Project Abstract

Contaminant Binding to the Humin Fraction of Soil Organic Matter

Principal Investigator

J.A. Rice
South Dakota State University
E-mail: RiceJ@ur.sdstate.edu

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

This project is on schedule, although the time line had to be adjusted to reflect the unexpected amount of time needed to complete the ultrafiltration of samples. To speed completion of this portion of work, an additional ultrafiltration cell is being purchased. A portion of the contaminant mass balance has been completed and a manuscript describing the distribution of PAHs and PCBs between the humic fractions and components of humin has been submitted. Studies on how organic components in humin affect binding capacity for different target compounds had to be modified when it became clear it might not be possible to get this information as the experiment was originally conceived. This work and the ultrafiltration studies should be completed in year three of the project. The analytical protocol for the BET soil analysis and the soil organic matter fraction surface area has been developed. The method for examining the binding of the target compounds to humin and its components has been refined. Delays in spending authorization have held up much of the project for year three but future research plans include describing the nature of bound contaminant residues and their chemical environment in order to begin to develop an understanding of the binding phenomena and the parameters that control it. This project is in its third 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.

Keywords

Contaminant binding, humin, soil organic matter, binding mechanisms.