Green synthesis of nanoscale zero-valent iron using a grape seed extract as a stabilizing agent and the application for quick decolorization of azo and anthraquinone dyes

Abstract

Grape seed-coated nanoscale zero-valent iron (GS-NZVI) was synthesized using grape seed extract as the stabilizing agent, and it was used to degrade Reactive Brilliant Red K-2G (RBR, azo dye) and Reactive Brilliant Blue KN-R (RBB anthraquinone dye). The GS-NZVI was characterized by scanning electron microscopy (SEM), X-ray energy-dispersive spectrometry (EDS), transmission electron microscopy (TEM), dynamic light scattering (DLS), X-ray diffraction (XRD), Brunauer–Emmett–Teller isotherm (BET) and Fourier transform infrared spectroscopy (FTIR). The grape seed extract played the role of a stabilizing agent to protect GS-NZVI from agglomeration and oxidation. Based on the decolorization efficiency and cost saving, the optimal grape seed extract concentration for GS-NZVI synthesis was 0.2 wt%. The effects of dye solution pH, GS-NZVI dosage and initial dye concentration on dye decolorization were investigated. The solution pH had little effect on dye decolorization. According to decolorization efficiency, reduction capacity and reaction time, the optimal GS-NZVI dosage was 2.0 g L⁻¹. The decolorization efficiency (96.08–98.22%) was achieved within 7 min with an initial RBR concentration ranging from 500 to 2000 mg L⁻¹; and decolorization efficiency of 97.45–99.08% was achieved within 17.5 min with an initial RBB concentration ranging from 250 to 1000 mg L⁻¹. The kinetic analysis demonstrated that the first-order kinetic model was more suitable to describe the decolorization processes of RBR and RBB dyes. The UV-vis spectra showed that the chromophores of the dyes and some functional groups were destroyed by GS-NZVI. GS-NZVI has good application prospects because of its low-cost, eco-friendly synthesis and significantly high decolorization efficiency.

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1. Introduction

Green synthesis has received more and more attention in the field of nanotechnology due to its low-cost, high-efficiency, non-toxic and environmental friendly characteristics.^1–3 The twelve principles of green engineering⁴ suggested that the rules of using non-toxic materials, waste recycling and energy saving should be applied. The exciting and innovative method of green synthesis is gaining momentum in the synthesis of nano-metals.^2,3,5 Due to its large specific surface area and high surface reactivity, nanoscale zero-valent iron (NZVI) can be used to degrade various contaminants efficiently through the redox reaction⁶ or as a solid form of iron instead of Fe²⁺ in a Fenton oxidation system.⁷ Therefore, the green synthesis of NZVI is gradually becoming the focus of researchers.

It has been reported that the agglomeration and stability are important factors affecting the behavior of NZVI particles. Bare NZVI particles tend to agglomerate rapidly to form larger aggregates, thereby diminishing their reactivity and mobility.⁸ In order to enhance dispersion or reduce aggregation of NZVI, stabilizing agents, such as soluble polymers or surfactants, should be attached onto the surface of NZVI particles, which can provide electronic repulsion and steric stabilization to stabilize nanoparticles.^8,9 Generally, without the presence of stabilizing agents, the inter-particle attractive forces prevail, and thus, nanoparticles agglomerate rapidly to form larger aggregates in water because they are sensitive to ionic strength and composition.⁹ In the presence of stabilizing agents, the surface of the nanoparticles could be coated with the charged stabilizer molecules, which provide strong and long-range electrostatic repulsion to maintain steric stabilization. For this reason, stabilizing agents can be applied to decrease NZVI aggregation and attachment.^8,9 Recently, various stabilizing agents have been reported to effectively prevent nanoparticles from agglomeration, including polyacrylic acid,¹⁰ carboxymethyl cellulose,⁸ polyvinylpyrrolidone,⁶ starch¹¹ and guar gum.² However, the stabilizing agents used in previous studies were expensive or used potentially toxic reagents, which limited their further application. Therefore, the focus of the present research is to find some agricultural or forestry waste materials as a stabilizing agent for the green synthesis of NZVI. They should be inexpensive, simple to use and eco-friendly, in order to promote the wide application of NZVI.

In 2010, the worldwide production of grape wine was more than 26 billion liters.¹² During the production of grape wine, about 25% of the grape weight resulted in waste, including grape skins and seeds.¹² To avoid environmental problems, any attempt to reutilize those waste materials will be useful. Grape seed extract can be used as the stabilizing agent because it is rich in oleic acids, linoleic acids, polyphenols, tocopherols, proanthocyanidins, etc.^13,14 The antioxidant properties of grape seed extract have been attributed to their polyphenol and proanthocyanidins, which can directly scavenge reactive oxygen species including hydroxyl and peroxyl radicals.^5,15 However, the grape seed extract has not been applied to the green synthesis of NZVI. In this study, grape seed extract, the agricultural waste material, was applied as a stabilizing agent for the synthesis of NZVI, which is called “grape seed-coated nanoscale zero-valent iron (GS-NZVI)”. The green synthesis of GS-NZVI could achieve low-cost, non-toxic, efficient and recycling of waste characteristics.

In a typical dyeing process, about 10–20% of dyes enter the environment through wastewater. The treatment of dye wastewater is urgently needed because low concentrations of dyes have a strong visibility, which will result in adverse effects on water transparency and sunlight penetration of water bodies, creating a serious threat to the health and survival of aquatic organisms.^16,17 In the textile industry, azo dyes and anthraquinone dyes make up about 60% and 15%, respectively, of all used dyes. Therefore, it is necessary to find an effective method for the treatment of these two kinds of dyes. The familiar azo dye of Reactive Brilliant Red K-2G (RBR) is widely used in wool, silk, leather and cotton dyeing processes,¹⁸ and the typical anthraquinone dye of Reactive Brilliant Blue KN-R (RBB) is usually applied for dyeing cellulosics and cotton fabrics.¹⁹ Hence, the RBR and RBB dyes were selected as the representatives to be degraded by the green synthesized GS-NZVI. The degradation of dyes by GS-NZVI achieved the purpose of waste control, and GS-NZVI can be evaluated by the decolorization efficiency.

