Photocatalytic Degradation of Azo Dyes by Zinc Oxide Nanoparticles Fabricated using Aqueous Flower Extract of Cassia alata

Introduction

Water is an inevitable resource for the sustenance of all life forms on Earth. The swift industrial growth in developing countries emanates fromwater pollution due to organic and inorganic pollutants. Dyes are one such organic compound that churns out colour due to its ability to absorb light when dissolved in water. They are enduring organic pollutants released from the textile, paper, pharmaceutical, tannery and dyeing industries. The textile industries extensively utilizes azo dyes which are categorized by the presence of functional group –N=N attached to aromatic moieties. The Azo dyes are synthetic colourants and approximately 20 to 25% of the dyes vanish during dyeing processes and settle in the environment when discharged without proper treatments. These dyes, when discharged into water bodies, even at very low concentrations, are persistent and recalcitrant. The complex structure and their stability make them non-biodegradable^1,2. Further, they are also mutagenic and carcinogenic. Hence, the removal of these organic dyes from water sources is highly important³. A number of physical and chemical methods are conventionally employed for the degradation of dyes. The physical processes end up in transferring the pollutants from one medium to another resulting in secondary pollutants. The chemical methods for the degradation of organic pollutants produce enormous amounts of sludge and are also expensive. The biological treatments of dye substances are ineffective because of their resistance to aerobic decolourization and anaerobic treatment of azo dyes results in the generation of aromatic amines that are carcinogenic^4,5. Hence, the search for an environmentally friendly and non-toxic alternative is essential.

The current research developments focus on the photocatalytic degradation of organic dyes utilizing semiconductor nanoparticles. In this study, the green chemistry approach for the synthesis of nanoparticles has been adopted. The n-type semiconductor, Zinc oxide nanoparticles (ZnO-NPs) was synthesized using aqueous flower extract of Cassia alata. This green mediated ZnO-NPs was employed for the degradation of azo dyes due to its high photo sensitivity, non-toxic nature, cost effectiveness in fabrication, wide band gap, high photocatalytic and quantum efficiency^6,7. Further, they can be easily scaled up for large scale production. In the present study, Methyl Orange, Congo Red, Malachite Green and Eriochrome Black T are used as model dye. Methyl orange is used as an indicator in titrations due to its ability to produce variations in colour at different pH⁸. It is used as colouring agents in food, pharmaceuticals, leather and textile industries. Congo red is an anionic diazo dye that produces carcinogens such as benzidine⁹. This dye is commonly employed in the silk manufacturing industries. Malachite green is a synthetic triphenyl methane dye and has applications in textile, fisheries and dyeing industries^10,11. Eriochrome Black T is an anionic dye used in the determination of water hardness. It is used in the paper, pharmaceutical, textile industries and also in research laboratories¹². These organic dyes are carcinogenic, mutagenic and are not easily degradable^13,14. Further, they are persistent and highly resistant to biological degradation due to the presence of aromatic and sulphonate groups^15,16.

C.alata (Fabaceae) is a medicinal plant used in traditional medicine in India and South East Asia¹⁷. The plant is rich in phytochemicals such as flavonoids, alkaloids, tannins, triterpenoids, anthroquinones and sterols^18,19. The presence of biomolecules such as proteins and carbohydrate (reducing sugars) together with secondary metabolites play an effective role of stabilizer in the fabrication of ZnO-NPs^20,21. Thus, the objective of the present study is to synthesis ZnO-NPs using aqueous flower extract of C.alata for the photocatalytic degradation of azo dyes.

Materials and methods

Materials

Bright yellow colouredC.alata flowers were collected from Thanjavur, Tamilnadu, India. They were taxonomically identified atthe department of botany, St. Joseph’s College, Tiruchirappalli, Tamilnadu, India.The chemicals used in this study are analytical grade chemicals procured from Sigma- Aldrich Pvt. Ltd.

