IN-SITU SPECTROSCOPIC INVESTIGATIONS OF MOLECULAR MECHANISMS ENABLING SORPTION OF DIOXINS AND PCBS BY SMECTITE CLAYS
Dioxins and poly-chlorinated biphenyl (PCB) compounds are high priority organic pollutants which are similar in structure and well known for their toxicity, bioaccumulation and persistence in the environment. Dioxins and PCBs have a high affinity for certain types of clay minerals. However, the molecular mechanism for the observed high affinity of these compounds to clay minerals is not well understood and has been the main focus of this research work. The mechanisms that govern dioxin-clay and PCB-clay interactions were investigated from two perspectives. First, the influence of selected properties clay minerals on dioxin sorption was investigated via in-situ spectroscopic techniques (ATR-FTIRand Raman) structural (XRD) and macroscopic batch sorption methods using dibenzo-p-dioxin (DD) as a model solute. Second, the influence of solute properties, especially position and degree of chlorination and molecular planarity, on sorption was investigated.
Smectites, especially, Cs-saponite effectively adsorbs dibenzo-p-dioxin (DD) from water with values reaching 10,000 mg kg-1, or one weight percent which greatly exceeds that by other naturally occurring sorbents such as soil organic matter. Adsorption was promoted by clay interlayer exchangeable cations with low hydration energies, and by smectites in which negative charge in the smectite originate from the tetrahedral siloxane sheets. IR-active bands of DD sorbed to saponite in the 1280 to 1500 cm-1 region were perturbed compared to the ‘reference’ IR spectra. Combined batch sorption, XRD and spectroscopic data confirm that the intercalation of DD occurred in the clay interlayer and site specific interactions occur between DD molecule and Cs+ cation.
Sorption of 1-chloro-dibenzo-p-dioxin (1-ClDD) and 2-chloro-dibenzo-p-dioxin (2-ClDD) on homoionic (Na+, K+, Rb+, and Cs+) smectites was evaluated to explore the effect of chlorine substitution position (and steric hindrance) of dioxin on sorption mechanisms. Similar to DD, adsorption was influenced by the hydration energy of exchangeable cations and the origin of negative charge in the smectite. XRD measurements revealed that 1-ClDD molecules were oriented nearly parallel to the siloxane surface of the clay while 2-ClDD adopted a tilted orientation, similar to DD. The location of the chlorine constituent in 1-ClDD prevents the molecule from its apparent energetically more favorable orientation. In-situ ATR-FTIR spectra revealed that sorption of 1-ClDD to Cs-saponite resulted in the loss of interfacial H2O and suggested that the sorption 1-ClDD displaces interlayer H2O and 2-ClDD is less sterically restricted in the clay interlayer.
Sorption of three dioxins (DD, 1-ClDD and 2-ClDD) was compared to three PCBs (PCB-1, PCB-4, and PCB47) with similar octanol-partition coefficients (log Kow) but varying molecular planarity and degree of chlorination onto Cs-saponite, which was shown to be representative of other smectites, revealed that despite having similar structure and hydrophobicity, dioxins have higher affinity for smectites than PCBs. Sorption studies also showed that sorption of PCBs is influenced by molecular planarity and hydrophobicity. Polarizability and dipole-moment were identified as important solute properties that affect the sorption behavior of dioxins and PCBSs. Linear relationships between these properties and log Kf’(subcooled liquid solubility normalized Freundlich sorption coefficient) values suggest that high sorption affinity of planar dioxins could be due to a combination of Van der Waals interactions with the siloxane surface, steric effects, and site-specific interactions between dioxin and exchangeable cations. In contrast, the sorption of PCBs was highly influenced by their molecular orientation.