GREEN SYNTHESIS OF NANOSTRUCTURED ZINC PARTICLES USING AQUEOUS LEAF EXTRACT OF SCHREBERA SWIETENIOIDES ROXB. AND THEIR CATALYTIC APPLICATION IN DEGRADATION OF METHYL ORANGE, CRYSTAL VIOLET DYES AND CHROMIUM METAL

Objective: The present work was aimed to synthesized the zinc nanoparticles (ZnO NPs) using aqueous leaf extract of Schrebera swietenioides Roxb., and further, the green-synthesized ZnO NPs were studied for its efficacy in the degradation of hazardous dyes like methyl orange, crystal violet and hazardous metal such as chromium.
Methods: The ZnO NPs were synthesized using aqueous leaf extract of S. swietenioides Roxb., as a green reducing agent and 0.1 M Zinc acetate as metal source and the NPs synthesis was completed within a short period of 6 h. The ZnO NPs synthesized were characterized using SEM, TEM, EDS, XRD, FT-IR and UV-visible spectrophotometer. Further, the synthesized NPs were applied for reduction of pollutant days such as methyl orange, crystal violet and pollutant metal chromium.
Results: The synthesis of NPs was monitored by observing the color change in the reaction mixture and UV visible spectral analysis. The UV spectral analysis shows a characteristic absorption wavelength at 379 nm. The synthesized NPs were hexagonal wurtzite form crystals having a spherical shape with rough surfaces with an average size of 68 nm and having 73.7 % of zinc content. At a NPs dose of 1.0 g/l the photocatalytic reduction was observed as 85.33±0.02 %, 86.82±0.095 % and 86.73±0.104 % for crystal violet dye, methyl orange dye and chromium metal, respectively. The NPs shows a high % photocatalytic reduction of chromium metal, crystal violet dye and methyl orange dye with less contact time confirms that the synthesized ZnO NPs were effectively catalyzed the degradation of methyl orange, crystal violet dyes and chromium metal. The NPs were observed to be recyclable and can shows high reduction activity after the completion of three cycles of degradation.
Conclusion: Hence it can be concluded that synthesized greener nanocatalyst was efficient for pollutant treatment and demonstrated the power of green biosynthesis for metallic nanoparticles.


INTRODUCTION
Nanoparticles have gained immense importance in recent times due to their diverse application in the field of science and technology. By definition, they are described as microscopic particles that have size smaller than 100 nm in at least one dimension [1]. Their microscopic size, morphology and distribution enable them to possess unique biological, chemical and physical properties that were distinctly different from those of individual atoms and molecules. Metal nanoparticles have been extensively explored due to their diverse applications in the areas of catalysis, biosensing, medicine, drug delivery etc. [2,3].
In this advanced and populated era, more comfort and ease of life have increased industrial zones and the rate of production is increased day by day because of the peoples' needs. For the need of wearing clothing and fashion changes, different types of textile industries, including dying, finishing, leather and weaving industries were using different types of chemicals and dyes for production [4]. Among the dyes that are used in various industries, few dyes namely Methyl orange, Methyl red, and Congo Red etc were the most common because of their ease of application and basic colors [5]. Due to the rapid growth in population and industrialization, increased quantities of toxic pollutants such as heavy metals and synthetic organic matters are being released into the aquatic system. Among these toxicants, chromium is one of the most toxic pollutants [6,7].
The Presence of dyes and metals in water would be very harmful to the aquatic environment as well as humans. Efficient dye and metals degradation has become a challenging task for environmental engineers as well as scientists. Different methods have been adopted by scientists for the degradation of dyes in effluent, which includes biological degradation, photocatalysis and adsorption tactics, etc [8]. But these processes are not sufficient for the removal of pollutants as the load is very high. So, there is a need to search for more advanced techniques to handle these types of pollutants.
ZnO nanoparticles (ZnO NPs) have drawn the attention of several researchers for their exclusive optical and chemical behaviors, which are strongly related to their nano size and morphology [9,10]. Within the large family of metal oxide nanoparticles, ZnO NPs have impressive properties, including large binding energy, wide band gap, and high piezoelectric features [11]. ZnO NPs have a highly promising prospective in biological functions such as drug delivery and nanomedicine [12], gene delivery [13], along with antibacterial [14], anti-biofilm [15], antifungal [16], larvicidal and anti-diabetic activities [17]. In view of the above, the present work intended to synthesis the Zinc oxide (ZnO) NPs using aqueous leaf extract of Schrebera swietenioides Roxb. Further, the synthesized NPs were studied for its effectiveness for the removal of pollutant dyes such as methyl orange and crystal violet. Further, the reduction of pollutant metal Chromium using synthesized ZnO NPs was evaluated.

