Increasing biohydrogen production with the use of a co-culture inside a microbial electrolysis cell

Review of cathodic electroactive bacteria: Species, properties, applications and electron transfer mechanisms

Cathodic electroactive bacteria (C-EAB) which are capable of accepting electrons from solid electrodes provide fresh avenues for pollutant removal, biosensor design, and electrosynthesis. This review systematically summarized the burgeoning applications of the C-EAB over the past decade, including 1) removal of nitrate, aromatic derivatives, and metal ions; 2) biosensing based on biocathode; 3) electrosynthesis of CH4, H2, organic carbon, NH3, and protein. In addition, the mechanisms of electron transfer by the C-EAB are also classified and summarized. Extracellular electron transfer and interspecies electron transfer have been introduced, and the electron transport mechanism of typical C-EAB, such as Shewanella oneidensis MR-1, has been combed in detail. By bringing to light this cutting-edge area of the C-EAB, this review aims to stimulate more interest and research on not only exploring great potential applications of these electron-accepting bacteria, but also developing steady and scalable processes harnessing biocathodes.

Harnessing recalcitrant lignocellulosic biomass for enhanced biohydrogen production: Recent advances, challenges, and future perspective

Biohydrogen (Bio-H2) is widely recognized as a sustainable and environmentally friendly energy source, devoid of any detrimental impact on the environment. Lignocellulosic biomass (LB) is a readily accessible and plentiful source material that can be effectively employed as a cost-effective and sustainable substrate for Bio-H2 production. Despite the numerous challenges, the ongoing progress in LB pretreatment technology, microbial fermentation, and the integration of molecular biology techniques have the potential to enhance Bio-H2 productivity and yield. Consequently, this technology exhibits efficiency and the capacity to meet the future energy demands associated with the valorization of recalcitrant biomass. To date, several pretreatment approaches have been investigated in order to improve the digestibility of feedstock. Nevertheless, there has been a lack of comprehensive systematic studies examining the effectiveness of pretreatment methods in enhancing Bio-H2 production through dark fermentation. Additionally, there is a dearth of economic feasibility evaluations pertaining to this area of research. Thus, this review has conducted comparative studies on the technological and economic viability of current pretreatment methods. It has also examined the potential of these pretreatments in terms of carbon neutrality and circular economy principles. This review paves the way for a new opportunity to enhance Bio-H2 production with technological approaches.

Date seed waste derived nanocatalyst and its application in production of hydrolytic enzyme, fermentative sugars and biohydrogen

Biofuel production from cellulosic biomass is a promising approach; however, the cost-intensive utilization of cellulolytic enzymes is a major roadblock to economic production. This study reports the preparation of a nanocatalyst using date seed and evaluates the impact of nanocatalysts on cellulolytic enzyme production using solid-state fermentation of date pulp waste through bacterial co-cultivation. Under optimized conditions, 30 IU/gds filter paper activity is produced in the presence of 2 mg of nanocatalyst. Cellulase showed thermal stability at 50 °C and pH 7 up to 10 h in the presence of nanocatalyst, and it produced 32.31 g/L glucose through the hydrolysis of acidic-pretreated date seeds in 24 h. Subsequently, 1788 mL/L of cumulative H2 in 24 h through cocultured dark fermentation could be produced. The approach presented in this study can be effective for multiple value additions, including nanocatalyst preparation, cellulase enzyme, and biohydrogen production.

Zinc recovery from bioleachate using a microbial electrolysis cell and comparison with selective precipitation

Metal recycling is essential for strengthening a circular economy. Microbial leaching (bioleaching) is an economical and environmentally friendly technology widely used to extract metals from insoluble ores or secondary resources such as dust, ashes, and slags. On the other hand, microbial electrolysis cells (MECs) would offer an energy-efficient application for recovering valuable metals from an aqueous solution. In this study, we investigated a MEC for Zn recovery from metal-laden bioleachate for the first time by applying a constant potential of -100 mV vs. Ag/AgCl (3 M NaCl) on a synthetic wastewater-treating bioanode. Zn was deposited onto the cathode surface with a recovery efficiency of 41 ± 13% and an energy consumption of 2.55 kWh kg-1. For comparison, Zn recovery from zinc sulfate solution resulted in a Zn recovery efficiency of 100 ± 0% and an energy consumption of 0.70 kWh kg-1. Furthermore, selective metal precipitation of the bioleachate was performed. Individual metals were almost completely precipitated from the bioleachate at pH 5 (Al), pH 7 (Zn and Fe), and pH 9 (Mg and Mn).

