Grantee Research Project Results
1998 Progress Report: The Influence of Nanoporosity in Soils from Contaminated Sites on Hydrocarbon Desorption Kinetics and Bioavailability
EPA Grant Number: R825959Title: The Influence of Nanoporosity in Soils from Contaminated Sites on Hydrocarbon Desorption Kinetics and Bioavailability
Investigators: Pignatello, Joseph J. , Neimark, Alexander V.
Institution: Connecticut Agricultural Experiment Station , Princeton University
Current Institution: Connecticut Agricultural Experiment Station , Tri / Princeton
EPA Project Officer: Aja, Hayley
Project Period: January 1, 1998 through December 31, 2000
Project Period Covered by this Report: January 1, 1998 through December 31, 1999
Project Amount: $436,399
RFA: EPA/DOE/NSF/ONR - Joint Program On Bioremediation (1997) RFA Text | Recipients Lists
Research Category: Hazardous Waste/Remediation , Land and Waste Management
Objective:
We are testing the hypothesis that resistant sorption/desorption commonly observed in soils and other geological sorbents is related to soil nanoporosity (pores ~1 nm). It further is suggested that nanopores exist in soil organic matter by virtue of its glassy character, as well as possibly in fixed pores of the inorganic matrix. The objectives of this project are to develop a methodology to characterize the nanoporosity of a number of soils and aquifer sediments of current interest, and then to determine the relationship, if any, between nanoporosity and certain physical-chemical and biological availability parameters for mono- and polycyclic aromatic hydrocarbon contaminants. The latter objective includes determining the influence of soil alteration and sorption competitive effects on bioavailability.Progress Summary:
Studies in the first year focused on: (1) developing protocol; and molecular-level theoretical models for assessing nanoporosity in soil specimens using experimental gas adsorption isotherms and molecular models; and (2) correlating desorption rates and bioavailability for polycyclic aromatic hydrocarbons, including assessing the effects of competitive sorption on sorption equilibria, sorption/desorption rates, and bioavailability.
1. Characterizing Nanoporosity. We have developed a new experimental protocol and molecular level theoretical models for assessing nanoporosity in soil specimens. The method is based on measurements of the CO2 adsorption isotherms at 0?C. We are interested in studies of micropores of molecular scales, 0.2-2 nm. The micropore size distributions are calculated from comparison of the experimental adsorption isotherms and the theoretical isotherms in model pores predicted by means of the density functional theory (DFT) model and grand canonical Monte Carlo (GCMC) simulations. The DFT model, theoretical foundations of which were formulated in our earlier work, has been modified by taking into account specifics of CO2 interaction with carbonaceous matrixes. A new set of intermolecular parameters for CO2/solid potentials was defined and verified against literature experimental data. The MC simulation model is under development. Preliminary results demonstrate a consistency of the results of DFT and MC models.
We have designed a new electric thermostat and fabricated its working prototype, which allows us to maintain 0?C in the adsorption cell with precision of 0.1 . This thermostat is capable of maintaining desired environment temperature in the adsorption cell in the range 20 to +40?C.
The CO2 adsorption isotherms at 0?C and N2 adsorption isotherms at 77K on selected samples of soils and reference sorbents were measured and the pore size distributions were constructed. Main attention was paid to the samples containing soil organic matter (SOM). The CO2 adsorption-desorption isotherms demonstrated hysteresis that presumably indicates irreversibility of sorption in nanapores of SOM. We have studied samples of peat and its fractions such as purified humin and humic acid. We observed an extreme hysteresis in CO2 sorption on these samples. CO2 desorption was hindered until the vapor pressure was reduced tenfold. The adsorption isotherm on peat and humic acid revealed a bend which was not observed in humin. This characteristic behavior of sorption isotherms probably indicates a phase transition in the humic acid fraction that occurs in the process of CO2 sorption. The hysteretic phenomenon requires further study.
The pore size distribution for the soil samples studied were constructed. We hypothesize that the nanopores of 0.3?1 nm revealed in these distributions may be responsible for a delayed desorption kinetics of organic contaminants sequestered in pores of soil particles. The results obtained justify that CO2 is a suitable molecular probe to study structural and sorption-desorption properties of soil particles, including SOM-containing samples.
2. Correlating Desorption and Bioavailability. A series of parallel desorption and bioavailability experiments were carried out on phenanthrene in Mount Pleasant silt loam or Pahokee peat soil. For both soils, the desorption rate in the presence of Tenax infinite sink, as well as the rate and extent of mineralization by two phenanthrene degraders, declined with the aging period. The aging period is defined as the contact time between phenanthrene and the sterile soil before desorption or biodegradation had commenced. For each soil, plots of normalized rates of mineralization or desorption, or fraction mineralized or desorbed during an arbitrary period, as a function of the aging period essentially coincided. This indicates for these organisms that biodegradation is rate-limited by desorption and that desorption to Tenax serves as an adequate predictor of the rate of biodegradation. The partial removal of organic matter from the peat by extraction with dilute base to leave the humin fraction reduced both the biodegradation rate and the desorption rate of phenanthrene as compared with the nonextracted peat. This is consistent with the expected greater glassy character of the humin fraction.
