Ag nanoshell catalyzed dedying of industrial effluents

Abstract

A rapid way to dedye industrial effluents is reported herein using silver nanoshells (Ag-NSs) as a green catalyst. Ag-NSs were synthesized using Crataeva religiosa leaf extract as both a reducing and stabilizing agent, and they were characterized by UV-visible spectroscopy, FT-IR spectroscopy, XRD, HR-TEM and EDX analysis. The green synthesized Ag-NSs catalyze in a minute the degradation of organic dye pollutants such as (1) methylene blue, (2) rhodamine-B, (3) safranin, (4) methyl orange, (5) eosin yellow, (6) rose bengal, (7) methyl red, (8) rhodamine-6G, (9) indigo, (10) crystal violet, (11) malachite green and (12) victoria blue from an aqueous environment using NaBH₄. Ag-NSs are able to be conveniently separated from an aqueous environment after catalytic dedying and reusable even after five cycles. The formation of Ag-NSs and dedying mechanism have also been investigated and discussed.

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Introduction

Over the past few decades, nanocatalysis has become a very important area in nanoscience and nanotechnology, where the high surface-to-volume ratio of nanoparticles (NPs) favors enhanced catalytic efficiency compared to the bulk counterpart. Particularly, silver nanoparticles (AgNPs) have many promising applications in nanotechnology because of their good electrical conductivity, chemical stability, catalytic and antibacterial properties.¹ These properties depend mainly on their size, shape, composition, structure and morphologies.² Efforts have been made to synthesize these tiny particles employing physical, chemical, and biological methods. The former two methods are expensive and are associated with high temperature, pressure and energy while most of the chemicals such as reactants, starting materials, and solvents used in the chemical synthetic routes are found to be toxic and potentially hazardous not only to the environment but also to the biological systems.^3,4 Hence, there is a growing need to develop eco-friendly processes for nanoparticle synthesis which do not use hazardous toxics. The green synthesis of NPs has received increasing attention due to a growing need to develop environmentally benign technologies in nanomaterial synthesis. Biological routes of NPs synthesis using plant extracts have been suggested as possible alternatives methods for physical and chemical process due to their solvent medium, environment, eco-friendliness and nontoxicity.⁵ There are many reports available for the synthesis of silver NPs using various plant extracts including Capsicum annuum L., Ziziphora tenuior, Nigella sativa, Grewia flavescens, Lantana camara, Justicia glauca and Acacia nilotica.^6–12 The uses of environment-friendly benevolent AgNPs are very important for its amazing and selective catalytic activity.

Today organic dyes, which form the foremost group of pollutants in wastewaters discharged from different industries including tannery, textile, paper, printing, paints, plastic, pharmaceutical, food, cosmetic, medicine and pulp industries are a global concern.^13–15 The textile industry plays a major role in the economy of Asian and other countries. In India, it accounts for the largest consumption of dyestuffs@80% taking in every type of dye and pigment produced, which amounts to close to 80 000 tons. India is the second largest exporter of dyestuffs, after China. Worldwide, 10⁶ tons of synthetic dyes are produced annually, of which 1–1.5 × 10⁵ tons are released into the environment as wastewaters.¹⁶ This release is because not all dye binds to the fabric during the dyeing processes. Depending on the class of the dye, the losses in wastewaters can vary from 2% for basic dyes to as high as 50% for reactive dyes, leading to severe contamination of surface and ground waters in the vicinity of such industries.^17–19 It is well-known that even 1 ppm of dye can cause serious problems in aquatic environments. It has been reported that dye-contaminated wastewater is hazardous, dangerous, poisonous, carcinogenic, allergenic and teratogenic to human beings.²⁰ Thus, the removal and/or reduction of organic hazardous dye wastes, for example, methylene blue (MB), rhodamine-B (R-B), safranin (SR), methyl orange (MO), eosin yellow (EY), rose bengal (RB), methyl red (MR), rhodamine-6G (R-6G), indigo (IG), crystal violet (CV), malachite green (MG) and victoria blue (VB) in waste water is a demanding task because they cause eye irritation, skin irritation, rhinorrhea, cough, dyspnea, respiratory inhibition, subsequent cell death, multi-organ tissue injury, ulcers, sore and painful nipples, several cancers, cerebrovascular disease and lung disease.^21,22 With the scarcity of fresh water resources, degradation of such dye pollutants in wastewater has become more and more crucial than ever. As international environmental standards are becoming more stringent technological systems for the removal of organic pollutants such as dyes are being developed day by day.²³

