Nonaqueous Phase Liquid Pool Dissolution as a Function of Average Pore Water Velocity

Bench-scale reactor experiments were performed to study the dissolution of a binary naphthalene-in-nonane mixture nonaqueous phase liquid (NAPL) pool over a wide range of average pore water velocities, vx (≈0.1–60 m/day). Experimental NAPL pool dissolution flux values were determined using a steady-state mass balance approach. The experimental flux data were compared to model predictions made assuming either local equilibrium or mass-transfer limited conditions. The local equilibrium model could describe the trends in the average effluent concentration and dissolution flux with 0.110?m/day. Data determined to be under mass-transfer limited conditions were fit to the nonequilibrium model to estimate values for an overall mass-transfer coefficient. The calculated overall mass-transfer coefficients had an average value of 0.407 m/day and showed no correlation with vx, probably due to mass-transfer resistance becoming dominated by the diffusional resistance in the NAPL. These results suggest that the nonequilibrium approach is better suited for describing high velocity (vx>10?m/day) dissolution of multicomponent NAPL pools, and that flushing of groundwater at very high velocities may not be an effective approach for enhancing NAPL-pool dissolution flux.     

Benzene Nonaqueous Phase Liquids Removal under Air-Sparged Conditions

The dissolution, diffusion, and volatilization of a free-phase benzene glob in the presence of air sparging were measured in laboratory-scale air sparging reactors and modeled using a dissolution–diffusion–volatilization (DDV) model. The estimated dissolution rate coefficients (Kf) from the DDV model ranged from 0.0050 to 0.017 mm/min while the estimated volatilization rate coefficients (KL) ranged from 0.012 cm/min to 0.029 mm/min. The DDV model fitted well the aqueous phase migration of benzene for air channels spaced at 45 and 60 mm. For air channels spaced at 30 mm, the model fitted the aqueous migration at most locations except above the benzene glob where the model underestimated the experimentally determined concentrations. However, the mass removed using the gas phase concentrations as predicted by the model were 65% of the actual mass removed. These observations suggest that other mass transport mechanisms may influence benzene mass removal, especially when the air channels are close to the benzene glob.     

Preferential Flow of a Nonaqueous Phase Liquid in Dry Sand

Geotechnical centrifuge testing is used to examine the preferential (fingered) flow of a nonaqueous phase liquid (NAPL) in a uniform dry sand. The results of nine experiments, containing a total of 87 observations of NAPL finger behavior, are analyzed. The observed finger tip velocities range from 0.01 to 0.3 cm/s, while the observed finger widths range from 0.3 to 3.6 cm. From the experimental data it is concluded that, asymptotically, the NAPL fingers are not fully saturated. For comparison, the behavior of water fingers is examined using the same experimental setup. In contrast to the NAPL fingers, and in agreement with other work reported in the literature, the water fingers are found to be fully saturated. In addition, it is confirmed that the water finger properties can be well predicted from known porous medium and fluid properties. A scaling analysis is presented that allows the NAPL finger properties to be inferred from models developed to describe water finger properties. The analysis predicts NAPL finger velocities to within 15% and NAPL finger widths to within 50% if both finger types are assumed saturated. By adjusting the analysis to account for the fact that the NAPL fingers are not fully saturated, NAPL finger widths can be predicted within to 10%, and NAPL finger velocities to within 30%.     

UNIFAC Modeling of Cosolvent Phase Partitioning in Nonaqueous Phase Liquid-Water Systems

In this study, an existing thermodynamic model was used to predict equilibrium phase partitioning behavior of a cosolvent in a two-phase nonaqueous phase liquid (NAPL)–water system. The activity coefficients are calculated using the universal quasichemical functional group activity coefficient (UNIFAC) method. We examined an assortment of cosolvent–NAPL pairs of environmental interest and compared the UNIFAC-predicted ternary phase diagrams against published experimentally derived ternary phase diagrams. Results show that the UNIFAC model is a promising method for predicting equilibrium cosolvent partitioning behavior in NAPL–water systems, and thus can be useful in estimating the potential for NAPL solubilization and mobilization in remediation processes. The cosolvent partitioning behavior is interpreted with regard to changes in the physical properties of the NAPL-water system. Changes in interfacial tension between the two phases were estimated using an existing correlation. A viscosity experiment was conducted for selected mixtures of ethanol, toluene, and water; and the viscosity was found to increase with increasing amounts of the cosolvent.     

