Improved Fermentative Hydrogen Production with the Addition

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Biofuels and Biomass

Improved fermentative hydrogen production with the addition of calcium lignosulfonate-derived biochar Lei Zhao, Jishi Zhang, Wenqian Zhao, and Lihua Zang Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.9b01380 • Publication Date (Web): 18 Jul 2019 Downloaded from pubs.acs.org on July 31, 2019

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Improved fermentative hydrogen production with the addition of calcium lignosulfonate-derived biochar Lei Zhao, Jishi Zhang*, Wenqian Zhao, Lihua Zang College of Environmental Science and Engineering, Qilu University of Technology (Shandong Academy of Science), Jinan 250353, China ABSTRACT: This study investigated the roles of calcium lignosulfonate (CL) and its derived biochar (BC) in biohydrogen production. CL was pyrolyzed at 250°C to produce BC, and the main characteristics of the two materials were compared. A series of anaerobic processes were carried out at 54±1°C, and the results suggested that CL remarkably lowered the H2 yield compared with that obtained with CL-derived BC. The highest H2 yield (262 mL/g glucose) was obtained at 20 g/L, corresponding to an increase of 50.9%, while the lowest H2 yield (110 mL/g glucose) occurred at 20 g/L and was 36.4% lower than that in the control group (173 mL/g glucose, without CL and BC). The CL-derived BC exhibited more pore structures than CL, providing more attachment sites for anaerobes. In addition, the high potassium, calcium and magnesium contents and low sulfur dosage enhanced the microbial activity and process stability. Keywords: Calcium lignosulfonate; Derived biochar; Biohydrogen production; Inhibition or enhancement 1. INTRODUCTION By far, economic development and modern lifestyles depend heavily on the use of fossil fuels. However, such fuels are nonrenewable, and their reserves on earth are limited. Therefore, more attention is being paid to the serious energy crisis and

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ACS Paragon Plus Environment

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environmental pollution.1 A consensus has been reached on promoting the coordinated development of economy, society and environment and implementing a strategy of sustainable development. Predictably, the development and utilization of renewable energy to replace fossil fuel has become an urgent and realistic research topics in recent years. Many countries are devoting themselves to developing alternative energy sources such as solar, wind, marine, geothermal, biomass, and hydrogen 2. Among such sources, H2 is considered as a promising energy carrier to reduce the use of fossil fuels because of its outstanding advantages: 3, 4 (1) H2 has a high

energy

density

compared

with

that

of

other

substances;

(2)

H2 can be used as a clean energy without any pollution; and (3) H2 has the lowest transportation cost and minimum loss through pipeline. Usually, the H2 generation process mainly includes physicochemical and biological methods. Large amounts of energy is consumed in the traditional H2 production technique, which results in a great waste of resources.5 In contrast, bio-H2 generation has many advantages like cleanliness, energy-savings, nonconsumption of mineral resources and recovery of biomass energy. The bio-H2 evolution process is a bioengineering method that employs the unique hydrogen metabolic system existing in some microbes to produce H2. Anaerobic fermentation is receiving increasing attention because of its high H2 yield, simple equipment, easy operation, raw material availability and low cost.6 This biogas production is often divided into three stages: hydrolysis,acidogenesis/acetogenesis and methanogenesis. Thermophilic fermentation is a more popular process for generating H2 because this technique can shorten the evolution time and produce more H2, although its stability and inhibition problems require further improvement.

7, 8

In addition, some carbon materials like activated

carbon (AC), biochar (BC) and carbon cloth (CC), in anaerobic fermentation

2


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processes have been verified to be effective in promoting H2 evolution.5, 9 Both AC and BC could be derived from the pyrolysis of bio-wastes such as wood and crop residues.

10

They are often used as a microbial carrier. These porous structures of the

carbon materials can increase microbial immobilization, which facilitates maintaining high biomass in biogas production process. It has been observed that the addition of 10 g/L BC enhanced the CH4 production rate by 86.6%, and shortened the lag period by 30.3%.11 Sharma and Melkania 12 also found that the lag time was reduced via BC amendation. Lignosulfonates are one of the main byproducts of acid sulfite and neutral sulfite pulping processes.13 According to the type of some cations, lignosulfonates mainly include calcium lignosulfonate (CL), sodium and ammonia lignosulfonates. Natural lignin is not soluble in water, but it gets soluble in water during sulphate or sulphite pulp process, due to the introduction of sulfonic groups and the partial degradation of lignin. Therefore, compared with natural lignin, the CL is low molecular weight. It has the unique structures of pherical three-dimensional network, which contains a hydrophilic sulfonic acid group on the outer chain.14 The presence of the sulfonic acid group on the surface of CL facilitates CL having strong micelle- forming ability and lowering the surface tension of a liquid surface.15 Some difficulties in the treatment of some wastewater containing CL lie in the nondegradability and toxicity of the CL. Although it may be feasible to extract CL from waste liquor and serve the CL as an anionic surfactant and concrete additive, the recycling rate of CL from some wastewater plants is generally low. The main wood component lignin gained as waste in the paper industry is an attractive raw material for a chemical agent such as resin based adhesives and anionic surfactant. Further expanding the value chain of CL is of great significance for effectively realizing the resource utilization of CL. At present,

