Dong Ju Kim

Bacteria transport through goethite-coated sand: Effects of solution pH and coated sand content

This study investigated the transport of bacteria through goethite-coated sand, focusing on the effects of solution pH and coated sand content on the transport of Escherichia coli ATCC 11105. The first set of column experiments was performed in columns (length 30 cm, diameter 5 cm) packed with quartz sand coated with goethite in solution having a pH in the range of 6-9. The second was carried out in columns (length 30 cm, diameter 2.5 cm) with varying coated sand contents ranging from 0 to 100%. Results indicate that the bacteria transport in the coated sand was influenced by solution pH. Around pH 6 and 7, bacterial mass recoveries were low at 2.4-6.7% while they were high at 76.3-81.6% around pH 8 and 9. Around pH 8, the positively charged coated sand may convert to being negatively charged, causing an electrostatically repulsive interaction between the coated sand and bacteria, thus effecting a sharp change in the mass recovery. Results also reveal that the mass recovery decreased from 76.7 to 2.7% as the coated sand content increased from 0 to 100%, showing the nonlinear dependency of mass recovery on the content of coated sand. This study demonstrates the importance of the solution pH and coated sand content in the adhesion of bacteria to goethite-coated sand and furthermore contributes to the knowledge of bacterial removal in positively charged porous media.

Unstable flow during redistribution: Controlling factors and practical implications

Unstable flow causes major uncertainties in the characterization of drainage in the vadose zone by inducing finger-like flow paths in soils with or without macropores. Recent studies have identified the major factors governing fingered flow to be the combined effects of capillary hysteresis, the existence of a threshold water-entry value in a porous medium, and a positive matric potential gradient behind the wetting front. This situation typically occurs during redistribution following high-rate infiltration, a common occurrence in hydrology. The conditions favoring instability can also develop during infiltration into a fine-over-coarse layered soil, into hydrophobic or air-entrapped soils, or even in a homogeneous coarse-textured soil if the infiltration rate is low. An analysis of the conditions necessary for the onset of unstable flow in a uniform soil is provided in this paper. We demonstrate that if the matric potential gradient (d h /d z ) becomes positive during redistribution, a perturbation at the wetting front will cause finger flow. However, if d h /d z remains negative, the perturbation will be dissipated. The analysis is used to predict a critical depth of irrigation ( I c ) beyond which the flow should become unstable. A series of point-source and line-source infiltration experiments were conducted using a slab-box filled with uniform sands. The results confirmed that as soon as I c is exceeded, a finger was formed at the bottom of the wetting front, channeling the flow and stopping water movement in the surrounding areas. We discuss this phenomenon9s implications for practical irrigation and leaching designs.

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.

Influence of Immobilization of Bacterial Cells and TiO2 on Phenol Degradation

We investigated the influence of immobilization of bacterial cells and photocatalytic material TiO2 on the degradation of phenol by conducting batch microcosm studies consisting of suspended, immobilized cells and immobilized TiO2 at various initial phenol concentrations (50–1,000xa0mgu2009L−1). Results showed that both suspended and immobilized cells were concentration-dependent, exhibiting the increasing degradation rate with the concentration of up to 500xa0mgu2009L−1 above which it declined. The degradation rate of 0.39–3.47xa0mgu2009L−1u2009h−1 by suspended cells was comparable with those of the literature. Comparison of the degradation rates between suspended, immobilized cells and immobilized TiO2 revealed that immobilized cells achieved the highest degradation rate followed by immobilized TiO2 and suspended cells due to the toxicity of phenol at the high concentration of 1,000xa0mgu2009L−1. This indicates that immobilization of bacterial cells or photocatalytic materials can serve a better alternative to offer the higher degradation efficiency at high phenol concentrations.

