Utilization of Pistia stratiotes (aquatic weed) for
fermentative biohydrogen: Electron-equivalent
balance, stoichiometry, and cost estimation
Nonsikelelo Precios Mthethwa
a
, Mahmoud Nasr
b
, Faizal Bux
a
,
Sheena Kumari
a,*
a
Institute for Water and Wastewater Technology, Durban University of Technology, Durban 4000, South Africa
b
Sanitary Engineering Department, Faculty of Engineering, Alexandria University, Alexandria 21544, Egypt
article info
Article history:
Received 14 January 2018
Received in revised form
13 March 2018
Accepted 15 March 2018
Available online xxx
Keywords:
Bacterial community
Biohydrogen cost
Dark-fermentation
Full stoichiometry
Pistia stratiotes substrate
abstract
This study investigated the utilization of Pistia stratiotes for biohydrogen production via a
dark-fermentation process. The aquatic plant was subjected to acid-hydrolysis using H
2
SO
4
:
3.0% (v/v) for 40 min, resulting in sugar yield: 122.2 ± 5.2 mg/g. The optimum culture pH was
5.5, achieving hydrogen yield (HY): 2.46 ± 0.14 mol-H
2
/mol-glucose (3.51 ± 0.20 mg-H
2
/g-dry
weight) at fermentation time 8 h, temperature 25
C, and substrate-to-biomass (S/X) ratio
1.0 g-COD/g-VSS. The organic mass balance (92e96%) and electron-equivalent balance
(92e98%) indicated the reliability of fermentation data. The dominant species included
Planctomycetales, Verrucomicrobiales, Clostridiaceae, and Gammaproteobacteria. The phyloge-
netic analysis confirmed the abundance of hydrogen-producing bacteria such as Bacillus,
Clostridium, and Enterobacter. The hydrogenase gene expression provided the highest activity
at pH: 5.5 with a cell number 2.53 10
4
copies/ng-DNA compared to pH: 4.5 (6.95 10
3
copies/ng-DNA) and pH: 8.5 (7.77 10
3
copies/ng-DNA). The total cost of the fermentation
system including the amortization cost of investment and operating cost was 0.08 $/kg-dry
weight (22.8 $/kg-H
2
produced).
© 2018 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Introduction
Hydrogen gas (H
2
) is considered as a potential green energy
carrier that can meet the increasing global energy demand [1].
H
2
has a higher energy content per unit weight (122e142 kJ/g)
compared to other fossil fuels such as coal, petroleum, oil, and
natural gas [2].H
2
produces only H
2
O during combustion [3]
and can be utilized in fuel cells and vehicle engines [4].H
2
can be generated by various methods such as steam reform-
ing, photoelectrochemical, photocatalytic, electrolysis, and
thermolysis [5]. However, many of these techniques employ
high quantities of heavy oil and fossil materials that increase
the energy input and cause environmental deterioration [6].
Biological fermentation is an efficient and environmentally
friendly method for H
2
production as it can be conducted under
ambient operating temperature and atmospheric pressure [7].
In this process, microorganisms ferment substrate in the form
of wastewater or biomass [8]. This trend results in achieving
net positive energy (i.e., producing hydrogen), and in parallel,
solving pollution problems (i.e., wastes removal) [9]. Dark- and
photo-fermentations are the main biological routes of H
2
gen-
eration [10]. In dark-fermentation, hydrogen-producing mi-
croorganisms utilize carbohydrate-rich substrates under an
* Corresponding author.
E-mail address: sheenas@dut.ac.za (S. Kumari).
Available online at www.sciencedirect.com
ScienceDirect
journal homepage: www.elsevier.com/locate/he
international journal of hydrogen energy xxx (2018) 1 e13
https://doi.org/10.1016/j.ijhydene.2018.03.099
0360-3199/© 2018 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Please cite this article in press as: Mthethwa NP, et al., Utilization of Pistia stratiotes (aquatic weed) for fermentative biohydrogen:
Electron-equivalent balance, stoichiometry, and cost estimation, International Journal of Hydrogen Energy (2018), https://doi.org/
10.1016/j.ijhydene.2018.03.099