Hydrothermal carbonization of holocellulose into hydrochar: Structural, chemical characteristics, and combustion behavior

Effective removal of Rhodamine B using the hydrothermal carbonization and citric acid modification of furfural industrial processing waste

In this study, the removal of RhB from water by furfural residue (FR) biochar was prepared by hydrothermal carbonization (HTC) and citric acid (CA) modification and named this biochar as CHFR (C refers to citric acid, H refers to hydrothermal carbonization and FR is furfural residue). The CHFR were characterized by SEM, FT-IR and XPS, and CHFR was investigated by the effects of initial concentration, adsorbent dosage, pH, and contact time on the removal of RhB, and the experimental data were analyzed using the adsorption isotherm models, the adsorption kinetic models and thermodynamics, et al. The results showed that CHFR has strong adsorption performance, and the theoretical maximum adsorption capacity of RhB was 39.46 mg·g-1 under the reaction conditions of pH3, the dosage of 1.5 g·L-1, and 120 min contact time, with a removal efficiency close to 100%. the adsorption of RhB by CHFR is spontaneous and endothermic, which is consistent with the Freundlich adsorption, and the isotherm model fits well with the pseudo-second-order model, and the adsorption rate could still be as high as 92.74% after five regenerations, therefore, CHFR is an environmentally friendly and efficient adsorbent with excellent adsorption regeneration performance.

Hydrothermal carbonization of Chinese medicine residues: Formation of humic acids and combustion performance of extracted hydrochar

Aiming at the sustainable management of high-moisture Chinese medicine residues (CMR), an alternative way integrating hydrothermal carbonization (HTC), humic acids (HAs) extraction and combustion of remained hydrochar has been proposed in this study. Effect of HTC temperature, HTC duration, and feedwater pH on the mass yield and properties of HAs was examined. The associated formation mechanism of HAs during HTC was proposed. The combustion performance of remained hydrochar after HAs extraction was evaluated. Results show that the positive correlation between hydrochar yield and HAs yield is observed. According to three-dimensional excitation emission matrix (3D EEM) fluorescence intensity, the best quality of HAs is achieved with a yield of 8.17 % at feedwater pH of 13 and HTC temperature of 200 °C. Fourier transform infrared spectroscopy and X-ray photoelectron spectroscopy analyses show abundant aromatic and aliphatic structure as well as oxygenated functional groups in HAs, which is like commercial HAs (HA-C). Besides, in terms of comprehensive combustion index (CCI), HTC can improve the combustion performance of CMR, while it becomes a bit worse after HAs extraction. Higher weighted mean apparent activation energy (Em) of hydrochar indicating its highly thermal stability. HAs extraction reduces Em and CCI of remained hydrochar. However, it can be regarded a potential renewable energy. This work confirms a more sustainable alternative way for CMR comprehensive utilization in near future.

Spent coffee ground torrefaction for waste remediation and valorization

Spent coffee grounds (SCGs) are a noticeable waste that may cause environmental pollution problems if not treated appropriately. Torrefaction is a promising low-temperature carbonization technique to achieve waste remediation, recovery, and circular bioeconomy efficiently. This study aims to maximize lipids retained in thermally degraded SCGs, thereby upgrading their fuel quality to implement resource sustainability and availability. This work also analyzes the lipid contribution to biochar's calorific value under various carbonization temperatures and times. Torrefaction can retain 11-15 wt% lipids from SCG, but the lipid content decreases when the pyrolysis temperature is higher than 300 °C. Extracted lipid content consisting of fatty acids echoed the results of diesel adsorption capacity. The lipid content in the biochar from SCG torrefied at 300 °C for 30 min is 11.00 wt%, and its HHV is 28.16 MJ kg^-1. In this biochar, lipids contribute about 14.84% of the calorific value, and the other carbonized solid contributes 85.16%. On account of the higher lipid content in the biochar, it has the highest diesel adsorption amount per unit mass, with a value of 1.66 g g^-1. This value accounts for a 22.1% improvement compared to its untorrefied SCG. Accordingly, torrefaction can sufficiently remediate SCG-derived environmental pollution. The produced biochar can become a spilled oil adsorbent. Furthermore, oil-adsorbed biochar (oilchar) is a potential solid fuel. In summary, SCG torrefaction can simultaneously achieve pollution remediation, waste valorization, resource sustainability, and circular bioeconomy.

