Grantee Research Project Results
2004 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. , Nayak, Arpan , 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, 2003 through August 25, 2004
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 our patented method. 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:
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 (NF) range after surface modification, which can reduce membrane permeability. Thus far, we have evaluated 1 kDa MWCO poly(ether sulfone) (PES) membranes (OS-001) obtained from Pall-Filtron Corporation (East Hills, NY) and 800 Da sulfonated PES membranes (NTR 7450) obtained from Nitto-Denko Incorporated (Fremont, CA). All of the membranes were washed several times, and then soaked in deionized (reagent grade) water overnight to remove surfactant used as preservative.
Chemicals
Three monomers were evaluated for membrane modification—acrylic acid (AA), 3-sulfopropyl methacrylate potassium salt (SPMA), and N-hydroxymethyl acrylamide (NHMA)—chosen as model weak (carboxylic) acid, strong (sulfonic) acid, and neutral monomers, respectively. The chemical structures of these monomers are shown in Figure 1.
Figure 1. Chemical Structures of Monomers Used for Membrane Surface Modification
The organic compounds used to prepare feed solutions for membrane filtration experiments were phenol and 2,4-dinitrophenol (2,4-DNP) purchased from Fisher Scientific and Acros Organics, respectively. Feed solutions were prepared at concentrations of 0.002 mol/L and 0.0001 mol/L for phenol and 2,4-DNP, respectively. Sodium perchlorate (HPLC grade, Fisher) was added directly to the feed solutions for selected experiments. Solution pH was adjusted by adding HCl or NaOH solutions 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 22oC, removed from the monomer solution, N2 purged for 10 minutes, and irradiated in water-saturated N2 for a 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 photo-reactor containing 16 300 nm lamps and sparged with N2 to remove oxygen.
Degree of Grafting
Attenuated total reflection Fourier transform infrared spectroscopy (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. After graft polymerization, the AA, SPMA, and NHMA molecules exhibited absorbance peaks for their carbonyl groups at wavenumbers 1717, 1724, and 1664 cm-1, respectively (Hx), in proportion to the amount grafted. The DG was calculated as:
DG = (Hx/H1487)Modified - (Hx/H1487)Unmodified
The peak at 1487 cm-1 was attributed to the benzene carbon-carbon double bond of PES membranes, which did not change as a result of grafting.
Surface Charge
The surface charge or electrical potential properties of as-received and modified membranes was measured by streaming potential using an Electro Kinetic Analyzer (Anton Paar, Graz, Austria). Streaming potential then was used to calculate zeta potential. A 10 mM KCl solution was used as the background electrolyte in all the experiments with pH maintained at the natural pH of 5.5 (±0.2). Zeta potential measurements of the as-received membrane, as well as the SPMA and NHMA modified membranes, confirmed that grafting of SPMA onto OS-001 membranes made the surface more negative under all modification conditions, although a maximum negative charge was observed between 20 and 40 seconds irradiation time. After this period, surface charge becomes less negative, possibly because of graft polymer or truck polymer cleavage. As expected, neutral NHMA did not change the zeta potential of the OS-001 membrane after monomer grafting.
Filtration Experiments, Membrane Permeability, and DG
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 600 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); membrane permeability was measured with reagent grade water. After stabilization, the test cell was filled with 250 mL 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. All experiments were conducted at 22oC.
Membrane permeability (Lp, flux per unit pressure) generally decreased with increasing irradiation time and degree of grafting. The permeability of NHMA-modified membranes decreased rapidly, which correlates with high DG values that result from the intrinsic reactivity of the NHMA with PES. Even for the same DG, however, the permeability of NHMA-modified membranes is still lower than that of SPMA-grafted ones. A possible explanation is that the smaller size of NHMA allows it to penetrate into membrane pores, causing a reduction in effective pore surface area, and hence permeability. In some experiments, it was possible to apply enough energy during modification to result in an increase in permeability, probably as a result of graft polymer of trunk polymer scission.
