Polyamide RO Membranes Research Articles

Understanding interfacial polymerization in the formation of polyamide RO membranes by molecular simulations

Since the water permeation mechanisms of polyamide membranes are highly dependent on the micro-structure of polyamide membranes, a deeper understanding of the formation process of them is necessary for the optimization of the membrane performance. As most simulation works construct the polyamide membranes in an ideal way, the interfacial diffusion of monomers, which is crucial for the formation of polyamide membranes, is usually ignored. To address this issue, we mimic the experimental conditions to develop an atomic model of a highly crosslinked polyamide membrane by conducting molecular dynamics simulations. Via tuning the concentration and molar ratio of monomers, the diffusion of monomers and its influence on the subsequent reaction are altered, and the final membrane structure consequently changed. Simulation results reveal that increasing the trimesoyl chloride concentration results in thicker membranes with a reduced specific surface area and consequently decreased water permeance. On the other hand, increasing the m-phenylenediamine concentration will accelerate the reaction rate and reduce the final crosslinking degree. A deeper understanding of the mechanism of polyamide-membrane formation is unveiled in this work, which can aid in the design of high-performance polyamide membranes in the future.

Piezoelectric reverse osmosis (RO) membrane: Fabrication and anti-fouling effect

In this study, a novel piezoelectric RO membrane with vibrating mechanism when exposed to an electric field was developed. The vibration due to the piezoelectric effect can disturb the concentration polarization layer on the membrane surface and minimize membrane fouling. Polyvinylidene fluoride (PVDF) which has piezoelectric properties was selected as the porous substrate for a thin film composite (TFC) polyamide RO membrane in this study. Electrical poling was performed to enhance the piezoelectric properties of the PVDF substrate. RO membranes fabricated via interfacial polymerization using different PVDF substrates (i.e., with and without pre-wetting the PVDF substrate, with and without polyethylene terephthalate (PET) non-woven fabric support (denoted as NWF and no-NWF, respectively), with and without electrical poling) were systematically investigated. The results indicated that RO membrane fabricated on poled PVDF-NWF substrate shows reasonable pure water permeability of ∼2 LMH/bar and sodium chloride (NaCl, 2000 ppm) rejection of ∼98.9 % when tested at 15 bar. In the accelerated fouling study using colloidal silica particles (∼20 nm in diameter, 200 ppm with background of 2000 mg/L NaCl), when the RO membrane fabricated using both unpoled and poled PVDF-NWF substrates were excited with AC signal, membrane fouling was significantly reduced. These findings suggested that electrical poling may not be necessary for such piezoelectric RO membrane.

Small and wide angle X-ray scattering to reveal nanoscale structure evolution of the polyamide RO membranes triggered by ethyl acetate

The intrinsic structure of the polyamide separation layer determines the transport of water and salt through a thin film composite membrane. In this work, we probed nanoscale structure variation of the polyamide layers generated by ethyl acetate through small/wide angle X-ray scattering. The gyration radius and physical radius of the polyamide colloidal particles were calculated from the small angle X-ray scattering (SAXS) curves, and the results are confirmed by AFM and SEM. The form factor derived from SAXS curves and horizontal distance between the centers of adjacent benzene rings calculated from the wide angle X-ray scattering (WAXS) curves showed quantitative relationships with membrane performance including water permeability coefficient and salt permeability coefficient. The results derived from SAXS and WAXS curves provided evidence for the formation mechanism of polyamide layers.

RO membrane with a surface tethered polymer brush layer for enhanced rejection of nitrate, boron, and arsenic

Thin-film composite (TFC) polyamide (PA) RO membrane modified with a layer of tethered poly(acrylic acid) (PAA) chains displayed intrinsic rejections of nitrate, boron, As(III), and As(V) of 98.0%, 90.7%, 96%, and 99.6%, respectively. The solute permeability coefficients for nitrate, boron, As(III), and As(V) by the SNS-PAA-PA membrane were lower by 31–38%, 49–63%, 57–72% and 87–93% relative to tested commercial membranes. The above results indicate that the SNS-PAA-PA membrane should be suitable for purification of brackish source water contaminated with nitrate, boron, As(III), and As(V) to levels 1219 ppm, 2 ppm, 95 ppb, and 948 ppb, respectively. The study results suggest that there is merit in further exploration of the potential of the present approach for enhancing RO membranes performance for targeted solute removal.

