ACTA DE POSESIÓN DEL ESTRECHO DE MAGALLANES 23

6.4.3.1. Effect of light intensity and wavelength

The light conversion efficiency (η) varies for different PNSB strains because of their different light harvesting antenna pigments, thus they have a different photosynthetically active radiation (PAR) range. However, η also depends on the light intensity, illuminated area of the PBR, reactor design and other operational conditions of the PF process. Generally, the intensity of light has a positive influence on the H2 production. There are some studies dedicated to assess the effect of

the light intensity on growth and H2 production by PNSB [36, 81–84].

Uyar et al. [81] studied the effect of intensity of light, light wavelength and illumination protocol on the growth and H2 production by Rhodobacter sphaeroides O.U. 001 in photobioreactors

124 (Figure 4). The hydrogen production increased with increasing the light intensity and the highest

production was reached at 270 W m-2_{. The results also showed the decrease in photo-production}

of hydrogen by 39% when there is a lack of infrared light (750-950 nm wavelength). The substrate conversion efficiency was increased and hydrogen production was stimulated when the light was illuminated after inoculation and no hydrogen was produced during the dark periods.

Figure 4. Effect of light intensity on biohydrogen production by Rhodobacter

sphaeroides O.U. 001 (based on the data obtained from [81]).

Sevinç et al. [82] studied the effect of temperature (20, 30 and 38 °C) and light intensity (1500, 2000, 3000, 4000 and 5000 lux) on the kinetic parameters and hydrogen production in PF of acetic and lactic acid using Rhodobacter capsulatus. The results of the study reported the maximum hydrogen production at 5000 lux for 20 °C and 3000 lux for 30 and 38 °C. In a more recent study, Androga et al. [84] established an optimal light intensity and temperature of 287 W

m-2 _{(4247.6 Lux) and 27.5 °C, respectively, in PF tests carried out using R. capsulatus DSM}

1710 in a medium containing acetate, lactate and glutamate. In accordance, Akman et al. [83]

reported an optimum light intensity of 263.6 W m-2 _{(3955 lux) in a PF study carried out with}

acetate as the carbon source and R. capsulatus.

Future development of PF systems requires an economical solution to provide the sources of light, so that outdoor systems utilizing natural sunlight become a practical option. Therefore, research interests have been growing to exploit the natural sunlight in PF processes [14, 85–87]. Even though sunlight cannot ensure continuous light conditions, there are some studies that have shown that the dark and light cycles might not have significant effects on photo-H2 production

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and dark conditions [89]. Montiel-Corona et al. [14] reported a 40.25% reduction in H2 yields

during PF using mixed PNSB in comparison to indoor conditions. However, H2 yields obtained

from outdoor reactors can be comparable to those under indoor conditions [87]. In addition to the type of light source, photofermentative H2 production also depends on other operating

conditions of the PBRs, such as mixing conditions that affects the distribution of light, culture temperature and pH. Furthermore, harnessing the natural light in upscale applications of PF might reduce the cost of long-term PBRs operation.

6.4.3.2. Culture temperature and pH

The operating temperature of a culture is one of the important parameters that affects the bacterial metabolism or metabolic pathways as well as substrate conversion efficiency and thus H2

production. Basak and Das [54] reported 31 to 36 °C as optimum temperature for Rhodobacter sp., while Androga et al. [84] reported 26.8 °C as optimum culture temperature for a higher H2

yield. Moreover, culture pH affects the biochemical reactions as it determines the ionic form of the active sites for enzymatic activity [16]. PF studies have been carried out in the pH range varying between 5.5 to 7.5 (Table 2 and 4). Akroum-Amrouche et al. [57] reported an optimum

pH of 7.5 (± 0.1) for the H2 production by Rhodobacter sphaeroides, while Nath and Das [90]

reported an optimum H2 production at pH 6.5 for the same PNSB species. This difference of

change in optimum pH can be attributed to the difference in substrate type used in PF experiments as lactate was used as a sole carbon source in the former, while DF spent medium was used in the latter study. In another study, Koku et al. [36] reported an optimum pH of 7.1- 7.3 for the activity of the nitrogenase enzyme, while the range of 6.5 to 7.5 is optimum for the activity of the hydrogenase enzyme.

During most of the PF tests, pH has shown an increasing trend which could be due to PHB production [11, 90]. Eroglu et al. [71] reported a slight decrease in pH during the bacterial growth

phase and pH increase during H2 production. The effluents from DF are generally in the acidic

pH range [4], and are required to be adjusted to a pH range 6.5-7.5 to ensure the optimum operating conditions in the PF process. However, the range of optimum pH seems to be

dependent on the PNSB species. Some other studies [66, 63] have shown the feasibility of H2

production by mixed PNSB at pH 5.5-6.0, which is generally an ideal pH range of DFE obtained from DF processes.

