Pyrolysis temperature-dependent electron transfer capacities of dissolved organic matters derived from wheat straw biochar

The critical role of electron donating rate of pyrogenic carbon in mediating the degradation of phenols in the aquatic environment

Phenols are the widely detected contaminants in the aquatic environment. Pyrogenic carbon (PyC) can mediate phenols degradation, but the specific properties of PyC or phenols influencing this reaction remain unknown. The present study investigated the kinetic process and mechanism of removal of various phenols by different PyC in aqueous phase system. To avoid the impact of the accumulated degradation byproducts on the overall reaction, we conducted a short-term experiment, quantified adsorption and degradation, and obtained reaction rate constants using a two-compartment first-order kinetics model. The adsorption rate constants (ka) of phenols by PyC were 10-220 times higher than degradation rate constants (kd), and they were positively correlated. Interestingly, no correlation was found between kd and common PyC properties, including functional groups, electron transfer capacities, and surface properties. Phenols were primarily attacked by •OH in the adsorbed phase. But neither the instantly trapped •OH, nor the accumulated •OH could explain phenol degradation. Chemical redox titration revealed that the electron transfer parameters, such as the electron donating rate constant (kED) of PyC, correlated well with kd (r>0.87, P < 0.05) of phenols. Analysis of 13 phenols showed that Egap and ELUMO negatively correlated with their kd, confirming the importance of the electronic properties of phenols to their degradation kinetics. This study highlights the importance of PyC electron transfer kinetics parameters for phenols degradation and manipulation of PyC electron transfer rate may accelerate organic pollutant removal, which contributes to a deeper understanding of the environmental behavior and application of PyC systems.

Dual role of dissolved black carbon in sensitized ofloxacin photooxidation: Mechanism and influential factors

Dissolved black carbon (DBC) is the more photoactive component of dissolved organic matter (DOM) pool, which plays a dual role in the photoconversion of aquatic contaminants, acting as both a photosensitizer and an inhibitor. However, little is known about the more systematic mechanism by which DBC exhibits a dual effect, which is closely related to the structure composition of DBC. In this study, the differences in characteristics of DBC obtained from 300 °C and 500 °C were compared via UV-vis absorption spectrum, Fluorescence excitation emission matrix spectra (3D-EEM), Fourier transform infrared (FT-IR), and X-ray photoelectron spectroscopy (XPS), and evaluated the promoting and inhibiting effects of DBC on ofloxacin (OFL) photodegradation. It was found that higher pyrolysis temperature reduced the UV absorbance, molecular weight, aromaticity, and phenolics of DBC while increasing the content of quinone/aromatic ketone and humic substances. Photochemical data showed that 3DBC*, 1O2 and ·OH were all participated in the DBC-mediated OFL photodegradation. Wherein, DBC300 (DBCT, where T = pyrolysis temperature) had strong light screening and dynamic quenching effect, but the formation ability of 3DBC*, 1O2 and ·OH was poor, which significantly retarded the photodegradation of OFL. While DBC500 exhibited a slight promotion effect due to its higher formation ability of reactive species and weak light screening effect. Moreover, DBC500 had higher steady-state concentration and (kOFL,3DBC⁎) than DBC300, which might be due to the higher contents of quinone/aromatic ketone and the lower contents of phenol in DBC500, thus enhancing the reactivity of 3DBC* and OFL. Our research systematically revealed the trade-off mechanism of DBC on the photodegradation of fluoroquinolones, and provided an important theoretical guidance for the photodegradation of fluoroquinolones under the evolution of DBC composition.

Influences of wildfire on the soil dissolved organic matter characteristics and its electron-donating capacity

Global increases in the intensity and frequency of wildfires are driving major changes in soil organic matter (SOM) characteristics, including soil dissolved organic matter (DOM). As the most crucial component of SOM, soil DOM plays a pivotal role in the carbon cycle and regulates the environmental fate of contaminants through its versatile reactivities, including electron-donating capacity (EDC). However, it is still being determined how wildfire influences key characteristics of soil DOM and subsequent effects on EDC in forest soils. Thus, we conducted our study to fill this gap with the forest soils of Jinyun Mountain Nature Reserve of China, which experienced an unprecedented wildfire event in 2022. The results from optical characterization, high-performance size-exclusion chromatography (HPSEC), and Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR-MS) showed decreasing molecular weight but elevating nitrogen-containing molecular formulas of soil DOM in the burned soils. This could be attributed to the Maillard reaction and microbial re-colonies. Additionally, wildfires increased the condensed aromatics and lignin components in soil DOM. In the burned soils, we observed increasing EDC of soil DOM, which accounts for an increase in lignin-derived phenolic components. Overall, the findings of this study demonstrate that eco-disturbances, such as wildfires, induce alterations in the properties of DOM, leading to variations in its reactivity and potentially influencing the fate of environmental pollutants beyond carbon dynamics alone. Thus, incorporating the dynamic properties of soil DOM, particularly in the context of climate change, can enhance the assessment of risks associated with contaminants in soil and water, providing valuable insights.

Temporal Variations in Fire Impacts on Characteristics and Composition of Soil-Derived Dissolved Organic Matter at Qipan Mountain, China

Dissolved organic matter (DOM), the most reactive fraction of forest soil organic matter, is increasingly impacted by wildfires worldwide. However, few studies have quantified the temporal changes in soil DOM quantity and quality after fire. Here, soil samples were collected after the Qipan Mountain Fire (3-36 months) from pairs of burned and unburned sites. DOM contents and characteristics were analyzed using carbon quantification and various spectroscopic and spectrometric techniques. Compared with the unburned sites, burned sites showed higher contents of bulk DOM and most DOM components 3 months after the fire but lower contents of them 6-36 months after the fire. During the sharp drop of DOM from 3 to 6 months after the fire, carboxyl-rich alicyclic molecule-like and highly unsaturated compounds had greater losses than condensed aromatics. Notably, the burned sites had consistently higher abundances of oxygen-poor dissolved black nitrogen and fluorescent DOM 3-36 months after the fire, particularly the abundance of pyrogenic C2 (excitation/emission maxima of <250/∼400 nm) that increased by 150% before gradually declining. This study advances the understanding of temporal variations in the effects of fire on different soil DOM components, which is crucial for future postfire environmental management.

Mechanisms of conjugative transfer of antibiotic resistance genes induced by extracellular polymeric substances: Insights into molecular diversities and electron transfer properties

Dissemination of antibiotic resistance genes (ARGs) has become a critical threat to public health. Activated sludge, rich in extracellular polymeric substances (EPS), is an important pool of ARGs. In this study, mechanisms of conjugation transfer of ARGs induced by EPS, including tightly bound EPS (TBEPS), soluble EPS (SEPS), and loosely bound EPS (LBEPS), were explored in terms of molecular diversities and electron transfer properties of EPS. Conjugation transfer frequency was increased by 9.98-folds (SEPS), 4.21-folds (LBEPS), and 15.75-folds (TBEPS) versus the control, respectively. Conjugation-related core genes involving SOS responses (9 genes), membrane permeability (18 genes), intercellular contact (17 genes), and energy metabolism pathways (13 genes) were all upregulated, especially in the presence of TBEPS. Carbohydrates and aliphatic substances in SEPS and LBEPS were contributors to ARG transfer, via influencing reactive oxygen species (ROS) formation (SEPS) and ROS and adenosine triphosphate (ATP) production (LBEPS). TBEPS had the highest redox potential and greatest lability and facilitated electron transfer and alternated respiration between cells, thus promoting ARG transfer by producing ATP. Generally, the chemical molecular characteristics and redox properties of EPS facilitated ARG transfer mainly by influencing lipid peroxidation and ATP, respectively.