Solute Rejection of As-Received and Modified Membranes
In preliminary experiments, it was found that surface modification of NTR 7450 membrane with AA monomer did not improve 2,4-DNP rejection. Because the permeability increased after grafting, it is possible that trunk polymer scission led to the observed changes in performance. In contrast, surface modification of OS-001 membranes with the SPMA monomer could significantly improve 2,4-DNP rejection, even in the presence of 0.1 mM NaClO4. The effects of grafting on rejection were evident at pH 4, but especially were dramatic at pH 10, as shown in Figure 3. The effects of enhanced charge repulsion can be further seen in Figure 4, which illustrates the ability of the modified membranes to reject perchlorate at pH 10. An interesting effect of pH is observed by comparing the perchlorate rejection by the as-received membrane at pH 4.3 and pH 10. Although neither the membrane charge nor the ionization of perchlorate is expected to change over this pH range, perchlorate rejection is higher at the lower pH. This effect has been observed in the literature and has been attributed to changes in membrane pore size as a function of pH. We will investigate this phenomenon in more detail in future experiments.
Figure 3. 2,4-DNP Rejection by As-Received and SPMA Modified OS-001 Membranes. Experimental condition: 0.1 mM 2.4-DNP and 0.1 mM NaClO4 solution filtration, pH 10.
Figure 4. Perchlorate Rejection by As-Received and SPMA-Modified OS-001 Membranes. Experimental condition: 0.1 mM 2.4-DNP and 0.1 mM NaClO4 solution filtration, pH 4.3.
Modeling Organic Molecules Transport Through Nanofiltration Membranes
At pH 10, 2,4-DNP feed solution concentration profiles during filtration through SPMA-grafted OS-001 membranes exhibited increasing concentration with time, as expected in a dead-end system, suggesting rejection mechanisms, including size or charge exclusion. In contrast, at low pH, the feed solution concentration kept nearly constant during the filtration process, suggesting that the 2,4-DNP rejection was not because of charge or size exclusion in these experiments. In addition, permeate concentrations exhibited breakthrough behavior, and more extensive flux decline was observed during filtration at low pH values. As a result of these observations, we hypothesized that adsorption was an important rejection mechanism, and we sought to evaluate a modeling approach to describe transport through NF membranes by convection, diffusion, and adsorption. The approach we have chosen views the membrane as a porous medium, with transport by hindered convection and diffusion. The liquid phase mass balance equation is the well-known advection diffusion equation:
(1)
where v is the unperturbed fluid velocity in pores [m s-1], C is the radial average solute concentration in the pores [mol L-1], Kc is the convective hindrance factor, D¥ is the free liquid diffusivity [m2 s-1], Kd is the diffusion hindrance factor, ε is the membrane porosity, ρs is the membrane density [kg m-3], and q is the amount adsorbed [mol kg-1]. Hindrance factors are available in the literature, usually expressed in terms of solute-to-pore size ratio (λ = rs/rp). Assuming local equilibrium and a linear isotherm, a closed analytical equation solution is available in the literature for flux-type boundary conditions:
(2)
where R is the adsorption retardation factor, and v* and D* are velocity and diffusivity corrected for both hindrance factor and retardation because of sorption. This model is suitable for the situation when the sizes of organic solutes and membrane pores are of the same order. Only the retardation factor, R, was treated as a fitting parameter; all other parameters are known, were measured experimentally, or were calculated using available correlations. The success of the proposed modeling approach is shown in Figure 5. To our knowledge, this is the first time this approach has been employed to describe organic solute transport in membranes combining hindered diffusion and convection with sorption.
Figure 5. Permeate and Feed Concentrations of Phenol (a) and 2,4-DNP (d) as a Function of Filtration Time
Future Activities:
New monomers, including 2-acrylamidoglycolic acid; 2-acrylamido-2-methyl-1-propanesulfonic acid; ethylene glycol diacrylate, and their possible synergistic combinations, will be evaluated for their efficacy in improving rejection characteristics and mitigating NOM fouling. Thus far, low MWCO membranes have been tested, yielding relatively low permeability. To improve permeability, higher MWCO membranes will be evaluated. 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, only two of them (2,4-DNP and perchlorate) have been evaluated. Future work will begin evaluation of metolachlor and arsenic.
Journal Articles:
No journal articles submitted with this report: View all 24 publications for this projectSupplemental Keywords:
drinking water, adsorption, absorption, chemical transport, chemicals, toxics, organics, 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, Engineering, Chemistry, & Physics, Drinking Water, 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.