Development and optimization of a bio-flotation process for hexavalent chromium removal using a microbial biosurfactant produced by Bacillus paramycoides

The release of hexavalent chromium (Cr(VI)) into ecosystems pose a serious threat to both environment and public health due to its high toxicity, mobility, and persistence in aqueous systems. Conventional remediation methods, including chemical reduction–precipitation, adsorption, and membrane processes, are often limited by high chemical consumption, operational costs, and the generation of secondary wastes. These limitations highlight the need for sustainable and cost-effective technologies, particularly for dilute Cr(VI)-contaminated effluents. In this context, the present study investigates ionic bio-flotation as an alternative approach to overcome these constrains. Cr(VI) removal was evaluated using a microbial biosurfactant (BioS) produced by a Bacillus paramycoides isolated from olive oil waste-contaminated soil. This strain exhibited non-pathogenic traits and was sensitive to common antibiotics (inhibition zones: amikacin 24 mm; gentamicin 20 mm), with no detectable protease or lipase activities, supporting safe handling during BioS production. The extracted BioS was characterized by surface tension measurements, FTIR spectroscopy, biochemical composition analysis, and CHNS elemental analysis, indicating its lipopeptidic nature. Microflotation experiments were conducted, and key process parameters were optimized using the Box-Behnken design. Under optimal conditions (air flow rate: 0.69 L min−1, BioS concentration: 172.69 mg l−1, pH 2), a maximum Cr(VI) removal efficiency of 93.67% was achieved. Mechanistic investigations based on zeta potential and UV–visible analyses revealed that the removal process is governed by electrostatic interactions between dichromate ions and protonated amine groups of the BioS. These results demonstrate the potential of BioS as an eco-friendly collector for the efficient Cr(VI) removal from wastewater.