Insights into Activation Pathways of Recovered Carbon Black (rCB) from End-of-Life Tires (ELTs) by Potassium-Containing Agents

This study explores the conversion of recovered carbon black (rCB) from end-of-life tires (ELTs) into activated carbons (ACs) using potassium-based activators, targeting enhanced textural properties development. The research focuses on the interaction between potassium and rCB, with the aim of understanding the underlying mechanisms of rCB activation. The study investigates several parameters of KOH activation, including the KOH/rCB mass ratio (1:3 to 1:6), activation temperatures (700–900 °C), activation time (1–4 h), and heating rate (5–13 °C/min). It also assesses the effects of different potassium salts (KCl, K2CO3, CH3COOK, and K2C2O4) on porosity and surface characteristics of the rCB/ACs. Furthermore, the role of the physical state of KOH as an activator (solid and gas–solid) was examined, alongside a comparative analysis with NaOH to evaluate the distinct effects of potassium and sodium ions. Optimal conditions were identified at an 800 °C activation temperature, a 7 °C/min heating rate, a 1:5 KOH/rCB ratio, and a 4 h activation period. X-ray diffraction analysis showed the formation of several K-phases, such as K2CO3, K2CO3·1.5H2O, K4(CO3)2·(H2O)3, KHCO3, and K2O. The effectiveness of the potassium salts was ranked as follows: KOH > K2C2O4 > CH3COOK > K2CO3 > KCl, with KOH emerging as the most effective. Notably, the gas–solid reaction of KOH/rCB was indicated as a contributor to the activation process. Additionally, it was concluded that the role of KOH in enhancing the textural properties of rCB was primarily due to the interaction of K+ ions with the graphite-like structure of rCB, compared to the effects observed with NaOH. This research introduces novel insights into the specific roles of different potassium salts and KOH activation conditions in optimizing the textural characteristics of rCB/ACs.