Photo-methanification of aquatic dissolved organic matters with different origins under aerobic conditions: Non-negligible role of hydroxyl radicals

Lingering inconsistencies in the global methane (CH4) budget and ambiguity in CH4 sources and sinks triggered efforts to identify new CH4 formation pathways in natural ecosystems. Herein, we reported a novel mechanism of light-induced generation of hydroxyl radicals (•OH) that drove the production of CH4 from aquatic dissolved organic matters (DOMs) under ambient conditions. A total of five DOM samples with different origins were applied to examine their potential in photo-methanification production under aerobic conditions, presenting a wide range of CH4 production rates from 3.57 × 10-3 to 5.90 × 10-2 nmol CH4 mg-C-1 h-1. Experiments of •OH generator and scavenger indicated that the contribution of •OH to photo-methanificaiton among different DOM samples reached about 4∼42 %. In addition, Fourier transform infrared spectroscopy and Fourier transform ion cyclotron resonance mass spectrometry showed that the carbohydrate- and lipid-like substances containing nitrogen-bonded methyl groups, methyl ester, acetyl groups, and ketones, were the potential precursors for light-induced CH4 production. Based on the experimental results and simulated calculations, the contribution of photo-methanification of aquatic DOMs to the diffusive CH4 flux across the water-air interface in a typical eutrophic shallow lake (e.g., Lake Chaohu) ranged from 0.1 % to 18.3 %. This study provides a new perspective on the pathways of CH4 formation in aquatic ecosystems and a deeper understanding on the sources and sinks of global CH4.

Crucial roles of soil inherent Fe-bearing minerals in enhanced Cr(VI) reduction by biochar: The electronegativity neutralization and electron transfer mediation

Biochar has been used for soil Cr(VI) remediation in the last decade due to its enriched redox functional groups and good electrochemical properties. However, the role of soil inherent Fe-bearing minerals during the reduction of Cr(VI) has been largely overlooked. In this study, biochar with different electron-donating capacities (EDCs) was produced at 400 °C (BC400) and 700 °C (BC700), and their performance for Cr(VI) reduction in soils with varied properties (e.g., Fe content) was investigated. The addition of BC400 caused around 14.2-36.0 mg g-1 Cr(VI) reduction after two weeks of incubation in red soil, paddy soil, loess soil, and fluvo-aquic soil, while a less Cr(VI) was reduced by BC700 (2.57-16.7 mg g-1) with smaller EDCs. The Cr(VI) reduction by both biochars in different soils was closely related to Fe content (R2 = 0.93-0.98), so red soil with the richest Fe (14.8% > 1.79-3.49%) showed the best reduction capability, and the removal of soil free Fe oxides (e.g., hematite) resulted in 71.9% decrease of Cr(VI) reduction by BC400. On one hand, Fe-bearing minerals could increase the soil acidity, neutralize the surface negative charge of biochar, enhance the contact between Cr(VI) and biochar, and thus facilitate the direct Cr(VI) reduction by biochar in soils. On the other hand, Fe-bearing minerals could also facilitate the indirect Cr(VI) reduction by mediating the electron from biochar to Cr(VI) with the cyclic transformation of Fe(II)/Fe(III). This study demonstrates the key role of soil Fe-bearing minerals in Cr(VI) reduction by biochar, which advances our understanding on the biochar-based remediation mechanism of Cr(VI)-contaminated soils.

Unveiling the effect of different dissolved organic matter (DOM) on catalytic dechlorination of nFe/Ni particles: Corrosion and passivation effect

Dissolved organic matter (DOM), which is ubiquitously distributed in groundwater, has a crucial role in the fate and reactivity of iron materials. However, there is a lack of direct evidence on how different DOMs interact with nFe/Ni in promoting or inhibiting the dechlorination efficiency of chlorinated aromatic contaminants. By comparing humic acid (HA), fulvic acid (FA), and biochar-derived dissolved organic matter (BDOM) at different pyrolysis temperatures, we first demonstrated that the dechlorination effect of nFe/Ni on 2,4-dichlorophenol (2,4-DCP) depended on the nature of DOMs and their adsorption on nFe/Ni. HA showed an enhancing effect on the dechlorination of 2,4-DCP by nFe/Ni, while the inhibition effect of other DOMs resulted in the following dechlorination order: BDOM300 ≈FA>BDOM700 ≈BDOM500. The C2 component with higher aromaticity and molecular weight promoted the corrosion of nFe/Ni and the production of reactive hydrogen atoms (H*). The effects of different DOMs on nFe/Ni include that (1) HA accelerates the corrosion and H* production of nFe/Ni, (2) FA and BDOM300 enhance the corrosion but inhibit H* production, and (3) Both nFe/Ni corrosion and H* formation are suppressed by BDOM500/BDOM700. Therefore, this study will provide a reference for understanding the nature of DOM-nFe/Ni interaction and improving the catalytic activity of nFe/Ni when different DOMs coexist in practical applications.

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.

Effect of pyrolysis temperature on the binding characteristics of DOM derived from livestock manure biochar with Cu(II)

Biochar-derived dissolved organic matter (BDOM) has the potential to influence the environmental application of biochar and the behavior of heavy metals. In this study, the binding properties of BDOM derived from livestock manure biochar at different pyrolysis temperatures with Cu(II) were investigated based on a multi-analytical approach. The results showed that the DOC concentration, aromatics, and humification degree of BDOM were higher in the process of low pyrolysis of biochar. The pyrolysis temperature changed the composition of BDOM functional groups, which affected the binding mechanism of BDOM-Cu(II). Briefly, humic-like and protein-like substances dominated BDOM-Cu(II) binding at low and high pyrolysis temperatures, respectively. The higher binding capacity for Cu(II) was exhibited by BDOM derived from the lower pyrolysis temperature, due to the carboxyl as the main binding site in humic acid had high content and binding ability at low-temperature. The amide in proteins only participated in the BDOM-Cu(II) binding at high pyrolysis temperature, and polysaccharides also played an important role in the binding process. Moreover, the biochar underwent the secondary reaction at certain high temperatures, which led to condensation reaction of the aromatic structure and the conversion of large molecules into small molecules, affecting the BDOM-Cu(II) binding sites.