A mechanistic model for salt and water transport in leaky membranes: Implications for low-salt-rejection reverse osmosis membranes

The transport of salt and water through “leaky” membranes is of fundamental importance in many disciplines of science and engineering. One important application is in low-salt-rejection reverse osmosis (LSRRO), an emerging membrane-based brine concentration technology with the potential to reduce the energy consumption and cost of brine management. For further development and optimization of the LSRRO technology, models that accurately describe salt transport through LSRRO membranes are critically needed. In this work, we use the solution-friction model to describe salt transport in LSRRO and demonstrate that the model can be simplified to the classic Spiegler-Kedem-Katchalsky model, where the phenomenological parameters of salt permeability and reflection coefficient are expressed in terms of friction and partitioning coefficients. We obtained membranes with different salt permeabilities by treating commercial polyamide RO membranes with an alkaline chlorine solution. Our results reveal the dependence of the phenomenological membrane parameters on the feed salt concentration, with salt permeability increasing and reflection coefficient decreasing as salt concentration increases. We further quantify the frictions among salt, water, and membrane for the chlorine-treated membranes with different salt permeabilities. We find that the friction between water and membrane is inversely proportional to water permeability, while friction between the salt ions and water is relatively insensitive to the feed concentration and chlorine treatment. Further, the salt-membrane friction coefficient is shown to be controlled by the friction between co-ions and membrane. Overall, our study presents a framework to quantify the frictions between water and salt ions as they move through LSRRO membranes, providing mechanistic insights and accurate model for salt transport in LSRRO.

Dealcoholization of Unfiltered and Filtered Lager Beer by Hollow Fiber Polyelectrolyte Multilayer Nanofiltration Membranes-The Effect of Ion Rejection.

Membrane-based beverage dealcoholization is a successful process for producing low- and non-alcoholic beer and represents a fast-growing industry. Polyamide NF and RO membranes are commonly applied for this process. Polyelectrolyte multilayer (PEM) NF membranes are emerging as industrially relevant species, and their unique properties (usually hollow fiber geometry, high and tunable selectivity, low fouling) underlines the importance of testing them in the food industry as well. To test PEM NF membranes for beer dealcoholization at a small pilot scale, we dealcoholized filtered and unfiltered lager beer with the tightest available commercial polyelectrolyte multilayer NF membrane (NX Filtration dNF40), which has a MWCO = 400 Da, which is quite high for these purposes. Dealcoholization is possible with a reasonable flux (10 L/m2h) at low pressures (5-8.6 bar) with a real extract loss of 15-18% and an alcohol passage of ~100%. Inorganic salt passage is high (which is typical for PEM NF membranes), which greatly affected beer flavor. During the dealcoholization process, the membrane underwent changes which substantially increased its salt rejection values (MgSO4 passage decreased fourfold) while permeance loss was minimal (less than 10%). According to our sensory evaluation, the process yielded an acceptable tasting beer which could be greatly enhanced by the addition of the lost salts and glycerol.

Preparation of antifouling TFC RO membranes by facile grafting zwitterionic polymer PEI-CA

In this work, we synthesized a zwitterionic polymer PEI-CA for the first time and then grafted it onto the surface of polyamide RO membrane via layer-by-layer interfacial polymerization (LbL-IP). XPS and ATR-FTIR techniques were used to verify the successful modification with PEI-CA. The zwitterionic hyperbranched polymer PEI-CA endowed the modified membrane with much improved surface hydrophilicity and decreased surface negative charge. The tailored M-PEICA exhibited a high water permeance up to 3.92 L m−2 h−1 bar−1 (increased by 62 %) while holding a good NaCl rejection (98.93 %). The dynamic fouling experiments proved that grafting of PEI-CA can help to effectively elevate its antifouling performance against most organic foulants except small molecular negatively charged surfactant SDS. Therefore, we further employed the mixed reagents modification strategy with arginine and PEI-CA, the resulting RO membrane M-mixed presented obviously improved antifouling performance towards SDS, the overall antifouling performances are even better than the well-known RO membrane CR100 of DOW FILMTEC™, that well illustrated multiple reagents modification strategy could be an efficient way that can meet all critical standards. Overall, the novel polymer PEI-CA and as-fabricated RO membranes with outstanding antifouling performances in this study have vast application potential in field of water treatment.

