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A preliminary study for the photolysis behavior of biodiesel and its

blends with petroleum oil in simulated freshwater

Zeyu Yang a,b,⇑, Bruce P. Hollebone a, Zhendi Wang a, Chun Yang a, Carl Brown a, Gong Zhang a,Mike Landriault a, Xinchao Ruan c,⇑

a Emergencies Science and Technology Section (ESTS), Science and Technology Branch, Environment Canada, Ottawa, ON, Canadab Key Laboratory of Catalysis and Materials Science of the State Ethnic Affairs Commission & Ministry of Education, Key Laboratory of Analytical Chemistry of the State Ethnic

Affairs Commission, College of Chemistry and Materials Science, South-Central University for Nationalities, Wuhan 430074, PR Chinac The Research Center of Environmental Science, Wuhan Textile University, Fangzhi Road, Hongshan District, Wuhan 430073, PR China

h i g h l i g h t s

For FAMEs, higher degree of saturation resulted in lower transformation. Water matrices, initial concentration of biodiesel affected degradation slightly. Biodiesel source effect can be neglected. Presence of biodiesel stabilized the small oil droplets in aqueous phase. Biodiesel inhibited the degradation of some heavy hydrocarbons.

a r t i c l e i n f o

Article history:

Received 13 June 2014Received in revised form 11 August 2014

Accepted 26 August 2014Available online 6 September 2014

Keywords:

PhotolysisBiodieselDieselBlendFreshwater

a b s t r a c t

With the increasing use of biodiesel and its blends with petroleumfuel, the corresponding environmentalissues also occur during its production, application and transportation. The photolysis behavior forbiodiesel and the impacts of biodiesel on the photo-oxidation of petroleum hydrocarbons in simulatedfreshwater was studied by irradiated with ultra violet (UV) and simulated sunlight in the present study.The results indicated that the photolysis rates of fatty acid methyl esters (FAMEs) were mainly dependedon their degree of saturation, slightly on water matrices and the initial concentration of biodiesel,regardless of biodiesel sources. Similar results were observed for total organic carbon (TOC) removalrates; however, TOC removal rates were slightly dependent on the initial concentration of biodiesel.The presence of humic acid and pyrogallic acid or lake water matrices slightly inhibited the removal ratesof TOC. The photolysis rates of individual petroleum hydrocarbons with and without the presence of biodiesel followed similar rules. In brief, alkanes with light molecular weights were transformed fasterthan those with heavy molecules, the removal of polycyclic aromatic hydrocarbons (PAHs) were moresignificantly than alkanes, and the removal of alkylated PAHs (APAHs) increased concurrently with thealkylation level in each family. The presence of biodiesel only inhibited the photolysis of some heavyalkanes and PAHs, not for all other petroleum hydrocarbons. Biodiesel, as a surfactant-like material, couldstabilize small oil droplets initially formed by agitation, therefore, these droplets experience longerlifetimes in the water phase before re-aggregating into larger globules and rising to the surface. Theapparent solubility of petroleum hydrocarbons, especially for those with heavier molecular weights,

has been enhanced in the presence of FAMEs. In this scenario, light needs to penetrate water phase todegrade these targets compared with diesel alone. The direct contacting opportunities between UV lightand targets, and radicals produced to attack targets were reduced, which finally resulted in the inhibitedphotolysis rates of some heavy molecular weight hydrocarbons.

2014 Elsevier Ltd. All rights reserved.

1. Introduction

Biodiesel consists of a mixture of long-chain fatty acid alkylesters processed from biological triglycerides. It has receivedintensive attention as one of the most significant supplement fuels

http://dx.doi.org/10.1016/j.fuel.2014.08.061

0016-2361/ 2014 Elsevier Ltd. All rights reserved.

⇑ Corresponding authors. Address: Emergencies Science and Technology Section(ESTS), Science and Technology Branch, Environment Canada, 335 River Road,Ottawa, ON K1A 0H3, Canada. Tel.: +1 (613)990 3219; fax: +1 (613)991 9485(Z. Yang), The Research Center of Environmental Science, Wuhan Textile University,Fangzhi Road, Hongshan District, Wuhan 430073, PR China (X. Ruan).

E-mail addresses: [email protected] (Z. Yang), [email protected] (X. Ruan).

Fuel 139 (2015) 248–256

Contents lists available at ScienceDirect

Fuel

j o u r n a l h o m e p a g e : w w w . e l s e v i e r . c o m / l o c a t e / f u e l

http://dx.doi.org/10.1016/j.fuel.2014.08.061mailto:[email protected]:[email protected]://dx.doi.org/10.1016/j.fuel.2014.08.061http://www.sciencedirect.com/science/journal/00162361http://www.elsevier.com/locate/fuelhttp://www.elsevier.com/locate/fuelhttp://www.sciencedirect.com/science/journal/00162361http://dx.doi.org/10.1016/j.fuel.2014.08.061mailto:[email protected]:[email protected]://dx.doi.org/10.1016/j.fuel.2014.08.061http://crossmark.crossref.org/dialog/?doi=10.1016/j.fuel.2014.08.061&domain=pdf

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and alternatives to petroleum diesel fuel, because of its compara-ble engine performance and environmental characteristics.

Spill accidents happen more often than ever with the increasinguse of biodiesel and its blends with petroleum fuel. Biodiesel spillstend to spread and form a sheer thick layer of clear to light milkywhite color on top of sea water, and fatty acid methyl esters(FAMEs) can be easily broken down to generate free fatty acidsand methanol after spillage [1,2]. Corresponding adverse effectswill occur to ecosystems and human health. Therefore, it is neces-sary to understand the fate of biodiesel exposed to the environ-ment, and the dominant degradation mechanisms, for developingsuitable remedial action and identifying the source in the case of a fuel spill.

