- Email: [email protected]
Effect of culture conditions on the kinetics of hydrogen production by photosynthetic bacteria in batch culture
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 6 ( 2 0 1 1 ) 1 4 0 0 4 e1 4 0 1 3
Available at www.sciencedirect.com
journal homepage: www.elsevier.com/locate/he
Effect of culture conditions on the kinetics of hydrogen production by photosynthetic bacteria in batch culture Yong-Zhong Wang a,b, Qiang Liao a,b,*, Xun Zhu a,b, Jun Li a,b, Duu-Jong Lee a,c a
Key Laboratory of Low-grade Energy Utilization Technologies and Systems, Chongqing University, Ministry of Education, China Institute of Engineering Thermophysics, Chongqing University, Chongqing 400030, China c Department of Chemical Engineering, National Taiwan University, Taipei 106, Taiwan b
article info
abstract
Article history:
Biochemical kinetic characteristics of photo-fermentative hydrogen production were
Received 23 January 2011
experimentally and numerically investigated to optimize the photo-fermentation
Received in revised form
hydrogen-producing process in this work. It is found that a maximum specific growth rate
29 March 2011
of 0.26 h1 was achieved under the optimal conditions of illumination intensity 6000 lux,
Accepted 2 April 2011
30 C culture temperature and pH 7.0 of culture medium. These experimental results also
Available online 29 April 2011
led to an empirical formula of the maximum specific microbial growth rate (mmax) as a function of illumination intensity, pH and temperature. With the empirical formula, the
Keywords:
modified Monod equation along with the kinetic equations for biomass growth, glucose
Photosynthetic bacteria
consumption and hydrogen production is then developed to simulate the photo-
Biohydrogen production
fermentation hydrogen-producing process. The modeling results are in good agreements
Reaction kinetics
with the experimental data, indicating that the developed kinetic models are able to
Illumination intensity
objectively describe the characteristics of hydrogen production by PSB under different
Culture temperature
culture conditions.
pH value
Copyright ª 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
1.
Introduction
Hydrogen is a clean and efficient fuel and has been recognized as the most promising alternative for fossil fuels by virtue of the fact that it is renewable and does not cause the ‘‘greenhouse effect” [1e3]. However, if hydrogen is to replace fossil fuels in the near future, it has to be produced renewably in large scale through environmentally friendly processes. Biological hydrogen production method stands out as an environmentally harmless process that converts renewable resources into hydrogen under mild operation conditions. Several types of microorganisms such as photosynthetic
bacteria, cyanobacteria, algae or fermentative bacteria are commonly utilized for biological hydrogen production [4e6]. Among these microorganisms, the photosynthetic bacteria are the most favorable candidates for large-scale hydrogen production due to the high substrate conversion efficiency and the capability to use a wide variety of substrates to produce hydrogen. Researches indicate that the growth and hydrogen production of PSB are not only dependent on intrinsic factors, but also dependent on extrinsic factors such as medium compositions, cell concentration, light intensity, carbon to nitrogen ratio, temperature, pH and so on [7e11]. Several
* Corresponding author. Key Laboratory of Low-grade Energy Utilization Technologies and Systems, Chongqing University, Ministry of Education, China. Tel./fax: þ86 0 23 65102474. E-mail address: [email protected] (Q. Liao). 0360-3199/$ e see front matter Copyright ª 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2011.04.005
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 6 ( 2 0 1 1 ) 1 4 0 0 4 e1 4 0 1 3
previous studies have confirmed that the temperature, pH and illumination intensity are relatively important environmental factors for the growth and metabolism of PSB [12e14]. However, most studies mainly focused on investigating the effects of these operation variables on the performance of hydrogen production of bioreactor rather than the kinetic characteristics of hydrogen production process. In the past few years, mathematical models, which describe the growth of microorganisms and effects of controlling factors on metabolism, have drawn a lot of attentions. An advantage of such models is that they could be used to simulate the influence of different environmental conditions on growth kinetics [15]. Another important feature of these models is the acquisition of the improved knowledge of these important factors that determine products formation. In terms of hydrogen production by photofermentation, these models could help to decrease the load of the experimental measurement that is time-consuming and costly, and to design more efficient hydrogen-producing reactors, and to control the photofermentation process effectively. Eventually, these models could be used for H2 production optimization. So far, many kinetic models have also been proposed and most of them are mainly based on Monod kinetic [16,17], Logistic lu et al. [21] models [18,19] or Gompertz equation [20]. Erog compared Logistic model with Monod model and Exponential model and concluded that bacterial growth rate obeys logistic model if the bacteria can make photosynthesis. Recently, the Anaerobic Digestion Model 1 (ADM1), a complex kinetic model describing the fermentative hydrogen process, has been proposed [22,23]. Mu et al. [24] evaluated the effect of pH, temperature and the ratio of initial substrate concentration to initial biomass concentration on the maximum specific microbial growth rate using a response-surface methodology. Obeid et al. [25] investigated the relationships between light intensity and hydrogen production by using a model to predict both the rate of hydrogen production and the amount of hydrogen produced in a period during batch cultures of R. capsulatus. Nath et al. [26]. researched the kinetics on growth, substrate inhibition and hydrogen production by twostage process combined dark and photo-fermentation in a sequential batch mode, and established a modified Gompertz equation to estimate the hydrogen production potential, rate and lag phase time in a batch process. Up to date, only a few elaborate researches have been reported about the complicated effects of operation variables on the kinetics of microbial growth, substrate consumption and product formation in the hydrogen production process [16,21,25,27]. However, no model has been proposed to describe the relationship of operation variables with the kinetic parameters of hydrogen production by photofermentation. In this work, effects of light intensity, culture temperature and pH of culture medium on hydrogen production by PSB were investigated in batch culture. It was expected to obtain the kinetic parameters of growth, H2 production and substrate consumption of PSB, and to establish the modified Monod equation on the growth of PSB. With these data, the kinetic models on hydrogen production and substrate consumption were proposed to describe the characteristics of hydrogen production by PSB. This work will be an essential part for developing full-scale bioreactor.
2.
Material and methods
2.1.
Microorganism and culture medium
14005
The strain used for hydrogen production in this study was originally isolated from the silt of sewages and identified as Rhodopseudomonas palustris [28]. The culture was kept in RCVBN Medium [29], with the exception that glucose (7.9 g/L) and CO(NH2)2 (1.35 g/L) were served as the carbon and nitrogen nutrients, respectively. Cells for further use were cultivated for 72 h in 150 mL of anaerobic sterile test bottle with 3000 lux illumination intensity from a tungsten filament lamp. Kinetic experiments of hydrogen production were conducted by using a basal medium, 1 L of which contained the following elements: C6H12O6$H2O (9.9 g); K2HPO4$3H2O (1.006 g); KH2PO4 (0.544 g); MgSO4$7H2O (0.2 g); FeSO4$7H2O (0.0417 g); (NH4)6Mo7O24$4H2O (0.0010 g); ZnSO4$7H2O (0.0010 g); NaCl (0.2 g); CaCl2 (0.010 g); CO(NH2)2 (1.35 g); yeast extract (1.0 g); growth factors solution (1 ml) which was composed of thiamin hydrochloride (1.0 g), riboflavin (1.0 g), nicotinic acid (1.0 g) and biotin (0.1 g) and 1 L water. The initial pH value of medium was adjusted by using 0.1 mol/L sodium hydroxide solution and 0.1 mol/L hydrochloric acid solution after autoclaving at 121 C for 12 min. Here, glucose was chosen as the sole carbon resource for growth and hydrogen production of PSB in the synthetic medium in the study.
2.2.
Photo-bioreactor and culture conditions
Five rectangle photo-bioreactors with 0.5 L volume made of transparent polymethylmethacrylate with 8 mm thickness were used for hydrogen production. Each bioreactor was filled with 0.3 L inoculated culture medium, the upper space was filled with gas. The front of the reactor illuminated by light was 100 mm in length and 100 mm in height, respectively. And the width of the reactor was 50 mm. Two holes were respectively drilled at the top of the reactor, one for charging culture medium and argon gas, another one for exhausting the gas sampling. In addition, one hole located on the left side of the bioreactor was for liquid sampling. Before loading the culture medium, the reactor must be disinfected with formaldehyde solution for 15 min to avoid bacteria contamination and washed three times using sterile distilled water. After autoclaving the pH value of culture medium was precisely adjusted to that of being set, using 0.1 mmol/L of the sodium hydroxide solution and 0.1 mmol/L of hydrochloric acid solution. In order to ensure the same activity and quantity of inoculum, the strain used for inoculum must be cultivated in a basal medium (the detailed components as above mentioned) up to exponential phase, and the quantity of inoculum was 10% (v/v). After being charged medium solution, the bioreactor was purged by argon gas to ensure an anaerobic atmosphere. Then, the bioreactor was placed in a culture device with a constant designated temperature for cultivation. And it was illuminated by LED lamps with the main light wavelength of 590 nm, the illumination intensity was adjusted from 2000 to 8000 lux by varying the distance between the LEDs and the bioreactor. The initial glucose
14006
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 6 ( 2 0 1 1 ) 1 4 0 0 4 e1 4 0 1 3
concentration of culture medium was 9.9 g/L. In this study, a range of cultures were incubated at different values of the operation variables and each of these cultures was subjected to the constant conditions in the whole photofermentation period. During the culture process, the liquid and gas samples were collected from the reactors at intervals of 4 h, respectively, until the biomass concentration and hydrogen production rate obviously decreased. Liquid samples were used to determine dry cell weight and residual glucose concentration in the culture medium, and the gas samples were adopted to detect hydrogen content in the discharged biogas. After sampling, the photo-bioreactor must be purged by argon gas again to obtain the environment without oxygen and hydrogen. In order to extract the model parameters of those particular conditions, the kinetic profile for each culture was thoroughly analyzed. Based on these analyses, an empirical equation of the maximum specific growth rate, which is a function of the operation variables, was achieved by fitting experimental data using MATLAB.
