Removal of As(III) from aqueous solutions through simultaneous photocatalytic oxidation and adsorption by TiO 2 and zero-valent iron

Role of the Sn-TiO2/Ti-SnO2 Heterojunction in Enhancing the Photocatalytic Oxidation of Arsenite (AsIII) through the Promotion of Charge Carrier Lifetime

This study proposes the heterojunction photocatalyst, Sn-doped TiO2/Ti-doped SnO2 (herein named Sn1Ti1O2), as a promising alternative to pure TiO2. Sn1Ti1O2 demonstrates improved light harvesting efficiency over TiO2 by generating longer-lived electron-hole (eCB--hVB+) pairs, while also displaying a smaller band gap compared to pure TiO2. Consequently, we show that it is a promising candidate for the photocatalytic oxidation (PCO) of AsIII to the less toxic and more readily removable form AsV. Transient absorption spectroscopy (TAS) shows increased eCB--hVB+ recombination half-lives from ∼0.5 ms in TiO2 to ∼1 ms in Sn1Ti1O2. The initial transient absorption signal for Sn1Ti1O2 is twice that of pure TiO2, suggesting early time scale (pre-μs) suppression of (eCB--hVB+) recombination. Moreover, TAS showed that Sn1Ti1O2 possesses more reactive charge carriers than TiO2 under reactions with chemical scavengers. For the first time, TAS experiments were conducted using both a colorimetric indicator (molybdate) and AsIII to determine the PCO kinetics from AsIII to AsV. The TAS molybdate─AsIII experiment results indicate that the oxidation process occurs on the sub-microsecond time scale, with a notable increase in absorption at ∼700 nm, providing evidence of the formation of the AsV─molybdate blue complex. PCO experiments showed that •OH radicals played the predominant role during PCO, followed by superoxide radicals (O2•─). •OH scavengers including isopropanol, rebamipide anhydrous, and dimethyl sulfoxide (DMSO) reduce the PCO yield of AsIII to 21, 30, and 23%, respectively. While O2•─ scavengers including superoxide dismutase (SOD) and p-benzoquinone suppressed the PCO yield of AsIII to a lesser degree, with yields of 35 and 49% seen, respectively. The effects of irradiance intensity, salinity, pH, AsIII concentration, and photocatalyst mass on both the quantum efficiency (QE) and PCO kinetics were investigated. The Sn1Ti1O2 catalyst exhibited effective recyclability, validating its economical reusability. Overall, the study demonstrates the potential of the Sn1Ti1O2 heterojunction photocatalyst for the PCO of AsIII to the less toxic AsV in water treatment, showing faster oxidation kinetics and improved charge separation compared to pure TiO2 as proven by TAS.

Core to concept: synthesis, structure, and reactivity of nanoscale zero-valent iron (NZVI) for wastewater remediation

Numerous technological advancements have been developed to tackle the issue of wastewater remediation effectively. However, the practical application of these technologies on a large scale has faced several challenges that have hindered their progress. These challenges include low selectivity, high energy requirements, and significant expenses. Nanoscale materials have demonstrated remarkable effectiveness in removing a wide range of contaminants. Nanoscale zero-valent iron (NZVI) exhibits a range of distinctive physical and chemical properties that have proven to be highly effective in various environmental remediation applications. These include its impressive surface area, remarkable reactivity, and its capacity to create stable colloidal suspensions. The paper explores the synthetic techniques for NZVI with special emphasis on green synthesis and the use of capping or support agents for maintaining stability and enhancing the reactivity of NZVI. The various structural and reactivity aspects of NZVI have been highlighted for its potential application in wastewater treatment sequestrating various categories of inorganic and organic contaminants. The discussion also delves into the limitations of NZVI, highlighting its dependence on water as a medium for contact reaction or electron transfer through the action mechanism of NZVI in adsorptive and photocatalytic sequestration of contaminants. The beneficial potential of NZVI-based composite systems in the field of environmental remediation has also been included which aids in the application of NZVI in environmental remediation.

