Effects of polycyclic aromatic hydrocarbons on the UV-induced fluorescence spectra of crude oil films on the sea surface

Marine Pollution Bulletin 146 (2019) 977–984

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Effects of polycyclic aromatic hydrocarbons on the UV-induced fluorescence spectra of crude oil films on the sea surface

T

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Yongchao Houa, Ying Lia, , Yu Liub, Guannan Lia, Zhenduo Zhanga a b

College of Navigation, Dalian Maritime University, Dalian 116026, China College of Environmental Science and Engineering, Dalian Maritime University, Dalian 116026, China

A R T I C LE I N FO

A B S T R A C T

Keywords: Oil spills Polycyclic aromatic hydrocarbons Fluorescence spectra Correlation analysis Oil identification

As the main fluorescent substances in oils, polycyclic aromatic hydrocarbons (PAHs) are the basis of ultraviolet (UV)-induced fluorescence spectroscopy methods to detect oil films on the sea surface. The relative contents of PAHs in six crude oil samples and their effects on ultraviolet fluorescence spectra were studied. The PAHs were divided into four categories according to their fluorescence characteristics. Naphthalene series dominated the fluorescence spectra, which led to a main peak at 320–350 nm, but this showed no relationship with PAH content. The six oil samples could not be distinguished by differences in the fluorescence spectra in this range, but could be distinguished by the fluorescence spectra in the 350–380 nm band. The relative contents of dibenzothiophene and phenanthrene series showed significant positive correlations (R2 = 0.96) with fluorescence intensity. Fluorescence spectroscopy combined with GC–MS can be used to distinguish and identify crude oils.

1. Introduction Oil spills often occur during the transportation or storage of petroleum products and can affect various aspects of the marine environment (Alves et al., 2016; Yang et al., 2019). Polycyclic aromatic hydrocarbons (PAHs) are a class of well-known organic pollutants in oils, which have significant fluorescence characteristics (Greene et al., 2017; John et al., 2016; Patra, 2003). Therefore, ultraviolet (UV)-induced fluorescence spectroscopy has been widely used as a timely and accurate monitoring method for remote sensing of oil spills (Christensen et al., 2005; Sikorska et al., 2004; Steffens et al., 2011). Highly sensitive fluorescence spectrometry offers a fast and reagent-free detection method for thin oil films on the sea surface (Beltran et al., 1998; Bublitz et al., 2009). Relative fluorescence intensity (RFI), excitation-emission matrix spectroscopy (EEMS), synchronous fluorescence spectrometry (SFS), and decay time spectra (DTS) are the most important techniques to detect and identify oil spills using UV-induced fluorescence (Girelli et al., 2017; Hou et al., 2018; Jiji et al., 1999; Niessner et al., 1991; Rohde et al., 2009). In most of the cases reported to date, accurate spectral main peak positions and intensities are common parameters required by the above four methods (Jiang et al., 2018). A major objective for the identification of oil species by fluorescence spectroscopy is the improvement of recognition accuracy. An oil slick consists of thousands of organic compounds (Fig. 1).

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Among them, saturated hydrocarbons and unsaturated hydrocarbons are the main substances (Salas et al., 2006; Stout and Wang, 2016). The application of UV-induced fluorescence spectroscopy in oil spill detection is possible due to the presence of the π-electron conjugated system (conjugated double bonds) in petroleum substances. Benzenes contain one benzene ring, which emits light of slightly longer wavelengths than the excitation light, which is difficult to detect in fluorescence spectra (Santana Rodríguez and Padrón Sanz, 2008). However, PAHs contain additional benzene rings, which emit longer wavelength light than benzenes (Pathiratne et al., 2010). PAHs are the most important fluorescent substances in oil slicks and dominate the fluorescence of oil slicks on the sea surface (Groner et al., 2001; Seo et al., 2019). The United States Environmental Protection Agency (US-EPA) has assigned a number of substances as priority pollutants. PAHs play an important role due to their mutagenic and/or carcinogenic potential (Dilkes-Hoffman et al., 2019), and these compounds have an important influence on the fluorescence of oil spills (Aeppli et al., 2012; Daniela and Magne, 2013). A lot of effort is being put into improving existing analytical methods and developing new techniques for analysis of PAHs in the marine environment, such as gas chromatography (GC) and gas chromatography–mass spectrometry (GC–MS) with extraction and fractionation, which could determine the types of PAHs present (Yang et al., 2017, 2014; Yim et al., 2011). The US-EPA has included some PAHs on its list of priority pollutants and summarized some of their

Corresponding author. E-mail address: [email protected] (Y. Li).

https://doi.org/10.1016/j.marpolbul.2019.07.058 Received 7 June 2019; Received in revised form 23 July 2019; Accepted 23 July 2019 Available online 31 July 2019 0025-326X/ © 2019 Elsevier Ltd. All rights reserved.

