Surface and Structural Characterization of Buckwheat Husk-Derived Activated Carbons: Correlation of SEM, Elemental, FTIR, Raman, and Porous Properties with Electrokinetic Behavior

Abstract

This study focuses on the synthesis and characterization of buckwheat husk-derived activated carbon, chemically activated with potassium hydroxide (KOH) and subsequently modified with urea and Prussian Blue (PB). The obtained carbons were evaluated in terms of particle-size distribution, surface morphology, structural features, and electrokinetic properties using scanning electron microscopy coupled with energy-dispersive X-ray spectroscopy (SEM–EDS), Fourier-transform infrared spectroscopy (FTIR), Raman spectroscopy, and electrophoresis, as well as N₂ adsorption–desorption (BET surface area and porosity analysis). The results confirmed that both pyrolysis conditions and the type of modifier significantly affect the physicochemical properties of the activated carbon and its behavior in electrolyte solutions. Colloidal stability and particle size were strongly dependent on pH and the type of anions present in solution, with sodium nitrate (NaNO₃) systems showing higher stability than sodium chloride (NaCl). Modification with KOH and urea imparted a more basic surface character, whereas PB introduced more acidic properties. All samples exhibited predominantly negative surface charges and mesoporous structures, which are favorable for adsorption processes and enhance affinity for heavy-metal cations. Among the tested materials, BH-KOH-Fe (Fe-modified KOH-activated carbon) showed the most favorable performance for the targeted application, while BH-KOH (KOH-activated buckwheat husk-derived carbon) exhibited high surface area and good colloidal stability. The prepared materials show promising applicability for water purification, including the removal of organic pollutants and radionuclides (e.g., ¹³⁷Cs and ⁹⁰Sr), as well as metal cations (K⁺, Na⁺, and Li⁺).

1. Introduction

The interface between carbon-based materials and electrolyte solutions plays a key role in various electrochemical processes, including energy storage, catalysis, and water purification. At this interface, an electrical double layer (EDL) forms, whose structure and properties govern electrostatic interactions and ion adsorption. The electric double layer (EDL) formed at the interface of carbon materials and electrolytes plays a critical role in ion adsorption and electrokinetic behavior, which directly influence water purification performance and colloidal stability. Previous studies have demonstrated that the accumulated interfacial charge and capacitance in carbonaceous systems are highly dependent on both the structural features and surface chemistry of the carbon materials [1]. The characteristics of the EDL depend both on the surface properties of carbon (porosity, presence of functional groups, conductivity) and on the composition and concentration of the electrolyte. A fundamental parameter for understanding surface charge phenomena is the point of zero charge (pHpzc), defined as the pH at which the net surface charge of the material is zero. At pH values above the pHpzc, the surface is negatively charged, favoring cation adsorption, while at pH values below the pHpzc, the surface becomes positively charged, promoting anion binding. A complementary parameter is the isoelectric point (pH_IEP), determined from electrokinetic measurements (e.g., zeta potential), which corresponds to the pH at which the electrokinetic potential at the slipping plane vanishes [2]. The difference between pHpzc and pH_IEP provides information on surface functional group heterogeneity, specific ion interactions, and the influence of electrolyte type on surface charge. Furthermore, it is well established that the characteristics of the EDL are influenced not only by porosity and specific surface area, but also by surface functional groups and morphology, which modulate surface charge density, zeta potential, and electrostatic interactions at the solid–liquid interface [1].

Carbon-based materials, including activated carbons, biochars, and graphene derivatives, exhibit complex surface chemistry, containing both acidic and basic functional groups [3,4]. The interaction of these groups with the electrolyte affects the structure and properties of the electrical double layer, and thus the ion adsorption capacity, capacitance, and electrochemical reactivity [5,6]. Despite extensive studies, the relationship between surface morphology, chemical functionality, and EDL characteristics particularly in the context of pHpzc and pH_IEP remains an active area of research [7].

The contamination of aquatic environments with radionuclides (e.g., ¹³⁷Cs, ⁹⁰Sr) and monovalent ions such as K⁺, Na⁺, Li⁺, and Cl⁻ poses significant risks to both ecosystems and human health. Developing effective, low-cost, and sustainable sorbents for their removal remains a pressing research challenge. Among various approaches, adsorption using activated carbon has attracted broad interest due to its high surface area, porosity, chemical stability, and tunable surface chemistry. Importantly, activated carbon derived from agricultural residues not only addresses waste valorization but also contributes to circular economy and environmental sustainability [8].

Activated carbons have been produced from many agricultural residues, and the precursor strongly affects both pore structure and surface chemistry after activation. For example, oil palm empty fruit bunches (EFBs) are a widely reported biomass source: KOH activation has been shown to significantly change the surface chemistry/morphology and porous texture of EFB-derived carbon materials, demonstrating how activation conditions control the final carbon properties [9]. In addition, EFB-based activated carbon has been integrated into practical water-treatment materials (e.g., ceramic membranes), confirming that such biomass carbons are relevant not only for lab studies but also for applied purification systems [10]. A broader overview of biomass-derived activated carbons for wastewater treatment is discussed in the review by Ullah et al., which highlights that adsorption-related performance is typically governed by a combination of pore size distribution/surface area and the type and density of surface functional groups [11]. Based on this background, our study focuses on a methodologically consistent comparison of buckwheat husk-derived activated carbons and links porous texture and surface functionality with electrokinetic/EDL-related parameters (ζ-potential, surface charge density, pHpzc/pH_IEP), which are often reported separately in the literature [12].

In addition to carbon-based sorbents, several alternative adsorption platforms are widely investigated for water remediation. Polymeric adsorbents and functional polymer particles offer tunable chemistry and, in many cases, convenient regeneration pathways, and their recent development for adsorption-based water treatment is summarized in a dedicated review [13]. Polyelectrolyte and hydrogel sorbents are also intensively studied because they can incorporate a high density of charged functional groups; for example, NaSS-based hydrogels containing sulfonate groups have demonstrated very high uptake for cationic pollutants [14]. Biomass biosorbents represent another active direction: marine algae such as Sargassum have been reported to bind metal ions efficiently, including in multi-metal solutions relevant to realistic water matrices [15,16]. Despite these advances, activated carbons remain a practical and scalable adsorbent class, and understanding how their porous texture and surface functional groups control electrokinetic/EDL-related behavior (ζ-potential, surface charge density, pHpzc/pH_IEP) is essential for predicting interfacial interactions and adsorption performance in electrolyte-containing waters [17].

Although the present manuscript focuses on surface/structural and electrokinetic characterization, these parameters are directly linked to adsorption behavior reported for biomass-derived carbons and related porous materials. In particular, a developed porous texture and oxygen-containing surface functional groups (e.g., carboxyl, lactone, phenolic) are repeatedly identified as key factors controlling Pb²⁺ uptake by biomass-derived activated carbons [18]. Similar structure–function relationships are also discussed for modified carbonaceous adsorbents such as hydrochars, where surface chemistry and porosity govern Pb²⁺/Cd²⁺ adsorption mechanisms [19]. Therefore, the characterization outcomes reported here provide the necessary basis for our subsequent application-oriented adsorption studies (including multi-component matrices and radionuclide-relevant ions).

