Valorization of spent coffee grounds using potassium hydroxide and concentrated leachate as activating agents in slow pyrolysis at 600°C: A comparative analysis of char properties

ABSTRACT

The coffee industry and landfill leachate treatment produce waste: spent coffee grounds (SCG) and landfill leachate membrane concentrate (LLMC). Current disposal methods, such as open burning, incineration, and landfilling, waste resources and hinder sustainability. Due to their high pollution potential, cost-effective management solutions are urgently needed. This research explores the slow pyrolysis of SCG using potassium hydroxide (KOH) and LLMC as activating agents (1:1 weight ratio). LLMC’s high alkali and alkaline earth metal content may enhance char activation and improve char quality. Using LLMC as an activating agent offers an alternative to value SCG and landfill waste, potentially replacing industrial-scale activators like steam, CO₂, and chemical additives. SCG had a low specific surface area (4.5 m² g^-1), contrasting with the notable surface areas observed in both activated chars. In particular, the KOH-activated char exhibited a higher surface area than the LLMC-activated char, measuring 1.960 m² g^-1 compared to 1.138 m² g^-1 - a difference of about 72%. On the other hand, the combustion enthalpy of the LLMC-activated material was estimated at 22.04 MJ kg^-1. The combustion enthalpy of LLMC-activated char was about 21.7% and 19.8% higher than that of SCG and KOH-activated chars, which had values of 18.11 and 18.40 MJ kg^-1, respectively. This work showed that both activated chars had superior energetic and morphological properties compared to non-activated chars made from SCG biomass. Among the activating agents, KOH led to better performance in terms of char yield and morphological properties. Meanwhile, utilizing LLMC as an activating agent highlights its potential for converting landfill waste into high-value material.

Keywords:
biomass residues; landfill leachate; membrane concentrate; thermogravimetric analysis; waste-derived char

INTRODUCTION

Landfill leachate treatment plants have employed nanofiltration and reverse osmosis to remove contaminants, complementing or replacing conventional methods. However, managing the landfill leachate membrane concentrate (LLMC) challenges operators (Chen et al., 2021; Li et al., 2023a). LLMC is a high-salinity residual stream containing recalcitrant organic contaminants like lignin-like substances, unsaturated hydrocarbons, and humic substances (conductivity of 16,130-98,000 μS cm^-1; 890-15,400 mg L^-1 sodium; 210-9,600 mg L^-1 potassium; 719-4,500 mg L^-1 total organic carbon; 1,393-1,501 mg L^-1 humic substances), making it highly polluting and complex (Almeida et al., 2023b). When used to handle LLMC, thermal processes like evaporation generate a mother liquor that requires further management (Fang et al., 2021). Likewise, evaporation ponds, used in warm regions to reduce LLMC volume, produce secondary pollutants such as air pollution and sludge (Keyikoglu et al., 2021). These residues, which consist of inorganic salts like K, Na, Mg, and Ca, pose environmental risks and contribute to equipment deterioration. At the same time, the disposal of secondary pollutants, such as sludge, into landfills exacerbates contamination, highlighting the need for more effective treatment and disposal methods (Wang et al., 2024).

Likewise, one of the emerging waste challenges is handling spent coffee ground (SCG) residues. SCG is a solid waste by-product from the coffee processing industry - the second-largest traded commodity after petroleum (Johnson et al., 2022). About 10 million tons of coffee beans were produced in 2024, with Brazil, Vietnam, and Colombia being the major producers. In Brazil, 30.5% of the coffee beans produced are directed to domestic consumers (approximately 1 million tons), while 2.28 million tons (69.5%) are exported worldwide (Brasil, 2024).

It is estimated that only about 30% of the mass of coffee beans is extracted into brewed coffee, with a large portion remaining as solid waste in the form of SCG. Individuals, coffee shops, and food services also generate it. Approximately six million tons of SCG are generated annually worldwide (Johnson et al., 2022; Wachter et al., 2022). If disposed of in the environment, SCG is a source of contamination because it contains caffeine, tannins, and polyphenols, making it a toxic residue (Mata et al., 2018). The SCG waste is typically landfilled, incinerated, burned with other coffee residues, or mixed into animal feed (Mussatto et al., 2011). Open burning is an unsustainable destination while landfilling and incineration release greenhouse gases (Saxena et al., 2024). Current SCG management practices contribute to a significant carbon footprint and raise sustainability concerns. Therefore, cost-effective and green management solutions are urgently needed.

