Biogenic synthesis of silver nanoparticles and its photocatalytic applications for removal of organic pollutants in water

Journal of Industrial and Engineering Chemistry 80 (2019) 247–257

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Journal of Industrial and Engineering Chemistry journal homepage: www.elsevier.com/locate/jiec

Biogenic synthesis of silver nanoparticles and its photocatalytic applications for removal of organic pollutants in water Jagpreet Singha,1, Vanish Kumarb,1, Sukhwinder Singh Jollyc, Ki-Hyun Kimd,* , Mohit Rawata,* , Deepak Kukkara , Yiu Fai Tsange a

Department of Nanotechnology, Sri Guru Granth Sahib World University, Fatehgarh Sahib - 140406, Punjab, India National Agri-Food Biotechnology Institute (NABI), S.A.S. Nagar, Punjab, 140306, India Department of Mechanical Engineering, Sri Guru Granth Sahib World University, Fatehgarh Sahib - 140406, Punjab, India d Department of Civil & Environmental Engineering, Hanyang University, Seoul, 04763, Korea e Department of Science and Environmental Studies, The Education University of Hong Kong, Tai Po, New Territories, Hong Kong b c

A R T I C L E I N F O

A B S T R A C T

Article history: Received 17 March 2019 Received in revised form 1 August 2019 Accepted 1 August 2019 Available online 9 August 2019

A facile/clean/sustainable route for the preparation of silver nanoparticles (Ag NPs) was investigated using Trigonella foenum-graecum (TF) leaf extract. The bio-reduced Ag NPs showed photocatalytic degradation potential of 88% and 86% for reactive blue 19 (RB19) and reactive yellow 186 (RY186), respectively (at 180 min). The complete mineralization of degraded medium was monitored by decline in chemical oxygen demand (COD). The role of activation energy in photocatalytic degradation process was investigated across different temperatures. In light of their photocatalytic efficiency, reusability, and environment friendly synthesis approach, biogenic Ag NPs were demonstrated as effective system for efficient water purification. © 2019 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights reserved.

Keywords: Green synthesis Silver nanoparticles Trigonella foenum-graecum Photocatalytic degradation Chemical oxygen demand

Introduction Freshwater resources are indispensable components of life due to their ability to support and sustain a large number of species and their role for supporting life activities (e.g., drinking, washing, and cultivation) [1]. The discharge of untreated industrial effluents into these pure resources has substantially deteriorated environmental conditions with significant health threats. These effluents may consist of many life-threatening emerging pollutants including cationic/anionic organic dyes, pesticides, and aromatic nitro compounds (such as reactive blue 19 (RB19), reactive yellow 186 (RY186), 4-nitrophenol (4-NP), and nitrobenzene). Many manmade production processes including paper mills, textile, plastic, leather, food, printing, and pharmaceutical industries are the primary sources of these emerging pollutants [2–5]. These pollutants adversely affect human life through various routes such as damage to multiple organs (e.g., nervous system, excretory system, and liver) [6,7]. In addition, the discharge of organic dyes

* Corresponding authors. E-mail addresses: [email protected] (K.-H. Kim), [email protected] (M. Rawat). 1 These authors are considered as co-first authors because they contributed equally to this work.

into the hydrosphere (or water bodies) is found to disturb environment by increasing undesirable turbidity in the water system, thus limiting the oxygen dissolving capability (in water) and sunlight penetration. These physicochemical phenomena can also harm various aquatic life forms [8]. The industrial revolution has disbursed various types of pollutants including organic dyes (especially with interlinked and tangled configurations). The presence of functionally reactive groups (~40 types of reactive groups) in the organic dyes is known to facilitate the formation of covalent bonds with various substrates such as cellulose, organic fiber, and protein molecules. As such, reactive dyes exhibit very slow biological degradation rate [9]. For example, the half-life of the hydrolyzed dye RB 19 is approximately 46 years (at a pH level of 7 and 25
C) [10]; hence, this pollutant persists in the environment over considerably long periods. As a result, it remains as one of the most difficult tasks to remove these compounds within short period and without producing any other harmful by-products. Numerous conventional methods (such as electrochemical process, chemical oxidation, coagulation, membrane separation, microbial degradations, ion exchange, and reverse osmosis) have been exploited to eliminate dyes from dye bearing effluents [11–13]. In general, the dye eliminating methods are categorized into three parts: physical, chemical, and biological processes. As described in Figure S1, these methods have inherent merits and

https://doi.org/10.1016/j.jiec.2019.08.002 1226-086X/© 2019 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights reserved.

