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 + PO43- + 3H+ = >FeO-H2PO4 + H2O (log K1int = 19.0) >FeOH + PO43- + 2H+ = >FeO-HPO4- + H2O (log K2int = 14.3 ± 0.17) >FeOH + PO43- + H+ = >FeO-PO42- + 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/ℓ) and total relative phosphate concentration (12–500 μmol 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.