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Decolorization of Reactive Red 2 in aqueous solutions using RPB-prepared nanoscale zero-valent iron
Chemical Engineering & Processing: Process Intensification 119 (2017) 1–6
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Decolorization of Reactive Red 2 in aqueous solutions using RPB-prepared nanoscale zero-valent iron
MARK
⁎
Chia-Chang Lina,b, , Shu-Ching Chena a b
Department of Chemical and Materials Engineering, Chang Gung University, Taoyuan, Taiwan, ROC Department of Psychiatry, Chang Gung Memorial Hospital, Linkou Branch, Taoyuan, Taiwan, ROC
A R T I C L E I N F O
A B S T R A C T
Keywords: Nanoscale zero-valent iron Rotating packed bed Reactive Red 2 Decolorization
This investigation assesses the feasibility of decolorizing Reactive Red 2 (RR2) in aqueous solutions using nanoscale zero-valent iron (nZVI). nZVI was prepared in a rotating packed bed (RPB) at a rotational speed of 1400 rpm and liquid flow rates of 0.6 L/min. The effects of pH, nZVI dosage, and temperature on the decolorization of RR2 were experimentally investigated. The results thus obtained importantly reveal that a lower pH, higher nZVI dosage, and higher temperature yielded more efficient decolorization of RR2. Furthermore, the decolorization of RR2 using the prepared nZVI during the initial process was consistent with the pseudo-first-order kinetic model. Additionally, the prepared nZVI was more reactive in decolorizing RR2 than ZVI. With an nZVI dosage of 0.1 g/L, the prepared nZVI effectively decolorized RR2, yielding a decolorization efficiency of around 88% in 45 min at pH 4 and 50 °C. These promising results obviously demonstrate the potential of the prepared nZVI for effectively decolorizing dyes in aqueous solutions.
1. Introduction Various dyes are used in many industries, such as textile dyeing, color photography, paper, plastic, pulp, cosmetic, food, and pharmaceutical [1]. More than 100,000 dyes are reportedly used, with an approximately estimated global production of 7 × 105–1 × 106 tons per year [1]. In typical manufacturing and dyeing processes, 10–20% of the dyes are discharged in industrial effluent, producing large amounts of dye-contaminated wastewater [1]. The presence of dyes in the wastewater from these industrial facilities represents a particular environmental concern because not only do they give the water an undesirable color [2], but also many of them are toxic, mutagenic, teratogenic, and carcinogenic [3]. Furthermore, oxidation, hydrolysis, or other chemical reactions of the dyes in wastewater can form dangerous by-products [2]. Adsorption and biological oxidation are the most frequently used methods for treating wastewater that is contaminated with dyes [4]. Adsorption transfers most dye wastes from wastewater to the solid phase in the form of sludge, which requires further disposal [4]. Although less expensive than other methods, biological oxidation is ineffective in removing dyes because many industrial dyes are toxic [4]. Therefore, more effective methods for removing dyes from wastewater must be developed. Over the past few years, various advanced oxidation processes (AOPs) have been developed to treat textile wastewater [5]. Almost all
⁎
AOPs are based on the production of extraordinarily reactive species, such as hydroxyl radicals (HO%), that can degrade quickly and nonselectively a large range of dye pollutants [4]. Of these approaches, the decolorization using zero-valent iron (ZVI) is a promising alternative means of treating wastewater that contains dyes [1,6,7]. Under acidic conditions, Fe0 reacts with O2 to form H2O2, according to Eq. (1) [8,9]. Then, H2O2 reacts with Fe2+ (Eq. (2), Fenton’s reaction) to generate hydroxyl radicals (HO%) in an acidic environment [8,9]. If Fe0 is present in the solution, then the Fe3+ can be reduced to Fe2+ through the antidismutation reaction, according to Eq. (3) [8,9]. The continuous formation of Fe2+ and H2O2 by Fe0 in the presence of O2 generates HO% [9]. Fe0 + O2 + 2H+ → H2O2 + Fe2+
(1)
Fe2+ + H2O2 + H+ → Fe3+ + H2O + HO%
(2)
2Fe3+ + Fe0 → 3Fe2+
(3)
Over the last few years, nanoscale zero-valent iron (nZVI) has been used to decolorize dyes because its particles are extremely small [10–13]. For example, Shu et al. [10] utilized nZVI with sizes of 50–80 nm to decolorize Acid Black 24 (AB24). Their results revealed that nZVI at a dosage of 0.3348 g/L decolorized AB24 (100 mg/L) with an efficiency of about 99%.
Corresponding author at: Department of Chemical and Materials Engineering, Chang Gung University, Taoyuan, Taiwan, ROC. E-mail address: [email protected] (C.-C. Lin).
http://dx.doi.org/10.1016/j.cep.2017.05.001 Received 2 December 2016; Received in revised form 2 May 2017; Accepted 4 May 2017 Available online 06 May 2017 0255-2701/ © 2017 Elsevier B.V. All rights reserved.
Chemical Engineering & Processing: Process Intensification 119 (2017) 1–6
C.-C. Lin, S.-C. Chen
Reactive Red 2 (RR2) was chosen in the present investigation, because it is extensively used for dyeing cotton, wood, and silk [14]. Only 60–70% of reactive dyes react with fibers during the dyeing process: the rest is hydrolyzed and discharged into the environment [14,15]. Deng et al. [6] used ZVI (< 100 mesh) for decolorizing RR2. Their results indicated that the efficiency of decolorization of RR2 was about 59% in 100 min at pH 3 with an initial RR2 concentration of 20 mg/L and a ZVI dosage of 2.5 g/L. However, very little work, if any, has been carried out on the performance of nZVI in decolorizing RR2 in aqueous solutions. Therefore, the goal of this investigation is to decolorize RR2 in aqueous solutions using nZVI. The effects of pH, nZVI dosage, and temperature on the efficiency of decolorization of RR2 were examined. 2. Experimental 2.1. Preparation of nZVI Fig. 2. FE-SEM image of nZVI.
