Microbial degradation of tannery chrome-solid waste using Bacillus thuringiensis: optimization of collagen hydrolysate extraction via response surface methodology

Abstract

A significant amount of chrome-containing shaving dust is generated by the leather industry during leather processing, posing a threat to both the ecosystem and human health. Its widespread disposal, including the mixing of leachate with groundwater, leads to a decline in water quality, while incineration can convert Cr(III) into the carcinogenic Cr(VI). Since shaving dust contains a substantial amount of protein, discarding it without recovering this valuable resource would be a considerable waste, as it could be repurposed to produce various profitable goods such as protein hydrolysate. This study presents an efficient and eco-friendly approach for managing chrome shaving dust through microbial degradation using the Bacillus thuringiensis strain SRL4A (PP802975). The bacterium exhibited optimal growth after 36 hours of incubation at 45 °C, at pH 8, and demonstrated high chromium resistance, tolerating up to 900 ppm of Cr(III) salts. Response Surface Methodology (RSM) was employed in conjunction with Central Composite Design (CCD) to investigate the impact of independent variables (seed volume, nutrient source, and time) on the response variable degradation. The optimum values of the independent process parameters for maximum degradation (94.80%) were obtained at 35% (v/w) seed volume, 12.86% (w/w) nutrient source, and 101.27 hours of incubation time. Analysis of Variance (ANOVA) confirmed that the degradation of chrome shavings was primarily influenced by seed volume, followed by nutrient source and time. Hydrolysates were collected at various intervals during proteolysis, chromium was removed, and the samples were characterized. The final protein and chromium content in the hydrolysate were 979.44 ± 4.88 mg L⁻¹ and 1.40 ± 0.37 mg L⁻¹, respectively. UV-vis and FTIR spectroscopy analysis demonstrated characteristic protein peaks. TGA indicated the higher thermal stability of the recovered collagen hydrolysate. SEM revealed a porous protein sample, and EDX confirmed the presence of C, N, O, and S elements. The findings of this research provide valuable insights into developing sustainable strategies for managing tannery waste per principles of circular economy and environmental stewardship.