Magnetic solid-phase extraction of strontium using core-shell structured magnetic microspheres impregnated with crown ether receptors: a response surface optimization

A new kind of core-shell structured magnetic microspheres impregnated with di-tert-butyl cyclohexano-18-crown-6 (DtBuCH18C6) receptors in the encapsulation layer was developed in this study, and was utilized for selective removal of strontium in strong HNO3 solutions via magnetic solid-phase extraction (MSPE). The magnetic microspheres, labelled as Fe3O4@SiO2@DtBuCH18C6, exhibited good magnetism (M (s) = 52.2 emu g(-1)), fast response under applied magnetic field and easy redispersion ability due to the superparamagnetic nature. Adsorption behaviors and mechanism were comprehensively studied, showing that the adsorption of Sr(II) by Fe3O4@SiO2@DtBuCH18C6 was most likely to be a monolayer chemisorption process via forming complexation between the crown ether receptors and Sr(II) (i.e. Langmuir model). More importantly, central composite design based on response surface methodology was utilized for optimizing the operational conditions of the adsorption process. A quadratic model was obtained to describe the relationship between adsorption capacity and the independent factors such as initial Sr(II) concentration, HNO3 concentration etc. The simulated model well predicted the optimum HNO3 concentration (similar to 1.7 mol L-1) to achieve the best Sr(II) adsorption capacity. On this basis, a conceptual MSPE process was proposed for an effective separation of strontium in simulated high level liquid wastes.