Bidentate Binuclear Complexes Research Articles

Respective role of Fe and Mn oxide contents for arsenic sorption in iron and manganese binary oxide: an X-ray absorption spectroscopy investigation.

In our previous studies, a synthesized Fe-Mn binary oxide was found to be very effective for both As(V) and As(III) removal in aqueous phase, because As(III) could be easily oxidized to As(V). As(III) oxidation and As(V) sorption by the Fe-Mn binary oxide may also play an important role in the natural cycling of As, because of its common occurrence in the environment. In the present study, the respective role of Fe and Mn contents present in the Fe-Mn binary oxide on As(III) removal was investigated via a direct in situ determination of arsenic speciation using X-ray absorption spectroscopy. X-ray absorption near edge structure results indicate that Mn atoms exist in a mixed valence state of +3 and +4 and further confirm that MnOx (1.5 < x < 2) content is mainly responsible for oxidizing As(III) to As(V) through a two-step pathway [reduction of Mn(IV) to Mn(III) and subsequent Mn(III) to Mn(II)] and FeOOH content is dominant for adsorbing the formed As(V). No significant As(III) oxidation by pure FeOOH had been observed during its sorption, when the system was exposed to air. The extended X-ray absorption fine structure results reveal that the As surface complex on both the As(V)- and As(III)-treated sample surfaces is an inner-sphere bidentate binuclear corner-sharing complex with an As-M (M = Fe or Mn) interatomic distance of 3.22-3.24 Å. In addition, the MnOx and FeOOH contents exist only as a mixture, and no solid solution is formed. Because of its high effectiveness, low cost, and environmental friendliness, the Fe-Mn binary oxide would play a beneficial role as both an efficient oxidant of As(III) and a sorbent for As(V) in drinking water treatment and environmental remediation.

Theoretical study of SO 2 adsorption on goethite (1 1 0) surface

Abstract Adsorption of SO 2 on fully hydrated and partially dehydrated (1 1 0) surface of goethite (α-FeOOH) has been investigated using density functional theory (DFT) and periodic conditions. Different degrees of dehydration were modeled by eliminating one or two water molecules from the fully hydrated surface. Calculations indicate that SO 2 shows preference to adsorb on dehydrated surface and the transformation to surface sulfite, bisulfite and sulfate was observed. In particular, surface sulfite can be formed over a variety of different dehydrated surfaces as monodentate and bidentate complexes. Theoretical vibrational frequencies of all the species have also been computed. Taking into account all the structures, we found frequency values within the 650–1030 cm −1 region due to S O Fe stretching, and between 1010 and 1190 cm −1 due to S O stretching. Furthermore, monodentate mononuclear and bidentate binuclear sulfite complexes present distinctive features at low frequencies (600–700 cm −1 ).

Mineralogy and speciation of Zn and As in Fe-oxide-clay aggregates in the mining waste at the MVT Zn–Pb deposits near Olkusz, Poland

Oxidation zones of ore deposits offer valuable insights into the long-term fate of many metals and metalloids. In this work, we have studied a paleo-acid rock drainage (ARD) system – the oxidation zone of Mississippi-valley type Zn–Pb deposits near Olkusz in southern Poland. The ARD systems exhausted their acid-generating capacity and have come almost to the conclusion of the mineral and geochemical transformations. Primary pyrite, marcasite, galena and sphalerite have been decomposed but the acidity was neutralized by the abundant carbonate host rocks. Zinc is stored in smithsonite, hemimorphite, and Zn-rich sheet aluminosilicates. Some of these minerals formed simultaneously with the oxidation zone but some precipitated in the soils in situ, thus documenting the mobility of Zn, Al, and Si in the soils. Iron oxides are represented mostly by goethite, either well-crystalline or nanocrystalline, as determined by a combination of powder X-ray diffraction, micro-X-ray diffraction, and Mössbauer spectroscopy. Iron oxides bind a substantial amount of arsenic, to a lesser extent also zinc, lead, and cadmium, as shown by electron microprobe analyses and sequential extractions. The X-ray absorption spectroscopy data of the local environment of arsenic in goethite suggest the existence of bidentate mononuclear complex, in addition to the more common bidentate binuclear complex. These results suggest that arsenic is incorporated in the crystal structure of goethite, in addition to adsorbed to the surface of the particles or occluded in the voids and pores. Zinc is bound in goethite as a mixture of tetrahedrally and octahedrally coordinated cations. This study shows that the mature system binds the metals from the primary sulfides relatively strongly. Yet, some release of the metals was observed in this study, either in the laboratory (by sequential extractions) and in nature (e.g., neoformed Zn phyllosilicates). The physical conditions in the oxidation zone and on the surface are largely similar but the metals, to a certain extent, are still mobile in the soils. We may speculate that their mobility near the surface, in the mining waste, may be enhanced by a higher water/rock ratio than in the oxidation zone. This result implies that although the studied material is relatively benign, it still has a potential to cause local environmental problems.

