1. Introduction
4.3 Results and Discussion
5.4.3 Batch assays with syngas/CO
Additional batch, bottles, assays, were performed, which allowed easy estimation of gas consumption data in such closed, constant volume systems and comparison with the bioreactor experiments. Syngas was used as substrate mixture and carbon and energy source in this case, and the results are compared with CO bioconversion data. In the absence of tungsten, and with syngas as the main substrate mixture, (Figure 5.3a), the acetic acid concentrations gradually increased from 0.1 g/L to 0.34 g/L on the 4th day. On 5th day, there was a sudden rise to 0.5 g/L acetic acid and it increased to 0.66 g/L on the 7th day. The concentrations remained then constant till the 10th day of experiment. On the other hand, the butyric acid concentration increased from 0.2 g/L to 0.3 g/L in 4 days and remained then stable at about 0.33 g/L till the 10th day. Though the ethanol concentration increased from 0.1 to 0.33 g/L, from the start till 3rd day of the experiment, it was readily consumed on the 4th day and no ethanol was found to be produced till the end of the experiment. The OD (600 nm) increased from 0.01 to 0.17 in five days, but remained unchanged later. On observing the gas consumption profile, the %
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removal of CO was found to increase from 0 to 41.9 after 8 days of operation and remained unchanged afterwards (Figure 5.3a.1). While there was a gradual increase in consumption of hydrogen reaching 28.6 % on the 10th day. The % removal of carbon dioxide fluctuated reaching 54% on 4th day of operation and 42.6 % on 10th day of operation; this decrease in removal was due to the fact that carbon dioxide can be produced through the consumption of carbon monoxide, meaning that at some stage more carbon dioxide was produced than originally present in the vials.
In absence of selenium and syngas as substrate (Figure 5.3b) the maximum concentrations of acids were similar (very slightly higher) as in the previous case, resulting in 0.8 g/L acetic acid and 0.46 g/L butyric acid, although the rate of acetogenesis was lower (in comparison to without W). A somewhat higher biomass concentration was reached, as indicated by an OD of 0.35 recorded on the 7th day of operation, when it then remained constant. In the course of 10 days of experiment, the acetic acid production rate increased from 0 to 0.13 g/L/day, till 4th day and decreased to 0.1 on 5th day of operation with a slight increase to 0.105 g/L /day on 7th day of operation. There was an abrupt fall in the acetic acid production to 0.01 g/L/day on 8th day, followed by slight consumption of about 0.01 g/L/day on the 9th day and finally the concentration of acetic acid became constant on 10th day of operation. On the other hand, butyric acid production rate increased from 0 to 0.1 on 1st day of operation and, followed by a small rise to 0.03 g/L/day on 2nd day of operation, remained constant at on 3rd and 4th day, and finally increased to 0.16 g/L/day on 5th day of operation. For the remaining course of the experiment, there were no more production of butyric acid. About 0.5 g/L of ethanol had accumulated on 5th day and its concentration then remained stable till the end of the experiment.
The gas removal rates were higher (in comparison to without W) reaching maximum values of 64%, 79%, and 67.2% for carbon monoxide, carbon dioxide, and hydrogen, respectively, on the 10th day of operation (Figure 5.3b.1).
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In the absence of both the trace elements, selenium and tungsten, the acids produced (Figure 5.3c), the gases consumed (Figure 5.3c.1) and the biomass growth were significantly lower compared to the previous set of results. No alcohols were produced at all (Figures 5.3c). They reached maximum concentrations of about 0.45 g/L acetic acid and 0.34 g/L butyric acid, when the biomass concentration ODreached a stable value. The rate of production of those acids were also lower than in the previous two experiments.
Pure CO is considered to be a better carbon and energy source for acetogens than syngas mixtures (CO, CO2, and H2). An additional experiment was performed in order to compare syngas and only CO as the substrates, and to confirm the inhibition of solventogenesis is in the absence of both trace elements with CO. Similarly as for syngas, only acids (acetic acid) and biomass growth was limited, with no production of alcohols (Figure 5.3d), confirming the effect of trace metals.
The lowest concentration of acids like acetic acid was 0.22 g/L and the initial amount of butyric acid which was about 0.2 g/L was totally consumed on the 1st day of operation.
(Figure 5.4d). The biomass growth was very limited and Butyric acid previously present in the medium was totally consumed. The OD reached 0.2 from a previously recorded data of 0.1 on the 1st day of operation and CO removal was only 12.5 % (Figure 5.4d.1).
The results obtained from the batch data indicates the cumulative role of selenium and tungsten on the production of acids and alcohols from gas fermentation. It is clearly observed that the metabolites production and biomass profile as well as the gas consumption are the lowest in the absence of both W and Se. The importance of tungsten+selenium> tungsten> selenium in the process of syngas fermentation in mixed culture to produce alcohols syngas can thus be inferred. When CO is the sole substrate, the production of acids, alcohols and biomass was observed to be the lowest as expected, due to the absence of H2, ( in comparison to syngas) which acts as the electron donor and facilitates the fermentation (Philips et al., 2017),
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Figure 5.3: Batch experiments in absence of tungsten and syngas as substrate. a) Metabolites and biomass, a.1) Gas removal profile
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Figure 5.3: Batch experiments in absence of selenium and syngas as substrate. b) Metabolites and biomass, b.1) Gas removal profile
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Figure 5.3: Batch experiments in absence of both selenium and tungsten and syngas as substrate. c) Metabolites and biomass, c.1) Gas removal profile
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Figure 5.3: Batch experiments in absence of both selenium and tungsten and CO as substrate.
(d) Metabolites and biomass, (d.1) Gas removal profile
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5.4.4 Predominance of tungsten over selenium as co-factor of solventogenic