PRECURSORS OF NDMA AND THMS: OCCURRENCE IN RAW WATER

Transcription

1 Proceedings of the 13 th International Conference on Environmental Science and Technology Athens, Greece, 5-7 September 2013 PRECURSORS OF NDMA AND THMS: OCCURRENCE IN RAW WATER PRZEMYSLAW ANDRZEJEWSKI 1, ANASTASIA NIKOLAOU 2*, SPYROS GOLFINOPOULOS 3 AND LYDIA UGENA GARCIA-CONSUEGRA 1 1 Adam Mickiewicz University, Faculty of Chemistry, ul. Umultowska 89B, Poznan, Poland 2* Department of Marine Sciences, University of Aegean, University Hill, GR-81100, Mytilene, Greece, corresponding author, nnikol@aegean.gr 3 Department of Financial and Management Engineering, University of the Aegean, Kountourioti 41, GR-82100, Chios, Greece EXTENDED ABSTRACT The occurrence of disinfection by-products (DBPs), especially trihalomethanes (THMs) in chlorinated drinking water has been well documented. Due to their adverse health effects, especially carcinogenicity, THMs have been regulated in drinking water by the American Environmental Protection Agency (US EPA), the World Health Organization (WHO) and the European Union (EU). In order to minimize their formation, chloramination has been applied in water treatment plants, leading to lower concentrations of THMs, but resulting in formation of N-nitrosamines, including N-nitrosodimethylamine (NDMA), a probable human carcinogen. Nitrosamines, mainly NDMA, are mutagenic compounds and are suspected to be carcinogenic towards humans. In 2002, Mitch and Sedlak as well as Choi and Valentine reported that NDMA is formed during disinfection of dimethylamine (DMA) containing water or treated wastewater with chloramines. Moreover in 2007 Andrzejewski and Nawrocki reported that not only chloramination but also other oxidants (particularly ozone and chlorine dioxide) applied in water/wastewater treatment leads to formation of NDMA, when dimethylamine is present in raw water. Currently, according to the results of many experiments, formation of NDMA during reactions of strong oxidants, particularly chloramines, with waters containing DMA seems to be proved. US EPA has classified NDMA into the group B2 i.e. compounds which are probably carcinogenic to humans. In 2008 the WHO placed NDMA on the list of DBPs related to application of chloramines in water/wastewater treatment. The precursors of DBPs have been the focus of recent research, in order to understand the mechanisms involved in their formation, in an effort to minimize it. Natural Organic Matter (NOM) components in raw water act as precursors for the formation of both THMs and NDMA. In addition, anthropogenic emerging pollutants (e.g. pharmaceuticals, herbicides), have been shown to result in THMs and NDMA formation, during chlorination or chloramination. Emerging pollutants are increasingly detected in raw water and they cannot be effectively removed with conventional treatment, creating concern for the water quality, in regard to the formation of THMs and NDMA. The aim of this paper is the presentation and critical review of recent findings in the sector of THMs and NDMA precursors research in raw water, in particular NOM, pharmaceuticals and herbicides. Relevant studies are reviewed, with emphasis on the formation mechanisms, the compounds that can act as DBP precursors, and the levels of THMs and NDMA detected in disinfected waters. Finally, applications and proposals for minimization of precursors occurrence and THMs and NDMA formation are compared and discussed in the light of the current scientific knowledge. KEYWORDS: DBPs, NDMA, THMs, precursors, strong oxidants, chlorination, chloramination, raw water

2 1. INTRODUCTION The problem of the occurrence of various categories of disinfection by-products (DBPs), especially trihalomethanes (THMs), in chlorinated drinking water has drawn increasingly scientific attention during the last decades. Numerous papers have been published regarding the presence of DBPs in disinfected waters, the factors affecting their formation, and application of techniques or modification of water treatment procedures in order to minimize their concentrations in finished waters. Chlorination, the most commonly used disinfection method of drinking water, has been proven to result in the formation of two main classes of halogenated organic DBPs: trihalomethanes (THMs) and haloacetic acids (HAAs). THMs and HAAs are formed when chlorine reacts with bromide (Br-) and natural organic matter (NOM) in source waters (Nikolaou, et al., 2002; 2004, Bekbolet et al., 2005). The issue of DBPs in drinking water obtained larger dimensions in global scale, as toxicological research revealed that THMs, HAAs and other DBPs are carcinogenic or cause adverse reproductive or