Basic methodology of solid-phase extraction

First, let us provide a more descriptive definition of solid-phase extraction for the purposes of this discussion. Solid-phase extraction refers to a liquid, semi-liquid or potentially solid complexing agent dispersed more or less homogeneously within an inert, solid medium. A solid-phase extraction material comprises three major components: a solid support or substrate (typically inert polymer), a stationary extractant phase, and a mobile fluid phase (e. g., conditioning, feed, wash, or strip solution). The three phases of a solid-phase extraction resin are depicted in Fig. 13.1.

Various materials have been used for the solid support, e. g. inorganic materials such as silica gel, organic polymers such as polystyrene-divinyl — benzene copolymers and polymethacrylate resins, or resins made of both organic and inorganic materials. The stationary phase is typically an organic extractant, many of which have been well characterized for their use in liquid-liquid extraction. Similar to liquid-liquid extraction, the target metals are converted from the hydrated ionic form to a neutral organophilic metal complex within the stationary phase. The solid-phase extraction material is made in the form of small beads. Although different in terms of functional­ity, the beads are similar in size and shape to those used in conventional ion exchange practice and can be used in a fixed bed or column arrange­ment. The solid-phase extraction resin has many of the operational advan­tages of ion exchange processes and similar methods for modeling and engineering scale-up may also be used as discussed in a later section. The solid-phase extraction resin may, however, provide certain benefits over conventional ion exchange resins that are attractive to the separations sci­entist or chemical engineer. Potential advantages of solid-phase extraction are summarized as follows:

image219

13.1 Three phases of solid-phase extraction resin.

• Selectivity: recent and continuing advances in the synthesis of organic extractants provide numerous options for highly selective An and Ln separations, most especially from dilute streams of high ionic strength.

• Solvent loading: the utilization of macroporous resins can yield higher specific mass loadings of the complexing molecule relative to functional­ized ion exchange resins.

• Cost: simple immobilization of complexing agents or extractants within an inert polymer may prove to be a less costly preparation route than the complex synthesis mechanisms of covalently linking a specific func­tional group to the backbone of the resin.

There are, however, some potential disadvantages of the solid-phase extrac­tion approach which are discussed in a later section.