4.2. Biochar aided PAH biodegradation by R. opacus
4.2.1. Biochar characterization
Fig. 4.18: FESEM micrographs of biochar prepared in this study (a) 1 µm and (b) 10 µm and (c) FESEM-EDX spectrum of the biochar (insert shows biochar FESEM)
In this study, contact angle of the biochar is found to be 78.4º. In general, hydrophilic surfaces have a contact angle less than 90° and impose positive capillary pressure, thus allowing water to easily enter into the pores, whereas hydrophobic surfaces have a contact angle value greater than 90° and imposes negative pressure, restricting water from entering into the pores (Gray et al., 2014).
4.2.1.2. Chemical properties
Fig. 4.19 (a-c) shows the results of chemical nature of the biochar by employing different techniques. Whereas Fig. 4.19 (a) shows the FTIR spectra of the biochar material, which depicts different vibrational frequencies due to several functional groups present in the biochar. Various types of biochar have shown to depict a wide band spectrum at around 3400 cm-1, which is characteristic of the hydroxyl groups (O-H) (Bind et al. 2018). At 2918 cm-1, another band of –CH2 is observed (Das et al., 2015b) followed by a band spectrum at 1744 cm-1 corresponding to
(c)
4000 3500 3000 2500 2000 1500 1000 500 30
40 50 60 70 80
Transmittance (%)
Wavelength (cm-1)
C-H
C=O -N-H
-C-C-
-SO3 -C-O
-C-N -C-H R-C=
R
-O-H
(a)
10 20 30 40 50 60 70
10 20 30 40 50 60 70
Intensity (a.u)
2
(b)
500 1000 1500 2000 2500 3000 3500
100 200 300 400 500 600 700 800
Intensity (a.u.)
Raman shift (cm-1) (D band) (G band) (c)
100 200 300 400 500 600 700 800 900
45 50 55 60 65 70 75 80 85 90 95 100 105
(Stage III) (Stage II)
(Total residue) (Volatile)
TGA (mass/%)
Temperature (oC) (1)
(2)
(3) (Moisture)
(Stage I) (d)
-15000 -10000 -5000 0 5000 10000 15000
-2.0 -1.5 -1.0 -0.5 0.0 0.5 1.0 1.5 2.0
Magnetic moment (emu g-1)
Magnetic field (Oe)
Hc = 178.32 G Ms = 1.6573 emu g-1 Mr = 0.30750 emu g-1 Squareness = 0.18 (e)
Fig. 4.19: (a) Fourier transform infrared spectroscopy; (b) X-ray diffraction; (c) Raman spectra; (d) Thermogravimetric analysis (TGA) curve; and (b) Vibrating sample magnetometer analyses of the biochar
the carbonyl of ester groups (Aran et al. 2016); a peak at 1631 cm-1 is assigned to the primary amine band spectra. The absorption band in the range 1615-1580 cm-1 signifies
the –C-C- band spectra of aromatic carbon atoms. A band spectrum between the position 1393 cm-1 and 1320 cm-1 with an intense peak at 1385 cm-1 signifies the presence of −SO3
stretching (Sinha et al 2015). In addition, the FTIR spectra reveals–C-O, -C-N, -C-H–and R2-C= band stretch corresponding to the functional groups alcohol/ carboxylic acid/
alkoxy, aliphatic amines, alkene and trisubstituted alkene, respectively.
Fig. 4.19 (b) shows XRD profile of the biochar in which the peak intensity in the range of 7–70° is mainly attributed to aromatic and graphite nature of the organic material.
A diffraction peak at around 21.0 is clearly attributed to the cellulose crystal plane (Li et al., 2016) with a crystallinity index (ICr) of 47.4 %. In addition, the peaks at 23 and 43 clearly depict the presence of turbostratic carbons (Das et al., 2016).
Fig. 4.19 (c) shows the first order Raman spectra of the biochar. In general, a peak around 1580 cm-1 reveals a perfectly ordered crystalline nature, whereas distorted amorphous carbon nature of the carbon is represented by two distinct peaks: (a) graphite band (G) at about 1600 cm-1, which is related to sp2 carbon atoms and (b) distorted carbon band (D) at around 1350 cm-1, signifying the sp3 carbon atoms (Lian et al., 2016; Frost et al., 2003). In this study, both graphite band (G) and distorted carbon band appear in the spectra (Fig. 4.19 (c)). In general, the ratio of intensity of D to G bands (ID/IG) (i.e. R value) represents a crystallographic structure, distorted degree and average size of the sp2 domains in graphite structure of a material (Akhavan et al., 2010). In this study, the biochar yielded two distinct peaks related to D band and G band in the Raman spectra with a ID/IG
intensity ratio (R-value) of 0.87. These results reveal amorphous nature of the biochar with reduced domains of sp2 in the carbon structure. All these results of functional group analysis using FTIR spectroscopy, XRD analysis and Raman spectroscopy clearly
demonstrate an excellent ability of the biochar for PAHs adsorption and in enhancing their bioavailability to degrading microorganism.
4.2.1.3. Thermal degradation analysis
TGA of the biochar is represented in Fig. 4.19 (d), which plots the residual sample mass (%) and the discrete time derivative of the residual mass (% min-1) vs. temperature.
This figure reveals three peaks indicating the following stages in the material weight loss:
Stage I (25°C-100°C), Stage II (100°C- 870°C) and Stage III (870°C -900°C).
The second stage, i.e., devolatilization region appears over a wide temperature interval and characterizes a major mass loss of the biochar. In Fig. 4.19 (d), Stage II indicates loss due to moisture and other volatile components just before the material reaches 100°C. A slight weight loss occurred in Stage I, probably due to the release of embedded volatiles within the biochar. Further weight loss during Stage II at a high rate indicates the formation of some pyrolytic products, primarily phenols, heterocyclic compounds, mono- and polycyclic compounds. Lastly, in Stage III, gradual weight loss (i.e., solid residue devolatilization) was observed which reveals that carbonaceous material present within the solid residue degraded continuously at a very mild rate before reaching a constant value (Peng et al., 2001). Approximately 10 % initial moisture is attributed to the weight loss in Stage I. The total volatile loss can be attributed to the weight loss observed in both Stage II and Stage III. In Stage III, negligible volatile loss is observed.
Overall, final residue content within the biochar at 900°C is determined to be 61.4% of the total initial biochar utilized for the TGA analysis, thereby indicating its potential for a long-term use.
4.2.1.4. Magnetic properties
Fig. 4.19 (e) is a plot of magnetization (M) Vs magnetic field (Oe) at room temperature showing the hysteresis loop obtained following the application of VSM technique on the biochar. The magnetization parameters such as saturation magnetization (Ms), remanent magnetization (Mr), coercivity (Hc) and squareness of the derived biochar are represented in Table 4.10. A low Hc value depicts that the material can be simply separated out with the help of a magnet or an externally applied magnetic field and, therefore, the material is easy to separate, recover and reuse in any process (Nguyen et al., 2011). Furthermore, following separation, the material can again be easily re-dispersed into the solution and reutilized owing to its low Mr value (Ahmed et al., 2015).