Optimization of dark fermentative biohydrogen production from rice starch by Enterobacter aerogenes MTCC 2822 and Clostridium acetobutylicum MTCC 11274

Dark fermentative biohydrogen production (DFBHP) has potential for utilization of rice starch wastewater (RSWW) as substrate. The hydrogen production of Enterobacter aerogenes MTCC 2822 and Clostridium acetobutylicum MTCC 11274, in pure culture and co-culture modes, was evaluated. The experiments were performed in a 2 L bioreactor, for a batch time of 120 h. The co-culture system resulted in highest cumulative hydrogen (1.13 L H₂/L media) and highest yield (1.67 mol H₂/mol glucose). Two parameters were optimized through response surface methodology (RSM)-substrate concentration (3.0-5.0 g/L) and initial pH (5.5-7.5), in a three-level factorial design. A total of 11 runs were performed in duplicate, which revealed that 4.0 g/L substrate concentration and 6.5 initial pH were optimal in producing hydrogen. The metabolites produced were acetic, butyric, propionic, lactic and isobutyric acids. The volumetric H₂ productions, without and with pH adjustments, were 1.24 L H₂/L media and 1.45 L H₂/L media, respectively.

Conversion of lignocellulosic biomass: Production of bioethanol and bioelectricity using wheat straw hydrolysate in electrochemical bioreactor

The present study evaluated efficiency of wheat straw (WS) hydrolysate obtained through fungal pre-treatment to produce ethanol and electricity in an electrochemical bioreactor. Three white rot fungi Phanerochaete chrysosporium, Phlebia floridensis and Phlebia brevispora were used to degrade WS for hydrolysate preparation, Lignocellulolytic enzyme production was also monitored during the pretreatment. Yeast Pichia fermentans was allowed to ferment all three hydrolysates up to 12 days. The yeast showed maximum electrochemical response as open circuit voltage (0.672 V), current density 542.42 mA m^-2, and power density of 65.09 mW m^-2 on 12th day in the hydrolysate prepared using Phlebia floridensis. Maximum ethanol production of 9.2% (w/v) was achieved on 7th day with a fermentation efficiency of about 62.1%. Further, the coulombic efficiency improved from 0.06 to 1.46% during 12 days of the experiment. Thus, the results indicated towards the possible conversion of lignocellulosic biomass into bioethanol along with bioelectricity generation.

Novel strategies towards efficient molecular biohydrogen production by dark fermentative mechanism: present progress and future perspective

In the scenario of alarming increase in greenhouse and toxic gas emissions from the burning of conventional fuels, it is high time that the population drifts towards alternative fuel usage to obviate pollution. Hydrogen is an environment-friendly biofuel with high energy content. Several production methods exist to produce hydrogen, but the least energy intensive processes are the fermentative biohydrogen techniques. Dark fermentative biohydrogen production (DFBHP) is a value-added, less energy-consuming process to generate biohydrogen. In this process, biohydrogen can be produced from sugars as well as complex substrates that are generally considered as organic waste. Yet, the process is constrained by many factors such as low hydrogen yield, incomplete conversion of substrates, accumulation of volatile fatty acids which lead to the drop of the system pH resulting in hindered growth and hydrogen production by the bacteria. To circumvent these drawbacks, researchers have come up with several strategies that improve the yield of DFBHP process. These strategies can be classified as preliminary methodologies concerned with the process optimization and the latter that deals with pretreatment of substrate and seed sludge, bioaugmentation, co-culture of bacteria, supplementation of additives, bioreactor design considerations, metabolic engineering, nanotechnology, immobilization of bacteria, etc. This review sums up some of the improvement techniques that profoundly enhance the biohydrogen productivity in a DFBHP process.

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

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