| The alkali effluent discharged from the chitin-processing is abundant in the protein-hydrolysates which can be effectively separated by alkali-tolerant nanofiltration membrane (NFM). Because of few researches reporting mechanism underlying filtration, membrane fouling and mass transfer of nanofiltration (NF) of the alkali effluent, this study focused on exploring the relevant mechanisms, which cloud benefit providing significant insights on our understanding of practical NF separations of protein hydrolysates in alkali solution.The traditional analysis methods, such as sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE), were conducted to determine the main competence in the effluent, which showed that molecular weights (MW) of protein hydrolysates were3313Da below. The influence of temperature and transmembrane pressure (TMP) on permeate fluxes and rejections of total proteins was also carried out, which suggested that NF of the effluent at40℃could achieve higher rejections of total proteins than at47℃despite higher flux at47℃and that permeate fluxes at high TMP were higher than at low TMP when the NF of the effluent was run continuously for8hours.Additionally, the resistance analysis was also conducted to achieve a better understanding of decline in fluxes. Hence the osmosis pressures of the feed and the permeate were detected by the freezing point osmotic pressure molar concentration meter in order to calculate the osmosis pressure gaps between the both sides of NFM, which was of benefit to figure up the membrane resistances generating during the NF of the effluent exactly. The results implied that the significant decline in the permeate flux during NF of the effluent was dominantly attributed to the increase of net driven force induced by the generation of dynamic filter gel and slightly affected by the membrane pore blocking and the irreversible peptide-membrane adsorption.Because the alkali effluent is a mixed solution varying in its composition and in isoelectric points (pI) and MW of the amino acids and peptides, the mechanism underlying NF of protein hydrolysates in the effluent was analyzed indirectly via Matlab simulating the parameters of the virgin and fouled NFM basing on the Teorell-Meyer-Sievers (TMS) model and the Steric-Hindrance Pore (SHP) model. The results demonstrated that the increase of the rejection of crude proteins during their NF might result from the enrichment of the NFM surface negative charge and the NFM pore blocking in the case of charged peptides both electrical repulsion effects and size are involved in their transmission.The present study also focused on the micro-analysis that the physicochemical characteristics of the protein-hydrolysates was identified by the matrix-assisted laser desorption/ionization time of flight mass spectrometry (MALDI-TOF-MS), being aimed at revealing the distinguished features which played a decisive role on the peptides-isolation. And our present work underlined the charge exclusion effect was predominant in the separations of peptides with MW of300-470Da, especially, with pI lower than4whereas the sieve effect was critical in the rejections of peptides (MW≥490Da) by NFM.Besides,60percent of peptides in the NF permeate (NF-P) were hydrophobic whereas70percent of peptides in the membrane adsorption cake were hydrophilic. And pI of DGL, TDT, LDT, ESLT, PEHAPVGID and EDLYSTPKLEEL were respectively3.80,3.80,3.80,4.00,4.35and4.00, namely, the hydrophobic peptides were with low pI, suggesting that there existed a possibility that acid peptides aggregating on the membrane surface because of peptide-membrane hydrophobic interaction enhanced the density of the surface electric charge compared to the virgin membrane, hence, being likely to strengthen the electrostatic exclusion and consequently, increasing the rejections of the major peptides and inorganic anion. |
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