CH4 Production Rate Research Articles

In-Situ Electrodeposition of Nickel on a Biocathode to Enhance Methane Production from Carbon Dioxide in a Microbial Electrosynthesis System

Power-to-gas (P2G) technologies convert electrical energy into renewable gaseous fuels or value-added molecules to be used for transportation, heat production, or as a feedstock for the chemical industry, respectively. Microbial Electrosynthesis Systems (MES) use electricity and microorganisms to produce such energy carriers, such as hydrogen (H2) or methane (CH4), Figure 1a. Also known as Bioelectrochemical power-to-gas (BEP2G), MES combines CO2 sequestration, renewable energy carrier production, and energy storage. The BEP2G technology presents several advantages such as i) single-step CO2 conversion into CH4, so that no separate process is needed to generate H2 for the reduction of CO2, ii) bioelectrochemical reactions take place at ambient conditions, which translates into lower costs of materials and of energy consumption, and iii) higher CH4 yields can be achieved, when compared to conventional (electro)catalytic reactions.In this study, a continuous conversion of CO2 to CH4 in a laboratory scale membraneless Microbial Electrosynthesis System (MES) is achieved. The MES cell (Figure 1b) for CH4 production was built using Plexiglass plates assembled to create two compartments divided by two layers of electrically insulating Nylon cloth, with a total area of 96 cm2 (anode-cathode interface area). The cathode compartment consisted of a stainless steel mesh (current collector) and it was filled with 45 g of carbon felt while the anode consisted of a Ti/IrO2 mesh current collector and it was filled with 20 g of carbon felt. The cathode compartment was maintained at a constant temperature of 30°C and continuously supplied with CO2 and a solution of nutrients. The cathode compartment was initially inoculated with 20 mL of homogenized anaerobic sludge. To further increase CH4 production, nickel was (in-situ) electrodeposited on the cathode carbon felt.The introduction of Ni at 0.2 g L-1 into the influent exhibited a remarkable improvement in CO2 conversion. At an applied voltage of 2.8 V, the MES with the Ni-modified carbon-felt cathode accomplished a CH4 production rate of 2.3 L d-1 which was 30% higher than that of this system before Ni was introduced to the influent stream (1.6 L d-1), Figure 1c. Similar improvements were noted with other performance parameters, such as the Coulombic efficiency and the energy consumption, which considerably improved from 56% to 73% and 11.5 Wh LH 2 −1 to 7.4 Wh LH 2 −1, respectively. Furthermore, once dissolved Ni was removed from the influent solution, the performance remained unchanged suggesting its successful in-situ electrodeposition on carbon felt. Electrochemical characterization using EIS and cyclic voltammetry techniques confirmed Ni electrodeposition. These results demonstrated a great potential of using in-situ Ni electrodeposition to improve catalytic properties of the MES biocathode. Also, with respect to the energy storage capacity of the system, a progressive increase of the MES internal capacitance from 2.6 F to 3.7 F was observed, suggesting that electroactive biofilms can be used to develop bioelectrochemical supercapacitors.It can be concluded that bioelectrochemical conversion of CO2 to CH4 can be exploited as a potential technology for long-term (power-to-gas conversion) and short term (bio-capacitor) energy storage combined with CO2 sequestration. This approach could be seen as a pathway for MES integration with the existing industrial processes producing significant amounts of CO2 as well as with new technologies based on the biorefinery concept.Figure 1: a) The concept of Microbial Electrosynthesis System (MES) for CH4 production; b) schematic representation of a MES cell for CH4 production from CO2, and c) summary of the results obtained before and after the in situ electrodeposition of nickel at the cathode. Figure 1

CO2 photocatalytic reduction with CNT/TiO2 based nanocomposites prepared by high-pressure technology

Previous works with titanium dioxide based photocatalysts synthesized with supercritical fluids led to improved catalyst properties in comparison to those prepared by traditional methods, but still insufficient CO2 reduction rates. In this work, in order to upgrade this type of catalysts, two supercritical methods have been used to prepare CNT/TiO2 and CNT/TiO2/Cu nanocomposites. The effect on photocatalytic performance of CNT/TiO2 ratio and Cu load has been investigated. Decreasing photocatalytic activities were observed when the CNT/TiO2 relationship separated from unit. Regarding the effect of copper doping, it was overshadowed by the effect of CNT support. Finally, because of the excellent properties of CNTs, synthesized composites showed greater catalytic activities than bare TiO2. Specifically, the catalyst with largest activity led to CO and CH4 production rates of 8.1 and 1.1 μmol g−1 h−1, which were much higher than those on P25 (4 times for CO and 15 times for CH4).

