Biochar Properties Research Articles

Customized High-Value Agricultural Residue Conversion: Applications in Wastewater Treatment

Wastewater treatment methods based on catalytic technology are receiving more and more attention in recent years. In order to achieve the high-value utilization of agricultural residues, the development of greener, more efficient, and economically favorable catalysts is the focus of this review. The main contents include: (i) discussions around the differences in surface morphology, chemical stability, electrical conductivity, porosity, and the specific surface area of the prepared biochar according to the biochemical composition of different agricultural residues; (ii) through the above comparison, the shortlisting and selection of suitable catalysts for the preparation of agricultural residues to improve the adsorption and catalytic performance; (iii) discussions around the interaction mode and catalytic mechanism between the surface properties of biochar and pollutants; (iv) based upon future needs, discussions around making full use of the distribution of the pyrolysis products of agricultural residue, and designing biochar-based catalysts to achieve the efficient degradation of pollutants in wastewater. The review aims to provide a waste disposal alternative for the alleviation of environmental pollution and the promotion of green chemistry as well as sustainable development.

Review on Characterization of Biochar Derived from Biomass Pyrolysis via Reactive Molecular Dynamics Simulations

Biochar is a carbon-rich solid produced during the thermochemical processes of various biomass feedstocks. As a low-cost and environmentally friendly material, biochar has multiple significant advantages and potentials, and it can replace more expensive synthetic carbon materials for many applications in nanocomposites, energy storage, sensors, and biosensors. Due to biomass feedstock species, reactor types, operating conditions, and the interaction between different factors, the compositions, structure and function, and physicochemical properties of the biochar may vary greatly, traditional trial-and-error experimental approaches are time consuming, expensive, and sometimes impossible. Computer simulations, such as molecular dynamics (MD) simulations, are an alternative and powerful method for characterizing materials. Biomass pyrolysis is one of the most common processes to produce biochar. Since pyrolysis of decomposing biomass into biochar is based on the bond-order chemical reactions (the breakage and formation of bonds during carbonization reactions), an advanced reactive force field (ReaxFF)-based MD method is especially effective in simulating and/or analyzing the biomass pyrolysis process. This paper reviewed the fundamentals of the ReaxFF method and previous research on the characterization of biochar physicochemical properties and the biomass pyrolysis process via MD simulations based on ReaxFF. ReaxFF implicitly describes chemical bonds without requiring quantum mechanics calculations to disclose the complex reaction mechanisms at the nano/micro scale, thereby gaining insight into the carbonization reactions during the biomass pyrolysis process. The biomass pyrolysis and its carbonization reactions, including the reactivity of the major components of biomass, such as cellulose, lignin, and hemicellulose, were discussed. Potential applications of ReaxFF MD were also briefly discussed. MD simulations based on ReaxFF can be an effective method to understand the mechanisms of chemical reactions and to predict and/or improve the structure, functionality, and physicochemical properties of the products.

Effect of pyrolysis temperature on the physiochemical properties of biochar and its potential use in anaerobic digestion: A critical review

A comprehensive study was conducted to assess the influence of pyrolysis temperature (PyT) on the properties of biochar (BC) and its potential application in anaerobic digestion (AD). PyT is an essential parameter for effectively utilizing BC in full-scale AD, because PyT can develop BC properties that are necessary for improving AD performance through (I) the increment of BC porosity, (II) the adsorption and conversion of metabolites, (III) the enrichment of microbial communities, and (IV) the enrichment of functional groups. Some features of BC, such as porosity, electrical conductivity, and pore structure, increase consistently with increasing PyT. These properties have a positive effect on AD and enhancement of methane yield. The adsorption of excessive metabolites correlates with the enrichment of BC functional groups. The appearance of functional groups of BC decreases when the PyT increases. No substantial correlation (R2 < 0.1) could be found between the enhancement of methane yield and the enrichment of BC functional groups and the adsorption capacities of BC. Microbial communities are positively correlated to the increment of PyT with a moderate correlation (R2=0.33) to the enhancement of methane yield. The review also identifies several knowledge gaps that necessitate further investigation.

