Environ. Sci. Technol. 2005, 39, 1283-1290
Use of Nanosized Catalysts for Transformation of Chloro-Organic Pollutants ISHAI DROR,* DANA BARAM, AND BRIAN BERKOWITZ Department of Environmental Sciences and Energy Research, Weizmann Institute of Science, Rehovot 76100, Israel
A new method to transform anthropogenic, chloro-organic compounds (COC) by use of nanosized molecular catalysts immobilized in sol-gel matrixes is presented. COC represent a serious threat to soil and groundwater quality. Metalloporphyrinogens are nanometer sized molecules that are known to catalyze degradation of COC by reduction reactions. In the current study, metalloporphyrinogens were immobilized in sol-gel matrixes with pore throat diameters of nanometers. The catalytic activity of the matrix arrays for anaerobic reduction of tetrachloroethylene (PCE), trichloroethylene (TCE), and carbon tetrachloride (CT) was examined. Experiments were performed under conditions pertinent to groundwater systems, with titanium citrate and zero-valent iron as electron donors. All chloroorganic compounds were reduced in the presence of several sol-gel-metalloporphyrinogen hybrids (heterogeneous catalysts). For example, cobalt-5,10,15,20-(4hydroxyphenyl)-21H,23H-porphine (TP(OH)P-Co) and cyanocobalamin (vitamin B12) reduced CT concentrations to less than 5% of their initial values in a matter of hours. Cyanocobalamin was found to reduce PCE to trace amounts in less than 48 h and TCE to less than 25% of its initial concentration in 144 h. The reactions were compared to their homogeneous (without sol-gel matrix) analogues. The reduction activity of COC for the homogeneous and heterogeneous systems ranged between similar reactivity in some cases to lower reduction rates for the heterogeneous system. These lower rates are, however, compensated by the ability to encapsulate and reuse the catalyst. Experiments with cyanocobalamin showed that the catalyst could be reused over at least 12 successive cycles of 24 h each.
Introduction Anthropogenic, chloro-organic compounds (COC) have been used intensively over the last century, and large amounts have been released to the environment. These compounds are known to be very toxic, carcinogenic, and mutagenic even through exposure to minute concentrations (1-5). Furthermore, these substances are extremely stable and may persist for decades, posing a continuing threat to soil and groundwater quality (6, 7). Effective remediation techniques are required, and particularly intensive efforts are being invested in the development of methods to detoxify these contaminants in groundwater (6, 8-16). * Corresponding author telephone: +972-8-9344230; fax: +9728-9344124; e-mail:
[email protected]. 10.1021/es0490222 CCC: $30.25 Published on Web 11/17/2004
2005 American Chemical Society
One method to abiotically detoxify chloro-organic pollutants involves their degradation, under anoxic conditions, using reductive dechlorination. This process forms less chlorinated compounds that are usually less toxic and more susceptible to further degradation (7, 13, 16-18). The approach is based on the utilization of electron-transfer mediators in reduction processes. Several compounds that function as effective mediators have been found, with the most common (and efficient) ones so far belonging to the metalloporphyrinogens group. Porphyrins are naturally occurring, organic tetrapyrrole macrocycles composed of four pyrrole-type rings joined by methylidene bridges. They form a nanosized (1-2 nm), nearplanar structure of aromatic macrocycles containing a total of 22 conjugated π-electrons, 18 of which are incorporated into the delocalization pathway in accord with Huckel’s rule of aromaticity [4n+2]. Metal complexed porphyrins and other biological porphyrin-type molecules (i.e., corroles, corrins, and corrinoid macrocycles) like vitamin B12 are referred to hereafter as metalloporphyrinogens. Metalloporphyrinogens have several properties that make them very appealing for the treatment of persistent organic pollutants: (i) They have been found to be effective as redox catalysts for many reactions, and they are known to be active catalysts over a long range of redox potentials (19-21). (ii) They have been found to be electrochemically