Search Results (415)

Biochar (BC) and hydrochar (HC) are carbon-rich materials derived from organic wastes through pyrolysis/gasification and hydrothermal carbonization (HTC), respectively, offering promising pathways for waste valorization and resource recovery within a circular economy framework. Owing to their porous structure and surface functionality, these materials have gained attention as additives in anaerobic digestion (AD), where they may enhance the microbial activity, improve the buffering capacity, and facilitate direct interspecies electron transfer (DIET), resulting in greater process stability and higher methane (CH₄) yields. This study evaluated the effect of BC and HC derived from aloe vera leaves and algae on methane production during the AD of food waste (FW). Batch experiments were conducted under mesophilic conditions (37 °C) for 60 days, using a 1:1 inoculum-to-substrate ratio and a dosage of 10 g L⁻¹ of each carbonaceous material. The results show that adding BC increased cumulative biogas production by 10–14%, while HC led to an 18–35% increase compared with the control. Additionally, dissolved chemical oxygen demand (d-COD) removal improved by up to 30%, and volatile solids (VSs) removal rose by 31% in the FW and aloe HC reactors, highlighting the greater enhancement potential of HCs in methane production. Overall, the findings demonstrate that integrating carbonaceous materials derived from organic residues into AD systems can enhance bioenergy recovery while reducing environmental impacts, supporting more sustainable and circular waste-to-energy processes. Full article

The sustainable valorization of food waste is essential for advancing the circular bioeconomy and reducing the environmental impacts of organic waste disposal. This study presents an integrated approach combining hydrothermal carbonization (HTC) and anaerobic digestion (AD) to recover renewable energy and valuable resources from food waste. The process was simulated in Aspen Plus^® version 14.1 using thermochemical and biochemical reaction models to evaluate the effects of feed moisture (60–85%) and HTC temperature (180–280 °C) on performance. Integration of HTC and AD increased overall energy recovery by 26–38% compared to standalone AD, with a feed moisture of 85%, organic loading of 4 kg VS m⁻³ d⁻¹, and mesophilic/thermophilic temperatures of 35 and 55 °C. Improvements resulted from higher methane yield (0.42 m³ CH₄ kg⁻¹ VS) from HTC liquor and energy-rich hydrochar (25–29 MJ kg⁻¹). The techno-economic assessment indicated a net energy ratio of 2.3, an Internal Rate of Return (IRR) of 18.6%, and a 4.8-year payback period, confirming economic viability. Sensitivity analysis highlighted energy prices and feedstock costs as key drivers, while Monte Carlo simulation demonstrated stability under ±20% uncertainty. Optimal conditions (HTC at 220 °C, 65% moisture, and 100 kg h⁻¹ solid loading) significantly enhanced profitability and carbon efficiency. Overall, the integrated HTC–AD process offers a technically, economically, and environmentally sustainable route for converting food waste into renewable energy and biochar, supporting circular bioeconomy and net-zero energy goals. Full article

Hydrothermal carbonization (HTC) of agricultural waste is a promising waste management technique. However, the use of different raw materials may produce hydrochars with varying efficiencies, both in yield and application, and environmental impacts, due to differences in composition and required processing conditions. To understand the influence of biomass type and acid-assisted HTC conditions, this study used sugarcane and agave bagasse to produce functionalized hydrochars and evaluated them for the removal of Reactive Orange 84; an azo dye used in the textile industry. Material characterization was performed using FT-IR, TGA, BET, and XRD analyses. In addition, a life cycle assessment was conducted to evaluate environmental impacts associated with hydrochars produced using H₂SO₄ at concentrations of 0.2 and 0.5 M. TGA and XRD results indicate that agave bagasse hydrochars (HBA) retain more crystalline lignocellulosic structures, whereas sugarcane bagasse hydrochars (HBS) exhibit predominantly amorphous structures after HTC. FT-IR analysis confirmed the presence of –SO₃H functional groups; however, HBA samples showed greater availability of these groups with increasing acid concentration. Adsorption experiments and LCA results demonstrated that the most favorable treatment, in terms of emission reduction and dye removal, was agave bagasse functionalized with 0.5 M H₂SO₄, achieving 75.7% mass yield and 94.5% dye removal. Full article

