Influence of pyrolysis temperature on the performance of cotton stalk biochar for hexavalent chromium removal from wastewater

Abstract

In recent years, biochar has gained popularity as green adsorbent when dealing with hexavalent chromium ions in the water environment. Herein, we prepared biochar at distinct pyrolysis temperatures, i.e., 300, 400, and 500 0C (denoted as CSB300, CSB400, and CSB500, respectively), using cotton stalks as biomass, and we investigated its impact on Cr (VI) removal. The characterization results based on SEM-EDS, BET, CHN, XRD, and FTIR analysis showed that the surface area, surface morphology, and elemental and functional group composition of CSB were considerably influenced by pyrolysis temperature. Despite having the lowest surface area, CSB500 presented superior adsorption performance. Batch sorption experiments with Cr (VI) solutions (20 mg/L) showed the optimal removal conditions to be pH (2), contact time (2 hours), and biochar dosage (3 g/L). Adsorption kinetics and isotherms were well defined by the Pseudo second order and Freundlich model. The thermodynamic studies showed the spontaneous and endothermic nature of the sorption process. The potential Cr (VI) removal mechanism mainly involved electrostatic attraction followed by Cr (VI) reduction to Cr (III) and complexation. Regeneration studies showed that the reusability aspect of the biochar could be improved, while the selected interfering ions had an insignificant effect on CSB500 adsorption capacity.