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Characterization of Motor Lubricating Oils and TheirOil–Water PartitionShan-Tan Lu a & Isaac R. Kaplan ba Zymax Forensics, Inc. , San Luis Obispo, CA, USAb Department of Earth and Space Sciences and IGPP , University of California , Los Angeles,CA, USAPublished online: 11 Dec 2008.

To cite this article: Shan-Tan Lu & Isaac R. Kaplan (2008) Characterization of Motor Lubricating Oils and Their Oil–WaterPartition, Environmental Forensics, 9:4, 295-309, DOI: 10.1080/15275920802119441

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Environmental Forensics, 9:295–309, 2008Copyright C© Taylor & Francis Group, LLCISSN: 1527–5922 print / 1527–5930 onlineDOI: 10.1080/15275920802119441

Contributed Article

Characterization of Motor Lubricating Oils and Their Oil–WaterPartition

Shan-Tan Lu1 and Isaac R. Kaplan2

1Zymax Forensics, Inc., San Luis Obispo, CA USA2Department of Earth and Space Sciences and IGPP, University of California, Los Angeles, CA USA

The purpose of the investigation was to determine the difference in chemical composition for unused and used (waste) motor oils andto understand their partitioning effect in water. Three brands of motor oil (unused and used) were selected for this study. Our analyticalresults show that the chemical composition of unused and used motor oil is significantly different, due in part to the presence of gasolinecombustion residues and polycyclic aromatic hydrocarbons (PAH) in the waste motor oil. No aromatic hydrocarbons were detectedin the unused motor oils, irrespective of the brand, whereas alkylbenzenes, naphthalene, and methylnaphthalenes are abundant in thewaste oils. Three and four-ring PAHs present as subordinate concentrations in used lubricants are probably synthesized during exposureof the motor oil to high engine temperature. Total ion chromatogram (TIC) chromatograms of extracts of water associated with unusedmotor oils, are dominated by a series of polar (N, S, O) compounds, which vary in composition with different brands of motor oils.The chromatographic TIC patterns of water extracts from waste motor oils are dominated by alkylbenzenes and naphthalenes, with N,S, and O polar compounds present as subordinated components. Furthermore, it is important to note that there is no unresolved humpin TIC chromatograms of the water phase extract of motor oil (unused and waste), although both non-aqueous phase liquid (NAPL)chromatograms have prominent unresolved humps. Therefore, the absence of an unresolved hump in a groundwater sample is not amarker which can be used to identify the presence or absence of motor oil in the contaminant NAPL.

Keywords: motor lubricating oils, partition, waste oils, polar compounds

Introduction

Motor lubricating oil is a common contaminant in water andsoils. Generally, motor oil comprises 80% of the hydrocar-bon lubricant, with the remainder being additives, which con-sist partly of zinc diaryl, molybdenum disulfide, zinc dithio-phosphate, metal soaps, and other organometallic compounds.Detergents and dispersants constitute 2–15% of the additives(Vasquez-Duhalt, 1989). The function of additives can be sum-marized as: 1) protect metal surface, such as antiwear and EPagents, corrosion and rust inhibitors, detergents, dispersants, andfriction modifiers; 2) extend the range of lubricant applicability,such as pour-point depressants, seal-swell agents, and viscos-ity modifiers; and 3) extend lubricant life with antifoamants,antioxidants, and metal deactivators.

Received 5 June 2007; accepted 7 December 2007.Address correspondence to Shan-Tan Lu, Zymax Forensics, Inc., A

DPRA company, 71 Zaca Lane, San Luis Obispo, CA 93401. E-mail:shantan@zymaxusa.com

Acknowledgment: We would like to express my sincere gratitudeto Ms. Carrie Walker for her review and proofreading on this article.Special thanks are due to John Pannis, Colleen Ryan, and Dan Goodwinfor their laboratory assistance. Furthermore, we appreciate the technicaldiscussion with Dwain Zsadayi for his valuable suggestions. Last, wealso thank the help of Jonathan Holt for his preparations of figures.

Engine lubricants generally are used to reduce friction be-tween moving parts within the interior engine. In addition to thelubricating function, motor oil also serves as a coolant, corro-sion protector, and a method of removing contaminants fromthe engine filter. As lubricants degrade, the physical properties(e.g., viscosity) change, leading to increased friction and wear.The degradation is primarily due to motor oil oxidation.

Lubricating oil normally ranges from about carbon (C20) toC35, but can be as low as C15 and as high as C50, depending on thedistillation process (Hunt, 1996). This range contains the nor-mal paraffin waxes (C22–C40) and some asphaltic compounds.Highly paraffinic crude oils frequently have a high wax contentin this range, and a correspondingly high pour point. The pourpoints of lubricating oils are lowered by removing waxes bymeans of solvents, for example, liquid propane or with ketonessuch as methylethylketone (Hunt, 1996).

