Removal of iodide anions in water by silver nanoparticles supported on polystyrene anion exchanger

The removal of iodide (I⁻) from source waters is an effective strategy to minimize the formation of iodinated disinfection by-products (DBPs), which are more toxic than their brominated and chlorinated analogues. In this work, a nanocomposite Ag-D201 was synthesized by multiple in situ reduction of Ag-complex in D201 polymer matrix, to achieve highly efficient removal of iodide from water. Scanning electron microscope /energy dispersive spectrometer characterization showed that uniform cubic silver nanoparticles (AgNPs) evenly dispersed in the D201 pores. The equilibrium isotherms data for iodide adsorption onto Ag-D201 was well fitted with Langmuir isotherm with the adsorption capacity of 533 mg/g at neutral pH. The adsorption capacity of Ag-D201 increased with the decrease of pH in acidic aqueous solution, and reached the maximum value of 802 mg/g at pH 2. This was attributed to the oxidization of I⁻, by dissolved oxygen under the catalysis of AgNPs, to I₂ which was finally adsorbed as AgI₃. However, the aqueous solutions at pH 7 – 11 could hardly affect the iodide adsorption. The adsorption of I⁻ was barely affected by real water matrixes such as competitive anions (SO₄²⁻, NO₃⁻, HCO₃⁻, Cl⁻) and natural organic matter, of which interference of NOM was offset by the presence of Ca²⁺. The proposed synergistic mechanism for the excellent performance of iodide adsorption by the absorbent was ascribed to the Donnan membrane effect caused by the D201 resin, the chemisorption of I⁻ by AgNPs, and the catalytic effect of AgNPs.