The extent and nature of humic acid (HA) bonding at the crystal edges relative to basal surfaces, in dried complexes of montmorillonite, illite and kaolinite, were studied by blocking the edges with sodium hexametaphosphate (HMP) or triethanolamine (TEA). Decrease in the fixation of HA caused by edge blocking was observed in every case except the HMP-Ca-montmorillonite complex, which showed an exceptional increase in fixation. It was concluded that direct bonding of HA to exposed structural ions at the crystal edges of kaolinite does not occur; the linkage is always through a cation bridge. In illite, the cations at the cleavage planes provide the major bonding sites for HA, with the crystal edges having only a minor role. The efficiency of HA fixation by the cations on illite is greater than that on montmorillonite as a result of the stronger attractive forces of the cation-illite bond. It has further been inferred that the unique behavior of HMP-Ca-montmorillonite is caused by its extensive swelling character, which makes available numerous basal surfaces for bonding with HA. Destruction of silicate layer stacking on HA complexation has been observed by XRD.
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