Grantee Research Project Results
1999 Progress Report: A Comprehensive Investigation of the Effects of Organic Geochemistry on the Sorption-Desorption, Sequestration, and Bioavailability of Mixed Organic Contaminants in Subsurface Systems
EPA Grant Number: R825962Title: A Comprehensive Investigation of the Effects of Organic Geochemistry on the Sorption-Desorption, Sequestration, and Bioavailability of Mixed Organic Contaminants in Subsurface Systems
Investigators: Weber, Walter J.
Institution: University of Michigan
EPA Project Officer: Aja, Hayley
Project Period: January 1, 1998 through December 31, 2000
Project Period Covered by this Report: January 1, 1999 through December 31, 2000
Project Amount: $440,748
RFA: EPA/DOE/NSF/ONR - Joint Program On Bioremediation (1997) RFA Text | Recipients Lists
Research Category: Hazardous Waste/Remediation , Land and Waste Management
Objective:
The objective of this research project is to: (1) obtain an understanding of mechanisms that cause slow desorption and contaminant sequestration, and (2) assess the implications of sorption-desorption hysteresis on contaminant mobility and bioavailability. This objective is being approached through systematic studies designed to determine, for multi-contaminant systems, the effects of soil organic matter (SOM) on:
- Sorption and desorption hysteresis of a probe contaminant in the presence of other organic compounds;
- Hydrophobic organic chemical (HOC) sorption-desorption rates, hysteresis, and sequestration;
- The subcritical water extractability of sorbed HOCs from soils;
- The bioavailability of sorbed HOCs and the relationship of their subcritical water extractability to bioavailability; and
- The effect of cumulative solute residence time on the sorbent (so-called "aging"), on each of the above issues.
Progress Summary:
We have characterized a number of different types of soils in our laboratories and have used these to evaluate relationships between SOM and sorption-desorption rates and hysteresis. We also have examined relationships between the bioavailability, sequestration, and transformation of sorbed phenanthrene and the SOM of the geosorbent. Finally, the subcritical water extractability of sorbed phenanthrene and its relationship to bioavailability have been studied. The results of these three areas of experimental work are presented below.Sorption and desorption equilibria, rates of desorption, and rates of biological mineralization of phenanthrene with respect to three different types of geosorbents were measured. The chemical nature of the organic matter associated with each geosorbent was characterized using solid state 13C-NMR spectrometry.
The effects of a chlorinated solvent and chlorinated benzene on the sorption and desorption behavior of phenanthrene were investigated. Trichloroethylene (TCE) and dichlorobenzene (DCB) were chosen as co-solutes for sorption and desorption experiments. Further experiments were conducted to investigate how sorbed polycyclic aromatic hydrocarbons (PAHs) could be displaced by other compounds. The results show that sorption isotherms for phenanthrene in the presence of TCE and DCB are more linear than the sorption isotherms for phenanthrene only. This is interpreted to be the result of swelling phenomena that increase the capacity of the SOM for the primary solute, phenanthrene. Competition was observed for phenanthrene sorption in the presence of DCB at low phenanthrene concentrations. In desorption experiments, some phenanthrene was entrapped at equilibrium, suggesting that abrupt collapses of the SOM cause phenanthrene to be sequestered in soil matrices. The displacement experiments indicate that TCE does not displace sorbed phenanthrene. However, it was observed that phenanthrene is displaced by DCB at low phenanthrene concentrations, and this result is consistent with that of phenanthrene sorption in the presence of DCB.
The bioavailability of phenanthrene sorbed to geosorbents was evaluated as a function of soil organic matter and aging period. The geosorbents were not sterilized prior to the experiments. The samples were prepared so that the equilibrium aqueous phase concentration of total phenanthrene was 800 mg/L. The geosorbents were first equilibrated for 1 month with unlabeled and radiolabeled phenanthrene. They then were aged for either 1 or 3 additional months. After aging, the soil and solution were transferred to slurry reactors and inoculated with Pseudomonas cepacia CRE7, a phenanthrene-degrading microorganism.