Marine Pollution Bulletin 146 (2019) 977–984

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Fig. 1. Polycyclic aromatic hydrocarbons (PAHs) represent the main components of crude oil that emit light of longer wavelengths than the excitation light; (a) fluorescence spectra of different oil samples measured using a spectrograph with a 200–300 nm filtered xenon lamp light source.

basic photophysical properties illustrating the absorption and fluorescence behavior of these compounds in solution (Table 1). However, an efficient and effective method to qualitatively/quantitatively analyzes the effects of PAHs on fluorescence spectroscopy of oil slicks is yet to be developed. Our goal was to determine the relationship between the relative content of different PAHs in crude oil slicks on the sea surface and their fluorescence spectra. The present study focuses on identifying the specific PAH compounds that dominate the fluorescence spectra of oil slicks under certain conditions. In this study, GC–MS was used to analyze the main types of PAHs in crude oil samples after extraction and fractionation; a UV laser source and a spectrograph were then used to collect fluorescence spectra of oil slicks on the sea surface. Based on the fluorescence spectra characteristic of PAHs from the US-EPA (Table 1), we identified the main types of PAHs that dominated the fluorescence spectra of the oil slicks. Fluorescence spectroscopy and GC–MS were used together to help identify crude oil species from UV-induced fluorescence.

2.2. GC–MS A previous GC–MS analysis study by our group demonstrated the variation in n-alkane content and PAH content of different types of oils during short-term weathering (Li et al., 2018; Liu et al., 2017). In this study, the content of PAHs in the six crude oils was analyzed using a similar method. Approximately 0.2 g of each crude oil sample was dissolved in n-hexane (8 mL) with ultrasonic concussion and 1 g of anhydrous sodium sulfate was added to it. The supernatant was collected in the column after centrifugation at a speed of 1500 r/min for 10 min. Total aromatics were eluted with an n-hexane/dichloromethane mixture (20 mL, 3:1 v/v) and were concentrated to 0.5 mL under a nitrogen stream prior to analysis. GC–MS (Thermo Fisher Scientific, Waltham, MA, USA) was performed with a DB-5MS quartz chromatography column (60 m × 0.25 mm × 0.25 μm); a 1 μL injected sample was used in splitless mode. Temperature was increased at a rate of 3 °C/min from 60 °C to 290 °C and then maintained for 10 min. The electron bombardment source for electron impact (EI) was 70 eV.

2. Materials and methods 2.3. UV-induced fluorescence 2.1. Oil samples An experiment that simulated an oil spill on the seawater surface was performed for each crude oil type. Seawater samples were collected in April 2019 at the port of Lingshui, China, on the northern Yellow Sea.

Six crude oil samples were selected for this study: Brazilian, Angolan, Venezuelan, Iranian, Omani, and Saudi (Table 2). 978

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to measure the fluorescence of samples and the fluorescence was detected over a spectral range of 180–850 nm with a resolution of 5 nm. The UV-induced source was a xenon lamp (L4634–01 synthetic silica glass; Hamamatsu Photonics, Hamamatsu, Japan) with a 200–300 nm band-pass filter. Ambient natural light was used as the environmental background to simulate the sea surface detection environment in the laboratory. All measurements were conducted under the same conditions. Fig. 2a shows the optical path of the UV-induced fluorescence experiment. Fig. 2b shows a photo of one crude oil sample in the experiment.

Table 1 Fluorescence properties of PAHs in six oil samples. λexmax, central wavelength of the excite source; λemmax, wavelength of maximum fluorescence intensity; τF, fluorescence lifetime; ΦFb, fluorescence quantum yield. Data for the fluorescence properties were taken from (Berlman, 2012; Kumke et al., 1995; Ligthart et al., 1985; Rudnick and Chen, 1998). Compound

Structure

λexmax (nm)

λemmax (nm)

τF (ns)

