Degradation of aniline with zero-valent iron as an activator of persulfate in aqueous solution

Article abstract of DOI:10.1039/c3ra43364a

Zero valent iron (ZVI) can activate persulfate to generate sulfate free radicals which are a strong oxidant to degrade organic pollutants. The oxidative degradation of aniline in aqueous solution by persulfate activated with zero valent iron was studied under laboratory conditions. Batch experiments were conducted to investigate the effects of different parameters such as pH, ZVI concentration, aniline concentration, persulfate concentration and reaction temperature on aniline degradation. The results showed that aniline degradation increased with increasing temperature. The optimum dosage of ZVI was 0.4 g L^-1 and 85% aniline degradation was observed. Maximum aniline degradation was observed at pH 4.0, whereas at pH above or below 4.0, aniline degradation efficiency was decreased. In the persulfate-ZVI system, the apparent energy of activation for aniline degradation was 14.85 kJ mol^-1. The existence of persulfate radicals and hydroxyl radicals produced during the degradation of aniline were identified with scavenger ethanol and tert-butyl alcohol. The reaction intermediates nitrobenzene, nitroso-benzene and p-benzoquinone were detected by gas chromatography-mass spectrometry and based on these intermediates obtained a probable pathway for aniline degradation has been proposed.

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PAPER
Degradation of aniline with zero-valent iron as an
activator of persulfate in aqueous solution†
Cite this: RSC Adv., 2014, 4, 3502
a
ab
and Shaobin Huangabc
*
Imtyaz Hussain, Yongqing Zhang
Zero valent iron (ZVI) can activate persulfate to generate sulfate free radicals which are a strong oxidant to
degrade organic pollutants. The oxidative degradation of aniline in aqueous solution by persulfate activated
with zero valent iron was studied under laboratory conditions. Batch experiments were conducted to
investigate the effects of different parameters such as pH, ZVI concentration, aniline concentration,
persulfate concentration and reaction temperature on aniline degradation. The results showed that
aniline degradation increased with increasing temperature. The optimum dosage of ZVI was 0.4 g Lꢀ1
and 85% aniline degradation was observed. Maximum aniline degradation was observed at pH 4.0,
whereas at pH above or below 4.0, aniline degradation efficiency was decreased. In the persulfate-ZVI
system, the apparent energy of activation for aniline degradation was 14.85 kJ molꢀ1. The existence of
persulfate radicals and hydroxyl radicals produced during the degradation of aniline were identified with
scavenger ethanol and tert-butyl alcohol. The reaction intermediates nitrobenzene, nitroso-benzene and
p-benzoquinone were detected by gas chromatography-mass spectrometry and based on these
intermediates obtained a probable pathway for aniline degradation has been proposed.
Received 2nd July 2013
Accepted 4th November 2013
DOI: 10.1039/c3ra43364a
www.rsc.org/advances
high reactivity; O3 has low water solubility and KMnO4 only
reacts with unsaturated compounds.11–13
1. Introduction
Recently, persulfate have received much attention as
common oxidant for the treatment of organic contami-
nants.14–18 Persulfate is strong and non-selective oxidant with
high redox potential of 2.01 V. Persulfate ions are able to oxidize
many organic substances because persulfate anion is the most
powerful oxidant of the peroxygen family and one of the
strongest oxidants used in remediation. In aqueous solution the
sulfate radical is a strong oxidant with redox potential of 2.6 V
similar to that of hydroxyl radical (2.8 V). The persulfate anion
activated by initiator, including UV light, heat, bases, soil
mineral or transition metals such as ferrous iron, produces
sulfate free radical (SO4_ꢀ), which is a stronger oxidant
(E0 ¼ 2.6 V) than persulfate anion.19–21
Aromatic amines such as aniline are some of the most toxic
pollutants present in the effluent of many industries. Aniline
has been widely used as a raw material in many industries such
as plastic, paint, dye manufacturing, pesticides, rubber chem-
icals, antioxidants and pharmaceuticals.1–3 Aniline has been
classied as a persistent pollutant by China and USA because of
its potent toxicity to humans and wildlife.4,5 It has been recog-
nized as a high priority contaminant. Therefore, efficient
treatment methods for aniline have been explored which are:
ozonation, electro-Fenton and uidized-bed Fenton processes,
chemical oxidation and electro-catalytic oxidation.6–10
Advanced oxidation processes (AOPs) are being considered
as an alternative to various treatment technologies for the
treatment of contaminants. AOPs are based on the utilization of
highly reactive oxidizing radicals to oxidize organic pollutants.
