(2007) 234–238www.elsevier.com/locate/microc

Microchemical Journal 85

Analysis of BTEX, PAHs and metals in the oilfieldproduced water in the State of Sergipe, Brazil

Haroldo S. Dórea ⁎, José R.L. Bispo, Kennedy A.S. Aragão, Bruno B. Cunha, Sandro Navickiene,José P.H. Alves, Luciane P.C. Romão, Carlos A.B. Garcia

Chemistry Department, Federal University of Sergipe, 49100-000, São Cristóvão, SE, Brazil

Received 3 February 2006; received in revised form 22 February 2006; accepted 1 June 2006Available online 12 July 2006

Abstract

During oil and gas exploitation, large amounts of produced water are generated. This water has to be analyzed with relation to the chemicalcomposition to deduce the environmental impact of its discharge after a treatment process. Therefore, a studywas carried out to evaluate preliminarily theBTEX (benzene, toluene, ethylbenzene and xylenes), polycyclic aromatic hydrocarbons (PAHs) and metals contents in produced water samples takenfrom effluents of the Bonsucesso treatment plant located in the city of Carmópolis, themost important oil and gas producer in the State of Sergipe, North-east of Brazil. Three methods were optimized to determine the target compounds. Polycyclic aromatic hydrocarbons were determined by gaschromatography with mass spectrometric detection (GC/MS), volatile aromatic hydrocarbons (BTEX) by gas chromatography with photoionizationdetector (GC/PID) and metals were analyzed by flame atomic absorption spectrometry (FAAS). The results showed that concentrations of the targetcompounds in these samples ranged from 96.7 to 1397 μg L−1 for BTEX, from 0.9 to 10.3 μg L−1 for PAHs and from 0.003 to 4540mg L−1 for metals.© 2006 Elsevier B.V. All rights reserved.

Keywords: Purge and trap GC/PID; GC/MS; FAAS; Produced water; PAHs; BTEX; Metals

1. Introduction

Petroleum exploitation began 1961 in the State of Sergipe inthe city of Riachuelo, with activities of which extended rapidlyto other regions. The State of Sergipe is the third biggest petro-leum producer in the Northeast region of Brazil. This activity isvery important to the local economy, being responsible for 40%of gross regional domestic product. Today, petroleum produc-tion is undertaken in both onshore and offshore operations [1].Petroleum exploration and production in the Sergipe/Alagoasregion produces approximately 8500 m3 of crude oil per day,5000 m3 of natural gas per day and 33,000 m3 of water per day.Approximately 85% of this produced water is extracted in theterrestrial platforms and is sent to the Bonsucesso treatment plantlocated in the city of Carmópolis. The State of Sergipe is Brazil's

⁎ Corresponding author. Universidade Federal de Sergipe, CCET-Departa-mento de Quimica, Av. Marechal Rondon, s/n. Jardim Rosa Elze CEP 49100-000 Sao Cristovao/SERGIPE, Brazil. Tel.: +55 79 3212 6654; fax: +55 79 32126681.

E-mail address: hdorea@ufs.br (H.S. Dórea).

0026-265X/$ - see front matter © 2006 Elsevier B.V. All rights reserved.doi:10.1016/j.microc.2006.06.002

largest terrestrial petroleum production field and the fourthbiggest producer in the world, with an estimated production of3600 m3 of crude oil per day and 28000 m3 of water per day.A part of this water is re-injected into wells to extend theproduction lifetime of the oilfield and the rest used to dissolvesalts generated by the Vale do Rio Doce Company, being sub-sequently discharged to subsurface waters after treatment.

