J. L. L. ANTONI et al.: 4-TERT-BUTYLBENZOIC ACID-INCORPORATED POLYSTYRENE: AN EFFECTIVE ...
485–490
4-TERT-BUTYLBENZOIC ACID-INCORPORATED POLYSTYRENE:
AN EFFECTIVE CATION-EXCHANGE POLYMER MATRIX FOR
THE REMOV AL OF METAL IONS FROM TANNERY EFFLUENT
ZDRU@ITEV 4-TERC BUTIL-BENZOJSKE KISLINE S POLISTIRENOM:
U^INKOVITA ODSTRANITEV KOVINSKIH IONOV V ODPADNIH VODAH
Z IZMENJA VO KATIONOV V POLIMERNI MATRICI
John Louis Louis Antoni
1*
, Senthilkumar Ganapathy
1
, Lekshmi Gangadhar
2
,
Mukesh Kumar Dharmalingam Jothinathan
3
, Lourdusamy Emmanuvel
4
1
Department of Civil Engineering, Annamalai University, Annamalai nagar, Chidambaram, TamilNadu – 608002, India
2
Department of Nanotechnology, NanoDot Research Private Limited, Nagercoil, TamilNadu-629001, India
3
Centre for Global Health Research, Saveetha Medical College and Hospitals, Saveetha Institute of Medical and Technical Sciences (SIMATS),
Saveetha University, Chennai-602105, India
4
Department of Civil Engineering, Government Polytechnic College, Valangaiman, Tiruvarur, TamilNadu-612804, India
Prejem rokopisa – received: 2024-04-23; sprejem za objavo – accepted for publication: 2024-06-24
doi:10.17222/mit.2024.1164
The preparation of a 4-tert-butylbenzoic acid-incorporated polystyrene matrix as the polymer matrix for removing the metal ions
present in water is reported. The hydrophobic interaction between the polystyrene and the tert-butyl group of carboxylic acid
was exploited to bind the carboxylic acid on polystyrene. Thus prepared polymer matrix was investigated for its ability to ex-
change and remove metal ions from water. Better efficiency was obtained with different metal ions, including copper, iron, cal-
cium, magnesium, chromium (III), etc., during a batch process as well as a continuous process. The carboxylic acid was not
leached out during the process and the hydrophobic force was strong enough to avoid leaching out. Further, it was employed to
remove metal ions, particularly chromium, from tannery effluent.
Keywords: chromium, hydrophobic interaction, ion exchange, metal removal, tannery treatment
V ~lanku avtorji opisujejo pripravo polistirenske matrice zdru`ene z 4-terc butil-benzojsko kislino. Tako izdelana polimerna
matrica naj bi delovala kot u~inkovit izmenjevalnik kovinskih ionov, ki so prisotni v odpadnih vodah strojarn `ivalskih ko`.
Avtorji so izkoristili hidrofobno interakcijo med polistirenom in terc-butilno skupino karboksilnih skupin za vezavo karboksilne
kisline na polistiren. Nato so avtorji raziskovali sposobnost izdelane polimerne matrice za izmenjavo in odstranitev kovinskih
ionov iz odpadne vode, uporabljene za strojenje ko`. Avtorji so ugotovili ve~jo u~inkovitost izmenjave za vrsto kovinskih ionov
vklju~no z Cu, Fe, Ca, Mg, Cr(III) itd. Izmenjava je potekala u~inkovito tako v {ar`nem kot tudi kontinuirnem procesu.
Karboksilno kislino niso izlu`ili med procesom in hidrofobna sila je bila dovolj mo~na, da ni pri{lo do izlu`enja (ekstrakcije).
Avtorji so uporabili izdelano matrico posebej uspe{no za odstranitev razli~nih kovinskih ionov, predvsem ionov kroma iz
odpadne vode v strojarnah `ivalskih ko`.
Klju~ne besede: krom, hidrofobna interakcija, izmenjava ionov, odstranitev kovin, obdelava odpadnih voda strojarn `ivalskih
ko`
1 INTRODUCTION
The usage of leather for making various products like
shoes and boots, belts, apparel, purses, luggage bags,
sports materials, etc., has been known since the iron
age.
