Photocatalytic oxidation of arsenite in TiO2 suspension: kinetics and mechanisms

Arsenite [As(III)] and arsenate [As(V)] are highly toxic aquatic contaminants. Since arsenite is more mobile in natural waters and less efficiently removed in adsorption/coagulation processes than arsenate, the oxidation of arsenite to arsenate is desirable in water treatment. We performed the photocatalytic oxidation of arsenite in aqueous TiO2 suspension and investigated the effects of pH, dissolved oxygen, humic acid (HA), and ferric ions on the kinetics and mechanisms of arsenite oxidation. Arsenite oxidation in UV-illuminated TiO2 suspension was highly efficient in the presence of dissolved oxygen. Homogeneous photooxidation of arsenite in the absence of TiO2 was negligibly slow. Since the addition of excess tert-butyl alcohol (OH radical scavenger) did not reduce the rate of arsenite oxidation, the OH radicals should not be responsible for As(III) oxidation. The addition of HA increased both arsenite oxidation and H2O2 production at pH 3 under illumination, which could be ascribed to the enhanced superoxide generation through sensitization. We propose that the superoxide is the main oxidant of arsenite in the TiO2/UV process. The addition of ferric ions also significantly enhanced the arsenite photooxidation. In this case, the addition of tert-butyl alcohol reduced the arsenite oxidation rate, which implied thatthe OH radical-mediated oxidation path was operative in the presence of ferric ions. Since both Fe3+ and HA that were often found with the arsenic in groundwater were beneficial to the photocatalytic oxidation of arsenite, the TiO2/UV process could be a viable pretreatment method. This can be as simple as exposing the arsenic-polluted water in a TiO2-coated trough to sunlight.     

Oxidation mechanism of As(III) in the UV/TiO2 system: evidence for a direct hole oxidation mechanism

Although it is well-known that As(III) is oxidized to As(V) in the UV/TiO2 system, the main oxidant for that reaction is still not clear. Accordingly, the present study aims at reinvestigating the TiO2-photocatalyzed oxidation mechanism of As(III). We performed a series of As(II) oxidation experiments by using UV-C/H2O2 and UV-A/TiO2, focusing on the effects of competing compounds. The experiment with UV-C/H2O2 indicated that HO2*/O2-* is not an effective oxidant of As(III) in the homogeneous phase. The effects of oxalate, formate, and Cu(II) on the photocatalytic oxidation of As(III) contradicted the controversial hypothesis that HO2*/ O2-* is the main oxidant of As(III) in the UV/TiO2 system. The effect of As(III) on the TiO2-photocatalyzed oxidations of benzoate, terephthalate, and formate was also incompatible with the superoxide-based As(II) oxidation mechanism. Instead, the experimental observations implied that OH* and/or the positive hole are largely responsible forthe oxidation of As(III) in the UV/TiO2 system. To determine which species plays a more significant role, the effects of methanol and iodide were tested. Since excess methanol did not retard the oxidation rate of As(III), OH* seems not to be the main oxidant. Therefore, the best rationale regarding the oxidation mechanism of As(III) in the UV/TiO2 system seems to be the direct electron transfer between As(III) and positive holes. Only with this mechanism, it was possible to explain the data of this study. Besides the mechanistic aspect, an application method for this technology was sought. The usage of UV/TiO2 for oxidizing As(II) requires a posttreatment in which both As(V) and TiO2 should be removed from water. For this objective, we applied FeCl3 and AIK(SO4)2 as coagulants, and the result implied that the combined usage of TiO2 and coagulation might be a feasible solution to treat arsenic contamination around the world.     

Photocatalytic oxidation mechanism of As(III) on TiO2: unique role of As(III) as a charge recombinant species

