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Solution Composition Effects on Cadmium Sorption by Forest Soil Profiles¹

ABSTRACT

Cadmium sorption can reduce Cd transport through sewage sludge-amended forest soils. Three mineral soil horizons from each of six western Washington State soil series were evaluated for their potential to sorb Cd. Horizons varied in pH from 3.6 to 6.4, in clay content from 100 to 470 g kg^–1, in organic C from 3 to 87 g kg^–1, and in cation exchange capacity from 1.8 to 35.8 cmol_c kg^–1. Batch equilibrium experiments were conducted in three background solutions (calcium nitrate, calcium sulfate, and sodium sulfate) that had ionic strengths of 0.016 and were maintained at pH values close to the natural soil pH. Between 4 and 97% of the initial 10 µM Cd was sorbed by 1 g soil in 0.1-L background solution. Sorption varied among the horizons in each soil profile, but there was no consistent trend with depth among the six profiles. Sorption did not differ significantly between calcium nitrate and calcium sulfate solutions. However, it was significantly lower in calcium sulfate than sodium sulfate solution, supporting the hypothesis that calcium competition lowers Cd sorption. The amount of Cd sorbed when a solution concentration of 1 µM free Cd²⁺ is maintained (Cd_sorbed) decreased 81% when free Ca²⁺ increased from 0.09 mM Ca²⁺ in sodium sulfate to 4.0 mM Ca²⁺ in calcium sulfate solution. The Cd_sorbed increased with increased natural soil pH. A horizons had higher Cd_sorbed than B horizons of similar natural soil pH, possibly due to their higher organic matter contents.

Key Words: Acidic soils • Competition by calcium • Complexation with sulfate • Sewage sludge leachate

¹ Contribution from the College of Forest Resources, Univ. of Washington, Seattle, WA 98195.

² Graduate research assistant and associate professor of forest soils, respectively; present address of second author is Department of Land, Air and Water Resources, Univ. of California, Davis, CA 95616.

Received for publication June 27, 1986.