Typical Run
A typical run is plotted between 1+ log of optical density (O.D.) and time. The observation data are given in Table 1 and graphically represented by figure 1. A straight line in the plot indicates the rate of reaction to follow pseudo first order kinetics. Kinetic models, pseudo-first-order and pseudo-second-order (Type-1, Type-2, Type-3, Type-4 and Type-5) were selected to explain the degradation data and it was found that pseudo-first-order kinetic model prevailed in degradation process. (Zulfiqar et al. 2021). The reaction conditions obtained for maximum degradation of the dye Azure A are at pH 8.0, dose of photocatalyst 0.22 g, concentration of Azure A 0.2 × 10-4 M and light intensity 74.0 mW/cm2.
Percentage of dye degradation at optimum conditions of parameters is given by:-
\(\text{D}\text{e}\text{g}\text{r}\text{a}\text{d}\text{a}\text{t}\text{i}\text{o}\text{n} \text{E}\text{f}\text{f}\text{i}\text{c}\text{i}\text{e}\text{n}\text{c}\text{y} \left(\text{D}\text{E}\right) \left(\text{\%}\right) =\frac{{\text{A}}_{0}-{\text{A}}_{\text{t}}}{{\text{A}}_{0}}\times 100\) = 75.31 (1)
It is observed that absorbance of the dye solution decreases with time. The maximum rate of degradation is found to be k= 7.67×10-4 (sec-1) and 75.31% degradation of the dye is observed in 40 minutes.
Effect of pH
One of the major factors affecting the rate of degradation of the dye is pH of the solution. Thus it was varied by adding pre standardized NaOH and HCl and all other factors were kept constant. It was observed that with change in pH, the initial optical density of the solution changes, which can also be observed in multiline graphs, suggesting the pH sensitive nature of dyes. The effect of pH was studied in range 5.0 to 10.0 with difference of 0.5, results of which are reported in figure 2 and table 2.
It was observed that rate of photocatalytic degradation increased with increase in pH up to 8.0 and then gradually decreased. It is explained by following reactions.
Electron hole pairs are generated at photocatalyst surface by absorption of photons from light.
ZrCdPbO4 + hⱱ → [e−(CB) + h+(VB)] (Electron-hole pair formation) (2)
The increase in rate is attributed to the availability of more concentration of OH‾ ions in the solution. Holes abstract electrons from hydroxyl anions generating hydroxyl free radicals which are the responsible species causing degradation.
h+(VB) + OH‾ → •OH (Radical formation) (3)
After pH 8.0, decrease in reaction rate is observed because repulsion amongst OH– ions and electrons on photocatalyst surface dominated due to crowd created at the surface. This repulsion forces the recombination of electrons and holes, besides abstracting electrons from OH‾ species. Thus a decrease in the rate of degradation is observed.
Effect of Dose of Photocatalyst
The effect of dose of photocatalyst is studied by varying its weight and with keeping all other factors constant. The range is considered from 0.06 g to 0.26 g having difference of 0.02 g. Figure 3 represents the effect of variation graphically and the data are given in Table 3.
Maximum rate of degradation is observed at 0.22 g of photocatalyst. Further increase in dose of photocatalyst, reduces the rate of reaction. It is explained on the basis that with increase in dose of photocatalyst surface area of particles exposed to light increases and thus formation of number of electron-hole pairs increases. These holes are found responsible for abstraction of electrons from the hydroxyl ion (OH–), producing hydroxyl free radicals (•OH), the responsible species for degradation of the dye. Thus increase in rate is observed. After attaining maximum value (0.22g), rate of reaction decreases because of formation of multilayer of photocatalyst and it forces the recombination of electrons and holes. Thus reduction in rate of degradation is observed.
Effect of Dye Concentration
The effect of concentration of dye on degradation is studied ranging from 0.08 × 10-4 to 1.4 × 10-4 M. The resulting data are shown graphically in figure 4 and tabulated in Table 4. All other factors were kept constant. Dye molecules absorb the light radiations and get excited to singlet state during the reaction and convert to triplet state by inter system crossing.
Dye + hⱱ (Photons) → Dye1 (Singlet excited state) (4)
Dye1 → Dye3 (Triplet excited state) (5)
Dye molecules are attacked by OH free radicals at weaker bond site and breakdown of the molecules is observed.
•OH + Dye3 → Leuco Dye → Degraded Products (6)
The observations show that with increase in concentration of dye, rate of reaction increases. The reason behind is with increase in concentration, more dye molecules are available to absorb photons from light and get excited. Thus rate of degradation increases. After attaining maximum value (0.2 x 10-4 M), increase in concentration of the dye decreases the rate of degradation because beyond the concentration, addition of dye imparts darker colour to the solution and starts acting as filter to the incident light. Thus the desired amount of incident light is not able to reach up to the photocatalyst surface at the base of the vessel; as a result the rate of reaction decreases.