1
The durability and elegant look make leather prod-
ucts a symbol of status. Leather production involves hide
preparation, tanning and finishing. The tanning process
can be carried out either with vegetable tanning or
chrome tanning. Chrome tanning is a more preferred
technique for the production of large quantities in indus-
tries since chrome tanning can be completed in one day
while vegetable tanning requires two weeks.
2
In addition,
chrome-tanned leather is softer than vegetable-tanned
leather; it also exhibits high hydrothermal stability and
good dying properties so that better finishing can be
achieved.
3,4
The skin from a slaughter house is subjected
to several step processes and treated with several organic
and inorganic chemicals to get better finished leather
products. The chemicals include chromium sulphate,
biocides like DDT, formaldehyde, a base like calcium
hydroxide, a reducing agent like sodium sulphide, ani-
line-based dyes and solvents like dichloromethane, ben-
zene, ethanol, etc.
5,6
During chrome tanning, chromium
binds to and stabilizes the collagen, a peptide containing
glycine, proline and 4-hydroxy proline in major along
with other amino acids including aspartic acids and
glutamic acids.
7
The carboxylic acid group of collagen
becomes bonded with chromium and thus chromium is
incorporated into the hide. The chemistry behind this
process is complex and it is given in Figure 1.
Chromium sulphate in a solution forms [Cr(H
2
O)
6
]
3+
which is an aqua acid (red violet in colour) and readily
deprotonates at a higher pH to give [Cr(H
2
O)
5
OH]
3+
Materiali in tehnologije / Materials and technology 58 (2024) 4, 485–490 485
UDK 577.11:546.3 ISSN 1580-2949
Original scientific article/Izvirni znanstveni ~lanek MTAEC9, 58(4)485(2024)
*Corresponding author's e-mail:
sllouis1966@gmail.com (John Louis Louis Antoni)

(green colour). Further, this complex polymerizes at a
higher pH, i.e., the basic condition allowing a
polychromium complex.
7
The carboxylic acid residue
present in the glutamic acid and aspartic acid of collagen
forms carboxylate ions upon treatment with lime
(Ca(OH)
2
). The carboxylate anion displaces the water
molecule from the polychromium complex via coordina-
tion and thus chromium is incorporated in the hide.
8
The chromium that is bound to the collagen of leather
gives softness, flexibility and suppleness to tanned
leather. A study shows that 90 % of leather is processed
with chrome tanning as it is quick and easy. In spite of
its advantages, chrome tanning also poses a serious
threat to the environment as chromium salt is not fully
absorbed by the collagen and a portion of chromium is
discarded in the effluent.
9
Fortunately, Cr(III) is less
toxic compared to Cr(IV) as Cr(III) is not readily ab-
sorbed by the body. While chromium (III) is essential for
the regular operation of human vascular and metabolic
systems as well as combating diabetes, too much chro-
mium (III) may result in a severe skin rash, or other more
serious symptoms. Also, Cr (III) can cause severe envi-
ronmental issues and pose a severe threat to aquatic
life.
10
Several research efforts have been carried out to re-
move chromium (III) from the tannery effluent.
11
The
techniques include adsorption
12
(rice husk, pumice, acti-
vated carbon from sugar industry, clay, zeolites, peat
moss, chitin and agricultural wastes like neem bark, wal-
nut shell, rice husk, tea waste, sugar industry waste, ba-
nana rachis charcoal, chitin–humic acid, etc.), chemical
precipitation (calcium hydroxide [Ca(OH)
2
], sodium hy-
droxide (NaOH), magnesium oxide (MgO), or calcium
magnesium carbonate [CaMg(CO
3
)
2
], coagulation and
flocculation (aluminium sulphate [Al
2
(SO
4
)
3
·18H
2
O] and
ferric chloride (FeCl
3
·6H
2
O), calcium carbonate
(CaCO
3
), ammonium aluminium sulphate
[NH
4
Al(SO
4
)
2
], aluminium sulphate [Al
2
(SO
4
)
2
], and so-
dium citrate (Na
3
C
6
H
5
O
7
), ferric chloride and organic
polymer), ion exchange
13
(zeolite, ion exchange resin,
macroporous carboxylic resin), membrane filtration tech-
niques like reverse osmosis (RO),
14
nanofiltration (NF),
ultrafiltration (UF), microfiltration (MF)
15
and
electrodialysis (ED) electrochemical treatment like elec-
tro-deposition,
16
electrocoagulation,
17
electro-disinfec-
tion, electro-oxidation,
1
electro-reduction, electro-osmo-
sis, and electro-flotation. These are some of the many
research methods for removing Cr from the tannery
waste.
