Determination of cadmium, zinc, copper, chromium and arsenic in crude oil cargoes

Environmental Pollution 107 (2000) 451±464

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Determination of cadmium, zinc, copper, chromium and arsenic in crude oil cargoes J.B. Stigter a, H.P.M. de Haan a, R. Guicherit a,*, C.P.A. Dekkers b, M.L. Daane c a

TNO Institute of Environmental Sciences, Energy Research and Process Innovation, Laan van Westenenk 501, PO Box 342, 7300 AH Apeldoorn, Netherlands b Ministry of Housing, Spatial Planning and Environment, Directorate General for Environmental Protection, The Hague, Netherlands c Shell Research and Technology Centre, Amsterdam, Netherlands Received 30 November 1998; accepted 13 May 1999

Abstract One of the sources of trace heavy metal elements in air is emission by the oil industry, either directly through stack emissions from re®neries or indirectly from emissions of combustion of hydrocarbons. Emission estimates are based mainly on the trace metal content of the crude oil processed. From a literature study carried out at the beginning of the 1990s it became clear that data on the trace metal content of crudes were scarce and showed a very large scatter. For this reason a measurement programme to assess the occurrence and concentrations of a number of trace metals, i.e. Cadmium (Cd), Zinc (Zn), Copper (Cu), Chromium (Cr), and arsenic (As), in crudes which are regularly processed in the Netherlands, was set up. By drafting strict sampling protocols and by constructing a special sampling device, as many as possible of the additional contamination sources were avoided. The study suggests that sample contamination may explain a signi®cant amount of the scatter and some of the high concentrations reported in the literature for certain metals. The measured variation in the concentrations of Cd, Zn, and Cu is thought to be due to associated water and/or sediment particles from the producing wells or that picked up during transport. The greater consistency in our measurements for Cr and As suggests that these metals are predominantly associated with the hydrocarbon matrix. Based on the results of this work, it can be concluded that emissions of Cd, Zn, Cu, Cr, and As by the oil industry in the Netherlands are most probably signi®cantly lower than hitherto assumed. # 2000 Elsevier Science Ltd. All rights reserved. Keywords: Trace metals; Crude oil; Chemical analysis; Emissions; Oil industry

1. Introduction Emission inventories are an important element of an environmental management system. One of the main aims of the inventory is to test the e€ectiveness of governmental environmental policies and to evaluate to what extent policy targets for emission reduction are achieved. Of crucial importance in this context is the quality/validity of the data on which the inventory is based. Most heavy metal emissions are the result of industrial activity, and one of the identi®ed potential sources is the oil industry; either directly through stack emissions from re®neries or indirectly from emissions resulting from combustion of hydrocarbon fuels. To

* Corresponding author. Tel./fax: +31-70-350-3651. E-mail address: [email protected] (R. Guicherit).

assess the magnitude of metal emission by the oil industry it is important to have an accurate knowledge of the metal content of the crudes which the industry processes. From a literature study carried out in the early 1990s (Veldt, 1993) it became clear that data on the metal content of crudes, except for vanadium and nickel, were scarce and often showed a very large scatter. This implies that emission assessment for the metals of interest, based on those literature data, has a very high degree of uncertainty. For this reason a programme to measure the occurrence and concentrations of the relevant metals for those crude oil cargoes, regularly processed in the Netherlands, was set up in the 1990s. The selected crudes are: Arabian Light; Iranian Heavy; Arabian Heavy; Iranian Light; Kuwait; Statfjord; Oseberg; and Ural. These eight crudes are representative of about 80% of the crude oil volume regularly processed in re®neries in the Netherlands.

0269-7491/00/$ - see front matter # 2000 Elsevier Science Ltd. All rights reserved. PII: S0269-7491(99)00123-2

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The metals of interest are Cd, Zn, Cu, Cr, and As. Since it was suspected that the large scatter in results reported in the literature could be due to contamination from external sources during production, transport, sampling, and analysis, much emphasis was placed on prevention of additional contamination during the whole project. Moreover, proper design of the sampling programme would possibly yield valuable clues regarding contamination sources and occurrence (speciation) of the metals. The sampling strategy and programme set-up was as follows: 1. Sampling to take place at the main import terminals in the Netherlands. Sampling at the production and/or loading side will introduce many logistic problems and unduly increase the costs and timing of the project. Hence it is accepted that the data will include any contamination introduced during production, storage, loading, and transport. 2. For each crude, one cargo would be sampled, except for the top two most processed crudes. For these crudes, ®ve cargoes would be sampled, preferably spread over the year. This would enable estimation of cargo-to-cargo variability and assessment the e€ect of possible seasonal in¯uences, such as ®eld maintenance and well workovers, on the metal levels. 3. From all relevant cargoes, a dedicated composite sample, the so-called in-line sample, to be taken as part of the normal cargo discharge procedure. This will yield an average value for the total cargo and be indicative of the amount of metal imported in the Netherlands through crude oils. 4. In addition, two randomly chosen ship tanks of each crude carrier will be sampled in order to have a back-up in case the in-line sample shows unexpectedly high values. Moreover, this will probably o€er the possibility of pin-pointing possible contamination sources; e.g. one of the ship tanks, the discharge pipeline, and/or related sampling set-up. To this end, from the two chosen ships' tanks, a top and bottom sample will be taken. It is logical to assume that (indigenous) metal organic complexes are homogeneously distributed through the tank, and inorganic `contaminant metals' (present as particles or dissolved in the associated water) will be inhomogeneously dispersed through the tank with the concentration increasing from top to bottom. The Cd, Cu and Cr concentrations in the samples were determined by means of graphite furnace atomic absorption spectrometry (AAS) with `Zeeman' background correction. The Zn concentration in the samples was measured by means of ¯ame AAS. For As, a portion of the samples was acid digested, followed by mea-

