How polyvinyl alcohol-based interlayers affect the performance of polyamide nanofiltration membranes prepared by polyethyleneimine

ABSTRACTPolyethyleneimine (PEI) based positive charge nanofiltration (NF) membranes have unique advantages in the removal of heavy metals. The regulation mechanism of the interlayer when PEI was used as monomer was still not clear. In this study, a PVA-based interlayer was constructed on the origin PES sublayer to tailor the interfacial polymerization in PEI systems. It was proved that the PVA interlayer can further elevate the PWP and rejections of heavy metals. Moreover, the absorption effect of interlayers could contribute to better removal of heavy metals. More negative charges of interlayer could result in a more enough and uniform absorption of PEI monomer as well as a slower diffusion rate of PEI monomer during the interfacial polymerization reaction, thus forming a loose top surface but dense sub-separating layer. This study provides new insight into the effect of interlayer on the performance of PEI-based nanofiltration membrane.KEYWORDS: PEI membranepositive membraneheavy metalmultifunctional membranethe charge of the interlayerdua-layer nanofiltration AcknowledgmentsThis study was supported by the National Natural Science Foundation of China (No. 22208091), the Natural Science Foundation of Hebei Province (No. B2022502009), and the Fundamental Research Funds for the Central Universities (No. 2022MS104).Disclosure statementNo potential conflict of interest was reported by the author(s).Statement of noveltyIn this work, PEI-based nanofiltration membranes with PVA-based interlayers for improved NF performance were fabricated and the tailoring mechanisms on interfacial polymerization and effect on heavy metal removal of interlayer were systematically investigated.Although many papers have been published about both PEI for interfacial polymerization and interlayer, most of these studies focus on evaluating the performance variation of nanofiltration membranes. The underlying mechanism that how interlayer affect the interfacial polymerization process when PEI is used as monomer is still not clear. Actually, this is important for a better fit-for tailoring on the PEI-based nanofiltration. Although there are many studies and some review papers have provided and summarized some reasonable explanation on how interlayer or sublayer modification influences the interfacial polymerization between PIP/MPD and TMC, the related study on PEI is still not enough. Because the different properties (larger molecule size and stronger positive charge) of PEI compared to PIP/MPD might lead to different influencing mechanisms on the interfacial polymerization process. More importantly, previous studies involving interlayer modification commonly targeted at removal or separation of NaCl and Na2SO4, which is a simple component. But how the interlayer affects the pollutants removal, for example heavy metals and micropollutants, is not yet elucidated. The novelty of this paper is as follows: The PVA based interlayer is for the first time applied for tailoring the PEI-based interfacial polymerization and the final prepared membrane was used for the heavy metal removal.It was found that the PVA interlayer can only slightly elevate the PWP and salt rejection of PEI NF membranes and the degree of improvement is much lower compared with that in PIP systems (where PIP is used as aqueous monomer).It was also found that the absorption effect of interlayers could contribute to better removal of heavy metals, playing the role of the supplementary defensive line to prevent the permeation of heavy metals.Moreover, it was demonstrated that the charge property of sublayers played a more important role than the hydrophilia of interlayers for the PEI-based interfacial polymerization. More negative charges of interlayer could result in the forming of a loose top surface but dense sub-separating layer due to the more enough and uniform absorption of PEI monomer as well as a slower diffusion rate of PEI monomer.This study not only helps better understand the role of interlayer in interfacial polymerization and NF performance during the filtration process but also inspires how to further promote the performance of PEI-based NF membranes. We believe that this paper is of both scientific and practical interest and falls within the aims and scope of Desalination.Additional informationFundingThe work was supported by the Fundamental Research Funds for the Central Universities [2022MS104]; National Natural Science Foundation of China [22208091]; Natural Science Foundation of Hebei Province [B2022502009].