An important goal in environmental chemistry is the extraction of metals that are toxic or radioactive from soils and waters. For many such metals, the problem is solved by designing compounds with coordination sites that are specific for that particular metal. For cases such as oxoanions, however, where the inorganic center is already fully coordinated by oxygens, different strategies need to be used. Chromate, phosphate, selenate, pertechnetate, and aluminate are such anions. These species are problematic contaminants in soils and waters because they are either toxic, environmentally undesirable, or radioactive. Furthermore, under both acidic and basic conditions, the coordination positions of these oxoanions are occupied by oxygens. Chromium(VI) as either chromate or dichromate presents a particular problem because it is a strong oxidizing agent. In this review the different methods of extracting this group of pseudotetrahedral oxoanions are discussed, along with the individual advantages and limitations of each strategy.

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† Satisfactory elemental analyses have been obtained for these calix[4]arenes. The calix[4]arenes were purified on a silica column using the eluents dichloromethane and ethyl acetate for 1 and ethyl acetate and light petroleum (bp 40-50 °C) for 2 and 3.

ChemInform Abstract (1H, 13C NMR examination of stereochemistry and conformer population in solution; (II): space group P21 with Z=2).