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A combined treatment approach using Fe0/air and Fenton's reagent for the treatment of delay explosive wastewater Bo Lai, Zhao-Yu Chen, Fang Shuping, and Yuexi Zhou Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.5b01360 • Publication Date (Web): 29 Jun 2015 Downloaded from http://pubs.acs.org on July 4, 2015

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Industrial & Engineering Chemistry Research

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3000

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Wavelength (nm)

The operation parameters of 1stFe0/air-Fenton-2ndFe0/air process were optimized, respectively. Then FTIR and EEM fluorescence spectroscopy were used to analyze the decomposition and transformation of the refractory pollutants, which could further confirm the high efficiency of the 1stFe0/air-Fenton-2ndFe0/air process.

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A combined treatment approach using Fe0/air and Fenton's reagent for the treatment of delay explosive wastewater Bo Lai1∗, Zhaoyu Chen1, Shuping Fang2, Yuexi Zhou3 1.Department of Environmental Science and Engineering, School of Architecture and Environment, Sichuan

University, Chengdu 610065, China

2. Chengdu Tianfu New Area Construction Investment Co., Ltd, Chengdu 610094, China

3.Research Center of Water Pollution Control Technology, Chinese Research Academy of Environmental Sciences,

Beijing 100012, China

Abstract: In order to improve the biodegradability of delay explosive wastewater (DEW),

a

combined

Fe0/air

and

Fenton

oxidation

process

(i.e.,

1stFe0/air-Fenton-2ndFe0/air) was developed to decompose the refractory organic pollutants and remove heavy metals (i.e., Pb and Cr) in this wastewater. Effect of the initial solution pH, Fe0 dosage, H2O2 dosage, aeration rate and treatment time on the treatment efficiency of the 1stFe0/air-Fenton-2ndFe0/air process were investigated, respectively. Meanwhile, two control experiments were setup to confirm the synergistic reaction between Fe0/air and Fenton. The results show that the maximum COD removal efficiency was approximately 50.0% under the optimal conditions. Also, Pb, Cr and polyvinyl alcohol (PVA) in the DEW could be completely removed. Therefore, its BOD5/COD ratio was enhanced from 0.20 to 0.56. Finally, FTIR and EEM fluorescence spectroscopy were used to analyze the decomposition and ∗

Corresponding authors. Tel./fax: +86 18682752302 E-mail address: [email protected] (Bo Lai) 1


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transformation of the refractory pollutants, which could further confirm the high efficiency of the 1stFe0/air-Fenton-2ndFe0/air process. Keywords Delay explosive wastewater (DEW); combined treatment approach; Fe0/air; Fenton process; Wastewater treatment 1. Introduction Explosives including 2,4,6-trinitrotoluene (TNT), royal demolition explosive (RDX), high melting explosives (HMX) are widely used for military and civil purpose. As TNT, RDX and HMX are toxic, carcinogenic, and mutagenic, the extensive use of these explosives poses environmental risks1, 2. Currently, zero valent iron (ZVI)3， activated carbon adsorption4, UV/H2O25, US-Fenton6, and other oxidation processes7 have been used to treat these explosive wastewater. In literature, however, the treatment process about delay explosive wastewater (DEW) was rarely reported. Therefore, it is necessary to study characteristics of DEW and develop feasible treatment method. In China, two types of civil delay explosives (silicon and tungsten delay explosives) usually are manufactured and used for detonator production. The main ingredients of silicon delay explosive include silicon powder, lead tetroxide and antimony sulfide, while tungsten delay explosive is mainly comprised of tungsten powder, barium chromate and potassium perchlorate. In addition, ethyl alcohol, acetic acid, polyvinyl alcohol (PVA) and other additive agents were used during the production processes of the two explosives. After these production processes, the

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production devices would be washed and generate washing wastewater. Since the production materials (especially for PVA, Pb, Cr and other unknown heavy metals or organics) are considered as recalcitrant pollutants to receiving water, these washing wastewater require adequate and sufficient treatment before being released back to the environment. In previous studies, UV/H2O28, electro-Fenton9, photo-Fenton10, persulfate11 and electro-coagulation12 usually were investigated to remove PVA in wastewater, and the results suggest that PVA could be decomposed by using advanced oxidation processes (AOPs). Cr(VI) in water and wastewater are commomly removed by absorption13, 14, meanwhile it also could be removed via predominant reduction and auxiliary adsorption when it was treated by nZVI or zero valent iron (ZVI)15, 16. Under acidic and oxic conditions, the dissolved oxygen (DO) can accept the electrons released from the corrosion of Fe0 and be reduced into H2O2 (Eqs. (1) and (2))17,

18

.

