Evaluation of highly active nanoscale zero-valent iron coupled with ultrasound for chromium(VI) removal

Chemical Engineering Journal 281 (2015) 155–163

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Chemical Engineering Journal journal homepage: www.elsevier.com/locate/cej

Evaluation of highly active nanoscale zero-valent iron coupled with ultrasound for chromium(VI) removal Xiaobin Zhou, Bihong Lv, Zuoming Zhou, Wenxin Li, Guohua Jing ⇑ Department of Environmental Science & Engineering, Huaqiao University, Xiamen, Fujian 361021, China

h i g h l i g h t s  Nanoscale zero-valent iron coupled with ultrasound was proposed for Cr(VI) removal.  The Cr(VI) removal rate and efficiency were greatly enhanced by ultrasound.  The mechanism of Cr(VI) removal in the US-nZVI system was discussed.  The reaction followed two-parameter pseudo-first-order model, and Ea was 21.53 kJ mol

a r t i c l e

i n f o

Article history: Received 21 April 2015 Received in revised form 18 June 2015 Accepted 19 June 2015 Available online 26 June 2015 Keywords: Ultrasound Cr(VI) removal Nanoscale zero-valent iron (nZVI) Mechanism

An integrated technology consisting of highly active nanoscale zero-valent iron (nZVI) and ultrasound (US) was proposed for chromium(VI) removal. The parameters affecting Cr(VI) removal, such as the ultrasonic frequency, power, reaction temperature, initial pH and Cr(VI) concentration, were investigated. Based on the investigation and the characterization of the nZVI particles, the removal mechanism of Cr(VI) in US-nZVI system is proposed. Ultrasound not only contributed to an increase on available surface area, but also activated the surface of nZVI particles to induce many new reactive sites for the chemical reaction, which greatly enhanced the rate and efficiency of Cr(VI) reduction. Moreover, ultrasound could also remove the products that covered the surface of the nZVI particles, thus helping them to retain adequate reactive sites for Cr(VI) removal. The apparent kinetics for Cr(VI) removal by this new system could be well expressed by two-parameter pseudo-first-order model, and the observed rate constant (kobs) under ultrasound was much higher than that under shaking. The activation energy (Ea) of the process was calculated to be 21.53 kJ mol1. The results indicated that the value of Ea was effectively reduced by introducing ultrasound into the nZVI/Cr(VI) reaction system, which was a benefit for Cr(VI) removal. Ó 2015 Elsevier B.V. All rights reserved.

Chromium and its compounds are extensively used in industrial processes such as leather tanning, metal electroplating and metal polishing [1]. Contamination of natural water by chromium has caused widespread public concern. Chromium mainly exists in two oxidation states in the environment, Cr(III) and Cr(VI), the latter of which is more toxic [2]. Therefore, the removal of chromium is mostly focused on Cr(VI). Conventional technologies, including chemical precipitation, solvent extraction, ion exchange, membrane technologies, adsorption and reduction, have been employed to remove Cr(VI) from aqueous environments [3]. Among these technologies, chemical reduction has been proven to be efficient and simple for the cleanup of Cr(VI) from contaminated ⇑ Corresponding author. Tel.: +86 592 6166216; fax: +86 592 6162300.

http://dx.doi.org/10.1016/j.cej.2015.06.089 1385-8947/Ó 2015 Elsevier B.V. All rights reserved.

.

a b s t r a c t

1. Introduction

E-mail address: [email protected] (G. Jing).

1

groundwater [4]. In the reduction process, Cr(VI) is converted to Cr(III), which can be easily removed by precipitation or co-precipitation [5]. In other words, reduction-precipitation is a potential method for Cr(VI) removal. As an effective reducing agent, nanoscale zero-valent iron (nZVI) has been increasingly utilized in groundwater remediation and hazardous waste treatment in recent years [6,7]. nZVI possesses a large removal capacity, fast kinetics and high reactivity for the degradation/removal of many chemical pollutants, such as orange II [8], crystal violet [9] and metronidazole [10]. It has also been confirmed that nZVI has a rapid rate of Cr(VI) removal [11] and could have an enhanced reactivity for Cr(VI) removal compared with other similar materials [12]. However, nZVI particles are prone to aggregation and inactivity in aqueous solution due to their properties of a high surface energy, magnetization and high reactivity [13], which result in a decrease in their reactivity limiting the application of nZVI particles in the remediation of contaminants. Some investigations had

