Waste to wealth: a solution to textile dyes related pollution

This study describes biosynthesis of Zinc oxide nanoparticles (ZnO NPs) using agro-waste durva grass aqueous extract without uses of any hazardous chemicals and stabilizing agents. Surface plasmon resonance (SPR) band of the synthesized ZnO NPs confirmed its conversion from its metal salt to NPs by UV–vis spectroscopy. Characterization of functional groups in durva grass aqueous extract and ZnO NPs surfaces were identified by Fourier transform infrared spectroscopy (FTIR), purity and crystallinity by power x-ray diffraction analysis (XRD), size and morphology by transmission electron microscopy analysis (TEM), elemental composition by Energy Dispersive x-ray spectroscopy (EDAX), stability by thermogravimetric analysis (TGA) and dynamic light scattering (DLS) study. The synthesized ZnO NPs was highly stable with average particle size of 17 nm. The photocatalytic activity of ZnO NPs was studied under UV irradiation to methylene blue (MB) and methyl orange (MO) degradation (10 mg l−1 each). 98.1% of MB was degraded within 60 min, while 97.5% of MO within 100 min. In addition, antioxidant activity of ZnO NPs showed 91.8% free radical scavenging capacity at the dose of 250 mg l−1 which implies its importance in biomedical applications.


Introduction
Nanotechnology is a fast growing arena in science and technology while nanomaterials having improved optical, magnetic and electrical properties compare to bulk materials are the promising materials in all these fields. Synthesis of nanomaterials has been widely studied in nanoscience due to enhanced physicochemical properties which leads to promising uses in different fields such as medicines, food sciences, agricultural pesticides, environmental clean-up, and nanocatalyst in organic synthesis, chemical reduction, biosensors and pollutants degradation [1,2].
Synthesis of metal oxide nanomaterials (MONPs) by different methods i.e. solvothermal method [3], hydrothermal method [4], precipitation method [5], sol-gel method [6] and thermal decomposition [7] needs toxic chemical and solvent as well as toxic stabilizing agents [8,9]. Effectively these methods can tune the shape and size of nanoparticles, but all these methods are expensive and required high temperature, pressure and energy. Now-a-days environmentally friendly MONPs are synthesized using simple green technologies and widely used in different applications such as photocatalytic degradation of water pollutants, nanocatalyst in synthesising organic compounds and biomedical application [10][11][12]. A few published articles have described the synthesis of ZnO nanoparticles (ZnO NPs) using green route [13,14]. Niranjan Bala et al [13] reported the biosynthesis of ZnO NPs by using Hibiscus subdariffa leaf extract, whereas Sundrarajan et al [14] synthesized ZnO NPs by using Pongamia pinnata leaf extract.
Organic dyes are used extensively by different industries which are well known toxicants to environment and ecosystem [15]. Moreover water containing dye of 1 mg l −1 is hazardous to environments and dyes contaminated wastewater is carcinogenic to human beings. Hence elimination of hazardous MB, MO and RB dyes from waste water is a challenging task to the scientists [16]. The international environmental standards have become more stringent day by day towards these pollutants. Several methods have been used for wastewater treatment and degradation of organic pollutants which are Fenton oxidation, chemical coagulation, aerobic, anaerobic, chemical oxidation, adsorption, membrane separation, electrochemical treatment, ion exchange and reverse osmosis [17,18]. These techniques have some drawbacks and hence lower cost & safer technologies are still needed for the practical applications. Among them, advanced catalytic degradation finds some importance in degrading pollutants of wastewater generated from industrial and living sources. Presently, nanocatalysts have undergone an enormous growth and seem to be an intensifying smart material in the field of remediation of environmental pollutants [19]. Not a single method is adequate to degrade highly stable aqueous dyes. Therefore it warrants effective remove methods for water contaminants. Among them photodegradation is the best alternative method to the conventional methods for remediation of toxic and hazardous pollutants from water and wastewater [20][21][22].
In the present study synthesise ZnO NPs using agro-waste durva grass aqueous extract was planned for the first-time, charactarized by different instrumental techniques and finally its applications to environmentally harmful MB and MO dye degradation and antioxidant study using DPPH assay against standard antioxidant ascorbic acid were performed. Collecting material and preparation of durva grass extract The green durva grass was collected, cleaned with water and after thorough washing durva grass was cut into small pieces. After hot water washing the durva grass pieces were dried at room temperature over night. The dried durva grass was powdered and sieved by 100 mesh sieve. 3 gm powder was added to 100 ml distilled water and then boiled in a water bath at 100°C for 30 min to prepare the aqueous extract. The cooled aqueous extract was filtered through Waterman filter paper and then the filtrate was used for the synthesis of ZnO NPs.

