Environ. Sci. Technol. 2000, 34, 3489-3494
Reduction of N-Nitrosodimethylamine with Granular Iron and Nickel-Enhanced Iron. 1. Pathways and Kinetics LAI GUI, ROBERT W. GILLHAM,* AND MAREK S. ODZIEMKOWSKI Department of Earth Sciences, University of Waterloo, Waterloo, Ontario, Canada N2L 3G1
Laboratory batch and column tests were conducted to examine the reduction pathways and kinetics of Nnitrosodimethylamine (NDMA) by iron (Fe) and nickelenhanced iron (Ni/Fe). A decrease in NDMA concentration and increases in dimethylamine (DMA) and ammonium were observed in both Fe and Ni/Fe columns. In the Fe column, the transformation process of NDMA appeared to follow pseudo-first-order kinetics with respect to NDMA, with an average half-life of 13(2 h. A small amount of nickel (0.25%) plated onto the iron greatly enhanced NDMA transformation rates. At early time the NDMA half-life in the Ni/Fe column was 2 min but as time progressed the halflife increased to 4 min, and departures from first-order kinetics were observed. The mass balances of carbon in DMA and nitrogen in DMA and ammonium improved over time and reached 100% and 90%, respectively, after NDMA had passed through the column for more than 50 pore volumes (PV). No 1,1-dimethylhydrazine, nitrous oxide, or methane were detected. Based on the electrochemical properties of NDMA, the transformation mechanism of NDMA with Fe and Ni/Fe is postulated to be catalytic hydrogenation, resulting in N-N bond breakdown to form DMA and ammonium as final products. Nickel, being a much stronger catalyst than Fe for catalytic hydrogenation, resulted in a much faster reduction rate of NDMA. Of several methods tested, flushing the Ni/Fe column with 0.01 N sulfuric acid proved to be the most effective in restoring the Ni/Fe activity. The rapid transformation rate on Ni/Fe and the formation of nontoxic products indicate that this material may be applicable for treating NDMA contaminated water, both in-situ and above ground.
NDMA is very soluble and volatile and therefore can be readily transported into various environmental compartments including groundwater. Recently NDMA, at concentrations of parts per billion, has been detected in groundwater samples in several locations, for example, in Elmira, Ontario, Canada (8), and in surface waters, seawaters, and soils (9). To date research on NDMA has focused mainly on its toxicity effects on humans and animals (10, 11) and its occurrence in foodstuffs such as fruit juices and beer (2). In natural environments or ambient conditions, NDMA appears to be recalcitrant under both aerobic and anaerobic conditions (7, 12) and is, therefore, difficult to remove from water. Currently NDMA in wastewaters is treated using physical or chemical methods, including UV destruction and adsorption by granular activated carbon (4). These methods are either very expensive or insufficient for achieving complete NDMA removal, and thus there is a need for developing an efficient and relatively inexpensive method that can destroy NDMA to innocuous compounds. An innovative technology, developed by Gillham and O’Hannesin, uses granular iron (Fe) as an electron source for reductively degrading chlorinated solvents (13). This technology has been widely accepted as a cost-effective method for treating contaminated groundwater. Subsequent research has extended the iron technology to include reduction of nitroaromatics (14), azo dyes (15), and nitrate (16) as well as the removal of chromium and uranium by reductive precipitation (17, 18). Reduction of nitroaromatic compounds proceeds stepwise, forming aromatic nitroso and hydroxylamines as intermediates. Although nitro reduction occurs much more easily on aromatic rings than on aliphatic moieties (19), it nevertheless suggests that the nitroso group in NDMA may be susceptible to reduction by iron, particularly with nickel-enhanced iron (Ni/Fe). Studies have shown that Fe plated with a small amount of a more noble metal, such as palladium or nickel, increases the dechlorination rates by at least a factor of 10 (20, 21). Other studies have shown that nitrosamines can be reduced by Raney-nickel and lithium aluminum hydride to corresponding amines (22) and by some low-valent titanium reagents to the corresponding hydrazines (23). The objectives of the present study were (1) to determine NDMA degradation kinetics and pathways with Fe and Ni/ Fe and (2) to examine factors affecting NDMA degradation rates by Ni/Fe. Column tests were conducted to compare the effectiveness of Fe and Ni/Fe in NDMA transformation. Supplementary batch experiments were conducted to facilitate the examination of potential intermediates/products and adsorption of products.