Differential photodegradation processes of adsorbed polychlorinated biphenyls on biochar colloids with various pyrolysis temperatures

Despite the crucial role of photodegradation in the environmental transformation of organic pollutants, the photodegradation process of organic pollutants irreversibly absorbed on biochar colloids (BCCs) remains poorly understood. This study investigated the photodegradation processes and mechanisms of 2,4,4'-trichlorobiphenyl (PCB28) adsorbed on BCCs released from bulk biochars derived from bamboo chips at pyrolysis temperatures of 300, 500, and 700 °C. Results show that BCCs-adsorbed PCB28 could be degraded under simulated solar illumination (95-105 mW·cm-2) but at decreased photodegradation rates compared to the dissolved PCB28. The inhibition effect of BCCs on the PCB28 photodegradation increased with increasing pyrolysis temperature. After adsorptive binding to BCCs, the half-life of PCB28 (0.1 mg/L) was prolonged from 2.65 h for the dissolved PCB28 alone in deionized water to 7.48, 40.67, and 81.82 h in the presence of BCC300, BCC500, and BCC700 (5.0 mg/L), respectively. Mechanistically, the photodegradation of adsorbed pollutants was regulated by the photogenerated free radicals and surface functional groups of the low-temperature BCCs, as well as the defects and direct electron transfer capabilities of the high-temperature BCCs; PCB28 adsorbed on the low-temperature BCCs accepted electrons from persistent free radicals under light illumination, which led to PCB28 dechlorination, followed by ring-opening oxidation through hydroxyl radical attack, ultimately resulting in progressive mineralization; singlet oxygen caused preferential ring opening of adsorbed PCB28 on the high-temperature BCCs, preceding dechlorination. The photodegradation of BCCs-adsorbed PCB28 remained significant though more or less being inhibited under the effects of water pH, ionic strength, dissolved organic matters (humic acid and fulvic acid), and in natural water samples. These findings contribute to a better understanding of the structural properties of BCCs that impact phototransformation processes of adsorbed pollutants and facilitate an accurate assessment of the environmental risk associated with biochar application.

Equilibrium, kinetic, and thermodynamic studies on the biosorption of lead by human metallothionein gene-cloned bacteria as a novel biosorbent

Heavy metals are the main pollutants in water and are an important global problem that threatens human health and ecosystems. In recent years, there has been an increasing interest in the use of genetically modified bacteria as an eco-friendly method to solve heavy metal pollution problems. The goal of this study was to generate genetically modified Escherichia coli expressing human metallothioneins (hMT2A and hMT3) and to determine their tolerance, bioaccumulation, and biosorption capacity to lead (Pb2+ ). Recombinant MT2A and MT3 strains expressing MT were successfully generated. Minimum inhibition concentrations (MIC) of Pb for MT2A and MT3 were found to be 1750 and 2000 mg L-1 , respectively. Pb2+ resistance and bioaccumulation capacity of MT3 were higher than MT2A. Therefore, only MT3 biosorbent was used in Pb2+ biosorption, and its efficiency was examined by performing experiments in a batch system. Pb2+ biosorption by MT3 was evaluated in terms of isotherms, kinetics, and thermodynamics. The results showed that Pb biosorption fits to the Langmuir isotherm model and the pseudo-first-order kinetic model, and the reaction is exothermic. The maximum Pb2+ capacity of the biosorbent was 50 mg Pb2+ g-1 . The potential of MT3 in Pb biosorption was characterized by Fourier-transform infrared spectroscopy (FT-IR), scanning electron microscopy (SEM), and scanning transmission electron microscopy (STEM) analyses. The desorption study showed that the sorbent had up to 74% recovery and could be effectively used four times. These findings imply that this biosorbent can be applied as a promising, precise, and effective means of removing Pb2+ from contaminated waters. PRACTITIONER POINTS: In this study, the tolerance levels, bioaccumulation, and biosorption capacities of Pb in aqueous solutions were determined for the first time in recombinant MT2A and MT3 strains in which human MT2A and MT3 genes were cloned. The biosorbent of MT3, which was determined to be more effective in Pb bioaccumulation, was synthesized and used in Pb biosorption. The Pb biosorption mechanism of MT3 biosorbent was identified using isotherm modeling, kinetic modeling, and thermodynamic studies. The maximum Pb removal percentage capacity of the biosorbent was 90%, whereas the maximum biosorption capacity was up to 50 mg Pb2+ g-1 . These results indicated that MT3 biosorbent has a higher Pb biosorption capacity than existing recombinant biosorbents. MT3 biosorbent can be used as a promising and effective biosorbent for removing Pb from wastewater.

The dual effect of disodium anthraquinone-2,6-disulfonate (AQDS) on the Cr(VI) removal by biochar: The enhanced electron transfer and the inhibited adsorption

Due to large specific surface area, abundant surface functional groups, and stable chemical structure, biochar has been widely used in many environmental fields, including the remediation of Cr pollution. Alternatively, electrochemically active organic matter (e-OM), which is prevalent in both natural environments and industrial wastewater, exerts an inevitable influence on the mechanisms underlying Cr(VI) removal by biochar. The synergistic interplay between biochar and e-OM in the context of Cr(VI) remediation remains to be fully elucidated. In this study, disodium anthraquinone-2,6-disulfonate (AQDS) was used as a model for e-OM, characterized by its quinone group's ability to either donate or accept electrons. We found that AQDS sped up the Cr(VI) removal process, but the enhancement effect decreased with the increase in pyrolysis temperature. With the addition of AQDS, the removal amount of Cr(VI) by BC300 and BC600 increased by 160.0% and 49.5%, respectively. AQDS could release more electrons trapped in the lower temperature biochar samples (BC300 and BC600) for Cr(VI) reduction. However, AQDS inhibited the Cr(VI) removal by BC900 due to the adsorption of AQDS on biochar surface. In the presence of the small molecule carbon source lactate, more AQDS was adsorbed onto the biochar surface. This led to an inhibition of the electron transfer between biochar and Cr(VI), resulting in an inhibitory effect. This study has elucidated the electron transfer mechanism involved in the removal of Cr(VI) by biochar, particularly in conjunction with e-OM. Furthermore, it would augment the efficacy of biochar in applications targeting the removal of heavy metals.

Assess the formation of disinfection by-products from pyrogenic dissolved organic matter (pyDOM): impact of wildfire on the water quality of forest watershed

Wildfires can release pyrogenic dissolved organic matter (pyDOM) into the forest watershed, which may pose challenges for water treatment operations downstream due to the formation of disinfection by-products (DBPs). In this study, we systematically assessed the physio-chemical properties of pyDOM (e.g., electron-donating and -accepting capacities; EDC and EAC) and their contributions to DBP formation under different disinfection scenarios using (1) ten lab samples produced from various feedstocks and pyrolysis temperatures, and (2) pre- and post-fire field samples with different burning severities. A comprehensive suite of DBPs-four trihalomethanes (THMs), nine haloacetic acids (HAAs), and seven N-nitrosamines-were included. The formations of THM and HAA showed an up to 5.7- and 8.9-fold decrease as the pyrolysis temperature increased, while the formation of N-nitrosamines exhibited an up to 6.6-fold increase for the laboratory-derived pyDOM. These results were supported by field pyDOM samples, where the post-fire samples consistently showed a higher level of N-nitrosamine formation (i.e., up to 5.3-fold), but lower THMs and HAAs compared to the pre-fire samples. To mimic environmental reducing conditions, two field samples were further reduced electrochemically and compared with Suwannee River natural organic matter (SRNOM) to evaluate their DBP formation. We found increased DBP formation in pyDOM samples following electrochemical reduction but not for SRNOM, which showed increased N-nitrosamines but decreased THMs and HAAs post-electrochemical reduction. Furthermore, this study reported for the first time the formation of two previously overlooked N-nitrosamines (i.e., nitrosodiethylamine (NDEA), N-nitrosodi-n-propylamine (NDPA)) in both laboratory and field pyDOM samples, raising concerns for drinking water safety given their higher toxicity as compared to the regulated counterparts. Results from this study provide new insights for DBP mitigation during post-fire recovery, which are particularly relevant to communities that rely on forest watersheds as their drinking water sources.

