Synthesis and evaluation of the oil removal potential of 3-bromo-benzimidazolone polymer grafted silica gel

Article abstract of DOI:10.1039/d0ra10848k

This work reports the synthesis of 3-bromo-benzimidazolone using melt condensation, its polymerization and functionalization on silica which was extracted from diatomaceous earth in our previous work. The synthesized compounds were characterized using FTIR, NMR, SEM-EDS and TEM. The FTIR and NMR spectra of the synthesized benzimidazolones showed the compounds to have several functional groups: A band due to Si-O-C at 1085.41 cm^?1, a broad band at 3380 cm^?1and chemical shifts: positive distortionless enhancement by polarization transfer (DEPT)¹³C peaks (indicating lack of CH₂and CH₃groups),¹H NMR - 11.053 ppm (N-H), 7.086 ppm (Ar-H);¹³C NMR - 155.34 ppm (CO), 101.04 ppm (C-Br) characteristic of benzimidazolones. SEM-EDS of the functionalized silica showed a rough irregular morphology with Si and O as the major elements. Carbon was also present indicating that silica was successfully functionalized with 3-bromo-benzimidazolone and TEM showed interconnected smear-like particles arranged irregularly. The functionalized silica was then applied in the treatment of oily wastewater and factors like initial oil concentration, adsorption dosage and time were optimized using the central composite design of response surface methodology in the design expert software. The amount of oil adsorbed was obtained by quantifying the total organic carbon using TOC test kits. Results showed that the optimum conditions for oil removal were 6650 mg L^?1oil concentration, with adsorbent dosage of 0.004 g and a contact time of 16 h. Under these conditions, the percentage adsorption was 97.9% with a desirability of 0.99. The materials were therefore seen to be applicable to field wastewaters.

Full text of DOI:10.1039/d0ra10848k

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Synthesis and evaluation of the oil removal
potential of 3-bromo-benzimidazolone polymer
Cite this: RSC Adv., 2021, 11, 11356
grafted silica gel†
a
a
Oisaemi Uduagele Izevbekhai, * Wilson Mugera Gitari,*
b
a
a
Nikita Tawanda Tavengwa, Wasiu Babatunde Ayinde and Rabelani Mudzielwana
This work reports the synthesis of 3-bromo-benzimidazolone using melt condensation, its polymerization
and functionalization on silica which was extracted from diatomaceous earth in our previous work. The
synthesized compounds were characterized using FTIR, NMR, SEM-EDS and TEM. The FTIR and NMR
spectra of the synthesized benzimidazolones showed the compounds to have several functional groups:
ꢀ1
ꢀ1
A band due to Si–O–C at 1085.41 cm , a broad band at 3380 cm and chemical shifts: positive
13
2 3
distortionless enhancement by polarization transfer (DEPT) C peaks (indicating lack of CH and CH
1
13
groups), H NMR – 11.053 ppm (N–H), 7.086 ppm (Ar-H); C NMR – 155.34 ppm (C]O), 101.04 ppm
C–Br) characteristic of benzimidazolones. SEM-EDS of the functionalized silica showed a rough irregular
morphology with Si and O as the major elements. Carbon was also present indicating that silica was
successfully functionalized with 3-bromo-benzimidazolone and TEM showed interconnected smear-like
particles arranged irregularly. The functionalized silica was then applied in the treatment of oily
wastewater and factors like initial oil concentration, adsorption dosage and time were optimized using
the central composite design of response surface methodology in the design expert software. The
(
amount of oil adsorbed was obtained by quantifying the total organic carbon using TOC test kits. Results
Received 26th December 2020
Accepted 8th March 2021
ꢀ1
showed that the optimum conditions for oil removal were 6650 mg L
oil concentration, with
adsorbent dosage of 0.004 g and a contact time of 16 h. Under these conditions, the percentage
adsorption was 97.9% with a desirability of 0.99. The materials were therefore seen to be applicable to
field wastewaters.
