3.1 Surface morphology and particle size characterization of Co nanoparticles
The surface morphology and structure of Co NPs were investigated using FESEM. As shown in Fig. 2, Co NPs are irregular block particles with individual sizes ranging from 70 to 104 nm.
3.2 Impacts of Co nanoparticles on biogas yield
When the substrate was exposed to 1, 2, and 3 mg/L of Co NPs, the starting of biogas yield was improved, as shown in Fig. 3. The addition of 1, 2, and 3 mg/L Co NPs, in particular, increased biogas yield startup by 23.0 %, 53.1 %, and 23.0.3 %, respectively, in the first 5 days of digestion. The addition of Co NPs speeds up the process of reaching peak biogas production. On day 6, the 2 mg/L Co NPs produced the highest daily biogas yield, with approximately 42.6 mL/g VS. However, the greatest daily biogas production of the control was 34.0 mL/g VS on day 19.
As shown in Fig. 4, the addition of 2 mg/L Co NPs increased biogas yield to 6767.5 mL/g VS, which is greater than the biogas yielded by the control, 1 mg/L Co NPs, and 3 mg/L Co NPs, which were 589.2, 629.5, and 602.3 mL/g VS, respectively. According to the statistical analysis, the addition of 1 and 2 mg/L Co NPs increased cumulative biogas yield by 6.83 % and 14.81 %, respectively, as compared to the control. However, there were no significant differences in cumulative biogas production with 3 mg/L Co NPs compared to the control (p-value = 0.430). The addition of Co NPs speeds up the process of reaching peak biogas production (Fig. 4). After 30 days of fermentation, the control treatment yielded 589.2 mL/g VS, but the Co NPs treatment (2 mg/L) yielded 595.8 mL/g VS after 22 days, indicating that 22 days may be sufficient (HRT).
3.3 Impacts of Co nanoparticles on methane yield
According to the cumulative CH4 production curves, adding 1 mg/L Co NPs to the substrate increased CH4 yield to 33.3 mL/g VS, which is significantly higher (p < 0.05) than the CH4 produced by control, 2 mg/L Co NPs, and 3 mg/L Co NPs, which produced 18.59, 29.07, and 28.75 mL/g VS of CH4, respectively. The addition of 1, 2, and 3 mg/L of Co NPs enhanced the CH4 yield by 79.12 %, 56.37 %, and 54.65 %, respectively, when compared to control (p < 0.05). Furthermore, as shown in Fig. 4, no significant variations in CH4 yield were attained when 2 mg/L Co NPs and 3 mg/L Co NPs were added to the substrate (p-value = 0.857).
The results of the experiment correspond with those of Zandvoort et al. (2006), who found that 0.8 mg/L Co is the best dosage. Furthermore, when the substrate was exposed to 1 mg/L Co NPs, the greatest CH4 yield was achieved, which accords with Abdelsalam et al. (2016), who observed that the presence of 1 mg/L Co NPs boosted CH4 production by 86 % when compared to the control condition (manure without NP additives). Furthermore, the increase in CH4 in our results using 1 mg/L Co was similar with the findings of Qiang et al. (2012), Demirel and Scherer (2011), and Feng et al. (2010), who all concluded that Co is an essential metal for methanogenesis because it acts as a metallic enzyme activator.
3.4 Effects of Co nanoparticles on hydrogen sulfide yield
As seen in Fig. 5, Co NPs significantly reduce cumulative H2S production (p < 0.05) for all concentrations evaluated when compared to control. When compared to control, cumulative H2S generation of 1, 2, and 3 mg/L Co NPs was reduced by 15.38 %, 13.20 %, and 57.89 %, respectively. Furthermore, the maximum H2S removal efficiency of 57.89 % was achieved with 3 mg/L Co NPs added. The removal efficiency of Co NPs at 1 and 2 mg/L was 15.38 % and 13.20 %, respectively.
The results are better than those of Hassanein et al. (2019), who reported that using 5.4 mg/L Co NPs improved H2S removal effectiveness by 6.79 %. It's possible that the variation in findings is due to the different substrate types. Co NPs effectively reduced H2S emissions in all treated bio-digesters, according to these findings. The precipitation of metal sulphide (Boer et al., 2014; Trifunovi et al., 2016) is one possible reason for the reduction of H2S emissions in our investigation. With 1 mg/L of Co NPs and 2 mg/L of Co NPs, there were no significant changes in H2S removal effectiveness (p-value = 0.548).
