Valorization of Birch Biochar: An Efficient and Sustainable Solution for Lead Decontamination of Water

Abstract

This study investigated the potential of a commercially available birch biochar, previously used as a soil amendment, for the adsorption of Pb²⁺ ions from aqueous solutions. For the first time, direct potentiometry with a lead ion-selective electrode was used for continuous in situ real-time monitoring of the adsorption process. The biochar demonstrated a maximum adsorption capacity of 14.21 mg/g (Langmuir model) and a high affinity for Pb²⁺. Kinetic analysis revealed a two-stage process limited by intraparticle diffusion. A significant decrease in pH and power-law dependencies between the adsorption parameters and the liquid/solid ratio confirmed ion exchange as the primary mechanism. Additionally, the biochar’s surface characteristics and accessibility for large molecules were evaluated by methylene blue adsorption, yielding a specific surface area of 4.0–6.6 m²/g. This value, being an order of magnitude lower than the BET surface area, highlighted the microporous nature of the biochar and its limited accessibility for bulky organic cations, providing crucial context for interpreting the lead adsorption mechanisms. The biochar effectively reduced the lead concentration to levels meeting the standards for irrigation water, demonstrating its dual application not only as an amendment but also as an effective and stable sorbent for water purification, while direct potentiometry proved to be a promising method for studying such processes.

1. Introduction

The contamination of water resources by heavy metals represents one of the most pressing global environmental challenges of our time [1]. The situation is exacerbated by the fact that only about 3% of the planet’s water reserves are suitable for human consumption, while approximately 4 billion people annually face the problem of clean water scarcity [1]. Intensive industrial production, agriculture, and urbanization lead to the release of highly toxic substances into water bodies, among which heavy metal ions, particularly lead, hold a special place due to their high toxicity and ability to accumulate in living organisms [2,3].

Lead exerts detrimental effects on virtually all systems of the human body: the nervous system (impaired synapse formation, reduced intelligence in children), the circulatory system (anemia), the reproductive system (reduced fertility), and the kidneys [4]. It is important to note that there is no safe threshold for lead exposure—any presence in the environment is potentially hazardous [4]. Consequently, extremely stringent regulations for its content in water have been established [5,6], necessitating the development of highly efficient purification methods.

A range of methods is employed for the removal of heavy metals from aqueous environments, including electrochemical (electrocoagulation, electroflotation), membrane (nanofiltration, reverse osmosis), physico-chemical (coagulation), and adsorption technologies [7]. Among these, adsorption is recognized as one of the most effective, technologically simple, and economically feasible approaches, capable of achieving up to 99% removal efficiency and offering application flexibility [1,8]. Key advantages of adsorption include relatively low operational costs, the possibility of sorbent regeneration, and the absence of toxic sludge formation [3,8].

Various materials are used as adsorbents, ranging from traditional activated carbons and zeolites to modern nanocomposites [7,9]. However, many of them possess significant drawbacks, such as high cost, complex production processes, or challenges in scaling up.

Materials derived from biomass hold particular promise as cost-effective and efficient adsorbents, among which biochar stands out [1,10]. It is important to emphasize that biochar is a material with a dual purpose. Beyond agronomy, it finds applications in the purification of water and air, as well as in the remediation of contaminated soils [11,12,13,14]. For instance, birch biochar has proven to be highly effective as a sorbent in stormwater treatment systems, where it reduces the concentration of heavy metals (including lead and zinc) and phosphorus, demonstrating stability even in cold climates [15,16]. Large-scale field experiments have confirmed its ability to reduce the leaching of nitrogen and heavy metals from urban soils by 44% and 50%, respectively [11], and to adsorb nitrogen compounds from drainage water in forest harvest areas [12]. Furthermore, birch biochar is successfully used for the reclamation of oil-contaminated soils, achieving 33–48% decomposition of petroleum products [14], and exhibits an ability to adsorb pesticides and herbicides, especially after appropriate modification [17].

Specifically, biochar from birch wood (Betula alba), produced via slow pyrolysis at temperatures of 360–380 °C, is a well-studied and effective soil amendment for agriculture. Numerous studies have proven its ability to significantly reduce CO₂ emissions from soils (by 28–57%) [18,19], improve their water-air properties, increase pH, and enhance crop yields [18,20]. Field studies have also demonstrated that the application of birch biochar at doses of 15–20 t/ha leads to significant improvement in the structure of degraded irrigated soils and increases the yield of soybeans, rice, and wheat [21]. Its physicochemical characteristics, such as high carbon content (~78%), alkaline reaction (pH ~8.1), developed porosity (specific surface area ~73 m²/g), and moderate ash content (~5–7%), have been detailed specifically in the context of agricultural application [18,22].

Thus, birch biochar is positioned as a versatile tool for the circular economy, transforming wood waste into a valuable resource for addressing environmental challenges [13,23]. However, despite extensive data on its soil-improving properties and successful use in sorbing other metals (such as aluminum, iron, cadmium, and zinc) [11,23], the potential of this specific type of biochar as a specialized adsorbent for immobilizing toxic metals, particularly lead, remains critically understudied. This constitutes a significant knowledge gap, as the mechanisms and efficiency of metal binding can differ fundamentally from the mechanisms of soil improvement. While its ability to modify pore structure and retain organic particles [18] indirectly suggests high adsorption potential, a direct assessment of its adsorption capacity and kinetics towards lead had not been conducted prior to this study.

The comprehensive characterization of an adsorbent’s textural properties is crucial for understanding its adsorption potential. Alongside the standard BET method, the methylene blue (MB) adsorption technique is widely used for the preliminary assessment of the specific surface area accessible to large organic molecules [24,25]. This method serves not only as a simple and accessible screening tool but also provides insight into the surface chemistry, specifically the presence of negatively charged groups capable of ion exchange [26]. For functionalized carbon materials like biochar, the MB method can offer a more relevant assessment of the surface available in aqueous solutions compared to gas adsorption methods [27]. Thus, the MB adsorption method was employed in this work to complement the BET analysis and contribute to a more comprehensive understanding of the biochar’s surface properties in the context of lead ion adsorption.

A comprehensive investigation of the adsorption process, especially its kinetics, is constrained by the limitations of traditional analytical methods. Most studies are conducted via discrete sampling followed by analysis using AAS or spectrophotometry. This approach has low temporal resolution, carries a risk of introducing artifacts during sampling, and is labor-intensive [28]. In this context, direct potentiometry using ion-selective electrodes (ISEs) presents a powerful alternative. The method is characterized by rapidity (analysis takes 1–2 min), operational simplicity, the ability to work with turbid and colored solutions, and, most importantly, the capacity for continuous real-time monitoring without disturbing the system integrity [28,29]. Despite these advantages and the high selectivity of modern ISEs for lead ions, the application of the potentiometric method for in situ monitoring of metal adsorption on biochars remains rare and methodologically underutilized, which defines the scientific novelty of this research.

The objective of this work was a comprehensive investigation of the sorption properties and mechanisms of lead ion adsorption on birch-derived biochar (already used as a soil amendment) employing the direct potentiometry method for the accurate determination of equilibrium concentrations and continuous in situ monitoring of the process kinetics.

2. Materials and Methods

2.1. Biochar Characterization

All chemicals used in this study, including nitric acid (HNO₃), potassium nitrate (KNO₃), potassium hydroxide (KOH), lead nitrate (Pb(NO₃)₂), and methylene blue, were of analytical grade or higher. They were purchased from NevaReaktiv (St. Petersburg, Russia) and used without further purification.

The starting material was a commercially available birch biochar produced by Krasilov & Co. (Krasnoyarsk, Russia) via slow pyrolysis at 360–380 °C. As characterized in our previous study [11], the biochar possesses a specific surface area of 73.25 m²/g, a pore volume of 0.048 cm³/g, and a high carbon content of 78%. The low atomic ratios (H/C = 0.0518, O/C = 0.1452) indicate a high degree of carbonization.

