Persulfate Oxidation of Gasoline Compounds
In situ chemical oxidation (ISCO) using persulfate is a promising remediation technology that can be potentially applied to a wide range of organic contaminants. Gasoline compounds are of particular interest because they extensively impact the soil and groundwater, and are highly persistent and toxic. In this investigation, destruction of specific gasoline compounds (benzene, toluene, ethylbenzenes, xylenes, trimethylbenzenes (TMBs) and naphthalene), and fractions (F1 and F2) by activated and inactivated persulfate was studied at the bench-scale. Aqueous phase batch reactors (25 mL) for inactivated systems employed persulfate at two concentrations (1 or 20 g/L), and activated systems were conducted with a persulfate concentration of 20 g/L. In the activated systems, the ability of hydrogen peroxide or chelated-ferrous as an activator was examined at two experimental conditions (peroxide molar ratio 0.1 and 1.0 with respect to persulfate; and citric acid chelated ferrous at 150 and 600 mg/L). All treatments and controls contained an initial gasoline concentration of approximately 25 mg/L and were run in triplicate. Sampling for gasoline compounds was conducted over <28 day reaction period. The controls showed insignificant degradation for all the gasoline compounds and fractions examined while inactivated persulfate at 1 g/L showed little (<10%) decrease in the concentration of gasoline compounds over the 28 day reaction period. Inactivated persulfate at 20 g/L demonstrated a significant decrease in the aqueous concentration of BTEX (>99%), TMB (>94%) and naphthalene (>71%). Oxidation of the F1 fraction (>94%) was more pronounced than the F2 fraction (>80%), and >93% TPH was oxidized. Use of peroxide as an activator at a molar ratio of 0.1 improved the destruction of TMBs (>99%) and naphthalene (>85%) while maintaining the high removal of BTEX (>99%) compounds. Increase in activator strength (molar ratio 1.0) decreased the destruction of xylenes (>86%) and TMBs (>81%). The decrease in concentration of all the compounds was higher for a molar ratio of 1.0 (<27%) as compared with a molar ratio of 0.1 (<11%). The activation by ferrous concentration resulted in higher oxidation of compounds (except naphthalene) as compared with unactivated or peroxide activated persulfate. 1,3,5-TMB was completed oxidized after 4 days using higher chelated ferrous concentration and after 12 days using lower chelated ferrous concentration for persulfate activation. In general, increase in chelated ferrous concentration resulted in higher oxidation of the gasoline compounds. While oxidation of F1 fraction was similar for two ferrous activation conditions, the oxidation of F2 fraction was lower when ferrous activation at 600 mg/L was employed. Use of persulfate at high dosages by itself or in combination with higher doses of chelated ferrous or optimum doses of peroxide as an activator seems to be a viable option for remediation of gasoline compounds examined in this study. Persulfate appears to be particularly effective in the oxidation of BTEX compounds, but may require ferrous activation for a complete oxidation of TMBs and peroxide activation for oxidation of naphthalene.
Electrostatic and Steric Contributions to nZVI Stability
Nanoscale zero-valent iron (NZVI) is a promising remediation technology for the transformation of subsurface halogenated hydrocarbons. However, one significant challenge for NZVI field application is that the nanoparticles rapidly agglomerate into larger clusters, which in turn limits the mobility of the particles through porous media. To prevent particle agglomeration, nanoparticles surface can be modified or functionalized to increase both mobility and reactivity. There are several reports in the literature indicating how the use of different polymers stabilizes iron nanoparticles. However the detailed reasons for the observed stability are still not well understood. We have investigated the stability of NZVI suspensions in water and their ability to degrade trichloroethylene using a family of carboxymethyl cellulose and polyvinyl pyrrolidone polymers of different molecular weights. These studies are carried out with the aim to isolate electrostatic effects in the polymer-iron nanoparticle interface from single steric effects. The interactions between the polymer and the NZVI surface have been characterized using electron microscopy, zeta potential measurements and infrared spectroscopy. The observed enhancements in stability are interpreted in terms of NZVI particle size, viscosity of the nZVI suspension, and the Fe/polymer ratio.