Methods Introduction N-Nitrosodimethylamine (NDMA) is one of the few proven potent carcinogens (1, 2). Consequently, it is listed as one of the U.S. EPA priority pollutants (3) and is a stringently regulated compound. The U.S. EPA drinking water standard for NDMA is set at 0.7 parts per trillion (4). NDMA was mainly produced in rocket fuel production (4, 5) and has also been used as an antioxidant, an additive for lubricants, and a softener of copolymers (5). In addition, NDMA can be formed naturally in the environment where precursors such as nitrite, nitrous oxides, and dimethylamine (DMA) coexist (6, 7). * Corresponding author phone: (519)888-4658; fax: (519)746-1829; e-mail:
[email protected]. 10.1021/es9909778 CCC: $19.00 Published on Web 07/14/2000
2000 American Chemical Society
Chemicals and Materials. NDMA, DMA‚HCl (99%), and dinitrofluorobenzene (DNFB, 99%) were purchased from Sigma, and 1,1-dimethylhydrazine (UDMH‚2HCl, g99%) was purchased from Fluka. All chemicals were used without further purification. The granular iron used in the study was obtained from Connelly-GPM Inc. (Chicago, IL). The material contains 89.8% metallic iron, and the surfaces were covered with various forms of iron oxides (data provided by Connelly GPM Inc., 1998). The surface area, measured by the BET method, was 1.8 m2/g. This material was used without prior cleaning. The Ni/Fe bimetal was produced by electroless plating in an acid hypophosphite solution. The plating was performed using the procedure given in ref 20. The iron was used as VOL. 34, NO. 16, 2000 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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TABLE 1. Experimental Conditions and the Apparent NDMA Reduction Kinetics in the Ni/Fe Columnc total PV
flow rate, mL/min
initial concn, mg/L
kinetic fita
73.2 102.5 272.8 307.2 312.4 565.2 592.5 788.8 812.7 912.4 921.8 992.1 1050
0.21 8.64 8.30 8.30 9.01 8.85 10.05 11.3 8.7 3.20 2.40 4.47 10.65
5.3 6.2 26.3 27.0 25.1 14.7 28.4 21.4 23.4 25.2 25.6 24.2 24.0
first-order first-order first-order zero-order first-order mixed-order zero-order zero-order zero-order mixed-order first-order mixed-order first-order
rateb constant
r2
note
0.35 ( 0.05 0.25 ( 0.03 0.17 ( 0.02 0.60 ( 0.01 0.17 ( 0.03
0.956 0.972 0.938 0.999 0.959
0.92 ( 0.08 0.19 ( 0.01 0.61 ( 0.07
0.992 0.994 0.985
23 PV after 43 PV distilled-H2O flush
0.035 ( 0.002
0.997
6 PV after 13 d shut off
0.16 ( 0.01
0.971
29 PV after 70 PV acid (pH ) 2) wash
before any treatment
5 PV after 1 wk shut off
28 PV after 86 PV distilled-H2O flush
a
Plots of NDMA concentration vs residence time were fitted with either exponential or linear regression models. The best fit with highest r2 (>0.9) were chosen to describe pseudo-first-order and pseudo-zero-order kinetics. b The dimensions for pseudo-first-order rate constant and pseudo-zero-order rate constant are min-1 and (mg/L)-1 min-1, respectively. c Prior to 73 PV NDMA concentration decreased to below detection limit at the first sampling port at a flow rate of 0.25 mL/min and an initial concentration of 5 mg/L. Therefore it was not included in the table.
