Treatment of ultra-high concentration 2-diazo-4,6-dinitrophenol (DDNP) industry wastewater by the combined Fe/Cu/air and Fenton process

Abstract

Treatment of 2-diazo-4,6-dinitrophenol (DDNP) industry wastewater by 1^stFe/Cu/air–2^ndFenton–3^rdFe/Cu/air was studied to degrade the toxic refractory pollutants and improve the biodegradability. Three control experiments (i.e., 1^stFe⁰/air–2^ndFenton–3^rdFe⁰/air, Fe/Cu/air, and Fenton) were set up to confirm the superiority of 1^stFe/Cu/air–2^ndFenton–3^rdFe/Cu/air and the synergistic reaction between Fe/Cu/air and Fenton process. Furthermore, the key operating parameters including initial pH (1.5–7.0), Fe/Cu dosage (5–50 g L⁻¹), aeration rate (0–2.0 L min⁻¹), reaction time (0–180 min) and H₂O₂ dosage (0–40 mmol L⁻¹) were optimized, respectively. The results showed that high COD removal (87.1%), decolority (99.9%), DDNP removal (100%) and B/C ratio (0.58) was obtained by 1^stFe/Cu/air–2^ndFenton–3^rdFe/Cu/air process. It has a higher treatment efficiency than 1^stFe⁰/air–2^ndFenton–3^rdFe⁰/air due to the high reactivity Fe/Cu bimetallic particles. Therefore, the developed method in this study is a promising process for treatment of DDNP industry wastewater. Finally, the analysis results of UV-vis and FTIR reveal that the main groups (e.g., benzene ring, nitro and azo groups) of the pollutants could be decomposed effectively after 4.5 h treatment by 1^stFe/Cu/air–2^ndFenton–3^rdFe/Cu/air. The results also further confirm the superiority of 1^stFe/Cu/air–2^ndFenton–3^rdFe/Cu/air and the synergistic reaction between Fe/Cu/air and Fenton process. Thus, it is a promising technology for the treatment of ultra-high concentrated DDNP industry wastewater.

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1. Introduction

2-Diazo-4,6-dinitrophenol (DDNP), which contains nitro (–NO₂) and diazo groups (–N=N–) in a benzene ring, has been widely used in military and commercial detonators as an efficient primary explosive.^1–3 DDNP is a green energetic compound without lead, and its good energy capability is comparable to that of high explosives such as octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine (HMX) and hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX).^4,5 In addition, DDNP with good flowability and controlled bulk density (0.65–0.95 g cm⁻³) has been prepared at factory scale by the modified method using 4-methylphenol as crystal control ingredient.¹ However, about 200 kg wastewater will be produced by preparing 1 kg DDNP.⁶ In particular, impurity, by-products and some residual raw materials in DDNP product must be removed by water scrubbing, which would cause a toxic, carcinogenic, refractory, and high colored DDNP wastewater. If DDNP wastewater was released into environment without treatment, it would pose a severe threat to human beings and wildlife.

In the literature, there are some studies on the treatment process of DDNP wastewater, such as supercritical water oxidation, electro-catalytic oxidation, Fenton oxidation, and adsorption resin.⁶ But they are not the cost-effective and robust processes for the DDNP wastewater. In our previous work, a combined ZVI and Fenton process had been developed to decompose or transform the toxic refractory pollutants in DDNP wastewater.⁷ The results show that BOD₅/COD ratio (0.27), COD (78%) and chromaticity (98%) removal efficiencies were obtained under the optimal conditions, when the DDNP wastewater (COD of 1250 ± 100 mg L⁻¹, colority of 12 500 ± 700 times) was treated by the combined ZVI and Fenton process. But the improvement of biodegradability was not enough for the subsequent biological treatment due to the lower B/C ratio (<0.30). In addition, the higher concentrated DDNP wastewater might not be treated effectively by the combined ZVI and Fenton process. Thus, it is necessary to develop a more effective process for the treatment of DDNP wastewater.

Meanwhile, there is no report on the treatment of ultra-high concentration 2-diazo-4,6-dinitrophenol (DDNP) industry wastewater, which contains high concentrated DDNP and other unknown pollutants (e.g., by-products). In addition, DDNP with a benzene ring, an azo group (–N=N–) and two nitro groups (NO₂–) was a special, toxic and refractory pollutant. This special pollutant was worth to be investigated thoroughly. Furthermore, this actual wastewater also contained some unknown surfactants, which would cause to form a large number of bubbles when the raw wastewater was treated under the aeration condition (see Fig. 1(a and b)). However, the surfactants in wastewater would be like a barrier to hinder the oxygen mass transfer at the interface. Moreover, there are many reports about the degradation of military explosives in wastewater (e.g., TNT, RDX and HXM),^8,9 but the treatment of civil explosive (i.e., DDNP) in wastewater was very few to be reported. Finally, the primary purpose of this study is to develop high effective process to treat the actual wastewater.