Combustion kinetics of hydrochar from cow-manure digestate via thermogravimetric analysis and peak deconvolution

Hydrothermal carbonization (HTC) can convert wet biomass into hydrochar (HC), a solid carbonaceous material exploitable as fuel. In this study, HTC was applied to anaerobic digestate from cow manure. HCs obtained at three HTC temperatures (180, 220, 250 °C) were characterized in detail and their combustion behavior was investigated by thermogravimetric analysis (TGA) coupled with peak deconvolution. Increasing HTC temperatures increased the fixed carbon content (17.9-20.7%), the ash content (27.2-32.5%) and the calorific value (14.3-18.2 MJ/kg), while decreased the hydrogen (5.01-4.54%) and oxygen content (24.09-12.35%) of HCs. DTG profiles peak deconvolution unveils the presence of five major components in the HCs. HCs combustion kinetics were studied applying the KAS method. Average apparent activation energy values of 100, 88, 67 kJ mol^-1 were obtained for HC180, HC220, HC250, respectively. HTC at 250 °C produced the HC with the best fuel characteristics.

Anaerobic digestion of sugarcane bagasse for biogas production and digestate valorization

Sugarcane bagasse is an abundantly available agricultural waste having high potential that is still underutilized and mostly burnt as fuel. There are various processes available for bagasse utilization in improved ways and one such process is anaerobic digestion (AD) of bagasse for biogas production. The complex structure of biomass is recalcitrant to degradation and is a major hindrance for the anaerobic digestion, so different pretreatment methods are applied to deconstruct the bagasse for microbial digestion. In this review, different processes developed for the pretreatment of bagasse and their effect on biogas production have been extensively covered. Moreover, combination of pretreatment methods, co-digestion of bagasse with other waste (nitrogen rich or easily digestible) for enhanced biogas production and biomethane generation along with other value-added products has also been reviewed. The digestate contains a significant amount of organics with partial recovery of energy and products and is generated in huge amount that further creates disposal problem. Therefore, integration of digestate valorization with AD through gasification, pyrolysis, hydrothermal carbonization and use of microalgae for maximum recovery of energy and value-added products have also been evaluated. Thus, this review highlights major emerging area of research for improvement in bagasse based processes for enhanced biogas production along with digestate valorization to make the overall process economical and sustainable.

Numerical Simulation Study on the Effects of Co-Injection of Pulverized Coal and Hydrochar into the Blast Furnace

To solve the energy crisis and slow down the greenhouse effect, it is urgent to find alternative energy sources for the iron and steel production process. Hydrochar is an auxiliary fuel and the only renewable carbon source that could reduce the injection of bituminous coal into the blast furnace. Numerical simulation is an effective method of understanding the combustion performance in the lower part of the blast furnace. A 3D blowpipe-tuyere-raceway model was established using the computational fluid dynamics (CFD) method to study the effects on combustion performance between pulverized coal and hydrochar. The results showed that co-injection of anthracite and hydrochar has a better combustion performance than co-injection of anthracite and bituminous coal, with a more appropriate distribution of temperature, velocity, and gas phase. With the co-injection of hydrochar, the total burnout rate and anthracite burnout rate increased, respectively, by 6% and 2.1%, which is caused by the interaction mechanism between anthracite and hydrochar. As a result, hydrochar as an auxiliary fuel for blast furnace injection not only can achieve low-carbon production and cut down carbon emission but also benefit the combustion process of anthracite coal.

A Sustainable Approach on Spruce Bark Waste Valorization through Hydrothermal Conversion

In the context of sustainable use of resources, hydrothermal conversion of biomass has received increased consideration. As well, the hydrochar (the solid C-rich phase that occurs after the process) has caused great interest. In this work, spruce bark (Picea abies) wastes were considered as feedstock and the influence of hydrothermal process parameters (temperature, reaction time, and biomass to water ratio) on the conversion degree has been studied. Using the response surface methodology and MiniTab software, the process parameters were set up and showed that temperature was the significant factor influencing the conversion, while residence time and the solid-to-liquid ratio had a low influence. Furthermore, the chemical (proximate and ultimate analysis), structural (Fourier-transform infrared spectroscopy, scanning electron microscopy) and thermal properties (thermogravimetric analysis) of feedstock and hydrochar were analyzed. Hydrochar obtained at 280 °C, 1 h processing time, and 1/5 solid-to-liquid ratio presented a hydrophobic character, numerous functional groups, a lower O and H content, and an improved C matter, as well as a good thermal stability. Alongside the structural features, these characteristics endorsed this waste-based product for applications other than those already known as a heat source.

