Enhanced biohydrogen production from macroalgae by zero-valent iron nanoparticles: Insights into microbial and metabolites distribution
Graphical abstract
Introduction
Dark fermentation is considered as a promising method for hydrogen generation because of the mild reaction conditions, easy operation and wide ranges of substrate sources (Wang and Yin, 2017a, Wang and Yin, 2017b). Among various feedstocks, macroalgae owns great potential for its high carbohydrates composition, high growth rate, easier hydrolysis without lignin, no occupation of farmland and low requirement of cultivation conditions. Besides, its excellent ability of fixing CO2 further promoted algae cultivation as energy crop (Wei et al., 2013, Ren et al., 2019). Furthermore, with the application of algae in adsorbing contaminants in marine environment (Wang and Chen, 2009), macroalgae cultivated in polluted seawater can also be a good source for energy recovery. However, the application of fermentative hydrogen production from macroalgae is restricted by the low hydrogen yield. Besides the explorations of pretreatment methods (Yin and Wang, 2018), enhancing hydrogen yield through process improvement seems more energy saving and environmentally friendly.
Iron has been widely reported to be helpful in enhancing the hydrogen production through dark fermentation (Yang and Wang, 2018a, Yang and Wang, 2018b). Ferrous ion (Fe2+) is the active site of functional proteins for hydrogen formation, like ferredoxins and hydrogenases. Fe2+ is also helpful to motivate the expression of genes responsible for hydrogenase synthesis (Zhao et al., 2017). In addition, recent studies have found that zero-valent iron (Fe0) could bring more benefit. Besides the beneficial Fe2+ obtained by iron corrosion, Fe0 could also help to eliminate the residual oxygen present in the system, and help to maintain a low oxidation–reduction potential (ORP), making it more favorable for hydrogen-producing metabolism (Yang and Wang, 2018b). Furthermore, Fe0 has also been reported to be able to catalyse the dissociation of propionate, forming hydrogen and acetate (Zhen et al., 2015). Comparing with Fe0 scraps, Fe0 nanoparticles (Fe0 NPs) can be better dispersed in fermentation broth, and much bigger surface for reactions. Yang and Wang (2018a) explored 0–600 mg/L Fe0 NPs on hydrogen generation from grass, obtained highest hydrogen yield of 64.7 mL/g dry grass with 400 mg/L Fe0 NPs addition. Camacho et al. (2018) studied hydrogen production from organic market waste with 0–2000 mg/L Fe0 NPs addition, enhanced hydrogen yield of 101.7 mL/g VS was attained at 2000 mg/L Fe0 NPs dose. Great variance of optimum Fe0 NPs dose and hydrogen yield indicates the effect of Fe0 NPs on hydrogen production is greatly affected by specific fermentation conditions, like substrate sources, inoculum, and operational conditions.
Unlike the widely studied hydrogen-producing feedstocks (sugars, lignocellulosic biomass), macroalgae is rich in triglyceride, glycogen, fucoidan and trehalose, etc. Its decomposition and conversion process during dark fermentation remains unclear, making it hard to enhance the hydrogen yield accurately targeting substrate treatment. Furthermore, macroalgae is characterized to be rich in sulfur (Suutari et al., 2015), which is the well-known inhibitor for fermentative hydrogen production. Thus, explorations of biomass conversion and enhancing the hydrogen yield through process improvement is worth to be explored.
This study explored the effect of Fe0 NPs on macroalgae fermentation. Hydrogen generation, products distribution and energy conversion were examined to reveal the influence of Fe0 NPs doses on energy recovery from macroalgal biomass. Changes of microbial distributions and microbial interactions were analyzed to explore the impacting mechanism. Furthermore, Principal Components Analysis was conducted to reveal the correlations among Fe0 NPs doses, hydrogen production, dominant microbes and volatile fatty acids formation.
Section snippets
Inoculum, macroalgae biomass and Fe0 NPs
Anaerobic sludge was taken from a primary digester of a sewage treatment plant. Before inoculation, heat treatment (100 °C, 15 min) was applied to eliminate the hydrogen consumers. (Yin and Wang, 2018). Total solids (TS) and volatile solids (VS) of inoculum are 868.8 ± 56.3 mg/L and 342.1 ± 10.2 mg/L, respectively. Macroalgae Saccharina japonica was used as substrate. Macroalgae was pretreated by heat-acid treatment (1% H2SO4, 121 °C, 30 min) to enhance the reaction efficiency during
Hydrogen production
Effect of Fe0 on CHP from macroalgae and the corresponding dynamic analysis are demonstrated in Fig. 1 and Table 1. Hydrogen production was significantly enhanced with the Fe0 NPs addition (Fig. 1). Both hydrogen production potential (P), maximum hydrogen production rate (Rmax) and hydrogen yield (HY) were increased, while lag time (λ) was expressively shortened (Table 1). Highest CHP of 26 mL/100 mL and HY of 20.25 mL H2/g VSadded were obtained in Fe0-200 mg/L group, which were 6.5 times of
Conclusions
This study firstly explored the effect of Fe0 NPs on macroalgae dark fermentation by process analysis. Products distribution and microbial interactions were in-depth analyzed. Results showed that with the supplementation of Fe0 NPs, mineralization effect of organics was impaired with the elimination of species Acinetobacter lwoffii. Both hydrogen generation and acids accumulation were enhanced with the enrichment of Clostridium sp. and Terrisporobacter sp. Positive correlations were more
Acknowledgement
The authors would like to gratitude the financial support from the China Postdoctoral Science Foundation (M640144) and the National Postdoctoral Program for Innovative Talents.
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