# Optimized wood-based activated charcoal for desulfurization under realistic biogas conditions

Luis Paz, Taher Selmi, Vanessa Fierro, Alain Celzard

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Luis Paz, Taher Selmi, Vanessa Fierro, Alain Celzard. Optimized wood-based activated charcoal for desulfurization under realistic biogas conditions. Carbon, inPress, 247, pp.120963. 10.1016/j.carbon.2025.120963. hal-05354476

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# Optimized wood-based activated charcoal for desulfurization under realistic biogas conditions

Luis Paz<sup>a, b</sup>, Taher Selmi<sup>b</sup>, Vanessa Fierro<sup>a, *</sup>, Alain Celzard<sup>a, c, **</sup>

## ARTICLE INFO

### Keywords:

Biogas upgrading

Biogas desulfurization

Activated carbon

$H_2S$ separation

Adsorption

## ABSTRACT

This study focuses on the development of a high-performance activated carbon (AC) derived from wood-based charcoal for biogas desulfurization. ACs were densified with eco-friendly binders (cane molasses, beet molasses and plant polyphenols) and impregnated with KI, $K_2CO_3$, NaOH and KOH at loadings of 5–15 wt%. Both processes enhanced $H_2S$ separation capacity, evaluated through dynamic adsorption tests using a gas mixture composed of $N_2$, $CO_2$, $O_2$ and $H_2S$, simulating realistic biogas conditions. The best-performing sorbent, AC densified with polyphenol-based binder and impregnated with 10 wt% KOH, was compared to commercial ACs used for biogas desulfurization under identical testing conditions. The developed AC achieved a high $H_2S$ separation capacity of $181 \text{ mg g}^{-1}$ and a competitive volumetric performance of $96 \text{ mg cm}^{-3}$, demonstrating strong potential for industrial application. A 1D model for simulating breakthrough curves was developed and validated, using densified AC with $CO_2$ and $O_2$ in the feed gas. The model provides accurate predictions and mass transfer coefficients, supporting process design and material optimization. Thermal regeneration at 500 °C under $N_2$ allowed approximately 50 % of the initial performance to be recovered indicating the need for improved regeneration strategies. The results demonstrate the potential of sustainable, wood-derived AC as a competitive sorbent for industrial biogas purification.

## 1. Introduction

Biogas, produced mainly from the anaerobic digestion of organic waste, is a clean, methane-rich energy source and an important sustainable alternative to fossil fuels. However, its composition often includes hydrogen sulfide ($H_2S$), a corrosive and toxic impurity that must be removed to avoid damaging equipment and ensure the safety of the atmosphere. Among the various desulfurization techniques, adsorption on activated carbon (AC) is known as a widely used industrial method due to its efficiency and cost-effectiveness [1].

However, pure adsorption may not be sufficient, and this is why impregnation of ACs is a widely used technique to enhance their performance for biogas desulfurization. By introducing chemical agents such as metal oxides (e.g. iron, zinc or copper) [2–4] or alkaline compounds (e.g. KOH or $K_2CO_3$) [5–7], the surface reactivity of AC is indeed significantly improved, allowing more efficient $H_2S$ removal.

Extensive research has focused on the impregnation of ACs to

improve their adsorption performance for biogas desulfurization, with results mainly reported per unit weight, such as milligram of $H_2S$ per gram of AC, as illustrated in literature reviews [1,8–10]. However, on a commercial scale, $H_2S$ removal relies on densified sorbents, which offer higher separation efficiency per unit volume, a metric preferred in industrial applications due to space constraints and the need to replace sorbents as less frequently as possible.

To our knowledge, no published research has evaluated the effect of AC densification on biogas desulfurization, despite its performance-enhancing potential. Additionally, impregnation of the densified AC is essential to further improve $H_2S$ separation efficiency. Commercial activated carbons (CAC) used for industrial-scale biogas purification already incorporate both optimization processes [1].

Furthermore, densified ACs are mandatory for industrial adsorption columns. Thus, ACs are generally pelletized to optimize particle size and structural integrity. Fine or low-density carbons can indeed lead to significant pressure drops, increasing process energy consumption [11].

<footnote><sup>a</sup> Université de Lorraine, CNRS, IJL, Epinal, F-88000, France</footnote>

<footnote><sup>b</sup> Groupe BORDET, Froidvent, F-21290, Leuglay, France</footnote>

<footnote><sup>c</sup> Institut Universitaire de France (IUF), F-75231, Paris, France</footnote>

<footnote>* Corresponding author.</footnote>

<footnote>** Corresponding author. Université de Lorraine, CNRS, IJL, Epinal, F-88000, France.</footnote>

<footnote>E-mail addresses: vanessa.fierro@univ-lorraine.fr (V. Fierro), alain.celzard@univ-lorraine.fr (A. Celzard).</footnote>
Pelletization increases particle size and uniformity, which improves bed packing, provides greater mechanical stability and facilitates gas flow with minimal pressure drop, which also extends the operational life of adsorption bed systems.

In this work, we investigated the densification of wood-based activated charcoal by compaction with binders. Three kinds of binders were considered: sugar cane molasses, beet molasses, and polyphenols of plant origin (not disclosed for reasons of industrial secrecy). Moreover, the densified ACs were impregnated with alkaline substances, including KI, K₂CO₃, NaOH and KOH, to improve their desulfurization performance. The study aimed to determine the optimal amounts of impregnating agents and binders, as well as the most effective modification sequence to maximize H₂S adsorption efficiency. Finally, the best-performing adsorbent developed was compared with CACs under identical evaluation conditions, enabling assessment of its potential for industrial application in biogas desulfurization.

H₂S separation was evaluated by a dynamic adsorption process using a gas mixture containing N₂, CO₂ and O₂. The presence of CO₂ is important, as it can compete with H₂S for adsorption sites and influence overall desulfurization performance. Moreover, the inclusion of O₂ allowed oxidation reactions that enhance sulfur retention on the AC

surface. Thus, this study ensured that the developed adsorbents were tested under relevant and practical operational parameters. A mathematical model for dynamic adsorption simulation of H₂S was also applied for mass transfer characterization of the adsorbent. In addition, thermal regeneration was tested on the optimum sorbents developed, revealing the challenges associated with their regeneration.

## 2. Experimental

### 2.1. Materials

Wood-based activated charcoal (particle size 3–8 mm) was supplied by the French company Groupe Bordet as part of a joint (LabCom "CarBioLab") laboratory. The charcoal was produced from oak and beech sawdust using Groupe Bordet's slow, continuous pyrolysis process, giving it high purity with a carbon content of over 90 %. It was steam-activated at 900 °C for 2 h with a heating rate of 5 °C min⁻¹ in a rotary reactor. These conditions were selected based on our previous work [12]. The resulting activated carbon, called Bordet AC, had a granular form and a bulk density of 263 kg m⁻³, measured by sieving the sorbent to retain particles between 0.5 and 1 mm, then filling a known

<figure><img src="image_3.png" /><figcaption>**Fig. 1.** (a) Bulk density, origin, modification and shape of Bordet and commercial ACs (CACs) for biogas desulfurization. (b) Synthesis of lab-made, densified impregnated AC. (c) Experimental test bench for dynamic H₂S adsorption. (A colour version of this figure can be viewed online.)</figcaption></figure>
volume and determining the mass it contained.

