Cauliflower stem-derived biochar for effective adsorption and reduction of hexavalent chromium in synthetic wastewater: A sustainable approach

Abstract

Contamination with hexavalent chromium (Cr(VI)) has drawn immense interest of national and global regulatory authorities due to its toxic and carcinogenic threat. Although various approaches have been applied for Cr(VI) removal from aqueous solution, adsorption has been proven to be the most promising method. Biochar, a cost-effective adsorbent, is one of the prominent adsorbents for environmental remediation that can be produced from different biowaste precursors. Cauliflower (Brassica oleracea) is one of the common vegetables around the world. The cauliflower stem is not commonly edible and therefore discarded as waste. In this work, the cauliflower waste-based biochar, with and without chemical modification, was prepared from a simple oxygen-limited pyrolysis. With the assistance of TGA, DSC, and SEM-EDS, the effect of pyrolysis temperature on the generation of biochar, reaction energy as well as reaction temperature, and morphology of the prepared biochar were analyzed. Pyrolytic temperature of 450°C was found to be the best for tailoring biochar. Chromium(VI) removal from the synthetic solution was performed by using biochar as an adsorbent. H3PO4-activated biochar (PBC) performs better than other biochar. The adsorption kinetics, isotherm, and thermodynamic parameters were studied in batch-wise experiments by varying the contact time, initial Cr(VI) concentration, and operating temperature. The Cr(VI) removal on PBC biochar follows a pseudo-second-order kinetics and fits with Langmuir isotherm resulting in Cr(VI) adsorption capacity of 64.10 mg/g. The thermodynamic parameters and XPS analysis of spent biochar adsorbent reveal that Cr(VI) reduction to Cr(III) happens simultaneously during the removal process. The spent biochar can be regenerated using 0.1 M NaCl and used for several cycles. Thus, the cauliflower stem-derived biochar can be effectively applied for Cr(VI) removal for multiple cycles through regeneration, which would contribute to the circular economy for environmental purposes.