persulfate system for the simultaneous removal of Cr(VI) and phenol from aqueous solutions

Chemical Engineering Journal 302 (2016) 213–222

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Bentonite-supported nanoscale zero-valent iron/persulfate system for the simultaneous removal of Cr(VI) and phenol from aqueous solutions Zeng-Hui Diao a, Xiang-Rong Xu a,⇑, Dan Jiang b, Ling-Jun Kong c, Yu-Xin Sun a, Yong-Xia Hu a,d, Qin-Wei Hao a,d, Hui Chen a,d a Key Laboratory of Tropical Marine Bio-resources and Ecology, Guangdong Provincial Key Laboratory of Applied Marine Biology, South China Sea Institute of Oceanology, Chinese Academy of Sciences, Guangzhou 510301, China b Research Resources Center, South China Normal University, Guangzhou 510631, China c School of Environmental Science and Engineering, Guangzhou University, Guangzhou 510006, China d University of Chinese Academy of Sciences, Beijing 100049, China

h i g h l i g h t s

g r a p h i c a l a b s t r a c t


Bentonite decreased the aggregation

of nZVI particles and increased their reactivity.
Persulfate had a negligible effect on Cr(VI) reduction in B-nZVI/PS system.
A synergistic effect between Cr(VI) reduction and phenol oxidation was achieved.
A positive correlation between persulfate decomposition and Fe2+ was found.
The intermediate products of both Cr (VI) and phenol were identified.

a r t i c l e

i n f o

Article history: Received 15 December 2015 Received in revised form 11 May 2016 Accepted 13 May 2016 Available online 16 May 2016 Keywords: Nano zero-valent iron Persulfate Cr(VI) Phenol Simultaneous removal Sulfate radical oxidation

⇑ Corresponding author. E-mail address: [email protected] (X.-R. Xu). http://dx.doi.org/10.1016/j.cej.2016.05.062 1385-8947/Ó 2016 Elsevier B.V. All rights reserved.

a b s t r a c t In this study, bentonite-supported nanoscale zero-valent iron (B-nZVI) was used as a catalyst to activate persulfate (PS) for the simultaneous removal of Cr(VI) and phenol from aqueous solutions. Experimental results indicated that the presence of bentonite could decrease the aggregation of nZVI and increase its reactivity. The removal rates of Cr(VI) and phenol by B-nZVI system were 99.90% and 6.50%, respectively. Whereas the corresponding values by B-nZVI/PS system were 99.30% and 71.50%, respectively. The presence of persulfate could not significantly decline B-nZVI reactivity toward Cr(VI) reduction but remarkably promote phenol oxidation. The addition of Cr(VI) did positively affect the oxidation rate of phenol, a significant synergistic effect between Cr(VI) reduction and phenol oxidation was achieved in B-nZVI/PS system. A positive correlation between persulfate decomposition and dissolved Fe2+ was found. Cr(III) species such as Cr2O3, Cr(OH)3 were identified in Cr(VI) reduction process, whereas the oxidation products such as catechol, 1,4-benzoquinone, propionic acid and formic acid were identified in phenol oxidation process. The reusability experiments of B-nZVI demonstrated that the structure of B-nZVI was relatively stable after four cycles of reuse. This is the first report for the feasibility of B-nZVI as a catalyst to activate persulfate for the simultaneous removal of heavy metal and organic pollutant. Ó 2016 Elsevier B.V. All rights reserved.

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1. Introduction In recent decades, nanoscale zero-valent iron (nZVI) has been extensively investigated to explore the potential for the remediation of various kinds of pollutants thanks to its advantages such as large surface area, high reactivity, strong reductive capacity and low cost [1–4]. Nevertheless, nZVI tends to aggregate due to its high surface energy and intrinsic magnetic interactions [5]. An oxide layer often forms on the surface of nZVI when exposed to the atmosphere and water [6,7]. The aggregation of nZVI can limit its dispersity and the oxidization of nZVI can significantly decrease its reactivity [8,9]. These disadvantages are the major challenges for nZVI use in environmental remediation [10,11]. To overcome these drawbacks, efficient dispersion of nZVI is thus critical in improving its reactivity, various types of modified nZVI have been developed, such as mesoporous silica microspheres-supported nZVI, kaolin-supported nZVI, bentonite-supported nZVI, carbonaceous materials-supported nZVI, polyacrylamide-stabilized nZVI and sodium carboxymethyl cellulose-stabilized nZVI [6–9,11–15]. These modified nZVI showed greater performance on the remediation of various kinds of pollutants (i.e., methyl orange and pentachlorophenol) compared to non-modified nZVI [5,16]. More recently, activation of persulfate (PS), based on the generation of strongly oxidizing sulfate radical anions, has emerged as an alternative oxidants [17–19], giving it the potential to degrade organic pollutants due to the advantages of high solubility and broad operative pH range [20,21]. The activation of persulfate are mainly carried out by UV radiation [18], heat [19,20], electrolysis [17], transition metal ions [22,23]:

S2 O2 8 S2 O2 8

þ UV=heat ! 

þe !

