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Environ Eng Res > Volume 24(1); 2019 > Article
Bian, Wang, Zhu, Ren, Zhang, and Hursthouse: Occurrence and control of N-nitrosodimethylamine in water engineering systems

Abstract

N-nitrosodimethylamine (NDMA) is a typical nitrogen disinfection by-product, which has posed a potential threat to human health during drinking water disinfection. Because of the well-known effects of mutagenesis, carcinogenesis and teratogenesis, the high detection rate in water engineering systems (such as coagulation, membrane filtration and biological systems), and difficulty to remove, it has received wide concern in the field of water engineering systems. The NDMA is a low molecular weight hydrophilic organic substance, which is difficult to remove. Also, the mechanism for NDMA formation is also recognized to be complex, and many steps still needed to be further evaluated. Therefore, the mechanistic knowledge on NDMA formation potential and their removal processes is of particularly interest. Few papers summarize the occurrence and control of NDMA in water engineering systems. It is for this reason that the content of this paper is particularly important for us to understand and control the amount of NDMA thus reducing the threat of disinfection by-products to drinking water. Four parts including the mechanisms for the NDMA formation potential, the factors affecting the NDMA formation potential, the technologies for removal of NDMA are summarized. Finally, some definite suggestions are given.

1. Introduction

Drinking water disinfection is an important step in guaranteeing good water quality and consumer safety. It is a process which is used to kill the pathogenic microorganisms in water, and prevent the outbreak and spread of many diseases [13]. Chlorine-based disinfection is one of the most widely used methods for water treatment plants [4, 5]. The observation of numerous disinfection by products (DBPs) (e.g. trihalomethanes (THMs), haloacetic acid (HAAs)) with known effects of mutagenesis, carcinogenesis and teratogenesis, a variety of alternative single or combined processes for replacement of disinfectants have appeared [610]. Among them, the use of chloramine disinfection dominates and aims to reduce the generation of THMs and HAAs [1114]. However, it was found that the use of a large number of chloramine disinfection products has produced more harmful nitrogenous disinfection byproducts (N-DBPS) [1517]. Because the content of the lower abundance of nitrogeous compounds in the aquatic environment, detection is challenging [1820], N-DBPs have previously not received wide attention. Recently, N-DBPs have become of interest, particularly in understanding their formation and the removal effect by selected water treatment processes [2123].
N-DBPs include the flowing four sub-categories: nitrosamines (NAMs), haloacetonitriles (HANs) and the halogenated nitromethane (HNMs) and halogen ethylene amide (HAcAms) [24, 25]. The NAMs are a new type of DBPs including HANs and HNMs, which have a higher mutagenic activity and higher carcinogenic activity [26, 27]. Nine species of NAMs have been detected in drinking water, which are N-nitrosodimethylamine (NDMA), nitrosomethylethylamine (NMEA), nitrosodiethylamine (NDEA), nitrosodi-npropylamine (NDPA), nitrosodi-n-butylamine (NDBA), nitrosodiphenylamine (NDPhA), nitrosopyrrolidine (NPyr), nitrosopiperidine(NPip) and nitrosomorpholine (NMor), respectively [28, 29]. Toxicology studies showed that, NDMA could induce human organs to produce tumors [28]. The chronic toxicity of NDMA may lead to liver cancer, lung cancer, and also damage the nervous system [30]. In Canada, the maximum NDMA value for tap water reached 67 ng/L [31]. In China, 46.9 ng/L [32] has also been observed. In municipal water supply pipe, the maximum detection values of NDMA observed in different contries are also different, which range between 180 ng/L [31] in Canada, and 78.93 ng/L [33] in China. In the swimming pool and hot bathing waters, NDMA can be detected from substances produced in water treatment plants using the combined chlorine for disinfection [17]. In some water plants using ion exchange treatment processes, NDMA can also be detected [17].
The content of NDMA in the wider aquatic environment has also been studied. NDMA can cause cancer, mutation, and distortion in humans and animals [34]. Corresponding to 1 × 10−6 level, the carcinogenic risk limit concentration for NDMA is 0.7 ng/L, and that for THMs is 6 μg/L [35, 36]. The standard regulatory limit for NDMA concentration is not uniform across many regions. The regulatory level provided by both health services department of California State in United States and in Germany is less than 10 ng/L NDMA in dringking water [37]. The office of health and environmental assessment requires that the NDMA corresponding to public health standard in dringking water is 3 ng/L [37]. The department of the environment and energy, Ontario, Canada, suggest the NDMA lower than 9 ng/L [37]. The NDMA regulatory level that is provided by world health organization and Australia is 100 ng/L [38]. Although the concentration limit of NDMA in drinking water is not uniform, but in order to protect the water quality and safety, it is an important task for water treatment workers to reduce the content of NDMA in drinking water.
The understanding for the NDMA formation mechanism is an important theoretical basis for controlling NDMA. In the aquatic environment, the formation of NDMA is influenced by many interrelated factors. This paper reviews the formation mechanism of NDMA, as well as parameters affecting and controlling the NDMA formation, in order to reduce the NDMA. Recommendations are made to provide technical support and theoretical guidance for researchers and practitioners involved in protecting water quality and safety.

2. Mechanisms for NDMA Formation

The yield of NDMA resulting from dimethylamine (DMA) chlorination only accounts for 0.6% of the total yields of that by from standard water chlorination [39]. Furthermore, the formation of DMA under natural conditions may produce natural photolysis and biodegradation. Therefore, the DMA is not likely to be the primary precursor of NDMA in waste-water effluent and surface waters. The structure of the DMA is simple but it is an effective precursor for NDMA formation. Therefore, during the discussion of the mechanism for NDMA formation, DMA is still as the important modeling precursor [4044]. In the mechanisms for the NDMA formation, those precusors containing a DMA group are subject to dealkylation, generating DMA following reaction with HClO, NH2Cl and NHCl2. Subsequently, the DMA is oxidized into NDMA. In general, NDMA formation is often achieved through two pathways: one is nitrosation and the other is the action of asymmetric N, N-Dimethylhydrazine (NDMH). The primary difference between two pathways is that during the transformation of DMA into NDMA, the introduction of N is different. For nitrosation pathway, the introduction of N can be achived via HNO2, NO2, NO3, N2O3, N2O4 and hydroxylamine alone or synergistic action, while for unsymmetrical dimethylhydrazine (UDMH) action, it is mainly through inorganic chloramines.

