Tailoring the structure of thin film nanocomposite membranes to achieve seawater RO membrane performance

Herein we report on the formation and characterization of pure polyamide thin film composite (TFC) and zeolite-polyamide thin film nanocomposite (TFN) reverse osmosis (RO) membranes. Four different physical-chemical post-treatment combinations were applied after the interfacial polymerization reaction to change the molecular structure of polyamide and zeolite-polyamide thin films. Both TFC and TFN hand-cast membranes were more permeable, hydrophilic, and rough than a commercial seawater RO membrane. Salt rejection by TFN membranes was consistently below that of hand-cast TFC membranes; however, two TFN membranes exhibited 32 g/L NaCl rejections above 99.4%, which was better than the commercial membrane under the test conditions employed. The nearly defect-free TFN films that produced such high rejections were achieved only with wet curing, regardless of other post-treatments. Polyamide films formed in the presence of zeolite nanoparticles were less cross-linked than similarly cast pure polyamide films. At the very low nanoparticle loadings evaluated, differences between pure polyamide and zeolite-polyamide membrane water and salt permeability correlated weakly with extent of cross-linking of the polyamide film, which suggests that defects and molecular-sieving largely govern transport through zeolite-polyamide thin film nanocomposite membranes.     

Depth heterogeneity of fully aromatic polyamide active layers in reverse osmosis and nanofiltration membranes

We studied the depth heterogeneity of fully aromatic polyamide (PA) active layers in commercial reverse osmosis (RO) and nanofiltration (NF) membranes by quantifying near-surface (i.e., top 6 nm) and volume-averaged properties of the active layers using X-ray photoelectron spectrometry (XPS) and Rutherford backscattering spectrometry (RBS), respectively. Some membranes (e.g., ESPA3 RO) had active layers that were depth homogeneous with respect to the concentration and pK(a) distribution of carboxylic groups, degree of polymer cross-linking, concentration of barium ion probe that associated with ionized carboxylic groups, and steric effects experienced by barium ion. Other membranes (e.g., NF90 NF) had active layers that were depth heterogeneous with respect to the same properties. Our results therefore support the existence of both depth-homogeneous and depth-heterogeneous active layers. It remains to be assessed whether the depth heterogeneity consists of gradually changing properties throughout the active layer depth or of distinct sublayers with different properties.     

Quantification of functional groups and modeling of their ionization behavior in the active layer of FT30 reverse osmosis membrane

A new experimental approach was developed to measure the concentration of charged functional groups (FGs) in the active layer of thin-film composite reverse osmosis (RO) and nanofiltration (NF) membranes as a function of solution pH. FT30 RO membrane, with a fully aromatic polyamide (PA) active layer sandwiched between a polysulfone support and a coating layer, was used. The experiments consisted of saturating charged FGs with heavy ion probes, and determining the ion probe concentration by Rutherford backscattering spectrometry (RBS). Deprotonated carboxylic groups were saturated with Ag+, and protonated amine groups with W04(2-). The ionization behavior of carboxylic and amine groups was modeled based on acid-base equilibrium theory. While the ionization behavior of amine groups was satisfactorily described by one dissociation constant (pKa = 4.74), two pKa values (5.23 and 8.97) were necessary to describe the titration curve of carboxylic groups. These results were consistent with the bimodal pore size distribution (PSD) of FT30 active layer reported in the literature. The calculated total concentrations of carboxylic and amine groups in the active layer of the FT30 RO membrane studied were 0.432 and 0.036 M, respectively, and the isoelectric point (IEP) was 4.7. The total concentration of carboxylic and amine groups revealed that the degree of cross-linking of the PA active layer of the FT30 RO membrane studied was 94%.     

