N-nitrosodimethylamine Photolysis Research Articles

N-Nitrosodimethylamine Formation during UV/Hydrogen Peroxide and UV/Chlorine Advanced Oxidation Process Treatment Following Reverse Osmosis for Potable Reuse.

Chloramines applied to control microfiltration and reverse osmosis (RO) membrane biofouling in potable reuse trains form the potent carcinogen, N-nitrosodimethylamine (NDMA). In addition to degrading other contaminants, UV-based advanced oxidation processes (AOPs) strive to degrade NDMA by direct photolysis. The UV/chlorine AOP is gaining attention because of its potential to degrade other contaminants at lower UV fluence than the UV/hydrogen peroxide AOP, although previous pilot studies have observed that the UV/chlorine AOP was less effective for NDMA control. Using dimethylamine (DMA) as a model precursor and secondary municipal wastewater effluent, this study evaluated NDMA formation during the AOP treatment via two pathways. First, NDMA formation by UV treatment of monochloramine (NH2Cl) and chlorinated DMA (Cl-DMA) passing through RO membranes was maximized at 350 mJ/cm2 UV fluence, declining at higher fluence, where NDMA photolysis outweighed NDMA formation. Second, this study demonstrated that chlorine addition to the chloramine-containing RO permeate during the UV/chlorine AOP treatment initiated rapid NDMA formation by dark breakpoint reactions associated with reactive intermediates from the hydrolysis of dichloramine. At pH 5.7, this formation was maximized at a chlorine/ammonia molar ratio of 3 (out of 0-10), conditions typical for UV/chlorine AOPs. At 700 mJ/cm2 UV fluence, which is applicable to current practice, NDMA photolysis degraded a portion of the NDMA formed by breakpoint reactions. Lowering UV fluence to ∼350 mJ/cm2 when switching to the UV/chlorine AOP exacerbates effluent NDMA concentrations because of concurrent NDMA formation via the UV/NH2Cl/Cl-DMA and breakpoint chlorination pathways. Fluence >700 mJ/cm2 or chlorine doses greater than the 3:1 chlorine/ammonia molar ratios under consideration for the UV/HOCl AOP treatment are needed to achieve NDMA control.

Rapid method for monitoring N-nitrosodimethylamine in drinking water at the ng/L level without pre-concentration using high-performance liquid chromatography-chemiluminescence detection

As a contaminant in drinking water, N-nitrosodimethylamine (NDMA) is of great concern because of its carcinogenicity; it has been limited to levels of ng/L by regulatory bodies worldwide. Consequently, a rapid and sensitive method for monitoring NDMA in drinking water is urgently required. In this study, we report an improvement of our previously proposed HPLC-based system for NDMA determination. The approach consists of the HPLC separation of NDMA, followed by NDMA photolysis to form peroxynitrite and detection with a luminol chemiluminescence reaction. The detection limit for the improved HPLC method was 0.2ng/L, which is 10 times more sensitive than our previously reported system. For tap water measurements, only the addition of an ascorbic acid solution to eliminate residual chlorine and passage through an Oasis MAX solid-phase extraction cartridge are needed. The proposed NDMA determination method requires a sample volume of less than 2mL and a complete analysis time of less than 15min per sample. The method was utilized for the long-term monitoring of NDMA in tap water. The NDMA level measured in the municipal water survey was 4.9ng/L, and a seasonal change of the NDMA concentration in tap water was confirmed. The proposed method should constitute a useful NDMA monitoring method for protecting drinking water quality.

Pump-and-Treat Groundwater Remediation Using Chlorine/Ultraviolet Advanced Oxidation Processes

