Effects of feedstock and inherent mineral components on oxidation resistance of biochars

A complete review on the oxygen-containing functional groups of biochar: Formation mechanisms, detection methods, engineering, and applications

Biochar is a porous carbon material generated by the thermal treatment of biomass under anaerobic or anoxic conditions with wealthy Oxygen-containing functional groups (OCFGs). To date, OCFGs of biochar have been extensively studied for their significant utility in pollutant removal, catalysis, capacitive applications, etc. This review adopted a whole system philosophy and systematically summarizes up-to-date knowledge of formation, detection methods, engineering, and application for OCFGs. The formation mechanisms and detection methods of OCFGs, as well as the relationships between OCFGs and pyrolysis conditions (such as feedstocks, temperature, atmosphere, and heating rate), were discussed in detail. The review also summarized strategies and mechanisms for the oxidation of biochar to afford OCFGs, with the performances and mechanisms of OCFGs in the various application fields (environmental remediation, catalytic biorefinery, and electrode material) being highlighted. In the end, the future research direction of biochar OCFGs was put forward.

Greenhouse gas mitigation and soil carbon stabilization potential of forest biochar varied with biochar type and characteristics

Biochar is increasingly used in climate-smart agriculture, yet its impact on greenhouse gas (GHG) emissions and soil carbon (C) sequestration remains poorly understood. This study examined biochar-mediated changes in soil properties and their contribution to C stabilization and GHG mitigation by evaluating four types of biochar. Soil carbon dioxide (CO2) and nitrous oxide (N2O) emissions, soil chemical and biological properties, and soil organic carbon (SOC) mineralization kinetics were monitored using greenhouse, laboratory, and modeling experiments. Three pine wood biochars pyrolyzed at 460 °C (PB-460), 500 °C (PB-500), 700 °C (PB-700), and one pine bark biochar from gasification at 760 °C (GB-760) were added into soil at 1 % w/w basis. Soils amended with biochar were used to cultivate sorghum for three months in a greenhouse, followed by three months of laboratory incubation. Data obtained from laboratory incubation was modeled using various statistical approaches. The PB-500 and PB-700 reduced cumulative N2O-N emissions by 68.5 % and 73.9 % and CO2 equivalent C emissions by 66.9 % and 72.4 %, respectively, compared to unamended control. The N2O emissions were positively associated with soil nitrate N, available P, and biochar ash content while negatively associated with SOC. The CO2 emission was negatively related to biochar C:N ratio and volatile matter content. Biochar amended soils had 49.2 % (PB-500) to 87.7 % (PB-700) greater SOC and 22.9 % (PB-700) to 48.1 % (GB-760) greater sorghum yield than the control. While PB-700 had more saprophytes than the control, the GB-760 yielded a greater yield than biochars prepared by pyrolysis. Microbial biomass C was 7.23 to 23.3 % greater in biochar amended soils than in control. The double exponential decay model best explained the dynamics of C mineralization, which was associated with initial soil nitrate N and available P positively and total fungi and protozoa biomass negatively. Biochar amendment could be a climate smart agricultural strategy. Pyrolysis pine wood biochar showed the greatest potential to reduce GHG emissions and enhance SOC storage and stability, and gasification biochar contributed more to SOC storage and increased crop yield.

Comparison of water-soluble organic matter (WSOM)-containing and WSOM-free biochars for simultaneous sorption of lead and cadmium

The effects of individual biochar constituents and natural environmental media on the immobilization behaviors and chemical activities of toxic heavy metals are still poorly understood. In this work, the physicochemical properties of raw corn straw (CS) and CS-derived biochar materials as well as their sorption abilities and retention mechanisms for lead (Pb) and cadmium (Cd) were evaluated by combining batch experiments and spectral approaches. According to the spectral analysis results and single variable principle, the setting of biochars after soaking in solution as the control group was suggested when evaluating their retention mechanisms for Pb and Cd. The rising of ionic strength did not apparently affect the immobilization of Pb by biochar prepared at 500 °C (i.e., CB500) and Pb/Cd by water-soluble organic matter (WSOM)-free CB500 (i.e., DCB500), while slightly inhibited the sorption of Cd by CB500. Pb and Cd exhibited a mutual inhibition effect on their sorption trends with a higher sorption preference of Pb. The dominant fixation mechanism of Pb by CB500 and DCB500 was identified to be mineral precipitation. In contrast, the main sorption mechanism of Cd changed from mineral precipitation in the single-metal system to surface complexation in the binary-metal system. The sorption ratios of Pb and Cd on CB500 were comparable to those on DCB500 with the coexistence of mixed natural organic matters (NOM) and ferrihydrite. The current experimental findings suggested that DCB500 was a suitable remediation agent for regulating the migration behaviors of toxic Pb and Cd in acidic and NOM-rich soil and water systems.

