence times in the top 5-cm layer of the water column are 90 and 45.5 d for the nonhumic and humic fractions and 75 d for unfractionated DOC. For the entire water column, -10% of the remineralization activity (bacterial respiration + photochcmical oxygen consumption) is due to photooxidation of the DOC, and the mean residence time of DOC is -80 d.
Article
Terrestrially derived dissolved organic matter (DOM) impacts the optical properties of coastal seawater and affects carbon cycling on a global scale. We studied sequential long-term photochemical and biological degradation of estuarine dissolved organic matter from the Satilla River, an estuary in the southeastern United States that is dominated by vascular plant-derived organic matter. During photodegradation, dissolved organic carbon (DOC) loss (amounting to 31% of the initial DOC) was much less extensive than colored dissolved organic matter (CDOM) or fluorescent dissolved organic matter (FDOM) loss (50% and 56% of the initial CDOM and FDOM), and analysis of kinetics suggested a reservoir of DOC that was resistant to photodegradation. In contrast, CDOM photodegra- dation closely followed first-order kinetics over two half-lives with no indication of a nondegradable component. FDOM loss was slightly biased toward fluorophores considered representative of terrestrial humic substances. Ad- ditional changes in optical properties included increases in spectral slope and shifts in fluorescence excitation/ emission maxima that were generally consistent with previous observations from field studies of photobleached DOM. Biological degradation of photobleached DOM was more rapid than that of unbleached material, and this net positive effect was evident even for extensively photodegraded material. Bacterial degradation caused shifts in the opposite direction from photochemical degradation for both spectral slope and excitation/emission maxima and thus dampened but did not eliminate changes in optical properties caused by photobleaching.
Article
Excitation of the Fe(CN)64− ion in aqueous solution into the 1T1u or 1T2g excited singlet state, below 313 nm, leads to hydrated electron formation in competition with internal conversion to the lowest excited singlet 1T1g state. The limiting quantum yield of hydrated electron formation is a linear function of quantum energy between 313 and 228 nm, reaching &phgr;e≃ 0.9 at 228 and 214 nm. Above 313 nm hydrated electron formation is not observed, but photoaquation is. Dependence of &PHgr;e on scavenger (N2O) concentration is observed. The involvement of CTTS character in the process, and the role of rapid solvent rearrangement in the dissociation of the excited state, are discussed.
Article
Complete optical absorption and fluorescence spectra were collected for a diverse suite of 0.2-pm- filtered marine, riverine, and estuarine waters, as well as for colored dissolved organic matter (CDOM) isolated from several of thcsc waters by solid-phase C,, extraction. Absorption and fluorescence parameters for these samples arc reported. For surface waters, variations in the fluorescence quantum yields obtained with 355- and 337-nm excitation fell within a narrow window (
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The efficiency of photoelectron ejection from the reduced form of nicotinamide adenine dinucleotide (NADH) in aqueous solution of pH 8.6 was studied as a function of the laser energy by measuring the transient eaq− absorption appearing on nanosecond laser excitation at 355 nm. It was found that this process is due exclusively to consecutive two-photon excitation at the laser energies used. Saturation effects were observed at the higher energies: their origin was discussed. The saturation probably accounts for the results of a recent picosecond study of NADH which seemed to indicate the existence of a monophotonic electron ejection of high quantum yield.
Article
Laser photolysis was used to investigate the production and quenching of hydrated electrons formed after absorption of 355 nm light by dissolved organic matter (DOM) in natural waters. Correlation between the hydrated electrons generated and dissolved organic carbon in seawaters was found. The DOM in land waters ejected more hydrated electrons than seawater DOM did. Land waters contained much natural fluorescent organic matter, so the DOM in the land waters was rich in photochemically sensitive chromophores. Quenching of the hydrated electrons by the inorganic ions, nitrite, nitrate, Cd2+, Ni2+, Co2+, and [Co(NH3)6]3+ were investigated by a first-order function and the Stern–Volmer model. With the former, the quenching rate constants were 10−9 to 10−10 dm3 mol−1 s−1, which was close to reported values. No regional differences were found among the seawater samples. This result suggested similar quenching mechanisms for hydrated electrons generated in the different seawaters. With the Stern–Volmer model, primary quenching was estimated. Quenching studies of land waters suggested that DOM had an aggregate nature in such waters.
