Adsorption is a physical process that involves the accumulation of molecules or ions on the surface of a solid material, called an adsorbent. Adsorption can be used for water purification by removing contaminants from water. By exploiting the specific interactions between the adsorbent and the adsorbate, adsorption has several advantages over other water purification methods, such as simplicity, high efficiency, selectivity, and recyclability.
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The aim of this article is to review the recent developments in the field of adsorption of contaminants onto novel materials, which refer to those that have unique properties or structures that enhance their adsorption performance or functionality. Also, we will discuss the synthesis, characterization, and application of these novel materials.
Fundamentals of Adsorption
What is Adsorption
Adsorption is a phenomenon in which molecules or ions from a fluid phase (gas or liquid) are attracted and retained on the surface of a solid phase (adsorbent). Adsorption is different from absorption, in which the fluid penetrates into the bulk of the solid. Adsorption is a surface phenomenon, and the amount of adsorbate (the substance that is adsorbed) on the adsorbent surface is determined by the equilibrium between the adsorption and desorption (the reverse process) rates.
Types of Adsorption Processes
Adsorption can be divided into two types: physical adsorption and chemical adsorption. In physical adsorption, the adsorbate is held on the adsorbent surface by van der Waals's forces. Physical adsorption is reversible, non-specific, and depends on the surface area, pore size, and temperature of the adsorbent.
In chemical adsorption, the adsorbate forms a chemical bond with the adsorbent surface, such as covalent or ionic bonds. Chemical adsorption is irreversible, specific, and depends on the surface chemistry, activation energy, and pressure of the adsorbent.
Factors Affecting Adsorption Efficiency
The efficiency of adsorption is affected by several factors, such as the properties of the adsorbent and adsorbate, the solution conditions, and the operational parameters. The adsorbent properties include the surface area, pore size, surface chemistry, functional groups, charge, and polarity.
The adsorbate properties include the molecular size, shape, polarity, charge, and solubility. The solution conditions include the pH, temperature, ionic strength, and presence of competing or co-adsorbing substances.
The operational parameters are contact time, agitation speed, adsorbent dosage, and initial concentration. These factors affect the adsorption capacity, selectivity, kinetics, and thermodynamics of the process.
The mechanisms of adsorption can also be affected by other factors, such as the adsorbent surface properties, the adsorbate concentration and size, the water pH and ionic strength, and the presence of competing or co-adsorbing substances. These factors can affect the adsorption equilibrium, kinetics, and thermodynamics, which determine the extent and rate of adsorption. Therefore, it is important to optimize the adsorption conditions and parameters to achieve the desired level of water purification.
Novel Adsorbent Materials and Their Applications for Water Purification
Graphene and Graphene Oxide
Graphene is a single layer of carbon atoms arranged in a hexagonal lattice, while graphene oxide (GO) is a derivative of graphene that has oxygen-containing functional groups on its surface, which make it hydrophilic, dispersible, and chemically reactive. Graphene and graphene oxide have attracted great attention as novel materials for water purification due to their high surface area, porosity, conductivity, and stability, which enable them to adsorb various contaminants from water, such as organic pollutants, pathogens, and microplastics.
The adsorption of contaminants onto graphene and graphene oxide depends on the type and modification of the materials, as well as the nature and concentration of the contaminants. The adsorption occurs through different interactions, such as electrostatic attractions, hydrogen bonding, π–π stacking, coordination bonding, and physical entrapment. The adsorption capacity and selectivity of graphene and graphene oxide can be enhanced by functionalizing, doping, or compositing them with other materials, such as metals, metal oxides, polymers, and biomolecules.
Contaminant adsorption onto graphene and graphene oxide can also be influenced by environmental conditions, such as the pH, ionic strength, temperature, and presence of competing or co-adsorbing substances. These factors can affect the adsorption equilibrium, kinetics, and thermodynamics, which determine the extent and rate of adsorption. Therefore, it is important to optimize the adsorption conditions and parameters to achieve the desired level of water purification using graphene and graphene oxide.
Metal-Organic Frameworks (MOFs)
Metal-organic frameworks (MOFs) are crystalline materials formed via bond coordination of multifunctional rigid organic ligands and metal clusters. MOFs have a set of characteristics that make them suitable for water purification, such as their high surface area, tunable pore size, shape, and functionality, and excellent stability and recyclability, which enable them to selectively capture, store, or release various contaminants from water by chemical mechanisms.
