Effects of biomass types and carbonization conditions on the chemical characteristics of hydrochars

Influence of different stalks on the metallization degree of FeCl3-derived magnetic biochar through pyrolysis behavior and compositional differences

To investigate the effect of stalk type on the metallization degrees in FeCl3-derived magnetic biochar (MBC), MBC was synthesized via an impregnation-pyrolysis method using six different stalks. The Fe0 content in MBC significantly influenced its magnetic properties and ostensibly governed its catalytic capabilities. Analysis of the interaction between stalks and FeCl3 revealed that the variation in metallization degrees, resulting from FeCl2 decomposition (6.1%) and stalk-mediated reduction (20.7%), was directly responsible for the observed differences in MBC metallization. The presence of oxygen-containing functional groups and fixed carbon appeared to promote metallization in MBC induced by reduction. A series of statistical analyses indicated that the cellulose, lignin, and hemicellulose content of the stalks were key factors contributing to differences in MBC metallization degrees. Further exploration revealed that hemicellulose and cellulose were more effective than lignin in enhancing metallization through FeCl2 decomposition and reduction. Constructing stalk models demonstrated that the variance in the content of these three biomass components across the six stalk types could lead to differences in the metallization degree attributable to reduction and FeCl2 decomposition, thereby affecting the overall metallization degree of MBC. A prediction model for MBC metallization degree was developed based on these findings. Moreover, the elevated Si content in some stalks facilitated the formation of Fe2(SiO4), which subsequently impeded the reduction process. This study provides a theoretical foundation for the informed selection of stalk feedstocks in the production of FeCl3-derived MBC.

Impacts of kaolinite enrichment on biochar and hydrochar characterization, stability, toxicity, and maize germination and growth

In this study, biochar (BC) and hydrochar (HC) composites were synthesized with natural kaolinite clay and their properties, stability, carbon (C) sequestration potential, polycyclic aromatic hydrocarbons (PAHs) toxicity, and impacts on maize germination and growth were explored. Conocarpus waste was pretreated with 0%, 10%, and 20% kaolinite and pyrolyzed to produce BCs (BC, BCK10, and BCK20, respectively), while hydrothermalized to produce HCs (HC, HCK10, and HCK20, respectively). The synthesized materials were characterized using X-ray diffraction, scanning electron microscope analyses, Fourier transform infrared, thermogravimetric analysis, surface area, proximate analyses, and chemical analysis to investigate the distinction in physiochemical and structural characteristics. The BCs showed higher C contents (85.73-92.50%) as compared to HCs (58.81-61.11%). The BCs demonstrated a higher thermal stability, aromaticity, and C sequestration potential than HCs. Kaolinite enriched-BCs showed the highest cation exchange capacity than pristine BC (34.97% higher in BCK10 and 38.04% higher in BCK20 than pristine BC), while surface area was the highest in kaolinite composited HCs (202.8% higher in HCK10 and 190.2% higher in HCK20 than pristine HC). The recalcitrance index (R50) speculated a higher recalcitrance for BC, BCK10, and BCK20 (R50 > 0.7), minimal degradability for HCK10 and HCK20 (0.5 < R50 < 0.7), and higher degradability for biomass and HC (R50 < 0.5). Overall, increasing the kaolinite enrichment percentage significantly enhanced the thermal stability and C sequestration potential of charred materials, which may be attributed to changes in the structural arrangements. The ∑ total PAHs concentration in the synthesized materials were below the USEPA's suggested limits, indicating their safe use as soil amendments. Germination indices reflected positive impacts of synthesized charred materials on maize germination and growth. Therefore, we propose that kaolinite-composited BCs and HCs could be considered as efficient and cost-effective soil amendments for improving plant growth.

Evolution of elemental nitrogen involved in the carbonization mechanism and product features from wet biowaste

Hydrothermal carbonization (HTC) represents elegant thermochemical conversion technology suitable for energy and resource recovery from wet biowaste, while the elemental nitrogen is bound to affect the HTC process and the properties of the products. In this review, the nitrogen fate during HTC of typical N-containing-biowaste were presented. The relationship between critical factors involved in HTC like N/O, N/C, N/H, solid ratio, initial N in feedstock, hydrothermal temperature and residence time and N content in hydrochar were systematic analyzed. The distribution and conversion of N species along with hydrothermal severity in hydrochar and liquid phase was discussed. Additionally, the chemical forms of nitrogen in hydrochar were elaborated coupled with the role of N element during hydrochar formation mechanism and the morphology features. Finally, the future challenges of nitrogen in biowaste involved in HTC about the formation and regulation mechanism of hydrochar were given, and perspectives of more accurate regulation of the physicochemical characteristics of hydrochar from biowaste based on the N evolution is expected.

Ecotoxicological characterization of engineered biochars produced from different feedstock and temperatures

Biochar (BC) engineering, which has recently gained a lot of interest, allows designing the functional materials. BC modification improves the properties of pristine biochar, especially in terms of adsorption parameters. An interesting type of modification is the introduction of metals into the BC's structure. There is a knowledge gap regarding the effects of modified BC (e.g., BC-Mg, BC-Zn) on organisms. The aim of this study was the ecotoxicological evaluation of BC-Mg and BC-Zn composites, received under diverse conditions from willow or sewage sludge at 500 or 700 °C. The ecotoxicological tests with bacteria Vibrio fischeri (V. fischeri) and invertebrates Folsomia candida (F. candida) were applied to determine the toxicity of BC. The content of toxic substances (e.g., polycyclic aromatic hydrocarbons (PAHs), heavy metals (HMs), environmentally persistent free radicals (EPFRs)) in BC were also determined and compared with ecotoxicological parameters. The ecotoxicity of studied BCs depends on many variables: feedstock type, pyrolysis temperature and the modification type. The Zn and Mg modification reduced (from 28 to 63 %) the total Ʃ16 PAHs content in willow-derived BCs while in SL-derived BCs the total Ʃ16 PAHs content was even 1.5-3 times higher compared to pristine BCs. The Zn modified willow-derived BCs affected positively on F. candida reproduction but showed inhibition of luminescence V. fischeri. BC-Mg exhibited harmful effect to F. candida. The ecotoxicological assessment carried out sheds light on the potential toxicity of BC-Zn and BC-Mg composites, which are widely used in the removal of heavy metals, pharmaceuticals, dyes from waters and soils.

