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Title:On Competitive Adsorption of Oxyanions by Soils (Freundlich Equation, Enthalphy of Adsorption, Selectivity Coefficient, Anion Mixtures, Anion)
Author(s):Roy, William Robert
Department / Program:Agronomy
Degree Granting Institution:University of Illinois at Urbana-Champaign
Abstract:The adsorption behavior of anionic mixtures (binary-solute combinations of arsenate, molybdate, and phosphate) by three soil samples (Cecil clay, EPA-14, and Catlin silt loam) was investigated using batch equilibrium techniques. The adsorption of arsenate and molybdate by all three soils was significantly reduced by the presence of phosphate, while the adsorption of phosphate was not greatly influenced by either arsenate or molybdate. The adsorption of arsenate by Cecil clay and EPA-14 was reduced in the presence of molybdate, while arsenate did not compete strongly in molybdate adsorption. In contrast, the adsorption of arsenate by Catlin appeared to be independent of molybdate, while the presence of arsenate lowered molybdate adsorption.
The usefulness of two multicomponent Freundlich-type isotherm equations to describe the adsorption of arsenate by Cecil clay in the presence of phosphate or molybdate was evaluated. These multicomponent equations were the DiGiano-Baldauf-Frick-Sontheimer Equation and the Sheindorf-Rebhun-Sheintuch (SRS) Equation. Both expressions were found to satisfactorily describe the isotherm data although there was no clear indication of which, if either, was the more accurate.
The application of the SRS equation yielded competitive coefficients for each binary-solute system on a concentration basis. These coefficients appeared to be consistent with qualitative observations, and were viewed as single-valued terms that indicated the relative affinity of each soil for a given anion from a binary-solute mixture.
These coefficients appeared to have a soil-specific, quasi-thermodynamic basis. Single-solute isotherms at 288(DEGREES), 298(DEGREES), and 308(DEGREES)K were constructed with arsenate, phosphate, and molybdate using all three soils. A Clausius-Clapeyron-type equation was used to estimate isosteric heats of adsorption values (q(,iso)). The adsorption of the three anions were characterized by different q(,iso) values, and were generally in the order of H(,2)PO(,4)('-) > H(,2)AsO(,4)('-) > MoO(,4)('2-). The magnitude of the SRS competitive coefficients derived from binary-solute mixtures were correlated with the ratios of the heats of adsorption of the individual anion-soil systems. Higher coefficients, indicating the preferential adsorption of solute i in a binary-solute mixture containing solutes i and j, tended to be associated with higher ratios of q(,iso) (i) to q(,iso) (j).
Issue Date:1985
Description:163 p.
Thesis (Ph.D.)--University of Illinois at Urbana-Champaign, 1985.
Other Identifier(s):(UMI)AAI8600298
Date Available in IDEALS:2014-12-16
Date Deposited:1985

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