Efficient degradation of Acid Orange 7 in aqueous solution by iron ore tailing Fenton-like process

Chemosphere 150 (2016) 40e48

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Efficient degradation of Acid Orange 7 in aqueous solution by iron ore tailing Fenton-like process Jianming Zheng a, b, Zhanqi Gao c, d, Huan He a, Shaogui Yang a, Cheng Sun a, * a

State Key Laboratory of Pollution Control and Resource Reuse, School of the Environment, Nanjing University, Nanjing 210046, PR China Jiangsu Entry Exit Inspection and Quarantine Bureau, Nanjing 210001, PR China c State Environmental Protection Key Laboratory of Monitoring and Analysis for Organic Pollutants in Surface Water, Nanjing 210036, PR China d Jiangsu Provincial Environmental Monitoring Center, Nanjing 210036, PR China b

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


The iron ore tailing was first applied as heterogeneous Fenton catalyst.
Acid Orange 7 was oxidized completely within 60 min by using the tailing and H2O2.
The mechanism of the tailing as an efficient heterogeneous Fenton catalyst has been explored.
$OH is the main active radicals for the Fenton-like reaction.
The tailing can be reused more than nine times without apparent decrease of the catalytic capacity.

a r t i c l e i n f o

a b s t r a c t

Article history: Received 27 June 2015 Received in revised form 8 January 2016 Accepted 1 February 2016 Available online xxx

An effective method based on iron ore tailing Fenton-like process was studied for removing an azo dye, Acid Orange 7 (AO7) in aqueous solution. Five tailings were characterized by X-ray fluorescence spectroscope (XFS), BrunnerEmmetTeller (BET) measurement, and Scanning Electron Microscope (SEM). The result of XFS showed that Fe, Si and Ca were the most abundant elements and some toxic heavy metals were also present in the studied tailings. The result of BET analysis indicated that the studied tailings had very low surface areas (0.64e5.68 m2 g1). The degradation efficiencies of AO7 were positively correlated with the content of iron oxide and cupric oxide, and not related with the BET surface area of the tailings. The co-existing metal elements, particularly Cu, might accelerate the heterogeneous Fenton-like reaction. The effects of other parameters on heterogeneous Fenton-like degradation of AO7 by a converter slag iron tailing (tailing E) which contains highest iron oxide were also investigated. The tailing could be reused 10 times without significant decrease of the catalytic capacity. Very low amount of iron species and almost undetectable toxic elements were leached in the catalytic degradation of AO7 by the tailing E. The reaction products were identified by gas chromatography-mass spectrometry and a possible pathway of AO7 degradation was proposed. This study not only provides an effective method for removing azo dyes in polluted water by employing waste tailings as Fenton-like catalysts, but also uses waste tailings as the secondary resource. © 2016 Elsevier Ltd. All rights reserved.

Handling Editor: Jun Huang Keywords: Iron ore tailing Heterogeneous Fenton-like process Acid Orange 7 Degradation Mechanism and pathway Catalyst stability

* Corresponding author. E-mail address: [email protected] (C. Sun). http://dx.doi.org/10.1016/j.chemosphere.2016.02.001 0045-6535/© 2016 Elsevier Ltd. All rights reserved.

