Content uploaded by Weizhou Jiao
Author content
All content in this area was uploaded by Weizhou Jiao on Nov 20, 2016
Content may be subject to copyright.
Degradation behavior and kinetics of dinitrotoluene in simulated wastewater
by iron–carbon microelectrolysis
Weizhou Jiao
a,b,
*, Lisheng Yu
a,b
, Youzhi Liu
a,b
, Zhirong Feng
a,b
, Wenli Liu
a,b
a
Shanxi Province Key Laboratory of Higee-Oriented Chemical Engineering, North University of China, Taiyuan 030051, Shanxi,
China, Tel. +86 351 3921986; Fax: +86 351 3921497; email: jwz0306@126.com (W. Jiao)
b
Research Center of Shanxi Province for High Gravity Chemical Engineering and Technology, North University of China, Taiyuan
030051, Shanxi, China
Received 8 January 2015; Accepted 6 October 2015
ABSTRACT
Dinitrotoluene (DNT) removal strategies suffer from high costs or slow conversion rates.
The effects of initial pH, mass ratio of iron–carbon, mass concentration of anhydrous
sodium sulfate, and residence time on DNT removal rate were investigated. Results showed
that the degradation efficiency of DNT reached 55.98% under the optimum conditions: an
initial value of pH 3, an iron dosage of 10 g/L, a mass ratio of iron–carbon of 1, and a
Na
2
SO
4
concentration of 200 mg/L. The degradation kinetics of DNT by iron–carbon
micro-electrolysis in the studied temperature range was a pseudo-first-order reaction.
Keywords: Dinitrotoluene; Wastewater; Iron–carbon micro-electrolysis; Kinetics; Pseudo-
first-order reaction
1. Introduction
Dinitrotoluene (DNT) is an industrial compound
widely used in the production of medicines, dyes,
high-polymer materials, explosives, and propellants.
Therefore, DNT is typically found as an environmental
contaminant [1] and has been listed as a priority pol-
lutant by the US Environmental Protection Agency [2].
In most cases, DNT-containing wastewater is always
characterized as complex composition, high toxicity
[3,4], high chromaticity, and high chemical oxygen
demand, so that it cannot be biodegraded readily.
Researches on various technologies used for the degra-
dation of DNT-containing wastewater have been con-
ducted. These technologies include active carbon (AR)
adsorption [5], advanced chemical oxidation processes
[6], electrochemical reaction [7], and biodegradation
[8]. However, many of these technologies are costly,
unefficient, and unable to treat highly concentrated
DNT-containing wastewater. Therefore, it is necessary
to develop an efficient and low-cost treatment strategy
of DNT-containing wastewater.
Iron–carbon micro-electrolysis is an effective water
treatment method which has been widely used in
treating poorly biodegradable wastewaters containing
nitrobenzene, pharmaceuticals, heavy metals, and dyes
as its low cost and high efficiency [9–13]. Based on the
electrochemical corrosion of metals, massive micro-
scopic galvanic cells will be formed by the iron chips
and a carbon component in the electrolyte solution
[14,15]. Electrode products released from the micro-
scopic galvanic cells reaction include hydroxyl,
nascent hydrogen, and Fe
2+
[16]. These products are
highly active in decomposing contaminants, especially
*Corresponding author.
1944-3994/1944-3986 !2015 Balaban Desalination Publications. All rights reserved.
