BTEX sorption by organo-clays: Cosorptive enhancement and equivalence of interlayer complexes

Aqueous organic contaminants, such as gasoline-derived aromatics, are effectively sorbed by organo-clays. Organo-clays were prepared from montmorillonite (SWy-1, SAz-1) and vermiculite (VSC) clay minerals by exchanging quaternary, alkylammonium cations for the inorganic exchange cations; thus, a variety of clay-charge/organiccation combinations were formed. Dodecyltrimethylammonium (DTMA), hexadecyltrimethylammonium (HDTMA), didodecyldimethylammonium (DDDMA), and dioctadecyldimethylammonium (DODMA) organic cations were used. An aqueous mixture of BTEX (benzene, toluene, ethylbenzene, xylenes) consistent with unleaded gasoline was used to relate organo-clay sorptivity to structure. Greater sorption of the BTEX mixture constituents than the pure compounds occurred when total sorbed BTEX loadings on the organo-clays exceeded 10 g/kg. A cosorption effect caused curvilinear isotherms and enhanced BTEX mixture sorption; BTEX sorption increased the organo-clay organic matter content, which promoted additional BTEX sorption. The sorptivities of the organo-clays were generally proportional to the organic C contents; although SAz-DDDMA and VSC-HDTMA, with 20 to 30% less C, had twice the BTEX sorptivity of SAz-DODMA and VSC-DDDMA. No relationship was found between surface area and BTEX sorption. A straight-chain alkyl cation conferred five times greater BTEX sorption to an organo-clay than did a comparable cyclic alkyl cation. This suggests that rearrangements in the alkyl groups may facilitate sorption in a manner analogous to solvation. Organo-clay BTEX sorptivity was governed by the type of alkylammonium complex; a paraffin complex with a 25.4 to 27.6 Angstrom basal spacing was optimal. Organo-clays of similar basal spacing prepared using different clays and alkylammonium cations had equivalent sorptive phases. Selection of an appropriate combination of clay and organic cation can optimize contaminant sorption by organo-clays.