Effect of pH on UV Photodegradation of N-Nitrosamines in...

Journal of Korean Society on Water Environment, Vol. 32, No. 4, pp. 357-366 (July, 2016)pISSN 2289-0971 eISSN 2289-098X http://dx.doi.org/10.15681/KSWE.2016.32.4.357

Journal of Korean Society on Water Environment, Vol. 32, No. 4, 2016

Effect of pH on UV Photodegradation of N-Nitrosamines in WaterJae-Goo Shim*․Afzal Aqeel**․Bo-Mi Choi**․Jung-Hyun Lee*․No-Sang Kwak*․Ho-Jin Lim**,†

*KEPCO Research Institute, 105 Munji-ro, Yuseong-gu, Daejeon 54056, Korea**Department of Environmental Engineering, Kyungpook National University, Daegu 702-701, Republic of Korea

수용액상 니트로스아민의 UV 광분해에서 pH 영향심재구*․아프잘 아킬**․최보미**․이정현*․곽노상*․임호진**,†

*한국전력공사 전력연구원**경북대학교 환경공학과

(Received 1 June 2016, Revised 15 July 2016, Accepted 25 July 2016)

AbstractN-nitrosamines are a class of carcinogenic chemicals that can pose significant hazards to the human life. Ultraviolet (UV) light irradiation is considered as one of the effective methods to reduce N-nitrosamines in the aqueous phase. This study aimed to investigate the pH influence on UV photodegradation of N-nitrosamines (i.e., N-nitrosodibutylamine (NDBA) and N-nitroso-pyrrolidine (NPYR)) closely related to water treatment. Photodegradation rate constants of NDBA and NPYR remained between 3.26×10-2 L/W-min to 5.08×10-3 L/W-min and 1.14×10-2 L/W-min to 2.80×10-3 L/W-min at pH2-10, respectively. This study also focused on the formation of oxidized products (i.e., primarily NO2- and NO3-). Under slightly acidic and neutral conditions, NO2- formation was more prevalent than NO3- formation, while under strong acidic conditions, NO3- was more pre-valent. There was no significant change in total organic carbon (TOC) and total nitrogen (TN), suggesting negligible loss of N-nitrosamines and degradation products from the system. NDBA was easily photodegraded than NPYR. This study also demon-strated that a lower pH is a favorable condition for photolytic degradation of N-nitrosamines in water.

Key words : Nitrosamine, pH effect, Photodegradation, UV photolysis, Water treatment

1. Introduction1)

It has been more than 100 years since N-nitrosamines have been studied by the scientific community. N-nitrosamines received much attention when Barnes and Magee discovered the carcinogenic properties of these compounds (Barnes and Magee, 1954). Moreover, with extensive research, most of N-nitrosamines (i.e., 90% of the total) are now classified as carcinogenic, mutagenic, and/or teratogenic compounds, which can ultimately cause detrimental effects on human health (Andrzejewski et al., 2005; Wang et al., 2011; Xu et al., 2010; Zhou et al., 2012). N-nitrosamines primarily target esophagus and liver sites for tumor formation. Other organs, which can also be affected by N-nitrosamines are bladder, brain, and lungs (Xu et al., 2009a). Therefore, many deve-loped countries established stringent laws for the regulation

†To whom correspondence should be [email protected]

This is an Open-Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License (http://creativecommons.org/ licenses/by-nc/3.0) which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited.

of acceptable level of nitrosoamines. Acceptable limit pro-posed by the Norwegian Public Health Institute for all N-nitrosoamines is 4 ng/L for drinking water and 0.3 ng/m3 for air (Sorensen et al., 2015; Zhou et al., 2012).

