Fouling of reverse osmosis and nanofiltration membranes by humic acid—Effects of solution composition and hydrodynamic conditions

Fouling of reverse osmosis (RO) and nanofiltration (NF) membranes by humic acid, a recalcitrant natural organic matter (NOM), was systematically investigated. The membrane flux performance depended on both hydrodynamic conditions (flux and cross-flow velocity) and solution composition (humic acid concentration, pH, ionic strength, and calcium concentration), and was largely independent of virgin membrane properties. While increasing humic acid concentration and ionic strength, and lowering cross-flow velocity affected flux performance moderately, severe flux reduction occurred at high initial flux, low pH, and high calcium concentration. At a calcium concentration of 1 mM, all the membranes exhibited an identical stable flux, independent of their respective intrinsic membrane permeabilities. The effect of solution composition was more significant at higher fluxes. Improved salt rejection was observed as a result of humic acid fouling, which was likely due to Donnan exclusion by humic material close to membrane surfaces. Greater rejection improvement was observed for membranes with rougher surfaces.     

Characterisation of zirconium/poly(acrylic acid) low pressure dynamically formed membranes by use of the extended Nernst-Planck equation

The rejection of a single electrolyte solution, by a hydrous zirconium oxide/poly(acrylic acid) (Zr/PAA) dual layer dynamically formed membrane (DFM), has been investigated. A flat sheet titania-coated sintered stainless steel (IMAS UK) was used as a substrate for DFM formation. Flux and rejection were recorded for a series of experiments at different transmembrane pressures, feed solution cross-flow rates and salt concentration. Experimental data was interpreted using a model based on the extended Nernst-Planck equation. This interpretation allows characterisation of the membrane in terms of two parameters, the effective membrane charge density and a structural parameter which combines porosity and membrane thickness. Good correlation between experimental data and theory has been obtained. Calculation of the effective membrane charge density and the structural parameter at pH 6.7 and 9.0 for a range of salt concentrations provides conclusive evidence that membrane pore size decreases with increasing salt concentration and that the degree of ionisation of the PAA contained within the membrane increases with increasing pH and salt concentration.     

Nanofiltration of biogenic amines in acidic conditions: Influence of operation variables and modeling

Biogenic amines are present in some fermented and non-fermented beverages and can cause diseases. This study analyzes the feasibility of separating biogenic amines by nanofiltration in acidic medium. Solutions of chloride salts of three biogenic amines: putrescine, histamine and tyramine were filtered through a nanofiltration membrane with a 1000 Da molecular weight cut-off (MWCO) and a positive electrical charge at pH 3. Increasing the transmembrane pressure or cross flow velocity led to an increase in solute rejection and permeate flow. Moreover, a higher electrical charge or lower concentration of amine cations caused a larger rejection indicating that membrane-solutes repulsion governs the filtration process. Finally, the experimental results were analyzed using the classic Donnan–Steric pore model. Values of 0.83 nm and 5.4 μm were estimated for pore radius and membrane effective ratio thickness-porosity from the filtration of neutral solutes. Membrane volumetric charge density and the proton diffusivity inside the pores were estimated from the experimental results.     

Adsorption of poly(acrylic acid) onto the surface of titanium dioxide and the colloidal stability of aqueous suspension

The adsorption of poly(acrylic acid) (PAA) in aqueous suspension onto the surface of TiO(2) nanoparticles was investigated. FTIR spectroscopic data provided evidence in support of hydrogen bonding and chemical interaction in the case of the PAA-TiO(2) system. Adsorption isotherms demonstrated that part of the PAA initially added to the suspension was adsorbed onto the TiO(2) surface, after which there was a gradual attainment of an adsorption plateau. The adsorption density of PAA was found to increase with an increase of PAA molecular weight, while it decreased with an increase of pH. The thickness of the PAA adsorption layer was calculated based on measurements of suspension viscosities in the absence and presence of PAA. It was shown that the thickness of the adsorption layer increased with the increase of pH, PAA molecular weight, and its concentration. The surface charge density, the diffuse charge density, and the zeta potential of TiO(2) varied distinctly after PAA adsorption. The shift of pH(iep) toward a lower pH value was observed in the presence of PAA. PAA was found to stabilize the suspension of TiO(2) nanoparticles through electrosteric repulsion. The influence of factors such as PAA molecular weight and its concentration on the colloidal stability of the aqueous suspension was also investigated.     

