Facile one step green synthesis of iron nanoparticles using grape leaves extract: textile dye decolorization and wastewater treatment

The world is facing a global water quality crisis. Population growth, urbanization and industrialization have accelerated the consumption of freshwater and thereby increased the unregulated or illegal discharge of contaminated water to the environment (Corcoran et al. 2010). An estimated 80% of all industrial and municipal wastewater are released to the environment without any prior treatment, resulting in a growing deterioration of overall quality with harmful impacts on human health and ecosystems (UNESCO 2018). One such industry is textile dyeing, which consumes enormous amount of water, dyes, chemicals and generates huge amount of colored wastewater (Ghaly et al. 2014; Pattnaik et al. 2018; Hynes et al. 2020). Most of the textile dyes utilized during the dyeing process get washed away in the wastewater and the conventional treatment methods failed to remove the dye color (Holkar et al. 2016; Paździor et al. 2019). Textile dyes are toxic, recalcitrant in the environment and therefore the degradation of textile dyes is more challenging (Ventura-camargo & Marin-morales 2013).

The application of nanotechnology in environmental remediation has been examined in the last decade and the potential of iron nanoparticles in removing various contaminants like chlorinated, halogenated aliphatic, nitrates, nitro aromatic carbons, heavy metals, phenols, inorganic species, explosives and pesticides has been successfully reported (Fu et al. 2014). Textile dye decolorization using nano size iron particles is emerging in the recent years as the particles are highly reactive, non-toxic, and cost-effective (Patil et al. 2016). Iron particles aggregate quickly with respect to time and addition of supports could enhance the stability. However, the dye decolorization in the presence of supports such as biochar, clay (kaolin, bentonite, montmorillonite), zeolite (rectorite, clinoptilolite, perlite), resins, heavy metals are observed as an environmental risk, as the supports transfer the adsorbed textile dye to various environments (Raman & Kanmani 2016; Hynes et al. 2020). In recent years, plant based green supports are introduced and acclaimed for cost-effective and eco-friendly treatment (Machado et al. 2013; Raman & Kanmani 2016; Ozkan et al. 2018; Lohrasbi et al. 2019; Bhuiyan et al. 2020; Ting & Chin 2020). The plant extract is comprised of polyphenols, flavonoids, etc. in which polyphenols are known to be strong reducers and effective metal chelators forming stable complexes with iron (Ozkan et al. 2018). Iron also possesses octahedral geometry, could coordinate upto three polyphenol groups (Yu et al. 2018). However, green iron particles are evolving and there is no adequate study to comprehend their competence on reactive dyes (widely used on cotton fabrics) decolorization.

In this study, the bare iron (B-nZVI) and green iron particles (GT-nZVI and GP-nZVI) were synthesized and utilized for the decolorization study of reactive dyes such as reactive red 195 (RR), reactive yellow 145 (RY), reactive blue 4 (RB), reactive black 5 (RBB). The green iron particles were synthesized using green tea leaves extract (GT-nZVI) and grape leaves extract (GP-nZVI). The synthesized particles were characterized for size, crystal structure, morphology, elements; functional groups attached and BET specific surface area. The dye decolorization was examined and the parametric study was conducted. The reaction kinetics and adsorption isotherm of iron particles were investigated. The intermediate products were monitored and the respective decolorization pathway was proposed. Then, the competence of green iron particles on real textile wastewater treatment was examined and reported.

Sodium borohydride (NaBH₄), ferric chloride (FeCl₃), hydrochloric acid, ethanol, sodium hydroxide, methanol were all purchased from Merck Private Ltd. Reactive textile dyes such as reactive red 195 (C₃₁H₁₉ClN₇Na₅O₁₉S₆), reactive yellow 145 (C₂₈H₂₀ClN₉Na₄O₁₆S₅), reactive blue 4 (C₂₃H₁₃Cl₂N₆O₈S₂), reactive black 5 (C₂₆H₂₁N₅Na₄O₁₉S₆) were purchased from the textile dyeing industries at Tiruppur, Tamil Nadu. The chemical structure of all purchased dyes is shown in Figure 1. The maximum wavelength (λ_max) and molecular weight for dyes are presented in Table 1. All the chemicals were of analytical grade and utilized as received without any further purification. The stock solution of 1,000 mg/L of RR, RY, RB, RBB were prepared by dissolving 1 g of each dye in 1 L of distilled water and serial dilutions were made as required from stock solutions. The standard calibration curves of various known concentrations of all dyes for maximum wavelength (λ_max) were prepared using UV-Vis spectrophotometer (Jasco Inc., Japan) and it was utilized to determine unknown dye concentration.