Evaluation of Babassu Cake Generated in the Extraction of the Oil as Feedstock for Biofuel Production

The growing demand for energy and the concern about environmental impacts reinforce the necessity for renewable energy sources such as biofuels. In this study, cake generated in the babassu oil extraction was evaluated as a potential feedstock for solid biofuel production, and it contains a blend of cashew nutshell, sugarcane bagasse, carnauba straw, and carnauba stalk. All samples were characterized by proximate analysis and Higher Heating Value. Carbonization was used to improve energy performance and compaction to understand the mechanism and the characteristics of the biomasses compacted. In the extraction of babassu oil, fresh and aged (90 days) kernel samples were used. The fresh samples reached a yield of 59.8%, and the aged samples reached a yield of 70.66%. The carbonization of babassu cake was carried out in a Muffle furnace at temperatures of 250, 300, 350, and 400 °C. The fresh babassu cake showed an HHV of 23.06 MJ kg−1 and after carbonization of 28.07 (250 °C), 30.69 (300 °C), 28.24 (350 °C), and 18.27 MJ kg−1 (400 °C). At 400 °C, a decrease in HHV of 20.8% occurred, and an increase in Ash (%) of 195% occurred. Proximate analysis showed that biomasses are compatible, with some having a higher compatibility than other biological materials already used as fuels in the industry.

Ecotoxicological characteristics and properties of zinc-modified biochar produced by different methods

Despite the dynamic progress of BC engineering, there is a lack of knowledge on the toxicity and environmental impact of modified BC. The aim of this study was the ecotoxicological evaluation of BC modified with zinc (Zn) using different methods: impregnation of feedstock with Zn before pyrolysis (PR), impregnation with Zn after pyrolysis (PS) and impregnation with Zn after pyrolysis with an additional calcination step (PST). The ecotoxicological assessment was based on tests with invertebrates (Folsomia candida, Daphnia magna) and bacteria (Aliivibrio fischeri). The post-treated and calcined composites had a higher content of total (C_tot) PAHs (144-276 μg kg^-1) than pre-treated BC-Zn (68-157 μg kg^-1). All BC-Zn treatments stimulated the reproduction of F. candida at the lowest BC dose (0.5%) by 4-24%. Increasing the biochar dose to 1% and 3% retained the stimulating effect of the pre-modified biochars (from 19 to 41%). Pre-modified BC-Zn reduced the luminescence of A. fischeri from 40% to 80%. Post-treated BCs reduced bacterial luminescence by 99%, but the calcination step limited the toxic effects to the level observed for the control. Post-treated BCs had a toxic effect on D. magna, with EC50 values ranging from 433 to 783 mg L^-1. The ecotoxicity of composites depends on modification methods, BC dose and pyrolysis temperature. The application of limiting conditions for HM leaching (i.e., pre-modification, calcination) increased the safety of using Zn-biochar composites.

From waste to fertilizer: Nutrient recovery from wastewater by pristine and engineered biochars

Biochar application for the recovery of nutrients from wastewater is a sustainable method based on a circular economy. Wastewater, food wastewater, and stormwater are valuable sources of nutrients (i.e., PO₄^3-, NO₃^-, and NH₄⁺). The unique properties of biochar, such as its large specific surface area, pH buffering capacity, and ion-exchange ability, make it a cost-effective and environmentally friendly adsorbent. Biochar engineering improves biochar properties and provide targeted adsorbents. The biochar-based fertilizers can be a sustainable alternative to traditional fertilization. The aim of the study was to compare the potential of pristine and engineered biochars to recover nutrients from wastewater and to determine the factors which may affect this process. Engineered biochar can be used as a selective adsorbent from multicomponent solutions. Adsorption on engineered biochar can be also regulated by additional mechanisms: surface precipitation and ligand/ion exchange. Metal modification (e.g. Mg, Fe) enhances PO₄^3- and NO₃^- adsorption capacity, and thus may provide the extra plant macro-/micronutrients. The desorption mechanism, which is the basis for nutrient release are strongly pH depended. The use of biochar-based fertilizer can have economic and agricultural benefits when using waste materials and reducing pyrolysis energy costs.

Acid-mediated hydrothermal treatment of sewage sludge for nutrient recovery

Hydrothermal carbonization allows material valorization and energy recovery from wet biomass waste. In this study, the hydrothermal treatment of dewatered waste-activated sludge (DWAS) was evaluated at several temperatures (170-230 °C) and reaction times (5-60 min) in an acid-free medium or in media such as citric acid or HCl (0.1-0.5 mol/L). Compared with the DWAS, an increase in the fixed carbon content (>45 wt%) and heating value (18.9-22.9 MJ/kg) was observed in the hydrochar; however, their ash content remained high, which is the main drawback hindering their direct use as a biofuel. The addition of acids during hydrothermal treatment favored the solubilization of N and P in the process water, which required strict control of the reaction time to avoid the recrystallization of P in the hydrochar. Under optimum operating conditions (230 °C, 15 min, 0.5 mol/L HCl), 94 % of P (as of PO₄) and almost 100 % of N (14 % as NH₄-N) present in the feedstock were concentrated in the process water.

Use of Hydrochar Produced by Hydrothermal Carbonization of Lignocellulosic Biomass for Thermal Power Plants in Chile: A Techno-Economic and Environmental Study

Hydrothermal carbonization makes it possible to transform lignocellulosic biomass into hydrochar, a carbon-rich material that can be used as fuel. Hydrochar has less calorific value than standard coal but generates less ashes during combustion. This study is a pre-feasibility analysis carried out to evaluate technically-economically and environmentally the use of hydrochar as fuel or co-fuel in thermal power plants in Chile. Until now there are no reports about it. The proposal of this work was to study the replacement of coal with a fuel that uses the same equipment and processes in power generation but with less air emission and with an economically profitable change. The results suggest that a plant with a supply of 104 t/h of bituminous coal could be replaced between 18 and 37 t/h of hydrochar, with a reduction of 8 and 27% in NOx and SO2 emissions, a reduction in 7 to 24% in ashes and a marginal increase in CO2 emission. The proposed use of hydrochar was economically profitable, with internal rates of return of up to 40% and with cash flows that reached USD 22 million.

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.

Hydrothermal Treatment of Residual Forest Wood (Softwood) and Digestate from Anaerobic Digestion—Influence of Temperature and Holding Time on the Characteristics of the Solid and Liquid Products

Hydrothermal treatment (HTT) offers the potential to upgrade low-value biomass such as digestate (DG) or forest residue (FR) by producing solids and liquids for material use or energetic utilization. In this study, microwave-assisted HTT experiments with DG and FR as feedstocks were executed at different temperatures (130, 150, 170 °C) and with different holding times (30, 60, 90 min) to determine the influences on product properties (ash and elemental concentrations, calorific values and chemical compounds). In general, DG and FR reacted differently to HTT. For the DG solids, for instance, the ash concentration was reduced to 8.68%DM at 130 °C (initially 27.67%DM), and the higher heating value increased from 16.55 MJ/kgDM to 20.82 MJ/kgDM at 170 °C, while the FR solids were affected only marginally. Elements with importance for emissions in combustion were leached out in both HTT solids. The DG and FR liquids contained different chemical compounds, and the temperature or holding time affected their formation. Depending on the designated application of HTT, less severe conditions can deliver better results. It was demonstrated that different low-temperature HTT conditions already induce strong changes in the product qualities of DG and FR. Optimized interactions between process parameters (temperature, holding time and feedstock) might lead to better cost–benefit effects in HTT.

