A critical comparison of Pb-loaded organosilica and ion-exchange resins, with bispicolylamine ligands, for adsorption of iodide: evidence of a ‘matrix effect’ on uptake properties

Ion-exchange resins currently rely upon petrochemicals for their manufacture. Alternative materials are under consideration to create adsorbents for selective removal of ions from water. However, there has been little investigation of the impact on these adsorbent chemistries on the resulting fundamental adsorption behaviour. Presented here is an investigative comparison of metal-loaded polymeric and silica-based adsorbents for the remit of selective radioiodine (in the form of iodide) capture, using lead ions, chelated by bispicolylamine (BPA) ligands to create iodide affinity. The results indicate marked differences in uptake behaviour between the two materials. The ion-exchange resin (M4195-Pb) displayed more thermodynamically-favourable iodide capture, with a calculated uptake of 338 ± 12 mg g−1, and retained some selectivity in the presence of acid, and 10 M equivalents of nitrate and molybdate. The silica had much lower capacity of 46.5 ± 7.6 mg g−1, but adsorbed extremely rapidly, reaching near capacity in <1 min. Generally, the binding of both Pb2+ and iodide were significantly destabilised by the silica matrix, which was attributed primarily to a H-bonding interaction between the BPA ligand and the adsorbent surface. It did however, result in generally better thermal stability of the bound iodide. This appears to be the first report of this nature into this ‘matrix effect’ phenomenon and demonstrates that great care is required in the future design of ion-exchange and adsorption media using alternate support chemistries.