3.5 Effects of Co nanoparticles on pH, volatile fatty acids and total alkalinity
To begin, the dynamic shift in pH during AD represented the effect of Co NPs on the stability of the AD process. As seen in Fig. 6, there are two stages to the pH variations. pH increased in the first stage (from the beginning of the experiment to day 20). Furthermore, the maximum pH value was 7.3 both on Day 20 and with 1 mg/L of Co NPs.
The increase in pH could be related to i) the widespread usage of VFAs in AD systems; ii) the reaction between Co and organic molecules in the medium, which could result in an increase in pH. As a result of the aforementioned reaction, the substrate under AD will be depleted of hydrogen ions (H+), causing the pH of the substrate to rise. Furthermore, collecting CO2 prevents the production of (H2CO3) inside the substrate, which raises pH (Amen et al., 2018). The pH drops throughout the second stage (from days 20 to shut down) due to the accumulation of VFAs (Chen et al., 2008).
Figure 7 shows the variation in volatile fatty acids (VFAs) concentrations of 1, 2, and 3 mg/L Co NPs. VFAs have two stages throughout the experimental period. The VFAs concentration showed a significant drop trend at the early stage (during the first 20 days of digestion time) for all Co NPs additions, with the highest VFAs degradation of 2800 mg/L with 2 mg/L Co NPs. This indicated that I once microorganisms adapted to the Co NPs environment, Co NPs offered an effective electron donor to boost microbial metabolism; and ii) the addition of track elements such Co may reduce the initial VFAs accumulation during the AD (Bayr et al., 2012; Wei et al., 2014).
The VFAs concentration increased with all Co NPs concentrations in the final stage (during the last 10 days of digestion), with the highest value of 4000 mg/L for 3 mg/L Co NPs. The increase in VFAs in the last stage could be related to total alkalinity decrees. The decrease in daily biogas production (Fig. 3) and pH value (Fig. 6) during the final stage could be attributed to the growth and accumulation of VFAs. Previous research has shown that increasing and accumulating VFAs causes a drop in pH and biogas production (Liu et al., 2014; Romsaiyud et al., 2009). This implies that the pH, VFAs, and biogas production are all in agreement.
Figure 8 shows the total alkalinity (TA) profile, which is identical to that of pH. Before 20 days (from the commencement of the experiment to day 20), the TA showed an upward trend. This meant that the rise in substrate TA was due to Co NPs having a beneficial influence on VFA consumption. The TA revealed a pattern of declines after 20 days (from day 20 until shut down), which could be due to VFA creation (Ahn et al., 2010).
The average TA concentrations were 4950, 4900, 4725, and 4475 mg CaCo3/L, respectively, when the substrate was treated to 1, 2, 3 mg/L Co NPs and control. With the addition of 1 mg/L Co NPs, the TA concentration was increased. Suanon et al. (2017) found that the TA concentration indicated the consumption of VFAs by methanogen bacteria, which resulted in increased CH4 production. These findings show that TA concentration and cumulative CH4 production in experiment results are in accord, especially when 1 mg/L Co NPs are introduced.
3.6 Effects of Co nanoparticles on total and volatile solids
When assessing the stability of the AD process, the degradation of TS and VS must be taken into account. To begin with, Fig. 9 shows the total solids (TS) content with 1, 2, and 3 mg/L Co NPs added in contrast to the control. During the digesting period, the TS content in all biodigesters and the control biodigester decreased. Control, 1, 2, and 3 mg/L Ni NPs have TS removal efficiencies of 12.04 %, 14.81 %, 16.25 %, and 14.81 %, respectively.
In comparison to control, Fig. 10 shows the VS content at varied concentrations of 1, 2, and 3 mg/L Co NPs. The volatile solids (VS) content curve followed a similar pattern throughout the experiment. Control, 1, 2, and 3 mg/L Co NPs have VS removal efficiencies of 11.55 percent, 12.16 percent, 11.85 percent, and 10.66 percent, respectively. Co nanoparticles increased the degradation of organic matter by boosting the capacity of methanogens bacteria to breakdown organics, as evidenced by the change in TS and VS content after the addition of Co NPs.
3.7 Effects of Co nanoparticles on the characterization of organic material and chemical composition of the effluent
The FTIR method is used to detect the characteristic vibrations of chemical bonds and chemical functions. Figure 11 shows the profile of the effluent FTIR spectrum after the substrates were exposed to 1, 2, and 3 mg/L Co NPs compared to the control. According to the literature, the main absorption peaks were: i) O-H stretching of the carboxylic and alcoholic groups at about 3450 cm− 1; ii) C-H stretching of the aliphatic at about 2900 cm− 1; iii) -COO- stretching of the carboxylic acid at about 1600 cm− 1; iv) C-O stretching of the carbohydrate at about 1100 cm− 1; v) C-O stretching of the carbohydrate at (Abdulla et al., 2010; Gamage et al., 2014). When varying concentrations of Co NPs were added, the intensity and shift of peaks changed when compared to the control peak. In particular, the prominent peak in the spectrum about 1100 − 950 cm− 1 corresponds to the carbohydrate's C-O stretching.