active with almost any core metal (21, 22). (iii) They have been shown to function well in aqueous solutions under conditions that are often found in the groundwater environment (2, 3, 7, 8, 14, 17). (iv) They are highly stable, which enables reactions under severe conditions (22) that prevent other treatment methods (e.g., bioremediation). Previous studies have shown that immobilized metalloporphyrinogens, formed by intercalation in layered minerals or other surfaces, have in some cases catalyzed electrontransfer-mediated reactions. For example TMPyP-Co on silica gel and double-layered clays were observed to proceed relatively similarly to their analogous homogeneous reactions (2). Carrado et al. (23) immobilized metalloporphyrinogens in layered silicate smectite clays and showed that the metalloporphyrinogens are incorporated intact. The immobilization of metalloporphyrinogens for reductive dechlorination has been performed in different matrixes such as sepharose, sephadex, and polystyrene (9). Ukrainczyk et al. (2) used hectorite, fluorohectorite, and amorphous silica gel surfaces that require permanent charge of the macrocycle in order to be adsorbed and retained on the charged surfaces of the supports. Much recent work has established that encapsulation of biofunctional molecules and porphyrins in sol-gel glasses is highly effective in the development of biocatalysts and biosensors (24-27). These studies led to the current effort to utilize biomimetic catalyst systems by fabrication of an inorganic-organic hybrid nanocomposite, via a sol-gel reaction in the presence of porphyrins, for the dechlorination of COC. The appeal of sol-gels as a matrix was best summarized by Avnir (28): “When one considers the unique properties of ceramics on the one hand and the vastness of organic chemistry on the other, it seems almost inevitable that overlapping these domains of chemical research should lead to novel materials and to novel reaction configurations”. The goal of sol-gel technology is to use low-temperature chemical processes to provide very high purity and homogeneity, with desirable properties of hardness, chemical durability, tailored porosity, and thermal resistance. Sol-gel processing can provide control of matrix microstructures in VOL. 39, NO. 5, 2005 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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the nanometer size range (i.e., 1-100 nm and pore throat diameters of >10 nm), which approaches the molecular level (28-30). The entrapped molecules are accessible to external reagents through the pore network; therefore, chemical reactions and interactions are possible (28). Nanotechnology generally allows use of nanometer size molecules in an array that gives enhanced performance to bulk materials. Harnessing such arrays to solve environmental problems is a promising field that may allow more efficient treatment of environmental pollutants. Therefore, the objectives of the current work were to synthesize composite material of nanosized metalloporphyrinogen catalyst molecules within a sol-gel porous matrix and to study the reductive dechlorination reaction of anthropogenic COC by these heterogeneous catalysts.
Materials and Methods Chemicals. All chemicals were received in sufficient purity and hence were used without any further purification. Tetraphenyl porphine (TPP)-Co, TPP-Ni, TPP-Fe, 5,10,15,20-(4-hydroxyphenyl)-21H,23H-porphine (TP(OH)P), 4,4′,4′′,4′′′-(21H,23H-porphine-5,10,15,20-tetrayl)terakis (benzoic acid) (T(benzoic)P), and 1-methyl-4 pyridinoporphyrin tetratosylate (TMPyP) were purchased from Aldrich Chemical Co. (Milwaukee, WI). The metalloporphyrinogens TP(OH)PCo, TP(OH)P-Ni, T(benzoic)P-Ni, T(benzoic)P-Co, TMPyPCo, and TMPyP-Ni were received from M. Schlautman and were synthesized according to published procedures (14). Cyanocobalamin (vitamin B12) was obtained from SigmaAldrich Israel. Tetrachloroethylene (PCE) (99%) and carbon tetrachloride (CT) (99.5%) were purchased from Frutarom (Haifa, Israel), and trichloroethylene (TCE) (99.5%) was purchased from Merck (Germany). Aqueous stock solutions (in concentrations of 50 ppm) were prepared by dissolving 15.4, 17.1, and 15.8 µL in 500 mL of DDW for PCE, TCE, and CT, respectively. For