A Mechanism of Phosphate Adsorption on Soil and Anion Exchange Resin Surfaces

Abstract

AbstractLangmuir plots of P adsorption isotherms of four soils were shown to fit two intersecting lines. The adsorption data were also found to fit the BET equation. The monolayer capacities computed from the BET equation corresponded closely with the adsorption maxima computed from the initial slopes of the Langmuir plots.Studies of P adsorption on the energetically homogeneous sites of the anion exchange resins Dowex 1 − X8 and Dowex 2 − X4 (Cl − forms) at 25°C, gave results similar to those obtained with soils. Measurements of Cl‐ released during adsorption indicated that initially 2 mmole of Cl‐ were released per mmole of P adsorbed, and this value decreased at higher equilibrium P concentrations approaching 1 mmole of Cl‐. Measurements of pH of the equilibrium P solutions suggested that there was deprotonation of the H2PO4‐ ion during the initial stages of adsorption. Similar results were obtained when the Dowex 1 − X8 was saturated with the IO3‐ ion and P adsorption on the resin studied.The differential isosteric heat of adsorption, ­ΔH, of P adsorption on Dowex 1 − X8 was computed between 11°C and 25°C. ­ΔH decreased sharply over the first region of the isotherm and became almost constant over the second.It was concluded that at low concentrations P was bonded by two points of attachment after deprotonation of the H2PO4‐ ion, followed by one point of attachment at higher P concentrations during adsorption on the resin surface. This resulted in the deviation from linearity predicted by the Langmuir equation. This hypothesis for the mechanism of adsorption is presented to explain the deviation observed on soil and pure soil mineral surfaces.