Sorption of Pb(II) and Cu(II) onto multi-amine grafted mesoporous silica embedded with nano-magnetite: Effects of steric factors

Steric factors affecting the mass transfer of Pb(II) and Cu(II) in mesocellular silica foams (MSU-F-S) functionalized with multi- (mono-, di-, tri-) amine groups and nano-magnetite were investigated through batch experiments. We observed that neither the sorption capacities nor the sorption rates were linearly proportional to the number of amine groups introduced to a ligand. Unexpectedly, the tri-amine grafted samples exhibited lower affinity for both metal cations. These results are mainly attributed to two important steric factors, pore blockage and a conformational change of available amine groups. The relatively large pore size (∼30. nm) of MSU-F-S could enable various functional molecules such as amines and nano-magnetite to be effectively loaded within the pores. However, their excessive densities in the limited pore structure could have adverse effects on the transport of metal ions into the mesopores. An intraparticle diffusion model was applied to elucidate the mechanisms involved in the sorption process. Our results showed that the diffusional mass transfer into the mesopores was significantly inhibited in tri-amine grafted samples. The present findings further the understanding of steric effects on the transport of cationic metals into functionalized mesoporous silica and designing efficient sorbents.