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limitations [14]. As an alternative to these conventional methods, the advanced oxidation processes (AOPs) have recently been developed. These processes are based on the generation of super oxides (e.g., hydroxyl radicals) or other highly reactive species that can induce quick and non-selective oxidation of various target organic pollutants [15,16]. Among the numerous applications available for nanomaterials, photocatalytic approaches have been frequently explored to effectively degrade pollutants. In general, a particular light source is commonly required to induce a photocatalytic oxidation. Such specific requirement may restrict the practical requisition of photocatalyst-based remediation processes [17–25]. The existing photocatalysts systems have many shortcomings such as use of expensive light sources and complex/harmful synthesis processes that may obstruct their demand in real world. Consequently, the utilization of direct sunlight irradiation (e.g., a visible light source) is highly desirable to expand the practical and effective implementation of photocatalytic systems. Generally, solar irradiation comprised with 45% of visible light (l > 400 nm) and 5% of UV-light (l < 400 nm) [26]. As the silver nanoparticles (Ag NPs) have an exceptional property to absorb the visible and UV light from solar irradiations due to surface plasmon resonance (SPR) effect and interband transitions, respectively. Thus, they have prodigious potential to combat with noxious dyes through effective photocatalytic process [27]. Therefore, the practical implementation (e.g., operation under visible light) of nanostructures-based photocatalysts (e.g., Ag NPs) should be one of the most effective and preferable system to treat noxious organic pollutants [21,28–31]. Among the nanomaterials, metal nanoparticles (such as Ag NPs) gained abundant attention in the scientific regime and industrial field because of their imperative physical, chemical, and optical properties [32]. The most commonly used approach for Ag NPs synthesis is the chemical reduction of silver salt by employing organic and inorganic molecules as reducing agents (e.g., trisodium citrate, dimethyl formaldehyde, hydrazine, m-hydroxy benzaldehyde, sodium borohydride, poly(ethylene glycol), and paraffin) [21,33–41]. Although chemical reduction methods offer several benefits, but they tend to suffer from many demerits like potentially menacing effects on the environment [42]. These deleterious effects of the chemical reduction method can be diminished by using green approaches to reduce metal salt. One of the green options to synthesize NPs is the use of a plant extract as a reducing agent and stabilizing agent as well. In numerous studies, the possibility for the preparation of biogenic Ag NPs was explored using a variety of plants (e.g., Acorus calamus, Azadirachta indica, Alternanthera dentate, Ocimum Sanctum, A. barbadensis Mill., Brassica rapa, Syzygium cumini, Coccinia indica, and Morus) [43–47]. The phytochemicals in plant extracts (e.g., terpenoids, polyphenols, tannins, reducing sugars, alkaloids, anthocyanins proteins, and phenolic acids) were responsible for the reduction of the metal salt into metallic nanoparticles and their capping as well [48–52]. Likewise, Trigonella foenum-graecum (TF) extract is rich in the aforementioned natural reducing agents that can be employed for biogenic NP synthesis. This plant stands as a self-pollinating annual leguminous bean, which belongs to the Fabaceae family and is universally known as Indian methi [53]. One of the major advantages of using TF extract is its fast growth rate, i.e.,~20 days for the first leaf [54]. The main phytochemicals present in TF leaf extracts are polyphenols and flavonoids that are responsible for the chemical reduction of silver nitrate [55]. In light of the importance of an efficient system for photocatalytic applications, biogenic Ag NPs were prepared to investigate their photocatalytic performance for the removal of two industrial dyes (e.g., RB19 and reactive yellow 186 (RY186)). The industrial dyes (e.g., RB19 and RY186) are not easy to degrade (e.g., as compared to lab dyes (e.g., methylene blue and congo red) due to

