Synthesis and characterization of polyaniline, polypyrrole and zero-valent iron-based materials for the adsorptive and oxidative removal of bisphenol-A from aqueous solution

Article abstract of DOI:10.1039/c9ra01666j

One pot synthesis of a polypyrrole, polyaniline and Fe⁰ nano-composite (Fe⁰-PPY/PANI) was achieved by polymerizing aniline and pyrrole with FeCl₃ followed by the reduction of Fe³⁺ to Fe⁰ with NaBH₄. PPY/PANI was synthesized the same way as Fe⁰-PPY/PANI, except that all the FeCl₃ was removed by rinsing. The presence of Fe⁰ was demonstrated using several analytical techniques; this was shown in comparison to materials that are without Fe⁰. A series of materials were screened as both adsorbents and catalyst for the activation of H₂O₂ towards bisphenol A (BPA) removal in batch experiments. Polymers performed better than composites containing Fe⁰ at adsorption, whereas Fe⁰ based materials were better catalysts for the activation of H₂O₂. BPA samples were then spiked with other contaminants including sewage water to test the performance of the various adsorbents and Fenton catalysts. PPY/PANI was found to be a better adsorbent than the rest, whereas Fe⁰-PPY/PANI was the best Fenton catalyst. The adsorption kinetics of BPA onto PPY/PANI was studied; it was found that the process was governed by the pseudo-second-order kinetic model. The adsorption isotherms revealed that the amount of BPA taken up by PPY/PANI increased with increasing temperature and was governed by the Langmuir adsorption isotherm. The mechanism in which Fe⁰-PPY/PANI and H₂O₂ degraded BPA was studied, it was found that surface-bound hydroxyl radicals were responsible for the degradation of BPA. It was also shown that the degradation process included the formation of smaller compounds leading to the reduction of the total organic content by 57%.

Full text of DOI:10.1039/c9ra01666j

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Synthesis and characterization of polyaniline,
polypyrrole and zero-valent iron-based materials
for the adsorptive and oxidative removal of
bisphenol-A from aqueous solution†
Cite this: RSC Adv., 2019, 9, 14531
a
a
a
Lerato Hlekelele, Nomvuyo E. Nomadolo, Katlego Z. Setshedi,
Lethula E. Mofokeng, Avashnee Chetty and Vongani P. Chauke *a
ab
a
0
0
One pot synthesis of a polypyrrole, polyaniline and Fe nano-composite (Fe -PPY/PANI) was achieved by
3+
0
polymerizing aniline and pyrrole with FeCl
3
followed by the reduction of Fe to Fe with NaBH
4
. PPY/
0
PANI was synthesized the same way as Fe -PPY/PANI, except that all the FeCl
3
was removed by rinsing.
0
The presence of Fe was demonstrated using several analytical techniques; this was shown in
0
comparison to materials that are without Fe . A series of materials were screened as both adsorbents
and catalyst for the activation of H
Polymers performed better than composites containing Fe at adsorption, whereas Fe based materials
were better catalysts for the activation of H . BPA samples were then spiked with other contaminants
2 2
O towards bisphenol A (BPA) removal in batch experiments.
0
0
2 2
O
including sewage water to test the performance of the various adsorbents and Fenton catalysts. PPY/
0
PANI was found to be a better adsorbent than the rest, whereas Fe -PPY/PANI was the best Fenton
catalyst. The adsorption kinetics of BPA onto PPY/PANI was studied; it was found that the process was
governed by the pseudo-second-order kinetic model. The adsorption isotherms revealed that the
amount of BPA taken up by PPY/PANI increased with increasing temperature and was governed by the
Received 5th March 2019
Accepted 26th April 2019
0
Langmuir adsorption isotherm. The mechanism in which Fe -PPY/PANI and H
O
2 2
degraded BPA was
studied, it was found that surface-bound hydroxyl radicals were responsible for the degradation of BPA.
It was also shown that the degradation process included the formation of smaller compounds leading to
the reduction of the total organic content by 57%.
DOI: 10.1039/c9ra01666j
rsc.li/rsc-advances
other EDC's (such as some hormones, pesticides, etc.) have been
Introduction
12,13
frequently detected in water and wastewater.
This suggests that
Bisphenol A, (BPA) (2,2-bis(4-hydroxyphenyl)propane), is one of the conventional drinking and wastewater treatment plants cannot
14
most frequently manufactured organic chemicals that is used in completely remove all the EDCs.
the production of polycarbonate plastics, epoxy resins and several
Various technologies have been developed and reported for
1,2
other types of plastic product. The persistence of BPA in our the removal of EDCs, these include photocatalytic degradation,
environment, as well as its exposure to humans and animals, biological treatment, advanced oxidation processes (AOPs), and
1
5–17
comes from its presence in baby feeding bottles, plastic containers, adsorption.
Adsorption technology has received a lot of
3,4
food cans and landll leachates. BPA has been classied as an attention due to its simplicity, cost-effectiveness, and high
1
8–20
endocrine disrupting compound (EDC) as it has been shown to efficiency.
Different adsorbents such as activated carbon,
display hormone-like properties as it is able to disrupt the func- agricultural waste, polymer resin, organic clay, membranes,
5
–8
tioning of thyroid processes, and nervous and immune systems.
Furthermore, BPA exposure has been associated with poor sperm nanocomposites have been used for the removal of EDCs and
polymeric/inorganic hybrid composites, and zero-valent iron
9–11
21–23
production and different kinds of cancer. Regrettably, BPA and various other contaminants.
Organic polymeric materials
such as polyaniline, polypyrrole, and composites of thereof have
been shown to be potent adsorbents for the removal of phar-
a
Polymers and Composites, Materials Science and Manufacturing, Council for
24–27
maceuticals, anions, organics, and cations.
Furthermore,
Scientic and Industrial Research, PO Box 395, 0001, Pretoria, South Africa. E-mail:
organic polymeric materials along with other types of carbon
Molecular Sciences Institute, School of Chemistry, University of the Witwatersrand materials such as carbon nanotubes, graphene, and nitrogen-
VChauke@csir.co.za; Vonganic@yahoo.com
b
(
Wits), Private Bag X3, Johannesburg 2050, South Africa
doped carbon nanotubes can be used as supports for active
nanomaterials for removing contaminants from water.
†
Electronic supplementary information (ESI) available. See DOI:
0.1039/c9ra01666j
28–30
1
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0
On the other hand, zero-valent iron (Fe ), is an environ- for a further 3 hours. Argon gas was then bubbled into the
mentally friendly material which in the past few years has been reaction mixture which contained residual FeCl and/or FeCl
3
2
3
1,32
0
exploited as a cheap catalyst in Fenton-like reactions.
