Clay minerals may be an important sorptive phase for dioxins in soils, and dioxins are often found in natural clay deposits. Sorption experiments demonstrated that smectites, especially Cs-saponite, effectively adsorbed dibenzo-p-dioxin /furan from water. Adsorption was promoted by exchangeable cations with low hydration energies, and negative charge in the smectite arising from the tetrahedral siloxane sheets. X-ray diffraction measurements revealed that as dibenzo-p-dioxin (DD) loading increased to ≥ 8,000 mg/kg the clay basal spacing increased abruptly from 12.3 to 15.2 Å, thereby demonstrating DD intercalation. The 12.3 Å spacing provides an interlayer distance that closely matches the molecular thickness of DD. The geometrical configurations of intercalated DD in the interlayer were investigated by a combination of ab initio calculations and Fourier transformation infrared spectroscopy. Molecular dynamics simulations of saponites saturated with different exchangeable cations (Na+, K+, Cs+ and Ca2+) with one, two and three layers of water in the interlayer regions further supported the hypothesis that Odd-interlayer cation complexation plays an important role in adsorption and the order of coordination numbers between Odd-interlayer cation follows Cs+>K+>Na+>Ca2+, in good agreement with the observed DD adsorption to the smectites from water.Octachlorodibenzodioxin (OCDD) forms spontaneously from pentachlorophenol (PCP) on the surfaces of Fe(III)-saturated smectite clay. Here, in situ FTIR methods and quantum mechanical calculations were used to determine the mechanism by which this reaction is initiated. As the clay was dehydrated, vibrational spectra showed new peaks that grew and then reversibly disappeared as the clay rehydrated. Density functional theory (DFT) calculations of hydrated Fe-PCP clusters reproduced these transient FTIR peaks when inner-sphere complexation and concomitant electron transfer produced Fe(II) and PCP radical cations. Thus, our experimental (FTIR) and theoretical (quantum mechanical) results mutually support the hypothesis that OCDD formation on Fe-smectite surfaces is initiated by the reversible formation of metastable PCP radical cations via single electron transfer from PCP to Fe(III). The negatively charged clay surface apparently selects for this reaction mechanism by stabilizing PCP radical cations. DFT methods were further applied to the calculations of 2,4,5-trichlorophenol (2,4,5-TCP) interacting with iron(III) species to evaluate the possible reaction pathways and the associated reaction energies and activation energies. The calculations support the formation of the direct precursors of the most toxic PCDD congener, 2,3,7,8-TCDD, and of 1,2,4,7,8-pentachlorodizenzodioxin, and two additional dimers, from the reaction of 2,4,5-TCP with Fe(III)-montmorillonite clay, which imply that soil contaminated with 2,4,5-trichlorophenol may have the potential risk for 2,3,7,8-TCDD contamination.
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