DOI: 10.1039/d0ra10848k
rsc.li/rsc-advances
Adsorption is another method that has been used in the
treatment of oily wastewater with advantages over other
Introduction
Petroleum is widely used globally, and its use is predicted to methods such as, high removal efficiency, low energy demand,
10
increase to about 107 million barrels daily by the year 2030. The less chemical investment and reusability. Over the years,
1,2
exploration of this mineral requires a lot of water just like all several adsorbents like activated carbon, zeolites, coal, y ash,
1
fossil fuel extraction and generates wastewater very rich in oils.
clays and nanomagnetic particles have been used. Moazed
11
Oily wastewater is ranked high in the global environmental et al. used bentonite clay to remove oil from water and re-
3
,4
issues list, hence treating it is of paramount importance.
ported 70% adsorption of its weight in oil. It was also reported
Coagulation, ltration with coagulation, precipitation, ozona- that the bentonite clay material had an oil removal capacity of
tion, ion exchange, reverse osmosis, biological methods and 100%, but the materials were not successful in the removal of
11
advanced oxidation processes have been used to treat oily organic particles. The other adsorbents listed above were
2
,10
wastewater in the past, but these methods have technical and found to have relatively low adsorption capacities.
economic limitations.
Fly ash
5–9
was reported to have an oil removal efficiency of 70–80% and
the adsorption data obtained in this study, tted best to
Freundlich isotherm indicating adsorption on a heterogenous
12–14
3
surface.
Li et al. used different particle sizes of three
Environmental Remediation and Nano Sciences Research Group, School of different types of coal – anthracite, lean coal and lignite to
a
Environmental Sciences, University of Venda, Private Bag X5050, Thohoyandou
remove oil from wastewater. The results of their experiments
indicated that the quantity of absorbed oil decreased with an
increase in particle size. They also found anthracite to have the
highest adsorption capacity (24.4 mg g ). Since the rst
synthesis of porous silica gels in 1968, different porous gels
0
950, South Africa. E-mail: gitariw@gmail.com; oisaemii@gmail.com
b
Department of Chemistry, University of Venda, Private Bag X5050, Thohoyandou 095,
0
, South Africa
ꢀ1
†
Electronic supplementary information (ESI) available. See DOI:
0.1039/d0ra10848k
15
1
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with different modications and pore sizes have been synthe-
16
sized by modifying the reaction conditions and parameters.
This makes the materials potentially applicable in a wide range
of areas. The modications have recently been extended to
organic polymers giving the materials characteristics of both
12,13,17
organic and inorganic moieties.
The benzimidazole group is an integral part of many thera- Scheme 1 Synthesis of 3-bromo-benzimidazolone.
peutic agents with many known applications as antiviral, anti-
histaminic, selective nucleotide-binding oligomerization
domain-containing protein
enzyme inhibitory agents.
important group of benzimidazoles and have been applied as
antipsychotic, antihistaminic, antihypertensive anti-
trichinellosis agents and antiviral agents, among others. This
group of compounds have only been applied in a few instances
in remediation of wastewater. For example, Samai et al.
1
and angiotensin-converting-
Benzimidazol-2-ones are an
functional group analysis was carried out using an ALPHA FT-IR
spectrophotometer from Bruker Pty (Sandton, South Africa).
Synthetic oily wastewater (SOWW) was prepared by modi-
fying the method described by Shoba et al. Sodium dodecyl
sulphate and vacuum pump oil (90 : 1 v/w) were added to a 1 L
18
24
19

ask and the volume made up with water. The mixture was then
20
sonicated for 5 min at an amplitude of 75% and cycle of 0.5 to
make the mixture homogenous.
successfully applied bis-benzimidazole compounds in the
sorption of dyes and its derivatives have been shown to be good
21
adsorbents of acids.
Synthesis of benzimidazolone
Given the frequency of use of petroleum products and the
associated spills and the increasing scarcity of water, there is
need for a reliable and efficient adsorbent to remove oil from
water. This work was aimed at the synthesis of 3-bromobenzi-
midazolone derivatives of porous silica gels which has never
been investigated before. A closely related compound, benz-
imidazoles, have however shown excellent sorption properties
because of the numerous functional groups it presents. 3-Bro-
mobenzimidazolone was chosen over the benzimidazoles for
this work because it presents an additional functional group,
the carbonyl group which is potentially useful in oil sorption.