In situ formation of CuxO/ZnO photocatalysts for efficient simultaneous oxidation of As (III) and adsorption of As (V): Effect of Cu loading

The introduction of Cu ions onto ZnO leads to alterations in the electrical, optical, and magnetic characteristics of ZnO. These transformations, in turn, result in heightened photocatalytic activity and enhanced stability when employed in the degradation of both organic and inorganic pollutants. Here, a novel photocatalytic-adsorbent system is developed using zinc oxide (ZnO) nanostructures modified with Cu (II) ions in an aqueous solution containing 40 mg/L of As (III). The system utilizes UV-A light (365 nm) as the irradiation source, and the weight percentage of Cu (II) in the composite varies from 0 to 20%. The experimental results reveal significant adsorption of As (III), ranging from 20 to 50%, depending on the solution's Cu (II) content. Remarkably, the ZnO10%Cu composite exhibits the highest photocatalytic activity, achieving 40% adsorption and complete oxidation of As (III) within 25 min of irradiation. Characterization of the composite after the photocatalytic treatment reveals the effective adsorption of As (V) within its structure. Furthermore, no traces of Cu (II) ions are detected in the solution after the reaction, indicating their successful adsorption onto the ZnO surface as Cu (I) and Cu (II) ions. This research marks a significant advancement in harnessing innovative materials for efficient arsenic removal, offering promising insights into the development of novel photocatalytic-adsorbent systems.

Removal of Toxic Metals from Water by Nanocomposites through Advanced Remediation Processes and Photocatalytic Oxidation

Purpose of Review

Heavy and toxic metals are becoming more prevalent in the water sources of the globe, which has detrimental repercussions for both human health and the health of ecosystems. The summary of recent findings on treatment possibilities of toxic metal species by nanomaterials should facilitate the development of more advanced techniques of their removal.

Recent Findings

The high concentrations of chromium, mercury, and arsenic identified in wastewater cause a hazard to human health. There is a wide variety of nanoadsorbents and nanophotocatalysts used for heavy/hazardous metal removal. Recent research has resulted in the production of advanced nanostructures that exhibit extraordinary heavy/hazardous metal adsorption effectiveness and photocatalytic diminution of metal ions. These nanostructures have physically and chemically tunable features.

Summary

In this review article, the use of carbon-based nanomaterials, polymer-based nanomaterials, and semiconductor-based nanomaterials are extensively discussed to remove mercury, chromium, and arsenic ions from wastewater by the adsorption process. Advanced nanomaterials involved in photocatalytic reduction are also comprehensively discussed.

Photocatalysis for arsenic removal from water: considerations for solar photocatalytic reactors

The following work provides a perspective on the potential application of solar heterogeneous photocatalysis, which is a nonselective advanced oxidation process considered as a sustainable technology, to assist in arsenic removal from water, which is a global threat to human health. Heterogeneous photocatalysis can oxidize trivalent arsenic to pentavalent arsenic, decreasing its toxicity and easing its removal with other technologies, such as chemical precipitation and adsorption. Several lab-scale arsenic photocatalytic oxidation and diverse solar heterogeneous photocatalytic operations carried out in different reactor designs are analyzed. It was found out that this technology has not been translated to operational pilot plant scale prototypes. General research on reactors is scarce, comprising a small percentage of the photocatalysis related scientific literature. It was possible to elucidate some operational parameters that a reactor must comply to operate efficiently. Reports on small-scale application shed light that in areas where other water purification technologies are economically and/or technically not suitable, and the solar energy is available, shed light on the fact that solar heterogeneous photocatalysis is highly promissory within a water purification process for removal of arsenic from water.

