Tanks of NAPL-phase benzene, hexachlorobenzene, and phenanthrene are spilled on

Question

Tanks of NAPL-phase benzene, hexachlorobenzene, and phenanthrene are spilled on top of the water table at a contaminated site. Using the physical parameters given in the table below, predict the relative distribution of the 3 contaminants into the air, water and soil. Are any of these chemicals likely to sink into the saturated zone?

Physical properties of various chemicals Chemical Mol. Wt. Density Solubility Log (H)*Log (Ko)oe (g/mol) (g/mL) (mg/L)(atm-L/mol) (mL/g) 83 3900 14,000 Benzene Hexachlorobenzene 284.76 Phenanthrene Log Henry's Law Constant at 25 °C Log Octanol-Water Partition Coefficient at 25 C 0.88 2.04 1.18 1780 0.006 0.74 1.51 1.45 2.12 5.23 4.46 78.1 178.2 Organic Carbon Partition Coefficient Constants and Conversions R = 0.082057 L-atm / K-mol 1 m3 1000 L

Explanation / Answer

To determine if any of the components will sink into the water table, we will evaluate the organic carbon partition coefficient Koc (ratio of the mass of a chemical that is adsorbed in the soil per unit mass of organic carbon in the soil). As we can see phenanthrene has the highest coefficient (14,000 mL/g) so you could say with some certainty that this compound will spread to the water table, and to a lesser extent so will the hexachlorobenzene (3.900 mL/g).

if these compounds have contact with water, its diffusion depend on their tendency to dissolve in polar compounds, for which we will evaluate the octanol-water partition coefficient (Kow), and its solubility in the water (solubility). These three compounds have Kow coefficients very similar, but have a big difference in solubility. Of the three compounds, benzene has the highest solubility (1.780 g/ml), so it will certainly be carried away by the currents of underground water, unlike the other two compounds.

Finally to predict the distribution of these compounds in the air, we use the henry law constant (ratio of a chemical's concentration in the air to its concentration in water at equilibrium). Of the three componenetes, hexachlorobenzene has the highest coneficiente, so this spread more easily to the air, followed by the benzene. The phenanthrene will not spread into the air.

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