J. Zheng et al. / Chemosphere 150 (2016) 40e48

1. Introduction Textile industries' production of dyestuffs creates huge volumes of wastewater. It is estimated that over 15% of the total world production of dyes is lost in the textile effluents (Liu et al., 2007). Azo dyes constitute the largest and the most important class of synthetic dyes for industrial applications (Yang et al., 2010). The discharge of azo dyes into water streams is undesirable not only because of their color but also their toxicity, non-biodegradability, and potential carcinogenic characteristics (Clarke and Johnson, 2010). Therefore, the urgent removal of azo compounds from wastewater has attracted worldwide attention. Among various treatment methods, advance oxidation processes (AOPs) are becoming important technologies for removing azo compounds (Yang et al., 2010; Stylidi et al., 2004; Guo et al., 2010; Yuan et al., 2011). AOPs based on the in situ generation of hydroxyl radical ($OH), which is a highly powerful oxidizer and can attack organic compounds in a non-selective way, leading them to be mineral end-products. AOPs, including ozonation (Muthukumar and Selvakumar, 2004), Fenton reaction (Guo et al., 2010; Gutowska et al., 2007), and photocatalysis (Bandara et al., 1999), have been applied to the degradation of azo dyes. Among them, Fenton process has its own unique advantages, including high degradation efficiency, simple operation, environmentally benign property, and low cost (Zhao and Hu, 2008). The classical Fenton reagent consists of a homogeneous solution of iron ions and hydrogen peroxide. However, its practical application in dealing with industry wastewater has some disadvantages. It is efficient only at low pH (pH < 4) (Hu et al., 2011), and it is costly to remove the iron ions from the system after the treatment (Pariente et al., 2008; Luo et al., 2009). In order to overcome the above shortages, developing heterogeneous Fenton-like catalysts such as iron-containing solid compounds, iron-coated particles, and supported iron compounds, is necessary. For heterogeneous processes, main oxidative reactions occur at the solideliquid interface where the iron remains substantially in the solid phase, either as a mineral or as an adsorbed ion (Hu et al., 2011). The heterogeneous Fenton-like reaction can effectively destroy organic pollutants in a wider pH range with much less iron loss than the homogeneous Fenton process. However, most of the procedures for preparing heterogeneous Fenton-like catalysts are tedious, time-consuming and costly. A continuous increase of hazardous industrial wastes like iron ore tailings generated by iron and steel industry, is becoming a serious problem. A total of 59.7 billion tons of tailings has been discarded as waste, and the production of iron ore tailings is estimated to be one third of all stockpiled tailings. The stockpiles of waste tailings not only occupy a large area of land and pollute the environment, but can also easily flow and topple, leading to vegetation deterioration and injuries, resulting even in natural disaster, like when a tailing dam collapses (Wu et al., 2013; Liakopoulos et al., 2010). At the same time, waste tailings as secondary resources are very important to all countries in the world. There is growing interest in possible alternative uses of solid wastes as adsorbents and catalysts or as the starting materials before their discharge into the environment (Zeng et al., 2004; Li et al., 2010; Santos et al., 2015; Lin et al., 2013). Waste tailings have already been used as absorbents or oxidants to remove dye compounds (Clarke et al., 2010, 2013; Wu et al., 2013). Clarke and Johnson (2010) found that Mn oxide (MnxOy) containing mine tailings from South Africa could decolorize a number of acid azo dyes including Acid Orange 7 (AO7), decolorizing 42% of AO7 after 2 h reaction. The waste tailings have rarely been applied as heterogeneous Fenton-like catalysts for the treatment of compounds. Ali et al. (2013) reported steel industry waste was effectively employed as a Fenton-like catalyst for the efficient decolorization

41

of methyl orange dye, but the iron-containing waste used in their study contained about 90% iron, which could not be considered as real waste. Therefore, it is important to investigate real waste tailings for application as Fenton-like catalysts to treat organic pollutants. In the present study, iron ore tailings were applied as the Fenton-like catalyst for the treatment of simulated dye wastewater. Acid Orange 7 was chosen as a model dye since it displays the “core-structure” of a number of commercial dyes and thus has similar physiochemical properties as more complex dye compounds (Coen et al., 2001). Reaction parameters on heterogeneous Fenton-like degradation of AO7 by iron ore tailings were investigated in detail. In addition, the intermediates were identified and the degradation pathway was proposed for AO7 in the Fenton-like process. 2. Experimental 2.1. Materials Iron ore tailings composite samples were collected from five different tailings' dumps. The tailings were dried after collection. Acid Orange 7 was obtained from SigmaeAldrich (China Division, Shanghai of China). H2O2 (30%, v/v) was purchased from Fisher Company (Utah, USA). HPLC-grade ethanol and dichloromethane (CH2Cl2) were obtained from Merck (Darmstadt, Germany). The spin-trapping agent 5,5-dimethyl-pyrroline-N-oxide (DMPO) was purchased from J&K Scientific Ltd (Beijing, China). Ultrapure deionized water produced by a Milli-Q water purification system (Millipore, Beldford, MA) was employed throughout this study. Stock solution was prepared by dissolving desired amount of AO7 into methanol and then stored at 4  C in a refrigerator. Working solutions were obtained by diluting stock solution with water to desired concentrations. 2.2. Degradation of AO7 by heterogeneous Fenton-like experiments The iron ore tailings were mixed thoroughly to guarantee uniformity in their composition and then 0.5 g of iron ore tailings were added to 100 mL of working solution containing 30 mg L1 of AO7 and 19.8 mM H2O2. The pH of the working solution was adjusted to 5.0. The above conditions were continued for the heterogeneous Fenton-like process, unless variables were examined during the investigation of reaction parameters. The mixture was then mechanically stirred at 100 rpm and 20  C. Aliquots (5 mL) of the sample were taken at different time intervals and filtered through syringe membrane filters (0.2 mm) for the determination of UVeVisible absorption. All experiments were carried three times. The recovery of AO7 in the filtered samples determined before heterogeneous Fenton-like experiments was 97.8% (RSD ¼ 3.2%, n ¼ 6), which shows adsorption of AO7 by the tailings is very low and can be negligible. 2.3. Analytical methods The components of the tailings were analyzed by X-ray fluorescence spectrometer (XFS) (S2 Ranger, Bruker AXS GmbH). The specific surface areas of the tailings were determined by multipoint N2-BET analysis using an accelerated surface area and porosimetry analyzer ASAP 2020 (Micrometrics, USA) with analysis bath temperature of 77 K. Surface properties of the tailings were investigated by S-4800N II scanning electron microscope (SEM) (Hitachi, Japan). EPR experiments were performed on a Bruker A300-10/12 spectrometer (Bruker, Bremen, Germany) with DMPO as a spintrapping agent for radical measurement.