2.3.
Analytical methods
The electronic analytical balance (Sartorius BP114, Germany) was used to determine the quantity of elements in basal culture medium. pH meter（Ecoscan-pH6, Singapore）was adopted to measure the pH value of medium solution. Illumination intensity of light was measured by a digital luxmeter (ST-85, China). The cell concentration of the suspension was obtained by optical density at a wavelength of 600 nm (i.e., OD600 nm) using a UVeVIS spectrophotometer (756 MC, China). Firstly, biomass in a culture solution with a certain OD600 nm value was collected by a centrifuge at 20,300 g for 10 min, then, was dried at 105 C till constant weight. A formula of dried weight of biomass as a function of the corresponding OD600 nm was then achieved as follows Dried cell weight ¼ 0:44 OD600nm þ 0:0226; R2 ¼ 0:9905 Hence, the cell concentration of culture medium could be regularly monitored by measuring OD600 value using a spectrophotometer. Hydrogen gas content was determined by gas chromatography (SC2000, China). A thermal conductivity detector (TCD) with a current of 80 mA was equipped. The flow velocity of carrier gas (i.e., argon) was adjusted at 25 mL/min and a 2.0 m stainless steel column was packed with porous polymer pellets of TDX. The operation temperatures of the column oven and detector were 55 and 100 C, respectively. The glucose concentration of culture medium was measured by 3, 5-dinitrosalicylic acid method. Dinitrosalicylic acid reagent of 1.5 mL was added into 1 mL of the liquid sample, the mixture of liquid was then heated in a boiling water bath for 5 min and diluted to 10 mL with distilled water. The color change of the prepared solution was measured by using a spectrophotometer at a wavelength of 540 nm. By the method, the glucose concentration of culture medium could be calculated according to the standard curve obtained before.
3.
Results and discussion
The microbial specific growth rate is one of the most important parameters in a biological process and the relationship between growth rate and substrate consumption rate or product formation rate is crucial for monitoring and controlling the process [30]. Therefore, an accurate estimation of the microbial specific growth rate is essential to characterize the hydrogen-producing process. Although the importance of these parameters like optimal illumination intensity, temperature, pH for industrially used strains is well understood, the quantitative relationships between growth rate and the environmental factors such as illumination intensity, temperature and pH are rarely reported.
3.1.
Effect of illumination intensity
Our previous work has shown that PSB can yield a better performance of hydrogen production with light illumination of 590 nm due to its maximal absorption peak [28]. In this study, yellow light with the main wavelength of 590 nm from LEDs was used as light source. Light intensity of LED lamps was adjusted between 2000 and 8000 lux by changing the distance between the light source and photo-bioreactor. Fig. 1 shows that the effect of illumination intensity on the performance of hydrogen production by PSB. As shown in Fig. 1a, the relationship between the maximum specific growth rate and illumination intensity was not monotonic. With an increase in illumination intensity from 2000 to 6000 lux, the maximum specific growth rate first increased and reached to the maximum value of 0.26 h1 at 6000 lux, then, with the further increase in illumination intensity to 8000 lux, the maximum specific growth rate decreased to 0.2 h1. Meanwhile, the specific growth rate initially increased with the increase in illumination intensity to 6000 lux at the same culture stage due to effect of the limited incident light, then, decreased with the further increase in illumination intensity to 8000 lux due to the effect of incident light saturation (Fig. 1b). It is also found that the specific growth rate dropped quickly within 36 h after inoculation, then, dropped slowly at the end of culture time due to nutrients depletion. On the other hand, when the illumination intensity increased from 2000 to 6000 lux, the specific substrate consumption rate (Fig. 1) and specific hydrogen production rate (Fig. 1d) of PSB at the same culture stage all increased, and then decreased with the increase of illumination intensity up to 8000 lux within 36 h after inoculation. According to the above results, the optimal illumination intensity for the growth and hydrogen production of this strain can be recommended to be 6000 lux. In addition, as shown in Fig. 1, the specific growth rate, specific substrate consumption rate and specific hydrogen production rate all decreased with increasing culture time. This can be attributed to limited substrate supply in batch culture. Furthermore, beyond 44 h after inoculation, the substrate consumption rate dropped to the minimum of 0.64 h1 with the increase of illumination intensity to 6000 lux, and then gradually increased to 0.68 h1 when the illumination intensity further increased to 8000 lux. The corresponding specific hydrogen production rate also achieved the peak
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 6 ( 2 0 1 1 ) 1 4 0 0 4 e1 4 0 1 3
14007
Fig. 1 e Effect of illumination intensity on the reaction kinetics characteristics of PSB for hydrogen production (points: Experimental data, lines: Predicted data).
at 6000 lux of illumination intensity. It can be explained by that the available glucose was almost depleted at the end of the culture when the illumination intensity remained at 6000 lux, but a quantity of intermediates produced in the solution could be utilized by PSB for growth and hydrogen production. Therefore, during the last phase, the specific rates of growth and hydrogen production at illumination intensity of 6000 lux were still higher than those at other illumination intensities. In summary, it can be concluded that the growth of PSB was nearly approaching to the decline phase at 44 h after inoculation.
3.2.
Effect of culture temperature
Operation temperature is always a critical factor influencing the performance of a fermentation process [31]. This is also valid for photo-fermentative H2 production by a pure culture bacteria strain. Here, the kinetic characteristics of hydrogen production was investigated in batch culture mode at six different culture temperatures under the illumination conditions of 590 nm and 6000 lux, where the initial glucose concentration was 9.9 g/L and the initial pH value of this medium was 7.0. The results are shown in Fig. 2. Effect of culture temperature on the maximum specific growth rate is presented in Fig. 2. It is found that the maximum specific growth rate (i.e., 0.26 h1) was achieved when the culture temperature was 30 C, the growth of PSB, however, became slow when the temperature was below or above 30 C. The result indicates that the optimal culture temperature for the growth of PSB should be at 30 C. The effects of culture temperature on the specific growth rate and specific substrate consumption rate are presented in Fig. 2b
and c, respectively. Within 24 h after inoculation, both of the specific growth rate and the specific substrate consumption rate first increased with an increase of culture temperature, and then decreased with the further increase of culture temperature. For instance, the specific growth rate and the specific hydrogen production rate at a culture temperature of 30 C achieved the maximum values within 24 h after inoculation. Conversely, both of them decreased with an increase or decrease in the culture temperature at the same culture stage. The maximum specific growth rate of 0.17 h1 for PSB was observed at 30 C culture temperature at 12 h after inoculation and the corresponding specific substrate consumption rate of 1.07 h1 was also maximal. Also, it is found that the biomass concentration of culture solution increased with the increase of culture time (the data not shown). This fact indicates that the growth of PSB at a culture temperature of 30 C had reached to the exponential phase within 24 h after inoculation. Beyond 24 h, both of the specific growth rate and specific substrate consumption rate initially decreased with the increase of culture temperature, then increased with further increase of culture temperature. And the specific growth rate and specific substrate consumption rate at 30 C were minimal. This indicates that the growth of PSB nearly approached the decline phase at the moment. For instance, the specific growth rate and specific substrate consumption rate at 30 C were 0.05 h1 and 0.64 h1, respectively, at 44 h after inoculation because of the decreasing cell activities of PSB due to the depletion of glucose. In addition, at 25 C and 40 C, the specific growth rates and specific substrate consumption rates were minimal and changed slightly during the entire period. It demonstrates that PSB cells used had no endurance for an extreme environment temperature.
14008
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 6 ( 2 0 1 1 ) 1 4 0 0 4 e1 4 0 1 3
Fig. 2 e Effect of culture temperature on the reaction kinetics characteristics of PSB for hydrogen production (points: Experimental data, lines: Predicted data).
As can be seen from Fig. 2d, the specific hydrogen production rate demonstrates a similar change to the specific growth rate. When the maximal specific growth rate occurred at 12 h at the culture temperature of 30 C, the maximal specific hydrogen production rate of 1.63 103 h1 was also observed at the same stage. Within 24 h after inoculation, the specific hydrogen production rate increased with the increase of culture temperature to 30 C; and then decreased to 1.11 103 h1 with further increase of culture temperature to 40 C. It can be inferred that the increase of culture temperature below 30 C could cause a decrease inactivation energy needed in these biochemical reactions and resulted in an increase of metabolic rate of PSB, while the increase of the culture temperature over 30 C could lead to the reversible inactivation of cellular enzymes and inhibited hydrogen formation. Lee et al. [32] had confirmed the dependence of volumetric production rate on the operation temperature in a carrier-induced granular sludge bed reactor. In addition, the specific hydrogen production rate varied irregularly at 44 h after inoculation across the entire range of culture temperature. It can be ascribed to that PSB cells near the decline phase mainly utilized the produced intermediates for hydrogen production, although a little of glucose remained in the solution. Furthermore, the specific hydrogen production rate varied slightly with culture time at the culture temperatures of 20 C and 40 C indicating that extreme culture temperature was unfavorable to hydrogen production.
3.3.
