Nano magnetite-loaded biochar boosted methanogenesis through shifting microbial community composition and modulating electron transfer

Effects of salinity and betaine addition on anaerobic granular sludge properties and microbial community succession patterns in organic saline wastewater

In this study, the effects of different salinity gradients and addition of compatible solutes on anaerobic treated effluent water qualities, sludge characteristics and microbial communities were investigated. The increase in salinity resulted in a decrease in particle size of the granular sludge, which was concentrated in the range of 0.5-1.0 mm. The content of EPS (extracellular polymeric substances) in the granular sludge gradually increased with increasing salinity and the addition of betaine (a typical compatible solute). Meanwhile, the microbial community structure was significantly affected by salinity, with high salinity reducing the diversity of bacteria. At higher salinity, Patescibacteria and Proteobacteria gradually became the dominant phylum, with relative abundance increasing to 13.53% and 12.16% at 20 g/L salinity. Desulfobacterota and its subordinate Desulfovibrio, which secrete EPS in large quantities, dominated significantly after betaine addition.Their relative abundance reached 13.65% and 7.86% at phylum level and genus level. The effect of these changes on the treated effluent was shown as the average chemical oxygen demand (COD) removal rate decreased from 82.10% to 79.71%, 78.01%, 68.51% and 64.55% when the salinity gradually increased from 2 g/L to 6, 10, 16 and 20 g/L. At the salinity of 20 g/L, average COD removal increased to 71.65% by the addition of 2 mmol/L betaine. The gradient elevated salinity and the exogenous addition of betaine played an important role in achieving stability of the anaerobic system in a highly saline environment, which provided a feasible strategy for anaerobic treatment of organic saline wastewater.

Comprehensive insights into the impact of magnetic biochar on protein hydrolysis in sludge anaerobic digestion: Protein structures, microbial activities and syntrophic metabolisms

The addition of composite conductive materials is being increasingly recognized as a promising strategy to enhance anaerobic digestion (AD) performance. However, the influence of these materials on protein hydrolysis has been poorly documented. Here, a novel magnetic biochar derived from oil sludge and straw was synthesized using different iron sources and successfully applied in sludge AD. Experimental results revealed that magnetic biochar modified by Fe2+ exhibited excellent electron transfer capacity, moderate magnetization, diverse functional groups (e.g. C=O, C-O=O-), and abundant iron distribution. These characteristics significantly enhanced the hydrolysis of tryptophan-like components, leading to increased methane production (144.44 mL gVS-1vs 79.72 mL gVS-1 in the control test). Molecular docking analysis revealed that the binding of magnetic biochar related Fe2+ and Fe3+, onto sludge proteins via hydrogen bond played a key role in promoting subsequent protein hydrolysis. Additionally, the noteworthy conservation of protein structures from α-helix and β-sheet to random coil, along with the breakdown of the amide I-associated C=O group and amide III-related CN and NH bonds following the addition of magnetic biochar, accelerated the degradation of sludge protein. Observation of variations in protease activity, coenzyme F420, electron transfer system (ETS), and conductivity within the AD systems, particularly the enrichment of Methanospirillum and Methanosaeta archaea, as well as the Petrimonas, Comamonas, and Syntrophomonas bacteria, suggested that magnetic biochar facilitated a conducive environment by improving hydrolysis-acidification and the direct interspecies electron transfer (DIET) process for acetoclastic methanogens. Moreover, metabolic pathways further proved that tryptophan metobalism and acetoclastic methanogenesis were both facilitated by magnetic biochar. This study provides an in-depth understanding of the impact of magnetic biochar on protein hydrolysis in sewage sludge AD.

One-step synthesis of a core-shell structured biochar using algae (Chlorella) powder and ferric sulfate for immobilizing Hg(II)

Mercury (Hg) pollution poses a significant environmental challenge. One promising method for its removal is the sorption of mercuric ions using biochar. FeS-doped biochar (FBC) exhibits effective mercury adsorption, however may release excess iron into the surrounding water. To address this issue, a novel magnetic pyrrhotite/magnetite-doped biochar with a core-shell structure was synthesized for the adsorption of 2-valent mercury (Hg(II)). The proposed synthesis process involved the use of algae powder and ferric sulfate in a one-step method. By varying the ratio of ferric sulfate and alga powder (within the range of 0.18 - 2.5) had a notable impact on the composition of FBC. As the ferric sulfate content increased, the FBC exhibited a higher concentration of oxygen-containing groups. To assess the adsorption capacity, Langmuir and Freundlich adsorption models were applied to the experimental data. The most effective adsorption was achieved with FBC-4, reaching a maximum capacity (Qm) of 95.51 mg/g. In particular, at low Hg(II) concentrations, FBC-5 demonstrated the ability to reduce Hg(II) concentrations to less than 0.05 mg/L within 30 min. Additionally, the stability of FBC was confirmed within the pH range of 3.8 - 7.2. The study also introduced a model to analyze the adsorption preference for different Hg(II) species. Calomel was identified in the mercury saturated FBC, whereas the core-shell structure exhibited excellent conductivity, which most likely contributed to the minimal release of iron. In summary, this research presents a novel and promising method for synthesizing core-shell structured biochar and provides a novel approach to explore the adsorption contribution of different metal species.

