Removal of methyl orange from aqueous solution using bentonite-supported nanoscale zero-valent iron

Journal of Colloid and Interface Science 363 (2011) 601–607

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Removal of methyl orange from aqueous solution using bentonite-supported nanoscale zero-valent iron Zheng-xian Chen a, Xiao-ying Jin a, Zuliang Chen a,b,c,⇑, Mallavarapu Megharaj b,c, Ravendra Naidu b,c a

School of Chemistry and Material Sciences, Fujian Normal University, Fuzhou 350007, Fujian Province, China Centre for Environmental Risk Assessment and Remediation, University of South Australia, Mawson Lakes, SA 5095, Australia c Cooperative Research Centre for Contamination Assessment and Remediation of Environments, Mawson Lakes, SA 5095, Australia b

a r t i c l e

i n f o

Article history: Received 3 April 2011 Accepted 15 July 2011 Available online 5 August 2011 Keywords: Nanoscale Zero-valent iron Bentonite Methyl orange Degradation

a b s t r a c t Zero-valent iron (ZVI) nanoparticles tend to agglomerate, resulting in a significant loss in reactivity. To address this issue, synthesized bentonite-supported nanoscale zero-valent iron (B-nZVI) was used to remove azo dye methyl orange (MO) in aqueous solution. Batch experiments show that various parameters, such as pH, initial concentration of MO, dosage, and temperature, were affected by the removal of MO. Scanning electron microscopy (SEM) confirmed that B-nZVI increased their reactivity and a decrease occurred in the aggregation of iron nanoparticles for the presence of bentonite (B). Using B-nZVI, 79.46% of MO was removed, whereas only 40.03% when using nZVI after reacting for 10 min with an initial MO concentration of 100 mg/L (pH = 6.5). Furthermore, after B-nZVI reacted to MO, XRD indicated that iron oxides were formed. FTIR showed that no new bands appeared, and UV–vis demonstrated that the absorption peak of MO was degraded. Kinetics studies showed that the degradation of MO fitted well to the pseudo first-order model. A degradation mechanism is proposed, including the following: oxidation of iron, adsorption of MO to B-nZVI, formation of Fe(II)–dye complex, and cleavage of azo bond. Finally, the removal rate of MO from actual wastewater was 99.75% when utilizing B-nZVI. Ó 2011 Elsevier Inc. All rights reserved.

1. Introduction In recent times, the chemical reduction of persistent pollutants using nanoscale zero-valent iron (nZVI) has received much attention due to its smaller particle size, larger specific surface area, higher density of reactive surface sites, and greater intrinsic reactivity of surface sites [1,2]. nZVI has become increasingly important in environmental remediation, especially of contaminants such as halogenated organics, heavy metals, pesticides, nitro-aromatic compounds and nitrates [1,2]. However, nZVI nanoparticles tend to either react with surrounding media or agglomerate, resulting in significant loss of reactivity [3]. Consequently, since efficient dispersion of the nZVI particles is a critical factor to improve their efficiency, various particle-stabilizing strategies have been reported [4–9]. A class of resin-supported nZVI particles (Ferragels) has been used to reduce Cr(VI) in aqueous solutions, and reduction of Cr(VI) was found to be 20–30 times faster than the commercial iron filings or iron powder per unit mass of Fe applied [4]. The feasibility of using carboxymethyl cellulose (CMC)-stabilized nZVI particles for in situ reductive immobilization of Cr(VI) ⇑ Corresponding author at: Centre for Environmental Risk Assessment and Remediation, University of South Australia, Mawson Lakes, SA 5095, Australia. Fax: +61 8 83023057. E-mail address: [email protected] (Z. Chen). 0021-9797/$ - see front matter Ó 2011 Elsevier Inc. All rights reserved. doi:10.1016/j.jcis.2011.07.057

