Plasma-Assisted Regeneration of Activated Carbon: Current Status and Prospects

Abstract

Activated carbon (AC) is widely used in pollution control, but it faces significant challenges in regeneration due to secondary pollution and structural degradation. Traditional methods, such as thermal and chemical regeneration, are energy-intensive and inefficient. Plasma-based regeneration, which includes high-voltage pulsed discharge and dielectric barrier discharge, provides an effective approach for restoring AC adsorption capacity with minimal environmental impact. While plasma techniques risk damaging AC’s porous structure, recent advances demonstrate their potential for efficient regeneration at lower energy costs. This review examines plasma-driven regeneration processes, focusing on optimizing reactivity control to maintain AC structural integrity while achieving high regeneration performance. The analysis highlights key mechanisms and operational parameters that influence plasma regeneration efficiency.

1. Introduction

With the global advancement of industrialization, environmental pollution has become increasingly severe. The latest Frontiers Report released by the United Nations Environment Programme highlights the urgent need to address critical environmental challenges, emphasizing the importance of pollution management and ecosystem restoration. In pollution management, AC is widely utilized owing to its excellent adsorption properties and low cost, particularly in the treatment of low-concentration alcohol and ketone exhaust gases, as well as industrial and domestic wastewater. The surface of AC contains numerous pores, resulting in an extremely high specific surface area [1,2]. Organic molecules can be adsorbed onto the AC surface via intermolecular forces. Additionally, the AC surface contains functional groups, such as hydroxyl (-OH) and carboxyl (-COOH), which can react with organic pollutants and lead to their accumulation. AC exhibits both physical and chemical adsorption capabilities [3]. Physical adsorption captures organic molecules smaller than the pore radius, whereas chemical adsorption involves reactions with various organic compounds (e.g., phenols, benzene homologs, and their derivatives). However, the adsorption capacity of AC is inherently limited. Once AC reaches its adsorption saturation, it loses its adsorption capacity, becoming solid waste and contributing to secondary pollution [4].

Plasma-enhanced electrostatic precipitation employing high-voltage nanosecond pulse discharges has been successfully applied to diesel exhaust treatment, demonstrating the capability of plasma technology to manage complex pollutants. However, the application of ultra-short (nanosecond) pulsed plasmas to enhance electrostatic precipitators has not been extensively studied for diesel exhaust treatment. One major advantage of nanosecond pulses over microsecond pulses is their superior power efficiency, which could significantly enhance the energy economy of plasma-based regeneration processes. This efficiency is particularly significant in the context of AC regeneration, where minimizing energy consumption and preserving structural integrity are critical factors [5]. Several research groups have explored the application of nonthermal plasmas to treat diesel exhaust emissions, achieving reductions in both particulate matter and NO_x (i.e., NO and NO₂). These studies typically involve small laboratory-scale reactors treating only a fraction of the total engine exhaust flow [6]. Therefore, from both economic and environmental perspectives, regenerating AC is essential (Figure 1).

The process of AC regeneration involves removing the organic compounds accumulated on the AC surface, essentially reversing the adsorption mechanism. Traditional regeneration methods include thermal regeneration and chemical solvent regeneration. However, thermal desorption or chemical oxidation regeneration can damage the carbon skeleton and consume the carbon content, leading to a reduced number of regeneration cycles [7]. During thermal regeneration, significant carbon loss occurs, and the high temperatures (400 °C to 600 °C) reduce the adsorption capacity [8]. The carbon recovered through chemical solvent regeneration exhibits low adsorption efficiency, only partially restores the AC’s adsorption capacity, and often leads to micropore clogging [9].

The concept of plasma, the fourth state of matter, emerged in the early 20th century. Its discovery can be attributed to Irving Langmuir in the 1920s, when he first recognized ionized gases in vacuum tubes and coined the term “plasma” (Figure 2) [10]. Over time, the theoretical foundations of plasma were established as researchers began to understand its unique behavior and applications, particularly in the context of ionized gases and astrophysics. Notably, plasmas were found to constitute the majority of visible matter in the universe, including the sun and stars. The 1950s represented a significant advancement in plasma research, with a focus on controlled nuclear fusion [11]. Scientists investigated the potential of plasma as a medium for achieving controlled thermonuclear reactions, which was pivotal for the development of fusion energy. However, despite promising theories, practical implementation remained a challenge due to the complex behavior of plasmas under extreme conditions.