Preparation of carbonyl, hydroxyl, and amino-functionalized microporous carbonaceous nanospheres from syrup-based waste to remove sulfamethazine

Sulfadiazine (SDZ) was a persistent sulfonamide antibiotic with a potential risk to human health. The waste dipping syrup was considered useless and environmentally unfriendly solution. In this work, carbonyl-, hydroxyl-, and amino-functionalized microporous carbonaceous nanospheres were synthesized using waste dipping syrup with glucose, fructose, and nitrogen, which was used as precursor for hydrothermal and pyrolysis process. The products were characterized using X-ray diffraction (XRD), scanning electron microscopy (SEM), Fourier-transformed infrared spectroscopy (FTIR), the point of zero charge (PZC), Xray photoelectron spectroscopy (XPS), and Brunauer-Emmett-Teller (BET). The carbonaceous nanospheres with large BET surface area (924.528 m²/g), micropores (2.127 nm), and high micro-porosity (89.54 %) allowed the rapid diffusion of SDZ (0.512nm×0.738 nm) into micropores of nanospheres. The majority SDZ (initial concentration = 20 mg/L) was removed (>96.8%) in the presence of 1.0 g/L nanoparticles after 40-min reaction at pH = 6.0. The adsorption capacity of SDZ onto nanospheres was 96.6 mg/g. The adsorption kinetic and equilibrium followed pseudo-first-order model and Langmuir isotherm, respectively. The intra-particle diffusion model indicated a three-step adsorption process. In addition, the regenerated nanospheres could be reused over four recycles. The optimal fabrication was realized at lower hydrothermal and pyrolysis temperature of 180 °C and 400 °C, respectively, which involved no additional chemical activating agent and had a high yield (70.8 %). Collectively, hydroxylation, carboxylation, amination, large specific surface area, and multi-microporosity may be responsible for improved adsorption performance of SDZ onto nanospheres. The findings provided a novel pathway for SDZ-loading wastewater treatment using waste syrup.

An Extensive Review and Comparison of Modern Biomass Torrefaction Reactors vs. Biomass Pyrolysis—Part 1

Major efforts are currently being made in the research community to address the challenges of greenhouse gas emissions from fossil fuel combustion by using lignocellulosic biomass, agricultural waste, and forest residues as cleaner energy sources. However, its poor qualities, such as low energy density, high moisture content, irregular shape and size, and heterogeneity, make it impossible to utilize in its natural state. Torrefaction, a simple heat treatment method, is used frequently with natural bioresources to improve their thermal characteristics so that they may be used as energy sources in domestic power plants. The quality of the resulting torrefied solids (biochar) is determined by the heat condition settings in the absence of oxygen, and it may be enhanced by carefully selecting and altering the processing parameters. The comprehensive overview presented here should serve as a useful toolkit for farmers, combined heat and power plants, pulp and paper installations, and other industrial plants that use biomass as a substrate for biofuel production. This research focuses on torrefaction product properties, reaction mechanisms, a variety of technologies, and torrefaction reactors. It is impossible to determine which torrefaction technology is superior as each reactor has unique properties. However, some suggestions and recommendations regarding the use of torrefaction reactors are given.

Valorization of sugar beet pulp to value-added products: A review

The processing of sugar beet in the sugar production industry releases huge amounts of sugar beet pulp as waste which can be considered a valuable by-product as a source of cellulose, hemicellulose, and pectin. Valorization of sugar beet pulp into value added products occurs through acid hydrolysis, hydrothermal techniques, and enzymatic hydrolysis. Biochemical conversion of beet pulp into simple fermentable sugars for producing value added products occurs through enzymatic hydrolysis is a cost effective and eco-friendly process. While beet pulp has predominantly been used as a fodder for livestock, recent developments in its biotechnological valorization have unlocked its value as a feedstock in the production of biofuels, biohydrogen, biodegradable plastics, and platform chemicals such as lactic acid, citric acid, alcohols, microbial enzymes, single cell proteins, and pectic oligosaccharides. This review brings forward recent biotechnological developments made in the valorization of sugar beet pulp into valuable products.

Structural, inorganic, and adsorptive properties of hydrochars obtained by hydrothermal carbonization of coffee waste

The hydrothermal carbonization process is a suitable process for the conversion of potentially harmful lignocellulosic waste into hydrochars. Defective coffee beans were the precursor raw material for hydrochar synthesis. Reactions were performed in a high-pressure reactor at 150, 200, and 250 °C, in autogenous pressure, for 40 min. Hydrochars were recovered by filtration and characterized by energy dispersive X-ray fluorescence spectroscopy, UV-Vis spectrophotometry, attenuated total reflection Fourier-transform infrared spectroscopy, differential thermal analysis, and scanning electron microscopy. Methylene blue adsorption tests were performed and analyzed by Langmuir and Freundlich adsorption isotherms. Adsorption mechanisms were investigated by computational calculations at DFT level. Results suggest that hydrochars from defective coffee beans can be applied as technological resources in the agronomic and environmental fields due to their inorganic composition, mainly to high magnesium content, the structural characteristics of porosity, biodegradation control, soil carbon-fixation and adsorption capacity. Important adsorption processes are caused by the development of oxygenated functional groups on the hydrochar surface.

Previous fire occurrence, but not fire recurrence, modulates the effect of charcoal and ash on soil C and N dynamics in Pinus pinaster Aiton forests

Understanding the effects of fire history on soil processes is key to characterise their resistance and resilience under future fire events. Wildfires produce pyrogenic carbonaceous material (PCM) that is incorporated into the soil, playing a critical role in the global carbon (C) cycle, but its interactions with soil processes are poorly understood. We evaluated if the previous occurrence of wildfires modulates the dynamic of soil C and nitrogen (N) and microbial community by soil ester linked fatty acids, after a new simulated low-medium intensity fire. Soils with a different fire history (none, one, two or three fires) were heat-shocked and amended with charcoal and/or ash derived from Pinus pinaster. Soil C and N mineralization rates were measured under controlled conditions, with burned soils showing lower values than unburned (without fire for more than sixty years). In general, no effects of fire recurrence were observed for any of the studied variables. Microbial biomass was lower in burned, with a clear dominance of Gram-positive bacteria in these soils. PCM amendments increased cumulative carbon dioxide (CO₂) production only in previously burned soils, especially when ash was added. This contrasted response to PCM between burned and unburned soils in CO₂ production could be related to the effect of the previous wildfire history on soil microorganisms. In burned soils some microorganisms might have been adapted to the resulting conditions after a new fire event. Burned soils showed a significant positive priming effect after PCM amendment, mainly ash, probably due to an increased pH and phosphorous availability. Our results reveal the role of different PCMs as drivers of C and N mineralization processes in burned soils when a new fire occurs. This is relevant for improving models that evaluate the net impact of fire in C cycling and to reduce uncertainties under future changing fire regimes scenarios.