Effects of Fe3+ on Hydrothermal Humification of Agricultural Biomass

Hydrothermal humification technology for the preparation of artificial humic matters provides a new strategy, greatly promoting the natural maturation process. Iron, as a common metal, is widely used in the conversion of waste biomass; however, the influence of Fe3+ on hydrothermal humification remains unknown. In this study, FeCl3 is used to catalyze the hydrothermal humification of corn straw, and the influence of Fe3+ on the hydrothermal humification is explored by a series of characterization techniques. Results show that Fe3+ as the catalyst can promote the decomposition of corn straw, shorten the reaction time from 24 h to 6 h, and increase the yield from 6.77 % to 14.08 %. However, artificial humic acid (A-HA) obtained from Fe3+ -catalysis hydrothermal humification contains more unstable carbon and low amount of aromatics, resulting in a significantly decreased stability of the artificial humic acid. These results provide theoretical guidance for regulating the structure and properties of artificial humic acid to meet various maintenance needs.

Highly efficient nanocomposite of Y2O3@biochar for oxytetracycline removal from solution: Adsorption characteristics and mechanisms

Nano Y₂O₃-modified biochar composites (Y₂O₃@BC600) were fabricated successfully and exhibited great adsorption toward oxytetracycline (OTC). The Langmuir adsorption capacity of Y₂O₃@BC600-1:4 for OTC reached 223.46 mg/g, 10.52 times greater than that of BC600. The higher dispersion of Y₂O₃ nanoparticles, increased surface area of 175.65 m²/g and expanded porosity of 0.27 cm³/g accounted for higher OTC adsorption by Y₂O₃@BC600-1:4. Y₂O₃@BC600-1:4 could resist the interference of co-existing cations (Na⁺, K⁺, Mg²⁺, Ca²⁺) and anions (Cl^-, NO₃^-, SO₄^2-) on OTC removal. Y₂O₃ coating changed surface charge property of BC600, favoring the contribution of electrostatic interaction. Synchrotron radiation-based Fourier transform infrared spectroscopy detected obvious peak shift and intensity change of surface -OH when OTC adsorption occurred. Accordingly, stronger H-bonding (charge-assisted hydrogen bond, OTC-H₂N⁺···HO-Y₂O₃@BC600-1:4) was proposed for OTC adsorption. Y₂O₃@BC600 exhibited renewability and stability in the adsorptive removal of OTC. Therefore, Y₂O₃@BC600 may be a novel and suitable adsorbent for antibiotic removal.

Co-pyrolysis of wood chips and bentonite/kaolin: Influence of temperatures and minerals on characteristics and carbon sequestration potential of biochar

Biochars have been highlighted as a means of carbon sequestration, which is significant for achieving carbon neutrality. Mixtures of wood chips and either bentonite or kaolin were co-pyrolysed at temperatures of 350 °C and 550 °C, and the microstructural characteristics and the carbon sequestration potential of the resultant biochar were explored in the study. The addition of minerals promoted the formation of a stable carbon structure in biochar, especially the proportion of SiC bonds in the high-temperature mineral-composited biochar increased by 3.56-3.82 times compared with the original biochar. After bentonite or kaolin was added to wood chips pyrolysed at 550 °C, the carbon loss after H₂O₂ oxidation was reduced to no more than 19.2%, and the Recalcitrance Index (R₅₀) of biochar increased to no less than 0.89. The combined action of high temperature and minerals promoted the formation of highly aromatic structures of biochar (H:C < 0.4) and reduced the amount of dissolved organic carbon to 4.89 mg g^-1. Furthermore, minerals directly covered the surface of biochar, and the content of SiC bond increased, thus strengthening the chemical and thermal stability of biochar. However, the addition of minerals had no significant effect on the biological stability of biochar. The study indicates that the pre-pyrolysis mineral addition is an effective way to increase the carbon sequestration potential of biochar.