2.1.1. Determination of Bulk Density

The bulk density was determined for the 0.3–0.5 mm fraction. A dried biochar sample was carefully loaded into a calibrated measuring tube, then compacted by tapping the tube walls with a glass rod until the volume stabilized. The mass of the compacted sample was determined on an analytical scale with an accuracy of ±0.0001 g. The bulk density (ρ, g/mL) was calculated using Equation (1):

$$\rho ={\displaystyle \frac{m}{V}}$$

(1)

where m is the mass of biochar (g), and V is the volume in the tube (mL). To ensure reproducibility, each sample was analyzed in six replicates, and the arithmetic mean value was recorded.

2.1.2. Determination of the Point of Zero Charge

The pH drift method was used to determine the point of zero charge (pH_PZC) of the biochar surface. All pH measurements were performed using a pH meter (Hanna Instruments HI2215, Hanna Instruments Inc., Woonsocket, RI, USA) equipped with a glass combination electrode and automatic temperature compensation. The electrode was calibrated daily using standard buffer solutions at pH 4.01 and 10.01. The experiment was conducted as follows: biochar samples weighing 0.05 g were contacted with 50 mL of 0.01 M KNO₃ solution for 92 h. The initial pH of the solutions varied from 3 to 11 by adding 0.5 M HNO₃ or KOH. To account for potential pH changes unrelated to the presence of the sorbent (e.g., due to CO₂ absorption), control experiments without biochar were conducted in parallel. The final pH value was measured after equilibrium was reached.

2.1.3. Evaluation of Methylene Blue (MB) Uptake

The adsorption of methylene blue (MB) on biochar was studied under static conditions using a batch experiments method. A stock MB solution (1000 mg/L) was prepared by dissolving an exact weight of the dye in distilled water. Working solutions with specified initial concentrations ranging from 5 to 130 mg/L were prepared by precise dilution of the stock solution in a 0.01 M KNO₃ background electrolyte. Biochar samples weighing 0.05 g were placed in polypropylene tubes and filled with 50 mL of an MB solution of a specified initial concentration, prepared in a 0.01 M KNO₃ background solution. The initial pH of the solutions was adjusted to the range of 6.3–6.8. The tubes were sealed tightly and incubated for 87 h at a constant temperature of 27 ± 4 °C on a vertical rotary shaker at 60 rpm to ensure constant mixing and achieve adsorption equilibrium. After incubation, the samples were centrifuged at 4000 rpm for 5 min to completely sediment the particles. The supernatant was carefully collected with a micropipette, avoiding the transfer of suspended particles into the analyzed sample, followed by filtration through a nylon membrane with a pore size of 0.45 µm. Control experiments without the sorbent were conducted in parallel to account for possible MB adsorption on the tube walls. pH values in the initial and equilibrium samples were monitored at all stages of the experiment using a pH meter. The absence of significant pH changes (ΔpH < 0.2) during the experiment indicated the buffer capacity of the system.

The quantitative determination of MB concentration in the solutions was performed by spectrophotometry using a UV mini-1240 instrument (Shimadzu, Kyoto, Japan) by measuring the optical density at the wavelength corresponding to the maximum absorption of methylene blue (λ = 660 nm). A calibration curve of optical density versus MB concentration was preliminarily constructed for the used concentration range.

The specific surface area accessible to methylene blue molecules (S_MB) was calculated using Equation (2), which assumes monolayer coverage:

$$S_{MB}={\displaystyle \frac{q_m\times N_A\times \sigma }{M}}$$

(2)

where q_m—calculated MB uptake capacity of the biochar (g/g), N_A—Avogadro’s number (6.022 × 10²³ mol⁻¹), σ—effective area occupied by one adsorbate molecule in a monomolecular layer (m²), M—molar mass of methylene blue (319.85 g/mol).

The value of σ is critical and depends on the molecule’s orientation on the adsorbent surface. Literature reports values ranging from approximately 66 Ų for an edge-on orientation to 130–135 Ų for a flat orientation [30,31]. To account for this uncertainty and provide a realistic range, the calculation was performed using σ values of 66 Ų and 130 Ų, yielding minimum and maximum estimates for the specific surface area.

2.1.4. Structural and Spectroscopic Analysis

The morphology of the biochar sample surfaces was studied by scanning electron microscopy (SEM) on a HITACHI TM 3000 scanning electron microscope (Hitachi High-Technologies Corporation, Tokyo, Japan). The analysis was performed at an accelerating voltage of 5–15 kV and an electron beam current of ~100 pA, achieving high resolution without significant thermal damage to the carbon material surface. Before microscopic analysis, the lead-saturated biochar samples underwent a multi-stage washing procedure specifically designed to remove weakly adsorbed and non-specifically bound Pb²⁺ ions, as well as any water-soluble salts that could form crystalline artifacts upon drying. This step is crucial to ensure that the lead detected via SEM-EDS analysis represents the fraction strongly bound to the biochar surface. The procedure involved repeated washing with an excess of 0.01 M KNO₃ solution, followed by vacuum filtration through an ash-free “white ribbon” filter. The samples were then dried at 60 °C to a constant mass. It is important to note that this preparation was not a regeneration treatment for sorbent reuse, but a necessary step for accurate analytical characterization.

Powder X-ray diffraction analysis was performed to characterize the crystalline phase composition of the birch biochar before and after lead adsorption. Measurements were carried out on a STADI P (STOE & Cie GmbH, Darmstadt, Germany) powder diffraction system operating in transmission mode with Bragg–Brentano geometry. The instrument was equipped with a copper anode (λ = 1.541874 Å) and nickel Kß-filter, using a MYTHEN2 1D (DECTRIS) detector (DECTRIS Ltd., Baden-Daettwil, Switzerland). Data collection was conducted at room temperature over the 2θ range from 5 to 80° with a step size of 0.5°.

Fourier transform infrared (FTIR) spectra were recorded on an IR-Affinity-1 spectrometer (Shimadzu, Japan) equipped with a single-reflection attenuated total reflectance (ATR) accessory QATR-10. Sample preparation involved compressing approximately 30 mg of material into pellets using a hydraulic press CY-PC-24 (CY-PC-24, Zhengzhou CY Scientific Instrument Co., Ltd., Zhengzhou, China) under 10 MPa pressure in a 5 mm diameter mold. Transmission spectra were collected and normalized to the total area in the 4000–400 cm⁻¹ range.

Elemental surface analysis was performed by energy-dispersive X-ray spectroscopy (EDS) using a Bruker spectrometer (Bruker Corporation, Berlin, Germany) installed on the SEM.

Parameters for the adsorption model equations were calculated using non-linear fitting of experimental values with corresponding function plots using the “Veusz” program (ver. 3.6.2, GNU GENERAL PUBLIC LICENSE).

2.2. Biochar Conditioning

Prior to investigations, biochar underwent multi-stage preparation to remove impurities and salts and bring the material into reproducible state for further experiments. The initial material was grounded in a porcelain mortar and then fractionated on a vibrating screen (20 cm in diameter), collecting the fraction of 0.3–0.5 mm. This range was selected as a compromise between high specific surface area needed for sorption and preservation of particle structural integrity under mechanical stress.

To remove water-soluble components, the fraction was sequentially treated by decantation with distilled water, followed by double boiling (10 min, water/biochar ratio = 10) to desorb salts and remove fine particles from the pore structure. Washing was completed by vacuum filtration using an ash-free “white ribbon” filter, ensuring retention of particles larger than 2–3 µm. The biochar was dried at 60 °C for 12 h until constant mass was achieved.

The obtained samples, represented by mechanically stable particles with a developed rough surface, were stored in sealed glass containers with ground-in stoppers to prevent sorption of atmospheric moisture and gases.