Impact of Ethanol on Benzene Plume Lengths: Microbial and Modeling Studies

Recent legislation in several states has called for the removal of methyl tert-butyl ether (MTBE) from gasoline. In order to comply with Federal Clean Air Act requirements for carbon monoxide and ozone attainment, ethanol is being considered as a replacement for MTBE. The objective of this study is to evaluate the potential impact of ethanol on benzene plume lengths in subsurface environments following accidental spills of ethanol-blended gasoline. Two types of studies were conducted here. First, laboratory studies were performed using a pure culture indigenous to a gasoline-contaminated aquifer to evaluate the effect of ethanol on the rate of benzene biodegradation under aerobic conditions. Results from microbial studies showed that the biodegradation of 25 mg/L benzene was severely inhibited in the presence of 25 mg/L ethanol. While the enzymes responsible for benzene biodegradation by the culture were inducible, ethanol degradation appeared to be constitutive. Second, a two-dimensional model was developed to quantify the impact of ethanol on benzene plume lengths using weighted-average aerobic and anaerobic biodegradation rates for benzene in the presence and absence of ethanol. Model simulations indicated that benzene plume lengths are likely to increase by 16–34% in the presence of ethanol.     

Role of Natural Attenuation in Life Cycle of MTBE Plumes

The natural life cycle of a plume of methyl tert-butyl ether from a spill of gasoline is controlled by the rate of attenuation of the source (due to partitioning from the residual gasoline to the flow of groundwater) and the rate of attenuation in the plume (due to dispersion and natural biodegradation). Rates of attenuation were extracted for plumes in California, Florida, North Carolina, New York, and New Jersey. The maximum rate of attenuation of the source was 0.75 per year. The rates of attenuation in the plumes varied from 0.56 to 4.3 per year. In all cases, the rate of attenuation of the plume exceeded the rate of attenuation of the source. As these plumes progress through their life cycle, they should recede back toward their source.     

Centrifuge Modeling of Nonaqueous Phase Liquid Movement and Entrapment in Unsaturated Layered Soils

Four geotechnical centrifuge tests with different soil layered systems were performed to investigate the movement and entrapment of water and of light nonaqueous phase liquids (LNAPLs) in unsaturated layered soil deposits. The tests were performed at 20 g and a vadose zone condition was created during the centrifuge tests by lowering the water table from the initially water saturated condition. During the water drainage stage, the water distribution within the models and the dynamic air–water capillary pressure saturation relationships of the three sands were obtained using tensiometers and resistivity probes. After achieving the unsaturated condition, a model LNAPL (Soltrol 220? or silicon oil) was injected near the soil surface and the movement and entrapment were monitored during the redistribution stage until the LNAPL reached the top of the water table. Complex LNAPL preferential flow and entrapment patterns were observed in the layered models with different textural interfaces due to the relative movement of all three phases [water, nonaqueous phase liquid (NAPL), and air]. The centrifuge tests data coupled with the numerical analyses show that NAPL properties, subsurface soil structures, initial water saturation, and NAPL infiltration rate affect the variation in entrapment conditions in heterogeneous unsaturated soil deposits.     

Destruction of a Carbon Tetrachloride Dense Nonaqueous Phase Liquid by Modified Fenton’s Reagent

Destruction of a dense nonaqueous phase liquid (DNAPL) by soluble iron (III)-catalyzed and pyrolusite (β-MnO2)-catalyzed Fenton’s reactions (hydrogen peroxide and transition metal catalysts) was investigated using carbon tetrachloride (CT) as a model contaminant. In the system amended with 5 mM soluble iron (III), 24% of the CT DNAPL was destroyed after 3 h while CT dissolution in parallel fill-and-draw systems was minimal, indicating that CT was degraded more rapidly than it dissolved into the aqueous phase. Fenton’s reactions catalyzed by the naturally occurring manganese oxide pyrolusite were even more effective in destroying CT DNAPLs, with 53% degradation after 3 h. Although Fenton’s reactions are characterized by hydroxyl radical generation, carbon tetrachloride is unreactive with hydroxyl radicals; therefore, a transient oxygen species other than hydroxyl radicals formed through Fenton’s propagation reactions was likely responsible for CT destruction. These results demonstrate that Fenton-like reactions in which nonhydroxyl radical species are generated may provide an effective method for the in situ treatment of DNAPLs.     

