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Article

Valorizing Biomass Waste: Hydrothermal Carbonization and Chemical Activation for Activated Carbon Production

by
Fidel Vallejo
1,*,
Diana Yánez
2,
Luis Díaz-Robles
3,
Marcelo Oyaneder
3,
Serguei Alejandro-Martín
4,
Rasa Zalakeviciute
5 and
Tamara Romero
6
1
Industrial Engineering, National University of Chimborazo, Riobamba 060108, Ecuador
2
Grupo de Inocuidad y Valorización de Recursos para la Agroindustria (INVAGRO), National University of Chimborazo, Riobamba 060108, Ecuador
3
Environmental Engineering and Management Particulas SpA, Santiago 9170022, Chile
4
Department of Process Engineering and Bioproducts, Faculty of Engineering, Universidad del Bío-Bío, Concepción 4030000, Chile
5
Grupo de Investigación en Biodiversidad, Medio Ambiente y Salud (BIOMAS), Facultad de Ingenierías y Ciencias Aplicadas, Universidad de Las Américas, Via Nayon, Quito 170525, Ecuador
6
Chemical Engineering Department, University of Santiago of Chile, Santiago 9170022, Chile
*
Author to whom correspondence should be addressed.
Biomass 2025, 5(3), 45; https://doi.org/10.3390/biomass5030045
Submission received: 16 June 2025 / Revised: 18 July 2025 / Accepted: 1 August 2025 / Published: 5 August 2025
(This article belongs to the Special Issue Recent Advances in Thermochemical Conversion of Biomass and Waste to Fuels, Chemicals and Materials)
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Abstract

This study optimizes the production of activated carbons from hydrothermally carbonized (HTC) biomass using potassium hydroxide (KOH) and phosphoric acid (H3PO4) as activating agents. A 23 factorial experimental design evaluated the effects of agent-to-precursor ratio, dry impregnation time, and activation duration on mass yield and iodine adsorption capacity. KOH-activated carbons achieved superior iodine numbers (up to 1289 mg/g) but lower mass yields (18–35%), reflecting enhanced porosity at the cost of material loss. Conversely, H3PO4 activation yielded higher mass retention (up to 54.86%) with moderate iodine numbers (up to 1117.3 mg/g), balancing porosity and yield. HTC pretreatment at 190 °C reduced the ash content, thereby enhancing the stability of hydrochar. These findings highlight the trade-offs between adsorption performance and process efficiency, with KOH suited for high-porosity applications (e.g., water purification) and H3PO4 for industrial scalability. The study advances biomass waste valorization, aligning with circular economy principles and offering sustainable solutions for environmental and industrial applications, such as water purification and energy storage.

1. Introduction

The increasing accumulation of waste represents one of the most pressing challenges modern society faces globally [1]. Rapid industrialization, urbanization, and population growth have accelerated waste generation, posing significant environmental and public health threats [2,3]. This phenomenon, closely linked to economic and social development, is an alarming indicator of inefficient resource management and significant environmental issues. Conventional waste management approaches, such as landfilling and incineration, often exacerbate environmental concerns by releasing greenhouse gases and toxic pollutants, underlining the urgent need for sustainable and innovative solutions [4]. Among the various waste streams, residual biomass constitutes a substantial portion, originating from agricultural, forestry, and industrial processes. This underutilized raw material, often discarded or incinerated, has the potential to be transformed into a diverse range of high-value products, such as activated carbon, thereby reducing landfill burden and mitigating environmental impacts [5]. The valorization of biomass reduces the burden on landfills and mitigates the environmental footprint of waste management, offering an economic opportunity to generate commercially viable products. Converting biomass waste into functional materials aligns with circular economy principles, promoting resource efficiency and sustainable development by minimizing waste and maximizing the utility of renewable resources.
Hydrothermal carbonization (HTC) has emerged as a promising pretreatment process for biomass valorization [6]. This process converts biomass into hydrochar under moderate temperatures and pressures, reducing its ash content and enhancing its structural properties. Hydrochar is a versatile precursor for producing advanced materials such as activated carbon, and it is widely used in water purification, air filtration, and energy storage applications [7,8]. The two-step process of HTC followed by chemical activation outperforms single-step activation by stabilizing biomass at moderate temperatures (180–260 °C) [9], reducing volatile content and impurities [10], and producing a uniform hydrochar that enhances activation efficiency.
Current research on activated carbon production highlights the efficacy of chemical activation using agents like potassium hydroxide (KOH) [11] and phosphoric acid (H3PO4) [12,13]. KOH is recognized for generating high-porosity materials with superior adsorption capacities, making them ideal for applications requiring a high surface area [11,14]. In contrast, H3PO4 offers a milder activation process, preserving structural integrity and achieving higher mass yields, which is advantageous for industrial scalability [15]. Studies have demonstrated the potential of HTC to enhance hydrochar properties [16,17], yet the optimization of activation conditions for HTC-treated biomass remains underexplored, particularly regarding the trade-offs between porosity, yield, and industrial applicability [14].
This study addresses this gap by investigating the chemical activation of HTC-treated hydrochar using KOH and H3PO4 to produce high-performance activated carbons. The optimization of activation parameters—agent-to-precursor ratio, dry impregnation time, and activation duration—to balance mass yield and iodine adsorption capacity was studied, thereby enhancing the suitability of activated carbons for industrial and environmental applications.

