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Solar Photocatalytic Degradation of Azo Dye: Comparison of Photocatalytic Efficiency of ZnO and TiO2
Abstract
The photocatalytic activity of commercial ZnO powder has been investigated and compared with that of Degussa P25 TiO2. Laboratory experiments with acid brown 14 as the model pollutant have been carried out to evaluate the performance of both ZnO and TiO2 catalysts. Solar light was used as the energy source for the photocatalytic experiments. These catalysts were examined for surface area, particle size and crystallinity. The effect of initial dye concentration, catalyst loading, irradiation time, pH, adsorption of acid brown 14 on ZnO and TiO2, intensity of light and comparison of photocatalytic activity with different commercial catalysts were studied. The progress of photocatalytic degradation of the acid brown 14 has been observed by monitoring the change in substrate concentration of the model compound employing HPLC and measuring the absorbance in UV–Visible spectrophotometer for decolourisation. The photodegradation rate was determined for each experiment and the highest values were observed for ZnO suggesting that it absorbs large fraction of the solar spectrum and absorption of more light quanta than TiO2. The complete mineralisation was confirmed by total organic carbon (TOC) analysis, COD measurement and estimation of the formation of inorganic ions such as NH4+, NO3−, Cl− and SO42−.
... According to global literature, 280,000 tonnes of synthetic dyes get wasted each year by industrial discharge into the environment. [133] Synthetic dyes are extremely toxic, carcinogenic, and mutagenic. [134] It is important to treat the dyed water effluents before released into the water bodies to reduce toxicity, water pollution and protect the environment. ...
... In photocatalytic systems, organic dyes may degrade completely upon light irradiation, or may efficiency levels off after a while. [133] This is possibly the consequence of numerous active sites being blocked on the catalyst surface, which can take place over time. [177] The amount and kind of catalyst utilized have a crucial impact on the photodegradation reaction, which can be increased by adding catalysts. ...
Green technology is gaining importance nowadays, it has become very important to synthesize various nanoparticles. This review mainly focuses on the phytosynthesis of silver nanoparti-cles (AgNPs) and shows their catalytic applications in the field of wastewater treatment. Plant-assisted synthesis of AgNPs has been documented by numerous studies. Plant-mediated synthesis has many advantages over conventional methods and is discussed comprehensively in this review. This critical review aims to help researchers to better understand and follow this rapid, cost-effective, and eco-friendly research area. Dye effluents in water bodies become a major environmental issue worldwide. This paper also offers a thorough understanding of the synthetic dyes such as Rhodamine B, Methylene Blue, Crystal violet, Methyl Orange, and Amido black 10B used in the textile industry, with a focus on the popular methods to degrade them from industrial wastewater, especially, chemical degradation, and photocatalytic dye degradation using silver nanoparticles and other Ag-systems. Additionally, the plausible mechanism involved in chemical and photocatalytic degradation was also provided. There are several review articles on all these topics, but such an in-depth study has not been investigated so far in the concerned research literature.
... In this study, to harness the UV radiation emitted by plasma discharge, two photocatalysts, ZnO and TiO 2 , were selected. Comparisons of these two catalysts are well documented in other applications and remediation methods [43][44][45][46]. The effect of catalyst loading on TMP degradation was thoroughly investigated by repeating the plasma experiments in the presence of different loadings of ZnO or TiO 2 ( Figure 4). ...
In this study, the synergetic action of nanopulsed plasma bubbles (PBs) and photocatalysts for the degradation/mineralization of trimethoprim (TMP) in water was investigated. The effects of ZnO or TiO2 loading, plasma gas, and initial TMP concentration were evaluated. The physicochemical characterization of plasma-treated water, the quantification of plasma species, and the use of appropriate plasma species scavengers shed light on the plasma-catalytic mechanism. ZnO proved to be a superior catalyst compared to TiO2 when combined with plasma bubbles, mainly due to the increased production of ⋅OH and oxygen species resulting from the decomposition of O3. The air–PBs + ZnO system resulted in higher TMP degradation (i.e., 95% after 5 min of treatment) compared to the air–PBs + TiO2 system (i.e., 87%) and the PBs-alone process (83%). The plasma gas strongly influenced the process, with O2 resulting in the best performance and Ar being insufficient to drive the process. The synergy between air–PBs and ZnO was more profound (SF = 1.7), while ZnO also promoted the already high O2–plasma bubbles’ performance, resulting in a high TOC removal rate (i.e., 71%). The electrical energy per order in the PBs + ZnO system was very low, ranging from 0.23 to 0.46 kWh/m³, depending on the plasma gas and initial TMP concentration. The study provides valuable insights into the rapid and cost-effective degradation of emerging contaminants like TMP and the plasma-catalytic mechanism of antibiotics.
... Hal tersebut menyebabkan kurang maksimalnya kontak antara katalis dengan sinar UV yang berakibat pada penurunan kemampuan penyisihan polutan (Rosariawari et al., 2012). Hal tersebut didukung oleh Sakthivel et al., (2003) yang hasil penelitiannya menyatakan bahwa terjadinya penambahan konsentrasi polutan awal akan mempengaruhi panjang jalur foton yang menyinari larutan serta sampai pada katalis akan menurun, hal ini juga yang menyebabkan terjadinya penurunan laju degradasi. ...
Nitrogen is one of the organic compounds contained in liquid waste produced by the tofu industry. In aquatic ecosystems, excessive amount of nitrogen can lead to eutrophication and increase the amount of ammonia, nitrate and nitrite which are poisonous to humans, livestock and wildlife. In order to deal with the issues, photo-catalyst technology is employed to process the organic liquid waste by converting themintoCO_2 and water which are relatively safer. Throughout the process, it requires a medium like resin immobilized photocatalyst-ZnO (RIP-ZnO) to absorb the pollutants. Based on the aforementioned background, this study aims to analyze the impact of RIP-ZnO capacity in reducing the total Nitrogen parameter. The analysis of total Ninthisstudy was conducted by using the Kjeldahl method consisting of three processes, namely destruction, distillation and titration. The research began with the process of making reactor and RIP-ZnO. Subsequently, once the total N concentration is known, the process of resin capacity calculation is continued. As a result, the research findings indicate that the highest resin capacity of 1.14542 reduces the total Nby 33%, while the lowest one of 0.00012 decreases it by 59%. In other words, the resin capacity has an impact on the percentage of total N removal. In conclusion, the lower the resin capacity is, the higher the removal percentage is, and vice versa.
... The methodology for the Photocatalytic measurement has been used as per our previously reported article. [28][29][30] The degradation percentage of RB5 dye was calculated using the formula percentage degradation = (C i -C f ) / C f × 100, where C i is the initial concentration of the dye solution (control), and C f is the final concentration. Using UV-Vis spectropotometer (1800 UV) at a wavelength of 595 nm, which corresponds to the peak absorption wavelength of RB5 dye, the absorption data in tap water was analysed for understanding the reaction kinetics. ...
... Photocatalyst and photoelectrochemical (PEC) cells are the main categories in water-splitting systems. Various nanostructures based on semiconductor oxides, including ZnO, SnO 2 , TiO 2 , V 2 O 5 , WO 3 , and Fe 2 O 3 , have been employed to enhance the PWS activity because of their structural simplicity, flexibility, relatively narrow band gaps, higher charge transport property, and good stability [2][3][4][5][6]. Among these oxides, SnO 2 nanostructures have several industrial applications owing to their high surface area, strong stability, and broad bandgap. ...
This study employs a simple and cost-effective technique to enhance the photoelectrochemical (PEC) water-splitting performance of melamine cyanaurate microrods (M), SnO2 nanostructures (S), and melamine cyanaurate microrods decorated with SnO2 quantum dots (MS) by optimizing NaOH and Na2SO3 electrolytes. Notably, the MS electrode demonstrates a remarkable improvement in PEC efficiency in Na2SO3 solution associated with NaOH solution. Specifically, the induced currents of the MS anode in the Na2SO3 electrolyte are approximately 6.28 mAcm−2 more than those observed in the NaOH electrolyte solution. It is revealed that SO32− anions effectively consume the holes, leading to improved separation of the generated charge pairs. This effective charge separation mechanism significantly contributes to the enhanced PEC performance observed in Na2SO3 electrolytes. The findings of this study suggest a capable approach for improving the PEC activity of the materials through the careful optimization of the supported electrolytes.
... For the removal of organic pollutants, nanomaterials have emerged as promising photocatalysts, which operate under the action of ordinary sunlight [7], owing to their special structure [8] and size (10 −9 nm) [9] as well as the variety of starting materials and synthesis methods [10]. In photocatalytic processes, visible light capping agents and stabilizing agents in nanomaterial synthesis processes [20]. ...
