Design and synthesis of water-soluble chelating polymeric materials for heavy metal ion sequestration from aqueous waste

Article abstract of DOI:10.1016/j.reactfunctpolym.2020.104687

Sequestration and removal of dissolved heavy metal ions from aqueous waste streams is a challenging task. Ethylenediaminetetraacetic acid (EDTA) is a hexadentate chelating ligand capable of forming 1:1 complex with various heavy metal ions while iminodiacetic acid (IDA) is analogous to half a unit of EDTA. A new styrene based monomer M1 bearing dimethyl iminodiacetate group was designed and synthesized in good yield to meet this objective. Free radical polymerization of M1 generated the homopolymer, which upon base hydrolysis generated the water-soluble chelating homopolymer P10, bearing sodium salt of IDA as the chelating group. The overall yield of P10 was 76.80% and the solubility was 5 mg/mL at room temperature. The water soluble polymer P10 was investigated for its ability to bind various heavy metal ions by UV–vis spectroscopy and was found to efficiently sequester Cu²⁺, Cd²⁺, Zn²⁺, Pb²⁺, Ni²⁺, Co²⁺, Cr³⁺, Fe²⁺ and Fe³⁺. The effect of pH on Cu²⁺ binding with P10 showed that every two IDA bearing monomeric repeat units binds with one Cu²⁺ ion at pH 7 suggesting that it forms complexes analogous to EDTA. Thermogravimetric analysis showed that the synthesized polymer possesses high thermal stability up to 400 °C. The potential for recovery and reuse of the polymer has been demonstrated with Cu²⁺ ion. The reported results suggest that this water-soluble chelating homopolymer is an excellent material with very high potential for application in wastewater treatment.

Full text of DOI:10.1016/j.reactfunctpolym.2020.104687

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Design and synthesis of water-soluble chelating polymeric
materials for heavy metal ion sequestration from aqueous waste
Reshu Tyagi, Josemon Jacob
PII:
S1381-5148(20)30446-6
DOI:
https://doi.org/10.1016/j.reactfunctpolym.2020.104687
REACT 104687
Reference:
To appear in:
Reactive and Functional Polymers
Received date:
Revised date:
Accepted date:
16 April 2020
17 June 2020
1 July 2020
Please cite this article as: R. Tyagi and J. Jacob, Design and synthesis of water-soluble
chelating polymeric materials for heavy metal ion sequestration from aqueous waste,
Reactive
and
Functional
Polymers
(2019),
https://doi.org/10.1016/
j.reactfunctpolym.2020.104687
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Design and synthesis of water-soluble chelating polymeric materials for heavy metal ion
sequestration from aqueous waste
Reshu Tyagi and Josemon Jacob*
Department of Materials Science and Engineering, Indian Institute of Technology, Hauz
Khas, New Delhi 110016, India.
*Corresponding author. Tel: +91 11 26591425; fax: +91 11 2659 1421
E-mail: jacob@mse.iitd.ac.in.
Abstract
Sequestration and removal of dissolved heavy metal ions from aqueous waste streams is a
challenging task. Ethylenediaminetetraacetic acid (EDTA) is a hexadentate chelating ligand
capable of forming 1:1 complex with various heavy metal ions while iminodiacetic acid
(IDA) is analogous to half a unit of EDTA. A new styrene based monomer M1 bearing
dimethyl iminodiacetate group was designed and synthesized in good yield to meet this
objective. Free radical polymerization of M1 generated the homopolymer, which upon base
hydrolysis generated the water-soluble chelating homopolymer P10 , bearing sodium salt of
IDA as the chelating group. The overall yield of P10 was 76.80% with and the solubility was
5 mg/mL at room temperature. The water soluble polymer P10 was investigated for its ability
to bind various heavy metal ions by UV–vis spectroscopy and was found to efficiently
sequester Cu2+, Cd2+, Zn2+, Pb2+, Ni2+, Co2+, Cr3+, Fe2+ and Fe3+. The effect of pH on Cu2+
binding with P10 showed that every two IDA bearing monomeric repeat units binds with one
Cu2+ ion at pH 7 suggesting that it forms complexes analogous to EDTA. Thermogravimetric
analysis showed that the synthesized polymer possesses high thermal stability up to 400 °C.
The potential for recovery and reuse of the polymer has been demonstrated with Cu2+ ion.
The reported results suggest that this water-soluble chelating homopolymer is an excellent
material with very high potential for application in wastewater treatment.
Keywords: Iminodiacetate; chelating polymer; water-soluble; heavy metal ion; wastewater
treatment.
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1. Introduction
Heavy metal ion pollution has become a major environmental issue facing most of the
developing countries today. Recent reports suggest that in many states in India, toxic metal
ion contamination of leafy vegetables and cereals exceed permissible levels set by the US
Environmental Protection Agency [1–3]. Removal of heavy metal ions such as Cu2+, Pb2+,
Ni2+, Cd2+, Zn2+, Co2+, Fe2+, Fe3+, Cr3+, Co3+ etc. from water is essential because of their
persistence, non-biodegradability, and toxicity for living organisms and the environment.
These contaminants, if left untreated, can enter the food chain and get accumulated in the
human body leading to serious health disorders [4–6]. The removal of heavy metal ions from
industrial waste is difficult due to their low concentration and presence in complex forms.