Once biodiesel/its blend with petroleum oil spills into the envi-ronment, various weathering processes, for example, biodegrada-tion, photo-oxidation, dissolution and evaporation, will occur.Because biodiesel components are more readily biodegraded thanfossil fuels, many studies have focused on microbial degradationfor remediation purposes [3–8]. Among them, a remediation studyhas shown that fatty acid methyl esters in biodiesel are degradedmuch faster than most components in fossil diesel [4]. This corre-sponds very well with the fact that biodiesel is more readily biode-gradable compared with fossil diesel [3].

The contribution of biodiesel to the physical property of dieselhas been investigated by several researchers [4,9]. It was foundthat the evaporation rate of fossil diesel components was notaffected by the presence of FAMEs in simulated seawater [4]. How-ever, the physical mobility of heavy oil mixtures was enhanced byFAMEs amendment in sand column [5]. Recently, one of ourresearch has reported that the evaporation process and stabilityof diesel would not alter with the presence of biodiesel in a simu-lated ambient condition [10].

The contribution of biodiesel to the biodegradation of dieselsuggested that the presence of biodiesel could facilitate the bio-degradation of some petroleum hydrocarbons due to microbialgrowth promotion [5,9] and increase bioavailability (emulsion)

[11]. Other researchers, however, have found that the presenceof biodiesel did not accelerate the biodegradation of hydrocar-bons [4,12,13]. For example, the degradation of benzene and tol-uene in anoxic and hypoxic conditions was hindered by thepresence of biodiesel [12]. It seemed that the relatively high vis-cosity of biodiesel limited the migration potential of target hydro-carbons, resulting in their relatively slow natural attenuationprocess.

Among the various weathering processes, photolysis behaviorof fuel is one of the important factors to control their transforma-tion and fate in the environment. It is well known that the photol-ysis behavior of organic compounds in aquatic phase is based onthe ability of solar/UV radiation to attack the target compounds.As a consequence, photochemical processes may take place, in

which different transient species are generated: e.g., photo-ioniza-tion, radicals generated by bond homolysis or bond heterolysis, aswell as a number of photo-physical processes (fluorescence, phos-phorescence, etc.) [14]. The photo-oxidation behavior of petroleumin water has attracted much attention in the past years [15–18].Generally, the photolysis rates of petroleum hydrocarbons dependon the composition and physical properties of the exposed parentoil, wavelength, turbidity levels of sample, suspended particulatematter concentration, and water matrices [17,19,20]. Till yet, dif-ferent mechanisms for the photo-oxidation of petroleum andFAMEs have been described, including free-radical oxidation inthe presence of oxygen, singlet oxygen initiation of hydro peroxideformation, and ground state triplet oxygen combining with freeradicals to form peroxides [17,18,21]. Recently, the photochemical

process of crude oil has also become better characterized due tothe development of analytical technologies [16,22].

Biodiesel, produced from different feedstocks, is generallyspiked with different antioxidants to extend its storage stability[23]. The environmental factors affecting the fate of biodiesel andits blends with diesel at spillage site may vary with the physicaland chemical properties of the spilled contaminants, the environ-mental matrices, climate, weather, topography and hydrology.Unfortunately, the impacts of environment matrices and physic-chemical properties of biodiesel on the photolytic behavior of bio-diesel have not been extensively studied till yet. The photolyticbehavior for individual petroleum hydrocarbons in biodiesel anddiesel blends has not been fully understood either.

The purpose of the present study is to investigate the effect of physic-chemical properties of biodiesel and environmental matri-ces on the photolysis of biodiesel. The impact of biodiesel on thephotolytic behavior of individual petroleum hydrocarbons wouldalso be discussed. Biodiesel samples with different sources and ini-tial concentration spiked into ultrapure (UP) water, and severalsimulated freshwater matrices (UP water, UP water with humicacid (HA) or pyrogallic acid (PY), and lake water) were irradiatedby UV and simulated sunlight firstly. The removal rates of mainFAMEs and total organic carbon (TOC) were measured and com-pared to evaluate the significant contributors to the photolysis of biodiesel in freshwater. Similarly, the depletion rates of petroleumhydrocarbons with and without the presence of biodiesel after irra-diated by UV light were measured and compared to investigate theinfluence of the presence of biodiesel on the photolytic behavior of petroleum hydrocarbons.

2. Experimental procedures

2.1. Chemicals and materials

Solvents including hexane, dichloromethane (DCM) and ace-tone were supplied by Spectrum Chemicals (Gardena, CA, USA) atthe highest purity and used without further purification. Silica

gel (100–200 meshes), humic acid and pyrogallic acid were pur-chased from Tianjin Chemical Co., Ltd. (Tianjin, China).Normal alkane calibration standards from n-C 9 to n-C 40,

5a-androstane, and polycyclic aromatic hydrocarbon (PAH) cali-bration certified standard mixtures were purchased from Restek(Bellefonte, PA, USA) and the US National Institute of Standardsand Technology (NIST, Gaithersburg, MD, USA), respectively.Deuterated internal and surrogates including [2H14] terphenyl(terphenyl-d14), [

2H50] n-C 24(C 24D50), and PAH surrogate standardsincluding [2H8] naphthalene (naphthalene-d8), [

2H10] acenaph-thene (acenaphthene-d10), [

2H10] phenanthrene (phenanthrene-d10), [

2H12] benz[a]anthracene (benz[a]anthracene-d12), and[2H12] perylene (perylene-d12), were supplied by Supelco (Belle-fonte, PA, USA).