The Effect of Arsenic on the Photocatalytic Removal of Methyl Tet Butyl Ether (MTBE) Using Fe2O3/MgO Catalyst, Modeling, and Process Optimization

MTBE is an aliphatic matter successfully removed from contaminated water by an advanced oxidation process. Additionally, arsenic is a toxic metalloid that is detected in some water supplies, such as in Iran. Concerning the oxidation potential of arsenic in an aqueous solution, it is expected that its interference in the photocatalytic removal of organic matter includes MTBE. Nevertheless, there is a lack of observation of this effect. In this study, the effect of arsenic on the photocatalytic removal of MTBE using an Fe2O3/MgO catalyst under UV radiation was investigated. Using an experimental design, modeling, and optimizing operational parameters, such as the arsenic and MTBE concentrations, catalyst dosage, pH, and reaction time, were studied. The synthesized nanocatalyst had a uniform and spherical morphological structure and contained 33.06% Fe2O3 and 45.06% MgO. The results indicate that the best model is related to the quadratic (p-value < 0.0001, R2 = 0.97) and that the effect of the MTBE concentration is greater than the others. The highest removal efficiency was taken in an initial concentration of 37.5 mg/L MTBE, 1.58 mg/L Fe2O3/MgO, pH 5, and a reaction time of 21.41 min without any As. The removal efficiency was negatively correlated with the initial MTBE concentration and pH, but it was positively associated with the Fe2O3/MgO dosage and reaction time. Finally, the presence of arsenic decreased the removal efficiency remarkably (90.90% As = 0.25 μg/L and 61% As = 500 μg/L). Consequently, MTBE was removed by the photocatalytic process caused by Fe2O3/MgO, but the presence of arsenic was introduced as a limiting factor. Therefore, pretreatment for the removal of arsenic and more details of this interference effect are suggested.

Advances in metal(loid) oxyanion removal by zerovalent iron: Kinetics, pathways, and mechanisms

Metal(loid) oxyanions in groundwater, surface water, and wastewater can have harmful effects on human or ecological health due to their high toxicity, mobility, and lack of degradation. In recent years, the removal of metal(loid) oxyanions using zerovalent iron (ZVI) has been the subject of many studies, but the full scope of this literature has not been systematically reviewed. The main elements that form metal(loid) oxyanions under environmental conditions are Cr(VI), As(V and III), Sb(V and III), Tc(VII), Re(VII), Mo(VI), V(V), etc. The removal mechanisms of metal(loid) oxyanions by ZVI may involve redox reactions, adsorption, precipitation, and coprecipitation, usually with one of these mechanisms being the main reaction pathway and the other playing auxiliary roles. However, the removal mechanisms are coupled to the reactions involved in corrosion of Fe(0) and reaction conditions. The layer of iron oxyhydroxides that forms on ZVI during corrosion mediates the sequestration of metal(loid) oxyanions. This review summarizes most of the currently available data on mechanisms and performance (e.g., kinetics) of removal of the most widely studies metal(loid) oxyanion contaminants (Cr, As, Sb) by different types of ZVI typically used in wastewater treatment, as well as ZVI that has been sulfidated or combination with catalytic bimetals.

In situ X-ray absorption spectroscopic studies of photocatalytic oxidation of As(III) to less toxic As(V) by TiO2 nanotubes

Arsenic in groundwater caused the black-foot disease (BFD) in many countries in the 1950-1960s. It is of great importance to develop a feasible method for removal of arsenic from contaminated groundwater in BFD endemic areas. Photocatalytic oxidation of As(III) to less toxic As(V) is, therefore, of significance for preventing any arsenic-related disease that may occur. By in situ synchrotron X-ray absorption spectroscopy, the formation of As(V) is related to the expense of As(III) disappearance during photocatalysis by TiO₂ nanotubes (TNTs). Under UV/Vis light irradiation, the apparent first-order rate constant for the photocatalytic oxidation of As(III) to As(V) is 0.0148 min^-1. It seems that As(III) can be oxidized with photo-excited holes while the not-recombined electrons may be scavenged with O₂ in the channels of the well defined TNTs (an opening of 7 nm in diameter). In the absence of O₂, on the contrary, As(III) can be reduced to As(0), to some extent. Cu(II) (CuO), as an electron acceptor, was impregnated on the TNTs surfaces in order to gain a better understanding of electron transfer during photocatalysis. It appears that As(III) can be oxidized to As(V) while Cu(II) is reduced to Cu(I) and Cu(0). The molecular-scale data are very useful in revealing the oxidation states and interconversions of arsenic during the photocatalytic reactions. This work has implications in that the toxicity of arsenic in contaminated groundwater or wastewater can be effectively decreased via solar-driven photocatalysis, which may facilitate further treatments by coagulation.

