Vietnam Journal of Agricultural Sciences - Green Synthesis and Utility of Nano Fe for Cr(VI) Treatment - Ngo Thi Thuong

Vietnam Journal of Agricultural Sciences ISSN 2588-1299 VJAS 2018; 1(1): 35-42 https://doi.org/10.31817/vjas.2018.1.1.04 35 Received: September 19, 2017 Accepted: March 9, 2018 Correspondence to lethithuhuong@vnua.edu.vn ORCID Thi Thu Huong Le https://orcid.org/0000-0002-3657- 8475 Green Synthesis and Utility of Nano Fe for Cr(VI) Treatment Ngo Thi Thuong, Le Thi Ngoc, and Le Thi Thu Huong Faculty of Environment, Vietnam National University of Agriculture, Hanoi 131000, Vietnam Abstract Zero valent iron (ZVI) nanoparticles have been considered as effective materials for environmental remediation because of their strong reducing ability, high reaction activity, and excellent absorption properties. In this study, we synthesized iron nanoparticles using an environmentally friendly method in order to treat Cr(VI) ions in an aqueous medium. Polyphenols from green tea leaf extracts were used as both the reducing agent and the stabilizer for ZVI nanoparticles. Modern techniques, including scanning electron microscopy (SEM), dynamic light scattering (DLS), X-ray diffraction (XRD), and infrared spectroscopy (FTIR), confirmed that ZVI nanoparticles were successfully prepared and surrounded by polyphenol molecules. Cr(VI) ion treatment of the nanoparticles was most favorable at pH 2.0, and 0.04 g ZVI nanoparticles for a 50 mg L-1 Cr(VI) solution. Under some treatment conditions, removal efficiency was 100%, suggesting that the synthesized ZVI nanoparticles can be used as materials for Cr(VI) ion removal. Keywords Cr(VI) treatment, green synthesis, green tea leaves extract, zero valent iron nanoparticles Introduction Recently, the developement of zero valent iron (ZVI or Fe0) nanoparticles for treatment of the environment has been highly investigated worldwide. ZVI nanoparticles are used instead of larger iron particles (> 50 µm) because they possess a large specific surface area, have high activity, and possess the ability to transfer to the ground or ground water (CityChlor, 2013). The synthesis of NZVI can be divided into two main groups: mechanical grinding or chemical reduction methods (Stefaniuk et al., 2016). According to the chemical reduction method, many authors have synthesized iron nanoparticles by reactions of Fe(II) or Fe(III) salts with the NaBH4 reductant (Meyer et al., 2004; Wu and Ritchie, 2006; Gunawardana and Swedlund, 2012). The nanoparticles obtained are often prone to accumulation due to electromagnetic interactions. The strong Ngo Thi Thuong et al. (2018) 36 Vietnam Journal of Agricultural Sciences reductivity of the particles makes them susceptible to oxidation and it can be difficult to separate them from the environment. To overcome these traits, researchers combined the iron nanoparticles with other metals by covering their surface, bringing them to polymer networks, or emulsifying them (Lu et al., 2016). In addition to NaBH4, plant anti-oxidant extracts can be used in iron nanoparticle preparation. It has been shown that this is a novel, environmentally friendly research direction resulting in increased reductivity or enhancement of the physical structure of the iron nanocrystals (Hoag et al., 2009; Oakes, 2013; Mystrioti et al., 2015; El-Kassas and Ghobrial, 2017; Devatha et al., 2016). All of these studies have confirmed the effectiveness of ZVI nanoparticles in the treatment of a wide spectrum of environmental contaminants such as Cr (VI), pigments, PCBs, and TCE. In Vietnam, ZVI nanoparticle preparations were also carried out by mechanical grinding or using the reducing agent NaBH4. The mechanical grinding method requires expensive equipment and techniques. In addition, the nanoparticles obtained are not uniform in size (Trung and Le, 2013). Meanwhile, the chemical method performed by reducing iron (II) or iron (III) using sodium borohydride NaBH4 is much less expensive (Dung, 2012; Huan and Quynh, 2013; Toan, 2014). These studies have used ZVI nanoparticles to treat NO3 - (Huan and Quynh, 2013), Cr(VI) (Dung, 2012; Trung and Le, 2013), Pb(II) (Dung, 2012), methylene blue (Toan, 2014), or DDT (Huan, 2011). The ability of the nanoparticles to remove the pollutants depends on the pollutant concentration, the pH of the solution, and