This is that served as a source of Fe to be reduced to nano Fe . Freshly
0
because Fe activates oxidants such as hydrogen peroxide prepared NaBH (0.75 M) solution (50 ml) was added dropwise
) to form highly reactive to the reaction mixture to reduce oxidized Fe species to Fe . The
oxidizing) radicals such as hydroxyl (HOc), sulfate (SO
4
2
ꢀ
0
(
(
H
2
O
2
) and persulphate (S
2
O
8
ꢀ
4
c ) and reaction mixture was then stirred for another 20 min to
ꢀ
superoxide (O
2
c ) radical species which are known to be highly complete the reduction. Aer this, acetone was added to stop
31,32
0
effective for degrading organic contaminants.
Other mate- the polymerization reaction, followed by ltration to collect Fe -
rials which undergo AOPs and have been exploited for water PPY/PANI. The material was then washed with distilled water
33,34
remediation include titanium dioxide and its derivatives,
until the ltrate became colorless and washed with acetone to
Liu et al. also demonstrated the removal of over 75% of BPA remove oligomers. The material was then dried in a vacuum
3
1
ꢁ
oven at 90 C for 24 h. The theoretical composition of this
within 12 hours using a zero valent aluminum-acid system.
0
0
The high efficiencies of AOPs in the removal of BPA from water material is PPY(13%) : PANI(40%) : Fe (47%). In the case of Fe -
have motivated the synthesis of a dual nanocomposite, where it PPY and Fe -PANI, the exact same procedure was used except for
0
was postulated that polypyrrole–polyaniline would adsorb BPA that only pyrrole (0.8 ml – 0.77 g and 6 g FeCl
3
) was polymerized
and Fe would degrade BPA into less-toxic degradation in the case of the former and the only aniline (0.8 ml – 0.82 g
products. and 6 g FeCl ) was polymerized when making the latter. The
Therefore, the aim of this work was to synthesize and char- theoretical composition of the two composites are PANI(55%)-
0
3
0
0
acterize materials consisting of polypropylene (PPY), polyani-
: (45%)Fe and PPY(52) : (48)Fe . When making materials that
0
0
line (PANI) and Fe . These materials were tested as adsorbents consisted of polymers only without Fe , i.e. PPY, PANI, and PPY/
and catalysts for the activation of H for Fenton-like reactions PANI, aer polymerizing all of the FeCl was rinsed away with
2
O
2
3
towards the removal of BPA. PPY and PANI are conducting water when ltering. The theoretical composition of PPY/PANI
polymers that have been used as mediums to root or house is PPY(24%) : (76%).
35
metal nanoparticles in nanocomposite synthesis. Moreover,
PANI and PPY have nitrogen based functional groups that can
assist in the removal of organic phenolic compounds through
Materials characterization
The surface morphology of the samples was investigated by
transmission electron microscopy (TEM, FEI Tecnai G2 Spirit
electron microscope at 120 kV) and scanning electron micros-
copy (SEM, FEI NOVA nanolab 600 FIB/SEM). For the powder X-
ray diffraction analysis, a PXRD, Bruker D2 PHASER diffrac-
tometer (Ni-ltered Co-Ka radiation) was used. The surface area
of the nanocomposite was determined by a Micromeritics
Tristar II 3020 Surface Area and Porosity BET analyzer using
nitrogen adsorption. Thermogravimetric analysis (TGA) was
conducted in an air atmosphere on a TGA, PerkinElmer Pyris 1
TGA. The wettability studies were conducted on a KRUSS DSA
36–38
0
hydrogen bonding.
In addition, immobilization of Fe
nanoparticles onto polymer matrices has been demonstrated to
0
be benecial in reducing aggregation of Fe nanoparticles by
Bhaumik et al., where they used PANI as a support material for
0
39
Fe nanoparticles.
Experimental
Materials
3
Pyrrole (PY, 99%), anhydrous iron(III)chloride (FeCl ), aniline
(
(
3
ANI, $99.5%), sodium hydroxide (NaOH) sodium borohydride
NaBH ), bisphenol A (BPA, 99+%), hydrochloric acid (HCl,
7%), humic acid, potassium iodide (KI), hydrogen peroxide
H O , 30 wt% in H O) and sodium chloride (NaCl) were all
1
00.
4
The point of zero charge (pzc) was determined by the solid
40
addition technique. Here, 5 mg of the solid sample was
dispersed in 25 ml of 0.1 M NaCl and equilibrated at room
temperature (ca. 23 C) for 48 hours. The initial pH (pH ) of the
suspensions was adjusted using 0.05 M HCl and 0.05 M NaOH.
The nal pH (pH
hours equilibration. The difference between pH
plotted against pH and the pzc, and the point where pH is equal
to 0, is the pzc of PPY/PANI.
(
2
2
2
purchased from Sigma-Aldrich, SA. Argon gas (AR) was
purchased from AFROX, South Africa. Phenol (99–100% assay)
was purchased from Parchem whereas methanol (99.8% assays)
was purchased from Protea Chemicals. Acetonitrile for HPLC,
gradient grade (>99.9%) was purchased from Merck. The water
used in this work unless stated otherwise is pure water with
a resistivity of 18.2 MU cm. Hypodermic polypropylene syringes
ꢁ
i
f
) of the suspensions was measured aer the 48
and pH was
f
i
(
5 ml) and 0.45 mm cellulose acetate membrane syringe lters
Removal experiments
were purchased from Sigma-Aldrich SA. Sewage water was ob-
tained from Marble Hall waste-water treatment plant in Lim-
popo province, South Africa.
Batch experiments were conducted in 3 sets of experiments, (i)
pH optimization, (ii) material screening experiments, and (iii)
assessing the effect of other contaminants on the removal effi-
ciencies of some of the materials synthesized. BPA solutions
were prepared in 5% acetonitrile in water. In pH optimization
Materials synthesis
Pyrrole (0.2 ml – 0.19 g), aniline (0.6 ml – 0.61 g) and water (80 experiments, 50 ml of 50 ppm solution of BPA was transferred
ml) were added into a 250 ml conical ask and stirred for 1 into a plastic bottle. To these solutions, 25 mg of the materials
hour. FeCl (6 g) was then added to the mixture and was stirred were added, closed and agitated at 200 rpm in a thermostatic
3
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shaker for 24 h. The pH of the solutions was varied using NaOH
or HCl. Also, in Fenton reactions, 15 ppm of H O was added
2
2
simultaneously with the catalyst to initiate the reaction. In
experiments where the effect of other contaminants was
studied, 0.12 M of NaCl, or 10 ppm phenol or 2 ppm humic acid
was added to the relevant BPA solutions.
Kinetics experiments were also performed where, typically,
200 ml BPA solution at a pH 6 for adsorption experiments and
pH 3 for Fenton-like reactions was transferred into a 500 ml
glass beaker. To this solution, 100 mg of the adsorbent or
Fenton catalyst was added to initiate the reaction and agitated
using an overhead stirred batch reactor stirring at 200 rpm.