Therefore, the design of polymeric materials with high affinity for metal ion binding have
gained great attention in recent years [7–9].
Water-soluble chelating polymers are used in various metal ion sequestration techniques such
as membrane ultrafiltration, adsorption, precipitation, coagulation/flocculation etc. [10–14].
The removal of heavy metal ions from wastewater can be accomplished by the introduction
of strong chelating functional groups on a polymer [8,15]. One of the biggest advantages of
water-soluble polymers is that the complexation takes place in the homogenous phase under
low contact time as compared to insoluble chelating resins (the reactions are in the
heterogeneous phase and needs long contact time) [13,16]. To increase the water solubility of
the polymers, more hydrophilic groups are introduced into the monomer units. Among the
various functional groups, carboxylic acids, aminocarboxylic acids, amines, thiols,
phosphonic acids, and imidazoles have been incorporated into polymers and tested for their
suitability [17–19]. Among these, aminocarboxylic acid groups are outstanding for their
chelating properties, EDTA which forms strong 1:1 complex with various transition metal
ions being a prime example and offers high scope if introduced in polymers [20]. The
incorporation of EDTA unit into vinyl monomers or polymer side chains is not easy and
straightforward. However, IDA comprising of half a unit of EDTA, can be incorporated into
a monomer or polymer easily. IDA can form complexes with metal ions by binding through
two carboxylate oxygen atoms and one amine nitrogen atom. [21]. Polymers with IDA
groups are synthesized either by polymerization of vinyl monomers bearing IDA group or by
chemical modification of polymers [22–24]. There are relatively few reports available in
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literature concentrating on the synthesis of vinyl monomers and polymers with IDA
functionalities for metal ion sequestration. Chen et al. synthesized a new monomer, glycidyl
methacrylate (GMA)-iminodiacetic acid by the reaction of GMA with IDA. Its homopolymer
and copolymers with other vinyl monomers were synthesized to test their suitability for the
removal of heavy metal ions from aqueous solution [25]. Albayrak et al. synthesized two new
iminodiacetate monomers from methacrylate derivatives, and the corresponding polymers
were investigated for Ni2+ and Cr3+ binding studies by UV–vis spectroscopy [26]. Morris et
al. synthesized N-(4-vinylbenzyl)iminodiacetic acid (VBIDA) by the reaction of
chloromethylated styrene with IDA and homopolymerized it [27]. VBIDA was used for the
synthesis of various ion imprinting and non-ion imprinting polymers. Complexes of VBIDA
with Cu2+, Co2+ and Ni2+ were polymerized and studied for chiral separation of underivatized
amino acids [28], radioactive waste removal [29], and heavy metal ion sequestration,
respectively [30].
Although few isolated reports on the use of IDA based polymers for heavy metal ion
sequestration are available, there is a growing demand to develop efficient and readily
available chelating polymers to address issues associated with metal ion contamination of
aqueous waste streams. In this article, we report on the design and synthesis of a styrene-
based polymer bearing iminodiacetate ester groups. The water-soluble polymer obtained
upon its base hydrolysis has been investigated for its ability to complex various metal ions
that are commonly found in aqueous waste. Also, using Cu2+ ion as an example, the effect of
pH on the complexation behavior was studied. To the best of our knowledge, the synthesis of
the new styrene monomer, its polymerization and the demonstrated ability of the water-
soluble polymer to efficiently sequester commonly found heavy metal ions in aqueous waste
have not been reported to date.
2. Experimental
2.1. Materials
Methyl bromoacetate and dry solvents were purchased from Sigma Aldrich and kept in a
glove box under nitrogen atmosphere. All other chemicals and solvents used in the reactions
were of reagent grade, procured from Merck, Spectrochem, CDH or Fisher Scientific and
used without purification unless reported. Benzoyl peroxide (BPO) was purified by
recrystallization from chloroform (CHCl3)/methanol (CH3OH) (1:1) and dried under vacuum.
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N,N'-Diisopropylethylamine (DIPEA) was purified by distillation over KOH and stored over
KOH under nitrogen atmosphere. Metal salts Cu(NO3)2·3H2O, Ni(NO3)2·6H2O,
Cd(NO3)2·4H2O, Pb(NO3)2, Zn(NO3)2·6H2O, FeSO4·7H2O, Co(NO3)2·6H2O, Fe(NO3)3·6H2O
and KCr(SO4)2·12H2O were used as the source for metal ions Cu2+, Ni2+, Cd2+, Pb2+, Zn2+,
Fe2+, Co2+, Fe3+ and Cr3+, respectively. Aqueous solutions of metal salts were prepared using
de-ionized (D.I.) water. 1-Nitro-4-vinylbenzene (2) and 4-vinylaniline (3) were synthesized
according to literature procedures with slight modifications (details are given in
supplementary information) [31].