received without acid wash. The surface area of the Ni/Fe, measured by the BET method, was 3.1 m2/g. Column Experiments. Four columns were prepared using Plexiglas tubing (50 cm × 3.81 cm ID) which were constructed according to Gillham and O’Hannesin (13). Seven sampling ports were located along each column at distances of 2.5, 5, 10, 15, 20, 30, and 40 cm from the influent end. Sampling ports, mounted along the side of the column, consisted of a nylon Swageloc fitting (0.16 cm O.D.) with a 16 gauge luer lock syringe needle. Columns were packed with either 100% Fe or 100% Ni/Fe in 2 cm increments to a porosity of 0.59. NDMA solution prepared with distilled water (DI-H2O) without deoxygenation was pumped through the columns from the bottom using a variable speed multichannel peristaltic pump. The average flow rate was 0.21 mL/min for the Fe columns and was varied between 0.2 mL/min and 10 mL/min for the Ni/Fe columns (Table 1). Flow rates were determined by measuring the discharge, and the variation in flow rates was accounted for in calculating residence time of solution in the columns. The initial concentration of NDMA solutions varied between 5 mg/L and 28 mg/L. The NDMA solution was stored in a 30-L collapsable Teflon bag to minimize volatile loss and was kept in the dark to prevent possible photolysis. One pair of Fe and Ni/Fe columns received only DI-H2O to serve as controls. The columns were sampled by clamping the effluent line to allow water to flow from a sampling port directly into a glass syringe. Batch Experiments. Two sets of batch experiments were conducted. One was to examine the formation of N2O during NDMA degradation with Fe and Ni/Fe as well as to determine NH4+ adsorption on Fe and Ni/Fe materials. In 160 mL serum bottles, 50 g of Fe or Ni/Fe and 100 mL of 50 mg/L NDMA solution were added, resulting in a headspace-to-solution ratio of 1:2. Two sets of controls were prepared. One set contained Fe or Ni/Fe and was filled with DI-H2O, while the second set contained only NDMA solution without metals. The bottles were sealed immediately with Teflon-faced silicone septa and aluminum seals and mixed daily by hand shaking. They were stored in the dark at room temperature for 7 days. Samples of 2 mL headspace from each bottle were injected onto a GC for N2O analysis. Thereafter the serum bottles were centrifuged at 2000 rpm for 15 min. The supernatant was carefully removed for NH4+ and organic analyses. Bottles were weighed to determine the amount of solution remaining. Then 50 mL of DI-H2O was added to each bottle to wash off NH4+ from metal particles by shaking at 250 rpm for 30 min. The bottles were centrifuged again, and the supernatant was collected for NH4+ analysis. The NH4+ concentrations in the wash water were calculated to account for the amount of unremoved solution. 3490
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The second batch experiment was prepared in a similar manner using UDMH as the starting material. Each bottle (60 mL), containing 10 g of Fe or Ni/Fe, was filled with a 20 mg/L UDMH solution. Periodically duplicate bottles were sacrificed, and aqueous samples were removed for UDMH, DMA, and ammonium analyses. Analytical Methods. Analyses for NDMA, DMA, and UDMH were performed using a series 1100 Hewlett-Packard high performance liquid chromatograph (HPLC). The HPLC consisted of a diode array detector, a quaternary pump, an autosampler, and a degasser. A C18 ODS Hypersil column (5 µm particles, 125 × 4 mm) equipped with a LiChrospher 100 RP-8 guard column (5 µm, 4 × 4 mm) was used. Aqueous samples collected from sampling ports along the column or from the batch bottles were filtered through 0.2 µm syringe filters (Gelman Sciences, Ann Arbor, MI). For NDMA analysis, 20 µL of filtrant was injected onto the HPLC, and the absorbance of NDMA was measured at a wavelength of 225 nm. The mobile phase composition was 80/20 water/ methanol, and the flow rate was 1 mL/min. For DMA and UDMH analyses, a precolumn derivatization method was developed using DNFB (Sanger’s reagent) as the derivatization reagent. DNFB reagent was prepared daily in acetonitrile. The molar ratio of DNFB to DMA or UDMH was greater than 50 to ensure complete derivatization. In 1.5 mL GC vials, 325 µL of filtered aqueous samples were combined with 