Determination of bromacil transport as a function of water and carbon content in soils.

This study was conducted to determine the significance of bromacil transport as a function of water and carbon content in soils and to explore the implications of neglecting sorption when making assessments of travel time of bromacil through the vadose zone. Equilibrium batch sorption tests were performed for loamy sand and sandy soil added with four different levels of powdered activated carbon (PAC) content (0, 0.01, 0.05, and 0.1%). Column experiments were also conducted at various water and carbon contents under steady-state flow conditions. The first set of column experiments was conducted in loamy sand containing 1.5% organic carbon under three different water contents (0.23, 0.32, and 0.41) to measure breakthrough curves (BTCs) of bromide and bromacil injected as a square pulse. In the second set of column experiments, BTCs of bromide and bromacil injected as a front were measured in saturated sandy columns at the four different PAC levels given above. Column breakthrough data were analyzed with both equilibrium and nonequilibrium (two-site) convection-dispersion equation (CDE) models to determine transport and sorption parameters under various water and carbon contents. Analysis with batch data indicated that neglect of the partition-related term in the calculation of solute velocity may lead to erroneous estimation of travel time of bromacil, i.e. an overestimation of the solute velocity by a factor of R. The column experiments showed that arrival time of the bromacil peak was larger than that of the bromide peak in soils, indicating that transport of bromacil was retarded relative to bromide in the observed conditions. Extent of bromacil retardation (R) increased with decreasing water content and increasing PAC content, supporting the importance of retardation in the estimation of travel time of bromacil even at small amounts of organic carbon for soils with lower water content.

Enantioselectivity in environmental safety of current chiral insecticides.

Chiral pesticides currently constitute about 25% of all pesticides used, and this ratio is increasing as more complex structures are introduced. Chirality occurs widely in synthetic pyrethroids and organophosphates, which are the mainstay of modern insecticides. Despite the great public concerns associated with the use of insecticides, the environmental significance of chirality in currently used insecticides is poorly understood. In this study, we resolved enantiomers of a number of synthetic pyrethroid and organophosphate insecticides on chiral selective columns and evaluated the occurrence of enantioselectivity in aquatic toxicity and biodegradation. Dramatic differences between enantiomers were observed in their acute toxicity to the freshwater invertebrates Ceriodaphnia dubia and Daphnia magna, suggesting that the aquatic toxicity is primarily attributable to a specific enantiomer in the racemate. In field sediments, the (-)enantiomer of cis-bifenthrin or cis-permethrin was preferentially degraded, resulting in relative enrichment of the (+)enantiomer. Enantioselective degradation was also observed during incubation of sediments under laboratory conditions. Enantioselectivity in these processes is expected to result in ecotoxicological effects that cannot be predicted from our existing knowledge and must be considered in future risk assessment and regulatory decisions.

Effect of sorption on benzene biodegradation in sandy soil.

The effect of sorption on benzene biodegradation in sandy soil was studied by conducting kinetic microcosm batch tests in soil-free solution and in the presence or absence of bacteria in soil materials with varying degrees of powdered activated carbon (PAC). In the soil-free experiment, benzene was added to a solution inoculated with Pseudomonas aeruginosa bacteria in order to achieve a potential or maximum biodegradation rate. In subsequent experiments, benzene was applied to a solution containing sandy soil and various PAC contents with and without inoculating P. aeruginosa. Benzene concentrations in the soil-free experiments decreased with time with two characteristic rates. A two-stage exponential decay model adequately represented the observed solution concentration pattern with time. Sorption experiments in bacteria-free soil also decreased monotonically, with the extent of sorption increasing as PAC content increased. The sorption data were represented well with a two-stage irreversible sorption model. A third set of experiments in the presence of both soil and bacteria showed more rapid concentration loss from solution than the set of experiments with bacteria-free soil. A model combining sorption and degradation greatly overestimated the loss when the rate coefficient from the bacteria-free experiments was used. Satisfactory agreement between model predictions and observed values was obtained when the degradation rate coefficients were decreased by factors ranging from 3 to 10, depending on the amount of PAC present. Model predictions of the percentage benzene mass remaining in the soil after 25 d of degradation ranged from 72 to 97%, depending on the PAC content, compared to only 2.5% remaining in soil-free solution.

