Synergetic effect of sustainable protic deep eutectic solvent and sonochemical extractor for extractive transesterification of biodiesel from Chlorella salina

[Display omitted] •One-pot sonochemical extractive transesterification with protic deep eutectic solvents investigated for first time.•Synergy of ultrasound and pDES yields more biodiesel (90.8% w/w).•Employing pDES results in biodiesel with low acid value (0.31) & iodine index (62 mg/kg BD).•Transesterification follows first order kinetics with lower activation energy (28.3 kJ/mol) than other DES catalysts.•pDES based transesterification results in sustainable biodiesel production with minimal energy consumption. Sustainable one-pot biodiesel production by sonochemical extractive transesterification (SET) in Chlorella salina was achieved with protic deep eutectic solvents (pDES). Six pDES were prepared with quaternary ammonium salts as hydrogen bond acceptors and organic acids as hydrogen bond donors. The eutectic mixture formed with choline chloride and lactic acid having moderate density and viscosity yields maximum FAME (90.8 % (w/w)) than the counterparts with higher density. Response surface methodology-based optimization was carried out and 25 % (w/v) of biomass load, 62.5 % (v/v) of DES/methanol ratio, 338 K of ultrasound temperature and 25 min of ultrasound time were identified as optimal conditions for biodiesel production. The resulting biodiesel was found to have an acceptable density (870 g cm−3), viscosity (4.24 mm2/s), acid value (0.31) and iodine index (62 mg/kg biodiesel) as per the EN 14214 standard. Further characterization by FTIR and GC–MS reveals that 90 % of methyl esters have lower carbon chains (