Thermal treatment-regulated decomposition and phase transformation of soda residue: product construction and mechanism of simultaneous removal of Cd2+ and Pb2+ from wastewater.

Cadmium (Cd) and lead (Pb) are recognized as priority pollutants owing to their high toxicity and environmental persistence. Concurrently, soda residue (SR), a by-product of the ammonia-soda process, is largely stockpiled due to ineffective utilization, posing significant environmental challenges. This study employs a "waste-treating-waste" strategy, achieving directional conversion of SR into an effective heavy metal removal material through controlled thermal treatment. The thermal transformation behavior of SR was systematically investigated using TG-DTG, XRF, XRD, and QXRD techniques, revealing a distinct three-stage evolution of mineral phases: low-temperature dehydration, medium-temperature decomposition, and high-temperature reconstruction. Under optimized conditions (calcination at 600°C for 3 h, initial pH 6, dosage 0.5 g/L, and reaction temperature 20°C), the thermally activated SR (SR-600) achieved simultaneous removal rates exceeding 99% for Cd2+ and 98% for Pb2+ from simulated wastewater. Multi-scale characterizations (SEM-EDS/BET/XRD/FTIR/Raman/XPS) revealed distinct mechanisms governing Cd2+ and Pb2+ removal. Pb2+ was immobilized primarily via chemical precipitation as stable Pb3(CO3)2(OH)2 crystalline phases. The removal of Cd2+ was synergistically achieved through four pathways: chemical precipitation, hydroxyl complexation, liquid-solid phase adsorption, and ion exchange. Specifically, the inherent phase CaCO3 and the thermal decomposition phase CaO in SR collectively promoted the precipitation/complexation/ion exchange of Cd2+ in the solution, while the reconstruction phases (Ca2SiO4 and CaAl2Si2O8·4H2O) after high-temperature phase transformation likely dominated the adsorption process of Cd2+ and its hydroxyl complexes. The final residue demonstrated excellent environmental safety in toxicity leaching tests. Furthermore, the estimated production cost is competitively low at approximately $1.58 per kilogram.