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dedicated studies on BC as product of low molecular lignin carbonization are scarce.16 Moreover, lignosulfonate characterized by a high sulfur content has been investigated less frequently,17 despite significantly affecting anaerobic digestion performance. To the best of our knowledge, the characteristics of CL and CL-derived BC and their opposite roles used in bio-H2 production are investigated here for the first time. In this work, to address resource utilization, CL and its derived BC were evaluated as the two additives in bio-H2 production process. The influences of the concentrations of CL and BC on bio-H2 yield and production rate were compared, and their effects on the H2 fermentation process were also compared with the results obtained with the control group (without CL and BC). Some possible mechanisms of the bio-H2 processes with the two additives were clarified. The techno-economic potential of hydrogen production using this technique was compared with other methods. This paper provides an effective way to promote H2 production and obtain highvalue utilization of kraft lignin. 2. MATERIALS AND METHODS 2.1. CL and Derived BC. The CL was industrial-grade lignosulfonate that was purchased from Tianjin Yeats Chemical Technology Co., Ltd, China. The essential properties of CL were described as follows: pH 5.5±0.1, water content (H2O, wt%) 4±0.1, water-insoluble content (wt%) 0.5±0.2, and reducing substances (wt%) 11±0.1. Besides, all chemicals were analytical reagent grade and purchased from the Sinopharm Chemical Reagent Co. Ltd, Beijing, China. The BC used in this study was pyrolyzed from the CL mentioned above, which is a suitable precursor for fabricating BC. To obtain BC, CL was first loaded into a temperature-programmed furnace (SX2-4-10GP, Jingrui Analytical Instrument Co. Ltd., China) and thermally treated to 250°C with a heating rate of 10°C/min and kept

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250°C for 2 h in a nitrogen atmosphere. Before that, the CL was crushed into 60-100 mesh powder and dried at 105°C for 24 h. After being cooled to room temperature (20-25°C), the CL sample was pulverized into a powder of 80 mesh. Then the obtained CL-derived BC was stored in glass bottles for further employment. The pH of the BC was determined to be 7.2. 2.2. Anaerobic Sludge. The sewage sludge (SS) could be served as an inoculum source, and it was dewatering sludge from a sewage treatment plant located in Jinan, China. The main physicochemical properties of the SS were presented as follows: pH 7.5±0.1, total solids (TS, wt%) 4.2±0.1, volatile solid (VS, wt% of TS) 78.2±0.1, ammonium (NH4+-N, mg/L) 165±1, and chemical oxygen demand (COD, mg/L) 1485±50. Firstly, the SS was cultured under thermophilic (54±1°C) and anaerobic conditions with 100 mg/L of glucose for approximately 25 d. Before anaerobic fermentation process, the SS sample was concentrated by settling for 6 h and stored at 4±1°C. Then, the SS was thermally pre-treated for 30 min at 90°C to hinder the activities of hydrogen-consuming bacteria and methanogenic archaea, decrease interspecies hydrogen transfer and increase hydrogen-producing bacteria. Next, the sludge sample was cooled to 54±1°C and cultured in a medium with 100 mg/L glucose for 36 h at 54±1°C. After precipitation and concentration for 8 h, the inoculum (seed sludge) used bio-H2 evolution test was obtained and stored at 4°C for further utilization. The major characteristics of the inoculum were as follows: pH 7.2±0.2, TS (wt%) 18.3±0.1, VS (wt%) 75.7±1, NH4+-N (mg/L) 142.8±5, and COD (mg/L) 16428±100. 2.3. Fermentation Procedures. Batch anaerobic fermentation was carried out in 5


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serum bottles with a working volume of 500 mL and a headspace of 125 mL. Then 200 mL of deionized water was added into each bioreactor, followed by the addition of 5 g glucose and different dosages of CL or BC sample: 0 g, 2.5 g, 5 g, 7.5 g, and 10 g. The pH value of each bioreactor was adjusted to 7.0±0.2 with 0.1mol/L sodium hydroxide. Subsequently, 150 mL inoculum was supplemented into the bioreactors, then mixed and diluted to 500 ml with deionized water. Consequently, the two factors of inoculation rate and organic load of each bioreactor were calculated to be 30% (V/V) and 10 g glucose/L, respectively. The space of each bioreactor was quickly sealed after being flushed with nitrogen gas for 30 s, and the bioreactor was incubated at 54±1°C for 36-42 h. Simultaneously, all bioreactors were equipped with a container containing 5.0-8.0 wt% of sodium hydroxide solution to absorb carbon dioxide and hydrogen sulfide from the produced biogas.18 The experiment was repeated three times, and the data presented in this paper were the average of three times. 2.4. Chemical Analysis. The pH values of liquid samples such as substrates, SS, inoculum and fermentation liquid were determined by pH meter (PHS-3C, Electric Science, Shanghai, China). The TS values of SS and inoculum were measured by drying at 105°C (weight method), and VS was determined by burning at 550°C (cauterization technique) using a programmed heating furnace (SX2-4-10GP, China). In addition, COD and NH4+-N of liquid samples were determined by using the dichromate method and Nessler reagent spectrophotometry, respectively. The pH values of CL and its derived BC samples were determined by mixing 5 g of samples with 25 mL deionized water at a solid:liquid mass ratio of 1:5, sealing with