Pyrolysis has become a promising solution for managing and converting various waste streams, such as sewage sludge, crop and agro-industrial residues, and industrial and municipal solid waste into valuable resources (Manyà et al., 2018; Li et al., 2023b; Manikandan et al., 2023). Pyrolysis is the thermochemical decomposition of carbon-based biomass under deficient concentration or absence of oxygen at a temperature higher than 400°C and atmospheric pressure. This process yields a solid carbon-rich substance known as biochar, volatile organic compounds that can be condensed into a liquid form (bio-oil), and various non-condensable gases (such as CO, CO₂, CH₄, and H₂) collectively referred to as syngas (Conte et al., 2021). Pyrolysis technology is classified into fast, intermediate, and slow types based on heating rate, peak temperature, and residence time. Slow pyrolysis, also known as carbonization, is the most widely used method for producing char due to its ability to achieve the highest recovery of carbon-based material (Manyà, 2012; Bertero; Sedran, 2015).

Char properties are affected by biomass characteristics and the activating method. The latter changes char porosity, surface area, and thermal stability (Qian et al., 2022). Conventionally, activated chars are produced using chemical and physical activation methods. An external activation agent, such as ZnCl₂, H₃PO₄, KOH, HNO₃, and NaOH is employed in chemical methods. In contrast, continuous CO₂ or H₂O gas incorporation into the pyrolysis reactor is performed in physical processes. As a disadvantage of these techniques, washing and neutralizing steps generate wastewater with toxic chemicals from the activation step, increasing costs for pollution control and equipment maintenance (Gurav et al., 2023). Thus, finding novel approaches to activate the carbon-based material produced in pyrolysis is imperative.

It is known that inorganic compounds, such as alkali and alkaline earth metals (mainly Na, Mg, and Ca), can activate carbon oxidation, enhancing the morphological properties of the resulting solid material produced by pyrolysis (Wang et al., 2006; Safar et al., 2019; Rosson et al., 2020; Wang et al., 2022; Wongmat; Wagner, 2022). However, the potential use of residual streams for this purpose has not been fully explored. Due to high Na and K content in LLMC residue, this landfill wastewater is a potential candidate for activating biomass conversion in pyrolysis. This approach is an innovative strategy to promote the valorization of LLMC and different kinds of biomass, including solid waste SCG.

Using leachate waste streams in pyrolytic processes is promising, but the literature on this topic is scarce. Therefore, our study contributes to this area of research by exploring the utilization of the LLMC residue in the pyrolysis of SCG. The concentrated leachate residue could be used as an activating agent to improve the quality of the char produced in the pyrolytic conversion due to the parallel biomass carbonization and oxidation of the resulting carbon structure.

In that direction, this research comparatively analyzes the slow pyrolysis of SCG using KOH and LLMC residue as activating agents. KOH is one of the most effective activating agents for preparing activated carbon with a high specific surface area (Gao et al., 2020). Therefore, KOH was used as the reference activating agent for comparative analysis. Morphological and thermal characterization were performed to discuss environmental benefits and potential applications. The experimental study addressed here represents the pioneering investigations into using LLMC as an activating agent in pyrolysis. This research extends and complements our previous findings (Almeida et al., 2023a), shedding light on possible alternatives and widening the spectrum for solid waste valorization.

MATERIALS AND METHODS

Biomass and activating agents

The LLMC was prepared following the procedure proposed by (Grossule et al., 2022). The sample was subsequently oven-dried at 105°C for 24 h.

A local Italian company supplied the SCG (100% Arabica blend). The sample was sieved using a 0.15 mm pore diameter (100 mesh) sieve to remove impurities, stirred to obtain a homogenous sample, oven-dried overnight at 110°C, and stored in glass bottles for pyrolysis tests. The LLMC sample was powdered and used in its current form.

The KOH powder (≥ 85%) was sourced from Sigma-Aldrich (Stainheim, Germany). KOH is commonly used as an activating agent to develop porosity during thermal processes (Rosson et al., 2020). This work employed KOH as the reference activating agent for comparative analysis. LLMC and KOH were not sieved, but a visual analysis revealed a particle diameter not exceeding that of the SCG sample used in the experiment.

After pyrolysis, the obtained chars were filtered through a Gooch 4 filter and washed with deionized water until the pH reached neutral. The chars were then oven-dried at 110°C for 24 hours.