N-nitrosamines cause severe problem in drinking water purification systems and in reprocessing of waste water to fulfill increasing water demands worldwide (Zhou et al., 2012). High concentration of organic nitrogen in industrial waste water effluents can serve as precursors for N-nitros-amines in the reaction with disinfectants (Krasner et al., 2009). Consequently, more efficient drinking water treat-ments are required to achieve the purity up to safe drinking levels. Recent studies have confirmed that chlorination and chloramination treatments of drinking water and industrial wastewater result in the formation of new disinfection by- products of N-nitrosamines (Mitch and Sedlak, 2002; Padhye et al., 2009; Schreiber and Mitch, 2006). N-nitrosopyrollidine (NPYR) and N-nitrosodibutylamine (NDBA) have been inc-luded in this new class of emerging disinfection by-products (Zhou et al., 2012). Charrois et al. (2004) detected NPYR in drinking water both in the water supply system and at the plant, where chloramination was used for disinfection

Jae-Goo Shim․Afzal Aqeel․Bo-Mi Choi․Jung-Hyun Lee․No-Sang Kwak․Ho-Jin Lim

한국물환경학회지 제32권 제4호, 2016

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(Charrois et al., 2004). NPYR has been reported in drinking water distribution systems up to 70.5 ng/L and up to several hundred ng/L in waste water and municipal sludge, respec-tively (Krasner et al., 2009; Padhye et al., 2009; Zhao et al., 2006). Wang et al. (2016) investigated 54 drinking water treatment plants and reported that NDBA was one of the most abundant N-nitrosamines. The occurrence of NPYR and NDBA in aquatic environment is estimated to be associated with 10-6 cancer risk at 20 ng/L concentration level (Gerrity et al., 2014). Conventional drinking water and waste water treatment plants are not designed to remove these emerging contaminants (Zhou et al., 2012).

N-nitrosamines are usually soluble in water due to their polar nature. These compounds are difficult to extract with organic solvents due to their low octanol/water partition coefficients (Kow). These compounds are not significantly adsorbed on non-polar surfaces. This high hydrophilicity and low adsorbability of N-nitrosamines contribute to a great risk of ground water contamination. Coagulation and filtration could not be used for the removal of N-nitrosamines in drinking water facilities due to the above discussed properties. The majority of N-nitrosamines precursors are also too small in size that could not be removed by coagulation (Xu et al., 2011). Furthermore, many polymers (e.g., poly-DADMAG, polyamines) used as coagulant in drinking water treatment plants, could be the source of nitrosamines precursors (Kohut and Andrews, 2003; Wilczak et al., 2003). Miyashita et al. (2009) concluded that nitrosamines cannot be efficiently removed even though nanofilters and reverse osmosis mem-branes are used. Nitrosamines cannot be removed from water by aeration due to low Henry constants, and are also hardly biodegradable (Nawrocki and Andrzejewski, 2011). Previous research groups have distinctly described that nitrosamines exhibit two absorption peaks (strong at ~230 nm and weak at ~340 nm) when expose to UV light. Therefore, this property can be exploited for photolytic degradation of N-nitrosamines in water (Andrzejewski et al., 2005; Sorensen et al., 2013; Stefan and Bolton, 2002). Furthermore, photo-degradation of nitrosamine results in less harmful products and conventional plants could also be retrofitted with this technology (Zhou et al., 2012).

Environmental hazards and measurement methods of both N-nitrosamines (i.e., NPYR and NDBA) have been widely studied in previous studies (Gushgari et al., 2016; Kodama-tani et al., 2009; Lee et al., 2013; Mahanama and Daisey, 1996; Ngongang and Duy, 2015; Pozzi et al., 2011; Sen et al., 1997; Wang et al., 2011; Zhang et al., 2016; Zhao et al., 2006). However, a few studies were published on treatment methods and degradation pathways of NPYR and NDBA (Plumlee and Reinhard, 2007; Xu et al., 2009b; Zhou et al.,

2012). The pH effects on photodegradation of NDBA have been studied in a mixture of nine N-nitrosamines in earlier study of Zhou et al. (2012). Therefore, it may have assumed that NDBA will follow the same behaviour alone as in mix-ture. To the best of authors’ knowledge, there is no study available where pH effect on photodegradation of NDBA was individually evaluated. It is important to evaluate the pH effect on photodegradation of NDBA in individual solution for proper understanding of kinetics and identification of the degradation products. Therefore, in this paper, photodegrada-tion of N-nitrosamines in aqueous solution by UV irradiation was carried out and influence of initial pH was investigated. Furthermore, formation of oxidized products (i.e., primarily NO2- and NO3-) was also investigated.

