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Perspectives on Potential Applications of Nanometal Derivatives in Gaseous Bioenergy Pathways: Mechanisms, Life Cycle, and Toxicity
... This decline in hydrogen production may be attributed to a possibility of toxicity to microorganisms caused by high amount of nanosized metals (Cu and Ag) added to the sludge which are known to be bactericidal [32]. Elsamadony et al. [33] stated that elevated concentration nanoparticle with metal derivatives, referring to metals and their oxides, can lead to toxicity and also the establishment of free radicals, which inhibit the growth of microorganisms and reduce the biohydrogen production. This was also confirmed by the results of the microbial structure analysis presented in Section 3.2.2. ...
... These results were superior to those of the batch system under optimal conditions, and this is confirming the possibility of success of the continuous system for the treatment of industrial sludge using the selected nanoparticles. The results obtained were comparable with similar investigations on H 2 production utilizing assisted with nanoparticles [12,33,45]. ...
Biohydrogen production from industrial waste has gained a significant attention as a sustainable energy source. In this study, the enrichment of biohydrogen production from pretreated dissolved air flotation (DAF) sludge, generated from food industry wastewater treatment plants, was investigated using SiO 2 @Cu-Ag dendrites core shell nanostructure (NS). The effect of NS on the changes of the microbial community and biohydrogen yield was evaluated through batch and continuous tests. In batch mode, various nanomaterial doses were investigated with several concentrations ranging from 20 to 50 mg/L for hydrogen production using glucose as a substrate. The optimum core-shell NS amount was 40 mg/L, achieving a maximum H 2 yield of 163 mL/g volatile solids (VS) compared to the control's 79 mL/g VS. However, 50 mg/L NS inhibited most bacteria in the sludge. The continuous experiment used a continuous stirring tank reactor (CSTR) with 40 mg/L SiO 2 @Cu-Ag core-shell NS and pretreated industrial sludge as substrate. The H 2 yield increased to 115 L/kg VS compared to the control reactor's 89 L/kg VS. The gas analysis showed compositional proportions of 83 % H 2 , 7 % CO 2 , and 4.5 % methane, while the microbial community analysis indicated the development of hydrogen-producing species such as Clostridium. In conclusion, SiO 2 @Cu-Ag core-shell NS addition enhanced anaerobic degradation of organic matter and its conversion to biohydrogen. The selected nanomaterial can be used for an effective continuous treatment system for industrial sludge while promoting dark fermentation.
... Recent approaches are applied for AD performance upgrading such as nanomaterials (NMs) supplementation (Elsamadony et al., 2021a) and electrical voltage (EV) application (Baek et al., 2020;Mostafa et al., 2022b). Both approaches mainly targeted the promotion of direct interspecies electron transfer pathway (DIET) (Mostafa et al., 2021b(Mostafa et al., , 2022a. ...
... A recent trend is to utilize NM in a composite form, built by two or more materials chemically bonded, to get the benefits of the building blocks. Composite of carbon and metal based NMs exhibited higher AD process with ability to degrade Eps (Elsamadony et al., 2021a). Further, metal counterpart in NMs catalyzes dissimilatory metal reduction reaction, this was demonstrated by Zhao et al. (2017), who discovered that magnetite (Fe 3 O 4 ) NMs supplementation to AD of wastewater enhanced methane production by 9-fold at OLR of 21.2 kg COD /m 3 /day, and acidification efficiency increased by 80%. ...
The unprecedented recent expansion in usage of paracetamol (AAP) has increased the need for suitable wastewater treatment technology. Furthermore, direct interspecies electron transfer promotion (DIET) offers simple and efficient approach for enhancing anaerobic digestion (AD). In this work, using AAP-containing domestic wastewater as feed, control AD reactor (RC) was operated, besides three DIET-promoted AD reactors (REV, RMC and REVMC, referring to electrical voltage "EV"-applied, nFe3O4-multiwall carbon nanotube (MCNT)-supplemented, and "EV applied + MCNT supplemented" reactor, respectively). Maximal treatable organic loading rates by RC, REV, RMC and REVMC were 3.9, 3.9, 7.8 and 15.6 g COD/L/d, corresponding to AAP loading rate of 26, 78, 156 and 312 μg/L/d, respectively. Methane production rate generated by RC, REV, RMC and REVMC reached 0.80 ± 0.01, 0.86 ± 0.04, 1.40 ± 0.07, and 3.01 ± 0.17 L/L/d, respectively. AAP expectedly followed hydroquinone degradation pathway, causing AD failure by acetate accumulation. However, this performance deterioration could be mitigated by DIET-promoted microbes with higher methanogenic activity and advanced electric conductivity. Economic evaluation revealed the favourability of MCNT addition over EV application, since payback periods for RC, REV, RMC and REVMC were 6.2, 7.7, 4.2 and 5.0 yr, respectively.
... Recently, an enormous interest has been focused on the application of NPs to increase the bioactivity of hydrogen-producing microorganisms in dark hydrogen fermentation with the aim to increase the yield and rate of hydrogen generation. Metal NPs supplementation has been shown to increase ferredoxin-oxidoreductase activity, resulting in increased biohydrogen generation [27]. Theoretically, the larger surface area and the quantum size impacts of NPs might increase the ferredoxin oxidoreductase activity by accelerating the transport of electrons between NADPH and hydrogenases, which would also improve hydrogen production [5,28]. ...
... According to Lin et al. [1], the supplementation of ferric oxide NPs to the dark fermentation system, could transfer more amounts of electrons to the E. aerogenes cells. Similarly, the NiFe 2 O 4 NPs could release Fe(II)/Fe(III), and Ni(II) ions, which served as an electron shuttle to accelerate the extracellular electron transfer process [27,43]. Based on the available reports and from the end metabolite analysis, it was suggested that the addition of magnetic NPs induced modification in the metabolic pathway of hydrogen production, resulting in improved acetate proportion and retarding the ethanol generation in the end metabolites. ...
... On the contrary, the above materials have negative environmental consequences. Elsamadony et al. (2021) reported that the biotic and abiotic relationship of electron flow takes place in both inside and outside of microbes. The inward flow leads to corrosion, whereas the outward anaerobes lead to dissimilatory metal reduction, implying that inward flow increases the reaction rate [14]. ...
... Elsamadony et al. (2021) reported that the biotic and abiotic relationship of electron flow takes place in both inside and outside of microbes. The inward flow leads to corrosion, whereas the outward anaerobes lead to dissimilatory metal reduction, implying that inward flow increases the reaction rate [14]. Feng et al. (2017) observed an enhanced CH 4 through DIET pathways activated by the electroactive bacteria enriched through the electrodes using a small applied voltage from a bioelectrochemical device [15]. ...
Increasing carbon footprint alters the carbon balance in nature, thereby worsening global climate change. The conversion of carbon dioxide (CO2) into value-added products through biological routes is the pathway of the future because of its ecological and sustainable character. The present study evaluated the conversion of CO2 into short-chain fatty acids (SCFA)/volatile fatty acids (VFA) and methane using four experimental conditions (R1-R4). The experimental conditions are R1 was an anaerobic fermenter (AF) operated as control, R2 consisted of an AF with electrodes operated in open circuit, R3 was an AF with electrodes operated in a closed circuit with 100 Ω as load and R4 was an electro-fermentation reactor with an applied cathodic potential of −0.8 V vs. Ag/AgCl. The results were assessed in terms of production of SCFA, methane, current density and inorganic carbon reduction. Electro-fermentation (R4) setup achieved the highest production of SCFA (2050 mg/L) and methane (41.2 mL/day) compared to other reactors. R3 reported 1800 mg/L and 24 mL/day, R2 reported 1560 mg/L and 15 mL/day and R1 reported 1430 mg/L and 10 mL/day of methane and SCFA production. The study-inferred that electro-fermentation could effectively catalyse the biochemical reactions and enhance the conversion of CO2 to organic compounds in a sustainable manner.
... Recent approaches are applied for AD performance upgrading such as nanomaterials (NMs) supplementation (Elsamadony et al., 2021a) and electrical voltage (EV) application (Baek et al., 2020;Mostafa et al., 2022b). Both approaches mainly targeted the promotion of direct interspecies electron transfer pathway (DIET) (Mostafa et al., 2021b(Mostafa et al., , 2022a. ...
... A recent trend is to utilize NM in a composite form, built by two or more materials chemically bonded, to get the benefits of the building blocks. Composite of carbon and metal based NMs exhibited higher AD process with ability to degrade Eps (Elsamadony et al., 2021a). Further, metal counterpart in NMs catalyzes dissimilatory metal reduction reaction, this was demonstrated by Zhao et al. (2017), who discovered that magnetite (Fe 3 O 4 ) NMs supplementation to AD of wastewater enhanced methane production by 9-fold at OLR of 21.2 kg COD /m 3 /day, and acidification efficiency increased by 80%. ...
... ENMs are released into the ambient environment at different stages, including during their synthesis, production, usage, and disposal [54]. Recently, it has been shown that biodigesters also release ENMs into the soil [55]. Subsequently, it has been determined that a greater toxicity risk exists since digesters fed with solid waste create more ENMs than those fed with liquid waste. ...
Various compounds that are emerging contaminants pose a significant risk to aquatic ecosystems and human health due to their potential to harm human health and the environment.Thus, there is an urgent requirement to use effective remediation methods and techniques to minimize the harmful impact of these contaminants on the environment. Biochar (BC) is a lightweight black residue that is made of carbon after the pyrolysis of biomass. BC is a product that is stable, rich in carbon, and exhibited improved properties. BC has come up with fascinating properties and results to remediate these pollutants from the soil effectively. Furthermore, it becomes possible to recover resources using BC because of the benefits such as (a) it offers in terms of cost, (b) the preservation of nutrients, and (c) the efficiency with which it absorbs pollutants. Consequently, it is necessary to have a knowledge of the interaction involving biochar and resource recovery to explore the applicability of BC in the cleaning up of the surroundings and the exploitation of wastewater. This review emphasize the physio-chemical and biological modification methods for the preparation of various types of engineered BC. Therefore, the present review aims: (i) provide an overview of emerging pollutants of human activities in soil (ii) synthesis and engineer BC for field application (iii) critically discuss and evaluate the factors affecting large-scale application techno-economic challenges. The review provided insight into the areas that need immediate attention in the upcoming investigation regarding the use of engineered biochar for wastewater treatment.
... In contrast, the discolouration during non-feeding periods (data did not showed) could be related to oxidation of Fe 2+ from magnetite delivering additional electrons for methanogenesis [15]. Furthermore, magnetite itself is a conductive mineral and has been shown to compensate for the deficiency of e-pilins, enabling probably direct electron transfer from fermenting organisms to methanogens [32,35,36]. ...
Background
Biological conversion of the surplus of renewable electricity and carbon dioxide (CO 2 ) from biogas plants to biomethane (CH 4 ) could support energy storage and strengthen the power grid. Biological methanation (BM) is linked closely to the activity of biogas-producing Bacteria and methanogenic Archaea . During reactor operations, the microbiome is often subject to various changes, e.g., substrate limitation or pH-shifts, whereby the microorganisms are challenged to adapt to the new conditions. In this study, various process parameters including pH value, CH 4 production rate, conversion yields and final gas composition were monitored for a hydrogenotrophic-adapted microbial community cultivated in a laboratory-scale BM reactor. To investigate the robustness of the BM process regarding power oscillations, the biogas microbiome was exposed to five hydrogen (H 2 )-feeding regimes lasting several days.
Results
Applying various “on–off” H 2 -feeding regimes, the CH 4 production rate recovered quickly, demonstrating a significant resilience of the microbial community. Analyses of the taxonomic composition of the microbiome revealed a high abundance of the bacterial phyla Firmicutes , Bacteroidota and Thermotogota followed by hydrogenotrophic Archaea of the phylum Methanobacteriota . Homo-acetogenic and heterotrophic fermenting Bacteria formed a complex food web with methanogens. The abundance of the methanogenic Archaea roughly doubled during discontinuous H 2 -feeding, which was related mainly to an increase in acetoclastic Methanothrix species. Results also suggested that Bacteria feeding on methanogens could reduce overall CH 4 production. On the other hand, using inactive biomass as a substrate could support the growth of methanogenic Archaea . During the BM process, the additional production of H 2 by fermenting Bacteria seemed to support the maintenance of hydrogenotrophic methanogens at non-H 2 -feeding phases. Besides the elusive role of Methanothrix during the H 2 -feeding phases, acetate consumption and pH maintenance at the non-feeding phase can be assigned to this species.
