Mechanism of synergistic enhancement in cesium extraction by organophosphorus extractants and 4-tert-butyl-2-(α-methylbenzyl) phenol under weak alkaline conditions

Cesium (Cs) is a critical strategic metal for advanced technologies, and its recovery from lepidolite leachate—an important byproduct of lithium mineral processing—holds significant value for mineral resource valorization. Traditional Cs extraction using 4-tert-butyl-2-(α-methylbenzyl) phenol (t-BAMBP) requires strong alkaline conditions, leading to excessive alkali consumption and extractant loss, which poses bottlenecks for sustainable metallurgical processes. This study focuses on elucidating the synergistic mechanism of Cs extraction by t-BAMBP combined with three representative organophosphorus extractants (D2EHPA, Cyanex272, TOPO) under weakly alkaline conditions, with an emphasis on the regulatory effects of electronic properties (electrostatic potential, ESP of P=O groups) and steric hindrance. Integrated characterization via Fourier transform infrared spectroscopy (FT-IR), proton nuclear magnetic resonance (1H NMR), and density functional theory (DFT) calculations reveals that synergistic efficacy depends on the balance between ESP strength and active site accessibility. Cyanex272, with a linear structure and low steric hindrance, forms robust hydrogen bonds with t-BAMBP’s phenolic –OH, exhibiting stable synergism across pH 6.0–12.5. Under optimal conditions (pH=10.5, Cyanex272=0.04 mol/L), it achieves 83.99% single-stage Cs extraction efficiency and a separation factor (βCs/Rb) of 22.74, reducing alkali consumption by over 90% compared to traditional processes. In contrast, TOPO’s bulky tri-octyl chains shield its strong ESP site, resulting in negligible synergism, while D2EHPA’s branched structure induces moderate steric hindrance and compromised selectivity. When applied to real complex lepidolite leachate (Cs+~3 g/L), the optimized t-BAMBP-Cyanex272 system realizes >97% Cs extraction efficiency via a two-stage countercurrent extraction-four-stage scrubbing-acid stripping integrated process. This work not only establishes a theoretical framework for the rational design of synergistic extractants in metallurgical separation by balancing electronic and steric effects but also provides a green and scalable technical route for Cs recovery from lepidolite leachate, advancing the comprehensive utilization of strategic mineral resources.