As industrialization accelerates and populations expand, finding sustainable solutions for escalating environmental pollution is crucial. The emergence of two-dimensional (2D) materials represents a significant advancement in addressing these environmental challenges. This article explores how these materials are revolutionizing environmental remediation and paving the way for a cleaner future.
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Properties and Characteristics of 2D Materials
2D materials such as graphene, transition metal dichalcogenides, boron nitride, and layered double hydroxides possess several advantageous properties that make them well-suited for environmental remediation applications. Their high surface area-to-volume ratio and planar structure provide a large interaction site and facilitate strong forces for removing analytes from contaminated water or air.
The tunable electronic structure of these materials enables modulation of their band gaps, with lower band gaps increasing the probability of oxidation and reduction reactions within water or air, potentially leading to more effective degradation of contaminants.1
How are 2D Materials Used in Environmental Remediation?
Water Purification with MXenes
The US Environmental Protection Agency (EPA) considers water contamination a global threat, exacerbated by industrial activities that release 300–400 million tons of pollutants into water sources annually. This prompts the development of effective remediation solutions.
MXenes, a class of 2D materials first identified in 2011, have emerged as prominent candidates for water purification. These materials belong to the family of transition metal nitrides, carbides, and carbonitrides with a chemical formula Mn+1XnTx, where ‘M’ represents an early transition metal, ‘X’ is carbon or nitrogen, and ‘T’ denotes surface terminal groups such as –OH, –F, –O, and –Cl.
This diverse surface chemistry enhances their hydrophilicity and enables various adsorption mechanisms, making MXenes suitable for removing a wide range of water pollutants, including heavy metals and organic compounds.
MXenes remove pollutants primarily through electrostatic interactions, ion exchange, and surface complexation. Their negatively charged surfaces attract positively charged metal ions through electrostatic interactions, with their high surface area offering numerous binding sites for these ions.
The functional groups on the surfaces of MXenes facilitate ion exchange, enabling them to exchange their ions with contaminant ions in the water. MXenes can also form complexes with metal ions at their surfaces, resulting in stable, non-soluble products that are easy to remove.2
Air Purification with Graphene
In air remediation, 2D materials are crucial for developing filtration technologies that capture and remove particulate matter and chemical pollutants from the air.
Graphene’s unique 2D hexagonal, honeycomb-like structure grants it excellent physicochemical, thermal, mechanical, and electrical properties, while its high surface area and porosity facilitate effective adsorption and absorption processes. This makes it highly attractive for air purification and carbon capture applications.
Graphene derivatives such as graphene oxide (GO) and reduced graphene oxide (rGO) are particularly effective in capturing CO2 from the environment. This is due to the presence of oxygen functionalities, which attract and hold acidic CO2 molecules on their surface.
The adsorption efficiency of graphene can be improved through hydrothermal reduction or the addition of amine groups, which alter oxygen functionalities and increase interlayer spacing. These changes enhance the electron donor-acceptor interactions crucial for selective CO2 capture.2
Soil Remediation with 2D Hydroxyapatite Nanocrystals
Hydroxyapatite (HAp) is a naturally occurring mineral form of calcium apatite. HAp 2D nanocrystals are notable for their non-toxicity, low water solubility, high adsorption capacity, and robust stability under various chemical conditions. These properties make them an effective and economical solution for removing heavy metal ions from contaminated water and soils.
In soil remediation, HAp nanocrystals function primarily through dissolution-precipitation and ion exchange. The dissolution-precipitation process involves the release of phosphate ions by HAp, which then form new apatitic structures with heavy metal ions, effectively immobilizing them. In contrast, the ion exchange mechanism allows Hap to replace calcium ions in its lattice with heavy metal ions, further aiding in the decontamination of soils.
Recent studies have shown that Hap nanocrystals, even when derived from waste materials like phosphogypsum waste or oyster shells, exhibit remarkable efficiency in adsorbing and immobilizing metals like cadmium, lead, and zinc, reducing their bioavailability significantly in contaminated soils.3
Advantages Over Traditional Remediation Methods
2D materials offer several advantages over traditional remediation techniques. Their high efficiency and selectivity in targeting specific pollutants lead to faster remediation times and lower secondary waste production. Additionally, the superior physical and chemical stability of 2D materials enables their use in a wide range of environmental conditions, often with superior performance.
