Selective adsorption of molybdate from water by polystyrene anion exchanger-supporting nanocomposite of hydrous ferric oxides.

Molybdenum is an essential trace element for humans but can be harmful with excess assimilations or chronic exposures. In this study a polymer-functionalized nanocomposite (HFO-PsAX) was fabricated for selective adsorption of molybdate from aqueous solution. HFO-PsAX was prepared by grafting hydrous ferric oxide nanoparticles (HFOs) into the porous structure of a polystyrene anion exchanger (PsAX) by in situ synthesis method. The resultant HFO-PsAX exhibited greatly enhanced selectivity toward molybdate as compared with the matrix, PsAX, which is also a fair adsorbent for scavenging molybdate. The competitive abilities of the ubiquitous anions, i.e., chloride, carbonate, sulfate, and phosphate, on the adsorption of molybdate by HFO-PsAX followed the order: chloride < phosphate < carbonate < sulfate. The unexpectedly weak competitive ability of trivalent phosphate may be due to incompletely dissociated state and formation of molybdate-phosphate complexes. The optimal pH for the adsorption of molybdate was determined as pH≈4, which is associated with the dissociation constants of molybdic acid; certain adsorption capacities were also observed even under extremely alkaline condition (pH=14) for single-component molybdate solution. Temperature (10, 25, and 40°C) has negligible effect on the adsorption capacities by HFO-PsAX, and Freundlich model and Dubinin-Radushkevich (D-R), Temkin model can describe the adsorption isotherms well. The adsorption potential of Temkin model is calculated as ≈100J/mol, which is between those of physisorption and chemisorption process. Fixed-bed column adsorption experiments validated the potential of HFO-PsAX in treating Mo(VI) contaminated water for practical application, and the exhausted HFO-PsAX can be regenerated by a binary NaOH-NaCl solution (both 5% in mass) without loss in adsorption capacities.