18
Though several methods for removing metals from
water are available, development of new methods that are
cheap and carried out with a readily accessible material,
is required. In this paper, we report on a simple and
cost-effective method for removing transition metals
from water, in particular Cr (III) ions from the secondary
treated tannery water. This method involves a polysty-
rene film incorporated with 4-tert-butylbenzoic acid
(TBBA) that binds to the surface of polystyrene through
J. L. L. ANTONI et al.: 4-TERT-BUTYLBENZOIC ACID-INCORPORATED POLYSTYRENE: AN EFFECTIVE ...
486 Materiali in tehnologije / Materials and technology 58 (2024) 4, 485–490
Figure 1: Chrome tanning process

a hydrophobic interaction and can exchange metal ions
with protons.
2 EXPERIMENTAL PART
Substrates were purchased from Avra synthesis, SD
fine, Spectrochem and Merck. AR grade methanol and
CH
2
Cl
2
were used as such without further purification. A
rotary evaporator was used for evaporation under a re-
duced pressure. A SEM analysis was carried out with a
TESCAN VEGA3. The analysis of metal ions was car-
ried out following the standard procedure. Double-dis-
tilled water was used for preparing a solution of known
salts. Complexometric titrations were carried out for the
estimation of Cu, Ca, Mg, Zn using suitable indicators
and buffer solutions. Fe (II) and Fe (III) were estimated
using permanganometry and spectrophotometry, respec-
tively.
2.1. General procedure for a TBBA-incorporated poly-
mer matrix
Commercially available polystyrene (1 g) and 4-tert-
butylbenzoic acid (0.1 mmol) were dissolved in 30 mL
of CH
2
Cl
2
and the solution was poured onto a Petri dish.
Over time, the organic solvents became evaporated, leav-
ing back flakes of TBBA-incorporated polystyrene. The
flakes were washed with more polar methanol (20 mL)
to remove any excess TBBA that was not tightly bound
to the polymeric matrix.
2.2. General procedure for the removal of metal ions
from the solution
A column with sintered disc (20 mm) is tightly
packed with 10 g of TBBA-incorporated polystyrene.
The height is approximately 8 cm. 20 mL of a metal-salt
solution (with a concentration of 1000 ppm or 1 mg/mL)
together with NaHCO
3
(0.05 mmol) is poured over the
column through the inner walls without disturbing the
column packing. The stopcock is partially open and the
solution is allowed to flow dropwise and be collected in
the conical. Thus collected solution is analysed for the
presence of metal ions.
2.3. Batch process
A beaker is charged with1go fT B B A-incorporated
polystyrene and 20 mL of the metal-salt solution (a con-
centration of 1000 ppm) together with NaHCO
3
(0.05 mmol) and a magnetic pellet. The mixture is stirred
with the help of magnetic stirrer for 10 minutes and the
mixture is decanted. Thus collected solution is analysed
for the presence of metal ions.
3 RESULTS
Cationic ion-exchange resin was successfully em-
ployed to remove the metals like Ca, Mg, Cr, etc., from
water. Though the process is promising, the cost of the
ion-exchange resin is the major concern. The carboxylic
acid incorporated onto the polystyrene through a hydro-
phobic interaction, rather than a covalent bond, could
play the role in the cationic ion-exchange resin and
might also be cost-effective. The choice was TBBA since
it has a large lyophilic/hydrophobic part oriented oppo-
site to hydrophilic carboxylic acid. Further, the aromatic
part can impart rigidity to the whole molecule once it is
bound to the polymer matrix. Initially, the carboxylic
acid (4-tert-butylbenzoic acid, TBBA)-incorporated
polystyrene flakes were prepared by simply dissolving
and evaporating. In the Petri dish, the polystyrene solu-
tion with 1 mmol of 4-tert-butylbenzoic acid was dried to
obtain flakes of a TBBA-incorporated polystyrene film.
