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Proceedings of the 10 th International Conference on Environmental Science and Technology Kos island, Greece, 5 7 September 2007 THE HAZARD OF N-NITROSAMINES FORMATION DURING SHORT CHAIN SECONDARY AMINES
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Proceedings of the 10 th International Conference on Environmental Science and Technology Kos island, Greece, 5 7 September 2007 THE HAZARD OF N-NITROSAMINES FORMATION DURING SHORT CHAIN SECONDARY AMINES (DMA, MEA AND DEA) REACTIONS WITH CATALYZED AND NON-CATALYZED HYDROGEN PEROXIDE P. ANDRZEJEWSKI 1 and N. KULIK 2 1 Adam Mickiewicz University, Faculty of Chemistry, Department of Water Treatment Technology, ul. Drzymaly 24, Poznan, Poland 2 Tallinn University of Technology, Department of Chemical Engineering, Ehitajate Tee 5, 19086, Tallinn, Estonia, EXTENDED ABSTRACT The aim of the paper is the evaluation of the possibility of N-nitrosodimethylamine (NDMA), N-nitrosomethylethylamine (NMEA) and N-nitrosodiethylamine (NDEA) formations as a result of dimethylamine (DMA), methylethylamine (MEA) and diethylamine (DEA), respectively, reactions with hydrogen peroxide and the Fenton reagent as well as influence of several parameters like ph, contact time and oxidant/amine ratio on N-nitrosamines formation. In 2002 Mitch et al. and Choi et al. reported that N-nitrosodimethylamine is formed during the disinfection of water or wastewater treatment plant effluents containing dimethylamine and ammonia with chlorine. Andrzejewski et al. proved that NDMA could be formed during reaction of DMA with chlorine dioxide or ozone. Experiments on dimethylamine, methylethylamine and diethylamine, respectively, reactions with hydrogen peroxide and the Fenton reagent were carried out in batch conditions at room temperature. Finally, in several experiments, the high purity water spiked with DMA was replaced with natural groundwater enriched with DMA. Water samples were withdrawn during the experiment and subsequently analyzed with HPLC-IE with UV-Vis detector at 230 nm in order to determine nitrosamines concentration as well as the main products of reactions. Formation of nitrosamines was also confirmed by GC-LRMS technique. The DMA concentration changes during reaction of DMA with hydrogen peroxide and the Fenton reagent were analyzed according GC-FID analytical procedure developed by Sacher et al. The aldehydes as well as nitrites and nitrates concentrations were also analyzed with GC-ECD and HPLC-IE-CD techniques respectively. N-nitrosamines are formed as a result of the reaction of non-catalyzed hydrogen peroxide with secondary amines only at ph higher than 11. The increase of reaction time as well as increase of H 2 O 2 /amine ratio results with significant increase of amine/nitrosamine conversion even to single percents. The formic acid was found as the main product of reaction of DMA with hydrogen peroxide; however DMA to formic acid molar conversion rate did not exceed 10%. When DMA was replaced with MEA or DEA, only NMEA formation was observed, however conversion rate of MEA to NMEA was significantly lower compared to results obtained for DMA. The mechanism of N-nitrosamines formation with H 2 O 2 is different to that described by Mitch and Choi. The presence of nitrites and nitrates in a post-reaction mixture suggests that NDMA is formed as result of DMA reaction with nitrites and the mechanism, probably, is similar to that observed previously for ClO 2 and ozone reaction with secondary amines. No oxidation of DMA by mean of the Fenton chemistry was observed. Both for the classical Fenton and Fenton-like oxidation no NDMA were found and no traces of formic acid were observed in post-reaction mixture. No changes in DMA concentration before and after Fenton oxidation of DMA were observed as well. Keywords: secondary nitrosamines, N-nitrosodimethylamine, NDMA, DBPs, hydrogen peroxide, Fenton reagent, water treatment. A-45 1. INTRODUCTION Secondary nitrosamines, mainly the short chain ones, like N-nitrosodimethylamine (NDMA), N-nitrosomethylethylamine (NMEA) and N-nitrosodiethylamine (NDEA) are highly mutagenic compounds that are suspected of carcinogenic activity to the human body. In 2002 Mitch et al. [1] and Choi et al. [2] reported that NDMA is formed during the disinfection of water or wastewater treatment plant effluents containing dimethylamine (DMA) and ammonia with chlorine. The mechanism proposed by Choi et al. [2] and Mitch et al. [1] is based on chlorination leading to the formation of 1,1-dimethylhydrazine, known as unsymmetrical dimethylhydrazine (UDMH). Furthermore, 1,1-dimethylhydrazine undergoes oxidation to yield many different by-products, including NDMA. The proposed mechanism was subsequently modified by Schreiber et al. The authors emphasized the role of dichloroamine and dissolved oxygen in NDMA formation due to the reaction of DMA with chlorine in the presence of ammonia ions [3]. In 2003 Choi et al. proposed other mechanism of NDMA formation during reaction of dimethylamine with chlorine. According proposed mechanism, HOCl reacts with nitrites presented in water to form very reactive nitrosating intermediate i.e. dinitrogen tetroxide (N 2 O 4 ), which subsequently reacts with dimethylamie to form NDMA [4]. Andrzejewski et al. [5] indicated the possibility of NMEA and NDEA formation as a result of respective amine reactions with chlorine in the presence of ammonia ions. During further investigations Andrzejewski et al. proved that NDMA could be formed during reaction of DMA with chlorine dioxide [6] or ozone [7]. The mechanism of NDMA formation with chlorine dioxide or ozone reaction with DMA is different from those postulated by Choi et al. [2] and Mitch et al. [1]. Based on nitrosamine formation pathway proposed by Keefer et al. [8], Andrzejewski et al. assume that NDMA, a product of ozone reaction with DMA, is formed as a result of a multi-step reaction. In the first stage formaldehyde and nitrites are formed, which subsequently react with DMA forming NDMA [7]. The compounds that contribute to the formation of NDMA are mainly: monochloramine and organic compounds containing nitrogen such as DMA or tertiary amines containing dimethyl functional groups. The other N-compounds such as proteins and aminoacids do not form significant concentrations of NDMA [9]. Gerecke et al. reported that natural organic matter, which is also a precursor of NDMA, has a significant influence on NDMA formation [10]. Hydrogen peroxide is a relatively strong oxidant and its efficient application for the oxidation of various inorganic and organic chemical compounds is well established. However, oxidation of certain refractory compounds is not always effective for hydrogen peroxide alone, due to low rates of reaction at reasonable hydrogen peroxide doses [11]. Mitch et al. preliminary investigated the hydrogen peroxide reactivity with DMA at ph=7 and hydrogen peroxide/dma molar ratio of 1:1, as compare test during their study on chlorine reactivity with DMA in presence of ammonia ions. No NDMA was found in post reaction mixture of DMA and hydrogen peroxide [1]. The improvement of hydrogen peroxide oxidative activity can be achieved by use of transition metal salts, ozone and ultraviolet radiation, which can activate hydrogen peroxide to form strong oxidants - hydroxyl radicals [11]. The activation of hydrogen peroxide by iron salts is usually referred as the Fenton chemistry. In the Fenton reagent ferrous ions cause the dissociation of the oxidant and the formation of hydroxyl radicals, which are short-lived reactive oxygen species with a high oxidation potential that can rapidly attack and destroy many organic compounds. Notably, to facilitate the generation of hydroxyl radicals a low ph range is preferred in the Fenton system, although the reaction is feasible up to neutral ph. Nowadays, the Fenton chemistry is successfully applied in treatment of industrial wastewater contaminated by various hazardous and recalcitrant organic pollutants, such as phenols, formaldehyde, BTEX, pesticides, wood preservatives and etc. A-46 The aim of the paper is the evaluation of the possibility of NDMA, NMEA and NDEA formations as a result of DMA, methylethylamine (MEA) and diethylamine (DEA), respectively, reactions with hydrogen peroxide and the Fenton reagent as well as influence of several parameters like ph, contact time and oxidant/amine ratio on N-nitrosamines formation. 