Jialin Wei,Zhiyuan Qiao,Dinghe Yan,Jingwen Chu,Xinyi Zheng,Dawei Ji,Changfa Xiao,Chun Wang
In this study, we deposited polyelectrolyte multilayers (PEMs) on Poly ( p -phenylene terephthamide) (PPTA) ultrafiltration (UF) substrates with strong polyelectrolytes (PEs): polystyrene sulfonic acid (PSS) and poly (diallyldimethylammonium chloride) (PDADMAC). The growth pattern of PEMs was shifted from linear to exponential by changing substrate potential, which was attributed to synergism and antagonism of dispersion of PEs on hydrophilic substrates and electrostatic interaction between PEs and substrates. Eventually, PEMs of layer-dominated and pore-dominated regimes were successfully obtained. We focused on the transition from the pore-dominated to the layer-dominated regime of PEMs. The PEMs nanofiltration (NF) membranes in this situation showed a better separation performance due to the together work of the interface and in-pore energy barriers. The retention rate of Na 2 SO 4 and MgSO 4 was 98 % and 91 %. The eosin Y retention rate was 99.9 % and only declined 2 % after 100 h. The hindered deposition of pore-dominated regimes led to denser and thinner layers, which achieved long-term acid resistance. It maintained a stable salt retention after 185 h cross-filtration of 0.1 M H 2 SO 4 feed solution. The performance of PEMs NF membrane was improved from a new perspective, and excellent acid-resistant nanofiltration membranes were obtained, which are expected to be used to treat industrial acidic wastewater.
Nanofiltration membranes (NFMs) are widely used in saline water desalination, wastewater treatment, and chemical product purification. However, conventional NFMs suffer from broad pore size distribution, which limits their applications for fine separation, especially in complete separation of molecules with slight differences in molecular size. Herein, defect-free composite NFMs with narrow pore size distribution are fabricated using a contra-diffusion method, with dopamine/polyethylenimine solution on the skin side and ammonium persulfate solution on the other side of the ultrafiltration substrate. Persulfate ions can diffuse through the ultrafiltration substrate into the other side and in situ trigger dopamine to form a codeposited coating with polyethylenimine. The codeposition is hindered on those sites completely covered by the polydopamine/polyethylenimine coating, although it is promoted at the defects or highly permeable regions because it is induced by the diffused persulfate ions. Such a “self-completion” process results in NFMs with highly uniform structures and narrow pore size distribution, as determined by their rejection of neutral solutes. These near electrically neutral NFMs show a high rejection of divalent ions with a low rejection of monovalent ions (MgCl2 rejection = 96%, NaCl rejection = 23%), majorly based on a steric hindrance effect. The as-prepared NFMs can be applied in molecular separation such as isolating cellulose hydrogenation products.
Thin-film composite (TFC) membranes can find broad uses in desalination and wastewater treatment . However, industrialized interfacial polymerization involves many organic solvents and toxic monomers . An aqueous fabrication process can meet the standards of green and sustainable production, but fabricating defect-free thin films in a homogenous phase remains challenging. Herein, we fabricate a series of TFC nanofiltration membranes through a contra-diffusion process based on the cross-linking of metal ions and alginate . The film formation is confined at the ultrafiltration substrate surface due to the competition between reaction and restricted diffusion. Among a series of metal ion/alginate composites, the Cu 2+ /alginate composite membrane displays superior nanofiltration performance because of the strong affinity between Cu 2+ and alginate. The highly negatively charged membrane performs a high rejection to divalent anions and anionic dyes based on the Donnan effect . The effects of alginate concentration and diffusion time on membrane performance were investigated to achieve optimal fabrication conditions. The as-prepared membrane shows a rejection of 96.2% to Na 2 SO 4 with permeation flux around 50.2 L·m −2 ·h −1 under 0.6 MPa, and 99.5% to several dye molecules. Moreover, the membrane performs outstanding anti-fouling and anti-bacterial properties because of its excellent hydrophilicity and the existence of Cu 2+ .
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