Environ. Sci. Technol. 2004, 38, 3161-3167
Free Radical Destruction of N-Nitrosodimethylamine in Water STEPHEN P. MEZYK* Department of Chemistry and Biochemistry, California State University at Long Beach, 1250 Bellflower Boulevard, Long Beach, California 90840 WILLIAM J. COOPER Department of Chemistry, University of North Carolina at Wilmington, 601 South College Road, Wilmington, North Carolina 28403 KEITH P. MADDEN Radiation Laboratory, University of Notre Dame, Notre Dame, Indiana 46556 DAVID M. BARTELS Chemistry Division, Argonne National Laboratory, 9700 South Cass Avenue, Argonne, Illinois 60439
Absolute rate constants for the reactions of the hydroxyl radical, hydrated electron, and hydrogen atom with N-nitrosodimethylamine (NDMA) in water at room temperature have been determined using electron pulse radiolysis and transient absorption spectroscopy (•OH and e-aq) and EPR free induction decay attenuation (•H) measurements. Specific values of (4.30 ( 0.12) × 108, (1.41 ( 0.02) × 1010, and (2.01 ( 0.03) × 108 M-1 s-1 were measured, respectively. DMPO spin-trapping experiments demonstrated that the hydroxyl radical reaction with NDMA occurs by hydrogen atom abstraction from a methyl group, and the rate constant for the subsequent reaction of this radical transient with dissolved oxygen was measured as (5.3 ( 0.6) × 106 M-1 s-1. This relatively slow rate constant implies that regeneration of the parent nitrosoamine from the oxidized transient could occur in natural waters containing dissolved organic compounds. The reaction of the hydrated electron with NDMA was to form a transient adduct anion, which could subsequently transfer this excess electron to regenerate the parent chemical. Such regeneration reactions would significantly reduce the effectiveness of any applied advanced oxidation technology remediation effort on NDMA-contaminated natural waters.
Introduction Nitrosoamines are ubiquitous within a number of environments and are of concern as they belong to a class of chemicals that has been shown to be carcinogenic, mutagenic, and teratogenic (1-5). Nitrosoamines, particularly N-nitrosodimethylamine (NDMA), have been detected in the air surrounding rubber, leather, metal, chemical, and mining industries (6); around factories producing secondary amines or the rocket fuel 1,1-dimethylhydrazine (unsymmetric * Corresponding author phone: (562)985-4649; fax: (562)985-4649; e-mail:
[email protected]. 10.1021/es0347742 CCC: $27.50 Published on Web 04/24/2004
2004 American Chemical Society
dimethylhydrazine, UDMH) (7); and in areas near industrial plants that use dimethylamine (DMA) in organic synthesis (8, 9). NDMA may also be present in foods and beverages that contain nitrite or that have been exposed to nitrous oxides (10, 11). Formation of gaseous NDMA occurs in the dark from reaction of dimethylamine and nitrous acid (HNO2) or nitrogen oxides (NOx) (12), or by oxidation of UDMH (13, 14). Gaseous NDMA can be degraded by reaction with the hydroxyl radical or ozone, or by photolysis (15); however, these loss reactions can be relatively slow, which means that airborne NDMA could enter aqueous systems. NDMA was also a common contaminant of UDMH, a component of rocket fuel, and prior waste management and general processing procedures have introduced both NDMA and UDMH into the groundwater supply (16, 17). In addition, recent studies have shown that NDMA can be formed in situ under water disinfection conditions from the reactions of monochloramine with dissolved dimethylamine (DMA) (18, 19) and UDMH (19). Minor NDMA formation can also result from chloramination of dissolved natural organic matter (20). The U.S. EPA classifies NDMA as a probable human carcinogen, with a 10-6 lifetime risk of contracting cancer from this chemical occurring at a concentration of 0.7 ng L-1 (∼9 pM) (21). The California Department of Health Services has set an action level of 10 ng L-1 for NDMA (22) in drinking water. To achieve such low NDMA concentrations, various water treatment technologies have been suggested. NDMA is not readily absorbed by carbon nor easily removed by air stripping (23, 24), thereby rendering these conventional treatments ineffective. Reduction of aqueous NDMA using granular iron and nickel-catalyzed granular iron has been reported (25, 26) with the major products being DMA and ammonium. However, the kinetics of this reduction were slow, and the reduction process exhibited catalytic poisoning. It is well-known that UV irradiation can reduce NDMA in water (27, 28), and direct UV photolysis has been used to remove NDMA from drinking water and treated wastewater. However, there are some concerns with this methodology; if the water is turbid, colored, or contains chemicals that can interfere with the short wavelength (