Fig. 1 Production and control of foam during the wastewater treatment process. (a) Plenty of bubbles generated after only 5 s aeration treatment (air flow rate of 1.5 L min⁻¹), (b) plenty of foam also generated after only 5 s treatment by Fe/Cu/air process (air flow rate of 1.5 L min⁻¹, Fe/Cu dosage of 40 g L⁻¹), (c) aeration was start after 60 min pretreatment by Fe/Cu process, and then only a little of foam was generated.

At present, a great deal of interest was focused on employment of Fe/Cu bimetallic particles, which are prepared by planting Cu on the surface of zero valent iron (Fe⁰) and have a much higher reactivity than Fe⁰.^8,10–14 Table 1 outlines the advantages of using Fe/Cu in place of Fe. In brief, because of the high reaction potential of 0.78 V between Cu and Fe,¹⁵ the corrosion of Fe⁰ could be improved by the planting Cu, which resulting in the higher reactivity of Fe/Cu bimetallic particles and higher rate of pollutants reduction.^15–17 Moreover, the Fenton-like reaction would be formed in Fe/Cu bimetallic system under oxic conditions.^11,18 In Fe/Cu bimetallic particles system, the chemical reactions occurring can be summarized in the following equations:

Table 1 Advantages of using Fe/Cu/air in place of Fe towards degradation of organics under the under oxygen environment

Factor	Fe^0/air	Fe/Cu/air	Reference
Kobs (min^−1)	0.011	0.113	18
Reaction equations	Fe^0 → Fe^2+ + 2e−, O2 + 2H^+ + 2e^− → H2O2, H2O2 + Fe^2+ → Fe^3+ + OH˙ + OH^−	, H2O2 + Fe^2+ → Fe^3+ + OH˙ + OH^−	39
Mechanism of iron corrosion	Spontaneous chemical oxidation by dissolved oxygen	Spontaneous chemical oxidation by dissolved oxygen and high reaction potential of 0.78 V between the planting Cu and iron	11 and 40

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Acidic without oxygen:

Acidic with oxygen:

H₂O₂ + Fe²⁺ → Fe³⁺ + OH˙ + OH⁻ (3)

In this study, to develop a highly efficient treatment process for the ultra-high concentrated wastewater from DDNP manufacturing (see Table S1, ESI†), a combined Fe/Cu/air and Fenton process (i.e., 1^stFe/Cu/air–2^ndFenton–3^rdFe/Cu/air) was set up and investigated thoroughly. In addition, a special pretreatment step (Fe/Cu process without aeration) for the removal of surfactants was performed before the subsequent aeration treatment (i.e., Fe/Cu/air process). So this combined process was different from the previous process due to the complex actual wastewater (i.e., DDNP industry wastewater).¹⁹ Besides Fe/Cu bimetallic particles, other parameters or conditions were similar to those of the combined Fe⁰/air and Fenton process in our previous work.⁷ Due the change of the key materials (i.e., Fe⁰ was replaced by Fe/Cu bimetal), the operating parameters including initial pH, Fe/Cu dosage, air aeration, treatment time, and H₂O₂ dosage should be optimized again. Otherwise, three control experiments (i.e., combined Fe⁰/air and Fenton, Fe/Cu/air, and Fenton) were setup to prove the superiority and synergistic reaction between Fe/Cu/air and Fenton. Finally, under the optimal operating conditions, the degradation and transformation of the pollutants in DDNP industry wastewater were analyzed and evaluated by COD (Chemical Oxygen Demand), BOD₅ (Determination of Biochemical Oxygen Demand after 5 days), BOD₅/COD ratio, colority, UV-vis and Fourier transform infrared (FTIR) spectrometry.

2. Experimental

2.1. Materials

CuSO₄·5H₂O (analytical reagent), zero valent iron (Fe⁰) particles, hydrogen peroxide (H₂O₂, 30% v/v), sulfuric acid (H₂SO₄, 98%, analytical grade) and sodium hydroxide (NaOH, analytical grade) purchased from Chengdu Kelong chemical reagent factory were used in the experiments. The Fe⁰ powders have a mean particle size of approximately 120 μm, and their iron content is above 98%. The Fe/Cu bimetallic particles were prepared through displacement plating Cu on the surface of Fe⁰. The preparation process and the optimal theoretical Cu mass loading (0.41 g Cu/g Fe) was the same as that in our previous work.²⁰ Deionized water was used throughout the whole experiment process.

2.2. Characteristics of DDNP industry wastewater

DDNP industry wastewater used in this study was obtained from a civilian explosive industry in northeast China. As listed in Table S1 (ESI†), COD, TOC, DDNP, pH, and colority of this raw wastewater were about 4740 mg L⁻¹, 1022 mg L⁻¹, 3131 mg L⁻¹, 4.8 and 50 000 times, and its concentration was about four times of the DDNP wastewater used in our previous work.⁷ The significant increase of wastewater concentration might be resulted from the change of operating parameters during the DDNP manufacturing process. Therefore, the ultra-high concentration DDNP wastewater was hard to be treated effectively by the 1^stFe⁰/air–2^ndFenton–3^rdFe⁰/air process, and it should be treated by using the Fe/Cu bimetal with high reactivity.