2.3. Adsorption Experiments

2.3.1. Determination of Equilibrium Lead Concentrations by Potentiometric Method

To construct the lead adsorption isotherm, working solutions with Pb²⁺ concentrations ranging from 1.6 to 50 mg/L were prepared by serial dilution of a concentrated Pb(NO₃)₂ stock solution in a 0.01 M KNO₃ background electrolyte. The pH of all working solutions was adjusted to 5.0 using 0.5 M HNO₃ or KOH.

Equilibrium lead concentrations after adsorption on biochar were determined by direct potentiometry using a lead ion-selective electrode. The choice of the potentiometric method was due to its key advantages for monitoring sorption processes: high selectivity of modern ISEs for Pb²⁺ ions even in the presence of other metals, a wide linear measurement range (covering up to 6 orders of concentration), low detection limit (down to the picomolar level), and the possibility of continuous monitoring without disturbing the system integrity [32,33]. Additional advantages of the method are low analysis cost and minimal sample preparation requirements [34].

The measuring system consisted of an ion-selective electrode with a crystalline membrane based on lead sulfide (ELIT-231) and a double-junction reference electrode (ESR-10101). Crystalline membranes are characterized by high selectivity due to a vacancy charge transfer mechanism and stable readings over a wide pH range [35]. The outer junction of the reference electrode was filled with a 0.1 M KNO₃ solution to prevent chloride contamination of the sample. Potential measurement was performed using a multi-channel “Expert-001” (Ekonix-Expert LLC, Moscow, Russia) measuring instrument with automatic data recording at fixed time intervals until stable values were established. Critical attention was paid to controlling the pH of the medium. All solutions were filtered through nylon membrane filters with a pore diameter of 0.45 µm before measurement, then acidified to pH 4.6–4.9 with nitric acid to ensure potential stability and prevent the formation of insoluble lead hydroxo-complexes [35].

Given the complex salt composition of the biochar water-salt extract, all calibrations and quantitative measurements were performed in the matrix of the analyzed solution. A 0.01 M KNO₃ solution was used as the background electrolyte. A multi-level control protocol was used to ensure accuracy.

An initial full calibration by the standard addition method in a “blank solution” (biochar extract without added lead) revealed two linear sections with characteristic slopes of 15.64 and 25.82 mV/decade (Figure 1), confirming significant matrix effects and justifying the use of a matrix-dependent methodology. The low-concentration segment (0.001–1 mg/L) was used for equilibrium concentrations below 1 mg/L, while the high-concentration segment (1–100 mg/L) was applied for concentrations above 1 mg/L to ensure measurement accuracy across the studied concentration range.

For each subsequent experimental series, an abbreviated calibration with 3–5 points in the corresponding concentration range was performed using a control experiment (solution with biochar without lead). The choice of equation for calculation was based on the measured potential value of the analyzed sample. The slope parameter (A) was considered constant for a given matrix, and the intercept (B) was adjusted to account for potential electrode drift. This approach combined the high accuracy of the standard addition method with operational control of the electrode system stability.

The potentiometric measurement methodology was validated by comparison with atomic absorption spectroscopy (AAS) measurements; the detailed comparative results are provided in Table S1 of the Supplementary Materials. The calculated (or equilibrium) amount of adsorbed substance per unit mass of biochar (q_e, mg/g) was determined by Formula (3), and the purification factor (PF) was determined by Formula (4).

$$q_e={\displaystyle \frac{\left(C_0-C_e\right)\times V_s}{m_s}}$$

(3)

$$PF={\displaystyle \frac{C_0}{C_e}}$$

(4)

where C₀—initial Pb²⁺ concentration (mg/L), C_e—equilibrium Pb²⁺ concentration (mg/L); V_s—solution volume (L), m_s—biochar weight (g).

2.3.2. Adsorption Isotherm Studies

Experiments to establish the Pb²⁺ adsorption isotherm on biochar were conducted under static conditions (batch mode). A 0.01 M KNO₃ solution was used as the background electrolyte to maintain constant ionic strength and minimize competitive adsorption of other cations. Working solutions with specified Pb²⁺ concentrations were prepared by diluting a concentrated solution, previously prepared from an exact weight of Pb(NO₃)₂ in this background electrolyte.

The lead adsorption isotherm on biochar was obtained as follows. A 0.01M KNO₃ solution with pH 5.0 and a specified lead concentration was used as the model solution; pH adjustment was performed with 0.5M HNO₃ or 0.5M KOH. The pH value was additionally adjusted because, with increasing lead concentration in the solution, the pH decreases due to lead hydrolysis, hence the need to create identical initial conditions for the entire series of experiments. This pH value was chosen to prevent lead ion hydrolysis, which intensifies significantly at pH > 6.0, and to ensure optimal conditions for the ion-selective electrode operation. The liquid-to-solid ratio (L/S) was 1000 mL/g, with a sorbent mass of 0.05 g. Control experiments without biochar were conducted in parallel under identical conditions to account for potential pH variations unrelated to the adsorption process.

A precise sample of biochar weighing 0.05 g was added to each experimental vial. The liquid-to-solid ratio (L/S) was kept constant at 1000 mL/g, achieved by adding 50 mL of the model solution to each sorbent sample. Such a high L/S ratio allows for obtaining a wide range of equilibrium concentrations and minimizes the dilution effect due to sorbent moisture. The exposure time was 3–4 days to guarantee the achievement of adsorption equilibrium, as confirmed in kinetic experiments. Throughout the contact time, the samples were continuously mixed on a vertical rotary shaker at 60 rpm to eliminate external diffusion limitations and ensure complete contact between the sorbent and the solution.

After equilibrium was reached, the solid phase was separated from the liquid phase by decantation followed by filtration of the supernatant through nylon membrane filters with a pore diameter of 0.45 µm. Filtration is necessary to completely remove fine biochar particles formed during stirring, which could distort the analysis results. Before measuring the equilibrium Pb²⁺ concentration by ionometry using the ion-selective electrode, all filtered solutions were acidified with 0.5 M HNO₃ to pH 4.8.

The obtained experimental adsorption data were fitted using three classical isotherm models: Freundlich (5), Langmuir (6), and Sips (7) equations. The nonlinear regression analysis was performed to determine the model parameters that best describe the experimental data. The model parameters obtained from nonlinear regression were used to calculate adsorption capacity across the studied concentration range.

$$q_e=K_f\times C_e^{1/n_F}$$

(5)

$$q_e={\displaystyle \frac{q_m\times K_l\times C_e}{1+K_l\times C_e}}$$

(6)

$$q_e={\displaystyle \frac{q_m\times K_{lf}\times C_e^{1/n_S}}{1+K_{lf}\times C_e^{1/n}}}$$

(7)

The dimensionless equilibrium parameter (R_L) was calculated using Equation (8).

$$R_L={\displaystyle \frac{1}{\left(1+K_l\times C_0\right)}}$$

(8)

where q_m—maximum adsorption capacity (mg/g); K_f—Freundlich constant characterizing the relative adsorption capacity and representing the adsorption value at unit equilibrium concentration; K_l, K_lf—adsorption equilibrium constants characterizing the adsorbent-adsorbate bond energy; n_F—heterogeneity index of exchange sites in Freundlich model, n_S—heterogeneity index in Sips model, characterizing the change in adsorption heat depending on their degree of filling.

2.3.3. Adsorption Kinetic Studies

The kinetics of lead ion adsorption on biochar were studied under static conditions with continuous potentiometric monitoring. To eliminate the influence of the biochar’s buffer capacity, preliminary sorbent preparation was conducted: a 200 mg biochar sample in a mesh bag made of chemically inert material was kept for 24 h in 200 mL of 0.01 M KNO₃ under constant stirring (150 rpm). After equilibrium was reached, the solution was acidified with 0.5 M HNO₃ to pH 4.80 ± 0.05, and conditioning continued for the next 24 h. The procedure was repeated until the buffer capacity was completely exhausted and the pH stabilized at 4.80 without subsequent shift.

