Research Article
Hydrogen Production by Green Alga GAF99 in Sea Water Bioreactor: II Modeling the Effect of Temperature
Royal Scientific Society, Amman, Jordan
Mohammed A. Ibrahim
Princess Sumaya University for Technology, Amman, Jordan
Algae are the fastest growing plants on the planet and have great potential to produce various forms of energy (Demirbas and Demirbas, 2010). They have been shown as an important source of renewable energy; these include biohydrogen (Chader et al., 2009; Beer et al., 2009), biogas (Vergara-Fernandez et al., 2008), bioethanol (Goh and Lee, 2010; Eshaq et al., 2010), biodiesel (Miao and Wu, 2004; Wiley et al., 2011; Pfromm et al., 2011) and bioelectricity (Rosenbaum et al., 2010). Moreover these photosynthetic organisms have been known for their other important economical products which are used in cosmetics, bioactive functional compounds, pigments, food and feed (Plaza et al., 2010; Spolaore et al., 2006; Del-Campo et al., 2007; Ming et al., 2012).
Biohydrogen production by algae is one of the promising potential biofuel in the future and is a hopeful approach for solving environmental problems associated with fossil fuels. It is viewed as an attractive energy source for transportation by virtue of the fact that it is renewable, considered friendly to environment since it does not produce the CO2 in combustion and as a consequence reduces air pollution (Balat, 2008; Nazlina et al., 2009; Vijayaraghavan et al., 2010). One important aspect of hydrogen production is the economic cost for production and the required process for utilization of hydrogen which are extremely high per unit of energy when compared to fossil fuel energy sources such as natural gas or gasoline. These issues require consideration in long term plans for high excellence research and development of hydrogen production. The reported results of research work and studies in this field have shown that the production of biohydrogen and other biofuels by biological systems and in particular by algae is an area of promising applications providing optimizing the cultural conditions for higher yield of biohydrogen (Benemann, 1997; Lindblad, 1999; Levin et al., 2004; Kotay and Das, 2008). Early studies on the ability of bacteria (Stephenson and Stickland, 1931) and algae (Gaffron and Rubin, 1942) to produce biohydrogen had been mostly a biological curiosity. Subsequently research activities in this subject have shown that biohydrogen can be produced by various genera of blue-green (Ghirardi et al., 2000; Dutta et al., 2005) and green algae (Guana et al., 2004; Markov et al., 2006). In this context, green algae or Chlorophyta are the largest phylum of algae can live in diverse habitats, including fresh and sea water (Sandgreen, 1991; Anderson, 2005; Norris, 2010). Recently, the authors were able to isolate and characterize marine green alga Chaetomorpha sp. GAF99 from Gulf of Aqaba which produces biohydrogen under anaerobic and dark conditions in seawater bioreactor (Ibrahim and Saeed, 2012). The study indicated the importance of light/dark periods, anaerobic condition and temperature of incubations on hydrogen productivity of Chaetomorpha sp. GAF99. The role of temperature was found critical in the process of biohydrogen production. Production was shown to drop drastically during production phase at higher incubation temperature 33°C. Thus for an efficient process of hydrogen production in seawater bioreactor requires generating mathematical kinetic model of the process. Such studies will provide useful information on optimization of critical factors for the process effectiveness of biohydrogen production (Palazzi et al., 2002; Akande et al., 2006). The main argument of present investigation is to provide the required information on the effect of temperature which will assist the simulation and design of future seawater bioreactor for bio-hydrogen production by marine green alga Chaetomorpha sp. GAF99.
Marine alga Chaetomorpha sp. GAF99: Marine green alga Chaetomorpha sp. GAF99 which was isolated originally in February 2008 from various shallow water niches (1-2 m deep) of Gulf of Aqaba shore (Ibrahim and Saeed, 2012) was used in present investigation. The cultures were maintained at room temperature in two liters flasks containing one liter sea water, the pH value of sea water is 8.3.
Biohydrogen production: Biohydrogen production by the marine green alga Chaetomorpha sp. GAF99 was followed according to reported method (Ibrahim and Saeed, 2012). Bio-hydrogen production was measured twice daily at 8.30 am (Tm) and at 17.30 pm (Te) to determine the influence of temperature on productivity at various temperatures. The volume of the generated gas from algal culture incubated in 2 L flask was collected in the graduated glass cylinders and was measured directly by water displacement. The measurements of the composition of produced gases were performed qualitatively by detection instrument CD100/Kane-May/Kane International Ltd. (EN61000-6-3) and quantitatively by GC-2014/Shimadzu.
Model generation: Regression analysis was used to develop a model for the collected data of biohydrogen produced during active production phase at various temperatures. The values of biohydrogen produced in active production phase were subjected to various mathematical models by using MATLAB software package to find the optimum model that defines the behavior of algae as a function to two temperatures. The statistical analysis based on ANOVA test (Montgomery and Runger, 2007) was used to show the significance of the model based on F and T tests and p-values with 95% confidence limit.
Effect of temperature on biohydrogen production: Biohydrogen production by Chaetomorpha sp. GAF99 in sea water bioreactor under anaerobic and dark conditions starts after a period of 10±2 days of adaptation period. The production phase, which continues for another 10±2 days, is influenced greatly by temperature of incubation. The higher incubation temperature (33°C) was found not favorable for hydrogen production. In order to find the optimum correlation that represents the production of biohydrogen (P) as function of temperatures, a statistical analysis was conducted to find an optimum model to show the role of temperatures on biohydrogen production by Chaetomorpha sp. GAF99.