The development of environmental remedies using ecofriendly and green technologies is attracting increasing research attention. Herein, the green synthesis of molybdenum oxide (MoO3) nanoparticles (NPs) using extract of selected Asteraceous flora species is reported for the first time along with the investigation of the degradation ability for organic pollutants and pollen morphological characteristics of wetland species of the family Asteraceae. An X-ray diffraction analysis confirmed the formation and crystalline structure of the MoO3 NPs. Fourier transform infrared spectra of Lactuca Serriola extract indicated the presence of capping agents, reducing agents, and phytochemicals. Scanning electron microscopy images revealed the almost spherical shape of the MoO3 NPs. A qualitative and quantitative palynological investigation of 21 species belonging to 12 genera of the Asteraceae from different wetlands of Azad Jammu and Kashmir is also reported. The MoO3 NPs degraded 99% of crystal violet dye within 30 minutes with a degradation rate of 0.022 min −1. The cost-efficient and green synthesis method for MoO3 NPs with high catalytic activity provides a new platform for the development of photocatalysts. ARTICLE HISTORY
... When BA was added, the photocatalytic effectiveness decreased, from 88.4 % to 75.7 %, and no decrease occurred with the use of EDTA. These results indicate that the influences of h + and •OH on the degradation of ER were negligible [29]. The electron (e − ) can produce active oxygen (•O 2 − ) when oxygen molecules exist. ...
... Oxidative stress is the principal mechanism to induce apoptosis in cancer cells by treatment with ZnO NMs. Previous studies demonstrated that active redox species (electrons and holes; e − /h + ) are present in ZnO NMs even without UV light activation [18]. Furthermore, Al doping increases the production of e − /h + pairs, enhancing the oxidative capacity of the AZO materials. ...
Bare and doped zinc oxide nanomaterials (ZnO NMs) are of great interest as multifunctional platforms for biomedical applications. In this study, we systematically investigate the physicochemical properties of Aluminum doped ZnO (AZO) and its bio-interactions with neuroblastoma (SH-SY5Y) and red blood (RBCs) cells. We provide a comprehensive chemical and structural characterization of the NMs. We also evaluated the biocompatibility of AZO NMs using traditional toxicity assays and advanced microscopy techniques. The toxicity of AZO NMs towards SH-SY5Y cells, decreases as a function of Al doping but is higher than the toxicity of ZnO NMs. Our results show that N-acetyl cysteine protects SH-SY5Y cells against reactive oxygen species toxicity induced by AZO NMs. ZnO and AZO NMs do not exert hemolysis in human RBCs at the doses that cause toxicity (IC50) in neuroblastoma cells. The Atomic force microscopy qualitative analysis of the interaction of SH-SY5Y cells with AZO NMs shows evidence that the affinity of the materials with the cells results in morphology changes and diminished interactions between neighboring cells. The holotomographic microscopy analysis demonstrates NMs' internalization in SH-SY5Y cells, changes in their chemical composition, and the role of lipid droplets in the clearance of toxicants.
Graphical Abstract
... Adding EDTA and BQ to the mixture, as shown in Fig. 10a and 10b, caused the photocatalytic degradation rate to drop from 89.0 % to 12.0 % and 22 %, respectively, proving that h + and • O 2 − are the common active species in photocatalytic reactions. The photocatalytic efficiency dropped from 89.0 % to 86.6 % (Fig. 10c) when BA was added to the mixture, showing that the effect of • OH on the deterioration of RhB was minor [44]. When oxygen molecules are present, the electron (e − ) can produce active • O 2 − , which can contribute to RhB decolorization [58]. ...
Zinc titanate (Zn2Ti3O8 , ZTO) was initially synthesized using a straightforward and inexpensive Pechini approach, and subsequently Zn2Ti3O8 /g-C 3 N 4 (ZTO/CN) nanocomposites with varied contents were created using an ultrasonic-assisted co-precipitation process. Based on the appropriate bandgaps (2.0 eV and 2.7 eV), the photodegradation of rhodamine B (RhB) was studied. This is the first time that ZTO/CN nanocomposites are studied for the degradation of organic pollutants. The results showed that the best performance belonged to the sample with a mass ratio of 0.25 to 1 among different ZTO/CN nanocomposites. Therefore, 50 mg of ZTO/CN (0.25:1) degraded 89.0 % of 10 ppm RhB. Scavenger testing revealed that superoxide radicals and holes played significant parts in photodegradation processes.
... Zinc oxide (ZnO) is a semiconductor with unique and excellent properties [1,2]. Recently, it attracted considerable attention in the photocatalytic activity of environmental pollutants, due to its direct band gap [3] and its high efficiency in the photodegradation of various organic dyes such as thionine, methylene blue, acridine orange, benzene and its derivatives [4][5][6][7][8]. It is important to highlight that ZnO has been extensively studied among other oxides because of its high photocatalytic efficiency, low cost and non-toxicity [9,10]. ...
... The rate of photocatalytic degradation decreased from 86.2% to 39.8 when BQ was added to the mixture, as shown in Fig. 7a, demonstrating that •O 2 − is the most frequent active species in photocatalytic processes. When BA and EDTA were added individually, the photocatalytic effectiveness of the mixture decreased from 86.2% to 81.1% and 83.8%, respectively, demonstrating that the effects of •OH and h + on the degradation of ER were negligible [44]. The electron (e) process can produce active oxygen (•O 2 − ), which can react in a variety of ways, when oxygen molecules are present [45]. ...
... The pollution of water sources by dyes has been increasing over the years, mainly because of industrial processes, endangering aquatic organisms and indirectly affecting humans. These dyes are frequently non-biodegradable, accumulating in the body over time [261]. Very often, industrial waste contains both dyes and heavy metals, which meaning that it is impractical to remove them with different processes [262]. ...
Contemporary technological and industrial advancements have led to increased reliance on chemicals for product innovation, leading to heightened contamination of water sources by traditional pollutants (organic dyes, heavy metals) and disease-causing microorganisms. Wastewater treatment processes now reveal “emerging pollutants”, including pharmaceuticals, endocrine disruptors, and agricultural chemicals. While some are benign, certain emerging pollutants can harm diverse organisms. Researchers seek cost-effective water purification methods that completely degrade pollutants without generating harmful by-products. Semiconductor-based photocatalytic degradation, particularly using titanium dioxide (TiO2), is popular for addressing water pollution. This study focuses on recent applications of TiO2 nanostructures in photocatalysis for eliminating various water pollutants. Structural modifications, like doping and nanocomposite formation, enhance photocatalyst performance. The study emphasizes photocatalytic elimination mechanisms and comprehensively discusses factors impacting both the mechanism and performance of nano-TiO2-based photocatalysts. Characteristics of TiO2, such as crystal structure and energy band-gap, along with its photocatalytic activity mechanism, are presented. The review covers the advantages and limitations of different TiO2 nanostructure production approaches and addresses potential toxicity to human health and the environment. In summary, this review provides a holistic perspective on applying nano-TiO2 materials to mitigate water pollution.
... Adding EDTA and BQ to the mixture, as shown in Fig. 10a and 10b, caused the photocatalytic degradation rate to drop from 89.0 % to 12.0 % and 22 %, respectively, proving that h + and • O 2 − are the common active species in photocatalytic reactions. The photocatalytic efficiency dropped from 89.0 % to 86.6 % (Fig. 10c) when BA was added to the mixture, showing that the effect of • OH on the deterioration of RhB was minor [44]. When oxygen molecules are present, the electron (e − ) can produce active • O 2 − , which can contribute to RhB decolorization [58]. ...
Zinc titanate (Zn2Ti3O8, ZTO) was initially synthesized using a straightforward and inexpensive Pechini approach, and subsequently Zn2Ti3O8/g-C3N4 (ZTO/CN) nanocomposites with varied contents were created using an ultrasonic-assisted co-precipitation process. Based on the appropriate bandgaps (2.0 eV and 2.7 eV), the photodegradation of rhodamine B (RhB) was studied. This is the first time that ZTO/CN nanocomposites are studied for the degradation of organic pollutants. The results showed that the best performance belonged to the sample with a mass ratio of 0.25 to 1 among different ZTO/CN nanocomposites. Therefore, 50 mg of ZTO/CN (0.25:1) degraded 89.0 % of 10 ppm RhB. Scavenger testing revealed that superoxide radicals and holes played significant parts in photodegradation processes.
... The photodecomposition rate is strongly influenced by the number of active sites and the photoabsorption ability of the catalyst (Sakthivel, et al., 2003). The increase in number of active sites available would have a positive impact to the increasing photocatalytic activity and efficiency. ...
The present study investigated the photodecomposition of an organic material during the partial (88.76%) solar eclipse phenomenon of 9 March 2016 in Bandung, Indonesia. Curcumin and anatase titanium dioxide (TiO 2) particles were used as models of organic material and photocatalyst, respectively. The influence of the process parameters (i.e., curcumin concentration and the amount of TiO 2) on the photodecomposition process was also investigated. The results showed that the curcumin was decomposed along with the solar light irradiation time. During the solar eclipse, the photodecomposition rate is suddenly down. The partial solar eclipse provided a slower photodecomposition process than the process under sunny days (conducted on 8 and 10 March 2016). The concentration of curcumin and the amount of TiO 2 also have played an important role in the photodecomposition, in which the lower concentration of curcumin and the higher amount of TiO 2 have a correlation to the obtainment of higher photodecomposition rate.
... A previous study reported high photocatalytic degradation stable for a long time of azo dyes using ZnO (Chen et al. n.d). Elsewhere, it was used in the degradation of acid brown 14 where it proved its performance by attaining full mineralization within 6 h thanks to its relatively good solar light harvesting (Sakthivel et al. 2003). On another hand, lots of research focused on using tin oxide (SnO 2 ) as anode in various applications due to its interesting properties, namely, a low reaction potential and a low cost. ...
