Improvement of hydrogen production from glucose by ferrous iron and biochar

Influence of adding strategy of biochar on medium-chain fatty acids production from sewage sludge

Production of medium-chain fatty acids (MCFAs) from sewage sludge has dual effects on valuable sludge disposal and renewable energy generation, while low efficiency limits its application. Biochar addition is considered an effective method to improve MCFAs production. In this study, the influence of biochar adding strategies (i.e., adding biochar in acidification or chain elongation (CE) processes) on MCFAs production was explored. Results showed that by adding biochar in the acidification process, MCFAs accumulation increased by over 114%, accompanied by the highest carbon conversion efficiency (134.66%) and electron transfer efficiency of MCFAs (94.22%) by the terminal CE. Adding biochar before the acidification process better enriched CE bacteria (e.g., Paraclostridium) and strengthened the dominant metabolic pathway. In contrast, the biochar added before the CE process priorly enriched the bacteria capable of degrading organics, like unclassified_f__Dysgonomonadaceae, norank_f__norank_o__OPB41, and Acetobacterium. The differences in excessive ethanol oxidation and short-chain fatty acids accumulation induced by varied adding strategies might be responsible for this.

Improved sequential production of hydrogen and caproate by addition of biochar prepared from cornstalk residues

This study proposes a new model in which ethanol and acetate produced by dark fermentation are processed by Clostridium kluyveri for chain elongation to produce caproate with an addition of biochar prepared from cornstalk residues after acid pretreatment and enzymatic hydrolysis (AERBC) in the dark fermentation and chain elongation processes. The results show a 6-25% increase in hydrogen production in dark fermentation with adding AERBC, and the maximum concentration of caproate in the new model reached 1740 mg/L, 61% higher than that in the control group. In addition, caproate was obtained by dark fermentation, using liquid metabolites as substrates with an initial pH range of 6.5-7.5. Finally, the electron balance and electron transfer efficiency in the new model were analyzed, and the role of AERBC in dark fermentation and chain elongation was investigated. This study provides a new reference for the use of dark-fermented liquid metabolites and cornstalk residue.

Effect of zero-valent iron nanoparticles on taxonomic composition and hydrogen production from kitchen waste

This study investigated the effects of varying zero-valent iron (ZVI) (0 to 5,000 mg/L) on fermentative hydrogen (H2) production, metabolic pattern, and taxonomic profile by using kitchen waste as substrate. The study demonstrated that the supplementation of 500 mg ZVI/L resulted in the highest H2 yield (219.68 ± 11.19 mL H2/g-volatile solids (VS)added), which was 19% higher than the control. The metabolic pattern analysis showed that acetic and butyric acid production primarily drove the H2 production. The taxonomic analysis further revealed that Firmicutes (relative abundance (RA): 80-96%) and Clostridium sensu stricto 1 (RA: 68-88%) were the dominant phyla and genera, respectively, during the exponential gas production phase, supporting the observation of accumulation of acetic and butyric acids. These findings suggest that supplementation of ZVI can enhance H2 production from organic waste and significantly influence the metabolic pattern and taxonomic profile, including the metalloenzymes.

Biochar applications in microbial fermentation processes for producing non-methane products: Current status and future prospects

The objective of this review is to encourage the technical development of biochar-assisted microbial fermentation. To this end, recent advances in biochar applications for microbial fermentation processes (i.e., non-methane products of hydrogen, acids, alcohols, and biofertilizer) have been critically reviewed, including process performance, enhanced mechanisms, and current research gaps. Key findings of enhanced mechanisms by biochar applications in biochemical conversion platforms are summarized, including supportive microbial habitats due to the immobilization effect, pH buffering due to alkalinity, nutrition supply due to being rich in nutrient elements, promoting electron transfer by acting as electron carriers, and detoxification of inhibitors due to high adsorption capacity. The current technical limitations and biochar's industrial applications in microbial fermentation processes are also discussed. Finally, suggestions like exploring functionalized biochar materials, biochar's automatic addition and pilot-scale demonstration are proposed. This review would further promote biochar applications in microbial fermentation processes for the production of non-methane products.

Magnetic nitrogen-doped activated carbon improved biohydrogen production

Low biological hydrogen (bioH2) production due to non-optimal metabolic pathways occurs frequently. In this work, magnetic nitrogen-doped activated carbon (MNAC) was prepared and added into the inoculated sludge with glucose as substrate to enhance hydrogen (H2) yield by mesophilic dark fermentation (DF). The highest H2 yield appeared in 400 mg/L AC (252.8 mL/g glucose) and 600 mg/L MNAC group (304.8 mL/g glucose), which were 26.02% and 51.94% higher than that of 0 mg/L MNAC group (200.6 mL/g glucose). The addition of MNAC allowed for efficient enrichment of Firmicutes and Clostridium-sensu-stricto-1, accelerating the metabolic pathway shifted towards butyrate type. The Fe ions released by MNAC facilitated electron transfer and favored the reduction of ferredoxin (Fd), thereby obtaining more bioH2. Finally, the generation of [Fe-Fe] hydrogenase and cellular components of H2-producing microbes (HPM) during homeostasis was discussed to understand on the use of MNAC in DF system.