25 µL of DNFB, 100 µL of acetonitrile, and 50 µL of 0.1 M NaOH (adjusting pH to 12). The GC vials were then vortexed for a few seconds. The derivatization reactions were completed in less than 2 min, and the derivatives were stable for at least 36 h at room temperature. The absorbance of the derivatives was measured at a wavelength of 380 nm. The mobile phase consisted of 40/60 0.2% acetic acid (pH 4)/ methanol. The flow rate was 1.0 mL/min, and the injection volume was 20 µL. Analysis for N2O gas was performed on a GOW-MAC series 350GP gas chromatograph (GC) equipped with a thermal conductivity detector and a Porapak Q (100/120) mesh column (2 m × 0.16 cm). The carrier gas was helium at a flow rate of 20 mL/min. The detector temperature was 140 °C, and the oven temperature was isothermal at room temperature. Two mL gas samples were injected onto the GC after shaking at 250 rpm for 2 h to allow equilibration between the aqueous and gas phases. The concentration of N2O in samples was calculated based on gas-phase concentrations using an Oswald Coefficient of 0.5937 at 25 °C (24). Analyses for methane and other hydrocarbons were performed on a Hewlett-Packard Model 5790A GC equipped with a flame ionization detector and a Megabore GS-Q capillary column. The carrier gas was air at a flow rate of 10
a
FIGURE 1. Decreases in NDMA concentration with time in Fe and Ni/Fe columns. In the Fe column, the flow rate was 0.2 mL/min, and the initial NDMA concentration was 5 mg/L. In the Ni/Fe column, the flow rate was 8.7 mL/min, and the initial NDMA concentration was 25 mg/L. mL/min. The detector temperature was 120 °C, and the injector temperature was 60 °C. The oven temperature was held at 60 °C for 5 min and then ramped up to 120 °C at 15 °C/min. A 2-mL aqueous sample collected with a glass syringe was transferred to a 4-mL vial. Samples were shaken for 15 min at 250 rpm to allow organic partitioning between the aqueous and gas phases. Headspace samples of 250 µL were injected onto the GC using a gastight syringe. Aqueous concentrations of hydrocarbons were calculated using the Henry’s law constants. Ammonium was determined by Philip Analytical Services Co. (Mississauga, Ontario), using the automated phenate method. Total aqueous concentrations of Ni and Fe were analyzed by Atomic Absorption Analysis by the Water Quality Laboratory at the University of Waterloo.
Results NDMA Transformation Using Fe and Ni/Fe. For tests conducted on the Fe column, NDMA concentration profiles along the column were measured after every 6 to 7 pore volumes (PV). After 30 PV of NDMA solution passed through the column, the concentration profiles appeared to reach a quasi steady state. At an initial concentration of 5 mg/L, the average NDMA removal was 83.0 ( 4.8% (n ) 11) by the effluent end. Figure 1 includes an example of NDMA disappearance along the column (73 PV). Using a pseudofirst-order kinetic model, the average rate constant between 30 PV and 100 PV was (5.7 ( 0.7) × 10-2 h-1 (n ) 11), hence the average half-life of NDMA in the Fe column was 13 ( 2 h. The pH and Eh profiles were relatively stable after 25 PV. From the influent end to the effluent end, the pH gradually increased from 6.3 to 9.2 and the Eh decreased from 400 mV to -126 mV. The changes in pH and Eh were similar in the control column which received only DI-H2O. In the Ni/Fe column, the rate of NDMA disappearance was much faster than in the Fe column (Figure 1). During the initial 273 PV, the decrease in NDMA concentration appeared to follow pseudo-first-order kinetics over an influent concentration range between 5 and 28 mg/L. The rate constant, however, decreased over time, for instance, from 0.35 min-1 after 73 PV to 0.25 min-1 after 102 PV and to 0.17 min-1 after 273 PV (Table 1). Thereafter, the kinetics appeared to change with both zero-order and mixed-order kinetics being observed (Table 1). The pH values along the column ranged between 6.5 and 8.1, with no clear trend along the column. The Eh dropped sharply from 300 mV in the influent to around zero mV at the first sampling port (2.5
b