The decreased carbohydrate is reflected in the intensity of this band in the spectra of the treated sample. In comparison to other treatments (2 and 3 mg/L Co NPs) and control, the presence of 1 mg/L Co NPs exhibited the lowest intensity of the C-O band. These findings indicated that i) the presence of 1 mg/L Co NPs may be enhancing the methanogenesis communities' ability to decompose the carbohydrate with the formation of VFAs, resulting in increased CH4 production; and ii) the highest cumulative CH4 production and the FTIR spectrum of effluent with 1 mg/L Co NPs added are in agreement.
The effect of different concentrations of Co NPs on the chemical composition of the digestate was observed in Table 2. The presence of Co NPs increased sulfide content. Sulfide content increased by 3%, 3.6%, and 12.12% with 1, 2, and 3 mg/L Co NPs added compared to control, respectively. The presence of 3 mg/L Ni NPs decreases the H2S production by 57.89% as shown in Fig. 5 compared with control.
Table 2 Chemical composition of the effluent with different concentrations of Co NPs.
Element
|
Before digestion
|
After digestion
|
Slurry
|
Control
|
1 mg/L Co NPs
|
2 mg/L Co NPs
|
4 mg/L Co NPs
|
C %
|
39.63
|
40.91
|
40.27
|
40.27
|
41.03
|
H %
|
5.93
|
40.91
|
40.12
|
40.12
|
39.80
|
S %
|
0.31
|
0.33
|
0.340
|
0.342
|
0.373
|
Fe %
|
0.18
|
0.20
|
0.18
|
0.24
|
0.19
|
Ni %
|
7.4*10-4
|
8.2*10-4
|
0.041
|
7.4*10-4
|
1.1*10-3
|
Co %
|
8.8*10-4
|
2.0*10-4
|
8.8*10-4
|
2.7*10-3
|
2.3*10-3
|
The bioavailability of organic matter and sulphate in AD stimulates the growth of SRB, which reduce sulphur to sulphide as a terminal electron receiver from a wide range of elements, including H2, ethanol, formate, succinate, pyruvate, and lactate, resulting in reduced H2S emission and increased sulphide content in the effluent in this study (Muyzer and Stams, 2008). In this case, Co NPs successfully reduced H2S emissions by killing SRB, directly absorbing sulphur during digestion, or changing the polluting materials via a biochemical mechanism (Sevcu et al., 2011).
3.8 Fertility evaluation of effluent containing Co NPs
As shown in Fig. 12, the total nutrient (NPK) content of the digestate with 1, 2, and 3 mg/L Co NPs is 5.32 %, 4.68 %, and 4.63 %, respectively. Since all Co NPs concentrations had a total nutrient content of close to 5%, they can be utilized in conjunction with an artificial compound fertilizer. The digestates were dewatered and dried to provide a high-quality organic compound fertilizer. The physical and chemical features of soil can be improved with NPK organic compound fertilizer, which promotes the creation of soil aggregate structure and increases the activation of soil nutrients. It is obvious that when NPK organic compound fertilizer is employed, the crops demand significantly less water, and the area's chronic water shortage problem may be remedied. As a result, the AD digestate combined with the three Co NPs can be utilized to make an NPK organic compound fertilizer that improves plant height, root length, root diameter, and dry weight (Möller and Müller, 2012).
3.9 Kinetic analysis of biogas yield
Tables 3 and 4 show the findings obtained from the modified Gompertz and Logistic Function models, respectively. Figures 13 and 14 show a comparison of experimental and expected biogas yield using 1, 2, and 3 mg/L Co NP additions, as well as a control. The addition of 1,2, and 3 mg/L Co NPs boosted maximum biogas production rate (Rmax) by 14.29 %, 26.80 %, and 4.60 %, respectively, using the modified Gompertz model. In addition, adding 1, 2, or 3 mg/L of Co NPs reduced the lag phase (λ) by 1.24, 0.59, and 0.89 days, respectively, as compared to the control (1.58 days).
Table 3 Parameters of modified Gompertz model at different concentrations of Co NPs.
