Competitive adsorption of phosphate and silicate on fine fraction granulated ferric hydroxide

Inga Hilbrandt, Water Quality Control, Technical University Berlin, Berlin, Germany

The fine fraction of granular ferric hydroxide (µGFH, <300 µm) is a promising adsorbent for the removal of heavy metals and phosphate but properties of µGFH were not known. The aims of the present study were to characterise µGFH physically and chemically through microscopy, XRD, particle size analysis and BET-method. Furthermore competitive adsorption of phosphate and silicate was studied as used conventional GFH shows loadings of up to 10 % by weight of silicate. Isotherms of phosphate and silicate adsorption were recorded at pH 6,7 and 8 with initial concentrations of 3 mg/L P and 10 mg/L Si in deionized water and drinking water. The fine fraction adsorbent reaches loadings of 22, 17 and 12 mg/g for phosphate and 16, 21 and 22 mg/g for silicate for pH 6,7 and 8 respectively. Thus adsorption capacity decreases at higher pH values for phosphate while the highest adsorption capacities of silicate are found at pH 8. Silicate and phosphate compete for the same adsorption sites on µGFH when adsorption sites are limited. Adsorption capacities in the two-solute system vary with pH and order of addition of the target pollutants. Through a shift in pH adsorption of one target pollutant can be favoured. Adsorption of silicate at pH 6 shifts the pH_PZC of the adsorbent to higher values thus decreasing the subsequent adsorption of phosphate by approx. 30 %. Adsorption of silicate is not influenced by the order of addition but decreased by up to 60 % in the presence of phosphate. Thus for constant removal of one target pollutant by adsorption on granular ferric hydroxide the water quality of the influent has to be closely monitored.