Hydrothermal Carbonization Treatment Research Articles

Degradation of toxic components in kitchen waste during hydrothermal carbonization process

Hydrothermal carbonization (HTC) is a promising technique for kitchen waste (KW) disposal. However, KW may contain heavy metals (HMs), aflatoxin, nitrites and sulfites, posing risk for hydrochar utilization. In order to clarify the migration of these toxic components during hydrochar formation process, 7 kinds of kitchen wastes were adopted to carry out HTC experiments at 180–240 °C. Concentration detections results showed that Fe, Mn, Cr, Ni, Cu, and Zn contents were relatively high with a range of 30–2500 mg/kg in KW feedstocks. After HTC treatment, Fe, Cr, Cu were mainly (>80 %) enriched in hydrochar, the enrichment factors of Mn, Ni, Zn was almost above 50 %. From the perspective of species and valence distributions, Mn, Fe, Ni, and Zn in hydrochar were below moderate risk level. No aflatoxin was detected in hydrochar, because AFB1 and AFB2 can be completely degraded into organic acids in the HTC environment at 180 °C. Part benzene rings in aflatoxin were reduced and the C-C bonds were further broken, the C-O bonds were oxidized to produce carboxylic acids. Some benzene rings in aflatoxin remained to form organic acids with aromatic structures. NO2− and SO32- ions with strong ecotoxicity were mostly converted into NO3− and SO42- and flowed into the liquid phase. Some NO2− ions were also converted to and ammonia N and pyridine N; some SO32- ions were converted to sulfonic and sulfinic acid organics, which were successively transformed into sulfonyl/sulfinyl and sulfone/sulfoxide organics. The above findings are of great significance for the transformation mechanism of toxic substances in HTC environment and environment-friendly utilization of hydrochar.

KHCO3 chemical-activated hydrothermal porous carbon derived from jackfruit inner skin for supercapacitor applications

The reuse of waste biomass resources had become a hot spot in the sustainable development of human society. Due to the complexity of biomass composition and microstructure, the quality reproducibility of biomass porous carbon was poor. Therefore, it was of great significance to develop a reliable method for preparing porous carbon from biomass. In this paper, a combination of hydrothermal carbonization treatment and mild activation of KHCO3 was used to prepare iron-modified porous carbon with carbon spheres/nanoplates structure, which promoted ion/electrolyte diffusion and increased the accessibility between surface area and electrolyte ions. Therefore, the active porous carbon from the Jackfruit inner skin has good specific capacitance (273.2 F/g at 1 A/g) and good cycle stability, and the capacitance loss is only 6.6 % after 10,000 charge discharge cycles. The above results showed that hydrothermal treatment and mild activation methods provided an effective way to transform waste biomass into high-performance electrode material.

KHCO3 Chemical-Activated Hydrothermal Porous Carbon Derived from Sugarcane bagasse for Supercapacitor Applications.

The reuse of waste biomass resources had become a hot topic in the sustainable development of human society. Biomass was an ideal precursor for preparing porous carbon. However, due to the complexity of biomass composition and microstructure, the quality reproducibility of biomass porous carbon was poor. Therefore, it was of great significance to develop a reliable method for preparing porous carbon from biomass. In this paper, the activated hydrothermal porous carbon was prepared by a combination of hydrothermal carbonization treatment and KHCO3 mild activation. The hydrothermal carbonization treatment could complete the morphology adjustment and iron doping of the carbon in one step, and the mild activation of KHCO3 could activate the porous carbon while maintaining the spherical morphology. Fe-modified porous carbon with carbon ball/nanosheet structure prepared from bagasse exhibited a high surface area (2169.8 m2/g), which facilitated ion/electrolyte diffusion and increased accessibility between surface area and electrolyte ions. Therefore, bagasse derived activated porous carbon had good specific capacitance (315.2 F/g at 1 A/g) and good cycle stability, with a capacitance loss of only 5.8 % after 5000 charge-discharge cycles, and the Na2SO4-based device showed the maximum energy density of 13.02 Wh/kg. This study showed that the combination of hydrothermal treatment and mild activation provided an effective way for the conversion of waste biomass into high-performance electrode materials.