2. Methodology

2.1. Biomass Utilized

The raw material for activated carbon production, consisting of agroforestry waste from the paper industry (sawdust, sludge, and unclassified pulp), was selected based on a detailed analysis of its suitability for hydrothermal carbonization (HTC) and chemical activation. The biomass consisted of sawdust (83.5%), bark (11.5%), residual sludge (4.32%), and pulp (0.68%), proportions representative of waste generated by the paper industry in southern Chile (Table S2). The high cellulose and moisture content make these biomasses ideal for HTC, while the low ash content in sawdust and bark, measured by ISO 1822:2016 [18], supports effective activation (Table S1). The regional abundance of paper industry waste (Table S2) ensures scalability for industrial applications [19]. Hydrothermal carbonization was performed at 190 °C for 30 min with a biomass-to-water ratio of 12%. A mixture of 400 g was fed per batch in a 1.5 L stainless steel reactor. The conditions were selected based on prior experimental work demonstrating optimal hydrochar stability and minimal ash content [6,20]. The resulting hydrochar, as shown in Table S3, presented a significant reduction in ash content, decreasing from 2.47% to 1.64% (33% reduction), attributed to the solubilization of inorganic fractions during HTC. This reduction enhances the quality of the hydrochar as a precursor for activated carbon by minimizing impurities that could interfere with the activation process.

2.2. Experimental Procedure

The experimental procedure, illustrated in Figure 1, aimed to produce and characterize activated carbons from hydrochar through a standardized chemical activation process. Hydrochar was first size-reduced using a mortar and sieve to achieve a particle size of 0.5–1 mm, ensuring uniform activation [21]. It was then washed with distilled water to remove residual impurities and dried in an oven at 105 °C for 24 h. Numbered ceramic crucibles were prepared to hold samples, ensuring accurate mass measurements throughout the process. The activation process comprised three phases: (1) ambient-temperature impregnation with the activating agent: KOH (85% purity, CAS 1310-58-3, USP/ACS grade, pellets) or H3PO4 (85% purity, CAS 7664-38-2, USP/ACS/ISO grade), (2) dry impregnation at 105 °C, and (3) thermal activation in a muffle furnace at 550 °C for a specified duration. The activation temperature was selected to ensure consistency in comparing KOH and H3PO4 as activating agents, as temperature was not a variable in the factorial design. The literature demonstrates that temperatures of 400–600 °C are sufficient for the effective activation of lignocellulosic precursors, particularly hydrochar, by KOH, achieving significant porosity. The use of hydrochar, pre-treated via hydrothermal carbonization at 190 °C for 30 min (Supplementary Table S3, ash content: 1.64 ± 0.13%), provides a precursor that enhances activation efficiency at lower temperatures. The mechanisms of KOH activation, including chemical reactions and physical activation by H2O and CO2 across a range of temperatures (500–800 °C), further support the suitability of 550 °C for hydrochar-derived activated carbons [22,23,24,25]. After activation, the activated carbons were washed with distilled water until a neutral pH (~7) was achieved, as confirmed using a digital pH meter (S400 SevenExcellence, Mettler Toledo, Muntinlupa, Philippines), to remove residual chemicals and then dried at 100 ± 5 °C for 24 h.
Product characterization focused on iodine adsorption capacity, measured using the ASTM D4607-14 standard [26]. Activated carbon samples were fragmented, heated to 105 °C to remove moisture, and treated with hydrochloric acid and iodine solution. The volume of sodium thiosulfate required for titration was recorded to calculate the iodine number, a key indicator of adsorption performance.