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been proposed to overcome these drawbacks. One proposed remedy was to coat the nZVI particles with surfactants, but the particle stability would be markedly reduced due to the rapid desorption of surfactants into the waste water [7]. Another method was to use porous materials to support the nZVI particles, such as multiwalled carbon nanotubes [2], montmorillonite [11] and bentonite [14]. Unfortunately, these supports increased the mass transfer resistance and decreased the reaction rate between nZVI and Cr(VI). It is for these reasons that new methods urgently need to be developed to avoid the aggregation and enhance the reactivity of nZVI particles for Cr(VI) removal. Recently, ultrasound has already been attracting a wide range of interest in nano-technology. It is considered to be a green technology because it only involves sound energy and requires no additional chemicals [15]. Compared with mechanical agitation, ultrasound can break up the aggregates of nanoparticles into a stable and homogeneous suspension in aqueous solution due to its high energy density [16]. Additionally, ultrasound acts like a mixer and can cause a strong mechanical effect in a heterogeneous system, intensifying the reaction mass transfer in solid–liquid interphases [17,18]. Furthermore, ultrasound can diminish the size of the particles, remove the corrosion products, and even destroy the passive film of metallic materials by producing intense shear forces [19,20]. It was demonstrated that after introducing ultrasound to nZVI/contaminant systems, the chemical reaction between nZVI and the contaminants was obviously accelerated [20–26]. For example, Liang et al. [21] reported that ultrasound could significantly improve the mixing and dispersion of nZVI and enormously accelerate the heterogeneous reaction between nZVI and nitrite. Weng et al. [23] showed that a positive synergistic effect was observed for the decolouration of direct blue 15 when Fenton’s reagent was combined with ultrasonic irradiation. Also, Lai et al. [24] suggested that the US-ZVI system was high-efficiency for the removal of p-nitrophenol. But to the best of our knowledge, the reported literatures mainly focused on organic pollutants, there has been no report concerning Cr(VI) removal by the integrated technology of ultrasound and nZVI. It seems to be attractive to introduce ultrasound into the reaction system of nZVI/Cr(VI). Therefore, the purpose of this work was to evaluate the technology of nZVI coupled with ultrasound for Cr(VI) removal. The nZVI particles were synthesized by a liquid-phase reduction method. The effects of different factors on the Cr(VI) removal using this new method were investigated in detail. Moreover, the analyses of products and the characterization of nZVI were performed to propose the Cr(VI) removal mechanism. Finally, the kinetics of Cr(VI) removal in the US-nZVI system was also studied.

Guante Ultrasonic Cleaning Instrument Co., China). Briefly, 100 mL of ethanol aqueous solution (7:3, V/V) was deoxygenated by bubbling nitrogen (N2) through it for 15 min in a three neck flask (500 mL), followed by dissolving 2.78 g of FeSO47H2O in the solution. Then, 100 mL of NaBH4 (0.5 mol L1, M) deoxygenated by N2 was gradually dripped (10 mL min1) into the ferrous solution under ultrasonic irradiation until no significant H2 was produced. The entire reaction process was carried out under a N2 atmosphere. The ferrous iron was reduced to zero-valent iron according to the following reaction:

Fe2þ þ 2BH4 þ 6H2 O ! Fe0 # þ2BðOHÞ3 þ 7H2 "

ð1Þ

The product was washed three times by anhydrous ethanol and then dried in a vacuum oven at 323 K for 24 h. The dried solid materials were preserved hermetically with the protection of N2 for further use. 2.3. Material characterizations The surface morphologies of the nZVI particles before and after the reaction were observed by a scanning electronic microscope (SEM, S-4800, Hitachi Ltd., Japan). The structure of the nZVI particles was investigated with a X-ray diffractometer (XRD, SmartLa, Rigaku Corp., Japan) with Cu Ka radiation (k = 1.54060 Å) at 45 kV and 30 mA with scanning in the range of 20–80°. The scan range covered all of the major species of iron and iron oxides. The surface composition of the nZVI particles was analyzed by X-ray photoelectron spectroscopy (XPS, PHI Quantum 2000 Scanning ESCA Microprobe, USA) using an Al Ka X-ray source operated at 15 kV and 30 W. 2.4. Cr(VI) removal experiments

2. Materials and methods

The effects of different factors on the Cr(VI) removal were investigated in conical flasks (150 mL) containing 0.030 g of nZVI and 100 mL of the aqueous solution in a water bath. The experiments were carried out under various ultrasonic frequencies (40, 50, and 59 kHz), ultrasonic powers (from 320 to 800 W), reaction temperatures (from 298 to 318 K), initial solution pH values (from 3 to 11) and initial Cr(VI) concentrations (from 10 to 60 mg L1) in the presence of ultrasound for 60 min. At certain time intervals, aliquots were taken out and filtered to remove particles by a 0.45 lm membrane filter. Then, the residual concentrations of Cr(VI) were measured. In the process of the experiments, one parameter changed while all of the others remained constant. An experiment was also conducted on a shaking bed in the absence of ultrasound for comparison. The removal efficiency of Cr(VI) was calculated as follows:

2.1. Chemicals

RE% ¼

Ferrous sulfate hepta-hydrate (FeSO47H2O), sodium borohydride (NaBH4) and potassium dichromate (K2Cr2O7) were all purchased from the Sinopharm Group Chemical Reagent Co., Ltd., China. All of the other chemicals were commercially available analytical grade regents, and they were used without further purification. Deionized water was used in all of the experiments. A stock solution of Cr(VI) was prepared by dissolving K2Cr2O7 in deionized water, and that solution was diluted to the desired concentration for actual use.

where RE (%) is the removal efficiency, and C0 and Ct (mg L1) are the concentrations of Cr(VI) in the aqueous solution at the initial time and time t (min), respectively. To clarify the mechanism of Cr(VI) removal in the US-nZVI system, two additional experiments in the presence and absence of ultrasound were carried out under the optimized reaction conditions.

2.2. Preparation of nZVI particles

The concentration of Cr(VI) was determined by the 1,5-diphenylcarbazide method at 540 nm using a UV–vis spectrophotometer (UV-1800 PC, Shanghai, China). The total concentration of Cr in the solution was determined by an atomic absorbance

The nZVI particles were synthesized by a liquid phase reduction method [27] in an ultrasonic instrument (SG7200HBT, Shanghai

ðC 0  C t Þ  100% C0

ð2Þ

2.5. Analytical methods

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spectrometer (AAS, 1100B, Perkin-Elm, USA). The concentrations of Fe(II) and total Fe were determined by the 1,10-phenanthroline method at 510 nm using a UV–vis spectrophotometer.

100

80

3. Results and discussion 3.1. Cr(VI) reduction in the US-nZVI system 3.1.1. Effect of the ultrasonic frequency The effects of various ultrasonic frequencies from 40 to 59 kHz on Cr(VI) removal were investigated. As shown in Fig. 1, there was no obvious influence on the Cr(VI) concentration in the solution when only the ultrasound irradiation was used. The removal efficiency of Cr(VI) increased with an increase in the ultrasonic frequency, and the chemical reaction reached equilibrium within 10 min in the presence of ultrasound. The final removal efficiency of Cr(VI) under 59 kHz ultrasound over a 60 min reaction time was 80.3% and was much higher than that of shaking (32.2%). Compared with common shaking, ultrasound was capable of enhancing the reaction rate due to its mass transfer effect and effective mixing [18]. Moreover, the transient localized hot spots (conditions of high temperature and pressure) derived from acoustic cavitations could enormously reduce the diffusion resistance and accelerate the reaction between nZVI and Cr(VI). Additionally, a higher frequency meant that more acoustic cycles were achieved and more numerous cavitations were generated per unit time [28], thus resulting in a higher Cr(VI) removal efficiency. 3.1.2. Effect of the ultrasonic power Because ultrasonic power affects the acoustic cavitations directly with a constant frequency, the removal efficiencies of Cr(VI) with various ultrasonic powers were investigated. As shown in Fig. 2, the removal efficiency of Cr(VI) increased greatly from 54.2% to 80.3% with an increase in the ultrasonic power from 320 to 800 W. Similar to the results in Fig. 1, the chemical reaction equilibrium was reached within 10 min in the presence of ultrasound under different ultrasonic powers. A higher ultrasonic power introduced a higher acoustic energy into the system, which led to a higher velocity and intensive microjets, and it resulted in a positive effect on the dispersion and mass transfer. In addition, an increased power would increase the corrosion rate of the surface of the material [20], which was advantageous for creating new active reaction sites on the nZVI particles and speeding up the release of

R E (%)

60

40 800 W 640 W 480 W 320 W

20

0 0

10

20

30 Time (min)

40

50

60

Fig. 2. Effect of the ultrasonic power on Cr(VI) removal. (Frequency: 59 kHz; temperature: 298 K; initial pH: 5.5; initial Cr(VI) concentration: 20 mg L1; nZVI dosage: 0.30 g L1.)

