3.1 Decolourization performance under Static, Shaking and MFC condition and removal of COD
Pseudomonas aeruginosa exhibited higher dye-decolourizing activity when operated under DCMFC condition (80.56%), whereas agitated culture showed less decolourization efficiency (49.25%). At static condition only 16% decolourization of Congo red (CR) was observed after 120 h (Fig. 3). At static condition bacterial growth rate 0.074 h− 1, this evident that the sulfonated diazo dye Congo red is recalcitrant to bio decolourization under static condition [21]. At shaking condition bacterial growth rate improved to 0.304 h− 1 which resulted in 50% of dye decolourization. The growth rate of bacteria was observed to be 0.688 h− 1 for MFC condition resulted in higher decolourization efficiency of CR dye. The time required for decolourization was decreased with MFC condition. This result proved that the decolourization was not to the adsorption of cells but due to the metabolic activity of bacterial cells [22]. The average rate of decolourization was increased up to 12 h in MFC condition. On continuous acclimatization in DCMFC 100% decolourization was observed within 48 h. Removal of COD was 85% under MFC condition. Hou et al., 2011 [23] reported 68.9 % removal of COD at MFC condition whereas Isik and Sponza (2003) [24] reported only 57% removal of COD under anaerobic condition while 87% of CR decolourization at aerobic condition.
3.2 Effect of glucose concentration on dye decolourization and power density
CR decolourization efficiency and power production by Pseudomonas aeruginosa were investigated under different glucose concentration (0 g/L,5 g/L,10 g/L, 15 g/L and 20 g/L). Initially all MFCs were operated under open circuit condition to evaluate its performance when no load was applied. The OCV was recorded at a time interval of 3 h and the recorded data was averaged for every 24h. Similarly, samples were withdrawn aseptically at regular time intervals from the inoculated MFC and analysed for cell density at OD 600 nm. As shown in Fig. 4, the growth curves of P. aeruginosa cultured under anaerobic condition in MFC displayed a typical three-stage growth cycle: an adaptive phase (0–4h), an exponential growth phase (5–14h), and a stationary phase (after 14h) approximately for all five sets.
The growth of bacteria was maximum at 10 g/L (0.858 OD at 11th hr). When the glucose concentration was 10g/L the maximum growth rate was expressed. Hence, this concentration was used for further study. Moreover, Fig. 5 shows the performance of MFC under different glucose concentration at 100 µM of CR concentration. Initially, an open circuit voltage of 0.184V, and a maximum power density of 1.3 mW m− 2 was observed after culturing time of 264 h. Later for 5 g/L of glucose concentration an OCV of 0.483 V, and a maximum power density of 1.7 mW m− 2 was observed for the culturing time of 480 h. Then for 10 g/L of glucose concentration an OCV of 0.691 V, and a maximum power density of 1.9 mW m− 2 was observed for the culturing time of 576 h. For 15 g/L of glucose concentration an OCV of 0.426V, and a maximum power density of 0.7 mW m− 2 was observed for the culturing time of 336 h. Finally, for 20 g/L of glucose concentration an OCV of 0.458 V, and a maximum power density of 0.4 mW m− 2 was observed for the culturing time of 216 h.
Concerning the working power densities at 220 ohm external resistance with the duration time of 72 h shows the variation in power densities at different culture time. Therefore, from the growth curve study a culture time of 3 h, corresponding to the middle of log phase would be suggested in this study. From Fig. 4 shows that the culture time of 4 h, corresponding to the transition of log phase to stationary phase in the growth curve. Therefore, higher the growth influences the bacteria metabolism and growth in Pseudomonas aeruginosa in MFC [25]. Better power performance was observed in stationary phase of the growth curve and can further be suggested.
Comparing the power performance among different concentration of glucose, 10 g/L of glucose concentration would produce an optimal performance for the studied cases. Further, the MFC operated with 10 g/L of glucose concentration had prolonged stationary phase which improves the electrical activity in the MFC. Previous study by Jafary et al., 2013 [26] investigated that the substrate concentration enhancement resulted in improved MFC performance. But, in this study, with 10 g/L of glucose the DCMFC will produce 1.9 mW m− 2 under the optimal condition of glucose.
