Microwave-assisted Hydrothermal Carbonization Research Articles

Negatively charged hydrochar from Aesculus indica waste: A sustainable material for enhanced Li-S batteries performance

This study presents a negatively charged hydrochar from Aesculus indica waste, employing a microwave-assisted hydrothermal carbonization (HTC) method at very low temperatures (120∼200 °C). The research shows the utilization of these negatively-charged-oxygen-containing hydrochars in lithium-sulfur (Li-S) batteries effectively mitigates polysulfide shuttling through electrostatic repulsive forces and simultaneously facilitate uniform lithium-ion transportation, leading to substantial improvement in battery performance. Through detailed investigations into the HTC reaction mechanism, the concentration of the negatively charged oxygen functional groups is tunable by controlling the carbonization process, which could further improve the enhancement effects. Furthermore, negatively-charged-oxygen-containing hydrochars were utilized as modified separator in Li-S batteries to enhance the electrochemical performance. The batteries assembled with hydrochar modified separator, has an initial discharge capacity of 1216 mAh g−1 at 0.1 C, and after 200 cycles at 1 C, its reversible discharge capacity was still 800 mAh g−1 with more than 80 % capacity retention. This research not only provides an efficient method for the utilization of waste biomass, but also offers a new approach for the preparation of negatively charged carbon for lithium-sulfur batteries.

Comparison of porous carbon prepared by conventional and microwave-assisted hydrothermal carbonization and its application in supercapacitors

Comparison of porous carbon prepared by conventional and microwave-assisted hydrothermal carbonization and its application in supercapacitors

Rare Earth Element Recovery and Hydrochar Evaluation from Hyperaccumulator by Acid Leaching and Microwave-Assisted Hydrothermal Carbonization

Phytomining is a sustainable approach that uses hyperaccumulators for critical element extraction from various substrates, such as contaminated soils, mine tailings, and aqueous solutions. In this study, grass seeds were fed with a solution containing Y, La, Ce, and Dy, resulting in around 510 mg/kg (dry basis) of total rare earth elements (TREEs) accumulated in grass leaves. Electron probe microanalyzer (EPMA) analysis showed that rare earth elements (REEs) in the grass leaves (GL) predominantly complexed with phosphorous (P). Around 95% of Y, 93% of La, 92% of Ce, and 93% of Dy were extracted from the GL using 0.5 mol/L H2SO4 at a solid concentration of 5 wt.%. Subsequently, microwave-assisted hydrothermal carbonization (MHTC) was used to convert the leaching residue into hydrochar to achieve a comprehensive utilization of GL biomass. The effect of temperature on the structural properties and chemical composition of the resulting hydrochar was evaluated. Scanning electron microscopy (SEM) analysis revealed that the original structure of GL was destroyed at 180 °C during MHTC, producing numerous microspheres and pores. As the reaction temperature increased, there was a concurrent increase in carbon content, a higher heating value (HHV), and energy densification, coupled with a decrease in the hydrogen and oxygen contents of hydrochar. The evolution of H/C and O/C ratios indicated that dehydration and decarboxylation occurred during MHTC. The results showed that the waste biomass of the GL after REE extraction can be effectively converted into energy-rich solid fuel and low-cost adsorbents via MHTC.

Nano graphene oxide creates a fully biobased 3D-printed membrane with high-flux and anti-fouling oil/water separation performance

Facile fabrication of green and renewable bio-based membranes with good anti-fouling and oil/water separation performance is of great importance to solve the oil spills and industrial oily wastewater threatening the ecological environment. Here, a fully biobased oil/water separation membrane with an ordered porous structure was 3D printed ultraviolet-assisted direct ink writing. The components of the bio-ink were obtained by methacrylation of chitosan (CS) and gelatin (GEL) to synthesize methacrylated chitosan (CSMA) and methacrylated gelatin (GELMA), while the nanographene oxide (nGO) was derived from CS through a simple microwave-assisted hydrothermal carbonization followed by oxidation step. The addition of nGO boosted the printability of the bio-ink, and the anti-fouling property and water permeation flux of the printed membranes. As a result, the membrane M−CSMA/GELMA/nGO-0.7 with the optimal performance possessed a low water contact angle in air of 0°, and high underwater oil contact angle of 161.5°, demonstrating a combination of superhydrophilic and underwater superoleophobic properties. M−CSMA/GELMA/nGO-0.7 has good corrosion resistance and long service life as evidenced from the separation efficiency of n-heptane/water, which kept above 99.5 % and a high water permeation flux above 38,300 L m−2h−1 after 20 cyclic tests in the harsh aquatic conditions containing 1 M NaCl, 1 M HCl, or 1 M NaOH, respectively. This shows promising potential for real-life applications.

