Many
human-made pollutants in the environment resist degradation through
natural processes, and disrupt hormonal and other systems in mammals and
other animals. Removing these toxic materials — which include
pesticides and endocrine disruptors such as bisphenol A (BPA) — with
existing methods is often expensive and time-consuming.
In a new paper published this week in Nature Communications,
researchers from MIT and the Federal University of Goiás in Brazil
demonstrate a novel method for using nanoparticles and ultraviolet (UV)
light to quickly isolate and extract a variety of contaminants from soil
and water.
Ferdinand Brandl and Nicolas Bertrand, the two lead authors, are
former postdocs in the laboratory of Robert Langer, the David H. Koch
Institute Professor at MIT’s Koch Institute for Integrative Cancer
Research. (Eliana Martins Lima, of the Federal University of Goiás, is
the other co-author.) Both Brandl and Bertrand are trained as
pharmacists, and describe their discovery as a happy accident: They
initially sought to develop nanoparticles that could be used to deliver
drugs to cancer cells.
Brandl had previously synthesized polymers that could be cleaved
apart by exposure to UV light. But he and Bertrand came to question
their suitability for drug delivery, since UV light can be damaging to
tissue and cells, and doesn’t penetrate through the skin. When they
learned that UV light was used to disinfect water in certain treatment
plants, they began to ask a different question.
“We thought if they are already using UV light, maybe they could use
our particles as well,” Brandl says. “Then we came up with the idea to
use our particles to remove toxic chemicals, pollutants, or hormones
from water, because we saw that the particles aggregate once you
irradiate them with UV light.”
A trap for ‘water-fearing’ pollution
The researchers synthesized polymers from polyethylene glycol, a
widely used compound found in laxatives, toothpaste, and eye drops and
approved by the Food and Drug Administration as a food additive, and
polylactic acid, a biodegradable plastic used in compostable cups and
glassware.
Nanoparticles made from these polymers have a hydrophobic core and a
hydrophilic shell. Due to molecular-scale forces, in a solution
hydrophobic pollutant molecules move toward the hydrophobic
nanoparticles, and adsorb onto their surface, where they effectively
become “trapped.” This same phenomenon is at work when spaghetti sauce
stains the surface of plastic containers, turning them red: In that
case, both the plastic and the oil-based sauce are hydrophobic and
interact together.
If left alone, these nanomaterials would remain suspended and
dispersed evenly in water. But when exposed to UV light, the stabilizing
outer shell of the particles is shed, and — now “enriched” by the
pollutants — they form larger aggregates that can then be removed
through filtration, sedimentation, or other methods.
The researchers used the method to extract phthalates,
hormone-disrupting chemicals used to soften plastics, from wastewater;
BPA, another endocrine-disrupting synthetic compound widely used in
plastic bottles and other resinous consumer goods, from thermal printing
paper samples; and polycyclic aromatic hydrocarbons, carcinogenic
compounds formed from incomplete combustion of fuels, from contaminated
soil.
The process is irreversible and the polymers are biodegradable,
minimizing the risks of leaving toxic secondary products to persist in,
say, a body of water. “Once they switch to this macro situation where
they’re big clumps,” Bertrand says, “you won’t be able to bring them
back to the nano state again.”
The fundamental breakthrough, according to the researchers, was
confirming that small molecules do indeed adsorb passively onto the
surface of nanoparticles.
“To the best of our knowledge, it is the first time that the
interactions of small molecules with pre-formed nanoparticles can be
directly measured,” they write in Nature Communications.
Nano cleansing
Even more exciting, they say, is the wide range of potential uses, from environmental remediation to medical analysis.
The polymers are synthesized at room temperature, and don’t need to
be specially prepared to target specific compounds; they are broadly
applicable to all kinds of hydrophobic chemicals and molecules.
“The interactions we exploit to remove the pollutants are
non-specific,” Brandl says. “We can remove hormones, BPA, and pesticides
that are all present in the same sample, and we can do this in one
step.”
And the nanoparticles’ high surface-area-to-volume ratio means that
only a small amount is needed to remove a relatively large quantity of
pollutants. The technique could thus offer potential for the
cost-effective cleanup of contaminated water and soil on a wider scale.
“From the applied perspective, we showed in a system that the
adsorption of small molecules on the surface of the nanoparticles can be
used for extraction of any kind,” Bertrand says. “It opens the door for
many other applications down the line.”
This approach could possibly be further developed, he speculates, to
replace the widespread use of organic solvents for everything from
decaffeinating coffee to making paint thinners. Bertrand cites DDT,
banned for use as a pesticide in the U.S. since 1972 but still widely
used in other parts of the world, as another example of a persistent
pollutant that could potentially be remediated using these
nanomaterials. “And for analytical applications where you don’t need as
much volume to purify or concentrate, this might be interesting,”
Bertrand says, offering the example of a cheap testing kit for urine
analysis of medical patients.
The study also suggests the broader potential for adapting nanoscale
drug-delivery techniques developed for use in environmental remediation.
“That we can apply some of the highly sophisticated, high-precision
tools developed for the pharmaceutical industry, and now look at the use
of these technologies in broader terms, is phenomenal,” says Frank Gu,
an assistant professor of chemical engineering at the University of
Waterloo in Canada, and an expert in nanoengineering for health care and
medical applications.
“When you think about field deployment, that’s far down the road, but
this paper offers a really exciting opportunity to crack a problem that
is persistently present,” says Gu, who was not involved in the
research. “If you take the normal conventional civil engineering or
chemical engineering approach to treating it, it just won’t touch it.
That’s where the most exciting part is.”