Photoelectrochemical setups based on semiconductor photoelectrodes are known for their effectiveness in wastewater treatment, powered by solar energy, which is a renewable and sustainable source. These systems require semiconductor photocatalysts with excellent light-absorbing properties and high stability in aqueous environments. In this regard, silicon is highly investigated in solar cells thanks to its narrow bandgap, making it a potential solar harvester. Metal oxides stand as promising semiconductors, which are non-toxic and thermodynamically stable. In this work, two high-efficiency silicon-based cells have been investigated via Solar Cell Capacitance Simulator (SCAPS-1D) software. Thickness and doping concentration, of each layer, have been scrutinized for multiple buffer propositions to investigate the physical feasibility and optimal values allowing maximal light harvesting. It was found that the overall cell performance is influenced by extremely high doping concentrations for some layers. The effect of temperature was investigated as well at temperatures ranging from 300 to 350 K; it was discovered that the cell demonstrates great performance at the ambient temperature. A maximum solar efficiency of about 25.44% was calculated. Our findings build the path towards fabricating highly efficient Si-based solar cells for photoelectrochemical wastewater treatment.
... For the removal of organic pollutants, nanomaterials have emerged as promising photocatalysts, which operate under the action of ordinary sunlight [7], owing to their special structure [8] and size (10 −9 nm) [9] as well as the variety of starting materials and synthesis methods [10]. In photocatalytic processes, visible light capping agents and stabilizing agents in nanomaterial synthesis processes [20]. ...
... However, zinc oxide is susceptible to photocorrosion in aqueous and acidic electrolytes and dissolves to form Zn(OH) 2 [13]. Despite this limitation, experimental results have shown that ZnO actually exhibits higher photocatalytic activity than TiO 2 and other semiconductor catalysts, such as CdS, WO 3 , Fe 2 O 3 , SnO 2 , ZrO 2 , and GaSe [14], especially in the degradation of dyes in aqueous solution [15][16][17][18][19]. These advantages make it a good candidate for a photoelectrode in photoelectrochemical cells for water oxidation. ...
Recently, heterostructured photocatalysts have gained significant attention in the field of photocatalysis due to their superior properties compared to single photocatalysts. One of the key advantages of heterostructured photocatalysts is their ability to enhance charge separation and broaden the absorption spectrum, thereby improving photocatalytic efficiency. Zinc oxide is a widely used n-type semiconductor with a proper photoelectrochemical activity. In this study, zinc oxide nanorod arrays were synthesized, and then the surfaces of ZnO nanorods were modified with the p-type semiconductor Co3O4 to create a p–n junction heterostructure. A significant increase in the photocurrent for the ZnO/Co3O4 composite, of 4.3 times, was found compared to pure ZnO. The dependence of the photocurrent on the morphology of the ZnO/Co3O4 composite allows for optimization of the morphology of the ZnO nanorod array to achieve improved photoelectrochemical performance. The results showed that the ZnO/Co3O4 heterostructure exhibited a photocurrent density of 3.46 mA/cm2, while bare ZnO demonstrated a photocurrent density of 0.8 mA/cm2 at 1.23 V. The results of this study provide a better understanding of the mechanism of charge separation and transfer in the heterostructural ZnO/Co3O4 photocatalytic system. Furthermore, the results will be useful for the design and optimization of photocatalytic systems for water splitting and other applications.
... Chen at al. [136] reported a sol-gel method for ZnO photocatalyst, and 99.70% removal of MO was observed under UV radiation. Sakthivel et al. [137] compared and investigated acid brown 14 degradation under solar light with commercially available TiO 2 . Te decolorization was achieved within 120 min of irradiation. ...
In recent decades, the textile industry has contributed to continuous pollution in the environment. Synthetic dyes which are commonly found in waste water are azo, sulfur, anthraquinone, triphenylmethyl, indigoid, and phthalocyanine derivatives. These pollutants block the light penetration in water bodies and prevent photosynthesis activity, thereby affecting aquatic life. As an environmental crisis, several technologies have been explored to control pollution. Among all the techniques, the photocatalysis process is considered as a green, simple, and economical process. To improve the photocatalytic activity, researchers worldwide have investigated various photocatalysts such as metal oxides, metal ferrites, and heterostructured nanocomposites. The major goal of this review article is to propose a high-performing, cost-effective hybrid photocatalyst reported to date for prospective azo dye pollutant remediation. This review article also aimed to highlight the challenges and uncertainties associated with dye degradation in the photocatalytic process.
... These observed results were in agreement with the previously reported reports. [48,49] Therefore, it was learned that the ideal amount to carry out the reaction was 10 ppm dye concentration. ...
In the present study, the synthesis of CFN‐800 photocatalyst (CdFe2O4, nanoparticles calcinated at 800 °C) has been achieved using ACM (auto‐combustion method). The confirmation of the formation of the CFN‐800 photocatalyst was done by using different characterization techniques such as FT‐IR, UV‐Vis, XRD, and SEM. Further, under environmental application, synthesized nanoparticles were utilized for the green photocatalytic degradation of industrial effluent Brilliant Blue FCF (BBF) in aqueous media using sunlight as a natural source of light. The degradation experiment was examined by using a UV‐vis spectrophotometer. Various parameters such as the amount of CFN‐800 photocatalyst (g), the concentration of dye (ppm), the amount of dye (ml), and the pH level (1–11) were analyzed to find the more efficient results in terms of % degradation of BBF dye solution. By experiments, the results found were that the photodegradation of BBF touched to 93 % in 100 min. with 0.010 g of CFN‐800 photocatalyst and 10 ppm dye solution (10 ml) at 5 pH. In addition, the prepared CFN‐800 photocatalyst showed excellent performance in recyclability. Even after recycling for up to four consecutive cycles, no significant decrease in the degrading efficiency of CFN‐800 was observed.
... Besides that, bimetallic tungstate has been one of the most potential and well-known semiconductor materials due to its low cost, biocompatibility, high chemical stability, and comparable optical properties with the standard ZnO and TiO 2 photocatalysts, and few reports are available in the literature regarding their uses as photocatalysts for environmental remediation. [12] In recent years, many researchers and scientists have focused mainly on WO 3 -based nanomaterials, which are widely used in different kinds of applications, such as gas sensors, [13] biosensors, [14] batteries, [15] supercapacitors, [16] explosive sensors [17] and photocatalysts. [18] WO 3 -based material absorbs light to increase its energy level and then transfers the energy to a reacting substance to initiate a chemical reaction. ...
Recently, the environmental contamination caused by toxic dyes has posed a severe threat to human health. Semiconductor‐based photocatalysis is gaining a lot of attention due to its potential applications in environmental remediation. Herein, photocatalytic reactive FeCoWO4/g‐C3N4 nanocomposites were prepared through a facile route hydrothermal process. The bare FeCoWO4, g‐C3N4 and FeCoWO4/g‐C3N4 nanocomposites possess specific features, including reactive surface sites, improved efficiency of charge transfer, and accelerated separation of photogenerated electron‐hole pairs. As a result, the photocatalytic activity towards methylene blue dye (MB) degradation activity of FeCoWO4/g‐C3N4 nanocomposites is significantly improved. Thus, photocatalysts exhibited the highest degradation activities and could remove 91.5 % of MB dye within 120 minutes of visible light irradiation. The enhanced photocatalytic activity with excellent recyclability shows potential for the practical application of FeCoWO4/g‐C3N4 nanocomposites for wastewater treatment.
... For the removal of acid brown 14, different photocatalysts were investigated, and ZnO performed best. The sequence of photocatalytic performance for acid brown 14 is summarized as follows: ZnO > TiO 2 > α-Fe 2 O 3 > ZrO 2 > CdS > WO 3 > SnO 2 [35,36]. However, ZnO presents certain challenges when used as a photocatalyst due to its fast electron-hole recombination and low activity under visible light [8,37]. ...
The present study successfully synthesized a rGO-modified Ag@ZnO nanocomposite using simple aqueous solution reactions conducted at low temperatures that successfully removed five emerging pollutants including nonylphenol, roxarsone, ciprofloxacin, rhodamine B, and methylene blue under visible light illumination. A comprehensive analysis encompassing the material optical properties, structure, chemical and surface characteristics was performed, revealing the presence of Ag@ZnO nanoparticles uniformly dispersed on reduced Graphene Oxide (rGO) layers, with particle sizes ranging from 100 to 200 nm. Important features include the material's high purity and a substantial surface area of 636.42 m2/g. The Ag@ZnO@rGO nanocomposite demonstrated significant efficacy of all the mentioned pollutants, achieving an effective removal rate exceeding 85 % after approximately 4 h of visible light irradiation. Systematic optimization of key process parameters, including catalyst dosage, methylene blue (MB) concentration, and pH levels, was performed. Further investigation into MB removal efficiency, using different scavengers, elucidated the crucial role of O2• during photodegradation, providing insightful mechanistic details. Significantly, sustained efficiency over five cycles underlined the reliability of this nanostructure composite.
