Science - USA (2020-09-04)

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SCIENCE sciencemag.org 4 SEPTEMBER 2020 • VOL 369 ISSUE 6508 1157

CREDITS: (PHOTO) DE BEERS GROUP; (GRAPHIC) PAN


ET AL., NATURE SUSTAINABILITY


,


DOI.ORG/10.1038/S41893-020-0486-9, ADAPTED BY N. DESAI/

SCIENCE

caustic leachates from mountains of steel
slag waste have wiped out fish populations
in Pennsylvania and the United Kingdom.
Reacting these wastes with CO 2 from the
air could make them safer by solidifying
them—and at the same time help the world
avert climate disaster. In the 2015 Paris cli-
mate agreement, most of the world’s coun-
tries resolved to limit climate warming to
below 2°C. For that to happen, the Inter-
governmental Panel on Climate Change
(IPCC) has determined, cutting greenhouse
gas emissions won’t be enough. Countries
will also need to employ “negative emis-
sions technologies” (NETs) to pull as much
as 10 billion tons (gigatons) of CO 2 out
of the atmosphere every year toward the
end of this century. Possible NETs include
planting vast forests, which suck carbon
out of the air as they grow; chemically ab-
sorbing CO 2 from the air or power plant
exhaust and pumping it underground; and
growing grasses or shrubs, burning them
for energy, and capturing and storing the
CO 2 (Science, 16 February 2018, p. 733).
But underground storage chambers can
leak, and forests can burn. Mineralization
is more permanent: Carbon-based miner-
als, or carbonates, are among the most
stable on Earth, adds Siobhan “Sasha”
Wilson, a biogeochemist at the University
of Alberta, Edmonton. “It’s a really robust
place to store CO 2 ,” she says.

And suitable rock waste is plentiful. Start
with ultramafic wastes, the calcium- and
magnesium-rich rock in which diamonds,
along with metals such as nickel, platinum,
and palladium are found. A 2019 report on
NETs by the U.S. National Academy of Sci-
ences (NAS) described CO 2 storage in ultra-
mafic mine wastes as “low-hanging fruit.”
Today, some 419 million tons of this and less
alkaline “mafic” wastes are produced annu-
ally. If fully carbonated, they could lock up
175 million tons of atmospheric CO 2 per year.
Then there are the alkaline wastes from alu-
minum, iron, steel, and cement production,
which could bring the total up to at least
310 million tons—and by some estimates
more than 4 gigatons (GTs)—of CO 2 trapped

each year. The somewhat less alkaline basalt
rock powder generated by coal production
could sequester another 2 GTs per year, Phil
Renforth of Heriot-Watt University and his
colleagues have calculated—meaning alka-
line wastes could in principle provide more
than half of the negative emissions that
IPCC called for.
But there are major hurdles. Govern-
ments will need to offer incentives for min-
eralization on the massive scale needed to
make a dent in atmospheric carbon. And
engineers will need to figure out how to
harness the wastes while preventing the
release of heavy metals and radioactivity
locked in the material. Still, “Alkaline
wastes have tremendous potential,” says
Liang-Shih Fan, a chemical engineer at
Ohio State University, Columbus. “It’s a po-
tential one should not overlook.”

THE NOTION of storing CO 2 in minerals isn’t
new. Plans to capture the gas from the air
or power plant exhaust often call for inject-
ing it into underground rock formations
that, like mine waste, react to form carbon-
ates. And certain rocks naturally capture
CO 2 in a process known as weathering. In
Oman, vast ridges of a mineral called peri-
dotite mineralize CO 2 from the air, forming
white veins resembling marbling in steak.
Similar smaller formations dot the globe.
Mine wastes behave the same way. In

Cement
waste
16.3%

Paper
industry
waste
11.4%
Mining
waste
13.5%

Coal
combustion
12.3%

Iron and
steel
slag
43.5%

Municipal
solid waste
2.4%

Trash to treasure
Industrial wastes could lock away 310 million tons
of carbon dioxide every year. Each category of waste
could mineralize the percentages shown.

NEWS

At Gahcho Kué (left), a diamond mine in Canada’s Northwest Territories, researchers bubble carbon dioxide
through a slurry of rock waste in a tube (right) to test how to lock away carbon from diesel exhaust.

Published by AAAS
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