Sustainable management of food waste; pre-treatment strategies, techno-economic assessment, bibliometric analysis, and potential utilizations: A systematic review

Food waste (FW) contains many nutritional components such as proteins, lipids, fats, polysaccharides, carbohydrates, and metal ions, which can be reused in some processes to produce value-added products. Furthermore, FW can be converted into biogas, biohydrogen, and biodiesel, and this type of green energy can be used as an alternative to nonrenewable fuel and reduce reliance on fossil fuel sources. It has been demonstrated in many reports that at the laboratory scale production of biochemicals using FW is as good as pure carbon sources. The goal of this paper is to review approaches used globally to promote turning FW into useable products and green energy. In this context, the present review article highlights deeply in a transdisciplinary manner the sources, types, impacts, characteristics, pre-treatment strategies, and potential management of FW into value-added products. We find that FW could be upcycled into different valuable products such as eco-friendly green fuels, organic acids, bioplastics, enzymes, fertilizers, char, and single-cell protein, after the suitable pre-treatment method. The results confirmed the technical feasibility of all the reviewed transformation processes of FW. Furthermore, life cycle and techno-economic assessment studies regarding the socio-economic, environmental, and engineering aspects of FW management are discussed. The reviewed articles showed that energy recovery from FW in various forms is economically feasible.

Wastewater-derived biohydrogen: Critical analysis of related enzymatic processes at the research and large scales

Organic-rich wastewater is a feasible feedstock for biohydrogen production. Numerous review on the performance of microorganisms and the diversity of their communities during a biohydrogen process were published. However, there is still no in-depth overview of enzymes for biohydrogen production from wastewater and their scale-up applications. This review aims at providing an insightful exploration of critical discussion in terms of: (i) the roles and applications of enzymes in wastewater-based biohydrogen fermentation; (ii) systematical introduction to the enzymatic processes of photo fermentation and dark fermentation; (iii) parameters that affect enzymatic performances and measures for enzyme activity/ability enhancement; (iv) biohydrogen production bioreactors; as well as (v) enzymatic biohydrogen production systems and their larger scales application. Furthermore, to assess the best applications of enzymes in biohydrogen production from wastewater, existing problems and feasible future studies on the development of low-cost enzyme production methods and immobilized enzymes, the construction of multiple enzyme cooperation systems, the study of biohydrogen production mechanisms, more effective bioreactor exploration, larger scales enzymatic biohydrogen production, and the enhancement of enzyme activity or ability are also addressed.

Enhanced biohydrogen production of anaerobic fermentation by the Fe3O4 modified mycelial pellets-based anaerobic granular sludge

To improve the catalytic efficiency and stability of hydrogen-producing bacteria (HPB), the Fe₃O₄ nanoparticles modified Aspergillus tubingensis mycelial pellets (AT)-based anaerobic granular sludge (Fe₃O₄@AT-AGS) was developed. The Fe₃O₄@AT-AGS protected flora with abundant extracellular polymeric substances, which increased diversity and stability of flora in early and late stage. The porous structure enhanced mass transfer efficiency, thus promoted dominant flora transferred from lactate-producing bacteria (LPB) to HPB in middle stage. The Fe₃O₄ improved biomass of mycelial by 19.5 %. The enhancement of dehydrogenase and conductivity of Fe₃O₄ increased the HPB proportion, electron transfer, and butyrate fermentation in early and middle stage. The Fe₃O₄@AT-AGS enhanced HPB abundance, dehydrogenase activity and stability, and significantly inhibited propionate fermentation. The biohydrogen production and yield respectively reached 2792 mL/L and 2.56 mol/mol glucose. Clostridium sensu stricto 11 as dominant microbes reached 77.3 %. This provided strategy for alleviating inhibition of LPB and improving competitiveness of HPB during biohydrogen production.

Understanding and mitigating the distinctive stresses induced by diverse microplastics on anaerobic hydrogen-producing granular sludge

This work comparatively studied the different stress responses of anaerobic hydrogen-producing granular sludge (HPG) to several typical MPs in wastewater, i.e., polyethylene (PE), polyethylene terephthalate (PET), and polyvinyl chloride (PVC) MPs. A new approach to mitigating the inhibition caused by MPs based on biochar was then proposed. The results displayed that microbe in HPG had diverse tolerances to PE-MPs, PET-MPs and PVC-MPs, with the hydrogen production downgraded to 82.0 ± 3.2 %, 72.3 ± 2.5 % and 66.6 ± 2.3 % (p < 0.05) of control respectively, due to the distinct leachates toxicities and oxidative stress level induced by different MPs. The discrepant mitigation reflected in the hydrogen yields of biochar-based HPGs raised back to 88.7 ± 1.4 %, 85.3 ± 3.8 % and 88.5 ± 3.5 % of control. The MPs induced disintegrated granule morphology, fragile microbial viability and impaired defensive function of extracellular polymeric substances were restored by biochar. The effective mitigation was revealed to be due to the strong adsorption of MPs by biochar, reducing direct contact between microbes and MPs. Biochar addition also enhanced protection for HPG by increasing EPS secretion and weakened the oxidative damage to anaerobes induced by MPs. Biochar manifested the disparate adsorption properties of three MPs. The most superior mitigation in HPG contaminated by PVC-MPs was attributed to the strongest affinity of biochar to PVC-MPs and effective alleviation of PVC leachates toxicity.

