Optimization of organosolv pretreatment of rice straw for enhanced biohydrogen production using Enterobacter aerogenes
Introduction
The world's growing population leads to the increase in demands for food and energy, and many environmental problems (Holdren, 1991, Lutz and Shah, 2002). Although about 80% of global energy demand is provided by fossil fuels, they are depleting and releasing large amounts of carbon dioxide into the atmosphere (Popp et al., 2014). Hydrogen is one of the potential alternative fuels widely used in several industries. Agricultural nitrogen fertilizers are mainly derived from ammonia synthesized by hydrogen. In addition, it is considered as the cleanest available fuel with the highest energy content (Suleman et al., 2015). Currently, hydrogen is commercially obtained from natural gas. Biological hydrogen production from crop residues is a renewable way to supply part of the global hydrogen demand, which might be utilized to provide cultivation energy or nitrogen fertilizer. Rice straw is one of the abundant agricultural residues in the world. Due to the nutrient deficiencies, it is a poor livestock feed (Kumar et al., 2015); however, because of the high carbohydrate content, it can be considered as an appropriate feedstock to produce biofuels (Binod et al., 2010). Before converting the carbohydrates into monomeric sugars through enzymatic hydrolysis, a pretreatment operation is needed to remove lignin from lignocellulosic biomass. Among different delignification technologies, organosolv pretreatment is an effective method not only to make cellulose more accessible to enzymes, but also to produce lignin with high purity and uniformity, which can be used to produce other chemicals (Cybulska et al., 2017, Goh et al., 2011).
Due to some considerable advantages, biohydrogen production by pure cultures has recently received great attention. For these cultures, the metabolic pathways and resulting metabolites can be detected and the optimum conditions to promote hydrogen production can be more easily adjusted. Simple sugars such as glucose have commonly been utilized in most studies that used a pure culture (Elsharnouby et al., 2013, Shah et al., 2016), while production of hydrogen from lignocellulosic materials comprises additional steps of pretreatment and hydrolysis that can significantly affect the performance of the pure culture system. Enterobacter aerogenes is an outstanding hydrogen producer. It is an anaerobic facultative and mesophilic bacterium that is able to consume different sugars and in contrast to cultivation of strict anaerobes, no special operation is required to remove all oxygen from the fermenter. E. aerogenes has a short doubling time and high hydrogen productivity and evolution rate (Martinez-Porqueras et al., 2013, Tanisho, 1998). Furthermore, hydrogen production by this bacterium is not inhibited at high hydrogen partial pressures; however, its yield is lower compared to strict anaerobes like Clostridia (Martinez-Porqueras et al., 2013, Roy and Das, 2016). A theoretical maximum of 4 mol H2 mol−1 glucose can be produced by strict anaerobic bacteria (Lee et al., 2014). Facultative anaerobic bacteria such as E. aerogenes have a theoretical maximum yield of 2 mol H2 mol−1 glucose (Lee et al., 2014, Vardar-Schara et al., 2008). It has been reported that the hydrogen yield by E. aerogenes is lower than the theoretical value (Lu et al., 2011). Based on the metabolism of E. aerogenes, Converti and Perego (2002) reported that by changing the glucose concentration from 9 to 72 g l−1 the yield coefficient of hydrogen varied from 0.56 to 0.66 mol H2 mol−1 glucose. A variety of methods such as using recombinant strains (Ma et al., 2012, Song et al., 2016), improving culture media and fermentation operation (Jitrwung et al., 2013, Satar et al., 2016, Tanisho et al., 1989), and employing co-cultures (Lu et al., 2007, Yokoi et al., 1998) has been proposed to improve the hydrogen yield of E. aerogenes. Moreover, statistical approaches such as response surface methodology (RSM) can be employed to maximize the hydrogen production by optimization of operational factors. In contrast to conventional methods, the interaction among process variables can be determined by statistical techniques (Reungsang et al., 2013). Jo et al. (2008) investigated the optimum conditions for hydrogen production from glucose by E. aerogenes. They used a Box–Behnken design to optimize the fermentation variables including glucose concentration, temperature and pH. In a study by Reungsang et al. (2013), RSM was applied to analyze the effect of initial pH, temperature, amount of vitamin solution, yeast extract and glycerol concentrations on the simultaneous hydrogen and ethanol production from glycerol. Pachapur et al. (2015) examined a co-culture of E. aerogenes and C. butyricum to obtain hydrogen from apple pomace hydrolysate co-fermented with crude glycerol. A central composite design (CCD) was employed to enhance hydrogen production by varying the crude glycerol concentration, apple pomace hydrolysate concentration, and inoculum size. Since pretreatment process can significantly influence the enzymatic hydrolysis and, consequently, fermentation operation, some studies developed statistical methods to investigate the optimum operational conditions for the pretreatment of lignocelluloses. Goh et al. (2011) studied the effect of sulfuric acid concentration, reaction temperature, and retention time on the pretreatment of empty palm fruit bunch by ethanol organosolv process using CCD method and the optimum values of parameters were determined for glucose recovery. In another investigation (Cybulska et al., 2017), RSM was applied to optimize temperature, catalyst content, and ethanol concentration for the organosolv pretreatment of date palm fronds. Lignin recovery and glucan digestibility were selected as the response variables.