SO2 4

2SO 4

ð1Þ

SO 4

ð2Þ

þ

 2 ðnþ1Þþ Menþ þ S2 O2 8 ! SO4 þ SO4 þ Me

ð3Þ

Among these methods, activation of persulfate with dissolved Fe2+ is the most preferable for environmental application since other metal ions are toxic [23–25]. However, the generation of sulfate radical anions by the reaction of dissolved Fe2+ with persulfate is too fast to control, and excessive amounts of the generated iron have been needed further treatment [25,26]. Therefore, it is necessary to find an alternative to make reaction smoothly continuous by controlling the release of dissolved Fe2+. It happened that nZVI was used as a slow-releasing source of dissolved Fe2+ in heterogeneous nZVI/PS system [25,27]. Actually, this heterogeneous nZVI/ PS system integrates nZVI reduction and Fenton-like oxidation. However, previous studies only have focused on the oxidation of organic pollutant by nZVI/PS system [25,27]. For example, nZVI was used as a catalyst to generate sulfate radicals anions for heterogeneous Fenton-like oxidation of organic pollutants such as p-chloroaniline and antibiotic sulfadiazine. The reductive capacity of nZVI had not been utilized in this nZVI/PS system. Nowadays, coexistence of organic pollutants and heavy metals in industrial wastewater is a serious environmental problem [28–32]. Cr(VI) ions and organic co-pollutants like phenol and naphthalene often originate from industrial sources of wastewater, such as leather tanning, photographic-film making, wood preservation, car manufacturing, petroleum refining and agricultural activity [31,32]. Thus, the simultaneous removal of organic pollutant and heavy metal from wastewater has a particular significance to both pollution control and remediation. In addition, Cr(VI) reduction by nZVI has been reported to be one of the most promising treatment due to its high surface area and strong reductive capability [2,8]. Therefore, Cr(VI) and phenol were selected as representative target pollutants in this study, and bentonite was used for the

immobilization of nZVI due to its abundance, chemical stability and adsorption capability [8]. It was attempted to use bentonitesupported nanoscale zero-valent iron (B-nZVI) as a catalyst to activate persulfate for the simultaneous removal of Cr(VI) and phenol from aqueous solutions. The main objectives were to: (1) evaluate the feasibility of B-nZVI/PS system for the simultaneous removal of Cr(VI) and phenol; (2) determine the removal of Cr(VI) and phenol at various conditions; (3) clarify the relationship between persulfate decomposition and Fe species; (4) identify the intermediate products of both Cr(VI) and phenol; and (5) elucidate the reusability and stability of B-nZVI. 2. Materials and methods 2.1. Materials The bentonite used in this study was obtained by Hongtai Co. Ltd., Henan, China, it contains a montmorillonite content >85% and has a specific surface area as measured by BET-N2 of 4.98 m2/g. After drying at 100 °C overnight, raw bentonite was ground and sieved through a 150 mesh sieve prior to use. Potassium dichromate (K2Cr2O7), ferric chloride hexahydrate (FeCl36H2O), potassium persulfate (K2S2O8) and sodium borohydride (NaBH4) obtained from the Tianjin Fuchen Chemical Reagent Factory, China. All chemicals were of analytical grade and used without further purification. Double distilled water was used to prepare all solutions throughout the experiment. 2.2. Synthesis of bentonite-supported nanoscale zero-valent iron Bentonite-supported nanoscale zero-valent iron (B-nZVI) was prepared by using conventional liquid-phase reduction method, and bentonite was used as a support material. The B-nZVI samples were synthesized such that the bentonite/iron mass ratio was 0, 1:2, 1:1 or 3:2. To obtain these B-nZVI samples, 9.66 g of ferric chloride hexahydrate was dissolved into a 50 mL mixture of ethanol and water (80%, v/v) in the three-neck flask, followed by addition of various amounts of bentonite (0, 1.00, 2.00 or 3.00 g) accordingly, with mechanical stirring to form a mixture of bentonite and iron for 10 min. Then a freshly prepared NaBH4 solution (3.54 g of NaBH4 in 100 mL) was dropwisely added into the stirred mixture solution to produce a black solid. The reduction reaction is given in Eq. (4).

4Fe3þ þ 3BH4 þ 9H2 O ! 4Fe0ðsÞ # þ3BðOHÞ3 þ 9Hþ þ 6H2ðgÞ "

ð4Þ

Subsequently, the mixture solution was stirred continuously for another 20 min under room temperature. Finally, the black solid produced separated from the mixture by vacuum filtration and quickly rinsed with ethanol for three times. The solid was then dried at 75 °C under vacuum overnight. The whole preparation was operated under a nitrogen atmosphere. 2.3. Experimental procedures To evaluate the reactivity of B-nZVI with various mass ratios (bentonite/iron) toward Cr(VI) removal, batch experiments were carried out using as-prepared B-nZVI samples in a conical flask containing solutions of Cr(VI) (30 mL, 0.38 mM, pH 5). In these treatments, as-prepared B-nZVI samples were used at the same dosage of 0.50 g/L (based on nZVI). A series of solutions were replaced on a rotary shaker with 150 rpm for 30 min. In addition, in order to investigate the simultaneous removal of Cr(VI) and phenol by B-nZVI, B-nZVI/PS or Fe2+/PS systems, the initial concentrations of persulfate, Fe2+, phenol and Cr(VI) were set at 1.00, 0.25, 0.11 and 0.38 mM, respectively.

