Optimization of the thermal pretreatment process for high lignin recovery from coconut husk by response surface methodology (RSM)

In this study, the influence of low nitric acid concentration, temperature, and residue time on lignin recovery from coconut husk using a thermal pretreatment process under optimal conditions is investigated along with the optimum conditions for lignin recovery using the RSM response surface methodology. Results after statistical analysis of independent variables such as nitric acid concentration, temperature, and time show that these parameters had a significant impact on lignin recovery. Analysis with ANOVA reveals that all model and independent parameters were statistically significant at the 95% confidence level. With respect to results in the current study, the highest lignin recovery yield was shown to be 85.31%; this was obtained from coconut under optimal conditions at low nitric acid concentration of 0.010 M and a temperature of 200 °C for 30 min. ln addition, scanning electron microscope (SEM) analysis showed the crystalline cellulose present in the coconut husk was all converted to an amorphous form during the thermal pretreatment process. Meanwhile, the crystallinity index of solid residue after thermal pretreatment showed a significant increase in crystallinity index of 76.65% from native coconut husk (64.12%). However, the recovered lignin is used as a precursor to convert into high-value products. Therefore, a thermal pretreatment process is very important to make the chemical conversion process more efficient.