EFFECTS OF IONIC STRENGTH ON CHEMISORPTION AND POTENTIAL-DETERMINING SORPTION OF PHOSPHATE BY SOILS

Amounts of inorganic phosphate (P) sorbed by two unfertilized soils, during times less than required to reach equilibrium, were affected by the ionic strength and cation species of the matrix solution. For non-equilibrium conditions the amounts of P sorbed increased with increasing ionic strength and were greater with Ca²⁺ than Na⁺. For higher P additions, resulting in equilibrium solution P concentrations greater than 30 to 40μrnole 1^?1, the effects of the matrix solution on P sorption were maintained at equilibrium, whereas at lower P additions the dependence of sorption on matrix solution composition was eliminated at equilibrium. Equilibrium sorption isotherms for each soil and matrix solution were described by three Langmuir equations, which corresponded to distinct concentration ranges or regions (I, II, and III) on the overall isotherm. The free energies of sorption (ΔG) for each region, were essentially independent of the soil matrix solution. The sorption maxima for regions I and II of the isotherm for a particular soil were also virtually independent of the matrix solution used. The sorption maximum for region III, however, was markedly dependent on the matrix solution, implying a potential-determining (p.d.) sorption mechanism.