Effective removal of fluorine ions in phosphoric acid by silicate molecular sieve synthesized by hexafluorosilicic acid

Silicone zeolite prepared from hexafluorosilicic acid is a good adsorbent for fluoride removal. The morphology and structure of the prepared composites were characterized by XRD, FT-IR and BET. XRD images showed that fluorine was adsorbed on the surface of zeolite and the FT-IR spectra showed that there are different kinds of silicon and aluminum in silica-based zeolite. Therefore, through the studied of regeneration-adsorption cycle of zeolite, fluoride desorption and regeneration mechanism of zeolite defluorination was explained, and the adsorption performance and regeneration adsorption performance of zeolite defluorination were evaluated. The effects of phosphoric acid concentration, operating condition and specific surface area of zeolite and Si/Al ratio of zeolite on adsorption performance were investigated. Silicate molecule sieve shows strong adsorption capacity for fluoride (F-), and the adsorption capacity reached 352.46 mg/g under the optimal conditions. The Freundlich and Langmuir isotherm models for aluminosilicate MCM-41 and NaA zeolite which experimental data of F- adsorption on in phosphoric acid were fitted to investigate the adsorption mechanism. In the mechanism of anion adsorption, F- or SiF62- is bound by electrostatic adsorption of hydroxyl on the surface of adsorbent with anion adsorption. The adsorbed SiF62− ions can be regenerated when the environmental pH increases to value higher than that of zero charge point of zeolite. There is a competitive adsorption between OH– and F- with negative charge, which reduces the adsorption efficiency of adsorbent for F-. The NaA zeolite regenerated with NH3·H2O showed better fluorine re-adsorption performance.