A recyclable rhamnolipid-modified magnetic bimetallic biochar demulsifier for oil-water separation in cold and saline environments

Stable oil-in-water emulsions in cold marine environments are difficult to separate because of low temperature, high salinity, and oil weathering. In this study, a recyclable rhamnolipid-modified magnetic bimetallic biochar demulsifier (FeCo-RBC) was developed for rapid oil-water separation under cold and saline conditions. Cobalt served as the main active metal, while iron enabled magnetic recovery. Response surface methodology showed that demulsifier dosage, initial oil volume fraction in emulsion, and salinity significantly affected the demulsification efficiency of the new demulsifier, whereas temperature was not significant within 4-20 °C. Under the optimal condition of 4 °C, 50 mg/L, 1.0‰ initial oil concentration, and 31.56‰ salinity from desirability-based optimization, the predicted demulsification efficiency was 94.53%, and the experimental value reached 100% in 5 min. FeCo-RBC showed pseudo-first-order kinetics (k_obs = 6.27 × 10⁻³ s⁻¹), a demulsification capacity of 311.00 ± 2.47 mg oil/mg demulsifier, and 93.91% efficiency after five cycles without washing. These results support its use in low-temperature marine oil spill response.