3.3 Effect of glucose concentration on different dye decolourization
The decolourization activity of Pseudomonas aeruginosa in MFC system was studied using CR at various initial concentrations with different glucose concentrations. DCMFC system inoculated with Pseudomonas aeruginosa can decolourize up to 200 µM of dye concentration, however higher concentration seemed to be toxic for the cell growth in MFC. The dye decolourization was strongly inhibited at 500 µM in the MSM medium. From Table 1, it was evident that a maximum OCV of 0.844 V was observed for the initial concentration 200 µM of dye and 10 g/L of glucose concentration. Further the dye decolourization was also reported to be higher (96.1%).
Table 1
Effect of dye concentration under different glucose concentration
Concentration of glucose (g/L) | Maximum OCV (mV) | Maximum decolourization (%) |
100 µM | 200 µM | 300 µM | 100 µM | 200 µM | 300 µM |
0 | 206 | 249 | 190 | 57.64 | 51.93 | 43.19 |
5 | 483 | 712 | 316 | 80.85 | 91.39 | 69.11 |
10 | 691 | 844 | 392 | 89.74 | 96.1 | 79.94 |
15 | 426 | 527 | 277 | 86.07 | 89.65 | 70.21 |
20 | 458 | 551 | 292 | 80.46 | 77.85 | 67.56 |
Table 2
Characteristics of dye wastewater before and after MFC treatment
TESTS | SAMPLE |
INITIAL | FINAL |
COD (mg/L) | 1036 | 148 |
TSS (mg/) | 923 | 452 |
TDS (mg/L) | 4211 | 2040 |
EC (mS/cm) | 8.422 | 4.08 |
As for the performance of MFC under 100 µM of dye using 10 g/L of glucose concentration a maximum OCV of 0.691 V and a maximum decolourization of 89.74% was reported. Moreover, for 300 µM of dye concentration, a maximum OCV of 0.392 and a maximum decolorization of 79.94% was observed. These results would indicate that the glucose concentration of 10 g/L and 200 µM seems to be a better choice for better decolourization in MFC. Stable OCV production was achieved during CR decolourization with the presence of 10 g/L of glucose. Further, the dye intermediates of CR might have enhanced the MFC performance after degradation [27]. The test for TSS, TDS and EC were conducted for samples obtained from MFC at optimum condition (200µM dye concentration and 10g/L glucose concentration) and the results are given in table. 2
3.4 Degradation analysis
CR colour removal in MFC was confirmed by UV-Visible analysis (200–800 nm) (Fig. 6). CR UV-Visible scanning depicted two peaks. One extended at 340 nm was due to the interaction between the aromatic hydrocarbon /polycyclic aromatic hydrocarbon groups and other chromophore group, and a peak at 495 nm, relative to the azo bond group and a large conjugated system of whole dye molecule. After 72 h treatment, a decrease in the major peak intensity at 495 nm was noted. This indicated that the dye structure especially the azo bond was broken and was degraded during MFC treatment. Surprisingly, the absorbance at 340 nm was also decreased which evident that interaction of the aromatic hydrocarbons and some chromophore group were partially destroyed. These results suggested that MFC treatment is able to degrade the CR dye with a complete breakdown of chromophore group. Unlikely Wang et al, 2017 [28] reported that complete breakdown of aromatic hydrocarbon seemed to be more difficult that the breakdown of azo double bond.
In order to elucidate the nature of the functional groups present in the untreated and treated sample, FTIR analysis was performed. The spectra of decolourized sample reveals shifting of some characteristic bands which depict changes in functional groups after decolourization compared to untreated dye sample (control) (Fig. 7). The FTIR spectrum of untreated sample and treated sample reveals several peaks and stretching as presented in Tables 3 and 4 respectively. FTIR analysis revealed the presence of aromatic compounds in untreated sample and alkenes in treated sample.