After pH stabilization, the bag with biochar was removed, and the prepared solution was transferred to the measurement cell. A standard Pb(NO₃)₂ solution was added to the solution to achieve an initial Pb²⁺ ion concentration of 5.0 mg/L (kinetic experiment No. 1) or 40.4 mgL (kinetic experiment No. 2). The electrode system was kept until the potential stabilized (drift < 0.1 mV/min) for 40 min, after which the bag with biochar was returned to the cell, and potential recording was started. A control experiment (“blank”) was conducted in parallel using an identical protocol but without lead addition to quantitatively assess the potential drift of the electrode system.

The main advantage of the used approach was the possibility of continuous potentiometric recording in real-time. This provided high temporal resolution of kinetic data, allowing detailed recording of the initial stages of the adsorption process. Continuous measurements eliminated artifacts associated with discrete sampling and ensured the recording of even minor concentration changes. This approach allowed for obtaining detailed kinetic curves suitable for building reliable mathematical models of the adsorption process.

The obtained kinetic data were processed using pseudo-first (9), pseudo-second order (10), Elovich (11), and Weber-Morris (12) models.

$$q_t=q_e\left(1-e^{-k_{1}t}\right)$$

(9)

$$q_t={\displaystyle \frac{k_2{q}_e^{2}t}{1+k_2{q}_{e}t}}$$

(10)

$$q_t={\displaystyle \frac{1}{\beta }}\mathit{ln}\left(1+\alpha \beta t\right)$$

(11)

$$q_t=k_i\times t^{0.5}+C$$

(12)

where q_t—amount of adsorbed substance per unit mass of adsorbent at time t (mg/g); t—time (days), k₁—pseudo-first order rate constant (1/day); k₂—pseudo-second order rate constant (g/(mg × day)); α—initial adsorption rate (mg/(g × day)); β—parameter related to activation energy and surface area (g/mg) in the Elovich equation, k_i—intraparticle diffusion rate constant (mg/g/min^1/2), C—parameter characterizing the boundary layer effect (mg/g).

2.3.4. Study of the Liquid-to-Solid Ratio Influence and Desorption

To evaluate the stability of sorbed lead and its potential for desorption under different L/S ratios, experiments were conducted at a constant initial lead ion concentration while varying the sorbent mass in the range from 10 to 500 mg at a constant solution volume (50 mL), corresponding to an L/S range from 100 to 5000 mL/g. After reaching adsorption equilibrium, the samples were processed and analyzed according to the methodology outlined in Section 2.3.1. The main process efficiency parameters, q_e and PF, were calculated using Equations (3) and (4), respectively.

To directly evaluate the strength of lead ion binding and potential desorption risk, a sequential extraction experiment was conducted. In the first stage, lead-saturated biochar samples, obtained under different L/S conditions, were separated from the solution by centrifugation and subjected to extraction with distilled water for 120 min. To stabilize the ionic strength necessary for the correct operation of the ion-selective electrode, the filtrate was acidified with nitric acid before analysis, and a KNO₃ solution was added to a final concentration of 0.01 M. In the second stage, the procedure was repeated using a 0.01 M calcium nitrate (Ca(NO₃)₂) solution as the extractant. Since the extractant solution already had sufficient ionic strength, the samples after filtration were only acidified. The lead concentration in all samples was determined by the potentiometric method. The desorbed fraction was calculated as the ratio of lead released into solution during extraction to the initially sorbed amount.

The nature of the dependencies of adsorption parameters on the L/S ratio indicated their non-linear power-law character. The linearization method was applied to determine the equation parameters. Logarithm of the variables q_e and PF relative to L/S was performed, followed by linear regression analysis of the transformed data using the least squares method.

3. Results

3.1. Main Characteristics of Biochar

The main physicochemical characteristics of the studied birch biochar are presented in

Table 1. Main characteristics of the studied biochar.

Parameter	Value
Fraction (mm)	0.3–0.5
Bulk weight (g/mL)	0.32 ± 0.01
Ash content (%)	9.27% ± 1.98%
Zero charge point	6.97

. The material is characterized by low bulk density and moderate ash content.

The point of zero charge (pH_PZC) was determined to predict the surface charge of the sorbent depending on the pH of the medium and to interpret the adsorption mechanisms. As can be seen in Figure 2, the pH_PZC value for the studied biochar was 6.97.

Diffraction patterns of both pristine and lead-saturated biochar samples revealed three distinct crystalline phases: calcite (CaCO₃), whewellite (CaC₂O₄·H₂O), and quartz (SiO₂) (Figure 3). All samples exhibited a characteristic amorphous halo in the 22–24° 2θ range, consistent with observations of amorphous carbon materials reported in the literature [36]. This amorphous halo, typically centered around 21–24° 2θ with corresponding interlayer spacing values greater than 3.35 Å, represents the disordered carbon matrix characteristic of biochars.

Quantitative phase analysis using the Rietveld method demonstrated exceptional stability of the crystalline composition following lead adsorption. The pristine biochar showed mass fractions of: calcite 50.5%, quartz 8.0%, and whewellite 41.4%. After lead adsorption, the composition exhibited minimal variation: calcite 48.8%, quartz 8.6%, and whewellite 42.7%. Crucially, no new diffraction maxima corresponding to lead-containing crystalline phases were detected in the post-adsorption samples.

Figure 4 presents the FTIR spectra of the original biochar and the sample after lead adsorption. The absorption infrared spectra, obtained in transmission mode and normalized to the total area in the 4000–400 cm⁻¹ range, revealed several changes. In the O–H stretching vibration region (3850–3600 cm⁻¹), an increase in intensity and shifts of the bands at 3860, 3740, and 3610 cm⁻¹ were observed. In the carbonyl and carboxylate vibration region, an increase in intensity of the band at 1568 cm⁻¹ and a decrease in intensity of the shoulder near 1684 cm⁻¹ were recorded. A new band appeared at 1064 cm⁻¹ in the C–O stretching vibration region. In the low-frequency region of the spectrum (718–400 cm⁻¹), a decrease in transmission relative to the original spectrum was observed. The intensity of the doublet in the 2350 cm⁻¹ region, characteristic of carbonate groups, increased after sorption. In the C–H stretching vibration region, the band at 2850 cm⁻¹ disappeared and a new band appeared at 2976 cm⁻¹. The spectral regions 2940–2408 cm⁻¹ and 2270–1740 cm⁻¹ showed a general increase in the transmission level while maintaining the shape of the original spectrum.

3.2. Methylene Blue Adsorption

The study of methylene blue (MB) adsorption was conducted for a comprehensive characterization of the sorption properties of the biochar and interpretation of the lead ion adsorption mechanisms. Figure 5 presents the adsorption isotherm of methylene blue (MB) on birch biochar.

To quantitatively describe the equilibrium, the obtained experimental data were approximated by three classical models: Langmuir, Freundlich, and Sips (Figure 5). The fitting results are presented in

Table 2. Parameters of Freundlich, Langmuir, and Sips models for methylene blue adsorption.

Equation	Parameter	Value
Freundlich	Kf	2.68 ± 0.29
	nF	5.89 ± 1.08
Langmuir	Kl	0.82 ± 0.22
	qm	5.18 ± 0.22
Sips	Klf	0.75 ± 0.21
	qm	5.70 ± 0.76
	nS	1.56 ± 0.63

.

Based on the methylene blue adsorption value, an approximate assessment of the specific surface area (S_MB) accessible for the sorption of large organic molecules was carried out. The calculation was performed assuming the formation of a monomolecular layer. Considering the ambiguity of molecular orientation, a range of possible values S_MB = 4.0–6.6 m²/g was calculated.