One important property for assessing good lubricating oil isthe change in viscosity with temperature, known as the viscosityindex (VI), which ranges from 0–100. A VI of 100 indicates thatthe oil does not tend to become viscous at low temperature orbecome thin at high temperature. Paraffin-based lubricating oils,which contain long-chain hydrocarbons, have a VI of nearly 100,whereas naphthene-based oils have VIs around 40, and can reach0 at high concentration of naphthenic and aromatic components.

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296 S.-T. Lu and I. R. Kaplan

Before additives were widely used, the best lubricating oil wasfrom Pennsylvania crude oil, whereas the worst lubricating oilwas from the aromatic rich California crude. However, the vis-cous index can be raised by the introduction of additives. Thus,the introduction of additives has allowed California-derived lu-bricating oils to become more competitive (Hunt, 1996).

The manufacture of lubricating oil consists of five basic steps:1) distillation; 2) de-asphalting to prepare the feedstock; 3) sol-vent or hydrogen refining to improve VI and quality; 4) solventor catalytic de-waxing to remove wax and improve the low tem-perature properties of paraffinic lubes; and 5) clay or hydrogenfinishing to improve color stability, and quality of the lube basestock (Sequeira, 1992).

Used motor oil or waste engine oil is another name for usedmineral-based crankcase oil. Previous studies have focused onpolycyclic aromatic hydrocarbons (PAH) in used motor oils dueto their human hazard (Cotton et al., 1977; Grimmer et al., 1981;Peake and Parker, 1980; Pruell and Quinn, 1988; Naughton andJesperson, 1991; Rostad and Hostettler, 2007). The chemicalcomposition of used motor oil varies widely and depends on thecomposition of original crude oil, the process used during refin-ing, the additives, the efficiency and type of engine that the oilis lubricating, the gasoline combustion products, patented addi-tives, and the length of time that the oil remains in the engine(United States [US] Department of Health and Human Services,1997). Previous studies concluded that waste oil is typically73%–80% weight/weight aliphatic hydrocarbon (primarily alka-nes and cycloalkanes with 1–6 rings); 11%–15% monoaromatichydrocarbons; 2%–5% diaromatic hydrocarbons; and 4%–8%PAH (Vasquez-Duhalt, 1989). Unused motor oils (lubricant) aregenerally composed of 75%–95% aliphatic hydrocarbons (pri-marily alkane and naphthenes). The remainder comprises addi-tives. Furthermore, per our experience, no detectable aromatichydrocarbons are present in modern unused motor oils sold inthe US.

The compositions of unused motor oil have not been ex-tensively studied and may differ between commercial products.Furthermore, the motor oil–water partition, which determinestotal discharges to the aqueous environment, has received lit-tle attention (Chen et al., 1994). Thus, three brands of motoroil were selected for this study: Pennzoil Motor Oil (PennzoilCorp., Houston, Texas), Mobil Motor Oil (Mobil Oil Corp., Fair-fax, Virginia), and Castrol Motor Oil (Castro Oil Corp., Wayne,New Jersey). Following the suggestion from Kaplan et al. (2001)that a significant difference in chemical composition exists be-tween unused motor oil and waste oil, we decided to conduct adetailed chemical analysis on both unused and used motor oilsfrom three manufacturers, in addition to identifying compoundsextracted by water washing from the six oil samples.

The purpose of this study is: 1) to determine the difference inchemical composition for unused and used motor oils, and 2) toqualitatively understand the partitioning of compounds amongunused and waste lubricating oil when exposed to a water phase.In order to identify peaks in the mass chromatograms, we had toconcentrate the water extracts to a low volume. Thus, we were

unable to quantify the concentration of dissolved components inthe extracts.

Materials and Methods

The motor oil (unused and waste) was passed through a silica-packed column to remove polar and asphaltene components. Theeluted aliphatic and aromatic fraction was concentrated and 1 µlof concentrate was injected directly into a HP 5890 (Hewlett-Packard, San Jose, California) gas chromatograph connected toa HP 5971A mode mass spectrometer.

The motor–water partitioning studies were performed as fol-lows: 1 mL of product was introduced to a 2-L separatory funnel,which was filled with 1 L of deionized water and seated in a hoodfor 1 week. Deionized water was used to minimize interferencefrom biodegradation and other chemical reactions. To avoid anyproduct colloids dispersed in water, the funnel containing theproduct–water mix was not agitated or shaken following prod-uct introduction. Furthermore, only approximately 900 mL (bot-tom part) of 1 L was collected for extraction. The non-aqueousphase liquid (NAPL) was separated from the rest of water (upperpart) and designated as product residue. The extraction of wa-ter phase of each sample was conducted by EPA method 3510(EPA, 1996). The water extract was concentrated and 1µl ofconcentrate was also injected directly into the same HP 5890gas chromatograph with mass spectrometer. This allowed us tobe more precise in identification of minor constituents.