in water has recently been investigated [5]. It has also been documented in batch kinetic tests that 0.08 g/L of nZVI particles was able to rapidly reduce 34 mg/L of Cr(VI). More recently, clays such as kaolin and bentonite-supported nZVI have been employed to remove heavy metals such as Cu(II), Co(II), Pb(II), and Cr(VI) [5–9]. The stabilized nZVI particles displayed much greater reactivity. Nonetheless, a few studies have been published on the removal of azo dyes using nZVI. Reducing Acid Black 24 in aqueous solution with nZVI indicated that more than 97% dye was removed in 30 min using an initial 100 mg/L dye solution [10]. Removal of methyl orange (MO) using nZVI suggested that the main roles played by nZVI particles in the decolorization process were adsorption reduction and catalysis [11]. MO is regarded as allergy materials after contacting by skin, which causes shin eczema and even be poisonous if swallowed. B, which is a low-cost and efficient adsorbent, has great potential in removing dyes from wastewaters [12,13]. The adsorption behavior of MO from aqueous solution onto raw B shows that the Langmuir adsorption capacity was found to be 34.34 mg/g at pH 4.0 [14]. For these reasons, using B as a supported material for nZVI could improve the degradation of MO due to the combination of the adsorption of MO onto B and the reduction of MO by nZVI on the B-nZVI. Hence, removal behaviors of MO and the relevant mechanism may help us to understand the removal of azo dyes using B-nZVI.

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Kaolin and B were used as support material for nZVI to remove Pb2+ and Cr2 O2 [8,9]. B-nZVI used to remove MO from aqueous 7 solutions was investigated to understand how B-nZVI removes MO. The present study focuses on the following four objectives: (1) synthesis and characterizations of B-nZVI; (2) various factors affecting the degradation of MO; (3) kinetics and possible mechanism; and (4) potential application of B-nZVI to remove MO from actual wastewater. 2. Experimental 2.1. Materials and chemicals B used in this experiment was supplied by Longyan Kaolin Co. Ltd., Fujian, China, which had the amount of montmorillonite (wt) <40% and a cation exchange capacity (CEC) of 32.4 meq/ 100 g. The chemical composition was 62.5% SiO2, 18.5% Al2O3, 1.75% Fe2O3, 4.25% MgO, 0.95% CaO, and 2.75% Na2O. After drying at 30 °C overnight, raw B was ground and sieved with a 200 mesh screen prior to use. FeCl36H2O was obtained from Tianjin Chemical Reagent Co. (Tianjin, China). NaBH4 and methyl orange (C14H14 N3NaO3S, C.I. 13025) were purchased from Sinopharm Chemical Reagent Co. (Shanghai, China). All the chemicals used in this study were of analytical grade and were used without further purification. MO solution of desired concentration was prepared by the dissolving required amount of the dye in a suitable volume of deionized water. Deionized water was used throughout this experiment. 2.2. Synthesis of bentonite-supported nZVI nanoparticles Zero-valent iron nanoparticles (nZVI) and bentonite-supported nZVI (B-nZVI) were synthesized using the liquid-phase reduction method where B acted as a support material as previously reported [8,9]:

4Fe3þ þ 3BH4 þ 9H2 O ! 4Fe0 þ 3H2 BO3 þ 12Hþ þ H2 "

ð1Þ

The preparation consisted of B-nZVI with an iron/B mass ratio of 1:1, ferric chloride hexa-hydrate (FeCl36H2O) (4.84 g) dissolved in 50 mL of miscible liquids (distilled water and absolute ethanol at a volume ratio of 4:1), and add treated B (1.00 g). The mixture was stirred with an electric rod for 15 min in a nitrogen atmosphere, and then 0.47 M NaBH4 solution (100 mL) was added at the speed of 1–2 drops per second drop by drop into this mixture and vigorously stirred continuously under nitrogen atmosphere. The mixture’s color turned from red brown to light yellow, and then eventually to black. At the same time, the mixture gradually produced more black grain particles in the three-neck flask. After all of the NaBH4 solution had been added, the mixture was stirred under the nitrogen atmosphere continuously for another 20 min to completely deplete NaBH4 and FeCl36H2O. Vacuum filtration was employed to collect the B-nZVI particles, and these were quickly rinsed three times with absolute ethanol. Doing so prevented the nZVI from oxidizing, and then it was dried at 333 K under vacuum overnight and kept retained as a powder (sieved about 200 mesh) in a nitrogen atmosphere prior to use. 2.3. Characterizations Scanning electron microscopy (SEM) images of B, nZVI, and zBnZVI were acquired using a Philips-FEI XL30 ESEM-TMP (Philips Electronics Co., Eindhoven, Netherlands). Images of samples were recorded at different magnifications at an operating voltage of 20 kV.