their high molecular masses and low diffusion rates. The efficiency of photocatalytic degradation against the industrial dyes was examined using Ag NPs as photocatalyst under direct sunlight environment to assess the role of activation energy across different temperatures. In addition to superior dye degradation efficiency, the developed photocatalysts (Ag NPs) are attractive because of their easy recovery and high reusability potential, which are essential for efficient photocatalysis. Therefore, the photocatalytic capabilities of Ag NPs firmly advocated their strong potential in the sanitization of polluted water by eradication of baleful contaminants. Materials and methods Materials TF leaves were obtained from a local market at Fatehgarh Sahib, Punjab. Silver nitrate (AgNO3, 99.9%), 4-nitrophenol (4-NP), and sodium borohydride (NaBH4) were purchased from Merck, Germany. RB 19 and RY 186 were purchased from Parswanath Dye Stuff Industries, Ahmedabad, India. Titanium dioxide (P-25) was purchased from Evonik Degussa India Pvt. Ltd., which consisted of about 70% anatase/30% rutile (Particle size: 30 nm and BET surface are: 50 m2/g). Before starting of the experiment, all glass wares were cleaned with freshly prepared piranha solution (H2SO4/H2O2 with ratio of 3:1). Methods Preparation of TF leaves extract The TF leaves were washed with distilled water (DI) water to eradicate particulate matter and dried in sunlight irradiations (60 min). Then, the 10 g of dried leaves were cut into fine pieces and boiled in 100 mL of DI water for 1 h to prepare leaf extract. Afterward, leaf extract was filtered by Whatman filter paper no:1 (1.5 mm pore size) for its separation from the mixture. The filtrate was then stored at 4
C for future use. Synthesis of Ag NPs The stepwise synthesis process of biogenic Ag NPs is illustrated in Fig. 1.To begin with, 0.0849 g of AgNO3 was added into 50 mL of DI water to make 102 M stock solution of AgNO3 and stirred for 20 min at 250 rpm for complete dissolution. To reduce silver nitrate into Ag0, 5 mL of prepared TF extract (optimum volume after various trails, i.e., 2, 3, 4, 5, and 6 mL as shown in Figure S2(a)) at pH value 10 (optimum value after various trails, i.e., 2, 4, 6, 8, and 10 pH as shown in Figure S2(b)) was added into AgNO3 solution (50 mL) with constant stirring (250 rpm for 60 min for complete reduction) at room temperature (RT) [46,56]. With the passage of time, the color of the solution was changed in the order: colorless, yellow, and reddish brown. The reddish brown color indicates the formation of Ag NPs [57]. Catalytic activity of Ag NPs To substantiate the catalytic potential of biogenic Ag NPs, the catalytic reduction of 4-nitrophenol (4-NP) to 4-aminophenol (4-AP) was initially investigated before proceeding to the photocatalytic reduction of organic dyes. For this purpose, the 20 mL of 102 M solution of 4-NP was mixed with 200 mL of 101 M solution of sodium borohydride in quartz cuvette and added into 3 mL DI water. After that, the different amounts of Ag NPs were added into the prepared solution for the analysis by the time dependent UVvisible spectra. The performance of 20–40 mL (~1.68 mg/mL) of Ag NPs on the reduction of 4-NP was evaluated (Fig. S3(c,d)). In our study, the optimized amount of Ag NPs required for 4-NP reduction

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Fig. 1. Flow chart for the synthesis of biogenic Ag NPs and characterization tools.

was 50 mg (30 mL of Ag NPs in solution) in light of its relatively shorter duration than that of other concentrations (Fig. S3(a)). [58–60]. A gradual change in the solution color from bright yellow to the colorless phase was observed during the reaction, indicating the formation of 4-AP [61]. At time t = 0, the absorption band (at 400 nm) was ascribed to the origination of nitrophenolate ions from 4-NP. As the absorption intensity (at 400 nm) decreased with the addition of biogenic Ag NPs into the reaction medium (4-NP + NaBH4+ water), a new peak at 298 nm appeared concurrently. This change in absorption intensity and the presence of new absorption band signifies the formation of 4-AP (Fig. S4(a)) [62]. In addition, blank experiment (e.g., without catalyst (Ag NPs)) was performed to evaluate the possible reduction of 4-NP in the absence of a catalyst. An insignificant decrease in the absorption intensity at 400 nm was observed in the absence of Ag NPs. A high activation energy barrier (e.g., between the acceptor molecules (nitrophenolate ions) and donor (NaBH4)) was attributed to the ineffectiveness of NaBH4 in the absence of Ag NPs (Fig. S4(b,d)) [63]. Moreover, Ag NPs alone were not able to reduce 4-NP (Fig. S4 (c)). The developed approach almost completely (~98%) reduced 4-NP to 4-AP within 15 min of reaction time. In addition, the kinetic study of the catalytic reduction was evaluated by plotting ln(C/C0) versus the reaction time (Fig. S3(b)). The pseudo first order rate equation was used to compute the apparent rate constant (K’app) for the reduction of 4-NP. The K’app of Ag NPs was determined to be 0.159 min1. Therefore, this enhanced catalytic potential of synthesized Ag NPs can be utilized further in the photocatalytic degradation of dyes like RB19 and RY186 dyes. The effectiveness of the present Ag NPs based 4-NP reduction system is compared with other Ag NPs systems in Table S1.

strength of solar irradiations when monitored by pyranometer was observed as 630 watt/m2 with air mass coefficient value (standard AM 1.5) of 1 and direct beam intensity (ID) of 0.947 kW/m2 [64,65]. After the reaction, the photocatalysts were separated from the solution using centrifugation at 10,000 rpm for 20 min. At last, 2 mL of supernatant was collected and the absorption spectra of the supernatant were recorded. The absorbance of the dye solution decreased with time. This phenomenon was suspected to be caused by the degradation of the dye. The absorbance spectra of RB 19 and RY 186 were recorded after a particular time interval to monitor the change in their absorption intensity at 582 and 425 nm, respectively. According to the Beer Lambert’s law, the absorbance is directly proportional to the concentration of colored moiety. Hence, the degradation efficiency (R) can be calculated as follows [66]: R={(C0-C)/ C0}100 ={(A0-A)/A0}100

(1)

where C0/C and A0/A correspond to the concentration and absorbance of dyes at time t = 0/t, respectively. Note that the degradation efficiency of the photocatalyst was also verified by monitoring chemical oxygen demand (COD) and cleavage of NH bonding in the FTIR spectra of RB19 dye. Biosafety analysis As per standard protocols, the leaching of silver ions has been checked through precipitation method [67]. After the photocatalytic process, the solution (dye + Ag NPs) was centrifuged at 15,000 rpm for 25 min to collect supernatant. Afterwards, hydrochloric acid (HCl) was added into supernatant and required observations were noted.

