Photodegradation in natural waters of nitrosamines and nitramines derived from CO2 capture plant operation

International Journal of Greenhouse Gas Control 32 (2015) 106–114

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Photodegradation in natural waters of nitrosamines and nitramines derived from CO2 capture plant operation Lisbet Sørensen 1 , Kolbjørn Zahlsen, Astrid Hyldbakk, Eirik Falck da Silva 2 , Andy M. Booth ∗ SINTEF Materials and Chemistry, Trondheim N-7465, Norway

a r t i c l e

i n f o

Article history: Received 3 May 2014 Received in revised form 17 October 2014 Accepted 5 November 2014 Keywords: Photolysis Hydrolysis Post combustion CO2 capture Nitrosamines Nitramine

a b s t r a c t Amine solvents used in post combustion CO2 capture (PCCC) plants react with NOx , forming carcinogenic nitrosamine and nitramine compounds. These can be emitted to the atmosphere, undergoing deposition to aquatic and terrestrial environments. In the current study, the hydrolytic and photolytic degradation of nine nitramines and nitrosamines identified as possible degradation products of 2-monoethanolamine and piperazine solvents are studied. Nitrosamines and nitramines are generally resistant to hydrolysis, although nitrosopiperazine and piperazine nitramine both undergo up to 50% degradation at 50 ◦ C and pH 7, but not at lower temperatures or other pH values. Owing to absorbance at ∼340 nm, the nitrosamines degrade rapidly in aqueous solution when exposed to sunlight (half-lives in the range 6–11 min), whilst nitramines are photolytically stable. Light screening by natural organic matter (1–100 mg/L) lead to an ∼3fold decrease in nitrosodiethanolamine (NDELA; 1 mg/L) degradation rate. A theoretical study indicates environmentally relevant concentrations of NDELA may be persistent due to competition with NOM for photons. The main photodegradation products are identified for each nitrosamine, and degradation mechanisms suggested. © 2014 Elsevier Ltd. All rights reserved.

1. Introduction In recent years, there has been development of carbon capture and storage (CCS) technologies that will limit CO2 release from large anthropogenic point sources such as fossil fuel fired power plants. Currently, amine solvent-based PCCC offers the most advanced approach in the near future (da Silva et al., 2013a; Rochelle, 2009; da Silva and Booth, 2013), with 30% monoethanolamine (MEA) in water the industry standard (Reynolds et al., 2012; Puxty et al., 2009). A number of alternative amines have been identified as being more efficient (e.g. piperazine) (Rochelle et al., 2011). As the realisation of full scale amine-based PCCC plants gets closer, there is a growing focus on the potential impact of their

∗ Corresponding author. Tel.: +47 93089510. E-mail addresses: [email protected] (L. Sørensen), [email protected] (K. Zahlsen), [email protected] (A. Hyldbakk), [email protected] (E.F.d. Silva), [email protected] (A.M. Booth). 1 Current address: Institute of Marine Research, Postboks 1870 Nordnes, 5817 Bergen, Norway. 2 Current address: Shell Technology Centre Amsterdam, Postbus 38000, 1030 BN Amsterdam, Netherlands. http://dx.doi.org/10.1016/j.ijggc.2014.11.004 1750-5836/© 2014 Elsevier Ltd. All rights reserved.

emissions (da Silva et al., 2013a). Among the possible and identified degradation products that may form in amine-based PCCC plants, nitramines (R2 NNO2 ) and nitrosamines (R2 NNO) are of particular concern due to their carcinogenicity (Fjellsbø et al., 2013; Låg et al., 2011; Richardson et al., 2007; Wollin and Dieter, 2005; Selin, 2011, and references therein). Several pathways of amine degradation within the PCCC unit have been identified, including reaction with SOx and NOx in the flue gas, which form a wide range of different degradation products (da Silva et al., 2013a; Rochelle, 2009; Reynolds et al., 2013). Whilst stable nitrosamines cannot be formed directly from primary amines (e.g. MEA), they can form from degradation products with secondary or tertiary amine functionalities (Reynolds et al., 2012; Masuda et al., 2000). Nitramines can form from primary, secondary or tertiary amines. Importantly, emissions of amine and degradation products to air have been observed (Reynolds et al., 2012; da Silva et al., 2013b; Veltman et al., 2010), but available data on nitrosamine and nitramine emissions from PCCC plants is limited. One study at a PCCC pilot plant in Maasvlakte, Netherlands, quantified the nitrosamines NDELA, NMOR and NDMA at concentrations ranging between 5 and 47 ng/N m3 dry gas (da Silva et al., 2013b). MEA-nitramine was also identified and present in similar concentrations to the nitrosamines.

