Reductive transformation of birnessite by aqueous Mn(II)

Reaction of aqueous Mn(II) with hexagonal birnessite at pH 7.5 causes reductive transformation of birnessite into feitknechtite (β-Mn(III)OOH) and manganite (γ-Mn(III)OOH) through interfacial electron transfer from adsorbed Mn(II) to structural Mn(IV) atoms and arrangement of product Mn(III) into MnOOH, summarized by Mn(II) + Mn(IV)O(2) + 2 H(2)O → 2 Mn(III)OOH + 2 H(+). Feitknechtite is the initial transformation product, and subsequently converted into the more stable manganite polymorph during ongoing reaction with Mn(II). Feitknechtite production is observed at Mn(II) concentrations 2 orders of magnitude below thermodynamic thresholds, reflecting uncertainty in thermodynamic data of Mn-oxide minerals and/or specific interactions between Mn(II) and birnessite surface sites facilitating electron exchange. Under oxic conditions, feitknechtite formation through surface-catalyzed oxidation of Mn(II) by O(2) leads to additional Mn(II) removal from solution relative to anoxic systems. These results indicate that Mn(II) may be an important moderator of the reductive arm of Mn-oxide redox cycling, and suggest a controlling role of Mn(II) in regulating the solubility and speciation of phyllomanganate-reactive metal pollutants including Co, Ni, As, and Cr in geochemical environments.     

Simultaneous removal of SO2 and trace As2O3 from flue gas: mechanism, kinetics study, and effect of main gases on arsenic capture

Sulfur dioxide (SO2) and trace elements are pollutants derived from coal combustion. This study focuses on the simultaneous removal of S02 and trace arsenic oxide (As2O3) from flue gas by calcium oxide (CaO) adsorption in the moderate temperature range. Experiments have been performed on a thermogravimetric analyzer (TGA). The interaction mechanism between As2O3 and CaO is studied via XRD detection. Calcium arsenate [Ca3(AsO4)2] is found to be the reaction product in the range of 600-1000 degrees C. The ability of CaO to absorb As2O3 increases with the increasing temperature over the range of 400-1000 degrees C. Through kinetics analysis, it has been found that the rate constant of arsenate reaction is much higher than that of sulfate reaction. SO2 presence does not affect the trace arsenic capture either in the initial reaction stage when CaO conversion is relatively low or in the later stage when CaO conversion is very high. The product of sulfate reaction, CaS04, is proven to be able to absorb As2O3. The coexisting CO2 does not weaken the trace arsenic capture either.     

In situ spectroscopic and solution analyses of the reductive dissolution of MnO2 by Fe(II)

The reductive dissolution of MnO2 by Fe(II) under conditions simulating acid mine drainage (pH 3, 100 mM SO4(2-)) was investigated by utilizing a flow-through reaction cell and synchrotron X-ray absorption spectroscopy. This configuration allows collection of in situ, real-time X-ray absorption near-edge structure (XANES) spectra and bulk solution samples. Analysis of the solution chemistry suggests that the reaction mechanism changed (decreased reaction rate) as MnO2 was reduced and Fe(III) precipitated, primarily as ferrihydrite. Simultaneously, we observed an additional phase, with the local structure of jacobsite (MnFe2O4), in the Mn XANES spectra of reactants and products. The X-ray absorbance of this intermediate phase increased during the experiment, implying an increase in concentration. The presence of this phase, which probably formed as a surface coating, helps to explain the reduced rate of dissolution of manganese(IV) oxide. In natural environments affected by acid mine drainage, the formation of complex intermediate solid phases on mineral surfaces undergoing reductive dissolution may likewise influence the rate of release of metals to solution.     