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J. Zheng et al. / Chemosphere 150 (2016) 40e48

The decolorization process of azo dye solution was monitored based on the decay of the absorbance (A) at the maximum wavelength (lmax ¼ 484 nm) of AO7, by measuring the spectra with a UV 1800 spectrophotometer (Shimadzu, Japan). Decolorization efficiency was calculated according to Eq. (1), where A0 and A were absorbance of AO7 solution at time 0 and t, respectively. The metal ions leached into the reaction solution after Fenton-like process were analyzed by NexIon 300 ICP-MS (PerkinElmer, USA). Decolorization efficiency (%) ¼ (A0A)/A0  100

(1)

The intermediate products during the reaction were detected by Trace gas chromatography coupled with an ISQ mass spectrometry (GC-MS). Prior to GC-MS determination, 80 mL of AO7 reactive solution (30 min) was extracted using 10 mL of CH2Cl2 three times. The extracts were combined and then concentrated by a rotary evaporator at 40  C to about 1 mL for detection of GCeMS (Wu et al., 2012). A DB-5MS capillary column (30 m  0.25 mm ID  0.25 mm film thickness) was employed for GC separation. The column oven was operated in a temperature programmed mode with an initial temperature of 40  C held for 2 min, first increased to 100  C with increments of 12  C min1, then increased again to 200  C with 5  C min1 increments, finally increased to 270  C with 20  C min1 increments and held for 10 min. Helium was used as a carrier gas. Electron impact (EI) mass spectra were scanned from 50 to 550 m/z. The compound analysis was undertaken with reference to the NIST02 mass spectral library database. The metal ions leached into the reaction solution after Fentonlike process were analyzed by inductively coupled plasma-mass spectrometry (ICP-MS) (NexIon 300 ICP-MS, PerkinElmer, USA). The removal of total organic carbon (TOC) in the reaction solution was determined by TOC-5000 analyzer (Shimadzu, Japan). 2.4. Toxicity measurement The potential toxicity of the treated solution was examined by means of luminescent bacteria (Photobacterium phosphoreum T3 spp.) test, following the procedures described by Farre et al. (2008) with modification. The inhibition of the luminescence (%) was calculated after 15 min of bacteria exposure to samples at 15  C with a DeltaTox Analyzer (SDI, USA). Each experiment was repeated three times.

