A method to determine silver partitioning and lability in soils

Abstract

There is increasing potential for pollution of soils by silver because of an increased use of this metal in consumer and industrial products. Silver may undergo reactions with soil components that mitigate its availability and potential toxicity, so that the total concentration of this metal in soil is not a useful indicator of potential risk. We developed an isotopic dilution method to simultaneously measure the partitioning (Kd-value) and lability (E-value) of Ag in soils, using the 110mAg isotope. An equilibration solution containing 10 mM Ca(NO3)2 was used along with a cation exchange resin to correct for possible interferences from non-isotopically exchangeable Ag associated with soil colloids in suspension (Er-value). The quantification limits for Kd and Er will depend on the amounts of radioisotope spiked and daily detection limits of inductively coupled plasma-mass spectrometry instrumentation but are typically >4000 L kg–1 and <0.92 mg kg–1. Measurement of Kd values for Ag in a range of soils indicated strong partitioning to the solid phase is positively associated with soil cation-exchange capacity or total organic carbon and pH. The concentrations of labile Ag in soils geogenically enriched in Ag were not detectable indicating occlusion of the Ag within poorly soluble solid phases. Measurement of labile Ag in soils spiked with a soluble Ag salt and aged for 2 weeks indicated rapid conversion of soluble Ag into non-isotopically exchangeable forms, either irreversibly adsorbed or precipitated in the soil. These results indicate that measurement of labile Ag will be important to estimate toxicity risks to soil organisms or to predict bioaccumulation through the food chain.