RBS characterization of arsenic(III) partitioning from aqueous phase into the active layers of thin-film composite NF/RO membranes

The main objective of this study was to apply Rutherford backscattering spectrometry (RBS) for characterizing the partitioning of arsenic(III) from aqueous phase into the active layer of NF/RO membranes. NF/RO membranes with active layer materials including polyamide (PA), PA-polyvinyl alcohol derivative (PVA), and sulfonated-polyethersulfone (SPES) were investigated. The partition coefficient was found to be constant in the investigated As-(III) concentration range of 0.005-0.02 M at each pH investigated. The partitioning of As(III) when predominantly present as H3AsO3 (pH 3.5-8.0) was not affected by pH. In contrast, the partition coefficient of As(III) at pH 10.5, when it was predominantly present as H2AsO3-, was found to be approximately 33-49% lower than that of H3AsO3. The partition coefficients of H3AsO3 and H2AsO3- for membranes containing PA in their active layers were within the respective ranges of 6.2-8.1 and 3.6-5.4, while the corresponding values (4.8 and 3.0, respectively) for the membrane with SPES active layer were approximately 30% lower than the average values for the PA membranes.     

Chemical speciation of arsenic-accumulating mineral in a sedimentary iron deposit by synchrotron radiation multiple X-ray analytical techniques

The comprehensive characterization of As(V)-bearing iron minerals from the Gunma iron deposit, which were probably formed by biomineralization, was carried out by utilizing multiple synchrotron radiation (SR)-based analytical techniques at BL37XU at SPring-8. SR microbeam X-ray fluorescence (SR-mu-XRF) imaging showed a high level of arsenic accumulation in the iron ore as dots of ca. 20 microm. Based on SEM observations and SR X-ray powder diffraction (SR-XRD) analysis, it was found that arsenic is selectively accumulated in strengite (FePO4 x 2H2O) with a concentric morphology, which may be produced by a biologically induced process. Furthermore, the X-ray absorption fine structure (XAFS) analysis showed that arsenic in strengite exists in the arsenate (AsO4(3-)) form and is coordinated by four oxygen atoms at 1.68 angstroms. The results suggest that strengite accumulates arsenic by isomorphous substitution of AsO4(3-) for PO4(3-) to form a partial solid-solution of strengite and scorodite (FeAsO4 x 2H2O). The specific correlation between the distribution of As and biominerals indicates that microorganisms seems to play an important role in the mineralization of strengite in combination with an arsenic-accumulating process.     

Sorption of arsenate and arsenite on RuO2 x xH2O: a spectroscopic and macroscopic study

The sorption reactions of arsenate (As(V)) and arsenite (As(III)) on RuO2 x xH2O were examined using macroscopic and spectroscopic techniques. Constant solid:solution isotherms were constructed from batch sorption experiments and sorption kinetics assessed at pH 7. X-ray absorption near edge spectroscopy (XANES) was employed to elucidate the solid-state speciation of sorbed As. At all pH values studied (pH 4-8), RuO2 x xH2O showed a high affinity for As regardless of the initial As species present. Sorption was higher at all pH values when the initial As species was As(III). Oxidation of As(III) (250 mg/L solution) to As(V) was virtually complete (98-100%) within 5 s. XANES results showed the presence of only As(V) on the RuO2 x xH2O regardless of the initial As oxidation state. There was no change in the As oxidation state on the solid phase for 4 weeks in both oxic and anoxic environments. It is speculated that changes in the RuO2 x xH2O structure, due to oxidation reactions, caused the higher total As sorption capacity when As(III) was the initial species. The As sorption capacity of RuO2 x xH2O is greater than that of other metal oxides reviewed in this study. The ability of RuO2 x xH2O to rapidly oxidize As(III) is much greater than other oxides, such as MnO2.     

Interaction of inorganic arsenic with biogenic manganese oxide produced by a Mn-oxidizing fungus, strain KR21-2

