Solar-Fenton removal of malachite green with novel Fe0-activated carbon nanocomposite

Article abstract of DOI:10.1016/j.apcata.2014.02.009

Zero valent iron-activated carbon nanocomposite (FeAC) was synthesized using liquid phase reduction method. Both FeAC and micro zero valent iron (ZVI) were characterized by scanning electron microscope (SEM), transmission electron microscopy (TEM), X-ray Diffraction (XRD), Fourier Transform Infrared (FTIR), UV-visible (UV-vis) spectral techniques. The size of Fe¹-AC was observed to be 50 nm. The catalytic efficiency of FeAC was explored for the removal of malachite green dye (MG) from aqueous phase. Fe¹-AC induced solar-Fenton removal of MG was investigated in the presence of H ₂1₂ and citrate (cit.).Solar FeAC system exhibited highest removal efficiency among investigated photocatalytic systems. The dye removal was strongly influenced by solar light, pH, Fe¹-AC dosage, Hand citrate concentration in reaction system. The reaction was maximal at pH 6.75. MG was completely decolorized in 60 min using solar/H₂1 ₂/cit./Fe¹-AC system. Hydroxyl radicals (OH²) were the main oxidizing specie during solar-Fenton process. Fe¹-AC proved to be a potential adsorbent for MG. Fe¹-AC showed high recycle efficiency due to easier separation from aqueous phase. The lower concentration of dissolved Fe(II) ions in reaction solution indicated the stability of Fe¹-AC during MG removal. On the basis of obtained results, the most plausible mechanism for oxidative removal MG of was proposed.

Full text of DOI:10.1016/j.apcata.2014.02.009

Applied Catalysis A: General 476 (2014) 9–18
Contents lists available at ScienceDirect
Applied Catalysis A: General
journal homepage: www.elsevier.com/locate/apcata
0
Solar-Fenton removal of malachite green with novel Fe -activated
carbon nanocomposite
a,∗
a,b
a
a
a
a
Pardeep Singh , Pankaj Raizada , Shailza Kumari , Amit Kumar , Deepak Pathania ,
Pankaj Thakur
a
School of Chemistry, Faculty of Basic Science, Shoolini University, Solan Himachal Pradesh, 173212, India
Center for Advanced Biomaterials for Health Care (Istituto Italiano Di Tecnologia) Largo Barsanti e Matteucci 53, 80125 Naples, Italy
b
a r t i c l e i n f o
a b s t r a c t
0
Article history:
Received 4 October 2013
Received in revised form
Zero valent iron-activated carbon nanocomposite (Fe -AC) was synthesized using liquid phase reduc-
0
tion method. Both Fe -AC and micro zero valent iron (ZVI) were characterized by scanning electron
microscope (SEM), transmission electron microscopy (TEM), X-ray Diffraction (XRD), Fourier Transform
Infrared (FTIR), UV-visible (UV-vis) spectral techniques. The size of Fe -AC was observed to be 50 nm. The
catalytic efficiency of Fe -AC was explored for the removal of malachite green dye (MG) from aqueous
phase. Fe -AC induced solar-Fenton removal of MG was investigated in the presence of H2O2 and citrate
cit.). Solar/H2O2/cit./Fe -AC system exhibited highest removal efficiency among investigated photocat-
alytic systems. The dye removal was strongly influenced by solar light, pH, Fe -AC dosage, H2O2 and citrate
concentration in reaction system. The reaction was maximal at pH 6.75. MG was completely decolorized
1
0 December 2013
0
Accepted 8 February 2014
Available online 17 February 2014
0
0
0
(
Keywords:
0
0
Fe -AC
Solar-Fenton reaction
Oxidative removal
Recycle efficiency
•
0
in 60 min using solar/H2O2/cit./Fe -AC system. Hydroxyl radicals (OH ) were the main oxidizing specie
during solar–Fenton process.
0
0
Fe -AC proved to be a potential adsorbent for MG. Fe -AC showed high recycle efficiency due to easier
separation from aqueous phase. The lower concentration of dissolved Fe(II) ions in reaction solution
0
indicated the stability of Fe -AC during MG removal. On the basis of obtained results, the most plausible
mechanism for oxidative removal MG of was proposed.
