Efficient photoreductive decomposition of N-nitrosodimethylamine by UV/iodide process
Graphical abstract
Introduction
N-nitrosodimethylamine (NDMA) is of great concern as a member of N-nitrosamines, a class of chemicals that have been demonstrated to be carcinogenic, mutagenic and teratogenic [1]. In 1989, NDMA was first detected as a disinfection byproduct (DBP) in drinking water in Ontario, Canada [2], and its cancer potency is much higher than those of the trihalomethanes. The U.S. EPA Integrated Risk Information System (IRIS) indicates that a concentration of 0.7 ng L−1 of NDMA in drinking water leads to an increased lifetime cancer risk of 10−6 [3]. In response to health concerns, NDMA has been listed as one of the U.S. priority pollutants, with a drinking water standard of 0.7 ng L−1 [4]. The California Department of Health Services set a NDMA action level of 10 ng L−1 [5]. Despite its high potential carcinogenicity and low maximum acceptable standards, NDMA has been detected at elevated levels all over the world. The median concentration of NDMA in untreated wastewater was approximately 80 ng L−1, with a maximum concentration up to 790 ng L−1 [6]. In authentic water, the value was found ranging from 2 to 180 ng L−1 [7]. NDMA forms during the process of chloramination and chlorination in water and wastewater treatment, especially in the presence of some nitrogen-containing precursors. Beyond that, other sources include food, beverages, consumer products, contaminated groundwater (from the use of rocket fuel), and polluted air (e.g. tobacco smoke) [1], [8].
The removal of NDMA is difficult by most conventional treatment processes. Volatilization or air stripping cannot remove NDMA from water effectively due to its relatively high vapor pressure (1.08 kPa at 25 °C) and low Henry’s law constant (3.34 Pa m3 mol−1 at 25 °C) [9]. Adsorption of NDMA on granular activated carbon is difficult because its log octanol-water partition coefficient (log Kow) is only −0.57 [10]. Ozonation is also ineffective because of the weak reactivity of NDMA with ozone (10 M−1 s−1) [11]. Other developed technologies such as reverse osmosis (RO) [12] and metals reduction [4], [13] have limited removal capacities as well. Biodegradation of NDMA is very slow [14], with a half-time of 11.6–38.5 days in soil slurries [10].
Ultraviolet (UV) light treatment is known to be an efficient method due to the strong photolability of NDMA. With its absorption peaks at 227 and 332 nm, direct UV photolysis using low-pressure or medium-pressure Hg lamps readily destroys NDMA and has been used commercially to date [12], [15], [16]. However, the cost of UV treatment systems is considerable. The UV radiation dosage required to achieve acceptable levels of NDMA is approximately an order of magnitude higher than those used for wastewater disinfection [6], [8], [14]. On the other hand, mechanistic studies indicated that the principal products of NDMA photolysis are dimethylamine (DMA) and nitrite (NO2−), which will in turn result in its re-formation if chloramination or chlorination is performed after UV or UV/hydrogen peroxide (H2O2) treatment [17]. UV/H2O2 is used to control NDMA degradation byproducts by using hydroxyl radicals (OH) generated during an advanced oxidation process (AOP) [17]. However, more NDMA than in the untreated water was found after UV/H2O2 process in some waters [18]. As a consequence, development of a more efficient technology against NDMA and its regeneration is still urgent and desirable.
As NDMA can be characterized as a reducible compound, reductive transformation to less harmful byproducts is a potential strategy. Mezyk et al. [19] reported that the reaction rate constant of NDMA with hydrated electrons (eaq−) was 30 times faster than that with OH. Aqueous solutions of iodide exhibit broad electronic bands in the UV corresponding to electron ejection from iodide into the solvent [20]. This process known as “charge-transfer-to-solvent (CTTS) states” could effectively generate eaq− on the femto-second time scale [20]. As a powerful nucleophile, eaq− is prone to attack some oxidants, with a standard reductive potential of −2.9 V [21]. Up to now, eaq− has exhibited distinctive capacity in the reductive degradation of various persistent contaminants including perfluorinated acids (PFAs) [22], nitro-aromatic compounds (NACs) [23], etc [24], [25], [26]. Thus eaq− is expected as a potent reductant for the destruction of NDMA in UV/iodide process.
To our best knowledge, there is no systematic study on the reduction of NDMA by eaq− so far. Little is known about the products, pathway and mechanism in this process. Consequently, in this study, we evaluate the efficacy of UV/iodide in NDMA degradation, which is compared with those of UV/H2O2 and direct UV irradiation. Meanwhile, a detailed pathway is proposed based on the identification and quantification of byproducts. The degradation profiles of NDMA and its products are correlated with the mass balances of carbon and nitrogen.
Section snippets
Chemicals
NDMA (99%), DMA (99%) and MA (97%) were obtained from Fluka (Buchs, Switzerland). The isotope internal standard substances (NDMA-D6, DMA-D6) were acquired from Wellington Laboratories Inc. (Guelph, ON, Canada). HPLC-grade formic acid (96%), glacial acetic (99.7%) and ammonium acetate (97%) were purchased from Fairfield, TEDIA, US. HPLC-grade methanol (≥99.9%) was obtained from Sigma-Aldrich. The other analysis reagents were purchased from Sino-pharm Chemical Reagent Co. Ltd.
Experimental setup
NDMA and KI stock
Decomposition efficiency of NDMA by UV/iodide
The result of the photodecomposition of NDMA in the presence of KI was shown in Fig. 1a. To study the effect of KI, two control experiments (1st and 2nd) were carried out. In UV/iodide process, NDMA degraded rapidly with 88.0% removed in the first 2 min, while most of the initial NDMA remained in the controls without KI or O2 pre-bubbling. At 10 min when nearly all NDMA (99.2%) was decomposed in UV/iodide process, the degradation ratios in the two controls were only 45.0% and 63.8%, respectively.
Conclusion
In this study, a novel photoreductive decomposition method by UV/iodide was developed for NDMA destruction. The results obtained suggest that UV/iodide was highly effective in NDMA decomposition with a higher reaction rate and a greater quantum yield compared with other treatments. NDMA was converted to benign products including HCOO−, NH4+-N and N2. Some refractory intermediates, such as DMA and NO2−, which could be generated in large quantities by other methods, was completely destructed by
Acknowledgement
This study has been supported by the National Natural Science Foundation of China (Project No. 21177094).
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2023, Chemical Engineering JournalCitation Excerpt :As a class of N-DBPs with high toxicity and low concentrations, N-nitrosamines (NMs) have attracted significant attention in recent years[41]. To date, water quality standards have been limited to only N-nitrosodimethylamine (NDMA), the first type of NMs detected in drinking water[53]. However, the carcinogenic effectiveness of N-nitrosodiethylamine (NDEA) is three times that of NDMA.
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2022, Water ResearchCitation Excerpt :Compared with oxidation process, eaq− based photochemical processes have higher decomposition efficiency of NDMA. For example, the degradation rate constant by reductive UV/iodide and UV/sulfite systems were 0.60 min−1 (Sun et al., 2017a) and 0.70 min−1 (Seid et al., 2020), respectively, which were higher than by oxidative UV/H2O2 (0.0613 min−1) (Zhou et al., 2012). In addition, eaq− based photochemical processes is able to generate non-toxic products when degrading NDMA, thus decreasing the potential secondary pollution.