In batch culture experiments we examined oxidation of As(III) and adsorption of As(III/V) by biogenic manganese oxide formed by a manganese oxide-depositing fungus, strain KR21-2. We expected to gain insight into the applicability of Mn-depositing microorganisms for biological treatment of As-contaminated waters. In cultures containing Mn2+ and As(V), the solid Mn phase was rich in bound Mn2+ (molar ratio, approximately 30%) and showed a transiently high accumulation of As(V) during the early stage of manganese oxide formation. As manganese oxide formation progressed, a large proportion of adsorbed As(V) was subsequently released. The high proportion of bound Mn2+ may suppress a charge repulsion between As(V) and the manganese oxide surface, which has structural negative charges, promoting complex formation. In cultures containing Mn2+ and As(III), As(III) started to be oxidized to As(V) after manganese oxide formation was mostly completed. In suspensions of the biogenic manganese oxides with dissolved Mn2+, As(III) oxidation rates decreased with increasing dissolved Mn2+. These results indicate that biogenic manganese oxide with a high proportion of bound Mn2+ oxidizes As(III) less effectively than with a low proportion of bound Mn2+. Coexisting Zn2+, Ni2+, and Co2+ also showed similar effects to different extents. The present study demonstrates characteristic features of oxidation and adsorption of As by biogenic manganese oxides and suggests possibilities of developing a microbial treatment system for water contaminated with As that is suited to the actual situation of contamination.     

Spectroscopic investigation of Cr(III)- and Cr(VI)-treated nanoscale zerovalent iron

The reaction of hexavalent chromium (Cr(VI)) with zerovalent iron (Fe0) during soil and groundwater remediation is an important environmental process. This study used several techniques including X-ray photoelectron spectroscopy (XPS) and X-ray absorption spectroscopy to investigate nanometer scale Fe0 particles (nano Fe0) treated with Cr(III) and Cr(VI). X-ray diffraction and XPS analyses of oxidized nano Fe0 showed the crystalline Fe(III) phase is composed of lepidocrocite (gamma-FeOOH). Results of XPS Cr 2p data and Cr K-edge X-ray absorption near edge spectroscopy (XANES) provided evidence that Cr(VI) was entirely reduced to Cr(III) by nano Fe0 with no residual Cr(VI) after reaction. In addition, XPS and XANES results of Cr(III) precipitated as Cr(OH)3 in the presence of corroding nano Fe0 were nearly identical to the Cr(VI)-nano Fe0 reaction product. Detailed analysis of XPS O 1s line spectra revealed that both Cr(III)- and Cr(VI)-treated nano Fe0 yielded a predominantly hydroxylated Cr(OH)3 and/ or a mixed phase CrxFe(1 - x)(OH)3 product. The structure of the Cr(III)- and Cr(VI)-treated nano Fe0 determined using extended X-ray absorption fine structure spectroscopy (EXAFS) revealed octahedral Cr(III) with Cr-O interatomic distances between 1.97 and 1.98 A for both Cr(III) and Cr(VI) treatments and a pronounced Cr-Cr second interatomic shell at 3.01 A. Our results suggest that the reaction product of Cr(VI)-treated nano Fe0 is either a poorly ordered Cr(OH)3 precipitate or possibly a mixed phase CrxFe(1 - x)(OH)3 product, both of which are highly insoluble under environmental conditions.     

Rapid reaction of nanomolar Mn(II) with superoxide radical in seawater and simulated freshwater

Superoxide radical (O2-) has been proposed to be an important participant in oxidation-reduction reactions of metal ions in natural waters. Here, we studied the reaction of nanomolar Mn(II) with O2- in seawater and simulated freshwater, using chemiluminescence detection of O2- to quantify the effect of Mn(II) on the decay kinetics of O2-. With 3-24 nM added [Mn(II)] and <0.7 nM [O2-], we observed effective second-order rate constants for the reaction of Mn(II) with O2- of 6×10(6) to 1×10(7) M(-1)·s(-1) in various seawater samples. In simulated freshwater (pH 8.6), the effective rate constant of Mn(II) reaction with O2- was somewhat lower, 1.6×10(6) M(-1)·s(-1). With higher initial [O2-], in excess of added [Mn(II)], catalytic decay of O2- by Mn was observed, implying that a Mn(II/III) redox cycle occurred. Our results show that reactions with nanomolar Mn(II) could be an important sink of O2- in natural waters. In addition, reaction of Mn(II) with superoxide could maintain a significant fraction of dissolved Mn in the +III oxidation state.     