Roles of Particle Properties, Subsurface Geochemical/Geophysical/ Hydrological Conditions, and Delivery Strategies on the Emplacement of Polymeric Modified Nanoscale Zerovalent Iron (NZVI) for In situ Subsurface Remediation
Polymer modified nanoscale zerovalent iron (NZVI) is used for in situ subsurface remediation of chlorinated solvents. Its small size, high reactivity with pollutants, and its potential to be transportable in the subsurface make it attractive. NZVI aggregation, the reaction kinetics of pollutants with various kinds of NZVI, and surface modification /functionalization to enhance remediation performance have been studied, however, tools for designing emplacement strategies for NZVI to the contaminated zones in a field scale are lacking. Here we develop a tool to aid the preliminary design of emplacemnt of polymer modified NZVI in the field by coupling semi-empirical correlations to predict aggregate size and aggregate deposition with a numerical simulation of the flow field in porous media at the scale and injection conditions analogous to field application of NZVI. The flow field in various subsurface geochemical/geophysical/hydrological conditions and NZVI injection strategies are simulated using COMSOL Multiphysics (formerly FEMLAB). We also use this modeling tool to evaluate the effects of particle properties, subsurface geochemical/geophysical/hydrological conditions, and delivery strategies on the delivery and emplacement of polymeric modified NZVI in saturated porous media. In addition, we investigate the alteration of groundwater flow due to pore blocking from emplacement of polymer-modified NZVI at various conditions.
Effects of Temperature Changes on Biodegradation of Petroleum Hydrocarbons in Contaminated Soils from an Arctic Site
Bioremediation is being considered as a cost-effective and a minimally disruptive remedial option at remote sites in the Arctic and sub-Arctic impacted by petroleum NAPL contamination. The implementation of on-site bioremediation in cold environments has been generally limited in the short, non-freezing summer months since ground remains frozen for 8-9 months of the year. This study evaluates the effect of different temperature regimes on petroleum hydrocarbon biodegradation rates and extent, as well as on the microbial activity. A series of pilot-scale landfarming bioremediation experiments (1 m×0.6 m×0.35 m soil tank dimension) was performed using aged, petroleum fuel-contaminated soils shipped from Resolution Island, Nunavut, Canada. These experiments were conducted under the following temperature conditions: (1) variable daily average field temperatures (1 to 10°C) representative of summers at the site; (2) constant mean temperature-mode with 6°C, representing typical stable laboratory incubation; and (3) under seasonal freeze-thaw conditions (-8°C to 10°C). Data to be presented include changes with time of petroleum hydrocarbons concentration fractionated by C-lengths, soil moisture (unfrozen water) contents, O2 and CO2 concentrations in soil pore gas, microbial population size and community composition in nutrient- amended and untreated landfarms. Hydrocarbon biodegradation and heterotrophic respiration activity was more rapid under the variable temperature cycle (1 to 10°C) than at a constant average temperature of 6°C, and total petroleum hydrocarbon (TPH) concentrations were reduced by 55% due to biodegradation over a 60 day test period under the variable temperature regime, compared to only 21% in soil tanks which were subjected to a constant temperature of 6°C. Shifts in microbial community were clearly observed in the both temperature modes using PCR-DGGE analyses and the emergence of a hydrocarbon-degrading population, Alkanindiges, was identified through 16S rRNA gene sequence analyses. Under the seasonal freeze-thaw conditions, microbial activity and hydrocarbon degradation was detected even when soils were frozen. The microbial activity was significantly correlated to residual unfrozen water contents.
