Eric Hoek

University of California, Los Angeles, Environmental Science and Engineering, Faculty Member

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Interfacial polymerization of thin film nanocomposites: A new concept for reverse osmosis membranes

by Dr A K Ghosh and Eric Hoek

Here, we report on a new concept for formation of mixed matrix reverse osmosis membranes by inter... more

Role of Specific Ion Interactions in Seawater RO Membrane Fouling by Alginic Acid

Environmental Science & Technology, 2009

Organic fouling plagues many environmental membrane processes. In this study, well-controlled lab... more Organic fouling plagues many environmental membrane processes. In this study, well-controlled laboratory experiments were performed to elucidate seawater RO membrane fouling by alginic acid. Interfacial free energies derived from multiple probe liquid contact angle analyses (including different seawater matrices) correlated strongly with the rates of membrane fouling. More importantly, the Lewis acid-base interfacial free energy quantitatively described the impacts of calcium-carboxylate complex formation and predicted membrane fouling and cleaning behavior. Calcium ions made polyamide composite RO membranes (and alginic acid) more hydrophobic, enhanced the rate and extent of flux decline, and reduced the effectiveness of chemical cleaning. The implications for seawater RO membrane fouling are clear. Selective removal of calcium ions via pretreatment can reduce the gel forming ability of carboxylate rich biomacromolecules and, hence, the extent to which they foul RO membranes. In addition, RO membranes should be produced with smooth, hydrophilic interfaces comprising monopolar electron-donor functionality and no carboxylic acid residue. More broadly, this paper presents a facile approach for quantifying the impacts of specific ion interactions on aquatic colloid stability, aggregation, and deposition.

Impacts of support membrane structure and chemistry on polyamide–polysulfone interfacial composite membranes

Journal of Membrane Science, 2009

Herein, we discuss the properties of polyamide thin films formed via identical interfacial polyme... more Herein, we discuss the properties of polyamide thin films formed via identical interfacial polymerization conditions over porous polysulfone supports with different physical and chemical properties. Polysulfone supports were formed by nonsolvent induced phase separation (i.e., immersion precipitation) employing several casting solution and precipitation bath chemistries. Hand-cast polysulfone films (and a commercial polysulfone membrane) exhibited a wide range of pure water permeabilities, dextran rejections, water contact angles, and surface roughness statistics. These results, combined with FTIR spectra and SEM images, confirm that each support offered a different skin layer pore morphology and chemistry. Polyamide composite membranes formed thereon displayed widely varying film morphology, separation performance, and interfacial properties. More permeable, hydrophilic supports produced low permeability polyamide-polysulfone interfacial composite membranes, whereas highly porous, relatively hydrophobic supports produced more permeable composite membranes. A conceptual model was proposed to explain the potential impacts of polysulfone support membrane properties during the polyamide interfacial polymerization reaction and the resulting characteristics of polyamide-polysulfone interfacial composite membranes.

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Crossflow membrane filtration of interacting nanoparticle suspensions

Journal of Membrane Science, 2006

A model describing the structure, resistance, and flux decline associated with interacting nanopa... more A model describing the structure, resistance, and flux decline associated with interacting nanoparticle cake layers formed during crossflow membrane filtration is used to interpret data from bench scale experiments using silica nanoparticles. Model predictions of permeate flux decline agree well with experimental flux data for all conditions tested and provide new insight into the delicate balance of interaction forces (van der Waals, acid-base, electrostatic, and permeate drag) that govern nanoparticle cake structures over a wide range of experimental conditions. At high flux and ionic strength, the porosity of nanoparticle cake layers was controlled by short-range acid-base and permeate drag forces, whereas at low ionic strength and flux, nanoparticle cake layer porosity was controlled by from electrostatic double layer and permeate drag forces. Use of an interfacial hydrodynamic drag correction suggests that colloidal cake structures may have a stronger than expected dependence on particle size, where smaller nanoparticles generally produce more porous cake layers. In addition, the model explains how microfiltration membranes can experience greater flux decline than ultrafiltration membranes without invoking pore blocking mechanisms.

format_quoteAverage intrinsic membrane hydraulic resistances were ∼2 × 10^11 and ∼9 × 10^11 m^-1 for tested membranes, affecting permeate drag force.format_quote

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Effect of mobile cation on zeolite-polyamide thin film nanocomposite membranes

Journal of Materials Research, 2009

... Titre du document / Document title. Effect of mobile cation on zeolite-polyamide thin film na... more

