Dynamic adsorption behaviors between Cu2+ ion and water‐insoluble amphoteric starch in aqueous solutions

Dynamic adsorption behavior between Cu2+ ion and water‐insoluble amphoteric starch was investigated. The sorption process occurs in two stages: external mass transport occurs in the early stage and intraparticle diffusion occurs in the long‐term stage. The diffusion rate of Cu2+ ion in both stages is concentration dependent. In the external mass‐transport process, the diffusion coefficient (D1) increases with increasing initial concentration in the low‐ (1 × 10−3‐4 × 10−3M) and high‐concentration regions (6 × 10−3‐10 × 10−3M). The values of adsorption activation energy (kd1) in the low‐ and high‐concentration regions are 15.46–24.67 and −1.80 to −11.57 kJ/mol, respectively. In the intraparticle diffusion process, the diffusion coefficient (D2) increases with increasing initial concentration in the low‐concentration region (1 × 10−3‐2 × 10−3M) and decreases with increasing initial concentration in the high‐concentration region (4 × 10−3‐10 × 10−3M). The kd2 values in the low‐ and high‐concentration regions are 9.96–15.30 and −15.53 to −10.71 kJ/mol, respectively. These results indicate that the diffusion process is endothermic in the low‐concentration region and is exothermic in the high‐concentration region for both stages. The external mass‐transport process is more concentration dependent than the intraparticle diffusion process in the high‐concentration region, and the dependence of concentration for both processes is about equal in the low‐concentration region.