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Nitrosamines and nitramines may also form in the atmosphere through reactions of amines with NOx , OH and Cl (Ge et al., 2011; Lee and Wexler, 2013; Nielsen et al., 2012). As a result, amines and other degradation products released from the PCCC plants may be transformed into nitrosamines and nitramines in the atmosphere. Nielsen et al. (2012), provide a detail review of atmospheric chemistry processes related to PCCC plant emissions. Nielsen et al. (2011) report that atmospheric degradation of MEA did not yield nitrosamines, but the nitramine form was observed; consistent for primary amines (Reynolds et al., 2012; Masuda et al., 2000). Nitrosamines are susceptible to atmospheric photodegradation and generally short-lived in the atmosphere (∼5 min), whilst nitramines are more stable having longer atmospheric residence times (2 days) (Maguta et al., 2014; Tuazon et al., 1984). The stability of nitramines indicates a higher potential for atmospheric accumulation than for nitrosamines. Nielsen et al. (2012), (and references therein) reviewed the atmospheric partitioning of amines, nitrosamines and nitramines. Amine compounds such as alkanolamines, piperazine and their derivatives typically exhibit small Henry’s Law constants, indicating a preferential partitioning to the atmospheric aqueous phase. In contrast, alkylamines and their derivatives will have limited partitioning to the atmospheric aqueous phase. The Henry’s Law constants for nitrosamines are typically an order of magnitude smaller than that of the corresponding amine, and whilst there is a lack of experimentally derived Henry’s Law data available for nitramines, it is expected they will typically exhibit values similar to that of the corresponding nitrosamine. Wet deposition is the most likely removal process to terrestrial and aquatic matrices (Knudsen et al., 2009; Karl et al., 2011). Photoreactions and hydrolytic stability of selected nitrosamines, in particular N-nitrosodimethylamine (NDMA), related to other sources (e.g. municipal wastewater effluent and reclaimed wastewater) have been studied previously under varying conditions (concentration, pH, reactants, irradiation intensity and wavelength cut-off range) (Chow et al., 1972; Grover et al., 1987; Lee et al., 2005a,b; Plumlee and Reinhard, 2007; Stefan and Bolton, 2002; Sørensen et al., 2013; Tate and Alexander, 1975; Williams et al., 2011; Horn and Nielsen, 2011; Chen et al., 2010; Herrmann and Weller, 2011; Xu et al., 2008). In general, studies have found nitrosamines and nitramines to be hydrolytically stable for >1 year under both laboratory (Williams et al., 2011; Horn and Nielsen, 2011) and natural conditions (Tate and Alexander, 1975). A number of studies have reported on the photolysis of a broad range of nitrosamine compounds in aqueous solution, although most work has focused on NDMA. Nielsen et al., provide a good summary of the available literature (Nielsen et al., 2012) (and refs therein). Briefly, most studies have reported rapid photolysis of nitrosamine compounds with quantum yields in the range ˚ = 0.28–0.61 and half-lives (t1/2 ) in the range 8–16 min depending on the study conditions (e.g. irradiation delivered, pH, analyte concentration) and the test compound (Lee et al., 2005a; Plumlee and Reinhard, 2007; Chen et al., 2010; Herrmann and Weller, 2011). The main photolysis products of NDMA and NDELA include methylamine, dimethylamine, nitrite, nitrate (Lee et al., 2005a; Plumlee and Reinhard, 2007; Xu et al., 2008). However, the aqueous phase processes following nitrosamine photo-excitation are still not clearly understood (Nielsen et al., 2012). Most nitrosamines are highly carcinogenic and, while less is known about nitramines, they appear mutagenic and carcinogenic although typically less potent than nitrosamines (Fjellsbø et al., 2013; Låg et al., 2011; Selin, 2011). Reviews of existing toxicity data and recommended exposure limits for nitrosamines and nitramines have recently been published, with most work focused on NDMA (Låg et al., 2011; Selin, 2011; Karl et al., 2011). The Norwegian Public Health Institute (NIPH) proposes acceptable exposure levels for all nitrosamines and nitramines of 4 ng/L for drinking

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water and 0.3 ng/m3 for air (Låg et al., 2011), based on a 10−6 lifetime risk of cancer following exposure to NDMA. The USA, Canada and several European countries have established, or are in the process of establishing, regulations for acceptable levels of NDMA in water (Selin, 2011). However, PCCC plants may potentially release complex mixtures containing many nitrosamine and nitramine compounds. Therefore, detailed knowledge about the environmental fate and effects of these compound classes, especially nitramines, is required to be able to assess risks accurately (da Silva and Booth, 2013). The current study investigates the hydrolytic and photolytic degradation of a suite of nitrosamines and nitramines in freshwater environments. All the selected test chemicals have previously been identified or theorised as PCCC process degradation and emission products of the amines MEA and piperazine (da Silva et al., 2013b). Specific emphasis is put on understanding NDELA photodegradation, as this has been identified as one of the primary nitrosamines formed in MEA-based PCCC plants (da Silva et al., 2013b) and is highly water soluble due to its hydroxyl functionality. Relevant conditions for natural waters were used in all tests (neutral pH and exposure to wavelengths corresponding to natural sunlight). The analyte concentrations selected, whilst higher than those expected in the environment, allowed for quantification and identification of degradation products. Hydrolytic stability was monitored to confirm that this process did not contribute to any observed degradation in the photolysis studies. The photolytic half-lives of the selected compounds are estimated from UV absorption and determined experimentally following exposure to synthetic sunlight. The formation of photodegradation products was also investigated and key compounds identified and quantified. The influence of dissolved natural organic matter (NOM) on the rate of photolysis is also investigated. 2. Experimental 2.1. Chemicals and materials All chemicals were of analytical grade, and MilliQ water was from a MilliPore system. Nitrosamines and nitramines, including deuterated analogues, were supplied by Chiron AS, Trondheim (Table 1). Stock solutions of each compound were prepared (200 mg/L) in sterilised MilliQ water and kept frozen until use. Potassium hydrogen phthalate (Merck), monopotassium phosphate (Merck) and boric acid (Merck) were used to prepare required buffer solutions. Working solutions were prepared daily by diluting the stock solutions into buffer solutions of pH 4 (potassium hydrogen phthalate buffer), pH 7 (monopotassium phosphate buffer) or pH 9 (boric acid buffer). A detailed description of the preparation of buffer solutions is provided in Supplementary information (Table S1). The buffers were sterilised by autoclaving for 20 min at 121 ◦ C before use in experiments. For the experiments investigating the influence of natural organic matter (NOM) on photolysis rates, Suwannee River NOM (SR-NOM, IHSS, St. Paul, USA) was dissolved in buffer pH 7 at an initial concentration of 100 mg/L. Particulates were then removed by filtering at 0.2 ␮m (Nalgene) and a dilution series was made in buffer pH 7. The final SR-NOM total organic carbon (TOC) concentration was determined using a Sievers 900 Portable Turbo instrument. 2.2. Hydrolysis study To ensure that the nitrosamine and nitramine compounds were stable in water at the selected pH-range used in the photolysis studies (pH 4 to 9), a two-tiered study looking at the hydrolytic stability of all selected nitrosamines and nitramines was undertaken

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Table 1 Summary of the nitramines and nitrosamines, including deuterated analogues, selected for use in this study. Chemicals Nitramines 2-Nitroaminoethanol Dimethyl nitramine Methyl nitramine 1-Nitropiperazine Nitrosamines n-Nitrosodimethylamine n-Nitrosodiethanolamine n-Nitrosomorpholine n-Nitrosopiperazine Dinitrosopiperazine

Abbreviation

CAS No.