2.1. Conventional Nitrosation

Nitrosation mainly refers to the reaction of amines such as DMA with nitrous acid to generate NDMA [45]. Lü et al. [46] studied the influence of cis and trans-configuration of HNO2 in the DMA reacting with HNO2 to form NDMA in conjunction with quantum chemical calculations. The main reaction process is that two molecules (HNO2) are firstly transformed into N2O3, and thereafter, N2O3 reacts with DMA to form NDMA. Some of N2O3 are able to generate other intermediates before NDMA formation. The HNO2 is likely to produce species such as NO+ and N2O3, and they can react with DMA to form NDMA [16, 34]. Mitch et al. [16] showed that less than 10−21 g/L NDMA would be generated by reaction of 100 μM NO2 and 100 μM DMA for 24 h at pH = 7. For most natural waters or treated in drinking water plants, the pH is around 7 and therefore conventional nitrosation appears to contribute little to NDMA formation. During the synthesis of NDMA in laboratory studies, controlling pH to an appropriate range could promote the reaction of dimethylamine hydrochloride with the NaNO2 to generate the NDMA. The equation for the reaction is as follows [47].
(1)
(CH3)2NH·HCl+NaNO2=(CH3)2N·NO+NaCl+H2O
That is
(2)
(CH3)2NH+HONO=(CH3)2N·NO+H2O
Sun et al. [48] indicated that: the trimethylamine (TMA) can be transformed into NDMA via nitrosation. During the transformation, TMA was transformed into the iminium ion. The iminium ion was then transformed into NDMA through three pathways. First, the iminium ion was hydrolyzed into DMA and then the NDMA was formed with DMA via nitrosation. Second, the NDMA was generated by decomposition and transformation of the intermediate products resulting from the reaction between iminium ion and NO2. Third, the iminium ion reacted with N2O3 to give NDMA.

2.2. Enhanced Nitrosation

Choi et al. [49] suggest that HClO can promote the formation of DMA to generate NDMA through nitrosation reaction during chlorine disinfection. This study showed that the formation of NDMA speed under conexistence of DMA, nitrite and HClO in the same system was very fast at pH 7. Within reaction of one hour, the NDMA amount had reached the maximum. Andrzejewski et al. [50] suggested that an ozonation of DMA would result in NDMA. Its yields were low (less than 0.4% relative to DMA) and it could increase with the increase of pH. The exposed times, ozone/DMA ratios, and free radical scavengers were other variables that affected the yield. The ozonation by-products indicated that nitrosation of DMA was possible pathway of nitrosamine formation. NDMA could be considered as a by-product of the ozonation of DMA in water, which was formed within a specific but reasonable ozone/DMA ratio range.

2.3. UDMH Pathways

Mitch et al. and Choi et al. [16, 17, 50, 51] studied the formation mechanism for NDMA in detail, suggesting that UDMH is the intermediate product resulting in the NDMA formation process. During the reaction, the DMA reacted with NH2Cl generating the UDMH, and finally UDMH was oxdized into NDMA via NH2Cl. The process of UDMH formation belongs to alkaline catalytic reaction. If the reaction occurred under neutral condition, it would obtain the maximum yield of NDMA while the NDMA formation needed a long time [51]. The above theory has been demonstrated through the fact that the NDMA content in water distribution network is much higher than that in the effluent from water plants [52, 53].
In the water containing ammoniacal nitrogen under action of free chlorine, the NDMA may be produced via UDMH pathway. For those water sources without ammonia nitrogen, the yield of NDMA formation by the NH2Cl disinfection was significantly higher that that by the HClO disinfection alone [54]. The primary reason for the increase is that the generation of the stable chloride dimethylamine (CDMA) as a result of a fast reaction of HCl with DMA with a reaction constant of 6.1×107 M−1·s−1. Because NH2Cl and CDMA are stable pro-electric substances, the NDMA generation would be inhibited [54].

2.4. Cl-UDMH Pathway

1, 1-dimethylhydrazine can be oxdized into various substances by NH2Cl, such as N, N-Dimethylformamide and dimethyl cyanamide. Among them, the rate of NDMA transformation accounted for about 5%. In addition, the generation of NDMA by UDMH was low. Therefore, there may be a more effective NDMA formation mechanisms than the UDMH approach [16]. Schreiber et al. [55] identified the UDMH chlorination pathway, called Cl-UDMH, suggesting that the concentration of dichloramine and dissolved oxygen were the key parameters determining amount of NDMA formation. The Cl-UDMH pathway includes the routes that the reaction between NHCl2 and DMA first generated Cl-UDMH, and then the product was oxidized into NDMA. A set of experiments demonstrated that the Cl-UDMH intermediate could explain most of NDMA formation process [56].

2.5. Chloramination Pathway

Partial NHCl2 formation significantly enhanced NDMA formation during chloramination [55]. Nonetheless, the predominance of NH2Cl under typical chloramination conditions might foster UDMH formation [56]. In addition to chlorination plus ammonia strategy and chlorination at breakpoints, pre-chlorination of pre-chlorinated source water can also be used to reduce the formation of NDMA; when chlorination time was increased before addition of ammonium chloride to the solution, NDMA formation from polyamine and polyDADMAC decreased during in situ chloramination [57]. Considering the high conversion of ranitidine to NDMA, the use of chloramine as a disinfectant for ranitidine waste-water can lead to the formation of important NDMA [58].

