title: Nature of the interlayer environment in an organoclay optimized for the sequestration of dibenzo-p-dioxin.
abstract: 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 31.4% 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. Shifts 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(l) sorption coefficients (log K(l)) 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(+).
authors.affiliation: Crop, Soil and Environmental Sciences, Purdue University, West Lafayette, Indiana 47907-2054, United States. clays@purdue.edu
authors.forename: Cliff T
authors.initials: CT
authors.lastname: Johnston
authors.forename: Bushra
authors.initials: B
authors.lastname: Khan
authors.forename: Edwin F
authors.initials: EF
authors.lastname: Barth
authors.forename: Sandip
authors.initials: S
authors.lastname: Chattopadhyay
authors.forename: Stephen A
authors.initials: SA
authors.lastname: Boyd
mesh_terms: D000327:Adsorption; D004147:Dioxins / Q000302:isolation & purification*; D008958:Models, Molecular; D000644:Quaternary Ammonium Compounds / Q000737:chemistry*; D017640:Silicates / Q000737:chemistry*; D017550:Spectroscopy, Fourier Transform Infrared; D014874:Water Pollutants, Chemical / Q000302:isolation & purification*
chemical_list: D004147:Dioxins; D000644:Quaternary Ammonium Compounds; D017640:Silicates; D014874:Water Pollutants, Chemical; C015831:dimethyldioctadecylammonium; C033214:Smectite; C020887:dibenzo(1,4)dioxin