Dual − acid doped PANI/ZnO composites: Adsorption mechanisms and dynamic adsorption for Pb (II) removal

Heavy metal contamination in aquatic environments represents a critical global threat to water resource security. In this study, polyaniline/zinc oxide (PANI/ZnO) composites doped with hydrochloric acid and sulfuric acid were successfully synthesized to overcome the instability of composites resulting from the rapid dissolution of ZnO under low-pH conditions. Batch static adsorption experiments were performed, and both adsorption kinetic and isotherm models were systematically fitted to elucidate the adsorption behavior. The sulfuric acid-doped PANI/ZnO (PANI-SO 4 /ZnO) exhibited a considerably higher adsorption capacity of 396.5 mg g −1 compared with the hydrochloric acid-doped composite (PANI-Cl/ZnO, 297.1 mg g −1 ). This enhanced performance was attributed to the abundant adsorption sites in PANI-SO 4 /ZnO and the chemical precipitation mechanism involving Pb 2+ . The experimental results aligned well with the pseudo-second-order kinetic model and the Freundlich isotherm model (typical thermodynamic isotherm models), indicating that chemisorption and heterogeneous surface interactions governed the adsorption process. In cyclic and dynamic adsorption tests, PANI-Cl/ZnO displayed better reusability and mass-transfer stability. This variation originated from the irreversible formation of PbSO 4 precipitates in PANI-SO 4 /ZnO, which hindered regeneration, whereas Cl – in PANI-Cl/ZnO contributed to preserving the composite’s structural integrity. Although PANI-SO 4 /ZnO is suitable for short-term, rapid treatment of high-concentration Pb 2+ wastewater, its recyclability is restricted by the irreversibility of precipitation. In contrast, the high stability and effective mass transfer of PANI-Cl/ZnO render it an ideal material for continuous industrial wastewater treatment systems requiring long-term operation.