No volatile compounds were detected for the unused motoroil (performed at our laboratory). Thus, we decided to focus onthe semi-volatile components. Consequently, we only applieddirect injection instead of the purge-and-trap method.

The gas chromatogram (GC) contained a 60-m DB1-ms,0.25-mm fused silica capillary column and was programmed at4◦C/min, starting from 40◦C (isothermal at 40◦C for 5 minutes)to 310◦C and held for 30 min at 310◦C. To identify unknowncompounds generated during passage of the lubricant through ahot engine, scanning was performed over a mass range of 50–550amu (full scan). Mass spectral data were stored and processedwith an HP ChemStation data system. In addition, selected ionmonitoring analysis was also applied to biomarker analyses (ter-panes and steranes).

Compounds were identified by computer spectral matchingtechniques, by comparing unknown spectra to those in the Na-tional Bureau of Standards library. Additionally, the biomarkercompounds (e.g., terpanes, steranes, and aromatic steranes) wereidentified by comparing their retention times and mass spec-tra with published data (Philp, 1985; Peters and Moldowan,1993).

Three brands of motor oils were selected to conduct the re-search: 1) Pennzoil 10W-40 motor oil; 2) Mobil 10W-30 motoroil; 3) Castrol 10W-40 motor oil. Each study includes unusedand waste oils for comparison. The used Pennzoil and Mobilmotor oils were run by a Toyota Corolla sedan (Toyota MotorCorp., Toyota City, Japan), and the Castrol motor oil was run bylight-duty Toyota pick-up truck using gasoline.

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Motor Lubricating Oils 297

Results and Discussion

Chemical Composition of Unused Motor Oil:General Characterization

Pennzoil 10W-40 Motor OilThe TIC of unused Pennzoil motor oil (Figure 1a) is domi-

nated by a large unresolved hump with a retention time rangingfrom 45 –80 min. A relatively high concentration of 2,6-bis (1,1-dimethylethyl) phenol (C16H22O) is present in the motor oil atelution time of 36 minutes, which probably was introduced as anadditive. The mass chromatogram of alkanes (m/z 85) displaysa series of peaks, ranging from n-C13–n-C30 in the unused motoroil (Figure 1b). A homologous mix of alkylcyclohexanes rang-ing from CH-10 –CH-23 also is present in the virgin lubricatingoil, optimizing at C16 (Figure 1c). No aromatic hydrocarbonswere detected in this brand of unused motor oil. However, smallconcentrations of terpanes and steranes were detected. The masschromatogram of terpanes (m/z 191) shows that a series of C23–C26 tricyclic terpanes and pentacyclic terpanes, such as C27, C29,and C30 hopanes, are most abundant, whereas C31–C35 homo-hopanes only are present in trace quantities (Figure 1d). Thekey to the labeling of the mass chromatograms is in Table 1.Furthermore, the mass chromatogram of steranes (not shown)indicates that the relative concentrations of C27–C29-diasteranesare higher than those of regular steranes. Thus, the distribu-tion patterns of terpanes and steranes provide evidence that themotor oil was manufactured at a relatively high temperature,which enrich tricyclic terpanes and diasteranes. This interpreta-tion confirms the previous finding (Peters et al., 1992; Kaplan etal., 2001) that those thermal resistant compounds are enrichedduring high temperature refining.

Mobil 10–30 Motor OilThe TIC of Mobil motor oil (Figure 2a) is similar to that of the

Pennzoil motor oil, both of which are dominated by a large unre-solved hump with a retention time in the range of 45–80 minutes.Nevertheless, the compound 2,6-bis (1,1-dimethylethyl) phenol,which is present in Pennzoil motor oil in relatively high concen-tration, is absent in Mobil motor oil, whereas diphenylamineis present in Mobil motor oil but absent in Pennzoil motor oil.No aromatic hydrocarbons were detected in Mobil motor oil.The mass chromatogram of alkanes (m/z 85) displays a series ofpeaks appearing at the top of the unresolved hump, ranging fromn-C16–n-C32 and optimizing around n-C24 (Figure 2b). The dis-tribution pattern of alkylcyclohexanes (m/z 83, Figure 2c) is verysimilar to the Pennzoil motor oil, optimizing at C16. However,the relative contents of terpanes (m/z 191) and steranes (m/z217) are higher in Mobil oil than those from Pennzoil oil. Thedistribution pattern of terpanes (Figure 2d) also appears slightlydifferent from that obtained from Pennzoil oil. It seems that thepentacyclic terpanes, i.e., C29-hopane, C30-hopane, and C31–C33

homohopanes, are more abundant in Mobil oil than in Pennzoiloil. Significant concentrations of steranes were detected in thisunused oil, and their distribution pattern was enriched with di-asteranes relative to regular steranes (m/z 217), similar to that in