In this study, the grape seed extract was used as a stabilizing agent to synthesize GS-NZVI with potassium borohydride (KBH₄) and ferrous ions (Fe²⁺). The effect of grape seed extract concentration on the synthesis of GS-NZVI was studied. The target product was characterized using scanning electron microscopy (SEM), X-ray energy-dispersive spectrometry (EDS), transmission electron microscopy (TEM), dynamic light scattering (DLS), Brunauer–Emmett–Teller isotherm (BET), X-ray diffraction (XRD) and Fourier transform infrared spectroscopy (FTIR). Then, the GS-NZVI was used to quickly degrade RBR and RBB dyes. The effects of initial pH value, GS-NZVI dosage and initial dye concentration on decolorization were investigated. Kinetics models were used to illustrate the decolorization processes. In addition, UV-vis spectroscopy was applied to monitor dye degradation and intermediate products during the decolorization process.

2. Materials and methods

2.1. Reagents and materials

The waste material, grape seed, was obtained from a local market in Beijing, China. RBR (C.I. Reactive Red 15, molecular formula = C₂₅H₁₄ClN₇Na₄O₁₃S₄, λ_max = 510 nm) and RBB (C.I. Reactive Blue 19, molecular formula = C₂₂H₁₆N₂Na₂O₁₁S₂, λ_max = 601 nm) were purchased from Tianjin Shengda Chemical Factory (China).

The following reagents were analytical grade and used in the experiments without further purification: FeSO₄·7H₂O, KBH₄, NaOH, HCl and (CH₃)₂CO. They were purchased from TianJin Fuchen Chemical Reagents Factory, China. Deionized water was used in all experiments.

2.2. Preparation of GS-NZVI

The grape seed was washed with deionized water and sun-dried. The dry grape seed was ground into powder by a homogenizer, and sieved to size range of 0.3–0.6 mm. An ultrasonic bath (KQ-5200 DE, Kun Shan Ultrasonic Instruments Co., Ltd, China), as an ultrasonic source, was used to prepare the grape seed extract. 3.0 g of dry grape seed powder was added to a beaker with 100 mL water followed by quick shaking. The extraction was then performed under a power rating of 100 W at 30 °C for 30.0 min. After cooling to room temperature, the extract was vacuum filtered with a filter paper (pore size 15–20 μm, diameter 7 cm).

By adding grape seed extract as a stabilizing agent, GS-NZVI was synthesized. The basic principle of the synthesis process is as follows:

Fe_(aq)²⁺ + 2BH_4(aq)⁻ + 6H₂O → Fe⁰_(s) + 2B(OH)_3(aq) + 7H_2(g)↑ (1)

At room temperature, the grape seed extract of six different concentrations (0, 0.1, 0.2, 0.3, 0.4, 0.5 wt%) was added to 100 mL 0.1 M ferrous sulfate solution in a conical flask with vigorous stirring until the extract was mixed evenly. No obvious change was seen. The value of dissolved oxygen (DO) was monitored using a DO-probe (WTW CellOx 325, Germany). Then, 100 mL 0.3 M potassium borohydride solution, which could ensure complete reduction of ferrous ions, was dropwise added into the solution mentioned above, and the mixture was vibrated in a water bath shaker (GFL 1086, Germany) with a shaking rate of 120 rpm at 20 °C until the mixture turned to black without rising bubbles. The GS-NZVI was washed with acetone twice and freeze-dried using a Labconco Freezone 1 L (Labconco, USA). The dried GS-NZVI samples, synthesized with six different concentrations of grape seed extract, were named as GS0, GS1, GS2, GS3, GS4 and GS5. The synthesis process was carried out without the protection of nitrogen gas.

2.3. Characterization

NZVI synthesized without stabilizing agent, GS-NZVI and GS-NZVI after use were characterized using SEM, TEM, DLS, XRD, BET and FTIR.

SEM (Hitachi S-4300, Japan), equipped with EDS, was used to qualitatively determine the morphology, size and composition of the samples. The dried particles were fixed on an electrical conductive resin (Three Bond 3350C, made in Japan) due to the magnetic properties of the samples.

TEM (Tecnai G2 F20 S-TWIN (200 kV), USA) was applied to show the particle size and morphology of samples. The dried particles were dispersed in ethanol, dropped onto the carbon net and evaporated to dryness.

DLS (Microtrac S3500, USA) was used to analyze the particle size distribution of the samples.

XRD (D8 Advance diffractometer, Bruker, Germany) was applied to measure the crystallinity of the dried particles in the scanning range between 30° and 90° using Cu Kα (λ = 1.54 Å) radiation at 25 °C.

BET isotherms (ASAP 2020 apparatus, Micromeritics, USA) measured the specific surface area of nanoparticles by nitrogen gas adsorption at 77 K.

FTIR (Bruker Vertex 70 FTIR spectrometer, Bruker, Germany) was used to compare the samples. The samples and KBr were mixed with a ratio of 1 : 100 and flattened under high pressure; afterwards they were immediately tested in the range 400–4000 cm⁻¹ with a resolution of 2 cm⁻¹.

