Treatment of industrial e�uent by synthesized Nano-composite ZrCdPbO4 through solar energy

Water is the most essential natural resource for living beings, animals and plants but unfortunately it is being polluted by various means. Coloured pollutants, mainly dyes contribute maximum to this. Thus an attempt is made to remove such pollutants in an eco friendly manner. Novel nano sized composite material ZrCdPbO 4 is synthesized by coprecipitation method, characterized by UV-vis, IR, FESEM, XRD, XPS and EDX analysis methods and is used for removing such pollutants from water. The emphasis of the present work is to use the prepared material as photocatalyst for degradation of various pollutants. Experimental details of degradation of a common industrial e�uent, Azure-A dye as role model are discussed. Number of factors are varied to obtain maximum degradation rate through pseudo �rst order kinetic study. Hydroxyl radicals are found to play important role in cleavage of various bonds of the pollutant and is endorsed by scavenger study. LCMS/MS analysis is carried to rule out the route of the degradation. Final products are ascertained by laboratory tests.


Introduction
Water is the essential natural resource for life on earth. Now a day, through release of different pollutants, the availability of potable water has decreased and if available, is adversely affecting the health of living beings. Some human activities like constructions, release of various industrial wastes like clothes, paper, printing, dye waste, agricultural and pharmaceutical waste, excavation, cutting of tree etc has harmed the nature and has disturbed the natural balance. One of such activities that had polluted water is synthesis of colours and dyes. These colour pollutants when excreted directly into water, without any treatment, cause harm to the resource ending into dangerous outcomes. Thus need of the hour is to treat such resources by any mean and remove these pollutants. Various treatment methods are available to treat pollutants in wastewater like foam separation, electrode materials, bio-adsorption technologies, Tailoring nano ltration membranes, Fenton-occulation process etc (Fei et  It is observed that hydroxyl radicals play a signi cant role in heterogeneous photocatalysis and in degradation of dyes (Daneshvar et al. 2004;Raizada et al. 2014 photocatalyst, concentration of dye, light intensity etc. are varied to extract the maximum degradation conditions. Participation of hydroxyl free radical is ruled out by scavenger study. A tentative mechanism for degradation is proposed through LCMS/MS peak analysis and intermediates are also determined.

Materials And Methods
Zirconyl nitrate, lead nitrate and cadmium nitrate were used as precursors (Merck) for synthesis of the photocatalyst, sodium hydroxide was used as precipitating agent (CDH) and Azure A (Otto) was used as the model pollutant. HCl (CDH) and NaOH (CDH) were used for adjusting the pH of the solutions and pH was recorded by pH meter (Hena imported pen type). Optical density of solutions at different time intervals was recorded by UV-Vis spectrophotometer (CHINO). A 200 Watt lamp (Philips) was used for irradiation and the intensity of light was measured by solarimeter (CEL-201). KI, EDTA and isopropyl alcohol (Merck) were used as scavenger to trap the active species. All chemicals were used in approximately 95-99% pure form and of LR grade. LCMS/MS technique 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 was employed.

Experimental
The photocatalyst ZrCdPbO 4 was synthesized by using precursors-zirconyl nitrate, lead nitrate and cadmium nitrate. Solid co-precipitation method was employed and sodium hydroxide was used as precipitating agent under controlled conditions. Solid off white coloured material was obtained after calcination with yield of 71.73%. The material was then characterized by XRD to obtain crystal size of 21.59±6.15 nm. EDX analysis proved the presence of Zr, Cd, Pb and O elements in ratio 1:1:1:4. XPS analysis determined the oxidation states as +4 for Zirconium, +2 for cadmium, +2 for lead and -2 for oxygen. The molecular formula of the prepared photocatalyst was thus con rmed to be ZrCdPbO 4 . FE-SEM analysis showed homogeneous cluster form of the material and IR study showed bonding and vibrations (bending and stretching) correlated to CdO, PbO and ZrO 2 molecules. UV-VIS spectral study was used to determine the band gap and peak at 229 nm suggested it to be 5 eV. The results are reported earlier (Lohar et al. 2021). Larger band gap ensured higher photo-quantum e ciency of the prepared material because of greater life time of photo-generated electrons and holes, thus having enough time to degrade pollutants. Participation of OH free radical was ascertained by scavenger study. Further the analysis for degradation of Azure A was carried out through LC/MS technique to determine the intermediated formed and path way of degradation.

Photocatalytic study
Stock solution of the dye of concentration 1× 10 -3 M (0.288 g/L) was prepared in doubly distilled water.
This was further diluted to obtain desired concentration. Dye solution of concentration 0.2×10 -4 M was taken, pH was adjusted to 8.0 by adding pre standardized NaOH solution and 0.22 g of ZrCdPbO 4 photocatalyst was added. The reaction mixture was then exposed to a tungsten lamp at the intensity of 74.0 mW/cm 2 and a water lter was used to cut off thermal side reactions. 2-3 mL of aliquot was drawn from the reaction mixture after particular time intervals during the progress of the reaction and optical density was recorded at λ max 600 nm. It was observed that presence of light and the photocatalyst both was necessary to bring the degradation of dye, suggesting the reaction to be a photocatalytic one.

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 gure 1. A straight line in the plot indicates the rate of reaction to follow pseudo rst order kinetics. Kinetic models, pseudo-rst-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-rst-order kinetic model prevailed in degradation process. Percentage of dye degradation at optimum conditions of parameters is given by:- (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 gure 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.
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.
After pH 8.0, decrease in reaction rate is observed because repulsion amongst OHions 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 gure 4 and tabulated in

Effect of Light Intensity
Variation of light intensity is carried out in the range 7.0 to 74.0 mW/cm 2 . The observation data are graphically represented in gure 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/cm 2 . Further rise in intensity of light causes heating of the solution and thus is avoided.

Scavenger study
Scavenger study is carried out to nd 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 gure 6. It is evident from the data that addition of isopropyl alcohol (hydroxyl radical scavenger) ceases the reaction and signi cant reduction in the rate of degradation (up to 82%) is observed proving that the dye degradation is caused by hydroxyl radicals.

Conclusions
It is concluded here by that a novel nanosized quaternary photocatalyst is synthesized and is characterized. Band gap energy of the photocatalyst comes out to be 5 eV, which makes the material suitable for photocatalytic activities as recombination of electrons and holes is reduced. The photocatalyst is further used for removal of dye Azure A from water. A mechanistic pathway is proposed where degraded fragments are identi ed by considering m/z peaks through LC-MS spectra. Complete degradation of the dye molecules into smaller harmless fragments is observed. The process is found bene cial for the society in numerous ways. Use of solar radiations, simple process of regeneration and reusability of photocatalyst, treatment of polluted water to make it useful, less time and energy consuming process etc. makes the process economic one. The process does not add any other material to the resource or environment and so is an eco-friendly process. Thus the process can be used at larger scale for puri cation of water in eco-friendly manner.    Scavenger study LCMS spectra of Azure A cleavage and peaks at higher m/z Figure 10 Mechanistic pathway of intermediates with high molecular mass fragments