Abstract
This study aims to synthesize activated magnetic biochar using a single-step approach and explore solute-solvent mechanisms for removing hexavalent chromium [Cr(VI)] from the aqueous solution. Agricultural waste rice husk was pre-treated in iron chloride and zinc chloride solution and pyrolyzed at 500°C. Obtained activated magnetic biochar were characterized by SEM, EDS, FTIR, XRD, VSM, and BET to analyze physicochemical features. Adsorption experiments indicated that activated magnetic biochar achieved an adsorption capacity of 9.97 mg/g with removal efficiency of 99.7% for an initial Cr(VI) concentration of 10 mg/L at pH 2 and 351 K. In aqueous solutions, Cr(VI) break down to HCrO4−, CrO42−, and Cr2O72− ions. However, at low pH (∼1–4), HCrO4− ion attributes electrostatic force of attraction, which encourages higher adsorption of Cr(VI) ions. Overall, batch experiments demonstrated that initial pH, Cr(VI) concentration, biochar dose, temperature, and co-existing ions significantly affected the Cr(VI) adsorption using activated magnetic biochar. Therefore, electrostatic interaction and ion exchange dominate Cr(VI) adsorption onto activated magnetic biochar at low pH. Experiments showed heterogeneous non-linear monolayer and spontaneous sorption, which were validated via Temkin isotherm and pseudo-second-order kinetic for Cr(VI) ions. As a result, Cr(VI) adsorption onto activated magnetic biochar occurred due to chemisorption on heterogeneous non-linear monolayers, mainly governed by electrostatic attraction and ion-exchange mechanisms due to enriched oxygen-containing surface groups with activated magnetic biochar. In terms of thermodynamics, the adsorption process for Cr(VI) ions using activated magnetic biochar was endothermic and spontaneous, as a positive ΔS° value indicates randomness during Cr(VI) adsorption at the solution-solute interface. In contrast, the negative value of ΔG° (−15855.3 kJ/mol) at 351 K shows that adsorption was spontaneous. Overall, the activated magnetic biochar synthesized from rice husk was more efficient than raw biochar in removing Cr(VI) ions from the aqueous solution.
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