2.4. Dye degradation experiments

Dye degradation experiments were carried out in 250 mL beakers by mixing 100 mL dye solution with GS-NZVI. The beakers were vibrated at 120 rpm at 25 °C in a water bath shaker (GFL 1086, Germany). During the process of dye decolorization, the supernatant samples were measured at given time intervals using UV-vis spectrophotometry (UV-2802PC, Unico, Shanghai, China). All the decolorization experiments were performed twice, and the average values were used.

To investigate the effect of grape seed extract concentration on the synthesis of GS-NZVI 2.0 g L⁻¹ GS-NZVI (GS0, GS1, GS2, GS3, GS4 and GS5) was added to 100 mL 1000 mg L⁻¹ RBR and RBB dye solutions. The initial pH of the dye solution was not adjusted; it was 8.23 and 7.85 for RBR and RBB solution, respectively. Based on the decolorization efficiency and cost saving, GS2 was applied for the subsequent experiments and characterized as the representative of GS-NZVI.

In order to study the effect of pH value on dye degradation, the pH value of the dye solution was adjusted with 1 mol L⁻¹ HCl and 1 mol L⁻¹ NaOH solutions using a pH-meter (WTW 340i, Germany). The initial pH values of 100 mL 750 mg L⁻¹ RBR dye solution were 2.00, 4.00, 6.00, 8.23 (the original pH value of RBR dye solution) and 10.00, respectively. The initial pH value of the 100 mL 600 mg L⁻¹ RBB dye solution was 2.00, 4.00, 6.00, 8.23 (the original pH value of RBB dye solution) and 10.00, respectively. 2.0 g L⁻¹ GS-NZVI (the sample GS2) was used to degrade them. A suitable pH value was chosen below.

The effect of GS-NZVI dosage was also studied. In a 100 mL dye solution, the amount of GS-NZVI dosage varied from 0.5, 1.0, 2.0, 3.0 to 4.0 g L⁻¹, and initial dye concentration was fixed at 1000 and 1000 mg L⁻¹ for RBR and RBB, respectively.

To investigate the influence of initial RBR and RBB concentration, the initial concentrations of 100 mL RBR and RBB solution ranged from 250 to 2000 mg L⁻¹ (RBR: 500, 750, 1000, 1500 and 2000 mg L⁻¹; RBB: 250, 500, 750, 1000 and 1250 mg L⁻¹).

The UV-vis spectra analysis was used to investigate the dye degradation and intermediate transformation. In the presence of GS-NZVI (0.5 g L⁻¹), the changes of UV-vis spectra of 50 mg L⁻¹ RBR and RBB dyes (100 mL) were measured from 200 to 800 nm with a fixed slit width of 1 nm.

3. Results and discussion

3.1. Characterization of GS-NZVI

3.1.1. SEM and TEM. Fig. 1 shows the SEM images of NZVI synthesized without stabilizing agent, GS-NZVI and GS-NZVI after use and the TEM image of GS-NZVI. NZVI synthesized without stabilizing agent showed serious agglomeration and poor dispersion, and only a few particles were spherical (Fig. 1(a)). In contrast, Fig. 1(b) shows that GS-NZVI was dispersed very well and the particles were almost spherical or elliptic spherical shapes. The mean particle size of GS-NZVI was about 172 nm (number = 100, standard deviation = 65 nm, minimum value = 63 nm, maximum value = 381 nm). Fig. 1(b) indicates that the grape seed extract acts as a stabilizing agent, playing an important role in the synthesis of GS-NZVI. The DO value decreased slightly (from 11.10 to 11.04 mg L⁻¹) in the ferrous sulfate solution over 1 min. However, after adding the grape seed extract, the DO value decreased quickly to 10.11 mg L⁻¹ over 1 min and then declined slowly. Meanwhile, the grape seed extract did not react with Fe²⁺, illustrating that the grape seed extract, which is full of polyphenols, proanthocyanidins and tocopherols,^3,13 can be used as an antioxidant to cover the surface of GS-NZVI to prevent it from being oxidized. Therefore, the GS-NZVI can be dried by vacuum freeze-drying and preserved in air without being oxidized. Normally, an inert/pseudoinert gas, such as Ar, N₂ and CO₂, is used to protect the freeze-dried NZVI from self-ignition.^20,21 However, the freeze-dried GS-NZVI can be directly stored in air, without being oxidized, and the activity of GS-NZVI was verified in the dye degradation experiments. It suggested that the green synthesis method was simple, feasible and economical, and can be stored in air, which is very beneficial for applications.

Fig. 1 SEM images: (a) NZVI particles without the stabilizing agent; (b) newly synthesized GS-NZVI particles; (c) GS-NZVI particles after use; TEM image: (d) newly synthesized GS-NZVI particles.

Fig. 1(c) shows that the shape and dispersion of GS-NZVI particles after use changed obviously. The spherical shape was destroyed, a filamentous morphology appeared and larger aggregates were formed. Meanwhile, it could be found that few particles in Fig. 1(b) presented a filamentous shape, similar to that in Fig. 1(c). It indicates that some GS-NZVI particles had not been coated by the grape seed extract which prevented it from being oxidized, because of the complex composition and uneven distribution of grape seed extract in the synthesis process. Thereby, some nanoparticles shown in Fig. 1(b) are oxidized and their morphology is similar to that in Fig. 1(c).

The TEM image in Fig. 1(d) shows the particle size and morphology of GS-NZVI. The particles of GS-NZVI show a core–shell structure, where spherical particles of NZVI were coated with grape seed extract, acting as a shell on the surface to protect the NZVI particles from being oxidized. The result obtained was consistent with a previous study,²² and as estimated from the SEM image (Fig. 1(b)), more than 90% of GS-NZVI particles were well coated by the grape seed extract. In addition, the particle size distribution in Fig. 2 shows that the size of GS-NZVI particles is mainly below 200 nm and 52.02% of particles was in the range 150–200 nm, which corresponded to the mean value of GS-NZVI observed in the SEM images.