PEG-coated reverse osmosis membranes: Desalination properties and fouling resistance

This study focuses on the use of surface-coated reverse osmosis (RO) membranes to reduce membrane fouling in produced water purification. A series of crosslinked PEG-based hydrogels were synthesized using poly(ethylene glycol) diacrylate as the crosslinker and poly(ethylene glycol) acrylate, 2-hydroxyethyl acrylate, or acrylic acid as comonomers. The hydrogels were highly water permeable, with water permeabilities ranging from 10.0 to 17.8 (L μm)/(m² h bar). The hydrogels were applied to a commercial RO membrane (AG brackish water RO membrane from GE Water and Process Technologies). The water flux of coated membranes and a series-resistance model were used to estimate coating thickness; the coatings were approximately 2 μm thick. NaCl rejection for both uncoated and coated membranes was 99.0% or greater, and coating the membranes appeared to increase salt rejection, in contrast to predictions from the series-resistance model. Zeta potential measurements showed a small reduction in the negative charge of coated membranes relative to uncoated RO membranes. Model oil/water emulsions were used to probe membrane fouling. Emulsions were prepared with either a cationic or an anionic surfactant. Surfactant charge played a significant role in membrane fouling even in the absence of oil. A cationic surfactant, dodecyltrimethyl ammonium bromide (DTAB), caused a strong decline in water flux while an anionic surfactant, sodium dodecyl sulfate (SDS), resulted in little or no flux decline. In the presence of DTAB, the AG RO membrane water flux immediately dropped to 30% of its initial value, but in the presence of SDS, its water flux gradually decreased to 74% of its initial value after 24 h. DTAB-fouled membranes had lower salt rejection than membranes not exposed to DTAB. In contrast, SDS-fouled membranes had higher salt rejection than membranes not exposed to SDS, with rejection values increasing, in some cases, from 99.0 to 99.8% or higher. In both surfactant tests, coated membranes exhibited less flux decline than uncoated AG RO membranes. Additionally, coated membranes experienced little fouling in the presence of an oil/water emulsion prepared from DTAB and n-decane. For example, after 24 h the water flux of the AG RO membrane fell to 26% of its initial value, while the water flux of a PEGDA-coated AG RO membrane was 73% of its initial value.     

Characterization of a planar poly(acrylic acid) brush as a materials coating for controlled protein immobilization

The adsorption of two different proteins at a planar poly(acrylic acid) (PAA) brush was studied as a function of the ionic strength of the protein solutions applying total internal reflection fluorescence (TIRF) spectroscopy. Planar PAA brushes were prepared with a grafting density of 0.11 nm(-2) and were characterized using X-ray reflectometry. Hen egg-white lysozyme and bovine serum albumin (BSA) were used as model proteins, which have a net positive and negative charge at neutral pH-values, respectively. It has been found that both proteins adsorb strongly at a planar PAA brush at low ionic strength. Whereas lysozyme interacts with a PAA brush under electrostatic attraction at neutral pH-values, BSA binds under electrostatic repulsion at pH > 5. Even at pH = 8, significant amounts of BSA are adsorbed to a planar PAA brush. In addition, the reversibility of BSA adsorption has been characterized. Dilution of a BSA solution leads to an almost complete desorption of BSA from a PAA brush at short contact times. When the ionic strength of the protein solutions is increased to about 100-200 mM, a planar PAA brush appears largely protein-resistant, regardless of the protein net charge. The results of this study indicate that the salt-dependent protein affinity of a PAA brush represents a unique effect that must be explained by a novel protein-binding mechanism. On the basis of a recent model, it is suggested that a release of counterions is the most probable driving force for protein adsorption at a PAA brush. In a general view, this study characterizes a planar PAA brush as a new materials coating for the controlled immobilization of proteins whose use in biotechnological applications appears to be rewarding.     