Using TiO(2) photocatalyst, arsenite, As(III), can be rapidly oxidized to arsenate, As(V), which is less toxic and less mobile in the aquatic environment. Therefore, the TiO(2)/UV process can be employed as an efficient pretreatment method for arsenic contaminated water. Since we first reported in 2002 that the superoxide (or hydroperoxyl radical) plays the role of main oxidant of As(III) in the TiO(2)/UV process, there has been much debate over the true identity of the major photooxidant among superoxides, holes, and OH radicals. The key issue is centered on why the much stronger OH radicals cannot oxidize As(III), and it has been proposed that the unique role of As(III) as an external charge recombination center on the UV-excited TiO(2) particle is responsible for this eccentric mechanism. Although the proposed mechanism has been supported by many experimental evidences, doubts on it were not clearly removed. In this study, we provided direct and undisputed evidence to support the role of As(III) in the charge recombination dynamics using time-resolved transient absorption spectroscopy. The presence of As(III) indeed mediated the charge recombination in TiO(2). Under this condition, the role of the OH radical is suppressed because of the null cycle, and the weaker oxidant, superoxide, should prevail. The role of the superoxide has been previously doubted on the basis of the observation that the addition of excess formic acid (hole scavenger that should enhance the production of superoxides) inhibited the photocatalytic oxidation of As(III). However, this study proved that this was due to the photogeneration of reducing radicals (HCO(2)·) that recycle As(V)/As(IV) back to As(III). It was also demonstrated that the 4-chlorophenol/TiO(2) system under visible light that cannot generate neither OH radicals nor valence band holes converted As(III) to As(V) through the superoxide pathway.     

Adsorption and photocatalyzed oxidation of methylated arsenic species in TiO2 suspensions

Monomethylarsonic acid (MMA) and dimethylarsinic acid (DMA) are used as herbicides in the agriculture industry. We have demonstrated that MMA and DMA are readily degraded upon TiO2 photocatalysis. DMA is oxidized to MMA as the primary oxidation product, which is subsequently oxidized to inorganic arsenate, As(V). The adsorption of MMA and DMA on Ti02 surface was measured as a function of initial arsenic concentration and solution pH. The pH of the solution influences the adsorption and photocatalytic degradation to a similar degree, due to the speciation of the arsenic substrates and surface charge of TiO2 as a function of pH. The mineralization of MMA and DMA by TiO2 photocatalysis follows the Langmuir-Hinshelwood kinetic model. Addition of tert-butyl alcohol, a hydroxyl radical scavenger, during TiO2 photocatalysis dramatically reduces the rate of degradation, indicating that *OH is the primary oxidant. For dilute solutions, TiO2 may also be applicable as an absorbent for direct removal of a variety of arsenic species, namely As(III), As(V), MMA, and DMA, all of which are strongly adsorbed, thus eliminating the need for a multistep treatment process.     

Effects of TiO2 surface modifications on photocatalytic oxidation of arsenite: the role of superoxides

Using TiO2 photocatalyst, arsenite [As(III)] can be rapidly oxidized to arsenate [As(V)], which is less toxic and less mobile in the aquatic environment. Superoxides have been recently proposed as a main photocatalytic oxidant of As(III) whereas OH radicals are dominant oxidants in most TiO2 photocatalytic oxidation (PCO) reactions. This study confirms that superoxides are mainly responsible for the As(III) PCO by investigating PCO kinetics in pure and modified TiO2 systems. The rate of As(III) oxidation drastically increased on Pt-TiO2, which could be ascribed to the enhanced superoxide generation through an efficient interfacial electron transfer from the conduction band (CB) to O2. Since the addition of tert-butyl alcohol (OH radical scavenger) had little effect on the PCO rate in both naked and Pt-TiO2 suspensions, OH radicals do not seem to be involved. The addition of polyoxometalates (POMs) as an electron shuttle between TiO2 CB and 02 highly promoted the PCO rate whereas the POM alone was not effective at all in oxidizing As(III). Fluorinated TiO2 that had a markedly reduced adsorptive capacity for As(III) did not show a reduced PCO rate, which indicates that the direct hole transfer path is not important. The arsenite oxidation proceeded under visible light with a similar rate to the case of Pt-TiO2/UV when dye-sensitized Pt-TiO2 was used. Since only superoxides can be generated as a photooxidant in this visible light system, their role as a main oxidant of As(III) is confirmed. In addition, the PCO rate was significantly reduced in the presence of superoxide dismutase.     