Influence of Reaction Conditions on Hydrothermal Carbonization of Fructose

Hydrothermal carbonization is a powerful way to convert cellulosic waste into valuable platform chemicals and carbonaceous materials. In this study, to optimize the process, fructose was chosen as the carbon precursor and the influence of reaction time, acid catalysis, feed gas and pressure on the conversion products is evaluated. 5-hydroxymethylfurfural (HMF) is produced in high amounts in relatively short time. Both strong and weak acids accelerate fructose conversion. Levulinic acid (LevA) formation is faster than that of hydrothermal (HT) carbon in acidic conditions. Strong acid catalysts should be considered to target preferentially LevA production, whereas milder conditions should be preferred for HMF production. Moreover, a slight initial overpressure of the reactor is always beneficial in terms of conversion. FT-IR and ¹³ C ss-NMR spectroscopy and SEM showed that HT carbon evolves through time from a furanic-based structure with alkylic linkers to an increasingly cross-linked condensed structure. MALDI-ToF mass spectrometry showed the existence of a series of oligomers in a mass range within 650 Da and 1500 Da formed by condensation of repeating units.

CO2 capture by adsorption on biomass-derived activated char: A review

Carbon capture and storage has been recognized as the most promising method for CO₂ control. Among the many sorbents, char derived from pyrolysis and hydrothermal carbonization (HTC) of biomass have demonstrated excellent CO₂ adsorption capability. This paper reviews the different parameters to produce a higher yield of biochar and hydrochar suitable for carbon sequestration. The mechanism of physisorption and chemisorption is briefly presented. The different kinetic models, diffusion models to describe adsorption mechanism, and adsorption isotherms for CO₂ uptake from biomass-derived hydrochar are reviewed. The different factors that affect the CO₂ uptake are the type of activation, surface area and porosity, the ratio of activation agent to char, activation temperature, adsorption pressure and temperature, additives, and other physicochemical properties. The optimal conditions for CO₂ uptake with chemical activation of KOH is a KOH/char ratio of 2-3, activation temperature of 700 °C, and an adsorption temperature below 50 °C.

Enhancement of hydrothermal carbonization of chitin by combined pretreatment of mechanical activation and FeCl3

In this work, a combined mechanical activation and FeCl₃ (MA + FeCl₃) method was applied to pretreat chitin to enhance the degree of hydrothermal carbonization. MA + FeCl₃ pretreatment significantly disrupt the crystalline region of chitin and Fe³⁺ entered into the molecular chain, resulting in the destruction of the stable structure of chitin. The chemical and structural properties of hydrochars were characterized by EA, SEM, FTIR, XRD, XPS, ¹³C solid state NMR, and N₂ adsorption-desorption analyses. The results showed that the H/C and O/C atomic ratios of HC-MAFCT/230 (the hydrochar derived from MA + FeCl₃ pretreated chitin with hydrothermal reaction temperature of 230 °C) were 0.96 and 0.34, respectively. Van Krevelen diagram indicated that the hydrothermal carbonization of chitin underwent a series of reactions such as dehydration, decarboxylation, and aromatization. HC-MAFCT/230 had abundant oxygen- and nitrogen-containing functional groups. HC-MAFCT/230 exhibited a porous structure, with the specific surface area of 128 m² g^-1, which was a promising carbon material.

A critical review on the application of biochar in environmental pollution remediation: Role of persistent free radicals (PFRs)

Biochar as an emerging carbonaceous material has exhibited a great potential in environmental application for its perfect adsorption ability. However, there are abundant persistent free radicals (PFRs) in biochar, so the direct and indirect PFRs-mediated removal of organic and inorganic contaminants by biochar was widely reported. In order to comprehend deeply the formation of PFRs in biochar and their interactions with contaminants, this paper reviews the formation mechanisms of PFRs in biochar and the PFRs-mediated environmental applications of biochar in recent years. Finally, future challenges in this field are also proposed. This review provides a more comprehensive understanding on the emerging applications of biochar from the viewpoint of the catalytic role of PFRs.