Synthesis of ZnO-NPs

The shade dried flower powder of C.alata (5gm) was dissolved in 100 ml of double distilled water to prepare a 5% aqueous flower extract by boiling for 10 min at 70^oC. 10 ml of this aqueous flower extract was filtered using Whatmann filter paper no.1 and added to 50 ml of zinc nitrate solution (0.1M). The flower extract and metal precursor solution was boiled for 2h at 75^oC. After boiling, the supernatant was discarded and the pale white precipitate formed by centrifuges to 1000 rpm for 10 min with the addition of distilled water. The pellets were collected, dried in an oven for 12h and calcinated at 350^oC for 2h²².

Characterization of ZnO-NPs

The surface plasmon resonance (SPR) of ZnO-NPs was monitored using UV-visible spectroscopy at wavelength between 200 and 800 nm. The FTIR absorption spectrum of ZnO-NPs was recorded between 400 and 4000 cm^-1 in diffuse reflectant mode to identify the functional groups present on the surface of synthesized nanoparticles. The crystallinity and particle size of the nanoparticles were determined using X-Ray diffraction spectroscopy. Debye-Scherrer’s equation (L=0.9λ/βcosθ) was used for calculating the particle size. The morphology and elemental composition of the nanoparticles were determined using SEM and EDAX analysis. The particle size and surface charge of ZnO-NPs was determined with DLS and zeta potential analysis. 

Photocatalytic activity of ZnO-NPs

Methyl orange, congo red, malachite green, and Eriochrome Black T dyes were used as a model system for studying the photocatalytic activity of the ZnO-NPs in the presence of direct sunlight. 1000 ml of distilled water was combined with 15 ml of dye. 0.1 g/L of plant extract-mediated nanoparticles received 100 mL of dye (ZnO-NPs). A control was maintained without the addition of nanoparticles. Before exposing the combination to direct sunlight irradiation, the mixture was magnetically agitated for 30 minutes in a dark environment. 2 mL of the solution was centrifuges at 10,000 rpm for 10 minutes at predetermined intervals (every 30 min) to get a clear supernatant, and the optical density was assessed using a UV-visible spectrophotometer in the wavelength range of 300 to 800 nm^23,24.  The λ_max of dye was detected at 460 nm (Methyl orange), 480 nm (Congo red), 618 nm (Malachite green), and 530 nm (Eriochrome Black T). By measuring the absorbance of dye, the concentration of dye following photocatalytic degradation was assessed using the following equation^25,26.

Result and Discussion

The development of industrialization at a rapid phase has led to enormous amounts of pollutants and, in turn to environmental toxicity. The discharge of synthetic dyes from various industries into drinking and fresh water sources causes water toxicity^27,28. The exorbitant use of coloured substances, particularly in the textile sector, has led to serious ecological problems²⁹. Among the different classes of coloured substances used in industries, azo dyes are the most important dye that is being utilized above 80% in comparison to other classes of dyes³⁰. Metal oxide nanoparticles act as catalysts in the degradation of dyes. ZnO nanoparticles are metal oxide nanoparticles that have a band gap of 3.3 eV and excitation binding energy of 60 meV^31,32. The photocatalytic activity of ZnO-NPs is due to their large surface area to volume ratio^33,34.

10 ml of 5% aqueous flower extract of C. alata was mixed with 50 ml of 0.1 N zinc nitrate solution and heated to 75^°C for 2h. The light brown colour of the solution turned into a pale white colour, indicating the formation of ZnO nanoparticles. A small fraction of the dried and calcinated powder of C.alata mediated ZnO-NPs was examined using UV-spectroscopyat a wavelength ranging between 200 to 800 nm exhibited a surface plasmon resonance peak at a wavelength measuring 372 nm (Fig.1).Kalopanaxseptemlobus mediated ZnO-NPs exhibited a surface plasmon resonance at wavelength 372 nm^35,36 was in agreement with the present study. The ZnO-NPs prepared using Eucalyptus globulus leaf extract exhibited an absorption peak at 375 nm, indicating the characteristics SPR of ZnO-NPs³⁷. The formation of a sharp peak confirms the presence of mono-dispersed ZnO-NPs³⁸. The absorption peak obtained with C.alata flower extract mediated ZnO-NPs was lower compared to its bulk material, ZnO given as 380 nm³⁹.

