Biochar boosts dark fermentative H2 production from sugarcane bagasse by selective enrichment/colonization of functional bacteria and enhancing extracellular electron transfer
Graphical abstract
Introduction
Biohydrogen is regarded as an attractive candidate due to its significant properties of CO2-neutral and high calorific value (142 KJ/g) (Arizzi et al., 2016, Sivagurunathan et al., 2017, Zhang et al., 2011). Conventional hydrogen production processes are usually unsustainable and energy consuming (Hallenbeck 2009). In this case, dark fermentative hydrogen production has drawn increasingly attentions due to its dual benefits of low operating cost, low energy demand and simple operation conditions (Wong et al., 2014, Yang and Wang 2018a). Moreover, a versatile range of organic wastes, such as wastewater, sewage sludge, food waste, agricultural waste, could be used as substrates for hydrogen production during dark fermentation. Among them, biological hydrogen production from lignocellulosic biomass is considered as the most promising mode (Kumari and Das 2016, Zhao et al., 2017a), which could simultaneously achieve the production of renewable hydrogen and the utilization of abundant organic waste.
Despite the advantages mentioned above, hydrogen production by dark fermentation is usually limited by the low yield. To solve this, many strategies have been developed and applied in dark fermentation to improve hydrogen production and yield (Yang and Wang 2018b). Recently, biochar was found to be an efficient additive in improving the performance of AD and have attracted more and more research interests. BC is produced by thermal decomposition or pyrolysis of carbonaceous biomass, such as agricultural residues and sewage sludge, in a temperature range of 300–900°C (Lee et al., 2017), with favorable properties such as high aromaticity, a large surface area and diverse surface functional groups on surface (Sathishkumar et al., 2020). The biomass immobilization function of BC by its excellent porosity with large surface area has been proven in many studies (Li et al., 2020b, Kyriakou et al., 2019). Also, trace minerals contained in biochar could serve as inexpensive nutrients to replace expensive additives (Sun et al., 2020). BC could also functioned as pH buffer to mitigate the decrease of pH and acid inhibition (Sunyoto et al., 2016). Besides, Biochar could improve electron transfer efficiency between exoelectrogenic microorganisms through two completely different pathways: 1. Biochar prepared at high temperature acts as an electrical conductor and could directly transfer electron through conjugated π-bond (Wang et al., 2021); 2. The biochar prepared at low temperature acts like a battery, using surface quinone/hydroquinone groups for reversible cycles for charging and discharging (Kluepfel et al., 2014, Sun et al., 2017). Although the benefits of BC on AD have been extensively confirmed, the influence of biochar on the process of hydrolysis/acidification during AD has been ignored and the potential mechanisms has not been explored.
The secretion of extracellular polymeric substances (EPS) during AD has been widely studied. EPS is mainly composed of proteins and polysaccharides (Cheng et al., 2020b, Yu et al., 2008). The composition and spatial distribution of EPS are vital to the metabolism of microorganisms and performance of AD (Yu et al., 2008). Nevertheless, the effects of BC on the composition and function during dark fermentation has been rarely studied, especially in lignocellulosic system. Furthermore, the correlation between the electrochemical property of EPS and the enhancement of dark fermentative hydrogen production by BC addition is unproven. Extracellular oxidation–reduction potential (ORP) of the fermentation system is an important parameter in controlling microbial metabolism by affecting the intracellular ORP through the oxidized/reduced NAD (NADH/NAD+) balance, which could subsequently control gene expression, enzyme synthesis and modify the whole metabolic pathways (Liu et al., 2013, Moscoviz et al., 2016).
To the best of the authors’ knowledge, few studies have been conducted to investigate the effect of BC on the process of hydrolysis/acidification during AD with lignocellulosics as substrate and to elucidate the underlying mechanisms. Therefore, this study aims to: (1) examine the effects of BCs with different physicochemical properties on mesophilic dark fermentative performance with PSCB as substrate; (2) systematically explore the effects of BC supplementation on multiple indicators, including the contributions on changes of cell biomass, extracellular ORP, cellulase/hydrogenase activity, NAD+/NADH, the characters of EPS, the electrochemical property of fermentation system; (3) propose hypothesis to explain the mechanisms of enhanced fermentative performance by BC amendment.
Section snippets
Anaerobic sludge and inoculum
Sewage sludge (SS) was taken from anaerobic pond bottom of Lijiao sewage treatment plant (Guangzhou, Guangdong province, China). The SS was acclimated with hydrogen-producing effluent and lignocellulosic residue in a long run. Before using, the SS was withdrawn into serum bottle and mixed well with sterilized saline (1:1, v/v). After purging with nitrogen, the bottle was heated at 90 °C for 15 min to restrain methanogenic activities. The bacterial suspension with cell number of 6.6 × 107
Biochar characteristics
BCs with different physicochemical properties were prepared under different temperatures. According to the elemental analysis in Table 1, total N, H and O contents decreased while total C contents increased with the pyrolysis temperature increased. BC600 had the lowest H:C (0.56) and O:C (0.03) molar ratios, suggesting a high degree of aromatic condensation and the loss of H and O-containing functional groups containing (). The conductivities and surface areas of BCs increased drastically as
Conclusions
This study demonstrated that mesophilic dark fermentative performance of biohydrogen production from PSCB could be significantly improved by BC amendment. Cell biomass, key enzyme activities, NADH/NAD+ ratio, and electron transfer efficiency of fermentation system were markedly improved by BC600 amendment. Additionally, cellulolytic Lachnospiraceae was efficiently enriched and electroactive bacteria such as Pseudomonadaceae, Clostridiaceae and Peptostreptococcaceae were selectively colonized on
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
This study was supported by the National Natural Science Foundation of China [grant nos. 51878291, 51908140 & 52070079], Guangzhou Science and Technology Program [grant no. 202002030137], and Open Project Funding of the Key Laboratory of Fermentation Engineering (Ministry of Education) [grant no. 202105FE01].
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