In this study, MCM-41 stabilized nano zero-valent iron (M-NZVI) is synthesized using the rheological phase reaction method. Characterization with transmission electron microscopy validates the hypothesis that the introduction of MCM-41 leads to a decrease in aggregation of iron nanoparticles. X-ray diffraction confirms the existence of Fe0 and the strong antioxidant activity of Fe0 nanoparticles. Batch Cr(VI) reduction experiments exhibit that solution pH, M-NZVI dosage, and reaction time have significant effects on Cr(VI) removal. A high removal efficiency of Cr(VI) (84.5%) is obtained within 60 min for 100 mg/L of Cr(VI) solution at an initial pH of 6.0 and M-NZVI dosage of 0.5 g/L at 35 °C. The Cr(VI) removal rates follow modified pseudo-first-order kinetic equations. The observed removal rate constant was 0.0168/min for the M-NZVI dosage of 1.0 g/L. Our study suggests that the introduction of an innocuous stabilizer such as MCM-41 can significantly improve the performance of Fe0 nanoparticles for environmental remediation applications.
Chromium compounds are widely applied in a variety of industrial processes such as textile dying, steel, ferro- and nonferrous alloys, refractories, pigments, electroplating, tanning, and others (Kotaś & Stasicka 2000). Thereupon, accidental leakage, unsuitable storage, and/or improper disposal practices often result in the release of chromium to the environment (Alidokht et al. 2011).
In the natural environment, chromium often exists in two highly stable oxidation states, namely trivalent, Cr(III), and hexavalent, Cr(VI). Cr(III) has a low solubility (<10−5M) and low toxicity, being considered an essential nutrient for many organisms (Kotaś & Stasicka 2000). In contrast, Cr(VI) is up to 1,000-fold more toxic than Cr(III), and it has been added to the Class A Human Carcinogens list by the US Environmental Protection Agency (US EPA) (Richard & Bourg 1991). Therefore, Cr(VI) must be removed from wastewater before discharge to the environment.
Several methods are available for removing Cr(VI) from contaminated wastewater, including membrane separation (Güell et al. 2008), chemical reduction (Jagupilla et al. 2009; Chrysochoou et al. 2010), electrokinetic remediation (Zhang et al. 2010), bioremediation (Masood & Malik 2011), and phytoremediation (Zazo et al. 2008), to mention a few. Among these methods, chemical reduction followed by precipitation has been considered the most common technique for removing Cr(VI) from wastewater (Alidokht et al. 2011).
In recent years, many researchers have demonstrated that nano zero-valent iron (NZVI) can serve as an effective reducing agent for in situ reductive removal of Cr(VI) (Wu et al. 2009; Wang et al. 2010; Alidokht et al. 2011; Fang et al. 2011; Li et al. 2012; Nahuel Montesinos et al. 2014; Wang et al. 2014). To prevent aggregation and facilitate the transport of NZVI in field conditions, NZVI has been modified with several kinds of organic and inorganic coatings, such as starch (Alidokht et al. 2011), Fe3O4 (Wu et al. 2009), bentonite (Li et al. 2012), and carboxymethyl cellulose (Wang et al. 2010; Wang et al. 2014). However, these kinds of iron-based materials prepared with traditional methodologies consume a lot of chemical reagents such as ferrous sulfate and ferrous chloride, which result in high production costs and limit further engineering applications. However, these kinds of NZVI particles were synthesized using traditional methodologies, which involve an inert reduction atmosphere and a series of more or less complicated technologies, resulting in high production costs and limitation in practical engineering application.
The rheological phase reaction method is a process for preparing compounds or materials from a solid–liquid rheological mixture, which has many advantages such as efficient utilization of surface area of solid particles, close and uniform contact between solid particle and fluid, good heat exchange, and good control of reaction temperature (Cong et al. 2005; Kima et al. 2011).
Mesoporous zeolite MCM-41 has received extensive research interest due to high porosity, large specific surface area (up to 1,000 m2/g or higher), and narrow pore size distribution. MCM-41 has been proved very useful as an adsorbent, a catalyst/support, and a nano-reactor for various materials synthesis due to its excellent properties (Yang et al. 2014).
