A review of polyelectrolytes and their complexes for lithium recovery from oilfield and geothermal brines

Lithium will continue to be in high demand for electric vehicle battery applications over the next few decades. Countries try to remain competitive in this emerging industry by contributing to the lithium raw material supply chain. Lithium from oilfields and geothermal brines is an important source of extracted lithium, but it needs to be concentrated before it can be recovered. This paper reviews the concentration stages during lithium recovery from brines and proposes using polyelectrolyte complex nanoparticles to improve or replace traditional concentration techniques, such as nanofiltration and solvent extraction. Key factors for optimizing nanoparticle performance, including charge density, pH, polyelectrolyte mass ratio, ionic strength, shear rate, order of addition, and multi-layer deposition, are discussed. In the proposed workflow to replace solvent extraction, release mechanisms such as pH or ionic strength-induced release are also presented to strip complexed lithium into the solution. Several cationic and anionic polymers have been compiled and screened based on criteria such as charge density, solubility, toxicity, selectivity, and their ability to form complexes with lithium through electrostatic interactions. The screening process has led to the recommendation of a specific polycation and polyanion to achieve an optimum nanostructure architecture for lithium extraction from brines containing high Mg2+ impurity. Polyelectrolyte complex nanoparticles could enhance positively charged nanofiltration membranes or enable lithium extraction from brines without needing organic solvents, as used in solvent extraction.