The production of oil and gas is usually accompanied by theproduction of water. This produced water consists of formationwater, which is water naturally present in the reservoir, flood-water previously injected into the formation and condensedwater, as a result of gas production [2]. Hence, produced wateris a by-product of the oil and gas exploitation and productionprocesses in terrestrial or offshore platforms. Large volumes ofthis produced water are often generated because many oildeposits reside in or around groundwater aquifers [3]. Duringthe lifetime of the oilfield, the amount of water may range fromseven to ten times the amount of oil produced [4]. The chemicalcomposition of this wastewater is dependent upon thegeological formation. It may contain various toxic compoundsof natural origin such as volatile aromatic fractions of the oil,

235H.S. Dórea et al. / Microchemical Journal 85 (2007) 234–238

including BTEX, polycyclic aromatic hydrocarbons (PAHs),organic acids, phenols and alkylated phenols, metals, radio-nuclides and a very high salt concentration.

Some studies have been described in the literature on chemicalcharacterization of produced water from offshore oilfield plat-forms in China, Norway, Austria and the United States [5–10]. InBrazil, limited monitoring has been carried out on naturally-occurring compounds such as organic acids, phenols, metals,PAHs and BTEX in produced water. Among them, there arestudies on the evaluation of different technologies for oilfieldwastewater treatments from the Campos Basin, State of Rio deJaneiro [11,12]. However, there is a general lack of data on thechemical composition of produced water from oilfields in theState of Sergipe. The chemical composition of produced water isfield-dependent. So, the field-specific detailed chemical charac-terization of produced water from each oilfield or platform isnecessary in order to predict the fate and effects of the producedwater discharged to the environment.

Taking into account the above considerations and consideringthat, to date, no data on chemical composition of the oilfieldproduced water in the State of Sergipe have been reported in theliterature, this paper provides a preliminary evaluation of the levelsof BTEX, PAHs andmetals in the effluent of producedwater fromthe Bonsucesso treatment plant located in the city of Carmopólis,State of Sergipe. This treatment station receives produced waterfrom a number of different petroleum production units.

2. Experimental

2.1. Chemicals and materials

Dichloromethane, acetonitrile and n-hexane were nanograde(Mallinckrodt Baker, Paris, KY, USA). Certified standards ofBTEX (benzene, toluene, ethylbenzene and m-xylene, o-xyleneand p-xylene) were purchased from ChemService at concentra-tion of 2000 μg mL−1 in methanol (West Chest, PA, USA).Intermediate solutions were prepared by diluting the standardmixture at concentrations of 5 and 50 μg mL−1 in methanol. Thedeionized water used for preparing working solutions was puri-fied by a Millipore (Bedford, MA, USA) Milli-Q water purifica-tion system. Certified standards of the 12 PAHs were purchasedfrom AccuStandard (New Haven, CT, USA): naphthalene,acenaphthene, phenanthrene, anthracene, fluoranthene, pyrene,benzo[a]anthracene, chrysene, benzo[a] pyrene, perylene, benzo[e] pyrene, benzo[g,h,i] perylene. The stock solutions of the PAHwere prepared in acetonitrile at 100 μg mL−1 and stored at−18 °C. Theworking standard solutions were prepared by dilutingthe stock solutions in dichloromethane as required. All standardswere at least 99% pure. All stock solutions of metals (1000 μgmL−1) were stored in nitric acid-washed polyethylene bottles.Analytical grade anhydrous sodium sulfate and silica-gel 60 (70–230 mesh) were supplied from Merck (Darmstadt, Germany).

2.2. Sampling

Produced water samples were collected from the Bonsucessotreatment station located in the city of Carmópolis (State of

Sergipe, Brazil). For PAHs, the volumes were 1 L in amberglass bottles pretreated with acetone and methanol. For BTEX,50 mL bottles were filled to capacity to avoid any headspace.After filling with water, the bottles were sealed with Teflon-lined screw caps and transported on the same day to the labo-ratory under refrigerated conditions. In the laboratory, sampleswere preserved at a temperature of 4 °C, and then diluted forfurther analysis of BTEX. For metal analysis, samples werecollected in 1 L acid-washed polyethylene bottles. After fillingwith water, the bottles were sealed with Teflon-lined screw capsand transported on the same day to the laboratory under refrig-erated conditions. In the laboratory, samples were preserved at atemperature of 4 °C before utilization.

