Heavy metal adsorption with zeolites: The role of hierarchical pore architecture

Zeolites have been widely used for heavy metal removal due to their high cation exchange capacity and surface sorption properties. However, the empirically determined adsorption capacity trend of Pb2+ > Cu2+ > Ni2+ was not well understood. Herein, we discovered that the adsorption of Ni2+ by zeolites was greatly influenced by hierarchical pore geometry, while the adsorption of Pb2+ or Cu2+ was dependent on cation exchange determined by the framework composition. This difference in adsorption mechanisms is manifested through the systematic study of heavy metal adsorption using low-silica LTA and FAU zeolites with different mesopore architectures and compositions. Synthesizing mesoporous zeolite A of MLTA-P using proline mesoporogen considerably enhanced the Ni2+ adsorption capacity to more than twice of that of conventional zeolite A synthesized without organics, while the adsorption capacities for Cu2+ and Pb2+ were almost unaltered at ∼170 and 510 mg/g. The study on the effect of zeolite synthesis time and adsorbate pH value revealed the significant influence of zeolite crystallinity, surface hydroxyl group, and hierarchical pore architecture on the Ni2+ uptake. The MLTA-P zeolite showed higher stability against pH variation in acid range and remarkably enhanced uptake in alkaline conditions, reaching 218 mg/g at a pH of 11. The full characterization of the adsorbent by scanning electron microscopy and X-ray photoelectron spectroscopy indicated the involvement of the surface reaction forming Ni phyllosilicate nanosheet species during the nickel metal removal process in which transport limitation played a crucial role. The uptake kinetics and isotherms could be perfectly reflected by a pseudo-second-order rate equation and the Langmuir model, confirming the nature of chemisorption.