2D material to clean up mercury pollution

Due to the alarming increase in mercury emissions into the atmosphere, researchers have been working for many years to develop systems to remove this heavy metal from water. Now, a team at Drexel University in Philadelphia might have found just the right material to efficiently catch the elusive quicksilver and clean up contaminated waters – MXene.

It is estimated that mercury emissions in the atmosphere have quadrupled since the Industrial Revolution.

The liquid-at-room-temperature metal, generated by burning fossil fuels and the disposal of industrial and medical waste, has become so pervasive in aquatic environments that the U.S. Food and Drug Administration states nearly half a dozen species of fish are so mercury-contaminated that people should avoid consuming them entirely.

Among many methods that exist presently to remove mercury from water, adsorption – a process that chemically attracts elements to remove contaminants – is the most promising due to its relative simplicity, efficiency, and lower cost.

"Modern adsorbents, such as resins, mesoporous silica, chalcogenides, and mesoporous carbons, have higher efficiencies than traditional adsorbents, such as activated carbon, clays, and zeolites that have low affinity toward mercury and low capacities," said Masoud Soroush, professor at Drexel College of Engineering and head of the lab developing this new adsorption technology. "However, the problem with all these materials, is that their mercury-removal efficiencies are still low, and they are unable to lower mercury level to less than 1 part per billion."

The team at Drexel, collaborating with Temple University, has been exploring the synthesis and use of a surface-modified titanium carbide called MXene for mercury removal.

Discovered more than a decade ago at Drexel, Mxenes are a family of two-dimensional nanomaterials that have demonstrated many exceptional properties.

Among these properties, aside from its high electrical conductivity, MXenes exhibit strong hydrophilic tendencies, meaning it is highly susceptible to mixing with water.

For mercury ion removal, titanium carbide MXene has several advantages over conventional adsorbents, its negatively charged surface and the "tunability" and versatility of its surface chemistry. Because of these properties and the layered structure of the MXene, titanium carbide Mxene-based materials have shown superior performance in gas separation, removing salt from water, killing bacteria, and even kidney dialysis.

"We knew that 2D materials, such as graphene oxide and molybdenum disulfide, had previously been effective in removing heavy metals from wastewater through adsorption because of their chemical functionalities/structures that attract metal ions," said Soroush. "MXenes are a similar type of materials but we estimated that titanium carbide MXene could have much greater uptake capacities than these other materials-therefore making it a better sorbent for mercury ions."

Redesigning 2D structures

However, Soroush's team needed to make a key alteration to titanium carbide MXene's chemical structure to further improve the material for one of its most challenging tasks.

"Mercury is called quicksilver for a reason-it's quite evasive once emitted into the environment, whether by burning fossil fuels, mining, or waste incineration," continued Soroush. "It quickly changes its chemical form-increasing its toxicity and making it tremendously difficult to remove from the bodies of water where it inevitably accumulates."

Turns out, there is a natural attraction between mercury ions and titanium carbide MXene's surface, as metal ions, like mercury, are positively charged, and the surface of MXene flakes is negatively charged. However, to pull mercury ions out of water more strongly, the team needed to give this attraction a boost.

"So, to attract mercury ions even faster we needed to modify the surface of titanium carbide MXene flakes," said Soroush.

To this end, the team treated MXene flakes with chloroacetic acid, which provides the MXene with highly mobile, strong carboxylic acid groups and increases the MXene-flakes surface negative charge, ultimately improving the ability of the flakes to attract and retain mercury ions.

Basically, the researchers treated MXene with carbon dioxide, which essentially created an effect similar to static electricity in a balloon when it clings to your hair or fabrics.

The result was a new sorbent material – carboxylated titanium carbide MXene – that demonstrated a faster mercury-ion uptake and greater capacity than all commercially available adsorbents, according to Drexel.

"Carboxylated titanium carbide MXene proved to be far superior to sorbent material currently being used for mercury-ion removal," continued Soroush. "Within one minute it was able to remove 95% of mercury ions from a water sample contaminated at a concentration of 50 parts per million, which means it could be effective and efficient enough for use in large scale wastewater treatment."

Within five minutes, the MXene removed 98% of the mercury ions from a 10-milliliter water sample contaminated at concentrations between 1 and 1000 parts per million.

"This indicates that both [MXene] and [carboxylated MXene] are effective adsorbents to remove mercury ions from wastewater due to their special structural properties and high density of surface functional group," the team wrote. "Generally, the adsorption mechanism of metal ions follows two steps; at first, the ions are quickly adsorbed on the available active sites, and the process is swift."

This development is monumental in the battle to contain mercury pollution, which has become so prevalent that health authorities have strongly cautioned against whole species of fish altogether.

While shifting away from the polluting energy sources is the ultimate solution to preventing the release of heavy metals like mercury, it is a slow and expensive process that would still require replacement technology of similar performance. Regardless, this technology could be the breakthrough that leads to new possibilities for cleaning up the pollution that has already been created.

"We envision that the use of the carboxylated MXene technology to remove all heavy metal ions," finished Soroush. "Besides using the carboxylated MXene as a sorbent, another way of achieving this is to fabricate filters coated or embedded with the carboxylated MXene."