2 2
Also, 15 ppm of H O was added in Fenton reactions. Addi-
Fig. 1 The PXRD patterns of the various materials.
tionally, in scavenging experiments, 5 mM KI or 350 mM
methanol was added to the BPA solutions. In experiments
where the effect of sewage-water was investigated, 25% of peak showed the amorphous nature of the polymer. In the case
ltered sewage water was added to the BPA solutions. In all of of PANI, several peaks were observed at 2q values of 12.3, 18.2,

these kinetics experiments, 1 ml aliquots were sampled using 25.3, 30.1, 32.3 and 35.3 and were assigned as (001), (011), (020),
a syringe at predetermined intervals and ltered using a syringe (200), (121) and (022) reections of the emeraldine salt form of
4
4
0

lter.
The samples in all of these experiments were analyzed using observed to have been similar to that of PPY (without PANI
high-performance liquid chromatography (Shimadzu HPLC, characteristic peaks) except for that the peaks were broader and
PANI. The PXRD patterns of PPY/PANI and Fe -PPY/PANI were
ꢁ
ꢁ
SPD-M20A, and Prominence with a UV/VIS Photodiode Array were centered at lower 2q values (2q ¼ 31.5 for PPY, 29.8 for
ꢁ
0
Detector). The analyst was eluted using a gradient solvent PPY/PANI and 27.8 for Fe -PPY/PANI). The broadness of these
system consisting of water and acetonitrile. The gradient was as peaks suggested polymerizing the two polymers simultaneously
follows: the gradient began with 30% acetonitrile for 6 min, compromised their crystallinity thus yielding a material that
followed by the increase of acetonitrile to 90% over 2 min and was more amorphous than the separate polymers. Furthermore,
kept like this for 1 min. The concentration of acetonitrile was two low intensity and narrow peaks were observed on the PXRD
0
then reduced back to 30% in 8 min and kept constant for 3 min. pattern of Fe -PPY/PANI, at 2q values of 37.3 and 53.7 and were
The gradient ow rate was 0.2 ml min using a Phenomenex attributed to the presence of Fe , similar peaks were observed by
LUNA 5 mm C18 reverse phase column (150 ꢂ 4.60 mm). The Hwang et al. and were attributed to nanoscale Fe . Important
BPA concentration was measured at 225 nm. Total organic information was deduced from PXRD analysis: (i) it was estab-
content (TOC) measurements were done using a TOC analyzer lished that PANI and PPY were successfully polymerized
Tekmar Dohrmann Apollo 9000) relying on CO2 for together (to form PPY/PANI composite), and (ii) Fe (used as an
initiator for the polymerization of pyrrole and aniline) was
In order to identify the degradation products, having per- reduced to Fe in the presence of the polymer composite to form
ꢀ
1
0
0
45
3
+
(
14,41
quantication.
0
0
formed kinetics experiments for Fenton-like reactions, the Fe -PPY/PANI.
remaining solution was ltered to remove the catalyst particles.
TEM and SEM imaging analyses were conducted on the
The liquid was then freeze-dried using a Telstar (LyoAlfa10) samples in order to study their morphological characteristics.
0
freeze drier. The dry samples were then dissolved in 10 ml The morphology of Fe -PANI was observed to be brous and
acetonitrile and analyzed on an LC/MS/MS system. The chro- intertwined (Fig. 2a and c), similar to that which was observed
46
0
matography was performed using the same column, solvent by Rahy et al. On the other hand, the morphology of Fe -PPY
gradient system, and ow rate as described above and the was observed to be spheres that were linked into a chain like
intermediates were identied using mass-spectrometry (3200 Q morphology, Fig. 2b and c. Similar morphological properties
47
TRAP LC/MS/MS). The MS was operated in the negative ion were observed by Chougule et al. Dark spots were observed on
0
mode at an m/z range of 50–300.
the TEM images of PANI and PPY; this was because of the Fe
nanoparticles.
0
The TEM image of the Fe -PPY/PANI composite revealed that
the material consisted of the morphological characteristics
which were observed for Fe -PANI and Fe -PPY separately
Results and discussion
Materials characterization
0
0
0
The PXRD analysis was conducted on the materials in order to (Fig. 2c). Here, also, the Fe nanoparticles were identied as the
differentiate the diffraction properties of PPY, PANI, PPY/PANI particles (dark spots) embedded in the polymeric material on
and Fe -PPY/PANI (Fig. 1). A broad peak was observed on the the TEM image (Fig. 2c). It was possible to differentiate between
PXRD pattern of PPY at a 2q value of 31 . This peak was assigned PANI/PPY and Fe by contrast because Fe nanoparticles are
as a characteristic peak for PPY that resulted from the p–p signicantly heavier than PPY or PANI and as such the inter-
interactions of partial PPY chains, comparable to those that actions with the beam of electrons differed. The particle size
0
0
0
ꢁ
42,43
0
occur in the case of aromatic groups.
The broadness of the distribution of the Fe nanoparticles was measured to have
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0
Fig. 3 Thermogram of (a) PPY/PANI and (b) Fe -PPY/PANI.
0
of Fe -PPY/PANI, it was observed that 68% of the material was
carbonaceous material (polymer) as it was combusted at
ꢁ
temperatures below 600 C (Fig. 3b). The remaining residue
0
which accounted for 32 wt% was attributed to Fe as it was the
only other material in the composite except for the polymer
materials as it was shown by PXRD. The mass of the sample was
observed to have increased at temperatures between 200 and
ꢁ
4
50 C, indicating that the material was oxidizing further
proving that iron in the material was in its zero valent state.
The BET surface area of PPY/PANI was found to be 27.5 m
2
ꢀ1
0
2
ꢀ1
g
whereas that of Fe -PPY/PANI was 20.1 m g . The reduc-
0
tion in surface area was attributed to that the smaller Fe
particles blocked the polymer substrate pores thus reducing the
surface area.
Removal experiments
pH optimization. The rst experiment performed in this
0
0
0
Fig. 2 TEM images of (a) Fe -PANI, (b) Fe -PPY, (c) Fe -PPY/PANI (d)
work was to investigate the optimum pH that the most basic
0
0
particle size distribution of Fe nanoparticles in Fe -PPY/PANI and SEM
0
0
adsorbent (PPY) and most basic Fenton catalyst (Fe ) can
image of (e) PPY/PANI and (f) Fe -PPY/PANI.
remove BPA. The inuence of solution pH on BPA adsorption
onto PPY/PANI was investigated at a pH range 2.0–10 and data is
shown in Fig. 4a. Solution pH plays a signicant role in
adsorption processes as it governs the protonation or deproto-
nation of target compounds in the aqueous phase and the
been between 60 and 180 nm at an average of 110 nm (Fig. 2d).
It was observed on the SEM image of PPY/PANI that the surface
of the material is smooth (Fig. 2e). The circled area on the SEM
image (Fig. 2e) better shows the smoothness of PPY/PANI. This
is also shown in Fig. S1a.† On the other hand, the morphology
37,48
surface charge density of the adsorbent.
