[Adsorption Mechanism of Cadmium by Superparamagnetic Nano-Fe₃ O₄@SiO₂ Functionalized Materials]

The treatment of cadmium-containing wastewater is of great significance for the emission control of the heavy metal cadmium. Here, a superparamagnetic nano-Fe₃O₄@SiO₂ functionalized material (MFS) was prepared via a co-precipitation method, and the adsorption thermodynamic and kinetic characteristics of Cd²⁺ were studied by isothermal adsorption tests and kinetic experiments. The adsorption process and mechanism of MFS with respect to Cd²⁺ were also studied using BET, XRD, and SEM. The Langmuir equation well described the isothermal adsorption characteristics of MFS, and the maximum adsorption capacity was 69.49 mg·g^-1. The standard free energy (ΔG), enthalpy (ΔH), and entropy changes (ΔS) showed that the adsorption reaction was a spontaneous, endothermic, and entropic process. The optimal initial pH of the adsorption reaction was 7. The four interfering ions (Mg²⁺, SO₄^2-, Ca²⁺, and NO₃^-) in the solution had a certain inhibitory effect on the adsorption reaction. The pseudo-second-order kinetic model showed that the adsorption process of Cd²⁺ was divided into two stages, namely a rapid external diffusion stage and a slow internal diffusion stage. The removal rate of Cd²⁺ was still>73% after using the MFS three times. The BET, XRD, FTIR, and VSM analyses showed that SiO₂ was successfully modified on the Fe₃O₄ surface. MFS is mainly spherical in structure with an average particle size of 38.7 nm and has a saturated magnetic intensity of 85.38 emu·g^-1. The XRD, EDS, and XPS analyses revealed that Cd²⁺ was successfully adsorbed by the material, and the main mechanism was the coordination reaction between Cd²⁺ and -OH on the surface of the material.