the sol-gel, zero-valent iron and titanium citrate synthesis, tetramethyl orthosilicate (98%) (TMOS), sodium citrate dihydrate (99.9%), and trizma base (tris(hydroxymethyl)aminomethane) (99.9%) were purchased from SigmaAldrich Israel. Titanium trichloride (15% solution in HCl) was purchased from RdH (Riedel-de Haen, Sleeze, Germany), and methanol (99.9%) was obtained from Biolab Ltd., Israel. NaBH4 (Merck, USA) (96%) and FeCl36H2O (98%) were obtained from Sigma-Aldrich Israel. Hexane (99%) for sample extractions was purchased from Sigma-Aldrich Israel. Titanium citrate was prepared according to a procedure published by Smith and Woods (31). In general, 25.707 g of titanium chloride mixed with 30 mL of DDW were added to 7.35 g of sodium citrate and 7.9926 g of trisma. The process was performed in an ice bath while purging argon. The pH of the solution was adjusted to 8.2 with NaOH, and DDW was added in order to dilute to 100 mL solution. The final solution of 250 mM titanium citrate in 660 mM tris buffer was then transferred to vials, which were sealed and stored at -20 °C until use. Nanosized Zero-Valent Iron Synthesis. FeCl36H2O (0.1 mol) was dissolved in water (100 mL) and charged in a 500 mL filter flask in an anaerobic chamber. NaBH4 (0.3 mol) was dissolved in water (100 mL) and charged in a 125 mL amber bottle in an anaerobic chamber. The reactants were transferred to a fume hood and placed on a magnetic stirrer. The reaction was initiated by addition of pumped NaBH4 into the Fe3+ solution while supplying N2 gas to the reaction bottle to prevent intrusion of O2. The reaction was completed upon delivering all of the NaBH4 solution, resulting in the precipitation of black solid. After reaction, the solid was transferred to an anaerobic chamber, collected by filtration, and washed with 1000 mL of water and 200 mL of acetone. 1284
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The solid was dried under a N2 atmosphere at 100 °C overnight and broken up to form a fine black powder in the anaerobic chamber. Preparation of Immobilized Metalloporphyrinogens in Sol-Gel Matrixes. TMOS, methanol, and metalloporphyrinogens dissolved in aqueous solution (2 mM) were mixed in a 1:5:8 molar ratio, respectively. The solution was mixed until gelation occurred and then dried in a hood for about 1 week until constant weight was achieved, and catalyst concentration was determined by calculating the amount of catalyst inserted per weight of matrix obtained. Dechlorination Reaction Systems. Batch reactor systems were prepared in an anaerobic chamber (Coy Laboratory Products Inc., Michigan) containing an atmosphere of 95% N2 and 5% H2. Each aliquot was prepared in an 8 mL clear bottle sealed with natural-colored PTFE/clear silicon septa. Each aliquot contained 50 µL of 2 mM aqueous metalloporphyrinogen solution (in the homogeneous catalyzed reaction systems) or an equivalent molar amount of immobilized metalloporphyrinogen in sol-gel (in the heterogeneous catalyzed reaction systems). A solution of 0.5 mL of titanium citrate or 30 mg of zero-valent iron (ZVI) powder was added to the bottles, followed by an addition of 7.5 mL of substrate stock solution of 50 mg/L, giving a final substrate concentration of 47 mg/L. The pH values of the titanium citrate and ZVI systems were 6.6 and 8.1, respectively. The last step initiated the reaction, and each reactor was sealed immediately. Control homogeneous systems consisted of titanium citrate or ZVI and COC stock solution. Control heterogeneous systems consisted of sol-gel without metalloporphyrinogen, titanium citrate/ZVI, and pollutant solution. The terms “heterogeneous system” and “homogeneous system” relate hereafter to the catalysts. Thus, for example, a system with solid ZVI and dissolved metalloporphyrinogen is considered an homogeneous system. The samples were then mixed in a shaker (250 rpm). As discussed in the Results and Discussion section, several metalloporphyrinogens were examined in a set of survey experiments. For these experiments, all metalloporphyrinogens were immobilized in sol-gel, and reaction systems were mixed in a shaker for 144 h before being examined for results. For the kinetic experiments, cyanocobalamin and TP(OH)P-Co were selected