New Gel-Like Polymers as Selective Weak-Base Anion Exchangers
New Gel-Like Polymers As Selective Weak- Base Anion Exchangers
Baej Gierczyk 0 1
Micha Cegowski 0 1
Maciej Zalas 0 1
0 Faculty of Chemistry, Adam Mickiewicz University in Poznan , Poznan , Poland
1 Academic Editor: Jason B. Love , Reader in Inorganic Chemistry, UNITED KINGDOM
A group of new anion exchangers, based on polyamine podands and of excellent ion-binding capacity, were synthesized. The materials were obtained in reactions between various poly(ethyleneamines) with glycidyl derivatives of cyclotetrasiloxane. The final polymeric, strongly cross-linked materials form gel-like solids. Their structures and interactions with anions adsorbed were studied by spectroscopic methods (CP-MAS NMR, FR-IR, UV-Vis). The sorption isotherms and kinetic parameters were determined for 29 anions. Materials studied show high ion capacity and selectivity towards some important anions, e.g., selenate(VI) or perrhenate.
Competing Interests: The authors have declared
that no competing interests exist.
The search for selective polymers or hybrid exchangers or sorbents of anionic species is a
fastdeveloping aspect of material chemistry [1,2,3,4], however the number of reports on this topic
is much lower than those on the sorption of cations by polymer materials. The anion-binding
and anion-exchanging polymers and composites are of interest as materials for harmful anion
removal from drinking water [5,6,7], waste-water management [8,9,10], analyte
preconcentration prior to anionic-species analysis [11,12], as carriers of anionic pharmaceuticals [13,14,15]
or for catalysts recovery . A variety of materials have been tested for anion sorption,
including the inorganic sorbents (such as alumina and other metal oxides) [6,17,18], synthetic resins
[19,20], chitosan [5,7,21,22,23] and a wide range of inorganic and organic supports grafted
with anion binding side-chains [11,12,24,25]. In most cases, the active anion-binding sites are
the quaternized amino groups (both exo- and endocyclic), which interact with anions through
electrostatic forces. For some oxoanions, more specific systems have been designed, e.g.
1,2-diaminoaryl moieties for Se(IV) separation  or organophosphonate-Zr(IV) adducts for
binding of tungstate(VI) and molybdate(VI) .
In our previous paper we have shown the preparation and application of novel gel-like
polyamine resins for removal of heavy metal cations . The synthesis of the polymer involves a
new strategy, based on simultaneous polymerization of polysiloxane precursor and its
modification by chelating units. We decided to use the same material for preparation of
anion-exchange polymers, containing ionized amino groups, i.e. weak-base ion exchangers. This class
of ion-exchange materials has some important advantages over strong-base ones (materials
containing quaternized ammonium and phosphonium centres). The most important one is the
increased selectivity towards some oxyanions. Strong-base exchangers are hydrophobic,
therefore the major parameter determining their selectivity towards various anions is the degree of
the hydration of separated ions, as long as their charge remains the same. The weak-base
exchangers interacts with anions adsorbed not only via the electrostatic forces, but by formation
of hydrogen and coordination bonds. This change their selectivity, e.g. increases the affinity to
multivalent anions or oxoanions containing larger number of oxygen atoms .
All reagents used are commercial products. The salts used for anion binding studies had the
purity grade p.a. or higher and were purchased from Sigma-Aldrich. They were used without
purification. The starting polymers (N2-N6) were obtained according to procedures described
earlier . All other compounds were purchased from Sigma-Aldrich and Merck and used as
received. The solvents used for measurements were SPECTRANAL grade.
Synthesis of protonated resins
One g of the starting polymer (N2-N6) was ground in a mortar and suspended in acetonitrile
(50 ml). To the obtained mixture, fivefold molar excess (calculated on the basis of the nitrogen
amount in the starting material) of perchloric or hexafluorophosphoric(V) acid in acetonitrile
(5% by weight) was added. The suspensions obtained were mixed overnight, then the solid
resins were filtered off, washed three times with acetonitrile and dried in vacuum at room
CAUTION. Solutions of perchloric acid in organic solvents may be explosive. Amine
perchlorates could detonate upon heating or hitting.