Hydrogen peroxide, ozone, potassium permanganate were used
as common oxidants in AOPs.11–13 These oxidants have some
limitations, like H2O2 and O3 have short life spans due to their
2ꢀ
2ꢀ
S2O8 + 2eꢀ / 2SO4
(1)
(2)
(3)
(4)
(5)
2ꢀ
S2O8 + heat/UV / 2SO4_ꢀ
2ꢀ
S2O8 + 2Fe2+ / 2Fe3+ + 2SO4_ꢀ
aCollege of Environment and Energy, South China University of Technology,
Guangzhou, 510006, PR China. E-mail: zhangyq@scut.edu.cn; Fax: +86-20-
39380508; Tel: +86-20-39380569
2ꢀ
2ꢀ
S2O8 + Fe2+ / Fe3+ + SO4 + SO4_ꢀ
SO4_ꢀ + Fe2+ / Fe3+ + SO4
2ꢀ
bGuangdong Provincial Key Laboratory of Atmospheric environment and Pollution
Control, Guangzhou, 510006, PR China
At neutral pH, sulfate radicals have higher redox potential,
furthermore; sulfate radicals are more selective for oxidation at
acidic pH when compared to hydroxyl radicals. Sulfate radicals
are more effective for the degradation of recalcitrant organic
cState Key Laboratory of Pulp and Paper Engineering, South China University of
Technology, Guangzhou, 510640, PR China
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c3ra43364a
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compound due to higher stability in water.22 Sulfate radical system.39 Zhao et al. also reported 88% degradation of 4-chlor-
oxidation has numerous advantages over other oxidizing ophenol in persulfate-ZVI system.34 In the persulfate-ZVI
systems such as high solubility, relatively stable at room system, 98.5% decolorization of anthraquinone dye Reactive
temperature, wide operating pH range and strong oxidizing Blue 19 was observed in 90 min due to generation of sulfate
potential.23–25 As a result, sulfate radical-based advanced radicals.42 These results clearly show that, in persulfate-ZVI
oxidation processes have been applied successfully for the system, mostly aniline was degraded through oxidation of per-
remediation of different contaminants.26,27 Sulfate radicals are sulfate rather than reduction of ZVI. The results also indicated
also produced by the activation of persulfate by visible light, that ZVI can activate persulfate to start organic oxidation,
microwave, natural organic compounds, quinones and chelated consistent with our previous result with PCA.17 Furthermore,
metal complex.28–32 Persulfate activation by ferrous iron has the degradation of contaminants by sulfate radicals in persul-
been used for waste water treatment including pollutants such fate-ZVI system also requires further active research work.43,44
as aniline and diuron.2,3 However, excess of ferrous iron in
The objectives of these studies were to determine the per-
solution reacts with sulfate radical which results in the sulfate activation efficiency with ZVI for aniline degradation and
consumption of sulfate radical and iron hydroxide produced to observe the effects of different reactions parameters on
decreasing the oxidation efficiency.33–35 In the persulfate aniline degradation by persulfate and ZVI.
activation, it is important to maintain suitable quantities of
Fe2+, preventing the rapid conversion of Fe2+ to Fe3+ and scav-
2. Materials and methods
2.1 Chemicals
enging of sulfate radicals. Liang et al.36 used chelating agents
such as EDTA and citric acid which maintain the concentration
of Fe2+ in solution.
All chemicals used in this study were reagent grade and ultra-
pure water produced by a Millipore milli-Q system. Sodium
persulfate (>99.0%), sodium thiosulfate pentahydrate (>99.0%),
disodium hydrogen phosphate dodecahydrate (>99.0%),
sodium phosphate monobasic dehydrate (>99.0%), concen-
trated sulfuric acid (>98.0%), sodium hydroxide (>96.0%), and
aniline (>98.0%) were purchased from Tianjin Kermel Chemical
Reagent Co. Ltd (Tianjin China). Methanol (>99.9%) were
obtained from Sigma-Aldrich USA. The ZVI (>99.0% pure,
particle size 0.21 mm and surface area 0.14 m2 gꢀ1) used in this
study was purchased from Sigma-Aldrich USA.17
Zero valent iron was used as an alternative source of Fe2+
through the oxidation of ZVI to activate persulfate37 as shown in
the following equations.
Fe0 / Fe2+ + 2eꢀ
(6)
(7)
2ꢀ
2ꢀ
Fe0 + S2O8 / Fe2+ + 2SO4
ZVI is readily oxidized to Fe2+ ion in the presence of oxidants.
In the aqueous systems this phenomenon leads to the disso-
lution of solid which is the primary cause of metal corrosion.38
In persulfate-ZVI system, persulfate also directly reacts with ZVI
to release Fe2+ (eqn (7)). The generation of ferrous iron and 2.2 Experimental procedures
recycling of ferric iron at the ZVI surface can prevent the accu-
mulation of excess ferrous iron and reduce the precipitation of
iron hydroxides during the reaction (eqn (8)).
