Environ. Sci. Technol. 2008, 42, 262–269
Rapid Reduction of N-Nitrosamine Disinfection Byproducts in Water with Hydrogen and Porous Nickel Catalysts A N D R E W J . F R I E R D I C H , †,§ J O H N R . S H A P L E Y , ‡,§ A N D T I M O T H Y J . S T R A T H M A N N * ,†,§ Department of Civil and Environmental Engineering, University of Illinois at Urbana–Champaign, Urbana, Illinois 61801, Department of Chemistry, University of Illinois at Urbana–Champaign, Urbana, Illinois 61801, and Center of Advanced Materials for the Purification of Water with Systems, Urbana, Illinois 61801
Received June 01, 2007. Revised manuscript received September 12, 2007. Accepted September 27, 2007.
There is a need for new technologies to rapidly and economically treat water contaminated with N-nitrosodimethylamine (NDMA) and related compounds because of their high toxicity and recent detection in drinking water sources as a consequence of industrial releases and chlorine disinfection of wastewater effluent. Treatment of N-nitrosamines with H2 in conjunction withahighsurfaceareaporousnickelmaterial,amodelnonprecious metal catalyst, has been evaluated. Experiments show that NDMA is reduced rapidly and catalytically to dimethylamine and N2 (e.g., t1/2 ) 1.5 min for 500 mg/L catalyst and PH2 ) 1 atm), and kinetic trends are consistent with a surface-mediated mechanism involving scission of the N-nitrosamine N-N bond and subsequent reactions with adsorbed atomic hydrogen. The metal-loading-normalized pseudo-first-order rate constant (77.9 ( 13.1 L gNi-1 h-1) exceeds values reported for Pd-based catalysts. Several related N-nitrosamines react at rates similar to those of NDMA, indicating a weak dependence on structure. The reaction rates for NDMA reduction are not significantly affected by changing pH, and the presence of high concentrations of many common water constituents (Na+, Ca2+, Mg2+, Cl-, SO42-, HCO3-, and NOM) exerts only a small effect on reaction rates. Nitrate is also reduced by the Ni catalyst, and high nitrate concentrations competitively inhibit the reduction of NDMA. (Bi)sulfide poisons the catalyst by strong chemisorption to the Ni surface. Cost-normalized rate constants for the Ni catalyst are highly favorable compared to Pd-based catalysts, indicating that, with further development, Ni-based catalysts may become attractive alternatives to precious metal catalysts.
Introduction N-nitrosodimethylamine (NDMA) and related N-nitrosamines are potent carcinogens (1), and dietary exposure to these * Corresponding author e-mail:
[email protected]; phone: 217244-4679; fax: 217-333-6968. † Department of Civil and Environmental Engineering, University of Illinois at Urbana–Champaign. ‡ Department of Chemistry, University of Illinois at Urbana– Champaign. § Center of Advanced Materials for the Purification of Water with Systems. 262
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compounds from items such as cured meats, fish, tobacco smoke, chewing tobacco, beer, cosmetics, and rubber products has been studied for many years (2, 3). As a result, N-nitrosamine levels in these products have been substantially reduced by improvements in production and preservation processes (4). Much of the current concern about NDMA and other N-nitrosamines is related to their occurrence in aquatic environments (5). Detection of high concentrations of NDMA (>10 µg/L) in groundwater near a rocket engine testing facility where 1,1-dimethylhydrazine was used as a propellant (NDMA is an oxidation product of 1,1-dimethylhydrazine (6)) raised concerns of environmental regulators (2). Subsequent field surveys detected NDMA in waters far from the reaches of rocket testing facilities (2), indicating that other sources were responsible. It was soon discovered that NDMA is produced as a byproduct of wastewater and drinking water chlorination and chloramination processes (2). Levels of NDMA as high as 460 ng/L have been detected in highly disinfected wastewater effluent (7), raising public health concerns about the indirect and direct potable reuse of the treated effluent. Consequently, there has been considerable effort undertaken to characterize the mechanisms and precursors responsible for NDMA formation during disinfection processes (5, 7–10). The U.S. Environmental Protection Agency has not yet established regulatory limits for NDMA, but