Fenton-like reaction. 3.2. AO 7 Fenton-like degradation The degradation efficiencies of AO7 by different tailings were evaluated and the result was shown in Fig. 1. The efficiencies were increased with the content of iron oxide and cupric oxide, and AO7 was completely degraded within 60 min by tailing E, which contains the highest amounts of iron oxide and cupric oxide. Tailing E is the waste produced in the initial smelting of copper concentrate. Calcium silicate in the tailing E is difficult to separate, so the iron content cannot be further improved through beneficiation step, although it is relatively high (~51.0%). In addition, the metals in tailings are hard to restore in the smelting reduction furnace. Therefore, tailing E is difficult to be used as a raw iron ore material for the secondary resource and has to be treated as waste. It might be used as a catalyst for treating environmental pollutants, however, because of its high content of iron. As shown in Fig. 1, AO7 could be efficiently degraded when tailing E was applied as heterogeneous Fenton-like catalyst, due to the fact that tailing E has the highest iron level among the 5 tested tailings (Table 1). It is worth noting that the catalytic degradation efficiencies of the tailings were positively correlated with iron level in the tailings, but were not consistent with the BET surface areas as shown in Table S1. Georgi and Kopinke (2005) found that the adsorbed fraction on the support was nearly unreactive and sorption on activated carbon had an adverse effect on the oxidation of organic contaminants via $OH. They claimed that the predominant degradation pathway was the attack of $OH species on the organic contaminant fraction that easily dissolved in the aqueous pore amount of activated carbon. The experimental results herein are consistent with their study. Fenton-like catalytic efficiency of tailing E was compared with that of Fe2O3 and Fe3O4 for degradation of AO7. Interestingly, the result showed that the catalytic efficiency of tailing E for degradation of AO7 was higher than Fe2O3 and Fe3O4 (Fig. 2). Obviously, the performance of tailing E was not only related with the content of Fe species. Other metal species, and specifically Cu, could affect the performance of the tailings. These metal species would generate $OH to oxidize AO7 (Kuan et al., 2015). It is proposed that the $OH radicals were mainly generated from the Fenton-like process. First, Fe3þ and Cu2þ ions can be produced from Fe2O3 and CuO. Fe2þ can be generated through the reaction of Fe3þ and H2O2.

3. Results and discussion 3.1. Tailing characterization The component analyses of the studied tailings were shown in Table 1, and the table indicates that Fe, Si and Ca are the three most abundant elements in the tailings, and the converter slag iron tailing (tailing E) contains the highest amount of iron. Also worth noting is these iron tailings all contain some toxic heavy metals. Therefore, it is necessary to investigate whether these toxic elements are released from the tailings into solution in the heterogeneous Fenton-like process. The BET surface area analyses (Table S1, Supporting Information) indicate the studied tailings have very low surface area (0.6e5.7 m2 g1) and their apertures are between 8.5 nm and 22 nm, indicating they are mesoporous materials. The scanning electron microscope (SEM) images of tailing E were displayed in Fig. S1 (Supporting Information). The sizes of tailing particles are quite different with the distribution of <1 mm to >100 mm (Fig. S1a). SEM image (Fig. S1b) clearly demonstrates the tailing particles possess some characteristics of lamellate structure with small interspace among flat pieces, which might result in the absorption of organic compounds and provide the active sites for

Fe3þ þ H2O2 4 Fe2þ þ Hþ þ $OOH

(2)

Then, $OH can be formed via the reaction of Fe2þ (Cuþ) and H2O2, as shown in Eq. (3) and Eq. (4). Fe2þ þ H2O2 4 Fe3þ þ OH þ $OH

(3)

Cuþ þ H2O2 4 Cu2þ þ $OH þ OH

(4)

In addition, Fe2þand Cuþ can be produced from Fe3þ and Cu2þ reacted with $OOH via Eq. (5) and Eq. (6) Fe3þ þ $OOH 4 Fe2þ þ Hþ þ O2

(5)

Cu2þ þ $OOH 4 Cuþ þ O2 þ Hþ

(6)

It is clear that in the presence of H2O2 both Fe and Cu ions would then produce $OH which can degrade organic contaminants (Kuan lu et al., 2015). Furthermore, the EPR technique et al., 2015; Cihanog with DMPO was used to validate the presence of $OH radicals during catalytic degradation of AO7 by tailing E (Fig. S2). The results

J. Zheng et al. / Chemosphere 150 (2016) 40e48

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Table 1 Component analysis of different tailings by XFS (percentage refers to in the oxide form). Component Mine tailing from Dexing (%) (tailing A)

Copper tailing from Wushan Copper-molybdenum tailings from (tailing B) Daye (tailing C)

Copper-iron tailings from Daye Converter slag iron tailing (tailing D) (tailing E)

Fe2O3 SiO2 CaO K2O Al2O3 SO3 P2O5 Cl TiO2 Nd2O3 CuO BaO MnO MoO3 MgO As2O3 Sb2O3 ZnO PbO Pr6O11 Cr2O3 V2O5 ZrO2 CoO Na2O La2O3 SrO

31.6 16.9 25.5 0.607 3.18 18 0.721 0.42 0.279 0.126 0.211 ND 0.357 ND 1.7 ND ND ND ND ND ND ND ND ND ND ND ND

38.8 14 37.5 0.681 2.01 3.85 0.607 0.405 0.2 0.15 0.335 0.618 ND ND 0.364 ND ND ND ND 0.106 ND ND ND 0.113 ND ND ND

a

21.2 45.6 1.79 7.24 12.1 5.34 1.19 0.635 1.43 NDa ND ND 1.3 ND ND 0.988 ND 0.432 0.254 ND ND ND ND ND ND ND ND

34.4 21.9 30.5 1.28 1.96 0.984 0.632 0.538 0.106 0.146 0.32 0.596 ND ND 2.62 ND ND ND ND ND 0.528 ND ND ND 2.51 0.186 0.184