INTRODUCTION
Despite the European Union's prohibition on the accumulation of end-of-life tires (ELTs) in landfills since 2009, challenges in enforcement and inadequate infrastructure persist.The number of vehicles increases annually, rising from 1.2 billion since 2014, and it is anticipated that the worldwide count will exceed 1.6 billion by 2024.This growth highlights the escalating challenge of managing the environmental impact of ELTs waste flow.Nowadays, more than 50% of ELTs are disposed of without undergoing any form of treatment, and it is projected that the annual number of discarded tires will reach 1.2 billion by 2030. 1 ELTs can be a source of environmental contamination, which is a result of the release of hazardous substances into soil and water systems during their degradation. 2Furthermore, tires contain natural (∼14−30%) and synthetic (∼14−27%) rubber, fabric/fillers/ accelerators antiozonants (∼10−17%), and a substantial amount of carbon in the form of carbon black (∼20−28%) and steel wire (∼14%). 3Due to ELTs relatively constant chemical content and their general availability, they are a wellsuited resource for the production of activated carbons (ACs) for which the presence of a high carbon content is a prerequisite.The pyrolysis technique has been recognized as the most effective method for recycling ELTs, with no economically viable alternative currently available.Pyrolysis facilitates the breakdown of the organic constituents included in tires, resulting in the formation of pyrolytic char that contains a high proportion of recovered carbon black (rCB) as well as residues of carbonization, as a solid raw material. 4herefore, a purification stage for char is essential to obtain high-value rCB with a large carbon content (up to 95%) and to maximize the yield of ACs.In view of this, the conversion of ELTs into rCB/ACs provides a sustainable approach not only to managing waste flows but also for producing value-added products.
−7 The synthesis of ACs from carbon-based materials involves a sequence of crucial procedures referred to as pretreatment and modification.Pretreatment techniques, including washing, drying, and thermal treatment, are essential for purifying carbonaceous precursors and enhancing their reactivity.−10 As a result, ACs are mainly composed of elemental carbon in an amorphous state and fine-crystalline graphite.Currently available commercial ACs remain costly owing to their reliance on nonrenewable precursors, which increases the demand for ACs that can be produced from a wide range of low-cost raw materials.The benefits of ACs are strictly related to their distinct physicochemical properties like a large specific surface area, a well-defined network of pores, significant mechanical strength, and abundant functional groups with various oxygencontaining compounds that can be tailored for the specific application by activation processes. 11,12n activation process is necessary for achieving the desired properties of ACs. 13,14Activation may be accomplished by several methods, including chemical or physical processes.Chemical activation is more favorable since it requires lower temperature, has higher efficiency, and requires shorter processing time compared to a physical one. 15It involves treatment with oxidizing acids (HNO 3 , H 2 SO 4 , H 3 PO 4 ), salt solutions (ZnCl 2 , MgCl 2 ), or alkalis (KOH, NaOH, K 2 CO 3 ).Different researchers 16−19 proved that the choice of activating agent such as H 2 SO 4 , H 3 PO 4 , HCl, NaOH, and KOH further modulates the properties of the ACs.Each activator operates through distinct mechanisms, impacting pore development and surface chemistry in unique ways.Furthermore, the activation parameters along with the carbonization process play a pivotal role in shaping the structure and characteristics of the resulting activated carbons. 20Factors such as temperature, heating rate, residence time, mass ratio of precursor:activator, and ambient atmosphere exert significant influence on the final product.While elevated temperatures generally lead to higher carbon content and surface area, they also affect pore size distribution and chemical composition.
−26 Furthermore, potassium tends to enhance the activation kinetics, leading to a more efficient thermochemical conversion. 27,28Previous research on KOH and K 2 CO 3 has suggested different chemical reactions to explain potassium effect. 8,29,30−37 This aims to reveal the different interactions of potassium and specific ionic groups and their effects on the AC production process, especially when using diverse precursors.Furthermore, each of these agents interacts with AC precursors in ways that could potentially optimize or enhance specific properties of the resulting ACs, justifying their investigation alongside traditional activators, such as KOH and K 2 CO 3 .
−43 The main goal of these investigations has been to examine the effectiveness of potassium hydroxide (KOH), potassium carbonate (K 2 CO 3 ), phosphoric acid (H 3 PO 4 ), zinc chloride (ZnCl 2 ), sodium hydroxide (NaOH), and sodium carbonate (Na 2 CO 3 ).Among the tested activating agents, the application of KOH has shown significant enhancements in specific surface area (180−820 m 2 /g), porosity development (0.28−1.32 cm 3 /g), and tailored pore size distribution of char/ ACs compared to those of other chemicals.Al-Rahbi and Williams 39 claimed that tires char-based ACs produced by the KOH activation with a mass ratio of 1:3 at a temperature of 900 °C had a more prominent porosity compared to those treated with alternative alkali activating agents, including K 2 CO 3 , NaOH, and Na 2 CO 3 .Hofman and Pietrzak 40 observed that the KOH has a notable positive influence on the creation of ACs from waste tires char that exhibit a highly developed porous structure, mostly composed of micropores, when activated at a temperature of 800 °C.Furthermore, the research performed by Nieto-Maŕquez et al. 38 proved that the rise in the proportion of the KOH to waste tires ratio subsequently resulted in a proportional augmentation in surface area, and total pore volume of ACs.
While these findings offer encouraging insights, their scope is limited, focusing solely on examining the dependence of temperature (550−900 °C) and char/activator mass ratio (1:0.5 to 1:4).It did not thoroughly examine how Kcontaining agents interact with char carbon atoms.Additionally, the studies neglected the impact of other critical activation factors (heating rate or activation time), the formation of specific chemical phases during the thermal process, and the effects of KOH phase transitions.This focus highlights a significant gap, underscoring the need for in-depth research into rCB activation mechanisms.−46 Most proposed mechanisms involve the presence of potassium in a metallic state, produced at high temperatures (800−900 °C).Metallic K is capable of intercalating into the carbon framework, creating intercalation compounds that leads to the expansion of graphitic lattice. 47,48his work aims to provide insights into the mechanism during the activation of rCB to ACs by potassium-containing activators.The main goal is to improve the textural properties of the rCB/ACs.KOH was selected as the baseline chemical to examine the effect of the KOH/rCB mass ratio, temperature, activation time, and heating rate influence on the characteristics of produced rCB/ACs, as suggested in the literature.It is recognized that different AC precursors might require tailored activation parameters to optimally develop their porous structures and surface characteristics.The research thoroughly explores these interactions, examining how changes in the KOH activation parameters impact rCB.Then, for the assumed standard parameters, different potassium salts, including KCl, K 2 CO 3 , CH 3 COOK, and K 2 C 2 O 4 •H 2 O, were employed to investigate the potential synergistic role of K + with specific functional groups (OH − , Cl − , CO 3 ) in the development of porosity and their effect on the rCB/AC surface.Additionally, the impact of the state of the activating agent (solid-state or gaseous-state reaction) and a comparative analysis with NaOH (Na + vs K + ) were explored.Furthermore, it is crucial to test industrially produced material to adapt laboratory findings for industrial-scale production, ensuring that the process is viable and effective on a larger scale.
Herein, the key objectives of this work were to fill this gap in knowledge by (1) revealing the optimal conditions for the rCB activation using K-based activators; (2) establishing the responsible factors in the conversion of rCB to ACs using potassium alkalis for porosity development; and (3) uncovering the generic mechanism driving rCB activation to efficiently convert the ELTs into ACs.Through laboratory experiments and material characterization, we provide potential activation pathways of rCB conversion to ACs with emphasis put on interactions with potassium.Understanding the potassium interaction with the rCB surface and optimizing the activation of potassium-containing agents of rCB has important implications as it can improve the production of ACs, aiding sorption application, and promote sustainability by using waste materials like ELTs.

Chemicals and Materials.
The reagents used for the synthesis of rCB/ACs were KOH, K 2 CO 3 , KCl, CH 3 COOK, K 2 C 2 O 4 •H 2 O, and NaOH with a purity of 97%.The activated carbon precursor employed in this study was tailor-made rCB obtained from a company in Poland.The production process involved treating char through devolatilization, milling, and, if necessary, pelleting to reduce the ash content, remove metal impurities, and normalize variations in the particle size.The chemical composition of rCB is notably variable and contingent on the composition of the ELTs subjected to industrial-scale pyrolysis.Carbon stands out as the principal component, ranging from 92% to 99.5 wt %, as shown in Table 1.Additionally, rCB may encompass volatile substances (>0.2 wt % content) and mineral matter (0.5−2 wt % content), including elements like Si, Al, Zn, S, and Ca.