Effect of pH value of culture medium
Different pH values of culture medium determine the ionic form of the active site and enzyme activity. Therefore, the pH
value of culture medium will affect the biochemical reaction characteristics. The effects of initial pH values of 4, 5, 6, 7, 8 and 9 on the characteristics of hydrogen production and substrate consumption in batch mode were investigated under the operating conditions of 50 mmol/L initial glucose concentration, 30 C culture temperature, 590 nm light wavelength and 6000 lux illumination intensity (Fig. 3). According to the data shown in Fig. 3a, operation at pH 7 achieved the peak maximum specific growth rate, 0.26 h1, in the study. The increase of pH to 9 or decrease to 4 would result in severe decrease of the maximum specific growth rate of PSB. Fig. 3b elaborately illustrates the effect of pH on the growth of PSB at different culture times. It is found that, within 36 h after inoculation, the specific growth rate initially increased with the increase of pH value of culture medium to 7 at the same culture stage, then, the specific growth rate decreased with the further increase of pH value to 9. It reveals that the strain used in the experiments had no natural tolerance to acidity or alkalinity, although the growth activity of R. palustris CQK 01 could be detected at the pH extremes of 4 and 9. This behavior can be attributed to that the acid or alkaline culture media deactivated the cellular enzymes and depressed the processes of substrate metabolism and electron transfer [33]. Furthermore, it is also found that effect of pH value of culture medium on the growth of PSB got abated beyond 36 h, i.e. the specific growth rate varied slightly with a variation of pH value of culture medium. Fig. 3c and d demonstrate that the variation of pH greatly affected the specific rates of substrate consumption and hydrogen production of PSB. In particular, at 12 h after inoculation, the specific rates of substrate consumption and hydrogen production increased to 1.07 and 1.63 103 h1 with an
14009
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 6 ( 2 0 1 1 ) 1 4 0 0 4 e1 4 0 1 3
Fig. 3 e Effect of initial pH value of culture medium on the reaction kinetics characteristics of PSB for hydrogen production (points: Experimental data, lines: Predicted data).
increase of pH value from 4 to 7, respectively; then, dropped to 0.88 h1 and 9.02 104 h1 with the further increase of pH value to 9. In general, the specific rates of substrate consumption and hydrogen production of PSB at a fixed pH value decreased due to nutrients depletion. For instance, the specific substrate consumption rate decreased rapidly beyond 36 h after inoculation and varied between 0.78 and 0.82 h1 at 44 h after inoculation across the entire range of pH values, it resulted from a reduction of metabolic activities of cells due to glucose depletion. While the corresponding specific hydrogen production rate descended slightly and varied irregularly with the increase of pH value of the culture medium due to the reutilization of intermediates produced. It is also found that both of the substrate consumption rate and the hydrogen production rate were low at an extreme acids or alkali and changed slightly with time. The experimental data also demonstrated that the concentration of glucose in medium decreased little (the data not shown). The results illustrate that the activities of cells were significantly inhibited and the synthesized biomass were small in extreme acid or alkaline environments. According to the principle of hydrogen production [34,35] and the above research results, it can be inferred that more available electrons in the substrate might have been transported to nitrogenase for H2 production at pH 7, which was consistent with the reference [36].
4.
Kinetic model development
4.1.
Calculation of kinetic parameters
In the experiment, when the specific growth rate, the specific substrate consumption rate as well as the specific hydrogen
production rate all achieved to the maxima during the exponential phase and stationary phase of the culture in the study, the culture conditions including the illumination intensity, culture temperature and pH value of medium solution were considered as the optimal conditions for hydrogen production. According to our experimental results, the optimal conditions included illumination intensity of 6000 lux, culture temperature of 30 C, initial pH value of 7.0 and an illumination wavelength of 590 nm. In the kinetic models, all these parameters (the maximum specific growth rates mopt, the Monod half saturation constant Ks, the maximum theoretical , cell maintenance coefficients m, yields of biomass YX=S constants a and b) under different operation conditions are estimated by regression analysis of LineweavereBurk equation. The parameter values under the optimal cultivation conditions are shown in Table 1.
4.2.
Model development
Batch culture of PSB used for hydrogen production was conducted at the initial glucose concentration of 9.9 g/L in the synthetic medium, implying that the substrate concentration is the rate-limiting factor for the hydrogen production. Here, the product is gaseous hydrogen, which will escape from the liquid medium to the upper space of the bioreactor, thus, the
Table 1 e Parameter values in models. Parameters
mopt
Ks
Results R2
0.26 0.964
5.20
YX=S
0.85 0.975
m
a
b
0.76
6.85 0.926
0.41
14010
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 6 ( 2 0 1 1 ) 1 4 0 0 4 e1 4 0 1 3
assumption was made that the inhibition by product will not take place in this case. Then, an empirical formula of the maximum specific growth rate, which is a function of illumination intensity, culture temperature and medium pH value, is obtained by fitting the experiment data using MATLAB, and a modified kinetic model based on Monod equation is also established to describe the cell growth of PSB. Based on it, the kinetic models of specific substrate consumption rate and specific hydrogen production rate, which are functions of these operation variables, are developed, respectively. The empirical formula on the maximum specific growth rates as a function of illumination intensity, culture temperature and pH value of medium solution are described as follows: 2 , mmax ¼ 0:9005mopt exp 0:3717 I= 1 Iopt exp 8:564 pH=pH
opt
2 ,exp 3:324 T=T 1
opt
2 1
qS ¼
(1)
opt
exp 3:324 T=T
opt
1
2 ,
2 1
Cs þm Ks þ Cs
dP ¼ arX þ bCX ¼ amCX þ bCX dt
qp ¼ 0:9005mopt exp 0:3717 I=I
opt
(6)
1
2 ,
2 , exp 8:564 pH= 1 pHopt 2 aCs exp 3:324 T= 1 þb Topt Ks þ Cs
2 , 1 m ¼ 0:9005mopt exp 0:3717 I= Iopt exp 8:564 pH=pH
opt
exp 3:324 T=T
opt
Analysis and comparison
1
The values of the specific growth rate, specific substrate consumption rate and specific hydrogen production rate predicted by these models are used as x-axis, respectively, and expressed as m0 , q0s , q0p . The predicted values as well as the corresponding experimental values within 44 h after inoculation are used as y-axis, respectively, and expressed as m, qs, qp. Thus, the corresponding rectangular coordinates are established to compare the predicted data with the corresponding experimental data. Comparison of the predicted data with the experimental data of the specific growth rates of PSB within 44 h after inoculation is illustrated in Fig. 4. And comparison of the
2 ,
2 1
Cs Ks þ Cs
(3)
Based on the principles of hydrogen production by PSB, it can be inferred that the consumed substrate was mainly used to the growth and maintenance energy of PSB cells. Therefore, the glucose consumption can be described by the maintenance energy model: 1 mCX þ mCX YX=S
(7)
(2)
Where, m is specific growth rate, Cs stands for the local glucose concentration and Ks represents the half saturation constant. When Eq. (1) is put into Eq. (2), the growth behavior of the used PSB for hydrogen production can be expressed by a modified Monod equation, in which the effects of illumination intensity, culture temperature and pH value of culture medium are considered.
rS ¼
(5)
LuedekingePiret proposed a model to calculate the production rate according to the kinetic parameters of biomass growth [32]:
4.3.
Cs Ks þ Cs
2 0:9005mopt , exp 0:3717 I= 1 Iopt YX=S exp 8:564 pH=pH
rP ¼
Where, mmax is the maximum specific growth rate, mopt the maximum specific growth rate under the optimal culture conditions, I illumination intensity and Iopt the optimal illumination intensity of 6000 lux for growth and hydrogen production, pH the initial value of medium solution and pHopt the optimal pH value of 7.0, T culture temperature and Topt the optimal culture temperature of 30 C. The Monod equation is described as m ¼ mmax
biomass concentration. Substituting Eq. (1) into Eq. (4) and then dividing both the sides of the quation by Cx, the specific substrate consumption rate qs can be given by:
(4)
Where rs is the glucose consumption rate, YX=S the theoretical cell yield coefficient, m the maintenance coefficient and Cx the
Fig. 4 e Comparison of the predicted data with the experimental data of specific growth rate.
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 6 ( 2 0 1 1 ) 1 4 0 0 4 e1 4 0 1 3
predicted data of the specific growth rates with the experimental data during different culture stages are shown in Figs. 1b, 2b and 3b. It is found that the predicted values of the specific growth rates under different operation conditions are almost consistent with the experimental data. Based on the above analysis, it is easy to say that the modified Monod model is relatively accurate to describe the growth behavior of this strain ahead of the decline phase. Fig. 5 shows the application of Eq. (5) to estimate the substrate consumption rate under different operation conditions, comparisons of the predicted values of the specific substrate consumption rate with the corresponding experimental data within 44 h after inoculation under different culture conditions are individually shown in Figs. 1c, 2c and 3c. The experimental data fluctuate around the predicted values. It can be concluded that the model reasonably described the substrate consumption during the culture except the decline phase. Fig. 6 illustrates comparison of the specific hydrogen production rates calculated from Eq. (7) with the experimental data within 44 h after inoculation. The corresponding comparisons of predicted data with experimental data at different culture time are shown in Figs. 1d, 2d and 3d. It is found that the reasonable agreement is achieved between predicted data of the proposed model and the experimental data during the culture except the decline phase. It denotes that the proposed model is capable to effectively predict the process of hydrogen production by the strain ahead of the decline phase. Regarding the batch culture mode, although the kinetic models based on the Monod equation has been successfully used to predict the behavior of PSB for hydrogen production, the deviation between the experimental data and the predicted data get increased during the last period of the culture, especially beyond 44 h after inoculation (the data not shown). This can be due to that the activities of PSB cells were declined due to nutrient depletion. The relative maximum errors of specific growth rate, specific substrate consumption rate and specific hydrogen production rate are 36.8%, 37.1% and 48.6%, respectively.
14011
Fig. 6 e Comparison of the predicted data with the experimental data of specific hydrogen production rate.
According to the above results, it can be concluded that the modified Monod model, which includes not only the limited effects of illumination intensity, culture temperature and pH value of culture medium, but also the inhibition to the growth of PSB for hydrogen production, is appropriate to describe the growth characteristics of PSB under these different culture conditions. And these kinetic models on substrate consumption and product formation based on Monod equation could reasonably describe the metabolic characteristics of PSB during hydrogen production. However, their applicability is limited to predict the growth phase and stationary phase with sufficient nutritious supply and high cell activities.
5.
