The decomposition of 1,1-dichloroethene on Pd(111) is investigated using conventional thermal desorption, laser-induced thermal desorption (LITD), and FT reflection absorption infrared spectroscopy (FT-RAIRS). The decomposition mechanism produces at least three hydrocarbon surface intermediates, including ethylidyne. Thermal desorption results differ between high and low coverages because of relative surface concentrations of Cl and H in combination with kinetic effects.

HCl adsorbed onto clean Pd(111) produces three distinct HCl desorption states in thermal desorption spectroscopy (TDS), all of which are populated simultaneously, even at exposures as low as 0.33 L. One desorption state emanates from molecularly adsorbed HCL (220 K), while the other two (300 and 470 K) emanate from recombination of H plus Cl atoms. The lowest temperature recombinative peak (beta at similar to 300 K) belongs to recombination of chlorine adatoms with surface hydrogen. This reaction is competitive with the recombinative desorption of H-2. Finally, the highest temperature peak with an onset at 470 K (alpha) is the recombination of the remaining Cl with hydrogen dissolved in the Pd crystal, which emerges from the bulk onto the surface at approximately 470 K. Some of the chlorine adatoms order on the Pd surface to yield a root3 x root3 R 30 degrees overlayer structure at coverages as low as theta = 0.13 ML, This structure persists through a range of temperatures from 100 to 320 K, when initially adsorbed at 100 K. The order-disorder transition at similar to 320 K is reversible.

The efficiency of biosurfactant-facilitated removal of soil-bound metals is affected by biosurfactant sorption to soil. In this study, batch and column experiments were performed to minimize rhamnolipid biosurfactant sorption and to optimize rhamnolipid application for removal of cadmium from four soils. In batch studies, rhamnolipid sorption to a model coarse loa my soil was found to vary with applied rhamnolipid and K+ concentration of the rhamnolipid matrix. The presence of solution-phase biosurfactant was correlated to the release into solution of a soil-bound metal (cadmium). A series of column experiments was performed to evaluate whether rhamnolipid could remove cadmium from soil under saturated flow conditions. Four different soils were contaminated with cadmium and treated first with an KNO3 electrolyte solution (3.5 or 7 mM K+) and then with a rhamnolipid-containing solution (5 or 10 mM). Results showed that between 15 and 36% of the cadmium was removed by the initial electrolyte treatment and an additional 8-54% of the cadmium was removed by rhamnolipid treatment. Rhamnolipid treatment was very effective for three of the soils tested, but for the soil with the highest clay content, rhamnolipid application caused soil dispersion and column plugging.

Column studies were used to investigate the fate of a representative nonaqeous-phase liquid (NAPL), hexadecane, with specific regard to (1) the effect of attached bacteria on the formation of residual saturation and (2) the role of biodegradation and biosurfactants on the removal of residual NAPL. Residual saturation of hexadecane was determined using sterile sand (40/50 mesh) columns and was found to be 19.0 +/- 4.8% of the pore volume. Columns loaded with bacterial biomass (Pseudomonas aeruginosa ATCC 15442, 10(9) cells g(-1)) showed no difference in residual hexadecane formation as compared to sterile sand columns. In further column studies examining the effect of ATCC 15442 and biosurfactants on the removal of hexadecane residual, results showed that biodegradation alone removed approximately 50% of the [C-14]hexadecane, in the form of (CO2)-C-14 and undefined cellular metabolites, during elution with at least 200 pore volumes of mineral salts medium. The columns were then eluted with 1 mM rhamnolipid biosurfactant which increased total removal to 65%. Rhamnolipid addition resulted in (1) the mobilization of hexadecane free product and (2) a transitory 3-12-fold increase in the rate of hexadecane mineralization. In a separate study, the column was eluted from the beginning with a low (0.1 mM) concentration of rhamnolipid. This lower concentration of biosurfactant enhanced the removal of hexadecane by mobilization, but had no effect on the rate of biodegradation of residual hexadecane. Analysis of residual radioactivity within two columns revealed only 2% remaining as intact hexadecane. These results suggest that a combination of biodegradation and rhamnolipid treatment could be used to maximize the removal of residual NAPL from porous media.

The objective of this research was to evaluate the effect of low concentrations of a rhamnolipid biosurfactant on the in situ biodegradation of hydrocarbon entrapped in a porous matrix. Experiments were performed with sand-packed columns under saturated flow conditions with hexadecane as a model hydrocarbon. Application of biosurfactant concentrations greater than the CMC (the concentration at which the surfactant molecules spontaneously form micelles or vesicles [0.03 mM]) resulted primarily in the mobilization of hexadecane entrapped within the sand matrix. In contrast, application of biosurfactant concentrations less than the CMC enhanced the in situ mineralization of entrapped hexadecane; however, this effect was dependent on the choice of bacterial isolate. The two Pseudomonas isolates tested, R4 and ATCC 15524, were used because they exhibit different patterns of biodegradation of hexadecane, and they also differed in their physical response to rhamnolipid addition. ATCC 15524 cells formed extensive multicell aggregates in the presence of rhamnolipid while R4 cells were unaffected. This behavior did not affect the ability of the biosurfactant to enhance the biodegradation of hexadecane in well-mixed soil slurry systems but had a large affect on the extent of entrapped hexadecane biodegradation in the sand-packed-column system that was used in this study.

Complexation of cadmium, lead, and zinc (singly and in a mixture) by a monorhamnolipid biosurfactant produced by Pseudomonas aeruginosa ATCC 9027 was studied in batch solution and soil experiments. Conditional stability constants (log K-L) for metal-rhamnolipid complexation in a buffered medium (0.1 M Pipes, pH 6.8) were determined in duplicate using an ion-exchange technique and averaged 6.5 (Cd2+), 6.6 (Pb2+), and 5.4 (Zn2+); these values are similar or slightly higher than literature values for Cd2+ and Pb2+ complexation with fulvic acid and activated sludge solids. To determine the ability of rhamnolipid to desorb soil-bound metals, rhamnolipid solutions (12.5, 25, 50, and 80 mM) were added to soil containing sorbed Cd2+ (1.46 mmol kg(-1)), Pb2+ (1.96 mmol kg(-1)), or a mixture of Pb2+-Cd2+-Zn2+ (3.4 mmol kg(-1)). At 12.5 end 25 mM rhamnolipid, rhamnolipid sorption to soil exceeded 78%, and less than 11% of soil-bound Cd2+ and Zn2+ was desorbed. However, ion exchange of bound metals with Kf present in the rhamnolipid matrix could account for the removal of between 16 and 48% of the sorbed Cd2+ and Zn2+. At 50 and 80 mM rhamnolipid, rhamnolipid sorption to soil decreased to between 20 and 77%, and the removal of Cd2+ and Zn2+ could exceed the removal by ion exchange by as much as 3-fold. The behavior of Pb2+ was quite different. Less than 2% of soil-bound Pb2+ was desorbed due to ion exchange, although up to 43% was desorbed by 80 mM rhamnolipid.