Buckwheat husk, an abundant agro-waste material, represents a promising precursor for activated carbon. Several recent studies have demonstrated the successful use of agricultural biomass, such as rice husk and buckwheat hulls, to prepare high-performance porous carbons with excellent adsorption properties for ions and organic pollutants [20,21,22]. Chemical activation, particularly with potassium hydroxide (KOH), is well known to significantly enhance surface area and pore development, improving adsorption efficiency toward metal ions and organic contaminants [23].

To further tailor the surface chemistry of carbon materials, nitrogen doping through urea modification has proven highly effective. Urea introduces nitrogen functionalities (–NH₂, –C=O) that enhance surface polarity and create additional coordination sites for cation binding. Recent studies confirmed that urea-modified activated carbons show improved sorption performance for toxic compounds due to the formation of stable surface complexes [24,25].

In addition, functionalization with Prussian Blue (Fe₄[Fe(CN)₆]₃) is recognized as one of the most efficient strategies for cesium removal [26]. The open-framework Fe–CN–Fe structure of PB provides ion-exchange sites with high selectivity toward Cs⁺ even in the presence of competing alkali ions. PB-modified composites have achieved remarkable capacities and fast kinetics for Cs⁺ adsorption in aqueous systems, making them promising candidates for the treatment of radionuclide-contaminated waters [27,28].

Beyond cesium and strontium, activated carbons and their modified derivatives also exhibit strong adsorption of Li⁺, Na⁺, K⁺, and Cl⁻, as well as heavy metals, dyes, and persistent organic pollutants, underscoring their multifunctionality. For instance, manganese dioxide (MnO₂)-decorated activated carbon has been used for lithium recovery from aqueous media [29]. Meanwhile, crown ether-functionalized graphene-based carbons (e.g., graphene oxide (GO) membranes and graphene oxide/chitosan/poly(vinyl alcohol) (GO/CS/PVA) nanofibers) exhibit selective sorption of alkali ions in competitive solutions [30]. More broadly, biomass-derived activated carbons are effective against heavy metals and diverse organic pollutants [18,31,32].

In this context, this work focuses on the synthesis of activated carbon derived from buckwheat hulls, activated with KOH and subsequently modified with urea and Prussian Blue. It also determined its structure and properties in dispersed systems, which are crucial for its application in the removal of radionuclides (Cs, Sr) and elemental ions (K⁺, Na⁺, Li⁺, Cl⁻). By combining waste valorization with advanced surface functionalization, this study aims to demonstrate the potential of a sustainable, multifunctional sorbent for integrated water treatment. The aim of this study is to investigate the surface charge properties of carbon materials in aqueous electrolytes, with emphasis on the relationship between the electrical double layer structure and key surface parameters such as pHpzc and pH_IEP. By combining surface characterization with electrokinetic measurements, we aim to achieve a comprehensive understanding of how chemical composition and surface modification influence charge distribution and ion adsorption at the carbon/electrolyte interface.

2. Results

2.1. Morphological and Elemental Characterization (SEM–EDS Analysis)

Morphological Analysis of BH After KOH Activation (SEM)

Figure 1a,b shows SEM micrographs of the pristine carbonized sample (BH 500). The surface is relatively compact and layered, with limited visible pore development at the micrometer scale, representing the baseline morphology prior to chemical activation. After KOH activation (Figure 1c,d), the material exhibits a markedly rougher and more heterogeneous surface with numerous cracks, cavities, and open pores, indicating intensive etching and pore formation. The urea-modified sample (BH+KOH+urea, Figure 1e,f) shows a more ordered and channel-like porous texture with improved structural uniformity compared to BH+KOH, suggesting that urea-assisted treatment promotes a more homogeneous porous network. In the Fe-modified sample (BH+KOH+Fe, Figure 1g,h), the carbon framework is preserved, while the surface is partially decorated with fine particulate/crystalline deposits, consistent with successful immobilization of the modifier and partial pore-wall coverage.

For clarity, the main SEM-derived morphological characteristics and their expected impact on material properties are summarized in

Table 1. Morphological comparison of activated carbon samples after different activation and modification treatments (SEM observations).

Sample	Morphological Features	Structural Characteristics	Expected Impact on Properties
BH 500	Dense/lamellar fragments; relatively smooth surface; limited visible macropores.	Compact carbon matrix; baseline porosity before activation.	Lower accessible surface area and sorption potential compared to activated samples.
BH+KOH	Irregular, rough surface; numerous cracks, channels, and randomly distributed pores.	Heterogeneous, fragmented structure with micro- and mesopores.	High surface area; good sorption capacity but less structural stability.
BH+KOH+urea	More ordered, channel-like pores; smoother fragmentation; higher integrity of the carbon matrix.	Homogeneous porous network with improved pore geometry.	Enhanced sorption performance due to uniform porosity; better mechanical stability.
BH+KOH+Fe	Porous carbon framework decorated with small crystalline/particulate deposits.	Partial pore blocking; immobilization of Prussian Blue domains on pore walls.	Reduced accessible porosity, but introduction of selective adsorption sites (e.g., for Cs+, heavy metals).

.

The EDS results (

Table 2. Comparison of the composition of the tested adsorbents.

Sample	C (wt. %)	O (wt. %)	N (wt. %)	K (wt. %)	Fe (wt. %)	Other Elements	Interpretation
BH+KOH	91.61	7.53	–	0.03	0.13	Mg, Al, Si, Zn (≤0.3%)	Highly carbonaceous, minor oxygen groups, residual inorganics
BH+KOH +urea	78.98	8.67	–	0.94	0.33	Zn (9.37%), Mg, Ca, Na	Decreased C, enriched in Zn and K; no detectable N in EDS
BH+KOH +Fe	60.12	11.96	17.66	1.17	6.65	Si, Al (≤1.6%)	PB immobilization confirmed: Fe and N enrichment, lower C

) support the SEM observations and are presented for the activated and modified samples (BH+KOH, BH+KOH+urea, and BH+KOH+Fe). The pristine carbonized sample (BH 500) was used as a morphological reference in SEM and was not included in the EDS comparison. The pristine BH+KOH sample contains mainly carbon (91.61 wt. %) and oxygen (7.53 wt. %), with trace quantities of Mg, Al, Si, K, and Fe, as shown in Table 1. In the BH+KOH+urea sample, carbon content is reduced to 78.98 wt. %, while oxygen increases slightly, and a significant amount of zinc (9.37 wt. %) is detected, which may be introduced from reagents or experimental conditions. Nitrogen was not detected in the BH+KOH+urea sample by EDS. This observation can be explained by the sequence of modification steps: nitrogen-containing groups introduced from urea are partially removed during subsequent high-temperature KOH activation due to thermal decomposition and volatilization of N-species, and/or their low concentration falling below the EDS detection limit. Similar behavior has been reported in the literature for urea-assisted N-doping followed by activation, where the final nitrogen content can significantly decrease after the activation stage.

The BH+KOH+Fe sample exhibits the most pronounced compositional shift, with carbon content decreasing to 60.12 wt. %, and a substantial increase in nitrogen (17.66 wt. %), oxygen (11.96 wt. %), and iron (6.65 wt. %). The presence of nitrogen, in combination with iron and potassium, confirms the successful immobilization of Prussian Blue on the carbon surface. Nitrogen here originates from the cyanide ligands (C≡N) in the coordination complex, which are structurally integrated into the modified surface. The combined SEM and EDS data clearly demonstrate the distinct morphological and chemical changes introduced by each modification approach, as shown in Table 2.