2.2. Instrumentation
Thin layer chromatography (TLC) was performed on Merck silica gel 60 sheets and analyzed
with a UV lamp (λmax = 254 nm). Compounds were purified by column chromatography on
silica gel (Merck 60–120 mesh). The melting point of the monomer was recorded on a Fisher-
Scientific melting point apparatus and is uncorrected. 1H and 13C NMR spectra were recorded
on a Bruker DPX-400 spectrometer operating at 400 MHz and 100 MHz, respectively, using
CDCl3 or D2O as solvents and tetramethyl silane (TMS) as an internal standard at room
temperature. High resolution mass spectra (HRMS) were recorded on Brucker MicrO-TOF-
QII model using ESI technique in positive ion mode. The monomer was analyzed for C, H
and N using a Perkin-Elmer 2400 Series II elemental analyzer. Fourier transform infrared
(FTIR) spectra were recorded using KBr pellets in the range of 4000 to 400 cm−1 on a
Thermo Nicolet IR200 spectrometer. Pellets were prepared by mixing the sample with dried
KBr. Molecular weights of the polymers were determined using a Waters series gel
permeation chromatography (GPC) equipped with WATERS 1515 column against
polystyrene standards. The data was recorded using THF as the mobile phase with a flow rate
of 0.5 mL min−1 at 30 °C and the concentration of the samples was 1 mg mL−1 in THF.
Thermogravimetric analysis (TGA) was performed using a Perkin-Elmer Pyris-6 TGA
thermo-balance at a heating rate of 20 °C min−1 under nitrogen atmosphere. Differential
scanning calorimetry (DSC) curves were recorded on a DSC Q200 calibrated with indium at
a heating/cooling rate of 10 °C min−1. UV–vis absorption studies were performed on a PG
Instruments T90+ UV/VIS spectrophotometer in D.I. water using a 1 cm quartz cell. The pH
of the metal ion solutions was measured with a LABMAN digital pH meter LMPH-10 in
aqueous solution.
2.3. Metal complexation studies
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For complexation studies of P10 with metal ions, the UV–vis absorption spectra were
recorded over a wavelength range of 190–500 nm using D.I. water was as a reference (no
absorption was observed above 500 nm for the metal ions investigated). The stock solutions
were prepared in D.I. water and the pH was adjusted with 2 M NaOH and 2 M HCl solutions.
The concentration of stock solutions was kept 2 × 10−2 M and 1 × 10−2 M for P10 and metal
ions, respectively. To record the absorption spectra of P10 and metal ion solutions, the
concentration used was 1 × 10−4 M. For P10-metal complexation studies, 2.5 mL of the metal
ion solution (1 × 10−4 M) was taken in a UV-quartz cell, then 100 µL of the polymer solution
(1 × 10−3 M) was added twice and spectra were recorded after each addition.
To investigate the effect of pH on the complexation behavior of P10 with Cu2+ ion, initial pH
of Cu2+ ion and P10 solutions were varied from 3–7. To record the spectra of P10-Cu2+
complexes at pH 7, 2.5 mL of Cu2+ (1 × 10−4 M, pH 7) solution was taken and small aliquots
(25.0 μL each, 7 times) of P10 (1 × 10−3 M, pH 7) solution were added and spectra were
recorded after each addition. A similar procedure was followed to record the absorption
spectra at pH 3–6 and the effect of complexation on pH was studied. To test the reusability of
P10, the absorption spectra of P10-Cu2+ complexes were recorded at pH 1.
2.4. Synthesis of dimethyl 2,2'-((4-vinylphenyl) azanediyl)diacetate (M1
)
To a solution of 4-vinylaniline (500 mg, 4.20 mmol) and t-butyl catechol (5 mg, 0.03 mmol)
in 10 mL of anhydrous acetonitrile, DIPEA (1.63 g, 12.59 mmol) was added under a nitrogen
atmosphere. After 5 min, methyl bromoacetate (3.21 g, 20.98 mmol) was added and the
mixture was stirred at 55 °C for 48 h. The reaction was monitored by TLC using 30% ethyl
acetate in hexane. Upon completion, the solvent was removed under vacuum, the product was
extracted into diethyl ether and washed with brine solution. The organic phase was collected
and dried over sodium sulphate. The solvent was evaporated, and the crude product was
purified by column chromatography with 15% ethyl acetate in hexane as the eluent to obtain
1
M1 as a white colored solid (1.0 g, 96%). Melting Point: 81–82 °C. H NMR (400 MHz;
CDCl3, Fig. S5): δ 7.30 (2H, d, J = 8.0 Hz, o-Ph), 6.65 (3H, m, J = 12, 8 Hz, -CH=CH2 + m-
Ph), 5.57 (1H, d, J = 16 Hz, -CH=CH2), 5.07 (1H, d, J = 12, -CH=CH2), 4.15 (4H, s, -
13
N(CH2COOCH3)2), 3.75 (6H, s, -N(CH2COOCH3)2). C NMR (100 MHz; CDCl3, Fig. S6):
δ 170.18, 146.34, 135.23, 127.17, 126.35, 111.38, 109.46, 52.30, 51.12. FTIR (KBr disk,
cm−1): 2952, 1742, 1610, 1519, 1438, 1267, 1180, 1007, 818. HRMS (ESI) m/z calc. for
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C14H17NNaO4 (M + Na)⁺: 286.1055, found 286.1059 (Fig. S7). Anal. calc. for C14H17NO4: C,
63.9%; H, 6.5%; N, 5.3%, found: C, 63.9%; H, 6.2%; N, 5.1%.
2.5. General synthetic procedure for polymers (P1–P8
)
M1 (200 mg, 0.76 mmol) and BPO (Table 1) were taken in a 50 mL Schlenk tube and purged
with nitrogen for 30 min. The reaction mixture was stirred at 90–100 °C for 22 h. It was then
cooled to room temperature, dissolved in 2 mL of tetrahydrofuran (THF) and added dropwise
into 200 mL of cold hexane to precipitate out the polymer. The polymer was isolated by
filtration and dried at 40 °C in a vacuum oven to obtain a white colored solid.