2. MATERIALS AND METHODS 2.1. Materials and chemicals All chemicals used, obtained from Fluka, were of reagent grade or analytical grade when available, and were used without further purification. Stock solutions were prepared in high quality pure deionized water (Millipore). The reaction solutions were prepared by spiking of high quality pure deionized water (Millipore) or nearly iron-free ground water with DMA (40%). The removal of iron from the natural ground water was done by aeration and filtration through an active sand filter. As a result, the average concentration of Fe in the ground water did not exceed 0.05 mg L -1. In addition to DMA, MEA ( 97%) and DEA ( 99.7%) were also used in some experiments. The initial concentration of DMA in reaction solutions were from 5 to 400 mg L -1. In the case of MEA and DEA, the initial concentration was 100 mg L -1. Tert-Butyl alcohol (tbuoh, 99.8%) was used as a hydroxyl radical scavenger in some experiments. The pure water samples spiked with DMA, MEA and DEA were buffered, before oxidation, at desired ph by adding Na 2 HPO 4 and/or NaH 2 PO 4 aqueous solutions. The natural ground water samples spiked with respective secondary amine were treated without buffering of solution. The ph of reaction solutions was adjusted using H 3 PO 4 and NaOH aqueous solutions Experimental procedure Experiments on DMA, MEA and DEA oxidation by catalyzed and non-catalyzed hydrogen peroxide were carried out under batch conditions at ambient temperature (20±1ºC). 0.5 L of reaction solution were treated in glass reactors with permanent agitation for period of hours. The experiments of Fenton/Fenton-like oxidation were performed according following procedure. The catalyst (FeSO 4 7H 2 O) was added first and the reaction was subsequently initiated by adding hydrogen peroxide all at once. The molar ratio of H 2 O 2 /Fe 2+ was kept invariable at 10:1, which is the optimal ratio between hydrogen peroxide and ferrous ions [12]. For the Fenton oxidation experiments, ph of secondary amine solution samples was adjusted to 3. Fenton-like experiments were carried out at ph values from 5.5 to 11. Samples of oxidized solution were withdrawn at regular interval times and reaction was terminated by adding of sodium sulfite anhydrous (Na 2 SO 3, 98%). After that, the ph of samples was adjusted to 11 (were needed) by adding 10% aqueous solution of NaOH. The samples were kept overnight at 4 ºC to allow the settling of solids. Precipitated iron hydroxocomplexes were separated from samples by paper (pores size µm) filter. The experiments of secondary amines oxidation with non-catalyzed hydrogen peroxide were performed in the same oxidation conditions as Fenton/Fenton-like treatment trials. The ph of reaction solution ranged from 3 to 11. To investigate the reactivity of MEA and DEA with catalyzed and non-catalyzed hydrogen peroxide the experiments were conducted at conditions, which resulted in the maximum conversion of DMA into NDMA. A-47 2.3. ANALYTICAL METHODS Apparatus 1. Hewlett-Packard 5890 (HEWLETT-PACKARD) gas chromatograph coupled with lowresolution Hewlett-Packard 5971A mass selective detector (MSD). GC HP-1 fused silica capillary column (25 m x 0.25 mm i.d. x 1,0 μm) was used. GC parameters: Injection mode: Splitt-splitless, injector temperature: 170 o C Carrier gas: helium, pressure 37.7 kpa, flow 0.7 ml/min. and linear velocity 30.8 cm/sek Temperature program: 40 o C(4 min)-15 o C/min.-280 o C(5 min.) MS parameters: Resulting voltage V. Mass range: Solvent delay: 4.00 min Data processing system: MS Chem Station 2. Carlo Erba GC 6000 coupled with FID detector. GC J&W DB-5 fused silica capillary column (30 m x 0.32 mm i.d. x 0,32 μm) was used. GC parameters: Injection mode: splitless, carrier gas: helium with pressure 80 kpa, Temperature program: 120 o C(2 min)- 13 o C/min.