Fermentative hydrogen production from macroalgae Laminaria japonica pretreated by microwave irradiation
Introduction
Algae-based biomass have gained increased attention as third-generation source of biofuel [1]. They do not compete for arable land comparing with first generation feedstock, like food crops. Neither do they have the limitations in utilizing the recalcitrant polymers, like lignocellulose. Algae-based biomass can be cultivated in marine environments, polluted ponds and even in wastewater; they can grow very fast, accumulating a variety of compounds and producing biomass by photosynthesis [2]. At present, algae have been used in many fields: seaweed cultivation for food products, like kelp and Gelidium [3]; extraction of high value compounds, like agar, various volatile fatty acids and alcohols [4]; wastewater remediation, like adsorption and ammonium removal [5], [6], [7]; biofuel production, like biohydrogen, bioethanol and biodiesel [8].
Energy crisis and environmental deterioration requires the development of clean and renewable fuel. Hydrogen owns great potential for its high energy content, non-polluting combustion product, and easy transportation [9]. Especially for the dark fermentative hydrogen production, significant environmental benefits could be achieved [10]. Concerning the excellent ability of algae biomass in easy cultivation and high efficient fixation of CO2, cultivating algae biomass as substrate for biohydrogen production can simultaneously contribute to CO2 remediation and fuel generation, achieving the alleviation of greenhouse effect [11].
Nevertheless, there lays challenges in applying algae biomass as substrate for biohydrogen production. Although algae biomass owns much simpler structure than lignocellulosic biomass, intact cell walls and the polymers in cell walls for mechanical strength show great restriction on its direct biodegradation [12]. To enhance the degradation of algae biomass by hydrogen producers, pretreatment is necessary to release the encapsulated organic matters and turn complex polymers to smaller molecules. A variety of pretreatment methods have been studied to enhance the solubilization of organic matters from algae biomass. Park et al. [13] applied heat-shock at 120 °C for 20 min in disintegrating macroalgae biomass, hydrogen yield was enhanced from 10.6 to 83.3 mL H2/g TS. Similarly, Jung et al. [14] enhanced the hydrogen yield from 69.1 to 109.6 mL H2/g COD by heat-shock at 170 °C for 20 min pretreatment. Jeong et al. [15] explored electric field in enhancing the hydrogen production from Laminaria japonica, found that the optimized hydrogen yield (102.7 mL H2/g TS) was attained at 58.5 V for 30 min, which was 1.7-fold of the control test. Nguyen et al. [15] found that both sonication and methanol treatment can significantly enhance the hydrogen production from Thermotoga neapolitana biomass. Because of the differences in algae characteristics and operational conditions in different studies, the most suitable treatment method for the enhancement of hydrogen production is uncertain. Liu and Wang [16] compared different treatment methods including acid, ultrasonication, heat-shock, and base in disrupting L. japonica cell walls, best hydrogen production of 66.7 mL H2/g TS was obtained when 121 °C, 20 min heat-shock treatment was adopted. Heat-shock showed great potential in enhancing hydrogen production from L. japonica biomass. Microwave has been used as a method for thermal treatment of waste activated sludge [17]. Then, studies found that microwave was more efficient in disrupting waste activated sludge than heat-shock, which means microwave owns both thermal and non-thermal effect in disrupting biomass cells [18], [19]. However, up to now, there is no report concerning the potential of microwave in enhancing the hydrogen production from algae biomass.
In this study, microwave was firstly used as pretreatment technology to disintegrate algae biomass for biohydrogen production. A kind of brown algae Laminaria japonica was chosen as substrate for hydrogen production because it owns high ratio of polysaccharides to protein, and it is a kind of waste in European coastal countries [20]. The objective of the present study was to examine the effects of microwave treatment at different temperatures on organics releasement, hydrogen production and energy conversion efficiency from macroalgae L. japonica biomass.
Section snippets
Algal biomass
Dried kelp (Laminaria japonica) was purchased from Jingdong (Beijing, China). The purchased kelp was washed by fresh water for 5 times to remove the salt on the surface, and dried at 80 °C for 24 h. The dried algal biomass was milled to a fine powder, and sifted by an 18-mesh screen, stored in Ziplock bags at room temperature for further use. Characteristics of L. japonica biomass are as follows: volatile solids (VS) (% TS) 82.81 ± 1.42, total chemical oxygen demand (TCOD) (mg/g TS) 998 ± 18,
Effects of microwave treatment on the disintegration of L. japonica
Microwave at different temperatures was explored to disintegrate the macroalgae cells, releasing the organic matters encapsulated inside the cell membranes. Fig. 1 shows the change of organic matters in the liquid phase after the pretreatment. As shown in Fig. 1A, SCOD in all test groups after the treatment were higher than the control group, indicating microwave can effectively disintegrate the macroalgae cells. The concentrations increased with the increasing temperature, and highest SCOD
Conclusion
Microwave pretreatment was used to enhance the solubilization of organic matters from macroalgae Laminaria japonica, hence promoted the hydrogen production process. 160 °C microwave pretreatment showed great potential by enhancing the hydrogen yield by 1.9 fold compared with the control test. However, energy analysis showed that over 90% of the energy remained in algae biomass and liquid metabolites. Thus, efforts on both biomass hydrolysis and soluble metabolites utilization are recommended in
Acknowledgement
The research was supported by the National Natural Science Foundation of China (51338005) and the Program for Changjiang Scholars and Innovative Research Team in University (IRT-13026).
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