Article
An experimentally determined limit function for long-range ferromagnetic and antiferromagnetic superexchange is proposed, the form of which comprises a quantitative statement of present knowledge of the "range" of superexchange effects in insulating solids. A possible functional form for this limit function, as well as for the general distance dependence of long-range superexchange, is also proposed. The fitting of this general function to experimental data for superexchange coupling vs. distance requires the fitting of three independent parameters. The possibility is considered that one of the parameters so obtained may be sensitive to the degree of extended character of the orbitals which mediate the superexchange effect. The superexchange limit function is shown to be important for characterizing the assignments of alternative paths of exchange interaction. It also serves to point out the need for improved theoretical methods for the analysis of the relative strength of competing paths of interaction in exchange coupled cluster complexes, and shows that there is a need for studies of superexchange interactions acting over distances greater than 6 Å.
Article
The optical absorption spectra of solvated electrons in HâO and DâO have been measured at 274, 298, 340, and 380 K. All the spectra were fitted very well with the Gaussian and Lorentzian shape functions at the low- and high-energy sides of the absorption maximum, respectively, excluding the high-energy tail. The spectrum does not shift uniformly with temperature. The temperature coefficient of absorption decreases rapidly with increasing energy on the low-energy side of the absorption maximum, while it changes only slightly on the high-energy side. When the temperature increases the Lorentzian width remains constant, the Gaussian width varies proportionally to TÂ¹Â², and the spectrum becomes more symmetrical. On going from HâO to DâO we found that the spectrum at a given A/A/sub max/ shows a shift of +0.05 eV in the low-energy wing. The shift decreases with increasing energy, reaching 0.03 eV at the absorption maximum. On the high-energy side of the band the shift becomes negative at h..nu.. > 2.2 eV. The shift on the low-energy side seems to be related to the difference of the zero-point energies of the inter- and intramolecular vibrations. The wavelength dependence of the temperature and isotope effects is consistent with the model that different types of excitation occur on the low- and high-energy sides of the absorption band. The temperature and isotopic dependence of the low-energy side are consistent with its width being due to phonon interactions.
Article
Photoreactions of dissolved organic matter can affect the oxidizing capacity, nutrient dynamics, trace gas exchange, and color of surface waters. This study focuses on factors that affect the photoreactions of the colored dissolved organic matter (CDOM) in the Satilla River, a coastal river that has high concentrations of dissolved organic carbon (DOC) (2.0 ± 0.2 mM C) and iron (12 ± 2 μM) in its freshwater reach. Quantum yields for the photoproduction of dissolved inorganic carbon (DIC) and carbon monoxide (CO) from Satilla CDOM decreased exponentially in the 300−450 nm spectral region. Photoreaction of the CDOM in sunlight caused a decrease in UV and visible absorbance that occurred most rapidly in the UV-B (280−315 nm) region, indicating that CDOM photoreactions can enhance exposure of aquatic organisms to DNA-damaging UV radiation. The role of iron in the photooxidation was investigated by adding fluoride ions or deferoxamine mesylate (DFOM) to the water to form unreactive Fe3+ complexes, thus inhibiting iron photoreduction and slowing CDOM photooxidation. Using this method, it was demonstrated that the photoformation of DIC, CO, and NH4+ is strongly affected by iron catalysis in the Satilla River water. Increasing the dioxygen content and lowering the pH accelerated the photoreaction of the CDOM. CDOM photoreaction was accompanied by conversion of dissolved iron and carbon to particulate forms. Such “photocoagulation” may contribute to the transport of iron and carbon from surface waters to sediments in sunlit, well-stratified aquatic ecosystems.
