Final Report: The Relationships Among Particle Size, Composition, and Partitioning Phenomena in Aqueous SystemsEPA Grant Number: R825398
Title: The Relationships Among Particle Size, Composition, and Partitioning Phenomena in Aqueous Systems
Investigators: Macalady, Donald L.
Institution: Colorado School of Mines
EPA Project Officer: Hiscock, Michael
Project Period: December 15, 1996 through December 14, 1999 (Extended to December 14, 2000)
Project Amount: $374,300
RFA: Exploratory Research - Water Chemistry and Physics (1996) RFA Text | Recipients Lists
Research Category: Water , Land and Waste Management , Engineering and Environmental Chemistry
Objective:The environmental behavior of contaminants having low water solubility (hydrophobic organic substances, trace metals, and radionuclides) is, in general, controlled by sorption/partitioning to solid phases. The objective of this research project was to provide a sound scientific basis for the prediction of the effects of partitioning reactions on the fate, transport, and exposure potential of anthropogenic chemicals in soil and aquatic systems. Recent investigations have shown that the characteristics of natural organic matter (aromaticity, polarity, etc.) influence the organic carbon normalized partition coefficients (Kocs) of organic contaminants. The key elements in the research were investigations of variations in the composition of natural organic matter (NOM) and concomitant effects on the partitioning of contaminant organic chemicals as a function of particle size. Particle-size separations provided soil fractions that displayed variations in NOM characteristics that could then be correlated to organic contaminant Kocs.
The project focused on extensive chemical and physical characterization of size fractions of soils and correlation of these characteristics to the results of equilibrium sorption experiments. The test compound used was 1,2-dichlorobenzene. One particular aspect of the work was the investigation of the role of soil aggregation in reducing the inherent differences among size fractions for both soil NOM characteristics and the associated Kocs.
Summary/Accomplishments (Outputs/Outcomes):The central theme of all the research was the investigation of the importance of soil/sediment particle size in environmental phenomena. There are many environmental observations that suggest the need for research to investigate particle-size effects. Three of these that were considered in this project are the composition of soil NOM as a function of particle size, the particle-size dependence on the adherence of soil to skin, and size effects in erosional transport of soil-associated contaminants. The first effect, the size dependence of NOM composition, also directly influenced the partitioning of 1,2-dichlorobenzene between soil and water. All of these particle-size effects were observed to be influenced by the degree of soil aggregation.
Wet and dry sieving were the primary methods used to provide the soil-size fractions under investigation. The size fractions that were prepared were: 2000-250, 250-125, 125-63, 63-25, and <25 µm. Two soils of differing total organic carbon content, a clay loam and a silty clay loam, were used in the 1,2-dichlorobenzene partitioning study and the soil adherence experiments.
The organic carbon content of soils has been shown by numerous studies to be the principal factor that controls the partitioning of hydrophobic organic compounds from aqueous solutions. Only more recently has a more subtle effect of the chemical nature of the NOM been observed to influence partitioning. For the clay loam, the percent organic carbon was found to be higher in the largest and smallest fractions than in the intermediate size fractions. This size variation in organic carbon content was even more apparent after dispersing a suspension of the soil aggregates by ultrasonication. A similar, but less obvious, trend was observed for the silty clay loam. The organic carbon present in the >250 µm fractions is believed to consist of detrital plant fragments. The smaller fractions, less than about 63 µm, show an increasing organic carbon content with decreasing size. This suggests a surface area effect consistent with the presumed presence of organic matter coatings on mineral grains.
The functional group content of NOM was investigated using solid-state 13C-CPMAS NMR. Differences in the NMR spectra were observed for different size fractions. The most obvious feature of the spectra was an increase in carbonyl content with decreasing soil particle size. This effect was more pronounced in the ultrasonicated samples. The size distribution of carbonyl content supports the assumption that organic matter coatings are more pronounced as specific surface area increases. The observed carbonyl content also may reflect the binding of organic matter to mineral surfaces through a ligand exchange mechanism. In the case of the non-sonicated samples (water-stable aggregates), the features of the NMR spectra show less variation with particle size. This is due to a "mixing" of the organic matter types by the presence of both organic matter-coated fine particles and larger detrital organic matter components in the aggregates. The presence of fine particles in the aggregates also explains the high surface areas found for the larger particles of the water-stable aggregates. The surface areas of the larger particles are dramatically reduced upon sonication, from approximately 40 m2/g to <5 m2/g for the clay loam. For the silty clay loam, a roughly threefold decrease in specific surface area occurs for the larger particles after sonication.
Partitioning of 1,2-dichlorobenzene to soil-size fractions revealed the importance of NOM content and composition on the sorption of organic contaminants to soil. For the water-stable aggregates of the silty clay loam, where more uniformity in the NOM composition exists between the largest (2000-500 µm) and smallest fractions (25-0.6 µm), similar Kps and Kocs were observed for these two fractions. Values of Kp for the coarse and fine fractions were 6.5 and 6.3, respectively. Values for Koc were 180 and 175, respectively. Upon sonication, greater differences in both the amount and composition of soil NOM are observed. This is manifested in a greater difference in the Kps and Kocs for the coarsest and finest fractions, 3.6 versus 7.3, and 206 versus 153, respectively. This greater difference between sonicated size fractions was observed in the clay loam soil as well, Kp = 11.7 and 3.4, and Koc = 236 and 201, for the 2000-500 µm and 25-0.6 µm fractions, respectively. The results suggest that in well-aggregated soil, differences in the NOM composition and the consequent partitioning behavior will be masked. However, should the degree of aggregation be disturbed, the inherent differences in the ability of different size fractions to partition contaminants may become important to issues such as erosional transport of contaminants and dermal exposure to soil-associated contaminants.
The potential for exposure to organic contaminants through soil adherence to skin also was examined. Specifically, the mass of adhered soil, the soil particle-size distribution, and organic matter content were examined. Soil that adheres to the skin is enriched in size fractions less than about 125 µm. Because these size fractions were found to be more water stable, differences in the nature of the NOM, and consequently the degree of contaminant partitioning, are minimal. Thus, the bulk properties (percent carbon) of the smaller size fractions need only be determined, making chemical analysis of size fractions unnecessary for this aspect of risk assessment.
Finally, the role of particle size and aggregation state was studied in relation to the size distribution and transport of radionuclides by soil erosion at the Rocky Flats environmental site. Between 60 and 70 percent of soil 239,240Pu inventory resides in the sand (>53 µm) fraction of undisturbed Rocky Flats soils. Because most Pu is expected to be associated with mineral surfaces, this again reflects the fact that aggregation results in fine particles being present in larger aggregate sizes. These aggregates are likely a consequence of the biological activity that occurs in a normal grassland soil. These aggregates are stable with respect to disaggregation upon emersion in water. Results of sonication analyses show that the water-stable aggregates can be mechanically dispersed. The response of soil aggregates to sonication is an indication of the mechanical stability of the soils. Sonication causes a substantial increase in the fraction of the 239,240Pu inventory in the <10 µm fractions. Hydrogen peroxide (H2O2) oxidation caused an even more substantial shift in the Pu size distribution than did sonication. After partial destruction of the soil organic matter by chemical oxidation, the percentage of 239,240Pu in the <10 µm fraction increased to 75 percent of the 239,240Pu soil inventory. The Pu distribution among the various size fractions of a runoff sample is substantially different from that found for the case of the water dispersion of the soils. In the runoff sample, 239,240Pu was primarily present in the smallest size fractions. Comparison of the soil and runoff results suggests that it is the <2 µm fraction of soil that has the greatest potential impact on Pu transport.