The research progress on the removal of heavy metals using carbon electrodes in capacitive deionization technology

1.1 Advantages and disadvantages of CDI in removing heavy metal

Over the recent decade, a variety of physicochemical methods have remained pivotal in addressing the challenges of heavy metal contamination. Table 1 juxtaposes the benefits and drawbacks of electrical adsorption in the treatment of heavy metal ions against traditional techniques such as chemisorption precipitation, membrane filtration, ion exchange, and conventional adsorption. Standard approaches to wastewater treatment typically incur high operational costs, generate considerable amounts of sludge, and carry the potential for secondary pollution. ⁸ Conversely, electro-sorption leverages electrode materials to adsorb, complex, and reduce heavy metal ions via Coulombic interactions, presenting several distinct advantages: ^{9 , 10}

1. Simplicity and cost-effectiveness: ^{7 , 11} CDI technology provides a straightforward, economically viable option for the removal of heavy metal ions from industrial wastewater. As shown in Figure 1, constituted of three primary components – an electrode, a fluid collector, and an ionic diaphragm – CDI systems demand minimal upkeep and circumvent the need for secondary water treatments. They are notably effective across a wide temperature spectrum above 0 °C, enhancing their utility.

2. Environmental sustainability: ⁴ electro-sorption is characterized by its eco-friendly approach, eschewing chemical additives and preventing the generation of noxious by-products. This attribute underscores its alignment with contemporary environmental sustainability goals.

3. Energy efficiency: ¹² operating within a low energy consumption framework, typically at a voltage range of 0.6–2 V, CDI units underscore efficient energy utilization primarily for ion migration. Notably, a portion of the energy expended during the sorption process can be recuperated during the capacitor’s discharge phase, bolstering its energy efficiency.

4. Durability and regenerability: ^{13 , 14} the electrodes employed within CDI systems are distinguished by their longevity and require no specialized maintenance or descaling, contributing to lower operational costs. The regenerative capability of these electrodes, achievable through circuit disconnection or the application of reverse voltage, further enhances their appeal.

Figure 1:

CDI device structure: Plexiglass plate (A, G); latex gasket (B, F); titanium plate (C, E); diaphragm (D); power supply (H).

CDI is a new water treatment technology that combines electrochemical and adsorption technologies. It offers advantages that are difficult to compare with other water treatment methods. It is also a significant solution for heavy metal pollution in the future.

1.2 Mechanism of heavy metal ions removal by CDI

In Capacitive Deionization (CDI), the two principal adsorption mechanisms at play are the electrical double-layers (EDLs) adsorption and the Faraday reaction, each utilizing distinct principles and materials to achieve ion removal from water. ^{19 , 20}

Electrical Double-Layers Adsorption (EDLs): EDLs adsorption is characterized by the accumulation of ions at the surface of carbon-based materials through electrostatic forces. This mechanism takes advantage of carbon materials that have a vast surface area and a rich distribution of micro-mesopores. ^{21 , 22} Such materials include carbon nanotubes, ²³ MOF-derived carbon, ^{24 , 25} biomass-derived carbon, ^{26 , 27} graphene oxide, ^{28 , 29} activated carbon fibers, ^{30 , 31} and carbon aerogels. ^{32 , 33} The effectiveness of EDLs adsorption is highly dependent on the surface characteristics of the carbon materials, which facilitate the adsorption of ions on their surfaces as shown in Figure 2(A). This process is efficient in removing a variety of ions from water, leveraging the physical properties of the carbon electrodes to create an electrostatic environment conducive to ion adsorption.

Figure 2:

Schematic representation of the bilayer adsorption process (A) with three types of Faraday processes (anodic oxidation and cathodic reduction (B); Faraday ion storage process (C)) ³⁴ (reprinted with permission).

Faraday reactions: Faraday reactions encompass both redox reactions and Faraday ion storage mechanisms. Redox reactions are driven by electric field forces, leading to the anodic oxidation and cathodic reduction of anions and cations, respectively. This can result in the transformation of heavy metal ions into less harmful valence states or their complete removal from the aqueous solution, as depicted in Figure 2(B). Meanwhile, Faraday ion storage mechanisms utilize the reversible structures of certain electrode materials to physically store metal ions within their crystal lattice. These stored ions can then be released and the electrode structure regenerated by applying a reverse voltage, as illustrated in Figure 2(C). ³⁵ Electrode materials that are commonly employed for these reactions include transition metal oxides, ^{36 , 37} Prussian blue analogs, ^{38 , 39} and conductive polymers, ^{40 , 41} which are selected for their ability to undergo reversible structural changes and store ions effectively. These mechanisms highlight the versatility of CDI technology in addressing water purification, offering multiple pathways for ion removal that can be tailored according to specific contamination profiles and treatment goals.