The influence of the autohydrolysis temperature of sunflower stems (SSs) and sunflower inflorescence (SI) on the changes in the composition of the pyrolysis products of their hydrochars (HCs) was investigated. This research was carried out using a TG/FT-IR analytical device, the semi-quantitative ATR technique, the quantitative XRD technique, and the SEM (EDS) technique. It was found that a rise in autohydrolysis temperature alarmingly increases the contribution of undesirable hydrocarbons in the volatile pyrolysis products of HCs calculated with respect to the emitted CO₂ and substantially decreases the yield of pyrolyzed solid products. The rise in autohydrolysis temperature not only changes the content of inorganics in HCs but also influences the migration of inorganics in these samples during pyrolysis: intensifies the migration of Mg and Ca and reduces the migration of K. This affects the secondary reaction between the volatile pyrolysis products. The addition of 2 wt% of paracetamol to pyrolyzed HCs inhibits the migration of Mg and Ca and increases the migration of K with volatile products, which positively influences the reduction in undesirable compounds in the composition of emitted volatile products. The addition of paracetamol decreases the yield of pyrolyzed SSHCs by circa 2% and increases the yield of pyrolyzed SIHC₁₈₀ by almost 5%. Full article

Agroindustrial residues represent an abundant and underutilized source of carbon-rich materials for environmental remediation. In this study, annatto processing waste (Bixa orellana), a largely unexplored lignocellulosic by-product generated during pigment extraction, was converted into hydrochar via hydrothermal carbonization at 200 °C for 3 h. The resulting hydrochar (HC-AW) exhibited a predominantly amorphous carbon structure with retained oxygen-containing surface functionalities, and a solid yield of 44%, indicating efficient biomass conversion under subcritical conditions. Adsorption performance toward tetracycline was evaluated through pH-dependent experiments, kinetic modeling, equilibrium isotherms, and thermodynamic analysis. Maximum adsorption occurred under near-neutral conditions (pH ≈ 7), consistent with the interplay between tetracycline speciation and the hydrochar surface charge (pH_PZC ≈ 6.3), highlighting its potential applicability under realistic water treatment conditions without pH adjustment. Kinetic data were well described by the pseudo-second-order model, while equilibrium results were best fitted by the Langmuir model, with a maximum adsorption capacity of 14.94 mg g⁻¹ at 30 °C. Thermodynamic analysis indicated a spontaneous and slightly endothermic adsorption process. Overall, the results highlight the potential of annatto-derived hydrochar as a low-cost adsorbent and provide insight into the relationship between surface properties and adsorption behavior governing antibiotic removal from aqueous systems. Full article

Triazine herbicides are widely used for weed control in agricultural systems, and their occurrence in water bodies has been frequently reported worldwide. This study assessed the efficiency of a hydrochar derived from the epicarp and mesocarp of passion fruit residues for the removal of three triazine herbicides (atrazine, ametryn, and metribuzin), with the aim of developing a material suitable for application in water remediation programs. The adsorption capacity of biomass and hydrochar derived from passion fruit residues was evaluated with and without activation using 0.5 mol L⁻¹ phosphoric acid. The adsorption of herbicides was not significantly affected by pH within the range of 4 to 8. The acid hydrochar, which exhibited the highest removal capacity among the evaluated adsorbents, presented adsorption capacities of 18.05, 10.83, and 5.05 µg g⁻¹ for atrazine, ametryn, and metribuzin, respectively. These values correspond to removal efficiencies of approximately 62%, 72%, and 52% at initial concentrations of 0.33, 0.25, and 0.15 mg L⁻¹. The adsorption equilibrium time varied among the herbicides, reaching 4 h for atrazine and ametryn and 5 h for metribuzin. The adsorption dynamics between the adsorbents and adsorbates were best described by the pseudo-second-order kinetic model for ametryn and metribuzin, while atrazine had a higher correlation with the Elovich equation. The Weber–Morris model did not adequately describe the adsorption process. Among the isotherms tested, the Freundlich model provided the best fit for all three herbicides. The desorption rates of the acid hydrochar were 51%, 13%, and 83% for atrazine, ametryn, and metribuzin, respectively. Therefore, hydrochar derived from passion fruit residues represents a promising alternative for the remediation of triazine herbicides. Full article