Adsorption properties of active biochar: Overlooked role of the structure of biomass

Vascular plants account for more than 80% of all biomass on earth and are potential precursors of biochar. However, the changes of vascular bundle have received less attention during the preparation of biochar. In this study, loofah sponge (LS), tangerine pith (TP), and rhodiola rosea (RR), were selected to show the role of vascular bundle in biochar through the pretreatment of vascular bundle. The results showed that the active biochar prepared with vascular bundle protection had high adsorption capacity for methylene blue (LS: 953.53 mg/g, TP: 714.77 mg/g, RR: 583.49 mg/g). The Brunauer-Emmett-Teller method was used to measure the specific surface area (SSA) of active biochar. The SSA of LS active biochar prepared by vascular bundle protection was 2262.67 m2/g, and has high adsorption properties under different conditions. In conclusion, the protection of vascular bundle during biochar preparation is important to improve the utilization of biological resources and environmental adsorption.

Synthesis of modified sludge biochar for flue gas denitration: Biochar properties, synergistic efficiency and mechanism

Biochar has been recommended as a carbon material with high porosity, rich functional groups, and low cost for flue gas denitration. In this study, five different biochar materials, including original sludge biochar (SC), the KOH-impregnated biochar (SCK), the biochar of SCK washing by ultra-water (SCK-W), the H2SO4-impregnated biochar (SCS), and the biochar of SCS washing by ultra-water (SCS-W), prepared using a one-step activation method were applied to remove NO from flue gases, acting as both reductants and catalysts. The results indicate that the SCK-700 can lead to the highest NO conversion of 100% at 325 °C, and can suppress the acidic gas generation compared with SCS. The order of reduction capacity at 300–400 °C was SCK-700 > SCK-W-700 > SC-700 > SCS-700 > SCS-W-700. Furthermore, the kinetics indicated that adsorption capacity enhanced with the rise in temperature, and that SCK-700 had the largest adsorption capacity, reaching 337.15 mg/g at 400 °C.

Pyrolysis temperature had effects on the physicochemical properties of biochar

Pyrolysis temperature had effects on the physicochemical properties of biochar

Sludge-derived biochar: Physicochemical characteristics for environmental remediation

The global production of fecal wastes is envisioned to reach a very high tonnage by 2030. Perilous handling and consequential exposition of human and animal fecal matter are inextricably linked with stunted growth, enteric diseases, inadequate cognitive skills, and zoonoses. Sludge treatment from sewage and water treatment processes accounts for a very high proportion of overall operational expenditure. Straightforward carbonization of sludges to generate biochar adsorbents or catalysts fosters a circular economy, curtailing sludge processing outlay. Biochars, carbonaceous substances synthesized via the thermochemical transformation of biomass, possess very high porosity, cation exchange capacity, specific surface area, and active functional sorption sites making them very effective as multifaceted adsorbents, promoting a negative carbon emission technology. By customizing the processing parameters and biomass feedstock, engineered biochars possess discrete physicochemical characteristics that engender greater efficaciousness for adsorbing various contaminants. This review provides explicit insight into the characteristics, environmental impact considerations, and SWOT analysis of different sludges (drinking water, fecal, and raw sewage sludge) and the contemporary biochar production, modification, characterization techniques, and physicochemical characteristics, factors influencing the properties of biochars derived from the aforestated sludges, along with the designing of chemical reactors involved in biochar production. This paper also manifests a state-of-the-art discussion of the utilization of sludge-derived biochars for the eviction of toxic metal ions, organic compounds, microplastics, toxic gases, vermicomposting approaches, and soil amelioration with an emphasis on biochar recyclability, reutilization, and toxicity. The practicability of scaling up biochar generation with multifaceted, application-accustomed functionalities should be explored to aggrandize socio-economic merits.