Optimized production, Pb(II) adsorption and characterization of alkali modified hydrochar from sugarcane bagasse

Today, sugarcane bagasse (SB) is used for bioethanol and biodiesel production, energy generation, and adsorbent synthesis. The goal of this project is to determine the optimized conditions for producing adsorbent from sugarcane bagasse using hydrothermal carbonization (HTC) and KOH activation. To optimize process parameters such as reaction temperature, residence time, ZnCl₂/SB mixing ratios, and water/SB mixing ratios, response surface methodology was used. The results revealed that the optimum modified adsorption occurred at 180 °C, 11.5 h, a water to biomass ratio of (5:1), and a ZnCl₂ to precursor ratio of (3.5:1). The physicochemical features of optimum activated hydrochar were investigated, as well as batch adsorption experiments. The pseudo-second-order kinetic model and the Langmuir isotherm model were found to fit the experimental results in batch adsorption studies [\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${q}_{max}=90.1$$\end{document}qmax=90.1 (mg/g)]. Thermodynamic experiments further confirmed the spontaneous and exothermic adsorption mechanism.

Improving nutrients removal and energy recovery from wastes using hydrochar

Hydrothermal carbonization (HTC) is an eco-friendly, flexible and efficient way to valorise wet solid wastes, producing a carbon-rich material named as hydrochar. Considerable efforts have been devoted to studying the feasibility of using hydrochar in waste management to achieve the goal of circular economy. However, a comprehensive evaluation of the impacts of hydrochar on energy recovery from anaerobic digestion (AD), nutrient reclamation, and wastewater treatment is currently lacking. To understand the influence of hydrochar type on its application, this review will firstly introduce the mechanisms and biomass treatment for hydrochar preparation. Most recent studies regarding the improvement of methane (CH₄) and volatile fatty acids (VFAs) production after dosing hydrochar in anaerobic digesters are quantitatively summarized and deeply discussed. The potential of using various hydrochar as slow-fertilizer to support the growth of plants are analysed by providing quantitative data. The usage of hydrochar in remediating pollutants from wastewater as effective adsorbent is also evaluated. Based on the review, we also address the challenges and demonstrate the opportunities for the future application of hydrochar in waste management. Conclusively, this review will not only provide a systematic understanding of the up-to-date developments of improving the nutrients removal and energy recovery from wastes by using hydrochar but also several new directions for the application of hydrochar in the future.

Chitosan-derived hydrothermally carbonized materials and its applications: A review of recent literature

Chitosan (CS) is a linear polysaccharide biopolymer, one of the most abundant biowastes in the environment. This makes chitosan a potential material for a wide range of applications. To improve CS's properties, chitosan has to be chemically modified. Hydrothermal carbonization (HTC) is a sustainable process for converting chitosan to solid carbonized material. This article presents a review on the applications of hydrothermally treated chitosan in different fields such as water treatment, heavy metals adsorption, carbon dioxide capturing, solar cells, energy storage, biosensing, supercapacitors, and catalysis. Moreover, this review covers the impact of HTC process parameters on the properties of the produced carbon material. The diversity of applications indicates the great possibilities and multifunctionality of hydrothermally carbonized chitosan and its derivatives. The utilization of HTC-CS is expected to further expand as a result of the movement toward sustainable, environmentally-friendly resources. Thus, this review also recommends a few suggestions to improve the properties of HTC chitosan and its comprehensive applications.

Uncatalyzed and acid-aided microwave hydrothermal carbonization of orange peel waste

Orange, one of the most important fruit categories to be consumed across the world, when processed produces 50% of its weight as waste. Current waste management options for orange peel waste are inadequate to use the waste in wholesome and its disposal might lead to other environmental concerns. Here, we present microwave hydrothermal carbonization as an alternative to utilize the orange peel waste. Further, using citric acid to catalyze the microwave hydrothermal carbonization resulted in 30% higher maximal yield of hydrochar, and the hydrochar produced had better elemental, proximate and energy properties than hydrochar made during uncatalyzed microwave hydrothermal carbonization. Further, structural analysis revealed that citric acid promoted the formation of nanospheres during microwave hydrothermal carbonization. Taken together, microwave hydrothermal carbonization of orange peel waste using citric acid as a catalyst might not only help address the waste management concerns for orange peel waste, but also can produce end products of potential commercial value.

Study of Thermal Behavior of Moxa Floss Using Thermogravimetric and Pyrolysis-GC/MS Analyses

Moxa floss is a type of biomass used as the main combustion material in moxibustion, a therapy that applies heat from moxa floss combustion to points or body areas for treatment. Safety concerns regarding moxa smoke have been raised in recent years. Since moxa floss is the source material in moxibustion, its thermal behavior and pyrolysis products would be related to the products formed in moxa smoke. This work aims to understand the thermal behavior of moxa floss and investigate the pyrolysis products generated from moxa floss combustion. Six commercial moxa floss samples of 3 storage years and 10 storage years, and of low, medium, and high ratios, were selected. The kinetic data from moxa floss combustion was carried out by a thermogravimetric analyzer. Pyrolysis-gas chromatography and mass spectroscopy using a gas chromatograph and mass spectrometer equipped with a pyroprobe were used to examine the pyrolysis products. Thermogravimetric profiles for all the samples were overall similar and showed a monotonic weight decrease. The range of intensive reaction temperature occurred between 150°C and 450°C, which was characterized by a major weight loss and accompanied by an exothermal degradation of the main components. The average ignition temperature for the samples of 3 and 10 storage years was 218.3°C and 222.6°C, respectively, which was lower than most herbaceous plants. The identified pyrolysis products include monocyclic aromatic hydrocarbons, polycyclic aromatic hydrocarbons, ketones, acids, and alkanes. All were of relatively low intensities of below 5% in relative abundance. No volatiles were detected in the samples of 10 storage years. The relatively low values of ignition temperature suggested that moxa floss is more combustible and can be ignited more easily than other herbaceous plants. This may explain why moxa floss has remained as the preferred material used for moxibustion over the years.