Carbon materials and their derivatives have become focal points of interest in the realm of CDI for heavy metal removal, attributed to their inherent properties such as expansive specific surface areas, superior electrical conductivity, adaptable structures, and diverse micro- and nanopore configurations. This paper endeavors to dissect the multifaceted challenges, deliberate on enhancement strategies, and forecast future advancements concerning the application of carbon-based electrodes in CDI, particularly focusing on five pivotal types: carbon nanotubes, MOF-derived carbon, biomass-derived carbon, graphene oxide, and activated carbon fibers.

2.1 Typical carbon electrodes and advantages of carbon materials

With the progression of material preparation methodologies, carbon materials that leverage EDLs adsorption mechanisms have risen to prominence in the realm of CDI. ¹⁴ Figure 3 delineates a variety of carbon materials pivotal for CDI, notably carbon nanotubes, MOF-derived carbon, biomass-derived carbon, graphene oxide, and activated carbon fibers. The allure of these carbon materials among the scientific community is attributed to their distinctive properties, which are instrumental in the efficacy of CDI processes.

Figure 3:

Carbon materials for CDI applications. ^{42 , 43 , 44 , 45} (reprinted with permission).

These properties include a vast surface area complemented by micro-nanoporous structures that facilitate the adsorption of ions, exceptional electrical conductivity that ensures rapid ion transport, and high ion mobility rates that enhance the removal efficiency. Additionally, the tunability of these materials allows for the optimization of their structure and functionality to target specific contaminants. At last, the ease of preparation and the broad availability of precursor materials further underscore the practicality and scalability of using carbon materials in CDI technologies. This synthesis of advantageous characteristics positions carbon-based electrodes at the forefront of research and application in CDI. ⁴⁶

2.2 Mechanism of heavy metal removal in CDI by carbon electrode

2.2.1 EDLs adsorption mechanism

Figure 4(A) presents a visual depiction of how carbon-based electrodes facilitate the adsorption of heavy metal ions from water through the establishment of EDLs structures. When the electrode is energized, it acquires a negative charge on its surface, which, due to enhanced electrostatic forces, attracts positively charged ions (cations) from the water toward the electrode surface, resulting in physical adsorption. This attraction creates a gradient in the concentration of cations, with the highest concentration immediately adjacent to the negatively charged electrode surface and progressively decreasing with distance from the surface. This gradient manifests as a double-layer structure around the electrode, with the ion concentration non-uniformly distributed near the solid surface. ^{47 , 48}

Figure 4:

Bilayer adsorption mechanism of carbon(A); soft-acid/soft-base interaction adsorption mechanism of N and S doped carbon derivatives (B); adsorption mechanism of carbon composite pseudocapacitive materials(C) ^{49 , 50} (reprinted with permission).

The ability of carbon-based electrodes to adsorb ions is significantly influenced by their surface area; electrodes with larger surface areas can hold more charge, leading to enhanced adsorption capacity. These electrodes are highly effective in capturing a wide range of cations due to their expansive surface area and the electrostatic mechanisms at play. ⁵¹ However, a limitation arises in their selectivity, especially in differentiating between ions of varying valences. The adsorption efficiency for low valence ions tends to be lower compared to that for high valence ions, posing challenges in selectively removing certain heavy metals from contaminated water. These characteristics underscores the importance of optimizing electrode material properties to improve selectivity and overall efficiency in the removal of heavy metals via CDI technology.

3.1 Carbon nanotube composite electrodes

Carbon nanotubes (CNTs) have attracted considerable interest for their outstanding electrical conductivity, superior adsorption characteristics, and unique one-dimensional nanostructures, positioning them as a material of choice for advanced electrode applications. ^{59 , 60} In work by Humair Hussain et al., ⁵⁹ composite electrodes that include CNTs have been shown to benefit from enhanced electrical conductivity and a significant specific surface area, attributes that are critical for effective adsorption processes. Despite these advantages, the reliance solely on EDLs adsorption mechanisms has frequently been found insufficient to meet the demands for high adsorption efficiency and selectivity. However, the adsorption efficiency and selectivity of EDLs from CNTs alone are not satisfactory. Therefore, researchers have begun to focus on the synergistic effect of multiple mechanisms.

The exploration of these synergistic effects in carbon nanotube-based electrodes represents a promising frontier in the development of more sophisticated and efficient water treatment technologies. ⁶¹ Materials that exhibit both significant pseudo-capacitance and selective adsorption properties, commonly referred to as Faraday materials, are increasingly capturing the attention of the scientific community. ³⁵ By combining CNTs with materials that facilitate additional adsorption mechanisms – such as pseudocapacitive reactions, Faraday reactions, or chemisorption – electrodes can achieve a much broader and more effective heavy metal ion removal. These combined mechanisms can offer enhanced performance by leveraging the high surface area and conductivity of CNTs while introducing specific interaction capabilities for targeted heavy metal ion removal, thus overcoming the challenges of selectivity and efficiency.

