Hydrothermal Carbonization Research Articles

Synthesizing a transesterification catalyst via a condensation reaction and assessment of the mechanism using density functional theory

AbstractBACKGROUNDA solid acid (SPEI–EN–OHCMs) having dendritic –NH2 groups was prepared via oxidation of hydrothermal carbon microspheres (OHCMs) followed by a condensation reaction with the –NH2 groups of polyethyleneimine and sulfonation. The mechanism by which –COOH groups on the surfaces of OHCMs reacted with these –NH2 groups was assessed using density functional theory (DFT).RESULTActivation using a combination of 1‐(3‐dimethylaminopropyl)‐3‐ethylcarbodiimide hydrochloride (EDC) and N‐hydroxysuccinimide (NHS) was found to promote the formation of amide bonds such that –NH2 groups were bonded to the microsphere surfaces. This effect provided more bonding sites for –SO3H groups. The DFT results indicated that the energy barrier on the potential energy surface in the presence of EDC and NHS was lowered by 18.1 kcal mol−1. X‐ray photoelectron spectroscopy analyses also confirmed the introduction of –NH2 groups followed by –SO3H groups onto the surfaces of the OHCMs. The concentration of surface acid sites on SPEI–EN–OHCMs was found to be as high as 11.8 mmol g−1. Thermogravimetric data and elemental analysis results showed that the –SO3H groups on the SPEI–EN–OHCM surfaces were thermally stable below 180 °C.CONCLUSIONThe SPEI–EN–OHCMs catalyst was used to promote the transesterification reaction of waste frying oil with methanol. A trial with a methanol‐to‐oil molar ratio of 10:1 at 110 °C for 4 h gave a fatty acid methyl ester (FAME) yield of 92.3%. After four reuses, an SPEI–EN–OHCMs specimen provided a FAME yield of 54.3%. © 2024 Society of Chemical Industry (SCI).

Green synthesis of silane-assisted fluorescent carbon dots based on sanding dusts and its application in flame-retardant and anti-leaching wood materials

Novel silane-assisted flame-retardant carbon dots (FR-CDs) were synthesized via the hydrothermal carbonization strategy using recycled wood sanding dusts, tris(hydroxymethyl) phosphine oxide (THPO), phosphoric acid (PA), and amino-silane as precursors. The highly transparent FR-CDs suspension appears pale yellow color under daylight and exhibits a strong fluorescence response to light excitations, which helps to keep the original color of the wood and also has broad application prospects for monitoring the leaching behavior of FR-wood. The as-prepared FR-CDs can be applied in FR-wood materials using the facile dipping process owing to their tiny nanoparticles, uniform dispersion, and numerous nitrogen–phosphorus groups. In the flame test, the FR-CDs treated wood composites emitted little smoke initially and maintained primary carbon layer that did not catch fire for over 180 s. The flame-retardancy of the FR-wood materials was confirmed by the thermo-analysis, limiting oxygen index, exposure to a high heat flux (50 kW/cm2), and the propane blowtorch tests (∼600 °C). Moreover, the formation mechanism of the silane-assisted FR-CDs during hydrothermal process and the FR-wood materials with dipping method was proposed. This study provides a facile and efficient strategy for developing highly transparent and cheap flame retardants for the wood materials, which could expand the application of the wood sanding dusts and the silane-assisted carbon-based nanomaterials in the design of anti-leaching flame-retardant materials.

Hydrothermal carbonization of pretreated pine needles: The impacts of temperature and atmosphere in pretreatment on structural evolution of hydrochar

