Ammonium molybdophosphate/metal-organic framework composite as an effective adsorbent for capture of Rb+ and Cs+ from aqueous solution

A new composite adsorbent (AMP@MOF-76(Sm)) grafted ammonium molybdophosphate onto MOF-76(Sm) was synthesized via a solvothermal method and well-characterized by SEM-EDS, particle size analysis, N-2 adsorption-desorption isotherms, TG, FT-IR, XRD and XPS analysis. The adsorption behaviors of Rb+ and Cs+ on synthesized AMP@MOF-76(Sm) were investigated depending on the parameters of solution pH, ionic strength, contact time, ion concentration and temperature. The adsorbent can adsorb Rb+ and Cs+ from the solution when the solution pH ranged from 4.0 to 10.0 and the coexisted ion (Na+) has little effect on the adsorption amounts of both Rb+ and Cs+. The adsorption kinetic data fitted well with pseudo-second order kinetic models. The adsorption equilibrium of Rb+ and Cs+ on AMP@MOF-76(Sm) is better explained by the Langmuir isotherm model and the maximum adsorption amounts for Rb+ and Cs+ are 0.490 mmol g(-1) and 0.434 mmol g(-1), respectively. Furthermore, the AMP@MOF-76(Sm) could be easily reused at least four times using 3.0 mmol L-1 ammonia nitrate as the eluent. The mechanism of the adsorption of Rb+ and Cs+ is proposed to be a combination of electrostatic attraction and ion exchange mechanism.