Environ. Sci. Technol. 2001, 35, 3713-3717
Synergistic Effect of Cationic Surfactants on Perchloroethylene Degradation by Zero-Valent Iron D A N I E L S . A L E S S I † A N D Z H A O H U I L I * ,†,‡ Geology Department and Chemistry Department, University of WisconsinsParkside, 900 Wood Road, Kenosha, Wisconsin 53141
Zero-valent iron (ZVI) as a permeable barrier material for degradation of chlorinated organic compounds has been extensively studied recently. One of the focal areas in ZVI studies is to increase the contaminant reduction rate. In this research, batch tests were performed to evaluate the synergistic effect of sorbed cationic surfactants on degradation of perchloroethylene (PCE). Sorption of cationic surfactants on ZVI was a function of hydrophobic chain length of the surfactant tail group. Minimal counterion sorption indicated that the sorbed surfactant molecules form a patchy monolayer on ZVI. Both PCE and trichloroethylene (TCE) degradation by ZVI with and without sorbed surfactant followed pseudo-first-order reaction kinetics. In general, the PCE degradation rate increases as the chain length of sorbed surfactant increases. Compared to unmodified ZVI, both apparent rate constants of PCE degradation and TCE accumulation increased by an order of magnitude when ZVI was modified by hexadecyltrimethylammonium. The rate of PCE degradation by ZVI modified to lower surfactant loading was relatively higher than that by ZVI modified to higher surfactant loading. It was speculated that longer chain length will result in better admicelle formations, and thus, promote PCE partition and increase PCE surface concentration or surface admicelle catalysis, while low surfactant loading makes significant amounts of surface reduction sites still available. The PCE reduction rate constants were not affected by solution ionic strength, but high initial solution pH, buffered by sodium carbonate and sodium bicarbonate, significantly reduced the PCE degradation rate.
Introduction Zero-valent iron (ZVI) as an inexpensive and effective material of permeable reactive walls for destruction of chlorinated solvents has drawn great attention in recent years (1-13). Laboratory studies indicated that ZVI was able to degrade carbon tetrachloride (CT) at a very fast rate with a half-life t1/2 of approximately 15 min based on pseudo-first-order kinetics (1). Compared to degradation of CT, the degradation of perchloroethylene (PCE) by ZVI was relatively slow, and the half-life was 20-40 d (2). Gillham and O’Hannesin (3) compiled the half-life of chlorinated methanes, ethanes, and ethenes in the presence of ZVI and found that, in general, chlorinated ethanes could be degraded more easily than chlorinated ethenes. The reported degradation rate constants * Corresponding author phone: (262)595-2487; fax: (262)595-2056; e-mail:
[email protected]. † Geology Department. ‡ Chemistry Department. 10.1021/es010564i CCC: $20.00 Published on Web 08/18/2001
2001 American Chemical Society
of specific chlorinated hydrocarbons varied widely due to different iron sources and treatment procedures; however, once the rates were normalized to the iron surface area, the variation in rate constants was reduced to within 1 order of magnitude for each halocarbon (4). For a reduction reaction to take place, PCE molecules are sorbed onto ZVI first (5, 6). The sorbed PCE, after formation of a di-σ-bonded intermediate, may undergo either reductive β-elimination producing dichloroacetylene or hydrogenolysis resulting in TCE formation (6). PCE could also sorb to nonreactive sites, hypothesized to be graphitic inclusions in the cast iron, which could prohibitively affect PCE reduction rate (7). Because of the slower reduction rate of chlorinated ethenes by ZVI, researchers have been focusing on studies related to enhancing their degradation by modifying ZVI. Coating iron with Pd resulted in more than 1 order of magnitude increase in rate constant for TCE reduction (8). Reducing the particle size, such as using nanometer-scale Pd/Fe, further increased the TCE reduction rate (9). Combining ZVI with other metals such as Ni and Zn enhanced the degradation of TCE (10). In a preliminary study, it was found that hexadecyltrimethylammonium (HDTMA), a cationic surfactant, enhanced the PCE dechlorination rate constant with ZVI by a factor of 3 while sodium dodecyl sulfate (SDS), an anionic surfactant, did not (11). However, prohibitive effects were also observed on pentachlorophenol degradation when the ZVI surface was modified by Pd, Pt, Ni, and Cu (12). In the presence of hydroxypropyl-βcyclodextrin, the PCE destruction rate decreased due to partitioning of PCE molecules into the hydrophobic interiors of the cyclodextrin molecules (5). Addition of corrosion inhibitors blocked the interaction sites and reduced the degradation rate of CT by iron metal due to strong complexation of ligands at the oxide-electrolyte interface, but a concomitant addition of chloride enhanced the reaction (13). Surfactants have long been used to change the surface or interfacial properties. In addition, surfactant micellar catalyses have been used extensively in industry. Studies on the hydrolysis of 5-nitro-2-(trifluoroacetylamino)benzoic acid in water and in the presence of micelles of cetyltrimethylammonium bromide (CTAB) showed that at pH 9 the pseudofirst-order rate constant increased by 20 times under the catalysis of micelles of CTAB (14). In a separate study, the greatest enhancement on complexation reaction between Ni2+(aq) and pyridine-2-azo-p-dimethylaniline was achieved under the catalysis of SDS solution (15). The rates of disappearance of dinitrotoluene, dinitrobenzene, and dinitrobenzenesulfonic acid from alkaline borohydride solution due to reduction and photolysis (>290 nm) were greatly enhanced in the presence of CTAB, both in the presence and in the absence of light (16). But a nonionic surfactant (Tween 80) had little or no effect on the photoreaction while an anionic surfactant (SDS) slowed the photoreaction slightly (16). Cationic surfactants have strong adsorption on solid surfaces, making them ideal for surface modification. Adsolubilization of hydrophobic compounds by adsorbed surfactant micelles (admicelles) can dramatically increase the surface concentration of hydrophobic compounds. Solubilization of toluene in surfactant bilayers formed in the interlayer space of vermiculite was responsible for the uptake of toluene by vermiculite (17). Admicelles have also been used as two-dimensional templates for reactions. Admicellecatalyzed hydrolysis of trimethyl orthobenzoate to methyl benzoate was achieved using SDS sorbed on high surface VOL. 35, NO. 18, 2001 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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area alumina (18). An increase of six in the chromate reduction rate was obtained under the influence of HDTMA admicelles on pelletized ZVI and zeolite surfaces, and it was speculated that the sorption of chromate increased the surface chromate concentration in the vicinity of the reactive ZVI surface (19). The objective of this study was to investigate whether a synergistic effect of PCE reduction by ZVI could be achieved using cationic surfactants sorbed on ZVI surface. Specifically, surfactant surface loading, surfactant chain length, solution pH, and ionic strength were considered when the reaction kinetics were evaluated.
Materials and Methods The ZVI was obtained from Fisher Scientific (Catalogue No. S71953) with particle sizes less than 100 mesh (