developmental effects in laboratory animals. As a result, the concentrations of THMs in drinking water were regulated by the United States Environmental Protection Agency (US EPA), the World Health Organization (WHO) and the European Union (EU) (Clark et al, 2001, WHO, 2004). In an effort to comply with the regulatory limits for DBPs, a number of techniques were adopted by water treatment plants aiming to minimize the formation of THMs. Among the alternative disinfection methods to chlorination, chloramination has been considered and applied in water treatment plants. However, subsequent research has shown that the use of chloramines for disinfection, although it leads to lower concentrations of THMs, can result in formation of N-nitrosamines, including N-nitrosodimethylamine (NDMA), a probable human carcinogen. Nitrosamines, mainly NDMA, are mutagenic compounds and are suspected to be carcinogenic towards humans. In 2002, Mitch and Sedlak as well as Choi and Valentine reported that NDMA is formed during disinfection of dimethylamine (DMA) containing water or treated wastewater with chloramines (Andrzejewski and Nawrocki, 2007). Moreover in 2007 Andrzejewski and Nawrocki reported that not only chloramination but also other oxidants (particularly ozone and chlorine dioxide) applied in water/wastewater treatment leads to formation of NDMA, when dimethylamine is presented in raw water. It means that contrary to THMs, which formation is contributed to application of one oxidant (chlorine), NDMA can be formed during application of several oxidants in water/wastewater treatment. Currently, according to the results of many experiments, formation of NDMA during reactions of strong oxidants, particularly chloramines, with waters containing DMA seems to be proved. US EPA has classified NDMA into the group B2 i.e. compounds which are probably carcinogenic to human, with concentrations as low as <1 ng/l associated with a 10-6 lifetime cancer risk. For example, the maximum admissible concentrations for the N- nitrosodimethylamine (NDMA) and N-nitrosodiethylamine (NDEA) in drinking water determined by the USEPA at a cancer risk level of 10-6 are 0.7 and 0.2 ng/l. Currently, California has a 10 ng/l notification level for NDMA, NDEA and N-nitroso-di-npropylamine (NDPA) (Padhye et al., 2013). In 2008 the WHO placed NDMA on the list of DBPs related to application of chloramines in water/wastewater treatment with a permissible concentration of 100 ng/l. One of the most important factors affecting the formation of various categories of DBPs during water treatment is the occurrence of organic compounds in raw water, known as DBP precursors. Research efforts have focused on the occurrence of precursors of DBPs in raw water, and have tried to identify the mechanisms involved in the formation of DBPs fro their precursors, aiming to develop techniques to minimize DBPs concentrations in drinking water (Farre et al., 2011, Bond et al., 2012). Natural Organic Matter (NOM) components in raw water act as precursors for the formation of both THMs and NDMA. In

3 addition, emerging pollutants from anthropogenic activities have been shown to result in THMs and NDMA formation, when subjected to chlorination or chloramination. Such pollutants-precursors of DBPs include compounds of the categories of pharmaceuticals and herbicides (Xu et al., 2011, Le Roux et al., 2011, Shen and Andrews, 2011). These pollutants are frequently detected in raw waters, and according to research results they cannot be effectively removed with conventional water treatment prior to disinfection (Nikolaou et al., 2007). Therefore these precursors create concern for the water quality, in regard to the concentrations of THMs and NDMA. In this paper, recent findings in the sector of THMs and NDMA precursors research in raw water are presented, regarding NOM as well as pharmaceuticals and herbicides. Emphasis is paid on the DBP formation mechanisms investigated, the compounds that were found to act as DBP precursors, and the levels of THMs and NDMA detected in disinfected waters. Recent research on THMs and NDMA precursor minimization techniques is also presented and discussed. 