Direct observation of bacterial deposition onto clean and organic-fouled polyamide membranes

Journal of Colloid and Interface Science, 2009

a b s t r a c t Nanofiltration (NF) and reverse osmosis (RO) membranes are commonly applied to pr... more a b s t r a c t Nanofiltration (NF) and reverse osmosis (RO) membranes are commonly applied to produce highly purified water from municipal wastewater effluents. In these applications, biofouling limits overall process performance and increases the cost of operation. Initial bacteria adhesion onto a membrane surface is a critical early step in the overall process of membrane biofouling. However, adsorption of effluent organic matter onto the membrane may precede bacterial deposition and change membrane surface properties. Herein we employed direct microscopic observation to elucidate mechanisms governing bacterial cell deposition onto clean and organic-fouled NF and RO membranes. Bovine serum albumin (BSA) and alginic acid (AA) were used as models for protein and polysaccharide rich organic matter in secondary wastewater effluents. In all experiments, organic fouling increased membrane hydraulic resistance and salt rejection, in addition to interfacial hydrophilicity and roughness. Even though surface hydrophilicity increased, the rougher surfaces presented by organic-fouled membranes produced nano-scale features that promoted localized bacterial deposition. An extended DLVO analysis of bacterial cells and membrane surface properties suggested that bacterial deposition correlated most strongly with the Lewis acid-base free energy of adhesion and root mean square (RMS) roughness, whereas van der Waals and electrostatic free energies were weakly correlated. This was true for both clean and organic-fouled membranes. Bacterial deposition rates were clearly influenced by an antagonistic interplay between macroscopic surface hydrophilicity and nano-scale surface roughness.

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Impacts of reaction and curing conditions on polyamide composite reverse osmosis membrane properties

Journal of Membrane Science, 2008

Here we report on the impacts of organic solvent properties, reaction conditions, and curing cond... more Here we report on the impacts of organic solvent properties, reaction conditions, and curing conditions on polyamide composite reverse osmosis membrane separation performance, film structure, and interfacial properties. We provide direct experimental evidence that: (1) MPD diffusivity in the organic phase governs MPD-TMC thin film water permeability, (2) MPD diffusivity and solubility influence MPD-TMC thin film crosslinking in competing ways, (3) water permeability correlates most strongly with MPD-TMC film structure (i.e., crosslinking), and (4) salt rejection correlates most strongly with MPD-TMC film thickness and morphology. Overall, higher flux membranes with good salt rejection appear to comprise thinner, more heavily crosslinked film structures. Such high performance RO membranes are obtained by (1) selecting high surface tension, low viscosity solvents, (2) controlling protonation of MPD and hydrolysis of TMC during interfacial polymerization, and (3) optimizing curing temperature and time based on organic solvent volatility. Finally, although more research is necessary, our results suggest the rugose morphology and relative hydrophobicity of high performance MPD-TMC membranes might enhance concentration polarization and exacerbate surface fouling.

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Modeling the impacts of feed spacer geometry on reverse osmosis and nanofiltration processes

Chemical Engineering Journal, 2009

Maximizing mass transfer and minimizing hydraulic losses in spiral wound elements is critical for... more Maximizing mass transfer and minimizing hydraulic losses in spiral wound elements is critical for designing effective and efficient reverse osmosis (RO) and nanofiltration (NF) processes. Herein we describe a multi-scale modeling approach, which links microscopic and macroscopic transport phenomena in spiral wound elements. Model simulations elucidate the impacts of feed spacer geometry on full-scale RO/NF system performance considering four representative water treatment scenarios: (1) RO membranes used to desalt ocean water, (2) low-pressure RO membranes used to desalt brackish water, (3) ultra-low pressure RO membranes used to purify wastewater, and (4) NF membranes used to soften a hard, fresh water. According to model simulations, feed spacer geometry had little impact on mass transfer; hence, engineering spacers to improve concentration polarization, trans-membrane osmotic pressure, or product water quality may prove difficult and yield limited benefits. In contrast, thinner filaments spread further apart significantly reduced hydraulic losses with negligible impacts to mass transfer. In addition, a few noncircular filament shapes produced even lower hydraulic losses, which might prove beneficial for RO/NF treatment of low salinity waters where hydraulic losses through spiral wound elements contribute significantly to the total process energy consumption. In high salinity waters, improved spacer designs may not significantly reduce energy consumption because hydraulic losses through spiral wound elements are relatively small.