For AC pelletization, four eco-friendly binders were considered: two sugar cane molasses, one beet molasses and polyphenols of plant origin, the exact name of which will not be revealed here. The sugar cane molasses had a viscosity of 1000–6000 cP at 20 °C, while that of the beet molasses was 500–1500 cP at the same temperature. Plant-based polyphenols were in the shape of a solid powder. Pelletization was carried out using an Atlas 15 T manual hydraulic press.

For chemical modification, activated charcoal was impregnated with analytical-grade potassium iodide (KI), potassium carbonate (K₂CO₃), sodium hydroxide (NaOH) or potassium hydroxide (KOH) (purity ≥99 %), using solutions prepared with distilled water.

To compare our materials, nine commercial activated carbons (CACs) used in industrial biogas desulfurization were analyzed. Fig. 1a shows their bulk densities (ρb), origins, chemical modifications and shapes. Most are coal-based, with the exception of CAC 1 and CAC 2, derived from wood and from coconut shell, respectively. All are chemically impregnated, although some suppliers do not specify impregnation details.

## 2.2. Densification

The densification procedures are illustrated in Fig. 1b as Method 1 (top route) and Method 2 (bottom route). In Method 1, Bordet AC was first mixed with a binder, either molasses or polyphenols, then sieved to a particle size of less than 1 mm. In the case of molasses, 3 wt% water (relative to the total mass of AC) was added to improve malleability. Similarly, 5 wt% water was added in the case of polyphenols. The mixture was then compacted into pellets under 87 MPa for 1 min. These pellets were dried overnight at 105 °C before being carbonized at 700 °C for 2 h under nitrogen flow (80 mL min⁻¹). The carbonization ramp included a heating rate of 2 °C min⁻¹ up to 400 °C, followed by 5 °C min⁻¹ up to 700 °C. In the final stage, chemical impregnation of the carbonized pellets was performed to obtain the densified and impregnated AC product. Impregnation will be described in detail in the next subsection 2.3.

In contrast, Method 2 began with an initial chemical impregnation of Bordet AC prior to pelletization. The impregnated AC was then mixed with a binder and compacted under the same conditions (87 MPa, 1 min), then dried and carbonized according to the same thermal profile as in Method 1.

The densified ACs were labelled as binder %, where the type of binder is followed by its weight percentage. The suffix “nc” indicates that the densified AC was prepared without undergoing carbonization.

In addition, pellets were also prepared by first sieving the Bordet AC to obtain very fine particles between 20 and 40 µm, followed by the same procedure as described above. These samples were labelled “20 µm” to distinguish this preparation method.

## 2.3. Impregnation

Fig. 1b also describes the impregnation procedures implemented in this study. In Method 1, impregnation was carried out after the densification and carbonization steps. Conversely, in Method 2, impregnation was performed prior to densification and carbonization. In both cases, impregnation was carried out using the incipient wetness method [13]. Prior to impregnation, the AC materials were dried at 105 °C for 24 h. The impregnation volume corresponded to the total pore volume of the ACs, estimated according to Eq. (1), using the skeletal density and grain density defined in the next subsection 2.4. The impregnation solutions contained specific amounts of KI, K₂CO₃, NaOH or KOH, selected to achieve a target impregnation ratio (wt.%). After impregnation, the samples were vacuum-dried overnight at 80 °C.

$$ \text{Pore volume} = \left( \frac{1}{\text{Grain density}} - \frac{1}{\text{Skeletal density}} \right) \times \text{Mass of AC} \quad (1) $$

## 2.4. Characterization

The binders used for densification were characterized in terms of ash content, and were also studied by thermogravimetric analysis (TGA), elemental analysis and, for the ashes obtained, by X-ray diffraction (XRD). To determine ash content, samples were heated in ceramic crucibles at a rate of 5 °C min⁻¹ up to 650 °C and held for 4 h to ensure complete combustion of the organic matter. After cooling, the residual ash was weighed. Each sample was tested in triplicate, and the average ash content was reported. The collected ash was subsequently analyzed by XRD to assess its crystalline composition.

Thermal decomposition behavior was analyzed using a Netzsch STA 449 F³ Jupiter thermogravimetric (TGA) analyzer. Binders were heated from room temperature to 900 °C at 5 °C min⁻¹ under argon flowing at 50 mL min⁻¹. TGA data revealed mass loss, volatile matter and fixed carbon contents. This information allowed us to choose the right temperature for carbonizing the pellets.

The bulk elemental composition (C, H, O, N and S) was analyzed using a Vario EL Cube (Elementar). Dried binder sample (≈2 mg) were burnt in a helium-driven O₂ stream, and the resulting gases were separated using trap columns and quantified using a thermal conductivity detector. The textural properties of the sorbent materials (pristine Bordet AC and densified (non-impregnated) ACs) were analyzed by N₂ adsorption–desorption isotherms at –196 °C using an ASAP 2020 apparatus (Micromeritics). Prior to analysis, the samples were degassed under vacuum at 110 °C for 24 h. The BET area (A<sub>BET</sub>) was calculated using the Brunauer–Emmett–Teller (BET) model [14], while the NLDFT specific surface area (S<sub>NLDFT</sub>) and pore size distribution were obtained with SAIEUS® software based on the non-local density functional theory (NLDFT) [15]. Pore volumes, including total (V<sub>tot</sub>), mesopore (V<sub>mes</sub>, pore width 2 < w < 50 nm), micropore (V<sub>mic</sub>, w < 2 nm), and ultramicropore (V<sub>ultra-mic</sub>, 0.7 < w < 2 nm) volumes were also obtained using the NLDFT. For comparison, the micropore volume was also calculated using the Dubinin–Radushkevich method (V<sub>DR</sub>). The total accessible pore volume was estimated from the N₂ adsorbed volume at –196 °C and a relative pressure of 0.97 (V<sub>0.97</sub>).

The skeletal density was measured using a Micromeritics AccuPyc II 1340 gas pycnometer with helium as the probe molecule to ensure accurate volume measurements by penetrating the narrowest pores. Samples were degassed beforehand to remove moisture and adsorbed gases. The skeletal density, calculated from the volume of He displaced, reflects the true density of the solid material excluding pore volumes.

The grain density was measured with a Micromeritics GeoPyc 1360 pycnometer using an incompressible DryFlow® medium. AC grains were embedded in this medium, whose grains are fine enough to fill all intergranular spaces, but not fine enough to penetrate intragranular porosity. The grain density was then calculated from the volume of DryFlow® displaced, accounting for internal pores but excluding inter-particle voids.

## 2.5. H₂S dynamic adsorption

Dynamic adsorption experiments were carried out using the fixed-bed setup shown in Fig. 1c. The AC pellets were ground and sieved to 0.5–1.0 mm particles, and a fixed volume was used to completely fill a stainless-steel column (0.77 cm internal diameter, 10 cm length). The column was maintained at 25 °C using a thermostated water bath and operated at atmospheric pressure. To standardize moisture content, the ACs were pre-humidified for 24 h at 50 % relative humidity (RH) and 25 °C before testing.

Dynamic adsorption tests used synthetic biogas containing either 400 or 4000 ppm H₂S, 60 % N₂, and the balance CO₂; 1 % O₂ was also included in some experiments to simulate oxidative conditions, even more representative of a real biogas. Outlet gas composition was analyzed with a Micro GC Fusion (Inficon). A constant flow rate of 80
mL min⁻¹ was maintained by mass flow controllers, and corresponded to a linear velocity of 2.9 cm s⁻¹.