Meanwhile, Fig. 4
0
shows that the adsorption of BPA onto PPY was the highest
when the pH of the solution was low i.e. 2.0–6.0, with slight
variations. A linear decrease in BPA adsorption was observed as
the solution pH was increased from pH 6.0–10.0. This obser-
vation was attributable to the postulation that, in alkaline
conditions, a negative charge is created on the PPY surface, and
correspondingly, the BPA hydroxyl groups become deproto-
of Fe -PPY/PANI is characterized by a harsher surface caused by
0
the Fe nanoparticles loaded on the surface of PPY/PANI (Fig. 2f
and S1b†). The EDS spectrum of PPY/PANI showed that the
material consisted of only carbon (nitrogen atoms were not
picked because of their low relative abundance) (Fig. S1c†)
whereas Fe was detected and observed on the EDS spectrum
0
0
Fe -PPY/PANI (Fig. S1d†). Furthermore, Fe -PPY/PANI was
ꢀ
2ꢀ
0
nated to form monoanions (HBPA ) and dianions (BPA ). This
mapped using EDS to study the dispersion of the Fe nano-
particles on the surface of PPY/PANI (Fig. S2a and b†). Here, it
0
was observed that the Fe nanoparticles are well distributed on
the surface of PPY/PANI. No evidence of agglomeration was
observed.
0
TGA measurements of PPY/PANI (Fig. 3a) and Fe -PPY/PANI
(Fig. 3b) were performed in order to study the thermal proper-
ties of the materials and to estimate the amount of non-
combustible iron species in the materials. The thermogram of
PPY/PANI started decreasing from the onset and stopped
ꢁ
decreasing at 600 C at which point, over 90% of the material
Fig. 4 Effect of pH on the (a) adsorption of BPA using PPY and (b)
Fenton degradation of BPA using Fe . Experimental conditions: 50 ml
of 50 ppm BPA, 25 mg of catalyst or adsorbent and 15 ppm H O for
2 2
had combusted. The residual non-combustible material which
was less than 10% of the material was attributed to Fe and Cl
0
species that remained when the material was rinsed. In the case only Fenton reactions.
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resulted in electrostatic repulsion between the PPY surface and
BPA ions. Indeed, the same phenomenon was observed by Li
et al. except for in their work, the repulsion was observed
between BPA and the negatively charged surface of CNTs/Fe O
3
4
49
at pH 10.4. Therefore, subsequent adsorption experiments
were conducted at pH 6, unless stated otherwise.
In the case of the removal of BPA by Fenton reaction using
0
Fe and H
2
O
2
, it was observed that the removal of BPA was at its
highest at pH values below 3 (Fig. 4b). At pH values higher than
, it was observed that the removal efficiency decreased with
Fig. 6 Removal efficiencies of bisphenol-A in the presence of other
contaminants using the various materials by (a) adsorption and (b)
3
increasing pH values. This was attributed to two factors, the Fenton reaction. Experimental conditions: 50 ml of 50 ppm BPA,
+
2
5 mg of catalyst or adsorbent and 15 ppm H
2 2
O for only Fenton
direct participation of H in the degradation reaction and mass
transport limitations as a result of a formation of hydroxide
precipitation on the surface of the material at higher pH
reactions.
50,51
efficiencies of the adsorbents in the presence of phenol and
humic acid were considerably lower than the efficiencies that
were observed in the absence of other contaminants (see Fig. 5a
and 6a). This was more pronounced in the case of samples
spiked with humic acid where it was observed that the least
amount of BPA was adsorbed followed by samples spiked with
phenol. This was not the case with BPA samples spiked with
NaCl. Here an increase in BPA uptake was observed indicating
that the presence of the salt promotes the uptake of BPA.
The attenuation of adsorption of various kinds of organic
contaminants on carbon-based adsorbents as a result of the
presence of humic acid has been reported in other publica-
values.
Consequently, succeeding Fenton reactions were
conducted at pH 2, unless specied otherwise.
Material performance as adsorbents and catalysts. Here it
was observed that the materials generally performed better in
Fenton reactions than they did in adsorption experiments
Fig. 5). This is because it is possible to activate H O to form
radical species capable of oxidizing organic contaminants.
This is particularly common when H
has also been shown that carbonaceous materials are capable of
It was also observed that PPY/PANI was the
best adsorbent, followed by PPY and Fe -PPY/PANI with every
other material removing less than 50% of BPA (Fig. 5a). On the
other hand, it was seen that Fe followed by Fe -PPY/PANI and
Fe -PPY were the most active materials for the activation of
for the degradation of BPA (Fig. 5b).
Effect of other contaminants on the removal of BPA. The
next study that was performed was to study the inuence of
other contaminants on the removal efficiency of BPA by
2 2
adsorption or through oxidation via the activation of H O . The
(
2 2
52,53
0
2 2
O is used with Fe but it
54
activating H
2
O
2
.
0
55–57
0
0
tions.
Jiyong et al. observed this when adsorbing 17-ethinyl
48
0
estradiol or BPA on single-walled carbon nanotubes. They
contributed their observations to two possible reasons: (i) the
two model contaminants were in competition with humic acid
for adsorption sites and (ii) the humic acid blocked the pores on
the adsorbents. These reasons also explain the adsorption
attenuation of BPA on the materials reported here in the pres-
ence of humic acid.
H
2
O
2
experiments were conducted using materials that were observed
The adsorption efficiencies of BPA in the presence of phenol
using the various materials were the second lowest observed in
our experiments (Fig. 6a). This was because phenol is a smaller
organic compound compared to BPPA and it has similar
structural properties as BPA except it looks like half of BPA. This
indicates that the two compounds competed for the adsorption
sites on the surface of the adsorbent materials and were
adsorbed by the same mechanism thus reducing the efficiency
at which BPA was adsorbed.
to be highly effective for the removal of BPA in the screening
0
experiments, i.e. PPY, PPY/PANI and Fe -PPY in the case of
0
0
0
adsorption and Fe , Fe -PPY and Fe -PPY/PANI in Fenton reac-
tions. Here it was also observed that more BPA was removed by
Fenton reaction than by adsorption (Fig. 6). Meanwhile, the best
performing adsorbent was generally PPY/PANI followed by PPY
0
and Fe -PPY was the least potent adsorbent (Fig. 6a). Indeed,
this was true for all 3 experiments where different contaminants
were added to the test solution. Nevertheless, the removal
In the case of samples spiked with NaCl, the adsorption of
BPA using the various materials increased. This experiment was
conducted to investigate the inuence that electrolytes have on
the materials synthesized. This observation was explained by
the screening effect phenomenon. This occurred because the
presence of a relatively high concentration of NaCl affected the
surface charge of the aromatic polymeric adsorbents to favor
58,59
the p–p dispersion interactions.
This then consequently
increased the adsorption of BPA onto the adsorbents.
The effect that phenol, humic acid, and NaCl had on the
2 2
removal of BPA through the activation of H O using the various
Fig. 5 Removal efficiencies of BPA using the various materials by (a)
adsorption and (b) Fenton reaction. Experimental conditions: 50 ml of
catalysts was also investigated and the results are plotted in
Fig. 6b. Here it was observed that the removal efficiencies of the
various materials were reduced signicantly by the various
5
2 2
0 ppm BPA, 25 mg of catalyst or adsorbent and 15 ppm H O for only
Fenton reactions.