Hydrothermal carbonization (HTC) is a process for enhancing energy content of biomass through deoxygenation reactions, effectiveness of which depends on composition of biomass. Thermal pretreatment could induce structural change of biomass and thus affects the chemical inertness/reactivity in HTC. This was investigated herein by HTC of the pine needles, thermally treated at 250 or 350 °C with/without presence of 11.5% O2, at 250 °C for 10 h. The results showed that pretreatment at 250 °C could increase overall yield of hydrochar in subsequent HTC, due to the enhanced aromatization and the resistance towards fragmentation in HTC. Nonetheless, the HTC worked on biochar formed at 250 °C, facilitating deoxygenation to remarkably decrease O/C ratio. In comparison, the pretreatment at 350 °C removed substantial amount of volatile fractions, significantly increasing chemical inertness of resulting char-350 during the followed HTC. Oxygen presence induced oxidation reactions in pretreatment, forming more bio-oil/gases and making resulting hydrochar more hydrophilic. Especially, oxidation at 250 °C formed more aliphatic structures that could be easier removed during followed HTC, enhancing thermal stability of resulting hydrochar. Although a portion of cellulose retained in the biochar obtained from pretreatment at 250 °C, the elimination of aliphatics suppressed formation of spheric structures of hydrochar, while retained original fibrous structures from pine needles.

Bis-amino modified magnetic molasses wastewater hydrochar adsorbent for chromium removal

The removal of hexavalent chromium (Cr(VI)) from industrial wastewater has always been an important issue in the field of environment and human health. However, there are still challenges in achieving effective adsorption of Cr(VI) by adsorbents in slightly neutral environments. The resource utilization of organic wastewater is also a current research focus and difficulty in the academic community. In this study, molasses wastewater was used as a carbon and nitrogen source to synthesize magnetic amino hydrochar (MMHC) through a hydrothermal carbonization process. In order to further enhance the adsorption performance of MMHC for Cr(VI) in slightly neutral environments, a two-step grafting strategy was employed to introduce quaternary ammonium groups with N+ sites onto the MMHC substrate, resulting in the surface modification of MMHC with dual amino groups (MMHCQA). The introduction of dual amino groups not only increases the adsorption sites on the surface of MMHCQA, but also raises the point of zero charge of MMHCQA, ensuring its excellent adsorption performance for Cr(VI) in higher pH environments. At pH 6, the adsorption capacity of MMHCQA are 3 times higher than MMHC. This work elucidated the adsorption process using adsorption kinetics, thermodynamics, and adsorption isotherms, and fitted the three major factors (pH, Cr(VI) concentration and temperature) influencing MMHCQA adsorption performance through response surface analysis, revealing that pH is the decisive factor affecting MMHCQA adsorption performance. Furthermore, MMHCQA can be stably reused for at least 5 cycles.

Kinetic and mechanistic study of CO2 adsorption on activated hydrochars

Four activated hydrochars (AHCs) were obtained by hydrothermal carbonization of pine sawdust followed by physical and chemical activation in mild conditions. The effects of the activating agents on the porous structure of AHCs were analysed using nitrogen adsorption isotherms. Their CO2 adsorption capacity was assessed by means of carbon dioxide isotherms at 25 and 50 ºC and thermogravimetry, and compared with a commercial activated carbon (AC). SBET values greater than 2000 m2/g were obtained. A narrow micropore volume of 0.392 cm3/g was obtained for the hydrochar activated with KHCO3. The highest CO2 uptake of 3.3 mmol/g (145.2 mg/g) was obtained at room temperature (25 ºC) for the hydrochar activated with KHCO3. Three kinetic models (pseudo-first, pseudo-second and fractional order Avrami’s models) were used to examine the rate parameters of CO2 uptake, with Avrami’s model giving the most accurate prediction of CO2 adsorption. The intraparticle diffusion model and Boyd’s film-diffusion model were applied to identify the CO2 adsorption mechanism.

Effect of accompanying anion on the formation pathway and photocatalytic performance of metal-modified hydrothermal carbon

Metal modification can elevate the photocatalytic ability of hydrothermal carbonization carbon (HTCC), but the effect of accompanying anions in precursor metal salts is unclear. Taking Ca2+, Mn2+, Fe3+, Zn2+, and Ni2+ as metal examples, HTCCs modified with metal chloride and metal nitrate were fabricated through a simple hydrothermal process using glucose as raw material. The photocatalytic performance for sulfamethoxazole (SMX) removal by chloride catalyzed HTCC was superior to its nitrate modified counterpart. Zn2+ catalysis was profitable for surface C = O generation and furan polymerization, which promoted the SMX removal by hydrothermal carbon via a photosensitization-like mechanism. In ZnCl2 catalyzed system, the promotion effect of Zn2+ was not influenced by anion due to Cl- absence in glucose transformation. While in Zn(NO3)2 catalyzed system, less levulinic acid (LA) led to less surface C = O, and NO3–, the powerful oxidant under acidic conditions was reduced to –NH2, which interrupted the original polymerization route of 5-hydroxymethylfurfural (HMF) by participating in the pseudo-Michael reaction, leading to the low polymerization of hydrothermal carbon, and these changes decreased the photocatalytic performance of nitrate-modified HTCCs, thus counteracting the promotion effect of Zn2+.

