Synthesis Of A Novel Silica-supported Chelating Adsorbent And Its Properties For Boron Removal

Boron is a naturally occurring element in the salt brine lake. Boric acid is usually prepared by acid precipitation and there is still about0.2mol/L boric acid in the brine after the acid precipitation. Because of easy coprecipitation with Mg in alkaline, the purity of the magnesium hydroxide was seriously influenced by residual boric acid in the subsequent production process from salt lake industry. The chelating adsorbent was synthesized through two different routes:one route was carried out by anchoring the chelating agent of N-methyl-d-glucamine(MG) to the y-chloropropyltrimethoxysilane (CPTS) or (3-glycidoxypropyl) trimethoxysilane(GPTMS) functionalized silica matrix(SiCl or Si-Epoxy), and another one involved the reaction of MG with GPTMS prior to immobilization on the silica support. It was demonstrated by elemental analysis and boron adsorption results that the post-grafting method allows the preparation of functionalized silica with higher loadings of organic functional groups and higher boron adsorption capacities onto the adsorbent. The surface characterization was performed by FT-IR, XPS and SEM indicating that the modification of the silica surface was successfully performed by the functional groups of MG.The adsorption properties of Si-MG for boron removal were studied by batch method. The effects of media pH, adsorption dynamics, adsorption isotherms and effect of ionic strength had been investigated. Adsorption capacities increased as the increase of pH value between2.5to9and decreses as the increase of pH value above9.3because of the formation of polyanionic species. The adsorption equilibriums of boron onto Si-MG was attained about30min, the adsorption process followed a second-order type reaction kinetics and the rate-limiting step of boron onto Si-MG may be chemical adsorption. The characteristics of the adsorption process could be described by the Freundlich adsorption isotherm model and adsorption capacities were found to be1.54mmol/g, indicated that the adsorption may be heterogeneous. The effect of ionic strength indicates that adsorption capacities increased as the increase of the ionic of Mg2+and Li+below0.1mol/L. It demonstrates that Si-MG has great potentialities for the boron removal in desalination of salt lake brine.