The geochemical release feature of Tl in Tl-rich pyrite mine wastes: a long-term leaching test

Identifying and revealing the geochemical behaviour of Tl during mine waste weathering are very important to assess the potential environmental impact of Thallium (Tl) from open mine-waste piles. Herein, two methods including the modified BCR sequential extraction and the long-term humidity cell tests (HCT) were employed to understand the Tl chemical fractions and to stimulate intense chemical weathering process, respectively. The results from BCR sequential extraction showed that the Tl concentration in the studied sample was 18.78 mg/kg, containing 1.878 mg/kg oxidisable, 0.282 mg/kg acid exchangeable and 1.596 mg/kg reducible Tl. The acid exchangeable fraction contributed to a particular potential risk to the aquatic marine life in the early stages and the Fe/Mn oxidisable fraction posed a potential risk being dissolved into solution at low pH (i.e. acidic conditions). The variations of Tl concentration in leachates were classified as two period as the pH values decrease. In the first period, the Tl concentrations decreased positively with pH value with poor correlation between pH value and SO₄^2- concentration in leachates. Drastic release of Tl was observed in the early period once the material was exposed to air and water, being ascribe to the acid exchangeable fraction bound to carbonate as dissolved by acid. During the second period, three increased peaks of Tl concentration (11.02, 16.03, 43.15 μg/L) and four increased peaks of SO₄^2- concentration (315, 390, 899.61 and 2670 mg/L) were observed. A good correlation (R ² = 0.8384) between the concentrations of Tl and SO₄^2- was observed, indicating the Tl was mainly released from the oxidation of sulphide.