Comparison of phosphate adsorption on kaolinite, illite and montmorillonite by surface complexation modelling

Abstract Kaolinite, montmorillonite, and illite are representative silicate minerals that play important roles in controlling the fate and mobility of phosphorus in soils. In this study, phosphate adsorption on these minerals was described using two surface complexation models, namely the Constant Capacitance Model (CCM) and the Triple Layer Model (TLM). The determination of surface site densities and surface complexation constants was evaluated and compared. Compared with the CCM, the TLM provided a better description of phosphate adsorption envelopes under different ionic strength conditions. In the TLM, the optimized equilibrium constants for the AlHPO₄⁻, SiOH₂⁺–PO₄³⁻, and AlPO₄²⁻ surface complexes were 24.53, 18.60, and 17.44 for kaolinite, 23.68, 17.83, and 17.56 for montmorillonite, and 23.09, 19.51, and 17.23 for illite, respectively. After normalization to mineral mass, the maximum phosphate adsorption capacities were 3.81×10 − 6 , 6.23×10 − 6 , and 1.88×10 − 6 mol/g for kaolinite, montmorillonite, and illite at same initial concentration 8.40×10 − 5 mol/L, respectively, indicating that montmorillonite exhibited the highest adsorption capacity, whereas kaolinite and illite reached approximately 61.1% and 30.1% of that of montmorillonite, respectively. ATR-FTIR spectra showed characteristic phosphate bands at 650, 960, 1027, and 1100 cm -1 , corresponding to the ν₄, ν₁, and ν₃ vibrational modes of phosphate. The spectroscopic results indicated that phosphate was adsorbed in similar surface species on all three minerals. Adsorption-envelope similarity analysis revealed that kaolinite and illite exhibited more comparable phosphate adsorption behaviors, whereas montmorillonite showed distinctly different adsorption characteristics. These findings highlight the importance of identifying and quantifying clay mineral compositions in soils and contribute to the development of consistent and realistic protocols for modelling ion adsorption reactions on soil minerals and soils.