Germanium isotope fractionation during Ge adsorption on goethite and its coprecipitation with Fe oxy(hydr)oxides

Isotopic fractionation of Ge was studied during Ge adsorption on goethite and its coprecipitation with amorphous Fe oxy(hydr)oxides. Regardless of the pH, surface concentration of adsorbed Ge or exposure time, the solution–solid enrichment factor for adsorption (Δ74/70Gesolution–solid) was 1.7±0.1‰. The value of the Δ74Gesolution–solid in Fe–Ge coprecipitates having molar ratio 0.1<(Ge/Fe)solid<0.5 remained constant at 2.0±0.4‰. For (Ge/Fe)solid ratio<0.1, the Δ74Gesolution–solid increased with the decrease of Ge concentration in the solid phase, with the value as high as 4.4±0.2‰ at (Ge/Fe)solid<0.001, corresponding to the majority of natural settings. These results can be interpreted based on available structural data for adsorbed and coprecipitated Ge. It follows that Ge(OH)4° adsorption occurring as bidentate binuclear complexes at the goethite surface is characterised by an enrichment factor of ∼1.7‰, likely related to the distortion of the GeO4 tetrahedron and the formation of Ge–O–Fe bonds at the goethite surface as compared to aqueous solution. In contrast, coprecipitation yields more distorted edge-sharing GeO4 tetrahedra and, in the case of the most diluted samples, part of the Ge is found in coordination 6, replacing Fe(III) in octahedral positions. This produces a greater enrichment of the solid phase in lighter isotopes, mostly due to the increase in Ge–O bond distances and coordination number compared to aqueous solution, which is in line with the basic principles of isotope fractionation. Discharge of hydrothermal fluids, leading to massive Fe(OH)3 precipitation in the vicinity of the springs should, therefore, represent an isotopically-heavy source of dissolved Ge to the ocean. Similarly, groundwater discharge and Fe(OH)3 precipitation at the Earth’s surface, Fe oxy(hydr)oxide formation in soils and riverine organo-ferric colloids coagulation, leading to iron hydroxide precipitation in estuaries, should produce an isotopically heavy Ge aqueous flux to the ocean compared to bedrock sources and particulate fluxes.

Arsenate and cadmium co-adsorption and co-precipitation on goethite

Arsenate (As(V), AsO43−) and cadmium (Cd) are among the toxic elements of most concern. Their sorption behaviors on goethite were studied by batch experiments (pH edges, isotherms and kinetics) and X-ray diffraction (XRD). Arsenic coordination environment was explored by X-ray absorbance fine structure (EXAFS) analysis. Sorption isotherms of both As(V) and Cd on goethite could be divided into the adsorption-dominated and precipitation-dominated parts, while their sorption showed different pH-dependency and sorption reversibility. Cadmium adsorption was enhanced in the presence of AsO43−, which could be explained by the decrease in the electrostatic potential due to the sorption of AsO43− and the formation of a ternary Cd–As(V)–goethite complex. Based on the EXAFS study, AsO43− adsorbed on goethite mainly formed bidentate–binuclear complex. The high loadings of Cd changed the As(V)–Fe distance and its coordination number. However, Cd did not affect the As(V) adsorption amount in the adsorption-dominated region. When As(V) and Cd formed co-precipitates, their sorption amounts were both increased. The formation of co-precipitates decreased the mobility of Cd but increased the mobility of As(V) because less As(V) was sorbed on goethite through surface complexation. This study will provide better understandings on As(V) and Cd transport and useful information on their remediation strategies.