Construction of pseudo-zwitterionic polyamide RO membranes surface by grafting positively charged small molecules

Membrane fouling is the major obstacle to hinder the extensive application of polyamide thin-film composite reverse osmosis (PA TFC RO) membranes in the water purification and reclamation fields. In this study, a series of hydrophilic and charge-tuned antifouling RO membranes were fabricated by grafting three kinds of small molecules of multi-grade amines N,N-dimethylethylenediamine (DMEDA), (2-aminoethyl) guanidine (AEGu) and (2-aminoethyl)triethylenediamine (AEDABCO) via layer-by-layer interfacial polymerization (LbL-IP). Detailed characterizations revealed that the three tailored membranes presented obviously diminished surface negative charge and reduced PA layer thickness, the water permeability increased by 67.4 %, 34.7 % and 75.6 % compared to the pristine membrane, reached 4.05 L m−2 h−1 bar−1, 3.26 L m−2 h−1 bar−1, and 4.28 L m−2 h−1 bar−1, respectively, meanwhile kept high NaCl rejection > 98.5 %. The dynamic fouling tests indicated that the tailored membranes exhibited much improved antifouling performance towards most of the model organic foulants. Meanwhile the membranes assembled with cationic groups exhibited an enhanced antibacterial property against E. coli. These functional small charged molecules and the convenient modification method provide a valuable pattern for designing antifouling PA TFC RO membranes, which is important for sustainable water purification.

Elucidating the Effect of Aliphatic Molecular Plugs on Ion-Rejecting Properties of Polyamide Membranes.

Polyamide RO membranes are widely used for seawater desalination owing to their high salt rejection and water permeability; however, improved selectivity-permeability trade-off is still desired. "Molecular plugs," small molecules immobilized within the polyamide structure, offer an attractive approach; however, their overall effect on polyamide physicochemical properties poses many questions. Here, we analyze the effect of decylamine, a promising plug, and a few charged and uncharged mimics on polyamide films using several in situ techniques. Electrochemical impedance spectroscopy (EIS) reveals a complex pH-dependent response, whereby, upon exposure to amine solution, conductivity first rapidly drops; however, under alkaline conditions, when amine is uncharged, the trend subsequently slowly reverses, and conductivity increases. This slow reversal was observed for noncharged alcohols of similar size as well, but not for larger surfactant molecules. The reversal was assigned to the uptake of plug molecules within polyamide, as opposed to the fast initial drop assigned to surface adsorption. EIS and quartz-crystal microbalance (QCM) results showed that exposure to decylamine under alkaline conditions ultimately led to an irreversible decrease in conductivity, that is, stronger ion rejection, remaining after re-exposure of polyamide to amine-free buffer. This suggests that plug uptake within polyamide resulted in polymer stress, indeed observed in surface stress measurements, and subsequent relaxation. The results indicate that the moderate size of decylamine and conditions minimizing its charge were optimal for irreversible change; however, charge interactions helped maximize its binding within polymer and induce the desired sustained change in selectivity. The results have many potential implications for improving current membrane desalination technology and increasing inherent membrane selectivity toward hard-to-remove species.

Novel RO membranes fabricated by grafting sulfonamide group: Improving water permeability, fouling resistance and chlorine resistant performance

Membrane fouling and degradation caused by free chlorine oxidation seriously hinder the long-term operation of aromatic polyamide (PA) thin-film-composite (TFC) membranes. In this study, sulfonamide monomers 4-aminobenzene sulfonamide (4-ABSA) and 2-aminoethanesulfonamide (2-AESA)) were facilely grafted onto the PA RO membranes surface via layer-by-layer interfacial polymerization (LbL-IP) method, endowing with much improved water flux, antifouling, and chlorine-resistance properties. The chemical compositions and surface properties of the RO membranes were systematically investigated by various techniques, as FTIR and XPS results clearly showed that sulfonamide groups were successfully grafted onto RO membranes’ surface. Compared to the pristine membrane, the water flux of the two modified membranes increased by 50.8% and 59.1%, reached 3.65 ± 0.15 L m−2 h−1 bar−1 and 3.85 ± 0.05 L m−2 h−1 bar−1, respectively, meanwhile their NaCl rejection remained above 99.25 ± 0.08%. Due to the improvement of surface hydrophilicity and decrease of surface roughness, the antifouling performance of the modified membranes was greatly improved. Moreover, the N–H moieties on sulfonamide can act as sacrificial units for free chloride attack to significantly improve the chlorine-resistance performances of the modified RO membranes compared to the pristine one, with NaCl rejection remain above 98.95 ± 0.09% after chlorination intensity of 8000 ppm⋅h under acidic condition of pH 4. This simple and efficient surface grafting strategy and modifying materials make it have great application potential in the field of water treatment.

