Competitive adsorption of humus acids and phosphate on goethite, gibbsite and two tropical soils

Competition in adsorption between humic acid (HA) or fulvic acid (FA) and phosphate on synthetic goethite, gibbsite and two tropical soils was studied. The results for both goethite and gibbsite showed that HA and FA competed strongly with phosphate for adsorption sites at low pH values. The soils showed a similar result with a reduction in phosphate adsorption resulting from the addition of HA at the pH of the soils. The competition between HA and phosphate at different pH levels is illustrated by comparing the adsorption envelopes for phosphate on goethite, gibbsite and the two soils in the presence and absence of HA. The trends observed may be explained by the relative positions of the maximum buffer-power (buffer capacity) of the organic acids and of phosphoric acid which are shown to lie in different pH ranges.