`Nanojars' capture dissolved carbon dioxide, toxic ions from water


Date:
August 25, 2021
Source:
American Chemical Society
Summary:
Carbon dioxide from the atmosphere dissolves in waterways, forming
bicarbonate ions and other compounds that change water chemistry,
with possible harmful effects on aquatic organisms. In addition,
bicarbonate can reenter the atmosphere as carbon dioxide later. Now,
researchers have developed tiny 'nanojars' that split bicarbonate
into carbonate and capture it, as well as certain toxic anions,
so they can be removed from water.



FULL STORY ========================================================================== Carbon dioxide from the atmosphere can dissolve in oceans, lakes
and ponds, forming bicarbonate ions and other compounds that change
water chemistry, with possible harmful effects on aquatic organisms. In addition, bicarbonate can reenter the atmosphere as carbon dioxide later, contributing to climate change.

Now, researchers have developed tiny "nanojars," much smaller than
the width of a human hair, that split bicarbonate into carbonate and
capture it, as well as certain toxic anions, so the ions can be removed
and potentially recycled.


==========================================================================
The researchers will present their results today at the fall meeting of
the American Chemical Society (ACS).

"We originally developed nanojars to extract harmful negatively charged
ions, like chromate and arsenate, from water," says Gellert Mezei, Ph.D.,
who is presenting the work at the meeting. "But it turns out that they
also bind strongly to carbonate." Carbonate or other ions captured in
the nanojars could later be disposed of or recycled into useful products,
he says.

Nanojars are tiny containers made up of multiple repeating units of a
copper ion, a pyrazole group and a hydroxide. The jars only form when an
ion with a - 2 charge, such as chromate, arsenate, phosphate or carbonate,
is present. When the proper ingredients are added to an organic solvent,
the repeating units form and assemble into nanojars, with the -2 charged
anion bound tightly at the center.

To remove anions from water, the researchers added the solvent containing
the nanojar components, which formed an organic layer on top of the
water. "The solvent doesn't mix with the water, but the anions from the
water can enter this organic layer," explains Mezei, who is at Western
Michigan University.

"Then, the nanojars form and wrap around the ions, trapping them in the
organic phase." Because the water and organic layers don't mix, they
can easily be separated. Treating the organic layer with a weak acid
causes the nanojars to fall apart, releasing the anions for disposal
or recycling.

The researchers have used nanojars to remove toxic anions from
water. "We've shown that we can extract chromate and arsenate to below
U.S. Environmental Protection Agency-permitted levels for drinking
water -- really, really low levels," Mezei says. The nanojars have
an even higher affinity for carbonate, and adding a molecule called 1,10-phenanthroline to the mixture produces nanojars that bind two
carbonate ions each instead of one.

The team has also made nanojars that are selective for certain
anions. "The original pyrazole building block makes nanojars that are
totally selective for -2 charged ions, but they can't discriminate among
these ions," Mezei says. By using two pyrazoles tethered by an ethylene
linker as a building block, the researchers made nanojars that bind preferentially to carbonate. More recently, they've shown that using
two pyrazoles with a propylene linker produces sulfate-selective
nanojars. These anion-selective nanojars will be important for
applications in which only certain -2 charged ions should be removed.

The researchers have also been working on making the process more suitable
for real-world applications. For example, they've swapped a weak base, trioctylamine, for the strong base, sodium hydroxide, originally used
to make nanojars. "Trioctylamine, unlike sodium hydroxide, is soluble
in the organic phase and makes the formation of the nanojars much more efficient," Mezei says.

Interestingly, trioctylamine causes nanojars to form with slightly
different structures, which he refers to as "capped" nanojars, but they
appear to bind carbonate just as tightly.

So far, all of the experiments have been conducted at the laboratory
scale.

Developing a system to treat large volumes of water, such as in a lake,
will require collaboration with engineers, Mezei says. However, he
envisions that contaminated lake water could be pumped into a station for treatment and then returned to the lake. Some ions, such as phosphate,
could be recycled for useful purposes, such as fertilizer. Carbonate
might be recycled to make "green" solvents, called carbonate esters,
for the nanojar extraction itself.

"Whether this process for removing carbon dioxide from water --
and indirectly, the atmosphere -- would be competitive with other
technologies, that I don't know yet," Mezei says. "There are many
aspects that have to be taken into account, and that's a tricky business." ========================================================================== Story Source: Materials provided by American_Chemical_Society. Note:
Content may be edited for style and length.


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Link to news story: https://www.sciencedaily.com/releases/2021/08/210825143106.htm

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