The bare iron particles (B-nZVI) were synthesized by sodium borohydride method (Raman & Kanmani 2019) and green iron particles using green tea leaves extract (GT-nZVI) and were synthesized as discussed in an earlier study (Raman & Kanmani 2018). The synthesizing approach of green iron particles using grapes leaves extract (GP-nZVI) is presented in Figure 2. The leaves of Vitis vinifera (black grapes) were collected, washed with deionized water, then dried under shade for 15–20 days and finely powdered. 250 mL of methanol was added to 5 g of the powder and the extract was vacuum filtered after an hour. The extract was then treated with 0.5M FeCl₃ at a volumetric ratio of 1:1. An intense black precipitate confirmed the formation of GP-nZVI. It was vacuum filtered, dried and kept in a desiccator until further use.

The particle size was observed based on dynamic light scattering technique using nano particle analyzer SZ-100 (Horiba, Japan). The crystal structure was examined using Ultima-IV X-ray diffractometer (Rigaku, Japan) with Cu Kα radiation. The operating voltage and current was maintained at 40 kV and 30 mA. The reflections were scanned from 2θ of 5° to 100° at a rate of 5° per minute with a step size of 0.02°. The size, morphology, and the elemental analysis were studied using transmission electron microscopy (TEM) coupled with energy-dispersive X-ray spectroscopy (EDS), which was acquired using Tecnai G2 Spirit (FEI, The Netherlands). The TEM images of different magnifications were recorded. Fourier transform infra-red (FTIR) spectrum of fresh and used iron particles were recorded using Perkin–Elmer (Instrument Co. Ltd, USA). The Brunauer–Emmett–Teller (BET) specific surface area of three synthesized iron particles was measured based on BET-N₂ adsorption method using Micromeritics ASAP 2020 (Micromeritics, USA).

The separation factor indicates the nature and the feasibility of the adsorption process.

The bare and green iron particles synthesized were characterized for size, shape, elemental composition, crystal structure, specific surface area and functional groups attached. The average particle size was 20–80 nm for bare iron and 120–160 nm for green iron respectively (Figure S1, supplementary data). In recent studies (Fan et al. 2009; Rahman et al. 2014; Sohrabi et al. 2016), the bare iron size was reported similarly as less than 100 nm, whereas the green iron size was reported as 172 nm (Gao et al. 2016). The morphology of B-nZVI is spherical and due to its magnetic property, with respect to time, a chain-like structure was observed (Figure S2); however, GT-nZVI and GP-nZVI showed irregular clusters of biomolecules capping individual iron particles (Figure S2 and Figure 3). The TEM images of green iron confirm the prevention of aggregation of iron by polyphenolic compounds and same pattern was reported earlier in a study (Hoag et al. 2009). The EDS revealed the presence of high carbon and oxygen content in green iron due to polyphenolic compounds.

The XRD spectrum of B-nZVI spectrum showed peaks at 2θ of 45°, 65.8°, 81.8° which confirmed the existence of body centered cubic crystal of iron and respective miller indices were 110, 200, and 211 ((Fan et al. 2009). In addition, the peak at 2θ of 35.5° and 62.7° indicated the formation of oxide layers (Fe₃O₄ and Fe₂O₃) on the bare surface as a signal of oxidation (Figure S3). As the morphology of green iron was amorphous in nature due to the preferred orientation of iron particles with the polyphenolic compounds, the XRD showed no peaks along 2θ both for GT-nZVI (Figure S4) and GP-nZVI (Figure 3), respectively, and similar observations for green iron were reported recently (Huang et al. 2014).

The FTIR spectrum (Figure 3) of B-nZVI showed a small peak at 542 cm⁻¹ and this may be due to the formation of oxide layers on the iron surface when exposed to atmospheric oxygen. The FTIR spectra of GT-nZVI and GP-nZVI showed a broad envelope due to OH stretching vibration of 3,600–2,500 cm⁻¹ (Huang et al. 2014; Luo et al. 2015). The broadening is due to hydrogen bonding of OH groups and the sharp peak at 1,606 cm⁻¹ and 1,605 cm⁻¹ is due to the vibrations of aromatic ring in epigallocatechin gallate and malonylglucoside compounds of GT-nZVI and GP-nZVI respectively (Figure 3). The peaks at 636 cm⁻¹ and 643 cm⁻¹ in the fingerprint region confirmed the presence of Fe-O bond in GT-nZVI and GP-nZVI. Hence, from this analysis it is confirmed that polyphenolic compounds are largely involved in complexation with Fe³⁺ in green iron and thus amorphous morphology.