In industrial desulfurization, ACs are replaced at partial saturation once the breakthrough point (BTP) is reached or H₂S content at the column outlet exceeds acceptable levels. ASTM D6646 [16] recommends a BTP of 50 ppm for an inlet of 10,000 ppm H₂S. In this study, adsorption capacity was measured at an outlet H₂S concentration of 40 ppm, equivalent to a 10% BTP for 400 ppm inlet concentration and 1% BTP for 4000 ppm. This threshold was chosen to approximate a BTP of 4 ppm, the limit for using biogas as a substitute for natural gas [17].

The H₂S separation capacity, qH₂S (mg), was calculated using Eq. (2) [12].

$$q_{H_2S} = Q C_0 \int_{t_0}^{t_m} \left(1 - \frac{C_m}{C_0}\right) dt \qquad (2)$$

where Q (mL min⁻¹) is the inlet flow rate, C₀ (mg mL⁻¹) is the H₂S concentration at the inlet, t₀ (min) is the time at the start of the experiment, t_m (min) is the time at which the separation capacity is measured, which can be at BTP or at full saturation, and C_m (mg mL⁻¹) is the concentration of H₂S at the outlet at t_m.

The separation capacity was normalized by either adsorbent mass (g) or volume (cm³) as shown in Eq. (3) and Eq. (4), respectively:

$$q_{H_2S}^{m} = \frac{q_{H_2S}}{AC \text{ mass}} \qquad (3)$$

$$q_{H_2S}^{v} = \frac{q_{H_2S}}{AC \text{ volume}} \qquad (4)$$

# 3. Results and discussion

## 3.1. Binder characteristics

The results of binder characterization by thermogravimetric analysis, ash content measurement determination and XRD analysis of these ashes are presented in Fig. 2a, b and 2c, respectively. The TGA profiles (Fig. 2a) show the thermal decomposition behavior of the four binders from room temperature to 900 °C. All binders exhibited significant weight loss between 100 °C and 300 °C, attributed to moisture evaporation and release of volatile organic compounds. Cane molasses 1 and cane molasses 2 displayed comparable thermal profiles, whereas the polyphenol-based binder showed lower weight loss over the whole temperature range (up to 900 °C), indicating better thermal stability.

The amount of water (mass loss up to 200 °C) in the binders is much higher for the cane molasses, presenting around 25 wt%, followed by beet molasses at 15.6 wt% and polyphenols with only 6.4 wt%. This is why more water is required for the preparation of the AC pellet when polyphenols are used. Above 200 °C, continued degradation of heavier organic substances is observed. The total mass losses follow the order: beet molasses (49.8%) > polyphenols (45.3%) > cane 1 molasses (33.9%) > cane 2 molasses (31.5%) (see Table S1 in the Supplementary Information).

This behavior reflects differences in volatile matter content and suggests that cane molasses and polyphenol-based binders produce a higher yield of fixed carbon during pyrolysis, an essential factor in maintaining structural integrity during pelletization. In particular, the fixed carbon content on a dry basis is higher in cane 1 (57.9%) and cane 2 (59.1%) molasses, and in polyphenols (53.3%), than in beet molasses (43.9%).

Furthermore, ash content influences the final purity of the densified AC, with a higher ash content introducing more residual inorganic matter, affecting the quality and performance of the final densified AC product. Among the samples, cane 2 molasses has the highest ash content (9.4%), followed by cane 1 molasses (8.2%) and beet molasses (6.3%). Polyphenols, on the other hand, have the minimum ash content (1.4%).

The XRD pattern compares the crystalline phases present in the ashes of the binder. The dots mark the diffraction peaks that could correspond to potassium chloride (KCl), present in all samples. The ashes of molasses and polyphenols contain inorganic components such as Ca, K, Mg, and Na, which remain in the densified AC after the carbonization process [18,19]. These residual elements are known to positively influence the separation of H₂S, as they play a catalytic role in promoting H₂S oxidation [1].

## 3.2. AC characteristics

Bordet AC was densified as explained in section 2.2. Pellets were produced when the cane molasses content reached at least 30 wt% relative to the amount of AC, but a higher content of 40 wt% produced pellets with improved mechanical properties, such as greater hardness and reduced generation of fine particles during handling. An even higher proportion of 50 wt% was also tested and identified as the most effective in producing high-quality pellets, based on our evaluation criteria, which include the release of fragments and fine particles during handling and after dropping the pellets from a height of 1 m onto the ground.

In contrast, the formation of pellets using beet molasses failed, even with a binder content of 50 wt%, as the resulting pellets were brittle and broke easily after carbonization. This poor performance is attributed to the lower thermal stability and reduced fixed carbon content of beet molasses (see Fig. 2a), which compromises the mechanical integrity of the pellets. Nonetheless, the material, even in its brittle state, was used for measurements of bulk density and H₂S adsorption performance. For the plant-based polyphenol binder, a lower binder content was sufficient, with just 10 wt% to achieve optimum pellet quality. In addition, a 20 wt% content was also evaluated, as it further enhanced pellet quality.

The bulk density of ACs densified with different binders is presented in Fig. 3a. To assess the impact of carbonization, the non-carbonized (nc) forms of ACs with cane molasses 2 and polyphenols were also considered. All densified samples show higher bulk densities than the non-densified Bordet AC, confirming the effectiveness of pelletization. In the carbonized form, density remained relatively constant for the same

<figure><img src="image_4.png" /><figcaption>Fig. 2. Results of binder characterization: (a) TGA curves, (b) ash contents, and (c) XRD patterns of the ashes. (A colour version of this figure can be viewed online.)</figcaption></figure>
<figure><img src="image_5.png" /><figcaption>Fig. 3. (a) Bulk density of densified ACs, compared to the pristine Bordet AC, and pore volume variation as a function of binder type and content: (b) mesoporosity, (c) microporosity, and (d) ultra-microporosity. (A colour version of this figure can be viewed online.)</figcaption></figure>

type of binder, whatever the binder content (30–50 % for molasses and 10–20 % for polyphenols). Non-carbonized samples presented higher densities; for example, Can2_50 % reached 437 kg m⁻³, while its non-carbonized form was at 591 kg m⁻³.

When the particle size was reduced to very fine particles ranging from 20 to 40 µm, the resulting sorbent became denser, as observed with cane molasses 2 (can2_50_%20µm) and polyphenol (pol_15_%20µm) binders, which exhibit densities of 600 and 531 kg m⁻³, respectively. This is because the finer particles pack more efficiently, reducing the voids between them and leading to a more compact structure.

Can2_50 % nc also showed high density (591 kg m⁻³) but, as it was not carbonized, its high content of binder-derived volatiles might have a negative effect on H₂S separation. When carbonized, AC with cane molasses 1 reached a density of around 430 kg m⁻³, similar to AC with cane molasses 2 at 440 kg m⁻³, both slightly higher than AC with polyphenols, which gave 402 kg m⁻³ when not carbonized and 389 kg m⁻³ when carbonized.

