Mangrove sediments host rich assemblages of microorganisms, predominantly mixed bacterial cultures, which can be efficiently used for biohydrogen production through anaerobic dark fermentation. The influence of process parameters such as effect of initial glucose concentration, initial medium pH, and trace metal (Fe2+) concentration was investigated in this study. A maximum hydrogen yield of 2.34, 2.3, and 2.6 mol H2 mol−1 glucose, respectively, was obtained under the following set of optimal conditions: initial substrate concentration—10,000 mg L−1, initial pH—6.0, and ferrous sulphate concentration—100 mg L−1, respectively. The addition of trace metal to the medium (100 mg L−1 FeSO4·7H2O) enhanced the biohydrogen yield from 2.3 mol H2 mol−1 glucose to 2.6 mol H2 mol−1 glucose. Furthermore, the experimental data was subjected to kinetic analysis and the kinetic constants were estimated with the help of well-known kinetic models available in the literature, namely, Monod model, logistic model and Luedeking-Piret model. The model fitting was found to be in good agreement with the experimental observations, for all the models, with regression coefficient values >0.92.
Fossil Fuels are the primary energy source for the world’s increasing energy consumption. According to a recent survey, total world energy use rises from 524 quadrillion British thermal units (Btu) in 2010 to 630 quadrillion Btu in 2020 and to 820 quadrillion Btu in 2040 [
Biohydrogen production through anaerobic fermentation is a sustainable alternate for the energy crisis and green environment [
A kinetic model can adequately describe the relationship among the different state variables and explain the behavior of fermentation quantitatively by providing useful information that can be subsequently used for analysis, design, and operation of any fermentation process [
The sediments were collected from the mangrove rhizosphere of Pichavaram, Tamil Nadu, India, at a depth of 100 cm, and later stored in sterile polythene bags. Heat-shock treatment was done on this sediment sample, by constant heating at 110°C for 2 h, in order to stimulate spore germination and eliminate all vegetative cells, particularly methanogens. The coarse particles were removed using a stainless steel mesh, while the finer fractions were stored at 4°C [
The nutrient medium (non-sterilized) used in this study had the following chemical composition (per litre): NH4Cl—0.5 mg, K2HPO4—0.25 mg, MgCl2·6H2O—0.3 mg, NiSO4—0.016 mg, CoCl2—0.025 mg, ZnCl2—0.0115 mg, CuCl2—0.0105 mg, CaCl2—0.005 mg, and MnCl2—0.015 mg.
Batch tests were conducted in duplicate, in 1 L Erlenmeyer flasks (working volume: 0.7 L), fitted air-tightly with rubber septum, and adequately sealed using commercially available fix gels. The effect of process parameters on biohydrogen yield, namely, the influence of initial substrate concentration (glucose), initial pH, and trace metal, Fe2+ concentration, was evaluated by carrying out experiments at different low to high levels of these parameters, and the average values of biohydrogen yield were presented. The pH of the growth medium was adjusted using 1N HCl or 1N NaOH during the start of the experiments. The growth medium was inoculated with 100 g of pretreated sediment under aseptic conditions, and the flasks were incubated at 35°C for fermentation.
The biohydrogen gas was measured using wet gas flow meter (Toshniwal, India). The gas content was analyzed using a gas chromatograph (Shimadzu, 221-70026-34, Japan) equipped with a thermal conductivity detector (TCD), and the column was packed with dual packed column. The operating temperatures of the column, detector and injector, were 40°C, 80°C, and 50°C, respectively. Biomass concentration was measured as volatile suspended solid (VSS) and analyzed according to Standard Methods [
Biohydrogen fermentation reached nearly constant values at the end of 120 h for each batch tests, including their duplicates. Glucose degradation efficiencies, cumulative biohydrogen gas, and hydrogen yields were calculated for each set of experimental condition.
For initial glucose concentrations of 4,000, 7,000, 10,000, 13,000, and 16,000 mg L−1, the values of cumulative biohydrogen production and glucose degrading efficiencies were 430, 1190, 2600, 2200, and 2099 mL and 75, 83, 90, 80, and 72%, respectively (Figure
(a) Profile of cumulative biohydrogen production at various initial glucose concentrations. (b) Dynamic profile of glucose degradation, biomass concentration, and cumulative biohydrogen production. (c) Biohydrogen yield and glucose degradation efficiency for various initial glucose concentrations.
The profile of cumulative biohydrogen gas production at various initial medium pH conditions is shown in Figure
(a) Profile of cumulative biohydrogen production at various medium pH. (b) Biohydrogen yield and glucose degradation efficiency for various medium pH.
Figure
(a) Profile of cumulative biohydrogen production at different Fe2+ concentrations. (b) Biohydrogen yield and glucose degradation efficiency for various Fe2+ concentrations.