Deuterated analogue for quantification

MEA-NO2 DMNA MNA Pz-NO2

74386-82-6 4164-28-7 598-57-2 42499-41-2

MEA-NO2 -d4

NDMA NDELA NMOR NPz DNPz

62-75-9 1116-54-7 59-89-2 5632-47-3 140-79-4

NDMA-d6

according to OECD Guideline 111 ‘Hydrolysis as a Function of pH’. Full details of the experimental method are provided in the Supplementary information. Briefly, in the Tier 1 test the compounds were dissolved in sterile, aqueous buffer solutions (pH 4, 7 and 9) at concentrations ranging between 10 and 100 ␮g/L. Samples were then incubated at 50 ◦ C for 5 days with samples collected at Day 0 and Day 5. Analytes exhibiting degradation in Tier 1 were subjected to the Tier 2 test. Here, samples were prepared as in Tier 1, and incubated at 10 ◦ C, 25 ◦ C and 50 ◦ C for a period of 30 days. Samples were collected for analysis after 0, 2, 7, 14, 21 and 30 days. In both tests, depletion of the target analytes was determined and quantified by liquid chromatography mass spectrometry analysis of the solutions.

2.3. Photodegradation study The photolytic stability of the suite of nitrosamine and nitramine compounds was investigated according to OECD Guideline 316 ‘Phototransformation of Chemicals in Water—Direct Photolysis’. In a Tier 1 test, sample solutions of each test chemical were prepared at a concentration of 100 mg/L (∼10−3 mol/L) by diluting stock solutions (200 mg/L) into a buffer solution at pH 7 (monopotassium phosphate). Two aliquots of each sample solution were then transferred to separate Quartz SUPRASIL® cuvettes (Hellma Analytics 100-QS, 10 mm path length). The UV–vis absorption spectra of the sample solutions were measured using a Hitachi U-2000 UV/VIS Spectrophotometer, scanning the wavelength range 250–800 nm. Duplicates of each compound were each analysed twice, and an average absorption value was calculated at each wavelength. Theoretical degradation rates were estimated based on molar absorption coefficients and daily radiation coefficients for a given latitude (50 ◦ ) as described in Supporting information. Based on their potential for photolysis, determined by their absorption peak within the solar wavelength range, the nitrosamines (NDMA, NDELA, NMOR and NPz) were selected for the full Tier 2 experimental study to determine degradation rates and identify degradation products. To allow analysis of both degradation rates and products, nitrosamine solutions of ∼1 mg/L (in buffer of pH 7, pH 9 for NPz) were irradiated using an Atlas Suntest CPS+ photosimulator instrument (Cromocol Scandinavia AB, Sweden) equipped with a 1.1 kV Xenon arc lamp and a natural daylight filter which blocks transmission of wavelengths below 290 nm, simulating the natural solar range (290–800 nm). The instrument was set to control the irradiation intensity between 300 and 400 nm to 60 W/m2 , which simulates the average annual radiation at approximately 50◦ latitude. This value corresponds to a total delivered irradiance of ∼575 W/m2 across the whole natural solar range (290–800). The exposure chamber temperature was controlled to 20 ± 2 ◦ C by external cooling. Nitrosamine samples (35 mL) were kept in capped quartz tubes (length 20 cm, diameter 1.5 cm; Quartz Scientific Inc., Fairport, USA) and exposed until 100% depletion of

Pz-NO2 -d6

NMOR-d8 NPz-d4 DNPz-d8

the analytes was observed (<2 h). The rate of photolytic degradation was determined by collecting samples at different time intervals (0, 10, 20, 30, 40, 50, 60 and 120 min). Each experiment was conducted in triplicate, including a triplicate set of dark control samples. To identify possible stable degradation products, samples from 0 min to 120 min were also analysed for the parent amine and nitramine corresponding to the individual nitrosamines tested. For one of the nitrosamines (n-nitrosodiethylamine, NDELA), the effect of light screening by natural organic matter (SR-NOM) present in the water was investigated by exposing triplicate samples of NDELA in buffer containing different concentrations of SR-NOM (1, 5, 10, 50 and 100 mg/L, corresponding to 0.4 to 42 mg/L total organic carbon, as measured by a Sievers 900 Portable Turbo TOC analyser). The samples were exposed for a total of 120 min with sampling intervals at 0, 10, 20, 40, 60 and 120 min. For each of the NOM concentrations used in the study, the absorption spectra (290–800 nm) of the water matrix (buffered NOM solution) was determined using UV–vis spectrometry and the theoretical degradation rate of an environmentally relevant concentration of NDELA (<10−4 M) under these conditions determined.

2.4. Chemical analysis All samples were analysed for the relevant nitrosamines, nitramines and amines by direct injection on an Agilent 1290 LC coupled with an Agilent 6490 QqQ MS system. The analytes were separated by reverse phase chromatography on various columns and mobile phases (See Table S2 in Supporting information). The mobile phases were based on methanol or acetonitrile solvents, and formic acid or ammonium acetate buffers. The analytes were ionised with APCI or jet-stream ESI. In the case of MNA, derivatisation was required prior to analysis. The derivatisation was done for both samples and standards using the reagent dibutylamine (DBA). All analytes were detected at their optimal transitions with target and qualifier fragment ions. Retention times were within the range of 1 to 10 min. The limits of quantification (LOQ) were within the range of 0.1 to 1 ng/mL. For all analytes deuterated internal standards were used. The precision (repeatability) of analysis was better than 5% RSD for all analytes. Analytical conditions for each analyte are summarised in Supplementary Information (Table S2). Where analytes were present in a mix, or degradation products were suspected, the samples were analysed by optimised methods for each analyte.