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Direct Observation of Microbial Adhesion to Membranes

Environmental Science & Technology, 2005

Direct microscopic observation and an interfacial force model were used to better understand and ... more Direct microscopic observation and an interfacial force model were used to better understand and control microbial adhesion to polymeric ultrafiltration membranes. The model was used to predict a &amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;quot;critical flux&amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;quot;, below which cells deposited reversibly, and direct observation was used to visually quantify cell deposition and removal. In preliminary direct observation experiments, permeate reversal (backpulsing) was more effective than cross-flow hydrodynamics at removing deposited cells. In experiments conducted below the critical flux, no cell accumulation was observed over repeated forward-reverse filtration cycles; however, a small fraction of cells deposited irreversibly regardless of the flux, membrane, or solution chemistry. The fraction of irreversibly deposited cells was consistent with the equilibrium surface coverage attained without permeation (i.e., due to heterogeneous adsorption). Although steric forces were not invoked to establish a critical flux, when operating above the critical flux, a balance between permeation drag and steric repulsion appeared to determine the strength of adhesion of cells to membranes. Direct observation also confirmed that above the critical flux fouling occurred and pressure losses accumulated over several backpulse cycles, whereas below the critical flux there were no observable pressure losses or fouling.

Dispersion and Stability Optimization of TiO 2 Nanoparticles in Cell Culture Media

Environmental Science & Technology, 2010

Accurate evaluation of engineered nanomaterial toxicity requires not only comprehensive physical-... more Accurate evaluation of engineered nanomaterial toxicity requires not only comprehensive physical-chemical characterization of nanomaterials as produced, but also thorough understanding of nanomaterial properties and behavior under conditions similar to those used for in vitro and in vivo toxicity studies. In this investigation, TiO 2 nanoparticles were selected as a model nanoparticle and bovine serum albumin (BSA) was selected as a model protein for studying the effect of protein-nanoparticle interaction on TiO 2 nanoparticle dispersion in six different mammalian, bacteria, and yeast cell culture media. Great improvement in TiO 2 dispersion was observed upon the addition of BSA, even though the degree of dispersion varied from medium to medium and phosphate concentration in the cell culture media was one of the key factors governing nanoparticle dispersion. Fetal bovine serum (FBS) was an effective dispersing agent for TiO 2 nanoparticles in all six media due to synergistic effects of its multiple protein components, successfully reproduced using a simple "FBS mimic" protein cocktail containing similar concentrations of BSA, γ-globulin, and apo-transferrin.

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The University of California Center for the Environmental Implications of Nanotechnology

Environmental Science & Technology, 2009

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Modeling concentration polarization in reverse osmosis processes

Desalination, 2005

Accurate prediction of concentration polarization (CP) phenomena is critical for properly designi... more Accurate prediction of concentration polarization (CP) phenomena is critical for properly designing reverse osmosis (RO) processes because it enhances trans-membrane osmotic pressure and solute passage, as well as surface fouling and scaling phenomena. The objective of this study was to compare available analytical CP models to a more rigorous numerical CP model and experimental CP data. A numerical concentration polarization model was developed to enable local description of permeate flux and solute rejection in crossflow reverse osmosis separations. Predictions of channel averaged water flux and salt rejection by the developed numerical model, the classical film theory model, and a recently proposed analytical model were compared to well-controlled laboratory scale experimental data. At operating conditions relevant to practical RO applications, film theory and the numerical model accurately predicted channel-averaged experimental permeate flux and salt rejection data, while the more recent analytical model did not. Predictions of local concentration polarization, permeate flux, and solute rejection by film theory and the numerical model also agreed well for realistic ranges of RO process operating conditions.

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Interactions controlling biopolymer fouling of reverse osmosis membranes

Desalination, 2007

Laboratory experiments and model calculations were performed to elucidate the fundamental interac... more Laboratory experiments and model calculations were performed to elucidate the fundamental interactions that control organic fouling in reverse osmosis (RO) processes. Bovine serum albumin and alginic acid were selected as model aquatic organic macromolecules (organic foulants). An extended Derjaguin-Landau-Verwey-Overbeek (DLVO) characterization analysis was used to elucidate mechanisms of organic matter fouling on a commercial, polyamide composite RO membrane. Surface tension parameters derived from contact angle analyses are used to demonstrate that the apparent thermodynamic stability of macromolecules determines and adhesive free energy between membranes and macromolecules explained the observed differences in flux decline. Further, foulantmembrane and foulant-foulant interfacial forces helped explain why hydrophilic macromolecules formed polarization layers causing minimal flux decline, while hydrophobic macromolecules formed gel (or cake) layers that led to severe flux decline.

