Thallium in the environment: a critical review of chemistry, fate, and ecological risks in soil and water systems

Thallium (Tl), a highly toxic trace metal, poses severe ecological and health risks yet remains understudied relative to arsenic (As) and lead (Pb). This review synthesizes Tl’s environmental chemistry, fate, and impacts in soil and water systems, emphasizing its global significance. Tl(I), resembling alkali metals, dominates in most environments, exhibiting high solubility and weak sorption to Fe/Mn oxyhydroxides, unlike As and Pb. Tl(III) forms complexes with organic ligands, but limited thermodynamic data hinder speciation modeling. Anthropogenic sources, such as mining and coal combustion, elevate Tl in hotspots (e.g., China, Poland), with soil concentrations reaching 500 ppm and water levels exceeding WHO guidelines. Tl’s bioaccumulation in plants and food chains, coupled with its neurological and renal toxicity, underscores its public health threat. Critical knowledge gaps particularly Tl interactions with organic matter and regional baseline data, necessitate advanced methods like X-ray absorption spectroscopy and isotopic tracing. This review highlights Tl’s unique behavior and advocates for enhanced research and regulation to mitigate its environmental and health impacts.