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0
contaminants, with Fe showing the best performance followed of adsorbent and Fenton catalyst (Fig. S4†). TOC measurements
0
by Fe -PPY/PANI. The activation of H O for the removal of were also recorded as a function of the mass of adsorbent and
2
2
contaminants usually follows the radical formation route; this Fenton catalyst. It was observed that the total organic content
is crucial to why the presence of other contaminants reduced also decreased with increasing mass of materials, achieving
the removal of BPA. Phenol competed directly with BPA for more than 50% removal efficiency in both adsorption and
radicals thus the attenuation of the removal efficiency. In the Fenton experiments (Fig. S4†). This showed that with the right
case of humic acid, it is possible that it may have combined with dosage, the materials can be applicable in real life samples.
the carbon-based catalyst to form complexes that hindered the
usual activation pathways. It was postulated that the decrease efficiency of BPA and total organic content by Fenton reaction
in performance as a result of the presence of chlorides was due using Fe -PPY/PANI was also investigated (Fig. S5†). These
2 2
The effect that the concentration of H O has on the removal
60
0
to that the chlorides quenched the radicals and formed less- experiments were conducted because H O plays an important
2
2
6
1
oxidative species as was reported for iodine atoms.
Effect of sewage water on the removal of BPA. Two best removal efficiency increased with increasing H
performing adsorbents (PPY/PANI and PPY, see Fig. 6a) and 20 mM. The removal efficiency decreased slightly when the
role in Fenton like experiments. Here, it was observed that BPA
2 2
O
from 2 to
0
0
catalysts (Fe and Fe -PPY/PANI, see Fig. 6b) were tested if they concentration of H
would still be able to efficiently remove BPA in the presence of brought upon by the surplus amount of H
actual sewage water and reduce the overall total organic content a slight decrease in BPA uptake (see eqn (S1) and (S2)†).
TOC) in the samples. It was observed that the performance of Nevertheless, the BPA and TOC uptakes reached 85 and 65%
2
O
2
was above 20 mM. The scavenging effect
2 2
O
possibly caused
(
the materials as adsorbents (Fig. 7a) and catalysts for the acti- respectively, showing that the matter in the sewage water used
vation of H O (Fig. 7b) were low. This indicated that the extra was in competition with BPA and that the materials have
2
2
contaminants in the sewage water hampered the performance a potential of being used in real life samples.
of the material probably because of competition with other
contaminants.
Adsorption kinetics of BPA onto PPY/PANI. The reaction
kinetics of BPA adsorption onto PPY/PANI was evaluated to
In order to investigate this claim, several other experiments determine the residence time required for equilibrium attain-
were conducted to determine the effect of different parameters ment as a function of initial BPA concentration (Fig. 8). The
on the removal of BPA and TOC attenuation. The parameters adsorption process was characterized by a rapid BPA uptake
investigated were the adsorbent/catalyst dosage, H O concen- which was observed in the rst 10 min, which subsequently
2
2
tration (only in Fenton reactions) and the amount of sewage slowed down as the reaction approached equilibrium. More-
water. These experiments were conducted using the better over, the amount of BPA adsorbed per unit mass of PPY/PANI
0
performing adsorbent (PPY/PANI) and catalyst (Fe -PPY/PANI). increased with increasing initial BPA concentration.
Data obtained when investigating the effect of sewage water Increasing the BPA concentration increased the concentration
0
concentration on the ability of PPY/PANI and Fe -PPY/PANI to gradient, and as a result, provided a driving force to overcome
remove BPA as an adsorbent and catalyst respectively are shown mass transfer resistance between the aqueous and solid pha-
37,62,63
in Fig. S3a and b.† Here it was observed that in both adsorption ses.
The equilibrium was attained at different times for the
and catalytic experiments, the amount of BPA removed various BPA concentrations, where it was found that the time
decreased with increasing concentration of sewage water. This required to reach the adsorption equilibrium increased with
demonstrates that the constituents of the sewage water
competed with BPA. A similar conclusion that the elements of
sewage water competed with BPA was reached having observed
0
how the mass of adsorbent (PPY/PANI) and Fenton catalyst (Fe -
PPY/PANI) affected the BPA uptake. Here, it was observed that
the amount of BPA uptake increased with an increasing amount
Fig. 7 Removal efficiencies of bisphenol-A and TOC using various
materials by (a) adsorption and (b) Fenton reaction. Experimental Fig. 8 Adsorption kinetics and the pseudo-second-order kinetic plot
conditions: 200 ml of 50 ppm BPA, pH 6 for adsorption experiments of BPA adsorption onto 50 mg of PPY/PANI. Experimental conditions:
and pH 3 for Fenton reactions, 100 mg of adsorbent or Fenton catalyst 200 ml of various concentrations of BPA (50, 75 and 100 ppm) at pH
ꢁ
and 25% of sewage-water.
6.0 and at 23 C.
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increasing BPA concentration. Further analysis of the graph determined. This is useful for the design and optimization of
64
(
Fig. 8) revealed that the q 's were approximately 55, 72 and adsorption system conditions. Adsorption dynamics can be
e
88 ppm for the 50, 75 and 100 ppm BPA solutions, respectively. described in four consecutive steps which are as follows: (i) lm
To explore the adsorption mechanism and reaction rates of BPA or surface diffusion where the adsorbate is transported from the
adsorption onto PPY/PANI, the kinetics data were subjected to the bulk solution to the external surface of adsorbent, (ii) inter-
linearized forms of the Lagergren pseudo-rst order (eqn (1)) and particle diffusion, (iii) intraparticle or pore diffusion, where the
Ho pseudo-second order (eqn (2)) kinetic models given by,
adsorbate molecules move into the interior of adsorbent parti-
64
cles, and (iv) chemical reaction on the sorbent surface. The
rst step is controlled by the stirring rate in the ask and the
surface chemical reaction step is usually fast. Thus, the two
steps cannot be the rate limiting steps and we can assume that
one of the two or both the diffusion processes (step ii and iii) are
the limiting steps and consequently. To describe the interpar-
ticle diffusion, the transfer of BPA onto a solid phase can be
modeled by relation eqn (3).
ln(q ꢀ q ) ¼ ln q ꢀ kt
(1)
(2)
e
t
e
t
1
t
¼ k
þ
2
q
t
2
q
e
q
e
ꢀ1
ꢀ1
ꢀ1
where k
1
(min ) and k
2
(g mg min ) are the pseudo-rst-
order and pseudo-second-order rate constants, respectively, q
t
ꢀ1
is the amount of BPA adsorbed at any time (mg g ) and q is
e
ꢀ
1
equilibrium adsorption capacity (mg g ). The linear plots of
ln(q ꢀ q ) versus t and t/q versus t (Fig. 8) were obtained and
dC
ꢀ dt ¼ k ðC ꢀ C Þ
t
a
V
e
t
t
(3)
t
e
their respective parameters were calculated and are shown in
2
ꢀ1
3
Table 1. Relatively higher linear regression coefficients (R
>
where k (m min ) is the kinetic constant, V (m ) is the initial
0
.994) were obtained for the pseudo-second-order model as volume of the solution and a is the interfacial area between the
2
opposed to the regression coefficients obtained for the pseudo- liquid and the solid phases (m ). C and C are the solution
t
e

rst-order model (Table 1). This observation along with the concentrations at any given time and at equilibrium, respec-
closeness of the equilibrium adsorption capacity obtained tively. The integrated form of this relation is given by eqn (4).