nZVI was prepared in the rotating packed bed (RPB) by reductive precipitation involved following chemical reaction (Eq. (4)). The use of the RPB and the experimental method for the preparation of nZVI were described in our previous study [9]. nZVI was prepared using FeCl2·4H2O (Alfa-Aesar, 98%) and NaBH4 (Alfa-Aesar, 98%) at concentrations of 0.1 mol/L and 0.2 mol/L, respectively, a rotational speed of 1400 rpm, liquid flow rates of 0.6 L/min, and a temperature of 21 °C [9]. Fe2+ + 2BH4− + 6H2O → Fe0↓ + 2B(OH)3 + 7H2↑
tration of 10 mg/L. The pH of this aqueous RR2 was maintained by adding concentrated aqueous HCl (Scharlau, 37%) or NaOH (Mallinckrodt, 99%). During the decolorization of RR2, the reactor was immersed in a temperature-controlled water bath to keep a fixed temperature. When the pH and temperature of aqueous RR2 reached constant values, a known dosage of nZVI was added. To ensure the homogeneity of the suspension, an agitator was used in the center of the reactor. The decolorization of RR2 was performed for 45 min. During that period, the total concentration of iron ions, dissolved oxygen (DO), and oxidation-reduction potential (ORP) were monitored using an ICPOES analyzer (Varian, Vista-Pro ICP-OES) and a DO meter (WTW, Oxi3210), and an ORP meter (WTW, pH3210), respectively. Analytic samples of 5 mL were extracted at known intervals and the amount of remaining RR2 was analyzed each time. Each sample was filtered through a 0.45 μm filter (Millipore) to remove nZVI. The concentration of RR2 in each sample was determined by measuring the absorbance of each sample at 535 nm using a UV–vis spectrophotometer (Jasco, V630) and a plotted calibration curve. The reproducibility test was performed in this investigation. The RR2 concentration in each sample was observed to be reproduced with a deviation of less than 5%.
(4)
XRD (Siemens, D5005) was used to identify the generated black particles, as shown in Fig. 1. The main peaks of the prepared nZVI matched the characteristic peaks in the standard pattern (JCPDS 870722) of cubic Fe, revealing that the prepared nZVI consisted of virtually pure crystalline Fe. The average size of the nZVI crystal was evaluated from its XRD pattern using the Debye-Scherrer equation. From the full width at half maximum (FWHM) of the most intense peak (44.8°), the average crystallite size of nZVI was calculated to be 11.5 nm [9], revealing that nZVI had been prepared by reductive precipitation in an RPB. Fig. 2 displays a representative FE-SEM (Hitachi, S5000) image of the morphology and structure of the prepared nZVI. As shown in Fig. 2, the prepared nZVI consisted of agglomerated spheres of uniform size [9]. The size of the prepared nZVI particles was in the range of 30–50 nm [9].
3. Results and discussion The efficiency of decolorization of RR2 using nZVI (E) is defined as,
2.2. Decolorization of RR2
E (%) =
RR2 (Sigma-Aldrich, 40%) was decolorized in an open glass reactor, which was filled with 500 mL of aqueous RR2 with an initial concen-
C0 − C × 100% C0
(5)
where C0 denotes the initial concentration of RR2 and C represents the concentration of RR2 at time t. A higher E represents more efficient decolorization of RR2. To evaluate the performance of nZVI in decolorizing RR2 in aqueous solutions, the efficiency is presented as functions of the main operating variables, which are pH, nZVI dosage, and temperature. 3.1. Effect of pH To examine the effect of pH on the decolorization of RR2 in aqueous solutions using nZVI, decolorization experiments were conducted at pH 4, 7, and 10 with an nZVI dosage of 0.1 g/L and a temperature of 30 °C. Fig. 3 indicates that the E values at pH 4, 7, and 10 after 45 min were 84%, 61%, and 29%, respectively; clearly, the efficiency of decolorization of RR2 after 45 min was higher at lower pH. This trend is attributable to the fact that alkaline conditions hindered the formation of Fe2+ ions on the surface of nZVI, as described by Eq. (1). Fe2+ ions and OH− ions in the alkaline solution precipitated Fe(OH)2 on the surface of nZVI, causing it to occupy the reactive sites, inhibiting the
Fig. 1. XRD pattern of nZVI.
2
Chemical Engineering & Processing: Process Intensification 119 (2017) 1–6
C.-C. Lin, S.-C. Chen
Fig. 3. Effect of pH on decolorization of RR2.
decolorization of RR2. Acidic conditions enhanced the formation of Fe2+ and Fe3+ ions, as shown in Fig. 4(a). The total concentration of iron ions that remained in the aqueous RR2 gradually increased, reaching 23 mg/L after 45 min at pH 4. However, at pH 7 and 10, the remaining concentrations of iron ions were much lower. During the decolorization of RR2 using nZVI at pH 4, the DO declined very rapidly to its lowest value of 4.1 mg/L at 5 min, before increasing slowly to 7.3 mg/L at 45 min, as shown in Fig. 4(b). However, the lowest values of DO at pH 7 and 10 were 5.3 mg/L and 6.0 mg/L, respectively. At pH 4, nZVI consumed more DO, as described by Eq. (1). Therefore, more HO% was generated, as described by Eq. (2), facilitating the decolorization of RR2. Fan et al. [12] investigated the effect of pH (4–10) on the decolorization of methyl orange (100 mg/L) using nZVI (20–80 nm) that had been prepared by reductive precipitation in batch mode. Their results revealed that the efficiency of decolorization of methyl orange increased as the pH value was decreased. They asserted that this trend followed from the fact that, at pH of less than the pH of the isoelectric point of nZVI, the surface of nZVI was positively charged, and the dye molecules were negatively charged, promoting the adsorption of dye onto the surface of nZVI. Under alkaline conditions, when the pH exceeded the pH of the isoelectric point of nZVI, the surface of nZVI became negatively charged, and its surface was easily covered by the corrosion products. According to our previous study [9], the pH of the isoelectric point of nZVI that was prepared in the RPB was found to be approximately 8.2, which was close to the value, 8.1, of nZVI with an average size of 105.7 nm that was obtained by Sun et al. [16], and the value, 8.3, of nZVI with an average size of 60 nm that was estimated by Sun et al. [17]. Furthermore, the zeta potential of nZVI was about 34 mV at pH 4 [9] and the RR2 molecules were negatively charged at that pH value, resulting in a higher efficiency of decolorization of RR2 than at either pH 7 or pH 10, consistent with the results of Fan et al. [12]. Deng et al. [6] studied the effect of pH (3–6) on the decolorization of RR2 using ZVI (< 100 mesh) with and without UV irradiation. Their results demonstrated that the efficiency of decolorization of RR2 increased as the pH was declined with and without UV irradiation. At pH 4 and a ZVI dosage of 2.5 g/L, the efficiencies of decolorization of RR2 (20 mg/L) with and without UV irradiation after 100 min were 23% and 15%, respectively, which were much lower than that at pH 4
Fig. 4. Variation of total concentration of iron ions (a) and DO (b) over time, showing effect of pH.
and an nZVI dosage of 0.1 g/L after 45 min in this investigation. This observation suggests that nZVI that was prepared in the RPB exhibited superior reactivity. Even at pH 7, the efficiency of decolorization of RR2 after 45 min exceeded 60%. This result has a very practical implication because it shows that no acid needs to be added during the decolorization process to maintain that process under acidic conditions. The following experiments were performed at pH 4 to evaluate the efficiencies of decolorization of RR2 using nZVI that was prepared in the RPB.