Is biochar a reliable catalyst for activating peroxydisulfate? Damage of biochar during catalytic process

The unsatisfactory reuse performance of biochar in the catalytic system has become an important factor limiting its application. Damage of biochar in the catalytic systems has been reported, but the mechanism of damage is still unclear. In this study, cotton stalk and walnut shell biochar were used to activate peroxydisulfate (PDS) for sulfamethazine degradation, and multiple characterization methods were used to determine the damage of biochar. The comparative experiments of biochar, recycled biochar, and the biochar treated by PDS in catalytic reaction showed that the performance of biochars decreased by 37.5%-65.3%. After the catalytic reaction, the unstable carbon in biochar was lost, and the biochar surface was oxidized, which reduced its affinity to organic matter. CO₂ physisorption showed that the pore structure of biochars barely changed. However, for walnut shell biochar, its adsorption capacity of nitrogen decreased obviously. Moreover, the electrochemical tests showed that the conductivity of walnut shell biochar decreased, which would lead to the reduction of its catalytic performance through the electron transfer path. Overall, the effects of catalytic process on the performance of biochar cannot be ignored. Therefore, more efforts are needed to improve the stability and homogeneity of biochar as an efficient and sustainable PDS activator.

Development of phosphorus composite biochar for simultaneous enhanced carbon sink and heavy metal immobilization in soil

As a porous and carbon material, biochar is focused on respectively in sequestrating carbon and stabilizing metals in soil, while few studies attempted to design biochar for simultaneously achieving these two targets. This study proposed to produce phosphorus-composite biochar for synchronously enhancing carbon sequestration and heavy metals immobilization. Two phosphorus materials from tailings, Ca(H₂PO₄)₂ and Ca₅(PO₄)₃(OH), were selected as modifier to load into biomass prior to pyrolysis. Results showed that incorporating P not only increased pyrolytic C retention in biochar by 36.1-50.1%, but also obtained biochar with higher stability by chemically formation of COP, C-PO₃ and C₂-PO₂. After 90-day incubation with soil, more C was sequestrated in the P-biochar amended soil (59.6-67.0%) than those pristine biochar (43.2-46.6%). Highly soluble Ca(H₂PO₄)₂ was more efficient than Ca₅(PO₄)₃(OH) in this regard. Meanwhile, these P-composite biochar exhibited more Pb/Cd immobilization (31.3-92.3%) compared with the pristine biochar (9.5-47.2%), which was mainly due to the formation of stable precipitates Pb₅(PO₄)₃Cl and Cd₃(PO₄)₂, especially for Ca₅(PO₄)₃(OH) modification. Additionally, P-composite biochar "intelligently" altered soil microbial community, i.e., they suppressed Actinobacteria proliferation, which is correlated to carbon degradation, while promoted Proteobacteria growth, facilitating phosphate dissolution for ready reaction with heavy metals to form precipitate, benefiting the Pb and Cd immobilization. A dual functions biochar was engineered via simply loading phosphorous prior to pyrolysis and simultaneously enhanced carbon sequestration and heavy metal immobilization.

Efficient removal of uranium from wastewater using pig manure biochar: Understanding adsorption and binding mechanisms

