Leaching of BTEX from Aged Crude Oil Contaminated Model Soils: Experimental and Modeling Results

It is generally assumed that soil properties such as organic matter content, porosity, and mineral surface area have a significant effect on the bioavailability and leachability of aged petroleum hydrocarbons. In order to test this hypothesis, nine model soils or sorbents (i.e., fine and coarse quartz sand, montmorillonite and kaolinite clay, peat, 60Å and 150Å silica gel, a loam soil, and non-porous glass beads) were spiked with a crude oil, aged for 27 months in the laboratory, and transferred to glass columns for the performance of continuous flow leaching experiments. The column effluents were periodically sampled for 43 days and analyzed for BTEX. A one-dimensional flow model for predicting the dissolution and dispersion of individual hydrocarbons from a multi-component NAPL such as crude oil was used to fit the leaching data (i.e., the BTEX concentration versus time curves) by adjusting the equilibrium oil-leachate partitioning coefficient (K ^ol ) for each respective hydrocarbon. The Peclet number, which is a measure of dispersion and a required modeling parameter, was measured in separate chloride tracer experiments for each soil column.

Results demonstrate that soil properties did not significantly affect the leaching kinetics of BTEX from the columns. Instead, BTEX leaching curves could be successfully fitted with the one-dimensional NAPL dissolution flow model for all sorbents with the exception of montmorillonite clay. The fitting parameter K ^ol for each hydrocarbon was found to be similar to the K ^ol values that were independently measured for the same crude oil by Rixey et al. (Journal of Hazardous Materials B, 65: 137–156, 1999 Rixey, W. G., Garg, S. and Nie, Y. 1999. Comparison of the fixed-bed and batch leaching characteristics of aromatic compounds in residually trapped crude oils and oily wastes. J. Hazard. Mat. B, 64: 137–156. [CSA][CROSSREF] [Google Scholar]). In addition, the fitted K ^ol values were very similar for BTEX leaching from aged compared to freshly spiked loam soil. These findings indicate that leaching of BTEX in the aged soils that are contaminated with crude oil at the high concentrations commonly found in the environment (i.e., > 20,000 mg/kg) was not affected by soil properties or aging but rather was governed by the equilibrium dissolution of these hydrocarbons from the crude oil NAPL that is coating the soil particles.     

Two-liquid-phase bioreactors for enhanced degradation of hydrophobic/toxic compounds

Two-liquid-phase culture systems involve the addition of a water-immiscible, biocompatible and non-biodegradable solvent to enhance a biocatalytic process. Two-liquid-phase bioreactors have been used since the mid-seventies for the microbial and enzymatic bioconversion of hydrophobic/toxic substrates into products of commercial interest. The increasing popularity of bioremediation technologies suggests a new area of application for this type of bioreactor. The toxicity and the limited bioavailability of many pollutants are important obstacles that must first be overcome in order to improve biodegradation processes. Two-liquid-phase bioreactors have the potential to resolve both limitations of biotreatment technologies by the enhancement of the mass-transfer rate of compounds with low bioavailability, and by the controlled delivery of apolar toxic compounds. This technology can also be useful in accelerating the enrichment of microorganisms degrading problematic pollutants. In this paper, we discuss the application of two-liquid-phase bioreactors to enhance the biodegradation of toxic/poorly bioavailable contaminants. Important microbial mechanisms involved in this type of system are described. Uptake of the substrates can be achieved by microorganisms freely dispersed in the aqueous phase and/or bound at the interface between the aqueous and the immiscible phases. Production of surface-active compounds and adhesion abilities are microbial features involved in the process. General guidelines for the design of two-liquid-phase bioreactors for biodegradation purposes are presented. Solvent selection should be established on specific criteria, which depend on the characteristics of target compound(s) and the microorganism(s) implicated in the biodegradation process. The central importance of maximizing the interfacial surface area is highlighted. The potential of this approach as an alternative to current biotreatment technologies is also discussed.     

Bacterial influence on partitioning rate during the biodegradation of styrene in a biphasic aqueous-organic system

The degradation by a consortium of slightly-halophile marine bacteria of styrene initially dissolved in silicone oil was monitored in batch reactors stirred at 75, 125 and 500 rpm, respectively. In the 75 and 125 rpm cases, the styrene biodegradation rate was higher than the rate of spontaneous partitioning of styrene from the oil to the water, determined under abiotic conditions. Abiotic transfer tests carried out after biodegradation runs revealed that bacterial activity had resulted in a significant increase in the rate of styrene partitioning between the two liquid phases. Even though bacterial adsorption was noticeable at the oil-water interface, this effect appeared to be due to the release by the bacteria of chemicals in the aqueous phase. Similarity with observations made with Triton X-100 suggested that the chemicals released may have been biosurfactants or solubilizing agents.     