2.2. Structural Analysis by Raman Spectroscopy

Raman spectroscopy was performed to evaluate the structural features of the carbon framework in the investigated materials (Figure 2). The spectra of all samples exhibit two characteristic peaks: the D band, located at approximately 1340 cm⁻¹, which is associated with defects and disorder in the sp²-hybridized carbon lattice and the G band, appearing near 1580 cm⁻¹, corresponding to the stretching vibrations of graphitic carbon domains.

In the BH+KOH sample, the G band intensity is higher relative to the D band, indicating a more ordered structure and a comparatively high degree of graphitization. Modification with urea (BH+KOH+urea) leads to a noticeable increase in the D band intensity and a higher I_D/I_G ratio, suggesting an increased defect density in the carbon lattice. Notably, nitrogen was not detected by EDS for the BH+KOH+urea sample (Table 2); therefore, oxygen functionalities and structural disorder are considered the main contributors.

The BH+KOH+Fe sample exhibits the most pronounced D band relative to the G band, along with a broadening of both peaks. These spectral changes point to significant disruption of graphitic ordering and partial amorphization of the carbon matrix. Such structural modifications are attributed to the deposition of Prussian Blue complexes on the carbon surface, which disturb the continuity of the sp² lattice and generate additional defect sites. Overall, the Raman results demonstrate that each modification pathway induces distinct alterations in the ordering and defect structure of the activated carbon framework.

2.3. FTIR Analysis

Figure 3 presents the FTIR spectra of BH+KOH, BH+KOH+Urea, and BH+KOH+Fe samples. In all cases, a broad band in the region of 3400–3200 cm⁻¹ is observed, which can be assigned to O–H stretching vibrations, indicating the presence of hydroxyl groups and adsorbed water. In the region around 2900–2850 cm⁻¹, weak bands corresponding to C–H stretching vibrations are detected, typical of organic components.

Significant differences between the samples appear in the range of 1700–1600 cm⁻¹, where the materials modified with urea and PB exhibit bands associated with C=O stretching vibrations. In the region of 1400–1000 cm⁻¹, intense bands are present and attributed to C≡N stretching vibrations and –COO⁻ groups. The intensity and presence of these bands strongly depend on the type of organic functional groups present on the surface. The most pronounced changes are observed for the BH+KOH+Fe sample, which shows a considerable decrease in transmittance at lower wavenumbers (1000–500 cm⁻¹), associated with C–O and C–C skeletal vibrations as well as deformation vibrations of aromatic rings.

These results clearly indicate that the incorporation of urea and PB leads to significant modifications in the chemical structure of the material compared to the BH+KOH sample. The observed spectral changes confirm the involvement of surface functional groups in the formation of new chemical bonds and structural reorganization within the investigated composites. In the spectrum of the BH+KOH+Fe sample, a distinct absorption band is observed at ~2100–2150 cm⁻¹, which can be assigned to the stretching vibrations of C≡N groups in the Prussian Blue complex. This band is absent in the spectra of BH+KOH and BH+KOH+urea, confirming the successful immobilization of Prussian Blue on the carbon surface.

2.4. ASAP- (Nitrogen Adsorption–Desorption Method) Analysis

Table 3. Surface characteristics of the tested adsorbents.

	BH+KOH	BH+KOH+urea	BH+KOH+Fe
Surface area from isotherms BET [m2/g]	1579	340	251
Total pore volume [cm3/g]	0.85	0.23	0.1
Average pore radius from the method BET [nm]	21.62	27.07	2.5

presents the surface characteristics of the tested adsorbents. The first sample has the largest surface area, indicating a highly developed porous structure. This may be due to chemical activation with potassium hydroxide (KOH), which effectively develops the surface of the carbonaceous material. The second sample, BH+KOH+urea, has a significantly lower specific surface area; it is a highly porous material, but less so than BH+KOH. The third sample, BH+KOH+Fe, has the lowest surface area, which is due to the presence of iron, which may partially block pores and reduce the accessible surface area. BH+KOH has the largest pore volume, indicating a well-developed mesoporous structure (with contributions from larger pores). It should be emphasized that the KOH activation step was performed under identical conditions for all samples. Therefore, the observed differences in BET surface area and pore volume mainly reflect the subsequent modification steps. Post-treatment with urea or Fe/PB may decrease the accessible surface area because newly formed surface species and deposits can partially block pore entrances and cover pore walls. In addition, inorganic/metal-containing phases contribute to the total mass but do not increase N₂-accessible porosity proportionally, which can lead to lower BET values. Thus, the reduced textural parameters should be interpreted as a result of post-modification rather than differences in the activation conditions. All samples contain mesopores.

2.5. Particle Size Analysis

Particle size values of the studied carbons measured in 0.001 mol/dm³ NaCl and 0.001 mol/dm³ NaNO₃ at different pH are summarized in

Table 4. Particle size measurement for BH+KOH+urea coal in 0.001 mol/dm³ NaCl and 0.001 mol/dm³ NaNO₃.

NaCl		NaNO3
pH	Size [nm]	pH	Size [nm]
2.28	3117	2.15	5317
7.78	2089	7.94	2522
11.81	1794	11.87	1544

,

Table 5. Particle size measurement for BH-KOH coal in 0.001 mol/dm³ NaCl and 0.001 mol/dm³ NaNO₃.

NaCl		NaNO3
pH	Size [nm]	pH	Size [nm]
2.14	4475	2.1	2956
7.96	3548	7.8	2266
11.83	2283	11.8	1627

and

Table 6. Particle size measurement for BH+KOH+Fe coal in 0.001 mol/dm³ NaCl and 0.001 mol/dm³ NaNO₃.

NaCl		NaNO3
pH	Size [nm]	pH	Size [nm]
2.15	1901	2.04	2082
7.97	1192	8.02	1594
11.88	1076	11.94	1343

.

