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COSOLVENT EFFECTS ON SORPTION ISOTHERM LINEARITY
Citation:
Bouchard, D C. COSOLVENT EFFECTS ON SORPTION ISOTHERM LINEARITY. JOURNAL OF CONTAMINANT HYDROLOGY 56(3-4):159-174, (2002).
Impact/Purpose:
The primary objectives of this research task are to measure the magnitude and kinetics of organic contaminant sorption and transport in soils and sediments, to apply and compare the utility of bicontinuum and distributed parameter models for describing contaminant release from soils and sediments, and to use the measured and estimated sorption/desorption kinetic descriptors developed for assessing long-term contaminant release from soils and sediments. Miscible displacement techniques with soil and sediment columns (with some supporting batch equilibration treatments) will be used in these studies, with analyses by high performance liquid chromatography (HPLC)and gas chromatography (GC). Liquid scintillation counting will be used to quantify 14C-labelled HOCs and the 3H2O used as a conservative tracer to characterize column hydrodynamics.
Description:
Sorption-desorption hysteresis, slow desorption kinetics, and other nonideal phenomena have been attributed to the differing sorptive characteristics of the natural organic polymers associated with soils and sediments. In this study, aqueous and mixed solvent systems were used to investigate the effects of a cosolvent, methanol, on sorption isotherm linearity with natural organic matter (NOM), and to evaluate whether these results support, or weaken, the rubbery/glassy polymer conceptualization of NOM. All of the sorption isotherms displayed some nonlinear character. Our data indicates that all of the phenanthrene and atrazine isotherms were nonlinear up to the highest equilibrium solution concentration to solute solubility in water or cosolvent ratios (Ce/Sw,c) used, approximately 0.018 and 0.070, respectively. Isotherm linearity was also observed to increase with volumetric methanol content (fc). This observation is consistent with the NOM rubbery/glassy polymer conceptualization: the presence of methanol in NOM increased isotherm linearity as do solvents in synthetic polymers, and suggests that methanol is interacting with the NOM, enhancing its homogeneity as a sorptive phase so that sorption is less bimodal as fc increases. When the equilibrium solution concentration was normalized for solute solubility in water or methanol-water solutions, greater relative sorption magnitude was observed for the methanol-water treatments. This observation, in conjunction with the faster sorption kinetics observed in the methanol-water sediment column systems, indicates that the increase in relative sorption magnitude with fc may be attributed to the faster sorption kinetics in the methanol-water systems, and hence greater relative sorptive uptake for the rubbery polymer fraction of NOM at similar time scales.