The Influence of Synthesis Parameters on the Porous Structure of Biochars and Their Adsorption Performance

Abstract

The growing volume of crustacean shell waste generated during seafood processing poses a serious environmental problem. However, this type of biowaste remains underutilized, despite being a promising renewable raw material for the production of functional carbon materials. This study aims to investigate how synthesis parameters influence the formation of a hierarchical porous structure in shrimp shell-based carbon materials and to optimize these parameters to improve CO₂ adsorption efficiency. Under optimal carbonization conditions (holding time: 2 h; temperature: 650 °C) and activation conditions (holding time: 2 h; temperature: 750 °C) with activator-to-carbon weight ratios (A/C) of 1/1, 2/1 and 4/1, the resulting porous carbon samples exhibited relatively high S_BET values (1175, 2708 and 3052 m²/g, respectively) and V_T (0.70, 1.55 and 2.60 cm³/g, respectively), as well as different pore size distributions. Notably, the resulting carbon materials demonstrated exceptional CO₂ adsorption performance at 298 K, reaching a maximum adsorption capacity of 40.03 mmol/g at 40 bar for sample SS_652_41752, 15.12 mmol/g at 15 bar for SS_652_21752, and 3.41 mmol/g at 1 bar for SS_652_11752. These values rank among the highest ever reported for biomass-derived porous carbon materials. The adsorption behavior of the most efficient sorbent, SS_652_41752, was further analyzed using Langmuir and Freundlich isotherm models over the temperature range of 298–318 K and at pressures up to 40 bar, and the isosteric heats of adsorption were calculated to elucidate adsorbent–adsorbate interactions. It was found that the differential molar isosteric heat of CO₂ adsorption decreased from approximately 20 to approximately 17 kJ/mol with increasing adsorption uptake, confirming the physisorption nature of the process. These results demonstrate that crustacean shell waste is a promising feedstock for producing carbon materials with tailored properties and significant potential for CO₂ adsorption applications.

1. Introduction

The accumulation of seafood waste, particularly crustacean shell waste, is a complex problem with environmental, economic, and social implications. One promising approach to utilizing this waste and mitigating its environmental impact is to use it as a precursor for the synthesis of nanoporous carbon materials. The exoskeleton of crustaceans is formed through a biomineralization process involving an organic matrix composed primarily of chitin fibrils. Since crustacean shells contain 20–40% protein, 20–50% CaCO₃, and 15–40% chitin, which is a polymer of (1 → 4)-β-linked N-acetyl-D-glucosamine [1], the bioconversion of this waste offers an opportunity to reduce the environmental burden while generating economic benefits by transforming problematic waste into a valuable product.

Over the past two decades, a substantial body of research worldwide has addressed the conversion of marine biomass waste into highly porous carbon materials [2]. Various types of carbon nanomaterials, such as carbon quantum dots, carbon nanotubes, porous carbon, carbon aerogels, and others, can be produced from crustacean waste [3,4,5]. However, the properties of the resulting materials depend on the synthesis methods and parameters [6,7]. Among the available methods, carbonization is the most studied and effective method for obtaining carbon materials from crustacean shell waste [8]. Carbonization is performed by heating in an inert atmosphere, typically in the temperature range of 400 °C to 900 °C. During this process, the organic components of the shells, primarily chitin and proteins, decompose to yield a carbon-rich residue. Thermal degradation of C–C bonds present in the organic fraction and removal of volatile species lead to the formation of carbon materials with an increased proportion of sp2-hybridized carbon atoms in the structures [9,10]. Carbonization of crustacean waste also induces pore formation, increases surface area [11], enlarges the size of polyaromatic ring systems [10], and promotes the development of graphitic structures [12]. Activation of the carbonized products obtained from crustacean shells is a crucial step for improving the characteristics of the final material, particularly for producing highly porous carbon structures. Activation enables carbon materials to attain a stable porous architecture and a well-developed specific surface area, thereby enhancing adsorption properties [13]. Compared with physical activation, chemical activation generally offers shorter reaction time, better control over the activation process, lower reaction temperatures, and higher activation efficiency [14]. In addition, chemical activation can effectively tailor the pore structure, pore size distribution, and specific surface area [15]. Various activating agents have been employed to produce carbon materials from crustacean waste, but the most common and effective agent for obtaining highly porous carbon materials is the alkaline activator KOH [16,17].