Photocatalytic activity of Ag NPs against organic dyes Characterization of biogenic Ag NPs As the main part of this study, the as-synthesized biogenic Ag NPs were used to degrade 30 ppm RB 19 and RY 186 in sunlight irradiations. The process began by liquefying the dye in DI water and stirring it for the duration comparable to that of the established adsorption-desorption equilibrium. The dye degradation was also tested without a photocatalyst under direct sunlight exposure. Afterward, 10 mg of Ag NP catalyst was mixed with 50 mL of 30 ppm RB 19 and RY 186 aqueous solution in 250 mL vessel on magnetic stirrer at constant stirring speed of 150 rpm (for 10 min) followed by sonication (for 10 min). Then, the solution was exposed to sunlight for a fixed reaction time (180 min for both dyes). The

The phase, nanocrystalline size, and inter planar spacing between crystallographic planes of the Ag particles were identified using X-ray diffraction (XRD) pattern performed on PANalytical Xray diffractometer, Japan. For XRD analysis, powder form of Ag NPs has been obtained through centrifugation and drying processes. The light absorption studies of Ag NPs suspension were performed on a Shimadzu-UV 2600 spectrophotometer using a quartz cuvette with a wavelength ranging from 200 to 800 nm. The intensity of the UV lamp used was 27 W/m2. The average particle size and polydispersity index (PDI) of Ag NPs suspension in diluted form were estimated using Zetasizer Nano (Malvern-ZEN-1690). The

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morphology and structure of the as-prepared samples were evaluated using Carl Zeiss field emission scanning electron microscopy (FE-SEM) and high- resolution transmission electron microscope (HR-TEM: Jeol JEM-2100) at 200 kV accelerating voltage. The Ag NPs were uniformly suspended in ethanol via sonication and drop casted on aluminium support and cupper grid for SEM and TEM analysis, respectively. The samples were appropriately dried before the microscopic analysis. The purity of the samples along with probable elemental chemical composition was determined by energy dispersive X-ray (EDX) spectroscopy (Oxford instruments: X-Max 51 –XMX0004). The surface chemistry of the samples (Ag NPs suspension) was examined on Alpha FTIR spectrophotometer (Bruker Corp.) in the range (4,000— 600 cm1). COD was monitored by Thermo Orion Aqua Fast II AQ 2040 automated COD colorimeter. After photocatalytic process, the required amount (2 mL in our case) of sample (photocatalyst treated dye) was added into the COD vial and placed in COD reactor at default temperature (i.e. 150
C). After that the readings was noted with time. Results and discussion Synthesis of Ag NPs The bioreduction of Ag+ ions to Ag NPs was investigated by measuring the light absorbance using UV-visible spectroscopy (Fig. 2(a)). The addition of TF extract into the AgNO3 solution changed the color for the reaction solution from colorless to light yellow, followed by a reddish brown color. The plasmon resonance in Ag NPs was attributed to the aforementioned color effect [68]. The absorption band at 428 nm in the absorption spectrum of TF mediated Ag NPs was attributed to the plasmon resonance of Ag NPs. The UV-visible spectra of the silver nitrate solution showed a peak around 217 nm (Fig. 2(a)), which was expected due to Ag+

ions; the absorption peaks around 278 nm were expected due to the presence of polyphenol in the TF extract [57]. The average diameter and PDI of bio-reduced Ag NPs were 71.7 nm and 0.336, respectively (Fig. 2(b)). These observations indicated the formation of monodispersed and non-aggregated Ag NPs. The subsequent XRD pattern of powdered Ag NPs was analyzed over 2u values of 0-808 and is illustrated in Fig. 2(c). The XRD patterns obtained for Ag NPs were in agreement with the facecentered cubic structure of Ag NPs (JCPDS 04-0783). The peaks for the crystal planes of (111), (200), (220), and (311) were observed at 2u values of 38.178, 44.278, 64.428, and 77.478, respectively. The lattice parameters of Ag NPs were as follows: a = 4.0862, b = 4.0862, and c = 4.0862. The crystal parameters of Ag NPs were calculated based on the Scherer equation [69]: D¼ 

Kl bcosu

where D is the mean crystal size (nm), l is the X-ray wavelength (1.54 Å for Cu Kα source), β is the full width of the half maxima (FWHM), k is the shape factor ð0:9Þ;  and u is the angle of X-ray diffraction peak. The crystalline size of as-prepared Ag particles was 24 nm. The FTIR spectra were recorded to identify the interactions between Ag NPs and the phytochemicals present in the TF extract. The FTIR spectrum in Fig. 2(d) indicates the absorption bands at 3841, 3740, and 3612 cm1 attributing to the hydroxyl (OH) stretching of alcohols and phenols. The absorption bands at 2927, 2313, 1522, and 1012 cm-1 specify the presence of C–H stretching, CRN stretching, N–O stretching, and C–O stretching, respectively. In particular, the absorption peaks at 1917, 894, and 769 cm1 represent the Ag NPs bonding with oxygen [70,71]. Therefore, the FTIR study confirmed the dual role of TF extract as a reducing and capping agent and the strong interactions of Ag NPs with the chemical groups in the TF extract.