2.3. Experimental Design

The chemical activation of hydrochar was analyzed using a 23 factorial design to systematically evaluate three variables: agent-to-precursor ratio (g/g), dry impregnation duration, and activation time, each at two levels (low: “−”, high: “+”). This design, detailed in Table 1, enabled the assessment of individual and interaction effects on mass yield (MY) and iodine adsorption capacity, which is critical for optimizing activated carbon performance. Two activating agents, potassium hydroxide (KOH) and phosphoric acid (H3PO4), were selected for their complementary properties and industrial relevance. KOH is widely recognized for producing activated carbons with exceptionally high porosity and adsorptive capacities [11,27], while H3PO4 was selected for its milder activation, preserving structural integrity and yielding higher mass retention [12,28].
The agent-to-precursor ratios were set at 2:1 (−) and 4:1 (+) for KOH and 1.3:1 (−) and 2:1 (+) for H3PO4, based on the literature, indicating these ranges balance porosity development and mass yield [29,30]. Excessive ratios can reduce yields and increase costs, while insufficient ratios limit activation efficiency [31]. No preliminary screening experiments were conducted; however, these ratios were selected based on the findings of prior studies on lignocellulosic biomass activation [12,27]. The dry impregnation duration was tested at 0 h (−) and 1 h (+), which are hypothesized to enhance agent penetration into the hydrochar, thereby improving porosity [32]. Activation time was evaluated at 1 h (−) and 2 h (+), which were selected to investigate the trade-off between porosity development and material loss, as longer durations increase porosity but reduce yield [23].
Mass yield (MY), defined as the percentage of activated carbon produced relative to the initial hydrochar mass on a dry and ash-free basis, and iodine adsorption capacity, measured in mg/g, were the response variables. These metrics were chosen for their relevance to industrial applications, such as water purification and air filtration [33,34]. Two separate factorial experiments were conducted—one for KOH and one for H3PO4—each comprising eight combinations of the three variables, with experiments performed in duplicate to ensure reliability.
Experimental runs were randomized to minimize bias and external influences, with samples coded to identify the activating agent, agent-to-precursor ratio, dry impregnation duration, and activation time (Table 2). Two control samples, CRUDO.KOH (4:1 ratio, 1 h impregnation, 1 h activation) and CRUDO.H3PO4 (2:1 ratio, 0 h impregnation, 1 h activation) were prepared using raw biomass without HTC pretreatment to assess the impact of HTC. This control comparison elucidated the role of HTC in enhancing the stability and activation efficiency of hydrochar. The systematic codification and factorial design provided a robust dataset for analyzing the effects of activation parameters on hydrochar-derived and raw biomass-derived activated carbons.

3. Results

Chemical activation of hydrochar using potassium hydroxide (KOH) and phosphoric acid (H3PO4) produced activated carbons with distinct properties, reflecting the influence of the activating agent, hydrothermal carbonization (HTC), and activation parameters (agent-to-precursor ratio, dry impregnation time, and activation time). These results, summarized in Figure 2, aim to optimize activation conditions to balance mass yield (MY) and iodine adsorption capacity for industrial applications.
Samples treated with KOH exhibited a substantial increase in volume during activation, filling up to 90% of the crucible. This expansion is primarily attributed to the pyrolytic reactions induced by KOH at elevated temperatures, which cause the lignin present in the hydrochar to expand [27]. These reactions are driven by the high reactivity of KOH, which not only catalyzes the decomposition of lignin but also promotes the generation of gas-phase products that further contribute to the observed swelling. Lignin, a complex aromatic polymer, undergoes significant structural transformations during pyrolysis, leading to pronounced mass loss [35,36]. This effect is more pronounced in lignocellulosic biomass and is further amplified in hydrochar due to its altered chemical structure following hydrothermal carbonization [37]. The unique characteristics of hydrochar, including its oxygenated functional groups and partially degraded cellulose and hemicellulose, render it highly susceptible to expansive reactions, distinguishing it from untreated biomass. While these reactions predominantly affect cellulose and hemicellulose, the residual lignin contributes significantly to the observed volume increase [38,39].
In contrast, activation with H3PO4 did not result in a noticeable expansion in volume of the hydrochar. This distinction arises because H3PO4 operates under fundamentally different chemical mechanisms. Phosphoric acid primarily facilitates dehydration and cross-linking reactions rather than the aggressive pyrolysis seen with KOH [40]. Cross-linking reactions help form stable phosphate–carbon linkages, preserving the hydrochar’s structural integrity while inducing microporosity [41]. These reactions enhance the structural stability of the carbon matrix without causing significant volumetric changes [9].
The lower thermal reactivity of H3PO4 refers to its milder chemical interaction with the hydrochar during thermal activation at 550 °C, causing less pronounced degradation of the lignocellulosic structure compared to KOH. This results in a higher mass yield (MY) by preserving more of the hydrochar’s mass while still developing adequate porosity and adsorption capacity, striking a balance critical for industrial applications. Furthermore, the lower thermal reactivity of H3PO4 minimizes the release of volatile compounds, enabling a more controlled activation process with reduced material loss. As a result, larger quantities of hydrochar can be treated during activation with H3PO4 without the constraint of excessive expansion, enhancing process efficiency. This characteristic makes H3PO4 a practical choice for processes that require the treatment of larger biomass volumes, particularly in industrial settings where maximizing throughput is crucial [42].

