SEAR技术修复土壤和地下水中NAPL污染的研究进展

就SEAR技术修复土壤及地下水中NAPL污染的原理及发展现状进行了综述.SEAR技术可以快速有效地去除土壤和地下水中的NAPL污染源,适于多种污染物.该技术通过增溶和增流2种途径提高NAPL污染物的去除率.表面活性剂的选择和微乳液体系的调配是SEAR技术实施的关键环节.将SEAR技术用于高浓度NAPL污染源的治理,并与生物修复和自然降解相结合,是经济高效的治理方案.     

Investigation of evaporation and biodegradation of fuel spills in Antarctica. I. A chemical approach using GC-FID

Little effort has been devoted to differentiating between hydrocarbon losses through evaporation and biodegradation in treatability studies of fuel-contaminated Antarctic soils. When natural attenuation is being considered as a treatment option, it is important to be able to identify the mechanism of hydrocarbon loss and demonstrate that rates of degradation are sufficient to prevent off-site migration. Similarly, where complex thermally enhanced bioremediation schemes involve nutrient addition, water management, air stripping and active heating, it is important to appreciate the relative roles of these mechanisms for cost minimisation. Following the loss of hydrocarbons by documenting changes in total petroleum hydrocarbons offers little insight into the relative contribution of evaporation and biodegradation. We present a methodology here that allows identification and quantification of evaporative losses of diesel range organics at a range of temperatures using successively less volatile compounds as fractionation markers. We also present data that supports the general utility of so-called biodegradation indices for tracking biodegradation progress. We are also able to show that at 4 degrees C indigenous Antarctic soil bacteria degrade Special Antarctic Blend fuel components in the following order: naphthalene and methyl-napthalenes, light n-alkanes, then progressively heavier n-alkanes; whereas isoprenoids and the unresolved complex mixture are relatively recalcitrant.     

Tracer test for the measurement of gas diffusion and non-aqueous phase liquid (NAPL) saturation in soil

During soil bioremediation, the diffusion of oxygen into the soil is an important prerequisite for aerobic biodegradation, and the decrease of petroleum products is the ultimate goal. Both processes need to be monitored. The aim of this work was to develop a gas tracer test that yields information on both, gas diffusion and residual saturation with non-aqueous phase liquids (NAPLs) in unsaturated soil heaps. One conservative tracer (methane) and 4 partitioning gas tracers (diethylether, methyl tert-butyl ether, chloroform and n-heptane) were injected as vapors into laboratory columns filled with unsaturated sand with increasing NAPL saturation. Breakthrough curves of gaseous compounds were measured at two points and compared to analytical solutions of an analytical diffusive-reactive transport equation. By fitting of methane data, robust results for effective diffusivity (tortuosity) were obtained. NAPL saturation was most accurately measured by the moderately water soluble tracers (ethers and chloroform). The hydrophobic tracer n-heptane did not partition into water-immersed NAPL. An easy and accurate way to assess air-NAPL partitioning constants from gas chromatography retention times is furthermore reported. It is concluded that gas tracer tests have the potential for measuring two important properties in soil bioremediation systems easily and quickly.     

Investigation of surfactant-enhanced mass removal and flux reduction in 3D correlated permeability fields using magnetic resonance imaging