3.2. Effect of nZVI dosage Fig. 5 presents the effect of nZVI dosage on the efficiency of decolorization of RR2 at pH 4 and 30 °C. According to the figure, after 2 min, the E values obtained with nZVI dosages of 0.1, 0.2, and 0.3 g/L were 40%, 60%, and 77%, respectively. After 45 min, the corresponding E values were 84%, 95%, and 98%, respectively. These results demonstrate that increasing the nZVI dosage increased the decolorization of RR2. This phenomenon follows from the fact that increasing the amount of nZVI improved the formation of HO% radicals, consistent 3
Chemical Engineering & Processing: Process Intensification 119 (2017) 1–6
C.-C. Lin, S.-C. Chen
Fig. 5. Effect of nZVI dosage on decolorization of RR2.
with Eqs. (1)–(3). More nZVI provided more iron surface-active sites with which the RR2 molecules could collide, accelerating the decolorization of RR2. Fan et al. [12] investigated the effect of nZVI dosage (0.2–0.5 g/L) on the decolorization of methyl orange (100 mg/L) using nZVI (20–80 nm) that had been prepared by reductive precipitation in batch mode. Their results revealed that a higher nZVI dosage provided a higher efficiency of decolorization of methyl orange. Shu et al. [10] studied the effect of nZVI dosage (0.0335–0.3348 g/L) on the decolorization of Acid Black 24 (100 mg/L) using nZVI (50–80 nm) that had been prepared by reductive precipitation in batch mode. They also verified that more efficient decolorization of Acid Black 24 was achieved at a larger nZVI dosage. These results were consistent with those presented in this investigation. The effect of the nZVI dosage on the total concentration of iron ions was monitored, as shown in Fig. 6(a). The concentrations of iron ions that remained in the aqueous RR2 after 45 min with ZVI dosages of 0.1, 0.2, and 0.3 g/L were 23 mg/L, 42 mg/L, and 74 mg/L, respectively. These results indicate that the total concentration of iron ions that remained in the aqueous RR2 at any time increased with nZVI dosage, verifying that more nZVI formed more HO% radicals, consistent with Eqs. (1)–(3). Another related result was that nZVI at a dosage of 0.3 g/L caused the DO to decline very promptly to its lowest value of 1.9 mg/L at just 5 min, before increasing slowly to 6.4 mg/L at 45 min, as shown in Fig. 6(b). However, the lowest values of DO at nZVI dosages of 0.1 g/ L and 0.2 g/L were 4.1 mg/L and 2.9 mg/L, respectively. nZVI at a dosage of 0.3 g/L consumed more DO, consistent with Eq. (1), and therefore generated more HO%, consistent with Eq. (2), favoring the decolorization of RR2. At an nZVI doage of 0.3 g/L, the ORP dropped from 306 mV to 132 mV quickly after 2 min of the decolorization [9], revealing the formation of H2O2, according to Eq. (1). The ORP further decreased to its lowest value of 73 mV after 18 min, before increasing slowly to 138 mV after 45 min [9], indicating that the decolorization of RR2 was dominated by oxidation owing to the presence of HO%. However, the lowest values of ORP at nZVI dosages of 0.1 g/L and 0.2 g/L were 159 mV and 118 mg/L, respectively. This observation further confirms that more HO% was obtained by more nZVI.
Fig. 6. Variation of total concentration of iron ions (a) and DO (b) over time, showing effect of nZVI dosage.
pH 4 and an nZVI dosage of 0.1 g/L. Fig. 7 displays the results. After 5 min, the E values at 30, 40, and 50 °C were 55%, 61%, and 70%, respectively; after 45 min, they were 84%, 83%, and 88%, respectively. These results show that increasing the temperature accelerated the decolorization of RR2. As noted by Fan et al. [12], temperature affected the decolorization of methyl orange using nZVI: a higher temperature supported faster decolorization. As the temperature was increased, more iron ions and HO% radicals were formed (Eqs. (1)–(3)) and the reaction of HO% radicals with RR2 was thereby accelerated. As shown in Fig. 8(a), the total concentrations of iron ions that remained in the aqueous RR2 after 45 min at temperatures of 30, 40, and 50 °C were 23 mg/L, 49 mg/L, and 59 mg/L, respectively, revealing that more iron ions accumulated at higher temperature. However, at all temperatures, DO fell by approximately 45% at 5 min, as shown in Fig. 8(b), implying that the same amount of HO% radicals may be formed at all temperatures. This observation demonstrates that the degree of decolorization of RR2 increased with temperature because more iron ions were formed and the HO% radicals reacted faster with
3.3. Effect of temperature To examine the dependence of the efficiency of decolorization of RR2 on the temperature using nZVI, an experiment was conducted at 4
Chemical Engineering & Processing: Process Intensification 119 (2017) 1–6
C.-C. Lin, S.-C. Chen
Fig. 7. Effect of temperature on decolorization of RR2.
RR2. A pseudo-first-order kinetic model describes the initial process of the decolorization of RR2, as follows.
dC = −kC dt
(6)
where C represents the concentration of RR2 at time t and k is the measured decolorization rate constant. Integrating Eq. (6) yields Eq. (7):
⎛C ⎞ ln ⎜ 0 ⎟ = kt ⎝C⎠
(7)
The slope of a plot of ln(C0/C) against time (0–8 min) yields the k value, where C0 denotes the initial RR2 concentration. Table 1 presents the k values for the decolorization of RR2 at various temperatures thus determined. The degree of consistency between experimental data and the results obtained using the model (Eq. (7)) was evaluated by obtaining coefficients of determination (R2). The high values of R2 at all temperatures considered herein revealed that the initial process of the decolorization of RR2 using nZVI was consistent with a pseudo-firstorder kinetic model at all of these temperatures. Furthermore, the k value increased by a factor of 1.5 as the temperature was increased from 30 to 50 °C. To elucidate the relationship between temperature and the rate of decolorization of RR2, the k values are assumed to be expressed by the Arrhenius equation, as follows.
⎛ E ⎞ k = A exp ⎜ − a ⎟ ⎝ RT ⎠
Fig. 8. Variation of total concentration of iron ions (a) and DO (b) over time, showing effect of temperature.
Table 1 Values of k for decolorization of RR2 at various temperatures.