as catalysts. In each experiment, batch reaction systems were prepared, each including samples containing immobilized metalloporphyrinogens with titanium citrate as the electron donor and the relevant chlorinated compound. Control samples contained the same solution with clean metalloporphyrinogens. Each batch was examined at different time intervals by extracting an aliquot of the organic compounds from the aqueous solution (i.e., no sol-gel catalyst) with hexane (1.5 mL of hexane to 6 mL of aqueous substrate solution). CT reduction with TP(OH)PCo was conducted with time intervals of 0, 20, 30, 60, 90, 200, and 260 min and 24 h). PCE reduction with cyanocobalamin was conducted with time intervals of 0, 3, 5, 24, 48, and 72 h. Catalyst Reuse Experiment. CT reduction catalyzed by cyanocobalamin was examined. Immobilized cyanocobalamin (0.178 g sol-gel matrix which was equivalent to 7.33 × 10-7 mol) was placed in a 60 mL bottle, and 3.6 mL of titanium citrate and 55 mL of a 50 ppm CT solution were added. The bottle was sealed and transferred into a shaker. Solution samples of 6 mL aliquots were extracted by 1.5 mL of hexane every 24 h and transferred for GC analysis. All residues of titanium citrate and COC solutions were then pumped out. The remaining sol-gel matrix was then immersed in a new solution with the same components. Analytical Analysis. Standards containing PCE, TCE, CT, chloroform, and DCM were injected separately into a GC equipped with a J&W DB-VRX column (30 m, 0.32 mm i.d.,
FIGURE 1. Scanning electron microscope (SEM) picture of panel A, undoped (clean) sol-gel matrix; panel B, cyanocobalamin-doped sol-gel; and panel C, TP(OH)P-Co-doped sol-gel. Side a is a magnification of 200-400×. Side b is a magnification of 25 000×. Agilent Technologies, USA) and an electron capture detector (ECD) to identify the retention times of the compound used and their expected products from the reactions. CT analysis protocol: 60 °C for 1.5 min, ramp of 5 °C/min to 80 °C and continue immediately with a 15 °C/min rate to 200 °C, hold for 1 min. PCE/TCE analysis protocol: 50 °C for 2 min, ramp of 17 °C/min to 190 °C, hold for 1 min.
Results and Discussion Sol-Gel Metalloporphyrinogen Hybrid Morphology and Activity. Clean sol-gel matrixes (no metalloporphyrinogen added) and sol-gel matrixes containing cyanocobalamin and TP(OH)P-Co were examined by scanning electron microscope (SEM). A panel of SEM pictures of the three matrixes is presented in Figure 1a,b. The clean sol-gel matrix shown
in the 100 µm scale pictures (Figure 1a, panel A) exhibits a large macro-porosity and a coral structure, which is not apparent for the case of both hybrid matrixes containing cyanocobalamin (Figure 1a, panel B) and TP(OH)P-Co (Figure 1a, panel C). For those matrixes, the surface appears much smoother with only some particles roughness. However with larger magnification (Figure 1b), which allows distinction of morphological structures of hundreds of nanometers, a similar porous configuration is apparent for all three materials. This result suggests that the addition of metalloporphyrinogens affects the large-scale porosity and the final morphology of the sol-gel matrixes; the presence of metalloporphyrinogens in the solutions may cause more rigorous polymerization of the silica network. Recent work by Sacco et al. (32) has demonstrated similar behavior, showing how VOL. 39, NO. 5, 2005 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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TABLE 1. Final Concentrations of Chloro-Organic Compounds after 144 ha
metalloporphyrinogen control sol-gel control homogeneous TMPyP-Fe TPP-Ni TP(OH)P-Fe TP(OH)P-Ni TPP-Co T(benzoic)P-Co TMPyP-Ni TP(OH)P-Co TPP-Fe T(benzoic)P-Ni cyanocobalamin
CT TCE PCE (initial conc. (initial conc. (initial conc. 47 mg/L) 47 mg/L) 47 mg/L) 100.00 84.83 82.42 72.33 50.57 40.81 10.81 0.33 0.21 0.12 0.10 0.05 0.00
100.00 103.50 102.15 108.26 97.83 102.53 94.22 0.47 25.66 89.79 100.08 95.41 23.94
100.00 92.53 89.50 84.86 82.19 84.04 83.18 0.55 24.32 23.39 91.76 76.30 0.46
a Expressed as percentage of initial amount of the parent compound, for batch experiments with sol-gel metalloporphyrinogen hybrids as catalysts and titanium citrate as reducing agent.