Characteristics of dissolved organic matter composition in biochar: Effects of feedstocks and pyrolysis temperatures

Biochar has widely used in soil pollution remediation due to its advantages of high efficiency and environmental sustainability. Dissolved organic matter (DOM) released by biochar plays a non-negligible role in the migration and transformation of pollutants in environment, and its composition was regarded as main impact factor. In this study, 28 biochar were investigated to detect the effect of pyrolysis temperature and feedstock on DOM content and components. Results showed that the content of DOM released from biochar at low pyrolysis temperatures (300-400 ℃) was higher than that from high pyrolysis temperatures (500-600 ℃). In addition, the specific UV-Visible absorbance at 254 nm (SUVA254) results expressed that DOM from peanut shell biochar (PSBC), rice husk biochar (RHBC) and bamboo biochar (BBC) had higher humification at high temperatures. Moreover, one fulvic acid-like (C2) and two humic acid-like (C1, C3) substances were main fluorescent components of biochar-derived DOM identified by parallel factor analysis based on excitation emission matrices fluorescence spectroscopies (EEM-PARAFAC). With the increase of pyrolysis temperature, humic acid substances content gradually decreased. The correlation analysis results revealed that pyrolysis temperatures and O/C, H/C, DOM content, the biological index (BIX), humification index (HIX), C1% and C3% was negatively correlated (p < 0.001). Thus, the pyrolysis temperatures take important roles in composition of DOM released from biochar, and this research would provide a reference for the application of biochar in the environment.

Electron exchange capacity of pyrogenic dissolved organic matter (pyDOM): complementarity of square-wave voltammetry in DMSO and mediated chronoamperometry in water

Pyrogenic dissolved organic matter (pyDOM) is derived from black carbon, which is important in the global carbon cycle and other biogeochemical redox processes. The electron-exchange capacity (EEC) of pyDOM has been characterized in water using mediated chronoamperometry (MCA), which gives precise results under specific operational conditions, but the broader significance of these EECs is less clear. In this study, we described a novel but complementary electrochemical approach to quantify EECs of pyDOM without mediation using square-wave voltammetry (SWV) in dimethyl sulfoxide (DMSO). Using both the SWV and MCA methods, we determined EECs for 10 pyDOMs, 6 natural organic matter (NOM) samples, and 2 model quinones. The two methods gave similar EECs for model quinones, but SWV gave larger EECs than MCA for NOM and pyDOM (by several-fold and 1-2 orders of magnitude, respectively). The differences in the EECs obtained by SWV and MCA likely are due to multiple factors, including the potential range of electrons sampled, kinetics of electron transfer from (macro)molecular structures, and coupling of electron and proton transfer steps. Comparison of the results obtained by these two methods should provide new insights into important environmental processes such as carbon-cycling, wildfire recovery, and contaminant mitigation using carbon-based amendments.

Conductive black carbon promoted biotransformation of undissolved 2, 2'-dinitrobiphenyl by mediating electron transfer

With low bioaccessbility, persistence of the undissolved organic pollutants in soil and sediments poses threat to health of the resident. Although ubiquitous black carbon catalyzes a wide range of biogeochemical reactions in nature, its role in biotransformation of the compounds in non-aqueous phase like 2, 2'-nitrobiphenyl remains unclear. Reduction rate constants of 2, 2'-dinitrobiphenyl by Shewanella oneidensis MR-1 increased from 0.0044 h^-1 by 7-fold to 0.035 h^-1 in the presence of black carbons produced at pyrolysis temperature of 250-900 °C. Accordingly, electrical conductivity of black carbon was enhanced from 0 to 5.56 S∙cm^-1. The reactivity of black carbon for catalyzing the biotransformation positively correlated with its electrical conductivity (R² > 0.89), which was strongly associated with conductive graphitic clusters in it. The surface oxygenated groups in black carbon were likely not involved in the bioreduction. This work attaches importance to role of the ubiquitous black carbon in natural biotransformation of the undissolved pollutants, and elucidates new mechanism for the biotransformation.

Biochar-derived dissolved black carbon accelerates ferrihydrite microbial transformation and subsequent imidacloprid degradation

The effects of an electron shuttle (dissolved black carbon (DBC) derived from biochar) on the microbial reduction of ferrihydrite and subsequent imidacloprid (IMI) degradation were studied. The results showed that DBC addition enhanced the microbial reduction of Fe(III) in ferrihydrite and increased the quantity of Fe(II) released into the liquid phase. The electron transfer capacity of DBC was significantly influenced by the content of redox-active oxygen-containing functional groups (e.g., quinone, hydroquinone, and polyphenol groups), which was dependent on the pyrolysis temperature. The electrochemical characteristics of DBC resulted in enhanced electron transfer, which promoted Fe(III) reduction and mediated the microbial transformation of ferrihydrite. The microbial transformation of ferrihydrite resulted in the formation of secondary minerals such as siderite and vivianite. The IMI degradation efficiency was related to the Fe(III) reduction rate and the pyrolysis temperature used in DBC production, and the degradation pathways were nitrate reduction and imino hydrolysis induced by the Fe(II) generated from the reduction of Fe(III) in ferrihydrite. The results obtained in this study provide new data for understanding the multifunctional roles of biochar-derived DBC in the redox and transformation processes of iron minerals induced by iron-reducing bacteria, the related biogeochemical cycles of iron and the fate of pollutants.

Compound-specific stable isotope analysis for characterization of the transformation of γ-HCH induced by biochar

The role of biochar as the redox catalyst in the removal of reductive pollutants from soil and water system has been extensively studied recently, but there is still a lack of qualitative description of its specific mechanisms in redox processes. In this study, the mechanism of biochar in the transformation process of γ-HCH under anoxic condition was revealed by the compound-specific isotope analysis. The concentration and carbon isotopic composition (δ¹³C) of γ-HCH were detected in the treatments with different initial concentrations of γ-HCH and biochar materials with different redox properties and varied doses. The surface functional groups and electrochemical properties of biochar before and after the reaction were also characterized. The addition amount of biochar could affect the reduction of γ-HCH concentration, which were 59.1%, 34.6% and 22.4% in treatments with the addition of 5%, 1% and 0.2% biochar, respectively. Meanwhile, the δ¹³C value of γ-HCH also increased from -26.6 ± 0.2‰ to -23.8 ± 0.2‰ with the addition amount of biochar, especially in the treatment with 5% biochar. As evidenced by X-ray diffraction analysis and electrochemical analysis, biochar promoted the adsorption and transformation of γ-HCH simultaneously, and the oxygen-containing functional groups on the surface of biochar played an important role in the redox process. The isotopic fractionation value (ε_C) of γ-HCH transformation by biochar was first reported as -3.4 ± 0.4‰. The results will enable the quantitative description of the transformation degree of organic pollutants induced by biochar, and provide a new approach for evaluating the in-situ remediation effects of biochar in a complex environment.

