Highly selective and efficient heavy metal capture with polysulfide intercalated layered double hydroxides

Abstract

Polysulfide [S_x]²⁻ (x = 2, 4) species were intercalated into magnesium–aluminium layered double hydroxide (MgAl–LDH) by a [S_x]²⁻/NO₃⁻ anion-exchange reaction. The resulting S_x–LDH materials exhibit excellent affinity and selectivity for heavy metal ions such as Cu²⁺, Ag⁺ and Hg²⁺. For the highly toxic Hg²⁺, the distribution coefficient K_d values can reach ∼10⁷ mL g⁻¹. The S_x–LDH materials rapidly reduce the concentrations of Hg²⁺ and Ag⁺ ions in testing solutions from ppm levels to trace levels of ≤1 ppb. A larger series of metal ions were investigated and the selectivity order of Ni²⁺, Co²⁺ ≪ Zn²⁺, Pb²⁺ < Cd²⁺ < Cu²⁺, Ag⁺, Hg²⁺ was observed. The S_x–LDH materials show higher selectivity for Cu²⁺/Zn² compared to Co²⁺/Ni²⁺, providing good separation for these transition metal ions. After ion capture, the LDH hybrid materials retained the original hexagonal prismatic shape and showed good stability under acidic conditions (pH ∼ 3). The adsorption process of the metals occurs via M–S bonding. The enhanced environmental stability of the [S_x]²⁻ groups provided by the LDH protective space, the confinement effect offered by the LDH layers, along with the easy accessibility of polysulfide ions to metal ions enable high capture ability and excellent selectivity. The S_x–LDH materials are thus promising as superior sorbents for the decontamination of polluted water.