Zinc Chloride as a Catalyst in Hydrothermal Carbonization of Cocoa Pods Husk

In this study, we have successfully synthesized a high surface area of activated carbons (ACs) from cocoa pod husk (CPH) by using a combination of hydrothermal carbonization (HTC) and chemical activation in the presence of ZnCl2 as both of catalyst as well as activating agent. During HTC, the effect of HTC temperature (200 and 225 °C) and ZnCl2 to biomass mass ratio (1:1, 2:1, 3:1) as a catalyst to the characteristics of the obtained hydrochar (HC) and ACs were investigated. The obtained hydrochar (HCs) was then processed by using ZnCl2 chemical activation with a fixed HC to ZnCl2 mass ratio of 1:4 and the mixture was pyrolyzed at 600 °C for 1 h under an inert atmosphere. The results showed that the addition of catalyst during HTC presented different AC morphology, where the carbon samples were decorated with carbon microspheres. An increase of surface area was observed, where the ACs synthesized from catalyzed HC at biomass to ZnCl2 ratio of 1:3 at temperature 200 °C gave a higher surface area of 1954 m2/g compared to that of without catalyst (1165 m2/g). The catalysis effect was more profound at the HTC temperature of 200 °C compared to 225 °C, as reflected in the significant increase of AC surface area. The addition of a catalyst creates ACs with narrow pore distribution compared to that of synthesized in the absence of a catalyst. It was also observed that the ACs from catalyzed hydrochar possessed higher oxygenated functional groups (OFG) than those without catalysts.

Boosting Solvent-Free Aerobic Oxidation of Benzylic Compounds into Ketones over Au-Pd Nanoparticles Supported by Porous Carbon

The exploitation of highly efficient solvent-free catalytic systems for the selective aerobic oxidation of benzylic compounds to produce corresponding ketones with molecular oxygen under mild conditions remains a great challenge in the chemical industry. In this work, Au-Pd nanoparticles supported on porous carbon catalysts were fabricated by the borax-mediated hydrothermal carbonization method and the chemical reduction method. The physicochemical properties of Au-Pd bimetallic samples were examined by XRD, N2 sorption, SEM, TEM, and XPS techniques. The Au-Pd nanoparticles have successfully immobilized on the spherical carbon support with a porous structure and large surface area. A solvent-free catalytic oxidation system was constructed to selectively convert indane into indanone with Au-Pd nanocatalysts and O2. In contrast with a monometallic Au or Pd catalyst, the resulting bimetallic Au-Pd catalyst could effectively activate O2 and exhibit improved catalytic activity in the controlled oxidation of indane into indanone under 1 bar O2. A total of 78% conversion and >99% selectivity toward indanone can be achieved under optimized conditions. The synergistic effect of Au and Pd and porous carbon support contributed to the high catalytic activity for aerobic benzylic compound oxidation. This work offers a promising application prospect of efficient and recyclable Au-Pd nanocatalysts in functional benzylic ketone production.

Catalytic Supercritical Water Gasification of Canola Straw with Promoted and Supported Nickel-Based Catalysts.