Microwave-Assisted hydrothermal carbonization and characterization of Amazonian biomass as an activated carbon for methane adsorption

Activated carbons were prepared from Amazonian biomass by means of microwave-assisted hydrothermal carbonization and activated with CO2. These materials were investigated as methane adsorption sorbents. The prepared samples were submitted to textural investigations and comprehensive characterizations using scanning electron microscopy, Transmission electron microscopy, energy dispersive X-ray diffraction, Raman and FTIR spectroscopy. Values of specific surface areas and pore volume ranged from 0.35 cm3/g to 0.50 cm3/g and from 840 to 1096 m2/g respectively, this value is likely attributed to the high selectivity of CO2/CH4 and methane adsorption by combining large surface area, narrow micropores. The results of the stability tests attested to the robustness of activated carbon as an adsorbent under the specified operating conditions The assessment, which included measurements after the first cycle as well as the fifth, tenth, and fifteenth cycles, revealed a remarkable consistency in its adsorption behavior, with a maximum recorded standard deviation of 0.05. Isosteric heat showed a decline in Qst from 21 to 20 as surface coverage increases from 0.1 to 0.5, indicating the energetic transformation occurring during the adsorption process. Activated carbon AC C shows the highest methane capture capacity of 150 V/V at 40 bar pressures and CH4/CO2 0.49 selectivity up to 1 bar.

Methylene Blue Dye Adsorption on Iron Oxide-Hydrochar Composite Synthesized via a Facile Microwave-Assisted Hydrothermal Carbonization of Pomegranate Peels' Waste.

The toxicity of dyes has a long-lasting negative impact on aquatic life. Adsorption is an inexpensive, simple, and straightforward technique for eliminating pollutants. One of the challenges facing adsorption is that it is hard to collect the adsorbents after the adsorption. Adding a magnetic property to the adsorbents makes it easier to collect the adsorbents. The current work reports the synthesis of an iron oxide-hydrochar composite (FHC) and an iron oxide-activated hydrochar composite (FAC) through the microwave-assisted hydrothermal carbonization (MHC) technique, which is known as a timesaving and energy-efficient method. The synthesized composites were characterized using various techniques, such as FT-IR, XRD, SEM, TEM, and N2 isotherm. The prepared composites were applied in the adsorption of cationic methylene blue dye (MB). The composites were formed of crystalline iron oxide and amorphous hydrochar, with a porous structure for the hydrochar and a rod-like structure for the iron oxide. The pH of the point of zero charge (pHpzc) of the iron oxide-hydrochar composite and the iron oxide-activated hydrochar composite were 5.3 and 5.6, respectively. Approximately 556 mg and 50 mg of MB dye was adsorbed on the surface of 1 g of the FHC and FAC, respectively, according to the maximum adsorption capacity calculated using the Langmuir model.

Recycling Pomelo Peel Waste in the Form of Hydrochar Obtained by Microwave-Assisted Hydrothermal Carbonization.

Pomelo peel (PP) is a kind of solid waste that is produced in the processing industry of honey pomelo. This study deeply explored the feasibility of recycling PP in the form of hydrochar (HC) by microwave-assisted hydrothermal carbonization (HTC) technology. Under the non-catalytic reaction conditions, the yield of hydrochar initially increased with the rise of reaction temperature (150-210 °C) until it remained relatively stable after 210 °C. Under the CaO-catalytical reaction condition, the yield of hydrochar did not change much at first (150-190 °C) but decreased significantly when the reaction temperature exceeded 190 °C. After the microwave-assisted HTC treatment, the PP-derived HC presented higher aromaticity, carbonization degree, porosity, and caloric value. Compared with raw PP, the nutrients in HC were more stable (conducive to being used as slow-release fertilizer). The application of CaO increased the pH value of HC and effectively promoted the accumulation of phosphorus in HC. The HC produced at 210 °C without any catalyst possessing a high devolatilization ability. Additionally, the HC obtained at 190 °C with CaO as the catalyst presented a high combustion property. In general, PP-derived HC showed great application potential in the field of soil remediation/improvement and solid fuels. This preliminary study would undoubtedly provide some important fundamental understanding of the microwave-assisted HTC of PP.