(8)
where A is the pre-exponential factor; Ea denotes the activation energy of the decolorization of RR2; R is the gas constant (=8.314 J/mol K), and T denotes the temperature. Therefore, the activation energy of decolorization of RR2 can be determined using Eq. (8). Taking the natural logarithm of both sides of Eq. (8) yields
ln(k ) = ln(A) −
Ea RT
Temperature (°C)
k (min−1)
R2
30 40 50
0.1375 0.1669 0.2040
0.956 0.931 0.960
decolorization of methyl orange (100 mg/L) using nZVI (20–80 nm) in the temperature range of 20–40 °C. As proposed by Lin et al. [11], a relatively high activation energy was obtained in the decolorization of Acid Black 24 (50 mg/L) using ZVI (531 nm) in the temperature range of 10–45 °C. Their calculated apparent activation energy for the decolorization of Acid Black 24 was 72.3 kJ/mol. These results implied a much stronger temperature-dependence for the decolorization of both methyl orange and Acid Black 24 than was obtained in this investigation. This discrepancy may be related to the species of iron, dye and other experimental conditions that were applied in these two investiga-
(9)
According to Eq. (9), a plot of ln(k) against 1/T should be a straight line with a slope of −Ea/R, as shown in Fig. 9. An activation energy of 16.1 kJ/mol was obtained from the slope of the fitted equation (R2 = 0.999). The activation energy that was obtained here was much lower than the value of 35.9 kJ/mol that was obtained by Fan et al. [12] for the 5
Chemical Engineering & Processing: Process Intensification 119 (2017) 1–6
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Acknowledgements The financial support of the Ministry of Science and Technology of the Republic of China, Taiwan (NSC 101-2628-E-182-003-MY2, MOST 104-2628-E-182-001-MY3) is gratefully appreciated. Ted Knoy is appreciated for his editorial assistance. References [1] Y. He, J.F. Gao, F.Q. Feng, C. Liu, Y.Z. Peng, S.Y. Wang, The comparative study on the rapid decolorization of azo, anthraquinone and triphenylmethane dyes by zerovalent iron, Chem. Eng. J. 179 (2012) 8–18. [2] A.B. Prevot, C. Baiocchi, M.C. Brussino, E. Pramauro, P. Savarino, V. Augugliaro, G. Marci, L. Palmisano, Photocatalytic degradation of Acid Blue 80 in aqueous solutions containing TiO2 suspensions, Environ. Sci. Technol. 35 (2001) 971–976. [3] S. Qadri, A. Ganoe, Y. Haik, Removal and recovery of acridine orange from solutions by use of magnetic nanoparticles, J. Hazard. Mater. 169 (2009) 318–323. [4] A.R. Khataee, V. Vatanpour, A.R. Amani Ghadim, Decolorization of C.I. Acid Blue 9 solution by UV/Nano-TiO2, Fenton, Fenton-like, electro-Fenton and electrocoagulation processes: a comparative study, J. Hazard. Mater. 161 (2009) 1225–1233. [5] A. Asghar, A.A. Abdul Raman, W.M.A. Wan Daud, Advanced oxidation processes for in-situ production of hydrogen peroxide/hydroxyl radical for textile wastewater treatment: a review, J. Clean Prod. 87 (2015) 826–838. [6] N. Deng, F. Luo, F. Wu, M. Xiao, X. Wu, Discoloration of aqueous reactive dye solutions in the UV/Fe0 system, Water Res. 34 (2000) 2408–2411. [7] M.C. Chang, H.Y. Shu, H.H. Yu, Y.C. Sung, Reductive decolourization and total organic carbon reduction of the diazo dye CI Acid Black 24 by zero-valent iron powder, J. Chem. Technol. Biotechnol. 81 (2006) 1259–1266. [8] G. Roy, P. de Donato, T. Görner, O. Barres, Study of tropaeolin degradation by ironproposition of a reaction mechanism, Water Res. 37 (2003) 4954–4964. [9] C.C. Lin, S.C. Chen, Enhanced reactivity of nanoscale zero-valent iron prepared by a rotating packed bed with blade packings, Adv. Powder Technol. 27 (2016) 323–329. [10] H.Y. Shu, M.C. Chang, H.H. Yu, W.H. Chen, Reduction of an azo dye Acid Black 24 solution using synthesized nanoscale zerovalent iron particles, J. Colloid Interface Sci. 314 (2007) 89–97. [11] Y.T. Lin, C.H. Weng, F.Y. Chen, Effective removal of AB24 dye by nano/micro-size zero-valent iron, Sep. Purif. Technol. 64 (2008) 26–30. [12] J. Fan, Y.H. Guo, J.J. Wang, M.H. Fan, Rapid decolorization of azo dye methyl orange in aqueous solution by nanoscale zerovalent iron particles, J. Hazard. Mater. 166 (2009) 904–910. [13] F.S. Freyria, B. Bonelli, R. Sethi, M. Armandi, E. Belluso, E. Garrone, Reactions of Acid Orange 7 with iron nanoparticles in aqueous solutions, J. Phys. Chem. C 115 (2011) 24143–24152. [14] C.C. Lin, Y.S. Lin, J.M. Ho, Adsorption of Reactive Red 2 from aqueous solutions using Fe3O4 nanoparticles prepared by co-precipitation in a rotating packed bed, J. Alloy Compd. 666 (2016) 153–158. [15] R. Maas, S. Chaudhari, Adsorption and biological decolourization of azo dye Reactive Red 2 in semicontinuous anaerobic reactors, Process Biochem. 40 (2005) 699–705. [16] Y.P. Sun, X.Q. Li, W.X. Zhang, H.P. Wang, A method for the preparation of stable dispersion of zero-valent iron nanoparticles, Colloid Surf. A—Physicochem. Eng. Asp. 308 (2007) 60–66. [17] Y.P. Sun, X.Q. Li, J.S. Cao, W.X. Zhang, H.P. Wang, Characterization of zero-valent iron nanoparticles, Adv. Colloid Interface Sci. 120 (2006) 47–56. [18] M.B. Goldhaber, Experimental study of metastable sulfur oxyanion formation during pyrite oxidation at pH 6–9 and 30 °C, Am. J. Sci. 283 (1983) 193–217. [19] P.L. Brezonik, Chemical Kinetics and Process Dynamics in Aquatic Systems, Lewis Publishers, 1994.
Fig. 9. Regression of Arrhenius equation for decolorization of RR2.
tions. The activation energy can be used to identify the rate-limiting step in the nZVI/H2O process, which may involve a chemical reaction at the surface or the diffusion of a reactant. Goldhaber [18] suggested that the activation energy of diffusion-controlled processes is very low – typically 17 kJ/mol or less. Brezonik [19] proposed that diffusioncontrolled processes in aqueous solutions have relatively low activation energies (8–21 kJ/mol). Therefore, the decolorization of RR2 using nZVI that was prepared in the RPB was diffusion-controlled. 4. Conclusions This investigation evaluates the effectiveness of the prepared nZVI in decolorizing RR2 in aqueous solutions. The nZVI was prepared herein by reductive precipitation in an RPB, rotating at 1400 rpm with liquid flow rates of 0.6 L/min. Experimental results indicate that the efficiency of decolorization of RR2 increased as the pH was decreased or the nZVI dosage or temperature were increased. A 500 mL volume of the aqueous RR2 was decolorized using an nZVI dosage of 0.1 g/L with an efficiency of approximately 88% in 45 min at pH 4 and 50 °C. The decolorization of RR2 using the prepared nZVI initially was consistent with a pseudo-first-order kinetic model. An activation energy of 16.1 kJ/mol was obtained for the decolorization of RR2 using the prepared nZVI, revealing that the diffusion of RR2 controlled the decolorization in the temperature range of 30–50 °C. The nZVI that was prepared in an RPB had much higher reactivity than ZVI in the decoloriztion of RR2. These results demonstrate that the prepared nZVI has great potential to decolorize dyes in the treatment of textile wastewater.
