To explore the photo-degradability of AB 10B dye with ZMT4 and ZMT4S2 catalysts under visible light irradiation, a series of experiments were designed in order to determine the reaction conditions for complete degradation of dye. Before establishing the reaction conditions, trial experiments were conducted to know the sensitivity of dye and catalyst.
In the first trial, 100 mL of 5 mg/L AB 10B dye solution was taken in a 150 mL pyrex glass vessel without catalyst and exposed to light for 60 min. 5 mL of aliquots were withdrawn at different time intervals and measured the absorbance at λmax of 619 nm. No appreciable change in the absorbance was observed for the dye solution before and after exposure to light implies that the insensitivity of dye towards light.
In 2nd trial, the reaction vessel containing dye solution (5 mg/L) with catalyst dosage (50 mg/L), maintained at pH=3 was stirred in the dark for 60 min and the aliquot taken was analyzed for absorbance at same λmax, the small change in absorbance of the dye was observed indicating the adsorption of dye molecules on the surface of charged catalyst.
In the last trial, the reaction vessels containing above said components kept in visible light irradiation for 60 min under continuous stirring and at different time intervals the aliquots samples were taken and absorbance was measured. It is noted that the progressive decrease in absorbance was occur which attributes to the interdependence of light and catalyst.
Based on the above said conditions it is necessary to optimize the reaction conditions.
4.1. Optimization of reaction conditions
To determine the influence of dopant concentrations and efficiency of the catalyst, experiments were conducted at different dopant concentrations by keeping other parameters constant such as catalyst dosage 0.05 g/L, solution pH 3, and initial dye concentration 5 mg/L. The experimental results depicted in Fig. 8 clearly revealed that the photocatalytic behaviour of all the co-doped TiO2 catalysts (ZMT1-ZMT5) are more pronounced than undoped-TiO2, attributed to the narrowing of band gap as discussed in sec. 3.5. But among all the catalyst ZMT4 exhibit elevated rate of degradation which is ascribed to utmost decrease in bandgap of TiO2 in ZMT4 and reduction in electron hole recombination. At higher concentration of dopants, they deposit on the surface of catalyst rather than substitution doping into TiO2 lattice. This can provoke the acceleration of electron hole recombination which retard the rate of degradation . Within these two dopants Mg2+ form an extra Fermi energy level by mixing of 2p orbitals of Mg and O. In the same way, Zn2+ facilitated as an electron trap by forming an extra energy level below the conduction band by mixing of 3d orbitals of conduction band and Zn2+ . In view of decrease in band gap of each particle, high quanta of visible radiations were absorbed leading to high quantum efficiency.
Based on the above data ZMT4 (1.00% Zn and 0.25% Mg) found to be an effective photocatalyst and for further enhancement in the photocatalytic activity of ZMT4 two factors decrease in particle size and increase in surface area favours the enhancement process. To achieve this criteria, ZMT4 was re-synthesized in presence of biogenic surfactant at 3 different concentrations and the process was discussed in Sec.2.2.2. After calcination the catalytic efficiency of these three catalysts (ZMT4S1, ZMT4S2 and ZMT4S3) were evaluated and the results were shown in Fig. 9. Among these ZMT4S2 exerts highest photocatalytic activity than the other two because of the detrimental effect on further increasing the surfactant concentration causes the restriction for coherent doping of metal ions . The rate degradation graph of these three catalysts is given in Fig. 9 as an inset. This result corroborated with the results obtained in XRD, TEM and BET surface area analysis (Sec 3.1,3.2 and 3.4). During the further course of catalysis ZMT4S2 fixed as a best catalyst and other parameters were varied for obtaining optimum conditions.
The impact of solution pH on the photocatalytic activity of ZMT4S2 for the degradation of AB 10B dye was studies by varying the pH from 2 to 9 and keeping the other parameters constant such as catalyst dosage (0.05 g/L) and dye concentration (5 mg/L). All the experimental results are shown in Fig. 10 shows that the degradation rate was observed to be high in acidic pH. Under acidic conditions <6.25, the surface hydroxyl group (Ti-OH) undergo protonation (Ti-OH2+) by making the surface positive which facilitate the adsorption of negatively charged AB 10B dye molecule by electrostatic interaction. Further, increase in pH from 5 to 6, the positivity of the surface decreases slowly and becomes negative by approaching 8 to 9 pH. On the whole, pH 3 is the better optimal condition where the surface of the catalyst is positive which is more favourable for the adsorption of negative dye molecule. Hence, at this condition, the rate of degradation is high. At pH 2, the approachability of the H+ towards Ti-OH on the surface is more competitive due to repulsion between protons [ 30].
The Fig. 11 illustrates the photocatalytic activity of ZMT4S2 at different catalyst loading varying from 0.05 g/100 mL to 0.20 g/100 mL keeping other parameters constant. It is clearly observed that the catalyst dosage at 0.1 g/L exhibited highest rate of degradation, later it decreases by increasing catalyst dosage. This condition may be attributed to the greater availability of catalyst up to optimum concentration (100 mgL-1) beyond which the increase in catalyst concentration the degradation rate decreases due to non-availability of the sufficient dye molecules to react with the active catalyst particle, also high catalyst dosage concentration, increases the turbidity impeding the penetration of light henceforth lowering the photocatalytic efficiency in the given working conditions .
After selection of the catalyst, pH and catalyst dosage, the final parameter initial dye concentration has to be optimized. Initial dye concentration varying from 5 mg/L, 10 mg/L and 15 mg/L by maintaining the other parameter constant. It can be inferred from the plot (Fig. 12) that the rate of degradation increases up to 10 mg/L dye concentration later it decreases (15 mg/L), this is may be attributed that up to 10 mg/L dye concentration, the surface area of photocatalyst is fully saturated with the monolayer deposition and simultaneous degradation of dye molecule but at high concentration (15 mg/L) due to blanket effect, multilayer adsorption penetration of light to the surface of the catalyst decreases . Moreover, confinement of •OH radicals at fixed catalyst dosage due to non-availability of catalyst particle confines the rate of degradation.
Optimum conditions for efficient degradation (99%) of Amido Black 10B by ZMT4S2 was arrived at catalyst dosage (0.1 g/L), dye concentration (10 mg/L) maintained at pH 3.