After drying, the film was washed with more polar meth-
anol to remove any TBBA present on the surface and
also washed with water before characterization. Upon
evaporation of the methanol, 0.07 mmol (12 mg) of
TBBA was obtained. This clearly indicated that the ma-
jor portion of the TBBA was incorporated in the polysty-
rene film. The surface of the polystyrene film was stud-
ied with SEM at various magnifications, as shown in
Figure 2. The TBBA-polystyrene flakes were soft, hav-
ing an irregular shape and surface. Under a high resolu-
tion (1 μm), one can observe several tiny humps on the
surface which are attributed to the binding of the TBBA
to the surface. Further, the sample was analysed with in-
frared spectroscopy, i.e., by dissolving the it chloroform
and using a NaCl crystal as the sample holder. The
J. L. L. ANTONI et al.: 4-TERT-BUTYLBENZOIC ACID-INCORPORATED POLYSTYRENE: AN EFFECTIVE ...
Materiali in tehnologije / Materials and technology 58 (2024) 4, 485–490 487
Figure 2: SEM images of 4-tert-butylbenzoic acid-incorporated poly-
styrene film

carboxylic acid group appears as the intense peak at
1754 cm
–1
.
As there is no possibility of chemical bonding be-
tween the polystyrene and TBBA, it is presumed that the
hydrophobic part of TBBA is bound to the hydrophobic
polystyrene while the more polar hydrophilic carboxylic
acid group is exposed at the surface, as shown in Fig-
ure 3.
After successful preparation of the TBBA-incorpo-
rated polystyrene, focus was given to the exploitation of
the ion exchange property of the carboxylic acid group,
removing various metal ions from the solution. The effi-
ciency in the batch process as well as the continuous pro-
cess were studied. As the continuous process required
more TBBA polystyrene it was limited to highly toxic
copper and chromium.
Table 1: Removal of metal ions by TBBA-incorporated polystyrene
a
S. No. Metal ions
b
Concentration of metal ions after
treatment (gm)
c
1 Cu (I) 0.7 (0.8)
d
2 Cu (II) 0.8
f
3 Fe (II) 1.1
h
4 Fe (III) 5.0 (4.5)
e, g
5 Cr (III) 0.6 (0.8)
d
6 Ni (II) 1.0
7 Zn (II) 0.3
f
8 Ca (II) 0.3
f
9 Mg (II) 0.3
f
a
refers to the experimental section,
b
metal sulphates with a concentra-
tion of 1 ppm/mL were used.
c
indicates the amount of metal ions after
the treatment with a batch process.
d
0.1 mol% NaHCO
3
was em-
ployed.
e
indicates a continuous process.
f
complexometric titration
was used to measure the metal ions.
g
analysed by a spectrophotometer
using KSCN.
h
permanganometry was used.
Initially, ion-exchange absorption with several most
common metal-ion solutions and without any base
(pH 7) was attempted. The absorption was very poor
which clearly indicates that –CO
2
H has a poor ligating
property with the metal. Inspired by the collagen binding
to Cr (III) under the basic condition, absorption with a
small quantity of weak base NaHCO
3
(pH 8) was car-
ried out. Surprisingly, the absorption increased
drastically and only a small amount of metal ions was
left unabsorbed in the water after treatment. This absorp-
tion property may be attributed to the formation of ben-
zoate which coordinates with the metal salt, as shown in
Figure 4. Encouraged by this result, several salt solu-
tions were prepared from their metal sulphates with a
concentration of 1 mg/mL and were studied for effi-
ciency of the ion exchange under batch and continuous
conditions, as shown in Table 1. From this table, it is
clear that the batch process is more effective than the
continuous process, which may be attributed to a shorter
contact time in the continuous process (the flow time is
only 2 min and 25 s).
After the batch process, the TBBA-incorporated
polystyrene flakes were washed with water, dried and
analysed with SEM. The surface had an entirely different
nature as compared to the film before the absorption
(Figure 5). The metal absorption was confirmed with
flame photometry studies of the polystyrene film in
CH
2
Cl
2
. Several attempts to increase the absorption pro-
cess by increasing the pH (up to 10) were not promising
as the concentration of Na ions increased in the waste-
water.