Conclusions
Taken together, the high adaptive potential of microbial communities contributes to the robustness of BM processes during discontinuous H 2 -feeding and supports the commercial use of BM processes for energy storage. Discontinuous feeding strategies could be used to enrich methanogenic Archaea during the establishment of a microbial community for BM. Both findings could contribute to design and improve BM processes from lab to pilot scale.
... A further enlightenment of the use of essential oils as packaging additives is their effect on the taste and smell of food (Marangoni Júnior, Vieira, Jamróz, & Anjos, 2021;Yong & Liu, 2021). The use of nanometals, although in trace amounts, also has its opponents -due to uncertainty about the accumulation of metals in the body or the eventual use of environmentally harmful compounds such as cadmium (Elsamadony et al., 2021;Ganguly, Breen, & Pillai, 2018;Zhu et al., 2019). These limitations are one of the many reasons for the further search for biopolymer films and active ingredients additives, which in the future could successfully replace the use of plastics in packaging. ...
In order to meet one of the greatest problems of food technology, i.e. unwanted food spoilage caused by the action of microorganisms, active or intelligent packaging is being created and used with increasing frequency. At the same time, substitutes for plastics are being sought in order to protect the environment. Biodegradable polymer composites seem to be an extremely promising material that could replace synthetic materials in the future. By encapsulating biologically active substances in a polymer matrix, directional action and controlled release of the substance are possible. The use of ozone, which has become more and more popular especially recently, allows to limit the growth of microorganisms, and the production of ozone derivatives of unsaturated fats allows to reduce the short time of its decomposition.
The aim of this study was to analyse the physicochemical and bacteriostatic properties of innovative chitosan-based films containing encapsulated nanocapsules of ozonated olive oil in two concentrations. The morphology and size of the obtained nanocapsules were determined using Scanning Electron Microscopy and DLS. The largest particle size was observed for the film containing the lower ozone concentration – up to 4900 nm, in the control film 3500 nm, for the film with the highest ozone concentration, the particle size was the smallest, with the majority in the range of 100–1000 nm. Ozone content was determined by peroxidation number. The properties of the composites were characterised by infrared (IR) which proved that chitosan is a good matrix for the formation of capsules and ultraviolet (UV) which showed that the obtained composite absorbs light in the range of blue and near ultraviolet light, and the addition of nanocapsules of ozonated olive oil increased the degree of absorption and decreased the transparency. We also performed photoluminescence spectroscopy before and after storage of pork meat proving that films are sensitive to the physico-chemical and biochemical transformation products formed as a result of meat spoilage.. The study of the microbiological properties included the analysis of the microbiological quality of pork meat stored under the obtained films as a function of time. The films have a bactericidal and bacteriostatic effect and the addition of ozonated olive oil capsules increases the bactericidal effect against Gram-negative bacteria. Contact angles, Water Vapour Transmission Rate and transparency were also determined and they were subjected to enzymatic hydrolysis. Water absorption, solubility and liquid absorption capacity were determined. All tested samples show high hydrophobicity and low solubility, and the ozone content directly influences these parameters.
... Furthermore, nanoparticles behave as O 2 scavengers, attempts to remove unwanted O 2 existence during fermentation and thus lowering the redox potential. This creates an appropriate anaerobic condition for such action of the hydrogenase enzyme, resulting in increased biohydrogen yield [10]. Furthermore, using nanomaterials in the pre-treatment phase of biomass (lignocellulose enriched) might very well enhance lignin removal, increasing carbohydrate yield as well as speeding up the processing time [3]. ...
Hydrogen has a negligible share on the global fuel market, yet it attracts a lot of investors. The main obstacle to the development of the hydrogen economy is its low cost-competitiveness. In order to meet energy demand and mitigate environmental damage, it is advisable to replace the existing fossil fuels with technologies that are more environmentally friendly and cost-competitive at the same time. Nowadays, some 97 % of hydrogen production comes from steam reforming of natural gas via energy that is obtained from fossil fuels. The production costs for 1 kg of hydrogen produced in this way, are between 2 and 4 €, while approximately 10 kg of CO2 is emitted. The production cost of hydrogen produced by electrolysis from water is about 7 €, 80 % of which is electricity cost. The production of (bio)hydrogen (via photobiological and dark fermentation techniques) from biowaste using renewable energy sources has recently come to the fore. This review discusses use of various types of nanoparticles (organic and inorganic) in (bio)hydrogen production. A diversity of organisms, in pure as well as mixed forms, could perhaps produce (bio)hydrogen using pure (preferably simple form) carbohydrates and biowaste as a feedstock in the existence of various forms of nanoparticles. Furthermore, the (bio)hydrogen production potential (and cost), have indeed been reported to change considerably depending on what type of nanoparticles used as well as their dosage.
... The presence of elements (K, Ca, Mg, Fe, Zn, Mn, S and P) in OSW could help in raising both enzymes sectretion and enzymatic activity (Elsamadony et al., 2021;Mostafa et al., 2021c;Zhang et al., 2012). Behaviour of elements (K, Ca, Mg, Fe, Zn, Mn, S and P) during digestion process is shown in Fig. 8a-h. ...
Onion skin waste (OSW) is common waste in developing countries, which can cause severe environmental pollution when not properly treated. Value-added products can be chemically extracted from OSW; however, that process is not economically feasible. Alternatively, dry anaerobic digestion (DAD) of OSW is a promising approach for both energy recovery and environment protection. The main hurdles during DAD of OSW can be the hydrolysis and acidification. In batch tests, sludge digestate (SD) rich with methanogens was co-digested with different fractions of OSW for enhancing hydrolysis and raising biogas productivity. The cumulative biogas production (CBP) was 36.6 ± 0.3 mL for sole DAD of SD (100% SD) and increased up to 281.9 ± 14.1 mL for (50% SD: 50% OSW) batch. Self-delignification of OSW took place by SD addition, where the lignin removal reached 75.3 ± 10.5% for (85% SD: 15% OSW) batch. Increasing the fraction of OSW (45% SD: 55% OSW) reduced the delignification by a value of 68.8%, where initial lignin concentration was 9.48 ± 1.6% in dry weight. Lignin breaking down resulted a high fraction of phenolic compounds (345.6 ± 58.8 mg gallic acid equivalent/g dry weight) in the fermentation medium, causing CBP drop (219.0 ± 28.5 mL). The presence of elements (K, Ca, Mg, Fe, Zn, Mn, S and P) in OSW improved the enzymatic activity, facilitated phenolic compounds degradation, shifted the metabolism towards acetate fermentation pathway, and raised biogas productivity. Acidogenesis was less affected by phenolic compounds than methanogenesis, causing higher H2 contents and lower CH4 contents, at batches with high share of OSW.
... In this study, a high concentration of NPs (2 g L −1 ) was selected according to previous studies (Dong et al., 2022;Gonzalez-Estrella et al., 2013;Sheng and Liu, 2017) due to the use of mixed cultures in cathodic conditions. In this study, the toxicity of Ni and Cu NPs could be resulting from the dissolution of toxic ions, which can bind to proteins or enzymes, damage the phospholipid membrane or genetic material, as well as elicit oxidative stress (Elsamadony et al., 2021;Zhu et al., 2021). In fact, the concentration of copper ion reached 10 mg L −1 in the catholyte. ...
Concerns about energy crisis and CO 2 emission have motivated the development of microbial electrosynthesis (MES); recent studies have showed the potential of novel slurry-electrode MES. In this study, the effect of non-precious metal nanoparticles (NPs) on the performance of slurry-electrode MES was systematically evaluated in terms of chemical production, physicochemical properties, electrochemical characterization, and microbial community. Ni and Cu NPs increased the lag period from 6 to 15 days for acetate production, while Mo NPs showed no apparent effect. However, these metal NPs slightly affected the final total acetate production (ca. 10 g L −1), Faradic efficiency (ca. 50%), net water flux across the anion exchange membrane (ca. 6 mL d −1), or electrochem-ical characterization of catholyte. BRH-c20a was enriched as the dominated microbe (>48%), and its relative abundance was largely affected by the addition of metal NPs. This study demonstrates that metal NPs affect the performance of biocathodes, mainly by shaping the microbial community.
... In the recent past, world is witnessing the development in the field of engineered nanomaterials, especially, nanoparticles and nanocomposites and their widespread applications in the field of science and technology. Although, nanomaterials have solved challenges in the fields of engineering, biotechnology, biomedical and life sciences, however the synthesis and disposal of nanomaterials is toxic to the environment as well as are uneconomical (Elsamadony et al., 2021;Rahman et al., 2018). Therefore, due to increasing array of applications of nanotechnology, it is mandatory to look for simple, economical and green process for the synthesis of nanoparticles and nanomaterials from bio-based substrates (Makarov et al., 2014). ...
A simple, time efficient process for valorization of lignin into lignin nanoparticles (LNPs) and further into nanogels was developed. LNPs were morphologically spherical with average size of ~136 nm as observed through SEM and particle size analysis. LNPs were applied for nanogel synthesis through UV-curing process in indigenously designed and developed curing cabinet by incorporating them into acrylamide, bis-acrylamide and 1-hydroxycyclohexyl phenyl ketone acting as monomer, cross-linker and photoinitiator, respectively. Nanogels were characterized through SEM, FTIR and compared with hydrogel (gel without LNPs). Mechanical properties of nanogel were analyzed through compression test. Nanogel was found to have better load bearing capacity and Young's modulus (0.8 kPa), which was 4× higher than hydrogel (0.2 kPa). Further, nanogel was used for water retention application and showed 1.6 times higher water retention capacity as compared to hydrogel. Developed nanogel has suitable application in drought prone areas due to its water retention capacity.
... Co, Fe, Cu, Se, and Ni) which are often used in anaerobic digesters for stimulating growth of microorganisms to enhance biogas production [26]. Elsamadony et.al [27] reported that the trace elements residues in the digestate could pose a potential threat due to its possibility of passing into food chain through plants when released in the environment. In addition, the higher amount of residual biodegradable organic material present in digestate can be further converted into an energy carrier [24,28]. ...
Digestate, residual organic matter produced as byproducts during the anaerobic digestion process, needs to be utilized because of its potential greenhouse gas emissions. An alternative solution is to valorize digestate for biogas production. The current study investigates feasibility of simultaneous thermal post-treatment of digestate and in-situ biogas upgrading in the trickling filter bed (TFB) reactor. Thermal post-treatment improved the solubilization of food waste-based digestate for biogas production. It was reported that the TFB reactor are efficient to facilitate gas-liquid mass transfer during production of methane (CH4) from carbon dioxide (CO2) and hydrogen (H2). The exogenous H2 injection in in-situ biogas upgrading decreased CO2 content from 51.35 to 16.72% and upgrade CH4 content from 48.65 to 81% in the output gas. The reactor CH4 production rate reached 0.89 L/LR•d and the H2 utilization efficiency of 98% was accomplished. Furthermore, the results showed that chemical oxygen demand removal efficiency of digestate reached to a maximum of 64% in 10 days hydraulic retention time. The microbial analysis indicated hydrogenotrophic Methanobacterium ferruginis was highly abundant in the liquid and biofilm phase, while acetoclastic Methanosarcina flavescens localized in the biofilm phase. The findings from this work showed that digestate can be simultaneously exploited for improving effluent quality and biogas upgrading.
... A similar observation was reported when SnO 2 nanoparticles were applied as a biocathode to upgrade the biogas where CO 2 was reduced to formate, stimulating the concentration of the 90% CH 4 in off-gas stream (Gao et al., 2021). These studies show that the electrode material development and its spatial surface modification are key strategies to optimize the electrode-microbe interactions, thereby reducing CO 2 fraction from biogas while upgrading (Elsamadony et al., 2021). ...