2.3. Batch experiments

Treatment of DDNP industry wastewater by combined Fe/Cu/air and Fenton process was carried out by batch experiments. The combined Fe/Cu/air and Fenton process was designed as a three-step process, (a) 1^stFe/Cu/air, (b) 2^nd Fenton oxidation process, (c) 3^rdFe/Cu/air, which was similar to our previous treatment work for the low concentration DDNP wastewater by 1^stFe⁰/air–2^ndFenton–3^rdFe⁰/air.⁷ In this study, there were two differences, (a) Fe⁰ was replaced by Fe/Cu bimetal with higher reactivity, (b) in the 1^stFe/Cu/air process, initial 1.0 h without aeration, and then aeration treatment. In other words, the 1^stFe/Cu/air process was composed of Fe/Cu process (initial 1.0 h, without aeration phase) and Fe/Cu/air process (aeration phase).

First, the wastewater was treated by Fe/Cu without air aeration, and then it began to be continuously treated by Fe/Cu with air aeration (i.e., 1^stFe/Cu/air process with two different operating phases). After the treatment by 1^stFe/Cu/air process, wastewater and residual Fe/Cu bimetallic particles were separated through 5 min precipitation. Furthermore, the separated effluent of 1^stFe/Cu/air process was treated by 2^ndFenton process. Finally, the effluent of 2^ndFenton process was further treated by 3^rdFe/Cu/air process. In addition, the whole experiment process was carried out in a 500 mL beaker and realized at 25 ± 2 °C by water batch heating, and the slurry was mixed by a mechanical stirrer (300 rpm). The NaOH and polyacrylamide (PAM) were added into the samples to remove iron corrosion products (e.g., Fe²⁺/Fe³⁺) by coagulation and flocculation.

During the 1^stFe/Cu/air batch experiments, effect of initial pH (1.5–7.0), Fe/Cu dosages (5–50 g L⁻¹), air flow rate (0–2.0 L min⁻¹) and treatment time (0–180 min) on the COD removal of DDNP industry wastewater were investigated, respectively. And then, effect of H₂O₂ dosage (0–40 mmol L⁻¹) in 2^ndFenton process on the COD removal of the whole combined process (1^stFe/Cu/air–2^ndFenton–3^rdFe/Cu/air) was investigated thoroughly. In addition, the operating conditions of 3^rdFe/Cu/air process were same to the optimal conditions obtained in 1^stFe/Cu/air batch experiments, but its whole process was carried out under the optimal air aeration conditions. Finally, under the optimal conditions (see Table S2, ESI†) of the combined process, measurements of the treatment effluent were routinely taken of COD, BOD₅, DDNP concentration, pH, colority, UV-vis and FTIR.

2.4. Control experiments

To confirm the superiority and synergistic reaction of combined Fe/Cu/air and Fenton process, three control experiments including (a) combined Fe⁰/air and Fenton, (b) Fe/Cu/air and (c) Fenton were setup. The operating conditions of combined Fe/Cu/air and Fenton process, and other three control experiments were listed in Table S2 (ESI†). During the whole treatment process, COD, BOD₅, DDNP concentration, pH, colority, and UV-vis of the obtained samples were analyzed, respectively.

2.5. Analytical method

Fourier transform infrared spectroscopy (FTIR) was used to assess the differences in the general functional groups of the influent and effluent of the combined Fe/Cu/air and Fenton process under the optimal conditions. The samples were analyzed by using a Perkin Elmer 100 FTIR spectrometer, and the samples were scanned four times between the wavelengths of 4000 to 400 cm⁻¹.⁷ UV-vis absorption spectra of the samples were carried out in 10 mm quartz cuvettes, and they were recorded from 190 to 500 nm. DDNP concentrations in the samples were achieved by reversed high-performance liquid coloritytography (HPLC) coloritytography (Agilent USA).⁷ Total iron ion concentration of the effluent of 1^stFe/Cu/air process was analyzed by using an inductively coupled plasma mass spectrometry (ICP-MS) (VG PQ ExCell, Thermo Elemental, America). The pH, BOD₅ and COD of the samples were determined by using pHS-3C meter (Rex, China), BOD₅ analyzer (OxiTop IS12, WTW, Germany) and COD analyzer (Lianhua, China), respectively. Colority of the samples was measured by using dilution multiple method.

3. Results and discussion

3.1. 1^stFe/Cu/air process

In literature, it is clear that dissolved oxygen (DO) could facilitate the formation of Fenton-like reaction in Fe/Cu or Fe⁰ system, which would significantly improve the degradation or mineralization of the toxic and refractory pollutants.^18,21–23 However, a large number of bubbles would be formed quickly when the raw wastewater was treated under the aeration condition (see Fig. 1(a and b)), which might be resulted from some surfactants in DDNP industry wastewater. In addition, surfactants in wastewater would be like a barrier to hinder the oxygen mass transfer at the interface.²⁴ Thus, it is necessary to remove these surfactants before the subsequent aeration treatment.