Materials
Synthesis of zinc oxide nanoparticles About 0.329 gm of zinc acetate dihydrate Zn(O 2 CCH 3 ) 2 .(H 2 O) 2 (0.05 M) was dissolved in 30 ml double distilled water and then 30 ml durva grass aqueous extract was added dropwise with stirring thoroughly at 80°C for 3h. The solution colour changed from light green to brown after 10 min heating due to the surface Plasmon resonance indicating the formation of ZnO NPs. The cooled reaction mixture was centrifuged at 5000 rpm for 30 min and the obtained precipitation was washed thrice with absolute ethanol followed by double distilled water, dried on hot air oven. The resulting dried powder was pulverized and heated at 500°C for 3h inside a furnace. The obtained calcined ZnO NPs was subjected to characterization followed by applications.
Test for phenolic compounds Qualitative determination of phenolic compounds by phenols-Ferric Chloride test A few drops of 5% neutral FeCl 3 solution was mixed with 3 ml durva grass extract and the appearance of intense dark green color indicated the participation of phenolic groups in the durva grass extract.

Quantitative determination of phenols-FC reagent test
Total phenolic content in Argo-waste durva grass aqueous extract was measured using Folin-Ciocalteu (FC) reagent with the help of UV-vis spectrophotometer. In briefly, 1 ml extract was added with 2 ml FC reagent (10%) and then 4 ml sodium carbonate solution (5%) was added. The absorbance of the reaction mixture (intense dark green colour) was monitored by spectrophotometer. Standard gallic acid was used as standard for phenolic content, The total phenolic contents in the plant extracts is expressed in terms of gallic acid equivalent (the standard curve equation: y=0.004X-0.0753) and it is found to be 81.45 mg g −1 . The plausible mechanism of ZnO NPs synthesis using durva grass aqueous extracts is shown in scheme 1.

Characterisation of ZnO NPs
The optical properties of ZnO NPs were characterized by using UV-visible spectroscopy. ZnO NPs was dispersed in water by using ultrasonic bath (Enertech ultrasonic bath) for 20 min and then was analysed for absorbance at room temperature using UV-visible (Shimadzu UV-2450). The functional groups of capped phytochemicals from durva grass extract on ZnO NPs surfaces were identified by Attenuated Total Reflection-Fourier Transform Infra-Red (FT-IR) spectroscopy (JASCO ATR-FT-IR 4100) against durva grass powder as control in wave number range of 500-4100 cm −1 at a resolution of 4 cm −1 using potassium bromide pellets. The bandgap energy (E g ) of ZnO NPs was determined by using UV-vis-NIR spectrophotometer (JASCO V-670). The x-ray diffractogram (powder XRD, Bruker D8 advance) of ZnO NPs was taken by x-ray diffraction analysis after coating ZnO NPs onto XRD grid and spectra were recorded for the 2θ values of 10°-90°with a scanning rate of 4°min −1 . The powder x-ray diffractometer used CuKα radiation with a λ max of 1.54 Å.
The size and shave of the prepared ZnO NPs were checked by dispersing sample dispersion on Cu grid by HR-TEM (JEOL JEM 2100 USA) with 0.1 nm resolution and acceleration voltage of 200 keV. The elemental composition of ZnO NPs was analysed by energy dispersive x-ray (EDX) analysis. The electrophoretic mobility of the samples was analysed by Nano particci (SZ-100 i.e. DLS) instrument in checking sedimentation of the NPs based on the charge distribution. In addition, the stability of the ZnO NPs was analysed by thermogravimetric analysis (2100F -TGA-JOEL). Its acceleration voltage was 100 kV.