one metalloporphyrinogens-doped sol-gel gave a “smooth surface composed of homogeneous aggregates of granules” and how other metalloporphyrins caused a “heterogeneous porous spongy structure of silica particles”. The ability of the resulting hybrid matrixes to reduce COC in aqueous solution under anaerobic reducing conditions was then examined. Several studies have shown that metalloporphyrinogens maintain and sometime even improve their catalytic activity over long times when used for oxidation reactions (e.g., refs 27, 33, and 34). Furthermore, it has been shown that the metalloporphyrinogens themselves are not usually affected by encapsulation in the sol-gel. For example, Dargiewicz et al. (35) showed that the absorption spectra of cationic water-soluble porphyrins in solutions and sol-gel matrixes indicate only small differences, which may imply very similar electronic behavior. The control heterogeneous systems include sol-gel without catalyst to ensure that effects of sorption or entrapment of contaminant in the matrix are eliminated. Eleven sets of immobilized metalloporphyrinogen matrixes were synthesized and tested for their activity as catalysts for reductive dechlorination after 144 h in batch reactions with titanium citrate as reducing agent. Table 1 shows the concentrations of COC expressed as a percentage of the initial concentration of the parent compounds. CT transformation was found in all cases, including some reduction in CT concentration for the homogeneous control experiment; this result might be explained by direct reduction of CT by the titanium citrate. The most effective catalysts for these reactions were T(benzoic)P-Co, TMPyP-Ni, TP(OH)P-Co, TPP-Fe, T(benzoic)P-Ni, and cyanocobalamin, reducing CT to less than 0.5% of its initial concentration. These results indicate that the reactions proceed with all metals complexed in the metalloporphyrinogens (Ni, Co, and Fe). However, when one considers a specific metalloporphyrinogen, different metals complexed within them show different reaction efficiencies. For example, reaction with CT and TMPyP-Fe resulted in 82.42% CT (relative to the initial concentration) similar to the homogeneous control (without any metalloporphyrinogen), while reaction with TMPyP-Ni resulted in 0.21% CT for the same reaction conditions. Reaction efficiencies for TPP complexes were found to proceed in the following order: TPP-Ni (72.33%) < TPP-Co (10.81%) < TPP-Fe (0.10%). For the TP(OH)P and T(benzoic)P complexes, the efficiency order was as follows: TP(OH)P-Fe (50.57%) < TP(OH)P-Ni (40.81%) < TP(OH)P-Co (0.12%) and T(benzoic)P-Co (0.33%) ≈ T(benzoic)P-Ni (0.05%). When examining a certain metal with 1286
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different metalloporphyrinogens, large differences in reaction efficiencies occur: reaction with TMPyP-Fe resulted in 82.42% CT, while reaction with TP(OH)P-Fe resulted in 50.57%, and reaction with TPP-Fe resulted in 0.10% CT. Significant differences appeared in reactions with metalloporphyrinogens complexed with Ni as well. All reactions with metalloporphyrinogens complexed with cobalt resulted in low concentration values of CT. In contrast, TCE and PCE transformations were more selective: TCE was transformed only in three cases (in all other cases 90% or more of the initial compound was found at the end of the reaction), while PCE was transformed successfully by four metalloporphyrinogens (the complexes showed between 0.5% and 25% reduction in PCE concentrations). Metalloporphyrinogens complexed with Fe did not show efficient reduction reactions with either TCE or PCE. Cobalt porphyrinogens showed effective reductive ability with PCE. T(benzoic)P-Co and cyanocobalamin catalyzed reactions in which less than 1% of PCE initial value remained. TP(OH)P-Co and TMPyP-Ni were also shown to reduce PCE with less than 25% left after 144 h. T(benzoic)P-Co was the most effective metalloporphyrinogen in terms of reducing all COC in one reaction system. It nearly completely catalyzed all reduction reactions, leaving only trace amounts of the initial COC. In previous work Dror and Schlautman (18) reported that reduction reactions of PCE with TP(OH)P-Fe and TPP-Co as a homogeneous catalyst did not proceed during 193 h. The current experiments with TP(OH)P-Fe and TPP-Co as heterogeneous catalysts resulted in 82.19% and 83.18% PCE after 144 h, respectively. These latter results, found for samples examined after 144 h, suggest that heterogeneous systems can in some cases improve reaction rates and efficiency. Furthermore, previous work (14, 18) indicates that the core metal, organic functional groups, and solvents of metalloporphyrinogens can influence the products, rates, and potentially the mechanisms of reductive dechlorination reactions. However, the metalloporphyrinogen should be considered as an integrated unit with multiple contributions from its different components. The effects of the various structures comprising the metalloporphyrinogen may