Subsequently, H2O2 reacts with Fe2+ and generates hydroxyl radicals (Eqs. (3)) which can non-selectively oxidize or mineralize the organic pollutants. It has been proved that Fe0/air process is much better than ZVI for the mineralization of the organic pollutants. Since Pb, Cr and PVA are the priority pollutants in DEW, therefore, the composed of Fe0/air and Fenton processes might be extremely promising for the treatment of DEW. Fe 0 → Fe 2+ +2e

(1)

O 2 +2H + +2e → H 2O2

(2)

H 2 O 2 +Fe 2+ → Fe3+ + • OH+OH -

(3)

3


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The aim of this work is to evaluate the feasibility of the composed of Fe0/air and Fenton processes on the treatment of a real industrial wastewater generated from an explosive manufacturing plant in China. The overall treatment was designed as a three-step process, 1stFe0/air as step I, Fenton oxidation process as step II, and 2ndFe/air as step III. In addition, effect of different crucial parameters (e.g., initial solution pH, Fe0 dosage, aeration rate, hydrogen peroxide dosage, etc.) of each step on the wastewater treatment will be evaluated, respectively. Furthermore, several control experiments were setup to confirm the superiority of the combined Fe0/air and Fenton processes. Finally, treatment process of DEW was further analyzed by Fourier transform infrared (FTIR) and excitation emission matrix (EEM) spectroscopy.

2. Materials and methods 2.1 Chemicals Micron-scale Fe0 powder, ferrous sulfate and H2O2 (30%, w/v) from Chengdu Kelong Chemical reagent factory were used in this study. The mean particle size of zero valent iron powder is approximately 120 µm, and their iron content reaches approximately 97%. Other chemicals used in this study were of analytical grade. Deionized water was used throughout the whole experiment process.

2.2 Characterization of raw wastewater Delay explosive wastewater (DEW) was obtained from a detonator manufacturing plant in southwest China. Characteristics of DEW were analyzed before the treatment process, and it is a refractory wastewater with high COD (5862±807 mg/L), low BOD5 (1110±15 mg/L) and low BOD5/COD ratio (0.19±0.02). In addition, its low

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biodegradability was mainly resulted from Pb (2.46±1.23 mg/L), Cr (1.61±0.92 mg/L), PVA (9.6±3.2 mg/L) and other unknown pollutants. Its pH was about 6.5. Therefore, it is necessary to remove these pollutants and improve the biodegradability of DEW.

2.3 Experimental procedure As shown in Fig.1, the combined Fe0/air and Fenton oxidation process (i.e., 1stFe0/air-Fenton-2ndFe0/air) included three steps: 1stFe0/air as step I, Fenton oxidation process as step II, and 2ndFe0/air as step III. All experiments were performed in a glass beaker (500 mL) at 30oC by water bath heating, and the slurry was mixed by an overhead stirrer (IKA, R20). First, DEW was treated by 1stFe0/air (i.e., step I), after which the slurry would be settled for 5 min and then taken the supernatant to the step II (i.e., Fenton oxidation process). Furthermore, the effluent of step I was further treated by the step II. Subsequently, the effluent of the step II was treated by the step III (i.e., 2ndFe0/air). Finally, NaOH was added into the effluent to remove Fe2+/Fe3+ by coagulating sedimentation.

Fig.1 Process flow diagram of 1stFe0/air-Fenton-2ndFe0/air and two control experiments

2.3.1 1st Fe0/air (step I) 1stFe0/air (step I) was started when the required Fe0 dosage was added into the beaker containing 400 mL DEW, meanwhile the slurry was stirred by a mechanical stirrer with a desired stirring speed (300 rpm). To obtain optimal operating parameters,

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effect of initial pH (2.0, 2.5, 3.0, 5.0, 7.0), Fe0 dosage (0, 2.5, 5, 10, 20, 50 g/L), aeration rate (0, 0.5, 1.0, 1.5, 2.0 L/min) and treatment time (0-90 min) on the COD removal efficiency of DEW were investigated, respectively. In addition, the initial pH of DEW was adjusted by adding diluted sulfuric acid (10%) or sodium hydroxide solutions (5 mol·L-1). Moreover, air is sparged into the beaker through a micropore plate at its bottom, and its aeration rate was adjusted by using a gas flow meter. Finally, COD of the effluent of each batch experiment was measured after coagulating sedimentation and filtration.

