Thin Film Composite RO Membranes Research Articles

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.

Effect of Vacuum UV Irradiation and PVA grafting on thin-film composite RO Membranes

Effect of Vacuum UV Irradiation and PVA grafting on thin-film composite RO Membranes

Ionic Liquid-Mediated Interfacial Polymerization for Fabrication of Reverse Osmosis Membranes.

This study revealed the effects of incorporating ionic liquid (IL) molecules: 1-ethyl, 1-butyl, and 1-octyl-3-methyl-imidazolium chlorides with different alkyl chain lengths, in interfacial polymerization (IP) on the structure and property (i.e., permeate-flux and salt rejection ratio) relationships of resulting RO membranes. The IL additive was added in the aqueous meta-phenylene diamine (MPD; 0.1% w/v) phase, which was subsequently reacted with trimesoyl chloride (TMC; 0.004% w/v) in the hexane phase to produce polyamide (PA) barrier layer. The structure of resulting free-standing PA thin films was characterized by grazing incidence wide-angle X-rays scattering (GIWAXS), which results were correlated with the performance of thin-film composite RO membranes having PA barrier layers prepared under the same IP conditions. Additionally, the membrane surface properties were characterized by zeta potential and water contact angle measurements. It was found that the membrane prepared by the longer chain IL molecule generally showed lower salt rejection ratio and higher permeation flux, possibly due to the inclusion of IL molecules in the PA scaffold. This hypothesis was supported by the GIWAXS results, where a self-assembled surfactant-like structure formed by IL with the longest aliphatic chain length was detected.

On the importance of selectivity and support layer compaction in pressure retarded osmosis

In this work, we use a modified thin film composite RO membrane to examine the impacts of selectivity and compaction on the performance of pressure retarded osmosis. RO membranes are promising platforms to study PRO because they are tolerant to typical PRO pressures and are well characterized for permeance and selectivity. Their availability across a range of selectivity (from nanofiltration to seawater RO) also allows us to systematically investigate the impact of selectivity on PRO power density under real PRO conditions. Parallel modeling efforts found that experimentally measured power densities for these membranes were less than 50% of predicted ideal values due to compaction. Interestingly, the highest selectivity membranes, which corresponded to the least permeable, exhibited the highest power densities.

Dissecting the Role of Substrate on the Morphology and Separation Properties of Thin Film Composite Polyamide Membranes: Seeing Is Believing.

Recent studies show that the surface morphology of a thin film composite (TFC) polyamide membrane depends strongly on its porous substrate. Nevertheless, the underlining mechanisms and the effects on membrane separation performance remain controversial. To dissect the exact role of pore properties, we synthesized TFC polyamide membranes on polycarbonate substrates with cylindrical track-etched pores (PCTE) of well-defined pore size ranging from 10 to 800 nm. Leaf-like roughness features were most prominent for polyamide films formed on substrates of intermediate pore sizes (80 and 100 nm). Smaller pores inhibited leaf-like features as a result of insufficient storage of m-phenylenediamine (MPD) monomers for the interfacial reaction, whereas larger pores resulted in diminished surface roughness due to the lack of confinement to the interfacially degassed nanobubbles. Substrate porosity plays a critical role on membrane water permeability, while smaller pores with greater pore density are favored to improve membrane rejection. TFC polyamide membranes prepared on sponge-like poly(ether sulfone) and polysulfone substrates exhibit better water permeability and salt rejection compared to the PCTE-TFC membranes thanks to the simultaneously enhanced confinement and MPD storage effects. The mechanistic insights gained in this study reveal the huge potential of substrate design toward high-performance TFC RO membranes.

A Novel Approach To Optimize the Fabrication Conditions of Thin Film Composite RO Membranes Using Multi-Objective Genetic Algorithm II.

This work focuses on developing a novel method to optimize the fabrication conditions of polyamide (PA) thin film composite (TFC) membranes using the multi-objective genetic algorithm II (MOGA-II) method. We used different fabrication conditions for formation of polyamide layer—trimesoyl chloride (TMC) concentration, reaction time (t), and curing temperature (Tc)—at different levels, and designed the experiment using the factorial design method. Three functions (polynomial, neural network, and radial basis) were used to generate the response surface model (RSM). The results showed that the radial basis predicted good results (R2 = 1) and was selected to generate the RSM that was used as the solver for MOGA-II. The experimental results indicate that TMC concentration and t have the highest influence on water flux, while NaCl rejection is mainly affected by the TMC concentration, t, and Tc. Moreover, the TMC concentration controls the density of the PA, whereas t confers the PA layer thickness. In the optimization run, MOGA-II was used to determine optimal parametric conditions for maximizing water flux and NaCl rejection with constraints on the maximum acceptable levels of Na2SO4, MgSO4, and MgCl2 rejections. The optimized solutions were obtained for longer t, higher Tc, and different TMC concentration levels.