Previous studies have shown that biomass-derived carbon materials possess excellent characteristics for efficient CO₂ adsorption. However, efforts to improve their performance still face challenges such as difficulty in controlling their structure [18]. In particular, changes in pore structure and pore volume significantly affect the properties of carbon materials. Therefore, in-depth investigation and optimization of the porous structure of biochars are crucial for further enhancement of their CO₂ adsorption efficiency.

Thus, the conversion of crustacean shell waste into value-added carbon materials is an important contribution to the circular bioeconomy. It allows spent crustacean shells to be used as precursors for advanced functional materials rather than being treated as waste.

The objectives of this study were: (a) to evaluate crustacean shells as a precursor for KOH-activated biochar for CO₂ adsorption; (b) to examine the influence of synthesis parameters, including pre-carbonization and alkali activation, on pore structure development; (c) to analyze the relationship between the pore structure of KOH-activated crustacean shell-derived biochar and its adsorption capacity; and (d) to investigate the adsorption mechanisms governing the performance of the biochar as a CO₂ adsorbent. Overall, this study provides a framework for valorizing biomass waste and for producing carbon materials with tailored porosity for efficient CO₂ adsorption.

2. Materials and Methods

2.1. Materials

The following materials and reagents were used to prepare the carbon materials: shrimp shells (SS), potassium hydroxide (KOH), hydrochloric acid (HCl), argon (Ar), and distilled water (H₂O). High-purity gaseous carbon dioxide was used in the study.

2.2. Biochar Synthesis

The carbon material synthesis comprised two main stages: carbonization and activation. First, shrimp shells were repeatedly washed with deionized water, dried, ground into powder, and sieved. The resulting solid sample with particle size < 0.5 mm was placed in a reactor and carbonized in an argon atmosphere for various times (1, 2, 3 h) at different final temperatures (550, 650 and 750 °C).

The carbonizate exhibiting the best porous-structure parameters was activated with potassium hydroxide, while varying the following process conditions: duration (1, 2, 3 h), temperature (650, 750, 850 °C), and the weight ratio of activating agent (KOH) to carbonizate (1/1, 2/1, 4/1). Thus, the obtained carbonizate powder (particle size < 0.5 mm) and pellets of 85%-potassium hydroxide were loaded into the experimental reactor in the specified weight ratio of potassium hydroxide (converted to 100%-KOH)/carbonizate. Under an argon flow, the reaction mixture was heated to the target temperature, held for the prescribed time, and then cooled to room temperature. After cooling, the reaction mixture was poured into water, the precipitate was washed until the alkali was removed and neutral pH was achieved, then treated for 24 h in hydrochloric acid solution to dissolve metal-containing impurities, after which it was washed again with water and dried at 110 °C to constant weight.

The nomenclature of each sample includes the feedstock name (SS), the carbonization temperature (for example, 650 °C corresponds to 65), the carbonization time in hours, the activation temperature (for example, 500 °C corresponds to 50), the activation time in hours, and the KOH/carbonizate weight ratio (for example, 2/1 corresponds to 21). Thus, an activated biochar prepared by carbonization at 650 °C for 2 h followed by activation with a KOH/carbonizate ratio of 2/1 at 500 °C for 2 h would be designated SS_652_21502.

2.3. Characterization Methods

Characterization of the porous structure of all samples was performed by physical gas adsorption (N₂ at 77 K) using the automated adsorption system Autosorb-iQ (Quantachrome Instruments, Boynton Beach, FL, USA). Samples (weight 0.02 g) were degassed at 350 °C for 5 h prior to porous structure analysis to remove air, free water, and other impurities. N₂ adsorption isotherms were obtained at 77 K for relative pressure (P/P₀) (gas pressure/saturated vapor pressure) in the range from 0.01 to 0.995. The specific surface area S_BET was calculated using the BET (Brunauer–Emmett–Teller) equation. The distribution of pore size and the volumes of micropores and mesopores were calculated using the density functional theory (DFT) under the assumption of slit-shaped/cylindrical/spherical pores (QSDFT model).

CO₂ adsorption–desorption isotherms were recorded on the iSorbHP gas adsorption analyzer (Anton Paar GmbH, Graz, Austria) at various temperatures (298.15, 303.15, 308.15, 313.15, 318.15 K) and pressures up to 40 bar. Prior to measurements, samples were degassed at 350 °C for 5 h to remove adsorbed water and impurities.

