THEORETICAL ASPECTS OF THE ADSORPTION OF ORTHO-PHOSPHATE ONTO IRON-HYDROXIDE

The adsorption of o-phosphate onto Fe(OOH) particles can be described by the Freundlich or by the Langmuir adsorption isotherm. If the adsor ption is written as a chemical equilibrium, it can be shown mathematic ally that between pH 6 and 10 the Freundlich isotherm, P-ads = A (o- P)(B), gives a satisfactory description with B = 0.333 or P-ads = A (3) root o-P. This can be explained chemically by assuming that two Fe (OOH) molecules are inactivated for each o-phosphate molecule adsorbed . This may be due to spheric hindrance or to shifts of the electric ch arge on the Fe(OOH) molecule by the phosphate adsorbed. The constant A depends on factors such as the pH and Ca2+ and Mg2+ concentrations. T he Langmuir isotherm does not give an acceptable description of this a dsorption process.