Role of biochar-based free radicals in immobilization and speciation of metals in the contaminated soil-plant environment

The structure of biochar produced at various pyrolysis temperatures influences metal geochemical behavior. Here, the impact of wheat straw-derived biochar (300, 500, and 700 °C) on the immobilization and transformation of metals in the contaminated soil-plant system was assessed. The findings of the sequential extraction revealed that biochar additives had a substantial influence on the speciation of Cr, Ni, Pb, and Zn in the contaminated soil. The lowest F1 (exchangeable and soluble fraction) + F2 (carbonate fraction) accounted for Cr (44%) in WB-300, Ni (43.87%) in WB-500, Pb (43.79%), and Zn (49.78%) in WB-700 with applied amendments of their total amounts. The characterization results indicated that high pyrolysis temperatures (300-700 °C) increased the carbon-containing groups with the potential to adsorb metals from the soil-plant environment. The bioconcentration and translocation factors (BCF and TF) were less than 1, indicating that metal concentration was restricted to maize roots and translocation to shoots. Reactive oxygen species (ROS) intracellularly influence metal interactions with plants. Electron paramagnetic resonance (EPR) was performed to determine hydroxyl radical generation (•OH) in plant segments to assess the dominance of free radicals (FRs). Consequently, the formation of •OH significantly depends on the pyrolysis temperature and the interaction with a contaminated soil-plant environment. Thus, metal transformation can be effectively decreased in the soil-plant environment by applying WB amendments.

Characterization of dissolved black carbon and its binding behaviors to ceftazidime and diclofenac pharmaceuticals: Employing the molecular weight fractionation

As the ubiquitous component of the aquatic environment, dissolved organic matter (DOM) readily bind with residual pharmaceutical contaminants (PCs) and influence their environmental behaviors. However, the binding mechanisms between dissolved black carbon (DBC), a vital part of the natural DOM pool, and PCs were poorly researched. In this study, the bulk DBC was divided into four fractions in molecular weight (MW) via an ultrafiltration system, and the properties of DBC and their binding interaction with two kinds of typical PCs (ceftazidime (CAZ) and diclofenac (DCF)) were explored concretely. The results showed that low MW component was the main contributor to bulk DBC, and the aromaticity increased with the increase of MW. The categories of chemical structures and fluorescent substances in different MW DBC were similar. Multispectral techniques showed that the oxygen-enriched compounds in DBC had the higher affinity to CAZ/DCF. The -NH-, -COOH, -NH₂ groups in CAZ molecules appeared to form the hydrogen bond with DBC. Fluorescence quenching experiments were analyzed, and the binding mechanisms were specifically expounded from the thermodynamic perspective. The fluorophore of fulvic acid-like compounds (FA) were quenched by both static and dynamic quenching mechanisms, while only static quenching occurred for humic acid-like compounds (HA). For bulk DBC, the hydrogen bond and van der Waals force were the major forces in the HA-CAZ system, while the hydrophobic force made the primary contribution to the HA-DCF system, which might be ascribed to the higher hydrophobic nature of DCF. Notably, with the increase of HA MW, the main binding mode of HA-CAZ/DCF changed from hydrophobic force to hydrogen bond and van der Waals force gradually, which also directly proved that various noncovalent interactions co-driven the binding processes. Our findings are beneficial to better assess the fate of DBC and PCs and the corresponding complexes in the aquatic environment.

Production of biochar from crop residues and its application for anaerobic digestion

Anaerobic digestion (AD) is a viable and cost-effective method for converting organic waste into usable renewable energy. The efficiency of organic waste digestion, nonetheless, is limited due to inhibition and instability. Accordingly, biochar is an effective method for improving the efficiency of AD by adsorbing inhibitors, promoting biogas generation and methane concentration, maintaining process stability, colonizing microorganisms selectively, and mitigating the inhibition of volatile fatty acids and ammonia. This paper reviews the features of crop waste-derived biochar and its application in AD systems. Four critical roles of biochar in AD systems were identified: maintaining pH stability, promoting hydrolysis, enhancing the direct interspecies electron transfer pathway, and supporting microbial development. This work also highlights that the interaction between biochar dose, amount of organic component in the substrate, and inoculum-to-substrate ratio should be the focus of future research before deploying commercial applications.

Deciphering physicochemical properties and enhanced microbial electron transfer capacity by magnetic biochar

Magnetic biochar is important for improving the electron transfer capacity (ETC) of microorganisms in wastewater treatment. In this study, three magnetic biochar under different pyrolysis temperatures (300, 500 and 700 °C) were prepared by co-precipitation, and their characteristics and impacts on mediating microbial ETC were investigated. Results indicated that magnetic biochar had a higher capacitance and conductivity than pyrolytic biochar, with the largest specific capacitance of 14.7F/g for FCS700 (magnetic biochar prepared at 700 °C). The addition of magnetic biochar could improve the nitrogen removal efficiency of a sludge-biochar system. The electron transfer resistance (R_ct) of magnetic biochar was lower than pyrolytic biochar by 25.5 % (300 °C), 19.7 % (500 °C), and 11.6 % (700 °C), respectively. The structure of the microbial community in the sludge-biochar system differed significantly. Spearman correlation suggested that the electrochemical properties of biochar were an important factor affecting the structure of the microbial community.

Efficient removal of aqueous Cr(VI) with ferrous sulfide/N-doped biochar composites: Facile, in-situ preparation and Cr(VI) uptake performance and mechanism

FeS nanoparticles loaded on nitrogen-doped biochar (FeS/BNC) were fabricated by pyrolyzing coffee husks pretreated with Mohr's salt. The nitrogen doping and FeS loading of biochar are simultaneously achieved in one-pot pyrolysis. The elemental analysis, SEM, TEM, XRD, XPS, Raman, FTIR and N₂ adsorption-desorption technologies were used to characterize the composition and structure of FeS/NBC. The appraisement for removing aqueous Cr(VI) testified that FeS/NBC offered a synergistic scavenging effect of Cr(VI) by FeS and NBC. The effect of crucial experimental conditions (FeS/NBC dosage, foreign ions, initial pH and concentration of Cr(VI) solution) were investigated. The Cr(VI) removal capacity was as high as 211.3 ± 26 mg g^-1 under the optimized condition. The practicability of FeS/NBC was examined by using simulated actual samples from tap water and lake water. The mechanism examination showed that surface adsorption/reduction and solution reduction were implicated in the removal of Cr(VI). The current work introduces a novel FeS/NBC composite prepared by an in situ pyrolysis method with excellent potential for chromium pollution remediation.