The impact of binder type and content on porosity was assessed by N₂ physisorption analysis, with adsorption isotherms at 77 K presented in Fig. S1. Fig. 3b, c and 3d show the evolution of mesopore (V<sub>mes</sub>), micropore (V<sub>mic</sub>), and ultramicropore (V<sub>ultra-mic</sub>) volumes, respectively, as a function of binder loading. The non-densified material exhibited the highest textural properties: BET area of 1120 m² g⁻¹, total pore volume of 0.56 cm³ g⁻¹, and V<sub>mic</sub> of 0.44 cm³ g⁻¹. As expected, increasing binder content in densified samples systematically reduced all porosity parameters (see Table S2), as shown in Fig. 3b–d. This decrease is due to partial obstruction or collapse of pores during densification and carbonization.

Among the densified materials, polyphenol-based ACs retained the highest porosity at comparable binder levels, with Pol_10 % reaching a BET area of 1012 m² g⁻¹ and V<sub>mic</sub> of 0.40 cm³ g⁻¹, close to the non-densified reference. In contrast, samples densified with 50 wt% cane molasses (e.g. Can1_50 %) exhibited significantly reduced porosity (BET

area of 595 m² g⁻¹ and V<sub>mic</sub> of 0.24 cm³ g⁻¹).

Carbonization proved essential for recovering microporosity. Non-carbonized samples (Pol_nc and Can2_nc) indeed showed drastically lower V<sub>mic</sub> and V<sub>ultra-mic</sub> values. For instance, Can2_50 % nc has V<sub>mic</sub> and V<sub>ultra-mic</sub> values of just 0.11 cm³ g⁻¹ and 0.05 cm³ g⁻¹, respectively, compared with 0.28 cm³ g⁻¹ and 0.14 cm³ g⁻¹ for the carbonized Can2_50 % sample.

ACs sieved to 20–40 µm exhibited higher packing densities, which also led to a more pronounced reduction in pore volume. Pol_15_%20 µm showed a BET area of 910 m² g⁻¹, compared with 1075 m² g⁻¹ for the sample sieved to a broader particle size range (pol_15 %). A similar trend was observed for the cane molasses 2 binder, where cane2_50 % 20 µm reached 550 m² g⁻¹, while cane2_50 % retained 700 m² g⁻¹. The greater loss in porosity with finer particles can be attributed to tighter packing, increased binder penetration, and possible pore blockage or collapse during densification.

Overall, these results highlight the importance of binder selection and carbonization in preserving microporosity and maximizing H₂S removal efficiency. Polyphenol-based binders offer the best compromise between structural integrity and porosity retention at lower binder dosages.

### 3.3. Effect of binder on H₂S separation

To evaluate the impact of binder type on the performance of densified ACs for H₂S separation, dynamic adsorption tests were run with 400 ppm H₂S in a gas composed of 60 % N₂ and the rest CO₂. Breakthrough curves are shown in Fig. 4a (cane 1- and beet-based AC), 4b (cane 2-based AC), and 4c (polyphenol-based AC). Separation capacities, normalized by AC mass and volume, appear in Fig. 4d and e, respectively, with the corresponding values in Table S3.

ACs densified with cane molasses 2 and beet molasses undergoing carbonization showed the longest durations (≈200 min). On the
<figure><img src="image_6.png" /><figcaption>**Fig. 4.** Breakthrough curves of ACs densified by different amounts of binders of different types for H₂S (400 ppm) separation: (a) Cane 1 and beet molasses, (b) Cane molasses 2 and (c) Polyphenols. Corresponding separation capacities at BTP per unit mass, and (e) per unit volume (f). AC ND means non-densified Bordet AC. (A colour version of this figure can be viewed online.)</figcaption></figure>

contrary, despite the higher densities of non-carbonized samples based on cane molasses (Can2_40 %_nc, Can2_50 %_nc), separation breakthrough times were reduced (<50 min).

Cane 1-based AC presents breakthrough times (≈159 min) shorter than cane 2-based AC, which may be due to difference in their compositions; for instance, cane molasses 2 contains a higher ash content (see Fig. 2b), indicating a greater presence of mineral compounds that contribute to enhanced H₂S adsorption.

Although polyphenol-based densified ACs retained higher porosity levels than molasses-based densified ACs (see Fig. 3b, c and 3d), they showed shorter breakthrough times (≈93 min) than carbonized molasses-based ACs and only better than non-densified AC (ND). This is due to the fact that, in molasses-based carbons, the material remaining after carbonization at 700 °C has been enriched in ashes, thus in metal compounds that act as catalysts, promoting H₂S separation by oxidation reactions and thus improving H₂S adsorption performance [1].

Non-carbonized, polyphenol-based ACs demonstrated comparable performance to their carbonized counterparts. For instance, the non-carbonized sample Pol_10 %_nc achieved an H₂S separation capacity of 1.9 mg g⁻¹ at breakthrough, while the carbonized form (Pol_10 %) reached 2.5 mg g⁻¹. Since the binder content of polyphenol-based ACs is lower (10–20 wt%) compared to cane-based ACs (50 wt%), the reduction in separation performance in the non-carbonized form is more favorable.

Overall, densification increased breakthrough times and therefore improved separation efficiency per unit volume, but it increased sample mass and could thus reduce separation capacity per unit mass. For example, the polyphenol-based AC (pol_10 %) adsorbed 2.4 mg g⁻¹ of H₂S, which is less than the 2.9 mg g⁻¹ of non-densified AC (AC ND). Despite this, pol_10 % achieved a higher volumetric separation capacity, 0.9 mg cm⁻³ versus 0.7 mg cm⁻³ for AC ND. In contrast, molasses-based ACs showed better performance, both per unit mass and volume, due to the binder-induced surface functionalization that improves H₂S separation.

The highest H₂S separation capacity was observed for can2_30 %, with 5.0 mg g⁻¹ and 2.1 mg cm⁻³ of H₂S removed. However, can2_50 %, with a slightly lower performance (4.2 mg g⁻¹ and 2.1 mg cm⁻³), is considered more suitable due to its higher hardness, making it more relevant for industrial applications. In contrast, although polyphenol-based ACs showed lower initial performance, their greater microporous volume makes them promising candidates for functionalization by impregnation.

### 3.4. Selection of the best impregnating agent in the presence of O₂

The effect of four impregnating agents (KI, K₂CO₃, NaOH and KOH) was tested at 5 and 10 wt%. These compounds either directly react with H₂S or function as catalysts to promote its oxidation [1]. Tests used a gas mixture containing 1.1 % O₂, 4000 ppm H₂S, 60 % N₂, and the rest CO₂. The positive influence of O₂ on enhancing H₂S adsorption capacity is well documented [1], and its effect was also observed and evaluated here. Non-carbonized polyphenol-based AC (Pol_15 %_nc) on the one hand, and cane 2-based AC on the other hand (Can_50 %), were used for this test, all synthesized by Method 2 with the density values reported in Table S4. Fig. 5a shows the H₂S breakthrough curves for non-carbonized polyphenol-based ACs, and Fig. 5b for cane-2-based ACs. Fig. 5c and d shows the separation capacities normalized by mass and volume, respectively; the corresponding values are presented in Table S5. Higher impregnation rates further extended the breakthrough curves. KOH gave the best results, with an optimum loading of 10 wt%. The polyphenol-based AC reached breakthrough time at 550 min, while cane-2-based AC did so at 640 min. NaOH performed best among the other agents at 10 wt% loading, with a 400-min breakthrough time for polyphenol-based AC, higher than KI (240 min) and K₂CO₃ (230 min), but still lower than KOH. For cane-2-based AC, all agents (NaOH, K₂CO₃, KI) gave similar breakthrough times of around 480 min for 10 wt% impregnation.