Boron removal with modified polyamide RO modules by cross‐linked glutaric dialdehyde grafting

AbstractBACKGROUNDReverse osmosis (RO) plays an increasingly important role in boron removal. In this study, novel polyamide RO membranes and modules with high boron rejection were fabricated by cross‐linked glutaric dialdehyde (GA) grafting. To evaluate the influence of GA grafting on membrane materials, the surface properties before and after modification were analyzed.RESULTSThe results showed that the membrane morphology changed distinctly after GA modification. All modified membranes became more hydrophilic as the water contact angle decreased from 60° to 35°. The zeta potential of RO membrane surface increased from −6.23 mV to 17.27 mV, which indicated that the surface charge altered from negative to positive. The rejection of boron increased from 76.65% to 90.14% and the boron permeability coefficient decreased from 3.71 to 0.79. Furthermore, the rejection of boron for modified module was >90% over a 5–22 ppm range of boron concentration. The rejection of boron also stabilized at ≈90% beyond 3000 min.CONCLUSIONThe modified RO membrane modules with high boron rejection and acceptable flux could be used in boron removal. The approach in this study can improve the separation properties of boron for commercially available modules without disassembling. © 2020 Society of Chemical Industry (SCI)

High-performance zwitterionic TFC polyamide nanofiltration membrane based on a novel triamine precursor

As known, piperazine (PIP) and m-phenylenediamine (MPD) are popularly employed as aqueous monomer to interfacially polymerize with trimesoyl chloride (TMC) for the fabrication of TFC polyamide NF and RO membrane, respectively. In this work, we designed a novel amine monomer 4-(piperazin-1-yl)benzene-1,3-diamine (PMPD) having the combined PIP and MPD units in one molecule and employing it with TMC to prepare the pristine nanofiltration membrane. Subsequently, zwitterionic antifouling PA surface was constructed by modification with 3-bromopropionic acid (3-BPA). The resulting membrane (PMPD-PA) exhibited a smoother surface with enhanced surface hydrophilicity than the membrane made from MPD, and the thickness of the PA active layer was down to ~35 nm. Meanwhile, its pure water permeability was ~19 L m−2 h−1 bar−1, with Na2SO4 rejection of 98.4%, and NaCl rejection as low as 24.1%, performing as a novel selective NF membrane. Furthermore, fouling tests using model foulants BSA and SA well illustrated the good antifouling properties of membrane PMPD-PA with the mitigated flux decline and enhanced flux recovery upon physical cleaning. Thus, this work further demonstrates the fabrication of high-performance TFC NF membrane from the perspective of molecular design.

Membrane Charge Weakly Affects Ion Transport in Reverse Osmosis

Polyamide RO membranes have been a “golden standard” in water desalination, with aromatic polyamide layer providing an excellent balance between salt rejection and water permeability. Yet, there st...

The impact of monochloramines and dichloramines on reverse osmosis membranes in wastewater potable reuse process trains: a pilot-scale study

Dichloramine has a strong oxidative effect on polyamide RO membranes, causing oxidative hydrogen bond breakage of the polyamide rejection layer.

The application of polyethylenimine grafting reverse osmosis membrane in treating boron‐containing low‐level radioactive wastewaters

AbstractBACKGROUNDReverse osmosis (RO), which is widely utilized in wastewater treatment, has been proven to be an effective tool for ion separation with high salts rejection. However, owing to the extremely low concentration of nuclides in boron‐containing low‐level radioactive wastewaters (LRWs), there is a significant reduction in rejection of nuclides by using commercial RO membranes. In this study, in order to obtain higher nuclides rejection under trace‐level condition, novel polyamide RO membranes were fabricated by grafting polyethylenimine (PEI) with different molecular weights on commercial RO membrane surfaces.RESULTSThe properties of PEI‐modified membranes were analyzed, such as surface charge, morphology and hydrophilicity. The results showed that zeta potential at pH 7 increased from −22.89 mV to 20.19 mV, which indicated that surface charge altered from negative to positive. Meanwhile, hydrophilicity of membranes was improved since the contact angle decreased from 65° to 36°. The decontamination factor (DF) of cesium(I) increased from 11.69 to 114.23 and the DF of cobalt(II) increased from 28.62 to 3306.88.CONCLUSIONThe laboratory‐scale experiments with cross‐flow demonstrated the effective rejection of nuclides without obvious membrane flux loss of PEI‐modified membranes. In addition, PEI‐modified membranes offered a stable nuclides removal under high‐level background boron and various pH conditions, which demonstrated a possible application in boron‐containing LRW treatment. © 2019 Society of Chemical Industry

