SITE INFORMATION AND CORRESPONDENCE_FILE 1

Home Browse Search

GEOMATRIX Mr. Don O. Culbertson Chevron Pipe Line Company S March 1995 Page 2 General chemistry of PHCs All PHCs are composed of carbon and hydrogen atoms; these atoms are arranged into an almost infinite number of discrete molecules. These molecules are classified as alkanes, alkenes, and aromatics based on their molecular structure. The molecular structure of these classes of molecules determines the solubility of the molecule in water. Water solubility is of primary importance relative to the potential for dissolved-phase transport of the molecule. Alkanes are linear (n-alkanes), branched, or cyclic in shape and consist of carbon single bonds. Alkanes are the most prevalent constituent of crude oil as well as most refined petroleum products and generally have very low toxicity and very low water solubility. Alkenes are byproducts of refining and are linear or branched in shape with carbon double bonds. Alkenes make up a limited fraction of gasolines and are usually not a component of higher-boiling products. Alkenes have very low toxicity and very low water solubility. Aromatics are ring structures with carbon double bonds. Aromatics may have one ring (monoaromatics such as benzene, toluene, etc.) or multiple rings (PNAs such as naphthalene, benzo(a)pyrene, etc.). Aromatics usually make up between approximately 10 and 35% of petroleum products, with some gasolines having as much as 50% aromatics. Aromatics have high to low toxicity and moderate to very low water solubility. The water-soluble fraction of PHCs Water solubility is a function of both the molecule class and the molecular weight (number of carbon atoms). Within a certain class (e.g. aromatics), lower molecular weight constituents usually tend to be more soluble. We know from published chemical research (Mackay and Shiu, 1992; Yaws, et al, 1990; Thomas and Delfino, 1991) that alkanes and alkenes have very low water solubilities (< 10 g/m3 or parts per million [ppm]) at molecular weights exceeding six carbon atoms (C6) either as discrete molecules or as a component of complex petroleum products or crude oil. The monoaromatics (BTEX) have moderate water solubilities (> 100 g/m3), with the lowest molecular weight (C6) benzene having the highest relative solubility (1750 g/m3). The PNAs have low to very low water solubilities (30 to < 1 g/m3), with the higher molecular weight PNAs (e.g. benzo(a)pyrene[C20]) being virtually insoluble in water (<0.01 g/m3). Based on this understanding of the chemical properties of PHCs, we would expect that the WSF of PHCs would be limited to a few constituents out of the thousands that make up the PHC. Further, the measurable portion of the WSF of a PHC is a function not only of the solubility of each constituent, but also the mole-fraction of the constituent within the PHC and the partitioning coefficient of the constituent between water and the other organics within the PHC.