Fig. 2. Spectroscopic and structural Characterizations of Ag NPs synthesized using TF extract: (a) UV-visible spectra of AgNO3 solution (black line), TF extract (green line), and Ag NPs (red line). Inset pictures show the visual appearance of synthesized biogenic silver nanoparticles at different time intervals: (A) 0, (B) 25, (C) 40, and (D) 60 min. (b) Zetasizer of biogenic Ag NPs. (c) XRD pattern of Ag NPs, and (d) FTIR spectra of Ag NPs (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article).

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Fig. 3. Morphological and elemental analysis of Ag NPs (a) FE-SEM images at 200 mm scale bar, (b) FE-SEM images at 1 mm scale bar, and (c) EDX spectrum.

The topographical and elemental analyses (quantitative as well as qualitative) of synthesized Ag NPs were elucidated from FE-SEM and EDX analysis. In Fig. 3(a–b), the synthesized Ag NPs have spherical morphology. The elemental analysis revealed high amounts of Ag (70.27%) and oxygen (29.73%) in the EDX spectrum of synthesized Ag NPs (Fig. 3c). The oxygen traces could have originated from the binding of phytochemicals present on the surface of Ag NPs or in the surrounding environment [72]. The morphology and particle size of the as-prepared Ag NPs were determined using HR-TEM. Fig. 4. shows that the biogenic Ag NPs exhibited a spherical shape with monodispersed size. The size of the synthesized Ag NPs was 5–20 nm. Thus, the electron microscopy studies were consistent with the particle size analyzer results that shows the formation of highly stable spherical Ag NPs with a narrow size distribution range. Photocatalytic degradation of organic dyes by Ag NPs The small size and uniformity of Ag NPs prompted the photocatalytic degradation of RB19 and RY186 dyes. Fig. 5a and 5b show the UV-visible absorption spectra of Ag NPs driven photocatalytic degradation profiles of RB19 and RY186 under direct

exposure of sunlight. As shown in Fig. 5a and 5b, the absorption bands at 582 and 425 nm corresponding to the RB19 and RY186 dyes, respectively, decreased with time after the addition of Ag NPs under light exposure. To confirm the effectiveness of Ag NPs and the effects of sunlight on the degradation of dyes, a control experiments (with and without catalyst) were also performed in both the dark and sunlight atmosphere. It was observed that under the direct sunlight exposure without catalyst, the removal of RB19 and RY186 was insignificant. The graph clearly depicts negligible depreciation in RB 19 and RY 186 concentrations in the absence of a catalyst (Ag NPs) as shown in Fig. 5c and 5d. In the dark phase, adsorption-desorption equilibrium was achieved to ensure the complete adsorption of both dyes on to the catalyst surface. However, no significant degradation was subsequently observed in dark period, suggesting that the dye molecules were not adsorbed or degraded considerably. Moreover, dye degradation experiments, conducted under direct sunlight in the absence of catalyst, showed almost negligible degradation of dye (Fig. 5 (c and d)). Thus, the degradation of dyes with only light irradiation (absence of catalyst) was insignificant. This observation indicated the strong dependency of dye degradation on biogenic Ag NPs. Also, the degradation performance of commercial P25-TiO2 has been investigated with same experimental conditions as depicted in

Fig. 4. HR-TEM images of Ag NPs at (a) 100 nm and (b) 5 nm scale bar.

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Fig. 5. UV-visible spectra of the degradation patterns for the two model dyes (RB19 and RY186). Time dependent decrease in the absorbance of (a) RB19 and (b) RY186 dye in presence of Ag NPs. (c) and (d) shows normalized concentration of RB19 and RY186 dyes, respectively in the presence and absence of Ag NPs/commercial P-25 TiO2 under dark and sunlight conditions.