Effect of metals on the regulation of acidogenic metabolism enhancing biohydrogen and carboxylic acids production from brewery spent grains: Microbial dynamics and biochemical analysis

The present study reports the mixed culture acidogenic production of biohydrogen and carboxylic acids (CA) from brewery spent grains (BSG) in the presence of high concentrations of cobalt, iron, nickel, and zinc. The metals enhanced biohydrogen output by 2.39 times along with CA biosynthesis by 1.73 times. Cobalt and iron promoted the acetate and butyrate pathways, leading to the accumulation of 5.14 gCOD/L of acetic and 11.36 gCOD/L of butyric acid. The production of solvents (ethanol + butanol) was higher with zinc (4.68 gCOD/L) and cobalt (4.45 gCOD/L). A combination of all four metals further enhanced CA accumulation to 42.98 gCOD/L, thus surpassing the benefits accrued from supplementation with individual metals. Additionally, 0.36 and 0.31 mol green ammonium were obtained from protein-rich brewery spent grain upon supplementation with iron and cobalt, respectively. Metagenomic analysis revealed the high relative abundance of Firmicutes (>90%), of which 85.02% were Clostridium, in mixed metal-containing reactors. Finally, a significant correlation of dehydrogenase activity with CA and biohydrogen evolution was observed upon metal addition.

Assessment of potential biotoxicity induced by biochar-derived dissolved organic matters to biological fermentative H2 production

Biochar is a widely used antecedent for improving bio‑hydrogen production. However, little is known about the impact of biochar-derived dissolved organic matter (DOM) on the performance of fermentative bio-H₂ production. Herein, we evaluated the impact of biochar-derived DOM on the fermentation performance of hydrogen-producing microflora. The pyrolysis temperature of biochar affected the DOM composition, with lower pyrolysis temperatures showing more serious inhibition on H₂ accumulation. When biochar was pyrolyzed at 500 °C, DOM prolonged the fermentation period and decreased H₂ production from 1330.41 mL L^-1 to 1177.05 mL L^-1 compared to the control group. The xylose utilization in mixed substrate decreased from 29.72% to 26.41%, which is not favorable for practical applications where lignocellulosic biomass is used as a substrate. Otherwise, DOM caused a 6% reduction in microbial biomass accumulation and less soluble metabolites formation. The potential mechanism of DOM inhibiting bio‑hydrogen production was verified by identifying an increase in reactive oxygen species (ROS) level (178.2%) and the microbial community shifted to containing fewer hydrogen-producing strains. The finding prompts a more precise design of biochar applications in fermentation systems to alleviate the potential hazards and maximum the fermentation performance, not limited to fermentative hydrogen production system.

Biomethane enhancement using reduced graphene oxide in anaerobic digestion of municipal solid waste

The present research investigated the impact of reduced graphene oxide (rGO) addition on the semi-continuous anaerobic digestion of the organic fraction of municipal solid waste (OFMSW) in the range of 0.5-10 gVolatileSolid(VS)/L_reactorday organic loading rates (OLR). Adding rGO enhanced the rate and yield of biomethane production, and the maximum biomethane increment rate was obtained as 110% at an OLR of 4.0 gVS/L_reactorday. However, after increasing the OLR to 6 gVS/L_reactorday, there was a dramatic decrease in biomethane production because of volatile fatty acid (VFA) accumulation. Methanotrix is the predominant archaeal genus at OLRs lower than 6 gVS/L_reactorday in reactors (89-97%). An increment in biomethane production was associated with the higher abundance of the Methanothrix genus in the rGO-supported reactor (rG) than in the control reactor (rC).

Sustainable biochar: A facile strategy for soil and environmental restoration, energy generation, mitigation of global climate change and circular bioeconomy

The increasing agro-demands with the burgeoning population lead to the accumulation of lignocellulosic residues. The practice of burning agri-residues has consequences viz. Release of soot and smoke, nutrient depletion, loss of soil microbial diversity, air pollution and hazardous effects on human health. The utilization of agricultural waste as biomass to synthesize biochar and biofuels, is the pertinent approach for attaining sustainable development goals. Biochar contributes in the improvement of soil properties, carbon sequestration, reducing greenhouse gases (GHG) emission, removal of organic and heavy metal pollutants, production of biofuels, synthesis of useful chemicals and building cementitious materials. The biochar characteristics including surface area, porosity and functional groups vary with the type of biomass consumed in pyrolysis and the control of parameters during the process. The major adsorption mechanisms of biochar involve physical-adsorption, ion-exchange interactions, electrostatic attraction, surface complexation and precipitation. The recent trend of engineered biochar can enhance its surface properties, pH buffering capacity and presence of desired functional groups. This review focuses on the contribution of biochar in attaining sustainable development goals. Hence, it provides a thorough understanding of biochar's importance in enhancing soil productivity, bioremediation of environmental pollutants, carbon negative concretes, mitigation of climate change and generation of bioenergy that amplifies circular bioeconomy, and concomitantly facilitates the fulfilment of the United Nation Sustainable Development Goals. The application of biochar as seen is primarily targeting four important SDGs including clean water and sanitation (SGD6), affordable and clean energy (SDG7), responsible consumption and production (SDG12) and climate action (SDG13).

