Desorption behavior of carbon tetrachloride and chloroform in contaminated low organic carbon aquifer sediments.

Slow release behavior of carbon tetrachloride (CCl(4)) and chloroform (CHCl(3)) in low organic carbon (<0.1%) deep aquifer sediments was quantified by 1-D column desorption studies with intact cores. The compounds had been in contact with the sediments for 30years. Comparison of the CCl(4) distribution coefficient (K(d)) from this study with those from short contact time experiments suggested that CCl(4)K(d)'s calculated from site contaminated sediments of long contact time are likely a factor of 10 or more higher than those calculated from short contact-time lab experiments. A significant portion of the CHCl(3) mass (55% to more than 90%) was resistant to aqueous desorption in sediments with clay contents ranging from 2.0% to 36.7% and organic carbon content ranging from 0.017% to 0.088%. In contrast, CCl(4) showed greatest mass retention (31% or more) only in the highest clay and organic carbon content sediment. Relatively easy solvent extraction of the residual masses of CCl(4) and CHCl(3) from the sediments indicated the compounds were not permanently sequestered. Tracer breakthrough in columns was well behaved, indicating interparticle diffusion was not causing the slow release behavior. Diffusion out of intraparticle pores is suggested to be the main process governing the observed behavior although, diffusion out of natural organic matter cannot be ruled out as a potential contributing factor. The half-life for release of the slow fraction of CHCl(3) mass from sediments was estimated to be in the range of weeks (100h) to months (1100h). Neither CCl(4) or CHCl(3) were detected at measurable levels in the column effluent of one of the sediments even though a significant mass fraction of CHCl(3) was found present on the sediment following desorption suggesting that our estimate of hundreds to thousands of hours for complete release of CHCl(3) masses from such sediment is conservative.

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

The long-term interactions of carbon tetrachloride (CCl(4)) and chloroform (CHCl(3)) with sediments that are low in organic matter (OM) are not well studied. In this study, CCl(4) and CHCl(3) were mixed with supercritical carbon dioxide (CO(2)) 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 K(d) values of CCl(4) or CHCl(3) 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 gamma-distribution function (beta and eta) 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.