Grantee Research Project Results
2005 Progress Report: Graft Polymerization as a Route to Control Nanofiltration Membrane Surface Properties to Manage Risk of EPA Candidate Contaminants and Reduce NOM Fouling
EPA Grant Number: R830909Title: Graft Polymerization as a Route to Control Nanofiltration Membrane Surface Properties to Manage Risk of EPA Candidate Contaminants and Reduce NOM Fouling
Investigators: Kilduff, James E. , Sharma, Ashish , Belfort, Georges , Zhou, Mingyan
Current Investigators: Kilduff, James E. , Belfort, Georges
Institution: Rensselaer Polytechnic Institute
EPA Project Officer: Aja, Hayley
Project Period: August 25, 2003 through August 25, 2007
Project Period Covered by this Report: August 25, 2004 through August 25, 2005
Project Amount: $349,000
RFA: Environmental Futures Research in Nanoscale Science Engineering and Technology (2002) RFA Text | Recipients Lists
Research Category: Nanotechnology , Safer Chemicals
Objective:
The objective of this research project is to develop new nanofiltration membranes by modifying the surface structure of commercial membranes at the molecular level via ultraviolet (UV)-assisted graft polymerization of hydrophilic monomers using Rensselaer Polytechnic Institute’s patented photooxidation method. UV-assisted graft polymerization of hydrophilic monomers is the modification technique chosen for this study because it is simple and easy to scale-up industrially. Our goal is to develop new materials that offer high flux compared to commercial membranes (by improving membrane porosity), enhanced rejection of inorganic anions and ionizable organic compounds (by controlling membrane pore size distribution and surface charge), and enhanced ability to resist fouling by natural organic matter (NOM) by reducing adhesion. In addition, we seek to understand the characteristics of NOM accumulated on membrane surfaces, both in terms of resistance to flow (which can influence the cost of membrane processes) and its affects on the transport and rejection of charged solutes.
Progress Summary:
This report highlights the following goals achieved to date. We have: (1) investigated the mechanism of the photoinduced grafting of poly(ether sulfone) (PES) and obtained the optimal irradiation conditions; (2) evaluated a series of hydrophilic vinyl monomers for grafting, polymerizing, and filtering (which appear to significantly enhance the properties of commercial membranes); (3) characterized photoinduced grafted PES membranes with measurements of contact angle (wettability), infrared spectra with attenuated total reflection Fourier transform infrared spectroscopy (ATR/FTIR) (chemical identification), surface topology (roughness and morphology with atomic force microscopy/scanning electron micrograph), and membrane filtration characteristics; and (4) developed new membrane materials on the basis of results observed from the steps above that offer high flux compared with commercial membranes (by improving membrane porosity), enhanced rejection of inorganic anions and ionizable organic compounds (by controlling membrane pore size distribution and surface charge), and enhanced ability to resist fouling by NOM by reducing adhesion.
Membranes
Commercially available poly(aryl sulfone) membranes were UV-irradiated in the presence of water-soluble monomers, which chemically bond to the surface by a mechanism involving a photochemically induced free radical polymerization. Low molecular weight cut-off (MWCO) membranes were chosen to operate in nanofiltration range after surface modification, which can reduce membrane permeability. Previously, we evaluated 1 kDa MWCO PES membranes (OS-001) obtained from Pall-Filtron Corporation (East Hills, NY). All the membranes were washed several times and then soaked in deionized (reagent grade) water overnight to remove surfactant used as preservative.
Chemicals
Figure 1 highlights the various monomers considered to date, of which various branches of poly(ethylene glycol) (PEG) monomer have been considered in detail; the other monomers are being considered for future work, with mono-amino propyl methacrylate (MAPM)/sulfopropyl methacrylate (SPMA) potassium salt in progress. Grafting with PEG having a range of monomer chain length from n equal to 1 to 22 (Figure 1) and molecular weight ranging from 144.17 to 2200 was evaluated (Table 1). Monomer concentration was varied from 0.002 weight percent to 2 weight percent.
Figure 1. Various Monomers Evaluated for This Research. Detailed work with PEG is described in this report. Work with MAPM and 3-SPMA is in progress. Future work will employ other monomers including N-2-vinyl pyrolidinone.