Biohydrogen production from microalgae for environmental sustainability

Hydrogen as a clean energy that is conducive to energy and environmental sustainability, playing a significant role in the alleviation of global climate change and energy crisis. Biohydrogen generation from microalgae has been reported as a highly attractive approach that can produce a benign clean energy carrier to achieve carbon neutrality and bioenergy sustainability. Thus, this review explored the mechanism of biohydrogen production from microalgae containing direct biophotolysis, indirect biophotolysis, photo fermentation, and dark fermentation. In general, dark fermentation of microalgae for biohydrogen production is relatively better than photo fermentation, biophotolysis, and microbial electrolysis, because it is able to consecutively generate hydrogen and is not reliant on energy supplied by natural sunlight. Besides, this review summarized potential algal strains for hydrogen production focusing on green microalgae and cyanobacteria. Moreover, a thorough review process was conducted to present hydrogen-producing enzymes targeting biosynthesis and localization of enzymes in microalgae. Notably, the most powerful hydrogen-producing enzymes are [Fe-Fe]-hydrogenases, which have an activity nearly 10-100 times better than [Ni-Fe]-hydrogenases and 1000 times better than nitrogenases. In addition, this work highlighted the major factors affecting low energy conversion efficiency and oxygen sensitivity of hydrogen-producing enzymes. Noting that the most practical pathway of biohydrogen generation was sulfur-deprivation compared with phosphorus, nitrogen, and magnesium deficiency. Further discussions in this work summarized the recent advancement in biohydrogen production from microalgae such as genetic engineering, microalgae-bacteria consortium, electro-bio-hydrogenation, and nanomaterials for developing enzyme stability and hydrolytic efficiency. More importantly, this review provided a summary of current limitations and future perspectives on the sustainable production of biohydrogen from microalgae.

Enhanced antibiotic degradation and hydrogen production of deacetoxycephalosporin C fermentation residue by gamma radiation coupled with nano zero-valent iron

Antibiotic fermentation residue (AFR) has been categorized as hazardous waste in China. Anaerobic biohydrogen fermentation may be a promising technology for handling AFR, which could achieve dual goals of waste treatment and clean energy production at the same time. However, the low hydrogen yield and low removal efficiency of residual antibiotics are two major factors limiting the AFR biohydrogen fermentation process. This work firstly applied gamma radiation (50 kGy) to remove the residual antibiotic in AFR and improve the bioavailability of organic matters, then adding nano zero-valent iron (nZVI) (100-1000 mg/L) to further enhance the AFR biohydrogen fermentation performance. Results showed that residual deacetoxycephalosporin C in AFR was removed with a high efficiency of 98.6%, and hydrogen yield achieved 20.45 mL/g-VS_added with the combined approach of gamma radiation pretreatment and 500 mg/L nZVI addition, which was 139.2% higher compared to the control experimental result. The combined approach also promoted the biohydrogen production rate, decreased the lag phase of hydrogen production, and increased the organics utilization. Microbiological analysis revealed that highly efficient hydrogen-producing genera Clostridium sensu stricto were enriched in much higher abundance with the combined approach, which might be the fundamental mechanism for the enhanced AFR fermentation performance.

Sustainable additives for the regulation of NH3 concentration and emissions during the production of biomethane and biohydrogen: A review

This study reviews the recent advances and innovations in the application of additives to improve biomethane and biohydrogen production. Biochar, nanostructured materials, novel biopolymers, zeolites, and clays are described in terms of chemical composition, properties and impact on anaerobic digestion, dark fermentation, and photofermentation. These additives can have both a simple physical effect of microbial adhesion and growth, and a more complex biochemical impact on the regulation of key parameters for CH₄ and H₂ production: in this study, these effects in different experimental conditions are reviewed and described. The considered parameters include pH, volatile fatty acids (VFA), C:N ratio, and NH₃; additionally, the global impact on the total production yield of biogas and bioH₂ is reviewed. A special focus is given to NH₃, due to its strong inhibition effect towards methanogens, and its contribution to digestate quality, leaching, and emissions into the atmosphere.

Revealing the mechanisms of alkali-based magnetic nanosheets enhanced hydrogen production from dark fermentation: Comparison between mesophilic and thermophilic conditions

In the present study, a dark fermentation system inoculated with mixed culture bacteria (MCB) was developed using prepared alkali-based magnetic nanosheets (AMNSs) to facilitate biohydrogen (BioH₂) production. The highest BioH₂ yields of 232.8 ± 8.5 and 150.3 ± 4.8 mL/g glucose were observed at 100 (mesophilic condition) and 400 (thermophilic condition) mg/L AMNSs groups, which were 65.4% and 43.3%, respectively, above the 0 mg/L AMNSs group. The fermentation pathway revealed that AMNSs enhanced the butyrate-type metabolic pathway and the corresponding nicotinamide adenine dinucleotides (NADHand NAD⁺) ratio, and hydrogenase activity was enhanced in mesophilic fermentation. The interaction of AMNSs and MCB suggested that AMNSs could assist in electron transfer and that the released metal elements might be responsible for elevated hydrogenase activity. AMNSs also promoted the evolution of the dominant microbial community and altered the content of extracellular polymers, leading to increased production of BioH₂.

