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.