-250 o C Data processing system: CSW (Czech Republic) 3. WATERS 2690 HPLC chromatograph coupled with WATERS 2487 UV-VIS detector, equipped with BIO-Rad Aminex HPX-87H ion-exclusion column (300 mm x 7.8 mm) was used. HPLC parameters: sulphuric acid solution at ph=1.25 as mobile phase with flow 0.75 ml/min Injection: 20 μl, wavelength: 230 nm, oven temperature: 35 o C Data processing system: CSW (Czech Republic). 4. Fisons 8000 gas chromatograph coupled with ECD detector (nitrogen as make-up gas was used), equipped with RTX-5MS (30 m x 0,25 mm x 0,25 μm) column, was used. The GC-ECD parameters: Injection mode: on-column, carrier gas: helium with pressure of 80 kpa. Temperature program: 80 o C(2min)-5 o C/min o C/min.-280 o C(3 min.) Data processing system: CSW (Czech Republic). 5. DIONEX ICS 2500 HPLC chromatograph coupled with CD detector ED 50A, equipped with IonPac AS19-HC (analytical column) and IonPac AG19-HC (guard column) (250 mm x 4.0 mm) was used. The HPLC parameters: KOH solution (10 mm) as mobile phase with flow rate of 1.0 ml min -1, injection volume of 80 μl Initial hydrogen peroxide concentration The initial hydrogen peroxide concentration in stock solutions was determined spectrophotometrically by measurement of the absorption of hydrogen peroxide in the ultraviolet region at 254 nm, using HACH DR/4000 U UV/VIS (Loveland, USA) NDMA analysis The main aim of the research was to verify whether NDMA, NMEA and NDEA are formed as a result of secondary amines oxidation by hydrogen peroxide and Fenton/Fenton-like system. Quantitative analysis was also carried out to assess the concentration of nitrosamines and other by-products formed. The necessity of both: resolving the problems with qualitative identification of reaction products as well as the necessity to analyze a high number of samples resulted in the application of two analytical methods GC-MS was the method of choice for the identification of NDMA, NMEA and NDEA, which are by-products of hydrogen peroxide/fenton chemistry reactions with secondary amines. Identification of nitrosamines was based on an identical retention times of the NDMA (5000 μg ml -1 in methanol, Fluka) standard and the compound in post-reaction mixture. The analytes from the post-reaction mixture were concentrated by means of liquid/liquid extraction with methylene chloride [1]. The extracts were analyzed with GC-MS technique. A-48 Apart from nitrosamines, the identification of other products with mass spectra library was also undertaken. Liquid-liquid extraction with methylene chloride does not ensure sufficient enrichment of the analytes due to low extraction efficiency. Therefore, this method despite being satisfactory for an identification of reaction by-products was not fast and repeatable enough to be used for quantitative analysis. That is why a modified ion-exclusion chromatography technique with UV-Vis detection at 230 nm, initially used for carboxylic acids determination, was successfully applied for determination of NDMA by Andrzejewski et al. [5-7]. This method is a simple and highly sensitive technique allowing a determination of nitrosamines at the level of a few micrograms per litre. The method also allows the verification of other catalyzed/non-catalyzed hydrogen peroxide by-products of such as: formic and acetic acid Secondary amines analysis The initial as well as remain concentrations of secondary amines in post reaction mixtures were analyzed according method developed by Sacher et al [13]. Prior to GC-FID analysis post-reaction mixtures were derivatized with benzenesulfonyl chloride. The method allows determining amines in water samples on sub-ppb level Aldehydes analysis. The formation of aldehydes, as products of hydrogen peroxide/fenton reagent reaction with DMA, MEA and DEA was also investigated with GC-ECD. Prior to GC-ECD analysis post-reaction mixtures were derivatized with