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Competitive Coefficients for the Adsorption of Arsenate, Molybdate, and Phosphate Mixtures by Soils¹

ABSTRACT

The adsorption behavior of anionic mixtures (binary-solute combinations of arsenate, molybdate, and phosphate) by three soils (Cecil clay [Typic Hapludults], EPA-14 [Ultisol], and Catlin silt loam [Typic Argiudolls]) was investigated using the Sheindorf-Rebhun-Sheintuch (SRS) equation. The SRS equation is a multicomponent Freundlich-type adsorption equation. Most of the isotherm data were found to conform to linear SRS expressions, making it possible to derive competitive coefficients on a concentration basis for each binary-solute system. The adsorption of arsenate and molybdate by all three soils were significantly reduced by the presence of phosphate, which was attributed to competitive interactions. In each solute-soil system, the adsorption of phosphate was not greatly suppressed by either arsenate or molybdate. The adsorption of arsenate by Cecil clay and EPA-14 was reduced in the presence of molybdate, whereas arsenate did not compete strongly with molybdate adsorption. In contrast, the adsorption of arsenate by Catlin appeared to be independent of molybdate, while the presence of arsenate lowered molybdate adsorption. In general, the associated competitive coefficient for each solute-soil system appeared to be consistent with qualitative observations, and was viewed as a single-valued term indicative of the relative affinity of each soil for a given anion with respect to a competing anion from a binary-solute mixture.

¹ Contribution from the Section of Geochemistry, Illinois State Geological Survey, 615 East Peabody, Champaign, IL 61820, and the Dep. of Agronomy, Univ. of Illinois, Urbana, IL 61801. This research was supported in part by: Cooperative Agreement CR-810245 from the U.S. EPA.

² Assistant Geochemist, Professor of Soil Chemistry, and Geochemist and Head of Geochemistry Section, respectively.

Received for publication December 10, 1985.

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