3.1. Effect of Hydrothermal Carbonization

Hydrothermal carbonization (HTC) at 190 °C for 30 min significantly influenced the mass yield (MY) of activated carbons, with a more pronounced effect when using phosphoric acid (H3PO4) as the activating agent, thereby optimizing precursor stability and yield. HTC-treated samples exhibited higher MY compared to non-HTC-treated controls; for example, CRUDO.H3PO4 achieved 44.47% ± 0.89% versus CRUDO.KOH at 18.07% ± 0.36% (Figure 2A). This enhancement, within a 2% error margin, underscores HTC’s efficacy in reducing ash content from 2.45% to ~1.64% and stabilizing biomass precursors [16]. H3PO4-activated samples consistently outperformed KOH, with AC.H3PO4.2 reaching 54.86% ± 1.10% compared to AC.KOH.8 at 18.49% ± 0.37% (Figure 2A), reflecting H3PO4’s milder activation that minimizes material loss. In contrast, KOH’s aggressive reactions enhanced porosity, as seen in AC.KOH.7, with an iodine number of 1289.00 ± 25.78 mg/g (Figure 2B) but a lower MY of 31.86% ± 0.64% (Figure 2A). This trade-off highlights HTC’s role in improving yield across conditions, with hydrochar-derived carbons outperforming non-HTC samples [43,44].
The interplay between activation conditions and yield was evident, particularly with KOH. Samples with extended activation times or higher agent-to-precursor ratios exhibited reduced MY; for example, AC.KOH.7 (2:1 ratio, 1 h impregnation, 2 h activation) balanced high IN (1289 mg/g) with a lower MY (31.86%). H3PO4 activation offered a more balanced outcome, with AC.H3PO4.4 (2:1 ratio, 1 h impregnation, 1 h activation) achieving 46.16% ± 0.92% MY and 1089.90 ± 21.80 mg/g IN. This balance arises from H3PO4-controlled dehydration and the formation of a stable phosphate–carbon complex, which reduces volatilization compared to KOH samples [45]. Optimizing activation parameters, such as ratio and duration, is critical; excessive times increase porosity but lower yields, while H3PO4’s versatility supports micropore development with retained integrity. These results suggest H3PO4’s potential for sustainable production, with yields competitive with ranges in the literature of 50–55% for lignocellulosic biomass [46], and emphasize a holistic process design to balance chemical, thermal, and precursor factors [47,48].