Benzimidazolone were prepared by modifying the method
described by Mavrova et al., Izevbekhai et al. and Yang et al.
25
26
27
1
mole of 3-bromo-1,2-benzenediamine, was mixed thoroughly
ꢁ
with 1 mole of urea by crushing in a mortar and heated at 190 C
for 1 h over a sand bath as shown in reaction Scheme 1. The
resulting solute was allowed to cool to room temperature and
dissolved in 10% NaOH, ltered and the ltrate extracted with
35% HCl, ltered, dried and then characterized using FT-IR and
NMR.
The synthesis was carried out by rst synthesizing the benzi- Polymerization of 3-bromo benzimidazolone
midazolone through a solvent free process followed by its
Prepared 3-bromo-benzimidazolone was polymerized by modi-
fying the method described by Mdlalose et al. Silica and
benzimidazolone (mole ratio 2 : 1) were dissolved in 50 mL,
copolymerization with silica. For the rst time, the usability of
these benzimidazolone–silica hybrids in the treatment of oily
wastewater is reported. The removal of oil from water using
these adsorbents were optimized using response surface
methodology and the optimum conditions for oil removal were
obtained using this soware.
28
0.1 M HCl and stirred in a beaker over ice bath for 3 h. A volume
of 25 mL 0.6 M ammonium persulphate in HCl was then added
dropwise into the reaction mixture and stirring was continued
for 24 h (Scheme 2). The reaction was then quenched by the
addition of acetone and ltered. The ltrate was evaporated to
ꢁ
dryness at 60 C to obtain solutes which were characterized and
Experimental
used in adsorption experiments.
Chemicals and reagents
Response surface optimization of oil removal
Silica extracted from diatomaceous earth in our previous study
22,23
The synthesized polymer was used in the optimization tests as
was used aer drying.
Sodium dodecyl sulphate, 3-bromo-
29,30
described by Izevbekhai et al.
Variable amounts of adsorbent
1,2-benzenediamine, urea and ammonium persulphate were
were put in sealed tea bags and placed in a constant volume of
SOWW. To test the effect of the tea bags, empty tea bags were
sealed and used in similar experiments. The setup was allowed
to stand for some time. Aer this, the solution was ltered and
of analytical grade and obtained from Sigma-Aldrich (St Louis,
USA), NaOH and HCl was obtained from Rochelle Chemicals
(
(
Johannesburg, South Africa). Milli-Q water from Millipore S.A.S
Molsheim, France) (18.2 ms cm at 25 C) was also used in all
ꢀ1
ꢁ
dilutions. Tea bags were bought from Shoprite, Thohoyandou,
South Africa. Vacuum pump oil was from Telstar technologies
(Barcelona, Spain). Freeze dryer (Telstar Lyoquest-55, Shanghai,
China) was used to freeze dry samples and spectroquant UV
spectrophotometer (Merk Group, Germiston, South Africa) was
used for total organic carbon measurement and Stuart recip-
rocating shaker (Staffordshire, UK) was used for shaking and Scheme 2 Polymerization of 3-bromo benzimidazolone.
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ꢀ
1
Table 1 Range of optimized parameters
distinctive bands, for example, the bands at 1458 cm and
ꢀ
1
1599 cm
are characteristic of the C]C stretching of the
benzene ring, band at 3117 cm is characteristic of N–H
stretching bands of benzimidazoles. This band was seen to be
Parameter
Low
High
9250
ꢀ1
31
32
ꢀ1
Oil concentration (mg L
Sorbent dosage (g)
Contact time (h)
)
192
0.0011
16.0
0.004 broad probably due to OH groups formed as a result of
60.5
tautomerism in the benzimidazolone ring and also as a result of
33
intra-molecular hydrogen bonding. Also worthy of note was
the fact that the band for asymmetric N–H stretching seen in the
spectrum for 3-bromo-1,2-diaminobenzene, was absent in the 3-
bromobenzimidazolone spectrum because the primary NH2
group (amine) in the former becomes a secondary NH group
the TOC of the treated and untreated SOWW was measured
using a spectroquant UV spectrophotometer. The percentage oil
removal was calculated using eqn (1).