The BET surface area of B-nZVI, GT-nZVI and GP-nZVI was found 31.04 m²/g, 14.6 m²/g and 15.08 m²/g respectively and this demonstrates that increase in green iron size showed reduced BET surface area. Increased BET surface area could enhance adsorption process and which is typically high (2,636 m²/g) for activated carbon, hence maximum adsorption capacity (Kumar & Jena 2016). Earlier studies reported 29–33.5 m²/g for bare iron (Chen 2005; Chompuchan et al. 2009; Shih et al. 2010) and 5.8–6.67 m²/g for green iron (Huang et al. 2014; Yu et al. 2018). Gao et al. (2016) reported 54.27 m²/g of surface area for 172 nm of green iron, which is objectionable as the increase in particle size automatically reduces specific surface area (Ponder et al. 2001; Sun et al. 2006). The observed BET surface area confirms the competence of adsorption of bare and green iron but is not as effective as activated carbon.

The reaction between dye molecules and the iron particles (B-nZVI, GT-nZVI, and GP-nZVI) were studied by varying iron dose (0.4–2.6 g/L), dye concentration (25–200 mg/L) and pH (3–11). The reactive dyes RR, RY, RB, and RBB have different molecular weights and complicated structure, hence the iron dose required for maximum decolorization were varied. The UV-Vis spectra of RR, RY, RB, and RBB before and after treatment with B-nZVI, GT-nZVI, GP-nZVI at optimized dose and 25 mg/L initial dye concentration is presented in Figure 4. The absorbance at λ_max of RR (542 nm), RY (418 nm), RB (597 nm), and RBB (597 nm) immediately disappeared within 30 min of treatment with bare and green iron particles. The decolorization efficiency of B-nZVI, GT-nZVI, and GP-nZVI in RR, RY, RB and RBB dyes for 30 min of contact time is illustrated in Figure 5. The decolorization attained close to 90–98% for all the dyes in the presence of bare and green iron particles. The results therefore illustrate the versatility of iron particles for treatment of textile dyeing wastewater.

The decolorization process was almost finished within 15 min of contact time. The ferric chloride solution, sodium borohydride solution, Camellia sinensis leaves extract and Vitis vinifera leaves extract were added to all dyes individually. The ferric chloride and sodium borohydride showed no influence in the decolorization efficiency and hence it is confirmed that these precursors do not have any role in reactive dye decolorization. Similarly, the components of Camellia sinensis and Vitis vinifera leaves extract as such are not adequate to decolorize any dyes. This demonstrates the key role of B-nZVI, GT-nZVI, and GP-nZVI in reactive dye decolorization process. The influence of dye concentration on reactive dye decolorization was studied at optimized dose of bare and green iron and the study was conducted with the initial pH 7 of reactive dyes. The dye concentration was varied as 25, 50, 100, 150, and 200 mg/L for RR, RY, RB, and RBB. GP-nZVI and GT-nZVI decolorized 95–98% whereas B-nZVI decolorized 90–92% of all reactive dye at 25 mg/L concentration; however, the potential of green iron particles in decolorization is reduced to 52% and 49% for RR, 64% and 61% for RY, 78% and 71% for RB, 76% and 68% for RBB at 200 mg/L dye concentration. The bare iron decolorized 46% of RR, 56% of RY, 65% of RB, 62% of RBB at 200 mg/L of dye concentration. It is evident that increase in dye concentration reduced decolorization efficiency of iron particles as all the reactive sites were utilized by dye molecules and there was a demand for more reactive sites of iron. Similarly, Sohrabi et al. (2016) reported 95% of Reactive Blue 21 decolorization was achieved at 30 mg/L of initial concentration; however, increase in dye concentration reduced the decolorization efficiency of 0.5 g bare iron particles.

