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Nature of the Interlayer Environment in an Organoclay Optimized for the Sequestration of Dibenzo-p-dioxin
Citation:
Johnston, C. T., B. Khan, E. F. BARTH, S. Chattopadhyay, AND S. A. Boyd. Nature of the Interlayer Environment in an Organoclay Optimized for the Sequestration of Dibenzo-p-dioxin. J.L. Schnoor (ed.), ENVIRONMENTAL SCIENCE AND TECHNOLOGY. ACS Publications, Washington, DC, 46(17):9584-9591, (2012).
Impact/Purpose:
The specific aims of this paper are three fold. The first is to characterize the interaction of DODA+ with Na-SWy-2 with emphasis on providing new insight into the physiochemical properties of the intercalated organophilic phase, and how these affect contaminant sorption, using a combination of structural, spectroscopic and thermal analysis tools. The second is to examine the sorption of dibenzo-p-dioxin (DD) by DODA-SWy-2 in aqueous suspension. The third aim is to compare the sorpiton of DD by DODA-SWy-2 smectite to that by three common geosorbents.
Description:
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 X 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 emphasized. Shifts in ν(CH) and the CH2 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 KI sorption coefficients (log KI)) 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 (Kom) was ~2.4 times the octanol-water partition coefficient (Kow). This indicates that DD has a higher affinity for the nonpolar interlayer DODA organic phase than for octanol. In contrast, the Kom for muck soil DD sorption was ~10 times less than Kow, which reflects the higher polarity of amorphous soil organic matter relative to octanol. Enhanced DD uptake by the DODA-derived lipophillic 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 +.