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Batch experiments were conducted to investigate effects of the initial pH, B-nZVI dosage, persulfate concentration and concentration ratio (Cr(VI)/phenol) on the simultaneous removal of Cr(VI) and phenol by B-nZVI/PS system. The experiments were carried out by putting the flask in a thermostatic shaker bath with a rotation speed of 150 rpm at atmospheric pressure. Unless stated otherwise, the initial concentrations of persulfate, phenol and Cr (VI) were set at 1.00, 0.11 and 0.38 mM, respectively. The dosage of B-nZVI was 0.50 g/L, the reaction temperature was at room temperature (25 ± 1 °C), and the initial pH was kept at 5 within the reaction time of 30 min. At the given time intervals, the sample aliquots were taken from the tubes and ethanol was added to quench any radical reactions [33]. The samples were centrifuged at 5000 rpm for 10 min, and then filtered through a 0.22 lm pore size filter to remove particles prior to analysis. All experiments were performed in duplicate. To investigate the effect of initial pH value on the simultaneous removal of Cr(VI) and phenol, different solution pH values (3, 5, 7, 9 and 11) were adjusted by 0.1 M HCl and NaOH solutions. No buffer solution was used to maintain a constant pH during the reaction. Different dosages of B-nZVI (0.25, 0.50, 0.75, 1.00 and 1.25 g/L) were added into solutions to evaluate the effect of B-nZVI dosage. In order to investigate the effect of persulfate concentration, the initial persulfate concentrations added were 0.33, 0.67, 1.00, 1.33 and 1.67 mM, respectively. To investigate the effect of concentration molar ratios (Cr(VI)/phenol), the initial concentration ratios added were set at 0.90, 1.80, 3.60, 5.40, 7.20 and 9.00, respectively. To investigate persulfate decomposition and Fe species leaching, the sampling interval time was set as 12 min within a-120 min reaction period. Meanwhile, the intermediate products of both Cr(VI) and phenol during the reaction were identified. To investigate the reusability of the B-nZVI, stability experiments were repeated by four times with the same B-nZVI sample. Upon the completion of each experiment, the BnZVI was centrifuged for 10 min, washed by double distilled water and recycled for the next run.

215

of <10 nm were analyzed by an X-ray photoelectron spectrometer (XPS) (Thermo-VG Scientific ESCALB-250 with Al–Ka radiation). 3. Results and discussion 3.1. Comparisons of Cr(VI) removal using synthesized B-nZVI samples The surface morphologies of B-nZVI samples with various mass ratios (bentonite/iron) were observed with SEM (Fig. 1). The bare nZVI sample was mostly spherical and formed prominent chainlike aggregates with the aggregate size varied between 100 and 120 nm (Fig. 1a). By contrast, nZVI immobilized on the bentonite was clearly discrete and well dispersed on the carrier with little aggregation (Fig. 1b–d). The aggregate size of nZVI decreased as the increase of mass ratios. In addition, the specific surface areas for bentonite, bare nZVI, B-nZVI (1:2), B-nZVI (1:1) and B-nZVI (3:2) samples were 4.98, 50.25, 39.41, 32.74 and 27.65 m2/g, respectively. The bare nZVI sample provided the largest specific surface area, while B-nZVI samples had a lower surface area due to the incorporation of bentonite as a support material. Given that the mass ratio was set initially, the increased specific surface area of B-nZVI suggested that bentonite played a role in dispersing and stabilizing nZVI. The smaller size of nZVI means larger surface area, which leads to more reactive sites and higher reactivity [8,9]. Since an appropriate bentonite/iron mass ratio is important for the B-nZVI application, the reactivity of as-prepared B-nZVI samples toward Cr(VI) reduction was evaluated. As shown in Fig. 2, it was observed that the removal of Cr(VI) increased with the increase of the bentonite/iron mass ratio, indicating that bentonite could not only prevent nZVI from aggregating together but also help the composite kept a higher reactivity. The removal of Cr(VI) was nearly 100% after 30 min at the mass ratio of 1:1, further increasing the mass ratio from 1:1 to 3:2 did not considerable improve the Cr(VI) removal. Thus, the ratio of 1:1 was used as the optimal mass ratio for the subsequent experiments.

2.4. Analytical methods Prior to analysis, phenol was converted into a complex by adding 1 mL of buffer (pH 9) followed by 1 mL of 0.05 M 4aminoantipyrene and 1 mL of 0.05 M potassium ferricyanide aqueous solution. Then as-obtained brownish red antipyrine dye was estimated with UV–vis spectrophotometer (SHIMADZU UV-1750, Japan) at 500 nm [34]. In addition, the evaluation of phenol oxidation products were followed using a HPLC system (Agilent 1100LC, USA) equipped with a RP C18 column (Acclaim 120, 25 cm  0.46 cm, 5 lm packing) and a UV–vis detector. Methanol and water (40:60, v/v) at a flow rate of 1.0 mL/min was used as the mobile phase, and the detection wavelength was set at 254 nm [35]. The oxidation products were confirmed by the coinjection of commercial standards under the same operating conditions. The concentration of Cr(VI) was analyzed using a 1,5diphenylcarbazide (DPC) colorimetric method by measuring the absorbance at 540 nm in acidic solution with a UV–vis spectrophotometer [36]. The concentration of persulfate anion was determined by iodometric titration with sodium thiosulfate [37]. The concentrations of dissolved iron and Fe2+ were measured at 510 nm by using a ferrozine method on a UV–vis spectrophotometer [34]. The specific surface areas of B-nZVI samples were determined by N2 adsorption at 77 K using a Micromeritics ASAP 2010 instrument. The surface morphologies of B-nZVI samples were analyzed by scanning electron microscopy (SEM) (ZEISS Ultra 55, Germany) with an operating voltage of 5 kV. X-ray diffraction (XRD) patterns of B-nZVI samples were collected on a D/Max-IIIA Powder X-ray Diffractometer (Rigaku Corp., Japan) equipped with Cu–Ka radiation. The surface products of B-nZVI samples within a depth

