Synthesis of highly porous chitosan microspheres anchored with 1,2-ethylenedisulflde moiety for the recovery of precious metal ions

The highly porous chitosan microspheres (HPCM) with throughpores were performed by mixing the oil-in-water-in-oil (chitosan O/W/O) emulsions containing chitosan in the water phase with the NaCl W/O emulsions containing 15% sodium chloride in the aqueous phase. The difference of the osmotic pressure between these emulsions causes the transportation of water from chitosan O/W/O emulsions to NaCl W/O emulsions to make HPCM. Both introduction of 1,2-ethylenedisulfide moiety and cross-linking were performed at the same time by activating HPCM with 2-(chloromethyl)oxirane, followed by introducing 1,2-ethylenedisulfide moiety with 1,2-ethanedithiol. These chitosan microspheres (EDSC) had an average diameter of 143 mu m and average pore size of 2-6 mu m. EDSC exhibited high selectivity for precious metals such as palladium(II), gold(III), and platinum(IV) over base metal ions in hydrochloric acid solutions. The adsorbed palladium(II) was completely desorbed using aqueous thiourea solution. Furthermore, the adsorption kinetics was investigated to elucidate the effect of large pores on the adsorption of palladium(II) with EDSC from acidic chloride media. The adsorption mechanism indicated that the mass transfer rate in the laminar film in the aqueous phase and the chemical reaction of palladium(II) with sulfur ligand in EDSC were the rate-determining steps at low and high concentrations of hydrochloric acid, respectively. This suggests that adsorption of palladium(II) on EDSC is very fast, which is due to the decrease in mass transfer resistance of intraparticle diffusion by the effect of large pores.