By the 1980s and 1990s, the industrial applications of plasma began to proliferate. Plasma etching was introduced as a method for precision microchip fabrication, revolutionizing the electronics industry. Simultaneously, applications of plasma in material processing, such as coatings and surface treatments, grew, laying the foundation for its widespread industrial use. In laser-driven scenarios, a strong laser pulse is fired into a pre-formed plasma. The pulse induces charge separation in the plasma, and the electric field from this charge configuration can accelerate trapped electrons [12]. The effects of atmospheric cold plasma-generated reactive oxygen species on problematic bacteria in chronic wounds have been explored. To specifically study the effects of reactive oxygen species, researchers employed a vacuum chamber equipped with a flowmeter to control the background gas composition and a mesh electrode within a dielectric barrier discharge (DBD) device [13].

In recent years, plasma research has expanded into diverse fields, including medicine and environmental technology. Today, plasma remains a critical area of scientific inquiry, with applications ranging from energy storage to space propulsion, and its future appears promising because of ongoing advancements in plasma-based technologies (Figure 2) [14,15,16,17,18,19,20]. Both thermal and electrochemical regeneration methods face significant limitations. (a) Regarding the main drawbacks of thermal regeneration, high temperatures may damage the carbon skeleton, reduce the carbon content, and degrade the adsorption capacity, thereby limiting regeneration cycles. (b) The main drawbacks of electrochemical regeneration include low efficiency, micropore clogging, and potential secondary pollution resulting from chemical solvents. Plasma technology has demonstrated significant potential in various environmental applications, including air pollution control and wastewater treatment.

As a result, plasma regeneration technology has become a global research focus in the field of AC regeneration. Several methods of plasma generation exist, including high-voltage pulsed discharge, DBD, microwave discharge, and electric corona discharge. Discharge plasmas can regenerate saturated AC under gaseous or liquid-phase conditions through the action of high-energy electrons. This work summarizes the mechanisms, application conditions, and effects of AC regeneration based on plasma technology. Particular emphasis is placed on high-voltage pulsed discharge and DBD techniques, which have gained considerable popularity in recent years. The outcomes of AC regeneration using plasma technology are presented, and the potential for coupling plasma technology with other methods to enhance regeneration efficiency, which focuses on thermal regeneration versus thermal reactivation, is discussed, highlighting future prospects.

2. Mechanism of AC Regeneration by Plasma

AC adsorbents are extensively utilized for the adsorption and elimination of volatile organic compounds, such as styrene, owing to their remarkable adsorption capacity and cost-effectiveness. However, the regeneration and reutilization of AC face significant limitations due to the incomplete cleavage of styrene and carbon accumulation during heated styrene cracking (Figure 3a) [21]. Sintered AC can be simultaneously utilized as both an adsorption filter and an electrode, enabling its electrochemical regeneration. The use of sintered AC improves the contact between AC particles and thus enhances the conductivity (Figure 3b) [22]. Plasma regeneration of AC employs high-energy electric fields to generate reactive species, such as ·OH, O₃, and H₂O₂, which degrade adsorbed pollutants and restore the adsorption capacity. This process occurs under gaseous or liquid-phase conditions, with plasma–water interactions that could enhance oxidative reactions. Additionally, plasma treatment introduces oxygen-containing functional groups onto the AC surface, further improving its adsorption properties. AC typically withstands 4–6 regeneration cycles before the adsorption capacity declines below 80%, as pore structure evolution and surface chemistry modification progressively alter its adsorption characteristics. Plasma, recognized as the fourth state of matter alongside solid, liquid, and gas, exhibits high electrical conductivity [23].

Free particles are energized in a high-energy electric field, forming high-energy electrons that can ionize, dissociate, excite, and radiatively recombine most molecules in wastewater. This process generates a substantial quantity of plasma species, such as H₂O₂, O₃, ·O, and ·OH [24]. This method falls under chemical activation. Chemical activation is a single-step process that involves impregnating raw materials with a chemical activating agent [25]. When humid air is used as the ambient discharge gas at atmospheric pressure, oxidizing species, including ·OH, can be efficiently generated via electron and/or photon impact dissociation of water molecules in the ambient gas. The chemical agents result in the incorporation of heteroatoms or surface carboxylic, phenolic, and hydroxyl groups, providing abundant functional active groups that facilitate higher adsorption rates [26]. The effect is attributed to the ability of porous carbon to trap and concentrate substances, prolonging the exposure of molecules to the plasma discharge zone and enhancing the production of ·OH radicals on granular activated carbon (GAC), thereby boosting the degradation of substances by plasma and in situ regeneration of exhausted GAC [27]. The decrease in O₃ and increase in ·OH in water after GAC addition demonstrate that O₃ can be catalyzed by GAC, leading to the formation of ·OH [28]. Plasma treatment could be applied to either chemically or physically modify carbon. In physical treatment, a change in the carbon textural property is realized. Some conducted studies claim that the use of O₂ plasma does not significantly change the textural structure of carbon, suggesting that only a superficial modification could be realized with this plasma. On the contrary, the use of argon gas plasma could change the topology and porosity of AC. Several studies have modified the surface of AC or biochar to enhance adsorption [29].