The mineralization profiles show that initial degradation rates are much faster for the younger geosorbents (Michigan peat and Chelsea soil) than for the older geosorbent (Lachine shale). For the younger geosorbents, mineralization was observed to slow after the initial degradation period; mineralization in the older geosorbent-water system continued at a nearly constant rate after the initial degradation period. Abiotic desorption experiments using the infinite-sink method provided similar trends, suggesting that desorption is the rate-limiting step in the system. After completion of the mineralization experiments, both combustion and methanol Soxhlet extraction were used to recover 14C-organics from the geosorbents. The amount of extractable 14C-organic material varied with the degree of diagenetic alteration of the soil: relatively small amounts of 14C-organics were extractable from the younger geosorbents while virtually all 14C-organics were extractable from the older geosorbents. The extraction results imply that biological activity alters the SOM and changes the nature of sequestration and transformation, and that this alteration is more pronounced for geosorbents that are younger, more chemically oxidized, and more biologically active. No significant differences between the two aging periods were found in these experiments.
The results of these studies reveal that desorption behavior, microbial bioavailability, and sequestration and transformation of sorbed contaminants are dependent on the physicochemical character of the geosorbent organic matter. The more reduced and condensed this organic matter, the greater the extent of sorption-desorption hysteresis, the slower the desorption rate, and the less readily bioavailable the sorbed contaminant.
Subcritical water extraction was developed as a technique for rapidly predicting the long-term phenanthrene desorption behavior from contaminated soils or sediments. It is expected that this technique will be invaluable for engineers and scientists planning remediation schemes and/or grappling with difficult alternative remediation endpoint decisions. Liquid-phase temperature programmed desorption (TPD) experiments were conducted to determine the apparent activation energies of phenanthrene desorption from contaminated soils and sediments. Apparent desorption activation energies also were measured by Arrhenius modeling of isothermal desorption rates at temperatures between 25?C and 150?C. The slow release rates of desorption resistant phenanthrene fractions at 25?C, which were measured for up to 455 days, correlated well with apparent desorption activation energies, which ranged from 30 to 80 kJ/mol, depending on sediment type and contamination level. Furthermore, isothermal phenanthrene desorption profiles at elevated temperatures matched those at 25?C, but time scales were reduced by several orders of magnitude. Time scaling factors to match 25?C desorption profiles to desorption at elevated temperatures were readily calculated from apparent activation energies. The current research reveals that superheated water desorption experiments can rapidly predict both rates and extents of long-term phenanthrene desorption. This is a development of great practical value because ambient desorption experiments can take years to measure, while superheated water extractions and TPDs are accomplished in only a few hours.
Future Activities:
Future activities planned for this research project include: (1) investigation of phenanthrene sorption enhancement in the presence of TCE and DCB due to the swelling of soil/sediment matrices; (2) verification of sorption competition between phenanthrene and DCB for high-energy adsorption sites on Lachine shale; (3) investigation of the SOM collapse model for desorption from Chelsea soil in the presence of TCE and DCB; (4) comparison of the microbial degradation kinetics in the presence and absence of geosorbent; (5) commencement of a new set of sorption-desorption and bioavailability experiments using diagenetically altered sediments; (6) development of experiments to determine biodegradation rates as a function of extent of desorption; and (7) development of experiments to determine the sequestration and transformation of phenanthrene metabolites.
We will continue to utilize the subcritical water extraction techniques we have developed to probe the kinetics, energetics, and mechanisms of hydrophobic organic contaminant desorption.
Journal Articles:
No journal articles submitted with this report: View all 13 publications for this projectSupplemental Keywords:
subsurface systems; contaminant transport, natural attenuation, alternative endpoints., RFA, Scientific Discipline, Waste, Water, Ecosystem Protection/Environmental Exposure & Risk, Bioavailability, Contaminated Sediments, exploratory research environmental biology, Environmental Chemistry, Ecosystem/Assessment/Indicators, Chemical Mixtures - Environmental Exposure & Risk, Ecosystem Protection, Health Risk Assessment, Chemistry, chemical mixtures, Ecological Effects - Environmental Exposure & Risk, Bioremediation, Biology, Geology, Environmental Engineering, risk assessment, fate and transport, risk-based decisions, contaminated sediment, chemical transport, subsurface systems, mixed organic contaminants, geochemistry, soil characterization, contaminant release, hydrocarbons, exposure assessment, chlorinated solvents, hydrophobic organic contaminantsProgress 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.