Solubility of Hydrocarbons in Seawater

Wasik SP, Brown RL (1973) Determination of hydrocarbon solubility in seawater and the analysis of hydrocarbons in water-extracts. In Proceedings of the conference on prevention and control of oil spills. American Petroleum Institute, Washington, DC, pp 223-237... 

The studies described above give evidence that the XAD-2 method provides a useful determination of the hydrocarbon components in dilute seawater-oil suspensions. The quantity of "total oil reported in Table I is in sharp contrast to the total hydrocarbons found in the water by the combined helium extraction/XAD extraction techniques. The discrepancy between total oil by IR and hydrocarbons found in water by component analysis was previously reported (5,11) and can be explained by the low contribution to the IR absorbance at 2927 cm 1 of the soluble aromatic constituents relative to the saturate hydrocarbons. The difference between IR analytical result and component analysis by GC becomes much greater in the filtered systems, where the total hydrocarbons found are three times that reported by the IR method. It is clear that the IR analytical technique is only useful in systems where there is a preponderance of particulate, bituminous petroleum or where it is used as a monitoring tool. It provides no information about actual levels of hydrocarbons in systems where there is a preponderance of water-soluble aromatic compounds. 

Concern about pollution of the oceans has stimulated measurements of the solubility of organic compounds in seawater. This table gives the solubility of several hydrocarbons in seawater. The data are derived from a review in the lUPAC Solubility Data Series (Reference 1). 

For most organic chemicals the solubility is reported at a defined temperature in distilled water. For substances which dissociate (e.g., phenols, carboxylic acids and amines) it is essential to report the pH of the determination because the extent of dissociation affects the solubility. It is common to maintain the desired pH by buffering with an appropriate electrolyte mixture. This raises the complication that the presence of electrolytes modifies the water structure and changes the solubility. The effect is usually salting-out. For example, many hydrocarbons have solubilities in seawater about 75% of their solubilities in distilled water. Care must thus be taken to interpret and use reported data properly when electrolytes are present. 

Rossi, S.S., Thomas, W.H. (1981) Solubility behavior of three aromatic hydrocarbons in distilled water and natural seawater. Environ. Sci. Technol. 15, 715-716. 

Particular attention has been focused on the toxic effects of aromatic hydrocarbons because these chemicals have proven highly carcinogenic to humans and marine life. Of greatest concern are the PAHs, which are toxic to the benthos at the ppb level. The most common compounds are shown in Figure 28.20 their structures are based on fused aromatic rings. These high-molecular-weight compoimds are very nonpolar and, hence, have low solubilities. Once in seawater, they tend to adsorb onto particles and become incorporated in the sediments. The toxicity of PAHs is enhanced by photochemical reaction with UV radiation. Photo-activated toxicity is especially problematic in shallow-water sediments, such as found in estuaries. 

The solubility of fuel oil no. 2, particularly the alkane and isoprenoid fractions, in seawater is increased by the presence of fulvic acid, although the solubilities of phenanthrene or anthracene, both polycyclic aromatic hydrocarbons, are unaffected by the presence of humic materials (Boehm and Quinn 1973). Unfiltered Narragansett Bay water was able to dissolve 1,560 g/L of fuel oil no. 2, although removal of... 

For an assessment of the global distribution of persistent volatile halogenated hydrocarbons, the solubility and activity coefficients of such compounds in natural waters need to be known. Warner and Weiss (1985) have determined the solubilities of dichlorodifluoromethane (Freon 12) at 1 bar partial pressure at various temperatures in freshwater and in seawater (35.8%o salinity) ... 

Sutton and Calder (9) have also measured the solubilities of several alkylbenzenes in distilled water and in seawater by a method based on GC. Saturated solutions were prepared by equilibrating water with aromatic vapor in an all-glass apparatus consisting of a 1-L Erlenmeyer flask with an insert tube. The insert tube was used to store the compound. It was capped with a ground-glass stopper. The liquid hydrocarbon did not come into contact with the water except through a perforation in the insert, which allowed hydrocarbon vapors to enter the headspace above the water in the flask. The flask was placed in a constant-temperature shaking bath controlled at 25.0 dt 0.1°C. The water was equilibrated for 48 hr prior to analysis. The solubilities were determined by solvent extraction of the saturated solutions with subsequent analyses of the extracts by GC. 

The analysis of Prudhoe Bay Crude Oil for the hydrocarbon components under study is presented in Table V, together with the percentage of the total hydrocarbons found represented by each of the hydrocarbon types. For comparison, the contribution of component types to the total hydrocarbon is listed for both a filtered and unfiltered seawater suspension. The comparison is somewhat biased because benzene was not determined in the crude oil, being poorly separated from the hexane solvent, and because C4-benzenes were not determined in the unfiltered sample. However, it can be readily seen from the results that while aromatic hydrocarbon types are present in the crude oil in roughly equal concentrations, the preponderance of the total hydrocarbons in the seawater suspension is composed of the low-molecular-weight aromatic hydrocarbons. In both unfiltered and filtered systems, 90% of the water-soluble aromatic hydrocarbons found are composed of benzene, toluene, ethyl benzene, and the xylenes. This is in contrast to their concentration in the whole crude oil, which is at most a few percent and where their contributions to the hydrocarbons analyzed for is probably less than 30%. 

Barium sulf ate scales form in situations where production of reservoir fluids causes mixing of incompatible aqueous fluids. For example, in North Sea (UK) offshore hydrocarbon production, seawater is injected into reservoirs to displace oil, and maintain reservoir pressure. When seawater, high in sulfate, contacts reservoir fluids that have high concentrations of Ba +, BaS04 scales result. Barium sulfate is an especially intractable scale mineral because of its physical hardness and very low solubility (i-6). 

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