reactive Fe(II) ions. Thus, to achieve a higher Cr(VI) removal efficiency, a higher level of ultrasound power is helpful. Similar results have been also found in previous reports [20,23–25]. 3.1.3. Effect of the temperature Fig. 3 shows the effects of various reaction temperatures from 298 to 318 K on Cr(VI) removal. The removal efficiency of Cr(VI) increased from 80.3% to 98.3% when the temperatures increased from 298 to 318 K. A higher temperature would increase the collision rate between nZVI and Cr(VI) and would result in a faster Cr(VI) removal rate. Meanwhile, the reaction between nZVI and Cr(VI) is an endothermic chemical process [29]; thus, a higher temperature was also helpful for Cr(VI) removal. 3.1.4. Effect of the pH The effects of various initial pH values from 3 to 11 on Cr(VI) removal are presented in Fig. 4. It was evident that the removal efficiency of Cr(VI) decreased and the time to reach equilibrium increased with increasing initial pH values. The efficiency was almost 100% after 5 min of reaction time when the initial pH was 3, and it decreased from 91.1% to 37.3% when the initial pH increased from 4 to 11. According to Eqs. (3) and (4), H+ is strongly needed and consumed during the reaction process [2]:

100 59 kHz 50 kHz 40 kHz Shaking 59 kHz Without nZVI

80

100 80

60 R E (%)

R E (%)

60 40

20

40 298 K 308 K 318 K

20

0

0 0

10

20

30

40

50

60

Time (min) Fig. 1. Effect of the ultrasonic frequency on Cr(VI) removal. (Power: 800 W; temperature: 298 K; initial pH: 5.5; initial Cr(VI) concentration: 20 mg L1; nZVI dosage: 0.30 g L1.)

0

10

20

30

40

50

60

Time (min) Fig. 3. Effect of the temperature on Cr(VI) removal. (Frequency: 59 kHz; power: 800 W; initial pH: 5.5; initial Cr(VI) concentration: 20 mg L1; nZVI dosage: 0.30 g L1.)

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100

100

80

80 60 R E (%)

R E (%)

60 pH=3 pH=4 pH=5 pH=7 pH=9 pH=11

40 20

-1 10 mg·L -1 20 mg·L -1 30 mg·L -1 40 mg·L -1 60 mg·L

40 20 0

0 0

10

20

30

40

50

0

60

10

20

30

40

50

60

Time (min)

Time (min) Fig. 4. Effect of the initial pH on Cr(VI) removal. (Frequency: 59 kHz; power: 800 W; temperature: 318 K; initial Cr(VI) concentration: 20 mg L1; nZVI dosage: 0.30 g L1.)

Fig. 5. Effect of the initial Cr(VI) concentration on Cr(VI) removal. (Frequency: 59 kHz; power: 800 W; temperature: 318 K; initial pH: 5.5; nZVI dosage: 0.30 g L1.)

2HCrO4 þ 3Fe0 þ 14Hþ ! 3Fe2þ þ 2Cr3þ þ 8H2 O

ð3Þ

HCrO4 þ 3Fe2þ þ 7Hþ ! 3Fe3þ þ Cr3þ þ 4H2 O

ð4Þ

However, the trends of the changes had significant differences between these two processes. As shown in Fig. 6a, the removal rate of Cr(VI) by nZVI under ultrasound was extremely rapid and efficient. The removal efficiency of Cr(VI) almost reached 100%, and the reaction equilibrium time was short (about 5 min), which is similar to the results

3.2.1. Cr(VI) removal performance with ultrasound and shaking As shown in the results presented above, Cr(VI) removal by nZVI was extremely fast in the presence of ultrasound. To further understand the Cr(VI) removal process, the performances of Cr(VI) removal by nZVI under ultrasound and shaking were compared. As seen in Fig. 6, the processes of these two systems share some similarities in appearance. The concentrations of Cr(VI) decreased while those of Cr(III), Fe(III) and Fe(II) increased in the solution.

35

25

-1

-1

25

Fe concentration (mg·L )

30

Fe(III) Fe(II)

Cr(VI) Cr(III)

20

20

15

15

10

10

5

5

0

0 0

10

20

30

40

50

60

Time (min)

(b)

35

35 Fe(II) Fe(III)

Cr(VI) Cr(III)

30

30 25

20

20

15

15

10

10

5

5

0

0

-1

25

Fe concentration (mg·L )

3.2. Cr(VI) removal mechanism of the US-nZVI system

35 30

Cr concentration (mg·L )

