Novel two-pot approach ultrasonication and deep eutectic solvent pretreatments for watermelon rind delignification: Parametric screening and optimization via response surface methodology

Sequential delignification of watermelon rind using ultrasonication and deep eutectic solvent (DES) pretreatment methods was demonstrated in this study. The effects of pretreatment factors on delignification were investigated using parametric screening and subsequent optimization of significant factors. Plackett–Burman Design was used for the screening of pretreatment variables, while optimization was performed adopting Central Composite Rotatable Design. For the range of variables considered in the study, the screening experiments revealed that the effects of ultrasonication power, ultrasonication frequency, ultrasonication time, DES reaction temperature and DES reaction time on delignification were significant, while the effects of ultrasonication solid-liquid ratio, ultrasonication temperature, hydrogen bond acceptor/hydrogen bond donor molar ratio, and DES solid-to-liquid ratio were not significant. The significant factors were further investigated and maximum lignin removal of 43.56 % was achieved at ultrasonication power 180 W, ultrasonication frequency 60 kHz, ultrasonication time 40 min, reaction temperature 120°C and reaction time 180 min. Synergistic effect between the two pretreatment methods was observed and it correlated positively with the severity of the ultrasound pretreatment. The SEM and FTIR analyses further established the effectiveness of the sequential combinative pretreatment methods on watermelon rind delignification.