Polyethyleneimine-Modified Magnetic Multivalent Iron Derived from Iron-Based Waterwork Sludge for Cr(VI) Adsorption and Reduction

In this study, activated carbon, iron-based waterwork sludge, and polyethyleneimine (PEI) were employed as the primary raw materials to synthesize the composite PEI@MMI(800) under the optimized conditions identified through experimental investigations. The resulting composite was employed as an adsorbent for static Cr(VI) adsorption tests. The results demonstrated that increasing the pH from 2 to 9 significantly decreased the Cr(VI) adsorption capacity from 41.09 mg/g to 15.75 mg/g. The adsorption process was well described by both the pseudo-second-order kinetic model and the Langmuir isotherm model. Thermodynamic analysis revealed that the adsorption process was spontaneous and endothermic in nature. The presence of anions (Cl−, SO42−, and PO43−) negatively impacted Cr(VI) adsorption, with their inhibitory effects following the order Cl− < SO42− < PO43−. Moreover, higher concentrations of these anions led to reduced Cr(VI) adsorption efficiency. After six cycles of use, PEI@MMI(800) retained 79.80% of its initial Cr(VI) adsorption capacity, indicating a loss of 20.20%. Based on the comprehensive characterization of the adsorbent and the results of the Cr(VI) adsorption tests, it was concluded that the removal of Cr(VI) by PEI@MMI(800) involved a combination of electrostatic adsorption, chelation of Cr(VI) by PEI, and reduction of Cr(VI) to Cr(III).