Magnetic resonance imaging (MRI) was used to visualize the NAPL source zone architecture before and after surfactant-enhanced NAPL dissolution in three-dimensional (3D) heterogeneously packed flowcells characterized by different longitudinal correlation lengths: 2.1 cm (aquifer 1) and 1.1 cm (aquifer 2). Surfactant flowpaths were determined by imaging the breakthrough of a paramagnetic tracer (MnCl(2)) analyzed by the method of moments. In both experimental aquifers, preferential flow occurred in high permeability materials with low NAPL saturations, and NAPL was preferentially removed from the top of the aquifers with low saturation. Alternate flushing with water and two surfactant pulses (5-6 pore volumes each) resulted in approximately 63% of NAPL mass removal from both aquifers. However, overall reduction in mass flux (Mass Flux 1) exiting the flowcell was lower in aquifer 2 (68%) than in aquifer 1 (81%), and local effluent concentrations were found to increase by as high as 120 times at local sampling ports from aquifer 2 after surfactant flushing. 3D MRI images of NAPL revealed that NAPL migrated downward and created additional NAPL source zones in previously uncontaminated areas at the bottom of the aquifers. The additional NAPL source zones were created in the direction transverse to flow in aquifer 2, which explains the higher mass flux relative to aquifer 1. Analysis using a total trapping number indicates that mobilization of NAPL trapped in the two coarsest sand fractions is possible when saturation is below 0.5 and 0.4, respectively. Results from this study highlight the potential impacts of porous media heterogeneity and NAPL source zone architecture on advanced in-situ flushing technologies.     

Parameters that control the cleanup of fractured permeable aquifers

This study develops a modeling approach for simulating and evaluating entrapped light nonaqueous-phase liquid (light NAPL-LNAPL) dissolution and transport of the solute in a fractured permeable aquifer (FPA). The term FPA refers to an aquifer made of porous blocks of high permeability that embed fractures. The fracture network is part of the domain characterized by high permeability and negligible storage. Previous studies show that sandstone aquifers often represent FPAs. The basic model developed in this study is a two-dimensional (2-D) model of permeable blocks that embed oblique equidistant fractures with constant aperture and orientation. According to this model, two major parameters govern NAPL dissolution and transport of the solute. These parameters are: 1) the dimensionless interphase mass transfer coefficient, K(f0), and 2) the mobility number, N(M0). These parameters represent measures of heterogeneity affecting flow, NAPL dissolution, and transport of the solute in the domain. The parameter K(f0) refers to the rate at which organic mass is transferred from the NAPL into the water phase. The parameter N(M0) represents the ratio of flow through the porous blocks to flow through the fracture network in regions free of entrapped NAPL. It also provides a measure of groundwater flow bypassing regions contaminated by entrapped NAPL. In regions contaminated by entrapped NAPL our simulations have often indicated very low permeability of the porous blocks, enabling a significant increase of the fracture flow at the expense of the permeable block flow. Two types of constitutive relationships also affect the rate of FPA cleanup: 1) the relationship between the saturation of the entrapped NAPL and the permeability of the porous blocks, and 2) the relationships representing effects of the entrapped NAPL saturation and the permeable block flow velocity on rates of interphase mass transfer. This study provides basic tools for evaluating the characteristics of pump-and-treat cleanup of FPAs by referring to sets of parameters and constitutive relationships typical of FPAs. The numerical simulations carried out in this study show that at high initial saturation of the entrapped NAPL, during initial stages of the FPA cleanup the contaminant concentration increases, but later it decreases. This phenomenon originates from significant groundwater bypassing the NAPL entrapped in the permeable blocks via the fracture network.     

Surfactant enhanced recovery of tetrachloroethylene from a porous medium containing low permeability lenses. 2. Numerical simulation

A numerical model of surfactant enhanced solubilization was developed and applied to the simulation of nonaqueous phase liquid recovery in two-dimensional heterogeneous laboratory sand tank systems. Model parameters were derived from independent, small-scale, batch and column experiments. These parameters included viscosity, density, solubilization capacity, surfactant sorption, interfacial tension, permeability, capillary retention functions, and interphase mass transfer correlations. Model predictive capability was assessed for the evaluation of the micellar solubilization of tetrachloroethylene (PCE) in the two-dimensional systems. Predicted effluent concentrations and mass recovery agreed reasonably well with measured values. Accurate prediction of enhanced solubilization behavior in the sand tanks was found to require the incorporation of pore-scale, system-dependent, interphase mass transfer limitations, including an explicit representation of specific interfacial contact area. Predicted effluent concentrations and mass recovery were also found to depend strongly upon the initial NAPL entrapment configuration. Numerical results collectively indicate that enhanced solubilization processes in heterogeneous, laboratory sand tank systems can be successfully simulated using independently measured soil parameters and column-measured mass transfer coefficients, provided that permeability and NAPL distributions are accurately known. This implies that the accuracy of model predictions at the field scale will be constrained by our ability to quantify soil heterogeneity and NAPL distribution.     