3.1.5. Effect of the initial Cr(VI) concentration The effects of various initial Cr(VI) concentrations in the range of 10.0–60.0 mg L1 on Cr(VI) removal are shown in Fig. 5. Obviously, the removal efficiency of Cr(VI) decreased with an increase in the initial Cr(VI) concentration. The Cr(VI) removal efficiency was almost 100% at low initial Cr(VI) concentrations (10–20 mg L1) and decreased to 50.5% when the initial Cr(VI) concentration reached 60 mg L1. A higher initial concentration of Cr(VI) meant a larger amount of Cr(VI) dissolved in the solution, and it could not be completely removed because of the limited active reaction sites on the nZVI [32]. However, the removal capacities of Cr(VI) were found to be 66.7 and 101.0 mgg1 while the initial Cr(VI) concentration were 20 and 60 mg L1, respectively. The results suggested that the active sites of nZVI would be well used at high concentration of Cr(VI).

(a)

-1

In acidic condition, a large quantity of H ions can enhance the corrosion rate of nZVI, which is also conducive simultaneously to the increase of reactive Fe(II) with the synergistic effect of ultrasound [24,26]. Both H+ and Fe(II) can accelerate the reduction of Cr(VI). On the contrary, the oxide and Fe(III)–Cr(III) hydroxide were easily generated and covered the surfaces of the nZVI particles at a higher pH values, thus acting as a physical barrier and reducing the reactive sites of the particles as well as blocking the access of Cr(VI) to the surfaces of the nZVI particles [30]. It was interesting to note that the Cr(VI) removal efficiencies were up to 90% over a 60 min reaction time at relatively high pH values, which might be attributed to the effect of continuous ultrasound irradiation or the entrapment of Cr(VI) in the structure of growing Fe(III)–Cr(III) hydroxides [31].

Cr concentration (mg·L )

+

0

10

20

30 Time (min)

40

50

60

Fig. 6. Changes in the concentrations of Cr and Fe in the aqueous solution during the reaction: (a) under ultrasound; and (b) under shaking. (Initial pH: 3; initial Cr(VI) concentration: 30 mg L1; temperature: 318 K; nZVI dosage: 0.30 g L1.)

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presented in Section 3.1. It was found that the concentrations of Cr(III) and Fe(III) reached their maximums during the reaction, and then they went down quickly. Since the intense redox reaction occurred between nZVI and Cr(VI) in US-nZVI system, a large amount of Cr(VI) is reduced by Fe0 to form Cr(III) and Fe(III). Cr(III) and Fe(III) were prone to form precipitation or co-precipitation due to their low solubility (the solubility products of Fe(OH)3 and Cr(OH)3 are 1.1  1036 and 5.4  1031, respectively) [31]. The previous reports also found the similar results [14,33]. However, Cr(VI) removal under shaking was inefficient, and the removal efficiency of Cr(VI) was only 65.2% after 60 min reaction (Fig. 6b). The concentration of Fe(III) was quite low (1.83 mg L1) and was hard to form co-precipitation with Cr(III) in the solution. Thus, the decreasing trend of Cr(III) and Fe(III) could not be seen visibly. The largest difference between these two processes was that the final concentration of Fe(II) under ultrasound was 25.48 mg L1, while the concentration of Fe(II) under shaking was only 1.75 mg L1. In both systems, Fe0 was oxidized by Cr(VI) to form Fe(II) in the solution. However, the reaction activity of Cr(VI) and Fe0 under ultrasound irradiation was higher and faster than that under shaking condition. Beside, ultrasound corrosion could activate the surface of iron particles to dissolve the Fe(II) ions [24,25]. From this point, ultrasound has a positive effect on enhancing the reactivity of nZVI particles. 3.2.2. Characterization The nZVI particles before and after the reaction were also characterized. The results of SEM showed that the fresh nZVI particles had a sheet shape (50–70 nm) and exhibited slight aggregation (Fig. 7a). After reaction under ultrasound (Fig. 7b), the nZVI particles became spherical or near-spherical in a narrower size range (40–50 nm). Their size was smaller than that of the fresh particles. However, the reacted nZVI particles under shaking were smoother than that under ultrasound. It seemed that there were flocculent clusters covered on the surface of the particles (Fig. 7c). Similar results were observed in previous studies [14,31,34], in which the flocculent clusters were confirmed to be the oxide and hydroxide precipitation of Cr(III) and Fe(III). As presented in Section 3.2.1, ultrasound corrosion helped to disrupt the oxide layer of iron particle surface and as a result smaller particles emerged. In contrast, the nZVI particles were covered with the oxide layer and Fe(III)– Cr(III) co-precipitation under common shaking, which hindered the reaction process. The XRD results (Fig. 8) present the changes in the characteristic diffraction patterns of the nZVI before and after the reaction. An apparent peak at 2h = 44.9° confirmed the existence of zero-valent iron (a-Fe) in fresh nZVI (Fig. 8a). The typical diffraction peaks displayed in Fig. 8b suggested the formation of FeO (2h = 26.9°), c-Fe2O3 (2h = 30.2°/35.6°), Fe3O4 (2h = 35.7°/43.1°/57. 1°/62.7°), and Cr2FeO4 (2h = 35.5°) after reaction under ultrasound agitation. The oxide products implied that a redox reaction occurred on the surfaces of the nZVI particles, as suggested in previous research [35]. However, all of the typical diffraction peaks of the reacted nZVI particles under shaking (Fig. 8c) became weaker, and some were missing. As mentioned in Section 3.2.1, shaking could not dissolve the layer covering the surfaces of the nZVI particles, which restricted the reaction and resulted in the generation of small quantity of oxide products. The XPS analysis of the composition of the surfaces of the nZVI particles before and after the reaction is presented in Fig. 9. Fig. 9a shows the wide scan XPS spectra for nZVI particles before and after Cr(VI) removal. New peaks at the binding energy (BE) about 580 eV emerged after Cr(VI) removal, suggested the uptake of chromium on the surface of iron particles. Detailed XPS surveys on the region of Fe 2p and Cr 2p are presented in Fig. 9b and c, respectively. For

