One-pot synthesis of Fe(III)-coordinated diamino-functionalized mesoporous silica: Effect of functionalization degrees on structures and phosphate adsorption

Fe(III)-coordinated mesoporous silica adsorbents functionalized with different loadings of ethylenediamine groups were prepared by a new NH 4 F-assisted co-condensation method and impregnation of Fe 3+ cations. Various characterization techniques, e.g. XRD , SEM, TEM, ICP-MS, elemental analysis , FT-IR spectroscopy, and nitrogen adsorption–desorption, were utilized to investigate the effect of functionalization degrees of adsorbents on their chemical composition, surface chemistry, pore structures and phosphate adsorption capacities in detail. In the batch adsorption tests, the functionalized adsorbents with increasing loadings of ethylenediamine groups possessed markedly enhanced adsorption capacities, although there was a gradual loss of ordered mesostructures accompanied. The adsorption isotherms were represented better by using Langmuir model than Freundlich model, which indicated the presence of monolayer adsorption. In particular, for the resulting adsorbent prepared with 0.5:1   M ratio of AAPTS and TEOS, the maximum phosphate capture capacity calculated from Langmuir model is 20.7   mg P/g. In the kinetic study , the phosphate adsorption followed pseudo-second-order equation well with a correlation coefficient of 0.999, suggesting the adsorption process be chemisorption . The phosphate adsorption efficiency of prepared adsorbent was highly pH-dependent and the high removal of phosphate was achieved within the pH between 3.0 and 6.0. The presence of Cl − and NO 3 - ${\text{NO}}_3^{-}$ exhibited small impacts on the phosphate adsorption by using our synthesized absorbent; whereas, there were significantly negative effects from HCO 3 - ${\text{HCO}}_3^{-}$ and SO 4 2 - ${\text{SO}}_4^{2-}$ on the phosphate removal. In 0.010   M NaOH, more than 90% of the absorbed phosphate anions on the spent adsorbent could be desorbed, suggesting the absorbent with a capacity of regeneration.