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Decolorization of Reactive Red 2 in aqueous solutions using RPB-prepared nanoscale zero-valent iron
MARK
⁎
Chia-Chang Lina,b, , Shu-Ching Chena a b
Department of Chemical and Materials Engineering, Chang Gung University, Taoyuan, Taiwan, ROC Department of Psychiatry, Chang Gung Memorial Hospital, Linkou Branch, Taoyuan, Taiwan, ROC
A R T I C L E I N F O
A B S T R A C T
Keywords: Nanoscale zero-valent iron Rotating packed bed Reactive Red 2 Decolorization
This investigation assesses the feasibility of decolorizing Reactive Red 2 (RR2) in aqueous solutions using nanoscale zero-valent iron (nZVI). nZVI was prepared in a rotating packed bed (RPB) at a rotational speed of 1400 rpm and liquid flow rates of 0.6 L/min. The effects of pH, nZVI dosage, and temperature on the decolorization of RR2 were experimentally investigated. The results thus obtained importantly reveal that a lower pH, higher nZVI dosage, and higher temperature yielded more efficient decolorization of RR2. Furthermore, the decolorization of RR2 using the prepared nZVI during the initial process was consistent with the pseudo-first-order kinetic model. Additionally, the prepared nZVI was more reactive in decolorizing RR2 than ZVI. With an nZVI dosage of 0.1 g/L, the prepared nZVI effectively decolorized RR2, yielding a decolorization efficiency of around 88% in 45 min at pH 4 and 50 °C. These promising results obviously demonstrate the potential of the prepared nZVI for effectively decolorizing dyes in aqueous solutions.
1. Introduction Various dyes are used in many industries, such as textile dyeing, color photography, paper, plastic, pulp, cosmetic, food, and pharmaceutical [1]. More than 100,000 dyes are reportedly used, with an approximately estimated global production of 7 × 105–1 × 106 tons per year [1]. In typical manufacturing and dyeing processes, 10–20% of the dyes are discharged in industrial effluent, producing large amounts of dye-contaminated wastewater [1]. The presence of dyes in the wastewater from these industrial facilities represents a particular environmental concern because not only do they give the water an undesirable color [2], but also many of them are toxic, mutagenic, teratogenic, and carcinogenic [3]. Furthermore, oxidation, hydrolysis, or other chemical reactions of the dyes in wastewater can form dangerous by-products [2]. Adsorption and biological oxidation are the most frequently used methods for treating wastewater that is contaminated with dyes [4]. Adsorption transfers most dye wastes from wastewater to the solid phase in the form of sludge, which requires further disposal [4]. Although less expensive than other methods, biological oxidation is ineffective in removing dyes because many industrial dyes are toxic [4]. Therefore, more effective methods for removing dyes from wastewater must be developed. Over the past few years, various advanced oxidation processes (AOPs) have been developed to treat textile wastewater [5]. Almost all
⁎
AOPs are based on the production of extraordinarily reactive species, such as hydroxyl radicals (HO%), that can degrade quickly and nonselectively a large range of dye pollutants [4]. Of these approaches, the decolorization using zero-valent iron (ZVI) is a promising alternative means of treating wastewater that contains dyes [1,6,7]. Under acidic conditions, Fe0 reacts with O2 to form H2O2, according to Eq. (1) [8,9]. Then, H2O2 reacts with Fe2+ (Eq. (2), Fenton’s reaction) to generate hydroxyl radicals (HO%) in an acidic environment [8,9]. If Fe0 is present in the solution, then the Fe3+ can be reduced to Fe2+ through the antidismutation reaction, according to Eq. (3) [8,9]. The continuous formation of Fe2+ and H2O2 by Fe0 in the presence of O2 generates HO% [9]. Fe0 + O2 + 2H+ → H2O2 + Fe2+
(1)
Fe2+ + H2O2 + H+ → Fe3+ + H2O + HO%
(2)
2Fe3+ + Fe0 → 3Fe2+
(3)
Over the last few years, nanoscale zero-valent iron (nZVI) has been used to decolorize dyes because its particles are extremely small [10–13]. For example, Shu et al. [10] utilized nZVI with sizes of 50–80 nm to decolorize Acid Black 24 (AB24). Their results revealed that nZVI at a dosage of 0.3348 g/L decolorized AB24 (100 mg/L) with an efficiency of about 99%.
Corresponding author at: Department of Chemical and Materials Engineering, Chang Gung University, Taoyuan, Taiwan, ROC. E-mail address: [email protected] (C.-C. Lin).
http://dx.doi.org/10.1016/j.cep.2017.05.001 Received 2 December 2016; Received in revised form 2 May 2017; Accepted 4 May 2017 Available online 06 May 2017 0255-2701/ © 2017 Elsevier B.V. All rights reserved.
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Reactive Red 2 (RR2) was chosen in the present investigation, because it is extensively used for dyeing cotton, wood, and silk [14]. Only 60–70% of reactive dyes react with fibers during the dyeing process: the rest is hydrolyzed and discharged into the environment [14,15]. Deng et al. [6] used ZVI (< 100 mesh) for decolorizing RR2. Their results indicated that the efficiency of decolorization of RR2 was about 59% in 100 min at pH 3 with an initial RR2 concentration of 20 mg/L and a ZVI dosage of 2.5 g/L. However, very little work, if any, has been carried out on the performance of nZVI in decolorizing RR2 in aqueous solutions. Therefore, the goal of this investigation is to decolorize RR2 in aqueous solutions using nZVI. The effects of pH, nZVI dosage, and temperature on the efficiency of decolorization of RR2 were examined. 2. Experimental 2.1. Preparation of nZVI Fig. 2. FE-SEM image of nZVI.