The effect of organic solvent washing on the structure of hydrochar-based dissolved organic matters and its potential environmental toxicity

With the increased interest in the practical use of hydrochar, concerns about the possible environmental biotoxicity of hydrochar and its released dissolved organic matters (DOM) have grown. As a common method for removing bio-oil on the surface of hydrochar, the effect of organic solvent washing on the properties of hydrochar released DOM remains unclear. In this study, we made a comprehensive comparison of hydrochar properties and molecule structure as well as biotoxicity of DOM released from HC (raw hydrochar) and THC (hydrochar washed by tetrahydrofuran). The results indicated that the mass loss of hydrochar was obvious after tetrahydrofuran (THF) washing, and a decline of H/C atomic ratio and increase of N/C and O/C atomic ratios was observed based on Van Krevelen (VK) diagram. This result was further confirmed by FTIR, ¹³C NMR, and XPS results. Meanwhile, the molecule structure of DOM was shifted to lower molecule weight with higher O-contain compounds after THF extraction due to the demethanation process. However, the biotoxicity experiments indicated that both extracted DOM had no significant impact on germination rate of wheat, and HC-treated sample even exhibited growth superiority. Nevertheless, potential toxicity was observed with the increase of the activity of antioxidant enzymes, and THF washing aggravated the potential oxidative damage through increasing the aromaticity of DOM. Such understanding highlights the importance of evaluating hydrochar and its released DOM before applications, so as to reduce the potential environment biotoxicity.

Greener Solution to Waste Corn Stalks and Shortage of Asphalt Resource: Hydrochar Produced by Hydrothermal Carbonization as a Novel Performance Enhancer for Asphalt Binder

Utilization of waste corn stalks (CS) has seized extensive attention due to high annual output and hazardous impact of piling aside or direct combustion on environment. However, previously there has been a lot of emphasis on improvement of its energy efficiency as solid fuel while limited investigations are available which explore the possibility of applying corn stalks as performance enhancer in asphalt binder. The purpose of this study is to examine the potential of employing hydrochar as modifiers in asphalt binder by a series of experimental tests. In this study, two hydrochar were produced from corn stalks by a novel process called hydrothermal carbonization at a different reaction temperature. The two hydrochar and their responding hydrochar-modified asphalt (HCMA) were tested by chemical and rheological tests. Chemical analysis detected the interaction between hydrochar and binder factions, resulting in poor compatibility but satisfying anti-aging property. Even though hydrochar increased the viscosity of bitumen, implying worse workability, and caused poor storage stability, ameliorated performance of asphalt binder at high temperature by incorporating hydrochar was verified by various criteria such as higher performance grade (PG) failure temperature and lower non-recoverable creep compliance (J_nr). Moreover, higher reaction temperature makes hydrochar’s particles smaller and more homogeneous, which results in slightly lower enhanced high temperature performance, more satisfying workability, better storage stability, and greater anti-aging effect of hydrochar-modified asphalt. Eventually, this study provided a promising win-win solution to environment problems concerning corn stalk treatment and shortage of asphalt binder. Further exploration of methods to improve HCMA’s storage stability, real-scale corroboration on trial section and life cycle assessment of asphalt pavement containing hydrochar modifiers will be followed in the future.

Hydrochar from corn stalk used as bio-asphalt modifier: High-temperature performance improvement

Biomass utilization, even for conversion products like hydrochar or biochar, has an increasing demand because improper disposal can cause intensive pollution. In this study, hydrochar obtained by hydrothermal treatment of corn stalk was added to virgin asphalt as a novel modifier by manual stirring and high-speed shearing. This hydrochar-modified asphalt (HCMA) showed a better high-temperature performance compared to unmodified asphalt, and the optimized dosage was 6 wt% with Rutting Index reaching 76 °C, and its penetration and softening point reaching 31.70 (0.1 mm) and 54.70 °C, respectively. The macroscopic representation of modified asphalt was conducted by microscopic characterization methods such as Fourier Transform Infrared Spectroscopy (FTIR) and Gel Permeation Chromatography (GPC). It was demonstrated that the performance was improved by the good blending state between hydrochar and asphalt. The application of hydrochar in modifying asphalt can reduce pollution and enhance its high-temperature performance, which has a potentially extensive application prospect in pavement engineering in subtropical and tropical climate.