The fundamental difference between adsorption phenomena occurring in micropores and on mesopore surfaces requires different theoretical approaches for their description and interpretation. In this regard, the experimental CO₂ adsorption isotherms were correlated with the Langmuir and Freundlich models according to the following equations [19,20]:

$${aL}_{mod}={\displaystyle \frac{AkLp}{1+kLp}}$$

(1)

$${aF}_{mod}=kF{p}^{{\displaystyle \frac{1}{n}}}$$

(2)

where ${aL}_{mod}$ is the value of equilibrium CO₂ adsorption; p is the equilibrium pressure of the gas phase; A is the limiting value of CO₂ adsorption; $kL$ is the Langmuir adsorption constant; where ${aF}_{mod}$ is the equilibrium CO₂ adsorption value; $kF$ is the equilibrium constant of the Freundlich model; n is an empirical parameter characterizing the interaction energy in the adsorbent–adsorbate system.

The reliability of these models was assessed by the regression coefficient R², which varies from 0 to 1, and by the normalized standard deviation $\mathsf{\Delta }a$ (%), found by:

$$∆a=100\sqrt{{\displaystyle \frac{{\displaystyle {\sum }_{i=1}^n}{\left[\left(a_{exp}-a_{mod}\right)/a_{exp}\right]}^2}{N-1}}}$$

(3)

where $a_{exp}$ and $a_{mod}$ are the CO₂ adsorption values obtained from experiments and theoretical models, respectively, and $N$ is the number of adsorption isotherm data points.

The differential molar isosteric heat of adsorption $q_{st}$ is an important thermodynamic parameter that describes the thermal effects of adsorption processes. To determine the isosteric differential heat of CO₂ adsorption on a sample with the best porous structure parameters, a method for calculating the heats of adsorption based on the Clausius–Clapeyron equation was used:

$${\left({\displaystyle \frac{\partial \ln p}{\partial \left(1/T\right)}}\right)}_{a=const}=-{\displaystyle \frac{q_{st}}{R}}$$

(4)

where $R$ is the gas constant; $T$ is temperature; $p$ is equilibrium pressure.

Specifically, the $q_{st}$ value was found from the slope of the straight lines after plotting the dependence of $ln p_{C{O}_2} \mathrm{o}\mathrm{n} 1/T$ for a fixed, specified adsorbed amount of carbon dioxide.

3. Results and Discussion

3.1. The Influence of Synthesis Parameters on the Porous Structure of Biochars

The efficiency of CO₂ adsorption by activated biochar is determined primarily by its porous structure [21]. Activated biochar is synthesized in two main stages: carbonization and activation [22].

Carbonization is an important stage in the production of activated biochar. As a result of this stage, the biomass undergoes a very important modification, converting the feedstock (biomass) into a carbon-rich product. The choice of process parameters for the carbonization process, in particular temperature and time, has a significant impact on the porous structure of the final product and, consequently, on the efficiency of CO₂ adsorption. However, without the activation stage, biomass-based carbonizate has a primitive porous structure and limited adsorption capacity compared to activated biochar [23]. The chemical activation method is an effective way to produce highly porous biochar, which involves controlled process parameters: (1) thermal treatment of the carbonizate in an inert atmosphere (time and temperature); (2) the weight ratio of the activating agent to the carbonate. During the decomposition process, the activating agent reacts with carbon, forming a well-defined porous structure [24]. The most effective activator for producing biochar with a given porous structure required for high CO₂ adsorption is KOH [25]. Carbon (C) and KOH react at elevated temperatures, typically in the range of 550 °C to 850 °C, producing metallic potassium (K), which intercalates into the carbon matrix, increasing the porosity of the final product, and also affects the pore size distribution, which improves the efficiency of CO₂ adsorption [26].

3.1.1. The Influence of Carbonization Parameters on the Porous Structure of Biochars

To study the effect of shrimp shell carbonization time on the porous structure of the final material, three types of samples were obtained at a fixed temperature and variable carbonization time, followed by KOH activation. Nitrogen adsorption–desorption isotherms were obtained for these samples at 77 K, shown in Figure 1a.

All isotherms for the samples obtained with different shrimp shell carbonization times exhibited type I hysteresis loops with hysteresis loops similar to type H4 according to the IUPAC classification [27]. This type of loop is confirmed by the micromesoporous structure of the obtained carbon materials (Figure 1b and Figure 2).

