Grantee Research Project Results
Final Report: Dissolved Humic Substances in Enhanced Dissolution of DNAPLs
EPA Grant Number: R826650Title: Dissolved Humic Substances in Enhanced Dissolution of DNAPLs
Investigators: Johnson, William P.
Institution: University of Utah
EPA Project Officer: Aja, Hayley
Project Period: September 1, 1998 through August 31, 2001
Project Amount: $285,595
RFA: Exploratory Research - Environmental Chemistry (1998) RFA Text | Recipients Lists
Research Category: Sustainable and Healthy Communities , Land and Waste Management , Air , Safer Chemicals
Objective:
The overall objective of this research project was to determine whether natural dissolved humic substances (DHS) offered advantages over commercial surfactants in specific remediation contexts. The specific objectives were to: (1) determine whether the absence of a critical micelle concentration for terrestrial DHS presents an advantage relative to commercial surfactants, deriving their ability to solubilize compounds irrespective of a critical minimum concentration (CMC); (2) determine the extent to which sorption behaviors of terrestrial DHS and surfactants on representative subsurface porous media differ, and determine which agent is more advantageous in terms of sorption losses to various subsurface media; (3) determine whether the kinetics of dense non-aqueous phase liquid (DNAPL) solubilization by terrestrial DHS versus surfactants with similar solubilization capacities differ significantly, in column experiments examining solubilization of DNAPL residual; and (4) characterize the equilibrium sorption isotherms for representative terrestrial DHS and commercial surfactants to representative porous media and DNAPL.
Summary/Accomplishments (Outputs/Outcomes):
Non-Ionic Surfactant Transport and Solubilization of Perchloroethylene (PCE). The work performed compared the fractionation of aldrich humic acid (AHA) and a nonionic surfactant mixture during transport through sediment contaminated with DNAPL. The bulk nonionic surfactant mixture was known to have a factor of four greater solubilizing capacity relative to bulk AHA for the DNAPL (tetrachloroethene). It was hypothesized that preferential sorption of components of the nonionic surfactant and AHA mixtures would alter their respective solubilization capacities for the DNAPL, possibly mitigating the greater solubilizing capacity of the surfactant.
At aqueous surfactant concentrations far below the CMC of the mixture, surfactant sorption to sediment was characterized by an initial steep isotherm for both high and low ethoxylate (EO) content oligomers, with somewhat greater uptake of high EO content (more polar) oligomers. This stage of sorption resulted in mild to negligible increases in the equilibrium constant, Kd,PCE, for distribution of PCE between solution (including surfactant) and sediment (including sorbed surfactant). As the aqueous surfactant concentration increased, surface aggregation of low EO content oligomers on the sediment commenced, with gradual incorporation of increasingly polar oligomers as the solution surfactant concentration increased still further. During surface aggregation, a dramatic increase in Kd,PCE was observed. At aqueous surfactant concentrations above the CMC, the formation of solution micelles caused the sorbed surfactant concentrations to plateau, starting with low EO content oligomers and progressing to high EO content oligomers, as was observed for surface aggregation. At the highest aqueous surfactant concentrations examined, surfactant sorption decreased. This decrease in sorbed surfactant, along with competition by micelles for contaminant, likely contributed to an observed rapid decrease in Kd,PCE toward zero. Surfactant sorption to PCE DNAPL was greater relative to sediment by one and two orders of magnitude for high and low EO content oligomers, respectively. Sorption of the lowest EO content oligomers to the PCE DNAPL was not terminated by micellization.
Comparison of fractionation of a nonionic surfactant mixture and AHA in low natural organic carbon content aquifer sediment was performed using high-pressure liquid chromatography (HPLC) and high-pressure size exclusion analysis (HPSEA) of surfactant and humic concentrations, respectively. The comparison was performed in batch experiments and a series of three sediment columns that represented the upgradient, residual-zone, and downgradient portions of a DNAPL-contaminated site. The flow was split between each column to analyze for surfactant/humic and DNAPL compounds, allowing monitoring of changes in the makeup of the humic and surfactant mixtures during transport.
In the upgradient column, greater retardation of high EO content (more polar) oligomers relative to low EO content oligomers was observed. In the residual-zone column, much greater sorption of low EO content oligomers relative to high EO content oligomers occurred. In the downgradient column, retardation of only the high EO content oligomers was observed. Surfactant losses to sediment and DNAPL in the upgradient and residual-zone columns delayed the solubilization of DNAPL by two system pore volumes. Sorption of high EO oligomers to sediment in the downgradient column increased the delay of solubilizate breakthrough to more than two system pore volumes. Surfactant losses from the aqueous phase increased the polarity of the aqueous mixture and decreased its solubilizing capacity for PCE. Increased solution flow rate decreased surfactant sorption, but resulted in an overall decrease in the mass of DNAPL solubilized due to kinetic limitations in DNAPL solubilization.
DHS Transport and Solubilization of PCE. Transport of DHS and DHS-solubilization of DNAPL PCE was examined using high-performance size exclusion chromatography (HPSEC), to allow tracking of the various molecular weight (MW) components of the humic acid mixture. The crux of developing this technique was to determine the relationship between molar absorptivity ultra-violet (UV) absorbance per mass of carbon and MW to convert the observed UV absorbance response for each component of the mixture to a carbon mass. The molar absorptivity was observed to increase with MW for MWs up to around 10K Daltons. Beyond this MW, the response factor was shown to decrease. To our knowledge, this is the first study to describe the response factor for various MW fractions of a soil humic acid. Batch and triple-column experiments examining AHA sorption to sediment and AHA solubilization of residual PCE were performed. It was observed that equilibration of the humic acid with sediment shifted the apparent MW of some fractions of the mixture, and that this resulted from increases in the molar absorptivity of some MW fractions of AHA in response to complexation of calcium or other cations originally associated with the sediment. Cyclic (temporary) alterations in molar absorptivity of AHA were observed during column breakthrough.