format_quoteInterfacial force balance approach combines hydrodynamic and XDLVO interactions to better understand macromolecule deposition.format_quote

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Influence of Crossflow Membrane Filter Geometry and Shear Rate on Colloidal Fouling in Reverse Osmosis and Nanofiltration Separations

Environmental Engineering Science, 2002

Modeling the effects of fouling on full-scale reverse osmosis processes

Journal of Membrane Science, 2008

Here, we model the effects of fouling on the performance of a reverse osmosis (RO) system treatin... more Here, we model the effects of fouling on the performance of a reverse osmosis (RO) system treating micro-filtered secondary effluent. The model considers system properties, water quality, and transport phenomena. The model is fitted to operating data from the Orange County Water District&amp;#x27;s Groundwater Replenishment System RO pilot plant, which experienced severe fouling over a period of 3–4 months. Modeling

Interfacial polymerization of thin film nanocomposites: A new concept for reverse osmosis membranes

Journal of Membrane Science, 2007

Here, we report on a new concept for formation of mixed matrix reverse osmosis membranes by inter... more Here, we report on a new concept for formation of mixed matrix reverse osmosis membranes by interfacial polymerization of nanocomposite thin films in situ on porous polysulfone supports. Nanocomposite films created for this study comprise NaA zeolite nanoparticles dispersed within 50-200 nm thick polyamide films. Hand-cast pure polyamide membranes exhibit surface morphologies characteristic of commercial polyamide RO membranes, whereas nanocomposite membranes have measurably smoother and more hydrophilic, negatively charged surfaces. At the highest nanoparticle loadings tested, hand-cast nanocomposite film morphology is visibly different and pure water permeability is nearly double that of hand-cast polyamide membranes with equivalent solute rejections. Comparison of membranes formed using pore-filled and pore-opened zeolites suggest nanoparticle pores play an active role in water permeation and solute rejection. The best performing nanocomposite membranes exhibit permeability and rejection characteristics comparable to commercial RO membranes. As a concept, thin film nanocomposite membrane technology may offer new degrees of freedom in tailoring RO membrane separation performance and material properties. (E.M.V. Hoek).

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A Novel Asymmetric Clamping Cell for Measuring Streaming Potential of Flat Surfaces

Langmuir, 2002

A novel asymmetric clamping cell is used to measure the potential of macroscopic sample surfaces.... more A novel asymmetric clamping cell is used to measure the potential of macroscopic sample surfaces. The unique design of the cell allows in-situ measurement of potentials of any flat surfaces, either solids (such as glass, plastics, metals, and ceramics) or flexible sheets (like polymer films, papers, foils, and membranes), without having to cut or shape the test surfaces to fit the measuring cell dimensions. The cell enables measurement of the streaming potential through several parallel rectangular channels formed by firmly pressing a grooved spacer against the test surface. The term "asymmetric" specifically refers to the unique configuration of the channels, where one of the walls (the test surface) bears a different charging property (or, more specifically, potential), compared to the other three surfaces of the channel (formed by the spacer). A mathematical formulation on the basis of Smoluchowski-Helmholtz approach reveals that the measured potential in such a cell represents the average potential of the test surface and the spacer material. Several surfaces, including clean glass disk, aminosilane modified glass, polymeric membrane, and poly(methyl methacrylate) (PMMA) plate, were tested using the cell. The potentials of the polymeric membrane obtained using the present cell were compared with the corresponding potentials measured using the traditional rectangular cell, and good agreement between the two measurements was observed. The asymmetric cell can be used in conjunction with standard commercially available streaming potential analyzers.

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Pressure, flow, and concentration profiles in open and spacer-filled membrane channels