©
2014 Elsevier B.V. All rights reserved.
1
. Introduction
Several heterogeneous photocatalysts such as Fe O hollow sphere
2 3
0
[
8], Cu modified iron oxide [9], silica supported iron oxide [10], Fe -
Advanced oxidation processes (AOPs) have gained immense
supported on resins [11] and pillared clays [12] have been prepared
for wastewater treatment.
importance owing to their applicability in wastewater treatment
in recent years. Habitually, AOPs are characterized by the forma-
For the past fifteen years, ZVI particles have been widely used
to reduce various contaminants present in wastewater [13]. Nano-
zero valent iron (nZVI) exhibits at least 25–30 times faster reactivity
than bulk ZVI [14]. nZVI is highly effective for the degradation of ␤-
lactam antibiotics (amoxicillin and ampicillin) [15], metronidazole
antibiotic [16], azo dyes [17], chlorinated solvents [18], chlorinated
pesticides [19], organophosphates [20], nitroaromatics [21] and p-
chlorophenol [22]. However, degradation rate is retarded by very
limited mobility of nZVI due to particle aggregation [23]. In order
to overcome these drawbacks, recent studies have been focused
on the incorporation of ZVI with different organic/inorganic sup-
ports. Xu et al. prepared nanoscale iron hydroxide doped granular
activated carbon for the adsorption of perchlorate in water [24].
Commercial activated carbon (CAC) seems to be an excellent sup-
porting material for nZVI because of its high chemical stability,
mechanical robustness, large specific surface area and commercial
availability [25].
•
•
−
tion of highly reactive species (H O , OH , O , OH 2) to degrade
2
2
2
refractory organic/inorganic compounds present in wastewater
1,2]. Among AOPs based remediation technology, homogenous
[
Fentonreactions have emerged as important way to generate highly
•
•
reactive and non-selective OH radicals [3,4]. The generated OH
radicals have high oxidizing ability and degrade persistent and
non-biodegradable pollutants present in aquatic environment [5].
The efficiency of Fenton process can be improved by the use of
visible light [6]. Moreover, heterogeneous photo-Fenton reactions
have attracted many researchers due to high reaction efficiency
and easier separation of photocatalyst from treated water [7].
∗ Corresponding author. Tel.: +91 1792 308000; fax: +91 1792 30800.
E-mail addresses: pardeepchem@gmail.com (P. Singh),
pankajchem1@gmail.com (P. Raizada).
http://dx.doi.org/10.1016/j.apcata.2014.02.009
0
926-860X/© 2014 Elsevier B.V. All rights reserved.

1
0
P. Singh et al. / Applied Catalysis A: General 476 (2014) 9–18
Most of ZVI based remediation techniques are reductive in
nature [26]. However, in the presence of H O /ZVI, hydroxyl radi-
2. Materials and methods
2
2
cals are generated through Fenton reaction leading to the oxidation
of organic compounds. The consecutive formation of Fe(II) is
needed to maintain the efficiency of Fenton process. However, the
rate of Fe(II) formation is comparatively low in H O and ZVI medi-
2.1. Chemicals
All the chemicals used in this study were of analytical grade. Fer-
rous sulphate (FeSO ·7H O) and powdered activated carbon (200
2
2
4
2
ated Fenton reaction (Eqs. (1) and (2)).
mesh) were purchased from Sigma-Aldrich, India. Polyethylene
glycol 6000 (PEG 6000), hydrogen peroxide, ethanol, hydrochlo-
ric acid (HCl), sodium borohydride (NaBH ), malachite green,
1,10-phenanthroline monohydrate, isopropanol and sodium citrate
4
•
Fe(II) + H O → Fe(III) OH + OH k = 40–60 L(mol s−1
+
−
)
(1)
2
2
(
Na C H5O7) were obtained from Merck, India. All the solutions
3 6
were prepared in doubly distilled water.
•
+
k = 2 × 10−3 L(mol s ) (2)
−1
Fe(III) + H O → Fe(II) + H + OH2
2
2
0
2
.2. Synthesis of Fe -AC
hꢀ
0
Fe(III)–organic acid complex−→ Fe(II) + organic radicals
(3)
The synthesis of Fe -AC was performed using liquid phase
reduction method [32]. In typical synthesis, powdered activated
carbon was washed with distilled water and dried at 60 C
◦
The combination of visible light and organic acid enhances
the regeneration of Fe(II) through photo-transformation of Fe(III)-
overnight. In the next step, 1.5 g of FeSO ·7H O and 1.2 g of dried
4
2
activated carbon were mixed in 100 mL distilled water and heated
◦
organic acid complex into Fe(II) and organic radicals(Eq. (3))
at 50 C for 20 min. To this mixture, 70% ethanol (100 mL) and 1.0 g
•
as well as resulting into additional OH radical formation [27].
of polyethylene glycol 6000 (PEG 6000) were added with constant
stirring. The reaction pH was adjusted to 7.5 using 1 M NaOH. In the
At the same, organic acid can be used as pH buffer to main-
tain constant pH of reaction system [28]. Katsumata et al.
next step, 0.5 M sodium borohydride (NaBH ) was added dropwise
4
introduced UV/Fe(II)–citrate/H O2 system at pH 8.0 for alachlor
with constant stirring for 3 h to obtain black colored precipitates.