Desalination and Water Treatment 57 (2016) 19975–19980
September
www.deswater.com
doi: 10.1080/19443994.2015.1106981
in transforming the original substituent group. The
iron–carbon micro-electrolysis reactions are proposed
as follows [17]:
Anode reaction:
Fe !2e !Fe2þAcidicðÞEh¼!0:44 V (1)
2OH!þFe2þ!FeðOHÞ2(2)
3OH!þFe3þ!FeðOHÞ3Neutral to alkalineðÞ(3)
Cathode reaction:
2Hþþ2e !2H½'!H2AcidicðÞEh¼0 V (4)
O2þ4Hþþ4e !2H2O AcidicðÞEh¼1:23 V (5)
O2þ2H2Oþ4e !4OH!Neutral to alkalineðÞ
Eh¼0:40 V (6)
This research aimed to establish the kinetic model of
iron–carbon micro-electrolysis reduction in DNT-con-
taining wastewater. The effects of initial pH, mass
ratio of iron–carbon, mass concentration of anhydrous
sodium sulfate (Na
2
SO
4
), and residence time on DNT
reduction were investigated. This article will provide
a definite theoretical basis and present potential appli-
cation for treating DNT-containing wastewater in
iron–carbon micro-electrolysis system.
2. Materials and methods
2.1. Materials
2,4-DNT (chemically Pure, CP), anhydrous Na
2
SO
4
(Analytical Reagent, AR), sodium hydroxide (NaOH,
AR), concentrated hydrochloric acid (HCl, AR), sulfu-
ric acid (H
2
SO
4
, AR), AR, and other reagents were all
purchased from Taiyuan Chemical Reagent Company
(Taiyuan, China). DNT-containing wastewater was
prepared by dissolving certain 2,4-DNT into deionized
water.
Cast iron chips were obtained from a metal
machining mill. The size of the chips was 0.3–2.5 mm
in length and 1.5 mm in width. The cast iron chips
were degreased in 10% NaOH solution, soaked in a
diluted 5% HCl solution, and then washed with
deionized water for several times. The AR were sieved
to 16–20 mesh, then immersed in raw wastewater to
reach saturated adsorption.
2.2. Experimental methods
The batch experiments were conducted with a
ZR4-6 Intelligent Jar Tester with six agitators
(Shenzhen Zhongrun Water Industry Technology
Development Co., Ltd). 300 mL certain concentration
of DNT-containing wastewater was added into 600-mL
beakers, then adjusted to a desired pH. For each bea-
ker, certain amount of cast iron chips, AR, and Na
2
SO
4
were added. The stirring speeds of the agitators ranged
from 10 to 1,000 rpm, and the reactions were pro-
ceeded at ambient temperature. The degradation kinet-
ics of DNT by iron–carbon micro-electrolysis was
investigated in a constant temperature water bath.
2.3. Analytical methods
The concentrations of DNT were analyzed by high-
performance liquid chromatography (HPLC; Dionex’s
Ultimate 3000, USA) with a C
18
reversed-phase col-
umn (250 mm ×4.6 mm, 5 μm). The detection wave-
length was at 246 nm, the mobile phase was methanol
and water (50/50, v/v). The flow rate, oven tempera-
ture, and sample volume were 1.5 ml/min, 40˚C, and
20 μL, respectively.
The removal efficiency of DNT was defined as the
following equation:
g¼C0!Ct
C0
(100%(7)
where C
t
represents the concentration of DNT at a
specific time tand C
0
represents the initial concentra-
tion of DNT.
3. Results and discussion
3.1. Effect of initial pH
The effect of initial pH on the removal efficiency of
DNT was investigated and pH was adjusted using
10% NaOH and 5% H
2
SO
4
. The mass concentration of
iron was 20 g/L, and the mass ratio of iron–carbon
was controlled at 2. During the experiment, the stir-
ring speed was 300 rpm and reaction temperature was
maintained at 25˚C. The residence time was controlled
at 60 min, the mass ratio of iron–carbon was con-
trolled at 2, and the mass concentrations of Na
2
SO
4
electrolyte was 200 mg/L. Six beakers with different
initial pH (2–7) were examined during the experiment.
The effects of pH on the removal efficiency of DNT
are presented in Fig. 1.
Fig. 1shows that the removal efficiency of DNT
decreased with increasing pH. The degradation
19976 W. Jiao et al. / Desalination and Water Treatment 57 (2016) 19975–19980
performance of DNT strongly depended on the pH.
