Size-Dependent Phase Transformation and Vacancy Formation Drive Highly Efficient Removal of Cadmium with ZnS

Abstract

Zinc sulfide (ZnS) is a promising material for removing cadmium from the environment. Here, we report a facile method to synthesize ZnS with mixed sphalerite and wurtzite phases and abundant zinc vacancies under ambient conditions for the highly efficient removal of cadmium. X-ray diffraction and high-resolution transmission electron microscopy confirmed that the reduction in the particle size of ZnS nanoparticles loaded on the surface of acid-thermal-modified sepiolite particles (ASP-ZnS) from ∼20 to ∼5 nm induced a phase transformation of ZnS from sphalerite to wurtzite. X-ray photoelectron spectroscopy showed that more zinc vacancies were generated with the decrease in ZnS particle size, and the Cd(II) incorporation into Zn vacancies was promoted accordingly. The Cd(II) removal capacity of ASP-ZnS was 293.25 mg·g^–1, which was ∼2 times that of bare ZnS. It was demonstrated that the enhancement of Cd(II) removal capacity was due to the synergistic effects of the Cd(II) incorporation into the zinc vacancies, isomorphous substitution of Zn(II) with Cd(II), and Cd(II) adsorption on ASP. Both wurtzite and sphalerite phases ZnS would transform into wurtzite Zn_0.78Cd_0.22S after Cd(II) adsorption. The particle-size-dependent phase transformation and vacancy formation provide a new strategy for designing highly efficient cadmium removal materials.