While widespread adoption of GAC offers a cost-effective solution, it also creates a pressing need for managing the accumulation of spent GAC [30]. The regeneration of GAC after adsorption of perfluoroalkyl and polyfluoroalkyl substances is an emerging challenge (Figure 3c,d) [31]. In one study, the plume of quenched plasma was directed toward a target, and the newly created oxidative species reacted at the target-plasma interface. Based on multi-point Brunauer–Emmett–Teller (BET) analysis, the surface areas of non-plasma-treated and plasma-treated samples were consistently around tens of m²/g, indicating that plasma treatment hardly influences the morphology and textural properties of porous carbon [32]. After the organics on the AC surface are decomposed by oxidation, the adsorption sites on the AC surface recover, thereby achieving AC regeneration. Plasma regeneration increases the number of oxygen-containing functional groups on the AC surface, thereby enhancing its adsorption performance. FT-IR analysis data indicate that cold plasma treatment can enhance the functional binding groups on the surface of the biomaterial, improving its adsorption performance (Figure 3e) [33].

Liu et al. [34] focused on investigating the physical and chemical structural transformations that occur during the conversion of AC into carbon quantum dots, elucidating the in situ regeneration mechanism of spent AC. The ultimate aim was to develop a durable, efficient, and cost-effective AC adsorbent. Kim et al. investigated how exposure to a nonthermal plasma sustained in a nonreactive argon atmosphere affects the desorption of carbon monoxide (CO) from platinum nanoparticles. Temperature-programmed desorption measurements indicated that the plasma reduced the effective binding energy of CO to Pt surfaces by approximately 0.3 eV [35].

3. Different Methods for Producing Plasma-Regenerated AC Technologies

Plasma-based regeneration technologies for AC have shown significant potential in addressing the limitations of traditional regeneration methods. High-voltage pulsed discharge, DBD, and microwave plasma are the primary techniques explored. High-voltage pulsed discharge, including gas-phase, liquid-phase, and gas–liquid two-phase systems, demonstrates high efficiency in organic pollutant degradation and AC regeneration. However, it faces challenges, such as electrode corrosion and energy inefficiency at higher voltages. DBD plasma offers stable and uniform plasma generation with minimal electrode damage, making it suitable for multiple regeneration cycles. Microwave plasma, leveraging rapid heating and high energy efficiency, preserves the porous structure of AC but requires high-temperature-resistant materials. Coupling plasma with complementary techniques, such as ozone or thermal regeneration, shows promise in enhancing efficiency and durability. Future research should focus on optimizing energy consumption, improving structural preservation, and scaling up these technologies for industrial applications. A comparative analysis of different plasma-regenerated AC technologies, including their operational parameters, advantages, and limitations, is presented in

Table 1. Comparative analysis of plasma-assisted AC regeneration methods. Key parameters include regeneration efficiency and advantages/disadvantages. The data highlights the trade-offs between oxidative capacity and structural preservation across different plasma technologies, with reference to conventional thermal/chemical methods as benchmarks.

Method	Efficiency	Advantages	Limitations	Key Parameters
High-voltage pulsed discharge	55–94%	Multi-phase compatibility Effective organics removal	Electrode degradation Structural damage Precise parameter control	Voltage: 18–28 kV Moisture: 10–31% Flow rate: 0.45 m3/h
Dielectric barrier discharge	65–90%	Electrode protection Multi-cycle durability	Significant power demand Potential pore occlusion	Field strength: 18–28 kV O2: 5–21%
Microwave plasma	~81%	Microstructure conservation Green process	Material thermal limits Cyclic structural changes	Power: 600 W Duration: 3–5 min N2: 300 cm3/min
Thermal regeneration	50–70%	Proven technology Broad applicability	Carbon attrition Energy intensive Capacity reduction	Temperature: 400–600 °C
Chemical regeneration	Partial restoration	Mild conditions Low energy input	Pore fouling Secondary contamination	Reagent selection Concentration optimization

.

