Direct Quantification of Sorption Thermodynamics of Phosphate on Four Soil Colloids through Isothermal Titration Calorimetry
Zhi‐neng Hong, Yan Jing, Jun Jiang, Jiuyu Li, Ren‐kou Xu
Abstract
Sorption thermodynamics of phosphate (P) on the soil colloid remains poorly known, although many previous studies have been done by macroscopic, microscopic, and spectroscopic methods. Here, we directly determined effective thermodynamic parameters and explored the underlying mechanisms for P sorption on colloids extracted from four soils (two Oxisols, one Ultisol, and one Alfisol) at different pHs using isothermal titration calorimetry (ITC), together with batch sorption experiment, solution modeling, and surface characterization. The obtained results showed sorption thermodynamics and mechanisms highly dependent on pH and soil type. Sorptions of P on all the four soil systems over pH 5.0–8.5 were probably mediated by the ligand exchange that resulted in exothermic enthalpy (ΔH) values. By contrast, those at pH 3.5 were initiated by the ligand exchange with exothermic ΔH and a high equilibrium constant (k) followed by surface precipitation with endothermic ΔH and low k. The affinity of the examined soils to P, indicated by the k values, generally decreased with the pH level. Stronger affinity to P and greater availability of the surface site were observed for the Oxisols relative to Ultisol and Alfisol, which probably resulted from their differences in mineralogy and zeta potential. This study provides thermodynamic information for P sorption on soil colloids and indicates that ITC is useful for mechanistic investigations on ion sorptions.