Heavy metal contamination in water sources poses significant risks to human health and ecosystems, with lead (Pb²⁺) being one of the most toxic pollutants. Conventional methods like activated carbon adsorption have limitations in efficiency and selectivity, prompting the exploration of advanced nanomaterials. Carbon-based nanomaterials, particularly graphene oxide (GO) and carbon nanotubes (CNTs), have emerged as promising candidates for Pb²⁺ removal due to their exceptional surface properties and tunable chemistry.

Graphene oxide, a derivative of graphene, possesses abundant oxygen-containing functional groups such as carboxyl, hydroxyl, and epoxy groups on its basal planes and edges. These groups enhance hydrophilicity and provide active sites for Pb²⁺ adsorption through mechanisms like electrostatic attraction, ion exchange, and surface complexation. The negatively charged oxygen groups attract Pb²⁺ ions, especially at pH levels above the point of zero charge (PZC) of GO, where the surface is deprotonated. Studies indicate that GO can achieve adsorption capacities exceeding 500 mg/g for Pb²⁺, significantly higher than traditional activated carbon, which typically ranges between 50–150 mg/g.

Carbon nanotubes, both single-walled (SWCNTs) and multi-walled (MWCNTs), also exhibit strong affinities for Pb²⁺. Their cylindrical structure and high aspect ratio provide large surface areas for metal ion interaction. However, pristine CNTs have limited adsorption capacity due to their hydrophobic nature. Surface modification through oxidation introduces carboxyl and hydroxyl groups, improving dispersibility in aqueous solutions and enhancing Pb²⁺ uptake. Functionalized CNTs have demonstrated adsorption capacities of 80–200 mg/g, depending on the degree of oxidation and the specific surface area.

The adsorption performance of these nanomaterials is highly dependent on solution conditions. pH plays a critical role, as it affects the surface charge of the adsorbent and the speciation of Pb²⁺. At low pH (below 3), competition between H⁺ ions and Pb²⁺ reduces adsorption efficiency. As pH increases, deprotonation of oxygen groups enhances electrostatic interactions, reaching optimal adsorption between pH 5–7. Beyond pH 7, precipitation of Pb(OH)₂ may occur, complicating the adsorption mechanism.

Ionic strength also influences Pb²⁺ removal. High concentrations of competing ions like Na⁺ or Ca²⁺ can reduce adsorption due to screening effects, where these ions occupy active sites or weaken electrostatic interactions. However, carbon-based nanomaterials often exhibit selectivity for Pb²⁺ over lighter ions due to the stronger affinity of heavy metals for oxygen-containing functional groups.

Dispersion stability is another critical factor. Aggregation of GO or CNTs reduces accessible surface area, diminishing adsorption capacity. Ultrasonication and the use of surfactants can improve dispersion, but excessive modification may block active sites. Balancing dispersion with functional group availability is essential for optimal performance.

Surface modifications beyond oxidation further enhance Pb²⁺ removal. Chelating agents like ethylenediaminetetraacetic acid (EDTA) or poly(amidoamine) dendrimers can be grafted onto GO or CNTs, creating tailored binding sites for Pb²⁺. These modifications enable selective and reversible adsorption, which is advantageous for regeneration and reuse. For instance, EDTA-functionalized GO has shown adsorption capacities over 600 mg/g, with the ability to desorb Pb²⁺ under acidic conditions for material recovery.

Despite their advantages, carbon-based nanomaterials face challenges in practical applications. The high cost of synthesis and purification limits large-scale deployment. Recovery of spent nanomaterials from treated water is another hurdle, as filtration or centrifugation adds operational complexity. Magnetic hybrids, such as GO or CNTs coated with iron oxide nanoparticles, offer a solution by enabling magnetic separation, but these composites may sacrifice some adsorption capacity.

Compared to activated carbon, GO and CNTs offer superior surface area and tunable chemistry, but their economic and operational drawbacks must be weighed. Activated carbon remains a cost-effective option for low-concentration Pb²⁺ removal, while nanomaterials excel in high-efficiency or selective extraction scenarios. Future research should focus on scalable synthesis, regeneration techniques, and hybrid systems that combine the strengths of nanomaterials with practical recovery methods.

In summary, carbon-based nanomaterials represent a significant advancement in Pb²⁺ removal technology. Their high adsorption capacities, selectivity, and potential for functionalization make them superior to conventional materials in many aspects. However, addressing cost, recovery, and scalability challenges will be crucial for their widespread adoption in water treatment systems.