2.6. Synthesis of polymer (P9
)
M1 (500 mg, 1.90 mmol) and BPO (0.5 mole %) were taken into a 50 mL Schlenk tube and
purged for 30 min under nitrogen flow. The reaction mixture was stirred at 90 °C for 22 h.
The polymer obtained was dissolved in 5 mL of THF and poured dropwise into 500 mL of
cold hexane. The crude polymer was redissolved in THF and precipitated again from cold
hexane. The polymer was isolated by filtration and dried at 40 °C in a vacuum oven to obtain
a white colored solid in 80% yield. 1H NMR (400 MHz, CDCl3, Fig. S8): δ 6.30 (br, 4H, Ph),
4.10 (br 4H, -N(CH2COOCH3)2), 3.63 (br, 6H, - N(CH2COOCH3)2), 1.6 (br, 3H, -CH2-CH).
13C NMR (100 MHz, CDCl3, Fig. S9): δ 171.69, 145.71, 135.85, 128.5, 112.32, 53.16, 51.89,
43.49 (br atactic), 39.43. FTIR (KBr disk, cm⁻1): 3008, 2921, 1741, 1612, 1518, 1442, 1268,
1181, 1004, 814.
2.7. Synthesis of water-soluble polymer (P10)
P9 (500 mg) was dissolved in a mixture of THF/ethanol (EtOH) (1:2 v/v, 15 mL),
and a solution of NaOH (250 mg in 5 mL of water) was added under nitrogen
atmosphere. The reaction mixture was stirred at 60 °C for 6 h and concentrated under
vacuum. The polymer was purified by washing with CH3OH to remove the excess
NaOH and dried at 40 °C in vacuum for 12 h to obtain a white colored solid (0.51 g,
1
96%). H NMR (400 MHz; D2O, Fig. S10): δ 6.37 (4H, br, Ph), 3.7 (4H, br s, -
13
N(CH2COONa)2), 1.35 (3H, br, -CH2-CH). C NMR (100 MHz; D2O, Fig. S11): δ
179.62, 146.77, 134.86, 128, 111.99, 56, 43.36 (br atactic), 39.27. FTIR (KBr disk,
cm−1): 3388, 2924, 1602, 1515, 1451, 825
3 Results and discussion
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3.1. Synthesis and characterization of monomer M1
The synthetic approach towards the synthesis of monomer, dimethyl 2,2'-((4-vinylphenyl)
azanediyl)diacetate (M1) is shown in Scheme 1. 1-Nitro-4-vinylbenzene (2) was synthesized
from 4-nitrobenzyl bromide (1) in 91% yield and further reduced to 4-vinylaniline (3) using
iron powder in 76% yield [31]. M1 was synthesized by alkylation of 3 with methyl
bromoacetate using DIPEA in 96% yield. All the intermediates and monomer M1 were
purified by column chromatography using ethyl acetate in hexane as eluent over silica gel.
13
The structures of the purified intermediates were characterized and confirmed by 1H and C
NMR spectroscopy. The 1H and 13C NMR spectra of 2 (Fig. S1–S2) and 3 (Fig. S3–S4) were
1
found to be in good agreement with reported literature [31]. In the aliphatic region of the H
NMR spectrum of M1 (Fig. S5), there are two singlets at 3.75 ppm (6H) and 4.15 ppm (4H),
corresponding to protons of OCH3 and CH2 of dimethyl iminodiacetate (DMIDA) moiety,
respectively and the disappearance of NH2 protons from intermediate 3 (Fig. S3), at 3.67 ppm
confirm the dialkylation of 3 with methyl bromoacetate. In the 13C NMR spectrum of M1
(Fig. S6), the characteristic ester carbonyl (C=O) peak at 170.18 ppm and the presence of 8
signals in the range of 146.34 ppm to 51.12 ppm corresponding to all aromatic and aliphatic
carbon atoms, further confirmed the structure of M1. The FTIR spectrum of M1 showed the
presence of characteristic ester carbonyl (C=O) stretching band at 1742 cm−1 and C‒O
stretching bands at 1180 and 1007 cm−1. Results from elemental analysis and mass spectrum
(Fig. S7) also agreed well with the calculated values for M1.
3.2. Synthesis and characterization of homopolymers
The route towards the synthesis of water-soluble polymer P10 is shown in Scheme 1. P10
was synthesized in two steps. In the first step, bulk polymerization of M1 was done using
BPO initiator to obtain the polymer P9, bearing DMIDA groups. In the second step, the
methyl ester groups of P9 were hydrolyzed with NaOH to generate the water-soluble polymer
P10. Water soluble polymers used for various metal ion removal processes such as polymer
assisted ultrafiltration require a molecular weight more than the molecular weight cut-off of
the membrane filters (generally 3, 5 and10 kDa).[10] Therefore, the targeted molecular
weight of P10 was more than 20 kg mol−1 for the metal complexation studies to ensure the
use of P10 as a novel polymer with the membrane ultrafiltration as an extension of this work
for future studies.