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a cap, oscillating continuously for 30 min (the oscillation frequency was 110±10 rpm), standing for 30 min, and measuring the pH value of the supernatant. Two parallel samples were collected from each sample to determine the pH. The surface functional groups of on the surfaces of the CL and BC were characterized with Fourier transform infrared-spectrometry (FT-IR) (IR prestige-21, Shimadzu, Japan) at wavenumbers scanning from 400 to 4000 cm-1. After being degassed in a vacuum at 150 ℃ for 2 h, the surface area of CL-derived BC was determined by a Brunauer-Emmett-Teller analyzer (Autosorb-iQ, Quanta Chrome, USA). An environmental scanning electron microscope equipped with an energy dispersive spectrometer (SEM/-EDS) (Regulus 8220, Hitachi, Japan) was employed to observe the surface structure and morphology, and to determine the elemental components of the CL and BC samples. The X-ray diffraction (XRD) spectra of the two samples of CL and BC were collected by using a powder diffractometer (D8-advanc, Germany) with Ni-filtered CuKα radiation (AXS, Bruker, Germany) and operated at 40 kV and 200 mA, and these samples were scanned from 2-theta 10° to 80° at 5°/min. Moreover, the thermal behavior of CL was analyzed by using thermogravimetric analysis (TGA) with differential scanning calorimetry (DSC) (STA 449F3-QMS403C, Netzsch, Germany), and the heat flow of the sample was determined according to the temperature during the pyrolysis process. CL sample was pyrolyzed from 100°C to 800°C with the heating rate of 10°C/min to obtain the CL thermogravimetric curve. The volume of H2 generated from anaerobic fermentation was measured by using a 1000 mL container connected to each bioreactor. The H2 content was analyzed by

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using a gas chromatograph (GC-2014C/TCD, Shimadzu, Japan) equipped with a thermal conductivity detector (TCD) and a column packed with Porapak Q. The measurement of H2 volume was carried out at standard temperatures and pressures (293.15 K, 101.325 kPa). Some volatile fatty acids (VFAs:acetate, propionate and butyrate) were measured by a GC system (GC-2010 Plus, Shimadzu, Japan) equipped with a KB-PLOT U capillary column and a flame ionization detector (FID). The operational temperatures of the injection port and detector were set at 220°C and 200°C, respectively. 2.5. Kinetic Modeling. To obtain the lag period, maximum production potential and rate of hydrogen production in each bioreactor, the modified Gompertz model (Eq.(1)) was employed. All parameters were fitted by Origin 8.5 software. The typical curve shape is "S" type, that is, there is an inflection point.

(1) where M is the cumulative hydrogen production (ml) at time t (h); P describes the hydrogen production potential (ml); Rm represents the maximum hydrogen production rate (mL/h); and λ refers to the lag period (h), which is the period delay before a culture responds to a new environment and begins to produce H2. P, Rm and λ can be obtained by fitting the experimental data. 3. RESULTS AND DISCUSSION 3.1. Characterization of CL and its Derived BC. To further understand some key differences in the main characteristics of CL and CL-derived BC, some analysis and testing techniques like FT-IR, XRD, TGA/DSC and SEM/EDS were employed. 8


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The two samples of CL and BC were analyzed by FT-IR method to confirm the organic-functional groups on the surfaces. The fluctuation spectra of the two samples were quite different (Fig.1a), indicating that the chemical structure of CL changed greatly after the dry pyrolysis process. The bands of CL and its derived BC (Fig.1a) can be assigned to -OH stretching vibrations (3500-3200 cm-1) and the vibrations of C=O (ketone) bands (1750-1520 cm-1). The bands at 1599 cm-1, and 1426 cm-1 are related to the C-C bonds stretching vibrations in the aromatic skeleton 19 while the two bands at approximately 1190 cm-1 and 650 cm-1 connect with sulfonic acid group on the surface of CL sample.19 Moreover, the two bands at 1218 cm-1 and 1033 cm-1 correspond to the stretching vibrations of the C-O-C and C=O ether groups, respectively. Fig.1a showed that the BC curve was remarkably different from the CL curve. Low temperature (250°C) pyrolysis caused a lower degree of cracking, which generated the presence of -CH2 (1453 cm-1, 2902 cm-1) and oxygen-containing C-O-C (1159 cm-1) functional groups in the biomass macromolecules. Besides, the C=O structure of carboxylic acids, and C=C and C=O of aromatic groups were retained in the CL-derived BC. The band at 1385 cm-1 was related to the sulfonic acid group (-SO3H) , while the intensity of band became weak after pyrolysis of CL (Fig. 1). The results demonstrated that pyrolysis treatment of CL sample significantly changed the macrostructure and surface organic functional groups of CL, which might further modified or changed the biohydrogen process. To determine the possibility of microbial carrier or inhibitor behavior during the anaerobic fermentation process, the particle morphologies of CL and BC samples were determined by SEM to obtain some important information about the microstructural variations in the two samples. The microstructures and elemental 9


