Immobilization or recovery of chlorinated hydrocarbons from contaminated groundwater using clathrate hydrates: A proof-of-concept

The objective of this study was to evaluate whether the formation of solid clathrate hydrates by certain chlorinated hydrocarbon compounds and water would be amenable to in situ immobilization and ex situ recovery of these contaminants from groundwater. Using help gases to enhance the formation of the solids, the melting points of clathrate hydrates of CCl4 1,1-DCE, PCE, and TCE were measured at 1 atm pressure. CCl4 hydrates had melting points between 16 and 20 degrees C; 1,1-DCE hydrates had lower melting points of between 11 and 13 degrees C. TCE and PCE hydrates could not be formed unless crystallizing seed material was introduced into the system. Concentrations of CCl4 and 1,1-DCE in the hydrate phase and the aqueous phase after hydrate formation were determined from experiments in which the chlorinated hydrocarbon compounds existed as a separate dense nonaqueous phase liquid (DNAPL) or as dissolved species in water. Enrichment factors (C-H/C-A, where C is the concentration of the organic in the hydrate [H] and aqueous [A] phase) were between 200 and 500 for 1,1-DCE and CCl4, respectively. These experiments provided a proof-of-concept suggesting that the in situ immobilization or ex situ recovery of some common hydrophobic groundwater contaminants may be possible in a wide range of subsurface environments.