Machine learning approach to predict adsorption capacity of Fe-modified biochar for selenium

The accurate prediction of environmental Se (selenium) adsorption levels is critical for sustainable development and management perception. The concept of sorting massive quantities of data to find important information using machine learning (ML) has recently been applied to environmental remediation, particularly the science-based design of a 'green' carbonaceous and an effective functional material (e.g., biochar and Fe modified biochar) with high Selenium (Se) removal capacity. The present study focuses on presenting ML models that utilize the random-forest (RF) support vector regression (SVR) and SHAP (SHapley Additive exPlanations) models to forecast the adsorption of Se by modified biochar. The RF, SVR and SHAP models, which were constructed using basic surface properties of Fe-modified biochar and environmental conditions showed accuracy and predictive performance for Se (removal capacity in the test group with R2 of 0.98, 0.98 and 0.95 and RMSE of 0.35, 0.14 and 0.23 mg-kg−1, respectively). The SVR model was highly effective for predicting Se adsorption, indicating potentially higher accuracy than the RF and SHAP models. This may be due to the small size of our data. According to the feature analysis and partial dependence plot analysis of all three models, the most significant component regulating Se adsorption was oxygen (%) followed by carbon (%), temperature, pH and Fe in all three models. The relative importance of variables may offer guidance for researchers to develop improved Se treatment of actual water and wastewater. Moreover, the ML models developed in this study took into account the surface functionalities of biochar and Fe-modified biochar to provide a more accurate prediction of Se removal, and offer a comprehensive guideline for the long-term development of biochar adsorbents for Se removal.Graphical

Production of bio-oil and biochar by the nitrogen-rich pyrolysis of cellulose with urea: Pathway of nitrile in bio-oil and evolution of nitrogen in biochar

The production of nitrogen-containing chemicals from renewable biomass under nitrogen-rich conditions represents a promising technological pathway. Urea, as a nitrogen carrier, offers advantages over gaseous ammonia due to its easier storage and transportation. In this study, cellulose and two of its model compounds (cellobiose and glucose) were used as raw materials. The effects of different urea concentrations on the types and relative contents of bio-oil components and the properties of nitrogen-doped biochar were analyzed using GC-MS, FT-IR, and elemental analysis. We further clarified the evolution of exogenous nitrogen in nitrogen-rich pyrolyzed bio-oil and biochar with different urea concentrations. Cellobiose was found to be a more suitable feedstock for the production of nitrogen-containing compounds from nitrile than cellulose or glucose. A significant increase in the relative content of nitrile compounds (44.45%) was observed at a ratio of 40% cellulose to nitrogen carrier. Analysis of the bio-oil fraction led to the conclusion that there may be two main pathways for the production of the two nitrile compounds, acetonitrile and propionitrile. This research provides a better understanding of the N transfer pathways during the nitrogen-rich pyrolysis of cellulose and its model compounds (cellobiose and glucose) when using urea as the nitrogen carrier. This will facilitate further improvements in the yield of nitrogen-containing compounds in bio-oil.

Effects of Heat Treatment on the Physicochemical Properties and Electrochemical Behavior of Biochars for Electrocatalyst Support Applications.

The present work reports the synthesis and the physicochemical characterization of biochar from the organic wastes of nopal (Opuntia Leucotricha), coffee grounds (Coffea arabica) and Ataulfo mango seeds (Mangifera indica) as alternative electrocatalyst supports to Vulcan XC-72 carbon black. The biochars were prepared using pyrolysis from organic wastes collected at three temperatures, 600, 750 and 900 °C, under two atmospheres, N2 and H2. The synthesized biochars were characterized using Raman spectroscopy and scanning electron microscopy (SEM) to obtain insights into their chemical structure and morphological nature, respectively, as a function of temperature and pyrolysis atmosphere. A N2 adsorption/desorption technique, two-point conductivity measurements and cyclic voltammetry (CV) were conducted to evaluate the specific surface area (SSA), electrical conductivity and double-layer capacitance, respectively, of all the biochars to estimate their physical properties as a possible alternative carbon support. The results indicated that the mango biochar demonstrated the highest properties among all the biochars, such as an electrical conductivity of 8.3 S/cm-1 at 900 °C in N2, a specific surface area of 829 m2/g at 600 °C in H2 and a capacitance of ~300 mF/g at 900 °C in N2. The nopal and coffee biochars exhibited excellent specific surface areas, up to 767 m2/g at 600 °C in N2 and 699 m2/g at 750 °C in H2, respectively; nonetheless, their electrical conductivity and capacitance were limited. Therefore, the mango biochar at 900 °C in N2 was considered a suitable alternative carbon material for electrocatalyst support. Additionally, it was possible to determine that the electrical conductivity and capacitance increased as a function of the pyrolysis temperature, while the specific surface area decreased for some biochars as the pyrolysis temperature increased. Overall, it is possible to conclude that heat treatment at a high temperature of 900 °C enhanced the biochar properties toward electrocatalyst support applications.