3.1. AC Regenerated by High-Voltage Pulsed Discharge Plasma

Pulsed discharge involves applying an impulse voltage to the two poles of a discharge tube, creating a cyclic discharge system. When voltage is applied, pulsed plasma is generated [36]. The pulsed discharge system contains highly oxidative high-energy particles that can oxidize organic pollutants attached to AC, leading to AC regeneration. Depending on the external voltage, high-voltage discharge pulses can be classified into pulsed, alternating, and direct types. Additionally, based on the phase state of the medium participating in the reaction, high-voltage discharge pulses can be classified into gas-phase discharge, liquid-phase discharge, and gas–liquid two-phase mixed discharge. Pulsed plasma exhibits excellent regeneration performance at any humidity [37].

3.1.1. Gas-Phase Pulse Discharge

Gas-phase pulsed discharge involves applying high-voltage to two groups of poles, creating a high electric field between them. Under the influence of the high electric field, the gas is ionized, producing plasma species such as ·OH, ·O, H₂O·, and O₃. The reactions for gas-phase discharge plasma generation are as follows (1)–(4) [38,39,40]:

O₂ + e⁻* → 2O· + e⁻

(1)

H₂O + e⁻* → H · + ·OH + e⁻

(2)

e⁻* + O₂ + O₂⁺ → 2e⁻

(3)

H· + O₂ → HO₂ ·

(4)

where e⁻* represents a high-energy electron; e⁻ represents an electron.

Qu Guangzhou et al. [41] investigated the regeneration of GAC saturated with the azo dye acid orange 7 using this method. The effects of parameters such as treatment time, electric field strength, and gas type on the re-adsorption rate were systematically studied. The results of structural property analyses showed that the surface area, micropore area, external surface area, micropore volume, and total volume of GAC decreased to varying extents after three cycles of DBD treatment.

3.1.2. Liquid-Phase Pulse Discharge

Pulsed discharges that form filamentary plasma channels propagating along the surface of a liquid interface are of interest for studies where the flow of liquid and its contact with plasma channels can be controlled [42]. Liquid-phase discharge can generate high concentrations of hydroxyl radicals but requires extremely high breakdown voltages (Figure 4a,b) [43]. An optimized design of a hybrid reactor, consisting of a stainless-steel needle enclosed in a conical tube, offers discharge stability by maintaining the gas and liquid phases separately. The main advantage of this hybrid reactor is its geometric design, which allows bubbles to slide upward along the tube wall without significantly perturbing the discharge plume. This design simplifies reactor construction and provides relatively stable discharge conditions, facilitating plasma diagnostics (Figure 4c) [44].

Compared to gas-phase pulsed discharge, liquid-phase pulsed discharge has a higher plasma density, higher temperature, and stronger oxidation capacity, enabling complete pyrolysis and chemical degradation of organic compounds attached to AC, thereby regenerating AC. At a certain discharge time, the amount of plasma generated can prolong AC exposure, improving regeneration efficiency. However, as discharge continues, the system temperature continues to rise, causing micropores on the AC surface to melt and form mesopores and macropores. This reduces the specific surface area of AC, decreasing its adsorption and regenerative efficiency (Figure 4d) [45].

Tang Shoufeng et al. [46] illustrated the main process of GAC regeneration by discharge plasma. They stated that the generation of H₂O₂ on GAC by DBD plasma is a critical step in the regeneration process, with active species, such as H₂O₂ and ·OH, playing a dominant role in phenol degradation and GAC regeneration. At the same time, the accompanying physical effects, such as ultraviolet radiation and strong electric fields in the discharge plasma, are further enhanced at higher pulse voltages, leading to an increased production of active species. In summary, due to the chemical and physical effects of DBD plasma, higher pulse voltages result in more H₂O₂ generation on GAC during the DBD process. Similar to H₂O₂ yields, they found that the rate constant also increased as the pulse voltage rose from 18 to 28 kV, rising from 0.14 × 10⁻² to 1.30 × 10⁻² 1/min. Additionally, energy efficiency, defined as the amount of H₂O₂ divided by the discharge power consumption, decreased from 0.340 to 0.212 µmol/kJ as the pulse voltage increased. This suggests that as voltage increases and treatment time extends, fewer H₂O molecules remain on the GAC, leading to a decline in energy efficiency (Figure 4e,f).

3.1.3. Gas–Liquid Two-Phase Mixed Pulse Discharge

Gas–liquid two-phase mixed pulsed discharge involves applying high-voltage pulses to the liquid phase, with the grounding electrode placed in the gas phase above the liquid phase. Pulsed electrical discharges in a gas–liquid mixture deposit energy into both phases [47]. High-voltage discharge in both liquid and gas phases generates plasma in both phases. Plasma produced in the liquid phase reacts directly with organic compounds, while plasma produced in the gas phase generates active species.