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3.3. Optimization studies (targeted number. average molecular weight >20 kg mol−1)
A series of polymers (P1–P8) were synthesized to optimize the conditions of polymerization
by varying the temperature and initiator concentration (Table 1) with a targeted number
average molecular weight of more than 20 kg mol−1. Polymers, P1–P4 were synthesized at 90
°C and P5–P8 were synthesized at 100 °C by varying the initiator concentration from 0.25–
2.0 mol% with respect to the monomer. Molecular weights (Mn and Mw) and the dispersity
(Ð) of the polymers were determined by GPC and tabulated in Table 1. The Mn of polymers
decreased from P2–P4, as well as from P5–P8, on increasing the initiator concentration. The
higher Mn of P1–P4 than P5–P8 with same initiator concentration, was due to the low
polymerization temperature for P1–P4. The dispersity of the polymers was found to vary
between 1.3–8.1, due to the lack of control in free radical polymerizations. The Mn of
polymers (P1–P8) were found to vary between 5.92 kg mol−1 to 23.23 kg mol−1 and Mw from
12.81 kg mol−1 to 79.22 kg mol−1. The highest Mn (23.23 kg mol−1) was obtained for P2 using
0.5 mole% initiator at 90 °C. This optimized condition was used to synthesize P9 in 80%
yield. P9 was found to be soluble in common organic solvents such as CH3OH, EtOH, THF,
CHCl3, ethyl acetate etc but insoluble in water. The methyl ester groups of P9 were
hydrolyzed with NaOH to generate the water-soluble polymer P10 in 96% yield. The overall
yield of P10 from M1 was 76.80%. The sodium salt form P10 was readily soluble while its
carboxylic acid form was insoluble in water, attributable to strong inter and intramolecular
hydrogen bonding. The solubility of P10 was 5 mg/mL at room temperature in D.I. water.
1
The polymers, P9 and P10 were characterized by H and 13C NMR, FTIR spectroscopy,
1
GPC, DSC and TGA. In the H NMR spectrum of P9 (Fig. S8), all peaks were broad and
peaks in the vinyl region disappeared. The peaks at 6.30 ppm and 1.6 ppm correspond to the
aromatic and aliphatic protons of the backbone, respectively. The peaks at 4.10 ppm and 3.63
ppm correspond to the CH2 and OCH3 protons of DMIDA moiety.
13
The measured peak intensities were in good agreement with the structure of P9. In the C
NMR spectrum of P9 (Fig. S9), the carbonyl carbon peak was at 171.69 ppm with 8 signals
in the region from 135.85 ppm to 39.43 ppm corresponding to all aromatic and aliphatic
carbons further confirming the structure of the P9. A broad peak at 43.49 ppm showed the
atactic nature of P9 [32]. The FTIR spectrum of P9 showed a characteristic ester carbonyl
stretching at 1741 cm−1. In 1H NMR spectrum of P10 (Fig. S10), the disappearance of OCH3
peak and a broad peak at 3.7 ppm confirmed the complete hydrolysis of P9. Peaks at 6.37
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ppm and 1.35 ppm corresponding to the aromatic and the aliphatic proton of the polymer
backbone. Measured peak intensities agreed well with P10 structure. In 13C NMR of P10
(Fig. S11) peak of carbonyl carbon at 179.62 ppm and 7 signals from 146.77 ppm to 39.27
ppm confirmed the structure of polymer P10. The broad signal at 39.27 ppm suggested that
P10 is atactic [32]. The FTIR spectrum of P10 showed the characteristic carbonyl stretch of
carboxylate salt at 1602 cm−1 and 1451 cm−1 corresponding to its asymmetric and symmetric
stretching [26,33]. The molecular weights of P9 were determined by GPC and Mn was found
to be 33.54 kg mol−1 with a Ð of 5.1 (Table 1). Although broad molecular weight distribution
is observed for the polymers, such high value of Ð are not uncommon for free radical
polymerization with BPO as initiator.
3.4. Thermal characterization of homopolymers
The thermal characterization data for the polymers are summarized in Table 2. The glass
transition temperature (Tg) of polymers, determined from DSC curves of second heating and
cooling (Fig. 1a) to remove any thermal history, was found to be at 87 °C for P9 and Tg was
not observed for P10 from room temperature to 250 °C.
The thermal stability of the polymers was analyzed by TGA under nitrogen
atmosphere (Fig. 1b). It was found that P9 showed a one-step degradation pattern due
to the decomposition of the polymer chain and revealed good thermal stability with an
onset degradation temperature of 330 °C at 5% weight loss (T5). P10 showed a two-
step degradation pattern, with an initial weight loss of 17% occurring from 0 °C to 150
°C was due to the absorbed water in P10 [26,34]. The second step weight loss was due
to the decomposition of the polymer chain, demonstrating good thermal stability for
the polymer with an onset degradation temperature of 400 °C.
3.5. Complexation studies with heavy metal ions
The sodium salt form of the homopolymer P10 was used for complexation studies with
various heavy metal ions (e.g. Cu2+, Co2+, Cd2+, Zn2+, Pb2+, Ni2+, Fe2+, Cr3+ and Fe3+), due to
its solubility in aqueous solution. UV–vis absorbance of polymer-metal complexes was
recorded in aqueous solution at 1 × 10−4 M concentration. The data obtained from UV–vis
absorbance spectra are summarized in Table 3. T. Li et.al. reported that the polystyrene
showed two peaks and maxima’s were 260 and 295 nm (THF/methanol), The absorbance at
260 nm was due to the isolated phenyl groups and absorbance at 295 was due to associative
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interactions between neighboring phenyl groups as the polystyrene chain is flexible and the
rotation about the carbon-carbon bond of the backbone is possible. While at lower
concentration polystyrene showed only a peak at 280 nm (spectrum was recorded in THF)
because of no associative interaction between phenyl groups. The derivative of polystyrene
with 4-methyl and 4-t-butyl group also showed a new peak at 295 nm, and the peak maxima
at 280 nm were blue-shifted to 260 nm, due to the presence of more electron density on
phenyl rings which exhibited increased interaction between neighboring phenyl groups [40].