X-ray diffraction (XRD) patterns of B-nZVI (2:1) before and after were performed using a Philips-X’Pert Pro MPD (Netherlands) with a high-power Cu Ka radioactive source (k = 0.154 nm) at 40 kV/ 40 mA. All samples were scanned from 5° to 90° 2h at a scanning rate of 3° 2h per minute. The specific surface areas (SSA) of B, nZVI, and B-nZVI (2:1) were tested using the BET-N2 adsorption method (Brunauer–Emmett–Teller isotherm). This was made possible with Micromeritics’ ASAP 2020 Accelerated Surface Area and Porosimetry Analyzer (Micromeritics Instrument Corp., GA). Fourier transforms infrared spectra (FTIR) showing B-nZVI before and after reacting with MO were obtained with a Fourier transform infrared spectroscope (FTIR Nicolet 5700, Thermo Corp., USA). Samples for FTIR measurement were prepared by mixing 1% (w/w) specimen with 100 mg of KBr powder and pressed into a sheer slice. An average of nine scans was collected for each measurement with a resolution of 2 cm1. 2.4. Batch experiments The concentration of MO solution was measured using a UV-Spectrophotometer (722N, Shanghai, China) at k = 464 nm. B-nZVI’s efficiency in removing MO was calculated using the following equation [12]:

Rð%Þ ¼

C0  Ct  100% C0

ð2Þ

where R (%) was the MO removal efficiency, C0 (mg/L) was the initial concentration of MO in the solution, and Ce (mg/L) was the concentration of MO at t min. To compare the efficiency of degradation of MO in aqueous solution, an experiment was carried out using nZVI (0.6667 g), B-nZVI (1.0000 g), and B (0.3333 g) added to 1000 mL solution of an initial concentration of 100 mg/L. The former two had the same mass as nZVI, while the latter two had the same mass of B. Mixed solutions were left at their initial pH level stirred at 60 rpm at 30 °C to the desired time intervals. Then, they were filtered through 0.45-lm membranes to measure the residual concentration of MO. Various parameters affecting the removal of MO using B-nZVI were investigated, such as solution of pH, dosage of B-nZVI, initial concentration, and temperature of the MO solution. Blank experiments (MO) were regarded as parallel control as well. Experiment results could have made negligible any loss adsorbed on the glassware wall and self-degradation. All these experiments were undertaken in quadruplicate. 3. Results and discussion 3.1. Comparison of removal efficiency of MO using various materials Fig. 1 shows the removal efficiency of MO from 1000 mL in aqueous solution with an initial concentration of 100 mg/L using B, nZVI, and B-nZVI. The results indicate that B-nZVI was proved to be the most efficient in that more than 79% of MO was removed after 10 min, while only 40% of MO was removed using nZVI and a mere 2% for B. The efficiency of removing MO in aqueous solution using B-nZVI was higher than nZVI in that nZVI (54.04 m2/g) not only had a higher superficial area than B-nZVI (39.94 m2/g), but B also could disperse and stabilize nZVI as well as increase the reactivity and decrease the aggregation of the nZVI in the presence of B as a support material. These results are consistent with our previous work on kaolin-supported nZVI in the removal of Pb(II) [8] and B-supported nZVI for the removal of Cr(V) from aqueous solution [9]. Natural B used in this experiment is a three-layered clay mineral, and it only slightly adsorbs MO as shown in Fig. 1, where

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Removal efficiency / %

100

80

60

40

B nZVI B-nZVI

20

0 0

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B could only absorb 2.10% of MO in aqueous solution after 30 min. This indicates that B demonstrated good suspensibility and dispersibility for nZVI in aqueous solution when adsorbing MO [13] and it enhanced the reactivity of B-nZVI [7,9]. In conclusion, nZVI played the dominant role in the reduction of MO using B-nZVI, while B (6.03 m2/g) acted as a dispersant of nZVI and partially absorbed MO [7,9,14].

