Polyamide-based Membranes Research Articles

Sodium hypochlorite-assisted osmotic backwashing for mitigating forward osmosis membrane fouling during pre-concentrating wastewater

Forward osmosis (FO) membrane has drawn substantial attention to pre-concentrating organic matter in wastewater for biogas production. However, membrane fouling control is an ongoing challenge for the feasibility of the FO system. Although chemical cleaning using a caustic solution (e.g., solution pH = 11) can remove organic foulants on the polyamide-based membranes, it can damage cellulose triacetate (CTA)-based FO membrane due to hydrolysis. This study assessed the efficacy of sodium hypochlorite (NaClO)-assisted osmotic backwashing to mitigate CTA-based FO membrane fouling during pre-concentrating primary wastewater effluent. The cleaning strategy employing a low concentration of NaClO achieved a higher restoration of water flux than conventional hydraulic flushing and sole osmotic backwashing. The water flux after NaClO-assisted osmotic backwashing (NaClO concentrations = 10–80 mg/L) reached 97–102%. Irreversible foulants on the membrane surface were fully removed by NaClO concentration from 20 to 80 mg/L. During four filtration cycles using a 40 mg-NaClO/L cleaning solution, the reverse salt flux increased from 4.3 to 6.3 g/m2h. This indicates that for a long-term operation, applying lower concentrations of NaClO (e.g., 20 mg/L) may be needed to avoid the changes in reverse salt flux. This study suggests that FO membranes during pre-concentrating primary wastewater effluent can be stably operated with NaClO-assisted osmotic backwashing.

Polyamide–based membranes with nanoscale homogeneity and asymmetric structure for ultrafast ion separation

Polymeric membranes are the most practical separation technology for extracting target ionic species necessary for water purification and clean energy, but are limited by the multi-scale heterogeneity, and transient and dynamic free volume pores. Modulated interfacial polymerization techniques are emergent concepts for preparing polyamide composite membranes with reduced thickness and homogeneous pore structures to satisfy the requirements for efficient ion separation applications. Herein, N–Cyclic cation-assisted kinetically regulated interfacial polymerization strategy to generate polyamide nanofilm composite membranes with tuned pore network and asymmetric structures for water purification is reported. Experimental characterization techniques and molecular dynamic simulations revealed that the N-Cyclic cations control the kinetic diffusion behaviour of the amine monomers to the aqueous/organic interface conducing to the formation of nanofilm membranes with fine-tuned polyamide network structures. Meanwhile, the N–Cyclic cation–regulated IP process not only affords to decrease the thickness of the formed polyamide nanofilm selective layers but also effectively narrows the effective pore size of the membranes. These membranes with thinner, homogeneous and asymmetric polyamide network structures exhibit enhanced water permeance of 55.7–60.42 L m−2 h−1 bar−1, and showed Na2SO4 rejection of 99.59–99.66% and MgCl2 rejection of 98.94–98.96% in nanofiltration separations, thereby outperforming the permeance–selectivity performance of the state-of-the-art polyamide-based membranes. The membranes also exhibit improved forward osmosis performances. We envision that this research will provide insightful foundations for the molecular tuning and construction of the next-generation structurally modulated polyamide nanofilms for various applications in molecular separation and water purification.

Surface room-temperature treatment of polyamide-based reverse osmosis membrane by diazotization reagent: Perm-selectivity regulation and mechanism

The use of acidic nitrite as diazotization reagent has been adopted previously for performance improvement of finished polyamide-based reverse osmosis membrane through post-treatment under a low temperature. However, the mechanism of diazotization reagent treatment on the membrane intrinsic permeability coefficients for both solvent and solute is still not clear and low-temperature treatment is energy intensive. This work focused on the mechanism and the efficient regulation of perm-selectivity by surface treatment of polyamide-based reverse osmosis membrane with diazotization reagent under room temperature. Polyamide based membranes were treated with diazotization reagent under temperatures between 5.0 and 25.0 °C. Membrane physico-chemical property was characterized via instrumental analysis and membrane intrinsic permeation coefficient was evaluated via cross-flow permeation test. It was found that the diazotization reagent treatments under 5.0, 15.0 and 25.0 °C could effectively enhance membrane intrinsic permeability coefficient for water (A) by 14.1, 17.7 and 21.7%, lower membrane intrinsic permeability coefficient for NaCl (B) by 60.1, 50.0 and 40.3%, and improve membrane perm-selectivity (A/B) from 5.55 bar-1 to 15.76, 13.00 and 11.31 bar-1, respectively. Mechanism exploration revealed that both the diazotization reaction occurred with end amino groups and the N-nitrosylation reaction occurred with amide bonds had a great influence on the membrane performance. Diazotization reagent treatment under different pH demonstrated that the perm-selectivity of polyamide-based membrane could be finely regulated by controlling an optimized pH of diazotization reagent at 1.5. The performance regulation efficiency of diazotization reagent treatment was also found to be related with the initial performance of the treated membrane. Under the same condition, salt rejections of 96.5% and 98.0% could be improved up to 98.3% and 98.8%, respectively. This work not only explains the mechanism of diazotization reagent treatment on regulation of membrane perm-selectivity, but also provides a room-temperature modification method suitable for performance enhancement of newly produced membrane and performance restoration of used membrane.