Hydrothermal carbonization of food waste after oil extraction pre-treatment: Study on hydrochar fuel characteristics, combustion behavior, and removal behavior of sodium and potassium

This study aims to convert oil extracted food waste (OEFW) into hydrochar as potential solid fuel via hydrothermal carbonization (HTC) process. The effect of HTC temperature and residence time on the physicochemical characteristic, combustion behavior, and the removal behavior of sodium and potassium were evaluated. The raw OEFW material was successfully converted into energy densified hydrochar with higher high heating value (HHV) (21.13-24.07 MJ/kg) and higher fuel ratio (0.112-0.146). In addition, carbon content in hydrochar increased to 46.92-51.82% after HTC at various operating conditions. Compared with OEFW, the hydrochar had more stable and longer combustion process with the higher ignition temperature and burnout temperature. Besides, the HTC process showed high removal rates of sodium and potassium. It was found that the HTC temperature resulted in a significant reduction of sodium and potassium in hydrochar as compared to the residence time. The highest removal rate of sodium (70.98%) and potassium (84.05%) was obtained. Overall, the results show that the HTC is a promising alternative for conventional technologies (e.g., incineration and landfill) for treatment and energy conversion of OEFW.

Hydrothermal Carbonization of Olive Tree Pruning as a Sustainable Way for Improving Biomass Energy Potential: Effect of Reaction Parameters on Fuel Properties

Hydrothermal carbonization (HTC) allows the conversion of organic waste into a solid product called hydrochar with improved fuel properties. Olive tree pruning biomass (OTP), a very abundant residue in Mediterranean countries, was treated by HTC to obtain a solid fuel similar to coal that could be used in co-combustion processes. Three different reaction temperatures (220, 250, and 280 °C) and reaction times (3, 6, and 9 h) were selected. The hydrochars obtained were extensively analyzed to study their behavior as fuel (i.e., ultimate, proximate, fiber and thermogravimetric analysis, Fourier-transform infrared spectroscopy (FTIR), activation energy, and combustion performance). The concentrations of cellulose, hemicellulose, and lignin in the samples depict a clear and consistent trend with the chemical reactions carried out in this treatment. Regarding O/C and H/C ratios and HHV, the hydrochars generated at more severe conditions are similar to lignite coal, reaching values of HHV up to 29.6 MJ kg−1. The higher stability of the solid is reflected by the increase of the activation energy (≈60 kJ mol−1), and ignition temperatures close to 400 °C. With this, HTC is a proper thermal treatment for the management of raw OTP biomass and its further conversion into a solid biofuel.

Activated hydrochar produced from brewer's spent grain and its application in the removal of acetaminophen

Acetaminophen has shown a gradual increase in detection in surface waters. Although present in low concentrations, it should be removed to prevent deleterious effects. Thus, adsorption onto activated carbon is emphasized. Adsorbents may be produced by hydrothermal carbonization (HTC), an environmental-friendly process. Therefore, this work aimed to investigate the use of HTC, verifying its application in acetaminophen removal. Brewer's spent grain (BSG), its hydrochar (HC-BSG) and its activated hydrochar (AHC-BSG) were characterized. HTC provided material with high carbon content. Lignocellulosic breakdown has been demonstrated in HC-BSG and AHC-BSG, but in the latter it was more intense as a result of activation with KOH. Also, a high surface area was found in AHC-BSG (1512.83 m² g^-1), resulting in an adsorption of 318.00 mg g^-1. The pseudo-second-order and Langmuir models were fitted to the experimental data. Therefore, HTC was effective as a pretreatment for AHC-BSG, resulting in significant acetaminophen removals.

Characterization and sulfonamide antibiotics adsorption capacity of spent coffee grounds based biochar and hydrochar