Effects of feedstock and pyrolysis temperature of biochar on promoting hydrogen production of ethanol-type fermentation

Biochar has been shown to benefit fermentative hydrogen production. However, the influencing factors and key characteristics of its promoting function remained to be elucidated. This study investigated the effects of two crucial factors, feedstock and pyrolysis temperature, on the hydrogen production-promoting function of biochar in ethanol-type fermentation. The physicochemical characteristics and promoting effects of biochars prepared with five biomass wastes (coffee ground, corn stalk, Ginkgo biloba leaf, mealworm frass, and sugarcane bagasse) were determined. Sugarcane bagasse-derived biochar (SBBC) showed the best hydrogen production-promoting effect in ethanol-type fermentation. The physicochemical properties of biochar, such as pH, element composition and surface features, were significantly affected by pyrolysis temperature, but the promoting effects were not significantly changed. The hydrogen production-promoting effect of biochar in ethanol-type fermentation was mainly affected by feedstock instead of pyrolysis temperature. A potential promoting mechanism was proposed that biochar prepared at low temperature boosted the hydrogen production with redox activity, while that at high temperature achieved the promotion via cell growth enhancement. This study revealed the key promoting factor of biochar in ethanol-type fermentative hydrogen production, and provided novel insights for the promoting mechanism of biochar.

Improvement of Biohydrogen and Usable Chemical Products from Glycerol by Co-Culture of Enterobacter spH1 and Citrobacter freundii H3 Using Different Supports as Surface Immobilization

Glycerol is a by-product of biodiesel production in a yield of about 10% (w/w). The present study aims to improve the dark fermentation of glycerol by surface immobilization of microorganisms on supports. Four different supports were used—maghemite (Fe2O3), activated carbon (AC), silica gel (SiO2), and alumina (γ-Al2O3)—on which a newly isolated co-culture of Enterobacter spH1 and Citrobacter freundii, H3, was immobilized. The effect of iron species on dark fermentation was also studied by impregnation on AC and SiO2. The fermentative metabolites were mainly ethanol, 1,3-propanediol, lactate, H2 and CO2. The production rate (Rmax,i) and product yield (Yi) were elucidated by modeling using the Gompertz equation for the batch dark fermentation kinetics (maximum product formation (Pmax,i): (i) For each of the supports, H2 production (mmol/L) and yield (mol H2/mol glycerol consumed) increased in the following order: FC < γ-Al2O3 < Fe2O3 < SiO2 < Fe/SiO2 < AC < Fe/AC. (ii) Ethanol production (mmol/L) increased in the following order: FC < Fe2O3 < γ-Al2O3 < SiO2 < Fe/SiO2 < Fe/AC < AC, and yield (mol EtOH/mol glycerol consumed) increased in the following order: FC < Fe2O3 < Fe/AC < Fe/SiO2 < SiO2 < AC < γ-Al2O3. (iii) 1,3-propanediol production (mmol/L) and yield (mol 1,3PDO/mol glycerol consumed) increased in the following order: γ-Al2O3 < SiO2 < Fe/SiO2 < AC < Fe2O3 < Fe/AC < FC. (iv) Lactate production(mmol/L) and yield (mol Lactate/mol glycerol consumed) increased in the following order: γ-Al2O3 < SiO2 < AC < Fe/SiO2 < Fe/AC < Fe2O3 < FC. The study shows that in all cases, glycerol conversion was higher when the support assisted culture was used. It is noted that glycerol conversion and H2 production were dependent on the specific surface area of the support. H2 production clearly increased with the Fe2O3, Al2O3, SiO2 and AC supports. H2 production on the iron-impregnated AC and SiO2 supports was higher than on the corresponding bare supports. These results indicate that the support enhances the productivity of H2, perhaps because of specific surface area attachment, biofilm formation of the microorganisms and activation of the hydrogenase enzyme by iron species.

Renewable biohydrogen production from lignocellulosic biomass using fermentation and integration of systems with other energy generation technologies

Biohydrogen is a clean and renewable source of energy. It can be produced by using technologies such as thermochemical, electrolysis, photoelectrochemical and biological, etc. Among these technologies, the biological method (dark fermentation) is considered more sustainable and ecofriendly. Dark fermentation involves anaerobic microbes which degrade carbohydrate rich substrate and produce hydrogen. Lignocellulosic biomass is an abundantly available raw material and can be utilized as an economic and renewable substrate for biohydrogen production. Although there are many hurdles, continuous advancements in lignocellulosic biomass pretreatment technology, microbial fermentation (mixed substrate and co-culture fermentation), the involvement of molecular biology techniques, and understanding of various factors (pH, T, addition of nanomaterials) effect on biohydrogen productivity and yield render this technology efficient and capable to meet future energy demands. Further integration of biohydrogen production technology with other products such as bio-alcohol, volatile fatty acids (VFAs), and methane have the potential to improve the efficiency and economics of the overall process. In this article, various methods used for lignocellulosic biomass pretreatment, technologies in trends to produce and improve biohydrogen production, a coproduction of other energy resources, and techno-economic analysis of biohydrogen production from lignocellulosic biomass are reviewed.

