Heteroatoms doped porous carbon derived from hydrothermally treated sewage sludge: Structural characterization and environmental application

Solid shrimp waste derived nanoporous carbon as an alternative bio-sorbent for oxytetracycline removal from aquaculture wastewater

Recently, it has been critical to effectively remove oxytetracycline (OTC) from aquaculture wastewater before releasing into the environment. The adsorption process is recognized as an efficient pathway for removing OTC since it is a simple, stable, and cost-effective method. This study aims to develop nanoporous carbon entirely from shrimp waste (SW) via hydrothermal carbonization assisted with KOH activation. Existing KOH significantly increases the porosity of SW nanoporous carbon. The optimal SW porous carbon was obtained using 5 wt%KOH for activation, which had the largest surface area of 679.51 m2/g with the total pore volume of 0.458 cm3/g. Moreover, the SW porous carbon with the highest porosity was selected for the OTC adsorption. The Langmuir isotherm model and the pseudo-second-order kinetic model match the experimental data, implying that the adsorption mechanism is mono-layered adsorption due to micropores by chemisorption interaction. The adsorption capacity significantly improved by increasing the dosage of SW nanoporous carbon. The SW nanoporous carbon adsorption for OTC is primarily regulated by pore filling affected by hydrogen bonding, and π-π* interaction also plays a significant role. The SW nanoporous carbon showed an efficient OTC adsorption after 5 regeneration cycles. This work demonstrates biomass waste recycling and emphasizes the potential of aquatic food processing waste-derived nanoporous carbon for antibiotic adsorption.

Multivariate and multi-interface insights into carbon and energy recovery and conversion characteristics of hydrothermal carbonization of biomass waste from duck farm

High lipid, high nitrogen duck manure (DM) with high lipid, high lignocellulosic litter materials (LM) are the main wet biomass wastes from duck farms and both are naturally abundant carbon resources. The synthesis of duck farming biomass waste into carbon-rich materials for high value utilization by hydrothermal carbonization (HTC), which can directly treat wet biomass, has not been investigated. In this study, the physicochemical properties of hydrochar derived from co-HTC of DM and LM and its carbon and energy recovery patterns were systematically investigated under multivariate conditions of raw materials ratios, solids contents, temperatures and residence times. The application of synchrotron-based near-edge X-ray adsorption fine structure technique (C K-edge NEXAFS) combined with gas chromatography-mass spectrometry (GC-MS) to the hydrochar and hydrothermal liquid, respectively. At multiple interfaces provided an in-depth analysis of the important material transformations of the co-HTC process and the structure of the hydrochar. Extending residence time (180 min) and increasing LM ratio (M@4%) in co-HTC reaction of DM and LM is beneficial to achieve hydrochar containing higher carbon content (44.84%) at lower reaction temperatures (180 °C). The heating value (HHV) of the hydrochar ranges between 17.12 and 25.05 MJ/kg. The carbon recovery rate of the co-HTC of DM and LM all exceeded 55% and was more closely related to the carbon content of the hydrochar than to its yield. Additionally, the model ERR=0.97±0.01CRR+2.40±0.71 (R2 = 0.99, P < 0.01) was developed to predict energy recovery rate (ERR) based on carbon recovery rate (CRR). Esters were an important intermediate during co-HTC of DM and LM, and the derived hydrochar consisted of a wide range of polycyclic aromatic hydrocarbons, alkanes and N-aromatic heterocycles as well as polyfuran, pyrrole and pyridine structures.