Table 1. Photografted PEG Vinyl Monomers Used in This Project
Monomer |
Branch chain, n |
Molecular weight, Da |
1 |
1 |
144.17 |
2 |
2 |
188.22 |
3 |
4 |
300 |
4 |
8 |
475 |
5 |
22 |
1100 |
NOM from two sources was used to prepare feed solutions for membrane filtration experiments. TMK NOM, which represented an aquatic NOM (surface water source) was collected from Tomhannock Reservoir, Troy, New York, using a reverse osmosis system. Elliott Humic Acid, a soil organic matter, was purchased from International Humic Substances Society. Feed solutions were prepared at total organic carbon concentrations of 10 and 50 mg/L. CaCl (Certified A.C.S Dihydrate, Fisher) was added directly to the feed solutions for selected experiments. Solution pH was adjusted by adding HCl or NaOH as necessary.
Preparation of Modified Membranes
A schematic diagram of the Rayonet photochemical chamber reactor system (Model RPR-100, Southern New England, Ultraviolet Co., Branford, CT), and the (patented) procedure used to modify membrane surfaces, is shown in Figure 2 below. The photochemical reactor contained 300 nm UV lamps (approximately 15% of the energy was at < 280 nm) and was used with the dip modification technique. In this method, membranes were dipped in monomer solution for 30 minutes with stirring at 22°C, removed from the monomer solution, N2 purged for 10 minutes, and irradiated in water-saturated N2 for specified time. After modification, the membranes were washed with reagent grade water by shaking for 2 hours to remove homopolymer and unreacted monomer in the membrane.
Figure 2. Schematic Diagram of the UV-Induced Graft Polymerization Procedure. This figure illustrates: (a) the membrane immersed in the monomer solution, (b) the membrane swatch affixed inside the quartz vessel, and then (c) placed in the photoreactor containing 16 300 nm lamps and sparged with N2 to remove oxygen.
Degree of Grafting
ATR/FTIR (Magna-IR 550 Series II, Thermo Nicolet Instruments Corp., Madison, WI) was used to obtain a measure of the degree of grafting (DG) to quantify the amount of polymer grafted to the membrane surface. The DG was calculated as:
DG = (Hx/H1487)Modified - (Hx/H1487)Unmodified
The peak at 1487 cm-1 was because of the benzene carbon-carbon double bond of PES membranes, which did not change as a result of grafting. The DG as a function of reaction time and monomer chain length is shown in Figure 3; the DG does not show any trend with reaction time for the range of reaction times (0 to 60 seconds) investigated. The DG, however, generally decreases with increasing chain length, although the highest degree of grafting was observed for the 188 Da monomer, not the 144 Da monomer. This finding can be explained by a decrease in the rate of diffusive transport to the membrane surface with increasing chain length, although differences in intrinsic reactivity cannot be ruled out.
Figure 3. Effect of Reaction Time and Monomer Chain Length on the DG
Filtration Experiments
Filtration was done using 82 mm diameter membrane coupons in a 270 mL stainless steel cell (Membrane Extraction Technology Ltd, London, UK) at a constant pressure of 80 psi (provided by a nitrogen gas cylinder). A magnetic stirrer mounted in the cell and operated at 500 rpm provided mixing to reduce concentration polarization. Permeate was not recycled back to the feed reservoir. Prior to each experiment, the membrane was cleaned thoroughly and then stabilized using reagent grade water at 160 psi for 1 hour (using an external reservoir); then membrane permeability was measured with reagent grade water. After stabilization, the test cell and external reservoir were filled with feed solution, the stirrer was turned on, and pressure (80 psi) was applied to the cell. Once the filtration started, samples were taken for analysis at regular time intervals to monitor the membrane performance with time. After filtration, the cell and membrane were rinsed with reagent grade water, and membrane permeability was measured again using reagent grade water. Then the membrane was taken out, and cake formed on the membrane coupon was removed with Kimwipes; membrane permeability was measured again using reagent grade water. All experiments were conducted at the 22°C.
Membrane Permeability and DG
In previous progress reports, it was shown that membrane permeability (Lp, flux per unit pressure) generally decreased with increasing monomer concentration, irradiation time, and DG. In some experiments, it was possible to apply enough energy during modification to result in an increase in permeability, likely as a result of trunk polymer scission. Modification with PEG was unique in this regard; permeability of modified membranes did not change with monomer concentration or UV irradiation time (Figure 4). The average permeability, however, decreased from 1.30 to 1.07 LMH/psi after modification.