Coupling hydrothermal carbonization and anaerobic digestion for sewage digestate management: Influence of hydrothermal treatment time on dewaterability and bio-methane production

Hydrothermal carbonization (HTC) technology is addressed in the framework of sewage digestate management. HTC converts digestate into a stabilized and sterilized solid (the hydrochar) and a liquor (HTCL) rich in organic carbon. This study aims to optimize the HTC operating parameters, namely the treatment time, in terms of hydrochar production, HTC slurry dewaterability, HTCL bio-methane yields in anaerobic digestion (AD), and process energy consumption. Digestate slurry was processed through HTC at different treatment times (0.5, 1, 2 and 3 h) at 190 °C, and the dewaterability of the treated slurries was addressed through capillary suction time and centrifuge lab-testing. In addition, biochemical methane potential (BMP) tests were conducted for HTCL under mesophilic conditions. Results show that by increasing the HTC treatment time the dewaterability was further improved, ammonium concentration in HTCL increased, and methane potential of HTCL decreased. 0.5 h HTCL had the highest bio-methane potential of 142 ± 3 mL CH₄/g COD yet the treatment time was not sufficient for improving the slurry's dewaterability. HTC treatment time of 1 h at 190 °C was identified as the optimum trade-off for improved dewaterability and utilisation of HTCL for biogas production. 1 h HTCL bio-methane potential can cover around 25% of the HTC and AD thermal and electrical energy needs without considering the eventual use of the hydrochar as a biofuel.

A review on nitrogen transformation in hydrochar during hydrothermal carbonization of biomass containing nitrogen

Biomass is a type of renewable and sustainable resource that can be used to produce various fuels, chemicals, and materials. Nitrogen (N) in biomass such as microalgae should be reduced if it is used to produce fuels, while the retention of N is favorable if the biomass is processed to yield chemicals or materials with N-containing functional groups. The engineering of the removal and retention of N in hydrochar during hydrothermal carbonization (HTC) of biomass rich in protein is a research hot spot in the past decade. However, the N transformation during HTC has not yet been fully understood. In order to mediate the migration and transformation of N in hydrochar, the present review overviewed i) the characteristics of hydrochar and the original feedstock, ii) the possible N transformation behavior and mechanisms, and iii) the effect of factors such as feedstock and pyrolysis parameters such as temperature on hydrochar N. The high temperature and high protein content promote the dehydration, decarboxylation, and deamination of biomass to produce hydrochar solid fuel with reduced N content, while the Millard and Mannich reactions for lignocellulosic biomass rich in carbohydrate (cellulose, hemicellulose, and lignin) at medium temperatures (e.g., 180-240 °C) significantly promote the enrichment of N in hydrochar. The prediction models can be built based on properties of biomass and the processing parameters for the estimation of the yield and the content of N in hydrochar.

Investigation on parameters optimization to produce hydrochar without carbohydrate carbon

Owing to its superior energy-saving function and strong potential to be applied as a soil amendment and fertilizer, hydrochar has gained wide attention in recent years. However, hydrochar contains greater amounts of labile fractions than traditional biochar and may exacerbate the short-term greenhouse effect. To lower the risk of greenhouse gas release due to labile fractions, optimize parameters must be determined to produce hydrochar without carbohydrate carbon. In addition, the effects of varying feedstocks and process conditions on hydrochar structure as well as its dissolved organic matter (DOM) must be investigated. Spartina alterniflora and pig manure were used to produce two hydrochars (HSAs and HPMs) and their corresponding DOM samples (DSAs and DPMs) at various production temperatures (T_p) and reaction times (t_r). The carbohydrate vanishing points (CVPs) were 265 °C-1 h, 250 °C-2 h, and 245 °C-4 h for the HSAs and 260 °C-1 h, 250 °C-2 h, and 250 °C-4 h for the HPMs. With the isolation of DOM, 1.09-4.33% organic carbon of the hydrochar was released. The aromaticity of DSAs decreased with increasing T_p and t_r. The molecular weights of the DSAs and DPMs decreased with increasing T_p and t_r. This study uncovered hydrochar's molecular structure as well as the content and properties of its labile fractions. Results can be used to help design specific hydrochars for potential applications, based on the trend of the molecular change under the condition of the studied parameters optimization to produce hydrochar in this study.

Comparative Studies on Water- and Vapor-Based Hydrothermal Carbonization: Process Analysis

Hydrothermal carbonization (HTC) reactor systems used to convert wet organic wastes into value-added hydrochar are generally classified in the literature as liquid water-based (HTC) or vapor-based (VTC). However, the distinction between the two is often ambiguous. In this paper, we present a methodological approach to analyze process conditions for hydrothermal systems. First, we theoretically developed models for predicting reactor pressure, volume fraction of liquid water and water distribution between phases as a function of temperature. The reactor pressure model predicted the measured pressure reasonably well. We also demonstrated the importance of predicting the condition at which the reactor system enters the subcooled compression liquid region to avoid the danger of explosion. To help understand water–feedstock interactions, we defined a new solid content parameter %S(T) based on the liquid water in physical contact with feedstock, which changes with temperature due to changes in the water distribution. Using these models, we then compared the process conditions of seven different HTC/VTC cases reported in the literature. This study illustrates that a large range of conditions need to be considered before applying the label VTC or HTC. These tools can help in designing experiments to compare systems and understand results in future HTC research.

Effect of pyrolysis temperature and correlation analysis on the yield and physicochemical properties of crop residue biochar

The aim of this study was to evaluate how pyrolysis temperature influences the yield and physicochemical properties of biochar. We produced biochar from four feedstocks (wheat straw, corn straw, rape straw, and rice straw) pyrolyzed at 300, 400, 500, and 600 °C for 1 h, respectively. The results showed that all biochar yields decreased consistently with increasing temperature during pyrolysis and showed a steady decrease over 400 °C. Rice straw derived biochar had high yield superiority due to its higher content of ash. Pyrolysis temperature has significant effects on the properties of biochar; demonstrating a negative relationship with H, O, H/C, O/C, (O + N)/C, and functional groups, whilst having a positive relationship with C, ash, pH, electrical conductivity, and surface roughness. Higher pyrolysis temperature was beneficial to the formation of a more recalcitrant constitutions and crystal structure, making it available for material application.

Valorisation of food waste via hydrothermal carbonisation and techno-economic feasibility assessment

Food waste constitutes a remarkable portion of municipal solid waste. About one-third of the global food waste produced is lost with the food supply chain. Food waste in many countries is still dumped of in landfill or incinerated simultaneously with other municipal wastes. Food waste requires proper management and recycling techniques in order to minimise its environmental burden and risk to human life. Despite considerable research on food waste conversion still, there is a shortage of comprehensive reviews of the published literature. In this review, we provide a mini global perspective of food waste with special emphasis on New Zealand and their conversion into the useful material through hydrothermal carbonisation (HTC). Other thermal technologies such as incineration and pyrolysis are also briefly discussed. The review discusses why HTC is more suitable thermal technology than others, which are currently available. Recognising the importance of techno-economic feasibility of HTC, we present a cost analysis on the production of value-added products via HTC with examples taken from the literature to gather information in the feasibility assessment process. Finally, key challenges and future directions for a better productive way of handling food waste are being suggested.