Electrokinetic enhancement of phenanthrene biodegradation in creosote-polluted clay soil

Given the difficulties caused by low-permeable soils in bioremediation, a new electrokinetic technology is proposed, based on laboratory results with phenanthrene, to afford bioremediation of polycyclic aromatic hydrocarbons (PAH) in clay soils. Microbial activity in a clay soil historically polluted with creosote was promoted using a specially designed electrokinetic cell with a permanent anode-to-cathode flow and controlled pH. The rates of phenanthrene losses during treatment were tenfold higher in soil treated with an electric field than in the control cells without current or microbial activity. Results from experiments with Tenax-assisted desorption and mineralization of 14C-labeled phenanthrene indicated that phenanthrene biodegradation was limited by mass-transfer of the chemical. We suggest that the enhancement effect of the applied electric field on phenanthrene biodegradation resulted from mobilization of the PAH and nutrients dissolved in the soil fluids.     

Biosurfactant-enhanced solubilization of NAPL mixtures

Remediation of nonaqueous phase liquids (NAPLs) by conventional pump-and-treat methods (i.e., water flushing) is generally considered to be ineffective due to low water solubilities of NAPLs and to mass-transfer constraints. Chemical flushing techniques, such as surfactant flushing, can greatly improve NAPL remediation primarily by increasing the apparent solubility of NAPL contaminants. NAPLs at hazardous waste sites are often complex mixtures. However, the equilibrium and nonequilibrium mass-transfer characteristics between NAPL mixtures and aqueous surfactant solutions are not well understood. This research investigates the equilibrium solubilization behavior of two- and three-component NAPL mixtures (containing akylbenzenes) in biosurfactant solutions. NAPL solubilization is found to be ideal in water (i.e., obeys Raoult's Law), while solubilization in biosurfactant solutions was observed to be nonideal. Specifically, the relatively hydrophobic compounds in the mixture experienced solubility enhancements that were greater than those predicted by ideal enhanced solubilization theory, while the solubility enhancements for the relatively hydrophilic compounds were less than predicted. The degree of nonideality is shown to be a nonlinear function of the NAPL-phase mole fraction. Empirical relationships based on the NAPL-phase mole fraction and/or micelle-aqueous partition coefficients measured in single-component NAPL systems are developed to estimate values for the multicomponent partition coefficients. Empirical relationships that incorporate both the NAPL-phase mole fraction and single-component partition coefficients yield much improved estimates for the multicomponent partition coefficient.     

Volatile hydrocarbon emissions from a diesel fuel-contaminated soil bioremediation facility

Concerns have been expressed that emissions of volatile hydrocarbons (HCs) from bioremediation facilities containing soils contaminated with petroleum HCs may negatively impact regional air quality or human health. Little information is available regarding the emission of HCs from bioremediation sites, and few field studies have been performed during which the flux of HCs has been directly measured during bioremediation. To aid in answering questions about the impact of bioremediation facilities on the atmospheric environment, a two-part field study was conducted over summer 1996 at a remote landfarm in northern Ontario where diesel fuel-contaminated soil was undergoing bioremediation. Volatile total hydrocarbon (THC) atmospheric flux measurements were successfully taken over 18 days using a flux gradient micrometeorological technique incorporating a THC detector constructed in-house. Peak THC emissions reached 131 microg C/m2/sec shortly after implementation and tilling of the landfarm soil. The influence of soil temperature and tillage on THC emissions was examined. Off-site inhalation exposure was considered with the aid of an areal source model and results from speciated air samples collected on sorbent tubes and analyzed via gas chromatography/mass spectrometry (GCMS) techniques.     