ꢀ
ꢁ
experimentally and by calculation (refer to Fig. 8 and Table 1),
showed that the adsorption of BPA onto PPY/PANI followed the
pseudo-second-order kinetic model.
C
o
ꢀ C
ꢀ C
e
¼ k ¼ k0t
a
V
(4)
ln
C
t
e
0
ꢀ1
where k is the global kinetic constant (min ) and C
o
is the
initial concentration of the adsorbate. The linear plots that
describe the diffusion model are tted below (Fig. 9a). It was
Diffusion processes
Information about the adsorption mechanism provides valu- observed from the curves that the rate limiting step for the mass
able insights on the overall rate of adsorption, where a rate- transfer of BPA is the external (interparticle) diffusion. This is
limiting step of the adsorption of BPA onto PPY/PANI can be
Table 1 Adsorption kinetics models for BPA adsorption onto PPY/PANI
Pseudo-rst order model
Pseudo-second order model
ꢀ1
ꢀ1
ꢀ1
2
ꢀ1
ꢀ1
ꢀ1
2
C
o
(mg L
)
k
1
(min
)
q
e
(mg g
)
R
k
2
(g mg min
)
q
e
(mg g
)
R
5
7
1
0
5
00
0.026
0.011
0.014
4.72
6.54
6.81
0.931
0.992
0.980
0.00152
0.00044
0.00072
60.6
89.3
97.1
0.998
0.994
0.996
Fig. 9 Diffusion models showing (a) interparticle and (b) intraparticle. Experiments conditions: 200 ml of various concentrations of BPA (50, 75
ꢁ
and 100 ppm) at pH 6.0 and at 23 C adsorbed onto 50 mg of PPY/PANI.
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2
because the relationship was linear (R z 0.99) and the inter-
cepts of each of the linear lines were close to zero (Table 2).
The intraparticle diffusion can also be a signicant step in
this adsorption diffusion process. The Weber–Morris relation
(eqn (5)) is used to dene the intraparticle diffusion mecha-
nism. This is denoted by the relationship between the time-
0
.5
t
dependent adsorption capacity (q ) and the t shown in eqn (5).
0
.5
q ¼ K t + C
(5)
t
P
ꢀ
1
where K
min
p
is the intraparticle diffusion rate constant (mg g
ꢀ
0.5
ꢀ1
) and C is the intercept (mg g ). It was observed from
investigating Fig. 9b that the plots of q versus t were all linear
t
0
.5
suggesting that intraparticle diffusion is or one of the rate-
limiting steps in the adsorption of BPA onto PPY/PANI. It was
also observed that the linear plots passed through the origin
further indicating that intraparticle diffusion is or one of the
Fig. 10 Effect of solution temperature on the removal of BPA using
PPY/PANI where the temperature is varied from 298 to 318 K. Exper-
imental conditions: 200 ml of 50 ppm BPA and 100 mg PPY/PANI.
rate-limiting steps. It was also observed that the plots of q
t
0
.5
versus t were multilinear; this meant that then two or more
33
steps govern the adsorption process. This is in agreement with
that the adsorption of BPA onto PPY/PANI was endothermic.
This was attributed to that, an increase in solution temperature
resulted in increased mobility of the BPA molecules thus
resulting in an increased driving force towards the adsorption
sites of PPY/PANI.
Furthermore, the relationship between the amount of BPA
adsorbed and the respective equilibrium concentration of BPA
molecules in the liquid phase was assessed by subjecting the
experimental isotherm data to the linearized forms of the
Langmuir and Freundlich isotherm models. The linearized
Langmuir isotherm, which is based on the monolayer adsorp-
tion on the adsorbent surface with identical adsorption sites, is
represented by eqn (6):
what was observed in Fig. 9a i.e. the rate-limiting steps were
28
both the interparticle and intraparticle steps. Also, given the
multilinearity of the plots, a three-stage adsorption process of
BPA onto PPY/PANI was thus suggested: (i) the rst steeper
segment represented the external surface adsorption or inter-
particle diffusion, (ii) the second linear segment represented an
adsorption stage or intraparticle diffusion and (iii) lastly, the
28,33
equilibrium stage was described by the third linear segment.
The slopes of the rst two linear segments of the plots in
Fig. 9b were also scrutinized to investigate the rates of reactions
during the diffusion steps. Here it was observed that although
both diffusion steps were rate limiting, the slopes of the rst
linear segments were steeper than the slopes of the second
linear segments. This meant that the intraparticle diffusion step
was slower than the interparticle diffusion step, in all the BPA
C
e
1
(6)
q
e
¼ q
o
b
64
concentrations investigated.
Meanwhile, the intraparticle diffusion constant K
ꢀ
1
where q
o
(mg g ) is the maximum amount of BPA molecules
p
and the
per unit mass of adsorbent to form a complete monolayer on
intercept (C) were calculated from the second linear segment of
the plots and were given in Table 2.
ꢀ
1
the adsorbent surface and b (L mg ) is the binding energy
66
constant.
Adsorption isotherms of BPA onto PPY/PANI. The effect
temperature had on the adsorption of BPA onto PPY/PANI was
investigated in order to study the thermodynamics of the
Moreover, an essential characteristic of the Langmuir model,
a dimensionless separation factor (R ) used for dening the
L
favourability of an adsorption process, was determined using
eqn (7):
65
process. The isotherms data for the adsorption of BPA onto
PPY/PANI is presented in Fig. 10.
1
It was seen that a signicant increase in BPA uptake by PPY/
PANI was observed with increasing temperature, suggesting
R
L
¼
(7)
ð1 þ bC
o
Þ
where average values of R
L
ranging between 0 and 1 for each of
Table 2 The interparticle and intraparticle diffusion model parameters the initial BPA concentrations used species favorable adsorp-
for BPA adsorption onto PPY/PANI
37
tion. The Freundlich isotherm is an empirical model that is
based on adsorption on a heterogeneous surface and is given by
eqn (8):
Interparticle diffusion
model
Intraparticle diffusion model
1
C
(mg g
ln q
¼ ln K
þ
ln C
(8)
e
F
e
ꢀ1
0
2
ꢀ1
ꢀ0.5
ꢀ1
2
n
C
o
(mg L
)
k
R
K
p
(mg g min
)
)
R
ꢀ1
where K (mg g ) and 1/n constants are related to the adsorp-
tion capacity and intensity of adsorption, respectively.