Microwave-coordinated KOH directionally modulated N/O co-doped porous biochar from Enteromorpha and its structure–effect relationships in efficient CO2 capture

To develop the CO2 adsorption capacity of Enteromorpha-based biochar, N/O co-doped porous biochar (NOBC) was directionally modulated by microwave-coordinated KOH, and the CO2 capture mechanism was explored by structure-effect relationship and density functional theory (DFT) calculations. NOBC with different porous structures and heteroatom contents were prepared by optimization of key parameters (microwave heating temperature, heating time, and KOH ratio) via Response Surface Methodology. The results showed that NOBC(700–20-3) had the maximum CO2 adsorption capacity of 6.09 mol/kg (273 K and 1 bar) and the CO2/N2 selectivity of 78.88 (298 K and 1 bar). After 5 cycles, the CO2 adsorption capacity of NOBC(700–20-3) was still up to 5.98 mol/kg (273 K and 1 bar), showing good cycling performance. The structure-effect relationship analysis showed that the CO2 capture performance of NOBC was proportional to the specific surface area, micropore ratios, graphitization degree, and heteroatoms contents, and inversely proportional to the mean pore diameter. DFT indicated that the synergistic effect between N and O elements developed the CO2 adsorption ability by boosting the affinity and electron transfer between NOBC and CO2. This work contributed to providing a theoretical basis and technical support for the targeted modulation of porous structures and improved surface properties of biochar for efficient CO2 adsorption.

Effects of biochar on soil microbial communities: A meta-analysis

Changes in soil microbial communities may impact soil fertility and stability because microbial communities are key to soil functioning by supporting soil ecological quality and agricultural production. The effects of soil amendment with biochar on soil microbial communities are widely documented but studies highlighted a high degree of variability in their responses following biochar application. The multiple conditions under which they were conducted (experimental designs, application rates, soil types, biochar properties) make it difficult to identify general trends. This supports the need to better determine the conditions of biochar production and application that promote soil microbial communities. In this context, we performed the first ever meta-analysis of the biochar effects on soil microbial biomass and diversity (prokaryotes and fungi) based on high-throughput sequencing data. The majority of the 181 selected publications were conducted in China and evaluated the short-term impact (<3 months) of biochar. We demonstrated that a large panel of variables corresponding to biochar properties, soil characteristics, farming practices or experimental conditions, can affect the effects of biochar on soil microbial characteristics. Using a variance partitioning approach, we showed that responses of soil microbial biomass and prokaryotic diversity were highly dependent on biochar properties. They were influenced by pyrolysis temperature, biochar pH, application rate and feedstock type, as wood-derived biochars have particular physico-chemical properties (high C:N ratio, low nutrient content, large pores size) compared to non-wood-derived biochars. Fungal community data was more heterogenous and scarcer than prokaryote data (30 publications). Fungal diversity indices were rather dependent on soil properties: they were higher in medium-textured soils, with low pH but high soil organic carbon. Altogether, this meta-analysis illustrates the need for long-term field studies in European agricultural context for documenting responses of soil microbial communities to biochar application under diverse conditions combining biochar types, soil properties and conditions of use.

