Synergetic Tl and As retention in secondary minerals: An example of extreme arsenic and thallium pollution

Abstract

Secondary minerals could be effective scavengers of toxic arsenic (As) and thallium (Tl). In environments polluted by mining, these elements are abundant both in acid rock/mine drainage scenarios, as well as in carbonate-buffered environments. In this study we have investigated the behavior of As and Tl during weathering in mine waste dumps and an associated technosol sample from the Crven Dol locality (Allchar Tl-As-Sb-Au deposit, North Macedonia) contaminated with up to 142 g kg􀀀 1 of As and 18 g kg􀀀 1 of Tl, making it an As- and Tlextreme environment. We identified As and Tl reservoirs and discuss their difference from those observed in other naturally As- and Tl-rich environments. The pore waters show high concentrations of As (up to 196 mg L􀀀 1) and Tl (up to 660 μg L􀀀 1). Mild extractions mobilized up to 46% of the total Tl and 11% of the total As, indicating that a large amount of these toxic elements is weakly bound and can be easily mobilized into the environment. Apart from the recognition of Tl storage in several secondary phases (mainly as Tl(I) in members of the pharmacosiderite and jarosite groups, as well as Mn oxides, but also as very minor Tl(III) in other secondary phases), this study also provides the first evidence of Tl uptake by previously unknown thallium arsenate phases (with Tl: As ratios ~ 2 and 4), detected in carbonate-buffered (near-neutral pH) As- and Tl-rich technosols and waste dumps. These results indicate the need for further studies on Tl speciation in extremely As- and Tl-rich environments.