Adsorptive Separation and Simultaneous Reduction of Highly Toxic Chromium Oxyanions by Agroforestry Biomass-Derived N-Rich Activated Carbon

Abstract

Hexavalent chromium, a potentially toxic micropollutant in industrial effluents due to anthropogenic activities, contaminates the water and poses severe health hazards. The present work demonstrates a scalable approach to synthesize nitrogen-doped activated carbon from agroforestry waste biomass for simultaneous adsorption and reduction of hexavalent chromium into remarkably less toxic Cr(III) species. Bhimal fiber (agroforestry waste biomass)-derived nitrogen-doped activated carbon (BFNAC) enriched with graphitic domains exhibits high surface area (1527 m²·g^–1) and plentiful nitrogen functionalities (12.6 at % nitrogen). The chemical characterization based on XPS analysis revealed the amine, pyrrolic, pyridinic, protonated nitrogen, hydroxyl, and carboxylic functionalities on the surface of BFNAC. These functionalities facilitated the adsorption of chromium oxyanions via electrostatic interactions under acidic pH. The BFNAC showed significantly higher adsorption of chromium oxyanions (255 mg·g^–1) compared to the corresponding activated carbon without nitrogen doping (BFAC; 168 mg·g^–1), revealing the role of nitrogen-based functionalities to aid the adsorption of chromium oxyanions. XPS analysis revealed the reduction of highly toxic hexavalent chromium oxyanions into notably less toxic trivalent species during the adsorption, mitigating the hazardous effect of Cr(VI) contaminants. The BFNAC exhibited pH swing-driven excellent recyclability and maintained >99% removal of chromium oxyanions even after 7 adsorption–desorption cycles. These findings promise lignocellulosic biomass-based nitrogen-doped carbons as potential adsorbents to remove the toxic and hazardous micropollutants from wastewater.