... According to the majority of the published research, ZnO nanoparticles are mainly used for self-cleaning purposes in cement mortars because of their photocatalytic capabilities that are similar to those of TiO 2 [25][26][27]; ZnO is usually thought of as a TiO 2 replacement since it can absorb a larger portion of the solar spectrum's energy and lighter quanta [28]. ...
The goal of this paper is to investigate the impact of nano-materials on the mechanical and electrochemical properties of self-cleaning concrete. Nano-titanium dioxide and nano-zinc oxide were used as additives for this purpose. Additionally, a comparative study on the effect of using these materials on the self-cleaning concrete’s characteristics was conducted. The dosages of nano-titanium dioxide (nps-TiO2) and nano-zinc oxide (nps-ZnO) used were 0, 0.5, 1, 1.5, 2, and 2.5% and 0, 1, 2, and 3% of the weight of the cement, respectively. The results showed that the optimum compressive strength and the lowest corrosion rate were fulfilled at 2.5% of nps-TiO2 and 1% of nps-ZnO, and using 2.5% of nps-TiO2 achieved the highest improvement in the corrosion rate. However, 1% for nps-TiO2 mixtures and 1% for nps-ZnO mixtures were the best ratios for flexural strength. On the other hand, for the corrosion rate, the samples were tested at 2 and 6 months. When nps-TiO2 and nps-ZnO samples were compared to the control sample, 2.5% and 1% of nps-TiO2 and nps-ZnO, respectively, showed the largest improvement in resistance to corrosion. Also, the self-cleaning property of the samples containing nano-materials (nps-TiO2 and nps-ZnO) was tested. As the results illustrated, the self-cleaning property of the samples was increased over time due to photocatalytic degradation. Furthermore, the results of the photocatalytic tests showed that nps-TiO2 samples outperformed nps-ZnO samples overall.
... Adding BQ to the mixture, as shown in Fig. 8a, caused the rate of photocatalytic degradation to drop from 83.9 % to 37.1 %, proving that •O 2 − is the most common active species in photocatalytic reactions. The photocatalytic efficiency of the combination reduced when BA and EDTA were added separately, going from 83.9 % to 82.9 % and 73.1 %, respectively, showing that the effects of •OH and h + on the deterioration of ER were insignificant (Sakthivel et al., 2003). When oxygen molecules are present, the electron (e − ) reaction can form active oxygen (•O 2 − ), which can react in a number of various manners (Wenderich and Mul 2016). ...
... Photocatalytic degradation of dyes is a green and environmentally friendly degradation method, which mainly Reynolds et al. 1999;Chen et al. 1998). Sakthivel et al. (2003) used sunlight as an irradiation source for the photocatalytic catabolism of azo dyes (acid brown 14) in a comparative photocatalytic degradation test and found that ZnO was the most active photocatalyst. However, the major drawbacks of ZnO are energy bandwidth and photocorrosion. ...
Zinc oxide has been of interest because of its efficient redox capacity in the UV spectral region. However, the high bandwidth limits its application in the visible region. Although synthesizing heterojunctions and doping with other elements have become the focus of the problem, it inevitably has an impact on the environment. In contrast, the template method is not only environmentally friendly but also can be used to increase the degradation rate by changing the nanoparticle mesoporous structure. Microporous/mesoporous zinc oxide with multi-level structure was synthesized using anhydrous ethanol as a green templating agent in a mild and energy-efficient method. The prepared nZnO was characterized using XRD, SEM, BET, and HR-TEM. XRD confirmed that the formation of hexagonal wurtzite zincite nZnO with good crystallinity. SEM results showed that the products were flower-like structures composed of nanosheets with a thickness of 20 nm and an average diameter of 400 nm. TEM and BET confirmed the presence of pits with diameters ranging from about 1 nm to 20 nm existed on the surface of the nanosheets, while the specific surface area of 28.05 m²/g and the pore volume of 0.069 cm³/g also provide advantages for nZnO as a photocatalytic material. The synthesized nZnO overcame the disadvantage of responding only in the UV region, and the photocatalytic degradation efficiency of MB reached 93.2% after 60 min of xenon lamp irradiation, and stabilized at 86.15% after five photocycling tests. Compared with other kinds of templates, anhydrous ethanol has the advantages of environmental friendliness and simple post-processing, and it also provides ideas for the synthesis of multilevel structures of other nanomaterials.
Graphical Abstract
Photocatalysis is considered a more effective form of wastewater purification due to its rapid oxidation process, cost effective and less toxicity.The photocatalysis mechanism involves using light to disintegrate the chemical bonds of dyes, leading to the decomposition of dyes into less harmful compounds. Photocatalysis includes the conventional and advanced (straddling-gap junctions, staggeredgap junctions, broken-gap junctions) processes for reducing the dyes. Additionally, the Z-scheme is an effective method for dye decolorization as it facilitates the separation of excitons and possesses efficient redox capabilities. This chapter provides a detailed discussion of the conventional and advanced photocatalysis mechanism. Moreover, a brief overview of the affecting parameters of photocatalysis, including irradiation time, UV light intensity, pollutant type and concentration, pH of the medium and reaction temperature are discussed. Various methods to increase the photocatalytic activity of the materials were also addressed.
High abundance and low toxicity make zinc oxide very attractive for broad application in the field of photocatalysis for remediation of toxic compounds from water. Its performance could be elegantly improved by inducing native defects into ZnO semiconductor nanocrystals. However, this typically requires post-processing at high temperatures, during which many of the benefits of the original batch of monodisperse small grains are lost due to agglomeration and/or crystal growth. Within this research we show how variations in defect population are inherent to recrystallization process and that they can be tuned to some extent by choosing optimal pathways. The defects were studied by Raman, photoluminescence and EPR spectroscopy, while the surface chemistry of the ZnO nanorods, solvothermally recrystallized from ZnO nanodots, was evaluated by X-ray photoelectron spectroscopy. The produced nanorods with different dimensions were also utilized to show how effective the samples are for degradation of a model pollutant, namely caffeine. Unexpectedly, we found that the surface chemistry of our samples is not the main factor in photocatalytic efficiency. Instead, we show that the differences in photocatalytic activity are mainly determined by the defect population within the bulk formed during the recrystallization process under altered solvothermal conditions.
Photocatalysis is an effective environment-friendly technique that has emerged as a viable option for the degradation of a wide range of contaminants. In most cases, natural sunlight has been used in photocatalysis which makes it cost-effective. Also, it has emerged as an efficient method for degrading harmful toxic chemicals which are hard to degrade by any other alternatives. Various metals and metal compounds like oxides, sulphides, and carbides are used as photocatalysts to increase the efficiency of degradation. The common metal compounds known as ceramics are ZnO, TiO2, MoS2, ZnS, Fe2O3, SiC, CdS, WO3, SrTiO3, SnO2, NiO, and ZrO2. These materials are also known as semiconductors and are used as effective photocatalysts to degrade harmful and toxic environmental pollutants. This review discusses various ceramic photocatalysts, their application, photocatalysis mechanism, and degradation of various organic and inorganic compounds in actual wastewater. The use of photocatalysts in the treatment of persistent organic and inorganic pollutants in wastewater, such as pharmaceutical compounds, pesticides, dyes, oil, microplastics, and heavy metals is also explored.
Polyethylene plastic bags end up in landfills where they take around 300 years to photodegrade and are difficult to recycle. They decompose into minute poisonous particles that pollute the land and streams and make their way into the food chain when animals eat them by mistake. To tackle these huge amounts of waste photoreforming process offers a simple and economical way that works at ambient conditions to produce value-added products through waste degradation and generate hydrogen (H2) via the water reduction process. Our research aims to produce effective catalysts by synthesizing cobalt oxide (Co3O4) on graphite carbon nitride (g-C3N4) layers to build a g-C3N4/Co3O4 Z-scheme hybrid using a green hydrothermal in situ synthetic technique. Specifically, this catalyst was used for forming a Z-scheme heterostructure composite which greatly enhances the photocatalytic performance of the catalysts when exposed to the visible range of light. From the morphological analysis, it was seen that Co3O4 nanoparticles are beautifully spread over g-C3N4 sheets making the composite stable and effective for catalytic applications. From the quenching experiment, it was found that the hydroxyl (•OH) and superoxide radicals (•O2‾) are responsible for the photocatalytic behaviour of the as-prepared Z-scheme catalyst. Using this composite, the photoreforming activity of the catalyst for the disintegration of polyethylene to create organic compounds and hydrogen (H2) at an evolution rate of 17249 μmol g−1 h−1 was investigated. The results obtained in photoreforming experiments to produce energy and value-added products using polyethylene as substrate are challenging as only a visible light source was used for the tests that degraded the polyethylene in only 6 h of irradiation. This result is far better than the others in comparison to the time, catalyst type, and light source. Through using the method reported herein, a promising photocatalyst for green and sustainable development can be designed.