There is no information, to the best of the authors’ knowledge, concerning the biohydrogen production from organosolv pretreated rice straw by E. aerogenes. In this study, a central composite design is employed to investigate the effect of critical parameters of organosolv pretreatment of rice straw including temperature, time, and ethanol concentration. In addition, variations in the residual solid, lignin recovery, and hydrogen yield by the severity factor are studied. Eventually, based on the experimental results, potential production of biohydrogen in the top ten rice producing countries and its application in providing nitrogen fertilizer for the rice cultivation are estimated.
Section snippets
Materials
Rice straw was collected from a rice planting area located in Lenjan, Isfahan, Iran. It was washed and air-dried and then crushed by a laboratory mill to reduce the size of particles and was passed through the 20–80 mesh sieve to achieve the desirable particle size. Then it was stored in air tight bags before use.
The cellulase complex Cellic CTec2 (Novozyme, Denmark) with the activity of 95 FPU ml−1 (measured by the standard method presented by Adney and Baker (2008)) was used for the enzymatic
Model fitting analysis
Based on the three-variable central composite design, the experimental results and predicted values of hydrogen overall yield (YH2 (ml g−1 straw)) and residual biomass after pretreatment (RB (g treated straw g−1 rice straw)) are shown in Table 1. The statistical analysis of variance (ANOVA) was used to determine the significant correlations. The insignificant terms (p-value > 0.05), except those required to maintain hierarchy, were removed to obtain the better responses. The fitted models in terms of
Conclusions
Ethanol concentration and temperature had a strong effect on the hydrogen yield and residual biomass, respectively. A higher severity factor could increase the enzymatic digestibility; however, under strict conditions, the amount of residual solid decreased significantly. Due to the more water activity, the hydrogen overall yield was more sensitive to the residence time at the lower ethanol concentration. Under the optimum pretreatment conditions, a higher glucose concentration was obtained
Conflict of interest
The authors declare that there are no conflicts of interest.
References (50)
- et al.
Bioethanol production from rice straw: an overview
Bioresour. Technol.
(2010) - et al.
Organosolv delignification of agricultural residues (date palm fronds, Phoenix dactylifera L.) of the United Arab Emirates
Appl. Energy
(2017) - et al.
A critical literature review on biohydrogen production by pure cultures
Int. J. Hydrogen Energy
(2013) - et al.
Techno-economic analysis of lignocellulosic ethanol: a review
Bioresour. Technol.
(2010) - et al.
Evaluation and optimization of organosolv pretreatment using combined severity factors and response surface methodology
Biomass Bioenergy
(2011) - et al.
Optimization of selected salts concentration for improved biohydrogen production from biodiesel-based glycerol using Enterobacter aerogenes
Renewable Energy
(2013) - et al.
Optimization of key process variables for enhanced hydrogen production by Enterobacter aerogenes using statistical methods
Bioresour. Technol.
(2008) - et al.
Pretreatment of lignocellulose: formation of inhibitory by-products and strategies for minimizing their effects
Bioresour. Technol.
(2016) - et al.
Biochemical production of bioenergy from agricultural crops and residue in Iran
Waste Manage.
(2016) - et al.
Global potential bioethanol production from wasted crops and crop residues
Biomass Bioenergy
(2004)