Degradation of pentachlorophenol by pure and mixed cultures in two different soils

GOAL, SCOPE AND BACKGROUND: Pentachlorophenol (PCP) is the second highest volume pesticide used in the United States. It is a mutagenic compound whose exposure poses significant health effects, One of the most desirable, environmentally friendly treatment methods is bioremediation. For soil-based contamination, the effectiveness of bioremediation will also be affected by the presence of an active indigenous population, sorption of the contaminant onto the soil, and environmental parameters. METHODS: Two pure strains and their mixed culture were used to evaluate PCP biodegradation in two different field soils, Columbia (CO) and New Mexico (NM). Biostimulation of the indigenous microbes was evaluated by adding nutrients. The efficiency of adding bacteria strains (bioaugmentation) for degrading PCP was determined with Arthrobacter sp., Flavobacterium sp. and a 50:50 mixture of the two bacteria strains. RESULTS: In CO soil, only 24%, 12% and 25% of the initial PCP concentration were degraded by Flavobacterium sp., Arthrobacter sp. and mixed culture, respectively. Arthrobacter sp. was used in NM soil with two initial concentrations and achieved degradation efficiencies of 57% and 61% for 361 and 95 mg kg- concentrations, respectively. Discussion. Analysis via statistical methods showed that the bacteria had different efficiencies on PCP degradation in each soil. 2 CONCLUSIONS: All bacteria catalyzed a higher PCP degradation when present in NM soil. Second, Flavobacterium sp. degraded more PCP than Arthrobacter sp. in CO soil. The mixed culture achieved the highest degradation efficiency regardless of the initial concentration or soil origin. RECOMMENDATIONS AND PERSPECTIVES: The effect of the soil properties, such as the soil organic matter (SOM) on PCP biodegradation should be investigated. Future work can also investigate the effect of aging time on biodegradation.     

The effects of surfactant formulation on nonequilibrium NAPL solubilization

Surfactant-enhanced aquifer remediation (SEAR) involves the injection of surfactant solutions into aquifers contaminated with nonaqueous phase liquids (NAPL). Batch and column experiments were used to assess the effect of surfactant formulation on the rate of NAPL solubilization. The experimental variables were surfactant type, surfactant concentration, electrolyte concentration, and cosolvent concentration. Model equations were proposed and solved to describe solubilization under the conditions of each type of experiment. Using these models, a solubilization rate constant, kappa(b), and an overall mass transfer rate coefficient, kappa, were estimated from the batch and column experiments, respectively. The solubilization rate constant was consistently sensitive to surfactant type, surfactant concentration, and electrolyte concentration. The estimated solubilization rate constants varied over two orders of magnitude. The results of the column experiments also were sensitive to the surfactant formulation. Variations in the fitted mass transfer rate coefficient parameter, beta(0), were related to variations in the surfactant formulations. A comparison between the results of the batch and column experiments yields an apparent relationship between beta(0) and kappa(b). This relationship suggests that the mass transfer rate coefficient is directly related to the formulation of the surfactant solution.     