Conclusion
The effects of illumination intensity, culture temperature and pH value of culture medium on the characteristics of photofermentative hydrogen production by Rhodobacter sphaeroides CQK 01 have been experimentally investigated in batch mode in this study. The experimental results led to an empirical formula of the maximum specific growth rate as a function of illumination intensity, pH and temperature of culture medium. And the modified kinetic equations on specific biomass growth rate, specific substrate consumption rate and specific hydrogen production rate are developed based on the empirical formula of the maximum specific growth rate, respectively. Research results indicate that good agreements between the experimental data and the modeled data are obtained, and the modified kinetic models are appropriate to describe the characteristics of hydrogen production by PSB under these experimental conditions.
Acknowledgment
Fig. 5 e Comparison of the predicted data with the experimental data of specific substrate consumption rate.
The authors gratefully acknowledge the financial support of the National Natural Science Foundation of China (No. 50825602, No. 20876183) and the Natural Science Foundation of Chongqing (CSTC, 2009BA6022).
14012
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 6 ( 2 0 1 1 ) 1 4 0 0 4 e1 4 0 1 3
references
[1] Akkerman I, Janssen M, Rocha J, Wijffels RH. Photobiological hydrogen production: photochemical efficiency and bioreactor design. Int J Hydrogen Energy 2002;27:1195e208. [2] Kokua H, Eroglu I`, Gu¨ndu¨z U, Yu¨cel M, Tu¨rker L. Aspects of the metabolism of hydrogen production by Rhodobacter sphaeroides. Int J Hydrogen Energy 2002;27:1315e29. [3] Basak N, Das D. The prospect of purple non-sulfur (PNS) photosynthetic bacteria for hydrogen production: the present state of the art. World J Microbiol Biotechnol 2007;23: 31e42. [4] Das D, Veziroglu TN. Hydrogen production by biological processes: a survey of literature. Int J Hydrogen Energy 2001; 26:29e37. [5] Ji CF, Legrand J, Pruvost J, Chen ZA, Zhang W. Characterization of hydrogen production by Platymonas Subcordiformis in torus photobioreactor. Int J Hydrogen Energy 2010;35:7200e5. [6] He DL, Bultel Y, Magnin JP, Willison JC. Kinetic analysis of photosynthetic growth and photohydrogen production of two strains of Rhodobacter Capsulatus. Enzyme Microb Technol 2006;38:253e9. [7] Akutsu Y, Lia YY, Harada Hideki, Yu Han-Qing. Effects of temperature and substrate concentration on biological hydrogen production from starch. Int J Hydrogen Energy 2009;34:2558e66. [8] Shi XY, Yu HQ. Response surface analysis on the effect of cell concentration and light intensity on hydrogen production by Rhodopseudomonas capsulate. Process Biochem 2005;40: 2475e81. [9] Kondo T, Arakawa M, Wakayama T, Miyake J. Hydrogen production by combining two types of photosynthetic bacteria with different characteristics. Int J Hydrogen Energy 2002;27:1303e8. [10] Antonopoulou G, Gavala HN, Skiadas IV, Lyberatos G. Influence of pH on fermentative hydrogen production from sweet sorghum extract. Int J Hydrogen Energy 2010;35: 1921e8. [11] Wu SY, Lin CN, Chang JS, Chang JS. Biohydrogen production with anaerobic sludge immobilized by ethylene-vinyl acetate copolymer. Int J Hydrogen Energy 2005;30:1375e81. [12] Idania VV, Poggi-Varaldo HM. Hydrogen production by fermentative consortia. Renew Sust Energy Rev 2009;13(5): 1000e13. lu I, Yu¨cel M, Gu¨ndu¨z U, Tu¨rker L. Effect of light [13] Uyar B, Erog intensity, wavelength and illumination protocol on hydrogen production in photobioreactors. Int J Hydrogen Energy 2007; 32:4570e677. [14] Liao Q, Wang YJ, Wang YZ, Zhu X, Tian X, Li J. Formation and hydrogen production of photosynthetic bacterial biofilm under various illumination conditions. Bioresour Technol 2010;101(14):5315e24. [15] Rosso L, Lobry JR, Bajard S, Flandrois JP. Convenient model to describe the combined effects of temperature and pH on microbial growth. Appl Environ Microbiol 1995;61(2):610e6. [16] Gadhamshetty V, Sukumaran A, Nirmalakhandan N, Myint MT. Photofermentation of malate for biohydrogen production - a modeling approach. Int J Hydrogen Energy 2008;33:2138e46. [17] Lin CN, Wu SY, Chang JS. Fermentative hydrogen production with a draft tube fluidized bed reactor containing siliconegel-immobilized anaerobic sludge. Int J Hydrogen Energy 2006;31:2200e10. ¨ zgu¨r E, Burg J, Yu¨cel M, Gu¨ndu¨z U, Eroglu I. [18] Boran E, O Biological hydrogen production by Rhodobacter capsulatus in solar tubular photobioreactor. J Clean Prod 2010;18:29e35.
[19] Jose GJ, Ray S, Das D. Hydrogen production using Rhodobacter sphaeroides (O.U. 001) in a flat panel rocking photobioreactor. Int J Hydrogen Energy 2011;36(5):3434e41. [20] Seifert K, Waligorska M, Laniecki M. Hydrogen generation in photobiological process from dairy wastewater. Int J Hydrogen Energy 2010;35:9624e9. _ lu E, Gu¨ndu¨z U, Yu¨celb M, Tu¨rker L, Erog lu I. [21] Erog Photobiological hydrogen production by using olive mill wastewater as a sole substrate source. Int J Hydrogen Energy 2004;29:163e71. [22] Ntaikou I, Gavala HN, Lyberatos G. Application of a modified Anaerobic Digestion Model 1 version for fermentative hydrogen production from sweet sorghum extract by Ruminococcus albus. Int J Hydrogen Energy 2010; 35:3423e32. [23] Gadhamshetty V, Arudchelvam Y, Nirmalakhandan N, Johnson DC. Modeling dark fermentation for biohydrogen production: ADM1-based model vs. Gompertz model. Int J Hydrogen Energy 2010;35:479e90. [24] Mu Y, Yu HQ, Wang G. A kinetic approach to anaerobic hydrogen-producing process. Water Search 2007;41: 1152e60. [25] Obeid J, Magnin JP, Flaus JM, Adrot O, Willison JC, Zlatev R. Modelling of hydrogen production in batch cultures of the photosynthetic bacterium Rhodobacter capsulatus. Int J Hydrogen Energy 2009;34:180e5. [26] Nath K, Muthukumar M, KumarA, Das D. Kinetics of twostage fermentation process for the production of hydrogen. Int J Hydrogen Energy 2008;33:1195e203. _ Gu¨ndu¨z U, Yu¨cel M, Tu¨rker L. Kinetics of lu I, [27] Koku H, Erog biological hydrogen production by the photosynthetic bacterium Rhodobacter sphaeroides O.U. 001. Int J Hydrogen Energy 2003;28:381e8. [28] Wang YZ, Liao Q, Zhu X, Tian X, Zhang C. Characteristics of hydrogen production and substrate consumption of Rhodopseudomonas palustris CQK 01 in an immobilized-cell photobioreactor. Bioresour Technol 2010;101:4034e41. [29] Li X, Wang YH, Zhang SL, Chu J, Zhang M, Huang MZ, et al. Enhancement of phototrophic hydrogen production by Rhodobacter sphaeroides ZX-5 using a novel strategy-shaking and extra-light supplementation approach. Int J Hydrogen Energy 2009;34:9677e85. [30] Adamberg K, Kask S, Laht TM, Paalme T. The effect of temperature and pH on the growth of lactic acid bacteria: a pH-auxostat study. Int J Food Microbiol 2003;85:171e83. [31] Karlsson A, Vallin L, Ejlertsson J. Effects of temperature, hydraulic retention time and hydrogen extraction rate on hydrogen production from the fermentation of food industry residues and manure. Int J Hydrogen Energy 2008; 33:953e62. [32] Lee KS, Lin PJ, Chang JS. Temperature effects on biohydrogen production in a granular sludge bed induced by activated carbon carriers. Int J Hydrogen Energy 2006;31: 465e72. [33] Fang HHP, Zhu HG, Zhang T. Phototrophic hydrogen production from glucose by pure and co-cultures of Clostridium butyricum and Rhodobacter sphaeroides. Int J Hydrogen Energy 2006;31:2223e30. [34] Cheong DY, Hansen CL. Feasibility of hydrogen production in thermophilic mixed fermentation by natural anaerobes. Bioresour Technol 2007;98:2229e39. [35] Jo JH, Lee DS, Park DH, Park JM. Biological hydrogen production by immobilized cells of Clostridium tyrobutyricum JM1 isolated from a food waste treatment process. Bioresour Technol 2008;99:6666e72. ¨ ztu¨rk Y, Yu¨cel M, Daldal F, Mandacı S, Gu¨ndu¨z U, Tu¨rker L, [36] O et al. Hydrogen production by using Rhodobacter capsulatus mutants with genetically modified electron transfer chains. Int J Hydrogen Energy 2006;31:1545e52.