In particular, the raw wastewater first treated by Fe/Cu process without aeration, and then its effluent was further treated by Fe/Cu/air process with aeration. Fig. 1(c) shows that after 1 h treatment by Fe/Cu process without aeration, there is only a few of bubbles generated in the subsequent aeration treatment by Fe/Cu/air process. The results suggest that the surfactants in raw wastewater could be removed effectively by Fe/Cu process without aeration. In Fe/Cu system, the removal of surfactants was mainly resulted from adsorption (e.g., Fe(OH)₂/Fe(OH)₃), reduction (e.g., Fe²⁺ or H˙)²⁵ or oxidation (i.e., Fenton-like reaction formed due to some DO from air through mechanical agitation).¹⁰ The similar results have been reported in literature, the surfactants in wastewater could be removed effectively by adsorption,^1,26 combined chemical coagulation–flocculation and ultraviolet photolysis process,²⁷ electro-coagulation and electro-Fenton.²⁸

After the removal of surfactants, the residual pollutants would be further decomposed by the subsequent Fe/Cu/air process. In other words, the 1^stFe/Cu/air process was composed of Fe/Cu (1 h, without aeration) and subsequent Fe/Cu/air (aeration). To obtain a higher degradation efficiency for the pollutants, effect of initial pH, Fe/Cu dosages, air flow rate and treatment time on the COD removal by 1^stFe/Cu/air process would be investigated, respectively.

(1) Effect of the initial pH on COD removal. Fig. 2(a) shows effect of initial pH (i.e., 1.5–7.0) on COD removal efficiencies of DDNP industry wastewater when it was treated by 1^stFe/Cu/air process. The other operating conditions were Fe/Cu dosage of 40 g L⁻¹, aeration rate of 1.5 L min⁻¹ (1.0–1.5 h), stirring speed of 300 rpm, and treatment time of 1.5 h. Obviously, the lower pH is beneficial to the degradation of DDNP industry wastewater. When the initial pH decreased from 7.0 to 2.0, the COD removal efficiency was enhanced rapidly from 11.5% to 68.4%. And then, it only further increased a little to 68.8% when the initial pH decreased from 2.0 to 1.5. Therefore, 2.0 should be chose as the optimal initial pH according to the consumption and cost of acid.

Fig. 2 Effect of (a) initial pH, (b) Fe/Cu dosage, (c) aeration and (d) treatment time on COD removal of DDNP industry wastewater by 1^stFe/Cu/air.

In the 1^stFe/Cu/air process, the contaminants removal was mainly resulted from adsorption, reduction and oxidation. In particular, adsorption and reduction played a leading role in the pollutants removal in the initial 1.0 h (without aeration, Fe/Cu); and then it was mainly attributed to the Fenton-like reaction (with aeration, Fe/Cu/air). In this process, the lower initial pH would enhance the corrosion rate of Fe⁰, which would improve the release rate of Fe²⁺/Fe³⁺ and electrons. In particular, effect of the lower initial pH could be explained from 3 aspects, (a) the increase of corrosion products (e.g., Fe(OH)₂/Fe(OH)₃) would enhance the pollutants removal by adsorption, (b) the pollutants was easy to be indirectly reduced by the generated H˙ under the acidic and anoxic conditions (eqn (1)),²⁵ (c) the pollutants also could be reduced directly through accepting the electrons at the catalytic activity sites (i.e., Cu⁰ on Fe⁰ surface).^10,12 However, the excess low initial pH (i.e., <2.0) could not significantly improve the pollutants removal because it was not a limiting factor. In addition, the excess H⁺ only could cause the excessive consumption of Fe/Cu bimetallic particles, which would increase the operating cost obviously. Therefore, the optimal initial pH of 2.0 was selected for the subsequent experiments.

(2) Effects of Fe/Cu dosages on COD removal. Effect of Fe/Cu dosage (i.e., 5–50 g L⁻¹) on COD removal efficiency of DDNP industry wastewater was evaluated thoroughly. The other operating conditions were initial pH of 2.0, stirring speed of 300 rpm, air flow rate of 1.5 L min⁻¹ (1.0–1.5 h) and treatment time of 1.5 h. Fig. 2(b) indicates that an increase in Fe/Cu dosage obviously enhanced COD removal efficiency of DDNP industry wastewater. In particular, COD removal efficiencies rapidly increased from 13.6% to 68.4% with the increase of Fe/Cu dosage from 5 to 40 g L⁻¹, while it increased slightly when the Fe/Cu dosage was above 40 g L⁻¹.