Model generation: The model was generated as a function of two temperatures measured at 8:30 AM (Tm) and at 5:30 PM (Te). The regression model statistics was utilized based on ANOVA test. The numerical estimates of the regression model coefficients were based on 95% confidence limit (significance level (α) = 1-0.95 = 0.05). The significance normally assessed using the t-test statistics (significance F or p-values) which must be <α = 0.05 to be significant. The results which are shown in Table 1-3 indicated that the coefficient of determination (R2 = 78.47%) is acceptable.
Table 1: | Regression model statistics |
Table 2: | Adopted model of hydrogen production as a function of two temperatures (Te and Tm) |
Table 3: | Numerical estimates of the regression model coefficients based on p-value with 95% confidence limit |
Fig. 1: | Three dimensional plot showing the surface response of biohydrogen production as a function of two temperatures Te and Tm as in Eq. 1 |
The obtained results also show the significance F value = 0.000193 which is <0.05 and the p value = 0.000126 which is also <0.05. Therefore the overall model given in Eq. 1 is significant.
Equation 1 represents an exponential form for the relation of biohydrogen production (P) as a function of two temperatures (Te and Tm).
(1) |
Furthermore, three dimensional mapping of the surface response (Fig. 1) of Eq. 1 shows clearly that the highest biohydrogen production was recorded at the morning [8:30 a.m.] (Tm) and the least production was recorded at the evening temperature (Te) which was measured at 5:30 p.m. In other word the main production of biohydrogen production is occurring during dark period at Tm. The statistical analysis showed that the optimum temperature for biohydrogen production is 23.2°C.
Ever since the discovery that the green alga Scenedesmus was able to produce hydrogen during anaerobic conditions (Gaffron and Rubin, 1942), efforts have been made to investigate the influence of various environmental factors on hydrogen production. Temperature is one of important environmental factors which have critical effect on biohydrogen production by various tested organisms. It had shown that biohydrogen production by Anabaena was highest at 27°C (Prabina and Kumar, 2010). Whereas other study investigated production by Chlamydomonas sp. MGA161, a marine green alga, when cultivated at a high CO2 concentration (15% CO2) and low temperature (15°C), under these conditions the total productivity was significantly improved (Miura et al., 1993). On the other hand, hydrogen production was studied using immobilized green alga Chlorella sp., the rate of hydrogen evolution was increased as temperature increased from 37-40°C (Song et al., 2011). In this context, our previous study (Ibrahim and Saeed, 2012) showed that production of hydrogen is almost ceased at 33°C. As part of our efforts to understand the role of environmental factors in biohydrogen production by Chaetomorpha sp. GAF99 culture, the collected data of biohydrogen produced at various temperatures during production phase were subjected to regression analysis to generate the model shown in Eq. 1 (see the result section) which represents the biohydrogen productivity of Chaetomorpha sp. GAF 99 population for various periods during 2010 and 2011. It was possible to demonstrate in the present investigation the correlation between hydrogen productivity of Chaetomorpha sp. GAF 99 in sea water bioreactor and incubation temperature. The given relationship in Eq. 1 showed a significant behavior of biohydrogen production in relation with the given data of Te and Tm. This was demonstrated by the R square value which is equivalent to 78.4767% (Table 1) and the p value which is equal to 0.000126 and was less than alpha value (α) of 0.05.
It is worth to mention that our study gave further support to utilize sea water for biohydrogen production. In addition and in contrast to the widely used freshwater green algae, the goal of our research was to ascertain utilization of marine green algae-based bioreactor system for biohydrogen production by developing a mathematical model to correlate biohydrogen production with incubation temperature. It is presumed that generating such system will demonstrate the cost feasibility effectiveness for sustained H2 photo-production. Thus by utilizing seawater as the substrate and solar light as the source of energy and right temperature beside other environmental conditions it might be possible to develop economically feasible system for biohydrogen production. The specific objectives of this study could be met by developing such mathematical models. These include higher production at lower cost in sustainable manner.
The results of this study and our other two studies (Ibrahim and Saeed, 2012; Saeed and Ibrahim, 2012) and those reported by other investigators (Palazzi et al., 2002; Prabina and Kumar, 2010; Song et al., 2011) show the importance of more standardized careful study which requires modeling and statistical analysis to define the effect of temperature as a crucial environmental factor in addition to other environmental factor (i.e., pH) in the production of biohydrogen by marine green alga Chaetomorpha sp. GAF99. Furthermore and taking in the account the influence of temperature on the activity of hydrogenases, key enzymes for biohydrogen production by green algae (Saeed and Ibrahim, 2012), it is possible to suggest designing improved hydrogenases for efficient production of biohydrogen at higher temperature by utilizing new molecular technologies (Ibrahim, 2012).
Marine green algae have been shown as promising candidates for future biohydrogen production. In this study Chaetomorpha sp. GAF99 was found dominant species in specimens obtained from habitats of Gulf of Aqaba. The green alga produced biohydrogen in bioreactor contained natural sea water and showed an interesting response to temperature. The study revealed the critical role of temperature on the amounts of biohydrogen produced during active production phase. The model and surface response curve obtained in this study showed clearly that the optimum biohydrogen production was recorded at the dark period at 23.2°C.
The authors would like to thank the Ministry of Higher Education of Jordan/Scientific Research Support Fund (SRSF) and the Royal Scientific Society (RSS) for funding this work. The support of the Princess Sumaya University for Technology (PSUT) is highly appreciated. Special acknowledgment to Eng. Amani N. Abdulhadi and Eng. Sarah R. Al Weissi for their technical assistance.