73.3 8.76 3.88 1.7 1.49 0.912 0.804 0.594 0.282 0.251 0.511 0.462 0.417 0.211 0.94 0.129 0.128 3.65 0.331 0.207 0.425 0.11 0.109 ND ND ND ND

ND means the concentration of element is below the detection limit.

Fig. 1. The catalytic degradation of AO7 by the different tailings ([iron ore tailing] ¼ 5 g L1, [H2O2] ¼ 19.8 mM, [AO7] ¼ 30 mg L1, pH ¼ 5.0).

clearly illustrate that the $OH radicals are the active species in the AO7 degradation process. Wu et al. (2012) proposed a process in which electrochemistry was coupled with ferrous ion activation of peroxydisulfate (Fe2þ/ 1 S2O2 8 ) for the decolorization of AO7 (C0 ¼ 35 mg L ). The removal of color was 96.5% after 60 min. The oxidative decolorization of AO7 by Mn oxide containing mine tailings was 42% within 2 h and 91% within 24 h (Clarke and Johnson, 2010). Compared with the other studies, the removal of AO7 in heterogeneous Fenton-like process using iron ore tailing is very rapid and effective. 3.3. Factorial effects of the heterogeneous Fenton-like degradation 3.3.1. Catalyst loading The influence of catalyst loading on heterogeneous Fenton-like

Fig. 2. The catalytic degradation of AO7 by the different catalysts. ([catalyst] ¼ 5 g L1, [H2O2] ¼ 19.8 mM, [AO7] ¼ 30 mg L1, pH ¼ 5.0).

degradation of AO7 by iron ore tailing was investigated with different catalyst dosage. As shown in Fig. 3A, the degradation efficiency of AO7 increased with the catalyst loading increasing from 1 to 5 g L1. This is due to the fact that the active sites of the catalyst greatly increased, which is in favor of the formation of $OH and the degradation of AO7.

3.3.2. H2O2 dosage Because H2O2 is a critical component of Fenton-like reagent which will generate the reactive $OH, the initial H2O2 concentration plays an important role in the AO7 degradation. The degradation of AO7 at pH 5.0 with different H2O2 concentrations was studied and the results were shown in Fig. 3B. The degradation of AO7 increased from 72.2% to 100% when H2O2 concentrations increased from 2.0

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J. Zheng et al. / Chemosphere 150 (2016) 40e48

Fig. 3. The influence of various parameters on the degradation of AO7 in iron ore tailing Fenton-like process (The conditions: tailing E ¼ 5 g L1, [H2O2] ¼ 19.8 mM, [AO7] ¼ 30 mg L1, pH ¼ 5.0; but variations with tailing E amount, [H2O2] concentration, pH value and AO7 level at A, B, C and D, respectively).

to 19.8 mM. Because the AO7 degradation is directly related to the concentration of the $OH produced by the catalytic dissolution of H2O2, more AO7 decomposition is expected with a higher dosage of H2O2. However, when the concentration of H2O2 was 39.6 mM, the degradation of AO7 did not improve and was even reduced because of a scavenging effect on $OH by high concentration of H2O2 (Xue et al., 2009) (Eq. (7) and (8)): $OH þ H2O2 / $OOH þ H2O

(7)

þ $OOH 4 $O 2 þ H

(8)

The oxidation potentials of $OOH and $O 2 are much lower than $OH (He et al., 2002), thus $OOH and $O 2 contribute less to the oxidation of certain organic compounds. Ethanol (EtOH), as a $OH scavenger (Li et al., 2009; Chen et al., 2004), was introduced into the reaction system to capture $OH in the process of AO7 degradation. As shown in Fig. S3, a consequence of addition of EtOH is a sharp decrease in removal of AO7, and the removal efficiency decreased from 100% to zero nearly. According to the above discussion, it is further confirmed that $OH makes a major contribution to degradation of AO7.