In this work, three kinds of biochars (PMBC-H₂O, PMBC-PP and PMBC-HP) with excellent adsorption performance were obtained by carbonizing pig manure pre-treated with different agents. These biochars had the ordered mesoporous structures and possessed abundant active functional groups on their surface. The adsorption behaviors of the biochars towards U_VI under various conditions were evaluated by batch experiment. The results showed that KMnO₄ and H₂O₂ could enormously improve the adsorption performance of PMBC to U_VI. After KMnO₄ and H₂O₂ pretreatment, the maximum adsorption capacities of PMBC-PP (979.3 mg/g) and PMBC-HP (661.7 mg/g) were about 2.6 and 1.8 times higher than that of PMBC-H₂O (369.9 mg/g), respectively, which was much higher than previously reported biochar-based materials. Obviously, KMnO₄ pretreatment leaded to a higher enhancement than that of H₂O₂. The removal mechanism of U_VI on PMBC-PP was discussed in-depth. The interaction between U_VI species and PMBC-PP was mainly ascribed to the abundant active sites on the surface of PMBC-PP. In a word, conversion of pig manure pre-treated with KMnO₄ into biochar not only demonstrates that PMBC-PP has great potential in the treatment of actual uranium-containing wastewater, but also provides a method for the rational utilization of pig manure to reduce the pollution.

Stability of biochar in mineral soils: Assessment methods, influencing factors and potential problems

Amendment of biochar into mineral soils has been reported a promising strategy for carbon sequestration and greenhouse gas mitigation due to its high stability. Currently, most studies on the stability of biochar are mainly focused on the assessment methods and influencing factors. The assessment methods include qualitative evaluation of physical and chemical properties, and utilization of kinetic mineralization models on the basis of laboratory incubation. As a result, these assessment methods are difficult to accurately reflect the real impact of the interaction between biochar and environmental factors. This article reviews the existing assessment methods, influencing factors, and the impact of environmental aging on the stability of biochar. It is found that under the influence of environmental factors, existing assessment methods are likely to overestimate the stability of biochar in mineral soils. Therefore, more emphases should be laid on the analyses of the deficiencies in the existing assessment methods on the stability of biochar in the consideration of practical applications. Long-term field experiment is strongly recommended to establish a more accurate assessment model on biochar stability for the evaluation of its carbon sequestration potential in mineral soils.

Slow pyrolysis of agro-food wastes and physicochemical characterization of biofuel products

Effective management and utilization of food waste and agricultural crop residues are highly crucial to mitigate the challenges of greenhouse gas generation upon natural decomposition and waste accumulation. Conversion of biogenic wastes to biofuels and bioproducts can address the energy crisis and promote environmental remediation. This study was focused on exploring the characteristics of food waste and agricultural crop residues (e.g., canola hull and oar hull) to determine their candidacy for slow pyrolysis to produce biochar and bio-oil. Process parameters of slow pyrolysis such as temperature, reaction time and heating rate were optimized to obtain maximum biochar yields. Maximum biochar yield of 28.4 wt% was recorded at optimized temperature, heating rate and reaction time of 600 °C, 5 °C/min and 60 min, respectively. Furthermore, the physicochemical, spectroscopic and microscopic characterization of biochar, bio-oil and gases were performed. The carbon content and thermal stability of biochar were found to increase at higher temperatures. Moreover, bio-oil generated at higher temperatures showed the presence of phenolics and aromatic compounds.

Mechanism of persulfate activation by biochar for the catalytic degradation of antibiotics: Synergistic effects of environmentally persistent free radicals and the defective structure of biochar

The abuse of antibiotics threatens the water environment and human health. Green treatment method is needed to degrade antibiotics such as biochar. Few studies have examined the environmentally persistent free radicals (EPFRs) and defective structure of biochar during the biochar-mediated catalytic degradation of antibiotics. In this study, biochar prepared from poplar and pine sawdust was used to activate peroxymonosulfate (PMS) to generate instant radicals (SO₄^•- and •OH) and degrade tetracycline (TC), chlortetracycline (CTC) and doxycycline (DOX). The preparation temperatures ranged from 300 °C to 900 °C. EPFRs were the main activator of PMS at 300-500 °C, and the defective structure of biochar was the main activator at 800-900 °C. The concentrations of EPFRs ranged from 1.75 × 10¹⁸ spins/g to 6.44 × 10¹⁸ spins/g. According to the electron paramagnetic resonance (EPR) parameter (g-factor), the main types of EPFRs were carbon-centered radicals (g₁ < 2.0030) or carbon-centered radicals with oxygen atoms (2.0030 < g₂ < 2.0040). Optimization of the degradation experiment revealed that the removal rate of antibiotics peaked when the preparation temperature was 500 °C and 900 °C. In the biochar/PMS system, the antibiotics removal rate of 90% was achieved in 40 min with an average apparent rate constant (k_obs) of 0.0588 min^-1. Analysis of the mechanism revealed that the free radical pathway (EPFRs and defective structure) can effectively activate PMS to generate SO₄^•- and •OH. However, control experiments suggested that the non-free radical pathway (singlet oxygen) had little effect on antibiotic degradation. After five cycles, the removal rate of antibiotics by biochar was still greater than 70%, indicating that biochar retains a high degradation ability. These results indicate that optimizing the preparation conditions can effectively expand the application range of the biochar/PMS system and improve the degradation of antibiotic wastewater.

