Study of phosphorus adsorption behaviour of soils of the Qazvin plain, NW Iran

Information of phosphorus adsorption characteristics in soils is of great importance for determining and applying proper management strategies in order to improve crop productivity and reduce environmental damages. A comprehensive equilibrium and kinetic adsorption study was designed to evaluate the adsorption behavior of phosphorus and the effect of soil properties on phosphorus retention in four different agricultural soils of Qazvin plain. The results showed that the Langmuir isotherm ( r 2 = 0.998, s = 0.007), and Kuo and Lotse equation ( r 2 = 0.946, s = 0.009) had the highest accuracy in equilibrium and kinetic modeling of phosphorus adsorption onto the soil surface. The results of linear regression analysis showed that the maximum adsorption capacity (q m ) was positively correlated with clay and calcium carbonate contents and negatively correlated with the percentage of organic matter (p < 0.05). Additionally, the effects of three distinct parameters including salinity (0 to 30 ppt), temperature (12 to 38°C), and pH (2.5 to 11.5) on phosphorus adsorption were found to be signi fi cant (p < 0.05; p < 0.01). According to these results, by changing the temperature from 12 to 25°C and 25 to 38°C, the maximum adsorption capacity increased in the range of 1.4–1.9 times. A rapid 82% increase in the adsorptive removal of phosphorus was observed at pH 5.36 to 7.5. However, with increasing salinity, the amount of adsorption decreased gradually and reached a plateau of about 0.189 mg g –1 over 10 ppt salinity. Furthermore, the results of thermodynamic test of the investigated soils showed that the process was endothermic and spontaneous.