F
5
7
1
0
5
00
0.053 0.990 4.56
0.030 0.991 7.68
0.036 0.977 9.64
1.93
0.09
2.31
0.995
0.996
0.998
37
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Table 3 Langmuir and Freundlich adsorption parameters for BPA removal efficiency of BPA decreased slightly when the pH was
removal using PPY/PANI
decreased from 2 to 7. At pH values higher than 7.5, a more
signicant decrease in removal efficiency was observed. The
solution pH inuences how BPA is adsorbed onto PPY/PANI in
a similar way it inuenced how BPA is adsorbed onto PPY
Langmuir isotherm
Freundlich isotherm
ꢀ
1
ꢀ1
2
ꢀ1
2
T (K)
q
o
(mg g
)
b (L mg
)
R
L
R
K
F
(mg g
)
1/n
R
(
Fig. 4). Here, also, the observed behavior of BPA and PPY/PANI
2
3
3
98 400
08 455
18 526
0.012
0.021
0.026
0.27 0.998 13.3
0.16 0.999 23.4
0.10 0.999 56.4
0.570 0.954
0.536 0.948
0.396 0.964
at different pH values was attributed to that the surface of PPY/
PANI becomes negatively charged at high pH values and it
repels BPA derived monoanions and dianions that also form at
high pH values. This was further supported by the point of zero
charge (PZC) studies conducted on the adsorbent (Fig. S7b†). It
was found that at higher pH values (above the pHpzc values), the
surface of PPY/PANI acquired a negative surface.
The Langmuir and Freundlich parameters were calculated
from their respective linear plots, they were determined from
the corresponding slopes and intercepts (Table 3). Higher
correlation coefficients were observed when the adsorption data
was tted into the Langmuir model (Fig. 10) as opposed to when
Adsorption mechanism

tted into the Freundlich model (linear plot not shown).
Moreover, the Langmuir maximum adsorption capacity, qo, The adsorption mechanism between organic contaminants and
corresponded adequately with the experimental adsorption carbon-based adsorbents usually involves electrostatic interac-
1
8,67
capacities, q , achieved for BPA adsorption onto PPY/PANI, for tions.
Electrostatic interactions can either be repellent or
at different attractive and this depends mostly on the charge densities of
temperatures were found to be in the 0–1 range, indicating that both the adsorbate and the adsorbent, which is inuenced by
the adsorption process was favorable.
the solution pH. In this work, we reported that the adsorption of
e
all the investigated temperatures. The values of R
L
Thermodynamic parameters of adsorption. Thermodynamic BPA onto PPY/PANI was inuenced by pH. This was further
considerations of an adsorption process are necessary to demonstrated by studying the pzc of PPY/PANI. It was found
conclude whether the process is spontaneous or not. Gibb's free that at high pH values the surface of the adsorbent accumulates
ꢁ
energy change, DG , is the fundamental criterion of sponta- a negative charge which is repellent to the negatively charged
neity. Reactions occur spontaneously at a given temperature if anions that BPA exist as at high pH values. Therefore, electro-
ꢁ
DG is a negative value. The thermodynamic parameters of static interactions were part of the adsorption mechanism. The
ꢁ
ꢁ
Gibb's free energy change, DG , enthalpy change, DH , and structure of BPA and that of both PPY and PANI consists of
ꢁ
entropy change, DS , for the adsorption processes are calculated aromatic rings with delocalized p electrons; therefore p–p non-
using the following equations:
electrostatic interactions were also part of the adsorption
ꢀ
ꢁ
mechanism. It was also found that PPY/PANI is a relatively
(9) hydrophobic material at pH 6.2 (contact angle of 77.3 , Fig. S8†)
ꢁ
mq
e
ꢁ
DG ¼ ꢀRT ln K
c
¼ ꢀRT ln
C
e
and BPA is also hydrophobic. This means that the hydrophobic
non-electrostatic interactions were also responsible for the
adsorption of BPA onto PPY/PANI.
ꢁ
ꢁ
ꢁ
DG ¼ DH ꢀ TDS
(10)
ꢀ1
where R is a gas constant, 8.314 J mol , m is the adsorbent
mass (g), K is the equilibrium constant of adsorption, T (in
Kelvin) is the temperature and nally, q and C are the equi-
c
Degradation of BPA
e
e
Oxidative radicals, hydroxyl radicals, in particular, are the
species responsible for the degradation of organic compounds
in Fenton reactions. In order to investigate if HOc were
responsible for the degradation of BPA using Fe -PPY/PANI and
2 2
H O in this work, BPA degradation experiments were carried
out in the presence and absence of methanol and iodide ions.
Methanol is capable of neutralizing hydroxyl radicals by scav-
enging them. The rate constant of the reaction between meth-
anol and hydroxyl radicals is high (eqn (11)).
librium adsorption capacity and concentration of adsorbate at
equilibrium, respectively. The thermodynamic parameters of
ꢁ
ꢁ
DH and DS were obtained from the ln Kc versus 1/T plot
(
0
ꢁ
Fig. S6†). The Gibbs free energies (DG ) were calculated from
eqn (10) and the results are given in Table S1.† The negative
ꢁ
ꢁ
ꢁ
values of DG at 35 C and 45 C temperatures studied are due to
the fact that the adsorption process is spontaneous. The nega-
ꢁ
tive value of DH suggests the exothermic nature of adsorption.
ꢁ
The positive value of DS suggests increased randomness at the
solid/solution interface during the adsorption of BPA onto PPY/
PANI-Fe composite.
8
ꢀ1 ꢀ1
s
CH
3
OH + HOc / CH
3
OHc + H
2
O, (k ¼ 9.7 ꢂ 10 M
(11)
0
Effect of pH on the adsorption behavior. The effect of solu-
tion pH on the adsorption of BPA onto PPY/PANI was investi-
gated (Fig. S7a†). This was done because the solution pH
inuences both the chemistry of BPA and the surface charge of
the adsorbent thereby governing the electrostatic interactions
between the adsorbent and adsorbate. It was observed that the
Iodide ions also scavenge hydroxyl radicals. Furthermore,
iodide anions get adsorbed onto the catalyst easier than
methanol. Therefore, when comparing methanol scavenging
and iodide scavenging, it is possible to differentiate if the
51
reaction occurs mostly on the material surface or in solution.
68
The rate constant for this reaction is dened as (eqn (12)):
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the experiments conducted in the presence of methanol and
potassium iodide (Fig. 11). This indicated that hydroxyl radicals
were responsible for the degradation of BPA in these experi-
ments. Furthermore, it also observed that iodide ions attenu-
ated the removal of BPA more than methanol, indicating that
69,70
hydroxyl radicals were primarily surface-bound.
In order to further investigate the mechanism at which BPA
was removed, HPLC-MS analysis was conducted on aliquots
sampled at 0 min, 30 min, and 180 min. This was from an
experiment where BPA was degraded in the absence of any other
additional contaminant or radical scavenger. It was observed
that the BPA chromatogram consisted of only 1 peak at
a retention time of 15.8 min and it was attributed to BPA
(Fig. 12a).