Fig. 2 Particle size distribution of GS-NZVI.

3.1.2. EDS. The data from the EDS spectra of NZVI, GS-NZVI and GS-NZVI after use are shown in Table 1. The main elements appearing in the EDS spectra were C, O, Cl, K, Ag and Fe. Because GS-NZVI was coated with antioxidant substances from grape seed extract, C and O existed. K is not only an essential element for plant growth, but also the composition of KBH₄. Ag is one of the main components of the electrical conductive resin used in the preparation of the SEM samples. According to the data from the EDS spectra, the weight percentage (wt%) of Fe decreased from 59.11% to 34.47% after the use of GS-NZVI, indicating some of the solid Fe particles were oxidized into dissolved Fe²⁺ or Fe³⁺ ions in the process of dye degradation. Because some dye molecules or intermediate products of dye degradation may be adsorbed or adhered on the surface of GS-NZVI, the weight percentage of O and C increased from 26.08% to 31.96% and from 7.81% to 31.16%, respectively.

Table 1 The main elements of GS-NZVI and GS-NZVI after use

Sample	C (wt%)	O (wt%)	Cl (wt%)	K (wt%)	Ag (wt%)	Fe (wt%)
NZVI	17.41	23.73	0.47	1.59	1.25	55.55
GS-NZVI	7.81	26.08	0.51	1.62	4.87	59.12
GS-NZVI after use	31.16	31.96	0.72	—	—	34.47

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3.1.3. XRD. Fig. 3 depicts the XRD patterns of NZVI synthesized without stabilizing agent (pattern A), GS-NZVI (pattern B) and GS-NZVI after use (pattern C). Compared with the sharp characteristic peak of α-Fe⁰ at 2θ = 44.7° in pattern (A) and (C), the intensity of α-Fe⁰ in pattern (B) (GS-NZVI) was weaker. Another two characteristic diffraction peaks of α-Fe⁰ at 2θ = 64.9° and 82.3° appeared in pattern (A) and (C), but not in pattern (B), which generally appeared in the NZVI synthesized without stabilizing agent.^23,24 The results supported the hypothesis^23,25 that the amorphous GS-NZVI nature may be caused by the coverage of antioxidant materials, such as polyphenols and proanthocyanidins.

Fig. 3 XRD patterns. Pattern (A): NZVI particles without stabilizing agent; pattern (B): GS-NZVI particles; pattern (C): GS-NZVI particles after use.

Pattern (A) showed the characteristic peaks of Fe₂O₃ (ref. 23) and Fe₃O₄ (ref. 25) at 2θ = 37.6° and 43.1°, respectively. But the two peaks did not appear in pattern (B). The result indicated that grape seed extract played the role of antioxidant, which can protect GS-NZVI from being oxidized by oxygen in the solution and air. Pattern (C) showed the characteristic peaks of Fe₂O₃/Fe₃O₄ (ref. 24) and FeOOH²⁶ at 2θ = 35.6° and 40.5°, respectively. However, the two peaks didn’t appear in pattern (B). Although the characteristic peak of α-Fe⁰ at 2θ = 44.7° in pattern (C) was sharper than that in pattern (B), the structure of the GS-NZVI remains amorphous.^27,28 Compared with the amorphous structure of GS-NZVI, the crystalline structure of NZVI synthesized without stabilizing agent, which showed in pattern (A), had low decolorization efficiency in the process of dye degradation. The results were illustrated in the studies of Choi et al.^20,22 Therefore, the antioxidant coated on the surface of GS-NZVI did not affect the activity of GS-NZVI, and they were broken during the process of dye degradation. The XRD spectra showed that grape seed extract was important for the green synthesis of GS-NZVI, which enabled GS-NZVI to possess both antioxidant capacity and high reductive activity.

3.1.4. BET. The BET surface area of GS-NZVI was 54.2790 m² g⁻¹, higher than the NZVI synthesized without the stabilizing agent (18.7221 m² g⁻¹) and GS-NZVI after use (36.5669 m² g⁻¹). Fig. 4 shows the N₂ adsorption/desorption isotherms of NZVI, GS-NZVI and GS-NZVI after use, respectively. All the samples displayed typical IUPAC type-IV isotherms with type H3 hysteresis loops for the relative pressure P/P₀ in the range of 0.5 to 0.97.

Fig. 4 Nitrogen adsorption/desorption isotherms of (a) the NZVI synthesized without stabilizing agent; (b) GS-NZVI and (c) GS-NZVI after use.

3.1.5. FTIR spectra analysis. To elucidate the ligand molecules of the samples, the FTIR spectra of NZVI synthesized without stabilizing agent, GS-NZVI and GS-NZVI after RBR decolorization were investigated. As shown in Fig. 5, the FTIR spectrum of NZVI synthesized without the stabilizing agent showed peaks at 3421, 3130, 1636 and 1400 cm⁻¹. The bands at 3421 cm⁻¹ and 3130 cm⁻¹ were due to the weak stretching vibration of surface hydroxyl groups and the bulk hydroxyl stretch,²⁹ respectively. The band at 1636 cm⁻¹ was responsible for the absorption of water,²⁹ and the band at 1400 cm⁻¹ was due to the asymmetric stretching vibration of –COO– functional groups.³⁰ Compared with NZVI synthesized without the stabilizing agent, the vibration of the cyclobenzene on GS-NZVI was confirmed by the new bands at 1578 and 1528 cm⁻¹, and the C–O vibration of phenols at 1020 cm⁻¹.⁵ In particular, the corresponding peak at 3428 cm⁻¹ was due to the stretching vibration of O–H, which was stronger than that of NZVI synthesized without the stabilizing agent. The new bands and stronger bands mentioned above belonged to the characteristic peaks of grape seed extract,⁵ indicating that the effective components in the grape seed extract can be coated onto the surface of GS-NZVI particles. After the degradation of the RBR dye, the protective layer on the surface of GS-NZVI was destroyed, and the O–H stretching vibration at 3414 cm⁻¹ was obviously weaker than GS-NZVI. Because the RBR dye molecules or intermediate products were adsorbed on the surface of GS-NZVI,⁶ a new band due to NH₂ stretching at 2362 cm⁻¹ appeared. The protective layer of grape seed extract did not affect the reductive capacity of GS-NZVI, which may be damaged during the degradation of dyes (as shown in SEM and XRD images).

