Chapter 35
Porous Membranes Containing Zero-Valent Iron Nanorods for Water Treatment 1,*
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S. M. C.Ritchie ,T. N.Shah ,L.Wu ,C. Claiborn, and J. C. Goodwin Downloaded by IMPERIAL COLL LONDON on September 1, 2013 | http://pubs.acs.org Publication Date: December 14, 2004 | doi: 10.1021/bk-2005-0890.ch035
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Chemical Engineering Department and Metallurgical and Materials Engineering Department, University of Alabama, Tuscaloosa, AL 35487
1. Introduction Zero-valent iron has long been known to be very active for the dechlorination of chlorinated organics (1). Early studies showed the efficacy of granular iron for degradation of trichloroethylene (2,3). These studies showed that although the reduction was effective, the reaction was slow, and could result in significant formation of degradation by-products (3). Reducing the size of the iron particles to the nano-range has been very effective for increasing the rate of reaction, and subsequently reducing the formation of by-products (4). However, with increased activity came decreased material stability. Zero-valent iron nanoparticles are subject to oxidation in air and hydrolysis in water. In both cases, these reactions significantly reduce the efficiency of these materials for large-scale application. That is, how do you store the materials and keep them active until they are needed? In this work, composite, polymeric materials are formed containing zero-valent nanoparticles. The nanoparticles have been formed in-situ and ex-situ, although the final reduction reaction occurs in the membrane-phase. In this paper, the formation of porous membranes containing zero-valent iron nanorods is discussed, and preliminary results are given to demonstrate the promise of these materials for degradation of chlorinated organics.
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© 2005 American Chemical Society
In Nanotechnology and the Environment; Karn, B., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2004.
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2. Methods Two methods were used to make nanoparticle containing membranes. In the first method, polystyrene grafts were grown in the pores of polyethersulfone membranes. The polystyrene grafts are formed by cationic polymerization from sulfonic acid sites in the membrane. The mechanism for polymerization does not allow branching of the polymer. The grafts are subsequently treated with sulfuric acid to created sulfonated polystyrene grafts. The grafts are used for adsorption of iron cations. The sorption step is followed by reduction of the iron with sodium borohydride. This results in iron nanorods immobilized in the pores of the polyethersulfone membrane. The nanorods are located in the flow paths of the membrane, so they may readily contact and react with chlorinated organics in a permeated aqueous solution. The second method involved solution formation of the nanoparticles similar to Li et al (5). Ferric chloride was dissolved in a 1:1:8 solution of cetyltrimethylammonium bromide (CTAB), n-butanol, and cyclohexane. The mixture was reduced using a 2 wt% solution of sodium borohydride in water to make the nanoparticles. The particles were diafiltered successively with water and methanol to remove residual surfactant and chloride. The nanoparticles were stored in methanol until needed. The membrane casting mixture was formed by adding the methanol slurry to a solution of cellulose acetate (CA) in acetone. There is no precipitation of the cellulose acetate, so nanoparticles can be dispersed throughout the mixture. Membranes were formed by casting the mixture on a glass plate and phase-inversion in ethanol. TCE degradation studies were performed in batch experiments. In a typical experiment, TCE was diluted to 0.75 - 2 mM with degassed water. The membrane was cut into pieces and was added to 40 mL of the TCE solution in a Teflon-cap sealed vial. Samples are allowed to react for up to 72 hours in a wrist-shaker. The membrane was removed from the reaction mixture prior to chloride ion analysis using an ion selective electrode. A material balance was used to determine maximum TCE degraded assuming complete dechlorination. TEM micrographs were obtained by application of the nanoparticle-methanol slurry to a T E M grid and subsequent evaporation of the alcohol.
3. Results and Discussion In-Situ Formed Nanorod Dechlorination. Studies with the nanorods formed on sulfonated polystyrene grafted membranes showed only minimal dechlorination.
In Nanotechnology and the Environment; Karn, B., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2004.
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264 A summary of these studies is shown in Table I. In both cases, the maximum available iron was only 5 mg. This is significantly lower than for comparable bulk metal studies. Long term studies yielded similar results to short-term studies, indicating that the reaction may be limited by the amount of iron. The starting pH was also less than 6, promoting side reactions for hydrolysis of the iron nanoparticles. The importance of reaction pH has been shown previously by Chen et al (6). Future work on these materials will focus on increasing the amount of iron in the membranes, along with better characterization of side products.
TCE (mM) 1.5 1.5
Max Fe° (mg) 5 5
Time (hr)
[CI] (mM)
Conversion
45 12
0.182 0.158
4.1 3.5
%
Ex-Situ Formed Nanorod Dechlorination. Membranes made with ex-situ formed nanoparticles were much more effective for TCE dechlorination. A summary of results for these membranes, including comparisons with iron powder, is given in Table II. The importance of having an active surface is shown with the first two entries. Long-term studies with a non-acid washed powder showed no dechlorination. However, the acid-washed metal provided almost 20% formation of the maximum chloride. Results for nanorod containing membranes gave better conversion with only a fraction of the iron. There is still not complete degradation of the chlorinated organic, but it does show the usefulness of incorporating nanoparticles in a polymer film. Table II: TCE Degradation with Granular Iron and Ex-Situ Formed Nanorods TCE Form Fe° F ? Time [CT] % Conversion (mM) (mg) (hr) (mM) 2
100 mesh powder
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0 (same as control) 18
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1.04
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In Nanotechnology and the Environment; Karn, B., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2004.
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These membranes are very stable and are gray to black in color. There is little to no oxidation of the films, unlike the particles, during storage for up to 1 month. During the degradation studies, the membranes turn orange which is indicative of iron oxidation. TEM micrographs of the nanoparticles used in this research are shown in Figure 1. The particles are very small ( 150 μηι) used for comparison.
Figure 1 : Nanorods of iron nanoparticles. References (1) Gotpagar, J.; Grulke, E.; Tsang, T.; Bhattacharyya, D. Environ. Prog. 1997, 16, 137-143. (2) Gillham, R.W.; O'Hannesin, S.F. Ground Water 1994, 32, 959-967. (3) Matheson, L.J.; Tratnyek, P.G. Environ. Sci. Technol. 1994, 28, 2045-2053. (4) Lien, H.-L.; Zhang, W.-X. Coll. Surf. A : Physicochem. Eng. Aspects 2001, 191, 97-105. (5) L i , F.; Vipulanandan, C.; Mohanty, K.K. Coll. Surf. A: Physicochem. Eng. Aspects 2003, 223, 103-112. (6) Chem, J.-L.; Al-Abed, S.R.; Ryan, J.A.; Li, Ζ. K. Haz. Mat'ls. 2001, B83, 243-254.
In Nanotechnology and the Environment; Karn, B., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2004.