recently placed six N-nitrosamines on the Unregulated Contaminant Monitoring Rule List 2 (UCMR 2) (11). Other regulatory agencies have taken more immediate action. The Ontario Ministry of the Environment set a regulatory guideline of 9 ng/L for NDMA (12), and the California Department of Health Services set a notification level of 10 ng/L for NDMA, NDEA (N-nitrosodiethylamine), and NDPA (N-nitrosodi-n-propylamine) (13). Concerns about contamination of drinking water by N-nitrosamines have also increased interest in the fate of these compounds during conventional treatment processes, as well as the development of new technologies to more efficiently remove these compounds from water. Physical removal processes, including air stripping, adsorption to activated carbon, and reverse osmosis, are ineffective due to the low volatilities, high polarities, and small sizes of most N-nitrosamines (2, 14, 15). Although NDMA is biodegradable, the potential for biological removal of NDMA during water treatment is unclear (16, 17), and biological water treatment processes are limited by poor public acceptance. Currently, UV photolysis is the major technology used to treat water contaminated with NDMA (2, 18). This technology is costly and energy intensive; the UV dosages used are more than an order-of-magnitude greater than dosages typically used for disinfection (2, 19). As a result, there continues to be strong interest in the development of more efficient treatment technologies that target N-nitrosamines. Since the structure of NDMA can be characterized as an oxidized compound, reductive transformation to less harmful byproducts is a potential treatment strategy. Previous studies report that NDMA is reduced to dimethylamine and NH4+ by granular Fe0 (20, 21),
(1) but the process is very slow (e.g., t½ ) 13 h). Addition of Ni 10.1021/es0712928 CCC: $40.75
2008 American Chemical Society
Published on Web 11/27/2007
surface coatings markedly improves reaction kinetics, but the enhancement is short-lived, and the role of the Ni as a stoichiometric reductant or catalyst has not been established (20, 21). At typical levels of NDMA contamination (30 min) and collected by vacuum filtration inside an anaerobic glovebox (95% N2, 5% H2; Pd catalyst changed weekly; Coy Laboratory Products). The catalyst was then dried under
vacuum with gentle heating and stored inside the glovebox until used. Tests showed no effects of storage on catalyst activity. Catalyst Characterization. Elemental composition of the catalyst was determined by inductively coupled plasma-mass spectrometry (ICP-MS; Perkin-Elmer SCIEX ELAN DRC-e) after microwave digestion in nitric acid (Multiwave 3000; Perkin-Elmer/Anton Paar). Surface area and pore structure analyses were determined by collecting N2 BET adsorption– desorption isotherms at 77 K using a Micromeritics ASAP 2010 Analyzer. Powder X-ray diffractograms were collected with a Bruker GADD system equipped with a four-circle diffractometer and HiStar multiwire area detector using Cu KR radiation. X-ray photoelectron spectroscopy (XPS) measurements were performed on a Physical Electronics PHI 5400 XPS system using Al KR radiation. XPS spectra were energy-normalized by assigning the carbon 1s peak from the support tape a value of 284.5 eV. Transmission electron micrographs were collected with a JEOL 2010 LaB6 system operated at 200 kV. The characterization procedures were all performed under strict anoxic conditions using specialized apparatuses to prevent Ni0 oxidation. Kinetic Experiments. Aqueous batch kinetic experiments were conducted at room temperature (23 ( 2 °C) in a 300mL 5-necked round-bottom flask that was sealed to maintain anoxic solution conditions and prevent oxygen intrusion. Individual anoxic batch suspensions of catalyst were initially prepared inside the anaerobic glovebox by adding the desired mass of catalyst and other solution constituents (e.g., NOM, inorganic ions) to the reactor containing deoxygenated water. The sealed reactor was then removed from the glovebox and connected to a gas sparging system inside a laboratory fume hood. The catalyst suspensions were then continuously mixed by magnetic stirring and sparged with the desired gas (H2 or N2). Typical gas sparging rates were 0.1 L min-1, and gas was released from the reactor through a mineral oil bubbler that maintained