Journal of Membrane Science, 2006

A finite element model was developed to study momentum and mass transport in cross flow membrane ... more A finite element model was developed to study momentum and mass transport in cross flow membrane filtration systems with open and spacerfilled channels. Simulated conditions represented a range of membrane desalination applications and considered both open and spacer-filled straight through channels, as well as a geometry representing lab-scale cross flow membrane filtration cells. The finite element model agreed with classical analytical models for pressure drop, fluid shear, and concentration polarization in straight-through open channels at low flux and high cross flow. In simulations of representative lab-scale cross flow membrane filters, stagnant regions at the entrance and exit regions, as well as developing flow lead to significant deviations between analytic model and finite element model results. In spacer-filled channels, axial pressure drops were substantially higher than in open channels, varying by a factor of 2 for different spacer configurations (cavity < zigzag < submerged), while shear rates were on average enhanced, but appeared to create stagnant zones where the filaments contacted the membrane. On average, feed channel spacers reduced the extent of concentration polarization because of the enhanced wall shear rates. However, the stagnant regions (in front and behind some spacer filaments) led to enhanced concentration polarization, which suggests certain spacer designs could promote local scale and cake formation in spiral wound elements. (E.M.V. Hoek). Concentration polarization is governed by a complex combination of solute properties, membrane properties, and hydrodynamics. From an operational perspective, the cross flow, permeate flux, module geometry, and array configuration determine the extent of concentration polarization. Both permeate flux and cross flow profiles may in turn be altered by the effects of concentration polarization . Accurate description of this complex transport problem, involving coupled momentum and mass transfer, requires simultaneous solution of the Navier-Stokes, continuity, and convection-diffusion equations for uncharged solutes . Modeling CP phenomena becomes even more complicated for charged and interacting solutes .

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Direct observation of initial microbial deposition onto reverse osmosis and nanofiltration membranes

Journal of Membrane Science, 2008

Physicochemical factors governing initial rates of microbial deposition onto reverse osmosis (RO)... more Physicochemical factors governing initial rates of microbial deposition onto reverse osmosis (RO) and nanofiltration (NF) membranes were studied using direct microscopic observation. Deposition rates for three different microorganisms onto four NF/RO membranes increased with (a) increasing permeate velocity, (b) decreasing crossflow velocity, (c) increasing salt rejection, (d) increasing cell size, (e) increasing membrane surface roughness, (f) decreasing (cell-membrane) free energy of adhesion, and (g) decreasing (cell-membrane) electrostatic double layer repulsion. Relatively hydrophilic, smooth surfaces produced macroscopically repulsive interfacial interactions (i.e., cells should not deposit); however, microscopic heterogeneities -which appear inherent to interfacially polymerized polyamide thin films -facilitated sufficient microbial deposition onto clean membranes that microcolonies formed within minutes to hours. At fluxes greater than ∼8 m s −1 (∼20 lmh or 16 gfd), permeate drag forces overwhelmed repulsive interfacial forces leading to purely convective deposition. Concentration polarization of rejected salt ions destabilized cells and enhanced deposition onto NF/RO membrane surfaces. The complex interplay between membrane surface properties, concentration polarization, and hydrodynamics was reasonably captured by a simple, analytical interfacial force model. These results help explain why polyamide NF/RO membranes are prone to bacterial adhesion and how the balance between crossflow and permeate hydrodynamics governs initial microbial deposition onto clean NF/RO membranes.

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Direct observation of biofouling in cross-flow microfiltration: mechanisms of deposition and release

Journal of Membrane Science, 2004

A laboratory-scale membrane filtration cell was constructed to enable direct microscopic observat... more A laboratory-scale membrane filtration cell was constructed to enable direct microscopic observation of microbial cell deposition and release in cross-flow microfiltration. Initial deposition rates determined from microscopic images were interpreted through an interaction force model. Experimentally derived deposition rates and model calculations confirmed that initial cell and particle deposition was governed by permeation drag and electrostatic double layer forces. Microbial deposition rates increased linearly with increasing permeation velocity over several orders of magnitude, but decreased dramatically with small increases to membrane and particle zeta potential. Cross-flow velocity had no effect on initial deposition up to cross-flow Reynolds numbers of 600. Cell deposition rates were significantly lower at high ionic strength and low pH due to cell aggregation in the bulk. Model equations suggested that cell aggregates were more strongly influenced by tangential shear and lift forces, which tend to reduce deposition. The interaction force model predicted at a set of "critical operating conditions" at which cells were reversibly deposited. Direct observation experiments verified that most cells deposited under the predicted "critical" conditions were easily removed when permeation ceased -without the need for back-pulsing or chemical cleaning. Beyond the fundamental knowledge gained from this study, the experimental and theoretical techniques presented may prove valuable in identifying particle and biological fouling potentials of new membrane materials, as well as in developing effective fouling control strategies for environmental membrane separations.

format_quoteCross-flow velocities had little impact on deposition rates up to a Reynolds number of 600, challenging existing assumptions about hydrodynamics.format_quote

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