Water purification has become a significant challenge in the last decades due to the increase of
pollutants present in water sources. Inorganic contaminants, as well as organic compounds, have
received special attention due to their impact on human health and the environment. The presence
of these species in aquatic ambient is increasing within the last few years due to industrialization
and population growth. For this reason, it is necessary to create an environment of human
conscience and develop methodologies for protecting and remediating the ambient. In this sense,
the development of novel technologies is emerging. Nanotechnology appears as a recourse for the
development of new nanomaterials, which could be implemented in different methodologies for
environmental remediation and especially for water purification. In this chapter, an overview of the
most novel nanomaterials based on inorganic nanoparticles and bio-based nanocomposites was
reviewed, and their methodology of synthesis, characteristics, and properties for water remediation
was exhausted analyzed due to give knowledge in terms of their impact on environmental
remediation.
Crystal violet (CV), a cationic dye, has been demonstrated to be a powerful carcinogen, a mitotic toxin and poisonous to mammalian cells such that the predominant presence of the dye invariably causes several effects of destruction to mankind and it necessitates a clarion call to do away with this nuisance before it can produce a snowball effect. The recent widespread reporting of the photocatalysis‐mediated degradation of CV as an emerging technique for removing this dangerous dye from aqueous mediums highlights its effectiveness. Numerous researchers have contributed substantially to describing how photocatalysis‐based nanomaterials could be used to degrade CV. The fundamentals and the factors that predominantly influence CV degradation are highlighted in this work. These factors include the nature of nanomaterials, catalyst concentration, dye dosage, pH of the solution in use, temperature and light intensity. Additionally, thorough kinetics and mechanistic descriptions are also included to give crucial insights into the light‐mediated reaction of CV. The present study's findings have the potential to alleviate practitioners and researchers of the burden of having to navigate through numerous studies under each category of parameters to get an exhaustive overview of all of the different variables that may affect the photocatalytic degradation of CV dye. In order to further enhance the photocatalytic degradation efficiency of CV, researchers can expand on this study by investigating novel strategies to lessen the impact of certain elements that adversely affect the photocatalytic reaction process. The potential prospects for extending the advancement of scientific study in this area are also explored. © 2024 Society of Chemical Industry (SCI).
We investigated the role of dynamic shock waves in perovskite SrTiO 3 (STO) material. XRD, FE-SEM, EDAX, FTIR, UV-DRS, XPS, and Raman spectroscopy were all used to examine the title material. When perovskite sample was loaded with shocks, its diffraction pattern did not show any crystal structure changes. The FE-SEM results suggest that the grain size increased linearly with the number of shocks. We used energy-dispersive X-ray spectroscopy to perform elemental analysis; results confirmed that SrTiO 3 NPs were indeed present. Although the impulse of the shock wave changed the optical characteristics, it did not affect the molecular structure. To find the optical band gap energies of untreated and shocked NPs, Tauc plot relationships were used. The band-gap energies got smaller as the shock pulse became more substantial. The impact of shock waves caused oxygen vacancies and surface defects, lowering band gap energy. The test for photocatalytic testing showed that SrTiO 3 NPs that are loaded with shock waves worked much better when they were exposed to visible light. The characteristics, including stress, strain, and bond length, were found to significantly influence photocatalytic applications. In addition, attempts were made to provide a viewpoint for future study. Overall, the objective of this research was to provide valuable insights for experts engaged in the field of SrTiO 3 .
There is no doubt that organic dyes currently play an indispensable role in our daily life; they are used in products such as furniture, textiles, and leather accessories. However, the main problems related to the widespread use of these dyes are their toxicity and non-biodegradable nature, which mainly are responsible for various environmental risks and threaten human life. Therefore, the elimination of these toxic materials from aqueous media is highly recommended to save freshwater resources, as well as our health and environment. Heterogeneous photocatalysis is a potential technique for dye degradation, in which a photocatalyst is used to absorb light (UV or visible) and produce electron–hole pairs that enable the reaction participants to undergo chemical changes. In the past, various metal oxides have been successfully applied as promising photocatalysts for the degradation of dyes and various organic pollutants due to their wide bandgap, optical, and electronic properties, in addition to their low cost, high abundance, and chemical stability in aqueous solutions. Various parameters play critical roles in the total performance of the photocatalyst during the photocatalytic degradation of dyes, including morphology, which is a critical factor in the overall degradation process. In our article, the recent progress on the morphological dependence of photocatalysts will be reviewed.
Removal of organic pollutants with green and efficient methods is an important goal for researchers studying environmental pollution. Photo-Fenton systems with composite catalysts are a promising alternative for efficient water treatment. In this study, Prussian blue modified magnetite decorated multi-walled carbon nanotubes (MW-CNT) nanocomposite was prepared and for the first time applied to the Photo-Fenton degradation of phenol. The catalyst was prepared by rapid hydrothermal method. XPS, SEM–EDX, XRD, FTIR, UV–Vis–NIR, EIS techniques were used for characterizing the functionalities and morphology of the catalyst. The effect of initial H2O2 concentration, catalyst mass, and initial pH of the phenol solution was investigated. Experimental results showed that 100 mg dm−3 of phenol could be degraded completely in 15 min at pH 7. This study describes the synthesis of the new Photo-Fenton catalyst for highly effective phenol treatment.
Binary nanocomposites are one of the promising photocatalysts for the photodegradation of toxic industrial organic dyes which are used as dying agents in different industries including garments and textiles, leather, paint and varnish industries. For this study, TiO2-ZnO nanocomposites were fabricated by the hydrothermal process; where ZnSO4.7H2O is used as a precursor and TiO2 is used as a supporting material. The prepared TiO2-ZnO nanocomposites were calcined at three distinct temperatures 300 °C, 400 °C, and 500 °C. These composite materials were characterized by Scanning Electron Microscope (SEM), X-Ray Diffraction (XRD), Energy Dispersive X-Ray (EDX), and Fourier Transform Infrared (FTIR) analyzes. Basic Violet 14 (BV14), an industrial dye, was modelled to examine the photocatalytic role of TiO2-ZnO under different experimental setups such as calcined temperatures, catalyst loading, concentrations of the BV14 dye, pH, and light sources. TiO2-ZnO prepared at 500 °C acted as the best photocatalyst among three nanocomposites and the prepared TiO2-ZnO worked better than solitary TiO2 and ZnO to decolorize the BV14 dye. In the presence of sunlight, UV light, and visible light the percentages of degradation of BV14 were found to be 81.78 %, 69.58 %, and 31.24 %, respectively. The maximum photodegradation corresponded to 0.175 g/100 mL of suspension of nanocomposite with an initial 3.0×10−5 M of BV14 having solution pH 6.88. The surface reaction constant and Langmuir-Hinshelwood adsorption constant were obtained to be 5.5×10−8 mol.L−1.min−1 and 1.7×108 L.mol−1, respectively. Copyright © 2023 by Authors, Published by BCREC Group. This is an open access article under the CC BY-SA License (https://creativecommons.org/licenses/by-sa/4.0).
Application of nano titanium dioxide (TiO2) in various fields such as advanced oxidation process (AOP) has led to the development of its preparation technologies. The sol–gel process is a widely used chemical wet method for preparing nanoscale TiO2 gels. This technique offers numerous advantages, such as the potential to produce large quantities of homogeneous materials with high purity, surface area, porosity, and reactivity, as well as being cost-effective, simple to implement, and capable of controlling the size and shape of the resulting particles. This review provides a comprehensive overview of the chemicals, reaction conditions, and procedures required for preparing nano TiO2 using the sol–gel method. It covers the selection of necessary compounds, such as TiO2 precursors, solvents, hydrolysis agents, and additives, along with their composition and sequences of adding, reaction order, and impact on the final product. Additionally, it provides detailed information on the routes of gel formation and ambient conditions, including temperature, humidity, stirring speed, injection rates of compounds, aging process, and storage conditions. This information serves as a basic reference for understanding the sol–gel process and the relative contribution rates of the influencing factors, which is essential for controlling the size, morphology, crystallinity, and other physicochemical properties of the resulting TiO2 gel/powder for targeted applications.
The TiO2 photoassisted degradation of the cationic dye rhodamine B (RhB) has been examined in aqueous dispersions under visible light irradiation at wavelengths longer than 470 nm in the presence and absence of the anionic surfactant sodium dodecylbenzenesulfonate (DBS). RhB degrades slowly via a pH-independent process in TiO2 dispersions containing no DBS. The surfactant DBS adsorbs strongly on the TiO2 particles and significantly accelerates RhB degradation with initial rates reaching maximal values at the critical micelle concentration of DBS (cmc = 1.2 mM). In the presence of DBS, rates decrease with increase in pH, an effect directly attributable to variations in the extent of adsorption of RhB with changes in the surface charge of TiO2 particles. The zeta (ζ)-potentials of TiO2 particles in RhB/DBS/TiO2 dispersions (pH 2.1) show that DBS significantly enhances RhB adsorption and correlates with an enhancement in the rate of photodegradation of RhB. The results confirm the heretofore presumed but valid notion that preadsorption on the surface of TiO2 particles is prerequisite for efficient photodegradation of RhB under visible light irradiation; moreover, the data infer that degradation occurs at the particle surface and not in the solution bulk. Present observations are consistent with a pathway in which excited RhB* injects an electron onto TiO2 (an electron-transfer mediator) that is subsequently scavenged by O2 to form the O2•- radical anion and ultimately the OH• radical, as evidenced by DMPO spin-trapping ESR experiments carried out under conditions otherwise similar to those in photodegradation which, we infer, participates in the RhB photodegradation.
