Charge mediated nano-filtration for recovering sinapic acid from a mustard bran hydrolysate

Abstract

The recovery of valuable chemicals, such as sinapic acid, from enzymatic hydrolysates is essential to enabling a sustainable biorefinery. In this study, we designed a system to recover sinapic acid produced chemo-enzymatically from waste mustard bran. The process involves sequential separation, beginning with size partitioning using ultrafiltration for enzyme removal and recovery, followed by charge-mediated size partitioning via nanofiltration to isolate sinapic acid from other extracts. Four polyethersulfone (PES) ultrafiltration membranes with molecular weight cut-offs (MWCOs) of 5000 and 10 000 Da and maximum allowable working pressures (MAWPs) of 3 and 10 bar were screened for their effectiveness in removing Bovine Serum Albumin (BSA) as a model compound. Subsequently, these membranes were applied to recover the feruloyl esterase enzyme. The membrane with an MWCO of 5000 Da and an MAWP of 10 bar achieved 97.93 % enzyme recovery with a permeate flow rate of 31.4 L/h/m² at 6 bar. Next, we evaluated the effects of zeta potential interactions on the preferential rejection of sinapic acid using various nanofiltration membranes with potentially charged surfaces, including polyimide, silicon-based thin film composite, and polypiperazine amide membranes with MWCOs ranging from 150 to 600 Da. The polypiperazine amide membrane demonstrated the highest recovery of sinapic acid, achieving 86 % recovery from a model solution and 64 % recovery from mustard bran hydrolysate. Compositional analysis of the permeate confirmed that the rejection rate (R) is influenced primarily by the pKa rather than molecular size, following the trend: sinapic acid (pKa = 4.58; 224.2 Da; R = 64.0 %), acetic acid (pKa = 4.76; 60.1 Da; R = 23.8 %), xylose (pKa = 12.15; 150.1 Da; R = 13.7 %), glucose (pKa = 12.28; 180.2 Da; R = 8.7 %), and arabinose (pKa = 12.34; 150.1 Da; R = 8.5 %). The zeta potential interactions across nanofiltration membranes enhanced sinapic acid recovery from the mustard bran hydrolysate, hence, charge mediation significantly influenced the membrane separation of these complex mixtures with varying pKa values.