RBS characterization of arsenic(III) partitioning from aqueous phase into the active layers of thin-film composite NF/RO membranes

The main objective of this study was to apply Rutherford backscattering spectrometry (RBS) for characterizing the partitioning of arsenic(III) from aqueous phase into the active layer of NF/RO membranes. NF/RO membranes with active layer materials including polyamide (PA), PA-polyvinyl alcohol derivative (PVA), and sulfonated-polyethersulfone (SPES) were investigated. The partition coefficient was found to be constant in the investigated As-(III) concentration range of 0.005-0.02 M at each pH investigated. The partitioning of As(III) when predominantly present as H3AsO3 (pH 3.5-8.0) was not affected by pH. In contrast, the partition coefficient of As(III) at pH 10.5, when it was predominantly present as H2AsO3-, was found to be approximately 33-49% lower than that of H3AsO3. The partition coefficients of H3AsO3 and H2AsO3- for membranes containing PA in their active layers were within the respective ranges of 6.2-8.1 and 3.6-5.4, while the corresponding values (4.8 and 3.0, respectively) for the membrane with SPES active layer were approximately 30% lower than the average values for the PA membranes.     

Temperature effects on the morphology of porous thin film composite nanofiltration membranes

Even though polymeric nanofiltration (NF) and reverse osmosis (RO) membranes often operate on surface waters and surficial groundwaters whose temperature varies over time and with season, very little detailed mechanistic information on temperature effects on membrane selectivity is available to date. Hence, a study was undertaken to investigate the effects of operating temperature (5-41 degrees C) on the morphology and structure of two commercially available thin film composite NF membranes. Application of hydrodynamic models to experimental rejection of dilute solutions of hydrophilic neutral alcohols, sugars, and poly(ethylene glycol)s revealed changes in both the sieving coefficient and permeability of solutes below the membrane glass transition temperature. The vast majority of pores were smaller than 2 nm for both membranes (network pores) even though evidence for a small fraction of larger aggregate pores (approximately 30 nm) was also obtained for one membrane. Increasing temperature appears to cause structural changes in network pores by increasing its pore size while simultaneously decreasing pore density. These increases in pore sizes partially explain reported reductions in contaminant (e.g. arsenic, salts, natural organic matter, hardness, etc.) removal by NF and RO membranes with increasing temperature.     

Performance characterization of nanofiltration membranes based on rigid star amphiphiles

The objective of this study was to develop new nanofiltration (NF) membranes capable of providing significantly greater water permeability and higher rejection of water contaminants compared to state-of-the-art NF membranes. The active layer of the new NF membranes is prepared with rigid star amphiphiles (RSAs) synthesized as part of this study. Performance characterization for a first generation of RSA membranes in a bench-scale apparatus reveals that most of the new membranes provide water permeability of 1.3-3.1 times that of two commercial NF membranes with polyamide active layers while providing comparable rejection of the organic contaminant surrogate Rhodamine WT. However, the rejection of arsenious acid (H3AsO3) by most new NF membranes was found to be lower than that by the two commercial NF membranes tested. Future research efforts of this study will focus on exploring if H3AsO3 rejection could be significantly increased, without negatively affecting water permeability and organic contaminant rejection, by addition of various chemical groups to RSA hydrophobic cores and hydrophilic branches, and by RSA cross-linking.     

Enhanced partitioning and transport of phenolic micropollutants within polyamide composite membranes

Aromatic phenols represent an important class of endocrine-disrupting and toxic pollutants, many of which (e.g., bisphenol A and substituted phenols) are known to be insufficiently removed by reverse osmosis (RO) and nanofiltration polyamide membranes that are widely used for water purification. In this study, the mechanism of phenol transport across the polyamide layer of RO membranes is studied using model phenolic compounds hydroquinone (HQ) and its oxidized counterpart benzoquinone (BQ). The study employs filtration experiments and two electrochemical techniques, impedance spectroscopy (EIS) and chronoamperometry (CA), to evaluate the permeability of an RO membrane SWC1 to these solutes in the concentration range 0.1-10 mM. In addition, combination of the permeability data with EIS results allows separately estimating the average diffusivity and partitioning of BQ and HQ. All methods produced permeability of the order 10(-7) to 10(-6) m s(-1) that decreased with solute concentration, even though the permeability obtained from filtration was consistently lower. The decrease of permeability with concentration could be related to the nonlinear convex partitioning isotherm, in agreement with earlier measurements by FTIR. The diffusivity of HQ and BQ was estimated to be of the order 10(-15) m(2) s(-1) and partitioning coefficient of the order 10. The high affinity of phenols toward polyamide and their high uptake may change membrane characteristics at high concentration of the solute. EIS results and hydraulic permeability indeed showed that permeability to ions and water significantly decreases with increasing concentration of organic solute.     

