Phosphate Desorption from Kaolinite Suspensions

Abstract

AbstractPhosphate desorption from soil minerals is one of the factors which determine the rate of P uptake by plants. Under laboratory conditions desorption is usually obtained by drastic dilution or leaching of the clay, thus inducing its dissolution. The objective of this work was to study the effect of the desorption method used, equilibration time and the dissolved silica on the desorption of P from kaolinite. Two desorption methods were used: (i) diluting 1% suspensions by various volumes of the same electrolyte, and (ii) immersing a dialysis tube containing 0.25% suspension (+P) in an identical suspension initially free of P. The desorption process in both cases could be divided into a rapid and a slow first‐order reaction. The rapid reaction rate constant was similar in both systems (about 4.65 × 10−3hours−1 at 25°C). The slow reaction constants were 0.3 × 10−3hours−1 and 1.15 × 10−3hours−1 for cases (i) and (ii), respectively. The activation energy of the desorption process in case (ii) was 16.2 Kcal/mole for the rapid and 4.8 Kcal/mole for the slow reaction. The amount of silica dissolved from kaolinite due to dilution with 0.01M KCl depended on the dilution ratio and reached 16 mg SiO2/g kaolinite when the suspension was diluted 100‐fold. Readsorption of part of the dissolved silica is stipulated to contribute to the fast P desorption process.