Effects of Soil Organic Matter on Kinetics and Thermodynamics of Phosphorous Sorption

It seems that the presence of soil organic matter (SOM) increases P availability to plants. This effect has commonly been ascribed as competition between the decomposition products of SOM and P for soil sorption sites resulting in P concentrations increase of soil solution. This study was carried out to obtain information about soil organic matter (SOM) influence on kinetics and thermodynamics of P sorption in three collected calcareous soil samples of east Azerbaijan province. Sorption kinetics was studied at 100 mg P L-1 solution as initial concentration before (B) and after (A) SOM removal and observations were carried out for 336 hours. SOM was removed with NaOCl solution. According to the results, P sorption increased rapidly up to about 48 hours beyond which a near steady-state condition was attained. Zero-order, first, second- and third-order equations did not adequately describe P sorption kinetics. Simple Elovich, parabolic diffusion and two-constant rate equations adequately described the data, but two-constant rate equation was the best. To evaluate the temperature effects, P sorption was measured at four temperatures, namely 10, 20, 30 and 400C. According to the results, P sorption, before and after SOM removal, increased with temperature raises indicating the endothermic nature of  the sorption reaction. The negative values of Gibbs free energy at different temperatures indicated the spontaneous nature of P sorption in the studied soils. Before SOM removal, ∆G0 in the first sorption site ranged from -32 to -38.2 kJ mol-1 and from -25.4 to -28.4 kJ mol-1 in the second site. While after the removal of organic matter in the first site ∆G0 ranged from -32.9 to -39.3 and in the second site from -30.9 to -36.9 kJ mol-1.