Carbon nanotubes (CNTs) possess unique structural and chemical properties that make them highly effective for adsorption and membrane filtration applications in environmental remediation. Their high surface area, porosity, and tunable surface chemistry enable efficient capture and removal of pollutants, including heavy metals and organic contaminants, from water and air. Surface modifications further enhance selectivity and performance, allowing tailored solutions for specific pollutant removal challenges.

The adsorption capacity of CNTs stems from their nanoscale dimensions and extensive surface area, which provide abundant active sites for pollutant binding. Pristine CNTs exhibit affinity for certain contaminants, but chemical functionalization significantly improves their selectivity and efficiency. Common modifications include oxidation, polymer grafting, and incorporation of functional groups such as carboxyl, hydroxyl, or amine. These alterations enhance interactions with target pollutants through electrostatic attraction, complexation, or hydrophobic effects.

For heavy metal removal, oxidized CNTs demonstrate high adsorption capacities due to the presence of oxygen-containing groups that form complexes with metal ions. Studies have shown that carboxyl-modified CNTs can adsorb lead (Pb²⁺) with capacities exceeding 50 mg/g, while mercury (Hg²⁺) removal efficiency improves with sulfur-functionalized CNTs due to strong Hg-S interactions. The adsorption mechanism often involves ion exchange, surface precipitation, or coordination bonding, depending on the metal and functional groups present.

Organic pollutant adsorption relies on hydrophobic interactions, π-π stacking, and electrostatic forces. CNTs effectively remove aromatic compounds, dyes, and pesticides due to their graphitic structure, which facilitates π-π interactions with conjugated molecules. For instance, multi-walled CNTs (MWCNTs) exhibit adsorption capacities of up to 100 mg/g for certain dyes, while single-walled CNTs (SWCNTs) show even higher affinities for polycyclic aromatic hydrocarbons (PAHs). Surface oxidation can alter selectivity, making CNTs more effective for polar organics by introducing hydrogen bonding or electrostatic interactions.

Membrane filtration incorporating CNTs enhances separation efficiency through size exclusion, adsorption, and antifouling properties. CNT-based membranes are fabricated by embedding CNTs into polymeric matrices or creating freestanding CNT arrays. The uniform pore structure of vertically aligned CNT membranes allows precise size-based separation, while functionalized CNT membranes combine sieving with selective adsorption. These membranes achieve high rejection rates for heavy metals and organics while maintaining permeability.

Surface modification plays a critical role in membrane performance. Hydrophilic functional groups reduce fouling by preventing organic adhesion, while charged groups improve heavy metal rejection via electrostatic repulsion. For example, amine-functionalized CNT membranes enhance arsenic removal by attracting anionic arsenic species, whereas carboxylated CNT membranes are more effective for cationic metals like cadmium (Cd²⁺). The controlled pore size and functionalization enable selective separation of pollutants without significant flux decline.

Comparative studies highlight the advantages of CNT-based adsorption and filtration over conventional methods. Activated carbon, a widely used adsorbent, typically has lower selectivity and slower kinetics compared to functionalized CNTs. Similarly, polymer membranes without nanomaterial incorporation often suffer from fouling and limited pollutant specificity. CNT-enhanced systems address these limitations by combining high adsorption capacity with mechanical stability and tunable surface properties.

Challenges remain in scaling up CNT-based technologies, including cost-effective production, regeneration, and long-term stability. However, advances in synthesis and functionalization continue to improve feasibility for large-scale applications. The ability to tailor CNT surfaces for specific pollutants positions them as a versatile solution for water and air purification, particularly in scenarios requiring high selectivity and efficiency.

In summary, CNTs offer a robust platform for pollutant removal through adsorption and membrane filtration. Surface modifications enable precise targeting of heavy metals and organic contaminants, while their structural properties ensure high capacity and durability. Continued research into functionalization strategies and scalable fabrication methods will further solidify their role in environmental remediation technologies.