Preparation of rCB-Based Activated Carbons.
The methodology employed in this study involves testing several key factors of KOH activation, including the reaction phase, mass ratio, temperature, heating rate, and activation time, as these are known to have a significant impact on the overall process.By systematic examination of these variables, the activation mechanism can be followed and optimized.The materials were exposed to tube furnace conditions.About 8 g of mixture of rCB and activator agent was placed in alumina crucibles and located in the center of the horizontal furnace, as presented in Figure 1.Two types of set rCB activators were tested: gas−solid reactions (activator salt as gas) and solid/ liquid−solid (activator salt as solid) reactions.In the case of a gas−solid reaction, the reactants were separated into two crucibles, accordingly.Then, the sample was heated under a continuous flow of N 2 gas at a rate of 200 cm 3 /min, based on the previous research related with 49 and in accordance with recommendations from the existing literature.50−52 The temperature, ranging from 700 to 900 °C, depending on the experimental setup was achieved by a heating rate of 5−13 °C/min and then maintained for a duration of 1−4 h (Table 2). Following that, the sample was exposed to a controlled cooling process, gradually reducing its temperature to correspond with 25 °C at a rate of 5 °C/min.
For the solid-state activation reaction, the rCB was dry mixed in a mortar with specific potassium-containing (KOH, K 2 CO 3 , KCl, CH 3 COOK, and K 2 C 2 O 4 •H 2 O) or sodiumcontaining (NaOH) activating agents in the solid−solid phase.The mass ratios between the precursor and the chosen salts ranged from 1:3 to 1:6.Following the completion of the activation step under the selected reaction conditions, half of the resultant solid material was rinsed with water until achieving a neutral pH of the washing solution in order to eliminate salt leftovers and evaluate the textural characteristics of the obtained activated carbons.Simultaneously, the other half of the material that was left untreated was used for the investigation of the carbon−potassium reaction mechanism through X-ray diffraction (XRD) analysis and scanning electron microscopy coupled with energy-dispersive X-ray spectroscopy (SEM-EDS).Finally, the rCB/ACs were subsequently dried at a temperature of 110 °C for a duration of 12 h.The samples were labeled using coding: activating agent, carbonization temperature, and residence time during activation (Tables 3 and 4).For example, the notation KOH_1:3_700_3_5 represents a rCB/AC sample that has undergone activation with KOH in the mass ratio 1:3 at 700 °C for 3 h with a heating rate of 5 °C/min.In line with the published literature concerning the reutilization of pyrolytic char to produce ACs, the baseline activation parameters for the study objectives were determined to be 800 °C, 1:4, 3 h, and 5 °C/min.It should be noted that these parameters were selected by assuming the ACs were not further manipulated to optimize their textural properties.

Characterization Techniques of Carbon Materials.
The analysis of the chemical composition of recovered carbon black was conducted using a FLASH 2000 CHNS Elemental Analyzer from Thermo Fisher Scientific.
The most important qualities of carbon materials are their textural characteristics, such as the surface area, pore volume, and pore size distribution, as well as their crystallographic structure.Thus, these aspects play a crucial role in defining the activation process and are the first parameters to follow in the rCB in order to evaluate a new activation procedure.
The enhancement of textural properties of prepared rCB/ ACs was measured by N 2 adsorption−desorption at 77 K using the fully automated, three-station surface area and porosity analyzer Tristar II 3020 Micromeritics.Prior to conducting adsorption tests, the samples were left in a degassed chamber overnight at a temperature of 250 °C.To calculate the specific surface area of samples (S BET ), the experimental data was fitted where Q represents the mass of gas adsorbed at a given relative pressure (P/P 0 ), P is the pressure of the nitrogen gas used in the adsorption experiment [mmHg], and P 0 is the saturation pressure of nitrogen gas [mmHg].Additionally, the selection of a proper P/P 0 range was based on a modification of the BET theory proposed by Rouquerol et al. 53,54 to obtain the highest linear relationship in regard to the coefficient of determination (R 2 ∼ 1).
The total pore volume (V TOT ) was determined from the N 2 adsorption isotherm at a high relative pressure, typically close to unity (0.99 or higher).The examination of micropore volume (V MIC ) within the range of pore diameters from 1.4 to 2 nm and pore size distribution (PSD) was conducted using the density functional theory (DFT) approach, specifically by applying the nonlocal density functional theory (NLDFT) model.Furthermore, the mesopore volume (V MES ) was estimated by subtracting the micropore volume from the total pore volume, utilizing the assumption that mesopores (2−50 nm) constitute the difference between these two volumetric measurements.
The crystallographic structure of the rCB/ACs was examined by X-ray diffraction analysis using a Bruker D8 Discover X-ray diffractometer.The particles were finely powdered before the patterns were recorded.Cu Kα radiation was applied (λ = 0.15418 nm) as the radiation source, and the   diffraction patterns were recorded at regular 2θ intervals covering 10−85°.The identification of the relevant phases created during activation was performed using the DIFFRA-C.EVA software, with reference to the PDF-4 + 2022 database (ICDD), which was also used for semiquantitative analysis.Scanning electron microscopy with an energy-dispersive Xray spectroscopy system was used to analyze the distribution of potassium on recycled rCB/ACs processed with different potassium-containing activators.The analysis was conducted using a JEOL 7800F Prime SEM, operating at 20 kV with a solid-state detector (SSD), utilizing backscattered electrons (BSE).The primary objective of the SEM-EDS analysis was to map the spatial distribution of K on the surfaces of rCB/ACs and to quantitatively evaluate the surface elemental composition.This approach provides insights into the effectiveness of the activation process in incorporating potassium into the carbon structure of the materials.