Various biological, physical, and chemical technologies including aerobic and anaerobic microbial degradation, coagulation, chemical oxidation, membrane separation, electrochemical treatment, dilution, filtration, flotation, softening, ion exchange and reverse osmosis have been employed in waste water treatments vis-à-vis removal of dye stuffs.^24–28 Factors affecting the technical and economic feasibility of each technique are wastewater composition, operation costs, dye type, and generated waste products. Therefore, the use of one individual technique is not sufficient to achieve complete decolorization or degradation.^29–31 These technologies have not been widely applied in wastewater treatments because of the fact that there always exist some drawbacks for their applications. For that reason, exploring simpler, lower cost and safer technologies is still needed for practical applications. Among them, advanced catalytic degradation finds importance for degrading concentrated wastewater from whatever industrial and living sources. Currently, nanotechnology has been extended to the area of wastewater treatment. Nanocatalysis has undergone a vast growth in recent years and seems to be a revolution in the field of catalysis.

This contribution aims to bring a green synthesis of silver-nanoshells (Ag-NSs) using Crataeva religiosa leaf extract (CRLE). The ability of synthesized Ag-NSs as an efficient green catalyst for the degradation of organic dye pollutants such as MB, R-B, SR, MO, EY, RB, MR, R-6G, IG, CV, MG and VB using NaBH₄ as a reducing agent has been investigated at room temperature. The formation and catalytic dedying mechanism of Ag-NSs are also discussed.

Experimental

The silver nitrate (AgNO₃) was purchased from Sigma-Aldrich, India and used as received. The fresh Crataeva religiosa (CR) leaf was collected in Thiagarajar College campus, Madurai, India. MB, R-B, SR, MO, EY, RB, MR, R-6G, IG, CV, MG, VB and NaBH₄ were purchased from Merck, India and used as received. All other chemicals were of analytical grade and used as such.

Instrumentation

UV-visible spectra were measured using a Jasco (V-560) model UV-visible double beam spectrophotometer. The sample measurements were performed in a 1 cm quartz cuvette at room temperature. The FT-IR spectral measurements were recorded using KBr disc on a JASCO FT-IR 460 Plus spectrophotometer. XRD analysis was carried out in X-ray diffraction unit, Cu Kα radiation (λ = 1.5418° Å) on JEOL JDX 8030 X-ray diffractometer. The size and morphology of the AgNPs were examined by transmission electron microscopy (TEM, JEOL JEM 2100 model instrument). Energy dispersive X-ray (EDX) spectrometer attached to the transmission electron microscope was used for elemental analysis. All experiments were carried out at room temperature.

Preparation of CRLE

Fresh leaves of CR were collected and washed thoroughly with running tap water followed by de-ionized water to remove all the impurities. The 100 mg of small pieces of CR leaves was boiled with 100 mL of deionized water at 90 °C for 15 min. The extract was filtered thrice using Whatmann filter paper no. 1 to remove organic contaminants. The filtered CRLE was pale yellow in color and it was used as reducing as well as stabilizing agent for the synthesis of Ag-NSs.

Green synthesis of Ag-NSs

5 mL of CRLE was mixed with 10 mL of aqueous solution of 1 mM AgNO₃ at room temperature. The pale yellow color solution becomes deeper brown within 1 h and 20 min and no further noticeable difference in the color of aqueous silver colloids is observed, which indicates that the bio-reduction process is over within 80 min. The brown color changes indicate the formation of Ag-NSs in aqueous solution due to excitation of surface plasmon vibration in the metal NPs. The synthesized Ag-NSs were collected by centrifugation at 2000 rpm for 15 min. Then the filtrate was redispersed in water and centrifuged for several times to remove excess amount organic contents present. Finally, the dark brown Ag-NSs were used for further studies.