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contents of CL and BC samples are shown in Fig. 2 and Table 1, respectively. The morphologies of CL and BC samples were clearly different. According to a previous study,20 the phenyl-propane structure has been observed in lignin, making it easy to form a larger structure regardless of the lignin modification method. Fig.2 revealed that CL surface was very smooth, and some small pore-like structures were intermittently distributed on its surface. When the carbonization temperature rose to 250°C, the CL structure was slightly damaged, and the pore structure of the BC surface was relatively developed (Fig. 2). This fact probably has a positive effect on the change in specific surface area of BC.21 Increased temperature could enhance the removal of aliphatic C-O, ester C=O and -OH groups from the surfaces, as well as phenolic -OH and the aromatic C-O groups linked to aromatic cores, which significantly increased the surface area of BC. This modification provided more attachment points on BC for anaerobic bacteria. The anaerobic fermentation system added with BC likely improved microbial activity and raised interspecies electron transfer.22 The elemental contents of CL and BC were qualitatively analyzed by using EDS technique. Carbon, oxygen, sulfur, chlorine, magnesium and calcium, in sequence, were observed in the two samples of CL and BC. The significant contents of carbon and oxygen in CL prove its phenyl-propane structure.23 Some elements such as carbon, oxygen, potassium, sulfur, chlorine, calcium and magnesium were observed in BC. The carbon element content was high in BC, confirming effective carbonization at 250°C.24 Both the oxygen and sulfur contents were lower in BC, indicating that the hydrophilicity of BC was decreased. Additionally, the presence of sulfites/sulfates in CL allows the competitive growth of sulfate-reducing-bacteria, and the hydrogen sulfide released by such bacteria must be removed from bio-H2 at considerable cost.

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The potassium and carbon contents in BC increased significantly, which was consistent with a previous report (Table 1).25 The carbonization of some organic matter in BC fabricated at low temperature ( about 200°C) was incomplete. The BC obtained at owl temperature contained not only incompletely carbonized organic carbon, but also highly carbonized organic carbon. Therefore, the decrease in oxygen content confirmed the decrease in oxygen-containing functional groups on the surface of CL-derived BC, showing that there were more pores on the BC surface to provide a large number of attachment sites for anaerobic microorganisms. Moreover, some trace elements, such as potassium and calcium are essential components of anaerobes and play an important role as buffering agents for anaerobic digestion systems.26 The XRD patterns showed obvious diffraction peaks of CaCO3 in CL-derived BC, as also confirmed by Wang et al.

25

The presence of CaCO3 facilitates BC being

used as a buffer for anaerobic digestion system because of its alkalinity. The accumulation of VFAs during H2 production would lead to an obvious decrease in the pH of the whole fermentation system, thus hindering the H2 generation process. The partial dissolution of CaCO3 under acidic conditions likely buffered the process acidification and increased the alkalinity of the bio-H2 process system.

27

Another

report showed that adding dolomite or calcite, which contain CaCO3 as their main component could effectively enhance the anaerobic fermentation efficiency treating brewery wastewater.

28

The CO2 and Ca2+ generated from the dissolution process of

CaCO3 could increase the buffering capacity of the bio-reaction system, and promote VFAs to convert into biogas.29 Furthermore, the CL sample was analyzed by using a TGA/-DSC system to evaluate the characteristics and behavior of CL during thermal pyrolysis process (Fig.3). It is very important to understanding the thermochemical properties of CL

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sample after pyrolysis as potential microbial carrier and buffering agent were particularly important when BC is added into thermophilic (54±1°C) anaerobic digestion process. The temperature-responsive mass evolution was a two-stage process for CL, as shown in Fig.3. The initial stage was accompanied by the loss of CL adsorbing/hydrating water, which occurs at about 100°C from 81.3-106.3°C.30 However, the stages of pyrolysis also depends on the types or physicochemical properties of lignins. Negligible mass loss was found when the pyrolysis temperature was below 200°C, and a main mass loss was observed between 200 and 500°C (Fig.3) Cui et al.

31

found that the molecular weight of lignosulfonates were easily changed

over 120°C. This further confirmed the thermal stability of CL after high carbonization (more than 250°C); therefore, CL-derived BC may be used as a good carbon material in the bio-H2 evolution process. The second stage was exothermic phase occurred between 250-570°C.

32

It was vital important to release sulfur and

produce porous BC. This phase was also divided into three sub-stages that were related to the volatilization of various volatile compounds: (1) CL began to depolymerize and lose weight (150-250°C), mainly due to CL vitrification; (2) The main stage of CL pyrolysis occurred at 250-450°C and the thermal weight loss was obviously observed in the stage. There was a weight loss peak on the DSC pattern, corresponding to approximately 350°C, while the volatile substances accounted for about 70% of the total CL; (3) After 450°C, pyrolysis process was basically completed, and the weight loss became very slow (Fig.3). The high temperature facilitated the sulfur (S) gas releasing, and retaining small quantity of S as polysulfides (chains of sulfur atoms) that were deeply embedded within BC matrix (Table 1 and Fig.1). This kind of phenomenon indicated that although the S of CL was nonconductive, it became highly conductive when combined with carbon at raised temperatures during

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CL pyrolysis process, which contributed to enhance the electron transport rate, thus improving hydrogen production. 3.2. Roles of CL and BC in Bio-H2 Production. To investigate the roles of added CL and BC in bio-H2 production, batch tests with CL or BC concentrations from 0-20 g/L were carried out under thermophilic conditions (Fig. 4). The cultures with added BC gave a higher H2 yield and shorter lag phase than the control group, while CL had the opposite effect on it. The high sulfur content of CL likely negatively affected microbial activity and metabolism, thus inhibiting glucose biodegradations. 17 However, S removal prior to microbial degradation could improve CL degradation.17 This facilitated obtaining the in-situ activation of CL-derived BC. Despite the inhibitory or negative effects of S from CL sample, glucose could be converted into VFAs and H2 when a small number of the samples were added into the H2 fermentation of glucose (Fig.4a). However, a higher dosage of CL caused less H2 yield and lower removal of glucose.This phenomena indicated that high S content of CL exhibited higher toxicity to anaerobes versus the CL-derived BC. Besides, CL maybe caused the microbe-in-CL liquid film, which prevented microorganisms from contacting glucose.The sulfonic acid group in CL is a hydrophilic group with negative electricity. The main characteristics indicated that CL could disperse and encapsulate microorganisms, thus blocking the electron transfer between microorganisms and reducing hydrogen production.These inferences were further confirmed by the following sections. According to previous studies on the effect of BC on anaerobic process, BC stimulated biogas production by promoting direct interspecies electron transfer (DIET) and interspecies electron transfer (IET) among microorganisms.