Pyrolysis experimental set-up

Slow pyrolysis experiments were conducted using a lab-scale pyrolyzer at a heating rate of 4.5°C min^-1. The pyrolyzer is an automated tubular alumina reactor (Carbolite) connected to a nitrogen cylinder. The bench-scale reactor was positioned within an exhaust system operating at atmospheric pressure and ambient temperature (± 20°C) (Rosson et al., 2020). Pyrolysis conditions included an isothermal temperature of 600°C, an inert gas flow of 100 cm³ N₂ min^-1, and a residence time of 1 h. The experimental conditions were established based on preliminary pyrolysis tests and a thorough review of the relevant literature (Manyà, 2012; Bertero; Sedran, 2015; Almeida, 2022). The peak temperature is a critical factor in slow pyrolysis, significantly influencing char characteristics. Elevating the peak temperature appears to result in carbon materials with increased aromaticity, fixed carbon (FC) content, and porosity (Manyà, 2012). At the same time, at temperatures above 800°C, the quantity of carbon left on char is minimal, as observed in our preliminary findings (Almeida, 2022). Below 500°C, biomass pyrolysis may produce biochar with low structural stability (Bertero; Sedran, 2015).

The SCG and activating agents (i.e., KOH and LLMC residue) were pyrolyzed in a 1:1 ratio by weight. In all pyrolysis runs, an amount of 1,000 mg of SCG was placed inside the reactor. The mass ratio of the activating agent to the carbon precursor is critical in the activation process. An equal mass ratio was adopted because our previous study confirmed that the chemical activation of SCG with KOH produced a carbon material with a high surface area (up to 1199 m² g-¹) (Rosson et al., 2020). The present study uses KOH as the reference activating agent for comparative analysis; therefore, the SCG and LLMC mass ratio follows the same proportion.

The chars were washed with deionized water on a paper filter at a neutral pH and left to dry at ambient temperature (± 20°C). The activated-carbon yield was determined as a mass fraction of the initial biomass (Equation 1). The activated-carbon materials were stored for morphological and thermal characterization.

$Yield\left(\%\right)=(m_i/m_0)\times 100\%$ (1)

Where:

• m₀ (in mass unit):   the initial mass of SCG/ activating agent;
• m_i (in mass unit):   the mass of activated carbon produced in slow pyrolysis.

Characterization of biomass and the activated materials

Moisture content (MC), volatile matter (VM), ash content, and FC were analyzed. MC was measured by heating a 1,000 mg sample at 105 ± 5°C for one hour in an oven. VM was determined by heating the remaining residue at 950°C for 6 minutes. Ash content was obtained by placing the samples in a furnace at 750°C for 6 hours. FC was calculated by difference according to ASTM method D1762 - 84/2021 (ASTM International, 2021).

Scanning electron microscopy with energy-dispersive X-ray spectroscopy (SEM/EDS) of biomass and prepared activated-carbon materials was performed with a FEI-QUANTA200 instrument (Milan, Italy). Proximate and ultimate analyses of LLMC, SCG, and activated chars were all obtained under the same conditions.

In the present work, energy-dispersive X-ray spectroscopy (EDS) was applied as a rapid and non-destructive screening method. EDS aims to provide compositional insights, particularly at the microscale, relevant to the surface features examined here; EDS results may not fully reflect the bulk elemental composition of the analyzed materials.

The Brunauer-Emmett-Teller (BET) surface area of SCG and prepared chars was calculated based on the N₂ adsorption-desorption isotherms measured using a gas sorption analyzer (ASAP 2010, Micromeritics, USA). The pore size distribution was derived from BET isotherms using the Density Functional Theory (DFT) method (Bardestani et al., 2019).

Thermogravimetric (TG) analysis and decomposition profiles were performed using TA Instruments equipment, model SDTQ600. The samples were weighed to around 5 mg. After that, they were heated from 20 to 1,000°C (in an alumina pan) at 20°C min^-1 under an air gas flow rate of 100 mL min^-1. Figure 1 illustrates a schematic diagram of the research steps.

Figure 1 -
Schematic diagram of the research steps conducted in this study.

RESULTS AND DISCUSSION

Char yield, proximate analysis, and elemental composition

The proximate and ultimate analyses of the LLMC residue are detailed in Table 1.

Table 1 -
Proximate and ultimate analyses of the LLMC residue.

	Proximate analysis (wt%)				*Ultimate analysis (dry basis, wt%)
	MC	VM	Ashes	FC	C	O	Na	K	Mg	Cl	S
LLMC residue	2.46	23.55	51.23	22.76	13.32 ± 2.23	33.99 ± 4.33	39.28 ± 1.99	8.09 ± 0.78	0.26 ± 0.08	3.83 ± 0.66	0.75 ± 0.44

• FC: fixed carbon; LLMC: landfill leachate membrane concentrate; MC: moisture content; VM: volatile matter; *Mean ± standard deviation of three determinations.
• Source: Elaborated by the authors.

