Heavy metal ion adsorption mechanism of polysulfide-modified layered double hydroxide

Abstract

A detailed study was conducted on the capture mechanism of heavy metals, especially cadmium (Cd), by polysulfide ion ([S_x]²⁻; x = 3, 4)-intercalated 3D rose-shaped layered double hydroxide (S_x-roseLDH). The roseLDH (Mg/Al = 2) was synthesized by homogeneous precipitation method using hexamethylene-tetramine (HMT) as an ammonia generator. Polysulfides were intercalated between the LDH layers by exchange with Cl⁻ ions. Polysulfide ions were introduced between the LDH layers by exchanging with Cl⁻ ions. The results showed that Sx-roseLDH effectively captured and immobilized Cd²⁺ ions from aqueous solutions. Additionally, this adsorbent demonstrated strong selectivity for Cd²⁺ and Zn²⁺ in simulated wastewater and mixed solutions containing Cd²⁺, Zn²⁺, and Ni²⁺. Such selectivity and high removal efficiency for Cd²⁺ were not observed with montmorillonite. The adsorption mechanism was investigated through solution analysis after adsorption and solid sample analysis using techniques such as FTIR, Raman spectroscopy, XRD, XPS, SEM-EDS, STEM-EDS, and HRTEM. These analyses revealed that during adsorption, Cd²⁺ formed CdS through SS bonds and was trapped in the cleavages of the LDH crystals, rather than between the layers. Hg²⁺ adsorption also resulted in the formation of HgS. This adsorbent efficiently immobilizes heavy metals as metal sulfide nanoparticles, with high selectivity for ions such as Cd and Hg. Furthermore, it was demonstrated that the particle shape of roseLDH is also beneficial for column applications. This suggests that S_x-roseLDH could be an effective adsorbent for industrial wastewater treatment, with the added benefit of reducing sludge generation.