Mesoporous carbon material prepared from sewage sludge hydrochar using Pluronic F127 as template for efficient removal of phenolic compounds: Experimental study and mechanism interpretation via advanced statistical physics model

Mesoporous carbon material (MCM) with rich ether surface group was prepared from sewage sludge hydrochar using Pluronic F127 as template under pyrolysis activation, which provided an energy-efficient method to promote the resource utilization of sewage sludge as adsorbents for phenols removal from water. The MCM possessed high surface area (549 m²/g), abundant mesopores (average width 3.81 nm) and well-developed graphite structure. Acidic conditions and low temperatures favored the adsorption of phenolic compounds. The quick adsorption process of reaching over 85% of the capacity in the first 10 min and intraparticle diffusion as primary rate-limiting step were observed for all phenolic compounds. Advanced statistical physics analysis was used successfully to interpret the adsorption mechanism of phenols onto MCM and revealed a multi-molecular monolayer adsorption process primarily through negative charge-assisted hydrogen bond interaction where the ether functional group contributed to the predominant active sites. The adsorption capacity of phenolic compounds depended upon the number of molecules adsorbed per ether active site and the available density of ether bond group on the surface of MCM. 2,4,6-trichlorophenol showed a highest adsorption priority to occupy the limited ether active sites and its adsorption capacity reached 0.49 mmol/g, while p-nitrophenol exhibited a maximum number of molecules adsorbed on the single ether active site, showing an adsorption capacity of 0.42 mmol/g. The synergistic effect of multi-interactions mechanisms resulted in phenolic compounds removal with adsorption energies lower than 30 kJ/mol. This prepared MCM adsorbent is promising for application in treatment of water polluted by phenols.

Valorization of orange peel waste to tunable heteroatom-doped hydrochar-derived microporous carbons for selective CO2 adsorption and separation

Constrained by the extortionately expensive carbon sources, low carbon yields, inadequate adsorption capacities, and corrosive chemical activating agents, the commercialization of carbonaceous CO₂ adsorbents remains a challenging task. Herein, potassium oxalate (K₂C₂O₄), an activating agent with less corrosive properties, was used for the synthesis of activated carbons from inexhaustibly available "orange peel biowaste." For the first time, a comprehensive report is presented on the effect of hydrothermal treatment, hydrochar/K₂C₂O₄ ratio, activation temperature, and melamine modification in tailoring the porosity and surface functionalization of activated carbons. The optimized sample, OPMK-900, exhibited large specific surface area ~2130 m²/g; micropore volume ~1.1166 cm³/g, and a high pyrrolic nitrogen content (~ 46.1 %). Notably, melamine played the dual role as a promoter to K₂C₂O₄ porosity generation and a nitrogen dopant, which synergistically led to an efficient CO₂ uptake of ~6.67 mmol/g at 273 K/ 1 bar via micropore-filling mechanism and Lewis acid-base interactions. Moreover, remarkably high IAST CO₂/N₂ selectivity (105 at 273 K and 96 at 298 K) surpasses most of the biomass-derived carbons. Furthermore, the moderately high isosteric heat of adsorption (∆H_ads ~ 38.9 kJ/mol) revealed the physisorption mechanism of adsorption with a limited energy requirement for the regeneration of the spent adsorbents.

Hydrochar: A Promising Step Towards Achieving a Circular Economy and Sustainable Development Goals

The United Nations 17 Sustainable Development Goals (SDGs) are a universal call to action to end poverty, protect the environment, and improve the lives and prospects of everyone on this planet. However, progress on SDGs is currently lagging behind its 2030 target. The availability of water of adequate quality and quantity is considered as one of the most significant challenges in reaching that target. The concept of the ‘Circular Economy’ has been termed as a potential solution to fasten the rate of progress in achieving SDGs. One of the promising engineering solutions with applications in water treatment and promoting the concept of the circular economy is hydrochar. Compared to biochar, hydrochar research is still in its infancy in terms of optimization of production processes, custom design for specific applications, and knowledge of its water treatment potential. In this context, this paper critically reviews the role of hydrochar in contributing to achieving the SDGs and promoting a circular economy through water treatment and incorporating a waste-to-value approach. Additionally, key knowledge gaps in the production and utilization of engineered hydrochar are identified, and possible strategies are suggested to further enhance its water remediation potential and circular economy in the context of better natural resource management using hydrochar. Research on converting different waste biomass to valuable hydrochar based products need further development and optimization of parameters to fulfil its potential. Critical knowledge gaps also exist in the area of utilizing hydrochar for large-scale drinking water treatment to address SDG-6.

