Die bydrae van die sandfraksie tot katioonadsorpsie in gronde
Sander, Carl Ludwig
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Models that have been set up to explain cation adsorption in soils and clay minerals are based on cation exchange. The applicability of the models depend to a great extent on the experimental terms in the adsorption equation. In many cases the models can only be applied in practice when certain assumptions have been made to simplify the models. A mechanism for the adsorption of a cation on an adsorbent is proposed. No assumption regarding the structure of the adsorbent is being made. The mechanism was tested with potassium as cation and the sand fraction of soils as adsorbent. The sand fraction is regarded as the least complicated adsorption complex of a soil. The adsorption of K+ by the sand fraction leads t o a "compound" with a specific "dissociation constant" between K+ and the sand fraction. The "dissociation constant" can be used as an indication of the adsorption energy of the "compound". The mechanism is simple because the experimental terms in the derived equation are easily determinable. The adsorption of K+ by the sand fraction of soils from solutions with cation concentrations which are comparable to that of soil solutions, could be explained with the mechanism. + 2+ + In the presence of Na and Ca as well as K in the equilibrium solutions, the adsorption of K+ is effected in accordance with the proposed mechanism. An equation for comparison of cation adsorption on different adsorbents in the same system as well as an equation to compare the contribution of one adsorbent to the total cation adsorption in an adsorption system, was also derived. The experimental results prove that K+-adsorption by the sand fractions of soils are comparable to K+-adsorption by die clay fractions. The sand fraction also contributes to the adsorption equilibrium in soils when the clay percentage is less that about 50.