The dried LLMC residue had a water content of 2.46% by weight and a volatile solids-to-total solids (VS/TS) ratio of 51%. The content of C, Na, and K was 13.32 ± 2.23, 39.28 ± 1.99, and 8.09 ± 0.78 wt%, respectively. The LLMC solid was powdered and used in co-pyrolysis tests with SCG.

Table 2 shows the char yield and proximate content (wt%) for SCG, biochar, and the activated chars produced.

Table 2 -
Yield and proximate analysis.

	SCG	Biochar	KOH-activated char	LLMC-activated char
Mass yield (wt%)	─	23.9	21.2	18.6
MC (wt%)	3.78	9.2	5.8	23.3
VM (wt%)	94.91	58.4	47.9	44.4
Ashes (wt%)	1.26	5.8	1.3	7.0
FC (wt%)	1.25	33.0	25.6	20.4

• FC: fixed carbon; LLMC: landfill leachate membrane concentrate; MC: moisture content; SCG: spent coffee ground; VM: volatile matter.
• Source: Elaborated by the authors.

The biochar yields produced from SCG and activated chars were 23.9, 21.2, and 18.6%, respectively. A low mass yield of activated chars is obtained because of the high release of VM catalyzed by oxidizing agents in slow pyrolysis (i.e., KOH and concentrated leachate residue). Alkali and/or alkaline earth metals shift the decomposition of biomass to lower temperatures while increasing char and gas yields at the expense of bio-oil. In particular, potassium additives have been shown to promote the yields of low-molecular compounds and gaseous species, which could justify the lower yield of KOH-activated char compared to SCG char in this study (Wang et al., 2022). For example, Wang et al. (2006) showed that potassium and sodium compounds promoted the reduction of char formation and made pyrolysis more exothermic.

Besides, the LLMC-activated char had a relatively lower mass yield than that obtained from KOH activation (18.6 vs. 21.2%). Alkali metals have been shown to induce a so-called synergistic effect that decreases the apparent activation energy of the pyrolysis reaction, promotes the yield of volatiles, and reduces the temperature of the maximum weight loss rate (Wang et al., 2022). The lower yield when concentrated leachate residue was employed could be due to the synergistic interactions between different LLMC metals (mainly Na and K) and the SCG (Rijo et al., 2023). Optimizing the mass ratio of SCG and LLMC could be a promising strategy to enhance char yields, as the interaction between these materials influences the activation process. By adjusting this ratio, it may be possible to maximize char yield while minimizing the loss of volatiles during the activation reaction.

Briefly, KOH catalyzes the decomposition of SCG volatile components, driving the reaction toward char formation. Meanwhile, the mixture of metals from LLMC enhances the carbonization of SCG by interacting with the biomass, increasing the thermal decomposition rate. These metals are supposed to facilitate oxidation reactions, which could lead to a faster loss of volatiles compared to KOH activation. They promote speedier carbonization but also result in lower char yield because the reaction shifts more toward volatile product formation. Studies have explored the kinetics of pyrolysis reactions in the presence of various activating agents (Montané et al., 2005; Tibola et al., 2020; Illingworth et al., 2022). Modeling pyrolysis kinetics and the activation process using computational methods could further explain the underlying interaction mechanisms between SCG and KOH or LLMC.

The proximate content of the SCG biomass is primarily dominated by VM (94.91 wt%), moisture (3.78 wt%), ashes (1.26 wt%), and fixed carbon (1.25 wt%). VM was significantly reduced to 44-47 wt% for both activated chars, producing materials with a high fixed carbon content (20-33 wt%). High fixed carbon, closely related to stable carbon content, represents a benefit, with higher stability against environmental oxidation and thermal degradation (Yek et al., 2021). On the other hand, the high ash content of LLMC-activated char indicates that it may not be appropriate for cofiring or use as boiler fuel, as that may lead to fouling and corrosion in combustors (Huang et al., 2019).

SEM/EDS was employed to observe the morphology of SCG, concentrated leachate residue, and prepared materials. EDS spectra were used to determine the composition of biomass and chars at the microscale (Table 3).

Table 3 -
Ultimate analysis of biomass and chars from EDS spectra.

Material	Elemental composition (dry basis, wt%)
	C	O	Na	K	Mg	Ca	Al	Cl	S
SCG	47.73	47.49	n.d	3.19	n.d	0.91	n.d	n.d	0.68
LLMC residue	14.55	37.04	40.86	2.26	n.d	n.d	0.78	2.63	1.55
SCG char	46.06	45.26	n.d	3.87	0.40	n.d	n.d	n.d	0.52
KOH-activated char	85.30	13.70	n.d	0.87	0.02	n.d	n.d	n.d	0.10
LLMC-activated char	63.97	20.16	4.32	4.14	1.00	2.87	n.d	0.47	1.43

• n.d.: not detected; KOH: potassium hydroxide; LLMC: landfill leachate membrane concentrate; SCG: spent coffee ground.
• Source: Elaborated by the authors.

