The Fate and Transport of Hazardous Chemicals in the Subsurface Environment

Abstract

Some volatile organic compounds (VOC's) such as 1,1,1-trichloroethane (TCA), trichloroethylene (TCE), and tetrachloroethylene (PCE) have been found as major groundwater pollutants which originate from the previous dump of these chemicals into unsealed ponds of landfill sites. One of the methods which has been proposed for cleaning these types of contaminants is to pump the groundwater out to the surface and then use air stripping to remove the VOC's. Knowledge of the fate and transport of these chemicals in the subsurface environment is necessary in predicting the time required for cleaning the groundwater. One of the major factors governing the transport and cleanup of VOC's in groundwater is the adsorption and desorption reactions of these compounds in the saturated and unsaturated soils. Thus, the objective of this study was to determine the rate and extent of adsorption and desorption of TCA, TCE. and PCE on clay and organic containing soils. The effects of solid concentration and sorbent characteristics were also investigated. It has been found that sorption and desorption are both very fast and reversible. The reactions fit well into the reversible pseudo first-order kinetic. The half time for the reversible adsorption is about 4 to 5 hrs, while that for desorption is around 15 hrs. The equilibrium adsorption follows the Henry's form of isotherm, or a simple partition model. The partition coefficients vary from .35 to 10 L/Kg at a test solid concentration of 40 g/L. When it is increased to 2700 g/L, the observed coefficients are reduced to only 5 to 50 percent. It has also been shown that for a sorbent having an organic fraction (foc) of 0.005 or greater, the partition coefficient, K, can be predicted from the following two models developed from this study:where Kow is the octanol-water partition coefficient and S is the solubility of VOC in mg/L.