33, 34

DIET is an

emerging phenomenon occurring during the syntrophic methanogenesis process of

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anaerobic digestion, but not very closely with H2 production.Generally, H2 production is related to IET.35, 36 In addition, as reported in recent studies, another possible reason for the efficient biogas production in systems amended with BC may be microbial aggregation and biofilm formation due to the porous structure of BC.37 With the elevation of BC concentration from 5 to 20 g/L, the cumulative H2 production gradually increased from 813 to 1309 mL. The highest yield of 262 mL/g glucose was obtained at 20 g/L (Table 2), similar to previous reports (Table 3).5,34,38-42 Moreover, the magnesium and calcium in CL-derived BC served as nutrients supplements to enhance biogas generation and avoid microbial substrate foaming.43 Magnesium can affect the transcription of extracellular enzymes and the synthesis of coenzymes.44 The retention of a certain quantity of biomass via these carbon-based materials would enable the bio-H2 system to operate at a high organic load and enhance H2 production rate.10 Nevertheless, there were still some differences in carbon type and concentration, microbes and process temperature that could have an important influence on the process of H2 fermentation. However, under the same conditions, CL addition produced opposite trends in H2 production. CL content from 5 to 20 g/L led to a decrease in cumulative H2 production from 813 to 552 mL in which the lowest H2 yield was observed at 20 g/L (Fig. 4). These phenomena could be explained by the following four reasons (a) liquid CL could cause the formation of foams and encapsulate anaerobes; (b) the high sulfur content from CL was toxic to the microbes; (c) the CL in liquid reduced the interspecies electron transfer among microorganisms; and (d) CL increased the distance between anaerobes and glucose, keeping anaerobes starvation. These four reasons likely inhibited and stopped the bio-H2 process in organic wastewater, as shown in Fig.5.

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The influences of adding CL and BC on COD removal rates of simulated glucose wastewater were also investigated as follows. After 39 h and 42 h, the COD concentrations in the groups with 20 g/L CL and BC added decreased from 12860 mL/g to 11150 and 5065 mL/g, respectively, with the corresponding removal rates of 13.29% and 60.61%. The COD removal rate in the control test was 37%, and the low value may be due to the conversion of most the COD to VFAs in the digestate. Compared with the control group, the COD removal rate with 20 g/L BC increased by 63.81%, which showed that BC considerably promoted the bioprocess and reduced the COD load for subsequent wastewater treatment. In contrast, the COD removal rate with CL addition was considerably lower than that of the control. The high sulfur content of CL has a negative influence on microbial metabolism and activity. In addition, the CL in liquid also segregated anaerobes from glucose. These facts could explain that the disadvantageous factors lowered CL and glucose degradation.17 3.3. Kinetics Analysis. The hydrogen production rates were fitted to modified Gompertz model (1). As given in Table 3, the bio-H2 production processes were well fitted (R2>0.99). Furthermore, the P value showed that the calculated values were not significantly different from the measured values, which reflected the rationality of the conclusion of these experiments, and the addition of BC promoted the bioprocess stability. However, the system stability was inhibited after the addition of CL. The lag phase is an important indicator of anaerobic fermentation performance. The lag phase with added BC was shortened, which indicated that BC was beneficial to the activities of anaerobic microorganisms and provided a carrier for the microorganisms, thereby improving the substrate conversion efficiency. However, the CL addition considerably prolonged this lag phase, suggesting that CL limited hinder microbial motility, did harm to anaerobes, and made microbes constantly hungry, thus inhibiting

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the microbial activity and reducing H2 yield. This was attributed to the chemical characteristics (e.g., pH, viscosity, cation and functional groups) of CL. However, anaerobic granular sludge could be easily developed in the bio-reactor added with BC. It was partly due to the bacterial microcolony in the granules that reduced the distance between anaerobes to increase the balance of the food chain, which improved process stability. 3.4. Anaerobic Process Stability. The system pH directly affects the microbial activity in H2 fermentation. Usually, the appropriate pH value for bio-methane production is 6.5-7.6; and low pH inhibited the activity of iron-containing hydrogenase

42, 45

The most suitable pH range for hydrogen producing bacteria is

6.8-7.2.42, 46 As shown in Fig.6(a), at the initial stage, the process pH (7.0) was within the optimum range for microorganisms, which favored more bacteria propagation comparatively. With the removal of COD from the substrate, VFA production increased while the system pH decreased rapidly. Then, the system pH fluctuated. After 33 h of anaerobic fermentation, the pH values of the control and the BC groups were stable at approximately 5.0, and the pH of the group with CL addition was slightly higher and stable at approximately 5.5. At this time, the production of a large amount of VFA led to the overacidification of the fermentation system, and the H2 evolution process effectively stopped. The concentration of VFAs is an important indicator because this value can reflect the operation state of anaerobic digestion.47 The changes in VFAs during the bio-H2 process are shown in Fig.6(b). The VFA concentration remained at a low level from the initial stage to 6 h in the BC and control groups and to 9 h in the CL test; these periods were the adaptive periods for microbial inoculation. At this stage, the microbial activity was not high, and thus gas production was low. In the BC and control groups, the cumulative VFA generation

