Phosphate removal by Ca(OH)2-treated natural minerals: experimental and modeling studies

Adsorption of phosphate phosphorus (PO ₄-P) from wastewater onto eco-friendly geosorbents has gained great attention aiming at recovering an essential nutrient for crop production. Notably, the literature on PO ₄-P aqueous-phase adsorption kinetics is limited to the application of either empirical reaction-based models lacking a physical significance or over-simplified diffusion-based models frequently used outside their applicability area. In this study, equilibrium and kinetic experiments are presented under a wide range of phosphate concentrations (50–500 mg P/L) using sustainable and low-cost modified adsorbents. The kinetics of PO ₄-P adsorption from aqueous solutions onto Ca(OH) ₂-treated zeolite (CaT-Z) and bentonite (CaT-B) was analyzed by a dimensionless two-phase homogeneous surface diffusion model (TP-HSDM) assuming constant diffusivity coupled with the double selectivity isotherm equation (DSM). The TP-HSDM fit to the data at four initial P concentrations (50, 100, 200 and 300 mg/L) resulted in an average relative error of 14.6% and 17.4% from the experimental data for CaT-Z and CaT-B, respectively. The average surface diffusion coefficient (D _s) ranged from 2.5 × 10 ^-10 to 8.7 × 10 ^-10 cm ²/s for CaT-Z and from 1.6 × 10 ^-10 to 4.78 × 10 ^-9 cm ²/s for CaT-B. The external mass transfer coefficient (k _f) ranged from 2.72 × 10 ^-4 to 8.38 × 10 ^-4 cm/s for CaT-Z and from 5.63 × 10 ^-4 to 2.24 × 10 ^-3 cm/s for CaT-B. The dimensionless Biot (Bi) number exhibited values in the order of magnitude of 10 ⁵ indicating that the intraparticle diffusion is the controlling mass transfer mechanism for both materials.