Table 1. Key for tricyclic, tetracyclic, and pentacyclic terpanesidentification (m/z 191 mass chromatograms)

Code Identity Carbon #

0 C20-Tricyclic Terpane 201 C21-Tricyclic Terpane 212 C22-Tricyclic Terpane 223 C23-Tricyclic Terpane 234 C24-Tricyclic Terpane 245 C25-Tricyclic Terpane 25Z4 C24-Tetracyclic Terpane 246a C26-Tricyclic Terpane 266b C26-Tricyclic Terpane 267 C27-Tricyclic Terpane 27A C28-Tricyclic Terpane #1 28B C28-Tricyclic Terpane #2 28C C29-Tricyclic Terpane #1 29D C29-Tricyclic Terpane #2 29E 18α-22,29,30-Trisnorneohopane (Ts) 27F 17α-22,29,30-Trisnorhopane (Tm) 27G 17ß-22,29-30-Trisnorhopane 27H 17α-23,28-Bisnorlupane 2810a C30-Tricyclic Terpane #1 3010b C30-Tricyclic Terpane #2 30I 17α-28,30-Bisnorhopane 2811a C31-Tricyclic Terpane #1 31J 17α-25-Norhopane 2911b C31-Tricyclic Terpane #2 31K 17α,21ß-30-Norhopane 29C29Ts 18α-30-Norneohopane 29C30* 17α-Diahopane 30L 17ß-21α-30-Normoretane 29Ma 18α-Oleanane 30Mb 18ß-Oleanane 30N 17α,21ß-Hopane 30O 17ß,21α-Moretane 3013a C33-Tricyclic Terpane #1 3313b C33-Tricyclic Terpane #2 33P 22S-17α,21ß-30-Homohopane 31Q 22R-17α,21ß-30-Homohopane 31R Gammacerane 3014a C34-Tricyclic Terpane #1 34S 17ß,21α-Homomoretane 3114b C34-Tricyclic Terpane #2 34T 22S-17α,21ß-30-Bishomohopane 32U 22R-17α,21ß-30-Bishomohopane 3215a C35-Tricyclic Terpane #1 3515b C35-Tricyclic Terpane #2 35V 17ß,21α-C32-Bishomomoretane 32WS 22S-17α,21ß-30,31,32-Trishomohopane 33WR 22R-17α,21ß-30,31,32-Trishomohopane 3316a C36-Tricyclic Terpane #1 3616b C36-Tricyclic Terpane #2 36XS 22S-17α,21ß-30,31,32,33-Tetrahomohopane 34XR 22R-17α,21ß-30,31,32,33-Tetrahomohopane 34YS 22S-17α,21ß-30,31,32,33,34-Pentahomohopane 35YR 22R-17α,21ß-30,31,32,33,34-Pentahomohopane 35

Pennzoil oil. The differences in concentrations and distributionsof terpanes and steranes between the Pennzoil and Mobil motoroils may result from differences in the initial feedstock as wellas refinery manufacturing processes.

Castrol Motor Oil 10W-40Minor differences in the distribution patterns of TIC, alkyl-

cyclohexanes (m/z 83), and terpanes (m/z 191) were observed

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between unused Castrol motor oil (Figures 3a, 3c, and 3d, re-spectively) and Pennzoil motor oil. The distribution pattern ofn-alkanes (m/z 85) obtained from Castrol oil (Figure 3b) moreclosely resembles Mobil oil than Pennzoil. Again, no aromatichydrocarbons were detected in Castrol motor oil. Alkylcyclo-hexanes extended from CH-12 to CH-22, optimizing at CH-18.Tricyclic terpanes were present at about the same relative con-centration levels as pentacyclic terpanes, but they were lowerthan those in the Mobil oil sample.

Differences in Brands of Unused Motor OilsSome differences in TIC were observed among those studied

Mobil, Pennzoil, and Castrol motor oils. For example, there aremore peaks (n-alkanes) seated on the top of the large unresolvedhump in the TIC of Mobil oil than Pennzoil or Castrol motor oils.The relative abundances of steranes and terpanes are also higherin Mobil motor oil than those from either Pennzoil or Castrolmotor oils. The reasons for theses differences are not known, butmay be attributed to the use of various feedstock (crude oils) orrefining procedures. A relatively high concentration of 2,6-bis(1,1-dimethylethyl) phenol is present in the Pennzoil motor oil,whereas diphenylamine is present in Mobil motor oil but absentin Pennzoil or Castrol motor oils.

