Immobilization of Rhodococcus sp. BEI-13A on grape-waste biochar for hexavalent chromium decontamination from aqueous solutions

This study investigates the immobilization of Rhodococcus sp. BEI-13A on grape-waste biochar (BC@R) as a sustainable decontamination strategy. Biochar acted as a microbial support and as a matrix contributing to chromium retention, while the immobilized microorganism contributed to Cr(VI) transformation. Batch assays were performed under varying pH, BC@R dosage, initial Cr(VI) concentration, temperature, and contact time using a fractional factorial design to screen the effects of these variables on Cr(VI) decontamination efficiency. The highest Cr(VI) decontamination efficiency observed among the experimental runs (18.55%) was obtained at pH 6, 3 g L -1 BC@R, 350 mg L -1 Cr(VI), 25 °C, and 48 h; this condition represents the best observed treatment within the tested design rather than a confirmed global optimum. Statistical analysis indicated that only Cr(VI) concentration and temperature significantly affected system performance (p < 0.05). Kinetic modeling of the apparent specific Cr(VI) depletion response showed that BC was better described by the pseudo-first-order model, whereas BC@R was slightly better described by the pseudo-second-order model. For BC@R, the PSO model yielded a model-estimated apparent equilibrium Cr(VI) depletion value of 107.46 ± 9.78 mg g -1 . Because q t was calculated from the decrease in aqueous Cr(VI), these kinetic values integrate Cr(VI) disappearance associated with both aqueous-phase chromium removal and Cr(VI)-to-Cr(III) transformation and should not be interpreted as pure adsorption capacities. BC@R exhibited the highest initial volumetric Cr(VI) depletion rates among the tested treatment configurations and maintained activity throughout the 32 h kinetic assay. Although the overall decontamination efficiency was relatively low, the results demonstrate the feasibility of combining grape-waste biochar and immobilized Rhodococcus sp. BEI-13A for Cr(VI) decontamination under comparatively high chromium concentrations.