Microbial electrochemical approach is an emerging technology for biogas upgrading through carbon dioxide (CO2) reduction and biomethane (or value-added products) production. There are limited literature critically reviewing the latest scientific development on the Bioelectrochemical (BES) based biogas upgrading technology, including CO2 reduction efficiency, methane (CH4) yields, reactor operating conditions, and electrode material tested in BES reactor. This review analyzes the reported performance and identifies the crucial parameters to be considered for future optimization, which is currently missing. In this review, the performances of BES approach of biogas upgrading under various operating settings in particular fed-batch, continuous mode in connection to the microbial dynamics and cathode materials have been thoroughly scrutinized and discussed. Additionally, other versatile application options associated with BES based biogas upgrading, such as resource recovery, are presented. The three-dimensional electrode materials have shown superior performance in supplying the electrons for the reduction of CO2 to CH4. Most of the studies on the biogas upgrading process conclude hydrogen (H2) mediated electron transfer mechanism in BES biogas upgrading.
Juncus effusus fibers (JEFs) are considered “natural aerogel materials,” which have been used in the textile field in China for > 2700 years. However, the brittle fracture of JEFs limits their wide application. Herein, the influences of different delignification pretreatments, including NaOH aq., NaClO2 aq., H2O2 aq., and deep eutectic solvents (DES), on the structure, morphology, and physical and chemical properties of JEFs have been investigated. The modified JEFs treated with NaOH aq. exhibited the most significant weight loss (30%), and their three-dimensional (3D) cylindrical shape was torn apart because of the strong delignification effect. However, the other three methods exhibit delignification processes for structural retention. Particularly, NaClO2 aq. could selectively remove the majority of lignin/hemicelluloses, resulting in modified JEFs whose tensile strength and specific surface area increased by 62.7% and 481.3%, respectively. On this basis, high efficiency oil-absorbing fiber and strain/humidity monitoring sensor were prepared by dip coating-drying method. The understanding of modified fibers gained in this study enables us to easily improve the JEFs properties of a given application.
The autochthonous microbial community from excess sludge was regulated for enhanced conversion of CO2 to acetate without exogenic H2. It was interesting that the acetate-fed system exhibited a surprising performance to regulate the microbial community for a high acetate yield and selectivity. As a result, some hydrogen-producing bacteria (e.g., Proteiniborus) and acetogenic bacteria with the ability of CO2 reduction were enriched by acetate feeding, 2-bromoethanesulfonate (BES) addition and CO2 stress. When the selected microbial community was applied to convert CO2, the accumulation of acetate was positively correlated to the concentration of yeast extract. Finally, the acetate yield reached up to 67.24 mM with a high product selectivity of 84% in the presence of yeast extract (2 g/L) and sufficient CO2 in semi-continuous culture for 10 days. This work should help get new insights into the regulation of microbial community for the efficient acetate production from CO2.
The synergistic impact of Tween 80 (T80) and
reduced graphene oxide (RGO) on the codigestion process of food
waste and biscuit wastewater via an anaerobic sequential batch
reactor was investigated in this study. The T80/RGO-amended
reactor exhibited a hydrogen yield of 313.2 ± 89.3 mL/gCODinitial,
which is 1.98-fold higher than that of the control. This increase was
a consequence of the increase of hydrogenase enzyme activity from
0.24 ± 0.11 to 0.46 ± 0.15 mg MBreduced/min, as well as the
stimulation of hydrolytic enzyme activities (i.e., α-amylase,
xylanase, CM-cellulase, polygalacturonase, protease, and lipase).
Likewise, microbial community analysis revealed that the relative
abundances of genera Acinetobacter, Syntrophomonas, and Clostridium (H2-producing species) were enhanced from 3.9, 2.3, and
2.8% to 10.8, 9.1, and 3.6%, respectively, when T80/RGO was supplemented. On the other hand, molecular docking analysis was
performed to explore the interaction of the hydrogenase protein with RGO (binding score: −10.1 kcal/mol) and T80 (binding
score: −4.1 kcal/mol) as possible targets with active site residues. Eventually, supplementation of the T80/RGO mixture could
stimulate the efficiency of the fermentative hydrogen process and, consequently, be used in practical engineering applications.
Biohydrogen productions from xylose and glucose under dark condition were enhanced by the presence of natural Fe3O4. The electron equivalent of H2 fractions accounted for 4.55% and 5.69% of the total given xylose and glucose in the experiments without Fe3O4, and that were correspondingly increased to 5.14% and 6.50% in the experiments with 100 mg L⁻¹ of Fe3O4, respectively. Moreover, Fe3O4 increased the total intracellular NAD(H) concentrations by 8.84% and 8.37%, and boosted the ratios of NADH/NAD⁺ by 8.33% and 17.72% in xylose and glucose fermentation, respectively, comparing to the corresponding control experiments. The formation of electron couples of Fe(III)/Fe(II) during the iron oxide respiration and more generation of active extracellular polymeric substances components were determined as the important reasons for the improved biohydrogen production performance. Thus, a promotion mechanism of the internal “driving forces” from extracellular iron oxide respiration on the biohydrogen production was proposed.
Despite having high-rate methanogenic performance, up-flow anaerobic sludge blanket reactor still has challenges regarding long-start up period (3-8 months) for granulation. In this study, “electrical voltage (EV, 0.3 V) application” was attempted for facilitating granulation in the continuous operation with increased organic loading rates (0.5-11.0 kg COD/m³/d). Up to 11.0 kg COD/m³/d, EV-reactor exhibited the stable performance, while the control failed. After 49 days of operation (at 7 kg COD/m³/d), the granules collected from EV-reactor had larger diameter (2.3 vs. 1.6 mm), higher settling velocity (2.6 vs. 1.9 cm/s), and higher hydrophobicity (52.1% vs. 34.5%), compared to the control. EV application also increased the specific methanogenic activity for propionate and hydrogen almost by two times. The relative abundance of Pseudomonas sp. (quorum sensing (QS)-related microbe) in EV-reactor was 17% higher than that in the control. In addition, EV application increased the expression of QS genes significantly by 27 times.
Anaerobic digestion (AD) encounters several challenges such as low digestibility of substances and biomethane production. Recently, extensive research has been conducted on biochar (BC) inclusion in the AD to mitigate inhibitors, promotes process stability, and increase biomethane production. However, a comprehensive review enclosing in-depth the effects of BC at each stage of AD, reduction of inhibitors, and microbial dynamics have not been reported. In this review, the influence of BC on the improvement of the AD system at hydrolysis, acidogenesis, acetogenesis, and methanogenesis along with microbial aggregation have been emphasized. The BC can provide a specific surface area for microbial colonization, enrich specific microbes, and enhance direct interspecies electron transfer (DIET). The BC has been reported to shorten the lag phase up to 20-64.4% and increase biomethane production up to 60-90%. The BC promoted the abundance of acetoclasts (including Methanosarcina, Methanosaeta, and Methanothrix) and hydrogenotrophs (Methanobacteria and Methanoculleus). A few reports have applied BC in pilot-scale AD. However, it still needs validation for both technical and economic feasibility. The BC combined AD digestate can serve as a soil conditioner to improve soil properties and promote crop production.
Anaerobic digestion provides an important approach for food waste treatment and valorization, yet a considerable amount of digestate is produced. The appropriate management and utilization of food waste anaerobic digestate is highly desirable for solving both environmental and economic concerns currently. This work innovatively develops a natural potential difference assisted landfill technology (shown as BESAL) for food waste digestate treatment and energy recovery. The results demonstrate the electrochemical assistant accelerates the stabilization of digestate, provides extra 14.89% of organic matter removal and 20.92 mW/m2 of electrical energy recovery over conventional treatment. BESAL promotes the removal of soluble matters in digestate extraction, prevents 13.07 mg/g ammonium-N and 32.87% of total VFAs from accumulation. BESAL also performs gene level stabilization by inhibiting/eliminating microbial and pathogenic gene to ensure the biosafety in its product. Integrated landfill with bioelectrochemical assistance provides a promising option for organic waste stabilization and valorization.
Microbial electrolysis cell (MEC) coupled anaerobic digestion (AD), named as MEC-AD system, can effectively promote methane production under ammonia inhibition, but the inherent mechanism is still poorly understood. This study comprehensively explored the MEC-AD performance and mechanism under high-concentration ammonia stress including using proteomic analysis. It was found that the methane generation rates in MEC-AD systems were 2.0–2.7 times that of AD ones under 5.0 g/L ammonia stress. Additionally, the experimental conditions for methane generation in MEC-AD systems were optimized using response surface methodology. Further analysis indicates that the activities of acetate kinase and F420 were improved, and particularly the direct interspecies electron transfer (DIET) was promoted in MEC-AD systems, as indicated by increased electroactive extracellular polymeric substance, decreased charge transfer resistance, and enrichment of electroactive microbes such as Geobacter on the bioelectrodes. Moreover, proteomic analysis reveals that the DIET associated proteins such as Cytochrome C was up-regulated, and ammonia transfer-related proteins were down-regulated and ammonium detoxification-related proteins were up-regulated in MEC-AD systems. This work provides us a better understanding on the MEC-AD performance especially for the treatment of wastewater containing high-concentration ammonia.
Fe2O3 nanoparticles have been reported to enhance the dechlorination performance of anaerobic systems, but the underlying mechanism has not been clarified. This study evaluated the technical feasibility, system stability, microbial biodiversity and the underlying mechanism involved in a Fe2O3 nanoparticle-coupled anaerobic system treating 4-chlorophenol (4-CP) wastewater. The results demonstrated that the 4-CP and total organic carbon (TOC) removal efficiencies in the Fe2O3-coupled up-flow anaerobic sludge blanket (UASB) were always higher than 97% and 90% during long-term operation, verifying the long-term stability of the Fe2O3-coupled UASB. The 4-CP and TOC removal efficiencies in the coupled UASB increased by 42.9±0.4% and 27.5±0.7% compared to the control UASB system. Adding Fe2O3 nanoparticles promoted the enrichment of species involved in dechlorination, fermentation, electron transfer and acetoclastic methanogenesis, and significantly enhanced the extracellular electron transfer ability, electron transport activity and conductivity of anaerobic sludge, leading to enhanced 4-CP biodegradation performance. A possible synergistic mechanism involved in enhanced anaerobic 4-CP biodegradation by Fe2O3 nanoparticles was proposed.
Microbial electrochemical system (MES) for enhancing methane production has gained significant interest during the recent years, but the practical applications of MES are still far away due to several limitations such as low efficiency of cathodic electrochemical kinetics. In this study, novel porous reduced-graphene oxide/hollow titania (rGO/TiO2) was successfully synthesized to be used as cathode catalyst for promoting electrochemical reduction of CO2 to methane. The MES operation with rGO/TiO2 catalyst exhibited 15.4% higher methane yield (0.383 ± 0.01 LCH4/gCOD) and 13.4% higher production rate (152.38 mL/L.d) compared to control MES with bare carbon cloth cathode. The MES-rGO/TiO2 produced around 33% higher in total Coulomb at 3837.9 ± 351.5C compared to the pristine cathode at 2887.92 ± 254.6C. Substrate degradation and volatile fatty acids conversion were significantly improved in the presence of rGO/TiO2 catalyst. By using cyclic voltammetry and electrochemical impedance spectroscopy analysis, rGO/TiO2 was proved to ease the electron transfer efficiency of working cathode for the conversion of electron to methane. The results suggest that porous rGO/TiO2 can be a promising cathode catalyst to upgrade the performance of a scalable methane-producing MES-AD system.