A developed hybrid fixed-bed bioreactor with Fe-modified zeolite to enhance and sustain biohydrogen production

In this study, we describe the development of a hybrid bioreactor with integrated chlorinated polyethylene (CPE) fixed-bed and zeolite as a microorganism nutrition carrier (MNC), aiming at enhancing and sustaining biohydrogen production during the anaerobic digestion (AD) process. In the batch test, the hybrid bioreactor achieved a maximum biohydrogen production of 646.3 mL/L. Accordingly, the hybrid bioreactor significantly enhanced biohydrogen production and maintained a stable performance for 50 days of semi-continuous operation. This result should be attributed to the CPE providing roughness surface and high porosity for microorganism immobilization, resulting in the enhancement of microbial quantity, confirmed by our scanning electron microscope and immobilized biomass analyses. Moreover, the element ratio significantly decreased, indicating that zeolite could provide metal cations for stimulating microbial bioactivity and growth, as well as contributing to superior biohydrogen productivity during the 50-day operation. In order to further enhance and sustain long-term biohydrogen production, raw zeolite was modified with iron. The hybrid-Fe bioreactor (CPE with Fe-modified zeolite) operated mainly following the acetate pathway and exhibited higher sustainability in improving biohydrogen production with a peak value of 1893.0 mL/L during a 72-day-lasting operation. The synergistic mechanism of the Fe-modified zeolite and CPE fixed-bed revealed that it could effectively induce favorable pathways and contribute to the synthesis of essential enzymes, micronutrient supplementation, electoral conductivity, and microbial immobilization for biohydrogen production. Therefore, a hybrid-Fe bioreactor could provide a unique alternative for the enhancement of hydrogen production for practical applications.

Mechanisms of enhanced hydrogen production from sewage sludge by ferrous ion: Insights into functional genes and metabolic pathways

Hydrogen production from sewage sludge was studied in the presence of Fe²⁺. The results showed that the highest cumulative hydrogen production of 26 mL/100 mL was achieved with 600 mg/L Fe²⁺ supplementation, which was 2 times of the control test. In depth analysis of organics in liquid phase revealed that Fe²⁺ addition promoted sludge disintegration and protein degradation during fermentation process. Functions prediction by PICRUSt analysis indicated the effect of Fe²⁺ on microbial metabolism and functional genes expression. The results showed that the expression of hydrogen-producing functions, like ferredoxin hydrogenase and formate dehydrogenase was activated by Fe²⁺ supplementation, while the hydrogen-consuming metabolism, like methane metabolism and homoacetogenic metabolism was inhibited. Furthermore, Fe²⁺ addition could stimulate organics utilization. This study explored the effect of Fe²⁺ on functional genes abundance, revealing the mechanisms of enhanced hydrogen production by Fe²⁺ from the perspective of microbial metabolism.

Residue cornstalk derived biochar promotes direct bio-hydrogen production from anaerobic fermentation of cornstalk

In this study, an innovative approach was proposed based on the implement of biochar derived from residue cornstalk left after anaerobic bio-hydrogen production (RCA-biochar) to improve direct bio-hydrogen production from anaerobic fermentation of cornstalk. The bio-hydrogen production potential and maximum bio-hydrogen production rate increased from 156.2 to 286.1 mL H₂/g substrate and 3.5 to 5.7 mL H₂/g substrate/h, respectively, following the added RCA-biochar increased from 2.5 to 15.0 g/L. Cornstalk chemical component analysis showed the cellulose and hemicellulose content decreased by 17.9-33.7% and 14.4-25.2%, and lignin content increased by 20.3-42.8%, respectively, after 96 h anaerobic fermentation with RCA-biochar 2.5-15.0 g/L. Further analyses revealed that RCA-biochar not only provided more specific surface area for hydrogen-producing bacteria attachment, but also promoted the cellulolytic enzyme activity, thereby resulted in increased substrate conversion to bio-hydrogen.The findings obtained in this study may provide supports for effective and sustainable lignocellulosic bio-hydrogen production in the future.

Advanced bioH2 and bioCH4 production with cobalt-doped magnetic carbon

In this work, a novel cobalt-doped magnetic carbon (CDMC) was prepared to boost hydrogen (H₂) and methane (CH₄) generation. A one-pot approach was employed to produce H₂ and CH₄ with an incompletely heat-treated mixed culture. A moderate amount of CDMC promoted biogas evolution, while excess CDMC eroded both H₂ and CH₄ productivity. The CDMC (600 mg L⁻¹) group achieved the highest biogas yields of 176 mL H₂ per g glucose and 358 mL CH₄ per g glucose, which were higher than those (102 mL H₂ per g glucose and 288 mL CH₄ per g glucose) found in the control group without CDMC. The mechanisms of H₂ and CH₄ production via the one-pot approach with CDMC were speculated to be as follows: CDMC provided beneficial sites and two elements (Co and Fe) for culture growth and boosted electron transfer, facilitating glucose degradation and conversion. Supplementation of carbon matrix composites and trace elements in biogas production has been shown to be an efficient strategy.

In this work, a novel cobalt-doped magnetic carbon (CDMC) was prepared to boost hydrogen (H₂) and methane (CH₄) generation.

Effects of biochar on ethanol-type and butyrate-type fermentative hydrogen productions

Low hydrogen yield was the bottleneck of dark fermentative hydrogen production. To solve this problem, the effects of rice straw-derived biochar on hydrogen production was investigated in different fermentation types. Ethanol-type and butyrate-type fermentations, two dominant types of dark fermentation, were carried out in batch fermentations with different concentrations of biochar. The results revealed that 3 g/L was the best concentration for both types of fermentations. Hydrogen production increased by 118.4% and 79.6% in ethanol-type and butyrate-type fermentations, respectively. The maximal hydrogen yields of ethanol-type and butyrate-type fermentations were 1.34 and 2.36 mol/mol-glucose, respectively. The addition of biochar buffered the broth pH, lowered the redox potential, and released mineral nutrients. The porosity of biochar boosted cell immobilization and thus improved the H2 productivity. This study demonstrated the enhancement effect of biochar on ethanol- and butyrate-type fermentative hydrogen productions, and enhanced the understanding of the functional mechanisms of biochar.

