Impact of Soil Organic Matter Heterogeneity on Sorption and Sequestration of Organic Pollutants in Environmental Systems
Dr. Weilin Huang
Drexel University
Philadelphia, PA 19104
 

Sorption by soils and sediments is a phase distribution process affecting the fate and transport of relatively hydrophobic organic contaminants (HOCs) in surface aquatic and groundwater systems. The process is driven fundamentally by the entropic effect and van der Waals interactions between organic solutes and soil/sediment organic matter (SOM). A linear partitioning model proposed in early 1980s has been widely used for mechanistically interpreting and quantitatively predicting the equilibrium sorption. This model is predicated on a hypothesis that SOM is relatively homogeneous, gel-like material and acts as a partitioning phase for HOCs.

Mounting data published in 1990s revealed that HOC sorption by soils and sediments are nonlinear, desorption isotherms are hysteretic, sorption and desorption rates are slow, and that there is competition among co existing HOCs for sorption. These observations have been attributed variously to the heterogeneity of SOM and are fundamentally and phenomenologically inconsistent with the linear partitioning model. A dual SOM domain model was invoked by several research groups based on an assumption that soils and sediments may contain two types of SOM; i.e., “soft” vs. “hard” carbon SOM domains. The soft carbon SOM may be highly amorphous, water-swollen, or gel-like material, and may exhibit linear partitioning behavior for uptake of HOCs. The hard carbon SOM may be relatively condensed, rigid, and chemicallyreduced material, and may exhibit site-limiting surface adsorption and slow matrix expansion and absorption when exposed to HOC-containing aqueous solutions.

While the dual SOM domain model can successfully explain a wide array of sorptiondesorption phenomena for HOCs, fundamental questions regarding the nature and quantification of hard carbon SOM still remain unanswered. This presentation will highlight the recent progress made by our research group on isolation, quantification, and characterization of different types of SOM using chemical, microscopic, and spectroscopic methods. The differential roles of the chemically-isolated SOM fractions in the sorption and sequestration of HOCs will be delineated in the context of their geochemical properties. The presentation will also include a brief discussion about the significance of our current findings in HOC sequestration and bioavailability and their potential applications in the selection of physical, chemical, or biological process based strategies for remedy of contaminated soils and sediments.

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