Research
Nanoporous Material Nets Contaminant from Water
By EMSL, 19/7/19
Jul 12, 2019 - 12:11:41 PM

Barely ​visible ​material ​that looks like ​tiny grains of ​sand may hold ​the key to ​removing an ​invisible ​health threat ​that has ​contaminated ​water supplies ​across the ​country. ​

Researchers at Pacific ​Northwest ​National ​Laboratory  (PNNL) ​have successfully ​tested highly ​porous ​materials and ​found they can ​absorb key ​components of a ​class of toxic ​chemicals found ​in 43 ​states. ​

Materials ​scientists at ​PNNL are ​experts in ​optimizing ​metal organic ​frameworks ​or MOFs. These ​nano-sized ​porous ​materials, with ​metal centers, ​can attract, ​hold, and then ​later release ​specific ​chemicals. PNNL ​researchers ​recently ​demonstrated a ​MOF that ​quickly takes ​up fluorinated ​compounds that ​were widely ​used in ​firefighting ​foam and non-​stick cookware. ​

These per- and ​polyfluoroalkyl ​substances (​PFAS) resist ​grease and ​water, so they ​have been used ​in some food ​packaging, ​stain-resistant ​fabrics and ​carpets, and ​cosmetics. ​Contamination ​of drinking ​water is ​typically ​associated with ​a specific ​facility like a ​manufacturing ​plant or ​military base. ​PFAS don’​t break down in ​the environment ​or in human ​bodies. ​

According to ​the ​Centers for ​Disease Control ​and Prevention, ​more research ​is needed ​regarding ​health effects, ​but certain ​human studies ​have shown that ​some exposure ​may affect ​growth, ​learning, and ​behavior of ​infants and ​older children, ​lower a ​woman’s ​chance of ​getting ​pregnant, ​interfere with ​the body’​s natural ​hormones, ​increase ​cholesterol ​levels, affect ​the immune ​system, and ​increase the ​risk of cancer. ​

The Environmental ​Protection ​Agency advises ​that the ​concentration ​in drinking ​water be ​limited to 70 ​parts per ​trillion for ​two of the most ​common ​compounds in ​the class: ​perfluorooctanesulfonate​ ​(PFOS) and ​perfluorooctanoic​ ​acid (PFOA). ​

With more than ​20 years’ ​experience ​working with ​MOFs, the PNNL ​research team ​built on ​past ​research ​and hypothesized ​that a MOF with ​just the right ​characteristics ​could adsorb ​PFOS from water.​

“We are ​excited to find ​a MOF that ​showed ​excellent ​capture ​capabilities at ​high concentrations ​of PFOS,” ​said Radha ​Motkuri, who ​leads the ​materials ​development ​team. ​“We began ​with two MOF ​candidates ​– one ​with an iron ​center and ​another with ​chromium. The ​internal ​structures act ​like little ​metal ‘​hooks’ ​that allow the ​material to ​hold PFAS ​compounds.​” ​

Both MOFs ​worked much ​better than ​existing ​technology, ​which consists ​mainly of ​granular ​activated ​carbon filters. ​But in lab ​tests reported ​recently ​in ​ Inorganic Chemistry  , the chromium-​based MOF had ​an extremely ​fast capture ​rate—​likely because ​of its uniform ​pores, highly ​active sorbent ​sites, and ​internal ​channels.  ​

Catch and release 

“There ​is a very high ​chemical ​affinity with ​the chromium ​attracting the ​sulfur ​headgroup and ​fluorine tail ​on PFOS as ​observed by our ​X-ray ​photoelectron ​spectroscopy ​measurements,​” said ​corresponding ​author Dev ​Chatterjee. ​“​That’s ​one reason that ​it works so ​well compared ​to existing ​activated ​carbon filters.​”  ​

Both the ​chromium- and ​iron-based MOFs ​are stable in ​water. ​That’s ​important ​because they ​can be flushed ​with fresh ​water to remove ​the PFOS, plus ​researchers ​estimate the ​material can be ​reused up to ​hundreds of ​times.  ​

MOFs can be ​thought of as a ​metallic ​mesh—​rather like a ​fish net—​scooping up ​PFOS while ​letting other ​molecules pass ​through based ​on specific ​parameters.​  ​

“​It’s the ​ability to ​tailor ​characteristics ​such as pore ​size, active ​sites, ​orientation and ​the MOF-​chemical ​interaction ​that make the ​PNNL team’​s expertise in ​MOFs so ​valuable.”​ said Jennifer ​Lee, PNNL ​commercialization​ ​manager. “​This means that ​other MOFs may ​be modified for ​many additional ​contaminants ​and uses.”​  ​

Absorbing results

To observe the ​performance, ​PFOS was ​combined ​independently ​in 5-millimeter ​vials with the ​iron MOF, the ​chromium MOF, ​or the granular ​activated ​carbon in an ​extremely ​sensitive ​spectrometer ​that showed the ​concentration ​of PFOS over ​time. The ​chromium-based ​MOF almost ​completely ​adsorbed the ​contaminant ​within seconds. ​There was ​little to no ​reduction when ​using the ​commercially ​available ​activated ​carbon. ​  ​

“Pure ​clean water is ​so important ​for all of ​humanity,”​ said Motkuri. ​“We ​envision a ​field-​deployable ​system where ​one day water ​would run ​through, ​contaminants ​would be ​captured by the ​MOF, and clean ​water would ​come out.”​

Chatterjee and ​the rest of the ​team are ​developing a ​patent-pending ​MOF-based ​electrochemical ​sensor that ​would detect ​ultratrace ​levels of PFAS ​contamination ​and would also ​help guide ​cleanup ​operations. ​

The nuclear ​magnetic ​resonance and X-​ray photoelectron ​spectrometer ​analyses were ​conducted ​at EMSL, ​the Environmental ​Molecular ​Sciences ​Laboratory, a ​DOE Office of ​Science user ​facility at ​PNNL. ​

The work was ​supported ​through ​PNNL’s ​Laboratory ​Directed ​Research and ​Development ​Program. ​Earlier ​materials ​development was ​funded by the ​Department of ​Energy’s ​Geothermal ​Technologies ​Office. ​

In addition to ​Motkuri and ​Chatterjee, the ​team included ​PNNL researchers ​Dushyant ​Barpaga, Jian ​Zheng, Kee Sung ​Han, Jennifer ​Soltis, ​Vaithiyalingam ​Shutthanandan (​EMSL), and B. ​Peter McGrail, ​with Sagnik ​Basuray of ​the ​New Jersey ​Institute of ​Technology .

This web ​feature is ​also available ​at ​PNNL News & Media .