Enhancing heavy metals removal from water by thiol-incorporation of UiO-66 based metal organic framework nanosheets with l-cysteine

Abstract

A new adsorbent, Cys-2D-UiO-66, was developed and applied to effectively eliminate heavy metal ions from water. The synthesis of 2D-UiO-66 involved developing a two-dimensional structure of UiO-66, created through an alkaline reaction medium, deviating from the normal three-dimensional structure. The advantage of 2D-UiO-66 nanosheet with a high aspect surface-to-thickness ratio is its ability to easily expose active sites on their surface to contaminants in solution, without diffusion limitations. Further attempts were made to enhance the adsorption capacity of the 2D-UiO-66 adsorbent by attaching l-cysteine to its surfaces through a simple one-pot process. This resulted in the modified adsorbent, Cys-2D-UiO-66, which exhibited exceptional adsorption performance. The adsorption experiments revealed that using Cys-2D-UiO-66 significantly improved the adsorption of Hg2+, Pb2+, and Cd2+ compared to 3D-UiO-66 (i.e. a 52 % increase for Hg2+, a 9.5-fold increase for Pb2+, and a 20-fold increase in Cd2+ adsorption). The adsorption equilibria of Cys-2D-UiO-66 align more closely with the Langmuir model, indicating that monolayer chemisorption mainly dominates the adsorption behavior. The thermodynamic study demonstrated that the adsorption of heavy metals onto the Cys-2D-UiO-66 favors higher temperatures. This fact is supported by thermodynamic calculations showing exothermic, favorable, and spontaneous chemisorption. The Cys-2D-UiO-66 adsorbent exhibited exceptional reusability with a marginal (less than 4 %) drop in adsorption performance after five successive cycles. Multicomponent adsorption results indicated a decline in the amount of adsorption for all species compared to the single component state due to competitive adsorption. FTIR analysis indicated that S–H and N–H groups aid in heavy metal ion adsorption, with Hg2+ ions being more easily adsorbed than Pb2+ and Cd2+ ions due to their higher electronegativity and ability to form covalent bonds with sulfur.