Article
The hydrophobic chlorocarbon mirex readily binds to aqueous humic acids (HAs). In HA solutions irradiated at sunlight wavelengths (λ ≥ 290 nm), mirex molecules bound to HAs are transformed to photomirex by dechlorination. The mechanism of this intrahumic dechlorination was investigated both in HA solutions and in model systems simulating select photochemical capabilities of HAs. In HA solution, the reaction was unaffected by methanol and pentanol; was accelerated by hydroxide; and was inhibited by oxygen, 2-chloroethanol, nitrate, and hydrogen ion, all four of which can scavenge hydrated electron. Additional experiments probing for involvement of hydrated electron were consistent with it as the reactant. In irradiated N,N-dimethylaniline solution, a model system for generating hydrated electron, mirex was dechlorinated to form photomirex, the same product that is found in HA solution. Relative rate experiments in HA solution, while inconsistent with the reaction with hydrated electron in homogeneous solution, were consistent with reaction with a purely intrahumic hydrated electron. However, other humic-generated reductants cannot be eliminated as possible reactants. The potential confounding influence of hydrophobic partitioning to HAs on investigations using molecular probes is evaluated in terms of current data and previous reports.
Article
A kinetic model was developed describing the effects of hydrophobic partitioning to humics on second-order humic-mediated photoreactions in aqueous solution. Model development and evaluation were in terms of the humic-mediated dechlorination of the hydrophobic chlorocarbon, mirex. Mirex dechlorination in irradiated (λ ≥ 290 nm) Aldrich humic acid (HA) solution was examined as a function of humic acid and scavenger concentration. Kinetic models in which HA solution is described as a homogeneous or single-phase solution were not able to describe the effects of HA and scavenger concentrations on mirex dechlorination rates. However, these effects were successfully described by including partitioning in the models to characterize the bound phase and dissolved phase mirex reactions separately. The resulting model uses homogeneous phase kinetics to describe reaction in the dissolved phase. Bound phase reaction is described as occurring within individual HA molecules with the distribution of scavengers within HA molecules following a Poisson distribution. Used to assess the relative reactivities of the bound and dissolved phases, the model indicated that the reaction of mirex in HA solution is confined to the humic phase. The general predic tions and utility of the model are evaluated both in terms of experimental data and pollutant photodegradation.
Article
Time-resolved photoacoustic spectroscopy in conjunction with magnetic circular dichroism (MCD) spectroscopy has been applied to determine energies and quantum yields for the formation of triplet states in aqueous solutions of two well-characterized (Laurentian and Armadale) fulvic acids. For the Laurentian sample, intersystem, crossing quantum yields range from 0.79 to 0.28 for the pH range 2.0-9.5. The average triplet energy is estimated as 1.8 X 10(2) kJ mol-1 from MCD spectra. For the Armadale sample, intersystem crossing quantum yields range from 0.82 to 0.35 for the pH range 2.0-9.5. The triplet energy is estimated as 1.7 X 10(2) kJ mol-1. With the primary photoproduct quantum yields, the overall photophysics and primary photochemistry of the fulvic acid may be described. The donor-acceptor model of humic spectra is also reinforced. Observed initial yields greatly exceed reported steady-state yields which appear to emphasize quenching and scavaging by humics themselves.
Article
The photochemical formation of carbon monoxide (CO) in water samples obtained from wetlands, lakes, and near-coastal/shelf areas and in aqueous solutions of soil organic matter was investigated. All of these samples contained dissolved organic matter that was largely derived from terrestrial sources. The studies show that, although the water samples had widely varying optical properties and CO photoproduction rates, the efficiencies for photochemical CO formation were remarkably similar in all waters examined. Model calculations further indicated that photodegradation of terrestrial dissolved organic matter (e.g., in wetland and near-coastal environments) may be an important global source of carbon monoxide and a key process in cycling of dissolved organic matter in these environments.
Chapter
Laser flash photolysis studies of the well characterised soil humic substance, Armadale Fulvic Acid (P.E.I., Canada), have been carried out with excitation at 355 nm on picosecond and nanosecond time scales. Three principal transient absorption signals have been observed in aqueous solution: a component with a maximum absorption at 675 nm and a lifetime of about 1 microsecond (at pH = 7.0), a second component with a maximum absorption at 475 nm and a lifetime of 1-10 microseconds, and a third component with a broad, featureless transient absorption spectrum and a lifetime in excess of 100 microseconds. The 675 nm signal is believed to be a solvated electron on the basis of lifetime and quenching data, and is observed 20 ps after excitation. The 475 nm signal is believed to be a radical cation on the basis of its concurrent appearance with the electron at 20 ps. The third featureless component emerges nanoseconds after excitation and is believed to correspond to the triplet states of the humic material.
