Characterization of the synthesized films
Porous P-PAN film was firstly synthesized to perform as the porous structure with highly efficient adsorption sites. The immobilized benzoic acid was removed by sodium hydroxide forming porous structure PAN film. The film of NH2[email protected] was obtained with 10% and 30% CP contents. The mechanism of NH2[email protected] synthesis could be hypothesized as suggested in Figure 1. The nitrile groups of PAN molecules can by chemically interacted with the Ti-CP through several interactions. The nitrogen atom of nitrile groups in PAN may form coordination and hydrogen bonds with the titanium atom and hydrogen of amino groups in the Ti-CP, respectively [38]. Moreover, π - π interactions can take a place between the groups of nitrile in PAN and aromatic nucleus of 2-aminoterephethalic acid in Ti-CP.
The content of Ti-CP within the P-PAN films were estimated and recorded in Table 1. The evaluated contents of Ti-CP were 106.5 mg/g for 10% NH2[email protected] film and 288.2 mg/g for 30% NH2[email protected] film. The real contents (10.65 % and 28.85%) of Ti-CP are quite close to that theoretically added. The amounts of Ti metal in the films were evaluated by using the chemical structure of NH2-MIL-Ti. Ti8C48H34O36N6 is the chemical formula for NH2-MIL-Ti and consequently, the percentage of Ti in the Ti-CP is 23.2%. Based on such calculations, the Ti contents were calculated to be 24.7 and 66.9 mg/g in case of 10% NH2[email protected] and 30% NH2[email protected] films, respectively.
The synthesized films (P-PAN, 10% NH2[email protected] and 30% NH2[email protected]) were examined under the electronic microscope. Figure 2 showed that highly porous structure was observed for PAN after removing of organic acid. The surface of P-PAN film is beehive like shaped. The diameter of the seen pores was measured and was ranged in 8.5 – 51.2 µm which confirmed the macro porous structure of the produced film. The pore diameter can be divided to two regions of 8.5 – 22.6 µm and 31.8 – 51.2 µm. For NH2[email protected] film, crystalline Ti-CP were densely distributed over the surface of the P-PAN film and filled the small pores. Crystalline disc structure of NH2-MIL-Ti was seen onto the P-PAN film surface with size dimensions of 84.0 – 98.0 x 167.9 – 654.1 nm. The amount of NH2-MIL-Ti onto the surface of P-PAN film was increased by increment of the impregnated ratio from 10–30%. The EDX analysis showed the signals of C and N for P-PAN film. While, the signals of O and Ti were both recorded besides those of C & N in case of NH2[email protected] film which further confirmed the incorporation of Ti-CP within the film.
The XRD for the as-synthesized films were shown in Figure 3 for more confirmation to the successive preparation of the as-required films. For P-PAN film, two diffractions at 2θ° = 17.2° (strong) and 29.3° (weak) characterized for (100) and (110) crystalline indices of PAN [39, 40]. NH2-MIL-Ti powder showed many characterized diffractions at 2θ° = 6.6°, 9.8° and 11.7°, 15.3°, 16.7°, 18.0°, 19.6° 21.6°, 22.7° and 25.4° which were strongly matched with the diffractions in literature [41, 42]. For NH2[email protected] films, the diffraction patterns of NH2-MIL-Ti were observably obtained beside those of PAN which further confirming the successive immobilized of NH2-MIL-Ti in the P-PAN film. The intensity of NH2-MIL-Ti diffraction was higher in case of impregnation higher Ti-CP contents.
The chemical interaction between P-PAN and NH2-MIL-Ti in the as-synthesized films was illustrated via FTIR spectra (Figure 4a). From the plotted spectra, it could be depicted that, P-PAN film was shown with three main absorption peaks at 2918-2844 cm−1, 2240/1728/1659 cm−1 and 1443-1226 cm−1. The detected peaks are attributed to the aliphatic C-H, the nitrile group and CH2 & CH3 groups which referring to the chemical structural of PAN [43, 44]. For NH2-MIL-Ti powder, four prime peaks were recorded for NH group at 3427 cm−1, OH groups at 3318 cm−1, carboxylate group of 2-aminoterephthalic acid at 1659/1537 cm−1 and Ti-O bond at 752 cm−1 [45, 46]. In case of NH2[email protected] film, the characterized absorption peaks of NH2-MIL-Ti were significantly observed beside that of PAN. The intensities of the absorption peaks for NH2-MIL-Ti became stronger at impregnation higher Ti-CP content in the film. These spectral mapping data are further support the data of XRD diffraction and microscopic observations.
