Polyamide Separation Layer Research Articles

Tailoring the internal void structure of polyamide films to achieve highly permeable reverse osmosis membranes for water desalination

The considerable permeability of fully-aromatic polyamide reverse osmosis (RO) membranes is believed to be strongly dependent on the presence of a mount of internal voids within the polyamide separation layer, which provides additional permeation channels in addition to diffusion through the polyamide matrix. Based on this understanding, herein, we reported a novel strategy of developing highly permeable RO membranes via introducing a void-tailoring agent, (3,3,3-trifluoropropyl)trichlorosilane (TFPTCS) into the traditional interfacial polymerization (IP) system composed of m-phenylenediamine and trimesoyl chloride. During IP process, the fast and violent hydrolysis and aminolysis of TFPTCS caused a significantly increasing pore size and porosity of the incipient polyamide film, which resulted in the formation of double- or multi-layer void structures with higher void fraction in the polyamide film. As a result, the TFPTCS added membrane showed a dramatic enhancement in water permeability, and the optimum condition was observed at the 0.03 wt% of TFPTCS concentration with 3.74 L m−2 h−1 bar−1, which was 2.8 times higher than that of the pristine membrane, while the salt rejection was almost unchanged (around 97.0%). These findings demonstrate the feasibility of void-tailoring inside the polyamide film, and encourage further exploration of new alternative void-tailoring agents to prepare highly permeable polyamide RO membranes for water desalination, which were seldom studied previously.

Manipulating the separation performance of nanofiltration membranes by coating thickness of organic phase during interfacial polymerization

ABSTRACTNanofiltration (NF) membranes have excellent separation capabilities and selective performance, and are widely applied in the field of concentration and separation of salt and inorganic salt mixtures. In this article, NF membranes with a higher comprehensive performance were fabricated using a thinner coated organic phase during interfacial polymerization, which greatly reduced the rejection of sodium chloride and significantly improved the water flux without reducing the rejection of magnesium sulfate. Structure comparison found that thinner polyamide separation layer resulted in the higher crosslinking degree, smaller particle, and relatively rough surface. Based on these analyses, we hypothesized that thermodynamically efficiency in the system varies with the coating thickness. Finally, the mechanism of the influence of the coating thickness of organic phase during heat treatment on the interfacial polymerization process and membrane structure was discussed. © 2019 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2020, 137, 48284.

Removal of polycyclic aromatic hydrocarbons by nanofiltration membranes: Rejection and fouling mechanisms

Polycyclic aromatic hydrocarbons (PAHs), as a group of micropollutants with high toxicity, are commonly detected in the environment and difficult to remove. In this work, five commercial polyamide (PA) nanofiltration (NF) membranes were used to treat three PAHs in the synthetic solution or coking wastewater and their removal mechanisms were discussed. By comparing water permeability and PAHs rejections of the NF membranes before and after PAHs adsorption, we found that the PAHs adsorption into the membrane was the dominant rejection mechanism at the initial filtration stage. Due to the limited adsorption capacity and increasing diffusion of PAHs in the membrane, the PAHs rejections would drop rapidly and then reach steady. Size exclusion mechanism was also important to PAHs removal by NF, especially for PAHs with weaker polarity and higher molecular weight. Although the polysulfone intermediate layer could adsorb substantial PAHs resulting in an increase of filtration resistance, the pore blocking and narrowing effects on the PA separation layer were still the main reasons for the permeability loss of NF membranes. However, the glucose rejection did not change obviously after the PAHs adsorption in the membrane. Through analyzing porosity and pore size distribution of the NF membranes before and after PAHs adsorption, we found that for the NF membrane with small pore size and narrow pore size distribution (e.g. NF270), PAHs adsorption produced indiscriminate pore blocking effect, and thus the average pore size and its distribution as well as glucose rejection did not change obviously after fouling formation; while for the NF membrane with large pore size and wide pore size distribution (e.g. NF10), PAHs adsorption occurred more seriously in the larger pores leading to a pore narrowing effect, but the reduced pore size was still too large to affect glucose rejections. The outcomes of this work suggested that the PAHs adsorption in both separation and intermediate layers of NF membranes should be avoided, and the NF membrane with narrow pore size distribution was preferred for PAHs removal.

Fabrication of a highly permeable composite nanofiltration membrane via interfacial polymerization by adding a novel acyl chloride monomer with an anhydride group

In this study, a novel acyl chloride monomer trimellitic anhydride chloride (TAC) with an anhydride group was blended with trimesoyl chloride (TMC) as organic phase monomers to fabricate high-flux nanofiltration membranes. Due to the reaction activity differences of organic phase monomers (TMC and TAC) with piperazine, the chemical compositions and structures of polyamide active layer could be tailored through varying the TAC content. It was found that the participation of TAC could yield a looser polyamide separation layer on the polysulfone support membrane, along with an increased surface roughness, hydrophilicity, pore size and negative charge density. As a result, the resultant membrane exhibited excellent water-salt separation. When the TAC concentration was increased to 0.04 wt%, the optimized membrane exhibited a high pure water permeability (PWP) of 13.2 L m−2 h−1 bar−1, up to 2-fold, in comparison with the pristine membrane, whereas the salt rejections of Na2SO4, MgSO4 and NaCl were 97.6%, 92.7%, and 34.0%, respectively. This work provided a novel and facile strategy to develop high-flux NF membrane.