Catalytic degradation of organic dye pollutants

In a typical experiment, 1.8 mL of dye (0.1 mM aqueous solution) was mixed with 0.7 mL of NaBH₄ (0.01 M aqueous solution) in a quartz cell (3.0 mL). Then, 0.001 mg of Ag-NSs catalyst was added to the mixture of dye and reductant. The color and absorbance intensity changes in the solution were monitored with a UV-visible spectrophotometer at different time intervals. After the reaction was over, the mixture was centrifuged and washed with deionized water thrice to recollect Ag-NSs to determine its reusability.

Results and discussion

UV-visible spectroscopy

The electronic spectroscopy is commonly employed for confirming the formation of metal NPs because the metal NPs exhibit strong absorption band due to surface plasmon resonance (SPR) in the visible region. The formation of Ag-NSs can be visualized through the color change from colorless to dark brown after the addition of CRLE (Fig. 1a, inset: (a–c)) due to collective oscillation of free conduction electrons induced by an interacting electromagnetic field results in SPR. Fig. 1a shows two absorption maxima at 254 and 304 nm corresponding to proteins and phenolic glycosides present in the leaf extract which are largely responsible for the rapid reduction of Ag⁺ to Ag⁰.⁶ After the addition of leaf extract the color of AgNO₃ turns from pale yellow to dark brown and a new absorption band at 413 to 417 nm is recorded in different time intervals (10, 20, 30, 40, 50, 60, 70 and 80 min) which correspond to SPR as shown in Fig. 1b (curve a–h). As the time increases the intensity of the SPR peak also increases, which indicates the increase in number of Ag-NSs. The shift in SPR peaks from 413 to 417 nm (red-shift) indicates the increase in size of the Ag-NSs.⁹ After 80 min, there is no change in the absorption peak which confirms that the reaction is completed within 80 min. The intensity of color does not intensify after 80 min which is established by UV-visible spectra as shown in Fig. 1c. The absorption recorded after 24 h shows the same intensity (Fig. 1d) which indicates that the synthesized Ag-NSs is stable. The stability is found to be retained even after one month.

Fig. 1 (a) UV-visible spectrum of CRLE and inset a–c shows the photographs of AgNO₃, CRLE and mixture respectively, (b) UV-visible spectra of formation Ag-NSs at different time intervals, (c) plot of the SPR intensity vs. time (min) and (d) UV-visible spectra of Ag-NSs in 80 min (green line) and after 24 h (red line).

FT-IR spectra

FT-IR measurement was used to identify the possible biomolecules responsible for the reduction of silver ion and stabilization of NPs. Fig. 2a shows the FT-IR spectrum of CRL powder and Fig. 2b green synthesized Ag-NSs respectively. The peak appears at 3415 cm⁻¹ can be assigned for the O–H stretching vibration indicating the presence of hydroxyl group in polyphenols. The peaks at 2925 and 2850 cm⁻¹ are characteristic of stretching vibrations of methyl groups or C–H of aldehydic amine groups and the peak observed at 1737 cm⁻¹ is attributed to a carbonyl stretching vibration of ester groups. The strong peak appearing at 1642 cm⁻¹ is assigned to the –OH bending mode or C=O stretching vibration of carbonyl and carboxylic group of amide I.^6,32 The weak band observed at 1392 cm⁻¹ is due to C–H deformation vibrations and the peak at 1236 cm⁻¹ is the bending vibration of C–N group of amide II and III or C–O stretching vibration of polyols. The peak at 643 cm⁻¹ is the plane bending vibration of N–H groups in the proteins. After reduction with silver ions the strength and intensity of all the peaks are obviously weakened and some peaks are shifted to reduced peak intensity.

Fig. 2 FT-IR spectra of CRLE (a) and Ag-NSs (b).