A large amount of spent coffee grounds is produced as a processing waste each year during making the coffee beverage. Sulfonamide antibiotics (SAs) are frequently detected in the environment and cause pollution problems. In this study, biochar (BC) and hydrochar (HC) were derived from spent coffee grounds through pyrolysis and hydrothermal carbonization, respectively. Their characteristics and sulfonamide antibiotics adsorption were investigated and compared with reference to adsorption capacity, adsorption isotherm and kinetics. Results showed BC possessed more carbonization and less oxygen-containing functional groups than HC when checked by Elemental Analysis, X-ray diffraction, X-ray photoelectron spectrometry and Fourier transform infrared. These groups affected the adsorption of sulfonamide antibiotics and adsorption mechanism. The maximum adsorption capacities of BC for sulfadiazine (SDZ) and sulfamethoxazole (SMX) were 121.5 μg/g and 130.1 μg/g at 25 °C with the initial antibiotic concentration of 500 μg/L, respectively. Meanwhile the maximum adsorption capacities of HC were 82.2 μg/g and 85.7 μg/g, respectively. Moreover, the adsorption mechanism for SAs adsorbed onto BC may be dominated by π-π electron donor-acceptor interactions, yet the SAs adsorption to HC may be attributed to hydrogen bonds. Further analysis of the adsorption isotherms and kinetics, found that physical and chemical interactions were involved in the SAs adsorption onto BC and HC. Overall, results suggested that: firstly, pyrolysis was an effective thermochemical conversion of spent coffee grounds; and secondly, BC was the more promising adsorbent for removing sulfonamide antibiotics.

Multianalytical characterization of biochar and hydrochar produced from waste biomasses for environmental and agricultural applications

Biochar (BC) and hydrochar (HC) are solid by-products obtained from various types of biomasses through the processes of pyrolysis and hydrothermal carbonization, respectively. Both BC and HC represent a sustainable solution for carbon sequestration and can be used as soil amendments or sorbents for organic and inorganic pollutants. However, the properties of BC and HC largely depend on feedstock and production parameters, which significantly affect their proper use. A detailed characterization of these materials is therefore needed to assess their suitability for environmental and/or agricultural applications. In this work, two BC samples and two HC samples were characterized with a multianalytical approach, including total reflection X-ray fluorescence (TXRF) spectroscopy, scanning electron microscopy (SEM), Fourier-transform infrared (FT-IR) spectroscopy, thermogravimetric analyses (TG), and pyrolysis coupled to gas chromatography and mass spectrometry (Py-GC/MS). By comparing BC and HC data, HC showed a higher content of mineral elements, including important plant nutrients and potentially toxic elements. HC produced from solid urban wastes contained also some potentially toxic organic molecules, like chlorinated aromatic compounds. BC samples were characterized by a higher porosity and hydrophobicity than HC, thus being potentially more suitable for the sorption of organic pollutants. HC samples showed a higher content of cellulose and hemicellulose, resulting in a more hydrophilic but less thermally stable material than BC. In conclusion, both BC and HC have interesting properties for environmental and agricultural applications but careful selection of feedstock is needed, especially for HC production.

Dye Adsorption and Electrical Property of Oxide-Loaded Carbon Fiber Made by Electrospinning and Hydrothermal Treatment

Our current study deals with the dye adsorption and electrical property of a partially carbonized composite fiber containing transition metal oxides including, iron oxide, nickel oxide, and titanium oxide. The fiber was made by electrospinning, carbonization, and hydrothermal treatment. During the electrospinning, titanium oxide particles were dispersed in polyacrylonitrile (PAN) polymer-dimethylformamide (DMF) solution. Nickel chloride and iron nitrate were added into the solution to generate nickel oxide and iron oxide in the subsequent heat treatment processes. The polymer fiber was oxidized first at an elevated temperature of 250 °C to stabilize the structure of PAN. Then, we performed higher temperature heat treatment at 500 °C in a furnace with hydrogen gas protection to partially carbonize the polymer fiber. After that, the oxide-containing fiber was coated with activated carbon in a diluted sugar solution via hydrothermal carbonization at 200 °C for 8 h. The pressure reached 1.45 MPa in the reaction chamber. The obtained product was tested in view of the dye, Rhodamine B, adsorption using a Vis-UV spectrometer. Electrical property characterization was performed using an electrochemical work station. It was found that the hydrothermally treated oxide-containing fiber demonstrated obvious dye adsorption behavior. The visible light absorption intensity of the Rhodamine B dye decreased with the increase in the soaking time of the fiber in the dye solution. The impedance of the fiber was increased due to the hydrothermal carbonization treatment. We also found that charge build-up was faster at the surface of the specimen without the hydrothermally treated carbon layer. Electricity generation under visible light excitation is more intensive at the hydrothermally treated fiber than at the one without the hydrothermal treatment. This result is consistent with that obtained from the dye adsorption/decomposition test because the charge generation is more efficient at the surface of the hydrothermally treated fiber, which allows the dye to be decomposed faster by the treated fibers with activated carbon.