Collection of plant material
The leaves of S. swietenioides plant were collected in Tirumala hills, Tirupati. The plant material was identified by Dr. Ch. Srinivasa Reddy, Assistant Professor, Department of Botany, SRR and CVR Government Degree College (A) Vijayawada and a dried specimen was stored in the department with specimen number SRR-CVR/2019-20/Bot/31. The fresh leaves were shade dried, powdered and the powdered plant material was used for the synthesis of NPs.

Chemicals and reagents
The chemicals used the study, such as Zinc acetate, sodium hydroxide, potassium dichromate, methylene blue, methyl orange, diphenyl carbazide etc were the reagent grade chemicals and were purchased from Merck chemicals, Mumbai.

Preparation of aqueous leaf extract
An accurately weighed 1 gram of the dried leaf powder was taken in a beaker containing 100 ml of water. Heat the content on a magnetic stirrer at 70 °C for 1 H. Then it was cooled and filtered using Whatman filter paper.

Green synthesis of zinc oxide nanoparticles
The solution 1 for the synthesis of NPs was prepared by stirring 9:1 (v/v) aqueous leaf extract of S. swietenioides and Zinc acetate (0.1 M) solution in a magnetic stirrer for 4 H. Sodium hydroxide solution was prepared in aqueous (0.8 M) ethanol and considered as solution B. The solution A and B were mixed and using a magnetic stirrer for 6 h. Then the zinc hydroxide peracetate formed at the bottom of the beaker was collected using centrifugation. Then the obtained particles were heated at 300 °C temperature for 45 min to evaporate the solvent in muffle furnace and to convert zinc hydroxide into ZnO NPs particle powder [18].

Photocatalytic experiment
The photocatalytic activity was evaluated by monitoring the degradation of the organic dyes (methyl orange, crystal violet) and chromium metal under visible light irradiation. The catalyst at selected fixed weight was dispersed in 50 ml of the aqueous dye solution at a concentration of 10 ppm of crystal violet, 50 ppm of methyl orange and aqueous metal solution at a concentration of 50 µg/ml separately. The reaction mixture was stirred to get homogenous mixture and then stirring continued in dark for 45 min to attain adsorption-desorption equilibrium. The reaction mixture was exposed to visible light irradiation (70 W mercury lamp) and then an aliquot was taken at 15 min intervals for 2 h. The solution was centrifuged, and the adsorption efficiency was monitored directly at 584 nm for crystal violet, 467 nm for methyl orange and diphenyl carbazide method at 540 nm for chromium metal using UV-Visible spectrometer [19,20]. The metal and dye adsorption capacity of NPs was calculated using the equations.

Recyclability and photostability of ZnO NPs
The recyclability of catalytic activity of the synthesized ZnO NPs was evaluated by repeating the catalytic activity as per the procedure described above. Each study cycle, the catalyst (ZnO NPs) was washed 3 times with distilled water using a magnetic stirrer to remove the dye completely from the particles. Then they are dried using vacuum 40 °C for 6 hours. The dried particle was used for a further cycle of catalytic activity [21].

