SOME ASPECTS OF CLAY-HUMUS COMPLEXATION - EFFECT OF EXCHANGEABLE CATIONS AND LATTICE CHARGE

The influence of exchangeable cations on montmorillonite, illite, and kaolinite and of lattice charge of montmorillonite on clay-humic acid complexation were studied with complexes which were subjected to drying. The amounts of humic acid (HA) nonextractable by water from the complexes (termed "fixed" humic acid) were determined. The fixation of HA by montmorillonite followed the order Li+ < Na+ < Mg2+ < K+ < Ca2+ < Ba2+ < Al3+. For illite and kaolinite, this was Na+ = Mg2+ = K+ < Li+ < Ba2+ < Ca2+ < Al3+ and Li+ = Na+ = K+ = K+ = Mg2+ < Ba2+ < Ca2+ < Al3+, respectively. A series of four reduced charge montmorillonites showed an initial increase in fixation with layer charge reduction. However, drastic reduction caused a significant loss of HA fixation ability. The following conclusions were drawn: 1) exchangeable cations form the predominant bonding link between HA and all three clay minerals, montmorillonite, illite, and kaolinite; 2) with montmorillonite, the bonding strength of cation to clay, as well as cation to HA, determines the order of effectiveness of the cations; 3) direct bonding of HA to crystal edges of kaolinite and illite is not significant; 4) electrostatic repulsive forces are a major obstacle to clay-humus bonding; 5) loss of swelling property of montmorillonite causes drastic reduction of HA fixation due to reduced availability of basal surfaces; and 6) interaction of HA with montmorillonite causes disruption of its stacking arrangement due to the prying open of the interlayers by HA molecules.