Streaming potential measurements as a characterization method for nanofiltration membranes

The streaming potentials of two different nanofiltration membranes were studied with several electrolyte solutions to investigate the influence of salt type and concentration on the zeta potential and kinetic surface charge density of the membranes. The zeta potentials decreased with increasing salt concentration, whereas the kinetic surface charge densities increased. The kinetic surface charge densities could be described by Freundlich isotherms, except in one case, indicating that the membranes had a negligible surface charge. The kinetic surface charge density observed was caused by adsorbed anions. Salt retention measurements showed different mechanisms for salt separation for the two investigated membranes. One membrane showed a salt retention that could be explained by a Donnan exclusion type of separation mechanism, whereas for the other membrane the salt rejection seemed to be a combination of size and Donnan excluion. Comparing the results obtained by the streaming potential measurements with those of the retention measurements, it could be concluded that the membrane with the highest kinetic surface charge density showed the Donnan exclusion type of separation, whereas the membrane with the lower surface charge density showed a separation mechanism that was not totally determined by Donnan exclusion, size effects seemed to play a role as well.     

Stabilization of gamma alumina slurry for chemical-mechanical polishing of copper

Stabilization of gamma-alumina suspension for chemical-mechanical polishing (CMP) of copper was investigated. Citric acid and poly(acrylic acid) (PAA) (M(w)=5000) were used as dispersant. The stability of suspension was evaluated from the changes in viscosity, particle size and zeta potential. It appears that metastable gamma-alumina mainly due to its high specific surface area and to the presence of aluminol groups on its surface is progressively transformed to bayerite (beta-Al(OH)(3)) by hydration procedure. Citric acid molecules were adsorbed onto gamma-alumina surface effectively and exhibited the excellent hydration inhibition effect. Although citrate-alumina surface complexes give barrier to the flocculation, the repulsion potential is based mainly on the electrostatic repulsion, thereby steric hindrance caused by the adsorption of these small molecules is very weak. The electrosteric repulsion, which provides more effective dispersion stability than electrostatic repulsion force, can be expected by using polyelectrolyte such as PAA; however, adsorbed layers of PAA onto solid/liquid interface are loosely formed. Therefore, a large amount of PAA was required to inhibit the surface hydration of gamma-alumina suspension, thereby the excess addition of PAA decreased the electrosteric repulsion and re-bridging of the dispersant between particles caused an increase in suspension viscosity. Therefore, synergistic effect can be expected in mixed dispersant system of citric acid and PAA, since small citric acid molecules are adsorbed faster than PAA, inhibiting the progress of surface hydration, and then adsorbed PAA layers exhibit the effective electrosteric repulsion interaction between particles with a small amount compared with PAA alone. It was revealed that the gamma-alumina slurry dispersed by mixed dispersant exhibited the improved removal rate of Cu layer by CMP polishing test.     

Ultrathin pH‐Sensitive Nanoporous Membranes for Superfast Size‐Selective Separation

Stimuli‐responsive nanoporous membranes have attracted increasing interest in various fields due to their abrupt changes of permeation/separation in response to the external environment. Here we report ultrathin pH‐sensitive nanoporous membranes that are easily fabricated by the self‐assembly of poly(acrylic acid) (PAA) in a metal hydroxide nanostrand solution. PAA‐adsorbed nanostrands (2.5–5.0 nm) and PAA‐Cu^II nanogels (2.0–2.5 nm) grow competitively during self‐assembly. The PAA‐adsorbed nanostrands are deposited on a porous support to fabricate ultrathin PAA membranes. The membranes display ultrafast water permeation and good rejection as well as significant pH‐sensitivity. The 28 nm‐thick membrane has a water flux decrease from 3740 to 1350 L m^?1 h^?1 bar^?1 (pH 2.0 to 7.0) with a sharp decrease at pH 5.0. This newly developed pH‐sensitive nanoporous membranes may find a wide range of applications such as controlled release and size‐ and charge‐selective separation.     

Synthesis and characterization of an amphoteric ion-exchange membrane

An amphoteric ion-exchange membrane was prepared by chemical grafting of acrylic acid and dimethyl amino-2-ethyl methacrylate on ozonized polyethylene. The effects of the variation of external pH on the physicochemical properties of this membrane were studied (exchange capacity, electrical resistance, water and electrolyte content, and membrane potential). The determination of the mean activity coefficient of the NaCl in the membrane phase using the Donnan equilibrium equation reveals that this coefficient increases with the NaCl concentration in the external solution. Fundamental electrochemical properties of the amphoteric membrane are discussed on the basis of the variation of the membrane selectivity with the external electrolyte concentrations and the pH. The properties at neutral pH are accounted for by a mutual neutralization of some acidic and basic groups in the membrane phase.     