ΦFb

Biphenyl

253.7

315.9

16

0.16

4-Methylbiphenyl

265.0

320.5

15.2

0.17

3,3-Dimethyltbiphenly

265.0

317.0

13.2

0.21

2,2-Dimethyltbiphenly

265.0

316.0

15.4

0.106

Fluorene

269

313.4

10

0.80

1-Methylfluorene

265

313.0

9.2

0.58

Naphthalene

265.0

334.4

96

0.23

1-Methylnaphthalene

265.0

337.8

67

0.25

2-Methylnaphthalene

265.0

335.5

59

0.32

2,2-Dimethylnaphthalene

265.0

336.1

78

0.38

2,6-Dimethylnaphthalene

265.0

340.7

38

0.45

289

336.7

46

0.5

253.7

361.2

0.9

0.09

Phenanthrene

275

366.9

12.5

0.36

3-Methyphenanthrene

275

355.9

13.5

0.35

4-Methyphenanthrene

275

358.2

14.1

0.38

Pyrene

270

396.8

45.0

0.65

Chrysene

279

381.6

44.7

0.14

Acenaphthene Dibenzothiophene

3. Results and discussion 3.1. Distribution and categories of PAHs Before exploring the effects of PAHs on the fluorescence spectroscopy of the crude oils, it was necessary to first analyze the PAH content of each crude oil species using GC–MS. The PAHs contained in the six crude oil samples were as follows: biphenyl (B); methylbiphenyl (MB); dimethylbiphenyl (DMB); fluorene (F); methylfluorene (MF); naphthalene (N); methylnaphthalene (MN); dimethylnaphthalene (DMN); trimethylnaphthalene (TMN); acenaphthene (A); dibenzothiophene (DBT); methyldibenzothiophene (MDBT); dimethyldibenzothiophene (DMDBT); phenanthrene (P); methyphenanthrene (MP); dimethylphenanthrene (DMP); trimethylphenanthrene (TMP); pyrene (PY); and chrysene (C). The content of each of these PAHs was different. PAHs were identified by matching the retention times to 16 PAH standards. Alkyl aromatic hydrocarbon compounds were identified by comparison with mass spectra and retention times against published references (Emsbo-Mattingly and Litman, 2016; Nabbefeld et al., 2010). To better understand the effects of PAHs on fluorescence spectroscopy, we attempted to find the relationship between the relative content of PAHs and the wavelength of maximum fluorescence intensity of the crude oil slicks. Using λemmax(the wavelength of maximum fluorescence intensity for each PAH's photophysical properties from the US-EPA), we established a three-dimensional coordinate system as shown in Fig. 3a. The x-axis is the type of PAH, the y-axis is the main peak position λemmax of the PAHs, and the z-axis is the relative content of each PAH. The naphthalene series had the highest relative content levels, contributing over 50% to all six crude oil samples. The relative contents of the other PAHs overlapped among the samples, suggesting that it is difficult to identify crude oil species based on their relative content. Our results are similar to the composition patterns seen in previous studies that examined PAH content after oil spills (Daling et al., 2002; Reddy and Quinn, 1999). Fig. 3b shows the relative content and main peak wavelength positions of PAHs in six crude oil samples, through the projection of Fig. 3a on the yz surface. It is obvious that PAHs with characteristic fluorescence wavelengths in the 320–340 nm range were the dominant contributors, contributing over 60% to the samples. The characteristic fluorescence wavelengths in the 300–320 nm and 340–380 nm ranges were the other main contributors, contributing approximately 10% and 20%, respectively.

Table 2 Characteristics of six crude oil samples. Number

1 2 3 4 5 6

Crude oil sample Brazilian Angolan Venezuelan Iranian Omani Saudi

API gravity (°) 18.1 30.7 11.2 35.0 34.6 33.7

Viscosity (50 °C) 119.6 15.7 16.3 5.2 8.0 7.0

Abbreviations

BX AGL VZL YL OM ST

3.2. Categories of PAHs The results of Section 3.1 indicated that the relative concentrations of PAHs in the six crude oil samples were complex and it was, therefore, difficult to analyze the effect of single ingredients on the fluorescence spectra. Moreover, the fluorescence spectrum of an oil film on the sea surface is the superposition of various fluorescent substances. Therefore, to qualitatively/quantitatively analyze the effects of PAHs on the fluorescence spectra, it was necessary to classify the PAHs according to the position of their main peak wavelength. Based on the photophysical properties of PAHs (Table 1), it can be seen that the main peak position of a PAH is linearly related to its own structure and the number of conjugate double bonds (Fig. 4a). Therefore, we categorized the PAHs

A small amount of each crude oil type (approximately 6.28 μL) was individually placed into six 40 mm × 25 mm glass bottles, along with 5 mL seawater, to form a 5 μm thick (the desired computational thickness) oil slick. Bottles and lids with ground glass joints were selected to prevent oil volatilization. A spectrograph (Shamrock 163; Andor Technology Ltd., Belfast, UK) equipped with fiber optics and a 300–400 nm band-pass filter was used 979

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Fig. 2. (a) Optical path of the UV-induced fluorescence experiment; (b) crude oil sample.