The major phytoconstituents present in the CRLE are hydrolysable tannins, polyphenols, terpenoids, alkaloids, saponins and glycosides and the functional groups like high concentrated alcohol or phenol, amine, amide (I) and (II), aldehydes and polyols. From the above results, it can be suggested that the mentioned functional groups are mainly responsible for reduction and stabilization of Ag-NSs. The multiple hydroxyl groups are considered to be hard ligands and take part in the reduction of soft metals, whereas compounds containing –OH groups present at ortho/para position act as good reducing agents. The silver ions can form intermediate complexes with phenolic –OH groups present in hydrolysable tannins which subsequently undergo oxidation to quinone forms with consequent reduction of Ag⁺ to Ag-NSs. A close observation of Fig. 2 shows that the reduction in intensity of band at 3415 cm⁻¹, corresponding to –OH groups in polyphenols is more pronounced which indicates that polyphenols are mostly responsible for reduction and stabilization. In general the chelating ability of phenolic compounds is probably related to the high nucleophilic character of the aromatic rings rather than to specific chelating groups within the molecule.³³Scheme 1 shows a possible mechanistic route for the formation and stabilization of Ag-NSs.

Scheme 1 Mechanistic route for the formation and stabilization of Ag-NSs.

X-ray diffraction

The XRD pattern of synthesized Ag-NSs at different time intervals using CRLE at room temperature is shown in Fig. 3a–c. The XRD pattern indicates four different diffraction peaks at 2θ values of 38.10, 44.30, 64.40 and 77.40 which can be attributed to (1 1 1), (2 0 0), (2 2 0) and (3 1 1) crystallographic planes of face centered cubic (fcc) structure of metallic silver and is consistent with Joint Committee on Powder Diffraction Standards (JCPDS) data [No. 04-0783]. Similar results have been documented in the earlier reports on the synthesis of silver nanoparticles using leaf extracts of Capsicum annuum L., Ziziphora tenuior, Nigella sativa, Grewia flavescens, Lantana camara, Justicia glauca and Acacia nilotica.^6–12 The sharp and strong signals confirm the nanosized and well crystallized Ag-NSs. Among the different diffraction planes, the peak corresponding to (1 1 1) plane is more intense than other planes suggesting a predominant orientation.

Fig. 3 XRD patterns of Ag-NSs synthesized at different time intervals of 20 min (a), 50 min (b) and 80 min (c).

Furthermore the size of synthesized Ag-NSs was calculated using Debye–Scherrer equation which shows that the nanoshells are in the average size of 17.2 nm.

TEM

Fig. 4a–c shows the HR-TEM image of Ag-NSs using CRLE at different time intervals. The TEM images clearly indicate that the Ag-NSs are nanosized with size ranges from 5 to 40 nm and unagglomerated shells. As the time increases, the number of Ag-NSs and size of Ag-NSs also increase. The conjugation length and intensities increase from 413 to 417 nm which indicate the increase in size and number of Ag-NSs.⁹

Fig. 4 HR-TEM images of Ag-NSs synthesized at different time intervals of 20 min (a), 50 min (b) and 80 min (c) and EDX spectrum of Ag-NSs (d).

Fig. 4d shows the EDX profile of the phytocapped Ag-NSs which confirms the existence of Ag. Furthermore Fig. 5a and c shows the HR-TEM images with lower and higher magnification of Ag-NSs. Fig. 5b shows the histogram of the green synthesized Ag-NSs which deduces the average size to be 20 nm. The crystallinity of Ag-NSs was observed by selected area emission diffraction (SAED) pattern as shown in Fig. 5d with bright circular rings corresponding to (1 1 1), (2 0 0), (2 2 0) and (3 1 1) planes, revealing the highly crystalline nature of nanoshells, which is consistent with XRD and TEM results.

Fig. 5 Lower magnification TEM image of Ag-NSs (a), histogram for the particle size distribution (b), higher magnification TEM image of Ag-NSs (c) and SAED patterns of Ag-NSs (d).

Catalytic degradation of organic dyes

The catalytic reduction of organic dyes such as MB, R-B, SR, MO, EY, RB, MR, R-6G, IG, CV, MG and VB with an addition of freshly prepared ice cold NaBH₄ was carried out to evaluate the catalytic performance of Ag-NSs. In the absence of either Ag-NSs or NaBH₄, no dedying reaction takes place. However, when Ag-NSs is used as a catalyst, the absorption intensity of organic dyes decreases successively. Ag-NSs effectively catalyzes the degradation of organic dyes by acting as an electronic relay system, wherein the electron transfer takes place from donor BH₄⁻ to acceptor dyes.^34–40 The Ag-NSs by virtue of the presence of large surface area enhances the rate of these reduction processes.