Effect of Light Intensity
Variation of light intensity is carried out in the range 7.0 to 74.0 mW/cm2. The observation data are graphically represented in figure 5 and tabulated in Table 5. All other factors are kept constant.
The rate of photocatalytic degradation increases with increase in intensity of light. This is explained by the fact that with increase in light intensity, number of photons striking per unit area per unit time increases. This increases the number of excited dye molecules and increase in generation of electron hole pair at the surface of photocatalyst is also observed. Thus rate of degradation increases. The rate of degradation is found maximum at light intensity 74.0 mW/cm2. Further rise in intensity of light causes heating of the solution and thus is avoided.
Scavenger study
Scavenger study is carried out to find out the role of active species that cause degradation. Different scavengers like benzoquinone, EDTA, KI and isopropyl alcohol are used and are graphically reported in figure 6. It is evident from the data that addition of isopropyl alcohol (hydroxyl radical scavenger) ceases the reaction and significant reduction in the rate of degradation (up to 82%) is observed proving that the dye degradation is caused by hydroxyl radicals.
LCMS/MS Analysis for Determination of Degradation Pathway
Liquid Chromatography-Tandem Mass Spectrometry (LC-MS/MS) is an analytical method in which fragmentation of any molecule is analyzed by trapping separated ions (Chen et al. 2008). The mechanistic path in degradation of dyes like Methylene Blue, Orange G etc was studied using LCMS technique (Rauf et al. 2010; Meetani et al. 2011; Hisaindee et al. 2013; Kushwaha et al. 2018; Sinha and Ahmaruzzaman. 2015).
LC/MS analysis for degradation of Azure A is carried out on instrument XEVO-TQD#QCA1232 with ESI type, source temperature 144°C, mass range 125 to 1000, duration time 15 minutes and collision energy 3.0. The dye solution with pH 8.0, dose of photocatalyst 0.22 g, concentration of dye 0.2×10-4 M, is exposed to the light with intensity 74.0 mW/cm2 and 10 mL of the solution at different time intervals (7, 14 and 21 minutes) are quenched and separated from the reaction system. These are then subjected to LC-MS analysis. The data are given in Table 6 and figure 7, 8, 9 and 10. The values of m/z are calculated to obtain possible degradation pathway and tentative fragmentation of dye molecule. Further formation of smaller gaseous fragments is ascertained through laboratory experiments.
LC/MS peak analysis suggested that during degradation process, the dye molecules are attacked by •OH free radicals several times, at the weaker bond site and a breakdown of ring as well as of conjugation is observed resulting in formation of smaller fragments.
Initially attack of hydroxyl free radicals at the parent dye molecule generates molecule 4 (m/z 311.05 via 2 and 3 of m/z 257 and 229 respectively, figure 8b). Further attack results in breakdown of the central ring into two fragments 5 and 6 (m/z 203.2 and 159.1 respectively, figure 8a, 8c). Molecule 5 is further attacked by hydroxyl free radicals converting into 5’’ (m/z 185.3, figure 8c) and total double bond break down (TDBB) is observed to form smaller molecules. Molecule 5 also gets converted to molecule 6 and further attack of •OH free radicals causes removal of H2O from the unstable fragment producing molecule 7 (m/z 159.1) followed by break down of ringed species to form molecule 8 (m/z 160, figure 8c) and into smaller fragments like CO2, NH3, H2O etc is observed.
Simultaneously, following other path, molecule 4 breaks into two fragments that are molecule 5’ (m/z 221.2) and molecule 6’ (m/z 130.3, figure 8a, 8c). Molecule 5’ then gets converted to molecule 7’ (m/z 195.2) via molecule 6’, by removal of unstable fragment (figure 8b). Further attack of free radical breaks the ring to form molecule 8’ (m/z 210.1, figure 9). Elemental test in the residual solution proved that no element is left behind, suggesting complete breakdown of the dye molecules in smaller fragments like CO2, SO2, NO2, NH3, H2O etc.
Numbers of peaks are observed at higher m/z values suggesting the formation of unstable polymeric molecules by recombination of fragments which readily break down into smaller ones. A tentative route is drawn for the same (figure 10). Some molecules with higher mass values 349, 456,762, 963 etc are constructed through azo linkage, ether linkage and through release of small fragments like H2O, CO2, H2 etc.(figure 9). Lower intensity of these peaks suggested their lower stability and their instantaneous breakdown into other fragments.