Hydrothermal carbonization of invasive plant biomass as a tool for its safe utilization and production of artificial humic substances

Abstract Invasive plants may cause significant environmental risks by affecting biological diversity, ecosystem services, agriculture, and forestry. Safe utilization of invasive plant biomass by obtaining new outcome products useful for bioeconomics is a challenging but promising solution. Hydrothermal carbonization (HTC) presents a sustainable and cost-effective approach to transforming invasive plant biomass into new products while simultaneously supporting carbon capture aims. This study utilized invasive plants such as lupine [Lupinus polyphyllus Lindl.], Sosnowsky’s hogweed [Heracleum sosnowskyi Manden.], and Japanese knotweed [Polygonum cuspidatum Siebold & Zucc.] as biomass sources for HTC to produce artificial humic substances (AHS). The greatest impact on the yield of AHS (maximum gained yield 62%) has the duration of the HTC process (up to 6 h) and temperature of the treatment (from 160 up to 250 °C), as well as the catalyst used (an alkaline medium is preferable). During the HTC treatment, significant changes in the invasive plant biomass composition occur as indicated by the removal of labile components of organic matter. Obtained AHS are essentially similar to natural humic matter and have biostimulatory properties; thus, they can find application in agriculture and other areas of bioeconomics.

Fate of Na & Cl in kitchen waste during hydrothermal carbonization

Hydrothermal carbonization (HTC) is a sustainable approach to recover nutrients from kitchen waste (KW) to realize the applications of hydrochar as fuel, soil amendment and so on. However, the salt content of KW can be as high as 5 wt%, which brings potential risks for product utilization. Thus, the migration path of endogenous (from ingredients) and exogenous (from condiments) Na & Cl in KW during HTC process was investigated. Results showed that the endogenous and total Na element in KW was 0.03–2.98 wt% and 2.97–4.82 wt%, respectively. The main form of sodium was water-soluble (48–82%). The KW feedstocks and cooking methods had little effect on the migration of Na & Cl. During HTC treatment, all sodium element was transformed into water-soluble state, subsequently about 97% was dissolved into the liquid phase. The content of Na element in hydrochar was only below 0.05 wt%, so its negative effect can be negligible. As for the fate of chlorine, the endogenous and total Cl element in KW was 0.31–1.62 wt% and 1.82–3.19 wt%, respectively, and the main form was inorganic Cl accounting for 83–91%. During HTC treatment, about 40–75% of endogenous organochlorine was converted into inorganic Cl by breaking C-Cl bonds, and all inorganic Cl was leached into the liquid phase. In this pattern, partial organochlorine remained in hydrochar with a content of 0.21–0.32 wt%, which need to be further reduced. Methods of adding Na2CO3 solution or HTC-pyrolysis were effective, and approximately 90% of the organic Cl can be removed. Based on the above findings, this study can provide theoretical guidance for the value-added conversion of solid waste with high moisture and high salinity.

Analysis of perfluorinated compounds in sewage sludge and hydrochar by UHPLC LTQ/Orbitrap MS and removal assessment during hydrothermal carbonization treatment

Wastewater treatment plants have been recognized as important sinks for per- and polyfluoroalkyl substances (PFAS) because of their ineffectiveness in removing them reflecting both water and sewage sludge discharge routes. Hydrothermal treatment represents an alternative technology for treating sludge to recover energy and other valuable products. In this study, 15 PFAS were determined in sludge and hydrochar substrates using sonication-solid phase extraction procedure and analyzed using LC-Orbitrap-High Resolution-MS/MS. The method was fully validated, exhibiting very good linearity, recoveries in the range of 48 to 126 %, low detection and quantification limits with expanded uncertainty and precision below 32 % and 21.9 %, respectively. The method was applied to sludge samples from the WWTP of Ioannina city (Greece), as well as to hydrothermally treated samples under various conditions. The most abundant PFAS were PFHxA (0.5–38.3 ng g−1) and PFOS (4.4–22.1 ng g−1). Finally, the hydrothermally treated sludge samples spiked with PFAS presented removal efficiencies for total PFAS of 86.9 %, 91.8 % and 95.7 % at three spiking levels namely 10, 50 and 200 ng g−1, respectively. Results indicated that PFCAs were almost completely removed, except for PFOA, while the concentrations of PFSAs increased in the produced hydrochar with the formation of several intermediates, as detected by HR-LC-MS/MS. The results of this study demonstrate the effect of hydrothermal treatment to the fate of PFAS in sewage sludge and contribute for further studies on design and scale up of hydrothermal carbonization technology as a management option for safer disposal of municipal wastewater sludge.

