Deep eutectic solvent-driven surface activation of montmorillonite enhances heavy metal uptake and electrochemical...

Montmorillonite clay was modified using a zinc chloride-urea metal salt-based deep eutectic solvent (MSDES) to probe how DES-induced surface reconfiguration governs metal-ion coordination and interfacial behaviour. XRD and FTIR confirmed retention of the 2 : 1 aluminosilicate framework and Si-O-Si bands, with redistribution of surface hydroxyl groups indicating surface-level rather than structural modification. XPS quantified Zn 2p at 0.74 atomic% on the DES-modified surface against no detectable signal on the unmodified clay, confirming zinc incorporation. N₂ physisorption showed a 61% increase in BET surface area (42.3 to 68.1 m² g^-1) and a 71% gain in total pore volume. Zeta potential remained negative across the working pH range, with the point of zero charge shifting from pH 2.5 to 3.2. Batch adsorption in real contaminated water under competitive multi-ion conditions gave a selectivity hierarchy of Pb²⁺ > Cr³⁺ > Cd²⁺. Single-component Langmuir capacities were 0.696, 0.154, and 0.105 mg g^-1 for Pb²⁺, Cr³⁺, and Cd²⁺, 158%, 927%, and 163% higher than unmodified montmorillonite. Non-linear kinetics revealed distinct rate-determining behaviour. Pb²⁺ followed pseudo-second-order kinetics indicative of chemisorption, Cr³⁺ followed pseudo-first-order kinetics, and Cd²⁺ showed rapid uptake followed by progressive displacement. Capacity retention exceeded 87% for Pb²⁺ and 68% for Cd²⁺ across five regeneration cycles. DES treatment reduced charge-transfer resistance from 2850 to 620 Ω and raised anodic peak current by 235% relative to unmodified montmorillonite. These results link DES-induced surface reconfiguration to the metal-stabilising and electrochemical properties of the material, supporting its use as a clay-based scaffold for catalytic and remediation applications.