The methanogenic performance and microbial community of the thermophilic anaerobic mono-digestion and co-digestion of food waste and sewage sludge in a high-solid membrane bioreactor were investigated by a continuous experiment. The methane recovery rate of the system reached 98.0% and 89.0% when the substrate was pure food waste and 25% sewage sludge substitution, respectively. Kinetics characterization showed that hydrolysis was the rate-limiting step in both mono-digestion and co-digestion while methanogenic performance and microbial community were significantly affected by feed condition. The dominant archaea for methane generation shifted from Methanothermobacter thermophilus (72.82%) to Methanosarcina thermophila (96.25%) with sewage sludge gradually added from 0% to 100% in the substrate. The relationships between digestion performance, such as the accumulation of soluble proteins in the reactor, and functional microbial groups were also carefully analyzed. Finally, reasonable metabolic pathways for mono-digestion and co-digestion were summarized.
Microbial electrochemical processes are primary platforms for generating electricity or value-added products by relying on the interaction between electroactive microorganisms and electrodes by utilizing electron carriers like hydrogen and enzyme through the oxidation–reduction reactions. Microbial electrosynthesis (MES), initially introduced as electricity-driven bioproduction from CO2, offered a novel pathway to produce biochemicals that eventually contribute to the CO2 sequestration. While most of the previous reviews concentrate on these microbial electrochemical platforms jointly referred to as MXC, such as Microbial fuel cell and microbial electrolysis cell, MES has grown tremendously in recent years, requiring a severe update on the scientific information on this topic. In this mini-review, the significant achievements in MES, specifically towards the production of a wide array of specialty chemicals, have been addressed by summarizing the recent scientific breakthroughs of the MES technology. Furthermore, improving MES's performance through modification of electrodes and membranes and outlook section with the technical challenges and probable solutions have been discussed. The review summarizes the technological drawbacks of the MES's towards a sustainable commercial platform for industrial commodities production and proposes ways to overcome the existing technical challenges in a nutshell towards turning MES in a full-fledged industrial-scale production platform.
This is a critical review regarding the enhancement of biomethane production through a syntrophic DIET process using conductive support materials focusing on the use of magnetite and granular activated carbon. These materials can accelerate electron transfer in methanogenic systems, relieving enzymatic activities required for hydrogen/formate transfer. However, the intrinsic limitations of DIET in batch and continuous reactors amended with conductive materials still require further investigations to understand which are the bottlenecks within these biosystems. On basis of this demand, this review raises a critical discussion on three methodological aspects regarding studies for DIET stimulation focused on using magnetite and activated carbon, two of the most easily accessible materials with high electrical conductivity used in methanogenic systems: i) the need for more investigation with long-term operation of biological reactors to identify possible inhibitory phenomena associated with conductive materials; ii) the importance of performing at least two different control tests in batch assays to disclose the potential effect of methanogenic DIET-based process; and iii) adopting a mass ratio between abiotic material and biomass to define the inhibitory range for conductive material dosage. The consideration of these three methodological aspects can lead to the development of more comprehensive and efficient strategies for the scale-up of methanogenic DIET-based systems amended with conductive materials.
Arsenic (As) contamination has emerged as a serious public health concern worldwide because of its accumulation and mobility through the food chain. Therefore, the current study was planned to check the effect of Bacillus subtilis-synthesized iron oxide nano particles (Fe3O4 NP) on rice (Oryza Sativa L.) growth against arsenic stress (0, 5, 10 and 15 ppm). Iron oxide nanoparticles were extracellular synthesized from Bacillus subtilis with a desired shape and size. The formations of nanoparticles were differentiated through UV-Visible Spectroscopy, FTIR, XRD and SEM. The UV-Visible spectroscopy of Bacillus subtilis-synthesized nanoparticles showed that the iron oxide surface plasmon band occurs at 268 nm. FTIR results revealed that different functional groups (aldehyde, alkene, alcohol and phenol) were present on the surface of nanoparticles. The SEM image showed that particles were spherical in shape with an average size of 67.28 nm. Arsenic toxicity was observed in seed germination and young seedling stage. The arsenic application significantly reduced seed germination (35%), root and shoots length (1.25 and 2.00 cm), shoot/root ratio (0.289), fresh root and shoots weight (0.205 and 0.260 g), dry root and shoots weight (6.55 and 6.75 g), dry matter percentage of shoot (12.67) and root (14.91) as compared to control. Bacillus subtilis-synthesized Fe3O4 NPs treatments (5 ppm) remarkably increased the germination (65%), root and shoot length (2 and 3.45 cm), shoot/root ratio (1.24) fresh root and shoot weight (0.335 and 0.275 mg), dry root and shoot weight (11.75 and 10.6 mg) and dry matter percentage of shoot (10.40) and root (18.37). Results revealed that the application of Fe3O4 NPs alleviated the arsenic stress and enhanced the plant growth. This study suggests that Bacillus subtilus-synthesized iron oxide nanoparticles can be used as nano-adsorbents in reducing arsenic toxicity in rice plants.
Anaerobic digestion was one of the first bioenergy strategies developed, yet the interactions of the microbial community that is responsible for the production of methane are still poorly understood. For example, it has only recently been recognized that the bacteria that oxidize organic waste components can forge electrical connections with methane-producing microbes through biologically produced, protein-based, conductive circuits. This direct interspecies electron transfer (DIET) is faster than interspecies electron exchange via diffusive electron carriers, such as H2. DIET is also more resilient to perturbations such as increases in organic load inputs or toxic compounds. However, with current digester practices DIET rarely predominates. Improvements in anaerobic digestion associated with the addition of electrically conductive materials have been attributed to increased DIET, but experimental verification has been lacking. This deficiency may soon be overcome with improved understanding of the diversity of microbes capable of DIET, which is leading to molecular tools for determining the extent of DIET. Here we review the microbiology of DIET, suggest molecular strategies for monitoring DIET in anaerobic digesters, and propose approaches for re-engineering digester design and practices to encourage DIET.
Various pretreatment methods have been combined and employed for maximizing the solubilization of waste-activated sludge (WAS). However, the question “by changing the series of applied combined pretreatments (CPs), can the solubilization efficiency of WAS be affected?” has never been addressed. In this study, firstly, thermal (T), alkaline (A), and ultrasonic (U) pretreatments were individually applied at broad strengths (T = 80–120 °C for 30 min, A = pH 9–12, and U = 5–60 min at 300 W). Then, pretreatment conditions that caused similar solubilization (13.0%) (120 °C, pH 11, and 30 min for T, A, and U, respectively), were adopted for CP with reverse sequences of T&A, U&A, and T&U. A similar disintegration degree was observed in U→A and A→U, while a meaningful difference was found in T&A and T&U: T→A (28.3%), A→T (42.9%), T→U (22.9%), and U→T (27.1%). The difference in pretreatment series also affected the characteristics of soluble matters, which was analyzed by excitation emission matrix and molecular weight distribution. Due to these differences, the highest methane yield of 68.8% (based on (chemical oxygen demand) CODinput) was achieved at A→T, compared to T→A (62.3%). Our results suggested a simple strategy for increasing solubilization, at the same expense of energy, which might be beneficial in the following treatment process, such as dewatering and transportation.
Biogas production via anaerobic digestion (AD) of waste is a very attractive and challenging task. A slow rate of biodegradation and the presence of impurities in biogas propose the whole process for several risks. However, the adoption of nanoparticles (NPs) to the AD process can influence its performance and stability. The present work has classified the NPs used in AD into three categories: metal oxide NPs, zero-valent metallic NPs, and carbon-based NPs. The addition of metal oxide NPs showed the mixed effects on biogas production as it is depending on the concentration, types, and size of NPs. Zero-valent metallic NPs could be regarded as the most promising for increasing biogas production especially the Ni and Co NPs. Carbon-based NPs showed an appositive effect on the concentration of ammonia. NP mixtures greatly decreased H2S production. Usage of the waste containing NPs generated by other industries and improve the methods to recover the NPs inside AD reactors could decrease the environmental risk of NPs. Decreasing NPs’ price and use low concentrations for enhancing biogas production (by over 90%, compared to control) would be more economically justified.
As a potential approach for enhanced energy generation from anaerobic digestion, iron-based conductive nanoparticles have been proposed to enhance the methane production yield and rate. In this study, the impact of two different types of iron nanoparticles, namely the nano-zero-valent-iron particles (NZVIs) and magnetite (Fe3O4) nanoparticles (NPs) was investigated, using batch test under mesophilic conditions (35 °C). Magnetite NPs have been applied in doses of 25, 50 and 80 mg/L, corresponding to 13.1, 26.2 and 41.9 mg magnetite NPs/gTS of substrate, respectively. The results reveal that supplementing anaerobic batches with magnetite NPs at a dose of 25 mg/L induces an insignificant effect on hydrolysis and methane production. However, incubation with 50 and 80 mg/L magnetite NPs have instigated comparable positive impact with hydrolysis percentages reaching approximately 95% compared to 63% attained in control batches, in addition to a 50% enhancement in methane production yield. A biodegradability percentage of 94% was achieved with magnetite NP doses of 50 and 80 mg/L, compared to only 62.7% obtained with control incubation. NZVIs were applied in doses of 20, 40 and 60 mg/L, corresponding to 10.8, 21.5 and 32.2 mg NZVIs/gTS of substrate, respectively. The results have shown that supplementing anaerobic batches with NZVIs revealed insignificant impact, most probably due to the agglomeration of NZVI particles and consequently the reduction in available surface area, making the applied doses insufficient for measurable effect.
Since the observation of direct interspecies electron transfer (DIET) in anaerobic mixed cultures in 2010s, the topic “DIET-stimulation” has been the main route to enhance the performance of anaerobic digestion (AD) under harsh conditions, such as high organic loading rate (OLR) and the toxicants’ presence. In this review article, we tried to answer three main questions: (i) What are the merits and strategies for DIET stimulation? (ii) What are the consequences of stimulation? (iii) What is the mechanism of action behind the impact of this stimulation? Therefore, we introduced DIET history and recent relevant findings with a focus on the theoretical advantages. Then, we reviewed the most recent articles by categorizing how DIET reaction was stimulated by adding conductive material (CM) and/or applying external voltage (EV). The emphasis was made on the enhanced performance (yield and/or production rate), CM type, applied EV, and mechanism of action for each stimulation strategy. In addition, we explained DIET-caused changes in microbial community structure. Finally, future perspectives and practical limitations/chances were explored in detail. We expect this review article will provide a better understanding for DIET pathway in AD and encourage further research development in a right direction.
Anaerobic digestion of municipal and other organic waste is a microbial process for conversion of complex organic substances to biogas (a renewable energy source) comprising a mixture of methane and CO2, and a stabilized sludge, which may be used as an organic fertilizer. Diverse groups of the methanogenic microbial community degrade complex organic compounds into simple fermentation products such as hydrogen, formate, acetate, short-chained volatile fatty acids, ethanol, etc. These low-molecular mass products act as the substrates and carriers involved in biogas production by syntrophic bacteria and methanogenic archaea at the methanogenesis stage, the last stage of the anaerobic process. The present review discusses syntrophic interactions between the microorganisms involved in anaerobic degradation of organic substances, as well as two types of interspecies electron transfer (IET): indirect IET (IIET, Indirect Interspecies Electron Transfer) and direct IET (DIET, Direct Interspecies Electron Transfer). DIET-based syntrophic interactions between microorganisms may be stimulated by adding conductive materials into anaerobic digesters, which may have the potential for practical applications.
This study investigated the impact of stimulating direct interspecies electron transfer (DIET), by supplementing nano-sized magnetite (nFe3O4, 0.5 g Fe/g VSS) and carbon nanotubes (CNT, 1 g/L), in anaerobic digestion of oleic acid (OA) at various concentrations (0.10 - 4.00 g chemical oxygen demand(COD)/L). Both supplementations could enhance CH4 production, and its beneficial impact increased with increased OA concentration. The biggest improvements of 114% and 165% compared to the control were achieved by nFe3O4 and CNT, respectively, at OA of 4 g COD/L. The enhancement can be attributed to the increased sludge conductivity: 7.1 ± 0.5 (control), 12.5 ± 0.8 (nFe3O4-added), and 15.7 ± 1.1 µS/cm (CNT-supplemented). Dissolved iron concentration, released from nFe3O4, seemed to have a negligible role in improving CH4 production. The excretion of electron shuttles, i.e., humic-like substances and protein-like substances, were found to be stimulated by supplementing nFe3O4 and CNT. Microbial diversity was found to be simplified under DIET-stimulating conditions, whereby five genera accounted for 88% of the total sequences in the control, while more than 82% were represented by only two genera (Methanotrix concilli and Methanosarcina flavescens) by supplementing nFe3O4 and CNT. In addition, the abudance of electro-active bacteria such as Syntrophomonas zehnderi was significantly increased from 17% to around 45%.