the internal headspace pressure at 1 atm and prevented oxygen intrusion (tests showed no effect of increasing gas sparging rates). A kinetic experiment was initiated by spiking the reactor with the target N-nitrosamine from a concentrated aqueous stock solution. Aliquots of suspension were then periodically collected for analysis by withdrawing 3 mL with a syringe through a serum-capped stopper on one of the reactor ports, and immediately filtering (0.2 µm Anatop inorganic membrane) to remove the catalyst particles and quench the reaction. Unless otherwise stated, reactions were performed at pH 7.0 with a catalyst loading of 500 mg/L and an initial NDMA concentration of 100 µM. Solution pH was maintained using an automated pH-stat controller (Radiometer Titration Manager, TIM854) equipped with a 50 mM HCl titrant solution. Dilutions of the reaction mixture from titrant additions were assumed to be negligible (i.e., NDPA (0.28 ( 0.02 min-1) ≈ NDBA (0.26 ( 0.01 min-1). The close agreement in rates indicates a weak dependence on structure and suggests that a wide variety of N-nitrosamines will be efficiently reduced by porous nickel catalysts. Influence of Water Quality Parameters. Natural waters and wastewater effluent conditions vary considerably and typically contain many other dissolved constituents at concentrations much higher than NDMA. As a result, it is important to assess the effects of water quality conditions and common nontarget water constituents on catalytic reduction processes. Figure 3A shows that varying pH from 4 to 10 has no effect on NDMA reactivity with porous Ni catalysts, and product analyses performed at pH 4 and 10 are consistent with results already described for pH 7 (eq 2). Although varying solution pH does not affect the initial rate of catalyst reactivity with NDMA, it does affect the tendency for Ni and Al leaching from catalyst surfaces (Figure 3B). Minimal leaching of Ni is observed at neutral and alkaline pH conditions, but significant levels of dissolved Ni are released under acidic conditions. Aluminum leaching increases at both acidic and alkaline pH. Continuous metal leaching from the catalyst would present undesirable health risks and would lead to losses in catalyst activity over time. Further studies in flow-through reactor systems are needed to address this subject in depth. Figure 4 shows the effects from the presence of several common water constituents on NDMA reduction and catalyst surface properties. Figure 4A compares the measured rate constant for NDMA reduction in deionized (DI) water with rate constants measured in batch reactors amended with common cations (10 mM chloride salts of Na+, Ca2+, and Mg2+), anions (10 mM sodium salts of Cl-, SO42-, HCO3-, NO3-, and S2-), and NOM (10 mg/L) for 10 min prior to introducing NDMA. With the exception of nitrate and (bi)sulfide (present predominantly as HS- and H2S species at pH 7.0), the solution amendments reduce the rate of NDMA reduction by no more than a factor of 2.2. In comparison, 266
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FIGURE 3. Effect of pH on the rate of NDMA reduction (A) and the extent of Ni and Al dissolution from the catalyst surface (B) during the corresponding batch reactions. Uncertainties in the measured rate constants represent 95% confidence levels. adding 10 mM nitrate or (bi)sulfide to batch reactors completely inhibits NDMA reduction. The small decrease in rates of NDMA reduction observed in solutions amended with redox-inert ions (Ca2+, Mg2+, Cl-, and HCO3-) and NOM may be caused by competitive adsorption of these substances to catalytic sites, thereby reducing the concentration available for reactions with NDMA (32). Many of these ionogenic substances have been shown to reversibly adsorb to metal-containing surfaces (34). Additional studies were performed to assess the mechanisms responsible for inhibition of NDMA reduction in batch reactors amended with nitrate and (bi)sulfide. A series of kinetic experiments was conducted in which different concentrations of nitrate or (bi)sulfide were added to batch reactors simultaneously with the addition of NDMA. Results from these experiments show that there is essentially no inhibitory effect of lower nitrate concentrations (Figure 4B), whereas catalyst activity is noticeably affected even at the lowest (bi)sulfide