Pincer ligand mesoporous material in heavy metal adsorption for environmental purposes

Based on our previous results obtained on the coordination of bis-pyrazolyl bis-acetate pincer ligand towards heavy metals, mesoporous silica (pore size: 60 Å) was functionalized with a new NNN pincer ligand prepared through three surface modification steps to obtain a novel mesoporous material @SiA₃. This study reports the synthesis and the application of @SiA₃ for the removal of heavy metal ions (Pb²⁺, Cu²⁺, and Cd²⁺) from aqueous media. The obtained material @SiA₃ was characterized by a series of analytical techniques including FT-IR, solid-state NMR ¹³C and ²⁹Si, BET, EA, SEM and BJH which all confirmed the successful grafting. Following the characterization, batch adsorption experiments were conducted to investigate the influence and the effect of various parameters: initial ions concentration, pH solution, equilibrium time, kinetics, temperature, thermodynamic properties and selectivity toward Pb²⁺, Cu²⁺ and Cd²⁺. The main findings from batch adsorption experiments demonstrated that @SiA₃ has a remarkable affinity and high selectivity toward Cu²⁺ achieving a maximum adsorption capacity of 127 mg/g in less than 15 min with stable efficiency after 5 cycles of regeneration/reusability maintaining over 98% of its initial efficiency. The adsorption kinetics were best described by the pseudo second order (PSO) model, while the equilibrium data fitted the Langmuir isotherm model, confirming a chemisorption mechanism involving monolayer adsorption onto a homogenous surface. Furthermore, thermodynamic studies revealed the adsorption to be spontaneous and endothermic with increased efficiency at higher temperatures. @SiA₃ practical applicability could be used for the adsorption of copper in real river water sample from the Oued Za river in Morocco without interferences of other transition metal ions. To further understand this performance, the mechanism of metal ion adsorption is discussed in detail, highlighting the role of ligand structure in driving selectivity. The proposed removal mechanism is chelation, where the nitrogen cavity NNN of the rigid pincer ligand selectively coordinate with Cu²⁺ ions, as supported by Hard-Soft-Acid-Base (HSAB) theory.