2.3.2 Fenton oxidation process After the treatment of the 1stFe0/air process (step I) under the optimal conditions, its effluent was continuously treated by Fenton oxidation process (step II) to further remove the refractory pollutants. In addition, dissolved Fe2+/Fe3+ generated from the Fe0 corrosion during the 1stFe0/air process could be used to react with H2O2, and generate •OH radical. Furthermore, it is clear that the optimal pH of Fenton reaction is approximately 3.019, so Fenton oxidation process (step II) was performed at pH 3.0. Moreover, effect of H2O2 dosage and treatment time on the pollutants removal of DEW was investigated, respectively.

2.3.3 2nd Fe0/air (step III) The effluent of Fenton oxidation process (step II) was directly treated by 2ndFe0/air process (step III). The operating conditions of 2ndFe0/air process were the same as the optimal conditions of 1stFe0/air (step I). In addition, residual acid and H2O2 in the effluent of Fenton oxidation process (step II) could be further consumed

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and utilized by 2ndFe0/air process (step III). Finally, under the optimal conditions of the three-step process (i.e., combined Fe0/air and Fenton processes), measurements of the treated effluent were routinely taken of COD, BOD5, pH, EEM and FTIR, and residual Pb and PVA were also measured.

2.3.4 Control experiments As shown in Fig.1, in order to investigate the synergistic reaction between Fe0/air and Fenton process, two control experiments were setup: (i) Fe0/air, (ii) Fenton process. The operating conditions (i.e., H2O2 dosage and initial pH) of Fenton control experiment were in accordance with the optimal conditions of Fenton process of the three-step process. Also, its Fe2+ dosage was in accordance with that in the effluent of 1stFe0/air process (step I). In addition, the operating conditions (i.e., initial pH, Fe0 dosage and air flow rate) of Fe0/air were in accordance with the optimal conditions of 1st Fe0/air process. Furthermore, total treatment time of the two control experiments all was 2 h. Finally, COD and BOD5of the effluent were determined, respectively.

2.4 Analytical methods FTIR spectra were obtained in the 4000-400 cm-1 wavelength range by a Perkin Elmer 100 FTIR spectrometer. 1 mg sample (dried and finely ground) and 300 mg KBr (spectrometry grade) were homogenized thoroughly in an agate mortar. The powder mixtures were pressed into pellets prior to analysis20. Fluorescence spectra were conducted on water solution of samples using a Hitachi F-7000 fluorescence spectrometer. Excitation emission matrix (EEM) spectra were detected by setting the excitation (Ex) wavelengths from 200 to 400 nm while the emission (Em)

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wavelengths from 200 to 500 nm. The concentrations of PVA were measured at λmax of 690 nm by the method of Finley16, which is based on the green color produced by the reaction of PVA and iodine in the presence of the boric acid. Total iron, Pb and Cr of the wastewater was measured by atomic absorption spectrophotometer (AA-6300, Shimadzu, Japan). COD and BOD5 of the influent and effluent were analyzed by COD analyzer (Lianhua, China) and BOD5 analyzer (OxiTop IS12, WTW, Germany), respectively. In addition, the remaining hydrogen peroxide should be neutralized before COD and BOD5 measurement. The samples were measured three times to calculate the average value and standard deviation.

3. Results and discussion 3.1 Parameters optimization of 1stFe0/air (step I) In literature, it is reported that initial pH, Fe0 dosage, aeration rate and reaction time usually affect treatment efficiency of Fe0/air process21. In this study, the Fe0/air processes (i.e., step I and step III) were the key of the combined Fe0/air and Fenton oxidation processes, so the operating parameter of the Fe0/air processes should be optimized first.

3.1.1 Effect of initial pH The acidic conditions can enhance the corrosion rate of Fe0, which would release a plenty of electrons and Fe2+. Then, the dissolved oxygen (DO) can accept the released electrons and be reduced into H2O2 (Eqs. (1) and (2))17, 18. Subsequently, H2O2 reacts with Fe2+ and generates •OH radical (Eqs. (3)). In fact, it is a Fenton-like reaction during Fe0/air process.

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Fig.2 Effect of (a) initial pH value;(b) Fe0dosage; (c) aeration rate; (d) reaction time on the treatment efficiencies of 1st Fe0/air process (stirring speed=300 r/min, temperature=25 oC)

Effect of initial pH on COD removal of DEW by Fe0/air was depicted in Fig.2(a). It shows that COD removal efficiency increased from 17.1% to 23.0% when the initial pH decreased from 7.0 to 3.0. And then, the obtained COD removal efficiency only increased a little to 24.5% when the initial pH further decreased from 3.0 to 2.0. The results suggest that the lower initial pH was favor for the treatment of DEW by Fe0/air. Although the excess acid (i.e., initial pH﹤3.0) could enhance the corrosion and consumption of Fe0, DO began to became a new limited factor because there was no enough DO to accept the released electrons. Furthermore, the excess H+ ions present in the solution can deplete

•

OH radical and then limit the removal of

pollutants (Eqs. (4))22. Since the excessive consumption of acid and Fe0 would increase the operating cost, the initial pH of 3.0 was selected in the sequential experiments to investigate the effects of Fe0 dosage, aeration rate and reaction time on the removal of pollutants.