Bioremediation process on Brazil shoreline

GOAL, SCOPE AND BACKGROUND: Bioremediation technique can be considered a promising alternative to clean oil spills using microbial processes to reduce the concentration and/or the toxicity of pollutants. To understand the importance of this work we must know that there is only little research performed to date using bioremediation techniques to clean oil spills in tropical countries. So, the main objective of this work is to analyze the behavior of a laboratory's bioremediation test using nutrients on coastal sediments. METHODS: The bioremediation process is followed through geochemical analysis during the tests. This organic material is analyzed by medium pressure liquid chromatography (MPLC), gas chromatography/flame ionization detection (GC/FID) and gas chromatography/ mass spectrometry. By microbial counting, the number of total bacteria and degrading bacteria is determined during the experiments, in order to confirm the effectiveness of the bioremediation process. The seawater obtained throughout the bioremediation process is analyzed for nutrients grade (phosphate and ammonium ions) and also for its toxicity (Microtox tests) due the presence of hydrocarbons and fertilizer. RESULTS: The results from the geochemical analyses of the oil show a relative decrease in the saturated hydrocarbon fraction that is compensated by a relative enrichment on polar compounds. It's confirmed by the fingerprint evaluation where it is possible to see a complete reduction of the normal alkanes followed by isoprenoids. Seawater analysis done by toxicity and nutrients analysis, such as microbial counting (total and degrading bacteria), confirm the fertilizer effectiveness during the bioremediation process. DISCUSSION: Results from simulating test using NPK, a low-price plant fertilizer, suggest that it's able to stimulate the degradation process. Results from medium pressure liquid chromatography (MPLC), done at two different depths (surface and subsurface), show different behavior during the biodegradation process where the later is seen to be more susceptible to microbial attack. Data from bioremediation unit shows a bigger reduction of the saturated fraction, followed by some smaller reduction of aromatic fractions, compensated by a relative increase from polar compounds (NSO). n-C17/pristane, n-C18/ fitane and pristane/fitane rates show constant values for the unity control, different from bioremediation samples which have a significant reduction, especially on subsurface areas, where a strong fall in the rates, seen to be reduced to zero over twenty days, had occurred during the first ten days. However, sample surfaces are reduced to zero in thirty days of experiments, proving that biodegradation is better on subsurfaces. Gaseous chromatography/mass spectrometry (CG/MS) analysis shows constant values to cyclic biomarker rates and aromatic compounds, suggesting that the biodegradation process is not strong enough to reduce these composites. Microbial analysis shows a reduction on heterotrophic (total bacteria) number from control unit, probably because the bacteria uses the spill oil like carbon source and energy. However, the number increases on bioremediation unit, because it uses NPK like a biostimulator. The hydrocarbonoclastic number isn't enough on the first moment, but it's detected after 30 days and quantified in all units, showing big values especially in bioremediation. Toxicity tests confirm that NPK fertilizer does not intoxicate the shoreline during the application of the bioremediation technique. Some nutrient concentration shows high values of ammonium and phosphate per bioremediation unit, reducing by the end of the experiment. CONCLUSIONS: Results reached the goal, finding a proper nutrient (NPK fertilizer) to stimulate the biodegradation process, growing bacteria responsible for reducing impact-contaminated coast ambient by oil spills. Chemical analysis of oil shows a reduction in the saturated fraction with a relative enrichment in polar composites (NSO) and the aromatic fraction from oil remaining constant. Subsurface samples show more biodegradation than surface samples, probably because the first one has higher humidity. Linear alcanes are more biodegraded than isoprenoids, confirming the biodegradation susceptibility order. Saturated cyclic biomarkers and aromatic compounds show constant behavior maybe because the nutrients or time was not enough for microorganismic attack. Fertilizer does not demonstrate any toxic effects in local biota so that it does not compromise the technique applicability and the environment is not saturated by nutrients during the simulation, especially since the coastal environment is an open system affected daily by tides. Therefore, bioremediation tests can be classified as moderate, reaching level 5 in the classification scale by Peters & Moldowan (1993). RECOMMENDATIONS AND PERSPECTIVES: The use of marine environment by the petroleum industry on exploration, production and transportation operation, transform this oil to become the most important pollutant in the oceans. Bioremediation is an important technique used to clean spilled oil impacting on shorelines, accelerating the biodegradation process by using fertilizer growing the microorganisms responsible for decontaminating the environment. We recommend confirming the efficiency of NPK nutrient used on bioremediation simulating experiments on beaches, while monitoring the chemical changes long-term. NPK fertilizer can be used to stimulate the biodegradation process on shoreline impacted by spilled oil.     

Bioremediation of a soil contaminated by hydrocarbon mixtures: the residual concentration problem

The phenomenon of residual concentration was investigated in the aerobic biodegradation of three different petroleum commercial products (i.e., kerosene, diesel fuel and a lubricating mineral oil) in static microcosms. Two different soils exhibiting different physical-chemical characteristics were used (i.e., a biologically treated hydrocarbon-contaminated soil and a pristine soil). Residual concentrations were observed and a simple way to take this phenomenon into account was proposed.     

