Restricting layer collapse enhances the adsorption capacity of reduced-charge organoclays

Abstract

By restricting layer collapse, and increasing the exposure of siloxane surfaces, in the interlayer space of organoclays, their capacity for adsorbing hydrophobic organic contaminants has been enhanced. The organoclays were prepared by replacing a proportion (20–80%) of the Li+ ions, occupying interlayer sites in montmorillonite, with tetramethylammonium (TMA) ions. The TMA-exchanged samples were then heated at 200°C for 12h to induce migration of most interlayer Li+ ions into the silicate layers, and as a result of which the layer charge was reduced. Finally, the remaining Li+ cations in the reduced-charge montmorillonite were exchanged with TMA. The structural and adsorptive characteristics of the novel reduced-charge organoclays (N-TMA-Mt) are compared with those of their traditional counterparts (T-TMA-Mt), obtained by direct intercalation of TMA into reduced-charge montmorillonites. As layer charge decreased, both the specific surface area and adsorption capacity (for nitrobenzene) of T-TMA-Mt increased to a maximum, and then declined. In the case of N-TMA-Mt, however, both parameters increased as layer charge decreased. When the layer charge of N-TMA-Mt decreased to approximately 60% of the value for the original montmorillonite, the adsorption capacity of the sample was greater than that of T-TMA-Mt. XRD analysis indicates that the layer structure of N-TMA-Mt is more rigid than that of T-TMA-Mt. The above results indicate that the pre-exchanged TMA cations in the interlayer space of N-TMA-Mt act as pillars, restricting layer collapse (during thermal treatment), and increasing the exposure of siloxane surfaces. As a result, the capacity of N-TMA-Mt for adsorbing hydrophobic organic contaminants is superior to that of traditional organoclays.