O-(2,3,4,5,6-pentafluorobenzyl) hydroxylamine (PFBOA) according procedure proposed by Yamada and Somiya [14]. The method allows determining aldehydes in water samples on ppb level Inorganic products analysis. Inorganic products of hydrogen peroxide/fenton reagent reaction with DMA, MEA and DEA were analyzed with HPLC-ion exchange chromatography (IEC) with conductometric detector (CD). This method allows the determination of nitrites and nitrates at micrograms per litre, after direct injection of post-reaction mixture. Limit of detection (LOD): 50 μg NO 2 - /L and 30 μg NO 3 - /L 3. RESULTS AND DISCUSSION 3.1. NDMA as a product of hydrogen peroxide/fenton reagent reactions with dimethylamine aqueous solutions In the preliminary experiments NDMA was found, on an ion-exclusion chromatogram, as a by-product of hydrogen peroxide and DMA reaction. Since the experiments have shown that the highest observed yield of the NDMA can be expected at ph 11, the hydrogen peroxide reaction with aqueous DMA solutions was repeated using 1,11 mm DMA and 30:1 hydrogen peroxide /DMA ratio after 24h contact time. Contrary to results obtained for hydrogen peroxide reaction with DMA, no trace of NDMA, on an ion-exclusion chromatogram, was found in post reaction mixture both in case of application of Fenton reagent (1,11 mm DMA and 30:1 Fenton reagent/dma ratio, ph=3,0) as well as application Fenton-like reagent (1,11 mm DMA and 30:1 Fenton-like reagent/dma ratio, ph=5,0; 7,0; 9,0 and 11,0). To verify the presence, or absence, of NDMA, the post-reaction mixtures were analyzed with GC-MS. The mass spectra of compound with retention time RT= 5.56 min found in the methylene chloride extract of post reaction mixture of hydrogen peroxide and DMA is shown on Fig.1 GC-MS analysis of post-reaction mixture of DMA and hydrogen peroxide confirmed that the compound characterized by retention time of 5.56 min is N-nitrosodimethylamine. The GC retention time of the NDMA standard was found the same and the mass spectrum of the A-49 by-product extracted from the reaction mixture is identical with the mass spectrum of NDMA from the MS library. GC-MS analysis also confirmed lack of NDMA in post-reaction mixture of Fenton/Fenton-like system and DMA. Abundance Average of to min.: FH34241.D\data.ms (-) m/z-- Abundance #762: N-Nitrosodimethylamine m/z-- Figure 1. GC-MS spectra of NDMA peak and library result 3.2. Influence of reaction conditions on NDMA formation As it is aforementioned, no NDMA formation and only negligible changes in DMA concentration of water samples were detected after Fenton/Fenton-like oxidation. However, in the case of non-catalyzed hydrogen peroxide oxidation of DMA, both decrease in DMA concentration and increase in NDMA concentration were observed. Thus, in the present part only results of non-catalyzed hydrogen peroxide application to DMA solutions are discussed Influence of ph The ph of the reaction mixture proved very significant parameter to manage NDMA formation during treatment of DMA solution by hydrogen peroxide. The results indicated, that NDMA formation from DMA by oxidation with hydrogen peroxide is affected by ph with maximum rates of formation occurring between ph 11 and 12. Apparently, this correlation between hydrogen peroxide reactivity towards DMA and ph of reaction mixture occurs because of pka values of DMA and hydrogen peroxide. First of all, the structure of DMA (pka=10.73) drastically depends on ph of medium. If ph value below pka, DMA is a quite resistant to oxidation protonated molecule. At ph values above pka DMA is the nonprotonated molecule and can be virtually easily oxidized. As a result, no NDMA formation was found for reaction mixture ph lower than 11. On the other hand, ph values above 12 demonstrated decrease in the NDMA yield because of hydrogen peroxide oxidative activity reduction. Hydrogen peroxide pka value is and at ph values higher than pka hydrogen peroxide undergoes autodecomposition, which, in its turn, results in diminution
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