Configurations of the Bentonite-Sorbed Myristylpyridinium Cation and Their Influences on the Uptake of Organic Compounds

Abstract

Variations in the configuration of a sorbed myristylpyridinium (MP+) surfactant cation with a loading level in bentonite were examined by X-ray diffraction (XRD) and Fourier transform infrared spectroscopy (FTIR); the attractive force from the clay basal surfaces to the sorbed MP+ and water molecule in the interlayers was estimated by thermogravimetric analysis (TG-DTG). At low loading levels (i.e., < CEC), MP+ sorbed mainly by cation exchange to clay basal surfaces to form tightly-adhered "organic films" (i.e., flat-layer). With increasing MP+ loading, sorbed MP+ gradually changed into a less confined "phase-like medium" (i.e., paraffin-type structures) via London attraction forces. Attractive forces between the mineral basal surfaces and the sorbed MP+ varied with the interlayer spacing and the stacking of MP+. Sorption of phenol and naphthalene to the resultant MP(+)--clay was a function of the configuration of MP+ aggregates on clay surfaces. At low MP+ densities, the sorbed MP+ film acted as an effective adsorbent for organic compounds. The carbon-normalized solute distribution coefficients (K(sf)) were exceptionally large and increased with MP+ densities up to approximately 340 (phenol) and approximattely 15000 mL g(-1) (naphthalene). At high MP+ loadings, the MP+ aggregates transformed into a partition-like medium, and the K(sf) values decreased sharply and leveled off. Nonetheless, because of the enhanced MP+ packing density within the clay interlayer, the solute K(sf) with a confined sorbed MP+ phase exceeded the corresponding aqueous micelle-water partition coefficients (k(mc)).