Integrating hydrothermal carbonization (HTC) and anaerobic digestion (AD) has the potential to improve energy conversion efficiency (ECE) of biomass with low energy density and high moisture content. This study aims to assess the influence of alkali metals and chlorides by comparing seawater and distilled water as a HTC reactant medium, treating Fucus serratus across a range of processing temperatures (150 °C, 200 °C and 250 °C). All HTC-AD integration options improved ECE of F. serratus compared to AD alone. ECE of F. serratus was similar across temperatures of 150 °C (84–88%) and 200 °C (75–77%) regardless of seawater or distilled water usage. However, HTC processing at 250 °C yielded a greater ECE from F. serratus using distilled water (78%), compared to seawater (57%), due to a higher hydrochar yield and biomethane generation from the process water. Higher HTC processing temperatures significantly reduced slagging and fouling propensity of hydrochars by selectively removing problematic alkali metals. This creates a compromise between process energetics and favourability of hydrochar properties in large-scale conversion systems. Overall, HTC of F. serratus in seawater at 250 °C produces hydrochar suitable for combustion, process water that generates biomethane during AD (168.4 mL CH₄/g COD) and a net energy-positive process (energy return on energy investment EROI = 1.53). Full article

The sustainable management of olive wastes represents an important environmental challenge. Biochars and hydrochars derived from biomass are promising adsorbents for removing emerging pollutants from water. In the present work, olive stone wastes were converted into biochar and hydrochar by using pyrolysis (500 °C for 30 min) and hydrothermal carbonization (HTC) processes (220 °C for 10 h). Then, the obtained materials were physically activated by using CO₂ gas (750 °C for 30, 60 and 180 min). Various analytical techniques were applied for the chemical, textural and structural characterization of these carbonaceous materials (i.e., ultimate and proximate analysis, scanning electron microscopy (SEM), BET surface area, Raman spectroscopy, X-ray diffraction, and Fourier transform infrared spectroscopy). Afterwards, the selected activated biochar and hydrochar were applied for the removal of amoxicillin from aqueous solutions. The experimental results show that the generated hydrochar has many microspheres on its surface and inside, while the produced biochar exhibits a porous structure with irregular forms. CO₂ physical activation has induced an important improvement of the biochar and hydrochar’s structural, textural, and surface chemistry properties. For instance, the activated biochar samples show a highly porous structure, with large specific surface areas that increase with the burn-off, reaching 1349.3 m² g⁻¹ following 3 h of activation. Regarding the activated hydrochar samples, they exhibit a spherical morphological structure with an important specific surface area, which increased to 846.7 m² g⁻¹ after 3 h of activation. Moreover, both activated materials have an amorphous structure with low oxygen surface groups. The selected novel CO₂-activated biochar and hydrochar efficiently remove amoxicillin from aqueous solutions under wide experimental conditions, with adsorption capacities of 386.4 and 215.9 mg g⁻¹, respectively. These efficiencies are higher than those reported for various activated biochars derived from lignocellulosic biomass, from sewage sludge, and from animal manure. Future research works are required to assess these materials’ effectiveness in treating real pharmaceutical effluents, to optimize the regeneration of the amoxicillin-loaded materials, and to design full-scale devices for a real application. Full article

The removal of water pollutants, specifically the organophosphorus pesticides chlorpyrifos (CHP) and azinphos-methyl (AZM), as well as the dye Rhodamine B (RB), was investigated through the valorization of grape pomace, an abundant agricultural byproduct. For the first time, hydrochars derived from grape pomace were utilized as adsorbents for these contaminants following KOH activation (HCK) and pyrolysis at 400 °C (PHC). The study aimed to evaluate the adsorption performance, determine the optimal conditions, and elucidate the adsorption mechanisms. Physicochemical characterization using SEM, FTIR, BET surface area analysis, stability, and pH_PZC measurements revealed distinct differences in surface morphology, functional groups, porosity, and surface charge. Under optimized conditions, maximum adsorption capacities reached 751.0, 3.98, and 1.39 mg g⁻¹ for RB, CHP, and AZM, respectively, on HCK, and 616.0 (RB), 30.10 (CHP), and 9.15 mg g⁻¹ (AZM) on PHC, indicating that the selected hydrochars efficiently removed the investigated pollutants from water. Kinetic modeling demonstrated pseudo-first-order adsorption for RB and CHP on HCK and pseudo-second-order adsorption for AZM on HCK and all pollutants on PHC. Thermodynamic analysis confirmed that adsorption processes were spontaneous and favorable, with enhancements dependent on temperature. These findings suggest that HCK is particularly effective for cationic dyes, while PHC exhibits greater affinity toward organophosphorus pesticides, offering complementary applications and providing new mechanistic insights into hydrochar-based pollutant removal. Full article