Effect of KOH Activation Parameters on rCB/ACs
Textural Properties.The optimization of KOH activation parameters, including the temperature (Figure 2a), mass ratio  (Figure 2b), residence time (Figure 2c), and heating rate (Figure 2d) emerges as a critical aspect of examining the limitations of the rCB precursor.Based on the obtained N 2 isotherms for each parameter, the development of the porosity can be followed, which indicates the type of structure of the material that evolves as a result of activation.According to the IUPAC classification, all curves exhibit a mixture of type I and type IV isotherms, indicating a bimodal pore system in the rCB/ACs.This combination suggests the existence of both micropores (<2 nm) and mesopores (2−50 nm). 55The presence of micropores was confirmed by the significant N 2 adsorption found at a P/P 0 below 0.05.Furthermore, the hysteresis loops have a nearly narrow, horizontal, and parallel shape throughout a broad range of P/P 0 (up to 0.85) with a pointed end, proving the H3-type of isotherm.The H3-type implies that the main pore morphology in all rCB/ACs samples involves slit-shaped pores with a well-developed mesoporous structure. 56The desorption isotherm exhibited a hysteresis loop as a result of capillary condensation occurring in mesopores. 57he N 2 adsorption−desorption isotherms were further applied for the determination of the textural properties of all rCB/AC samples along with the related impact of the activation parameters on them, including the specific surface area, total volume of the pores, and mesoporosity and microporosity content (Table 5).The rCB had a very low BET surface area of 55 m 2 /g, lacking micropores, and possessing a limited total pore volume of 0.170 cm 3 /g.The KOH-activated rCB exhibited both micro-and mesoporous characteristics, with the mesopore percentage (determined by Brunauer, Emmett, and Teller method using the Rouquerol criteria.b V TOT calculated by the N 2 adsorption isotherm at a high relative pressure (∼0.99).c DFT method by the NLDFT model.the V MES /V TOT ratio) mostly exceeding 60%.These findings demonstrate that the KOH benefits increased porosity with the BET surface of rCB/ACs.Based on Table 5, each examined activation parameter (temperature, mass ratio, activation time, and heating rate) is expected to fall within a certain range to influence the obtained textural properties of the rCB/ACs optimally.Furthermore, through the experiments performed, one can gain an understanding of how each of the varied parameters influences the overall activation mechanism, as shown in Table 3.−40 As demonstrated previously, the optimal conditions, detailed in Figure 3 highlight the critical role of activation parameters in enhancing the quality and efficiency of rCB/ACs production.
The temperature dependence can be followed when the temperature increased from 700 to 800 °C.As a result of the increased temperature, the KOH activation reaction and diffusion rate with carbon atoms become more intense, leading to the creation of the pore skeleton (0.605−0.743 cm 3 /g), the development of microporosity (0.225−0.303 cm 3 /g), and increase in the specific surface area (688−945 m 2 /g).In the range of 700−800 °C, K + ions can penetrate the carbonized surface more effectively, which leads to the growth of micropores.On the other hand, the temperature dependence observed during the activation of rCB between 800 and 900 °C can be related to the intensified activation of carbonaceous rCB that forms wide interconnected mesopores (0.901 cm 3 /g), which partially degrades the specific area to 915 m 2 /g and decreases the microporous structure to 0.288 cm 3 /g.This observation aligns with the conclusions reported by Serafin et al. 58 In addition, Hofman and Pietrzak 40 also proposed that the ideal temperature for the pyrolytic char is 800 °C, with a KOH/rCB ratio of 1:4.Other authors, Al-Rahbi and Williams 39 indicated that a temperature of 900 °C leads to higher effectiveness in achieving the desired S BET , V TOT , and V MIC , despite the use of a lower mass ratio.That fact potentially implies a significant correlation between these two factors.
Further, when considering the impacts of the rCB/KOH mass ratio (1:6, 1:5, 1:4, and 1:3), the optimized values of S BET (1025 m 2 /g), V TOT (0.901 cm 3 /g), and V MIC (0.901 cm 3 /g) were found to be 1:5.The 1:4 ratio shows a decrease in all properties compared to 1:5, reinforcing the importance of the correct amount of KOH for achieving optimal activation.The performance decreases further at a 1:3 ratio, indicating insufficient activation, while the 1:6 ratio suggests diminishing returns with excess KOH.At ideal levels, KOH efficiently reacts with rCB, ensuring a balanced reaction and the appropriate quantity of KOH for the formation and stabilization of the desired pore structure.However, beyond a certain threshold (≥1:6), excessive KOH can lead to the blockage of pores due to deposition of potassium compounds within them. 59or different activation periods (1−4 h), 4 h was the most effective time interval for producing rCB/ACs, resulting in an enhanced surface area (1028 m 2 /g), micropore volume (0.332 cm 3 /g), and a more evenly distributed range of pore sizes (0.874 cm 3 /g).A longer period of time (3−4 h) allows for the complete realization of chemical processes, such as carbonate formation and gasification, required for opening and expanding pores in rCB/ACs.This is directly connected to the overall reaction rate. 60Conversely, shorter activation durations (≤3 h) may not provide enough time for these processes to occur fully (788−945 m 2 /g, 0.699−0.743cm 3 /g).
Upon comparing the heating rates in the range of 5−13 °C/ min, it is evident that the best results for S BET and developed pore structure, of 970 m 2 /g and 0.774 cm 3 /g, respectively, were attained at 7 °C/min.The consistent trend across different properties indicates a measurable, yet low, impact of the heating rate on rCB/ACs properties.This particular rate achieves an optimal equilibrium between the dispersion of heat and improved kinetics.It is fair to assume that heating rates below 7 °C/min would provide improved contact between the carbon and the molten KOH (>360 °C) before the desired reaction temperature is attained.Moreover, KOH activation leads to the formation of surface oxygen complexes, causing carbon gasification and the release of gases like CO 2 and CO.The use of a particular heating rate threshold contributes to a more regulated release of gas, which might possibly compensate for the enhanced formation of micropores (0.314 cm 3 /g), as described by Lozano-Castello et al. 61 Whereas a rate of 5 °C/min may be insufficient, impeding the growth of pores, 10 and 13 °C/min may be excessive, posing a danger of irregular porosity and significant harm to the structure.
The pore size distribution of rCB/ACs for each KOH activation parameter is schematically shown in Figure 4a−d, which proves the previous observations.A comparable PSD has been identified for all carbon samples that were analyzed.The application of KOH as an activator led to an enhancement in microporosity that confirmed previous observations (∼30− 40% enhancement).The content of micropores in rCB/AC was significantly improved at a temperature of 800 °C, rCB/ KOH ratio of 1:5, heating rate of 7 °C/min, and a time of 4 h, illustrating that the pore size was mainly concentrated in three distinct peaks, situated in the ranges of 0.99−1.09,1.09−1.30,and 1.43−1.63nm.Furthermore, it was found that the mesopores had a dominating size located within the range of 2.0−13.49nm.
In continuation of previous discussions, a comparison of activated carbons synthesized from various precursors via KOH activation is presented in Table 6, showing the textural properties, including BET surface area and porosity, as reported in the literature.It is observed that the synthesized rCB/ACs compare favorably with others in terms of S BET , V TOT , and V MIC values.In the case of V MIC /V TOT , a significant distinction exists, which may be related to the fact that rCB features a rigid, spherical, and aggregated structure. 49This configuration starkly contrasts with the "model" texture that was earlier identified as optimal for KOH activation, which should ideally develop a high content of micropores. 42The closed nature of the rCB structure likely restricts potassium accessibility, thereby hindering its penetration and subsequent reactions within the particles.Nevertheless, limited amounts of potassium can still infiltrate the interior of these particles, fostering porosity through both carbon consumption and intercalation.

