A laboratory method for investigation of diffusion and transformation of volatile organic compounds in low-permeability media

Abstract

A laboratory diffusion cell technique is presented that provided effective diffusion coefficients (D6) and first order rate constants (k) of volatile organic compounds (VOCs) in one sample of reactive low permeability media based on the temporal VOC concentration measurements of both a reservoir source and a porous medium columns. A sealed stainless steel cylinder contained a vapour reservoir overlying an artificial, low permeability medium under slight negative porewater pressure. Vapour-filled horizontal "mini-boreholes" were established along the length of the porous medium. Nonreactive experiments (Ottawa sand crushed to find silt and clay size) with 5 VOCs (carbon disulphide (CS2), chloroform (CCl3H), 1,2-dichloroethane (1,2-DCA), carbon tetrachloride (CCl4) and perchloroethylene (PCE)) and reactive experiments (the nonreactive material amended with 17% (w/w) pyrite) with CCl4 transforming to CCl3H and CS2 were conducted in duplicate, each for one month.

Reservoir concentrations and borehole profiles were obtained within a few minute of collecting a 1 minute sample utilizing Solid Phase Microextraction fibres coated with 1 or 2 tested poly(dimethylsiloxane) thicknesses (7 um and 100 um). For VOCs with greater fibre coating partitioning values (CCl4 and PCE), the thinner coating was better suited because it reduced the mass extracted from the reservoir and the porewater. For application to field samples, the core container itself could be utilized as the diffusion cell to ensure a tight seal with the cylinder walls.

D6 values, which ranged from 4 x 10^-6 to 7 x 10^-6 cm2/sec, are within the range of literature values for nonsorbing and nonreactive compounds in natural silt and clay deposits. The 4 estimated first order rate constraints for the CCl4-pyrite reactions were within the same order of magnitude as results by others. The overall rate constant for CCl4 was within experimental precision in the duplicate diffusion cells. The rate constant for CCl4 was within experimental precision in the duplicate diffusion cells. The rate constant for the formation of the transformation products were up to 1.6 times different in the two diffusion cells and this may reflect the reaction mechanism's sensitivity to slight differences in reaction conditions, as reported by others. CCl4 transformation is overpredicted in the first few cm of diffusive transport with a first order reaction kinetic model (with respect to CCl4 concentration model) and may be better predicted with a zero order kinetic model and/or a decrease in assumed pyrite reactivity with time.