3.2. Characterizations Fig. 2 obtained from SEM displays the morphology and distribution of B, nZVI, and B-nZVI. Raw B (Fig. 2a) is shown as a sheet and glossy layer, where there are plenty of small ravines among different interlaminations. These make it a well-supported material. As shown in Fig. 2b, generally, the morphology of synthesized nZVI existed as chain-like aggregates, and most of them were spherical [1]. The diameter of nZVI particles ranged from 30 to 90 nm. However, B-nZVI was prepared with mass ratios of B to nZVI being 2:1. The aggregation of nZVI seemed to decrease, and nZVI particles were less chain-like and better distributed in the B (Fig. 2c) compared to nZVI alone. A similar finding was reported for removing heavy metals such as Cu(II), Co(II), and Pb(II) by kaolin-supported nZVI [7,8]. Fig. 2d shows that the dimensions of nZVI supported on the B had clearly increased and the surface became scabrous after reacting with MO in aqueous solution. This was due to the fact that the formation of iron oxide layers such as Fe3O4, Fe2O3, Fe (OH)3, and FeOOH became gradually covered with nZVI particles on the surface [7–9]. The XRD patterns of B-nZVI before and after reacting with MO are indicated in Fig. 3. When comparing Fig. 3a and b, the largest diffraction peaks appeared at 2h = 26.67°, which was the characteristic peak of B as well as some little peaks representing the intrinsic structure of B. However, the peak at 2h = 44.75° represented the characteristic peak of zero-valent iron (a-Fe) and confirmed its

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existence in freshly prepared B-nZVI (Fig. 3a) [9,15]. After reacting to the MO, the B-nZVI peak as shown in Fig. 3b indicated that maghemite (c-Fe2O3) (2h = 35.68°) and magnetite (Fe3O4) (2h = 35.45°) in the B-nZVI were produced and formed iron oxide layers, while their identical peaks did not appear before reaction occurred [7,9,15]. It could be concluded that the iron oxides gradually formed when B-nZVI reduced the amount of MO present [7,9,15]. The FTIR spectra for natural B, nZVI, and B-nZVI before and after reaction (Fig. 4) were obtained in the range of 400–4000 cm1. In Fig. 4a, the bands show that 1034 and 910 cm1 represent the bending vibrations of SiAO and AlAO, respectively [16]. The SiAO coordination bands at 1102 cm1 are observed as a result of the SiAO vibrations. The absorption peaks were observed at 1635 cm1, which is due to HAOAH stretching vibration bands of water molecule weakly hydrogen bonded to the SiAO surface in B, while the bands at 3622 cm1 suggest the presence of AlAOAH bands. The bands at 523 and 470 cm1 are due to SiAOAAl (octahedral) and SiAOASi bending vibrations, respectively [17]. The deep band at around 1034 cm1 represents the stretching of SiAO in the SiAOASi groups of the tetrahedral sheet. In addition, the bands at 467 and 532, 796, 911, 1035 cm1 corresponding to the FeAO stretch regarding Fe2O3 and Fe3O4 were observed in Fig. 4c and d. These agreed well with the bands from the nZVI spectra (Fig. 4b), indicating that nZVI in B-nZVI was partially oxidized. The SiAO stretching vibration at around 1034 cm1 shifts 1035 cm1 after reacting with MO. However, no new band was observed before and after B-nZVI had reacted with MO. This can be explained by most of the MO being adsorbed on the surface of BnZVI since MO from aqueous solution can be absorbed onto the B and iron oxides in B-nZVI [14,18]. However, new products from MO partly reduced by B-nZVI are not observed due to their concentration being too low to be detected using FTIR. Fig. 5 illustrates the UV–vis spectra of MO obtained before and after B-nZVI reacted with the MO solution containing an initial concentration of 20 mg/L. The band at 464 nm (Fig. 5a) originated from the azo bond and bands at 264 nm relating to aromatic rings were observed before being treated with B-nZVI [13]. However, the band at 464 nm was significantly reduced, and a new band at 239 nm appeared (Fig. 5b) when the B-nZVI was added to the solution containing MO. This indicates that the degradation of MO by B-nZVI was thought to cleave the azo bond and hence led to degradation of MO. The band appearing at 239 nm is possibly explained by the presence of sulfanilic acid, which was also observed by nZVI degrading the azo dye methyl orange [13]. 3.3. Parameters affecting the removal of MO nZVI nanoparticles were easily oxidized by oxygen in solution to form maghemite (c-Fe2O3) and magnetite (Fe3O4) as confirmed by XRD. Consequently, this made it possible to reduce the removal of MO from the solution [1,2]. To understand how oxygen in solution influences the removal of MO, batch experiments that passed N2 and did not pass N2 to the solution containing MO were

Fig. 2. SEM images of various materials (10,000): (a) B; (b) nZVI before reaction; (c) B-nZVI before reaction; (d) B-nZVI after reaction.

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2.5

Fe

0

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Fe 3 O4

Fe 2O3

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a Relative transmittance %

3622

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Fig. 3. XRD pattern: (a) B-nZVI before reaction; (b) B-nZVI after reaction.