Changing sediment and surface water processes increase CH4 emissions from human-impacted estuaries

Coastal waters are known to emit globally significant quantities of CH4, a potent greenhouse gas, but the potential of the rapid and ongoing human alterations to coastal areas to alter these emissions remains undefined. Here we addressed this gap by quantifying water-to-air CH4 fluxes and δ13C-CH4 values in sub-tropical estuaries at Low (n = 3), Moderate (n = 2), and High (n = 3) levels of human modification (agricultural land use, wastewater discharge), and sediment-to-water CH4 fluxes from the major benthic habitats in representative Low, Moderate, and High systems. An increase in water-to-air CH4 fluxes from 9.7 µmol m−2 d−1 (Low) to 28 µmol m−2 d−1 (Moderate) to 47 µmol m−2 d−1 (High) was accompanied by a shift from hydrogenotrophic to acetoclastic production pathways. Unexpectedly, benthic CH4 production, which ranged from −48 µmol m−2 d−1 to +180 µmol m−2 d−1 between habitats, estuaries, and seasons, was not the primary driver of this shift. Sediments produced more CH4 (∼600%) than emitted from the Low estuary, ∼90% of CH4 emitted from the Moderate estuary, but only 9% of CH4 emitted from the High estuary. Instead, a combination of wastewater, groundwater, and apparent water column production caused a ∼ 3-fold increase in estuary CH4 emissions. Our findings indicate that human alterations to the source, rate, and pathways of CH4 production are driving a net increase in emissions from estuaries, demonstrating a need to redefine how we quantify ‘anthropogenic’ CH4 emissions.

Influence of Carbon Dioxide on the Adsorption of Methane by Coal Using Low-Field Nuclear Magnetic Resonance

The adsorption capacity of coal with respect to carbon dioxide (CO2) is greater than that of methane (CH4). Studying the CO2 replacement of CH4 in coal is essential to understanding the mechanism behind CO2-enhanced coalbed methane. In this paper, scanning electron microscopy was used to qualitatively study the influence of CO2 injection pressure on the fracture characteristics of coal. The time effect of the CH4/CO2 adsorption and the effect of the CO2 injection on the CH4 adsorption were quantitatively studied using low-field nuclear magnetic resonance. The results suggest three different CH4 states in the coal samples: CH4 adsorbed on the pore surface, free CH4 in the pore center, and free CH4 between the coal particles. Indeed, the greater the CO2 injection pressure, the more developed the fracturing and network, which, in turn, increases connectivity. As the adsorption time of CH4/CO2 increases, the adsorption rate of CO2/CH4 gradually decreases. In essence, CO2 preferentially replaces the CH4 of the minipores. As the CO2 injection pressure increases, the difference between the CO2 equilibrium pressure and the initial injection pressure increases. Moreover, the CH4 production rate increases, the transverse relaxation time (T2) spectrum of the adsorption-state area decreases, the T2 spectrum of the free-state area increases, and the number of adsorption holes gradually decreases, whereas the number of seepage holes increases.

Effect of biogas recirculation strategy on biogas upgrading and process stability of anaerobic digestion of sewage sludge under slightly alkaline condition

In this study, in-situ biogas upgrading was examined and compared in lab-scale semi-continuous anaerobic digestion (AD) systems coupled with biogas recirculation under neutral and slightly alkaline conditions. Results showed that neutral pH (~7.5) condition yielded 6–7% higher CH4 content and 52–53% less H2S production, while slightly alkaline condition (pH ~8.0) achieved 14–15% higher CH4 content and 76–77% less H2S production under biogas recirculation in comparison to the control (no biogas recirculation). Little effect was noticed on daily CH4 production rate at slightly alkaline condition under biogas recirculation. Endogenous free ammonia nitrogen-dissolved inorganic carbon-volatile fatty acids (FAN-DIC-VFAs) buffer system was found to regulate the pH and alkalinity in the bulk liquor. Therefore, biogas recirculation under slightly alkaline condition is proposed as an efficient and sustainable strategy for biogas upgrading during AD of sewage sludge.