TiO2-photocatalyzed As(II) oxidation in aqueous suspensions: reaction kinetics and effects of adsorption

Oxidation of arsenite, As(III), to arsenate, As(V), is required for the efficient removal of arsenic by many water treatment technologies. The photocatalyzed oxidation of As(III) on titanium dioxide, TiO2, offers an environmentally benign method for this unit operation. In this study, we explore the efficacy and mechanism of TiO2-photocatalyzed As(III) oxidation at circumneutral pH and over a range of As(III) concentrations approaching those typically encountered in water treatment systems. We focus on the effect of As adsorption on observed rates of photooxidation. Adsorption (in the dark) of both As(III) and As(V) on Degussa P25 TiO2 was examined at pH 6.3 over a range in dissolved arsenic concentrations, [As]diss, of 0.10-89 microM and 0.2 or 0.05 g L(-1) TiO2 for As(III) and As(V), respectively. Adsorption isotherms generally followed the Langmuir-Hinshelwood model with As(III) exhibiting an adsorption maxima of 32 micromol g(-1). As(V) adsorption did not reach a plateau under the experimental conditions examined; the maximum adsorbed concentration observed was 130 micromol g(-1). The extent of As(III) and As(V) adsorption observed at the beginning and end of the kinetic studies was consistent with that observed in the adsorption isotherms. Kinetic studies were performed in batch systems at pH 6.3 with 0.8-42 microM As(III) and 0.05 g L(-1) TiO2; complete oxidation of As(III) was observed within 10-60 min of irradiation at 365 nm. The observed effect of As(III) concentration on reaction kinetics was consistent with surface saturation at higher concentrations. Addition of phosphate at 0.5-10 microM had little effect on either As(III) sorption or its photooxidation rate but did inhibit adsorption of the product As(V). The selective use of hydroxyl radical quenchers and superoxide dismutase demonstrated that superoxide, O2-, plays a major role in the oxidation of As(III) to As(V).     

Photocatalytic oxidation of arsenite on TiO2: understanding the controversial oxidation mechanism involving superoxides and the effect of alternative electron acceptors

Previously we have reported that superoxide plays the primary role as oxidant of As(III) in the UV/TiO2 system, however, since then there has been a controversy over the true identity of the major As(III) oxidant. This study aims to establish a comprehensive understanding of the oxidative mechanism which satisfactorily explains all of the observed results during the photocatalytic oxidation (PCO) of As(III). The key step that has masked the true oxidative mechanism is related to the fact that the adsorbed As(III) on TiO2 serves as an external charge-recombination center where the reaction of As(III) with an OH radical (or hole) is immediately followed by an electron transfer to make a null cycle. This was confirmed by the observation that the photoanodic current obtained with a TiO2 electrode immediately decreased upon spiking with As(III), portraying the superoxide-mediated PCO as the dominant pathway. The degradation of competitive substrates (benzoic acid and formic acid) was delayed until As(III) was fully converted into As(V) since the normal PCO mechanism that is based on the action of adsorbed OH radicals (or holes) is not working as long as As(III) is present on the TiO2 surface. However, the As(III) PCO mechanism is entirely altered when alternative electron acceptors (Ag+, Cu2+, polyoxometalate) are present. When these alternative electron acceptors are more efficient than 02 they are able to intercept the CB electron, impeding the recombination pathway and enabling an anoxic oxidation mechanism in which OH radicals and holes play the role of main As(III) oxidant. In the presence of polyoxometalate or Cu2+, the above-mentioned photoanodic current immediately increases upon spiking As(III), indicating that the PCO mechanism has changed in the presence of more efficient electron acceptors. Comprehensive mechanisms of As(III) PCO and experimental factors that alter the mechanism are discussed.     

Extended X-ray absorption fine structure analysis of arsenite and arsenate adsorption on maghemite

Arsenic sorption onto maghemite potentially contributes to arsenic retention in magnetite-based arsenic removal processes because maghemite is the most common oxidation product of magnetite and may form a coating on magnetite surfaces. Such a sorption reaction could also favor arsenic immobilization at redox boundaries in groundwaters. The nature of arsenic adsorption complexes on maghemite particles, at near-neutral pH under anoxic conditions, was investigated using X-ray absorption fine structure (XAFS) spectroscopy at the As K-edge. X-ray absorption near edge structure spectra indicate that As(III) does notoxidize after 24 h in any of the sorption experiments, as already observed in previous studies of As(III) sorption on ferric (oxyhydr)oxides under anoxic conditions. The absence of oxygen in our sorption experiments also limited Fenton oxidation of As(III). Extended XAFS (EXAFS) results indicate that both As(III) and As(V) form inner-sphere complexes on the surface of maghemite, under high surface coverage conditions (approximately 0.6 to 1.0 monolayer), with distinctly different sorption complexes for As(III) and As(V). For As(V), the EXAFS-derived As-Fe distance (approximately 3.35 +/- 0.03 A) indicates the predominance of single binuclear bidentate double-corner complexes (2C). For As(III), the distribution of the As-Fe distance suggests a coexistence of various types of surface complexes characterized by As-Fe distances of approximately 2.90 (+/-0.03) A and approximately 3.45 (+/-0.03) A. This distribution can be interpreted as being due to a dominant contribution from bidentate binuclear double-corner complexes (2C), with additional contributions from bidentate mononuclear edge-sharing (2E) complexes and monodentate mononuclear corner-sharing complexes (1V). The present results yield useful constraints on As(V) and As(III) adsorption on high surface-area powdered maghemite, which may help in modeling the behavior of arsenic at the maghemite-water interface.     