The effect of pH for all reactive dyes of 25 mg/L initial concentration at optimized dose is studied. Slight decrease in dye decolorization efficiency was observed when pH was increased from 3 to 11. Hence, pH has less effect on reactive dye decolorization (Chen et al. 2018; Trevizani et al. 2018). This was further demonstrated by the point of zero charge (pH 8) studies conducted on the bare and green iron particles. It was found that at higher pH values (above the point of zero charge); the surface of iron acquired a negative surface. All the reactive dyes contain azo groups and protonation of azo groups was expected in the presence of H⁺ ions when pH is lower than pH of point of zero charge. Once all the reactive sites of iron particles were utilized by dye molecules, the passive layer of oxides and hydroxides on iron surface induced adsorption. From the results, it is evident that the order of the reactivity of iron particles in dye decolorization is GP-nZVI > GT-nZVI > B-nZVI. Similarly, the order of reactive dyes achieved maximum decolorization at an optimized iron dose is RB > RBB > RY > RR. This might be ascribed to the bulky nature of RR and RY. Increase in iron dose increased the reactive sites for the dye decolorization and once all the sites were utilized by the reactive dye molecules, a greater number of basic nitrogen sites in RR, RY, RB enhanced adsorption with iron particles. Thus, reduction and adsorption processes are involved in the reactive dye decolorization (Huang et al. 2014; Luo et al. 2015).

The dye decolorization kinetics was investigated using pseudo-first-order and pseudo-second-order models. Pseudo-first-order model is fit better for RY, RBB decolorization using B-nZVI, and then RR, RBB decolorization using GT-nZVI, and RR, RY decolorization using GP-nZVI and the respective rate constants were computed and presented in Table 2. Other decolorization fits well with pseudo-second-order model. The pseudo-first-order and second-order correlations varied less than 5%. The pseudo-first-order fit confirms the reaction between iron particles and dye molecules with rate constant 0.317–0.422 and it is followed by adsorption, which was confirmed with data fit pseudo-second-order model. Similar observations were reported by Chompuchan et al. (2009) and Gao et al. (2016) for reactive dye decolorization.

The adsorption isotherms were investigated for reactive dye decolorization using bare and green iron particles. From Table 3, it is observed that the correlation of determination R² value of Langmuir isotherm model is favorable to experimental data than Freundlich isotherm model. This demonstrates the monolayer adsorption of dye molecules on the homogenous surface of iron was achieved (Lemraski & Tahmasebi 2018). The computed maximum adsorption capacity of bare iron and green iron was 4.704–9.901 mg/g and 11.575–14.749 mg/g, respectively. The adsorption capacity of bare and green iron is low when compared to activated carbon from coconut shell (51.56–62.06 mg/g). Similar adsorption behavior was observed for methylene blue and vat green dye decolorization using bare iron in previous studies and the adsorption capacity of bare iron was reported as 5.53 mg/g (Arabi et al. 2013; Hamdy et al. 2018). The n value greater than one indicates physical adsorption process was achieved between dye molecules and iron particles.

The adsorption between reactive dye molecules and iron particles involves electrostatic interactions and which can be either attractive or repellent, based on the surface charge of the dye molecules and iron particles. This is influenced by dye solution pH and confirmed by point of zero charge studies. It was found that at high pH, the surface of iron accumulates negatively charged dye molecules which are repellent in nature, thus decrease in decolorization efficiency. Therefore, electrostatic interactions were part of the adsorption mechanism (Chi et al. 2017; Hlekelele et al. 2019). The structure of reactive dye molecules consists of aromatic rings with delocalized π electrons; therefore π–π non-electrostatic interactions were also part of the adsorption mechanism. The separation factor value 0.013–0.112 demonstrates that the adsorption of dye molecules on the iron surface is favorable (Hamdy et al. 2018).

The intermediates and products formed during RR, RY, RB and RBB dye decolorization were determined using GC-MS and the detected molecular ions in the mass spectrum are presented in Table 4. This provides insight on the underlying decolorization mechanism and the respective decolorization pathway using green iron particles (GP-nZVI) was proposed and presented in Figures 6 and 7. The azo bond present in the parent molecule of RR dye (m/z 1136) was initially broken by B-nZVI (Figure S4) and the compound was reduced to m/z 534 and m/z 334. However, green iron (Figures S4 and 6) reduced the parent molecule m/z 1136 into individual aromatic structures m/z 246, m/z 232, and m/z 214 respectively. Especially, GP-nZVI demonstrated RR decolorization into smaller molecular compounds compared to GT-nZVI, thus the decolorization efficiency order GP-nZVI > GT-nZVI > B-nZVI was observed. Most of the degraded products are aromatic structures and no azo bond was perceived, hence the complete decolorization of RR dye was observed. Benzene, phenyl sodium, 2-chloro-1,3,5-triazine, di hydro naphthalene, sodium benzene sulfonate, methyl cyclo hex enol were the major intermediates observed in RR decolorization using bare and green iron particles. The azo bond and the weak –NH– bonds present in the parent molecule of RY dye (m/z 1026) were initially broken and the compound was reduced to m/z 434, m/z 230 and m/z 229 in the presence of B-nZVI (Figure S5). However, green iron reduced the parent molecule m/z 1026 into smaller aromatic structures m/z 128, m/z 115, m/z 108, m/z 100, m/z 93 and m/z 78 (Figures S5 and 6). Hence, the decolorization efficiency of green iron particles is better than bare iron. Intermediates and products such as naphthalene, 2-chloro-1,3,5-triazine, benzene 1,2 di amine, phenyl sodium, aniline and benzene were observed in RY decolorization using bare and green iron particles.

