Optimisation of BTX removal from simulated refinery effluent using hydrophobic DES: a response surface methodology approach

Abstract Petroleum refining generates effluents rich in monocyclic aromatics with toxic benzene, toluene and xylene (BTX) persisting in water, so their removal is an environmental priority. Here, a hydrophobic deep eutectic solvent (HDES) of tetrabutylammonium bromide and decanoic acid (1:2) was evaluated for liquid-liquid BTX extraction. A four-factor central composite design varied stirring speed, solvent mass fraction, contact time and temperature across 30 runs, analysed by response surface methodology (RSM). Benzene and toluene efficiencies were modelled on a logit scale to keep predictions physically bounded, and xylene on the untransformed response. The models returned R² values of 0.73 for benzene, 0.78 for toluene and 0.74 for xylene, with adjusted R² values of 0.65, 0.72 and 0.68 respectively. Contact time was not a significant factor and solvent mass fraction was the dominant one. Individually optimised predicted efficiencies were 98.7% for benzene, 97.6% for toluene and 82.2% for xylene, obtained at closely similar conditions near 446 rpm, a solvent mass fraction of 0.20 and 55 °C. At independent test conditions within the design space, the model-to-experiment deviations were 2.1% for benzene and 10.0% for toluene, at or within the 10% acceptance criterion, while the xylene model deviated by 19.7% and is not considered validated for prediction. Benzene extraction stayed high in real refinery wastewater, above 95.8% by UV-Vis estimate. The observed extraction order, benzene > toluene > xylene, may reflect differences in the aqueous phase availability of the three analytes rather than hydrophobicity alone. Efficiencies are reported as aqueous phase depletion measured by UV-Vis, without HDES-free controls or chromatographic confirmation. Within these limits, the HDES shows promise for benzene removal from refinery effluent, although its environmental performance and its stability on reuse were not assessed in this study.