Comparative biosorption of hexavalent chromium by Enterobacter sp. Z11 and Bacillus sp. S7: Isotherm, kinetic, and mechanistic insights.

This study compares the hexavalent chromium [Cr(VI)] biosorption capacities of dried biomass from a Gram-negative bacterium (Enterobacter sp. Z11) and a Gram-positive bacterium (Bacillus sp. S7), given their inherent differences in their cell wall structure. The effects of pH, biomass dosage, contact time, and initial metal concentration were systematically examined. Both biomasses exhibited maximum Cr(VI) removal at pH 1.0, achieving removal efficiencies of 94% for Enterobacter sp. Z11 and 93.3% for Bacillus sp. S7 at an initial concentration of 200 mg/L. Biosorption equilibrium was achieved within 120 min, characterized by a rapid initial uptake followed by a slower diffusion-controlled phase. Increasing biomass dosage reduced biosorption capacity per gram due to aggregation of active sites and decreased surface availability. Isotherm studies revealed that the Freundlich and Temkin models provided the best fit, indicating multilayer biosorption on heterogeneous surfaces and suggesting that chemisorption-type interactions may contribute to the dominant mechanism. Fitting the full equilibrium dataset to the Langmuir model gave a theoretical maximum binding capacity for Enterobacter sp. Z11 (303 mg/g) and Bacillus sp. S7 (50 mg/g), notably higher than the capacity measured a single optimized concentration. Kinetic data fitted well to the pseudo-second-order model (R2 = 0.999), consistent with, though not proof of, chemically driven interactions. SEM-EDX and elemental mapping confirmed surface-localized Cr accumulation, with the involvement of hydroxyl, amino, carboxyl, phosphate, and carbonyl functional groups as revealed by FTIR. Overall, the results demonstrated that the biosorption performance of the Gram-negative Enterobacter sp. Z11 was significantly superior to that of the Gram-positive Bacillus sp. S7, indicating that Enterobacter sp. Z11 is a highly promising and efficient biosorbent for high efficiency Cr(VI) removal in sustainable bioremediation applications.