The weak –NH– bonds present in the parent molecule of RB dye (m/z 636) were initially broken and the compound was reduced to m/z 288, m/z 158 and m/z 150 in the presence of B-nZVI (Figure S6). However, green iron reduced the parent molecule m/z 636 into smaller aromatic structures m/z 223, m/z 208, m/z 158, m/z 115, and m/z 93 respectively (Figures S6 and 7). Naphthalene, 2-chloro-1 3,5-triazine, anthracene-9,10 dione, 1,3,5 triazine, aniline and benzene were the intermediates and products observed during RB decolorization using bare and green iron particles. The azo bonds present in the parent molecule RBB (m/z 992) were initially broken and the compound was reduced to m/z 159, m/z 158 and m/z 128 in the presence of B-nZVI (Figure S7). However, green iron reduced the parent molecule m/z 636 was further degraded into smaller aromatic structures m/z 110, m/z 94, m/z 86, and m/z 78, and m/z 93 respectively (Figures S7 and 7). Naphthalene, aniline, benzene, phenol, naphthalene-1-amine, benzene sulfonic acid and benzene-1, 3-di ol, hexane were the intermediates and products observed during RBB decolorization using bare and green iron particles. Thus, the green iron could disintegrate RB and RBB dye molecules into lower compounds better than bare iron. This confirms not only adsorption but also reduction process are involved in the reactive dye decolorization (Huang et al. 2014; Luo et al. 2015).

The green iron forms chelate bond with polyphenolic compounds and the bonds were dissolved once the green iron particles are released into dye solution and iron acts as zero valent instantly. Then, the zero valent iron is oxidized to ferrous ion, and then undergoes further oxidative transformation to ferric ion. The oxidation process emitted enormous amount of electrons, those involved in the reaction. The ferric ions in the presence of water forms ferric hydroxide and generate hydronium (H⁺) ions. The emitted electrons are accepted by the dye molecules and thereby dye molecules were reduced and intermediates, products were formed. Further reactions of bare iron with atmospheric dissolved oxygen and water generate hydrogen peroxide (Donadelli et al. 2018). Similarly, the generated electrons in the presence of H⁺ ions also react with dissolved oxygen and produce hydrogen peroxide. Ferrous ions catalyze hydrogen peroxide and generate hydroxyl radicals (Wu et al. 2014), which in turn react with dye molecules and metabolites are formed. In addition, the electrostatic interaction between dye molecules and iron particles, π–π non-electrostatic interaction are also involved in the adsorption process. The ferrous and ferric ions are converted into ferric hydroxides and could also be involved in physical adsorption of dye molecules and metabolites (Mu et al. 2017).

The FTIR spectrum of used bare and green iron particles is presented in Figure S8. The spectrum of B-nZVI showed peaks at 1,021 cm⁻¹, 828 cm⁻¹, and 820 cm⁻¹, thus confirming hydroxide layers on the iron surface. Also, the small peaks observed at 546 cm⁻¹, 656 cm⁻¹, and 660 cm⁻¹ represented the existence of oxide layers on the iron surface, those were exposed to oxidation and reduction during reaction. All the strong and broad C-OH, C=C, O-H bonds observed due to polyphenolic compounds in freshly synthesized green iron (Figure 3) were diminished after use (Figure S8). A similar observation was reported (Shahwan et al. 2011) for methyl orange dye decolorization using green iron particles synthesized from green tea leaves extract. The disappearance of polyphenolic groups from used green iron particles confirmed the dissociation of polyphenolic compounds during dye decolorization.