Improved photocatalytic oxidation of arsenic (III) with WO3/TiO2 nanomaterials synthesized by the sol-gel method

Photocatalytic oxidation of arsenite (As(III)) to arsenate (As(V)) was studied in aqueous solution using a series of WO₃/TiO₂ semiconductors readily synthesized through sol-gel method with WO₃ content in the range of 1-5 wt%. The resulting materials showed enhanced photocatalytic activity towards As(III) photo-oxidation compared to their individual counterparts under UV radiation. The amount of As(III) and As(V) species in the irradiated solution was determined using the molybdenum blue method. The efficiency of photoinduced carriers separation was further affirmed by electrical impedance spectroscopy (EIS) and photocurrent tests. The maximum catalytic efficiency was observed when the binary oxide 3%WO₃/TiO₂ (TW3) was used, reaching a 99% conversion of As(III) to As(V) within the first 25 min under UV irradiation. The enhanced photocatalytic performance of the heterostructures could be explained as consequent to an improved charge separation due to the migration of photoproduced holes in TW3 photocatalyst. Based on the electric band structure of WO₃ and TiO₂, a reasonable mechanism for the photo-oxidation of As(III) over TW3 novel catalyst has been proposed.

Present applications of titanium dioxide for the photocatalytic removal of pollutants from water: A review

The evolution of modern technology and industrial processes has been accompanied by an increase in the utilization of chemicals to derive new products. Water bodies are frequently contaminated by the presence of conventional pollutants such as dyes and heavy metals, as well as microorganisms that are responsible for various diseases. A sharp rise has also been observed in the presence of new compounds heretofore excluded from the design and evaluation of wastewater treatment processes, categorized as "emerging pollutants". While some are harmless, certain emerging pollutants possess the ability to cause debilitating effects on a wide spectrum of living organisms. Photocatalytic degradation has emerged as an increasingly popular solution to the problem of water pollution due to its effectiveness and versatility. The primary objective of this study is to thoroughly scrutinize recent applications of titanium dioxide and its modified forms as photocatalytic materials in the removal and control of several classes of water pollutants as reported in literature. Different structural modifications are used to enhance the performance of the photocatalyst such as doping and formation of composites. The principles of these modifications have been scrutinized and evaluated in this review in order to present their advantages and drawbacks. The mechanisms involved in the removal of different pollutants through photocatalysis performed by TiO2 have been highlighted. The factors affecting the mechanism of photocatalysis and those affecting the performance of different TiO2-based photocatalysts have also been thoroughly discussed, thereby presenting a comprehensive view of all aspects involved in the application of TiO₂ to remediate and control water pollution.

Nano-Scale Drinking Water Treatment Residuals Affect Arsenic Fractionation and Speciation in Biosolids-Amended Agricultural Soil

An incubation experiment was conducted to determine the effects of nanoscale drinking water treatment residuals (nWTRs) on arsenic (As) fractionation and speciation in agricultural soil amended with biosolids. The soils were treated with biosolids of 3% (w/w), along with nWTR application rates of 0, 0.25, 0.50, or 1.00% (w/w). The results revealed that the As adsorption rate increased with increasing the As treatment level from 50 to 800 mg/L. The maximum efficiency of As adsorption was 95%–98% in the soil treated with nWTRs of 1%, while the least As adsorption was 53%–91% in the soil treated with nWTRs of 0.25%. The overall As bioavailability in the biosolids-amended soil followed a descending order of nWTRs treatment: (0%) > 0.25% nWTRs, >0.50% nWTRs, and >1% nWTRs. The addition of nWTRs significantly changed As speciation in biosolids-amended soil. The X-ray absorption near-edge structure spectroscopy (XANES) and MINEQL+4.6 analyses showed that most of As was in a oxidized form of As5+ that likely incorporated in As pentoxide, and thus, with low mobility, bioavailability, and toxicity. This study demonstrated that nWTRs were effective in adsorbing and immobilizing As in biosolids-amended agricultural soils by forming stable As-nWTR surface complexes.

