To address the challenges posed by highly toxic Cr(VI)-containing wastewater from hydrometallurgical industries, three biochars (BC-C, BC-M, and BC-S) were prepared from coffee grounds, corn cobs, and sugarcane bagasse via oxygen-limited pyrolysis at 450°C. Batch adsorption experiments demonstrated that corn cob-derived biochar (BC-M) exhibited the highest Cr(VI) adsorption capacity of 76.8 mg/g at pH 1, an initial concentration of 100 mg/L, and 298.15 K, outperforming BC-C and BC-S. BC-M possessed the largest specific surface area and a unique three-dimensional interconnected honeycomb-like mesoporous structure, as confirmed by SEM and BET analyses. The Langmuir isotherm model predicted a maximum adsorption capacity of 223.30 mg/g at 318.15 K. Solution pH significantly influenced adsorption performance, with pH 1 identified as optimal. Characterization analyses revealed that C=O-containing functional groups on BC-M surfaces reduced Cr(VI) to Cr(III) through electrostatic attraction coupled with electron transfer; protonation of surface functional groups under strongly acidic conditions further enhanced Cr(VI) anion capture. The adsorption process followed the Freundlich model at 298.15 K and 308.15 K, indicating multilayer adsorption on heterogeneous surfaces, while tending toward Langmuir-type monolayer chemisorption at 318.15 K. This study provides theoretical insights into the valorization of agricultural waste biomass as efficient adsorbents for Cr(VI) removal from aqueous solutions.
Liu et al. (Sun,) studied this question.