2.3. Sample preparation

The samples for PAHs and BTEX analyses were not filtered.Before metal analysis, the samples were filtered through a0.45 μm Millipore filter. Determination of ammonia was madeusing the indophenol blue reaction, while the azide-Winklermethod and electrical conductivity were used to measure dis-solved oxygen and salinity, respectively.

2.4. Apparatus

2.4.1. Purge and trap conditionsATekmar–Dohrmann velocity XPT purge-and-trap (Mason,

Ohio, USA) was used to pre-concentrate the BTEX in producedwater. The P and T unit consists of a 5 mL stripping vessel. Nocryogen was needed for BTEX trapping, using Tenax TA asadsorbent trap. The water sample was purged with helium(99.995% purity) at 40 mL min−1 during 11 min at 26 °C. Thevolatiles were thermally desorbed (180 °C, 4min), and transferredto the GC/PID-FID via a transfer line heated to 100 °C. Afterdesorption, the trap was cleaned by heating at 200 °C for 8 min.

2.4.2. GC/PID-FID conditionsA Shimadzu 17A gas chromatograph (Kyoto, Japan) equipped

with photoionization (PID) and flame ionization (FID) detectorsand a split/splitless injector was used for quantification of BTEX.A fused-silica megabore column, DB-624 (6% cyanopropyl-phenyl–94% dimethylpolysiloxane; 75 m×0.53 mm I.D.,3 μm), supplied by J and W Scientific (Folsom, CA, USA),was employed, with helium (purity 99.995%) as carrier gas at aflow-rate of 10 mL min− 1. The column temperature wasprogrammed as follows: 30 °C for 1 min, increasing to 100 °Cat 5 °Cmin−1 and directly to 220 °C at 8 °Cmin−1 then holding for5 min. The injection port and detector were operated at 180 and220 °C, respectively. The PID system was operated with a 10 eVbulb and at an intensity of 1 mA.

2.4.3. Flame atomic absorption spectrometric conditionsA Shimadzu (Kyoto, Japan) AA-6800 flame atomic

absorption spectrophotometer equipped with BCG-D2 back-ground correction was used for determining the metals. Theinstrumental adjustments were determined according to therecommendations of the manufacturer.

236 H.S. Dórea et al. / Microchemical Journal 85 (2007) 234–238

2.4.4. GC/MS conditionsA Shimadzu system consisting of a QP-5050A mass spec-

trometer equipped with a GC-17A gas chromatograph with asplit/splitless injector was used for the quantification andconfirmation of the polycyclic aromatic hydrocarbons. A fused-silica column, DB-5 ms (5% phenyl-95% dimethylpolysilox-ane; 30 m×0.25 mm I.D., 0.25 μm), supplied by J and WScientific (Folsom, CA, USA), was employed with helium(purity 99.995%) as carrier gas at flow-rate of 1.5 mL min-1.The column temperature was programmed as follows: 40 °C for1 min, increasing to 160 °C at 25 °C min-1 and directly to300 °C at 5 °C min-1 and holding for 6 min. The solvent delaywas 5 min. The injector port was maintained at 250 °C and 1 μLvolume was injected in splitless mode (1.0 min). The data wereacquired and processed by the Shimadzu Class 5000 software.The effluent from the GC column was transferred via a transferline held at 280 °C and fed into a 70 eV electron impactionization source held at 280 °C. The analysis was performed inthe selected ion monitoring (SIM) mode.

2.5. Methods

2.5.1. Procedure for BTEX analysisAn analytical aliquot of 5 mL of produced water with 10 μL

of α, α, α-trifluorotoluene (surrogate) was transferred to the Pand T port without air bubbles. The sample was purged withhelium and the volatiles were trapped on the Tenax adsorbentand thermally desorbed into the GC/PID system.