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 6 ( 2 0 1 1 ) 1 4 0 0 4 e1 4 0 1 3
14013
Nomenclature Cs: local glucose concentration, g/L Cx: biomass concentration, g/L Ks: Monod half saturation constant, g/L m: cell maintenance coefficient, h1 I: illumination intensity, lux Iopt: the optimal illumination intensity, lux pH: initial pH value of medium solution pHopt: the optimal pH value qp: specific hydrogen production rate, h1 qs: specific substrate consumption rate, h1
rp: hydrogen production rate, g/L/h rs: glucose consumption rate, g/L/h T: culture temperature, C Topt: the optimal culture temperature, C YX=S : theoretical cell yield coefficient m: specific growth rate, h1 mmax: the maximum specific growth rate, h1 mopt: the maximum specific growth rate under the optimal culture conditions, h1 a: constant b: constant
Available at www.sciencedirect.com
journal homepage: www.elsevier.com/locate/he
Effect of culture conditions on the kinetics of hydrogen production by photosynthetic bacteria in batch culture Yong-Zhong Wang a,b, Qiang Liao a,b,*, Xun Zhu a,b, Jun Li a,b, Duu-Jong Lee a,c a
Key Laboratory of Low-grade Energy Utilization Technologies and Systems, Chongqing University, Ministry of Education, China Institute of Engineering Thermophysics, Chongqing University, Chongqing 400030, China c Department of Chemical Engineering, National Taiwan University, Taipei 106, Taiwan b
article info
abstract
Article history:
Biochemical kinetic characteristics of photo-fermentative hydrogen production were
Received 23 January 2011
experimentally and numerically investigated to optimize the photo-fermentation
Received in revised form
hydrogen-producing process in this work. It is found that a maximum specific growth rate
29 March 2011
of 0.26 h1 was achieved under the optimal conditions of illumination intensity 6000 lux,
Accepted 2 April 2011
30 C culture temperature and pH 7.0 of culture medium. These experimental results also
Available online 29 April 2011
led to an empirical formula of the maximum specific microbial growth rate (mmax) as a function of illumination intensity, pH and temperature. With the empirical formula, the
Keywords:
modified Monod equation along with the kinetic equations for biomass growth, glucose
Photosynthetic bacteria
consumption and hydrogen production is then developed to simulate the photo-
Biohydrogen production
fermentation hydrogen-producing process. The modeling results are in good agreements
Reaction kinetics
with the experimental data, indicating that the developed kinetic models are able to
Illumination intensity
objectively describe the characteristics of hydrogen production by PSB under different
Culture temperature
culture conditions.
pH value
Copyright ª 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
1.
Introduction
Hydrogen is a clean and efficient fuel and has been recognized as the most promising alternative for fossil fuels by virtue of the fact that it is renewable and does not cause the ‘‘greenhouse effect” [1e3]. However, if hydrogen is to replace fossil fuels in the near future, it has to be produced renewably in large scale through environmentally friendly processes. Biological hydrogen production method stands out as an environmentally harmless process that converts renewable resources into hydrogen under mild operation conditions. Several types of microorganisms such as photosynthetic
bacteria, cyanobacteria, algae or fermentative bacteria are commonly utilized for biological hydrogen production [4e6]. Among these microorganisms, the photosynthetic bacteria are the most favorable candidates for large-scale hydrogen production due to the high substrate conversion efficiency and the capability to use a wide variety of substrates to produce hydrogen. Researches indicate that the growth and hydrogen production of PSB are not only dependent on intrinsic factors, but also dependent on extrinsic factors such as medium compositions, cell concentration, light intensity, carbon to nitrogen ratio, temperature, pH and so on [7e11]. Several
* Corresponding author. Key Laboratory of Low-grade Energy Utilization Technologies and Systems, Chongqing University, Ministry of Education, China. Tel./fax: þ86 0 23 65102474. E-mail address: [email protected] (Q. Liao). 0360-3199/$ e see front matter Copyright ª 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2011.04.005
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 6 ( 2 0 1 1 ) 1 4 0 0 4 e1 4 0 1 3
previous studies have confirmed that the temperature, pH and illumination intensity are relatively important environmental factors for the growth and metabolism of PSB [12e14]. However, most studies mainly focused on investigating the effects of these operation variables on the performance of hydrogen production of bioreactor rather than the kinetic characteristics of hydrogen production process. In the past few years, mathematical models, which describe the growth of microorganisms and effects of controlling factors on metabolism, have drawn a lot of attentions. An advantage of such models is that they could be used to simulate the influence of different environmental conditions on growth kinetics [15]. Another important feature of these models is the acquisition of the improved knowledge of these important factors that determine products formation. In terms of hydrogen production by photofermentation, these models could help to decrease the load of the experimental measurement that is time-consuming and costly, and to design more efficient hydrogen-producing reactors, and to control the photofermentation process effectively. Eventually, these models could be used for H2 production optimization. So far, many kinetic models have also been proposed and most of them are mainly based on Monod kinetic [16,17], Logistic lu et al. [21] models [18,19] or Gompertz equation [20]. Erog compared Logistic model with Monod model and Exponential model and concluded that bacterial growth rate obeys logistic model if the bacteria can make photosynthesis. Recently, the Anaerobic Digestion Model 1 (ADM1), a complex kinetic model describing the fermentative hydrogen process, has been proposed [22,23]. Mu et al. [24] evaluated the effect of pH, temperature and the ratio of initial substrate concentration to initial biomass concentration on the maximum specific microbial growth rate using a response-surface methodology. Obeid et al. [25] investigated the relationships between light intensity and hydrogen production by using a model to predict both the rate of hydrogen production and the amount of hydrogen produced in a period during batch cultures of R. capsulatus. Nath et al. [26]. researched the kinetics on growth, substrate inhibition and hydrogen production by twostage process combined dark and photo-fermentation in a sequential batch mode, and established a modified Gompertz equation to estimate the hydrogen production potential, rate and lag phase time in a batch process. Up to date, only a few elaborate researches have been reported about the complicated effects of operation variables on the kinetics of microbial growth, substrate consumption and product formation in the hydrogen production process [16,21,25,27]. However, no model has been proposed to describe the relationship of operation variables with the kinetic parameters of hydrogen production by photofermentation. In this work, effects of light intensity, culture temperature and pH of culture medium on hydrogen production by PSB were investigated in batch culture. It was expected to obtain the kinetic parameters of growth, H2 production and substrate consumption of PSB, and to establish the modified Monod equation on the growth of PSB. With these data, the kinetic models on hydrogen production and substrate consumption were proposed to describe the characteristics of hydrogen production by PSB. This work will be an essential part for developing full-scale bioreactor.
2.
Material and methods
2.1.
Microorganism and culture medium
14005
The strain used for hydrogen production in this study was originally isolated from the silt of sewages and identified as Rhodopseudomonas palustris [28]. The culture was kept in RCVBN Medium [29], with the exception that glucose (7.9 g/L) and CO(NH2)2 (1.35 g/L) were served as the carbon and nitrogen nutrients, respectively. Cells for further use were cultivated for 72 h in 150 mL of anaerobic sterile test bottle with 3000 lux illumination intensity from a tungsten filament lamp. Kinetic experiments of hydrogen production were conducted by using a basal medium, 1 L of which contained the following elements: C6H12O6$H2O (9.9 g); K2HPO4$3H2O (1.006 g); KH2PO4 (0.544 g); MgSO4$7H2O (0.2 g); FeSO4$7H2O (0.0417 g); (NH4)6Mo7O24$4H2O (0.0010 g); ZnSO4$7H2O (0.0010 g); NaCl (0.2 g); CaCl2 (0.010 g); CO(NH2)2 (1.35 g); yeast extract (1.0 g); growth factors solution (1 ml) which was composed of thiamin hydrochloride (1.0 g), riboflavin (1.0 g), nicotinic acid (1.0 g) and biotin (0.1 g) and 1 L water. The initial pH value of medium was adjusted by using 0.1 mol/L sodium hydroxide solution and 0.1 mol/L hydrochloric acid solution after autoclaving at 121 C for 12 min. Here, glucose was chosen as the sole carbon resource for growth and hydrogen production of PSB in the synthetic medium in the study.
2.2.
Photo-bioreactor and culture conditions
Five rectangle photo-bioreactors with 0.5 L volume made of transparent polymethylmethacrylate with 8 mm thickness were used for hydrogen production. Each bioreactor was filled with 0.3 L inoculated culture medium, the upper space was filled with gas. The front of the reactor illuminated by light was 100 mm in length and 100 mm in height, respectively. And the width of the reactor was 50 mm. Two holes were respectively drilled at the top of the reactor, one for charging culture medium and argon gas, another one for exhausting the gas sampling. In addition, one hole located on the left side of the bioreactor was for liquid sampling. Before loading the culture medium, the reactor must be disinfected with formaldehyde solution for 15 min to avoid bacteria contamination and washed three times using sterile distilled water. After autoclaving the pH value of culture medium was precisely adjusted to that of being set, using 0.1 mmol/L of the sodium hydroxide solution and 0.1 mmol/L of hydrochloric acid solution. In order to ensure the same activity and quantity of inoculum, the strain used for inoculum must be cultivated in a basal medium (the detailed components as above mentioned) up to exponential phase, and the quantity of inoculum was 10% (v/v). After being charged medium solution, the bioreactor was purged by argon gas to ensure an anaerobic atmosphere. Then, the bioreactor was placed in a culture device with a constant designated temperature for cultivation. And it was illuminated by LED lamps with the main light wavelength of 590 nm, the illumination intensity was adjusted from 2000 to 8000 lux by varying the distance between the LEDs and the bioreactor. The initial glucose
14006
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 6 ( 2 0 1 1 ) 1 4 0 0 4 e1 4 0 1 3
concentration of culture medium was 9.9 g/L. In this study, a range of cultures were incubated at different values of the operation variables and each of these cultures was subjected to the constant conditions in the whole photofermentation period. During the culture process, the liquid and gas samples were collected from the reactors at intervals of 4 h, respectively, until the biomass concentration and hydrogen production rate obviously decreased. Liquid samples were used to determine dry cell weight and residual glucose concentration in the culture medium, and the gas samples were adopted to detect hydrogen content in the discharged biogas. After sampling, the photo-bioreactor must be purged by argon gas again to obtain the environment without oxygen and hydrogen. In order to extract the model parameters of those particular conditions, the kinetic profile for each culture was thoroughly analyzed. Based on these analyses, an empirical equation of the maximum specific growth rate, which is a function of the operation variables, was achieved by fitting experimental data using MATLAB.
