One-Step Preparation of Thiol-Functionalized Porous Clay Heterostructures: Application to Hg(II) Binding and Characterization of Mass Transport Issues

Naturally occurring smectite clays have been expanded by a surfactant (cetyltrimethylammonium) and a cosurfactant (dodecylamine), which were then used in combination with the sol-gel process to prepare porous clay heterostructures (PCHs). A variety of thiol-functionalized PCHs have been obtained in one step by surfactant-directed co-condensation of tetraethoxysilane (TEOS) and 3-mercaptopropyltrimethoxysilane (MPTMS) in various molar ratios (ranging from 0 to 35% MPTMS) in the interlayer region of the clay. These materials have been characterized by several physicochemical techniques. They featured large specific surface areas and pore volumes (after template removal), due to the presence of regular mesopore channels between the clay sheets. but the mesostructural order and porosity were found to decrease upon rising the functionalization level. Hg(II) binding to these materials was then analyzed to get insight in the accessibility to the active centers and to characterize mass transfer rates to the binding sites. The experiments have been carried out at two distinct pH values (2 and 4) to point out the influence of speciation on these processes. Full accessibility was always observed at pH 4 where Hg(II) was adsorbed under a neutral form, giving rise to very high sorption capacities (up to about 600 mg g(-1)). The rate of access to the binding sites was usually faster in the more ordered materials, but some limitations were observed at pH 2 due to unfavorable electrostatic interactions. Finally, hydrodynamic electrochemistry has been applied to demonstrate that mesostructuration of the interlayer region of the clay has positive impact in terms of enhancing diffusion rates through PCH thin films in comparison to nonmodified clay coatings. The layered Structure of the materials creates their direct arrangement as mechanically stable layers onto electrode surfaces, contrary to other powdered mesoporous materials which require the use of a polymeric binder.