The objectives of this study were: (1) to synthesize MCM-41 stabilized NZVI (M-NZVI) by using the rheological phase reaction method; (2) to study the effects of different parameters (such as nanoparticle dosage, solution pH, and reaction time) on Cr(VI) reduction in aqueous solution by M-NZVI; (3) to investigate the kinetics model fitting with Cr(VI) removal by M-NZVI nanocomposites; and (4) to explore the possible mechanism for the reduction of Cr(VI) by M-NZVI.
MATERIALS AND METHODS
Synthesis of MCM-41
MCM-41 was prepared by a direct hydrothermal synthesis method using cationic surfactant cetyltrimethylammonium bromide (CTAB) as a template and silica sol (25%, ρ = 0.15 g/cm3) as silicon source under basic conditions. Typically, 2.184 g of CTAB was dissolved into a solution containing 54 mL of deionized water and 0.48 g of NaOH, 10.43 mL of silica sol was then added dropwise under continuous stirring. The final molar composition of the gel was SiO2/CTAB/NaOH/H2O = 1:0.12:0.24:70. The mixture was stirred at 50 °C for another 2 h, and then aged at 50 °C overnight. Afterwards, the gel was transferred to a Teflon-lined autoclave and crystallized at 85 °C for 48 h. After cooling to room temperature, the crystalline solid was taken out of the autoclave and filtered, repeatedly washed with deionized water until the filtrate was neutral, and then dried at 80 °C for 2 h. To remove the organic template, the samples were annealed at 550 °C in air for 6 h, and then cooled in a desiccator to constant weight.
Preparation of MCM-41 stabilized NZVI
M-NZVI particles were prepared in water by reducing Fe(II) to Fe(0) using KBH4 in the presence of MCM-41 as a stabilizer. M-NZVI was prepared based on the final mass ratio of MCM-41/NZVI = 1:1, 2:1, or 4:1. Typically, for 4:1 ratio sample, 4.96 g FeSO4·7H2O was dissolved in a mixture of 30 mL ethanol and 20 mL deionized water. Then 4 g MCM-41 was added to this solution and the mixture was stirred with a magnetic stirrer for 2 h and then oscillated by ultrasound for 0.5 h. Afterwards, under continuous stirring, 50 mL of 1.0 M KBH4 solution was added dropwise to the Fe(II)/MCM-41/water/ethanol mixture to obtain a rheological body. The mixture was stirred for another 10 min. The final molar ratio of the mixture was BH4–/Fe2+ = 3:1. All the reactions were conducted under continuous stirring at room temperature under the continuous purge of nitrogen gas. The resultant M-NZVI particles were collected by magnetic separation, washed with deionized water and ethanol three times, and finally dried at 60 °C under vacuum.
Characterization of materials
X-ray diffraction (XRD) patterns were obtained from a Bruker D8 advance diffractometer (Bruker, Germany) with Cu Kα radiation from 5 to 70° at a rate of 0.2°/s. The tube current was 100 mA with a tube voltage of 40 kV. The micromorphology was characterized by transmission electron microscope (TEM) on a high-resolution transmission electron microscope (HR-TEM, JEOL JEM-2010, Japan) with an acceleration voltage of 200 kV.
Batch Cr(VI) removal experiments
The Cr(VI) reduction experiments were carried out in batch conditions at 35 °C and atmospheric pressure in stoppered 250 mL Erlenmeyer flasks, containing 100 mL of Cr(VI) solution on a temperature-controlled shaker with continuous stirring at 250 rpm. Single-factor test was used for investigating the effects of reaction time, initial concentration of Cr(VI), initial solution pH, and dosage of M-NZVI particles on Cr(VI) removal efficiency. The initial solution pH was adjusted with 0.1 M HCl or 0.1 M NaOH solution.
To investigate the role that MCM-41 and Fe0 played in the M-NZVI system, MCM-41 and NZVI were also used independently in batch experiments examining Cr(VI) removal from aqueous solution (pH 6.0) at an initial concentration of 100 mg/L at 35 °C and 250 rpm. As the optimum mass ratio of MCM-41/Fe0 was 4:1 in the M-NZVI system, the dosages of MCM-41 and NZVI were set at 0.4 and 0.1 g/L, respectively.