Hu et al. ⁶² successfully demonstrated the creation of Co–N active sites through the integration of Co–Co₃O₄ nanoparticles within nitrogen-doped carbon nanotubes (CNTs). This innovative approach resulted in electrode materials characterized by remarkable stability and an impressive specific capacitance of 319.3 F g⁻¹. In a parallel advancement, Feng et al. ⁴² engineered intertwined 3D interoperable nanopore structures using CNTs as a scaffold, onto which nano-TiO₂ was loaded. These specially designed carbon nanotubes, with their extensive specific surface area and hydroxyl active sites, showcased selective adsorption efficiencies exceeding 85 % for total Cr and more than 90 % for Cr (VI) across multiple cycles, as shown as an inset of Figure 5(A).

Figure 5:

Cyclic adsorption and desorption of CNT@TiO₂ material with mechanism (A). ⁴² (reprinted with permission); scanning electron microscopy and performance images of Fe₇S₈@NCNT materials with different initial concentrations of Pb²⁺ ions, as well as removal of Pb²⁺/Na⁺ and Pb²⁺/Ca²⁺ competing ions and recovery performance images in 10 mg L⁻¹ Pb²⁺ solution (B). ⁶³ (reprinted with permission).

Additionally, Fe₇S₈, known for its rich reversible pseudocapacitive storage valence states and sulfur vacancies, demonstrates a pronounced affinity for heavy metals. However, it is prone to challenges such as volume expansion and structural decomposition. To counter these issues, Gao et al. ⁶³ encapsulated Fe₇S₈ nanoparticles within nitrogen-doped carbon nanotubes to explore their effectiveness in removing Pb²⁺ ions. The encapsulation technique yielded an electrode material with extraordinary adsorption capacity for Pb²⁺ ions (223.1 mg g⁻¹) and high selectivity for Pb²⁺ ions in the binary system Pb²⁺/Na⁺, leveraging the pseudocapacitive properties of the Fe²⁺/Fe³⁺ redox reaction in conjunction with the sulfur active sites’ unique affinity for Pb²⁺ ions. Remarkably, this encapsulated nanocarbon tube (NCNT) electrode maintained 91 % of its original adsorption capacity even after 16 absorption-desorption cycles, highlighting its exceptional cycling stability as shown in an inset of Figure 5(B).

While metal oxides and metal sulfides offer a plethora of active sites and unique pseudocapacitive characteristics, they often fall short due to limitations such as poor electrical conductivity, sluggish ion diffusion kinetics, and susceptibility to structural volume expansion. Contrastingly, CNTs emerge as superior substrate materials owing to their high conductivity and potential to act as a buffer against structural changes. The concept of encapsulating CNT structures further amplifies their appeal, offering enhanced stability and performance in adsorption applications. This synergy between the high surface area and structural resilience of CNTs and the pseudocapacitive properties of Faraday materials represents a potent combination for developing advanced electrode materials for environmental purification and energy storage applications.

Carbon nanotubes (CNTs) are highly regarded in the field of capacitive deionization (CDI) for their exceptional specific capacitance, outstanding electrical conductivity, and effective electro-sorption capabilities. These attributes render them promising candidates for the enhancement of CDI technologies, particularly in the efficient removal of contaminants from water. Despite these advantages, the practical deployment of CNTs in CDI systems is currently restrained by several significant hurdles. The high cost associated with CNTs production, intricate processing requirements, and challenges related to scaling up production processes effectively limit their widespread application. In response to these impediments, the focus of current research has shifted towards the development of innovative modification techniques aimed directly at electrodes comprised of industrially produced carbon nanotubes. By devising suitable methods for the direct modification of these electrodes, researchers aim to sidestep the complexities and high costs traditionally associated with CNTs utilization in CDI systems. Such strategies may include the functionalization of CNTs surfaces to improve their affinity for specific ions, the incorporation of CNTs into composite materials that enhance their electrochemical properties, or the simplification of CNTs processing techniques to facilitate easier integration into CDI systems. The pursuit of these modification methodologies holds the promise of overcoming the prevailing challenges, thereby paving the way for the large-scale application of CNTs in CDI technologies.