surement of As in the diluted digest by hydride generation AAS. 2. Materials and methods 2.1. Sampling For this project, dedicated protocols were developed regarding sampling and sample treatment at every stage. The in-line samples were obtained by normal procedures and by equipment for automatic in-line sampling from the discharge pipeline at the terminal, where the crude oil was discharged. Some cargoes were partially discharged at two di€erent terminals. In such a case, two in-line samples were available. The only deviation from normal in-line sampling procedures was that it was con®rmed prior to each sampling exercise that the sample receptacle was indeed clean. The large (20-l) in-line samples were subsequently transferred to the terminal laboratory, where the sample was homogenized. From this homogenized sample, a glass sample bottle was ®lled immediately to obtain a 1-l sub-sample on which metal analysis was performed. Again, cleanliness of the homogenization equipment was checked on the spot prior to each sub-sampling exercise. Sampling from the ships' tanks was done manually, guided by strict protocols. As standard manual tank sampling equipment is almost always constructed of copper (and other metals) containing alloys, this equipment could not be used. Therefore, a special sampling device was developed and constructed allowing for contamination-free manual top-and-bottom sampling, directly into a glass sample bottle. The sampling device consists of a 1-l colourless boron-silicate glass sample bottle mounted in a Te¯on1-coated stainless steel holder (Fig. 1). All sample bottles were cleaned inside and outside with nitric acid (Merck, Suprapur1), followed by rinsing with Milli-Q1 water until acid-free. After samples were taken, the bottles were closed by a plastic screwcap with Te¯on1 inlay. Samples were stored in the dark at 2±8 C until needed for analysis. Test portions for analysis were withdrawn from the sample bottles directly after homogenization with a high shear `Ultra Turrax Mixer', equipped with a titanium shaft. For eciency reasons and to limit equipment variability as much as possible, metal analysis started after the last crude carrier had been sampled. This implies that the duration of the storage of the samples could be an important variable. It was, therefore, needed to establish whether the time-lapse between sampling and analysis a€ected the metal concentration in the crude samples. Conceivably, the metals may be adsorbed by the wall of the glass storage bottles, or settle to the

J.B. Stigter et al. / Environmental Pollution 107 (2000) 451±464

453

Fig. 1. Sampling device for crude oil.

bottom. To test if this was the case for Cd, the following experiments were conducted. A dedicated reference subsample was spiked with approximately 10 mg (ionic) Cd/ kg. Directly after preparation and homogenization of the sample with the Ultra Turrax Mixer, two sub-samples were withdrawn and analysed in duplicate (week 0). This duplicate sub-sampling and analysis was repeated subsequently after 1, 2, 6 and 8 weeks. The results are given in Table 1. From Table 1 it follows that prolonged storage of the samples did not result in any loss of Cd. The apparent increase in Cd content is thought to be due to di€erences in homogenization eciency at the time of subTable 1 Cadmium (Cd) content of the original sample stored for several time periods Week

Cd concentration (mg/kg) Sub-sample 1

Cd concentration in (mg/kg) Sub-sample 2

0 1 2 6 8

11.8/12.2 15.7/17.3 15.9/13.7 15.7/15.4 18.7/18.2

11.9/13.6 14.6/15.3 14.0/15.4 16.6/16.0 17.1/16.7

sampling and/or analytical uncertainties caused by slight instrument variations at the time of analysis. 2.2. Analytical procedure An overview of the analytical methods used is given in Table 2. Several methods for the determination of Cd, Zn, Cu, Cr, and As in a hydrocarbon matrix are described in the literature. The methods fall into two di€erent categories, i.e. direct methods by which dilution of the sample to be analysed is the only pre-treatment and indirect methods by which a diluted digest of the sample is being analysed. Because digestion of the oil matrix is vulnerable to contamination and, moreover, usually leads to high detection limits (with the exception of As), a direct method was used during this work. For the dilution of the samples, carbon-tetrachloride (pro analysis) was used for the analysis of Cd. Because of the toxicity of carbon-tetrachloride the solvent was originally changed to xylene for Zn, Cu and Cr. The use of xylene as solvent for samples and calibration standards led to instability, especially during the analysis of Cu. A mixture of toluene±glacial acetic acid (4/1 v/v) (pro analysis) was used instead to dilute the samples.

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Table 2 Analytical methods Metal

Method

Cd

Graphite Furnace AASb with L'vov platform and Zeeman correction; (direct injection)

Cu Cr

Detection limit (3s) (mg/kg)a

Solvent

0.4

Carbon-tetrachloride

As for Cd

9

Glacial acetic acid±toluene

As for Cd

19

Glacial acetic acid±toluene

Zn

Flame AAS (direct aspiration)

63

Glacial acetic acid±toluene

As

Acid digestion followed by hydride AAS

10

a b

Detection limit in oil, corrected for dilution. AAS, atomic absorption spectrometry.

This procedure has been described by Wittman (1979). In this respect it proved to be crucial that calibration solutions and samples were prepared fresh before every measurement series. All the solutions were considered to be potentially unstable after 48 h storage. Hence, where possible, solutions were prepared daily, but they were always analysed within 48 h of preparation. For As, an indirect method was used. It involved digestion of the crude oil sample with sulphuric acidhydrogen peroxide under re¯ux, followed by measurement of As in the diluted digest by hydride generation AAS. As pre-treatment procedure of the samples, the crude oil under investigation was homogenized with the Ultra Turrax mixer for 10 s. After this homogenization, step a sub-sample was taken for either determination of Cd, Cu, Cr, Zn or As. For the determination of As, the subsample as such was directly digested. For the determination of Cd, Cu, Cr and Zn, the sub-sample was diluted as described above and analysed. The main performance ®gures of the analytical methods are described in Section 3. 2.3. Method used for the determination of Cd, Cu and Cr The determination of Cd, Cu and Cr was carried out using a direct method. The metal concentrations in the diluted crude oil were measured with graphite furnace AAS with `Zeeman' background correction. For the analysis, a Perkin Elmer 5100 PC Atomic Absorption Spectrophotometer, ®tted with a pyrolytically coated graphite tube with L'vov platform was employed. The analytical sequence for the AAS measurements comprises: injection of 10 ml of the diluted sample on the platform followed by a temperature-programmed drying/ashing stage for digestion of the hydrocarbon matrix using oxygen as internal ¯ow gas, after which the temperature of the platform was brought to the required atomization temperature for the elements, using argon as internal ¯ow gas. Prior to the actual measurements, the temperature programme for the AAS was stepwise optimized for each element individually. The optimized parameters are listed in Table 3.

