Differential Retention or Release Behavior of Thallium Driven by Oxygen-Dependent Transformation of Iron Sulfides

The oxidation of ubiquitous iron sulfides during the transition from anoxic conditions to oxic conditions is among the primary sources of heavy metal thallium (Tl) release into the environment. However, the migration behavior and transformation mechanism of Tl during the oxidation process of Tl-bearing iron sulfides are ambiguous. In the strongly acidic environments of this study, substantial Tl release under suboxic conditions is controlled by iron sulfide dissolution, whereas intense Tl release under oxic conditions is affected by a combination of iron sulfide dissolution and oxidation. In a circumneutral environment (pH 5–7), the abundant active sulfide groups in the Tl–FeS system facilitate the gradual conversion of surface-adsorbed Tl into Tl sulfide precipitates under suboxic conditions, which ensures the long-term stable retention of Tl (>98.8%). In comparison, under oxic conditions, FeS is converted to iron sulfates and iron oxides during intense oxidation processes. While these oxidation products can retain partial Tl through coprecipitation and surface adsorption, 60.6%–72.3% of the initially adsorbed Tl is subsequently released into the solution because of the decreased binding affinity. These findings provide more comprehensive insights into the geochemical behavior of Tl in environments related to sulfur and iron cycling.