H + + • OH+e → H 2 O

(4)

3.1.2 Effect of initial Fe0 dosage At the optimal initial pH of 3.0, effect of Fe0 dosage on the reduction of contaminants by the 1stFe0/air process was evaluated and shown in Fig. 2(b). It is clear that COD removal efficiency increased from 8.3% to 23.0% when the Fe0 dosage

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increased to 10 g/L. However, the continuous increase of Fe0 dosage could not further improve the removal of pollutants. In particular, only 22.2% COD removal efficiency was obtained when the Fe0 dosage reached 50 g/L. It is well known that the more Fe0 dosage was added, the more active sites on the surface of Fe0 could be obtained, which could improve the treatment efficiency. However, when excess Fe0 dosage was added, other operating conditions including aeration rate, initial pH or reaction time would become the limiting factors. In addition, the excessive Fe0 dosage in the solution would act as

•

OH radical and H2O2 scavenger (Eqs. (5) and (6))23-25.

Furthermore, since the excess Fe0 dosage would lead to the aggregation or overlapping of Fe0 particles, the active sites of the unit mass Fe0 would decrease26. From an economic point of view, optimal Fe0 dosage could not only improve treatment efficiency but also save cost. Therefore, the optimal Fe0 dosage of 10 g/L was chosen for the treatment of DEW in the subsequent experiments.

Fe0 +H 2 O 2 +2H + → Fe2+ + 2H 2 O

Fe0 +2 • OH+H+ → Fe2+ + H 2 O+OH-

(5) (6)

3.1.3 Effect of aeration rate During the Fe0/air treatment process, air flow with different aeration rate was employed to 400 mL DEW which could provide DO and improve the fluidized state of Fe0 particles in solution. Obviously, the increase in DO concentration can facilitate the formation of H2O2 (Eqs. (2)) and enhance degradation of the pollutants17. Meanwhile the fluidized state of Fe0 particles also can affect its treatment efficiency due to the mass transport according to our previous work27. As shown in Fig. 2(c),

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effect of aeration rate on COD removal of DEW by Fe0/air process was evaluated. It is clear that while an increase in aeration rate (＜1.0 L/min) enhanced COD removal efficiency in Fe0/air process, its further increase (＞1.0 L/min) did not obviously improve COD removal. For example, COD removal efficiencies of 12.8%, 22.6% and 23.4% were achieved at aeration rate of 0, 1.0 and 2.0 L/min, respectively. Since DEW contains some volatile organic compounds (i.e., ethyl alcohol, acetic acid, etc), stripping control experiments without Fe0 particles were setup to confirm the treatment efficiency of Fe0/air. Fig. 2(c) shows that COD removal efficiency increased to 9.3% with an increase of aeration rate from 0 to 2.0 L/min during the stripping control experiments. Its removal efficiency was much lower than that of Fe0/air. At aeration rate of 1.0 L/min, for example, its COD removal efficiency was 7.3%, while COD removal efficiency of Fe0/air reached 22.6%. Finally, the optimal aeration rate of 1.0 L/min was chosen for the subsequent experiments.

3.1.4 Effect of reaction time Under the above optimal conditions (stirring rate of 300 rpm, initial pH of 3.0, Fe0 dosage of 10 g/L and aeration rate of 1.5 L/min), DEW was treated 90 min by Fe/air process. Fig. 2(d) shows that COD removal efficiency rapidly increased to 21.1% after only 30 min treatment. However a further increase in treatment time to 90 min did not obviously enhance the pollutants removal. For example, COD removal efficiency obtained at 90 min (25.5%) only increased 4.4% compares with that obtained at 30 min (21.1%). As pH is a crucial factor to the Fe0/air process, in this study the variation of

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solution pH was detected and shown in Fig. 2(d). It is apparent that the pH of solution increased quickly within the first 30 min and then slowed down. During the first 30 min the pH increased from 3.0 to 5.8, after that the pH slightly rose up to 6.4 until 90 min reaction. Note that as the pH increased, the removal of COD slowed down. As indicated in the previous section, lower pH was favor for the treatment of DEW. At higher pH after 30 min treatment, however, the process of hydrolysis of iron ions would led to the deposition of precipitated iron hydroxides on the Fe0 surface, which blocks the further electron transfer process between Fe0 and pollutants and as a result reduces the production of ·OH in the solution18. In addition, when the pH is above 5.0, the overall removal of the pollutants was found to be mainly due to the adsorption/precipitation24. The prolonged treatment time did not significantly increase COD removal of DEW. Therefore, the optimal parameters (i.e., treatment time of 30 min, aeration rate of 1.0 L/min, initial pH of 3.0, Fe0 dosage of 10 g/L and stirring rate of 300 r min-1) of 1st Fe/air process were chosen for the treatment of DEW in the subsequent experiments.