Kinetics parameter
|
Treatments
|
Control
|
1mg/L Co NPs
|
2 mg/L Co NPs
|
3 mg/L Co NPs
|
R2
|
0.998
|
0.998
|
0.997
|
0.998
|
Mmax (mL/g VS)
|
663.9
±10.37
|
664.86
±7.29
|
695.86
±8.27
|
647.92
±8.26
|
Rmax (mL/g VS/d)
|
27.35
±0.51
|
31.26
±0.56
|
34.68
±0.81
|
28.61
±0.54
|
λ (d)
|
1.58
±0.19
|
1.24
±0.17
|
0.59
±0.21
|
0.89
±0.18
|
Predicted biogas yield
(mL/g VS)
|
593.13
±13.74
|
620.67
±19.15
|
661.50
±18.08
|
596.58
±19.39
|
Measured biogas yield
(mL/g VS)
|
589.2
±13.65
|
629.50
±19.43
|
676.50
±18.50
|
602.30
±19.58
|
Difference between measured and predicted biogas yield (%)
|
0.66
|
1.42
|
2.26
|
0.95
|
AIC
|
135.88
|
134.00
|
151.31
|
134.80
|
Mmax: Maximum biogas potential yield; Rmax: Maximum biogas yield rate; λ: Lag phase; R2: Correlation Coefficient; AIC: Akaike’s information criterion.
Table 4 Parameters of logistic function model at different concentrations of Co NPs.
Kinetics parameter
|
Treatments
|
Control
|
1 mg/L Co NPs
|
2 mg/L Co NPs
|
3 mg/L Co NPs
|
R2
|
0.996
|
0.996
|
0.991
|
0.995
|
Mmax (mL/g VS)
|
603.56
±8.62
|
620.12
±7.01
|
660.19
±9.88
|
600.42
±8.02
|
Rmax (mL/g VS/d)
|
28.69
±0.74
|
32.23
±0.85
|
34.58
±1.35
|
29.47
±0.84
|
λ (d)
|
2.43
±0.28
|
1.90
±0.26
|
0.95
±0.39
|
1.55
±0.30
|
Predicted biogas yield
(mL/g VS)
|
580.8
±13.45
|
607.12
±18.73
|
649.29
±17.75
|
584.22
±18.99
|
Measured biogas yield
(mL/g VS)
|
589.2
±13.65
|
629.50
±19.43
|
676.50
±18.5
|
602.30
±19.58
|
Difference between measured
and predicted biogas yield (%)
|
1.43
|
3.68
|
4.19
|
3.09
|
AIC
|
157.08
|
158.41
|
183.81
|
161.15
|
Mmax: Maximum biogas potential yield; Rmax: Maximum biogas yield rate; λ: Lag phase; R2: Correlation Coefficient; AIC: Akaike’s information criterion.
The maximum biogas production rate (Rmax) for the control, 1, 2, and 3 mg/L Co NPs was 28.69, 32.23 (12.33 % increase), 34.58 (20.52 % increase), and 29.47 (2.71 % increase) mL/g VS/d, respectively, for the logistic function model. In addition, the presence of 1, 2, and 3 mg/L Co NPs reduced the lag phase (λ) to 1.90, 0.95, and 1.55 days, respectively, as compared to the control group (2.43 days). Both kinetic models show that Ni NPs increased the rate of biogas production and decreased the lag phase. Tables 3 and 4 show the results of the Akaike Information Criterion (AIC) test for the modified Gompertz and logistic models. The Akaike Information Criterion (AIC) value of the modified Gompertz model is lower, indicating that it is a superior model to utilize in this circumstance.
To derive the hydrolysis rate constant (K) and R2 in Eq. (4), the hydrolysis of organic materials was assessed using the First-order Kinetic model (Table 5). In general, the model suited the experimental results well, with a coefficient of determination of (0.991–0.993). The presence of 1, 2, and 3 mg/L Co NPs increased the cow dung hydrolysis rate by 72.22 %, 144 %, and 66.66 %, respectively, as compared to the K value of the control treatment. The modified Gompertz model (R2 = 0.997–0.998) and the logistic function model (R2 = 0.991–0.996) match the biogas production data well.
Table 5 Parameters of first-order kinetic model at different concentrations of Co NPs.
Kinetics parameter
|
Treatments
|
Control
|
1mg/L Co NPs
|
2 mg/L Co NPs
|
3 mg/L Co NPs
|
R2
|
0.991
|
0.989
|
0.993
|
0.993
|
k (d-1)
|
0.018
±0.003
|
0.031
±0.003
|
0.044
±0.003
|
0.030
±0.003
|
R2: Correlation Coefficient; k: hydrolysis rate constant.