., "Arsenic(III) Oxidation and Arsenic(V) Adsorption Reactions on Synthetic Birnessite" (2002). Publications from USDA-ARS / UNL Faculty. 503. http://digitalcommons.unl.edu/usdaarsfacpub/503 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 (MnO 2 ). Stirred reactor experiments indicate that As(III) is oxidized by MnO 2 followed by the adsorption of the As(V) reaction product on the MnO 2 solid phase. The As(V)-Mn interatomic distance determined by EXAFS analysis for both As(III)-and As(V)-treated MnO 2 was 3.22 Å, giving evidence for the formation of As(V) adsorption complexes on MnO 2 crystallite surfaces. The most likely As(V)-MnO 2 complex is a bidentate binuclear corner sharing (bridged) complex occurring at MnO 2 crystallite edges and interlayer domains. In the As(III)-treated MnO 2 systems, reductive dissolution of the MnO 2 solid during the oxidation of As(III) caused an increase in the adsorption of As(V) when compared with As(V)-treated MnO 2 . This suggested that As(III) oxidation caused a surface alteration, creating fresh reaction sites for As(V) on MnO 2 surfaces. Arsenic(III) Oxidation and Arsenic(V) Adsorption Reactions on Synthetic Birnessite

Abstract--Surface adsorption mechanisms of dissolved inorganic carbon species on soil minerals are not well understood. Traditional infrared (IR) study of adsorbed species of inorganic carbon using air-dried samples may not reveal true species in the solid/water interface in suspension. The ptu'pose of this study was to obtain information on interracial carbonate speciation between solid and aqueous phases. The interaction of bicarbonate and carbonate ions with X-ray amorphous (am) A1 and Fe oxides, gibbsite (~/-AI(OH)3 ) and goethite (a-FeOOH) was examined by electrophoresis and in situ attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectroscopy. The presence of carbonate lowered the electrophoretic mobility and decreased the point of zero charge (PZC) of all minerals, implying specific adsorption. Inner-sphere complexation of bicarbonate and carbonate was supported by a lowering in the anion symmetry due to the interaction with A1 and Fe oxide surfaces. Only complexed monodentate carbonate was identified in am-Al(OH)3/aqueous solution at pH 4.1-7.8 when the solid was reacted with either NanCO 3 or Na2CO 3 solutions. Am-AI(OH)3 was transformed to a crystalline sodium aluminum hydroxy carbonate, dawsonite [NaAl(CO3)(OH)2], and bayerite (ct-AI(OH)3) after reacting with 1.0 M Na2CO3 for 24 h. Gibbsite adsorbed much less carbonate than am-AI(OH)3 such that adsorbed carbonate on gibbsite gave weak IR absorption. It is probable that monodentate carbonate is also the complexed species on gibbsite. Evidence suggesting the presence of both surface complexed bicarbonate and carbonate species in the interfacial region of am-Fe(OH)3 in suspension and the dependence of their relative distribution on solution pH is shown. Only monodentate carbonate was found in the interracial region of goethite in 1.0 M NaHCO3. A ligand exchange reaction was proposed to describe the interaction of bicarbonate and carbonate with the surface functional groups of A1 and Fe oxides.

Knowledge of the CO2 concentration in the unsaturated zone is essential for prediction of solution chemistry in the vadose zone and groundwater recharge as well as for quantifying carbon source/sink terms as part of the global CO2 mass balance. In this paper we present a predictive simulation model, SOILCO2, based on process‐oriented relationships. The model includes one‐dimensional water flow and multiphase transport of CO2 utilizing the Richards and the convection‐dispersion equations, respectively, as well as heat flow and a CO2 production model. The transport of CO2 in the unsaturated zone can occur in both the liquid and gas phases. The gas transport equation accounts for production of CO2 and uptake of CO2 by plant roots associated with root water uptake. The CO2 production model considers both microbial and root respiration which is dependent on water content, temperature, growth, salinity and plant and soil characteristics. Heat flow is included, since some gas transport parameters, partitioning coefficients and production parameters are strongly temperature dependent. The resulting set of partial differential equations is solved numerically using the finite element and finite difference methods.

