A kinetic approach to evaluate the association of acid volatile sulfide and simultaneously extracted metals in aquatic sediments.

The acid volatile sulfide (AVS) and simultaneously extracted metals (SigmaSEM) method is widely used for evaluating potential bioavailability of heavy metals in soil and sediment. It is also criticized, because the requirement that AVS and SEM metals (i.e., Cd, Cu, Ni, Pb, and Zn) are associated in the same phase is not always met. Here, we propose a dissolution-kinetics-based approach to assess whether AVS and SigmaSEM originate from the same phase, as a prescreening tool for SigmaSEM-AVS-based risk assessment or site characterization. Acid volatile sulfide and SEM metals from the same phase are assumed to yield equal dissolution rates. Therefore, dissolution rates for AVS and SEM metals were measured using a modified purge-and-trap method. Results were interpreted in terms of a shrinking particle model and a first-order model, which performed equally well. Of the SEM metals, only Cu showed reaction kinetics similar to those of AVS. Extraction of Fe and SEM-Zn (which constituted more than 90% of SigmaSEM) was much faster than AVS and did not fit to the models. This suggests that they are not associated with AVS but also that AVS is probably not present as sulfide minerals. These data illustrate that the SigmaSEM-AVS risk assessment concept would not be applicable for the studied sediments.