Chapter
Natural water samples and humic substance solutions were probed for their phototransient behavior. Laser flash kinetic spectroscopy was used to study two transients common to most samples. One transient with a maximum around 720 nm was quenched by decreasing pH and nitrous oxide. It was present in all waters with DOC and had a spectrum which resembled that of a solvated electron. The signal was linear with laser power. The quantum yield for this transient was measured. Samples with higher ground state absorbance yielded a transient with a maximum at 475 nm that was quenched by oxygen. This transient seemed to be a photophysical hybrid with triplet and radical cation character. Additional work done to characterize these transients and predict their environmental fates is discussed.
Article
The contribution of dissolved organic matter (DOM) to alkalinity in estuarine waters and its relationship to CO2 chemistry has not received much previous attention. In this paper, we present some of the first organic alkalinity measurements in the context of estuarine mixing, focusing on three rivers in the Southeastern US. The simple model presented here illustrates that the organic contribution to alkalinity in estuarine waters is largely controlled by the dramatic pH change in the early stage of the mixing. As mixing progresses, organic alkalinity becomes nearly conservative with respect to salinity change. Although no DOM removal during estuarine mixing was detected here, this work provides an alternative approach to evaluate the issues of colloid and small particle formation and coagulation during mixing.This paper demonstrates that a large fraction of the proton binding sites of humic substances is either completely protonated or deprotonated during the estuarine mixing processes and, therefore, that these sites contribute neither to alkalinity nor to proton transfer reactions. One class of chargeable sites may be adequate to model the acid-base properties of humic substances during the mixing process.
Article
The biological turnover of riverine dissolved organic carbon (DOC) discharged into five southeastern United States estuaries was examined in long-term respiration bioassays. Measures of bacterial oxygen consumption indicated surprisingly large differences in the inherent biodegradability of DOC among the five estuaries, despite their close geographic proximity. Differences of up to 13-fold in biodegradation rates were also found temporally within a single estuary. For most of the southeastern United States estuaries, measured rates of riverine DOC biodegradation were low relative to rates reported for other freshwater and marine environments. This was particularly true for the coastal plain (“blackwater”) rivers that contribute about 35% of the riverine DOC exported to coastal marine environments in this region; extrapolation of biodegradation rates to the adjacent continental shelf predict biodegradation of a maximum of 11% of exported blackwater DOC within the estuary-shelf system (with transit times of up to 140 d). DOC from Piedmont rivers was more biologically labile, with maximum losses of 30% predicted within the estuary and adjacent shelf. Short exposures to natural sunlight increased the lability of the riverine DOC and enhanced biodegradation rates by over 3-fold in some cases, although significant inter-estuary differences in susceptibility of riverine DOC to photolysis were also evident. *** DIRECT SUPPORT *** A01BY085 00007
Article
The effects of monochromatic and polychromatic UV and visible (VIS) radiation on the optical properties (absorption and fluorescence) of chromophoric dissolved organic matter (CDOM) were examined for a Suwannee River fulvic acid (SRFA) standard and for water from the Delaware and Chesapeake Bays. The primary (direct) loss of absorption and fluorescence occurred at the irradiation wavelength(s), with smaller secondary (indirect) losses occurring outside the irradiation wavelength(s). The efficiency of both direct and indirect photobleaching decreased monotonically with increasing wavelength. Exposure to polychromatic light increased the CDOM absorption spectral slope (S), consistent with previous field measurements. An analysis of the monochromatic photobleaching kinetics argues that a model based on a simple superposition of multiple chromophores undergoing independent photobleaching cannot apply; this conclusion further implies that the absorption spectrum of CDOM cannot arise solely from a simple superposition of the spectra of numerous independent chromophores. The kinetics of CDOM absorption loss with the monochromatic irradiation were employed to create a simple, heuristic model of photobleaching. This model allowed us to examine the importance of the indirect photobleaching losses in determining the overall photobleaching rates as well as to model the photobleaching of natural waters under polychromatic light fields. Application of this model to natural waters closely predicted the change in the CDOM spectral shape caused by photodegradation. The time scale of this process was consistent with field observations acquired during the summertime for coastal waters in the Middle Atlantic Bight (MAB). The results indicate that the ratio of the photodegradation depth to the mixed layer depth is a key parameter controlling the rate of the photobleaching in surface waters.