Kinetics of Cr(VI) removal
The kinetic process of Cr(VI) removal was studied in stoppered 250 mL Erlenmeyer flasks, containing 100 mL of 100 mg/L Cr(VI) solution and 0.1 g M-NZVI. The initial solution pH was 6. The kinetics of Cr(VI) reduction was determined at 7-min interval (0–42 min). In this study, all reduction experiments were performed in triplicate to get reliable data, and results represent the average of three parallel experiments.
RESULTS AND DISCUSSION
XRD patterns of freshly synthesized MCM-41 and M-NZVI particles are presented in Figure 1.
As shown in Figure 1(a), the product has a distinct broad peak at 2θ = 2.1–2.5°, which is typical of the hexagonal structure of MCM-41 material (Kresge et al. 1992). In addition, the sample exhibits several small diffraction peaks at 2θ = 3–6°, which can be indexed on a hexagonal lattice as (100), (110), (200), and (210). The 2θ position of the diffraction angle of the (100) plane is varied, indicating different interplanar spacings among the (100) plane of the MCM-41 sample.
Figure 1(b) shows the powder XRD patterns of a series of M-NZVI with varied Fe0 content. A characteristic diffraction peak at 44.7° 2θ degrees indicates the crystallization of Fe0 nanoparticles. In the low 2θ region of 1–10°, three diffraction peaks indexed as (100), (110), and (200) lattice planes can be observed, which are the characteristic peaks of hexagonal MCM-41 mesoporous material. However, it can be clearly observed that the intensity of these peaks becomes lower with the increase of the iron content. This may be due to the excess of iron destroying the mesoporous structure of MCM-41. Moreover, in the high 2θ region of 10–80°, all of the three types of M-NZVI with varied Fe0 content exhibit the presence of body-centered cubic α-Fe (110) (2θ = 44.83°) and no signal for iron oxides (2θ = 35.46°, 43.12°, 53.50°, 56.98°, and 62.64°) is observed. This demonstrates that the prepared M-NZVI has a strong antioxidant activity. Thus, M-NZVI particles with a (MCM-41):Fe0 mass ratio of 4:1 were used in the subsequent batch Cr(VI) removal experiments.
Figure 2(a) shows TEM images of the synthesized nanoscale Fe0. NZVI has a smooth surface, and a spherical shape for single particle with diameters of 20–50 nm. The spherical nanoparticles aggregate to form larger dendritic structures, due to the geomagnetic forces between nanoscale particles and small particles as well as their surface tension interactions (Wu et al. 2009).
The TEM image of the MCM-41 stabilized NZVI particles as shown in Figure 2(b). It can be observed that NZVI particles are encapsulated into the microspheres by a thin layer of mesoporous structure and are isolated from each other. No aggregation of NZVI particles is observed. In addition, lattice fringes can be observed on the surface of the thin layer, indicating the MCM-41 sample possesses high crystallinity. Round-shaped pores with mean diameter of 3–5 nm distributed regularly on the aggregate surface can be observed in Figure 2(b). These results also demonstrate that the mechanical alloying results in a significant dispersion of NZVI and the covering of the MCM-41 surface with very small NZVI particles.
Effect of reaction time
As shown in Figure 3, at a given experimental condition, Cr(VI) removal by M-NZVI and bare NZVI proceeds rapidly within the first 30 min, and then slows down and reaches equilibrium at 60 min with a removal efficiency of 84.5% and 72.3%, respectively. This indicates that the dominant mechanism for Cr(VI) removal is most likely due to the adsorption to the surface of Fe0 nanoparticles and then reduction rather than the direct reduction transformation in the aqueous phase (Geng et al. 2009; Wang et al. 2010). The removal efficiency of Cr(VI) is only 4.6% for MCM-41 after 60 min, indicating the low adsorption of Cr(VI) by the zeolite.
