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

摘要

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 Cu~(2+), Ag~+ and Hg~(2+). For the highly toxic Hg~(2+), the distribution coefficient K_d values can reach ～10~7 mL g~(-1). The S_x-LDH materials rapidly reduce the concentrations of Hg~(2+) 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 Ni~(2+), Co~(2+) Zn~(2+), Pb~(2+) < Cd~(2+) < Cu~(2+), Ag~+, Hg~(2+) was observed. The S_x-LDH materials show higher selectivity for Cu~(2+)/Zn~2 compared to Co~(2+)/Ni~(2+), 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.