IngentaConnect BTEX Sorption by Montmorillonite Organo-Clays: TMPA, Adam, HDTMA

Abstract:

Organo-clay can be utilized for the containment of environmental pollutants originating from waste sites or accidental spills. A batch study was conducted using organo-clays produced from a Wyoming montmorillonite (SWy) and three organic cations (trimethylphenylammonium (TMPA), trimethylammonium adamantane (Adam), and hexadecyltrimethylammonium (HDTMA)) to characterize BTEX (benzene, toluene, ethylbenzene, o-, m-, p-xylene) sorption. Sorption data were fitted to two models, with Freundlich resulting in greater correlations of the data than the Langmuir model (R^2 at P 0.001-0.05). The Freundlich conditional index (n_f), which describes the experimental sorption characteristics, decreased curvilinearly with organic-cation molecular weights, thus suggesting organo-clays with smaller cations had greater hydrocarbon retention. Sorption of BTEX followed the order of TMPA > Adam > HDTMA organo-clays. A similar sequence in the magnitudes of log K_d and log K_omsupported this finding. Positive log K_om/K_ow values for TMPA and Adam derivatives indicated there was a greater retention of BTEX by these organo-clays than octanol. The order of log K_om for SWy-HDTMA, although concentration-dependent, was analogous to the log K_ow order, indicating partitioning was the dominant sorption mechanism for the HDTMA-clay. Isotherms for SWy-TMPA and SWy-Adam followed a convex up pattern. In contract, a concave up curvature, noted for SWy-HDTMA isotherms, was probably caused by a cosorptive enhancement process resulting from an effective increase in organic matter content of the organo-clay due to further hydrocarbon sorption, in concurrence with a decrease in adsorbate activity coefficients. Values of binding affinity constant, K_f, for SWy-TMPA were consistently higher than SWy-Adam. The K_f values determined for total BTEX sorption by TMPA and Adam derivatives were higher than those for the individual hydrocarbons. With SWy-HDTMA, the same order was observed for benzene and toluene; however, ethylbenzene and xylenes had greater K_f values than that for the BTEX mixture, possibly due to higher partitioning affinity of the larger alkylbenzenes. With SWy-HDTM, the sequence of K_f values was: ethylbenzene > m-xylene > p-xylene > o-xylene > toluene >benzene. Trends for SWy-TMPA and SWy-Adam were in contrast to that of the partitioning order, suggesting that adsorption, rather than partitioning, was the primary sorption mechanism for these two organo-clays. With respect to the equilibrium concentrations, the sorbed amounts for total BTEX mixture were generally higher than those for the individual compounds. As compared to benzene and toluene, the large-size alkylbenzenes showed greater partitioning affinity due to their high hydrophobicity.

Keywords: adsorption; BTEX; hexadecyltrimethylammonium; hydrocarbon; montmorillonite; partitioning; organo-clay; trimethylammonium adamantane; trimethylphenylammonium

Language: English

Document Type: Regular paper

Affiliations: 1: Soil and Environmental Sciences Group, Department of Renewable Resources, University of Wyoming, Laramie, WY 82071-3354, U.S.A. (author for correspondence, e-mail: shankar@uwyo.edu) 2: Plant and Soil Science Department, Texas Tech. University, Lubbock, TX 79409-2122, U.S.A. 3: Soil and Environmental Sciences Group, Department of Renewable Resources, University of Wyoming, Laramie, WY 82071-3354, U.S.A.