Arsenic(III) and arsenic(V) reactions with zerovalent iron corrosion products

Zerovalent iron (Fe0) has tremendous potential as a remediation material for removal of arsenic from groundwater and drinking water. This study investigates the speciation of arsenate (As(V)) and arsenite (As(III)) after reaction with two Fe0 materials, their iron oxide corrosion products, and several model iron oxides. A variety of analytical techniques were used to study the reaction products including HPLC-hydride generation atomic absorption spectrometry, X-ray diffraction, scanning electron microscopy-energy-dispersive X-ray analysis, and X-ray absorption spectroscopy. The products of corrosion of Fe0 include lepidocrocite (gamma-FeOOH), magnetite (Fe3O4), and/or maghemite (gamma-Fe2O3), all of which indicate Fe(II) oxidation as an intermediate step in the Fe0 corrosion process. The in-situ Fe0 corrosion reaction caused a high As(III) and As(V) uptake with both Fe0 materials studied. Under aerobic conditions, the Fe0 corrosion reaction did not cause As(V) reduction to As(III) but did cause As(III) oxidation to As(V). Oxidation of As(III) was also caused by maghemite and hematite minerals indicating that the formation of certain iron oxides during Fe0 corrosion favors the As(V) species. Water reduction and the release of OH- to solution on the surface of corroding Fe0 may also promote As(III) oxidation. Analysis of As(III) and As(V) adsorption complexes in the Fe0 corrosion products and synthetic iron oxides by extended X-ray absorption fine structure spectroscopy (EXAFS) gave predominant As-Fe interatomic distances of 3.30-3.36 A. This was attributed to inner-sphere, bidentate As(III) and As(V) complexes. The results of this study suggest that Fe0 can be used as a versatile and economical sorbent for in-situ treatment of groundwater containing As(III) and As(V).     

Catalyzed oxidation of arsenic(III) by hydrogen peroxide on the surface of ferrihydrite: an in situ ATR FTIR study

Knowledge of arsenic redox kinetics is crucial for understanding the impact and fate of As in the environment and for optimizing As removal from drinking water. Rapid oxidation of As(III) adsorbed to ferrihydrite (FH) in the presence of hydrogen peroxide (H2O2) might be expected for two reasons. First, the adsorbed As(III) is assumed to be oxidized more readily than the undissociated species in solution. Second, catalyzed decomposition of H2O2 on the FH surface might also lead to As(III) oxidation. Attenuated total reflection-Fourier transform infrared (ATR-FTIR) spectroscopy was used to monitor the oxidation of adsorbed As(III) on the FH surface in situ. No As(III) oxidation within minutes to hours was observed prior to H2O2 addition. Initial pseudo-first-order oxidation rate coefficients for adsorbed As(III), determined at H2O2 concentrations between 8.4 microM and 8.4 mM and pH values from 4 to 8, increased with the H2O2 concentration according to the equation log k(ox) (min(-1)) = 0.17 + 0.50 log [H2O] (mol/L), n = 21, r2 = 0.87. Only a weak pH dependence of log k(ox) was observed (approximately 0.04 logarithm unit increase per pH unit). ATR-FTIR experiments with As(III) adsorbed onto amorphous aluminum hydroxide showed that Fe was necessary to induce As(III) oxidation by catalytic H2O2 decomposition. Supplementary As(III) oxidation experiments in FH suspensions qualitatively confirmed the findings from the in situ ATR-FTIR experiments. Our results indicate that the catalyzed oxidation of As(III) by H2O2 on the surface of iron (hydr)oxides might be a relevant reaction pathway in environmental systems such as surface waters, as well as in engineered systems for As removal from water.     