The biochar obtained from SCG presented 46% wt% of carbon, and both activated chars were highly C-rich. The results showed similar carbon content in activated chars (> 60 wt%). The final elemental composition of the LLMC-activated sample was the following (wt%): carbon (C) content of 63.97%, oxygen (O) content of 20.16%, sulfur (S) content of 1.43%, chloride (Cl) content of 0.47%, sodium content of 4.32%, potassium (K) content of 4.14%, calcium (Ca) content of 2.87%, and magnesium (Mg) content of 1%. Foreign elements were observed in the three prepared materials (i.e., Na, K, or S, < 5% wt%).

It should be noted that EDS results may not fully reflect the bulk elemental composition of the materials analyzed and, therefore, these results should be interpreted with caution. More robust techniques such as X-ray fluorescence (XRF), inductively coupled plasma optical emission spectrometry (ICP-OES), or atomic absorption spectrometry (AAS) are recommended for precise bulk quantification.

Despite the samples being washed with deionized water under neutral conditions, the residual salts from the LLMC activator may have persisted in the structure of the activated carbon. Due to the high salt content of the LLMC residue, primarily Na and K, the washing process may not have been thorough enough to remove all the residual salts. The remaining chemical constituents in the carbon network can affect the material’s surface area, chemical reactivity, electrochemical properties, and adsorption capabilities, thus limiting its effectiveness in various applications. Therefore, purification steps (e.g., acid or alkaline washing) may be necessary to remove these residues and enhance the material’s functionality, especially when purity and specific surface properties are critical (Gao et al., 2020; Heidarinejad et al., 2020). In addition, the sulfur content in the LLMC-activated char (1.43 wt%) raises concerns about potential sulfur dioxide (SO₂) emissions during combustion. Likewise, addressing sulfur removal through additional purification steps may be crucial to ensuring the material’s environmental sustainability when used as a fuel or in combustion processes.

Scanning electron microscopy images and porosity

Figure 2 illustrates SEM images of SCG, KOH-activated, and concentrated leachate-activated chars. The SCG char was darker than the precursor. Larger porous structures are observed in the SEM images. In the LLMC-activated char, larger dispersed pores were also observed, and discontinuous structures impregnated with inorganic elements were seen. This suggests a unique structural arrangement compared to the other char samples.

Figure 2 -
Scanning electron microscopy images of SCG char and activated chars.

Table 4 shows the BET surface area and average pore diameter of SCG and prepared chars at isothermal pyrolysis (600°C) for 1 h.

Table 4 -
BET surface area and average pore diameter of SCG and prepared chars.

Sample	BET surface area (m² g-1)	Average pore size (nm)	Isotherm type
SCG	4.5	10.7	Type IV (mesoporous, low surface area)
SCG char	463	4.8	Type IV (mesoporous)
KOH-activated char	1960	1.8	Type I (microporous)
LLMC-activated char	1138	5.9	Type IV (mesoporous with micropores)

• BET: Brunauer-Emmett-Teller; LLMC: landfill leachate membrane concentrate; SCG: spent coffee grounds.
• Source: Elaborated by the authors.

The N₂ physisorption isotherms revealed that the SCG had a low BET surface area (4.5 m² g^-1). On the other hand, both activated chars exhibited high surface areas. In particular, the KOH-activated char displayed a higher specific surface area than the LLMC-activated char (1960 vs. 1,138 m² g^-1). KOH is one of the most effective activating agents for preparing activated carbon with a high specific surface area. The BET surface areas for SCG and KOH-activated chars in this study were consistent with values reported in the literature (4.3 to 2,230 m² g^-1) (Ballesteros et al., 2014; Alcaraz et al., 2019).

The mechanism of KOH activation involves converting KOH into potassium oxide (K₂O) at 400°C and then completely converting it into potassium carbonate (K₂CO₃) at 600°C (Equations 2 and 3) (Gao et al., 2020). K₂CO₃ is also produced during the activation process (Equation 4). Afterward, metallic potassium may be generated by K₂CO₃ or K₂O reduction (Equations 5 and 6).