2.6. O3 Pathway

Ozone (O3) has been widely used in water treatment due to its highly effective disinfection [5963], decolorization [6466] and deodorant [67]. NDMA was found to occur in deionized water containing DMA after the addition of O3 and with the increase of pH, its yield increased [50]. Experimental studies oxidizing eight substances using O3 indicated that those materials that contained the DMA group were the precursors of NDMA and the presence of the NO2 could improve its production [68]. During oxidization by O3, the hydroxyl radicals can cause NDMA decomposition and two kinds of reaction then take place, in which the reaction trend was detemined by the structure of oxidized material and the pH value [68]. A mechanism for DMA oxdization into NDMA by O3 [54] indicated that the hydroxylamine might be the inorganic precursor of NDMA. Therefore there are three possible mechanisms for NDMA formation. The first one is that the precursor of the secondary amines and hydroxylamine react to form asymmetric two alkyl hydrazine intermediates, which are subsequently oxidized into nitrosamines substences through O3. Taking NDMA formation by DMA as an example, the DMA may react with the hydroxylamine to form UDMH and subsequently oxidized into NDMA (see Eq. (3)(4)). The second is that the hydroxyl radical produced by O3 hydrolysis react with NO2 generate NO2•, by which the N2O4 generated oxdizes the DMA into NDMA (see the enhanced nitrosation shown in Eq. (5)(7)). The last one is that the NDMA formation by those HCHO compounds generated during the oxidation of organic matter by O3 when DMA is available (enhanced nitrosation [50]).
(3)
NH2OH+(CH3)2NH(CH3)2NNH2+H2O
(4)
(CH3)2NNH2+O2(CH3)2NNO+H2O
(5)
OH+NO2-NO2+OH-
(6)
2NO2N2O4
(7)
(CH3)2NH+ONONO2(CH3)2NNO+HONO2

3. Factors Affecting NDMA Formation

3.1. Disinfectants

Commonly used disinfection methods for dringking water often include the use of chlorination, chloramine, ClO2 and O3 disinfection [69]. There are great differences in the mechanisms for NDMA formation by the precursors derived from different disinfectants [16]. The route from chloramine disinfection has been considered to produce the highest NDMA, followed by chlorination [16], in which the yields of NDMA by reaction between NHCl2 and DMA were higher than that from NH2Cl and DMA [55, 56]. The route via O3 disinfection because of the relatively small, the amount of NDMA generated is relatively limited. In recent years, there are also reports on the generation of NDMA by ClO2 [70, 71]. Mitch et al. [72] showed that the yields of NDMA by the NaClO disinfection were less than that by the chloraminated route and the results of NDMA precursor analyses and dimethylamine analyses on those waters from two surface waters and two wastewater treatment plant process units. Chen et al. [73] showed that the phenylurea herbicide diuron might be an NDMA precursor, which formed NDMA under varied chlorine and chloramine conditions. The degree of NDMA generation with viarous disinfectants in an ascending order was as follows: ClO < NH2Cl < NHCl2. Charrois et al. [74] quantified NDMA of two full-scale chloraminating water treatment plants in Alberta between 2003 and 2005 and conducted bench-scale chloramination/breakpoint experiments to assess NDMA formation. It was shown that the ratio of Cl to NH3-N was a significant parameter affecting NDMA formation. The molar ratio in the range of 0.6:1 to 0.8:1 could produce the highest yield. Farre et al. [75] indicated that the simultaneous introduction of NHCl2 also had an impact on NDMA output; pre-prepared NHCl2 could generate more NDMA than the online NDMA formation. The NDMA formation by O3 and DMA indicated that with the increase of the O3 dosage, the NDMA yield increased and even presented a linear relationship with the O3 dosage [54]. The NDMA yield was greater with a combination of O3 and NaClO, than that with O3 alone. Lee, et al. [76] showed that the combination of NaClO and O3 also had more positive effect in NDMA formation than O3 alone. It would be attributed to the O3 pretreatment which possibly resulted in the release of the DMA from relevant substances and subsequently generated NDMA during further NaClO disinfection. Applying eleven kinds of disinfection to seven different surface waters, Zhao et al. [70] showed that the amount of NDMA formed by the chloramine disinfection alone was higher than that by the chlorination. Environmental concentrations and mixtures of unknown nitrosamine precursors in source waters can form NDMA and other nitrosamines. In chlorine-based water treatment, Ranitidine (RNT) has been an important tertiary amine precursor of NDMA. Mingizem et al. [77] examined the effects of nitrite ions on the kinetics of NDMA formation during the chloramination of RNT with variable dissolved oxygen (DO), RNT, NH2Cl, NO2 or NO3 and pH. In the absence of the NO2, the molar yield of NDMA after 6 h of reaction was primarily influenced by DO and pH, and marginally affected by initial RNT and NH2Cl.

3.2. Reaction Conditions

The reaction variables significantly affecting NDMA generation include pH, temperature and time [7880]. The best pH for NDMA generation by UDMH and by enhanced nitrosation is different [17]. During NDMA formation by UDMH, the main influences include the form of inorganic chloramines, proportion and reaction rate [34]. The best pH for nitrasation to form NDMA was condiered to be 3.4 and under neutral and alkaline conditions, the reaction was slow [34]. Andrzejewski et al. [81] indicated that the NDMA yield by the reaction of NO2 with DMA decreased with the increase of pH. The highest NDMA formation could be achieved between pH 7 and 8 through disinfection of water or sewage containing dimethylamine, ammonia and chlorine [82]. The results also suggested that the generation of UDMH and other intermediate products were largely affected by pH, and that at pH 6–8, it could reach the maximum of NDMA generation [16, 17, 51], which was consistent with that obtained by Mitch et al. [72]. The non-protonated DMA could react with NHCl2, resulting in maximum NDMA FP [56]. With either NH2Cl or NHCl2 and DMA at pH 6–11, the NDMA achieved the maximum at pH 8–10 for 2 h of reaction [56]. Another study showed that those such as DMA, diuron and ranitidine after chloramine disinfection test in the range of pH 7 to 9 could generate the maximum amount of NDMA [16, 58, 83]. The reaction temperature could lagely influene the chloramine species as well as the reaction rate. A low temperature could reduce molecular collision, possibly leading to a reduction in the level of the reaction rate; however, a relatively higher temperature would cause a monochloramine decomposition or an unstable volatilization. Farre et al. [75] used three kinds of models to investigate the NDMA yield through NHCl2 disinfection, which was found the yields of the NDMA low than 10 ng/L for 6 h of disinfection. Overall, according to the need for disinfection and water protection, adjusting the dosing position of disinfectant and time interval can effectively control NDMA generation.