With an increase in carbonization time from 1 to 2 h, the nitrogen adsorption capacity increased significantly, but decreased after 3 h. According to the same scenario, all porosity parameters (specific surface area (S_BET), total pore volume (V_T) and micropore volume (V_mic)) changed, except for the mesopore volume (V_mes), the value of which, in turn, continuously increased with increasing exposure time (Figure 2). However, it should be noted that changing the carbonization duration did not have a significant effect on the type of porosity, which is confirmed by the pore size distribution curves (Figure 1b). Thus, 2 h is the optimal time for shrimp shell carbonization. Sample SS_552_11752 had the highest porosity parameters: S_BET = 1087 m²/g, V_T = 0.55 cm³/g, V_mic = 0.5 cm³/g and V_mes = 0.05 cm³/g. In addition, it can be seen from Figure 2 that this material exhibited the highest adsorption capacity (a) for CO₂ (13.06 mmol/g) in comparison with the samples obtained with a carbonation time of 1 and 3 h.

Increasing the shrimp shell carbonization temperature from 550 °C, at which the effect of carbonization time was assessed, to 650 °C affected the nature of both the nitrogen adsorption isotherms and the pore size distribution curves. Figure 1c shows that the SS_652_11752 and SS_752_11752 materials are characterized by similar reversible adsorption isotherms, corresponding to a combination of types I and II with an H4 hysteresis loop with different values of nitrogen adsorption uptake. This type is inherent to micromesoporous materials with a wide range of pore size distributions. This statement is confirmed by Figure 1d, which shows that with an increase in carbonization temperature from 550 to at least 650 °C, the pore size range becomes wider. With the increase in carbonization temperature from 550 to 650 °C, the S_BET, V_T and V_mes values increased, and those of V_mic decreased (Figure 3). The increase in carbonization temperature from 650 to 750 °C led to a decrease in all porosity parameters. Based on the above, it can be concluded that 650 °C is the optimal temperature for shrimp shell carbonization. Thus, the SS_652_11752 sample had the highest porosity parameters: S_BET = 1175 m²/g, V_T = 0.7 cm³/g, V_mic = 0.4 cm³/g and V_mes = 0.3 cm³/g. In addition, Figure 3 shows that this material exhibited the highest adsorption capacity (a) for CO₂ (15.38 mmol/g) in comparison with the samples obtained at carbonization temperatures of 550 and 750 °C.

3.1.2. The Influence of Activation Parameters on the Porous Structure of Biochars

To study the effect of the carbonizate activation time on the porous structure of the final material, three types of samples were selected; all obtained at a fixed temperature and KOH/carbonizate weight ratio, but with a variable activation time. Nitrogen adsorption–desorption isotherms at 77 K were also obtained for these samples, shown in Figure 4a. All isotherms for the samples obtained with different carbonizate activation times demonstrated a combination of types I and II with an H4 hysteresis loop with different values of nitrogen adsorption uptake. With an increase in the activation time from 1 to 2 h, the nitrogen adsorption capacity showed a slight tendency to increase, and with an increase in the holding time to 3 h, it decreased. The porosity parameters (S_BET, V_T, V_mes) varied according to the same algorithm (Figure 5). The pore size distribution curves also had a similar nature (Figure 4b). Based on the highest values of porosity parameters and adsorption capacity for CO₂, the optimal activation time is 2 h.

Changing the carbonizate activation temperature did not significantly affect the nature of the nitrogen adsorption isotherms or pore size distribution (Figure 4c,d). However, it did significantly affect the porosity parameters and the CO₂ adsorption capacity of the samples (Figure 6). With an increase in activation temperature from 650 to 750 °C, the S_BET, V_T, V_mes, V_mic and a values increased by 43, 36, 33, 38, and 55%, respectively. However, a further increase in temperature (to 850 °C) led to a decrease in all porosity parameters (Figure 6). Based on the above, it can be concluded that 750 °C is the optimal carbonizate activation temperature.

It is known that changing the excess of the activating agent relative to carbon significantly affects not only the specific surface area and pore volume, but also their size distribution. Taking this fact into account, changes in the pore structure of biochars with an increase in the KOH/C ratio were studied.

Nitrogen adsorption–desorption isotherms and pore size distribution curves of biochar samples obtained by carbonization (2 h; 650 °C) followed by alkaline activation carried out for 2 h at 750 °C and different KOH/carbonate weight ratios (from 1/1 to 4/1) are shown in Figure 4e,f.