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.

A novel approach to bind graphene oxide to polyamide for making high performance Reverse Osmosis membrane

We report the novel thin film composite RO membrane modified by graphene oxide. The thin film composite RO membrane was exposed to 2000 mg/l sodium hypochloride; thereafter it was subjected to different graphene oxide concentration ranging from 50 mg/l to 1000 mg/l in water. The resultant membrane was crosslinked with 5000 mg/l N-hydroxysuccinimide. The performance of different membranes were analysed by solute rejection and water-flux measurement. It was found that 100 mg/l graphene oxide exposure followed by 5000 mg/l N-hydroxysuccinimide treatment resulted in the membrane with the highest solute rejection of 97.78% and water-flux of 4.64 Liter per sqm per hour per bar g. The membranes were characterized by contact angle for hydrophilicity, scanning electron micrographs for surface morphology, energy dispersive X-Ray for chemical composition of the surface, Atomic force microscope for surface roughness, ATR-FTIR for chemical structure identification. It was found that the graphene oxide modified membrane increases the salt rejection performance after exposure to high-fouling water containing albumin. Highly hydrophilic, antifouling surface formation with the nanomaterial led to the improved membrane performance. Moreover, the protocol of incorporating nanomaterial by this post-treatment is simple and can be applied to any RO membrane after it is manufactured.

Surface modification of thin film composite reverse osmosis membrane by glycerol assisted oxidation with sodium hypochlorite

Need for improvement in water flux of thin film composite (TFC) RO membrane has been appreciated by researchers world over and surface modification approach is found promising to achieve higher water flux and solute rejection. Thin film composite RO membrane was exposed to 2000mg/l sodium hypochlorite solution with varying concentrations of glycerol ranging from 1 to 10%. It was found that there was a drop in concentration of sodium hypochlorite after the addition of glycerol because of a new compound resulted from the oxidation of glycerol with sodium hypochlorite. The water flux of the membrane treated with 1% glycerol with 2000mg/l sodium hypochlorite for 1h was about 22% more and salt rejection was 1.36% greater than that of only sodium hypochlorite treated membrane for the same concentration and time. There was an increase in salt rejection of membrane with increase in concentration of glycerol from 1% to 5%, however, increasing glycerol concentration further up to 10%, the salt rejection declined. The water flux was found declining from 1% glycerol solution to 10% glycerol solution. The membrane samples were characterized to understand the change in chemical structure and morphology of the membrane.

Improving fouling resistance and chlorine stability of aromatic polyamide thin-film composite RO membrane by surface grafting of polyvinyl alcohol (PVA)

Improving fouling resistance and chlorine stability of aromatic polyamide (PA) thin-film composite (TFC) reverse osmosis membrane is still of considerable need in desalination membrane technology. Here, we reported the chemical linkage of neutral hydrophilic polymer polyvinyl alcohol (PVA) on the surface of a commercial PA TFC membrane through a single step of grafting with potassium persulfate as thermal dissociation initiator and its role on the improvement of membrane resistance to both chlorine and fouling. Membrane characterization was conducted through ATR-FTIR spectroscopy, SEM, AFM, measurements of streaming potential and contact angle and cross-flow permeation tests. It was found that membrane surface became smoother, more hydrophilic and less charged after modification and the modified membrane exhibited an increased slat rejection, a slightly declined water flux and improved fouling resistances to the model foulants of bovine serum albumin (BSA), sodium dodecyl sulfate (SDS) and dodecyltrimethyl ammonium bromide (DTAB). The chlorine exposure tests under accelerated conditions also indicated that the membrane chlorine stability has been enhanced effectively. The PVA molecules on the membrane surface could effectively enhance membrane anti-adsorption capability and prevent the underlying polyamide backbones from chlorine attack, and thereby improving membrane resistance to both fouling and chlorine.

A novel high-flux, thin-film composite reverse osmosis membrane modified by chitosan for advanced water treatment

This work demonstrates a novel method for improving the performance of thin-film composite RO membrane by making a polyamide–chitosan composite.