The amount of active species generated by discharge plasma increases significantly when fine bubbles are introduced into an aqueous solution [48]. Therefore, injecting bubbles into the liquid phase enhances the production of oxidative species during discharge plasma application [49]. In a gas–liquid two-phase slug flow, bubbles continuously flow in the gaseous phase within a microchannel, completely separated from the liquid phase [50]. Organic compounds in the liquid phase are primarily degraded by miscible phases. In the application of pulsed discharge plasma for regenerating bio-carbon, studies on single gas-phase and liquid-phase discharges are limited. Most studies focus on gas–liquid two-phase mixed pulsed discharge, as it offers higher regeneration efficiency than single-phase discharges.

Wei Song et al. [51] confirmed that the proper tuning of the textural structure and surface chemistry of activated carbons (ACs) could efficiently promote the production rate of H₂O₂ via NH₂OH oxidation. The textural features and surface chemical properties of ACs were significantly modified by gas-phase or liquid-phase oxidation. Minor variations in pore size and pore size distribution (PSD) were detected for AC samples obtained by gas-phase oxidation, while a significant decrease in total pore volume was observed for AC samples obtained by liquid-phase oxidation.

Gas-phase oxidation of AC under mild conditions efficiently enriched surface quinoid groups, promoting the yield and selectivity of H₂O₂ to 55% and 87%, respectively, on AC. This corresponded to an H₂O₂ concentration of 0.47 wt%, which is superior to that obtained on supported noble metals (0.05–0.1 wt%) and comparable to that obtained by homogeneous manganese complex catalysts. Liquid-phase oxidation of AC generated more carboxyl groups, lowering the yield of H₂O₂. The quantitative correlation between the reaction rate and the number of surface oxygen-containing groups verified that quinoid groups acted as active species for NH₂OH oxidation to H₂O₂. While plasma treats organic compounds attached to AC, it can also oxidize the AC surface, forming new micropores and increasing the specific surface area of AC [52]. Therefore, the adsorption ability of AC after primary regeneration is similar to that of fresh carbon. However, after several regeneration cycles, due to the constant total volume, newly formed micropores connect with existing ones to form larger micropores, reducing the specific surface area and adsorption capacity of GAC. Conversely, larger specific surface areas and more exposed active sites can enhance adsorption [53].

Tang et al. [54] utilized a bipolar pulse power-driven zoom-in medium-scale discharge reactor to regenerate GAC. By controlling parameters such as pulse voltage, discharge time, air flow rate, and moisture content, they investigated the efficiency of phenol degradation by AC to evaluate GAC regeneration performance. Their study showed that under optimal conditions (pulse voltage of 21 kV, GAC moisture content of 31%, air flow rate of 0.45 m³/h, and regeneration time of 60 min), the GAC regeneration rate reached 94%. However, after four regeneration cycles, the regeneration rate decreased to 55%.

3.1.4. Experimental Results and Discussion of High-Voltage Pulsed Discharge Plasma

The deactivation of GAC during regeneration may be attributed to the collapse of the surface structure caused by the chemical and physical effects of DBD plasma, as well as the accumulation of phenol and its decomposition intermediates, which occupy adsorption sites on GAC. The difference between the cumulative adsorbed phenol and the residual phenol on GAC across regeneration cycles provides strong evidence for this explanation. Furthermore, the destruction of GAC porosity and pore blockage during the DBD process may contribute to the decrease in adsorption capacity.

Plasma generated by high-voltage discharge can regenerate AC to a certain extent. An advantage of this method is that bubbles in the discharge channel can carry away organic compounds attached to AC during the later stages of discharge, enhancing the regeneration process. However, the method requires strict operating conditions, and reaction time critically influences the outcome—neither excessively long nor too short treatment times achieve optimal regeneration. Additionally, electrodes are prone to corrosion during liquid-phase discharge, potentially damaging the reactor.

3.2. DBD Plasma-Regenerated AC

DBD plasmas find applications in various fields, including material synthesis and functionalization, plasma catalysis for gas conversion, pollution control, and biological sample treatment [55]. A typical DBD reactor mainly consists of an alternating current high-voltage power supply, high-voltage electrodes, a grounding electrode, and a dielectric barrier. [56].

