Phosphorus adsorption maximum of sands for use as media in subsurface flow constructed reed beds as measured by the Langmuir isotherm

The P-adsorption capacities of 13 Danish sands were studied by short-term isotherm batch experiments and related to the physico-chemical characteristics of the sands. The maximum P-adsorption capacities ( Q) and P-binding energy constants ( b) were calculated using the Langmuir-isotherm model. The Freundlich model was also used, but it was not useful for the description of adsorption phenomena per se since it fitted well P-removal data even if precipitation of P-salts occurred simultaneously. The Langmuir model described the data well ( R 2=0.90−0.99) when precipitation of phosphates did not occur and seems therefore to be useful for describing the adsorption processes per se. The relationships between maximum sorption capacities and physico-chemical characteristics of the sands were investigated using classical univariate and partial least squares regression analyses. Among the physico-chemical properties of the sands, Ca and Mg content, grain size, porosity, bulk density and hydraulic conductivity were significantly related ( P