KOH at 10 wt% showed the highest H₂S separation capacity,
<figure><img src="image_7.png" /><figcaption>c) Separation per unit mass</figcaption></figure>

<figure><img src="image_8.png" /><figcaption>b) Method 2: Cane molasse 2</figcaption></figure>

<figure><img src="image_9.png" /><figcaption>d) Separation per unit volume</figcaption></figure>

<figure><img src="image_10.png" /></figure>

**Fig. 5.** Breakthrough curves for the separation of H₂S (4000 ppm), in the presence of O₂, of densified ACs with (a) non-carbonized polyphenol binder and (b) cane molasses 2 binder, all prepared by Method 2. Corresponding separation capacities: (c) per unit mass, and (d) per unit volume. NI means non-impregnated, still densified. (A colour version of this figure can be viewed online.)

confirming its superior performance. In non-carbonized, polyphenol-
based AC, it achieved 111 mg g⁻¹ and 54 mg cm⁻³, outperforming NaOH
(76 mg g⁻¹, 37 mg cm⁻³). With cane-2-based AC, it allowed reaching

146 mg g⁻¹ and 62 mg cm⁻³, well above NaOH (105 mg g⁻¹ and 44 mg cm⁻³).

Cane 2 binder performed better than the non-carbonized

<figure><img src="image_11.png" /></figure>

**Fig. 6.** Breakthrough curves for the separation of H₂S (4000 ppm), without O₂, of densified ACs with binders: (a) non-carbonized polyphenols, (b) carbonized polyphenols, and (c) cane molasses 2, prepared by Method 1 (full symbols) or Method 2 (empty symbols). Corresponding separation capacities: (d) per unit mass, and (e) per unit volume. (A colour version of this figure can be viewed online.)
polyphenols. However, the non-carbonization aspect is attractive, as it could lead to substantial energy savings in industrial applications. Therefore, in the next test (see subsection 3.5.2), the non-carbonized polyphenol binder will be reconsidered to compare its performance with that of the carbonized version.

## 3.5. Selection of the best impregnation method with KOH

### 3.5.1. In the absence of O₂

Two binders were chosen for AC impregnation: can2_50 % for its high separation capacity without impregnation, and pol_15 % (carbonized and non-carbonized) for its microporosity offering potential performance after impregnation with KOH. Impregnation levels of 5, 10, and 15 wt% were tested. H₂S separation was evaluated at 4000 ppm in a gas mixture of 60 % N₂ and the rest CO₂.

The two methods of impregnation explained in subsections 2.2 and 2.3, and summarized in Fig. 1b, were applied. It is useful to recall here that these methods differ in whether KOH was added before or after densification. Adding KOH prior to carbonization at 700 °C can cause chemical activation of the carbon matrix, forming potassium-based compounds (e.g. K, K₂O, K₂CO₃) [20]. Without washing after carbonization, these residues (or the products of their oxidation and reaction with moisture) remain on the carbon surface and improve H₂S removal, which is in line with the intended purpose.

Fig. 6 shows the breakthrough curves obtained by the two methods for non-carbonized polyphenol-based AC (6a), carbonized polyphenol-based AC (6b) and cane 2-based AC (6c). The separation capacities are presented per unit mass and unit volume in Fig. 6d and e, respectively. Tabulated values of separation capacities at saturation are presented in Table S6.

Cane-based ACs showed the longest breakthrough time (45 min), followed by carbonized polyphenol-based AC (41 min). Breakthrough time improved until impregnation reached 10 wt%, but dropped at 15 wt% due to excessive pore blocking.

Method 1 performed slightly better than Method 2: for instance, at

10 wt% KOH on the cane-based AC, it achieved 8.75 mg g⁻¹ and 4.30 mg cm⁻³ against 8.62 mg g⁻¹ and 4.11 mg cm⁻³ with Method 2. This trend is found in all comparisons carried out with identical impregnation levels and the same binder.

Impregnated, polyphenol-based AC performed similarly to impregnated, cane-based AC. At 10 wt% KOH (method 1), the former removed 8.74 mg g⁻¹ and 4.10 mg cm⁻³ of H₂S versus 8.75 mg g⁻¹ and 4.30 mg cm⁻³ for the latter. This small difference contrasts with the larger discrepancy observed in non-impregnated samples, and highlights the high microporosity of the polyphenol-based AC for functionalization by impregnation. Overall, the two methods gave similar results, with method 1 being slightly superior. The same comparison is made in the next subsection 3.5.2 under O₂, which improves separation and allows for more definite conclusions.

### 3.5.2. In the presence of O₂

KOH at 10 wt%, identified as the most effective impregnating agent, was used to select the best method and type of binder for the fabrication of AC pellets, in a gas mixture containing O₂ (1.1 % O₂, 4000 ppm H₂S, 60 % N₂ and the rest balanced with CO₂). A higher impregnation level of 15 wt% was also tested to determine whether oxygen content could further improve performance. Fig. 7 shows the H₂S breakthrough curves for Method 1 (7a) and Method 2 (7b). Fig. 7c and d presents H₂S separation capacities per unit mass and unit volume, respectively.

Among polyphenol and cane 2 binders, fabricated in the same way (by sieving the granular AC to a particle size of less than 1 mm), the longest breakthrough time occurs for the cane-2-based sample with 10 wt% KOH using Method 1 (880 min), followed by the carbonized polyphenol-based sample with the same method (820 min). Increasing KOH up to 15 wt% reduced performance, confirming that 10 wt% is optimal. For example, cane-2-based AC reduced breakthrough time to 500 min using Method 1, while polyphenol-based ACs also showed a decrease (from 820 to 670 min by Method 1). Non-carbonized polyphenols remained less competitive with a breakthrough time of 580 min using Method 1.

<figure><img src="image_12.png" /><figcaption>**Fig. 7.** Breakthrough curves for the separation of H₂S (4000 ppm), in the presence of O₂, of densified ACs impregnated with KOH at 10 wt% (full symbols) and 15 wt % (empty symbols), synthesized by (a) Method 1 and (b) Method 2. Corresponding separation capacities normalized by (c) unit mass and (d) unit volume of adsorbent. (A colour version of this figure can be viewed online.)</figcaption></figure>
Under these optimum preparation conditions (Method 1), the separation capacity per unit mass was nearly identical for the polyphenol-based AC (178 mg g⁻¹) and the cane molasses 2-based AC (181 mg g⁻¹). This is due to the higher density of cane molasses-based AC, which reduces separation capacity when normalized per unit mass. However, when considering H₂S separation capacity per unit volume, the cane molasses 2-based AC demonstrated slightly higher performance (85 mg cm⁻³) compared to the polyphenol-based AC (80 mg cm⁻³).

KOH impregnation at 10 wt% using Method 1, identified as the most optimal, was applied to pellets made from AC sieved to a particle size of 20–40 µm, resulting in denser sorbents. However, the dynamic adsorption test could only be performed on pol_15_%_20 µm. For can2_50_%_20 µm, the test was not possible due to excessive pressure drop. This was caused by high flow resistance due to the high packing density of the bed, primarily due to the high binder content (50 %), which reduced porosity and limited gas flow. Pol_15_%_20 µm showed the longest breakthrough curve, lasting 1000 min. Its capacity per unit mass, 181 mg g⁻¹, was nearly the same as that of the version synthesized with AC < 1 mm, 178 mg g⁻¹. However, the volumetric capacity increased significantly from 80 mg cm⁻³ to 96 mg cm⁻³. This confirms that the polyphenol binder performs better than cane molasses 2.