2.5.2. Procedure for polycyclic aromatic hydrocarbon analysisAn analytical aliquot of 1000 mL of produced water sample

was placed in a separating funnel. 30 mL of n-hexane wereadded and the mixture was extracted for 20 min in a manualshaker. The residue was re-extracted with another 2×30 mL ofn-hexane. The extract was concentrated using a rotaryevaporator (35 °C) and was transferred to at chromatographiccolumn filled with 0.5 g of silica-gel (200–325 mesh) and 5 g ofanhydrous sodium sulfate. The PAHs were eluted with 20 mL ofn-hexane:dichloromethane (70:30, v/v) mixture and evaporatedto nearly 0.1 mL under a gentle stream of nitrogen. The residuewas dissolved in dichloromethane (1 mL) and an aliquot (1 μL)was analyzed by GC/MS.

2.5.3. Procedure for heavy metal analysisAfter the metals had been extracted from the matrix using the

ammonium pyrrolidine dithiocarbamate (APDC) in methylisobutyl ketone (MIBK) method [13], the metals were measuredin the organic phase by FAAS.

3. Results and discussion

3.1. General comments

This study reports on results obtained from a survey ofwastewater from the Bonsucesso produced water treatmentplant for the levels of BTEX, PAHs and metals in the effluent.This treatment plant was selected because it contributes a

high percentage of the total amount of produced water dis-charged into subsurface waters from the oilfields of the Stateof Sergipe.

3.2. BTEX analysis

The above described extraction procedure was developed onthe basis of EPA method 524.2 (purgeable organic compoundsby the PT technique) [14]. To find the extraction conditionsproviding the highest yields of BTEX from the produced waterand to validate the method for the simultaneous determinationof these analytes, some parameters of the purge and trap systemand of the chromatograph were optimized: Purge and desorptiontimes and temperatures; transfer line and column temperatures;detector selection (PID or FID). The m-and p-xylenes were notseparated and were determined together.

After the definition of the analytical conditions for the PT-GC/PID-FID, a study of comparison was performed between thetwo gas chromatographic detection systems (PID and FID) tofind the detector providing the highest sensitivity using standardsolutions at 4 ng mL−1. Considering the results, PID showedhigher sensitivity ranging from 5.6 (for the ethylbenzene) to 7.2(for the m+p-xylenes) times greater than FID. Hence, PID waschosen for determining BTEX in produced water.

Recovery assays were done in triplicate by spiking producedwater samples at two levels (4 and 40 μg L−1). The meanrecoveries obtained with spiked samples at the 4 μg L−1 levelranged from 105.5% to 138.6%, with relative standard devia-tions (RSD) between 10% and 19.8%, while at 40 μg L−1

recoveries ranged from 99.9% to 104.3%, with RSD between1.2% and 3.5%. Surrogate standard was added to each extractionto control any losses during the analysis procedures. Addition-ally, each analytical sequence included quality control standardsand blanks to check noise and background levels, possiblecarryover effects and to cover small retention time variations.

Six point (average of three injections) calibrations (based onpeak areas) were obtained over the concentration range ofinterest (2–200 μg L−1 for m+p-xylenes and 1–100 ng mL−1

for other species). Calibration curve correlation coefficientswere higher than 0.9998 in all cases. The analytical calibrationcurves were used for quantification purposes using the externalstandards procedure. The limits of detection (LODs)were verifiedby analyzing the BTEXmixture at 0.5μg L−1. LODs ranged from0.06 μg L−1 for xylenes to 0.12 μg L−1 for benzene, while thelimits of quantification (LOQs) ranged from 0.21 μg L−1 forxylenes to 0.41 μg L−1 for benzene.

3.3. Polycyclic aromatic hydrocarbon analysis

The liquid–liquid extraction procedure described above wasdeveloped on the basis of the EPA 3510 and 8270C methods[15,16] and the Reddy and Quinn procedure [17]. The pro-cedures given in these sources were modified and further im-proved for produced water analysis. Positive identification ofthe total PAHs was achieved first by mass spectral identificationand then by acquisition of standard PAHs and comparison withunknown GC retention times.