Analyzing the Interplay of Factors in the KOH Activation Process. Figure 5a−d displays the XRD patterns
of rCB/ACs prior to the washing treatment, providing a thorough examination of the evolution characteristics of KOH throughout activation.The spectra of the rCB/AC products reveal that the primary phases identified in rCB/ACs are K 2 CO 3 , K 2 CO 3 •1.5H 2 O, K 4 (CO 3 ) 2 •(H 2 O) 3 , KHCO 3 , and K 2 O, confirming the presence of carbonates/bicarbonate and oxides as crucial components.According to the existing literature, it is well-acknowledged that carbon elements in precursor materials may initially engage in redox solid−solid interactions, KOH dehydration at 400 °C, and subsequently progress through a solid−liquid state, as detailed in eq 2. 71 Furthermore, the thermal treatment (<700 °C) might result in an excess of compounds from carbonaceous materials, such as carbon monoxide (CO), hydrogen (H 2 ), carbon dioxide (CO 2 ), and water vapor (H 2 O) as described in eqs 3−5, supporting previous reported findings. 72Ultimately, the procedure can involve metallic potassium obtained from decomposed K 2 O and K 2 CO 3 at temperatures over 750 °C that intercalates into the carbon matrix and is mainly responsible for generating the pore network (eqs 6 and 7).The initial interaction of K with the C-matrix is followed by many supplementary reactions between KOH and carbon, including distinct active intermediates, as proposed by Otowa et al. + + (5) To delineate the trajectory of chemical evolution more clearly during the KOH treatment of rCB/ACs, the mechanism of rCB activation is outlined based on the applied experimental setup.This confirms and expands on existing proposals in the field as following.system can influence the overall process as well, potentially altering the temperature required for effective decomposition.K 2 O can further react with atmospheric CO 2 , leading back to the formation of K 2 CO 3 .
• Formation of KHCO 3 The presence of the KHCO 3 phase is notably indicated in Figure 5a,b,d.Theoretically, its formation in a reversible reaction and stability can be influenced by the partial pressure of CO 2 and the moisture content in the environment.Reversibility mainly refers to the ability of KHCO 3 to undergo decomposition into KOH and CO 2 (eq 14), resulting in the establishment of a dynamic equilibrium KOH CO KHCO  7 provides an overview of the textural properties of rCB/ACs activated through different K-containing salts, such as CH 3 COOK, K 2 CO 3 , K 2 C 2 O 4 , and KCl.Despite the KOH_1:4_800_3 sample, K 2 C 2 O 4 _1:4_800_3 stood out with the highest BET surface area (299 m 2 /g), total pore volume (0.369 cm 3 /g), and micropore volume (0.093 cm 3 /g) among the samples.On the other hand, KCl_1:4_800_3 exhibited the lowest values among the tested samples, such as 56.7 m 2 /g, 0.189 cm 3 /g, and 0.011 cm 3 /g, for BET, pore volume, and micropore volume, respectively.Moreover, all rCB/ACs share the same type of N 2 isotherm as KOH, emphasizing uniform adsorption behavior across the studied activators (Figure 6a).It should be noted that their pore size distribution is characterized by a high proportion of mesopores, with clear peaks in size that vary from 2.8 to 13.5 nm, and a broader peak extending up to 30 nm and further to 48 nm (Figure 6b).In addition, rCB/ACs synthesized with potassium oxalate used as an activating agent have exhibited the most developed microporosity among all activators, with the range identified to be between 1.2 and 1.94 nm.
By examining the synergetic role of both potassium ions and its related anions (OH , and COOK − ) in several chemical compounds, it becomes evident that K-based compounds serve as a powerful activator of rCB/ACs.The effectiveness of potassium-containing agents in generating pore structures within a carbon matrix can be influenced by several factors, the types and amounts of gases released during decomposition, the control over the pore generation step, and the specific chemical reactivity of the compound under the activation conditions.Among the four activators, the development of textural properties can be ranked in the following order: KOH > K 2 C 2 O 4 > CH 3 COOK > K 2 CO 3 > KCl.The observed findings prove the collaborative specific impact of K and its related anion groups on the formation of a porous Brunauer, Emmett, and Teller method using the Rouquerol criteria.b V TOT calculated by N 2 adsorption isotherm at a high relative pressure (∼0.99).c DFT method by the NLDFT model.structure and intercalated phases during the overall activation reactions, as given in Figure 7.
Upon reviewing the data of rCB/ACs from Table 7, the efficiency of KOH is clearly noticed, as suggested by the existing literature. 75The high molar content of K and the presence of OH − groups are likely responsible for the enhancement of textural characteristics, especially microporosity.The potassium hydroxide undergoes dissociation during high-temperature activation, resulting in the formation of potassium cations (K + ) and hydroxide (OH − ) ions.Hydroxyl ions have the ability to chemically attack and weaken the bonds in the carbon substrate, releasing volatile substances, thus efficiently converting the rCB into more porous ACs. 76his is also consistent with the quantitative energy-dispersive X-ray spectroscopy analysis (Table 8) and the corresponding SEM-EDS elemental mapping of potassium images (Figure 8).The EDS image of KOH_1:4_800_3_5 shows a high-intensity potassium signal, suggesting a high concentration of K (29.5 wt %) uniformly distributed throughout the rCB/AC sample.
In contrast, CH 3 COOK, K 2 CO 3 , and K 2 C 2 O 4 have lower K/ C and K/O ratios.As reported by Yang et al., 77 this enables the production of a greater amount of CO 2 and H 2 O during activation, leading to the formation of larger pores and a decrease in specific surface areas.For K 2 CO 3 and CH 3 COOK, the EDS mapping of AC/rCB shows a variance in the K distribution, suggesting inhomogeneity due to the local formation of new K-enriched compounds or the dispersal of volatile products.K 2 CO 3 has lower reactivity toward carbon materials compared to KOH and a much higher melting point.The pure form of K 2 CO 3 melts at 891 °C.At 800 °C, K 2 CO 3 is not expected to be completely liquefied but still can soften, releasing a significant amount of CO 2 (above 650 °C) and K 2 O (below 650 °C), with possible subsequent formation of salt complexes (∼475 °C) on the surface of rCB/ACs (eqs 7−9).On the other hand, the BSE image and EDS map of CH 3 COOK_1:4_800_3_5 show a more heterogeneous distribution of potassium with areas of higher and lower K concentration.Smaller particles and some individual bright spots can be identified, suggesting possible agglomeration or coagulation of potassium-rich phases upon thermal decomposition of CH 3 COOK to K 2 CO 3 (>303 °C), which undergoes a series of reactions with the carbon structure to produce K-species.In the case of K 2 C 2 O 4 , the EDS mapping reveals a more intense and patchy spread of potassium, indicative of its higher overall presence (27.5 wt %) than the smoother distributions observed for K 2 CO 3 and CH 3 COOK.Lastly, KCl has been found to be ineffective as an activator, which is primarily attributed to its melting point (∼770 °C), and weak reactivity with rCB, according to the XRD patterns in Figure 7 and SEM-EDS image in Figure 8.