3.2. Effect of Activating Agent and Agent-to-Precursor Ratio

The activating agent’s effect on the properties of activated carbon was evident in the stark differences between samples treated with potassium hydroxide (KOH) and phosphoric acid (H3PO4). Hydrochar treated with KOH reached a maximum mass yield (MY) of 35.50% ± 0.71% (AC.KOH.4), while H3PO4-treated samples showed greater consistency, with only three conditions below 40% and a peak MY of 54.86% ± 1.10% (AC.H3PO4.2) (Figure 2A). This contrast reflects the divergent activation mechanisms, with KOH’s aggressive reactivity driving porosity and H3PO4’s milder approach favoring yield retention [23]. Activation with KOH resulted in significant mass losses due to its aggressive reactions, which fragmented the biomass via reduction processes and released carbon oxides, forming potassium carbonate within the carbon matrix—for instance, non-hydrochar-treated CRUDO.KOH yielded 18.07% ± 0.36%, while the hydrochar sample AC.KOH.8 achieved 18.49% ± 0.37%, underscoring the trade-off between KOH’s high reactivity and enhanced porosity [49] (Figure 2A). This porosity was evident in iodine numbers (INs), with AC.KOH.7 (2:1 ratio, 1 h impregnation, 2 h activation) reaching a value of 1289.00 ± 25.78 mg/g, the highest recorded, despite a MY of 31.86% ± 0.64%.
In contrast, H3PO4 acted more gently, inducing dehydration and condensation reactions that targeted cellulose, reducing aliphatic content and increasing aromaticity to improve thermal stability. H3PO4-activated samples maintained higher MY, e.g., AC.H3PO4.6 at 45.99% ± 0.92% (no impregnation, 2 h activation) and AC.H3PO4.4 at 46.16% ± 0.92% (1 h impregnation, 1 h activation) (Figure 2A). However, IN values were generally lower than those of KOH, with AC.H3PO4.6 at 929.29 ± 18.59 mg/g compared to AC.KOH.7 under similar conditions (Figure 2B). A key mechanism for H3PO4 was the formation of phosphate esters, which facilitated cross-linking and the creation of a stable, moderately porous carbon network [49]. The highest IN for H3PO4, 1117.30 ± 22.35 mg/g (AC.H3PO4.5), demonstrated its capacity for adequate adsorption (Figure 2B). While KOH excelled in porosity (IN > 1200 mg/g), it incurred greater mass loss; in contrast, H3PO4 offered a balanced approach suitable for applications requiring structural stability [50].
In this way, the agent-to-precursor ratio distinctly influenced mass yield and iodine number of activated carbons, with opposing trends for potassium hydroxide (KOH) and phosphoric acid (H3PO4), as shown in Figure 2. For KOH, increasing the ratio from 2:1 to 4:1 significantly reduced MY. KOH.1 yielded 28.58% ± 0.57% at 2:1, dropping to 22.58% ± 0.45% at 4:1 (AC.KOH.2). Under dry impregnation and 2 h activation, the yield fell from 31.86% ± 0.64% (AC.KOH.7, 2:1) to 18.49% ± 0.37% (AC.KOH.8, 4:1), a 13% reduction. This loss correlated with KOH’s heightened reactivity [15], causing extensive precursor degradation, yet it enhanced porosity, with IN rising from 902.35 ± 18.05 mg/g (AC.KOH.1) to 1138.75 ± 22.78 mg/g (AC.KOH.2) and peaking at 1289.00 ± 25.78 mg/g (AC.KOH.7). For H3PO4, higher ratios increased MY; AC.H3PO4.1 yielded 44.35% ± 0.89% at 1.3:1, rising to 54.86% ± 1.10% at 2:1 (AC.H3PO4.2). This trend held with longer activation, e.g., AC.H3PO4.5 (1.3:1, 2 h) at 27.84% ± 0.56% versus AC.H3PO4.6 (2:1, 2 h) at 45.99% ± 0.92%, an 18% increase. The IN also improved with higher ratios, from 521.20 ± 10.42 mg/g (AC.H3PO4.2) to 1117.30 ± 22.35 mg/g (AC.H3PO4.6), reflecting enhanced porosity via structural reordering [51]. This reveals a trade-off for KOH (yield vs. porosity) and a synergy effect for H3PO4 (yield and adsorption), and suggests tailoring ratios to applications, e.g., high-porosity needs (KOH) or balanced performance (H3PO4), with H3PO4’s higher yields, aligning with values in the literature [46].