Fig. 5c and 5d. It was observed that the synthesized Ag NPs showcased high degradation efficiency for both dyes relative to P25-TiO2, which clearly indicates the advantages and efficacy of Ag NPs over commercial photocatalysts. It was suggested that the localized surface plasmon resonance (LSPR) property and interband transition in Ag NPs are responsible for their activity in visible and UV region of sunlight irradiations, respectively to degrade baleful dyes through photocatalysis process [27,73]. Absorption of visible light by the sp band electrons of the Ag NPs can cause the excitation of the electrons to higher energy state [27,74–76]. Afterwards, the heating of electron gas occurred due to release of energy by plasmon. These high energy or high temperature electrons yielded sufficient energy to generate oxygen free radicals (O2 ) after the interaction with environment oxygen (O2) molecules. The interactions of generated free radicals with dye molecules led to

the degradation of dye molecules. Moreover, the holes created in the 5 sp orbital further improved degradation of dye by accepting electrons from dye molecules. Likewise, in the presence of UV-light, the inter-band excitation of electrons from the 4d orbital to 5sp orbital takes place. Because of inter-band transition, a number of photogenerated electrons become excited to higher state. These highly energetic electrons interact with the oxygen and hydroxyl ion to produce oxygen (O2) and hydroxyl radical (OH), respectively. Consequently, theses free radicals are responsible for the degradation of the dye. Also, the holes formed in the d orbital of the Ag NPs accept the electrons from the adsorbed dye molecule to facilitate the dye degradation process. This dye degradation mechanism is based on direct plasmon photocatalysis [73,77–81]. The above explained dye degradation mechanism driven by Ag NPs is illustrated in Figs. 6 and 7. Additionally, the role of activation

Fig. 6. Schematic for the proposed mechanism of photocatalytic dye degradation by biogenic Ag NPs and also describing the role of activation energy.

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Fig. 7. Schematic depicting the response of inner energy bands of Ag NPs under UV and visible light in photocatalysis process.

Fig. 8. Plot of the degradation patterns of RB19 and RY186 dyes. Line-Weaver Burk plots for (a) RB19 and (b) RY186.Relationship expressed for (c) RB19 and (d) RY186 in terms of lnK versus temperature.

energy was also described in Fig. 6. The photocatalytic activity of the NPs is enhanced with decrease in activation energy barrier by increasing temperature (discussed in the kinetic study section) [82]. Interestingly, the exact mechanism of dye degradation by Ag NPs is yet unclear. A few other mechanisms are also proposed to understand to activity of Ag NPs for the degradation of organic molecules. For instance, LSPR induced hot electrons based

mechanism has been also investigated [83]. Due to the resonance effect, the hot electrons are generated in plasmonic metals to be transferred into the semiconductor support (as this transfer occurred within a few femtoseconds). Then these high energetic electrons were responsible for the degradation of organic molecules. However, in the present study, none of any extra semiconductor support was used. Further, the developed system was

Table 1 K’app and R2 values for RB19 and RY186. S.No:

Dye

CAS Registry number

K’app (first order) (102 min1)

R2

1. 2.

Reactive blue 19 (RB19) Reactive yellow 186 (RY186)

2580-78-1 84000-63-5

1.23 1.03

0.98446 0.96881

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Table 2 Variations in ln K for RB19 and RY186 with respect to the reciprocal value of temperature (T1) at a constant amount of biogenic Ag NPs. Dye

Temperature (K)

(1/T)

lnK

Ea (kJ mol1)

Reactive blue 19 (RB19)

301 305 309 313 301 305 309 313

0.00332 0.00327 0.00323 0.00319 0.00332 0.00327 0.00323 0.00319

4.50 4.39 4.32 4.26 4.76 4.57 4.44 4.31

19.61 13.84 11.84 – 36.7 29.9 20.7 –

Reactive yellow 186 (RY186)

Fig. 9. Histogram showing the photocatalytic degradation efficiency of Ag NPs for the removal of (a) RB19 and (b) RY186 dyes.

efficient enough to degrade the organic molecules without any support material. The kinetic study of dye degradation was scrutinized with the help the pseudo-first order rate equation (ln(C/C0) = -kt), where k is the apparent rate constant of the reaction, C0 and C are concentration of dye at time t = 0 and at any time t, respectively. From the kinetic study, the rate constants (K’app) and R2 values of RB19 and RY186 dyes were calculated to be 0.01230 and 0.01034 min1, and 0.98446 and 0.96881, respectively. The reaction rate constants were calculated from the pseudo-first order liner fit graph (Fig. 8(a, b)). The time required to remove both the aforementioned dyes was 180 min. The values of K’app and R2 of RB19 and RY186 were obtained from the pseudo-first order linear fit graph (Fig. 7(a) and (b)) and are summarized in Table 1. In the next step, the photocatalytic degradation of RB19 and RY186 dyes was studied as a function of temperature in the range of 301–313 K with an increase of 4 K (Table 2). Fig. 8 (c, d) shows that the ln K values with respect to 1/T (K1) followed a linear trend with a negative slope. Such a relationship alludes towards the Arrhenius nature of Ag NPs driven photocatalytic reaction in the given temperature range. Thus, it suggests that the number of effective collisions between molecules has increased with an increase in temperature. Accordingly, the activation energy (Ea) of the photocatalytic dye degradation was derived based on the Arrhenius equation as follows [84]:     K2 Ea 1 1   ¼  ð2Þ ln  T1 K1 R T2 where A is constant, Ea is the activation energy, K2 and K1 are the rate constants at temperatures T2 and T1, respectively, and R (8.314 J/K mol) is the universal gas constant. The values of Ea were estimated from the slope of the linear plot of ln k versus 1/T, as shown in Fig. 8(c,d). The values of Ea for the RB19 and RY186 dyes as determined from equation (ii) are described in Table 2. The obtained activation