TREATMENT OF PRODUCED WATER FROM CRUDE OIL DESALTER BY COMPOSITE POLYAMIDE RO MEMBRANE

Produced water generated from oil refinery desalting unit requires proper treatment to adhere to stringent regulations before discharging or recycling into the processing area. In this research, desalter’s produced water from PP(M)SB (PETRONAS Penapisan (Melaka) Sdn. Bhd.) was treated via AFC99 tubular thin film composite polyamide RO membrane and were investigated concerning TMP, feed pH and feed-solution concentration. The experimental results indicated that AFC99 membrane removed 100% of TSS and salinity, and more than 95% of COD, salts ions and TDS. This work shows that the RO membrane was feasible and technically viable to be used for optimum salts and contaminants removal from produced water of the desalting unit. Besides, the treated water (permeate) fulfills the watering standards and can be used for recycling and other purposes.
 Keywords: produced water, reverse osmosis, polyamide membrane, operating conditions, watering standards

Surface coating of polyamide reverse osmosis membranes with zwitterionic 3‐(3,4‐dihydroxyphenyl)‐l‐alanine (l‐DOPA) for forward osmosis

AbstractTo overcome low permeability and fouling problems of membranes used in FO processes, modification is needed to improve the hydrophilicity, permeability and selectivity of membranes. In this work, thin film composite (TFC) commercial polyamide RO membranes (BW30‐LE, SW30‐HR, AG and AC) were functionalized with zwitterionic l‐DOPA. The effect of l‐DOPA on the morphology of membranes was determined via SEM, FT‐IR, AFM and contact angle analysis. The l‐DOPA modified BW30‐LE membrane showed excellent properties with 46° contact angle and 3.8 L/m2hbar water permeability and 0.83 L/m2h salt permeability. Although, l‐DOPA modified BW30‐LE membrane had the highest water flux and hydrophilicity, l‐DOPA modified SW30‐HR membrane showed higher FO flux with 9.38 L/m2h than BW 30 membrane with 3.5 L/m2h at 50 g/L NaCl draw solution. Introducing hydroxyl and carboxyl ionic groups on the membrane surface with l‐DOPA coating enhanced the FO performance and water permeability which provide a new insight in FO applications.

Performance of Acacia Gum as a Novel Additive in Thin Film Composite Polyamide RO Membranes.

Novel thin film composite (TFC) polyamide (PA) membranes blended with 0.01–0.2 wt.% of Acacia gum (AG) have been prepared using the interfacial polymerization technique. The properties of the prepared membranes were evaluated using contact angle, zeta potential measurements, Raman spectroscopy, scanning electron microscopy, and surface profilometer. It was found that the use of AG as an additive to TFC PA membranes increased the membrane’s hydrophilicity (by 45%), surface charge (by 16%) as well as water flux (by 1.2-fold) compared with plain PA membrane. In addition, the prepared PA/AG membranes possessed reduced surface roughness (by 63%) and improved antifouling behavior while maintaining NaCl rejection above 96%. The TFC PA/AG membranes were tested with seawater collected from the Arabian Gulf and showed higher salt rejection and lower flux decline during filtration when compared to commercial membranes (GE Osmonics and Dow SW30HR). These findings indicate that AG can be used as an efficient additive to enhance the properties of TFC PA membranes.

Enhancement in permselectivity and antibacterial performances of polyamide RO membranes via surface modification of AgCl nanoparticles

Enhancement in permselectivity and antibacterial performances of polyamide RO membranes via surface modification of AgCl nanoparticles

Modification of polyamide membranes by hydrophobic molecular plugs for improved boron rejection

Commercial polyamide RO membranes, though effective in terms of flux and salt removal, have a few drawbacks, in particular, poor rejection of boron (B), thus they are often unable to remove B to the required level in a one-pass sea desalination process. This complicates the process and available solutions increase the cost of desalinated water significantly. Here we explore in-situ modification procedure that can significantly increase their boron rejection by incorporating suitable modifying molecules in the selective polyamide layer. We propose that aliphatic amines be used as such “plug” molecules that combine a bulky hydrophobic moiety with a reactive group that can chemically or physically bind to polyamide layer, tighten its structure and increase selectivity. Based on previous results, we hypothesize that hydrophobicity of the selective layer increased by the immobilized amine “plug”, along with reduced pore size, may help disrupt water-boric acid association and decouple water and boron permeation. The results show that the proposed treatment with sufficiently long (up to 10–12 carbons) aliphatic amines, using either covalent bonding or simple sorption, may indeed reduce the boron passage by a factor of 2–4, without impairing the salt rejection of the membrane. The improved selectivity comes at the expense of some flux reduction, but the flux-selectivity tradeoff improves compared with commercial polyamide membranes.