Unravelling the Potential Cytotoxic Effects of Metal Oxide Nanoparticles and Metal(Loid) Mixtures on A549 Human Cell Line

Humans are typically exposed to environmental contaminants' mixtures that result in different toxicity than exposure to the individual counterparts. Yet, the toxicology of chemical mixtures has been overlooked. This work aims at assessing and comparing viability and cell cycle of A549 cells after exposure to single and binary mixtures of: titanium dioxide nanoparticles (TiO2NP) 0.75-75 mg/L; cerium oxide nanoparticles (CeO2NP) 0.0.75-10 μg/L; arsenic (As) 0.75-2.5 mg/L; and mercury (Hg) 5-100 mg/L. Viability was assessed through water-soluble tetrazolium (WST-1) and thiazolyl blue tetrazolium bromide (MTT) (24 h exposure) and clonogenic (seven-day exposure) assays. Cell cycle alterations were explored by flow cytometry. Viability was affected in a dose- and time-dependent manner. Prolonged exposure caused inhibition of cell proliferation even at low concentrations. Cell-cycle progression was affected by TiO2NP 75 mg/L, and As 0.75 and 2.5 μg/L, increasing the cell proportion at G0/G1 phase. Combined exposure of TiO2NP or CeO2NP mitigated As adverse effects, increasing the cell surviving factor, but cell cycle alterations were still observed. Only CeO2NP co-exposure reduced Hg toxicity, translated in a decrease of cells in Sub-G1. Toxicity was diminished for both NPs co-exposure compared to its toxicity alone, but a marked toxicity for the highest concentrations was observed for longer exposures. These findings prove that joint toxicity of contaminants must not be disregarded.

Arsenic removal from highly-acidic wastewater with high arsenic content by copper-chloride synergistic reduction

A synergistic combination of chloride and copper powder was proposed as a new method to reductively remove arsenic from highly-acidic wastewater with high arsenic content (HAWA). As(III) was reduced to As(0) by copper powder in the presence of chloride and were effectively removed from HAWA. The procedure to remove arsenic was optimized as follows: initial H⁺ concentration of 5 mol L^-1, Cu-to-As molar ratio of 8, Cl-to-As molar ratio of 10, a reaction temperature of 60 °C, copper powder particle size of 68-24 μm, and a stirring speed of 300 r min^-1. Under these optimal conditions, the removal rate of arsenic was close to 100%. Kinetics results suggested that the arsenic removal process was controlled by both diffusion and chemical reactions with an apparent activation energy of 29.78 kJ mol^-1. The XRD results showed that the removed arsenic in the residue existed primarily in the form of AsCu₃ alloy.

Treatment of olive mill effluent by adsorption on titanium oxide nanoparticles

Olive mills wastewater (OMW) causes a serious environmental problem in the olive oil producing countries. This is due to its high organic matter content (COD), acidic pH values, suspended solids and high content of phytotoxic and antibacterial phenolic compounds. In this study, titanium dioxide (TiO₂) as an adsorbent to reduce the COD value of the olive mill wastewater was investigated. Several variables were studied including the removal efficiency, effect of the initial COD value, amount of TiO₂, temperature and pH value. The results revealed that the adsorption reached equilibrium within <120 min. Isotherm studies showed that the adsorption equilibrium data is in agreement with Freundlich isotherm. In addition, the results showed that the adsorption process was spontaneous and exothermic. The kinetic study indicated that adsorption did follow a pseudo-second order reaction. Variation of the amount of the TiO₂ showed that using of 1.5 and 2 g/L of TiO₂ caused the COD to drop from 1000 ppm to about 100 ppm (equilibrium concentration) in about 120 min. However, the use of 1 g/L of TiO₂ exhibited almost the same effect on the COD-uptake, and the equilibrium concentration was about 400 ppm. The COD uptake was found to be inversely proportional with the temperature, pH value and the addition of salts such as sodium chloride (NaCl) and potassium chloride (KCl).