Effect of water-table fluctuation on dissolution and biodegradation of a multi-component, light nonaqueous-phase liquid

Light nonaqueous-phase liquids (LNAPLs) such as gasoline and diesel fuel are among the most common causes of soil and groundwater contamination. Dissolution and subsequent advective transport of LNAPL components can negatively impact water supplies, while biodegradation is thought to be an important sink for this class of contaminants. We present a laboratory investigation of the effect of a water-table fluctuation on dissolution and biodegradation of a multi-component LNAPL (85% hexadecane, 5% toluene, 5% ethylbenzene, and 5% 2-methylnapthalene on a molar basis) in a pair of similar model aquifers (80 cm x 50 cm x 3 cm), one of which was subjected to a water-table fluctuation. Water-table fluctuation resulted in LNAPL and air entrapment below the water table, an increase in the vertical extent of the LNAPL source zone (by factor 6.7), and an increase in the volume of water passing through the source zone (by factor ~18). Effluent concentrations of dissolved LNAPL components were substantially higher and those of dissolved nitrate lower in the model aquifer where a fluctuation had been induced. Thus, water-table fluctuation led to enhanced biodegradation activity (28.3 mmol of nitrate consumed compared to 16.3 mmol in the model without fluctuation) as well as enhanced dissolution of LNAPL components. Despite the increased biodegradation, fluctuation led to increased elution of dissolved LNAPL components from the system (by factors 10-20). Hence, water-table fluctuations in LNAPL-contaminated aquifers might be expected to result in increased exposure of downgradient receptors to LNAPL components. Accordingly, water-table fluctuations in contaminated aquifers are probably undesirable unless the LNAPL is of minimal solubility or the dissolved-phase plume is not expected to reach a receptor due to distance or the presence of some form of containment.     

Influence of cation type, ionic strength, and pH on solubilization and mobilization of residual hydrocarbon by a biosurfactant

This study investigated the effect of cation type, ionic strength, and pH on the performance of an anionic monorhamnolipid biosurfactant for solubilization and removal of residual hexadecane from sand. Three common soil cations, Na⁺, Mg²⁺, and Ca²⁺, were used in these experiments and hexadecane was chosen to represent a nonaqueous phase liquid (NAPL) less dense than water. Results showed that hexadecane solubility in rhamnolipid solution was significantly increased by the addition of Na⁺ and Mg²⁺. Addition of up to 0.2 mM Ca²⁺ also increased hexadecane solubility. For Ca²⁺ concentrations greater than 0.2 mM there was little effect on hexadecane solubility due to competing effects of calcium-induced rhamnolipid precipitation and enhanced hexadecane solubilization. Efficiency of NAPL solubilization can be expressed in terms of molar solubilization ratios (MSR). The results showed that MSR values for hexadecane in rhamnolipid solutions increased 7.5-fold in the presence of 500 mM Na⁺, and 25-fold in the presence of 1 mM Mg²⁺. The presence of cations also reduced the interfacial tension between rhamnolipid solutions and hexadecane. For example, an increase in Na⁺ from 0 to 800 mM caused a decrease in interfacial tension from 2.2 to 0.89 dyn cm⁻¹. Similarly, decreasing pH caused a reduction in interfacial tension. The lowest interfacial tension value observed in this study was 0.02 dyn cm⁻¹ at pH 6 in the presence of 320 mM Na⁺. These conditions were also found to be optimal for removal of hexadecane residual from sand columns, with 58% of residual removed within three pore volumes. The removal of residual NAPL from the packed columns was primarily by mobilization, even though solubilization was significantly increased in the presence of Na⁺.     