Integrated anaerobic digestion and CO2 sequestration for energy recovery from waste activated sludge by calcium addition: Timing matters

Anaerobic digestion (AD) with in-situ biogas upgrading is a technology that integrates methane production and selective CO2 sequestration in a one-pot process. This study investigated the effect of Ca2+ addition timing on thermophilic AD of waste activated sludge (WAS). CaCl2 solution was added to the digester on day 0, 2, 4, 6 and 8 at concentration of 3000 mg∙L−1. Ca2+ addition on day 4, after which exponential methane production started, achieved the best AD performance with cumulative CH4 production, maximum CH4 potential and maximum CH4 production rate increased by 20.8%, 20.8% and 50.2%, respectively. This treatment also remarkably reduced free ammonia inhibition and enhanced Methanosarcina abundance in the digester. A substantial increase (up to 58.4%) in the gas-phase CH4 to CO2 molar ratio was observed in the Ca2+-supplemented digesters, demonstrating the effectiveness of Ca2+ addition for CO2 sequestration. The relative change in the CH4 to gross CO2 ratio indicated the promoted hydrogenotrophic methanogenesis, possibly as a pertinent microbial mechanism behind the improved methane production.

Clumped Isotopes Link Older Carbon Substrates With Slower Rates of Methanogenesis in Northern Lakes

AbstractThe release of long‐stored carbon from thawed permafrost could fuel increased methanogenesis in northern lakes, but it remains unclear whether old carbon substrates released from permafrost are metabolized as rapidly by methanogenic microbial communities as recently produced organic carbon. Here, we apply methane (CH4) clumped isotope (Δ18) and 14C measurements to test whether rates of methanogenesis are related to carbon substrate age. Results from culture experiments indicate that Δ18 values are negatively correlated with CH4 production rate. Measurements of ebullition samples from thermokarst lakes in Alaska and glacial lakes in Sweden indicate strong negative correlations between CH4 Δ18 and the fraction modern carbon. These correlations imply that CH4 derived from older carbon substrates is produced relatively slowly. Relative rates of methanogenesis, as inferred from Δ18 values, are not positively correlated with CH4 flux estimates, highlighting the likely importance of environmental variables other than CH4 production rates in controlling ebullition fluxes.

Coupled effects of ferroferric oxide supplement and ethanol co-metabolism on the methanogenic oxidation of propionate

Direct interspecies electron transfer (DIET) is a new electron-transfer strategy for enhanced propionate degradation. Ethanol can enrich the DIET species of Geobacter and conductive ferroferric oxide (Fe3O4) can promote DIET. Therefore, coupled effects of ethanol and Fe3O4 on propionate degradation were investigated. The maximum CH4 production rate was increased by 81.4% by adding Fe3O4 when simultaneously fed with ethanol and propionate, while the improvement could not be observed without ethanol. The sludge conductivity and the electron transfer system activity by adding Fe3O4 were increased by 2.66 and 2.73 times, respectively. Besides, the relative abundance of functional microbes such as Geobacter, Syntrophobacter, Smithella, and Methanosaeta, and their functional genes were increased by the supplement of Fe3O4. The improvement of propionate degradation by adding Fe3O4 was largely attributed to the co-existence of ethanol degradation. The DIET between Geobacter and Methanosaeta might provide more energies or rapidly consume the oxidation products to promote the propionate degradation.

Assembling ultrafine TiO2 nanoparticles on UiO-66 octahedrons to promote selective photocatalytic conversion of CO2 to CH4 at a low concentration

The improvement of CO2 conversion efficiency over semiconductor photocatalysts is usually restricted by their poor CO2 adsorption capacity. In this work, a zirconium-based organic framework (UiO-66) was introduced as an effective CO2 adsorbent to couple with TiO2 photocatalyst. The designed two-step strategy endowed the TiO2/UiO-66 composite with hierarchical porous structure, which ensured the sufficient exposed catalytic sites and a high CO2 uptake of 78.9 cm3 g−1. In consequence, a high CH4 production rate of 17.9 μmol g-1 h-1 with 90.4 % selectivity was achieved in a mild gas-solid system using water as electron donors. Interestingly, the photocatalytic efficiency reached the similar level to that of pure CO2 atmosphere even under diluted CO2 condition (≤ 2%). Further, the enhanced mechanism was discussed in detail to present the superiority of such designed composite structure from the aspects of CO2 enrichment and exposure of sufficient catalytic sites.

