Insight into a novel composite biosorbent by incorporating chitosan into electrolytic manganese residue for Cr(VI) removal from aqueous solutions

A novel adsorbent was obtained by using a typical solid waste electrolytic manganese residue (EMR) and a green material chitosan (CS) for the Cr(VI) removal from the aqueous environment. The tagged mineral phases and elements in the EMR were successfully detected in the synthesized composites according to the XRD and SEM-EDS results. Furthermore, the characteristic peak of metal and hydroxyl group binding appeared at 779 cm−1, and the Si-O bending vibration at 454 cm−1 in the FT-IR pattern of the synthesized composites indicating the functional Fe and Mn hydroxyl groups and the silicates in EMR were all successfully grafted onto the CS. This obtained composite showed a high Cr adsorption capacity with a maximum adsorption capacity of 133.64 mg·g−1, and the Cr(VI) removal rate could maintain a high level of 94.6 % after five adsorption–desorption cycles. Its adsorption isotherm matched with the Langmuir model, and the adsorption kinetics followed the pseudo-first-order model. According to the response surface CCD model, the optimal conditions for Cr(VI) removal were pH at 2, contact time at 53.0 min, and initial concentration at 100 mg·L-1. For the removal mechanism, Cr(VI) anions could be easily electrostatic adsorbed by –NH3+, or chelated with the Fe and Mn oxides. Meanwhile, partial Cr(VI) (∼35.7 %) was reduced into Cr(III) by the abundant hydroxyl group of chitosan on the surface of the as-obtained materials, and then the free Cr(III) could be easily chelated with three chitosan monomers. This work opens up a novel and green methodology to modify chitosan and reuse the solid waste EMR.