The growing demand for sustainable soil management strategies has intensified interest in hydrochar (HC), a waste-derived amendment produced via hydrothermal carbonization (HTC). This review synthesizes recent advances in HC production, characterization, and agri-environmental applications within a waste-to-resource framework. It covers studies conducted mainly over the last decade, encompassing a wide range of feedstocks, including agricultural residues, sewage sludge, animal manures, and food waste. HTC is typically performed at 130–280 °C under autogenous pressure (2–15 MPa), generating HCs with low intrinsic surface area (<50 m²g⁻¹) and oxygen-containing functional groups that govern nutrient dynamics and soil interactions. Reported application rates vary broadly between 10 and 60 t ha⁻¹, with most experiments conducted under greenhouse conditions. Positive effects on soil pH, cation exchange capacity, water retention, and phosphorus availability are frequently observed. However, plant responses vary according to the type of stimulation promoted by HC, as well as its processing conditions, application rates, and the soil characteristics in which it is applied. Advanced molecular-level analyses (e.g., FT-ICR-MS, GC-MS, and ¹³C-NMR) have provided mechanistic insights into carbon stability, nutrient release, and interaction with soil organic matter. Reusing HTC process water offers an additional pathway for nutrient recovery, although concerns about phytotoxic compounds remain. Despite promising short-term results, long-term field evaluations and standardized assessment protocols are still limited. This review integrates structural, functional and agri-environmental perspectives to identify critical knowledge gaps and guide the optimized and context specific use of hydrochar in sustainable agricultural systems. At the same time, it emphasizes its role in advancing carbon sequestration and in operationalizing resource-circular strategies, thereby underscoring its broader practical and strategic relevance. Full article

This study explores the valorisation of apple pruning (AP) residues into sustainable carbonaceous adsorbents for dye-contaminated wastewater. Activated biochars (ABCs) were produced via single-step (ABC-1S) and two-step (ABC-2S) KOH activation, while activated hydrochar (AHTC) was obtained through hydrothermal carbonization followed by H₃PO₄ activation. The materials were comprehensively characterized using proximate analysis, FTIR spectroscopy, SEM imaging, and N₂ adsorption–desorption to evaluate surface chemistry, morphology, and textural properties. Batch adsorption experiments using MB (5–100 mg/L) demonstrated the superior performance of ABCs compared to AHTC. At low dye concentrations, adsorption on ABCs was partially influenced by external mass transfer, while kinetic data were best described by the Avrami model, indicating complex adsorption mechanisms. Isotherm analysis showed that ABC-2S exhibited heterogeneous adsorption behaviour, whereas AHTC poorly conformed to conventional isotherm models. The Langmuir model indicated higher monolayer capacities for ABCs (up to 22.9 mg/g) relative to AHTC (9.7 mg/g), reflecting a greater density of accessible adsorption sites induced by alkaline activation. Notably, nearly complete methylene blue (MB) removal was maintained over three regeneration cycles, confirming the stability, reusability, and practical potential of AP-derived ABCs and AHTC for sustainable wastewater treatment. Full article

The invasive aquatic plant Myriophyllum aquaticum represents both an ecological threat and a wet biomass disposal challenge. This study investigates hydrothermal carbonization (HTC) as a strategy for its valorisation into energy-dense hydrochar. A Design of Experiments–Response Surface Methodology (DoE-RSM) approach was applied to elucidate the combined influence of temperature (200–260 °C), residence time (30–210 min), and solid load (5–25 wt%) on hydrochar yield and properties. Hydrochar yields ranged from 48.8% to 65.6%, with the highest yields achieved at 200 °C, 30 min, and 25 wt% solids. Higher heating values of hydrochars spanned from 12.14 to 14.53 MJ/kg, corresponding up to +19% energy densification at higher process severity. Carbon and energy yields reached 69.7% and 68.6%, respectively, with maximum values attained under low-severity, high-solid-load conditions. The predictive models exhibited strong agreement with experimental data, enabling optimisation of HTC parameters for targeted hydrochar applications. Two hydrochars, “peat-like” and “lignite-like”, were further characterised for their potential use as soil amendments. The lignite-like hydrochar complied with EU contaminant limits and showed no phytotoxicity, confirming its suitability for agronomic use. Overall, HTC of M. aquaticum provides an effective waste-to-resource pathway, transforming wet invasive biomass into value-added carbon materials. Full article

