DISPERSION OF KAOLINITE BY DISSOLVED ORGANIC MATTER FROM DOUGLAS-FIR ROOTS

Abstract

The organic constituents of water extracts from Douglas-fir (Pseudotsuga menziesii [Mirb.] Franco var. menziesii) roots that cause kaolinite dispersion were investigated. The dissolved organic matter was fractionated according to molecular size and chemical characteristics into acids, neutrals, and bases of the hydrophilic and hydrophobic groups. The dominant fraction causing dispersion included the hydrophobic acids and organics with molecular sizes greater than 104 nominal molecular weight. Partial oxidation of the dissolved organic matter increased its carboxylic acid content and dispersion potential. Organic acids promoting kaolinite dispersion included aliphatic and aromatic carboxylic acids. The dispersing ability of a carboxylic acid was related to its charge and charge density. Kaolinite has a pH-dependent surface charge; specific adsorption of carboxyl groups makes the clay more negative and promotes dispersion. Polycarboxylic acids appear to play the major role in kaolinite dispersion by dissolved organic matter in forests. Key words: Fulvic acid, specific adsorption, carboxylic acid, fractionation