Stock solutions of aniline (10 mM) and persulfate (50 mM) were
prepared in deionized water prior to each batch experiment.
The buffer solution used was prepared by mixing 1.85 g Lꢀ1 of
dibasic sodium phosphate (Na2HPO4) and 1.52 g Lꢀ1 of
monobasic sodium phosphate (NaH2PO4). The initial pH of the
solution was 7.0. The pH values of all solutions were adjusted
with 1 M sodium hydroxide (NaOH) or sulfuric acid (H2SO4). All
of the reactions were carried out in 250 mL Erlenmeyer asks
operated as completely mixed batch reactor systems. Reaction
mixtures were obtained by mixing an appropriate amount of
aniline (0.5 mL, 0.05 mM) from stock solution and phosphate-
buffer solution (94.5 mL) on a rotary shaker at 125 rpm and
25 ꢁC. Aer 10 min, 0.5 mL solution was collected from the
reactor to determine the initial aniline concentration and then
predetermined amount of ZVI particles were added followed by
2Fe3+ + Fe0 / 3Fe2+
(8)
Oh et al.19 observed the activation of persulfate by ZVI
without involving aqueous Fe2+. Heterogeneous activation of
persulfate may be an alternative mechanism in which there is
direct electron transfer from ZVI or surface bound Fe2+ to
persulfate.39–41
0
Fe(s) + 2S2O8
2ꢀ
2ꢀ
+ 2SO4
/ Fe2+ + 2SO4_ꢀ
(9)
(aq)
(aq)
(aq)
Fe(surf) + S2O8 (aq) / Fe(surf) + 2SO4_ꢀ
+ SO4
(10)
2+
2ꢀ
3+
2ꢀ
(aq)
(aq)
In Fenton's oxidation process, ZVI was used as iron source. the addition of sodium persulfate (5 mL, 2.5 mM). At regular
As a result, hydroxyl radicals were produced which degrade time intervals, the sample aliquots were taken from asks and
environmental pollutants. The degradation of contaminants by ltered through 0.45 mm membrane. Aer ltration all samples
sulfate radical in persulfate-ZVI system is promising. Oh et al.19 were quenched immediately with sodium thiosulfate, to stop
investigated that the complete oxidation of polyvinyl alcohol the chemical oxidation reaction.
was obtained by ZVI-activated persulfate in 2 h due to produc-
Several sets of the experiments were conducted to determine
tion of sulfate radicals. Furthermore, 91% removal of 2,4-dini- the effects of various parameters on aniline degradation. To
trotoluene in 300 min was observed which shows fast and determine the effect of pH on the aniline degradation, ve pH
distinct degradation of 2,4-dinitrotoluene in persulfate-ZVI regimes of aqueous solutions at 2.0, 4.0, 7.0, 9.0, and 11.0 were
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studied, respectively. The effect of ZVI dosages on the aniline
degradation was studied at 0.1, 0.2, 0.3, 0.4 and 0.5 g Lꢀ1 ZVI,
respectively. To investigate the effect of temperature on the
oxidation of aniline in the persulfate-ZVI system, the tempera-
ture was maintained at 20, 40, 60 and 80 ꢁC. The effects of
aniline and persulfate concentration on the degradation of
aniline were also studied. For all experiments, duplicate reactor
systems were run simultaneously.
2.3 Analytical methods
The concentration of aniline in the aqueous phase was
measured by HPLC (Shimadzu LC-20) equipped with UV detector
at 244 nm. The HPLC column used was a reversed-phase C18
column (250 mm ꢂ 4.6 mm) and the temperature of the column
was maintained at 40 ꢁC. The mobile phase used was methanol–
water (70 : 30, v/v) with a ow-rate at 0.7 mL minꢀ1. The
concentration of Fe2+ was determined colorimetrically using a
UV-vis spectrophotometer at 510 nm aer adding 1,10-phenan-
throline to form a colored complex of Fe2+–phenanthroline.45
Persulfate ion was determined by iodometric titration with
sodium thiosulfate.46 The intermediates were identied on a
VARIAN 4000 gas chromatography tandem mass spectrometer
(GC-MS) with a weak polarity chromatographic column (0.15 mm
ꢂ 0.25 mm ꢂ 60 m). The temperature program used was: 0 min
at 40 ꢁC, raised to 100 ꢁC at 5 ꢁC minꢀ1, then increased to 300 ꢁC
at 15 ꢁC minꢀ1. The injector temperature was set at 250 ꢁC. The
surface morphology and chemical composition of ZVI was
analyzed using EVO LS10 (Car Zeiss) scanning electron micro-
scope (SEM) being operated at an acceleration voltage of 10 kV
and equipped with an Oxford Energy 400 X-ray energy dispersive
spectrometer (EDS, INCA Energy). X-ray powder diffraction
(XRD) pattern was obtained on a diffractometer with Cu KR
radiation (D8 ADVANCE, Bruker).