perhaps best be explained in terms of their influence on the gap between the highest occupied molecular orbital and the lowest unoccupied molecular orbital (HOMO-LUMO) (which may or may not allow a particular reaction) as well as how they may influence the catalytic mechanism. Carbon Tetrachloride Reduction Reaction Catalyzed by TP(OH)P-Co. TP(OH)P-Co was chosen for more detailed investigation of CT reduction over time. The possible reduction products of CT are chloroform (CF), dichloromethane (DCM), and non-chlorinated methane. In the first reaction system, immobilized TP(OH)P-Co was mixed with CT and titanium citrate. Concentrations of CT, chloroform, and DCM were measured throughout the experiment after 0, 20, 30, 60, 90, 200, and 260 min and 24 h from the beginning of the experiment. Figure 2 shows the normalized concentration of CT versus time and exponential decay fit. The total mass concentration of the chlorinated compounds (i.e., CT, chloroform (CF) and dichloromethane (DCM)) examined in the experiment was reduced completely within 24 h. CT was reduced to around 10% of its initial concentration after less than 100 min and completely after 260 min, with only a small amount of CF evolving in parallel and with no other chlorinated compound in detectable concentrations being found in the system (i.e., transformation of COC to nonchlorinated methane). An exponential decay was fit to the concentration degradation of CT with a decay constant of -0.022 min-1. This exponential decay indicates a pseudofirst-order reaction and degradation rate proportional to the CT concentration in solution. These results are in accordance
FIGURE 2. Normalized concentration of CT (initial concentration and control was 47 mg/L) vs time for immobilized TP(OH)P-Co in sol-gel matrixes using titanium citrate as an electron donor. CT concentrations normalized to the control sample; error bars present standard deviation values from four analyses; exponential decay parameters N(t) ) N(0)e-λt; N(0) ) 100.4; λ ) 0.022 min-1. with the pseudo-first-order reaction found by Ukrainczyk et al. (2) with TMPyP-Co derivatives immobilized on mineral supports. Thus under the current conditions, the reduction reaction rate depends on the substrate (CT) concentration and the catalyst is not a limiting factor. In other words, the metalloporphyrinogen is accessible and does not limit the reaction. Similar transformations were also found for homogeneous solutions of the same catalyst with complete transformation of CT within about 3 h. This result also suggests that the sol-gel matrix posed no major interference in the reaction. Product analysis during the experiment indicates evolution of CF (i.e., 1.5, 4.0, 6.1, and 9.97% out of the total chlorinated compounds after 20, 30, 60, and 90 min, respectively); no other chlorinated compounds were found. Therefore, it is assumed that at least some of the CT is first transformed to CF and is then reduced to non-chlorinated species in a fast reaction that does not leave any other byproduct apparent in the analysis. Carbon Tetrachloride Reduction Reaction Catalyzed by Cyanocobalamin. Cyanocobalamin has been used frequently as catalyst for reductive dechlorination, in general, and for CT, in particular, with promising results (36-39). Therefore, encapsulation of cyanocobalamin in a sol-gel and examination of the activity of the hybrid material as heterogeneous catalyst is of special interest. Four treatments for CT reduction were examined: homogeneous and heterogeneous systems with titanium citrate and ZVI as electron donors. Results for the systems treated with titanium citrate as the electron donor are shown in Figure 3. Chlorinated compounds (i.e., CT, CF (and DCM, if identified)) concentrations decreased to less than 10% of their original values in both experiments during the first 10 h. The dechlorination reactions in the homogeneous system occurred faster than in the heterogeneous system. However, both systems resulted in major dechlorination within the first 3 h, and very low chlorinated compound concentrations remained after less than 24 h. For the heterogeneous system, a reduction in the transformation rate is shown when the chlorinated compound concentration is close to 10% of the initial value. This reduced rate might be due to the fact that the sol-gel matrix interferes in the transport and mixing of the three components (catalyst, electron donor, and pollutant); therefore, the rate of reaction is reduced. In an additional experiment (data not shown) after a longer time, complementary measurements showed no chlorinated compounds in the reaction solution.
FIGURE 3. Concentration of total chlorinated compounds found in the reaction (CT + CF) vs time for (A) homogeneous and (B) heterogeneous systems catalyzed by cyanocobalamin using titanium citrate as an electron donor (0.5 mL of titanium citrate for each reactor of 7.5 mL solution of 50 mg/L CT). (9) Electron donor plus catalyst; (b) control electron donor only (no catalysts).