Comparative studies of the use of chlorine/ultraviolet (Cl2/UV) and hydrogen peroxide/ultraviolet (H2O2/UV) Advanced oxidation processes (AOPs) to remove trichloroethylene (TCE) from groundwater in a pump-and-treat application were conducted for the first time at the full-scale operational level at two water treatment facilities in Northern California. In these studies, aqueous chlorine replaced hydrogen peroxide in the AOP treatment step, where the oxidant is exposed to UV light to produce highly reactive radical species that degrade groundwater contaminants. TCE removal rates as a function of initial chlorine dose and pH were then determined. At the site where the natural pH of the water was 7.1, TCE was removed (to a concentration of less than 0.5 µg/L) for nearly every chlorine dose point tested, and pH adjustment slightly enhanced the treatment process at this facility. The second site had a high natural pH of 7.7, and here, TCE was not completely removed for any chlorine dose up to 5.7 mg/L, although TCE removal did increase when the chlorine dose increased between 0.9 and 3.6 mg/L. Residual TCE remaining in the water post-Cl2/UV was readily removed using active carbon filtration, which is part of the overall treatment train at this facility. These studies also verified that Cl2/UV AOP did not interfere with the photolysis of N-nitrosodimethylamine or result in an effluent acutely toxic toward Ceriodaphnia dubia. Comparative economic analysis revealed that the chemical costs associated with Cl2/UV AOP were 25 to 50% of the costs associated with in place H2O2/UV AOP treatment.

N-nitrosodimethylamine (NDMA) formation at an indirect potable reuse facility

Full-scale experiments to evaluate N-nitrosodimethylamine (NDMA) formation and attenuation were performed within an advanced indirect potable reuse (IPR) treatment system, which includes, sequentially: chloramination for membrane fouling control, microfiltration (MF), reverse osmosis (RO), ultraviolet irradiation with hydrogen peroxide (UV/H2O2), final chloramination, and pH stabilization. Results of the study demonstrate that while RO does effectively remove the vast majority of NDMA precursors, RO permeate can still contain significant concentrations of NDMA precursors resulting in additional NDMA formation during chloramination. Thus, it is possible for this advanced treatment system to produce water with NDMA levels higher than regional requirements for potable applications (10 ng/L). The presence of H2O2 during UV oxidation reduced NDMA photolysis efficiency and increased NDMA formation (∼22 ng/L) during the secondary chloramination and lime stabilization. This is likely due to formation of UV/H2O2 degradation by-products with higher NDMA formation rate than the parent compounds. However, this effect was diminished with higher UV doses. Bench-scale experiments confirmed an enhanced NDMA formation during chloramination after UV/H2O2 treatment of dimethylformamide, a compound detected in RO permeate and used as model precursor in this study. The effect of pre-ozonation for membrane fouling control on NDMA formation was also evaluated at pilot- (ozone-MF-RO) and bench-scale. Relatively large NDMA formation (117–227 ng/L) occurred through ozone application that was dose dependent, whereas chloramination under typical dosages and contact times of IPR systems resulted in only a relatively small increase of NDMA (∼20 ng/L). Thus, this research shows that NDMA formation within a potable water reuse facility can be challenging and must be carefully evaluated and controlled.

Formation of Reactive Species Enhanced by H2O2Addition in the Photodecomposition of N-Nitrosodimethylamine (NDMA)

This study noted that the actual mechanism of N-nitrosodimethylamine (NDMA) photodecomposition in the presence of H2O2 is missing from the previous works. This study investigated a key unknown reactive species (URS) enhanced by the addition of H2O2 during the photolysis of NDMA with H2O2, not hydroxyl radicals. In order to provide experimental evidences in support of URS formation, we have mainly used p-nitrosodimethylaniline, methanol, and benzoic acid as well-known probes of ∙OH in this study. Both loss of PNDA and formation of hydroxybenzoic acids were dependent on NDMA concentrations during the photolysis in a constant concentration of H2O2. In particular, competition kinetics showed that the relative reactivity of an URS was at least identical with ∙OH-like reactivity. In addition, the decay of NDMA was estimated to be about 65% by the direct UV light and about 35% by the reactive species or URS generated through the photolysis of NDMA and H2O2. Therefore, our data suggest that a highly oxidizing URS is formed in the photolysis of NDMA with H2O2, which could be peroxynitrite (ONOO-) as a potent oxidant by itself as well as a source of ∙OH.