Conductive magnetite (Fe3O4) has been applied into some anaerobic bioprocesses to accelerate direct interspecies electron transfer (DIET), however, Fe3O4 is usually dissolved by iron-reducing bacteria under anaerobic conditions, resulting in the loss of magnetite. Therefore, submicron magnetite particles were added to the sequencing batch reactor (SBR) to build a Fe3O4/SBR system, which could alleviate magnetite dissolution and simultaneously remove tribromophenol (TBP) effectively. The average removal efficiencies of chemical oxygen demand (COD) and TBP in Fe3O4/SBR system were 81% and 91%, respectively, which were 51% and 18% higher than those of the control group without Fe3O4 (SBR system). The enhanced TBP biodegradation was likely related to potential DIET, which was supported by the scanning electron microscopy (SEM) analysis, the increase of dehydrogenase and heme c (fivefold and 1.7-fold), and the enrichment of iron-redoxing bacteria (Geobacter and Thiobacillus). Furthermore, magnetite mainly remained intact in structure as indicated by X-ray diffraction (XRD), which might be ascribed to in situ iron redox cycle and magnetite biosynthesis via Magnetospirillum. Notably, the content of hydrogen peroxide (H2O2) and hydroxyl radical (⋅OH) in Fe3O4/SBR system was 4–5 times higher than that of SBR system. These findings could provide insights into the development of cost-effective strategy for the removal of refractory organic pollutants.
The study of electrically conductive protein nanowires in Geobacter sulfurreducens has led to new concepts for long-range extracellular electron transport, as well as for the development of sustainable conductive materials and electronic devices with novel functions. Until recently, electrically conductive pili (e-pili), assembled from the PilA pilin monomer, were the only known Geobacter protein nanowires. However, filaments comprised of the multi-heme c-type cytochrome, OmcS, are present in some preparations of G. sulfurreducens outer-surface proteins. The purpose of this review is to evaluate the available evidence on the in vivo expression of e-pili and OmcS filaments and their biological function. Abundant literature demonstrates that G. sulfurreducens expresses e-pili, which are required for long-range electron transport to Fe (III) oxides and through conductive biofilms. In contrast, there is no definitive evidence yet that wild-type G. sulfurreducens express long filaments of OmcS extending from the cells, and deleting the gene for OmcS actually increases biofilm conductivity. The literature does not support the concern that many previous studies on e-pili were mistakenly studying OmcS filaments. For example, heterologous expression of the aromatic-rich pilin monomer of Geobacter metallireducens in G. sulfurreducens increases the conductivity of individual nanowires more than 5,000-fold, whereas expression of an aromatic-poor pilin reduced conductivity more than 1,000-fold. This more than million-fold range in nanowire conductivity was achieved while maintaining the 3-nm diameter characteristic of e-pili. Purification methods that eliminate all traces of OmcS yield highly conductive e-pili, as does heterologous expression of the e-pilin monomer in microbes that do not produce OmcS or any other outer-surface cytochromes. Future studies of G. sulfurreducens expression of protein nanowires need to be cognizant of the importance of maintaining environmentally relevant growth conditions because artificial laboratory culture conditions can rapidly select against e-pili expression. Principles derived from the study of e-pili have enabled identification of non-cytochrome protein nanowires in diverse bacteria and archaea. A similar search for cytochrome appendages is warranted. Both e-pili and OmcS filaments offer design options for the synthesis of protein-based “green” electronics, which may be the primary driving force for the study of these structures in the near future.
IMPORTANCE The anaerobic corrosion of iron structures is expensive to repair and can be a safety and environmental concern. It has been known for over 100 years that the presence of anaerobic respiratory microorganisms can accelerate iron corrosion. Multiple studies have suggested that there are sulfate reducers, methanogens, and acetogens that can directly accept electrons from Fe(0) to support sulfate or carbon dioxide reduction. However, all of the strains studied can also use H2 as an electron donor for growth, which is known to be abiotically produced from Fe(0). Furthermore, no proteins definitely shown to function as extracellular electrical contacts with Fe(0) were identified. The studies described here demonstrate that direct electron transfer from Fe(0) can support anaerobic respiration. They also map out a simple genetic approach to the study of iron corrosion mechanisms in other microorganisms. A better understanding of how microorganisms promote iron corrosion is expected to lead to the development of strategies that can help reduce adverse impacts from this process.
Auto-generative high pressure digestion (AHPD) and hydrogen-injecting digestion (HID) have been introduced to directly produce high CH4-content biogas from anaerobic digester. However, each approach has its own technical difficulties (pH changes), and practical issues (high cost of H2) to obtain > 90% CH4 containing biogas, particularly, from the high-strength waste like food waste (FW). To overcome this problem, in this study, AHPD and HID were integrated, which can offset each drawback but maximize its benefit. Substrate concentration of FW tested here was 200 g COD/L, the highest ever applied in AHPD and HID studies. At first, the reactor was operated by elevating the autogenerative pressure from 1 to 3, 5, and 7 bar without H2 injection. With the pressure increase, the CH4 content in the biogas gradually increased from 52.4% at 1 bar to 77.4% at 7 bar. However, a drop of CH4 production yield (MPY) was observed at 7 bar, due to the pH drop down to 6.7 by excess CO2 dissolution. At further operation, H2 injection began at 5 bar, with increasing its amount. The injection was effective to increase the CH4 content to 82.8%, 87.2%, and 90.6% at 0.09, 0.13, and 0.18 L H2/g CODFW.fed of H2 injection amount, respectively. At 0.25 L H2/g CODFW.fed, there was a further increase of CH4 content to 92.1%, but the MPY was dropped with pH increase to 8.7 with residual H2 being detected (4% in the biogas). Microbial community analysis showed the increased abundance of piezo-tolerant microbe with pressure increase, and direct interspecies electron transfer contributors after H2 injection. In conclusion, the integration of two approaches enabled to directly produce high calorific biogas (90% > CH4, 180 MJ/m³ biogas) from high-strength FW at the lowest requirement of H2 (0.18 L H2/g CODFW.fed) ever reported.
Conductive materials can serve as biocatalysts during direct interspecies electron transfer for methanogenesis in anaerobic reactors. However, the mechanism promoting direct interspecies electron transfer in anaerobic reactors, particularly under environments in which diverse substrates and microorganisms coexist, remains to be elucidated from a scientific or an engineering point of view. Currently, many molecular microbiological approaches are employed to understand the fundamentals of this phenomenon. Here, the direct interspecies electron transfer mechanisms and relevant microorganisms identified to date using molecular microbiological methods were critically reviewed. Moreover, molecular microbiological methods for direct interspecies electron transfer used in previous studies and important findings thus revealed were analyzed. This review will help us better understand the phenomena of direct interspecies electron transfer using conductive materials and offer a framework for future molecular microbiological studies.
Minerals are ubiquitous in the natural environment and have close contact with microorganisms. In various scenarios, microorganisms that harbor extracellular electron transfer (EET) capabilities have evolved a series of beneficial strategies through the mutual exchange of electrons with extracellular minerals to enhance survival and metabolism. These electron exchange interactions are highly relevant to the cycling of elements in the epigeosphere and have a profound significance in bioelectrochemical engineering applications. In this review, we summarize recent advances related to the effects of different minerals that facilitate the EET process and discuss the underlying mechanisms and outlooks for future applications. The promotional effects of minerals arise from their redox-active ability, electrical conductivity and photocatalytic capability. In mineral-promoted EET processes, various responses have concurrently arisen in microorganisms, such as stretching of electrically conductive pili (e-pili), upregulated expression of outer-membrane cytochromes (Cyts) and production of specific enzymes, and secretion of extracellular polymeric substances (EPSs). This review synthesizes the understanding of electron exchange mechanisms between microorganisms and minerals and highlights potential applications in development of renewable energy production and pollutant remediation, which are topics of particular significance to future exploitation of biotechnology.
Nanoparticle (NP) use can increase biological activity and adversely impact the environment. This study was the first to quantify biogas increases with NP mixtures during continuous anaerobic digestion (AD) of poultry litter and NP uptake in crops through tracking: 1) CH4 and H2S production from a NP mixture (Fe, Ni, and Co) in 30 L continuous digester (AD1) for 278 days compared to a control digester (AD2) without NP addition, 2) NP degradation during digestion, 3) using AD effluent with and without NP addition as a fertilizer, and 4) plant uptake of NPs. With NP inclusion, CH4 production increased by 23.7%, and H2S was reduced by 56.3%. The AD1 effluent had 1,160–19,400% higher NP concentrations and the lettuce biomass had 21.0–1,920% more NPs than lettuce fertilized with the AD2 effluent. This study showed that the effects of NPs remaining in the AD effluent must be considered.
The inhibition of the anaerobic digestion (AD) process, caused by long chain fatty acids (LCFAs), has been considered as an important issue in the wastewater treatment sector. Proper understanding of mechanisms behind the inhibition is a must for further improvements of the AD process in the presence of LCFAs. Through analyzing recent literature, this review extensively describes the mechanism of LCFAs degradation, during AD. Further, a particular focus was directed to the key parameters which could affect such process. Besides, this review highlights the recent research efforts in mitigating LCFAs-caused inhibition, through the addition of commonly used additives such as cations and natural adsorbents. Specifically, additives such as bentonite, cation-based adsorbents, as well as zeolite and other natural adsorbents for alleviating the LCFAs-induced inhibition are discussed in detail. Further, panoramic evaluations for characteristics, various mechanisms of reaction, merits, limits, recommended doses, and preferred conditions for each of the different additives are provided. Moreover, the potential for increasing the methane production via pretreatment using those additives are discussed. Finally, we provide future horizons for the alternative materials that can be utilized, more efficiently, for both mitigating LCFAs-based inhibition and boosting methane potential in the subsequent digestion of LCFA-related wastes.
Fe-based nanoparticles (Fe-based NPs) have great potential as a substitute for traditional Fe-fertilizer; however, their environmental risk and impact on plant growth are not fully understood. In this study, we compared the physiological impacts of three different Fe-based NP formulations: zero-valent iron (ZVI), Fe3O4 and Fe2O3 NPs, on hydroponic rice after root exposure for 2 weeks. Fe-normal (Fe(+)) and Fe-deficiency (Fe(-)) conditions were compared. Results showed that low dose (50 mg L-1) of ZVI and Fe3O4 NPs improved the rice growth under Fe(-) condition, while Fe2O3 NPs did not improve plant growth and caused phytotoxicity at high concentration (500 mg L-1). Under Fe(+) conditions, none of the Fe-based NPs exhibited positive effects on the rice plants with plant growth actually being inhibited at 500 mg L-1 evidenced by reduced root volume and leaf biomass and enhanced oxidative stress in plant. Under Fe(-) condition, low dose (50 mg L-1) of ZVI NPs and Fe3O4 NPs increased the chlorophyll content by 30.7% and 26.9%, respectively. They also alleviated plant stress demonstrated by the reduced oxidative stress and decreased concentrations of stress related phytohormones such as gibberellin and indole-3-acetic acid. Low dose of ZVI and Fe3O4 NPs treatments resulted in higher Fe accumulation in plants compared to Fe2O3 NPs treatment, by down-regulating the expression of IRT1 and YSL15. This study provides significant insights into the physiological impacts of Fe-based NPs in rice plants and their potential application in agriculture. ZVI and Fe3O4 NPs can be used as Fe-fertilizers to improve rice growth under Fe-deficient condition, which exist in many rice-growing regions of the world. However, dose should be carefully chosen as high dose (500 mg L-1 in this study) of the Fe-based NPs can impair rice growth.
