The enthalpy of the adsorption of orthophosphate onto iron hydroxide w
as measured with isothermal calorimetry and found to decrease with dec
reasing ratios of Fe/P. The Langmuir adsorption isotherm seems, theref
ore, not to be appropriate to describe this phenomenon. For the moment
the Freundlich adsorption isotherm seems to be a superior descriptor
of this adsorption process and should, therefore, be applied in phosph
ate flux models taking sediments into account. The Ca2+ concentration
of the water and pH enforce the adsorption process, but do not change
the enthalpy.