Enhanced N-nitrosamine formation in pool water by UV irradiation of chlorinated secondary amines in the presence of monochloramine

N-Nitrosamines, in particular N-nitrosodimethylamine (NDMA), are carcinogens, which occur as chlorine disinfection by-products (DBPs) in swimming pools and hot tubs. UV treatment is a commonly used technique in swimming pools for disinfection and DBP attenuation. UV irradiation is known to efficiently degrade N-nitrosamines. However, UV irradiation (at λ = 254 nm) of chlorinated dimethylamine (CDMA) and monochloramine, two NDMA precursors present in swimming pool water, resulted in a substantial UV-induced NDMA formation (∼1–2% molar yield based on initial CDMA concentration) simultaneously to NDMA photolysis. Maximum NDMA concentrations were found at UV doses in the range used for advanced oxidation (350–850 mJ cm−2). Very similar behaviour was found for other chlorinated secondary amines, namely diethylamine and morpholine. Effectiveness of UV irradiation for N-nitrosamine abatement depends on initial N-nitrosamine and precursor concentrations and the applied UV dose. N-Nitrosamine formation is hypothesized to occur via the reaction of nitric oxide or peroxynitrite with the secondary aminyl radical, which are products from the photolysis of monochloramine and chlorinated secondary amines, respectively. Experiments with pool water showed that similar trends were observed under pool water conditions. UV treatment (UV dose: ∼360 mJ cm−2) slightly increased NDMA concentration in pool water instead of the anticipated 50% abatement in the absence of NDMA precursors.

The formation of reactive species having hydroxyl radical-like reactivity from UV photolysis of N-nitrosodimethylamine (NDMA): Kinetics and mechanism

This study focuses on the detailed mechanism by which N-nitrosodimethylamine (NDMA) is photolyzed to form oxidized products, i.e., NO2− and NO3−, and reveals a key reactive species produced during the photolysis of NDMA. Under acidic conditions, NO2− formed from the photodecomposition of NDMA was more prevalent than NO3−. In this result, key species for the formation of NO2− are presumably N2O3 and N2O4 as termination products as well as NO and O2 as reactants. Conversely, under alkaline conditions, NO3− was more prevalent than NO2−. For this result, a key species for NO3− formation is presumably peroxynitrite (ONOO−). A detailed mechanistic study was performed with a competition reaction (or kinetics) between NDMA and p-nitrosodimethylaniline (PNDA) probe for hydroxyl radical (OH). It is fortuitous that the second-order rate constant for NDMA with an unknown reactive species (URS) was 5.13×108M−1s−1, which was similar to its published value for the reaction of NDMA+OH. Our study results showed that a key reactive species generated during NDMA photo-decomposition had hydroxyl radical-like reactivity and in particular, under alkaline conditions, it is most likely ONOO− as a source of nitrate ion. Therefore, for the first time, we experimentally report that an URS having OH-like reactivity can be formed during photochemical NDMA decomposition. This URS could contribute to the formations of NO2− and NO3−.

Photochemical Attenuation of N-Nitrosodimethylamine (NDMA) and other Nitrosamines in Surface Water

The aqueous photolysis of seven alkyl nitrosamines was studied by irradiation in a solar simulator. Nitrosamines included N-nitrosodimethylamine (NDMA), N-nitrosomethylethylamine (NMEA), N-nitrosodiethylamine (NDEA), N-nitrosodi-n-propylamine (NDPA), N-nitrosodi-n-butylamine (NDBA), N-nitrosopiperidine (NPip), and N-nitrosopyrrolidine (NPyr). Direct photolysis at irradiations of 765 W/m2, representing Southern California midsummer, midday sun, resulted in half-lives of 16 min for NDMA and 12-15 min for the other nitrosamines. The quantum yield for NDMA was determined to be phi = 0.41 and phi = 0.43-0.61 for the other nitrosamines. Quantified products of NDMA photolysis included methylamine, dimethylamine, nitrite, nitrate, and formate, with nitrogen and carbon balances exceeding 98 and 79%, respectively. Indirect photolysis of nitrosamines in surface water was not observed; increasing dissolved organic carbon (DOC) slowed the NDMA photolysis rate because of light screening. Removal of NDMA measured in tertiary treated effluent flowing in a shallow, sunlit engineered channel agreed with photolysis rates predicted based on the measured quantum yield and system parameters. Because biodegradation is relatively slow, aquatic photolysis of NDMA is generally expected to be more significant even at relatively low levels of solar irradiation (t(1/2) = 8-38 h at 244-855 W/m2, 51 degrees N latitude, 1 m depth).