MOFs can adsorb heavy metals, organic pollutants, gases, and ions from water by coordination bonding, which is the formation of a strong bond between the metal centers of the MOFs and the electron pairs of the contaminants. Modifying metal ions, organic ligands, pore size, and surface groups in MOFs can improve their adsorption capacity and selectivity. These alterations influence the coordination number, geometry, and affinity of the metal centers within MOFs.
Layered Double Hydroxides (LDHs)
LDHs are two-dimensional (2D) nanostructures composed of stacked layers consisting of mixed hydroxides of di- and trivalent cations with hydrated anions in the spaces between the positively charged lamellae.
LDHs are capable of adsorbing heavy metals, organic pollutants, oxyanions, and emerging pollutants from water by anion exchange, which is the replacement of the interlayer anions of the LDHs by the anionic contaminants.
Enhancing the adsorption capacity of LDHs involves modifying metal cations, interlayer anions, layer charge, and surface groups. These adjustments can alter the affinity and accessibility of the interlayer sites within LDHs.
Carbon Nanotubes (CNTs)
Carbon nanotubes (CNTs) are defined as cylindrical nanostructures composed of rolled-up graphene sheets. They come in single-walled (SWCNTs) or multi-walled (MWCNTs) forms.
CNTs have emerged as a promising material for water purification due to their high adsorption capabilities for various organic compounds, such as DDT and its metabolites, dioxin, polynuclear aromatic hydrocarbons (PAHs), PBDEs, chlorophenols, chlorobenzenes, trihalomethane, dyes, phthalate esters, pesticides (thiamethoxam, imidacloprid, and acetamiprid), and herbicides such as sulfur derivatives, dicamba, atrazine, bisphenol A, and nonylphenol.
The adsorption performance of CNTs is affected by factors such as surface area, pore density, functionality, and purity. CNTs provide four main adsorption sites for water contaminants: interstitial channels, inner CNT holes, grooves, and outer surfaces.
Nanocomposites with Magnetic Components
Nanocomposites with magnetic components are a type of materials that consist of magnetic nanoparticles, such as iron oxide, cobalt, or nickel, embedded or attached to other materials, such as carbon, polymer, or metal-organic frameworks. Their characteristics, such as high surface area, porosity, conductivity, and stability, enable them to adsorb various contaminants from water by physical and chemical mechanisms.
Nanocomposites with magnetic components can adsorb heavy metals, organic pollutants, pathogens, and microplastics from water by various interactions, such as electrostatic attractions, hydrogen bonding, π–π stacking, coordination bonding, and physical entrapment.
One of the main advantages of nanocomposites with magnetic components is the easy separation and recovery from water by applying an external magnetic field, which reduces the operational cost and environmental impact of water purification. Moreover, they can be regenerated and reused by various methods, such as washing, heating, or irradiation, which increases their economic and environmental viability. Furthermore, they can be integrated with other techniques, such as photocatalysis, electrochemistry, and biotechnology, to improve their adsorption efficiency and functionality.
Challenges and Limitations
Novel adsorbents face some challenges and limitations that need to be addressed and overcome to realize their full potential and application in water purification. These challenges and limitations include:
Synthesis and Modification
The synthesis and modification of novel adsorbents require advanced techniques, equipment, and expertise, which can increase the cost, time, and complexity of the production process. Moreover, the synthesis and modification of novel adsorbents may involve the use of toxic or hazardous chemicals, which can pose environmental and health risks. Therefore, there is a need to develop simple, green, and scalable methods for the synthesis and modification of novel adsorbents, as well as to ensure the safety and quality of the products.
Application and Performance
The application and performance of novel adsorbents depend on several factors, such as the type and concentration of the contaminants, the environmental conditions, the adsorption conditions and parameters, and the integration and optimization of the adsorption process.
Moreover, the application and performance of novel adsorbents may not be satisfactory or sustainable in real or complex water systems due to the presence of interfering or co-adsorbing substances, the fouling or degradation of the adsorbents, and the regeneration and reuse of the adsorbents. Therefore, there is a need to conduct extensive and rigorous studies on the application and performance of novel adsorbents in different water systems, as well as to improve and maintain the efficiency and functionality of the adsorbents.
Separation and Recovery
The separation and recovery of novel adsorbents from water are crucial steps that affect the operational cost and environmental impact of water purification. However, the separation and recovery of novel adsorbents may not be easy or feasible due to the small size, low density, or high dispersibility of the adsorbents.