3.2. Simultaneous removal of Cr(VI) and phenol by B-nZVI, B-nZVI/PS or Fe2+/PS system The differences among B-nZVI, B-nZVI/PS and Fe2+/PS systems for the simultaneous removal of Cr(VI) and phenol were investigated (Fig. 3). It was observed that removal rates of Cr(VI) and phenol by B-nZVI were 99.90% and 6.50% after 30 min, respectively, suggesting that B-nZVI exhibited a high reactivity toward Cr(VI) reduction while a quite low reactivity toward phenol oxidation. In contrast, the corresponding values by B-nZVI/PS system were 99.30% and 71.50%, respectively. Actually, Cr(VI) was reduced into Cr(III) by nZVI and dissolved Fe2+ via Eqs. (5) and (6), while phenol was oxidized by sulfate radical anions and hydroxyl radicals via Eqs. (7)–(10) [38,39]. 3þ þ 3Fe0 þ Cr2 O2 þ 3Fe3þ þ 7H2 O 7 þ 14H ! 2Cr

ð5Þ

3þ þ 6Fe2þ þ Cr2 O2 þ 6Fe3þ þ 7H2 O 7 þ 14H ! 2Cr

ð6Þ

2Fe0 þ O2 þ 4Hþ ! 2Fe2þ þ 2H2 O

ð7Þ

 2 3þ Fe2þ þ S2 O2 8 ! SO4 þ SO4 þ Fe

ð8Þ

  H2 O þ SO 4 ! HSO4 þ HO

ð9Þ

 phenol þ SO 4 =HO !! oxidation products

ð10Þ

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Fig. 1. SEM images of B-nZVI at various ratios (bentonite/iron mass): (a) 0; (b) 1:2; (c) 1:1; (d) 3:2.

80

1.0 60

0.8 3:2 1:1 1:2 0

40

20

removal of phenol by B-nZVI removal of Cr(VI) by B-nZVI 2+

removal of phenol by Fe /PS system

0.6

2+

removal of Cr(VI) by Fe /PS system removal of phenol by B-nZVI/ PS system removal of Cr(VI) by B-nZVI/ PS system

Ct/C0

Cr(VI) removal efficiency (%)

100

0.4 0

0

6

12 18 Reaction time (min)

24

30

Fig. 2. Removal of Cr(VI) by B-nZVI with various ratios (bentonite/iron mass). Reaction conditions: [Cr(VI)]0 = 0.38 mM, [nZVI]0 = 0.50 g/L, [B-nZVI]0 = 0.50 g/L, pH = 5.

0.2

0.0

0

6

12 18 Reaction time (min)

24

30

Fig. 3. Simultaneous removal of Cr(VI) and phenol by B-nZVI, B-nZVI/PS or Fe2+/PS system. Reaction conditions: [Cr(VI)]0 = 0.38 mM, [phenol]0 = 0.11 mM, [BnZVI]0 = 0.50 g/L, [PS]0 = 1.00 mM, [Fe2+]0 = 0.25 mM, pH = 5.

Z.-H. Diao et al. / Chemical Engineering Journal 302 (2016) 213–222

It is interesting to note that the removal of phenol increased from 6.50% to 71.50% when persulfate was added. The enhanced removal of phenol was mainly attributed to catalytic persulfate oxidation in B-nZVI/PS system. In addition, compared to the reduction of Cr(VI) by B-nZVI, the presence of persulfate could not significantly decline B-nZVI reactivity toward Cr(VI) reduction. These results clearly demonstrated that the simultaneous removal of Cr (VI) and phenol could be successfully achieved in B-nZVI/PS system. However, a relatively poor removal performance was observed in Fe2+/PS system compared with B-nZVI/PS system, the removal rates of Cr(VI) and phenol by Fe2+/PS system were 14.70% and 63.90% after 30 min, respectively. Moreover, the removal of phenol by Fe2+/PS and B-nZVI/PS systems were further investigated in the absence of Cr(VI). It was observed that a negative effect of Cr(VI) on the removal of phenol by Fe2+/PS system (Fig. 4). This may be mainly attributable to the rapid depletion of dissolved Fe2+ since dissolved Fe2+ is used as an activator of persulfate for the production of sulfate radical anions. Reduction of Cr(VI) by dissolved Fe2+ into Cr(III) species led to a decrease in the amount of dissolved Fe2+ via Eq. (6). Furthermore, the phenol reaction kinetic data calculated from the obtained results of phenol oxidation could be fitted to a pseudo-second order reaction. Similar result was observed for photocatalytic removal of Cr(VI) and phenol over TiO2-based catalyst [29]. An increase in kinetic rate constant of phenol oxidation was observed when Cr(VI) was added into B-nZVI/PS system, indicating that the addition of Cr(VI) did positively affect oxidation rate of phenol. Actually, the corrosion rate of nZVI can be accelerated in the presence of Cr(VI) and lead to the release of more dissolved Fe2+ for activation of persulfate, which favored to phenol removal by B-nZVI/PS system. However, the presence of Cr(VI) consumed dissolved Fe2+, which was not favorable to phenol removal by Fe2+/PS system. Thereby a decrease in kinetic rate constant of phenol oxidation was observed when the Cr(VI) was added into Fe2+/PS system. Particularly, the change of pH values as a function of time in BnZVI and B-nZVI/PS systems was also investigated. It was observed that pH value of solution gradually increased with the increase of reaction time in B-nZVI system, whereas the corresponding value gradually decreased in B-nZVI/PS system (Fig. S1). A gradual pH increase was due to the consumption of hydrogen ions in solution via Eq. (4), there may be resulted in formation of precipitation. Whereas a gradual pH drop was due to decomposition of persulfate