Preparation of Citric Acid-Sewage Sludge Hydrochar and Its Adsorption Performance for Pb(II) in Aqueous Solution

In order to seek the value-added utilization method of sewage sludge and develop low-cost and high-efficient adsorbents, a hydrochar was prepared by the co-hydrothermal carbonization of sewage sludge and citric acid and then characterized. The differences in Pb(II) adsorption performance between the citric acid–sewage sludge hydrochars (AHC) and the hydrochar prepared solely from sewage sludge (SSHC) were also investigated. When citric acid dose ratio (mass ratio of citric acid to dry sewage sludge) is 0.1, the obtained hydrohcar (AHC0.1) has the highest specific surface area (59.95 m²·g⁻¹), the most abundant oxygen-containing functional groups, the lowest pH_pzc (5.43), and the highest equilibrium adsorption capacity for Pb(II). The maximum adsorption capacity of AHC0.1 for Pb(II) is 60.88 mg·g⁻¹ (298 K), which is approximately 1.3 times that of SSHC. The potential mechanisms can be electrostatic attraction, co-precipitation, complexation, and cation-π interaction. It was demonstrated that by incorporating citric acid into the hydrothermal carbonization, resource utilization of sewage sludge can be accomplished effectively.

Low Temperature One-Pot Hydrothermal Carbonization of Corn Straw into Hydrochar for Adsorbing Cadmium (II) in Wastewater

Corn straw, a typical agricultural waste, was directly converted into hydrochar with a yield of 77.56% by hydrothermal carbonization at 140–230 °C for 2 h with a solid–liquid ratio of 1:20. The morphology and surface properties were characterized by elemental analysis, specific surface area and pore size analysis and Fourier transform infrared spectroscopy. The results showed that with the increase of hydrothermal reaction temperature, some physical and chemical properties such as the increase of hydrocarbon content, crystallinity, and specific surface area of hydrochar changed significantly. A series of chemical reactions such as dehydration, decarboxylation, and aromatization occurred in the hydrothermal carbonization process so that the prepared hydrochar had rich oxygen-containing functional groups (-HO, C-O-C, C=O) and unique porous structure made the hydrochar prepared at 170 °C had the best removal effect on Cd2+ in solution (5.84 mg/g). These specific conditions could remove Cd2+ and greatly improve the adsorption performance. The pseudo-second-order kinetic model and Freundlich isotherm model could better describe the adsorption behavior of Cd2+. Therefore, corn straw hydrochar as a potential adsorbent for removing Cd2+ from water.

Hydrochar derived from municipal sludge through hydrothermal processing: A critical review on its formation, characterization, and valorization

Additional options for the sustainable treatment of municipal sludge are required due to the significant amounts of sludge, high levels of nutrients (e.g., C, N, and P), and trace constituents it contains. Hydrothermal processing of municipal sludge has recently been recognized as a promising technology to efficiently reduce waste volume, recover bioenergy, destroy organic contaminants, and eliminate pathogens. However, a considerable amount of solid residue, called hydrochar, could remain after hydrothermal treatment. This hydrochar can contain abundant amounts of energy (with a higher heating value up to 24 MJ/kg, dry basis), nutrients, and trace elements, as well as surface functional groups. The valorization of sludge-derived hydrochar can facilitate the development and application of hydrothermal technologies. This review summarizes the formation pathways from municipal sludge to hydrochar, specifically, the impact of hydrothermal conditions on reaction mechanisms and product distribution. Moreover, this study comprehensively encapsulates the described characteristics of hydrochar produced under a wide range of conditions: Yield, energy density, physicochemical properties, elemental distribution, contaminants of concern, surface functionality, and morphology. More importantly, this review compares and evaluates the current state of applications of hydrochar: Energy production, agricultural application, adsorption, heterogeneous catalysis, and nutrient recovery. Ultimately, along with the identified challenges and prospects of valorization approaches for sludge-derived hydrochar, conceptual designs of sustainable municipal sludge management are proposed.