Table 3
FTIR Peaks (cm− 1) of CR before MFC treatment
Sl. No. | Absorption (cm− 1) | Appearance | Group | Compound class |
1. | 3452.30 | weak, sharp | - | -- |
2. | 3057.99 | weak, broad | O-H stretching | Alcohol |
3. | 2292.49 | weak, broad | CΞN stretching | Nitrile |
4. | 2070.22 | weak, broad | - | - |
5. | 1988.63 | weak, sharp | C-H bending | aromatic compound |
6. | 1899.28 | weak, broad | C-H bending | aromatic compound |
7. | 1581.53 | medium, sharp | - | - |
8. | 1550.09 | weak, sharp | - | - |
9. | 1500.85 | weak, sharp | - | - |
10. | 1445.59 | weak, sharp | - | - |
11. | 1399.10 | weak, sharp | - | - |
12. | 1351.03 | medium, sharp | O-H bending | Alcohol |
13. | 1322.18 | weak, sharp | - | - |
14. | 1221.71 | strong, sharp | C-O stretching | vinyl ether |
15. | 1175.35 | strong, sharp | C-O stretching | Ester |
16. | 1121.71 | medium, sharp | - | - |
17. | 1059.22 | strong, sharp | C-O stretching | primary alcohol |
18. | 919.72 | medium, sharp | - | - |
19. | 860.71 | weak, sharp | - | - |
20. | 832.11 | medium, sharp | C = C bending | Alkene |
21. | 783.62 | weak, sharp | - | - |
22. | 750.72 | medium, sharp | - | - |
23. | 724.55 | weak, sharp | - | - |
24. | 695.18 | medium, sharp | - | - |
25. | 638.02 | strong, sharp | - | - |
26. | 591.72 | medium, sharp | - | - |
27. | 552.55 | weak, sharp | - | - |
28. | 524.06 | weak, sharp | C-Br stretching | halo compound |
Table 4
FTIR Peaks (cm− 1) of CR after MFC treatment
Sl. No. | Absorption (cm− 1) | Appearance | Group | Compound class |
1. | 3277.83 | strong, broad | O-H stretching | carboxylic acid |
2. | 2112.40 | weak, broad | CΞC stretching | Alkyne |
3. | 2012.27 | weak, broad | - | - |
4. | 1636.52 | medium, sharp | C = C stretching | conjugated alkene |
5. | 1080.26 | weak, sharp | - | - |
6. | 988.06 | weak, broad | - | - |
7. | 922.49 | weak, broad | - | - |
8. | 862.41 | weak, broad | - | - |
9. | 662.11 | weak, broad | - | - |
10. | 606.27 | weak, broad | - | - |
11. | 567.38 | weak, sharp | - | - |
3.5 Bacterial adhesion on anode
The surface of the anode electrode was visualized by scanning electron microscope SEM (FEI-Quanta FEG 200F) to determine microbial attachment and formation of a biofilm on the anode electrode surface. It was removed after 29 days incubation period. SEM Images of bacterial attachment on Anode from MFC Setup used for decolourization of dye concentration (200µM) along with glucose concentration (10g/L) is represented in Fig. 8. The average size of the microbe was obtained as 1µm.
The formation of a biofilm begun when bacteria sense certain environmental cues and then trigger the transition to life; these environmental signals vary among microbial communities. P. aeruginosa biofilms can be found on any substrate surface suitable for its growth [29]. Some studies [30] have shown that bacteria develop biofilms on surfaces when nutrients are provided at sufficient levels in the solution; the cells will then enter back into the solution when the nutrients are scare. This starvation response is a key factor regulating biofilm formation [31]. Changes in environmental factors cause changes in microbial state. Xu found that Desulfovibrio vulgaris adhered to carbon steel surfaces under starvation conditions to acquire extra energy from steel [32]. Additionally, microorganisms are capable of complex behaviours that best benefit their reproduction. Mutual interactions in biofilms result in biofilm formation, which enables sessile organisms to stay on surfaces and exhibit improved growth.
Ilamathi et al., 2019 [17] reported that the presence of P. aeruginosa biofilm on electrode surface strongly improved the redox behaviour when compared to uncolonized electrodes. From the SEM analysis (Fig. 8) and OCV generation (Fig. 5) it is understood that the presence of P. aeruginosa biofilm on electrode surface improved the redox behaviour.