Boron plays unexpected role in lithium brine chemistry

Tech-savvy readers may know that much of the world’s lithium, the lightweight metal at the heart of lithium-ion batteries, comes from South American brines. They may be surprised, however, by a new study that shows that the buffer system in these brines doesn’t rely primarily on carbonate, which buffers other saline waters, but rather on boron (Sci. Adv. 2025, DOI: 10.1126/sciadv.adw3268). The findings broaden our understanding of these regions’ unique geochemistry and may play a role in reducing the environmental impact of lithium production.

For years, the waters of the Salar de Uyuni in Bolivia, an ecosystem characterized by high salinity and crusts of precipitated minerals, have been the focus of considerable research for two main reasons: because Salar de Uyuni is the largest known lithium brine source and because of its “extraterrestrial” chemistry, says Avner Vengosh, a specialist in earth and climate sciences at Duke University who led the study.

The unique chemical feature of these lithium brines is the way they regulate their pH value. Other natural saline bodies of water maintain acid-base neutrality by relying on a series of chemical reactions known as the carbonate system. The interactions between CO₂, carbonic acid, and bicarbonate help buffer the pH of these waters when environmental factors would otherwise influence their acidity or alkalinity.

But the brines studied by Vengosh and coworkers, even though they are also saline waters, do not rely on the carbonate system to remain neutral. The researchers found that the main driver of pH buffering in Salar de Uyuni is boron, which, along with lithium, is found in abundance in these brines. Through its chemical speciation, boron maintains neutrality in this ecosystem.

The researchers also studied what happens to these brines after lithium is extracted. They found that the high concentration of boron also drives acidification of the residual waters from the mining process.

After studying the Salar de Uyuni, they also analyzed brines in Argentina and Chile, where these lithium-rich ecosystems also exist. They again found that boron is the main actor in the pH buffer system.

For Vengosh, understanding the mechanisms through which boron drives the pH of these regions is important to avoid repeating mistakes of the fossil fuel industry. “One of the missions in my lab is to learn to do critical mineral extraction and mining, which is so important for renewable energy, in a way that does not affect the environment,” he says. “To do that, you need to know what's in the water, and the chemistry of the water, and how it could affect the environment.”

To mine lithium, brines are extracted from the ground and stored in pools exposed to the sun. In a process that can take up to a year, the water evaporates, the concentration of lithium rises, and enriched salts precipitate. Then, the lithium is extracted, leaving residual waters with high concentrations of boron in the form of boric acid.

After evaporation, the pH level of these brines can drop to about three. The high acidity means “that if you take this brine and put it in the ground, it could dissolve rocks or change rock properties or sediment properties,” Vengosh says.

For Jorge Campanini, an engineer and researcher at the environmental nongovernmental organization (NGO) the Center for Documentation and Information of Bolivia, these findings raise concerns about the best management of these mining residues.

Companies such as Uranium One have proposed reinjecting the residual brines as a strategy to mitigate the risk of the basin sinking as a result of extracting water in Salar de Uyuni.

The company’s proposal to manage the brines via this method, which has not yet been deployed at a large scale, is currently under evaluation by the Bolivian government. Campanini explains that the possible consequences of reinjecting these acidic brines are hard to predict and need further study.

A team of environmental consultants at the Autonomous University Tomás Frías that is evaluating the new method warns of “the lack of clear mechanisms” for the safe management of residual waters in the proposed contracts.

This uncertainty causes Campanini to ask, “What will be the effect of an activity of which we have no technical, scientific certainty? We do not know the hydrogeological and geochemical functioning of our Salar. What will happen with this alteration of the pH?”

Roberto González is a freelance writer based in Mexico City.