Nuclear magnetic resonance spectra of the solids (13C, 15N, 29Si) were obtained at 298 K, on a
Varian VNMR-S 400 MHz spectrometer, operating at frequencies 101.25, 40.84 and 80.00 MHz,
respectively. The standard CP-MAS technique was used (tancpx sequence with 1H decoupling).
The 13C spectra were referred to the methylene signal of external glycine (43.30 ppm),
acquisition time40 ms, cross-polarization contact time2 ms, sample rotation5 kHz. The 15N
spectra were referred to the signal of external glycine (-347.60 ppm), acquisition time35 ms,
cross-polarization contact time2 ms, sample rotation6 kHz. The 29Si spectra were referred
to external neat TMS (0.00 ppm), the acquisition parameters were the same as for 15N
Infrared spectra were recorded on a Bruker IFS 66s spectrometer, using KBr pellets (2 mg of
sample in 200 mg of KBr). The complexes for these measurements were prepared by
immersion of a polymer sample in 1 102 M solution of the corresponding sodium salt in water for
10 h, followed by decantation, washing of a solid material with water and methanol and drying
in vacuum at room temperature.
Elemental analysis, chemical analysis
The contents of C, H and N were determined using a Vario EL III elemental analyser. The
content of silicon in the polymers was determined on an EDXRF spectrometer MiniPal 2
(PANalytical) after sample solubilisation with hydrofluoric acid. Iodine and phosphorus content in
final polymers, after its solubilisation in 0.5 M NaOH, were determined by the ICP-OES
method on a Varian Vista-MPX spectrometer with argon plasma ionization. The perchlorates were
determined in the same solution spectrophotometrically, as an ion-pair with Brilliant Green
according to the procedure described by Fogg et al . Ion-chromatographic analyses were
carried out on Methom Vario system.
Isotherms of adsorption, adsorption kinetics, sorption-regeneration
The adsorption measurements were performed for water solutions of sodium salts (except
tellurate, which was used as a potassium salt) of studied ions (F-, Cl-, Br-, I-, NO3-, NO2-, ClO3-,
ClO4-, BrO3-, SO32-, SO42-, SeO32-, SeO42-, TeO32-, TeO42-, PO43-, HPO32-, H2PO2-, P2O74-,
P3O93-, AsO2-, HAsO42-, MnO4-, ReO4-, CrO42-, WO42-, MoO42-, VO3- and VO43-). To make
each isotherm, a series of samples containing 0.01 g of polymer and 5 ml of salt solution were
used. The adsorption properties at eight concentrations were measured for each system (0.1,
0.5, 1, 2, 3, 4, 5 and 10 mM). The pH of the solutions was adjusted with NaOH and HClO4 or
HPF6 (depending on the polymer counter ion) to 5.0. The mixtures were equilibrated for 24 h
at 298 1 K. The starting (C0) and final, equilibrium (CA) concentrations of anions were
determined with the method dependent on the character of the counter ion in the polymer
(Table 1). The amount of the anion adsorbed (qA; mmol/g) in equilibrium was calculated from
the difference between these concentrations (Eq 1):
where m is the mass of the adsorbing polymer (g) and V is the volume of salt solution (dm3).
The relations between the sorption properties of the polymers studied and the pH of the
solution were studied in various pH ranges, taking into regard the complex pH-dependent
equilibriums known for some anions studied (e.g. MoO42- or WO42-) or anion instability (e.g.
SO32-). The pH of the solutions was adjusted with NaOH and HClO4 (or HPF6). To calculate
the maximal amount of the metal ions adsorbed by the polymer, the measurements were
made for five salt concentrations. Other experimental details were as for the isotherms
The adsorption kinetics was studied for 1 102 M salt concentration, using 20 mg of finely
ground adsorbent (150200 mesh) and 20 ml of adsorbate solution at 298 1 K; pH was
adjusted at 5.0. This stoichiometry guarantees large excess of the ions over the binding sites,
therefore the pseudo second-order kinetic model could be used. The reaction mixtures were
magnetically stirred vigorously at a stirring speed of 800 rpm. The same volume aliquots
(0.1 ml) were withdrawn periodically and the metal concentration was determined by the
methods mentioned above. The first-order rate constants were calculated from the equation
while the second-order rate constants were calculated from the equation (Eq 3):
where qt and qA are the amounts of anion adsorbed after time t and at equilibrium, respectively,
(in mmol of adsorbate per 1 g of sorbent); k1, k2 are the rate constants.
