The overlooked oxidative dissolution of silver sulfide nanoparticles by thermal activation of persulfate: Processes, mechanisms, and influencing factors.

The widely occurring silver sulfide nanoparticles (AgS-NPs) are regarded as stable Ag species in subsurface environments, where are often disturbed by human activities, such as the application of advanced oxidation technologies (e.g. persulfate based in situ chemical oxidation (PS-ISCO)) in the remediation of contaminated soil and groundwater. However, stability of AgS-NPs was rarely investigated referring to these processes. Here, we systematically investigated the dissolution process of AgS-NPs in thermal activation of PS system. Results showed that dissolution of AgS-NPs fitted the pseudo-first-order kinetics and the k increased from 0.017 h to 0.249 h with increasing PS concentration from 2 mM to 10 mM (36 h, 40 °C). Quenching experiments and EPR results showed that sulfate radical (SO) and hydroxyl radical (OH) were the dominant oxidants in inducing the oxidative dissolution of AgS-NPs. XPS analysis showed that surface-bound S in AgS-NPs was oxidized and transformed into aqueous sulfur species. The released Ag may also act as effective catalysts to activate PS and therefore promote the oxidation process. These findings suggest that stability of AgS-NPs should be reevaluated to better understand its risk to the ecological system in the subsurface environment where ISCO was widely applied.