Comparison of Liquid and Gas-Phase Photooxidation of MTBE: Synthetic and Field Samples

The feasibility of photooxidation treatment of methyl tert-butyl ether (MTBE) in water was investigated using two systems: (1) a slurry falling film photoreactor and (2) an integrated air stripping with gas phase photooxidation system. Methyl tert-butyl ether-contaminated synthetic water and field samples from contaminated sites were used for these studies. Using a TiO2 slurry (0.1 g/L; Degussa P25) flowing down at a rate of up to 0.26 L/min over the inner surface of a glass tube surrounding a 1-kW medium pressure mercury lamp, more than 99% of MTBE in the synthetic samples, initially at 1 mg/L, was degraded within 90 min. The major degradation products from MTBE were tert-butyl alcohol, tert-butyl formate, and small amounts of acetone. However, the degradation of MTBE and its byproducts in contaminated groundwater samples was hindered significantly by dissolved metals such as Fe2+, chloride ions, and aromatic organic species. Integrating air stripping with gas-phase photocatalysis is an an effective alternative that would not be affected by the water chemistry. The reaction rates for MTBE degradation in the gas phase are orders of magnitude faster than in aqueous solution.     

Aqueous and Mineral Intrinsic Bioremediation Assessment: Natural Attenuation

An approach to evaluating intrinsic bioremediation, aqueous and mineral intrinsic biodegradation assessment (AMIBA) is described. AMIBA is based on the microbial reduction of Fe3+ and SO42?, forming reduced Fe and S mineral species in amounts stoichiometrically equivalent to the contaminant mass oxidized by microbial processes. Mineral data from sediment cores are emphasized rather than the aqueous data used in existing protocols. AMIBA was demonstrated at Westover Air Force Base, Chicopee, Massachusetts. Over 300 times more reduced Fe2+ was in mineral form, compared to aqueous. The distribution of mineral Fe3+ and Fe2+ marked the historic extent of the plume, confirming plume retreat. Aqueous SO42? reduction resulted in the deposition of equivalent amounts of iron sulfide minerals near the source area. The total mass of fuel degraded by intrinsic bioremediation and the rate of source depletion was estimated. Thus, the past and future performance of intrinsic bioremediation was assessed using one sampling event rather than relying on protracted monitoring, as is the current practice.     

Cation Effects on Chromium Removal in Permeable Reactive Walls

Permeable reactive walls have proven to be successful in laboratory and pilot-scale field applications. However, the long-term efficacy of reactive permeable walls has not been established due to the novelty of the technology. Also, the impact of common groundwater ions such as calcium and magnesium (i.e., hardness) on permeable reactive walls is unknown. In theory, the ions may react competitively with chromium in solution and/or other materials on the surface of the zero-valent iron. The ions may also form precipitates that could clog the reactive zone over time, resulting in decreased contaminant removal and a shorter wall lifetime. The purpose of this research was to determine the effects of common groundwater ions on permeable reactive walls. A range of calcium and magnesium concentrations was tested in laboratory columns to determine the effect of these ions on removal of a constant chromium concentration (100 mg/L). Results from the laboratory tests indicated that calcium and magnesium had a significant impact on chromium removal. The most dramatic effects were witnessed at hardness levels up to 140 mg/L as CaCO3 where zero-valent iron capacity was reduced by 45%.     

Transport of Colloids and Associated Hydrophobic Organic Chemicals through a Natural Media Filter

In this project, the ability of natural media filtration (NMF) to remove colloidal particles and associated hydrophobic organic compounds from the aqueous phase was evaluated by performing sorption and transport experiments with leaf compost media. Phenanthrene sorption isotherm experiments for compost and model colloidal (latex) particles found that phenanthrene has a greater affinity for the colloidal particles than for the compost materials. In column experiments, the transport of phenanthrene through the NMF in the presence and absence of two colloidal particles with different hydrophobicities [sulfate (more hydrophobic) and carboxylate (less hydrophobic)] showed that the effluent phenanthrene concentration in the presence of colloids, particularly sulfate latex particles, is much higher than that in the absence of colloids. The results from a mathematical model used to evaluate data from the column experiments suggest that enhancement of contaminant transport can be significant under the following conditions: high colloidal concentrations, high partition coefficient between contaminant and colloids, or a slow desorption rate of contaminant from colloids.     