Catalytic activity of ZnO NPs
Degradation of MB and MO dyes by photocatalyst ZnO NPs was carried out using Heber photoreactor (Heber photoreactor HML MP 88) attached mercury lamp (0.170 Å, 8 W, 220-230 V at 50 Hz) with UV exposure (254 nm). Basically, 60 ml of MB and MO dye aqueous solution (10 mg l −1 ) was taken in a quartz tube and ZnO NPs (20 mg) was added with stirring for 30 min in the dark condition without UV-light exposure. In a similar way effects of varying catalyst and dye concentration were studied on the degradation of MB and MO dyes. The progress of degradation process was monitored using UV-visible spectroscopy by collecting samples time to time and measuring absorbance of the collected solution.

Antioxidant activity of ZnO NPs by DPPH assay
The antioxidant activity of ZnO NPs (50, 100, 150, 200, 250 mg l −1 ) was checked after mixing in 1 ml methanol solution of DPPH (50 mg l −1 ) in 2 ml Eppendorf tube for incubation at 37°C for 30 min in dark. The colour change (purple to yellow) was monitored by a UV-visible spectrophometry at λ max 517 nm in triplicate against std. ascorbic acid as reference antioxidant. The free radical scavenging capacity (%) of ZnO NPs was calculated as: Inhibition (%) of DPPH Scavenging activity=[(A 0 −A t ) * 100]/A 0 Where, A 0 is absorbance value of control sample (DPPH) and A t is absorbance value of the test sample (DPPH+ZnO NPs) at time 't'.