the amount of nanoparticles used. In spite of the effective pollutant removal, using sodium borohydride in a preparation of ZVI nanoparticles has several disadvantages. NaBH4 is a relatively expensive chemical that has to be used in surplus quantities, which can generate highly flammable gas (hydrogen). High levels of toxic NaBH4 residue may affect safety during work and require further separation (Soliemanzadeh et al., 2016). Therefore, this study aimed to synthesize ZVI nanoparticles in a green process, in which no hazardous chemicals are used or generated, in order to treat Cr(VI) ions in an aqueous medium. The synthesis used the reductive source of polyphenols, a type of antioxidant, found in green tea leaves. This is a cheap, popular, environmentally safe source of polyphenol that is completely non-toxic and does not need to be removed. In particular, polyphenols are capable of forming complexes with iron so they are capable of stabilizing the ZVI nanoparticles, preventing them from aggregating (Truskewycz et al., 2016). At the same time, with their antioxidant properties, polyphenols are capable of protecting the iron nanoparticles from oxidation in the environment, thus helping the particles maintain activity (Lin et al., 2017). Materials and Methods Materials Analytical grade ferrous sulfate heptahydrate (Mohr salt FeSO4.7H2O), potassium dichromate (K2Cr2O7), and 1,10- phenanthroline were used without further purification. Green tea leaves were purchased from Trau Quy, Gia Lam, Hanoi, Vietnam. Double distilled water was used throughout all experiments. Preparation of ZVI nanoparticles Green tea leaf extract was obtained by boiling 20 g of green tea leaves with 1 L of water at 80oC for 1 h. Then, 50 mL of 0.1 M FeSO4 solution was quickly added to 150 mL of the tea extract at room temperature. The color of the reaction mixture turned from brown to dark blue and black precipitates appeared. Solid samples for further characteristics were obtained by centrifugal settling and dried in a desiccator. Characterization of ZVI nanoparticles ZVI nanoparticle characterization was performed according to similar reports on ZVI nanoparticles (Kumar et al., 2013; Weng et al., 2016). Phase structure of the materials was determined by X-ray diffraction (D8 ADVANCE Green synthesis and utility of nano Fe for Cr (VI) treatment 37 (a) (b) Figure 1. (a) The green tea extract turned to black when reacting with iron (II) solution; (b) ZVI nanoparticles Table 1. Experimental conditions Series pH Time (h) Cr (VI) concentration (mg L -1 ) ZVI mass (g) Effect of pH 2.0 – 10.0 2 99.04 0.02 Effect of time 2.0 1 - 6 99.04 0.02 Effect of initial Cr(VI) concentration 2.0 6 50 - 300 0.02 Effect of ZVI mass 2.0 2 99.04 0.01 - 0.04 Bruker). Molecular structure of the materials was characterized by Fourier transform infrared spectroscopy (FTIR, SHIMADZU spectrophotometer) using KBr pellets in the wave number region of 400 - 4000 cm-1. Size and shape of the particles were investigated by Field Emission Scanning Electron Microscopy (SEM) on a Hitachi S-4800 system. Size distribution was measured by the dynamic light scattering (DLS) method in a Nano Zetasizer, Malvern UK. Determining the efficiency of Cr (VI) treatments The effects of different factors on Cr(VI) treatment efficiency were investigated by applying a certain amounts of ZVI nanoparticles to 50 mL of different Cr (VI) solutions for a set period of time. The detailed conditions of the experiments are listed in Table 1. The Cr(VI) concentration at equilibrium was analyzed by chemical titration with Mohr salt with 1,10 - phenanthroline. The experiments were conducted three times to determine the mean value. The treatment efficiency was calculated using the formula: Efficiency (%) = (C0 - Ce)/C0*100% in which C0 is the initial Cr(VI) concentration, and Ce is the Cr(VI) concentration at equilibrium or at the end of each experiment. Data processing was completed using Microsoft Excel software (2010). Results and Discussion Characteristics of the obtained sample Size and size distribution The Fe-SEM images (Figure 2) show that the particles are flattened, smooth, and fairly uniform in size. The particle size ranges from 50 to 60 nm. At this size, they have the advantage of increased contact potential, so faster, easier, and more efficient surface