Moreover, the separation and recovery of novel adsorbents may not be efficient or effective due to the loss or leakage of the adsorbents, the contamination or damage of the adsorbents, and the disposal or recycling of the adsorbents. Therefore, there is a need to develop novel and practical methods for the separation and recovery of novel adsorbents from water, as well as to minimize and mitigate the adverse effects of the adsorbents on the environment.
Applications of Novel Materials in Water Purification
Heavy Metal Removal
Graphene-based materials, MOFs, and LDH composites have shown good ability in removing heavy metals such as lead, cadmium, and mercury. Their high adsorption capacities and selectivity make them ideal for addressing metal contamination in water sources.
Organic Pollutant Removal
Graphene and CNTs excel in adsorbing organic pollutants due to their hydrophobic nature and large surface area. Emerging contaminants like pharmaceuticals and dyes are effectively removed by these novel materials.
Arsenic Remediation
LDH composites, graphene oxide, and magnetic nanoparticles have been employed for the efficient removal of arsenic from water. Their ability to selectively adsorb arsenic species makes them valuable in regions facing arsenic contamination challenges.
Tailored Adsorbents for Specific Contaminants
Metal-organic frameworks can be designed with specific ligands to target particular pollutants. This tailoring makes MOFs versatile adsorbents for addressing diverse contaminants in water.
Challenges and Future Directions
While novel adsorbent materials show great promise, challenges remain. Aggregation of graphene nanosheets limits available adsorption sites, prompting the need for strategies to prevent aggregation. The efficient separation of nanomaterials from water is also a concern, with magnetic separation emerging as a promising technique.
Future research should focus on enhancing the dispersion properties of graphene and improving the efficiency of magnetic separation methods. Additionally, exploring the adsorption capabilities of new materials and tailoring them for specific pollutants will contribute to the development of advanced adsorbents.
Conclusion
Novel materials used for adsorption present a sustainable and effective approach to addressing water pollution. Graphene, metal-organic frameworks, layered double hydroxides, and other materials exhibit good adsorption capacities, making them invaluable for water purification. As research advances, these materials hold the potential to revolutionize water treatment practices, offering scalable and eco-friendly solutions to meet the growing demand for clean water worldwide.
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References and Further Reading
Jain, A., Kumari, S., Agarwal, S., & Khan, S. (2021). Water purification via novel nano-adsorbents and their regeneration strategies. Process Safety and Environmental Protection. [Online] Available at: https://www.sciencedirect.com/science/article/abs/pii/S0957582021003220.
Raut, B. (2022). Adsorption – Mechanism, Types of Adsorption, and Applications. Chemist Notes. [Online] Available at: https://chemistnotes.com/physical/adsorption-mechanism-types-of-adsorption-and-applications/.
Huawen Hu, Wu Wen, & Jian Zhen Ou. (2022). Construction of adsorbents with graphene and its derivatives for wastewater treatment: a review. Environmental Science: Nano, Issue 9. [Online] Available at: https://pubs.rsc.org/en/content/articlelanding/2022/en/d2en00248e.
Arora, B., & Attri, P. (2020). Carbon Nanotubes (CNTs): A Potential Nanomaterial for Water Purification. [Online] Available at: https://www.mdpi.com/2504-477X/4/3/135.
Villar da Gama, B. M., Selvasembian, R., Giannakoudakis, D. A., Triantafyllidis, K. S., McKay, G., & Meili, L. (2022). Layered Double Hydroxides as Rising-Star Adsorbents for Water Purification: A Brief Discussion. Molecules. [Online] Available at: https://www.mdpi.com/1420-3049/27/15/4900.
Dubey, R., Dutta, D., Sarkar, A., & Chattopadhyay, P. (2021). Functionalized carbon nanotubes: synthesis, properties and applications in water purification, drug delivery, and material and biomedical sciences. Nanoscale Advances, Issue 20. [Online] Available at: https://pubs.rsc.org/en/content/articlelanding/2021/na/d1na00293g.
Rashid, R., Shafiq, I., Akhter, P., Iqbal, M. J., & Hussain, M. (2021). A state-of-the-art review on wastewater treatment techniques: the effectiveness of adsorption method. Environmental Science and Pollution Research. [Online] Available at: https://link.springer.com/article/10.1007/s11356-021-12395-x.
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