The performance assessment of green iron particles on real textile wastewater treatment is required before it is opted for any industrial applications. The wastewater consists of mixture of reactive dyes, inorganic salts and it is a combined wastewater from the pretreatment, dyeing and rinsing process. Five samples of textile wastewater (TWW1, TWW2, TWW3, TWW4, and TWW5) were collected (Figure 8) and analyzed for pH, electrical conductivity (EC), total dissolved solids (TDS), chlorides (Cl), sulphates (SO₄), sodium (Na), chemical oxygen demand (COD), and total organic carbon (TOC). The characterized results are summarized in Table 5, which confirms the presence of organics due to textile dyes and high inorganic salts. The wastewater was scanned at multiple wavelengths of 436, 525, 620 nm (ISO 7887: 1985) after removing the turbidity by centrifugation for color measurement as the wastewater contains more than one reactive dye and no single peak was observed.

The bare and green iron dose of 2 g/L was utilized for the treatment of textile wastewater. The decolorization of wastewater was monitored at every 5, 10, 15, 30, and 60 min. Although the equilibrium was attained in 30 min of contact time, to ensure steady state, the reaction was monitored until 60 min. The pH 8.68–10.2 of textile wastewater was reduced to 7.4–8.2 and 7.2–7.6 in the presence of bare and green iron respectively. The decolorization efficiency of B-nZVI was 72–82%. However, GT-nZVI decolorized 80–89% whereas GP-nZVI decolorized 83–92% of textile wastewater (Figure 8). Bare iron removed only 19–32% of COD; however, GT-nZVI and GP-nZVI removed 35–48% and 43–52% of COD respectively (Table 6). This demonstrates that the reactivity of green iron is better than bare iron and the order of reactivity is GP-nZVI > GT-nZVI > B-nZVI. It is significant to discuss the reduction observed in the quantity of inorganic salts (Cl, SO₄, and Na) during the treatment. The reduction was less than 10% using B-nZVI and it was around 12% for GT-nZVI and 14% for GP-nZVI. This might be due to the adsorption of inorganic salts on the oxidized iron surface.

The cost for textile wastewater treatment using bare and green iron particles were computed based on the chemicals utilized for synthesis. This confirms that textile wastewater treatment using GT-nZVI (0.18 $/m³) and GP-nZVI (0.29 $/m³) was found to be very cheap compared to B-nZVI (2.32$/m³). The estimated cost of the proposed textile wastewater treatment technique is compared with the existing treatment technologies (Table 7) and observed to be economical. As the reactivity of GP-nZVI is also better than other iron particles, it is highly recommended to use the green iron particles from grape leaves in order to decolorize textile wastewater effectively.

1.4–2.0 g/L of green iron decolorized 95–98% of RR, RY, RB, RBB dye at 25 mg/L concentration. However, the potential of green iron particles (GT-nZVI and GP-nZVI) in decolorization is reduced to 52% and 49% for RR, 64% and 61% for RY, 78% and 71% for RB, 76% and 68% for RBB at 200 mg/L dye concentration. Increase in green iron dose increased the reactive sites for dye decolorization, and once all the reactive sites were utilized by the dye molecules, basic nitrogen sites in RR, RY, RBB, and RB as well the oxidized iron surface enhanced adsorption process. The adsorption isotherms fit with Langmuir isotherm model and this demonstrates the monolayer physical adsorption of dye molecules on the homogenous iron surface. Thus the reduction and adsorption processes are involved in the dye decolorization. Aromatic compounds were detected as intermediates/products and the respective decolorization pathway was proposed. The green iron of 2 g/L dose was optimized for textile wastewater treatment and the decolorization efficiency was 83–92% (GP-nZVI). The pH 8.68–10.2 of textile wastewater was reduced to 7.2–7.6 and 43–52% of COD was removed. This demonstrates that the performance of green iron (GP-nZVI) is remarkable for textile wastewater treatment and found to be economical. The organic residue from green iron has high biocompatibility and, after treatment managing, the residue would be an ecofriendly practice. This study confirms the competence of green iron particles synthesized from grape leaves in reactive dye decolorization and further recommendation to textile wastewater treatment.

The authors would like to acknowledge the Nano Science & Technology Department of Tamil Nadu Agricultural University, Coimbatore, the Department of Energy Environment, National Institute of Technology, Tiruchirappalli and the Council of Scientific and Industrial Research – National Metallurgical Laboratory, Chennai to provide the characterization facilities for the bare and green iron particles. The authors also would like to thank the Research Centre, Sathyabama University, Chennai and the Department of Biotechnology, Periyar Maniammai University, Tanjavore, to provide the characterization facilities for reactive dye and textile wastewater samples.

We declare that there is no conflict of interest concerning this publication.

All relevant data are included in the paper or its Supplementary Information.