Role of clay minerals in controlling the fate of exceptionally toxic organic contaminants in the environment

摘要

Understanding the chemical mechanisms of interaction of dioxins with soils and sediments are critical to understanding their environmental fate, transport and bioavailability. Recently, expandable clay minerals have been shown to have a higher-than-expected affinity for these exceptionally toxic, nonpolar compounds. To gain additional perspective, this study focused on the interaction of dioxin congeners with expandable clay minerals by integrating macroscopic batch sorption experiments, spectroscopic analysis and computational methods. In addition, preliminary data on the bioavailability of dioxin sorbed to clay minerals will be presented. Maximum dioxin sorption occurred on clay minerals exchanged with weakly hydrated monovalent cations (e.g., Cs~+). Regarding clay specificity, highest sorption was observed on saponite, a trioctahedral smectite with isomorphous substitution in the tetrahedral sheet. Of the cation-smectite variables explored in this study, the nature of the exchangeable cation wasthe most significant determinant followed by the type of clay. In order to investigate the influence of chlorine substitution, we compared the sorption of dibenzo-p-dioxin (DD) and 1-chloro-dibenzo-p-dioxin (1CIDD). For all of the smectites studied, sorption of 1CIDD was greater than that of DD. Polarized FTIR and Raman spectra of sorbed DD and 1CIDD on the clay minerals revealed information about the orientation of the sorbed species and, in the case of DD on saponite, specific information about the interaction of the sorbed DD species with the interlayer cation. Specifically, Raman spectra of dioxin associated with Cs-saponite showed that the molecular symmetry of the interlayer species was reduced. The exception toxicity of vertebrate animals to chlorinated dioxins occurs through activation of the aryl hydrocarbon receptor (AhR). Working in collaboration with the Center for Integrative Toxicology at Michigan State University, we at the early stages of examining the bioavailability of dioxins sorbed to clay minerals.