The Kinetics of Adsorption of Calcium and Magnesium Ions on to an Insoluble Sodium Polyphosphate

Abstract

The adsorption kinetics of calcium and magnesium on to an insoluble sodium polyphosphate were determined at 25°, 37°, and 49°C. The adsorption process has two distinct phases, an initial rapid phase lasting 10-20 min and a slow phase which occurs over several hours. The data were fitted to a two-site adsorption model by nonlinear least squares regression and the rate constants of the rapid and slow adsorption phases were estimated. Experimental adsorption rates for magnesium are lower than for calcium and this may be because the greater energy of dehydration of magnesium ions hinders adsorption. The high experimental adsorption densities for both ions suggest that the effective surface area is substantially larger than the estimate obtained by helium adsorption. It is suggested that the large apparent surface area may be due to the deposition of a gel layer of insoluble calcium or magnesium polyphosphates on the surface.