The results can be explained from 2 aspects, (a) the total number of adsorption sites, catalytic sites and galvanic couples increase seriously with increasing Fe/Cu dosage, which could improve the pollutants removal; (b) when excess Fe/Cu dosage was added in reactor (e.g., >40 g L⁻¹), the pollutants removal efficiency would be limited by other factors, especially for the mass transportation rate of pollutants, intermediates, corrosion products at the interface.²⁹ The similar phenomenon also occurred in our previous work and other experimental results.^7,11,30 Therefore, a Fe/Cu dosage of 40 g L⁻¹ was selected in the sequential experiments to investigate the effect of air flow rate and treatment time on the pollutants removal.

(3) Effect of air flow rate on COD removal. Effects of air flow rate (i.e., 0–2.0 L min⁻¹) on COD removal efficiency of DDNP industry wastewater by 1^stFe/Cu/air were evaluated. The operating conditions were Fe/Cu dosage of 40 g L⁻¹, initial pH of 2.0, stirring speed of 300 rpm, and treatment time of 1.5 h (i.e., initial 1.0 h without aeration, and then aeration treatment of 0.5 h). Fig. 2(c) shows that an increase in the air flow rate from 0 to 1.5 L min⁻¹ (at 1.0–1.5 h) made the COD removal efficiency improve from 60.0% to 68.4%. However, a further increase in air flow rate to 2.0 L min⁻¹ did not significantly enhance the contaminants removal. In Fe/Cu/air system, DO was a key limiting factor for the generation of H₂O₂ that would trigger Fenton-like reaction (eqn (2) and (3)) and produce strong oxidants such as hydroxyl radical.³¹ In addition, COD removal efficiency obtained by Fe/Cu without aeration (0–1.5 h) still reached 60.0%, which mainly attributed from 3 aspects: (a) adsorption of micro-size Fe/Cu bimetallic particles and their corrosion products (e.g., Fe(OH)₂/Fe(OH)₃); (b) some pollutants would be reduced by Fe/Cu bimetallic particles or their products (e.g., Fe²⁺ or H˙), (c) some oxygen in air could be dissolved into solution by mechanical agitation (300 rpm) which could facilitate the formation of Fenton-like reaction.

Thus the optimal air flow rate of 1.5 L min⁻¹ (at 1.0–1.5 h) was selected in the following experiments to investigate the treatment time on the pollutants removal.

(4) Effects of treatment time on COD removal. Under the above optimal conditions (initial pH of 2.0, Fe/Cu dosage of 40 g L⁻¹, air flow rate of 1.5 L min⁻¹), effect of treatment time on COD removal of the DDNP industry wastewater was evaluated. Fig. 2(d) shows that COD removal efficiency reached 58.9% after 60 min treatment by Fe/Cu without aeration, and then it did not increase with the increase of treatment time (60–180 min) if there was no aeration during the whole treatment process. However, COD removal efficiency could be further increased to 68.4% during the subsequent 30 min aeration treatment (i.e., at 60–90 min, air flow rate of 1.5 L min⁻¹). In addition, the further increase of aeration treatment time still could not improve the COD removal (i.e., 90–180 min). Therefore, the optimal treatment time of 1^stFe/Cu/air process was 90 min (i.e., 0–60 min: without aeration; 60–90 min: air flow rate of 1.5 L min⁻¹).

3.2. Effects of H₂O₂ dosages on the total COD removal efficiency of 1^stFe/Cu/air–2^ndFenton–3^rdFe/Cu/air

When DDNP industry wastewater was treated by 1^stFe/Cu/air process under the above optimal conditions, its effluent was continuously treated by Fenton process to further decompose its toxic and refractory pollutants. In addition, the initial pH of 2^ndFenton process was adjusted to 3.0 by adding diluted sulfuric acid (30%),^32,33 and its treatment time was 2 h. Finally, the effluent of 2^ndFenton process was directly treated by 3^rdFe/Cu/air process, and its operating conditions were initial pH of 3.0, Fe/Cu dosage of 40 g L⁻¹, air flow rate of 1.5 L min⁻¹, and treatment time of 1 hour.

Effects of the H₂O₂ dosages (i.e., 0–40 mmol L⁻¹) on the total COD removal efficiency of the 1^stFe/Cu/air–2^ndFenton–3^rdFe/Cu/air process were investigated in the following experiments. Fig. 3 shows that the total COD removal was quickly increased from 71.0% to 87.1% with the increasing of H₂O₂ dosage from 0 to 10 mmol L⁻¹, and then it only increased a little when the H₂O₂ dosage was above 10 mmol L⁻¹. Thus the optimal operating condition of Fenton process was 10 mmol L⁻¹ H₂O₂. With 2.6 mmol L⁻¹ iron ions (the total iron ion concentration of the effluent of 1^st Fe/Cu/air process was 890.1 mg L⁻¹, which contained 145.6 mg L⁻¹ ferrous ions and 744.5 mg L⁻¹ ferric ions) and 10 mmol L⁻¹ H₂O₂, homogeneous Fenton system was to be formed in the second stage, and the probable reactions were presented as following:³⁴

H₂O₂ + Fe²⁺ → Fe³⁺ + OH˙ + OH⁻ (4)

H₂O₂ + OH˙ → HO₂˙ + H₂O (5)

Fe²⁺ + OH˙ → Fe³⁺ + OH⁻ (6)

HO₂˙ + Fe³⁺ → Fe²⁺ + O₂ + H⁺ (7)

HO₂˙ + Fe²⁺ → Fe³⁺ + HO₂⁻ (8)

Fig. 3 Effect of H₂O₂ dosage of Fenton process on COD removal of DDNP industry wastewater by 1^stFe/Cu/air–2^ndFenton–3^rdFe/Cu/air.

