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Separation of silkworm proteins in cocoon cooking wastewaters via nanofiltration: Effect of solution pH on enrichment of sericin

Cocoon cooking wastewater contains waste silkworm proteins including sericin, which is a valuable raw material for many industries including cosmetics and pharmaceuticals. Sericin can be recovered via nanofiltration (NF) with high efficiency; however the process conditions need to be optimized to maximize separation. In this study, the effect of solution pH on NF performance was investigated for separation of silkworm proteins towards sericin enrichment. Solution pH (3.5-9.0) slightly influenced sericin rejection, which was higher than 80% in all cases. On the other hand, solution pH significantly influenced transmission of contaminant proteins, which was highest (0.52) at pH 3.5 and lowest (0.06) at pH 9.0. Increased transmission of contaminant proteins at acidic pH was attributed to possible effects of concentration polarization and increased ionic strength. Although acidic conditions (pH 4.5) favored sericin enrichment, the instantaneous purity of sericin in the retentate remained at 0.48-0.52 at pH 3.5-9.0. Furthermore, working at acidic pH caused severe fouling problems; more than threefold increase occurred in flux decline; with an increase of flux decline from 20% to 69% and fouling resistance from 3.8 x 10(13) m(-1) to 15.5 x 10(13) m(-1) (almost fourfold) when pH was reduced from 6.4 to 4.5. The contribution of fouling resistance to total resistance increased from 32-40% (pH 6.4) to 70% and 53% in acidic and alkaline conditions, respectively. Concentrating the wastewater at pH 4.5 had adverse impacts on rejection performance of NF membrane; rejection of sericin and contaminant proteins decreased from 78% to 52% and from 33% to 9%, respectively, as volume reduction factor increased from 1.00 to 1.70. Correspondingly, sericin purity in the retentate decreased from 0.59 to 0.55. The pH adjustment had no benefit for separation of sericin from contaminant proteins, and hence it was decided not to apply pH adjustment in NF stage. Instead, a two-stage process, NF (at original pH of 6.4) plus ethanol-induced precipitation were adopted, which enabled enrichment of sericin 1 from 80.0% to 92.9%. The product was considered to be of high quality as it contained greater than 90% sericin.