3.2 MOF-derived carbon composite electrode

Metal-organic frameworks (MOFs) renowned for their molecularly porous three-dimensional structures and an abundance of functional groups on their surfaces, have become central to innovative research initiatives. ^{64 , 65 , 66} Compared with the original MOF materials, MOF derivatives inherit the advantages of the original MOF materials. It has stronger structural stability, stronger electronic conductivity, and excellent electrochemical properties. Wang et al. ⁶⁷ prepared Co/Fe co-doped MOF-derived carbon electrodes using ZIF-8 as a template, which exhibited excellent Cu²⁺ ion electro-adsorption performance with a maximum adsorption capacity of 91.31 mg g⁻¹. The integration of MOFs and their derivatives in the design of electrode materials thus presents a promising avenue for the development of advanced adsorption systems. ^{68 , 69 , 70} By harnessing the unique structural properties and functional versatility of MOFs, researchers can engineer electrode materials that offer superior adsorption capabilities, paving the way for innovative solutions in the removal of heavy metals and other contaminants from aqueous environments.

Among such studies, Li et al. ⁷¹ leveraged ZIF-8 as a precursor to developing a highly N-doped ZnO@N-PCNM-10 electrode material via the electrostatic spinning technique, as depicted in Figure 6(A). The thermal decomposition of ZIF-8 facilitated the creation of interconnected hierarchical porous structures, culminating in an electrode material with an exceptionally large specific surface area of 488.6 m² g⁻¹, showcased in Figure 6(B). These intricate porous architectures significantly enhance the interfacial transfer capabilities between the electrode material and the electrolyte, thereby providing a plethora of favorable adsorption sites for heavy metal ions. Subsequent electro-sorption tests revealed that the fabricated electrode material exhibits remarkable adsorption capacities for Pb²⁺, Cu²⁺, and Cd²⁺ ions, quantified at 32.87 mg g⁻¹, 23.81 mg g⁻¹, and 20.85 mg g⁻¹, respectively, as illustrated in Figure 6(C). This elevated adsorption performance is largely ascribed to the presence of nitrogen doping within the electrode’s structure, specifically the formation of Pyridinic-N and Pyrrolic-N functionalities. These nitrogen configurations act as electron donors, thereby contributing additional pseudo-capacitance which works in tandem with the EDLs adsorption mechanism to enhance overall adsorption efficiency, as shown in Figure 6(D).

Figure 6:

ZnO@N-PCNM-10 materials: ⁷¹ schematic illustration of the synthesis (A); N₂ adsorption/desorption isotherm (B); the electric adsorption capacity of different ions over time (C); N 1s spectra (D) (reprinted with permission).

Xu et al. ⁷² took an innovative approach by intertwining CNTs with a MOF skeleton, creating three-dimensional nanostructures that boast a high specific surface area. This unique assembly, described as a “ZIF-67-CNT-ZIF-67” structure, was designed to establish specialty electronic conduction pathways that significantly improve the electrical conductivity of the composite material. The integration of CNTs within the MOF matrix not only enhances the structural robustness and conductivity but also leverages the synergistic properties of both materials to optimize the adsorption and electrochemical performance of the electrodes.

Despite the promising attributes of MOF-derived carbons, such as their extensive surface area, porosity, and functional versatility, which make them excellent candidates for CDI applications, there are notable limitations to their widespread adoption. ⁷³ The synthesis of these advanced materials often involves intricate procedures that can be costly and require specific conditions, factors that pose significant barriers to their large-scale production, and practical application. These challenges underscore the need for ongoing research to streamline the fabrication process, reduce costs, and develop more accessible synthesis methods that can unlock the full potential of MOF-derived carbons in CDI and other environmental remediation technologies.

3.3 Biomass-derived carbon composite electrodes

Biomass-derived carbon materials, celebrated for their abundant availability, straightforward preparation methods, and cost-effectiveness, have found widespread utility across a spectrum of applications, including gas adsorption and storage, electrocatalysis, supercapacitors, and notably, in CDI materials. ⁷⁴ An innovative approach by Chang et al. ⁷⁵ demonstrated the potential of utilizing waste polyvinyl chloride (PVC) as a source of biomass carbon to fabricate nitrogen and sulfur co-doped carbon electrode materials. The investigation revealed through SEM images and pore size distribution analyses that the resultant biomass-derived carbon boasts a multi-scale hierarchical pore structure – (macro/meso/microporous) and a large pore volume, as depicted in Figure 7(A). This structural complexity contributes to a high removal efficiency, achieving a remarkable 94–99 % elimination rate for various heavy metals at low concentrations, including Fe²⁺, Co²⁺, Ni²⁺, Cu²⁺, Pb²⁺, and Cd²⁺.

Figure 7:

SEM, pore width distribution with heavy metal ion adsorption capacity of NS-C materials (A); ⁷⁵ N₂ adsorption/desorption isotherms, FT-IR with Cu²⁺ and Zn²⁺ adsorption capacity of AC-KOH material (B) ⁴³ (reprinted with permission).

