Operational problems related to the preparation of the seawater soluble fraction of crude oil

Abstract

Owing to the importance of dissolution and weathering processes following oil spills, this work focused on the operational (quantitative) aspects related to the dissolution of petroleum-derived products, as well as the influence of solar light on both dissolution and the photoproduction of hydrogen peroxide. Four Brazilian crude oil samples were used to study the transfer process of organic compounds from the crude oil film to the aqueous phase (natural seawater) over a period of up to 45 days. Dissolved organic carbon (DOC), measured by non-dispersive infrared spectroscopy followed by high temperature catalytic combustion, was used to follow the partitioning between the two phases. Aqueous DOC values increased as a function of time (up to 15 days) until equilibrium was reached at concentrations ranging from 5 to 45 mg C L⁻¹. The final DOC concentration as well as the rate of dissolution depends on the nature of the crude oil. When exposed to sunlight, the dissolution was enhanced by up to 67.3%, and inorganic peroxides were generated in the concentration range from 4.5 up to 8.0 µmol L⁻¹ after 7.3 h irradiation. These results indicate that there is a need for a standard procedure for the production of the WSF in order to generate a more reliable tool to assess the impact of oil spills on the marine environment.

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Introduction

Problems associated with oil pollution of navigable waters have reached critical proportions. Marine waters are nowadays constantly receiving a wide range of contaminants, some of them in acute events. Marine pollution caused by continuous inputs is basically associated with sewage discharge, offshore oil drilling and from ships. The long-term ecological impact of such input depends largely on the dispersion of the in the seawater and on the ultimate fate of the spilled petroleum products.¹

Following any disaster involving an oil tanker, the oil slick is normally removed from the surface seawater by either mechanical or absorption methods. In terms of the environmental and ecological effects of the oil pollution, the degree to which the crude oil components dissolve in the seawater before the removal of the crude oil slick is very important. For example, it has been established that it is the dissolved, rather than the emulsified or the adsorbed fraction of the oil that is acutely toxic to the marine biota. One reason for this is that the dissolved fraction is readily ingested by organisms at the lower end of the food chain, thereby concentrating and accumulating in organisms at higher trophic levels.²

Because of its importance, researchers have been studying the dissolution of petroleum-derived products in water in order to evaluate the toxicological characteristics of seawater contamination caused by crude oil. The water soluble fraction (WSF, also referred to as SWSF if dissolved in seawater) is the organic enriched aqueous phase in contact with the oil spill. This enrichment is caused by the dissolution of the relatively low molecular weight compounds present in crude oils, mainly aromatic hydrocarbons (with relatively high water solubility), and small amounts of high molecular weight polar compounds. However, in the literature there is no uniformity in the experimental steps used by different authors to obtain the WSF from a crude oil sample and the experiments are conducted in different ways. This results in considerable difficulties in comparing experimental data. This lack of consistency may be partially explained by the lack of a standard procedure to ensure reliable, reproducible and comparable results. Another important limitation is that the experimental details are often poorly documented.

Unfortunately, there has been very little systematic experimental work aimed at providing a comparison of WSF from crude oils. This makes it both difficult and of debatable value to use laboratory data to evaluate environmental damage or to select mitigating strategies. Several laboratories have been involved in the problem of preparing a WSF of a crude oil, as presented by the National Academy of Sciences,³ but the solubility reported varies according to a number of key factors, including the composition of the oil,^4,5 the ratio of the volume of oil to the water brought into contact,⁶ the mixing rate⁷ and time,^8,9 the seawater temperature,^6,10 the salinity,^6,11 the pH,¹² and to other effects, such as separation time,^8,13 filtration,^7,8 volatilisation,¹⁴ dilution,^5,8 illumination during preparation,¹⁵ flask dimensions,⁸etc.

The present work shows the results of the production of WSF in seawater for four different Brazilian crude oils. The amount of dissolved organic carbon (DOC) was monitored at different time intervals until an equilibrium concentration (monitored by DOC) was reached. Some experiments were also carried out under solar light and the photoproduction of hydrogen peroxide in the WSF was also investigated. The results are discussed taking into consideration experimental procedures normally found in the literature.