Optimization of microwave-assisted hydrothermal carbonization and potassium bicarbonate activation on the structure and electrochemical characteristics of crop straw-derived biochar

Conventional pyrolysis biochar generally has underdeveloped pore structure, low surface activity and poor electrochemical performance, which seriously limit its application in supercapacitors. Herein, biochar was fabricated through KHCO3 activation coupling microwave-assisted phosphoric acid hydrothermal carbonization (MPHTC). The results revealed that both MPHTC and KHCO3 activation can further improve the physicochemical structure and electrochemical characteristics of crop straw-derived biochar. Biochar obtained by KHCO3 activation coupling MPHTC (HBCK) exhibited the highest surface oxygen content and the most adsorbed O2/H2O and aromatic CO, as well as a larger proportion of defect structures. Moreover, CS-HBCK displayed advantageous pore structure, with the highest specific surface area (1058.36 cm2/g), the largest total pore volume (0.79 cm3/g), and a superior micropore proportion of 59.21 %. Hence, CS-HBCK exhibited the highest specific capacitance (317.01 F/g) with better rate performance and lower resistance, whose capacity retention rate could reach 87.33 % after 10,000 cycles of charge-discharge. Furthermore, the CS-HBCK symmetric capacitor could achieve an excellent energy density of 14.82 Wh/kg at the power density of 225 W/kg, whose coulombic efficiency could still be maintained at a high level (98.38 %) after 10,000 cycles of charge-discharge, indicating a great potential of recycled waste crop straw in energy storage systems.

Microwave-Assisted Hydrothermal Carbonization of Pomegranate Peels into Hydrochar for Environmental Applications

Several studies have reported that the hydrothermal carbonization method (HTC) of agricultural waste is able to produce a solid residue with interesting properties for the adsorption of organic pollutants from contaminated water. This work represents a facile method to prepare hydrochar (HC) from pomegranate peels’ waste using the microwave-assisted hydrothermal carbonization method (MHTC) at 200 °C for 1 h with a mass ratio of peel to water = 1:10. Activated hydrochar (AHC) was prepared by in situ chemical activation using ZnCl2 and MHTC. Several techniques have been applied to characterize the prepared samples as FTIR, XRD, TEM and SEM. The samples were investigated for their possible use as adsorbents of methylene blue (MB) dye. The results confirm the formation of amorphous hydrochar with a porous structure. The pH of zero point charge (pHzpc) is 4.3 and 4.6 for HC and AHC samples, respectively. The maximum adsorption capacity of HC and AHC samples are 194.9 and 12.55 mg/g (i.e., mg of adsorbate/g of adsorbent), respectively.

Preparation of rice husk hydrochar as an atrazine adsorbent: Optimization, characterization, and adsorption mechanisms

This study aims to develop rice husk hydrochar as an alternative adsorbent for application in atrazine removal. Hydrochar was obtained by microwave-assisted hydrothermal carbonization under different conditions: temperature (150 – 200°C), residence time (20 – 60 min), and liquid to solid ratio (5:1 – 15:1 mL g-1). Surface activation of hydrochar was conducted using various concentrations of KOH and H2O2. The results indicated that hydrochar prepared at higher temperatures, lower liquid to solid ratios, and longer residence times had significantly higher atrazine adsorption capacities. Atrazine adsorption by pristine and activated hydrochars could be best fitted to the pseudo-second-order and Freundlich models. The adsorption capacity of KOH-activated hydrochar was the highest (4.06 mg g-1), which is even greater than that of biochar with higher surface areas. The results of FTIR and XPS demonstrated that activated rice husk hydrochars contained abundant oxygen functional groups, which played a dominant role in atrazine adsorption via hydrogen-bonding interaction.