2922

a

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2-Theta-Scale

3704

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1.0

Fe 2 O3

b

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Fig. 4. The FTIR spectra of various materials: (a) B; (b) nZVI; (c) B-nZVI before reaction; (d) B-nZVI after reaction.

conducted. Fig. 6a shows that it slightly reduced the removal of MO at a low concentration of 100 mg/L. For example, 99.85% of MO was removed by passing N2, while 99.79% of MO was removed without passing N2 after 60 min. In addition, the removal of MO declined significantly in the presence of oxygen at a high concentration of 800 mg/L after 10 min. A difference of 8% was observed between the presence of oxygen in the solution and with passing N2. This is due to the fact that the precipitation of iron oxides and hydroxides was rapidly formed at a high concentration of MO in the presence of oxygen in the solution. Consequently, it reduced the reactivity of nZVI [19]. For example, iron corrosion was reduced during the precipitation of iron oxides and hydroxides, leading to block iron corrosion [19]. To investigate whether pH value affected the removal of MO, batch experiments were carried out using 1.0 g/L B-nZVI at 30 °C at various values of pH from 3.0 to 6.0 with an initial concentration of 100 mg/L. As shown in Fig. 6b, removing MO decreased when the initial pH values rose from 3.0 to 6.0, and nearly 100% MO was removed within 20 min. The increase in the rate of removing MO when pH fell is due to the ionization of the surface of nZVI and deprotonating of MO. This leads to the favorable adsorption of MO onto B and iron oxide on the surface of B-nZVI [14,18,20]; MO is an anionic azo dye with a sulfuric group and was therefore negatively charged. In addition, H+ at a low pH promotes the reduction of MO through the formation of Fe(II)–MO complex [20].

Fig. 5. UV–vis spectral making absorbance before and after treatment compared: (a) addition of MO (20 mg/L) before reaction; (b) addition of MO (20 mg/L) after reaction.

However, hydroxide precipitation was gradually formed when the pH solution was at a high level, which blanketed the nZVI’s shell and thus reduced the degradation of MO [21]. This was consistent with the formation of maghemite (c-Fe2O3) and magnetite (Fe3O4) in the B-nZVI after reacting with MO as confirmed by SEM images and XRD patterns in the previous section. The initial concentrations of MO on its removal were tested, and these concentrations ranged from 100 to 800 mg/L with a 1.0 g/L BnZVI at 30 °C. As shown in Fig. 6c, the removal of MO decreased when the initial concentration of MO increased, where more than 93.75% of MO was removed from 100 to 400 mg/L, while only 81.56% of MO was removed from 800 mg/L after 30 min. However, more than 59.29% of MO was removed during the first 10 min even when the initial concentration stood at 800 mg/L. This can be ascribed to the actions of B and nZVI and maghemite (c-Fe2O3) and magnetite (Fe3O4) in the B-nZVI, which can strongly adsorb and reduce MO from aqueous solution [13,14,18]. The reduction of MO using B-nZVI represented not only a heterogeneous reaction, but it also referred to adsorption of MO on the surface of B-nZVI and subsequent surface reduction [20,21]. These results are in accordance with the previous ones recorded for the FTIR, where no new peak appeared before and after reduction, suggesting that the adsorption of MO occurred on the surface of B-nZVI. This is also confirmed by the previous results from UV–vis spectra, where the azo band at 464 nm was reduced due to the degradation of MO by B-nZVI, which was thought to cleave the azo bond. Dosage is an important parameter influencing the degrading procedure of the dye in aqueous solution. This is because it determines the degrading capacity of a catalytic agent for a given initial concentration of the dye in operating conditions [22,23]. The removal profile of MO using B-nZVI versus different dosages in the range of 0.3–1.0 g L1 is shown in Fig. 6d. It can be seen that the removal efficiency of MO increased as the dosage of B-nZVI rose; 99.80% of MO was removed after 40 min. using more than 0.5 g/L dosage of B-nZVI, while the other dosage was only 75.12% when the dosage of B-nZVI decreased to 0.3 g/L. It can be interpreted that the dosage of B-nZVI increased in the adsorptive surface area and the number of active sites corresponding increased since the degradation process had occurred at the Fe0–H2O interface [13]. The equilibrium degrading time of B-nZVI in removing the MO was prolonged when the dosage of B-nZVI increased for a given initial concentration at the same solution temperature. This may be attributed to the decrease in total degradation surface area and active sites available to dye molecules resulting from intermolecular competition and formation of iron oxide on the surface of nZVI particles among B-nZVI [7–9,13]. Furthermore, the unit