Pyrolysis Kinetics of Hydrochars Produced from Brewer’s Spent Grains

The current market situation shows that large quantities of the brewer’s spent grains (BSG)—the leftovers from the beer productions—are not fully utilized as cattle feed. The untapped BSG is a promising feedstock for cheap and environmentally friendly production of carbonaceous materials in thermochemical processes like hydrothermal carbonization (HTC) or pyrolysis. The use of a singular process results in the production of inappropriate material (HTC) or insufficient economic feasibility (pyrolysis), which hinders their application on a larger scale. The coupling of both processes can create synergies and allow the mentioned obstacles to be overcome. To investigate the possibility of coupling both processes, we analyzed the thermal degradation of raw BSG and BSG-derived hydrochars and assessed the solid material yield from the singular as well as the coupled processes. This publication reports the non-isothermal kinetic parameters of pyrolytic degradation of BSG and derived hydrochars produced in three different conditions (temperature-retention time). It also contains a summary of their pyrolytic char yield at four different temperatures. The obtained KAS (Kissinger–Akahira–Sunose) average activation energy was 285, 147, 170, and 188 kJ mol−1 for BSG, HTC-180-4, HTC-220-2, and HTC-220-4, respectively. The pyrochar yield for all hydrochar cases was significantly higher than for BSG, and it increased with the severity of the HTC’s conditions. The results reveal synergies resulting from coupling both processes, both in the yield and the reduction of the thermal load of the conversion process. According to these promising results, the coupling of both conversion processes can be beneficial. Nevertheless, drying and overall energy efficiency, as well as larger scale assessment, still need to be conducted to fully confirm the concept.

Hydrothermal carbonization of yard waste for solid bio-fuel production: Study on combustion kinetic, energy properties, grindability and flowability of hydrochar

Yard waste is either dumped or is being openly burned to get rid of it, instead of using it as a valuable renewable energy source. In this study, hydrothermal carbonization of yard waste was conducted to valorize it as a solid bio fuel, using a batch reactor. The effect of process parameter on yield, energy and physicochemical properties of the valorized solid bio fuel (hydrochar) was examined in this study by varying reaction temperature (160-200 °C for 2 h) and reaction time (2-24 h at 200 °C). The calorific value of hydrochar was within a range of 17.72-24.59 MJ/kg as compared to 15.37 MJ/kg for untreated yard waste. Hydrochar mass yield decreased from 78.6% at operating temperature - time of 160 °C -2 h to 45.6% at 200 °C -24 h. The plot of atomic ratios (H/C and O/C) demonstrates improvement in the coalification process which was mainly governed by decarboxylation and dehydration reactions. The grindability of the prepared hydrochar was comparable to that of coal. Hydrochar produced at lower reaction condition (160-200 °C at 2 h) have better flowability as compared to that produced at higher reaction condition (4-24 h at 200 °C). The reaction time longer than 12 h has a minimal effect on the yield, energy and physicochemical properties of hydrochar. Increasing reaction time and temperature improved the ignition and burnt temperature of hydrochar. All reaction condition has an energy ratio (energy output to energy input) of more than one making HTC process a net energy producer.

Characterization and quantification of electron donating capacity and its structure dependence in biochar derived from three waste biomasses

Biochar has shown a unique electrochemical property being involved in various redox reactions in soil and water. In this study, the electron donating capacities (EDCs) of biochar pyrolyzed at 200-800 °C from pine wood, barley grass and wheat straw were investigated by using the mediated electrochemical oxidation method. The EDC values for all biochar were in the range of 0.18-1.83 mmol e^- (g biochar)^-1, showing the increase as the temperature increased from 200 °C to 400 °C, the decrease from 400 °C to 650 °C, and then increase from 650 °C until to 800 °C. At low and intermediate temperatures of 200-650 °C, the EDCs were mainly attributed to the phenolic hydroxyl groups, while the conjugated π-electron system associated with aromatic structure dominated the EDCs of biochar at the high temperatures of over 650 °C. The barley grass- and wheat straw-derived biochar had higher EDCs than the pine wood-derived biochar, resulting from the higher phenolic hydroxyl groups in the former samples than the latter one. In conclusion, the reductive property of biochar was mainly attributed to both phenolic hydroxy group and conjugated π-electron system associated with aromatic structure, depending on the pyrolytic temperature and feedstock source. The results will help us to obtain a complete view on the role of biochar in biogeochemical redox reactions and consider developing biochar with controlled redox properties for specific environmental applications such as electron shuttle and catalyst material.

Design and Preparation of Biomass-Derived Carbon Materials for Supercapacitors: A Review

The synthesis and application of biomass-derived carbon in energy storage have drawn increasing research attention due to the ease of fabrication, cost-effectiveness, and sustainability of the meso/microporous carbon produced from various biological precursors, including plants, fruits, microorganisms, and animals. Compared to the artificial nanostructured carbons, such as fullerene, carbon nanotube and graphene, the biomass-derived carbons may obtain superior capacitance, rate performance and stability in supercapacitor applications ascribing to their intrinsic nanoporous and hierarchical structures. However, challenges remain in processing techniques to obtain biomass-derived carbons with high carbon yield, high energy density, and controllable graphitic microstructures, which may require a clear understanding over the chemical and elemental compositions, and the intrinsic microstructural characteristics of the biological precursors. Herein we present comprehensive analyses over the impacts of the chemical and elemental compositions of the precursors on the carbon yield of the biomass, as well as the mechanism of chemical activation on the nanoporous structure development of the biomass-derived carbons. The structure–property relationship and functional performance of various biomass-derived carbons for supercapacitor applications are also discussed in detail and compared. Finally, useful insights are also provided for the improvements of biomass-derived carbons in supercapacitor applications.

Quantifying the sensitivity of feedstock properties and process conditions on hydrochar yield, carbon content, and energy content

Hydrothermal carbonization (HTC) is a wet, low temperature thermal conversion process that continues to gain attention for the generation of hydrochar. The importance of specific process conditions and feedstock properties on hydrochar characteristics is not well understood. To evaluate this, linear and non-linear models were developed to describe hydrochar characteristics based on data collected from HTC-related literature. A Sobol analysis was subsequently conducted to identify parameters that most influence hydrochar characteristics. Results from this analysis indicate that for each investigated hydrochar property, the model fit and predictive capability associated with the random forest models is superior to both the linear and regression tree models. Based on results from the Sobol analysis, the feedstock properties and process conditions most influential on hydrochar yield, carbon content, and energy content were identified. In addition, a variational process parameter sensitivity analysis was conducted to determine how feedstock property importance changes with process conditions.

