Synthesis of polyhydroxyl-capped PAMAM dendrimer/silica composites for the adsorption of aqueous Hg(II) and Ag(I)

Water pollution caused by Hg(II) and Ag(I) poses deleterious effects to environmental safety. Adsorption is one of the promising methods to decontaminate aqueous metal ions. Herein, polyhydroxyl-capped poly(amidoamine) (PAMAM) dendrimer/silica composites (G1-OH and G2-OH) were prepared for decontaminating aqueous Hg(II) and Ag(I). The maximum adsorption capacity of G1-OH and G2-OH for Hg(II) are 0.45 and 0.76 mmol/g, while that for Ag(I) are 0.66 and 0.81 mmol/g. The optimum solution pH for the adsorption of Hg(II) and Ag(I) are both 6. The adsorption for Hg(II) and Ag(I) can reach equilibrium at 150 and 120 min, respectively. Pseudo-second-order model can be used to describe the adsorption kinetic process and the rate-controlling step is film diffusion process. Adsorption isotherm indicates the adsorption can be promoted by increasing concentration and temperature, and the adsorption process could be described by Langmuir model with chemical mechanism. G1-OH and G2-OH exhibit excellent adsorption selectivity and they can 100% adsorb Hg(II) or Ag(I) with the coexisting of Fe(III), Co(II), Cu(II) or Ni(II). Adsorption mechanism confirms C-N, OH and CONH groups play critical role for the adsorption of the two ions. The work may provide efficient adsorbents for the decontamination of aqueous Hg(II) and Ag(I) with practical value.