Lignocellulosic biomass such as canola straw is produced as low-value residue from the canola processing industry. Its high cellulose and hemicellulose content makes it a suitable candidate for the production of hydrogen via supercritical water gasification. However, supercritical water gasification of lignocellulosic biomass such as canola straw suffers from low hydrogen yield, hydrogen selectivity, and conversion efficiencies. Cost-effective and sustainable catalysts with high catalytic activity for supercritical water gasification are increasingly becoming a focal point of interest. In this research study, novel wet-impregnated nickel-based catalysts supported on carbon-negative hydrochar obtained from hydrothermal liquefaction (HTL-HC) and hydrothermal carbonization (HTC-HC) of canola straw, along with other nickel-supported catalysts such as Ni/Al2O3, Ni/ZrO2, Ni/CNT, and Ni/AC, were synthesized for gasification of canola straw on previously optimized reaction conditions of 500 °C, 60 min, 10 wt%, and 23-25 MPa. The order of hydrogen yield for the six supports was (10.5 mmol/g) Ni/ZrO2 > (9.9 mmol/g) Ni/Al2O3 > (9.1 mmol/g) Ni/HTL-HC > (8.8 mmol/g) Ni/HTC-HC > (7.7 mmol/g) Ni/AC > (6.8 mmol/g) Ni/CNT, compared to 8.1 mmol/g for the non-catalytic run. The most suitable Ni/ZrO2 catalyst was further modified using promotors such as K, Zn, and Ce, and the performance of the promoted Ni/ZrO2 catalysts was evaluated. Ni-Ce/ZrO2 showed the highest hydrogen yield of 12.9 mmol/g, followed by 12.0 mmol/g for Ni-Zn/ZrO2 and 11.6 mmol/g for Ni-K/ZrO2. The most suitable Ni-Ce/ZrO2 catalysts also demonstrated high stability over their repeated use. The superior performance of the Ni-Ce/ZrO2 was due to its high nickel dispersion, resilience to sintering, high thermal stability, and oxygen storage capabilities to minimize coke deposition.

Fe3C-modified carbon composites for boosted peroxymonosulfate activation and organic pollutant decontamination: Contributions of dual radical and nonradical pathways

In this study, we prepared a hydrothermal carbonation carbon (HTCC) coated with Fe3​C nano-catalyst (Fe/C nano-catalyst) for water decontamination. We first fabricated HTCC via a hydrothermal carbonization process with abundant Fe3+ dopants, and the subsequent calcination transformed iron species into magnetic Fe3​C nanoparticles. The Fe/C nano-catalyst was employed for activating peroxymonosulfate (PMS) and degrading phenolic pollutants. The Fe3​C species significantly enhanced the activity of HTCC to generate reactive species, achieving 100 % phenol degradation in 30 min. Radical capture and electrochemical tests revealed that organic pollutant was oxidized via two pathways: sulfate radical oxidation and direct electron transfer pathway over the Fe/C nano-catalyst. The findings will inspire the development of green and high-performance purification system for organic wastewater treatment.

Significance and Optimization of Operating Parameters in Hydrothermal Carbonization Using RSM–CCD

To ascertain the significance of temperature and residence time of hydrothermal carbonization (HTC) in controlling hydrochar production, multiple regression was employed based on central composite design (CCD) to model the responses of mass yield (MY) and higher heating value (HHV). The hydrothermal reaction was explored at temperatures and times ranging from 150 to 250 °C and 0.5 to 3.5 h. Sorghum bagasse (SB) and microalgae (MA) were used to complex the reaction due to their differences in organic constituents. Simultaneously, the operating parameters were optimized by maximizing the response values under domain constraints in the HHV models. The results show that at least temperature and time in the linear system played a significant role in determining the solids recovery and the energy generation of hydrochars (p-values = 0.00), regardless of the biomass type. Moreover, the optimum conditions of SB and MA hydrochars can be achieved by increasing the temperature to the limit of 250 °C and prolonging the time to 3.5 and 3.25 h, respectively. Both respective conditions resulted in maximum HHVs of 27.54 and 35.83 MJ kg−1.

Understanding heavy metal in the conversion of biomass model component: Migration and transformation characteristics of Cu during hydrothermal carbonization of cellulose

Heavy metals (HMs), with their high toxicity and propensity to accumulate in the human body, pose a significant threat to public health. Simultaneously, their resistance to degradation leads to persistent environmental damage. Hydrothermal carbonization (HTC) emerges as a promising method for reducing the ecological toxicity of HMs. However, the effects of various factors on the immobilization efficacy of HMs remain unclear. This study aimed to investigate the Cu distribution of cellulose in the HTC process. The temperature, retention time, and Cu categories were selected as the factors for the orthogonal experiment. The results showed that HTC produced hydrochars with a complex surface structure, promoting the adsorption, complexation, and precipitation of HMs. Moreover, the HTC process reduced the exchangeable/acid-soluble and reducible fraction of Cu, decreasing the potential risk of Cu. The optimal reaction conditions were 275 °C and 90 min for cellulose added with Cu(NO3)2. It was particularly observed that the increase in temperature enhanced Cu immobilization.