Article
The photoreactivity to UV light of ultrafiltered dissolved organic matter (DOM) collected during cruises along salinity transects in the Mississippi and Atchafalaya River plumes was examined by measuring photogenerated free radicals and singlet molecular oxygen () photosensitization. Singlet oxygen was detected by its infrared phosphorescence at 1270 nm using both steady-state and time-resolved techniques. The quantum yields were corrected for self-quenching of by the DOM substrates. Photogenerated free radicals were monitored by electron paramagnetic resonance (EPR). Two size fractions of the dissolved organic matter were examined: material retained with a 3 kDa cut-off filter and material retained with a 1 kDa cut-off filter. The highest quantum yields were found in the lower molecular mass material. There was little change in the quantum yields with increasing salinity, indicating that the photosensitizing ability of the estuarine DOM does not decrease as terrestrial DOM is transported to sea and mixes with marine DOM. In contrast to formation, the steady-state levels of photoproduced free radicals did not significantly differ between high and low molecular mass DOM, and the levels were substantially higher in riverine DOM than along plume salinity transects. This rapid transition in free radical level suggests that terrestrially-derived DOM experiences significant changes in this aspect of its photoreactivity in low (<10 ppt) salinity waters.
Article
The direct photooxidation of coloured dissolved organic matter (CDOM) to dissolved inorganic carbon (DIC) may provide a significant sink for organic carbon in the ocean. To calculate the rate of this reaction on a global scale, it is essential to know its quantum yield, or photochemical efficiency. We have determined quantum yield spectra, φ(λ), (moles DIC/mole photons absorbed) for 14 samples of seawater from environments ranging from a turbid, eutrophic bay to the Gulf Stream. The spectra vary among locations, but can be represented quite well by three pooled spectra for zones defined by location and salinity: inshore φ(λ)=e−(6.66+0.0285(λ−290)); coastal φ(λ)=e−(6.36+0.0140(λ−290)); and open ocean φ(λ)=e−(5.53+0.00914(λ−290)). Production efficiency increases offshore, which suggests that the most highly absorbing and quickly faded terrestrial chromophores are not those directly responsible for DIC photoproduction.
Article
In a comprehensive study of the distribution of organic and inorganic constituents in natural waters, the chemical compositions of several rivers flowing through the Coastal Plain of southeast Georgia (the Satilla River, in particular) have been investigated. These streams are generally characterized by low suspended load, low ionic strength, low pH values, and a predominance of organic over inorganic constituents. The dominance of sodium and chloride among inorganic ions indicates that rainfall is a major factor in determining the distribution of major elements in these waters. The low pH values and the relatively high concentrations of iron and aluminum appear to result from the high concentrations of organic matter. Chemical analysis (total acidity, carboxyl groups, amino acid residues, and CHN analysis) and spectroscopic analysis (i.r. and NMR) indicate that river water organic matter is chemically similar to soil fulvic acids.The dominance of organic matter over inorganic constituents and the resultant low pH values of these waters are in direct contrast with the chemical compositions of most of the rivers of the world which have been previously studied. Many large tributaries draining from the low relief tropical basins of the Amazon and other tropical rivers are expected to show similar characteristics; however, none of these has been studied extensively.