3.3. Effect of Activation Time and Operational Conditions

The adsorption capacity of activated carbons, as reflected by iodine numbers, is intricately shaped by the characteristics of the precursor, the behavior of the activating agent, and processing variables, revealing a complex interplay that is critical to optimization. Hydrochar, enriched with porosity through hydrothermal carbonization (HTC), offered a promising starting material; however, its inherent stability and high fixed carbon content often limited the development of porosity compared to raw biomass after activation [52]. Chemical activation reduced reactivity and enhanced structural integrity, limiting micropore formation—a key factor in adsorption performance. For example, raw biomass-derived activated carbon (CRUDO.KOH) achieved an iodine number of 856.70 ± 17.13 mg/g, while hydrochar-derived activated carbon (AC.KOH.5) reached 786.56 ± 15.73 mg/g. Potassium hydroxide (KOH) emerged as a potent driver of porosity, leveraging its aggressive reactivity to dismantle volatile components and forge micropores and mesopores. The pinnacle of this effect was seen in AC.KOH.7, where a 2 h activation at 550 °C with 1 h impregnation yielded 1289.00 ± 25.78 mg/g, outstripping H3PO4’s best at 1117.30 ± 22.35 mg/g for AC.H3PO4.5 [30]. Nevertheless, this came at a cost, as evidenced by the AC.KOH.8 value of 18.49% ± 0.37% yield alongside 971.51 ± 19.43 mg/g, highlighting KOH’s trade-off between adsorption capacity and material retention. In contrast, phosphoric acid (H3PO4) adopted a gentler approach, inducing dehydration and cross-linking through phosphate esters, which enhanced structural stability and reduced porosity [53]. This mechanism yielded higher results, as evident in AC.H3PO4.2, with 54.86% ± 1.10% and 521.20 ± 10.42 mg/g, and in AC.H3PO4.6, with 45.99% ± 0.92% and 929.29 ± 18.59 mg/g, reflecting a balanced outcome suitable for applications that value integrity over maximal porosity. These values are similar to those of commercial activated carbons, which exhibit iodine numbers ranging from 800 to 1300 mg/g [54].
Activation time proved a double-edged sword, amplifying adsorption capacity at the expense of yield. Extended durations at 550 °C, such as the 2 h activation in AC.KOH.7, boosted iodine numbers by 64% over the 1 h AC.KOH.5 (1289.00 vs. 786.56 mg/g), yet eroded mass retention. Dry impregnation further enhanced this effect, with KOH’s 1 h step increasing iodine numbers by over 30% (e.g., AC.KOH.3 vs. AC.KOH.1), driven by deeper agent penetration [22,31]. For H3PO4, 2 h activation in AC.H3PO4.5 elevated performance to 1117.30 mg/g, though yields dipped to 27.84% ± 0.56%, suggesting a threshold beyond which benefits plateau. The agent-to-precursor ratio added another layer of complexity. KOH at a 4:1 (AC.KOH.2) ratio increased iodine numbers to 1138.75 ± 22.78 mg/g but reduced the yield to 22.58% ± 0.45%. In contrast, a 2:1 ratio (AC.KOH.3) offered a compromise at 1217.22 ± 24.34 mg/g with 27.64% ± 0.55% yield. H3PO4 at 1.3:1 (AC.H3PO4.1) yielded 44.35% ± 0.89% with 698.13 ± 13.96 mg/g, improving to 54.86% ± 1.10% at 2:1 (AC.H3PO4.2) but dipping to 521.20 ± 10.42 mg/g, indicating a nuanced balance between ratio and porosity.
These dynamics position KOH as ideal for high-adsorption needs, such as water purification or gas storage, where its porosity (exceeding 1200 mg/g) rivals commercial standards despite higher processing costs and mass loss. H3PO4, with its higher retention (up to 54.86%) and moderate porosity (up to 1117.30 mg/g), is suitable for applications requiring structural stability, such as filtration media, with potential energy savings in production [34]. Optimization of time, impregnation, and ratio can elevate hydrochar-based carbons to compete with commercial products, necessitating a strategic approach to balance chemical reactivity, thermal exposure, and precursor properties for sustainable industrial scalability.

3.4. ANOVA and Contour Analysis for Mass Yield and Iodine Number

The influence of activation parameters—agent-to-precursor ratio (APR), dry impregnation time (DI), activation time (AT), and activating agent (AGENT)—on mass retention and adsorption capacity was rigorously assessed using analysis of variance (ANOVA), building on the experimental design outlined earlier. Contour visualizations (Figure 3a–d), derived from response surface models at DI = 0 for KOH and H3PO4, complement these findings by illustrating parameter interactions, with gradients reflecting statistically significant effects. For mass yield, the first-order model (MY = 34.20 + 1.21∙APR − 1.62∙AT + 3.77∙AGENT − 0.55∙APR:DI + 1.58∙APR:AGENT − 0.81∙DI:AGENT − 0.66∙AT:AGENT) yielded an F-value of 7.0949 (p = 0.004461, df = 4, 11, R2 = 0.84), indicating a highly significant fit, with a residual mean square of 41.389. The variable chemical AGENT emerged as the dominant factor, explaining a substantial portion of variance (estimated η2 ≈ 0.45), consistent with the steeper gradients for KOH and reflecting KOH’s aggressive degradation versus H3PO4 retention. The activation time (AT) and the ratio (APR) also contributed significantly (p < 0.01), as indicated by the rising contours with increasing ratio, aligning with the observed yield drops (e.g., AC.KOH.8 at 18.49% vs. AC.KOH.7 at 31.86% in Figure 2.
For iodine number, a parallel model (IN = 905.78 − 49.47∙APR + 42.95∙DI − 21.36∙APR:DI − 19.41∙APR:AT + 16.49∙APR:AGENT + 27.95∙AT:AGENT) produced an F-value of 8.0125 (p = 0.0056325, R2 = 0.81), confirming statistical significance across the design space. This higher F-value suggests a stronger parameter effect on porosity, as supported by contour peaks for KOH (800–1289 mg/g in Figure 3c) and a more uniform range of 800–1117 mg/g for H3PO4 (Figure 3d). DI and AT (time) effects, although less pronounced (p > 0.05 for DI), align with contour stability, suggesting secondary roles unless optimized with impregnation. Residual analysis confirmed model validity, with Shapiro–Wilk tests (p = 0.21 for MY, p = 0.20 for IN) indicating normality and Breusch–Pagan tests (p = 0.19 for MY, p = 0.18 for IN) confirming homoscedasticity, as visualized in Supplementary Figures S1 and S2.
Mechanistically, these results reflect KOH’s volatile-driven micropore formation, contrasting with H3PO4’s cross-linking stability, a trend validated by the contour gradients. The statistical significance supports optimizing APR and AGENT for specific goals—high porosity for KOH or high yield for H3PO4—with industrial scalability enhanced by minimizing activation time where yield is critical. Future work could explore higher-order terms or other experimental block designs, such as central composite designs.