UV-A induced photochemical formation of N-nitrosodimethylamine (NDMA) in the presence of nitrite and dimethylamine

The photochemical nitrosation of dimethylamine (DMA) has been suggested as a possible pathway for the occurrence of N-nitrosodimethylamine (NDMA) in aquatic environments. The aim of this study was to investigate the formation of NDMA from the nitrosation of DMA during nitrite (NO 2 −) photolysis in aqueous solution by varying several of the experimental parameters. NDMA was formed at neutral pH, in the presence of DMA and NO 2 − under UV-A irradiation, and exhibited an increase in concentration with irradiation time. Increasing the concentrations of DMA and NO 2 − led to enhanced NDMA formation. Through experiments employing OH scavenger ( t-BuOH), OH was found to play an important role in the production of nitrosating agents during the photolysis of NO 2 −. The optimum pH for the formation of NDMA was around 10. This pH dependence was explained by the dual role of deprotonated DMA as a nucleophile, which enhances the nitrosation reaction between nitrosating agents and DMA, and a hydroxyl radical ( OH) scavenger, which reduces the production of nitrosating agents. A higher concentration of NDMA was obtained under N 2 condition due to the lower quantum yield for NDMA photolysis in the absence of oxygen. Various anions, including phosphate ions in the buffer solution, retarded the formation of NDMA, possibly due to degradation of the nitrosating agents.

UV Photolytic Mechanism of N-Nitrosodimethylamine in Water: Roles of Dissolved Oxygen and Solution pH

The photolytic mechanism of N-nitrosodimethylamine (NDMA) in aqueous solution was investigated under ultraviolet (UV) irradiation at 253.7 nm, by determining the quantum yield of NDMA photolysis and analyzing the photolysis products in the pH range of 2-11 under O2 or N2 saturation. Several important photolytic behaviors of NDMA photolysis regarding the roles of dissolved oxygen and solution pH were newly observed, and novel photolytic mechanisms were proposed based on these observations. First, the presence of dissolved oxygen clearly increased the quantum yield of the NDMA photolysis, and promoted the photolytic pathway leading to the formation of methylamine and nitrate. The direct oxidation of photoexcited NDMA by dissolved oxygen (photooxidation) was suggested as a novel photolytic pathway to explain the observed effect of dissolved oxygen. This photooxidation mechanism was supported by the detection of superoxide radicals during the photolysis of NDMA under O2 saturation. Second, the dependence of the quantum yield of NDMA photolysis and the yields of the known photolysis products (methylamine, dimethylamine, nitrite, and nitrate) on the solution pH were different under O2 and N2 saturation. N-Methylformamide was observed as the novel photolysis product of NDMA in the neutral pH region under N2 saturation. These pH-dependent photolytic behaviors of NDMA were successfully explained by the interaction of the photoexcited NDMA with a proton, and the pH-dependent secondary reactions between the primary photolysis products.

UV Photolytic Mechanism of N-Nitrosodimethylamine in Water: Dual Pathways to Methylamine versus Dimethylamine

The direct ultraviolet (UV) photolysis of N-nitrosodimethyl-amine (NDMA), a well-known potential carcinogen, was investigated in aqueous solution with its degradation products analyzed quantitatively. NDMA is known to be photolyzed either to dimethylamine (DMA) or to methylamine (MA) by two distinct pathways. However, the mechanism through which NDMA is photolyzed to DMA is still not clearly understood. This study reveals a new mechanistic pathway of NDMA photolysis to DMA by identifying the factors influencing the photolysis pathway. The two pathways of NDMA photolysis were found to be strongly dependent on the initial NDMA concentration and solution pH. Increasing the initial NDMA concentration clearly favored the DMA formation path. DMA production was optimized in the region of pH 4-5. The nitrite ion (NO2-) produced from the NDMA photolysis was identified as a key reagent in directing the NDMA photolysis toward DMA production. The observed photolytic behaviors of NDMA photolysis could be successfully explained in terms of the new mechanism involving the role of NO2-.