3.3. Precursors

Research on the classification and characteristics of NDMA precursors in the water environment are beneficial to the analysis of formation mechanism, effective regulation and removal from the water environment. The previous research has indicated that the NDMA precursors include the inorganic compounds (such as N2O3, NH2Cl, NHCl2, hydroxylamine, etc.) and a range of organic compounds [57, 8489].

3.3.1. DMA

The dimethylamine (DMA, HN(CH3)2), and tertiary amines containing DMA group and quaternary ammonium compounds, and those polymers containing DMA groups have been considered to be the most direct and effective precursors of NDMA [9093]. Some materials such as primary amine, amino acid, dimethylamino compounds, proteins and other nitrogenous substances cannot generate detectable NDMA [94]. Those organic coagulants containing DMA group in water purification process and the strong anion exchange resin containing amine can produce a large amount of NDMA during the disinfection process, which are also considered as the precursors of NDMA [95]. Kemper et al. [91] indicated that the soap containing quaternary amine monomer, mouthwash shampoo containing quaternary amine polymer, and commercial detergents have the potential to generate NDMA, but the yield was less than 1%. Le et al. [58] systematically studied the formation potential of NDMA (NDMA FP) from three pesticides (diuron, isoproturon, trifluralin) and five kinds of drugs (ranitidine, doxepin, amitriptyline, mifepristone, minocycline). Eight kinds of medications contained of the DMA group, among them four of the medications had higher NDMA FP than DMA. Ranitidine at pH 7.9 had a 59.6% molar yield of NDMA. Shen et al. [96] studied 18 kinds of drugs containing DMA group, and found that the molar NDMA yields of 8 kinds of drugs were only above 1%.

3.3.2. DOM

Mitch et al. [97] confirmed that with DMA under the action of chloramine, the therotical formation of NDMA content was far below that detected in real water, suggesting that there must be other NDMA precursors existing in the water environment. Natural organic matter (NOM) has been taken into account as a heterogeneous mixture of the organics. The concentration of dissolved organic nitrogen (DON) is much higher than the detection limit for NDMA in water [83]. Chen et al. [98] separated the DOM from the river water into different fractions and after chloramination, the NDMA FP of the fractions were examined (see Table 1). Among them, the hydrophilic base (HPIB) had the highest NDMA FP. As the DOC content of the hydrophobic acid (HPOA) is up to 72%, the HPOA should have significant contribution to the formation of NDMA.
Among the seven different water sources, three components of NDMA FP were identified and average values were listed in descending order as the hydrophilic component (HPI) of 23.8 ng/mgDOC > transitional component (TPI) of 11.20 ng/mg DOC > hydrophobic components (HPO) 4.95 ng/mgDOC. Combined with the study of Chen et al. [98], it can be concluded that the NDMA FP of the hydrophilic component was greater than that of the hydrophobic component; the NDMA FP of the alkaline component was greater than that of the acidic component; the hydrophilic alkaline component had the highest NDMA FP.
One of the most important sources of natural organic matter is the release of the algal organic matter (AOM). The AOM exists mainly in the form of the hydrophilic organic matter, which has great contributions to the formation of the NDMA [99, 100]. Li et al. [99] found that the Microcystis aeruginosa during exponential growth period had the NDMA FP of intracellular organic matter being 9.65 ng/mgDOC with NDMA FP of the extracellular organic matter being 18.91 ng/mgDOC. The Microcystis aeruginosa often grows in the lakes, ponds and other water environments containing rich organic matter, so the eutrophic water is likely to have higher NDMA FP.
Those components which contained DMA materials and DON from the water body are the important precursors of NDMA. The main reasons for the differences in NDMA yields are attributed to variation in the concentration and the structural characteristics of DOM in the water resource. During the reaction of precursors with DMA, the formation of NDMA may be affected due to the concentration of individual precursors in the reaction process and relevant reaction rate [101]. Different precursors have different release characteristics for DMA, which can introduce different effects on the formation of DMA and the conversion of DMA to NDMA. This will affect the formation of NDMA. Junghoon et al. [17, 51] studied the reaction of DMA and NH2Cl, and found that the generation of NDMA varied with DMA dosage and the maximum NDMA could be acchieved at the 1:1 molar ratio of NH2Cl to DMA, while the NH2Cl at a ratio > 1 was overdosed and numorous NDMA could be generated via UDMH. At a ratio < 0.1, the DMA was relatively overdosed, thus generating CDMA while inhibiting NDMA formation. Therefore, the structure, molecular mass and concentration of the precursor and the release characteristics of DMA are an important influence on the formation of NDMA.

3.4. Background Water Quality Parameters

Water quality parameters had a significant influence in NDMA production, more specifically the inorganic nitrogen (e.g., NH4+, NH3, NO2 and NO3), DON, bromide ion and dissolved oxygen concentration [16, 17, 70, 102].
The disinfection of DOM components showed that individual DOM components had a larger difference in the level of NDMA FP [83, 98, 99, 103]. DON was an important component of DOM in the water, which was found an important source of the precursor of N-DBPs [83]. Mantas et al. [104] showed that the sewage plant effluent free DON is the main precursor of NDMA. Lee et al. [105] undertook a NDMA FP test for different water samples, indcaiting that the yield of the NDMA increased as the DOC/DON value decreased. After the NDMA FP test, it was found that HPO had the smallest NDMA FP, and overall, research on the speciation and concentration level of DON and inorganic nitrogen parameters (including NH4+-N, NO2-N and NO3-N) in water and their relationship between them and NDMA formation potential, are destined to be essential for exploration of mechanisms for N-DBPs.

3.4.1. Inorganic nitrogen

The NO2, NO3 and other inorganic salts play a critical role during nitrosation. The best pH for nitrosation reaction was at 3.4 while between pH 6 to 8, there was lower NDMA formation in drinking water treatment. In the presence of NH3 and NH4+, the chloramine generated during chlorine disinfection in which NDMA is produced was via UDMH or Cl-UDMH. Using chloramine disinfection generates a higher quantity of NDMA, the presence of NH3 or NH4+ rapidly converts DMA to NDMA because the presence of ammonia can control the hydrolysis of monochloramine.
Fig. 1 presents a scheme for NDMA formation by DMA and NH2Cl [17]. The NDMA was found to increase slightly with the decrease of ammoniacle nitrogen in the molar concentration range of 0.14–1.0 mmol/L (Cl/N molar ratio was in the range of 0.1 to 0.7) [17].
In the presence of the ammoniacle nitrogen in secondary effluent from wastewater treatment plants, the use of chlorine disinfection could yield up to 460 ng/L NDMA; however, only 20 ng/L could be generated in the absence of ammoniacle nitrogen [106].

