InfrastructureNew material shows promise for nuclear waste clean-up

A team of
Northwestern University chemists is the first to focus on
metal sulfide materials as a possible source for nuclear waste remediation
methods. Their new
material is extremely successful in removing strontium from a sodium-heavy
solution, which has concentrations similar to those in real liquid nuclear
waste. Strontium-90, a major waste component, is one of the more dangerous
radioactive fission materials created within a nuclear reactor. By taking
advantage of ion exchange, the new method captures and concentrates strontium
as a solid material, leaving clean liquid behind. In the case of actual nuclear
waste remediation, the radioactive solid could then be dealt with separately —
handled, moved, stored or recycled — and the liquid disposed. “It is a
very difficult job to capture strontium in vast amounts of liquid nuclear
waste,” said Mercouri Kanatzidis, Charles E. and Emma H. Morrison Professor
of Chemistry in the Weinberg College of Arts and Sciences and the paper’s
senior author of a 3 March PNAS paper titled “Layered Metal Sulfides:
Exceptionally Selective Agents for Radioactive Strontium Removal.” He
adds: “Sodium and calcium ions, which are nonradioactive, are present in
such enormous amounts compared to strontium that they can be captured instead
of the radioactive material, interfering with remediation.”

Strontium
is like a needle in a haystack: sodium ions outnumber strontium ions by more
than a million to one. The material developed at Northwestern — a layered
metal sulfide made of potassium, manganese, tin and sulfur called KMS-1 —
attracts strontium but not sodium. “The metal sulfide did much, much
better than we expected at removing strontium in such an excess of
sodium,” said Kanatzidis. “We were really amazed at how well it
discriminates against sodium and think we have something special. As far as we
can tell, this is the best material out there for this kind of application.”
KMS-1 works at the extremes of the pH scale — in very basic and very acidic
solutions, the conditions common in nuclear waste — and everywhere in between.
Metal oxides and polymer resins, the materials currently used in nuclear waste
remediation, perform reasonably well but are more limited than KMS-1: each
typically works in either basic or acidic conditions but not both and
definitely not across the pH scale. In earlier work, Kanatzidis and his team
had found KMS-1 to be very quick and facile at ion exchange (the material gives
up an ion and takes another to maintain charge balance). Knowing this and also
that the ion exchange process is a removal process, the researchers decided
that strontium was an interesting ion with which to test their new material. The
solution the researchers used in the lab contained strontium and two
“interfering” ions, sodium and calcium, in concentrations like those
found in the nuclear waste industry (nonradioactive strontium, which works the
same as the radioactive version, was used in the experiments). KMS-1, a free
flowing black-brown powder, was packaged like tea in a teabag and then dropped
into the solution. The all-important ion exchange followed: the metal sulfide
“teabag” soaked up the strontium and gave off potassium, which is not
radioactive, into the liquid.

KMS-1
does its remarkable work targeting only strontium by taking advantage of two
things: strontium is a heavier ion than calcium, and sulfur (a component of
KMS-1) attracts heavier ions; and KMS-1 attracts ions with more charge so it
attracts strontium, which has a charge of 2+, and doesn’t attract sodium, which
only has a charge of 1+. So, as Kanatzidis likes to say, “Our material
beats both sodium and calcium.” He adds that “The nuclear power process generates
enormous amounts of radioactive liquid waste, which is stored in large tanks.
If we can concentrate the radioactive material, it can be dealt with and the
nonradioactive water thrown away. I can imagine our material as part of a
cleansing filter that the solution is passed through.” Looking to the
future, to be a scaleable and affordable remediation method, the metal in the
metal sulfide needs to be inexpensive and readily available and also make a
stable compound. “We focused on potassium, manganese and tin because we
have been working with them for some time,” said Manolis Manos, a
postdoctoral fellow at Northwestern and lead author of the paper. “All
three metals make stable compounds and are common and abundant. Our next step
is to do systematic studies, including using an actual waste solution from the
nuclear power industry, to learn how KMS-1 works and how to make even better
metal sulfides,” he added.

In
addition to Kanatzidis and Manos, Nan Ding, a former graduate student in
Kanatzidis’ group, now at Claflin College in South Carolina, is the other author of the PNAS paper