Acetate turnover and methanogenic pathways in Amazonian lake sediments

Abstract. Lake sediments in Amazonia are a significant source of CH4, a potential greenhouse gas. Previous studies of sediments using 13C analysis found that the contribution of hydrogenotrophic versus acetoclastic methanogenesis to CH4 production was relatively high. Here, we determined the methanogenic pathway in the same sediments (n=6) by applying 14Cbicarbonate or 2-14Cacetate and confirmed the high relative contribution (50 %–80 %) of hydrogenotrophic methanogenesis. The respiratory index (RI) of 2-14Cacetate, which is 14CO2 relative to 14CH4+14CO2, divided the sediments into two categories, i.e., those with an RI < 0.2 consistent with the operation of acetoclastic methanogenesis and those with an RI > 0.4 showing that a large percentage of the acetate-methyl was oxidized to CO2 rather than reduced to CH4. Hence, part of the acetate was probably converted to CO2 plus H2 via syntrophic oxidation, thus enhancing hydrogenotrophic methanogenesis. This happened despite the presence of potentially acetoclastic Methanosaetaceae in all the sediments. Alternatively, acetate may have been oxidized with a constituent of the sediment organic matter (humic acid) serving as oxidant. Indeed, apparent acetate turnover rates were larger than CH4 production rates except in those sediments with a R<0.2. Our study demonstrates that CH4 production in Amazonian lake sediments was not simply caused by a combination of hydrogenotrophic and acetoclastic methanogenesis but probably involved additional acetate turnover.

CO2 injectability and CH4 recovery of the engineering test in qinshui Basin, China based on numerical simulation

An important CO2-ECBM (CO2 enhanced coalbed methane) engineering test will be carried out in #3 coal seam of Qinshui basin, China this year. This study is a preliminary work of CO2-ECBM engineering test with an aim at optimizing the engineering proposal and evaluating the effectiveness of CO2-ECBM. In this study, considering the competitive adsorption and diffusion of CO2 and CH4, seepage of gas and water, coal deformation, the dynamic evolution of coal seam porosity and permeability, and heat conduction and convection between coal and fluid, a fully coupled THM (thermo-hydro-mechanical) mathematical model for CO2-ECBM was established. The modelling was followed by a detailed discussion of the engineering well group’s CO2 injection and CH4 output characteristics under different CO2 injection pressures. The results confirm that CO2 injection has a positive effect on improving CH4 production. Within the simulated time, the CH4 recovery was enhanced by 4.26%–12.80%. Although the CO2 injectability of the engineering well group reduced significantly due to the drop in permeability near the CO2 injection well, the engineering well group has considerable CO2 storage potential. Under simulation conditions, the CO2 storage capacity for single CBM well is up to 0.73×105 - 2.54×105 m3 (standard temperature and pressure, STP). Increasing CO2 injection pressure can expand the coverage of CO2, thereby significantly improving the CH4 production rate and CO2 storage capacity. In addition, the permeability decrease caused by CO2 injection mainly affects the CO2 injection rate and storage capacity with weak influence on CH4 production and recovery.

Clarithromycin affect methane production from anaerobic digestion of waste activated sludge

Anaerobic digestion is widely identified as a sustainable technology for the treatment of waste activated sludge (WAS). However, various antibiotics enriched in WAS making the anaerobic digestion process full of uncertainty. This experiment studied the potential impact of a shock load of macrolide antibiotic clarithromycin (CLA) on CH4 production from anaerobic digestion of thermal pretreatments (55 °C, 10 h) WAS. Experimental results showed that the CH4 production rate was significantly suppressed in the existence of CLA during the first 10 days of anaerobic digestion, but gradually recovered thereafter. Although the total cumulative CH4 yield in the presence or absence of CLA showed little difference after 36 days of digestion, the maximal CH4 production rate decreased from 22.3 ± 0.7 mL/(g volatile suspended solids (VSS)·d) to 15.0 ± 0.4 mL/(g VSS·d) with CLA concentration elevated from 0 to 2000 mg/kg total suspended solids. Mechanism investigations showed that CLA had negative effect on the process of hydrolysis, acidogenesis, acetogenesis, homo-acetogenesis and methanogenesis, but its longer-time and more-severely inhibitory effect on acetogenesis and methanogenesis than other process was the main reason for the lower CH4 production rate in the CLA-added reactors. IonS5TMXL sequencing analysis showed that the relative abundance of typical syntrophic acetogens, such as Syntrophomonas and Syntrophorhabdus, and the methanogens, such as Methanosaeta and the hydrogenotrophic methanogens all increased with the addition of CLA at the later period of digestion. Their changes were consisted with the variations of volatile fatty acids and CH4 in digesters.

