Prominent role of mineralogy in thallium and arsenic environmental cycling during weathering of subsurface mine waste: From rock to soil.

Weathering of sulfide-bearing mine waste poses long-term environmental risks through coupled acid generation and release of toxic elements, notably thallium (Tl) and arsenic (As). Despite their high toxicity, the mineralogical controls on Tl and As sequestration during progressive weathering remain poorly constrained. Here, we elucidate the divergent migration and attenuation pathways of Tl and As by identifying their dominant hosts across distinct weathering stages. In unweathered mine waste, LA-ICP-MS reveals enrichment of Tl and As in pyrite structural defects and grain margins, rendering them susceptible to preferential release during oxidative dissolution. In weakly weathered layers, Tl and As concentrations reach up to 710 and 2090 mg kg-1, respectively, with both elements predominantly retained in the residual fraction. Combined XRD, SEM-EDS, and EPMA confirm that jarosite serves as the dominant host for Tl and As, with their association consistent with structural incorporation via Tl+-K+ and arsenate-sulfate substitution. During advanced weathering, rising pH initiates gradual jarosite dissolution. In contrast to Tl, which is efficiently scavenged by pedogenic Mn oxides in the regolith, As undergoes progressive depletion, reflecting its limited retention by Fe-(hydr)oxides under near-neutral pH conditions. These findings demonstrate that the dynamic evolution of secondary mineral assemblages governs the decoupling of Tl and As cycling during weathering, providing a predictive mineralogical framework for stage-specific remediation strategies to mitigate their release.