Fig. 5 FTIR spectra: (a) NZVI particles without stabilizing agent; (b) GS-NZVI particles; (c) GS-NZVI particles after use (represented by the degradation of RBR dye).

3.2. Effect of grape seed extract concentration on the synthesis of NZVI

The grape seed extract concentration is an important factor affecting the property of GS-NZVI particles. Fig. 6 depicts the decolorization processes of RBR and RBB dyes by GS-NZVI synthesized under different grape seed extract concentrations. The decolorization efficiency was compared to determine an optimal grape seed extract concentration. Fig. 6(a) shows that the degradation of RBR dye by sample GS0 (NZVI synthesized without adding grape seed extract) stopped at 4.0 min. The decolorization efficiency was very low, only 10.77% (at 4.0 min). On the contrary, for sample GS1 to GS5, the degradation of RBR dye stopped at 7.0 min, and the decolorization efficiency reached 94.53, 95.45, 93.22, 92.40 and 91.50% (at 7.0 min), respectively, which was higher than for sample GS0. The results suggested that GS-NZVI synthesized with different grape seed extract concentrations had little influence on the RBR dye decolorization.

Fig. 6 The degradation processes of RBR and RBB dye by GS-NZVI synthesized under different grape seed extract concentrations: (a) RBR degradation processes by sample GS0, GS1, GS2, GS3, GS4 and GS5, dosage = 2 g L⁻¹, C_RBR = 1000 mg L⁻¹, initial pH = 8.23; (b) RBB degradation processes by sample GS0, GS1, GS2, GS3, GS4 and GS5, dosage = 2 g L⁻¹, C_RBB = 1000 mg L⁻¹, initial pH = 7.85.

The decolorization process of RBB dye by GS-NZVI synthesized with different grape seed extract concentrations is shown in Fig. 6(b). On the whole, the degradation of RBB was slower than for RBR. For GS0, the degradation process stopped at 17.5 min, and for GS1 to GS5, it was 25.0 min. The decolorization efficiency of RBB dye by sample GS0 was only 25.50% at 17.5 min. The decolorization efficiency of sample GS1, GS2 and GS3 reached 92.75, 97.17 and 99.07% at 25.0 min, respectively, higher than that of samples GS4 and GS5 (only 85.80% and 63.47% at 25.0 min, respectively). The results indicated that GS-NZVI synthesized with different grape seed extract concentrations did indeed influence the RBB decolorization.

Based on the degradation process of RBB and RBR dyes, it was found that the optimal grape seed extract concentration was 0.2 wt% (sample GS2) and 0.3 wt% (sample GS3). To save costs, the concentration of 0.2 wt% was selected to synthesize GS-NZVI, which was applied in subsequent experiments.

3.3. Effects of initial pH value on decolorization

The initial pH value of the dye solution is an important factor affecting dye removal. The real dye wastewater had a wide range of pH values, so it is necessary to investigate the effect of initial pH value on the decolorization of RBR and RBB by GS-NZVI (Fig. 7). As shown in Fig. 7(a), 750 mg L⁻¹ RBR dye was degraded by 2 g L⁻¹ GS-NZVI under different initial pH values, and the degradation rate decreased with increasing pH value. Under an initial pH value of 2.00, the degradation of RBR dye was quickly completed in 4.0 min and the decolorization efficiency was 92.94%. The decolorization process finished at 7.0 min for initial pH values of 4.00, 6.00, 8.23 and 10.00, and higher decolorization efficiencies of 97.08, 97.74, 98.09 and 97.81% were achieved, respectively, which was higher than that using a pH value of 2.00. The decolorization efficiency indicated that the initial pH value had little effect on the degradation of RBR dye by GS-NZVI.

Fig. 7 Effect of initial pH values on decolorization: (a) RBR (GS-NZVI dosage = 2 g L⁻¹, C_RBR = 750 mg L⁻¹, initial pH = 2, 4, 6, 8.23, 10); (b) RBB (GS-NZVI dosage = 2 g L⁻¹, C_RBB = 600 mg L⁻¹, initial pH = 2, 4, 6, 7.85, 10).

The decolorization process of 600 mg L⁻¹ RBB dye with different initial pH values using 2.0 g L⁻¹ GS-NZVI is shown in Fig. 7(b). At pH values of 2.00, 4.00, 6.00, 7.85 and 10.00, the degradation processes finished at 25.0 min with decolorization efficiencies of 90.96, 93.70, 95.46, 97.79 and 95.45%, respectively. The best decolorization efficiency was achieved at the original pH value for both RBR and RBB dyes (RBR pH = 8.23, RBB pH = 7.85). And the original pH value was used in the subsequent experiments.

Under the strong reducibility of GS-NZVI (eqn (2)–(4)),³¹ the decolorization of RBR and RBB dyes occurred by the cleavage of the azo bond of RBR and anthraquinone ring of RBB.

