Simultaneous immobilization of arsenic and cadmium in paddy soil with co-application of birnessite and lime

The remediation of arsenic (As) and cadmium (Cd) co-contaminated paddy soils is challenging due to their contrasting biogeochemical behaviors. This study innovatively used the combination of birnessite and lime to mitigate the release of As and Cd in paddy soils. Soil incubation experiments revealed that anaerobic conditions mobilized As, whereas aerobic conditions released Cd. The birnessite-lime treatment effectively addressed both processes, reducing anaerobic porewater As by 29.49 %-31.75 % and aerobic porewater Cd by 70.76 %-99.45 % on the last day. Lime application enhanced Cd immobilization by elevating pH, promoting retention of Cd in carbonate precipitation (T2) and Fe/Mn oxide phases (T3). However, lime application stimulated organic matter dissolution and Fe-reducing bacteria, decreasing Fe oxide-bound As (F3 +F4) but increasing specifically adsorbed As (F2), thus rendering the net effect on As dissolution dependent on the balance of these processes. Birnessite counteracted the lime's negative aspect on F3 +F4 via inhibiting anaerobic reductive dissolution of Fe oxides. Pot experiments confirmed that the combined treatment (0.1 % birnessite_0.5 % lime) reduced As and Cd uptake in rice seedlings by 13.26 %-37.57 % and 43.76 %-70.89 %, respectively. Notably, excessive lime (0.1 % birnessite_1 % lime) elevated As bioavailability, highlighting the importance of optimizing lime application dosing in soil-rice system. This redox-adsorption coupled strategy offers a cost-effective solution for As-Cd co-contaminated paddies and similar waterlogged ecosystems.