Results and discussion
Absorption spectrum of agro-waste durva grass aqueous extract shows three absorption peaks at 274 nm, 322 nm and 416 nm due to π→π * transitions ( figure 1(A)). These absorption peaks may develop due to the presence of phytochemicals in the aqueous extract [23]. Normally polyphenolic, carboxylic and nitrile compounds appear at 270-274 nm in the absorption spectrum. Formation of ZnO NPs turned solution colour from yellow to brown with a SPR band at 362 nm by spectroscopy ( figure 1(B)). Similar result was reported for ZnO NPs synthesized by using tea leaf aqueous extract [24]. UV-vis diffuse reflectance spectroscopy (UV-Vis-DRS) study was carried out using pellets form of ZnO NPs to determine band gap (E g ) value between 200-800 nm. The band gap energy was calculated by using the following formula as (αhυ) 2 =A(hυ−Eg) n , Scheme 1. Plausible mechanism for the synthesis of the Argo-waste durva grass aqueous extract mediated ZnO NPs.
where α is the absorption coefficient, A is the constant, hʋ as photon energy, n=2 for direct and n=½ for indirect transition, Eg is the band gap energy. Figure 1(C) shows the graphical plot of hʋ versus (αhʋ) 2 and it gives the optical band gap energy of 2.89 eV which is similar to other reports [25].
X-ray diffraction (XRD) analysis was performed to investigate the crystalline nature and purity of the calcined ZnO NPs.   cellulose, terpenoids and proteins compounds. Khodadadi et al [27] reported the presence of different functional groups in Achillea millefolium L. extract which was responsible for the synthesis of Ag NPs.
Normally plant based synthesized metal and metal oxide NPs possess higher stability due to capping of phytochemicals on their surfaces. Hence FT-IR analysis was performed to identify the possible functional groups adhere to synthesized ZnO NPs surfaces. Figure 2(B) shows the FTIR spectrum of ZnO NPs. Phytochemicals such as alkaloids, flavonoids, cellulose and phenols, proteins and polyphenols having functional groups of carboxylic acid, ketons, amines and alcohol may reduce to zinc salts to ZnO NPs. However polyphenols exhibited more active role in the biosynthesis of ZnO NPs [28]. A significant peak at 1037.70 cm −1 is referred to -C-O-H bending vibration. The new stretching vibration peak appeared at 437.84 cm −1 corresponding to metal and oxygen (Zn-O) stretching vibration. No other peaks appeared which suggests purity of ZnO NPs.
GC-MS analysis was carried out to screen and identify phytochemicals of durva grass extract which were responsible for the formation of ZnO NPs. The two major peaks appeared at retention time of 21.32 and 22.94 min in the GC-MS chromatogram ( figure S2). The 21.32 min peak indicated aliphatic aster compound and 22.94 min peak for aromatic ester compound. Other major peaks i.e. 15.52 min peak could be assigned to aromatic heterocyclic compound, the peak at 17.92 min to boron compounds, the peak at 19.82 min represent to fatty acids and the peak at 27.68 min could be assigned to aliphatic aster compounds (table S1). These compounds i.e. phytochemicals in durva grass extract were identified by matching with NIST library and could be responsible for the synthesis of ZnO NPs.
Size and morphology of synthesized ZnO NPs were checked by HR-TEM.   Thermogravimetric analysis (TGA) shows any loss of weight during annealing of ZnO NPs at high temperature ( figure 4(A)). Total two weight losses were observed in the range of 50°C-800°C. Initial 4.9% of the total weight loss at 50°C-400°C was due to the expulsion of surface adsorbed water molecule and 2nd wt. loss (1.8%) occurred at 400°C-800°C, because terminal hydroxyl groups bonded to the surface of ZnO NPs were lost. No further weight loss was observed even after high temperature heating (>800°C). Wang et al [31] reported a weight loss of 6.7% on heating in between 50°C-800°C due to removal of volatile products from ZnO NPs. All organic residues was lost after annealing at and above 500°C and no further weight loss was there within 400°C-800°C in the TGA curve of ZnO NPs, but pure crystallinity was achieved by heating at 800°C.
The dispersion stability of ZnO NPs was checked by measuring Zeta potential using photon correlation spectroscopy (PCS) i.e. dynamic light scattering (DLS) study. Figure 4(B) shows Zeta potential of −46.9 mV which describes the high stability of ZnO NPs dispersion. Similar results were reported by Suganthi et al [32] and Moghaddam et al [33]. The higher zeta potential value indicates its higher surface charge on ZnO NPs surfaces.
Photocatalytic activity of ZnO NPs depends on its surface area. Figure 4(C) shows Brunauer-Emmett-Teller (BET) analysis of ZnO NPs surface area and it is 44.84 m 2 g −1 . Similar BET surface area of ZnO NPs was reported elsewhere as 34 m 2 g −1 [34]. Figure 4(D) shows the Barret, Joyner and Halenda (BJH) model for pore size calculation and average pore diameter is 12.42 nm and pore volume is 0.16 cc g −1 . Figure 4(C) represents the hysteresis loop of adsorption and desorption isotherm which is very similar to type IV isotherm [35]. The specific surface area of nanomaterials is correlated to its particle size in BET model by the following formula as S = 6/ρd, where ρ is the ZnO NPs density (5.67700 g cm −3 ) taken from XRD analysis, d is the particles size; S is the specific surface area from BET-adsorption and desorption equilibrium (44.84 m 2 g −1 ). The average size of ZnO NPs is 23.19 nm, which is in good agreement with TEM (17.25 nm) and powder XRD (12.42 nm) results. The slight difference in particle size is because all particles are assumed to be spherical in nature by BET model.

