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Contaminant Binding to the Humin Fraction of Soil Organic Matter

Principal Investigators
J.A. Rice, South Dakota State University


Goal: The goal of this research is to understand contaminant binding to soil organic matter, particularly the fraction known as humin.

Rationale: Most previous work on the nature of contaminant binding to soil organic matter has utilized 14C-labeled compounds to reconstruct the fate of contaminants introduced into a soil system. Essentially all of these studies have stopped at the point of assigning a fraction of the bound-radioactivity to one of the humic fractions of soil organic matter; no studies have been able to characterize the actual nature of bound-residues or the nature of their interaction with a humic material. The humin fraction of humic substances is usually the predominant organic material in most soils; humin organic-carbon typically represents more than 50% of the total organic-carbon in a soil, and a significant fraction of most anthropogenic organic compounds bind rapidly and, in many cases, irreversibly to it. Yet, despite these compelling reasons for a detailed understanding of the nature of contaminant binding to humin, very little is known about its environmental chemistry.

Approach: This study will utilize a new technique that not only isolates humin but, for the first time, permits the separation of humin's organic components from its inorganic component and fractionates the organic components into recognized compound classes. Carbon-14 and carbon-13 labeled contaminants; the polynuclear aromatic hydrocarbons napthalene, phananthrene, and benzo[a]pyrene; and the polychlorinated biphenyls 4,4'-dichlorobiphenyl and 2,2',5,5'-tetrachlorobiphenyl will be incubated with two soils of different composition in separate experiments. Organic components of the soil will be isolated by a combination of traditional and MIBK methods. Humin will be fractionated into its components using the MIBK method. Using ultrafiltration, scintillation counting, and 13C CPMAS NMR, the organic matter will be fractionated and the qualitative and quantitative nature of contaminant binding to humin assessed. The role of lipids in contaminant binding to humin will be investigated utilizing column adsorption studies with humin from which first the lipids and then the humic component have been selectively removed. These results will be evaluated in light of the partitioning model of contaminant sorption to soil organic matter.

Status: Incubation experiments have been started, and preliminary characterizations of humic material fractions in the soils have been performed. The first major results-oriented research milestone, construction of the mass balances to describe distribution of contaminants among the different soil organic matter fractions, has just begun. Preliminary results have shown that the adsorption of PAH and PCB to soil organic matter in the presence of water is rapid and essentially complete. This project is in its first year.

Clients/Users: Results of this project could be used by regulatory agencies, individuals conducting research into the fate and transport of environmental contaminants, or those attempting to produce more effective herbicides or pesticides. The U.S. Department of Defense will also be interested in this research.

Key words: contaminant binding, humin, soil organic matter, binding mechanisms.

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