Surface modification of nanofiltration membranes to improve the removal of organic micro-pollutants (EDCs and PhACs) in drinking water treatment: Graft polymerization and cross-linking followed by functional group substitution

A commercially available thin film composite (TFC) polyamide (PA) nanofiltration (NF) membrane was chemically modified to improve its rejection capacity for selected organic micro-pollutants categorized as endocrine disrupting chemicals (EDCs) and pharmaceutically active compounds (PhACs): bisphenol-A (BPA), ibuprofen, and salicylic acid. The raw NF membrane was altered using the following modification sequence: graft polymerization (methacrylic acid (MA)-membrane); cross-linking of grafted polymer chains (ethylene diamine (ED)-membrane); and, substitution of functional groups (succinic acid (SA)-membrane). Attenuated total reflective Fourier transform infrared (ATR-FTIR) was used to verify each modification in the sequence: the formation of amide bonds; graft polymerization and cross-linking; and, increased carboxylic acids on the modified membrane. Based on zeta-potential and contact angle measurements, graft polymerization increased the negative charge and hydrophilicity of the raw membrane, while cross-linking replaced carboxylic acid with amide bonds, which made the modified membrane almost neutral at pH 6.5. The water fluxes of the ED- and SA-membranes were similar to that of the raw membrane; however, the water flux of the MA-membranes varied with polymerization time (the membrane polymerized for 15 min revealed ≥20% higher flux than the raw membrane). BPA rejection by the raw membrane was substantially improved from 74% to ≥95% after this series of modifications. However, the rejection capacity of the ED-membrane for ibuprofen and salicylic acid was slightly reduced compared with those of the MA-membrane, which was polymerized for 15 min, due to the lack of an electrical repulsion mechanism. The SA-membrane recovered its negative surface charge and showed a clear enhancement in the rejection of all pollutants.     

Chitosan–poly(acrylic acid) nanofiber networks prepared by the doping induction of succinic acid and its ammonia‐response studies

Chitosan–poly(acrylic acid) (CS–PAA) composite membrane with a 3D network nano‐structure was prepared using an electrostatic interaction process by adding succinic acid as a branch promoter. Variations of the final solution pH values, concentration of CS, and PAA/CS volume ratio were examined systematically for their effects on average fiber diameter size, intensity of surface charge, and tendency of network formation. It was found that nanofiber size was affected by the mixing ratio of PAA and CS, the concentration of CS, and the final pH of the CS–PAA solution. The smallest diameter size distribution of the scaffold can be obtained when the PAA/CS ratio is in the range of 2:1–1:2 in a pH 3 environment. Negative charge nanofibers prepared using PAA and CS in a ratio of 2:1 in pH 3 environments had an average diameter of 215 nm. The formation of the interconnecting 3D self‐organized network structure can be built up with limited parasitic branching by crystallized succinic acid. The gas response to ammonia, including sensitivity and response time, was evaluated using impedance spectroscopy at room temperature. The results of sensing experiments indicate that the sensitivity of nanofibrous membrane (NM)‐coated sensors was eight times higher than that of continuous film‐coated sensors. NM‐coated sensors exhibited high sensitivity towards a low concentration of ammonia, as low as 50 ppm at a relative humidity of 45%. Copyright © 2008 John Wiley & Sons, Ltd.     