Monod kinetics was applied to study the cell growth kinetics during biohydrogen production. Monod kinetics is given by the following equation:
Monod model for substrate utilization kinetics.
Table
Comparison of kinetic parameters for Monod model.
Process | Type of culture | Substrate | µmax |
|
|
Author |
---|---|---|---|---|---|---|
Batch | Mixed anaerobic culture | Sucrose | 0.078 h−1 | — | — | [ |
Batch |
|
Sucrose | 0.31 h−1 | 4.39 g COD L−1 | 0.935 | [ |
Batch | Mixed sludge | Glucose | 0.03 g biomass/g biomass/day | — | — | [ |
Batch | Mixed culture | Xylose | 0.17 h−1 | 0.75 g/L | — | [ |
Sequential batch | Activated sludge | Glucose | 0.125 h−1 | — | — | [ |
Batch | Acidogenic mixed culture | Glucose | 0.163 h−1 | — | — | [ |
Batch | Acidogenic mixed culture | Fructose | 0.108 h−1 | — | — | [ |
Batch | Anaerobic acclimatized banana stem sludge | Banana stem waste | 0.111 h−1 | 0.330 g/L | 0.902 | [ |
Batch | Sediments of Pichavaram mangroves | Glucose | 0.166 h−1 | 0.112 g/L | 0.971 | Present study |
The specific growth rate for the logistic curve relates the change of specific growth rate with respect to change in cell concentration (
On integrating and applying the limits,
However, for the purposes of batch hydrogen production experiments, where the initial substrate concentrations and the inoculation volume are kept constant, the logistic model is only a fair approximation of the growth curve. From Figure
Comparison of kinetic parameters of logistic model.
Process | Type of culture | Substrate |
|
|
Author |
---|---|---|---|---|---|
Batch |
|
Malic acid | 0.098 | 0.98 | [ |
Batch | Sludge | Glucose | — | 0.99 | [ |
Batch | Sediments of Pichavaram mangroves | Glucose | 0.034 | 0.943 | Present study |
Logistic model for cell growth kinetics.
The Luedeking-Piret model shown in (
Table
Comparison of kinetic parameters of Luedeking-Piret model.
Process | Type of culture | Substrate |
|
|
Author |
---|---|---|---|---|---|
Batch |
|
Xylose | 0.041 | 0.910 | [ |
Batch | Mixed microflora | Wheat stalk | — | >0.855 | [ |
Batch | Sediments of |
Glucose | 11.04 | 0.999 | Present |
Luedeking-Piret model for product formation kinetics.
Scanning electron micrographs showed that the granules had multiple cracks with cavities on the surface (Figure
SEM image of typical hydrogen-producing granule.
Furthermore, considering the practicality of this research work, microbiological analyses are warranted at this stage to characterize the dominant anaerobic consortium responsible for biohydrogen production. In general, kinetic models are applied in order to study and assess the metabolic features of defined cultures. Further studies in this field should be aimed at the following aspects: optimization studies with different innocula, substrates and process parameters, evaluation of the performance, and economics of a continuous biohydrogen production processes (bioreactors).
The results from batch tests showed that initial substrate (glucose) concentration, medium pH, and Fe2+ concentration had influence on the biohydrogen yield. Maximum biohydrogen yields were found to be 2.34, 2.3, and 2.5 mol H2 mol−1 glucose at the following conditions: initial substrate concentration—10,000 mg L−1, medium pH—6.0, and Fe2+ concentration—100 mg L−1, respectively. The addition of trace metal to the medium at a concentration of 100 mg L−1 was found to enhance biohydrogen production although higher metal ion concentrations reduced biohydrogen production. The kinetics of batch anaerobic hydrogen production was estimated by fitting the experimental data to the well-known unstructured kinetic models. The Monod model, logistic model, and Luedeking-Piret model were used to describe the kinetics of cell growth rate as a function of substrate, cell concentration, and product formation, respectively, in the hydrogen production process, and the corresponding kinetic constants were estimated. The results showed that high regression co-efficient values (
Specific growth rate (h−1)
Maximum specific growth rate (h−1)
Microbial concentration (g VSS L−1)
Initial microbial concentrations (g VSS L−1)
Substrate consumption rate (g L−1)
Apparent specific growth rate (h−1)
Maximum microbial concentration (g VSS L−1)
Cumulative biohydrogen production (mL)
Growth associate product yield coefficient
Non-growth associated product yield coefficient.
The authors declare that there is no conflict of interests.
The authors of this research article contributed to a similar extent overall and agreed to submit the paper.
The authors thank the Ministry of Earth Sciences, Government of India, New Delhi, for funding this research project (No: MOES/MRDF-11/1/25/P/09-PC-III) and Pollution Control Research Laboratory, Department of Chemical Engineering, Annamalai University, India, for laboratory and analytical facilities.