The precipitates were filtered and washed with degassed water and
finally vacuum dried. The obtained product was leveled as Fe -AC
and placed in anaerobic chamber filled with pure N2 gas prior to
use. This methodology was also adopted for the preparation of ZVI
with no addition of activated carbon powder.
2
degradation [29]. Silva et al. investigated degradation of her-
bicide tebuthiuron using UV-Fenton and ferric citrate complex
system at neutral pH [30]. Lucas and Peres explored UV/Fenton and
ferrioxalate/H O /solar light based processes with dissolved iron to
0
2
2
degrade reactive black dye [31].
0
In precedent work, we preferred the use of Fe -AC instead of
nZVI powder to reduce the difficulties in separating ZVI particles.
Solar light was used throughout the experiment rather than expen-
2.3. Adsorption and photocatalytic experiments
sive artificial source of light. The foremost objective of our research
Double walled pyrex vessel (ht. 7.5 cm × dia. 6 cm) was used
0
was to explore the photocatalytic efficiency of solar/H O /cit./Fe -
0
2
2
to investigate adsorptional and photocatalytic activity of Fe -AC.
The pyrex vessel was surrounded by thermostatic water circulation
AC system for MG degradation in aqueous medium. The effect of
0
reaction parameters such as Fe -AC dosage, pH, citrate and H O
◦
2
2
arrangement to keep temperature in the range of 30 ± 0.3 C (Fig. 1).
concentration on MG removal was also investigated. The adsorp-
During adsorption experiments, slurry composed of dye solution
and catalyst suspension was stirred magnetically and placed in
0
tion behavior of Fe -AC was also explored for the MG removal.
Fig. 1. Schematic diagram of photoreactor.

P. Singh et al. / Applied Catalysis A: General 476 (2014) 9–18
11
0
dark to equilibrate adsorption and desorption of MG molecule onto
catalyst surface. Prior to photocatalytic studies, suspension com-
posed of dye and catalyst was stirred for ten min in dark. Then
suspension was exposed to solar light with continuous stirring.
At specific time intervals, aliquot (3 mL) was withdrawn and cen-
trifuged for 2 min to remove catalyst particles form aliquot. The
concentration of MG in supernatant was determined on double-
beam UV-Vis spectrophotometer at 620 nm. The average intensity
of solar light between 11 am to 2 pm was measured by a digital
2.5. Characterization of Fe -AC
SEM micrographs have been obtained through a Quant-250,
model 9393 by attaching the powder samples onto adhesive car-
bon tapes supported on metallic disks. Sample surfaces were then
observed at different magnifications and images were recorded.
Energy dispersive X-ray (EDX) analysis was performed at randomly
selected locations on the solid surfaces. EDX mapping was car-
ried out at 1000× magnification under vacuum conditions. The
3
0
lux-meter (35 × 10 ± 1000 lx). All the photocatalytic experiments
microstructure of Fe -AC was analyzed using TEM (Tecnai 20
were performed in March to May 2012 between 11 am to 2 pm. All
the experiments were undertaken in triplicate with errors below
G2 Plate/CCD Camera). Fourier-transform Infrared Spectra (FTIR)
was obtained using Perkin Elmer spectrometer (Spectrum 400,
USA). XPERT-PRO diffractometer system was used for powder XRD
analysis. UV-visible analysis was performed using UV-Visible spec-
trophotometer (Systronics 117). Cu K␣ radiations at wavelength
of 1.54 A˚ were used as the source of X-rays. Fe -AC particles were
placed in glass holder and were recorded over 2ꢁ range of 10 –100 .
