Rare earth elements in the 21st century: A critical review of novel chemical extraction pathways from conventional hydrometallurgy to emerging greener methodologies

Rare earth elements (REEs) are essential components of modern technologies, including permanent magnets, catalysts, and energy systems, yet their extraction remains largely dependent on energy-intensive and chemically aggressive hydrometallurgy processes based on inorganic acids. Growing environmental concerns, together with the increasing importance of low-grade ores and secondary resources, have stimulated interest in alternative extraction strategies, particularly bioleaching and organic-acid-based systems. These emerging approaches differ fundamentally from conventional routes, as metal dissolution is influenced not only by proton activity but also by ligand coordination, solution speciation, and competitive complexation equilibria. This review provides a critical analysis of REE recovery pathways developed in the 21st century, spanning conventional inorganic acid leaching, industrial hydrometallurgical methods, direct and indirect bioleaching, and organic-acid-based processes. Special emphasis is placed on the mechanistic evolution from proton-driven dissolution to systems where complexation, microbial metabolites, and metal–ligand speciation play a central role in controlling extraction efficiency and selectivity. The influence of ligand structure, pH-dependent speciation, and conditional stability constants is examined across primary ores and secondary resources. By integrating coordination-chemical principles with process-level and environmental considerations, this review aims to clarify the mechanisms governing modern REE leaching and to support the rational development of greener and more selective extraction strategies.