Characterization of thin film
SEM micrographs of Ag0(NP)/TiO2 are shown in Figure 1(a). The result showed that small sized TiO2 particles are distributed uniformly onto the silicate glass and formed a uniform thin film of nanocomposite. However, at places the cracks are observed onto the surface. Further, it is interesting to observe that the titanium dioxide is not agglomerated on the surface. This is because of the template synthesis to titania. The EDX analysis of the thin film sample was conducted and shown in Figure 1(a)(Inset). It is evident from the figure that the silver is incorporated within the titania network. Similarly, the TEM image of the nanocomposite is shown in Figure 1(b). The high resolution TEM image clearly showed the fringes of Ag nanoparticles and it is uniformly distributed with TiO2 structure. Moreover, the interplanar distance of the Ag0(NP) is estimated as 0.15 nm.
Similarly, the AFM image of nanocomposite thin film is shown as in Figure 1(c). AFM image inferred that the surface of thin film of photocatalystis highly uneven or heterogeneous in nature and the surface is possessed with small-sized pillars of titanium dioxide having maximum height of Ca. 350 nm. Further, the root mean square roughness (Rq) and mean roughness (Ra) of thin film is 16.952 nm and 12.250 nm, respectively.
Photocatalytic removal of MO1
pH dependence removal
Effect of pH is an useful parameter that demonstrates the mechanism of photocatalytic elimination of pollutants. This is mainly due to the pH dependence transitions in pollutant species as well as the surface properties of active sites of photocatalysts (Lalhriatpuia et al. 2015). Therefore, the pH influenced the overall efficiency of the photocatalyst in the degradation of pollutant molecules (Zucca et al. 2008). Photocatalytic elimination of MO1is carried out for a wide range of pH (4.0-10.0) and results are depicted as in Figure 2. The percentage removal of the MO1 was obtained after the 2 hr irradiation. Results clearly showed that the decrease in pH i.e., pH 10.0 to 4.0 has caused to increase the elimination of MO1. Quantitatively, with an decrease in solution pH from 10.0 to 4.0, the corresponding increase in percent removal of MO1 is from 29.15 to 61.3% (for UV-A) and 16.65 to 35.81% (for LED light) using nanocomposite thin film photocatalyst. The MO1 molecule contains two dissociable hydrogens one from the carboxylic group and second is due to the phenolic group which are having the pka values of 5.0 and 11.0 (Nazar et al. 2010).Therefore, within the pH region 6.0-10.0, the MO1 molecule usually exists as mono-anionic species. However, the phenolic and carboxylic groups forming the hydrogen bonding resulting a non-ionic species. Hence, in this pH range the MO1 molecule exists in equilibrium between the the mono-anionic species (Cf Figure 2 (Inset)) (Nazar et al. 2010).Further,pH >10.0, both the protons of MO1 molecule are dissociated hence, the mono-anionic MO1 molecule becomes di-anionic species. Thus, the net charge of the MO1 species greatly becomes negative at and above pH 10.0. On the other hand, the pHpzc of nanocomposite was obtained to be 6.8 (Tiwari et al. 2020). This infers that the thin film possesses net positive charge pH <6.8 whereas its surface possesses negative charges pH >6.8. These studies showed that increasing the pH gradually enhances the net negative charges both on the nanocomposite surface and MO1 species. This eventually caused an enhanced coulombic repulsions between the MO1 species and nanocomposite surface which resulted in gradual decrease of MO1 removal at pH>6.0 (pH 6~10). Photocatalytic removal of Alizarin Yellow using the catalyst Au0(NPs)/TiO2 thin film showed similar trends (Lalliansanga et al. 2019b). Further, reaction was conducted employing UV-A and LED illuminations without using nanocomposite thin films at a wide range of pH (pH 4.0-10.0). It was observed that a negligible amount of MO1 molecules was degraded after 2 hr of contact. These results clearly inferred that the nanocomposite catalyst greatly favoured the elimination of MO1 in aqueous media by UV-A or LED lights. Further, compared to the UV-A and LED light illumination, the UV-A light showed relatively higher degradation of MO1 throughout the studied pH.
Concentration dependence removal of MO1
Concentration dependence elimination of MO1 is analysed for varied concentration of MO1 (0.5 - 20.0 mg/L; pH 6.0). Removal efficiency of MO1 is illustrated in figure 3. Figure 3 demonstrates that decrease in concentration greatly favoured removal efficiency of MO1. Decreasing concentration of MO1 from 20.0 to 0.5 mg/L had enabled to increase the percentage degradation of MO1 from 13.7 to 60.6% (for UV-A) and 6.5 to 38.14% (LED light), respectively. The decrease in degradation efficiency with an increase in concentration of MO1 is, perhaps, due to the reason that the contact possibility of MO1 at the photocatalyst surface was relatively less at higher concentration of MO1 dye (Tiwari et al. 2015). It is also evident that at high concentration of MO1, the scavenging effect increases which possibly results in decrease of percentage removal of the MO1 (Nasseri et al. 2017).Furthermore, dark reaction is performed employing the catalyst Ag0(NPs)/TiO2 at various concentrations of MO1 (0.5–20.0 mg/L; pH 6.0). It was observed that almost negligible amount of MO1 was removed even after 12 hr of contact. Therefore, this indicates that no surface adsorption is taking place using the nanocomposite thin film for at least for MO1.