Despite these impressive attributes, biomass-derived carbon materials are not without their challenges. Drawbacks such as a wide size distribution of particles, suboptimal electrical conductivity, particle non-uniformity, and significant gaps between particles have been identified as areas requiring improvement. These limitations can impact the efficiency and consistency of biomass-derived carbon materials in their various applications, including their performance in CDI systems. ²² Addressing these drawbacks is crucial for enhancing the utility and effectiveness of biomass-derived carbon materials. Potential strategies for improvement might include advanced processing techniques to refine the pore structure and size distribution, doping with other elements to improve electrical conductivity, and developing methods to achieve more uniform particle sizes and tighter packing. ^{76 , 77 , 78 , 79} Through such optimizations, the full potential of biomass-derived carbon materials can be unlocked, further extending their applicability and performance in CDI and beyond, while maintaining the benefits of sustainability and cost-effectiveness.

Enhancing the performance of biomass-derived carbon electrodes for applications such as CDI has been a focal point of recent research efforts. The primary strategies to achieve this involve the incorporation of highly conductive materials and the innovative design of the electrode’s structure. A notable study by Le et al. ⁴³ utilized coconut shells, an abundant biomass source, to create electrode materials with a significant specific surface area of 581 m² g⁻¹ and a pore size of 1.204 nm. The modification of these electrodes with KOH introduced polar groups (-OH) and oxygen-containing functional groups onto the surface, improving the hydrophilicity of the electrodes. This modification was instrumental in achieving a maximum adsorption capacity of 50.2 mg g⁻¹ for Cu²⁺ ions and 28.2 mg g⁻¹ for Zn²⁺ ions at an operating voltage of 1.2 V, as illustrated in Figure 7(B). Further exploration by Le et al. ⁸⁰ into enhancing the porosity of activated carbon derived from coconut shells through high-temperature CO₂ activation demonstrated that this treatment significantly boosted the electrode’s capacitance to 112 F g⁻¹ at a current density of 0.1 A g⁻¹, with an adsorption capacity of 5.32 mg g⁻¹ for Ni²⁺ removal. These findings highlight the potential of such modifications to improve both the electrical and adsorption performance of biomass-derived carbon materials in CDI applications.

However, while the addition of highly conductive materials is a straightforward and effective method to enhance electrode performance, it is not without its challenges. Issues such as the aging and shedding of conductive agents can arise during prolonged use, adversely affecting the durability and service life of the electrodes. Addressing these challenges requires a balanced approach that not only aims to improve the immediate performance metrics of biomass-derived carbon electrodes but also considers the long-term stability and reliability of these materials in practical applications.

In an innovative approach to address these challenges, Wang et al. ⁸¹ developed hierarchical porous carbon electrode materials modified with Fe₃O₄ nanoparticles using waste oil tea husk as a biomass carbon source. SEM images revealed that Fe₃O₄ nanoparticles were uniformly distributed within the hierarchical porous carbon structure, effectively filling the gaps between carbon particles and contributing to a more integrated electrode structure as shown in Figure 8(A). This design, characterized by a layered porous carbon structure with a large specific surface area and multiple active sites, facilitated efficient adsorption sites for heavy metals. Notably, while the adsorption efficiency for Pb²⁺ ions slightly decreased with an increase in Pb²⁺ concentration, the adsorption capacity experienced a significant increase, reaching a peak of 39.52 mg g⁻¹, as illustrated in Figure 8(B). The surface of the layered porous carbon was rich in oxygen-containing functional groups such as C=O, C–O–C, and C–OH, which interact with Pb²⁺ ions to form compounds like PbO, Pb(OH)₂, and PbCO₃, as depicted in Figure 8(C). Furthermore, the presence of Fe₃O₄ nanoparticles introduced a redox reaction mechanism between Fe³⁺ and Fe²⁺ ions, along with a unique affinity for Pb²⁺ ions. This contributed to the formation of Fe–O–Pb⁺ and Fe–O–…Pb²⁺ complexes, ⁸² enhance the selectivity of the electrode material towards Pb²⁺ ions, especially in a Pb²⁺/Na⁺ binary co-expression system, as shown in Figure 8(D). The structural design not only addresses the issues of uneven distribution and aggregation often associated with activated carbon but also improves the selective adsorption capabilities of the material by integrating pseudo-capacitive components. Wu et al. ⁸³ used walnut shells as a biomass carbon source and Zn-doped and found up to 99 % removal of Pb²⁺ ions by triple synergistic action of physisorption, EDLs adsorption Faraday reaction.

Figure 8:

Fe₃O₄ NPs/HPC materials: ⁸¹ SEM (A); plot of removal rate and electrosorption at different Pb²⁺ concentrations (B); plot of removal curve and removal efficiency of Pb²⁺ (C); plot of Pb 4f XPS spectra after adsorption of Pb²⁺ ions (D) (reprinted with permission).