Co-production of hydrochar, levulinic acid and value-added chemicals by microwave-assisted hydrothermal carbonization of seaweed

Seaweed is considered a third-generation biomass for transport biofuel production with great potential for bioenergy and biorefinery applications. Microwave-assisted hydrothermal carbonization (HTC) offers a conversion route for seaweed valorization to multiple value-added products. However, the HTC products (such as hydrochar and process liquor) are highly dependent on feedstock properties and HTC conditions. This study investigated the valorization of Laminaria digitata to a range of products (primarily including hydrochar, levulinic acid and 5-hydroxymethylfurfural) through microwave-assisted HTC at a range of low temperatures (160, 180 and 200 °C) and levels of dilute acid addition (1, 2 and 4% H2SO4 concentration). The work suggests an optimized process at a temperature of 200 °C with 4% H2SO4, with levulinic acid and 5-hydroxymethylfurfural in the process liquor achieved the highest yields of 12.5% (12.5 g per 100 g dry L. digitata) and 5.8%, respectively. The obtained hydrochar had a higher heating value of 25.5 MJ/kg volatile solids, comparable to lignite coal. At such a condition the total carbon recovery in the produced hydrochar and process liquor accounted for 84% of carbon in the original seaweed. The 200 °C derived hydrochar presented the highest carbon content of 65.4%, but the lowest abundance in oxygen-containing functional groups compared to the hydrochars obtained under other conditions (160 and 180 °C). The economic benefits of the process might be significantly increased if hydrochar was upgraded into activated carbon instead of being used as a solid fuel.

Nitrogen migration in products during the microwave-assisted hydrothermal carbonization of spirulina platensis

Nitrogen has a vital influence on the properties of the microwave-assisted hydrothermal carbonization (MHTC) products of Spirulina platensis (SP). The effects of hydrothermal temperature (140–220 °C) and time (1–4 h) on the product distribution and nitrogen migration of SP in MHTC were studied. Increasing temperature led to an increase in the carbon content, and a decrease in the nitrogen content in hydrochar. Protein-N was the major nitrogen-containing species in hydrochar. The total nitrogen in liquid phase increased significantly with increasing temperature. Carbon dots were found to be one of the valuable products in the liquid phase. Higher temperatures improved the amine-N level and reduced the quaternary-N content in carbon dots. A close correspondence was found between the N-containing species and the luminescence centers of carbon dots. A possible nitrogen migration mechanism was proposed to provide guidance for the potential application of the products.

Pyrolysis kinetics of the hydrothermal carbons derived from microwave-assisted hydrothermal carbonization of food waste digestate

The treatment of food waste digestate (FWD) is a major obstacle to the development of industrial biogas field. To mitigate the drying cost and promote energy recovery, a strategy combining microwave-assisted hydrothermal carbonization (MHTC) pretreatment with subsequent pyrolysis for FWD was investigated in this work. Firstly, MHTC was performed at different reaction variables, including reaction temperature, reaction time and solid loading, to convert FWD to hydrothermal carbons (HCs). The fuel property of raw FWD could be effectively promoted by the MHTC pretreatments and reaction temperature was the most influential factor affecting the properties of FWD-derived HCs. Afterwards, thermogravimetrc experiments were conducted to study the pyrolysis behavior of FWD and three representative FWD-derived HCs. Thermal decomposition of FWD-derived HCs is consistent with that of the platform biomass with cellulose-lignin structures. Finally, the kinetic parameters and probable reaction mechanism for the pyrolysis of FWD and representative FWD-derived HCs were determined. The average values of apparent activation energy for FWD and the three HCs were in the range of 85.181–106.499 kJ/mol. The pyrolysis of FWD and the three HCs can be described by the model of three-dimensional diffusion and reaction order, respectively.