2.3.
Analytical methods
The electronic analytical balance (Sartorius BP114, Germany) was used to determine the quantity of elements in basal culture medium. pH meter（Ecoscan-pH6, Singapore）was adopted to measure the pH value of medium solution. Illumination intensity of light was measured by a digital luxmeter (ST-85, China). The cell concentration of the suspension was obtained by optical density at a wavelength of 600 nm (i.e., OD600 nm) using a UVeVIS spectrophotometer (756 MC, China). Firstly, biomass in a culture solution with a certain OD600 nm value was collected by a centrifuge at 20,300 g for 10 min, then, was dried at 105 C till constant weight. A formula of dried weight of biomass as a function of the corresponding OD600 nm was then achieved as follows Dried cell weight ¼ 0:44 OD600nm þ 0:0226; R2 ¼ 0:9905 Hence, the cell concentration of culture medium could be regularly monitored by measuring OD600 value using a spectrophotometer. Hydrogen gas content was determined by gas chromatography (SC2000, China). A thermal conductivity detector (TCD) with a current of 80 mA was equipped. The flow velocity of carrier gas (i.e., argon) was adjusted at 25 mL/min and a 2.0 m stainless steel column was packed with porous polymer pellets of TDX. The operation temperatures of the column oven and detector were 55 and 100 C, respectively. The glucose concentration of culture medium was measured by 3, 5-dinitrosalicylic acid method. Dinitrosalicylic acid reagent of 1.5 mL was added into 1 mL of the liquid sample, the mixture of liquid was then heated in a boiling water bath for 5 min and diluted to 10 mL with distilled water. The color change of the prepared solution was measured by using a spectrophotometer at a wavelength of 540 nm. By the method, the glucose concentration of culture medium could be calculated according to the standard curve obtained before.
3.
Results and discussion
The microbial specific growth rate is one of the most important parameters in a biological process and the relationship between growth rate and substrate consumption rate or product formation rate is crucial for monitoring and controlling the process [30]. Therefore, an accurate estimation of the microbial specific growth rate is essential to characterize the hydrogen-producing process. Although the importance of these parameters like optimal illumination intensity, temperature, pH for industrially used strains is well understood, the quantitative relationships between growth rate and the environmental factors such as illumination intensity, temperature and pH are rarely reported.
3.1.
Effect of illumination intensity
Our previous work has shown that PSB can yield a better performance of hydrogen production with light illumination of 590 nm due to its maximal absorption peak [28]. In this study, yellow light with the main wavelength of 590 nm from LEDs was used as light source. Light intensity of LED lamps was adjusted between 2000 and 8000 lux by changing the distance between the light source and photo-bioreactor. Fig. 1 shows that the effect of illumination intensity on the performance of hydrogen production by PSB. As shown in Fig. 1a, the relationship between the maximum specific growth rate and illumination intensity was not monotonic. With an increase in illumination intensity from 2000 to 6000 lux, the maximum specific growth rate first increased and reached to the maximum value of 0.26 h1 at 6000 lux, then, with the further increase in illumination intensity to 8000 lux, the maximum specific growth rate decreased to 0.2 h1. Meanwhile, the specific growth rate initially increased with the increase in illumination intensity to 6000 lux at the same culture stage due to effect of the limited incident light, then, decreased with the further increase in illumination intensity to 8000 lux due to the effect of incident light saturation (Fig. 1b). It is also found that the specific growth rate dropped quickly within 36 h after inoculation, then, dropped slowly at the end of culture time due to nutrients depletion. On the other hand, when the illumination intensity increased from 2000 to 6000 lux, the specific substrate consumption rate (Fig. 1) and specific hydrogen production rate (Fig. 1d) of PSB at the same culture stage all increased, and then decreased with the increase of illumination intensity up to 8000 lux within 36 h after inoculation. According to the above results, the optimal illumination intensity for the growth and hydrogen production of this strain can be recommended to be 6000 lux. In addition, as shown in Fig. 1, the specific growth rate, specific substrate consumption rate and specific hydrogen production rate all decreased with increasing culture time. This can be attributed to limited substrate supply in batch culture. Furthermore, beyond 44 h after inoculation, the substrate consumption rate dropped to the minimum of 0.64 h1 with the increase of illumination intensity to 6000 lux, and then gradually increased to 0.68 h1 when the illumination intensity further increased to 8000 lux. The corresponding specific hydrogen production rate also achieved the peak
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 6 ( 2 0 1 1 ) 1 4 0 0 4 e1 4 0 1 3
14007
Fig. 1 e Effect of illumination intensity on the reaction kinetics characteristics of PSB for hydrogen production (points: Experimental data, lines: Predicted data).
at 6000 lux of illumination intensity. It can be explained by that the available glucose was almost depleted at the end of the culture when the illumination intensity remained at 6000 lux, but a quantity of intermediates produced in the solution could be utilized by PSB for growth and hydrogen production. Therefore, during the last phase, the specific rates of growth and hydrogen production at illumination intensity of 6000 lux were still higher than those at other illumination intensities. In summary, it can be concluded that the growth of PSB was nearly approaching to the decline phase at 44 h after inoculation.
3.2.
Effect of culture temperature
Operation temperature is always a critical factor influencing the performance of a fermentation process [31]. This is also valid for photo-fermentative H2 production by a pure culture bacteria strain. Here, the kinetic characteristics of hydrogen production was investigated in batch culture mode at six different culture temperatures under the illumination conditions of 590 nm and 6000 lux, where the initial glucose concentration was 9.9 g/L and the initial pH value of this medium was 7.0. The results are shown in Fig. 2. Effect of culture temperature on the maximum specific growth rate is presented in Fig. 2. It is found that the maximum specific growth rate (i.e., 0.26 h1) was achieved when the culture temperature was 30 C, the growth of PSB, however, became slow when the temperature was below or above 30 C. The result indicates that the optimal culture temperature for the growth of PSB should be at 30 C. The effects of culture temperature on the specific growth rate and specific substrate consumption rate are presented in Fig. 2b
and c, respectively. Within 24 h after inoculation, both of the specific growth rate and the specific substrate consumption rate first increased with an increase of culture temperature, and then decreased with the further increase of culture temperature. For instance, the specific growth rate and the specific hydrogen production rate at a culture temperature of 30 C achieved the maximum values within 24 h after inoculation. Conversely, both of them decreased with an increase or decrease in the culture temperature at the same culture stage. The maximum specific growth rate of 0.17 h1 for PSB was observed at 30 C culture temperature at 12 h after inoculation and the corresponding specific substrate consumption rate of 1.07 h1 was also maximal. Also, it is found that the biomass concentration of culture solution increased with the increase of culture time (the data not shown). This fact indicates that the growth of PSB at a culture temperature of 30 C had reached to the exponential phase within 24 h after inoculation. Beyond 24 h, both of the specific growth rate and specific substrate consumption rate initially decreased with the increase of culture temperature, then increased with further increase of culture temperature. And the specific growth rate and specific substrate consumption rate at 30 C were minimal. This indicates that the growth of PSB nearly approached the decline phase at the moment. For instance, the specific growth rate and specific substrate consumption rate at 30 C were 0.05 h1 and 0.64 h1, respectively, at 44 h after inoculation because of the decreasing cell activities of PSB due to the depletion of glucose. In addition, at 25 C and 40 C, the specific growth rates and specific substrate consumption rates were minimal and changed slightly during the entire period. It demonstrates that PSB cells used had no endurance for an extreme environment temperature.
14008
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 6 ( 2 0 1 1 ) 1 4 0 0 4 e1 4 0 1 3
Fig. 2 e Effect of culture temperature on the reaction kinetics characteristics of PSB for hydrogen production (points: Experimental data, lines: Predicted data).
As can be seen from Fig. 2d, the specific hydrogen production rate demonstrates a similar change to the specific growth rate. When the maximal specific growth rate occurred at 12 h at the culture temperature of 30 C, the maximal specific hydrogen production rate of 1.63 103 h1 was also observed at the same stage. Within 24 h after inoculation, the specific hydrogen production rate increased with the increase of culture temperature to 30 C; and then decreased to 1.11 103 h1 with further increase of culture temperature to 40 C. It can be inferred that the increase of culture temperature below 30 C could cause a decrease inactivation energy needed in these biochemical reactions and resulted in an increase of metabolic rate of PSB, while the increase of the culture temperature over 30 C could lead to the reversible inactivation of cellular enzymes and inhibited hydrogen formation. Lee et al. [32] had confirmed the dependence of volumetric production rate on the operation temperature in a carrier-induced granular sludge bed reactor. In addition, the specific hydrogen production rate varied irregularly at 44 h after inoculation across the entire range of culture temperature. It can be ascribed to that PSB cells near the decline phase mainly utilized the produced intermediates for hydrogen production, although a little of glucose remained in the solution. Furthermore, the specific hydrogen production rate varied slightly with culture time at the culture temperatures of 20 C and 40 C indicating that extreme culture temperature was unfavorable to hydrogen production.
3.3.