Photodegradation of methylene blue (MB)
Degradation of MB dye was performed by ZnO NPs as photocatalyst under UV irradiation (λ=254 nm). Figure 5(A) shows degradation of MB dye within 0 min to 60 min and MB dye concentration decreased with time by UV irradiation and photocatalyst ZnO NPs. The degradation progress of MB dye was followed by UV-vis spectrophotometer and the dye degradation (%) was calculated by using the following formula: Degradation of dye (%)=(C 0 −C t ) * 100/C 0 where, C 0 is the initial concentration of dye absorbance and C t is the concentration of dye at time 't'. Initially degradation of MB dye was monitored without ZnO NPs and only 11% degradation occurred within 60 min ( figure 5(A)), whereas 98.11% of MB dye was degraded with photocatalyst ZnO NPs within 60 min under UV irradiation ( figure 5(B)). Diallo et al [36] also reported degradation of MB and Congo red dyes in the presence of UV irradiation using ZnO NPs as photocatalyst. The present degradation results are compared with other reported literatures (table 2). Figure 5(C) shows degradation of MB dye (10 mg l −1 ) with varying catalyst dose (10 mg, 20 mg, 30 mg). The photodegradation efficiency of MB dye was increased from 97.98 to 99.42 with increasing catalyst doses (10 to 30 mg). Photocatalytic dye degradation followed a pseudo first order kinetics model as calculated by the following equation as Where, r is the degradation rate of dye, C is the concentration of dye solution, K is the adsorption coefficient of dye, t is the time of sampling during the degradation, and k is the reaction rate constant. Also, Venkata Reddy et al [37] reported degradation of Rh-B dye under UV irradiation, whereas Ranjan et al [38] reported photodegradation of MB dye by ZnO NPs as photocatalyst and found that the value of rate constant increased with increasing catalyst dose (table 3). When the initial concentration of dye solution was increased from 5 mg l −1 to 15 mg l −1 , the removal rate of dyes decreased from 98.84% to 97.63% for MB which is not so significant ( figure 5(D), table 3). Figure 6(A) shows the photocatalytic dye degradation of MO in the presence of ZnO NPs as photocatalyst under UV irradiation (λ=254 nm) and λ max of MO dye is 462 nm in UV-visible spectrum. Only 8% degradation of MO happened without ZnO NPs photocatalyst within 100 min under UV irradiation ( figure 6(A)), whereas 97.45% MO was degraded within 100 min with ZnO NPs as photocatalyst under UV irradiation ( figure 6(B)). The obtained results of this study are resembled well with other reported literatures on MO dye degradation study (table 2). Figure 6(C) shows degradation of MO dye (10 mg l −1 ) with varying catalyst dose (10 mg, 20 mg, and 30 mg). The photodegradation efficiency of MO dye was increased from 97.09% to 98.12 respectively (figure 6(C)). When the initial concentration of dye solution was increased from 5 mg l −1 to 15 mg l −1 , the removal rate of dyes decreased slightly from 98.04% to 96.54% for MO (figure 6(D), table 3). Our results supported the findings of Kaur et al [51], because high concentrated dye molecules blocked majority active sites of catalyst, the adsorption of O 2 and OH on the photocatalyst was decreased leading to less generation of radicals. Also, the present study results are similar to Eucalyptus globules L. plant mediated synthesized ZnO NPs based MB and MO dyes degradation in the presence of UV light irradiation [52].

Possible mechanism of dye degradation
The plausible mechanism of dye degradation using semiconductor ZnO NPs as photocatalyst towards degradation of MB or MO is predicted as follows: When UV light was irradiation on ZnO NPs (2.89 eV), reactive oxygen species (ROS) was generated in situ which sequentially interacted with dyes and degraded to carbon dioxide and water molecules. Ajmal et al [53] proposed similar mechanism for TiO 2 NPs photocatalyst mediated dye degradation. When ZnO NPs is irradiated with UV light, conduction band gets electrons from valence band (equation (1)). The conduction band electron (e ̅ CB ) reacts with oxygen to produce the super oxide radical ions ( · O̅ 2 ) (equation (2)) which react with hydrogen ions (H + ) to produce peroxide ions (-· OOH) (equation (4)), whereas valence band holes (h + VB ) react with water molecules to release hydroxyl ions which then releases hydroxyl radicals (equations (3) and (5)). On the other hand the -· OOH reacts with CB electrons and H + ions to generate H 2 O 2 which consequently produce hydroxyl radical ( • OH). The photoelectrons reduce O 2 adsorbed on the photocatalyst surface into · O̅ 2 . Finally dyes were degraded by interacting with hydroxyl radicals ( • OH) and superoxide radicals ( · O̅ 2 ) to produce carbon dioxide and water molecules (equation (6)-(9)). Figure S3 shows possible reaction mechanism of MB and MO dye degradation in the presence of photocatalyst ZnO NPs under UV irradiation. Here the semiconductor ZnO NPs catalysed the redox reaction in the presence of air/O 2 and water leading to complete mineralization of organic pollutants. Zangeneh et al [54] proposed similar radical mechanism for photocatalyst mediated dye degradation.

