Synthesis of novel lignosulfonate-modified graphene hydrogel for ultrahigh adsorption capacity of Cr(VI) from wastewater

Heavy metal contamination has prevented the sustainable and efficient utilization of wastewater. Biosorption is a promising process which means technical innovation and biomaterials preparation to remove pollutants from water solutions. In this study, a novel lignosulfonate-modified graphene hydrogel (LGH) was successfully prepared to remove Cr(VI) from aqueous solution. Results indicated the combination of lignosulfonate and graphene could significantly improve the adsorption performance of LGH. The obtained biomaterials LGH were characterized by FT-IR, XPS, BET, TGA, and SEM. Results showed that LGH has a high specific surface area, interconnected porous structure, and plenty of oxygen-containing groups, which are particularly suitable to adsorb heavy metals. Experimental results indicated an ultrahigh adsorption capacity of 1743.9 mg/g for Cr(VI) capture, and has been found to be the highest among the previously reported values on Cr(VI) adsorption by various lignin/lignosulfonate-based adsorbents. Systematic kinetic and isotherm studies were performed to investigate the adsorption behaviors of Cr(VI) by LGH. Results showed that the adsorption of Cr(VI) by LGH follows the pseudo-second-order model and the equilibrium data consistent with Langmuir isotherm model. The active groups presented in LGH were related to the binding of Cr(VI), and electrostatic adsorption and chemisorption could be the major adsorption mechanisms. The recycling and reuse of LGH was successfully achieved by using 0.1 M NaOH without destroying its original hydrogel structure, and more than 88% reuse efficiency of Cr(VI) (300 mg/L) was obtained after 5 cycles. The results suggest a promising approach by utilizing low-cost lignosulfonate for the high-potential purification and sustainable utilization of wastewater, especially for the high concentrations of heavy metal polluted wastewater treatment.