DFT+U Study of Arsenate Adsorption on FeOOH Surfaces: Evidence for Competing Binding Mechanisms

On the basis of periodic density functional theory (DFT) calculations including an on-site Coulomb repulsion term U, we study the adsorption mechanism of arsenate on the goethite (101), akaganeite (100), and lepidocrocite (010) surfaces. Mono- and bidentate binding configurations of arsenate complexes are considered at two distinct iron surface sites—directly at 5-fold coordinated Fe1 and/or 4-fold coordinated Fe2 as well as involving ligand exchange. The results obtained within ab initio thermodynamics shed light on the ongoing controversy on the arsenate adsorption configuration, and we identify monodentate adsorbed arsenate complexes as stable configurations at ambient conditions with a strong preference for protonated arsenate complexes: a monodentate mononuclear complex at Fe1 (dFe1–As = 3.45 Å) at goethite (101) and a monodentate binuclear complex at Fe2 (dFe2–As = 3.29 Å) at akaganeite (100). Repulsive interactions between the complexes limit the loading capacity and promote configurations with maximized distances between the adsorbates. With decreasing oxygen pressures, a mixed adsorption of bidentate binuclear complexes at Fe1 (dFe1–As = 3.26–3.34 Å) and monodentate binuclear arsenate at Fe2 (dFe2–As = 3.31–3.50 Å) and, finally, rows of protonated bidentate complexes at Fe1 with dFe1–As = 3.55–3.59 Å are favored at α-FeOOH(101) and β-FeOOH(100). At lepidocrocite (010) with only Fe2 sites exposed, the surface phase diagram is dominated by alternating protonated monodentate binuclear complexes (dFe2–As = 3.38 Å) and hydroxyl groups. At low oxygen pressures, alternating rows of protonated bidentate mononuclear complexes (dFe2–As = 3.10 Å) and water are present. Hydrogen bond formation to surface hydroxyl groups and water plays a crucial role in the stabilization of these adsorbate configurations and together with steric effects of the oxygen lone pairs leads to tilting of the arsenate complex that significantly reduces the Fe–As distance. Our results show that the Fe–As bond length is mainly determined by the protonation state, arsenate coverage, steric effects, and hydrogen bonding to surface functional groups and to a lesser extent by the adsorption mode. This demonstrates that the Fe–As distance cannot be used as a unique criterion to discriminate between adsorption modes.

A real time in situ ATR-FTIR spectroscopic study of glyphosate desorption from goethite as induced by phosphate adsorption: Effect of surface coverage

The desorption of glyphosate from goethite as induced by the adsorption of phosphate was investigated by attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectroscopy in combination with adsorption isotherms. Desorption of glyphosate was very low in the absence of phosphate. Addition of phosphate promoted glyphosate desorption. At low initial surface coverages, added phosphate adsorbed on free surface sites, mainly, displacing a small amount of glyphosate. At high initial surface coverages, on the contrary, phosphate adsorption resulted in a significant glyphosate desorption. In the latter conditions, the ratio desorbed glyphosate to adsorbed phosphate was 0.60. The desorption process can be explained by assuming that phosphate adsorbs first forming a monodentate mononuclear complex, which rapidly evolves into a bidentate binuclear complex that displaces glyphosate.

