Calix[4]arene urea derivatives: The pathway from fundamental studies to the selective removal of fluorides and phosphates from water.

ABSTRACT

Wet processes of phosphoric acids produce untreated wastewater containing large amounts of fluoride leading to serious environmental problems. This paper reports fundamental studies on two lower rim functionalised calix[4]arene based receptors namely 5, 11, 17, 23 tetra-tert-butyl, 25, 27 bis [diethylphenylurea]ethoxy, 26, 28 dihydroxycalix[4]arene, 1 and 25, 27 bis[diethylphenylurea]ethoxy, 26, 28 dihydroxycalix[4]arene, 2 and their ionic interactions. It is shown that these receptors only interact with fluoride and phosphate in acetonitrile. Their receptive properties are higher for phosphate (21 anionreceptor complex) relative to fluoride (11 complex). However, thermodynamics shows that these receptors are more selective for fluoride relative to phosphate in the formation of the 11 complex. The pathway from fundamental studies to the use of these receptors for removing these anions from water has been tested. The receptive properties of 1 for phosphate are held when the extraction involves aqueous solutions containing individual ions. However, in mixtures containing both anions, the kinetics of the process and the selectivity of 1 for fluoride predominate and as a result, fluoride is better extracted than phosphate. The counter-ion effect on the removal process is assessed from Molecular Simulation studies. The removal of fluoride from phosphate is discussed taking into account existing technologies.