nZVI was prepared in the rotating packed bed (RPB) by reductive precipitation involved following chemical reaction (Eq. (4)). The use of the RPB and the experimental method for the preparation of nZVI were described in our previous study [9]. nZVI was prepared using FeCl2·4H2O (Alfa-Aesar, 98%) and NaBH4 (Alfa-Aesar, 98%) at concentrations of 0.1 mol/L and 0.2 mol/L, respectively, a rotational speed of 1400 rpm, liquid flow rates of 0.6 L/min, and a temperature of 21 °C [9]. Fe2+ + 2BH4− + 6H2O → Fe0↓ + 2B(OH)3 + 7H2↑
tration of 10 mg/L. The pH of this aqueous RR2 was maintained by adding concentrated aqueous HCl (Scharlau, 37%) or NaOH (Mallinckrodt, 99%). During the decolorization of RR2, the reactor was immersed in a temperature-controlled water bath to keep a fixed temperature. When the pH and temperature of aqueous RR2 reached constant values, a known dosage of nZVI was added. To ensure the homogeneity of the suspension, an agitator was used in the center of the reactor. The decolorization of RR2 was performed for 45 min. During that period, the total concentration of iron ions, dissolved oxygen (DO), and oxidation-reduction potential (ORP) were monitored using an ICPOES analyzer (Varian, Vista-Pro ICP-OES) and a DO meter (WTW, Oxi3210), and an ORP meter (WTW, pH3210), respectively. Analytic samples of 5 mL were extracted at known intervals and the amount of remaining RR2 was analyzed each time. Each sample was filtered through a 0.45 μm filter (Millipore) to remove nZVI. The concentration of RR2 in each sample was determined by measuring the absorbance of each sample at 535 nm using a UV–vis spectrophotometer (Jasco, V630) and a plotted calibration curve. The reproducibility test was performed in this investigation. The RR2 concentration in each sample was observed to be reproduced with a deviation of less than 5%.
(4)
XRD (Siemens, D5005) was used to identify the generated black particles, as shown in Fig. 1. The main peaks of the prepared nZVI matched the characteristic peaks in the standard pattern (JCPDS 870722) of cubic Fe, revealing that the prepared nZVI consisted of virtually pure crystalline Fe. The average size of the nZVI crystal was evaluated from its XRD pattern using the Debye-Scherrer equation. From the full width at half maximum (FWHM) of the most intense peak (44.8°), the average crystallite size of nZVI was calculated to be 11.5 nm [9], revealing that nZVI had been prepared by reductive precipitation in an RPB. Fig. 2 displays a representative FE-SEM (Hitachi, S5000) image of the morphology and structure of the prepared nZVI. As shown in Fig. 2, the prepared nZVI consisted of agglomerated spheres of uniform size [9]. The size of the prepared nZVI particles was in the range of 30–50 nm [9].
3. Results and discussion The efficiency of decolorization of RR2 using nZVI (E) is defined as,
2.2. Decolorization of RR2
E (%) =
RR2 (Sigma-Aldrich, 40%) was decolorized in an open glass reactor, which was filled with 500 mL of aqueous RR2 with an initial concen-
C0 − C × 100% C0
(5)
where C0 denotes the initial concentration of RR2 and C represents the concentration of RR2 at time t. A higher E represents more efficient decolorization of RR2. To evaluate the performance of nZVI in decolorizing RR2 in aqueous solutions, the efficiency is presented as functions of the main operating variables, which are pH, nZVI dosage, and temperature. 3.1. Effect of pH To examine the effect of pH on the decolorization of RR2 in aqueous solutions using nZVI, decolorization experiments were conducted at pH 4, 7, and 10 with an nZVI dosage of 0.1 g/L and a temperature of 30 °C. Fig. 3 indicates that the E values at pH 4, 7, and 10 after 45 min were 84%, 61%, and 29%, respectively; clearly, the efficiency of decolorization of RR2 after 45 min was higher at lower pH. This trend is attributable to the fact that alkaline conditions hindered the formation of Fe2+ ions on the surface of nZVI, as described by Eq. (1). Fe2+ ions and OH− ions in the alkaline solution precipitated Fe(OH)2 on the surface of nZVI, causing it to occupy the reactive sites, inhibiting the
Fig. 1. XRD pattern of nZVI.
2
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Fig. 3. Effect of pH on decolorization of RR2.
decolorization of RR2. Acidic conditions enhanced the formation of Fe2+ and Fe3+ ions, as shown in Fig. 4(a). The total concentration of iron ions that remained in the aqueous RR2 gradually increased, reaching 23 mg/L after 45 min at pH 4. However, at pH 7 and 10, the remaining concentrations of iron ions were much lower. During the decolorization of RR2 using nZVI at pH 4, the DO declined very rapidly to its lowest value of 4.1 mg/L at 5 min, before increasing slowly to 7.3 mg/L at 45 min, as shown in Fig. 4(b). However, the lowest values of DO at pH 7 and 10 were 5.3 mg/L and 6.0 mg/L, respectively. At pH 4, nZVI consumed more DO, as described by Eq. (1). Therefore, more HO% was generated, as described by Eq. (2), facilitating the decolorization of RR2. Fan et al. [12] investigated the effect of pH (4–10) on the decolorization of methyl orange (100 mg/L) using nZVI (20–80 nm) that had been prepared by reductive precipitation in batch mode. Their results revealed that the efficiency of decolorization of methyl orange increased as the pH value was decreased. They asserted that this trend followed from the fact that, at pH of less than the pH of the isoelectric point of nZVI, the surface of nZVI was positively charged, and the dye molecules were negatively charged, promoting the adsorption of dye onto the surface of nZVI. Under alkaline conditions, when the pH exceeded the pH of the isoelectric point of nZVI, the surface of nZVI became negatively charged, and its surface was easily covered by the corrosion products. According to our previous study [9], the pH of the isoelectric point of nZVI that was prepared in the RPB was found to be approximately 8.2, which was close to the value, 8.1, of nZVI with an average size of 105.7 nm that was obtained by Sun et al. [16], and the value, 8.3, of nZVI with an average size of 60 nm that was estimated by Sun et al. [17]. Furthermore, the zeta potential of nZVI was about 34 mV at pH 4 [9] and the RR2 molecules were negatively charged at that pH value, resulting in a higher efficiency of decolorization of RR2 than at either pH 7 or pH 10, consistent with the results of Fan et al. [12]. Deng et al. [6] studied the effect of pH (3–6) on the decolorization of RR2 using ZVI (< 100 mesh) with and without UV irradiation. Their results demonstrated that the efficiency of decolorization of RR2 increased as the pH was declined with and without UV irradiation. At pH 4 and a ZVI dosage of 2.5 g/L, the efficiencies of decolorization of RR2 (20 mg/L) with and without UV irradiation after 100 min were 23% and 15%, respectively, which were much lower than that at pH 4
Fig. 4. Variation of total concentration of iron ions (a) and DO (b) over time, showing effect of pH.
and an nZVI dosage of 0.1 g/L after 45 min in this investigation. This observation suggests that nZVI that was prepared in the RPB exhibited superior reactivity. Even at pH 7, the efficiency of decolorization of RR2 after 45 min exceeded 60%. This result has a very practical implication because it shows that no acid needs to be added during the decolorization process to maintain that process under acidic conditions. The following experiments were performed at pH 4 to evaluate the efficiencies of decolorization of RR2 using nZVI that was prepared in the RPB.