Formation of hydroxylapatite from co-sorption of phosphate and calcium by boehmite

The interaction of calcium with phosphate at mineral/water interfaces is of importance for understanding both P sequestration and phosphate mineral formation. We investigated the effect of dissolved calcium on phosphate uptake by boehmite in batch sorption studies as a function of pH. Examination of the solids by 31P NMR spectroscopy and powder X-ray diffraction (XRD) shows evidence for formation of hydroxylapatite from pH 7 to pH 9, which is supported by correlation of Ca and P on particle surfaces at pH 9 observed by scanning electron microscopy with energy-dispersive X-ray spectroscopy (SEM/EDX) analysis. At pH 6, two major 31P NMR peaks are observed at δP–31=0 and −6ppm, indicating the presence of bidentate binuclear complexes with surface Al atoms, similar to those found in the absence of dissolved Ca. At higher pH, an additional 31P peak at δP–31=2.65ppm is observed, consistent with hydroxylapatite (Hap). The NMR data indicate that after 30days most of the phosphate (75%) remained as adsorption complexes at pH 7, but that Hap accounts for most of the phosphate at higher pH, although surface complexes were still evident in CP/MAS NMR spectra. The identification of crystalline Hap is further supported by 31P{1H} heteronuclear correlation (HetCor) experiments in which the 2.65ppm 31P peak correlates to a narrow 1H peak at δH–1=0.2ppm that is diagnostic of the hydroxyl groups of Hap. In powder X-ray diffraction patterns, two small peaks are observed at slow scan rates that match the major reflections of Hap. Nonetheless, Hap crystals could not be identified in SEM images suggesting small particle size, in agreement with broad XRD peaks. At short reaction times only adsorbed phosphate is observed at pH 7, whereas Hap forms within 15min at pH 9. These results indicate that the crystallization rate of Hap is enhanced by the boehmite surface, although the detailed mechanisms could not be discerned from these data.

Fabrication, Characterization, and Application of a Composite Adsorbent for Simultaneous Removal of Arsenic and Fluoride

Coexisting arsenic (As) and fluoride (F) in groundwater poses severe health risks worldwide. Highly efficient simultaneous removal of As and F is therefore of great urgency and high priority. The purpose of this study was to fabricate a novel composite adsorbent and explore the mechanism for concurrent removal of As(V) and F at the molecular level. This bifunctional adsorbent with titanium and lanthanum oxides impregnated on granular activated carbon (TLAC) exhibits a pronounced As(V) and F adsorption capacity over commercially available iron- and aluminum-based adsorbents for synthetic and real contaminated groundwater samples. Synchrotron-based X-ray microfluorescence analysis demonstrates that La and Ti were homogeneously distributed on TLAC. Extended X-ray absorption fine structure spectroscopic results suggest that As(V) formed bidentate binuclear surface complex as evidenced by an averaged Ti-As bond distance of 3.34 Å in the presence of F. Adsorption tests and Fourier transform infrared spectroscopy analysis indicate that F was selectively adsorbed on lanthanum oxides. The surface configurations constrained with the spectroscopic results were formulated in the charge distribution multisite complexation model to describe the competitive adsorption behaviors of As(V) and F. The results of this study indicate that TLAC could be used as an effective adsorbent for simultaneous removal of As(V) and F.

A novel application of H 2O 2–Fe(II) process for arsenate removal from synthetic acid mine drainage (AMD) water