The chromatogram of the aliquot sampled at 30 min had 3
Fig. 11 Effect of methanol and potassium iodide on the removal of
BPA. Experimental conditions: 200 ml of 50 ppm BPA at pH 6 with peaks, the original BPA peak, and two others at 6.22 and
0
1
00 mg Fe -PPY/PANI and 5 mM KI or 350 mM methanol.
9.69 min. The peak at 6.22 min was identied as 3-(4-
hydroxyphenyl)-3-methyl-2-oxobutanoic acid (m/z 207 negative
(12) mode) whereas the peak at 9.69 was identied as 4-ethynyl-
phenol (m/z 118 negative mode) (Fig. 12b). The intensity of BPA
ꢀ
ꢀ
10
ꢀ1 ꢀ1
M s )
HOc + I / Ic + OH , (k ¼ 1.02 ꢂ 10
It was detected that the experiment conducted without aer 30 minutes was signicantly smaller than that of BPA
a scavenger yielded high BPA removal efficiency as compared to before the reaction commenced. This showed that BPA was
Fig. 12 Chromatograms of the aliquots sampled at (a) 0 min, (b) 30 min, (c) 180 min and (d) is the MS spectrum of the aliquot sample at 180 min.
0
Experimental conditions: 200 ml of 50 ppm BPA at pH 6 with 100 mg Fe -PPY/PANI.
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0
2+
ꢀ
removed from the sample and that smaller compounds were
formed as a direct result of its degradation.
Fe -PPY/PANI + H O / Fe -PPY/PANI + 2OH
(13)
2
2
The chromatogram of the aliquot sampled at 180 min con-
It was also shown by the use of methanol and potassium
sisted of several peaks, including the 3 compounds identied iodide as scavengers that hydroxyl radicals were responsible for
above. The other compounds at 13.33 and 8.31 min were the degradation of BPA. Therefore the next step aer the
0
2+
identied as 1-(-4-hydroxyphenyl)ethanone (m/z 135 negative oxidation of Fe -surface to Fe -surface is the reaction between
mode and 4-(prop-1-en-2-yl)phenol (m/z 133), respectively iron species and H to form the hydroxyl radicals. From here
Fig. 12c). Furthermore, the mass spectrum of the aliquot on, a series of chemical reactions occurred until BPA was
sampled at 180 min was recorded (Fig. 11d). Here, the peaks degraded into smaller products and subsequently CO . Some of
O
2 2
(
2
representing peaks that were identied on the corresponding the chemical reactions that occurred are represented by the
chromatogram (Fig. 11c) were observed and identied with following chemical equations. The general mechanism is
asterisks on Fig. 11d. Unfortunately, the rest of the peaks, both conceptualized graphically in Fig. 13.
on the chromatogram (Fig. 12c) and mass spectrum (Fig. 12d)
2
+
3+
Fe -surface + H O / Fe -surface + 2HOc
(14)
2
2
were not identied. Nevertheless, it is clear that BPA was
degraded and other compounds were formed as a result. The
degradation by-products that were identied in this work were
previously identied in other reports, were BPA was degraded
using different techniques like the one reported in this work.
For instance, in one article they reported the ultrasonic degra-
dation of BPA and they identied compounds that were iden-
2
+
3+
ꢀ
2 2
Fe -surface + H O / Fe -surface + HO + HOc
(15)
HOc + BPA / degradation by-products / CO
2
+ H
2
O (16)
71
tied in this work. In another study by Fukahori et al. their
Conclusions
BPA photodecomposition experiments also yielded compounds
72
that were identied in this work. These reports also attributed
the degradation of BPA and formation of these intermediates to
the action of hydroxyl radicals, further supporting the postula-
tion made in this report. Furthermore, it was found from TOC
measurements that the amount of organic carbon had been
reduced by 57%, showing that some of the BPA had been turned
A series of polymeric and/or iron zero-based materials were
synthesized and used as adsorbents and as Fenton catalysts
along with H O towards the removal of BPA in water. It was
2 2
seen that polymeric materials were more potent at adsorption
0
whereas materials that contained Fe were more active at Fen-
ton reactions. The presence of other organic contaminants
reduced the BPA in both adsorption and Fenton reactions,
whereas increasing the ionic strength of the solution favored
the removal of BPA in Fenton reactions. In these experiments, it
was seen that PPY/PANI was the most potent adsorbent, while
2
into possibly CO . The possible degradation mechanism from
looking at the paper holistically is as follows. The rst step is
0
that BPA gets adsorbed onto Fe -PPY/PANI (see Fig. 5a, where it
0
was shown that Fe -PPY/PANI removed more than 50% of BPA
0
by absorption). This is crucial as in advanced oxidation
processes adsorbed pollutant molecules get degraded quicker
than the pollutant molecules in solution. In the presence of
Fe -PPY/PANI was found to be the best Fenton catalyst. Unfor-
tunately, the adsorbents and Fenton catalysts were not as
effective for removing BPA in the presence of sewage-water, even
the TOC removal efficiency was low but it was shown that at the
mass of catalyst or adsorbent and H O concentration, the
2 2
efficiencies could be increased. The adsorption of BPA onto
0
H O in an acidic medium, Fe immobilized on PPY/PANY was
2
2
2
+
0
oxidized to Fe on the surface of Fe -PPY/PANI (eqn (13)). This
is represented as Fe -PPY/PANI. Further evidence for that Fe
was oxidized to Fe on the surface of the composite was
deduced from that iodide ions reduced the removal efficiency of
BPA better than methanol.
2
+
0
2
+
PPY/PANI was characterized by the pseudo-second-order
0
kinetics model whereas Fe -PPY/PANI activated H O to form
2
2
hydroxyl radicals that were then responsible for the removal of
BPA. This report demonstrates the synthesis of potent adsor-
bent and Fenton catalyst for the removal of BPA. The synthesis
procedure is a relatively easy one and the materials are relatively
eco-friendly.
Conflicts of interest
The authors declare that there is no conict of interest.
Acknowledgements
The authors would like to express their gratitude to the Council
of Scientic and Industrial Research for providing the facilities
and resources under project number HP1312P. The University
of Witwatersrand, the Microanalysis and Microscopy (MMU) in
particular is acknowledged for their assistance with
Fig. 13 Advanced Fenton reaction mechanism for the degradation of
BPA using Fe -PPY/PANI.
0
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characterization. Prof. Ndinteh from the University of Johan- 25 F. V. A. Dutra, B. C. Pires, T. A. Nascimento and K. B. Borges,
nesburg assisted with TOC measurements; his contribution is
appreciated by the authors.
J. Environ. Manage., 2018, 221, 28–37.
26 T. A. Do Nascimento, F. V. A. Dutra, B. C. Pires and
K. B. Borges, New J. Chem., 2018, 42, 7030–7042.
2
7 B. C. Pires, F. V. A. Dutra, T. A. Nascimento and K. B. Borges,
React. Funct. Polym., 2017, 113, 40–49.
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