5
% and average values were reported. The removal efficiency was
calculated using following equation:
0
C − Ct
0
%
removal efficiency =
× 100
(4)
◦
◦
C
0
0
Crystalline size of Fe -AC was calculated using Debye-Scherer for-
mula (Eq. (6)):
where C0 is the initial concentration and Ct is instant con-
centration of dye sample. Fe(II) concentration was determined
spectrophotometrically (absorbance measurement at 510 nm) by
kꢂ
D =
(6)
ˇ cos ꢁ
1
,10-phenanthroline method [33]. The kinetics of MG degradation
was explained by pseudo first order kinetics. The rate constant (k)
were calculated using Eq. (5):
where D = the thickness of the nanocrystal, k (constant) = (0.9),
ꢂ = wavelength of X-rays (1.54 A), ˇ = width at half maxima of
reflection at Bragg’s angle 2ꢁ (ꢁ = Bragg’s angle).
˚
k = 2.303 × slope
(5)
3
3
3
. Results and discussion
where the slope was obtained from the plot of ln(absorbance)
versus t.
0
.1. Characterization of Fe -AC, AC and ZVI
0
.1.1. SEM, TEM, EDX, XRD and UV-vis analysis of AC, Fe -AC and
2.4. Determination of pH of zero point charge
ZVI
Fig. 2 a and b depicts the uniform porous nature of activated
The pH of zero point charge (pHzpc) was estimated by pH drift
carbon. These pores (10 ␮m) provided a good possibility for ZVI
particlesto be trapped inside them. Fig. 2c and d indicates that
in the absence of activated carbon, ZVI particles were found to
agglomerate more rapidly to microsphere of 20 ␮m. Fig. 2e and f
confirms the presence of both zero valent iron and activated carbon
method [34]. For this purpose, 50 mL solutions of 0.01 M sodium
chloride were adjusted to initial pH values between 2 and 12 in.
0
Then, AC/Fe -AC (0.10 g) was added into this solution. After 48 h,
final pH of solution was measured and plotted against initial pH.
The pH at which the curve crossed the line of equality was taken as
in Fe0−AC. Fe0 aggregates were uniformly dispersed over activated
carbon matrix.
0
the pHpzc of AC and Fe -AC.
0
Fig. 2. (a–f) SEM images of activated carbon (a and b), zero valent iron (c and d) and Fe -AC (e and f) at 2000 (a), 5000 (b), 5000 (c), 10,000 (d), 500 (e) and 5000 (d)
magnifications.

1
2
P. Singh et al. / Applied Catalysis A: General 476 (2014) 9–18
0
Fig. 3. TEM images of Fe -AC and zero valent iron (ZVI).
TEM images demonstrate that Fe0 was distributed in chain like
of Fe-O were observed at 420 cm
−1
and 618 cm , which con-
−1
0
−1
manner over activated carbon (Fig. 3a). Fe -AC comprised of rod
like and spherical particles arranged in irregular fashion. The aver-
firmed the presence of zero valent iron [36]. The band at 3393 cm
ascribed to OH stretching vibration and one at 1636 cm to the OH
bending vibration of surface-adsorbed water of AC [37]. The peaks
at 2805 and 1108 cm were due to C–H and C–O stretching vibra-
tion of activated carbon. The peak at 1384 cm was assigned to
deformation vibration of hydroxyl groups [37,38]. MG dye was suc-
−
1
0
age size of Fe -AC carbon was found to be 50 nm while ZVI particles
−
1
tend to form aggregation in the range of 200 nm. TEM results clearly
indicated the agglomeration of ZVI in the absence of activated car-
bon and were in agreement with SEM analysis.
−
1
0
0
0
Fig. 4a–c demonstrates EDX images of AC, ZVI and Fe -AC. In Fe -
AC, zero valent iron (45%) and activated carbon (51%) are the main
cessfully adsorbed onto Fe -AC (Table 1). The characteristic peaks
of MG loaded Fe -AC are shown in Table 1.
0
0
constituents of nanocomposite. XRD pattern of Fe -AC shows char-
◦
◦
acteristics peaks of zero valent iron at 2ꢁ = 44.9 and 73.3 (Fig. 4d).
◦
3.1.3. pH of zero point charge (pHzpc
)
pHzpc plays very important role in adsorption process as it refers
to pH at which the electrical charge density on a surface is zero. At
higher pH than pHzpc the surface of Fe -AC and AC is negatively
charged and attracts cations. Conversely, below pHpzc, the surface
is positively charged and repels cations[33]. Therefore, this param-
The broad peak at 2ꢁ = 22 confirmed the presence of activated
0
0
carbon (AC) in Fe -AC [35]. Crystalline size of the Fe -AC was cal-
culated using Debye-Scherer formula and found to be 50 nm. These
results are in approximation with TEM analysis.
