Syntheses, characterization, and adsorption properties for metal ions of silica-gel functionalized by ester- and amino-terminated dendrimer-like polyamidoamine polymer

A series of ester- and amino-terminated dendrimer-like polyamidoamine (PAMAM) grafted silica-gel (SiO2-G0–SiO2-G4.0) was prepared with a divergent method by repeating two processes: (1) Michael addition of methyl acrylate (MA) to surface amino groups; and (2) amidation of the resulting esters with ethylenediamine (EDA) from γ-aminopropyl silica gel core. Their structures were characterized by FTIR, XRD, TG, porous analysis, and SEM. FTIR method was employed to monitor amidation reaction in order to judge the optimum reaction time. The results indicated it needed at least 5, 7, 8, and 9 days for SiO2-G0.5, SiO2-G1.5, SiO2-G2.5, and SiO2-G3.5 to be converted into SiO2-G1.0, SiO2-G2.0, SiO2-G3.0 and SiO2-G4.0, respectively at 25 °C. The porous structure analysis showed the BET surface area, BJH desorption average pore diameter and BJH desorption cumulative volume of pores for the products decreased after the series of grafting reactions. Au3+, Pd2+, Pt4+, Ag+, Cu2+, Zn2+, Hg2+, and Cd2+ were chosen as representatives to investigate the relationship between adsorption capacities and percentage of grafting of polymer onto the surface of silica-gel. The experiments showed that both ester- and amino-terminated dendrimer-like polyamidoamine (PAMAM) grafted silica-gel exhibited better adsorption capabilities for Au3+ and Pd2+ than for base metal ions Cu2+, Zn2+, Hg2+, and Cd2+ and noble metal ions Pt4+ and Ag+. One interesting conclusion could be drawn from the above investigation, that is, a high percentage of grafting of polymer onto the surface of silica-gel did not mean a high adsorption capacity.