Comprehensive insights into the application of graphene-based aerogels for metals removal from aqueous media: Surface chemistry, mechanisms, and key features

Toxic metal pollutants in aqueous environments pose a severe threat due to their toxicity, solubility, and mobility. While various remediation methods exist (chemical, physicochemical, biological, and electrochemical), adsorption stands out due to its efficiency, tailorability, practicality, cost-effectiveness, scalability, and environmental friendliness. Carbonaceous adsorbents, particularly graphene-based materials, have shown excellent potential due to their low cost, scalable fabrication, and exceptional physicochemical and mechanical properties. Graphene, a two-dimensional nanomaterial, exhibits high thermal conductivity, chemical stability, optical transparency, electron mobility, and specific surface area (SSA). Its derivatives, graphene oxide (GO) and reduced graphene oxide (rGO), offer additional advantages through their rich surface chemistry featuring oxygen-containing functional groups. Aerogels, known for their low density, high porosity, and large SSA, further enhance the performance of graphene-based adsorbents when combined. This review addresses the need for a comprehensive analysis of GBAs' application in removing metal pollutants from aqueous media, focusing on the last six years of research.

The literature review section discusses various existing adsorbents for metal removal, categorized by composition (polymers, biopolymers, MXenes, covalent organic frameworks, metal-organic frameworks, zeolites, minerals, low-cost materials, and biosorbents). Carbonaceous adsorbents, including activated carbon, biochar, carbon nanotubes, graphitic carbon nitride, and carbon dots, are highlighted for their advantageous properties. Graphene-based materials are then presented as a subset of carbonaceous materials, emphasizing their unique properties that lead to impressive metal removal performance. Finally, aerogels are introduced as a superior class of adsorbents due to their advantageous morphological features. The review then focuses on graphene-based aerogels (GBAs) which combine the benefits of both graphene derivatives and the aerogel structure.

The review details various synthesis strategies for GBAs, which primarily involve the use of GO or rGO as building blocks. Four main synthesis approaches are discussed: 1. **Reduction-Induced Assembly:** This method involves removing oxygenated groups from GO nanosheets, increasing π-π attraction and facilitating 3D network formation. Hydrothermal and chemical reduction methods are compared. 2. **Chemical Crosslinking:** Various crosslinking agents (metal/inorganic ions, small organic molecules, polymers, biopolymers) are used to enhance the structural integrity and mechanical properties of GBAs. 3. **Sol-Gel Methods:** These methods involve the hydrolysis and polycondensation of monomers (resorcinol-formaldehyde, silica/silane precursors) in the presence of graphene derivatives. Graphene contributes to network formation through binding with polymeric components. 4. **Template-Directed Methods:** Methods such as chemical vapor deposition (CVD) and freeze-casting are described, highlighting their ability to control the aerogel's microstructure. The role of different drying techniques (freeze-drying, ambient-pressure drying, supercritical drying) in influencing the properties of the final GBAs is also discussed. Following synthesis, characterization techniques (FTIR, XPS, EDX/EDS), as well as computational methods (MD simulations and DFT calculations), are used to investigate the surface chemistry, adsorption mechanisms, and selectivity of GBAs. Adsorption isotherms (Langmuir, Freundlich, Dubinin-Radushkevich, Temkin, Sips) and kinetic models (pseudo-first order, pseudo-second order, Elovich, intra-particle diffusion) are employed to analyze experimental data.

GBAs exhibit diverse metal removal pathways including physisorption (van der Waals forces, electrostatic attraction, porous structure), chemisorption (surface coordination/complexation, ion exchange), and precipitation. Surface functional groups (oxygen-, nitrogen-, sulfur-, phosphorus-based, and hybrid groups) significantly influence these mechanisms. Electrostatic interactions between charged functional groups and metal ions are key, influenced by the solution pH. Surface coordination involves electron sharing between metal cations and functional groups acting as Lewis bases. Ligand exchange involves the replacement of existing ligands (e.g., hydroxyl groups on metal oxides) with metal pollutants. Cation-π and anion-π interactions are also discussed. Ion exchange involves replacing exchangeable ions (H+, K+, Na+, Ca2+) on the GBA surface with metal pollutants. Ion trapping involves metal entrapment in lattice vacancies. Assisted co-precipitation involves releasing agents from GBAs to react with metal ions. Hydrogen bonding further assists adsorption. Adsorption isotherms and kinetics analysis typically indicate chemisorption dominated processes, with high SSA and porosity facilitating fast adsorption. Spectroscopic (FTIR, XPS, EDX/EDS) and computational (MD simulations, DFT calculations) studies support the proposed mechanisms. Competitive adsorption in multi-component systems is influenced by factors such as ion size, hydration energy, charge, speciation, concentration, and HSAB principles. Ion-imprinted GBAs (II-GBAs) demonstrate enhanced selectivity. Synergistic adsorption-reduction involves GBAs acting as electron donors, reducing metal ions (Cr(VI), U(VI), Au(III), Ag(I)) to less toxic forms. Photocatalytic reduction using GBAs with photocatalytically-active species enhances Cr(VI) and U(VI) removal. Scavenging agents further improve the photocatalytic process. pH significantly impacts both adsorption and reduction. Electrochemical removal using GBAs as electrodes is also discussed, highlighting electrosorption (ES) which leverages the electrical double layer (EDL). GBAs exhibit excellent stability and recyclability, with chemical (acid, alkali, chelating agents) and electrochemical methods used for desorption and regeneration. Magnetic GBAs (MGBAs) facilitate easy recovery. Continuous metals removal in fixed-bed columns demonstrates the scalability of GBAs, with performance influenced by flow rate, influent concentration, and column length. GBAs also exhibit multi-pollutant removal capabilities, simultaneously removing metals, dyes, phenols, antibiotics, microorganisms, and oil/organic solvents, often with synergistic effects.

The findings demonstrate the significant potential of GBAs as high-performance adsorbents for metal removal from aqueous media. The versatility of GBAs in employing multiple removal pathways, combined with their excellent stability and reusability, addresses critical limitations of traditional methods. The ability to tailor the surface chemistry and structure of GBAs enables both high adsorption capacity and selectivity, making them suitable for complex real-world applications. The integration of GBAs into continuous processes further highlights their scalability and potential for industrial implementation. The synergistic effects observed in multi-pollutant removal demonstrate the broad applicability of GBAs in comprehensive water treatment strategies.

This review highlights the remarkable progress in utilizing GBAs for metal removal from wastewater. GBAs offer a unique combination of high adsorption capacity, selectivity, stability, and reusability, surpassing many traditional methods. Future research should focus on systematic mechanistic studies, exploring beyond conventional adsorption methods (electrochemical reduction, advanced catalysis), addressing the removal of metal complexes, enhancing practicality and scalability (using real-world samples, optimizing synthesis, and integrating in situ regeneration), and reducing synthesis and operation costs.

While this review provides a comprehensive overview, several limitations exist. The focus is primarily on literature from the last six years, potentially overlooking earlier relevant contributions. Furthermore, the lack of standardized testing protocols and the diversity of GBAs and experimental conditions can make direct comparisons challenging. Future research should strive for greater standardization to facilitate broader comparisons and conclusions.