3.2 Parameters optimization of Fenton (step II) After DEW was treated by 1stFe/air process (step I) under above optimal conditions, its effluent was subsequently treated by Fenton process (step II) to further degrade its refractory pollutants. Since it has been reported that the optimal pH of Fenton process is approximate 3.022, the influent pH of step II was hold to 3.0 with diluted sulfuric acid (10%). Finally, the effluent of Fenton process (step II) was directly

treated

by

2ndFe/air

process

(step

III).

12


With

respect

to

the


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1stFe0/air-Fenton-2ndFe0/air process (three-step process), the treatment efficiency of Fenton process would affect the subsequent 2ndFe0/air. Hence, influence of Fenton process on the overall pollutants removal of three-step process was investigated thoroughly.

3.2.1 Effect of H2O2 dosage

Fig.3 Effect of (a) H2O2 dosage and (b) reaction time of Fenton (step II) on COD removal efficiency of 1stFe0/air-Fenton-2ndFe0/air.

Since chemical reagents (i.e., H2O2) are major operational cost item for the wastewater treated by Fenton oxidation process, the effect of H2O2 dosage on the COD removal efficiencies of DEW was investigated significantly. Fig. 3(a) shows that COD removal of the effluent of 1stFe0/air-Fenton-2ndFe0/air process increased from 36.1% to 51.7% when the H2O2 dosage was changed from 0 to 10 mmol/L. Nevertheless, its treatment efficiency would not be obviously improved when H2O2 dosage was above 10 mmol/L. In particular, COD removal efficiency (55.3%) obtained with 50 mmol/L H2O2 only increased 3.6% compares with that (51.7%) of 10 mmol/L

H2O2.

Note

that

COD

removal

efficiency

(51.7%)

of

1stFe0/air-Fenton-2ndFe0/air with 10 mmol/L H2O2 is much higher than that (36.1%) of 1stFe0/air-2ndFe0/air without H2O2. The results indicate that Fenton process is an inseparable part of the three-step process. Moreover, an excess of H2O2 dosage (i.e.,

＞10 mmol/L) would act as

•

OH radical scavenger (Eqs. (7))22, which has adverse

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effect on the treatment of DEW. Therefore, 10 mmol/L H2O2 was selected as the optimal dosage.

H 2O 2 + ⋅ OH → HO2 ⋅+H 2O

(7)

3.2.2 Effect of treatment time Fig. 3(b) shows that influence of the reaction time (i.e., 0-3.0 h) of Fenton process on the overall pollutants removal of three-step process was investigated under initial pH of 3.0 and H2O2 dosage of 10 mmol/L. In addition, both 1stFe0/air and 2ndFe0/air were carried out under above optimal parameters. It could be seen from Fig. 3(b) that the overall COD removal efficiency of the three-step process quickly increased from 34.6% to 50% when the treatment time of Fenton process increased to 1.0 h, while it only increased a little to 52.8% when treatment time further increased to 3.0 h. In other words, the excess treatment time of Fenton process could not efficiently improve the overall COD removal of the three-step process. Since the shorter treatment time can reduce occupied area and save investment costs and, the optimal treatment time of Fenton process was 1.0 h, and the optimal total time of the three-step process was 2.0 h.

3.3 Control experiments Since Fe2+/Fe3+ of Fenton process (step II) was from the corrosion of Fe0 in the 1stFe0/air (step I), the total dissolved iron ions (Fe2+/Fe3+) concentration in the effluent of the 1stFe0/air (step I) should be measured. Fig. 4(a) shows that total dissolved iron ions (Fe2+/Fe3+) concentration of Fe0/air process rapidly increased with the decrease of initial pH because H+ could accelerate the corrosion rate of Fe0. At the optimal initial

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pH of 3.0, the total dissolved iron ions (Fe2+/Fe3+) concentration in the effluent of the 1stFe0/air (step I) was approximate 101.4 mg/L. Thus, the Fe2+ dosage of Fenton control experiment was 101.4 mg/L, its other conditions were H2O2 dosage of 10 mmol/L, initial pH of 3.0 and treatment time of 2.0 h according the above optimal conditions of Fenton process (step II). Meanwhile, the operating conditions of Fe0/air control experiment were initial pH of 3.0, Fe0 dosage of 10 g/L, aeration rate of 1.0 L/min and treatment time of 2.0 h according to the above optimal conditions of 1stFe0/air (step I).

