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 (Psub 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 Psub i described by current models (log K = 5.99 plus minus 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 pHsub zpc values of 3 pH units. Sediments were slow to reach equilibrium (\textgreater54 h), especially at high pH and low initial Psub i, consistent with a conceptual model of rapid adsorption followed by a period of intraparticle diffusion.