2.5. Nitrosamine and nitramine safety Nitrosamines are highly carcinogenic, and the utmost precaution must be taken when handling them. Necessary health and safety precautions were followed when handling all carcinogenic materials during the study. Concentrated nitrosamine and

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Fig. 1. Molar extinction coefficients for nitrosamines and nitramines (dotted lines) in the solar wavelength range (290–800 nm). Spectra recorded at pH 7 for all compounds.

nitramine samples were stored either in a vented hood or in a refrigerator. 3. Results 3.1. Hydrolytic stability The results from the Tier 1 hydrolytic stability test are presented graphically in the Supplementary information (Fig. S1). Briefly, the study revealed that nitrosamines and nitramines are generally resistant to hydrolytic degradation in the pH-range 4–9. However, the nitrosamine and nitramine derivatives of piperazine (NPz and Pz-NO2 ) both exhibited degradation at pH 7 and 50 ◦ C, which according to OECD guideline 111 corresponds to an approximate depletion of 20 and 30%, respectively, over one year at 25 ◦ C. Both NPz and Pz-NO2 contain an amine group in their structure a feature absent from the other test chemicals. It is generally considered that nitrosamines and nitramines are not susceptible to hydrolysis (da Silva et al., 2013a), but other structural properties might influence this resistance. To investigate the sensitivity of the observed degradation process to pH, degradation of NPz and Pz-NO2 was also tested in MilliQ water at pH 6.1 at 50 ◦ C (data not shown). Under these conditions neither compound exhibited any degradation after 5 days. Importantly, dinitrosopiperazine (DNPz) which does not contain an amine functionality, did not degrade under any of the pH conditions tested. This indicates that the degradation observed is related to the presence of the secondary amine group and is pH sensitive. An extended 30 day study was conducted for both NPz and Pz-NO2 at three different temperatures (10, 25 and 50 ◦ C). The results of this Tier 2 study are presented graphically in the Supplementary information (Fig. S2). No depletion was evident at the two lower temperatures (10 and 25 ◦ C), but at 50 ◦ C, a steady decrease of both compounds was observed until approximately 50% remained at day 30. 3.2. Potential for photolytic degradation Results from an initial screening using UV–vis spectrometry (previously reported in Sørensen et al., 2013) show that nitrosamines absorb radiation with an absorbance peak at approximately 340 nm while nitramines does not have an absorbance peak for the sunlight range (Fig. 1). Absorption peaks for nitrosamines (∼230 nm and ∼330 nm) have also been reported by others (Chow et al., 1972; Lee et al., 2005b; Plumlee and Reinhard, 2007; Stefan and Bolton, 2002). As the nitrosamines exhibit an absorbance peak at ∼340 nm an estimation of their maximum possible direct photolysis rate constant and corresponding t1/2 can be calculated

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Fig. 2. Photolytic decay (expressed as nitrosamine concentration in ␮g/L) as a function of exposure time (minutes) for NPz, NDMA, NDELA and NMOR. The figure demonstrates the validity of first-order kinetics fitting for NDMA, NDELA and NMOR but not for NPz under the studied conditions; pH 7 (9 for NPz), ∼10−5 M solutions and irradiation at 60 W/m2 (controlled at 300–400 nm). Error bars represent standard deviation (n = 3).

(Sørensen et al., 2013). The t1/2 of the test compounds under summer and winter conditions at 50◦ latitude are presented in Table 2. The t1/2 are estimated to be less than 20 min under summer conditions and less than 2 h under winter conditions. These values are relevant for the surface layers of natural freshwaters receiving standard sunlight levels. 3.3. Photodegradation rates for nitrosamines A detailed investigation to determine more accurate photolytic degradation rates for the nitrosamines was conducted according to the OECD Guideline 316 Tier 2 test. A summary of the experimental data is provided in Supplementary information (Table S3). In the current study, initial nitrosamine concentrations were ∼1 mg/L (10−5 M) and first-order photodegradation rate kinetics were observed for all compounds (except NPz) at pH 7 and irradiation at 60 W/m2 (controlled at 300–400 nm) (Fig. 2). NPz studies were conducted at pH 9 (to avoid any contribution from hydrolysis) and this compound appears to undergo approximately zero order degradation rate kinetics. Individual photodecay plots, together with non-linear regression of direct photolysis rate constants are presented in Supplementary information (Fig. S3). The t1/2 values were derived through the first order direct photolysis rate constants determined for each compound using equations (1) and (2) in Supplementary Information. The experimentally determined t1/2 for the test nitrosamines are presented in Table 2 whilst a complete summary of t1/2 and direct photolysis rate constants are presented in Supplementary information (Table S4). 3.4. Influence of natural organic matter on photodegradation In order to gain a more relevant insight in the photolytic degradation of nitrosamines in surface waters, the effect of light screening by SR-NOM was assessed using NDELA as a representative compound. The results show that a linear relationship exists between increasing SR-NOM concentration in the water and the t1/2 of NDELA (Fig. 3A). Increasing the NOM concentration from 1 mg/L to 100 mg/L gave a three-fold increase in the t1/2 of NDELA. The calculated rate constants and t1/2 for NDELA in each SR-NOM concentration are presented in Supplementary information (Table S4). Whilst the results give a clear indication that increasing amounts of dissolved NOM decreases nitrosamine degradation rates, experimental conditions might underestimate the

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Table 2 Experimentally (current study) and theoretically determined half-lives (t1/2 ) (Sørensen et al., 2013). Experimental t1/2 values represent average sunlight conditions at 50◦ latitude. Estimated t1/2 values are presented for both summer and winter conditions at 50◦ latitude, and based on quantum yields (˚) of both 1 (maximum estimated t1/2 ; OECD 316) and 0.5 (expected experimental t1/2 ; average value determined from Plumlee and Reinhard, 2007). Exp t1/2 (min) Summer (˚ = 1)

Summer (˚ = 0.5)

0.5 0.2 .02 0.6

15 10 17 15

30 20 34 30

60 W/m NDMA NDELA NMOR NPz

7.5 6.4 6.1 10.6

± ± ± ±

Est t1/2 (min)

2

significance of shading for lower, environmentally relevant concentrations of nitrosamines (1–10 ng/L). At such concentrations, nitrosamines may not be able to compete with the NOM for available photons. A theoretical estimation of the influence of NOM shading at low nitrosamine concentrations can be calculated using the UV–vis absorption spectra of the NOM and nitrosamine. Fig. 3B shows how this theoretical approach can be used to estimate degradation rates of NDELA at an environmentally relevant concentration under the same NOM concentrations used in the experimental study (0–100 mg/L). The estimated NDELA half-lives are clearly longer than those determined experimentally ones using a relatively high concentration of NDELA. At an NOM concentration of 100 mg/L it is no longer possible to calculate the degradation. Whilst no substitute for experimental data, this approach offer a useful indication of the possible half-lives for nitrosamines under environmentally relevant concentrations.