Table 4, Table 5 and Table 6 present the particle size measurements of the tested adsorbents in two electrolytes. Particle size measurements of different activated carbons in the same electrolyte show a significant dependence on pH, primarily due to changes in the surface charge density of the particles and their tendency to aggregate or disperse in solution. At low pH, the surface of the activated carbon particles can become protonated, reducing the surface charge, and the zeta potential approaches zero, a consequence of the weaker electrostatic repulsion between particles. This results in particle aggregation, which manifests itself in larger particle sizes than at higher (alkaline) pH. Functional groups (e.g., –OH) are deprotonated, increasing the negative surface charge. The negative zeta potential increases, a consequence of the stronger electrostatic repulsion between particles. This bicarbon/electrolyte system is more colloidally stable; the particles remain more dispersed, resulting in smaller particle sizes. Particle size measurements for the same activated carbon differ depending on the type of electrolyte, such as NaCl and NaNO₃, even though both are monovalent electrolytes (Na⁺). These differences are mainly due to the influence of anions (Cl⁻ vs. NO₃⁻) on the colloidal stability and double layer structure around the activated carbon particles. The first important phenomenon in such systems is surface charge shielding: both electrolytes reduce the thickness of the electrical double layer, which reduces electrostatic repulsion between particles. The second important difference is based on the anions: Cl⁻ vs. NO₃⁻. Cl⁻ and NO₃⁻ have different adsorption abilities on the activated carbon surface and different polarities and hydration radii. NO₃⁻ has a larger hydration radius than Cl⁻, resulting in the formation of a thicker hydration layer and greater colloidal stability of the system. Cl⁻ can adsorb more easily to the surface, which changes the surface charge and leads to stronger aggregation. For our activated carbons presented in Table 4, Table 5 and Table 6, we see that for BH+KOH, larger grains are present in the NaCl electrolyte than for NaNO₃. This is a result of the aggregation process and weaker hydration of Cl⁻ ions. For the BH+KOH+urea and BH+KOH+Fe samples, the opposite is true, resulting from the interaction of functional groups on the surface with these electrolytes. Measuring activated carbon particle size depends on the type of modification, as each modification affects: surface structure (porosity, presence of functional groups), surface charge (and therefore zeta potential), colloidal stability (i.e., susceptibility to aggregation), and interaction with the electrolyte (e.g., ion adsorption, shielding). Activated carbons modified with KOH (BH+KOH) have a highly developed surface area and numerous basic groups (-OH), and high porosity. This results in smaller particle sizes (reduced aggregation). The system is more stable in both electrolytes, but NaNO₃ may provide slightly greater stability due to better NO₃⁻ hydration. BH+KOH+urea samples modified with urea have nitrogen-containing groups (–NH₂), which increase the hydrophilicity of these samples and have a moderate effect on porosity. As a result, the average particle size may depend on interactions with ions and, therefore, may be pH-dependent, and amine groups may undergo protonation. Therefore, their particle size is smaller in NaCl (lower stability) than in NaNO₃. BH+KOH+Fe samples have a surface coated with an inorganic ferrocyanide complex (Fe₄[Fe(CN)₆]₃), which increases the charge and mass of the particles and reduces their porosity. Prussian Blue can interact with Cl⁻ and NO₃⁻ ions, affecting the stability of the more complex system.

2.6. Electrical Double Layer (EDL) Analysis

The surface charge density trends as a function of pH in NaCl and NaNO₃ are shown in Figure 4, Figure 5 and Figure 6.

In the adsorption of ions on solid surfaces, phenomena occurring at the solid-solution interface play a key role. The effectiveness and mechanism of adsorption depend on the way in which solid-phase particles interact with ions and molecules in solution. An ordered structure of matter, known as an electric double layer (EDL), forms at this interface, constituting a fundamental element in describing the electrokinetic and surface properties of disperse systems. Among the most important experimental quantities characterizing EDL are surface charge density and zeta potential. In recent years, these parameters have been increasingly considered in the scientific literature as significant factors describing the behavior of modern adsorbents, alongside their surface and structural characteristics [33,34].

Figure 4 and Figure 5 show the changes in surface charge density as a function of pH for the tested materials: BH+KOH, BH+KOH+urea, and BH+KOH+Fe, in a 0.001 mol/dm³ NaNO₃ and NaCl solutions. In all cases, a significant decrease in surface charge density was observed with increasing pH, particularly below pHpzc for the tested samples, which is consistent with the expected behavior for materials containing dissociable hydroxyl groups on their surfaces. For the adsorbents modified with KOH and urea (BH+KOH and BH+KOH+urea), the surface charge density is very similar throughout the tested pH range. As pH increases, a decrease in surface charge values is observed, reaching negative values in the alkaline pH range. This indicates that deprotonation of functional groups occurs and negative charges appear. In the case of the BH+KOH+Fe adsorbent, the surface charge density is practically negative from pH = 5.8. This result is consistent with the presence of iron oxides or hydroxyoxides, which impart a permanently acidic character and a strongly negative charge to the surface. The minimum σ value for this material reaches approximately −10 mC/cm² for NaNO₃ and −20 mC/cm² for NaCl, indicating a potentially high electrostatic cation adsorption capacity.

Figure 5 and Figure 6 present changes in surface charge density (σ) as a function of pH for the tested materials: BH+KOH, BH+KOH+urea, and BH+KOH+Fe in a 0.001 mol/dm³ NaCl solution. This parameter is an important indicator of the electrostatic nature of the adsorbent surface and directly influences its ability to adsorb ions. In all cases, a significant decrease in surface charge density was observed with increasing pH, which is consistent with the expected behavior for materials containing dissociable hydroxyl groups on the surface. For adsorbents modified with KOH and urea (BH+KOH and BH+KOH+urea), the surface charge density remains positive in the pH range < 6–7, indicating the presence of protonated groups (–OH₂⁺, –NH₃⁺). As pH increases, a decrease in surface charge is observed, reaching negative values in the alkaline pH range. This indicates deprotonation of functional groups and the appearance of negative charges (e.g., –O⁻, –COO⁻). The x-axis intercept corresponds to the point of zero charge density (pHpzc), which for these materials is approximately, for BH+KOH, pH ~8.5–9, and for BH+KOH+urea, pH ~8–8.5, respectively. In the case of the BH+KOH+Fe adsorbent, the surface charge density is negative throughout the analyzed pH range, suggesting a very low pHpzc, below pH 5. This result is consistent with the presence of iron oxides or hydroxyoxides, which impart a permanently acidic character and a strongly negative charge to the surface. The minimum σ value for this material reaches approximately ~(−35 mC/cm²), which indicates a potentially high electrostatic adsorption capacity for cations (e.g., heavy metals) under neutral and alkaline conditions. In Figure 6a–c, the plots for all three samples (BH+KOH, BH+KOH+urea, BH+KOH+Fe) show a similar trend. For each sample, the values of surface charge density in the NaCl solution are more negative than those in NaNO₃ at the same pH. This difference arises from the distinct adsorption behavior of Cl⁻ and NO₃⁻ ions. The Cl⁻ ion exhibits a higher affinity for the surface, leading to a more negative charge. This indicates that the chloride anion has a stronger influence on the surface potential compared to the nitrate anion, suggesting the occurrence of specific Cl⁻ adsorption. The BH+KOH+Fe sample shows the highest negative charge at elevated pH values, implying that this material may have the greatest adsorption potential toward metallic cations (e.g., Ag⁺, Pb²⁺). The observed trend indicates a gradual increase in surface acidity with modification (KOH → Urea → Fe).

The ζ-potential dependences on pH in the studied electrolytes are presented in Figure 7.

Although NaCl and NaNO₃ at a concentration of 0.001 mol dm⁻³ exhibit identical ionic strength and comparable bulk properties, differences in anion size, hydration energy and interfacial behavior may lead to subtle but measurable variations in electrokinetic parameters such as zeta potential, pHpzc and pH_iep. To determine the electrostatic nature of the surfaces of the studied adsorbents, zeta potential measurements were performed as a function of pH in a 0.001 mol/dm³ NaNO₃ solution. The obtained curves are presented in Figure 7. All analyzed materials—BH+KOH, BH+KOH+urea, and BH+KOH+Fe—exhibit a negative zeta potential over a wide pH range, indicating the dominance of functional groups that impart a negative surface charge, such as –O- or –COO⁻. For the BH+KOH and BH+KOH+urea adsorbents, zeta potential values close to zero were observed at pH ~2.0 and ~2.5, respectively, indicating isoelectric points (pH_iep) in this range. With increasing pH, the zeta potential becomes increasingly negative, reaching values of approximately −30 mV at pH 9–10, suggesting increasing colloidal stability and a strongly negative surface charge. The BH+Fe adsorbent exhibits different characteristics: the zeta potential from pH ~3 onwards assumes significantly more negative values (approximately −20 mV), not reaching positive values throughout the entire pH range studied. This indicates the acidic nature of the material’s surface and the isoelectric point below pH 2. The presence of iron oxides may contribute to a higher negative charge density on the surface and a greater capacity for adsorption of positive ions, especially in the neutral and alkaline pH range. It is worth noting that at pH > 6, all materials exhibit zeta potentials less than −20 mV, indicating good colloidal stability of the suspension systems. The highest negative zeta potential value was achieved for the BH+KOH+Fe sample, suggesting its potentially highest efficiency in cation adsorption processes over a wide pH range.