The continuous reduction processes and kinetics can be monitored UV-visible spectrophotometrically. Fig. 6 & 7 are the UV-visible spectra for the degradation of dyes with respect to time as indicated by colour lines. Fig. 6a–f and 7a–f show the strong absorption peaks at 664, 552, 464, 462, 514, 545, 427, 554, 604, 585, 621 and 605 nm corresponding to MB, R-B, SR, MO, EY, RB, MR, R-6G, IG, CV, MG and VB respectively. In the absence of Ag-NSs, there is no change in the absorption spectra, which indicates that there is no possible degradation by visible light.

Fig. 6 UV-visible spectra for the degradation of MB (a), R-B (b), SR (c), MO (d), EY (e) and RB (f) using NaBH₄ in the presence of AgNPs.

Fig. 7 UV-visible spectra for the degradation of MR (a), R-6G (b), IG (c), CV (d), MG (e) and VB (f) using NaBH₄ in the presence of AgNPs.

Fig. 6a–f and 7a–f represent the compiled absorption spectra of degradation of organic dyes by NaBH₄ in the presence of green synthesized Ag-NSs as a catalyst. The reduction process is found to be accelerated in the presence of Ag-NSs colloid which shows a decrease in the absorption intensity of dye solutions. The absorption peak for dyes is found to decrease with the increase in the reaction time indicating the dedying process, which takes minimum 30 s to maximum 100 s. Here Ag-NSs acts as an electron transfer mediator; it accepts the electrons from BH₄⁻ and donates to organic dyes by acting as a redox catalyst which shows electron relay effect.^41–45

The possible mechanisms of degradation of organic dyes using NaBH₄ in the presence of Ag-NSs are validated in Scheme 2. The reaction kinetics was monitored easily from the time-dependent absorption spectra. The reaction kinetics can be described as −ln(C/C₀) = kt, where k is the rate constant at a given temperature and t is the reaction time.

Scheme 2 The mechanism for the degradation of organic dyes catalyzed by Ag-NSs in the presence of NaBH₄.

C ₀ and C are the dye concentration at the beginning and at time t, respectively. The kinetic rate constant k is calculated from the slopes of the linear sections of the plots as shown in Fig. 8. The plot of ln(C/C₀) vs. time has been given here for the degradation kinetics of MB and R-B. Similar plots (not shown) were obtained for other organic dye pollutants. The rate constants k calculated from the slope are tabulated in Table 1.

Fig. 8 The plot of ln(C/C₀) against the reaction time for degradation kinetics of MB (a), R-B (b) using NaBH₄ in the presence of Ag-NSs.

Table 1 Comparison of the catalytic activity of various silver nanomaterials for the degradation of organic dyes at room temperature