Fig. 1 Effect of ZVI, persulfate (PS) and persulfate-ZVI on the degra-
dation of aniline. [Aniline]0 ¼ 0.05 mM; temp. ¼ 25 ꢁC; [PS]0 ¼ 2.5 mM;
[ZVI] ¼ 0.3 g Lꢀ1; pH ¼ 7.0.
at xed oxidant concentration (2.5 mM). The removal of aniline
was signicant as shown in Fig. 2. The aniline degradation
efficiency increased with increasing ZVI doses. Higher amount
of ZVI provides more sites for sulfate radical generation thereby
increasing the rate of reaction. When the ZVI concentration was
varied from 0.1 to 0.4 g Lꢀ1, the aniline degradation efficiency
was 34%, 48%, 72% and 85%, respectively. The higher aniline
degradation at higher ZVI concentration might be due to higher
sulfate radical production. However, the degradation of aniline
decreased with the increase in ZVI concentration from 0.4 to
0.5 g Lꢀ1. The aniline degradation efficiency in the persulfate-
ZVI system followed this order 0.4 g Lꢀ1 > 0.5 g Lꢀ1 > 0.3 g Lꢀ1
>
0.2 g Lꢀ1 > 0.1 g Lꢀ1. As mentioned earlier, the activation of
persulfate might be through the electron transfer at the ZVI
surface or by Fe2+ in aqueous solution. As a result, persulfate
activation and degradation of aniline were increased. Another
set of experiment was conducted to compare the degradation of
aniline in persulfate-Fe2+ system. The degradation efficiency of
aniline was lower in persulfate-Fe2+ system (data shown in
ESI†). The reaction of aniline degradation was rapidly termi-
nated in persulfate-Fe2+ system because all the persulfate acti-
vated with excess Fe2+ quenched the sulfate radicals produced.
Based on the results and reaction rate constants, it can be
3. Results and discussion
3.1 Comparison experiments
Batch experiments were conducted to investigate the degrada-
tion of aniline by persulfate, ZVI and the combination of per-
sulfate and ZVI, respectively. The results showed that lower
aniline degradation was observed by persulfate or ZVI alone
when compared to persulfate-ZVI system. As shown Fig. 1, only
18.22% removal of aniline was observed in the ZVI system aer
5 h of reaction. The degradation of aniline occurred due to ZVI
surface adsorption and reduction. In the persulfate alone
degradation experiment, 38.50% aniline removal was observed.
In addition, oxidation of persulfate was also very slow. While in
the persulfate-ZVI system, 72% aniline degradation was ach-
ieved due to the generation of sulfate radicals. The aniline
decomposition efficiency in persulfate-ZVI system was much
higher when compared to ZVI and persulfate only.
3.2 Effects of ZVI dosage on aniline degradation
The effect of ZVI concentration on aniline degradation effi-
ciencies of persulfate-ZVI system was evaluated by conducting
Fig. 2 Effect of ZVI dosages on aniline degradation in persulfate-ZVI
system. [Aniline]0 ¼ 0.05 mM; temp. ¼ 25 ꢁC; [PS]0 ¼ 2.5 mM; pH ¼ 7.0.
experiments using different ZVI concentrations (0.1–0.5 g Lꢀ1
)
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concluded that higher Fe2+ concentration can also act as a
sulfate radical scavenger as mentioned by eqn (5) and caused
the decrease in sulfate radical concentrations.41 Fe2+ was rapidly
consumed and its concentration decreased sharply because of it
short lifespan. The previous studies also indicated that
destruction of sulfate radical might occurred in the presence of
excess Fe2+ due to the rapid conversion of Fe2+ to Fe3+, which
reduced the ultimate oxidizing capability.47–49 In persulfate-ZVI
system, a continuous supply of Fe2+ could be provided to the
persulfate system during the oxidation of ZVI by persulfate. The
activation of persulfate to generate sulfate radicals is more
effective when Fe2+ is supplied continuously when compared to
its one-time dose. The results showed that the constant activity
of ZVI in treatment or remediation processes can control
oxidation process in a better way.
