Enhanced removal of Cr(VI) by reductive sorption with surface-modified Ti3C2Tx MXene nanocomposites

Being one of the most toxic pollutants commonly present in wastewater as soluble hexavalent chromium (Cr(VI)) specie, its concomitant adsorption and chemical reduction to relatively insoluble and less poisonous Cr(III) would be an effective strategy for remediation of Cr-polluted water. As such, a novel MXene-polyelectrolyte nanocomposite was typically synthesized for enhanced Cr(VI) removal. In detail, we applied polyethylenimine (PEI) to modify the Ti3C2Tx surfaces and synthesized Ti3C2Tx/PEI nanocomposites. The as-prepared composites were characterized by various methods such as SEM-EDX, N2 sorption-desorption isotherm, XRD, FTIR, and XPS. Further, the Ti3C2Tx/PEI was used to adsorb Cr(VI) for clarifying the kinetics, isotherms, and the effects of solution pH and coexisting ions on Cr(VI) removal by batch experiments. Our results show that coating of Ti3C2Tx nanosheets with PEI regulates surface charge of the as-synthesized adsorbent and creates it with a threedimensional network, allowing the fast electrostatic adsorption of Cr(VI) anions to positively charged surface over wide pH range of 1-10 and promoting to achieve improved removal capacity maximum up to 186.5 mg/g. The Cr(VI) adsorption data were well fitted by the pseudo-second-order kinetic and Langmuir isotherm models. In addition, Ti3C2Tx/PEI also exhibits excellent selectivity for Cr(VI) anions over competing ones such as NO3-, Cl-, ClO4-, and SO42-. Time-dependent reaction routes of Ti3C2Tx/PEI nanocomposite toward Cr(VI) was studied by XPS, which confirmed the slow reduction of all sorbed Cr(VI) to Cr(III) in the dozens of hours and final in-situ Cr(III) immobilization, clarifying the sorption-reduction mechanism for Cr(VI) elimination. All these results suggest that Ti3C2Tx/PEI nanocomposite prepared by reported surface-modification approach available for developing other functionalized adsorbents, may be a potential candidate for decontamination of Cr(VI)-polluted wastewater.