Influence of coexisting calcium ions during on-column phosphate adsorption and desorption with granular ferric oxide

Abstract Adsorption and desorption of phosphate by hydrated granular ferric oxide (GFO) were investigated using model phosphate wastewater (2 mg P/dm3) containing that also contained calcium and magnesium ions. The phosphate adsorption isotherm of GFO was fit with the Langmuir equation. GFO particles with an average size of 0.75 mm took up a maximum of 14.2 mg P/g. The intraparticle surface diffusivity (Ds) of phosphate was 1.0 × 10−11 m2/h. Phosphate adsorption breakthrough curves were obtained with a solution that contained alkaline earth metals (Ca, Mg) and one that did not. The breakthrough curves were compared to examine the effects of coexisting alkaline earth metal ions on-column phosphate adsorption. The column breakthrough volume (3800 times volume of the adsorbent bed, BV) with the solution containing Ca and Mg was over four times larger than that (860 BV) obtained with the Ca- and Mg-free solution at the same space velocity (SV 4.3/h). Greater uptake of P in the presence of Ca and Mg was ascribed to the precipitation of Ca salts in the GFO bed. Pretreatment with acid increased the efficiency of phosphate desorption in NaOH to regenerate phosphate-loaded GFO owing to the dissolution of precipitated Ca salts.