3.4.2. Organic nitrogen

NDMA precursors are predominantly found in DON sources [102, 107110]. NDMA formation is correlated with the ratio of DOC to DON (DOC/DON) [89, 105]. In general, the generation of NDMA is inversely proportional to the ratio. The contents of DON and DOC in the surface water source change with the seasonal temperature. DON concentration (0.02–0.17 mg/L) increased with the increase of water temperature (6–22) (r = 0.77) [111]. In the winter dry season, despite the reduction of both the rain water and surface runoff, the treated industrial wastewater and domestic sewage and waste emissions did not change, which weaked the water dilution and self purification effects on lowing pollutants concentration thus resulting in the increase in the level of DOC and DON [112]. When the DOC/DON decreased, the formation of NDMA would be increased. Conversly, the formation of NDMA would reduce in summer wet-season.

3.4.3. Bromine ion and dissolved oxygen

The formation of NDMA can be influenced by the presence of the bromide ion and dissolved oxygen in a particular environment or the presence of specific substances. Bromide ion as a common trace component in drinking water can be rapidly oxidized by free chlorine and NH2Cl. In the presence of an excess of NH3 or NH4+, the hypochlorite when added into water is able to generate the bromamines. The bromamines has similar chemical properties with chloramine, but has more chemical activity, thus it is more easily to interact with DMA to generate NDMA [113115]. Only in the presence of bromide in the water, the N, N-Dimethylsulfamide (DMS) can be oxidized to form NDMA via the catalytic effect of ozone [116]. The dissolved oxygen in water plays a key role in the formation of NDMA in Cl-UDMH pathway [55, 56]. Under low oxygen concentration, the inhibition of NDMA formation by the reaction of the ranitidine with chloramine was significant [83]. Dissolved oxygen was also found an influential factor during the catalytic conversion of DMA to NDMA via the activated carbon [117].

4. Technologies for Removal of NDMA

The effective control of NDMA may be divided into four steps. First, conducting timely and effective analysis of trace NDMA in water is required, which is the basis for the effective control of NDMA. Second, the precursor of NDMA and others affecting NDMA formation needs to be effectively removed or controlled prior to disinfection. Third, during water treatment, the amount of NDMA is to be reduced by optimizing the disinfection process. Finally, with advanced treatments, removal of any NDMA formed can be carried out.

4.1. Monitoring Methods

Effective detection is an important for the effective control of NDMA. It can be divided into two parts including sample pretreatment and analysis. The detection and analysis were carried out by using combination of chromatography and mass spectrometry after the pretreatment of sample through extraction and enrichment. Due to the hydrophilic and strong polarity of NDMA, using a highly efficient sample preconcentration technique and high sensitivity detection method is the key the quantitative analysis of trace NDMA [118, 119].
The pretreatment of NDMA water samples can be used ascorbic acid or Na2S2O3 to prevent the residual chlorine from reacting with DMA to generate NDMA, to eliminate the interference of the reaction to the detection of NDMA [120]. Given the NDMA degradation occurs under solar radiation [34], the water samples need to be stored at low temperature in the dark. The commonly used methods for extraction and enrichment include liquid-liquid extraction (LLE), solid-phase extraction (SPE) and solid phase micro extraction (SPME) [121, 122]. LLE method requires larger volumn of water sample and the extraction agent is generally toxic and volatile, in addition, sample recovery rate is not high [16, 123]. Overall, it cannot reach the dringking water standard of NDMA detection in the ng/L level [16, 123]. It is for this reason that the LLE method is limited in practice. The SPME is a relatively recent development of sample pretreatment technology, which integrates sampling, extraction and concentration [124]. It can reduce blank values, shorten the pretreatment time and deliver the selective enrichment of the sample [124]. However, the SPME method is affected by many factors. Its detection limit is difficult to meet drinking water requirements (1–10 ng/L), thus it is more commonly applied to the monitoring of wastewater [125]. The SPE method is a more mature and effective method for the detection of trace NDMA, which has the advantages of being suited to large volumes of water sample, convenient operation and only small amount of organic solvent. It is the latest NDMA standard detection method used in the United States and Canada [126]. There are many differences between SPME and SPE.
The combined use of chromatography and mass spectrometry (MS) is an important analytical tool, which benefits from the high sensitivity and strong qualitative capability of MS. Due to the low NDMA content in samples, it has higher sensitivity and specificity requirements for analysis. The traditional gas chromatography and thermal conductivity detector (GC/TCD) [129], GC and nitrogen-phosphorus detector (GC/NPD) [130] are difficult to meet these requirements for reliable trace detection. However, gas chromatography-mass spectrometry (GC/MS/MS, GC/MS) can show high sensitivity and accuracy [98, 131]. The most widely used method for determination of tracing NDMA is GC/MS/MS, which uses the selected ion monitoring mode (SIM) and the positive chemical ionization (PCI) as ion source [98, 131]. Liquid chromatography and mass spectrometry (LC/MS, LC/MS/MS) has a unique advantage for measuring those N-DBPs that have poor thermal stability, of which the NDMA detection limit proposed by the United States and Canada in 2007 was to 0.1–10.6ng/L [126]. In the case of LC/MS/MS improved detection limits can reach lower than 2.5 ng/L [70, 132].
There is no uniform standard method, which can quickly and accurately determine ng/L concentration NDMA. The existing methods can only rely on new instruments or related accessories to achieve the accuracy of detection [133137]. The operation is complicated and not cost-effective. Therefore, it is necessary to optimize the NDMA detection method regarding the sample pretreatment procedure and the testing equipment. At present, there is an urgent need to develop new technology that is more accurate and convenient, as well as the rapid detection and real-time monitoring method without complex sample pretreatment. For NDMA sample pretreatment, new type or combination of solid phase extraction agent, or other strongly applicable, higher efficient and low cost pretreatment technologies needs to be developed. For NDMA analysis, it can also be considered to improve the operating performance and technical index of instrument by using the new technology in other scientific fields, and establish the standard method for determining NDMA.

