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.
Source: Unique chemistry discovered in critical lithium deposits
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