Figure 8 shows the dependence of the zeta potential on pH for three adsorbents: BH-KOH, BH+KOH+Urea, and BH+KOH+Fe, in a 0.001 mol/dm³ NaCl electrolyte. The aim of the measurements was to determine the nature of the zeta potential of the tested materials and to identify their isoelectric points (pH_iep), which are key for predicting their adsorption properties and colloidal stability. All materials exhibit negative zeta potential over a wide pH range, indicating the dominance of acidic functional groups capable of dissociating and generating a negative charge. The KOH-activated adsorbent exhibits a zeta potential close to zero at pH ~2.5, suggesting an isoelectric point in this range. As pH increases, the zeta potential value decreases, reaching a minimum of approximately −25 mV at pH ~9. In the pH range of 6–10, the material exhibits a relatively stable, negative zeta potential, indicating favorable colloidal stability and the presence of persistent –OH⁻ groups on the surface. The BH+KOH+urea sample exhibits a similar curve to BH+KOH, but with slightly less negative values in the alkaline pH range. The isoelectric point of the material is observed at pH ~2. The minimum zeta potential reaches a value of approximately −22 mV at pH ~9. The adsorbent surface remains negatively charged throughout the tested pH range, which may be due to the presence of amide groups or urea residues (e.g., –NH₂), which modify the electrostatic properties. The iron-modified material (BH+KOH+Fe) exhibits significantly more negative zeta potential than the other samples throughout the pH range. At pH 3, it reaches a value of approximately −20 mV, and the minimum zeta potential is observed at pH ~9 and is approximately −35 mV. The isoelectric point lies outside the studied pH range (<2), indicating the strongly acidic nature of the surface. The presence of iron oxides or hydroxyoxides (e.g., FeOOH) may promote strong dissociation of –OH groups and the generation of a negative charge. This characteristic makes BH+KOH+Fe potentially the most effective adsorbent of cations over a wide pH range.

The increase in zeta potential observed for some samples at very high pH values may be attributed to compression and reorganization of the electric double layer as well as specific interactions of OH⁻ ions with surface functional groups. In the case of the BH+KOH+urea sample, the presence of nitrogen-containing surface functionalities introduces additional acid–base equilibria, leading to a non-linear pH dependence of the zeta potential. The observed maximum may result from complete deprotonation and subsequent reorientation of nitrogen groups, combined with changes in the position of the shear plane and possible aggregation effects under strongly alkaline conditions.

Table 7. pHpzc and pH_iep points depending on the type of electrolyte and adsorbent.

Adsorbent	BH+KOH		BH+KOH+urea		BH+KOH+Fe
electrolyte	NaCl	NaNO3	NaCl	NaNO3	NaCl	NaNO3
pHpzc	9.74	9.24	9.15	8.85	3.5	5.8
pHiep	2.4	2.4	2.5	3.1	2	2

presents the pHpzc and pH_iep points depending on the type of electrolyte and adsorbent. pHpzc (point of zero charge) is the pH at which the total surface charge of the adsorbent is zero. Above pHpzc, the surface is negatively charged. Below pHpzc, the surface is positively charged. pH_iep (isoelectric point) is the pH at which the zeta potential = 0, meaning the particle has no electrophoretic mobility. This is the point at which the particle does not migrate in an electric field, which is important for colloid stability. The RH–KOH sample has a very high pHpzc; the material has a positive surface charge over a wide pH range < 9.7. A large difference between pHpzc and pH_iep (>7 units) means that despite the positive charge, the surface is not necessarily electrokinetically neutral. Colloidal stability for this system is most likely only at pH close to pHiep (~2.4), and agglomeration may occur in other ranges. The BH+KOH+urea adsorbent also has a high pHpzc; the surface is strongly positive at lower pH. The preferential adsorption behavior of different anions on carbonaceous surfaces has been reported in the literature. For instance, activated carbon has been shown to exhibit markedly different affinities toward nitrate and chloride ions, with nitrate often being adsorbed more strongly under comparable conditions due to differences in ion hydration and solvation structure [35].

The pHpzc shifts slightly higher in NaNO₃ than in NaCl, influenced by the type of anion. The material will be negatively charged only at pH > 9. The BH+KOH+Fe material has significantly lower pHpzc values than for BH+KOH and BH+KOH+Urea; the BH+KOH+Fe surface is negatively charged over most of the pH range. This indicates the presence of acidic groups on the surface (e.g., –OH groups bound to iron oxides). The pHpzc increases in the presence of NO₃⁻, likely due to adsorption of the anion, which changes the surface potential. pH_iep < 2. Additionally, the surface is almost always charged, which means high colloidal stability, and this is beneficial for dispersive materials. For example, Figure 5 and Figure 6 present changes in surface charge density (σ) as a function of pH for the tested materials, BH+KOH, BH+KOH+urea, and BH+KOH+Fe, in a 0.001 mol/dm³ NaCl solution. This parameter is an important indicator of the electrostatic nature of the adsorbent surface and directly influences its ability to adsorb ions. In all cases, a significant decrease in surface charge density was observed with increasing pH, which is consistent with the expected behavior for materials containing dissociable hydroxyl groups on the surface. For adsorbents modified with KOH and urea (BH+KOH and BH+KOH+urea), the surface charge density remains positive in the pH range < 6–7, indicating the presence of protonated groups (–OH₂⁺, –NH₃⁺). As pH increases, a decrease in surface charge is observed, reaching negative values in the alkaline pH range. This indicates deprotonation of functional groups and the appearance of negative charges (e.g., –O⁻, –COO⁻). The x-axis intercept corresponds to the point of zero charge density (pHpzc), which for these materials is approximately, for BH+KOH, pH ~8.5–9, and for BH+KOH+urea, pH ~8–8.5, respectively. In the case of the BH+KOH+Fe adsorbent, the surface charge density is negative throughout the analyzed pH range, suggesting a very low pHpzc, below pH 5. This result is consistent with the presence of iron oxides or hydroxyoxides, which impart a permanently acidic character and a strongly negative charge to the surface. The minimum σ value for this material reaches approximately −35 mC/cm², which indicates a potentially high electrostatic adsorption capacity for cations (e.g., heavy metals) under neutral and alkaline conditions.

3. Discussion

3.1. Chemical and Physical Interactions During Carbon Formation and Modification

During pyrolysis of the buckwheat husk precursor, dehydration and aromatization reactions progressively convert biomass into a carbonaceous framework, while residual oxygen-containing surface groups and structural defects remain and later serve as anchoring/active sites for further activation and functionalization [36].

