Solvent extraction of rhenium(vii) from aqueous solution using an ionic liquid system

Abstract

The application of ionic liquids in rhenium extraction not only enhances rhenium extraction efficiency and mitigates the challenges posed by the surging demand for rhenium in high-tech industries, but also minimizes pollution within the extraction process. In this study, trioctylamine (TOA) was used as the extractant, and two imidazolium-based ionic liquids (1-pentyl-3-methylimidazolium hexafluorophosphate ([C₅mim][PF₆]) and 1-pentyl-3-methylimidazolium bis((trifluoromethyl)sulfonyl)imide ([C₅mim][NTf₂])) were used as diluents to extract rhenium from hydrochloric acid solutions. Under optimal conditions (m_TOA = 0.02 mol kg⁻¹, pH = 1, T = 293.15 K, I = 0.2 mol kg⁻¹), the extraction efficiency of both systems reached 96%. Moreover, the extraction efficiency of the TOA–[C₅mim][PF₆] system was even higher. The study revealed that after four cycles of recycling and reuse, the extraction efficiency of ReO₄⁻ remained at 95%. Based on the Pitzer electrolyte solution theory, the standard equilibrium constants (K⁰) under different conditions were obtained. The thermodynamic parameters (Δ_rH⁰_m < 0, Δ_rS⁰_m > 0, Δ_rG⁰_m < 0) of the extraction process were calculated by the polynomial fitting method, which clarified the microscopic mechanism of the extraction process from the thermodynamic perspective, and the results were consistent with the optimal conditions obtained from the experiment. The results showed that in practical operation, low temperature (293.15 K) and low ionic strength (0.2 mol kg⁻¹) are beneficial to the extraction experiment, and the reuse of ionic liquids conforms to the concept of green chemistry.