4. Conclusions

This study highlights the crucial role of activating agents, processing conditions, and hydrothermal carbonization (HTC) in determining the performance of biomass-derived activated carbons, thereby laying the groundwork for the optimized production of these materials. The distinct behaviors of potassium hydroxide (KOH) and phosphoric acid (H3PO4) shape material properties, with KOH driving porosity to iodine numbers up to 1289.00 ± 25.78 mg/g (e.g., AC.KOH.7) at the cost of mass loss as low as 18.49% ± 0.37% (e.g., AC.KOH.8), while H3PO4 balances yield at 54.86% ± 1.10% (e.g., AC.H3PO4.2), with adsorption of up to 1117.30 ± 22.35 mg/g (e.g., AC.H3PO4.5). This interplay highlights tailored strategies for specific needs, aligning to enhance activation efficiency.
Dry impregnation proves essential, particularly for KOH, as it boosts adsorption capacity by over 30% (e.g., AC.KOH.7 vs. AC.KOH.5) through enhanced agent penetration, especially in high-temperature processes. Its effect is less pronounced with H3PO4 due to its gentler cross-linking, suggesting a need to adjust this step based on agent choice. Optimal conditions include a 2:1 KOH ratio with 1 h impregnation and 2 h activation for maximum porosity, as well as 2 h H3PO4 activation without impregnation for balanced performance, supporting industrial scalability.
HTC pretreatment significantly stabilizes hydrochar, reducing ash content to approximately 1.6% and improving yield consistency (e.g., 44.47% ± 0.89% for CRUDO.H3PO4); however, its lower reactivity requires more aggressive KOH conditions to achieve high porosity. This stability enhances structural integrity but limits micropore development, as evident in the lower iodine numbers for hydrochar (e.g., 786.56 ± 15.73 mg/g for AC.KOH.5) compared to raw biomass, necessitating a tailored activation intensity. The dominance of agent type and its interaction with the ratio suggests customizing these parameters, with KOH suiting high-adsorption applications, such as water purification, and H3PO4 excelling in yield-focused uses, such as filtration media.
These insights advance biomass valorization by offering a framework for sustainable activated carbon production, with H3PO4’s higher yields reducing resource loss. Future research should explore alternative feedstocks (e.g., agricultural residues) and novel agents to diversify material properties and enhance their performance. Advanced techniques, such as the BET model, could also be used to quantify micropore contributions, thereby addressing current gaps in porosity assessment. Additionally, refining activation models with replicates could enhance reliability, paving the way for large-scale, eco-efficient processes.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/biomass5030045/s1, Figure S1: Q-Q plot of residuals for the Mass Yield (MY) model; Figure S2: Q-Q plot of residuals for the Iodine Number (IN) model; Table S1: Characterization of Paper Industry Biomasses for Activated Carbon Production; Table S2: Amount of waste produced in a papermill industry (southern Chile); Table S3: Mass Yield (MY), Higher Heating Value (HHV), Energy Densification Ratio (EDR), Energy Yield (EY), and Ash Content (ASH) for Biomass Mixture; Table S4: Mass Yield, Ash Content, and Iodine Number of Activated Carbons.

Author Contributions

Conceptualization, F.V.; methodology, F.V., D.Y. and M.O.; software, F.V. and M.O.; validation, F.V. and T.R.; formal analysis, F.V. and S.A.-M.; investigation, F.V., R.Z. and T.R.; resources, L.D.-R.; data curation, T.R. and D.Y.; writing—original draft preparation, F.V.; writing—review and editing, F.V., L.D.-R., M.O., S.A.-M. and R.Z.; visualization, F.V. and R.Z.; supervision, L.D.-R. and F.V.; project administration, L.D.-R. and F.V.; funding acquisition, L.D.-R. All authors have read and agreed to the published version of the manuscript.