Enhanced degradation of bisphenol A by mixed ZIF derived CoZn oxide encapsulated N-doped carbon via peroxymonosulfate activation: The importance of N doping amount

In this study, mixed metal cobalt zinc oxide embedded nitrogen enriched porous carbon composites (CoZnO-PC) were prepared via pyrolyzing polyvinylpyrrolidone (PVP) encapsulated Co, Zn-bimetal centered zeolitic imidazolate frameworks (ZIF). The prepared composites were then used to activate peroxymonosulfate (PMS) for bisphenol A (BPA) removal in water. When mole ratio of Co/Zn was 2/1, the resulted Co₂Zn₁O-PC possessed spinel structure with prominent degradation capability, in which the introduction of Zn accelerated the PMS activation performance of Co through establishing bimetal synergistic interactions. Both radical and non-radical activation pathways were existed in the Co₂Zn₁O-PC/PMS system, in which Co₂Zn₁O dominated the radical pathway whereas PC dominated the non-radical way. Since PVP contained abundant nitrogen atoms and could form strong coordination interactions with the ZIF precursor, the introduction of PVP in the ZIF precursor prevented pore collapsing during pyrolysis process, as well as enhancing the nitrogen content in the pyrolzed composites, which significantly promoted the generation of singlet oxygen. With combined pathways, the Co₂Zn₁O-PC/PMS system showed a wide pH application range with promising mineralization rate. Meanwhile, the spinel-structured Co₂Zn₁O-PC was magnetically separable with desirable recyclability. This study presents a novel composite with remarkable performance for the removal of refractory organic pollutants in municipal wastewater.

Impacts of different activation processes on the carbon stability of biochar for oxidation resistance

Biochar modification is widely used to improve its capability for environmental application, while its impact on carbon sequestration potential is unknown. Herein, the oxidation-resistance stability of biochar with different activation processes was first evaluated, which is crucial for sustainable production of engineered biochar. Thermal activation enhanced the thermal stability of biochar with a higher R₅₀ as 61.5-62.7%, whereas a higher carbon loss of 15.2-17.2% was revealed after chemical oxidation. Physical activation of biochar had marginal effect on thermal stability, but it still weakened its chemical stability. By contrast, chemical activation with H₂SO₄ improved the stability in terms of chemical-oxidation (6.7% carbon loss) and thermal-oxidation (R₅₀ as 66.2%). Further analysis revealed that the thermal stability of engineered biochar was controlled by aromaticity, while the surface area was a vital factor correlating to the chemical stability. Our findings serve as an important reference to understand trade-off between biochar stability and broader application.

Towards a Soil Remediation Strategy Using Biochar: Effects on Soil Chemical Properties and Bioavailability of Potentially Toxic Elements

Soil contamination with potentially toxic elements (PTEs) is considered one of the most severe environmental threats, while among remediation strategies, research on the application of soil amendments has received important consideration. This review highlights the effects of biochar application on soil properties and the bioavailability of potentially toxic elements describing research areas of intense current and emerging activity. Using a visual scientometric analysis, our study shows that between 2019 and 2020, research sub-fields like earthworm activities and responses, greenhouse gass emissions, and low molecular weight organic acids have gained most of the attention when biochar was investigated for soil remediation purposes. Moreover, biomasses like rice straw, sewage sludge, and sawdust were found to be the most commonly used feedstocks for biochar production. The effect of biochar on soil chemistry and different mechanisms responsible for PTEs' immobilization with biochar, are also briefly reported. Special attention is also given to specific PTEs most commonly found at contaminated soils, including Cu, Zn, Ni, Cr, Pb, Cd, and As, and therefore are more extensively revised in this paper. This review also addresses some of the issues in developing innovative methodologies for engineered biochars, introduced alongside some suggestions which intend to form a more focused soil remediation strategy.