Chemical Composition of Used Motor Oil (Waste Oil):General Characterization

A similar profile of unresolved hump, as appeared in the unusedoil, is also present in the used waste oil in the same retention timerange. Many peaks, however, elute earlier in the more volatilerange, at retention times of 10–50 min. The relative abundanceof those volatile peaks look similar for all three used waste oils,e.g., Pennzoil (Figure 4a), Mobil (Figure 5a), and Castrol (Fig-ure 6a). The volatile-range prominent peaks specific to used oilshave been identified as one to two-ring aromatics, such as alkyl-benzenes, naphthalene, and alkyl-naphthalenes. The presenceof these compounds is the most significant difference in hydro-carbon composition observed between unused and used motoroils. As previously discussed, no aromatic hydrocarbons weredetected in the unused motor oil, whereas relatively high con-centrations of aromatic hydrocarbons (one to two rings) wereobserved in the used motor oil (4d, 5d, and 6d; Table 2). Fur-thermore, significant concentration of three to four-ring PAHs,such as phenanthrenes, pyrenes, and chrysenes are also presentin used motor oils, but in much lower abundance than thosemore volatile mono- and di-aromatic compounds. The mostprobable source for the aromatic hydrocarbon content is fromgasoline residues, which include un-combusted and combustedgasoline residues. It is also possible that the PAHs may be syn-thesized either from naphthenes or from naphthenic componentsin the unresolved complex materials because motor oils alwaysassociate with a huge hump. However, we did not attempt todetermine the source of the PAH in this study.

In addition, the aromatic hydrocarbon composition of used(waste) oils are similar to each other for Pennzoil and Mobil oils,

whereas three to four-ring aromatic hydrocarbons are relativelymore abundant in Castrol than the former two oils. The reasonsfor these differences are unclear, though they may be attributedto different type of engines that the oil is lubricating and possiblythe different length of time that each oil remained in the engine.Furthermore, it is interesting to note that trace amounts of five-rings pyrogenic aromatic hydrocarbons were detected in the usedPennzoil and Castrol motor oils, but they were not detected inused Mobil motor oil.

Comparison between unused and used motor oils of the samebrand, display similarities in both relative abundance and dis-tribution patterns of various classes of aliphatic hydrocarbons.Nevertheless, the carbon ranges for n-alkanes are wider in usedmotor oil than their corresponding unused oil. For example, thedistribution pattern of n-alkanes (m/z 85) is from n-C10–n-C33

for used waste oils (Figures 4b, 5b, and 6b), and from n-C16 to n-C30–C33 for unused oils (Figures 1b, 2b, and 3b). The source ofn-C10–n-C15 may result from either of two possible processes:first, by cracking of C–C bonds in long carbon chains duringcombustion in the engine chamber or, second, from partiallycombusted gasoline residues, because gasoline contains n-C6–n-C12 alkanes.

A homologous series of CH-10 to CH-23 alkylcyclohexanesis present and similar to each other for all studied unused andused motor oils. However, a small amounts of CH-1 to CH-9 arepresent in Pennzoil and Mobil waste oils (Figures 4c and 5c),whereas they are absent in both Pennzoil and Mobil unused mo-tor oils. The relative concentration is the highest for CH-1 anddecrease toward CH-9. This finding again supports previous con-clusion that they were formed as gasoline combustion residues,because CH-1 is the most abundant alkylcyclohexane in gaso-line. However, CH-1–CH-9 alkylcyclohexanes were absent inCastro waste oil (Figure 6c). The possible explanation for thismay be attributed to the higher engine combustion temperaturefor the pick-up truck than that for sedan vehicle. Unfortunately,we did not predict that the different engine would cause problem.Because each sampling took 3 months per vehicle, we decidedto use two different vehicles to perform the study. In retrospect,this plan was wrong.