Relating fouling behavior and cake layer formation of alginic acid to the physiochemical properties of thin film composite and nanocomposite seawater RO membranes

In this study, fouling behavior of alginic acid onto four types of hand-cast and one commercially-available seawater reverse osmosis membranes was investigated. Hand-cast membranes included a polyamide thin film composite and a zeolite-polyamide thin film nanocomposite, as well as poly(vinyl alcohol) coated versions of both. Flux decline due to fouling and the structural features of the fouling layers formed during seawater desalination experiments were analyzed using classic Kozeny–Carman cake filtration theory along with a more recently developed crossflow model. Initial rates of flux decline correlated strongly with membrane permeability and root-mean-squared roughness and moderately with alginate–membrane interfacial free energy. The porosity (i.e., hydraulic resistance) of the fouling layers correlated strongly with deionized water contact angle and moderately with surface area difference of the membranes. The contact angle of the membranes also heavily impacted the fouling layer mass and thickness. Membranes with high permeability, strong alginate–membrane interfacial energy of adhesion, and hydrophobic surfaces had the highest propensity for fouling by low porosity, high specific resistance cake layers. Results indicate that reverse osmosis membranes should be produced with hydrophilic, smooth, and large peak-to-peak surfaces in order to reduce flux decline due to cake layer formation.

Surface modification of thin film composite RO membrane for enhanced anti-biofouling performance

Anti-adhesion and antimicrobial coatings were prepared and applied on commercial thin-film-composite (TFC) polyamide (PA) membrane to enhance anti-biofouling performance. Polyvinyl alcohol (PVA) coating was modified with cationic polyhexamethylene guanidine hydrochloride (PHMG) polymer to obtain antimicrobial performance. ATR-FTIR, SEM and AFM investigated the surface chemistry and morphology of the coated membranes. The contact angle measurement was used to determine hydrophilicity and surface energy. All coated membranes revealed more hydrophilic and lower surface roughness compared to uncoated membrane. Lower number of adhered Pseudomonas aeruginosa (P. aeruginosa) bacteria was detected on coated membranes, indicating anti-adhesion performance. The colony forming unit (CFU) and diffusion inhibition zone (DIZ) tests determined antimicrobial activity of the coated membranes against Escherichia coli (E. coli) and Bacillus subtilis (B. subtilis), showing the antimicrobial performance of PHMG. The results suggested that an optimal anti-fouling surface could be obtained applying a coating, which combines anti-adhesion and antimicrobial performance.

Interfacially synthesized chlorine-resistant polyimide thin film composite (TFC) reverse osmosis (RO) membranes

Chlorine-resistant polyimide thin film composite (TFC) RO membranes were prepared via interfacial polymerization of m-phenylene diamine (MPD) and 1,2,4,5-benzene tetracarbonyl chloride (BTC) and subsequent thermal imidization. Thermogravimetric analysis, differential scanning calorimetry and attenuated total reflection-Fourier transform infrared spectroscopy confirmed that the use of a tertiary amine catalyst allowed the preparation of the polyimide films at below the glass transition temperature of the polysulfone support sublayer, thus preventing its degradation. The effects of annealing on the membranes performance were investigated. Increasing temperature and duration during the thermal treatment accelerated the imidization of the poly(amic acid) membranes and converted their amorphous structures to semi-crystalline structures, deteriorating their performance. The addition of a cross-linking agent, trimesoyl chloride (TMC), during the interfacial polymerization improved the membrane's performance. The polyimide thin film composite, reverse osmosis membranes prepared from MPD, BTC and TMC showed significantly enhanced chlorine resistance due to the elimination of chlorine-sensitive sites by the replacement of amide linkage with imide linkage.

Rejuvenation of discarded RO membrane for new applications

Abstract The sensitivity of thin film composite RO membrane (TFCs) with chlorine has been widely reported. In the present paper, attempt has been made to modify the chemical structure of thin film composite RO membrane by exposing the membrane to free chlorine in order to evaluate potential of utilizing the discarded membrane after its useful life. The different variants of thin film composite RO membrane in terms of flux and rejection could be prepared by altering the exposure time and concentration of sodium hypochlorite solution. Tailoring the membrane for solute rejection offers advantages over the conventional process for instance: (1) It avoids re-mineralization and directly produces the water of required quality; (2) It enhances flux of membrane and thereby improves the productivity of the membrane; (3) Improved recovery leads to lower capacity pre-treatment and thereby improves the operational efficiency; (4) Number of membrane required for the given capacity will be less and the associated unit cost, e.g. pumping cost will also decrease. Successful attempt has been made to enhance permeability seven times that of the terminally fouled membrane by sequential hypochlorite treatment in controlled fashion, i.e. fouled seawater RO membrane with flow 3.36 l/min and solute rejection of 97.21% has been converted to the membrane with flow 24 l/min and solute rejection 82.37%.Moreover, the thin film composite RO membrane has been degraded totally till its solute rejection efficiency falls to approximately zero in order to convert it to microfiltration/ultra-filtration membrane, i.e. the membrane with pure water permeability 257 gfd @50 psig pressure has been made by exposing 3286 mg/l free chlorine for 96 h to TFC RO membrane. The characterization of this membrane was performed by measuring molecular weight cut off of the membrane and pure water permeability of the membrane to evaluate its performance as an ultra-filtration membrane. These membranes were also tested for micro-organism removal and found that their performance resembles the new ultra-filtration membrane. The hypochlorite treated membranes were also evaluated for their permeability after bio-film formation with different micro-organisms where it manifested higher permeability as compared to untreated membrane.

