Nanocrystalline cellulose production from unbleached Ceiba pentandra (L.) Gaertn (Kapok) seed pod fibres via multiple acid hydrolysis: 'More is Less' or 'Less is More'?

Nanocrystalline cellulose (NCC) isolation from non-woody biomass is attractive to replace the current reliance on wood as feedstock. The predominant use of single acid hydrolysis cycle in existing literature may overlook the residual sediment. In this work, we explored the use of multiple acid hydrolysis to investigate the NCC isolation from unbleached kapok pulp. Initial screening using factorial design showed the significance of acid concentration, reaction time and temperature as crucial variables in determining the NCC characteristics, among five variables examined which also included sonication time and acid-to-pulp ratio. Subsequent optimisation using central composite design demonstrated that the optimal reaction condition for NCC recovered from supernatants entails a single cycle employing 63%w/w of sulphuric acid at 45 degrees C for 50 min with acid-to-pulp ratio of 30:1 mL/g and sonication time of 5 min. The NCC yield obtained (supernatant) was similar to 19 % with PDI of 0.4, zeta potential of -39.7 mV and crystallinity index of 72.6 %. When combining with the further hydrolysis of sediments, the data showed comparable responses for NCC produced (yield, size and zeta potential) using either 3 cycles or 1 cycle. Nevertheless, opting for 3 cycles resulted in a lighter NCC colour owing to lower acid concentration and temperature. This study demonstrated that the feasibility of adjusting the number of hydrolysis cycles based on the desired outcome for NCC isolation.