Eqn (4) is the initiation reaction. When one of the reactants (H₂O₂ of Fe²⁺) was overdosed, OH˙ could be depleted through the scavenging of OH˙ by eqn (5) and (6). The scavenging effect will decrease the organics removal rate. The ferric ions would be reducing to ferrous ions by eqn (7). As a result, corrosion products (Fe²⁺/Fe³⁺) of Fe/Cu bimetallic particles from the 1^stFe/Cu/air process could be used as catalyst for the subsequent 2^ndFenton reaction.

Meanwhile the residual H⁺ and H₂O₂ of Fenton process could be completely consumed and utilized by the subsequent 3^rdFe/Cu/air process. In other words, it was combined Fenton and Fenton-like reaction, which could improve the degradation of the toxic refractory pollutants. The similar case had been found when the combined Fe⁰/air and Fenton process was used to treatment a lower concentrated DDNP wastewater (COD = 1250 ± 100 mg L⁻¹) in our previous work.⁷

3.3. Control experiments

To investigate the high reactivity of Fe/Cu bimetallic particles and the synergistic reaction between Fe/Cu/air and Fenton process, three control experiments were setup, (a) combined Fe⁰/air and Fenton, (b) Fe/Cu/air and (c) Fenton. The operating conditions of three control experiments were listed in Table S2 (ESI†), which were similar to the optimal conditions of 1^stFe/Cu/air–2^ndFenton–3^rdFe/Cu/air.

Fig. 4(a) shows the colority removal during the whole treatment process (4.5 h) by 1^stFe/Cu/air–2^ndFenton–3^rdFe/Cu/air. In particular, the colority was rapidly decreased from 50 000 to 16 000 times after 1.5 h treatment by the 1^stFe/Cu/air process (0–1.0 h, without aeration; 1.0–1.5 h, air flow rate of 1.5 L), and then it further decreased to 2000 times after 2.0 h treatment by the 2^ndFenton process. Finally, the colority decreased to about 50 times after the 3^rdFe/Cu/air process. The results indicates that the ultra-high colority problem of DDNP wastewater could be resolved effectively by 1^stFe/Cu/air–2^ndFenton–3^rdFe/Cu/air. In addition, Fig. 4(b) shows that decolorization rate of 1^stFe/Cu/air–2^ndFenton–3^rdFe/Cu/air (99.9%) was much higher than that of 1^stFe⁰/air–2^ndFenton–3^rdFe⁰/air (60.2%), Fe/Cu/air control (76.1%) or Fenton control (40.5%).

Fig. 4 Colority removal of the DDNP wastewater by different treatment processes, (a) changes of colority during 4.5 h treatment process by the combined Fe/Cu/air and Fenton, (b) colority removal after 4.5 h treatment by the different processes (experiment conditions were listed in Table S2, ESI†).

Also, Fig. 5(a) shows that COD removal efficiency obtained by the 1^stFe/Cu/air–2^ndFenton–3^rdFe/Cu/air process (87.1%) was much higher than that of 1^stFe⁰/air–2^ndFenton–3^rdFe⁰/air (60.0%), Fe/Cu/air control (77.8%) or Fenton control (39.5%). In addition, DDNP removal efficiency obtained by the 1^stFe/Cu/air–2^ndFenton–3^rdFe/Cu/air process (100%) was much higher than those (60.3%, 75.2% and 44.6%) of the three control experiments (see Fig. 5(b)). Finally, Fig. 5(c) shows that its B/C ratio increased from 0 to 0.58 after 4.5 h treatment by 1^stFe/Cu/air–2^ndFenton–3^rdFe/Cu/air process under the optimal conditions, which was much higher than that of 1^stFe⁰/air–2^ndFenton–3^rdFe⁰/air (0.17), Fe/Cu/air control (0.19) or Fenton control (0.15). Meanwhile, B/C ratio (0.58) of the effluent of the combined Fe/Cu/air and Fenton process was much higher than the sum (0.34) of Fe/Cu/air alone and Fenton alone. The results suggest that obvious improvement of biodegradability was mainly attribute to the synergistic reaction between Fe/Cu/air and Fenton process.

Fig. 5 COD removal, DDNP removal and B/C ratio obtained by four different treatment process, (a) COD removal efficiency, (b) DDNP removal efficiency, and (c) B/C ratio (experiment conditions were listed in Table S2, ESI†).