An increase in the alkali excess during carbonization promotes a significant increase in the nitrogen sorption capacity of the samples. Biochar samples obtained at KOH/carbonization ratios from 1/1 to 2/1 exhibited isotherms of a combination of types I and II with an H4-type hysteresis loop. The isotherm for the sample obtained at a higher alkali excess (4/1) had features inherent to H3-type hysteresis loops, since the adsorption branch resembles a type II isotherm, and the lower limit of the desorption branch was in a narrow P/P₀ range (~0.4–0.5 for nitrogen at 77 K). This type of loop is often found in mesoporous carbon materials, as confirmed by the data in Figure 4f. As shown in Figure 4f, with an increase in the KOH/carbonizate ratio from 2/1 to 4/1, the micropore size of the first and second modes of biochar shifted from ~0.9 to ~1.1 nm and from 1.2 to 2.0 nm, respectively. The mesopore size also changed with an increase in the alkali from 2/1 to 4/1. First, modes 3–5 merged into a single mode, and the pore volume of these ranges increased from 0.779 to 2.069 m³/g. Second, this pore size shifted toward larger values from the range of ~2.0–6.0 nm to ~3.0–9.0 nm. Third, the peak intensity itself increased significantly.

Thus, the S_BET and V_T of the biochar samples increased from 1175 to 3052 m²/g and from 0.707 to 2.614 cm³/g, respectively, as the KOH/carbonizate ratio increased from 1/1 to 4/1 (Figure 7).

The sample obtained with a KOH/carbonizate ratio of 2/1 was micromesoporous: the mesopore volume to the total pore volume was 60%. The material formed with a lower alkali excess was also micromesoporous (mesopore volume was 42% of the total pore volume) with a narrower pore size distribution (S_BET: 1175 m²/g, V_T: 0.707 cm³/g, S_DFT: 1480 cm³/g, V_mic: 0.409 cm³/g (58%), V_mes: 0.298 cm³/g).

Biochar obtained with a KOH/carbonizate ratio of 4/1 was mesoporous (mesopore volume was 89% of the total pore volume) and also had the highest values of specific surface area and total pore volume (S_BET: 3052 m²/g, V_T: 2.614 cm³/g, S_DFT: 2450 cm³/g, V_mic: 0.295 cm³/g, V_mes: 2.319 cm³/g).

Based on the highest values of the porosity parameters and adsorption capacity for CO₂ (Figure 7), the optimal KOH/carbonizate weight ratio is 4/1 (a = 40.03 mmol/g).

3.2. CO₂ Adsorption of Biochars

Since changing the excess of the activating agent relative to carbon significantly affected the specific surface area, volume, and pore structure, these samples were used to study the influence of these parameters on the adsorption process and their adsorption properties. Figure 8 shows the CO₂ adsorption isotherms for the SS samples at 298 K and pressures up to 40 bar.

As noted above, the maximum CO₂ adsorption (40.03 mmol/g at 40 bar and a temperature of 298 K) was demonstrated by the SS_652_41752 sample, obtained with a KOH/carbonizate ratio of 4/1. Notably, the SS_652_21752 sample demonstrated the highest CO₂ adsorption in the pressure range from 2 to 10 bar (15.12 mmol/g). However, at low pressures (up to 1 bar), the SS_652_11752 sample (3.41 mmol/g) proved to be the most effective CO₂ adsorbent. The effect of different adsorption capacities of the materials should be associated with their different porous structures. Thus, the SS_652_11752 sample had a bigger ultramicropore volume (less than 0.7 nm) than the SS_652_21752 and SS_652_41752 samples by 7 and 90%, respectively, which determined the sorption behavior at 1 bar. The SS_652_21752 sample had a bigger micropore volume (up to 2 nm) and narrower mesopore volume (from 2 to 3 nm) than the SS_652_11752 sample (by 37 and 92%, respectively) and the SS_652_41752 sample (by 54 and 94%, respectively), which determined the sorption behavior up to 10 bar. Finally, the SS_652_41752 sample had a bigger pore volume up to 10 nm (2.24 m³/g) than the SS_652_11752 sample (0.60 m³/g) and the SS_652_21752 sample (1.44 m³/g), which determined the sorption behavior at high pressure (up to 40 bar).

