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

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 Sx-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 similar to 10(7) 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+/Zn-2 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 similar to 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.