Subsequent KOH chemical activation proceeds through coupled redox/gasification pathways (formation–decomposition/reduction of potassium-containing intermediates and evolution of CO/CO₂) together with metallic K intercalation/etching, which opens the carbon matrix and generates abundant micro/mesopores, thereby markedly increasing accessible surface area and transport pathways [37,38].

Urea treatment acts primarily as a nitrogen source: upon thermal decomposition, reactive N-containing species (e.g., NH₃/HCNO-derived fragments) interact with defect-rich carbon sites and surface functionalities, forming N-containing groups and shifting the surface chemistry toward a more basic character; in combination with KOH, urea can also contribute to pore widening and a more interconnected hierarchical pore network. [24,39].

Prussian Blue (PB) is immobilized on the porous carbon via interfacial interactions with surface functional groups and confinement within the pore network, yielding well-dispersed PB domains, and the PB lattice provides ion-exchange/cage-type sites responsible for high selectivity toward Cs⁺ (and related monovalent cations), while the carbon support ensures rapid mass transfer and additional adsorption sites for metal ions [27,40].

3.2. Analysis of SEM Images in Correlation with Surface Properties (pHpzc and pH_IEP)

Chemical and Physical Interactions During Carbon Formation and Modification

SEM observations revealed significant morphological differences among the activated carbon samples, correlating with their points of zero charge (pHpzc) and isoelectric points (pH_IEP). The BH+KOH sample shows a highly irregular surface with numerous cracks, channels, and pores, typical of KOH-activated carbons with well-developed micro- and mesoporosity. High pHpzc values (~9.2–9.7) indicate a predominantly basic surface due to oxygen-containing groups (–C–O⁻, –O⁻) and residual alkaline species. For BH+KOH+urea, SEM images reveal a more ordered, stable porous structure. Urea introduces –NH₂ and –C–N groups, slightly lowering pHpzc (8.85–9.15) and balancing acidic and basic surface functionalities. The BH+KOH+Fe sample retains its porous framework but features fine nanocrystalline Prussian Blue deposits that partially block pores while creating Fe–CN–Fe adsorption sites. This results in lower pHpzc and pH_IEP (3.5–5.8 and 2.0–2.4), reflecting a more acidic surface that enhances cationic species adsorption, particularly heavy metals and cesium. Overall, morphology and surface charge clearly depend on the chemical composition and modification route: KOH activation generates high porosity and basicity, urea stabilizes the structure and adds nitrogen groups, and Prussian Blue imparts acidity and selective adsorption properties.

3.3. Analysis Between Elemental Composition in Correlation with Surface Properties (pHpzc and pH_IEP)

EDS analysis combined with pHpzc and pH_IEP measurements highlight the effects of activation and modification on carbon adsorbents. The BH+KOH sample is highly carbonaceous (91.6%) with low oxygen (7.5%), displaying a basic surface (pHpzc 9.24–9.74) that favors anion adsorption but offers limited sites for metal binding. Incorporation of urea (BH+KOH+urea) reduces carbon content (79%) and slightly increases oxygen (8.7%), introducing additional polar groups and stabilizing the porous framework. Nitrogen was not detected due to pyrolytic decomposition, while enrichment in K and Zn adds active sites. The most pronounced modification occurs with Prussian Blue immobilization (BH+KOH+Fe), decreasing carbon (60%) and increasing oxygen (12%), nitrogen (18%), and iron (6.7%), shifting the surface from basic to acidic (pHpzc 3.5–5.8, pH_IEP ~2), and creating Fe–CN–Fe centers that enhance adsorption of cationic species such as Cs⁺, Pb²⁺, and Cu²⁺. Overall, KOH activation ensures high carbon content and basicity, urea stabilizes and polarizes the structure, and Prussian Blue imparts acidity and selective adsorption capacity.

3.4. Analysis of FTIR Spectra in Correlation with Surface Properties (pHpzc and pH_IEP)

FTIR spectra of BH+KOH, BH+KOH+Urea, and BH+KOH+Fe (Figure 3) show broad O–H stretching bands (3400–3200 cm⁻¹) from hydroxyl groups and adsorbed water, and weak C–H bands (2900–2850 cm⁻¹) from residual organics. Significant differences appear at 1700–1600 cm⁻¹, where urea- and Prussian Blue-modified samples display C=O bands. In the 1400–1000 cm⁻¹ region, C–N and carboxylate signals vary with the modifier type. BH+KOH+Fe additionally shows reduced transmittance at 1000–500 cm⁻¹ due to skeletal C–O/C–C and aromatic ring vibrations. These spectra indicate that urea and Prussian Blue introduce new oxygen-, nitrogen-, and iron-containing functional groups, reshaping the surface chemistry. Accordingly, BH+KOH exhibits a basic surface (pHpzc 9.24–9.74) dominated by hydroxyls, BH+KOH+Urea shows additional polar groups and slightly lower pHpzc (8.85–9.15), and BH+KOH+Fe presents –C≡N and –Fe–CN– motifs with markedly acidic character (pHpzc 3.5–5.8, pH_IEP ~2), favoring cation adsorption.

3.5. Analysis of Raman Spectra in Correlation with Surface Properties (pHpzc and pH_IEP)

Raman spectra confirm these trends: BH+KOH shows a strong G band (~1580 cm⁻¹) reflecting high graphitic order and basicity, BH+KOH+Urea exhibits increased D band intensity, indicating more defects and nitrogen-functional groups with slightly lower basicity, and BH+KOH+Fe has the highest D/G ratio, signifying amorphous, defect-rich carbon and an acidic surface. Together, FTIR and Raman analyses correlate structural defects and functional group formation with surface acid–base properties, explaining the enhanced affinity of modified carbons toward cationic species.

3.6. Analysis of Porous Structure in Correlation with Surface Properties (pHpzc and pH_IEP)

Correlation of surface parameters (Table 3) with pHpzc and pH_IEP values (Table 7) highlights the effects of activation and chemical modification on the porosity and surface chemistry of the activated carbons. The BH+KOH sample has the highest specific surface area (1579 m²/g) and pore volume (0.85 cm³/g), dominated by mesopores (average radius 21.6 nm), reflecting efficient KOH activation. High pHpzc (9.24–9.74) indicates a basic surface, favoring anion adsorption but limiting cation binding. Incorporation of urea (BH+KOH+Urea) reduces surface area (340 m²/g) and pore volume (0.23 cm³/g) while increasing average pore radius (27.1 nm), producing a more ordered structure with slightly enhanced polarity (pHpzc 8.85–9.15) and moderate cation adsorption potential. The BH+KOH+Fe sample shows the lowest surface area (251 m²/g) and pore volume (0.10 cm³/g) due to partial pore blockage by Prussian Blue, with mesopores around 2.5 nm. The average pore radius (2.5 nm) indicates the predominance of mesopores in the range of approximately 2.5 nm for BH+KOH+Fe and mesopores in the range of approximately 21–27 nm for BH+KOH and BH+KOH-urea. Its acidic surface (pHpzc 3.5–5.8, pH_IEP ~2.0) favors adsorption of cationic species such as Cs⁺, Pb²⁺, and Cu²⁺. Overall, KOH activation produces highly porous, basic carbons; urea stabilizes the structure; and Prussian Blue imparts acidity and selective adsorption sites, enhancing cation removal efficiency.