energy (Ea) values were significantly low, especially in 301 to 309 K. As the low activation energy facilitated the completion of reaction [85], the present finding signifies the prodigious potential of Ag NPs as a green photocatalyst for the degradation of RB19 and RY186. The degradation efficiencies for the removal of RB19 and RY186 were calculated with the Lambert-Beer law using eq. (i), and were determined to be~88% and 86%, respectively (Fig. 9). In light of the kinetics and the thermodynamics data, we found the enhanced potential of biogenic Ag NPs as adsorbent and photocatalyst. The degradation and complete mineralization of degraded medium can be monitored by reduction in COD [86]. This is because the mineralization of organic compounds can lead to the formation of carbon dioxide (CO2) and water, which results in reduction of oxygen content [87]. Hence, in the present work, COD analysis of dyes has been investigated after the photocatalytic process. It was observed that the mineralization of organic species

Fig. 10. COD reduction studies of RB19 and RY186 dyes after photocatalytic process.

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Fig. 11. Reusability of the Ag NPs photocatalyst after four successive runs.

has been completed after 4 h (Fig. 10). The degradation efficiency of RB19 and RY186 was~88% and 86% after 180 min, but the complete mineralization of degraded medium demanded more time. This may be due to the formation of some organic intermediates [88]. Note that the analysis of intermediates was not carried out, because it was not the main motive our present work. Also, to confirm the degradation of these azo dyes (RB19 and RY186), FTIR analysis of RB19 dye has been carried out after photocatalytic process (Fig. S5). It was observed that the peak at 1520 cm1 was absent in FTIR spectra, indicating the cleavage of N H bonding and accordingly the degradation of dye [89]. For the practical application of the photocatalyst, its reusability is one of the momentous factor. As the dye molecules are not

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permanently adsorbed on the surface of catalyst, the catalyst can be easily separated from the dye solution through centrifugation process. As shown in Fig. 11, even after four successive experiments, the photocatalytic activity of the photocatalyst remained almost constant. A minor reduction in photo activity might be caused by the removal of photocatalyst particles during experiments. As shown in Fig. S6, no significant deviation in the PXRD profile of the Ag NPs was noticed except for slight drop in peak intensities (in comparison with pure Ag NPs). Therefore, the Ag NPs were seen to have a noticeable photostability and recyclability against dyes. As this process is related to water purification, biosafety analysis of the process has also been conducted. In this context, the presence or absence of silver ions (toxic in nature) has been monitored in solution after photocatalytic process [90,91]. (Note that the presence of silver ions may be expected in the reaction phase due to the leaching of silver ions from Ag NPs catalyst.) In general, the silver ions formed the white precipitates of silver chloride after reacting with HCl [92]. Hence, the leached silver ions can be monitored by observing the precipitates in the reaction mixture after the addition of HCl. As shown in Figure S7, before and after addition of HCl, the solution remains transparent without the formation of any visible precipitates. Thus, it confirms that no leaching of silver ions occurred after photocatalytic reaction. Finally, we compared our results with previous findings to analyze the effectiveness of Ag NPs as photocatalysts (summarized in Table 3). Based on this comparison data, it was observed that easily degradable laboratory dyes (e.g., congo red, methyl red, methylene blue, nile blue, methyl orange, and crystal violet) were used as the model in most studies [93]. Such as, Kumar et al. [73] and Sumi et al. [78] have prepared the Ag NPs by using Ulva lactuca and cocos nucifera extract respectively. They employed the synthesized NPs for the degradation of malachite green (MG) and methyl orange (MO) dyes, which are easily degradable [73,78]. This is because the diffusion rates of their dyes are higher than

Table 3 Performance comparison of different nanoparticles for the degradation of various pollutant dyes. S. No

Catalyst

Dye

Concentration of dye

Catalyst loading

Catalytic efficiency (%)

Reaction Time (min)

Rate constant (min1)

Ea (kJ mol1)

Reference

1.

Ag NPs

Ag Ag Ag Ag

Ag NPs

7.

Ag NPs

0.1 mg/mL 0.1 mg/mL 0.1 mg/mL 0.1 mg/mL 0.25 mg/mL 10 mg/100 mL 10 mg/200 m L 0.5 mg/mL 0.5 mg/mL 2 g/100 mL 2 g/100 mL 2 g/100 mL 10 mg/200 m L

— — — — 86.0 95.0 98.0 — — 99.0 97.0 95.0 98.2

1,440 1,440 1,440 1,440 150 4,320 240 360 20 60 60 60 240

1.44  103 1.86  103 1.03  103 1.02  103 0.268 — 1.72  102 — — 6.3  103 6.5  103 6.7  103 1.77  102

— — — — — — — — —

6.

