Selective separation and recovery of au(III) and pd(II) from acidic chloride solutions via cooperative O/N-rich binding sites on a tannin-based phenolic resin

Selective separation of precious metals from chloride-rich acidic media remains challenging because of the complex aqueous speciation of metal ions and the interference of competing ions. In this study, a polyethyleneimine (PEI)-functionalized tannin-based phenolic resin (TPR@PEI), enriched with O/N binding sites, was synthesized via Schiff base reactions for the selective separation and recovery of Au(III) and Pd(II) from complex acidic chloride solutions. The effects of key synthesis parameters, including tannic acid (TA) dosage, formaldehyde (FA) volume and crosslinking time, on adsorbent structure and Au(III)/Pd(II) adsorption performance were systematically investigated. Under the optimized synthesis conditions (1 g TA, 5 mL FA, and 2 h crosslinking time), TPR@PEI exhibited rapid adsorption kinetics, reaching equilibrium within 40 min for both metal ions. The adsorption behavior was well described by the pseudo-second-order kinetic model and the Langmuir isotherm model, with maximum adsorption capacities of 416.67 mg g −1 for Au(III) and 270.27 mg g −1 for Pd(II) at pH 2. More importantly, TPR@PEI showed superior selectivity toward Au(III) (> 99%) and Pd(II) (> 87%) in both binary and multicomponent systems, and maintained removal efficiencies remaining above 90% over five adsorption-desorption cycles. Combined characterization and density functional theory calculations (DFT) revealed that the excellent separation performance was governed by the synergistic effects of electrostatic attraction, redox interaction, and coordination/chelation enabled by the cooperative O/N-rich binding sites. These results demonstrate that TPR@PEI is a promising adsorbent for the highly selective separation and recovery of precious metals from complex chloride-rich acidic solutions.