Microemulsion as solvent for naphthoquinones extraction from walnut (Juglans mandshurica Maxim) green husk using high voltage electrical discharge

Walnut green husks are a good source of natural naphthoquinones (NQs). In this study, a novel extraction strategy combining microemulsion and high voltage electric discharge (HVED) was proposed to extract the NQs from walnut green husks and maximize the value of this agro-byproduct. Firstly, the optimum microemulsion formula (ME8) for NQs extraction was determined among 17 microemulsions prepared with various contents ratios. Then the single factor experiments were conducted to explore the effect of discharge parameters (voltage, treatment time, liquid-solid ratio) on NQs yield. These parameters were further optimized by response surface methodology with Box-Behnken design, and the highest yield of NQs (67.1 ± 0.3 mg/g) was obtained under the predicted optimal conditions. The combination of HVED and microemulsion showed favorable synergistic preponderance and the NQs yield was 1.52 times higher than heating reflux extraction. Moreover, this strategy presented a competitive option to extract hydrophobic compounds in a less-organics-consumption way.