P10 showed 3 absorption peaks from 190 to 900 nm range, the absorbance maxima were
found to be at 204, 249 and 301 nm (Fig. 2). The absorbance at 204 nm was the solvent
absorption peak. The absorption maxima at 249 nm was related to the isolated phenyl groups
of P10 due to the increased electron density on phenyl rings, and the band at 301 nm was due
to the associated interactional between neighboring monomeric units attributable due to the
iminodiacetate functional groups. The increased electron density on the phenyl ring was
attributable to the conjugation from the lone pair of electrons on nitrogen of iminodiacetate
functional group as a result blue shift was observed from 260-249 nm for P10 while
comparing with polystyrene [40]. Metal ions, Pb2+ and Co2+ also showed a single peak in this
range and their corresponding absorbance maxima were found to be at 209 nm (Fig. S12d)
and 206 nm (Fig. S12a), respectively. At low concentrations (1 × 10⁻4 M) other metal ions do
not show any absorptions peaks from 200-500 nm. P10-metal complexes and pure metal ions
have not shown any peak above 500 nm at low concentrations, so all the spectra were
recorded from 190–500 nm range. For polymer-metal complex formation analysis, two
spectra of each metal ion with P10 in 1:1 and 1:2 (metal:P10) ratio were recorded at pH 5 for
all metal ions studied except Fe3+ which was studied at pH 2 because Fe3+ starts precipitation
above pH 2 [8][41] and compared with pure P10 spectrum. The details of coordination
interaction with metal ions are discussed below. with their corresponding spectra.
In the spectra of P10-metal2+ complexes peak at 301 nm was not observed due to the
formation of complexes between P10 and metal ions. The complexation of metal2+ ions with
iminodiacetate functionalities of P10 restricts the associated interaction between the
neighboring phenyl rings. The absorption peak at 249 nm was blue-shifted in the spectra of
P10-metal2+ complexes in the range of 228-240 nm (Table 3). It may be due to the increased
electron density on phenyl rings attributable to the conjugation with phenyl ring from the lone
pair of electrons on nitrogen of iminodiacetate functional group [40]. In the P10-metal
complexes, the charge transfer interactions take place between the iminodiacetate functional
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groups of P10 and metal ions. So, the electrons from oxygen of the carboxylate group of P10
was donated to the metal ions. Higher the blue shift was observed from 249 nm, stronger the
charge transfer interaction between P10 and metal ions. The P10-Cu2+ complexes showed the
highest blue shift from 249 nm to 228 nm (Fig 2), indicating the strongest interaction between
P10 and Cu2+ metal ions. Earlier studies also reported that the iminodiacetate functionalized
polymers show the highest removal efficiency with Cu2+ suggesting the strongest complexes
formation of Cu2+ ions with iminodiacetate functionalized polymers [20]. T. Li et.al. reported
that a new peak appeared only if the strong charge-transfer interaction occurred [40]. In our
studies, a new absorbance peak at 382 nm was observed in the spectra of P10-Cu2+
complexes (Fig. 2) indicating the strong charge-transfer interaction between iminodiacetate
functional group of P10 and Cu2+ ions, the intensity of which increased with increase in P10
concentration from 1:1 to 1:2 (Cu2+:P10). It is reported that only the broadening of
absorption maxima or a shoulder peak was observed in the case of weak charge-transfer
interactions [42][40]. Only a slight blue shift at 249 nm was observed in the case of other
metal ions studied as compared to Cu2+ ion, indicating the weak charge-transfer interaction
between P10 and these divalent metal ions (Co2+, Cd2+, Zn2+, Pb2+, Ni2+ and Fe2+) compare to
Cu2+. The absorption peak at 249 nm showed broadening and an enhancement in intensity
upon complexation in the case of P10-metal3+ with an increase of P10 concentration from
1:1 to 1:2. Although no obvious shift was noticed in the absorption spectra of P10-metal3+
complexes, with the increase of P10, precipitations of the polymer-metal complexes were
observed, attributable to the crosslinking between polymer chains with the formation of
expected 1:2 complexes. The observed changes in the spectra of P10-metal complexes
confirmed the formation of polymer-metal complexes in aqueous solution. From the UV–vis
studies, it was found that a new peak at 382 nm was observed in P10-Cu2+complexes,
whereas no new peaks were noticed in this range for P10-metal complexes with other metal
ions. The shifts at 249 confirmed the coordination interaction between the metal ions and the
P10. As the P10 was mixed with metal ion solution, two types of polymer-metal complexes
were formed in an aqueous system, either with one IDA monomeric unit (Tridentate) or
EDTA type (hexadentate) with two IDA units of P10 (Fig. 3). So, the aqueous solution
contained the metal ions, polymer P10, and P10-metal complexes. A tridentate complex as
well as an intramolecular hexadentate complex will lead to a soluble complex whereas
complexation between polymer chains will lead to an insoluble crosslinked complex and
hence precipitation of metal complexes will occur. To avoid precipitation, dilute
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concentrations (maximum concentration used without precipitation of P10-metal complexes
was 4 × 10⁻4 M) were used in the study.