Effect of nonionic surfactant partitioning on the dissolution kinetics of residual perchloroethylene in a model porous medium

At concentrations above the critical micelle concentration, surfactants can significantly enhance the solubilization of residual nonaqueous phase liquids (NAPL) and, for this reason, are the focus of research on surfactant-enhanced aquifer remediation (SEAR). As a consequence of their amphiphilic nature, surfactants may also partition to various extents between the organic and aqueous phases, thereby affecting SEAR performance. We report here on the observation and analysis of the effect of surfactant partitioning on the dissolution kinetics of residual perchloroethylene (PCE) by aqueous solutions (1000 mg/L) of the non-ionic surfactant Triton X-100 in a model porous medium. For this fluid system, batch equilibration experiments showed that the surfactant partitions strongly into the NAPL (NAPL-water partition coefficient equal to 12.5). Dynamic interfacial tension (IFT) measurements were employed to study surfactant diffusion and interfacial adsorption. The dynamic IFT measurements were consistent with partitioning of the surfactant between the two liquid phases. PCE dissolution experiments, conducted in a transparent glass micromodel using an aqueous surfactant solution, were contrasted to experiments using clean water. Surfactant partitioning was observed to delay significantly the onset of micellar solubilization of PCE, an observation reproduced by a numerical model. This effect is attributed to the reduction of surfactant concentration in the immediate vicinity of the NAPL-water interface, which accompanies transport of the surfactant into the NAPL. Accordingly, it is suggested that both the rate and the extent of diffusion of the surfactant into the NAPL affect the onset of and the driving force for micellar solubilization. While many surfactants do not readily partition in NAPL, this possibility must be considered when selecting non-ionic surfactants for the enhanced solubilization of residual chlorinated solvents in porous media.     

Effect of expandable clays and cometabolism on PAH biodegradability

Pyrene and phenanthrene degradation was examined in both single and binary slurry systems for three different natural soils. It was found that the amount of total expandable clays (smectite and vermiculite) was in a good agreement with the achieved rate and extent of biodegradation. For instance, the intrinsic phenanthrene biodegradation rate was 626 microg/L/day for the soil with the largest expandable clay and 3203 microg/L/day for the soil with the least. Similarly, the smallest total pyrene biodegradation (65%) was found for the soil rich in expandable clays, compared to an 82% pyrene reduction in the soil that had the lowest amount. Mass transfer limitation after compound sorption to the clays was more pronounced for the more hydrophobic pyrene. In the presence of phenanthrene, total pyrene biodegradation increased by 2 to 7% due to cometabolism, while the total phenanthrene biodegradation was only enhanced by 0.5 to 5% in the binary system. This research demonstrated that expandable clays might govern the substrate availability to microorganisms and microbial accessibility to substrates. Therefore, the contribution of organic matter and expandable clays to sorption, desorption and biodegradation should be taken equally into account in order to better understand complex bioremediation issues.     

Removal of DNAPL contamination from the saturated zone by the combined effect of vertical upward flushing and density reduction

A common aspect of innovative remediation techniques is that they tend to reduce the interfacial tension between the aqueous and non-aqueous phase liquids, resulting in mobilization of the organic contaminant. This complicates the remediation of aquifers, contaminated with Dense Non-Aqueous Phase Liquids (DNAPLs), as they are likely to migrate downwards, deeper into the aquifer and into finer layers. A possible solution is the use of swelling alcohols, which tend to reduce the density difference between the aqueous phase and the DNAPL. To avoid premature mobilization upon the initial contact between the DNAPL and the alcohol, several researchers have proposed the use of vertical upward flow of the alcohol. In this paper, we present an equation, which describes the upward mobilization of both continuous and discontinuous DNAPLs and so the important parameters governing the upward controlled mobilization of the DNAPL. The need and required magnitude of this specific discharge was investigated by conducting four column experiments in which the initial density of the DNAPL and the permeability was varied. It was shown that the required flow velocities increase with the permeability of the porous medium and the initial density difference between the aqueous phase and the DNAPL. Whenever the specific discharge falls below the critical value, the DNAPL moves downward. A second set of column experiments looked at the impact of permeability of porous medium on the solubilization and mobilization of DNAPL during alcohol flooding. Columns, packed with coarse or fine sand, containing a residual trichloroethylene (TCE) or perchloroethylene (PCE) saturation were flushed with the alcohol mixture at a fixed specific discharge rate. The induced pressure gradients in the aqueous phase, which were higher in the fine sand, resulted for this porous medium in extensive mobilization of the DNAPL against the direction of the buoyancy force. The density of the first NAPL coming out of the top of the fine sand was close to that of the pure DNAPL. In the coarser sand, the pressure gradients were sufficient to prevent downward migration of the DNAPL, but upward mobilization was minimal. The predominant removal mechanism in this case was the much slower solubilization.     