3.3.3. Effect of pH Effects of pH on the degradation of AO7 in iron ore tailing Fenton-like process were investigated and the results were presented in Fig. 3C. The pH is very important for the efficiency of Fenton-like reaction. Decoloration of AO7 was completed within 60 min at pH 3.0, 4.0 and 5.0. The rates of reaction were relatively

slow and the conversions of AO7 were 66.7% and 21.6% at pH 6.8 and 8.1, respectively. The degradation of AO7 at pH 3.0 was faster than that at other pH. The result was consistent with previous works in which the reaction efficiencies of the heterogeneous Fenton-like process were highest around pH 3.0 and decreased with the increasing of pH (Hu et al., 2011). In homogenous systems pH z 3 is also known to be the optimal value for organic pollutant degradation by H2O2/iron (Arnold et al., 1995). The higher pH with more OH will move the reaction backwards (Eq. (3)) and reduce the activity of Fenton reagent. However, when the pH is below 3, generation of $OH will be constrained as shown in the following reaction (Eq. (9)) (Lucas and Peres, 2006): H2O2 þ Hþ / H3Oþ 2

(9)

Many researchers reported that the heterogeneous Fenton-like reaction could also be influenced by pH. He et al. (2002) investigated effect of pH on photodegradation of azo dye MY10 in H2O2/aFeOOH system. The degradation efficiency decreased from about 100% to 65% within 120 min with pH increasing from 5 to 9. The present study also found pH can significantly affect the degradation of AO7 in iron ore tailing heterogeneous Fenton-like process. However, it is highly significant that degradation of AO7 was completed within 60 min at pH 5.0 by the iron ore tailing heterogeneous Fenton-like process in comparison with homogenous processes at pH 2.0e4.0.

3.3.4. Initial AO7 concentration Influence of initial concentration on the oxidation removal of

J. Zheng et al. / Chemosphere 150 (2016) 40e48

AO7 was also investigated, since pollutant concentration is an important parameter in wastewater treatment. From Fig. 3D it is possible to see that the extent of degradation decreases with the increase in the initial dye concentration. Increase of dye from 5 to 30 mg L1 decreases the decolorization from the beginning to ~50 min. The increase in dye concentration increases the number of dye molecules and not the $OH radical concentration and so the removal rate decreases, but the $OH is produced by the iron ore tailing heterogeneous Fenton-like process and to react with AO7 and byproducts continuously. Fortunately, the results showed that AO7 at different concentrations generally degraded under tested conditions and completed at the 60 min, which illustrates that the iron ore tailing heterogeneous Fenton-like reaction is efficient enough to decompose 5e30 mg L1 AO7 completely within the proposed time.

3.4. Catalyst stability Evaluating the stability of a heterogeneous catalyst is necessary. The performance of the catalyst may decrease because of the soluble iron species generated by the action of $OH on the catalyst surface. The reusability of the catalyst was evaluated under reaction conditions identical with the first oxidation cycle, but the catalyst was obtained by centrifugation of the tailing E after the Fenton-like reaction. As illustrated in Fig. S4, degradation of AO7 was also completed within 60 min in the tenth oxidation cycle, which illustrates that the tailing E can be reused more than ten times without apparent decrease of the catalytic capacity. The concentrations of the dissolved iron species in the aqueous solution during the degradation were detected by ICP-MS. Table 2 shows the amount of the leached iron species did not increase with the increasing of the reused time, and very low amount of iron species was leached in the catalytic degradation of AO7 by tailing E. The proposed reason for the above phenomenon is because the iron species are recycled directly on the catalyst without significant diffusion in the solution phase in heterogeneous Fenton-like oxidation reaction (He et al., 2002). The amount of other elements leached into the aqueous solution with the reuse of the tailing E was also investigated. As shown in Table 2, toxic elements such as As, Cd, Cr, Pb, etc. were below the limit of quantitation (LOQ) of ICP-MS. Although LOQs are very low, these elements are undetectable due to the fact that they are tightly coupled with the tailing. The amount of Cu leached into the aqueous solution was in the range of 0.003e0.006 mg L1. These results demonstrated the application of the tailings as the Fentonlike catalyst did not bring any secondary pollution and should be safe to the environment.