processing is achieved. In solution, ZVI nanoparticles had an average size of 72 nm with a narrow size distribution (small polydispersity index of 0.1). The hydrodynamic particle size measured by the DLS method was larger than that in Fe-SEM images because the ZVI nanoparticles were bounded by hydrophilic polyphenol molecules that expand the particle size due to their interaction with an aqueous medium. XRD analysis In the X-ray diffraction diagram of the ZVI nanoparticle sample, it can be seen that the peaks Ngo Thi Thuong et al. (2018) 38 Vietnam Journal of Agricultural Sciences for Fe (0) typically occur with the greatest intensity at 44.8° (corresponding to the red line). In addition, the presence of additional peaks of Fe3O4 and Fe2O3 suggested that the Fe nanoparticles were partly oxidized at room temperature (t = 26°C). However, their appearance was negligible. This result is quite similar to that of Dung (2012). Thus, the ―green‖ synthesis can produce ZVI nanoparticles with an equivalent crystalline structure compared to the NaBH4 utility synthesis. FTIR spectra In the FTIR spectrum of the particles, the stretching vibrations of the O-H groups appear at 3437.50 cm- 1 (Figure 5). Other peaks are also characteristic for bonds in organic compounds of green tea extract: 1634.2 cm-1 for the C=C, 1366.38 cm-1 and 1207.63 cm-1 for the CN and C-O-C bonds, respectively. The formation of the Fe-O bond of ZVI with the polyphenols at the peak of 606.78 cm-1 showed that the Fe nanoparticles formed were bound by the polyphenols from the tea leaves, helping to increase the stablity of the particles. Cr(VI) treatment of ZVI nanoparticles It was reported by Fang et al. (2011) that nano Fe reacts with Cr(VI) in 3 steps: Step 1: The Cr(VI) ion is exposed to the environment consisting of ZVI nanoparticles and reduction occurs at the solid-liquid surface. Cr(VI) was reduced to Cr(III) and Fe was oxidized to Fe2+: 3Fe0 + Cr2O7 2 - + 14H+ → 3Fe2+ + 2Cr3+ + 7H2O Step 2: Fe0 reacted with H+ ions in the solution to form Fe2+, and then the Fe2+ ions transfered their electrons to Cr(VI) to form Cr3+ and Fe3+: Figure 2. Fe-SEM images of ZVI nanoparticles obtained in the experiment Figure 3. Size distribution of ZVI nanoparticles obtained in the experiment Green synthesis and utility of nano Fe for Cr (VI) treatment 39 Figure 4. XRD diagram of the ZVI sample Figure 5. FTIR spectrum of ZVI nanoparticles obtained in the experiment 6Fe2+ + Cr2O7 2 - + 14H+ → 6Fe3+ + 2Cr3+ + 7H2O Step 3: Both Cr3+ and Fe3+ formed an (oxy) hydroxide of Cr or Fe in precipitation form and adhered to the surface of the particles: (1 - x)Fe3+ + xCr3+ + 3H2O → (CrxFe1-x) (OH)3 ↓ + 3H + (1 - x)Fe2+ + xCr3+ + 3H2O → CrxFe1-xOOH ↓ + 3H+ Another study also reported that the reduction of Cr(VI) by Fe depends on the pH of the solution, reaction time, Cr(VI) concentration, and the amount of ZVI nanoparticles (Kunwar et al., 2011). In this study, the effects of similar factors on Cr(VI) removal via ZVI nanoparticles were examined and the results are described below. pH solution Figure 6 shows that at the pH level of 2.0, the Fe nanoparticles achieved the highest treatment efficiency of 63.95% and this rate gradually decreased to 14.94% at pH 10.0. The treatment efficiency increased as the pH decreased and vice versa. In an acidic environment the reaction mechanism by the ZVI nanoparticles is: 4 47 .3 6 6 06 .7 8 1 09 1. 7 11 20 7. 6 3 1 36 6. 3 8 1 42 6. 8 7 1 63 4. 2 0 3 43 7. 5 0 14 16 18 20 22 24 26 28 30 32 34 36 38 40 42 44 % T 1000 2000 3000 4000 Wavenumbers (cm-1) % T Ngo Thi Thuong et al. (2018) 40 Vietnam Journal of Agricultural Sciences 2 4 6 8 10 0 20 40 60 80 100 E ff e c ie n c y o f C r( V I) r e m o v a l (% ) pH 63.95 40.50 26.49 18.45 14.94 1 h 2 h 4 h 6 h 0 20 40 60 80 100 E ff ic ie n c y o f C r( V I) r e m o v a l (% ) Time 50.03 63.05 88.45 94.04 Figure 6. Efficiency of Cr(IV) removal dependence on pH Figure 7. Efficiency of Cr(IV) removal dependence on reaction time Cr2O7 2- + 3Fe0 + 14H+ → 2Cr3+ + 3Fe2+ + 7H2O As more H+ was added, the formed Fe2+ ions continued to react with Cr (VI) according to the reaction: Cr2O7 2 - + 6Fe2+ + 14H+ → 2Cr3+ + 6Fe3+ + 7H2O Therefore, when H+ concentration increases, it facilitates the molecules to react continuously and rapidly, stimulating the surface activity