The TiO2-assisted photodegradation of anionic eosin has been examined in TiO2 aqueous dispersions under illumination by visible light. Eosin is easily decomposed photochemically by visible light in the presence of TiO2 particles. The degradation kinetics followed a Langmuir-Hinshelwood type equation. The rate was greater in acid media than in neutral and alkaline media, which correlates with the adsorption behavior of eosin on the TiO2 surface. Adsorption of eosin is a prerequisite for the TiO2-assisted photodegradation. The evolution of CO2 occurred concomitantly with the photodegradation of eosin. A plausible mechanism of degradation is discussed.
Highlights of the current state-of-the-art of research activities in photocatalytic transformations of organic molecules on semiconductor particles are presented. These include the specific reactivity of such molecules due to the stabilization of intermediate free radicals at the particle surface. The effect of the size of the catalyst grain on the product distribution is expounded. Examples from time-resolved studies are given to further elucidate mechanistic details. The influence of surface charge and zeta-potential as well as the possible intermediacy of hydroxyl radicals are discussed. It is shown that variations of the particle morphology or its environment (solvent effect) often result in drastic differences in the photocatalytic reactivity. Similar observations are made when different materials are tested for the same reaction. Results from recent publications are given to illustrate areas of common knowledge, but also to draw the attention to obvious contradictions.
Photocatalytic degradation of Acid Green 16 has been investigated in aqueous heterogeneous solutions containing TiO2 as photocatalyst. The influence of irradiation period, dye concentration, catalyst loading, light intensity, H2O2, FeCl3 and Fenton's reagent has been systematically studied. Chemical Oxygen Demand (COD) determination and OD measurement (spectroscopy) have been used to study the degradation and decolorisation, respectively. A solution containing 9 × 10-4 M of the dye could be completely decolourised in 30 min and 90 per cent degradation could be achieved in 3 h.
The photocatalytic degradation of leather dye, Acid green 16 was studied in aqueous solution in the presence of ZnO catalyst by solar oxidation process. The effects of various parameters like concentration of dye, amount of catalyst, irradiation time and pH on the efficiency of decolourisation and degradation were studied. The degradation of Acid green 16 and organic intermediates was found to be more effective in alkaline pH range. The decolourisation and degradation were followed by UV Visible spectrophotometric and COD methods respectively. It is suggested that photocatalytic degradation will be a useful technique for the removal of dye in wastewater from leather industries.
The photocatalytic degradation of Acid Green 16 has been investigated in the presence of an aqueous suspension of ZnO irradiated with near UV light (254 nm). The influences of irradiation time, initial concentration of the dye, catalyst loading, light intensity, pH, particle size, presence of oxygen, nitrogen, hydrogen peroxide, FeCl3 and Fenton's reagent on the degradation efficiency were systematically studied. The decolourisation and extent of degradation of the dye were determined by UV-Vis spectroscopy and COD reflux methods respectively. Complete mineralisation was confirmed by COD analysis as well as high performance liquid chromatography (HPLC).
Biocatalysts, heterogeneous catalysts and homogeneous catalysts are applied as catalytic materials in artificial photosynthetic assemblies. Photoinduced CO2-fixation to formate proceeds in the presence of a Pd-colloid. A photogenerated hydrido-rhodium complex is applied for the regeneration of NAD(P)H cofactors and nicotinamide model compounds. Photogenerated artificial electron carriers are electrically communicated with redox enzymes.
The current status of potential applicability of solar-assisted photocatalysis to water purification is considered by summarizing data germane to nine issues in reactor engineering design: these are (1) photocatalyst action spectrum, (2) rate dependence on intensity, (3) normal vs. concentrated solar photons, (4) reactant concentration influence, (5) catalyst disguises, (6) simultaneous homogeneous and heterogeneous photochemistry, (7) oxidant concentration, (8) mass transfer influences, and (9) simultaneous mass transfer and photon influences.
The photocatalytic decolorization of municipal waste-water contaminated with textile dyes was studied using a batch reactor. Degussa P25 titanium dioxide was used as the photocatalyst and proved to be effective for dye degradation when irradiated with UV light in
the presence of air. In addition to removing the color from the wastewater, the photocatalytic reaction simultaneously reduced the COD which suggests that the dissolved organics had been oxidized. The activation energy for the photocatalytic decolorization reaction was only about 3 to 6 kJ/mole
indicating a weak temperature dependence of the rate. These results suggest that the photocatalytic degradation of textile dyes may be a viable method for decolorizing and oxidizing organics in wastewater.
Photoactive catalysts, when illuminated with UV-light, generate highly reactive radicals that can oxidize the organic contaminants in water. One method to increase the efficiency of the process, and thereby reduce the light energy requirements, is by developing
more active catalysts. Several catalysts that were obtained commercially and/or prepared in the laboratory were examined for their photoactivity, and they are: Ti02, Pt-Ti02 with platinum loading varying from 0.5% to 10% by weight, SrTi03, and
1.5% NiO-SrTi03. The organic compounds used to identify the best catalyst were trichloroethylene (TCE), toluene, methyl ethyl ketone (MEK), salicylic acid, and 2,4-dichlorophenol, with initial concentration varying from 0.1 to 10.0 mg/L. This study also examined the impact of catalyst
dosage, organic compound and its initial concentration, and electron acceptor concentration on the reaction kinetics. The process efficiency for mineralization of organic compounds is also evaluated. The results demonstrate that the activity of photocatalysts can be improved by approximately
2–4 times over commercially available catalysts.
Heterogeneously dispersed semiconductor surfaces provide both a fixed environment to influence the chemical reactivity of a wide range of adsorbates and a means to initiate light-induced redox reactivity in these weakly associated molecules. Upon photoexcitation of several semiconductors nonhomogeneously suspended in either aqueous or nonaqueous solutions or in gaseous mixtures, simultaneous oxidation and reduction reactions occur. This conversion often accomplishes either a specific, selective oxidation or a complete oxidative degradation of an organic substrate present. The paper discusses the following: survey of reactivity (functional group transformations and environmental decontamination); mechanism of photocatalysis (photoelectrochemistry, carrier trapping, inhibition of electron hole recombination by oxygen, involvement of the hydroxy radical, adsorption effects, Langmuir-Hinshelwood kinetics, pH effects, temperature effects, and sensitization); and semiconductor pretreatment and dispersion (photocatalytically active semiconductors, photocatalyst preparation, and surface perturbation). 215 refs.
We have investigated the sensitization of nanoporous titanium dioxide, zinc oxide, tin dioxide, niobium oxide, and tantalum oxide by quantum-sized cadmium sulfide, lead sulfide, silver sulfide, antimony sulfide, and bismuth sulfide. It was shown that the relative positions of the energetic levels at the interface between the quantum size particles and the oxide substrate can be optimized for an efficient charge separation by utilizing the size quantization effect and by varying the materials of the particles and the substrate. Photocurrent quantum yields of up to nearly 80% and open circuit voltages up to the I-V range were measured. We showed that the photostability of the electrodes could be significantly enhanced by surface modification of the particles. 37 refs., 7 figs., 1 tab.
Aqueous suspensions of titanium dioxide (anatase) containing benzoic acid or sodium benzoate have been illuminated with near-ultraviolet light under a variety of experimental conditions. In the presence of oxygen, o-, m- and p-hydroxybenzoic acids are formed in the same isomeric distribution found for OH-radical attack on benzoic acid. When OH-radical scavengers are present the yield of o-hydroxybenzoic acid is depressed. Ratios of rate constants evaluated on the basis of competition kinetics for the OH radical are in agreement with published rate constants. In the absence of oxygen the yield of the hydroxy compound is quite low, but high yields can be restored by the addition of iron(III). In oxygenated slurries, peroxides are formed in initial yield equivalent to about half the yield of total hydroxybenzoic acids. The addition of hydrogen peroxide to the slurries makes no significant difference to the yield of o-hydroxybenzoate. The results are consistent with the generation of OH radicals via the positive holes of the TiO2 particles, followed by OH-radical attack on the aromatic ring and subsequent oxidation of the hydroxy adduct with oxygen or iron(III) to give the corresponding phenol.
Light and dark processes involving Fenton reagent are effective in the degradation of Orange II solutions. The degradation time is shown to be strongly dependent on the initial pH of the solution used. Experimental results show that the mineralization rates for Orange II become significant only at pH 6~5. The H2O2 added and O2 evolved were monitored during the degradation cycles alIowing optimization for the times for oxidant addition. About 88% dye mineralization in 40 minutes under light irradiation. A model for the degradation has been developed from the available kinetic rates for radical reactions. The model predicts an H2O2 consumption time of ~100 seconds consistent with the experimental results obtained. A turnover number of 4.7 was estimated for light induced processes in the model system used. Near surface radical formation is shown to be important during the observed photocatalysis. No activation energy was detected during the mineralization suggesting a radical mechanism for this reaction.