This innovative approach demonstrates the potential for advancing the development of biomass-derived carbon electrodes by focusing on structural design and material compounding. By optimizing the interaction between biomass-derived carbon structures and pseudo-capacitive materials, researchers can create more efficient, selective, and stable electrode materials for various applications, including the removal of heavy metals from wastewater.

Biomass-derived carbon materials have emerged as promising candidates for use as substrate materials in CDI electrodes, primarily due to their straightforward and efficient preparation process, the abundance and renewability of biomass sources, and the minimal pre-treatment required to convert biomass into useful carbon materials. Despite these advantages, there are inherent limitations associated with biomass-derived carbons, such as their variability in pore size distribution, the potential for lower electrical conductivity compared to synthetic materials, and a general reliance on physical adsorption mechanisms that may not offer the selectivity needed for certain applications. The integration of highly conductive materials into biomass-derived carbons has been recognized as a mature approach to address some of these limitations, enhancing the electrical conductivity and, thereby, the overall performance of CDI electrodes. However, this strategy predominantly focuses on enhancing the EDLs adsorption capacity of the electrodes without adequately addressing the need for selective adsorption.

Selective adsorption is crucial for the effective removal of specific ions from solutions, particularly in applications requiring the targeted removal of contaminants such as heavy metals or specific ionic species. The recognition of this gap – the reliance on EDLs adsorption mechanisms without adequately leveraging selective adsorption effects, such as those arising from soft-soft interactions observed in pseudocapacitive adsorption mechanisms – points to a significant area for further research and development. ²⁶ Future efforts in the field should aim to develop composite materials that incorporate both adsorption typologies, effectively combining the broad adsorption capacity enabled by EDLs mechanisms with the specificity and selectivity afforded by pseudocapacitive and other chemisorptive interactions. ⁸⁴ By exploring and developing such composite materials, researchers can create more versatile and effective CDI electrodes. These advanced electrodes would not only benefit from the environmental sustainability and cost-effectiveness of biomass-derived carbons but also achieve higher levels of specificity in ion removal, opening new avenues for the application of CDI technology in water purification and beyond.

3.4 Graphene oxide composite electrode

The distinctive functional groups, two-dimensional layered structures, and inherent hydrophilicity of graphene oxide (GO) render it an exemplary material for the adsorption of heavy metal ions. ⁸⁵ This assertion is supported by Yang et al., ⁸⁶ who observed that GO exhibits an adsorption capacity of up to 46.6 mg g⁻¹ for Cu²⁺, a figure that significantly surpasses the adsorption capabilities of activated carbon by nearly an order of magnitude. In parallel, research conducted by Mi et al. ⁸⁷ highlights GO superior efficacy in adsorbing Fe³⁺ ions when compared to activated carbon, further evidencing its potential in heavy metal ion removal. Despite these advantageous properties, GO practical application is hindered by its propensity for interlayer aggregation, attributed to π-π interactions, which poses a challenge to its performance in adsorption processes.

To mitigate the aggregation tendency of GO, strategies such as increasing layer spacing and crafting lamellar 3D structures are predominant. ^{88 , 89 , 90 , 91} Zhang et al. ⁹² developed a monolithic structure via 3D printing techniques to circumvent GO aggregation, employing it as an adsorbent for the removal of Cu²⁺ ions. Utilizing the Langmuir model, the maximum adsorption capacity was determined to be 179.32 mg g⁻¹. This remarkable efficiency in Cu²⁺ ion adsorption is primarily attributed to the complexation with oxygen functional groups present in GO.

Mao et al. ⁹³ engineered electrode materials by integrating W₁₈O₄₉ with graphene to form a spherical flower layer structure, as depicted in Figure 9(A). This novel configuration demonstrated exceptional selectivity in a binary mixture of heavy metal salts and NaCl, achieving over 90 % removal efficiency for heavy metal ions, while the extraction of Na⁺ ions remained below 30 %, as shown in Figure 9(B). The incorporation of W₁₈O₄₉ not only ameliorates the aggregation issue inherent to graphene but also enhances the composite’s specific surface area and conductivity. The synergistic interaction between the composite components results in improved electrochemical and adsorption properties. Furthermore, the layered structure of graphene, in conjunction with the valency variability of W⁴⁺, W⁵⁺, and W⁶⁺ ions, facilitates the reversible intercalation and release of heavy metal ions within the W₁₈O₄₉ lattice, as shown in Figure 9(C) and (D). This mechanism preserves the crystal structure, ensuring excellent cyclic performance. By leveraging pseudocapacitive materials and innovative structural designs, these approaches address the challenge of GO aggregation, opening new avenues for graphene’s application in capacitive deionization technologies.

Figure 9:

W₁₈O₄₉@GO materials: ⁹³ SEM (A); plot of removal efficiency of heavy metal ions and Na⁺ in the binary solution of metal nitrate with NaCl (B); comparison of W 4f XPS spectra of pristine and adsorbed heavy metal ions and Na⁺ (C); plot of heavy metal ion adsorption mechanism (D) (reprinted with permission).