Experiments conducted in the presence of O₂ resulted in greater H₂S separation capacities, more than 20 times higher than those observed under O₂-free conditions (see Fig. 6). Although Methods 1 and 2 showed comparable performance in the absence of O₂, the substantial increase in H₂S uptake under oxidative conditions makes the differences between the two methods more obvious. Oxygen, in gaseous or adsorbed form, promotes the oxidation of H₂S and its decomposition into sulfur compounds. The O₂ present in the inlet gas drives these reactions, as shown in Eq. (5) [1]:

$$H_2S_{\text{gas}} + O_2 \rightarrow SO_2 + H_2 \quad (5)$$

The H₂S bound to the surface of activated carbon oxidizes to form solid sulfur (S, S₈), sulfur dioxide (SO₂) and H₂O, as shown in the following reactions:

$$H_2S_{\text{gas}} + \frac{1}{2}O_2 \rightarrow S + H_2O \quad (6)$$

$$2H_2S_{\text{gas}} + O_2 \rightarrow \frac{1}{4}S_8 + 2H_2O \quad (7)$$

The deposited sulfur can oxidize further to form sulfur dioxide:

$$S + O_2 \leftrightarrow SO_2 \quad (8)$$

$$\frac{1}{4}S_8 + 2O_2 \leftrightarrow 2SO_2 \quad (9)$$

Alternatively, an elemental sulfur site, fixed at the surface (Sf) can react with O₂ to form oxidized sulfur species, S(O):

$$S_f + \frac{1}{2}O_2 \rightarrow S(O) \quad (10)$$

S(O) can also react with H₂S to produce the original sulfur Sf, sulfur (Sx) and H₂O:

$$S(O) + H_2S \rightarrow S_f + S + H_2O \quad (11)$$

It is useful to recall here that, in Method 1, the AC is first pelletized and carbonized before being impregnated, thus allowing the structural integrity of the pellets to be preserved during high-temperature treatment. Method 2, on the other hand, involves impregnation of the AC prior to pelletization and carbonization, which presents certain disadvantages. During compaction, the impregnating agent may indeed not effectively penetrate the microporous structure, resulting in uneven distribution throughout the material and compromising the uniformity of surface functionality. Additionally, subjecting the impregnated AC to carbonization at 700 °C may degrade or volatilize some of the chemical

agents, reducing impregnation efficiency and altering surface properties. By isolating the impregnation step after heat treatment, Method 1 maintains both chemical functionality and structural stability, leading to greater separation efficiency.

Method 1 is in line with industrial practice, as it avoids introducing reactive chemicals into high-temperature furnaces, an approach preferred for safety, operational and environmental reasons. Chemicals such as KOH are highly corrosive and reactive, especially at 700 °C, presenting risks to equipment and personnel. In addition, the management of waste and effluents, such as potassium vapors, is a major concern.

### 3.6. Comparison with CACs for biogas desulfurization

As we have seen, bulk density significantly affects H₂S separation efficiency: higher density provides more adsorbent per unit volume, which increases H₂S uptake. While low-density AC can give good results per gram due to its intrinsic properties, it offers lower volumetric efficiency due to limited packing. As shown in Fig. 8a, our best-performing sorbent (Pol_15_%_20 µm_KOH 10 %) has an acceptable bulk density (531 kg m⁻³), comparable to CAC 1, the only wood-based CAC (518 kg m⁻³), highlighting the disadvantage of wood-based ACs compared with mineral-coal based ACs. CAC 6 and CAC 7 have the highest densities at 753 kg m⁻³ and 764 kg m⁻³, respectively, while CAC 2, CAC 8 and CAC are around 700 kg m⁻³.

Fig. 8b shows the relationships between pore volumes (ultra, micro and mesopores), which were determined in a previous study, considering only the substrate material without impregnation [12]. Non-coal-based CACs (CAC 1 and CAC 2) have the highest BET areas (1125–1145 m² g⁻¹), while coal-based CACs range from 830 to 940 m² g⁻¹ with similar microporosity (~0.35 cm³ g⁻¹) and ultramicroporosity (0.08–0.17 cm³ g⁻¹). Our AC made with polyphenol binder show average textural properties, with a BET area of 910 m² g⁻¹, a micropore volume of 0.33 cm³ g⁻¹, and an ultra-micropore volume of 0.17 cm³ g⁻¹. Remarkably, it features a high mesopore volume of 0.12 cm³ g⁻¹, greater than that of CACs (max 0.07 cm³ g⁻¹, CAC 6).

The CACs were evaluated under the same conditions as in subsection 3.5.2 (1.1 % O₂, 4000 ppm H₂S, 60 % N₂, the rest CO₂) and the resulting breakthrough curves are presented in Fig. 8c, along with the calculated H₂S separation capacities shown in Fig. 8d (per unit mass) and Fig. 8e (per unit volume) [12].

CAC 7, CAC 8 and CAC 9 have the longest breakthrough times (1180–1280 min), outperforming all other sorbents. With bulk densities above 650 kg m⁻³ and probably optimal surface functionalization, confirmed for CAC 7 (10 wt% NaOH), they combine the three key factors for high performance: high microporosity, effective functionalization and high bulk density. Other dense CACs such as CAC 6 (753 kg m⁻³) and CAC 2 (687 kg m⁻³) perform less effectively, likely due to less efficient functionalization. CAC 2, for instance, uses only 5 wt% KOH. CAC 1, CAC 2 and CAC 5, form a second group of high-performance adsorbents, with breakthrough times of between 770 and 870 min. The sorbent developed here (Pol_15_%_20 µm_KOH 10 %) outperforms them, achieving a breakthrough time of 1000 min.

Despite the lower bulk density of our developed AC, it is competitive with 6 of the 9 CACs in terms of separation efficiency per unit volume (96 mg cm⁻³). In fact, when normalized by mass, our ACs rank among the best (181 mg g⁻¹), surpassed only by CAC 9 (193 mg g⁻¹). Such high performance from our relatively low-density AC offers the advantage of easier handling and transport, which is especially beneficial for mobile systems or small-scale digesters in rural or off-grid facilities.

## 4. Mathematical model

The mathematical model used to simulate dynamic H₂S adsorption and its constitutive equations are detailed in the SI. To implement it, the data in Table 1 were used. Adsorption isotherms were obtained by
<figure><img src="image_13.png" /><figcaption>**Fig. 8.** (a) Bulk density of Pol_15_%_20 µm_KOH 10% and CACs for biogas desulfurization investigated here. (b) Pore volume vs BET area of Pol_15_%_20 µm (circles) and CACs (crosses). (c) H₂S breakthrough curves and separation capacities of CACs and Pol_15_%_20 µm_KOH 10% per (d) unit mass and (e) unit volume of sorbent. (A colour version of this figure can be viewed online.)</figcaption></figure>

<table><caption>**Table 1**

Parameters used for the simulation of H₂S dynamic adsorption.</caption><thead><tr><th>Gas properties</th><th></th><th>Bed properties</th><th></th></tr></thead><tbody><tr><td>Feed gas flow rate (m³·s⁻¹)</td><td>1.33 × 10⁻⁶</td><td>Length (m)</td><td>0.1</td></tr><tr><td>Gas density (kg·m⁻³)</td><td>1.41</td><td>Diameter (m)</td><td>7.7 × 10⁻³</td></tr><tr><td>Gas viscosity (Pa·s)</td><td>1.66 × 10⁻⁵</td><td>Void fraction</td><td>0.21</td></tr><tr><td>Reynolds number</td><td>8.5</td><td>Activated carbon properties</td><td></td></tr><tr><td>Peclet number</td><td>146.4</td><td>Particle diameter (m)</td><td>7.5 × 10⁻⁴</td></tr><tr><td>D<sub>L</sub> (m²·s⁻¹)</td><td>9.08 × 10⁻⁵</td><td>Bulk density (kg·m⁻³)</td><td>m<sub>ads</sub>/V<sub>bed</sub></td></tr></tbody></table>

M<sub>ads</sub>: Mass of adsorbent.