Removal of carbofuran
The so-prepared films (PAN, P-PAN, 10% NH2[email protected], 30% NH2[email protected]) were employed in the adsorptive removing of carbofuran insecticide. Firstly, FTIR spectral mapping data (Figure 4b) were illustrated for approval of the adsorption processing using the prepared films. After adsorption of carbofuran onto the NH2[email protected] film, three new base absorption peaks were observed at 3356 cm−1, 2972 cm−1, 1519 cm−1 and 868 cm−1. These peaks are characterized for NH, CH aliphatic, C=C of aromatic and CH aromatic for the carbofuran molecule [26, 28]. Moreover, the amide-C=O was overlapped with that for CN of PAN. These interoperations affirmed the adsorption of carbofuran onto the as-applied NH2[email protected] film.
The adsorptive removal of carbofuran was systematically monitored via the investigation of the characteristic absorption peak at 290 nm. Adsorption capacities of the carbofuran were detected with time and plotted in Figure 5. The plotted data showed that, adsorption of carbofuran using the synthesized films was enhanced by prolonging the duration and the adsorption was fast in the first adsorption hour, to be subsequently slowed down with prolonged duration. The adsorption capacities of carbofuran onto the applied films were followed the order of PAN < P-PAN < 10% NH2[email protected] << 30% NH2[email protected] Whereas, within only 2.5 hours, almost all carbofuran were removed by using 30% NH2[email protected] film. In the first hour, the removal amounts of carbofuran were 86.6, 126.6, 180.0 and 300.0 mg/g when PAN, P-PAN, 10% NH2[email protected] and 30% NH2[email protected] films were applied, respectively. The removal amounts of carbofuran onto the corresponding film were increased up to 110.0, 153.3, 220.0 and 360.0 mg/g, respectively after 2.5 hours of contact time. The lowest removal capacity was recorded for PAN due to the limited interaction with carbofuran and absence of porous structure. While, the removal was slightly improved by formation of porous film of P-PAN. The adsorption of carbofuran onto the exploitable P-PAN film was progressively increased by immobilization of NH2-MIL-Ti. Removal was observably enhanced by increment the impregnated NH2-MIL-Ti, owing to increasing of the reactive adsorptive sites.
Eventually, the kinetic parameters for adsorption removal of carbofuran onto the synthesized films were carried out for nonlinear model of pseudo-1st ordered and pseudo-2nd ordered (Figure 5). For examination the fitting of adsorption data, the value of correlation coefficient (R2) and Chi-squared test (x2) were measured and all parameters were tabulated in Table 2. Attributing to higher values of R2 and smaller values of x2, the adsorption of carbofuran onto the films was well described by pseudo-2nd ordered modelling which means that the removal of carbofuran depended on the concentration of carbofuran and amount of applied film. Additionally, attributing to the observable increasing of adsorption sites, the rate constant (k2) was considerably reduced from 2.9 x 10−4 L/mg.min for P-PAN film to 1.3 x 10−4 L/mg.min for 30% NH2-MIL[email protected] film. This observation could mean that the rate of carbofuran removal using the synthesized films was accelerated by 55% when 30% of NH2-MIL-Ti immobilized within the P-PAN film.
Table 2
Adosprtion parameters of adsorption kietics and isotherm for carbofruan onto the synthesized films.
Model
|
Parameter
|
PAN
|
P-PAN
|
10% NH2[email protected]
|
30% NH2[email protected]
|
|
Qe exp.