2. PRECURSORS OF THMs AND NDMA The precursors of THMs and NDMA formation are organic compounds included in NOM existing in raw water (Samios et al., 2007), while in more recent research it was shown that anthropogenic emerging pollutants, for instance pharmaceuticals and herbicides, can also act as precursors for the formation of both THMs and NDMA, upon chlorination or chloramination (Xu et al., 2011, Aydin et al., 2012). Taking into account the increasing occurrence of emerging pollutants in raw waters, as a result of worldwide pollution from agricultural, industrial and domestic activities, their potential to contribute to DBPs formation during the treatment surface waters becomes important. Relevant studies focused on the individual compounds that act as DBP precursors, and on the formation mechanisms and levels of THMs and NDMA formed in disinfected waters. The analysis of NOM has shown the presence of carboxylic acids and other functional groups such as alcohols, aromatic rings and aliphatic chains. Chloroform and chloropropanones detected as DBPs can be considered to be products of humic acid, that are formed due to the reaction of chlorine with hydroxy-benzoic acid and β-ketocarboxylic acid (Samios et al., 2007). THMs and other volatile DBPs are small molecules with 1-3 carbon aliphatic saturated chains. It is considered that the origin of these chains is, mainly, from aromatic carbons of humic substances molecules. This has been indicated by a good correlation (coefficient of determination R2 = 0.7) between TTHMFP and UV absorption at 254 nm (wavelength where benzene rings absorb) (Singer et al., 1989, Heller-Grossman et al., 1993). Benzene rings of humic substances are bonded with functional groups such as carboxylic acid, hydroxide, and carbonyl group that make benzene rings more chemically active than benzene itself. It has been reported that reactions of dihydroxybenzoic acids with aqueous hypochlorite yield chloroform in reaction time less than an hour. In particular, the conversion of 2,4-2,6- and 3,5- dihydroxybenzoic acids to chloroform involves decarboxylation. 3-ketoglutaric acid (acetone-di-carboxylic acid COOHCH 2CH 2(CO)CH 2COOH) also reacts rapidly with hypochlorite at near neutral PH giving quantitative yields of chloroform within a few minutes due to two carboxylate groups close to one carbonyl group. Acids with similar structures (acids with β-keto groups) can yield also dichloro- and trichloroacetones which, after enolization, hydrolize to corresponding halomethanes and acetic acid ion (CH 3COCCl 3 + OH CH 3COO + CHCl 3) (Samios et al., 2007). Another precursor for chloroform is citric acid (COOH)(OH)C(CH 2COOH) 2, because of its easily enolized structure: three carboxylate groups available close to one hydroxide on appropriate PH conditions. Chloroform, formation from humic substances chlorination is due to their similar moieties such as carbonyl or hydroxide close to carboxylates bonded to benzene rings. Products of humic substances degradation (after chlorine addition) can also act a THMs precursors. Such compounds are benzenemono- di- or tricarboxylic acids (e.g. benzoic acid), succinic acid, butanoic acid, propanoic acid, other aliphatic acids, acetic

4 acid and their chloroderivatives hydroxy-methylbenzoic acids, 1,2,4-triazolidine-1- carboxylic acid, butyl-1,2-benzenedicarboxylic acid, 3-nitro-1,2-benzenedicarboxylic acid (Richardson et al., 2005). The vast majority of the products mentioned above seem not to be stable in a chlorine solution, and after 2 days of reaction the final products could be chloroform, chloral hydrate, chloropicrin, chloro-, and dichloropropanones and mono-, diand trichloroacetic acids (Samios et al., 2007). The NDMA precursors typically contain dimethylamino moiety that generates the nitrosamine upon chloramination, ozonation or other oxidants applying. Dimethylamine (DMA) was the first NDMA precursor identified. This precursor is present in surface waters as it is the typical catabolic product of proteins in animals and plants with concentration in urine 40 mg/l (Farre et al., 2011) and it is also produced by the chemical industry. However, the conversion of DMA to NDMA by monochloramination is rather inefficient, with yields of 0.5% and % reported, and it appears that DMA levels are too low to account for significant amounts of NDMA (Mitch et al., 2003, Bond et al., 2012). By measuring the concentrations of DMA (ND 410 ng L 1) in 31 German surface water samples it was concluded that NDMA formation was largely caused by other precursors (Nawrocki and Andrzejewski, 2011), such as group of secondary or tertiary amines containing a DMA moiety. For instance, trimethylamine (TMA) present in primary treated wastewater at 900 nm, is converted to NDMA at yields of % (Mitch et al., 2008). Τhree mechanisms of nitrosamine formation from amines have been suggested (Keefer et al., 1973, Schreiber et al., 2006, Bond et al., 2012): (i) an amine reacts with monochloramine (NH 2Cl) to give a hydrazine intermediate, which is subsequently oxidised to a nitrosamine, (ii) dichloramine (NHCl2) and an amine can generate a chlorinated hydrazine which is then converted to the equivalent nitrosamine, (iii) an