V<sub>bed</sub>: Bed volume.

testing different H₂S concentrations and calculating the total saturation capacity from the corresponding breakthrough curves for a gas composed of 1.1 % O₂, 4000 ppm H₂S, 60 % N₂ and the rest CO₂, as shown in Fig. 9a. The sample used was Pol_15_%_KOH10. Langmuir equilibrium constants were determined by fitting the experimental results for adsorption capacity (circles in Fig. 9b) by Eq. (S4). The calculated equilibrium constants were q<sub>s</sub> = 7.1 mol kg⁻¹ (245.4 mg g⁻¹) and b = 38.1 m³ mol⁻¹ with a coefficient of determination (R²) of 0.94.

The Langmuir model fitted the experimental data well but overestimated adsorption, predicting a q<sub>s</sub> of 245.4 mg g⁻¹ against an experimental value of 203.1 mg g⁻¹ at 4000 ppm. This discrepancy stems from the model's idealized assumptions, which do not fully reflect the complexities of the real system. Thus, while useful for isotherm shape, it may not accurately predict the maximum capacity. The isotherm used is not a pure-component isotherm, as the gas mixture contains CO₂ and O₂. CO₂ causes competitive adsorption by occupying sites available for H₂S, while O₂ promotes its oxidation, thereby improving its removal. These effects are taken into account in the measured adsorption behavior and separation capacities, making the isotherm suitable for modeling real biogas mixtures. In contrast, pure-component isotherms underestimate performance and would require additional equations in the mathematical model to account for oxidation and chemisorption reactions.

The simulation (dashed lines) presented in Fig. 9a closely follows the

experimental data (dots), showing good agreement. A slight deviation is observed at the 4000 ppm concentration; however, the predicted value still follows the general trend. The mass transfer coefficient *k* or LDF coefficient (see Table 2), was determined by fitting the experimental data to the simulation results shown in Fig. 9a. *K* tends to stabilize (0.0002 s⁻¹) as soon as the concentration exceeds 400 ppm, meaning that the resistance to mass transfer becomes similar. This is confirmed by the comparable slopes observed in the adsorption curves above 1000 ppm H₂S. To fit the experimental data with the simulation at 400 ppm, a higher *k* was required (0.0004 s⁻¹).

The coefficient *k* is inversely related to breakthrough time, with a longer breakthrough time corresponding to a lower *k* value. In this experiment, including densified AC and O₂ in the feed gas, high separation capacities were obtained, with a q<sub>s</sub> (Langmuir coefficient) of 245.4 mg g⁻¹ and a k of 0.0002 s⁻¹. On the other hand, other studies on H₂S adsorption simulation reported lower capacities and higher *k* values: q<sub>s</sub> = 11.9 mg g⁻¹ (k = 0.008 s⁻¹) [21], q<sub>s</sub> = 15.0 mg g⁻¹ (k = 0.0019 s⁻¹) [22], and q<sub>s</sub> = 5.9 mg g⁻¹ (k = 0.128 s⁻¹) [23]. These results demonstrate the importance of simulating realistic biogas conditions.

The H₂S concentration distribution in the adsorption column at different H₂S inlet concentration is shown in Fig. 9c. At the lowest concentration (400 ppm), the adsorption front moves slower through the bed due to the lower driving force for mass transfer. In contrast, at higher concentrations, particularly at 2000 and 4000 ppm, the front advances more rapidly and breakthrough occurs sooner. In addition, the mass transfer zone (MTZ) becomes broader at higher H₂S concentrations. Ideally, a lower inlet H₂S concentration would enhance the efficiency of the separation process by extending the lifetime of the adsorbent. However, the actual H₂S concentration is dictated by the composition of the biogas, which depends on its production conditions.

## 5. Regeneration

Table 3 summarizes the regeneration methods investigated, after using a gas mixture of 1.1 % O₂, 4000 ppm H₂S, 60 % N₂, and ~39 % CO₂ for saturating the sorbents. The spent sorbents evaluated were
<figure><img src="image_14.png" /><figcaption>**Fig. 9.** (a) Experimental (dots) and simulated (dashed lines) breakthrough curves at different H₂S concentrations. (b) H₂S equilibrium adsorption isotherms at 25 °C based on experimental breakthrough curves (circles), along with Langmuir fit (dashed lines). (c) Breakthrough curves and H₂S concentration profiles from simulations. (A colour version of this figure can be viewed online.)</figcaption></figure>

<caption>**Table 2**</caption>

Mass transfer coefficients estimated by fitting experimental and simulation data at different H₂S feed concentrations.

<table><thead><tr><th>H₂S concentration (ppm)</th><th>400</th><th>1000</th><th>2000</th><th>3000</th><th>4000</th></tr></thead><tbody><tr><td>Mass transfer coefficient (s⁻¹)</td><td>0.0004</td><td>0.0002</td><td>0.0002</td><td>0.0002</td><td>0.0002</td></tr></tbody></table>

densified impregnated ACs: Pol_15_%_KOH 10 and Can_50_%_KOH 10. Unlike previous studies that ignore CO₂ [1], this work includes CO₂ levels typical of real biogas, requiring its desorption during regeneration and adding complexity compared to systems with only H₂S and inert gases [1]. Fig. 10 shows the breakthrough curves and separation capacities of the regenerated sorbents.

Soxhlet washing, air at 150 °C and N₂ at 300 °C failed to regenerate Pol_15_%_KOH 10, as its adsorption capacity lost, indicating that the sorbent remained saturated (Fig. 10a). Some studies report successful regeneration with N₂ at 120–200 °C using simplified gas streams (H₂S in N₂). For example, a UiO-66/graphene oxide composite retained 87 % of its capacity after three cycles [24], and a N-doped porous carbon maintained 20.1 mg g⁻¹ of its initial capacity of 27.6 mg g⁻¹ after four

regeneration cycles [25]. In contrast, our sorbent operates under real biogas conditions and achieves >175 mg g⁻¹, demonstrating greater complexity.

Thermal regeneration at 500 °C with N₂ improved the results. Fig. 10b–e shows breakthrough curves and separation capacities for Pol_15_%_KOH 10 and Can_50_%_KOH 10. The impregnating agent probably lost its catalytic activity by reacting into pore-blocking compounds, such as potassium sulfate, with local accumulation. Additionally, H₂S removal on KOH-impregnated ACs in the presence of O₂ generates strongly bound sulfur species (e.g., S₈, SO₂, HS⁻, polysulfides), as well as reaction products between KOH and H₂S such as K₂S, K₂SO₄, KHS and KHCO₃ (the latter formed in the presence of CO₂) [26]. These compounds are difficult to desorb or even decompose, further complicating regeneration. To investigate whether the loss of performance was due to KOH depletion or degradation during the 1st adsorption test, a second impregnation step with 10 % KOH was applied after heat treatment at 500 °C (500 °C_KOH 10). The amount of KOH was calculated based on the AC mass after regeneration, without confirming how much active KOH remained. As a result, the actual KOH content may have exceeded 10 wt%.