3.3. Fluorescence of each oil type

according to their main peak wavelength positions. Specifically, the PAHs were divided into four categories (I, II, III, and IV) according to the fluorescence wavelength ranges shown in Table 3. Fig. 4b presents the relative content of PAHs in six oil samples in the four categories based on GC–MS analysis. Category II PAHs were the main components of the six samples, with a content of more than 60%. The content of category II PAHs in Brazilian and Angolan crude oil (more than 80%) was significantly higher than in the other four samples. However, the relative contents of category I and III PAHs in Brazilian and Angolan crude oil were relatively small. The relative content of category IV PAHs was generally low in all six oil samples (lower than 0.05). The Venezuelan crude oil had the highest content of category I PAHs compared with the other oil samples. In conclusion, category II PAHs were the main components of the six samples, suggesting that the main peaks of the fluorescence spectra of the oil films should be in the 320–340 nm range. Category I and III PAHs had much lower relative contents than category II PAHs in the six samples, suggesting that they should produce smaller peaks in the fluorescence spectra in the corresponding bands.

The fluorescence spectra of six crude oil samples were achieved using an optical measuring device that we designed (Fig. 2). Fig. 5a and b present the fluorescence spectra of the six crude oil samples determined using the laboratory experiments and normalization and Savitzky-Golay (S-G) smoothing (Betta et al., 2015; Romo-Cárdenas et al., 2018; Staggs, 2005). The suppression of fluorescence signals outside the 300–400 nm band by the 300–400 nm band-pass filter is clearly visible, showing enhanced fluorescence intensity in the desired region. The 300–400 nm wavelength range was selected considering that most PAHs have a main fluorescence peak within this wavelength band. In addition, the fluorescence spectrum is strongly affected by the excitation source at wavelengths below 300 nm, where benzenes emit fluorescence and the water Raman signal exists. Therefore, we selected the 300–400 nm wavelength of the fluorescence spectra for further analysis (Fig. 5b). The fluorescence spectra of the six oil samples exhibited one main peak, between 320 and 340 nm (centered at 330 nm) (Fig. 5a). The peaks detected correspond with UV-fluorescence spectra typical of soil and seawater polluted by oils (Bublitz et al., 2004; Hou et al., 2018).

Fig. 3. (a) Relative content of PAHs in six crude oil samples and the maximum fluorescence intensity wavelengths. (b) Projection of Fig. 3a on the yz surface. 980

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Fig. 4. (a) Main peak wavelength position of PAHs is linearly related to its own structure and the number of conjugate double bonds; (b) relative content of four categories of PAHs in six oil samples based on GC–MS analysis.

Table 3 PAHs divided into four categories. The PAH abbreviations are shown in Table 1. Category

Fluorescence spectra range (nm)

I II III IV

300–320 320–350 350–380 380–400

B; MB; DMB-1; DMB-2; F; MF N; 1-MN; 2-MN; 2,3-DMN; 2,6-DMN; TMN; A DBT; MDBT; DMDBT; P; MP; DMP; TMP PY; C

350–380 nm range) are clearly visible. Fig. 5d presents the fluorescence spectra after Gaussian fitting using Shirley baseline mode to enhance the features of the peaks (Dao et al., 2014; Yamashita and Hayes, 2006). The fluorescence spectra of the six crude oil samples exhibited two peaks at 335 nm and 355 nm. Thus, the six crude oil samples produce intensity differences in the peak of the fluorescence spectrum from 350 to 360 nm and the six crude oil samples could be identified using the differences in this range. In summary, through the above qualitative and quantitative analysis, we found that the six crude oil samples could be identified from the fluorescence spectra in the 350–360 nm range, when combined with the classification of PAHs using GC–MS.