NATURAL RUBBER COMPOSITES WITH HYDROCHAR AS A PARTIAL FILLER: INVESTIGATION OF KINETIC PARAMETERS

Reducing the environmental impact of the rubber industry has emerged as a major challenge, and one potential solution that has garnered significant attention is incorporating hydrochar as a partial filler in natural rubber composites. Hydrochar was obtained through hydrothermal carbonization treatment of hardwood waste biomass, and it has been found to have potential as a sustainable alternative to traditional filler carbon black. The aim of this study was to investigate the effect of varying hydrochar and carbon black content in natural rubber composites, while keeping the total filler amount constant at 50 phr. The study's findings indicated that higher hydrochar content resulted in a greater curing activation energy, facilitating the manufacture of largernatural rubber vulcanizates that required extended curing periods at reduced temperatures. The rubber industry's stringent environmental regulations have created a pressing need for sustainable alternatives, and incorporating hydrochar as a partial fillercould offer a promising solution by repurposing waste materials into a valuable component for rubber composites. KEYWORDS:hydrochar, hydrothermal carbonization, natural rubber composites, kinetic parameters

Insights into pyrolysis process of coconut shell waste hydrochar: In-situ structural evolution and reaction kinetics

Coconut shell (CS) can be efficiently utilized by coupling hydrothermal carbonization (HTC) pretreatment with pyrolysis to meet the goal of global green development. In-situ Raman spectroscopy and thermogravimetric analyzer (TGA) were adopted to investigate the structural evolutions and reaction kinetics characteristics of hydrochar during the pyrolysis process. According to the results, the intensity of peak D and peak G in Raman spectrum decreased with increasing hydrothermal degree. Mild HTC treatment (≤200 °C) led to an increase in ID/IG, indicating the formation of large aromatic rings through small ring condensation. In contrast, severe HTC treatment (≥220 °C) reduced the ID/IG ratio, indicating an increased polycondensation of aromatic compounds and a higher degree of aromatization, resulting in larger molecules sizes. The spectral area below 400 °C reduced with increasing pyrolysis temperature due to the reduction of oxygen-containing functional groups and thermal radiation. Above 400 °C, the total area remains stable as the Raman spectrum scattering ability increases due to char formation and aromatization reactions. HTC-200 exhibited an increase in ID/IG from 0.54 to 0.72 and ID/I(Me + Ke + GL) from 0.58 to 0.68 with increasing pyrolysis temperatures. The activation energy during pyrolysis ranged from 194.50 to 238.34 kJ/mol for CS and 217.51–259.51 kJ/mol for HTC-200. Hydrothermal carbonization had a significant effect on pyrolysis behavior of hydrochar, as it was found that the mild HTC hydrochar followed D-type diffusion models when conversion rate was less than 0.65, and transitioned to higher reaction order in the late-stage of pyrolysis.

Hydrothermal carbonization of corncob for hydrochar production and its combustion reactivity in a blast furnace.

A key factor restricting the application of biochar in the steel industry is its high-quality upgrading. This paper evaluated the characteristics of hydrochar produced by HTC (hydrothermal carbonization) process of corncob to be used as a solid fuel. HTC temperatures (240-300°C) and HTC water-reused times (1-3 times) were examined for their effects on hydrochar yield, physicochemical characteristics, and combustion properties. The results showed hydrochar yields, O/C, and H/C parameters decreased as HTC temperature and water-reused times increased, while its high heating value increased. Due to dehydration and decarboxylation, hydrochar showed similar characteristics to those in bituminous coal. The removal efficiency of alkali metal K reached 99% after HTC treatment. Carbonaceous hydrochar had become more compact, orderly, and stable with increasing amounts of aromatic functional groups, C = C, and C = O. Hydrochar, as a biofuel, has higher ignition energy and is more stable than corncob due to its high carbonaceous order degree. To calculate combustion kinetic parameters, the Kissinger-Akahira-Sunose (KAS) and Flynn-Wall-Ozawa (FWO) methods were applied. The results revealed that Eα (average activation energy) was quite similar between the two models. HC-300 had an Eα of 262kJ/mol. HTC could be an efficient way to reutilize corncob biomass into clean biofuels with high calorific value.

