Nitrate removal from aqueous solutions by magnetic cationic hydrogel: Effect of electrostatic adsorption and mechanism

Excessive nitrate (NO3−) is among the most problematic surface water and groundwater pollutants. In this study, a type of magnetic cationic hydrogel (MCH) is employed for NO3− adsorption and well characterized herein. Its adsorption capacity is considerably pH-dependent and achieves the optimal adsorption (maximum NO3−-adsorption capacity is 95.88 ± 1.24 mg/g) when the pH level is 5.2–8.8. The fitting result using the homogeneous surface diffusion model indicates that the surface/film diffusion controls the adsorption rate, and NO3− approaches the center of MCH particles within 30 min. The diffusion coefficient (Ds) and external mass transfer coefficient (kF) in the liquid phase are 1.15 × 10−6 cm2/min and 4.5 × 10−6 cm/min, respectively. The MCH is employed to treat surface water that contains 10 mg/L of NO3−, and it is found that the optimal magnetic separation time is 1.6 min. The high-efficiency mass transfer and magnetic separation of MCH during the adsorption–regeneration process favors its application in surface water treatment. Furthermore, the study of the mechanism involved reveals that both –N+(CH3)3 groups and NO3− are convoluted in adsorption via electrostatic interactions. It is further found that ion exchange between NO3− and chlorine occurs.