Understanding chemical transformations of contaminants and the resulting products is extremely important in devising proper monitoring methods for such contaminants and in assessing potential human exposure to the transformation products in the environment. Ultraviolet (UV) light from the sun can induce various photochemical transformations of contaminants in the environment. Alkylnaphthalenes are light-molecular-weight polycyclic aromatic hydrocarbons (PAHs) which are one of the most widespread organic pollutants present in ambient air as a result of a variety of incomplete combustion sources. In this study, 1-methylnapthalene,a typical example of an alkylnaphthalene, was subjected to UV irradiation to investigate its transformation in the presence and absence of air. Twenty-one products were detected in the reaction mixtures. Some photo-oxidation products were identified, including both ring-opened and ring-retained oxygenated compounds, such as 1-naphthaldehyde, 1-naphthoic acid, 1-naphthalenemethanol and phthalic anhydride. Although dimeric products were observed in the presence of air, more were found in the presence of helium or argon gas, indicating a different photo-oxidation pathway from those commonly observed in other media, such as water. Under just 48 hours of exposure to the UV light in the presence of air, three major products were formed with a production yield of about 10% each. Compared to 1-methylnapthalene, the UV induced transformation products observed in this study are more volatile, acidic, water soluble or toxic. The formation of these products may significantly change our understanding of the risks assessed solely from the parent compound in contaminants research and supports the inclusion of airborne transformations of the parent compound in risk assessment.
Methylnaphthalenes (MNs), dimethylnaphthalenes (DMNs) and ethylnaphthalenes (ENs) are common polycyclic aromatic hydrocarbons (PAHs) emitted into ambient air from a variety of incomplete combustion sources, including diesel engines [1-7], and wood burning [8,9]. These substances are predominantly present in the gas phase [
Although numerous studies have focused on photooxidation of alkylnaphthalenes in the presence of oxygen, the UV induced transformations of alkylnaphthalenes in various reaction conditions, such as in the presence of various gases, in contrast, have not yet been systematically investigated and compared. In the presence of an inert gas such as helium or argon gas, the transformation process is thought to be a photolysis process due to a lack of oxygen in the system. In this study, degradation of 1-methylnaphthalene and formation of transformation products under various reaction conditions, including in the presence of dry or humid air, were systematically investigated. The degradation rate of 1-methylnaphalene was also investigated to evaluate the contributions of UV exposure in the presence of air to the formation of oxygenated transformation products. The implications of the transformation products on air quality and the environment were also discussed.
1-Methylnaphthalene (95%), 1-naphthaldehyde (95%), 1-naphthalenemethanol (98%), 1-naphthoic acid (96%), 1-naphthaleneacetic acid (98%), 2-naphthaleneacetic acid
(99%), phthalic acid (99.5+%), phthalic anhydride (99+%), 2,2’-dimethyl 1,1’-biphenyl (Aldrich compound number T206571), and 2,2’-dimethyl-1,1’-binaphthalene (Aldrich compound number T330272) were purchased from Sigma-Aldrich Canada (Oakville, Ontario, Canada). Hexane (99.98%), methanol (99.9%) and dichloromethane (99.96%) were purchased from Omnisolv (Swedesboro, NJ, USA).Solutions of 1-methylnaphalene and other standards were prepared by dissolving them inhexane to make 1 mg/mL stock solutions. The working solutions were prepared by diluting the stock solutions to certain concentrations with hexane.
The photo-oxidation experiments were carried out in a cylindrical quartz chamber. The chamber was fitted with a Teflon air-lock at both ends, one end was further screwed with a septum thread cap (
to allow the 1-methylnaphthalene to be naturally evaporated in the chamber. The UV lamp (UV A + B) was then turned on, and the time was recorded. The strength of the UV radiation was set at 26 mW·cm–2. An identical chamber loaded with same amount of 1-methylnaphthalene in the same way was wrapped with aluminium foil to block any light as the dark control. After a fixed time of exposure, the UV lamp was turned off. Both chambers were rinsed twice with 2.5 mL of hexane, twice with 2.5 mL of dichloromethane, and twice with 2.5 mL of methanol, in sequence. The rinses of solvents were combined and stored at 4˚C for GC/MS analysis.