Table 1Concentrations of PAHs and BTEX in produced water collected from theBonsucesso treatment plant, State of Sergipe

Compound Concentration (μg L−1)

Range Average⁎

Minimum Maximum

Naphthalene 9.9 10.7 10.3Acenaphthene 1.6 2.4 2.0Phenanthrene 2.2 2.4 2.3Anthracene 0.8 1.7 1.3Fluoranthene 2.7 6.2 4.4Pyrene 0.9 1.0 0.9Benzo[a]anthracene 1.6 1.9 1.7Chrysene 5.9 9.9 7.9Perylene 1.6 2.6 2.1Benzo[e] pyrene 2.4 2.6 2.5Benzo[g,h,i] perylene 0.9 3.1 2.0Benzene 1291 1511 1397Toluene 1167 1357 1263Ethylbenzene 136 158 148m+p-xylenes 194 242 216o-xylene 89 103 96

⁎ n=3 replicates.

Table 2Concentrations of metals, and hardness, salinity and density of produced watercollected from the Bonsucesso treatment plant, State of Sergipe, and from theNorth Sea

Parameter Carmópolis, Sergipe (mg L−1) North Sea⁎⁎

(mg L−1)Range⁎

Minimum Maximum Average

Pb 0.003 0.003 0.003 <0.500Co 0.003 0.004 0.003 <1.00Cu 0.001 0.001 0.001 <0.001–0.100Fe 4310 4770 4540 4.50–6.00Mn 0.058 0.068 0.062 Not analyzedNi 0.015 0.017 0.016 <0.300Zn 0.027 0.028 0.027 0.005– 35.0Cl 16,100 19,500 17,800 44,600K 3100 4900 4000 Not analyzedNa 8800 9600 9200 Not analyzedHardness of water,CaCO3

4890 5130 5010 7000

Salinity, NaCl 31,700 4110 36,400 58,500Density, 20°C 1010 1070 1040 Not analyzed

⁎n=3 replicates; ⁎⁎Ref. [21].

237H.S. Dórea et al. / Microchemical Journal 85 (2007) 234–238

Five point calibrations were performed at 10, 15, 20, 25and 50 μg L−1. The regression coefficients show good linear-ities in the concentration range under consideration, allowingquantification of these compounds by the method of externalstandardization.

The LODs were defined as a response three times theaverage height of the blank baseline noise and the LOQs weredefined as a response ten times the average height of the blankbaseline noise. So, the results were between 5 and 15 μg L−1 forLOD and between 15 and 40 μg L−1 for LOQ.

The herein reported results are based on an unfortifiedsample and three samples fortified with 1.0 μg L−1 of eachPAH. All experiments showed mean recoveries in the rangefrom 62.1% to 113.6%, except for the phenanthrene, for whichrecoveries exceeded 120%. Relative standard deviations werebetween 14.3% and 29.5%.

3.4. Heavy metal analysis

The predominant inorganic components associated withproduced waters are dissolved salts, which many times exceedthat of seawater. For instance, the concentration of chloride ex-ceeds the concentrations of other anionic produced water species(for the greater part by a factor of 1000 and more). Therefore,while quantification of major ionic components such as chlorideor sodium exhibits no significant problem, it is far more chal-lenging to monitor anions or cations occurring in produced waterat lower and especiallymuch lower concentrations. Regarding thisanalytical task, produced water can be categorized as a complexsample and hence often forces an analytical method to operatevery close of the performance limit.

In this work, recovery assays of seven metals were done byspiking water samples at 5 μg mL−1. Recoveries ranged from97% for Ni to 104% for Cr.