3.3 Effect of persulfate and aniline concentration on aniline
degradation
Persulfate ions are able to oxidize many organic substances
because persulfate anion is the most powerful oxidant of the
peroxygen family and one of the strongest oxidants used in
remediation. In aqueous solution the sulfate radical is a strong
oxidant with redox potential of 2.6 V similar to that of hydroxyl
radical (2.8 V). In this study, the effect of persulfate concen-
trations (1–5 mM) on the degradation of aniline was studied at
xed concentration of ZVI (0.3 g Lꢀ1) (Fig. 3a). The higher per-
sulfate concentration resulted in higher production of sulfate
free radicals as well as higher degradation of contaminant. The
Fig. 3 (a) Effect of persulfate concentration on the degradation of
aniline; (b) effect of aniline concentration on the degradation of
aniline. Temp. ¼ 25 ꢁC; [ZVI] ¼ 0.3 g Lꢀ1; pH ¼ 7.0.
results also showed that aniline degradation increased with persulfate-ZVI system at different aniline concentration 0.05,
increasing amount of persulfate (1–2.5 mM). Furthermore, 72% 0.10, 0.20, 0.40 mM were 0.0039, 0.003, 0.018 and 0.011 minꢀ1
aniline degradation was achieved within 5 h of reaction with respectively. Increase in the concentration of aniline from 0.05–
2.5 mM persulfate concentration. However, further increase in 0.40 mmol Lꢀ1 decreased the degradation of aniline from 72%
persulfate concentration above certain limits (critical concen- to 29% aer 300 min (Fig. 3b). The lower degradation of aniline
tration), decreased the degradation rate of aniline because was due to excess concentration of aniline that might cover the
persulfate acts as a sulfate radical scavenger instead of a free- iron surface and active sites for persulfate, retarding the
radical generator.42
degradation reaction.
SO4_ꢀ + S2O8 / SO4 + S2O8_ꢀ
(11)
(12)
2ꢀ
2ꢀ
3.4 Effect of pH on aniline degradation
2ꢀ
SO4_ꢀ + SO4_ꢀ / S2O8
The solution pH is an important parameter affecting the
degradation of organic pollutants. Therefore the effect of pH on
the aniline degradation efficiency was studied. The experiment
The equation (11) showed that the addition of excess per- was carried out by changing in the initial pH of persulfate-ZVI
sulfate in persulfate-ZVI system, persulfate can compete with system from 2–11. The results showed that the degradation of
aniline for sulfate free radicals which can decrease the sulfate aniline was signicantly inuenced by solution pH. The
free radicals in the solution and then decreasing the aniline degradation of aniline decreased with the increase in the initial
removal efficiency. The optimum persulfate concentration for pH value (Fig. 4). As shown in Fig. 4, rapid degradation of
aniline degradation was 2.5 mM (Fig. 3a).
aniline was observed at pH 4.0. At pH 4.0, aniline was
The degradation efficiency also depends on the initial completely removed aer 10 min. These results are similar to
concentration of the pollutant. The effect of initial concentra- our recent study in which complete p-chloroaniline (PCA)
tion of aniline on the rate of degradation in persulfate-ZVI degradation was observed at pH 4.0.17 The degradation of
system was investigated in the range of 0.05–0.40 mmol Lꢀ1 aniline observed at pH 2.0, was 71% aer 1 h and 76% aer 5 h.
while maintaining the other parameters constant. It was The rate of aniline degradation at pH 7.0, 9.0 and 11 were 72%,
observed that initial concentration of aniline had a signicant 56.49% and 43.97%, respectively. The results obtained in this
effect on the degradation of aniline. It was found that increase study were also in agreement with the previous ndings which
in aniline concentration decreased the rate constant of aniline showed the rate of PCBs, propachlor, diphenylamine and azo
degradation. The rate constant of aniline degradation in dye orange G degradation by persulfate decreased with the
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2ꢀ
(SO4 ).47,51 The degradation efficacy of aniline decreased due to
recombination of SO4_ꢀ and _OH as shown in eqn (19).
SO4_ꢀ + _OH / HSO4ꢀ + 0.5O2
(19)
3.5 Effect of temperature on aniline degradation
In order to elucidate the effect of temperature on the degra-
dation of aniline in persulfate-ZVI system a series of experi-
meꢁnt were conducted at different temperature from 20 to
80 C. As the temperature of the system increased, the degra-
dation of aniline was faster. The results show (Fig. 5a) that
aniline degradation efficiency increases with increase in
temperature of persulfate-ZVI system. Complete degradation
of aniline was observed for reaction carried out at 80 ꢁC and 60
ꢁC over 1 h and 2 h, respectively. However, the aniline degra-
dation efficiencies were about 69 and 84% at 20, 40 ꢁC,
respectively. The results revealed that more generation of
sulfate radicals occurred at higher temperature. This is
because higher temperature increased the reaction rate
between persulfate and ferrous iron. The rate constant of
aniline degradation in persulfate-ZVI system at 2ꢀ01, 40, 60,
80 ꢁC were 0.0037, 0.006, 0.0147, and 0.0319 min respec-
tively. Oh et al.19 observed that temperature is an important
parameter to promote the oxidation of polyvinyl alcohol in
Fig. 4 Effect of pH on the degradation of aniline in persulfate-ZVI
system. [Aniline]0 ¼ 0.05 mM; [PS]0 ¼ 2.5 mM; temp. ¼ 25 ꢁC; [ZVI] ¼
0.3 g Lꢀ1
.