$2KOHàK_{2}O+H_{2}O$ (2)

$K_{2}O+C{O}_2àK_{2}C{O}_3$ (3)

$4KOH+CàK_{2}C{O}_3+K_{2}O+2H_2$ (4)

$K_{2}O+Cà2K+CO$ (5)

$K_{2}C{O}_3+Cà2K+3CO$ (6)

The interaction between SCG and KOH occurs due to the strong ionic and electrostatic forces influencing the carbon surface structure (Heidarinejad et al., 2020). As a result of intermolecular force compensation, the energy of molecules inside is lower than that of surface molecules (Huang et al., 2022; 2024). Potassium ions from KOH can break the π-bonds in the aromatic layers of SCG, further activating the carbon surface and increasing the available reaction sites. It penetrates the internal structure of the carbon lattice to create new pores and expand the char surface area (Bhat et al., 2023). Additionally, the enhanced wettability and reduced surface tension facilitate better penetration of KOH into the carbon matrix, thereby promoting the overall reactivity of the system (Gao et al., 2020). Besides, the activation promoted by the LLMC residue could be attributed to its high sodium content (approximately 40 wt%), in addition to the potassium content of the LLMC residue (approximately 8 wt%) (similar to the KOH activation pathway). Upon heating, sodium salts are thought to decompose and transform into oxides, such as Na₂O and Na₂CO₃, in the presence of CO₂ released from SCG devolatilization (Gao et al., 2020). Sodium carbonate is reactive toward carbon and may be reduced at elevated temperatures, generating metallic sodium together with CO gas (Equation 7). The intercalation of metallic alkali into the carbon lattice disrupts the graphitic structure, enlarges interlayer spacing, and contributes to the formation of new pores (Wang et al., 2006; Gao et al., 2020).

${Na}_{2}C{O}_3+2Cà2Na+3CO$ (7)

While the pore structure and surface area achieved with the waste-derived alkali mixture are lower than those obtained with pure KOH - likely due to the lower potassium concentration, heterogeneous salt distribution, and the presence of low reactivity elements bound in oxides, such as Mg and Al, the resulting textural properties are still relevant.

Notably, the activation performance of LLMC demonstrates that an otherwise problematic leachate residue can act as a waste-derived activating agent, providing both porosity development and environmental benefits.

Still, due to the complex nature of the LLMC residue and its diverse alkali content, the LLMC activation pathways cannot be fully elucidated in this work; therefore, the waste-derived alkali mixture activation mechanisms should be further investigated to clarify the formation of alkali oxides/carbonates, their catalytic role in SCG pyrolysis, gasification reactions, and other related processes.

On the other hand, the higher ash content of LLMC-char might justify why the surface area was smaller than that of KOH-activated char (7.0 vs. 1.3 %wt). The inorganic matter formed by the ash component occupies the material’s pore structure, reducing the surface area (Thue et al., 2022). Therefore, the subsequent washing step is essential to remove remaining chemical constituents from the carbon network, improving the porous structure and material surface area (Gao et al., 2020).

Wang et al. (2006) suggested that sodium compounds can act catalytically during pyrolysis by breaking intermolecular hydrogen bonds in biomass, thereby facilitating earlier devolatilization and influencing the weight loss rate. This effect could help explain the lower char yield of SCG activated with LLMC compared to pure KOH. In addition, part of the inorganic sodium may react with ash components to form silicates or aluminosilicates, which can hinder pore development (Tang et al., 2017).

Regarding pore structure, KOH- and LLMC-activated materials showed average pore diameters of 1.8 and 5.9 nm, respectively. According to the International Union of Pure and Applied Chemistry (IUPAC), KOH-activated char is classified as microporous. In contrast, LLMC-activated char is a mesoporous material (Rouquerol et al., 1994). N₂ absorption/desorption analysis of KOH- and LLMC-activated chars is shown in Figure 3. The KOH-activated material is characterized by microporosity, which is consistent with the steep uptake at low relative pressure (P/P₀) and the slight hysteresis observed. In contrast, LLMC-activated chars exhibit a clear mesoporous contribution. The more noticeable H4 hysteresis and improved uptake at intermediate to high P/P₀ (0.5-0.9) are attributed to the mesopore contribution (Liang et al., 2019). The obtained values agree with the typical type I/IV N₂ adsorption-desorption isotherms anticipated for microporous/mesoporous activated carbons.

Figure 3 -
BET isotherms and DFT pore size distribution. (A, C) N₂ adsorption-desorption isotherms of KOH and LLMC-activated chars. (B, D) Corresponding pore size distribution curves calculated by the DFT method.

Thermal analysis

Figure 4 presents the TG, derivative thermogravimetry (DTG), and differential scanning calorimetry (DSC) profiles for SCG and the resulting biochar. The thermal analysis of SCG indicates an initial loss of free water between 20 and 150°C, followed by the combustion of organic matter around 580°C. The first DTG peak and its corresponding endothermic DSC response at 150°C are associated with the evaporation of free water. Subsequent DTG peaks and exothermic DSC signals observed from 150 to 580°C correspond to the combustion of organic compounds in SCG.

