Experimental evaluation and chemical modeling of hexavalent chromium adsorption, desorption, and reduction in a natural soil Public Deposited

http://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/s4655k06d

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  • Chromium and its compounds are widely used by modern industries, resulting in large quantities of this element being discharged into the environment. To remove chromium from contaminated soils and ground water, it is necessary to predict chemical and physical processes that control the rate of reactions and transport of chromium in soils and aquifers. The goals of this experimental study were to determine (i) kinetics and equilibrium adsorption of chromium(VI) in a natural soil, (ii) reduction of Cr(VI) to Cr(III) in the soil, and (iii) the effect of competing oxyanions on Cr(VI) adsorption in the soil. The TLM was used to interpret surface complexation reactions of the chromate ions in the soil. A laboratory investigation of reactions between hexavalent chromium, Cr(VI), and a natural soil was conducted to evaluate factors that influence sorption and reduction of Cr(VI) in natural soils. Both batch and soil column experiments were conducted to study the chemical behavior and transport of Cr(VI) in the soil. Results indicated that adsorption and reduction of Cr(VI) are the major processes that control the rate of transport and mobility of chromium in natural soils. Cr(VI) removal from solution increased with increasing solute concentration and with decreasing solution pH. This experimental study provides insight on how the residual amount of ferrous ions in minerals such as magnetite can effect the redox speciation of chromium in natural soils. Experimental results indicated that the small amounts of magnetite Fe3O4 contained in the soil caused reduction of Cr(VI) to Cr(III) even at pH above 8. The ferrous iron contained in magnetite provides a source of electrons for the reduction of Cr(VI) to Cr(III). Competing oxyanions, phosphate (H2PO4-/HPO4^2-) and sulfate (SO4^2-), increased Cr(VI) desorption by direct competition for adsorption sites. The equilibrium adsorption capacity of the soil was described with the Langmuir model, while a triple layer model (TLM) was employed to describe the surface complexation reactions. Outer-sphere surface complexation reactions and two-site (FeOH and AIOH) modeling were used to simulate adsorption of the chromate (CrO4^2-) and bichromate (HCrO4-) ions.
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