Batch experiments were carried out to investigate the influences of H 2O 2/Fe(II) molar ratio, pH, sequence of pH adjustment, initial As(V) concentration, and interfering ions on As(V) removal in H 2O 2–Fe(II) process from synthetic acid mine drainage (AMD). The optimum H 2O 2/Fe(II) molar ratio was one for arsenate removal over the pH range of 4–7. Arsenate removal at pH 3 was poor even at high Fe(II) dosage due to the high solubility of Fe(III) formed in situ. With the increase of Fe(II) dosage, arsenate removal increased progressively before a plateau was reached at pH 5 as arsenate concentration varied from 0.05 to 2.0 mg L −1. However, arsenate removal was negligible at Fe/As molar ratio <3 and then experienced a striking increase before a plateau was reached at pH 7 and arsenate concentration ≥1.0 mg L −1. The co-occurring ions exerted no significant effect on arsenate removal at pH 5. The experimental results with synthetic AMD revealed that this method is highly selective for arsenate removal and the co-occurring ions either improved arsenate removal or slightly depressed arsenate removal at pH 5–7. The extended X-ray absorption fine structure (EXAFS) derived As–Fe length, 3.27–3.30 Å, indicated that arsenate was removed by forming bidentate–binuclear complexes with FeO(OH) octahydra. The economic analysis revealed that the cost of the H 2O 2–Fe(II) process was only 17–32% of that of conventional Fe(III) coagulation process to achieve arsenate concentration below 10 μg L −1 in treated solution. The results suggested that the H 2O 2–Fe(II) process is an efficient, economical, selective and practical method for arsenate removal from AMD.

Performance and mechanism of simultaneous removal of chromium and arsenate by Fe(II) from contaminated groundwater

The feasibility and the mechanisms of simultaneous removal of chromium and arsenate by Fe(II) were investigated. In the absence of arsenate, chromium removal by Fe(II) increased to the maximum with increasing pH from ∼4 to ∼7 and then decreased with further increase in pH. Chromium removal by Fe(II) was controlled by the rate of Cr(VI) reduction by Fe(II) and the solubility of Fe 0.75Cr 0.25(OH) 3 at pH ≤ 7, but by the extent of Cr(VI) reduction under alkaline conditions. The presence of arsenate resulted in a decrease in chromium removal by Fe(II) under neutral and alkaline conditions as a result of the depression in the magnitude of Cr(VI) reduction by Fe(II) and sequestration of the Fe 0.75Cr 0.25(OH) 3 and FeOOH precipitation by HAsO 4 2 - . Arsenate removal by Fe(II) alone was trivial but was improved significantly at pH 4–9 due to the presence of 10–30 μmol L −1 chromate. It was the oxidative property of chromate that resulted in the oxidization of Fe(II) to Fe(III) concomitantly facilitating the removal of arsenate. Arsenate was removed by both adsorption and co-precipitation with Fe 0.75Cr 0.25(OH) 3 and FeOOH precipitates. EXAFS results revealed that arsenate mainly coordinated with Fe(III) rather than Cr(III) and arsenate formed bidentate-binuclear complexes with FeO(OH) octahydra as evidenced by an average Fe–As(V) bond distance of 3.25–3.26 Å.

Multiscale Assessment of Methylarsenic Reactivity in Soil. 1. Sorption and Desorption on Soils

Methylated forms of arsenic (As), monomethylarsenate (MMA), and dimethylarsenate (DMA) have historically been used as herbicides and pesticides. Because of their large application to agriculture fields and the toxicity of MMA and DMA, the persistency of these compounds in the environment is of great concern. MMA and DMA sorption and desorption were investigated in soils, varying in mineralogical and organic matter (OM) contents. Sorption studies showed that the MMA sorption capacity and rate were greater than DMA sorption. Al/Fe-oxyhydroxides were the main sorbents in the soils, and the sorption capacity was proportional to the Al/Fe concentration in the soils. Extended X-ray absorption fine structure (EXAFS) studies showed that both MMA/DMA-Fe interatomic distances were around 3.3 Å, which were indicative of bidentate binuclear inner-sphere complex formation. Desorption studies showed that not all of the sorbed MMA or DMA was desorbed due to the strong binding between MMA/DMA and Al/Fe-oxyhydroxide surfaces via possible inner-sphere complex formation. The amount of the desorbed MMA and DMA decreased as the sorption residence time increased. For example, 77% of sorbed MMA was desorbed from the Reybold subsoil after 1 day residence time, while 66% of sorbed MMA was desorbed from the soil after six months of residence time. The decreases in desorption were likely due to As speciation changes from MMA/DMA to inorganic arsenate, which was more strongly bound to the surface.

