Biogeochemical interaction between thallium (Tl) and schwertmannite in acidic environment and the anti-dissolution mechanisms of Tl(I)-coprecipitated schwertmannite.

Highly toxic thallium (Tl) can be released into the environment through acid mine drainage (AMD). However, our knowledge on the biogeochemical processes of Tl in such acidic, iron (Fe)-rich environments is limited. Here, we show that schwertmannite, a naturally formed Fe(III) mineral in AMD, can effectively immobilize Tl(I) through coprecipitation and adsorption. Tl(I) coprecipitation into schwertmannite removed a large portion of Tl(I) under a wide range of initial Tl(I) concentrations (0.01-1.0 mg/L) and within a short duration (48 h). The saturated adsorption capacities of the biosynthetic and chemically synthesized schwertmannite for Tl(I) (1.0 mg/L) were 1.96 and 1.59 mg/g, respectively, under acidic conditions (pH=3.0). The kinetic dissolution results indicated that biogenic Tl-coprecipitated schwertmannite exhibited greater stability, which was attributed mainly to the elevated extent of Tl oxidation and enhanced crystallinity of Tl-bearing schwertmannite. The extended X-ray absorption fine structure (EXAFS) analyses revealed that the incorporation of Tl into schwertmannite involves the heterovalent substitution of Fe(III) by the formation of double-corner sharing linkages between the Tl-O tetrahedra and Fe-O octahedra. These results suggested that coprecipitation combined with adsorption can achieve retention of Tl in acidic environment throughout the entire mineralization process of schwertmannite, which provides a comprehensive understanding of biogeochemical fate of Tl in AMD-affected areas.