Effect of flux (transmembrane pressure) and membrane properties on fouling and rejection of reverse osmosis and nanofiltration membranes treating perfluorooctane sulfonate containing wastewater

Perfluorooctane sulfonate (PFOS) is an emergent contaminant of substantial environmental concerns. In this study, reverse osmosis (RO) and nanofiltration (NF) membranes were used to remove this toxic and persistent compound from PFOS-containing wastewater. Five RO membranes and three NF membranes were tested at a feed concentration of 10 ppm PFOS over 4 days, and the PFOS rejection and permeate flux performances were systematically investigated. PFOS rejection was well correlated to sodium chloride rejection. The rejection efficiencies for the RO membranes were > 99%, and those for the NF membranes ranged from 90-99%. Improvement in PFOS rejection, together with mild flux reduction (< 16%), was observed at longer filtration time. Such shifts in rejection and flux performance were probably due to the increased PFOS accumulation at longer duration, as shown by X-ray photoelectron spectroscopy and liquid chromatograph and tandem mass spectrometry results. A fraction of PFOS molecules might be entrapped in the polyamide layer of the composite membranes, which hindered the further passage of both water and other PFOS molecules. In a similar fashion, PFOS rejection and fouling were enhanced for greater initial flux and/or applied pressure, where PFOS accumulation was promoted probably due to increased hydrodynamic permeate drag. Flux reduction was also shown to correlate to membrane roughness, with the rougher membranes tend to experience more flux reduction than the smoother ones.     

黄浆水中功能性成分和主要污染物在组合膜分离中的再分布

为了进一步开展豆制品加工高浓废水的再资源化研究,对黄浆水进行超滤、纳滤和反渗透组合膜处理,并分析各个膜分离单元功能性成分的再分布情况和主要污染物指标的变化。结果显示,超滤单元可以实现糖与蛋白质的分离、不同分子量蛋白质的分离,纳滤单元可截留原液中绝大部分的大豆低聚糖,大豆异黄酮和大豆皂苷的再分离贯穿了所有膜分离单元,纳滤截留液中活性成分多、相对含量高,可进一步分离纯化,反渗透单元出水COD累计去除率达99.6%。     

Biofilm formation characteristics of bacterial isolates retrieved from a reverse osmosis membrane

High-quality water purification systems using reverse osmosis (RO) membrane separation have faced a major challenge related to biofilm formation on the membrane surface, or biofouling. To understand this issue, the biofilm formation characteristics of four bacterial isolates previously retrieved from an RO membrane treating potable water were investigated. Biofilm formation of all four isolates occurred to different extents in microtiter plates and could be related to one or more cell properties (hydrophobicity, surface charge, and motility). For Dermacoccus sp. strain RO12 and Microbacterium sp. strain RO18, bacterial adhesion was facilitated by cell surface hydrophobicity, and for Rhodopseudomonas sp. strain RO3, adhesion was assisted by its low surface charge. Sphingomonas sp. strain RO2 possessed both twitching and swarming motilities, which could be important in mediating surface colonization. Further, strains RO2, RO3, and RO12 did not exhibit swimming motility, suggesting that they could be transported to RO membrane surfaces by other mechanisms such as convective permeate flow. The biofilm formation of RO2 was further tested on different RO membranes made of cellulose acetate, polyamide, and thin film composite in continuous flow cell systems. The resultant RO2 biofilms were independent of membrane surface properties and this was probably related to the ex-opolysaccharides secreted bythe biofilm cells. These results suggested that RO2 could colonize RO membranes effectively and could be a potential fouling organism in RO membranes for freshwater purification.     