Acetotrophic methanogens are sensitive to long-term nickel contamination in paddy soil.

Paddy soil accounts for approximately one-fifth of the world's cultivated area and faces a serious threat from nickel (Ni). Ni pollution has an impact on the activity, composition and emission of methanogens in paddy, which is a major natural source of methane (CH4) emissions. We combined a high-throughput sequencing approach and laboratory incubation methods to evaluate the impact of long-term Ni pollution on the methanogenic archaeal community in paddy soil. The highest rate of CH4 production was 697 mg kg-1 of dry soil per d with the addition of sodium acetate at 50 mg kg-1 of Ni, which was significantly negatively correlated with the total and available Ni (p < 0.05). While the highest CH4 production rates were 485 and 544 mg kg-1 of dry soil per d with the addition of sodium formate and methanol, respectively, there was no significant difference in the CH4 production rate and maximum CH4 accumulation between the different Ni additions. Heavy pollution with 500 mg kg-1 of Ni unexceptionally inhibited the relative abundance of various genera of methanogens (22.2% in total). The abundance of acetotrophic Methanosaeta decreased with an increasing concentration of Ni (3.25-1.11%). The diverse nutrient types of species belonging to Methanosarcina were the highest under treatment with Ni200 (18.0%), and lowest in the soil with 500 mg kg-1 of Ni (2.8%). Similarly, the abundances of the most abundant hydrogenotrophic methanogens of Methanocellales were relatively high with Ni200 (26.2%) compared with those with Ni500 (5.4%). The mcrA gene was enriched under the light pollution treatment (50 mg kg-1 of Ni, 6.73 × 107 ± 9.0 × 106 copies per g of soil) compared with the control (4.18 × 107 ± 5.1 × 106 copies per g of soil). These results indicate that the long-term pollution by Ni has an impact on the activity and composition of methanogens with heavy Ni pollution, and in particular, acetotrophic methanogens are sensitive to Ni pollution in paddy soil.

Formation of a mixed-valence Cu(i)/Cu(ii) metal-organic framework with the full light spectrum and high selectivity of CO2 photoreduction into CH4.

Based upon the hetero-N,O ligand of pyrimidine-5-carboxylic acid (Hpmc), a new semiconductive Cu(i)/Cu(ii) mixed-valence MOF with the full light spectrum and a novel topology of {43·612·86}2{43·63}2{63}6{64·82}3, {(Cu4I4)2.5[Cu3(μ4-O) (μ3-I) (pmc)3(Dabco)3]·2.5DMF·2MeCN}∞ (NJU-Bai61, NJU-Bai for Nanjing University Bai group; Dabco = 1,4-diazabicyclo [2.2.2] octane), was synthesized stepwise. NJU-Bai61 exhibits good water/pH stabilities and a relatively large CO2 adsorption capacity (29.82 cm3 g−1 at 1 atm, 273 K) and could photocatalyze the reduction of CO2 into CH4 without additional photosensitizers and cocatalysts and with a high CH4 production rate (15.75 μmol g−1 h−1) and a CH4 selectivity of 72.8%. The CH4 selectivity is the highest among the reported MOFs in aqueous solution. Experimental data and theoretical calculations further revealed that the Cu4I4 cluster may adsorb light to generate photoelectrons and transfer them to its Cu3OI(CO2)3 cluster, and the Cu3OI(CO2)3 cluster could provide active sites to adsorb and reduce CO2 and deliver sufficient electrons for CO2 to produce CH4. This is the first time that the old Cu(i)xXyLz coordination polymers' application has been extended for the photoreduction of CO2 to CH4 and this opens up a new platform for the effective photoreduction of CO2 to CH4.