Adsorption mechanism of arsenic on nanocrystalline titanium dioxide

Arsenate [As(V)] and arsenite [As(III)] interactions at the solid-water interface of nanocrystalline TiO2 were investigated using electrophoretic mobility (EM) measurements, Fourier transform infrared (FTIR) spectroscopy, extended X-ray absorption fine structure (EXAFS) spectroscopy, and surface complexation modeling. The adsorption of As(V) and As(III) decreased the point of zero charge of TiO2 from 5.8 to 5.2, suggesting the formation of negatively charged inner-sphere surface complexes for both arsenic species. The EXAFS analyses indicate that both As(V) and As(III) form bidentate binuclear surface complexes as evidenced by an average Ti-As(V) bond distance of 3.30 A and Ti-As(III) bond distance of 3.35 A. The FTIR bands caused by vibrations of the adsorbed arsenic species remained at the same energy levels at different pH values. Consequently, the surface complexes on TiO2 maintained the same nonprotonated speciation at pH values from 5 to 10, and the dominant surface species were (TiO)2AsO2- and (TiO)2AsO- for As(V) and As(III), respectively. The surface configurations constrained with the spectroscopic results were formulated in the diffuse layer model to describe the adsorption behavior of As in the pH range between 4 and 12. The study suggests that TiO2 is an effective adsorbent for As removal due to its high surface area and the presence of high affinity surface hydroxyl groups.     

Iron-catalyzed oxidation of arsenic(III) by oxygen and by hydrogen peroxide: pH-dependent formation of oxidants in the Fenton reaction

The oxidation kinetics of As(III) with natural and technical oxidants is still notwell understood, despite its importance in understanding the behavior of arsenic in the environment and in arsenic removal procedures. We have studied the oxidation of 6.6 microM As(II) by dissolved oxygen and hydrogen peroxide in the presence of Fe(II,III) at pH 3.5-7.5, on a time scale of hours. As(III) was not measurably oxidized by O2, 20-100 microM H2O2, dissolved Fe(III), or iron(III) (hydr)-oxides as single oxidants, respectively. In contrast, As(III) was partially or completely oxidized in parallel to the oxidation of 20-90 microM Fe(II) by oxygen and by 20 microM H2O2 in aerated solutions. Addition of 2-propanol as an *OH-radical scavenger quenched the As(III) oxidation at low pH but had little effect at neutral pH. High bicarbonate concentrations (100 mM) lead to increased oxidation of As-(III). On the basis of these results, a reaction scheme is proposed in which H2O2 and Fe(II) form *OH radicals at low pH but a different oxidant, possibly an Fe(IV) species, at higher pH. With bicarbonate present, carbonate radicals might also be produced. The oxidant formed at neutral pH oxidizes As(III) and Fe(II) but does not react competitively with 2-propanol. Kinetic modeling of all data simultaneously explains the results quantitatively and provides estimates for reaction rate constants. The observation that As(III) is oxidized in parallel to the oxidation of Fe(II) by O2 and by H2O2 and that the As(III) oxidation is not inhibited by *OH-radical scavengers at neutral pH is significant for the understanding of arsenic redox reactions in the environment and in arsenic removal processes as well as for the understanding of Fenton reactions in general.     

