Interactive effects of hydroxy-aluminum polycations, polysaccharides, and humic acid on soil/clay suspension and aggregate stability

Soil clay dispersion and aggregation are of prime importance in the problems of water penetration in soil. Surface modification with hydroxy-Al and organic polymers is known to cause significant changes in the flocculation-dispersion characteristics of clays. This study was undertaken to examine the interactive effects of hydroxy-Al polycations (Al-p), polysaccharides (PSS), and humic acid (HA) on soil/clay suspension and aggregate stability.; Al-p was shown to have a strong affinity to clay surfaces due to electrostatic forces between the two components. The adsorption of PSS by the clays was influenced by adsorbed cations on clay surfaces, pH and ionic strength. Polyvalent cations effectively screened the surface negative charge on clay surfaces and acted as bridges between the clay surface and the negatively charged PSS, thereby increasing the PSS adsorption.; Small amounts of Al-p effectively caused flocculation of the soil/clay suspensions, whereas excessive Al-p reversed the surface charge on clays and restabilized illite and soil suspensions. However, negatively charged organic polymers were shown to be effective dispersing agents for Na-clays. The CCC values of soils/clays were significantly increased by adding small amounts of the polymers, with the effect of HA {dollar}>{dollar} soil PSS {dollar}>{dollar} anionic PSS. Polyvalent cations were found to be necessary for flocculating the soil/clay suspensions by those organic polymers. The structural organizations of dilute clay suspensions added with or without Al-p and organic polymers were studied by the freeze-etch replica electron microscopy, and shown to be related to the flocculation-dispersion behavior of the clay suspensions.; Measurements of soil aggregate stability and hydraulic conductivity further indicated that addition of HA to soils resulted in the lowest aggregate stability and hydraulic conductivity. Treatment with the anionic PSS increased soil aggregate stability and only the initial hydraulic conductivities of the soils. On the other hand, treatments with Al-p plus the PSS or HA were the most effective in producing stable soil aggregates as well as maintaining high hydraulic conductivity. This study strongly suggests that polyvalent cations are essential for stable soil aggregation by negatively charged HA and the anionic PSS.