FAME mixtures, including 14 FAME standards ranged from C6 to

C24 with different saturated degree, surrogate of 13-methyl,methyl myristate (13-methyl, C14:0), and internal standard of methyl heptadecanoate (C17:0) purchased from Sigma–Aldrich(Bellefonte, PA, USA) were employed for identifying and quantify-ing FAMEs. Detailed chemical information for FAME standardsused for calibration and identification are shown in Ref. [24].

2.2. Preparing fuel mixtures

Diesel purchased from a gas station in Wuhan, China, was usedin this study as the reference oil to be blended with biodiesel.Three pure biodiesel samples sourced from soybean oil, canolaoil and animal fat was designated as Bsoy, Bca and Ban, respectively.

All samples were diluted with hexane to stock solution with a finalconcentration of 80 mg/mL.

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2.3. Exposure experiments

2.3.1. Biodiesel exposure

The diagram of UV or visible light exposure for biodiesel and itsblends with diesel is depicted in Fig. S1. In detail, exposures wereconducted by pouring 130 mL of ultrapure water (with a pH of 7,18.2 MX of resistivity, 0.01 lg/mL of TOC, and undetectable cat-ions, anions and total dissolved solids) into a 7-cm (i. d.) 10-cm(height) glass reactor. Biodiesel diluted by acetone was added intothe water phase to a final concentration ranged from 26.5, 62.5 to125lg/mL. Water samples were then exposed to UV light with alight intensity of 33.7 mW/cm2, and Xenon lamp as a simulatedsunlight system with a light intensity of 55.8 mW/cm2. Becauselight irradiation will heat the exposed water, an outside coolingwater recycle was employed to avoid the alternations of tempera-ture during UV and sunlight exposure. The temperature in the irra-diated water was maintained at 25 ± 2 C. Biodiesel with differentsources and varied spiking concentration, and different watermatrices were tested to assess the factors affecting the photolysisbehavior of biodiesel. The water matrices included UP water, UPwater spiked with 10lg/mL of humic acid or 8 lg/mL of pyrogallicacid, and lake water. It is noted that the original TOC of water withpyrogallic acid was determined to be close to 40lg/mL. It began todegrade after 2 h UV irradiation, and decreased to 16lg/mL after5 h. The TOC of water with humic acid was determined to be10.4lg/mL with a standard deviation of 0.9 lg/mL. It almost keptconstant within 5 h UV irradiation. The TOC and total nitrogen of lake water was determined to be 18.7 ± 0.6 and 11.0 ± 0.8lg/mL,respectively. The UV irradiation within 5 h did not alter the TOCfor lake water. All TOC data were corrected based on their corre-sponding blank sample after exposed to UV light at same time.

For every test, two same samples were prepared. One samplewas exposed to light; the other one was kept at room temperaturein a dark area as a control sample to avoid microbial degradationduring UV/sunlight exposure. Sub-samples were taken at 0, 0.5,1, 1.5, 2, 3, 4, and 5 h, respectively, for further analysis.

Similarly, the simulated solar irradiation within 24 h was testedfor all above mentioned conditions. In this scenario, samples weretaken at 0, 4, 8, 12, 16, 20 and 24 h, respectively, for furtheranalysis.

2.3.2. Exposure of biodiesel and diesel blends

The effect of the presence of biodiesel on the photolysis of indi-vidual petroleum hydrocarbons was evaluated by exposing dieseland the blends of biodiesel and diesel with a mass ratio of 1:1 toUV for 1, 2, 3, 4, and 5 h, respectively. The spiked concentrationof diesel and the blends was set as 40 lg/mL. After exposure, allirradiated water samples were taken for liquid–liquid extraction(LLE), and DCM was used to rinse the glassware of the reactor forfurther analysis. To eliminate the biodegradation and evaporation

loss, the one-to-one control samples were set up in the darknessto act as the original standard without UV exposure for estimatingdepletion rates of individual petroleum hydrocarbons.

2.4. Analytical procedures

2.4.1. Liquid–liquid extraction (LLE)

For pure biodiesel samples, ten milliliter of water samplesspiked with 100 lL of 200 lg/mL 13-methyl, C14:0 as surrogatewere extracted with DCM three times for FAME analysis. Theextracts were combined and concentrated to about 0.9 mL, then100lL of 200 lg/mL C17:0 was added as the internal standardfor GC/MS analysis.

For blended samples, all water samples spiked with 100 lL

200lg/mL of C 24D50 (surrogate for n-alkanes), 100 lL 10lg/mL of PAH surrogate standards (surrogates for PAHs and their

alkylated homologues), and 100lL 200lg/mL of 13-methyl,C14:0 (Surrogate for FAMEs), were extracted with DCM threetimes. The LLE extracts combined with above glassware rinsingsolution were concentrated and solvent exchanged to hexane forthe following fractionation procedures.