0
0
3
.1.2. FTIR analysis
eter determines the potential of Fe -AC to adsorb MG. In precedent
study, pHzpc of Fe -AC and AC were 6.00 and 6.10, respectively
0
0
0
Table 1 depicts FTIR analysis of Fe -AC, MG-loaded Fe -AC and
0
Fe -AC after MG degradation. The characteristic absorption bands
(Fig. 5a and b).
Table 1
0
0
0
FTIR analysis of Fe -AC, MG loaded Fe -AC and Fe -AC after six cycles.
Band (cm 1
−
)
Inference
Reference
Catalyst
0
Fe -AC
420, 618
Fe–O stretching vibrations
C–H stretching vibration and C–O stretching vibration of AC
O–H stretching and bending of vibration of H2O
[36]
[37]
[38]
2
3
805, 1108
393, 1636
0
MG loaded Fe -AC
405
C–H bending vibrations due to aromatic structure
C–N stretching vibrations of aromatic structure
C–H bending vibrations due to aromatic structure
C–H bending vibrations of asymmetric CH3 group
C–N stretching of aromatic tertiary amine
[36]
[38]
[38]
[38]
[38]
[38]
[37]
[36]
[37]
[37]
[37]
5
8
1
1
1
1
1
2
2
3
87
50–670
156
368
478
586
640
756
917
402
C–H bending vibrations of asymmetric CH3 group
C
C stretching vibration of benzene
O–H bending of vibration of H2O
C–H stretching vibration of AC
C–H stretching of asymmetric CH3 group
O–H stretching vibrations of AC
0
Fe -AC after six cycles
415
O–H stretching vibrations of AC
C–O stretching vibration of AC
bending of vibration of H2O
Fe–O stretching vibration
[36]
[37]
[37]
[36]
1
1
3
150
610
409

P. Singh et al. / Applied Catalysis A: General 476 (2014) 9–18
13
0
Fig. 4. EDX (a–c) and XRD (d) spectra of activated carbon, zero valent iron and Fe -AC.
0
UV-visible analysis was performed by dispersing AC and Fe -AC
in ethanol (Fig. 5c). The characteristic peaks of zero valent iron were
observed at 200 nm and 260 nm [39].
nearly colorless. The main chromophoric peak at 620 nm decreased
rapidly and disappeared in 60 min of radiation time. The blue-shift
of 10 nm (ꢃꢂ = 620–610 nm) was observed during decolorization
of MG. The decrease at 620 nm was due to the destruction of
conjugated aromatic ring system. While blue shift from 620 to
610 nm depicted de-ethylation of MG [40]. Therefore, it can be
inferred that both cleavage of chromophoric ring and de-ethylation
process proceeded during MG removal. The decolorization of MG
0
3
.2. Degradation of MG by solar/H O /cit./Fe -AC system at
2
2
circumneutral pH
In precedent study, malachite green was selected as target
organic substrate to explore photocatalytic efficiency of Fe -AC. MG
was photo-stable and citrate-ferric complexes did not influence the
absorption spectrum at 620 nm. The pH of solution was kept at 6.75
for solar/H O /cit./Fe -AC system. Fig. 6a displays the changes in
0
2
followed pseudo first order kinetics (R , 0.95) with rate constant of
−
2
−1
9 × 10
s
(Fig. 6b).
0
During Fe and H2O2 mediated degradation processes, corrosive
dissolution of Fe is a matter of concern for the researchers [22]. The
erosion of Fe should be lowfor the efficacy of process. In case of Fe -
0
0
2
2
0
0
0
the temporal absorption of spectra of MG in solar/H O /cit./Fe -
2
2
AC system. The dye color was changed from initial blue to
AC, 0.9 ␮mol of Fe (II) was found in reaction solution after 60 min.
0
0
Fig. 5. (a–c) pHzpc of Fe -AC (a) and AC (b). (c) UV spectrum of ZVI and Fe -AC.

1
4
P. Singh et al. / Applied Catalysis A: General 476 (2014) 9–18
Fig. 6. (a–d) (a) UV-visible spectrum of MG degradation. (b) Log(absorbance) vs Time plot. (c) Concentration of dissolved Fe(II). (d) Effect of isopropanol on MG removal in
0
−5
−4
−5
using solar/H2O2/cit./Fe -AC system. Reaction conditions: [MG] = 3.5 × 10 M, [H2O2] = 7.0 × 10 M, [cit.] = 4 × 10 M, catalyst dosage = 60 mg/50 mL, pH = 6.75, solar light
3
−3
◦
intensity = 35 × 10 ± 1000 lx, isopropanol = 3 × 10 M, temperature = 30 ± 0.3 C and time = 60 min.