4.2. Precursor Removal

According to generation mechanisms for NDMA, combined with the operation of water plant, source control is focused on removal of the NDMA precursors as well as the key substances (e.g., organic nitrogen compounds as DON and inorganic nitrogen compounds as ammoniacl nitrogen, nitrate and soluble nitrogenous salts) by optimizing conventional plant operation processes.Table 2

4.2.1. Coagulation

The DON component in the source water is complex, with a wide molecular weight range distribution, and most of them are 1–50 kD [138]. Conventional coagulation can remove the organic substances of molecular weight > 3 kD, while enhanced coagulation has a considerable removal efficiency for organic matter in the 0.5–3 kD range [139141]. As the DON is mainly hydrophilic, low molecular weight (< l kD) organic matter [83], the conventional treatment processes (e.g., coagulation, sedimentation, filtration and disinfection) find it difficult to remove [142145]. From the analysis of molecular weight distribution, enhanced coagulation is likely to be the most effective approach in reducing DON.
The removal effect on DON by the conventional treatment process in 28 drinking water plants in the United States was statistically analyzed as shown in Table 3 [146]. The DON concentration of raw water was 0.2 mg/L.
Table 3 showed that the removal efficiency of DON by conventional coagulation is lower than that of the combination of coagulant and cationic polymer, and the removal of DON after O3 is enhanced by biological filters. Previous research shows that the removal rate of 5% – 20% can be improved by the combination of polymer and alum [146]. Using aluminum sulfate as coagulant for DON, the best removal rate of 30% – 45% was achieved [146]. Li et al. [147] indicated that after pre ozonation, the DON removal efficiency by the conventional coagulation flocculation is maintained at around 20%. The raw water of the Huangpu River in China was coagulated by the aluminum sulfate, of which the removal efficiency of DON was up to 35% with coagulant dosage of 100 mg/L, while increasing the coagulant dosage the effect on removal is not obvious. The combined process of ozone and biologically activated carbon (O3/BAC) had a significant effect on the removal efficiency of DON, which can reach 60%. Lee et al. [130] selected ferrate to pre-oxidize DMA and some materials containing DMA groups, suggesting that ferrate can effectively reduce the NDMA FP value. After reaction of 30 min, except for DMA, the removal efficiency of NDMA FP of the other precursors were over 95%. The pre oxidation of the NDMA precursor could decrease the NDMA FP during chloramine disinfection and O3 oxidation was more effective than ClO2 [76]. Enhanced coagulation improves the removal efficiency of DON to a certain extent, but the total removal was not satisfactory. The combined process of preoxidation and enhanced coagulation has a significant effect on lowering FP NDMA value. Therefore, research on new enhanced treatment technology should attract more attention on the application to drinking water treatment.

4.2.2. Membrane treatment

DON has a low molecular weight, polar characteristic [148]. The acid-base difference of DON is larger and the range of molecular mass distribution is wide. There is no obvious effect on removal of DON via conventional microfiltration (MF) and ultrafiltration (UF) membrane treatment technology [148]. Yeomin et al. [149] selected 6 kinds of reverse osmosis (RO) membrane and 2 kinds of nanofiltration (NF) membrane to study the raw water DON removal effect by membrane treatment and the influencing factors. The RO membrane had a pore size ranging from 0.33 to 0.35 nm and a normal pressure from 1,035 to 1,550 kPa, and NF membrane pore size from 0.43–0.45 nm and a normal pressure from 518 to 690 kPa. The results showed that the average removal efficiency of 6 RO membrane (65%) was higher than that of NF (50%), suggesting that the removal effect of DON was related to the ratio of solute particle diameter to membrane pore size. In general, the ratio (> 0.8) implied a better removal efficiency of DON while it was difficult to remove DON with a smaller ratio (< 0.4) [149]. DON molecular weight and its structure characteristics are the main factors that affect the membrane treatment effect. Usually, those having the complex structure and charged molecules were easily removed. Tomaszewska et al. [148] studied the removal effect of the DON in 12 water treatment plants, which indicated that the combined process (coagulation/UF) had a larger impact on the DON removal, by which the removal efficiency was increased by nearly 45%. The used coagulant (50–100 mg/L) was poly aluminium chloride (PAC), which could adsorb those DON small molecular components that have penetrated the ultrafiltration membrane. This also reduces the adhesion of pollutants on membrane thus reducing the membrane fouling [150]. The membrane also has better rejection effect on nitrate, nitrate and nitrogous salts. Overall, the membrane technology can effectively control the NDMA precursor and the inorganic nitrogen. How to improve DON rejection efficiency, control membrane fouling and reduce economic cost is one of the important contents of membrane industry development in the future.

4.2.3. Disinfection control

The yields of the NDMA could be reduced via disinfection process optimization. During disinfection process there is a need to avoid generating inorganic chloramines. In the presence of ammonia, using break point chlorination to determine chlorination dosage was effective for reducing NDMA formation [39]. When the chloramine disinfection is used or there is an unavoidable chloramine generation occuring, adjusting Cl/N to one would reduce the NDMA yields [39]. Extending free chlorine contact time before dosing ammonia was able to effectively reduce NDMA formation in drinking water during chloramine disinfection [55]. For example, Charrois et al. [151] increased the free chlorine contact time of 30 s to 2 h before adding ammonia, finding that the NDMA yields decreased from 180 ng/L to 5 ng/L. The NDMA yields in the disinfection process increased rapidly in the first few hours, especially in the first 6 h. There was few NDMA yields produced less than 10 ng/L [76]. Thus, according to actual needs of disinfection and water quality guarantee, adjusting disinfectant dosing position and time interval can effectively control the formation of NDMA.