The use of Microarrays to Elucidate Biomarkers for use in Site Assessment and Monitoring of Reductive Dechlorination
Background and Objectives: Chlorinated solvents are prevalent groundwater contaminants that often exist as DNAPLs, and as such serve as long-term sources of contamination. Bioaugmentation with Dehalococcoides- containing mixed cultures is an effective remediation strategy that can significantly increase DNAPL dissolution rates and thus mass flux, or can be used in conjunction with other physical and chemical remediation technologies to constrain the remaining DNAPL mass. Molecular biological tools are being used to assess the need for, and to monitor the progress of, bioaugmentation. Unfortunately, the current tools have limitations and provide an incomplete picture of the reductively dechlorinating bacterial community. To overcome these limitations and more accurately assess, predict, monitor and manage reductive dechlorination processes at contaminated sites, our research is focused on identifying novel reductive dechlorination biomarker genes and developing tools that improve our understanding of target gene presence, abundance, and expression, and thus, contaminant detoxification. Methods. To accomplish this, microarrays were used to investigate transcription within the Dehalococcoides- containing microbial community KB1. A 19,200 feature Shotgun Metagenome Microarray was created by amplifying and spotting random DNA fragments from a KB1 clone library, as well as 100 known reductive dehalogenase genes. In initial experiments the microarrays were used to investigate differential gene expression during dechlorination of chlorinated ethenes. Statistical analysis indicated differential expression of about 500 spots which were then sequenced. Subsequently, all of the spots on the microarray were sequenced. Differential expression was also verified using reverse transcriptase -quantitative PCR. Results: The KB1 Shotgun Metagenome Microarrays were used to identify genes within the community which are functionally important during dechlorination. Dehalococcoides-respiratory genes were identified such as hydrogenases and reductive dehalogenases. In addition, experiments with different chlorinated ethenes revealed that a different suite of reductive dehalogenase genes is important for each step of dechlorination, and has identified specific reductive dehalogenase genes as indicators for degradation of specific compounds. Furthermore, transcription of genes from non-dechlorinating organisms has revealed the importance of many other microorganisms, and highlighted the dynamic interaction between phage and other microorganism in the community. Moreover, analyses indicated possible inter-organism interactions including the transfer of cofactors. Conclusions: These microarrays provided important transcription and gene sequence information for this uncultivated and remediation-relevant microbial culture. This has allowed for the development of quantitative PCR techniques that can track different dehalogenase genes, micro-organisms, and even strains, within a dechlorinating community. New molecular biology tools will describe the dechlorinating community in much greater resolution, and will allow science-based site management decisions to achieve contaminant removal and plume control in the most efficient manner.
Bacterial Adhesion to Hexadecane (Model NAPL)-Water Interfaces
The rates of biodegradation of NAPLs have been shown to be influenced by the adhesion of hydrocarbon- degrading microorganisms as well as their proximity to the NAPL-water interface. Several studies provide evidence for bacterial adhesion or biofilm formation at alkane- or crude oil-water interfaces, but there is a significant knowledge gap in our understanding of the processes that influence initial adhesion of bacteria on to NAPL-water interfaces. In this study bacterial adhesion to hexadecane, and a series of NAPLs comprised of hexadecane amended with toluene, and/or with asphaltenes and resins, which are the surface active fractions of crude oils, were examined using a Microbial Adhesion to Hydrocarbons (MATH) assay. The microorganisms employed were Mycobacterium kubicae, Pseudomonas aeruginosa and Pseudomonas putida, which are hydrocarbon degraders or soil microorganisms. MATH assays as well as electrophoretic mobility measurements of the bacterial cells and the NAPL droplet surfaces in aqueous solutions were conducted at three solution pHs (4, 6 and 7). Asphaltenes and resins were shown to generally decrease microbial adhesion. Results of the MATH assay were not in qualitative agreement with theoretical predictions of bacteria- hydrocarbon interactions based on the extended Derjaguin-Landau-Verwey-Overbeek (XDLVO) model of free energy of interaction between the cell and NAPL droplets. In this model the free energy of interaction between two colloidal particles is predicted based on electrical double layer, van der Waals and hydrophobic forces. It is likely that the steric repulsion between bacteria and NAPL surfaces, caused by biopolymers on bacterial surfaces and aphaltenes and resins at the NAPL-water interface contributed to the decreased adhesion compared to that predicted by the XDLVO model.
