Dual-functional amino-carboxyl co-modified Fe3O4 nanoparticles for synergistic selective adsorption of lead and cadmium ions from aqueous solutionss

Heavy metal contamination in aquatic environments constitutes a critical environmental and public health challenge necessitating advanced remediation technologies. This investigation presents a novel dual-functional adsorbent design achieving synergistic selective adsorption through amino-carboxyl cooperative binding mechanisms. Dual-functional amino-carboxyl co-modified Fe₃O₄ nanoparticles (Fe₃O₄-NH₂-COOH) were synthesized via sequential silanization and carboxylation reactions, yielding functional group densities of 1.62 mmol/g (amino) and 1.23 mmol/g (carboxyl) while preserving strong magnetic properties (Ms = 58.4 emu/g) enabling efficient separation. The synthesized nanoparticles exhibited superior maximum adsorption capacities following Langmuir isotherm behavior: qm = 125.3 mg/g for Pb²⁺ and qm = 98.7 mg/g for Cd²⁺ in single-component systems, representing 27-30% enhancement compared to single-functional modifications. In competitive binary systems, selectivity coefficients (αPb/Cd = qe, Pb·Ce, Cd/qe, Cd·Ce, Pb) ranged from 2.8 to 4.7, demonstrating preferential Pb²⁺ binding governed by hard-soft acid-base (HSAB) coordination preferences. Comprehensive characterization (XRD, TEM, FTIR, VSM, BET, XPS) confirmed successful dual-functionalization while maintaining the inverse spinel crystal structure (space group Fd3m, a = 8.392 Å). Kinetic studies revealed pseudo-second-order behavior (R² > 0.99) with rate constants k₂ = 0.00147 g·mg⁻¹·min⁻¹ for Pb²⁺ and k₂ = 0.00089 g·mg⁻¹·min⁻¹ for Cd²⁺, indicating chemisorption-dominated mechanisms. Activation energies calculated from Arrhenius plots (Ea = 24.7 kJ/mol for Pb²⁺, 28.3 kJ/mol for Cd²⁺) confirmed chemical binding processes. Thermodynamic parameters (ΔG° = -18.7 to -24.3 kJ/mol, ΔH° = 12.4 kJ/mol, ΔS° = 98.5 J·mol⁻¹·K⁻¹ for Pb²⁺) indicated spontaneous endothermic adsorption with increased entropy at the solid-liquid interface. Langmuir isotherm modeling (qe = qmbCe/(1 + bCe)) yielded excellent correlation (R² > 0.98) across concentration ranges of 5-300 mg/L. The synergistic mechanism between amino and carboxyl groups enhanced selectivity through differential coordination preferences, with quantified synergy factors (S = qe, dual/(qe, amino + qe, carboxyl)) of SPb = 1.34 and SCd = 1.18 demonstrating cooperative enhancement exceeding simple additive contributions. DFT calculations at B3LYP/6-31G(d, p) level revealed ΔEbind = -198.5 kJ/mol for mixed-ligand chelate formation, providing 41.7 kJ/mol additional stabilization compared to single-site binding. The dual-functional magnetic adsorbent demonstrated > 85% capacity retention after five regeneration cycles, < 5 min magnetic separation time under 0.3 T field, and excellent performance in multi-metal systems with interference resistance (< 15% capacity reduction at 200 mg/L Ca²⁺/Mg²⁺), showing significant potential for sustainable heavy metal remediation with enhanced selectivity in competitive adsorption scenarios.