Kinetics and Thermodynamics of Phosphorus Adsorption onto a Clay Soil Influenced by Various Environmental Parameters

 Phosphorus is an essential soil nutrient that plays key roles in plant growth and development. Limited availability of P is the main constraint for crop production in many soils. Long-term phosphate fertilizers application in agricultural areas to increase the physiological efficiency of crops can lead to a significant P accumulation. The process of P fixation or sorption includes precipitation and adsorption onto mineral and organic surfaces. Various factors such as clay content, organic matter, exchangeable Al, Fe, Ca content and pH soil affect P sorption capacity. In order to achieve the proper management of P fertilization, it is necessary to understand the mechanism of the sorption process and the contributing factors, as well as how to influence these factors. Qazvin plain is one of the most important agricultural plains in Iran, playing a pivotal role in maintaining national food security. Cultivating crops such as wheat, barley, alfalfa and corn in different areas of this plain is widespread. Therefore, high amounts of phosphate fertilizers are applied in this plain every year. In this study, the kinetic and equilibrium adsorption of P in a heavy textured agricultural soil sample in Qazvin plain were investigated under the influence of some different environmental parameters.

In order to conduct the kinetic adsorption experiment, one gram soil samples were placed in the shaker in contact with 25 ml of 0.01 M CaCl2 solution containing 20 mg P l-1. Time intervals were 0.17, 0.5, 1, 2, 4, 8, 16, 24, 48 and 72 hours. The effects of temperature (12, 25, 38 °C), salinity (0, 8.96, 17.02, 32.09, 46.25 dS m-1), pH (2.5, 3.5, 5.36, 7.5, 9.5, 11.5) and the type of background solution (distilled water and 0.01 M CaCl2 solution) were also investigated on P equilibrium adsorption. In the equilibrium batch experiments, the soil samples were placed in contact with the background solutions containing 0, 15, 20, 30, 50, 80 and 100 mg P l-1 (ratio 1:25) for 24 hours. The concentration of P in the samples was determined by a spectrophotometer after passing through the filter. The amount of P adsorption to each soil sample was then calculated based on the concentrations. The experiments were carried out in the factorial and completely randomized designs with three replications for each treatment. Using CurveExpert 1.4 software, the Langmuir and Freundlich isotherms, as well as the pseudo-first-order, pseudo-second-order, the Elovich and Intra-particle diffusion models were fitted to the obtained laboratory data. Statistical analysis of experimental data was done based on the Tukey test at 5% level using Minitab software. The thermodynamics of P adsorption was also determined by examining parameters of the Gibbs free energy, enthalpy and entropy changes.

 According to the results, the highest amount of adsorption occurred in the first 8 hours of soil contact with P solution, and approximate time of achieving the equilibrium conditions was 24 to 48 hours. The process of P adsorption onto soil particles consisted of two fast and slow stages until the equilibrium was reached. The kinetic adsorption properties of the studied soil was best described by the Elovich equation (r2=0.964). The Freundlich model showed better fit than the Langmuir equation to the equilibrium data. The effects of all four parameters of temperature, salinity, pH and background electrolyte solution on the P equilibrium adsorption were significant. By changing the temperature from 25 to 38 °C, qm (Langmuir coefficient) was 2.1 times. It was also 7.5 times under the conditions of using CaCl2 solution instead of distilled water. Increasing pH caused an increase in adsorption rate and the highest amount of adsorption changes occurred in the pH varying between 5.36 and 7.5. However, the highest and lowest P adsorption percentage with the values of 45 and 37% were related to zero and 46.25 dS m-1 salinity, respectively. The results also indicated that the sorption process was endothermic and spontaneous.

 Adjusting and controlling the studied parameters in the soil during the application of phosphate fertilizers can optimize P use efficiency and increase crop yield in the studied area. Based on the results of the present study, it is recommended to add sulfur, ammonium sulfate, ammonium nitrate fertilizers and organic compounds to the studied calcareous soil with high pH and low salinity. Application of this method can reduce soil pH, which leads to a decreased P sorption onto the soil particles and an enhanced P availability for plants. Adjusting the P fertilization time with the crop growth and uptake is also recommended due to the high adsorption of P onto the soil particles in a short period of time.