Arsenate adsorption on an Fe–Ce bimetal oxide adsorbent: EXAFS study and surface complexation modeling

The mechanism of arsenate (As(V)) adsorption on an Fe–Ce bimetal (hydrous) oxide (Fe–Ce) was investigated using complementary analysis techniques including extended X-ray absorption fine structure (EXAFS) and surface complexation modeling. The As K-edge EXAFS spectra showed that the second peak of Fe–Ce after As(V) adsorption was the As–Fe shell, which supported the finding that As(V) adsorption occurred mainly at the Fe surface active sites. Two As–Fe distances of 3.30 Å and 3.55 Å were observed from the EXAFS spectra of As(V) adsorbed samples at three pH levels (5.0, 7.6, and 9.0) and two initial surface loadings of 70 and 130 mg/g, which indicated that monodentate mononuclear and bidentate binuclear As surface complexes coexisted. When compared with the reported dominant species of bidentate binuclear complex for As existing on iron (hydro)oxides, the existence of Ce atoms in the bimetal oxide and the high surface loading were the likely reasons for the existence of the monodentate complex. A Charge Distribution-Multi-site Sites Complexation (CD-MUSIC) model showed that protonated monodentate (MH) and deprotonated bidentate (B) complexes preferred to exist on the Fe–Ce surface in a high surface loading range ( Γ = 5.11–14.4 μmol/m 2). The MH complex was shown to be dominant at pH < 8. Based on the results from EXAFS analysis and the CD-MUSIC model, the adsorptive behavior of As(V) on Fe–Ce with high surface loadings was satisfactorily interpreted and understood.

Molecular Scale Assessment of Methylarsenic Sorption on Aluminum Oxide

Methylated forms of arsenic (As), monomethylarsenate (MMA) and dimethylarsenate (DMA), have historically been used as herbicides and pesticides. Because of their large application to agriculture fields and the toxicity of MMA and DMA, the sorption of methylated As to soil constituents requires investigation. MMA and DMA sorption on amorphous aluminum oxide (AAO) was investigated using both macroscopic batch sorption kinetics and molecular scale extended X-ray absorption fine structure (EXAFS) and Fourier transform infrared (FTIR) spectroscopic techniques. Sorption isotherm studies revealed sorption maxima of 0.183, 0.145, and 0.056 mmol As/mmol Al for arsenate (As(V)), MMA, and DMA, respectively. In the sorption kinetics studies, 100% of added As(V) was sorbed within 5 min, while 78% and 15% of added MMA and DMA were sorbed, respectively. Desorption experiments, using phosphate as a desorbing agent, resulted in 30% release of absorbed As(V), while 48% and 62% of absorbed MMA and DMA, respectively, were released. FTIR and EXAFS studies revealed that MMA and DMA formed mainly bidentate binuclear complexes with AAO. On the basis of these results, it is proposed that increasing methyl group substitution results in decreased As sorption and increased As desorption on AAO.

Influence of pH on Initial Concentration Effect of Arsenate Adsorption on TiO2 Surfaces: Thermodynamic, DFT, and EXAFS Interpretations

Under the same thermodynamic condition where the total mass of arsenate was fixed, when the initial arsenate was added to TiO2 suspension by multiple batches, adsorption isotherms declined as the multi-batch increased, which was termed initial concentration (C0) effect. The extent of C0 effect decreased gradually as pH decreased from 7.0 to 5.5. Extended X-ray absorption fine structure analysis of 1-batch and 3-batch isotherm samples showed that the relative proportion of bidentate binuclear (BB) and monodentate mononuclear (MM) complex was rarely affected by pH change from 5.5 to 7.0, indicating that the dependence of C0 effect on pH was not due to inner-sphere chemiadsorption. The influence of pH on adsorption was simulated by density functional theory through changing the number of H+ in model clusters. Calculation of adsorption energy showed that BB surface complex was the most thermodynamically favorable mode (−244.5 kJ mol−1) at low pH, but MM surface complex was the most thermodynamically favorable mode (−135.6 to 27.5 kJ mol−1) at intermediate and high pH, which indicated the influence of surface functional groups (−H2O and −OH) on adsorption reaction pathways. As pH decreased, C0 effect weakened gradually because outer-sphere H-bond adsorption became thermodynamically favorable (−203.1 kJ/mol). The dependence of C0 effect on pH showed that traditional equilibrium adsorption constants could not accurately describe the real adsorption equilibrium at solid−liquid interface, because real equilibrium adsorption state is generally a mixture of various outer-sphere and inner-sphere metastable-equilibrium states.