2. Materials and Methods

2.1. Nitrosamines Reagents

N-Nitrosodibutylamine (analytical grade) and N-nitrosopyr-rolidine (purity=99%) were purchased from Sigma-Aldrich (St. Louis, MO, USA). Stock solutions (i.e., 500 mg/L) were prepared in ultra-pure water (>18.3 µΩ-cm) taken from a Milli-Q water system (Power 1+, Human, Korea). These stock solutions were stored in the refrigerator before photo-degradation experiments and reaction solutions (i.e., 50 mg/L) were prepared by further dilution with ultra-pure water. Glass bottles containing stock solutions were properly covered with the aluminum foil to avoid from light. These stock solutions were kept in the refrigerator and used within 30 days. Con-centrations of stock solutions were rechecked throughout storage period with UV absorbance calibration. All glassware was cleaned thoroughly by sonicating in deionized water with a cleaning agent (CIP 100, Steris, USA), then followed by rinsing with reverse osmosis water in an ultrasonic cleaner (8510R-DTH, Branson, USA) and dried in a drying oven (SW-90D, Sang Woo, Korea) at 50°C before use.

2.2. Experimental Procedure

Photodegradation experiments were performed in a cylin-drical water jacketed glass batch reactor under controlled conditions as shown in Fig. 1. Initial pH of the solution was pre-adjusted with 2 M HCl and NaOH using a pH meter (Orion 4 star, Thermo Scientific, USA) after calibration. Then, 700 mL of 50 mg/L N-nitrosamine solution was exposed to UV irradiation. A low pressure Hg lamp (GL4WP, UV Nature, Korea) of 4 W was installed to the reactor. Solution was heated on a hot plate equipped with magnetic stirrer (HMS100, Yhana, Korea) and a temperature controller (TZ4ST, Autonics, USA) with a K-type thermocouple to maintain temperature at 40°C. A fraction collector (2110, Bio-Rad,

Effect of pH on UV Photodegradation of N-Nitrosamines in Water


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Fig. 1. Experimental design for direct photolysis of N-nitrosamine.

USA) was installed to collect samples at fixed intervals. A peristaltic pump (BT 100-2J, Longer Pump, China) was used to transport reaction solution to the fraction collector. After stabilizing the system, UV-lamp and peristaltic pump were switched on to start irradiation and drawing sample solution. Samples were collected in 7 mL vials with 0.5 min interval keeping the flow rate of 10 mL min-1 during first 10 minutes. Afterwards samples were collected in 2.5 min interval kee-ping the flow rate of 2 mL/min up to next 20 minutes. Then, every two consecutive vials were combined in 20 mL amber colored vial to get cumulative volume of 10 mL for each sample. These collected samples were stored at 6°C in the refrigerator for further analysis. Remaining N-nitrosamine solution in the reactor was further degraded for 2 hours before disposal to ensure complete degradation of nitrosamines for the safety of environment. Samples were analyzed within three weeks after collection.

2.3. Analysis

Degradation of N-nitrosamines was monitored by UV-Vis spectrophotometer (8453, Agilent, USA). The removal of N-nitrosamines was quantified by correlating the absorbance at specific wavelengths. A rectangular quartz cuvette of 10 mm path length (5061-3387, Agilent) was used for absor-bance measurements between 190-800 nm with 1 nm interval. Every time the cuvette was washed three times with Milli-Q water and one time with sample solution before absorbance measurement. Collected samples at different time intervals were diluted two times with ultra-pure water (LiChrosolv, Merck, USA) before analysis of NO2- and NO3-. Dionex ICS-3000 ion chromatography (Sunnyvale, CA, USA) equip-ped with self-regenerating suppressor and conductivity detector was used for the measurement of NO2- and NO3-. Dionex Ionpac AS14 analytical column (I.D. 4 × L 250 mm) coupled to AG14 guard column (I.D. 4 × L 50 mm) was used in the analysis. The ion analysis unit was operated in autosuppres-

sion mode using an eluent comprised of 3.5 mmol Na2CO3 + 1 mmol NaHCO3 at a flow rate of 1.2 mL/min. The accu-racy and precision for the IC analysis was ensured by placing the repeated set of samples after every 10th sample and standard sample was added after this repeated set of samples. In order to remove zero error, blank samples were also placed after each standard sample. The minimum detec-tion limit of NO2- and NO3- was 8.9×10-3 ppm and 8.3×10-3 ppm, respectively. It was determined as 3 times of standard deviation of blank measurements.