Approximate correction for the shifts in molar absorptivity during interaction with sediment allowed the fractionation of AHA during transport in DNAPL-contaminated sediment to be assessed and compared to the nonionic surfactant. The comparison showed that although nonionic surfactants and humic acids are highly distributed mixtures of molecules of varying MW and polarities, the differences between MW fractions with respect to adsorption in the surfactant mixture were much greater than those in the AHA mixture. Much greater fractionation of the surfactant mixture was observed relative to AHA during transport in DNAPL-contaminated sediment. Fractionation was shown to compromise the solubilizing capacity of the surfactant mixture to the extent that the efficiency of DNAPL removal at low numbers of pore volumes was similar to AHA, despite the nominal factor of four greater solubilizing capacity of the surfactant relative to AHA for the DNAPL.
Transport of Anionic Surfactant-Trichloroethylene (TCE) Mixture. A series of column experiments was performed to examine microemulsion transport and stability during transport through quartz sand, and to determine the feasibility of applying surfactant enhanced aquifer remediation (SEAR) to lesser-constrained sites. This investigation involved injecting a stable microemulsion into a column packed with medium quartz sand and monitoring the breakthrough of tracer, surfactant, and contaminant. The experiments in this investigation were modeled after a recent SEAR flush conducted at Hill Air Force Base, in which the trichloroethylene (TCE)-contaminated aquifer was preconditioned with a 1 percent NaCl flush before injecting an anionic surfactant microemulsion containing 4 weight percent Aerosol MA-80I and 1 percent NaCl. The tritium tracer test showed symmetrical breakthrough-elution and little dispersion, indicating an absence of physical nonequilibrium in the system lacking microemulsion. In contrast, the surfactant-TCE microemulsion experiments showed significant tailing of tritiated water during elution and loss of TCE from the microemulsion during transport. The non-aqueous phase retained in the column impeded flow, thereby producing the observed physical non-equilibrium. Surfactant dilution during transport of the microemulsion caused a portion of the NAPL contaminants to drop from the microemulsion. This study reinforced the need for the present protocol of implementing SEAR in highly controlled, well-constrained systems.
Solubilization of Coal Tar Mixtures by MW Fractions of DHS. Experiments examined fractionation effects on DHS solubilization of coal tar mixtures using the methodologies described earlier in this report, but with addition of fluorescence detection for some of the PAH compounds. A coal tar mixture was developed, and refinement of the HPLC method for analysis of coal tar components was completed. The solubilization of coal tar mixtures by DHS was examined. The coal tar mixtures were developed following Catharine Peters, whose work examined solidification of less-soluble coal tar components in response to dissolution into groundwater of the more soluble components. A synthetic coal tar was equilibrated with DHS solution in batch experiments conducted over a period of 60 days. During the course of the experiments, PAH degradation was indicated in experiments with relatively low mass of coal tar despite the lack of exhaustion of coal tar in all experiments. Experiments also were performed to determine whether increased salt concentrations might enhance the solubilizing capacities of DHS, as occurs for nonionic surfactants. Only very slight increases in enhanced solubility were observed, and only for the most hydrophobic compounds. The presence of salt did appear to affect the timing of onset of apparent degradation.
Journal Articles on this Report : 3 Displayed | Download in RIS Format
Other project views: | All 8 publications | 3 publications in selected types | All 3 journal articles |
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Bao G, John WW, Johnson WP. Chromatographic alteration of a nonionic surfactant mixture during transport in dense nonaqueous phase liquid contaminated sediment. Environmental Science & Technology 2000;34(4):680-685. |
R826650 (2000) R826650 (Final) |
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John WW, Bao G, Johnson WP, Stauffer TB. Sorption of nonionic surfactant oligomers to sediment and PCE DNAPL: effects on PCE distribution between water and sediment. Environmental Science & Technology 2000;34(4):672-679. |
R826650 (2000) R826650 (Final) |
Exit |
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Johnson WP, Bao G, John WW. Specific UV absorbance of Aldrich humic acid: changes during transport in aquifer sediment. Environmental Science & Technology 2002;36(4):608-616. |
R826650 (2000) R826650 (Final) |
Exit |
Supplemental Keywords:
remediation, groundwater, chromatography, dissolution., RFA, Scientific Discipline, Air, Toxics, Waste, Remediation, Environmental Chemistry, Chemistry, HAPS, chemical mixtures, Groundwater remediation, Engineering, Chemistry, & Physics, fate and transport, DNAPL, hydrocarbon, exposure, chemical composition, chemical transport modeling, PAH, Cresols/Cresylic acid (isomers and mixture), chemical kinetics, groundwater contamination, surfactants, creosoteProgress and Final Reports:
Original AbstractThe perspectives, information and conclusions conveyed in research project abstracts, progress reports, final reports, journal abstracts and journal publications convey the viewpoints of the principal investigator and may not represent the views and policies of ORD and EPA. Conclusions drawn by the principal investigators have not been reviewed by the Agency.