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

This review addresses the pressing challenge of toxic metals (heavy metals, metalloids, radionuclides) in aqueous environments and evaluates graphene-based aerogels (GBAs) as promising solutions. It situates adsorption among remediation methods (chemical, physico-chemical, biological, electrochemical), underscoring adsorption’s efficiency, scalability, and cost-effectiveness. Graphene and its derivatives (GO, rGO) offer high specific surface area, exceptional electron mobility, and tunable surface chemistry (oxygenated groups), enabling strong interactions with metal ions. Aerogels contribute 3D open-cell porosity, ultralow density, high pore volume, and mechanical/chemical robustness, improving mass transport and site accessibility. Combining these features, GBAs are positioned as advanced, tailorable, and reusable adsorbents for metal removal. The review focuses on literature mainly from 2018–2023 to articulate mechanisms, surface-chemistry–performance relationships, selective adsorption, electrosorption, reductive detoxification, stability/recycling, magnetic recovery, continuous (column) operation, and multi-pollutant capabilities, with the goal of guiding practical wastewater treatment implementations.

The article surveys: (1) Synthesis routes for GBAs (reduction-induced assembly via hydrothermal or chemical reducers; chemical crosslinking with ions/molecules/polymers; sol–gel incorporation; template-directed methods including CVD and freeze-casting) and the impact of drying (freeze-, ambient-pressure, supercritical) on porosity, SSA, and mechanics. (2) Surface chemistry of GO/rGO and functionalized GBAs (O-, N-, S-, P-containing groups; hybrid moieties such as amidoxime, thiourea, aminothiazole, dopamine derivatives) as key sites for metals binding. (3) Metal-removal mechanisms: physisorption, electrostatic attraction, surface coordination/complexation, ligand exchange (metal–OH), cation–π and anion–π interactions, ion exchange and trapping, co-precipitation/assisted precipitation, and hydrogen bonding. (4) Effects of pH on zeta potential, metal speciation, competitive H+/OH−, and precipitation; amphiprotic surfaces for dual charges. (5) Isotherms (often Langmuir dominance; Freundlich in heterogeneous systems) and kinetics (pseudo-second-order typically best; multi-step intraparticle diffusion). (6) Spectroscopy (FTIR, XPS, EDX/EDS) and computation (DFT, MD) to validate binding sites, energies, selectivity, diffusion, and desorption behavior. (7) Selectivity in complex matrices: roles of hydrated radius/geometry, hydration/binding energy, charge interactions, speciation, concentration, HSAB principle; and ion-imprinted GBAs for templated selectivity. (8) Synergistic adsorption–reduction (Cr(VI)→Cr(III), U(VI)→U(IV), Ag(I)→Ag(0)) and photocatalysis (TiO2, CQDs, g‑C3N4, CuS, BiVO4, etc.) leveraging graphene’s electron mediation. (9) Electrosorption (capacitive deionization) with GBA electrodes for Pb(II), Zn(II), Li(I), U(VI), including electro-reduction and electro-Fenton routes. (10) Stability, desorption (acids, bases, salts, chelants, electro-desorption), and multi-cycle reuse; magnetic GBAs for facile recovery; continuous fixed-bed columns for Cu, Pb, Cd, As, Sb, U, Cs; and multifunctional removal of dyes, phenols, antibiotics, microorganisms, oils/solvents alongside metals.

This is a critical review synthesizing and analyzing recent (primarily 2018–2023) literature on GBAs for metals removal. The authors compare synthesis strategies and processing–property relationships; collate mechanistic insights from equilibrium/kinetic modeling, spectroscopic characterization (FTIR, XPS, EDX/EDS), and computational studies (DFT, MD); and evaluate performance under competitive conditions (co-existing ions), specialized selectivity strategies (ion-imprinting), and alternative operating modes (electrosorption, photocatalysis, adsorption–reduction). Practical considerations are examined through reported regeneration/desorption protocols, magnetic recovery, and continuous column studies using synthetic and real wastewater. No new experimental data were generated; the approach emphasizes structure–function correlations and scalability implications from published reports.

