Phosphorus desorption kinetics in four mineralogically variant soils showed that Elovich equation better described the data with variation in equilibrium time from 2 h to 16 h during successive extractions. At this juncture of change in equilibrium time, first-order kinetic and parabolic diffusion equations exhibited an inflection point splitting the isotherm into two segments. However, with constant equilibrium time, parabolic diffusion equation better described P desorption kinetics in all the four soils. The P desorption data suggested that in smectite dominant soil most of the added P was sorbed as outer-sphere surface complexes with weak bonding energies and as inner-sphere surface complexes in other soils. This was attributed to weakening of P bonding forces due to turbostratic disorder of smectite layers in smectite dominant soil, wherein the positive charge on the edge face of one layer stays in close proximity to the negative charge on the siloxane surface of adjacent layers. The interstratified smectite dominant soils, in comparison, sorbed much lower P as outer-sphere surface complexes. The kinetic data suggested that mineralogy of the soils, especially the smectite content determines the extent of spilling-over of charge and hence the sorption of P as outer or inner-sphere surface complexes.
P desorption, kinetics, c/ayminerals, inner-and outer-sphere complexes, charge spill-over