Nanoscale zero valent iron supported on MgAl-LDH-decorated reduced graphene oxide: Enhanced performance in Cr(VI) removal, mechanism and regeneration

The scaled application of nanoscale zero-valent iron nanoparticles (nZVI or Fe° NPs) in environmental remediation is challenged by easy surface passivation and particle aggregation. To improve this situation and enhance their performance in Cr(VI) removal from water phase, we present one novel strategy to hybridize nZVI with layered double hydroxide (LDH) decorated reduced graphene oxide (rGO). The as-prepared ternary (Fe@LDH/rGO) composites possess better dispersibility, improved hydrophilicity and more positive surfaces that allows higher removal efficiency and capacity for Cr(VI) oxyanions. Composition proportion are optimized and influences of surroundings (solution pH, Cr(VI) concentration and temperature) are evaluated. Also, we demonstrate that Fe@LDH/rGO can be reused with suitable post-treatments, which combines alkaline solution desorption and NaBH₄ revivification possess. Cr desorption and Fe leaching ratio during regeneration should be critical indicators that determine the recovery efficiency. Synergistic effect within this ternary system not only contributes to its superiorities in stability, but also continuous iron corrosion via the formation of micro Fe-C batteries, where rGO acts as cathode and alternative electron conductor. The present work suggests great potentials of Fe@LDH/rGO composites in groundwater remediation.

Synergistic deep removal of As(III) and Cd(II) by a calcined multifunctional MgZnFe-CO3 layered double hydroxide: Photooxidation, precipitation and adsorption

A high removal rate (>99.7%) of combined arsenite (As(III)) and Cd (Cd(II)) in low concentration (1000 μg/L) from contaminated water was achieved by a calcined MgZnFe-CO₃ layered double hydroxide (CMZF) adsorbent. Batch control studies and a series of spectroscopy detection technologies were employed to investigate the removal mechanism and interactions between As(III) and Cd(II) on the interface of water/CMZF. Synergistic adsorption and photooxidation occurred based on the systematical kinetic and isotherm studies. The enhanced removal of As(III) was achieved by the photooxidation, formation of ternary As(III)Cd(II) surface complexes and enhanced hydrogen bond. Meanwhile, oxidative formed negative charged As(V) could reduce the electrostatic repulsion force between Cd(II) cations and play a role as anion bridging, consequently resulted in a stronger attraction between CMZF and Cd(II). Combined with the verdicts of relevant characterizations such as XRD, XPS and EPR, it was assumed that the deep co-removal mechanism could be attributed to the coupling of various processes including intercalation, complexation, photooxidation of As(III) and precipitation of CdCO₃. Moreover, the successful removal of As(III) and Cd(II) from real water matrix qualified the CMZF a potentially attractive adsorbent for both As(III) and Cd(II) deep treatment in practical engineering.

Well-dispersed TiO2 nanoparticles anchored on Fe3O4 magnetic nanosheets for efficient arsenic removal

Magnetic iron-titanium binary oxide as an effective adsorbent for arsenic contaminant is a challenge primarily because of their bulk structure and agglomeration effect. Herein, a novel and uniform sandwich-like magnetic Fe₃O₄@TiO₂ sheets were synthesized by utilizing a facile strategy involving amorphous-to-crystalline transformation and reduction in H₂, to achieve dispersed anatase TiO₂ nanoparticles with a small size of ∼8 nm anchored on Fe₃O₄ sheets. The resultant Fe₃O₄@TiO₂ sheets nanocomposite possessing a high specific surface area of ∼89.4 m² g^-1 and available magnetic susceptibility of ∼20.0 emu g^-1, significantly enhanced the photocatalytic oxidation property of arsenite and considerable adsorption capability for arsenic removal. The adsorption capacities of As(V) and As(III) with UV-assisted from adsorption experimental results were 36.36 and 30.96 mg g^-1, respectively, while the residual concentrations for both As(V) and As(III) were lower than the strict limit of 10 μg L^-1. Adsorption equilibriums were almost reached within 45 min. In addition, the adsorbent exhibited excellent stability over a broad pH range of 3-9 and still maintained great removal efficiency after five time regeneration cycles. Furthermore, except for silicate and phosphate, the extremely weak inhibiting influences of common co-existing ions in arsenic removal process, demonstrated that the developed magnetic Fe₃O₄@TiO₂ sheets with unique nanostructure could be a promising efficient adsorbent for arsenic removal.

