ON THE LANGMUIR PHOSPHATE ADSORPTION MAXIMUM

The use of the Langmuir isotherm equation to calculate phosphate (P) a dsorption maximum was investigated by studying P adsorption by boehmit e (a soil mineral) at pH 3.25, 4.00, and 5.50. This well-defined simpl e system was designed to eliminate other influencing reactions and fac tors in an attempt to study only the adsorption process. The Langmuir adsorption maximum was compared to three other methods (K-d, ''concent ration null'', and graphical) for determining the value of this consta nt. There was generally good agreement between methods at the differen t pH's, except the ''concentration null'' at pH 3.25 and the Langmuir at pH 4.00. It appears that at least three of the methods (Langmuir in cluded) can be used independently to develop the adsorption envelope, which peaked at pH 4.00. At maximum adsorption, approximately 20% of t he surface was covered by P, suggesting monolayer adsorption at specif ic sites on the surface. Apparently the simple Langmuir isotherm equat ion, if used correctly, may give valid adsorption maximum values.