Physical control on CCl4 and CHCl3 desorption from artificially contaminated and aged sediments with supercritical carbon dioxide

The long-term interactions of carbon tetrachloride (CCl4) and chloroform (CHCl3) with sediments that are low in organic matter (OM) are not well studied. In this study, CCl4 and CHCl3 were mixed with supercritical carbon dioxide (CO2) and loaded onto columns packed with two sediments with low OM and different textures, to establish contamination and achieve expedited artificial aging. The columns were subsequently leached with a simulated groundwater under hydraulically saturated conditions. Scanning electron microscopy was used to inspect the morphology of sediment single particles, determine the degree of particle association in aggregates and qualitatively estimate porosity and the possible diffusional pathways that might affect the overall contaminant desorption rates. Results demonstrated that most of contaminant inventories were rapidly released in the first pore volume of effluent, although a small portion of contaminants’ total mass exhibited time-dependent desorption. The calculated Kd values of CCl4 or CHCl3 partition were negligibly small. Both contaminants had similar transport behavior which was simulated well with a distributed (multiple)-rate (DR) statistical model. The model accounted for the apparent contaminant mass transfer through diffusional pathways of different lengths, towards the advective pores. The distribution of contaminant mass between equilibrium and kinetic fractions, the distribution of the individual rate constants, and the average rate constants calculated with the parameters of the γ-distribution function (β and η) of the DR model, were sediment (texture) dependent. This indicated that contaminant desorption during the late stage of leaching was driven by concentration gradients (i.e., diffusion) within sediment matrix porosity.