By Elemental analysis
More than 80% of swine blood waste is protein. mainly containing C, N, O, H and other elements. The elemental analysis of precursor and BN-AC is shown in Table 1. According to Table 1, the carbon, nitrogen, oxygen and hydrogen content in the precursor were 49.443%, 11.846%, 26.014% and 12.697% respectively. The protein was unstable at high temperature. After the BN-AC was prepared from blood powder, the element content changed, and the proportion of nitrogen element retained by BN-AC was proportionately high. The presence of nitrogen content indicated that N was successfully doped in BN-AC sample. Due to the high nitrogen content in the protein of blood powder, a part of nitrogen was retained during the pyrolysis process, indicating that there may be some nitrogen-containing functional groups on the surface of BN-AC. There is a small amount of magnesium because BN-AC is prepared by impregnation 2:1 with magnesium chloride as the activator. Some magnesium ions enter into the structure of BN-AC in the activation process, which is not easy to clean, so they are deposited on the surface of BN-AC or in the pores.
Table 1
Elemental Analysis of precursor and BN-AC
Sample
|
C/dw%
|
N/dw%
|
O/dw%
|
H/dw%
|
Mg/dw%
|
precursor
|
49.443
|
11.846
|
26.014
|
12.697
|
-
|
BN-AC
|
54.625
|
18.244
|
15.833
|
9.282
|
2.016
|
From the elemental analysis of precursor: - indicates that the ingredient is not detected |
By Boehm titration analysis
The surface chemistry of the functional groups in this study, which determines the acidity and basicity of the surface oxygen groups of the BN-AC. The Boehm titration results of BN-AC is shown in Table 2. Total basicity of 0.720 mmol/g, and acidity of 0.313 mmol/g of the BN-AC was obtained from the analysis. Thus, there was 2.3 times the number of basic groups as acidic groups. The content of acid and base indicated that the surface of BN-AC contained acidic functional groups containing oxygen and alkaline functional groups containing nitrogen. With alkaline functional groups such as C = N, NH, amino, cycloamide, nitrile and pyrrole groups (Huang et al. 2020; Thue et al. 2017). The existence of basic functional groups is due to the fact that the precursor containing a large number of proteins as raw materials retains more nitrogen content.
Table 2
Boehm titration results of BN-AC
Sample
|
Total amount of acid/mmol g− 1
|
The total amount of alkali/mmol g-1
|
Carboxyl/
mmol g-1
|
Ester base carbonyl/
mmol g-1
|
BN-AC
|
0.313
|
0.720
|
0.357
|
0.175
|
By FTIR spectroscopy
Infrared spectra are generated by the transition of vibrational energy levels (accompanied by rotational energy levels) of molecules. It can be widely used in the characterization of the type and number of surface functional groups on activated carbon. The surface chemical properties of activated carbon are mainly determined by the type and number of surface functional groups, while the functional groups on the microporous surface of activated carbon are mostly oxygen-containing functional groups, such as carboxyl, phenolic hydroxyl, carbonyl, ester, ketone, ether, etc (Nina et al. 2020; Yousif Mohammed et al. 2018.).
For more investigation on the adsorbents surface properties, Fig. 2 shows the infrared spectrum curve of precursor and BN-AC. The precursor and BN-AC display a number of spectral features in the range of wavenumber between 4000 − 400 cm− 1. The BN-AC show that the characteristic broad band at ca 3415 cm− 1 can be assigned to N-H and/or O-H stretching vibration. The weak band at 1624 cm− 1 is attributed to the distinctive absorbance of C-H bonds of benzene rings as well as the C = N bonds from the carbon framework (Alabadi et al. 2016). The peaks at 1233 cm− 1 suggest the presence of the BN-AC stretching vibration. The broad peak of 1099 cm− 1 is associated with C-N stretching vibration. The FTIR analysis, therefore, confirms the existence of N-H and C-N species in the carbon samples. It still has some nitrogen-containing groups. -NO2 symmetric stretching vibration at wave number 1387 cm− 1of precursor (Tadepalli et al. 2021). The precursor and BN-AC had similar peaks, with significant differences at the peaks of 3415 cm− 1 and 1624 cm− 1, related to the O-H, N-H, NH2, C = C, C = N, C-N (Wang et al. 2018) which is caused by the gradual increase of carbon net and the appearance of carbon-containing functional groups under the catalysis of magnesium chloride at the end of activation of precursor.
By XPS analysis
X-ray photoelectron spectroscopy (XPS) is an effective method for detecting surface chemical structure. XPS was used to qualitatively analyze the functional groups on the surface of activated carbon (Liu et al. 2020). Figure 3 (a) is the wide full XPS spectra of precursor and BN-AC. As seen from the graphs, characteristic peaks of C1s, O1s, N1s are found in precursor and BN-AC, witnessing that N have been successfully retain on BN-AC. The peaks located in 285.56, 399.26 and 532.35 eV are corresponding with C1s, N1s and O1s, respectively. The Mg1s peak appeared in raw material at 1 305.52 eV. The Mg1s peak appears because BN-AC is activated by magnesium chloride with a concentration of 65% as the activator. During the activation process, magnesium ions enter into the structure of BN-AC and are not easy to be cleaned, so they are deposited on the surface or pores of BN-AC. This is because at relatively high temperatures, the aromatic rings of nitrogen are partially stable at the edges of the graphite layer of the carbon material. With the further increase of temperature, nitrogen gradually enters into the skeleton of BN-AC and plays a dominant role (Yao et al. 2020).
