Sequential acclimatization of Bacillus subtilis and Candida rugosa for sustainable bioremediation of wastewater containing cadmium, chromium, and synthetic dyes

Heavy metals and synthetic dyes in industrial wastewater represent a major environmental and public health concern due to their toxicity, persistence, and non-biodegradability. This study evaluates the synergistic bioremediation of heavy metals (cadmium and chromium) and recalcitrant synthetic dyes (methyl orange, crystal violet, and commercial textile dyes) externally added to Najafgarh drain wastewater using two different microbial systems comprising Bacillus subtilis and Candida rugosa . A 33 h, five-stage successive batch strategy exposed the microbial strains to increasing pollutant concentrations, up to 100 mg∙ L −1 for heavy metals and 10 mg∙ L −1 for dyes, to simulate acute real and synthetic industrial effluent stress. Despite pollutant-induced biomass suppression, progressive acclimatization enhanced the maximum specific growth rate from 0.326 h −1 to 0.768 h −1 . Both strains demonstrated exceptional detoxification capacities, sustaining >96% removal efficiency for heavy metals, with specific removal rates peaking at 2.05 mg∙g −1 ∙h −1 . Dye decolorization remained robust at peak loads (10 mg∙ L −1 ), achieving >80% for methyl orange and >96% for commercial dye mixtures. Acclimatization triggered a vital metabolic shift from cellular proliferation to intensified per-cell detoxification. This was evidenced by profound intracellular and extracellular enzymatic upregulations. Under maximal stress, specific superoxide dismutase activity in Bacillus subtilis surged to 5815 U∙ mg −1 , while specific laccase activity reached 203 U ∙mg −1 . These quantitative physiological adaptations confirm the complementary robustness of bacterial biosorption and yeast oxidative degradation under complex mixed-pollutant constraints. By validating an eco-friendly, highly adaptable, and scalable bioprocess for holistic wastewater management, this research directly advances the United Nations Sustainable Development Goals 6, 9, 12, and 13.