Preparation of Reduced Graphene Oxide (RGO) Modified Titanium Dioxide Nanotube (TNTs) as Visible Light Effective Catalyst for the Conversion of CO2 to CH4

In this study, a modified visible light active photocatalyst has been prepared where titanium dioxide nanotube (TNTs) was coupled with reduced graphene oxide (RGO) through a facile synthesis process. The photoactivity of the RGO-TNTs has been evaluated by converting CO2 to CH4 under visible light irradiation. In order to justify the photo-effectivity of the catalysts, physical and optical characterization were performed through FESEM, EDX, UV-Vis absorption spectra and PL spectra. The morphological analysis shows homogeneous RGO distribution on the TiO2 nanotube whereas elemental compositional analysis revealed the presence of all elements in the prepared catalyst. Visible light enhancement activity of the prepared catalyst after the incorporation of RGO was exposed by UV-Vis analysis due to its enhanced light absorption properties. Improved electron-hole separation rate was investigated for the RGO incorporated TNTs through PL analysis. The fruitful incorporation of the RGO with TNTs further affirmed by the increased photocatalytic activity by converting CO2 to CH4 that acquired enhanced CH4 production (9.27%) which is 1.81 times higher than the CH4 production rate obtained through TNTs (5.12%). Thus, this study induces a simple synthesis procedure to modify TNTs as a visible light active photocatalyst with the integration of RGO. Moreover, RGO-TNTs enhanced visible light activity were evaluated through the conversion of CO2 to CH4.

Localized surface plasmon resonance enhanced visible-light-driven CO2 photoreduction in Cu nanoparticle loaded ZnInS solid solutions.

Visible-light-driven photocatalysts have shown tremendous prospects in solving the energy crisis and environmental problems, thanks to their wide spectral response and high quantum efficiency. Several strategies including the expansion of visible light response and the improvement of solar energy utilization and photocatalytic quantum efficiency via more effective separation of photogenerated carriers are the current focuses of research that direct the design and fabrication of viable photocatalysts. Herein, a series of composite photocatalysts assembled from plasmonic Cu nanoparticles (NPs) and Zn3In2S6 (ZIS) solid solutions were synthesized by means of a simple solvothermal method. In comparison with the pristine ZIS semiconductor, Cu NP loaded ZIS solid solutions showed greatly enhanced photocatalytic activity, selectivity and stability towards CO2 reduction under visible irradiation. Of note was that the optimized ZIS-Cu2 exhibited an enhanced CH4 production rate of ca. 292 μL g-1 h-1 and a selectivity of ca. 71.1%, which were among the highest numbers reported hitherto. The localized surface plasmon resonance (LSPR) effect, shown by surface Cu NPs, was believed to play a critical role in the enhanced CO2 photoreduction efficiency. More importantly, the introduction of plasmonic Cu NPs could restrain the recombination of photogenerated electron-hole pairs and promote the migration of photogenerated electrons to better participate in the photocatalytic CO2 reduction in the presence of water vapor. This work thus provides a facile means to design robust and flexible composite photocatalysts for visible-light-driven CO2 photoreduction with high efficiency.

Photocatalytic CO2 reduction of C/ZnO nanofibers enhanced by an Ni-NiS cocatalyst.

The photocatalytic reduction of CO2 into valuable hydrocarbon fuels via solar energy is a promising strategy for carbon utilization. In the present paper, a hierarchical Ni-NiS/C/ZnO photocatalyst was prepared via the in situ photodeposition of compact Ni-NiS nanosheets onto C/ZnO electrospun nanofibers. The existence of metallic Ni and NiS was confirmed by X-ray photoelectron spectroscopy. Photoluminescence (PL) and time-resolved PL spectra revealed that the cocatalyst Ni-NiS enhanced the charge separation efficiency of the C/ZnO nanofibers. The as-prepared Ni-NiS/C/ZnO showed enhanced CO2 reduction activity, with CO and CH4 production rates 10 and 15 times greater than those of pristine C/ZnO under 350 W visible light illumination. The intermediates of CH3O-, HCHO, and HCOO- were detected by in situ Fourier transform infrared spectroscopy, confirming that CO2 reduction is a complex reaction with multiple steps. The 13C isotopic tracer method proved that CH4 and CO were obtained from the reduction of CO2 rather than from other carbon species in the environment. The amorphous carbon in C/ZnO could promote optical absorption, improve conductivity and reduce the interfacial charge transport resistance. Ni-NiS improved the electron-hole-pair separation of the C/ZnO nanofibers. The observed enhancement in photocatalytic activity was largely attributed to higher light utilization and effective electron-hole separation. This work proves that Ni-NiS is a promising cocatalyst to ZnO for photocatalytic CO2 reduction.