1.0 B-nZVI/PS in the presence of Cr(VI) 2+

Fe /PS in the absence of Cr(VI) 2+

Fe /PS in the presence of Cr(VI) B-nZVI/PS in the absence of Cr(VI)

0.8 Phenol (Ct/C0)

1.6

-ln C t/C 0

1.2

0.6

0.8 2

0.0

0

0.4

0.2

0

6

2

y= -0.00313x +0.130x+0.0986, R =0.939 2 2 y= -0.00266x +0.117x+0.0780, R =0.955 2 2 y= -0.00181x +0.0854x+0.0516, R =0.928 2 2 y= -0.000332x +0.0533x+0.0248, R =0.995

0.4

12 18 Reaction time (min)

6

12 18 Reaction time (min)

24

24

30

30

Fig. 4. Kinetic of phenol degradation by B-nZVI/PS or Fe2+/PS system in the absence or present of Cr(VI). Reaction conditions: [Cr(VI)]0 = 0.38 mM, [phenol]0 = 0.11 mM, [B-nZVI]0 = 0.50 g/L, [PS]0 = 1.00 mM, [Fe2+]0 = 0.25 mM, pH = 5.

217

by nZVI, there may be resulted in formation of byproduct (i.e., HSO 4 ) via Eqs. (11) and (12) [40,41].   2H2 O þ 2S2 O2 8 ! 4HSO4 þ O2

ð11Þ

þ HSO4 ! SO2 4 þH

ð12Þ

3.3. Simultaneous removal of Cr(VI) and phenol by B-nZVI/PS system at various conditions It was observed that the Cr(VI) reduction and phenol oxidation were dependent on varying pH conditions (Fig. 5a). The removal of Cr(VI) reached the maximum at the pH range of 3–5, but a dramatic decrease occurred at pH 5–7, followed by an increase in the pH range of 7–11. Approximately 97% of Cr(VI) was removed after 30 min at pH 11, whereas nearly 100% of Cr(VI) removal were obtained at pH 3–5, suggesting that an acidic condition favored Cr (VI) removal. It was observed that 71.90%, 71.70%, 45.20%, 50.00% and 54.10% of phenol removal were obtained at pH of 3, 5, 7, 9 and 11, respectively. The results indicated that phenol removal decreased with the increase of pH value. This may be due to the generation of more sulfate radical anions by persulfate activated with B-nZVI in the acidic conditions. Actually, more dissolved Fe2+ and electrons are generated from the B-nZVI under the acidic conditions, thereafter as-formed dissolved Fe2+ could activate persulfate to the generation of more sulfate radical anions. Nevertheless, the precipitation of dissolved Fe2+ and Fe3+ could occur under neutral and alkaline conditions, resulting in the formation of iron hydroxides on the surface of B-nZVI, which maybe inhibit the release of dissolved Fe2+ from B-nZVI. Fig. 5b shows that Cr(VI) removal increased as the increase of the B-nZVI dosage, and the Cr(VI) removal maintained nearly 100% when the B-nZVI dosage exceeded 0.50 g/L. Phenol removal was increased with an increase in B-nZVI dosage up to 0.50 g/L and decreased thereafter. Increasing the amount of B-nZVI from 0.50 to 1.25 g/L resulted in decreasing the phenol removal from 72.00% to 41.8% after the reaction time of 30 min, respectively. One main explanation for the negative effect was that B-nZVI at a relatively high dosage provides excess Fe2+ that might scavenge the generated sulfate radical anions, resulting in decreased phenol removal. Furthermore, more BnZVI dosage would also induce greater aggregation of the B-nZVI and decrease the total active surface area. Fig. 5c indicates that no obvious difference was observed in Cr(VI) reduction (nearly 100%) when persulfate concentrations were in the range of 0.33– 1.00 mM, suggesting that B-nZVI had sufficient strong activity for reduction of Cr(VI) completely at 1.00 mM persulfate. However, a decrease in Cr(VI) removal was observed when the concentration of persulfate exceeded 1.00 mM, meaning that reduction activity of nZVI was not sufficient to maintain a high Cr(VI) removal. This should be attributed to the generated of more sulfate radical anions and hydroxyl radicals as the increase of persulfate concentration. Thus, the formed Cr(III) species would be reconverted into Cr(VI) in the presence of presulfate, sulfate radical anions and hydroxyl radicals. The enhanced phenol removal was due to the presence of the above oxidizing agents. In addition, there is no obvious difference in Cr(VI) reduction within the concentration ratio range of 0.90–9.00 (Fig. 5d), it was found that the Cr(VI) reduction sharply decreased when the concentration ratio exceeded 3.60. In contrast, the phenol removal gradually increased with the increase of the concentration ratio. This may be mainly due to the depassivation of the aged nZVI surface by a high Cr (VI) concentration.

