Elucidation of Mechanisms and Evaluation of Multicomponent Sorption Models for 2,2',4,4'-Tetrachlorobiphenyl, Humic Substances, and Natural Sorbent Systems.

Abstract

Some earlier sorption studies of hydrophobic contaminants with soils and sediments have exhibited apparent sorption-desorption hysteresis and a dependency of the partition coefficient on the sorbent concentration. These have led to uncertainties and controversy in data interpretation. Analysis of these apparent anomalies has shown that they may be the result of either oversights of complex physico-chemical phenomena or errors in methodology. These are expressed as "caveats" for experimentalists to heed. The caveats were used as a guide to design a methodology to investigate sorption and desorption processes at 25 C and pH = 6.9 for 2,2$\\prime$,4,4$\\prime$-tetrachlorobiphenyl (TeCB). The sorbed concentrations were measured concurrently by a modified solution concentration difference method and an intra-vessel extraction method. TeCB adsorption to a well-crystallized kaolinite was reversible with partition coefficients ($K\\sb{A}$) of 129 $\\pm$ 17 (99% c.i.) and 132 $\\pm$ 34 L/kg for adsorption and desorption. No significant dependency of $K\\sb{A}$ on the kaolinite suspension concentration was observed. A weak sorbent-concentration effect ($<$10%) was measured with lake sediment, between 2 and 10 g/L, which was attributed to residual colloidal organic matter in the aqueous phase. An apparent hysteresis effect for the sediment was shown to be caused by the lack of attainment of sorption equilibrium. The sorption of TeCB to two kaolinites, hematite and two montmorillonites in the presence of a filtered Aldrich humic acid (HA) was measured at pH 4.6 and 6.9 to study the role of humic substances in soil/water partitioning of contaminants. The immobilization of an organic contaminant to a mineral surface by humic acid, previously unstudied, was investigated as well as the hypothesis of competitive TeCB and humic acid adsorption. Characterizations of the binary interactions between TeCB, HA and the mineral adsorbents at pH = 6.9 gave $K\\sb{A}$ values of 224 $\\pm$ 21 L/kg for poorly crystallized kaolinite (KGa-2) and 67.1 $\\pm$ 7.8 L/kg for hematite; a complexation equilibrium constant between TeCB and HA, $K\\sb{AB}$, of 48,300 $\\pm$ 15,000 L/kg-carbon; and a HA saturation of KGa-2 at 1.35 mg-carbon/g. Multicomponent sorption results were predicted satisfactorily, using measured binary interaction parameters ($K\\sb{A}, K\\sb{AB}$), by a model that accounts for non-competitive TeCB and humic acid adsorption, TeCB/humic acid complex adsorption and TeCB complexation in solution. The mechanism of complex adsorption can impede contaminant mobility in agricultural runoff and infiltration from l and fills. Conversely, dissolved or desorbed organic matter can act as macromolecular carriers which facilitate transport.