3.4.
Examining the Phase Impact of KOH Activation: Solid/Liquid State vs Gas State Reactions.The process of activating rCB with KOH requires a complex combination of chemical reactions, including not only the conventional solid− solid (<380 °C) and solid−liquid (≥406 °C) interactions 78 but also a substantial solid−gas reaction.The use of KOH vapor contributes to the enhancement of the rCB/ACs characteristics, as shown in Table 9 and Figure 9a,b.The gaseous KOH etches the carbon surface and creates pores when the temperature exceeds 600 °C, resulting in the release of gases such as CO 2 and H 2 O.This phenomenon induces a substantial modification in the surface and pore morphology of the rCB.The S BET value increased from 55 m 2 /g to 118, while the V TOT increased to 0.239 cm 3 /g.Although the growth of V MIC was not significant (0.033 cm 3 /g), the total change in adsorbed quantity of N 2 and PSD suggests a modification in the texture of the rCB.
The EDS mapping of the rCB sample postreaction with vapor-phase KOH (Figure 10) presents a homogeneous distribution of oxygen, indicating the oxygenation of the carbon structure.Moreover, the rCB/ACs appear to have an irregular K compound distribution from the reaction with vapor-phase KOH.EDS findings directly corroborate with XRD analysis (Figure 11), identifying the formation of KHCO 3 , K 2 O, and K 2 CO 3 within the sample, which are the expected products of the gas−solid reaction.The presence of KHCO 3 (60 wt %), K 2 O (10 wt %), and K 2 CO 3 (30 wt %) in the unwashed rCB/ACs samples indicates that they may be involved in the alterations in textural qualities, including specific surface area and porosity.The formation of KHCO 3 , K 2 O, and K 2 CO 3 is based on the reaction between gaseous KOH and carbon oxidation.The relative content of C (75 wt %), O (17.8 wt %), and K (7.2 wt %) further supports the impact of the gas−solid reaction on rCB/ACs textural properties.
3.5.Comparative Analysis: Potassium (K + ) vs Sodium (Na + ) Ions.The isotherms and textural characteristics presented in Figure 12a and Table 10 indicate that KOH outperforms NaOH in the chemical activation of rCB/ACs.The utilization of NaOH as an activator led to the development of rCB/ACs that possessed S BET , V TOT , and V MIC of 480 m 2 /g, 0.523 cm 3 /g, and 0.146 cm 3 /g, respectively.Furthermore, the improvement in microporosity (Figure  12b,c) may be attributed to the nature of alkali metal cation (K + vs Na + ), as studied by Guo et al. 79 KOH demonstrated a greater propensity to interact with the carbon atoms from rCB, resulting in the formation of micropores in the range of 1.1− 1.64 nm and mesopores that vary from 2 to 8.7 nm.On the other hand, NaOH exhibited a greater tendency to generate    activation process.Yahya et al. 80 and Rambabu et al. 81 have both reported similar findings.Additionally, the intercalation of alkali metals (Na or K) depends upon the crystallinity of the precursor used during the activation procedure, since the structural arrangement, which was formerly overlooked, plays a crucial role.Linares-Solano, Lillo-Rodenas, Marco-Lozar et al. 82 stated that NaOH is more suitable for carbons with poor ordered atom arrangement (nongraphitic structure), whereas KOH produces better results for those displaying some organizational regularity (graphite-like structure).Based on this understanding, it can be assumed that rCB/ACs tend to have a more crystalline phase than an amorphous one.This makes KOH a preferred choice as an activator due to its effectiveness in engaging with and enhancing the organized, graphite-like arrangement of atoms.
The comparison of EDS mapping between NaOH-activated rCB (Figure 13) and KOH-activated rCB (Figure 10) and semiquantitative analysis suggest that NaOH activation results in a lower distribution of alkali metal (Na: 19.8 wt %) compared to KOH (K: 29.5 wt %) but similar oxygen content (O: 37.8 wt % vs O: 38.8 wt %).The XRD analysis presented in Figure 14, which detected only Na 2 CO 3 as the primary phase in the NaOH-treated rCB/ACs, points to a more limited set of chemical reactions in the case of NaOH compared to that obtained with KOH treatment, where KHCO 3 , K 2 O, and K 2 CO 3 were identified.This is likely because K + , with its larger ionic radius (152 pm) and higher reactivity than Na + (116 pm), is more effective at incorporating in the carbon structure of rCB and reacting with CO 2 .On the contrary, the stronger ionic bonds and lower polarizing power of Na + relative to K + might potentially affect the transport and interaction with the carbon matrix.Therefore, potassium not only promotes more reactions but also leads to more pronounced porosity and surface complexity.

