Interaction of Dissolved Organic Matter with Mineral Soil Matrices: Sorption-Desorption and Fractionation.

The sorptive interaction of dissolved Organic matter (DOM) with soil clays and oxides is a highly reactive and dynamic process, influential upon the carbon cycle and critical for the fate of contaminants and other organic and inorganic species. Most reported data on DOM sorption have focused on soils with significant amount of organic matter (SOM) as sorbents, thus intermingling the effects of both the organic and mineral constituents on DOM sorption. The goals of the current study were: (i) to elucidate the interactions of DOM in natural, untreated soil systems and to link sorptive and desorptive behaviors to soil mineral matrix characteristics; and (ii) to study molecular-level physico-chemical DOM fractionation processes that occur during both sorption and desorption.

The experimental approach involved batch sorption and desorption experiments with the concurrent analysis of DOM solutions in the various experimental stages using FTIR spectroscopy. Both the affinity and capacity of soil for DOM sorption were demonstrated to closely correlate with the content of iron oxides, with a less prominent control exerted by the soil clay content. While several major DOM sorption mechanisms may concurrently operate in soil, the involvement of carboxylic and phenolic functional groups in ligand exchange complexation reactions with hydroxylated Fe and Al oxide surfaces has been considered as a major pathway of interaction by several researchers. Furthermore, this mechanism has been associated with high-energy bonding of DOM. The FTIR data gained in the current study was intimately linked with the finding that iron oxides dominate DOM sorption and support sorption irreversibility.