The contribution of lignocellulosic constituents to Cr(VI) reduction capacity of biochar-supported zerovalent iron

Biochars (BCs) derived from individual and blending lignocellulosic constituents were prepared to harbor zerovalent iron (ZVI/BC) in an effort to discriminate significance of each constituent or combination in ZVI/BC for Cr(VI) removal. BCs and ZVI/BC were characterized by TGA/GSC, XRD, Raman and BET analyses. Cellulose (BC_C) and hemicellulose (BC_H)-derived BCs has greater C content, H/C ratio, surface area and mass loss than BCs derived from lignin or lignin-containing biopolymer blends (BC_LX). As per sorption and XPS analysis, ZVI/BC demonstrated greater Cr(VI) removal capacity than respective BCs, in which reduction accounted for over 77% Cr(VI) detoxification. Cr(VI) reduction by ZVI harbored by BC_C and BC_H was 19.72-16.54 g kg^-1, compared to 5.97-4.26 g kg^-1 for BC_LX. ZVI/BC prepared by three-biopolymer blends with (12.63 g kg^-1) or without (12.32 g kg^-1) mineral approximated pinewood-BC (BC_P) (13.02 g kg^-1) for Cr(VI) reduction, suggesting minerals are not important constituent. Tafel analysis showed BC_C and BC_H, with lower I_D/I_G ratio owing to greater graphitization, were more conducible to transfer electron of ZVI in Cr(VI) reduction than BC_LX. Thus, cellulose, hemicellulose and lignin can offer a good prediction of property of natural biomass, in which BC_C and BC_H favor electron transfer of ZVI but BC_L is not electroactive.

Pinewood outperformed bamboo as feedstock to prepare biochar-supported zero-valent iron for Cr6+ reduction

In this work, pinewood and bamboo were pyrolyzed at 600 °C to prepare PBC and BBC-supported zerovalent iron (ZVI), respectively. Raman spectra suggested PBC was more intensively carbonized than BBC as indicated by higher I_D/I_G ratio. XRD and TEM confirmed nanoscaled ZVI was well dispersed in PBC but soldered in chain-structure in BBC. Maximal chromate (Cr(VI)) sorption capacity followed the order of PBC/ZVI (5.93 g kg^-1)>BBC/ZVI (3.61 g kg^-1)>BBC (3.55 g kg^-1)>PBC (2.59 g kg^-1). Desorption and XPS of four Cr-spent sorbents suggested reduction accounted for 79-88% of overall Cr(VI) detoxification. Greater Cr(VI) reduction of BBC than PBC indicated greater tendency of BBC to donate electrons. However, Cr(VI) reduction by PBC/ZVI was 1.7 times greater than BBC/ZVI, corresponding to greater electron transfer of PBC/ZVI (2.5 μA e^-) than BBC/ZVI (0.5 μA e^-). Thus, PBC is more conducible to transfer electrons as evidenced by Tafel and Amperometric analyses. Demineralization of pristine BC enhanced the difference between PBC/ZVI and BBC/ZVI regarding Cr(VI) reduction, suggesting the dominant role of biopolymers in biomass in terms of electron transfer capacity. Three model biopolymers were compared which indicated lignin-BC had lower electron transfer rates than cellulose-BC and hemicellulose-BC. BC prepared by lignin extracted from pinewood exhibited higher corrosion rate and lower electrical resistance than that from bamboo. Thus, unfavorable lignin in bamboo compromised electron transfer of BBC and Cr(VI) reduction by BBC/ZVI.

Mechanism of phosphate removal from aqueous solutions by biochar supported nanoscale zero-valent iron

The purpose of this study was to investigate the removal mechanism of phosphate by rape straw biochar (RSBC) supported nanoscale zero-valent iron (nZVI).