| Summary: | Abstract The elimination of hazardous metal ions from contaminated water has been an important procedure to improve the quality of the water source. Hence, this study presents the fabrication of Allium cepa extract-coated biochar for the elimination of Cr (VI) from wastewater. The synthesized biochar (SBCH) and modified biochar (BMOJ) were characterized by making use of FTIR, BET, XRD, TGA and SEM. Optimum Cr (VI) removal was achieved at solution pH 2, 0.05 g adsorbent dosage and 180 min agitation period. The adsorptive removal of Cr (VI) onto SBCH and BMOJ followed the pseudo-second-order kinetic model with a satisfactory sum of square residuals (SSR) of 3.874 and 5.245 for SBCH and BMOJ, respectively. Meanwhile, Freundlich isotherm was found to best describe the uptake of Cr (VI) SBCH and BMOJ. Experimental data showed an adsorption capacity of 37.38 and 25.77 mg g−1 and a maximum efficiency of 85.42% and 51.63% for BMOJ and SBCH, respectively. BMOJ also showed good antioxidant characteristics. Thermodynamic data revealed that the uptake of Cr (VI) onto the SBCH and BMOJ was an exothermic and endothermic (ΔH: SBCH = − 16.22 kJ mol−1 and BMOJ = 13.74 kJ mol−1), entropy-driven (ΔS: SBCH = 40.96 J K−1 mol−1 and BMOJ = 93.26 J K−1 mol−1) and spontaneous process. Furthermore, BMOJ demonstrated excellent reusability and promising characteristics for industrial applications.
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