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100

100

(a)

80 removal of Cr(VI) removal of phenol

70

(b)

90 Removal efficiency (%)

Removal efficiency (%)

90

60

80 70 60 removal of Cr(VI) removal of phenol

50 40

50 2

3

4

5

6

7

8

9

10

11

30

12

0.25

pH

100

0.50

0.75 1.00 B-nZVI dosage (g/L)

1.25

100

(c)

(d)

80 70 removal of Cr(VI) removal of phenol

60

Removal efficiency (%)

Removal efficiency (%)

90

90

80

removal of Cr(VI) removal of phenol

50 40 0.00

70 0.30

0.60 0.90 1.20 1.50 persulfate concentration( mM)

1.80

0.90

1.80 3.60 5.40 7.20 concentration molar ratio (Cr(VI)/phenol)

9.00

Fig. 5. Simultaneous removal performance of Cr(VI) and phenol by B-nZVI/PS system: (a) effect of pH; (b) effect of B-nZVI dosage; (c) effect of persulfate concentration; (d) effect of concentration ratio (Cr(VI)/phenol). Reaction conditions: [Cr(VI)]0 = 0.095–0.95 mM, [phenol]0 = 0.11 mM, [B-nZVI]0 = 0.25–1.25 g/L, [PS]0 = 0.33–1.67 mM, pH = 5.

3.4. Persulfate decomposition and Fe species leaching Persulfate decomposition as a function of time at pH of 5 is shown in Fig. 6. Approximately 68% of persulfate were decomposed after 48 min. Persulfate was decomposed rapidly in the initial 48 min, and subsequently began to decrease gradually to relatively stable. That may be due to the rapid depletion of dissolved Fe2+. Meanwhile, it was observed that the Cr(VI) removal dramatically increased as the reaction time increased in the initial 12 min. The reduction of Cr(VI) reached the maximum (nearly 100%) at

36 min, and then remained constant up to 48 min. Subsequently, the reduction of Cr(VI) began to decrease gradually and about 80% of Cr(VI) was measured after 120 min. Actually, the conversion of chromium species is reversible. Thus, in the presence of presulfate, sulfate radical anions and hydroxyl radicals, the lower valence states of chromium species might be reconverted into Cr(VI) according to Eqs. (13) and (14).

Cr3þ þ 2H2 OCrðOHÞþ2 þ 2Hþ 2CrðOHÞþ2 þ 3O2 þ H2 O

!

þ 2CrO2 4 þ 6H

ð14Þ

However, the phenol removal dramatically increased as the reaction time increased in the initial 12 min, and slightly increased as the reaction time increased, the removal of phenol increased from 68.50% to 74.20% as reaction time increased from 12 to 120 min. A slow increase in phenol oxidation was due to excess of radical species [42], the generated radicals self-scavenging can take place via Eq. (15).

1.0 persulfate Cr(VI) phenol

0.8

2  SO 4 ;S2 O8 ;HO

ð13Þ

0.6 Ct/C0

 2 SO 4 þ SO4 ! S2 O8

ð15Þ 2+

0.4

0.2

0.0

0

20

40 60 80 Reaction time (min)

100

120

Fig. 6. Conversion of persulfate, Cr(VI) and phenol as a function of time at pH of 5. Reaction conditions: [Cr(VI)]0 = 0.38 mM, [phenol]0 = 0.11 mM, [B-nZVI]0 = 0.50 g/L, [PS]0 = 1.00 mM, pH = 5.

In addition, the changes of dissolved iron and Fe concentrations of solution were investigated (Fig. 7). Dissolved Fe2+ concentration dramatically increased as the reaction time increased in the initial 48 min, and subsequently began to decrease gradually to reach a relatively stable concentration. This observation corresponded well with the decomposition of persulfate (Fig. 6). Since dissolved Fe2+ is regarded as an activator of persulfate decomposition, the decomposition rate of persulfate dramatically increased with the increase of the generated dissolved Fe2+ concentration at the beginning of reaction. Then, with the decrease of the generated dissolved Fe2+, persulfate decomposition began to decrease gradually to relatively stable. Actually, some dissolved Fe2+ could

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Z.-H. Diao et al. / Chemical Engineering Journal 302 (2016) 213–222 Table 1 The intermediate products of phenol during the reaction.