Quantitative XANES Studies on Metastable Equilibrium Adsorption of Arsenate on TiO2 Surfaces

Quantitative information on the electronic and geometric structures of adsorption complexes of arsenate on TiO2 surfaces was obtained on the basis of XANES analysis combined with DFT calculation. The density of states (DOS) showed that the As(V) K-absorption edge corresponded to the dipole transition from the As Is electron into the As(4p)-O(2p) antibonding molecular orbital, suggesting that the bonding between arsenate and TiO2 was primarily due to the interaction of As 4p and O 2p orbitals. Previous EXAFS studies indicated that both bidentate binuclear (BB) and monodentate mononuclear (MM) surface complexes coexisted in the As(V)-TiO2 adsorption system. DFT calculation showed that the XANES transition energy of the BB complex was significantly higher than that of the MM complex. Thus, quantitative information on adsorption microstructures call be obtained by measuring the blue-shift of the XANES absorption edge. XANES results of adsorption samples at different kinetic stages indicated that there was a structural evolution from the MM complex to the BB complex as the reaction processed to adsorption equilibrium. Therefore, the final proportion between BB and MM complexes in the real equilibrium state was fundamentally affected by surface coverage and kinetic pathways. This fact implied that the real equilibrium constant, when defined by macroscopic parameters of concentration and adsorption density (mol/m(2)), are generally inconstant in nature and hence cannot describe the real equilibrium properties of adsorption, because different microscopic metastable equilibrium adsorption structures that construct the real adsorption equilibrium cannot be counted for both mass and energy by the macroscopic parameter of surface concentrations (mol/m(2)).

Copper and arsenate co-sorption at the mineral–water interfaces of goethite and jarosite

The co-sorption reaction products of arsenate (As(V)) and copper (Cu(II)) on goethite ( α-FeOOH) and natro-jarosite (Na 3Fe 3(SO 4) 2(OH) 6) were investigated with extended X-ray absorption fine structure (EXAFS) spectroscopy to determine if Cu(II) and As(V) would form precipitates or compete with each other for surface sites. The reaction products were prepared by mixing 250 μM Cu(SO 4) with 10, 25, or 50 μM Na 2HAsO 4 at pH 5.65 and allowing the mixture to react in 10 m 2 L −1 goethite or jarosite suspensions for 12 days. In addition, EXAFS data of Cu(SO 4) and As(V) sorbed on goethite and jarosite were collected as control species. All reaction conditions were under-saturated with respect to common copper bearing minerals: tenorite (CuO), brochantite (Cu 4(OH) 6SO 4), and hydrated clinoclase (Cu 3(AsO 4) 2⋅2H 2O). The extents of the As(V) and Cu(II) surface adsorption reactions showed a strong competitive effect from Cu(II) on As(V) adsorption for a nominal Cu:As mole-ratio of 25:1. With increasing nominal As(V) concentration, As(V) sorption on goethite and jarosite increased without diminishing the amount of Cu(II) sorption. In the absence of either co-sorbate, As(V) and Cu(II) formed the expected surface adsorption species, i.e., bidentate binuclear and edge-sharing surface complexes, consistent with previously published results. In each other's presence, the local bonding environments of As(V) and Cu(II) showed that the co-sorbates form a precipitate on the goethite and jarosite surface at nominal concentrations of 10:1 and 5:1. At nominal Cu:As mole-ratios of 25:1, Cu(II) did not form significantly different surface complexes on goethite or jarosite from those in the absence of As(V), however, As K-edge EXAFS results distinctly showed Cu(II) atoms in As(V)'s local bonding environment on the goethite surface. The structures of the two precipitates were different and depended on the anion-layer structure and possibly the presence of structural oxyanions in the case of jarosite. On goethite, the copper–arsenate precipitate was similar to hydrated clinoclase, while on jarosite, a euchroite-like precipitate (Cu 2[AsO 4](OH)⋅3H 2O, P 2(1)2(1)2(1)) had formed. Despite under-saturated solution conditions, the formation of these precipitates may have occurred due to a seed-formation effect from densely surface adsorbed Cu(II) and As(V) for which the “new” saturation index was significantly lower than homogeneous values would otherwise suggest. Synergistic reactions between two co-sorbates of fundamentally different surface adsorption behaviour can thus be achieved if the number of available sites for surface adsorption is limited.

