Probing the microscopic hydrophobicity of smectite surfaces. A vibrational spectroscopic study of dibenzo-p-dioxin sorption to smectite

Abstract

The interaction of dibenzo-p-dioxin (DD), from aqueous suspension, with smectite was investigated using in situ vibrational spectroscopy (FTIR and Raman), structural and batch sorption techniques. Batch sorption isotherms were integrated with in situ attenuated total reflectance (ATR)-FTIR and Raman spectroscopy and X-ray diffraction. Sorption isotherms revealed that the affinity of DD for smectite in aqueous suspension was strongly influenced both by the type of smectite and by the nature of the exchangeable cation. Cs-saponite showed a much higher affinity over Rb-, K- and Na-exchange saponites. In addition, DD sorption was found to depend on clay type with DD showing a high affinity for the tetrahedrally substituted trioctahedral saponite over SWy-2 and Upton montmorillonites. A structural model is introduced to account for the influence of clay type. Raman and FTIR data provided complementary molecular-level insight into the sorption mechanisms. In the case of Cs-saponite, the selection rules of DD based on D(2h) symmetry were broken indicating a site-specific interaction between DD and intercalated Cs(+) ions in the interlayer of the clay. Polarized in situ ATR-FTIR spectra revealed that the molecular plane of sorbed DD was tilted with respect to the clay surface which was consistent with a d-spacing of 1.49 nm. Finally, cation-induced changes in both the skeletal ring vibrations and the asymmetric C-O-C stretching vibrations provided evidence for site specific interactions between the DD and exchangeable cations in the clay interlayer. Together, the combined macroscopic and spectroscopic data show a surprising link between a hydrophilic material and a planar hydrophobic aromatic hydrocarbon.

Nature of the Interlayer Environment in an Organoclay Optimized for the Sequestration of Dibenzo-p-dioxin

“…61 This is especially evident for Cs + -exchanged saponite. 54,55 The use of K-saponite was included in this study to evaluate the role of mineral-only surfaces. Although significant, the overall sorption affinity of DD for K-saponite is several orders of magnitude less than that for DODA–SWy-2 or GAC.…”

“…Physicochemical details about this clay have been reported previously. 52–54 The clay was exchanged with Na + by placing 40 g of the clay in 1 dm 3 of 0.5 M NaCl for 24 h. The resulting Na–smectite suspension was “washed” with 0.5 M NaCl twice. Excess salts were removed by repeated centrifugation with Millipore water until a negative Cl − test using AgNO 3 was obtained.…”

“…In related work we have shown that K-exchanged saponite (and Cs-saponite) has a high affinity for dibenzo- p -dioxin, 54,55 hence it was included in this study. The Houghton organic muck soil (low mineral content, 46.5% OC, and a CEC of 214 cmol c kg −1 ) was provided by Prof. Hui Li at Michigan State University and used to represent natural soil organic matter.…”

Reconstitution of cutin monomers on smectite surfaces: adsorption and esterification

“…Higher extent of isomorphic substitution in tetrahedral layers of NAu2, results in stronger localized charge on the clay surface with respect to SWy1. Rana et al (2009) suggested that localized charge leaves more "charge-neutral" siloxane surfaces that are not influenced directly by isomorphic substitutions. In nontronite these sites may be more favorable for hydrophobic interactions.…”

In situ particle film ATR FTIR spectroscopy of poly (N-isopropyl acrylamide) (PNIPAM) adsorption onto talc

“…One of the most useful ways to study molecular adsorption onto talc (and other minerals/particles) is to use the methodology of particle film attenuated total reflection (ATR) FTIR spectroscopy. [36][37][38][39][40][41][42] The detection of polymer adsorbing from a dilute solution onto a thin (sub-micron thickness) layer of mineral particles on top of an ATR crystal allows direct determination of important parameters such as adsorption kinetics and polymer binding mechanisms. [43][44][45][46] The data presented below for PNIPAM on talc reveal information of the polymer conformation at the mineral surface, the rapidity of adsorption, tendency for desorption, and an inference of the most likely binding mechanism for the polymer-mineral interaction.…”