Figure 4. Permeability of Modified OS-001 Membranes With the Modification Condition of PEG 144 at Different Monomer Concentrations (left), Different MW PEGs at 2 weight percent Monomer Concentration (right)
NOM Fouling of As-Received and Modified Membranes
In preliminary experiments, it was found that TMK NOM was too hydrophilic to foul as-received membranes during short duration screening runs, even when the NOM concentration was increased up to 50 mg/L. Therefore, to provide a greater challenge to the membranes, the experiments in this section were done with the more hydrophobic Elliott Soil Humic Acid, at a concentration of 10 mg/L. To accelerate the fouling process, 1 mM Ca<sup>2+</sup> was added to feed solution. The solution pH was adjusted to 7 with HCl and NaOH solutions, and ionic strength (I.S.) was adjusted to 0.01 M with NaCl. NOM fouling experiments of as-received membranes were repeated six times, flux and normalized flux as a function of cumulative permeate volume are shown in Figure 5. It was found the flux and flux decline varied among different membrane coupons, and generally the membranes that had smaller flux also showed a smaller flux decline (experiment sets of 3 and 4). This is because the transport of foulant species to the membrane surface is reduced when the flux is reduced. In contrast, the membranes in experiments 5, 1, and 6 showed similar flux decline, even though the initial flux was different. These results illustrate the importance of basing changes that result from modification and/or fouling on the same membrane coupon.
Figure 5. Flux (left) and Normalized Flux (right) of As-Received OS-001 Membranes. Experimental conditions: 10 mg/L NOM, 1 mM Ca<sup>2+</sup>, I.S. = 0.01 mM, pH 7.
Flux decline curves during filtration of NOM solutions with PEG modified membranes are shown in Figure 6. Comparing the membranes modified with 2 weight percent PEGs, we can see that the membrane grafted by PEG 475 was the best from both the point of view of flux and flux decline results, followed by PEG 300, 188, and 144. A possible explanation is that some fraction of the PEGs with smaller sizes could penetrate into membrane pores more easily, causing a reduction in effective pore surface area, and hence permeability, whereas the bigger PEGs were excluded from the pores and remained on the membrane surface.
The optimum monomer concentration was evaluated for PEG 144 and PEG 475. The monomer concentrations investigated were 0.002, 0.02, 0.2 and 2 weight percent. For PEG 144, although 2 weight percent PEG improved membrane surface hydrophilicity the most as indicated by the smallest flux decline, membrane pores also were plugged significantly by the monomer, resulting in a lower flux. Membranes modified with 0.002 and 0.02 weight percent PEGs showed relatively high flux, and a small flux decline over the entire filtration process; therefore, these two concentrations were the best for PEG 144 in our study.
For PEG 475, the monomer concentration of 0.2 weight percent performed the best in both the flux (high) and flux decline (relatively low). A weight percent of 0.002 corresponds to a molar concentration of 4 mM for PEG 475. This concentration was not sufficiently high enough to significantly alter membrane properties; therefore, the modified membrane showed similar flux and flux decline to the as-received membranes.
The effects of graft polymerization on flux, as shown in Figures 5 and 6 for as-received and modified membranes, suggest that modification may reduce the number of defects in as-received membranes. These defects essentially are large pores that make a disproportionate contribution of flux (because flow is proportional to pore size to the fourth power). Membrane surface modification reduced the maximum flux observed from about 100 to about 70 LMH at the same operating pressure, which can be interpreted as a reduction in the size or number of the largest pores.
Figure 6. Flux (left) and Normalized Flux (right) of PEG Modified OS-001 Membranes. Experimental condition: 10 mg/L NOM, 1 mM Ca<sup>2+</sup>, I.S. = 0.01 mM, pH 7.
Solute Rejection of As-Received and Modified Membranes
NOM rejection by as-received and PEG-modified membranes is shown in Figure 7. Most of the modified membranes had NOM rejection (>90%) with the exception of membranes modified with high concentration (2 wt %) of PEG. This result suggests that PEG may partially solubilize or soften the PES membranes. This finding will be investigated further.
Figure 7. NOM Rejection by As-Received and PEG Modified OS-001 Membranes. Experimental condition: 10 mg/L NOM, 1 mM Ca<sup>2+</sup>, I.S. = 0.01 mM, pH 7.