Photochemical oxidation of As(III) in ferrioxalate solutions

Photochemical reactions involving aqueous Fe(III) complexes are known to generate free radical species such as OH* that are capable of oxidizing numerous inorganic and organic compounds. Recent work has shown that As(III) can be oxidized to As(V) via photochemical reactions in ferric-citrate solutions; however, the mechanisms of As(III) oxidation and the potential importance of photochemical oxidation in natural waters are poorly understood. Consequently, the objectives of this study were to evaluate oxidation rates of As(III) in irradiated ferrioxalate solutions as a function of pH, identify mechanisms of photochemical As(III) oxidation, and evaluate the oxidation of As(III) in a representative natural water containing dissolved organic C (DOC). The oxidation of As(III) was studied in irradiated ferrioxalate solutions as a function of pH (3-7), As(III), Fe(III), and 2-propanol concentration. Rates of As(III) oxidation (0.5-254 microM h(-1)) were first-order in As(III) and Fe(III) concentration and increased with decreasing pH. Experiments conducted at pH 5.0 using 2-propanol as an OH* scavenger in light and dark reactions suggested that OH* is the important free radical responsible for As(III) oxidation. Significant rates of As(III) oxidation (4-6 microM h(-1)) were also observed in a natural water sample containing DOC, indicating that photochemical oxidation of As(III) may contribute to arsenic (As) cycling in natural waters.     

TiO₂-photocatalytic reduction of pentavalent and trivalent arsenic: production of elemental arsenic and arsine

Heterogeneous photocatalytic reduction of As(V) and As(III) at different concentrations over TiO(2) under UV light in deoxygenated aqueous suspensions is described. For the first time, As(0) was unambiguously identified together with arsine (AsH(3)) as reaction products. As(V) reduction requires the presence of an electron donor (methanol in the present case) and takes place through the hydroxymethyl radical formed from methanol oxidation by holes or hydroxyl radicals. On the contrary, As(III) reduction takes place through direct reduction by the TiO(2)-conduction band electrons. Detailed mechanisms for the photocatalytic processes are proposed. Although reduction to solid As(0) is convenient for purposes of As removal from water as a deposit on TiO(2), attention must be paid to formation of AsH(3), one of the most toxic forms of As, and strategies for AsH(3) treatment should be considered.     

TiO2-photocatalyzed As(III) oxidation in a fixed-bed, flow-through reactor

Compliance with the U.S. drinking water standard for arsenic (As) of 10 microg L(-1) is required in January 2006. This will necessitate implementation of treatment technologies for As removal by thousands of water suppliers. Although a variety of such technologies is available, most require preoxidation of As(III) to As(V) for efficient performance. Previous batch studies with illuminated TiO2 slurries have demonstrated that TiO2-photocatalyzed AS(III) oxidation occurs rapidly. This study examined reaction efficiency in a flow-through, fixed-bed reactor that provides a better model for treatment in practice. Glass beads were coated with mixed P25/sol gel TiO2 and employed in an upflow reactor irradiated from above. The reactor residence time, influent As(III) concentration, number of TiO2 coatings on the beads, solution matrix, and light source were varied to characterize this reaction and determine its feasibility for water treatment. Repeated usage of the same beads in multiple experiments or extended use was found to affect effluent As(V) concentrations but not the steady-state effluent As(III) concentration, which suggests that As(III) oxidation at the TiO2 surface undergoes dynamic sorption equilibration. Catalyst poisoning was not observed either from As(V) or from competitively adsorbing anions, although the higher steady-state effluent As(III) concentrations in synthetic groundwater compared to 5 mM NaNO3 indicated that competitive sorbates in the matrix partially hinder the reaction. A reactive transport model with rate constants proportional to incident light at each bead layer fit the experimental data well despite simplifying assumptions. TiO2-photocatalyzed oxidation of As(III) was also effective under natural sunlight. Limitations to the efficiency of As(III) oxidation in the fixed-bed reactor were attributable to constraints of the reactor geometry, which could be overcome by improved design. The fixed-bed TiO2 reactor offers an environmentally benign method for As(III) oxidation.     

Coadsorption of arsenic(III) and arsenic(V) onto hydrous ferric oxide: effects on abiotic oxidation of arsenic(III), extraction efficiency, and model accuracy

Single solute adsorption and coadsorption of As(III) and As(V) onto hydrous ferric oxide (HFO), oxidation of As(III), and extraction efficiencies were measured in 0.2 atm O2. Oxidation was negligible for single-adsorbate experiments, but significant oxidation was observed in the presence of As(V) and HFO. Single-adsorbate As(III) or As(V) were incompletely extracted (0.5 M NaOH for 20 min), but all As was recovered in coadsorbate experiments. Single-adsorbate data were well-simulated using published surface complexation models, but those models (calibrated for single-adsorbate results) provided poor fits for coadsorbate experiments. An amended model accurately simulated single- and coadsorbate results. Model predictions of significant change in As(III) surface complex speciation in coadsorbate experiments was confirmed using zeta potential measurements. Our results demonstrate that mobility of arsenic in groundwater and removal in engineered treatment systems are more complicated when both As(III) and As(V) are present than anticipated based on single-adsorbate experimental results.     

