Introduction

Successful synthesis of silica-based nanoporous materials using liquid crystal templating was achieved about a decade ago [1,2]. Since then, use of these nanoporous materials in diverse applications such as catalysis, sensor technology, and sorbents has proved to be feasible. In sorbent technology applications, the nanoporous materials offer a significant advantage over conventional sorbents, due to their high surface areas (~500 to 1000 m2/g). However, the pore surfaces of these novel materials need to be activated before they can be deployed as effective sorbents.

Typically, the nanoporous materials are synthesized through a combination of oxide precursors and surfactant molecules in solution reacted under mild hydrothermal conditions. Under these conditions, the surfactant molecules form hexagonally ordered rod-like micelles, and the oxide materials precipitate on these micellar surfaces to replicate the organic templates formed by the rod-like micelles. Subsequent calcination at 500°C removes the surfactant templates and leaves a high surface area nanoporous ceramic substrate. The pore size of these ceramic substrates can be controlled by using surfactants of different chain lengths.

The authors have developed a method to activate the pore surfaces of silica-based nanoporous materials so that these materials can be used as effective sorbents. This process consists of synthesizing, within pores, self-assembled monolayers of adsorptive functional groups selected to adsorb specific groups of contaminants. Molecular self-assembly is a unique phenomenon in which functional molecules aggregate on an active surface, resulting in an organized assembly that has order and orientation.

In this approach, bifunctional molecules containing a hydrophilic head group and a hydrophobic tail group adsorb onto a substrate or an interface as closely packed monolayers (Figure 19.1). The driving forces for the self-assembly are the intermolecular interactions between the functional molecules (such as van der Waals forces). The tail group and the head group can be chemically modified to contain certain functional groups to promote covalent bonding between the functional organic molecules and the substrate on one end, and the molecular bonding between the organic molecules and the metals on the other. For instance, populating the head group with alkylthiols (which are well known to have a high affinity for various soft heavy metals, including mercury) results in a functional monolayer that specifically adsorbs heavy metals such as Ag, Cd, Cu, Hg, and Pb.

If the head group consists of Cu-ethylenediamine complex, the monolayer will sorb oxyanions (As, Cr, Se, Mo) with high specificity. Additional monolayers with head groups designed by the authors include acetamide and propinamide phosphonates for binding actinides (Am, Pu, U, Th); Hg- and Ag-thiol for sorbing radioiodine; and a ferricyanide Cu-EDA complex for selectively bonding radiocesium. The functionalized monolayer and substrate composite (Figure 19.1) was designated as SAMMS. Various self-assembled monolayer functionalities and the contaminants that they were designed to target are shown in Figure 19.2.

A. Self-assembled monolayers

A. Self-assembled monolayers

B. Ordered measoparous oxide

FIGURE 19.1 Technological basis of novel nanoporous sorbents.

C. Self-assembled monolayers on mesoparous supports (SAMMS)

FIGURE 19.1 Technological basis of novel nanoporous sorbents.

Cu-EDA SAMMS

Ag, Hg thiol SAMMS

Cu-EDA SAMMS

Ag, Hg thiol SAMMS

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