Chemical and Biological Sci., 2014, Vol. 59, No. 9, pp. 59-65 This paper is available online at http://stdb.hnue.edu.vn
N I T R O B E N Z E N E DEGRADATION INDUCED BY OXYGEN ACTIVATION IN A ZERO-VALENTIRON/AIR/EDTAAVATER SYSTEM T r a n D u e L u o n g ^ , N g u y e n H o a i N a m ^ a n d T r a n V a n C h u n g ^
^Faculty of Biochemistry, Nam Dinh University of Nursing
^Institute of Material Science, Vietnamese Academy of Science and Technology
^Institute of Chemistry and Material, Academy of Military Science and Technology Abstract. Nitrobenzene (NB) is a recalcitrant organic compound that can be degraded and mineralized by oxygen activation induced in a zero-valent iron/air/EDTA/water system. The degradation and mineralization efficiency of NB in the systems looked at in this study were determined by measuring the NB concentration and COD values before and after reaction. The influence of pH, Fe^^^-mass and EDTA on die degradation and mineralization efficiencies were also investigated.
Keywords: Nitrobenzene, activation of oxygen, zero-valent iron.
1. Introduction
Nitrobenzene (NB) and its derivative compounds are widely used in practices that can harm ecological systems and human health [11]. NB is a toxic recalcitrant organic compound and a dangerous environmental pollutant that is present in industrial wastewater [ 1 , 3 - 5 , 9 , 12]. NB from wastewater can be treated using various methods [9].
Of these methods, an advanced oxidation process that removes NB with high efficiency is presented in this paper. This method, which destroys NB that is present in wastewater using oxygen activated with ZVI and EDTA, is described in detail. An aqueous system consisting of oxygen, ZVI and EDTA can producing free radical O H ' which can oxidize NB and other recalcitrant organic compounds [1, 3, 5-8, 10], The process of oxygen activation using ZVI with EDTA to produce free radical O H ' is suggested in [5]. It is hypothesized that the mechanism of the process is the production of reactive oxygen intermediates. Intermediates such as hydrogen peroxide are postulated to be continuously produced by the reduction of aqueous oxygen which may take place either on the iron surface or in solution [5]. In another way, F e " L complexes (L denoted EDTA) might be Received September 14, 2014. Accepted November 28, 2014.
Contact Tran Van Chung, e-mail address: [email protected]
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Tran Due Luong, Nguyen Hoai Nam and Tran Van Chung
reacting with O2 to form the superoxide radical which leads to the production of H2O2 and eventually to a Fenton reaction as follows [4]:
Fe(o) -> Fe'+ + 2eE?3d = -0,44V d-l) Fe^+-FL = Fe"LKf-10^'-^' (1-2) Fe"L + 0 2 ^ F e " ' L + 0 r tl.3) Fe"L -\- o r + 2H-H -^ Fe"^L + H2O2 (1-4) Fe"L + H2O2 -> Fe"^L -F OH" -\- OH", kp = lO^Mr^s"^ (1.5)
The NB molecules (R) can be oxidized with OH' to produce CO2, H2O and mineral salt:
R-t- OH* -J- CO2 + H2O -I- meneral salt (NO3 ) (1.6) During the reaction, the oxidation efficiency of NB by reaction with 02-activationwas determined by measuring the change of NB-concentration and COD (Chemical Oxygen Demand) values. The main factors influencing the oxidation efficiency of NB, such as dose of Fe°, EDTA and pH, were investgated.
2. Content
2.1. Materials and methods
* Materials
All of the chemicals used in this work were of reagent grade. EDTA (> 99.0%), NB (yellowish liquid, d = 1.199 g/cm^), concentrated sulfuric acid (> 98.0%), sodium hydroxide (> 96.0%) and u-on powder (cubie-Fe > 98%, particle size 0.50 pm) were purchased from Shanghai Chemical Reagent Co., Ltd, China. All solutions were prepared with distilled water.
* Experiment
Batch experiments were carried out in a 500 mL glass vessel using a 200 mL solution. The solutions were continuously stirred with a mechanical stirrer. Stock solutions of NB (80 mg/L) were prepared in glass jars using 66.7 mL NB (liquid) diluted to 1000 mL by adding distillated water. The pH of all solutions was adjusted with 0.1 M NaOH or H2SO4. A pre-determined mass of Fe'*'' and EDTA was added to the NB - solution to investigate oxygen activation. During the reaction, a flow of atmospheric oxygen was passed through the solution to ensure that the saturated oxygen concentration in the sample was 8 mg/L. Samples of a certain volume of solution collected from each reaction vessel at regular time intervals were filtered to determine the concentration of NB and COD value remained.