Fig.4 (a) Effect of initial pH on the total dissolved iron concentration in the effluent

of

1stFe0/air

(step

I);

(b)

COD

removal

efficiencies

of

1stFe0/air-Fenton-2ndFe0/air process (optimum conditions), Fenton control (pH=3.0, [Fe2+] = 101.4 mg/L , [H2O2]=10 mmol/L) and Fe0/air control (initial pH=3.0, [Fe0]=10 g/L, aeration rate=1.0 L/min).

Fig.4(b) shows that COD removal efficiency of 1stFe0/air-Fenton-2ndFe0/air process was much higher than those of two control experiments during 2.0 h treatment process. After 2.0 h treatment, in particular, COD removal efficiency of 1stFe0/air-Fenton-2ndFe0/air process (50.0%) was approximate two times of that of Fenton control (16.3%) or Fe0/air control (27.7%). In addition, COD removal efficiency of 1stFe0/air-Fenton-2ndFe0/air process (50.0%) was still higher than the sum (44%) of two control experiments. In 1stFe0/air-Fenton-2ndFe0/air process, Fenton

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process (step II) could use the corrosion products (Fe2+/Fe3+) of Fe0 in the 1stFe0/air (step I) as catalyst. In addition, 2ndFe0/air (step Ⅲ) also could utilize the residual acid and H2O2 of Fenton process (step II). In fact, 2ndFe0/air (step Ⅲ) became a heterogeneous Fe0/air/H2O2 Fenton-like system. Meanwhile, Zhou and his colleagues have found that the treatment efficiency of Fe0/H2O2 Fenton-like system is much higher than that of zero valent iron (ZVI)28. Also, Segura and his colleagues found that the capacity of Fe0/air/H2O2 system is much stronger than that of Fe0/air process25. Therefore, the results confirm the synergistic reaction between Fe0/air and Fenton process. In addition, complete consumption of the residual acid and H2O2 of Fenton process (step II) could avoid the bacteria in the sequent biochemical treatment system to be inhibited by the excess acid and H2O2. Thus, 1stFe0/air-Fenton-2ndFe0/air may be a promising and effective treatment process for refractory industrial wastewater.

3.4 FTIR spectra characteristics In this study, FTIR and EEM spectra were applied to gain a better insight into the transformation characteristics of the organic pollutants in 1stFe0/air-Fenton-2nd Fe0/air process. After the 2.0 h treatment, the two FTIR adsorption spectra of the influent and effluent of the 1stFe0/air-Fenton-2nd Fe0/air process are illustrated in Fig.5. The absorbance bands are interpreted by information from prior reports20, 29-31.

Fig.5 FTIR adsorption spectra of the influent (a) and effluent (b) of 1stFe0/air-Fenton-2ndFe0/air process under the optimal conditions (initial pH=3.0, [Fe0]=10 g/L, aeration rate=1.0 L/min, [H2O2]=10mmol/L, stirring speed=300

16



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

r/min, temperature=25℃ ℃)

Fig.5(a) is the FTIR adsorption spectrum of the influent of the 1stFe0/air-Fenton-2nd Fe0/air process. The band at about 3405 cm-1 is attributed to the overlap of O-H stretching and hydrogen-bonded O-H; the bands at 2923 and 2859 cm-1 are attributed to stretching of CH2; the bands between 1300 and 1800 cm-1 are attributed to the vibration of C=O, C-H, -CH2-, CH3, and aromatic C=C skeletal; the band at about 1114 cm-1 is attributed to the C-O stretching of alcohol or C-O-C stretching of ether or ester.; the band at 873 is attributed to out-of-plane bending of =CH or aromatic =CH. The results imply that the main organic pollutants of the DEW might be the saturated alkane, unsaturated alkane, alcohols, ether, ester, or aromatics, etc. Fig.5(b) is the FTIR adsorption spectrum of the effluent of the 1stFe0/air-Fenton-2nd Fe0/air process. The bands at 2923, 2859 and 873 cm-1 in the effluent disappeared completely. The intensity of the bands between 1300 and 1800 cm-1 decreased obviously, while the intensity of the bands at about 1137 cm-1 enhanced obviously. In addition, a new band at 1677 cm-1 (C=O stretching in carboxyls, acids and ketones) is observed in the effluent. The results suggest that the aromatic pollutants have been completely decomposed, and other organic pollutants are also degraded or transformed. Meanwhile, some smaller molecular compounds (e.g., carboxylic acid and alcohol) are produced after the decomposition of aromatic compounds and macromolecular organic pollutants.