Reusability of the synthesized ZnO NPs
Recyclability test for the degradation of MB and MO dyes was done to evaluate the stability or performance of the prepared ZnO NPs under similar experimental conditions. The photocatalytic dye degradation experiments were repeated five times with 20 mg catalyst in 60 ml dye solution (10 mg l −1 ) under UV irradiation. For reusability experiments, the catalyst was collected from reaction mixture, carefully washed with deionised water and dried after each experiment. The results of recyclability experiments are shown in figure 7. The degradation efficiencies of MB for five consecutive cycles were 98.11%, 97.15%, 96.68% and 96.20%, 95.73% respectively and degradation efficiencies of MO were 97.45%, 95.76%, 94.06% and 93.22%, 92.37% respectively. The results show a slight decrease in photodegradation efficiency after each cycle which matched to the results of Ban et al [55]. However, the biosynthesized ZnO NPs exhibited good photocatalytic activity with high stability under UV irradiation after 5 cycles of experiment.
Antioxidant activity of ZnO nanoparticles Nagajyothi et al [56] reported antioxidant activity of ZnO NPs using DPPH assay. Banerjee et al [57] performed antioxidant activity of bulk and nanostructured polyaniline nanofibers and nanostructured polyaniline fibres and showed higher free radical scavenging activity since ZnO NPs possess higher surface to volume ratio compared to its bulk form. Herein antioxidant activity of ZnO NPs and extract was checked by using standard DPPH test. The results obtained in the antioxidant assay show effective free radical scavenging capacity of ZnO NPs.   figure 8(A)), whereas in the presence of extract scavenging capacity (%) increased from 11.94% to 54.79%. In a similar way we performed with standard ascorbic acid which shows the scavenging capacity (%) increased from 31.63% to 94.95% ( figure 8(A)). The colour change of methanolic DPPH solution from purple to yellow was observed by the addition of ZnO NPs, because electron density of oxygen atoms in ZnO NPs was transferred to odd electron located at nitrogen atom in DPPH molecule i.e. this process increased the stability of DPPH molecule as well as increased the scavenging capacity of ZnO NPs. Also, the absorbance of the peak at 517 nm decreased which indicated the increasing free radical scavenging activity of ZnO NPs. Actually ZnO NPs being an electron donor, can convert free radicals to more stable products by terminating radical chain reaction [58]. Hence the reducing power of ZnO NPs is correlated well with the radical scavenging activity. Figure 8(B) shows the possible antioxidant mechanism of ZnO NPs in DPPH assay. In dark electron transfer from nanoparticles to 2,2-diphenyl-1-picrylhydrazyl molecules leads to stable 2,2-diphenyl-1picrylhydrazine which proportionally varies with ZnO NPs dose [59].

Conclusion
This reports 1st time on synthesis of ZnO NPs by using agro-waste durva grass aqueous extract. The synthesized nanocatalysts were characterized by UV-Vis, FTIR, powder XRD, HR-TEM, DLS and TGA analysis. The photocatalytic activity of ZnO NPs was studied for MB and MO dyes degradation with UV irradiation which was monitored by using UV-vis spectroscopy. MB degraded by 98.1% within 60 min, while 97.5% MO degraded within 100 min at a photocatalyst dose of 10 mg l −1 . As-synthesized ZnO NPs showed good photocatalytic activity as well as good antioxidant activity using DPPH assay against ascorbic acid as antioxidant standard. Hence the synthesized ZnO NPs could be considered as a good photocatalyst and antioxidant.