Formation, characteristics, and applications of environmentally persistent free radicals in biochars: A review

Due to abundant biomass and eco-friendliness, biochar is exemplified as one of the most promising candidates to mediate the degradation of environmental contaminants. Recently, environmentally persistent free radicals (EPFRs) have been detected in biochars, which can activate S₂O₈^2- or H₂O₂ to generate reactive oxygen species for effective degradation of organic and inorganic contaminants. Comprehending the formation mechanisms of EPFRs in biochars and their interactions with contaminants is indispensable to further develop their environmental applications, e.g., direct and indirect EPFR-mediated removal of organics/inorganics by biochars. With reference to the information of EPFRs in environmental matrices, this article critically reviews the formation mechanisms, characteristics, interactions, and environmental applications of EPFRs in biochars. Synthesis conditions and loading of metals/organics are considered as key parameters controlling their concentrations, types, and activities. This review provides new and important insights into the fate and emerging applications of surface-bound EPFRs in biochars.

Efficient removal of several estrogens in water by Fe-hydrochar composite and related interactive effect mechanism of H2O2 and iron with persistent free radicals from hydrochar of pinewood

Recently, hydrochar (HC) with existed persistent free radicals (PFRs) has attracted researches' attention for the potential application in heterogeneous Fenton-like reactions, but studies on the interactive effects of H₂O₂, iron, and HC in removal of organic pollutants are still limited. In this paper, magnetic iron (hydr)oxides immobilized hydrochar composite (Fe/HC) derived from hydrothermal carbon (HTC) of pinewood were synthesized and characterized. The interactive effects of H₂O₂, iron, and HC in the removal of several estrogens were systematically investigated to understand the removal performance and related mechanism, especially at a pH range close to natural water environment. Batch experiments results showed that estrogens could be efficiently removed over Fe/HC material under a wide pH range of 4-9. Based on the analysis of electron spin resonance, X-ray photoelectron spectroscopy, Mössbauer spectroscopy, and electrochemical impedance spectroscopy, mechanism study indicated that the carbon-centered PFRs on the surface of hydrochar can act as electron donors, and transfer the electrons on adsorbed O₂ to generate O₂^- rapidly, while the addition of H₂O₂ enhanced the transmission ability of electron to produce OH_(ads) on the material surface. The iron and hydrochar components contributed to the desirable removal of estrogens via the synergistic effect between catalysis and adsorption. This study provides a promising application for the use of Fe/HC materials on remediation of pollution with trace estrogens in water environment.

Effect of pretreatment on chemical characteristic and thermal degradation behavior of corn stalk digestate: Comparison of dry and wet torrefaction

In this study, the dry torrefaction (DT) and wet torrefaction (WT) were compared at different temperatures to explore the effect of these pretreatments on the characteristic of corn stalk digestate (CSD) and the thermal degradation behaviors. The results indicated the both torrefactions improved the fuel properties of CSD, including the lower volatiles content, higher carbon content and HHV. The WT showed higher removals of organics and alkali metals, while DT retained more carbon and ash. The SEM, FTIR and XRD analyses indicated the existence of organic coverings of WT samples, and showed richer surface functional groups, relative complete lignin structure and higher crystallinity compared to DT samples. The thermogravimetric analysis displayed the torrefied temperature above 260 °C (DT) or 220 °C (WT) was not suitable for CSD pyrolysis. Besides, the WT samples showed the more concentrated combustion range, while DT tends the burning of CSD to the behavior of coal.

Effects of aqueous phase recirculation in hydrothermal carbonization of sweet potato waste

Aqueous phase recirculation was investigated in hydrothermal carbonization of sweet potato waste at 220 °C for 60 min. The result showed that the aqueous phase reuse significantly increased the hydrochar yield. The lower H/C and O/C ratios indicated that decarboxylation reaction was promoted. The CC vibration of the benzene backbone became intense, suggesting the occurrence of aromatization and polymerization reactions. Thus, the carbon content and HHV were improved. After recirculation, hydrochar showed a decrease in combustion ignition temperature whereas an increase in pyrolysis initial decomposition temperature. The burnout temperatures in combustion and terminated temperature in pyrolysis both showed an increase trend. The hydrochars obtained from the recirculation step possessed lower emissions of NO_X or SO₂ than that from reference step. The pyrolysis emission result showed that more high thermal stability components were formed during recirculation step. Overall, aqueous phase recirculation was a feasible way to improve hydrothermal carbonization process.