Spectral characteristics of dissolved organic carbon (DOC) derived from biomass pyrolysis: Biochar-derived DOC versus smoke-derived DOC, and their differences from natural DOC

Biochar-derived dissolved organic carbon (BDOC) and smoke-derived dissolved organic carbon (SDOC) are two different biomass-pyrogenic DOCs. They inevitably enter soil and water, then potentially pose different impacts on the chemistry of these media. This study systemically investigated the emissions and spectral characteristics of BDOC and SDOC as well as their differences from natural DOC. The results showed that the emission of SDOC was 1-3 orders of magnitude greater than that of BDOC after biomass pyrolysis. UV-vis spectra indicated that BDOC had higher aromaticity and molecular weight as well as lower polarity than SDOC. The two-dimensional correlation infrared spectrum (2D-PCIS) matrix indicated that BDOC contained more chemical groups with stronger temperature-dependence than SDOC. Fluorescence EEM-PARAFAC analysis showed that BDOC was dominated by macromolecular humic-like substances, while SDOC was primarily composed of small molecules of aromatic protein/polyphenols-like compounds. The fluorescence indicators including humification index (HIX) (0.08-0.76) and biological index (BIX) (1.18-1.72) of SDOC were significantly different from those of BDOC (HIX: 1.64-12.68, and BIX: 0.17-1.62). The higher BIX and more small molecules of aromatic protein/polyphenols-like compounds indicated SDOC had potentially higher bioavailability and turnover rate in the environment than BDOC. Furthermore, the UV-vis spectral indicator (S_275-295) and fluorescence spectral indicators (HIX, and BIX) of BDOC were equivalent to those of natural DOC, whereas these indicators of SDOC were significantly different from those of natural DOC. This study demonstrated that BDOC and SDOC had significantly different components and properties and they might present different environmental behaviors and effects.

Biochar raw material selection and application in the food chain: A review

As one of the largest carbon emitters, China promises to achieve carbon emissions neutrality by 2060. Various industries are developing businesses to reduce carbon emissions. As an important greenhouse gas emissions scenario, the reduction of carbon emissions in the food chain can be achieved by preparing the wastes into biochar. The food chain, as one of the sources of biochar, consists of production, processing and consumption, in which many wastes can be transferred into biochar. However, few studies use the food chain as the system to sort out the raw materials of biochar. A systematic review of the food chain application in serving as raw materials for biochar is helpful for further application of such technique, providing supportive information for the development of biochar preparation and wastes treating. In addition, there are many pollution sources in the food production process, such as agricultural contaminated soil and wastewater from livestock and aquatic, that can be treated on-site to achieve the goal of treating wastes with wastes within the food chain. This study focuses on waste resource utilization and pollution remediation in the food chain, summarizing the sources of biochar in the food chain and analyzing the feasibility of using waste in food chain to treat contaminated sites in the food chain and discussing the impacts of the greenhouse gas emissions. This review provides a reference for the resource utilization of waste and pollution reduction in the food chain.

Redox properties of nano-sized biochar derived from wheat straw biochar

Nano-sized biochar (NBC) has received increasing attention due to its unique physicochemical characteristics and environmental behaviour, but an understanding of its redox properties is limited. Herein, the redox properties of NBC derived from wheat straw were investigated at two pyrolysis temperatures (400 and 700 °C). These NBC materials were prepared from bulk-biochar by grinding, ultrasonication and separation treatments. The resulting NBC had average particle sizes of 78.8 ± 1.9 and 122.0 ± 2.1 nm after 400 and 700 °C treatments, respectively. The physicochemical measurements demonstrated that both the NBC prepared at 400 °C (NBC-400) and the NBC prepared at 700 °C (NBC-700) were enriched in carboxyl and phenolic oxygen-content groups. Electrochemical analyses showed that both NBC-400 and NBC-700 were redox active and had an electron transfer capacity (ETC) of 196.57 μmol⁻¹ g_C⁻¹ and 363.47 μmol⁻¹ g_C⁻¹, respectively. On the basis of its redox activity of NBC, the NBC was capable of mediating the reduction of iron and manganese minerals as well as the degradation of methyl orange (MO) by sulfide. The NBC-700 could stimulate these reactions better than the NBC-400 due to its higher redox activity. Meanwhile, the NBC was more active in stimulating these reactions than bulk-biochar. Our results highlight the importance of size in evaluating the redox reactivity of biochar and related environmental processes and improve our understanding of the redox properties of biochar.

NBC exhibit significant efficiency in mediating MO or minerals reduction by accelerating electron transfer. NBC-700 has higher SSA, ETC and stronger redox property than NBC-400.

Enhanced microbial reduction of aqueous hexavalent chromium by Shewanella oneidensis MR-1 with biochar as electron shuttle

Biochar, carbonaceous material produced from biomass pyrolysis, has been demonstrated to have electron transfer property (associated with redox active groups and multi condensed aromatic moiety), and to be also involved in biogeochemical redox reactions. In this study, the enhanced removal of Cr(VI) by Shewanella oneidensis MR-1(MR-1) in the presence of biochars with different pyrolysis temperatures (300 to 800 °C) was investigated to understand how biochar interacts with Cr(VI) reducing bacteria under anaerobic condition. The promotion effects of biochar (as high as 1.07~1.47 fold) were discovered in this process, of which the synergistic effect of BMBC700(ball milled biochar) and BMBC800 with MR-1 was noticeable, in contrast, the synergistic effect of BMBCs (300-600 °C) with MR-1 was not recognized. The more enhanced removal effect was observed with the increase of BMBC dosage for BMBC700+MR-1 group. The conductivity and conjugated O-containing functional groups of BMBC700 particles themselves has been proposed to become a dominant factor for the synergistic action with this strain. And, the smallest negative Zeta potential of BMBC700 and BMBC800 is thought to favor decreasing the distance from microbe than other BMBCs. The results are expected to provide some technical considerations and scientific insight for the optimization of bioreduction by useful microbes combining with biochar composites to be newly developed.

Bioreduction and bioremoval of hexavalent chromium by genetically engineered strains (Escherichia coli MT2A and Escherichia coli MT3)

The number of studies on the removal of hazardous metals from water using genetic engineering technologies is growing. A high rate of metal ion removal from the environment is ensured, particularly through the expression of cysteine and thiol-rich proteins such as metallothioneins in bacterial cells. In this study, we used recombinant strains created by cloning the human metallothioneins MT2A and MT3 into Escherichia coli Jm109 to assess the removal and reduction of hexavalent chromium (Cr(VI)) from aqueous solutions. MT2A was the most effective strain in both Cr(VI) removal (89% in 25 mg/L Cr(VI)) and Cr(VI) reduction (76% in 25 mg/L Cr(VI)). The amount of Cr adsorbed per dry cell by the MT2A strain was 22 mg/g. The biosorption of total Cr was consistent with the Langmuir isotherm model. Scanning electron microscope (SEM) images revealed that the morphological structures of Cr(VI)-treated cells were significantly damaged when compared to control cells. Scanning transmission electron microscope (STEM) images showed black spots in the cytoplasm of cells treated with Cr(VI). Shifts in the Fourier transform infrared spectroscopy analysis (FTIR) spectra of the cells treated with Cr(VI) showed that the groups interacting with Cr were hydroxyl, amine, amide I, amide II, phosphoryl and carbonyl. When all of the experimental data was combined, it was determined that both MT2A and MT3 were effective in removing Cr(VI) from aqueous solutions, but MT2A was more effective, indicating that MT2A may be employed as a biotechnological tool.