The most significant peaks in the DTG thermal decomposition of SCG include cellulose, lignin, and hemicellulose degradation (Safar et al., 2019). Wachter et al. (2022) also described three thermogravimetric peaks of the SCG in airflow. The first DTG peak was assigned to hemicellulose degradation, the second (lower) peak to cellulose, and the third peak to lignin decomposition. DTG peaks were observed at 337, 451, and 505°C. These values are very similar to those shown in Figure 3A (300, 425, and 520°C).

Figure 4 -
TG, DTG, and DSC curves for (A) SCG and (B) the produced char.

This study utilized the SCG biomass derived from Arabica coffee, and three distinct phases were observed during combustion. The first two peaks have almost the same heat flow, about 12.5 W g^-1, and the last peak has a higher heat flow of about 40 W g^-1. This finding corroborates that of Bejenari et al. (2021), who investigated the combustion of SCG from Arabica and Robusta coffee varieties. The combustion of Arabica coffee occurs in three stages. In addition, similar heat flows were recorded in their study.

Similar behavior was observed for the char produced from that biomass. However, DTG and DSC peaks from 150 to 390°C are absent for SCG char, which can be explained by hemicellulose loss during SCG pyrolysis. Hemicellulose has a temperature decomposition of around 220°C (Johnson et al., 2022). Thus, SCG pyrolysis (600°C) provided the total decomposition of hemicellulose and modified it thermally. In the char, it can be assumed that the first peak is linked to the thermo-oxidation of cellulose and lignin. The last stage was probably the thermal decomposition of carbonaceous residues, that is, stable components with high molecular weight and refractory carbon (Cárdenas-Aguiar et al., 2019). Both the width of the curves and the width of the peaks are higher in the SCG char. It should be highlighted that the char FC content was more than 25-fold higher than that in the SCG (Table 2). Materials with high FC content often exhibit a heterogeneous structure with different types of bonds and functional groups. Thus, the decomposition process may break various chemical constituents over various temperatures, contributing to broader peaks in the DTG curve, as observed in the work of Protásio et al. (2013).

The amount of free water in SCG char was 9.18%. The raw SCG is 3.87%, indicating that the SCG char can absorb more water on its external and internal surfaces due to its higher porosity. By DSC peak areas, the combustion enthalpy of SCG char is 18.11 MJ kg^-1. For SCG, it is 11.55 MJ kg^-1, showing that biochar will release more energy upon combustion. The enhanced SCG char stability and higher surface area than the raw biomass can explain this. Char formation is frequently promoted by intramolecular and intermolecular rearrangement processes, culminating in a material characterized by enhanced thermal stability (Amalina et al., 2022). The high stability results in a more complete combustion and higher energy release during combustion. Besides, the augmented surface area of SCG char (463 vs. 4.5 m² g^-1) enhances combustion kinetics, enabling more effective utilization of its carbon content and releasing more energy (Leng; Huang, 2018).

TG, DTG, and DSC curves of SCG + LLMC residue (1:1) and LLMC-activated char are illustrated in Figure 5. For the LLMC-activated char, water loss occurred at 20 to 160°C. Organics were combusted at 160 to 650°C. The curves showed that hemicellulose degradation shifts toward lower temperatures (c.a. 280 °C). This indicates that the presence of the LLMC residue influences the thermal oxidation of the SCG. Inorganic salts, mainly in the LLMC residue, melt and vaporize above 650°C. For example, MgCl₂, KCl, and NaCl have melting points of around 700, 770, and 800°C, respectively (Haynes, 2014).

Figure 5 -
TG, DTG, and DSC curves for (A) SCG+LLMC residue (1:1) and (B) the LLMC-activated char.

The LLMC-activated char exhibited loss of free water from room temperature up to 150°C, identified by the DTG peak and endothermic DSC peak, followed by the combustion of pyrolyzed organic products between 150 and 580°C, determined by the two DTG peaks and two exothermic DSC peaks. It was observed that the SCG char underwent combustion in a single DTG peak with two overlapping stages, peaking at around 500°C (Figure 3B). In contrast, the LLMC-activated char exhibited combustion in two stages, identified by two DTG peaks, peaking at temperatures of 360°C and 445°C (Figure 4B). It has been underlined that alkali metals, such as Na and K, alter the char reactivity and shift the thermal oxidation of carbon materials to lower temperatures (Wang et al., 2006). These elements on the carbon surface could act as the active sites for oxygen chemisorption, weakening C=C surface bonds and promoting the desorption of monoxide and carbon dioxide (Safar et al., 2019). We assumed that the content of alkali and alkaline metals in the LLMC residue contributed to the results obtained in this work. From thermal analysis, the water content and combustion enthalpy in the LLMC-activated char were estimated at 23.25 wt% and 22.04 MJ kg^-1.