3.5 EEM fluorescence spectra characteristics

17


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The three-dimensional EEM fluorescence spectra of the aromatic pollutants in DEW is shown in Fig.6(a). Two peaks were identified in this study. The first peak was located at the excitation/emission wavelengths (Ex/Em) of 275/305 nm (Peak A), while the second peak was observed at the Ex/Em of 225/305 nm (Peak B). The Peak B was the fluorescence center of the aromatic pollutants, and the peak intensity ratio A/B was 0.81. After 2.0 h treatment by the 1stFe0/air-Fenton-2ndFe0/air process, the three-dimensional EEM fluorescence spectrum of the effluent is illustrated in Fig.6(b). Each EEM gave spectral information about the chemical compositions of dissolved organic matter.

Fig.6 Three-dimensional fluorescence spectra of the influent (a) and effluent (b) of 1stFe0/air-Fenton-2ndFe0/air process under the optimal conditions (initial pH=3.0, [Fe0]=10 g/L, aeration rate=1.0 L/min, [H2O2]=10mmol/L, stirring speed=300 r/min, temperature=25℃ ℃).

After 2.0 h treatment under the optimal conditions, total fluorescence intensity of Peak A and Peak B decreased from 8785 to 1081 a.u., and its removal efficiency reached 87.7%. In addition, the A/B ratio also decreased from 0.81 to 0.72. Furthermore, The location of Peak B of the effluent was blue-shifted by 5 nm along the excitation axis and 10 nm along the emission axis. The location of Peak A of the effluent was blue-shifted by 5 nm along the excitation axis. A blue shift is associated with a break-up of the large molecules into smaller fragments and a reduction of

18



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conjugated bonds in a chain structure or an elimination of particular functional groups including carbonyl, hydroxyl and amine32. The results suggest that all macromolecular and aromatic pollutants in DEW could be decomposed or transformed by the 1stFe0/air-Fenton-2ndFe0/air process.

3.6 Improvement of biodegradability The above results of FTIR and EEM spectra show that the mainly pollutants in DEW could be decomposed or transformed by 1stFe0/air-Fenton-2ndFe0/air process. Therefore, not only did the COD removal efficiency reach 50.0%, but also the B/C ratio increased from 0.20 to 0.56. The improvement of biodegradability was resulted from two aspects, (a) PVA, Pb and Cr in wastewater had been completely removed through reduction, adsorption and co-precipitation by Fe0/air process, (b) other unknown refractory and toxic pollutants might be degraded or transformed. The results also reveal that the combined Fe0/air and Fenton process was an effective pretreatment for DEW.

4. Conclusions This study shows that the developed 1stFe0/air-Fenton-2ndFe0/air process can be considered as an effective solution for the removal of contaminants in delay explosive wastewater. Under the optimal conditions (initial pH of 3.0, Fe0 dosage of 10 g/L, H2O2 dosage of 10 mmol/L, aeration rate of 1.0 L/min, total treatment time of 2.0 h), not only did the COD removal efficiency reach 50.0%, but also the B/C ratio increased from 0.20 to 0.56. The improvement of biodegradability was resulted from the removal PVA, Pb, Cr, aromatics and other refractory pollutants. According the

19


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results of FTIR and EEM spectra, it could be concluded that the macromolecular and aromatic pollutants could be effectively decomposed or transformed by the 1stFe0/air-Fenton-2ndFe0/air process. This study shows that this process can be considered as an effective, robust and feasible pretreatment method for delay explosive wastewater.

Acknowledgement The authors would like to acknowledge the financial support from National Natural Science Foundation of China (No. 21207094), Fundamental Research Funds for the Central Universities (No. 2015SCU04A09) and Special S&T Project on Treatment and Control of Water Pollution (No. 2012ZX07201-005).