Effect of pH value of culture medium
Different pH values of culture medium determine the ionic form of the active site and enzyme activity. Therefore, the pH
value of culture medium will affect the biochemical reaction characteristics. The effects of initial pH values of 4, 5, 6, 7, 8 and 9 on the characteristics of hydrogen production and substrate consumption in batch mode were investigated under the operating conditions of 50 mmol/L initial glucose concentration, 30 C culture temperature, 590 nm light wavelength and 6000 lux illumination intensity (Fig. 3). According to the data shown in Fig. 3a, operation at pH 7 achieved the peak maximum specific growth rate, 0.26 h1, in the study. The increase of pH to 9 or decrease to 4 would result in severe decrease of the maximum specific growth rate of PSB. Fig. 3b elaborately illustrates the effect of pH on the growth of PSB at different culture times. It is found that, within 36 h after inoculation, the specific growth rate initially increased with the increase of pH value of culture medium to 7 at the same culture stage, then, the specific growth rate decreased with the further increase of pH value to 9. It reveals that the strain used in the experiments had no natural tolerance to acidity or alkalinity, although the growth activity of R. palustris CQK 01 could be detected at the pH extremes of 4 and 9. This behavior can be attributed to that the acid or alkaline culture media deactivated the cellular enzymes and depressed the processes of substrate metabolism and electron transfer [33]. Furthermore, it is also found that effect of pH value of culture medium on the growth of PSB got abated beyond 36 h, i.e. the specific growth rate varied slightly with a variation of pH value of culture medium. Fig. 3c and d demonstrate that the variation of pH greatly affected the specific rates of substrate consumption and hydrogen production of PSB. In particular, at 12 h after inoculation, the specific rates of substrate consumption and hydrogen production increased to 1.07 and 1.63 103 h1 with an
14009
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 6 ( 2 0 1 1 ) 1 4 0 0 4 e1 4 0 1 3
Fig. 3 e Effect of initial pH value of culture medium on the reaction kinetics characteristics of PSB for hydrogen production (points: Experimental data, lines: Predicted data).
increase of pH value from 4 to 7, respectively; then, dropped to 0.88 h1 and 9.02 104 h1 with the further increase of pH value to 9. In general, the specific rates of substrate consumption and hydrogen production of PSB at a fixed pH value decreased due to nutrients depletion. For instance, the specific substrate consumption rate decreased rapidly beyond 36 h after inoculation and varied between 0.78 and 0.82 h1 at 44 h after inoculation across the entire range of pH values, it resulted from a reduction of metabolic activities of cells due to glucose depletion. While the corresponding specific hydrogen production rate descended slightly and varied irregularly with the increase of pH value of the culture medium due to the reutilization of intermediates produced. It is also found that both of the substrate consumption rate and the hydrogen production rate were low at an extreme acids or alkali and changed slightly with time. The experimental data also demonstrated that the concentration of glucose in medium decreased little (the data not shown). The results illustrate that the activities of cells were significantly inhibited and the synthesized biomass were small in extreme acid or alkaline environments. According to the principle of hydrogen production [34,35] and the above research results, it can be inferred that more available electrons in the substrate might have been transported to nitrogenase for H2 production at pH 7, which was consistent with the reference [36].
4.
Kinetic model development
4.1.
Calculation of kinetic parameters
In the experiment, when the specific growth rate, the specific substrate consumption rate as well as the specific hydrogen
production rate all achieved to the maxima during the exponential phase and stationary phase of the culture in the study, the culture conditions including the illumination intensity, culture temperature and pH value of medium solution were considered as the optimal conditions for hydrogen production. According to our experimental results, the optimal conditions included illumination intensity of 6000 lux, culture temperature of 30 C, initial pH value of 7.0 and an illumination wavelength of 590 nm. In the kinetic models, all these parameters (the maximum specific growth rates mopt, the Monod half saturation constant Ks, the maximum theoretical , cell maintenance coefficients m, yields of biomass YX=S constants a and b) under different operation conditions are estimated by regression analysis of LineweavereBurk equation. The parameter values under the optimal cultivation conditions are shown in Table 1.
4.2.
Model development
Batch culture of PSB used for hydrogen production was conducted at the initial glucose concentration of 9.9 g/L in the synthetic medium, implying that the substrate concentration is the rate-limiting factor for the hydrogen production. Here, the product is gaseous hydrogen, which will escape from the liquid medium to the upper space of the bioreactor, thus, the
Table 1 e Parameter values in models. Parameters
mopt
Ks
Results R2
0.26 0.964
5.20
YX=S
0.85 0.975
m
a
b
0.76
6.85 0.926
0.41
14010
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 6 ( 2 0 1 1 ) 1 4 0 0 4 e1 4 0 1 3
assumption was made that the inhibition by product will not take place in this case. Then, an empirical formula of the maximum specific growth rate, which is a function of illumination intensity, culture temperature and medium pH value, is obtained by fitting the experiment data using MATLAB, and a modified kinetic model based on Monod equation is also established to describe the cell growth of PSB. Based on it, the kinetic models of specific substrate consumption rate and specific hydrogen production rate, which are functions of these operation variables, are developed, respectively. The empirical formula on the maximum specific growth rates as a function of illumination intensity, culture temperature and pH value of medium solution are described as follows: 2 , mmax ¼ 0:9005mopt exp 0:3717 I= 1 Iopt exp 8:564 pH=pH
opt
2 ,exp 3:324 T=T 1
opt
2 1
qS ¼
(1)
opt
exp 3:324 T=T
opt
1
2 ,
2 1
Cs þm Ks þ Cs
dP ¼ arX þ bCX ¼ amCX þ bCX dt
qp ¼ 0:9005mopt exp 0:3717 I=I
opt
(6)
1
2 ,
2 , exp 8:564 pH= 1 pHopt 2 aCs exp 3:324 T= 1 þb Topt Ks þ Cs
2 , 1 m ¼ 0:9005mopt exp 0:3717 I= Iopt exp 8:564 pH=pH
opt
exp 3:324 T=T
opt
Analysis and comparison
1
The values of the specific growth rate, specific substrate consumption rate and specific hydrogen production rate predicted by these models are used as x-axis, respectively, and expressed as m0 , q0s , q0p . The predicted values as well as the corresponding experimental values within 44 h after inoculation are used as y-axis, respectively, and expressed as m, qs, qp. Thus, the corresponding rectangular coordinates are established to compare the predicted data with the corresponding experimental data. Comparison of the predicted data with the experimental data of the specific growth rates of PSB within 44 h after inoculation is illustrated in Fig. 4. And comparison of the
2 ,
2 1
Cs Ks þ Cs
(3)
Based on the principles of hydrogen production by PSB, it can be inferred that the consumed substrate was mainly used to the growth and maintenance energy of PSB cells. Therefore, the glucose consumption can be described by the maintenance energy model: 1 mCX þ mCX YX=S
(7)
(2)
Where, m is specific growth rate, Cs stands for the local glucose concentration and Ks represents the half saturation constant. When Eq. (1) is put into Eq. (2), the growth behavior of the used PSB for hydrogen production can be expressed by a modified Monod equation, in which the effects of illumination intensity, culture temperature and pH value of culture medium are considered.
rS ¼
(5)
LuedekingePiret proposed a model to calculate the production rate according to the kinetic parameters of biomass growth [32]:
4.3.
Cs Ks þ Cs
2 0:9005mopt , exp 0:3717 I= 1 Iopt YX=S exp 8:564 pH=pH
rP ¼
Where, mmax is the maximum specific growth rate, mopt the maximum specific growth rate under the optimal culture conditions, I illumination intensity and Iopt the optimal illumination intensity of 6000 lux for growth and hydrogen production, pH the initial value of medium solution and pHopt the optimal pH value of 7.0, T culture temperature and Topt the optimal culture temperature of 30 C. The Monod equation is described as m ¼ mmax
biomass concentration. Substituting Eq. (1) into Eq. (4) and then dividing both the sides of the quation by Cx, the specific substrate consumption rate qs can be given by:
(4)
Where rs is the glucose consumption rate, YX=S the theoretical cell yield coefficient, m the maintenance coefficient and Cx the
Fig. 4 e Comparison of the predicted data with the experimental data of specific growth rate.
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 6 ( 2 0 1 1 ) 1 4 0 0 4 e1 4 0 1 3
predicted data of the specific growth rates with the experimental data during different culture stages are shown in Figs. 1b, 2b and 3b. It is found that the predicted values of the specific growth rates under different operation conditions are almost consistent with the experimental data. Based on the above analysis, it is easy to say that the modified Monod model is relatively accurate to describe the growth behavior of this strain ahead of the decline phase. Fig. 5 shows the application of Eq. (5) to estimate the substrate consumption rate under different operation conditions, comparisons of the predicted values of the specific substrate consumption rate with the corresponding experimental data within 44 h after inoculation under different culture conditions are individually shown in Figs. 1c, 2c and 3c. The experimental data fluctuate around the predicted values. It can be concluded that the model reasonably described the substrate consumption during the culture except the decline phase. Fig. 6 illustrates comparison of the specific hydrogen production rates calculated from Eq. (7) with the experimental data within 44 h after inoculation. The corresponding comparisons of predicted data with experimental data at different culture time are shown in Figs. 1d, 2d and 3d. It is found that the reasonable agreement is achieved between predicted data of the proposed model and the experimental data during the culture except the decline phase. It denotes that the proposed model is capable to effectively predict the process of hydrogen production by the strain ahead of the decline phase. Regarding the batch culture mode, although the kinetic models based on the Monod equation has been successfully used to predict the behavior of PSB for hydrogen production, the deviation between the experimental data and the predicted data get increased during the last period of the culture, especially beyond 44 h after inoculation (the data not shown). This can be due to that the activities of PSB cells were declined due to nutrient depletion. The relative maximum errors of specific growth rate, specific substrate consumption rate and specific hydrogen production rate are 36.8%, 37.1% and 48.6%, respectively.
14011
Fig. 6 e Comparison of the predicted data with the experimental data of specific hydrogen production rate.
According to the above results, it can be concluded that the modified Monod model, which includes not only the limited effects of illumination intensity, culture temperature and pH value of culture medium, but also the inhibition to the growth of PSB for hydrogen production, is appropriate to describe the growth characteristics of PSB under these different culture conditions. And these kinetic models on substrate consumption and product formation based on Monod equation could reasonably describe the metabolic characteristics of PSB during hydrogen production. However, their applicability is limited to predict the growth phase and stationary phase with sufficient nutritious supply and high cell activities.
5.