amine reacts directly with a reactive nitrosating agent (e.g. NO+, HNO2, N2O4), forming the corresponding nitrosamine. Additionally to NOM components, emerging pollutants such a pharmaceuticals, herbicides and pesticides can act as precursors for THMs and NDMA. Pharmaceuticals can serve as THMs precursors, as shown by Acero et al. (2010) who observed a positive influence of the increasing initial chlorine concentration on the pharmaceuticals removal. According to these researchers, the pharmaceutical elimination trend for the chlorination process in natural waters and secondary effluents was: amoxicillin > naproxen > phenacetin > metoprolol. In parallel, regarding the THMs formation, chloroform was the dominant THM (around weight-% of TTHMs), followed by bromodichloromethane (Acero et al., 2010). Herbicides are another category of THMs precursors. According to Xu et al. (2011), chlortoluron chlorination kinetics follow a second-order kinetics model, first-order in chlorine and first-order in chlortoluron. The main chlortoluron chlorination byproducts identified were chloroform (CF), dichloroacetonitrile, 1,1-dichloropropanone, 1,1,1-trichloropropanone, dichloronitromethane and trichloronitromethane as well as NDMA (Xu et al., 2011) (Figure 1). Recently, it has also been shown that pharmaceuticals with substituted dimethylamino groups, and herbicides, can serve as NDMA precursors during chloramine disinfection (Lee et al., 2007; Kemper et al., 2010; Shen and Andrews, 2010, Aydin et al. 2012). The pharmaceutical ranitidine (occurring in Spanish primary effluent at a range of ng L 1) (Radjenovic et al., 2009) has been found to be a potent precursor, with a conversion yield of 63 89%, implying a NDMA formation potential of ng L 1. Bond et al (2012) conclude that the fact that NDMA yields from such precursors are much higher than from DMA implies the existence of key mechanisms which do not involve DMA. The herbicide diuron during dichloramination was shown to be an NDMA precursor (20 μg L 1 of diuron generated 170 ng L 1 of NDMA) (Bond et al., 2012). According to Padhye et al (2013) dithiocarbamate (DTC) pesticides can serve as nitrosamine precursors, by release of secondary amines through hydrolysis or through reactions with oxidants. Shen and Andrews (2011) demonstrated

5 that a group of pharmaceuticals containing amine groups are precursors of NDMA during chloramination. In particular, they reported molar yields higher than 1% for eight pharmaceuticals (18.5 ng/l of NDMA formed), with ranitidine showing the strongest potential to form NDMA (conversion 89.9e-94.2%), followed by doxylamine ( %), sumatriptan (6.1%), chlorphenamine ( %), nizatidine ( %), diltiazem ( %), carbinoxamine ( %) and then tetracycline ( %). These eight pharmaceuticals have the DMA group bound to an electron-rich moiety. Ranitidine and nizatidine, both have the DMA group bound to the C2 site of a fiveelement heterocyclic ring. Sumatriptan and diltiazemhave have the DMA bound to an electron-donating indole and benzothiazepine structure, respectively. Carbinoxamine, chlorphenamine and doxylamine have the electron-rich bulky aromatic system in their structures, but the distance between the aromatic structure and the DMA group is farther than that of ranitidine, resulting in the overall lower molar conversions (Shen and Andrews, 2011). Figure 1. Proposed mechanisms for the formation of chloroform (CF), NDMA and other DBPs after chlorination of chlortoluron (Xu et al., 2011) Pharmaceuticals and herbicides as precursors for NDMA and other halogenated DBPs as haloketones, have also been studied by Le Roux et al. (2011). Several compounds investigated formed NDMA in greater amounts than DMA, revealing the importance of structural characteristics of tertiary amines for NDMA formation. Le Roux et al. (2011), found that ranitidine showed the highest molar conversion to NDMA, results in accordance to those obtained by Shen and Andrews (2011). Shen and Andrews (2011) attributed the higher yield observed for ranitidine to the electron-donating effect of furan group that increases electron density on the nitrogen atom and thus enhance electrophilic substitution of chlorine atom. However Le Roux et al (2011) support that this mechanism would involve the formation of dimethylchloramine (DMCA), DMA and then NDMA, which could not explain the high yield obtained with ranitidine, and therefore they suggest an alternative mechanism that would involve the nucleophilic substitution of NH 2Cl on nitrogen atom instead of electrophilic substitution (i.e. chlorine transfer with formation of a DMCA group), underlining the necessity of further research (Le Roux et al., 2011).