3.2. Effect of nZVI dosage Fig. 5 presents the effect of nZVI dosage on the efficiency of decolorization of RR2 at pH 4 and 30 °C. According to the figure, after 2 min, the E values obtained with nZVI dosages of 0.1, 0.2, and 0.3 g/L were 40%, 60%, and 77%, respectively. After 45 min, the corresponding E values were 84%, 95%, and 98%, respectively. These results demonstrate that increasing the nZVI dosage increased the decolorization of RR2. This phenomenon follows from the fact that increasing the amount of nZVI improved the formation of HO% radicals, consistent 3
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Fig. 5. Effect of nZVI dosage on decolorization of RR2.
with Eqs. (1)–(3). More nZVI provided more iron surface-active sites with which the RR2 molecules could collide, accelerating the decolorization of RR2. Fan et al. [12] investigated the effect of nZVI dosage (0.2–0.5 g/L) on the decolorization of methyl orange (100 mg/L) using nZVI (20–80 nm) that had been prepared by reductive precipitation in batch mode. Their results revealed that a higher nZVI dosage provided a higher efficiency of decolorization of methyl orange. Shu et al. [10] studied the effect of nZVI dosage (0.0335–0.3348 g/L) on the decolorization of Acid Black 24 (100 mg/L) using nZVI (50–80 nm) that had been prepared by reductive precipitation in batch mode. They also verified that more efficient decolorization of Acid Black 24 was achieved at a larger nZVI dosage. These results were consistent with those presented in this investigation. The effect of the nZVI dosage on the total concentration of iron ions was monitored, as shown in Fig. 6(a). The concentrations of iron ions that remained in the aqueous RR2 after 45 min with ZVI dosages of 0.1, 0.2, and 0.3 g/L were 23 mg/L, 42 mg/L, and 74 mg/L, respectively. These results indicate that the total concentration of iron ions that remained in the aqueous RR2 at any time increased with nZVI dosage, verifying that more nZVI formed more HO% radicals, consistent with Eqs. (1)–(3). Another related result was that nZVI at a dosage of 0.3 g/L caused the DO to decline very promptly to its lowest value of 1.9 mg/L at just 5 min, before increasing slowly to 6.4 mg/L at 45 min, as shown in Fig. 6(b). However, the lowest values of DO at nZVI dosages of 0.1 g/ L and 0.2 g/L were 4.1 mg/L and 2.9 mg/L, respectively. nZVI at a dosage of 0.3 g/L consumed more DO, consistent with Eq. (1), and therefore generated more HO%, consistent with Eq. (2), favoring the decolorization of RR2. At an nZVI doage of 0.3 g/L, the ORP dropped from 306 mV to 132 mV quickly after 2 min of the decolorization [9], revealing the formation of H2O2, according to Eq. (1). The ORP further decreased to its lowest value of 73 mV after 18 min, before increasing slowly to 138 mV after 45 min [9], indicating that the decolorization of RR2 was dominated by oxidation owing to the presence of HO%. However, the lowest values of ORP at nZVI dosages of 0.1 g/L and 0.2 g/L were 159 mV and 118 mg/L, respectively. This observation further confirms that more HO% was obtained by more nZVI.
Fig. 6. Variation of total concentration of iron ions (a) and DO (b) over time, showing effect of nZVI dosage.
pH 4 and an nZVI dosage of 0.1 g/L. Fig. 7 displays the results. After 5 min, the E values at 30, 40, and 50 °C were 55%, 61%, and 70%, respectively; after 45 min, they were 84%, 83%, and 88%, respectively. These results show that increasing the temperature accelerated the decolorization of RR2. As noted by Fan et al. [12], temperature affected the decolorization of methyl orange using nZVI: a higher temperature supported faster decolorization. As the temperature was increased, more iron ions and HO% radicals were formed (Eqs. (1)–(3)) and the reaction of HO% radicals with RR2 was thereby accelerated. As shown in Fig. 8(a), the total concentrations of iron ions that remained in the aqueous RR2 after 45 min at temperatures of 30, 40, and 50 °C were 23 mg/L, 49 mg/L, and 59 mg/L, respectively, revealing that more iron ions accumulated at higher temperature. However, at all temperatures, DO fell by approximately 45% at 5 min, as shown in Fig. 8(b), implying that the same amount of HO% radicals may be formed at all temperatures. This observation demonstrates that the degree of decolorization of RR2 increased with temperature because more iron ions were formed and the HO% radicals reacted faster with
3.3. Effect of temperature To examine the dependence of the efficiency of decolorization of RR2 on the temperature using nZVI, an experiment was conducted at 4
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Fig. 7. Effect of temperature on decolorization of RR2.
RR2. A pseudo-first-order kinetic model describes the initial process of the decolorization of RR2, as follows.
dC = −kC dt
(6)
where C represents the concentration of RR2 at time t and k is the measured decolorization rate constant. Integrating Eq. (6) yields Eq. (7):
⎛C ⎞ ln ⎜ 0 ⎟ = kt ⎝C⎠
(7)
The slope of a plot of ln(C0/C) against time (0–8 min) yields the k value, where C0 denotes the initial RR2 concentration. Table 1 presents the k values for the decolorization of RR2 at various temperatures thus determined. The degree of consistency between experimental data and the results obtained using the model (Eq. (7)) was evaluated by obtaining coefficients of determination (R2). The high values of R2 at all temperatures considered herein revealed that the initial process of the decolorization of RR2 using nZVI was consistent with a pseudo-firstorder kinetic model at all of these temperatures. Furthermore, the k value increased by a factor of 1.5 as the temperature was increased from 30 to 50 °C. To elucidate the relationship between temperature and the rate of decolorization of RR2, the k values are assumed to be expressed by the Arrhenius equation, as follows.
⎛ E ⎞ k = A exp ⎜ − a ⎟ ⎝ RT ⎠
Fig. 8. Variation of total concentration of iron ions (a) and DO (b) over time, showing effect of temperature.
Table 1 Values of k for decolorization of RR2 at various temperatures.