Some properties of electrolyte solutions in nanoconfinement revealed by the measurement of transient filtration potential after pressure switch off

We have demonstrated that with a composite nanoporous ceramic membrane in a batch membrane cell it is technically feasible to switch off the trans-membrane hydrostatic pressure difference within tens of milliseconds. That enabled us to resolve practically the whole time evolution of transient filtration potential. Measurements of the latter have been complemented by measurements of steady-state salt rejection by the composite membrane and by measurements of the streaming potential and hydraulic permeability of membrane supports available separately. A theory has been developed in terms of network thermodynamics for the electrical response of a bilayer membrane to a pressure perturbation. In combination with the results of salt rejection measurements, from the time transients of filtration potential we could determine the ion transport numbers within the nanoporous layer. Besides that, from the dependence of steady-state salt rejection on the trans-membrane volume flow, we have determined the diffusion permeability of and the salt reflection coefficient in the nanoporous layer. This has enabled us to estimate the contributions of Donnan and non-Donnan mechanisms to the rejection of ions by the nanoporous membrane used in this study. It has been unexpectedly found that the Donnan exclusion played only a secondary role. Our hypothesis is that the non-Donnan exclusion of ions from the nanopores might be caused by changes in water properties in nanoconfinement. Proceeding from the results of steady-state filtration experiments with the membrane and the support, we also concluded that the nanoporous layer was imperfection-free and had a quite narrow pore size distribution, which made it a suitable object for fundamental studies of ion transfer mechanisms in nanopores.     

Membrane potential across reverse osmosis membranes under pressure gradient

Membrane potential measurement has been widely used for the characterization of ionic membranes such as ion-exchange membranes without solvent permeability. However, there have been few studies on membrane potentials across pressure-driven processes such as reverse osmosis (RO) membranes with solvent permeability. In the present study, the membrane potential across RO membranes in NaCl and MgCl2 under the pressure gradient, DeltaP=0-0.3 MPa, was measured. The experimental results were analyzed by the theoretical model based on the Donnan equilibrium and the extended Nernst-Planck flux equation considering the pressure effect. The theoretical values agreed well with the experimental ones. This indicates that membrane potential is useful for characterizing the effective charge density of the active layer of RO membranes under pressure gradient.     

Surfactant modified ultrafiltration for nitrate ion removal

It is confirmed that small concentrations of cationic surfactant radically modify cellulose acetate membrane properties concerning nitrate ion rejection. This modification is produced by a strong adsorption of surfactant molecules on the membranes which acts on the Donnan potential difference. A mechanism for nitrate rejection is proposed, which accounts for the important parts played by counterion valence and operating pressure drop in the phenomenon.     

Reverse osmosis filtration for space mission wastewater: membrane properties and operating conditions

Reverse osmosis (RO) is a compact process that has potential for the removal of ionic and organic pollutants for recycling space mission wastewater. Seven candidate RO membranes were compared using a batch stirred cell to determine the membrane flux and the solute rejection for synthetic space mission wastewaters. Even though the urea molecule is larger than ions such as Na+, Cl-, and NH4+, the rejection of urea is lower. This indicates that the chemical interaction between solutes and the membrane is more important than the size exclusion effect. Low pressure reverse osmosis (LPRO) membranes appear to be most desirable because of their high permeate flux and rejection. Solute rejection is dependent on the shear rate, indicating the importance of concentration polarization. A simple transport model based on the solution-diffusion model incorporating concentration polarization is used to interpret the experimental results and predict rejection over a range of operating conditions. Grant numbers: NAG 9-1053.     

Study on hypochlorite degradation of aromatic polyamide reverse osmosis membrane

A serious limitation of most commercial polyamide reverse osmosis (RO) membranes is their sensitivity to chlorine attack. By studying the hypochlorite degradation of aromatic polyamide RO membrane, this work was to get some understandings in the prevention of membrane depreciation and develop membranes with improved chlorine resistance. Membrane performances, including water flux and salt rejection, were evaluated before and after hypochlorite exposure under different pH and concentration conditions. The results showed that chlorination destroyed hydrogen bonds in polyamide chains, causing a notable decline of membrane flux especially in acid environment; however, membrane performance was slightly improved after the treatment of alkaline hypochlorite solution for a certain time, which was probably due to the effect of amine groups in barrier layer. Based on the attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR) characterizations and performance measurements, the results indicated that N-chlorination reaction of aromatic polyamide was also reversible, in other words, the N-chlorinated intermediate could be regenerated to initial amide with the alkaline treatment before ring-chlorination reaction. This conclusion provided several relative suggestions for membrane cleaning procedures. Finally, a method adopting surface coating was proposed to develop membranes with good chlorine resistance, and the preliminary results showed its potential for applications.     