–N=N– + 2H⁺ + 2e⁻ → –NH + HN– (2)

Benzoquinone + H⁺ + e⁻ → semiquinone + H⁺ + e⁻ → hydroquinone (3)

Fe⁰ + 2H⁺ → Fe²⁺ + H₂ (4)

Generally, as GS-NZVI was oxidized, a surface layer consisting of γ-Fe₂O₃ and Fe₃O₄-like oxides formed, which prevented GS-NZVI from further reaction.³² So, the degradation speed will be faster at the beginning. In theory, the lower pH value of dye solution is favorable for decolorization. The reason may be that with the increase of pH value, the ferrous hydroxide precipitates accumulated on the surface of GS-NZVI particles to cover the reactive sites,⁶ so the decolorization speed will become slow. Another possible reason may be that, according to eqn (2) and (3), a large amount of H⁺ can react with azo bonds and anthraquinone rings quickly under strong acid conditions. However, according to the dye degradation profiles shown in Fig. 7, when the initial pH was 2.00, the final decolorization efficiency was lower (both RBR and RBB) than the higher initial pH value. When pH value was 2.00, the GS-NZVI corroded quickly into the dye solution, leading to an insufficient release of ferrous ions and electrons for dye degradation.^33,34

3.4. Effects of GS-NZVI dosage on decolorization

Fig. 8 shows the effects of GS-NZVI dosage on the decolorization of RBR and RBB dyes. For 1000 mg L⁻¹ RBR (Fig. 8(a)), the decolorization process by 2.0, 3.0 and 4.0 g L⁻¹ GS-NZVI samples achieved a decolorization efficiency of 96.08, 98.03 and 98.78% at 4.0 min, respectively; while the lower GS-NZVI dosage of 1 g L⁻¹ needed a longer reaction time of 7.0 min with a decolorization efficiency of 92.23%. Particularly, the RBR decolorization efficiency was only 69.71% at 10.0 min by 0.5 g L⁻¹ GS-NZVI. As shown in Fig. 8(c), the decolorization process of 1000 mg L⁻¹ RBB dye by 0.5 g L⁻¹ GS-NZVI was similar to the RBR dye; the decolorization efficiency was only 42.17% at 17.5 min, indicating that a GS-NZVI dosage of 0.5 g L⁻¹ was insufficient for 1000 mg L⁻¹ RBR and RBB dyes. The decolorization reactions accomplished at 30.0, 17.5, 15.0 and 12.5 min by GS-NZVI dosages of 1.0, 2.0, 3.0 and 4.0 g L⁻¹ had decolorization efficiencies of 95.30, 98.07, 98.80 and 99.52%, respectively. On the whole, the decolorization efficiency of RBR and RBB dyes increased with increasing GS-NZVI dosage.

Fig. 8 Effects of GS-NZVI dosage on the degradation of dyes: (a) RBR degradation process (pH = 8.23, C_RBR = 1000 mg L⁻¹, GS-NZVI dosage = 0.5, 1, 2, 3, 4 g L⁻¹); (b) variations of RBR decolorization efficiency, reduction capacity and reaction time with increasing GS-NZVI dosage. (c) RBB degradation process (pH = 7.85, C_RBB = 1000 mg L⁻¹, GS-NZVI dosage = 0.5, 1, 2, 3, 4 g L⁻¹); (d) variations of RBB decolorization efficiency, reduction capacity and reaction time with increasing GS-NZVI dosage.

Although the increase of GS-NZVI dosage was beneficial to improve the decolorization efficiency, the reduction capacity of GS-NZVI was weakened. The reduction capacity was 922.30 mg g⁻¹ for RBR (Fig. 8(b)) at 7.0 min and 943.50 mg g⁻¹ for RBB (Fig. 8(d)) at 25.0 min when the GS-NZVI dosage was 1.0 g L⁻¹. When the GS-NZVI dosage was increased to 4.0 g L⁻¹, the reduction capacity for RBR and RBB dyes reduced to 246.95 mg g⁻¹ at 4.0 min and 248.40 mg g⁻¹ at 12.5 min, respectively.

The decolorization time was also an important factor to evaluate the decolorization process. As shown in Fig. 8(b) and (d), except for a dosage of 0.5 g L⁻¹, with the increase of GS-NZVI dosage, the decolorization time decreased. The reaction times were 7.0, 4.0, 4.0, 4.0 min using 1.0, 2.0, 3.0 and 4.0 g L⁻¹ GS-NZVI for RBR, respectively. And the reaction times were 25.0, 17.5, 15.0 and 12.5 min for RBB dye, respectively.

Based on the data mentioned above, it could be summarized that decolorization efficiency increased with the increasing GS-NZVI dosage, while reduction capacity and reaction time decreased. On the one hand, the increased GS-NZVI dosage can improve the availability of GS-NZVI in the dye solution, which may reduce reaction time and increase decolorization efficiency. On the other hand, the excess GS-NZVI dosage will decrease the reduction capacity. Based on degradation efficiency, reduction capacity, decolorization time and cost saving, the optimal GS-NZVI dosage of 2.0 g L⁻¹ was chosen for the subsequent experiment.