45

3.5. Possible degradation pathway To investigate the degradation of AO7 in iron ore tailing Fentonlike process, representative UVeVis spectral changes in the aqueous solution as a function of reaction time were recorded (Fig. 4). One main band in the visible region located at 484 nm was attributed to an extended chromophore (Wu et al., 2012), with both aromatic rings connected through the azo bone (Zhang et al., 2008, 2009). The peaks at 310 and 228 nm in ultraviolet region are associated with naphthalene ring and benzene ring structures in the dye molecule (Ji et al., 2009). As the reaction proceeded, the visible band at 484 nm decreased and finally disappeared, indicating at least the azo link was broken down by the reaction. The absorptions at 310 and 228 nm also decreased as the reaction proceeded, indicating the total dye molecule including aromatic rings was broken down. In contrast, the aromatic intermediates could not be further degraded by many other chemical treatments since the peaks at 310 and 228 nm increased with the reaction proceeding (Zhang et al., 2009; Drozd et al., 2012; Chen et al., 2004). No new absorption band appeared in either the visible or the ultraviolet spectral regions during the Fenton-like degradation of AO7, which further illustrates that the iron ore tailing Fenton-like process can rapidly and effectively remove AO7. To further understand the degradation process, GCeMS analysis was employed to identify the intermediate products. As illustrated at Table S2, there are total 19 compounds detected, including pentanoic acid, 4-nitroisoindoline-1,3-dione, naphthalene-1,4dione, 1a,7a-dihydronaphtho[2,3-b]oxirene-2,7-dione, 4-

Fig. 4. UVeVis spectral variations with reaction time.

Table 2 The amount of toxic elements evaluated in the solution (unit, mg L1). LOQ

Al As Ca Cd Co Cr Cu Fe Mn Pb Sr a

0.002 0.001 0.005 0.0005 0.0005 0.0005 0.0005 0.002 0.0005 0.0005 0.0005

Cycle 1th

2th

3th

4th

5th

6th

7th

8th

9th

10th

NDa ND 0.144 ND ND ND 0.004 0.018 0.004 ND ND

ND ND 0.079 ND ND ND 0.003 0.039 0.004 ND ND

ND ND 0.066 ND ND ND 0.006 0.045 0.002 ND ND

ND ND 0.058 ND ND ND 0.003 0.044 0.001 ND ND

ND ND 0.027 ND ND ND 0.003 0.043 0.001 ND ND

ND ND 0.087 ND ND ND 0.003 0.044 ND ND ND

ND ND 0.084 ND ND ND 0.006 0.056 0.001 ND ND

ND ND 0.093 ND ND ND 0.006 0.028 ND ND ND

ND ND 0.084 ND ND ND 0.006 0.037 ND ND ND

ND ND 0.087 ND ND ND 0.006 0.032 ND ND ND

ND means the element level is lower than limit of quantitation.

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J. Zheng et al. / Chemosphere 150 (2016) 40e48

Fig. 5. Proposed degradation pathway of AO7 in iron ore tailing Fenton process.

J. Zheng et al. / Chemosphere 150 (2016) 40e48

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of 81.1% of luminescence. After 2 min treatment in the process, the inhibition of luminescence decreased to 50.2%. The increase of the inhibition ratio was observed after 2 min treatment, in relation to the formation of toxic intermediates such as 4-nitroisoindoline-1,3dione, naphthalene-1,4-dione, catechol, quinol, etc. Catechol and quinol easily transform to 1,2-benzoquinone and 1,4benzoquinone, being more toxic. The toxicity decreases with disappearance of aromatics. At the end of the treatment, the inhibition is lower than 5%, which shows that the solution toxicity is in relation with aromatic compounds and the mineralization leads to the detoxification of treated solution (Hammami et al., 2008). 4. Conclusion

Fig. 6. The removal of TOC and toxicity in iron ore tailing Fenton-like process.