(a)

(b)

(c)

Fig. 7. SEM images of the particles: (a) fresh nZVI; (b) reacted nZVI under ultrasound; and (c) reacted nZVI under shaking.

0

Fe

a FeO Fe2O3

Cr2FeO4 Fe3O4

Fe3O4

b

c

20

30

40

50 60 Degree 2 Theta

70

80

Fig. 8. XRD patterns of nZVI. (a: fresh nZVI; b: reacted nZVI under ultrasound; c: reacted nZVI under shaking.)

fresh nZVI (Fig. 9b), the peak at around 706.5 eV of Fe0 was weak in this study. Because XPS is a truly surface analysis technique with

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only 2–5 nm probing depth, the inherent oxide layer on the surfaces of iron particles resulted that Fe0 was not distinguishable [36,37]. All samples presented the Fe 2p photoelectron peaks at BE of 724.4 and 710.7 eV, which implied the oxidized iron [Fe(III)] covered on the surface of the samples. As shown in Fig. 9c, the peaks at BE of around 586.5, 577.5 and 576.2 eV corresponded to the Cr 2p photoelectrons, which could be assigned to Cr(OH)3 and/or Cr2O3 [12,29]. The result indicated the inexistence of Cr(VI) on the surface of iron particles, Montesinos et al. [12] also suggested that Cr(VI) was instantaneously reduced when absorption occurred on the iron surface. The results of XRD and XPS proved that Cr(VI) was effectively reduced to Cr(III) by nZVI.

nZVI particles, and the equations could be represented as follows:

ð5Þ

ð1  xÞFe3þ þ xCr3þ þ 2H2 O ! Crx Fe1x ðOOHÞ þ 3Hþ

ð6Þ

Ultrasound instantly simultaneously expelled the co-precipitant from the nZVI particles, and helped clean the surface for continuous reaction. Moreover, Fe0 would also react with Fe(III) and regenerate the reactive product of Fe(II) (Eq. (7)), which seemed to be an alternative pathway for Cr(VI) removal [39]. Therefore, the reaction rate and the removal efficiency of Cr(VI) in the US-nZVI system were strongly enhanced.

3.2.3. Cr(VI) removal mechanism Based on the results described above, the ultrasound-enhanced reaction mechanism could be outlined as shown in Fig. 10. When exposed to ultrasound irradiation, nZVI aggregates were shredded into smaller monomers to form a highly dispersed suspension (Fig. 10a). Every pits created instantly by ultrasound on the surface of nZVI particles can disrupt the oxide layer and created a mass of new reactive sites and nascent Fe(II) (Fig. 10b). The mass transfer rate of Cr(VI) was greatly accelerated by ultrasound, and Cr(VI) migrated rapidly to the surfaces of the nZVI particles. On the surfaces of the nZVI particles, Cr(VI) easily reacted with Fe0 or Fe(II) with the assistance of hot spots, which could release heat and pressure as high as 5000 K and 1800 atm [38]. Then, mixed Fe(III)– Cr(III) oxy(hydroxides) were formed on the surfaces of the

(a)