Electrochemical Performance of Metal-Free Carbon-Based Catalysts from Different Hydrothermal Carbonization Treatments for Oxygen Reduction Reaction.

This research investigates the difference between products obtained through two hydrothermal carbonization treatments. Our aim is to synthesize metal-free, carbon-based catalysts for the oxygen reduction reaction (ORR) to serve as efficient and cost-effective alternatives to platinum-based catalysts. Catalysts synthesized using the traditional hydrothermal approach exhibit a higher electrocatalytic activity for ORR in alkaline media, despite their more energy-intensive production process. The superior performance is attributed to differences in the particle morphology and the chemical composition of the particle surfaces. The presence of functional groups on the surfaces of catalysts obtained via a traditional approach significantly enhances ORR activity by facilitating deprotonation reactions in an alkaline environment. Our research aims to provide a reference for future investigations, shifting the focus to the fine-tuning of surface chemical compositions and morphologies of metal-free catalysts to enhance ORR activity.

Enhancing Fuel Properties of Napier Grass via Carbonization: A Comparison of Vapothermal and Hydrothermal Carbonization Treatments

Napier grass is a herbaceous biomass that can be used as biofuel; however, its high ash, potassium, sulfur and chlorine content may cause problems when combusted. Napier grass was submitted to vapothermal carbonization (VTC) and hydrothermal carbonization (HTC) processes at 190 and 220 °C to compare their ability to enhance its fuel properties. The different water distribution between phases in the two processes was verified: up to 14.5% of the water vaporized to steam in the VTC ran at 220 °C, while over 99% of the water remained in the liquid state and in contact with the solids during all HTC runs. Both processes improved the calorific value of the Napier grass (up to 20.6% for VTC220 and up to 29.8% for HTC220) due to the higher C content in the chars. Both processes reduced the sulfur content, removing up to 15.3% of it with VTC190 and 28.5% of it with HTC190 compared to that of Napier grass. In contrast, the two processes had different effects on the ash and chlorine content. While HTC removed both ash and Cl from the Napier grass, VTC concentrated it in the chars (ash: 5.6%wt. Napier grass, 3.3%wt. HTC chars, 7.1%wt. VTC; chlorine: 1.08%wt. Napier grass, 0.19%wt. HTC chars, 1.24%wt. VTC). Only the HTC process leached high percentages of Cl (up to 80%), S (up to 70%), sodium (Na, up to 80%) and potassium (K, up to 90%) into the process water. This may prevent fouling and slagging problems when burning HTC char. The biofuel qualities of the raw Napier grass, VTC, and HTC chars were evaluated using two standards: the international standard for solid biofuels, EN ISO 17225, and the Korean regulation for biomass solid recovered fuels (Bio-SRF). Napier grass and VTC chars presented problems regarding Cl content thresholds for both EN ISO 17225 and Bio-SRF. Both VTC and HTC chars along with the Napier grass fulfilled the requirements for heavy metals (Pb, Ni, Cr, and Cd) except for copper. The choice of process in practical applications will depend on the goal; HTC improves fuel quality and VTC has higher high solid, carbon and energy yields.

Tailoring the porosity of chemically activated carbons derived from the HTC treatment of sewage sludge for the removal of pollutants from gaseous and aqueous phases