Kinetic study of MO1 degradation
The time dependence kinetics is carried out to assess the performance of nanocomposite photocatalyst in the elimination of MO1.The kinetic results are at varied concentrations of MO1 is shown in Table 1. It is observed that degradation of MO1 is proceeds through pseudo-first-order rate equation (Lalliansanga et al. 2019b). Further, the time dependent degradation kinetics of MO1 is favoured with the dilution i.e., increasing the concentration of the dyes result in decrease in the rate constant values. Moreover, the rate of degradation of MO1 is faster using the UV-A light irradiations compared to the LED light irradiations. Similar data is reported earlier where methylene blue was degraded using graphene decorated TiO2 (Acosta-Esparza et al. 2020).
Table 1 Pseudo-first order rate constants in the photo-catalytic removal of MO1 employing nanocomposite thin film employing under UV-A and LED light irradiations [pH of solution: 6.0].
Initial Concentration of MO1
(mg/L)
|
UV-A Irradiation
|
LED Light Irradiation
|
Rate constant
k1x10-3
|
R2
|
Rate Constant
k1x10-3
|
R2
|
0.5
|
7.0
|
0.965
|
4.1
|
0.998
|
1.0
|
5.9
|
0.993
|
3.4
|
0.982
|
5.0
|
4.0
|
0.987
|
2.5
|
0.999
|
10.0
|
2.9
|
0.9895
|
1.8
|
0.998
|
15.0
|
2.0
|
0.979
|
1.1
|
0.995
|
20.0
|
1.4
|
0.952
|
0.5
|
0.994
|
Additionally, the removal of MO1 is modelled to the Langmuir–Hinshelwood (L-H) rate kinetics using the standard equations (Lalliansanga et al. 2019b). Therefore, the Langmuir–Hinshelwood (L-H) adsorption constant (kr;mg/L/min) and the reaction rate constant (K; L/mg) are computed as 0.038 and 0.202 (R2: 0.997 for UV-A) and 0.018 and 0.255 (R2: 0.976 for LED), respectively.
Mineralization of MO1
Theextent of mineralization of pollutants in the photocatalytic treatment demonstrates the efficiency of operation. The percentage mineralization of MO1 is obtained at a wide range of concentration (1.0 to 20.0 mg/L; pH~6.0). Results are shown as in Figure 4(a). Results indicated that decrease in concentration of MO1 from 20.0 to 1.0mg/L had caused to increase the percentage mineralization of MO1 from 7.72 to 27.38% (for UV-A light) and from 4.12 to 17.1% (for LED light), respectively. The single reactor operation enabled to mineralize significantly the percentage mineralization of MO1. The finding are similar to the concentration dependent studies conducted previously in the removal of MO1. On the other hand, the photolysis using the UV-A or LED irradiation showed almost negligible degradation of MO1. Therefore, the photocatalytic operations favoured the mineralization of MO1 using the nanocomposite thin film photocatalyst (Tiwari et al. 2015).
Repetitive use of thin film photocatalyst
The efficiency of the photocatalyst largely depends on its reusability for successive reactor operations. Therefore, the reusability of nanocomposite thin film photocatalyst is carried out for repeated reactor operations in the elimination of MO1 (MO1 concentration: 5.0 mg/L; pH 6.0). The percentage elimination of MO1 with effect of number of cycles of reactor operations are shown as in Figure 4(b). The degradation efficiency of the photocatalyst is almost unaffected even at the end of six successive cycles of reactor operations,. Quantitatively, the percentage degradation of MO1 is decreased only from 40.29% to 40.23% (i.e., 0.06%). The results showed that thin film catalyst is reasonably stable towards the reactor operations in photocatalytic degradation of MO1. Hence, the photocatalyst is shown to be employed for prolonged and sustainable operations.
Simultaneous presence of co-ions
Applicability of thin film catalyst is further assessed in presence of variety of co-ions viz., glycine, oxalic acid, NaNO3, NaCl, CuSO4, NiCl2 and EDTA. The initial concentration of MO1 and co-existing ions was taken to be 5.0 mg/L and 50.0 mg/L, respectively (pH 6.0 and UV-A illumination for 2h). The removal efficiency of MO1 for simultaneous presence of co-ions is shown in Figure 5(a). The removal efficiency of MO1 is affected to by the NaCl, EDTA and glycine. However, the other ions introduced have not affected significantly the degradation of MO1 in the photocatalytic reactor operations.
