Design and development of ion-selective polymer-supported reagents: The immobilization of heptamolybdate anions for the complexation of silicate through Keggin structure formation

A silicate-selective polymer-supported reagent has been developed that utilizes the reactivity of silicate to generate polyanionic Keggin structures. Heptamolybdate and heptatungstate anions are immobilized onto trimethylammonium ligands bound to microporous poly(vinylbenzyl chloride) beads. The heptamolybdate complexes >90% of the silicate from a 20 ppm solution at pH 7; the heptatungstate has a lower affinity, complexing 40% of the silicate. Complexation by the heptamolybdate remains high throughout the pH range 3.8-10.7. Sorption is unaffected by the presence of chloride, sulfate, and nitrate ions. The apparent rate of reaction is maximized by immobilizing the ligand on an expanded gel support: whereas the microporous polymer requires 24 h to complex all of the silicate from a 100 ppm solution, the expanded gel attains that level in 4 h. The rate-limiting step is thus identified as accessibility of the silicate to the heptamolybdate rather than the rate of reaction to form the silicomolybdate. FTIR spectra confirm silicomolybdate formation: The heptamolybdate polymer has four characteristic bands at 715, 845, 912 and 945 cm−1 and these bands get weaker as silicate reacts with the heptamolybdate; at complete reaction, the band at 845 cm−1 disappears. The spectrum of the silica-saturated polymer has strong bands at 900-904 cm−1 and 790-800 cm−1, consistent with the spectrum of tert-butylammonium silicododecamolybdate.