Hypochlorite treatment on thin film composite RO membrane to improve boron removal performance

Controlled hypochlorite treatment was conducted on three commercial RO membranes to improve their boron removal performance. The effects of hypochlorite treatment on membrane performance, including permeate flux, NaCl rejection as well as boron removal were investigated. The effect of pH of feed water on both original and chlorinated membranes was also investigated. In order to better understand the membrane performance changes related to membrane structure and physicochemical property changes caused by hypochlorite treatment, the membranes before and after chlorination were characterized by XPS, ATR-FTIR, SEM, AFM, surface contact angle, zeta potential and isopropanol (IPA) rejection. The results showed that all the chlorinated membranes had better performance than the original ones, including boron removal, NaCl rejection and permeate flux. Boron can be removed efficiently at pH of 11 after hypochlorite treatment and boron concentration of permeate water was less than 0.5 mg·L − 1 . Chlorine had been bound to the surface of the membrane, which led to a more hydrophilic, more negatively charged surface and without change of the morphology. The negative shift of zeta potential and increase of IPA rejection indicated that both electrostatic repulsion and size exclusion effects contributed to the enhanced boron and NaCl rejection.

Flux enhancement of thin film composite RO membrane by controlled chlorine treatment

A major stumbling block in polyamide thin film composite RO membrane performance is its incompatibility with chlorine and oxidizing agents. The amide bond of the membrane is highly vulnerable to chlorine attack. Two reactions are possible with chlorine exposure i.e. N–H bond chlorination and/or aromatic ring chlorination. In this way, chlorine may cause degradation/modification in the membrane leading to deterioration in performance. However, low concentration of chlorine up to a certain time may give synergistic effect on membrane and improve its performance. Chlorine solution, if exposed to membrane for certain time gives enhancement in trans-membrane flux of the membrane. The same solution if exposed for more time deteriorates ultra-thin polyamide layer of TFC membrane. Conspicuously, the membrane with poor salt rejection and flux benefited more as compared to the membrane with better performance. In the present study, membranes with different salt rejection and flux were taken and exposed to the inorganic chlorine solution. The inorganic chlorine solutions were made by dissolving sodium hypochlorite in pH buffer. The different solutions were made by varying pH to investigate the pH dependence. The membrane samples were kept in solution for different time durations. The exposure time was monitored and the exposure level was taken in terms of ppm h (ppm chlorine solution exposed to membrane for a fixed time in h). With the same chlorine concentration, effect of varying pH was studied. Spiral wound TFC membrane modules were also subjected to chlorine solution to study its effect.

Preparation and characterization of thermally crosslinked chlorine resistant thin film composite polyamide membranes for reverse osmosis

Currently, polyamide reverse osmosis membranes are highly effective for desalination, industrial process water, and home drinking water. However, they have poor resistance to strong oxidants especially chlorine due to chain cleavage of aromatic polyamide. In general, aromatic polyamide RO membranes are essentially random copolymers consisting of the linear and crosslinked structures. The amide ring is sensitive to attack by chlorine because it is an electron-rich region. Therefore, the activated carbon or sulfite addition processes are essential to remove the chlorine in the separation processes. Many research groups have studied to improve the chlorine-resistance RO membrane having hydrophilic groups (− SO 3H and − COOH) or nitro groups (− NO 2) such as electron acceptors. In this study, thin film composite polyamide RO membranes were prepared by interfacial polymerization method including cross-linking agents having hydroxyl groups to improve the chlorine-resistance. The chlorine-resistance of polyamide RO membrane was influenced by the thermal cross-linking conditions (temperature and time) and cross-linking density of polyamide membranes.