6 Padhye et al (2013) recently examined the contribution of dimethyldithiocarbamate (DMDTC) and diethyldithiocarbamate (DEDTC), as potential precursors of nitrosamines, in contact with various water disinfection oxidants. These two compounds are commonly used DTC pesticides and their monochloramination and ozonation yielded highest amounts of nitrosamines compared to free chlorine and chlorine dioxide. N- Nitrosodiethylamine (NDEA) yield from DEDTC was lower, by different degrees, than NDMA yield from DMDTC for all four oxidants, which was attributed to the steric hindrance associated with bulkier reaction intermediate that are more difficult to be further oxidized to form nitrosamine. The yield of nitrosamines increased with the oxidant dosage for both monochloramination and ozonation. Results for nitrosamine formation from DTCs at varying ph were consistent with the ph trend of nitrosamine formation from ozonation and monochloramination of secondary amines. According to the kinetic study, the DTCs were not significant direct precursors of nitrosamines during mono-chloramination or ozonation, but rather nitrosamines formed were primarily from reaction of oxidants with the amine which may be generated either through hydrolysis or through oxidation of DTCs. The results for the effect of ph on the nitrosamine yield from monochloramination of DMDTC and DEDTC, showed a decrease of nitrosamine yield with increasing ph from 5 to 7, a slight increase from ph 7 to 9 and a decrease with increasing ph. The effect of ph from ozonation of DMDTC and DEDTC showed that nitrosamine yield was highest at basic ph and it decreased with decrease in ph (Padhye et al., 2013). 3. MINIMIZATION OF DBP PRECURSORS IN WATER Different applications and proposals for minimization of DBP precursors occurrence, in order to control THMs and NDMA formation have been reported in the literature (Clark et al, 1994). During conventional water treatment, the removal of precursors occurs during coagulation, sedimentation and filtration, while for better control of DBPs it is suggested to move the point-of-chlorination to the end of the treatment process in order to minimize the time for by-product formation, while the treatment process will remove precursors more effectively. Another effective but more expensive technique is the removal by Granular Activated carbon (GAC). GAC is very efficient at removing both volatile and non-volatile compounds from water by surface attraction forces. GAC has been widely used in columns for continuous removal of a wide range of organics, including synthetic organics, total organic carbon and DBP precursors. Factors affecting the effectiveness of GAC are the surface area and hydraulic loading rate, the pore size distribution and bed depth, Surface Chemistry, ph and Temperature variation, Presence of Competing Organics for the adsorption sites, the presence of metals and the biogrowth in GAC bed. A pilot column study of GAC adsorption capacity for THMs and HAAs in Athens waterworks, Greece, showed a very good removal efficiency for dissolved organic matter (Babi et al, 2007). Advanced oxidation with UV/TiO 2 is also applied for precursors' removal. Kent et al. (2011) separated river water into six different natural organic matter (NOM) fractions, including hydrophobic acids, bases and neutrals and hydrophilic acids, bases and neutrals, and applied UV/TiO 2 treatment with a nanostructured thin film (NSTF), coated with TiO2 which is compared with the use of a TiO 2 suspension. The results have shown that for the raw river water, removal of total trihalomethane formation potential (TTHMFP) was approximately 20%, with 50 mj/cm 2 UV exposure and 1 mg/l TiO 2. For the fractionated samples, approximately 75% of DBP precursors were found to be associated with the hydrophobic acid fraction, for which the same UV/TiO 2 treatments exhibited approximately 20 25% removal of TTHMFP (Kent et al., 2011). In their recent paper, Alpatova et al. (2013) describe the removal of the DBP precursors, antibiotics dicloxacilline and ceftazidime by hybrid ozonation membrane filtration (HOMF). Three surface waters were studied (Lake Huron, Lake Lansing and Huron River) and the results revealed that, compared to membrane filtration, HOMF significantly improves the removal of dicloxacilline, ceftazidime, and DBP precursors. In particular, at a sufficiently

