Solution Composition Effects on Cadmium Sorption by Forest Soil Profiles

Abstract

AbstractCadmium 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 cmolc 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 (Cdsorbed) decreased 81% when free Ca2+ increased from 0.09 mM Ca2+ in sodium sulfate to 4.0 mM Ca2+ in calcium sulfate solution. The Cdsorbed increased with increased natural soil pH. A horizons had higher Cdsorbed than B horizons of similar natural soil pH, possibly due to their higher organic matter contents.