Removal of cadmium(II) from aqueous solutions by a novel sulfide-modified nanoscale zero-valent iron supported on kaolinite: Treatment efficiency, kinetics and mechanisms

Abstract

Nano zero-valent iron (nZVI) had great potential in water pollution remediation, but the instability and easy agglomeration limited its application. To improve the dispersion and antioxidant properties of the nZVI, kaolinite-supported sulfide-modified nZVI (S-nZVI@Kaol) was synthesized for the removal of cadmium (II) (Cd(II)) in aqueous solution. The structure and morphology of the samples were characterized by field emission scanning electronic microscopy (FE-SEM) and X-ray diffraction (XRD), the elemental distribution and microstructure of the S-nZVI@Kaol were accomplished by transmission electron microcopy (TEM) and energy dispersive X-Ray spectroscopy (EDX), Fourier transform infrared (FTIR) and X-ray photoelectron spectrum (XPS). The results showed that S-nZVI was successfully loaded on kaolinite with a “core–shell” structure. The S-nZVI had higher specific surface area and lower magnetism compared with nZVI. The adsorption process of S-nZVI@Kaol for Cd(II) conformed to the pseudo-second-order kinetic model and Langmuir adsorption isotherm model better, and the maximum adsorption capacity was 104.7 mg·g−1. When the S/Fe molar ratio (MS/Fe) was 0.3, and the Fe/Kaol mass ratio (mFe/Kaol) was 50:50, S-nZVI@Kaol had the best removal efficiency for Cd(II). A higher solution pH (3.0–9.0) was beneficial for the removal of Cd(II). Adsorption thermodynamics studies showed that removing Cd(II) by composites was a spontaneous endothermic process. The mechanisms of the removal of Cd(II) involved electrostatic gravitation, chemical precipitation of CdS, and surface complexation.