3.3. Adsorption Isotherm of Lead Ions

Figure 6 presents the experimental data on the adsorption of lead ions on biochar, as well as the results of mathematical modeling using Freundlich, Langmuir, and Sips equations.

According to the results of nonlinear regression (

Table 3. Parameters of adsorption models for Pb²⁺ ions.

Equation	Parameter	Value
Freundlich	Kf	9.94 ± 0.49
	nF	7.71 ± 1.10
Langmuir	Kl	9.32 ± 1.30
	qm	14.21 ± 0.43
Sips	Klf	16.40 ± 8.23
	qm	13.94 ± 0.44
	nS	0.81 ± 0.13

), the best description of the experimental data in the studied concentration range is achieved using the Sips model, which is confirmed by the highest values of the coefficient of determination (R²).

For a quantitative assessment of the adsorption favorability, the dimensionless equilibrium parameter R_L was used. The calculated R_L values range from 0.002 to 0.063 for the entire range of initial concentrations (1.6–50 mg/L). Figure 7 shows the model-calculated and experimental values of the equilibrium lead concentration in the model solution.

During the experiment, the change in pH value of the solutions after reaching adsorption equilibrium was recorded. As shown in Figure 6, a clear dependence was observed between the initial concentration of lead ions and the equilibrium pH value. The observed systematic decrease in the equilibrium pH with increasing initial lead concentration (Figure 8) provides evidence for the ion exchange mechanism. Control experiments without biochar showed negligible pH variation (≤0.1 units), confirming that the observed acidification specifically results from biochar–lead interaction.

3.4. Kinetic Studies

Kinetic studies of Pb²⁺ ion adsorption on birch biochar revealed a two-stage nature of the process, well described by the Weber-Morris intraparticle diffusion model (Figure 9).

Pseudo-first (PFO), pseudo-second order (PSO) and Elovich models were also applied to quantitatively describe the kinetic curves (Figure 10).

An important aspect for the practical application of the sorbent is not only q_e, but also the removal efficiency. As demonstrated by the dependencies presented in Figure 10a*,b*, a pronounced decrease in the removal efficiency of lead ions is observed with an increase in their initial concentration.

A state close to equilibrium is achieved within 48 h for both studied initial concentrations. The parameters of the kinetic models are presented in

Table 4. Parameters of kinetic models.

Model	Parameter	Kinetic Experiment No. 1	Kinetic Experiment No. 2
PFO	qe	4.39 ± 0.03	14.3 ± 0.1
	k1	0.0018 ± 0.0001	0.0015 ± 0.0001
	R2	0.99457	0.99542
PSO	qe	4.97 ± 0.03	17.1 ± 0.2
	k2	0.00052 ± 0.00002	0.00010 ± 0.00001
	R2	0.99838	0.99543
Elovich	a	0.038 ± 0.001	0.046 ± 0.001
	β	1.089 ± 0.006	0.232 ± 0.002
	R2	0.99984	0.99943

.

3.5. Influence of Liquid-to-Solid Ratio and Desorption Studies

The conducted study revealed key patterns in the process of lead ion adsorption on birch biochar, determined by the liquid-to-solid ratio (L/S). A compromise between the efficiency of metal removal from the solution and the degree of utilization of the material’s sorption capacity was established (Figure 11).

Power-law approximation revealed quantitative dependencies (

Table 5. Parameters of power regression equations for qₑ and PF versus L/S.

Dependent Variable	Equation	Parameter a	Parameter b	R2
qe	$q_e=a\times {\left(L/S\right)}^b$	0.046 ± 0.007	0.716 ± 0.110	0.979
PF	$PF=a\times {\left(L/S\right)}^b$	(4.41 ± 0.70) × 105	−1.418 ± 0.229	0.986

).

$q_e=a\times {\left(L/S\right)}^b$$PF=a\times {\left(L/S\right)}^b$ A critically important aspect for the practical application of the sorbent is the stability of the adsorbed metal and its resistance to being released back into the environment. Sequential extraction experiments showed that only a small portion of lead (0.07–0.69%) is desorbed by distilled water (Figure 12), and this value is statistically independent of the degree of saturation. In contrast, desorption with a 0.01 M Ca(NO₃)₂ solution showed a pronounced linear dependence on the degree of saturation.

3.6. Microscopy and Elemental Analysis

Scanning electron microscopy (SEM) was used to analyze the microstructure and assess the structural stability of the obtained biochar before and after the adsorption process (Figure 13). As can be seen in Figure 13a,a*, the pyrolysis of birch wood leads to the formation of a carbon material with a developed hierarchical pore structure.

A critically important aspect is the preservation of the integrity of this hierarchical structure after the Pb²⁺ adsorption process, which is clearly demonstrated in Figure 13b,b*.

The presence of adsorbed lead on the surface was confirmed by EDS analysis (Figure 8). The results revealed significant heterogeneity in the distribution of the element, which is typical for porous materials. The lead content varied in the range from 0.06 to 0.64 at.%. An estimate of the approximate lead content by converting atomic percentages to mass percentages yielded a value of 44.7 mg/g.

Analysis of the element distribution map (Figure 14a) demonstrates a clear correlation between morphology and element localization. Lead is localized in the form of separate clusters. The EDS spectrum (Figure 14b) confirms the presence of characteristic peaks of carbon, oxygen, and lead.

4. Discussion

4.1. Sorption Properties and Adsorption Mechanisms

The pH_PZC value for the studied biochar was 6.97. This suggests that at pH values below 6.97, the biochar surface likely has a net positive charge, favoring the adsorption of anionic compounds. Conversely, at pH > 6.97, the surface presumably acquires a net negative charge, which may promote the sorption of cationic substances, such as methylene blue. However, it is important to note that at pH > 6.0, lead hydrolysis processes intensify, which can distort the actual adsorption results due to precipitate formation.

The choice of methylene blue as a model adsorbate was motivated not only by its classic use for assessing sorption capacity but also by the goal of providing an approximate evaluation of the specific surface area accessible to large organic molecules and indirectly characterizing the ion-exchange potential of the biochar surface [24,25,37,38]. Methylene blue is a cationic dye whose molecules in monomeric form have dimensions of ~1.7 × 0.76 × 0.325 nm. The effective projection area of the molecule on the surface depends on its orientation and varies from ~66 Ų at an angled orientation to ~130–197 Ų at a flat orientation [30,31,38,39]. Traditionally, MB adsorption is associated with electrostatic attraction to negatively charged surface sites and dispersion interactions (van der Waals forces) with the carbon matrix [38,40,41,42].

For the studied biochar, adsorption experiments were conducted at an initial pH of 6.3–6.8, i.e., under conditions close to or slightly below the pH_PZC. This means the sorbent surface had a weak positive or neutral net charge. Despite this, significant adsorption of the cationic dye was recorded. Based on the coefficient of determination, it can be concluded that the Sips model best describes the experimental methylene blue adsorption data in the studied concentration range. The calculated maximum adsorption capacity value (q_m) obtained from the Sips model was 5.70 ± 0.76 mg/g. The possibility of methylene blue adsorption on a neutrally charged surface, along with the significant surface heterogeneity (n_S > 1), suggests that the dominant contribution to the process comes from non-specific interactions (dispersion forces) with the carbon matrix, as well as possible ion-exchange processes, rather than electrostatic attraction [38].