3.5. Identified degradation products and degradation mechanisms Following the photodegradation studies, samples were analysed to determine the amount of parent amine, corresponding nitramine, MEA and methylamine (MA) formed for each of the nitrosamines investigated (Table 3). The major photolytic degradation product (amine nitrogen balance) could be identified for three of the four nitrosamine compounds. In the case of NDELA, a 95% conversion to the amine MEA (based on molar calculations) was observed, whilst for NDMA a 65% conversion was observed. MEA and MA were not major degradation products for either NMOR or NPz. In the case of NPz, the parent amine (piperazine) was the main degradation product with 55% conversion. A 10% conversion of NMOR to the parent amine (morpholine) was observed, but the main degradation product was not identified. The corresponding nitramine was not observed as a degradation product for any of the nitrosamines studied.

Winter (˚ = 1) 92 61 111 90

Winter (˚ = 0.5) 184 122 222 180

4. Discussion 4.1. Hydrolytic stability The results of this study are consistent with previous reports on the hydrolytic stability of nitrosamine and nitramines (Tate and Alexander, 1975; Williams et al., 2011; Horn and Nielsen, 2011; Chen et al., 2010). Williams et al. (2011), reported that a suite of 10 nitrosamines were found to be stable to hydrolysis during a five day exposure at 50 ◦ C at pH values 4, 7 and 9 and at concentrations of 100 ng/L and 10 ␮g/L. However, the authors reported high variability in their measured nitrosamine concentrations due to limitations with their analytical chemical methodology, and were unable to analyse all test chemicals. Tate and Alexander (1975) found that N-nitroso dipropylamine (NDPA) did not degrade in lake water incubated at 30 ◦ C for 108 days. Horn and Nielsen recently reported that DMNA and MEA-NO2 had hydrolysis lifetimes in pure water and salt water of >1 year and >4 years, respectively, and was independent of pH in the range 5–9 (Horn and Nielsen, 2011). Chen et al. (2010), concluded from dark control samples in their photolysis studies that other degradative processes (e.g. hydrolysis) were not occurring. Whilst most nitrosamine and nitramine compounds did not undergo hydrolysis, NPz and Pz-NO2 both exhibited degradation, but only at pH 7 and 50 ◦ C. This indicates the process is linked to the presence of an amine group and is dependent on both pH and temperature. Conditions required for degradation to occur are not typical of those encountered in the environment or in the laboratory. The temperature dependence indicates that thermal degradation may be occurring and that it is the amine group on the two molecules which is susceptible. Thermal degradation of NPz was investigated in a previous study where CO2 concentration and analyte concentration were found to influence the degradative process, although degradation only occurs at temperatures above 100 ◦ C (Fine and Rochelle, 2013). The pH dependence of thermal decomposition of nitrosamines have been previously reported (Fan and Tannenbaum, 1972), but a more recent study did not observe

Fig. 3. Influence of light screening on the t1/2 of NDELA (min) determined (A) experimentally by increasing concentrations (0–100 mg/L) of natural organic matter (SR-NOM) in water at an NDELA concentration of 1 mg/L, and (B) theoretically by calculations based on molar extinction coefficients of NOM (0–100 mg/L) and NDELA. In (A) error bars represent standard deviation (n = 3).

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Table 3 Identified degradation products of the studied nitrosamines. Nitrosamine

MEA (molar %)

MA (molar %)

Parent amine (molar %)

Corresponding nitramine (molar %)

NDMA NDELA NMOR NPz


∼65


n.d. not determined.

significant pH dependence for the thermal decomposition of NPz (Fine et al., 2014). Unfortunately, one of the compounds Williams et al. (2011), were unable to measure analytically in their study was NPz, as it would have been interesting to see if they observed a similar degradation to that in the current study. Based on the data generated in this study, it was decided that the piperazine-derived compounds (NPz and Pz-NO2 ) would be held in buffer solutions at pH 9 to ensure no complications with the photolytic degradation studies. 4.2. Potential for photolytic degradation Natural sunlight emits radiation in the wavelength range 290–800 nm, and in our study we observe that only nitrosamines and not nitramines have an absorption peak in this range (∼330–340 nm; Fig. 1). Our data show that the value of the molar absorption peak for nitrosamines only varies slightly with structural features of the compounds. However, a shift towards higher wavelengths in the peak position is observed for the secondary nitrosamines (NDELA, NPz and NMOR) compared to NDMA. The common peak for nitrosamines and nitramines at the lower wavelength (∼230 nm) is explained by a ␲–>␲* transition in the NOx group of the molecule. The absorption peak at 330–340 nm, which is specific to the nitrosamines, is explained by an n–>␲* transition by electron excitation from an oxygen lone-pair to an anti-bonding ␲ orbital in the N O group (Plumlee and Reinhard, 2007; Chow et al., 1972; Ditchfield et al., 1972). Nitramines do not have the chromophoric and unsaturated N O group, but rather a NO2 group which makes the n–>␲* impossible. As natural sunlight emits radiation in the wavelength range 290–800 nm, only the nitrosamines and not the nitramines will degrade photolytically when released to the environment. Whilst there are no reports of the photodegradability of PCCC-related nitramines in aqueous matrices, the preliminary results in this study are consistent with the observations of nitramine stability in the atmosphere (Nielsen et al., 2011). 4.3. Photodegradation rates for nitrosamines The conditions used in the current study simulate the average annual radiation at approximately 50◦ latitude, and the calculated half-lives (t1/2 ) are therefore comparable to the estimated halflives from the Tier 1 study (Table 2). The maximum estimated t1/2 are calculated according to OECD Guideline 316 by using a quantum yield (˚) of 1, whilst experimentally determined values indicate quantum yields for nitrosamines in the range 0.28–0.61 (Lee et al., 2005a; Plumlee and Reinhard, 2007; Herrmann and Weller, 2011). Plumlee and Reinhard (2007) investigated the photolysis of 7 alkyl nitrosamines and determined the quantum yield for each compound (˚ = 0.41–0.61; average 0.50) using a simulate sunlight irradiance of 765 W/m2 (300–800 nm) in Milli-Q water (pH 6) and analyte concentrations of 100 ␮g/L. Based on an average quantum yield of 0.5 for alkyl nitrosamines, the t1/2 values (˚ = 1) are underestimated by approximately 50% compared to those expected to be observed experimentally (Table 2). The experimentally determined t1/2 for the nitrosamines in the