(
amine) in the later. There was also an observed red shi in
ꢀ
À
Á
ꢁ
ꢀ1
ꢀ1
band for the C]O group in urea from 1678 cm to 1629 cm
.
C
i
ꢀ C
f
%
removal ¼
ꢂ 100
(1)
This could have been as a result of increased electronegativity of
the oxygen atom (with contributions from tautomerism) and
a subsequent increase in intra-molecular hydrogen bonding
leading to the observed red shi in wavelength. FT-IR analysis
of the 3-bromo benzimidazolone polymer showed the presence
C
i
ꢀ
1
i f
where C and C are the oil concentrations (mg L ) before and
aer treatment with adsorbent respectively.
ꢀ
1
34,35
Experimental design
of Si–O–C bond at about 1085.41 cm
.
The transmission
band was broad with shoulders indicating that there was also
Percentage oil removal was optimized by varying the weight of 3-
bromo benzimidazolone polymer, initial oil concentration and
contact time using the central composite design in the response
surface methodology (RSM) soware. The range of optimized
parameters are listed in Table 1.
contribution from C–H out of plane band. There was also
ꢀ1
a broad band of N–H bond between 3172–3380 cm (ref. 32)
and there was a shi and a decrease in the intensity of the
ꢀ
1
ꢀ1
carbonyl band from 1629 cm to 1680 cm as well as a change
from C]O to C–O possibly because of the formation of a bond
with Si. As a result of increased conjugation, a decrease in
ꢀ
1
Results and discussion
Functional group analysis
intensity and a shi in the C–N band from 1454 cm
1
to
ꢀ1
36
499 cm was also observed. A decrease in the intensity and
number of CH out of plane bands of the benzene ring was
noticed. This could be attributed to the formation of phenylene
units through polycondensation.
Fig. 1 shows the FT-IR spectra of 3-bromo-1,2-diaminobenzene,
urea and the synthesised 3-bromo-benzimidazolone. The spec-
trum for 3-bromo-1,2-diaminobenzene exhibited bands at
36
ꢀ
1
ꢀ1
ꢀ1
6
1
20 cm due to C–Br, 797 cm , 1060 cm (CH out of plane),
ꢀ
1
ꢀ1
ꢀ1
ꢀ1
Structure elucidation of 3-bromo benzimidazolone polymer
234 cm , 1295 cm (CH plane), 1367 cm , 1625 cm (C–N
ꢀ1
ꢀ1
ꢀ1
1
stretching vibration), 1456 cm , 1573 cm and 1738 cm
The H NMR peaks in Fig. 2(a) shows chemical shis of Ar-H at
ꢀ
1
ꢀ1
37
(
aromatic C]C) as well as 3282 cm and 3371 cm due to 6.9 ppm and 7.0 ppm. Chemical shis at this position is
38
N–H bonds. The product (3-bromo-benzimidazolone) showed usually due to immobile protons. This therefore suggested
Fig. 1 FTIR spectra for urea, 3-bromo-1,2-phenylenediamine, 3-bromobenzimidazolone and 3-bromobenzimidazolone polymer.
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1
13
Fig. 2 (a) H, (b) C and (c) DEPT NMR of 3-bromo-benzimidazolone.
electron redistribution in the benzimidazole ring and Morphological studies
conrmed that tautomerism exists in the benzimidazolone ring
As seen in the TEM image (100 nm) of the modied silica
and could have caused the broad transmission band observed
on the IR spectrum for the 3-bromo benzimidazolone. It also
(Fig. 3(d)) shows an oily like surface as a result of its recovery
from solution. On the SEM image (magnication of 5000ꢂ) in
Fig. 3(b) pores are evident on the surface of razor-like crystals of
silica while the SEM image of the 3-bromo benzimidazolone
modied silica (Fig. 3(e)) showed a rough surface with clearly
dened pores which shows it could be a good adsorbent. The
EDX of the unmodied silica (Fig. 3(c)) shows silicon and
oxygen as major elements while the benzimidazolone modied
silica (Fig. 3(f)) shows the presence of silicon and oxygen from
silica, nitrogen from the benzimidazolone ring and other
unreacted elements from the synthesis. This served as extra
conrmation of the of the nal product.
