The conductivity and redox properties of pyrolyzed carbon mediate methanogenesis in paddy soils with ethanol as substrate

Impacts of the quinone-functionalized biochar on anaerobic digestion: Beyond the redox property of biochar

Recent developments in biochar materials have led to renewed interest in biochar modification for environmental applications, however, much uncertainty still exists about the impact of engineered biochar on a given biotechnological process. The redox properties of biochar were considered to be the key property for enhancing the methanogenic process, and the redox activity of biochar was closely related to the type and amount of oxygen-containing functional groups, especially quinone groups. Therefore, anthraquinone-2-sulfonate (AQS) was immobilized on algal biochar (ABC) by surface doping method, and the impacts of the quinone-functionalization process on algal biochar for regulating methane production were investigated in this study. Results showed that the immobilization capacity of AQS on ABC (ABC-AQS) reached 0.289 mmol/g. The acidogenesis rate was improved by 26.3% with the addition of ABC-AQS during anaerobic digestion test. However, methane production was inhibited rather than enhanced by the ABC-AQS, which could be attributed to the strong acid treatment stage involved in the biochar modification process. pH interferences, the generation and/or dissolution of inhibitory substances, and the release of Zn2+ should be the major mechanisms of microbial inhibition by ABC-AQS. The findings of this study give us an important clue that when designing a biochar modification procedure for anaerobic digestion, attentions should be paid to the possible influences of chemical side reactions during biochar modification process on subsequent microbial metabolism, which would be valuable in designing engineered biochar for practical applications.

Outstanding enhancement of caproate production with microwave pyrolyzed highly reductive biochar addition

The integration of biochar into microbial Chain Elongation (CE) proves to be an effective tool of producing high-value bio-based products. This study innovatively applied biochar fabricated under microwave irradiation with carbon fiber cloth assistance into CE system. Results highlighted that microwave biochar achieved maximal CE efficiency yielding 8 g COD/L, with 3-fold increase to the blank group devoid of any biochar. Microwave biochar also obtained the highest substrate utilization rate of 94 %, while conventional biochar group recorded 90 % and the blank group was of 74 %. Mechanistic insights revealed that the reductive surface properties facilitated CE performance, which is relevant to fostering dominant genera of Parabacteroides, Bacteroides, and Macellibacteroides. By metagenomics, microwave biochar up-regulated functional genes and enzymes involved in CE process including ethanol oxidation, the reverse β-oxidation pathway, and the fatty acid biosynthesis pathway. This study effectively facilitated caproate production by utilizing a new microwave biochar preparation strategy.

Geobatteries in environmental biogeochemistry: Electron transfer and utilization

The efficiency of direct electron flow from electron donors to electron acceptors in redox reactions is significantly influenced by the spatial separation of these components. Geobatteries, a class of redox-active substances naturally present in soil-water systems, act as electron reservoirs, reversibly donating, storing, and accepting electrons. This capability allows the temporal and spatial decoupling of redox half-reactions, providing a flexible electron transfer mechanism. In this review, we systematically examine the critical role of geobatteries in influencing electron transfer and utilization in environmental biogeochemical processes. Typical redox-active centers within geobatteries, such as quinone-like moieties, nitrogen- and sulfur-containing groups, and variable-valent metals, possess the potential to repeatedly charge and discharge. Various characterization techniques, ranging from qualitative methods like elemental analysis, imaging, and spectroscopy, to quantitative techniques such as chemical, spectroscopic, and electrochemical methods, have been developed to evaluate this reversible electron transfer capacity. Additionally, current research on the ecological and environmental significance of geobatteries extends beyond natural soil-water systems (e.g., soil carbon cycle) to engineered systems such as water treatment (e.g., nitrogen removal) and waste management (e.g., anaerobic digestion). Despite these advancements, challenges such as the complexity of environmental systems, difficulties in accurately quantifying electron exchange capacity, and scaling-up issues must be addressed to fully unlock their potential. This review underscores both the promise and challenges associated with geobatteries in responding to environmental issues, such as climate change and pollutant transformation.

