Irreversible adsorption of naphthalene and tetrachlorobiphenyl to Lula and surrogate sediments

Several unique features of sorption irreversibility have been investigated in this paper. Adsorption has been found to be biphasic with about 30-50% of the adsorbed mass residing in the irreversibly sorbed compartment, until this compartment is filled, and the rest of the mass resides in the labile compartment. Naphthalene in the reversible compartment follows a linear adsorption isotherm with a normal organic carbon-based partition coefficient. A finite fixed total compartment size is observed for the irreversible fraction, q(max)(irr) (mu g/g), on both natural and surrogate solids. In multiple batch adsorption/desorption experiments, the maximum concentrations that resist desorption are q(max)(irr) approximate to 10 mu g/g for naphthalene on Lula sediment and q(max)(irr) approximate to 0.36 mu g/g for 2,2',5,5'-tetrachlorobiphenyl (2,2',5,5'-CB) on both Lula and surrogate solids. The concentration in the irreversibly sorbed compartment varied with the initial naphthalene concentration available for adsorption. In addition, the amount in the irreversibly sorbed compartment increases linearly with the number of adsorption steps until the maximum concentration q(max)(irr) is reached. After the maximum concentration of the irreversibly sorbed compartment is satisfied, the adsorption/desorption of naphtha I ene and 2,2',5,5'-CB becomes reversible. The irreversibly sorbed compartment appears to be at equilibrium with the aqueous phase when the labile naphthalene or 2,2',5,5'-CB is removed, but the equilibrium concentration is much lower than would be predicted with conventional hydrophobic partitioning theory. The aqueous phase concentration in equilibrium with the irreversibly sorbed compartment is about 2-5 mu g/L for naphthalene and 0.05-0.8 mu g/L for 2,2',5,5'-CB. Similar adsorption/desorption phenomena are observed with both a natural sediment and a well-characterized sorrogate solid.