Biochar for agronomy, animal farming, anaerobic digestion, composting, water treatment, soil remediation, construction, energy storage, and carbon sequestration: a review

In the context of climate change and the circular economy, biochar has recently found many applications in various sectors as a versatile and recycled material. Here, we review application of biochar-based for carbon sink, covering agronomy, animal farming, anaerobic digestion, composting, environmental remediation, construction, and energy storage. The ultimate storage reservoirs for biochar are soils, civil infrastructure, and landfills. Biochar-based fertilisers, which combine traditional fertilisers with biochar as a nutrient carrier, are promising in agronomy. The use of biochar as a feed additive for animals shows benefits in terms of animal growth, gut microbiota, reduced enteric methane production, egg yield, and endo-toxicant mitigation. Biochar enhances anaerobic digestion operations, primarily for biogas generation and upgrading, performance and sustainability, and the mitigation of inhibitory impurities. In composts, biochar controls the release of greenhouse gases and enhances microbial activity. Co-composted biochar improves soil properties and enhances crop productivity. Pristine and engineered biochar can also be employed for water and soil remediation to remove pollutants. In construction, biochar can be added to cement or asphalt, thus conferring structural and functional advantages. Incorporating biochar in biocomposites improves insulation, electromagnetic radiation protection and moisture control. Finally, synthesising biochar-based materials for energy storage applications requires additional functionalisation.

A review of biochar in anaerobic digestion to improve biogas production: Performances, mechanisms and economic assessments

Anaerobic digestion (AD) technology still faces some challenges including low methane productivity, instable operation efficiency and undesired refractory substances degradation. Biochar has recently been recognized as a promising alternative addition in AD process to enhance methane production. Based on VOSviewer analysis, this review presents a comprehensive summarizing of the applications, performances, and economies of biochar strategies in AD system. Firstly, typical production processes of biochar and its main characteristics including adsorption and immobilization ability, buffering ability and electron transfer ability were evaluated. Then, the applications of biochar in AD and its improving effects on biogas production/purification were summarized. Accordingly, the corresponding mechanisms of biochar addition in AD for digestion efficiency improvement were elucidated. Finally, the economic and environmental feasibilities of application biochar in AD, as well as prospective future studies were summarized. Through an overview of biochar in AD system, this paper aims to promote its widely practical applications.

Bioengineered biochar as smart candidate for resource recovery toward circular bio-economy: a review

Biochar's ability to mediate and facilitate microbial contamination degradation, as well as its carbon-sequestration potential, has sparked interest in recent years. The scope, possible advantages (economic and environmental), and future views are all evaluated in this review. We go over the many designed processes that are taking place and show why it is critical to look into biochar production for resource recovery and the role of bioengineered biochar in waste recycling. We concentrate on current breakthroughs in the fields of engineered biochar application techniques to systematically and sustainable technology. As a result, this paper describes the use of biomass for biochar production using various methods, as well as its use as an effective inclusion material to increase performance. The impact of biochar amendments on microbial colonisation, direct interspecies electron transfer, organic load minimization, and buffering maintenance is explored in detail. The majority of organic and inorganic (heavy metals) contaminants in the environment today are caused by human activities, such as mining and the use of chemical fertilizers and pesticides, which can be treated sustainably by using engineered biochar to promote the establishment of a sustainable engineered process by inducing the circular bioeconomy.

Photochemical Behavior of Microbial Extracellular Polymeric Substances in the Aquatic Environment

Microbially derived extracellular polymeric substances (EPSs) occupy a large portion of dissolved organic matter (DOM) in surface waters, but the understanding of the photochemical behaviors of EPS is still very limited. In this study, the photochemical characteristics of EPS from different microbial sources (Shewanella oneidensis, Escherichia coli, and sewage sludge flocs) were investigated in terms of the production of reactive species (RS), such as triplet intermediates (³EPS*), hydroxyl radicals (^•OH), and singlet oxygen (¹O₂). The steady-state concentrations of ^•OH, ³EPS*, and ¹O₂ varied in the ranges of 2.55-8.73 × 10^-17, 3.01-4.56 × 10^-15, and 2.08-2.66 × 10^-13 M, respectively, which were within the range reported for DOM from other sources. The steady-state concentrations of RS varied among different EPS isolates due to the diversity of their composition. A strong photochemical degradation of the protein-like components in EPS isolates was identified by excitation emission matrix fluorescence with parallel factor analysis, but relatively, humic-like components remained stable. Fourier-transform ion cyclotron resonance mass spectrometry further revealed that the aliphatic portion of EPS was resistant to irradiation, while other portions with lower H/C ratios and higher O/C ratios were more susceptible to photolysis, leading to the phototransformation of EPS to higher saturation and lower aromaticity. With the phototransformation of EPS, the RS derived from EPS could effectively promote the degradation of antibiotic tetracycline. The findings of this study provide new insights into the photoinduced self-evolution of EPS and the interrelated photochemical fate of contaminants in the aquatic environment.

Redox Properties of Pyrogenic Dissolved Organic Matter (pyDOM) from Biomass-Derived Chars

Chars are ubiquitous in the environment and release significant amounts of redox-active pyrogenic dissolved organic matter (pyDOM). Yet, the redox properties of pyDOM remain poorly characterized. This work provides a systematic assessment of the quantity and redox properties of pyDOM released at circumneutral pH from a total of 14 chars pyrolyzed from wood and grass feedstocks from 200 to 700 °C. The amount of released pyDOM decreased with increasing pyrolysis temperature of chars, reflecting the increasing degree of condensation and decreasing char polarity. Using flow-injection analysis coupled to electrochemical detection, we demonstrated that electron-donating capacities (EDCpyDOM; up to 6.5 mmole-·gC-1) were higher than electron-accepting capacities (EACpyDOM; up to 1.2 mmole-·gC-1) for all pyDOM specimens. The optical properties and low metal contents of the pyDOM implicate phenols and quinones as the major redox-active moieties. Oxidation of a selected pyDOM by the oxidative enzyme laccase resulted in a 1.57 mmole-·gC-1 decrease in EDCpyDOM and a 0.25 mmole-·gC-1 increase in EACpyDOM, demonstrating a largely irreversible oxidation of presumably phenolic moieties. Non-mediated electrochemical reduction of the same pyDOM resulted in a 0.17 mmole-·gC-1 increase in EDCpyDOM and a 0.24 mmole-·gC-1 decrease in EACpyDOM, consistent with the largely reversible reduction of quinone moieties. Our results imply that pyDOM is an important dissolved redox-active phase in the environment and requires consideration in assessing and modeling biogeochemical redox processes and pollutant redox transformations, particularly in char-rich environments.