Green synthesis of silver nanoparticles (Ag NPs) by using plants extracts has provided an eco-friendly alternation for industry and agriculture application. Here, we prepared Ag NPs by using the cucumber leaves and rice husk extracts, and further assessed the antimicrobial activity and phytotoxicity of green synthesized Ag NPs (g-Ag NPs) comparing with chemically synthesized Ag NPs (chem-Ag NPs). The chem-Ag NPs had strong antibacterial activity on the growth of Escherichia coli, while g-Ag NPs by rice husks (gr-Ag NPs) exhibited long-term antibacterial effects. In terms of phytotoxicity, the chem-Ag NPs induced over-generation of ROS and activated plant antioxidant defense systems, thus resulting in the upregulation of MDA and Zn contents and downregulation of antioxidant capacity, carotenoid, globulin and Mo contents. However, g-Ag NPs significantly promoted cucumber photosynthesis by increasing chlorophyll contents. Besides, the green synthesized Ag NPs by cucumber extracts (gc-Ag NPs) increased protein contents and gr-Ag NPs stimulated the upregulation of Mn and the downregulation of Al, which were all positive effects. Overall, compared with chem-Ag NPs, g-Ag NPs exhibited long-tern antimicrobial properties and attenuated toxicity to plants, which could be used as potential nanopesticide or nanoscale growth regulator in agriculture.
The lignocellulose of cow manure hinders a good methane yield in anaerobic digestion. • Biological pretreatment (composting) of cow manure is promising in full-scale application. • Selection of a lignin-poor co-substrate is vital when conducting co-digestion trials. • Fe-based nano-particles are excellent additives in lab and full-scale applications. • Bioelectrochemical reactor represents future reactor module treating cow manure. Cow manure represents a surplus manure waste in agricultural food sectors, which requires proper disposal. An-aerobic digestion, in this regard, has raised global interest owing to its apparent environmental benefits, including simultaneous waste diminishment and renewable energy generation. However, dedicated intensifications are necessary to promote the degradation of recalcitrant lignocellulosic components of cow manure. Hence, this manuscript presents a review of how to exploit cow manure in anaerobic digestion through different incentives extensively at lab-scale and full-scale. These strategies comprise 1) co-digestion; 2) pretreatment; 3) introduction of additives (trace metals, carbon-based materials, low-cost composites, nanomaterials, and microbial cultures); 4) innovative systems (bio-electrochemical fields and laser irradiation). Results imply that co-digestion and pretreatment approaches gain the predominance on promoting the digestion performance of cow manure. Particularly, for the co-digestion scenario, the selection of lignin-poor co-substrate is highlighted to produce maximum synergy and pronounced removal of lignocellulosic compounds of cow manure. Mechanical , thermal, and biological (composting) pretreatments generate mild improvement at laboratory-scale and are proved applicable in full-scale facilities. It is noteworthy that the introduction of additives (Fe-based Science of the Total Environment xxx (xxxx) xxx
Nanoscale zero-valent iron (nZVI) is the main nanomaterial used in environmental remediation processes. The present study aims to evaluate the life cycle sustainability of nZVI production methods applied in environmental remediation. Three production methods of nZVI were selected for analysis: milling, liquid reduction with sodium borohydride, and chemical reduction with hydrogen gas (in two approaches: considering the goethite and hematite synthesis and after using in nZVI production and, using goethite and hematite particles already synthesized for nZVI production). The life cycle sustainability assessment was carried out based on a multi-criteria and multi-attribute analysis. The multi-criteria analysis was used to determine impact category preferences of different specialists in sustainability and remediation, and calculate the sustainability score through a linear additive model. Finally, a Monte Carlo simulation was used to quantify the results uncertainty. The functional unit considered was 1.00 kg of nZVI produced. The milling method and the hydrogen gas method in approach considering the use goethite and hematite particles already synthesized were the most sustainable. Moreover, the sustainability index was found to be influenced by the considered location scenarios as well as the perspectives of the different specialists, which was essential in producing a more accurate and comprehensive evaluation of the aforementioned sustainability methods. Overall, this study significantly contributed to applications of the state-of-the-art life cycle sustainability assessment in studies regarding nanomaterials, employing a simple methodology that included an analysis of different specialists. In addition, this is the first article that uses life cycle sustainability assessment in nanomaterials.
In this study, we investigated the potentials of nanomaterials to enhance anaerobic ammonium oxidation (anammox) process, in terms of nitrogen removal, microbial enrichment, and activity of key enzymes. Graphene nanosheets (GNs) and γ-Fe 2 O 3 nanoparticles (NPs) were selected due to their catalytic functions as conductive material and electron shuttles, respectively. The obtained results revealed that the optimum dosage of GNs (10 mg/L) boosted the nitrogen removal rate (NRR) by 46 ± 3.1% compared to the control, with maximum NH 4 +-N and NO 2 −-N removal of 86.5 ± 2.7% and 97.1 ± 0.5%, respectively. Moreover, hydrazine dehydrogenase (HDH) enzyme activity was augmented by 1.1-fold when using 10 mg/L GNs. The presence of GNs promoted the anammox granulation via enhancement of hydrophobic interaction of extracellular polymeric substances (EPS). Regarding the use of γ-Fe 2 O 3 NPs, 100 mg/L dose increased NRR by 55 ± 3.8%; however, no contribution to HDH enzyme activity and a decrease in EPS compositions were observed. Given that the abiotic use of γ-Fe 2 O 3 NPs further resulted in high adsorption efficiency (~92%), we conclude that the observed promotion due to γ-Fe 2 O 3 NPs was mainly abiotic. Moreover, the 16S rRNA analysis revealed that the relative abundance of genus C. Jettenia (anammox related bacteria) increased from 11.9% to 12.3% when using 10 mg/L GNs, while declined to 8.3% at 100 mg/L γ-Fe 2 O 3 NPs. Eventually, nanomaterials could stimulate the efficiency of anammox process, and this promotion and associated mechanism depend on their dose and composition.
Hydrogen production from waste activated sludge (WAS) was widely considered and intensively investigated as a promising technology to recover energy from wastewater treatment plants. To date, no efforts have been made on either systematic summarization or critical thinking of the application niche of hydrogen production from WAS treatment. It is therefore time to evaluate whether and how to recover hydrogen in a future paradigm of WAS treatment. In this critical review, the principles and potentials, microorganisms, possible technologies, and process parameters of hydrogen generation were analyzed. Microbial electrolysis cell shows high theoretical hydrogen yield and could utilize a variety of organic compounds as substrates, which is regarded as a prospective technology for hydrogen production. However, the poor organics utilization and rapid consumptions of produced hydrogen hindered hydrogen recovery from WAS. Based on the analysis of the current state of the literatures, the opportunities and challenges of hydrogen production from WAS are rethought, the detailed knowledge gaps and perspective of hydrogen production from WAS were discussed, and the probable solutions of hydrogen recovery from WAS treatment are figured out. To guide the application and development of hydrogen recovery, a more promising avenue through rational integration of the available technologies to form a hybrid process is finally proposed. The integrated operational paradigm of WWTPs could achieve substantial technical, environmental and economic benefits. In addition, how this hybrid process works is illustrated, the challenges of this hybrid process and future efforts to be made in the future are put forward.
Serious inhibition of methane production in an anaerobic digestion (AD) system can be caused by propionic acid, which is derived from lactic acid degradation. Nanoscale zero-valent iron (nZVI) was used in this study to improve conversion of propionic acid into acetic acid, thereby promoting methane production. The methane yield was markedly enhanced when nZVI concentration increased from 0 to 2 g/L; however, it decreased when nZVI concentration further increased to 8 g/L. At an nZVI concentration of 2 g/L, the methane yield increased by 37% from 398.5 to 546.4 mL CH4/g TVS. The abundance of Candidatus Cloacamonas in the bacterial community increased from 2.17% to 3.78%, which facilitated conversion of propionic acid into acetic acid. Meanwhile, the abundances of Methanomassiliicoccus and Methanosarcina in archaeal community increased, which was beneficial to methane production. Cyclic voltammetry showed that the electron transfer coefficient in the AD system increased from 0.029 to 0.034 s⁻¹.
Methanogenesis can be promoted by the addition of conductive materials. Although stimulating effects of conductive materials on methane (CH4) production has been extensively reported, the crucial roles on recovering methanogenic activities under inhibitory conditions have not been systematically discussed. This critical review presents the current findings on the effects of conductive materials in methanogenic systems under volatile fatty acids (VFAs), ammonia, sulfate, and nano-cytotoxicity stressed conditions. Conductive materials induce fast VFAs degradation, avoiding VFAs accumulation during anaerobic digestion. Under high ammonia concentrations, conductive materials may ensure sufficient energy conservation for methanogens to maintain intracellular pH and proton balance. When encountering the competition of sulfate-reducing bacteria, conductive materials can benefit electron competitive capability of methanogens, recovering CH4 production activity. Conductive nanomaterials stimulate the excretion of extracellular polymeric substances, which can prevent cells from nano-cytotoxicity. Future perspectives about unraveling mitigation mechanisms induced by conductive materials in methanogenesis processes are further discussed.
Anaerobic digestion is a globally used biochemical process to convert the organic matter present in wastes into an energy rich biogas with methane as the major constituent. Implementing additives in the anaerobic digestion process enhances biogas production. Recent studies have revealed that nanoparticles additives influence the anaerobic digestion. Interspecies electron transfer (IET) is the main mechanism behind methane production. Apart from traditional IET routes, like IET through hydrogen and formate, direct interspecies electron transfer (DIET) through conductive materials have gained importance. This paper reviews DIET via abiotic conductive materials and describes the impact of various nanoparticle additives on anaerobic digestion. The paper also discusses the positive and negative impacts of nanoscale materials on biogas production. This study confirms that proper screening of nanoparticles based on physicochemical characteristics and other factors supportive of DIET can enhance the performance of the anaerobic digestion process, thereby increasing biogas generation.
Biomethanation through anaerobic digestion (AD) is the most reliable energy harvesting process to achieve waste-to-energy. Microbial communities, including hydrolytic and fermentative bacteria, syntrophic bacteria, and methanogenic archaea, and their interspecies symbioses allow complex metabolisms for the volumetric reduction of organic waste in AD. However, heterogeneity in organic waste induces community shifts in conventional anaerobic digesters treating sewage sludge at wastewater treatment plants globally. Assessing the metabolic roles of individual microbial species in syntrophic communities remains a challenge, but such information has important implications for microbially enhanced energy recovery. This review focuses on the alterations in digester microbiome and intricate interspecies networks during substrate variation, symbiosis among the populations, and their implications for biomethanation to aid stable operation in real-scale digesters.
The effects on the activity of anaerobic digestion (AD) of interactions between extracellular polymeric substances (EPS), a protective barrier of microorganisms towards toxic compounds, and nanoscale zero valent iron (nZVI) remain incompletely understood. In this work, EPS induced a dosage-dependent dispersion of nZVI clusters due to their effective accumulation on the nZVI surface. The small size of nZVI clusters and the formation of stable Fe-EPS complex promoted the dissolution of nZVI with a final increase of 15–20% H2 yield. Further characterizations of EPS demonstrated the presence of some semiquinones, like riboflavin, which may work as a sink to accept electrons from nZVI. This likely explains the EPS dosage-related reduction of H2 release rate in the initial stage and the possible decrease in nZVI reducibility responsible for disrupting cell integrity. Interactions between nZVI and EPS could improve the electrochemical activity of EPS, favoring microbial extracellular electron transfer. Therefore, the presence of EPS at relatively higher concentrations may 1) reduce the inhibition of nZVI to AD process by avoiding the fast accumulation of H2 and restricting damage to cell integrity; 2) benefit the methanogenesis process by providing more exogenous H2 from complete nZVI dissolution with higher electrochemical activity of EPS. This study provides insight into the interactions between EPS and nanoparticles with strong reducibility in biological wastewater treatment systems.