The system was calibrated using commercially available metallo-organic compounds (Conostan1, 5000 mg metal/kg), diluted with metal-free solvent to yield at least three points (not including zero) in the concentration range of interest. For Cd the Conostan1 standard was pre-diluted with cyclohexanol (Baker1, reagent grade) and for Cu and Cr the Conostan1 standard was pre-diluted with `base oil 20' (Conostan1). The prediluted solution was ®nally injected as a 1:4 solution in the appropriate solvent, also used for the actual sample analysis. Cd was calibrated over the range 0±10 mg/kg (on pre-dilution), Cu over the range 0±160 mg/kg and Cr over the range 0±320 mg/kg. For all metals the signals proved to vary linearly with concentration over the concentration range tested. 2.4. Method used for the determination of Zn The determination of Zn was also carried out using a direct method. However, because of the higher concentration range expected and the incompatibility of Zn with the graphite L'vov platform, the diluted crude oil was directly measured with ¯ame AAS instead of graphite furnace AAS. The gas mixture used was air±acetylene, the proportions of these gasses being set in such a way that the ¯ame was close to the point of extinction. The absorption for Zn was measured at 213.9 nm, with background correction, using a hollow cathode lamp for Zn. After each calibration standard or sample aspiration, the system was rinsed by aspiration of the toluene±glacial acetic acid solvent to exclude signal suppression and memory e€ects. Thorough rinsing between the two sample analyses was found to be crucial for the determination. Also, after every 15 measurements, the burner head had to be cleaned to avoid clogging. The system was calibrated using a commercially available metallo-organic Zn compound (Conostan1, 5000 mg Zn/kg), pre-diluted with `base oil 20' (Conostan1). The pre-diluted solutions were ®nally aspirated as a 1:4 solution in the toluene±glacial acetic acid solvent. Zn was calibrated over the range 0±1.2 mg/kg (on

J.B. Stigter et al. / Environmental Pollution 107 (2000) 451±464

pre-dilution). The signals proved to vary linearly with concentration over the concentration range tested. 2.5. Method used for the determination of As Approximately 2.5 g of homogenized crude oil was transferred to a round-bottomed ¯ask and 10 ml of sul-

455

phuric acid (Merck, Suprapur1) was added through a connected condenser. The mixture was ®rst gently heated for about 15 min. After this period the temperature was gently increased to a maximum and cooling of the condenser was turned o€. Hydrogen peroxide (Merck, pro analysis1) was added drop by drop, until the charring disappeared and a clear colourless solution

Table 3 Parameters for the graphite furnace measurement of cadmium (Cd), copper (Cu) and chromium (Cr) Cd Solvent Sample size Atomic absorption line Slit

Carbon tetrachloride 10 ml 228.8 nm 0.7 nm Temperature ( C)

Stage

Step

Evaporation

1 2 3

150 250 450

10 60 100

0 0 0

300 O2 300 O2 50 O2

Ashing

4 5 6

650 650 650

100 1 1

100 39 5

50 O2 300 Ar 0 Ar

Atomisation

7 8

1900 2400

1 1

5 3

0 Ar 300 Ar

Clean-out

9

20

1

9

300 Ar

Ramp time (s)

Cu Solvent Sample size Atomic absorption line Slit

Hold time (s)

Int. gas ¯ow (ml/min)

Toluene±glacial acetic acid (4/1) 10 ml 324.8 nm 0.7 nm Temperature ( C)

Stage

Step

Evaporation

1 2 3

150 250 375

20 99 99

0 0 1

300 O2 300 O2 50 O2

Ashing

4 5

650 1000

99 15

99 15

50 O2 300 Ar

Atomisation

6

2350

0

8

0 Ar

Clean-out

7

2600

1

10

300 Ar

8

20

1

9

300 Ar

Ramp time (s)

Cr Solvent Sample size Atomic absorption line Slit

Hold time (s)

Int. gas ¯ow (ml/min)

Toluene±glacial acetic acid (4/1) 10 ml 357.9 nm 0.7 nm Temperature ( C)

Stage

Step

Evaporation

1 2 3

150 250 375

10 99 99

0 0 1

300 O2 300 O2 300 O2

Ashing

4 5

650 1550

99 15

99 15

300 Ar 300 Ar

Atomisation

6

2700

0

8

0 Ar

Clean-out

7

2700

1

5

300 Ar

8

20

1

1

300 Ar

Ramp time (s)

Hold time (s)

Int. gas ¯ow (ml/min)

456

J.B. Stigter et al. / Environmental Pollution 107 (2000) 451±464

resulted. After cooling down, the digest was transferred to a 50-ml volumetric ¯ask and brought to volume with Milli-Q1 water. Sixteen hours before analysis the following reagents were added to a 10-ml aliquot of the digest solutions: 1 ml of hydrochloric acid (J.T. Baker `Baker instra-analysed'1), 0.2 ml of hydroxyl ammonium chloride solution (J.T. Baker `Baker analysed'1), 0.3 ml of potassium iodide solution (J.T. Baker `Baker analysed'1), and 0.2 ml of ascorbic acid solution (J.T. Baker `Baker analysed'1). For the analysis of As a Varian Spectra 300 Atomic Absorption Spectrophotometer with Hydride system VGA 76 was used. As hydride solution, a solution of 0.6 g sodium borohydride (J.T. Baker `Baker analysed'1) and 0.5 g sodium hydroxide (pro analysis) in 100 ml Milli-Q1 water was used. The calibration solutions, to which 0±20 mg As/kg (as aqueous solution) was added, were prepared in the same way as the digest solution. The absorption was measured at a wavelength of 193.7 nm with background correction. 2.6. Quality assurance of the measurements During the analyses of the samples for Cd, Zn, Cu, Cr and As, after every ®fth sample, a calibration standard solution with a metal concentration in the middle of the calibrated range was measured. The measured concentration of this standard had to be within 10% of its nominal value. The validation of all techniques used and sample pretreatment was conducted according to ISO 5725-2 (1995). Ten repeat measurements for each sample were carried out for the Cd analysis. For the other metals, ®ve repeat measurements were carried out for each sample. The methods were tested for their correctness by measurement of a sample using a di€erent sample preparation technique and/or a di€erent measurement technique. During the project, one test sample, i.e. Arabian Heavy, was taken as a `reference sample'. In the course of the project, this sample was repeatedly analysed for the elements mentioned. For each analysis the sample pre-treatment was also performed. Table 4 shows the results of the measurements during the course of the project. From these results it can be concluded that in Table 4 Concentration (mg/kg) found in the reference sample during the course of the projecta Analysis