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.

Removal of arsenic from water using granular ferric hydroxide: Macroscopic and microscopic studies

Removal of arsenate from water using granular ferric hydroxide (GFH) was investigated under different pH and As(V) loading conditions, using batch equilibrium adsorption, FTIR, and EXAFS methods. The arsenate adsorption envelopes on GFH exhibited broad adsorption maxima when the initial As(V) concentration was less than 500 mg/L at sorbent concentration of 10 g/L. As the initial As(V) concentration increased to 500, 1000 or 2000 mg/L for the same sorbent concentration, distinct adsorption maxima appeared and shifted to lower pH. Acidimetric–alkalimetric titration and arsenic adsorption isotherm data indicated that the surface of GFH is high heterogeneous. FTIR spectra revealed that complexes of two different structures, bidentate and monodentate, were formed upon the adsorption of arsenate on GFH, and bidentate complexes were only observed at pH values greater than 6. The EXAFS analyses confirmed that arsenate form bidentate binuclear complexes with GFH at pH 7.4 as evidenced by an average Fe-As(V) bond distance of 3.32 Å.

Formation of Metal−Arsenate Precipitates at the Goethite−Water Interface

Little information is available concerning cosorbing oxyanion and metal contaminants in the environment, yet in most metal-contaminated areas, cocontamination by arsenate [AsO4, As(V)] is common. This study investigated the cosorption of As(V) and Zn on goethite at pH 4 and 7 as a function of final solution concentration. Complimentary extended X-ray absorption fine structure (EXAFS) spectroscopic data were collected at the As and Zn K-edges in order to glean information about the coordination environment of As and Zn at the goethite-water interface. Macroscopic sorption studies revealed that As(V) and Zn sorption on goethite increased in cosorption experiments beyond that suggested by single sorption isotherms. At pH 4 and 7, As(V) surface saturation was 3.2 and 2.2 micromol m(-2), respectively, and Zn surface saturation was absent at pH 4 and approximately 1.0 micromol m(-2) at pH 7. Arsenate sorption on goethite increased in the presence of Zn by 29% and by more than 500% at pH 4 and 7, respectively. In the presence of As(V), Zn sorption on goethite increased by 800 and 1300% at pH 4 and 7, respectively. More As(V) than Zn sorbed on goethite below surface saturation at pH 7. Above surface saturation, the Zn:As surface density ratio (SDR) remained constant at 0.91 +/- 0.03. At pH 4, the Zn:As SDR was less than 1 throughout the concentration range. Below As(V) surface saturation on goethite, As(V) formed bidentate binuclear bridging complexes on Fe and/or Zn octahedra, while Zn mainly formed edge-sharing complexes with Fe at the goethite surface. Above surface saturation, Zn was increasingly complexed by AsO4, gradually forming an adamite-like [Zn2(AsO4)OH] surface precipitate on goethite. Precipitated contaminants are more stable due to the limited dissolution kinetics of their solid phase. This study may therefore prove useful in remediation strategies of sites knowingly contaminated with oxyanions and metals.