MEMBRANES FOR WATER TREATMENT

MEMBRANES FOR WATER TREATMENT
The modern era of membrane technologies for water purification was launched in the late 1950s with the development of asymmetric cellulose acetate (CA) membranes for reverse osmosis (RO).1The commercial implementation of water treatment systems has grown steadily with further development of new membrane materials, configurations, and applications. Membranes are used for a wide range of commercial applications due to their small modular size, low energy usages, and low operating costs. While membranes are manufactured from a variety of materials, polymeric membrane materials dominate the commercial products because of low cost and ease of processibility. The materials that followed from CA and its derivatives (cellulose diacetate and cellulose triacetate) are polyamide (PA), aromatic polyamides, polyetheramides, polyetheramines, poly-etherurea, polysulfone, polyethersulfone, polyvinyli-dene fluoride (PVDF), and polypropylene. Thin-film composite (TFC) membranes may be made from a variety of polymers consisting of several different materials for the substrate, the thin film, and other functional layers.

Membrane technologies offer great promise to meet the increasingly stringent regulatory requirements for potable water production. While other technologies can achieve similar treatment objectives, membranes offer notable advantages particularly with respect to the EPA’s Enhanced Surface Water Treatment Rule and Disinfection/Disinfection By-products Rule. For example, microfiltration (MF) and ultrafiltration (UF) membranes can be configured to provide high levels of pathogen removal without dependence on chemical pretreatment and provide a smaller pore size absolute barrier, in contrast to media filtration which relies on chemical pretreatment for adequate pathogen removal. Providing additional pathogen removal credits at lower disinfectant dosages reduces disinfection by-product formation. Moreover, nanofiltration (NF) and RO membranes have made alternative water reclamation (i.e., brackish water and seawater) and wastewater reuse possible solutions to address the growing global scarcity of traditional water sources.

The presence of microorganisms in feed water can further exacerbate fouling due to the accumulation of microorganisms on the membrane surface and on the feed spacer between the envelopes, or biofoul-ing. Microorganisms transported to the membrane element can attach to the feed side of the membrane and the spacer. Attachment depends on Van der Waals forces, hydrophobic interactions, and electrostatic interactions between the microorganisms and the surface. Biofouling control has been attempted via biocide additions; however, while a biocide may kill the biofilm organisms, it usually will not remove the bio-fouling layer2 and may cause bacteria that survive disinfection to potentially become more resistant.3 It is critical to both detect potential fouling bacteria or pathogens and reduce fouling of water treatment membranes to reduce operating cost, extend membrane life, and allow application in challenging environments. This could extend the commercial application of membrane-based water treatment systems.

IMPROVING MEMBRANE FOULING CONTROL
Membrane replacement due to fouling is the single largest operating cost when membranes are used in water separation applications4,5 and, thus, the greatest hindrance to the widespread use of membranes. Fouling [the irreversible (adhesive) macromolecular adsorption] refers to specific intermolecular interactions between macrosolutes present in the feed water and the membrane that occur even in the absence of filtration. These materials on the membrane surface, which cannot be removed by cross-flow operation, backflushing, or back-pulsing, result in permanent flux decline and lead to fouling. Many researchers agree that organic matter is a major contributor to abiotic membrane fouling in water separation applications.6-14

As described below, several methods have been used to modify the membrane surface chemistry which has led to various claims of “low-fouling” membranes. The surface properties that have been targeted for modification are hydrophilicity, roughness, and charge.