(mg/g)
|
110.0
|
153.3
|
220.0
|
360.0
|
Pseudo-first order
|
Qe (mg/g)
|
105.3 ± 1.9
|
148.8 ± 1.8
|
211.4 ± 3.1
|
347.4 ± 6.9
|
K1 x 10−3
(min−1)
|
32.4 ± 2.3
|
35.6 ± 1.7
|
36.1 ± 2.2
|
40.9 ± 3.6
|
R2
|
0.98
|
0.99
|
0.99
|
0.98
|
χ2
|
12.6
|
11.6
|
37.4
|
213.7
|
Pseudo-second order
|
Qe (mg/g)
|
127.1 ± 1.4
|
177.3 ± 1.1
|
250.8 ± 1.6
|
403.4 ± 4.9
|
K2 x 10−4
(L/mg.min)
|
2.9 ± 0.2
|
2.4 ± 0.1
|
1.7 ± 0.1
|
1.3 ± 0.1
|
R2
|
0.99
|
0.99
|
0.99
|
0.99
|
χ2
|
1.5
|
1.1
|
2.6
|
29.2
|
Freundlich isotherm
|
n
|
2.9 ± 0.5
|
2.5 ± 0.2
|
2.2 ± 0.1
|
2.7 ± 0.3
|
KF
|
12.5 ± 2.2
|
12.8 ± 2.9
|
15.5 ± 2.8
|
45.2 ± 5.4
|
R2
|
0.92
|
0.97
|
0.98
|
0.96
|
χ2
|
223.5
|
225.1
|
326.2
|
2005.9
|
Langmuir isotherm
|
Qmax
(mg g−1)
|
160.8 ± 4.2
|
292.2 ± 5.8
|
489.5 ± 13.3
|
690.1 ± 13.1
|
KL x 10−3
(mg/L)
|
4.7 ± 0.4
|
3.0 ± 0.2
|
2.6 ± 0.2
|
5.1 ± 0.3
|
R2
|
0.99
|
0.99
|
0.99
|
0.99
|
χ2
|
19.7
|
21.6
|
82.5
|
157.6
|
The nonlinear isotherm fitting for carbofuran adsorption onto the as-synthesized films was investigated in accordance to Langmuir and Freundlich modelling (Figure 6). While, the calculated parameters of both isotherms and the estimated R2 & x2 were all presented in Table 2. According to the values of x2 and R2, the adsorptive removal of carbofuran was well fitted to Langmuir isotherm rather than Freundlich modelling. Langmuir isotherm clarified that the adsorption of carbofuran onto the applied films performed through monolayer formation [47]. The calculated maximum capacities (Qmax, mg/g) for carbofuran adsorptive removal were greatly enlarged from 160.8 mg/g for PAN to 292.2 mg/g for P-PAN and to 489.5 – 690.1 mg/g for NH2[email protected], respectively. The maximum carbofuran adsorption capacity of P-PAN film was enhanced by 1.7 and 2.4 times at incorporation of 10% and 30% NH2-MIL-Ti, respectively. The obtained findings approved the superiority of 30% NH2[email protected] film in the removal of carbofuran.
Adsorptive removal of carbofuran onto the applied films can be described by physical and chemical adsorption. The physical adsorptive removal might be carried out by the pores of P-PAN film and pores of NH2-MIL-Ti as a kind of coordination polymer which characterized by its porous framework. The higher adsorption capacity of P-PAN than that of PAN confirmed this hypothesis. For chemical adsorption, the reactive groups of films (CN of P-PAN and COO & Ti in CP) could be chemically interacted with the functional groups of carbofuran (N, O) as suggested in Figure 7. Coordinating bonds formed between Ti in film and N & O in carbofuran, while, N of CN and O of COO in the film interacted by H- bonds with H of N in carbofuran [26, 37, 38]. Furthermore, the aromatic ring of 2-aminoterphethalic acid can compose π – π interactions with the benzene ring of carbofuran [26, 48, 49]. Due to the several functional groups as reactive sites, NH2[email protected] film exhibited extremely greater adsorptive removal of carbofuran comparing to P-PAN film. This postulation further emphasized by the observable increment in the adsorptive removal capacity of carbofuran with increasing the immobilized content of NH2-MIL-Ti from 10–30% in the film. In accordance to Langmuir isotherm, monolayer adsorption was implemented by a limit of adsorption in the active sites of film including pores and the accessible functional groups [50]. Consequently, each active site in the applied film was full by only one carbofuran molecule resulting of one layer. The pseudo 2nd ordered model declared that the carbofuran adsorption onto the applied film was relied on the contents of the active sites (pores and functional groups) and the concentration of carbofuran [47]. Subsequently, the adsorptive removal capacity of carbofuran could be further improved by immobilization much higher NH2-MIL-Ti content within the film.