3.7. Sorption Performance of Buckwheat Husk-Derived Activated Carbons Toward Cesium-137

3.7.1. Effect of Sorbent Mass on ¹³⁷Cs Removal

This section presents the experimental results on the removal of cesium-137 (¹³⁷Cs) from real contaminated groundwater using buckwheat husk-derived activated carbon materials. Two aspects of sorption performance were investigated: the influence of sorbent dosage and the effectiveness of various modified carbon sorbents. The data were obtained under identical experimental conditions, as described in Section 4.5.

The results are presented in

Table 8. Residual ¹³⁷Cs activity depending on sorbent mass.

Sorbent Mass (g)	Residual 137Cs (Bq/L)
0.2	110 ± 22
0.4	101 ± 20
1.0	106 ± 21

.

The data demonstrate that increasing the sorbent dose from 0.2 g to 0.4 g results in a clear reduction in cesium activity in solution, indicating improved sorption. However, a further increase to 1.0 g does not lead to additional enhancement and even results in a slightly higher residual value. This may be attributed to equilibrium saturation or agglomeration effects that reduce accessible surface area.

Therefore, 0.4 g was selected as the optimal sorbent dosage for subsequent experiments due to its superior removal efficiency and practical material usage.

3.7.2. Sorption Efficiency of Modified Buckwheat Husk Carbons

The sorption efficiency of various modified buckwheat husk-derived activated carbons was tested under the same experimental conditions using the previously optimized sorbent mass of 0.4 g. The modifications included activation with KOH, urea treatment, and Prussian Blue loading, which is known to enhance selectivity toward cesium ions.

The results of ¹³⁷Cs removal by different sorbents are presented in

Table 9. Residual ¹³⁷Cs after sorption with BH-based sorbents.

Sorbent	Residual 137Cs (Bq/L)
BH 500	117 ± 23
BH+KOH	100 ± 20
BH+KOH+urea	110 ± 22
BH+KOH+Fe	10 ± 2

.

A graphical representation of the data is shown in Figure 9.

The high efficiency of this adsorbent can be attributed to the lowest point of zero charge (pHpzc) among all materials investigated in this study, determined to be 3.5 (measured in NaCl solution) and 5.8 (measured in NaNO₃ solution). Such low pHpzc values result in a negatively charged adsorbent surface over a wide pH range, which favors electrostatic interactions and promotes the efficient adsorption of cesium cations. Among the tested materials, the BH+KOH+Fe sample (carbon modified with KOH and loaded with Prussian Blue) showed exceptional performance, reducing the ¹³⁷Cs activity to 10 ± 2 Bq/L, corresponding to a removal efficiency of over 90%. This result confirms the high selectivity of Prussian Blue toward cesium ions and the synergistic effect of chemical activation and ion-exchange functionality.

4. Materials and Methods

4.1. Materials

Buckwheat husk (BH) was obtained from local farmers in Kazakhstan. The raw biomass was washed with distilled water, dried at 105 °C for 2–4 h, and stored in sealed containers until use. Analytical-grade reagents were used without further purification: potassium hydroxide (KOH, Sigma-Aldrich, St. Louis, MI, USA), ferric chloride (FeCl₃, Meryer, Shanghai, China), potassium ferrocyanide (K₂[Fe(CN)₆], Shandong, Jinan China), and urea (Sigma-Aldrich, St. Louis, MI, USA). Nitrogen (99.6%, 300 mL/min), carbon dioxide (99.8%), and compressed air were supplied by local distributors.

4.2. Synthesis of Activated Carbons

4.2.1. Carbonization

Buckwheat husk was carbonized in a chemical vapor deposition (CVD) furnace (BS-HTF-1200C, Almaty, Kazakhstan, 2023) under nitrogen flow (300 mL/min) at 500 °C for 1 h, yielding the intermediate product BH 500 with a yield of 30–36%.

The yield of BH 500 (%) was determined gravimetrically according to the following relation:

$$\mathrm{Y}\mathrm{i}\mathrm{e}\mathrm{l}\mathrm{d} (\%)={\displaystyle \frac{{\mathrm{m}}_{\mathrm{B}\mathrm{H} 500}}{{\mathrm{m}}_{\mathrm{r}\mathrm{a}\mathrm{w}}}}\times 100\%$$

where m_{BH 500} is the mass of the carbonized product and m_raw is the initial dry mass of buckwheat husk.

4.2.2. Chemical Activation

For chemical activation, the carbonized mass was mixed with KOH powder at a 1:2 (w/w) KOH-to-carbon ratio and then pyrolyzed at 800 °C for 2 h under a nitrogen flow of 300 mL/min (heating rate: 10 °C/min). The 1:2 ratio was selected because it is a commonly used activation condition for biomass-derived carbons and provides efficient pore development without overly severe burn-off. The resulting product was denoted BH+KOH.

4.3. Functionalization of Activated Carbons

4.3.1. Nitrogen Doping

To introduce nitrogen functionalities, 15 g of BH+500 was immersed in 150 mL of 10% urea solution and treated hydrothermally in an autoclave at 180 °C and 5 MPa for 24 h. The obtained material was dried at 105 °C for 4–6 h and subsequently activated with KOH in a 1:2 ratio (dry mixing).

4.3.2. Prussian Blue Modification

For ferrocyanide functionalization, 20 mM aqueous solutions of FeCl₃ (6.448 g in 200 mL) and K₂[Fe(CN)₆] (16.8952 g in 200 mL) were prepared. The BH+KOH sample (5 g) was impregnated with 50 mL of 20 mM K₂[Fe(CN)₆] and hydrothermally treated at 180 °C for 24 h in a microreactor. Afterwards, FeCl₃ solution was added dropwise under stirring (300 rpm). The Prussian Blue-modified product (BH+KOH+Fe) was washed thoroughly with distilled water and dried at 105 °C for 4 h.

4.4. Characterization Methods

4.4.1. Scanning Electron Microscopy (SEM-EDS)

The morphology of the obtained carbons (BH 500, BH+KOH, BH+KOH+urea, BH+KOH+Fe) was examined by SEM (Quanta 3D 200i, Thermo Fisher Scientific, Waltham, MA, USA). Elemental composition was analyzed using energy-dispersive X-ray spectroscopy (EDS).

4.4.2. Raman Spectroscopy

Raman spectra were recorded using a handheld Raman spectrometer BRAVO (Bruker Optics, Ettlingen, Germany). The instrument is equipped with DuoLASER™ excitation (785 and 852 nm) and Sequentially Shifted Excitation (SSE™) technology for fluorescence suppression. The spectral range was 3200–300 cm⁻¹ with a spectral resolution of ~10–12 cm⁻¹.

4.4.3. Fourier-Transform Infrared Spectroscopy (FTIR)

The chemical structure was studied using FTIR spectroscopy (JSM-6490LA, JEOL, Tokyo, Japan). Samples were mixed with KBr at a 0.5:99.5 ratio, pressed into pellets under 10 t/cm², and analyzed in the range of 4000–400 cm⁻¹ with a resolution of 1 cm⁻¹. Each spectrum was obtained by averaging 32 scans.