Other experiments were conducted to determine cosolute competitive effects on the biological and physical availability of PAHs using phenanthrene as the principal solute and pyrene as the cosolute. Sterile suspensions of Mount Pleasant silt loam and Pahokee peat soils were spiked with phenanthrene and allowed to age for 3 to 123 days before inoculation with a phenanthrene-degrading bacterium in the presence or absence of pyrene, which was not biodegradable by this organism. Mineralization decreased with aging in the samples not amended with pyrene, as expected. However, addition of pyrene just before inoculation at 123 d significantly mitigated this decrease (i.e., the extent of mineralization was greater in the 123-d pyrene-amended samples than in the 123-d nonamended samples). Parallel experiments on sterile soils showed that pyrene increased the physical availability of phenanthrene by competitive displacement of phenanthrene from sorption sites. Further experiments (described below) indicated that pyrene exerted a competitive effect on both magnitude and the rates of phenanthrene sorption.
These experiments were conducted on Pahokee peat and Cheshire fine sandy loam. The dual-mode model predicts that a competing solute will (a) suppress sorption; and (b) accelerate the rates of sorption and desorption of the primary solute by blocking nanopore sites, thus facilitating diffusion of the principal solute through SOM. Two and 33-day isotherms of phenanthrene were nonlinear, indicating a heterogeneous distribution of sites. Pyrene suppressed phenanthrene sorption (by as much as 80 percent in the apparent Koc under some conditions) and also increased the linearity of its isotherm significantly. This is expected if pyrene were blocking phenanthrene sites in the adsorption domain, while having no effect on the dissolution domain.
Kinetic experiments were carried out by preloading the sorbent with pyrene at
various concentrations 14 days prior to addition of phenanthrene. Phenanthrene
uptake normalized to final uptake in short- and long-term experiments was
slightly faster in the pyrene-containing soils and in proportion to pyrene
concentration. Desorption was carried out by successive supernatant replacement
experiments at constant pyrene concentration (i.e., by using as the dilution
water, water from dummy flasks that had been in contact with pyrene-contaminated
soil for the same length of time in the absence of phenanthrene). In all cases,
the rate of phenanthrene desorption was greater in the presence of pyrene and
increased in proportion to pyrene concentration.
Studies in the first year focused on: (1) developing protocol; and molecular-level theoretical models for assessing nanoporosity in soil specimens using experimental gas adsorption isotherms and molecular models; and (2) correlating desorption rates and bioavailability for polycyclic aromatic hydrocarbons, including assessing the effects of competitive sorption on sorption equilibria, sorption/desorption rates, and bioavailability.
1. Characterizing Nanoporosity. We have developed a new experimental protocol and molecular level theoretical models for assessing nanoporosity in soil specimens. The method is based on measurements of the CO2 adsorption isotherms at 0?C. We are interested in studies of micropores of molecular scales, 0.2-2 nm. The micropore size distributions are calculated from comparison of the experimental adsorption isotherms and the theoretical isotherms in model pores predicted by means of the density functional theory (DFT) model and grand canonical Monte Carlo (GCMC) simulations. The DFT model, theoretical foundations of which were formulated in our earlier work, has been modified by taking into account specifics of CO2 interaction with carbonaceous matrixes. A new set of intermolecular parameters for CO2/solid potentials was defined and verified against literature experimental data. The MC simulation model is under development. Preliminary results demonstrate a consistency of the results of DFT and MC models.
We have designed a new electric thermostat and fabricated its working prototype, which allows us to maintain 0?C in the adsorption cell with precision of 0.1 . This thermostat is capable of maintaining desired environment temperature in the adsorption cell in the range 20 to +40?C.
The CO2 adsorption isotherms at 0?C and N2 adsorption isotherms at 77K on selected samples of soils and reference sorbents were measured and the pore size distributions were constructed. Main attention was paid to the samples containing soil organic matter (SOM). The CO2 adsorption-desorption isotherms demonstrated hysteresis that presumably indicates irreversibility of sorption in nanapores of SOM. We have studied samples of peat and its fractions such as purified humin and humic acid. We observed an extreme hysteresis in CO2 sorption on these samples. CO2 desorption was hindered until the vapor pressure was reduced tenfold. The adsorption isotherm on peat and humic acid revealed a bend which was not observed in humin. This characteristic behavior of sorption isotherms probably indicates a phase transition in the humic acid fraction that occurs in the process of CO2 sorption. The hysteretic phenomenon requires further study.
The pore size distribution for the soil samples studied were constructed. We hypothesize that the nanopores of 0.3?1 nm revealed in these distributions may be responsible for a delayed desorption kinetics of organic contaminants sequestered in pores of soil particles. The results obtained justify that CO2 is a suitable molecular probe to study structural and sorption-desorption properties of soil particles, including SOM-containing samples.