Influence of a nonaqueous phase liquid (NAPL) on biodegradation of phenanthrene

A series of batch reactor experiments was carried out to examine the effect of a nonaqueous phase liquid (NAPL) on the biodegradation of a hydrophobic solute. A mathematical program model that describes physical processes of solute solubilization and partitioning between the NAPL and aqueous phases as well as microbial degradation and oxygen utilization was used to analyze the test data. The model calculates the cumulative changes in concentration of substrate, cell mass, carbon dioxide, and dissolved oxygen as a function of time. The equations incorporate the effects of solute solubilization, partitioning, biodegradation, as well as oxygen availability. Hexadecane was used as the model NAPL and was not biodegraded in the timeframe of the experiments performed. The model solute was the polyaromatic hydrocarbon, phenanthrene. In agreement with several previous studies, experimental measurements showed that hexadecane increased rates of mineralization of 15 mg phenanthrene when present at low mass but decreased rates at high mass. Model results suggest that partitioning of the phenanthrene into the hexadecane phase limits bioavailability at high NAPL mass. Further the model suggests that mineralization rates were higher with the low NAPL mass because aqueous phenanthrene concentrations were higher in those treatments from ca. 20 to 40 h than in other treatments. Finally, experiments showed that the presence of hexadecane, at all masses tested, resulted in a lower cell yield, effectively increasing the amount of CO₂ produced during the experiment. Model results suggest that this is due to changes in phenanthrene metabolism that are induced by the presence of the hexadecane phase. Model studies aimed at increasing rates of biodegradation by modifying operating conditions are described along with practical approaches to implementing these modifications.     

Biodegradation of phenanthrene and analysis of degrading cultures in the presence of a model organo-mineral matrix and of a simulated NAPL phase

Two mixed bacterial cultures (C_B-BT and C_I-AT) degraded phenanthrene when it was: (i) in the presence of either hexadecane as a non aqueous phase liquid or a montmorillonite–Al(OH)x-humic acid complex as a model organo-mineral matrix; (ii) sorbed to the complex, either alone or in the presence of hexadecane. The cultures had different kinetic behaviours towards phenanthrene with or without hexadecane. The degradation of Phe alone as well as that of Phe in hexadecane ended in 8 and 15 days with C_B-BT and C_I-AT cultures, respectively. Hexadecane increased Phe bioavailability for C_I-AT bacteria which degraded Phe according to first-order kinetics. The same effect was observed for C_B-BT bacteria, but with an initial 2 days lag phase and in accordance with zero-order kinetics. The presence of hexadecane did not affect the degradation of phenanthrene sorbed and aged on the complex by C_I-AT culture. This capability was exhibited also after experimental aging of 30 days. The dynamics of the bacterial community composition was investigated through PCR-DGGE (denaturing gradient gel electrophoresis) of 16S rRNA gene fragments. Individual bands changed their intensity during the incubation time, implying that particular microbe’s relative abundance changed according to the culture conditions. Isolation of phenanthrene and/or hexadecane degraders was in accord with cultivation-independent data. Growth-dependent changes in the cell surface hydrophobicity of the two cultures and of the isolates suggested that modulation of cell surface hydrophobicity probably played an important role for an efficient phenanthrene assimilation/uptake.     

Biodegradation of Residual Dense Nonaqueous-Phase Liquid Tetrachloroethene: Effects on Mass Transfer

The mass emissions rate of contaminants from nonaqueous-phase liquids (NAPLs) is a driving factor in remediation efforts, whether those efforts are designed to remove, transform, or stabilize the entrapped NAPL or down-gradient aqueous concentrations. Enhancement of mass flux from NAPL source zones has been previously reported in the presence of microbial reductive dechlorination activity in systems containing NAPL with a low proportion of tetrachloroethene (PCE) or a low residual saturation (e.g., 2%). The results reported here demonstrate reductive dechlorination of PCE at residual saturations of 35%, obtained under two different column flow velocities and NAPL configurations. Mass flux in biotic columns was approximately 45% greater than that in uninoculated columns, due to both the presence of daughter products and higher concentrations of PCE in the effluent from biotic columns. Daughter product concentrations were greater in columns with NAPL emplaced only in the lower quarter compared to those with NAPL throughout, and in columns run at the slower velocity. The elevated PCE concentrations in biotic column effluents suggest the influence of microbially generated surfactants, which was supported by surface tension measurements. These results demonstrate the potential significance of bioactivity within NAPL source zones on NAPL longevity and down-gradient aqueous concentrations.     

