Photochemical ROS-Driven Autocatalytic Transformation of Heavy Metal–Xanthate Complex Pollutants in Tailings Pond

Heavy metal-xanthate complexes (HMXs), formed during processing in nonferrous metal mines, have attracted increasing attention due to their complex transformation pathways and ecological risks. Solar radiation plays a key role in driving the transformation of HMXs in tailings ponds. This study revealed an overlooked transformation pathway and environmental impact of mining pollutants─the photochemical ROS-Driven autocatalytic transformation of typical HMXs such as cadmium-xanthate (CdX). CdX conversion pathways and their kinetic characteristics were investigated by the comprehensive material characterization and substance analysis. Charge transferring and radical formation-attacking were demonstrated through the joint survey of electrochemical measurements, oxygen activation monitoring, and quantum chemical calculations. Results shows that 94.93% of CdX transformed after 28 days of solar exposure. The autocatalytic kinetic matches the accelerated characteristics described by the logistic model, with its half-lives varied significantly among different types of HMX: zinc-xanthate (ZnX) (4.3 days) < CdX (19.2 days) < plumbum-xanthate (PbX) (23.6 days). Light-induced charge carriers in CdX initiated the ROS production and its conversion to cadmium sulfide (CdS). CdS accumulation enhanced the photochemical activity and the ROS generation, resulting in the autocatalytic conversion. This work provides molecular-level insights into the transformation mechanisms of HMXs, and offers theoretical foundations for controlling mine-related pollutants.