Thallium in Pb-Zn smelting waste: Speciation, stability, and environmental risks

Thallium (Tl) pollution has become an increasing concern in non-ferrous metal smelting. Understanding Tl speciation and release behavior in smelting waste is essential for risk control, but relevant research remains limited. This study investigates typical heavy metal sulfide sludge (HMSS) from two smelters in China, which is generated by the lime-sulfide precipitation. The speciation and distribution of Tl in HMSS was determined using X-ray diffraction, X-ray photoelectron spectroscopy, scanning electron microscopy-energy dispersive spectroscopy, as well as sequential extraction protocol. The release behavior and ecological risk were assessed using pH-dependent tests and Risk Assessment Code. It was found that: (1) HMSS consisted of heavy metal sulfides/oxides and gypsum; (2) the solids content of Tl in HMSS was extremely high (up to 35,098.81 mg/kg). Toxicity leaching tests showed that Tl concentration in leachate reached 1127.20 mg/L, far exceeding other heavy metals (e.g., Pb, Zn, Cd); (3) a modified sequential extraction revealed that Tl existed in water-extractable (15.45 %-31.11 %), gypsum-incorporated (30.13 %-34.72 %), and stable (34.18 %-52.79 %) forms; (4) pH-dependent tests showed extensive Tl release across a wide pH range. Even at neutral pH (6-7), Tl was released at 9002.84 mg/kg, while Pb, Cd, and Zn remained below 20 mg/kg and leached mainly under strong acidity (pH < 2); (5) risk assessment indicated that Tl posed the highest ecological risk (60.83 %-67.38 %) than other heavy metals. This study reveals the easily overlooked environmental risks of Tl in HMSS, points out the urgency of harmless disposal of Tl-containing sludge wastes.