Investigating the effects of iron and nickel nanoparticles on dark hydrogen fermentation from starch using central composite design

Highlights

• Dark hydrogen fermentation from starch was performed using mesophilic sludge.

• Effects of iron and nickel nanoparticles on biohydrogen production were studied.

• Response surface method was used to design the experiments and analyse the results.

• Yield of 149.6 mL H₂/g-VS was obtained by adding 37.5 mg/L of each nanoparticle.

Abstract

The effects of Fe⁰ and Ni⁰ nanoparticles on mesophilic dark hydrogen fermentation from starch were investigated using heat-shock pretreated anaerobic sludge in batch reactors. Starch concentration in the range of 0–20 g/L was used in the presence of 0–50 mg/L of each Fe⁰ and Ni⁰ nanoparticles while the initial pH of all experiments was adjusted to 7. The hydrogen production was optimized using response surface methodology (RSM) in a central composite design (CCD) mode. The results showed the significance of the linear effect of starch and Fe⁰ concentrations, the interactive effect between Fe⁰ and Ni⁰ concentrations, and the squared effect of starch concentration on the biohydrogen production. The maximum yield of hydrogen production 149.8 mL/g-VS was obtained at starch, Fe⁰ and Ni⁰ concentrations of 5.37 g/L, 37.5 mg/L, and 37.5 mg/L, respectively. This was nearly 200% higher than those in the corresponding control test. Moreover, the analysis of metabolites indicated the hydrogen production through acetic acid pathway.

Introduction

Global warming, greenhouse gas emission, and environmental pollution caused by fossil fuel consumption, along with the depletion of fossil fuel sources and the rise of their prices, have increasingly directed the attentions towards energy production from clean and renewable energy sources [1], [2]. Hydrogen, with the highest energy density among other known fuels (142 MJ/kg), has become a promising energy carrier for the future as it is carbon-free and its combustion produces only energy and water. It could also be efficiently produced via biological methods under ambient conditions [3], [4].

Dark hydrogen fermentation is considered more practical in comparison with the other biological hydrogen production methods [5], [6]. It is technically simpler and more flexible. It can deal with a wide variety of biomass wastes including wastewaters and solid wastes. Solid wastes such as agricultural and domestic wastes have a high starch content which could be efficiently hydrolyzed into glucose and maltose by enzymes involved in the dark fermentation process [6], [7], [8], [9].

Hydrogenases play a key role in the dark fermentation process by catalyzing the simplest chemical reaction, 2H⁺ + 2e⁻ ↔ H₂. The released electrons from substrate oxidation, together with protons (electron acceptor), are converted to hydrogen in the presence of hydrogenases under anoxic conditions [10]. Two types of hydrogenases are known to be evolved in the dark fermentation process: [Ni–Fe]-hydrogenases and [Fe–Fe]-hydrogenases (Fig. 1). [Ni–Fe]-hydrogenases are usually expressed by a wide variety of bacteria while a few bacteria have the potential to express [Fe–Fe]-hydrogenases [11]. Moreover, substrate affinity of [Ni–Fe]-hydrogenases is higher than that of [Fe–Fe]-hydrogenases. The chemical structures related to the active sites for [Ni–Fe]-hydrogenase and [Fe–Fe]-hydrogenase are shown in Fig. 1[12].

The catalytic activity of hydrogenase usually depends on metal cofactors (Fe and Ni) in the active sites of their polypeptide structure. Therefore, iron and nickel are expected to influence the activity of hydrogenases and therefore can enhance the efficiency of the dark fermentation process. However, at higher concentrations, iron and nickel may inhibit the process [11]. Alshiyab et al. (2008) investigated the impact of Fe²⁺ ions concentration (0–1000 mg/L) on the batch hydrogen fermentation from glucose using Clostridium acetobutylicum, and reported that the hydrogen yield was enhanced from 391 to 408 mL/g-VS at Fe²⁺ ions concentration of 25 mg/L, while the higher concentrations of Fe²⁺ ions negatively affected the yield [13]. Wang and Wan (2008) investigated the impact of Ni²⁺ ions on the batch hydrogen fermentation using mixed culture and reported that Ni²⁺ ions concentrations below 0.1 mg/L could increase the hydrogen yield. Addition of 0.1 mg/L Ni²⁺ ions to the system led to the maximum hydrogen yield of 296.1 mL/g-VS, while further increase in Ni²⁺ ions concentration reduced the hydrogen yield [11].

When Fe⁰ nanoparticles (NPs) are exposed to oxygen in an aqueous phase, they are oxidized to Fe²⁺ by the following reaction [14]:

2 Fe⁰ + O₂ + 2H₂O → 2 Fe²⁺ + 4 OH⁻, ΔE⁰ = 1.67 V

This reaction (in the presence of nano-size iron powder) is completed in less than 1 min. After the consumption of the available oxygen, Fe⁰ NPs is oxidized by the water, which has a lower oxidation potential than oxygen, and then hydrogen is produced as the sole product of the reaction:

Fe⁰ + 2H₂O → Fe²⁺ + 2 OH⁻ + H₂

Since the redox potential of the Fe⁰∖Fe²⁺ couple is low (−0.44 V), the above equation occurs readily at the ambient temperature and pressure. It must be noted that the reduction of iron particle sizes from micro to nano-scale can increase the rate of the above reactions by 10,000 times [14], [15]. Therefore, the addition of iron NPs to the anaerobic fermentation medium could decrease the availability of undesired oxygen in the system like the oxygen consumption by the facultative anaerobic bacteria, thereby leading to a better performance of the oxygen-sensitive hydrogenases. Moreover, after the depletion of oxygen, it produces hydrogen gas and Fe²⁺ ions required for hydrogenases. Mullai et al. (2013) investigated the impact of glucose concentration (10–30 g/L), the initial pH (5–6) and Ni⁰ NPs concentration (0–30 mg/L) on the dark hydrogen fermentation. They obtained the maximum yield of 316.1 mL/g-VS at glucose concentration of 14.01 g/L, initial pH 5.61, and Ni⁰ NPs concentration of 5.67 mg/L [16]. Mohanraj et al. [17] compared the impacts of iron oxide NPs and iron sulfate on the efficiency of dark hydrogen fermentation from glucose using Enterobacter cloacae. They observed that the supplementation of iron oxide NPs, at the concentration of 125 mg/L, led to the maximum hydrogen yield of 257.6 mL/g-VS, while supplementation of iron sulfate at their optimum concentration (25 mg/L) led to the hydrogen yield of 211.6 mL/g-VS. To the best of our knowledge, no study has ever investigated the effect of nickel and iron NPs on the efficiency of the dark fermentation of starch for biohydrogen production.

The aim of this work was to investigate the impacts of starch, Fe⁰ NPs, and Ni⁰ NPs concentrations on the yield of the dark hydrogen fermentation using the mesophilic mixed culture. The response surface methodology (RSM) in a central composite design (CCD) mode was applied for the optimization.