Spectroscopic and Modeling Investigation of Sorption of Pb(II) to ZSM-5 Zeolites

Methyl tert-butyl ether (MTBE) was used as a replacement for Pb in gasoline before it was found to be harmful, and for this reason, these two contaminants often coexist in groundwater at older fueling facilities. ZSM-5 is a well-characterized synthetic zeolite that shows promise in the removal of organic contaminants from water. While numerous studies have quantified the adsorption of metal to hydrophilic zeolites, few have investigated the mechanisms for adsorption to hydrophobic zeolites such as ZSM-5. In this study, batch adsorption tests and synchrotron-based extended X-ray absorption fine structure (EXAFS) analyses were conducted to investigate the adsorption of Pb to ZSM-5 in the presence and absence of MTBE. Batch adsorption studies showed that MTBE did not significantly impact Pb adsorption. EXAFS analyses revealed two Pb binding mechanisms: (1) precipitation as a PbO·(H2O) type surface coating and (2) adsorption of Pb at Si surface sites. The PbO·(H2O) type surface coating is more stable at pH 6 due to the formation of solid-phase hydroxide minerals, while the Pb to Si surface site occupancy is constrained by the availability of silanol sites on the surface. Our study provides an understanding of the mechanisms of precipitation and/or adsorption of Pb to hydrophobic zeolites and new insights into the synthesis and/or modification of zeolites to target the removal of co-contaminants from water and wastewater.