Progress 12/01/00 to 11/30/03
Outputs
A three-dimensional molecular model was constructed based on elemental and group composition of humic acid (HA) and fulvic acid (FA). Molecular dynamics (MD) simulations at constant pressure were used to obtain equilibrium configurations and to estimate flexibilities of different model HA fragments for use in large-scale lattice Monte-Carlo simulations. To predict dissolution of organic compounds we employed UNIFAC-FV (free volume) group-contribution model. Calculated partition coefficients of typical contaminants in dry and hydrated model HA compared favorably with their octanol-water partition coefficients. Measured partition coefficients of probes (cyclohexane, benzene, methyl benzoate, acetophenone, and anisole) in Leonardite HA and HA extracted from a peaty soil are compared with calculated values. To study mechanisms of hole-filling, melting and glass transition of HA by Monte Carlo simulations, a coarse-grained lattice model was constructed based on the MD
simulation results. Sorption of carbon dioxide was studied on soils, humin and HA fractions. Carbon dioxide sorption was strongly hysteretic, especially for soils with a high organic matter content. Sorption could be described as a two-stage process characterized by different short-time and long-time rates. The first stage (hours to several days) is controlled by diffusion through rubbery regions of SOM and adsorption in (desorption from) accessible micropores in glassy regions of SOM. A Diffusion-Controlled Hysteresis (DCH) model is introduced to extract equilibrium isotherms and diffusion coefficients from nonequilibrium hysteretic measurements. The shapes of the equilibrium isotherms unambiguously demonstrate the microporous nature of SOM. Second stage uptake or release scales linearly with the square root of time, and the characteristic time scale is much larger (months). Using the DCH model, we show that the long-time kinetics is correlated with the increasing sorption of CO2 in
micropores. We hypothesize that the second stage is caused by a slow diffusion into remote structural domains, which become accessible via structural swelling and/or restructuring of the solid matrix. These domains are thought to be responsible for irreversible sorption and sequestration of organic molecules. Conditioning effect experiments are used to investigate history-dependent sorption behavior of organic solutes in natural organic matter solids. The conditioning effect was observed in a soil humic acid in the hydrogen ion form and in the same humic acid in the aluminum-ion exchanged form. The conditioning effect is consistent with the presence of deformable pores. The conditioning effect for 1,2,4-trichlorobenzene in a conditioned peat soil decayed with sample heating. The decay was biphasic and the rate constants increased exponentially with temperature. The results are tentatively interpreted in terms of a two-compartment (elastic and viscoelastic) model of matrix
relaxation.
Impacts
The objective of this project is to use molecular models and physical measurements to characterize sorption of hydrophobic organic contaminants (HOC) in soils. The short and long term hysteresis of CO2 is highly significant in that it suggests that sorption to soil organic matter is subject to continued slow changes over months even for small gaseous molecules. The hysteresis observed for CO2 and the conditioning effect observed for trichlorobenzene signifies the existence of deformable pores in humic substances and lends support to the glassy polymer paradigm for humic substances. The DCH model of sorption kinetics and corresponding experimental protocols constitute a basis for characterization of microporosity in soils. The theoretical work suggests UNIFAC-FV may be useful as predictive tool for sorption.
Publications
- Ravikovitch, P.I., A. V. Neimark, W.J. Braida, and J.J. Pignatello, Sorption of Carbon Dioxide in Soils and Soil Organic Matter Fractions. Diffusion Controlled Hysteresis Model, Pore Structure Characterization, and Long Time Kinetics., Environ. Sci. Technol., 2004, under review.
- Lu,Y. and J.J. Pignatello, Sorption of Apolar Aromatic Compounds to Soil Humic Acid Particles Affected by Polyvalent Metal Ion Crosslinking, J. Environ. Qual., 2004, in press.
