Tuesday, 11 July 2006 - 5:00 PM

Molecular-Level Studies of Organo-Clay Complexes and their Role in the Sorption of Polycyclic Aromatic Hydrocarbons.

Myrna J. Simpson1, Xiaojuan Feng1, Andre Simpson1, Seunghun Kang2, and Baoshan Xing2. (1) Dept. of Physical and Environmental Sciences, University of Toronto, Scarborough College, 1265 Military Trail, Toronto, ON M1C 1A4, Canada, (2) Department of Plant, Soil and Insect Sciences, University of Massachusettes at Amherst, Amherst, MA 01003

Sorption of organic chemicals is central to the fate, toxicity, bioavailability, and transport of these compounds in soil environments. Early research with hydrophobic, nonionic organic chemicals, such as Polycyclic Aromatic Hydrocarbons (PAHs), recognized the dependence of sorption distribution coefficients (Kd values) to the quantity of organic carbon in the soil. Consequently, Organic Matter (OM) chemistry has been focused upon heavily during attempts to elucidate sorption mechanisms and in most cases, OM fractions, such as Humic Acids (HA), were used. In addition, the application of molecular-level methods, namely NMR, necessitated the removal of the mineral phase prior to the analysis of soil OM structure. Therefore, the role of clay minerals in regulating sorptive processes at the soil-water interface has not been addressed to the same extent. Our research has demonstrated that kaolinite and montmorillonite indirectly govern sorptive processes by governing the accessibility of OM structures at the surfaces of organo-clay complexes. In addition, these studies have been carried out with those of whole soils and soil fractions (HA and humin). Our results suggest that the clay minerals indirectly govern sorption through regulating the accessibility of organic matter structures at the soil-water interface. NMR studies with constructed organo-clay complexes demonstrate that HA structures are selectively fractionated by clay mineral surfaces. For instance, aliphatic components of HA were preferentially sorbed to kaolinite and montmorillonite (1). Using a novel NMR method, High Resolution Magic Angle Spinning (HR-MAS) to probe the surfaces of constructed organo-clay complexes, we observed that mainly CH2 groups were associated with the kaolinite surface whereas montmorillonite preferentially sorbed both aromatic (peptide) and CH2 carbon (2). Studies with organo-clays constructed under different solution (pH and ionic strength) conditions resulted in varying phenanthrene organic-carbon normalized distribution coefficients (Koc values) also suggesting that clay minerals are important for regulating the quality and quantity of sorption sites at the soil-water interface. Furthermore, the data suggest that not all organic matter structures are “available” for sorption and some structures may be hidden or buried on the clay mineral surface. This hypothesis is also supported by studies with whole soils and soil fractions, and studies with clay-associated biopolymers. For instance, phenanthrene Koc values decreased when arachidic acid, cellulose, collagen, and lignin were bound to montmorillonite. Analysis by 1H HR-MAS also suggests that the physical orientation of these biopolymers is constrained when sorbed on clay surfaces. Comparison of 1-naphthol and phenanthrene sorption values to whole soils with that of soil humin suggest that chemical fractionation reveals more or more favorable sorption sites (ie: the Koc increases). Solid-state 13C NMR studies of soil humin reveal that the prevalence of polymethylene carbon (3,4), likely derived from the preservation of plant cuticular material), which has demonstrated to sorb appreciable amounts of PAHs (5,6). Our experimental approach includes both macroscopic and molecular-level data which both suggest that organic matter physical conformation is an important consideration in sorption processes. These studies collectively suggest that clay minerals play an integral role in the sorption of organic contaminants and organo-mineral complexes should be included in more sorption studies. References: (1) Wang, K., B. Xing. 2005. J. Environ. Qual. 34:342-349. (2) Feng, X., A. J. Simpson, M. J. Simpson. 2005. Org. Geochem. 36:1553-1566. (3) Simpson, M. J., C. S. Johnson, in press, Environ. Toxicol. Chem. (4) Kang, S., B. Xing. 2005. Environ. Sci. Technol. 39:134-140. (5) Salloum, M. J., B. Chefetz, P. G. Hatcher. 2002. Environ. Sci. Technol. 36:1953-1958. (6) Chefetz, B., 2003. Environ. Toxicol. Chem. 22:2492-2498.

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