increase in the pH value.50–53 In case of pH 2.0, however,
degradation of aniline decreased when compared to pH 4.0,
most likely because of the scavenging reactions of SO4_ꢀ. Under
strong acidic conditions (i.e., pH 2.0) the generation of SO4_ꢀ
increased due to acid-catalyzation via eqn (13) and (14), and
then higher SO4_ꢀ radical concentrations, which favored scav-
enging of SO4_ꢀ by itself, or SO4_ꢀ/S2O82ꢀ reaction in accordance
with eqn (11) and (12).22
2ꢀ
S2O8 + H+ / HS2Oꢀ8
(13)
(14)
2ꢀ
HS2Oꢀ8 / SO4_ꢀ + SO4 + H+
Nevertheless, the aniline degradation efficiency at pH 2.0
was as high as 76.0% comparable to that at near neutral and
basic pH. An optimum pH, 4.0 is appropriate and either
decreasing or increasing the pH would result in the decrease of
the degradation process. The precipitation of Fe3+ ions occurred
whenever the pH was higher than 4.0. The oxyhydroxides of Fe3+
had low sulfate radical production efficiency by activating per-
sulfate. The following equations indicate the formation of Fe3+
oxyhydroxides:
Fe3+ + H2O / FeOH2+ + H+ (k ¼ 2.3 ꢂ 107 sꢀ1
)
(15)
(16)
Fe3+ + 2H2O / Fe(OH)2+ + 2H+ (k ¼ 4.7 ꢂ 103 sꢀ1
)
4+
2Fe3+ + 2H2O / Fe2(OH)2 + 2H+ (k ¼ 1.1 ꢂ 107 sꢀ1) (17)
The degradation of aniline at high pH values may be due to
base activation of persulfate for the generation of sulfate free
radicals in persulfate-ZVI system. The base activation mecha-
nism of persulfate was shown by the following equation.20,51
2ꢀ
2ꢀ
2S2O8 + 2H2O / SO4_ꢀ + 3SO4 + O2ꢀ_ + 4H+
(18)
Fig. 5 (a) Effect of temperature on the degradation of aniline in per-
sulfate-ZVI system. [Aniline]0 ¼ 0.05 mM; [PS]0 ¼ 2.5 mM; [ZVI] ¼ 0.3 g
Moreover, under strong basic conditions sulfate radicals can
react with hydroxyl anion to generate cOH and the reactivity of
cOH is inhibited due to the presence of various anions pH ¼ 7.0.
L
ꢀ1; pH ¼ 7.0. (b) Effect of temperature on the degradation of aniline
with persulfate. [Aniline]0 ¼ 0.05 mM; [PS]0 ¼ 2.5 mM; [ZVI] ¼ 0.3 g Lꢀ1
;
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persulfate-ZVI system. A set of experiments without ZVI were
conducted to investigate the effect of temperature on aniline
degradation because persulfate can also be activated by heat-
ing to generate sulfate radicals.8,14,19 Aniline degradation
increased with increasing temperature from 20 to 80 ꢁC
without ZVI (Fig. 5b), but aniline degradation efficiency is less
when compared to persulfate-ZVI system. The activation
energy of the reaction was obtained by using Arrhenius
equation.
ln k ¼ ln A ꢀ Ea/RT
where A is the frequency factor, R is the universal gas constant,
Ea is the activation energy and T is the temperature in Kelvin. As
shown inset Fig. 5, a good linear relationship was observed
between ln k vs. 1/T. The activation energy for the degradation
of aniline in persulfate-ZVI system was 14.85 kJ molꢀ1. This
lower value of activation energy indicates that aniline degra-
dation requires low energy in persulfate-ZVI system when
Fig.
6
Effect of EtOH and TBA as radical scavengers on the
degradation of aniline in persulfate-ZVI system. [Aniline]0 ¼ 0.05 mM;
[PS]0 ¼ 2.5 mM; temp. ¼ 25 ꢁC; [ZVI] ¼ 0.3 g Lꢀ1; pH ¼ 7.0, EtOH ¼ 1
M, TBA ¼ 1 M.
compared to thermal reaction energy of 60–250 kJ molꢀ1 53 The
.
EtOH + SO4ꢀ_ ꢃ 1.6–7.7 ꢂ 107 Mꢀ1 sꢀ1
activation energy of catalytic degradation of aniline by immo-
bilized iron oxide catalysts was reported to be 13.65–15.65 kJ
molꢀ1
.