concentrations added (Figure 4C). NDMA reduction is completely inhibited when introduced with 10 mM nitrate, similar to the result shown in Figure 4A (where the catalyst was pre-exposed to nitrate for 10 min prior to introducing NDMA). When NDMA is added with 3 mM nitrate, 25% of the former is reduced before onset of complete inhibition. Separate measurements show that nitrate is also being reduced by the porous Ni catalyst, consistent with earlier reports (24, 31). Therefore, it is likely that the trends shown in Figure 4B result from competitive adsorption and surface reactions of nitrate with Hads. No catalyst inhibition is observed at lower nitrate concentrations because the availability of catalytic sites exceeds the total concentration of NDMA and nitrate adsorbed to the surface (i.e., the 2 reducible contaminants do not feel each other’s presence). In the presence of 10 mM nitrate, no NDMA reduction is observed because the much higher nitrate concentration is swamping the available catalytic surface sites and blocking NDMA and H2 access to the surface. The small amount of
FIGURE 4. Effects of nontarget water constituents on Ni catalyst activity and surface properties (All reactions conducted with 100 µM NDMA, 500 mg/L catalyst, pH 7, PH2 ) 1 atm). (A) Effect of common ions (10 mM) and NOM (10 mg/L) on kobs for NDMA reduction. Uncertainty represents 95% confidence level. (B) Effects of varying nitrate concentration on NDMA reduction. (C) Effects of varying (bi)sulfide concentration on NDMA reduction. (D) Sulfur 2p region of XPS spectra of catalysts collected from batch reactions performed when (bi)sulfide is present. NDMA reduction observed when introduced with 3 mM nitrate is due to reactions with Hads initially present on the catalyst surface. Once the Hads is rapidly consumed by reactions with NDMA and nitrate, the high concentration of the latter saturates the surface and inhibits regeneration of Hads by blocking access to H2(aq). A competitive inhibition mechanism is supported by XPS spectra that show no incorporation of nitrogen into the surface of catalysts collected from nitrate-inhibited reactions. Previous work shows that sulfur compounds deactivate Ni catalysts by irreversible binding to the active surface sites (35). The apparent rate of NDMA reduction decreases with increasing (bi)sulfide concentration. The logarithmic timecourse plots in Figure 4C are linear, indicating that the extent of inhibition does not increase during the batch reactions. This is presumably because (bi)sulfide reactions with the catalyst are extremely fast and proceed to completion within a few seconds at most. Figure 4D shows the sulfur 2p region of XPS spectra of Ni catalysts following exposure to aqueous solutions containing different concentrations of (bi)sulfide. The spectra show incorporation of sulfur into the catalyst surface, with the level of incorporation increasing with initial (bi)sulfide concentration. The nickel 2p region of the XPS spectra also show features consistent with Ni0 conversion to Ni2+ when (bi)sulfide is added to solution, supporting a deactivation mechanism involving conversion of Ni0 catalytic sites to inactive NiS-like surface sites. Although the findings presented in Figure 4 indicate that the presence of some nontarget constituents may be problematic to catalytic treatment of NDMA, it should be kept in mind that the amendment concentrations used in these survey experiments are much greater than typical concen-
trations in natural water matrices. Furthermore, batch experiments like those shown in Figure 4 only provide a preliminary indication of the short-term effects of high concentrations of such constituents; long-term studies in flow-through reactors that better represent real treatment systems (e.g., packed column reactors) need to be conducted to thoroughly assess how catalyst activity and longevity will be affected by continuous exposure to different water constituents. Technology Development. This study and work by Davie and co-workers (22) indicate that metal-catalyzed reduction is effective for treating NDMA and related N-nitrosamine contaminants, and such processes may become viable treatment technologies in the future. Furthermore, reductive approaches are also effective in treating other contaminants, including nitrate, nitrite, perchlorate, and halogenated organic