Consideration of Treatment Performance Assessment Metrics for a TCE Source Area Bioremediation (SABRe project)
Techniques for optimizing the removal of NAPL mass in source zones have advanced at a more rapid rate than strategies to assess treatment performance. Informed selection of remediation approaches would be easier if measurements of performance were more directly transferable. We developed a number of methods based on data generated from multilevel sampler (MLS) transects to assess the effectiveness of a bioaugmentation/biostimulation trial in a TCE source residing in a terrace gravel aquifer in the East Midlands, UK. In this spatially complex aquifer, treatment inferred from long screen monitoring well data was not as reliable as that from consideration of mass flux changes across transects installed in and downgradient of the source. Falling head tests were conducted in the MLS ports to generate the necessary hydraulic conductivity (K) data. Combining K with concentration provides a mass flux map that allows calculation of mass turnover and an assessment of where in the complex geology the greatest turnover occurred. Five snapshots over a 600-day period indicate a marked reduction in TCE flux, suggesting a significant reduction in DNAPL mass over that expected due to natural processes. However, persistence of daughter products suggested that complete dechlorination did not occur. The MLS fence data also revealed that delivery of both carbon source and pH buffer were not uniform across the test zone. This may have lead to the generation of niches of iron(III) and sulphate reduction as well as methanogenesis, which impacted on dechlorination processes. In the absence of this spatial data, it is difficult to reconcile apparent treatment as indicated in monitoring well data to on-going processes.
Limitation of Empirical Koc Value in Sorption Predictions for Chlorinated Solvents in Low foc Sediments
In this study, as part of the remedial investigation of a former dry cleaning facility in New York State, site-specific perchloroethene (PCE) sorption behavior was measured and used to develop target soil cleanup levels. Batch equilibrium sorption of PCE for site-representative low carbon subsurface sediment samples was measured at different concentrations such that isotherms could be developed. In contrast, most site investigations employ the empirical organic carbon-water partition coefficient (Koc) to predict the distribution of a solute between organic carbon and water. The empirical Koc is estimated by available relations to the compounds octanol-water partition coefficient or aqueous solubility. The Koc parameter so determined assumes a partitioning process only, i.e., the variability in structure and composition of the organic matter has no effect on it. Our results indicate that the experimentally observed Koc (=Kd/foc; where Kd and foc are the observed sorption distribution coefficient at a particular aqueous concentration and organic carbon content, respectively) strongly depended upon the aqueous concentration and attributes of the subsurface material. At lower concentrations, i.e., near the drinking water standard of 5 μg/L, the observed Koc was as much as 100-times greater than the empirical Koc, whereas the empirical Koc value was reasonable at aqueous concentrations approaching the compound solubility (at 7% S, i.e., 10000 μg/L in this study). In this study, the use of site-specific observed PCE Koc values allowed a target soil cleanup level seven-fold higher in concentration and resulted in a 65% reduction in the area of the remediation zone compared to the empirical Koc. Numerous research studies have observed elevated experimental (termed 'excess') compared to empirical Koc values for low polarity organic compounds and especially so at low aqueous concentrations. This phenomenon has been explained by the extra significant surface adsorption contribution of thermally altered carbonaceous matter (TACM) that is most evident at aqueous concentrations <=∼0.01 to 0.1 of the solubility. Indeed, TACM was identified in sediment from this study site. The circumstance of a persistent solvent plume in a low foc sedimentary aquifer is the environment in which excess sorption is most likely. It is clear that site-specific determinations are preferable to the empirical Koc approach at sites in which the TACM exerts significant impact on compound behavior. Based on the results of this study and other reports in the literature, similar circumstances may prevail at other field sites with chlorinated solvents.