Efficient removal of Hg2+ by L-cysteine and polypyrrole-functionalized magnetic kaolin: condition optimization, model fitting and mechanism

To enhance the adsorption capacity and improve agglomeration properties of kaolin during the adsorption process of Hg 2+ from aqueous solution, kaolin was bi-functionally modified using L-cysteine and polypyrrole. The as-synthetized adsorbent of L-Ppy-Fe 3 O 4 /kaolin was characterized before and after modification. The removal conditions were optimized by response surface methodology, and the optimized results show that the optimal adsorption capacity of Hg 2+ reaches 482.71 mg/g at pH of 7.9, temperature of 315 K, initial concentration of 40 mg/L and dosage of 0.05 g/L. And the effects of adsorption conditions on Hg 2+ removal follow the order of solution pH > temperature > initial concentration > dosage. The adsorption process of Hg 2+ in aqueous solution can be well described by pseudo-second-order kinetic model and Langmuir isothermal model. Moreover, the thermodynamic fitting indicates that the adsorption of Hg 2+ is an endothermic and spontaneous process. The adsorption mechanism of Hg 2+ onto L-Ppy-Fe 3 O 4 /kaolin is mainly the complexation between divalent Hg 2+ and sulfhydryl/amino groups to form stable metal complexes, and electrostatic attraction is also a part of the reaction mechanism. In addition, L-Ppy-Fe 3 O 4 /kaolin is proved to have satisfactory dispersion ability and chemical stability and can be easily separated and recovered after adsorption in aqueous solution. Therefore, L-Ppy-Fe 3 O 4 /kaolin is a promising adsorbent for the efficient removal of mercury ions. Graphical abstract