Chemical Composition of Water Extracts (Unused Motor Oils)

Aqueous Phase of Unused Pennzoil Motor OilThe large unresolved hump, which is dominant in the un-

used Pennzoil motor oil, is essentially absent in the TIC ofthe corresponded water extract (Figure 7a). The mass chro-matograms of alkanes (m/z 85) shows that only trace amount ofn-alkanes, ranging from n-C16–n-C21, are present in the waterphase. Further, apart from methyl styrene, no aromatic hydrocar-bons were detected in the associated water phase of the unusedmotor oil. However, the TIC of the water extract is dominatedby numerous polar compounds (Table 3), such as 2,6-bis (1,1-dimethylethyl)phenol,C14H22O (which is also present in the un-used motor oil), 2-hexanol (C6H14O), 4-methyl-2-pentanethiol(C6H14S), diphenylamine (C12H11N), which was present in

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Table 2. Key for identifying aromatic hydrocarbons

No. m/z Abbreviation Compound

1 120 AB C3-alkylbanzens2 134 C4-alkylbenzens3 148 C5-afkyl benzenes4 162 C6-aiky I benzenes5 128 NAPH C0-naphthalene6 142 C1-naphthalenes7 156 C2-naphthalenes8 170 C3-naphthalenes9 184 C4-naphthalenes10 166 FL C0-fluorene11 180 C1-fluorenes12 194 C2-fluorenes13 208 C3-flucrenes14 222 C4-flucrenes15 154 BP C0-biphenyl16 168 Ci-biphenyls + dibenzofuran17 182 C2-biphenyls + C1-dibenzofurar18 178 PHEN C0-phenanthrene19 192 C1-pheranthreres20 206 C2-phenanthfenes21 220 C3-phenanthrenes22 234 C4-phenanthrenes23 202 PY C0-pyrene/fluoranthene24 216 C1-pyrenes/fluoranthenes25 230 C2-pyrenes/fluoranthenes26 244 C3-pyrenes/fluoranthenes27 258 C4- pyre nes/fluora nthenes28 228 CHR C0-chrysens29 242 C1-chrysenas30 256 C2-chrysenes31 270 C3-chrysenes32 284 C4-chrysenas33 148 BT C1-berizothiophenes34 162 C2-benzothiophenes3S 176 C3-benzolhiophenes36 190 C4-benzothiophenes37 204 C5-benzothiophenes28 184 DBT C0-dibenzothiophena39 198 C1-dibenzothiophenes40 212 C2-dibenzottiiophenes41 226 C3-dibenzothiophenes42 240 C4-dlbenzothioptienes43 234 NBT C0-naphthobenzothiophene44 248 C1-naphthobenzothiophenes45 262 C2-naphthobenzothiophenes46 276 C3-naphthobenzothiophenes47 280 C4-naphthobenzothiophenes48 253 MAS Monoaromatic steranes49 267 Monoaromatic Eteranes50 239 Monoaro malic steranes51 231 TAS Tri aromatic steranes52 245 Triaromatic steranes

Mobil virgin oil, and some unidentified compounds, followedby smaller quantities of α-methylstyrene (C9H10), acetophenone(C8H18O), α,α-dimethylbenzenemethanol (C9H12O), limonene(C10H16), and 1,3,5-trithiane (C3H6S3). Most, if not all of thesecompounds are probably additives to the motor oil. Their pres-ence in the water phase is due in part to the higher solubility ofthe N, O- and S-substituted compounds and their lower concen-tration compared with alkanes, as well as the analytical ability toconcentrate them and to detect them in the absence of paraffinicinterference.

Table 3. Polar compounds from water extracts of unused motor oils

Compound name Molecular Pennzoil Mobil Castro

2-Hexanol C6H14O xxx xxx4-methyl-2-pentanethiol C6H14S xxxx xx2-butanethiol C4H10S xxx2-methyl-1-butanethiol C5H12S x1-pentanethiol C5H12S xx4-pyridinamine C5H6N2 xMethylstyrene C9H10 xxAcetophenone C8H18O xxIsooctanol C8H18O x4-methyl-1-hepanol C8H18O x2,2’-thiobis-butane C8H18S xQuinoline C9H7N xIsoquinoline C9H7N xxDimethyl-benzenemethanol C9H12O xx1,2,4-Trithiolane C2H4S3 x xx1,3,5-trithiane C3H6S3 xx xBis(1,1-dimethylethyl)-phenol C14H22O xxxxButylesterbenzoic Acid C11H14O2 xxxDiphenylamine C12H11N xxx xxxxUnidentified polar compounds xx xx xxxx

xxxx: Very abundance.xxx: abundance.xx: presence.x: present in low concentration.