We studied selenate and selenite sorption by amorphous Fe oxide [am-Fe(OH) 3 ] and goethite (a-FeOOH) as a function of time (25 min-96 h), pH (3-12), ionic strength (0.01-1.0 M NaCl), and total Se concentration (0.0001-1.0 M). We examined sorbed selenate and selenite by in situ attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectroscopy, diffuse reflectance infrared Fourier transform (DRIFT) spectroscopy, and electrophoresis to deduce sorption mechanisms. Sorption of both selenate and selenite reached equilibrium in <25 min and the sorption isotherm was not reversible. Increasing ionic strength decreased selenate sorption but did not affect selenite sorption. The presence of either selenate or selenite lowered the electrophoretic mobility (EM) and decreased the point of zero charge (PZC) of both sorbents, suggesting inner-sphere complexation for both selenate and selenite species. Both in situ ATR-FTIR and DRIFT difference spectra showed bidentate complexes of selenate with am-Fe(OH) 3. The structure of selenite complexes in am-Fe(OH) 3-solution interface was uncertain due to insensitivity of the in situ ATR-FTIR technique. The DRIFT spectra of selenite on am-Fe(OH) 3 showed i'j splitting as evidence of complexation. The DRIFT spectra of selenite on goethite showed bridging bidentate complex of selenite. We conclude that the influence of ionic strength on Se sorption cannot be used as a criterion for distinguishing outer-vs. inner-sphere complex formation. A THOUGH SE is ESSENTIAL to humans and animals, it is toxic at high concentrations (NRC, 1983). Soil Se is derived from parent rocks in which Se often occurs together with sulfides in reduced forms. Weathering of parent materials causes Se oxidation and mobilization (Ihnat, 1989; McNeal and Balistrieri, 1989). Selenium deficiency has been linked to certain endemic diseases in China (Tan and Huang, 1991), whereas Se enrichment in soils and waters has been implied as a major factor resulting in severe teratogenic symptoms in wildlife at Kesterson National Wildlife Refuge in the western San Joaquin Valley, California. The source of Se to the Kesterson Wildlife Refuge is from agricultural drainage from seleniferous soils (Presser and Barnes, 1984,1985; Letey et al., 1986). A major effort has been directed to the study of Se sorption phenomena because it plays an important role in mobility, transport, transformation, and the ultimate fate of Se in soil and aquatic systems. The interaction of the inorganic Se species, selenate

W e studied B adsorption on amorphous aluminum and iron hydroxides, allophane, and kaolinite as a function of pH and initial B concentration. Boron adsorption lowered the point of zero charge of all four adsorbents, implying specific adsorption (inner-sphere complexation) of B. W e provided novel information on the coordination of B adsorbed at mineral-water interfaces by attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectroscopy. The ATRFTlR spectra of interfacial B species were influenced by pH and mineral type. Strong trigonal B and weak tetrahedral B bands of the asymmetric stretching mode were observed on the difference spectra at pH 737 for amorphous iron hydroxide, whereas both strong trigonal and tetrahedral B bands were found at pH 4 0 . A strong IR band of asymmetric stretching of tetrahedral B shifted to higher frequencies in am-Fe(OH)s paste at both pH's relative to that of boric acid solution at pH 11. Trigonal B asymmetric stretching bands shifted to higher frequencies on the difference spectra for am-AI(OH)s and allophane at both pH's compared to that of boric acid solution at pH 7. Polymerization of B on mineral surfaces is shown to be possible. The results provide spectroscopic evidence that both B(OH)3 and B(OH)4-are adsorbed via a ligand exchange mechanism.

The adverse effects of exchangeable sodium on soil hydraulic conductivity (K) are well known, but at present only sodicity and total electrolyte concentration are used in evaluating irrigation water suitability. In arid areas, high sodicity is often associated with high dissolved carbonate and thus high pH, but in humid areas high sodicity may be associated with low pH. To evaluate the effect of pH (as an independent variable) on A", solutions with the same SAR and electrolyte level were prepared at pH 6, 7, 8, and 9. Saturated A' values were determined at constant flux in columns packed at a bulk density of 1.5 Mg m' 3. At pH 9, saturated K values were lower than at pH 6 for a montmorillonitic and a kaolinitic soil. For a vermiculitic soil with lower organic carbon and higher silt content, pH changes did not cause large K differences. Decreases in A" were

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Understanding the speciation of the multioxidation states of selenium is vital to predicting the mineralization, mobilization, and toxicity of the trace element in natural systems. A sequential extraction scheme (SES) was developed for identification of Se oxidation states that first employed 0.1 M (pH 7.0) K 2 HPO 4 -KH 2 PO 4 (P-buffer) to release soluble selenate (Se +VI ) and selenide (Se -II ) and ligandexchangeable selenite (Se +IV ). The second step involved oxidation of organic materials with 0.1 M K 2 S 2 O 8 (90°C) to release Se -II and Se +IV associated or occluded with organic matter. The final step used HNO 3 (90°C) to solubilize insoluble Se remaining in the sample. The solubilized Se compounds were speciated by a selective hydride generation atomic absorption spectrophotometry technique. Accuracy of the developed SES method (96-103% recovery) was verified by use of prepared Se compounds of known speciation, NIST standard reference materials, and existing seleniferous soils. The average precision (relative standard deviation) for the P-buffer extraction ranged from 5.5 to 7.7% (n ) 12); the precision of the persulfate extraction ranged from 2.6 to 8.4% (n ) 12); and the precision of the nitric acid extraction ranged from 2.8 to 7.4% (n ) 12) for three soils extracted at four different time periods.The method was applied to analyze Se species in seleniferous plant, soil, and sediment samples.