of the ZVI particles by removing the oxy hydroxide surface (Rivero- Huguet and Marshall, 2009), and resulting in a higher Cr(VI) treatment efficiency. In an OH- containing atmosphere, it is difficult for Cr(VI) to be reduced to Cr(III) because of the formation of Fe(OH)3 precipitate which reduces the treatment efficiency. A similar trend was observed by Liu et al. (2005). Reaction time To determine the affect of reaction time, the experiment was run for 1 h to 6 h at pH 2.0 and 0.02 g ZVI nanoparticles (Figure 7). The results revealed that under the same conditions of material weight, pH of the solution, and concentration, the reaction time affected the efficiency of Cr (VI) treatment. The treatment efficiency increased as the reaction time increased. For the reaction time of 6 h, the efficiency reached nearly 100% (94.04%), so it is not essential to prolong the time reaction beyond 6 h. Initial Cr(VI) concentration The effects of initial Cr(VI) concentration on the treatment efficiency are shown in Figure 8. The results showed that the lower the Cr(VI) concentration was, the higher the efficiency was. At concentrations lower than 50 mg L-1, approximately 100% of the Cr(VI) was removed. When the concentration was doubled to 99.04 mg L-1, the efficiency decreased to 94.04%. At the concentrations of 155.64 mg L-1 and 300.68 mg L-1, the efficiency significantly decreased. It is possible to explain this phenomenon by the fact that the Fe0 content is only able to remove Cr(VI) within a certain range. If the concentration of Cr(VI) is higher, complete removal requires a higher Fe0 content. Other authors also reported that the removal efficiency of Cr(VI) and the initial pollutant concentration were inversely proportional. According to Liu et al. (2005), the removal efficiency of Cr(VI) in nano Fe0 fluid was negatively correlated with the initial Cr(VI) concentration. At the initial concentration of 25 mg L-1, the researchers found that removal efficiency was 42% and reached 100% when the initial concentration was decreased to 10 mg L-1. Thus, our results and those of others suggest that the concentration of pollutants treated by Fe0 nanoparticles should be tested before treatment. Moreover, our green synthesized ZVI nanoparticles were more effective within a larger range of pollutant concentrations. Green synthesis and utility of nano Fe for Cr (VI) treatment 41 0 20 40 60 80 100 E ff c ie n c y o f C r( V I) r e m o v a l (% ) Cr(VI) concentration (mg/L) 100 94.04 67.25 38.13. 50 100 150 300 Figure 8. Efficiency of Cr (IV) removal dependence on initial Cr(VI) concentration 0.01 0.02 0.03 0.04 0 20 40 60 80 100 E ff c ie n c y o f C r( V I) r e m o v a l (% ) ZVI mass (g) 35.94 63.95 93 100 Figure 9. Efficiency of Cr (IV) removal dependence on ZVI mass The amount of ZVI nanoparticles The amount of ZVI nanoparticles was adjusted from 0.01 g to 0.04 g to examine how this rate impacted Cr(VI) removal. As aforementioned, if the reaction time was 6 h and Cr(VI) concentration was 49.53 mg L-1 or lower, the predicted efficiency would be approximately 100% at the Fe0 content of 0.02 g. Therefore, the reaction time was set to be 2 h and the Cr(VI) concentration was set to be 99.04 mg L-1 to be able to more clearly see the effect of Fe0 content on the efficiency of the treatment. The results showed that an increase in Fe0 content resulted in a higher processing efficiency of Cr(VI) removal (Figure 9). The amount of 0.04 g ZVI nanoparticles completely eliminated Cr(VI) in 50 mL of 99.04 mg L-1 solution in 2 h. The effectiveness of ZVI nanoparticles in treating Cr(VI) is confirmed. Conclusions In conclusion, ZVI nanoparticles were successfully synthesized by using polyphenols in an extract from green tea leaves with a simple procedure. Analyses showed that the synthesized ZVI particles were about 50 - 60 nm in size, had a lattice structure of iron, and were bounded by the polyphenols which helped prevent oxidation. The results revealed that within the range of our experimental variables, the Cr(VI) treatment was most favorable at the conditions of pH 2.0, reaction time 6 h, Cr(VI) concentration of 49.52 mg L-1, and Fe0 content of 0.04 g. Under certain conditions, the efficiency of Cr(VI) treatment by ZVI nanoparticles can reach 100%. 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