However, 5.0 ␮mol of Fe(II) was released in case of ZVI (Fig. 6c). It
where qe is amount of adsorbate adsorbed per unit weight of adsor-
bent (mg/g), stands for adsorption capacity and Ce is the initial
0
clearly indicated that corrosive dissolution of Fe was quite lower
0
0
in Fe -AC. About 15% of MG was removed in the absence of Fe .
amount of adsorbate. K stands for Freundlich constant and was
f
•
To investigate the role of OH radicals in photo-degradation,
calculated from the plot ln qe versus ln Ce (Fig. 7b). The obtained Kf
experiments were performed with isopropanol. Isopropanol con-
was found to be 0.25 (Table 2). Langmuir isotherm is given by Eq.
(9):
•
taining ␣-hydrogen is highly reactive with OH where as poorly
−
with O2 . Buxton et al. has described high second order rate
•
9
−1 −1
constant (6 × 10 M
s
) of isopropanol with OH radicals [41].
q0bCe
qe =
(9)
Fig. 6d depicts the effect of isopropanol on the degradation of
MG. Only 15% of MG was removed in the presence of isopropanol
1 + bCe
where qe is amount of adsorbate adsorbed per unit weight of adsor-
due to quenching of hydroxyl radicals. These results indicated that
•
bent (mg/g), q is the monolayer adsorption capacity (mg/g) and b is
MG degradation predominantly occurred via OH radical assisted
0
the Langmuir constant (L/mg). Langmuir isotherms for MG adsorp-
oxidative pathway.
0
tion onto Fe -AC and ZVI plots were plotted in Fig. 7c. Langmuir
0
isotherm model exhibited better fit to data for MG removal by Fe -
3.3. Adsorption studies for MG removal
AC and ZVI than Freundlich isotherm. The value of qmax was found
to be 100 mg/g (Table 2). This suggests that monolayer adsorption
occurred uniformly on the active sites of the adsorbents [42]. These
results indicated that adsorption of MG was an important parame-
0
The adsorption of MG onto Fe -AC is an important parameter
for efficient dye removal. The amount of MG adsorbed per gram of
adsorbent at time t (min) can be found by Eq. (7) [42,43]
0
ter in Fe -AC based solar Fenton’s reaction for efficient degradation
of dye.
(
C − Ct)V
0
gt =
(7)
m
where qt (mmol g−1) is the amount of adsorbed MG/CR per gram of
Table 2
adsorbent at time t (min), C is the initial concentration of MG in
Langmuir and Freundlich isotherm parameters for MG adsorption: MG con-
0
−
5
−3
◦
−
3
−3
centration = 5 × 10 mol dm , contact time = 50 min, temperature = 30 ± 1 C and
solution (dm ), Ct is the concentration of MG (mol dm ) at time
t (min), V is the volume of the solution (50 mL) and m is the mass
of the adsorbent (g). The percentage of MG removal in dark k are
pH = 6.75.
0
Parameter
Fe -AC
ZVI
0
plotted in Fig. 7a. 75% of MG was removed in 45 min using Fe -AC
Langmuir
qmax (mg/g)
b (L/mg)
as an adsorbent. While in case of ZVI, only 10% of MG was removed
in 30 min. In the present study, adsorption data was modeled using
Langmuir and Freundlich isotherms. Freundlich isotherm model is
given by following equation [42,43]:
100.12
3.56
0.985
25.5
1.2
0.984
2
R
Freundlich
N
9.12
0.25
0.95
3.2
0.05
0.94
1
n
Kf
ln qe = ln k +
ln Ce
(8)
2
f
R

P. Singh et al. / Applied Catalysis A: General 476 (2014) 9–18
15
0
Fig. 7. Adsorption of MG onto Fe -AC, ZVI: (a) Percentage removal of malachite green in dark, (b) Freundlich isotherm model for MG adsorption and (c) Langmuir isotherm
−
5
◦
model for MG adsorption. Reaction condition: [MG] = 3.5 × 10 M, adsorbent dosage = 60 mg/50 mL, pH = 6.75, temperature = 30 ± 0.3 C and time = 60 min.
3.4. Control experiments for MB degradation
radicals could be produced via H O2 and solar light interactions
2
through dye sensitization process (Eqs. (10) and (11)) [44].