current study ranged between 6.1 and 10.6 min, which is shorter than expected based on the estimated values (Table 2). However, this observation is typically consistent with experimental values determined in other studies (Lee et al., 2005a; Plumlee and Reinhard, 2007; Chen et al., 2010; Herrmann and Weller, 2011) which also reported shorter experimental t1/2 values than the maximum estimated t1/2 values. This indicates that one or more additional loss processes (e.g. radical chemistry) are occurring which increase the photolysis rate of nitrosamine compounds. However, the estimated lamp output does not take into account the configuration of exposure vessels and their impact on processes such as light scattering and reflection. The exposure vessel could therefore be contributing to the discrepancies observed between predicted and experimental values. The use of chemical actinometers may help to account for this influence. Whilst Tier 1 estimated t1/2 values can be a useful guide for predicting nitrosamine photodegradation rates, they appear to be generate t1/2 values which are consistently longer than those determined experimentally in this and other studies. The t1/2 values generated in this study (6–11 min) are generally consistent, although slightly shorter than those observed in previous studies of nitrosamine photolysis, which report t1/2 values in the range 8–16 min depending on the study conditions (e.g. irradiation delivered, pH, nitrosamine concentration) and the test compound (Lee et al., 2005a; Plumlee and Reinhard, 2007; Chen et al., 2010; Herrmann and Weller, 2011). The current study employed experimental conditions most similar to those reported by Plumlee and Reinhard (2007). A previous study has shown that that sunlight intensity (actinic flux) plays an important role in the degradation kinetics of these compounds (Chen et al., 2010). Plumlee and Reinhard (2007) used an irradiance 765 W/m2 (300–800 nm) to generate nitrosamine t1/2 values of 12–16 min. The current study used an irradiance of 60 W/m2 (equivalent to 575 W/m2 over 300–800 nm) suggesting longer t1/2 values could be expected. However, this was not the case and indicates that other parameters must also be influencing the photolysis rate. In addition to the different irradiance levels, a number of other exposure conditions also differed between the two studies, including test compound concentration, pH, the use of a water bath and the use of buffers to control pH. The photodegradation kinetics of NDMA is dependent on concentration, with first-order kinetics dominating at lower concentrations (<10−5 M) (Stefan and Bolton, 2002). In the current study, exposure concentrations of ∼10−5 M (1 mg/L) were used, which is on the border between first order and second order kinetics for NDMA. In the study of Plumlee and Reinhard (2007), exposure concentrations an order of magnitude lower were employed. Photolysis of nitrosamines has also been shown to proceed more quickly under acidic conditions compared to neutral conditions (Stefan and Bolton, 2002), however there appears to be little influence from pH within the neutral pH range (e.g. pH 6–8) and acidic pH range (e.g. pH 1–4.5) (Plumlee and Reinhard, 2007; Herrmann and Weller, 2011). Plumlee and Reinhard conducted their experiments at pH 6 whilst pH 7 was used in the current study. Therefore, it appears that neither nitrosamine concentration nor pH of the exposure solution are responsible for the observed differences in photolysis rate.

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A water bath was used in the Plumlee and Reinhard study to control temperature, whilst in the current study temperature was maintained by air cooling. It is therefore possible that the presence of a water layer above the exposure samples is reducing the irradiance intensity in the Plumlee and Reinhard study. It has also been reported that organic components and nitrate/nitrite in water can act as competing absorbers, reducing the photolysis efficiency (Stefan and Bolton, 2002; Hutchings et al., 2010). In the current study, monopotassium phosphate used in the preparation of pH 7 exposure solutions absorbs in the UV region and may therefore have a role in the measured photolysis rate of the nitrosamine compounds. However, the shorter t1/2 values in this study compared to those of Plumlee and Reinhard who did not buffer the pH of their exposure solutions, indicate that the monopotassium phosphate is not acting as a significant absorber. It is possible that the presence of the monopotassium phosphate is contributing to the enhanced photolysis rate observed in the current study. It is therefore suggested that the use of monopotassium phosphate buffered exposure solutions and the absence of a water bath might be increasing nitrosamine photolysis rates compared to those reported by Plumlee and Reinhard (2007). Directly extrapolating the degradation rates determined in the laboratory to surface waters is challenging given the differences in optical path length. With increasing depth in the water column the degradation rate will be expected to decrease due to light filtering. However, this study provides a good indication that nitrosamines have the potential for rapid degradation in the upper reaches of the photic zone of natural waters. The neutral pH value (7) and temperature (20 ◦ C) used in the current study are considered environmentally realistic, though it could be expected that the photodegradation process would proceed more rapidly where lower pH values are encountered in the environment. Given the results by Stefan and Bolton (2002) showing that nitrosamine degradation at concentrations below 10−5 M obey first-order kinetics, it is believed that the degradation rates obtained in the current study are also representative for lower (and more environmentally realistic) concentrations. However, it should be noted that the concentration used appears to be on the limit between first order and zero order kinetics for NDMA (Stefan and Bolton, 2002). Importantly, the data determined in this study support the evidence from other studies that nitrosamines present in surface waters exposed to sunlight will rapidly degrade. 4.4. Influence of natural organic matter on photodegradation In the environment, surface waters often contain relatively high concentrations of particulate and dissolved natural organic matter (NOM) whilst oxygen content can vary significantly. The concentration of dissolved organic carbon in natural waters typically ranges from 0.5 to 50 mg/L (Urrestarazu Ramos et al., 1998), corresponding to a NOM concentration range of 1 to 100 mg/L. The standard OECD Guideline 316, does not take into account the influence that NOM concentration can have on photodegradation rates. In order to gain a more accurate insight in the photolytic degradation of nitrosamines in surface waters, the effect of light screening by SRNOM was assessed using NDELA as a representative compound. NDELA was also selected over NDMA as comparative data for NDMA has been previously reported (Plumlee and Reinhard, 2007). In an emission study completed at the MEA operated PCCC pilot plant in Maasvlakte, Netherlands, NDELA was the identified nitrosamine present in the highest concentration (47 ng/N m3 dry gas) (da Silva et al., 2013b). In their study, Plumlee and Reinhard (2007) also found that increasing concentrations of Aldrich humic acid (measured in mg DOC/L), significantly decreased the photolysis rate of NDMA (due to light screening). They also compared the decrease in photodecay rate to predicted photodecay rates determined based