Figure 1: UV absorption spectrum of C.alata mediated ZnO-NPs Click here to View Figure

The FTIR spectrum of C.alata aqueous flower extract medicated ZnO-NPs (Fig.2) exhibited prominent peaks at the stretching vibrations of the OH group was assigned with the peak at 3417.01 cm^{−1 40,41}. The peak representing the C-H stretching vibrations due to CH₂ group was shown at 2919.58 cm^{−1 42}. The presence of C=O group was indicated by the peak at 2850.88 cm^{−1 43}. The peak at 1605.97 cm⁻¹ shows the carboxyl group symmetric stretching vibrations of aminoacids present in the protein molecules⁴⁴. The C=O stretching vibrations of the carboxylic group was represented with the peak at 1518.80 cm^{−1 45}. The peak at 1410.79 cm⁻¹indicates the C-N stretching present in aminoacids⁴⁶. The C-N stretching of aromatic amines was indicated with the peak at 1252.63 cm^{−1 47,48}. The synthesis of ZnO-NPs and the zinc oxide bond was shown with peaks at 1193.24cm⁻¹, 693.15 cm⁻¹, 658.03 cm⁻¹, 643.82 cm⁻¹, 602.00 cm⁻¹, 536.44 cm⁻¹ and 470.79 cm^{−1 49,50}. The C-O stretching vibrations and the C=C stretching vibrations of aromatic amines was represented with peak at 1098.64 cm⁻¹and 1036.23cm^{−1 51}. The C-H vibration was indicated with the peak at 836.71cm^{−1 52}. The presence of aliphatic chloro compounds was shown with the peak at 777.58 cm^{−1 16,20,48,52}. Thus, from the FTIR spectrum it was identified that functional groups OH, NH, –CH2, C=C, C-O of aromatic amines and alcohols and are involved in the reduction of zinc ions and in the stabilization of ZnO-NPs.

Figure 2: FTIR spectrum of C.alata mediated ZnO-NPs Click here to View Figure

C. alata aqueous flower extract mediated zinc oxide nanoparticles were subjected to XRD analysis for the confirmation of nano synthesis and to identify the crystallinity and size. The zinc oxide nanoparticles exhibited prominent De Bragg’s reflection values of 31.66^o, 34.33^o, 36.21^o, 47.63^o, 56.48,  62.61^o, 67.95^oand 69.17^owhen recorded at 2θ angles from 10 to 90^o. The De Bragg’s reflections corresponded to the crystalline plane index (100), (002), (101), (102), (110), (103), (112) and (201) (Fig.3). These planes of indices confirm the synthesis of hexagonal phase (Wurtzite) zinc oxide nanoparticles. The broadening of peaks and noise visualized might be probably due to nanosized particles and biomolecules. The XRD spectrum matched the standard diffracted powder card no. JCPDS No 089-0511.it has reported a XRD peaks for ZnO-NPs synthesized using bark extract of Aglaiaelaeagnoidea that was in agreement with this study. The average size of the nanoparticles calculated with major diffraction peak using Scherrer’s equation was 9.93 nm.

Figure 3: XRD Diffractogram of C.alata mediated ZnO-NPs Click here to View Figure

The SEMmicrograph of the synthesized ZnO-NPs was found to contain hexagonal wurtzite structures with high crystallinity and uniform distribution (Fig.4). The particles were found to be agglomerated and the EDX pattern revealed the composition of nanoparticles which reported 64.81% of zinc, and 24.12% of oxides (Fig.5).The SEM micrograph shows that the particles were agglomerated, a typical phenomenon observed in green synthesis of nanoparticles. The aggregation of nanoparticles might be attributed to the large surface area to volume and due to the affinity of nanoparticles towards each other^4,6,48,52. The ecological factors are highly influential for the stability and agglomeration of nanoparticles during their synthetic process and form asymmetric clusters^20,40.