Reduced graphene oxide (rGO) enhances the inherent advantages of GO by providing effective adsorption centers for heavy metal ions through a conjugated sp² heterostructure that incorporates π-electrons and oxygen-containing functional groups. ⁹⁴ This unique configuration facilitates the adsorption of heavy metals. L. Bautista-Patacsil et al. ⁹⁵ augmented rGO with titanate nanotubes (TNT) to expand the interlayer spacing, effectively mitigating rGO aggregation. This modification resulted in a material with a specific surface area of up to 511.226 m² g⁻¹ and 99.83 % mesoporosity, as determined by surface area and pore size analysis, as shown in Figure 10(A). The enhanced structure exhibited maximum adsorption capacities of 3.99 mmol g⁻¹ (253.25 mg g⁻¹) for Cu²⁺ and 1.17 mmol g⁻¹ (241.65 mg g⁻¹) for Pb²⁺ at a concentration of 80 ppm, as depicted in Figure 10(B). The synergistic interaction between rGO sp and sp² hydrocarbons with electron-rich functionalities and the oxygen-containing groups (–OH/Ti–O) in TNT provides a versatile adsorption site that acts variably as acidic or alkaline, optimizing Cu²⁺ adsorption, as indicated in Figure 10(C). Further advancements were made by Gil Stefan S. Mamaril et al., ⁴⁴ who developed a nitrogen-fluorine co-doped 3D structure via a 16-h hydrothermal reaction to decrease rGO aggregation and create a multitude of layered mesopores ranging from 2.1 nm to 5.6 nm, as depicted in Figure 10(D) and (E). This structure facilitates rapid ion transport and, through nitrogen-fluorine co-doping, enhances charge transfer and affinity for Cu²⁺, achieving a maximum adsorption capacity of 52.4 mg g⁻¹ for Cu²⁺ ions, as indicated in Figure 10(F).

Figure 10:

rGO/TNT materials: ⁹⁵ BET and pore volume-pore diameter plots (A), adsorption capacity of Cu²⁺ and Pb²⁺ (B), and FT-IR plots(C) (reprinted with permission); NFrGO materials: ⁴⁴ SEM (D), pore volume-pore size plots (E), Cu²⁺ adsorption capacity (F) (reprinted with permission).

Despite these innovative approaches significantly addressing the aggregation issue of GO and improving the adsorption efficiency of rGO-based materials, ^{96 , 97} challenges such as higher production costs and complex process modifications persist. These obstacles limit the broader application of graphene and related electrode materials in CDI technology. Research efforts in China and elsewhere have yielded noteworthy results, indicating ongoing interest and potential for overcoming these barriers in the future.

3.5 Activated carbon fibers composite electrodes

Activated carbon fibers (ACFs) and their derivatives are heralded as among the most eco-friendly materials of the 21st century, drawing considerable attention within the realm of carbon materials research. ^{98 , 99 , 100} ACFs are distinguished by their exceptional mechanical properties and flexibility, positioning them as an ideal substrate for electrode applications. The innovative application of electrostatic spinning and electrochemical deposition techniques in the fabrication of flexible, self-supporting monolithic electrodes represents a significant advancement. ^{101 , 102 , 103} These methods eliminate the need for adhesives, thereby enhancing the adsorption performance of the materials and contributing to the sustainability of the technology. However, their limited adsorption capacity and lack of active sites have been notable drawbacks. These limitations have spurred interest in utilizing ACFs primarily as electrode skeletons, aiming to diminish the reliance on adhesives and extend the lifespan of electrodes. ^{104 , 105}

To address these challenges, Men et al. ¹⁰⁶ explored the coating of monolayer GO on ACFs, examining its impact on the adsorption properties of CDI electrode materials under binder-free conditions. The SEM images and BET analysis revealed that the unique interstitial structure, resulting from the interlocking shape of the ACFs, furnished a substantial specific surface area (1,532.73 m² g⁻¹) along with active sites, as depicted in Figure 11(A) and (B). The integration of single-layer GO not only augmented the specific surface area but also modified the pore structure of ACFs, leading to an increase in mesopore formation, as indicated in Figure 11(C). This alteration improved the solid/liquid interface between the electrode and the solution, diminishing the EDLs overlap effect typically induced by micropores. Consequently, these modifications bolstered the adsorption capacity, presenting a novel approach for the development of CDI electrodes. This work underscores the potential of combining ACFs with nano-engineered materials like GO to enhance electrode performance for water purification technologies.

Figure 11:

SGO@ACF materials: ¹⁰⁶ SEM (A); BET (B); pore volume-pore width plots (C) (reprinted with permission).