FIGURE 4. Concentration of total chlorinated compounds (CT + CF) vs time for (A) homogeneous and (B) heterogeneous systems catalyzed by cyanocobalamin using ZVI as an electron donor (30 mg of ZVI for each reactor of 7.5 mL solution of 50 mg/L CT). (9) Electron donor plus catalyst; (b) control electron donor only (no catalysts). Figure 4 shows the reaction catalyzed by cyanocobalamin with ZVI as the electron donor. ZVI represents an environmentally acceptable reducing agent that has been shown to directly reduce organohalides in laboratory and field experiments (e.g., refs 40 and 41) as well as to act as an electron source for other redox catalysts (37). Consequently in the case of experiments with ZVI it is expected that the reduction of CT will proceed from both the direct ZVI pathways and by catalysis with the cyanocobalamin. As a result the control presented in Figure 4 (for controls the same reaction setup were used but without the metalloporphyrinogens) also shows reduced concentrations with time. For the reductive dechlorination of ZVI, three pathways were proposed (40): “1. direct electron transfer from iron metal at the metal surface; 2. reduction by Fe2+, which results from corrosion of the metal; 3. catalyzed hydrogenolysis by the H2 that is formed by reduction of H2O during anaerobic corrosion”. The first pathway requires movement of the nanosized ZVI particles in the matrix to reduce the metalloporphyrinogen, which may be possible but probably has a steric limitation that slows the reaction. The last two pathways result in species that can readily move in solution and through the matrix to reduce the catalyst. VOL. 39, NO. 5, 2005 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 5. PCE concentrations vs time for (A) homogeneous and (B) heterogeneous catalysis of PCE by cyanocobalamin, using titanium citrate as an electron donor (0.5 mL of titanium citrate for each reactor of 7.5 mL solution of 50 mg/L PCE). (9) Electron donor plus catalyst; (b) control electron donor only (no catalysts). Inset: Linear fit for results of concentration vs time in PCE transformation reaction for heterogeneous catalysis. The total concentration of chlorinated compounds in the heterogeneous catalysts system decreased to 33.7 µM (i.e., 5.18 mg/L. ∼10% of initial concentration), while the control sample decreased to 135.9 µM (i.e., 20.9 mg/L ∼40% of initial concentration). Similar (and even somewhat better results) were obtained for the homogeneous catalyst systems. In both experiments (the homogeneous and heterogeneous catalyst with ZVI), the addition of metalloporphyrinogen improved both the rate of CT reduction and the final CT degradation levels. Supporting results showing similar behavior in homogeneous catalyst systems (only) were presented by Morra et al. (37); they found that for some COC compounds, such a combination of ZVI and catalyst may provide a remediation solution that is not given by ZVI alone. The effect of the sol-gel matrix on the reduction of chlorinated compounds (mainly CF and DCM) for the heterogeneous catalyst with ZVI (Figure 4) is shown to be more pronounced when the total concentration is reduced to about 10% of the initial concentration. This pattern is similar to the behavior found for the parallel reaction with titanium citrate as electron donor (see Figure 3B). From that stage (∼10%) the reduction reaction rate is almost stopped. Because the only difference between the homogeneous and the heterogeneous systems is the sol-gel matrix, it is suggested that, in order to achieve complete dechlorination of the CT, a longer residence time is needed to allow the contaminant to react with the reduced catalyst. PCE Reduction Reaction Catalyzed by Cyanocobalamin. PCE reduction reactions catalyzed by cyanocobalamin were studied by comparing heterogeneous and homogeneous reactions with titanium citrate as the electron donor. As shown in Figure 5, the cyanocobalamin-catalyzed reaction 1288
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resulted in (almost) complete reduction of PCE in the homogeneous and heterogeneous systems, and only low concentrations of PCE (less than 5%) remained. In the homogeneous system, 2.20% of the PCE remained after 3 h, while in the heterogeneous system, 1.63% was left after 48 h. This behavior is consistent with the other experiments in which homogeneous reactions were faster than the corresponding heterogeneous reactions. The major (and the only chlorinated) byproduct of PCE reduction was TCE. In a separate set of experiments (data not shown), TCE concentrations decrease to 39.3% of initial concentration during 72 h for the homogeneous system and to 82.7% and 23.9% of initial concentration after 72 and 144 h, respectively, for the heterogeneous system. Because the two systems differ only in the presence of the inert sol-gel matrix, it is reasonable to assume that the reaction rate is affected by the transport of the reducing agent and contaminant in the sol-gel matrix. For the reduction, PCE electrons must be transported from the electron donor (titanium citrate) to the contaminant. In a well-mixed homogeneous system with a sufficient amount of electron donors, the transformation of electrons from and to the catalysts is very fast. This can be seen from the fact that at the first sampling point in Figure 5A the PCE concentration is almost zero. In the heterogeneous system, however, the sol-gel matrix hinders the electron transformation; hence, the rate of PCE reduction was reduced. Additional comparison of the reduction of PCE and CT of the heterogeneous systems (Figures 3B and 5B) shows also that the reduction of the larger PCE molecule is slower while the reduction in the homogeneous systems (Figures 3A and 5A) shows complete reduction after 3 h. This suggests that the sol-gel matrix does play a role by posing a steric