TOC/TN analyzer (Shimadzu, TOC-L CPH, Japan) coupled with autosampler was used for total organic carbon (TOC) and total nitrogen (TN) of the collected samples after two times dilution. A multi-point calibration was carried out from a mixed standard solution of 100 mgC/L TC and 100 mgN/L TN acidified with 0.05 M HCl. The standard solutions were diluted in the range of 1-50 mg/L (i.e., 1, 2, 5, 10, 25, and 50 mg/L) and then these were analyzed for TOC and TN. The accuracy was checked and zero error was removed during the analysis of samples by introducing check standard (5 mg/L) and blank sample after every 10th sample. The minimum detection limit of TOC and TN was 7.0×10-2 ppm and 9.4×10-3 ppm, respectively. It was determined as 3 times of standard deviation of blank measurements.

3. Results and Discussions

3.1. UV-Vis Absorption Properties

N-nitrosamines exhibit two primary absorption peaks in UV-Vis range. Absorption spectra of N-nitrosamines along with UV lamp emission spectra are shown in Fig. 2. The maximum peak intensity was observed at λmax = 234 nm and λmax = 231 nm for NDBA and NPYR, respectively. Cross- sectional absorptivity (єmax) at λmax was determined to be 5.94×106 and 4.20×106 cm2/mol for NDBA and NPYR, respectively as shown in Table 1. The strong absorption band is due to π→π* intramolecular charge transfer (Lee et al., 2005; Stefan and Bolton, 2002). Weak absorption bands were observed at λ = 338 nm and λ = 333 nm for NDBA and NPYR, respectively. The weak absorption band is associated with n→π* transition. These are consistent with earlier reports of a strong peak at ~230 nm and a weak peak at ~340 nm for N-nitrosamines (Nawrocki and Andrzejewski, 2011; Stefan and Bolton, 2002; Xu et al., 2008).

The homolytic cleavage of N-NO bond in N-nitrosamine is associated with π→π* electronic transition when irradiated under UV light as shown in R1 below (Lee et al., 2005; Xu et al., 2009). Hydrogen ions present in water attach to the oxygen of nitroso group via hydrogen bonding. Due to the sharing of electron in hydrogen bonding, N-N bond becomes



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Table 1. Physicochemical and optical properties of N-nitrosamines at λmax and 254 nm

Species Formula Molecular weight λmax єλmax єλ254 Solubility at 20°CVP*

at 20°Cg/gmol nm cm2/mol cm2/mol mg/mL mm Hg

NDBA C8H18N2O 158.24 236 5.94×106 2.57×106 100 0.0005NPYR C4H8N2O 100.11 231 4.20×106 2.54×106 soluble 0.06

*VP denotes saturation vapor pressure.

Fig. 2. UV-Vis absorption spectra of 50 mg/L nitrosamine solutions at pH6 along with emission spectrum of UV lamp.

weaker to result in degradation of N-nitrosamines. Acidic solutions have large quantities of H+ available for this hydro-gen bonding, which could accelerate the degradation of N-nitrosamines.

(R1)

This photolytic cleavage of N-nitrosamines produces cor-responding ammonium radical and nitric oxide. In addition, the heterolytic cleavage of N-NO bond in nitrosamines is induced by n→π* electronic transition as shown in R2 below (Lee et al., 2005).