The results implied an acid-promoted reaction
between cast iron chips (Fe
0
) and DNT. A more rapid
and complete reaction may be due to the lower pH,
which could enhance the efficiency of the micro-elec-
trolysis system. Under acidic conditions, higher con-
centration of hydrogen ion resulted in higher potential
generated at the cathode. This phenomenon leads to
an increase in electromotive force and concentration of
Fe
2+
and [H]. And these nascent Fe
2+
and [H] have
much higher reduction activity than Fe
0
and H
+
, and
they can convert nitro aromatic compounds into
amino compounds efficiently [16,18]. It is clear that
the lower the pH, the higher the removal efficiency of
DNT. However, a lower pH may lead to a faster cor-
rosion rate of the iron and the reaction apparatus.
Take into account of these aspects, the pH of the
iron–carbon system was chosen at 3.
3.2. Effect of iron dosage
The effect of iron dosage on the removal efficiency
of DNT is shown in Fig. 2under the following operat-
ing conditions: pH of 3, mass ratio of iron–carbon of
2, Na
2
SO
4
concentration of 200 mg/L, the reaction
temperature of 25˚C, the stirring speed of 300 rpm,
and the residence time of 60 min. As shown in Fig. 2,
the removal efficiency of DNT increased with increas-
ing iron dosage and increased sharply to 35.3% from
20.3% when iron dosage increased from 2 to 10 g/L.
This phenomenon may be ascribed to the fact that
higher iron dosage results in larger surface area; thus,
more DNT could be absorbed on the reactive sites of
the iron [19,20], then the redox reactions of DNT were
accelerated. When the dosage of iron exceeded 10 g/L,
the removal efficiency of DNT tended to be flat. This
might due to that with the dosage of iron increased to
a certain amount, accumulation between iron and car-
bon exhibited obviously, then the exposure of reactive
sites on the surface of iron were reduced. Therefore,
the removal efficiency of DNT did not increase with
an increase in iron dosage. The dosage of iron was
fixed at 10 g/L for the following investigation.
3.3. Effect of mass ratio of iron–carbon
Fig. 3shows the removal efficiency of DNT as a
function of mass ratio of iron–carbon that ranged from
0.25 to 3 at a Na
2
SO
4
concentration of 200 mg/L, an
iron dosage of 10 g/L, a pH of 3, a stirring speed of
300 rpm, a reaction temperature of 25˚C, and the resi-
dence time of 60 min. As shown in Fig. 3that with an
increasing mass ratio of iron–carbon, the removal effi-
ciency of DNT initially increased and then decreased.
In general, the mass ratio mainly affected the numbers
of microscopic galvanic cells, more microscopic
galvanic cells result in quicker and more efficient
redox reaction [21]. When the mass ratio of iron–
carbon was controlled to 1, the removal efficiency of
DNT reached a peak value. This might be due to that
with the mass ratio of iron–carbon approximated to 1,
the mass concentration of iron and carbon were
optimum to form the greatest numbers of microscopic
galvanic cells, so the reduction of DNT was enhanced.
The mass ratio of iron–carbon was fixed at 1 for the
following investigation.
Fig. 1. Effect of initial pH on the removal efficiency of
DNT.
Fig. 2. Effect of dosage of iron on the removal efficiency of
DNT.
