Boosting electrochemical reduction-recovery of trace gold(I) with ferric-modified walnut shell charcoal

Abstract

Enhancing the electrochemical reduction-recovery of trace-level gold(I) in thiosulfate media is essential for advancing environmentally friendly, cyanide-free hydrometallurgical technologies. In this work, walnut shell charcoal (WSC) was modified using various ferric compounds to enhance its performance as an electrode. Ferric modification led to stable surface coating which significantly improved electrical conductivity and electrochemical behavior by increasing the specific capacitance and reducing the charge transfer resistance during gold(I) recovery. Among the different ferric precursors tested, ferric sulfate (Fe₂(SO₄)₃) modification exhibited the most outstanding performance, increasing the reduction capacity from 36.52 mg/g (pristine WSC) to 97.70 mg/g, a 2.67-fold enhancement. Under optimized electrochemical conditions, this modified electrode achieved a 95.72% recovery for trace gold(I) (5 mg/L), significantly outperforming electrodes modified with FeCl₃. Systematic experiments and material characterizations revealed that Fe₂(SO₄)₃ modification led to the formation of surface-bound sulfate groups and oxygen-rich functional sites, which enhance the adsorption affinity toward gold species and facilitated pre-concentration near active centers. Moreover, density functional theory (DFT) simulations confirmed strong interactions between sulfur-derived oxygen atoms and gold(I), contributing to the improved recovery. Chloride from ferric chloride also showed ability to coordinate with oxygen atoms, contributing to a moderate recovery. These results collectively highlight the importance of ferric source selection in optimizing carbon-based electrodes for noble metal recovery, providing a promising strategy for sustainable hydrometallurgical processes.

Keywords

Trace gold(I) recovery / Thiosulfate leaching / Electrochemical reduction-recovery / Ferric modification / Fe2(SO4)3-WSC electrodes

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