Cd

Cu

Cr

Zn

As

Average SD (s) RSD (%)

0.58 0.07 12

73 5 7

63 6 9

517 36 7

23 4 18

a

RSD, relative standard deviation.

the course of the project all samples of crude oil were analysed under nominally the same analytical conditions. 3. Validation of the analytical methods 3.1. General validation parameters For the validation study of the analytical methods, the following two samples were used: 1. Test sample No. 1: Arabian Heavy, in-line sample (not included in the ®nal measurement programme). For validation of the method for Zn, a top sample (not included in the ®nal measurement programme) of Arabian Heavy was used. 2. Test sample No. 2: Statfjord, top sample (not included in the ®nal measurement programme). For Cd, Cu, Cr, and Zn, the following parameters were determined: 1. distribution homogeneity (homogenization procedure); 2. intra-laboratory repeatability; 3. reproducibility; 4. detection limit; and 5. accuracy (method validation). For As, the following parameters were determined: 1. 2. 3. 4.

intra-laboratory repeatability; detection limit; method blank value; and recovery.

3.2. Validation of the homogenization procedure For the determination of the correctness of the applied homogenization procedure, the content of Cd, Cu, Cr and Zn, was determined for sub-samples taken from evenly spaced positions from top to bottom throughout the sample bottle. The results of these measurements are listed in Table 5. These ®gures con®rm that the applied method of homogenization for Cd, Zn Cu and Cr is satisfactory. Di€erences between the metal content at various positions are small and can all be explained by analytical uncertainty (single measurements) and/or slight remaining inhomogeneities. This test was not performed for As, but the standard deviation listed in Table 4 shows that the applied homogenization for this element is also satisfactory. 3.3. Performance assessment: precision and detection limit The main performance data for the individual methods for Cd, Cu, Cr, Zn and As are listed in Table 6. The

J.B. Stigter et al. / Environmental Pollution 107 (2000) 451±464

457

Table 5 Metal content in test sample No. 1 after homogenization (mg/kg) Position

1 (top)

2

3

4

5

6

7

8

9

10 (bottom)

Average content (mg/kg)

Cd Cu Cr Zn

0.95 63 34 519

1.02 64 35 505

0.99 62 57 569

1.02 60 26 541

1.43 62 31 583

1.04 69 31 548

1.44 62 35 541

0.89 61 37 579

1.15 63 48 582

1.91 59 50 570

1.18 63 38 554

detection limit is de®ned as three times the standard deviation of the element in a blank solution. For Cd, Zn and As, test sample No.1 was used, and for Cu and Cr, test sample No.2, which was spiked with, respectively 20 mg/kg Cu and 40 mg/kg Cr (as metallo-organic compounds dissolved in `base oil 20' Conostan1). 3.4. Validation of method accuracy For the validation of the accuracy of the applied methods, each element was also analysed using either a di€erent preparation method or a di€erent detection method. The results of the applied methods and those of the alternative methods are listed in Table 7. Taking into account the relative precision of the applied methodologies it can be concluded that results for the standard method and the alternative method are on a par. The degree of agreement of the results provides a measure of con®dence in the correctness of the analysis.

Table 6 Main performance ®gures for the individual methodsa RSD RSD Limit of Linearityb repeatability reproducibility detectionb (mg/kg) (%) (%) (3s) (mg/kg) Cd Cu Cr Zn As (recovery: 89‹4%) a b

6 5 7 8 8

10 29 12 10 NA

0.4 9 30 63 10

0.4±60 9±800 30±600 63±1200 10±400

NA, not available; RSD, relative standard deviation. In oil, corrected for dilution.

‹ ‹ ‹ ‹

0.32 2.7 10 27

4. Results Each sample analysed was analysed in 10-fold for Cd and in ®ve-fold for the other metals. In Table 8, the average metal content (mg/kg) and the standard deviation of the in-line samples of each cargo are given. Table 9 lists the average metal content (mg/kg) of the various crudes and, when available, the concentration range of di€erent cargoes of the same crude. In the following sections the individual elements will be discussed. 4.1. Cd The average Cd content of the crude oils analysed is low: it averages 1.58 mg/kg over a range from below the detection limit (0.4 mg/kg) to 5.3 mg/kg. Moreover, the range covered by the results of multiple cargoes (Iranian Heavy, Arabian Light) equals approximately the range covered by all individual crude oils. This suggests that probably Cd is no part of the hydrocarbon matrix, but present as insoluble (probably inorganic) contaminant. When comparing the results of the in-line samples with those of the top and bottom samples (Table 10) further evidence is found for the conclusion that at least part of the Cd in the crudes is present as a `contaminant' rather than organic bound Cd indigenous to the hydrocarbon matrix. From the Table 10 it follows that the Cd concentration is not homogeneously distributed through the tanks and increases from top to bottom, indicating that Cd is either present as solid particles or dissolved in the water phase. The data obtained in this work fall in the lower part of the range of those published by others (Table 11). Also included in the literature data in Table 11 is the

Table 7 Accuracy of the applied methodsa Element

Method 1 (standard)

Method 2 (alternative)

Results method 1 (mg/kg)

Results method 2 (mg/kg)

Cd Cu Cr Zn As

GFAAS, direct GFAAS, direct GFAAS, direct FAAS, direct HGAAS, wet ashing

ICP±MS, wet ashing GFAAS, wet ashing GFAAS, wet ashing ICP-AES, wet ashing GFAAS, direct

0.58‹0.07 73‹5 63‹6 517‹5 23 ‹ 4

1.0‹0.10 85‹28 67‹16 466‹25 <20

a

GF-, graphite furnace; F-, ¯ame; HG-, hydride generation; AAS, atomic absorption spectrometry.