EVALUATION OF THE PROPERTIES OF BIOCHAR OBTAINED FROM RICE HUSK FOR ITS APPLICATION IN AGRICULTURAL SOILS

The increasing world population boosted the demand for basic needs such as energy, water, and food. In turn, agricultural production has increased, generating a large amount of agro-industrial waste every year. Globally, Brazil is the seventh-largest rice producer, and the State of Rio Grande do Sul is responsible for 70% of the rice production in Brazil. Rice husks are composed of organic compounds (cellulose, hemicellulose, lignin, and extractives) and inorganic elements that include potassium, calcium, magnesium, and sulfur. These inorganic elements are essential nutrients in the soil. Since natural degradation is slow due to the aromaticity of lignin, rice husk (RH) waste accumulates and poses an environmental threat, causing air and water pollution. In this context, in order to return the biochar to the soil, the properties of rice husk and rice husk biochar were evaluated. Parameters such as pH, true density, bulk density, porosity, identification of the composition and concentration of the elements, analysis of the functional groups, total carbon (TC), and cation exchange capacity (CEC) were performed on RH biochar samples. These samples were produced in a laboratory-scale pyrolysis rig, under a con-trolled atmosphere of Nitrogen (N), using three peak temperatures (350 °C, 450 °C, and 550 °C) and three soak times (30, 60, and 120 min).Based on the results obtained in the characterization analyses of the biochar, as well as the yield values found, it is concluded that the best pyrolysis temperature for the production of biochar from rice husks, for application in soils, is 550 °C.

Catalytic pyrolysis of biomass using fly ash leachate to increase carbon monoxide production and improve biochar properties to accelerate anaerobic digestion

Biomass-derived biochar is attracting growing interest as an effective additive for anaerobic digestion (AD). To improve energy yield and digestion performance through an integrated process of biochar production and AD, biochar upgrading using biomass fly ash leachate as a sustainable metal catalyst was investigated. The results indicate that the bark soaked in the leachate improved the pyrolysis reactivity of biomass with CO2 and reduced the temperature at which the reaction rate reached its maximum from 943 °C to 801 °C. This resulted in a doubling of CO production during CO2-assisted pyrolysis at 800 °C. In addition, the combined use of fly ash leachate and CO2 in bark pyrolysis improved not only the porosity of the resulting biochar but also its wettability and liming potential. Moreover, biochar amended with fly ash leachate and CO2 significantly reduced the lag time in the anaerobic digestion of grease trap waste.

Effect of Fenton pre-oxidation on the physicochemical properties of sludge-based biochar and its adsorption mechanisms for ammonia nitrogen removal

Fenton pre-oxidation, as a promising technology for sludge deep dehydration, has been used intensively; however, the outputs (i.e., low-water-content sludge cakes) need to be further disposed for a full line sludge resource utilization. Carbonization presents a promising approach for disposing of the sludge cake within the process. Here, the physicochemical properties of sludge-based char obtained after Fenton pre-oxidation were studied. Both Fenton pre-oxidized sludge-based char (FSC) and raw sludge-based char (RSC) were produced within the temperature range of 400–600 ℃, and the adsorption performance of the prepared chars, exemplified by their ability to adsorb ammonia nitrogen (NH4+), was tested under various conditions. FSC exhibited superior char yield and adsorption performance compared to RSC, with chemical adsorption being the primary controlling factor. The optimum conditions for FSC were observed at 400 ℃, resulting in a char yield of 72.2 %, and an adsorption capacity of 221.7 mg/g. Notably, FSC demonstrated an increase in C-C, C-N, OC-O, C-O and CC functional groups in comparison to RSC, which suggests that pre-oxidation increases organic matter decomposition and chemical reactivity of the sludge. The introduction of Si-O-Si, Si-O-C and Fe-O-H functional groups in the FSC facilitated ion exchange with NH4+ in an aqueous solution. The key findings indicate that Fenton pre-oxidation not only enhances the adsorption performance of FSC but also reduces the required carbonization temperature for excellent-char preparation. These results demonstrated the potential to integrate sludge deep dewatering, including pre-oxidation, with char preparation.