3.2.1. Mechanism of DBD Plasma

Due to the low noise generated during discharge, DBD is also referred to as silent discharge [57]. Unlike pulsed discharge, in DBD reactors, the electrodes are covered with an insulating medium (e.g., glass, quartz, alumina) or suspended within the discharge area. When a sufficiently high alternating current voltage (typically between 50 Hz and 1 MHz) is applied to the electrodes, gas breakdown occurs in the discharge area, resulting in plasma generation (Figure 5a). Owing to the presence of the dielectric barrier, the current is limited, leading to the formation of homogeneous and stable plasma with a high electron density [58]. Pulse voltage is a critical control parameter in discharge plasma processes, directly influencing both the discharge energy input and the generation of chemically active species during DBD-based regeneration. Atmospheric DBD plasma has been demonstrated to effectively generate reactive oxygen species (ROS) [59].

Song Wei [51] investigated the changes in H₂O₂ formation at different pulse voltages during DBD plasma treatment. Experiments were conducted at pulse voltages of 18, 23, and 28 kV, with other operating parameters including an air flow rate of 0.8 L/min and a GAC moisture content of 10%. The production of H₂O₂ increased with higher pulse voltages because of the greater energy input into the DBD reactor. After 100 min, the H₂O₂ yield reached 260.9 µmol/kg GAC at 28 kV, compared to 245.6 µmol/kg GAC at 23 kV and 221.7 µmol/kg GAC at 18 kV.

Several studies have shown that plasma regeneration of AC can undergo multiple cycles, although the regeneration efficiency tends to exhibit a clear declining trend. However, some researchers have reported achieving high regeneration efficiencies after multiple regeneration cycles. For example, Chen et al. [60] studied the regeneration process of AC saturated with dimethyl sulfide (DMS) using DBD. When the oxygen density was 21% and the energy density was 288 J/L, the AC regeneration efficiency increased from 65% at 0% humidity to 67% at 1% humidity (volume fraction). This enhancement was attributed to the generation of hydroxyl radicals (·OH) under DBD conditions, which promoted the decomposition of DMS and improved the AC regeneration efficiency.

Under humid conditions, AC accelerates the decomposition of ozone (O₃), resulting in the formation of a greater amount of highly oxidative hydroxyl radicals (·OH) [61]. Under optimal operating parameters (energy density of 761 J/L, humidity of 0–1%, and oxygen density of 5%), the regeneration efficiency of AC reached 90% after 10 regeneration cycles. This improvement was primarily attributed to the presence of surface oxygen functional groups, especially quinoid groups (including phenolic and carbonyl-quinone groups), which played a crucial role in promoting H₂O₂ production. The impact of quinoid groups on the rate of H₂O₂ formation was particularly significant. However, plasma regeneration efficiency can vary considerably depending on the type of AC and the nature of the adsorbates. Structural changes during regeneration have been observed, such as the generation of voids within penetrated and dislocated channels [62], which facilitate plasma penetration. When the carbon structure is divided into distinct regions—intervoids (voids between carbon granules), intravoids (macropore volume), the carbon skeleton volume, and micropores [63]—it is found that plasma cannot easily penetrate into micropores. This difficulty is mainly due to the irreversible uptake of adsorptive molecules in pores whose widths are comparable to those of the adsorbate molecules and/or the swelling of non-rigid pore walls. At relatively low pressures, micropore filling and high uptakes were observed, with narrow pore widths inducing strong adsorption potentials (Figure 5b) [64].

Qu Guangzhou [65] demonstrated that the regeneration of spent GAC exhausted with acid orange 7 (AO7) via DBD plasma treatment was feasible. After multiple regeneration cycles, the adsorption rates of AO7 on GAC remained stable, with regeneration efficiencies exceeding 73% even after five cycles. The DBD plasma treatment not only decomposed the adsorbed contaminants on GAC but also altered the texture and chemical characteristics of the GAC itself. Despite a significant increase in the specific surface area and pore volume after the first DBD treatment, the adsorption capacity of GAC did not return to its initial value. On the one hand, the longer diffusion paths in micropores resulted in a higher probability of pore blockage during diffusion. On the other hand, the increased carboxylic groups and by-product molecules or atoms polarized under the stronger electric field in the regenerated carbon weakened the affinity between the adsorbates and the surface chemistry of GAC and AO7 in the bulk solution. The adsorption isotherms were well fitted to the Freundlich equation in all cases, indicating that while DBD did not modify the adsorption process itself, it shifted the adsorption equilibrium to lower concentrations. The plasma regeneration mechanism of AC in DBD systems involves complex nitrogen chemistry, where NOx species are generated through electron-impact dissociation of molecular nitrogen, followed by subsequent oxidation pathways. Zhang et al. demonstrated the plasma nitrogen fixation for NO_x synthesis from N₂ and O₂ using a MnO_x/Al₂O₃ packed-bed DBD reactor, along with an enhanced catalytic effect of MnO_x/Al₂O₃. At a N₂ content of 50% and an SEI of about 16 kJ/mol, the NO_x production rates were 0.28 SCCM for Al₂O₃ and 0.42 SCCM for MnO_x/Al₂O₃, showing an improvement of approximately 60% due to the enhanced effect of MnO_x/Al₂O₃ [66].