0
MG removal was explored under solar/H O /cit./Fe -
2
2
0
0
MG + hꢀvisible → 1MG ∗ or MG∗
(10)
(11)
AC,
solar/cit./Fe -AC,
solar/H O /Fe -AC,
solar/H O /cit.,
2
2
2
2
2
solar/cit./AC and solar/cit/Fe0 treatment systems (Fig. 8a
and b). About 40% of MG was removed in both
•
MG ∗ or MG ∗ +H O → 2OH + MG
1
3
2
2
0
0
solar/H O /Fe -AC and solar/cit./Fe -AC systems. The MG
removal efficiency followed the order: solar/H O /cit./Fe -
2
2
0
In solar/cit./Fe -AC, 66% of MG was removed in 60 min. It
0
2
2
shows that MG was mainly removed through reductive pathway
0
0
0
AC > solar/H O /Fe -AC ≈ solar/cit./Fe ≈ solar/cit./Fe -AC > solar/
2
2
in absence of H O2 via following reactions (Eq. (12) and (13)) [45].
2
H O /cit./AC > solar/cit./AC (Fig. 8a). Solar/cit./H O system was
2
2
2
2
•
0
able to remove MG in the absence of F -AC. It indicated that OH
0
Fe + MG → Fe(II) + MG(
(12)
red)
Fig. 8. (a–b) MG removal under different reaction systems (a) solar (b) in dark. Reaction condition: MG = [MG] = 3.5 × 10 M, [H2O2] = 7.0 × 10−4 M, [cit.] = 4 × 10−5 M, catalyst
−5
3
◦
dosage = 60 mg/50 mL, pH = 6.75, solar light intensity = 35 × 10 ± 1000 lx, temperature = 30 ± 0.3 C and time = 60 min.

1
6
P. Singh et al. / Applied Catalysis A: General 476 (2014) 9–18
0
0
Fig. 9. (a–d) Effect reaction parameters, pH (a), H2O2 dosage (b), citrate dosage (c), and Fe -AC (d) dosage on MG removal using solar/H2O2/cit./Fe -AC system.
−5
−4
−5
3
Reaction conditions: [MG] = 3.5 × 10 M, [H2O2] = 7.0 × 10 M, [cit.] = 4 × 10 M, catalyst dosage = 60 mg/50 mL, pH = 6.75, solar light intensity = 35 × 10 ± 1000 lx, tem-
◦
perature = 30 ± 0.3 C and time = 60 min.
hꢀ
visible
2+
•
•
Fe(II) + O + H O −→ Fe(OOH) + OH
(13)
be ascribed to OH radical extinguishing, and self-decomposition of
2
2
H O due to excessive addition of H O (Eqs. (14) and (15)) [44,47].
2
2
2
2
The removal experiments were also performed in the
absence of solar light (Fig. 8b). The removal efficiency followed
•
•
H O + OH → H O + HO
(14)
(15)
2
2
2
2
0
0
0
0
•
•
the sequence: H O /cit./Fe -AC ≈ H O /Fe -AC > cit./Fe -AC ≈ Fe -
OH + HO2 → H O + O2
2
2
2
2
2
0
0
AC/Fe > Fe > AC > H O /Cit. Without irradiations, 23% of MG
removal was achieved using H O /cit./Fe -AC system. Very low
removal efficiency was observed for AC, Fe -AC and H O treat-
2
2
0
Fig. 9c shows the effect of citrate ion on MG removal in
solar/H2O2/cit./Fe -AC system. MG removal efficiency was found
2
2
0
0
2
2
−
4
−4
to increase with an increment in 2 × 10 to 4 × 10 M of citrate
ment systems. In general, from the view point of individual factor
importance, the achieved prohibitive order followed the trend:
ion, which could be due to higher absorption of visible light by
−
0
0
Fe(III)citrate-OH complex in photoactive volume. However, fur-
ther increase in citrate concentration reduced the rate reaction. This
solar light > Fe -AC > Fe > H O > AC > cit.
2
2
reduction was attributed to the consumption of hydroxyl radicals
0
•
3
.5. Reaction parameter affecting solar/H O /cit./Fe -AC
2−
2
2
by excessive citrate ions form 3-hydroxo-glutrate
[45].
0
The effect of Fe -AC dosage on MG removal was investigated in
range of 20 mg/50 mL to 100 mg/50 mL (Fig. 9d). The rate constant
0
Since reaction pH, Fe -AC, H O and citrate concentration
were the most important parameters influencing MG removal in
solar/H O /cit./Fe -AC system. So, all experiments were carried out
2
2
0
−2 −1
was optimal at 60 mg/50 mL of Fe -AC loading (9 × 10
s
). The
0
•
2
2
increased rate constant was due to higher rate of OH formation.
individually to check extent of MG degradation.