Zinc adsorption effects on arsenite oxidation kinetics at the birnessite-water interface

Arsenite is more toxic and mobile than As(V) in soil and sediment environments, and thus it is advantageous to explore factors that enhance oxidation of As(III) to As(V). Previous studies showed that manganese oxides, such as birnessite (delta-MnO2), directly oxidized As(III). However, these studies did not explore the role that cation adsorption has on As(III) oxidation. Accordingly, the effects of adsorbed and nonadsorbed Zn on arsenite (As(III)) oxidation kinetics at the birnessite-water interface were investigated using batch adsorption experiments (0.1 g L(-1); pH 4.5 and 6.0; I= 0.01 M NaCl). Divalent Zn adsorption on synthetic delta-MnO2 in the absence of As(II) increased with increasing pH and caused positive shifts in electrophoretic mobility values at pH 4-6, indirectly suggesting inner-sphere Zn adsorption mechanisms. Arsenite was readily oxidized on birnessite in the absence of Zn. The initial As(III) oxidation rate constant decreased with increasing pH from 4.5 to 6.0 and initial As(III) concentrations from 100 to 300 microM. Similar pH and initial As(III) concentration effects were observed in systems when Zn was present (i.e., presorbed Zn prior to As(III) addition and simultaneously added Zn-As(III) systems), but As(III) oxidation reactions were suppressed compared to the respective control systems. The suppression was more pronounced when Zn was presorbed on the delta-MnO2 surfaces as opposed to added simultaneously with As(III). This study provides further understanding of As(III) oxidation reactions on manganese oxide surfaces under environmentally applicable conditions where metals compete for reactive sites.     

Arsenic(III) oxidation and arsenic(V) adsorption reactions on synthetic birnessite

The oxidation of arsenite (As(III)) by manganese oxide is an important reaction in both the natural cycling of As and the development of remediation technology for lowering the concentration of dissolved As(III) in drinking water. This study used both a conventional stirred reaction apparatus and extended X-ray absorption fine structure (EXAFS) spectroscopy to investigate the reactions of As(III) and As(V) with synthetic birnessite (MnO2). Stirred reactor experiments indicate that As(III) is oxidized by MnO2 followed by the adsorption of the As(V) reaction product on the MnO2 solid phase. The As(V)-Mn interatomic distance determined by EXAFS analysis for both As(III)- and As(V)-treated MnO2 was 3.22 A, giving evidence for the formation of As(V) adsorption complexes on MnO2 crystallite surfaces. The most likely As(V)-MnO2 complex is a bidentate binuclear corner sharing (bridged) complex occurring at MnO2 crystallite edges and interlayer domains. In the As(III)-treated MnO2 systems, reductive dissolution of the MnO2 solid during the oxidation of As(III) caused an increase in the adsorption of As(V) when compared with As(V)-treated MnO2. This suggested that As(III) oxidation caused a surface alteration, creating fresh reaction sites for As(V) on MnO2 surfaces.     

Arsenic sequestration by ferric iron plaque on cattail roots

Typha latifolia (cattail) sequesters arsenic within predominantlyferric iron root coatings, thus decreasing mobility of this toxic element in wetland sediments. Element-specific XRF microtomographic imaging illustrated a high spatial correlation between iron and arsenic in root plaques, with little arsenic in the interior of the roots. XANES analyses demonstrated that the plaque was predominantly ferric iron and contained approximately 20% As(III) and 80% As(V), which is significant because the two oxidation states form species that differ in toxicity and mobility. For the first time, spatial distribution maps of As oxidation states were developed, indicating that As(III) and As(V) are both fairly heterogeneous throughoutthe plaque. Chemical extractions showed that As was strongly adsorbed in the plaque rather than coprecipitated. Iron and arsenic concentrations ranged from 0.03 to 0.8 g Fe g(-1) wet plaque and 30 to 1200 microg As g(-1) wet plaque, consistent with a mechanism of As adsorption onto Fe(III) oxyhydroxide plaque. Because this mechanism decreases the concentrations of both As(III) and As(V) in groundwater, we propose that disruption of vegetation could increase the concentrations of mobile arsenic.     

