Positive role of sulfonated PEEK coating for PEEK membrane in mass transfer of lithium extraction

Membrane extraction (ME) is a promising technology for efficiently separating lithium from salt-lake brine with high Mg/Li ratio. By utilizing a solvent resistant semicrystalline poly(ether ether ketone) (PEEK), an extraction-stripping loop is envisioned, but significant improvement in the lithium extraction flux is required. We report in this paper a new finding of strong interaction of the extractant, Tributyl phosphate (TBP), to PEEK, leading to reduction in Li extraction flux. Phosphorous element inside PEEK substrate supported the finding; significantly higher Li flux was observed when the top surface faces the organic extractant than that when the bottom surface faces the organic. Mass transfer model detailed the consequence of infiltration by TBP to the ion flux due to the low diffusion coefficient of the bulky ion-extractant complex. We further coated a sulfonated PEEK (SPEEK) layer to effectively improve the lithium extraction flux by 154 %. This new finding provides a coherent understanding for previous results that morphology optimization did not show significant changes in ME performance. With current findings, membrane extraction is one step further to practical applications.