Degradation of polyamide nanofiltration and reverse osmosis membranes by hypochlorite

The degradation of polyamide (PA) nanofiltration and reverse osmosis membranes by chlorine needs to be understood in order to develop chlorine-resistant membranes. Coated and uncoated fully aromatic (FA) and piperazine (PIP) semi-aromatic PA membranes were treated with hypochlorite solution and analyzed by X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared (FTIR). XPS results showed that in chlorine treated FA PA membranes the ratio of bound chlorine to surface nitrogen was 1:1 whereas it was only 1:6 in the case of PIP PA membranes. Surface oxygen of uncoated FA and PIP membranes increased with increasing hypochlorite concentration whereas it decreased for coated FA membranes. High resolution XPS data support that chlorination increased the number of carboxylic groups on the PA surface, which appear to form by hydrolysis of the amide bonds (C(O)-N). FTIR data indicated the disappearance of the amide II band (1541 cm(-1)) and aromatic amide peak (1609 cm(-1)) in both coated and uncoated chlorinated FA membranes, consistent with the N-chlorination suggested by the XPS results. Furthermore, the surface charge of chlorinated membranes at low pH (<6) became negative, consistent with amide-nitrogen chlorination. Chlorination appeared to both increase and decrease membrane hydrophobicity depending on chlorination exposure conditions, which implied that N-chlorination and hydrolysis may be competing processes. The effects of property changes on the membrane performance were also observed for NF90, BW30, and NF270 membranes.     

Ion implantation: effect on flux and rejection properties of NF membranes

Nanofiltration (NF) membranes typically carry a net electric charge, enabling electrostatic interactions to play a pivotal role in the rejection of species such as metals, nitrates, and other charged contaminants. In this study, two types of polymeric NF membranes, polyamide and cellulose acetate, were modified by ion implantation to increase the effective surface charge of the membranes. The modified membranes contain implanted ions in the membrane matrix, inducing a discrete, permanent charge in the active membrane layer. The presence of a permanent charge in the membrane matrix allows for increased electrostatic repulsive forces throughout the entire pH range. Streaming potential measurements were conducted as a function of pH for the modified and unmodified membranes to determine the effect of ion implantation on the zeta potential of the membranes. Rejection experiments were performed in order to quantify the effect of increased electrostatic repulsion on ion rejection, and flux measurements quantified the effect of the modification on permeability. Results indicate that electrostatic interactions near the membrane surface can affect rejection; however, the extent of the effect of increased membrane charge depends on physical-chemical characteristics of the membrane. Increased negative zeta potential of the modified membranes resulted in slightly higher rejection of salts with divalent co-ions from the membrane, with less increase observed with salts of monovalent co-ions. Modified membranes were less permeable than the unmodified membranes. Results of this research hold implications in membrane synthesis and modification studies as well as choice of membranes for water treatment applications.     

Recovery and purification of potato proteins from potato starch wastewater by hollow fiber separation membrane integrated process

Potato starch wastewater contains high-concentration potato proteins which have great potential in the fields of food and health care. Most researches on potato protein recovery by membrane separation technique are focused on flat sheet or tubular ultrafiltration (UF) and reverse osmosis (RO) membranes and lack the further protein purification and the in-depth discussions on the fouling behavior. In this laboratory-scale study, potato proteins were recovered and purified from the simulated potato starch wastewater by the self-made hollow fiber (HF) UF and nanofiltration (NF) separation membrane integrated process. 85.62% potato proteins with high molecular weight in the potato starch wastewater could be retained by UF membrane and 92.1% potato proteins with low molecular weight were rejected by NF membrane. The concentrated solution after UF and NF filtration was desalinated and purified by diluting the solution eight times and filtering the diluted solution with UF membrane. Both types of HF membranes, UF and NF, suffered the inevitable membrane fouling. After the traditional physical washing and chemical cleaning, water flux of UF and NF membranes can be effectively recovered. The corresponding recovery rates of UF and NF membranes can reach 93.5% and 84.7%, respectively. The hollow fiber UF-NF separation membrane integrated process was proved to be a promising technique of high-purity potato protein recovery from potato starch wastewater.     