(8)
where A is the pre-exponential factor; Ea denotes the activation energy of the decolorization of RR2; R is the gas constant (=8.314 J/mol K), and T denotes the temperature. Therefore, the activation energy of decolorization of RR2 can be determined using Eq. (8). Taking the natural logarithm of both sides of Eq. (8) yields
ln(k ) = ln(A) −
Ea RT
Temperature (°C)
k (min−1)
R2
30 40 50
0.1375 0.1669 0.2040
0.956 0.931 0.960
decolorization of methyl orange (100 mg/L) using nZVI (20–80 nm) in the temperature range of 20–40 °C. As proposed by Lin et al. [11], a relatively high activation energy was obtained in the decolorization of Acid Black 24 (50 mg/L) using ZVI (531 nm) in the temperature range of 10–45 °C. Their calculated apparent activation energy for the decolorization of Acid Black 24 was 72.3 kJ/mol. These results implied a much stronger temperature-dependence for the decolorization of both methyl orange and Acid Black 24 than was obtained in this investigation. This discrepancy may be related to the species of iron, dye and other experimental conditions that were applied in these two investiga-
(9)
According to Eq. (9), a plot of ln(k) against 1/T should be a straight line with a slope of −Ea/R, as shown in Fig. 9. An activation energy of 16.1 kJ/mol was obtained from the slope of the fitted equation (R2 = 0.999). The activation energy that was obtained here was much lower than the value of 35.9 kJ/mol that was obtained by Fan et al. [12] for the 5
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Acknowledgements The financial support of the Ministry of Science and Technology of the Republic of China, Taiwan (NSC 101-2628-E-182-003-MY2, MOST 104-2628-E-182-001-MY3) is gratefully appreciated. Ted Knoy is appreciated for his editorial assistance. References [1] Y. He, J.F. Gao, F.Q. Feng, C. Liu, Y.Z. Peng, S.Y. Wang, The comparative study on the rapid decolorization of azo, anthraquinone and triphenylmethane dyes by zerovalent iron, Chem. Eng. J. 179 (2012) 8–18. [2] A.B. Prevot, C. Baiocchi, M.C. Brussino, E. Pramauro, P. Savarino, V. Augugliaro, G. Marci, L. Palmisano, Photocatalytic degradation of Acid Blue 80 in aqueous solutions containing TiO2 suspensions, Environ. Sci. Technol. 35 (2001) 971–976. [3] S. Qadri, A. Ganoe, Y. Haik, Removal and recovery of acridine orange from solutions by use of magnetic nanoparticles, J. Hazard. Mater. 169 (2009) 318–323. [4] A.R. Khataee, V. Vatanpour, A.R. Amani Ghadim, Decolorization of C.I. Acid Blue 9 solution by UV/Nano-TiO2, Fenton, Fenton-like, electro-Fenton and electrocoagulation processes: a comparative study, J. Hazard. Mater. 161 (2009) 1225–1233. [5] A. Asghar, A.A. Abdul Raman, W.M.A. Wan Daud, Advanced oxidation processes for in-situ production of hydrogen peroxide/hydroxyl radical for textile wastewater treatment: a review, J. Clean Prod. 87 (2015) 826–838. [6] N. Deng, F. Luo, F. Wu, M. Xiao, X. Wu, Discoloration of aqueous reactive dye solutions in the UV/Fe0 system, Water Res. 34 (2000) 2408–2411. [7] M.C. Chang, H.Y. Shu, H.H. Yu, Y.C. Sung, Reductive decolourization and total organic carbon reduction of the diazo dye CI Acid Black 24 by zero-valent iron powder, J. Chem. Technol. Biotechnol. 81 (2006) 1259–1266. [8] G. Roy, P. de Donato, T. Görner, O. Barres, Study of tropaeolin degradation by ironproposition of a reaction mechanism, Water Res. 37 (2003) 4954–4964. [9] C.C. Lin, S.C. Chen, Enhanced reactivity of nanoscale zero-valent iron prepared by a rotating packed bed with blade packings, Adv. Powder Technol. 27 (2016) 323–329. [10] H.Y. Shu, M.C. Chang, H.H. Yu, W.H. Chen, Reduction of an azo dye Acid Black 24 solution using synthesized nanoscale zerovalent iron particles, J. Colloid Interface Sci. 314 (2007) 89–97. [11] Y.T. Lin, C.H. Weng, F.Y. Chen, Effective removal of AB24 dye by nano/micro-size zero-valent iron, Sep. Purif. Technol. 64 (2008) 26–30. [12] J. Fan, Y.H. Guo, J.J. Wang, M.H. Fan, Rapid decolorization of azo dye methyl orange in aqueous solution by nanoscale zerovalent iron particles, J. Hazard. Mater. 166 (2009) 904–910. [13] F.S. Freyria, B. Bonelli, R. Sethi, M. Armandi, E. Belluso, E. Garrone, Reactions of Acid Orange 7 with iron nanoparticles in aqueous solutions, J. Phys. Chem. C 115 (2011) 24143–24152. [14] C.C. Lin, Y.S. Lin, J.M. Ho, Adsorption of Reactive Red 2 from aqueous solutions using Fe3O4 nanoparticles prepared by co-precipitation in a rotating packed bed, J. Alloy Compd. 666 (2016) 153–158. [15] R. Maas, S. Chaudhari, Adsorption and biological decolourization of azo dye Reactive Red 2 in semicontinuous anaerobic reactors, Process Biochem. 40 (2005) 699–705. [16] Y.P. Sun, X.Q. Li, W.X. Zhang, H.P. Wang, A method for the preparation of stable dispersion of zero-valent iron nanoparticles, Colloid Surf. A—Physicochem. Eng. Asp. 308 (2007) 60–66. [17] Y.P. Sun, X.Q. Li, J.S. Cao, W.X. Zhang, H.P. Wang, Characterization of zero-valent iron nanoparticles, Adv. Colloid Interface Sci. 120 (2006) 47–56. [18] M.B. Goldhaber, Experimental study of metastable sulfur oxyanion formation during pyrite oxidation at pH 6–9 and 30 °C, Am. J. Sci. 283 (1983) 193–217. [19] P.L. Brezonik, Chemical Kinetics and Process Dynamics in Aquatic Systems, Lewis Publishers, 1994.
Fig. 9. Regression of Arrhenius equation for decolorization of RR2.
tions. The activation energy can be used to identify the rate-limiting step in the nZVI/H2O process, which may involve a chemical reaction at the surface or the diffusion of a reactant. Goldhaber [18] suggested that the activation energy of diffusion-controlled processes is very low – typically 17 kJ/mol or less. Brezonik [19] proposed that diffusioncontrolled processes in aqueous solutions have relatively low activation energies (8–21 kJ/mol). Therefore, the decolorization of RR2 using nZVI that was prepared in the RPB was diffusion-controlled. 4. Conclusions This investigation evaluates the effectiveness of the prepared nZVI in decolorizing RR2 in aqueous solutions. The nZVI was prepared herein by reductive precipitation in an RPB, rotating at 1400 rpm with liquid flow rates of 0.6 L/min. Experimental results indicate that the efficiency of decolorization of RR2 increased as the pH was decreased or the nZVI dosage or temperature were increased. A 500 mL volume of the aqueous RR2 was decolorized using an nZVI dosage of 0.1 g/L with an efficiency of approximately 88% in 45 min at pH 4 and 50 °C. The decolorization of RR2 using the prepared nZVI initially was consistent with a pseudo-first-order kinetic model. An activation energy of 16.1 kJ/mol was obtained for the decolorization of RR2 using the prepared nZVI, revealing that the diffusion of RR2 controlled the decolorization in the temperature range of 30–50 °C. The nZVI that was prepared in an RPB had much higher reactivity than ZVI in the decoloriztion of RR2. These results demonstrate that the prepared nZVI has great potential to decolorize dyes in the treatment of textile wastewater.
6