aminobenzenesulfonic acid, benzoic acid, maleic acid, succinic acid, catechol, quinol, glutaric acid, 4-nitroisobenzofuran-1,3-dione, 2hydroxybenzoic acid, isoindoline-1,3-dione, naphthalene-1,3-diol, phthalic acid, 2-hydroxynaphthalene-1,4-dione, naphthalene-1,5diol and naphthalene-1,2,4-triol. The intermediate products clearly demonstrate that the cleavage ofeN]Ne(AO7) was caused by $OH attack, and even the opening of benzene ring and naphthalene ring. Based on frontier orbital theory, chemical reactions preferentially occur at the position where mutual overlap of their frontier orbits is most effective. Nucleophilic reactions may occur at the atom where the electron density of the lowest unoccupied molecular orbit (LUMO) is the largest, whereas electrophilic reactions may happen at the position where the density of the highest occupied molecular orbit (HOMO) is the largest (Zhang et al., 2005). The geometry and Frontier Electron Densities (FED) on AO7 molecule were calculated and shown in Fig. S5 and Table S3. The neucleophilic reaction of AO7 may primarily occur at the N atom bonding with the naphthalene ring according to the electron densities of HOMO and LUMO (Hihara et al., 2003). The value of FED2HOMOþ FED2LUMO (0.425) for the N atom is the maximum among all the atoms of AO7 molecule. Correspondingly, the reactions initiated by $OH should occur at the N atom bonding with benzene ring and at the C atom where the naphthalene ring and the azo group link up, since the electron densities of HOMO at the two positions are about the same (Hihara et al., 2003). Therefore, the oxidative degradation mechanisms of AO7 might be nonexclusive. Based on the intermediate products and the previous studies (Zhang et al., 2009; Zhao et al., 2010; Hammami et al., 2008), a possible pathway for AO7 degradation in the iron ore tailing Fenton-like system was proposed in Fig. 5. The $OH generated in the system attacks eN]Ne bond to decolorize the azo dye in aqueous solution. The AO7 molecule is split to form intermediates containing various oxygenated benzene and naphthalene rings. The intermediate compounds could be further oxidized to open their rings, and finally mineralized to CO2 and H2O. 3.6. TOC removal and toxicity change of AO 7 solution The removal of TOC (Fig. 6) in heterogeneous iron ore tailing system was 58.6% after 60 min reaction. The toxicity of the treated solution in the T-SRO process was also evaluated by luminescent bacteria test and expressed in luminescence inhibition ratio. As shown in Fig. 6, the initial solution of AO7 dye leads to an inhibition

An effective iron ore tailing Fenton-like process was developed to remove AO7 from aqueous solution. AO7 was completely degraded within 60 min when the converter slag iron tailing, containing 73.7% iron oxide and a small quantity of Cu and other metal elements, was used as the catalyst. The iron ore tailing can be reused more than nine times with little iron leaching and without obvious decrease of catalytic activity. In addition, the application of the tailing as the Fenton-like catalyst will not add secondary pollution into the environment, and the toxicity of the treated AO7 solution was reduced effectively. The intermediate products were identified by GC-MS and a possible pathway for AO7 degradation in iron ore tailing Fenton-like system was proposed. The iron ore tailing Fenton-like process can be applied to remove dyestuff from wastewaters cost-effectively. Further study will be proposed to employ the iron ore tailing Fenton-like system for treatment of other organic pollutants, as well as its application to the treatment of real wastewaters. Acknowledgments The authors acknowledge the financial support from National Natural Science Foundation of China (51578279, 50938004), and the Sixteenth Batch of Scientific and Technological Development Plan of Suzhou City (ZXG201441). Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.chemosphere.2016.02.001. References Ali, M.E.M., Gad-Allah, T.A., Badawy, M.I., 2013. Heterogeneous Fenton process using steel industry wastes for methyl orange degradation. Appl. Wat. Sci. 3, 263e270. Arnold, S.M., Hickey, W.J., Harris, R.F., 1995. Degradation of atrazine by Fentons reagent-condition optimization and product quantification. Environ. Sci. Technol. 29, 2083e2089. Bandara, J., Mielczarski, J.A., Kiwi, J., 1999. Photosensitized degradation of azo dyes on Fe, Ti, and Al oxides. Mechanism of charge transfer during the degradation. Langmuir 15, 7680e7687. lu, A., Gündüz, G., Dükkancı, M., 2015. Degradation of acetic acid by hetCihanog erogeneous Fenton-like oxidation over iron-containing ZSM-5 zeolites. Appl. Catal. B Environ. 165, 687e699. Chen, Y.X., Wang, K., Lou, L.P., 2004. Photodegradation of dye pollutants on silica gel supported TiO2 particles under visible light irradiation. J. Photoch. Photobio. A 163, 281e287. Clarke, C.E., Johnson, K.L., 2010. Oxidative breakdown of Acid Orange 7 by a manganese oxide containing mine waste: insight into sorption, kinetics and reaction dynamics. Appl. Catal. B Environ. 101, 13e20. Clarke, C.E., Kielar, F., Johnson, K.L., 2013. The oxidation of acid azo dye AY 36 by a manganese oxide containing mine waste. J. Hazard. Mater. 246e247, 310e318. Clarke, C.E., Kielar, F., Talbot, H.M., Johnson, K.L., 2010. Oxidative decolorization of acid azo dyes by a Mn oxide containing waste. Environ. Sci. Technol. 44, 1116e1122. Coen, J., Smith, A.T., Candeias, L.P., Oakes, J., 2001. New insights into mechanisms of

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