Comparison of Calcium Oxide and Calcium Peroxide Pretreatments of Wheat Straw for Improving Biohydrogen Production

Wheat straw was pretreated with either CaO₂ or CaO to improve biohydrogen production. Both CaO and CaO₂ pretreatments improved the biodegradability of the wheat straw. CaO pretreatment raised the H₂ yield by between 48.8 and 163.9% at CaO contents ranging from 2 to 4%. The highest H₂ yield [144 mL/g total solid (TS)] was obtained at 121 °C and 6% CaO. In addition, the highest H₂ yield from wheat straw pretreated at the same temperature and dosage of CaO₂ was 71.8 mL/g TS, which was higher than that of the control group (43.2 mL/g TS), with hot water (121 °C) treatment. Considering pretreatment costs and H₂ production potential, CaO was a better pretreatment agent than CaO₂.

Exploring the roles of zero-valent iron in two-stage food waste anaerobic digestion

This research investigated the roles of zero-valent iron (ZVI) in a two-stage food waste digestion process. ZVI was added separately to hydrolytic-acidogenic (HA) and methanogenic (MG) stages to understand its impacts on FW hydrolysis-acidification, methanogenesis and bioenergy recovery efficiency. Results showed that ZVI effectively enhanced the overall performance of digestion as compared with the controls without ZVI. Supplementing with ZVI could facilitate the HA process along with faster hydrogen generation. In addition, ZVI shortened the lag phase of MG stage by 42.43-57.23% and raised the maximum methane production rate and yield by 33.99-38.20% and 11-13%, respectively, compared with the controls. Supplementing ZVI to the HA stage could simultaneously raise the bioenergy recovery efficiency of the HA and MG stages by 71.92% and 96.96%, respectively. Further studies demonstrated that iron corrosion contributed little to hydrogen and methane production. The enrichment of syntrophic bacteria, Pseudomonas, and methanogens, and the enhancement of electron transfer among those microbes was supposed to be the main possible mechanism for the enhancement of methanogenesis with ZVI assisted.

Feasibility of enhancing hydrogen production from cornstalk hydrolysate anaerobic fermentation by RCPH-biochar

This study presents a novel approach based on addition of biochar generated from residue of cornstalk left after pretreatment and hydrolysis (RCPH-biochar) to improve hydrogen production from cornstalk hydrolysate. RCPH-biochar at concentration of 15 g L^-1 substantially enhanced hydrogen generation during batch tests, with the highest cumulative hydrogen volume (3990 mL L^-1) being 1.7 times that without RCPH-biochar. Then, continuous hydrogen production performance demonstrated that RCPH-biochar was capable of retaining biomass in the reactor, at 6 h hydraulic retention time, hydrogen production rate (22.8 mmol H₂ L^-1 h^-1) was tripled compared to the control, meanwhile, glucose and xylose utilization reached to 82.3% and 54.6%, respectively. Overall material balance indicates continuous hydrogen production with RCPH-biochar enabled 63.4% higher cornstalk transfer to H₂ and 53.3% more cornstalk utilization. The findings reported is a closed-loop process and is economically and environmentally attractive, which might support comprehensive cornstalk utilization with less energy input in the future.

Biochar facilitated bioprocessing and biorefinery for productions of biofuel and chemicals: A review

Biochar is traditionally used to improve soil properties in arable land and as adsorbent or precursor of activated carbon in wastewater treatment. Recent advances have shown biochar potentials in enhancing productions of biofuels and chemicals such as bio-ethanol, butanol, methane, hydrogen, bio-diesel, hydrocarbons and carboxylic acids. The properties of biochar such as high levels of porosity, functional groups, cation exchange capacity, pH buffering capacity, electron conductivity, and macro-/micro- nutrients (Na, K, Ca, Mg, P, S, Fe, etc.) provide appropriate conditions to relieve physicochemical stresses on microorganisms through pH buffering, detoxification, nutrients supply, serving as electron carrier and supportive microbial habitats. This paper critically reviewed biochar production and characteristics, biochar utilization in anaerobic digestion, composting, microbial fermentation, hydrolysate detoxification, catalysis in biomass refinery and biodiesel synthesis. This review provides novel vision of biochar application, which could guide future research towards cleaner and more economic production of renewable fuels and bio-based chemicals.

Mechanisms of enhanced biohydrogen production from macroalgae by ferrous ion: Insights into correlations of microbes and metabolites

This study explored the mechanisms of the enhanced hydrogen production from macroalgae by Fe²⁺ supplementation. Highest hydrogen yield of 19.47 mL/g VS_added was achieved at Fe²⁺ supplementation of 400 mg/L, which was 6.25 times of the control test. In depth analysis of substrate degradation, microbial distribution and metabolites formation was conducted. The results showed that Fe²⁺-supplemented group was dominated by Clostridium butyricum (67.2%) and Ruminococcus gnavus (24.2%), which stimulated hydrogen generation and volatile organic acids accumulation. In contrast, Fe²⁺-deficient group had a microbial community dominated by Exiguobacterium sp. (29.0%), Acinetobacter lwoffii (24.5%) and Clostridium stricto 13 (23.4%), which induced higher efficiency of both biomass hydrolysis and mineralization. Microbes from a single system were mutually cooperative, while microbes from Fe²⁺-deficient and those from Fe²⁺-supplemented systems were mutually exclusive. This study suggested that Fe²⁺ is critical in macroalgae fermentation system to affect the microbial community structure and subsequently switch the metabolic pathways.