Enhancement of electron transfer via magnetite in nitrite-dependent anaerobic methane oxidation system

Nitrite-dependent anaerobic methane oxidation (n-DAMO) is a novel denitrification process that simultaneously further removes and utilizes methane from anaerobic effluent from wastewater treatment plants. However, the metabolic activity of n-DAMO bacteria is relative low for practical application. In this study, conductive magnetite was added into lab-scale sequencing batch reactor inoculated with n-DAMO bacteria to study the influence on n-DAMO process. With magnetite amendment, the nitrogen removal rate could reach 34.9 mg N·L-1d-1, nearly 2.5 times more than that of control group. Magnetite significantly facilitated the interspecies electron transfer and built electrically connected community with high capacitance. Enzymatic activities of electron transport chain were significantly elevated. Functional gene expression and enzyme activities associated with nitrogen and methane metabolism had been highly up-regulated. These results not only propose a useful strategy in n-DAMO application but also provide insights into the stimulating mechanism of magnetite in n-DAMO process.

Role of KOH-activated biochar on promoting anaerobic digestion of biomass from Pennisetumgianteum

Pennisetum giganteum is a promising non-food crop feedstock for biogas production due to its high productivity and bio-methane potential. However, the accumulation of volatile fatty acids (VFA) usually restricts the conversion efficiency of P. giganteum biomass (PGB) during anaerobic digestion (AD). Here, the role of KOH-activated biochar (KB) in improving the AD efficiency of PGB and the related mechanisms were investigated in detail. The results revealed that KB exhibited excellent electrical conductivity, electron transfer capacity and specific capacitance, which might be related to the decrease in the electron transfer resistance after adding KB to the AD process. In addition, the KB addition not only reinforced metabolisms of energy and VFAs but also promoted the conversion of VFAs to methane, leading to a 52% increase in the methane production rate. Bioinformatics analysis showed that Smithella and Methanosaeta were key players in the KB-mediated AD process of PGB. The stimulatory effect of methanogenesis probably resulted from the establishment of direct interspecies electron transfer (DIET) between VFA-oxidizing acetogens (e.g., Smithella) and Methanosaeta. These findings provided a key step to improve the PGB-based AD process.

A comprehensive review on the preparation of biochar from digestate sources and its application in environmental pollution remediation

The preparation of biochar from digestate is one of the effective ways to achieve the safe disposal and resource utilization of digestate. Nevertheless, up to now, a comprehensive review encompassing the factors influencing anaerobic digestate-derived biochar production and its applications is scarce in the literature. Therefore, to fill this gap, the present work first outlined the research hotspots of digestate in the last decade using bibliometric statistical analysis with the help of VOSviewer. Then, the characteristics of the different sources of digestate were summarized. Furthermore, the influencing factors of biochar preparation from digestate and the modification methods of digestate-derived biochar and associated mechanisms were analyzed. Notably, a comprehensive synthesis of anaerobic digestate-derived biochar applications is provided, encompassing enhanced anaerobic digestion, heavy metal remediation, aerobic composting, antibiotic/antibiotic resistance gene removal, and phosphorus recovery from digestate liquor. The economic and environmental impacts of digestate-derived biochar were also analyzed. Finally, the development prospect and challenges of using biochar from digestate to combat environmental pollution are foreseen. The aim is to not only address digestate management challenges at the source but also offer a novel path for the resourceful utilization of digestate.

Improved Short-Chain Fatty Acids Production and Protein Degradation During the Anaerobic Fermentation of Waste-Activated Sludge via Alumina Slag-Modified Biochar

As the by-product in the biological sewage treatment, waste-activated sludge (WAS) always suffers from the difficulty of disposal. Anaerobic fermentation to achieve valuable carbon sources is a feasible way for resource utilization of WAS, whereas the process is always restricted by its biochemical efficiency. Hence, the WAS was used as the feedstock in this study. Alumina slag-modified biochar (Al@BioC) respectively from pine wood (PW) or fresh vinegar residue (FVR) was employed to stimulate the process of short-chain fatty acids (SCFAs) production during the anaerobic treatment of WAS. The results indicate that the addition of Al@BioC could facilitate the distinct increase in SCFAs yield (42.66 g/L) by 14.09% and acetate yield (33.30 g/L) by 18.77%, respectively, when compared with that in regular fermentation without Al@BioC addition. Furthermore, protein degradation was also improved. With the Al@BioCPW added, the maximum concentration of soluble protein reached 867.68 mg/L and was 24.39% higher than the initial level, while the enhancement in the group with Al@BioCFVR and without biochar addition was 12.49% and 7.44%, respectively. According to the results of 16S rDNA sequencing, the relative abundance of acid-producing bacteria (Bacteroidota and Firmicutes) was enriched, enhancing the pathways of protein metabolisms and the ability to resist the harsh environment, respectively. Moreover, Proteiniphilum under Bacteroidota and Fastidiosipila under Firmicutes were the main microorganisms to metabolize protein. The above results might provide a novel material for harvesting the SCFAs production, which is conducive to harmless disposal and carbon resource recovery.