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* Analytical methods
- The pH solution was monitored using a Toledo pH meter.
- The concentration of NB was determined by anodic stripping Voltammetry with a mercury hanging drop electrode (HMDE) using Metrohom 797 VA computrace. The solution consisting of NB in acid acetic-sodium acetate buffer 0.1 M as electrolyte was removed oxygen for 60 s by N2 gas, then pre-concentrated onto HMDE for 60 s at a potential of -0.90 V, then an anodic stripping Voltammetric current was determined in the potential as being from -0.90 to -0.10 V. The current peak height appeared at -0.442 V, this being proportional to the NB concentration used for analysis of this compound. The degradation efficiency (ENB) of NB for the reaction time was determined based on the following expression:
no CMP
'•- X 100% (2.1)
here C^g is the initial concentration of NB in the sample and C^^g is the NB concentration for the t-reaction time.
COD analysis was determined hy the usual method using K2Cr207 and concentrated sulfuric acid [2]. The mineralization efficiency (ECOD) of NB was evaluated hy the expression:
ECOD = ^COD "
C%. ^ ^ ^ ^ X 100% (2.2)
'COD
here CCQD' ^S^ denoted initial COD and C ^ O D ' "ig/L is for t-reaction time.
2.2. R e s u l t s a n d d i s c u s s i o n
2.2.1. The degradation and mineralization of NB by oxygen activation in a zero-valent air/EDTA water system
The initial experiments were carried out using various concentrations of components to demonstrate the capacity of NB degradation and its mineralization. The experimental results are hsted in Table 1.
Table 1. Result of degradation and mineralization ofNB under the condition:
Cpe = 0.15 g/L; CEDTA = 6.72 mg/L, 02=8 mg/L, pH = 3.0, reaction time: 150 minutes
No.
1 2 3
CcoDn>g/L 50.0 30.0 20.0
CfjBing/L 0.56 0.24 0.10
CODo 96 55 38
CODj 29 15 7
E C O D ( % ) 69.8 72,7 81,6
E N B % 98.9 99.2 99.5
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Tran Due Luong, Nguyen Hoai Nam and Tran Van Chung
The data from Table 1 indicate that in the system consisting of Fe"", EDTA and Oj, there is a process of degradation and mineralization of NB. The NB degradation process takes place much more quickly than does the mineralization. This shows that in the system, during a 150 minute time period, NB was converted into intermediate compounds and most of them mineralized to produce CO2, H2O and mineral salts.
2.2.2. Factors influencing the degradation and mineralization of NB
* Influence ofpH
The influence of pH (from 2.5 to 9.5) on NB degradation and mineralization for a 150 minute reaction time is presented in Table 2.
Table 2. Influence ofpH in the reaction condition:
C°„„ = 30 mg/L; CODo= 55 mg/L; Cpe = 0-15 g/L; EDTA = 6.72 mg/L O; =8 mg/L pH
COD, mg/L EcOD%
CNBmg/L E N B %
2.5 6 89.1 0.15 99.5
3.5 3 92.5 0.081 99.7
4.5 3 92.5 0.081 99.7
5.5 5 90.9 0.18 99.4
6.5 8 85.,5 0.33 98.9
7.5 20 63.6 0.36 98.8
8.5 35 36.3 0.46 98.4
9.5 47 14.5 0.82 97.3 The experimental data in Table 2 indicate that in the range of pH from 3.5 to 4.5, the degradation and mineralization efficiencies of NB were highest, then they decreased slowly. This phenomenon can be explamed as the formation of free radical OH' in situ from reaction (5) - Fentonreaction. In an acid medium, H"*" ions will enhance the formation of free radicals and tiiis will increase NB degradation and mineralization efficiency.
However, at pH < 3.5, the corrosion rate of iron in the sample will be higher than at pH > 3.5, and this leads to a production of more Fe^+. These Fe^+ ions will consume free radical OH*and reduce the degradation and mineralization of NB. This is consistent with the work [2].