Fig. 4. Proposed mechanism for the observed degradation of NDELA to MEA (∼95 mol%), similar to proposals for NDMA by Lee et al. (2005a) and Stefan and Bolton (2002).

on the measured light screening factor for each DOC solution, finding good agreement and indicating indirect photolysis does not increase photodegradation rates (Plumlee and Reinhard, 2007). Previous studies (Plumlee and Reinhard, 2007; Stefan and Bolton, 2002) have also shown that the oxygen content of the water has no significant effect on the photolysis rate of NDMA. Furthermore, no significant oxygen consumption is observed during nitrosamine photodegradation experiments, indicating that the availability of oxygen (dependent on energy and depth of water column) is not a limiting factor (Stefan and Bolton, 2002). The results from this study and that of NDMA by Plumlee and Reinhard (2007) clearly indicate that nitrosamine t1/2 will be significantly influenced by the concentration of NOM present in surface waters, with high NOM concentrations leading to longer residence times. However, even under conditions considered to represent the highest NOM concentrations expected to be observed in freshwater environments (100 mg/L), the experimental t1/2 of NDELA was only ∼17 min in the current study. In theory this could represent a very short residence time, indicating nitrosamines would be persistent in natural surface waters. However, it is important to consider that the waters will constantly be replenished with more nitrosamine from emissions. As a result the final environmental concentration will depend upon the balance between decay and nitrosamine emissions. One study has suggested that this may could lead to a steady an expected steady state nitrosamine concentration in the water (de Koeijer et al., 2013). At the relatively high concentration used in the current study (∼1 mg/L), NDELA appears able to compete effectively with NOM for photons (Fig. 3A). However, environmental concentrations of nitrosamines are predicted to be in the range 1–10 ng/L. At these low concentrations NOM might outcompete nitrosamines for photons, effectively reducing their degradation rate. The t1/2 of NDELA at an environmentally relevant concentration in increasing NOM concentrations (0–100 mg/L) was estimated theoretically using the absorption spectra of background water matrix (NOM) and the molar extinction coefficient of NDELA. At low NDELA concentrations, NOM appears to significantly reduce NDELA t1/2 (Fig. 3B). Degradation rates were significantly hindered at a NOM concentration of 10 mg/L and become negligible at a NOM concentration of 100 mg/L NOM (under summer conditions). This theoretical approach, whilst limited compared to experimentally determined values, indicates that environmentally relevant concentrations of nitrosamines may persist in natural waters, especially where the NOM concentration is quite high. 4.5. Identified degradation products and degradation mechanisms One proposed mechanism for the direct photolytic transformation of NDELA is given in Fig. 4, and is based on the degradation

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Fig. 5. Proposed mechanism for the observed degradation of NDELA to MEA (∼95 mol%), based on Mezyk et al. (2004).

mechanisms of NDMA suggested by Lee et al. (2005a), and Stefan and Bolton (2002) where partial transformation (65 molar %) to methylamine (MA) was observed at neutral pH and relatively low concentrations (∼10−4 M). A total amine balance also showed that DMA and MA were the only amine degradation products of NDMA, with DMA becoming the dominant degradation product at higher concentrations of NDMA (10−2 M) or lower pH (3) (Lee et al., 2005a). It is possible that the proposed HNO formed during both these reactions may further react to form N2 O. Alternative mechanisms for aqueous degradation of NDMA, based on hydroxyl radical and hydrated electron initiated reactions have been proposed by Mezyk et al. (2004,2006), and appear to be common to most nitrosamine compounds. Hydroxyl radical initiated degradation leads to carbon-centred radical formation whilst hydrated electron initiated degradation leads to radical formation on the nitroso group. Unlike the hydroxyl radical mechanism, the hydrated electron reaction rate appears to be independent of alkyl substitution. Furthermore, the hydrated electron reduction rate of nitrosamines is much faster than the oxidation rate by hydroxyl radicals. The process forms an electron adduct, which might be capable of transferring the extra electron to another compound present in the water, thus regenerating the parent nitrosamine species. This back reaction could (at least partially) account for the observed quantum yields below 1 observed for alkyl nitrosamines, and may be the more likely of the two mechanisms proposed. This mechanism proposed by Mezyk et al. (2004) for NDMA has been modified for NDELA and is shown in Fig. 5. Plumlee and Reinhard (2007) also quantified products of NDMA photolysis, identifying MA, DMA, nitrite, nitrate, and formate as the main degradation products contributing to nitrogen and carbon balances exceeding 98 and 79%, respectively. It was suggested that formaldehyde could account for the remaining carbon. This is consistent with the mechanisms proposed in Figs. 4 and 5, which both suggest formaldehyde as a major degradation product for NDMA, and subsequently glycoaldehyde for NDELA. The study by Lee et al. suggests that DMA, the parent amine of NDMA, should also be observed as a degradation product under the conditions employed in the current study (∼35%). However, chemical analysis did not identify the formation of DMA in this study (Table 3). This suggests that one or more other degradation products are also formed, and it is suggested these would comprise, nitrite, nitrate, formate and formaldehyde as reported by Plumlee and Reinhard (2007). Stefan and Bolton (2002) showed that while UV treatment of NDMA gave formation of DMA, DMA did not degrade further due