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

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

Carbon-rich and low-ash hydrochar formation from sewage sludge by alkali-thermal hydrolysis coupled with acid-assisted hydrothermal carbonization

The production of carbon-rich and low-ash hydrochar from sewage sludge is attracting interest due to its great application prospect in high value-added carbon materials fields, but which is impossible through direct hydrothermal carbonization. In this study, alkali-thermal hydrolysis followed by acid-assisted hydrothermal carbonization was thus proposed. Thermal hydrolysis at strong alkaline environment was more effective than acid one to promote the dissolution of organic matters and restrain the release of inorganic matters from sludge, which created a favorable condition for hydrochar formation in a carbon-rich and low-ash way. Alkali-thermal hydrolysis began to show a positive effect on the dissolution of organics in sludge when temperature exceeded the threshold of 90 °C, and an increase of 9.77 % was found at 150 °C when compared to 30 °C. Acid-assisted hydrothermal carbonization of alkali-thermal hydrolysate (ATH) at pH 1.0 strongly promoted condensation polymerization of dissolved organics to form hydrochar and meanwhile inhibited introduction of dissolved inorganics. The nanosized microparticulate hydrochar derived from ATH-30 had a carbon and ash content of 50.98–61.31 % and 10.76–12.09 %, while the micro-sized microspheric hydrochar with multiple deposition layers formed from ATH-150 showed a better performance in carbon-rich and low-ash aspect where a carbon and ash content of 58.24–70.07 % and 0.40–3.24 % was realized, both of which were obviously superior to the direct hydrochar (carbon 34.86 % and ash 46.11 %). The condensation of dissolved organics during alkali-thermal hydrolysis stage is important to the carbonization degree of hydrochar. This study provides a new perspective in sludge disposal and production of advanced carbon materials.

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.

A comparison of hydrothermal carbonization versus pyrolysis-activation for sludge-derived carbon materials on physiochemical properties and electrochemical performance

Sludge-derived carbons were prepared by hydrothermal carbonization and pyrolysis-activation. In this work, the physiochemical properties and electrochemical performance of these carbons were compared in detail. The structure of pyrochar (PC) has been fully developed that layered porous skeleton was formed during activation process. Thus, obtained the pyrolysis-activated carbon (PAC) had excellent specific surface area (463.2 m2/g) and pore volume (0.36 m3/g), which were higher than these of hydrothermal carbonization-activated carbon (HCAC-300) (364.1 m2/g, 0.18 m3/g). Due to the high nitrogen content of sludge, there were abundant N functional groups (N-5, N-6, N-Q) on the surface of PAC and HCAC. The self-doping of nitrogen was beneficial for improving the carrier concentration and charge transport capacity of carbons, and forming the pseudocapacitance via the redox reaction. Owing to the remarkable physiochemical properties, PAC exhibited highest specific capacitance of 114.7 F/g at 1 A/g, which led to a large amount of charge storage and rapid diffusion of electrons/ions. However, hydrothermal carbonization performed a high economic advantage in disposing the high-humidity feedstock such as sludge. Therefore, the exploration on improving the pore structure in hydrothermal carbonization process thus obtaining high performance carbon material would be of significance and of prospects.

Towards Negative Emissions: Hydrothermal Carbonization of Biomass for Sustainable Carbon Materials.