ð1  xÞFe3þ þ xCr3þ þ 3H2 O ! Crx Fe1x ðOHÞ3 þ 3Hþ

2Fe3þ þ Fe0 ! 3Fe2þ

ð7Þ

3.3. Kinetics of Cr(VI) removal in the US-nZVI system Generally, the pseudo first-order reaction model was used to simulated the Cr(VI) removal in base-nZVI system, and the normalized reaction kinetics could be expressed as follows [14]:

dC t ¼ kSA as wC t dt

ð8Þ

where Ct (mg L1) is the concentration of Cr(VI) in the solution at time t (min), kSA (L min1 m2) is the surface-area normalized rate coefficient, as (m2 g1) is the specific surface area and w (g L1) is the mass concentration of the material (here was nZVI) in the

(b)

Fe 2p 724.4 eV

C 1s

Intensity (CPS)

O 1s

Fe 2p

Intensity (CPS)

710.7 eV

A

Cr 2p

0

Fe

A

B

Cr 2p

B C

C

1200

1000

800

600

400

200

0

740

735

730

Binding Energy (eV)

(c)

Cr 2p

720

715

710

705

700

576.2 eV

586.5 eV

Intensity (CPS)

725

Binding Energy (eV)

577.5 eV

B

C

597

594

591

588

585

582

579

576

573

570

Binding Energy (eV) Fig. 9. XPS spectra for nZVI before and after Cr(VI) removal: (a) full survey; (b) Fe 2p; (c) Cr 2p. (A: fresh nZVI; B: reacted nZVI under ultrasound; C: reacted nZVI under shaking.)

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to be 1.3025 min1 under ultrasound and was over 23 times that under shaking (0.0553 min1). It was known that the Cr(VI) reduction was generally regarded as being mediated by the nZVI surface [10]. According to the results presented above, the available surface area, the active sites of nZVI and the nascent reactive Fe(II) for Cr(VI) removal were greatly increased under ultrasound. Moreover, the diffusion layer of solid–liquid interphases between the nZVI and Cr(VI) was reduced. Thus, a faster reaction rate was achieved. Similar results were observed when ultrasound was used to remove nitrite and organic pollutants by nZVI [21,22]. Compared to the reaction rates reported in other studies (Table 1), although there were some differences in reaction conditions, it was clear that ultrasound was efficient to increase the reaction rate. According to Eq. (10), the observed rate constants at 298, 308, and 318 K were calculated as 0.2076, 0.2678 and 0.3585 min1, respectively. When the observed rate constant was plotted logarithmically versus the reciprocal of the temperature, the activation energy (Ea, kJ mol1) for the reaction could be calculated using the Arrhenius equation as follows:

ln kobs ¼ 

Ea þ ln A0 RT

ð11Þ

where R (8.314 J mol1 K1) is the universal gas constant and A0 is a pre-exponential factor with the same dimension as kobs. The regression equation was obtained when the observed rate constant was plotted logarithmically versus the reciprocal of the temperature.

(a)

0.5 0.0 -0.5

solution. Since kSA, as and w were constant in this specific reaction, they could be replaced by one parameter, kobs. After integration, the kinetics of the removal process can be expressed as a pseudo-first-order reaction model [40] (Eq. (9)):

 ln

Ct C0

 ¼ kobs  t

ð9Þ

ln(Ct /C0)

-1.0 Fig. 10. Schematic of the Cr(VI) removal process under ultrasound: (a) dispersion of the nZVI particles; and (b) the reaction of the US-nZVI/Cr(VI) system.

-1.5 -2.0 -2.5

2

Ultrasound k=1.180 min-1, R =0.8833 2 Shaking k=0.0203 min-1, R =0.9286

-3.0 -3.5 0

10

ln



C t  C ultimate C 0  C ultimate



1

¼ kobs  t

ð10Þ

where Ct (mg L ) is the concentration of Cr(VI) in the solution at time t (min), Cultimate (mg L1) is the residual nonreactive Cr(VI) concentration at equilibrium of reaction, C0 (mg L1) represents the initial concentration of Cr(VI), kobs is the reaction rate constant. In present work, these two kinetics models were both employed to study the kinetics of Cr(VI) removal by nZVI under ultrasound and shaking (the date shown in Fig. 6). As shown in Fig. 11, it is found that the two-parameter pseudo-first-order decay model fitted the experimental date better. The value of kobs was calculated

(b) ln((Ct-Cultimate)/(C0-Cultimate))

In this model, C0 (mg L ) is the initial Cr(VI) concentration, and kobs (min1) is the observed rate constant of the pseudo-first-order reaction. However, some researchers reported that a two-parameter pseudo-first-order decay model, which took the presence of nonreactive pollutants into account, could better describe the reaction process than pseudo-first-order kinetics model [41]. The two-parameter pseudo-first-order decay model could be expressed as:

20

30

40

50

Time (min)

1

0 -1 -2 -3 -4 -1

2

Ultrasound k=1.3025 min , R =0.932 -1 2 Shaking k=0.0553 min , R =0.997

-5 -6 0

10

20

30

40

50

time (min) Fig. 11. Reaction kinetics plots: (a) pseudo-first-order reaction model; (b) twoparameter pseudo-first-order decay reaction model.