4.3. Removal of NDMA

4.3.1. Physical process

NDMA is a pale yellow oily liquid, whose density is similar to water. It has a high solubility, belonging to low molecular weight hydrophilic organic material [39, 78, 152154]. Because the octanol/water partition coefficient is low −0.57 [152], it is hard to bio-enrich or adsorb by particles during migration and transformation. In addition, removing NDMA from natural water via the evaporation and stripping is also difficult. The efficiency of physical removal is relatively low, but promising physical methods could be through adsorption and membrane process.
The physical and chemical properties of the NDMA make its adsorption on soil, activated carbon or other hydrophobic materials poor, while the hydrophilic materials (such as zeolite and Ambersorb572 resin) have stronger performance for NDMA adsorption. Kommineni et al. [155] compared adsorption effects of NDMA among a range of materials, and found that Ambersorb572 resin and shell derived activated carbon has good adsorption for NDMA, but the adsorption capacity is relatively low overall which potentially increases the processing costs. The NDMA concentration lower than 50 ng/L was also difficult to treat by adsorption. Dai et al. [156] selected Lignin and coconut shell as adsorbents, finding that the adsorption capacity for NDMA by activated carbon is much higher than that by molecular sieve. After modification, the polar functional groups were decreased, and the hydrophobic capacity of activated carbon was enhanced, and the NDMA with nonpolar methyl end was adsorbed in the pores. As a result, the amount of NDMA adsorption increases exponentially. In addition, solid surfaces containing aldehyde or ketone functional groups also contribute to the adsorption of NDMA. With the development of activated carbon fiber as adsorption material for adsorption of NDMA and modified activated carbon [157], the adsorption capacity of NDMA needs to be re-evaluated.
NDMA molecules are very small. Except for reverse osmosis, the ultrafiltration and nanofiltration processes have no effects on removal of them and in contrary, it would lead to an increase in the level of NDMA [157]. Reverse osmosis membrane can effectively remove secondary and tertiary amines from water, because they have a positive charge at neutral pH. In the general water environment, NDMA does not hold charge so the removal efficiency of NDMA by reverse osmosis is not high [157]. Eva et al. [158] studied the effect of several kinds of RO membranes on the removals of seven types of alkyl nitrosamines containing the NDMA in deionized water. The results showed that as the molecular weight increased the better the interception of molecules and the removal efficiency of NDMA by the reverse osmosis membrane can reach up to 56% – 70%. Plummee et al. [157] used a membrane (EPSA2, HAIDELUN Com., USA) to remove NDMA in water treatment plants. It was observed that NDMA removal by RO (Hydranautics ESPA2, composite polyamide) at the Interim Water Purification Facility is between 24–56% and 59–75% was removed across RO and ultraviolet radiation (UV) treatment together [157]. It is noted that the characteristic variations of inlet water and polarization possibly leads to membrane fouling when the water passes through the membrane, which makes the removal efficiency of NDMA unstable.

4.3.2. Chemical processes

NDMA is sensitive to light and photochemical dissociation, thus photolysis is most widely used technique to remove NDMA, especially using UV photolysis. NDMA is able to strongly adsorp UV radiation within wavelength range of 200–275 nm and the strongest absorption band occurs at 227 nm. It results in the electron transition from π to π*, followed by the broken of N - N key fracture [159]. The primary products resulting from the photolysis are DMA and NO2 salt and secondary products including NO3 salt, HCHO and TMA [160163]. The DMA is not further photodegraded but NO2 salt can be oxidized to NO3 salt [164]. Lee et al. [165] found that pH and dissolved oxygen have an influence on UV removal of NDMA. It is not effective when high water turbidity and chromophores or other chemicals interfere with short wavelength UV.
Sharpless et al. [166] made a comparison between degradation effect of low pressure mercury lamp (LP) and effect of medium pressure mercury lamp (MP) on removals of NDMA, it showed that the LP and MP had the same photon efficiency, and no relationship with the wavelength in the short-wave ultraviolet region was found. Xu et al. [167] showed that with UV degradation of NDMA for reaction of 5 min, the NDMA was reduced up to 97.5%. With lower pH, enhanced illumination area and radiation intensity higher removal of NDMA occurred. At pH 2.2, the NDMA has the maximum photodegradation rate. The UV removal efficiencies of NDMA in tap water and river water reached 96.7% and 94.8%, respectively, which showed that the environmental conditions have an influence on NDMA degradation by photolysis. NDMA also has a second light absorption region between 300 and 350 nm where it produces a transition from π to π*, thus photolysis of NDMA can occur under sunlight [34]. Solar radiation is an important pathway in degradation of NDMA [34, 132]. Fan et al. [168] studied the effects of solar light and temperature on degradation of NDMA and N-diethylnitrosamine (NDEA), and found that 60.88 ng/mL of NDMA and 60.06 ng/mL of NDEA could be reduced to 10.88 ng/mL and 7.57 ng/mL for photolysis of 4 h. Degradation by solar radiation appears to be time-consuming. Plumlee et al. [132] showed that with modeled natural light to observe photochemical decay of NDMA from surface water it showed a half-life period of NDMA to be 16 min, the presence of light shielding and more DOC would delay the photolysis of NDMA. The half-life period of NDMA from the typical rivers and wetlands would take a few hours or a few days [132]. Therefore, the application of solar radiation to effectively degrade NDMA is important task in the future.
AOPs have received wide attention in the degradation of organic matter [169], and processes assessed include UV, O3, H2O2, Fenton reagent, Fenton-like reagent, ultrasound and others. AOPs has been found to remove DBPs and the hydroxyl radical (·OH) produced in AOPs is an important factor affecting the removal efficiency of NDMA. Hiramoto et al. [170] used Fe/H2O2 to degrade NDMA in phosphate buffer, showing a good removal efficiency, but the degradation of NDMA eventually produces NO, which is potentially damaging to DNA and protein molecules thus a threat to human health. The main drawback was that the Fe/H2O2 was often used within strong acidic environment whilst the real water environment is often at pH 6–8.
There is considerable interest in the development of O3 oxidation [171]. The influence of pH on oxidation is significant: at neutral condition, the oxidation rate is 13% while at pH 8, the oxidation rate was increased to 55% [171]. In the O3/H2O2 coupling process, the NDMA oxidation rate was about 85% at pH 7 and 8, which significantly increased NDMA oxidation rate [171]. The main reason for the increase was that the HO2 produced by H2O2 hydrolysis promoted the O3 to generate ·OH, thereby increasing the oxidation rate of NDMA. In the O3 /UV process, the O3 generated H2O2 in the presence of UV radiation [169], the subsequent reactions similar to those occuring in the O3/H2O2 process. The advantage of this process in addition to the increase in NDMA removal rate, includes degradation of NDMA by partial oxidation to DMA and the generation of NO3 salts [172], and a controable DMA yield [167, 173]. At pH 4, the UV/H2O2 had a better effect on removing NDMA with a degree close to 100% for reaction of 5 min. Compared with O3 and UV alone the DMA yields were more controlable in the UV/H2O2 process [174].
Catalytic metal reduction is also another method for degradation of NDMA, and can be a single metal catalysis system or double metal catalytic system, with the later appearing to be more efficient [175177]. Frierdich et al. [175] applied porous Ni material with high specific surface area in catalytic reduction of NDMA. As a result, the NDMA was rapidly reduced to DMA and N2. However, NO3 salts can also be reduced by Ni material, which possibly influence the degradation of NDMA due to the competitive use of Ni material. Lai, et al. [176] applied the zero-valent Fe in catalytic reduction NDMA, which generated DMA and NH4+, for which the half-life is up to 13 h. Matthew et al. [178] applied metal powder in catalytic H2 to reduce NDMA and form DMA. A Fe-Ni catalytic system to degrade NDMA [176, 177] showed Fe reduced water to monatomic hydrogen and subsequently reacted with Ni-excited NDMA producing DMA and NH4+ [176, 177]. The half-life period of NDMA by Fe-Ni catalytic degradation was 2–20 min [176, 177]. The Fe-Ni system was not active or stable for long periods [176, 177]. A Pd-In catalytic system was applied in the removal of NDMA from drinking water, which was degraded into DMA and NH4+ [179]. The H2S resulting from the raction of sulfate reducing bacteria and H2 have strong poisoning effect for metal catalysts, and subsequently treatment to inhibit the growth of sulfate reducing bacteria would increase the cost and complextity of operation process [179]. Finally, the precious metals used in metal catalytic reduction are mostly expensive, limiting its wide use in industry. To ensure the reproducibility and stability of metal catalysts is also another challenge.