RESULTS
The UV-visible absorption spectra ( fig. 1A) give the primary information for the formation of NPs and shows the characteristic sharpened nature of the absorption peak at a wavelength of 379 nm proves that the NPs were mono-dispersed with narrow size distribution of particles [22]. The type of bioactive molecules or functional groups that are actively involved to bind the metal and formation of NPs was evaluated by shifting in wavenumber in FT-IR spectrum. The FT-IR spectral analysis ( fig. 1B) confirms the presence of various bioactive functional groups in the NPs such as free-OH in alcohols (3668 cm -1 ), intramolecular bonded alcohols (3461 cm -1 ), N-H stretching in amine salts (2800-3000 cm -1 ), C-H stretching in alkanes (3005 cm -1 ), C-N stretching in aromatic amines (1314 cm -1 ) and aromatic esters (1287 cm -1 ). These functional group corresponding to bioactive compounds that are actively involved in the formation of NPs. The SEM analysis ( fig. 1C) confirms that the NPs were spherical shape with rough surfaces. The Zn metal composition of 73.7 % with Zn was identified at 8.61 (Kα) and 1.09 keV (Lα) in EDS analysis data ( fig. 1D). It was also confirmed that Oxygen was observed at 0.5 keV, which is another key element in the formation of ZnO NPs. The shape of the synthesized NPs was further characterized using TEM analysis.   2D). The degradation reaction follows a pseudo-first-order reaction kinetics with respect to crystal violet. The rate constant (k) was calculated as 2.29×10 -3 , 5.07×10 -3 , 1.21×10 -2 and 2.50×10 -2 respectively for the crystal violet dye reduction study conducted at a nanocatalyst dosage of 0.25, 0.5, 0.75 and 1.0 g/l, respectively.
The methyl orange dye at a concentration of 50 ppm was selected for the dye reduction study. The reduction of methyl orange dye in the aqueous solution was monitored from time to time using a spectrophotometer at a maximum absorption wavelength of 467 nm ( fig. 3A). The absorption spectra show the decline in the concentration of methyl orange dye was observed to be increased with increase in time as well as an increase in the dose of the NPs. At 1 h of the catalytic study, the % degradation was observed to be 41.62±0.040, 48.55±0.052, 75.79±0.156 and 86.82±0.095 % for the nano catalyst dose of 0.25, 0.5, 0.75 and 1.0 g/l, respectively ( fig. 3C). The proved that the metal reduction was very effective at initial time of the reduction study and by increase in the time, the dye reduction efficiency was decreased due to the blockage of the active sites in the NPs with the dye. The FT-IR spectrum ( fig. 3B) also confirms that the dye molecules adsorbed on the surface of the NPs. The kinetics of the reaction was study by plotting the graph by considering ln (A0/At) on the y-axis and reduction time in minutes on x-axis ( fig.  3D) and the plot confirms that the degradation follows the pseudo first-order reaction. The rate constant (k) was calculated and found to be 6.61×10 -3 , 1.07×10 -2 , 1.64×10 -2 and 2.68×10 -2 respectively for the methyl orange dye reduction study conducted at a nanocatalyst dosage of 0.25, 0.5, 0.75 and 1.0 g/l, respectively. Hence it can be confirmed that the NPs were very effective for the reduction of methyl orange dye from aqueous solution. The recyclability of the photocatalyst treated with both dyes crystal violet, methyl orange and chromium metal was evaluated in three photocatalysis process. After three cycles of study, the photocatalytic efficiency was observed to be 98.71 %, 98.27 % and 98.06 for methyl orange, crystal violet and chromium metal, respectively ( fig. 5). The results confirm that after three cycles of study, there is the negligible decline in the photocatalytic degradation efficiency for both dyes and metal in the study.

DISCUSSION
The study aimed to utilize the phytochemical constituents present in the aqueous leaf extract of S swietenioides as biological reducing agents for the formation of ZnO NPs. In the initial stage of the synthesis process, the formation of NPs was assessed by the color change in the reaction mixture from light green to dark brown.
The NPs were formed by the reaction of Zinc acetate with the phytochemical compounds present in the aqueous leaf extract of S. swietenioides. The formed NPs were preserved and further the characterization of NPs and its applications as pollutant adsorbents was evaluated. The preliminary identification of NPs was done by observing characteristic UV-visible absorption maxima at 379 nm, which was in correlation with reported results [23,24].  [23,25]. The average size of the NPs was observed to be 68 nm and the % metal content in the synthesized NPs was observed to be 73.7 % which is very higher than the few findings reported [24], which confirms that the metal content was very high in the present study. Hence it can be confirmed that the NPs synthesized in the present study high metal content and significantly less size.
The synthesized ZnO NPs were studied for its efficiency on the reduction of crystal violet dye, methyl orange dye and chromium metal. The NPs were proved to be having efficient dye and metal reduction activity. At a high dose of NPs studied, the photocatalytic efficiency of 94.90±0.031 %, 86.82±0.095 % and 86.73±0.104 % was observed respectively for crystal violet, methyl orange and chromium metal within a very less contact time of less than 2h. The photocatalytic efficient of ZnO NPs reported in the present study was very higher than the many findings reported in the literature [26][27][28]. Hence it can be confirmed that the NPs synthesized in the study will be the best choice for the photocatalytic degradation of crystal violet, methyl orange dyes and chromium metal.

CONCLUSION
In summary, the biological method for synthesis of ZnO NPs was achieved using aqueous leaf extract of S. swietenioides. The method was very fast, simple and eco-friendly due to the non-involvement of harmful chemicals and involvement of active molecules present in aqueous leaf extract of S. swietenioides. The synthesized NPs were spherical in shape with rough surfaces, hexagonal wurtzite form crystals with an average size of 68 nm. Based on the EDS elemental analysis, the metal composition was confirmed as 73.7 %. The NPs were applied for the reduction of pollutant dyes crystal violet, methyl orange and metal pollutant chromium. The NPs were proved to be having high photocatalytic activity and were effectively used for the reduction of dyes and metal in the study. Therefore, this study provides an eco-friendly method for the synthesis of ZnO NPs, which can be used in effluent treatment (dye and metal degradation) of different types of industries.