Revisiting the alkali hydrolysis of polyamide nanofiltration membranes

High flux membranes are required for energy-efficient separation processes. Chemical treatments of conventionally prepared polyamide thin film composite (TFC) membranes, including alkali hydrolysis of polyamide, have been found suitable for achieving higher membrane permeance without significant loss in solute rejection. However, polyamide-based membranes are known to be stable in alkaline conditions during chemical cleaning. Hence, we revisited the possibility of alkali hydrolysis post-treatment of the polyamide layer of a commercial nanofiltration membrane upon short-term exposures at a wide range of pH exceeding the recommended value (up to pH 13). We optimized the post-treatment parameters using the Box Behnken design (BBD) of Response Surface Methodology (RSM) to achieve maximum pure water permeance. We observed that the pure water permeance (PWP) of the post-treated (optimum) membrane increased to 21.1 ± 0.4 LMH/bar from 12.3 ± 0.4 LMH/bar of the pristine membrane while retaining the same salt rejection. However, the chemical composition, thickness, surface charge, and pore size of the polyamide layer in the pristine and post-treated membranes were found identical. Hence, it was concluded that the increase in PWP might be attributed to the physical restructuring of the polyamide layer or restructuring of the support layer and interface, making the membrane more permeable, rather than hydrolysis of the polyamide layer.

Polyamide-based membranes with structural homogeneity for ultrafast molecular sieving

Thin-film composite membranes formed by conventional interfacial polymerization generally suffer from the depth heterogeneity of the polyamide layer, i.e., nonuniformly distributed free volume pores, leading to the inefficient permselectivity. Here, we demonstrate a facile and versatile approach to tune the nanoscale homogeneity of polyamide-based thin-film composite membranes via inorganic salt-mediated interfacial polymerization process. Molecular dynamics simulations and various characterization techniques elucidate in detail the underlying molecular mechanism by which the salt addition confines and regulates the diffusion of amine monomers to the water-oil interface and thus tunes the nanoscale homogeneity of the polyamide layer. The resulting thin-film composite membranes with thin, smooth, dense, and structurally homogeneous polyamide layers demonstrate a permeance increment of ~20–435% and/or solute rejection enhancement of ~10–170% as well as improved antifouling property for efficient reverse/forward osmosis and nanofiltration separations. This work sheds light on the tunability of the polyamide layer homogeneity via salt-regulated interfacial polymerization process.

Antifouling and antibacterial β-cyclodextrin decorated graphene oxide/polyamide thin-film nanocomposite reverse osmosis membranes for desalination applications

• β-cyclodextrin-functionalized graphene oxide enhanced membrane properties. • Membrane with improved surface hydrophilicity and pure water permeability obtained. • Superior antibacterial and antifouling properties imparted on membrane. • Potentially a robust reverse osmosis membrane with an improved life-span. Incorporating functional nanomaterials in polyamide-based membranes has opened up new alternative ways to develop reverse osmosis membranes with excellent performance, biological stability, chemical stability and longer life-span. In this study, β-cyclodextrin functionalized graphene oxide nanosheets (β-CD-f-GO) were synthesized and used to fabricate polyamide thin film nanocomposite membranes possessing excellent reverse osmosis performance, antifouling and antibacterial properties. The β-CD functionalized graphene oxide nanosheets were prepared via an amide-mediated coupling reaction between ethylenediamine-grafted β-CD and graphene oxide. The fabricated nanosheets were then embedded in the polyamide thin film composite membranes via interfacial polymerization of m-phenylenediamine in an aqueous solution containing β-CD-f-GO nanosheets with trimesoyl chloride. The influence of the nanosheets on the membrane surface chemistry, permselectivity performance, antibacterial and antifouling properties was investigated. The results revealed that the water permeation flux and antibacterial activity of the β-CD-f-GO incorporated membranes showed an improvement by approximately 38% and 45.9%, respectively, relative to an unmodified polyamide membrane, with an insignificant change in salt rejection. In addition, the nanocomposite membranes exhibited improved antifouling properties with a lower total fouling ratio of 26.2% and a higher flux recovery ratio of 88.3% in comparison to the respective 46.8% and 74.1% obtained with the unmodified polyamide membrane. Additionally, the nanomposite membranes showed long term stability over a period of 144 h of reverse osmosis testing and improved chlorine resistance. With this study, it can be concluded that the β-cyclodextrin-functionalized graphene oxide nanocomposite has a good potential in the fabrication of membranes with improved stability and lifespan for desalination applications.