Conclusion
The effects of illumination intensity, culture temperature and pH value of culture medium on the characteristics of photofermentative hydrogen production by Rhodobacter sphaeroides CQK 01 have been experimentally investigated in batch mode in this study. The experimental results led to an empirical formula of the maximum specific growth rate as a function of illumination intensity, pH and temperature of culture medium. And the modified kinetic equations on specific biomass growth rate, specific substrate consumption rate and specific hydrogen production rate are developed based on the empirical formula of the maximum specific growth rate, respectively. Research results indicate that good agreements between the experimental data and the modeled data are obtained, and the modified kinetic models are appropriate to describe the characteristics of hydrogen production by PSB under these experimental conditions.
Acknowledgment
Fig. 5 e Comparison of the predicted data with the experimental data of specific substrate consumption rate.
The authors gratefully acknowledge the financial support of the National Natural Science Foundation of China (No. 50825602, No. 20876183) and the Natural Science Foundation of Chongqing (CSTC, 2009BA6022).
14012
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 6 ( 2 0 1 1 ) 1 4 0 0 4 e1 4 0 1 3
references
[1] Akkerman I, Janssen M, Rocha J, Wijffels RH. Photobiological hydrogen production: photochemical efficiency and bioreactor design. Int J Hydrogen Energy 2002;27:1195e208. [2] Kokua H, Eroglu I`, Gu¨ndu¨z U, Yu¨cel M, Tu¨rker L. Aspects of the metabolism of hydrogen production by Rhodobacter sphaeroides. Int J Hydrogen Energy 2002;27:1315e29. [3] Basak N, Das D. The prospect of purple non-sulfur (PNS) photosynthetic bacteria for hydrogen production: the present state of the art. World J Microbiol Biotechnol 2007;23: 31e42. [4] Das D, Veziroglu TN. Hydrogen production by biological processes: a survey of literature. Int J Hydrogen Energy 2001; 26:29e37. [5] Ji CF, Legrand J, Pruvost J, Chen ZA, Zhang W. Characterization of hydrogen production by Platymonas Subcordiformis in torus photobioreactor. Int J Hydrogen Energy 2010;35:7200e5. [6] He DL, Bultel Y, Magnin JP, Willison JC. Kinetic analysis of photosynthetic growth and photohydrogen production of two strains of Rhodobacter Capsulatus. Enzyme Microb Technol 2006;38:253e9. [7] Akutsu Y, Lia YY, Harada Hideki, Yu Han-Qing. Effects of temperature and substrate concentration on biological hydrogen production from starch. Int J Hydrogen Energy 2009;34:2558e66. [8] Shi XY, Yu HQ. Response surface analysis on the effect of cell concentration and light intensity on hydrogen production by Rhodopseudomonas capsulate. Process Biochem 2005;40: 2475e81. [9] Kondo T, Arakawa M, Wakayama T, Miyake J. Hydrogen production by combining two types of photosynthetic bacteria with different characteristics. Int J Hydrogen Energy 2002;27:1303e8. [10] Antonopoulou G, Gavala HN, Skiadas IV, Lyberatos G. Influence of pH on fermentative hydrogen production from sweet sorghum extract. Int J Hydrogen Energy 2010;35: 1921e8. [11] Wu SY, Lin CN, Chang JS, Chang JS. Biohydrogen production with anaerobic sludge immobilized by ethylene-vinyl acetate copolymer. Int J Hydrogen Energy 2005;30:1375e81. [12] Idania VV, Poggi-Varaldo HM. Hydrogen production by fermentative consortia. Renew Sust Energy Rev 2009;13(5): 1000e13. lu I, Yu¨cel M, Gu¨ndu¨z U, Tu¨rker L. Effect of light [13] Uyar B, Erog intensity, wavelength and illumination protocol on hydrogen production in photobioreactors. Int J Hydrogen Energy 2007; 32:4570e677. [14] Liao Q, Wang YJ, Wang YZ, Zhu X, Tian X, Li J. Formation and hydrogen production of photosynthetic bacterial biofilm under various illumination conditions. Bioresour Technol 2010;101(14):5315e24. [15] Rosso L, Lobry JR, Bajard S, Flandrois JP. Convenient model to describe the combined effects of temperature and pH on microbial growth. Appl Environ Microbiol 1995;61(2):610e6. [16] Gadhamshetty V, Sukumaran A, Nirmalakhandan N, Myint MT. Photofermentation of malate for biohydrogen production - a modeling approach. Int J Hydrogen Energy 2008;33:2138e46. [17] Lin CN, Wu SY, Chang JS. Fermentative hydrogen production with a draft tube fluidized bed reactor containing siliconegel-immobilized anaerobic sludge. Int J Hydrogen Energy 2006;31:2200e10. ¨ zgu¨r E, Burg J, Yu¨cel M, Gu¨ndu¨z U, Eroglu I. [18] Boran E, O Biological hydrogen production by Rhodobacter capsulatus in solar tubular photobioreactor. J Clean Prod 2010;18:29e35.
[19] Jose GJ, Ray S, Das D. Hydrogen production using Rhodobacter sphaeroides (O.U. 001) in a flat panel rocking photobioreactor. Int J Hydrogen Energy 2011;36(5):3434e41. [20] Seifert K, Waligorska M, Laniecki M. Hydrogen generation in photobiological process from dairy wastewater. Int J Hydrogen Energy 2010;35:9624e9. _ lu E, Gu¨ndu¨z U, Yu¨celb M, Tu¨rker L, Erog lu I. [21] Erog Photobiological hydrogen production by using olive mill wastewater as a sole substrate source. Int J Hydrogen Energy 2004;29:163e71. [22] Ntaikou I, Gavala HN, Lyberatos G. Application of a modified Anaerobic Digestion Model 1 version for fermentative hydrogen production from sweet sorghum extract by Ruminococcus albus. Int J Hydrogen Energy 2010; 35:3423e32. [23] Gadhamshetty V, Arudchelvam Y, Nirmalakhandan N, Johnson DC. Modeling dark fermentation for biohydrogen production: ADM1-based model vs. Gompertz model. Int J Hydrogen Energy 2010;35:479e90. [24] Mu Y, Yu HQ, Wang G. A kinetic approach to anaerobic hydrogen-producing process. Water Search 2007;41: 1152e60. [25] Obeid J, Magnin JP, Flaus JM, Adrot O, Willison JC, Zlatev R. Modelling of hydrogen production in batch cultures of the photosynthetic bacterium Rhodobacter capsulatus. Int J Hydrogen Energy 2009;34:180e5. [26] Nath K, Muthukumar M, KumarA, Das D. Kinetics of twostage fermentation process for the production of hydrogen. Int J Hydrogen Energy 2008;33:1195e203. _ Gu¨ndu¨z U, Yu¨cel M, Tu¨rker L. Kinetics of lu I, [27] Koku H, Erog biological hydrogen production by the photosynthetic bacterium Rhodobacter sphaeroides O.U. 001. Int J Hydrogen Energy 2003;28:381e8. [28] Wang YZ, Liao Q, Zhu X, Tian X, Zhang C. Characteristics of hydrogen production and substrate consumption of Rhodopseudomonas palustris CQK 01 in an immobilized-cell photobioreactor. Bioresour Technol 2010;101:4034e41. [29] Li X, Wang YH, Zhang SL, Chu J, Zhang M, Huang MZ, et al. Enhancement of phototrophic hydrogen production by Rhodobacter sphaeroides ZX-5 using a novel strategy-shaking and extra-light supplementation approach. Int J Hydrogen Energy 2009;34:9677e85. [30] Adamberg K, Kask S, Laht TM, Paalme T. The effect of temperature and pH on the growth of lactic acid bacteria: a pH-auxostat study. Int J Food Microbiol 2003;85:171e83. [31] Karlsson A, Vallin L, Ejlertsson J. Effects of temperature, hydraulic retention time and hydrogen extraction rate on hydrogen production from the fermentation of food industry residues and manure. Int J Hydrogen Energy 2008; 33:953e62. [32] Lee KS, Lin PJ, Chang JS. Temperature effects on biohydrogen production in a granular sludge bed induced by activated carbon carriers. Int J Hydrogen Energy 2006;31: 465e72. [33] Fang HHP, Zhu HG, Zhang T. Phototrophic hydrogen production from glucose by pure and co-cultures of Clostridium butyricum and Rhodobacter sphaeroides. Int J Hydrogen Energy 2006;31:2223e30. [34] Cheong DY, Hansen CL. Feasibility of hydrogen production in thermophilic mixed fermentation by natural anaerobes. Bioresour Technol 2007;98:2229e39. [35] Jo JH, Lee DS, Park DH, Park JM. Biological hydrogen production by immobilized cells of Clostridium tyrobutyricum JM1 isolated from a food waste treatment process. Bioresour Technol 2008;99:6666e72. ¨ ztu¨rk Y, Yu¨cel M, Daldal F, Mandacı S, Gu¨ndu¨z U, Tu¨rker L, [36] O et al. Hydrogen production by using Rhodobacter capsulatus mutants with genetically modified electron transfer chains. Int J Hydrogen Energy 2006;31:1545e52.
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 6 ( 2 0 1 1 ) 1 4 0 0 4 e1 4 0 1 3
14013
Nomenclature Cs: local glucose concentration, g/L Cx: biomass concentration, g/L Ks: Monod half saturation constant, g/L m: cell maintenance coefficient, h1 I: illumination intensity, lux Iopt: the optimal illumination intensity, lux pH: initial pH value of medium solution pHopt: the optimal pH value qp: specific hydrogen production rate, h1 qs: specific substrate consumption rate, h1
rp: hydrogen production rate, g/L/h rs: glucose consumption rate, g/L/h T: culture temperature, C Topt: the optimal culture temperature, C YX=S : theoretical cell yield coefficient m: specific growth rate, h1 mmax: the maximum specific growth rate, h1 mopt: the maximum specific growth rate under the optimal culture conditions, h1 a: constant b: constant