The photocatalytic decomposition of phenol in oxygenated aqueous suspensions of lightly-reduced anatase TiO2, being the most satisfactory among the semiconductors investigated from the standpoint of the photocatalytic activity and stability, has been investigated at the optimum pH 3.5. The products at the initial stage of the reaction were hydroquinone, pyrocatechol, 1,2,4-benzenetriol, pyrogallol, and 2-hydroxy-1,4-benzoquinone. These intermediates underwent further photocatalytic oxidation via acids and/or aldehydes finally into CO2 and H2O. A reaction scheme involving hydroxyl radicals as real reactive species has been proposed. Although H2O2 was formed via O2\
ewdot produced by electron trapping of adsorbed oxygen, its concentration remained constant at a low value during the reaction. About 0.7 mole of O2 was consumed for the consumption of one mole of phenol at the initial stage of the reaction. These results indicated that hydroxyl radicals were formed not only via holes but also via H2O2 from O2\
ewdot. It was interesting from the viewpoint of wastewater treatment that phenol was completely mineralized to CO2 in the presence of TiO2 powder under solar irradiation without both aeration and mixing of the solution.
The photoreduction of methyl orange sensitized by ZnS sols was studied. The reaction is diffusion controlled under the experimental conditions without stirring. A steady state equation is proposed which successfully describes the kinetic process. It can be deduced from the kinetic analysis that the reduction rate is first order with respect to the concentration of methyl orange if the diffusion effect is eliminated. The effects of the surface charge of the ZnS particles on the reaction kinetics were investigated. A positive surface charge formed by adsorbed cations, such as methylviologen (MV2+), Cd2+ and Cu2+, favours photoreduction, whereas a negative surface charge formed by adsorbed anions (e.g. S2−) decreases the reaction rate. This is due to the influence of the surface charge on the electron transfer from the ZnS particle to the methyl orange molecule. In addition, the pH dependence of the reaction rate was studied. The optimum pH for the photoreduction is pH 4.0.
The photocatalytic reaction of xylidine ponceau on zinc oxide power was carried out. Photocatalytic bleaching of the dye was observed spectrophotometrically. The effects of the variation of various parameters, such as the concentration of xylidine ponceau, pH, amount and particle size of the semiconductor and light intensity, on the rate of photocatalytic bleaching were observed. A tentative mechanism for the photocatalytic bleaching of xylidine ponceau is proposed.
The detoxification of water has been performed at a pilot scale under solar irradiation at the Plataforma Solar de Almeria (Spain). The photoreactor consisted of a tank (247l) connected to compound parabolic collectors (CPCs) (147l), through which the aqueous suspension of titania (0.2g/l of TiO2 Degussa P-25) was circulated at a flow-rate of 3.5m3/h. Two model molecules have been chosen: 2,4-D (2,4-dichlorophenoxyaceticacid), a common herbicide and benzofuran (BZF). Both pollutants disappeared following a first-order kinetics and were completely mineralized according to the total organic carbon (TOC) analysis within a residence time
Over the past two decades there has been a large effort to establish a relationship between the catalytic and electrical properties of solids.
Visible light induced degradation of the textile diazo dye Naphthol Blue Black (NBB) has been carried out on TiO2 semiconductor nanoparticles. Diffuse reflectance transient absorption and FTIR techniques have been used to elucidate the mechanistic details of the dye degradation. The failure of the dye to degrade on insulator surfaces such as Al2O3 or in the absence of oxygen further highlights the importance of semiconducting properties of support material in controlling the surface photochemical processes. The primary event following visible light excitation is the charge injection from the excited dye molecule into the conduction band of the semiconductor TiO2, producing the dye cation radical. This was confirmed by diffuse reflectance laser flash photolysis. The surface-adsorbed oxygen plays an important role in scavenging photogenerated electrons, thus preventing the recombination between the dye cation radical and photoinjected electrons. Diffuse reflectance FTIR study facilitated identification of reaction intermediates and end products of dye degradation. By comparison with the degradation products from other azo dyes such as Chromotrope 2B and Chromotrope 2R we conclude that the NBB is degraded to a colorless naphthaquinone-like end product.
The photocatalytic oxidations of CN- and SO32- were studied at several semiconductor powders including TiO2, ZnO, CdS, Fe2O3, and WO3 using a xenon light source. TiO2, ZnO, and CdS were active photocatalysts for cyanide oxidation, while no oxidation was seen for Fe2O3 and WO3. The catalyzed oxidation of CN- at TiO2 using sunlight was also investigated. The rate constant for CN- oxidation at TiO2 in sunlight was 3.1 × 10-6 mol day-1 cm-2 illuminated surface. The product of the oxidation of CN- at TiO2 was quantitatively determined to be OCN-. TiO2, ZnO, CdS, and Fe2O3 photocatalyzed the oxidation of SO32-. The order of the catalytic activity was Fe2O3 ∼ ZnO ∼ CdS > TiO2. The rates of the photocatalytic oxidations were greater for SO32- than for CN- in the cases of TiO2, ZnO, and CdS. The chemical and photochemical stabilities for the most active catalysis were determined.
The photocatalytic oxidation of phenol in oxygenated solution with suspensions of titanium dioxide powders has been investigated. The experimental results indicate that there is an optimum value for the TiO2 content. Sufficient O2 supply is needed, but a too high gas flow rate induces large bubbles that eliminate the gas residence time and the gas/liquid contact interface. The initial concentration of phenol demonstrates a negative effect on the pseudo-first-order reaction rate constant. A mechanism of the phenoxide ions being adsorbed on the TiO2 surface has been proposed to account for this inverse effect. Moreover, the different pH regions affect the competitive adsorption between the phenoxide ions and OH- ions on the TiO2 surface, which in turn influences the generation of the hydroxyl radicals on the TiO2 surface.
The photocatalytic degradation of chloroform has been investigated in aqueous suspensions of TiOâ over the wavelength range of 310-380 nm. A detailed reaction mechanism has been proposed in which the rate-determining step is the reaction of surface-bound {sup {sm bullet}}OH with adsorbed CHClâ. A pH-stat titration technique was developed for the measurement of the rates of degradation of chlorinated hydrocarbons. The quantum efficiency (Φ = 0.56 at λ = 330 nm) of the degradation of CHClâ was found to be inversely proportional to the square root of the incident light intensity. This relationship can be explained in terms of a direct competition between a second-order recombination of surface-bound {sup {sm bullet}}OH and the rate-determining reaction of surface-bound {sup {sm bullet}}OH with CHClâ. The rates of degradation of several electron donors have been correlated with their computed surface speciation. The results of this study show that the adsorption of electron donors and acceptors to the TiOâ surface plays a more important role in determining the rate of the photocatalytic reactions than the effect of pH-dependent Fermi-level shifts.
Colloidal TiO2 particulates have been prepared using a low-temperature (0-degrees-C) method by controlled hydrolysis of TiCl4; TiO2 particulates doped in the lattice with Cr3+ (0.5, 5, and 10 wt % -relative to weight of TiO2-from aqueous CrCl3), with Fe3+ (10 wt % from aqueous FeCl3) and with V5+ ( 10 wt% from aqueous NH4VO3) were also synthesized by an analogous procedure. These particulates were examined spectroscopically and by photoconductivity measurements to assess the photosensitization of titania (anatase) by incorporated metal dopants. Visible absorption bands in metal-doped particulates parallel those observed for aqueous metal cations at all levels of doping (up to 20 wt %); these bands are not reproduced in the action spectra of photoconductivity vs wavelength, except for the absorption threshold of the TiO2 system in all cases. Their photocatalytic activity was determined by standard photoreduction (of water, H-2 evolution) and photooxidation (of oxalic acid) reactions to assess the influence of metal dopants in heterogeneous photocatalysis.
This paper studies the feasibility of photocatalytic degradation of organophosphorus pesticides using thin films of TiO2. The results show that 0·65 × 10−4mol dn−3 of dichlorvos, monocrotophos, phorate, parathion can be completely photocatalytically degraded into PO within a short time under illumination with a medium pressure mercury lamp of 375 W. The effect of parameters such as he amount of TiO2 attached, initial concentration of organphosphorus pesticides, initial pH, amount of air flow, concentrations of H2O2 and Fe3+ on the photocatalytic degradation are studied. The possible mechanisms of photocatalytic degradation are discussed. It is our assumption that the degradation reactions occur on the surface of TiO2 via primary produced oxidizing species such as the OH radical and O, after 80 h illumination, there is no significant loss of the photocatalytic activity of TiO2.
The relative concentrations of ammonium and nitrate ions were measured during the photocatalytic oxidation of theophylline, proline, pyridine, and piperidine. It was found that the formation of inorganic nitrogeneous species occurred almost instantaneously upon illumination and at a significantly greater rate than carbon dioxide production, irrespective of whether the nitrogen heteroatom was contained in a saturated or an aromatic ring. The opening of the ring systems was postulated not to be the rate determining step in the formation of these species.The concentration versus time profiles for the formation of carbon dioxide, ammonium and nitrate for each compound indicated that a number of reaction intermediates, referred to here as organic and inorganic nitrogenous intermediates, have to be involved for the photocatalytic conversion of initial organic nitrogen to the inorganic species. A framework is thereby proposed for the formation pathway of these species. Indirect evidence was used to support the existence of such intermediates.