Materials and methods

Crude oil samples and seawater

The Brazilian crude oil samples (named A, B, C, and D for simplicity) originated from the continental shelf of Rio de Janeiro State (Enchova (A), an unknown field (B) and Marlin (D)), as well as from Rio Grande do Norte State (Terra (C)). Seawater was collected at São Sebastião (São Paulo State), along the coastal line of the Ilha Bela channel.

Obtaining the WSF

Pyrex flasks (2.0 L) with a Teflon tap at the bottom were filled with 1.5 L of seawater. Crude oil was added at a ratio of 1∶20 v/v, and the solution was magnetically stirred for 30 min. For experiments in the absence of light, the flasks were left to equilibrate for up to 45 days in the dark at room temperature. The Pyrex flasks remained stoppered for the entire period, except when water samples were drawn through the Teflon tap, this was done without disturbing the oil/seawater surface. Sampling was carried out in duplicate.

Solar light irradiation

The samples (about 1200 mL) were exposed to solar light irradiation in Pyrex flasks in batch experiments, for periods of up to 7.3 h. The solar light intensity was measured using a 9811-56 radiometer (Cole-Parmer Instrument Co., Chicago, IL) fixed at 365 nm.

DOC analysis

The amount of DOC present in the aqueous phase samples (about 10 mL) was measured by non-dispersive infrared spectroscopy followed by high temperature catalytic combustion, using a Shimadzu TOC 5000 Total Organic Carbon Analyzer (Shimadzu Corporation, Kyoto, Japan).

Determination of hydrogen peroxide

Photoproduction of hydrogen peroxide was investigated during irradiation of the WSF. Samples (about 20 mL) were collected during radiation exposure and the determination, including the speciation of organic and inorganic peroxide, was carried out using a photometric method based on the peroxidase catalyzed oxidation of N,N-diethyl-p-phenylenediamine.¹⁶

Results and discussion

DOC values resulting from the dissolution of crude oil derived products in seawater are presented in Fig. 1. For all experiments monitoring the DOC, the initial values for each crude oil sample were taken immediately after the stirring finished (time zero), and the values plotted were corrected taking into consideration the DOC background values present in the seawater. The equilibrium concentration and steady state concentration are defined here as the arithmetic average DOC values obtained after the time required until no further variation in the DOC concentration could be observed (up to 45 days).

Fig. 1 Dissolution of the WSF of four Brazilian crude oils (A, B, C, and D) from a surface film for a period of 45 days, by DOC measured using a Shimadzu TOC 5000 Total Organic Carbon Analyzer.

Maximum enrichment of the aqueous phase with crude oil products was observed for the Enchova Field crude oil (A), which reached an equilibrium concentration after 5.8 days, producing a WSF with an average concentration of 46.8 mg C L⁻¹. On the other hand, minimum dissolution was observed for the Terra Field cruse oil (C), whose WSF reached a steady state with an average concentration of 4.9 mg C L⁻¹ after 7 days. Dissolution from crude oil B was quite stable, yielding an aqueous phase with an average concentration of 8.6 mg C L⁻¹ after 17 days. However, crude oil D apparently did not achieve a steady state in the test period used in this work (45 days). Based upon these results, one can see that dissolution and the resulting WSF concentrations vary markedly and depend on the nature of the crude oil, on the formation of emulsions and on the time of contact of the oil/seawater surface.

It is important to emphasize that laboratory procedures to generate the WSF of organic compounds present in crude oils, deviate significantly from natural conditions, where the floating oil slick rarely remains in contact with the same water mass for such long periods of time. This means that the equilibrium concentrations obtained under laboratory conditions are expected to be much higher than the ones observed in the field, and represent the worst-case scenario.

When the experimental procedure for the production of the WSF from crude oil A was carried out in the dark (control) as well as under sunlight, an increase of 67.3% in the DOC concentration, from 15.9 (dark) up to 26.6 mg C L⁻¹ (irradiated) after 7.3 h of solar exposure, was observed (Fig. 2). Differences in the average solubilisation rate varied from 0.61 mg C L⁻¹ h⁻¹ in the dark sample to 2.00 mg C L⁻¹ h⁻¹ for the irradiated samples. Considering that the light intensities observed in these experiment varied from 1.20 up to 2.70 mW cm⁻², typical for mid-south latitudes, and taking into consideration that oil spills often occur in areas exposed to sunlight, this data further emphasizes the need for more realistic procedures to obtain the WSF, so the results can be extrapolated to natural conditions. In this context, WSF procedures incorporating photoperiodicity using artificial light as surrogate sources of irradiation have to be considered.

