Extraction of Anions and Oxyanions

Abstract

The synthesis of complexants and hosts for specific metals is an important goal for high value toxic metals. For cations, the metal can be directly bound to a ligating group on the host, but for anions or uncharged molecules it may not be possible to get a direct interaction between the metal center and the atoms in the host. Although numerous molecules act as hosts and complexants for cations, fewer molecules function as hosts for anions.^1–4 Recently, however, a number of chemically modified calixarenes have been synthesized that can be used as hosts for simple anions, and this has helped to stimulate renewed interest in developing hosts for anions.^{5, 6} From an environmental viewpoint, a series of anions for which selective hosts would be useful are the oxyanions. Three such oxyanions are chromate, selenate and phosphate. Under basic solution conditions each of these anions has a pseudo-tetrahedral structure, therefore conceptually they can be viewed as a group where hosts may be interchangeable. Chromium(VI) is somewhat of an exception, however, because of the reversible formation of a dimer at low acidity. Furthermore, since remediation of chromate frequently involves its reduction to chromium(III), it is necessary to be aware of methods for extracting this reduced state of chromium when considering the removal of chromium(VI). The selenate, chromate, and dichromate oxyions are all environmentally important, both because of their high toxicity,^7–12 and because of their localized presence in soils and waters.¹³