Production of bioemulsifier by Serratia marcescens and its potential application in oily sludge washing remediation.

Oily sludge treatment remains a critical challenge in the petrochemical industry. Surfactants are widely used for oily sludge washing, but chemical surfactants induce secondary pollution, whereas bioemulsifiers (high-molecular-weight biosurfactants) enable eco-friendly treatment. However, their washing mechanisms remain unclear, and their application is hindered by limited washing efficiency, high extraction and purification costs, insufficient parameter optimization, unclear applicability to different types of oily sludge, and underexplored potential in froth flotation. To address these gaps, we screened and isolated a bioemulsifier-producing strain (Serratia marcescens SM1), which could produce a bioemulsifier (named SM1-BE). Emulsification tests verified its superior emulsifying activity across wide ranges of pH, temperature and salinity. Mechanistic studies revealed that SM1-BE efficiently washed oily sludge through three key functions: facilitating oil droplet detachment, emulsifying and stabilizing the dispersed oil droplets, and mitigating the re-adsorption risk of detached oil droplets. In thermochemical cleaning, the cell-free fermentation broth from strain SM1 achieved an oil removal rate 18% higher than that of sophorolipid. In froth flotation, the oil content of eluted solids was reduced to 17.86 mg/g, meeting local industrial control limits. Furthermore, applicable oily sludge types and feasible disposal pathways for flotation products were determined. This study produced a high-performance bioemulsifier, elucidated its mechanism of action, and established an efficient oily sludge washing system, providing new insights into bioemulsifier-enhanced sludge deoiling.