Energy Densification of Groundnut Shell through Microwave-Assisted Hydrothermal Carbonization

Modern technology of microwave synthesis was utilized for hydrochar production from groundnut shell biomass. Hydrochar was produced at three different temperatures (180, 200, and 220°C) with 20 minutes of holding time and analyzed for the physicochemical characteristics. The yield of hydrochar and hydrocrude were in the range of 45 to 61 % and 51 to 56 % respectively. The maximum value of the higher heating value of 19.11 MJ/kg was obtained for hydrochar produced at 220°C, which is similar to the higher heating value of lignite (19 MJ/kg). The energy densification ratio of respective hydrochar was 1.13 with 51.77 % energy yield. The result implied that microwave-assisted hydrothermal carbonization was an energyefficient method to produce hydrochar. Hydrochar produced from groundnut shells was an eco-friendly, energy enriched product that can be used as a solid fuel.

Improved citric acid-derived carbon dots synthesis through microwave-based heating in a hydrothermal pressure vessel.

Carbon dots (CDs) are a diverse and wide-reaching field of study which encompasses multiple scientific disciplines. This, along with their extensive applications, calls for the need to continually seek to improve their synthesis efficiency. To this end, we focused upon the effects of altering the heating method on the characteristics and quality of the final CD product. In our approach, two different versions of heating techniques were used including the common oven-based hydrothermal method and a microwave (MW)-assisted hydrothermal method. The MW-assisted heating involved the implementation of a pressure vessel (similar to oven hydrothermal vessel) for improved yield of luminescent CDs. We found that this MW method yielded CDs of bright intensity with comparable shape, size, and dispersity to that produced using conventional hydrothermal methods. Specifically, the CDs average particle diameters were determined to be between 1.8 and 2.0 nm for either method. The MW method also demonstrated certain advantages over the oven method in terms of required reaction time and particle yield. The total reaction and cooldown time was reduced by ∼75% (with ideal MW heating time) and an estimation of the average CDs particle density revealed that the MW method was able to produce approximately 1.2 times more particles than the oven method in less time. Correcting for the same processing time, larger yields are available in microwave synthesis. We were able to also demonstrate that both versions of synthesized CDs can serve as viable fluorescence probes for in vitro imaging experiments due to the fact that, under the determined working conditions, their fluorescence emission is significantly greater than that of cell autofluorescence [oven: p = 1.47 × 10-27 (blue), p = 6.98 × 10-4 (green); 60 second MW: p = 7.59 × 10-10 (blue), p = 3.03 × 10-4 (green)] and the CDs were not cytotoxic. This rapid, simple synthesis approach can be applied to other forms of carbon dots and serves as a high-yield alternative to hydrothermal synthesis.

Optimization of rice husk hydrochar via microwave-assisted hydrothermal carbonization: Fuel properties and combustion kinetics

The optimization of rice husk hydrochar was carried out via microwave-assisted hydrothermal carbonization to find the optimum parameters for hydrochar production. Three optimized hydrochar was selected to find the best properties for solid biofuel application. The percentage difference between predicted and experimental values was below 10% for all optimized hydrochar. Hydrochar B (optimized for maximum high heating value) has the highest fixed carbon (29.86%), lowest volatile matter (54.6%), lowest ash (8.87%) and highest higher heating value (18.9 MJ/kg) compared to raw rice husk, hydrochar A (maximum solid yield) and hydrochar C (maximum solid yield and higher heating value). The result suggests that the optimized hydrochar produced is comparable to lignite and can be used as direct solid biofuel.

Comparative study on fuel characteristics and pyrolysis kinetics of corn residue-based hydrochar produced via microwave hydrothermal carbonization

Corn residues are an important source of bioenergy. Due to their highly diverse lignocellulosic structures, the hydrochar produced from microwave-assisted carbonization of different corn residues may have distinct fuel properties and pyrolysis kinetics. This study comprehensively investigated the effect of processing temperature on the basic fuel properties of hydrochar and examined the pyrolysis behavior of hydrochar as a precursor through kinetic analysis. The results indicate that the fuel quality of corn straw hydrochar prepared by microwave-assisted hydrothermal carbonization at 230 °C was significantly improved over that of its feedstock, with a higher heating value of approximately 20.7 MJ/kg. Hydrochar prepared by microwave-assisted hydrothermal carbonization of corn cob at 230 °C presents noticeable environmental advantages because it contains the lowest ash and nitrogen contents (0.5% and 0.5%, respectively) and lower sulfur content (0.05%). Moreover, regarding the kinetic modeling, the Doyle and Coats-Redfern models, which are both first-order and single-step kinetic models, were identified as satisfactory in interpreting the key pyrolysis kinetic parameters. Additionally, the microwave-assisted hydrothermal process increased the apparent activation energy of hydrochar due to the increase in crystallinity and the increase in the number of CC and CO bonds.