Advanced oxidation process based on the Cr(III)/Cr(VI) redox cycle

Oxidative degradation of aqueous organic pollutants, using 4-chlorophenol (4-CP) as a main model substrate, was achieved with the concurrent H(2)O(2)-mediated transformation of Cr(III) to Cr(VI). The Fenton-like oxidation of 4-CP is initiated by the reaction between the aquo-complex of Cr(III) and H(2)O(2), which generates HO(?) along with the stepwise oxidation of Cr(III) to Cr(VI). The Cr(III)/H(2)O(2) system is inactive in acidic condition, but exhibits maximum oxidative capacity at neutral and near-alkaline pH. Since we previously reported that Cr(VI) can also activate H(2)O(2) to efficiently generate HO(?), the dual role of H(2)O(2) as an oxidant of Cr(III) and a reductant of Cr(VI) can be utilized to establish a redox cycle of Cr(III)-Cr(VI)-Cr(III). As a result, HO(?) can be generated using both Cr(III)/H(2)O(2) and Cr(VI)/H(2)O(2) reactions, either concurrently or sequentially. The formation of HO(?) was confirmed by monitoring the production of p-hydroxybenzoic acid from [benzoic acid + HO(?)] as a probe reaction and by quenching the degradation of 4-CP in the presence of methanol as a HO(?) scavenger. The oxidation rate of 4-CP in the Cr(III)/H(2)O(2) solution was highly influenced by pH, which is ascribed to the hydrolysis of Cr(III)(H(2)O)(n) into Cr(III)(H(2)O)(n-m)(OH)(m) and the subsequent condensation to oligomers. The present study proposes that the Cr(III)/H(2)O(2) combined with Cr(VI)/H(2)O(2) process is a viable advanced oxidation process that operates over a wide pH range using the reusable redox cycle of Cr(III) and Cr(VI).     

Arsenite oxidation by a poorly crystalline manganese-oxide. 2. Results from X-ray absorption spectroscopy and X-ray diffraction

Arsenite (As(III)) oxidation by manganese oxides (Mn-oxides) serves to detoxify and, under many conditions, immobilize arsenic (As) by forming arsenate (As(V)). As(III) oxidation by Mn(IV)-oxides can be quite complex, involving many simultaneous forward reactions and subsequent back reactions. During As(III) oxidation by Mn-oxides, a reduction in oxidation rate is often observed, which is attributed to Mn-oxide surface passivation. X-ray absorption spectroscopy (XAS) and X-ray diffraction (XRD) data show that Mn(II) sorption on a poorly crystalline hexagonal birnessite (δ-MnO?) is important in passivation early during reaction with As(III). Also, it appears that Mn(III) in the δ-MnO? structure is formed by conproportionation of sorbed Mn(II) and Mn(IV) in the mineral structure. The content of Mn(III) within the δ-MnO? structure appears to increase as the reaction proceeds. Binding of As(V) to δ-MnO? also changes as Mn(III) becomes more prominent in the δ-MnO ? structure. The data presented indicate that As(III) oxidation and As(V) sorption by poorly crystalline δ-MnO? is greatly affected by Mn oxidation state in the δ-MnO? structure.     

Arsenic adsorption by Fe(III)-loaded open-celled cellulose sponge. Thermodynamic and selectivity aspects

Nowadays there is a great concern on the study of new adsorbent materials for either the removal or fixation of arsenic species because of their high toxicity and the health problems associated to such substances. The present paper reports a basic study of the adsorption of arsenic inorganic species from aqueous solutions using an open-celled cellulose sponge with anion-exchange and chelating properties (Forager Sponge). Consequences of preloading the adsorbentwith Fe(III) to enhance the adsorption selectivity are discussed and compared with the nonloaded adsorbent properties. The interactions of arsenic species with the Fe(III)-loaded adsorbent are accurately determined to clarify the feasibility of an effective remediation of contaminated waters. Arsenate is effectively adsorbed by the nonloaded and the Fe(III)-loaded sponge in the pH range 2-9 (maximum at pH 7), whereas arsenite is only slightly adsorbed by the Fe(III)-loaded sponge in the pH range 5-10 (maximum at pH 9), being that the nonloaded sponge is unable to adsorb As(III). The maximum sorption capacities are 1.83 mmol As(V)/g (pH approximately 4.5) and 0.24 mmol As(lII)/g (pH approximately 9.0) for the Fe(III)-loaded adsorbent. This difference is explained in terms of the different acidic behavior of both arsenic species. The interaction of the arsenic species with the Fe(III) loaded in the sponge is satisfactorily modeled. A 1:1 Fe:As complex is found to be formed for both species. H2AsO4- and H3AsO3 are determined to be adsorbed on Fe(III) with a thermodynamic affinity defined by log K = 2.5 +/- 0.3 and log K = 0.53 +/- 0.07, respectively. As(V) is, thus, found to be more strongly adsorbed than As(III) on the Fe(III) loaded in the sponge. A significant enhancement on As(V) adsorption selectivity by loading Fe(III) in the sponge is observed, and the effectiveness of the Fe(III)-loaded sponge for the As(V) adsorption is demonstrated, even in the presence of high concentrations of interfering anions (chloride, nitrate, sulfate, and phosphate).     