(R2)

Mechanistic pathways are further described by different research groups. Xu et al. (2010) suggested that homolytic cleavge of N-N bond in NDEA photodegradation resulted in diethylaminium radical. Two diethylaminium radicals could combine to form diethylamine. Acid-catalyzed hydrolysis of intermediates resulted in the formation of ethylamine and aldehyde. When the heterolytic cleavage takes place in the acidic solution, diethylamine is exposed to favorable condi-tions for the nucleophile attack. Stefan and Bolton (2002)

reported that secondary amines and nitrite ions were pro-duced during photodegradation in weakly acidic and neutral pH conditions. Chow (1973) showed that acid complex of NDMA (or protonated NDMA) was photodegraded into dimethylamine and HNO2 by photo-hydrolysis from the hete-rolytic cleavage of N-NO bonds. Whereas, aminium radical and NO . produced by photoelimination from the homolytic cleavage of N-NO bonds.

The interference of absorbance was observed due to bypro-ducts at shorter wavelength (i.e., λmax). There was a minimal interference for weak peak at λ = 338 nm and λ = 333 nm for NDBA and NPYR, respectively. This wavelength was selected for the determination of nitrosamines, because inter-ference at this wavelength was apparently appeared only at the later part of the reaction. Therefore, the absorbance values of N-nitrosamines during the first 10 min were used to deter-mine the reaction rate constants.

3.2. Effect of pH on kinetics of N-nitrosamines photode-

gradation

The decay of N-nitrosamines under UV irradiation was studied over a wide range of pH2-10 as shown in Fig. 3. The photodegradation depends on photon flux and concentration of N-nitrosamine. A uniform photon flux was supposed in the reactor, so reaction rate should depend only on the con-centration of N-nitrosamine. Therefore, reaction rate constants were obtained assuming pseudo-first order kinetics for the degradation. For more reasonable comparison of rate con-stants presented in this study with the literature data, these were normalized to the intensity of UV lamp and volume of the reactor (i.e., L/W-min). In case of NDBA, degradation rate constants were 3.26×10-2 L/W-min, 2.38×10-2 L/W-min, 9.98×10-3 L/W-min, 7.53×10-3 L/W-min, and 5.08×10-3 L/W-min at pH2, 4, 6, 8, and 10, respectively. Degradation rate con-stants increased with decrease in pH for NDBA. Similar trends for degradation rate constants were observed in pre-vious studies (Lee et al., 2005; Stefan and Bolton, 2002; Xu et al., 2008; Xu et al., 2010). In case of NPYR, degradation rate constant of 1.14×10-2 L/W-min at pH4 was higher than 9.45×10-3 L/W-min of pH2. Such irregular trend of NPYR is not clearly known. NPYR showed negligible change in rate constant at alkaline pH. This negligible change in rate con-stant could be explained by corresponding similar formation



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(a) (b)

Fig. 3. Photodegradation of nitrosamies (expressed as C/Co) as a function of reaction time at different pH. Reaction rate constantsare obtained assuming pseudo-first order reaction for the first 10 min. a) NDBA b) NPYR.

Table 2. Kinetic studies comparison of NDBA and NPYR with the available literature, on the basis of different parameters

Species Concentration RateconstantWorkingvolume Power

Normalizedrate constant pH Temperature References

mM 1/min L W L/W-min °CNDBA 3.16×10-1 1.86×10-1 0.7 4 3.26×10-2 2±0.05 40 This study

3.16×10-1 1.36×10-1 0.7 4 2.38×10-2 4±0.05 40 This study3.16×10-1 5.70×10-2 0.7 4 9.98×10-3 6±0.05 40 This study3.16×10-1 4.30×10-2 0.7 4 7.53×10-3 8±0.05 40 This study3.16×10-1 2.90×10-2 0.7 4 5.08×10-3 10±0.05 40 This study6.32×10-4 1.20×10-1 0.2 75 3.20×10-4 5 Room temp. Zhou et al. (2012)6.32×10-4 8.60×10-2 0.2 75 2.29×10-4 7 Room temp. Zhou et al. (2012)6.32×10-4 7.40×10-2 0.2 75 1.97×10-4 9 Room temp. Zhou et al. (2012)