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Table 8 Average metal content of the in-line samples for each cargo sampled Type of oil/date

Terminala

Cd (mg/kg)

Cu (mg/kg)

Cr (mg/kg)

Zn (mg/kg)

As (mg/kg)

Iranian Heavy December 1993 Iranian Heavy March 1994 Iranian Heavy 2 May 1994 Iranian Heavy 9 May 1994 Iranian Heavy 22 May 1994 Arabian Light March 1994 Arabian Extra Light April 1994 Arabian Light July 1994 Arabian Light August 1994 Arabian Light December 1994 Arabian Heavy May 1994 Iranian Light June 1994 Ural Crude February 1994 Statfjord Crude May 1994 Kuwait Crude December 1994 Oseberg Crude February 1995

TEM

0.46‹0.26

100‹3.0

155‹4.7

310‹34

17.1‹1.4

TEM MOT MOT TEM TEM

0.82‹0.21 1.76‹0.38 0.55‹0.07 5.86‹0.89 1.03‹1.02

65‹2.0 26‹1.8 21‹1.5 43‹4.3 45‹2.3

152‹1.5 128‹3.8 167‹6.7 240‹9.4 162‹8.1

990‹20 470‹10 470‹19 1090‹33 420‹21

16.3‹3.4 13.4‹2.3 15.9‹1.9 18.0‹4.2 15.2‹1.8

MOT TEM TEM MOT TEM

0.57‹0.12 0.44‹0.35 0.48‹0.09 0.92‹0.39 0.50‹0.15

<10 <10 25‹0.8 33‹1.3 21‹1.1

139‹5.6 152‹3.0 27‹1.6 26‹0.8 102‹2.0

240‹34 270‹25 420‹42 230‹6.9 91‹37

<10 14.7‹1.4 <10 <10 <10

MOT TEM TEM

5.32‹1.70 2.08‹0.30 5.25‹1.09

37‹3.7 13.4‹3.8 16.8‹3.6

29‹1.2 32‹2.6 26‹6.0

220‹24 180‹20 750‹38

12.9‹3.7 <10 13.3

TEM

<0.40

15.3‹1.5

19.2‹2.3

<63

<10

TEM

0.54‹0.18

31‹1.2

140‹5.6

500‹25

14.4‹2.9

MOT TEM TEM

0.68‹0.11 0.84‹0.36 0.69‹0.18

24‹2.2 86‹4.3 51‹1.0

58‹2.3 65‹5.9 21‹5.0

194‹62 630‹19 380‹19

<10 11.2‹1.7 37‹6.0

MOT TEM MOT

1.98‹0.35 2.77‹1.71 <0.40

17.9‹2.0 33‹1.3 35‹2.5

<19 <19 24‹3.8

136‹62 290‹43 420‹42

<10 <10 14.6‹2.1

Shell

2.12‹1.00

195‹19

<19

116‹39

<10

a

TEM, Texaco±Esso Maatschap Europoort Terminal; MOT, Maasvlakte(NL) Olie Terminal; Shell, Shell Europoort Terminal.

estimated Cd content of crude oils which have been used to calculate the Cd emission due to the oil industry in the Netherlands for 1995 (Veldt, 1993). In particular, the data by (Al-Swaidan, 1988, 1990, 1994; Al-Swaidan et al., 1988) seem to be exceptionally high. That this is not caused by the selection of the suite of crude oils analysed in the various studies is borne out by the data on Arabian Light crude oil only from four di€erent

studies, also listed in Table 11. Again the data reported by Al-Swaidan di€er by approximately three orders of magnitude from those of the other investigators. This leads to the conclusion that the data of Al-Swaidan are most probably incorrect, although a speci®c cause for the deviation cannot be identi®ed from the appropriate references (Al-Swaidan, 1988, 1990, 1994; Al-Swaidan et al., 1988).

Table 9 Average metal content and concentration range of the in-line samples for each crude oil type sampled Type of oil

Cd (mg/kg)

Cu (mg/kg)

Cr (mg/kg)

Zn (mg/kg)

As (mg/kg)

Iranian Heavy (range of 5 cargoes) Arabian Light (range of 5 cargoes) Arabian Heavy Iranian Light Ural Crude Statfjord Crude Kuwait Crude Oseberg Crude

1.44 (0.46±3.2) 2.1 (<0.40±5.3) 0.54 0.76 0.69 2.4 <0.40 2.1

47 (<10±100) 23 (13.4±37) 31 55 51 25 35 195

161 (128±240) 41 (19.2±102) 140 62 21 <19 24 <19

499 (240±1090) 280 (<63±750) 500 412 380 213 420 116

15.8 (<10±18.0) 10.9 (<10±13.3) 14.4 11.2 37 <10 14.6 <10

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459

Table 10 Cadmium (Cd) analysis (mg/kg) of in-line, top and bottom samples

Table 12 Copper (Cu) analysis (mg/kg) of in-line, top and bottom samples

Crude

Top

In-line

Bottom

Crude

Top

In-line

Bottom

Arabian Light (August 1994) (Water content, mg/kg)a Iranian Heavy (May 1994) (Water content, mg/kg)a Statfjord (May 1994) (Water content, mg/kg)a

1.7‹1.0 (390) <0.4 (575) <0.4 (260)

5.3‹1.1 (ND) 0.55‹0.1 (ND) 2.3‹0.4 (ND)

10.1‹2.0 (1390) 8.5‹1.4 (520) 10.3‹2.7 (1290)

Arabian light (December 1994) Oseberg (February 1995)

<10

15.3‹1.5

<10

<10

195‹19

161‹4.6

a

Karl Fischer: ASTM D4377. ND, not determined.