Solid solutions between CrO4- and SO4-ettringite Ca6(Al(OH)6)2[(CrO4)x(SO4)(1-x)]3*26 H2O

Chromate is a toxic contaminant of potential concern, as it is quite soluble in the alkaline pH range and could be released to the environment. In cementitous systems, CrO4(2?) is thought to be incorporated as a solid solution with SO4(2?) in ettringite. The formation of a solid solution (SS) could lower the soluble CrO4(2?) concentrations. Ettringite containing SO4(2?) or CrO4(2?) and mixtures thereof have been synthesized. The resulting solids and their solubility after an equilibration time of 3 months have been characterized. For CrO4-ettringite at 25 °C, a solubility product log K(S0) of ?40.2 ± 0.4 was calculated: log K(CrO4?ettringite) = 6log{Ca2+} + 2log{Al(OH)4(?)} + 3log{CrO4(2?)} + 4log{OH?} + 26log{H2O}. X-ray diffraction and the analysis of the solution indicated the formation of a regular solid solution between SO4- and CrO4-ettringite with a miscibility gap between 0.4 ≤ XCrO4 ≤ 0.6. The miscibility gap of the SO4- and CrO4-ettringite solid solution could be reproduced with a dimensionless Guggenheim fitting parameter (a0) of 2.03. The presence of a solid solution between SO4- and CrO4-ettringite results in a stabilization of the solids compared to the pure ettringites and thus in an increased uptake of CrO4(2?) in cementitious systems.     

Photodegradation of RDX in aqueous solution: a mechanistic probe for biodegradation with Rhodococcus sp

Recently we demonstrated that Rhodococcus sp. strain DN22 degraded hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX) (1) aerobically via initial denitration followed by ring cleavage. Using UL 14C-[RDX] and ring labeled 15N-[RDX] approximately 30% of the energetic chemical mineralized (one C atom) and 64% converted to a dead end product that was tentatively identified as 4-nitro-2,4-diaza-butanal (OHCHNCH2NHNO2). To have further insight into the role of initial denitration on RDX decomposition, we photolyzed the energetic chemical at 350 nm and pH 5.5 and monitored the reaction using a combination of analytical techniques. GC/ MS-PCI showed a product with a [M+H] at 176 Da matching a molecular formula of C3H5N5O4 that was tentatively identified as the initially denitrated RDX product pentahydro-3,5-dinitro-1,3,5-triazacyclohex-1-ene (II). LC/MS (ES-) showed that the removal of RDX was accompanied by the formation of two other key products, each showing the same [M-H] at 192 Da matching a molecular formula of C3H7N5O5. The two products were tentatively identified as the carbinol (III) of the enamine (II) and its ring cleavage product O2NNHCH2NNO2CH2NHCHO (IV). Interestingly, the removal of III and IV was accompanied by the formation and accumulation of OHCHNCH2NHNO2 that we detected with strain DN22. At the end of the experiment, which lasted 16 h, we detected the following products HCHO, HCOOH, NH2CHO, N2O, NO2-, and NO3-. Most were also detected during RDX incubation with strain DN22. Finally, we were unable to detect any of RDX nitroso products during both photolysis and incubation with the aerobic bacteria, emphasizing that initial denitration in both cases was responsible for ring cleavage and subsequent decomposition in water.     