The obtained low S_MB values, however, are inconsistent with the developed macroporous structure revealed by SEM. This contradiction is explained by the fact that the methylene blue method characterizes not the pore morphology, but the functional accessibility of the surface for large molecules [24,42]. The low SMB value indicates limited accessibility of the biochar’s internal surface for bulky dye molecules, which is typically associated with the presence of a significant volume of narrow micropores (<~1.5 nm) inaccessible to MB [25,37,40,43]. Furthermore, MB adsorption on a surface with a net charge near its point of zero charge suggests that non-specific interactions (dispersion forces) and ion exchange play a more significant role than pure electrostatic attraction for this cationic dye [26]. Conducting the MB adsorption study in the context of lead removal work has important methodological significance. It demonstrates that the biochar surface retains significant sorption potential even under conditions unfavorable for electrostatic cation attraction (pH ≈ pH_PZC). This indicates the key role of non-specific interactions and ion exchange in the sorption process, which is fundamentally important for explaining the high Pb²⁺ adsorption indicators obtained in this work.

The obtained adsorption capacity value for birch biochar with respect to methylene blue (5.70 ± 0.76 mg/g) is in the lower range of values reported in the literature for biochars from various origins. For instance, biochar from mixed municipal waste [44] and wheat straw [45] showed similar values (7.2 and 12.03 mg/g, respectively), while biochars from rapeseed straw [46], date pits [47], and eucalyptus bark [48,49] have capacities that are one to two orders of magnitude higher (97.3–104.2 mg/g). Chemically activated sorbents demonstrate the highest capacity (e.g., 268.46 mg/g for NaOH-activated rapeseed straw carbon [46] and 290.71 mg/g for NaOH-modified corn straw biochar [49,50]).

It is fundamentally important that high adsorption capacity is typically achieved through the use of additional activation stages that significantly increase the specific surface area and mesopore volume [49,50]. This study used unprepared birch biochar obtained via traditional pyrolysis simulation, without an additional activation step. Therefore, direct comparison of its capacity with activated materials may be incorrect.

Despite the relatively low capacity, birch biochar, like other unmodified biochars [45,47], is of significant practical interest from the standpoint of economic efficiency and resource conservation.

Thus, the obtained adsorption capacity values adequately reflect the properties of unmodified wood biochar. It is advisable to consider it not as a high-capacity sorbent for deep purification, but as an economical and environmentally friendly material for use as a sorbent loading in barrier technologies, for soil melioration, or pre-treatment of wastewater, where the combination of acceptable efficiency and low cost is critically important [45,47].

The conducted study of methylene blue adsorption allows for a comparative analysis of the sorption properties of the obtained birch biochar in the context of modern literature data. The obtained adsorption capacity value according to the Sips model (5.70 ± 0.76 mg/g) demonstrates characteristics consistent with unmodified biochars from mixed municipal waste (7.2 mg/g) [44] and wheat straw (12.03 mg/g) [45]. At the same time, this indicator is significantly inferior to values characteristic of biochars from rapeseed straw (97.3 mg/g) [46], eucalyptus bark (104.2 mg/g) [48,49], and date pits (42.57 mg/g) [47]. As evidenced by literature data, alkali-activated biochars possess the greatest efficiency (up to 290.71 mg/g) [49,50], which is due to the targeted modification of the porous structure during chemical activation.

The low adsorption capacity of the studied material with respect to methylene blue, confirmed by the calculated S_MB values, is determined by the structural features of its surface. The presence of a developed macropore system, performing primarily a transport function, combined with the probable predominance of micropores inaccessible to bulky dye molecules, limits the effective area available for sorption of large organic compounds.

It should be noted that, as demonstrated by the results of other studies [45,47], the key advantage of unmodified biochars is not high adsorption capacity, but the economic feasibility of their application and the technological simplicity of production. In this context, birch biochar can be considered as an accessible and eco-friendly sorbent for solving the problems of preliminary water treatment from cationic pollutants under conditions where achieving maximum sorption capacity indicators is not required [49,50].

4.2. Adsorption Characteristics of Biochar with Respect to Lead

The observed scatter of experimental points in the region of high equilibrium concentrations may be due to the heterogeneity of the sorbent surface, increasing as the state of complete saturation of active centers is approached.

The interpretation of the Sips model parameters is the most justified, since this model best describes the experimental data. According to the calculated values, the maximum adsorption value (q_m) is 14.21 and 13.94 mg/g for the Langmuir and Sips models, respectively. The value of the heterogeneity parameter n_S in the Sips model (0.81 ± 0.13), being statistically significantly less than unity, convincingly indicates the energetic heterogeneity of the biochar surface and the favorable nature of the adsorption (high affinity at low concentrations).

Regarding the Freundlich model, despite its limited applicability for describing this system (as indicated by the lower R²), the obtained parameter value n_F > 1 (7.71 ± 1.10) may indirectly indicate the presence of complex effects not accounted for by other models, such as cooperative adsorption or changes in the surface state during sorption.

It is worth noting that despite the relatively low maximum capacity, the K_l value characterizes high binding energy and indicates high affinity of the sorbent for Pb²⁺, making it particularly useful for purifying waters with low concentrations of this toxic metal.

The calculated R_L values are in the range of 0.002–0.063 for the entire range of initial concentrations (1.6–50 mg/L), indicating a highly favorable nature of adsorption in the studied concentration range.

It is worth noting that despite the high R² values, the Langmuir model fit yields systematically lower equilibrium concentrations compared to experimental values, especially in the region of medium and high C₀. This can be explained by the presence of a small fraction of weak or non-specific adsorption centers that bind ions less strongly, or kinetic limitations that prevented the system from reaching ideal equilibrium for these centers, as well as the limitation of the lower detection limit of the ionometry method.

The conducted study revealed a noticeable gap between the theoretical potential of the sorbent, predicted by adsorption models, and the practical purification results (Figure 7). Although extrapolation based on the Langmuir isotherm parameters suggests the possibility of reducing the lead concentration to strict drinking water standards (0.01 mg/L according to WHO recommendations [5] and SanPiN 1.2.3685-21 [6]), the experimentally achieved range of equilibrium concentrations (0.022–0.178 mg/L) corresponds to standards for non-potable water. Thus, the practically obtained values satisfy the requirements for irrigation water (up to 0.5 mg/L at an application rate of 10,000 m³/ha according to FAO recommendations [51]) and are within the limits established for the discharge of wastewater into the centralized sewerage system (no more than 0.25 mg/L according to the Rules for Cold Water Supply and Sanitation [52]). This allows us to recommend biochar for the effective treatment of lightly contaminated waters (with an initial lead concentration up to 7 mg/L) for their subsequent use in agriculture or discharge into the sewer system, where it provides a competitive and economical solution. However, to achieve drinking water standards, as well as for fisheries water bodies (MPC 0.006 mg/dm³ according to RD 52.24.448-2009 [53]), further process optimization aimed at overcoming limitations associated with sorption kinetics and the heterogeneity of the material’s active centers is necessary.

The observed systematic decrease in the equilibrium pH value with an increase in the amount of sorbed lead (Figure 8) is convincing evidence in favor of ion exchange with protons of acidic oxygen-containing functional groups (carboxyl, phenolic) being one of the dominant mechanisms of Pb²⁺ sorption on this biochar [54,55,56]. This phenomenon is explained by the competition between Pb²⁺ and H⁺ for active sites on the biochar surface. Thus, the release of H⁺ into the dissolved phase is the direct cause of the observed acidification of the medium. The intensity of this process correlates with the amount of adsorbed metal ions.

The obtained data are in good agreement with literature data, according to which ion exchange contributes significantly (more than 50%) to the total adsorption of heavy metal cations on oxygen-rich biochars [57,58]. In particular, studies show that this mechanism plays a key role for unmodified biochars derived from palm leaves and bamboo dust [59,60,61]. In addition to ion exchange, complexation and coordination of lead ions with carboxylate and other oxygen-containing groups, as well as interaction with π-electrons of the carbon matrix, may play a certain role, especially in the later stages of the process [58,62]. However, unlike biochars derived from agricultural wastes rich in carbonates and silicates, or those intentionally modified with alkaline earth metal salts [57,63], where the primary mechanism is precipitation as insoluble compounds [64], the contribution of the precipitation mechanism for the studied birch biochar is likely minor. This is supported by the absence of significant medium alkalinization and morphological changes in the SEM micrographs (Figure 13).