Fig. 2 DOC levels when a film of crude oil A was exposed to sunlight for a period of 7.3 h. Average solubilisation rate was defined as the angular coefficient of the linear regression for both dark and sunlight data.

Another important environmental factor contributing to the variations in both quality and quantity of the WSF is the presence of dissolved oxygen in the water column, which might act as a precursor for many reactive intermediate species, such as hydrogen peroxide, in light-driven reactions. Photoproduction of peroxide was also investigated in this work and, for all experiments, the initial peroxide concentration in the WSF of crude oils samples was determined immediately after stirring (time zero), and the values presented were corrected, taking into consideration the background peroxide concentration in the seawater. Fig. 3 presents the results of the photoproduction of peroxides for the Enchova crude oil (A) WSF exposed to sunlight and kept in the dark (samples obtained in the experiment shown in Fig. 2 for DOC monitoring). The peroxide concentration reached 4.5 µmol L⁻¹ for the sample exposed to sunlight after 7.3 h of irradiation whilst in the dark no photoproduction was observed (below 0.2 µmol L⁻¹). Since hydrogen peroxide is a powerful oxidant of a wide class of organic compounds, the photoproduction of the peroxides can also contribute to the WSF carbon pool. The real implications of this photooxidation and its effect on the crude oil components is not clearly understood, but it certainly alters the toxicity to the biota due to the generation of this transient species. Although the protocols currently used for preparation of WSF from petroleum hydrocarbons do not allow for the generation of peroxides, this new parameter should also be considered. Peroxide concentrations ranging from 4.5 to 7.9 µmol L⁻¹ were observed for all the WSFs obtained from the crude oil samples (A, B, C, and D) exposed to 5.0–7.3 h of sunlight irradiation. The significant differences in the results can be attributed to the irradiation intensity and also to the nature of the oils. On the other hand, no correlation was found between the peroxide generation and the DOC values.

Fig. 3 Photoproduction of peroxides into the WSF when film crude oil A on seawater was exposed to sunlight for a period of 7.3 h.

The above-mentioned experiments of peroxide photoproduction are expressed in terms of total peroxides (organic plus inorganic peroxides). The speciation of organic and inorganic peroxide was carried out using catalase enzyme to inhibit inorganic peroxide formation. As shown in Fig. 4, no organic peroxides were detected when the WSF of crude oil B was exposed to sunlight, thus confirming that molecular oxygen is the major species acting as electron scavenger in this organically enriched seawater.

Fig. 4 Speciation of organic and inorganic peroxide using catalase enzyme to inhibit inorganic peroxide formation. Photoproduction of inorganic and organic peroxides in the WSF when a film of crude oil B on seawater was exposed to sunlight for a period of 5 h.

To better understand the role of the oil slick in the photoproduction of hydrogen peroxide, a sample of the WSF from the Enchova crude oil (A) was exposed to sunlight. The only difference was that in this new experiment the oil slick was physically removed just before exposure to the sunlight. According to Fig. 5, it can be seen that under these new conditions the final concentration of H₂O₂ (2.8 µmol L⁻¹) was 62% lower than the value obtained when in the presence of the oil slick. This confirms the role of the DOC as a precursor of H₂O₂ in waters, once it is exposed to sunlight.^17,18

Fig. 5 Photoproduction of peroxides in the WSF of crude oil A when the oil slick was removed, before exposure to sunlight for a period of 5.8 h.

Conclusion

The development of standard protocols for producing WSFs for oil spill research is an important issue for assessing the environmental impacts associated with the oil industry as a whole, from drilling to refining. Although it has been known for some time that variations in protocols will cause significant differences in results, and also that different oils produce different WSFs, this paper adds new parameters to the discussion. The effects of sunlight and peroxide photoproduction are explored, pointing out the urgent need to develop more realistic laboratory procedures for obtaining a WSF that will incorporate the possible effects caused by such variables.

Acknowledgements

The authors are grateful to the Coordenação de Aperfeiçoamento de Pessoal e Ensino Superior (CAPES) for financial support and they also thank Dr. C. Collins for revising the manuscript.

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