The role of superoxide anion to Cr(VI) reduction by pine biochar

It has been reported that Cr(VI) can be reduced by biochar because of its redox activity. Considering the anionic form of Cr(VI), we hypothesize that the reduction in aqueous phase is significant. However, the contribution of different reactive oxygen species in the biochar-Cr(VI) reaction system has not been distinguished. Herein, we quantitatively identified Cr(VI) adsorption and reduction in biochar systems. The reduction content of Cr(VI) was 1.5 times higher in untreated conditions than in anaerobic conditions. The disappearance of·O2- under anaerobic conditions illustrated that·O2- may be involved in the reduction of Cr(VI). Quenching of·O2- resulted in a decrease of Cr(VI) reduction by 34%, while 1O2 was negligible, probably due to the stronger electron-donating capacity of·O2-. The degradation of nitrotetrazolium blue chloride (quenching agent of·O2-) confirmed that the reduction process of·O2- mainly occurred in the liquid-phase. Boehm titration and quantification of·O2- further elucidated the significant correlation (P < 0.05) between phenolic groups and the formation of·O2-, which implied that phenolic groups acted as the primary electron donors in generating·O2-. This study highlights the importance of the liquid-phase reduction process in removing Cr(VI), which provides theoretical support for biochar conversion of Cr(VI).

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.

Anodic ammonium oxidation in microbial electrolysis cell: Towards nitrogen removal in low C/N environment

Biological nitrogen removal in low C/N environment is challenging in wastewater treatment for a long time. Autotrophic ammonium oxidation is promising due to the no need of carbon source addition, but alternative electron acceptors other than oxygen has to be widely investigated. Recently, microbial electrolysis cell (MEC), which applies a polarized inert electrode as the electron harvester, has been proved effective to oxidize ammonium with electroactive biofilm. That is, anodic microbes stimulated by exogenous low power can extract electron from ammonium and transfer electron to electrodes. This review aims to consolidate the recent advances in anodic ammonium oxidation in MEC. Various technologies based on different functional microbes and mechanisms of these processes are reviewed. Thereafter, the crucial factors influencing the ammonium oxidation technology are discussed. Challenges and prospects of anodic ammonium oxidation in ammonium-containing wastewater treatment are also proposed to provide valuable insights on the technologic reference and potential value of MEC in ammonium-containing wastewater treatment.

The promoted degradation of biochar-adsorbed 2,4-dichlorophenol in the presence of Fe(III)

Organic pollutant degradation by biochar could be promoted by Fe because of the Fenton-like reaction. However, studies have also confirmed that reactive oxygen species (ROS) play only a limited role in organic pollutant degradation by biochar. Herein, we quantitatively identified 2,4-dichlorophenol (2,4-DCP) adsorption and degradation in Fe-biochar systems and obtained degradation (k₁) and adsorption rate constants (k₂) by two-compartment first-order kinetics modeling. The k₁ was approximately 7-10 times lower than the corresponding k₂ and the positive correlation between k₁ and k₂ illustrated that adsorption and degradation were kinetically associated. ROS quenching only slightly inhibited 2,4-DCP degradation. Chemicals with similar structures to ROS quenchers (without quenching ability) also inhibited 2,4-DCP degradation, probably because of the competition of the active degradation sites on biochars. Electrochemical analysis and pH-impact experiments further elucidated that 2,4-DCP underwent oxidation-dominated degradation in the adsorbed phase via direct electron transfer. Fe(III) obviously increased 2,4-DCP adsorption through cation bridging and enhanced electron density by Fe-O conjugations on the biochar surface, which facilitated subsequent degradation. This study emphasized the importance of degradation on the biochar solid phase and that a breakthrough of the mass transfer bottleneck of adsorption will greatly promote degradation.

