In this study, a biochar-supported polycarboxylic acid-functionalized nanoiron oxide-encapsulated composite (BC@nFe-CA) was synthesized by the pyrolysis of rice husk biomass, nZVI reductive deposition and surface functionalization under mild conditions for Cd(II), EDTA and Cd-EDTA wastewater treatment. Physical and chemical properties of BC@nFe-CA were characterized by various spectral techniques; the effects of initial pH, contact time, initial concentration, etc., and the mechanism on Cd(II) and EDTA adsorption by BC@nFe-CA were explored by batch experiments and spectral techniques. The isotherm results indicated that Langmuir model can well describe the equilibrium data with the maximum BC@nFe-CA adsorption capacities of 63.84 mg g −1 toward Cd(II) and 50.27 mg g −1 for EDTA at pH 5.5 (± 0.5) and 40 °C, and the Cd(II) and EDTA adsorption process was quick, endothermic and spontaneous. Furthermore, Cd-EDTA decomplexation experiment was carried out in the presence of H 2 O 2 and BC@nFe-CA to evaluate the dominant catalytic role of reactive oxygen species (ROS). It was found that ROS generated in the BC@nFe-CA/H 2 O 2 system was responsible for Cd-EDTA decomplexation. The stabilization energies of BC@nFe-CA loaded with Cd(II) and EDTA were analyzed by density functional theory (DFT) calculation, and the possible decomplexation pathway of Cd-EDTA was proposed. Finally, high-purity (approximately 99.8%) cadmium salt crystals were obtained through the pickling-oxidation-evaporation crystallization (POEC) process. This work develops a feasible route for the application of rice husk biomass, unravels the interaction of BC@nFe-CA with Cd(II), EDTA and Cd-EDTA under various conditions, and provides deep insight into Cd-EDTA decomplexation in the presence of BC@nFe-CA and ROS. Graphical abstract