The management of sewage sludge is currently an open issue due to the large volume of waste to be treated and the necessity to avoid incineration or landfill disposal. Hydrothermal carbonization (HTC) has been recognized as a promising thermochemical technique to convert sewage sludge into value-added products. The hydrochar (HC) obtained can be suitable for environmental application as fuel, fertilizer, and sorbent. In this study, activated hydrochars (AHs) were prepared from sewage sludge through HTC followed by chemical activation with potassium hydroxide (KOH) and tested for the removal of pollutants in gaseous and aqueous environments, investigating carbon dioxide (CO2) and ciprofloxacin (CIP) adsorption capacity. The effects of activation temperature (550–750 °C) and KOH/HC impregnation ratio (1–3) on the produced AHs morphology and adsorption capacity were studied by Response Surface Methodology (RSM). The results of RSM analysis evidenced a maximum CO2 uptake of 71.47 mg/g for mild activation conditions (600–650 °C and KOH/HC = 1 ÷ 2), whereas the best CIP uptake of 628.61 mg/g was reached for the most severe conditions (750 °C, KOH/HC = 3). The prepared AHs were also applied for the removal of methylene blue (MB) from aqueous solutions, and the MB uptake results were used for estimating the specific surface area of AHs. High surface areas up to 1902.49 m2/g were obtained for the highest activation temperature and impregnation ratio investigated. Predictive models of CO2 and CIP uptake were developed by RSM analysis, and the optimum activation conditions for maximizing the adsorption performance together with high AH yield were identified: 586 °C and KOH/HC ratio = 1.34 for maximum yield (26.33 %) and CO2 uptake (67.31 mg/g); 715 °C and KOH/HC ratio = 1.78 for maximum yield (18.75 %) and CIP uptake (370.77 mg/g). The obtained results evidenced that chemical activation of previously HTC-treated sewage sludge is a promising way to convert waste into valuable low-cost adsorbents.

Effect of aqueous phase recycling on iron evolution and environmental assessment during hydrothermal carbonization of dyeing sludge

Iron is usually added as a flocculant in the wastewater treatment process, indicating that the terminal product-sludge will contains. Aqueous phase (AP) is the main byproduct produced during hydrothermal carbonization (HTC) of sludge. Thus, HTC treatment of sludge produces wastewater containing Fe. Therefore, it is necessary to study the migration behavior of Fe during AP recycling process. This study aims to investigate the effect of AP recycling (APR) and catalytic AP recycling (CAPR) on hydrochar properties, iron evolution and environmental assessment during HTC of sludge. The yield of hydrochar increased with the increase of recycle number and CAPR process was more significant than APR (13.96 % for CAPR and 9.9 % for APR). After four AP recycling process, the approximate equilibrium moisture content (AEMC) of WHC (hydrochar obtained in APR) and HHC (hydrochar obtained in CAPR) decreased by 4.62 % and 6.27 %, respectively, indicating AP recycling process was beneficial in improving the dewatering performance of hydrochar. The dehydration and demethylation reaction were strengthened during APR process, while the CAPR process showed enhanced decarboxylation reaction. The AP recycling process facilitated the recovery of Fe from the liquid phase and reached stability after the third cycle. The XRD pattern showed that Fe in the WHC was mainly in the form of lepidocrocite (FeO(OH)) and vivianite (Fe3(PO4)2), while in HHC it was present in magnetite (Fe3O4) and butlerite (Fe(SO4)(OH)•2H2O) forms. The XPS spectra indicated that most of Fe3+ entered the interior of the hydrochar during the AP recycling process. Life cycle assessment (LCA) results showed that AP recycling process was more eco-friendly, but it will be upto at least three cycles. In theory, the environmental impact categories would continue to decline as AP recycling proceeded in the laboratory scale.

Hydrothermal carbonization of sewage sludge coupled with Fenton oxidation pretreatment: Moderate oxidation to enhance hydrochar yield and properties

Hydrothermal carbonization (HTC) is a promising technology for sewage sludge (SSL) utilization. However, the yield and performance of hydrochar need to be further improved produced when SSL is used as a feedstock must be improved further. Herein, a process combining physicochemical pretreatment, namely, Fenton oxidation (FO) and HTC was developed for SSL utilization. Moderate oxidation was found to improve the yield and performance of the produced hydrochar. The product distribution, yield, and properties of the hydrochar produced using the proposed combined process were compared with those of the hydrochar produced via direct HTC treatment. The pretreatment affected the surface structure and organic composition of SSL and promoted the carbonization of the intermediate products. Thus, compared with the yield of hydrochar (50.7 %) obtained via the direct HTC treatment, the yield of the hydrochar obtained using the combined process increased to 55.0–65.2 % (depending on the pretreatment condition). Hydrochar properties were enhanced using the combined process, and the energy density of hydrochar slightly decreased after pretreatment (1.2–13.1 %); however, the energy yields increased by 0.6–30.1 % due to the enhanced hydrochar yield. The carbonization degree of hydrochar improved, the carbon in the feedstock distributed to the hydrochar increased from 33.40 % to 46.09 %. Moreover, the hydrochar showed more distinct pore channels, slightly improving its combustion activity. This study shows the efficacy of the combined pretreatment–HTC process and provides an experimental basis for the combined hydrothermal treatment of SSL.