The sustainable conversion of agricultural waste biomass, particularly crop residues such as corn stover, into high-value products is vital for reducing their open-field burning and mitigating environmental hazards. The hydrothermal carbonization (HTC) process integrated with natural deep eutectic solvents (NADES) presents an alternative approach for valorizing biomass into lignin-rich solid fuels and fermentable sugars for bioethanol production. In this study, corn stover was subjected to HTC using deionized (DI) water, a xylose-based NADES (ChCl:Xy:W), and an oxalic acid-based NADES (ChCl:OA:W) in a 150–300 °C temperature range to optimize both solid fuel and sugar stream yields. Characterization, including fiber analysis, SEM, FTIR, EDS, and bomb calorimetry, was conducted to evaluate structural, compositional, and energetic transformations. The results explored the HTC process, restructuring the biomass, promoting extensive hemicellulose solubilization and cellulose depolymerization, as well as substantially enriching lignin and polymerized compounds with increasing temperature. In addition, the DI water at 300 °C generated a lignin-rich residue, the Xy-based NADES effectively removed ash and extractives, and the OA-based NADES produced the most carbon-dense hydrochar with the highest calorific value. Collectively, these findings demonstrate that solvent-assisted HTC may be employed as a possible strategy for the valorization of agricultural residues into high-energy solid fuels. Full article

Organic residues from households and food-service facilities, such as orange peels, spent coffee grounds, banana peels and potato skins, represent abundant biomass resources that can release undesirable compounds during degradation. Their conversion into carbonized materials through thermochemical processes offers a sustainable route for waste valorization. In this study, residues were characterized by proximate and elemental analyses, density, porosity, and calorific value. Valorization was performed using microwave-assisted pyrolysis and two hydrothermal carbonization (HTC) routes. Pyrolysis experiments were conducted at 450, 600 and 800 W with residence times of 20–70 min. Conventional HTC was carried out at 180, 200 and 220 °C for 20 h, while autoclave HTC was performed at 134 °C for 2 and 4 h. The resulting biochars and hydrochars were evaluated for their physicochemical and energetic properties and ANOVA was applied to assess the influence of operating conditions. Conventional HTC at higher temperatures produced the highest calorific values, whereas microwave-assisted pyrolysis at 800 W provided competitive HHVs with high solid yields. Autoclave HTC enhanced solid retention and carbon preservation. Among the investigated residues, spent coffee grounds exhibited the most favorable solid-phase energetic performance. These findings demonstrate that thermochemical conversion enables the transformation of common residues into carbon-rich materials with physicochemical and energetic properties relevant for comparative assessment and future application-oriented studies. It should be noted that conventional hydrothermal carbonization experiments were conducted using pre-dried biomass, which represents a methodological limitation of the comparative assessment. Full article

This study investigates the impact of ethanol as a co-solvent in hydrothermal carbonization (HTC) of sewage sludge, a process referred to here as ethanothermal or solvothermal carbonization. Experiments were conducted at 180 °C, 200 °C, 220 °C, and 240 °C, comparing two sets of conditions: one using water (S/W) and the other using ethanol (S/E) as the reaction medium. The focus was placed on the composition of the aqueous phase, particularly the formation of volatile fatty acids (VFAs). Ethanol-assisted experiments consistently produced more alkaline process water (pH 7.6–8.2) compared to water-based runs. COD values in S/W samples ranged from 9358 mg/L to 19,756 mg/L, indicating significant organic loading. Hydrochar derived from the ethanol experiments exhibited higher energy content, with a peak high heating value (HHV) of 21.9 MJ/kg at 240 °C, compared to 19.9 MJ/kg in S/W samples. VFA concentrations were also enhanced under ethanothermal conditions, especially at lower temperatures: formic acid (30.4–34.8 mg/L), acetic acid (8.7–9.6 mg/L), and propionic acid (10.8–14.6 mg/L). These results demonstrate ethanol’s potential to enhance both the yield and quality of liquid and solid products in HTC of sewage sludge. Full article