A novel activation-hydrochar via hydrothermal carbonization and KOH activation of sewage sludge and coconut shell for biomass wastes: Preparation, characterization and adsorption properties

A two-stage method of hydrothermal carbonization and chemical activation technology was applied to prepare a novel, large surface area and rich-pore structure activation-hydrochar from sludge sewage and coconut shell due to its mild, low-cost, and well-developed merits. The pore-making mechanism of activation-hydrochar was discussed by FT-IR, XPS, SEM, TG, TG-MS, XRD, and BET characterization. These results illustrated that the first stage of hydrothermal carbonization achieved the rich-pore structure hydrochar via dehydration, decarboxylation, deamination, and rearrangement reactions. The subsequent KOH activation was conducive to the pore-forming process. Specifically, the pore structure of activation-hydrochar was ameliorated and abundant active adsorption sites were obtained by the modification. The adsorption properties of activation-hydrochar on Methylene Blue (MB) and Congo Red (CR) were systematically investigated, and the max adsorption capacities of those were obtained with 623.37 mg/g and 228.25 mg/g, respectively. The pseudo-second-order kinetics and Langmuir models were both fit to elucidate the adsorption process for both dyes. Thermodynamics revealed adsorption performance accompanied by the spontaneous and endothermic processes. In general, the research clearly indicated the synthesis route for activation-hydrochar, and its further adsorption performance, capacity, and mechanism on MB and CR. This research demonstrated that activation-hydrochar with the abundant surface area and rich-pore structure made it a candidate for the production of effective adsorption material. It is prospective to achieve the utilization of wastes and its further application in wastewater treatment.

Biochar catalyzed dechlorination - Which biochar properties matter?

Bone char catalyzed dechlorination of trichloroethylene (TCE) by green rust (iron(II)-iron(III) hydroxide, GR) has introduced a promising new reaction platform for degradation of chlorinated solvents. This study aimed to reveal whether a broader class of biochars are catalytically active for the dechlorination reaction and to identify which biochar properties are the most important for the catalytic activity. Biochars produced by pyrolysis of animal, plant, and sewage waste substrates at 950 °C were prepared for catalytic dechlorination of TCE by GR tested in batch experiments with 0.15 g L^-1 biochar, 3.2 g L^-1 GR, and ~ 20 µM TCE. The results showed that the biochar substrate significantly affects its catalytic activity, with the highest TCE reduction rate observed for bone and shrimp-based biochars (k ≥ 0.18 h^-1), whereas no reactivity was seen for graphite and activated carbon references. Multivariate regression indicated that the biochar catalytic activity is controlled by multiple biochar properties - biochar surface area, TCE sorption, abundance of C-O groups, and pore size are the properties that impact the catalytic activity most. Derivation of biochar reactivity relationship for a broad spectrum of biochars provides a new approach for identifying proper biochar catalysts for pollutant degradation.

Distribution and toxicity of polycyclic aromatic hydrocarbons during CaO-assisted hydrothermal carbonization of sewage sludge

Hydrothermal carbonization (HTC) of sewage sludge (SS) with and without calcium oxide (CaO) introduction was conducted at 160-240 °C, and the yield and distribution of polycyclic aromatic hydrocarbons (PAHs) were evaluated for the first time. PAHs (2972.99 μg/kg) and toxic equivalent quantity (TEQ) (373.09 μg/kg) yields in SS decreased by 13.61% and 14.65%, respectively, after treatment at 160 °C and substantially increased as temperatures increased. More PAHs were distributed in the hydrochar than in the aqueous products. Hydrochar yields decreased linearly with temperature, thus increasing PAH concentration in hydrochar; 6221.98 μg/kg of PAHs in hydrocar at 240 °C exceeded agricultural use standard limits. PAH and TEQ yields at 200 °C decreased by 5.55-15.98% and 2.88-3.54%, respectively, when 3-9% CaO was added, which could be ascribed to CaO inhibition in the free radical reaction for PAH generation. Additionally, 6% CaO addition substantially weakened the acceleration effect of high temperatures on PAH formation; the decrease of PAH yield at 240 °C was 22.14%, which is higher than that at other temperatures. Consequently, the PAH concentration in hydrochar declined by 2.33-22.37%. PAH content in hydrochar obtained from CaO-assisted HTC of SS fell within agriculture use standard limit and exhibits potential for use as a soil conditioner. However, condition with a CaO amount of 15% would significantly increase TEQ yields. Considering both PAH and TEQ yields and the ecological risks of PAHs in hydrochar derived from HTC of SS, the appropriate reaction conditions were found to be 200 °C with 3-6% added CaO.