Response of electron transfer capacity of humic substances to soil microenvironment

The humic substances (HS) - mediated electron transfer process is of great significance to the reduction and degradation of pollutants and the improvement of soil quality. Different soil conditions lead to different characteristics of HS, resulting in differences in the electron transfer capacity (ETC) of HS. It is unclear how the environmental conditions in soil affect the ETC by affecting on HS. In this study, the response relationship of soil microenvironment, HS and ETC has been studied. The results show that the ETC follows the descending order of: Langshan > Nanchang > Anqing > Beijing > Guilin. There were significant differences in ETC in soil HS in different regions. There were significant differences in electron-donating capacity (EDC) in soil HS in different regions and depths. EDC in soil was higher than electron-accepting capacity (EAC), and on average, are 22.4 times higher than the EAC. The HS components of soils in different regions are different. The most significant differences were in tyrosine-like substances and soluble microbial by-products (SMPs). The five components of the soil HS from Langshan were the most different from those in other regions. There were differences in SMPs and humic-like substances in soils of different depths in Anqing and Guilin. ETC can be affected by the composition of HS components in different regions. The composition of HS at different soil depths in the same regions had little effect on ETC. SMPs can promote ETC and EDC, and tyrosine-like substance can promote EDC. Moisture content, pH and TOC are the main factors affecting the composition of HS components. This results can provide a research basis for the sustainable and safe utilization of agricultural soil.

Combined remediation effects of biochar, zeolite and humus on Cd-contaminated weakly alkaline soils in wheat farmland

Threats posed by Cd-contaminated arable soils to food security have attracted increasing attention. The combination of organic and inorganic amendments has been extensively applied to immobilize Cd in paddy soils. However, the regulatory mechanism of Cd fractionation under these combined amendments and the effect on wheat Cd accumulation remain unclear in upland soils. In this work, different combinations of organic and inorganic amendments were prepared with biochar, zeolite and humus, and the Cd-immobilization mechanism was also investigated in field experiments. The results demonstrated that the mixture of biochar, zeolite and humus had excellent Cd immobilization performance in highly Cd-contaminated (4.26 ± 1.25 mg kg^-1) weakly alkaline soils, resulting in 76.5-84.8% decreases in soil available Cd. The contribution of single components to Cd immobilization in the combined amendment follows the order of humus > biochar > zeolite. The combined amendment converted the acid soluble Cd to the Cd bound to the reducible fraction with higher stability, thereby decreasing Cd bioavailability. The maximum Cd decrease rate in wheat roots, straw and grains could reach 68.2%, 45.0% and 59.3%, respectively, and the Cd content in grains (0.098 mg kg^-1) was lower than the food security standards of China (0.1 mg kg^-1). Wheat planting for two successive years in a large-scale field further verified the superior Cd immobilization performance and stability of the combined amendment in moderately to slightly Cd-contaminated soil. The present study provides references for the remediation of Cd-contaminated weakly alkaline upland soils and certain guidance for safe food production.

Roles of humic substances redox activity on environmental remediation

Humic substances (HS) as representative natural organic matters and the most common organic compounds existing in the environment, has been applied to the treatment and remediation of environmental pollution. This review systematically introduces and summarizes the redox activity of HS for the remediation of environmental pollutants. For inorganic pollutants (such as silver, chromium, mercury, and arsenic), the redox reaction of HS can reduce their toxicity and mobilization, thereby reducing the harm of these pollutants to the environment. The concentration and chemical composition of HS, environmental pH, ionic strength, and competing components affect the degree and rate of redox reactions between inorganic pollutants and HS significantly. With regards to organic pollutants, HS has photocatalytic activity and produces a large number of reactive oxygen species (ROS) under the light which reacts with organic pollutants to accelerate the degradation of organic pollutants. Under the affection of HS, the redox of Fe(III) and Fe(II) can enhance the efficiency of Fenton-like reaction to degrade organic pollutants. Finally, the research direction of HS redox remediation of environmental pollution is prospected.