Symbiotic relationship between hydrothermal carbonization technology and anaerobic digestion for food waste in China

Food waste (FW) is traditionally disposed through landfills and incineration in China. Nowadays, there are some promising methods, such as anaerobic digestion (AD) or feeding and composting, which are being applied in pilot cities. However, the inherent characteristics of Chinese FW may be regarded as a double-edged sword in the practical applications of these disposal methods. To overcome these challenges, two modes of the hydrothermal carbonization (HTC) process were reviewed as innovative strategies in this article. Meanwhile, the "symbiotic relationship" between Chinese FW and HTC technologies was highlighted. To improve treatment efficiency of FW, we should not only try different methods and develop existing technologies, but also pay more attention to the utilization and "1 + 1 > 2" synergistic effect of their combinations, such as the combination of HTC and AD as a co-treatment method for saving on the construction cost and avoiding redistribution of social resources.

Oxidation resistance of biochars as a function of feedstock and pyrolysis condition

Assessing biochar's ability to resist oxidation is fundamental to understanding its potential to sequester carbon. Chemical oxidation exhibits good performance in estimating the oxidation resistance of biochar. Herein, oxidation resistance of 14 types of biochars produced from four feedstocks at different pyrolysis conditions (hydrothermal versus thermal carbonization) was investigated via hydrogen peroxide oxidation with varying concentrations. The oxidation resistance of organic carbon (C) of hydrochars was relatively higher than that of 250°C pyrochars (P250) but was comparable to that of 450°C pyrochars (P450). Both hydrochars and P450 from ash-rich feedstocks contained at least three different C pools (5.9-18.3% labile, 43.2-56.5% semi-labile and 26.9-45.9% stable C). Part (<33%) of aromatic C within 600°C pyrochars (P600) was easily oxidizable, which consisted of amorphous C. The influence of pyrolysis temperature upon oxidation resistance of biochars depended on the feedstock. For ash-rich feedstock (rice straw, swine manure and poultry litter), the oxidation resistance of biochars was determined by both aromaticity and mineral components, and mineral protection was regulated by pyrolysis conditions. The amorphous silicon within hydrochars and P450 could interact with C, preventing C from being oxidized, to some extent. Nevertheless, this type of protection did not occur for P250 and P600.

Evaluation of engineered hydrochar from KMnO4 treated bamboo residues: Physicochemical properties, hygroscopic dynamics, and morphology

In this study, a novel approach was developed to prepare engineered hydrochar from KMnO₄ treated bamboo residues through hydrothermal carbonization. The hydrochar yields were within a specified range of 61.8-67.8% at 180 °C and 39.8-45.0% at 260 °C, respectively. The higher temperature led to the higher C content, lower H/C and O/C ratio, whereas the ash content increased with increasing KMnO₄ concentration, causing the increase of solid yield as well as the decrease of C content. Pseudo-second kinetic model was optimal to describe bamboo hydrochar's hygroscopic dynamic, and the engineered hydrochar produced at 260 °C and 1.0 wt% concentration obtained the better hydrophobicity of 0.82%. SEM-EDS and XRD analysis confirmed the existence of manganese carbonate on the surface of engineered hydrochar, from which we inferred the chemical complexation between KMnO₄ and hydrochar.

Biochar as an adsorbent for inorganic nitrogen and phosphorus removal from water: a review

Biochar is the solid product of biomass pyrolysis that can be used for carbon sequestration, soil amendment, and pollution remediation. The use of biochar as an adsorbent for the removal of water contaminants has elicited increasing interest due to the multifunctional properties of this material. The application of biochar in the adsorption of inorganic nutrients from eutrophic water has not been reviewed. This review focuses on recent research on the use of biochar for the adsorption of inorganic nitrogen (ammonium and nitrate) and phosphorus (phosphate) from water, especially for the main influence factors and mechanisms for nitrogen and phosphorus adsorption on biochar.

Hydrothermal carbonization of food waste for nutrient recovery and reuse

Food waste represents a rather large and currently underutilized source of potentially available and reusable nutrients. Laboratory-scale experiments evaluating the hydrothermal carbonization of food wastes collected from restaurants were conducted to understand how changes in feedstock composition and carbonization process conditions influence primary and secondary nutrient fate. Results from this work indicate that at all evaluated reaction times and temperatures, the majority of nitrogen, calcium, and magnesium remain integrated within the solid-phase, while the majority of potassium and sodium reside in the liquid-phase. The fate of phosphorus is dependent on reaction times and temperatures, with solid-phase integration increasing with higher reaction temperature and longer time. A series of leaching experiments to determine potential solid-phase nutrient availability were also conducted and indicate that, at least in the short term, nitrogen release from the solids is small, while almost all of the phosphorus present in the solids produced from carbonizing at 225 and 250°C is released. At a reaction temperature of 275°C, smaller fractions of the solid-phase total phosphorus are released as reaction times increase, likely due to increased solids incorporation. Using these data, it is estimated that up to 0.96% and 2.30% of nitrogen and phosphorus-based fertilizers, respectively, in the US can be replaced by the nutrients integrated within hydrochar and liquid-phases generated from the carbonization of currently landfilled food wastes.

Bio-butanol sorption performance on novel porous-carbon adsorbents from corncob prepared via hydrothermal carbonization and post-pyrolysis method

A series of porous-carbon adsorbents termed as HDPC (hydrochar-derived pyrolysis char) were prepared from corncob and used for the 1-butanol recovery from aqueous solution. The influences of pyrolysis temperature on properties of the adsorbents were systematically investigated. The results showed that hydrophobicity, surface area, and pore volume of HDPC samples increased with an increase in pyrolysis temperature. Furthermore, the adsorption mechanism of 1-butanol on the adsorbents was explored based on correlation of the samples properties with adsorption parameters extracted from the 1-butanol adsorption isotherms (K_F and Q_e12). Overall, the 1-butanol adsorption capacity increased with a decrease in polarity and an increase in aromaticity, surface area and pore volume of HDPC samples. However, at different pyrolysis temperature, the factors causing the increase of 1-butanol adsorption on the adsorbents are variable. The kinetic experiments revealed that the pores played a vital role in the 1-butonal adsorption process. The intraparticle diffusion model was used to predict the adsorption kinetic process. The simulation results showed that intraparticle diffusion was the main rate-controlling step in the 1-butanol adsorption process.