Catalyst/concentration	Catalyst stability (degradation cycles)	Size (nm)	Shape	Dyes	Degradation time	Rate constant (k)	References
AgNPs/30 μL mL^−1	1	16–24	Spherical	Methylene blue	9 min	—	34
AgNPs/—	1	20–35	Spherical	Methylene blue	20 min	0.22417 min^−1	35
Cef-AgNPs/0.5 mg mL^−1	6	—	Spherical	Rose bengal	60 s	7 × 10^−2 s^−1	36
AgNPs/—	1	18–24	Spherical	Methylene blue	12 min	0.3415 min^−1	37
				Eosin yellow	8 min	0.3659 min^−1
				Methyl orange	8 min	0.3758 min^−1
AgNPs/0.03 mg mL^−1	1	15–25	Spherical	Methylene blue	5.5 min	0.2758 min^−1	38
				Methyl orange	9 min	1.3080 min^−1
AgNPs/—	1	10	Spherical	Methylene blue	3 min	—	39
				Methyl orange	3 min	—
AgNPs/10–100 μg mL^−1	1	50–80	Spherical	Methylene blue	8 min	—	40
AgNPs/79.78 μL	1	25	Spherical	Methylene blue	60 min	—	41
Ag-NPs/—	1	150–200	Spherical	Rhodamine-B	30 min	—	42
Fe3O4@PDA–Ag 5 mg mL^−1	26	25	Spherical	Methylene blue	22 min	0.430 min^−1	43
Fe3O4@Ag/1.6 mg mL^−1	1	52	Core–shell	Methylene blue	6 min	0.410 min^−1	44
Ag–Fe3O4@C core–shell NCs/-	5	300	Spherical	Methylene blue	10 min	0.340 min^−1	45
AgNPs/10 mg/100 mL	1	6–35	Spherical	Methylene blue	180 min	1.72 × 10^−2 min^−1	46
				Malachite green	180 min	1.51 × 10^−2 min^−1
Ag-NSs/0.001 mg mL^−1	5	20	Nanoshells	Methylene blue	100 s	8.935 × 10^−3 s^−1	Present work
				Rhodamine-B	100 s	2.855 × 10^−3 s^−1
				Safranin	100 s	5.481 × 10^−3 s^−1
				Methyl orange	100 s	5.296 × 10^−3 s^−1
				Eosin yellow	100 s	7.001 × 10^−3 s^−1
				Rose bengal	100 s	5.089 × 10^−3 s^−1
				Methyl red	100 s	4.168 × 10^−3 s^−1
				Rhodamine-6G	100 s	5.481 × 10^−3 s^−1
				Indigo	20 s	18.769 × 10^−3 s^−1
				Crystal violet	40 s	9.096 × 10^−3 s^−1
				Malachite green	40 s	16.005 × 10^−3 s^−1
				Victoria blue	60 s	6.102 × 10^−3 s^−1

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Up to 99% degradation of organic dye pollutants takes place in 30 s to 100 s as shown in Fig. 9a and b. To check the reusability and stability which are important characteristics of a good catalyst and any possible saturation effect, the same catalyst was utilized by the degradation reaction repeatedly. After each use, the catalyst was centrifuged, washed and dried for the next cycle of catalysis. Up to five cycles studied, there is no weight loss observed. The catalyst exhibits high activity for the degradation even up to five cycles as shown in Fig. 10a and b. Ag-NSs with high activity and specific surface area accelerates the reduction rate of dyes, thus increasing the dedying efficiency. The complete degradation can also be established by the perfect color change of the solution from color to colorless as shown in Fig. 11. Fig. 12 clearly shows the HR-TEM image of Ag-NSs after five cycles, which results suggest that there is no noticeable structural change observed after degradation of Ag-NSs. A detailed comparison has been made on the catalytic activity of various Ag nanomaterials for the degradation of organic dyes and is given in Table 1. From Table 1, it can be observed that the synthesized Ag-NSs performs better than any other Ag nanomaterials reported elsewhere for the degradation of dye stuffs.

Fig. 9 (a) The plot of degradation% vs. reaction time for (1) MB, (2) MO, (3) R-B, (4) EY, (5) SR and (6) RB and (b) the plot of degradation% vs. reaction time for (1) IG, (2) CV, (3) MG, (4) VB, (5) R-6G and (6) MR.

Fig. 10 (a) The plot of degradation% vs. cycle numbers for (1) MO, (2) MB, (3) R-B, (4) SR, (5) EY and (6) RB and (b) the plot of degradation% vs. cycle numbers for (7) MR, (8) R-6G, (9) CV, (10) IG, (11) MG, and (12) VB.

Fig. 11 Photographic images of visually identified colored solution (before degradation) to colorless solution (after degradation).

Fig. 12 HR-TEM image of Ag-NSs after five cycles.

Conclusions

In summary, a simple and green reduction route for the synthesis of Ag-NSs for the degradation of organic dye pollutants has been demonstrated. The Ag-NSs are uniformly shaped with the average size of 20 nm. The synthesized Ag-NSs shows excellent catalytic activity in the degradation of different organic dyes namely methylene blue, rhodamine-B, safranin, methyl orange, eosin yellow, rose bengal, methyl red, rhodamine-6G, indigo, crystal violet, malachite green and victoria blue using NaBH₄ as reducing agent. The Ag-NSs catalyst retains its high activity for the degradation even after five cycles. The Ag-NSs act through an electron relay system and influences the degradation of organic dyes. A very efficient and economic route for environmental remediation of waste waters from industries can be hopefully obtained from study of this kind.

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