Dissolved black carbon induces fast photo-reduction of silver ions under simulated sunlight

Silver has been associated with significant environmental challenges. This study aimed to investigate the photo-reduction of silver ions (Ag+) mediated by dissolved black carbon (DBC), a vital dissolved organic matter released from black carbon. Suwannee River humic acid (SRHA) was used as a comparison group. DBC was observed to effectively mediate the photo-reduction of Ag+ to the silver nanoparticles (nAg) under simulated sunlight. The initial rate of formation of nAg mediated by DBC was approximately 3.0–9.5 times higher than that of nAg mediated by SRHA. The Ag+ concentration required to form nAg in the DBC solution was observed to be only 0.5 mg L−1, which was one-tenth of the concentration in the SRHA solution. Further, the mediation efficiency of DBC was higher than that of SRHA, which was attributed to the facilitated ligand-to-metal electron transfer and reduced generation of the oxidative phototransients. The Ag+ sorption played a crucial role during the photo-reduction process by altering the reaction pathway and local Ag+ concentration as well as affecting the initial rate of formation of nAg. Furthermore, the silver seeds generated by DBC are able to trigger the formation of nAg by SRHA at low Ag+ concentrations. Overall, the findings from this study highlight the vital role of DBC in mediating the sunlight-induced speciation of the silver ions.