Kinetics and mechanism of hydrolysis of PF6− accelerated by H+ or Al3+ in aqueous solution†

Treatment of wastewater containing PF₆⁻ is required during hydrometallurgical recycling of lithium-ion batteries. Because of the kinetic stability of PF₆⁻ in aqueous solution, the decomposition study into PO₄³⁻ or F⁻ is required for wastewater treatment. In our previous report, the hydrolysis of PF₆⁻ was shown to be accelerated by adding Al³⁺ and elevating the solution temperature. In this work, the kinetics and mechanism of the hydrolysis of PF₆⁻ at several pH and Al³⁺ concentrations were investigated for more efficient wastewater treatment. The solutions containing LiPF₆ at various pH and AlCl₃ concentrations were kept at 90 °C, and the concentration changes of PF₆⁻, PO₂F₂⁻, PO₃F²⁻, PO₄³⁻, and F⁻ were measured by ion chromatography. The measurement results were analyzed assuming pseudo-first-order kinetics. The results showed that Al³⁺ and H⁺ accelerated the hydrolysis of PO₂F₂⁻ and PO₃F²⁻, but the levels of accelerating effects were different. More specifically, the accelerating effects of Al³⁺ are higher in the order PF₆⁻ > PO₂F₂⁻ > PO₃F²⁻, while the accelerating effects of H⁺ are in the opposite order. Based on the discussion, a more efficient treatment process for wastewater containing PF₆⁻ was proposed. The proposed process is expected to reduce heating costs and processing time compared to previously reported ones.