Dubinin-Radushkevich Multiphase Adsorption Isotherms for Phosphorus Sorption by Alkaline Soils

Dubinin-Radushkevich (DR) multiphase adsorption isotherm using Polanyi potential for sparingly soluble solute was applied to phophate sorption by eight alkaline soils of Punjab, north-western India. Phosphate sorption by soils followed a three region DR equation, i.e. the plot showed three distinct linear regions. These multi-linear regions of the DR plots correspond closely to saturation with respect to different calcium phosphate compounds on the solubility diagram. Adsorption maxima recorded for different regions on DR isotherm gave positive relationship with clay and CaCO₃ content of the soils. The mean energy of P sorption in soils forregion I, II and III varied from 3.25 to 4.60.4.19 to 7.96 and 6.62 to 8.91 kJ mol⁻¹ P, respectively. The mean energy of P sorption decreased with increasing amount of P sorbed by soils. It is concluded that deviations from linearity in the DR isotherm plots of P sorption by alkaline soils of Punjab were due to the formation of different isomorphically substituted amorphous calcium phosphate compounds.