Article
Hydrogen peroxide (H2O2) has been observed in significant concentrations in many natural waters. Because hydrogen peroxide can act as an oxidant and reductant, it participates in an extensive suite of reactions in surface waters. Hydrogen peroxide is produced as a secondary photochemical product of chromophoric dissolved organic matter (CDOM) photolysis. Apparent quantum yields for the photochemical production of hydrogen peroxide were determined in laboratory irradiations of filtered surface waters from several locations in the Chesapeake Bay and in Arctic coastal waters with varying levels of CDOM. The apparent quantum yield for H2O2 decreases by about an order of magnitude from 280 nm to 500 nm, and the majority of H2O2 production occurs at wavelengths less than 340 nm. The apparent quantum yield for H2O2 production at 290 nm ranged from 4.2 × 10− 4 to 2.1 × 10− 6 mol H2O2 (mol photons)− 1 from freshwater to marine waters. A linear relationship was found between the production of H2O2 and change in CDOM absorbance characterized as photobleaching (loss of absorbance). No significant relationship was observed between DOC concentration and peroxide production. Methylhydroperoxide (CH3O2H) was the only short chain peroxide produced during the irradiations, and its production is at least an order magnitude less than that of hydrogen peroxide. Peroxide production was greatest in waters containing significant amounts of terrigenous C in the form of humic substances. Surface waters whose synchronous fluorescence spectra indicated the presence of polyaromatic and/or extensive conjugated compounds exhibited the greatest peroxide production. CDOM photobleaching is not significantly linked to apparent quantum yields for peroxide production.
Article
Partial pressures of carbon dioxide (pCO2) were measured in river and coastal waters on two cruises in November 1999 and June 2000 on the Southwest Florida Shelf. Supersaturation with respect to the atmosphere was observed for most river and near-shore waters in November 1999. pCO2 ranged from 403 μatm in the Gulf of Mexico to 1280 μatm in the Shark River. The coastal waters of the Southwest Florida Shelf had unusually low salinities at this time due to high inputs of freshwater runoffs after Hurricane Irene. In general, pCO2 levels decreased with increasing salinity, but at different gradients for the different river systems. In June 2000, salinity gradients were considerably reduced during this drought year, with hypersalinity indicating reduced freshwater inputs. However, high pCO2 levels were still observed in and near the mouth of the Shark River (pCO2=383–1280 μatm). A positive correlation of pCO2 levels with colored dissolved organic matter (CDOM) and chlorophyll was observed in all systems examined. CO2 in natural waters may be produced from the photochemical degradation of CDOM, microbial respiration or via shifts in the carbonate equilibrium. Some evidence for a small contribution from photochemical production was observed in the Shark River mouth in a dry season diel study, but not in the wet season. Differences between the rivers are primarily attributed to the significantly higher total alkalinity and lower pH values in the Shark River, and associated higher pCO2 levels. In general, spatial variability in pCO2 is dominated by the chemical characteristics of the river inputs, with temporal variability modulated by changes in pH, photochemical production in low flow seasons and draw down by primary production.
Article
Fluorescence-based observations provide useful, sensitive information concerning the nature and distribution of colored dissolved organic matter (CDOM) in coastal and freshwater environments. The excitation–emission matrix (EEM) technique has become widely used for evaluating sources and sinks of CDOM. Water scattering peaks, however, can create problems for quantitative analysis and display of the EEMs, especially for samples with low CDOM concentrations. Here we report a new method for eliminating Rayleigh and Raman scatter peaks from EEMs during post-processing of the data in MATLAB®. An algorithm was developed to excise scatter peaks (i.e. peak emission±10–15 nm at each excitation wavelength) from the scan data and replace the excised values using three-dimensional interpolation of the remaining data (Delaunay triangulation method). The interpolated surface was constrained to pass through the non-excised values so that only data in excised portions were replaced. Tests of the algorithm in non-scatter regions indicate expected deviations of 0–4% for interpolated regions of DOM fluorescence peaks (i.e. difference between measured and interpolated intensity after removal/interpolation), which is within machine error for the primary observations. This new scattering correction method is shown to provide much improved results in the quantitative analysis of EEMs compared to the conventional blank-subtraction procedure. The method is used to process EEMs and fluorescence quantum yields for water samples obtained along a salinity transect in a river located on the coast of the southeastern United States. Results of this analysis demonstrate observed shifts in EEM peak positions along most of the transect cannot be accounted for by a simple model that computes EEMs as a function of salinity assuming that the dominant driver of EEM spectral change is mixing between riverine and marine waters within the estuary. Other results show that fluorescence apparent quantum yields (AQYs) increased with increasing salinity and pH in the estuary and that the highest quantum yields are observed on excitation by 350–380-nm light. Modeling results and the observed EEM spectral changes indicate that photoreactions had an important effect on the fluorescent dissolved organic matter (FDOM) optical properties in the estuary. The increase in fluorescence quantum yields with increasing salinity and pH in the estuary likely were caused by reactions involving magnesium or hydroxide ions that reduced fluorescence quenching by chelated iron and possibly other paramagnetic ions. The results of the study indicate that EEM analysis with careful scatter correction can provide a powerful tool for evaluating pathways for carbon cycling in estuaries.