While the initial cost of implementing 2D materials may be higher, their durability, reusability, and ability to target multiple pollutants simultaneously can translate into long-term cost-effectiveness. Integrating 2D materials into existing remediation technologies can also enhance overall effectiveness, making them valuable to environmental remediation strategies.
Challenges and Considerations
Despite their potential, 2D materials present particular challenges in pollutant treatment.
For instance, graphene has a tendency to stack, which can impair its effectiveness. Similarly, transition metal dichalcogenides (TMD/TMO) and carbon nitrides (CN) often exhibit poor adsorption capabilities.
MXenes, while highly hydrophilic, can face difficulties separating oil from water. Finally, the synthesis of boron nitride can often prove challenging.4
Recent Advances and Case Studies
Graphene-Enhanced MXene Membranes for Chromium Removal in Water Treatment
Chromium (Cr (VI)) poses a significant threat to plant metabolic activities, inhibiting crop growth and quality. It is, therefore, critical to monitor Cr(VI) levels in water, soil, and crop production systems.
A study in Nature Sustainability incorporated reduced graphene oxide into Ti3C2Tx-based membranes, increasing membrane adsorption ability, wettability, and metal ion reduction.
While only 44 % of the initial Cr (VI) was removed after 150 minutes using the pristine Ti3C2Tx membrane, the graphene composite membrane achieved a removal efficiency of 91 % in the same timeframe.
The researchers also observed a shift in binding energy for C-Tiδ+-Tx moieties after HCrO4− removal, suggesting electron transfer from Ti3C2Tx to HCrO4−, resulting in the release of Cr(III) that can be retained in Ti3C2Tx-based membranes.5
Enhanced Volatile Organic Compounds Adsorption with Graphene-Iron Oxide Heterostructures
Recently, Korea Institute of Science and Technology (KSI) researchers developed a new adsorbent technology using graphene-iron oxide to capture amphiphilic volatile organic compounds (VOCs) found in paints and cosmetics. The results are published in the Chemical Engineering Journal.
Traditional air purifiers primarily utilize activated carbon and struggle with polar substances like ketones and aldehydes. To address this, the research team developed a graphene-iron oxide heterostructure with controlled surface oxidation of graphite and iron. This innovation significantly improved the adsorption of amphiphilic VOCs, achieving an adsorption efficiency up to 15 times higher than conventional activated carbon adsorbents.
Their research also uncovered correlations between ketone chain length and adsorption efficiency, providing valuable insights into electron transfer phenomena between the adsorbent and VOC molecules. This breakthrough not only exceeds the limits of existing adsorbents but also offers customizable solutions for controlling various air pollutants, utilizing accessible materials like graphite and iron.6
The Future of 2D Materials in Environmental Remediation
The future of 2D materials in environmental remediation looks promising, with ongoing research exploring their potential in more advanced and niche applications. Material design and manufacturing innovations will likely overcome current limitations, leading to more effective and economically feasible solutions.
However, comprehensive regulatory frameworks and thorough safety assessments are crucial to ensure the responsible use of 2D materials without introducing new risks.
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References and Further Reading
- Chauhan, D., Ashfaq, M., Talreja, N., Managalraja, RV. (2021). 2D materials for environment, energy, and biomedical applications. Journal ISSN. doi.org/10.37871/jbres1340
- Kumar, N., Gusain, R., Ray, SS. (2023). Two-Dimensional Materials for Environmental Applications. Springer. https://doi.org/10.1007/978-3-031-28756-5
- Ibrahim, M., Labaki, M., Giraudon, JM., Lamonier, JF. (2020). Hydroxyapatite, a multifunctional material for air, water and soil pollution control: A review. Journal of hazardous materials. doi.org/10.1016/j.jhazmat.2019.121139
- Kong, H., et al. (2022). Two-dimensional material-based functional aerogels for treating hazards in the environment: synthesis, functional tailoring, applications, and sustainability analysis. Nanoscale Horizons. doi.org/10.1039/D1NH00633A
- Xie, X., Chen, C., Zhang, N., Tang, ZR., Jiang, J., Xu, YJ. (2019). Microstructure and surface control of MXene films for water purification. Nature Sustainability. doi.org/10.1038/s41893-019-0373-4
- Lee, S., Kim, S., Han, SS., Kim, DW., Lee, J., Oh, Y. (2023). Effect of adsorbate geometry and hydrogen bonding on the enhanced adsorption of VOCs by an interfacial Fe3O4–rGO heterostructure. Chemical Engineering Journal. doi.org/10.1016/j.cej.2023.145346