14

14 2+

Fe iron

12

Reaction time (min)

Oxidation products

Molecular formula

Molecular weight

10

10

60 60

C6H6O C6H4O2

110.11 108.09

8

8

6

6

Catechol 1,4Benzoquinone Maleic acid Hydroquinone Propionic acid Formic acid

C4H4O4 C6H6O2 C3H6O2 CH2O2

116.07 110.11 74.08 46.03

4

4

2

2

dissolved iron pH

60 120 120 120

pH

[Fe species] (mg/L)

12

0

0

20

40 60 80 Reaction time (min)

100

120

0

Fig. 7. Concentration of Fe species and changes of pH values as a function of time at pH of 5. Reaction conditions: [Cr(VI)]0 = 0.38 mM, [phenol]0 = 0.11 mM, [BnZVI]0 = 0.50 g/L, [PS]0 = 1.00 mM, pH = 5.

react with persulfate for the generation of sulfate radical anions, but other dissolved Fe2+ could react with the generated sulfate radical anions to form dissolved Fe3+, result in decreased persulfate decomposition rate. These data indicate that there was a positive correlation between persulfate decomposition and dissolved Fe2+. However, the presence of sulfate radicals could induce nZVI to generate more dissolved iron. Thus, a gradual dissolved iron increase was observed during reaction process. Moreover, a gradual pH drop was observed, which was due to decomposition of persulfate by nZVI, leading to the formation of HSO 4. 3.5. Composition of intermediate products In order to further investigate the simultaneous removal of Cr (VI) and phenol by B-nZVI/PS system, the intermediate products of both Cr(VI) and phenol during the reaction were identified. The detailed XPS surveys on the region of Cr (2p) and the oxidation products of phenol during the reaction are presented in Fig. 8 and Table 1. For Cr(VI) reduction process, the XPS spectra of Cr (2p) at the initial 60 min showed three peaks at the binding energies of 577.10, 586.80 and 588.70 eV (Fig. 8a). The peaks observed at 586.80 and 577.10 eV are corresponded to the characteristics binding energy for the Cr(III) species (Cr2O3, Cr(OH)3) [43,44]. Although the peaks observed at 588.70 eV is attributed to Cr(VI) species, the presence of Cr(III) species confirmed that most of Cr(VI) were reduced by nZVI (97.5% of Cr(VI) removal at the initial 60 min).

However, a decrease in intensity of peak at 586.80 and 577.10 eV and an increase in intensity of peak at 588.70 eV were observed for the XPS spectra of Cr (2p) at 120 min (Fig. 7b). Addition to the above three main peaks, a peak at 580.10 eV corresponded to Cr(VI) species was also observed [45], meaning that the presence of sulfate radical anions and hydroxyl radicals might reoxidize Cr (III) to Cr(VI) and slow down the Cr(VI) reduction process. Similar result was observed for photocatalytic reduction of Cr(VI) over ZnO nanoplates in the presence of phenol [44]. For phenol oxidation process, the generated sulfate radicals and hydroxyl radicals attack on phenol lead to the formation of some intermediates such as catechol and 1,4-benzoquinone at the initial 60 min (Table 1). Further oxidation results in the cleavage of the benzene ring and emergence of ring opening products such as propionic acid and formic acid were observed at 120 min, which were in agree with the results of previous study [46].

3.6. Reusability and stability of B-nZVI Repetitive experiments were carried out to evaluate the reusability and stability of B-nZVI on the simultaneous removal of Cr(VI) and phenol from aqueous solutions. The results are shown in Figs. 9–12 and Figs. S2 and S3, it was found that the reactivity of B-nZVI gradually decreased with the reuse time (Fig. 9). After four cycles of reuse, the removal of Cr(VI) and phenol decreased from 99.60% to 39.20% and 72.60% to 40.50%, respectively. As shown in Fig. S2, it was observed that the removal of Cr(VI) by B-nZVI in the absence of phenol decreased from 99.83% to 61.44% after four cycles of reuse, whereas the removal of phenol by B-nZVI/PS system in the absence of Cr(VI) decreased from 73.21% to 23.83%. Compared with either Cr(VI) or phenol removal alone, the simultaneous removal of Cr(VI) and phenol by B-nZVI/PS system exhibited a great application superiority. These results implied that the BnZVI displayed a better reusability after four cycles of reuse. In order to further investigate the stability of B-nZVI on the simultaneous removal of Cr(VI) and phenol, the decomposition of

8100

8600

Cr 2p at 60 min

(a)

(b)

Cr 2p at 120 min 8500

8000

Counts/s

Counts/s

8400

7900

7800

8300 8200

7700

7600

8100

590

585

580

Binding Energy (eV)

575

570

8000

590

585

580

575

Binding Energy (eV)

Fig. 8. XPS survey of Cr (2p) on the surface of B-nZVI during the reaction, (a) 60 min; (b) 120 min.

570

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Z.-H. Diao et al. / Chemical Engineering Journal 302 (2016) 213–222

100

80 Removal efficiency (%)

before reaction after reaction

Cr(VI) phenol

Fe2O3

0

Fe

Fe3O4

60

40

20

0

Run 1

Run 2 Run 3 Reuse time

Run 4

Fig. 9. Reuse performance of B-nZVI on the simultaneous removal of Cr(VI) and phenol at different reuse cycles. Reaction conditions: [Cr(VI)]0 = 0.38 mM, [phenol]0 = 0.11 mM, [B-nZVI]0 = 0.50 g/L, [PS]0 = 1.00 mM, pH = 5.