W. Jiao et al. / Desalination and Water Treatment 57 (2016) 19975–19980 19977
3.4. Effect of mass concentration of Na
2
SO
4
electrolyte
The effect of mass concentration of Na
2
SO
4
elec-
trolyte on the removal efficiency of DNT was investi-
gated at an iron dosage of 10 g/L, a pH of 3, a mass
ratio of iron–carbon of 1, a temperature of 25˚C, a stir-
ring speed of 300 rpm, and the residence time of
60 min. Fig. 4shows the effect of mass concentration
of Na
2
SO
4
on the removal efficiency of DNT. In the
micro-electrolysis system, adding electrolytes could
improve the electrical conductivity of wastewater and
then accelerate the electrochemical oxidation corrosion
[20]. The electron transfer between metal iron and
nitro compounds during the reaction process was rein-
forced and then the degradation efficiency of DNT
was improved. As shown in Fig. 4that the removal
efficiency of DNT increased with the increasing mass
concentration of Na
2
SO
4
electrolyte. But the variation
tendency of the removal efficiency of DNT tended to
be flat when the Na
2
SO
4
concentration exceeded
200 mg/L. This might be due to that though the pres-
ence of electrolyte can improve the transfer of electron
between iron and nitro compounds, the production of
electron during a certain period was limited. There-
fore, the transfer of electron did not increase linearly
with an increase in the concentration of electrolyte, so
the variation tendency of the removal efficiency of
DNT tended to be flat while the Na
2
SO
4
concentration
exceeded 200 mg/L.
3.5. Effect of residence time
The effect of residence time (20–160 min) on the
removal efficiency of DNT was observed under the
following conditions: an iron dosage of 10 g/L, a mass
ratio of iron–carbon of 1, a pH 3, a stirring speed of
300 rpm, a Na
2
SO
4
concentration of 200 mg/L, and a
reaction temperature of 25˚C. Fig. 5shows that the
removal efficiency of DNT increased with the increas-
ing residence time. These results might attribute to the
variation in the concentration of hydrogen electrons
under an acidic environment. In the first 60 min,
numerous electrons were released from the anode
reaction to reduce DNT. Under an acidic environment,
the system generated a large amount of nascent acti-
vated hydrogen [H] and other active groups [22].
When the residence time exceeded 60 min, an increase
in the removal efficiency of DNT became slightly as
Fig. 3. Effect of mass ratio of iron–carbon on the removal
efficiency of DNT. Fig. 4. Effect of Na
2
SO
4
concentration on the removal
efficiency of DNT.
Fig. 5. Effect of residence time on the removal efficiency of
DNT.
19978 W. Jiao et al. / Desalination and Water Treatment 57 (2016) 19975–19980
the concentration of hydrogen ion gradually reduced.
With the residence time prolonged, Fe
2+
, Fe
3+
, and
OH
−
easily formed a hydroxide layer on the iron sur-
face [23,24], thereby, the degradation of DNT was
inhibited.
3.6. Degradation kinetics of DNT
The reduction kinetics of DNT in aqueous systems
was investigated to elucidate the mechanism of the
iron–carbon micro-electrolysis reaction. The DNT con-
centration obtained at different times for the experi-
ments was plotted on a semilogarithmic plot of C
t
/C
0
vs. the residence time, where C
t
represents the concen-
tration at a specific time tand C
0
represents the initial
concentration of DNT. From the previous study
[25,26], DNT can be reduced to 2,4-diaminotoluene
2,4-DAT with 2-amino-4-nitrotoluene and 4-amino-2-
nitrotoluene as intermediates, the degradation path-
way can be described as the following mechanism
(Fig. 6).
Fig. 7shows the analysis of the first-order reaction
linear regression. According to the fitting results, the
value of the linearly dependent coefficient Rwas 0.995
and the rate coefficient was 0.00203. However, this
reaction not only included redox, but also adsorption
and deposition. Water, hydrogen ions, dissolved oxy-
gen, and iron hydroxides were also involved in the
reaction. Therefore, the DNT removal by iron–carbon
micro-electrolysis followed pseudo-first-order reaction
at room temperature. The kinetic equation was:
Ct¼C0e!kt ¼200 (e!0:00203t(8)
4. Conclusions
The factors influenced the iron–carbon micro-elec-
trolysis system for treating DNT-containing wastewa-
ter were investigated. The appropriate conditions for
treating DNT-containing wastewater are as follows: an
initial value of pH 3, an iron dosage of 10 g/L, a mass
ratio of iron–carbon of 1, and a Na
2
SO
4
concentration
of 200 mg/L. Under these process conditions, the
removal efficiency of DNT reached 55.98%. The study
on the reduction kinetics of DNT indicated that the
removal of DNT by iron–carbon micro-electrolysis
followed a pseudo-first-order reaction at room
temperature.