Enhanced Dissolution of Trichloroethene: Effect of Carbohydrate Addition and Fermentation Processes

Remediation of source areas is challenging because lingering contaminants are often present as nonaqueous phase liquid (NAPL) and sorbed mass, and therefore difficult to remove via biodegradation or other commonly used remedial methods. Experimental results indicate that enhanced dissolution of a model NAPL, trichloroethene (TCE), can occur through the addition and/or subsequent fermentation of a dilute molasses solution. Enhanced mass transfer occurs by two mechanisms, depending upon whether the molasses solution is fresh or has fermented. The addition of fresh molasses worked to increase TCE solubility (>200%), thereby increasing mass transfer from the NAPL phase. Mixing TCE NAPL with a fermented molasses solution, however, increased TCE mass flux via the formation of a NAPL/aqueous phase emulsion. In addition, fermented liquid may have also decreased the soil partitioning coefficient (Kd) of TCE, indicating that enhanced transfer of sorbed mass to the aqueous phase could also occur in the presence of fermented molasses. These results provide guidance on how remedial systems may be optimized to increase NAPL and sorbed-mass dissolution and are therefore important, particularly when bioremediation is used to polish residual source zones.     

Design and Operation of Point-of-Use Treatment System for Arsenic Removal

A point-of-use (POU) system was designed and constructed using commercially available activated alumina to remove arsenic from drinking water. Testing with City of Albuquerque chlorinated tap water containing an average of 23 ug/L arsenic found that 1 L of adsorbent would provide water for direct consumption by a family of four for 435 days. It was estimated that the POU system constructed for this study could be sold for $162, and the arsenic adsorption columns were estimated to cost $4. A monthly cost to the customer of $10/month was estimated to purchase, install, and operate this POU system, assuming annual replacement of adsorption media cartridges. The implications of relying upon POU systems to comply with a new drinking water standard for arsenic are discussed.     

Effects of Ethanol versus MTBE on Benzene, Toluene, Ethylbenzene, and Xylene Natural Attenuation in Aquifer Columns

The increased use of ethanol as a replacement for the gasoline oxygenate, methyl tert-butyl ether (MTBE), may lead to indirect impacts related to natural attenuation of benzene, toluene, ethylbenzene, and the three isomers of xylene (BTEX compounds). Ethanol could enhance dissolved BTEX mobility by exerting a cosolvent effect that decreases sorption-related retardation. This effect, however, is concentration dependent and was not observed when ethanol was added continuously (at 1%) with BTEX to sterile aquifer columns. Nevertheless, a significant decrease in BTEX retardation was observed with 50% ethanol, suggesting that neat ethanol spills in bulk terminals could facilitate the migration of pre-existing contamination. MTBE (25 mg/L influent) was not degraded in biologically active columns, and it did not affect BTEX degradation. Ethanol (2 g/L influent), on the other hand, was degraded rapidly and exerted a high demand for nutrients and electron acceptors that could otherwise have been used for BTEX degradation. Ethanol also increased the microbial concentration near the column inlet by one order of magnitude relative to columns fed BTEX alone or with MTBE. However, 16S-ribosomal ribonucleic acid sequence analyses of dominant denaturing gradient gel electrophoresis bands identified fewer species that are known to degrade BTEX when ethanol was present. Overall, the preferential degradation of ethanol and the accompanying depletion of oxygen and other electron acceptors hindered BTEX biodegradation, which suggests that ethanol could increase the length of BTEX plumes.     

Influence of COD:N:P Ratio on Nitrogen and Phosphorus Removal in Fixed-Bed Filter

This study examined the effects of COD:N:P ratio on nitrogen and phosphorus removal in a single upflow fixed-bed filter provided with anaerobic, anoxic, and aerobic conditions through effluent and sludge recirculation and diffused air aeration. A high-strength wastewater mainly made of peptone, ammonium chloride, monopotassium phosphate, and sodium bicarbonate with varying COD, N, and P concentrations (COD: 2,500–6,000, N: 25–100, and P: 20–50 mg/L) was used as a substrate feed. Sodium acetate provided about 1,500 mg/L of the wastewater COD while the remainder was provided by glucose and peptone. A series of orthogonal tests using three factors, namely, COD, N, and P concentrations, at three different concentration levels were carried out. The experimental results obtained revealed that phosphorus removal efficiency was affected more by its own concentration than that of COD and N concentrations; while nitrogen removal efficiency was unaffected by different phosphorus concentrations. At a COD:N:P ratio of 300:5:1, both nitrogen and phosphorus were effectively removed using the filter, with removal efficiencies at 87 and 76%, respectively, under volumetric loadings of 0.1?kg?N/m3?d and 0.02?kg?P/m3?d.     