Functional biochar in enhanced anaerobic digestion: Synthesis, performances, and mechanisms

Anaerobic digestion technology is crucial in bioenergy recovery and organic waste management. At the same time, it often encounters challenges such as low organic digestibility and inhibition of toxic substances, resulting in low biomethane yields. Biochar has recently been used in anaerobic digestion to alleviate toxicity inhibition, improve the stability of anaerobic digestion processes, and increase methane yields. However, the practical application of biochar is limited, for the properties of pristine biochar significantly affect its application in anaerobic digestion. Although much research focuses on understanding original biochar's fundamental properties and functionalization, there are few reviews on the applications of functional biochar and the effects of critical properties of pristine biochar on anaerobic digestion. This review systematically reviewed functionalization strategies, key performances, and applications of functional biochar in anaerobic digestion. The properties determining the role of biochar were reviewed, the synthesis methods of functional biochar were summarized and compared, the mechanism of functional biochar was discussed, and the factors affecting the function of functional biochar were reviewed. This review provided a comprehensive understanding of functional biochar in anaerobic digestion processes, which would be helpful for the development and applications of engineered biochar.

A review of application of combined biochar and iron-based materials in anaerobic digestion for enhancing biogas productivity: Mechanisms, approaches and performance

Strengthening direct interspecies electron transfer (DIET), via adding conductive materials, is regarded as an effective way for improving methane productivity of anaerobic digestion (AD). Therein, the supplementation of combined materials (composition of biochar and iron-based materials) has attracted increasing attention in recent years, because of their advantages of promoting organics reduction and accelerating biomass activity. However, as far as we known, there is no study comprehensively summarizing the application of this kind combined materials. Here, the combined methods of biochar and iron-based materials in AD system were introduced, and then the overall performance, potential mechanisms, and microbial contribution were summarized. Furthermore, a comparation of the combinated materials and single material (biochar, zero valent iron, or magnetite) in methane production was also evaluated to highlight the functions of combined materials. Based on these, the challenges and perspectives were proposed to point the development direction of combined materials utilization in AD field, which was hoped to provide a deep insight in engineering application.

Quantitative analysis of the current status and research trends of biochar research - A scientific bibliometric analysis based on global research achievements from 2003 to 2023

Biochar has excellent physical and chemical properties such as porosity, high carbon content, high cation exchange capacity, and rich surface functional groups and has been widely used in environmental remediation. Over the past 20 years, although various reviews have described the application of biochar as an environmentally friendly multifunctional material in environmental remediation, no comprehensive summary and analysis of the research trends in this field exists. To promote the rapid and stable development of the field of biochar, the current state of research on biochar is clarified using the bibliometric method in this report, and potential development directions and challenges for the future are identified. All relevant biochar literature from 2003-2023 was collected from the Chinese National Knowledge Infrastructure and Web of Science Core Collection. A total of 6,119 published Chinese papers and 25,174 English papers were selected for the quantitative analysis. CiteSpace, VOSviewer, and Scimago graphics software was used to summarize the numbers of papers published over the years, as well as the countries, institutions, and authors that published the most articles. Secondly, using keyword co-occurrence and emergence analysis, the recognized research hotspots in different areas such as adsorbents, soil remediation, catalytic oxidation, supercapacitors, and "biochar-microbial" synergy were analyzed. Finally, the prospects and challenges of biochar were assessed to provide new perspectives for further promoting its development in technological, economic, environmental, and other aspects.

Application of Bioelectrochemical Systems and Anaerobic Additives in Wastewater Treatment: A Conceptual Review

The interspecies electron transfer (IET) between microbes and archaea is the key to how the anaerobic digestion process performs. However, renewable energy technology that utilizes the application of a bioelectrochemical system together with anaerobic additives such as magnetite-nanoparticles can promote both direct interspecies electron transfer (DIET) as well as indirect interspecies electron transfer (IIET). This has several advantages, including higher removal of toxic pollutants present in municipal wastewater, higher biomass to renewable energy conversion, and greater electrochemical efficiencies. This review explores the synergistic influence of bioelectrochemical systems and anaerobic additives on the anaerobic digestion of complex substrates such as sewage sludge. The review discussions present the mechanisms and limitations of the conventional anaerobic digestion process. In addition, the applicability of additives in syntrophic, metabolic, catalytic, enzymatic, and cation exchange activities of the anaerobic digestion process are highlighted. The synergistic effect of bio-additives and operational factors of the bioelectrochemical system is explored. It is elucidated that a bioelectrochemical system coupled with nanomaterial additives can increase biogas-methane potential compared to anaerobic digestion. Therefore, the prospects of a bioelectrochemical system for wastewater require research attention.