0
However, further increase in Fe -AC loading caused reduction in
The effect of pH on MG removal was investigated in range of
–11 using solar/H O /cit./Fe -AC system (Fig. 9a). The experimen-
0
rate constant. The excessive Fe -AC loading reduced the photoac-
0
2
2
2
tive volume due to screening of solar light [44,45,67].
2
tal data were fitted to pseudo first order kinetics (R = 0.97). The
MG degradation enhanced with increase in pH from 1 to 7.5. How-
ever, further increase in pH caused a reduction in rate constant.
The adsorption of dye is prerequisite parameter in the effective
photo degradation process. At lower pH below pHpzc, positively
charged surface of Fe -AC was resulted in low adsorption of MG on
the catalyst surface. With increase in pH above pHpzc, more neg-
atively charged surface of Fe -AC resulted in higher adsorption of
MG. However, excessive MG adsorption resulted in lowering of rate
constant due to screening of solar light leading to reduction in photo
active volume [44,46].
Fig. 9b depicts the effect of H O concentration on MG removal
in solar/H O /cit./Fe -AC system. The rate constant increased from
3
.6. Proposed mechanism of MG degradation in
0
solar/H O /citrate/Fe -AC
2
2
Based on the above results and discussion, plausible mecha-
nism for MG removal was deduced. First of all, MG was adsorbed
0
0
0
onto surface of Fe -AC in dark and surface of Fe -AC became
0
blue (Eq. (16)). After this, solution was exposed to solar light. Fe0
was converted into Fe(II) under aerobic condition (Eqs. (17) and
(18)) also trough dye sensitization process (Eq. (11)). Photo-Fenton
reaction was initiated by Fe(II) and H O to produce to hydroxyl
2
2
•
2
2
radicals(OH ) and Fe(III)(Eq. (16)–(19)) [27].
0
2
2
0
0
−
−
2
4
−2 −1
−4
to
Fe -AC + MG → Fe -AC––MG (adsorption)
(16)
(17)
(18)
2
9
× 10
to 9 × 10
s
with increase in H O2 from 3 × 10
2
× 10 M. The increased rate constant was due to enhanced for-
0
+
Fe -AC + O + 2H → Fe(II)-AC + H O2
•
2
2
mation of hydroxyl radicals (OH ). However, further increase in
H O concentration reduced MG removal. This phenomenon might
0
Fe -AC + H O + 2H → Fe(II)-AC + H O
2
2
2
2
2

P. Singh et al. / Applied Catalysis A: General 476 (2014) 9–18
17
catalytic cycles (Fig. 11). On the other hand, no significant changes
0
in FTIR spectrum of Fe -AC were recorded after 6 cycles (Table 1)
0
Fe -AC was easily reused with 75% efficiency for six cycles.
4. Conclusion
0
A new solar/H O /cit./Fe -AC system was developed for the
2
2
removal of MG at circum neutral pH. It showed high removal effi-
ciency for dye removal. MG was completely decolorized in 60 min
of reaction time. Control test and factor experiments indicated that
•
MG removal mainly occurred through OH radicals. The removal of
MG was higher at circumneutral pH 6.75, which was quite differ-
ent from Fenton reaction. Hydroxyl radical formation was largely
enhanced by the citrate ions and solar light. Fe -AC had higher recy-
Fig. 10. Diagrammatic representation of proposed mechanism for MG degra-
0
0
dation from aqueous phase using solar/H2O2/cit./Fe -AC system. Inset is
0
cle efficiency compared to ZVI. The enhanced oxidative treatment
MG under solar/H2O2/cit./Fe -AC system at 0 min and 60 min. Reaction con-
−
5
−4
−5
0
ditions: [MG] = 3.5 × 10 M, [H2O2] = 7.0 × 10 M, [cit.] = 4 × 10 M, catalyst
is the main advantage of investigated system. Solar/H2O2/cit./Fe -
3
dosage = 60 mg/50 mL, pH = 6.75, solar light intensity = 35 × 10 ± 1000 lx, temper-
AC system would provide an alternative for tedious dye removal
treatment.
◦
ature = 30 ± 0.3 C and time = 60 min.
•
−
Fe(II)-AC + H O + Fe(III)-AC + OH + OH
(19)
2
2
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formed into Fe(II) and citrate-OH through visible light absorption.
The produced Fe(II) could participate in OH formation (Eq.
−
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−5
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