Table S1 is shown in supporting information comparing the various iminodiacetate and
EDTA functionalized chelating resins for their ability to remove the heavy metal ions. It can
be seen that the polymer P10 reported in this study is comparable in terms of its ability to
complex commonly found metal ion contaminants present in the aqueous waste. However,
due to the insufficient details available in reported literature, a direct comparison of P10 with
commercially available resins is not possible.
3.6. Effect of pH on complexation behaviour of P10 with Cu2+ metal ions
The effect of pH value on the complexation behavior of P10 with Cu2+ metal ion was
investigated in the range of pH 3–7. All the IDA binding sites of P10 were protonated below
pH 3 and did not bind with metal ions while above pH 7 the metal ions were hydrolyzed and
precipitated out as metal-hydroxides [23,35–36]. The initial pH value of polymer and metal
ion solutions are the most important factor which affects the formation of polymer-metal
complexes. The initial pH values of Cu2+ ion and P10 solutions were varied (3, 4, 5, 6 and 7)
and the concentration of Cu2+ ion solution (CM, 1 × 10−4 M) was kept constant. The binding
capacity was analyzed using reference peak at 382 nm for P10-Cu2+ complexes therefore, the
absorptions spectra were recorded from 280–550 nm (Fig. S13). The increase in intensity was
observed at 382 nm as the ratio of IDA monomeric units to metal ions (CL:CM, CM:
concentration of Cu2+ ion and CL: concentration of P10) increased and after a certain CL/CM
ratio it became constant (Fig. 4a), due to the completion of complexation. The constant
CL/CM value was taken from the Fig. 4a to calculate the chelating capacity of P10 at that pH.
Each monomeric unit of P10 has one IDA group, so each metal ion can bind with one IDA
unit or with two IDA units of polymer chains. At lower pH values the coordination sites
available to bind with metal ions were less, due to the protonation of IDA units in acidic
conditions and at pH 7 all IDA sites were deprotonated and formed complexes with the metal
ion [37-38]. The chelating capacity (no of metal ions bound per IDA unit of P10) increased
from 0.28 to 0.5 as the pH was increased from 3–7, due to the formation of strong P10-Cu2+
(2:1) complexes with the increased no of deprotonated IDA sites of P10 at higher pH values
(Fig. 4b). The chelating capacity was found to be highly pH dependent and maximum was
achieved at pH 7, which is the natural pH of the water and best for purifications. Each IDA
bearing monomeric unit of P10 was bound with 0.5 equivalents of Cu2+ ion at pH 7 and
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formed the EDTA type complexes. The P10 was precipitate out in its diacid form at pH 1,
due to the inter and intramolecular hydrogen bonding between polymer chains. When the pH
of the solution of P10-Cu2+ complexes (complex formed at pH 7) was adjusted to pH 1 with
dilute HCl, P10 was precipitate out and then filtered. The absorbance peak at 382 nm was not
observed in these filtrates, due to the decomplexation of metal ions. This confirms the release
of Cu2+ from P10-Cu2+ complexes at pH 1. This indicates the possibility of recovery of P10
at pH 1. In fact, it was found that by lowering the pH to 1, decomplexation occurs and P10
can be recovered in its diacid form by filtration followed by washing with water. Hence,
recycling of the polymers is possible although it has been established only with Cu2+ ion at
the moment.
4 Conclusions
In this work, a new styrene-based monomer M1 and a water-soluble homopolymer P10 were
designed and synthesized in good yields with an objective for application in heavy metal ion
removal. M1 was polymerized in bulk using BPO as free radical initiator, which upon base
hydrolysis generated a water-soluble chelating polymer. The polymerization conditions were
optimized for temperature and initiator concentration. A polymer P9, bearing DMIDA
functionalities was synthesized using the optimized conditions (0.5 mole% of BPO at 90 °C
for 22 h) and the Mn was found to be 33.54 kg mol−1. The methyl ester groups of P9 were
hydrolyzed with NaOH to generate P10 bearing carboxylate chelating groups which showed
high thermal stability up to 400 °C. P10 was investigated for metal complexations behavior
with various heavy metal ions such as Cu2+, Cd2+, Zn2+, Pb2+, Ni2+, Co2+, Cr3+, Fe2+and Fe3+
by UV–vis absorption spectroscopy and found to be a strong chelator for these metal ions.
The effect of pH on the complexation formation behavior of P10 with metal ions were also
investigated using Cu2+ as an example and found to form strong 2:1 (P10:Cu2+) complex at
pH 7. The highest chelating capacity was obtained at pH 7, which is the natural pH of water
and best for purifications because no pH adjustments are required for removal of metal ion
from wastewater. Also, the potential for recovery and reuse of the polymer has been
demonstrated using Cu2+ ion as an example. This new monomer M1 can be used to
synthesize copolymers with diverse functionalities and tunable complexation behavior which
are currently under investigation. Water soluble homopolymer P10 could be used in various
metal ion separation processes such as membrane ultrafiltration and adsorption and has been
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shown as a promising new material for efficient removal of heavy metal ions from aqueous
solution.
