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.

Quantification of volatile organics in soil aging experiments using fourier transform infrared spectroscopy.

On-line Fourier transform infrared (FT-IR) spectroscopy was applied to monitor the concentration of halogenated volatile organic compounds in a sample-preparation process that simulates long-term, slow accumulation of contaminants in soils (i.e., aging). Artificial aging is conducted by circulating a supercritical fluid solution containing the contaminant(s) of interest through a packed soil column. Mid-infrared spectra of several volatile halocarbons were measured in supercritical Xe and CO(2) to evaluate possible interferences from the strong absorption of CO(2). Although some of the C-X bands were partially masked in supercritical CO(2), all of the compounds studied had distinct spectral features in the region 1400-700 cm(-1) and could be monitored in either solvent. Quantitative measurements of halogenated volatile organics in supercritical CO(2) were demonstrated with CCl(4). Excellent results were obtained over the range 7-280 mM. Representative artificial aging experiments were conducted on two test soils using CCl(4) as the contaminant. On-line (FT-IR) estimates of the aged soil concentrations were 1.3-4.4 times higher than off-line concentrations obtained by gas chromatography/mass spectrometry. The discrepancies were primarily ascribed to post-aging losses that occurred during depressurization and subsequent sample handling. FT-IR spectroscopy is shown to be a powerful tool for monitoring soil loading behavior and for developing artificial aging protocols.

The retention of 14C-labelled poly(acrylic acids) on gastric and oesophageal mucosa: an in vitro study.

Polymers that bind from solution onto gastric mucosa can be used either as a means of facilitating localised drug delivery, or can act as therapeutic agents in their own right (e.g. by forming a protective layer or by inhibiting enzymes). In our previous study [Int. J. Pharm. 236 (2002) 87], the binding and retention of labelled poly(acrylic acid)s on sections of gastric mucosa from pigs was evaluated using 'dynamic flow' conditions and a high molecular weight poly(acrylic acid) was found to bind most avidly. In the current study, 3% solutions of 'low', 'high' and 'ultra high' molecular weight polymers were evaluated in the 'dynamic flow' model for their ability to bind to tissues from the fundic and pyloric regions of the stomach and the oesophagus of pigs. All the polymers tested were retained on each mucosa for extended periods; the high and ultra high molecular weight polymers showed the greatest retention. Examination of the kinetics of polymer elution suggested that two fractions exist, 'bound' and 'unbound' polymer, showing differing retention profiles. The high molecular weight polymer showed the greatest retention on pyloric tissue, particularly on the upper sections. The retention of the ultra high and high molecular weight polymer was similar on the fundic and oesophageal mucosa, and the distribution was even across the tissue. It was concluded that poly(acrylic acid) binding from solution presents a therapeutic opportunity, and the differences in binding and retention of the polymers on the different mucosae could present an opportunity for targeting.

An in vitro model for investigating the gastric mucosal retention of 14C-labelled poly(acrylic acid) dispersions.

Polymers that bind from solution onto gastric mucosae can be used as a means of facilitating localised drug delivery, or act as therapeutic agents in their own right (e.g. by forming a protective layer or by inhibiting enzymes). Previous workers have used semi-quantitative methods to identify the ability of commercially available poly(acrylic acid)s to bind to gastric mucosa. In this study, the binding and retention of labelled poly(acrylic acid)s to sections of gastric mucosa from the pyloric region of pigs stomach were evaluated using 'static' and 'dynamic flow' test systems. Dispersions (3%) of 'low', 'high' and 'ultra high' (cross-linked) polymers were seen to adhere to porcine pyloric mucosa after exposure and rinsing in the 'static' system. The high molecular weight polymer showed the greatest retention in the 'dynamic' test system when washing continuously with simulated gastric acid. Changing the pH of the dispersions from 4.3 to 6.2 had little effect on polymer retention. It was concluded that polymers that were sufficiently mobile in solution to spread on, and interact with, the mucosal surface, but had a sufficiently high molecular weight to form viscous solutions and/or bioadhere to the mucosa, may be retained on the mucosal surface for the longest periods.