In aqueous solution, NZVI particles will react with water to generate Fe(II), which will then react with Cr(VI) rapidly, resulting in the disappearance of Cr(VI) from the aqueous phase in the second stage (Wang et al. 2010). It has been demonstrated that the dissolution of ferrous ions and electrons to react with Cr(VI) by NZVI in the aqueous phase is a fairly fast process (Wu et al. 2009). During the early stages of the reaction, Cr(VI) ions can be easily transported to the Fe0 surface because of the strong adsorption and reduction ability of NZVI. As the reaction continues, the surface reactivity sites of NZVI particles are gradually taken over by corrosion products including iron oxides layers (Fe3O4, Fe2O3, Fe(OH)3, and FeOOH) (Mielczarski et al. 2005) and Fe(III)–Cr(III) hydroxides (Alowitz & Scherer 2002). The formation of this surface passivation layer on NZVI particles will accordingly block Cr(VI) reduction. Figure 3 also shows that bare NZVI particles have a lower reactivity than M-NZVI particles, which may be due to aggregation of bare NZVI particles in aqueous solution. M-NZVI particles apparently do not aggregate as much, as demonstrated in Figure 2(b), yielding a greatly enhanced removal efficiency. Compared with other investigations related to Cr(VI) reduction by NZVI (Wu et al. 2009; Wang et al. 2010; Alidokht et al. 2011; Fang et al. 2011; Li et al. 2012; Nahuel Montesinos et al. 2014; Wang et al. 2014), the results of the present study are in good agreement.
Effect of solution pH
As shown in Figure 4, the Cr(VI) removal efficiency increases significantly with decreasing pH. The final Cr(VI) removal by M-NZVI declines from 94.0 to 64.9% when the initial pH increases from 2.0 to 12.0. This indicates that the reduction of Cr(VI) by NZVI is strongly pH-dependent. At low pH, iron is readily soluble and oxide films will not be formed immediately after the start of the experiment, leading to increment in the reduction rate of Cr(VI) (Wu et al. 2009). In addition, the increase in H+ concentration left the surface of MCM-41 less negatively charged, which will reduce the electrostatic repulsion between MCM-41 and Cr(VI) anions. But the increase in pH value will promote the formation of mixed iron oxides and Fe(III)–Cr(III) hydroxides on the iron surfaces, resulting in a decrease in the reaction rate and extent (Wu et al. 2009).
Effect of M-NZVI dosage
Figure 5 shows that increasing the dosage of M-NZVI enhances the Cr(VI) removal efficiency. With M-NZVI dosage increasing from 0.3 to 1.2 g/L, the Cr(VI) removal efficiency increases from 77.2 to 98.3% within 60 min. It is believed that the Cr(VI) reductive reaction occurs on the surfaces of NZVI (Wu et al. 2009). As the NZVI mass concentration increases, the reactive iron sites increase proportionally, leading to the increment of Cr(VI) removal efficiency. At higher dosage of M-NZVI (>1.2 g/L), however, the Cr(VI) removal efficiency decreases slightly as the iron dosage increases, which is ascribed to the splitting effect of flux (concentration gradient) and mass transfer barrier between adsorbate and adsorbent (Nandi et al. 2009).
The removal process of Cr(VI) and the reaction mechanism of M-NZVI are discussed in this study. In the reaction process of Cr(VI) reduction on M-NZVI, M-NZVI particles are evenly dispersed in aqueous solution. After a period of time, the MCM-41 layer of iron surface is completely dissolved in aqueous solution, which exposes Fe0 to the water. The pathway for removing Cr(VI) in aqueous solution is summarized in Figure 7.
In this study, ZVI nanoparticles were prepared by using the rheological phase reaction method, employing MCM-41 as a stabilizer. The synthesized M-NZVI particles have higher reactivity and stronger antioxidant activity than bare NZVI due to reduction of aggregation and increased specific surface area. Batch experiments indicate that the Cr(VI) removal increases as the M-NZVI dosage (≤1.2 g/L) increases, and falls as the initial pH increases. The kinetics of Cr(VI) reduction is developed as a modified pseudo-first-order reaction. These results demonstrated that the M-NZVI prepared in this study can be an efficient, effective, and promising remediation material to remove Cr(VI) from wastewater.
This work has been funded by the National Natural Science Foundation of China (No. 51268018). The authors are grateful to the National Engineering Research Center for Domestic and Building Ceramics (JCU) for assistance in the analytical measurements.