Characterization of organo-montmorillonites and comparison for Sr(II) removal: Equilibrium and kinetic studies

Organo-montmorillonites (organo-Mts) were prepared by grafting with 3-aminopropyl triethoxysilane (APTES), and surface modification which was composed of hexadecyl trimethyl ammonium bromide (HDTMAB) and sodium dodecyl sulfate (SDS) solutions. XRD, BET, FTIR, SEM and Zeta potential analysis were used to characterize the surface and structure properties of the as-obtained products. The XRD and FTIR results showed that APTES was successfully grafted to calcium-saturated montmorillonite (Ca-Mt) and the BET data illustrated that the surface area and microporous volume decreased largely after modification, while the pore diameter increased. Moreover, batch experiments were conducted to investigate the adsorption behavior of Sr(II) on organo-Mts. The experimental results showed that the adsorption capacity of APTES-Mt was 65.6   mg/g, much larger than those of Ca-Mt, SDS-Mt and HDTMAB-Mt (13.23   mg/g, 26.85   mg/g and 3.91   mg/g, respectively) under the chosen conditions. Pseudo-second-order model fitted well with the adsorption kinetic studies for all materials with an equilibrium time of 24   h. Further, the isotherm adsorption could be described well with both Langmuir and DKR models for SDS-Mt, Freundlich model for Ca-Mt and APTES-Mt as well. The Sr(II) removal by Ca-Mt was mainly ion exchange, nevertheless, that by SDS-Mt and APTES-Mt were surface adsorption and ligand adsorption, respectively.