4.2. Cu Data for Cu are higher than for Cd. The average concentration was 41 mg/kg over a range from below the detection limit of 10 to 195 mg/kg. If we compare top, bottom and in-line sample results, the following picture emerges (Table 12): 1. For the Arabian light crude no di€erence in Cu content for top and bottom samples can be distinguished. Concentrations are low and even below our detection limit. 2. For the Oseberg crude a signi®cant di€erence between top and bottom sample exists. This points to the likelihood that Cu also might be present as an inorganic `contaminant' in the crudes. 3. The Cu content of the in-line samples is higher than the Cu content of the bottom samples, indicating that further contamination of the in-line samples with Cu has probably taken place during cargo discharge at the terminals. The results obtained in this work are much lower by at least a factor 10 than those published elsewhere (Ball et al., 1960; Colombo et al., 1964; Veal, 1966; Shah et al., 1970a; American Petroleum Institute, 1973; Hall et al., 1973; Yen, 1975; Block and Dams, 1978; Ali et al.,

1983; GonzaÂlez et al., 1987; Jones, 1988; Anwar et al., 1990; Chai Chif Ang et al., 1991; Speight, 1991; Olajire and Oderinde, 1993; Filby and Olsen, 1994; Dewakar et al., 1995; Olsen et al., 1995; Petrenko and Dorogochinskaya, 1995). (Table 13). A plausible explanation for the higher data published before could be that samples analysed were possibly contaminated by the sampling equipment. The traditional oil sampling equipment is made of copper or brass and pumps, sampling cans, etc., usually contain Cu as an alloying element. 4.3. Zn The Zn content of the crude oils is higher than the Cu content and signi®cantly higher than the Cd content. Analytical results in this work range from less then 63 to 1090 mg/kg. From Table 14 it can also be seen that Zn is not homogeneously distributed through the tanks but that the concentration increases from top to bottom, indicating that Zn is either present as (inorganic) particles or dissolved in the water phase. Like Cu, the Zn content of the in-line sample is higher than the Zn content of the bottom sample, indicating that probably additional (sample) contamination has taken place during cargo discharge at the terminals. Further evidence for this conclusion is that the Zn content of the in-line samples taken from the same cargo but at di€erent terminals, is signi®cantly di€erent (Table

Table 11 Cadmium (Cd) content (mg/kg) of crude oils, literature data Range

Average

Typical valuea

References

All crudes This work Emission Inventory 1995 Literature study 1998

<0.40±5.3 <0.10±500 <0.10±140

1.58 ± ±

1.05 50 2.4

Al-Swaidan

450±4600

±

980

± Veldt, 1993 Filby and Shah, 1993; Araktingi et al., 1974; Robbins and Walker, 1975; Hofstader et al., 1976; KaÈgler et al., 1983; KaÈgler and Kotzel, 1984; Narres et al., 1984; Filby and Olsen, 1994; Olsen et al., 1995 Al-Swaidan, 1988, 1990, 1994; Al-Swaidan et al., 1988

Arabian Light only This work Narres et al. KaÈgler et al. Al-Swaidan

<0.40±5.3 ± ± 460±1250

2.1 0.1 0.8 710

1.26 ± ± 650

± Narres et al., 1984 KaÈgler et al., 1983; Anwar et al., 1990 Al-Swaidan, 1988, 1990, 1994; Al-Swaidan et al., 1988

a

The `typical value' is de®ned as the log average value of the analytical results of this study or of the data quoted in the literature.

460

J.B. Stigter et al. / Environmental Pollution 107 (2000) 451±464

Table 13 Copper (Cu) content (mg/kg) of crude oils, literature data Range

Average

Typical valuea

References

All crudes This work Emission Inventory 1995 Literature study 1998

<10±195 30±7180 <10±31500

41 ± ±

31 480 375

Al-Swaidan

620±6550

±

2100

± Veldt, 1993 Ball et al., 1960; Colombo et al., 1964; Veal, 1966; Shah et al., 1970a; American Petroleum Institute, 1973; Hall et al., 1973; Yen, 1975; Block and Dams, 1978; Ali et al., 1983; GonzaÂlez et al., 1987; Jones, 1988; Anwar et al., 1990; Chai Chif Ang et al., 1991; Speight, 1991; Olajire and Oderinde, 1993; Filby and Olsen, 1994; Dewakar et al., 1995; Olsen et al., 1995; Petrenko and Dorogochinskaya, 1995 Al-Swaidan, 1988, 1994; Al-Swaidan et al., 1988

Arabian Light only This work Ali et al. Beg et al. Al-Swaidan

13.4±37 ± ± 1330±6550

23 200 210 3880

22 ± ± 3200

± Ali et al., 1983 Beg et al., 1989 Al-Swaidan, 1988, 1994; Al-Swaidan et al., 1988

a

The `typical value' is de®ned as the log average value of the analytical results of this study or of the data quoted in the literature.

15). Overall, the Zn content of the in-line samples taken at the Texaco±Esso Maatschap Europort Terminal (TEM) are on the average signi®cantly higher (up to a factor of more than 2) than those taken at the Maasvlakte (NL) Olie Terminal (MOT). Similar to Cu, and probably for the same reason(s), the results obtained in this work are lower than those published elsewhere (Table 16). 4.4. Cr The average Cr content of the crude oils analysed amounts to 83 mg/kg, ranging from less than 19 to 240 mg/kg. In Table 17 the Cr analysis of top, bottom and in-line samples are given. From Table 17 it is clear that the Cr content of the top and bottom samples is approximately on a par. From this we might conclude that Cr in crude oil is mainly present in the (indigenous) hydrocarbon matrix. However, from Table 17 it can also be concluded that in some cases (e.g. for the Iranian Heavy crude) additional Cr might be picked up during handling and transportation of the crudes. The latter is also borne out by the Iranian Heavy data in Table 18 where it can be observed that the Cr content of the in-line samples of the Iranian Heavy cargoes taken at the TEM-terminal is higher than in the samples taken at the MOT-terminal. Table 14 Zinc (Zn) analysis (mg/kg) of in-line, top and bottom samples Crude