* Influence ofFe^^^ mass
Table 3. Influence ofFe'^^^-mass Nr
1 2 3 4 5 6 7
Fe("), g/L 0.25 0.50 0.70 1.0 1.5 2.0 2.5
COD, mgOj/L 31 15 9 3 5 11 26
ECOD(%) 44.4 71.3 83.6 92.5 90.8 80.1 52.7
CNBtmg/L 3.81
1.86 0.45 0.081
0.26 0.72 0.94
E N B % 87.3 93.8 98.5 99.7 99.1 97,6 96.7
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The influence of Fe '°^ mass on the degradation and mineralization of NB was determined under the experimental condition: C^B = 30 mg/L; CODo= 55 mg/L; EDTA
= 6.72 mg/L; O2 = 8 mg/L, pH = 3.5; Fe <°) mass changing from 0.25 to 2.5 g/L, with a reaction time of 150 minutes. The obtained result is presented in Table 3.
The experimental data from Table 3 shows that the degradation and mineralization efficiencies of NB increase when Fe^^Wass increases from 0.25 to 1.0 g/L, then they decreases when Fe'°^ mass is increased. This can be explained by the reaction of free radicals with Fe^°' or Fe^^ available in the system as follows:
OH* + Fe^+ - 20H* + Fe'°' -
. Fe'+ - . Fe^+ -
OH- 20H-
(2.3) (2.4)
These lead to reduced efficiency of NB degradation and mineralization.
* Influence of EDTA
The influence of EDTA on the degradation and mineralization of NB was determined under the experimental condition: C^B= 30 mg/L; CODo= 55 mg/L;
Fe(°> = 1.0 g/L; O2 = 8 mg/L, pH = 3.5; EDTA changing from 0.83 to 11.7 mg/L, with a reaction time of 150 minutes. The obtained results are presented in Table 4.
Nr 1 2 3 4 5 6 7
Fe"" g/L^
1.0 1.0 1.0 1.0 1.0 1.0 2.5
Table 4. Influence of EDTA EDTA mg/L
0.83 1.66 3.36 6.72 8.38 10.04 11.70
COD, mg02/L 37 16 8 3 3 6 8
ECOD * 68.4 71.5 85.2 92.5 92.8 90.1 86.2
CNBI mg/L 0.57 0.36 0.15 0.081 0,082 0.093 0.98
E N B % 98.1 98.8 99.5 99.7 99.7 99.6 96.7 The obtained data in Table 4 show that the degradation and mineralization efficiencies of NB increase when the EDTA concentration increases from 0.83 to 11.70 mg/L. When the concenU-ations of EDTA were higher than 8.38 mg/L, the efficiencies of NB degradation decreased, perhaps due to the reaction of free radicals with EDTA in the sample.
2.2.3. Degradation kinetics of NB by oxygen activation in a zero-valent iron/air/EDTA water system
By experimentation, the optimal conditions for oxygen activation in a zero-valent iron/air/EDTA water system were estabhshed. The optimal conditions were pH = 3.5 -
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Tran Due Luong, Nguyen Hoai Nam and Tran Van Chung
4.5, EDTA concenti-ation = 6.72 - 8.38 mg/L; Fe^^'-mass = 1.0 g/L, and a reaction time of 150 minutes. Under tiiese conditions, the change of COD and NB concentrations versus the reaction time of from 0 to 210 minutes are presented in Table 5.
Table 5. The change of COD and NB concentration vs. time Time, mins
COD, mgOj/L NB, mg/L
0 55 30.0
30 32 9.1
60 18 2.79
90 10 0.85
120 6 0.265
150 3 0.081
180 2 0.025
210 0.012
-
By simple calculation, the integral reaction rate expressions corresponding to the reduction of NB and COD versus time were found. These expressions obey the pseudo first order reaction with die following forms: For NB concentration: YNB = kNst + b;
*:NB = -0.03812 ± 7.3 X 10-Vmin;6 = 3.31652 ± 0.092. For the COD reduction:
YcoD = kooDt + b; kcoo = -0.01897 ± 3.46 x IQ-Vmin; b = 4.04032 ± 0.03745.
These results show that the reaction constant of the NB degradation process is always higher than the reaction constant of NB mineralization. This is consistent with the experimental data shown in Tables 1, 2, 3 and 4.
3. Conclusion
The system consisting of zero-valent air/EDTA/water was successfully applied to show the degradation and mineralization of NB. The degradation and mineralization of NB are taken to be due to the appearance of free radical OH" in the samples. By experimentation, the optimal conditions for oxygen activated by Fe'"' + EDTA were established: pH =3.5 - 4.5, EDTA concentration = 6.72 - 8.38 mg/L; Fe(")-mass = 1.0 g/L, and a reaction time of 150 minutes. Under these conditions, the degradation and mineralization of NB can reach > 99% for NB and > 92% for COD.
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