Purification, Characterization, and Oil-Displacement Performance of Rhamnolipid Biosurfactant Produced by Bacillus sp. DQ-4.

Rhamnolipids are attractive biosurfactants for enhanced oil recovery, but the commonly used producing strains may raise biosafety concerns. In this study, a rhamnolipid-producing isolate, designated Bacillus sp. DQ-4, was obtained from oily sludge and cultivated in a glucose-based fermentation medium. The purified product, DQ-Rha, was characterized by TLC, FTIR, MALDI-TOF MS, and 1D/2D NMR (1H, 13C, 1H-1H COSY, 1H-13C HSQC, and HMBC). The combined spectroscopic results were consistent with a rhamnolipid structure, and no obvious conflicting signals were detected. DQ-Rha reduced the surface tension to 33.4 mN/m, and the DQ-4 supernatant showed an oil-spreading diameter of 89.3 mm and an emulsification index of 72.3% against diesel. In oil-displacement-related evaluations, DQ-Rha gave an oil-washing efficiency of 54.62%. In etched micromodel experiments, the total recovery factor reached 60.06%, which was comparable to that of commercial rhamnolipid (59.53%). In core flooding experiments at 85°C, injection of DQ-Rha after primary water flooding further increased the recovery factor by 10.56%, again showing performance comparable to commercial rhamnolipid. Acute oral toxicity testing in ICR mice showed no mortality or obvious toxic symptoms at 5040.6 mg/kg, and the acute oral LD50 was greater than 5000 mg/kg under the test conditions. These results suggested that DQ-Rha was a rhamnolipid biosurfactant with favorable oil-displacement-related performance and low acute oral toxicity.