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X. Zhou et al. / Chemical Engineering Journal 281 (2015) 155–163

Table 1 Comparison of nZVI based system for Cr(VI) removal. Conditions Cr(VI) (mg L

1

)

20.0 20.0 6.0 20.0 30.0 30.0 a

pH

T (K)

Agents

4.0 3.0 5.0 6.4 3.0 3.0

308 RTa RT 293 313 313

Bentonite-nZVI Resin-nZVI Silica-nZVI Chitosan-nZVI nZVI nZVI

Dosage (g L1)

Mode

Rate constants (k)

Correlation coefficients (R2)

Refs.

3.0 20.826 0.180 12.0 0.3 0.3

Shaking Stirring Stirring Shaking Shaking Ultrasound

0.2275 min1 0.0572 min1 0.332 h1 0.0908 min1 0.0553 min1 1.3025 min1

0.983 0.962 0.97 0.98 0.997 0.932

Shi et al. [14] Fu et al. [34] Petala et al. [40] Liu et al. [42] This work This work

Room temperature.

ln kobs ¼ 

2589:5 þ 9:7187 T

ð12Þ

Covering all of the reaction steps associated with Cr(VI) removal, the Ea value for Cr(VI) removal in this new system was 21.53 kJ mol1 (R2 > 0.99) and was lower than the values of 24.29 and 33 kJ mol1 obtained by Shi et al. [14] and Geng et al. [43]. This result indicated that the activation energy of Cr(VI) reduction could be effectively reduced in the US-nZVI/Cr(VI) system, which suggested an alternate method by using ultrasound to enhance this chemical reaction. 4. Conclusions The nZVI particles synthesized in this work were coupled with an ultrasound technology to remove Cr(VI). It was confirmed that positive effects occurred on Cr(VI) removal when nZVI was coupled with ultrasound. The removal efficiency of Cr(VI) increased with increasing ultrasonic frequency, power and reaction, but it decreased with increasing pH and Cr(VI) concentration. Ultrasound helped to enhance the dispersion of the nZVI and induced a larger available surface area for the chemical reaction to occur. On the other hand, ultrasonic pitting on the surfaces of the nZVI particles disrupted the oxide layer and simultaneously expelled Fe(III)–Cr(III) co-precipitant from the surfaces of nZVI particles, thus creating adequate reactive sites for Cr(VI) removal. Therefore, compared with shaking, the removal rate and efficiency of Cr(VI) were significantly enhanced under ultrasound. The chemical reaction of Cr(VI) followed two-parameter pseudo-first-order model, and the observed rate constant under ultrasound was much higher than that under shaking. The activation energy of the new system was found to be 21.53 kJ mol1, which was lower than that of the most existing works. The results indicated that the reduction reaction of Cr(VI) removal by nZVI could easily occur with the assistance of ultrasound. Acknowledgement This work was sponsored by the Promotion Program for Young and Middle-aged Teacher in Science and Technology Research of Huaqiao University (ZQN-YX104). References [1] M. Bhaumik, A. Maity, V.V. Srinivasu, M.S. Onyango, Enhanced removal of Cr(VI) from aqueous solution using polypyrrole/Fe3O4 magnetic nanocomposite, J. Hazard. Mater. 190 (2011) 381–390. [2] X.S. Lv, J. Xu, G.M. Jiang, X.H. Xu, Removal of chromium(VI) from wastewater by nanoscale zero-valent iron particles supported on multiwalled carbon nanotubes, Chemosphere 85 (2011) 1204–1209. [3] W.J. Jiang, M. Pelaez, D.D. Dionysiou, M.H. Entezari, D. Tsoutsou, K. O’Shea, Chromium(VI) removal by maghemite nanoparticles, Chem. Eng. J. 222 (2013) 527–533. [4] G. Wang, Q. Chang, X.T. Han, M.Y. Zhang, Removal of Cr(VI) from aqueous solution by flocculant with the capacity of reduction and chelation, J. Hazard. Mater. 248–249 (2013) 115–121.

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