Role of soluble microbial products (SMP) in membrane fouling and flux decline

Soluble microbial products (SMP), a significant component of effluent organic matter (EfOM), play an important role in membrane fouling and flux decline in wastewater reclamation/reuse applications. The SMP compounds of a microbial origin are derived during biological processes of wastewater treatment. They exhibit the characteristics of hydrophilic organic colloids and macromolecules. These high molecular weight compounds play an important role in creating high resistance of the membrane, leading to a reduction of permeate flux. The SMP fouling of RO, NF, and tight UF membranes is associated with formation of a cake/gel layer due to size (steric) exclusion. FTIR spectra of SMP- and EfOM-fouled membranes exhibited foulants' composition, consisting of polysaccharides, proteins, and/or aminosugar-like compounds. This finding reveals the important role of the SMP components as factors in membrane fouling and flux decline associated with EfOM source waters. Solids retention time (SRT) affects the characteristics and amounts of SMP, however, SRT did not affect flux decline trends of RO and NF membranes.     

Toward improved boron removal in RO by membrane modification: feasibility and challenges

Membrane modification by concentration polarization (CP)-enhanced radical graft polymerization using a dilute aqueous solution of appropriate monomer was examined as a method for increasing rejection of boric acid by reverse osmosis (RO) membranes. On the basis of suggested physicochemical rationales a number of monomers were examined in order to determine those with the lowest affinity toward boric acid as compared to water. The improvement in the modified membrane performance was mainly attributed to sealing less selective areas ("defects") inherently present in the original low pressure RO (LPRO) membranes. However, the effect clearly differed for different monomers. Among the examined monomers glycidyl methacrylate (GMA) exhibited the lowest affinity and the largest improvement in removal of boric acid along with a moderate loss of permeability and slightly improved NaCl rejection. Modification of LPRO membrane thus resulted in a membrane with a permeability in the brackish water RO (BWRO) range but with removal of boric acid and salt superior to those reported for most commercial BWRO membranes.     

Thin-film composite pressure retarded osmosis membranes for sustainable power generation from salinity gradients

Pressure retarded osmosis has the potential to produce renewable energy from natural salinity gradients. This work presents the fabrication of thin-film composite membranes customized for high performance in pressure retarded osmosis. We also present the development of a theoretical model to predict the water flux in pressure retarded osmosis, from which we can predict the power density that can be achieved by a membrane. The model is the first to incorporate external concentration polarization, a performance limiting phenomenon that becomes significant for high-performance membranes. The fabricated membranes consist of a selective polyamide layer formed by interfacial polymerization on top of a polysulfone support layer made by phase separation. The highly porous support layer (structural parameter S = 349 μm), which minimizes internal concentration polarization, allows the transport properties of the active layer to be customized to enhance PRO performance. It is shown that a hand-cast membrane that balances permeability and selectivity (A = 5.81 L m(-2) h(-1) bar(-1), B = 0.88 L m(-2) h(-1)) is projected to achieve the highest potential peak power density of 10.0 W/m(2) for a river water feed solution and seawater draw solution. The outstanding performance of this membrane is attributed to the high water permeability of the active layer, coupled with a moderate salt permeability and the ability of the support layer to suppress the undesirable accumulation of leaked salt in the porous support. Membranes with greater selectivity (i.e., lower salt permeability, B = 0.16 L m(-2) h(-1)) suffered from a lower water permeability (A = 1.74 L m(-2) h(-1) bar(-1)) and would yield a lower peak power density of 6.1 W/m(2), while membranes with a higher permeability and lower selectivity (A = 7.55 L m(-2) h(-1) bar(-1), B = 5.45 L m(-2) h(-1)) performed poorly due to severe reverse salt permeation, resulting in a similar projected peak power density of 6.1 W/m(2).     

Positron annihilation spectroscopic evidence to demonstrate the flux-enhancement mechanism in morphology-controlled thin-film-composite (TFC) membrane