Grafting of hydroxymethylacrylamide and acrylic acid copolymer onto polyvinylidene fluoride membrane by supercritical carbon dioxide and its application in dye separation

We report a thermally induced graft copolymerization of acrylic acid and N‐hydroxymethylacrylamide onto polyvinylidene fluoride microporous membrane by using supercritical carbon dioxide as a solvent and carrier agent. The effects of monomer mole ratio, pressure, reaction temperature, time, and initiator concentration on the degree of grafting were investigated. The morphology of the grafted membrane was studied via scanning electron microscopy. Attenuated total reflectance infrared spectroscopy spectroscopy and X‐ray diffraction proved the existence of grafting monomers. The results of contact angle measurement and water filtration at different pH levels indicated pronounced pH‐sensitive behavior and increased hydrophilicity of the polyvinylidene fluoride grafted membrane. In particular, the grafted membrane was applied to the removal of methylene blue with a relative rejection of 98.1% compared with 30.5% of rejection observed from the pristine polyvinylidene fluoride membrane. Copyright © 2014 John Wiley & Sons, Ltd.     

Factors influencing reverse osmosis rejection of inorganic solutes from aqueous solution

Reverse osmosis (RO) rejection is strongly influenced by the distribution of solute between the membrane and solvent phases. For this reason, we examined the partition coefficients of inorganic compounds between water and cellulose acetate (CA) membranes. Cation and anion partition coefficients were determined by independent analyses. Effects of fixed (negative) membrane charges on CA are clearly apparent at low solute concentrations. The mean cation/anion partition coefficients decrease with the product of the cation and anion valence, and increase with increasing ionic size. Un-ionized inorganic compounds, HgC1₂ and HAuC1₄, are strongly sorbed by CA membranes. All of these observations are consistent with electrostatic theory.Experimental membrane/water partition coefficients are influenced by temperature, pH, and ion-pairing. CA membranes exhibit swelling and shrinkage when exposed to certain aqueous solutions. Swelling and shrinkage influence solute partition and diffusion coefficients, the water content of the membranes, and their RO rejection.The present results provide a comprehensive experimental basis for understanding the mechanism of RO rejection by CA membranes. Moreover, these results can be used to predict RO behavior under a wide variety of experimental conditions. The potential use of reverse osmosis in a variety of wastewater applications is considered in some detail.     

聚多巴胺在强负电型微球表面的形貌调控

多巴胺(DA)已被证实可在多种材料的表面进行氧化自聚而形成聚多巴胺(PDA),但其在带强负电荷的表面上的聚合机理和所形成的形貌却还不太明确。 为考察材料表面的电负性、氧化条件等对DA氧化自聚速度和聚多巴胺层形貌的影响,本文通过无皂乳液聚合制备了以聚苯乙烯(PS)为核、聚丙烯酸(PAA)为壳的纳米粒子(PS/PAA NPs),探究其表面的阴离子在不同pH缓冲液、反应时间下与DA的加入量对聚合过程及其形貌的影响。 通过扫描电子显微镜(SEM)、透射电子显微镜(TEM)和Zeta电势对所得纳米粒子的结构、形貌与尺寸等进行分析。 结果发现,在pH值为8.5的三羟甲基氨基甲烷(Tris)缓冲溶液中,当DA与PS/PAA NPs的质量比为1∶1时,在反应24 h后,PDA以纳米颗粒的形式存在于PS/PAA NPs的表面,即所形成的PS/PAA/PDA NPs为明显的树莓状结构。 随着DA含量的增加和反应时间的延长,所形成的树莓状粒子粒径增大;当DA远远过量时,PDA最终在微球表面形成致密、均匀的壳层。 Zeta电势结果表明树莓状结构是由于DA在PAA表面聚合过程中受到静电相互作用和电荷排斥作用间的竞争而形成的,随着DA量的增大和反应时间的延长,形成的静电作用增大,使较多的PDA粘附至PAA表面而形成较致密的均匀壳层。 同时,DA在电负性较小的PS/SDS微球和非离子的PS微球表面聚合时,均形成致密的PDA壳层。 因而,材料表面的电负性大小可以调控DA在其表面的沉积,以制备不同形貌的PDA复合材料。