FIGURE 2. Decreases in NDMA concentration, increases in DMA and ammonium concentrations, and mass balances for (a) the Fe column after 73 PV and (b) the Ni/Fe column after 273 PV. cm). From the first sampling port to the effluent end Eh fluctuated slightly with a lowest observed value of -151 mV. Again the changes in pH and Eh were similar in the control column which received only DI-H2O. NDMA Transformation Products. In both the Fe and Ni/ Fe columns, the NDMA transformation products detected were DMA and ammonium. As NDMA disappeared, DMA and NH4+ formed accordingly. The carbon mass balance was about 80% at low pore volumes and increased gradually. After 56 PV in the Fe column and 38 PV in the Ni/Fe column, a carbon mass balance near 100% was obtained as DMA. Examples of mass balances in the Fe column and the Ni/Fe column are shown in Figure 2 where the carbon mass balance reached 98 ( 2% in the Fe column (73 PV, Figure 2a) and 93 ( 7% (273 PV, Figure 2b). This suggests that at early time a portion of the DMA was adsorbed onto the particle surfaces. Analyses of aqueous samples collected from columns and batch bottles showed formation of very low amounts (sub µg/L, data not shown) of methane and other hydrocarbons. Similarly low levels of hydrocarbons were detected in the Fe and Ni/Fe controls. Thus the hydrocarbons detected are unlikely a result of the presence of NDMA in the solution. The total nitrogen mass balance in the Fe column was 90 ( 4% in which nitrogen as DMA accounted for 49 ( 1% and as NH4+ accounted for 41 ( 3% (73 PV, Figure 2a). In the Ni/Fe column nitrogen as DMA accounted for 46 ( 4% and as NH4+ accounted for 42 ( 5%, resulting in a total nitrogen mass balance of 88 ( 9% (273 PV, Figure 2b). No other nitrogen containing intermediates or products of NDMA reduction were detected. UDMH was not detected in aqueous samples taken from batch and column experiments with VOL. 34, NO. 16, 2000 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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either Fe or Ni/Fe. A three-week batch test showed an insignificant decrease in UDMH concentration in the presence of Fe or Ni/Fe. Thus UDMH is ruled out as an intermediate of NDMA reduction in our experimental systems. N2O, which was examined in detail in a batch experiment, was not detected in the headspace samples with either Fe or Ni/Fe at a metal to solution-to-headspace ratio of 1:2:1 (wt:vol:vol). Ammonium adsorption on metal surfaces was examined in a batch experiment in which the iron particles were washed with DI-H2O at the end of the experiment. A net recovery of 0.09 µg NH4+/g Fe and 0.26 µg NH4+/g Ni/Fe was obtained in the batch experiment, respectively, which accounted for 17.4% and 20.2% of the total NH4+/N mass balances, respectively. This suggests that some nitrogen loss unaccounted for in the column experiment may be attributed to ammonium adsorption. Furthermore, some ammonium may not be detected due to its complexation with dissolved iron. Improvement of Ni/Fe Column Performance. A loss of reactivity in the Ni/Fe column was observed as time progressed (Table 1). Consequently several experiments were conducted to examine ways that may help to regain the reactivity. These experiments included shutting off the column for a few days, flushing the column with distilled water, and flushing with acid solution (H2SO4, pH 2). The results of these sequential experiments, summarized in Table 1, indicate that first, the recovery of Ni/Fe reactivity (i.e. NDMA reduction rates) was achieved after each treatment. The extent of the recovery, however, decreased after each treatment with the exception of acid wash. Second, the rate of reactivity loss increased over time. Detailed studies showed that shutting the column down not only recovered the Ni/Fe reactivity but also changed NDMA reduction kinetics from zero- or mixed-order to pseudo-first-order kinetics, as initially observed (Figure 3a, Table 1). Compared to the initial pseudofirst-order rate constants after 73 PV (0.35 min-1), the pseudofirst-order rate constants after a 7-day shut off at 307 PV and a 13-day shut off at 912 PV were 0.17 min-1 and 0.031 min-1, respectively, suggesting loss of recoverable active sites by this means. In addition, it was noticed that by shutting the column off, the brownish rust that had formed at the very bottom part of the column disappeared after a few days. Flushing with DI-H2O also regained some activity of the Ni/Fe for NDMA reduction but to a lesser extent (Figure 