37
13
shows chemical shis of N–H at 11.0 and 10.9 ppm. The
C
NMR peaks on Fig. 2(b) indicated chemical shis for a total of
seven carbon atoms. This is in line with the chemical formula
C
7
H
5
BrN
were assigned as follows: C]O at 155.3 ppm, C–N at
31.09 ppm and 129.6 ppm, Ar C–H at 123.5 ppm, 122.4 ppm
2
O, of 3-bromo-benzimidazolone. The chemical shis
1
39
and 108.1 ppm and C–Br at 101.04 ppm. The chemical shis
on the C NMR are conrmed by the DEPT (Fig. 2(c)) chemical
shis. These had all positive amplitudes which indicated that
all seven carbon atoms present in the C NMR were CH. The
absence of negative amplitudes shows the absence of CH
peaks conrming that all C atoms in the bromo benzimida-
1
3
1
3
2
40
zolone are unsaturated.
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Fig. 3 TEM, SEM and EDX of unmodified silica (a)–(c) and 3-bromo-benzimidazolone modified silica (d)–(f).
Response surface experiments and model tting
among a large set of data by plotting a series of bar graphs in
descending order with the most important factor on the far le.
The Pareto chart in Fig. 5 shows that the most signicant factors
affecting oil adsorption is the initial oil concentration and its
quadratic function. Table S2† showed that the quadratic
The quadratic model of the central composite design of the
response surface methodology (RSM) was used for modelling
experiments altering three reaction parameters: initial oil
ꢀ
1
concentration (mg L ), adsorbent dosage (g) and contact time
h). The actual percentage removal as calculated using eqn (1)
(
are presented in Table S1.† The colour of the adsorbent treated
water had an inverse relationship with the percentage oil
adsorption. The more intense the colour, the lower the
percentage removal and vice versa. However, in experiments
carried out with empty tea bags, the colour of the treated water
remained the same. This is consistent with results reported by
30
Izevbekhai et al. who studied the effect of empty tea bags on oil
adsorption. The trends observed in the table are presented in
the 3D surface plots.
A graph of model predictions (Fig. 4) of oil removal
percentage from synthetic oily wastewater was plotted against
the values obtained from experiments. A straight line, with most
points distributed on the line and a few very close to the line,
2
was obtained. The coefficient of determination R was found to
be 0.99 indicating that the predicting response of the model is
41,42
good.
Regression model and analysis of variance (ANOVA)
Analysis of variance (ANOVA) was used to nd out which of the
model terms were statistically signicant. The values are pre-
sented in Table S2† and summarized in the Pareto chart in
Fig. 4 Comparison between values predicted by response surface
Fig. 5. Pareto charts typically highlight the most important methodology (RSM) model and experimentally determined values.
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Fig. 5 Pareto charts of contribution of model terms to percentage
yield.
Fig. 7 Response surface plot for the effect of initial oil concentration
and time on percentage adsorption.
polynomial model employed in the representation of
percentage adsorption was signicant (p-value less than 0.05
and high F-value) which means that the model was sufficient to
represent the design space just like a line of best t. The lack of
but as these get saturated, the adsorption capacity begins to
44–46
reduce.
With adsorbent dosage, there is a slight increase in
adsorption capacity as the adsorbent dosage is increased
because of an increase in the binding sites.