Carbon sequestration potential and physicochemical properties differ between wildfire charcoals and slow-pyrolysis biochars

Pyrogenic carbon (PyC), produced naturally (wildfire charcoal) and anthropogenically (biochar), is extensively studied due to its importance in several disciplines, including global climate dynamics, agronomy and paleosciences. Charcoal and biochar are commonly used as analogues for each other to infer respective carbon sequestration potentials, production conditions, and environmental roles and fates. The direct comparability of corresponding natural and anthropogenic PyC, however, has never been tested. Here we compared key physicochemical properties (elemental composition, δ¹³C and PAHs signatures, chemical recalcitrance, density and porosity) and carbon sequestration potentials of PyC materials formed from two identical feedstocks (pine forest floor and wood) under wildfire charring- and slow-pyrolysis conditions. Wildfire charcoals were formed under higher maximum temperatures and oxygen availabilities, but much shorter heating durations than slow-pyrolysis biochars, resulting in differing physicochemical properties. These differences are particularly relevant regarding their respective roles as carbon sinks, as even the wildfire charcoals formed at the highest temperatures had lower carbon sequestration potentials than most slow-pyrolysis biochars. Our results challenge the common notion that natural charcoal and biochar are well suited as proxies for each other, and suggest that biochar’s environmental residence time may be underestimated when based on natural charcoal as a proxy, and vice versa.

Optimization and characterization of hydrochar produced from microwave hydrothermal carbonization of fish waste

Fish processing results in large amounts of solid and liquid wastes that are unsustainably dumped into oceans and landfills. Alternative sustainable technologies that completely utilize seafood wastes are needed. Hydrothermal carbonization (HTC) that converts moisture-rich biomass into hydrochar is mostly employed for pure lignocellulosic biowaste. However, the suitability of HTC for pure non-lignocellulosic waste is unknown. Here, for the first time, a response surface design guided optimization of microwave hydrothermal carbonization (MHTC) process parameters, holding temperature (150-210°C) and time (90-120min), showed that a temperature of approximately 200°C and a time of approximately 119min yielded maximal hydrochar (∼34%). The atomic carbon and ash content, and calorific value of hydrochar were approximately 25-57%, 20-28%, and 19-24.5MJ/kg respectively, depending on the MHTC operating conditions. Taken together, these results confirm that MHTC produces hydrochar from fish waste of quality comparable to one produced from certain lignocellulosic, sewage and municipal wastes. Therefore, this strategy presents an exciting alternative technology that can be used either independently or in combination with other valorization techniques to completely utilize fish wastes irrespective of their quality.

Removal of antimony (III) and cadmium (II) from aqueous solution using animal manure-derived hydrochars and pyrochars

In this study, hydrochars and pyrochars prepared from animal manures were characterized and were used to remove Sb (III) and Cd (II) from aqueous solution. Fourier transform infrared spectroscopy (FTIR) analysis revealed the interaction between Cd (II) and CO and CO groups within biochars and between Sb (III) and CO, CO and OH groups, respectively. Additionally, the lower absolute value of zeta potential of biochar after loading Sb (III) and Cd (II) suggested the occurrence of surface complexation. Existing primarily in the form of Sb (OH)₃, the maximum adsorption capacities (Q_max) for Sb (III) were lower than those for Cd (II). Due to the lower contents of surface polar functional groups and less negative surface charge, hydrochars exhibited lower Q_max for Sb (III) and Cd (II) than pyrochars. However, hydrochars in this study had higher sorption capacities for Cd (II) than most of plant-based pyrochars reported by other literature.

New Evidence for High Sorption Capacity of Hydrochar for Hydrophobic Organic Pollutants

This study investigated the sorption potential of hydrochars, produced from hydrothermally carbonizing livestock wastes, toward organic pollutants (OPs) with a wide range of hydrophobicity, and compared their sorption capacity with that of pyrochars obtained from conventional dry pyrolysis from the same feedstock. Results of SEM, Raman, and ¹³C NMR demonstrated that organic carbon (OC) of hydrochars mainly consisted of amorphous alkyl and aryl C. Hydrochars exhibited consistently higher log K_oc of both nonpolar and polar OPs than pyrochars. This, combined with the significantly less energy required for the hydrothermal process, suggests that hydrothermal conversion of surplus livestock waste into value-added sorbents could be an alternative manure management strategy. Moreover, the hydrochars log K_oc values were practically unchanged after the removal of amorphous aromatics, implying that amorphous aromatic C played a comparable role in the high sorption capacity of hydrochars compared to amorphous alkyl C. It was thus concluded that the dominant amorphous C associated with both alkyl and aryl moieties within hydrochars explained their high sorption capacity for OPs. This research not only indicates that animal-manure-derived hydrochars are promising sorbents for environmental applications but casts new light on mechanisms underlying the high sorption capacity of hydrochars for both nonpolar and polar OPs.

Multiscale characteristics dynamics of hydrochar from hydrothermal conversion of sewage sludge under sub- and near-critical water

Dewatered sewage sludge was upgraded to hydrochar using hydrothermal conversion in sub- and near-critical water. Three characteristic temperature regimes responsible for the upgrading were identified. Drastic hydrolysis of carbohydrates, amide II or secondary amines occurred at 200°C while noticeable decarboxylation initiated above 260°C. Elevated temperature improved porosity but did not induce higher surface area. Aliphatic C was mainly transformed to aromatic hydrocarbon rather than aromatic C-O in subcritical water, whereas COO/N-CO and aromatic C-O were decomposed to carbohydrate C at 380°C. Below 300°C, carbon functionalities in hydrochars were thermally stable and faster decomposition of N than C-(C,H) resulted in dramatic decline of N/C. Above 300°C, C-H was gradually polymerized to aromatic C-(C,H) which was considerably transformed to C-(O,N) and C-H at 380°C. CaO favored intense destruction of aromatic C-C/C-H, anomeric O-C-O, C-H and C-(O,N) functionalities but introduced more aromatic C-O and OC-O.

Sorption of four hydrophobic organic contaminants by biochars derived from maize straw, wood dust and swine manure at different pyrolytic temperatures

Sorption behavior of acetochlor (ACE), dibutyl phthalate (DBP), 17α-Ethynyl estradiol (EE2) and phenanthrene (PHE) with biochars produced from three feedstocks (maize straw (MABs), pine wood dust (WDBs) and swine manure (SWBs)) at seven heat treatment temperatures (HTTs) was evaluated. The bulk polarity of these biochars declined with increasing HTT while the aromaticity and CO2-surface area (CO2-SA) rose. The surface OC contents of biochars were generally higher than bulk OC contents. The organic carbon (OC)-normalized CO2-SA (CO2-SA/OC) of biochars significantly correlated with the sorption coefficients (n and logK(oc)), suggesting that pore filling could dominate the sorption of tested sorbates. SWBs had higher logK(oc) values compared to MABs and WDBs, due to their higher ash contents. Additionally, the logK(oc) values for MABs was relatively greater than that for WDBs at low HTTs (≤400 °C), probably resulting from the higher CO2-SA/OC, ash contents and aromaticity of MABs. Surface polarity and the aliphatic C may dominate the sorption of WDBs obtained at relatively low HTTs (≤400 °C), while aromatic C affects the sorption of biochars at high HTTs. Results of this work aid to deepen our understanding of the sorption mechanisms, which is pivotal to wise utilization of biochars as sorbents for hazardous organic compounds.