While the adsorption capacity of ACFs is enhanced by the addition of graphene oxide, the inherent non-selectivity of these carbon-based materials towards heavy metal ions remains a significant limitation. To overcome this, researchers have turned their attention to modifying ACFs with elements such as nitrogen (N) and sulfur (S) to introduce selective adsorption capabilities through soft-soft interaction and pseudo-capacitance. ¹⁰⁷

Sun et al. ⁴⁵ advanced the field by fabricating self-supported electrodes with flower-like frameworks, integrating N-doped carbon nanolayers (N–C) and ultrafine titanium nitride (TiN) onto carbon cloth. This novel electrode design, characterized by a three-dimensional open framework and hierarchical pore arrangement, not only served as an efficient carrier for TiN but also significantly enhanced the adsorption of heavy metal ions, as indicated in Figure 12(A) and (B). The CDI tests on heavy metal ions, including Cr³⁺, Fe³⁺, Pb²⁺, Cd²⁺, Ni²⁺, and Cu²⁺, demonstrated exceptional selective adsorption, achieving more than 98 % electro-sorption removal in single-component systems and over 95 % in binary systems containing NaCl, as shown in Figure 12(C). The remarkable performance of these TiN nanorod arrays can be attributed to the synergistic effects of Faradaic pseudocapacitive reactions and the bilayer electro-adsorption of porous N-doped carbon nanolayers, as depicted in Figure 12(D).

Figure 12:

N–C@TiN@CC materials: ⁴⁵ SEM (A); BET and pore volume-pore diameter plots (B); removal efficiency of heavy metal ions and Na⁺ in binary solutions containing NaCl (C); heavy metal ion adsorption mechanism (D) (reprinted with permission).

Despite these advancements, the application of ACFs in creating flexible and self-supporting monolithic electrodes is still predominantly at the research stage. The challenges lie in the treatment of ACFs and the precise control over the loading of electrode materials, which remain key obstacles to the widespread adoption of these technologies in practical applications. ¹⁰⁸ These findings underscore the potential of doped and modified ACFs in enhancing the selectivity and efficiency of adsorption in capacitive deionization processes, paving the way for future innovations in water treatment technologies.

3.6 Problems and improvement methods of carbon material electrodes

The utilization of carbon-based electrodes in CDI technologies is predominantly governed by the mechanism of EDLs adsorption, presenting inherent limitations in achieving selective adsorption of contaminants, such as heavy metal ions. Additionally, challenges including non-uniform particle distribution, suboptimal hydrophilicity, and the dependency on bonding agents impede the effectiveness of these electrodes. To address these issues, functional modifications of carbon materials are crucial for enhancing their performance as CDI electrodes. The following summary elucidates the primary challenges, proposed improvement strategies, and underlying mechanisms pertinent to carbon-based materials in CDI applications, ¹⁰⁹ as shown in Table 2:

1. Selective adsorption enhancement:

   Carbon materials primarily rely on EDLs adsorption, lacking selective adsorption capabilities for heavy metal ions. Incorporating transition metal oxides and sulfides, known for their selective adsorption properties, onto the carbon base to facilitate selective adsorption in multi-component ion scenarios. Transition metal oxides and sulfides offer selective adsorption through Faradaic reactions, presenting a viable route to improve specificity towards particular ions. ¹¹⁰

2. Electrical conductivity and particle distribution:

   Biomass-derived carbon materials often suffer from poor electrical conductivity and large gaps between particles. Utilizing conductive agents and binders initially, with a long-term strategy focusing on the integration of metal oxides and sulfides with lamellar or spherical structures to create a unique conductive network. The formation of a conductive network through the composite of carbon with metal oxides and sulfides enhances electrochemical performance and mitigates the effects of particle gaps.

3. Aggregation and specific surface area:

   Electrostatic forces cause carbon materials to aggregate, reducing the specific surface area and active sites. Designing unique 3D structures and incorporating transition metal oxides to increase layer spacing. The introduction of 3D structures and layer spacing expansion addresses aggregation, thereby preserving surface area and active sites for improved adsorption. ¹¹¹

4. Hydrophilicity and contact angle:

   The inherent hydrophobicity of carbon materials leads to a large contact angle with water, diminishing the effective adsorption surface area and reducing wastewater retention time. Enhancing hydrophilicity through acidification, doping, and the addition of metal oxides or sulfides, as well as introducing polar functional groups (N, S, O). Metal oxides/sulfides and polar functional groups improve the hydrophilicity of carbon materials, increasing the solid/liquid contact area and facilitating better adsorption and wastewater retention. ⁵³

These strategies underscore the multi-faceted approach required to address the inherent limitations of carbon-based electrodes for CDI. By integrating materials science and electrochemical engineering principles, these modifications aim to advance CDI technologies toward higher efficiency, selectivity, and sustainability in water purification applications.