DOM-affected transformation of contaminants on mineral surfaces: A review

This review analyzes the role and reactivity of dissolved organic matter (DOM) in oxidation, reduction, hydrolysis, and photochemical reactions of contaminants occurring on mineral surfaces. DOM affects transformation via competition for adsorption sites on the mineral surface, dissolution of minerals and exposing new reactive surface sites on the mineral surface, and by electron shuttling. Most of the data suggest that DOM reduces oxidation and hydrolysis, and increases reduction of contaminants by minerals. Alternatively, mineral surfaces can enhance redox transformations of contaminants due to interactions with DOM. DOM impact on transformation of contaminants varies as a function of its molecular composition and chemical properties. In some cases, the influence of dissolved small organic molecules on the transformation of contaminants by minerals may be opposite to the bulk DOM effect. In addition, fractionation of DOM on the mineral surface can also influence the contaminant-mineral interactions. Based on the vast reviewed data, we suggest that the evaluation of DOM effects on contaminant transformations needs to be based on the chemistry and concentration of the DOM functional groups and the overall physicochemical properties of DOM. Moreover, the self-fractionation of DOM upon interactions with minerals must be considered in order to elucidate the holistic effect of DOM in the contaminant-mineral system. In addition, we suggest that "natural DOM" should be used to elucidate DOM impact on the mineral surface reactions and not dissolved humic acids, which exhibit quite different chemical structure and properties.