A review on fabricating functional materials by heavy metal-containing sludges

With the development of industry, sustainable use of natural resources has become a worldwide hot topic. Heavy metal-containing sludge (HMS) is a hazardous waste after wastewater treatment. At present, HMS is still treated by landfill or landfill after incineration. Considering the components, HMS usually contains various heavy metals and organic compounds, which is potentially used as a raw resource for catalyst production. This review thus concludes recent reports and developments in this field. First, basic technologies are summarized as component regulation, precursor formation, and structure transformations. Second, prepared materials are applied in various catalytic fields, such as gas purification, photocatalysis, electrocatalysis, and Fenton catalysis. During these processes, key factors are multi-metallic components, metal doping, temperature, and pH. They not only influence the formation of HMS-derived catalyst but also the catalytic activity. Furthermore, catalytic activities of HMS-derived catalysts are compared with those synthesized by pure reagents. An assessment and accounting are also supplied if raw resources are substituted by HMS. Finally, in order to apply HMS in a real application, more works must be devoted to the influence of trace metal doping on catalytic activities and stabilities. Besides, more pilot experiments are urgently necessary.

Comparison of porous carbons derived from sodium alginate and calcium alginate and their electrochemical properties

Here, sodium alginate and calcium alginate which have the same carbon-forming component (alginic acid) and different regulation component (sodium/calcium) were used to prepare porous carbons, and comparisons were made of the microstructures and electrochemical properties of the obtained charcoals. The morphology was characterized by Scanning electron microscopy (SEM), and the results show that porous carbons can inherit plane or concave structures from their corresponding carbonized samples. The Horvath–Kawazoe (HK) method was used to analyze micropore size distributions, and the results show that, under the same mass ratio of potassium hydroxide to carbonized sample (KOH/C), the positions of extreme points on the two curves are similar, but the extreme values are different, and new extreme points appear at larger pore sizes with increases in the KOH/C ratio. The results of cyclic voltammetry (CV) and galvanostatic charge and discharge (GCD) tests show that the capacitance of sodium alginate-derived porous carbon is greater than that of porous carbon derived from calcium alginate when the KOH/C ratios are 2 and 4, and the size relationship is reversed when the KOH/C ratio is 3. The results of cycling performance tests show that the cycle numbers corresponding to the three stages on the two curves are similar under the same KOH/C ratio, but the cycle numbers at the same stage are significantly different under different KOH/C ratios.

Comparisons of the microstructures and electrochemical properties of porous carbons derived from the carbon-forming component alginic acid under the action of Na/Ca.

Hydrothermal carbonization of sewage sludge: A critical analysis of process severity, hydrochar properties and environmental implications