In this study, positron annihilation lifetime spectroscopy (PALS) is applied to explain the flux-enhancement mechanism in thin-film-composite (TFC) membranes prepared by using dimethyl sulfoxide (DMSO) as an additive in the interfacial polymerization. The TFC membranes show a large increase in water flux, up to 5-fold, compared to nonadditive membrane. Atomic force microscopy (AFM) shows that surface roughness and surface area increase when DMSO in the aqueous phase solution phase works to increase miscibility of the aqueous and the organic phase by reducing the solubility difference of two immiscible solutions. X-ray photoelectron spectroscopy (XPS) reveals the variation of the chemical compositions to the extent that there is a considerable increase in the cross-linked amide linkages of the flux-enhanced TFC membranes. The effects of these structural changes on the molecular-size free volume properties are evaluated by PALS studies. The PALS results are the first to experimentally show that the thin films of cross-linked aromatic polyamide RO membranes are composed of two types of pores having radii of about 2.1-2.4 A from tau3, network pore, and 3.5-4.5 A from tau4, aggregate pore. The increase in the size and number of network pores by means of DMSO addition during interfacial polymerization enhances the water flux notably. The size of aggregate pores also increases and may contribute to enhance water flux, although their number inevitably decreases as the number of network pores becomes increased. Details on the correlations between RO performances and o-Ps lifetime parameters are clearly described based on the pore-flow model of reverse osmosis developed by Sourirajan et al.     

Cake-enhanced concentration polarization: a new fouling mechanism for salt-rejecting membranes

Results from well-controlled colloidal fouling experiments with reverse osmosis (RO) and nanofiltration (NF) membranes suggest the existence of a new source of flux decline for salt-rejecting membranes-cake-enhanced osmotic pressure. The physical mechanisms leading to this enhanced osmotic pressure are a combination of hindered back-diffusion of salt ions and altered cross-flow hydrodynamics within colloidal deposit layers, which lead to an enhanced salt concentration polarization layer. A model that accounts for both hindered diffusion of salt ions and altered hydrodynamics within colloidal deposit ("cake") layers is presented. The model successfully links permeate flux and salt rejection to cake-enhanced concentration polarization and provides new insight into the mechanisms through which salt-rejecting membranes foul. Experimental data support the model calculations and highlight the role of enhanced concentration polarization phenomena in the performance (i.e., water flux and salt rejection) of polymeric thin-film composite RO/NF membranes in environmental applications.     

Reverse Osmosis and Flavor Retention in Apple Juice Concentration

Plate-and-frame (PF) and spiral-wound (SW) membrane configurations were used to study influence of reverse osmosis (RO) on flavor compounds in apple juice permeate and retentate. Low temperature (20°C) and high pressure (50 bar) were most effective operating parameters for flavor recovery. Non-volatile flavor component RO losses were due to permeation. However, loss of volatile compounds through permeation was minor compared with equipment processing losses. Flavor compounds showed more rejection by polyamide than poly-ether-urea thin-film composite membranes. The SW configuration demonstrated greater flavor losses than did PF. Chemical structure of aldehydes and esters appeared to affect flavor compound retention. Volatile losses modified the flavor profile of RO concentrated apple juice.     

Fouling of nanofiltration, reverse osmosis, and ultrafiltration membranes by protein mixtures: the role of inter-foulant-species interaction

Protein fouling of nanofiltration (NF), reverse osmosis (RO), and ultrafiltration (UF) membranes by bovine serum albumin (BSA), lysozyme (LYS), and their mixture was investigated under cross-flow conditions. The effect of solution chemistry, membrane properties, and permeate flux level was systematically studied. When the solution pH was within the isoelectric points (IEPs) of the two proteins (i.e., pH 4.7-10.4), the mixed protein system experienced more severe flux decline compared to the respective single protein systems, which may be attributed to the electrostatic attraction between the negatively charged BSA and positively charged LYS molecules. Unlike a typical single protein system, membrane fouling by BSA-LYS mixture was only weakly dependent on solution pH within this pH range, and increased ionic strength was found to enhance the membrane flux as a result of the suppressed BSA-LYS electrostatic attraction. Membrane fouling was likely controlled by foulant-fouled-membrane interaction under severe fouling conditions (elevated flux level and unfavorable solution chemistry that promotes fouling), whereas it was likely dominated by foulant-clean-membrane interaction under mild fouling conditions. Compared to nonporous NF and RO membranes, the porous UF membrane was more susceptible to dramatic flux decline due to the increased risk of membrane pore plugging. This study reveals that membrane fouling by mixed macromolecules may behave very differently from that by typical single foulant system, especially when the inter-foulant-species interaction dominates over the intra-species interaction in the mixed foulant system.