Impact of Nitrate Dose on Toluene Degradation under Denitrifying Condition

In this study, we investigated the impact of nitrate dose on toluene degradation by Pseudomonas putida to elucidate the upper limit of nitrate concentration and whether an optimum ratio of nitrate to toluene concentration exists. Batch microcosm studies were conducted in order to monitor toluene degradation for various ratios (2–20) of nitrate to toluene with nitrate concentrations ranging from 0 to 700xa0mgu2009L−1 for a given toluene concentration of 50 and 25xa0mgu2009L−1 during 4-day (short term) and 14-day (long term) incubation time, respectively. The short-term study revealed that nitrate concentration of 500xa0mgu2009L−1 was toxic to bacteria and the optimum concentration was 300xa0mgu2009L−1 yielding the highest toluene degradation rate (0.083xa0mgu2009L−1u2009h−1). In the batch study of long term, toluene degradation was limited to 6xa0days after which the nitrate at 50xa0mgu2009L−1 was depleted, indicating that nitrate was a necessary electron acceptor. For both batch studies, an optimum ratio of 6 was found yielding the highest toluene degradation rate. This indicates that an appropriate nitrate dose is essential for efficient degradation of toluene when bioremediation of groundwater contaminated with toluene is under consideration.

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.

A Laboratory Column Study on the Biodegradation of Toluene and Methyl tert–Butyl Ether: The Effect of Substrate Interactions

We investigated the effects of substrate interactions on the degradation of toluene and methyl tert-butyl ether (MTBE) by Pseudomonas putida during transport through quartz sand because of the coexistence of toluene and MTBE in aquifer systems. A laboratory test was conducted for a pulse injection of a toluene and/or MTBE solution with and without bacteria into a saturated sand column. We found that the effect of toluene on MTBE was negative because the mass recovery of MTBE increased by 30xa0% when toluene was added, whereas the effect of MTBE on toluene degradation was positive because the mass recovery of toluene decreased by 7xa0%. These results were comparable with those of a previous batch study on substrate interactions which reported that toluene can be more negative than MTBE at concentrations higher than 25xa0mg/L. This finding indicates that substrate interaction is also an important mechanism, controlling the fate of contaminants during transport through aquifer systems. In addition, bacteria-facilitated transport was also observed for both substrates. Therefore, for an efficient bioremediation scheme, care should be taken for substrate interaction as well as for bacteria-facilitated transport in porous media.

Deposition and transport of Pseudomonas aeruginosa in porous media: lab-scale experiments and model analysis

In this study, the deposition and transport of Pseudomonas aeruginosa on sandy porous materials have been investigated under static and dynamic flow conditions. For the static experiments, both equilibrium and kinetic batch tests were performed at a 1:3 and 3:1 soil:solution ratio. The batch data were analysed to quantify the deposition parameters under static conditions. Column tests were performed for dynamic flow experiments with KCl solution and bacteria suspended in (1) deionized water, (2) mineral salt medium (MSM) and (3) surfactant+MSM. The equilibrium distribution coefficient (Kd) was larger at a 1:3 (2.43 mL g−1) than that at a 3:1 (0.28 mL g−1) soil:solution ratio. Kinetic batch experiments showed that the reversible deposition rate coefficient (katt) and the release rate coefficient (kdet) at a soil:solution ratio of 3:1 were larger than those at a 1:3 ratio. Column experiments showed that an increase in ionic strength resulted in a decrease in peak concentration of bacteria, mass recovery and tailing of the bacterial breakthrough curve (BTC) and that the presence of surfactant enhanced the movement of bacteria through quartz sand, giving increased mass recovery and tailing. Deposition parameters under dynamic condition were determined by fitting BTCs to four different transport models, (1) kinetic reversible, (2) two-site, (3) kinetic irreversible and (4) kinetic reversible and irreversible models. Among these models, Model 4 was more suitable than the others since it includes the irreversible sorption term directly related to the mass loss of bacteria observed in the column experiment. Applicability of the parameters obtained from the batch experiments to simulate the column breakthrough data is evaluated.

Novel method for determination of phenol degradation kinetics

In this study, we proposed a new method for estimating biokinetic parameters in phenol degradation kinetics. The new method relies on the new formulation of q–S relation where degradation rate q is calculated from the changes of substrate concentration S for each time segment during the course of entire degradation, while in the conventional method q is obtained from the slope of the straight line that is given as substrate concentration changes with time in a semi-logarithmic scale. Thus, this new method provided more data points than the conventional method. The q–S relations obtained from the new method and the conventional method were fitted with three inhibitory kinetic models of Haldane, Yano and Edwards. Simulation of degradation profile with each kinetic model and comparison with the observed profile revealed that the new method offered a better prediction with Edwards model as the best inhibitory model.