- Synthesis–structure control: Reduction-induced assembly, crosslinking, sol–gel, and templating (CVD, freeze-casting) yield GBAs with tunable porosity, SSA, conductivity, and mechanics, impacting adsorption and transport. - Surface functionality governs mechanisms: O/N/S/P groups and hybrid ligands (e.g., amidoxime, thiourea) drive electrostatic attraction, coordination/complexation, ligand exchange (metal–OH), ion exchange, hydrogen bonding, and cation–π/anion–π interactions. Functionalization markedly enhances capacity and selectivity. - pH and speciation: Zeta potential switches with pH, altering cation/anion uptake; extreme pH can degrade sites or induce hydroxide precipitation (beneficial or fouling). Sensitive species (Cr(VI), U(VI), Sb, As) require pH-optimized charge matching. - Isotherms and kinetics: Langmuir and pseudo-second-order models commonly fit best, indicating monolayer chemisorption on specific sites; intraparticle diffusion is often a rate-limiting step among multiple transport stages. - Spectroscopy and computation: FTIR/XPS/EDS confirm functional-group participation and metal capture; DFT quantifies adsorption energies and preferential binding sites; MD elucidates diffusion, hydration structures, and selectivity trends. - Selectivity factors: Hydrated/ionic radius and complex geometry, hydration/binding energy, charge interactions, speciation/concentration, and HSAB matching (e.g., amidoxime–U(VI), phosphonate–Th(IV), amino-phosphonic–Cr(III)) dictate competitive uptake; ion-imprinted GBAs provide templated recognition and strong selectivity. - Adsorption–reduction and photocatalysis: GBAs (with TiO2, CQDs, g‑C3N4, CuS, BiVO4, etc.) enable Cr(VI)→Cr(III) and U(VI)→U(IV) reductions; graphene facilitates charge separation and electron transfer, boosting photo-reduction and concurrent dye/phenol degradation. - Electrosorption and electrochemistry: GBA electrodes remove Pb(II), Zn(II), Li(I), U(VI); applied potential enhances directional ion migration and site accessibility; surface sites add chemisorption; electro-reduction (e.g., Cr(VI), Cu(II)) and electro-Fenton routes expand remediation to metal complexes (e.g., EDTA–Ni). - Regeneration and reuse: Effective desorption via acids (HCl/HNO3), bases (NaOH/Na2CO3), salts (NaCl), chelants (EDTA), hybrids (thiourea-acid), and electro-desorption; multi-cycle stability with minor losses due to partial site deactivation or structural relaxation. - Magnetic recovery: Embedding Fe3O4/CoFe2O4/NiFe2O4 affords easy magnetic separation with sustained performance across cycles for Hg(II), Cu(II), Pb(II), Cd(II), Cr(III/VI), Cs(I). - Continuous operation: Fixed-bed columns (downflow/upflow) with GBAs show high capacities and efficiencies for Cu, Pb, Cd, As, Sb, U, Cs; performance governed by flowrate, influent concentration, and bed height; aligned pore structures improve flux and mass transfer. - Multifunctionality and synergy: GBAs simultaneously remove metals with dyes/phenols/antibiotics/microorganisms/oils. Examples include rGO/MMT aerogel qmax: Cd(II) ≈ 104.8 mg/g; MB ≈ 139.0 mg/g; GO/CMC/CS aerogel qmax: Cr(VI) ≈ 127.4 mg/g; MB ≈ 3824 mg/g; rGO/CuS aerogel photo-reduction of Cr(VI) ≈ 98.9% with dye degradation >97%; several systems show >99% Cr(VI) reduction and >90% BPA degradation.

The review consolidates evidence that GBAs integrate the advantages of graphene derivatives (high SSA, electron mobility, rich/engineerable chemistry) with aerogel architectures (3D interconnected porosity, low density, robustness) to deliver fast diffusion, abundant accessible sites, and multi-modal removal pathways. Mechanistic understanding (supported by spectroscopy and computation) explains observed selectivity and capacity trends, guiding rational functionalization (HSAB-compliant ligands, ion imprinting) and pore-structure design. Alternative operation modes (electrosorption, adsorption–reduction, photocatalysis) expand applicability to diverse matrices and enable detoxification and even resource recovery. Demonstrated recyclability, magnetic separability, and promising column results collectively address practical deployment. Remaining gaps (e.g., complex real-water matrices, long-term stability under harsh conditions, cost/scalability) are identified, and strategies (in situ column regeneration, aligned pores, tailored amphiprotic surfaces) are proposed to improve industrial relevance.

GBAs emerge as high-performance, versatile materials for metals remediation, offering: (i) tunable structures and rich surface chemistries enabling multiple adsorption mechanisms; (ii) synergistic adsorption–reduction and photocatalysis for detoxifying redox-active metals; (iii) electrochemical capabilities (electrosorption, electro-reduction) enhancing efficiency and selectivity; (iv) strong stability, recyclability, and magnetic recoverability; and (v) efficacy in continuous columns and multi-pollutant contexts. Future directions include: systematic mechanistic protocols (in situ/operando characterization; rigorous kinetic/isotherm modeling), expanded electrochemical and catalytic routes (electrodeposition, hybrid processes for resource recovery), targeting metal complexes and organometallics, cost and scale optimization (low-cost precursors, mild synthesis), real wastewater validation under harsh conditions, pore alignment and device integration, and standardized in situ regeneration in continuous systems.

As a review, no new experiments were conducted. Reported performances depend on study-specific conditions (pH, ionic strength, co-contaminants) and may not directly translate to all real effluents. Mechanistic assignments often rely on ex situ spectroscopy and model fitting, leaving room for ambiguities without in situ confirmation. Many studies are lab-scale; field-scale validations, long-term fouling/aging assessments, and full techno-economic analyses remain limited. Extreme pH and repetitive regeneration can partially deactivate sites or alter structures, causing performance decay in some systems.