This study investigated the effect of supplementing nano-sized magnetite (Fe3O4 NPs), multi-wall carbon nanotubes (MWCNTs) and Fe3O4-MWCNTs composite on bioconversion of waste activated sludge to hydrogen, in batch systems. Substrate degradation efficiency (SDE) increased from 28±3.8 (control) to 49±5.9, 46±4.8 and 52±6.3% at optimal doses of 200 (Fe3O4 NPs), 300 (MWCNTs) and 200 mg/L (Fe3O4-MWCNTs), respectively. Based on dissolved iron and sludge conductivity measurements, superior SDE in Fe3O4 and MWCNTs batches have been assigned to enhanced dissimilatory iron reduction (DIR) and high sludge conductivity, respectively. Combined impacts for sludge conductivity and DIR were revealed in Fe3O4-MWCNTs system. In 200 mg/L (Fe3O4-MWCNTs) batch, catalytic activities of hydrogenase, protease and α-amylase peaked to 596, 146 and 131% (relative to control), respectively; as well as, highest volumetric H2 production of 607±59 mL/L was acquired. Performance deteriorations at high concentrations of nanoparticles were caused by cellular oxidative stress induced by generated reactive oxygen species.
In the present work, effects of direct interspecies electron transfer (DIET)-promoting condition on anaerobic treatment of phenol, in up-flow anaerobic sludge blanket reactors (UASB) were investigated. One reactor (C-UASB) was operated as a control, while two reactors (EA-UASBs) were operated with electrical energy input (EEI) at 0.3 V and 0.6 V. The organic loading rate was gradually increased from 0.5 to 14 kg COD/m³/d at a fixed hydraulic retention time of 1 d. A drastic decline in the biogas production was started to be seen from 7.0 kg COD/m³/d in C-UASB, while a stable performance was achieved up to 11 kg COD/m³/d with maintaining a CH4 production yield of 0.3 m³ CH4/g CODadded in EA-UASBs. There was no difference in the performance between two voltages applied, but higher energy recovery value of 68% was achieved at 0.3 V (49% at 0.6 V). EEI changed the physicochemical properties of granules in a favorable way, increasing diameter, settling velocity, porosity, permeability, hydrophobicity, and integrity coefficient from 2.2, 1.4, 0.7, 0.0025, 23, and 87 to 2.7–2.9 mm, 1.9–2.0 cm/s, 0.9, 0.0039–0.0053 cm², 36–60%, and 94–95%, respectively. EEI also affected the microbial community, increasing the dominance of DIET-related microorganisms including Methanobacterium, Methanothrix, and Syntrophorhabdus.
A shift from petrochemical processes toward a bio-based economy is one of the most advocated developments for a sustainable future. To achieve this will require the biotechnological production of platform chemicals that can be further processed by chemical engineering. Bioelectrochemical systems (BESs) are a novel tool within the biotechnology field. In BESs, microbes serve as biocatalysts for the production of biofuels and value-added compounds, as well as for the production of electricity. Although the general feasibility of bioelectrochemical processes has been demonstrated in recent years, much research has been conducted to develop biocatalysts better suited to meet industrial demands. Initially, mainly natural exoelectrogenic organisms were investigated for their performance in BESs. Driven by possibilities of recent developments in genetic engineering and synthetic biology, the spectrum of microbial catalysts and their versatility (substrate and product range) have expanded significantly. Despite these developments, there is still a tremendous gap between currently achievable space-time yields and current densities on the one hand and the theoretical limits of BESs on the other. It will be necessary to move the performance of the biocatalysts closer to the theoretical possibilities in order to establish viable production routines. This review summarizes the status quo of engineering microbial biocatalysts for anode-applications with high space-time yields. Furthermore, we will address some of the theoretical limitations of these processes exemplarily and discuss which of the present strategies might be combined to achieve highly synergistic effects and, thus, meet industrial demands.
Considering their antimicrobial, electrical and optical properties, silver nanoparticles (AgNPs) are the most common type of engineered nanomaterials found in consumer products. AgNPs may be synthesized through multiple methods, including chemical, biological and physical techniques; however, literature suggests that the manufacturers prefer to use physical and chemical methods (85 %) rather than biological. This work presents cradle-to-gate life cycle impact assessments in order to evaluate global environmental impacts of six different AgNPs synthesis routes (two chemical and four physical) along with thirteen different inventories and a mass based functional unit of 1 kg of AgNPs. Results are then combined with the annual global AgNPs production estimates, and global environmental impact calculations are performed based on both optimistic and skeptical estimations. Since AgNPs production volumes are forecasted to increase drastically, industrial scale AgNPs syntheses are modeled and future life cycle impacts are projected using three different scale-up factors. Furthermore, given that each industry has specific preferences for properties of AgNPs (i.e. size, surface area) and those are dependent on the synthesis methods, industry based environmental impact projections are developed for industries where the majority of AgNPs are used such as textiles; coatings, paints and pigments; consumer electronics and optics; cosmetics; medical and packaging. Results show that scaling up may reduce the environmental emissions up to 90 % globally, and up to 83 % per industrial sector which suggest that the global environmental impact of AgNPs may vary significantly as a function of the synthesis method, scale, and desired product application.
In this study, a Fe2O3 supported on conductive carbon cloth (FC) was prepared and supplemented into anaerobic digestion reactors to improve propionate degradation. In the FC-supplemented reactors, the cumulative methane production and propionate degradation increased by 15.4% and 19.67% compared with those of the control, respectively. Less methane production with H2/CO2 as the sole substrate in the culture taken from the FC reactors suggested that interspecies hydrogen transfer in the FC reactors was weaker. These results suggested that direct interspecies electron transfer (DIET) was established in the FC reactors to improve the performance. Fe2O3 increased the secretion of electron shuttle components of extracellular polymeric substances to increase electron exchange capacity of biomass of the FC reactors, which further facilitated the DIET. Analysis on microbial communities confirmed that the abundance of microorganisms-related DIET in the FC reactors was higher than that in the control.
Recently, the influence of metal oxide nanoparticles (NPs) on methanogenesis in anaerobic digestion has drawn much attention, however, the changes in NPs and functioning consortia within the methanogenic community are usually not investigated. Therefore, the methanogenesis performance, NPs transformation and methanogenic community development in anaerobic digester sludge under MnO2 NP supplementation were demonstrated in this study. MnO2 NPs (400 mg/gVSS) stimulated the methane (CH4) yield by 42% for a final CH4 proportion of 81.8% of the total gas production. Meanwhile, the coenzyme F420 and INT-electron transport system activities showed positive correlation with MnO2 concentration. Microbial Mn reduction and oxidation occurred in conjunction with methanogenesis, resulting in transformation of the shape of the MnO2 NPs from wire-like to globular particles. Microbial community analysis indicated that the relative abundances of genera Methanobacterium, Methanosaeta, and Methanosarcina were higher in the presence of MnO2 NPs. Moreover, a new and different crucial synergy within the methanogenic community was formed with low-abundance consortia driving Mn respiration coupled to methanogenesis in anaerobic digestion. To our knowledge, this is the first report on transformation of metal oxides NPs combined with syntrophic community development in studies focusing on methanogenesis in response to NPs.
Direct interspecies electron transfer (DIET) between exoelectrogenic fatty acid oxidizers and electrotrophic methanogens plays an important role in keeping the overall anaerobic digestion (AD) process well-balanced. This study examined the individual and combined effects of two different DIET-promoting strategies, i.e., magnetite addition (20 mM Fe) and external voltage application (0.6 V), in continuous digesters treating dairy wastewater. Although the strategies were both effective in enhancing the process performance and stability, adding magnetite had a much greater stimulatory effect. External voltage contributed little to the methane yield, and the digester with magnetite addition alone achieved stable performance, comparable to that of the digester where both strategies were combined, at short hydraulic retention times (down to 7.5 days). Diverse (putative) electroactive microorganisms were significantly enriched under DIET-promoting conditions, particularly with magnetite addition. The overall results suggest that magnetite addition could effectively enhance AD performance and stability by promoting DIET-based electro-syntrophic microbial interactions.
Fe3O4 supported on water hyacinth biochar (Fe3O4/WHB) was successfully used in anaerobic degradation of 2,4,6-trichlorophenol and coal gasification wastewater (CGW). Chemical oxygen demand removal efficiency and methane production were significantly improved to 98.9% and 2.0 L with Fe3O4/WHB assisted. Fe3O4/WHB facilitated the conversion of CO2 to methane and reduce H2 production. A higher coenzyme F420 concentration of 1.32 μmol/(g-mixed liquor volatile suspended solids) was found with the presence of Fe3O4/WHB, which might result in a faster conversion of acetate to methane. More interspecific signal molecules, lower diffusible signal factor, and higher mean particle size indicated that Fe3O4/WHB accelerated the sludge granulation process. Microbial community analysis revealed that enriched bacteria Geobacter along with archaea Methanothrix and Methanosarcina may be involved in direct interspecies electron transfer by Fe3O4/WHB stimulation, enhancing the performance of 2,4,6-trichlorophenol fermentation. It is shown that use of Fe3O4/WHB is feasible for enhanced CGW treatment.
Recently, the use of nanomaterials as biostimulators for the methanogenic bacteria has been commonly deployed. This is to maximize the biogas production from livestock manure through the anaerobic digestion processes. Yet, the environmental impact of the nanomaterials as manure additives has not been evaluated. In this respect, different nanoparticles (NPs) of nickel (Ni), cobalt (Co), iron (Fe) and iron oxide (Fe3O4) were used in biogas production to study their environmental impact using life-cycle assessment (LCA) methodology. Global warming, greenhouse gas (GHG) emissions, acidification, eutrophication, resource depletion, human toxicity potential, and ozone layer depletion potential were investigated. The results showed that Co NPs was the most effective in reducing the greenhouse gas emissions through electricity production. The greenhouse gas emissions were 0.0366, 0.0276, 0.0225, 0.0336 and 0.0290 kg CO2 eq./MJ elect. for the control, Ni NPs, Co NPs, Fe NPs and Fe3O4 NPs, respectively. Furthermore, Co NPs delivered the lowest acidification, human toxicity potential and eutrophication values. While, Ni NPs delivered the lowest resource and ozone layer depletion values.
Anaerobic digestion (AD) of organic wastes is among the most promising approaches used for the simultaneous treatment of various waste streams, environment conservation, and renewable bioenergy generation (biomethane). Among the latest innovations investigated to enhance the overall performance of this process both qualitatively and quantitatively, the application of some nanoparticles (NPs) has attracted a great deal of attention.Typically, the NPs of potential benefit to the AD process could be divided into three groups: (i) zerovalentiron (ZVI) NPs, (ii) metallic and metal oxides NPs, and (iii) carbon-based NPs. The present review focuses on the latest findings reported on the application of these NPs in AD process and presents their various mechanisms of action leading to higher or lower biogas production rates. Among the NPs studies, ZVI NPs could be regarded as the most promising nanomaterials for enhancing biogas production through stabilizing the AD process as well as by stimulating the growth of beneficial microorganisms to the AD process and the enzymes involved. Future research should focus on various attributes of NPs when used as additives in biogas production,
including facilitating mixing and pumping operations, enriching the population and diversity of beneficial microorganisms for AD, improving biogas release, and inducing the production and activity of AD-related enzymes. The higher volume of methane-enriched biogas would be translated into higher returns on investment and could therefore, result in further growth of the biogas production industry. Nevertheless, efforts should be devoted to decreasing the price of NPs so that the enhanced biogas and methane production (by over 90%, compared to control) would be more economically justified, facilitating the large-scale application of these compounds. In addition to economic considerations, environmental issues are also regarded as major constraints which should be addressed prior to widespread implementation of NP-augmented AD processes. More specifically, the fate of NPs augmented in AD process should be scrutinized to ensure maximal beneficial impacts while adverse environmental/health consequences are minimized.
