Reductive transformation of aqueous Tl(III) at the pyrite-water interface: Reactive site dynamics-controlled kinetics and electron transfer mechanisms

The interfacial redox transformations of toxic thallium (Tl) at mineral–water interfaces play a critical role in governing its speciation and environmental fate. While pyrite serves as both a major source and potential sink for Tl in sulfide-rich environments, the redox reaction kinetics of Tl and the underlying mechanisms remain poorly understood. In this study, biphasic kinetics were observed for the reduction of Tl(III) at the pyrite-water interface under acidic oxic conditions. It was identified that surface disulfide (S(-I)) rather than Fe(II) acts as the predominant reactive site responsible for Tl(III) reduction, with kinetic transitions governed by a dynamic change of pyrite reactivity due to the site consumption and exposure. The inner-sphere electron transfer from S(-I) to Tl(III) induced the oxidative dissolution of pyrite and the turnover of S(-I) sites. Furthermore, superoxide generated during pyrite oxidation was also demonstrated to contribute secondarily to homogeneous Tl(III) reduction. Molecular-scale evidence indicates that preferential interaction of Tl(III) with S(-I) arises from lower adsorption energy, enhanced electronic state occupancy, and stronger Tl-S bonding within the disulfide bridge group. These findings establish a mechanistic link between Tl interfacial transformation with pyrite reactivity dynamics, offering critical insights into the pyrite-mediated redox chemistry of Tl in sulfide-rich aquatic environments.