High-hydrophilic and salt rejecting PA-g/co-PVP RO membrane via bionic sand-fixing grass for pharmaceutical wastewater treatment

In this study, a novel PA-g/co-PVP reverse osmosis (RO) composite membrane with high-hydrophilic, antifouling, high salt rejection properties, was prepared according to the principle of sand-fixing grass effect for pharmaceutical wastewater treatment. N-vinyl-2-pyrrolidone (NVP) was used as a “seed agent” that was seeped into a low cross-linked polyamide (PA) film. A hydrophilic molecular brush of polyvinylpyrrolidone (PVP) grafted on the PA surface (PA-g-PVP) similar to grass growing on sand and a cross-linked copolymer layer of PA and NVP (PA-co-NVP) similar to grass roots distributed in sand, as well as an interlayer cross-linking between PA separation layer and PES ultrafiltration membrane with a highly stable interfacial structure, synchronously occurred through γ-ray radiation. Upon incorporation of 1.0% of NVP, the resultant PA-g/co-PVP membrane achieved 18.8° water contact angle and high flux recovery ratio (FRR) values of 91.23%, 96.28%, and 93.67% for the foulants of bovine serum albumin (BSA), sodium alginate (SA), and sodium dodecyl sulfate (SDS), respectively. The FRR value of pharmaceutical wastewater-polluted membrane after cleaning was 94.30%. The PA-g/co-PVP RO membrane showed outstanding separation stability due to the radiation cross-linking reactions, which limit matrix swelling and interface delamination similar to the action principle of sand-fixing grass. These merits prove that the RO membrane developed in this study can be used in many applications, including small organic molecular wastewater separation.

Polyamide thin-film composite membrane fabricated through interfacial polymerization coupled with surface amidation for improved reverse osmosis performance

In this study, the novel technique, namely interfacial polymerization (IP) coupled with surface amidation, was proposed to fabricate polyamide thin-film composite (TFC) membranes for improved reverse osmosis performance. TFC membrane with loose polyamide separation layer was firstly prepared through conventional IP process. Surface amidation was then performed through covering the surface of the loose polyamide layer with aqueous 1-ethyl-3-(3-dimethylaminopropyl) carbodiimide (EDC) solution. Amidation reaction between the free carboxylic and amino groups on membrane surface was confirmed through ATR-FTIR and XPS analyses. Permeation tests revealed that the step of surface amidation could effectively improve the ability of membrane rejection to solute, showing significant increases of NaCl, NaNO3 and glycerol rejections from about 96.4%, 90.5% and 78.4% to 97.8%, 93.7% and 91.3%, respectively. The water permeability of the membrane obtained via IP followed with surface amidation was at least 22.3% higher than the membrane of the same NaCl rejection obtained via conventional IP process. Thus, the method developed was attractive for the fabrication of TFC reverse osmosis membranes with improved permeability to water and rejection ability to solutes. Additionally, the step of surface amidation of present work can be potentially applied to the in-situ rejuvenation of used polyamide reverse osmosis membranes.

The morphology of fully-aromatic polyamide separation layer and its relationship with separation performance of TFC membranes

TFC membranes with various morphologies of fully aromatic polyamide (PA) separation layer were prepared by just changing the monomer concentrations in a wide range. The morphologies of PA top surface, cross-section and back surface were systematically studied by using SEM and TEM. The PA layer thickness and its chemical structure were also characterized by using FTIR and XPS. A corresponding relationship of PA layer morphology was found, i.e. if there are obvious pores on the back surface of PA layer, there must be void structure on the PA cross-section and rough leaf-like structure on the PA top surface. The relationship between PA layer morphology and membrane separation performance, especially water flux, was elucidated. The porous structure on the back surface of PA layer is a necessary condition for TFC membranes to achieve high separation performance. For PA layer with back surface pores and single-layer void structure, its effective thickness is only the thickness of the top surface of PA layer. A three-stage formation mechanism of PA layer was proposed, which can explain the variation of pore size of PA back surface with the change of monomer concentration very well.

Exploring the dependence of bulk properties on surface chemistries and microstructures of commercially composite RO membranes by novel characterization approaches

Two kinds of novel reverse-osmosis (RO) membrane characterization approaches were firstly proposed for observing the backside and cross-sectional morphologies of polyamide separation layers and their extents of cross-linking, by using scanning electronic microscopy (SEM) and X-ray photoelectron spectroscopy (XPS) techniques, respectively. The surface chemistries (streaming potential and functional groups) and microstructures (polyamide thickness, morphology, and roughness) of three kinds of new commercial RO membranes (LP21, ULP21, and FR11) were comprehensively investigated. In addition, their salt rejections, and permeate fluxes were investigated in 2000ppm NaCl solution under different operating pressures ranging from 125 to 250psi; and their fouling resistances to insoluble CaCO3, MgCO3, and SiO2 colloids were also detailedly investigated. Based on the above results, the dependence of bulk properties (salt rejection, permeate flux, fouling resistance, and hydrophilicity) on surface chemistries and microstructures was carefully analyzed. Most importantly, it was observed that both of the top surface and backside of polyamide separation layer exhibit loose structures. The inner of separation layer should play an important role for the salt-rejection property of RO membrane. These results also provide important pieces of evidence for some previous reports on the microstructures of polyamide separation layers and desalting mechanism of RO membranes.