Performance and enhancement mechanism of corncob guiding chromium (VI) bioreduction

Chromium-contaminated groundwater has drawn extensive attention due to its high toxicity and wide application. Although bioremediation is considered to be an effective approach for Cr(VI) removal, a better method is still urgently needed. In this study, corncob-guided Cr(VI) reduction achieved the highest removal efficiency due to the highest amount of total carbon and available carbon emissions. After verifying the sustainability and operational feasibility of this approach, the broad-spectrum applicability of corncob to guide Cr(VI) bioreduction was further explored under various operating conditions. In addition, it suggested that the carrier effect, nutrient element release and electron shuttle effect were the main mechanisms enhancing the reduction process, with approximate contribution rates of 12.5%, 7.5% and 75%, respectively. Microbiological analysis demonstrated that the addition of solid-phase carbon sources increased the abundance of microbes related to carbon metabolism and promoted the expression of glycolytic metabolic pathway. Furthermore, the addition of corncob led to an elevation of expression level of the electron transport pathway, which is consistent with the function of the electron shuttle. This study provides theoretical and technical support for the practical application of corncob-mediated Cr(VI) bioreduction.

Application of biochars in the remediation of chromium contamination: Fabrication, mechanisms, and interfering species

Chromium (Cr) is one of the most toxic pollutants that has accumulated in terrestrial and aqueous systems, posing serious risks towards living beings on a worldwide scale. The immobilization, removal, and detoxification of active Cr from natural environment can be accomplished using multiple advanced materials. Biochar, a carbonaceous pyrolytic product made from biomass waste, is considered as a promising material for the elimination of Cr contamination. The preparation and properties of biochar as well as its remediation process for Cr ions have been well investigated. However, the distinct correlation of the manufacturing, characteristics, and mechanisms involved in the remediation of Cr contamination by various designed biochars is not summarized. Herein, this review provides information about the production, modification, and characteristics of biochars along with their corresponding effects on Cr stabilization. Biochar could be modified via physical, hybrid, chemical, and biological methods. The remediating mechanisms of Cr contamination using biochars involve adsorption, reduction, electron shuttle, and photocatalysis. Moreover, the coexisting ions and organic pollutants change the pattern of the remediating process of biochar in actual Cr contaminated water and soil. Finally, the present limitations and future perspectives are proposed.

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.

P-nitrophenol degradation by pine-wood derived biochar: The role of redox-active moieties and pore structures

Biochar can both adsorb and degrade p-nitrophenol (PNP); however, the PNP degradation mechanism has not been well investigated. We prepared two biochars at pyrolysis temperatures of 500 °C (B500) and 700 °C (B700). Although B500 showed much stronger free radical signals (which are associated with organic degradation, according to previous studies), the apparent PNP degradation was approximately 3 times higher in the B700 system. The degradation increased significantly after the biochars were washed with water. According to a quantitative analysis of the sorption and degradation and two-compartment first-order kinetics modeling of the apparent removal kinetics, sorption occurred mainly in the initial period, whereas degradation continued throughout the removal process. The PNP degradation rate constant depended mainly on the external surface area at a relatively low concentration (200 mg/L) and was controlled by the microporous surface area at a relatively high concentration (800 mg/L). In addition, the apparent degradation did not depend on the biochar particle size. Therefore, PNP degradation may be related to the three-dimensional structure of the biochar in addition to the exposed external surface. The well-developed pore structure, more accessible surface, and larger electron exchange capacity of B700 may promote electron transfer between the biochar and PNP, and thus accelerate PNP degradation. This study demonstrates that various properties of the biochar may contribute to PNP degradation.

The comparison of dissolved organic matter in hydrochars and biochars from pig manure

Dissolved organic matter (DOM) has an important effect on soil fertility, activity of microorganisms and transport of contaminants. In this study, DOM released by the hydrochar and biochar prepared under various conditions from pig manure, was assessed using a combination of UV-Visible spectroscopy, fluorescence excitation-emission (EEM) spectrophotometry and ¹H-nuclear magnetic resonance (¹H NMR). The dissolved organic carbon (DOC) extracted from the hydrochar and biochar ranged from 3.34-11.96% and 0.38-0.48%, respectively, and the highest DOM was released by HCK0.5 (180 °C and 0.5% KOH). The aliphatic compounds were most common in DOM which mainly included three humic acid-like and one protein-like substance. The hydrochar-DOM had a larger molecular weight and lower aromaticity than biochar-DOM, but the effect of temperature on the DOM characteristics of hydrochar and biochar was opposite. The acidic treatment increased the content of functional groups containing oxygen and nitrogen in hydrochar-DOM, and alkaline treatment increased the content of aliphatic compounds. The results obtained are beneficial to select carbonation process and guide the rational application of hydrochar and biochar from pig manure in soil remediation field.

Autochthonous N-doped carbon nanotube/activated carbon composites derived from industrial paper sludge for chromate (VI) reduction in microbial fuel cells

The performance of microbial electrochemical system for hexavalent chromium (Cr(VI)) contaminant has been a severe challenge remaining active for further development. In this study, we developed a novel biochar material from industrial paper sludge for microbial fuel cell cathode fabrication to reduce aquatic Cr(VI) to non-toxic Cr(III). With additive melamine as nitrogen source and self-containing small portion of Fe as catalyst, the sludge evolved into electroactive biochar (BC-M) rendering a unique N-doped carbon nanotubes/activated carbon (N-CNT/AC) frame after pyrolyzed at 900 °C for 2 h. Electrochemical analysis revealed enhanced electron transference capacity of this composite material, such effectiveness was attributed to the increased surface area and superior electroconductivity of N-doped CNTs. For performance of Cr(VI) reduction, a 55.1% reduction efficiency was achieved in an microbial fuel cell equipped with BC-M cathode while it reduced to about 41.8% when the cathode was replaced by electrode modified with no-melamine-involved biochar. The strategy of biochar upgrading from industrial paper sludge proposed in this work is expected to not only bring technical solution for low-cost CNT materials preparation for Cr(VI) reduction, but also put forward further research on value-added chemical synthesis from waste in various fields of energy and environment.

Bone Char Mediated Dechlorination of Trichloroethylene by Green Rust

Biochars function as electron transfer mediators and thus catalyze redox transformations of environmental pollutants. A previous study has shown that bone char (BC) has high catalytic activity for reduction of chlorinated ethylenes using layered Fe(II)-Fe(III) hydroxide (green rust) as reductant. In the present study, we studied the rate of trichloroethylene (TCE) reduction by green rust in the presence of BCs obtained at pyrolysis temperatures (PTs) from 450 to 1050 °C. The reactivity increased with PT, yielding a maximum pseudo-first-order rate constant (k) of 2.0 h^-1 in the presence of BC pyrolyzed at 950 °C, while no reaction was seen for BC pyrolyzed at 450 °C. TCE sorption, specific surface area, extent of graphitization, carbon content, and aromaticity of the BCs also increased with PT. The electron-accepting capacity (EAC) of BC peaked at PT of 850 °C, and EAC was linearly correlated with the sum of concentrations of quinoid, quaternary N, and pyridine-N-oxide groups measured by XPS. Moreover, no TCE reduction was seen with graphene nanoparticles and graphitized carbon black, which have high degrees of graphitization but low EAC values. Further analyses showed that TCE reduction rates are well correlated with the EAC and the C/H ratio (proxy of electrical conductivity) of the BCs, strongly indicating that both electron-accepting functional groups and electron-conducting domains are crucial for the BC catalytic reactivity. The present study delineates conditions for designing redox-reactive biochars to be used for remediation of sites contaminated with chlorinated solvents.