NPYR 4.99×10-1 5.40×10-2 0.7 4 9.45×10-3 2±0.05 40 This study4.99×10-1 6.50×10-2 0.7 4 1.14×10-2 4±0.05 40 This study4.99×10-1 2.70×10-2 0.7 4 4.73×10-3 6±0.05 40 This study4.99×10-1 1.60×10-2 0.7 4 2.80×10-3 8±0.05 40 This study4.99×10-1 1.60×10-2 0.7 4 2.80×10-3 10±0.05 40 This study9.99×10-4 7.80×10-2 0.2 75 2.08×10-4 Room temp. Zhou et al. (2012)9.99×10-4 6.00×10-2 0.2 75 1.60×10-4 Room temp. Zhou et al. (2012)9.99×10-4 5.40×10-2 0.2 75 1.44×10-4 Room temp. Zhou et al. (2012)1.00×10-4 1.92×10-1 0.25 40 1.20×10-3 Chen et al. (2015)

Table 3. Formation rate of NO2- and NO3- along with degra-dation rate of N-nitrosamines

Species pH Degradation rates Formation ratesd/dt

(mmol/L-min)d(NO2-)/dt

(mmol/L-min)d(NO3-)/dt

(mmol/L-min)NDBA 2±0.05 2.9×10-2 4.0×10-4 2.5×10-2

4±0.05 2.8×10-2 1.8×10-2 1.4×10-2

6±0.05 1.6×10-2 2.0×10-2 1.0×10-2

8±0.05 1.3×10-2 8.3×10-3 8.3×10-3

10±0.05 7.0×10-3 6.0×10-3 4.0×10-3

NPYR 2±0.05 2.3×10-2 8.6×10-4 2.6×10-2

4±0.05 2.4×10-2 1.2×10-2 8.7×10-3

6±0.05 1.4×10-2 8.9×10-3 6.6×10-3

8±0.05 6.0×10-3 8.2×10-3 6.1×10-3

10±0.05 6.0×10-3 8.2×10-3 6.1×10-3

rates of NO2- (i.e., 8.2×10-3 mmol/L-min) and NO3- (i.e., 6.1× 10-3 mmol/L-min) at pH8-10 as shown in Table 3. Because, it has been observed that the formation of NO2- and NO3- follow degradation of N-nitrosamines.

There are two different explanations about the mechanism of N-nitrosamines photodegradation. The stretching of N-N double bond induces partial dipole and negative charge at nitroso oxygen, which is a good site for protonation (Chow, 1973). According to Xu et al. (2010), protonated species of nitrosamines are more photolabile than unprotonated species. Lee et al. (2005) suggest that acid-catalyzed complex rather than protonated or unprotonated species are responsible for the photodegradation. Polar character of N-nitrosamines is good explanation to conclude that acidic conditions promote photo-lability of nitrosamines through weakening of N-N bonding.



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Fig. 5. Time profiles of TOC, TN, NNitrosamine, NNO2-, NNO3-, and NOthers for NDBA.

Fig. 4. Reaction rate constants of NDBA and NPYR at different pH.

Photodegradation rate constant of NDBA was higher than that of NPYR at all pH values. Zhou et al. (2012) reported similar trend of rate constants for NDBA and NPYR. The difference in degradation rate constants might be primarily due to their chemical structures (Ohwada et al., 2001; Salvo et al., 2008). The higher degradation rate of NDBA might be caused by the fact that organic species produced also affect the dissociation energies such as benzyl radicals clearly con-tribute to the C-N bond breakage (Salvo et al., 2008). The lower degradation rate of NPYR might be due to its unique cyclic structure with high electron density that might tolerate the weakening of N-NO bonding by protonation via electron donation. The pH effect is caused by the weakening of N-NO



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Fig. 6. Time profiles of TOC, TN, NNitrosamine, NNO2-, NNO3-, and NOthers for NPYR.

bond by increased protonation at lower pH. This makes NPYR more stable than other straight chain nitrosamines (i.e., NDBA) (Xu et al., 2009b). Hence, chemical structures might influ-ence degradation rate along with pH effect.

