Surface-complexation modelling for describing adsorption of phosphate on hydrous ferric oxide surface

Abstract

Adsorption of dissolved phosphate onto synthetic hydrous ferric oxide (HFO) was measured in the laboratory as a function of pH, ionic strength, and phosphate relative concentration. Experimental data were used to constrain optimal values of surface complexation reactions using a geochemical modeling code JCHESS according to the diffuse layer model. The results provide a consistent set of model equilibrium constant (log K) values at 25‹ C and 100 KPa for the following reactions:
>FeOH + PO₄^3- + 3H⁺ = >FeO-H₂PO₄ + H₂O (log K1int = 19.0)
>FeOH + PO₄^3- + 2H+ = >FeO-HPO4- + H2O (log K2int = 14.3 « 0.17)
>FeOH + PO₄^3- + H⁺ = >FeO-PO₄^2- + H₂O (log K3int = 8.32 « 0.27)
These results differ significantly from previously-published estimates of log K2int and log K3int [1.3], and provide a more accurate fit to experimental measurements over a broad range of pH (3.12), ionic strength (0.001.0.1 mol/.) and total relative phosphate concentration (12.500 ƒÊmol phosphate/g HFO). The results provide a close fit to the experimental data within a wide range of conditions, and should be adopted in modelling the chemical speciation of phosphate in natural systems containing HFO as a predominant adsorbing material.

Keywords: phosphate, adsorption, hydrous ferric oxide, surface complexes, equilibrium constants