3b). The NDMA reduction appeared to follow zero-order-kinetics after a 43 PV flush at 595 PV and an 86 PV flush at 789 PV (Table 1) with half-lives of 16.3 and 19.2 min, respectively. In contrast, flushing with pH 2 H2SO4 for 67 PV dramatically increased the Ni/Fe reactivity, and NDMA transformation kinetics appeared to be pseudo-first-order again (Figure 3b). After 29 PV following the acid wash, the pseudo-first-order rate constant increased to 0.16 min-1, which was similar to the values measured at 273 PV (before any column activation treatment) and after one week shutting off for the first time (Table 1). Analysis of effluent samples collected during flushes with water and acid showed that NDMA concentration dropped rapidly and complete removal was achieved after 3 PV (Figure 4), noticing that the decrease in NDMA concentrations was due not only to water replacement but also concurrent degradation. DMA was also flushed out by water and acid solution, however, with some retardation (Figure 4). A relatively fast removal of DMA occurred during the initial 10 PV, and a slow but continuous removal was observed thereafter. During the acid wash, the pH of the effluent dropped from 8.5 to approximately 5 during the first 3.6 PV then remained at this level with small fluctuations (Figure 4). Accordingly dissolution of Fe and Ni was observed in the effluent and quickly reached the maximum concentrations of 240 mg/L and 3.5 mg/L, respectively (Figure 4). The total Fe concentration remained above 200 mg/L with 3492
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FIGURE 3. Recovery of lost reactivity of Ni/Fe after (a) shutting the column off for 7 days after 307 PV and 13 days after 912 PV and (b) flushing with distilled water for 86 PV after 789 PV and with pH 2 H2SO4 solution for 67 PV after 1021 PV. The symbols are measured concentrations, and the dash lines are calculated from kinetic models.
FIGURE 4. Effluent concentrations of NDMA, DMA, total dissolved Fe and Ni, and changes in pH during Ni/Fe column wash with pH 2 H2SO4 solution. The flow rate used was 10.3 mL/min for the initial 21 PV and was reduced to 3.5 mL/min thereafter. some fluctuation, while the total Ni concentrations declined gradually to 0.7 mg/L at the end of the acid wash. The cumulative amount of Ni that was washed out of the column was less than 1% of the total plated Ni in the column. No Ni was detected in the effluent during the water flush.
Discussion NDMA can be reduced by both Fe and Ni/Fe to form DMA and ammonium, which are nontoxic. The NDMA reduction rate is significantly enhanced by plating a small amount of Ni (0.25%) onto the Fe surface. For instance, at 73 PV NDMA reduction by Ni/Fe was 340 times faster than that by Fe (Figure 1). The much faster reduction rate of NDMA observed with Ni/Fe than with Fe can be explained by the proposed catalytic hydrogenation reaction mechanism since Ni is a stronger hydrogenation catalyst than Fe (25). As discussed in detail in Part 2 of this paper (26), this mechanism is proposed based on comparison of the results obtained from column and batch experiments to the electrochemical experiments. In this mechanism, the charge-transfer reaction does not take place between Fe or Ni and the organic contaminant but rather between metal and water molecules. This produces adsorbed atomic hydrogen (FeHads and NiHads) at local cathodes (20, 26) which in turn takes part in catalytic hydrogenation reactions, resulting in NDMA reduction. Furthermore, atomic H can be absorbed onto Ni and then diffuse into the metal lattice to form NiHabs in the presence of P (20). The adsorbed or absorbed atomic H is a very powerful reducing agent leading to fast NDMA reduction as evidenced in Figure 1. The use of strong hydrogenation catalysts such as Pd have also been used to enhance reductive dehalogenation reactions (21, 27). The proposed catalytic hydrogenation reaction mechanism can also be used to postulate the NDMA transformation pathway. Previous studies have proposed several NDMA transformation pathways (28-31). The first NDMA transformation pathway involves stepwise reductions to form UDMH and then methane and N2 (31). The reduction of UDMH across the C-N bond was observed in the presence of transition metals in UDMH vapor phase at above room temperature. Our