3.5. Effects of initial dye concentration on decolorization

In fact, the initial dye concentration of dye wastewater was unstable. Thus, it is necessary to investigate the effect of different initial dye concentration, and the results are depicted in Fig. 9. For RBR, as shown in Fig. 9(a), concentrations of 500, 750, 1000, 1500 and 2000 mg L⁻¹ RBR dye can be decolorized efficiently within 4.0, 4.0, 4.0, 6.0 and 7.0 min, with decolorization efficiencies of 97.67, 97.62, 96.08, 98.22 and 98.16%, respectively. Generally, reduction capacity and specific decolorization rate will increase with the increasing initial dye concentration. For the RBR dye, the reduction capacity increased from 244.18 to 981.60 mg g⁻¹ and the specific decolorization rate increased from 61.04 to 140.23 mg g⁻¹ min⁻¹ with the initial RBR concentration increasing from 500 to 2000 mg L⁻¹. Fig. 9(a) illustrates that the RBR dye can be readily decolorized by 2.0 g L⁻¹ GS-NZVI in a wide concentration range (up to 2000 mg L⁻¹). For RBB (Fig. 9(c)), when the initial RBB dye concentration was 250, 500, 750, 1000 and 1250 mg L⁻¹, 2.0 g L⁻¹ GS-NZVI could achieve decolorization efficiencies of 99.08, 97.90, 97.45, 98.07 and 94.00% at 10.0, 15.0, 17.5, 17.5 and 25.0 min. With the increasing RBB dye concentration, reduction capacity and specific decolorization rate showed a rising trend. The reduction capacity increased from 12.39 to 28.02 mg g⁻¹ min⁻¹ with the initial dye concentration increasing from 250 to 1000 mg L⁻¹. At the initial RBB dye concentration of 1250 mg L⁻¹, the specific decolorization rate decreased to 23.50 mg g⁻¹ min⁻¹. It could be found that 2.0 g L⁻¹ GS-NZVI was insufficient to degrade 1250 mg L⁻¹ RBB dye, because the decolorization efficiency decreased.

Fig. 9 Kinetic modeling under different initial dyes concentration: (a) first-order kinetic modeling under different RBR concentrations; (b) second-order kinetic modeling under different RBR concentrations (pH = 8.23, GS-NZVI dosage = 2 g L⁻¹, initial C_RBR = 500, 750, 1000, 1500 and 2000 mg L⁻¹); (c) first-order kinetic modeling under different RBB concentrations; (d) second-order kinetic modeling under different RBB concentrations (pH = 7.85, GS-NZVI dosage = 2 g L⁻¹, initial C_RBB = 250, 500, 750, 1000, 1250 mg L⁻¹).

3.6. Kinetic analysis

The RBR and RBB dye decolorization by GS-NZVI under different initial dye concentrations can be represented by the following nth-order reaction kinetics:³⁵

dC/dt = −kCⁿ (5)

where C represents the dye concentration, n is the order of the reaction, t is the reaction time, k is the reaction rate constant. According to eqn (5), the first-order and second-order reaction can be represented as follows (eqn (6) and (7)):

C = C₀ exp(−kt) (6)

C = C₀/(1 + kC₀t) (7)

where C₀ represents the concentration of RBB dye at time t = 0. The fitted curves of RBR and RBB dye degradation processes by the first-order and second-order reactions are shown in Fig. 9, and the parameters of the two kinetic models are presented in Table 2.

Table 2 Constants of first-order and second-order kinetic models for the decolorization of RBR and RBB by GS-NZVI particles

Type of dye	Concentration of dye (mg L^−1)	First-order		Second-order
		k (min^−1)	R^2	k (L mg^−1 min^−1)	R^2
RBR	500	0.8631	0.9844	1.8230	0.9052
	750	0.8523	0.9828	1.8026	0.9020
	1000	0.6638	0.9721	1.3396	0.8893
	1500	0.7669	0.9916	1.6391	0.9146
	2000	0.6890	0.9915	1.4366	0.8965
RBB	250	0.4850	0.9620	1.1823	0.9921
	500	0.2499	0.9894	0.5634	0.9715
	750	0.1881	0.9837	0.4229	0.9586
	1000	0.1438	0.9627	0.3326	0.9370
	1250	0.1219	0.9769	0.2670	0.9590

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As shown in Fig. 9(a) and (b) and Table 2, the regression coefficients of the second-order kinetic model for RBR are in the range 0.8893–0.9146, which did not fit the data well. However, the regression coefficients of the first-order kinetic model are in the range 0.9721–0.9961, indicating that the decolorization process of RBR was fitted well by the first-order kinetic model. As seen from Table 2, with the RBR concentration increasing from 500 to 2000 mg L⁻¹ (except 1000 mg L⁻¹), the k value of the first-order kinetic model showed a downward trend, decreasing from 0.8631 to 0.6890 min⁻¹. The value of k at a dye concentration of 1000 mg L⁻¹, which did not conform to the overall trend, was lower than that at other dye concentration. The GS-NZVI sample was not completely homogeneous which may be the reason; this was confirmed in the SEM image (Fig. 1).

For RBB, as shown in Fig. 9(c) and (d) and Table 2, the regression coefficients of the first-order and second-order kinetic model are in the range 0.9629–0.9894 and 0.9370–0.9921, respectively. At the same RBB concentration (except 250 mg L⁻¹), the regression coefficients of the first-order kinetic model are higher than the second-order kinetic model. Thus, the decolorization of the RBB dye fitted well for the first-order kinetic model. Generally, the GS-NZVI–contaminant system was a surface-mediated process; the main influencing factors of the reduction reaction was the nature of contaminant and the active surface sites on the GS-NZVI particles. If the number of active surface sites on GS-NZVI particles is constant, it can be assumed that the azo bond of the RBR dye degrades quickly through a reduction reaction, and the degradation of the anthraquinone ring on the RBB dye was relatively difficult due to the longer decolorization time.

