[Image above] Sign warning visitors of mercury contamination in the Florida Everglades. Credit: Tom Friedel, Wikimedia (CC BY 3.0)
By Laurel Sheppard
As governments around the world look to reduce their reliance on China for rare earth minerals and other critical materials, there is increasing attention given to identification, development, and operation of new mines and processing facilities. But as these projects and downstream industrial operations progress, discussions on potential environmental impacts grow louder as well, such as heavy metal pollution.
Heavy metals such as mercury and cadmium cause negative effects on human health by binding to cells and preventing them from performing their functions. Heavy metals show up regularly in mining and industrial operations. The common practice of establishing such operations near water sources can lead to these metals entering the aquatic ecosystem, contaminating the fish we eat and water we drink.
Most heavy metals do not undergo microbial or chemical degradation. Thus, they must be removed from the water through various developed remediation technologies.
Adsorption is a widely used process for removing heavy metals from water because of its low cost, availability, and ecofriendly nature. It involves placing a solid substance called the adsorbent into contaminated water to attract and bind the heavy metal particles to its surface.
Traditional sorbents such as activated carbons, clays, and zeolites are not efficient for heavy metal removal due to their weak affinity with most metal ions, low surface area, and pH instability. In contrast, 2D materials with special structural features and abundant functional groups can be ideal candidates for this purpose.
MXenes are one 2D material family with great potential as adsorbents for heavy metals. These 2D transition metal carbides, nitrides, and carbonitrides feature high specific surface area and functional groups on the MXene surface, which not only provide sites for direct ion exchange but also reduce some organic molecules and cations. Additionally, compared to other 2D materials such as graphene oxide and molybdenum disulfide, MXenes feature a larger d-spacing (distance between planes of atoms), which aids in ion diffusion into the pores and interlayer.
Titanium-based MXenes, particularly titanium carbide, (Ti3C2Tx), are the most widely studied MXenes for environmental applications due to element abundance and nontoxic decomposition products. However, Ti3C2Tx decomposes to TiO2 crystals in aqueous media due to surface oxidation, hindering its use in water remediation.
Researchers have investigated functionalization of the Ti3C2Tx surface to delay degradation and improve rate of adsorption. In a recent paper, researchers from Drexel University and Temple University developed a simple one-step method to functionalize Ti3C2Tx MXene for mercury removal.
For this study, functionalization meant introducing carboxyl functional groups on the surface of the Ti3C2Tx MXene. This choice stems from the belief that “carboxyl groups stabilize MXene layers, owing to higher energy formation compared to the OH–terminated Ti3C2Tx,” the researchers write.
They created the carboxylated Ti3C2Tx MXenes by adding 1.7 grams of chloroacetic acid to a 1 mg/mL delaminated Ti3C2Tx colloidal solution at 0°C. They stirred the solution for two hours to convert –OH groups on the MXene surface into carboxyl groups. The mixture was then neutralized with distilled water.
Analysis of the carboxylated Ti3C2Tx MXene revealed its surface exhibited a more negative charge than pristine Ti3C2Tx MXene, over a pH range of 2.0–8.5. Such negativity increased the electrostatic interactions between mercury ions and the carboxylated Ti3C2Tx MXene. Plus, addition of the carboxyl groups increased interlayer spacing of the Ti3C2Tx nanosheets and their oxidation stability.
Compared to pristine Ti3C2Tx MXene (pH range values of 2–6), the carboxylated sample demonstrated increased stability and an improved rate of mercury ion adsorption. For example, at an initial concentration of 200 ppm, pristine Ti3C2Tx removed about 70% of mercury ions after 20 minutes; the carboxylated version removed the same amount in just 1 minute.
Additionally, leaching of adsorbed mercury ions from the carboxylated Ti3C2Tx MXene was much lower than that from the pristine Ti3C2Tx MXene. However, the leached mercury for both samples remained much lower than the safe limit defined by the U.S. Environmental Protection Agency.
Based on these results, the carboxylated Ti3C2Tx MXene “has industrial potential for efficient removal of heavy metal ions, as it has a higher mercury-ion uptake capacity than commercially available adsorbents reported in the literature,” the researchers conclude.
The paper, published in Journal of Hazardous Materials, is “Efficient mercury removal from aqueous solutions using carboxylated Ti3C2Tx MXene” (DOI: 10.1016/j.jhazmat.2022.128780).