Photocatalytic oxidation of arsenite over TiO2: is superoxide the main oxidant in normal air-saturated aqueous solutions?

TiO(2) photocatalytic oxidation (PCO) of As(III) in the normal air-saturated aqueous solutions has been widely studied. Yet no consensus has been achieved on the mechanism whether superoxide is the main oxidant, although many approaches have been taken. (Photo)electrochemical method can minimize changes to TiO(2) surface and could therefore not alter the normal mechanism. In this Article, both this approach and As(III) oxidation kinetic measurements were performed to clarify the disputed mechanism. Under a sufficient cathodic bias potential, the dark oxidation of As(III) by superoxide could occur, but both the reaction rate and the columbic efficiency were rather low, suggesting that it is a weak oxidant. However, under UV light, both the reaction rate and the columbic efficiency were greatly enhanced even at potentials negative enough to eliminate photohole participation, indicating that more efficient oxidants than superoxide were produced. Under UV illumination and enough positive potential where superoxide was absent, the As(III) oxidation was the most highly efficient. The columbic efficiency of photoholes was much higher than that of superoxide. In the normal aerated aqueous solutions and at open circuit, although the total contribution of superoxide and its derivates to the PCO of As(III) was considerably high (up to 43%), it was not more than that of photohole (57%). In addition, the reported various approaches taken to elucidate the mechanism were discussed, and the resulting disputes can be clarified by these findings. It was demonstrated that (photo)electrochemical method could provide direct and undisputed evidence to reveal the truth mechanics issues.     

Iron(II)-catalyzed oxidation of arsenic(III) in a sediment column

Arsenic contamination in aquatic systems is a worldwide concern. Understanding the redox cycling of arsenic in sediments is critical in evaluating the fate of arsenic in aquatic environments and in developing sediment quality guidelines. The direct oxidation of inorganic trivalent arsenic, As(III), by dissolved molecular oxygen has been studied and found to be quite slow. A chemical pathway for As(III) oxidation has been proposed recently in which a radical species, Fe(IV), produced during the oxidation of divalent iron, Fe(II), facilitates the oxidation of As(III). Rapid oxidation of As(III) was observed (on a time scale of hours) in batch systems at pH 7 and 7.5, but the extent of As(III) oxidation was limited. The Fe(II)-catalyzed oxidation of As(III) is examined in a sediment column using both computational and experimental studies. A reactive-transport model is constructed that incorporates the complex kinetics of radical species generation and Fe(II) and As(III) oxidation that have been developed previously. The model is applied to experimental column data. Results indicate that the proposed chemical pathway can explain As(III) oxidation in sediments and that transport in sediments plays a vital role in increasing the extent of As(III) oxidation and efficiency of the Fe(II) catalysis.     

Arsenic(III) oxidation by iron(VI) (ferrate) and subsequent removal of arsenic(V) by iron(III) coagulation

We investigated the stoichiometry, kinetics, and mechanism of arsenite [As(III)] oxidation by ferrate [Fe(VI)] and performed arsenic removal tests using Fe(VI) as both an oxidant and a coagulant. As(III) was oxidized to As(V) (arsenate) by Fe(VI), with a stoichiometry of 3:2 [As(III):Fe(VI)]. Kinetic studies showed that the reaction of As(III) with Fe(VI) was first-order with respect to both reactants, and its observed second-order rate constant at 25 degrees C decreased nonlinearly from (3.54 +/- 0.24) x 10(5) to (1.23 +/- 0.01) x 10(3) M(-1) s(-1) with an increase of pH from 8.4 to 12.9. A reaction mechanism by oxygen transfer has been proposed for the oxidation of As(III) by Fe(VI). Arsenic removal tests with river water showed that, with minimum 2.0 mg L(-1) Fe(VI), the arsenic concentration can be lowered from an initial 517 to below 50 microg L(-1), which is the regulation level for As in Bangladesh. From this result, Fe(VI) was demonstrated to be very effective in the removal of arsenic species from water at a relatively low dose level (2.0 mg L(-1)). In addition, the combined use of a small amount of Fe(VI) (below 0.5 mg L(-1)) and Fe(III) as a major coagulant was found to be a practical and effective method for arsenic removal.