Enhanced Adsorption of Hexavalent Chromium from Aqueous Solution by Citric Acid-Modified Biochar from Invasive Plant Biomass

The Mimosa pigra, a noxious invasive plant, was used to produce biochar (MPBC), which was then modified with citric acid to synthesize engineered biochar (CABC) with improved adsorption properties. The effects of solution pH (3.0–9.0), Cr(VI) initial concentration (2–25 mg L⁻¹), contact time (up to 24 h), and temperature (25, 35, and 45 °C) on the efficacy of Cr(VI) removal by biochars were examined through batch sorption experiments. Furthermore, the mechanisms of the sorption process were revealed by spectroscopic analysis performed with X-ray photoelectron spectroscopy (XPS). Results demonstrated that both biochars have strong adsorption potential toward Cr(VI) in an acidic medium (pH 3.0) whereas CABC showed reasonably high adsorption selectivity for Cr(VI) (78%). The incorporation of O–C = O and -OH functional groups onto the adsorbent surface through the esterification reaction was found to increase the Cr(VI) sorption capacity of modified biochar by 15%. Experimental kinetics and isotherm data were well described by fractional power kinetic and Hill isotherm models with a maximum adsorption capacity of 14.5 mg g⁻¹. Thermodynamic studies prove the endothermic and spontaneous nature of the adsorption process. The post-adsorption spectroscopic analysis performed with XPS confirmed the coexistence of Cr(VI) and Cr(III) on the CABC surface. A three-step reaction mechanism involving electrostatic attraction, reduction, and complexation was responsible for Cr(VI) adsorption onto CABC. This work presents a win–win strategy that will limit the expansion of noxious plants by converting them into biochar and simultaneously removing the top priority pollutant (Cr) from water. Graphical Abstract: [Figure not available: see fulltext.]