An Agilent GC/MS system (6890N GC and 5973 MSD, Agilent Technologies (Canada) Inc., Mississauga, Ontario) was used for both measuring the degradation of 1-methylnaphthalene and identifying reaction products. A DB-5MS GC column (30 m × 0.25 mm i.d. × 0.25 μm film thickness, J & W Scientific, Folsom, USA) was used. The injection port temperature was 280˚C, the oven temperature was set 45˚C for 5 minutes, increased to 210˚C at 15˚C /min; to 270˚C at 8˚C/min; to 310˚C at 30˚C/min, and then kept at this temperature for 25 minutes. The MSD was operated in full scan mode with a scan range of 30 - 700 m/z. The peaks were also analyzed with high resolution mass spectrometer. A COC inlet with track over mode for the temperature was used in coupling with the Waters Autospec Premier double focusing magnetic sector instrument. The magnet scan in EI + mode was from 60 to 450 m/z with parameters: 5000 resolution(at 5% peak height), 0.5 s/decadescan time, 0.2 s interscan time and 0.64 s total cycle time.
When the chamber containing 1-methylnaphthalene was exposed to UV light in a clean dry air environment, the photo-reaction process was observed visually by the appearance of a light brown colour on the entire internal wall of the quartz chamber. The same colour was also observed on the bottom wall of the chamber, where the starting material was initially placed, indicating that the starting material might not be completely evaporated. The multiple peaks in the total ion chromatogram (TIC) of the GC/MS showed many potential photo-oxidation transformation products in the extracted reaction mixture (
The identification of peak 9 proved to be challenging. That peak was further analyzed with high resolution mass spectrometry and the spectrum indicated a molecular mass of 186.0681 with a formula of C12H10O2, its mass spectrum was similar to either 2-naphthaleneacetic acid or 1-naphthaleneacetic acid according to the NIST library (
The mass spectra of peak 4 and peak 5 matched the spectra of 3-methyl-1(3H)-isobenzofuranone and 3, Xdimethyl-1(3H)-isobenzofuranone (X denoting the unknown position of the second methyl group), respectively in the NIST library. The two chemicals were previously reported in the photo-oxidation of 1-methylnaphthalene in seawater [
A recent study revealed that the photo-transformation of naphthalene in atmospheric conditions under UV light resulted in the formation of two groups of products: ring-opened products and ring-retained products [
hydroxymethyl group. Moreover, this molecular weight of 174 equals to the sum of 1-methylnaphthalene and two oxygen atoms. Therefore, it would be reasonable to assign this compound as (E)-3-(2-acetylphenyl)acrylaldehyde.
So far, we have discussed identification of peaks eluted between 13 and 17 minutes. Additional peaks (peaks 12 to 22) were also observed in the late elution times after 24 minutes (
retention time range have not been identified due to the complexity of peaks. However, as their molecular weights are all over 282 and the loss of 141 mass units was indicated in all of their mass spectra, they could be assumed to be all from further photo-oxidation of dimers under UV irradiation in the presence of air. Nevertheless, the compounds represented by peaks 12 to 22 have demonstrated the complexity of the photo-oxidation of
1-methylnaphthalene in the dry air environment.
In order to further understand if the OH group from water in the reaction environment would promote the generation of oxygenated products, moisturised air was used in the UV irradiation. Similar products were observed except that the yield of 1-naphthoic acid, 1-naphthalenemethanol and phthalic acid increased. The increase of phthalic acid is not surprising because the water in the reaction environment could act as an OH donor to promote the oxidation of the naphthalene skeleton, which is in agreement with results from a previous study on UV photo-oxidation of the same material dissolved in seawater [
As shown from the results above, in UV promoted photo-oxidation processes, oxygen may play an important role as either a precursor of the hydroxy or superoxo radicals. However, it is not clear how a dimer was formed in the presence of dry air in the photo-oxidation process of airborne 1-methylnaphthalene. Therefore, an experiment using an inert gas environment was conducted to investigate the transformation of 1-methylnaphthalene under UV radiation using helium or argon gas in the reaction chamber. Under this reaction environment, it was found that only three peaks, peak 1, 2 and 7 were observed in the first product zone of the extracted mixture. In the second product zone, a new peak with a retention time of 24.8 min appeared (peak 14,
It was anticipated that the oxygenated compounds would not be generated from the photo-oxidation under the inert gas environments. However, two of the side chain oxygenated compounds, phthalic anhydride and 1-naphthaldehyde represented by peak 1 and 7, respectively, were still observed in both the helium and argon gas environments, though at significantly reduced levels compared to the dry air environment. The small quantity of oxygenated products observed in this case could be attributed to the small amount of water present in the helium or argon gas as an impurity, and its involvement in oxygenation reactions. This might also explain the observation of peak 16, a possible oxygenated dimer product, in the absence of air. However, the disappearance of the alcohol and acid products, on the other hand, could demonstrate that the lack of oxygen in the air in the system significantly reduced the formation of further oxygenated compounds.
