Biosorption of Chromium (VI) in Aqueous Solution by Fermented Sweet Sorghum Residues: Kinetics, Isotherm and Mechanism

Fermented sweet sorghum residues (FSSR) were developed as an eco-friendly and low-cost biosorbent for the removal of chromium(Cr)(VI) from aqueous solutions, and their adsorption kinetics, isotherm and mechanism, and practical application potential were systematically investigated. The results showed that under the conditions of an initial Cr(VI) concentration of 50 mg/L, pH = 2.0, adsorbent dosage of 0.1 g, and temperature of 303 K, the equilibrium adsorption capacity (qe) of FSSR for Cr(VI) reached 7.37 ± 0.44 mg/g, which was 61.6% higher than that of original sweet sorghum residues (SSR, 4.56 ± 0.19 mg/g); an independent-samples t-test confirmed that the difference was statistically significant (p < 0.05). Kinetic analysis revealed that Cr(VI) adsorption by FSSR followed the pseudo-second-order model (R2 > 0.91; k2 = 0.021–0.079 g/(mg·h)) and the Elovich model (R2 > 0.95; α = 0.306–0.502 mg/(g·h), β = 0.625–1.010 g/mg), indicating that chemisorption dominated the adsorption process, while equilibrium data were best described by the Freundlich isotherm model (R2 > 0.95, KF = 0.83–4.76 mg/g, n = 2.01–3.64), suggesting heterogeneous multi-layer adsorption. Advanced characterization techniques, including X-ray adsorption near-edge structure (XANES) and energy-dispersive spectroscopy (EDS), demonstrated a dual removal mechanism in which Cr(VI) was first reduced to Cr(III) on the surface of FSSR, followed by chemical adsorption of Cr(III). A comparison with biosorbents reported recently showed that FSSR exhibited competitive adsorption performance among the chemically modified biosorbents; although its maximum adsorption capacity was lower than that of some carbonized biosorbent, FSSR offers advantages in terms of cost-effectiveness and environmental sustainability, providing a feasible strategy for the valorization of agricultural residues and a sustainable alternative for the treatment of Cr(VI)-contaminated wastewater.