Acknowledgements
This work was supported by the Excellent Youth
Science and Technology Foundation of Province
Shanxi of China (2014021007) and Program for the
Outstanding Innovative Teams of Higher Learning
Institutions of Shanxi (201316).
References
[1] J.D. Rodgers, N.J. Bunce, Treatment methods for the
remediation of nitroaromatic explosives, Water Res. 35
(2001) 2101–2111.
[2] D.K. Kenneth, A.P. Khodadoust, K.R. Reddy, Reactiv-
ity of lactate-modified nanoscale iron particles with
2,4-dinitrotoluene in soils, J. Hazard. Mater. 182 (2010)
177–183.
[3] H. Wintz, L.J. Yoo, A. Loguinov, Y.Y. Wu, J.A.
Steevens, R.D. Holland, R.D. Beger, E.J. Perkins, O.
Hughes, C.D. Vulpe, Gene expression profiles in fat-
head minnow exposed to 2,4-DNT: Correlation with
toxicity in mammals, Toxicol. Sci. 94 (2006) 71–82.
Fig. 6. Degradation pathway of DNT by micro-electrolysis.
Fig. 7. Degradation kinetics of iron–carbon micro-electroly-
sis reaction.
W. Jiao et al. / Desalination and Water Treatment 57 (2016) 19975–19980 19979
[4] S. Rocheleau, R.G. Kuperman, M. Simini, J. Hawari,
R.T. Checkai, S. Thiboutot, G. Ampleman, G.I.
Sunahara, Toxicity of 2,4-dinitrotoluene to terrestrial
plants in natural soils, Sci. Total Environ. 408 (2010)
3193–3199.
[5] C. Rajagopal and J.C. Kapoor, Development of adsorp-
tive removal process for treatment of explosives con-
taminated wastewater using activated carbon, J.
Hazard. Mater. B87 (2001) 73–98.
[6] S.Y. Oh, S.G. Kang, Degradation of 2,4-dinitrotoluene
by persulfate and steel waste powder, Geosyst. Eng.
13 (2010) 105–110.
[7] R. Doppalapudi, D. Palaniswamy, G. Sorial, S. Maloney,
Electrochemical pilot scale study for reduction of
2,4-DNT, Water Sci. Technol. 9 (2003) 173–178.
[8] S. Zita, N. Ales, T. Safieh, V. Jaco, V. Tomas, D.V.D.
Lelie, Biological remediation of explosives and related
nitroaromatic compounds, Environ. Sci. Pollut. Res.
Int. 1 (2002) 48–61.
[9] L.M. Wu, L.B. Liao, G.C. Lv, F.X. Qin, Y.J. He, X.Y.
Wang, Micro-electrolysis of Cr(VI) in the nanoscale
zero-valent iron loaded activated carbon, J. Hazard.
Mater. 254–255 (2013) 277–283.
[10] Z.R. Feng, W.Z. Jiao, Y.Z. Liu, C.C. Xu, L. Guo, L.S.
Yu, Treatment of nitrobenzene-containing wastewater
by iron-carbon micro-electrolysis, CIESC J. 3 (2015)
1150–1155.
[11] K. Wang, S. Liu, Q. Zhang, Y.L. He, Pharmaceutical
wastewater treatment by internal micro-electrolysis–
coagulation, biological treatment and activated carbon
adsorption, Environ. Technol. 30 (2009) 1469–1474.