Aqueous Phase of Unused Mobil Motor OilThe large unresolved hump, which is dominant in the un-

used Mobil motor oil, is absent in the TIC of its water extract(Figure 7b). Numerous polar compounds dominate the TIC.Diphenylamine (C12H11N), which is present in the virgin mo-tor oil, is still the most prominent compound in the water ex-tract. Some of the polar compounds identified are similar tothose appearing in the water extract of the Pennzoil motor oil:2-hexanol (C6H14O), 4-methyl-2-pentanethiol (C6H14S), anddiphenylamine (C12H11iN; Table 3). However, other compoundssuch as iso-octanol (C8H18O), 4-methyl-heptanol (C8H18O), andbutylester-benzoic acid (C11H14O2) are present only in the waterextract of Mobil motor oil, and absent in the Pennzoil motor oil.Further, no n-alkanes or aromatic hydrocarbons were detectedin the water phase of Mobil oil. The method detection limit wasapproximately 5 ppm for water extracts in the gas chromatogra-phy/mass spectrometry full scan analysis.

Aqueous Phase of Unused Castrol Motor OilNumerous unidentified polar compounds, with retention

times ranging from 42–55 min, predominate in the TIC of theaqueous extract obtained from the unused Castrol oil (Figure7c). In addition, a significant number of identified polar com-pounds, such as 2-butanethiol (C4H10S), 2-methyl-1-butanethiol(C5H12S), 1-pentanethiol (C5H12S), s-(1-methylpropyl)ester-ethanethioic acid (C6H12OS), 4-pyridinamine (C5H6N2), 2,2-thiobis-butane (C8H18S), D-limonene (C10H16), s-phenylester-ethanethioic acid (C7H14OS), 1,2,4-trithiolane (C2H4S3), andquinoline (C9H7N) were detected in the extract, but at muchlower concentrations than that of the unidentified compounds.

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Figure 7. Polar compound in water extracts of unused a) Pennzoil, b) Mobil, and c) Castrol motor oils.

Most of these polar compounds probably were introduced asadditives during the manufacturing processes. However, we can-not eliminate the possibility that some of these polar compoundswere inherited from the initial feedstock (crude oil). The polarcompound composition from aqueous extract of Castro motor oil

differs significantly from those identified from either Pennzoilor Mobil oil. Furthermore, trace amounts of naphthalene andmethylnaphthalenes were detected in the unused Castro motoroil water extract, which were absent in the water extracts of bothunused Pennzoil and Mobil motor oils.

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Figure 8. Polar compound in water extracts of used a) Pennzoil, b) Mobil, and c) Castrol motor oils.

Chemical Composition of Water Extracts (Used Motor Oils)

Water Extract of Used Pennzoil Motor OilThe large unresolved hump present in the waste oil is not

present in the water extract of the oil. The TIC of the Pennzoilused motor oil (Figure 4a) is dominated by a series of volatile aro-matic (one to two-ring aromatics) hydrocarbons with retention

times between 14.5–37.5 minutes. The series of one to two-ringaromatics was also detected in the chromatogram of the waterextract (Figure 8a). However, PAH with larger molecular weightthan methylnaphthalenes, which are present at subordinate con-centrations in the waste oil, are only present at trace levels in thewater extract.

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Table 4. Polar compounds from water extracts of used motor (waste) oils

Compound name Molecular Pennzoil Mobil Castro

C1-C3 Phenols C6-C9H12O xx xx xxC0-C1-Benzofuranones C8-C9H8O2 x xC1-C2-benzaldehyde C8-C9H10O xx xx xxBenzyl alcohol C7H8O x1H-phenalen-1-one C13H8O x x xcarbazole C12H9N x2-naphthalenecarboxaldehyde C11H18O xx2,6-bis(1,1-dimethylethyl)-4- C16H26O x

ethyl-phenol2-hydroxy-benzaldehyde C7H6O2 xMethoxy-benzaldehyde C8H8O2 x2,6-dimethyl-4-nitro-phenol C8H9NO3 x9,10-anthracenedione C14H8O2 xPhenindione C15H10O2 xUnidentified polar compounds xx xxx xx

xxx: abundance.xx: presence.x: present in low concentration.

The primary aromatic hydrocarbons are toluene, xylenes,ethylbenzene, C3and C4-benzenes, naphthalene, and methyl-naphthalenes. There are many polar aromatic compounds(Table 4), such as C1-C3 phenols, benzofuranones (m/z 134),C2-benzaldehyde (m/z 134), C1-benzofuranones (m/z 148), 1H-phenalen-1-one (m/z 180), and carbazole that are present atmuch lower concentrations. Furthermore, some hydrocarbons,such as indane, C1-indanes, indene, C1-indenes, which arepresent in trace amounts in the waste oil, become concentratedin the aqueous extract.

Water Extract of Used Mobil Motor OilThe primary compounds present in the water phase from used

Mobil motor oil (Figure 8b) are similar to those from the wa-ter phase of the used Pennzoil motor oil. They are toluene,xylenes, ethylbenzene, C3 to C4-benzenes, naphthalene andC1-naphthalenes. C2–C4-naphthalenes, C0 to C1-phenanthrenes,C0-pyrene, and fluoranthene are present at very low concentra-tions. The n-alkanes were not detected in the water phase, al-though they are present in both unused and used motor oils.