Article
For prediction of the rate with which organic compounds are oxidized and photoproducts formed in the aquatic environment by the /sup 1/O/sub 2/ route, the steady-state concentrations of O/sub 2/ in various waters were determined by using furfuryl alcohol as a trapping agent. Under noon, summer sunlight (Switzerland) in lake, river, and wastewaters, surface concentrations were (0.3-3) x 10/sup -14/ M /sup 1/O/sub 2/ per mg of DOC/L. Separation of natural aquatic organic compounds by gel permeation chromatography resulted in fractions showing little correlation between /sup 1/O/sub 2/ production efficiency and molecular weight, indicating that large macromolecular structures are not a prerequisite for the sensitization of /sup 1/O/sub 2/ formation. 26 references, 8 figures, 3 tables.
Article
A bulk sample of oil shale from the Julia Creek deposit in Queensland was retorted under Fischer assay conditions at temperatures ranging from 250 to 550 Â°C. The distributions of the trace elements detected in the shale oil and retort water were determined at each temperature. Oil distillation commenced at 300 Â°C and was essentially complete at 500 Â°C. A number of trace elements were progressively mobilized with increasing retort temperature up to 450 Â°C. The following trace elements partitioned mainly to the oil: vanadium, arsenic, selenium, iron, nickel, titanium, copper, cobalt, and aluminum. Elements that also partitioned to the retort waters included arsenic, selenium, chlorine, and bromine. Element mobilization is considered to be caused by the volatilization of organometallic compounds, sulfide minerals, and sodium halides present in the oil shale. The results have important implications for shale oil refining and for the disposal of retort waters. 22 references, 5 tables.
Article
Laser flash photolysis was used to investigate the transients formed on absorption of 355-nm light by dissolved organic matter (DOM) from natural water bodies and from soil. Absorption spectra and quenching studies of the transients provided confirming evidence that hydrated electrons were formed by all of the DOM that were studied. The DOM from the Suwannee River in Georgia and from the Greifensee, a Swiss lake, exhibited great variability in light-absorbing properties. Despite this high variability in absorption coefficients, the primary quantum yields for electron ejection from the Greifensee and Suwannee DOM fell in a narrow range (0.005-0.008). Steady-state irradiations (355 nm) of the DOM with 2-chloroethanol (0.02 M) present as an electron scavenger produced chloride ions with quantum yields that were about 2 orders of magnitude lower than the primary quantum yields. This result indicates that most of the photoejected electrons recombine with cations before escaping into bulk solution. Irradiations of DOM solutions under sunlight (April, latitude 34° N) photoproduced electrons at rates falling in the range of 0.2-0.4 μmol/[(mg of DOC) h]. These results indicate that hydrated electrons can play a significant role in the environmental photoreduction of persistent, electronegative pollutants but may be relatively unimportant in the environmental production of hydrogen peroxide.
Article
Aqueous solutions of isolated humic substances were investigated by laser flash photolysis. Kinetics of the decay of excited states were measured in emission and absorption. From the absorption data at least two transients with different lifetimes (order of 10 -5 and 10 -4 s) could be deduced. Quenching effects were determined for oxygen and Ni(II), Co(II), and Cd(II). The quenching efficiency of oxygen is similar to the efficiency of the paramagnetic metal ions. The quantum yield of the singlet oxygen production sensitized by humics was determined to be 1-3% with the furfuryl alcohol method. The results are discussed with respect to the photochemistry of aquatic ecosystems.