results showed that no UDMH and CH4 above background level were detected, indicating that this pathway did not occur under our experimental conditions. The formation of a small amount of background hydrocarbons can be attributed to the presence of iron carbides and surface carbon (32). The second NDMA reduction pathway usually occurs under alkaline conditions and involves two electron transfer to form DMA and N2O without production of ammonia (24, 29). The lack of N2O formation indicates that this pathway did not occur under our experimental conditions. A more convincing argument can be made using results obtained from electrochemical reduction experiments (see Part 2 for details) which showed that reduction of NDMA to DMA and N2O only occurred at potentials as low as -1.3 V (SHE). The third pathway, which usually occurs under acidic conditions, involves initial reduction across the NdO bond to form UDMH (28, 29). Subsequently, UDMH is reduced to DMA and ammonia (28). Although such a reaction pathway may occur for a number of N-nitrosamines such as Nnitrosodiphenylamine (30), UDMH was not detected in our experiments despite our various efforts to do so. Furthermore, the batch test using UDMH as the starting material in the presence of Fe and Ni/Fe did not result in a significant reduction of UDMH over a period of three weeks. Alternatively, the initial reduction can occur across the N-N bond to form DMA and NO- (12). NO- may be subsequently reduced to ammonia through sequential electron transfer steps (33). Hence both reduction processes ultimately result in the formation of DMA and ammonia. Formation of the same products makes it difficult to distinguish these two reduction processes. Both of the above proposed pathways, however, do not provide satisfactory answers when the NDMA structure is carefully inspected. The two methyl groups attached to the amine nitrogen might result in a shift in charge
distribution since methyl groups are inductively electron donating. The amine nitrogen might have a partial positive charge, while the nitroso nitrogen might have a partial negative charge. Consequently it is suggested that the initial addition of hydrogen adsorbed on the iron surface would occur at the nitroso nitrogen as shown in eq 1:
This surface bound intermediate might react further with another surface adsorbed hydrogen to give N-N bond cleavage as shown in eq 2 and consequently to produce DMA and ammonia as final products as observed in this study.
Most previous studies of bimetals have been conducted in batch experiments over short periods of time (hours to days, e.g. 21, 34). The limited information that is available indicates a loss of reactivity over time (35, 36). On the other hand, a gain of reactivity has also been reported (20). Such a discrepancy may be due to differences in the method of catalyst preparation, for example the use of acid washed (21) versus unwashed (20) iron material or to differences in the catalyst support such as Al2O3 (27) versus Fe3O4 or Fe2O3 (20). In the 5-month column experiment with Ni/Fe, we observed that while NDMA degradation rates with Ni/Fe are consistently faster than with Fe, the Ni/Fe reactivity decreased over time and the reduction kinetics changed from pseudofirst-order to mixed-order or zero-order. The loss of reactivity may have been caused by (1) adsorption of NDMA reduction products (DMA and NH4+), (2) surface coverage by corrosion products, and (3) possible loss of galvanic coupling (20). Washing with DI-H2O removes adsorbed DMA and NH4+, resulting in a partial recovery of Ni/Fe reactivity. The adsorbed DMA and NH4+ may compete with NDMA for active sites on the metal surface, as suggested by a competition batch experiment which showed that with an addition of 3 mg/L DMA to 25 mg/L NDMA solution, the reduction rate of NDMA decreased with both Fe and Ni/Fe (data not shown). Being a weak base, DMA is considered to be an inhibitor in catalytic hydrogenation processes (19). Therefore, despite the small amount of DMA that was removed during flushing, a significant portion of the reactivity was restored. When an acid solution passed through the column, it dissolved and thus removed some of the corrosion products that covered the metal surfaces. The high concentration of total dissolved Fe in the