Mild pre-heating of organic cation-exchanged clays enhances their interactions with nitrobenzene in aqueous environment

Aqueous sorption kinetics and equilibrium isotherms of nitrobenzene were studied on two series of sorbents that were prepared by (i) replacing inorganic exchangeable cations in Wyoming bentonite with tetraethylammonium (TEA) and benzyltrimethylammonium (BTMA) and (ii) heating synthesized complexes in air at different temperatures (between 150 and 420°C). The aim of this work was to examine recently observed enhancement of aqueous sorption of a probe organic sorbate on organoclays after mild thermal pre-treatment of sorbents. Thermal pretreatment of TEA- and BTMA-clays at 150°C results in the maximal enhancement of nitrobenzene-sorbent interactions as compared with treatment of original bentonite and its exchange complexes formed with long-chain quaternary ammonium (n-hexadecyltrimethylammonium, HDTMA). Based on C, N content data and FTIR spectra of TEAand BTMA-clay complexes, no indications of decomposition of organic matter were found in organoclays heated at 250°C (and below this temperature). Suppressed hydration of pre-heated sorbents resulting in a lessening of water-organic sorbate competition for sorption sites is considered to be responsible for thermally induced enhancement of nitrobenzene-sorbent interactions. In the HDTMA-based organoclays, the long-chain aliphatic groups of the quaternary ammonium can additionally interact with clay surface thus competing with organic sorbate-sorbent surface interactions and, in this way, mitigating the enhancement of nitrobenzene sorption on thermally treated sorbents.