Synthesis, solution and solid-state fluorescence of nitrogen self-doped carbon dots derived from Chlorella pyrenoidosa

In this work, we report a simple and green approach for the synthesis of nitrogen-doped carbon dots from Chlorella pyrenoidosa, hereafter referred to as CP-CDs, via microwave-assisted hydrothermal carbonization without any additives. The effects of temperature (170–210 °C) and time (1–3 h) on the structure, functional groups, optical properties and formation route of the CP-CDs were assessed in detail. FTIR, XPS, and XRD results showed that high temperatures promoted the nitrogen doping level, resulting in good fluorescence intensity and high quantum yields (QYs). As the reaction temperature increased from 170 °C to 210 °C, the amount of nitrogen doped into the CP-CDs increased significantly from 4.1% to 11.9% and the absolute QY improved from 1.79% to 2.31%. The CP-CDs solutions exhibited excitation-dependent blue fluorescence from 360 to 527 nm with the excitation from 280 to 460 nm. The optimal preparation temperature and time for CP-CDs aqueous solution were 210 °C and 2 h, respectively. The solid-state CP-CDs without any further processing exhibited self-quenching resistant performance. The obtained CP-CDs as fluorescence inks could be used in security printing.

Efficient conversion of sewage sludge into hydrochar by microwave-assisted hydrothermal carbonization

The treatment of sewage sludge (SS) is an environmental problem worldwide. In recent years, hydrothermal carbonization (HTC) of SS for hydrochar (HC) has attracted extensive attention. This study preliminarily explored the microwave-assisted HTC of SS for the first time. Increasing the reaction temperature (150–250 °C) and reaction time (0–120 min) resulted in a decrease in the HC yield, and it gradually increased with the rising solid–liquid ratio (0.03–0.25 g/mL). Compared with raw SS, the HC products possessed higher aromaticity, carbonization degree, porosity, and polarity, and lower content of soluble nutrients (N/P/K) and leachable heavy metals (Cu, Zn, Pb, Cd, Cr, and Ni), indicating a lower risk of nutrient and heavy metal loss. Attention should be paid to the total contents of Zn and Cd in HC exceeded the permitted value for use in cultivated land with edible crops. The use of CaO as a catalyst improved the yield of HC, made the HC and process water (PW) weakly alkaline, and further passivated the heavy metals in the HC. In the case of H3PO4, although the conversion of SS was enhanced (lower content of volatile organic matter in HC), the contents of soluble nutrients (N/P/K) in HC/PW increased, and the migration of Zn and Cd into process water was enhanced. The HCs obtained in this study had poor combustion properties, but higher ignition temperatures than raw SS. PW must be properly treated or recycled because it still contained high contents of organic matter and nutrients. This fundamental study provides basic insights into the microwave-assisted HTC of SS.

Microwave-assisted catalytic hydrothermal carbonization of Laminaria japonica for hydrochars catalyzed and activated by potassium compounds

There are limited investigations describing preparation and application of alga-based hydrochars via microwave–assisted catalytic hydrothermal carbonization (MA-CHTC). Therefore, hydrochars were successfully prepared from macroalgae biomass Laminaria japonica impregnated with KH2PO4, KCl, K2CO3, and KOH as acidic, neutral salt, and alkaline catalysts, respectively, via the MA-CHTC. Comprehensive characterization of physicochemical properties of the hydrochars, including yields, elemental and phase composition, specific surface areas, functional groups, and morphology, confirmed different catalytic effects of these catalysts on hydrochar formation. Adsorption kinetics and isotherms of Pb(II) revealed significant improvement of adsorption capacities for Pb(II) due to synergetic chemical activation of the spiked catalysts. Therefore, the synergetic catalytic effects and chemical activation is benefic for tailored design of engineered hydrochars with different properties for special application through selection of catalysts during the MA-CHTC process.