Treatment of Acidic Groundwater in Acid Sulfate Soil Terrain Using Recycled Concrete: Column Experiments

Acidic groundwater generated from pyrite oxidation in acid sulfate (AS) soil is a major geoenvironmental problem in Australia. This study aims to evaluate recycled concrete as a reactive material in permeable reactive barriers (PRBs) for the remediation of acidic groundwater in low-lying AS soil floodplains. Laboratory experiments were systematically conducted to investigate the acid neutralization behavior of recycled concrete and its potential to remove dissolved Al and Fe. The results confirmed that recycled concrete could effectively treat acidic groundwater from an AS soil terrain, resulting in near neutral effluent over a long period with complete removal of Al and Fe. The major mechanisms involved in neutralizing acidic groundwater are thought to be the precipitation of Al and Fe as oxides, oxyhydroxides, and hydroxides. However, the accumulation of secondary minerals could decrease the reactivity of the recycled concrete. For example, chemical armoring could decrease the neutralizing capacity of recycled concrete by up to 50% compared with the theoretical acid neutralization capacity of this material. The results reported here also show that the neutralization capacity and reactive efficiency of recycled concrete are dependent on the initial pH value and also the concentration of Al and Fe in acidic groundwater.     

Assessing of Natural Attenuation and Intrinsic Bioremediation Rates at a Petroleum-Hydrocarbon Spill Site: Laboratory and Field Studies

Natural attenuation is a passive remedial approach that depends upon natural processes to degrade and dissipate contaminants in soil and groundwater. Intrinsic bioremediation is believed to be the major process among the natural attenuation mechanisms that account for the reduction of contaminant concentrations. In this study, a mass flux approach was used to calculate the contaminant mass reduction at a petroleum-hydrocarbon spill site. The mass flux technique is a simplified mass balance procedure, which is accomplished using the differences in total contaminant mass flux across two cross sections of the contaminant plume. The mass flux calculation results show that up to 86% of the dissolved total benzene, toluene, ethylbenzene, and xylene (BTEX) isomers removal was observed via natural attenuation at this site. Evidence for the occurrence of natural attenuation was the decreased contaminant mass flux through the plume cross sections along the transport path and limited spreading of the BTEX plume. Evidences for the BTEX biodegradation include: (1) decreased BTEX concentrations along the transport path; (2) depletion of dissolved oxygen, nitrate, and sulfate; (3) production of dissolved ferrous iron, sulfide, methane, and CO2; (4) deceased pH in the spill source area and increased pH in iron-reducing area; (5) increased alkalinity and microbial populations; and (6) preferential removal of certain BTEX components along the transport path. The effectiveness of intrinsic bioremediation on BTEX removal was evaluated by the in situ tracer method. Results reveal that approximately 74% of the BTEX removal was due to the intrinsic biodegradation process. The first-order decay model was applied for the natural attenuation and intrinsic bioremediation rates calculation. Results show that the biodegradation capacity (34.5 mg/L) for BTEX was much higher than the detected contaminants within the plume. The calculated total BTEX first-order natural attenuation and intrinsic bioremediation rates were 0.025 and 0.017% 1/day, respectively. Results of polymerase chain reaction, denaturing gradient gel electrophoresis, and nucleotide sequence analysis reveal that some petroleum-hydrocarbon degraders (Flavobacterium capsulatum, Xanthobacter sp., Xanthobacter flavus, Xanthomonas codiaei, Pseudomonas boreopolis, Methylobacterium sp., Reichenowia pictae) might exist at this site, which might contribute to the BTEX biodegradation. Results suggest that the natural attenuation mechanisms can effectively contain the plume, and the mass flux method is useful in assessing the occurrence and efficiency of the natural attenuation and intrinsic bioremediation processes.     

Removal of Antibiotics from Surface and Distilled Water in Conventional Water Treatment Processes

Conventional drinking water treatment processes were evaluated under typical water treatment plant conditions to determine their effectiveness in the removal of seven common antibiotics: carbadox, sulfachlorpyridazine, sulfadimethoxine, sulfamerazine, sulfamethazine, sulfathiazole, and trimethoprim. Experiments were conducted using synthetic solutions prepared by spiking both distilled/deionized water and Missouri River water with the studied compounds. Sorption on Calgon WPH powdered activated carbon, reverse osmosis, and oxidation with chlorine and ozone under typical plant conditions were all shown to be effective in removing the studied antibiotics. Conversely, coagulation/flocculation/sedimentation with alum and iron salts, excess lime/soda ash softening, ultraviolet irradiation at disinfection dosages, and ion exchange were all relatively ineffective methods of antibiotic removal. This study shows that the studied antibiotics could be effectively removed using processes already in use in many water treatment plants. Additional work is needed on by-product formation and the removal of other classes of antibiotics.