effluent during the acid wash, particularly at early time (Figure 4) may be attributed, to a large extent, to the dissolution of the surface films, thereby making iron and nickel more accessible to the NDMA molecules in the solution. As a result, we observed a dramatic improvement in Ni/Fe reactivity and a reappearance of the pseudo-first-order kinetics after the acid wash. Muftikian et al. (37), using X-ray photoelectron spectroscopy, showed a progressive growth of hydroxylated oxide films on the Pd/Fe surface in the course of TCE reduction in aqueous solution. Upon acid wash using 3 M HCl, the thick surface film was removed, and the original activity of the Pd/Fe was nearly restored. In contrast to Muftikian et al. (37) who reported no loss of Pd upon acid wash, we observed a small removal of VOL. 34, NO. 16, 2000 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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Ni (1%) in the effluent. In the present study, Ni was plated on the oxide-covered granular iron without prior cleaning. It is possible that a small percent of Ni was still in cation form and was incorporated in the surface oxides. Furthermore, we cannot rule out the possibility that Ni would lose contact with the catalyst support which may undergo dissolution by acid. This may partially explain the lack of full recovery of Ni/Fe reactivity after the acid wash. Further study is needed to determine the long term use of Ni/Fe materials. Shutting the column off for several days resulted in partial recovery of the Ni/Fe activity. NDMA solution that was pumped through the column contained dissolved oxygen, resulting in the formation of brownish patches at the bottom of the column. The brownish patches associated with Fe(III) disappeared during the shut down period, indicating reduction of the three-valent film on the metal. The reduced iron products were mainly magnetite as we observed by its distinctive black color. Magnetite will not prevent hydrogen evolution (38, 39). There may be other reasons for the observed gain of reactivity of the Ni/Fe; however, further studies are needed to examine the surface changes of the Ni/Fe during NDMA reduction in aqueous solution. An understanding of the gain of reactivity may provide a simple but useful means to regenerate Ni/Fe, for example, in canisters for above ground water treatment.
Acknowledgments This research was funded by the NSERC/Motorola/ETI Industrial Research Chair in Groundwater Remediation held by Dr. R. W. Gillham.
Literature Cited (1) Haley, T. J. In Handbook of Carcinogens and Hazardous Substances: Chemical and Trace Analysis; Bowman, M. C., Ed.; Marcel Dekker: NY, 1982; Chapter 1, pp 1-18. (2) Massey, R. C. In Nitrosamines: Toxicology and Microbiology; Hill, M. J., Ed.; Ellis Horwood Series in Food Science and Technology. VCH Publishers: NY, 1988; Chapter 2, pp 16-47. (3) National Primary and Secondary Drinking Water Regulation. Fed. Regist. 1989, 54, 22062. (4) Fleming, E. C.; Pennington, J. C.; Wachob, B. G.; Howe, R. A.; Hill, D. O. J. Hazard. Mater. 1996, 51, 151-164. (5) The Merck Index, 12th ed.; Budavari, S., Ed.; 1996. (6) Mills, A. L.; Alexander, M. J. Environ. Qual. 1976, 5, 437-440. (7) Mosier, A. R.; Torbit, S. J. Environ. Qual. 1976, 5, 465-468. (8) Sen, N. P.; Baddoo, P. A.; Water, D.; Boyle, M. Inter. J. Environ. Anal. Chem. 1994, 56, 149-163. (9) Jenkins, S. W. D.; Koester, C. J.; Taguchi, V. Y.; Wang, D. T.; Palmentier, J.-P. F. P.; Hong, K. P. Environ. Sci., Pollut. Res. 1995, 2, 207-210. (10) Guttenplan, J. B. In Genotoxicology of N-nitroso compounds; Rao, T. K., Lijinsky, W., Epler, J. L., Eds.; Plenum Press: NY, 1984; Chapter 4, pp 59-90. (11) Leach, S. In Nitrosamines: Toxicology and Microbiology; Hill, M. J., Ed.; Ellis Horwood Series in Food Science and Technology. VCH Publishers: NY, 1988; Chapter 4, pp 69-87. (12) Larson, R. A.; Weber, E. J. In Reaction Mechanisms in Environmental Organic Chemistry; Lewis Publishers: 1994; Chapter 3, pp 169-215. (13) Gillham, R. W.; O’Hannesin, S. F. Ground Water 1994, 32, 958967. (14) Matheson, L. J.; Tratnyek, P. G. Environ. Sci. Technol. 1994, 28, 2045-2053.
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Received for review August 18, 1999. Revised manuscript received May 10, 2000. Accepted May 17, 2000. ES9909778