Applicability of magnetic biochar derived from Fe-enriched sewage sludge for chromate removal from aqueous solution

In this work, inexpensive biochar derived from different feedstock was synthesized, characterized, and tested for Cr(VI) removal from synthetic wastewater. The physical properties of the sludge-based magnetic biochars (SSB) were analyzed using FT-IR, XRD, BET surface area, SEM-EDS, and XPS techniques. The effects of pyrolysis temperature (300–900 ℃), initial Cr(VI) concentration (2–30 mg/L), rotation speed (50–200 rpm), pH (3–11), and co-ions (Na+, Ca2+, Mg2+, K+, Cl−, HCO3− and SO42−) on Cr(VI) removal by the SSB were determined in batch modes. It was found that the SSB had the highest Cr(VI) removal (43.7%), as compared to the other feedstock-derived biochars (p ≤ 0.05; Anova test). The characterization data indicated that with an increasing pyrolysis temperature from 300 to 900℃, the surface area, pore volume, and pore size decreased, while the Fe3O4 on SSB was gradually reduced to α-Fe0. With respect to Cr(VI) removal, under the optimized conditions of 900℃ of pyrolysis temperature, 2 mg/L of Cr(VI) concentration, 200 rpm of agitation speed, and pH 6.8, a complete Cr(VI) removal could be attained by the SSB900. Its treated effluents could comply with the required Cr(VI) discharge limit of <0.01 mg/L (GB3838-2002) mandated by local legislation. Therefore, further treatment was not required, avoiding additional cost. Coexisting ions experiments showed that all of them inhibited the Cr(VI) removal by the adsorbent after 1 h of reaction. The Cr(VI) removal followed the pseudo-second-order kinetics and the Langmuir isotherm model. The removal mechanism was based on the reduction of Cr(VI) by nZVI on the SSB900 and the Fe leakage might have facilitated the Cr(VI) removal. Overall, iron-enriched sewage sludge biochar is promising and cost-effective for treatment of low-level Cr(VI)-contaminated wastewater.

Investigation on Pb2+ adsorption characteristics by AAEMs-rich biochar in aqueous solution: Performance and mechanism

Biochar derived from soybean straw with AAEMs (alkali and alkaline earth metals) enrichment could efficiently remove heavy metals from contaminated water. In this study, the influences of pyrolysis temperature on the physicochemical property and adsorption performance of soybean straw biochar were investigated. The contributions of different adsorption mechanisms were analyzed quantitatively. The results show that the soybean straw biochar exhibits excellent Pb2+ adsorption performance (157.2–227.2 mg g−1), with an order of BC800 > BC400 > BC600 > BC700 > BC500. The mechanisms of metal ion exchange (37.49%–72.58%) and precipitation with minerals (22.38%–58.03%) mainly control the Pb2+ adsorption, whereas complexation with organic functional groups (OFGs) and cation-Cπ interaction make the less contribution. The order of cation exchange capacity (CEC) is BC400 > BC800 > BC700 > BC600 > BC500, showing a high correlation (0.965) with the contribution of metal ion exchange with AAEMs. Moreover, Ca exhibits the strongest exchange capacity. The contribution of precipitation is consistent with the variation of soluble CO32− content in biochar. These results suggest that soybean straw biochar rich in AAEMs is a prospective adsorbent for Pb2+ elimination.