10 mg/L 10 mg/L 10 mg/L 10 mg/L 1 104 M 10 mg/L 104 M 10 mg/L 100 mg/L 30 mg/L 30 mg/L 30 mg/L 102 M

[99]

2. 3. 4. 5.

Methylene blue Methyl orange Methyl red Safranine O Crystal violet Methylene blue Methyl violet 6B Methylene blue Congo red Methylene blue Malachite green Rhodamine B Rose bengal Methyl violet 6B Methylene blue Rose bengal Methylene blue Methyl violet 6B Methyl orange Methylene blue Methylene blue Reactive black-5 Methyl orange Reactive violet 1 Reactive black 5 Methylene blue Reactive blue 19 Reactive yellow 186

102 M 102 M 102 M

10 mg/200 m L 10 mg/200 m L 10 mg/200 m L

98.4 97.0 97.3

240 150 90

1.54  102 2.08  102 4.19  102

— — —

102 M 102 M 5.0  105 M 10 mM 5–15 ppm 50 mg/L 50 mg/L 50 mg/L 100 mg/L 50 mg/L 30 ppm 30 ppm

10 mg/200 m L 10 mg/200 m L 30 mg — 10 mg 200 mg 2.4 g 1g 0.1 mg/mL 0.2 mg/mL 10 mg 10 mg

96.0 97.6 — 30.0 — 85.0 92.0 85.0 97.9 86.3 88.5 86.6

90 105 60 60 100 105 420 20 20 20 180 180

4.00  102 3.36  102 1.9  101 1.0  105 5.3  103 — — 0.012 — — 1.23  102 1.03  102

— — — 13.86 14.03 — — — — — 13.84 29.9

8.

NPs NPs NPs NPs

Au-Ag core shell NPs

9. 10. 11. 12.

Ag/ZnO Au-ZnO NRs TiO2 NPs Commercial TiO2 (P25)

13.

Ag NPs

[100] [101] [102] [103] [104]

— —

[105]

[106] [107] [108] [109] [110] [111] [112] [112] Present Study Present Study

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those of the textile dyes (used in present work), due to their low molecular weight (i.e., MG = 364.91 g/mol, MO = 327.33 g/mol, MB = 319.85 g/mol > RB19 = 626.53 g/mol) [94]. Also, in their studies, the size of Ag NPs was higher (30–50 nm) with irregular morphology relative to the present work (5–20 nm with spherical morphology). Moreover, in many studies, nanocomposites were prepared to alleviate the dyes from water. However, these techniques are costly and complex while requiring very sophisticated tools for monitoring [95–98]. We successfully employed Ag NPs based photocatalysis system to degrade model textile dyes (e.g., RB19 and RY186), which are difficult for degradation (e.g., in comparison to the laboratory dyes) due to their complex structure and high molecular weight. Importantly, sunlight irradiations were utilized in our work instead of expensive artificial light sources. We also explored the role of activation energy in photocatalysis process and monitored the mineralization of dyes by determining the content of COD. Nonetheless, the results indicated the excellent photocatalytic performance and precise control on size and morphology of Ag NPs in comparison to that of the other methods. Thus, this finding implies that Ag NPs should be efficient potential candidates for the photocatalytic degradation of textile dyes. Conclusions In conclusion, we demonstrated a facile and economic means to prepare the biogenic Ag NPs using renewable source (TF leaves). The ramifications of various optical and electron microscopic techniques confirmed the formation of face-centered cubic structured Ag NPs with the absorption edge (428 nm) and the average diameter (5–20 nm). The optimum amount and pH value of TF extract was found to be 5 mL and 10, respectively. The synthesized biogenic Ag NPs showcased the higher photocatalytic degradation efficiency on RB19 and RY186 dyes relative to commercial photocatalysts. The kinetics of the reaction followed the pseudo first order with K’app values for RB19 and RY186 as 0.0123 and 0.0103 min1, respectively. Thus, the overall results of this study suggest the futuristic opportunities for the purification of wastewater using Ag NPs prepared via inexpensive and green approach. Acknowledgments This study was supported by financial assistance received from the Shromani Gurdwara Prabhandak Committee (SGPC), Amritsar. The authors are thankful to the Vice-Chancellor of the SGGSW University for providing the necessary laboratory facilities. KHK acknowledges the support made in part by grants from the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT & Future Planning (No. 2016R1E1A1A01940995) and the support of "Cooperative Research Program for Agriculture Science & Technology Development (Project No. PJ014297)" Rural Development Administration, Republic of Korea. VK acknowledges the support from Department of Science and Technology, New Delhi, India for INSPIRE Faculty Award. Conflicts of Interest The authors declare that there are no conflicts of interest regarding the publication of this manuscript. Appendix A. Supplementary data Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.jiec.2019.08.002.

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