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Progress 01/01/02 to 12/31/02
Outputs
In 2002, our research aimed at the relationship between the molecular structure and composition of soil organic matter and its sorption properties. We followed the dual-mode model of sorption to SOM based on a glassy polymer model. In this model, sorption occurs by concurrent dissolution and hole-filling mechanisms. Dissolution of a probe sorbate (CO2) and hydrophobic organic contaminants (trichloromethane, trichloroethylene, dichlorobenzene and trichlorobenzene) in humic substances were studied by the UNIFAC group contribution model. Using the UNIFAC approach, we calculated partitioning coefficients of the hydrophobic organic contaminants in several samples of soil organic matter. The influence of the stage of metamorphism of soil organic matter and the water content on the partitioning coefficients of aromatic and non-aromatic contaminants has been established. The resulting partitioning coefficients of the contaminants in several samples of soil organic matter were
employed in modeling of the sorption-desorption hysteresis of hydrophobic organic contaminants using the diffusion-controlled hysteresis model. Sorption hysteresis of CO2 was studied experimentally with example of Pahokee Peat soil. We found short-term and long-term kinetics of sorption, which are attributed to different sorption mechanisms. The observations were explained with the diffusion-controlled hysteresis model. The conditioning effect was observed in a soil humic acid in the hydrogen ion form and in the same humic acid in the aluminum-ion exchanged form. In this experiment, naphthalene and 1,2,4-trichlorobenzene were the test compounds and chlorobenzene was the conditioning agent. The conditioning effect is consistent with the presence of deformable pores in these materials.
Impacts
The conditioning effect signifies the existence of deformable pores in humic substances and lends support to the glassy polymer paradigm for humic substances. This introduces a new level of understanding of the sorption process that, because of the importance of sorption in the fate and bioavailability of chemicals in soil, is likely to have important implications for future work.
Publications
- Y. Lu and J.J. Pignatello. Sorption hysteresis of organic compounds in soils: Demonstration of the conditioning effect in soil organic matter. Environ. Sci. Technol. 2002, 36, 4553-4561.
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Progress 01/01/01 to 12/31/01
Outputs
In 2001, our research aimed at the relationship between molecular structure and composition of soil organic matter (SOM) and its sorbent properties. We followed the dual-mode model of sorption to SOM, where sorption occurs by concurrent dissolution and hole-filling mechanisms. Dissolution of probe sorbate CO2 and hydrophobic organic contaminants (trichloromethane, trichloroethylene, dichlobenil, and chloranil) in humic substances is studied by UNIFAC group contribution model. The UNIFAC method is based on the concept that a mixture may be considered a solution of structural units from which the molecules are formed rather than a solution of the molecules themselves. The parameters for individual subgroups are obtained by fitting to experimental data on vapor-liquid and liquid-liquid equilibria in relatively simple systems such as individual compounds and simple mixtures which contain particular groups. Three-dimensional models of different fractions of SOM (humin,
humic acid, fulvic acid) have been constructed according to experimental data on elementary and functional composition of SOM. The molecular-level models of SOM will be used for group contribution modeling and the following molecular dynamics simulations of sorption and diffusion of CO2 and organic contaminants in SOM. The software for UNIFAC modeling has been developed at TRI/Princeton. The computer program has been tested against experimental data on phase equilibrium in styrene-polystyrene, hexane-polybutadiene systems. Glassy polymers exhibit a type of hysteresis known as the "conditioning effect". The conditioning effect refers to the observed enhancement in sorption of a compound following brief exposure of the sorbent to high concentrations of the same or a similar compound. It is thought to be caused by the irreversible enlargement of existing holes or creation of new holes by the conditioning chemical. We have now demonstrated the conditioning effect for trichloromethane
(TCM) in IHSS soil using dichloromethane (DCM) as the conditioning agent. Conditioning was performed by treating the soil with a nearly-saturated aqueous solution of a DCM for 6 - 7 days followed by N2 sparging of the DCM. A non-conditioned sample was treated identically except for the absence of DCM. Adsorption isotherms of TCM were constructed over a concentration range of 4 powers of ten. The TCM isotherm in the conditioned soil was clearly shifted upward and its nonlinearity was increased compared to the non-conditioned soils. The ratio of the apparent soil-water distribution ratio (D) for conditioned and non-conditioned soils reached a maximum (1.7) at very low TCM concentration and tended to approach unity as TCM concentration approached its water solubility. Aging studies showed that the memory of the conditioning effect persists practically undiminished for at least three months, and disappears when the samples are heated to 90 oC overnight. The results suggest that the
relaxation of the glassy matrix upon desorption is very slow, and may be rate-determining to desorption. The results also provide support for the glassy-polymer hypothesis of SOM.
Impacts
Sorption to soil organic matter (SOM) is fundamental to the bioactivity, leaching potential, and bioavailability of agrichemicals. This study will help assign mechanism of pesticide interaction, lead to breakthroughs in molecular modeling, and promote wider application of theoretical physics to environmental problems.
Publications
- No publications reported this period
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