Effect mechanism of iron conversion on adsorption performance of hydrochar derived from coking sludge

In this study, Fe conversion during hydrothermal carbonization (HTC) of coking sludge were investigated, and the effect mechanism of Fe component on the adsorption performance of coking sludge hydrochar (CHC) was explored. The results showed that after HTC treatment, more than 95 % of Fe remained in the CHC. Fe3+ was reduced to Fe2+ by sugar and amino acids. Fe was stabilized during the HTC process and was still predominantly in the Fe manganese oxidation state. The CHC prepared at 270 °C exhibited excellent adsorption capacities for Congo red (CR), tetracycline (TC), and Cr (VI). Their maximum adsorption capacities were 140.85, 147.06, and 19.92 mg/g, respectively. Quantitative adsorption mechanism experiments, XRD and VSM characterization revealed that Fe component played a significant role in adsorption, and CHC with more Fe3O4 exhibited better adsorption capacity. The results of the XPS characterization of CHC before and after adsorption showed that Fe3O4 provided rich Fe adsorption sites on the surface of CHC to strengthen the adsorption efficiency of pollutants through Fe3+/Fe2+ reduction and complexation of Fe-O/N. In addition, the formed Fe3O4 also imparted CHC with magnetic properties (Ms = 4.12 emu/g) to facilitate the subsequent separation and recovery. These results demonstrated that the prepared CHC has great potential for treating actual wastewater containing CR and TC.

Hydrothermal carbonization coupled with fast pyrolysis of almond shells: Valorization and production of valuable chemicals

In this study, it was found that hydrothermal carbonization (HTC) can be an effective method for almond shell (AS) valorization. The severity of HTC treatment had a significant effect on hydrochar yields, with higher severity promoting carbonization but reducing yields. Furthermore, the work found that HTC treatment effectively demineralized biomass samples by removing inorganic material that could catalyze carbonization. As residence time or temperature increased, the amount of carbon increased, while the amount of oxygen decreased. An acceleration in thermal degradation was detected for hydrochars after pretreating for 4 h. The hydrochars showed they had a higher volatile content than untreated biomass, making them potentially useful for producing quality bio-oil through fast pyrolysis. Finally, HTC treatment led to the production of valuable chemicals such as guaiacol and syringol. For syringol production, HTC residence time had more effect than HTC temperature. However, high HTC temperatures benefited levoglucosan production. Overall, the results demonstrated the potential for HTC treatment to be an effective method for valorizing agricultural waste, offering the possibility of producing valuable chemicals.

Manure-derived hydrochar superior to manure: Reducing non-point pollution risk by altering nitrogen and phosphorus fugacity in the soil–water system

Hydrothermal carbonization (HTC) technology is an emerging technology for the disposal of manure-based wet wastes. However, the effects of manure-derived hydrochar inputs to agricultural soils on nitrogen (N) and phosphorus (P) morphology and conversion in soil–water systems remain largely unexplored. In this study, pig and cattle manure (PM and CM), and their derived hydrochar (PCs and CCs) were applied to agricultural soils, with changes in nutrient morphology and enzyme activities related to N and P transformation in the soil–water systems observed through flooded incubation experiments. The results showed that floodwater ammonia N concentrations were reduced by 12.9–29.6% for PCs relative to PM, and 21.6–36.9% for CCs relative to CM, respectively. Moreover, floodwater total P concentrations of PCs and CCs were reduced by 11.7–20.7% relative to PM and CM. Soil enzyme activities closely related to N and P transformations in the soil–water system responded differently to manure and manure-derived hydrochar application. Compared to manure, the application of manure-derived hydrochar inhibited soil urease and acid phosphatase activity by up to 59.4% and 20.3%, respectively, whereas it had significant promotion effects on soil nitrate reductase (∼69.7%) and soil nitrite reductase (∼64.0%). The products of manure after HTC treatments have the characteristics of organic fertilizers, and the fertilization effects of PCs are more prominent than CCs, which are subject to further verification in field trials. Our findings improve the current understanding of manure-derived organic matter affecting N and P conversions in soil–water systems and the risk for non-point source pollution.