The contemporary production of carbon materials heavily relies on fossil fuels, contributing significantly to the greenhouse effect. Biomass is a carbon-neutral resource whose organic carbon is formed from atmospheric CO2. Employing biomass as a precursor for synthetic carbon materials can fix atmospheric CO2 into solid materials, achieving negative carbon emissions. Hydrothermal carbonization (HTC) presents an attractive method for converting biomass into carbon materials, by which biomass can be transformed into materials with favorable properties in a distinct hydrothermal environment, and these carbon materials have made extensive progress in many fields. However, the HTC of biomass is a complex and interdisciplinary problem, involving simultaneously the physical properties of the underlying biomass and sub/supercritical water, the chemical mechanisms of hydrothermal synthesis, diverse applications of resulting carbon materials, and the sustainability of the entire technological routes. This review starts with the analysis of biomass composition and distinctive characteristics of the hydrothermal environment. Then, the factors influencing the HTC of biomass, the reaction mechanism, and the properties of resulting carbon materials are discussed in depth, especially the different formation mechanisms of primary and secondary hydrochars. Furthermore, the application and sustainability of biomass-derived carbon materials are summarized, and some insights into future directions are provided.

Hydrochar from dairy sludge as phosphorus fertiliser affects greenhouse gas emissions and maize yield

ABSTRACT Dairy processing sludge is a phosphorus (P) rich waste with a high potential to replace mineral phosphorus fertiliser in crop production, with possible enhancement of greenhouse gas emissions to the environment. Hydrothermal carbonisation is a technology that transforms the sludge into a hydrochar. The objective of this study is examining P availability of two hydrochars produced from Danish and Irish dairy sludge and their influence on greenhouse gas emissions and maize yields. The trial assessed (i) Danish dairy sludge; (ii) hydrochar derived from Danish sludge; (iii) hydrochar made from Irish dairy sludge; (iv) mineral phosphorus fertiliser; and (v) control. Emissions of nitrous oxide and carbon dioxide, soil pH, mineral nitrogen contents and crop yields were measured. Treatment with Danish dairy sludge had significantly higher cumulative nitrous oxide emissions while the emissions from both hydrochars were not significantly different compared to mineral phosphorous feriliser. Statistical modelling showed that temperature, soil nitrate content, interactions both between temperature and precipitation, and between soil moisture and precipitation were drivers for nitrous oxide emissions. There was no difference in emissions among all treatments when scaled for yield. Hydrochar may alleviate the enhanced nitrous oxide emissions in soil without constraining P availability and maize crop yields.

Analysis of Carbon Dots Synthesized at Different Hydrothermal Carbonization Temperatures and Their Ferric Ion Detection Performances

In this article, carbon dots (CDs) are fabricated via hydrothermal carbonization (HTC) at different temperatures ranging from 120 to 210 °C. Regarding the temperature variations, they demonstrate a crucial role in specific identification in the ability to differentiate the chemical structures and optical properties of the resulting CD features. Both heteroatoms‐doped core structure, and the functionalized surface state display a changed that correlates with the temperature variations mentioned. The CDs which are synthesized at 180 °C contain the appropriate balance of doped heteroatoms and surface functional groups which further display the most promising performance of the quenching effect of up to 17% toward ferric ions. This work contributes to the growing understanding of CDs and provides valuable insights for the future development of applications that may favor certain interest in the creation or designing process in various applications.

Electrochemical oxidation of refractory compounds from hydrothermal carbonization process waters

Hydrothermal carbonization (HTC) is an emerging technology for treating sewage sludge. However, the resulting HTC process water is heavily contaminated with various carbonaceous and nitrogenous components, some of them being non-biodegradable. To implement HTC as a full-scale treatment alternative for sewage sludge, effective concepts for treating process water are crucial. This study focuses on the electrochemical oxidation (EO) using a boron-doped diamond electrode to treat one HTC process waters with different pretreatments: (i) without pretreatment, (ii) biologically pretreated with chemical oxygen demand (COD) removal, (iii) biologically pretreated with nitrification and denitrification. The EO removed COD of all HTC process waters by over 97%, but as COD concentrations decreased, the instantaneous current efficiency (ICE) dropped below 5% and energy consumption increased. The organically bound and refractory nitrogen was completely mineralized and converted to mainly NO3–N. After EO of process waters without nitrification/denitrification, nitrogen was present as NO3–N with up to 730 mg/L and NH4–N with up to 1813 mg/L. Such high ammonium concentrations treatment could be interesting for nitrogen recovery. In addition, the toxicity towards Vibrio fischeri could be reduced to a large extent. The findings suggest that EO after a biological step with COD removal is a viable solution for HTC process water treatment.