3.2.2. Experimental Results and Discussion of DBD Plasma

To better understand the effect of the electrochemical regeneration process on the surface of AC under optimal conditions, a physicochemical characterization was performed before and after ten regeneration cycles of AC-2h. The morphological structure of the AC before and after the regeneration cycles was examined using FESEM. Small impurities along the surface of the AC were visible in the micrographs before treatment, but they were no longer visible after the treatment. The analysis also showed that the structural integrity of the regenerated AC was preserved (Figure 5c) [67]. Due to the presence of a dielectric medium, uniform DBD, in contrast to high-voltage pulsed discharge, generates more stable plasma. Furthermore, the dielectric barrier prevents the electrodes and plasma from coming into direct contact with the organic compounds adsorbed on the AC, thereby avoiding electrode corrosion. However, several factors, such as the distance between electrodes, discharge voltage, and discharge frequency, must be strictly controlled to generate the most stable plasma.

It was observed that the decrease in AC adsorption capacity resulted from changes in the pore size distribution, surface chemistry, and residual by-products on the AC. Energy consumption analysis suggests that AC regeneration by DBD plasma is a viable technology with significant potential for industrial applications. However, it requires more energy, and efforts to reduce energy costs must be considered for large-scale applications.

3.3. Microwave Plasma-Regenerated AC

In one study, a simple and highly efficient method initiated by microwave plasma discharge was presented, wherein iron-based catalysts were used as microwave susceptors, and metal tips were employed to induce plasma discharge [68]. Microwaves serve as an alternative energy source in chemical reactions and processes by providing energy through dielectric heating, allowing the reaction mixture to be homogeneously mixed without contact with the vessel walls. Microwave heating differs from conventional heating because internal heat is produced by the vibrations of elements or molecules directly heated by microwaves. This form of heating is more efficient than conventional heating as it enables volumetric heating with minimal heat loss to the surroundings (no wall effect) [69].

Experimental Results and Discussion of Microwave Plasma

Foo KY conducted an experiment in which methylene blue (MB)-loaded ACs were immersed in a 500 mg/L MB solution for 24 h to fully saturate the active sites. The saturated ACs were subsequently washed with double-distilled water for 2 h to remove excess MB and then dried overnight at 120 °C. Regeneration was performed in a 600 W domestic microwave oven at a heating frequency of 2.45 GHz, using a Pyrex glass reactor fixed inside the microwave chamber. Nitrogen gas was purged into the reactor at a preset flow rate (300 cm³/min) before the start of microwave heating and maintained throughout the activation stage. The heating times for PFAC, EFBAC, and PSAC were 3, 4, and 5 min, respectively [70]. Xin et al. [71] investigated the regeneration of waste ACs containing anthraquinone dye through microwave irradiation. The adsorption isotherms and kinetics were analyzed by batch adsorption experiments, with MB dye and malachite green (MG) dye selected as model adsorbates.

The effects of microwave heating power, heating temperature, and holding time on the adsorption capacity of regenerated AC were investigated. The BET equation, scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), and Raman spectroscopy were used to characterize the spent waste AC and the regenerated AC.

The results showed that microwave heating efficiently preserved the porous structure of the regenerated AC, restoring the original activation sites and adsorption capacity. The specific surface area of the regenerated AC was 634.73 m²/g, and the maximum adsorption capacities for MB and MG were 265.65 mg/g and 366.89 mg/g, respectively. The C–O functional groups on the surface of the AC increased significantly after regeneration. The adsorption isotherm conformed well to the Langmuir model, and the adsorption kinetics were accurately described by the pseudo-second-order kinetic model.

Microwave-assisted activation of carbon can be divided into two parts. During physical activation, reactive carbon atoms from the char are gasified and removed in the form of CO, resulting in the creation of micro- and mesopores, which drastically increase the specific surface area of the material. Microporous carbons with narrow pore size distributions are typically obtained under CO₂ activation, whereas broader pore size distributions with a predominance of larger pores are usually formed under steam treatment. The accessibility of pores for activating agent diffusion and the dominance of reactive control over diffusional control are essential factors for the development of porosity and surface area.

