Effects of dissolved oxygen, pH, salinity and humic acid on the release of metal ions from PbS, CuS and ZnS during a simulated storm event.

Metal sulfides serve as the sinks of toxic heavy metals in anoxic sediments. Once exposed to fluctuations in environmental conditions, dissolution of metal sulfides can occur and release toxic heavy metal ions into water column. In this study, we investigated the effects of dissolved oxygen (DO), pH, salinity and humic acid (HA) on the dissolution of CuS, PbS and ZnS using batch experiments with continuous aeration to simulate the re-suspension of these metal sulfides during a storm event. Experimental results indicated that oxidative dissolution of CuS, PbS and ZnS occurred in the presence of DO. The rate was the fastest for PbS, followed by CuS and ZnS. Soluble metal concentrations also increased with decreasing pH under oxic conditions. Compared to metal sulfides dissolution rates in freshwater without HA, the presence of saline conditions and HA generally accelerated the dissolution of CuS but inhibited the dissolution of PbS and ZnS. A higher saline or HA concentration, however, did not always lead to a stronger enhanced or inhibited dissolution rate, which could be a collective effect resulting from ligand-enhanced dissolution, complexation, and decreased oxygen and proton attacks due to HA adsorption on the metal sulfide surfaces.