Polypyrrole confined in network of composite hydrogel as adsorbent for enhanced adsorption of Cr(VI) in aqueous solutions

The treatment of Cr(VI) wastewater containing hexavalent chromium is a difficult problem. The World Health Organization (WHO) "Drinking Water Quality Standard" stipulates that the maximum allowable limit of chromium in drinking water is 0.05 mg/L. It is very necessary to develop an adsorbent for efficiently removing Cr(VI) in water. In this paper, a composite porous hydrogel adsorbent of sepiolite/humic acid/polyvinyl alcohol@polypyrrole (SC/HA/PVA@PPy) was prepared by Pickering emulsion template-in-situ oxidative polymerization, and the enhancement effect of this design idea on the adsorption performance of PPy was studied with heavy metal ion hexavalent chromium Cr(VI) as the target adsorbate. The morphology and structure of the composite adsorbent were characterized by FESEM, EDX and FT-IR, which confirmed the successful compounding of PPy with SC/HA/PVA and effectively avoided the agglomeration of PPy. The adsorption performance of Cr(VI) was systematically studied through the experiments of solution pH, adsorption dosage, adsorption isotherm and adsorption kinetics. The adsorption capacity of SC/HA/PVA@PPy for Cr(VI) can reach 970.31 mg/g-PPy, which is about 44 times higher than that of pure PPy (21.87 mg/g), indicating that the experimental design can effectively avoid PPy agglomeration and exert its potential adsorption performance. The fitting results of adsorption isotherm are more in line with Langmuir model (R 2 = 0.912, 0.975, 0.991 at three temperatures), and the thermodynamic parameters reveal that the adsorption process is a spontaneous endothermic process. The adsorption kinetics proved that the adsorption process of Cr(VI) mainly followed the pseudo-second-order kinetic model (R 2 = 0.957). The regeneration study shows that the adsorbent can be recycled. In addition, according to the FESEM-EDX, FTIR and XPS analysis of the adsorbent before and after adsorption, the removal of Cr(VI) was mainly enhanced by ion exchange, electrostatic attraction and chemical reduction. The peak of Cr element in EDX spectrum after adsorption provide direct evidence for the adsorption of Cr(VI). SC/HA/PVA@PPy adsorbent has good adsorption performance and strong stability, which has great development prospects in wastewater treatment.