Funding

This research was supported by the Universidad de Santiago de Chile and funded by the FONDEF project ID18I182. The authors are grateful for the invaluable assistance and resources that enabled them to complete this study.

Data Availability Statement

The original contributions presented in this study are included in the article. Further inquiries can be directed to the corresponding author.

Conflicts of Interest

Authors Luis Díaz-Robles and Marcelo Oyaneder were employed by the company “Environmental Engineering and Management Particulas SpA”. The remaining authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

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Biomass 05 00045 g001
Figure 1. Hydrochar activation process.
Figure 1. Hydrochar activation process.
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Figure 2. Experimental results for (A) mass yield and (B) iodine number of activated carbons (ACs) vs. controls (CRUDOs).
Figure 2. Experimental results for (A) mass yield and (B) iodine number of activated carbons (ACs) vs. controls (CRUDOs).
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Figure 3. Mass yield and iodine number contours for experiments with KOH and H3PO4.
Figure 3. Mass yield and iodine number contours for experiments with KOH and H3PO4.
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Table 1. Experimental design for hydrochar activation.
Table 1. Experimental design for hydrochar activation.
Precursor AgentAgent/Precursor Ratio (g/g)Dry Impregnation (h)Activation Time (h)
−+−+−+
KOH2:14:10112
H3PO41.3:12:10112
Table 2. Codification for experimental samples.
Table 2. Codification for experimental samples.
Sample NameActivating AgentAgent/Precursor RatioDry Impregnation (h)Activation Time (h)
AC.KOH.1KOH2:1 (−)0 (−)1 (−)
AC.KOH.2KOH4:1 (+)0 (−)1 (−)
AC.KOH.3KOH2:1 (−)1 (+)1 (−)
AC.KOH.4KOH4:1 (+)1 (+)1 (−)
AC.KOH.5KOH2:1 (−)0 (−)2 (+)
AC.KOH.6KOH4:1 (+)0 (−)2 (+)
AC.KOH.7KOH2:1 (−)1 (+)2 (+)
AC.KOH.8KOH4:1 (+)1 (+)2 (+)
AC.H3PO4.1H3PO41.3:1 (−)0 (−)1 (−)
AC.H3PO4.2H3PO42:1 (+)0 (−)1 (−)
AC.H3PO4.3H3PO41.3:1 (−)1 (+)1 (−)
AC.H3PO4.4H3PO42:1 (+)1 (+)1 (−)
AC.H3PO4.5H3PO41.3:1 (−)0 (−)2 (+)
AC.H3PO4.6H3PO42:1 (+)0 (−)2 (+)
AC.H3PO4.7H3PO41.3:1 (−)1 (+)2 (+)
AC.H3PO4.8H3PO42:1 (+)1 (+)2 (+)
CRUDO.KOHKOH4:1 (+)1 (+)1 (−)
CRUDO.H3PO4H3PO42:1 (+)0 (−)1 (−)
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MDPI and ACS Style

Vallejo, F.; Yánez, D.; Díaz-Robles, L.; Oyaneder, M.; Alejandro-Martín, S.; Zalakeviciute, R.; Romero, T. Valorizing Biomass Waste: Hydrothermal Carbonization and Chemical Activation for Activated Carbon Production. Biomass 2025, 5, 45. https://doi.org/10.3390/biomass5030045

AMA Style

Vallejo F, Yánez D, Díaz-Robles L, Oyaneder M, Alejandro-Martín S, Zalakeviciute R, Romero T. Valorizing Biomass Waste: Hydrothermal Carbonization and Chemical Activation for Activated Carbon Production. Biomass. 2025; 5(3):45. https://doi.org/10.3390/biomass5030045

Chicago/Turabian Style

Vallejo, Fidel, Diana Yánez, Luis Díaz-Robles, Marcelo Oyaneder, Serguei Alejandro-Martín, Rasa Zalakeviciute, and Tamara Romero. 2025. "Valorizing Biomass Waste: Hydrothermal Carbonization and Chemical Activation for Activated Carbon Production" Biomass 5, no. 3: 45. https://doi.org/10.3390/biomass5030045

APA Style

Vallejo, F., Yánez, D., Díaz-Robles, L., Oyaneder, M., Alejandro-Martín, S., Zalakeviciute, R., & Romero, T. (2025). Valorizing Biomass Waste: Hydrothermal Carbonization and Chemical Activation for Activated Carbon Production. Biomass, 5(3), 45. https://doi.org/10.3390/biomass5030045

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