16


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Energy & Fuels

increased rapidly between 6 and 12 h, while the VFA yields for the CL group increased between 15 and 21 h. The accumulation of VFAs caused a rapid decrease in system pH. In addition, the types of VFA have a strong influence on H2 production, as was shown in Fig, 6(c,d). Acetic acid and ethanol were the main components in the liquid phase during thermophilic digestion, which indicates that the main metabolic pathway of microorganisms in this process was acetic acid fermentation. Fig.6 showed that the final concentrations of acetate and butyrate accounted for 57.8%, 63.3% and 64.3% for the control, CL (20g/L) and BC (20g/L) groups, respectively. High concentration of BC facilitated the acetate and butyrate metabolic pathways (Fig.6d), which indicated that there was a correlation between H2 yield and VFAs. The two metabolic pathways were described as Eqs.(2) and (3). C 6 H12 O 6  2H 2 O  2CH 3COOH  2CO 2  4H 2

(2)

C 6 H12 O 6  CH 3CH 2 CH 2 COOH  2CO 2  2H 2

(3)

2CO 2  4H 2  2H 2 O  CH 3COOH  4H 2 O

(4)

According to Eqs.(2) and (3), more H2 would be produced with acetate generation, but this phenomenon was not found. In addition, there was no significant generation of propionate, thus acetate generation based on Eqs.(2) and (3) would be main pathway by which H2 was consumed. As described in Eqs.(4), acetogens could also utilize glucose, thus competing with H2-producing bacteria. The high yield of propionic acid in the mixed acid fermentation process was responsible for the low hydrogen production capacity (Fig.6c and d). Acetate and butyrate generations favor H2 production, while propionic acid consumes hydrogen. Therefore, the bio-reactors amended with the CL-derived BC showed mixed acid (acetate and butyrate) type fermentation, and maintain to obtain more H2 (Fig.6d). The concentration of VFAs was high due to the effect of acid-producing bacteria, and the pH of the system was 17


Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 18 of 32

maintained in the range of 4.5-6.0. Within a certain range, the higher the VFA content in the system was, the more conducive the conditions were to hydrogen generation. The pH of the BC group increased at 12 h, which was likely due to the dissolution of carbonate and other substances from BC; this finding was consistent with a previous report.48 The plateau period of the BC group in Fig.6(b) also validated this finding. The BC derived from CL contained higher contents of microelements (e.g., K, Ca and Mg) than did CL (Table 1, Fig.5). The elements take part in microbial metabolism, and the observed concentrations provided a suitable microenvironment for anaerobic bacteria, which included inorganic nutrients, and buffering agents. For instance, these elements were likely responsible for the BC buffering capacity, as described in Eq. (5) where CxHyCOOH served as VFAs.49 Ca(Mg)CO 3 + 2C x H y COOH= [C x H y COO] 2 Ca(Mg) + H 2 O + CO 2

(5)

In addition, compared with CL, dense microbial films were formed on the surface of CL-derived BC at the end of hydrogen generation since BC, with a suitable surface area (3.68 m2/g), was capable of providing support carriers for microbial immobilization.50 The bio-H2 process is more likely to maintain system stability and higher microbial activity than the process without BC.51 The efficiency of converting glucose into bio-H2 relies on the fermentation pathway employed by the microbes. The theoretical yields of H2 range from 248.9 to 497.8 mL/g glucose.52 The theoretical values are different from experimental results and depend on the substrate, microbes, and microenvironment conditions, such as pH, oxidation/reduction potential and presence of trace elements (e.g., Mg2+ and Fe2+). Therefore, these synergistic effects can further maintain anaerobic process stability. In addition, the experimental H2 yield was lower than the theoretical value due to utilizing part of the carbon source for the growth and maintenance of the microbes.52 18


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Energy & Fuels

3.5. Techno-economic potential. The most important factors determining the economic feasibility of hydrogen production process are the performance of anaerobic sludge and the cost of hydrogen production. However, Sludge performance depends on the concentration and activity of hydrogen-producing bacteria. Therefore, the cost of additives may be more conducive to comparing the economic benefits of hydrogen production. As mentioned previously, carbon materials have been the most popular additives for hydrogen production.10,

34

However, the high cost and energy consumption of

commercial carbon materials make them less economic attractive in industrial scale. Commercial carbons normally cost about 3.0 $/kg.

53

However, the industrial grade

calcium lignosulfonate is quite cheap and their cost is about 0.46-1.45 $/kg. The electricity cost for preparing BC is 0.62 $/kg. Therefore, BC prepared in this study cost about 1.2 $/kg. The electricity cost of water bath pot during the incubation remains very low, which could be neglected. In conclusion, the total cost of hydrogen production by anaerobic fermentation is 1.2 $/kg. It can be seen that if CL were used to prepare BC, the price would be reduced by 1.8 $/kg. From the point of view of hydrogen production process, this process also has great advantage in cost. Hydrogen from steam methane reforming, gasification of coal, biomass, nuclear energy and water electrolysis costs 2.27, 1.48, 2.05, 4-7, 10-23 $/kg, respectively.54-58 The results indicated that hydrogen production by anaerobic fermentation could be a suitable alternative to many hydrogen production processes. 4.

CONCLUSION CL inhibited H2 fermentation while CL-derived BC promoted the bio-H2 process.

