From Surface Energetics to Environmental Functionality: Mechanistic Insights into Hg(II) Removal by L-Cysteine-Modified Silica Gel

The development of oxidation-resistant and regenerable materials remains a major challenge for mercury removal from contaminated waters and industrial effluents. In this study, a zwitterionic mesoporous silica gel functionalized with L-cysteine (SG-3PS-Cys) was synthesized, where the thiol group is covalently anchored to the silica framework, preventing oxidative degradation while preserving -NH₃⁺ and -COO^- groups for Hg(II) coordination. Spectroscopic analyses (FTIR, XPS, and ¹³C NMR) confirmed the formation of a stable, thiol-free binding environment in which mercury interacts through carboxylate oxygen atoms, electrostatically stabilized by neighboring ammonium groups. The material exhibited a high surface area (134 m² g^-1) and uniform mesoporosity (9.8 nm), achieving a maximum Hg(II) uptake of 82.7 mg g^-1 at pH 3 with rapid kinetics and cooperative S-type isotherms. The adsorbent retained 72% of its capacity after five regeneration cycles and maintained 38.7% selectivity toward Hg(II) in multicomponent solutions. DFT-based surface energy distribution analysis supported the zwitterionic coordination mechanism, revealing energetically homogeneous and high-affinity binding domains. Beyond its chemical stability, the material introduces a sustainable route for mercury remediation, linking surface energy, electrostatic effects, and porosity to achieve durable performance under acidic and complex aqueous conditions. These findings provide a mechanistic and design framework for the next generation of non-thiol adsorbents capable of selective and reusable Hg(II) removal in environmentally relevant scenarios.