Synthesis of magnetic biochar with high iron content and large specific surface area: Synergistic effect of Fe doping and KOH activation

Magnetic biochar has promising applications in environmental remediation, catalysts, electrode materials, etc. The influence of the sequence of iron doping process and KOH activation process on the properties of magnetic biochar is significant and has not been studied yet. In this work, the properties of magnetic biochar were evaluated by changing the order of iron doping and KOH activation, accompanied by the comparison of simultaneous these two processes. Following the doping of biochar with iron, the oxide clusters of iron distributed in the surface of the carbon skeleton protect the interior from etching by KOH, resulting in low specific surface area (SSA). The advantages of the synergy strategy were evident in the high surface iron content and large SSA of the synthesized magnetic biochar. Further, batch adsorption and recovery experiments were performed on Fe/KOH-C obtained by a simple and low-cost synergistic synthesis strategy, and the results showed that the maximum adsorption capacity of tetracycline was up to 375 mg g−1, with advantages in terms of recovery. Pore filling, electrostatic attraction, π-π interaction, and complexation reaction of the surface functional groups were proved to be the adsorption mechanism of Fe/KOH-C.

A novel composite hard carbon from waste Camellia oleifera shell modified by phenol-formaldehyde resin for supercapacitor electrode with high specific capacitance

The lower specific capacitance of biochar electrode material limits its practical application. Therefore, this study attempts to develop a novel supercapacitor biochar electrode material with high specific capacitance from waste camellia oleifera shell (COS). COS shell powder was pre-impregnated with phenol-formaldehyde (PF) resin then carbonization-KOH activation. An orthogonal experiment (L9(33)) was designed to explore the influence of mass mixing ratio of PF/COS, activation time, and temperature on the electrochemical property of COS-based biochar. The optimal process parameters were: 800 °C, 1 h and mass mixing ratio of 5:1. Compared with c. oleifera shell-activated carbon (COS-AC) and phenol-formaldehyde-activated carbon (PF-AC) controls, the phenol-formaldehyde modified c. oleifera shell-activated carbon (PMC-AC) exhibited micro-morphology of carbon microspheres and a three-dimensional network pore. The internal orientation of PMC-AC tended to be disordered, and had a superior specific surface area of 3037.68 m2 g−1 and total pore volume of 1.7 cm3 g−1. Electrochemical tests used PMC-AC as the negative electrode, with high specific capacitance (375.9 F g−1 at 0.5 A g−1), good rate property (49.3 % of capacitance retention at 0.5–10 A g−1), and charging/discharging stability. It is supposed that it was a convenient and feasible way to modify biochar into high-quality hard carbon for supercapacitors.

CO2 and H2O as sweep gases elevated carbon stability and decreased phytotoxicity of biochars

As a crucial pyrolysis factor for biochar production, the mechanistically effect of different sweep gases on the inherent multi-level chemical structure of biochars is poorly understood, impeding the widespread use of biochars as sustainable carbon-negative materials. This study investigated the effect of CO2 and H2O as sweep gases relative to N2 on the physic-chemical properties of biochars derived from sawdust (SBC-N/C/H) and pig manure (PBC-N/C/H) at 300–700 °C. Meanwhile, carbon sequestration potential and potential environmental risks of SBC-N/C/H and PBC-N/C/H were comprehensively evaluated. Unlike N2, CO2 showed a superior effect on biochar (especially SBCs with more carbonaceous substances), decreased the oxygenated-functional groups content in SBC-C (14.6%–28.1%) relative to SBC-N (25.5%–37.4%) via cracking reaction. Besides, CO2 stimulated the formation of pore structure in biochar (surface area increased from 15.2 m2/g in SBC700-N to 24.1 m2/g in SBC700-C) by boudouard reaction. H2O exhibited a similar but less pronounced effect with CO2, leading to a lower H/C ratio in biochar by aggravated demethylation of methoxyl groups and facilitating a more developed pore structure with water–gas reaction. Both CO2 and H2O enhanced the carbon stability of biochar and carbon sequestration potential in SBC500-C (30.1%) and SBC500-H (28.7%) was higher than SBC500-N (25.6%). Moreover, CO2 and H2O exhibited excellent performance in mitigating phytotoxicity of biochars (especially for PBCs) to radish and cucumber seeds via the reduced release of polycyclic aromatic hydrocarbons and heavy metals. These results proposed an effective approach to improve long-term carbon sequestration and environmental safety of biochars.