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Fig. 6. Conditions affecting the degradation of MO: (a) effect of O2 in aqueous solution; (b) effect of pH; (c) effect of initial concentration; (d) effect of dosage; (e) effect of temperature; (f) effect of inorganic salt Na2SO3.

degraded amount of dye gradually decreased as reaction time increased. A similar result has been reported for degradation of azo dye acid Black 24 [12] and MO [13] when using nZVI. The optimum degraded dosage was found to be 0.5 g L1 in consideration of cost and efficiency, and consequently, it was used for the successive experiments. Temperature is generally regarded as another important parameter significantly affecting the degradation process of organic materials including dyes such as azo dye by nZVI nanoparticles. Various temperatures ranging from 20 to 30 °C were tested for batch experiments (Fig. 6e) to evaluate the influence of temperature on the degradation of MO. The percentages of degradation after 20 min were 79.89% for 20 °C, 90.01% for 30 °C, and 96.75% for 40 °C, respectively. It is evident that the temperature did affect the removal of MO when the temperature climbed from 20 to 30 °C. It also indicated that a reaction temperature equal to or more than 20 °C favors of the degradation efficiency of MO through B-nZVI. This is explained by the degradation efficiency of MO via BnZVI improving when the temperature rises. In addition, the efficiency of removal was more than 99.35% for 60 min at these temperatures. When temperature increased, the degradation rate was improved by the acceleration of the process of degradation [24]. These results indicate that the degraded process of MO by

B-nZVI is endothermic in character. In higher temperature, the more efficient is the degradation of MO by B-nZVI. This is due to the dye molecules that increase tendency to transfer from the solution phase to the B-nZVI particle surface. Similar results were previously reported by Fan [11] on the degradation of MO using nZVI particles. Since dye wastewater usually contains many inorganic salts such as Na2SO3, the influence of Na2SO3 was also investigated (see Fig. 6f). A negative influence on the removal of MO using BnZVI was observed when the initial concentration of Na2SO3 was increased. The efficiency of removal declined from 72.58% to 57.68% as the concentration of Na2SO3 increased from 0.1 to 0.4 mol/L within 3 min. The explanation is that sulfate competed with the molecule of MO in reactive sites on the surface of B-nZVI [13]. In addition, the solution of pH value will increase when the concentration of Na2SO3 also increases to enhance the amount of hydroxyl. This leads to reducing the activity of B-nZVI, and in turn, this leads to the formation of iron oxide on the surface of B-nZVI. 3.4. Kinetics of degradation of MO In order to investigate the mechanism of degradation, the pseudo first-order kinetics model was generally used to test the

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0 -1

ln (c/c0)

-2 -3 -4 2

100 mg/L R =0.989 2 200 mg/L R =0.963 2 400 mg/L R =0.999 2 800 mg/L R =0.998

-5 -6 -7 0

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t / min Fig. 7. Kinetic parameters of degradation.

degradation of azo dye using nZVI nanoparticles [9,20], which can be expressed as the following equation:

v¼

dc ¼ kSA as qm c dt

ð3Þ

where c is the concentration (mg/L) of MO in solution, kSA is the specific reaction rate constant bounded to the SSA of the materials (L/ h m), as is the specific surface area (m2/g), and qm is the mass concentration (g/L). Since kSA, as, and qm are constant for a given reaction, they can be replaced by another parameter kobs. After an evaluation of integrals, Eq. (2) is as follows [9,19]:

ln

c ¼ kobs t c0

ð4Þ

where kobs is the rate constant of a pseudo first-order reaction (min1) and can be calculated from the slope of the line by plotting ln(c/c0) versus t (min). The regression lines can be achieved by plotting ln(c/c0) against t (min) with a correlation coefficient (r2) higher than 0.963 as shown in Fig. 7, which shows that the degradation of MO by B-nZVI is suited to the pseudo first-order kinetics model. The rate constants for a pseudo first-order reaction were 0.1083/min for 100 mg/L, 0.0879/min for 200 mg/L, 0.0677/min for 400 mg/L, and 0.0341/min for 800 mg/L, respectively. These were calculated from the slope of the line by plotting ln(c/c0) versus t (min). The result indicates that the degradation of MO occurs in the interface of B-nZVI, and hence, the rate of degradation was closely linked to the initial concentration of MO and the active surface sites of BnZVI as discussed in the previous section. When the rate constant versus reciprocal of temperature was plotted logarithmically, a distinct linear relationship resulted, which allows computation of Ea from the Arrhenius formula to be expressed as follows [21]:

