Original Synthesis and Supramolecular Chemistry of Calixarene-Based Receptors for Selective Recognition and Extraction of Uranium from Nigerian Phosphate Processing Streams

Nigerian phosphate ore deposits, particularly those in Sokoto State, contain naturally occurring uranium concentrations that partition into phosphoric acid processing streams during fertiliser manufacture, creating both a radioactive contamination management challenge and a potential critical mineral recovery opportunity, yet no selective receptor chemistry has been developed and evaluated for uranium extraction from the specific ionic matrix conditions of Nigerian phosphate processing liquors. This study performs the original synthesis and comprehensive supramolecular characterisation of a series of calixarene-based macrocyclic receptor molecules functionalised with phosphinoyl, amidoxime, and hydroxamic acid pendant arms designed for selective uranium(VI) binding over competing cations including thorium, radium, rare earth elements, calcium, and phosphate. Receptor synthesis employs multi-step convergent routes from para-tert-butylcalix[4]arene and calix[6]arene scaffolds, with all intermediates and final products characterised by nuclear magnetic resonance spectroscopy (1H, 13C, 31P), mass spectrometry, elemental analysis, and single-crystal X-ray diffraction. Uranium binding constants and selectivity coefficients are determined by potentiometric titration, fluorescence spectrophotometry, and isothermal titration calorimetry in aqueous and ionic liquid media representative of phosphoric acid process stream conditions. Density functional theory and molecular dynamics simulations provide mechanistic insight into host-guest geometry and binding energy for each receptor-uranyl complex. Solvent extraction performance tests in synthetic and real Sokoto phosphoric acid samples quantify extraction efficiency, stripping performance, and uranium product purity. An original supramolecular design principle for calixarene-based actinide receptors under high-ionic-strength acidic conditions constitutes the primary theoretical contribution. Keywords: calixarene receptors, uranium extraction, supramolecular chemistry, phosphate processing, Sokoto State