2. Correlating Desorption and Bioavailability. A series of parallel desorption and bioavailability experiments were carried out on phenanthrene in Mount Pleasant silt loam or Pahokee peat soil. For both soils, the desorption rate in the presence of Tenax infinite sink, as well as the rate and extent of mineralization by two phenanthrene degraders, declined with the aging period. The aging period is defined as the contact time between phenanthrene and the sterile soil before desorption or biodegradation had commenced. For each soil, plots of normalized rates of mineralization or desorption, or fraction mineralized or desorbed during an arbitrary period, as a function of the aging period essentially coincided. This indicates for these organisms that biodegradation is rate-limited by desorption and that desorption to Tenax serves as an adequate predictor of the rate of biodegradation. The partial removal of organic matter from the peat by extraction with dilute base to leave the humin fraction reduced both the biodegradation rate and the desorption rate of phenanthrene as compared with the nonextracted peat. This is consistent with the expected greater glassy character of the humin fraction.
Other experiments were conducted to determine cosolute competitive effects on the biological and physical availability of PAHs using phenanthrene as the principal solute and pyrene as the cosolute. Sterile suspensions of Mount Pleasant silt loam and Pahokee peat soils were spiked with phenanthrene and allowed to age for 3 to 123 days before inoculation with a phenanthrene-degrading bacterium in the presence or absence of pyrene, which was not biodegradable by this organism. Mineralization decreased with aging in the samples not amended with pyrene, as expected. However, addition of pyrene just before inoculation at 123 d significantly mitigated this decrease (i.e., the extent of mineralization was greater in the 123-d pyrene-amended samples than in the 123-d nonamended samples). Parallel experiments on sterile soils showed that pyrene increased the physical availability of phenanthrene by competitive displacement of phenanthrene from sorption sites. Further experiments (described below) indicated that pyrene exerted a competitive effect on both magnitude and the rates of phenanthrene sorption.
These experiments were conducted on Pahokee peat and Cheshire fine sandy loam. The dual-mode model predicts that a competing solute will (a) suppress sorption; and (b) accelerate the rates of sorption and desorption of the primary solute by blocking nanopore sites, thus facilitating diffusion of the principal solute through SOM. Two and 33-day isotherms of phenanthrene were nonlinear, indicating a heterogeneous distribution of sites. Pyrene suppressed phenanthrene sorption (by as much as 80 percent in the apparent Koc under some conditions) and also increased the linearity of its isotherm significantly. This is expected if pyrene were blocking phenanthrene sites in the adsorption domain, while having no effect on the dissolution domain.
Kinetic experiments were carried out by preloading the sorbent with pyrene at
various concentrations 14 days prior to addition of phenanthrene. Phenanthrene
uptake normalized to final uptake in short- and long-term experiments was
slightly faster in the pyrene-containing soils and in proportion to pyrene
concentration. Desorption was carried out by successive supernatant replacement
experiments at constant pyrene concentration (i.e., by using as the dilution
water, water from dummy flasks that had been in contact with pyrene-contaminated
soil for the same length of time in the absence of phenanthrene). In all cases,
the rate of phenanthrene desorption was greater in the presence of pyrene and
increased in proportion to pyrene concentration.
Future Activities:
Work on this research project will continue; the results will be published in scientific journals and presented at relevant scientific conferences.Journal Articles on this Report : 4 Displayed | Download in RIS Format
Other project views: | All 61 publications | 19 publications in selected types | All 19 journal articles |
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Type | Citation | ||
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Vishnyakov A, Ravikovitch PI, Neimark, AV. Molecular level models for CO2 sorption in nanopores. Langmuir 1999;15:8736-8742. |
R825959 (1998) R825959 (1999) R825959 (Final) |
Exit Exit |
|
White JC, Hunter M, Pignatello JJ, Nam KP, Alexander M. Correlation between the biological and physical availabilities of phenanthrene in soils and soil humin in aging experiments. Environmental Toxicology Chemistry 1999;18(8):1720-1727. |
R825959 (1998) R825959 (1999) R825959 (Final) |
not available |
|
White JC, Hunter M, Pignatello JJ, Alexander M. Increase in the bioavailability of aged phenanthrene in soils by competitive displacement with pyrene. Environmental Toxicology and Chemistry 1999;18(8):1728-1732. |
R825959 (1998) R825959 (1999) R825959 (Final) |
not available |
|
White JC, Pignatello JJ. Influence of bisolute competition on the desorption kinetics of polycyclic aromatic hydrocarbons in soil. Environmental Science & Technology 1999;33(23):4294-4298. |
R825959 (1998) R825959 (1999) R825959 (Final) |
not available |
Supplemental Keywords:
soil organic matter, polycyclic aromatic hydrocarbons, slow desorption, density functional theory, Grand Canonical Monte Carlo, molecular models, dual-mode sorption., Scientific Discipline, Waste, Water, Ecosystem Protection/Environmental Exposure & Risk, Bioavailability, Environmental Chemistry, Geochemistry, Contaminated Sediments, Chemistry, Environmental Microbiology, Microbiology, Bioremediation, Environmental Engineering, Geology, sorption, Toluene, contaminated sites, density functional theory, contaminated sediment, aquifer sediments, sorption kinetics, PAH, soil characterization, bioremediation of soils, sediments, nonoporosity, hydrocarbon desorption kinetics, PhenanthreneProgress 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.