Figure 4: SEM micrograph of C.alata mediated ZnO-NPs Click here to View Figure

Figure 5: EDAX spectrum of C.alata mediated ZnO-NPs Click here to View Figure

The determination of particle size using Debye-Scherrer’s equation is best suited for semispherical and spherical structures. Hence, to obtain a more precise particle size, Dynamic Light Scattering (DLS) analysis was performed. DLS analysis indicated that the particle size was 78.18 nm (Fig.6).DLS provides a much larger value of particle size due to its hydrodynamic shell, which is dependent on the structure, shape and roughness of the synthesized nanoparticles (Katzelet al., 2008). The surface charge and the stability of ZnO-NPs were evaluated with zeta potential analysis. The surface charge of the nanoparticles was -14.6 mV (Fig.7) as observed with zeta potential analysis.The results indicated that the ZnO-NPs were coated with negatively charged groups, which account for its moderate stability^16,35.

Figure 6: DLS analysis of C.alata mediated ZnO-NPs Click here to View Figure

Figure 7: Zeta potential analysis of C.alata mediated ZnO-NPs Click here to View Figure

C.alata mediated ZnO-NPs exhibited 76.65%, 72.76%, 65.07% and 60% of degradation of methyl orange, congo red, malachite green and Eriochrome Black T dye respectively (Fig.8 and Table 1). The absorption bands with high intensity are found at 460 (M.O), 486 (CR), 618 (MG) and 530 nm (EBT). Among the evaluated dyes, Congo red was degraded to 72.76% within 120min. Maximum intensity of absorption peak was observed at 0 min followed by a drastic decrease in intensity of peak after 30 min, which might have resulted in complete saturation of dye molecules within 30 min of exposure and due to affinity of nanoparticles towards dye molecules^20,51. The Photocatalytic acceleration of MO degradation was 85% in 180 min of exposure to nanoparticles^34,35,41 reported 100% degradation of MO in 240 min by ZnOnanoflowers synthesized using precipitation method. The ZnO-NPs synthesized using aqueous extract of Amomumlongiligulare showed a decrease in the concentration of malachite green dye with a decrease in its characteristic peak at 619 nm. After 60 min of photocatalytic activity, 38.1% of dye degradation³⁷was in agreement with the present study. Similarly, ³⁵reported 90% of EBT degradation in 5h by ZnO-NPs, where as the present study reported 60% within 150 min. When the nanoparticles are irradiated by a light source, electrons present on the surface of  the nanoparticles get excited from valance band to conduction band utilizing the energy of light source (Photoexcitation). The dissolved oxygen in the reaction mixture utilizes  photo excited electrons for the formation of oxygen free radicals. Simultaneously, the photoexcited holes produced in the valence band oxidises the water molecules that have been absorbed and produce OH radicals. Both the superoxide and hydroxyl radicals with high oxidation co-efficient react with dye molecule and degrade it into their by-products^20,34,35.

Methyl Orange

Congo red

Malachite Green

Eriochrome Black T

Figure 8: Photocatalytic degradation of azo dyes by C.alata mediated ZnO-NPs Click here to View Figure

Table 1: Photocatalytic degradation (%) of azo dyes by ZnO-NPs.

Sl.No	Dye	Reaction time (Min)	% of Degradation
1	Methyl orange	150	76.65
2	Congo Red	120	72.76
3	Malachite Green	150	65.07
4	Eriochrome Black T	150	60

 

Conclusion

The study witnessed the successful synthesis of ZnO-NPs using aqueous flower extract of C.alata. The synthesized nanoparticles possessed hexagonal wurtzite shapes with an average particle size of 78.18 nm as given by DLS measurements. The Green chemistry mediated ZnO-NPs was effective as the fabrication of nanoparticles with a particle size of less than 100 nm was achieved. Due to the non-toxic and biocompatibility nature of ZnO-NPs it could be utilized in a wide range of applications. In the present work, ZnO-NPs exhibited effective Photocatalytic activity as > 60% of degradation of dyes was achieved with the evaluated dyes. ZnO-NPs showed maximum activity against methyl orange and congo red. Thus, the study suggests that the green mediated ZnO-NPs could be used as an effective and eco-friendly alternative in the removal of chemical contaminants, especially the azo dyes, from the polluted water.

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