4 DISCUSSION
Though only a few ppm of Cr were present in tannery
wastewater, attempts were made to study the efficiency
of the TBBA polystyrene (1 g) by employing the solu-
tions with different concentrations (10–70 ppm). At a
lower concentration the absorption was greater than
J. L. L. ANTONI et al.: 4-TERT-BUTYLBENZOIC ACID-INCORPORATED POLYSTYRENE: AN EFFECTIVE ...
488 Materiali in tehnologije / Materials and technology 58 (2024) 4, 485–490
Figure 4: Metal-ion exchange with TBBA polystyrene
Figure 3: Proposed structure for the TBBA incorporation in polysty-
rene
Figure 5: SEM images of 4-tert-butylbenzoic acid-incorporated poly-
styrene film after treatment

90 % and it decreased considerably with higher concen-
trations. This is attributed to the decrease in the available
CO
2
-
Na
+
sites on the surface (Table 2).
Table 2: Absorption of Cr (III) by TBBA polystyrene
S. No.
Initial Cr (III)
concentration
(ppm)
Cr (III) in the
treated water
(ppm)
Absorbed Cr
(III) concentra-
tion (ppm)
11 019
22 031 7
33 042 6
44 073 3
55 01 04 0
66 01 64 4
With these promising preliminary results, the re-
moval of Cr from the secondary treated tannery waste-
water was carried out. The preliminary studies were con-
ducted to measure the common parameters like pH,
TDS, etc., as shown in Table 3. The tannery effluent was
turbid with soil particles and several other chemicals. So
it was centrifuged to remove the suspended solids and
the supernatant was taken for analysis. A systematic
analysis showed that the effluent contained several other
chemicals in small quantities (Table 2). The pH of the
tannery effluent was 8.7 which was enough for the ben-
zoate ions to form during the absorption process. The ab-
sorption in the tannery wastewater was complex as there
were several chemicals that could interact with the poly-
mer matrix.
Table 3: Common parameters of the secondary treated effluent
S. No. Parameters
Measured values
with units
Permissible val-
ues for potable
water
18
1 pH 8.8 6.5–8.5
2
Electrical con-
ductivity
6.32 mmho/cm 800–2500 μS/cm
3 Colour brown colourless
4
Total dissolved
solids (TDS)
4260 mg/L 50–150 ppm
5 COD 1470 mg/L 1–2 mg/L
7 Chromium 7.26 mg/L trace
8 Chloride 1730 mg/L 250 mg/L
The tannery effluent (100 mL) was being stirred with
the flakes of TBBA-incorporated polystyrene (5 g) and
0.1 mmol of NaHCO
3
for 10 minutes (the batch process).
The effluent was then decanted and analysed for Cr(III)
ions. The analysis clearly showed that the solution con-
tained a negligible amount of chromium. The polymer
matrix was not selective towards Cr (III) ions. In addi-
tion to chromium, other metals like Ca and Mg were also
trapped during this process. The study was not complete
without studying the leaching of TBBA from the poly-
meric matrix. An HPLC chromatogram of the effluent
showed that the organic residues in the effluent were not
absorbed by the polymeric matrix. From the HPLC stud-
ies of the solution, it was clear that TBBA was not
leached out from the polymeric matrix during this opera-
tion.
5 CONCLUSIONS
In conclusion, the absorption of metal ions from wa-
ter using a polystyrene matrix that incorporated TBBA
was demonstrated. The benzoic acid derivative bound on
the surface via a non-covalent hydrophobic interaction.
Hence, the hydrophilic carboxylic acid moiety was ex-
posed on the surface and could bind with several metals.
The ligation was more facile under the basic condition
than in a neutral medium. This polymeric matrix was
successfully used in both the continuous and batch pro-
cesses to remove metal ions. Further, the scope of this
process was extended to the removal of chromium ions
from the tannery effluent. The studies showed that
4-tert-butylbenzoic acid was not leached out during the
process and it is thus safe to handle.
Acknowledgement
The Authors thank the central facilities, SASTRA
deemed to be university, Thanjavur for their support to
analyze sample and the Head of the department, Civil
Engineering, Annamalai University for constant support
and infrastructure to carry out this research work.
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J. L. L. ANTONI et al.: 4-TERT-BUTYLBENZOIC ACID-INCORPORATED POLYSTYRENE: AN EFFECTIVE ...
490 Materiali in tehnologije / Materials and technology 58 (2024) 4, 485–490