Calcium-Based Amendments Mitigate Thallium and Cadmium Transfer from Mining-Impacted Paddy Soils to Rice (Oryza sativa L.)

Thallium (Tl) and cadmium (Cd) are highly toxic heavy metals that frequently co-occur in sulfide ores, posing a serious food safety risk through accumulation in rice. Although calcium-based (Ca-based) amendments have been widely applied to remediate heavy metal-contaminated soils, their effectiveness in Tl–Cd co-contaminated paddy soils remains unclear. A pot experiment was conducted to evaluate four Ca-based amendments—limestone powder, dolomite powder, hydrated lime, and oyster shell powder—on Tl and Cd bioavailability and uptake in paddy soil near a mining area. Ca-based amendments effectively reduced Tl and Cd bioavailability, with DTPA-Tl reducing by 11.2–17.2% and DTPA-Cd by 8.9–21.3%. These reductions were attributed to increased soil pH and decreased DOC, Fe, and Mn in the pore water. Additionally, Ca-based amendments shifted Tl and Cd from acid-extractable to residual fractions, reducing mobility. Additionally, Ca-based amendments promoted Fe/Mn plaque formation on rice roots, reducing Tl and Cd uptake. Consequently, Tl and Cd concentrations in brown rice decreased by over 14%, with the lowest levels observed under oyster shell powder. However, Cd concentrations still exceeded the maximum permissible limit, indicating that, although Ca-based amendments show considerable potential for in situ remediation of Tl–Cd co-contaminated paddy soils, further optimization and additional measures are required to achieve safe production.