[12] X.S. Lv, J. Xu, G.M. Jiang, X.H. Xu, Removal of chro-
mium(VI) from wastewater by nanoscale zero-valent
iron particles supported on multiwalled carbon nan-
otubes, Chemosphere 85 (2011) 1204–1209.
[13] M.R. Samarghandi, M. Zarrabi, A. Amrane, M.N.
Sepehr, M. Noroozi, S. Namdari, A. Zarei, Kinetic of
degradation of two azo dyes from aqueous solutions
by zero iron powder: Determination of the optimal
conditions, Desalin. Water Treat. 40 (2012) 137–143.
[14] C.H. Xu, L.J. Zhu, X.H. Wang, S. Lin, Y.M. Chen, Fast
and highly efficient removal of chromate from aque-
ous solution using nanoscale zero-valent iron/
activated carbon (NZVI/AC), Water, Air, Soil Pollut.
225 (2014) 1845–1857.
[15] G. Qin, D. Gong, Pretreatment of petroleum refinery
wastewater by microwave-enhanced Fe
0
/GAC micro-
electrolysis, Desalin. Water Treat. 52 (2014) 2512–2518.
[16] J.H. Luo, G.Y. Song, J.Y. Liu, G.R. Qian, Z.P. Xu,
Mechanism of enhanced nitrate reduction via micro-
electrolysis at the powdered zero-valent iron/acti-
vated carbon interface, J. Colloid Interface Sci. 435
(2014) 21–25.
[17] M. Li, D.L. Zou, H.C. Zou, D.Y. Fan, Degradation of
nitrobenzene in simulated wastewater by iron-carbon
micro-electrolysis packing, Environ. Technol. 32 (2011)
1761–1766.
[18] A. Agrawal, P.G. Tratnyek, Reduction of nitro aro-
matic compounds by zero-valent iron metal, Environ.
Sci. Technol. 30 (1996) 153–160.
[19] D. Kladekova
´, M. Hez
ˇelova
´, L. Lux, Evaluation of
surface reactivity of Fe powders: Reaction with
nitrobenzene, Part. Sci. Technol. 25 (2007) 139–146.
[20] J.H. Fan, H.W. Wang, D.L. Wu, L.M. Ma, Effects of
electrolytes on the reduction of 2,4-dinitrotoluene by
zero-valent iron, J. Chem. Technol. Biotechnol. 85
(2010) 1117–1121.
[21] B. Lai, Y.X. Zhou, J.L. Wang, Y.H. Zhang, Z.Q. Chen,
Passivation process and the mechanism of packing parti-
cles in the Fe
0
/GAC system during the treatment of ABS
resin wastewater, Environ. Technol. 35 (2014) 973–983.
[22] Y.H. Huang, T.C. Zhang, Effects of low pH on nitrate
reduction by iron powder, Water Res. 38 (2004)
2631–2642.
[23] Y.H. Huang, T.C. Zhang, Effects of dissolved oxygen
on formation of corrosion products and concomitant
oxygen and nitrate reduction in zero-valent iron sys-
tems with or without aqueous Fe
2+
, Water Res. 39
(2005) 1751–1760.
[24] Y.H. Huang, T.C. Zhang, Nitrite reduction and forma-
tion of corrosion coatings in zerovalent iron systems,
Chemosphere 64 (2006) 937–943.
[25] J.H. Fan, H.W. Wang, D.L. Wu, Z.G. Liu, L.M. Ma,
Scrap cast iron and copper-modified cast iron for
reductive degradation of 2,4-dinitrotoluene, J. Chem.
Technol. Biotechnol. 86 (2011) 1295–1302.
[26] K.R. Reddy, K. Darko-Kagya, C. Cameselle, Electroki-
netic-enhanced transport of lactate-modified nanoscale
iron particles for degradation of dinitrotoluene in
clayey soils, Sep. Purif. Technol. 79 (2011) 230–237.
19980 W. Jiao et al. / Desalination and Water Treatment 57 (2016) 19975–19980
