Sustainable metal extraction and recovery using amino acid-based leaching systems: A review approach

Amino acid-based lixiviants are emerging as credible alternatives to cyanide and strong mineral acids for sustainable hydrometallurgy, yet their industrial relevance depends on more than demonstrating individual leaching recoveries. This review critically evaluates how amino acid chemistry, metal–ligand complex stability, oxidant selection, mineralogy, downstream recovery and reagent recycling collectively determine process performance for base and precious metals. Unlike earlier summaries that mainly catalogue leaching studies, this article integrates evidence from primary ores, tailings, electronic waste, batteries and spent catalysts to identify where amino acids offer genuine technical advantages and where important limitations remain. The review shows that glycine, glutamate, cysteine, histidine and related amino acids can provide selective metal dissolution under mild alkaline conditions because their amine, carboxylate and side-chain donor groups stabilise metal ions in solution while suppressing many gangue reactions. However, performance varies strongly with mineral type, oxidative pathway, pulp density, pyrite content, ligand concentration and recovery method. High recoveries reported for copper oxides and selected secondary resources contrast with slower kinetics for refractory sulphides and precious metals in cyanide-free systems, demonstrating that amino acid leaching should be viewed as a feed-specific platform rather than a universal replacement for established lixiviants. The most promising near-term opportunities are secondary resources and environmentally sensitive operations where selectivity, low toxicity, reagent recyclability and reduced effluent burdens can offset slower kinetics and higher reagent cost. Key research priorities include mechanistic kinetic modelling, ligand and oxidant optimisation, high-solid-density testing, integrated recovery flowsheets, amino acid degradation control, techno-economic analysis and pilot-scale validation. Overall, amino acid lixiviants represent an important pathway toward circular and lower-impact metal extraction and recovery, provided future research moves from descriptive leaching demonstrations toward integrated, mechanism-based process design.