The role of boron in controlling the pH of lithium brines

The global clean energy transition requires the development of alternative energy technologies that rely on critical raw materials including lithium. Closed-basin brines, which generate ~40% of global lithium production, often have a circumneutral pH; however, during the evaporative concentration required for lithium production, the evaporated brines become acidic. Using primary geochemical and boron isotope data from the Salar de Uyuni (SDU), Bolivia combined with a modeling approach, we show that boron enrichment, which commonly co-occurs with lithium in closed-basin brines, is the primary factor in controlling the pH of brines from the SDU. We demonstrate that boron in global lithium- and boron-rich brines from closed basins exerts a similar influence on brine pH. The unique boron enrichments and its speciation can explain large proportions of alkalinity in these brines (~98% at the SDU), where evaporation alters the dissociation of boric acid, which triggers the formation of acidic evaporated brines.