The above results indicate that the toxic and refractory pollutants (e.g., DDNP) could be decomposed effectively by 1^stFe/Cu/air–2^ndFenton–3^rdFe/Cu/air, which could cause an obvious improvement of biodegradability. The high treatment efficiency was mainly attribute to the synergistic reaction between Fe/Cu/air and Fenton process. In addition, the treatment efficiencies of 1^stFe/Cu/air–2^ndFenton–3^rdFe/Cu/air (i.e., decolority rate of 99.9%, COD removal of 87.1%, DDNP removal of 100%, B/C ratio of 0.58) was much higher than those of 1^stFe⁰/air–2^ndFenton–3^rdFe⁰/air (i.e., decolority rate of 60.2%, COD removal of 60.0%, DDNP removal of 60.3%, B/C ratio of 0.17). The results also suggest that Fe/Cu bimetallic particles have a higher reactivity than that of Fe⁰.^8,13,35 In particular, the coating Cu could accelerate the corrosion of iron, which could facilitate the formation of Fenton-like reaction under oxic condition (eqn (2) and (3)).

It could be concluded that 1^stFe/Cu/air–2^ndFenton–3^rdFe/Cu/air is a promising technology for the treatment of ultra-high concentrated DDNP industry wastewater.

3.4. UV-vis spectral analysis

The benzene ring, nitro (NO₂–) and azo (–N=N–) groups of DDNP or its by-products can be observed by using UV-vis analysis. Fig. 6 shows the changes in UV-vis absorbance characteristics of the influent and effluent of 4 different experiments (i.e., 1^stFe/Cu/air–2^ndFenton–3^rdFe/Cu/air, 1^stFe⁰/air–2^ndFenton–3^rdFe⁰/air, Fe/Cu/air control and Fenton control) from 190 to 500 nm under the optimal conditions (see Table S2, ESI†). With regard to the UV-vis spectrum of the raw ultra-high concentration DDNP wastewater (the influent), it was similar to the lower concentrated DDNP wastewater used in our previous work.⁷ In particular, a maximal peak between 200 and 250 nm is mainly attributed to the π–π* transition of benzene ring and nitro group. The broad peak between 280 and 500 nm represents conjugation of azo group (–N=N–) of DDNP or its by-products. Fig. 6(a and c) show that the broad peak (280–500 nm) could be removed completely after 4.5 h treatment by 1^stFe/Cu/air–2^ndFenton–3^rdFe/Cu/air or Fe/Cu/air control. But intensity of the adsorption peak (200–250 nm) of the effluent of 1^stFe/Cu/air–2^ndFenton–3^rdFe/Cu/air was much lower than that of the Fe/Cu/air control. The results indicates that azo group (–N=N–) of DDNP or its by-products could be decomposed completely by the two methods, but the benzene ring and nitro group of the pollutants were easier to be decomposed by the 1^stFe/Cu/air–2^ndFenton–3^rdFe/Cu/air process. It also confirmed the synergistic reaction between Fe/Cu/air and Fenton. According to Fig. 6(b and d), it is clear that the main groups of the pollutants were hard to be decomposed effectively by 1^stFe⁰/air–2^ndFenton–3^rdFe⁰/air or Fenton control. Finally, Fig. 6(e) shows the UV-vis spectra of the effluent after 4.5 h treatment by 4 different methods, and they reveal the following intensity trend of adsorption peaks: Fenton control > 1^stFe⁰/air–2^ndFenton–3^rdFe⁰/air > Fe/Cu/air > 1^stFe/Cu/air–2^ndFenton–3^rdFe/Cu/air. The results reveal the superiority of 1^stFe/Cu/air–2^ndFenton–3^rdFe/Cu/air, and they are also consistent with the results of COD removal (87.1%), decolority (99.9%), B/C ratio (0.58) and DDNP removal (100%).

Fig. 6 Variation of UV-vis spectra during the treatment process (a) 1^stFe/Cu/air–2^ndFenton–3^rdFe/Cu/air, (b) 1^stFe⁰/air–2^ndFenton–3^rdFe⁰/air control, (c) Fe/Cu/air control, (d) Fenton control, and (e) UV spectra of the effluent after 4.5 h treatment by four different treatment process under the optimal conditions (experiment conditions were listed in Table S2, ESI†).

