Interactions of hydrophobic fractions of dissolved organic matter with Fe³⁺- and Cu²⁺-montmorillonite

Interactions of dissolved organic matter (DOM) with clays can significantly affect a variety of soil processes. We studied adsorption and fractionation of hydrophobic acid (HoA) and hydrophobic neutral (HoN) fractions of DOM on Cu ²⁺- and Fe³⁺-montmorillonite. Adsorption of both samples was higher on Fe³⁺-montmorillonite than on Cu²⁺- montmorillonite. A pH increase of about one unit was recorded followed by HoA adsorption by Fe³⁺-montmorillonite. This suggested exchange of negatively charged DOM groups on surface hydroxyl groups of Fe ³⁺-montmorillonite surfaces. Adsorption of HoA on Cu ²⁺-montmorillonite and HoN on Fe³⁺- and Cu ²⁺-montmorillonites was governed mainly by van der Waals interactions. Spectroscopic analyses showed a distinct HoA fractionation by molecular size and aromaticity only by Fe³⁺-montmorillonite. On the basis of the pH measurements (increase in pH following adsorption of acid components) and enhanced DOM fractionation by molecular size and aromaticity we suggest that DOM reacted with Fe³⁺-montmorillonite similar to goethite.