ln kobs ¼ 

Ea þ ln A0 RT

ð5Þ

where Ea (kJ/mol) is the Arrhenius activation energy or apparent activation energy and A0 is pre-exponential factor with the same dimension as kobs. The degradation of MO proved to be an endothermic reaction, since the removal rates increase as temperature increases. The apparent activity energy (Ea) of reacting to MO has been calculated by applying the Arrhenius formula; the value is obtained at 54.65 kJ/mol. The apparent activity energy (Ea) is more than 42.0 kJ/mol, indicating the surface-limiting reaction [25]. However,

Fig. 8. Proposed mechanism for the degradation of MO using Fe0 nanoparticles.

the high apparent activity in this study suggests that the ratedetermining step of MO degradation was breaking the azo bond at the B-nZVI surface. Hence, it is concluded that surface reactions including sorption and chemical reaction controlled the degradation under experimental conditions. The pathway for removing MO in aqueous solution is summarized in Fig. 8 as follows: the oxidation of nZVI nanoparticles, the adsorption of MO to the surface of B-nZVI by B and iron oxide, the formation of chelate complex of Fe(II)–dye, and breakage of the AN@NA bond. The nZVI in B-nZVI offered two electrons, while the MO molecule accepted one. The radicals of H were generated by the reaction nZVI nanoparticles and H2O or hydrogen ion, which caused the azo bond to open (AN@NA) and consequently the visible absorption peaks at 464 nm vanished. The first step is that the MO molecule was firstly adsorbed onto the surface of B-nZVI, and then it was combined with Fe0 in the second step. It took the form of an intermediate phase that allowed the exchange of electron transfer to be completed and combined with the first two radicals of H. Meanwhile, another Fe0 also united the former mid-body and integrated another two radicals of H again. Finally, the azo-doublebond (AN@NA) was disconnected from the different amines [12,26].

Fe0 þ 2H2 O ! Fe2þ þ H2 þ 2OH þ 2e ðin basic solutionÞ

ð6Þ

Fe0 þ 2Hþ ! Fe2þ þ H2 þ 2e ðin acid solutionÞ

ð7Þ

MO þ B ! MO  B ðadsorptionÞ

ð8Þ

MO þ Fe3 O4 =c-Fe2 O3 ! MO  Fe3 O4 =c-Fe2 O3 ðadsorptionÞ

ð9Þ

3.5. Removal of MO from dyeing wastewater To evaluate whether or not the B-nZVI can degrade MO in real wastewater, a test was done using such wastewater collected from a textile printing and dyeing company based in Fuzhou, China. A 50 mL centrifugal glass containing 25 mL wastewater with pH at 8.31 was added to 0.025 g B-nZVI and then shaken at 303 K for

Z.-x. Chen et al. / Journal of Colloid and Interface Science 363 (2011) 601–607

6 h. The concentration of MO was determined using a spectrophotometer at 200–800 nm. The results showed that more than 99.75% of MO in wastewater was removed, thus demonstrating that the B-nZVI has the potential to remediate MO in wastewater.

4. Conclusion It has been shown that B has the potential to act as a dispersant and stabilizer during the synthesis of B-nZVI, which decreases in aggregation and enhanced activity of nZVI. Batch experiments show that various parameters such as pH, initial concentration of MO, dosage, and temperature did affect the removal of MO. Kinetics studies demonstrate that the degradation of MO by B-nZVI followed a pseudo first-order model, suggesting that this degradation occurs at the interface of B-nZVI and hence the rate of degradation is closely linked to the initial concentration of MO and the active surface sites of B-nZVI. Moreover, characterizations show the adsorption and reduction of MO onto the B-nZVI occurred. Hence, it is proposed that the degradation mechanism of MO using B-nZVI should include adsorption and redox mechanisms, namely the oxidation of iron, adsorption of MO to B-nZVI, formation of Fe(II)–dye complex, and cleavage of azo bond. Finally, it has been demonstrated that B-nZVI has the potential as a remediation material for removing azo dyes from wastewater.

Acknowledgment The financial support of the Fujian ‘‘Minjiang Fellowship’’ from Fujian Normal University is gratefully acknowledged.

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