Zhou et al. (2012) showed the influence of various factors (e.g., Initial nitrosamine concentration, UV intensity, pH, H2O2 dosage, and inorganic anions) on photodegradation of the mixture of nine N-nitrosamines. Rate constants obtained in our study for NDBA and NPYR were much higher rela-tive to the former study data. These higher rate constants might be due to difference in experimental design. In our study nitrosamine solutions were directly irradiated under UV light, while baffles with different helix angles between

lamp shade and nitrosamine solution were used in the former study. On the other hand, competition for available UV light might be increased for the mixture of nine N-nitrosamines irradiated together in the previous study. Photodegradation rate constant of NPYR presented in this study (i.e., 4.73× 10-3 L/W-min) at pH6 was 3.94 times higher than that of Chen et al. (2015) at pH7 (i.e., 1.20×10-3 L/W-min). The discre-pancy can be explained by the fact that photodegradation rate constant increases with decrease in pH. Solution was kept at a distance of 30 cm from lamp in their study, whereas UV lamp was submerged in the solution in our study. This might be another reason for the discrepancy in rate constants.



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3.3. NO2-, NO3

-, TN, and TOC

Linear regression lines were obtained from the concen-tration (mmol) and reaction time (min) data for NO2- and NO3- ions. Slopes of these regression lines were reported as rates (dC/dt) as shown in Table 3. The influence of pH on TOC, TN, NNitrosamine, NNO2-, NNO3-, and NOthers is shown in the Fig. 5. NNitrosamine, NNO2-, NNO3-, and NOthers denote nitrogen in nitrosamine, nitrogen in NO2-, nitrogen in NO3-, and nitro-gen in other compounds, respectively. Concentrations of NNO2- and NNO3- increased gradually with the reaction time during the photolysis of N-nitrosamines. NNO2- concentration was too low to be detected at pH2 in both N-nitrosamines. NO2- might be unstable at strong acidic conditions (Fan and Tannenbaum, 1972) or react to form stable organic compounds (i.e., organic nitrate) (Polo and Chow, 1976). Hydroxide ion concentration might be too low to form nitrite at lower pH (Fan and Tannenbaum, 1972). This may well be another possible reason for low concentration of nitrite at pH2. Hence, NO2- was present at lower level than NO3- at pH2. Xu et al. (2008) also observed similar results for NDEA photodegradation in strong acidic conditions. A slight dec-rease in NNO2- after the complete removal of nitrosamine at lower pH was associated with increase in NNO3-. NO3- pro-duced by further oxidation of NO2- (Stefan and Bolton, 2002). Yield of NNO2- rapidly increased with increasing pH from acidic to neutral solution and decreased in alkaline solution. These results are consistent with the previous studies (Xu et al., 2009; Xu et al., 2010). Owing to different patterns in NO2- and NO3- formation, it might be concluded that NO2- and NO3- production were dependent on degradation rates of parent N-nitrosamines along with other control factors. Solu-tion pH might be one of these factors which influences the yield of NO2- and NO3- (Fan and Tannenbaum, 1972). Higher concentration of NO2- indicates that main degradation path-ways (i.e., R1 and R2) were the principal source of NO2- production under neutral conditions. NO3- formation mecha-nism has been proposed in a previous study under acidic and alkaline conditions as shown in R3 and R4 (Lee et al, 2005).

(R3)

(R4)

forms as an intermediate and reacts with NO to formperoxynitrile. Peroxynitrile finally transforms to NO3- by spon-taneous isomerization or its reaction with NO2-. NO2- and NO3- formation could also be explained by the following reactions

(R5)

(R6)