2.4.2. Column chromatographic fractionation

For pure biodiesel, the above concentrated extracts were pre-pared with hexane to a final concentration of approximate200lg/mL, and spiked with methyl heptadecanoate (C17:0) witha final concentration of 20 lg/mL as internal standard for directGC/MS analysis of FAMEs. However, for the blended biodieseland diesel extracts, column clean-up procedures were performedprior to GC/MS analysis. This fractionation procedure is to fraction-ate extracts into petroleum hydrocarbons and FAMEs based ontheir polarity difference, and avoid the interference of FAMEs dur-ing GC/MS analysis. Column clean-up and fractionation procedureswere adapted from one of our methods for chemical fingerprintingof blends of biodiesel and diesel [24]. In brief, a 3-gram silica gelcolumn topped with a 1-cm layer of anhydrous sodium sulfatewas preconditioned with about 20 mL of hexane. Hexane (12 mL)was used to elute aliphatic hydrocarbons (F 1), and 15 mL mixtureof 1:1 hexane: DCM by volume was used to elute aromatic hydro-carbons (F 2). F 1 and F 2 fractions were finally concentrated to about1 mL, spiked with 100 lL of 200 lg/mL of 5a-androstane and10lg/mL of d14-terphenyl as the internal standards for GC–MSanalysis of n-alkanes, petroleum-characteristic alkylated PAHhomologues and other US Environmental Protection Agency (USEPA) priority PAHs, respectively. It is noted that the variation of FAMEs in blends was not investigated in the present study, sothe elution of FAMEs was not mentioned here.

2.4.3. GC/MS and TOC analysis

Characterizations of n-alkanes, PAHs, petroleum biomarkers,and FAMEs were performed on an Agilent 6890 GC system inter-faced to an Agilent 5973 mass spectrometer. The n-alkanes, PAHs

and petroleum biomarker compounds were separated on a HP-5MS capillary column (30 m 0.25 mm I.D., 0.25lm filmthickness)with the following temperature program: 50 C for 2 min, heatedto 300 C at 6 C/min and held for 15 min at 300 C.

A DB-225 MS GC column (30 m 0.25 mm i. d., film thickness:0.25lm) from Agilent was employed to separate FAME and sterolcompounds with the following settings: initial temperature of 50 C, held for 1 min, then increased at a rate of 7 C/min to185 C, held for 10 min, then increased at a rate of 15 C/min to230 C, with a final hold for 5 min.

For all GC/MS analysis, ultrahigh purity helium was employedas carrier gas at a flow rate of 1 mL/min. The temperatures of injec-tor, transfer line, ion source, and MS quadrupole analyzer wereheld at 280, 280, 230, and 150 C, respectively. Samples were

injected in splitless/split mode. The mass-selective detector(MSD) was operated at an electron impact mode (70 eV) forselected ion monitoring (SIM) runs.

For TOC analysis, samples were acidified by hydrochloric acid,and total inorganic carbon was removed by purging the acidifiedsample with an inert gas prior to TOC measurement. All acidifiedsamples were rinsed with UP water till a pH of 7. Then organic car-bon present in the pre-treated samples was oxidized to carbondioxide by catalytic combustion in the TOC analyzer. The relativestandards for TOC analysis were less than 10% for allmeasurements.

2.5. Data analysis

Except for the analysis of the variation of FAMEs for purebiodiesel and petroleum hydrocarbons for blends by GC/MS, the

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variation of TOC after exposure to UV or visible light was alsorecorded at the determined time points. The removal rate of TOCcan be expressed as TOC (%); it can be described as the followingEq. (1):

TOC ð%Þ ¼ TOC0 TOCt

TOC 0 100 ð1Þ

where TOC0 and TOCt are the measured TOC values at the beginningand pre-determined reaction time, respectively. Similarly, thetransformation rates of FAMEs and petroleum hydrocarbons wereestimated from the following Eq. (2), where C c and C t are the mea-sured concentration for one of the targets at the control and UVexposure samples at pre-determined time points, respectively.Therefore, all data reported in this study were control corrected.

Transformation ð%Þ ¼ C c C t

C c 100 ð2Þ

3. Results and discussion

3.1. Chemical composition analysis of biodiesel and diesel

The dominant FAMEs detected in Bca, Ban and Bsoy have a carbonnumber of 18. Methyl oleate (cis-9, C18:1) comprises 64.0 ± 4.2%(m/m) of Bca. The other FAMEs from high to low concentration inthe Bca sample are methyl linoleate (cis-9, cis-12, C18:2), methyllinolenate (cis-9, cis-12, cis-15, C18:3), methyl palmitate (C16:0),methyl vaccinate (cis-11, C18:1), and methyl stearate (C18:0).Methyl linoleate (cis-9, cis-12, C18:2) makes up 41.8 ± 2.0% (m/m) of the Bsoy sample. Similarly, the other FAMEs for the Bsoy fromhigh to low abundance are ranked as: cis-9, C18:1, C16:0, cis-9,cis-12, cis-15, C18:3 and C18:0. For Ban, the FAMEs distributionfrom high to low is followed as: cis-9, C18:1 (35–42%) > cis-9,cis-12, C18:2 or C18:0 (8.0–11.2%) > C16:0 (5.2–7.5%). The totalFAME composition in Bca, Bsoy and Ban were measured to be

102 ± 4%, 95 ± 3% and 96 ± 4%, respectively. The chemical composi-tion of all these biodiesel samples did not show significantvariation in all control blank samples.

The chemical composition of the present used diesel is depictedin Fig. S2. It can be seen that the distribution profiles of n-alkanesrange from C 9 to C 30 with a bell shape, and their concentrationlocates from 21.2 to 14,563 ng/g. The measured alkylated polycy-clic aromatic hydrocarbons (APAHs) included 0–4 alkylatednaphthalene, phenanthrene, fluorene and chrysene, they wereabbreviated as C i-N, P, F and C (i = 0–4), respectively. It can be seenthat C i-N are the most dominant components, followed by C i-F,C i-P, and C i-C. For all control blank samples, most of hydrocarbonsdid not change significantly, but some of light molecular weighthydrocarbons (e.g., n-C 9, n-C 10, and the naphthalene family)

showed a decreasing tendency with the extension of exposed toair. It is obvious that the loss is ascribed to the evaporation of lightmolecular hydrocarbons.