Biohythane production via single-stage anaerobic fermentation using entrapped hydrogenic and methanogenic bacteria

This study demonstrates the continuous biohythane production in a single-stage anaerobic digester using a biomass mixture of separately entrapped hydrogenic and methanogenic bacteria (H2- and CH4-producing bacteria, respectively). The entrapped hydrogenic/methanogenic bacteria biomass ratios of 1/4, 2/3, 3/2 and 4/1 were tested and shown to have a great effect on the single-stage biohythane production performance. At steady-states, the cultivations had biohythane production rates in the range of 381–480 mL/L-d, with H2 content in biohythane (HCH) varying from 1% to 75% (v/v) and chemical oxygen demand removal efficiencies (TCODre) of 57.6–81.9%. Biomass ratio 2/3 (weight ratio 1/1.5) resulted in peak biohythane production with H2 and CH4 production rates being 64.6 and 395 mL/L-d, respectively, HCH 15% and TCODre 74.4%. The novelty of this work is to show the potential of producing biohythane from an innovative single-stage dark fermentation system using entrapped hydrogenic and methanogenic bacteria.

Fabrication of hierarchical ZnIn2S4@CNO nanosheets for photocatalytic hydrogen production and CO2 photoreduction

Photocatalytic H2 production and CO2 reduction have attracted considerable attention for clean energy development. In this work, we designed an efficient photocatalyst by integrating lamellar oxygen-doped carbon nitride (CNO) nanosheets into ZnIn2S4 (ZIS) microflowers by a one-step hydrothermal method. A well-fitted 2D hierarchical hybrid heterostructure was fabricated. Under visible light irradiation, the ZIS@CNO composite with 40 wt% CNO (ZC 40%) showed the highest hydrogen evolution rate from water (188.4 μmol·h−1), which was approximately 2.1 times higher than those of CNO and ZIS (88.6 and 90.2 μmol·h−1, respectively). Furthermore, the selective CO production rates of ZC 40% (12.69 μmol·h−1) were 2.2 and 14.0 times higher than those of ZIS (5.85 μmol·h−1) and CNO (0.91 μmol·h−1), respectively, and the CH4 production rate of ZC 40% was 1.18 μmol·h−1. This enhanced photocatalytic activity of CNO@ZIS is due mainly to the formation of a heterostructure that can promote the transfer of photoinduced electrons and holes between CNO and ZIS, thereby efficiently avoiding recombination of electron-hole pairs.

Methane production from rice straw carbon in five different methanogenic rice soils: rates, quantities and microbial communities

The input of organic substances (e.g., rice straw) in rice field soils usually stimulates the production and emission of the greenhouse gas methane (CH4). However, the amount of CH4 derived from the applied rice straw, as well as the response of bacterial and archaeal communities during the methanogenic phase, are poorly understood for different rice field soils. In this study, samples of five different rice soils were amended with 13C-labeled rice straw (RS) under methanogenic conditions. Immediately after RS addition, the RS-derived CH4 production rates were higher in soils (Uruguay, Fuyang) that possessed a stronger inherent CH4 production potential compared with other soils with lower inherent potentials (Changsha, the Philippines, Vercelli). However, soils with higher inherent potential did not necessarily produce higher amounts of CH4 from the RS applied, or vice versa. Quantitative PCR showed copy numbers of both bacteria and methanogens increased in straw-amended soils. High-throughput sequencing of 16S rRNA genes showed distinct bacterial communities among the unamended soil samples, which also changed differently in response to RS addition. Nevertheless, RS addition generally resulted in all the rice field soils in a relative increase of primary fermenters belonging to Anaerolineaceae and Ruminococcaceae. Meanwhile, RS addition also generally resulted in a relative increase of Methanosarcinaceae and/or Methanocellaceae. Our results suggest that after RS addition the total amounts of RS-derived CH4 are distinct in different rice field soils under methanogenic conditions. Meanwhile, there are potential core bacterial populations that are often involved in primary fermentation of RS under methanogenic conditions.