SCIENCE sciencemag.org 4 SEPTEMBER 2020 • VOL 369 ISSUE 6508 1159their metals and boosts their affinity for
CO 2. She and her colleagues are exploring
whether adding compounds called oxalates
will speed this process further by weaken-
ing chemical bonds in the tailings. And
they are trying to encourage the growth of
CO 2 -hungry magnesium carbonate crystals
by dispersing tiny crystallites of a mineral
in the tailings. The crystallites, Wilcox
says, are “like a blueprint for making more
of what you want.”
CO 2 mineralization could help remedi-
ate environmental problems that min-
ing creates, such as the release of heavy
metals from pulverized rock. On 1 March
in Economic Geology, Wilson and her col-
leagues reported that techniques that
accelerate weathering, such as
adding acid, effectively trap heavy
metals inside newly formed car-
bonate minerals, keeping them
out of groundwater. Other teams
have shown that the carbonates
can also trap hazardous residual
asbestos fibers in chrysotile mine
tailings. “You can lock away just
about anything,” Wilson says.
Other industrial wastes, such
as red mud from aluminum pro-
duction and “slags” leftover from
making steel and iron, also harbor
plenty of chemical reactivity to bind
and store CO 2. However, accord-
ing to NAS, fully carbonating these
wastes could require building costly
plants to speed the reactions.
The rock dust created by pulver-
izing basalt rock, which is already
mined for construction aggregate,
could do the job more cheaply, ac-
cording to Renforth’s team. The
researchers suggest adding this dust to ag-
ricultural soils around the world, where it
would be exposed to the air, could capture
up to 2 GTs of CO 2 per year. The basalt dust
could also fortify soils with nutrients, such
as potassium and zinc, depleted by inten-
sive agriculture. And as a bonus, they say,
the dust would react with CO 2 , generat-
ing bicarbonate, much of which over time
would flow through rivers to the sea; once
there, bicarbonate, which is alkaline, could
counteract ocean acidification.
In another environmental plus, the NAS
panel said, carbonated wastes of all kinds
could serve as raw material for concrete
and road aggregate. The report noted that
replacing 10% of building materials with
carbonated minerals could reduce CO 2
emissions by 1.6 GTs per year by lowering
emissions from cement production. Nu-
merous companies around the globe have
already jumped into the field to make and
sell the new materials (see table, p. 1158).YET EVEN IF LARGE-SCALE mineralization
works, scaling it up will carry daunting
costs, both financial and environmental.
Quarrying, crushing, and grinding ultra-
mafic rocks would cost only about $10 per
ton of CO 2 absorbed, Wilson and her team
estimate. Moving the rock, stirring it, and
other steps to speed mineralization would
likely boost the cost to between $55 and
$500 per ton of stored CO 2. That’s similar
to the cost of more traditional direct air
capture using liquid amines, which has
already gained widespread attention and
commercial interest.
But it would take mind-boggling quan-
tities of rock to budge global CO 2 levels.
According to a report published online on6 May in Chemical Geology led by Peter
Kelemen, a geologist at Columbia Univer-
sity, consuming 1 GT of CO 2 would require
10 to 100 GTs of tailings—5 to 50 cubic
kilometers of material. That’s enough to
bury Washington, D.C., 30 to 300 meters
deep—but it could only capture roughly
one-fortieth of the CO 2 humans spew into
the atmosphere every year. “We don’t make
anything on the scale that we make CO 2 ,”
says Klaus Lackner, a physicist who runs a
center at Arizona State University, Tempe,
that is evaluating all types of NETs.
All that mining, grinding, and transpor-
tation would itself generate CO 2 , unless
it were powered with renewable energy.
And if even a tiny bit of the heavy met-
als from the pulverized rock leached out,
mountains of rock waste could risk con-
taminating groundwater. “Not all rocks
are environmentally friendly if you spread
them all over,” Lackner says.
“We’re trying to understand the trade-offs,” Dipple says. “The size of the problem is
tens of billions of tons of CO 2 per year. The
only way to deal with that is to create an in-
dustry on the scale of the oil and gas indus-
try. There is more than enough rock to do
that. The question is how do you do that in
a way that is a net environmental benefit?”
A compromise requiring less land but
also less assurance that carbon will remain
locked up could come from a hybrid be-
tween carbon mineralization and direct air
capture. In Chemical Geology as well as a
recent patent, Kelemen and his colleagues
propose using a mineral called magnesite
that, when heated, gives off pure CO 2 , which
could be captured in tanks and pumped
underground. That reaction would leave
magnesium oxide powder, which
when spread thin would rapidly re-
act with CO 2 from the atmosphere,
re-forming magnesite, complet-
ing a cycle that could be repeated
over and over. Kelemen and his
colleagues calculate that mining
and processing 2 GTs of magne-
site would enable capture and
injection underground of 1 GT of
CO 2 every year. The cost would be
between $24 and $98 per ton of
CO 2 , which is less than traditional
direct air capture methods cost.
And it would likely require only
4500 to 6100 square kilometers of
land, or about four times the size
of Gahcho Kué.
Looming just as large as cost
is the question of how to entice
companies to build a vast carbon-
capture industry. Existing govern-
ment incentives to reduce carbon
are little help. The United States
offers a tax credit of $50 per ton of CO 2
that gets stored underground. California’s
low carbon fuel standard also rewards
companies that sequester carbon. And car-
bon taxes in place in 29 countries encour-
age carbon reductions. But none of those
incentives rewards mineralization as a way
to lower atmospheric carbon.
There’s reason to hope that could change,
says Noah Deich, executive director of
Carbon180, a nonprofit firm that is pushing
Congress to increase funding and incen-
tives for NETs, including mineralization. If
regulators verified mines and other alka-
line waste producers as CO 2 sequestration
sites, Lackner adds, incentives would sky-
rocket, companies could claim tax benefits,
and industry might start to tackle climate
change on the grand scale that’s necessary.
To avert the worst damage from climate
change, Lackner says, “we need to throw
everything we can at it.” Including, per-
PHOTO: VINCENT FOURNIER/ haps, a lot of rocks. j
THE NEW YO
RK TIMES
/REDUX
In Oman, carbon dioxide reacted naturally with rock, forming white
streaks of carbonate.NEWS | FEATURESPublished by AAAS