3.2. Impact of biodiesel source on TOC removal rates

To investigate the effect of biodiesel source on its photolysisbehavior, the variation of TOC removal rates for biodiesel sourcedfrom soybean oil, canola oil and animal fat with exposure time isdepicted in Fig. 1. It can be seen that the accumulated TOC removalrates increased with the irradiation time regardless of biodieselsource. The final total TOC removal rates reached up to 87.0% witha relative standard derivation of 5.2% after 5 h UV irradiation.Therefore, FAMEs and most of the produced intermediates have

been degraded within 5 h by UV irradiation. The accumulatedTOC removal rates followed the zero order kinetics with

correlation coefficients ranged from 0.945 to 0.971, the averageslope of the linear regression varied from 11.0, 11.8 to 12.6 for

Bca, Ban and Bsoy, respectively. As the slope represents the degrada-tion rates of different biodiesel, it seems that Bsoy was degradedfaster than Ban and Bca. It is reasonable because the main FAMEcomponent in Bsoy is cis-9, cis-12, C18:2; the most dominant com-ponent is cis-9, C18:1 in Ban and Bca. However, the difference of theslopes can be neglected due to experimental errors. Therefore, thecontribution of biodiesel sources to the mineralization of biodieselcan be neglected in the present study.

3.3. Effect of Bsoy concentration on TOC removal rate

As the removal of TOC is independent on the biodiesel source,Bsoy was selected as the representative biodiesel to study its pho-tolysis behavior in the following sections. The concentration of Bsoyranged from 25.0, 62.5 to 125 lg/mL was investigated to explorethe dependence of the TOC removal rates on the loading concentra-tion of biodiesel in water (Fig. 2). It can be seen that all the TOCremoval rates increased with the extensionof exposure time. How-ever, they increased with the decrease of the initial Bsoy loadingconcentration when the irradiation time was less than 2 h. Afterthat, similar TOC removal rates were observed for the above threeloading amounts. It is obvious that the energy to degrade FAMEsand their produced intermediates are limited, therefore, the highinitial concentration of Bsoy resulted in the relatively low TOC

0 1 2 3 4 5 6

T O C r e m o

v a l r a t e ( % )

0

20

40

60

80

100

120BcaBanBsoy

UV irradiation time (h)

Fig. 1. Transformation of different biodiesel formulation vs. UV exposure time.


0 1 2 3 4 5 6

T O C r e m o v a l r a t e ( % )

0

20

40

60

80

100

12025 ppm62.5 ppm

125 ppm

Fig. 2. Effect of Bsoy concentration on TOC removal rate.

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removal at the beginning of the reaction. However, the energy wasenough to remove the residual targets, when most of FAMEs andintermediates have been removed after 2 h exposure. Linearregression of the kinetics of TOC removal rates indicated that thecorrelation coefficients ranged from 0.940 to 0.982; and averageslopes varied from 13.1, 12.6 to 12.6 for 25.0, 62.5 and 125 lg/mL of Bsoy, respectively. It is obvious that the TOC removal pro-cesses obey the zero kinetics. The spiking level of 25.0

lg/mL

was degraded faster than the other two levels. However, they didnot change significantly when Bsoy concentration increased from62.5 to 125lg/mL. Considering the analytical detection limits of FAMEs, a concentration of 62.5 lg/mL was selected in the follow-ing sections.

3.4. Effect of sample matrices on TOC removal rates

Humic acid and PY, representing NOM and synthetic antioxi-dant, respectively, were spiked into UP water to test the effect of sample matrices on TOC removal efficiency. A natural lake fresh-water was also selected as the representative of real water matri-ces. The background for these sample matrices have beenreported in Section 2.3.1. It can be found that although similar lev-els of humic acid and PY have been spiked, samples with humicacid were determined to own lower TOC than PY. It is reasonablebecause humic acid is a sodium salt, which is water soluble, andcontains around 20% inorganic residue. The variation of TOCremoval rates with different sample matrices is depicted inFig. 3. Similarly, all TOC removal rates increased greatly with theextension of UV exposure till 3–4 h, a relatively slight increasingtendency was observed after 4 h exposure. The final TOC removalrates reached up to 81.293.1% after 5 h UV exposure. However,the TOC removal rates also slightly depended on the sample matri-ces. In detail, the TOC removal rates in UP water were generallyhigher than the other sample matrices, followed by UP water withPY and lake water, then UP water with humic acid. The linearregression of the kinetics of TOC removal rates indicated that thecorrelation coefficients ranged from 0.952 to 0.981, and averageslopes/degradation rates per unit time varied from 12.6, 11.9,11.8 to 11.0 for UP water, UP water with PY/lake water and UPwater with humic acid, respectively.

It is obvious that the presence of other materials in waterslightly inhibited the photo-degradation of biodiesel and theirintermediates, because these chemicals may absorb part of lightor scavenge some free radicals [17]. The chemical composition inlake water should be more complex than all the other three sam-ples, however, similar TOC removal rates were observed in lake

water and UP water with PY. The presence of humic acid in UPwater inhibited the photo-degradation behavior of biodiesel andits intermediates more obviously. It was reported that humic mate-rials are UV-absorbing species, which can compete UV with targetpollutants to reduce the efficiency of PAH degradation [25]. There-fore, humic acid may compete UV light from FAMEs and/or theirintermediates, thus resulting in less TOC removal rates. Again, Millet al. [26] found that humic acid, as a light screening material,decreased the light intensity in water, resulting in the decreaseddegradation rates of polycyclic aromatic hydrocarbons.

