Competitive Sorption of Dissolved Organic Matter Fractions to Soils and Related Mineral Phases

Experiments with soils, Al(OH) 3 gel, and goethite were conducted to investigate the competitive sorption of hydrophilic and hydrophobic dissolved organic matter (DOM) fractions and inorganic anions such as SO 2− 4 and H 2 PO 4 − . The DOM for the experiments was extracted from a mor layer. The ratio of hydrophilic to hydrophobic dissolved organic carbon was 37/63. The sorption of the hydrophobic DOM fraction was strongly favored over the binding of the hydrophilic fractions on soils and hydrous oxides. Under conditions of a limited number of available binding sites, the preferential sorption of hydrophobic DOM suppressed the binding of hydrophilic DOM or even resulted in a displacement of indigenous hydrophilic organic substances. Favorable chemical properties were assumed to be responsible for the strong sorption of hydrophobic DOM. The SO 2− 4 induced little reduction of the DOM sorption, whereas H 2 PO 4 − caused a strong decrease. The effects of both inorganic anions were more pronounced for the hydrophilic than for the hydrophobic DOM fraction. As the sorption of hydrophilic DOM was strongly affected by inorganic anions and hydrophobic DOM, hydrophilic DOM seems to be the most mobile organic component in soil solutions. The results give evidence that competitive interactions among the chemically heterogeneous DOM constituents need to be considered in DOM transport process in soils.