The decolorization efficiency of RBR and RBB dyes by GS-NZVI in this study was compared with other studies of NZVI/ZVI. 0.2 mM acid orange 7 (azo dye) was degraded by 0.3 g L⁻¹ NZVI with the pseudo first-order rate constant of 0.1850 min⁻¹.³⁶ 5.0 g L⁻¹ NZVI was used to reduce 100 mg L⁻¹ acid orange II (azo dye) with the first-order kinetic constant of 0.0180 min⁻¹.³⁷ In this study, the k value (first-order kinetic model) of 500–2000 mg L⁻¹ initial RBR dye concentration was higher than the results above. For 1000 mg L⁻¹ Reactive Blue 4, an anthraquinone dye, the pseudo first-order rate constant was 0.0290 h⁻¹ by 55.9 g L⁻¹ ZVI.³⁸ In this study, the k value (first-order kinetic model) of 250–1250 mg L⁻¹ RBB dye is much higher than the results mentioned above. GS-NZVI not only conformed to the concept of green synthesis, but also has a good stability and a better application potential.

Decolorization of RBR and RBB dye by different methods was compared with this study. A 50 mg L⁻¹ RBR dye solution needed 10 days to achieve a degradation efficiency of 93.80% in an up-flow anaerobic bioreactor under saline conditions.³⁹ A 6 mg L⁻¹ RBR dye solution was degraded by decorated TiO₂ nanotube arrays with a degradation efficiency of 60.40%.⁴⁰ Degradation of a 50 mg L⁻¹ RBB dye solution by a TiO₂ assisted process under UV irradiation reached a degradation efficiency of 97.70% at 3.0 h,⁴¹ but the specific decolorization rate was only 0.60 mg g⁻¹ min⁻¹, which is far less than the specific decolorization rate of GS-NZVI (28.02 mg g⁻¹ min⁻¹) in this study. 4.0 g L⁻¹ Fe-containing Y and ZSM-5 zeolites as heterogeneous catalysts with 30.0 mmol L⁻¹ H₂O₂ were applied to degrade a 250 mg L⁻¹ RBB dye solution; 90.00% of the dye was degraded after 20.0 min and the specific decolorization rate was only 2.81 mg g⁻¹ min⁻¹.⁴² Compared with the decolorization methods mentioned above, using GS-NZVI to decolorize RBR and RBB had a better degradation efficiency, reduction capacity and specific decolorization rate; and it features mild conditions, low-cost and is eco-friendly. Therefore, the green synthesized GS-NZVI, using grape seed extract as a stabilizing agent, exhibited good application prospects for decolorizing azo and anthraquinone dyes.

3.7. UV-vis spectra analysis

The changes of UV-vis spectra of 50 mg L⁻¹ RBR and RBB dyes decolorized by 0.5 g L⁻¹ GS-NZVI are shown in Fig. 10. For the RBR dye in Fig. 10(a), the characteristic peaks attributed to the azo bond, aromatic rings⁴³ and benzene components³¹ were at 510, 278 and 243 nm, respectively. When the decolorization reaction proceeded, the chromophores from the azo bond at 510 nm and aromatic rings at 278 nm were destroyed, as shown by a decrease in their absorption peak. On the contrary, the peak from benzene at 243 nm gradually increased. It indicated that the RBR dye was degraded into benzene components by GS-NZVI, and a further treatment process would be necessary to mineralize or degrade the products. For the RBB solution, the characteristic peaks at 601 and 256 nm were related to the anthraquinone group and aromatic/reactive dichloropyrazine groups. According to Fig. 10(b), the absorption peak at 601 nm became flat, and the peak at 256 nm decreased gradually. It demonstrated that the anthraquinone group and aromatic/reactive dichloropyrazine groups³¹ in the RBB dye were destroyed by GS-NZVI. The dye solution can be decolorized rapidly at the end of reaction because the chromophores of RBR and RBB dyes in the visible range, azo bond and anthraquinone group, were destroyed by GS-NZVI.

Fig. 10 UV-vis spectra of the dye degradation process: (a) RBR (pH = 8.23, C_RBR = 50 mg L⁻¹, GS-NZVI dosage = 0.5 g L⁻¹); RBB (pH = 7.85, C_RBB = 50 mg L⁻¹, GS-NZVI dosage = 0.5 g L⁻¹).

4. Conclusions

GS-NZVI was synthesized in one step using a grape seed extract and potassium borohydride to reduce ferrous ions under mild conditions. The grape seed extract, an agriculture and forestry waste material, played the role of stabilizing agent during the process of green synthesis, which was confirmed by SEM, EDS, TEM, DLS, XRD, BET and FTIR spectra of GS-NZVI. The GS-NZVI was mainly amorphous in nature and can be preserved in air. The grape seed extract was coated on the surface of GS-NZVI, and almost did not affect the activity of GS-NZVI. The grape seed extract concentration influenced the properties of GS-NZVI, and 0.2 wt% was the optimal concentration. The pH value of the dye solution had little effect on the decolorization by GS-NZVI. The decolorization efficiency increased with the increasing GS-NZVI dosage, while the reduction capacity and reaction time declined. Under the optimal dosage of 2.0 g L⁻¹ GS-NZVI, the decolorization efficiency can achieve more than 96.08% within 7 min and 97.45% within 17.5 min with RBR dye concentrations between 500 and 2000 mg L⁻¹ and RBB dye from 250 to 1000 mg L⁻¹, respectively. The pseudo first-order kinetic model can well explain the RBR and RBB dye degradation process. The UV-vis spectra indicated that chromophores of dyes were degraded by GS-NZVI. Therefore, the green synthesized GS-NZVI with grape seed extract as a stabilizing agent can be applied in dye wastewater treatment because of its simple and mild synthesis conditions, low-cost, easy storage and high surface reactivity.

Acknowledgements

We would like to thank the NSFC (51078007, 51378027, 51578015) and Beijing Talent Foundation of BJUT (2013-JH-L06) for the financial support of this study.

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