Modeling Flux Decline Using a Pore Blockage-Cake Filtration Model
A pore blockage-cake filtration model, Equations 1 and 2, was applied to model the flux decline. The reduced flux J/Jo , where Jo is the initial flux through the clean (unfouled) membrane is described in a pore blockage parameter, α:
(1)
Where ΔP is the transmembrane pressure, Cb is the bulk NOM concentration, t the filtration time, and µ is the solution viscosity. When t is small, flux decline is mainly caused by pore blockage, but as t gets large, a cake layer forms over the membrane surface, and it is the resistance of the cake layer, Rc, which acts in series with the membrane resistance, Rm, that causes the majority of the flux decline. The cake layer resistance is found from:
(2)
Where Rco is the resistance of the first layer of the NOM deposit, f’ the fraction of NOM in the bulk solution that contributes to the growth of the cake deposit, and R’ is the specific resistance of the cake layer. Experimental data also were plotted using the general filtration equation to provide insight into fouling mechanisms (Figure 8):
(3)
where the parameter n characterizes the fouling mechanism: for cake filtration, for intermediated blocking, for pore constriction, and for pore blockage. The required derivatives were evaluated in the filtrate flux as:
(4)
(5)
with dJ/dt numerically determined from Equation 1 after fitting experimental results to Equations 1 and 2. In most of the as-received membrane filtration experiments, the NOM fouling of membranes was caused by pore constriction and pore blocking followed by cake formation. Of the PEG-modified membranes, only the membrane modified with 2 weight percent PEG 144 exhibited fouling by a pore constriction and blockage fouling mechanism; all of the other membranes exhibited a rapid transition to fouling by cake formation. The cake formed on the membrane surface could be removed much more easily than the NOM adsorbed into the membrane pores, thus the modified membranes performed better in flux recovery than the as-received membranes.
Figure 8. Flux Decline Analysis for NOM Filtration by As-Received (left) and PEG-Modified (right) OS-001 Membranes. Experimental condition: 10 mg/L NOM, 1 mM Ca<sup>2+</sup>, I.S. = 0.01 mM, pH 7.
Future Activities:
New monomers, including 3-MAPM and 3-SPMA, and their possible synergistic combinations, will be evaluated for their efficacy in improving rejection characteristics and mitigating NOM fouling. Additional membrane characterization will be performed, including air-water contact angle measurements and dextran rejection experiments, to obtain pore size information. In addition, topographical images of unmodified and modified membranes will be taken using an atomic force microscope to determine surface roughness, which will be correlated to NOM fouling potential. Among the four proposed representative compounds selected for the project, two of them (2,4-DNP and perchlorate) have been evaluated thus far. Future work will begin evaluation of metolachlor and arsenic.
Journal Articles on this Report : 1 Displayed | Download in RIS Format
Other project views: | All 24 publications | 5 publications in selected types | All 4 journal articles |
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Type | Citation | ||
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Kilduff JE, Mattaraj S, Zhou M, Belfort G. Kinetics of membrane flux decline: the role of natural colloids and mitigation via membrane surface modification. Journal of Nanoparticle Research 2005;7(4-5):525-544. |
R830909 (2005) R830909 (Final) |
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Supplemental Keywords:
drinking water, nanofiltration, natural organic matter, fouling, innovative technology, remediation, environmental chemistry, engineering, chemical contaminants, drinking water contaminants, graft polymerization, membranes, monitoring, nanocomposite filter, nanoporous membranes,, RFA, Scientific Discipline, Water, POLLUTANTS/TOXICS, Sustainable Industry/Business, Sustainable Environment, Arsenic, Technology for Sustainable Environment, Environmental Monitoring, Water Pollutants, Drinking Water, Engineering, Chemistry, & Physics, Environmental Engineering, monitoring, public water systems, Safe Drinking Water, graft polymerization, nonocomposite filter, natural organic material, membranes, nanotechnology, chemical contaminants, community water system, treatment, nanofiltration, nanoporous membranes, contaminant removal, drinking water contaminants, drinking water treatment, water treatment, nanocomposite filter, nanofiltration membranes, green chemistry, drinking water systemProgress and Final Reports:
Original AbstractThe perspectives, information and conclusions conveyed in research project abstracts, progress reports, final reports, journal abstracts and journal publications convey the viewpoints of the principal investigator and may not represent the views and policies of ORD and EPA. Conclusions drawn by the principal investigators have not been reviewed by the Agency.