to its lack of chromophores. This rules out the possibility of DMA being formed and further photodegraded during the experiments. In another study, Lee et al. (2005b) describe the formation of Nmethylformamide (NMF) at pH > 4 and under N2 saturated water conditions (low oxygen content). Both NPz and NMOR show photodegradation into the parent amines (piperazine and morpholine, respectively), but at very different yields. In the case of NMOR, only 10% (molar) formation of the amine is observed. No other degradation products have been identified in the current study. NPz shows 55% conversion into piperazine, but again no other significant degradation product was observed for this compound. The results indicate the presence of the alicyclic ring in NMOR and NPz results in a different degradation pathway to that of the alkyl nitrosamines (e.g. NDMA and NDELA). Furthermore, only NPz appeared to form the corresponding nitramine (Pz-NO2 ), and then only at a molarity corresponding to <1%. This indicates that nitramine formation is not a primary photodegradation mechanism for nitrosamines as any nitramine formed would be resistant to further photodegradation and should accumulate. Importantly, this study indicates that carcinogenic nitrosamine compounds can potentially be photodegraded rapidly in aquatic environments. Furthermore, the photodegradation products of all nitrosamines studied appear to include amines (to a greater or lesser extent depending on the individual nitrosamine), which pose much less of a problem from an ecotoxicological and toxicological perspective (Låg et al., 2011). 5. Conclusion Both nitrosamines and nitramines have been identified in the emissions from pilot PCCC plant studies showing there is a direct route into the environment. Whilst information about their environmental fate and effects can be inferred from previous studies into other related nitrosamines (and to a lesser extent nitramines), this study provides data on compounds specifically linked to PCCC plant operation and emissions. In aquatic systems, nitrosamines degrade rapidly by photolysis under natural sunlight although the degradation rate can be significantly impacted by normal environmental concentrations of NOM. At environmentally relevant concentrations, theoretical approaches indicate that nitrosamines may persist for long periods. It is also important to consider that degradation will decrease with increasing depth in the water column and be limited when nitrosamines are rapidly transported to environmental compartments where there is little or no light

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penetration (e.g. deeper waters and groundwater). The release of nitrosamines at night or in parts of the world where there are long periods of the year with no daylight can also decrease the importance of this degradation pathway. Although nitramines exhibit resistance towards photodegradation, the potential concern regarding their accumulation in environmental waters appears to be unfounded as they are formed in sufficiently low quantities and disperse quickly enough that they will most likely reach environmental concentrations significantly below the NIPH limits (de Koeijer et al., 2013). Whilst a variety of amine compounds can be formed through nitrosamine photodegradation, they appear to be of less concern than the nitrosamines or nitramines from a toxicological perspective. In order to establish a complete overview of nitramine and nitrosamine fate in terrestrial and aquatic environments, other relevant processes such as adsorption to soil and biodegradation should also be investigated in more detail. Acknowledgements The SOLVfate project has received financial support from Gassnova and the Research Council of Norway (throught the CLIMIT Programme; Grant Agreement number 203095), Mitsubishi Heavy Industries and ENEL. The authors wish to acknowledge Kai Vernstad of SINTEF Biotechnology and Nanomedicine for his work on the development of analytical methods for nitrosamines and nitramines. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.ijggc.2014.11.004. References Chen, B., et al., 2010. Solar photolysis kinetics of disinfection byproducts. Water Res. 44 (11), 3401–3409. Chow, Y.L., et al., 1972. Photoreactions of nitroso compounds in solution. XX. Photoreduction, photoelimination, and photoaddition of nitrosamines. Can. J. Chem. 50, 1044–1050. da Silva, E.F., Booth, A.M., 2013. Emissions from postcombustion CO2 capture plants. Environ. Sci. Technol. 47 (2), 659–660. da Silva, E.F., Hoff, K.A., Booth, A., 2013a. Emissions from CO2 capture plants; an overview. Energy Procedia 37 (0), 784–790. da Silva, E.F., et al., 2013b. Emission studies from a CO2 capture pilot plant. Energy Procedia 37, 778–783. de Koeijer, G., et al., 2013. Health risk analysis for emissions to air from CO2 Technology Centre Mongstad. Int. J. Greenhouse Gas Control 18 (0), 200–207. Ditchfield, R., Del Bene, J.E., Pople, J.A., 1972. Molecular orbital theory of the electronic structure of organic compounds. IX. A study of n–␲* transition energies in small molecules. J. Am. Chem. Soc. 94 (3), 703–707. Fan, T.Y., Tannenbaum, S.R., 1972. Stability of N-nitroso compounds. J. Food Sci. 37 (2), 274–276. Fine, N.A., Rochelle, G.T., 2013. Thermal Decomposition of N-nitrosopiperazine. Energy Procedia 37 (0), 1678–1686. Fine, N.A., Nielsen, P.T., Rochelle, G.T., 2014. Decomposition of nitrosamines in CO2 capture by aqueous piperazine or monoethanolamine. Environ. Sci. Technol. 48 (10), 5996–6002. Fjellsbø, L.M., et al., 2013. Screening for potential hazard effects from four nitramines on human eye and skin. Toxicol. in Vitro 27 (4), 1205–1210. Ge, X., Wexler, A.S., Clegg, S.L., 2011. Atmospheric amines—Part I. A review. Atmos. Environ. 45 (3), 524–546. Grover, T.A., Ramseyer, J.A., Piette, L.H., 1987. Photolysis of nitrosamines and nitrosamides at neutral pH: a spin-trap study. Free Radical Biol. Med. 3 (1), 27–32. Herrmann, H., Weller, C., 2011. Atmospheric Chemistry—Aqueous Phase Chemistry. Tel-Tek, Porsgrunn, Norway. Horn, A., Nielsen, C.J., 2011. Hydrolysis of Nitramines—Final Report. University of Oslo, Oslo, pp. 9. Hutchings, J.W., et al., 2010. N-nitrosodimethylamine occurrence: formation and cycling in clouds and fogs. Environ. Sci. Technol. 44 (21), 8128–8133.

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