Chemical activation is usually performed simultaneously with the carbonization step. The mechanism of microstructure formation is complex and largely dependent on the activating agent. ZnCl₂ and H₃PO₄ act as dehydrating agents, facilitating the carbonization process and catalyzing the depolymerization/repolymerization reactions of various biopolymers. These agents also serve as templating species, promoting the development of mesoporosity [72]. The generation of microwave plasma relies on microwave energy, which can rapidly elevate the temperature throughout the entire system to approximately 1000 °C. However, successive adsorption/desorption cycles may alter the internal structure of AC because of the partial collapse of the porous framework (structural annealing) or the formation of coke deposits inside the pores, thereby reducing the active surface area and pore volume. This degradation is reflected as a decline in adsorption capacity.

Surprisingly, previous comparative studies showed that microwave regeneration maintained the original adsorption capacity better than conventional regeneration methods [73]. The regenerated graphite exhibited excellent electrochemical performance, with a specific charge capacity of 352.2 mAh/g at 0.2 C, with approximately 81% capacity retention after 400 cycles, comparable to commercially available fresh materials (Figure 5d). Additionally, a specialized device was developed for the continuous plasma regeneration of spent graphite anode materials (Figure 6) [74], demonstrating the potential of this new method for scalable and practical applications. Therefore, this regeneration approach is particularly suitable for AC materials capable of withstanding high temperatures, such as honeycomb AC and coconut shell AC.

Overall, the majority of researchers have utilized oxygen plasma as the discharge gas. However, ammonia, carbon dioxide, atmospheric air, specific gases, such as chlorine and hydrogen sulfide, and neutral gases, such as nitrogen and argon, have also been used as the discharge gas. These plasma activations were conducted under different power conditions (W to kW) and varying treatment times (seconds to hours) using different plasma sources, such as DBD, arc, radio frequency, and microwave, for surface functionalization. Most researchers have uncovered both positive and negative co-relationships between principal parameters and surface functional groups, surface area, porosity, and other surface features, such as roughness and hydrophilicity [75].

4. Summary and Prospects

Plasma-based regeneration of AC offers a promising solution for sustainable waste management and resource recovery. Techniques such as high-voltage pulsed discharge, DBD, and microwave plasma have shown promise, but challenges, such as high energy consumption, structural degradation, and limited scalability, remain. Future research should focus on coupling plasma-based methods with complementary techniques, such as ozone treatment or thermal regeneration, to enhance regeneration efficiency and material durability. Optimizing energy consumption, improving structural preservation, and developing scalable processes for industrial applications are critical steps toward the widespread adoption of plasma-assisted AC regeneration. Overall, plasma-driven regeneration represents a transformative approach, offering a balance between reactivity control and material sustainability in the recycling of AC.

AC saturated with adsorbates is considered hazardous waste. Waste AC not only poses significant environmental risks but also represents a considerable loss of valuable resources. Therefore, regenerating AC for reuse is crucial for both environmental protection and resource conservation. In studies on plasma-based regeneration of AC, techniques such as high-voltage pulsed discharge and DBD have demonstrated high regeneration efficiency with minimal environmental impact under small-scale experimental conditions. However, when processing large volumes of waste AC, plasma technologies exhibit high energy consumption, and the overall energy efficiency remains relatively low. Moreover, while plasma treatment effectively removes organic pollutants attached to the AC surface, it can also damage the pore structure, potentially reducing the material’s adsorption capacity. Recent research by scholars worldwide has primarily focused on the use of standalone plasma technologies for AC regeneration, which, although effective, tends to be costly and often results in unstable adsorption performance after regeneration. To address the challenges of high energy consumption and structural degradation, future strategies should explore the coupling of plasma treatment with complementary regeneration methods to maximize regeneration efficiency and enhance material durability. For example, (a) when using the DBD–Ozone–Biological method, ozone-biofilm treatment can form a thick biofilm on the AC surface, protecting it from plasma-induced damage, maintaining regeneration efficiency, and extending the service life of AC. Additionally, ozone can activate and clear blocked AC pores while oxidizing organic compounds into H₂O and CO₂, thereby reducing environmental pollution. (b) When using thermal regeneration and high-voltage pulsed discharge coupling, considering that high temperatures may degrade AC, relatively low temperatures (100–200 °C) can be applied. Low-temperature conditions do not impair high-voltage discharge effectiveness, while the added thermal energy can synergistically enhance plasma-based regeneration efficiency. (c) When using UV irradiation and microwave plasma coupling, UV irradiation can improve the oxidation processes, thereby further enhancing the regeneration efficiency of AC.

Given the diversity of AC regeneration technologies, and considering factors such as economic feasibility, practicality, and operability, coupling plasma treatment with other regeneration methods emerges as a highly promising direction for future research in plasma-assisted regeneration of AC.