Shahrezaei et al.55 observed the activation energy of
54
On the basis of the above mentioned rate of constants for
TBA and EtOH, it can be inferred that in TBA system the
degradation of aniline was due to the formation of sulfate
radicals as TBA quenched hydroxyl radicals only. While in the
presence of EtOH, the efficiency of aniline degradation was
much lower (16%) because both radicals were scavenged.
Sulfate radicals are more selective for electron transfer reac-
tion while hydroxyl radicals rapidly undergo by hydrogen
addition or abstraction. Therefore, we observed that sulfate
radicals are dominant active species for the degradation of
aniline.
20.337 kJ molꢀ1 for the degradation of aniline by photocatalytic
using TiO2 nanoparticles. As compared to above mentioned
activation energy, the relative lower value of aniline degradation
in persulfate-ZVI system implies that such reaction can be easily
attained.
3.6 Identication of predominant radicals in persulfate-ZVI
system
To identify the dominating radical species in persulfate-ZVI
system, alcohols were used to quench the hydroxyl and sulfate
radicals. Therefore, ethanol (alcohol with alpha hydrogen) was
used as hydroxyl and sulfate radical scavenger and tert-butyl
alcohol (TBA) with no alpha hydrogen were effective quenching
agents for hydroxyl radicals. In this study, alcohol (EtOH, TBA)
concentration used was 1 M. The results clearly showed that
aniline degradation was signicantly decreased by the addition
of alcohols. As shown in Fig. 6, the degradation of aniline in
persulfate-ZVI system was 85% without any radical scavenger.
However, in the presence of ethanol (EtOH), only 16% aniline
degradation was observed. While in the presence of TBA, the
degradation of aniline was 44%. The results revealed that the
reaction was completely quenched by the addition of EtOH as
_OH radical and SO4ꢀ_radical scavengers, while with the addition
of TBA, the degradation rate of aniline was moderately inu-
enced. According to previous literature, the second order rate
constants of the EtOH and TBA with hydroxyl radicals and
sulfate radicals are following.48,56–58
3.7 Removal of TOC in persulfate-ZVI system
The TOC removal efficiency indicates the effective degradation
of organic pollutants. The experiment was conducted to
investigate the removal of TOC. Experiment was carried out at
aniline initial concentration 0.05 mM Lꢀ1, persulfate 2.5 mM
Lꢀ1, dosage of ZVI 0.3 g Lꢀ1 and pH 7.0. Aniline mineralization
by persulfate-ZVI process was quantied by measuring TOC
contents. As shown in Fig. 7, aniline mineralization was
signicant, where 25% TOC removal efficiency was achieved
within 1 h. TOC removal efficiency increased with increasing
time, 61.5% TOC was removed aer 5 h reaction time. The
results showed that aniline was not directly oxidized to CO2
and H2O in one step. Therefore, intermediates were produced
and then open-ringed and organic acids were produced.
Sulfate radicals were considered as major oxidant for the
degradation of aniline and its intermediates. At pH 7.0, aniline
degradation was higher when compared to TOC removal effi-
ciency. The degradation of aniline was 72% and TOC removal
was 61.5% in ZVI-activated persulfate system. TOC removal
efficiency of aniline was more when compared to p-chlor-
oaniline.17 In other study only 24% TOC removal was observed
during degradation of aniline.59 The results showed that
t-BuOH + HO_ ꢃ 3.8–7.6 ꢂ 108 Mꢀ1 sꢀ1
t-BuOH + SO4ꢀ_ ꢃ 4.0–9.1 ꢂ 105 Mꢀ1 sꢀ1
EtOH + HO_ ꢃ 1.2–2.8 ꢂ 109 Mꢀ1 sꢀ1
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oxidation of benzoquinonimine produced p-benzoquinone.
Further degradation of nitrobenzene, as well as p-benzoqui-
none produced maleic acid, and further maleic acid trans-
formed into oxalic acid. The maleic acid directly or via oxalic
acid mineralized to CO2 and H2O. The generation of hydroxyl
and persulfate radicals is essentially important in organic
oxidation because these are powerful oxidizing agents. Sulfate
and hydroxyl radicals are electrophilic radicals whom attack
on benzene ring result into mineralization of aniline and its
intermediates to CO2 and H2O.
Fig. 7 Removal of TOC in persulfate-ZVI system. [Aniline]0 ¼ 0.05
mM; [PS]0 ¼ 2.5 mM; [ZVI] ¼ 0.3 g Lꢀ1; pH ¼ 7.0.