3.5. Factors affecting the photolysis of individual FAME

Except for measuring the variation of TOC, the variation of sev-eral representative FAMEs was also recorded. The effects of samplematrices on the removal of the representative FAMEs are shown inFig. 4. It can be seen that the FAME photolysis rates are mainlydependent on their degree of saturation. However, sample matri-ces also affect their transformation rates. Firstly, the saturated levelof FAMEs controls their transformation rates. In detail, around 90–100% cis-9, cis-12, C18:2 was transformed within 1.5 h UV irradia-tion, then it almost kept constant after 2 h. Cis-9, C18:1 wasremoved by 60–80% within 1.5 h, it continued to be transformedeven after 5 h irradiation. Both of them have been transformed90–100% after 5 h UV exposure. It is noted that the variations of cis-9, cis-12, cis-15, C18:3 were not monitored because its contentin the tested biodiesel are relatively low. It can be detected in theoriginal samples; however, it disappeared after 0.5 h UV irradia-tion, which indicates that cis-9, cis-12, cis-15, C18:3 can be trans-formed by UV irradiation very easily. Similarly, C16:0 and C18:0were removed very quickly at the initial UV exposure stage, fol-lowed by a slow removal process till they were removed about40–60% after 5 h UV irradiation. It is obvious that the saturatedFAMEs of C16:0 and C18:0 are more photo stable compared withcis-9, cis-12, C18:2 and cis-9, C18:1. Similar conclusions have beenreported by Khoury et al. [1].

Secondly, samples matrices also contributed to the photolysis of FAMEs. Generally, FAMEs in lake water exhibited the lowestremoval rates, followed by UP water plus PY/humic acid, and thenin UP water. This conclusion is consistent with the variation rulesof TOC removal rates. For different FAME congeners, it seems thatC16:0 and C18:0 were affected by sample matrices more signifi-cantly than cis-9, cis-12, C18:2 and cis-9, C18:1. It is reasonableas unsaturated FAMEs are more vulnerable to photo-oxidationthan saturated ones [1]. Therefore, even sample matrices partiallyabsorb light or scavenge part of free radicals [17,25,26], the resid-ual energy and radicals are strong enough to break the unsaturatedFAMEs, but not for saturated ones.

Fig. 4 also demonstrates that FAMEs, especially for unsaturatedFAMEs, were transformed very quickly at the initial UV irradiation,

followed by a platform or a slow increasing photolysis process.However, the removal rates of TOC (Figs. 1–3) increased with theextension of UV irradiation, no obvious platform was formed forthe removal rates of TOC. It is reasonable because cis-9, 12,C18:2 and cis-9, C18:1 should have been transformed into someintermediates prior to mineralization to carbon dioxide and water.Although they could not be detected after they have been damagedto other intermediates, these intermediates still contributed to themeasured TOC results. Therefore, different removal profiles havebeen achieved.

Similarly, a simulated sunlight irradiation within 24 h wastested for all above mentioned scenarios. Unfortunately, the TOCfor all simulated samples did not decrease significantly within24 h irradiation. Individual FAME did not exhibit significant

decrease either. Khoury et al. [1] studied the photolysis behaviorof FAMEs by exposure seawater with a floating layer of biodiesel

Irradiation time

0 1 2 3 4 5 6

T O C r e m o v a l r a t e s ( % )

0

20

40

60

80

100

UP water

UP water with HA

UP water with PY

Lake water

Fig. 3. Effect of sample matrices on TOC removal rates.

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to sunlight. It was reported that cis-9, cis-12, cis-15, C18:3 was

removed completely after 3 days sunlight exposure, cis-9, cis-12,C18:2 decreased dramatically in 3 days and disappeared totallyafter 10 days. Therefore, 24 h sunlight irradiation was not longenough for studying the photolysis behavior of biodiesel. The fur-ther investigation will be performed by extending exposure timelonger than 24 h for sunlight exposure.

3.6. Photolysis of diesel and blends of biodiesel and diesel

3.6.1. Removal kinetics of representative petroleum hydrocarbons

As mentioned in the above sections, most of FAMEs can beremoved within 5 h by UV irradiation. The removal kinetics of petroleum hydrocarbons with and without the presence of biodie-sel was simulated within 5 h UV exposure (Figs. 5 and 6, the origi-

nal GC/MS data are shown Tables S1 and S2 in the SupportingMaterial). Where the left panel depicts the variation of alkanes indiesel samples, right panel demonstrates those in the blends of biodiesel and diesel. It is obvious that all alkanes showed anincreasing removal rates with the extension of UV exposure intwo simulated samples, although some light molecular weightalkanes almost completely removed within 1 h exposure. It seemsthat alkanes with long carbon chain length are more reluctant to betransformed compared with short ones. Unlike bio-degradation,where n-alkanes can be degraded easier than branched alkanes,cyclic alkanes, substituted/unsubstituted aromatics. For example,n-C 17 can be depleted preferably to pristine, and n-C 18 is preferableto phytane through biodegradation [27]. Herein, the pairs of n-C 17/pristine and n-C 18/phytane were transformed at similar rate, which

suggests photo-oxidation treatment gives a completely differentresult compared with biodegradation. Fig. 5 also illustrates that

the depletion of alkanes in blended samples is more random than

diesel sample. This suggests the presence of biodiesel affects thephoto-oxidation of alkanes (see detailed discussion in Section3.6.2). It is noted that several depletion rates are lower than 0%,which are acceptable as they locate in the limits of experimentalerrors.

