nFeO decorated wood biochar as an adsorbent for aqueous Cr(VI) ions: Hyphenated, coagulation-column treatment of tannery effluent

Tannery effluent harbors an extensive array of pollutants, whose direct release into ecosystems could potentially endanger the environment. One of the most concerning pollutants found in tannery effluent is Cr(VI). This study endeavors to assess the efficacy of adsorbent materialfor the adsorption of toxic Cr(VI) from synthetic water, subsequently employing an integrated coagulation-adsorption system to treat actual tannery effluent. The biochar adsorbent was prepared by thermal conversion of Bauhinia variegata wood at high temperatures. The results were corroborated with isotherm and kinetic modeling with the characterization of prepared biochar by the analytical techniques including SEM-EDAX, BET–N_2, XRD, FTIR, and LIBS. SEM results showed the porous structure of biochar while EDAX and LIBS analysis confirmed the presence of chromium (Cr) and iron (Fe) over the surface of the adsorbent material. The data obtained from the adsorption experiments conducted under optimum conditions revealed that the Langmuir isotherm exhibited a remarkable maximum adsorption capacity of 98.56 mgg⁻¹.The study proposed that the adsorption mechanism of Cr(VI) onto both adsorbents could involve chemisorption or ion exchange processes, given the effective fit of the adsorption data to the PSO model. The experimental results from the column adsorption investigation were well fit by the Thomas model. The repeated adsorption and desorption of Cr ions for a total of 10 cycles demonstrated the recyclability of the utilized biochar. A comprehensive investigation was conducted to examine the impact of coagulant dosage (FeSO₄ and Ca(OH)₂) and adsorbent parameters in the effective elimination of toxic Cr(VI) and organic contaminants from the tannery effluent sample. The removal efficiency for the major contaminants e.g. COD, Cl⁻ and Cr(VI) in tannery effluent by the combined process was 95, 89, and 96 %, respectively. Thus this study suggests that coagulation followed by biochar filtration can be a low-cost alternative to energy-intensive conventional tannery waste treatment.