3.5. FTIR spectral analysis

Fig. 7 is the FTIR adsorption spectra of the raw wastewater and effluent of 1^stFe/Cu/air–2^ndFenton–3^rdFe/Cu/air under the optimal conditions. According to the previous works,^36–38 the main characteristics of these spectra are the following: about 3450 cm⁻¹ (stretching vibration of O–H and hydrogen-bonded O–H); around 1610, 1550, 1490 cm⁻¹ (stretching vibration of –NO₂ and the aromatic C=C skeletal vibration); about 1350 and 1380 cm⁻¹ (stretching vibration of N–O and bending vibration of N–H); about 1120 cm⁻¹ (stretching vibration of C–O–H). It is clear that the FTIR spectrum of the ultra-high concentrated DDNP wastewater was similar to that of the lower concentrated DDNP wastewater used in our previous work.⁷ It could be observed that the bands at 1266, 1490, 1539, 1562 cm⁻¹ almost removed completely, and the intensity of the band at about 1612 cm⁻¹ was decreased seriously. In addition, the band at 1349 cm⁻¹ was shifted to 1385 cm⁻¹, and two board new bands at 1134 and 619 cm⁻¹ were generated in the effluent of 1^stFe/Cu/air–2^ndFenton–3^rdFe/Cu/air. The results further confirm that the main groups (e.g., benzene ring, nitro and azo groups) of the pollutants in DDNP wastewater could be decomposed effectively by the 1^stFe/Cu/air–2^ndFenton–3^rdFe/Cu/air process. Meanwhile, these pollutants could be transformed into the smaller molecule intermediates that were easy to be degraded by the subsequent biological treatment process. Therefore, B/C ratio of the DDNP wastewater could reached up to 0.58 after the 4.5 h treatment under the optimal conditions.

Fig. 7 FTIR adsorption spectra of 400–4000 cm⁻¹ region of (a) raw wastewater and (b) effluent of 1^stFe/Cu/air–2^ndFenton–3^rdFe/Cu/air (experiment conditions were listed in Table S2, ESI†).

3.6. Analysis of operating cost

The operating cost of 1^stFe/Cu/air–2^ndFenton–3^rdFe/Cu/air process was mainly from the consumption of Fe/Cu bimetallic particles and H₂O₂. Under the optimal conditions, it could be calculated that about 1.12 kg Fe/Cu bimetallic particles and 1.2 L H₂O₂ (27.5%, v/v) would be consumed per treatment 1 ton DDNP wastewater. The Fe/Cu bimetallic particles with a theoretical Cu mass loading of 0.41 g Cu/g Fe were prepared by planting Cu on the surface of Fe⁰,²⁰ and their cost was about 1500 USD/t. In addition, the market prices of H₂O₂ (27.5%, v/v) is about 150 USD/t. Therefore, it could be calculated that the main operating cost of 1^stFe/Cu/air–2^ndFenton–3^rdFe/Cu/air process was about 1.9 USD per treatment 1 ton DDNP wastewater. Although the cost of Fe/Cu bimetallic particles (1500 USD/t) is much higher than that of Fe⁰ (230 USD/t), the ultra-high concentrated DDNP wastewater could be pretreated effectively by 1^stFe/Cu/air–2^ndFenton–3^rdFe/Cu/air process due to the high reactivity of Fe/Cu bimetallic particles. Therefore, the 1^stFe/Cu/air–2^ndFenton–3^rdFe/Cu/air process is a promising pretreatment method for the ultra-high concentrated DDNP industry wastewater.

4. Conclusions

DDNP industry wastewater was treated by 1^stFe/Cu/air–2^ndFenton–3^rdFe/Cu/air process, and the optimal operating parameters (1^stFe/Cu/air process: initial pH = 2.0, Fe/Cu dosage = 40 g L⁻¹, air flow rate = 1.5 L min⁻¹, treatment time = 1.5 h; 2^ndFenton oxidation: initial pH = 3.0, H₂O₂ dosage = 10 mmol L⁻¹, treatment time = 2.0 h; 3^rdFe/Cu/air: treatment time = 1.0 h, Fe/Cu dosage = 40 g L⁻¹, air flow rate = 1.5 L min⁻¹) were obtained in this study. First, the bubbles problem of the raw wastewater was resolved through the removal of surfactants by using Fe/Cu process without aeration. Under the optimal conditions, furthermore, high COD removal (87.1%), decolority (99.9%), DDNP removal (100%) and B/C ratio (0.58) was obtained by 1^stFe/Cu/air–2^ndFenton–3^rdFe/Cu/air process. Meanwhile, its treatment efficiency was much higher than that of three control experiments. For example, DDNP removal efficiency (100%) obtained by 1^stFe/Cu/air–2^ndFenton–3^rdFe/Cu/air process was much higher than that of 1^stFe⁰/air–2^ndFenton–3^rdFe⁰/air process (60.3%), Fe/Cu/air control (75.2%) and Fenton control (44.6%). According to the analysis results of UV-vis and FTIR, it could further confirm that the main pollutants in DDNP wastewater could be decomposed through the synergistic reaction between Fe/Cu/air and Fenton process. Although the cost of Fe/Cu bimetallic particles (1500 USD/t) is much higher than that of Fe⁰ (230 USD/t), the ultra-high concentrated DDNP wastewater could be pretreated effectively by 1^stFe/Cu/air–2^ndFenton–3^rdFe/Cu/air process due to the high reactivity of Fe/Cu bimetallic particles. Therefore, 1^stFe/Cu/air–2^ndFenton–3^rdFe/Cu/air could be considered as a promising process for the pretreatment of the ultra-high concentrated DDNP industry wastewater.

Acknowledgements

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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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra05371h

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