4.4.4. Nitrogen Adsorption–Desorption Method (ASAP)

ASAP analyzers are widely used in the industries. The porosimetry method was used to measure physisorption using the ASAP 2420 instrument from Micromeritics Inc., Norcross, GA, USA. Nitrogen adsorption–desorption is a fundamental, very frequently used method for determining parameters of porous materials, such as specific surface area, pore volume and diameter, pore distribution and size. Therefore, based on the tested adsorption isomers, pore size distribution functions or surface area of the test substance are determined. Nitrogen adsorption–desorption measurements were carried out using an ASAP 2420 instrument (Micromeritics Inc., USA) at liquid nitrogen temperature (77 K). Prior to the measurements, the samples were degassed under vacuum (~10⁻³–10⁻⁴ Pa) at 200 °C for 8 h until a stable pressure was achieved, in order to remove adsorbed gases and moisture from the material surface. The specific surface area (S_BET) was calculated using the Brunauer–Emmett–Teller (BET) method in the relative pressure range of p/p₀ = 0.05–0.30, in accordance with IUPAC recommendations. The total pore volume was determined from the amount of nitrogen adsorbed at p/p₀ (~0.99. The mesopore size distribution was calculated using the Barrett–Joyner–Halenda (BJH) method based on the desorption branch of the isotherm. All calculations were performed using the software provided by the instrument manufacturer (Micromeritics ASAP 2420 software).

4.4.5. Potentiometric Titration

Surface charge density measurements were performed simultaneously in the suspension of the same solid content, to keep the identical conditions of the experiments in a thermostated Teflon vessel at 25 °C. To eliminate the influence of CO₂, all potentiometric measurements were performed under nitrogen atmosphere. pH values were measured using a set of glass REF 451 (Radiometer) and calomel pHG201-8 (Radiometer, Copenhagen, Denmark) electrodes with the Radiometer assembly. Surface charge density was calculated from the difference in the amounts of added base to obtain the same pH value of suspension as for the background electrolyte. As a background electrolyte, NaNO₃ and NaCl solutions were used at concentrations of 0.001 mol/dm³.

4.4.6. Particle Size and Zeta Potential Measurements

Particle size and zeta potential measurement using the Zetasizer Nano-ZS90 from Malvern is based on two basic techniques. Particle size measurement is performed using dynamic light scattering (DLS). To prepare the measurement solution, 0.01 g of individual samples was measured and transferred to separate beakers. Then, 100 cm³ of 10⁻³ mol/dm³ NaCl electrolyte solution was added to each sample. Samples of individual carbons were prepared in a 10⁻³ mol/dm⁻³ NaNO₃ electrolyte solution in the same manner. Each sample was then ultrasonified for 10 min. Sample dispersions were prepared using a Sonics Vibra-Cell sonicator operating at 20 kHz. Sonication was performed at 45% amplitude for 15 min in pulsed mode (5 s ON/5 s OFF), while the sample was cooled in an ice bath to limit temperature increase. The resulting systems were characterized by good dispersion stability. Zeta potential measurement is based on electrophoretic light scattering (ELS). pH measurement to determine zeta potential was performed with a PHM Beckman pH meter using a combination electrode. To disperse the system, individual samples were subjected to ultrasound using a Misonix, (Farmingdale, NJ, USA) Sonicator XL2020 device. Measurements were performed according to the following procedure: 0.1 g of sample was weighed, added to a 10⁻³ mol/dm⁻³ NaCl electrolyte solution, and then ultrasonicated. A similar procedure was followed using a 10⁻³ mol/dm⁻³ NaNO₃ electrolyte solution. The resulting suspension was poured into five 125 cm⁻³ flasks. The tested pH range was 2 to 12, and it was adjusted using 0.1 mol/dm⁻³ HCl and NaOH. For suspensions in the NaNO₃ electrolyte solution, 0.1 mol/dm⁻³ HNO₃ was used instead of 0.1 mol/dm⁻³ HCl.

4.5. Sorption Experiment for Cesium-137 Removal

To evaluate the sorption performance of buckwheat husk-derived activated carbon materials toward cesium-137 (¹³⁷Cs), batch adsorption experiments were conducted using real groundwater contaminated with this radionuclide. The water sample was collected from adit No. 104, located at the Degelen site of the National Nuclear Center of the Republic of Kazakhstan (NNC RK, Kurchatov, Kazachstan).

The initial specific activity of ¹³⁷Cs in the sample was measured as 118 ± 24 Bq/L, using high-purity germanium (HPGe) gamma spectrometry. The concentration of the selected radionuclides was determined using a gamma ray spectrometer. The sealed container was placed in a measuring chamber composed of a 10 cm layer of lead. The measurements were performed using an Ortec spectrometer, Oak Ridge, TN, USA equipped with an HPGe GEM S-7030 germanium crystal with a polycarbonate window. The detector with an extended energy range exhibited the following parameters: a crystal diameter of 70 ± 2 mm, a relative efficiency of 28%, an FWHM- full width at half maximum, resolution of 1.9 keV (for the 1.33 MeV Co-60 line) and a peak/Compton ratio of 40:1. Energy and efficiency calibration was performed using a standard source, i.e., a mixture of radioactive gamma isotopes covering the range of 59 keV–1330 keV dispersed in a matrix of density 1.5 g/cm³. The measurement uncertainty was determined at the 1 sigma level (one standard deviation). The measurement time for one sample was 172,800 s (2 days), while the background of the device itself was measured over a period of 7 days. Maestro-PRO V9.00.02 GammaVision ver. 9 software (both Ortec-Ametek, USA) was used to analyze the collected data. The sample-specific activity calculations were subjected to correction on the spectrometer background radiation. The obtained results were related to the unit mass of the delivered material. The activity of Cs-137 has been calculated on the basis of the measurement of the number of peak counts for energies of 661.66 keV and 1460.83 keV.

The experiment was performed by adding a known mass of sorbent (0.2 g, 0.4 g, or 1.0 g) into a 1 L conical flask containing 1 L of the contaminated water. The flasks were sealed and placed on an orbital shaker operating at 120 rpm. The suspensions were stirred for 24 h at room temperature to ensure sufficient contact time and sorption equilibrium.

After the contact period, the solid–liquid separation was carried out using paper filters placed in a porcelain Büchner funnel. The collected filtrates were analyzed to determine the residual ¹³⁷Cs activity using an HPGe gamma spectrometer (Ortec spectrometer, Oak Ridge, TN, USA).

All experiments were conducted in duplicate, and the results were averaged to ensure reproducibility. These measurements were used to assess the influence of sorbent dosage on cesium removal under static batch conditions.

5. Conclusions

This study demonstrates that the surface properties and adsorption performance of activated carbon are strongly influenced by the type of chemical activation and modification. KOH activation effectively develops a highly porous, basic carbon structure, while urea addition stabilizes and partially orders the pore network, introducing nitrogen-containing functional groups. Prussian Blue modification significantly alters the surface chemistry, lowering pHpzc and pH_IEP, increasing acidity and polarity, and creating Fe–CN–Fe active sites. Electrokinetic analyses show that surface charge depends on pH and electrolyte type, with BH+KOH+Fe exhibiting the most persistent negative charge across the pH range, favoring the adsorption of cationic species such as heavy metals and radionuclides. Particle size and colloidal stability are also governed by surface charge and specific ion interactions, with NaNO₃ providing greater suspension stability than NaCl. The correlation between structural characterization (FTIR, Raman) and electrochemical properties confirms that a more graphitic structure leads to a basic surface, whereas amorphous, defect-rich activated carbons promote acidic functional groups. Overall, these findings highlight the tunability of activated carbon surfaces for selective and efficient removal of cationic pollutants.