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6.4.3.3. Effect of mixing

Mixing is required in PBRs to keep the PNSB biomass suspended and uniformly distribute the substrates and nutrients in the culture medium. Moreover, since, the light source is non mobile, mixing would only ensures the uniform distribution of light for the suspended microorganisms throughout the PBRs, thus, avoiding light gradients. It also helps to maintain sufficient mass transfer, which generally includes the exchange of gases, i.e. H2 and CO2. Akroum-Amrouche et

al. [57] found unstable H2 production with a 13.0% and 60.8% reduction of the average and

maximum H2 production rate when mixing was stopped during the exponential phase of PF. In

another study, Li et al. [88] reported that mixing during the H2 production phase of the PNSB

stationary growth phase as vital for higher H2 yields than during the exponential cell growth

phase. Moreover, the type of mixing system may also affect the photo-H2 production

performance. Zhang et al. [67] showed that baffled PBRs can outperform magnetic-stirred PBRs

as supported by higher H2 yields as well as faster cell growth and substrate conversion. This

higher H2 production can be attributed to enhanced gas transfer and distribution of light in the

PBRs due to well mixing conditions.

6.4.3.4. Inhibition of photo-H2 production

Nitrogenase plays an important role in the hydrogen generation. Thus, the presence of chemical substances that disrupt the nitrogenase activity decreases the photo-H2 production. Koku et al.

[36] reported that the presence of N2 and NH4+ inhibit the H2 production. Also CO, EDTA and

O2 are likely to inhibit the nitrogenase activities. Similarly, an elevated level of CO2 inside the

reactor inhibits the photo-H2 production, while lower levels (4-18% w/v) favour the growth phase

of PNSB and thus H2 production (See section 6.4.2). Furthermore, a lower C/N ratio does not

favour photo-H2 production as it could result in the accumulation of ammonium and inhibition

127 Table 4. Variation of different operational parameters in PF studies.

PNS strains Carbon (& nitrogen)

source Reactor type) Culture Culture Temp. °C pH Light intensity (Lux if not specified) Maximum H2 yield mL H2 g COD-1 b Maximum H2 Production Rate mL H2 L-1 _h-1 Reference R. sphaeroides

O.U.001 (DSM586) DFE of glucose Batch 30 6.4 38-50

a

(Tungsten lamp) - 26.4 [53]

R. capsulatus

(DSM1710) DFE of potato steam peels hydrolysate Batch 30 6.4 38-50

a

(Tungsten lamp) - 12.3 [53]

R. capsulatus (hup-_{) Acetic acid (glutamate) Continuous}

Tubular PBR < 40 < 8.0 Natural sunlight 122.5 8.9 [91]

R. sphaeroides CIP

60.6

Lactate (glutamate) Batch 30 7.0 4500-8500

(Tungsten lamp)

- 39.8 [57]

R. capsulatus

YO3(hup- ₎ Acetate (glutamate) _{panel PBR} Fed-batch 35 7.0 Natural sunlight - 11.4 [87]

Rhodopseudomonas

palustris WP 3-5 Formic, acetic, butyric, lactic acid (glutamate) Column PBR Continuous 28-35 6.8 4000-7000 - 13.2 [92]

R. sphaeroides

O.U.001 Malate (glutamate) Batch annular PBR 32 6.8 15 W m

-2

equivalent 1050 6.5 [93]

Mixed culture Acetate & glucose

(glutamate) Batch 34 6-7 (Fluorescent 4000

light)

78.6c _{- [94]}

Butyrate & glucose

(glutamate) 74.5

c _-

R. capsulatus (DSM

155) Dark fermented effluents Batch 30-33 6.6-6.8 4000 - 19.0 [76]

R. sphaeroides

O.U.001 (DSM 5864)

Malate Flat panel PBR 32 6.8 50a

(Tungsten lamp)

1073.3 10.0 [95]

Rhodopseudomonas

palustris WP3-5 Butyrate (glutamic acid) Batch 32 7.1 (Tungsten lamp) 10,000 803.6 24.9 [96]

a _{Calculated using the available data and conversion values from the URL: http://www.egc.com/useful_info_lighting.php} b _{COD values are calculated from theoretical oxygen demand of main carbon sources}

c _{mL H2 g COD}-1 _d-1 - Data not available

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