Hydrothermal carbonization (HTC) of sewage sludge reduces the waste volume and can be source of energy and valuable products. Furthermore, HTC offers several advantages over conventional dry-thermal pre-treatments, as no prior drying is requested, and the high quality of the char produced promotes applications as energy production and storage, wastewater remediation, and soil amendment. Relationships between char yields, physicochemical properties and process parameters are here analysed, with the aim to provide insight into the choice of the process severity required to fit the desired application. Moreover, presence and fate of heavy metals and organic contaminants are discussed. The highest reaction temperature is the main parameter affecting the physicochemical characteristics of the char produced, while the heating rate governs the heat mass transfer and the rate of intermediates formation. Depolymerization of the biomass results in a reduction of the oxygen to carbon ratio and, therefore, in augmented high heating values, further increased by deposition of 5-(hydroxymethyl)furfural. Recirculation of process water may enhance dehydration reactions and the deposition of degraded polymers, increasing dewaterability and yield, but field trials are recommended to assess the feasibility of this option. An overuse of chars for energy generation purposes would be deleterious for the environmental life cycle. Further research is encouraged to assess the pollutants abatement and their degradation pathways when incorporated in the carbonaceous product, to promote the application of hydrochars as soil amendment, as well as for environmental remediation purposes.

Evaluation of activated carbon synthesized by one-stage and two-stage co-pyrolysis from sludge and coconut shell

Waste biomass and sewage sludge were used to obtain an adsorbent material with excellent performance qualities by adopting a KOH activation process via one-stage (AC_one) or two-stage (AC_two) co-pyrolysis. The main purpose of this work was to investigate the effects of both methods in terms of the physicochemical properties and adsorption capacities for methylene blue (MB). Textural analyses revealed that the surface area (S_tot= 683.82 m²/g) and total pore volume (V_tot= 0.72 cm³/g) of AC_two were more than two-fold compared with AC_one (S_tot= 285.33 m²/g; V_tot= 0.35 cm³/g). Thus, two-stage co-pyrolysis produced activated carbon with increased porosity, which was favorable for MB adsorption. Nevertheless, the intensity of the surface functional groups of AC_two was weaker than for AC_one, which could be due to the pore-forming mechanism. Two-stage co-pyrolysis increased the yield and aromaticity of activated carbon, but sufficient activation caused more functional groups to decompose. For the adsorbate MB, the maximum adsorption capacity of AC_two (602.80 mg/g) was more than five-fold greater than that of AC_one (101.88 mg/g), due to its excellent porosity properties. Furthermore, the interactions of MB molecules with activated carbon were via hydrogen bonds and electrostatic attraction. The adsorption process of MB onto activated carbon was accurately described by the pseudo-second-order kinetic model. Adsorption equilibrium evaluated Langmuir isotherms demonstrated that MB formed a monolayer by adsorption onto the activated carbon. Adsorption thermodynamics was used to investigate the influence of temperature on the adsorption process. Thermodynamic parameters indicated that MB adsorption onto activated carbon was spontaneous and endothermic. In conclusion, our results showed that two-stage co-pyrolysis improves the adsorption capabilities of activated carbon, so achieving better economic value from waste materials.

Removal of methylene blue from aqueous solution by modified bamboo hydrochar

Four hydrochars (labeled as HC, AHC, MHC, and MAHC, respectively) were prepared by hydrothermal carbonization of bamboo with and without chlorane or followed by NaOH modification. Various techniques were adopted to characterize the physicochemical properties of hydrochars and the removal of methylene blue (MB) by these hydrochars from aqueous solution was investigated. The MB adsorption isotherm and kinetic onto MAHC and MHC can be preferably interpreted by Langmuir adsorption mode and the pseudo-second-order model, respectively. Thermodynamic parameters implied that adsorption was a spontaneous and exothermic process. The adsorption capacity of MAHC was 655.76 mg g^-1, which was 2-3 times compared to that of MHC (268.93 mg g^-1) at 303 K. The high adsorption capacity of MAHC for MB suggests that hydrothermal carbonization in acidic medium followed by alkaline treatment has the potential application to produce efficient MB adsorbents used in wastewater treatment.

Magnetic N-containing carbon spheres derived from sustainable chitin for the selective oxidation of C–H bonds

Magnetic N-containing carbon spheres were synthesized using sustainable N-acetyl-d-glucosamine (NAG) and iron nitrate as raw materials. This carbon material exhibited excellent catalytic performance in the C–H bond oxidation.