CONCLUSIONS
In this study, the activation mechanism of rCB using potassium-based activators was explored to efficiently transform ELTs into ACs.The objectives were to determine the optimal activation conditions and analyze textural changes, explore the interactions between rCB and potassium alkalis, and unveil a universal process for efficient rCB activation.Techniques such as N 2 adsorption−desorption at 77 K, XRD, and SEM-EDS were used to investigate the structural and chemical properties of the rCB/ACs.The study identified the below.
• The most effective KOH activation parameters for enhancing the textural properties of ACs were determined, which include an activation temperature of 800 °C, a heating rate of 7 °C/min, a KOH to rCB ratio of 1:5, and a process duration of 4 h.These Brunauer, Emmett, and Teller method using the Rouquerol criteria.b V TOT calculated by the N 2 adsorption isotherm at a high relative pressure (∼0.99).c DFT method by the NLDFT model.conditions were found to be optimal for achieving the desired improvements in specific surface area, total pore volume, and micro/mesoporosity.• XRD patterns of the rCB/ACs products revealed the presence of key phases such as K 2 CO 3 , K 2 CO 3 •1.5H 2 O, K 4 (CO 3 ) 2 •(H 2 O) 3 , KHCO 3 , and K 2 O.This confirmed the significant role of carbonates, bicarbonates, and oxides in the activated products, indicating a complex chemical transformation during the activation process.This order of importance was shown to be correlated with the potassium content and its distribution, as revealed using SEM-EDS analysis.The rank of activation potency, from highest to lowest, was as follows: KOH > K 2 C 2 O 4 > CH 3 COOK > K 2 CO 3 > KCl.
• The impact of the KOH gas−solid interaction with rCB suggested that improvements are not solely attributed to liquid/solid−solid reactions.• The KOH activation was primarily attributed to the nature of the alkali metal cation and graphite-like structure of rCB.This emphasizes the importance of the specific chemical characteristics of the activating agent in influencing the final properties of the rCB/ACs.
In essence, the research highlights effective strategies for producing rCB/ACs with superior textural properties, underscoring the significance of carefully chosen activation conditions and the role of potassium-containing activating agents.Moreover, it sets the stage for future studies to explore the potential environmental benefits of utilizing such materials in specific sorption applications, such as water and air purification.

Figure 1 .
Figure 1.Schematic diagram of the experimental process setup applied for solid/liquid-solid state activation (a) and gas-solid state activation (b).