Declaration of competing interest
An Indian Patent application has been filed on this work [39].
Acknowledgements
The authors would like to gratefully acknowledge Science and Engineering Research Board,
Department of Science and Technology, India (grant no CRG/2018/001488) for partial
financial support and the Department of Chemical Engineering, Indian Institute of technology
Delhi for elemental analysis. Reshu Tyagi acknowledges a research fellowship from Ministry
of Human Resources Development, India.
Credit author statement
Reshu Tyagi: Investigation, formal analysis, conceptualization, writing - Original Draft.
Josemon Jacob: Supervision, conceptualization, resources.
Declaration of interests
The authors declare that they have no known competing financial interests or personal
relationships that could have appeared to influence the work reported in this paper.
Appendix A. Supplementary data
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Scheme 1 Reaction scheme for the synthesis of monomer M1 and water-soluble polymer
P10.
Fig. 1 Thermal characterization of homopolymers (a) DSC traces and (b) TGA curves of P9
and P10.
Fig. 2 UV–vis absorption spectra of P10 and P10-metal complexes in aqueous solution with
(a) Cu2+, (b) Co2+ metal ions. The concentrations of metal ions: 1 × 10⁻4 M, and P10: 1 ×
10⁻4–2 × 10⁻4 M.
Fig. 3 (a) Schematic illustration for the formation of P10-metal complexes in aqueous
medium, (b) and (c) UV–vis absorptions spectra and effect of CL/CM ratio on absorbance of
P10-Cu2+ complexes at pH 7 (CM (concentration of Cu2+ ion): 1 × 10⁻4 M, and CL
(concentration of P10): 1 × 10⁻4–4 × 10⁻4 M).
Fig. 4 (a) Effect of CL/CM ratio on the absorbance of P10-Cu2+ complexes in aqueous
solution at different pH (CM (concentration of Cu2+ion): 1 × 10⁻4 M; CL (concentration of
P10): 1 × 10⁻4–4 × 10⁻4 M) and (b) Effect of pH on the chelating capacity of P10 (no of metal
ions adsorbed/IDA repeat unit of P10) from pH 3–7.
Table 1 Reaction conditions and characterization data of polymers (P1–P9).
Table 2 Thermal characterization data of polymers P9 and P10.
Table 3 UV-vis spectroscopy data of P10, metals, and P10-metal complexes.
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Scheme 1
Fig. 1
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Fig. 2
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Fig. 3
Fig. 4
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Tables
Table 1
b
c
Polym
BPOa
(mole %)
0.25
0.5
Temperatu Time Yield Mn (1 × 10⁻3
Mw (1 × 10⁻3
Ðd =
Mw/Mn
3.3
ers
P1
P2
P3
P4
P5
P6
P7
P8
P9
re (°C)
90
(h)
22
22
22
22
22
22
22
22
22
(%)
60
70
45
25
55
60
25
30
80
gmol⁻1)
22.20
23.23
8.07
gmol⁻1)
74.26
79.22
12.81
51.28
64.32
45.97
15.77
48.45
169.65
90
3.4
1.0
90
1.5
2.0
90
7.40
6.9
0.25
0.5
100
100
100
100
90
19.51
18.84
8.65
3.2
2.4
1.0
1.6
2.0
5.92
8.1
0.5
33.54
5.1
a Benzoyl peroxide initiator.
b Number-average molecular weight.
c Weight average molecular weight.
d Dispersity.
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Table 2
Absorbed water
a
b
Polymer
Tg (°C)
T5 (°C)
(wt%)
-
87
-
330
400 (2nd step)
P9
17%
P10
a Glass transition temperature determined by DSC.
b Onset decomposition temperature (5 % weight loss)
Table 3
Polymer/Metal Fig. No.
ion
Peak maxima of P10-metal
metal ions (nm) complexes
Peak maxima of P10-
metal complexes (nm)
205, 249, 301
205, 228, 382
205, 234
Fig 2
P10
-
Neat P10
P10-Cu2+
P10-Co2+
P10-Zn2+
P10-Ni2+
Cu2+
Co2+
Zn2+
Ni2+
-
Fig. 2
Fig. S12a
Fig. S12b
Fig. S12c
206
-
-
205, 232
205, 242
Pb2+
Cd2+
Fe2+
Fe3+
P10-Pb2+
P10-Cd2+
P10-Fe2+
P10-Fe3+
Fig. S12d
Fig. S12e
Fig. S12f
Fig. S12g
209
205, 238
205, 238
-
-
-
205, 240
205, 249, 301
Cr3+
-
P10-Cr3+
Fig. S12h
205, 249, 301
Preference for color: Online only.
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Graphical Abstract
Highly efficient water-soluble chelating polymer capable of sequestering commonly found
toxic metal ions from aqueous solution.
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Highlights
 A new styrene-based monomer bearing dimethyl iminodiacetate group was synthesized in
good yields.
 Free radical polymerization of monomer followed by base hydrolysis generated water-
soluble chelating polymer.
 Water soluble polymer was investigated for metal complexation behavior with various
heavy metal ions such as Cu2+, Cd2+, Zn2+, Pb2+, Ni2+, Co2+, Cr3+, Fe2+and Fe3+.
 The effect of pH on the complexation behavior with Cu2+ ion was studied, and found to
form strong EDTA type complexes.
26