CL encapsulated microbes, made microbes hunger, interrupted the electron transfer among the microbes, and was toxic to bacteria. The CL-derived BC provided

19


Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

considerable attachment sites for microbes and released some elements (e.g., K, Ca, and Mg) to enhance microbial activity and buffer the system pH. Besides, CL supported mixed acid type fermentation while BC facilitated ethanol. Under the same conditions, the yield of the former was higher than that of the latter. The yield of 262 mL H2/g glucose in the sample amended with 20 g/L BC represented an increase of 50.9% compared to the control group (173 mL/g), while the yield decreased by 36.4% in the CL test with the same dosage. ■ AUTHOR INFORMATION Corresponding Author E-mail Address: [email protected] Fax & Tel.: +86 531 89631680 Notes The authors declare no competing financial interest. ■ ACKNOWLEDGMENTS This work was accomplished under the supports of the Shandong Province Natural Science Foundation, China (ZR2016EEM33) and the Foundation of Key Laboratory of Pulp and Paper Science and Technology of Ministry of Education/Shandong Province, Qilu University of Technology (Shandong Academy of Science), China (KF201720). ■ REFERENCES (1) Qiu, C.; Zheng, Y.; Zheng, J.; Liu, Y.; Xie, C.; Sun, L. Mesophilic and thermophilic biohydrogen production from xylose at various initial pH and substrate concentrations with microflora community analysis. Energy Fuels 2016, 30(2), 1013−

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(30) Lu, X.; Feng, X.; Li, X.; Zhao, J. The Adsorption Properties of Endoglucanase to Lignin and their Impact on Hydrolysis. Bioresour. Technol.2018, 267, 110−116. (31) Cui, C.; Sadeghifar, H.; Sen, S.; Argyropoulos, D.S. Toward thermoplastic lignin polymers; part II: thermal & polymer characteristics of kraft lignin & derivatives. Bioresources 2013, 8(1), 864−886. (32) Qiao, Y.; Chen, S.; Liu, Y.; Sun, H.; Jia, S.; Shi, J.; Hou, X. Pyrolysis of chitin biomass: TG–MS analysis and solid char residue characterization. Carbohyd. Polym. 2015, 133, 163−170. (33) Jang, H.M.; Choi, Y.K.; Kan, E. Effects of dairy manure-derived biochar on psychrophilic, mesophilic and thermophilic anaerobic digestions of dairy manure. Bioresour. Technol.2017, 250, 927−931. (34) Zhang, J.; Fan, C.; Zang, L. Improvement of hydrogen production from glucose by ferrous iron and biochar. Bioresour. Technol. 2017, 245, 98−105. (35) Lovley, D.R. Syntrophy goes electric: direct interspecies electron transfer. Annu. Rev. Microbiol. 2017, 71, 643-664. (36) Liu, F.; Rotaru, A.; Shrestha, P.; Malvankar, N.; Nevin, K.; Lovley, D. Magnetite compensates for the lack of a pilin-associated c-type cytochrome in extracellular electron exchange. Environ. Microbiol. 2015, 17(3), 648-655. (37) Sunyoto, N.M.S.; Zhu, M.; Zhang, Z.; Zhang, D. Effect of biochar addition on hydrogen and methane production in two-phase anaerobic digestion of aqueous carbohydrates food waste. Bioresour. Technol.2016, 219, 29−36. (38) Wu, S.Y.; Hung, C.H.; Lin, C.Y.; Lin, P.J.; Lee, K.S., Lin, C.N.; Chang, F.Y.; Chang JS 2008. HRT-dependent hydrogen production and bacterial community structure of mixed anaerobic microflora in suspended, granular and immobilized sludge systems using glucose as the carbon substrate. Int. J. Hydrogen Energy 2008,

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(47) Zamanzadeh, M.; Hagen, L.H.; Svensson, K.; Linjordet, R.; Horn, S.J. Anaerobic digestion of food waste – Effect of recirculation and temperature on performance and microbiology. Water Res.2016, 96, 246−254. (48) Xu, S.; He, C.; Luo, L.; Lü, F.; He, P.; Cui, L. Comparing activated carbon of different particle sizes on enhancing methane generation in upflow anaerobic digester. Bioresour. Technol. 2015, 196, 606−612. (49) Wang, D.; Ai. J.; Shen, F.; Yang, G.; Zhang, Y.; Deng, S.; Zhang, J.; Zeng, Y.; Song, C. Improving anaerobic digestion of easy-acidification substrates by promoting buffering capacity using biochar derived from vermicompost. Bioresour.Technol. 2017, 227,286−296. (50) Wimonsong, P.; Nitisoravut, R. Biohydrogen enhancement using highly porous activated carbon. Energy Fuels 2014, 28, 4554−4559. (51) Zhang, Z.P.; Show, K.Y; Tay, J.H.; Liang, D.T.; Lee, D.J. Enhanced continuous biohydrogen production by immobilized anaerobic microflora. Energy Fuels 2008, 22, 87−92. (52) Bao, M.; Su, H.; Tan, T. Biohydrogen production by dark fermentation of starch using mixed bacterial cultures of Bacillus sp and Brevumdimonas sp. Energy Fuels 2012, 26, 5872−5878. (53) Ahmaruzzaman M. Industrial wastes as low-cost potential adsorbents for the treatment of wastewater laden with heavy metals. Adv. Colloid Interface.2011, 166(1-2), 36-59. (54) Barbir, F. Transition to renewable energy systems with hydrogen as an energy carrier. Energy 2009, 34, 308-312. (55) Ahmed, A.; Al-Amin, A.Q.; Ambrose, A.F.; Saidurc, R. Hydrogen fuel and transport system: a sustainable and environmental future. Int. J. Hydrogen Energy