Figure 2 .
Figure 2. N 2 adsorption−desorption isotherm curves at 77 K of the rCB/ACs in relation to the activation effect of temperature (a), mass ratio (b), time (c), and heating rate (d).

Figure 4 .
Figure 4. Pore size distribution of the rCB/ACs based on the DFT method of N 2 adsorption at 77 K on carbon slit pores by the NLDFT model in relation to the activation effect of temperature (a), mass ratio (b), time (c), and heating rate (d).

K 4 (
CO 3 2 •(H 2 O) 3 , it is most likely produced through a series of reactions involving KOH, CO 2 , and water vapor, under the influence of temperature, KOH concentration, and humidity.• Decomposition of K 2 CO 3 to K 2 O.The XRD patterns of rCB/ACs in Figure 5a unequivocally prove the detection of K 2 O within the rCB/AC samples KOH post-thermal treatment at 800−900 °C.The presence of K 2 O, identified in this specific temperature range, provides empirical evidence supporting the breakdown of K 2 CO 3 into K 2 O and CO/CO 2 .This reaction is a thermally driven process that often occurs at temperatures higher than the standard melting point of K 2 CO 3 (>700 °C), as shown in eqs 11−13.In addition, the transformation of KOH into K 2 O and H 2 O occurs at elevated temperatures.The presence of other impurities in the activation

Figure 5 .
Figure 5. XRD patterns for rCB/ACs before washing treatment related to the effect of KOH activation parameters: temperature (a), mass ratio (b), activation time (c), and heating rate (d).

Figure 6 .
Figure 6.N 2 adsorption−desorption isotherm curves at 77 K (a) and PSD based on the DFT method on carbon slit pores by the NLDFT model (b) of the rCB/ACs obtained through the utilization of CH 3 COOK, K 2 CO 3 , KCl, and K 2 C 2 O 4 , as potassium-containing activating agents.

Figure 8 .
Figure 8. SEM-EDS elemental mapping of potassium for rCB/ACs before washing treatment obtained through KOH, K 2 CO 3 , CH 3 COOK, KCl, and C 2 K 2 O 4 activation.

Figure 9 .
Figure 9. N 2 adsorption−desorption isotherm curves at 77 K (a) and PSD based on the DFT method on carbon slit pores by the NLDFT model (b) of the rCB and rCB/ACs produced through gas−solid reaction during KOH activation.

Figure 11 .
Figure 11.XRD pattern for unwashed rCB/ACs produced through a gas−solid reaction during KOH activation.

Figure 12 .
Figure 12.N 2 adsorption−desorption isotherm curves at 77 K (a), PSD based on DFT method on carbon slit pores by the NLDFT model for the pore diameter in the ranges of 1.1−2 nm (b) and 2−30 nm (c) of the rCB/ACs obtained by KOH/NaOH activation.

Figure 13 .
Figure 13.EDS analysis of the NaOH-activated rCB; (a) representative EDS image and corresponding elemental mapping analysis: C (b), Na (c), and O (d).

Table 1 .
Chemical Composition of rCB in wt % to the Brunauer−Emmett−Teller (BET) equation in a relative pressure range of P/P 0 from 0.05 to 0.3, as presented below

Table 2 .
Experimental Conditions for Activation Processes type of activation heating rate [°C/min] temperature [°C] activation time [h] flow rate of N 2 [cm 3 /min] cooling rate [°C/min]

Table 3 .
Overview of rCB/ACs Sample Symbol Sets for Porosity Development aThe study's base activation conditions were set at 1:4 ratio, 800 °C, 3 h, and 5 °C/min.

Table 4 .
Description of rCB/ACs Sample Symbol Sets for Research Goals

Table 5 .
Textural Properties Associated with the Produced rCB/ACs by Specific Activation Parameters a

Table 6 .
Comparison of Textural Characteristics of ACs Prepared from Various Precursors by KOH Activation • Multistage transformation to K 2 CO 3 , K 2 CO 3 •1.5H 2 O, and K 4 (CO 3 ) 3 •(H 2 O) 3 .The resulting peaks of K 2 CO 3 , K 2 CO 3 •1.5H 2 O, and K 4 (CO 3 ) 3 •(H 2 O) 3 phases for each activation parameters are clearly distinguishable in Figure5a−d.For all of the examined activation parameters, the reaction might occur at 400 °C with KOH consumption by majority of the carbon atoms in rCB.This leads to a release of H 2 , K, while carbon is oxidized to form carbonates, as presented in eq 8.As a second step, pores are generated through the process of carbon gasification in the presence of CO 2 , effective at temperatures below 700 °C.Concurrently, this phase facilitates the formation of K 2 CO 3 , a reaction notably influenced by the textural properties of the raw material.

Table 7 .
Textural Properties Associated to rCB/ACs Produced via Chemical Activation with CH 3 COOK, K 2 C 2 O 4 , K 2 CO 3 , and KCl

Table 8 .
EDS Semiquantitative Analysis of rCB/AC Samples

Table 9 .
Textural Properties Associated with rCB/ACs Produced through Gas−Solid Reaction during KOH Activation Brunauer, Emmett, and Teller method using the Rouquerol criteria.b V TOT calculated by N 2 adsorption isotherm at a high relative pressure (∼0.99).c DFT method by the NLDFT model. a

Table 10 .
Textural Properties Associated with rCB/ACs Obtained by NaOH Activation abc a