Much of the world's lithium occurs in salty waters with fundamentally different chemistry than other naturally saline waters like the ocean, according to a study published on May 23 in Science Advances. The finding has implications for lithium mining technologies and wastewater assessment and management.
Lithium is a critical mineral in the renewable energy sector. About 40% of global lithium production comes from large salt pans, called salars, in the central Andes Mountains in South America and the Tibetan Plateau in Asia. In these arid, high-altitude regions, lithium exists below surface salt deposits, dissolved in extremely saline water called brine.
"We discovered that the pH of brines in these regions is almost entirely driven by boron, unlike seawater and other common saline waters. This is a totally different geochemical landscape, like studying an extraterrestrial planet," said Avner Vengosh, distinguished professor of environmental quality and Chair of the Division of Earth and Climate Sciences at Duke University's Nicholas School of the Environment, who oversaw the research.
A solution's pH is a measure of how acidic or alkaline it is. In most natural waters, chemical reactions involving a molecule called carbonate primarily govern a solution's ability to control changes in pH — a measure known as alkalinity. But the Duke team uncovered a dramatically different scenario at the Salar de Uyuni, a giant salt pan situated on a Bolivian plateau, where the world's largest known lithium brine deposit exists underground.
The researchers analyzed the pH and chemistry of brines and salts associated with a pilot mining operation at the Salar de Uyuni. Mining lithium from salt pans traditionally involves pumping natural brine from underground into a series of shallow, above-ground ponds. Liquid evaporates from successive ponds, leaving behind increasingly concentrated brine containing lithium and boron, plus undesirable salts. Lithium is eventually extracted at a processing facility.
The team found that pH levels in natural brine samples from the salar hovered around neutral. By contrast, brine samples from evaporation ponds were highly acidic. Computer modeling showed that high concentrations of boron were the primary drivers of pH in both cases.
Specifically, the natural brines contain high levels of boron in different forms — including the molecule boric acid and compounds called borates — whose relative distribution controls pH. Evaporation in the ponds increases the overall concentration of boron and triggers the breakdown of boric acid, generating hydrogen ions that reduce the pH.
"Through a chain of geochemical reactions, the carbonate alkalinity is diminished in the brine from the Salar de Uyuni, while boron alkalinity becomes predominant," said lead author Gordon Williams, a Ph.D. student in the Vengosh Lab.
"The integration of the chemical analysis with geochemical modeling helped us to quantify the different molecular structures of boron that contribute to alkalinity in these lithium brines," added Paz Nativ, a postdoctoral researcher in the Vengosh Lab.
To corroborate their findings, the team gathered data on more than 300 analyses of lithium-rich brine from various salt pans, including in Chile, Argentina and Bolivia — collectively known as the Lithium Triangle — and the Tibetan Plateau. Modeling showed that boron exerted the most influence on alkalinity, and therefore pH, in most of those brines as well.
"In addition to the new data we generated, we compiled a geochemical database of lithium brines from around the world and consistently found that boron is often the predominant component in brine alkalinity and controls brine pH, reinforcing the results from the Salar de Uyuni in Bolivia," Williams explained.
The research is the first to demonstrate the role of boron in controlling the chemical changes that occur during lithium brine evaporation in salt pans, according to the researchers. The findings could inform future lithium mining technologies as operators explore ways to more efficiently extract lithium and safely manage wastewater, they added.
Funding: This study was supported by the Duke University Climate Research Innovation Seed Program (CRISP), the Duke University Josiah Charles Trent Memorial Foundation Endowment Fund, and the Duke University Graduate School Dissertation Research Travel Award.