Ion Beam Irradiation
Ion beam irradiation was used to modify the surface of a sulfonated polysulfone water treatment membrane. A beam of 25 keV H+ions with three irradiation fluences (1 1013ions/cm2, 5x1013ions/cm2, and 1 1014ions/cm2) was used for membrane irradiation. Sulphonic and C-H bonds were broken and new C-S bonds were formed after irradiation; further, membrane roughness decreased after irradiation. A significant increase in flux after ion beam irradiation was also observed, while the amount of cake accumulation on the membrane was decreased after ion beam irradiation. Hydrophobicity, pore size distribution, and selectivity of the membrane were not affected by ion beam irradiation. Results are described in Chennamsetty et al.15,16 and King et al.17

Graft Polymerization
In order to reduce hydrophobic interactions between natural organic matter (NOM) and the membrane surface, and, thereby, fouling due to NOM, hydrophilic poly ethylene glycol (PEG) monomer chains were attached to a commercially available membrane via in situ graft polymerization. Controlling the density and length of these monomer chains is equally important as binding them to membrane surface. Excessive polymerization leads to poor control of density and length of grafted monomer chains and causes pore blockage. Thus, a chain transfer agent (CTA) was used to terminate the propagation of grafted chain with PEG monomer units. Thus, free radical graft polymerization of the membrane was carried out using an oxidizing agent as initiator, PEG monomer, and a CTA as a polymerization terminating agent. Graft polymerization led to carbonyl attachment and OH stretching, as well as to an increase in permeability with lower cake accumulation. Detailed descriptions are provided in Morao et al.18 and Morao and Escobar.19

Stimuli-responsive Brushes
The work presented here produced a novel fouling-resistant membrane by attaching a stimuli-responsive polymer brush (hydroxypropyl cellulose) on the surface, which offers the potential to reversible change the membrane surface chemistry from hydrophilic to hydrophobic by controlled collapse or expansion of the polymer brush (Figure 1). The phase change arose from the existence of a lower critical solution temperature (LCST) such that the polymer becomes hydrophobic and precipitates from solution as the temperature was increased. This capability can be exploited to control adsorption/des-orption of molecules that can result in membrane fouling. A temperature decrease caused the brush to expand into a hydrophilic state while a temperature increase caused a collapse into a hydrophobic state. NOM adsorption is reduced in the expanded, hydrophilic state relative to the collapsed, hydrophobic state. This paper will expand on successful functionalization of water treatment membrane with thermally responsive polymer brushes to modify fouling resistance.

IN-SITU DETECTION OF BACTERIA
In situ detection of bacteria in membrane-based water treatment systems is critical since biofouling can significantly impact membrane efficiency. Moreover, there is a keen interest in tracking and eliminating potential pathogens in these systems. Admittedly, much attention has been given to sensor development in the past several years; however, sensors that exhibit strong and selective binding for biological targets are still needed. In addition, biomolecule detection requires isolation and concentration of the target biomolecule to mitigate interference in complex water samples and cross-reactivity from competing analytes. Therefore, separations processes are needed with adjustable affinity properties specific to the analyte of interest.

Biorecognition Molecules
Most rapid detection assays are affinity-based, where organism-specific biomolecules, such as artifacts (e.g., exocellular proteins, fatty acid composition) or genom-ic material (e.g., DNA, rRNA) are targeted. The effectiveness of affinity-based sensors is dependent upon: (i) the biorecognition element; (ii) the target biomolecule; (iii) the method to separate bound target from unbound reactants and the matrix; and (iv) the detection method. Once the target biomolecule is identified, the sensor must have the specificity to identify a target biomolecule in a complex system and the sensitivity to detect its presence, even at low concentrations. Immunochemical assays, which rely on antibody (Ab) affinity to target ana-lytes, are arguably the most frequently used biosensors due to their simplicity, rapid response, and financial viability.20,21For specific detection, Abs can be immobilized on surfaces for immunocapture of target bacterial species and subsequent separation of the target species from complex water samples (i.e., process water). Previously, support media for antibody-based sensors have included the surfaces of magnetic beads, microplates, and glass slides, and their applications include natural waters and sediments.22-26

Challenges to In Situ Detection
The overall quality of a biological sensor will be determined by its cross-reactivity. It is imperative to select a biorecognition molecule that exhibits minimal cross-reactivity. Several approaches can be taken to minimize cross-reactivity: (i) modify the protocol to incorporate more stringent conditions for target binding such as shorter incubation time; (ii) collect and characterize the competitive molecule(s) and pre-select for that molecule prior to detection; or (iii) develop a new biorecognition molecule specific for the target in the presence of a more complex mixture that includes the competitive molecule.