Synthesis of Bi2WO6/Na-bentonite composites for photocatalytic oxidation of arsenic(iii) under simulated sunlight

Novel Bi₂WO₆/bentonite (denoted as BWO/BENT) composites were prepared via a typical hydrothermal process and employed for the photocatalytic oxidation of arsenic(iii) (As(iii)). The properties of the prepared samples were characterized through X-ray diffraction, transmission and scanning electron microscopy, UV-visible diffuse reflectance spectroscopy, X-ray photoelectron spectroscopy, and photoluminescence spectroscopy. Effects of the BENT ratio on the As(iii) removal were explored under simulated sunlight, and the best photocatalytic effect was observed for the composite with BWO : BENT = 7 : 3 w/w. Compared with the pure BWO, the BWO/BENT composites exhibited an improved photocatalytic ability in the removal of As(iii), which was mainly ascribed to the enlarged specific surface area and the suppressed electron–hole recombination by the incorporated BENT. Furthermore, photo-generated holes (h⁺) and superoxide radicals ·O₂⁻ were confirmed to be the major contributors to the oxidation of As(iii), and an associated mechanism of photocatalytic oxidation of As(iii) over BWO/BENT composites was proposed.

Novel Bi₂WO₆/bentonite (denoted as BWO/BENT) composites were prepared via a typical hydrothermal process and employed for the photocatalytic oxidation of arsenic(iii) (As(iii)).

Facile synthesis of ZrO2 coated BiOCl0.5I0.5 for photocatalytic oxidation-adsorption of As(III) under visible light irradiation

ZrO₂ modified BiOCl_0.5I_0.5 composites (ZBCI), synthesized via a facile precipitation method at room temperature, were utilized to photocatalytically oxidize and adsorb arsenite from water under visible light irradiation. The composites were well characterized by using various techniques. With visible light irradiation, 5 mg L^-1 of As(III) could be completely removed by ZBCI (0.25 g L^-1) in 90 min. Particularly, we found that ZBCI composites not only could oxidize As(III) into As(V) with visible light irradiation, but also could effectively capture the generated As(V), leading to the negligible residual As(III) or As(V) in aqueous solutions after 90 min treatment. In the fabricated composites, ZrO₂ acted as the main adsorption sites while BiOCl_0.5I_0.5 served as the primary photocatalysis center. Because of the heterostructure of ZBCI, e^- generated by BiOCl_0.5I_0.5 would be transferred to ZrO₂ and inhibited e^--h⁺ recombination rate, contributing to the improved photocatalytic efficiency. ZBCI could effectively remove As(III) over a broad range of pH from 3 to 11. Chloride and nitrate did not obviously affect the photocatalytic As(III) removal, while sulfate and phosphate yet reduced the capture of As(III). Moreover, ZBCI composites exhibited high photocatalytic As(III) removal efficiency during the fourth reused cycles. The facile synthesized ZBCI could be employed to capture and oxidize As(III) from water.

Effective elimination of As(iii)viasimultaneous photocatalytic oxidation and adsorption by a bifunctional cake-like TiO2derived from MIL-125(Ti)

A bifunctional cake-like TiO₂was successfully synthesizedviacalcination of MIL-125(Ti) at a suitable temperature by a one-step method and was applied for As(iii) removal by simultaneous photocatalytic oxidation and adsorption.