4.3.3. Biological degradation

Biodegradation, unlike photolysis, leads to the disppearance of NDMA precursors, and will not form NDMA during disinfection, with complete degradation of NDMA. Compared with photolysis, the biodegradation speed is slower. However, biological degradation is still a great potential in removal of NDMA. The half-life of NDMA in lawn soil by biodegradation was 4–6 d [180] while those in silt soil were 11–39 d [181]. Under aerobic and anaerobic conditions, NDMA in soil and the water environment can be degraded by microorganisms in which the aerobic biodegradation rate is faster than anaerobic [175]. Zhou et al. [182] monitored NDMA in groundwater over 7 y and found that about 90% of NDMA was degraded by microorganisms. Sharp et al. [183] used aerobic bacterial to deal with NDMA in 15 kinds of sewage, the removal rate is more than 50% within the 30 d. Yifru et al. [184] found that some aquatic plants was able to absorb NDMA. NDMA has a high solubility in water and cultivating aquatic plants has more positive economic impacts and great potential to absorb NDMA. Chung et al. [131] studied the effect of the hydrogen matrix biofilm reactor on the removal of NDMA, which showed a high removal efficiency up to 96%. The generation of H2 was considered as determinant factor affecting NDMA removal.

5. Conclusions

The NDMA has wide range of sources and high carcinogenic risk. It is important to evaluate and control its formation potential. The present work reviewed the NDMA formation and controlable methods. There are still some issues to address. For example, identifying the precursor of NDMA is important because the proportion of NDMA generated by DMA is small while research on NDMA formation by other precursors is relatively rare, especially on the DON and other DMA-containing materials. The mechanism for formation of NDMA in disinfection process is still to be further revealed because the existing mechanisms cannot completely explain the generation of NDMA. In the disinfection process, it is necessary to study new NDMA formation mechanisms. The migration and transformation of N-containing compounds in NDMA formation process are the current focus of interest, especially the role of DON on the formation of NDMA. The DOC, DON and other parameters, for example, disinfectant type and dosages, environmental conditions affecting the water source, have important impact on the NDMA formation potential, which requires timely on-line monitoring. Conventional methods are difficult to control NDMA. UV radiation and AOPs have good influence on the removal of NDMA, and the equipment is simple, but the higher operating cost limits its application. The AOPs with low energy consumption of hydroxyl radical are therefore expected to improve the degradation efficiency of NDMA. The use of metal reduction has some limitations and the cost is high, so it needs to further process. Microbial and plant treatments have certain potential. The coupling these processes may be a better pathway to ensure removal of NDMA from water.

Acknowledgments

We acknowledge the financial support of the National Natural Science Foundation of China (No. 51408215), Natural Science Foundation of Hunan Province of China (No. 2018JJ2128), China Postdoctoral Science Foundation (No. 2017M622578) and Hunan Province Graduate research and innovation projects in China (CX2017B638).

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Fig. 1
Scheme for NDMA formation by DMA and NH2Cl [17].
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Table 1
NDMA FP of the Separated Components of DOM from the Rivers of Iowa, USA [98]
DOM Hydrophilic acid (HPIA) Hydrophilic neutrality (HPIN) Hydrophilic base (HPIB) Hydrophobic acid (HPOA) Hydrophobic neutrality (HPON) Hydrophobic base (HPOB)
DOC contribution (%) 7.90 0.80 4.60 72.00 1.10 1.98
NDMA FP (ng/mg DOC) 43.50 25.76 77.50 27.47 22.44 31.43
Table 2
Comparison of SPE and SPME [37, 125, 127, 128]
Method Sample (mL) Organic solvents (mL) Time (h) Detection limit (ng/L) Rate of recovery (%)
SPE 250–1,000 45–84 1–5 0.26–2 71–110
SPME 4.5–10 0 0.5–1.5 3.2–2,000 46–259
Table 3
Removal of DON by Conventional Treatment Process [146]
Process DON removal efficiency (%)
Conventional Coagulation 9
Coagulation + Cationic polymer 23
Biofilter with O3 34
Biofilter without O3 16
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