PHOSPHORUS SORPTION BY SEDIMENTS FROM A SOFT-WATER SEEPAGE LAKE .1. AN EVALUATION OF KINETIC AND EQUILIBRIUM-MODELS

Models describing kinetics and equilibria of sorption were compared by using phosphorus sorption data for sediments from Little Rock Lake (Vilas County, WI) at pH 4.5-6.0. A maximum sorption capacity was not exhibited by the sediments, but sorption continued to increase (slowly) as solution [P(i)] increased. Sorption equilibria and kinetics thus did not fit well to simple models that assume homogeneous binding sites but were described by models that incorporate either a distribution of intrinsic binding energies or variable binding caused by a change in electrostatic potential with increasing surface coverage. The variance in sediment affinity for P(i) described by current models (log K = 5.99 +/- 5.00) realistically cannot be ascribed solely to variation in intrinsic binding energies among different sites, but can be accounted for by a range in pH(zpc) values of 3 pH units. Sediments were slow to reach equilibrium (> 54 h), especially at high pH and low initial P(i), consistent with a conceptual model of rapid adsorption followed by a period of intraparticle diffusion.