Figure 1: Stimuli-responsive polymer brushes acting as the support medium for bacterial sensing.

FOULING RESISTANT MEMBRANES WITH IN-SITU DETECTION
This paper presents a novel project that combines a stimuli responsive brush bound to the membrane for fouling resistance with a biosensor component to detect potential biofouling species (Figure 1). There were three main goals of this work: (i) develop chemistries to bind the thermally responsive polymer brush to the membrane surface, (ii) determine if the lower solution critical temperature of the bound brush was maintained, and (iii) demonstrate covalent binding of a model biorecogntiion molecule to the brushes on membrane surface. The membrane used for the preliminary work was a hydrophilic CA ultrafiltration membrane with a molecular weight cutoff (MWCO) of 20,000 Da.

The membrane surface was modified with the polymer brush, hydroxy-propyl cellulose (HPC) via a divinylsulfone spacer (DVS). The first primary task of the project was to confirm this functionalization and verify if the HPC LCST would occur near 43 °C when it is attached to the membrane. In this case, the hydrophilic brushes would expand into solution at temperatures below the LCST but would collapse at elevated temperatures as brush became hydrophobic.

Atomic force microscopy (AFM) is very sensitive to changes in surface roughness and is an ideal method to monitor the swelling/collapse of membrane bound brushes in presence of solutions of varying temperatures. As shown in Figure 2, the unmodified membrane displayed a roughness of 2.242 nm and 2.245 nm at 25 °C and 60 °C, respectively. The negligible difference between the cold and hot temperature measurements indicates that temperature had no effect on the as received CA membrane. On the other hand, the HPC modified membrane displayed a roughness of 7.94 nm at 25 °C and 0.915 nm at 60 °C. The difference in roughness demonstrated that the change in surface chemistry of the HPC-modified membrane could be activated by temperature. The increase in roughness at the cold temperature was attributed to the extension of the surface attached brushes. The significant decrease in roughness at 60 °C shows that the brush collapses at high temperatures (i.e., in its hydrophobic stage). This is a key first step to developing simple fouling resistant membranes.

A model biorecognition molecule (i.e., antibody) was attached to the membrane and to verify the ability of the membrane-based sensor to detect bacteria. While a number of chemistries are available to attach the antibody, the surface-bound HPC, a carbodiimide (CDI) was chosen. Fourier transform infrared spectroscopy was used to monitor the membrane following each reaction step. The only peak that was affected was at 2129 cm-1. Since the CDI is acting as a zero-length linker, we hypothesize that the appearance of the peak at 2129 cm-1is due to the brush binding to the antibody.


Figure 2: AFM images used to determine the roughness of the
unmodified and modified membranes at high temperatures.

The antibody-HPC modified membrane was tested for its ability to detect mycobacteria. The membranes (25 25 mm) were manipulated in 6-well cell culture plates. One milliliter of concentrated bacteria (7.7 105/mL) was added to the surface of the membrane and incubated for up to 90 min covered on a shaker table. After incubation, the membrane was rinsed 3 with and resuspended in 1 mL laboratory grade water. Samples were processed on a fluorescent microscope. Approximately 10% (8.05 104/mL) of the original bacteria applied to the membrane were recovered.

Studies with model brushes and biorecognition molecules demonstrated that the membranes could be readily modified with a combined fouling resistant layer and detection method. While this paper has focused on use of surface modified membranes for water treatment, this method could be extended to developing selective detection media for a wide range of analytes. In addition, improving fouling resistance of membranes could lead to their use in a wide range of applications.

Acknowledgements
This project was funded by NSF CBET SGER 0610624. The students who have participated in this project are Colleen Gorey, Olga Mileyeva-Biebesheimer, Brook Urban, and Natalie Bailey.

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