Top

In-line

Bottom

Arabian Light (August 1994) Arabian Light (December 1994)

<63

750‹38

425‹16

<63

<63

<63

In Table 19 the results of this work are compared with those published before. It can be observed that the present data are amongst the lowest. However, unlike Cd, Cu and Zn, the Cr content is thought to originate for the main part from indigenous organic Cr compounds. Hence, a signi®cant part of the di€erences noted between the various studies is likely due to inherent compositional di€erences from oil to oil. Nevertheless, even for nominally the same crude oils (Table 19), the di€erences are larger than can reasonably be explained from additional contamination through sampling and sample handling. Hence, di€erences in analytical technique and/or problems due to the technique are also a possible explanation for part of the signi®cant di€erences between the various studies. For example, Olsen et al. (1995) and Filty and Olsen (1994) demonstrated (Table 19) that the variation of 40 Ar12C1H interference, which varied depending on oil type, rules out the use of ICP-MS for the analysis of Cr in oils with 53Cr. Table 15 Zinc (Zn) content (mg/kg) of in-line crude oil samples, sampled at different terminalsa Crude oil

Sample

MOT

TEM

Iranian Heavy Iranian Heavy Iranian Heavy Arabian Light Arabian Light Iranian Light Statfjord

March, 1994 2 May 1994 22 May 1994 March, 1994 July, 1994 June, 1994 May, 1994

470 470 240 230 220 194 136

990 1090 270 420 180 630 290

280

553

Average a

MOT, Maasvlakte (NL) Olie Terminal; TEM, Texaco-Esso Maatschap Europoort Terminal.

J.B. Stigter et al. / Environmental Pollution 107 (2000) 451±464

461

Table 16 Zinc (Zn) content (mg/kg) of crude oils, literature data Range

Average

Typical valuea

References

All crudes This work Emission Inventory 1995 Literature study1998

<63±1090 200±19500 <10±62800

385 ± ±

305 1000 815

Al-Swaidan

1080±6555

±

4560

± Veldt, 1993 Veal, 1966; Shah et al., 1970a,b; American Petroleum Institute, 1973; Filby, 1973; Filby and Shah; 1973 Hall et al., 1973; Hitchon et al., 1973; Araktingi et al., 1974; Robbins and Walker, 1975; Ali et al., 1983; KaÈgler and Kotzel, 1984; GonzaÂlez et al., 1987; Hirner, 1987; Beg et al., 1989; Chai Chif Ang et al., 1991; Filby and Olsen, 1984; Dewakar et al., 1995 Al-Swaiden, 1988, 1994; Al-Swaidan et al., 1988

Arabian Light only This work Ali et al. Al-Swaidan

63±750 ± 1290±4600

280 250 2490

210 ± 2110

± Ali et al., 1983 Al-Swaiden, 1988, 1994; Al-Swaidan et al., 1988

a

The `typical value' is de®ned as the log average value of the analytical results of this study or of the data quoted in the literature.

4.5. As The average As content of the crude oils amounts to 13.7 mg/kg, ranging from less than 10 to 37 mg/kg. In Table 20, analysis of top, bottom and in-line samples is given. From Table 20 it follows that the concentrations of As in the in-line samples as well as the top and bottom samples are on a par. From this it might be concluded that As is homogeneously distributed through the tanks and consequently possibly present as part of the indigenous hydrocarbon matrix. A comparison of the As data from this work with those published in literature is given in Table 21. From Table 21 it can be seen that data obtained in this work fall in the lower range of those published by others. However, we feel that As Table 17 Chromium (Cr) analysis (mg/kg) of in-line, top and bottom samples Crude

Top

In-line

Bottom

Arabian Light (December 1994) Iranian Heavy (May 1994)

25‹4.8

19.2‹2.3

36‹1.2

129‹5.0

167‹6.7a

122‹4.4

a In-line sample taken at the Maasvlakte (NL) Olie Terminal (MOT).

Table 18 Chromium (Cr) content (mg/kg) of in-line crude oil samples, sampled at di€erent terminalsa Crude oil

Sample

MOT

TEM

Iranian Heavy Iranian Heavy Iranian Heavy

March, 1994 2 May 1994 22 May 1994

167 128 139

240 152 152

145

181

Average a

MOT, Maasvlakte (NL) Olie Terminal; TEM, Texaco-Esso Maatschap Europoort Terminal.

probably exists in crude oil as a metal organic complex and consequently the As content might be quite di€erent in di€erent suites of samples. In comparison to Cr, the di€erences between the results of the various studies are much less pronounced, probably because it is very unlikely that As is picked-up as additional contamination in sampling and sample handling. Problems with the analytical technique may be another factor that accounts for some of these discrepancies. 5. Conclusions This study has shown the need for stringent sampling and sample handling protocols to ensure that the sample taken and the analytical results obtained are truly representative of the crude oil cargo analysed for trace elements. It further shows that the e€ect of additional, sometimes uncontrollable, contamination on the ®nal results might be signi®cant. Despite rigorous precautions being taken to ensure representative sampling and preservation of sample integrity, some sample contamination has, nevertheless, taken place during crude oil discharge at the terminals. Hence, it may be concluded that sample contamination might explain, to a large extent, the highly variable results in (trace) metal content of crude oils reported in the literature for some of the metals investigated. The results obtained indicate that Cd, Zn, and Cu are predominantly not indigenous to the crude oil hydrocarbon matrix, but the result of contamination with associated water and/or particles from the producing wells and/or sampling procedures. On the other hand, analytical results for Cr, and most probably also for As, indicate that these trace elements are probably predominantly associated with the hydrocarbon matrix. Some Cr, however, might also be present as a contaminant.