7 high ozone dosage, the concentrations of the two antibiotics in the permeate were reduced to below the detection limits in the three waters studied, while significant reductions in total organic carbon (TOC), specific UV absorbance (SUVA), and chlorinated DBPs were also achieved. The degree of fouling was also studied, and it was greater in waters with a high TOC and/or alkalinity. Alkalinity inhibited both the removal of the antibiotics and the ability of ozone to control fouling, a fact attributed by the authors to the scavenging of hydroxyl (OH) radicals by carbonate species (Alpatova et al, 2013). Magnetic Ion Exchange (MIEX ) technology has been recently applied by Gan et al. (2013) for removing nitrogenous disinfection byproduct (N-DBP) precursors while minimizing carbonaceous DBP (C-DBP) precursors. Both surface waters and effluent impacted waters were investigated. Formation potential tests were performed for DBPs, including THMs and NDMA before and after the MIEX treatment. The results were encouraging regarding THM precursors removal, showing that the MIEX process substantially lowered UV absorbance, total organic carbon, and THMFPs in all water samples (39-87% reduction). On the other hand, regarding the NDMA precursors, an increase in N-nitrosodimethylamine (NDMA) FP was observed after the MIEX process, but only for the effluent impacted waters, while there was no effect of MIEX treatment on the removal of other nitrosamine species precursors. The authors performed simulations of typical water treatment and distribution systems scenarios resulting in NDMA concentrations below 10 ng/l, when chlorine alone or 40 min chlorine contact time prior to ammonia addition were employed for post-disinfection, while when chlorine and ammonia were added simultaneously, NDMA concentration in water reached 36 ng/l (Gan et al., 2013). As it concerns the minimazion of DTCs as precursors of nitrosamines, emphasis should be placed on reducing secondary amine formation from DTCs. Increasing the ph would decrease hydrolysis rate of DTCs; however it increases nitrosamine formation through ozonation of secondary amines. Decreasing the ph is beneficial to remove toxicity associated with DTCs themselves since decreasing the ph results in more hydrolysis of DTCs. However, in addition to more amine release from hydrolysis, lower ph may result in more nitrosamine formation through monochloramination due to more disproportionation of monochloramine to dichloramine, a more potent nitrosamine precursor (Padye et al., 2013). 4. CONCLUSIONS Recent research on the precursors of THMs and NDMA has focused on the identification of compounds and mechanisms involved in DBP formation, shedding more light in a previously unknown field. Apart from the naturally occurring organic materials, research results show the importance of emerging pollutants as well for the formation of THMs and NDMA during water treatment. Pharmaceuticals, herbicides and pesticides have been shown to act as precursors for these reactions. Techniques for the removal of precursors from raw water in order to minimize DBP formation are, in addition to conventional treatment and moving the chlorination point later in the process, the shift of ph, GAC, UV/TiO2, HOMF and MIEX. Most of these techniques have shown promising results, while their optimization to increase their effectiveness is the aim of current research. REFERENCES Acero J.L., Javier Benitez F., Real F.J., and Roldan G. (2010) Kinetics of aqueous chlorination of some pharmaceuticals and their elimination from water matrices, Water Research 44, Alpatova A.L., Davies S.H. and Masten S.J. (2013) Hybrid ozonation-ceramic membrane filtration of surface waters: The effect of water characteristics on permeate flux and the removal of DBP precursors, dicloxacillin and ceftazidime, Separation and Purification Technology, Gan X., Karanfil T., Sule S., Bekaroglu K. and Shan J. (2013) The control of N-DBP and C-DBP precursors with MIEX, Water Research, 47, Andrzejewski P. and Nawrocki J. (2007) N-nitrosodimethylamine formation during treatment with strong oxidants of dimethylamine containing water, Water Science and Technology, 56,

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