Aphoto-oxidation mechanism for 1-methylnaphthalene in atmospheric conditions was previously proposed for the formation of ring-opened transformation products including (E)-3-(2-acetylphenyl)acrylaldehyde and the major dicarbonyl products through an initial reaction with an OH radical followed by the reaction with oxygen and NO or RO2 radicals [
However, the formation of dimeric products likely indicates a different photo-oxidation mechanism. To the best of our knowledge, no mechanism has yet been proposed for the formation of dimers and oxygenated dimer products. According to the findings in this study, it is reasonable to believe that radical reactions would be the initial pathway for UV photo-oxidation of 1-methylnaphthalene in the presence of air. Either excited oxygen or hydroxyl radical mediated the formation of two types of methylnaphthalene-based radicals. We therefore propose a plausible pathway for the formation of dimers under photo oxidation of 1-methynaphthalene (
presence of oxygen, the dimers (M.W. 282) could further pick up a molecular oxygen or another hydroxyl radical to form oxygenated dimeric products with a molecular weight of 296, 298, 304, 312, 314 and 346 (
When the primary contaminants transform in the environment through the reactions with environmental factors, they may form similarly or even more mobile, persistent, or toxic transformation products than the parent compound [
From the discussion above, we can see that 1-methylnaphthalene can be transformed by UV light to more than 21 products. Among those transformation products, phthalic anhydride is a volatile chemical that can have an irritative effect on mucous membranes and the skin [21-23], and repeated exposure may cause allergic skin rash, rhinitis, bronchitis and asthma [
In addition to the one-ring and two-ring transformation products, this is the first study to show 1-methylnaphthalene dimerizing under environmental conditions. Compared to earlier studies on the transformation of 1-methylnaphthalene in either water or air in which no dimerization was detected, this study found two dimers and eight possible oxygenated dimeric products. It is clear that methyl naphthalene can be transformed into a larger number of products. This work shows that it is important to continue to investigate such transformations in order to better understand any risk arising from the presence of these compounds in the environment.
The presence of more than 21 transformation products observed in this study indicates that the formation of transformation products in the environment can add further complexity to risk assessment. Those transformation products may contribute to the risk posed by the parent compound. The three major compounds, 1-naphthaldehyde, 1-hydroxymethylnaphthalene, and naphthoic acid, with a yield of as high as around 10% each in the transformation of 1-methylnaphthalene, are either more volatile or water soluble than the parent compound. It has been reported that these alkyl side chain hydroxylation products were more toxic than the parent methylnaphthalenes [
result of this study indicates that this light-molecular-weight PAH is able to be transformed in the air environment.
The results of the current study are only subject to UV light induced degradation products. The photo-oxidative processes observed in this study lead to the formation of oxygenated products that are more polar and consequently, more water soluble than the parent compounds. However, the formation of dimers and oxidized dimeric products under the same environmental conditions indicates that it can yield more complex transformation products in the environment. The presence of dimers and oxidized dimers suggests that a mixture of contaminants could lead to a complex mixture of transformation products. Regardless of the type of transformation pathway, the results from the present study demonstrate an important fact that 1-methylnapthalene can be transformed to as many as 21 products in the presence of air. Although some of those compounds have not yet been identified in the present study, their mass spectra indicate that most of them are oxygenated compounds from 1-methylnaphthalene. More importantly, the formation of dimers and the possibility of further oxygenated compounds forming from the dimers were first demonstrated in this study. It reveals the complexity of photo-oxidation in the presence of air, which indicates the potential sources of secondary pollutants in the environment. Further study is secured to confirm the identity of some of the dimers and their oxygenated compounds. These compounds need to be characterized in order to better understand any potential risks. The results of this study indicate the importance of understanding transformation of environmental pollutants in contaminants research. Methylnaphthalene is one of many PAH compounds that can be found in both outdoor and indoor environments, therefore, the fate and transformation products of such environmental contaminants should be examined.
This study has revealed that 1-methylnaphthalene in the air environment was photo-transformed via reactions with oxygen and/or water primarily to ring-opened products, such as (E)-3-(2-acetylphenyl)acrylaldehyde, and side chain oxidized compounds, such as 1-naphthaldehyde, 1-naphthoic acid and 1-naphthylmethanol. Significant dimerization occurred when the reaction was carried out in the inert atmosphere. Dimers and their oxygenated products in the presence of oxygen were first observed in this study. A radical reaction mechanism was proposed for the formation of dimers and oxygenated dimers. The 21 transformation products resulted from only one compound, 1-methylnaphthalene, under UV radiation indicates the importance of further study on the environmental fate of airborne contaminants and the impacts of transformation products to the environment.
This project was financially supported by the Canadian government under the Chemicals Management Plan (CMP).