compounds (24, 36, 37). Despite some kinetic advantages of Ni-based catalysts, Davie and co-workers (22) have chosen to focus on developing catalytic NDMA treatment systems using precious metal catalysts because of safety concerns related to the pyrophoric properties of dry porous Ni (however, wetted catalyst is not pyrophoric). Although we acknowledge that safety concerns may place limits on the practical application of traditional porous Ni catalysts of the kind used in this study, the high kinetic activity of these catalysts combined with the low cost of Ni suggests that nonpyrophoric Ni catalysts currently being developed (24, 38) will be economically attractive alternatives to precious metal catalysts. A simple comparison between cost-normalized rate constants for NDMA reduction by different catalysts illustrates the potential cost savings from using nonprecious VOL. 42, NO. 1, 2008 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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metals like Ni. By using market prices for Ni ($1.66/t oz, $0.05/g) and Pd ($364/t oz, $11.70/g) on May 11, 2007, the cost-normalized rate constants for NDMA reduction using porous Ni and Pd-Cu bimetal catalysts is 1500 L $-1 h-1 and 5.7 L $-1 h-1, respectively (only Ni and Pd costs considered in calculation; costs of other components neglected). Although such comparisons ignore many important costs associated with implementation and operation of treatment systems, the results illustrate the significant savings potential if viable and sustainable nonprecious metal catalysts can be developed. Further research on the activity and longevity of both precious and nonprecious metal catalysts in more complex matrices (e.g., groundwater, highly treated wastewater effluent) is needed to overcome current limitations of these technologies. Results presented in Figure 4 illustrate a clear need to develop a much better understanding of the interactions between catalysts and nontarget water constituents that can act to inhibit reactions with target contaminants or contribute to catalyst fouling and deactivation. Improved understanding of these interactions can be used to develop both preventative and regenerative strategies for improving the sustainability of catalytic treatment processes. For example, recent studies show that (bi)sulfidefouled Pd catalysts can be regenerated by treatment with strong oxidizing agents like hypochlorite (36), so catalytic processes can be sustained by periodic regeneration procedures that can be automated like membrane cleaning processes. Catalyst leaching measurements (Figure 3B) raise concerns about the long-term stability of nanophase metal catalysts and the potential risks for exposure to dissolved metals and nanoparticles that may be released into the treated product water. Thus, while heterogeneous catalytic treatment technologies show great promise for removing a growing number of recalcitrant micropollutants, work needs to be done to address these critical issues and obstacles to widespread technology adoption.
Acknowledgments This work was partially supported by WaterCAMPWS, a Science and Technology Center of Advanced Materials for the Purification of Water with Systems under the National Science Foundation agreement number CTS-0120978. XPS and TEM analyses were performed at the Center for Microanalysis of Materials, University of Illinois, which is partially supported by the U.S. Department of Energy under grant DEFG02-91-ER45439. Tias Paul, Brian Chaplin, Lindsay Knitt, Dongwook Kim, Rick Haasch, Mike Marshall, Scott Wilson, and Shaoying Qi (UIUC) provided technical assistance and valuable discussion.
Supporting Information Available A figure showing characterization of the catalyst (N2 adsorption–desorption isotherms, X-ray diffractogram, XPS spectrum, and electron micrograph), plots showing the catalytic reduction of 1,1-dimethylhydrazine and a kinetic simulation demonstrating that 1,1-dimethylhydrazine is not an important reaction intermediate, a figure showing the effect of catalyst loading on kobs, and detailed calculations of the potential influences of aqueous/solid and intraparticle mass transfer processes on measured rate constants. This material is available free of charge via the Internet at http:// pubs.acs.org.
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