Synthesis of Hybrid Adsorbents Combining Sol-Gel Processing and Molecular Imprinting Applied to Lead Removal from Aqueous Streams

Two high-capacity thiol functionalized adsorbents are prepared, using sol-gel processing, and applied to the removal of lead(II) from aqueous streams. The first adsorbent (SN) is prepared by co-condensing oligomers of tetraethoxysilane (TEOS) and 3-mercaptopropyltrimethoxysilane (MPS); the second adsorbent (MI) is synthesized by a combined co-condensation/molecular imprinting route of TEOS and MPS. The resulting physicochemical properties of adsorbents are investigated by nitrogen sorption measurements, elemental analysis, Fourier transform infrared spectroscopy (FTIR), solid-state C-13 and Si-29 cross-polarization magic angle spinning nuclear magnetic resonance (C-13 and Si-19 CPMAS NMR, respectively), and X-ray photoelectron spectroscopy (XPS). The adsorbents exhibit high ligand densities (1.19 mmol/g for SN and 1.03 mmol/g for MI), improved Brunauer-Emmett-Teller (BET) surface areas (S-BET = 129 m(2)/g for SN and 464 m(2)/g for MI), and highly developed mesoporosity (D-p = 15.1 nm for SN and 8.3 nm for MI). Si-29 CPMAS NMR measurements indicate that the silicon oxide solid structure of adsorbents is not modified by lead adsorption. XPS results indicate the presence of lead acetate species on the surface of adsorbents. Batch adsorption data are explained by a mechanism in which a hydrated species (Pb(OOCCH3)(H2O)(5)+) forms a monodentate complex with thiol surface groups. Further characterization of the adsorbents shows rapid adsorption kinetics and equilibrium lead(II) adsorption capacities of 1.13 and 0.715 mmol/g for SN and MI. Lead adsorption dynamics in a packed column indicates high lead uptakes (155 and 80 mg Pb/g-adsorbent for SN and MI, respectively). Combined and simple sol-gel synthesis routes for preparation of adsorbents with high ligand densities and mesoporous structures are demonstrated here.