mentation of the technique. It is noted here that no leaching of cyanocobalamin (which is detectable at concentrations of 1 µM; 14) was observed during the experiment. The significant reductions in COC concentrations in the systems, together with the ability to reuse the materials, leads to the potential for creating in situ hybrid systems consisting of nanosized metalloporphyrinogen catalysts encapsulated in porous sol-gel matrix. Specifically, extrapolating results of the current study that show the reduction of PCE and TCE, especially with application of ZVI as an electron donor, suggest that such hybrid systems may provide a new groundwater remediation technique. To facilitate the application of this new catalyst array for groundwater remediation, further investigation should be directed toward obtaining field measurements and to exploring the mechanistic pathways of the reaction and the effect of environmental conditions on the reaction.
Acknowledgments FIGURE 6. Carbon tetrachloride concentration (in %) in solution vs cycle number (24 h each) of successful reduction reaction, using cyanocobalamin immobilized in sol-gel matrixes as the catalyst and titanium citrate as electron donor. perturbation that affects the reduction rate. In the heterogeneous system, PCE transformed completely into less chlorinated compounds after 48 h, in a reaction that might be interpreted as a pseudo-zero-order reaction (i.e., the reaction does not depend on the concentration of the compound). The reaction order was determined by plotting concentration versus time for the heterogeneous system for the sampling points during the first 48 h, as presented in the inset in Figure 5. This reaction order means that the reaction rate does not depend on the PCE concentration, which suggests that here the effect of the matrix is evident (i.e., the rate-determining step for the reaction is limited by the transport rate of PCE in the matrix). Burris et al. (7) examined reduction reactions of PCE immobilized on agarose and showed that such reactions act upon pseudo-first-order reaction rates. This difference in reaction order for similar reaction conditions can be explained by the role of the matrix, which can affect the reaction rate. As explained above for the heterogeneous system, the sol-gel matrix hinders the rate of electron transfer from and to the catalysts and hence here the rate determining step is now the transport in the matrix. Reuse of Sol-Gel Metalloporphyrinogen Hybrids for the Reductive Dechlorination of CT. The advantage of heterogeneous catalysis over homogeneous catalysis is the ability to easily separate the catalytic system from the reaction solution. The ability to transfer the sol-gel-metalloporphyrinogen matrixes from one solution to another or the ability to immobilize the catalyst in a way that will enable, for example, polluted groundwater to flow (in situ) through it, will result in much better reaction efficiencies than can be achieved for the same amount of catalyst in an homogeneous system. Therefore, the ability to reuse the sol-gel-cyanocobalamin hybrid was examined in order to test the possibility of reusing the catalytic matrixes. Immobilized cyanocobalamin was added to titanium citrate and CT solutions. Solution samples were taken every 24 h for analysis, and residual solutions in the reaction bottles were replaced with new solutions. Figure 6 shows the initial CT concentration added to the sample and the measured concentration of CT after 24 h reaction. The ability to transfer and reuse the catalyst while maintaining the same reactivity over time was illustrated for the reductive dechlorination of CT in 12 cycles of 24 h each. In all cases, the CT was consumed completely after each cycle and transformed mainly to (nonchlorinated) methane. The turnover ratio ) CT (mol)/catalyst (mol) ) 269. Repetitive usage is important for the imple-
The authors thank Dr. Hanan Serchook for discussions and for technical assistance. The Marcus and Lily Sieff Postdoctoral Fellowship and the P. and A. Guggenheim-Ascarelli Foundation are thanked for generous financial support. B.B. holds The Sam Zuckerberg Professorial Chair.
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Received for review June 29, 2004. Revised manuscript received August 25, 2004. Accepted August 26, 2004. ES0490222