It is worth noting that the obtained value of adsorption capacity (q_m = 14.21 ± 0.43 mg/g) is in the lower part of the wide range characteristic of unmodified biochars (2.4–147 mg/g) [55] and is consistent with data for biochars from wastewater sludge (18.56 mg/g) [65] and sunflower husks (~33 mg/g) [66]. At the same time, it is significantly lower than the indicators for activated or functionalized sorbents, where the capacity can reach 150–395 mg/g due to chemical modification, porous structure development, or synthesis of composite materials [60,67,68,69]. This is expected, since this study used unprepared biochar without a stage of chemical or physical activation aimed at increasing the specific surface area and the number of functional groups. Nevertheless, the high value of the Langmuir constant (K_l = 9.32 ± 1.30 L/mg) indicates a high affinity of the surface specifically for lead ions, despite possible competition from other cations. Literature data on the selectivity of biochars vary: a number of studies indicate higher efficiency of lead extraction compared to copper, cadmium and zinc (Pb²⁺ > Cu²⁺ > Cd²⁺ ≈ Zn²⁺) [70], while others note the intermediate position of lead in the series Cd > Zn > Pb > Cu [71]. The high affinity of the studied sorbent for Pb²⁺, confirmed by low R_L parameter values, allows us to assume that its selectivity is close to the first variant.

4.3. Structural Transformations, Surface Chemistry and Morphology

As can be seen in Figure 13a,a*, the pyrolysis of birch wood leads to the formation of a carbon material with a developed hierarchical pore structure, which is a direct legacy of the original biomass anatomy. A structure characteristic of hardwood species is observed, with alternating structural elements: macropores with a size of 40–100 μm, corresponding to former conducting vessels (tracheae), as well as a network of pores with a diameter of 5–15 μm, morphologically corresponding to tracheids and parenchyma cells [72]. The pore length significantly exceeds their diameter, which is typical for wood biochar and indicates the preservation of the original wood texture after thermal treatment [71,73].

A critically important aspect is the preservation of the integrity of this hierarchical structure after the Pb²⁺ adsorption process, which is clearly demonstrated in Figure 13b,b*. Comparative analysis reveals no signs of destruction or pore collapse. The absence of morphological changes indicates high mechanical strength and chemical stability of the biochar framework and excludes destructive sorption mechanisms.

The obtained data are in good agreement with literary sources. As noted in works [71,72], despite the smaller specific surface area of birch biochar (5.92–7.17 m²/g) compared to coniferous analogues, its microstructure is characterized by a more developed system of meso- and macropores, which ensures effective transport of ions to active centers. It is this combination—a developed macropore system performing the role of transport channels, and the presence of micropores with functional groups—that determines the high cation exchange capacity and efficiency of heavy metal sorption, despite the relatively low specific surface area [71].

An estimate of the approximate lead content by converting atomic percentages to mass percentages yielded a value of 44.7 mg/g, which exceeds the volumetric adsorption capacity calculated using the Langmuir model (14.21 mg/g). This discrepancy can be explained both by the surface nature of EDS analysis, which records an increased concentration in the surface layer, and by the limitation of the process due to diffusion of ions deep into the sorbent particles.

Analysis of the element distribution map (Figure 14a) demonstrates a clear correlation between morphology and element localization. Lead is localized in the form of separate clusters, visually confirming its heterogeneous surface distribution. Such a pattern may be associated with an uneven distribution of active centers, as well as with surface nucleation processes.

The crystalline phase composition identified in the birch biochar, consisting of calcite, whewellite and quartz, shows consistency with various plant-derived biochars reported in the literature. Similar mineral phases have been identified in olive branch biochar [74] and Syzygium cumini stem biochar [75], suggesting this phase assemblage represents a common characteristic of biochars derived from lignocellulosic biomass. The stability of whewellite in the 360–380 °C pyrolysis temperature range aligns with observations by Kieush et al. [76], who reported whewellite presence in biochar produced at 400 °C but noted its absence at 600 °C. Majee et al. [77] similarly documented whewellite persistence in cotton stalk biochar across 300–500 °C pyrolysis temperatures.

The absence of detectable crystalline lead phases following adsorption, coupled with the minimal variation in phase composition and mass fractions, suggests that crystalline phase transformations do not represent a dominant mechanism in lead immobilization under these experimental conditions. This behavior differs from systems involving chemical activation, where Sharma et al. [75] observed dissolution of original mineral phases during KOH treatment. The structural characteristics of low-temperature biochar, including the relatively disordered carbon matrix noted by Kieush et al. [76], may contribute to the prevalence of surface-mediated adsorption mechanisms. The phase stability observed in this study, along with similar reports in the literature, indicates that low-temperature biochars can maintain structural integrity during aqueous applications, which may have implications for their long-term performance in water treatment scenarios.

The observed changes in the FTIR spectra after lead sorption are consistent with literature data, indicating a complex interaction mechanism. The enhancement and shifts of bands in the O–H region (3850–3600 cm⁻¹) correlate with the results of studies [59,60], where similar changes were explained by the involvement of hydroxyl groups in ion exchange and the formation of coordination bonds. The transformation of bands at 1568 and 1684 cm⁻¹, characteristic of carboxylate and carbonyl groups, corresponds to data [65,78], where such changes were interpreted as the formation of chelating complexes with metal ions. The appearance of a new band at 1064 cm-1 (C–O bonds) is consistent with observations [61,78] on the involvement of ether and alcohol groups in the sorption process.

Of particular interest is the enhancement of the doublet at 2350 cm⁻¹, which, combined with XRD data indicating the presence of calcite, suggests the participation of carbonate groups in the sorption process. This aligns with studies [61,69], which noted the formation of surface lead carbonate complexes. However, the absence of peaks characteristic of crystalline cerussite (PbCO₃) in our data suggests that the interaction with carbonate groups occurs primarily through a surface complexation mechanism without the formation of a separate crystalline phase.

Changes in the C–H vibration region (disappearance of the 2850 cm⁻¹ band and appearance of the 2976 cm⁻¹ band) may be associated with conformational rearrangement of aliphatic chains near sorption centers, as noted in [78]. The observed increase in transmission in broad spectral regions (2940–2408 cm⁻¹ and 2270–1740 cm⁻¹) while maintaining the overall spectral shape may be due to changes in the surface optical properties after sorption.

The totality of the observed changes indicates a multi-level sorption mechanism, including complexation with oxygen-containing functional groups of the biochar’s organic matrix, ion exchange, and surface interactions with carbonate groups. The obtained results are consistent with data [67] on the importance of surface modification for the adsorption properties of materials.

4.4. Discussion of Kinetic Studies

The presence of two clearly defined kinetic stages, identified using the Weber-Morris intraparticle diffusion model, is typical for the process of metal ion adsorption on biochars and porous carbon materials [54,55,79]. The first, fast stage (up to t^1/2 ≈ 16–17 min^1/2), characterized by a high rate constant k_i1, corresponds to mass transfer in macropores and adsorption on the most accessible active centers of the external surface. The second, slow stage, with constant k_i2, represents the limiting stage of the process, due to slow diffusion into meso- and micropores and the very act of chemosorption [54].

The invariance of the time boundaries of the transition between stages to changes in the initial adsorbate concentration (Figure 9) indicates that the geometry of the adsorbent’s pore structure is a key factor determining the process chronology. This observation is in good agreement with the conclusions of other researchers that for high-temperature biochars (similar to those obtained from birch), in which the number of surface functional groups is reduced, it is intraparticle diffusion that becomes the dominant mechanism controlling the kinetics at later stages [54,55].