Comparing both activated chars, the DSC peaks of the LLMC-activated char were more substantial, meaning that the thermal oxidation of the LLMC-activated char is more exothermic than the KOH-activated char (Figure 6). Besides, the energy of the KOH-activated char was similar to that of the SCG char (18.40 MJ kg^-1). These results are consistent with the literature (Bejenari et al., 2021; Ben Abdallah et al., 2023). In sum, combustion enthalpies of SCG (raw biomass), SCG char, KOH-, and LLMC-activated chars were 11.55, 18.11, 18.40, and 22.04 MJ kg^-1, respectively.

Figure 6 -
TG, DTG, and DSC curves for KOH-activated char and LLMC-activated char.

Activated carbons with a well-developed microstructure (i.e., balanced porosity and graphitic domains) exhibit high thermal stability and a favorable energy enthalpy. However, a well-developed microstructure may also reduce energy enthalpy by increasing the proportion of oxygenated functional groups and decreasing carbonization efficiency (El-Hendawy et al., 2008; Li et al., 2014). El-Hendawy et al. (2008) confirmed the presence of an open-pore structure and various functionalities on the carbon surfaces of cotton stalk chars prepared with KOH, H₃PO₄, and steam as activating agents. KOH produced a carbon with higher microporosity and the highest specific surface area among the prepared chars (1,307 m² g^-1). In contrast, despite the lower surface area of the H₃PO₄-activated char (841 m² g^-1), it exhibited higher thermal stability. Fourier Transform Infrared (FTIR) analysis confirmed a high amount of oxygen-functional groups on the surface of the KOH-activated carbon, which could contribute to its lower energy enthalpy (El-Hendawy et al., 2008).

Moreover, the material may become less dense as microporosity increases, and the carbon content per unit mass can decrease. Microporous carbons may also release more volatile organic compounds during combustion. These volatiles can reduce the combustion enthalpy of the carbon, as they might not contribute as much energy as the solid carbon structure itself (El-Hendawy et al., 2008; Li et al., 2014; Plavniece et al., 2022). Despite that, all prepared chars had combustion enthalpy values higher than the lowest limit of 16.50 MJ kg^-1 stated in ISO 17225-1:2021, being possible to use them as solid biofuels (ISO, 2021).

CONCLUSIONS

This research focused on the slow pyrolysis of SCG using KOH and LMMC residue as activating agents. The KOH-activated char exhibited a higher surface area than the LLMC-activated char, with a surface area of 1,960 m² g^-1, approximately 72% greater than the 1,138 m² g^-1 surface area of the LLMC-activated char. In addition, the elemental composition of the KOH-activated sample identified the material as C-rich (85.30 wt%). These features could render it invaluable for applications such as pollutant adsorption, catalyst support, and energy storage devices. In contrast, the LLMC inorganic components catalyzed the carbonization of SCG, speeding up thermal decomposition. The combustion enthalpy of the activated char was estimated at 22.04 MJ kg^-1, making it a viable biofuel option. Comparing KOH and LLMC as activating agents, slow pyrolysis was more effective in the presence of KOH. It provided a higher char yield and a material with higher carbon content and surface area. Meanwhile, using LLMC residue as an activating agent could value landfill waste by producing a potential biofuel. Future studies for this research are crucial, including optimizing the pyrolysis process and, most importantly, exploring analytical techniques to understand the char surface chemistry and functional groups. Studies on practical applications for the prepared activated chars and life cycle analysis are also on the horizon.

DATA AVAILABILITY STATEMENT

The data supporting this study’s findings are available from the corresponding author upon reasonable request.

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• Funding:
  FAPERJ - Fundação de Amparo à Pesquisa do Estado do Rio de Janeiro (Grant numbers E-26/200.065/2020; E-26/205.842/2022; E-26/205.843/2022; E-26/204.425/2024) and IILA - Organização Italo Latino Americana (Grant number 40/1621).

Edited by

• Editor:
  Maurício Alves da Motta Sobrinho, Universidade Federal de Pernambuco, Recife, Pernambuco/PE, Brasil. https://orcid.org/0000-0003-2638-9096

Publication Dates

• Publication in this collection
  03 Apr 2026
• Date of issue
  2026

History

• Received
  21 Apr 2025
• Accepted
  07 Nov 2025