In this study, NO2- and NO3- formed at comparable levels during initial five minutes of degradation possibly due to above reactions R5 and R6 at all pH except at pH2. The formation rates of NO2- remained between 4.0×10-4 - 2.0×10-2 mmol/L-min and 8.6×10-4 - 1.2×10-2 mmol/L-min for NDBA and NPYR at pH2-10, respectively. The formation rates of NO3- remained between 4.0×10-4 - 1.2×10-2 mmol/L-min and 6.1×10-3 to 2.6×10-2 mmol/L-min for NDBA and NPYR at pH2-10, respectively. Formation rates of NO2- and NO3- are presented in Table 3. It is evident that mechanistic pathways of UV photodegradation are pH dependent. Total nitrogen almost remained constant throughout the reaction. Further-more, it was observed that the TN concentration was greater than the collective sum of NNitrosamine, NNO2-, and NNO3- throughout the reaction. Hence, it could be concluded that some undetected nitrogen species also produced. These might be organic (i.e., alkylamines) or inorganic (i.e., NH4+) species. The amount of these species has been specified as NOthers in the Fig. 5. The total organic carbon (TOC) also remained at a constant level, suggesting negligible loss of N-nitrosamines and degradation products from the system. Xu et al. (2009b) identified pyrrolidine as main degradation product of NPYR. Moreover, low molecular weight aliphatic amines were also identified as further degradation products of NPYR i.e., (methy-lamine, dimethylamine, ethylamine, diethylamine, n-propyla-mine and n-methylethylamine). In case of NDBA, it could be proposed that n-butylamine would be the main degrada-tion product. Further low molecular weight degradation pro-ducts might be similar to those of NPYR.

4. Conclusions

Degradation rate constants of NDBA and NPYR increased with the decrease in pH between pH2-10. The pH effect is caused by the weakening of N-NO bond by increased proto-nation at lower pH. Overall NDBA degradation rate constants were higher relative to NPYR. Chemical structures might influence degradation rate along with pH effect. NDBA photodegradations showed gradual decrease with the increase in pH. The lower degradation rate of NPYR might be due to its unique cyclic structure with high electron density that might tolerate the weakening of N-NO bonding by proto-nation via electron donation. The formation rates of NO2- remained between 4.0×10-4 - 2.0×10-2 mmol/L-min and 8.6× 10-4 - 1.2×10-2 mmol/L-min for NDBA and NPYR at pH2-10, respectively.The formation rates of NO3- remained between 4.0×10-4 - 1.2×10-2 mmol/L-min and 6.1×10-3 to 2.6×10-2 mmol/ L-min for NDBA and NPYR at pH2-10, respectively. Pro-



365

ductions of these ions are also pH dependent. To sum up, the results show that acidic condition is an effective option for the removal of N-nitrosamines by UV photolysis, because in acidic pH both nitrosamines degraded in less than 20 minutes. Although, N-nitrosamines have been removed efficiently, detailed analysis of degradation products is required to sug-gest this process for drinking water facility. It is remained as our future study.

국문초록

N-니트로스아민은 인체에 매우 위험한 발암성 화합물이다. 수용액상 N-니트로스아민을 효과적으로 감소시킬 수 있는 방법 중 하나로 자외선 조사가 고려되고 있다. 본 연구의 목적은 수처리와 연관성이 높은 N-니트로스아민(즉, N-nitrosodibutylamine (NDBA)과 N-nitrosopyrrolidine (NPYR))의 UV 광분해에서 pH 영향을 규명하는 것이다. NDBA과 NPYR의 광분해 속도상수는 pH2-10 사이에서 각각 3.26× 10-2 L/W-min에서 5.08×10-3 L/W-min와 1.14×10-2 L/W-min 에서 2.80×10-3 L/W-min로 나타났다. 한편 산화 생성물인 NO2-와 NO3- 이온의 생성에 대해서도 연구하였다. 약산성에서 중성의 조건에서는 NO3-에 비해 NO2-이 주로 생성되었고, 강한 산성에서는 NO3-이 더 많이 생성되었다. 총유기탄소(TOC)와 총질소(TN)은 거의 변화가 없었으며, 이것으로부터 N-니트로스아민과 생성물이 광분해 시스템에서 거의 손실이 없었던 것으로 볼 수 있다. BDBA가 NPYR에 비해 상대적으로 용이하게 광분해되었다. 또한 수용액상 N-니트로스아민을 UV 광분해로 제거할 때 pH가 낮을수록 효과적인 것으로 나타났다.

Acknowledgment

This work was supported by the Korea Institute of Energy Technology Evaluation and Planning (KETEP) granted finan-cial resource from the Ministry of Trade, Industry & Energy, Republic of Korea (No. 20142010201810) through POSCO E&C.

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