Valorization of Face Masks Produced during COVID-19 Pandemic through Hydrothermal Carbonization (HTC): A Preliminary Study

The COVID-19 pandemic has led to the increased use of disposable face masks worldwide, resulting in a surge of potentially infectious waste. This waste must be safely managed and disposed of to prevent the spread of the virus. To address this issue, a preliminary study explored the use of hydrothermal carbonization (HTC) as a potential method for converting surgical mask waste into value-added carbonaceous materials. The HTC treatments were conducted at 220 °C for 3 h with or without the addition of acetic acid. The resulting hydrochar was characterized using several techniques, including thermogravimetric analysis (TGA), scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FTIR), and N2-physisorption analyzers. The study found that the masks formed a melt with reduced mass (−15%) and volume (up to −75%) under the applied conditions. The carbon content and higher heating value (HHV) of the produced hydrochars were higher than those of the original masks (+5%). Furthermore, when acetic acid was added during the HTC experiment, a new crystal phase, terephthalic acid, was produced. This acid is a precursor in surgical mask production. The study suggests that hydrothermal carbonization could potentially achieve sanitization and volume reduction in non-renewable and non-biodegradable surgical masks while also producing a solid fuel or a raw material for terephthalic acid production. This approach offers an innovative and sustainable solution to manage the waste generated by the increased use of disposable face masks during the pandemic.

Comprehensive insights into the application strategy of kitchen waste derived hydrochar: Random forest-based modelling

Due to the complex and variable composition of kitchen waste (KW), it is difficult to master the product properties after hydrothermal carbonization treatment. In this study, random forest method was adopted to firstly predict the application performance of hydrochar from the perspectives of fuel, soil amendment and adsorbent. A total of 9 correlation models were established, with organic components and reaction parameters taken as input variables. The training set was supported by 132 literature data and 153 experimental data, with 18 experimental data as testing set. Results showed that the correlation coefficients of 9 models were up to 0.89–0.99. The relative significance of input variables was also acquired by mean decrease impurity method. It was found that the effects of reaction temperature/time on hydrochar properties were relatively negligible. Thus, hydrochar properties can be generally predicted based on the composition of KW feedstocks. The high heating value of hydrochar was mainly in the range of 20–30 MJ/kg, which was comparable to that of coal. To obtain hydrochar with good fuel performance, KW with high lipid (>50%) and low carbohydrate (<25%) is suitable. The pH value and cation exchange capacity of hydrochar was 3.23–5.69 and 1.08–42.37 cmol+/kg, respectively, which can be used for the improvement of saline-alkali land. KW of high protein (>35%), high lipid (>25%) and low carbohydrate (<35%) was potential to be converted into excellent adsorbents with specific surface area over 2000 m2/g.

Migration and Conversion of Phosphorus in Hydrothermal Carbonization of Municipal Sludge with Hydrochloric Acid

Phosphate ore is a non-renewable resource, so finding a replacement is necessary. Municipal sludge has significant recycling potential because of its high phosphorus content and large discharge characteristics. The migration and transformation of phosphorus in municipal sludge treated with different concentrations of HCl were studied using the standards, measurements, and testing phosphorus extraction protocol from two aspects: phosphorus complexation and mineral form. After the hydrothermal carbonization treatment without HCl, the hydrochar retained 99.7% of phosphorus in the sludge, and the organophosphorus percentage was about 30%. In the hydrothermal carbonization treatment with the addition of 0.5–2.5% HCl, the phosphorus content in the hydrochar decreased gradually from 99.5% (46.18 mg/g) to 91.8% (64.17 mg/g) that of the original sludge, and the proportion of non-apatite inorganic phosphorus increased from 34% to 94%. Hydrochloric acid provides a low-pH environment and promotes the dissolution of calcium-related phosphorus precipitates and enhances the dehydration reaction. This study provides technical support for the recovery of phosphorus resources from municipal sludge.