In order to highlight the superiority of the currently prepared NH2[email protected] film in the adsorptive removal of carbofuran (690.1 mg/g), comparative overview for series of adsorbents in literature was presented in Table 3. Many of the summarized adsorbent materials (indian soils, slow pyrolyzed sugarcane bagasse biochar, animal bone meal, tea waste biochars, magnetic peanut shell, granular activated carbon, commercial activated carbon) showed very low adsorption capacity for carbofuran (0.9 – 97.1 mg/g) [22–28]. Much high carbofuran adsorption was obtained (137.0 – 312.5 mg/g) at using date seed activated carbon, modified maize cobs waste, steam-activated biochar, palm oil fronds activated carbon, magnetic sugarcane bagasse, mesoporous activated carbon, steel industry wastes, activated carbon from rice straw and rice straw-derived activated carbon [28, 51–58]. However, the presently synthesized 30% NH2[email protected] film exhibited extremely greater carbofuran adsorption than that of all the adsorbents reported in Table 3 by factor of more than 2. Additionally, NH2[email protected] film was advantageously characterized by the recyclability efficiency. Therefore, the currently obtained NH2[email protected] film is an excellent adsorptive film for removal of carbofuran with higher applicability and hence it could be more desirable for removal of different insecticides.
Table 3
Adsorption capacity for carbofuran onto different adsorbents reported in literature.
Adsorbent
|
Qmax
(mg/g)
|
Reference
|
Reference
|
30% NH2[email protected]
|
690.1
|
Current work
|
Current work
|
Indian soils
|
0.9 – 4.9
|
18
|
Krishna, K. R., & Philip, L., 2008
|
Slow pyrolyzed sugarcane bagasse biochar
|
3.6 – 18.9
|
21
|
Vimal, V., et al., 2019
|
Animal bone meal
|
18.5
|
20
|
Roudani, A., et al., 2014
|
Tea waste biochars
|
22.8 – 54.7
|
22
|
Mayakaduwa, S., et al, 2016
|
Magnetic peanut shell
|
89.3
|
23
|
Toledo-Jaldin, H. P., et al., 2020
|
Granular activated carbon
|
96.2
|
17
|
Salman, J., & Hameed, B., 2010
|
Commercial activated carbon
|
97.1
|
19
|
Salman, J., et al., 2011a
|
Date seed activated carbon
|
137.0
|
52
|
Salman, J., et al., 2011b
|
Modified maize cobs waste
|
149.2
|
51
|
Foo, K., 2016
|
Steam-activated biochar
|
160.8
|
56
|
Mayakaduwa, S., et al., 2017
|
Palm-oil-fronds-activated carbon
|
164.0
|
54
|
Salman, J. M., 2013
|
Magnetic sugarcane bagasse
|
175.0
|
23
|
Toledo-Jaldin, H. P., et al., 2020
|
Mesoporous activated carbon
|
193.1 – 205.0
|
50
|
Njoku, V., et al., 2014
|
Steel industry wastes
|
208.0
|
49
|
Gupta, V. K., et al., 2006
|
Activated carbon from rice straw
|
296.5
|
55
|
Chang, K.-L., et al., 2011
|
Rice straw-derived activated carbon
|
222.2 – 312.5
|
53
|
Chang, K.-L., et al., 2014
|
Recoverability
The four synthesized films (PAN, P-PAN, 10% NH2[email protected] and 30% NH2[email protected]) were utilized in the study the recyclability for carbofuran adsorption up to six repetitive cycles. Figure 8 showed that carbofuran adsorption by the applied films was slightly reduced after reusing which is due to the leaching of some of NH2-MIL-Ti from the film during the adsorption/desorption process. For 30% NH2[email protected] film, the adsorptive removal capacity was decreased from 360 mg/g to 315 mg/g after six reusing cycles which means that the adsorption of carbofuran was lowered by only 12.5 % when re-using for six repetitive cycles. These could explain that the employed NH2[email protected] film exhibited a superior recyclability in carbofuran removal that approved its wide exploitability in removing of insecticides. The synthesized NH2[email protected] film might be promisingly employed in removal of toxic & hazardous materials from water with highly applicability which support its application in environmental purposes.