Similarly, the photolytic kinetics of representative alkylatedPAHs in diesel and blended samples are shown in Fig. 6. The pho-tolysis rates for most of APAHs increased with the extension of UVirradiation within 12 h, but thefamily of naphthalenes (C i-N, i = 0–4)were transformed quicker than the other high molecular weightPAHs. More random variation was also observed for C i-N series,this discrepancy can be partially ascribed to the random evapora-tion loss of these targets. For most of phenanthrene (C i-P) and flu-orene (C i-F) families, they were transformed dramatically at the

first 2 h exposure in both diesel and blended samples, followedby an almost consistent photolysis rates in the following 3 h expo-sure. It can be found that APAHs were depleted more significantlythan alkanes in the same sample by comparing Figs. 5 with 6. Indetail, almost 100% APAHs, but 80–100% of alkanes have beentransformed in diesel sample after 5 h UV exposure. In blendedsamples, all APAHs, except for C 0-F, have been removed by 80–100%, but the removal rates of alkanes ranged from 20% to 100%.It is obvious that aromatic hydrocarbons are more sensitive tophoto-oxidation than alkanes. In diesel samples, the photolysisefficiency was generally increased concurrently with the alkylationlevel in each family of APAHs after 1 h UV exposure. It is reasonableas the alkyl groups enrich the electron density of the p system of the molecule, facilitating electron excitation and its resulting pho-

tolysis [28,29]. This effect should also be enhanced by the numberof aromatic rings in the molecule [30], but higher depletion of

cis-9, 12, C18:2

20

40

60

80

100

120

UP water UP water + HAUP water + PYLake water

cis-9, C18:1

0

20

40

60

80

100

120

C16:0


0

20

40

60

80C18:0


0 1 2 3 4 5 6 0 1 2 3 4 5 6

0 1 2 3 4 5 60 1 2 3 4 5 60

20

40

60

80

F A M E p h o t o l y s i s r a t e s ( % )

Fig. 4. Effect of sample matrices on the degradation rates of several representative FAMEs.

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UV exposure time (h)

-20

0

20

40

60

80

100

120n-C18Phytanen-C22n-C26n-C30

-20

0

20

40

60

80

100

120n-C18Phytanen-C22n-C26n-C30

(a)

P h o t o l y s i s r a t e ( % )

-20

0

20

40

60

80

100

120

n-C10n-C12

n-C14n-C16n-C17Pristane

(b)

0 1 2 3 4 5 6 0 1 2 3 4 5 6

0 1 2 3 4 5 6 0 1 2 3 4 5 6-20

0

20

40

60

80

100

120

n-C10n-C12

n-C14n-C16n-C17Pristane

Fig. 5. Photolysis kinetics of representative n-alkanes for diesel and the blends of diesel and biodiesel. (a), Diesel; (b), blends.

Naphthalene family

20

40

60

80

100

120

C0-N2-ME N1-ME NC2-NC3-N

C4-N

Phenanthrene family

20

40

60

80

100

120

C0-PC1-PC2-PC3-PC4-P

Fluorene family


20

40

60

80

100

120

C0-FC1-FC2-FC3-F

20

40

60

80

100

120

20

40

60

80

100

120

UV exposure time (h)

0 1 2 3 4 5 6

0 1 2 3 4 5 6

0 1 2 3 4 5 6

0 1 2 3 4 5 6

0 1 2 3 4 5 6

0 1 2 3 4 5 620

40

60

80

100

120

Fig. 6. Photolysis kinetics of representative APAHs for diesel and the blend of diesel and biodiesel. Left panel, diesel; right panel, blends.

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naphthalene families have been observed due to the evaporationloss during the test, which canbe confirmed by the control samplesin the dark (Table S2). For blended samples, the photolysis extentvaried randomly. This also indicates the presence of biodieselaffects the photo-oxidation of petroleum hydrocarbons. Thedetailed discussion involving in the contribution of biodiesel tothe photolysis of petroleum hydrocarbons will be discussed in

the following section.

3.6.2. Contribution of biodiesel to the photolysis of individual

petroleum hydrocarbons

Although data at different time points within 5 h UV exposureare available in the present study, only data with 5 h irradiationare compared to investigate the effect of the presence of biodieselon photolysis rates of individual petroleum hydrocarbons (Fig. 7).It can be seen that the removal of individual petroleum hydrocar-bons with and without the presence of biodiesel exhibits withsome discrepancy. In detail, the photolysis for most of alkaneswas inhibited, especially for low and high molecular ones, butnot for middle ones (e.g., n-C 13n-C 17). For APAHs, the effect of bio-

diesel is not as significant as alkanes, but obvious inhibition effectcan be found for fluorene and chrysene series, especially for

chrysene. As the discussion in the upper section, the presence of aromatic structure of PAHs enhances the electron excitement dur-ing UV irradiation, which makes them to be more favorable to betransformed compared with alkanes [28,29]. Therefore, the photol-ysis of most of alkanes was significantly inhibited, but not forAPAHs with the presence of biodiesel.