A novel photocatalytic reactor design for water treatment is characterized by the use of new extremely narrow diameter lamps, thus allowing for much higher surface area for catalyst coating per unit reactor volume and consequently for much higher specific reactor capacity. Experiments in a reactor containing 21 novel U-shaped lamps coated with catalyst showed a 695% increase in efficiency of the reactor performance in comparison with a classical annular reactor and 259% in comparison with a slurry reactor. Both a classical annular reactor and a slurry reactor cannot be scaled up for large-scale applications due to the low values of illuminated catalyst surface area per unit volume of liquid treated inside the reactor while this configuration is flexible enough for large-scale applications.
The destruction of toxic halogenated hydrocarbons in waste water effuents or ground water wells is a problem of growing importance in our industrial society. Since conventional methods such as chemical oxidation or microbial treatment are often not efficient for the destruction of these toxins, alternative routes for detoxification are required. It has been shown that semiconductor particles can be used as photocatalysts which are capable of inducing the complete mineralization of many of these hazardous compounds. Detailed mechanistic studies are presented here which have been carried out to investigate the potential use of concentrated solar illumination as the source of irridiance in those photocatalytic systems. Thus the influence of light intensity, temperature and pH on the overall yield of the destruction of halogenated hydrocarbons in aqueous suspensions containing titanium dioxide powder has been studied in detail. Models are presented to explain the observations made with chloroform as the probe molecule and to enable predictions of the efficiency of this method for “real world” applications.
Nitrogen-containing substrates such as amino acids, amides, succinimide, imidazole, hydroxylamine and urea were photodegraded in illuminated TiO2 suspensions, and the temporal course of formation of NH4+ and NO3− ions was monitored. The hydroxylated nitrogen moiety in a molecule was predominantly converted to NO3− ions, whereas a primary amine and/or an amide were exclusively transformed to NH4+ ions under the prevailing conditions. Heterocyclic nitrogen groups in imidazole were converted to both NH4+ and NO3− ions via the intermediates whose structures were the primary amine and hydroxylamine, respectively. The influence of chemical structure on formation of NH4+ and NO3− ions is discussed from the view point of adsorption behaviours of substrates onto the TiO2 surface.
The organochlorine pesticide permethrin can be photodegraded into Cl− and CO2 with TiO2 semiconductor catalyst. The aromatic moiety in permethrin is easily cleaved approximately via apparent first-order kinetics. The aromatic ring opening rate (1.73 × 10−3 min−1) is nearly identical with the dechlorination rate (1.82 × 10−3 min−1). The presence of the TiO2 catalyst, UV irradiation and oxygen gas are essential for photo-oxidation at a reasonable rate. The insoluble permethrin (in water) can be efficiently photodegraded in a TiO2 slurry of hexane-water mixture under solar exposure with high conversion (more than 90%), even at high concentration (17 000 ppm), in 8 h on a sunny day. The hydrophobic TiO2 catalyst (T-805 modified by octyltrimethoxyl silane gives more effective photocatalytic activity than the naked TiO2 (P-25).
UV light induced degradation of textile dye, reactive red M5B has been carried out on TiO2 and ZnO semiconductor particles. Spectrophotometer and COD techniques have been used to elucidate the details of dye decolourisation and degradation. The experiments have been carried out with different amount of catalyst, various concentration of dye solution, different irradiation time and in the presence of air. The reactive red M5B has been degraded to colourless end products. The results suggest that the photocatalytic degradation of textile dye may be a viable method for the safe disposal of wastewater.
The photocatalytic oxidation of methylene blue, Rhodamine B, methyl orange and salicylic acid have been studied in the presence of immobilized titanium dioxide coated sand in a flat bed configuration illuminated with a 100 W medium pressure mercury lamp and sunlight. The disappearance of solute in each case approximately obeys first order kinetics with the apparent first order decay constant increasing with decreasing solute concentration. Flow rate data, total organic carbon analyses and the effect of hydrogen peroxide are also given. The marked dependence of degradation rates on flow rates indicated a substantial mass transfer limitation but the rates were much greater in the photocatalytic system than for the natural u.v. degradation rates. The geometric arrangement lends itself to scaling up of the process in continuous operation without major engineering cost. Times for removal of 50% of solute for 151. of each solution of initial concentration 10 μM at a flow rate of 13.81/min per m2 of sunlight at the equinox were 12–22 min. The corresponding times for 50% mineralization were 21–46 min.
This paper reports the photocatalytic degradation of sodium dodecyl sulphate (SDS) in a bubble column reactor containing suspended irradiated (λ = 250–310 nm) titania particles. The effects of six major process variables were investigated. Reaction rate increased linearly with light intensity and the variation in temperature revealed an activation energy of about 41 kJ mol−1. Although an increase in solution pH enhanced the hydrolysis of SDS and hence an increase in free dodecyl sulphate anion for adsorption, a downturn in rate was observed beyond pH = 7.5 probably due to reduced surface concentration of the dodecyl sulphate species on the predominantly negatively charged titania surface at pH values higher than the isoelectric point, IEP = 6. The optimum seen in the rate vs catalyst loading curve was also attributed to the compromise between increase in number of sites (particles) at higher catalyst concentration and the occurrence of light scattering and hence, reduced light-harvesting efficiency at these higher values. The data on the role of SDS concentration and oxygen partial pressure similarly suggest nonlinear dependency of rate on these reactants. The empirical models derived were consistent with the proposition of a dual-site (photogenerated positive and electron-rich sites) Langmuir–Hinshelwood mechanism where the rate-controlling step is the surface interaction between the adsorbed dodecyl sulphate and peroxy radicals. Highly reactive peroxy radicals were presumably formed from the protonation of surface superoxide anion, O2−, produced via adsorption of oxygen on the electron-rich site.
Degussa TiO2 nanoparticles have been immobilized on a foamed polyethylene by thermal bonding to produce a stable catalyst sheet containing 0.7 mg TiO2 cm−2 and retaining 40–50% of active surface area of the particles. On such a catalyst sheet, exposed to radiation of a 125 W mercury vapour lamp, decomposition of about 0.3 mg of methylene blue (MB) is obtained per cm2 in 1 h at ambient temperature from an aqueous solution of 200 ppm MB. The rate data fit well to classical Langmuir—Hinshelwood (L—H) rate form. This rate form also results from mechanisms based on the assumption of hydroxyl radical formation on the irradiated catalyst and a reaction between the hydroxyl radical and the organic dye molecule, either or none of them being adsorbed on the catalyst surface. An activation energy of 14.5 kcal mol−1 is obtained for the photocatalytic decomposition of methylene blue following the L—H rate laws.
Benzoic acid, salicyclic acid, phenol, 2-chlorophenol, 3-chlorophenol, 4-chlorophenol, nitrobenzene, methanol, ethanol, acetic acid and formic acid in aerated, aqueous suspensions of TiO2 were illuminated with near u.v. light. Each solute was present over a range of concentration; generally 1–50 mg 1−1. The rate of oxidation to carbon dioxide was measured at each concentration. The dependence of the rate on concentration obeyed simple Langmuir-Hinshelwood (L-H) kinetics for each solute. Empirical constants have been determined enabling the prediction of mineralization rates at any solute concentration in 0.1% (w/w) TiO2 suspensions. The integrated L-H rate expression gives reasonable agreement with observed time-dependent CO2 formation curves. A comparison was made between CO2 formation rate using 4-chlorophenol illuminated with artificial and natural u.v. light. The results indicate that 90% mineralization of all the solutes except formic acid initially present as 50 mg solute 1−1 suspension occurs within 2–3 h with 1 m2 of sunlight. Formic acid was mineralized at approximately twice the rate of the other solutes.
Surface doping of colloidal TiO/sub 2/ particles with chromic ions precipitated from aqueous H/sub 2/SO/sub 4/ solution produces very small (<0.1 ..mu..m) mixed-oxide particles which absorb light in the 400-550-nm region in addition to the band-gap absorption of anatase. Sustained water cleavage by visible light is observed in aqueous solutions of these particles. Ultrafine deposits of Pt or RuO/sub 2/ are necessary to promote water decomposition. A pronounced synergistic effect in catalytic activity is noted when both RuO/sub 2/ and Pt are codeposited onto the particle. Wavelength dependency and kinetics of H/sub 2/ and O/sub 2/ evolution are examined.
The photocatalytic degradation of leather dye, Acid green 16, has been investigated over a ZnO catalyst supported on two different materials, namely alumina and glass beads (3-5 mm diameter). Sunlight was used as the energy source. The alumina-supported ZnO outperformed the glass-supported ZnO under identical operational conditions suggesting that the dye molecules are adsorbed on the alumina supports to make a high concentration environment around the loaded ZnO. The degradation efficiency was greater at pH = 4 compared to other acidic and neutral pH. Also, the degradation efficiency was a little bit higher in alkaline medium, which correlates with the adsorption behaviour of acid green 16 on the alumina supported ZnO. The influence of inorganic oxidants like H2O2, FeCl3 and Fenton reagent on the degradation efficiency were systematically studied. The decolourisation and extent of degradation of the dye were determined by UV-VIS spectroscopy and COD reflux methods, respectively. Complete mineralisation of the dye was conformed by High performance liquid chromatography (HPLC) analysis.
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