Improving the yield and methane content of biogas is of great concern for wastewater treatment by anaerobic digestion. Herein we developed a nanomagnetite-enhanced electromethanogenesis (EMnano) process for the first time, the sustainable utilization of which improved the biomethane production rate from dairy wastewater. The maximum CH4 production rate in the EMnano process is 2.3 ± 0.3-fold higher than it is in the conventional methanogenesis (CM) process, and it is accompanied by an almost delay-free start-up. The technical-economic evaluation revealed that an 82.1 ± 5.0% greater net benefit was obtained in the third generation of the EMnano process compared with the CM process. The improved methanogenesis was attributed to the formation of dense planktonic cell co-aggregates that are triggered by nanomagnetite, which facilitated the interspecies electron exchange during acetoclastic methanogenesis. Simultaneously, a cathode biofilm with high viability and catalytic activity was also formed in the EMnano process that decreased the biofilm resistance and facilitated the electron transfer during electromethanogenesis. This study is a worthwhile attempt to combine recyclable conductive materials with an electromethanogenesis process for wastewater treatment, and it effectively achieves energy recovery with high stability and cost-effectiveness.
The effects of nanoscale zero-valent iron (nZVI) on the performance of food waste anaerobic digestion and the fate of antibiotic resistance genes (ARGs) were investigated in thermophilic (TR) and mesophilic (MR) reactors. Results showed that nZVI enhanced biogas production and facilitated ARGs reduction. The maximum CH4 production was 212.00 ± 4.77 ml/gVS with 5 g/L of nZVI in MR. The highest ARGs removal ratio was 86.64 ± 0.72% obtained in TR at nZVI of 2 g/L. nZVI corrosion products and their contribution on AD performance were analyzed. The abundance of tetracycline genes reduced significantly in nZVI amended digesters. Firmicutes, Chloroflexi, Proteobacteria and Spirochaetes showed significant positive correlations with various ARGs (p < 0.05) in MR and TR. Redundancy analysis indicated that microbial community was the main factor that influenced the fate of ARGs. nZVI changed microbial communities, with decreasing the abundance bacteria belonging to Firmicutes and resulting in the reduction of ARGs.
Direct interspecies electron transfer (DIET) via electrically conductive pili (e-pili) and c-type cytochrome between acetogens and methanogens has been proposed as an essential pathway for methane production. Supplements of conductive materials have been extensively found to promote methane production in microbial anaerobic treatment systems. This review comprehensively presents recent findings of DIET and the addition of conductive materials for methanogenesis and summarizes important results through aspects of electron flux, organic degradation, and microbial interaction. Conductive materials improve DIET and methanogenesis by acting as either substitute of e-pili or electron conduit between e-pili and electron acceptors. Other effects of conductive materials such as the change of redox potential may also be important factors for the stimulation. The type and organic loading rate of substrates affect the occurrence of DIET and stimulating effects of conductive materials. Geobacter, which can participate in DIET, were less enriched in anaerobic systems cultivated with non-ethanol substrates, suggesting the existence of other syntrophs with the capability of DIET. The coupling of communication systems such as quorum sensing may be a good strategy to achieve the formation of biofilm or granule enriched with syntrophic partners capable of DIET.
Methane production of chicken litter in anaerobic digestion (AD) was evaluated with different concentrations of magnetite (Fe3O4) nanoparticles (NPs). The Fe3O4 NPs were synthesized by co-precipitation, where the effect of different concentrations and dissolution temperatures of the passivant and precursors, as well as the sonication time, on NPs size was determined. The best NPs were selected based on the smallest hydrodynamic diameter (79.37 nm) with the highest absolute value of zeta potential (−18.06 mV). These NPs were analyzed in a transmission electron microscope (average size of 4.2 nm) and by X-ray diffraction (to corroborate their structure). These 4.2 nm NPs were used to evaluate their effect on AD of chicken litter under mesophilic conditions. The treatments for AD were the control (only chicken litter) and the addition of different concentrations of Fe3O4 NPs (20, 40 and 60 mg L⁻¹). The treatment with 20 mg L⁻¹ of NPs had the highest methane production rate (2.55 mL CH4∙gvsf⁻¹∙d⁻¹) and the highest cumulative methane yield (137.23 mL CH4∙gvsf⁻¹), the last one with an increase of 73.9% compared to the control, while the concentration of volatile fatty acids was similar in all treatments. In general, NPs had a biostimulating effect on methanogenic activity.
Increasing studies indicate that magnetite addition could accelerate the methanogenesis via enhancing direct interspecies electron transfer (DIET)-based anaerobic syntrophy. However, magnetite is found to run off in continuous bioreactor, and the effect of magnetite loss on syntrophic aggregates is still underreported. In this study, two EGSB reactors (RM with magnetite-enhanced sludge, and RB as a control) were operated to investigate the magnetite behavior in continuous bioreactor and the corresponding response of syntrophic aggregates. Results showed that magnetite in RM was washed out gradually in form of iron ions, and a slightly acidic niche was supposed to be the major cause. Nevertheless, candidate DIET partners like Geobacter and Methanothrix along with syntrophic volatile fatty acids (VFAs)-degrading microbes were enriched in RM. In addition, the improved redox activity of extracellular polymeric substance (EPS), higher sludge conductivity and electron transport activity suggested that the DIET ability of sludge in RM was still enhanced, which favors the syntrophic metabolism of VFAs. Interestingly, syntrophic partners were loosely combined under the condition of high organic loading rate (OLR) in the presence of magnetite, but with gradual loss of magnetite, dense and active anaerobic granular sludge (AGS) was formed in RM. This study provided a comprehensive understanding of magnetite behavior in continuous bioreactor and the response of syntrophic aggregates. The robust DIET-based syntrophy after magnetite adding could favor the high-efficient anaerobic wastewater treatment and resource recovery in the future, and further investigations on magnetite resupply and the mechanism of magnetite enriching candidate DIET partners are recommended.
The potential applications of electrically conductive protein nanowires (e-PNs) harvested from Geobacter sulfurreducens might be greatly expanded if the outer surface of the wires could be modified to confer novel sensing capabilities or to enhance binding to other materials. We developed a simple strategy for functionalizing e-PNs with surface-exposed peptides. The G. sulfurreducens gene for the monomer that assembles into e-PNs was modified to add peptide tags at the carboxyl terminus of the monomer. Strains of G. sulfurreducens were constructed that fabricated synthetic e-PNs with a six-histidine ‘His-tag’ or both the His-tag and a nine-peptide ‘HA-tag’ exposed on the outer surface. Addition of the peptide tags did not diminish e-PN conductivity. The abundance of HA-tag in e-PNs was controlled by placing expression of the gene for the synthetic monomer with the HA-tag under transcriptional regulation. These studies suggest broad possibilities for tailoring e-PN properties for diverse applications.
The raw fresh leachate from municipal solid waste (MSW) incineration plants contains high concentrations of volatile fatty acids (VFAs), ammonia and metals, all compounds that severely limit anaerobic digestion treatment efficiencies. These inhibitory compounds make reactor systems unstable, causing reactor start-up periods to take more than 100 days, even when the leachate is diluted significantly. In this study, granular activated carbon (GAC) was incorporated into a bioreactor fed with raw incineration leachate. Addition of GAC allowed direct treatment of raw incineration leachate without any start-up acclimation period, while the non-amended control reactor soured immediately and collapsed within 17 days. When hydraulic retention time (HRT) of the GAC-amended reactor was stepwise decreased to increase organic loading rates (OLR) to 25.0 kgCOD/(m3·d), COD removal efficiencies remained stable at >90%. Metagenomic analysis of the GAC-amended reactor revealed that Geobacter and Methanosarcina, species known to participate in direct interspecies electron transfer (DIET), were more abundant in the GAC-amended reactor than the seed sludge. In addition, the abundance of genes coding for proteins thought to be involved in DIET such as electrically conductive pili and the outer membrane c-type cytochrome, OmcS, increased significantly, while genes involved in fermentation, and nitrate (narG) and sulfate (dsrA) reduction dropped significantly as the experiment progressed. These results are significant because this is the first detailed investigation into the metabolic capabilities of microbial communities involved in efficient treatment of raw incineration leachate within biomethanogenic reactors that did not require a long start-up period.
In this work, iron (II, III) oxide (Fe3O4) was synthesized and incorporated onto multi-walled carbon nanotubes (MWCNTs) to prepare Fe3O4-MWCNTs composite in a quest for evaluating its performance and reusability in both Fenton-like and photo-Fenton processes. The characterization of Fe3O4-MWCNTs by X-ray diffraction (XRD), transmission electron microscopy (TEM), Fourier-transform infrared (FTIR) spectroscopy, and energy dispersive X-ray (EDX) spectroscopy showed that the prepared catalyst can behave as a composite. Methylene blue (MB) was used as a substrate for evaluation of Fenton-like and photo-Fenton processes. The reusability of the catalyst and the influence of operation parameters such as pH, H2O2 dosage, and catalyst loading were investigated. Complete degradation of MB was attained by Fe3O4-MWCNTs in the aforementioned processes, whereas the removal efficiency of MB by using bare MWCNTs under the same conditions was 52%, which suggests that the generated oxidant species due to the reactions between H2O2 and leached iron contribute to the degradation of MB. A degradation pathway was proposed based on the oxidation intermediates that have been detected by mass spectrometry. The reusability of Fe3O4-MWCNTs has been examined in four consecutive cycles. The final removal of MB in the fourth cycle was 94%. The optimization of MB removal was investigated by response surface methodology (RSM) based on central composite design (CCD). Moreover, an artificial neural network (ANN) of type feed-forward back propagation was employed to model the influence of operating conditions. The ANN model revealed a high correlation in the prediction of the removal efficiency (R² = 0.9934).
Granular activated carbon (GAC) was modified by the addition of conductive nano-Fe3O4 for use in the anaerobic digestion of low-strength wastewater. This new material, called magnetic granular activated carbon (MGAC), was fabricated by the facile co-precipitation method. The MGAC exhibited superior electro-conductivity, electron transfer rate, and methane production. The observed conductivity of the MGAC (17.5 ± 0.6 mS cm⁻¹) was 2.03 times that of the GAC (8.6 ± 0.3 mS cm⁻¹). With the MGAC, the effluent total chemical oxygen demand concentration (44.5 ± 1.5 mg L⁻¹) was 33% lower than that obtained with the GAC and 43% lower than that obtained in the control experiment (with no conductive additive). The methane production obtained with the MGAC (4.7 ± 0.2 mL per cycle) was 3.6 times that of the control and 1.57 times that of the GAC. The anaerobic system with the MGAC exhibited stronger electrochemical response to riboflavin and cytochrome C. The peak current at 0 V (reduction peak, cytochrome C) of the MGAC was 45.1 μA, which was 54.0% higher than that of the GAC and 54.8% higher than that of the control. The electron accepting capability [56.71 ± 0.70 μmol e⁻ (g char)⁻¹] was 2.75 times that of the GAC. The MGAC also exhibited a higher abundance of functional microorganisms. These results demonstrated that the MGAC enhanced the electron transfer between bacteria and archaea, improved methane production, and produced high-quality effluent.
The sole, dual and multi-fermentations of fruit and vegetable peels (FVPs) were investigated in order to balance nutrition hierarchy for maximizing hydrogen potential via Batch experiments. The highest volumetric hydrogen production of 2.55 ± 0.07 L/L and hydrogen content of 64.7 ± 3.7% were registered for multi-fermentation of M-PTBO (25% pea +25% tomato + 25% banana +25% orange). These values outperformed sole and dual fermentation. The multi-fermentation of FVPs provided sufficient nutrients and trace elements for anaerobes, where C/N and C/P ratios were at levels of 24.7 ± 0.2 and 113.2 ± 9.4, respectively. In specific, harmonizing of macro and micro-nutrients remarkably maximized activities of amylase, protease and lipase to 4.23 ± 0.42, 0.035 ± 0.002 and 0.31 ± 0.02 U/mL, respectively, as well as, substantially incremented counts of Clostridium and Enterobacter sp. up to 5.81 ± 0.23 × 10 ⁵ and 2.17 ± 0.09 × 10 ⁶ cfu/mL, respectively. Furthermore, multi-fermentation of M-PTBO achieved the maximum net energy gain and profit of 1.82 kJ/g feedstock and 4.11 $/kg feedstock , respectively. Nutrients balance significantly develops bacterial activity in terms of hydrogen productivity, anaerobes reproduction, enzyme activities and soluble metabolites. As a result, overall fermentation bioprocess performance was improved.