Solid-solution reactions in As(V) sorption by schwertmannite

Sorption behavior of As(V) by synthesized schwertmannite was examined under pH 3.3 as a function of As(V) concentration in the initial solution and interpreted in term of solid-solution reactions. Results showed that schwertmannite released 0.62 mmol of SO4(2-) for every 1 mmol of H2AsO4- and 0.24 mmol of OH- that has been sorbed. As(V) replaced SO4 up to half of the total SO4 in schwertmannite. The quantitative relationship among the three chemical compositions indicated that As(V)-sorbed schwertmannite would behave as a solid solution between the As(V) free schwertmannite and schwertmannite containing the maximum level of As(V). The equilibrium constant for the anion exchange in the solid-solution reaction estimated from the reacted solution chemistry depicts the As(V) content found in precipitates formed in acid mine drainage and laboratory experiments. Although schwertmannite is metastable with respect to goethite, the transformation is significantly inhibited by sorption of As(V). The solid-solution reactions also explain the stabilization of schwertmannite by sorption of As(V).     

Removal mechanism of As(III) by a novel Fe-Mn binary oxide adsorbent: oxidation and sorption

A novel Fe-Mn binary oxide adsorbent was developed for effective As(III) removal, which is more difficult to remove from drinking water and much more toxic to humans than As(V). The synthetic adsorbent showed a significantly higher As(III) uptake than As(V). The mechanism study is therefore necessary for interpreting such result and understanding the As(III) removal process. A control experiment was conducted to investigate the effect of Na2SO3-treatment on arsenic removal, which can provide useful information on As(III) removal mechanism. The adsorbent was first treated by Na2SO3, which can lower its oxidizing capacity by reductive dissolution of the Mn oxide and then reacted with As(V) or As(III). The results showed that the As(V) uptake was enhanced while the As(III) removal was inhibited after the pretreatment, indicating the important role of manganese dioxide during the As(III) removal. FTIR along with XPS was used to analyze the surface change of the original Fe-Mn adsorbent and the pretreated adsorbent before and after reaction with As(V) or As(III). Change in characteristic surface hydroxyl groups (Fe-OH, 1130, 1048, and 973 cm(-1)) was observed by the FTIR. The determination of arsenic oxidation state on the solid surface after reaction with As(III) revealed that the manganese dioxide instead of the iron oxide oxidized As(III) to As(V). The iron oxide was dominant for adsorbing the formed As(V). An oxidation and sorption mechanism for As(III) removal was developed. The relatively higher As(III) uptake may be attributed to the formation of fresh adsorption sites at the solid surface during As(III) oxidation.     

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.     

Kinetic studies on Sb(III) oxidation by hydrogen peroxide in aqueous solution

Knowledge of antimony redox kinetics is crucial in understanding the impact and fate of Sb in the environment and optimizing Sb removal from drinking water. The rate of oxidation of Sb(III) with H2O2 was measured in 0.5 mol L(-1) NaCl solutions as a function of [Sb(III)], [H2O2], pH, temperature, and ionic strength. The rate of oxidation of Sb(III) with H2O2 can be described by the general expression: -d[Sb(III)]/dt= k[Sb(III)][H2O2][H+](-1) with log k = -6.88 (+/- 0.17) [kc min(-1)]. The undissociated Sb(OH)3 does not react with H2O2: the formation of Sb(OH)4- is needed for the reaction to take place. In a mildly acidic hydrochloric acid medium, the rate of oxidation of Sb(III) is zeroth order with respect to Sb(III) and can be described by the expression -d[Sb(III)]/dt = k[H2O2][H+][Cl-] with log k = 4.44 (+/- 0.05) [k. L2 mol(-2) min(-1)]. The application of the calculated rate laws to environmental conditions suggests that Sb(III) oxidation by H2O2 may be relevant either in surface waters with elevated H2O2 concentrations and alkaline pH values or in treatment systems for contaminated solutions with millimolar H2O2 concentrations.