Use of pulsed‐UV processes to destroy NDMA

Bench‐scale experiments were conducted to determine the effectiveness of using pulsed‐ultraviolet (UV) irradiation and pulsed‐UV/hydrogen peroxide (H2O2) processes to destroy N‐nitrosodimethylamine (NDMA). The effects of various UV and H2O2 dosages and source waters, as well as nitrate (NO3–) and initial NDMA concentrations, were investigated as control parameters for both completely mixed batch reactor and continuously stirred tank reactor tests. The presence of compounds that interfere with UV light (e.g., NO3–) and the formation of total trihalomethanes after pulsed‐UV treatment were also studied. Pulsed‐UV technology was highly effective for destroying NDMA. The pseudo–first‐order rate constants were calculated to be in the range of 1.4 to 12.2 min–1. This technology offers other benefits (e.g., disinfection) and can be applied directly to drinking water treatment. However, potential concerns in pulsed‐UV photolysis of NDMA include (1) the formation of undesirable chemicals as reaction intermediates and (2) possible reformation/regeneration of NDMA after chlorination of pulsed‐UV–treated effluent. Pulsed‐UV with a small amount of H2O2 could be used to control the reaction by‐products and to inhibit the reformation of NDMA by using hydroxyl radicals generated during an advanced oxidation process. In contrast, pulsed‐UV with a larger amount of H2O2 could inhibit NDMA decay by direct photolysis.

Quantum Yields for the Photodegradation of Pollutants in Dilute Aqueous Solution: Phenol, 4-Chlorophenol and N-Nitrosodimethylamine

AbstractA broadband method has been applied to determine the quantum yields for the photochemical removal of three common pollutants: phenol, 4-chlorophenol and N-nitrosodimethylamine (NDMA) in dilute aqueous solution. Flash photolysis (xenon flash lamps) was used to cause a significant amount of photolysis without photolyzing intermediates. The analysis of reactant depletion following a single flash was carried out by high- performance liquid chromatography (HPLC) or UV/visible absorption spectroscopy. The method for determining quantum-yields employed p-benzoquinone as an actinometer and was validated by determining the average (200-400 nm) quantum yield for the generation of hydroxyl radicals from the photolysis of hydrogen peroxide (0.90 ± 0.10) and the quantum yields for the photolysis of phenol (0.13 ± 0.02) and 4-chlorophenol (0.24 ± 0.04). The values determined agree very well with the literature ones obtained with monochromatic radiation. The quantum yield for the direct photolysis of NDMA was found to be 0.11 ± 0.03 at neutral pH and 0.27 ± 0.02 at pH 2-4. Under conditions where hydrogen peroxide is the principal absorber, the NDMA quantum yield is 0.32 ± 0.04, independent of pH in the range 2-8.

Atmospheric reactions of N-nitrosodimethylamine and dimethylnitramine

The major atmospheric reactions of N-nitrosodimethylamine (NDMA) and dimethylnitramine (DMN) have been investigated in an approx.3500-L environmental chamber utilizing in situ long-path Fourier transform infrared spectroscopy. Rate constants for the reactions of OH radicals with NDMA and DMN were measured relative to that for dimethyl ether at 298 +/- 2K and resulted in values of k(OH + NDMA) = (3.0 +/- 0.4) x 10/sup -12/ cm/sup 3/ molecule/sup -1/ s/sup -1/ and k(OH + DMN) = (4.5 +/- 0.5) x 10/sup -12/ cm/sup 3/ molecule/sup -1/ s/sup -1/. Rate constants for the reactions of O/sub 3/ with NDMA and DMN were determined to be less than or equal to 1 x 10/sup -20/ and less than or equal to 3 x 10/sup -21/ cm/sup 3/ molecule/sup -1/ s/sup -1/, respectivley. The photolysis rate NDMA measured in the presence of excess O/sub 3/ indicated the photodissociation quantum yield to be unity for lambda >/approx.290 nm. The photolysis of NDMA in O/sub 3/ resulted in the formation of approx.65% DMN together with lesser yields of HCHO, CH/sub 3/NO/sub 2/, and CO. A mechanism accounting for the observed products is presented. The rapid photolysis of NDMA leads to a tropospheric half-life of onlymore » approx.5 min, while a half-life of approx.2 days is predicted for DMN, with its major removal process being reaction with OH radicals.« less