It should be noted that the adsorption capacities obtained for the SS_652_11752, SS_652_21752 and SS_652_41752 materials at 1, 10 and 40 bar, at a temperature of 298 K, are among the highest values in the existing literature and research.

Having selected the carbon material with the highest adsorption capacity for CO₂ at 40 bar, further research was focused on its change depending on temperature, as shown in Figure 9. According to the isotherm data, an increase in temperature contributed to a decrease in the adsorption value of CO₂ on the SS_652_41752 sample, which indicates the exothermic nature of the process. The change in the adsorption capacity of biochar depending on pressure, approximated by the Langmuir and Freundlich models, is also shown. The parameters calculated using these models are presented in

Table 1. Results of processing CO₂ adsorption isotherms on the SS_652_41752 sample using the Langmuir and Freundlich models.

Temperature (K)	298.15	303.15	308.15	313.15	318.15
$a_{exp}$	40.03	34.31	30.92	28.02	25.82
Langmuir constants
${aL}_{mod}$	35.36	30.98	28.79	25.28	24.72
$A$	65.79	57.80	54.95	48.78	48.54
$kL$	0.0290	0.0288	0.0274	0.0269	0.0258
$R^2$	0.9810	0.9838	0.9829	0.9823	0.9844
$\mathsf{\Delta }a$	9.01	8.60	8.56	8.99	8.17
Freundlich constants
${aF}_{mod}$	40.00	35.29	31.92	29.02	26.62
$kF$	2.0473	1.7988	1.6248	1.4696	1.3406
$n$	1.241	1.239	1.231	1.223	1.205
$R^2$	0.9979	0.9976	0.9974	0.9971	0.9970
$\mathsf{\Delta }a$	4.63	4.52	4.58	4.89	4.79

.

Taking into account the obtained values of normalized standard deviations ($\mathsf{\Delta }a$ = less than 10%) and correlation coefficients $R^2$ = (more than 0.9), it can be stated that both models provide an adequate description of the adsorption isotherms at the five studied temperatures. However, comparing the results, it is obvious that the experimental data for all temperatures studied are more consistent with the Freundlich model, with $\mathsf{\Delta }a$ values below 5% and $R^2$ above 0.99 (Table 1, Figure 9).

In addition, Table 1 shows that the adsorption values (${aL}_{mod},{aF}_{mod}$) and adsorption isotherm constants ($kL, kF$), calculated using the models, decreased with increasing temperature, which is due to the exothermic nature of CO₂ adsorption on the sample and its greater thermodynamic favorability at low temperatures. Comparing the experimentally obtained values of the CO₂ adsorption capacity with the values obtained using the models, it is evident that the deviations were less than 12 and 3% for Langmuir and Freundlich, respectively, which also indicates a good agreement between the models and the experimentally obtained data. The $n$ parameter of the model is related to the affinity between the adsorbate and the adsorbent. Values of $n$ greater than 1 indicate favorable adsorption, and the higher the $n$ value, the stronger the adsorption. In all cases, $n$ values greater than 1 were obtained, and they decreased with increasing temperature.

The isosteric heat of CO₂ adsorption was calculated using the Clausius–Clapeyron equation for isothermal data at different temperatures.

Adsorption isosteres, the curves showing the equilibrium phase pressure versus temperature at a constant adsorption value, play an important role in adsorption thermodynamics. Figure 10 shows the adsorption isosteres of CO₂ on SS_652_41752 sample for adsorption values of 0.5 (1), 3.0 (2), 6.0 (3), 9.0 (4), 12.0 (5), 14.0 (6), 17.0 (7), 19.0 (8), 21.0 (9), and 24.0 (10) mmol/g, which remain linear over the entire pressure range.

Figure 11 shows the dependence of the differential molar isosteric heat of CO₂ adsorption on the adsorption value on the SS_652_41752 sample at pressures up to 40 bar.

The isosteric heat of CO₂ adsorption decreased with increasing filling (Figure 11), indicating a heterogeneous distribution of site energy in the SS_652_41752 sample with high-energy sites being filled first and then, at higher filling levels, weaker sites becoming active. Low values of $q_{st}$ (initial value ~20 kJ/mol) for the studied sample indicate a physisorption process of CO₂ adsorption.

Table 2. Comparison of CO₂ adsorption characteristics at 40 bar by different carbon adsorbents.