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Fig.1 Process flow diagram of 1stFe0/air-Fenton-2ndFe0/air and two control experiments


Page 26 of 31

Page 27 of 31

26 25

24

(a)

(b)

22 20

0

[Fe ]=10 g/L Aeration rate=1.5 L/min Reaction time=60 min

23 22

COD removal (%)

21 20 19

Initial pH=3.0 Aeration rate=1.5 L/min Reaction time=60 min

16 14 12 10

18

8

17 16

18

6 2

3

4

5

6

7

0

10

pH 25

30

(d)

25 0

1 Fe /air (Reaction time=60 min) 0 ([Fe ]=10 g/L, Initial pH=3.0)

10

5

Air stripping (Control experiment) (Reaction time=60 min)

COD removal (%)

20 st

30

40

50

Fe dosage (g/L)

(c)

15

20 0

8

COD removal Variation in solution pH

7

20 6 15 0

5

[Fe ]=10 g/L Initial pH=3.0 Aeration rate=1.0 L/min Reaction time=60 min

10 5

4

0

0 0.0

0.5

1.0

1.5

2.0

Solution pH

COD Removal (%)

24

COD Removal (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


3 0

15

30

Aeration rate (L/min)

45

60

75

90

Time (min)

Fig.2 Effect of (a) initial pH value;(b) Fe0dosage; (c) aeration rate; (d) reaction time on the treatment efficiencies of 1st Fe0/air process (stirring speed=300 r/min, temperature=25 oC)



(a)

55

(b)

50

50

Fenton: initial pH=3.0 reaction time=2 h

45

1stFe0/air: [Fe0]=10 g/L, initial pH=3.0 aeration rate=1.0 L/min reaction time=0.5 h

40

2ndFe0/air: [Fe0]=10 g/L aeration rate=1.0 L/min reaction time=0.5 h

35 0

10

20

30

40

COD removal (%)

55

COD removal (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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Fenton: initial pH=3.0 [H2O2]=10 mmol/L

45

1stFe0/air: [Fe0]=10 g/L, initial pH=3.0 aeration rate=1.0 L/min reaction time=30 min

40

2ndFe0/air: [Fe0]=10 g/L aeration rate=1.0 L/min reaction time=30 min

35

50

H2O2 dosage of Fenton (mmol/L)

30

0.0

0.5

1.0

1.5

2.0

2.5

3.0

Reaction time of Fenton (h)

Fig.3 Effect of (a) H2O2 dosage and (b) reaction time of Fenton (step II) on COD removal efficiency of 1stFe0/air-Fenton-2ndFe0/air.


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(a)

400 Total dissolved iron [Fe0]=10 g/L Aeration rate=1.0 L/min Reaction time=30 min Stirring rate=300 r/min

300

200

1stFe0/air (0.5 h)-Fenton (1 h)-2ndFe0/air (0.5 h) Fe0/air control (2.0 h) Fenton control (2.0 h)

50

(b)

40

COD removal (%)

Total dissolved Fe (mg/L)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


100

0

30 20 10 0

2

3

4

5

6

7

0.0

0.5

Initial pH

1.0

1.5

2.0

Time (h)

Fig.4 (a) Effect of initial pH on the total dissolved iron concentration in the effluent of 1stFe0/air (step I); (b) COD removal efficiencies of 1stFe0/air-Fenton-2ndFe0/air process (optimum conditions), Fenton control (pH=3.0, [Fe2+] = 101.4 mg/L , [H2O2]=10 mmol/L) and Fe0/air control (initial pH=3.0, [Fe0]=10 g/L, aeration rate=1.0 L/min).



4000

458 464

626 615

669

1114

1384

1137

3552

1621

1677

3405

(b)

711

873 1620

2117

(a)

1300-880

1803

2923 2859

2516

1850-1300

3411

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 30 of 31

3500

3000

2500

2000

1500

1000

500

Wavelength (nm) Fig.5

FTIR

adsorption

spectra

of

the

influent

(a)

and

effluent

(b)

of

1stFe0/air-Fenton-2ndFe0/air process under the optimal conditions (initial pH=3.0, [Fe0]=10 g/L,

aeration

rate=1.0

L/min,

[H2O2]=10mmol/L,

stirring

temperature=25℃ ℃)


speed=300

r/min,

Page 31 of 31

400

400

(a)

380

(b)

0

380

312.5

0 312.5 625.0

625.0

360

360

1125

1125 1938

1938

340

340

2750

2750 3563

3563

320

320

4375 5188

300

6000

280

A

Ex (nm)

Ex (nm)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


4375 5188

300

6000

280 260

260 240

240

B 220 200 200

220

250

300

350

400

450

500

200 200

250

300

350

400

450

500

Em (nm)

Em (nm)

Fig.6 Three-dimensional fluorescence spectra of the influent (a) and effluent (b) of 1stFe0/air-Fenton-2ndFe0/air process under the optimal conditions (initial pH=3.0, [Fe0]=10 g/L,

aeration

rate=1.0

L/min,

[H2O2]=10mmol/L,

stirring

temperature=25℃ ℃). .


speed=300

r/min,

(PDF) A Combined Treatment Approach Using

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