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USING METHANOL-WATER SYSTEMS TO INVESTIGATE PHENANTHRENE SORPTION-DESORPTION ON SEDIMENT
Citation:
Bouchard, D C. USING METHANOL-WATER SYSTEMS TO INVESTIGATE PHENANTHRENE SORPTION-DESORPTION ON SEDIMENT. Presented at 223rd American Chemical Society National Meeting, Orlando, FL, April 7-11, 2002.
Impact/Purpose:
Elucidate and model the underlying processes (physical, chemical, enzymatic, biological, and geochemical) that describe the species-specific transformation and transport of organic contaminants and nutrients in environmental and biological systems. Develop and integrate chemical behavior parameterization models (e.g., SPARC), chemical-process models, and ecosystem-characterization models into reactive-transport models.
Description:
Sorption isotherm nonlinearity, sorption-desorption hysteresis, slow desorption kinetics, and other nonideal phenomena have been attributed to the differing sorptive characteristics of the natural organic matter (NOM) polymers associated with soils and sediments. A conceptualization of NOM sorptive character that has appeared frequently in the literature recently draws on synthetic polymer chemistry concepts in describing NOM as a rubbery/glassy polymer where sorption in the expanded NOM rubbery domain occurs by partitioning, which is linear with solute concentration and rapid; and where sorption in the more condensed NOM glassy domain is nonlinear and characterized by slow sorption-desorption kinetics (1-3). This conceptual model has been given some credence in that naturally occurring humic substances have also been described as having both condensed and expanded regions (4), and humic polymers molecular weights are similar to those of many synthetic polymers.
If NOM behaves similarly (with respect to sorption) to more homogeneous synthetic polymers, with both a rubbery and glassy nature, then one would expect that organic solvents would have similar effects on the sorptive properties of both NOM and synthetic polymers. The objective of the research presented here is to test this hypothesis; i.e., to determine if solute sorption on NOM in the presence of an organic solvent is consistent with the rubbery-glassy polymer construct. Methanol was selected for this study because of the large database for its use in soils, its presence in the polymer science literature, and because of its high dissolution capacity in NOM (5).
The batch sorption-desorption techniques used were similar to those used previously by the author (6). The Upper Call's Creek sediment was air-dried and passed through a 1-mm sieve prior to use, this fraction having a total organic carbon content of 0.75% and being dominated by the sand and silt fractions with <1% clay content. All aqueous solution components were 0.01N in CaCl2 and contained 200 mg/L sodium azide as a biocide. The batch reactors were 20-mL borosilicate glass vessels with foil-lined caps. Based on the anticipated sorption magnitude, differing amounts of air-dry sediment were weighed into batch reactors. The amount of phenanthrene sorbed to the sediment (S, nmol/g) was determined by the difference between the initial solution concentration (C0, nmol/ml) and the equilibrium solution concentration (Ce, nmol/ml). The equilibrium sorption coefficient (K) was defined as the slope of the Ce vs S plots. The linear sorption model was used in this study as it described the data quite well (a fairly narrow range of C0 was used) and because it allows direct comparison of K values. To close the phenanthrene mass balance at the end of the desorption study, the sediment was extracted with methanol at 55C using a method similar to that of Huang and Pignatello [7].
One sorption and two desorption isotherms are presented in Figure 1. The lower two isotherms represent phenanthrene sorption-desorption in aqueous systems. As has often been noted in prior studies, the system exhibited some hysteresis with sorption magnitude being greater during the desorption phase than in the sorption phase. Of particular interest, however, is the upper isotherm for the treatments that were equilibrated with phenanthrene in a 40% (v/v) methanol/water solution, and then desorbed with an aqueous solution. The sorption coefficient in this treatment was nearly double that of the purely aqueous treatments. These results were also corroborated by studies conducted at 60% methanol/water concentrations. The higher degree of phenanthrene retention in the mixed solvent equilibrated system may have been due to methanol induced relaxation of the rigid structure of glassy NOM domains, forcing transit to a more rubbery state that acts more like a partitioning medium. Since sorption in the glassy domain is slow, and partitioning is known to be rapid, the end result is faster sorption kinetics in the rubbery domain. Hence, the sorption coefficient in the mixed solvent equilibrated system may reflect a sorption measurement more characteristic of longer solute-sorbent contact times than the 1-week equilibrations used in this study.
All of the above studies were performed using 3H-labelled phenanthrene. After the initial 3H phenanthrene desorption step, 14C-labelled phenanthrene in aqueous solution was added. This procedure constituted the second 3H-phenanthrene desorption step (a total of three were performed) and the initial 14C-phenanthrene sorption step used to determine the reversibility of the postulated NOM changes elicited by the methanol. Results indicated that any changes in the NOM sorptivity were at least partially reversible as the 14C-phenanthrene sorption-desorption isotherms (not shown) exhibited somewhat greater phenanthrene retention than the purely aqueous systems, but significantly less retention than the mixed solvent equilibrated systems. Finally, extraction recoveries to close the phenanthrene mass balance at the end of the desorption studies were all >87%, thus demonstrating the ultimate reversibility of the sorption-desorption process, and also, consistency with the concept of a reversible and deformable NOM phase.