462

J.B. Stigter et al. / Environmental Pollution 107 (2000) 451±464

Table 19 Chromium (Cr) content (mg/kg) of crude oils, literature data Study

Region

Samples Range

Inventory 1995 Literature 1998

Average Reference

± ±

100±3170 1000a <10±34,400 2650

Veldt, 1993 Colombo et al., 1964; Veal, 1966; Shah et al., 1970a,b; Filby, 1973; Hitchon et al., 1973; Yen, 1975; Block and Dams, 1978; Ali et al., 1983; Hirner, 1987; Jones, 1988; Anwar et al., 1990; Chai Chif Ang et al., 1991; Filby and Olsen, 1994; Frankenberger et al., 1991; Dewakar et al., 1995; Olsen et al., 1995; Petrenko and Dorogochinskaya, 1995

10 88 9 14 4 12 10

2±17 9 ± 93 <12±240 83 <50±1180 620 10±870 320 1450±12,050 5550 570±34,400 7460

Shah et al., 1970b Hitchon et al., 1973 ± Ali et al., 1983 Dewaker et al., 1995 Anwar et al., 1990 Chai Chif Ang et al., 1991

Shah et al. Hitchon et Al. This work Ali et al. Dewakar et al. Anwar et al. Ang et al.

USA Canada Middle East Middle East India Pakistan China

Study

Region

Crude oil

Samples Range

Average Reference

This work Ali et al. This work Ali et al.

Middle East Middle East Middle East Middle East

Arabian Light Arabian Light Arabian Heavy Arabian Heavy

7 2 1 2

37 545 140 1115

± Ali et al., 1983 ± Ali et al., 1983

Study

Technique

Crude oil

Samples

Result

Reference

Filby et al. Filby et al.

INAA ICP-MS

171-4 171-4

1 1

a

19±102 520±570 ± 1050±1180

± ±

1120 3170

Filby and Olsen, 1994 Filby and Olsen, 1994

Log average.

Table 20 Arsenic (As) analysis (mg/kg) of in-line, top and bottom samples Crude

Top

In-line

Bottom

Arabian light (December 1994) Ural (February 1994)

<10

<10

<10

42‹2.8

37‹6.0

46‹4.5

The presented data are all signi®cantly lower than those hitherto published, which has signi®cant consequences for emission estimates for these metals and the contribution of the oil industry to these emissions. A more detailed discussion on the occurrence of the metals investigated in crude oil and a detailed discussion on the environmental implications regarding emissions

Table 21 Arsenic (As) content (mg/kg) of crude oils, literature data Study

Region

Samples Range

Inventory 1995 Literature 1998a

Various Various

± ±

20±26200 <0.2±1630

This work Vealb Veal Filby/Olsenc Filby API Shah et al. Ang et al.a Ang et al. (Shengli)

North Sea/Middle East Various California North Sea California Various California China China

9 7 1 6 2 24 5 7 1

<10±37 5±92 ± 12±60 656±1630 <1±1100 63±1112 40±630 ±

a b c d

Minus `Shengli' data from (Chai Chif Ang et al., 1991) Minus `California' data (Veal, 1966) Minus one sample, 280 mg/kg (Olsen et al., 1995) Log average

Average References 50d 190

14 35 142 29 1140 138 380 255 26200

Veldt, 1993 Veal, 1966; Shah et al., 1970b; American Petroleum Institute, 1973; Filby, 1973; Hitchon et al., 1973; Smith et al., 1975; Bergerioux and Zikovsky, 1978; Block and Dams, 1978; Hirner, 1987; Jones, 1988; Chai Chif Ang et al., 1991; Olajire and Oderinde, 1993; Filby and Olsen, 1994; Frankenberger et al., 1994; Olsen et al., 1995; Petrenko and Dorogochinskaya, 1995 ± Veal, 1966 Veal, 1966 Filby and Olsen, 1994; Olsen et al., 1995 Filby, 1973 American Petroleum Institute, 1973 Shah et al., 1970b Chai Chif Ang et al., 1991 Chai Chif Ang et al., 1991

J.B. Stigter et al. / Environmental Pollution 107 (2000) 451±464 Table 22 Emission inventory for the Netherlands Metal

Emission kg/year (1995)

Emission kg/year (1995) based on this work

Cd Cr Cu Zn As

124 2530 1262 2482 130

2.6 127 78 757 34

will be published elsewhere. In the framework of this article we will give only a brief summary regarding the implications for the emission inventory in the Netherlands. When comparing the results given in the emission inventory by the oil manufacturing industry for the Netherlands for 1995 (Veldt, 1993) with the new data, the picture given in Table 22 emerges. It is evident that the new data will lead to signi®cantly lower emission estimates than hitherto assumed. This most probably holds for other European countries as well. Acknowledgements This research project was commissioned to TNO by the Ministry of Housing, Spatial Planning and Environment. The authors would like to thank the members of the Steering Committee for their stimulating discussions and the Dutch Petroleum Federation (OCC) and the individual oil companies for providing the samples and allowing their sta€ members to participate in the Steering Committee. References Ali, M.F., Bukhari, A., Saleem, M., 1983. Trace metals in crude oils from Saudi Arabia. Ind. Engng. Chem. Prod. Res. Dev 22 (4), 691± 694. Al-Swaidan, H.M., 1988. Simultaneous determination of trace metals in Saudi Arabian crude oil products by inductively coupled plasma mass spectrometry (ICP/MS). Analytical Letters 21 (8 ), 1487±1497. Al-Swaidan, H.M., 1990. Trace metals determination by wet ashing ICP/MS in Saudi Arabian crude oils. Analytical Letters 23 (7), 1345±1356. Al-Swaidan, H.M., 1994. Simulteneous multielement analysis of Saudi Arabian petroleum by microemulsion inductively coupled plasma mass spectrometry (ICP/MS). Analytical Letters 27 (1), 145±152. Al-Swaidan, H.M., Al-Gadi, A.A., Abdalla, M.A., 1988. Optimization and determination of trace metals in Saudi petroleum and petroleum products by atomic absorption spectrometry. Oriental J. Chem 4 (3), 221±229. American Petroleum Institute, 1973. Validation of Neutron Activation Technique for Trace Element Determination in Petroleum Products. (Publication No. 4188, August 1973). API. Anwar, J., Nahid, F., Kirmani, Z.U., 1990. Determination of trace metals in crude oil samples by atomic absorption spectroscopy using a mixed solvent system. J. Chem. Soc. Pak 12 (3), 249±252.

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