Further, there are some unidentified polar compounds foundin the Mobil extract (Table 4), with retention time between35–70 minutes, which do not appear in the water extract ofPennzoil oil. Other hydrocarbons (e.g., indane, indene, C1-C2-indanes) and polar compounds (e.g., C1-C3- phenols, C1-C2-benzaldehydes, and 1H-phenalen-1-one [C13H8O]) are similarto those in the water extract of used Pennzoil motor oil, whereas2-naphthalenecarboxaldehyde (C11H18O) is only present in thewater extract of Mobil motor oil.

Water Extract of Used Castrol Motor OilIn general, the primary compositions of the water extracts

are similar to each other among Pennzoil, Mobil, and Castrolmotor oils (Figure 8c). They are dominated by one to two ringsaromatic hydrocarbons (Figure 8c). However, the distribution of

polar compounds is somewhat different for the water extract ofCastro and their relative abundances are much lower than thealkylbenzenes and naphthalenes (Table 4).

More polar compounds were detected in water extracts ofCastrol motor oil than those from the water extracts of either usedPennzoil or Mobil motor oil. For example, C1-C3 phenols and1H-phenalen-1-one (m/z 180) are present in the aqueous extractsof all studied motor oils, whereas benzaldehyde, benzy alcohol,2,6-dimethyl-4-nitro-phenol, 9,10-anthracenedione (m/z 208),C1-C2-anthracenediones, and phenindione (m/z 222) are onlypresent in the water extract of used Castrol motor oil. In contrast,carbazole is only present in the water extract of Pennzoil oil,but absent in the extracts of both Mobil and Castrol oils. Thedifference in polar compounds may be attributed to be refineryprocedures (different additives) and compositional differencesamong starting crude oil blends, which can be used as tracers todifferentiate the manufacturer of the lubricating oil.

Oil–Water Partitioning RemarksThe reason for the absence of the unresolved hump in the

water phase is most probably due to the absence of the insolubleheavy polycyclic naphthenic hydrocarbons in the water extract.Initially, the period of extraction was carried out for 1 week,which, after review of the results, was thought to have been tooshort to allow the unresolved hump to dissolve into the waterphase. Thus, we extended the extraction period from one weekto one month and utilized extraction agitation to facilitate themotor oil to partition into the water phase. However, the resultsremained identical to those of 1-week data. We therefore con-clude that the constituents composing the unresolved hump arehighly insoluble. No significant peak sits on top of the unresolvedhump, making it difficult to identify compounds by looking atany particular spectrum, This would probably be the same at alower method detection limit.

Conclusions

1. It is important to note that there is no hump in the waterphase of motor oil (unused and used), although the unre-solved hump predominated in both product TICs. However,the absence of the unresolved hump in any water extract of anaqueous sample should not be taken as evidence that waterhas not been in contact with motor oils. The unresolved humpmainly represents insoluble naphthenic hydrocarbons, whichare not extracted by water. Our experience has shown that itis common to have an inseparable sheen floating on top of awater column in the natural environment. As the laboratoryis obliged to analyze the entire sample, unless otherwise in-structed, it is possible for analysts to report an anomalouslyhigh value for total dissolved hydrocarbon.

2. The chemical compositions of unused and used motor oils aresignificantly different due to either the addition of partiallycombusted gasoline residues or the residues of high tem-perature cracking in the used waste motor oil. Nevertheless,

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the retention time of dominant peaks and the shape of theunresolved hump are similar for all of them.

3. No aromatic hydrocarbons were detected in any of the un-used motor oils analyzed, whereas alkylbenzenes, naph-thalene, and methylnaphthalenes are abundant in the usedmotor oils (waste oil) and three and four-ring aromatics arealso present at subordinate concentrations.

4. The primary compounds present in the water phase ofused motor oils are toluene, xylenes, ethylbenzene, C3 toC4-benzenes, naphthalene and C1-naphthalenes. C2 to C4-naphthalenes, C0 to C1-phenthalenes, C0-pyrene, and fluo-ranthene are also present at very low concentration.

5. There are numerous polar (N, S, O) compounds present inthe water phase of various brands of unused and waste oils.The composition of polar compounds varies with differentbrands of motor oil. These represent a mixture of refinery-specific additives, crude oil derived compounds and possiblycompounds generated during exposure to air in a hot engine.

6. The presence of polar compounds in the ground water may beused to differentiate different lubricating oil manufacturers,and we recommend that a catalogue of such compounds beestablished for the different commercial products.

References

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