Impact of the plant rhizosphere and augmentation on remediation of polychlorinated biphenyl contaminated soil.

This study investigated the interactive effects of bioaugmentation, biostimulation, and the rhizosphere during remediation of Aroclor 1242-contaminated soil. Treatments were repeatedly augmented with polychlorinated bipheny (PCB)-degrading bacteria, inducers (carvone and salicylic acid), surfactant (sorbitan trioleate), minimal salts medium in a 20-cm high soil column, or a combination of these elements. Soils containing a single Brassica nigra plant achieved 61% PCB removal in the 0 to 2 and 2 to 6 cm depths after 9 weeks of bioaugmentation, whereas only 43 and 14% PCB removal, respectively, was achieved in unplanted controls. Gas diffusion coefficients of 13.0 and 5.0 x 10(-7) m2 s(-1) were calculated from a methane diffusion assay for planted and unplanted soils respectively, indicating the positive effect of plant roots on gas diffusion into the soil. A second, modified column study removed 87, 73, 63, and 45% of PCB after 12 weeks in the 0 to 5, 5 to 11, 11 to 26, and 26 to 35 cm depths, respectively, in planted-bioaugmented soils, whereas 65, 54, 53, and 47% of PCB was removed from the planted-minimal salts treatment, respectively. Shifts in the soil microbial community structure were demonstrated by denaturing gradient gel electrophoresis of bacterial 16S ribosomal DNA. Results support that Brassica nigra directly contributed to accelerated PCB removal by increased oxygen diffusion, amendment infiltration, and microbial enrichment.

Kinetics of benzene biodegradation by Pseudomonas aeruginosa: parameter estimation.

This study determined the model parameters describing biodegradation of benzene by conducting kinetic microcosm batch tests in both pure solution and saturated aquifer material conditions for various initial benzene (100-700 mg/L) and microbial concentrations (10(7)-10(9) colony-forming units [CFU]/ml) using Pseudomonas aeruginosa as benzene-degrading bacteria. In both tests, benzene and microbial concentrations were monitored over time in order to investigate which of two Monod kinetic equations, the Monod-with-growth or the Monod-no-growth model, was more suitable for describing benzene biodegradation and to estimate the associated model parameters. Parameter estimation was performed by fitting the numerical solution of each model obtained by the fourth-order Runge-Kutta integration to the measured data of benzene and/or microbial concentrations. For the Monod-with-growth model, the best fit of the numerical solution was significantly different than the measured benzene concentrations, especially at early times, because of the gradual increase of microbial population in the growth curve. In contrast, the solution based on the Monod-no-growth model produced reasonable agreement with the measured benzene data. The estimated parameters of maximum substrate utilization rate (kmax) and half-saturation constant (Kc) were in the range of 61 to 105 mg/L/d and about 270 mg/L, respectively, which differ significantly from values previously reported in the literature. We attribute the differences observed in our study to our experimental conditions of initial substrate and bacterial concentrations and oxygen and nutrient supply. Our results imply that an appropriate model type and reasonable values of kinetic parameters should be chosen to model the biodegradation of hydrocarbons in the subsurface environment.

Dechlorination of chloroacetanilide herbicides by thiosulfate salts.

Halogenated organic compounds (XOCs) are among the most widely used synthetic chemicals. Many XOCs are recalcitrant to natural degradation and have become prominent environmental contaminants. One group of such XOCs are the heavily used chloroacetanilide herbicides. We have found that chloroacetanilide herbicides are rapidly dechlorinated in water, sand, and soil by thiosulfate salts under ambient conditions. Structural and kinetics analysis suggests that the reaction occurred by S(N)2 nucleophilic substitution, in which the chlorine was replaced by thiosulfate and the herbicide was detoxified. Laboratory studies showed that this reaction could be used for removing residues of chloroacetanilide herbicides in water, soil, and sand. Our findings also suggest that some other XOCs may be subject to this reaction. Because common thiosulfate salts are innocuous products (e.g., fertilizers) and the reaction selectively detoxifies XOCs at low thiosulfate levels, this discovery may lead to a new way for safe removal of certain XOCs from the environment.