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Control of Nickel and Strontium Sorption by Free Metal Ion Activity¹

ABSTRACT

Formation of soluble complexes usually lowers sorption of metals by soils. The purpose of this study was to determine if changes in free Ni²⁺ or Sr²⁺ activity could explain differences in isotherms obtained in highly complexing SO^2-₄ systems vs. slightly complexing Cl^- systems. Sorption of Ni and Sr by homoionic montmorillonite and soil was studied using the batch isotherm technique. The Na⁺ (0.1N) and Ca²⁺ (0.02N) solutions of Cl^- and SO^2-₄ with Ni or Sr (10^–6 to 10^–4M) yielded systems in which up to 45% of soluble metal (M) was present as MSO⁰₄. Sorption was greater in the Cl^- vs. the SO^2-₄ systems at the same total M (Ni or Sr) concentration at equilibrium. Plotting data in terms of free M²⁺ activity at equilibrium eliminated significant differences between corresponding isotherms. Sorption thus appeared to be controlled by M²⁺ activity. The data supported the hypothesis that MSO⁰₄ complexes were not sorbed and MCl⁺ complexes sorbed only slightly if at all. Nickel sorption was greater than Sr sorption under similar conditions. The results illustrate the importance of correcting for ionic strength/ion pairing effects before making comparisons among isotherms.

¹ Journal Article no. 923, Agric. Exp. Stn. New Mexico State Univ., Las Cruces, NM 88003.

² Soil Scientist, U.S. Water Conservation Laboratory, Phoenix, Ariz. (formerly Graduate Research Assistant, Dep. of Agronomy), and Professor of Agronomy, respectively, New Mexico State University.

Received for publication February 9, 1982. Accepted for publication May 3, 1982.