Multimedia fate of petroleum hydrocarbons in the soil: Oil matrix of constructed biopiles

A dynamic multimedia fugacity model was used to evaluate the partitioning and fate of petroleum hydrocarbon fractions and aromatic indicator compounds within the soil: oil matrix of three biopiles. Each biopile was characterised by four compartments: air, water, soil solids and non-aqueous phase liquid (NAPL). Equilibrium partitioning in biopile A and B suggested that most fractions resided in the NAPL, with the exception of the aromatic fraction with an equivalent carbon number from 5 to 7 (EC₅₋₇). In Biopile C, which had the highest soil organic carbon content (13%), the soil solids were the most important compartment for both light aliphatic fractions (EC₅₋₆ and EC₆₋₈) and aromatic fractions, excluding the EC₁₆₋₂₁ and EC₂₁₋₃₅. Our starting hypothesis was that hydrocarbons do not degrade within the NAPL. This was supported by the agreement between predicted and measured hydrocarbon concentrations in Biopile B when the degradation rate constant in NAPL was set to zero. In all scenarios, biodegradation in soil was predicted as the dominant removal process for all fractions, except for the aliphatic EC₅₋₆ which was predominantly lost via volatilization. The absence of an explicit NAPL phase in the model yielded a similar prediction of total petroleum hydrocarbon (TPH) behaviour; however the predicted concentrations in the air and water phases were significantly increased with consequent changes in potential mobility. Further comparisons between predictions and measured data, particularly concentrations in the soil mobile phases, are required to ascertain the true value of including an explicit NAPL in models of this kind.     

Effects of source zone heterogeneity on surfactant-enhanced NAPL dissolution and resulting remediation end-points

The effectiveness of removal of nonaqueous phase liquids (NAPLs) from the entrapment source zone of the subsurface has been limited by soil heterogeneity and the inability to locate all entrapped sources. The goal of this study was to demonstrate the uncertainty of degree of source removal associated with aquifer heterogeneity. In this demonstration, source zone NAPL removal using surfactant-enhanced dissolution was considered. Model components that simulate the processes of natural dissolution in aqueous phase and surfactant-enhanced dissolution were incorporated into an existing code of contaminant transport. The dissolution modules of the simulator used previously developed Gilland-Sherwood type phenomenological models of NAPL dissolution to estimate mass transfer coefficients that are upscaleable to multidimensional flow conditions found at field sites. The model was used to simulate the mass removal from 10 NAPL entrapment zone configurations based on previously conducted two-dimensional tank experiments. These entrapment zones represent the NAPL distribution in spatially correlated random fields of aquifer hydraulic conductivity. The numerical simulations representing two-dimensional conditions show that effectiveness of mass removal depends on the aquifer heterogeneity that controls the NAPL entrapment and delivery of the surfactant to the locations of entrapped NAPLs. Flow bypassing resulting from heterogeneity and the reduction of relative permeability due to NAPL entrapment reduces the delivery efficiency of the surfactant, thus prolonging the remediation time to achieve desired end-point NAPL saturations and downstream dissolved concentrations. In some extreme cases, the injected surfactant completely bypassed the NAPL source zones. It was also found that mass depletion rates for different NAPL source configurations vary significantly. The study shows that heterogeneity result in uncertainties in the mass removal and achievable end-points that are directly related to dissolved contaminant plume development downstream of the NAPL entrapment zone.