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Page 28 of 32

Figure and Table Captions Table 1 CL and BC components Table 2 Comparisons of H2 yields from various types of carbon Table 3 Key parameters from the Gompertz model of the bio-H2 process

Table 1 CL and BC components Elemental content (wt%) Sample C

O

K

S

Cl

Ca

Mg

CL

52.32

40.4

0.02

5.16

0.98

0.45

0.67

BC

61.97

30.82

3.53

1.38

1.02

0.58

0.7

Table 2 Comparisons of H2 yields from various types of carbon Additives/dosage

Temp.

Seed sludge

H2 yield Initial pH

Reference

g/L

°C

mL/g glucose

Enterobacter aerogenes

CC, 1.0

37

N.A.

242

Cheng et al.3

Heart treated sludge

BC, 0.6

37

7.1

204.0

Zhang et al.31

Sewage sludge

AC, N.A.

40

6.5

191.64

Wu et al.33


BC, 0.5

37

7.0

192.3

Sun et al.34


BC, 0.6

37

5.5

206.8

Sun et al.34


BC, 0.4

37

5.5

197.4

Sun et al.34


BC, 0.8

37

6.0

184.1

Zhang et al.35


Fe2 O3 BC, 0.3

37

6.6

218.63

Zhang et al.35

Anaerobic sludge

AC, 0.1

37

6.5

176.71

Liu et al.36

E. cloacae and R. sphaeroides

-

30

7.0

233.95

Zhang et al.37


BC, 20

53

7.0

262

This work


CL, 20

53

7.0

110

This work

N.A.: not available

28


Page 29 of 32 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

Table 3 Key parameters from the Gompertz model of the bio-H2 process The parameter with CL

CL or BC

The parameter with BC

P

Rm

λ

R2

P

Rm

λ

R2

(mL)

(mL/h)

(h)

(%)

(mL)

(mL/h)

(h)

(%)

0

867.2

57.39

2.82

0.993

867.2

57.39

2.82

0.993

5

750.8

55.32

4.66

0.994

973.7

60.86

2.78

0.993

10

638.4

51.4

5

0.997

1080.4

63.55

2.78

0.993

15

564.5

47.44

7.54

0.991

1193

65.79

2.74

0.993

20

537.4

39.33

7.98

0.997

1299.9

66.9

2.75

0.993

(g/L)

29


Energy & Fuels

Fig. 1 FTIR spectra (a) and XRD patterns (b) of CL and BC Fig.2 CL and BC morphologies Fig.3 Thermogravimetric analysis of CL Fig.4 Bio-H2 production with added CL (a) or BC (b) Fig. 5 Roles of CL-derived BC in bio-H2 evolution Fig. 6 Changes in pH (a) and VFAs (b) during bio-H2 evolution and the final metabolite distribution with CL addition (c) and BC addition (d). 100 95

CL BC

(a)

CL

(b)

Relative intensity

90 Transmittance (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 30 of 32

85 80 75 70

BC CaCO3

65 60 55 4000 3600

3200 2800 2400 2000 1600 1200 Wavenumber (cm-1)

800

400

10

20

Fig. 1 FTIR spectra (a) and XRD patterns (b) of CL and BC

Fig.2 CL and BC morphologies

30


30

40 2-Theta (°)

50

60

70

Page 31 of 32

100

10 9

90

8

80 DSC

TG

6

70

5 4

60

TG (%)

DSC (mW/mg)

7

3 50

2 1

40

0 0

200

400 Temperature (°C)

600

800

Fig.3 Thermogravimetric analysis of CL

(a)

1400

Control 5 g/L 10 g/L 15 g/L 20 g/L

1200 1000

Cumulative hydrogen production (mL)

1400 Cumulative hydrogen production (mL)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

800 600 400 200 0

0

6

12

18

24 Time (h)

30

36

42

(b)

Control 5 g/L 10 g/L 15 g/L 20 g/L

1200 1000 800 600 400 200 0

0

6

Fig.4 Bio-H2 production with added CL (a) or BC (b)

31


12

18

24 Time (h)

30

36

42

Energy & Fuels

Fig. 5 Roles of CL-derived BC on bio-H2 evolution 7.5

2000

(a)

(b)

1800 7.0 1600 Control CL BC

6.0

VFAs (mg/L)

pH

6.5

5.5

1200 1000

4.5

800

2000

6

12

18

24

30

36

2000

(c) Propionic acid

Butyrate

Ethanol

VFAs+Ethanol (mg/L)

1500

1000

6

12

18

24

30

36

42

Time (h) (d)

Acetic acid

Propionic acid

Butyrate

Ethanol

1500

1000

500

500

0

0

42

Time (h) Acetic acid

Control BC CL

1400

5.0

0

VFAs+Ethanol (mg/L)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 32 of 32

0 0

5

10 15 Concentration of CL (g/L)

20

0

5

10 15 Concentration of BC (g/L)

20

Fig. 6 Changes in pH (a) and VFAs (b) during bio-H2 evolution and the final metabolite distribution with CL addition (c) and BC addition (d).

32


Improved Fermentative Hydrogen Production with the Addition

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