Material	BET Surface Area (m2/g)	Total Pore Volume (cm3/g)	Temperature (°C)	Adsorption Pressure (bar)	CO2 Uptake (mmol/g)	Reference
SS_652_41752	3052	2.60	25	40	40.0	This study
AC-2–700	1569	0.81	25	40	~8.5	[28]
MC-B-40	1722	1.21	25	40	~8.5	[29]
MCM-41_MW60-0.5	941	0.84	25	50	12.8	[30]
LACO	1773	1.07	26	40	~5.1	[31]
NaSB31	3024	1.30	25	40	∼26.0	[32]
KL31	2540	1.02	25	40	∼21.0	[32]
KA21	2156	1.02	25	40	∼17.5	[32]

and

Table 3. Comparison of CO₂ adsorption characteristics at 1 bar by different carbon adsorbents.

Material	BET Surface Area (m2/g)	Total Pore Volume (cm3/g)	Temperature (°C)	Adsorption Pressure (bar)	CO2 Uptake (mmol/g)	Reference
SS_652_11752	1175	0.70	25	1	3.41	This study
MFC-700-0.2	1658	0.66	25	1	3.30	[33]
AC-AND-10 h	1937	0.93	25	1	3.20	[34]
APC-500–700	1211	0.63	25	1	2.89	[35]
PC	2370	0.91	25	1	2.90	[36]
Cu-MOF/AC(700)	1111	0.85	25	1	2.58	[37]
HDDS-AC (850−120)	2000	1.24	25	1	3.0	[38]
ChAB	643	0.29	25	1	1.93	[39]
Ultramicroporous carbon	1580	-	25	1	3.05	[40]
APC-300-800	1750	0.77	25	1	2.63	[41]
WSCPM-900-2	683	0.30	25	1	3.15	[42]
AC-AV-P	1272	0.75	25	1	3.30	[43]

present the results of CO₂ adsorption, using the carbon adsorbent obtained in this study and the achievements of other research groups.

Based on the above discussion, it was found that the effective CO₂ adsorption capacity is determined not only by the specific surface area but also by the pore volume and pore size distribution.

4. Conclusions

This study discusses the key role of pore structure in the significant CO₂ adsorption capacity of biochars obtained from shrimp shell waste, a byproduct of the food industry. Furthermore, changes in the pore structure of the biochars were investigated by varying synthesis parameters, specifically the temperature and time of carbonization and activation, as well as the mass ratio of activating agent to carbonizate. The results showed that all studied parameters influenced the pore structure of the resulting biochars and, consequently, their CO₂ adsorption capacity. It is noteworthy that changing the excess of activating agent relative to carbon significantly affected the specific surface area, volume, and pore structure of the resulting biochars. The relationship between the volume of ultramicropores (less than 0.7 nm) of biochar and the value of CO₂ adsorption at 1 bar, the volume of micropores (up to 2 nm) and narrow mesopores (from 2 to 3 nm) of biochar with the value of CO₂ adsorption up to 10 bar, as well as the volume of pores up to 10 nm of biochar with the value of CO₂ adsorption up to 40 bar is shown.

The optimized SS_652_41752, SS_652_21752 and SS_652_11752 samples possessed high S_BET (1175, 2708 and 3052 m²/g, respectively) and V_T (0.70, 1.55 and 2.60 cm³/g, respectively) with an effective adsorption capacity for CO₂ at 298 K of 40.03 (at 40 bar), 15.12 (at 15 bar) and 3.41 mmol/g (at 1 bar), respectively. Sorption processes on the surface of synthesized biochar exhibiting the highest CO₂ adsorption capacity were simulated using the Langmuir and Freundlich adsorption models in the temperature range of 298–318 K up to a pressure of 40 bar. It was shown that the differential molar isosteric heat of CO₂ adsorption decreases from ~20 to ~17 kJ/mol with increasing adsorption magnitude, demonstrating the physical nature of the adsorption process. In conclusion, this study confirmed the need for targeted production of the surface morphology of carbon materials obtained from crustacean shells by varying the synthesis parameters to achieve maximum adsorption capacity for CO₂.

Due to the lack of data on changes in biochar morphology and structure with varying synthesis parameters, this study represents a preliminary investigation of the properties of biochars obtained from shrimp shell waste. Future research should utilize a range of modern analytical methods to thoroughly study the structural and physicochemical properties of the resulting biochars, allowing for a precise determination of the relationship between synthesis parameters, pore architecture, and the sorption capacity of the resulting material.