NAPL mobility of OPA-containing sediments

ABSTRACT

In situ deposited non-aqueous phase liquid (IDN) sediments have unique characteristics that inherently mitigate the movement of separate phase liquids. IDN sediments are composed of oil-particle aggregates (OPAs). OPAs consist of an oil bead or globule with attached solid particles, such as clay platelets, silt and sand granules, and/or organic materials. IDN sediments develop at locations where a continual or near continual discharge of non-aqueous phase liquids (NAPLs) have occurred over a period of time. IDN sediments consist of an open network of small pores where fluids are retained. Although the pore structure is very open, the pore openings are relatively small, which appears to inhibit fluid movement. In particular, capillary pressure analyses indicate that NAPL was not generally released until pressures of at least 15 pounds per square inch (psi) were induced. In addition, centrifuge testing at 1,000 G shows that NAPL immobility is observed in samples at NAPL saturations as high a 12%. These data suggest that NAPL is retained within the smallest pores and is encapsulated within a network of larger pores filled with water. Although the sediment contains NAPL, this original OPA structure appears to inhibit the oil beads from coalescing, preventing NAPL flow.     

Bacterial chemotaxis toward a NAPL source within a pore-scale microfluidic chamber

Chemotaxis toward chemical pollutants provides a mechanism for bacteria to migrate to locations of high contamination, which may improve the effectiveness of bioremediation. A microfluidic device was designed to mimic the dissolution of an organic-phase contaminant from a single pore into a larger macropore representing a preferred pathway for microorganisms that are carried along by groundwater flow. The glass windows of the microfluidic device allowed direct image analysis of bacterial distributions within the vicinity of the organic contaminant. Concentrations of chemotactic bacteria P. putida F1 near the organic/aqueous interface were 25% greater than those of a nonchemotactic mutant in the vicinity of toluene for a fluid velocity of 0.5 m/d. For E. coli responding to phenol, the bacterial concentrations were 60% greater than the controls, also at a velocity of 0.5 m/d. Velocities in the macropore were varied over a range from 0.5 to 10 m/d, the lower end of which is typical of groundwater velocities. The accumulation of chemotactic bacteria near the NAPL chemoattractant source decreased as the fluid velocity increased. Good agreement between computer-based simulations, generated using reasonable values of the model parameters, and the experimental data for P. putida strains confirmed the contribution due to chemotaxis. The experimental data for E. coli required a larger chemotactic sensitivity coefficient than that for P. putida, which was consistent with parameter values reported in the literature.     

Effects of Heterogeneity and Experimental Scale on the Biodegradation of Diesel

Biodegradation of petroleum hydrocarbon contamination is a common method forremediating soils and groundwater. Due to complexities with field-scale studies,biodegradation rates are typically evaluated at the bench-scale in laboratory studies.However, important field conditions can be difficult to mimic in the laboratory. Thisstudy investigates three scaling factors that can impact laboratory biodegradation ratesand that are frequently unaccounted for in typical laboratory experimental procedures.These factors are soil heterogeneity, morphology of petroleum hydrocarbon non-aqueous phase liquids (NAPLs) and soil moisture distribution. The effects of these factors on the biodegradation rate of diesel NAPL is tested under a variety of experimental procedures from well-mixed batch studies to four-foot static soil columns. The results indicate that a high degree of variability results from even small-scale heterogeneities. In addition, it appears that as the experimental scale increases, the measured biodegradation rates slow. The results indicate that diesel biodegradation rates derived from small-scale experiments are not necessarily representative of field-scale biodegradation rates.     

The effect of soil: water ratios on the mineralisation of phenanthrene: LNAPL mixtures in soil

Contamination of soil by polycyclic aromatic hydrocarbons is frequently associated with non-aqueous-phase liquids. Measurement of the catabolic potential of a soil or determination of the biodegradable fraction of a contaminant can be done using a slurried soil respirometric system. This work assessed the impact of increasing the concentration of transformer oil and soil:water ratio on the microbial catabolism of [(14)C]phenanthrene to (14)CO(2) by a phenanthrene-degrading inoculum. Slurrying (1:1, 1:2, 1:3 and 1:5 soil:water ratios) consistently resulted in statistically higher rates and extents of mineralisation than the non-slurried system (2:1 soil:water ratio; P<0.01). The maximum extents of mineralisation observed occurred in the 1:2-1:5 soil:water ratio microcosms irrespective of transformer oil concentration. Transformer oil concentrations investigated displayed no statistically significant effect on total mineralisation (P>0.05). Soil slurries 1:2 or greater, but less than 1:5 (soil:water), are recommended for bioassay determinations of total contaminant bioavailability due to greater overall mineralisation and improved reproducibility.