3.5. Real produced water samples

The developed methods were applied to the determination oftarget compounds in produced water samples collected from theBonsucesso treatment plant effluent. Target compounds levelsabove the detection limits were found. Tables 1 and 2 contain asummary of the occurrence and concentrations of target com-pounds in produced water samples. These data reflect mainlythe presence, at the highest concentration, of BTEX, althoughthe presence of PAHs was also detected. Individual PAHconcentrations ranged from 0.9 to 10.3 μg L−1, with total PAHsof 37.4 μg L−1. In earlier work, UV-fluorescence was used todetermine PAHs in seawater from different coastal areas inBrazil, with a range from <0.04 to 52.43 μg L−1 [20]. Utviket al [18] evaluated concentrations of produced water originatedPAHs in the marine environment of the North Sea. Measurableconcentrations of PAHs in produced water included a notableproportion of alkylated PAHs, in addition to the parent PAHcompounds, from the Norwegian Brage field, North Sea, with atotal range from 0.2 to 350 μg L−1. Discharged PAHs werereadily diluted by receiving waters, so that PAH concentrationsin the ocean typically reach near background levels a shortdistance from the discharge point. In addition, processes such asevaporation, sedimentation, adsorption, chemical oxidation,photo-oxidation and biodegradation contribute to lowering PAHconcentrations in the surrounding seawater. The total PAHsevaluated at that site were in the range of 0.002–0.01 μg L−1 forthe non-impacted locations.

There is scarce information available on the concentrationlevels of PAHs in produced water from Brazilian oilfields,however the levels recorded in this study for the PAHs in pro-duced water are higher than those found in some other locations,such as Western Xiamen Bay in China (0.106–0.945 μg L−1 insub-surface marine water) or the Baltic Sea and the Seine River

238 H.S. Dórea et al. / Microchemical Journal 85 (2007) 234–238

and Estuary (France), where values ranged from 0.0003 to0.0005 ng L−1 and 0.004 to 0.036 μg L−1, respectively. How-ever, the present concentrations are lower than found in DayaBay, China, (range 4.2–29.3 μg L−1) [19].

BTEX mean levels found in our study were between 96.7and 1397 μg L−1. Results of these samples were given as thesum of m- and p-xylenes, because these isomers show similarrecovery and response. These findings are of the same order ofmagnitude as those reported by other authors, such as in theeffluent of produced water from platforms of two differentpetroleum production units located in the Campos Basin, Stateof Rio de Janeiro, Brazil (283–1855 mol L−1 for benzene,87.04–2224 mol L−1 for toluene, 16.77–1220 mol L−1 forethylbenzene and 67.35–5969 mol L−1 for xylenes) [11]. Forcomparison, 1318 μg L−1 for benzene and 1065 μg L−1 fortoluene were detected in oilfield wastewater from platforms ofthe Gulf of Mexico [7], and in Norway total BTEX of 8000 μgL−1 was measured [5].

Metal mean levels found in our study are of the same order ofmagnitude as those reported by Ray and Rainer [21].

4. Conclusions

This paper describes the evaluation of PT-GC/PID, GC–MSand FAAS for the determination of BTEX, polycyclic aromatichydrocarbons andmetals of environmental interest, respectively,in produced water samples. The reliability and performance ofthe analytical methods were checked by determining the detec-tion and quantification limits, precision and accuracy. Concen-tration levels ranged from 96.7 to 1397 μg L−1 for BTEX, from0.9 to 10.3 μg L−1 for PAHs and from 0.003 to 4540 mg L−1 formetals, in produced water samples collected from the Bonsu-cesso treatment station. The salinity of produced water wasabout 36 g L−1. The work was restricted to produce watersamples only from the city of Carmópolis. More detailed moni-toring studies have to be carried out on different oil productionunits, covering a wider spectrum of chemical classes, to obtain adetailed chemical characterization of the produced water fromoilfields in the State of Sergipe.

Acknowledgements

The authors gratefully acknowledge Dr. Andrew Allen(University of Birmingham) for his assistance with the Englishin this manuscript and the Petrobras/Unseal, CNPq/PIBIC,FAPITEC-SE/FUNTEC and the FINEP for financial support ofthis study. We also would like to thank the Bonsucesso treat-ment station located in the city of Carmópolis, State of Sergipe,for the produced water samples.

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