B-nZVI in each reuse process was also investigated (Fig. S3). It was observed that the concentration of dissolved iron decreased with the reuse time. The concentration of dissolved iron reached maximum (9.25 mg/L) at the first cycle of reuse, thereafter decreased to 5.12 mg/L after four cycles of reuse. Actually, nZVI was greatly oxidized in the presence of sulfate radical anions, hydroxyl radicals, persulfate and dissolved oxygen. Thus the loss of nZVI increased with the increase of the cycle of reuse. To better understand morphology affecting the performance of B-nZVI, the morphologies of B-nZVI samples before and after reaction were characterized by SEM (Fig. 10). It was found that the SEM images of B-nZVI before reaction consisted of spherical particles (Fig. 10a), whereas some stripe structure particles were observed after four cycles of reuse (Fig. 10b), indicating that the B-nZVI was damaged after four cycles of reuse. Actually, both the generated sulfate radical anions and hydroxyl radicals not only participated as reactive species in phenol removal, but also were used for the B-nZVI oxidation, resulting in a relatively large degree of oxidation on the B-nZVI surface. Consequently, its reactivity toward Cr(VI) reduction and phenol oxidation decreased with the increase of reuse cycle. Furthermore, the XRD patterns of B-nZVI before and after reaction are presented in Fig. 11. Before reaction, the largest diffraction peak observed at 26.67° is the characteristic peak of bentonite as well as some little peaks representing the intrinsic structure of bentonite. Moreover, a large characteristic peak at 44.7° corre-

20

30

40

50 60 2-Theta-Degrees

70

80

Fig. 11. XRD patterns of B-nZVI before and after reaction.

sponding to the formation of nZVI could be observed [5], indicating that nZVI particles were incorporated with bentonite [5,16]. Some other small peaks such as Fe2O3 (2h at 35.60°) and Fe3O4 (2h at 56.62°/62.31°) were also observed [5,47,48]. This may be due to the formation of an oxidation layer on the surface of nZVI when exposed to the atmosphere. After four cycles of reuse, the characteristic peak of nZVI (2h at 44.71°) became much weaker and peaks of Fe2O3 and Fe3O4 appeared more remarkable. This result may be resulted from the Fe2+ being leached from the surface of nZVI, resulting in a decrease of nZVI concentration. Additionally, XPS technique was employed to analyze elements composition and the oxidation state of B-nZVI surface, thereby XPS spectra of B-nZVI were recorded before and after reaction, and the detailed XPS surveys on the region of Fe (2p) are presented in Fig. 12. The XPS spectra of Fe (2p) before reaction showed four peaks at the binding energies of 706.70, 710.82, 711.51 and 725.13 eV (Fig. 12a). According to early reports [47,49,50], a strong intensity peak observed at 706.7 eV corresponds to nZVI, and the peak at 711.51 eV corresponds to FeOOH, while broad peaks at 710.82 and 725.13 eV are attributed to iron oxides (a-Fe2O3). These results explained the existence of nZVI and a thin oxidation layer on the particle surface before reaction, which was in agree with the results of XRD analysis (Fig. 11). However, after four cycles of reuse, a decrease of intensity peak at 706.70 eV and an increase intensity of peaks at 710.82, 711.51 and 725.13 eV were observed from the Fe (2p) spectrum of B-nZVI (Fig. 12b). Particularly, the peak observed at 706.70 eV became much weak, indicating that

Fig. 10. SEM images of B-nZVI. (a) Before reaction; (b) after reaction.

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4400

3000

(a)

Fe 2p

2800

3600

2600

Counts/s

Counts/s

4000

3200

2800 730

(b)

Fe 2p

2400

2200 725

720 715 Binding Energy (eV)

710

705

730

725

720 715 Binding Energy (eV)

710

705

Fig. 12. XPS survey of Fe (2p) for the B-nZVI. (a) Before reaction; (b) after reaction.

B-nZVI has lost some reactivity after four cycles of reuse, resulting in a decrease in removal of Cr(VI) and phenol. These results demonstrated that B-nZVI was relatively stable to activate persulfate for the simultaneous removal of Cr(VI) and phenol after four cycles of reuse. 4. Conclusions The feasibility of bentonite-supported nanoscale zero-valent iron (B-nZVI) as a catalyst to activate persulfate (PS) for the simultaneous removal of Cr(VI) and phenol was investigated. It was demonstrated that bentonite could effectively decrease the aggregation of nZVI and increased its reactivity toward Cr(VI) reduction. B-nZVI/PS system had a good synergistic effect on the simultaneous removal of Cr(VI) and phenol from aqueous solutions. A significant enhancement of phenol oxidation was attributable to the presence of persulfate. Addition of Cr(VI) did accelerate the oxidation rate of phenol, a significant synergistic effect between Cr(VI) reduction and phenol oxidation was achieved in B-nZVI/PS system. A positive correlation between persulfate decomposition and dissolved Fe2+ was found. Cr(III) species such as Cr2O3 and Cr(OH)3 were found in Cr(VI) reduction process, whereas the oxidation products such as catechol and 1,4-benzoquinone, propionic acid and formic acid were found in phenol oxidation process. The reusability and stability performance of B-nZVI was demonstrated that B-nZVI was relatively stable in B-nZVI/PS system. The further study on the synergistic mechanism between Cr(VI) reduction and phenol oxidation, the toxicity evaluation and the potential application in real leather tanning wastewater will be carried out in our next period. Acknowledgments This study was financially supported by the National Natural Science Foundation of China (Nos. 21407155, 51378488, 51508116 and 41401576) and Guangdong Province Public Welfare Research and Capacity Building Project (No. 2015A020215022). Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.cej.2016.05.062. References [1] R.A. Crane, M. Dickinson, I.C. Popescu, T.B. Scott, Magnetite and zero-valent iron nanoparticles for the remediation of uranium contaminated environmental water, Water Res. 45 (2011) 2931–2942.

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