Effect of Exchangeable Sodium Percentage on the Sorption of Orthophosphate and Pyrophosphate

A Typic Ustochrept having 9.6 exchangeable sodium percentage (ESP) was sodicated to 32.8 and 48.0 ESP to study the sorption of orthophosphate and pyrophosphate. Phosphate sorption (x/m) progressively increased with increasing solution P concentration and decreased with ESP. Distribution coefficient (Kd) and percentage of added phosphate sorbed (X_Ad) decreased with increasing solution P concentration and ESP. Phosphate sorption was satisfactorily described by two surfaces Langmuir equation and Freundlich equation. Soils had higher sorption capacity and lesser binding energy for orthophosphate than for pyrophosphate but the differences were more marked on sorption parameters at higher concentration than that at the lower concentration range. The contribution of sorption by lower energy sites (b₂) to total sorption maxima was 97.0 to 98.2% indicating that very less number of reactive sites are specific which have very high affinity for phosphate. Sorption maxima (h₁ & b₂) decreased and binding energy constants (k₁ & k₂) increased with ESP but the effect was more pronounced on b₂ and k₂ than on b₁ and k₁. Freundlich parameter ‘K’ decreased and ‘n’ increased with increasing ESP. Surface energy is thus not homogeneous but decreases exponentially with fractional surface coverage with phosphate.