As shown in Fig. 3 (c), there is only peaks that locate at 399.48 eV (pyrrolic N) in precursor. The N1s XPS spectra of BN-AC could be deconvoluted into three types of N-containing compounds, and results are depicted in Fig. 3 (b). The peaks of N1s located in 398.709 (pyridinic N), 399.45 (pyrrolic N) and 400.15 (graphitic N) (Yang et al. 2019), respectively in BN-AC. Compared with the N1s XPS spectra of precursor and BN-AC, it was found that the preparation of BN-AC from blood containing protein would increase the types and number of nitrogen-containing functional groups in BN-AC. Pyrrolic N is converted into three nitrogen-containing groups, namely 398.709 eV (pyridinic N), 399.45 eV (pyrrolic N) and 400.15 eV (graphitic N). The O1s XPS spectrum of the BN-AC shown exhibits three peaks in Fig. 3 (d). at 530.44 eV, 531.539 eV and 533.04 eV, corresponding to (C = O), (C-O) and (C-O-C). Figure 3 (e) shows high-resolution C1s XPS spectrum of BN-AC, which can be separated into two peaks at 284.662 eV(C-C) and 286.739 eV(C = N) (Oluwatosin et al. 2019).
Table 3 shows the binding energy and specific area of precursor and BN-AC. From Table 3, in the specific functional groups in BN-AC, the proportion of nitrogen-containing groups increased significantly. The N contents of pyrrolic N,pyridinic N and graphitic Nincreased by 42.5%, 31.03%, 5.14%.Using waste blood (containing a lot of protein) as raw material to prepare activated carbon will increase the types and quantity of nitrogen-containing functional groups in activated carbon. In the presence of precursor, which can act as a nitrogen source, in turn cause nitrogen content to increase. For this reason, the N content of the precursor was successfully preserved.
Table 3
Binding energy and proportion area of BN-AC and precursor functional groups
Functional groups
|
Precursor Binding Energy/eV
|
Precursor Accounted for area/%
|
BN-AC
Binding Energy/eV
|
BN-AC
Accounted for are /%
|
C-C
|
285.75
|
56.15
|
284.662
|
42.99
|
C = N
|
286.75
|
9.69
|
286.739
|
13.16
|
C-O-C
|
287.58
|
4.88
|
287.61
|
8.01
|
pyrrolic N
|
399.48
|
18.58
|
399.45
|
61.08
|
pyridinic N
|
-
|
-
|
398.709
|
31.03
|
graphitic N
|
-
|
-
|
400.15
|
5.14
|
C = O
|
531.76
|
3.15
|
530.44
|
25.49
|
C-O
C-O-C
|
531.059
533.26
|
74.30
3.70
|
531.539
533.04
|
63.20
11.32
|
- Indicates that the sample is not tested
|
Textural characterization
Surface area
The pore size of BN-AC is 1.688 nm, belonging to micropore (< 2 nm). The specific area of blood powder was 0.826 cm3/g, and the specific surface area of BN-AC and micropores were 283.719 m2/g and 135.036 m2/g, respectively. The precursor has almost no pore size and specific surface area, and the specific surface area increases by 343.5 times after activation with magnesium chloride solution to prepare BN-Ac. Table 4 shows that the waste blood containing protein can successfully prepare bio-nitrogen doped activated carbon.
Table 4. Specific surface area and pore structure parameters ofprecursor and BN-AC
Sample
|
SBET/m2g-1
|
Smicro/m2 g-1
|
Vtotal/cm3g-1
|
Vmicro /cm3 g-1
|
D/nm
|
precursor BN-AC
|
0.826
283.720
|
-
135.036
|
-
0.289
|
-
0.128
|
-
1.688
|
SBET, total specific surface area;Smicro, specific surface area of micropores;Vtotal, total pore volume;
Vmicro, micropore volume;-, is not tested.
|
By SEM analysis
The morphology including the porosity of the prepared BN-AC could be clearly observed from their SEM micrographs, as shown in Fig 1. Without any activation, BN-AC from blood(a) shows compact and smooth surface with little pores. On the surface of BN-AC (b) activated by magnesium chloride, there are small particles and unevenly distributed, and a small amount of floc structure with loose structure. The loose structure will make the microporous structure become developed.Results of SEM analysis indicate that BN-AC with different porosity have been successfully prepared.
Determination of adsorption performance
The methylene blue adsorption value represents the decolorization ability of the adsorbent, and the methylene blue adsorption value is also an important index for the characterization of the liquid adsorption performance of activated carbon. Iodine adsorption value indicates the developed degree of micropores greater than 1.0 nm of activated carbon, which is the performance of the adsorption capacity of activated carbon to small molecule impurities.
Table 5. Determination results of BN-AC adsorption values
Sample
|
Methylene blue adsorption value (mg/g)
|
Iodine adsorption value(mg/g)
|
precursor
BN-AC
Ordinary AC
|
0
600.00
450.00
|
0
734.95
680.24
|
Table 5 is the adsorption value measurement results of the precursor, BN-AC and ordinary activated carbon.The precursor has no adsorption property due to its small specific surface area.The methylene blue adsorption capacity of BN-AC is 600 mg/g, and the iodine adsorption capacity is 734.95 mg/g.The adsorption capacity of methylene blue on normal activated carbon is 450 mg/g, and the adsorption capacity of iodine is 680.24 mg/g. The methylene blue and iodine adsorption values of BN-AC were higher than those of ordinary AC, which indicated that the prepared bio-nitrogen doped activated carbon had potential adsorption properties.