The category II PAHs formed their main peaks from 320 to 340 nm, and the fluorescence intensity reached its maximum in this range of wavelengths. However, the main peaks for the six crude oil samples partially overlapped each other, due to the fluorescence intensity spectrograph reaching output saturation in the 320–340 nm range, making the identification of the crude oil species difficult. Although the content of the category II PAHs varied among the six types of oil, there were few differences in fluorescence intensity. The fluorescence spectra were noisier and overlapped in the 300–320 nm wavelength band as they were affected by the excitation source and other interference. The fluorescence intensity in the 380–400 nm region was weaker than in the other regions (Fig. 5b), due to the fact that the category IV PAHs had the lowest contents among the crude oil samples. In addition, all six crude oil samples exhibited another small peak at around 360 nm (Fig. 5b), where the fluorescence intensity of the six crude oil samples differed. Moreover, the fluorescence intensities corresponded to the contents of the category III PAHs in the samples. This small peak was sufficiently pronounced to differentiate between the different types of crude oil. In view of these results, it was necessary to find a method to enhance the differences between the fluorescence intensities of the six crude oil samples in the peak band (350–380 nm), so as to effectively identify oil species. To this end, we employed the average fluorescence intensity and peak fitting method to enhance the differences between the fluorescence intensities of six crude oil samples. The average fluorescence intensity was calculated as follows:

3.4. Correlation analysis Correlation analysis based on the average fluorescence intensities was conducted to examine the correlation between the categories of PAHs and their fluorescence intensities; the results are shown in Fig. 6. The relative content of each category was different from the distribution of the average fluorescence intensity. The PAHs in categories I, II, and IV overlapped in the correlation analysis, probably because each of these three categories of PAHs had similar relative contents. On the other hand, category III showed a positive correlation between the relative content of PAHs and the average fluorescence intensity (Fig. 6). Fig. 6 shows the category III average fluorescence intensity values against the relative content, which had a strong positive correlation (R2 = 0.96). The PAH content of crude oils and their fluorescence spectra were closely related, providing evidence that PAHs are the main fluorescent ingredient of oils. Analytical data from fluorescence spectra and correlation analysis implied that the six crude oils can be distinguished and identified in the scattered plots. In conclusion, we have provided evidence that the distinction and identification of crude oils can be advanced by fluorescence spectra analysis combined with GC–MS.

n

Favr (x ) =

Contains

∑i = 1 f (x ) n

where n is the spectrograph sample number in the 300–320; 320–350; 350–380; and 380–400 nm bands, respectively; f(x) is the fluorescence intensity value. As seen in Fig. 5c, the fluorescence intensities for the six crude oil samples were averaged across the four categories (Table 3). The average fluorescence intensities showed significant overlap within the 320–350 and 380–400 nm wavelength ranges. The differences between the average fluorescence intensities of the six crude oil samples (in the 981

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Fig. 5. Fluorescence spectra (full data) of six crude oil samples determined using laboratory experiments after data normalization and smoothing: (a) full data; (b) fluorescence spectra of 300–400 nm wavelength; (c) average fluorescence spectra of six crude oil samples in the 300–400 nm range based on the PAH categories; (d) fluorescence spectra of six crude oil samples after peak fitting processing using the data in the 300–400 nm band.

4. Conclusions

However, the influence of category II PAHs was notably smaller than that of the dibenzothiophene and phenanthrene series (Category III) in the fluorescence spectra. The biphenyl and fluorene series (Category I) may have a large influence on the fluorescence spectra; however, the spectra were quite noisy at 300–320 nm, preventing the retrieval of

GC–MS and fluorescence spectra analysis of six crude oils indicated that the naphthalene and acenaphthene series (Category II) led to the main peak in the fluorescence spectra, in the 320–350 nm range.

Fig. 6. Correlation analysis of PAH relative content and average fluorescence intensities in the six crude oil samples based on four categories. 982

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useful information. The relative content of the category IV PAHs was much lower than that of the other categories, and the fluorescence intensity was very low at 380–400 nm. Hence, the naphthalene and acenaphthene series were the most influential, and dibenzothiophene and phenanthrene series were the most useful to distinguish crude oil species. Correlation analysis showed a positive correlation between the relative content and average fluorescence spectra for category III PAHs (R2 = 0.96), which could be used to distinguish different types of crude oil. The fluorescence spectra analysis using Gaussian fitting to enhance the intensities of the six crude oil samples in the peak band (350–380 nm) could also distinguish the crude oil samples. This study establishes the relationship between fluorescence spectra and the relative content of four categories of PAHs and provides reference methods for the identification of crude oil products. In addition to identifying crude oil samples, this effective method could be used to identify light fuel oil, heavy fuel oil, and gasoline based on differences in the content of category III PAHs. However, this method is limited to the identification of oil species with obvious differences in PAH content. A detailed database of the PAH content of different oil species is needed. Such a database, used in combination with the fluorescence measuring device developed in this work, could be used to distinguish and identify oil species.

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