Biohydrogen and polyhydroxyalkanoate production from original hydrolyzed polyacrylamide-containing wastewater

This work aimed to study biohydrogen (H₂) and polyhydroxyalkanoate (PHA) production from original hydrolyzed polyacrylamide (HPAM)-containing wastewater. NH₄⁺-N from HPAM hydrolysis was removed efficiently through short-cut nitrification and anoxic ammonia oxidation (anammox). Carbon/Nitrogen (C/N) ratios of effluent reached 51-97, and TOC decreased only 2%-4%, providing potential for subsequent H₂ and PHA production. The maximum yields of H₂ (0.833 mL·mg^-1_substrate) and Volatile Fatty Acid (VFA) (465 mg·L^-1) occurred at influent C/N ratio of 51. Substrate removal increased linearly with the activities of dehydrogenase and hydrogenase (R² ≥ 0.990), and H₂ yield rose exponentially with enzyme activities (R² ≥ 0.989). The maximum PHA yield (54.2% VSS) occurred at the 42nd hour and influent C/N ratio of 97. PHA yield was positively correlated with substrate uptake. The change of H₂-producing, PHA-accumulating and HPAM-degradating bacteria indicated that those functional microorganisms had synergistic effects on H₂ production and substrate uptake, as well as PHA accumulation and substrate uptake.

Stimulation of ferrihydrite nanorods on fermentative hydrogen production by Clostridium pasteurianum

Conversion of organic matter to biohydrogen possesses promising application potential. In this study, low-cost ferrihydrite nanorods were used to enhance hydrogen production by Clostridium pasteurianum. The maximum cumulative hydrogen production and the hydrogen yield were 1.03 mmol and 3.55 mol H₂/mol glucose, respectively, which were 68.9% and 15.6% higher than those of the batch groups without ferrihydrite addition. Moreover, in comparison with magnetite and hematite nanoparticles, ferrihydrite presented the best stimulation for hydrogen evolution. The enhancement mechanisms were explored based on metabolic distribution, gene expression, enzymatic activity, and metabolite determination, such as Fe(II) concentration and pH value. The potential stimulation mechanisms are summarized as follows: ferrihydrite improves glucose conversion efficiency and promotes cell growth; ferrihydrite elevates the transcripts and activity of hydrogenase; and ferrihydrite reduction via its buffer function could ease acidification. This study demonstrates that ferrihydrite addition is an effective and green strategy to enhance fermentative hydrogen production.

A Review of the Enhancement of Bio-Hydrogen Generation by Chemicals Addition

Bio-hydrogen production (BHP) produced from renewable bio-resources is an attractive route for green energy production, due to its compelling advantages of relative high efficiency, cost-effectiveness, and lower ecological impact. This study reviewed different BHP pathways, and the most important enzymes involved in these pathways, to identify technological gaps and effective approaches for process intensification in industrial applications. Among the various approaches reviewed in this study, a particular focus was set on the latest methods of chemicals/metal addition for improving hydrogen generation during dark fermentation (DF) processes; the up-to-date findings of different chemicals/metal addition methods have been quantitatively evaluated and thoroughly compared in this paper. A new efficiency evaluation criterion is also proposed, allowing different BHP processes to be compared with greater simplicity and validity.

Improving mechanisms of biohydrogen production from grass using zero-valent iron nanoparticles

This paper investigated the improving mechanisms and microbial community dynamics of using zero-valent iron nanoparticles (Fe⁰ NPs) in hydrogen fermentation of grass. Results showed that Fe⁰ NPs supplement improved microbial activity and changed dominant microbial communities from Enterobacter sp. to Clostridium sp., which induced a more efficient metabolic pathway towards higher hydrogen production. Meanwhile, it is also proposed that Fe⁰ NPs could accelerate electron transfer between ferredoxin and hydrogenase, and promote the activity of key enzymes by the released Fe²⁺. The maximal hydrogen yield and hydrogen production rate were 64.7 mL/g-dry grass and 12.1 mL/h, respectively at Fe⁰ NPs dosage of 400 mg/L, which were 73.1% and 128.3% higher compared with the control group. Fe⁰ NPs also shorten the lag time and facilitated the hydrolysis and utilization of grass. This study demonstrated that Fe⁰ NPs could effectively improve hydrogen production and accelerate the fermentation process of grass.

Recent achievements in enhancing anaerobic digestion with carbon- based functional materials

Carbon-based materials such as graphite, graphene, biochar, activated carbon, carbon cloth and nano-tube, and maghemite and magnetite carbons are capable for adsorbing chemicals onto their surfaces. Currently, this review is to highlight the relevance of carbons in enhancing hydrogen or methane production. Some key roles of carbons in improving cell growth, enrichment and activity, and accelerating their co-metabolisms were elaborated with regard to their effects on syntrophic communities, interspecies electron transfer, buffering capacity, biogas upgrading, and fertilizer nutrient retention and land application. Carbons can serve as a habitation for microbial immobilization, and a provision for bioelectrical connections among cells, and provide some essential elements for anaerobes. Besides, an outlook on the possible options towards the large scale and improvement solutions has been provided. Further studies in this area could be encouraged to intend and operate continuous mode by designing carbon-amended bioreactor with stability and reliability.