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

摘要

A Na-smectite clay (Na-SWy-2) was exchanged with various amounts of dimethyldioctadecylammonium bromide (DODA-Br) up to twice the cation exchange capacity (CEC). The organoclay (DODA-SWy-2) with DODA-Br added at 2 × CEC exhibited a maximum 4.2 nm d-spacing and a 31.496 carbon content, which demonstrates DODA~+ intercalation. DODA- SWy-2 was evaluated as an archetype of commercial products used to sequester hydrophobic contaminants, and the nature of the primarily C18 alkylhydrocarbon-chain interlayer environment was emhasized. Shirts in ν(CH) and CH_2 rocking band positions in DODA-SWy-2-complex FTIR-spectra indicate that DODA C18 chains were more ordered as DODA surface coverage was increased. Differential scanning calorimetry analysis indicated a DODA- SWy-2 gel-to-liquid transition temperature much lower than the melting point of crystalline DODA-Br and similar to that of aqueous DODA-Br vesicles. This suggests that the transition was governed by C18 alkyl tail-tail interactions in the clay interlamellar region. Dibenzo-p-dioxin (DD) sorption from water by DODA- SWy-2 was compared to DD sorption by the geosorbents granular activated carbon (GAC), K- exchanged saponite, and a muck soil. The linear K_1 sorption coefficients (log K_1) from a linear fit of the sorption isotherms were 4.37 for DODA-SWy-2, 5.55 for GAC, 3.19 for muck soil, and 2.46 for K-saponite. The DD-organic- matter-normalized sorption coefficient (K_(om)) was ～2.4 times the octanol-water partition coefficient (K_(ow)). This indicates that DD has a higher affinity for the nonpolar interlayer DODA organic phase than for octanol. In contrast, the K_(om) for muck soil DD sorption was ～10 times less than K_(ow), which reflects the higher polarity of amorphous soil organic matter relative to octanol. Enhanced DD uptake by the DODA-derived lipophilic phase in the organoclay is attributed to the low polarity, "open" C18 alkyl structure due to the physical dimensions of "v-shaped" DODA~+ molecular, and low density of the interlamellar phase (～0.50 g/ cm3) density of intercalated DODA~+.