t P-value, which was high and therefore not signicant, further
proved that the model was good. As seen from the Pareto chart,
Table S2† also showed that only initial oil concentration and its
Effect of oil concentration and time
quadratic function had an effect on the percentage oil removal. The 3D graph in Fig. 7 shows an increase in percentage oil
removal with time. This could be as a result of an increase in the
number of positive interactions between the 3-bromo benzi-
midazolone polymer adsorbent and oil, as the adsorbent stayed
As the response surface plot (Fig. 6) shows, an increase in initial in longer contact with the oily solution.29,30,45
oil concentration caused an increase in percentage adsorption
Effect of oil concentration and sorbent dosage
up to a maximum and started to decrease. This could be
because as the oil concentration is increased, larger oil droplets
Effect of adsorbent dosage and time
are formed by the coming together of smaller droplets due to As can be seen from the response surface contour plot in Fig. 8,
the coagulative effect of the adsorbents. These large oil droplets which represents the effect of adsorbent dose and time on
become trapped in the pores of the adsorbent and settle at the
29,30,43
base, leaving the water clear.
This could also be because of
the introduction of new binding sites as the dosage is increased
Fig. 6 Response surface plot for the effect of initial oil concentration Fig. 8 Response surface contour plot for the effect of contact time
and adsorbent dosage on percentage adsorption.
and adsorbent dosage on percentage adsorption.
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Table 2 Comparison of adsorption capacity for different adsorbents
Sorption capacity
ꢀ1
Adsorbent
Type of oil studied
(mg g
)
Reference
Silica aerogels
Carbon nanotube polymer
composite
Dibutylphthalate
Phenol
3500
262
47
48
Polypyrrole–Si complex
Carbon–silica composite
Synthetic oily wastewater (SOWW)
n-Hexane
8451
442
30
49
3
-Bromo-benzimidazolone
SOWW
5196
This study
polymer
percentage adsorption, as time and adsorbent dosage benzimidazolone modied silica adsorbent can be applied in
increased, the percentage adsorption also increased. The reason eld wastewaters. In addition, preliminary studies suggests that
ꢁ
for this is, as the dosage was increased, the number of binding mechanical pressure followed by drying at 60 C gives the
sites were also increased and with increase in time, there was adsorbents a potential for regeneration.
prolonged interaction between the oil molecules and the
29,30,46
binding sites.
Author contributions
Conceptualization, O. U. I. and W. M. G.; methodology, O. U. I.;
soware, O. U. I.; investigation, O. U. I.; resources, W. M. G.;
writing—original dra preparation, O. U. I., W. M. G. and N. T.
T.; writing—review and editing, O. U. I., W. M. G., N. T. T., W. B.
A., R. M.; supervision, W. M. G. and N. T. T.; funding
acquisition, W. M. G. All authors have read and agreed to the
published version of the manuscript.
Comparison to other similar
adsorbents
The adsorption capacity of 3-bromo-benzimidazolone polymer
has been compared with other adsorbents in Table 2. As can be
seen from the table, the adsorption capacity of 3-bromo-
benzimidazolone is comparable to other adsorbents employed
in literature. This makes 3-bromo-benzimidazolone a prom-
ising adsorbent in the treatment of oily wastewater.
Conflicts of interest
There are no conicts to declare.
Conclusions
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fully synthesized by rst synthesizing the benzimidazolone via
a solvent free process. The synthesized bromo benzimidazolone
was then functionalized on pre-extracted silica gels as an oil
sorbent material. The oil adsorption performance of this poly-
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ology. It was found that the central composite model chosen
was adequate to represent the relationship between the opti-
mized parameters: initial oil concentration, adsorbent dosage
and contact time. The results demonstrated that oil removal by
adsorption onto benzimidazolone modied silica is feasible.
The adsorption and percentage removal were found to be
dependent on the adsorbent dose, initial oil concentration and
contact time with initial oil concentration being the major
determinant of oil adsorption. The optimum conditions for
removal of oil from synthetic oily wastewater using silica func-
ꢀ1
tionalized with benzimidazolone were as follows 6650 mg L
oil concentration, with adsorbent dosage of 0.004 g and
a duration of 16 h. The percentage oil adsorption using these
optimum conditions was found to be 97.9%. Given the
percentage oil removal, then optimum duration of adsorption
and the fact that oil adsorption takes place with stirring or 10 M. K. Dinker and P. S. Kulkarni, J. Chem. Eng. Data, 2015, 60,
shaking or any form of mechanical agitation, the
2521–2540.
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RSC Adv., 2021, 11, 11356–11363 | 11363