Fig. 10b and c shows breakthrough curves for Pol_15_%_KOH 10 and
<table><caption>**Table 3**
Regeneration methods and H₂S separation capacities of densified, impregnated ACs: Pol_15_%_KOH 10 and Can_50_%_KOH 10.</caption><thead><tr><th>Sample</th><th>Regeneration method</th><th>Cycle</th><th>H₂S separated at BTP C/C₀ 1 % (mg·g⁻¹)</th></tr></thead><tbody><tr><td rowspan="10">Pol_15 %<br>_KOH 10</td><td>Fresh</td><td>—</td><td>178</td></tr><tr><td>Soxhlet washing during 24 h</td><td>1</td><td>≈0</td></tr><tr><td>Air flow (100 mL min⁻¹) at 150 °C during 2 h, ramp temperature 5 °C·min⁻¹</td><td>1</td><td>≈0</td></tr><tr><td>N₂ flow (100 mL min⁻¹) at 300 °C during 2 h, ramp temperature 5 °C·min⁻¹</td><td>1</td><td>≈0</td></tr><tr><td>N₂ flow (100 mL min⁻¹) at 500 °C during 2 h, ramp temperature 5 °C·min⁻¹</td><td>1</td><td>13</td></tr><tr><td>N₂ flow (100 mL min⁻¹) at 500 °C during 2 h, ramp temperature 5 °C·min⁻¹, followed by impregnation with KOH 10 wt% – Cycle 1</td><td>2</td><td>31</td></tr><tr><td>N₂ flow (100 mL min⁻¹) at 500 °C during 2 h, ramp temperature 5 °C·min⁻¹, followed by impregnation with KOH 10 wt% – Cycle 1</td><td>1</td><td>71</td></tr><tr><td>N₂ flow (100 mL min⁻¹) at 500 °C during 2 h, ramp temperature 5 °C·min⁻¹, followed by impregnation with KOH 10 wt% – Cycle 1</td><td>2</td><td>79</td></tr><tr><td>Can_50 %<br>_KOH 10</td><td>Fresh</td><td>—</td><td>181</td></tr><tr><td></td><td>N₂ flow (100 mL min⁻¹) at 500 °C during 2 h, ramp temperature 5 °C·min⁻¹ – Cycle 1</td><td>1</td><td>62</td></tr><tr><td></td><td>N₂ flow (100 mL min⁻¹) at 500 °C during 2 h, ramp temperature 5 °C·min⁻¹, followed by impregnation with KOH 10 wt% – Cycle 1</td><td>2</td><td>58</td></tr><tr><td></td><td>N₂ flow (100 mL min⁻¹) at 500 °C during 2 h, ramp temperature 5 °C·min⁻¹, followed by impregnation with KOH 10 wt% – Cycle 1</td><td>1</td><td>104</td></tr><tr><td></td><td>N₂ flow (100 mL min⁻¹) at 500 °C during 2 h, ramp temperature 5 °C·min⁻¹, followed by impregnation with KOH 10 wt% – Cycle 1</td><td>2</td><td>75</td></tr></tbody></table>

Can_50_%_KOH 10, respectively, after regeneration at 500 °C for the first (500C_1) and second cycle (500C_2), as well as after regeneration followed by re-impregnation with 10 wt% KOH for the first (500C_KOH10_1) and second cycle (500C_KOH10_2). For both samples, re-impregnation improved performance, but it remained difficult to achieve the same adsorption capacity as the fresh, pristine sample.

Iodine number tests (ASTM D4607-94) were conducted on fresh and thermally regenerated samples at 500 °C. The iodine number of Pol_15

%_KOH 10 decreased from 1138 to 1022 mg·g⁻¹, and Can_50_%_KOH 10 from 1020 to 964 mg·g⁻¹. Although partial H₂S adsorption capacity was recovered after re-impregnation, the initial iodine number was not fully restored. Since the iodine number reflects micropore volume, this decline suggests irreversible pore blockage or structural changes.

Despite the challenges associated with regenerating impregnated ACs, they are the preferred choice for industrial applications due to their high performance compared with non-impregnated ACs. In industry, saturated AC columns are replaced by fresh ones rather than being re-generated in situ.

## 6. Conclusions

Wood-based activated charcoal was densified with eco-friendly binders and chemically impregnated for efficient H₂S removal from biogas. Despite reduced microporosity, densification improved performance through higher bulk density (263 → 531 kg·m⁻³). Among the impregnating agents evaluated, KOH impregnation at 10 wt% gave the best results. Cane molasses and polyphenols binders performed similarly when impregnated with AC grains of less than 1 mm. However, sieving AC to 20–40 µm and using polyphenols produced denser and more efficient sorbents. The optimal preparation sequence was densification → carbonization → chemical impregnation, giving performance (H₂S separation capacity: 181 mg·g⁻¹ and 96 mg·cm⁻³) comparable to or exceeding most commercial ACs, while offering a sustainable, wood-based alternative.

A mass transfer model accurately described adsorption dynamics in columns using densified AC with O₂ containing feeds. Under these conditions, much lower mass transfer coefficients and longer breakthrough times were obtained, resulting in markedly higher separation capacities than those reported in the literature. Regeneration of impregnated AC pellets at 500 °C under N₂ restored approximately 50% of the initial performance, indicating that further optimization of regeneration strategies is required.

<figure><img src="image_15.png" /><figcaption>**Fig. 10.** (a) Breakthrough curves of sample Pol_15_%_KOH 10 before (fresh) and after regeneration by Soxhlet washing or thermal regeneration with either air (150 °C) and or N₂ (300 °C). Breakthrough curves of sample (b) Pol_15_%_KOH 10 and (c) Can_50_%_KOH 10 after thermal regeneration with N₂ at 500 °C and thermal regeneration with impregnation with KOH 10 wt%. Separation capacities at BTP C/Co 1 % corresponding to the regeneration methods of (d) Fig. 10b, and (e) Fig. 10c. (A colour version of this figure can be viewed online.)</figcaption></figure>
CRediT authorship contribution statement

**Luis Paz:** Writing – original draft, Visualization, Methodology, Investigation. **Taher Selmi:** Writing – review & editing, Validation, Project administration, Methodology, Funding acquisition. **Vanessa Fierro:** Writing – review & editing, Supervision, Methodology, Funding acquisition, Formal analysis, Conceptualization. **Alain Celzard:** Writing – review & editing, Validation, Supervision, Project administration, Methodology, Funding acquisition, Conceptualization.

Funding

This research was funded by the French National Research Agency (ANR) through the "CarBioLab" project (ANR-21-LCV3-0001), as part of the LabCom "Joint Laboratories" program 2021 edition, and the "Catalpa" project NR-22-PESP-0007 (France 2030).

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

The authors thank Dr. Jimena Castro Gutierrez and Mr. Philippe Gadonneix for their invaluable help in the laboratory.

Appendix A. Supplementary data

Supplementary data to this article can be found online at. https://doi.org/10.1016/j.carbon.2025.120963.

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