A remediation approach to chromium-contaminated water and soil using engineered biochar derived from peanut shell

Hexavalent chromium (Cr[VI]) is one of the major environmental concerns due to its excessive discharge through effluents from the leather tanning industry. Peanut production leads to the generation of residual shells as waste calling for sustainable disposal. In this study, we employed an innovative approach of applying peanut-shell-derived pristine and engineered biochar for the remediation of Cr-contaminated wastewater and soil. The peanut shell waste was converted to biochar, which was further engineered with cetyltrimethylammonium bromide (CTAB, a commonly used cationic surfactant). The biochars were then used for the adsorption and immobilization of Cr(VI) in water and soil, respectively. The adsorption experiments demonstrated high Cr(VI) removal efficiency for the engineered biochar (79.35%) compared with the pristine biochar (37.47%). The Langmuir model best described the Cr(VI) adsorption onto the biochars (R² > 0.97), indicating monolayer adsorption. Meanwhile, the adsorption kinetics indicated that chemisorption was the dominant mechanism of interaction between the Cr(VI) and the biochars, as indicated by the best fitting to the pseudo-second-order model (R² > 0.98). Adsorption through the fixed-bed column also presented higher Cr(VI) adsorption onto the engineered biochar (q_eq = 22.93 mg g^-1) than onto the pristine biochar (q_eq = 18.54 mg g^-1). In addition, the desorption rate was higher for the pristine biochar column (13.83 mg g^-1) than the engineered biochar column (10.45 mg g^-1), indicating that Cr(VI) was more strongly adsorbed onto the engineered biochar. A higher immobilization of Cr(VI) was observed in the soil with the engineered biochar than with the pristine biochar, as was confirmed by the significant decreases in the Cr(VI) bioavailability (92%), leachability (100%), and bioaccessibility (97%) compared with the control (soil without biochar). The CTAB-engineered biochar could thus potentially be used as an efficient adsorbent for the removal and the immobilization of Cr(VI) in water and soil, respectively.