Transport of Cationic Silver in Soils

Abstract

Silver (Ag) is a broadly toxic element, and the potential for its release into the soil environment is increasing with its increasing use in consumer products. The first objective of this study was to quantify the transport of Ag in three soils having different properties. Miscible displacement experiments were carried out using pulses of AgNO3 (200 mg Ag L in 0.005 M Ca(NO3)2 background) applied to uniformly packed, water-saturated soil columns. Silver was mobile in all soils, with Ag breakthrough curve (BTC) having sharp to moderate influent fronts, high maximum concentrations relative to input, extensive tailing during leaching, and showing recoveries ranging from 65% to 84% of the amount applied. Differences in Ag mobility among the soils were consistent with results from sorption isotherm experiments. Another objective was to assess the influence of the presence of Zn on Ag transport using mixed pulses of 200 mg L Ag and Zn (as nitrates in 0.005 M Ca(NO3)2). For all soils, the presence of Zn resulted in earlier arrival of Ag in the effluent solution, indicating reduced affinity of Ag due to competition and leading to very high recovery Ag and Zn (95%) for the least retentive soil. The BTC were approximately described using a transport model with linear reversible and first-order irreversible sorption but were well described with a multireaction model, which accounts for nonlinear kinetic reversible and irreversible reactions. Consistent with high mobility, irreversible processes were of minor importance. As with BTC, the nonlinear transport model gave better description of residual Ag and Zn concentrations in the soil columns than the linear model.