Degradation mechanism
It is known that the photons carrying enough energy (hυ) may generate electron and hole pairs
(Selvaraj and Li 2006). However, titanium dioxide possessed wide band gap energy (3.2 eV) hence, excited by the photons at UV region only. However, the doping of titanium dioxide by noble metals (Ag or Au) lowers apparent band gap energy. Noble metal nanoparticles generates a phenomenon called localised surface plasmon resonance which enables TiO2 photocatalyst to absorb light within the visible region and enhances the photo-excitation of electrons (Zangeneh et al. 2015). It also traps the newly generated electrons thus helping in reducing the charge recombination rate and allow it to proceed further for photocatalytic reaction. Therefore, in order to demonstrate the possible mechanism involved in the elimination of MO1 using the nanocomposite photocatalyst, the investigation was extended in presence of several scavengers.2-propanol and HCO3− compounds are known •OH radical scavengers (Xu et al. 2015; Lalhriatpuia et al. 2016); whereas EDTA scavenges the h+ of photocatalyst (Jia et al. 2016). Similarly, the sodium azide traps singlet oxygen which are generated in reaction of O2−• with h+ (Xu et al. 2015). Thus, the degradation of MO1 (5.0 mg/L; pH 6.0) in presence of these scavengers is carried out using the UV-A irradiation for 2h. The degradation efficiency of MO1 is shown as in Figure 6(b). The results inferred that the 2-propanol, HCO3− and sodium azide are greatly hampered the percentage degradation of MO1. This indicated that hydroxyl radicals are primarily involved in the degradation process. Additionally, NaN3 suppressed the removal efficiency of MO1, inferred that singlet oxygen is taking part in photocatalytic degradation reaction. Hence, it infers that the elimination mechanism of MO1 proceeds in two different possible pathways. First, the Ag(NPs) traps the newly generated electrons which helps in inhibiting the charge recombination and allow it to go further for reaction to form peroxide radical and hydroxyl radicals (Ahmad et al. 2016). Peroxide radicals are generated at the conduction band through interaction of trapped electrons and oxygen in presence of water while hydroxyl radicals are generated at the valence band through the interaction of the H2O and oxygen (Akpan and Hameed 2009). Both the radicals then interacted with the pollutant on the vicinity of the thin film photocatalyst which results in the degradation of MO1 (Vogna et al. 2004; Tiwari et al. 2019). The other possible pathway is that Ag(NPs) absorbs light radiations resulting the generation of electromagnetic field as because of localized surface plasmon resonance (Lee et al. 2014). Further, Schottky barrier present with metal-semiconductor causes the excited electrons to move toward the electric field and the holes towards the opposite direction of the electric field. This inhibits the recombination of electron-hole pairs (Z et al. 2014). Further, the electron-hole pairs undergo further reaction to form peroxide radical and hydroxyl radical which take part in the degradation of MO1.
Real water treatment
Applicability of fabricated Nano catalyst depends greatly on its utility to real water samples. Hence, intended performance of Nano catalyst thin film in the elimination of MO1 was performed using the MO1 tagged real water samples. Previously, physico/chemical parameters are obtained for this water and shown is Table 2. The real water contained high concentrations of Fe, Ca and Zn. On the other hand, the TOC data showed that real water is contained with high inorganic carbon value with less NPOC value. MO1 was tagged with real water having various concentrations of MO1 (0.5 to 20.0 mg/L; pH 6.0). Further, degradative removal of MO1 is carried out using nanocomposite thin film catalyst under UV-A and LED light illuminations for 2h. Percentage degradation of MO1 was obtained and compared with the result obtained with the purified water samples (Cf Figure 6). The study revealed that the percentage degradation of MO1 is not significantly decreased in real water samples. This signifies the high applicability of nanocomposite thin film catalyst in the removal of MO1.
Table 2Various Physico-chemical Parametric Analysis of Reiek Kai Site, Tlawng River Water.
Parameters Studied
|
Analytical Results
|
pH
|
7.6
|
Conductivity
|
0.0172 S/m
|
Resistivity
|
0.0138 Mohm.cm
|
Salinity
|
0.12 PSU
|
Ox. Red. Potential
|
210.7 mV
|
Elements studied (AAS)
|
(mg/L)
|
Ni
|
0.0
|
Zn
|
0.535
|
Pb
|
0.078
|
Mn
|
0.01
|
Fe
|
0.198
|
Ca
|
2.341
|
Cu
|
0.0
|
TOC Analysis
|
(mg/L)
|
Inorganic Carbon
|
13.94
|
NPOC
|
2.493
|
Anions studied
|
(mg/L)
|
Nitrate
Fluoride
Sulphate
Phosphate
|
11.43
0.0
6.12
0.09
|