4. Observation of ZVI surface
The changes in the iron surface were observed by conducting
SEM with corresponding EDS and XRD. The result showed
that different surface texture was seen due to corrosion of
iron by persulfate oxidation on the surface of ZVI. An amor-
phous plate-like porous structure (Fig. 9b) was formed in ZVI/
PS system (in the absence of aniline) while coarse and
grooving aggregate formation was seen in ZVI/PS/aniline
system (Fig. 9c, in the presence of aniline).37 As shown in
persulfate-ZVI combined system was not only effective for the
degradation of aniline, but also for the mineralization of
aniline.
3.8 Mechanism of aniline degradation
To explore the formation of intermediates during the degra- Fig. 10, SEM-EDX analysis showed the changes occurred in
dation of aniline in persulfate-ZVI was identied by gas the elemental composition of ZVI. The ZVI surface exhibited
chromatography-mass spectrometry (GC-MS). The decompo- the presence of oxygen aer oxidation with persulfate.
sition of persulfate in aqueous solution produced hydroxyl and Atomic weight-based quantitative SEM-EDX peak area anal-
sulfate radicals which can oxidize many organic pollutants ysis showed original ZVI having Fe : C ¼ 99.00 : 1.00%
into carbon dioxide.13 The results revealed that during the (Fig. 10a). The elemental composition changed to Fe : O : C ¼
degradation process of aniline, the color of aniline solution 58.77 : 40.23 : 1.00 aer reaction with persulfate (Fig. 10b).
varied from colorless to yellow and brown respectively and This
%
atomic ratio further changed to Fe : O : C
¼
then ultimately became colorless. During the process of 19.31 : 55.24 : 25.45 in ZVI/PS/aniline system (Fig. 10c). The
degradation, some complex intermediate products were XRD diffractogram of original ZVI is shown in Fig. 11a. The
developed causing a change in the color of aniline solution. Fig. 11a indicates that iron is mainly in its Fe0 state, char-
The possible intermediate products were p-benzoquinone, acterized by the basic reection appearing at a 2-theta value
nitroso-benzene and nitrobenzene. Based on the intermedi- of 44.9ꢁ. Signal of the iron oxides (magnetite, Fe3O4) were
ates detected in this study and previous literature,6,10
a
observed at a 2-theta value of 35.4 in the XRD patterns of
pathway might be proposed for the degradation of aniline in persulfate-ZVI system. It indicated that the reections of
persulfate-ZVI system (Fig. 8). The degradation of aniline magnetite in the spectrum were very weak, but magnetite
process initiated by the attack of sulfate and hydroxyl radicals reection was not observed in the XRD spectrum in ZVI/PS/
and yield either benzoquinonimine or nitrobenzene. The aniline system (Fig. 11c).
Fig. 8 Proposed reaction pathway for the degradation of aniline degradation in persulfate-ZVI.
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Fig. 9 SEM images of the ZVI surface from the systems of (a) original
ZVI, (b) ZVI/PS, and (c) aniline/ZVI/PS.
Fig. 11 XRD spectrum of ZVI particles for (a) original ZVI, (b) ZVI/PS,
and (c) aniline/ZVI/PS.
5. Conclusions
The oxidation system consisting of persulfate-ZVI has been
demonstrated to be an effective oxidant reagent to degrade
aniline in aqueous solution. In the persulfate-ZVI system the
decomposition of persulfate occurred more rapidly due to the
release of Fe2+ from ZVI. Increased ZVI dosages resulted in
enhanced aniline degradation due to increased activation of
persulfate by ZVI. Aniline was successfully degraded in the
persulfate-ZVI system within 1 h at a temperature of 80 ꢁC. The
oxidation reaction showed excellent Arrhenius behavior with an
activation energy of about 14.84 kJ molꢀ1. The results showed
that high temperature and acidic pH values e.g. 4.0 were more
favorable than neutral and basic pH for complete degradation
of aniline. The optimum pH for degradation of aniline was 4.0
at which 100% degradation efficiency was achieved aer 10
min. Radical scavengers such as EtOH and TBA demonstrated
the responsibility of sulfate radicals as well as hydroxyl radicals
in the degradation of aniline. Nitrobenzene, p-benzoquinone
Fig. 10 Results of EDS analysis for residual elements on the ZVI
surface (a) original ZVI, (b) ZVI/PS, and (c) aniline/ZVI/PS.
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´
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Acknowledgements
This study was nancially supported by the Foundation of
Science and Technology Planning Project of Guangdong Prov-
ince (2010B050200007), the Fundamental Research Funds for
the Central Universities (2011ZM0054) and the Research Fund
Program of Guangdong Provincial Key Laboratory of Environ-
mental Pollution Control and Remediation Technology
(2011k0013) and Research Funds of Guangdong Provincial Key
Laboratory of Atmospheric environment and Pollution Control.
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