The potential reason for the inhibition effects of biodiesel ondiesel can be explained by the following two possibilities. Firstly,the dominant components of biodiesel are fatty acid alkyl esters,one end of these components are hydrophobic alkyl groups, theother end is hydrophilic ester groups. They exhibit with the char-acters of surfactants when biodiesel is spiked into water. The pres-ence of FAMEs could stabilize oil droplets in the water phase bydecreasing the oil–water surface tension and therefore reducingoil droplet re-aggregation. These droplets experience longer life-times in the water phase before re-aggregating into larger globulesand rising to the surface. Therefore, oil stabilization by FAMEs mayincrease the rate of petroleum hydrocarbons, especially for thosewith heavier molecular weight, dissolution into water phase, dueto the increased surface area to volume ratio of smaller oil droplets[4]. The direct contact probabilities between UV light and targets,and radicals to attack targets would be reduced significantly whenmore petroleum hydrocarbons dissolution into water phase.Accordingly, the photolysis of some heavy petroleum hydrocar-bons in the presence of biodiesel was significantly inhibited com-pared with pure diesel.

One home-made biodiesel without complete purification wasutilized to verify the above assumption firstly. More obvious inhi-bition effects have been observed in this scenario. As the removalof glycerol is limited in this product, obvious micelles have beenvisualized once it was spiked into aqueous phase. It is obvious thatmore surfactant-like materials present in this product than thecommercial Bsoy, which resulted in more petroleum hydrocarbonsdispersing into water phase, and thus more significant inhibitioneffects were observed compared with the above commercialbiodiesel.

Based on this explanation concerning with the dissolution of petroleum hydrocarbons into water phase, the inhibited photolysisrates of alkanes with light molecular weights is an exception. Thesealkanes may have been generated by photo-oxidative cleavage of the alkyl side chains present in unresolved compound mixture(e.g., the photo-oxidation of nonylbenzene [31], or the photo-oxi-dative decarboxylation of n-alkanoic acids or by the Norrish type1 rearrangement via an alkyl radical of ketones [27], which canbe generated by the photo-oxidation of n-alkanes.

As mentioned in Section 2.3.2, 40lg/mL of diesel and blendsamples were used to investigate the impact of biodiesel on thephotolysis of petroleum hydrocarbons. This means extra 40 lg/mL of biodiesel presented in blends, except for 40 lg/mL of diesel.Accordingly, the second possible reason for the inhibition effects

may be ascribed to the competitive consumption of UV light fromFAMEs. Blends with a concentration of 20 lg/mL (the mixture of 20lg/mL biodiesel and 20lg/mL diesel) are prepared to verify thisconsumption (Fig. 7). Comparing the blends with two spiking lev-els, it can be seen that petroleum hydrocarbons with 20 lg/mL spiking dosage were depleted more significantly than 40 lg/mL.However, comparing the 20 lg/mL of blends with the 40 lg/mL of diesel samples, it can be found that the photolysis rates of petro-leum hydrocarbons in the 20 lg/mL of blends were lower than in40lg/mL of diesel. As FAMEs are more vulnerable to be photo-oxi-dized than petroleum hydrocarbons, the photolysis rates of petro-leum hydrocarbons in 20lg/mL of blends should have beenremoved more significantly than that in 40 lg/mL of pure dieselif the competition of FMAEs is the main reason for the inhibited

effects. This reversed phenomenon indicates that the competitionof UV light from FAMEs can be neglected. Therefore, the dominant

(a)

Individual n-alkanes

n - C 1 0

n - C 1 1

n - C 1 2

n - C 1 3

n - C 1 4

n - C 1 5

n - C 1 6

n - C 1 7

P r i s t a n e

n - C 1 8

P h y t a n e

n - C 1 9

n - C 2 0

n - C 2 1

n - C 2 2

n - C 2 3

n - C 2 4

n - C 2 5

n - C 2 6

n - C 2 7

n - C 2 8

n - C 2 9

n - C 3 0

n - C 3 1

n - C 3 2


0

20

40

60

80

100

120

14040 µg/mL diesel40 µg/mL blend20 µg/mL blend

(b)

Individual APAHs

C 0 - N

2 - M E N

1 - M E N

C 2 - N

C 3 - N

C 4 - N

C 0 - P

C 1 - P

C 2 - P

C 3 - P

C 4 - P

C o - F

C 1 - F

C 2 - F

C 3 - F

C h r y

C 1 - C

C 2 - C

C 3 - C

0

20

40

60

80

100

120

140

Fig. 7. Effect of biodiesel on the photolysis rates of individual petroleum hydro-carbons. Conditions: 5 h UV irradiation. Blends of diesel and Bsoy (1:1, m/m) with afinal concentration of 40 lg/mL and 20 lg/mL in water phase. (a), n-Alkanes; (b),alkylated PAHs. Where C i-N, C i-P, C i-F and C i-C represent the 0–4 alkylatednaphthalene, phenanthrene, fluorene and chrysene, respectively.

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possibility for the inhibited photolysis behavior for some heavymolecular weight petroleum hydrocarbons can be ascribed to theirincreased apparent solubility in the aqueous phase with the pres-ence of FAMEs.

4. Conclusions

The photolysis of FAMEs and their mineralization efficiency infreshwater mainly depended on their degree of saturation, slightlyon water matrices and their initial concentration, regardless of biodiesel sources. The presence of biodiesel stabilized small oildroplets in the aqueous phase to increase the apparent solubilityof petroleum hydrocarbons, especially for heavy molecular weightones. Therefore, the direct contacting opportunities between UVlight and targets and radicals produced to attack targets werereduced, which finally resulted in the inhibited photolysis ratesfor some heavy molecular weight hydrocarbons.

Acknowledgement

This work was funded and supported by the National NaturalScience Foundation of China (Project No. 41373133), and theMinistry of Science and Technology of China (No. 2012ZX07503-003-002).

Appendix A. Supplementary material

Supplementary data associated with this article can be found, inthe online version, at http://dx.doi.org/10.1016/j.fuel.2014.08.061.

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