The observed decrease in the pseudo-second order rate constant (k₂) by more than 5 times with increasing initial concentration (Table 5) is also consistent with literature data [79] and can be explained by the filling of the most accessible and high-energy centers at the beginning of the process, while subsequent adsorption on less accessible centers, also requiring intraparticle diffusion, proceeds significantly slower.

Thus, the kinetics of lead ion adsorption on birch biochar is a multi-stage process, initiated by rapid surface adsorption (likely due to ion exchange and complexation) and limited by the rate of subsequent intraparticle diffusion and chemosorption in the pore depth, which is fully consistent with modern concepts of sorption mechanisms on carbon materials [55,58].

Pseudo-first and pseudo-second order models were also applied to quantitatively describe the kinetic curves (Figure 10). Despite high coefficients of determination (R² > 0.99), these models demonstrate a discrepancy in the estimates of the calculated equilibrium capacity (q_e) within each experiment. For example, for the experiment with an initial concentration of 40.4 mg/L, the q_e estimates were 14.3 and 17.1 mg/g for the PFO and PSO models, respectively (Table 5).

It is important to emphasize that the observed discrepancies (about 20%) are within the limits typical for heterogeneous materials like biochar. The natural heterogeneity of wood raw materials (density, texture, microstructure) naturally leads to variability in the properties of the final product, including the porous structure and distribution of active centers, which, in turn, can affect kinetic parameters.

In kinetic experiment No. 1 (C₀ = 5.0 mg/L), the biochar provided almost complete metal removal (94.8%), while in kinetic experiment No. 2 (C₀ = 40.4 mg/L) the removal efficiency at equilibrium was 37.3%. This effect is fundamental and expected for an adsorption process described by the Langmuir isotherm. It is explained by the limited and fixed number of active centers on the sorbent surface. At low initial concentrations, the amount of metal ions in the solution is small and sufficient for their complete binding by the available centers. At high concentrations, the amount of Pb²⁺ ions significantly exceeds the sorbent capacity. As a result, although q_e increases (Figure 10a,b), the fraction of its total amount in the system that can be removed naturally drops.

The observed decrease in the pseudo-second order rate constant (k₂) by more than 5 times with increasing initial concentration (Table 5) is consistent with the idea of filling the most accessible and high-energy centers at the beginning of the process, while subsequent adsorption on less accessible centers proceeds more slowly.

A state close to equilibrium is achieved within 48 h for both studied initial concentrations. The equilibrium adsorption value is naturally determined by the initial concentration and the maximum capacity of the sorbent. For the experiment with an initial concentration of 40.4 mg/L, the equilibrium amount of adsorbed substance (q_e) approaches the value of the maximum adsorption calculated using the Langmuir and Sips equations (q_m), while at a concentration of 5 mg/L the q_e value is limited by the amount of lead in the system.

The conducted kinetic analysis is in good agreement with the results of the adsorption isotherm study, SEM data on the hierarchical porous structure, and the observation of the pH shift, collectively confirming a mechanism involving rapid surface adsorption and subsequent slow diffusion with ion exchange and chemosorption on the heterogeneous biochar surface.

Thus, the kinetics of lead ion adsorption on birch biochar is a multi-stage process, limited by the rate of intraparticle diffusion, and is determined primarily by the pore structure of the sorbent. The observed scatter of kinetic parameters is a reflection of the natural heterogeneity of the material and does not exceed values typical for this class of sorbents.

4.5. Influence of Liquid-to-Solid Ratio and Lead Desorption

The conducted study revealed key patterns in the process of lead ion adsorption on birch biochar, determined by the liquid-to-solid ratio (L/S). A compromise was established between the efficiency of metal removal from the solution and the degree of utilization of the material’s sorption capacity (Figure 11). As the results show, PF decreases exponentially with increasing L/S, reaching maximum values (on the order of 10²) in the region of low L/S values (100–200 mL/g), indicating high metal removal efficiency under conditions of sorbent excess. In contrast, q_e demonstrates the opposite trend, increasing with increasing L/S and reaching maximum values (15.3 mg/g) at L/S = 5000 mL/g, indicating the most complete use of sorption capacity under conditions of metal excess.

Power-law approximation revealed their quantitative characteristics (Table 1). The obtained exponents are statistically significantly different from the expected values for an ideal Langmuir system (+1 and −1 for q_e and PF, respectively), expected for an ideal system obeying the Langmuir isotherm under conditions C_e << C₀. The analysis of the PF dependence is complicated in the region of low L/S values (<500 mL/g). High confidence interval values and the nature of the dependence (reaching a “plateau”) unambiguously indicate that under these conditions, the equilibrium concentration of lead ions reaches the lower detection limit of the applied analytical method. Thus, the calculated PF values in this region are estimates and are underestimated, not true, which leads to an overestimation of the exponent in the power model. This effect is a measurement artifact and does not reflect the fundamental properties of the system.

In contrast, the q_e dependence is not subject to this artifact, since its calculation is based on the concentration difference (C₀ − C_e). The deviation of the exponent for q_e from the expected ideal value may be associated with a more complex adsorption mechanism. The most likely competing process is the precipitation of lead ions in the form of poorly soluble compounds on the biochar surface or in the solution volume.

The obtained equations allow predicting the process parameters over a wide range of L/S values and optimizing the adsorption conditions depending on the target process indicator—maximum degree of purification or maximum load on the sorbent.

A critically important aspect is the stability of the adsorbed metal and its resistance to release (desorption). Sequential extraction experiments showed that only a small portion of lead (0.07–0.69%) is desorbed by distilled water (Figure 12), and this value is statistically independent of the degree of saturation.

In contrast, desorption with a 0.01 M Ca(NO₃)₂ solution showed a pronounced linear dependence on the degree of saturation, indicating a significant contribution of the ion-exchange sorption mechanism. This result has important practical significance, since calcium ions are the dominant cations in soil solutions.

The obtained results allow us to formulate practical recommendations for the application of biochar. For tasks of concentrating metal from solutions, it is advisable to use regimes with high L/S values (1000–5000 mL/g), ensuring maximum load on the sorbent. For the purposes of soil remediation and long-term metal stabilization, regimes with low L/S values (100–200 mL/g) are recommended, which ensure strong metal binding and minimal risk of its desorption even in the presence of competing ions.

Thus, birch biochar is an effective sorbent for the extraction and immobilization of lead ions, characterized by a significant adsorption capacity and strong binding, which minimizes its subsequent release. The established quantitative dependencies allow optimizing the parameters of its application for various tasks of environmental remediation.

5. Conclusions

The conducted research allows us to conclude that biochar from birch wood, produced by slow pyrolysis at temperatures of 360–380 °C, demonstrates significant potential as a sorbent for lead(II) ions from aqueous solutions, effectively adsorbing and strongly retaining the metal. The established adsorption capacity of 14.21 mg/g according to the Langmuir model, along with high values of the adsorption equilibrium constant, indicates a pronounced affinity of the material for the target contaminants. The observed changes in the acid–base characteristics of the system during sorption and the kinetic patterns suggest that the ion exchange process involving protonated surface functional groups plays a leading role in the metal ion removal mechanism, although the contribution of other mechanisms, particularly chemosorption, also likely plays a certain role. An important result of the work is the demonstration of the effectiveness of the direct potentiometry method using an ion-selective electrode for real-time process monitoring, which opens up possibilities for more detailed study of the kinetics of sorption processes on carbon materials. From a practical standpoint, the obtained data indicate that the studied biochar can be recommended for use in wastewater polishing technologies and metal immobilization in soils, particularly for achieving standards established for irrigation water. Thus, the work expands the understanding of the functional properties of biochar and substantiates the promise of its use not only as a soil amendment but also as a component of treatment systems.