Sorption-regeneration experiments were made for 0.05 g samples of resins. The sample
studied was suspended in 5 ml of the appropriate adsorbate solution (1 102 M), pH was
adjusted to 5.0 and the suspension was stirred vigorously for over 5 h. Then the solid material
was filtered off. The amount of exchanged ions was determined in the filtrate by the method
indicated in Table 1. The resins were regenerated in two ways. The first method was a two-step
process. The solid material was suspended in 10 ml of NaOH solution (1 102 M), mixed
over 5 h, separated by centrifugation, washed with water and protonated with HClO4 (or
HPF6) as described in section 2.1. The amount of desorbed ions was determined in alkaline
supernatant. The second method of resins regeneration consisted of mixing it with 10 ml of 10%
solution of NaClO4 (or NaPF6) and separation of the solid material by centrifugation. The
resins were re-used after washing with water. The concentration of desorbed ions was determined
Ten cycles of ions binding/sorbent regeneration were performed. The efficiency of the
anion retention in the n-th cycle was expressed as the retention factor Rn (Eq 4):
while the ability to ions release in alkaline/brine solution in the n-th cycle was described by the
release factor (Eq 5):
where n1A, nnA are the amounts of adsorbed ions in the first and n-th adsorption cycle (in
mmol), nnRthe amount of anions released during the n-th regeneration.
The swelling of the resins was studied in water. The polymers used for this study were vacuum
dried at 373 K and the particles of 2050 mesh were used. The dry, weighted material (about
250 mg) was allowed to swell in water for 24 h at 298 K, filtered off, the excess of water was
removed by blotting and the sample was weighted again. The water uptake was calculated from
equation (Eq 6):
where m0 and ms are the masses of dry and swelled samples respectively.
Derivative scanning calorimetry and thermogravimetric measurements were performed on a
Setoram TGA thermoanalyser in nitrogen atmosphere at the constant heating rate of 10 K min-1.
The samples were dried in vacuum (1 mmHg) at 353 K for 5 h before analysis. Due to violent
decomposition of perchlorate-containing resins, only hexafluorophosphate(V) derivatives were
Results and Discussion
The new polymeric ion adsorbents with polyamine podand arms were synthesized according
to the synthetic route presented in Fig 1. After formation of cross-linked
polyamine/polysiloxane resin, the nitrogen atoms were quaternalized by protonation with perchloric or
hexafluorophosphoric acid. These acids were chosen due to their chemical properties. Both are strong,
monoprotonic acids (pKa < -5), therefore they do not undergo anionic hydrolysis, which may
influence the sorption properties in more acidic solution. Perchlorate anion forms strong
hydrogen bonds with polyamine net of the studied materials and is effectively bonded by them.
Contrary, PF6- anion is weakly adsorbed and easily exchanged. The use of these anions as
counter-ions in the resins studied permits the studies and comparison of the sorption of both
weakly adsorbed anions (e.g. halides) as well as strongly bonded ones (e.g. perrhenate, selenate
(VI)). Moreover, they are stable in oxidizing and reducing conditions, therefore they do not
Fig 1. Synthetic route to studied ion-exchange resins.
Elemental composition [%]
react with anions used for the sorption studies (e.g. permanganates or hypophosphites).
Finally, both show strong bands in IR spectra, which allows monitoring of their exchange by this
technique. The elemental compositions of the polymers and their structural features are
presented in Tables 2 & 3. The amount of protonated nitrogen atoms was calculated from
N1:N2:N3ratio of primary:secondary:tertiary nitrogen atoms in polymer; N:Cl or N:Pmoles of counter ion (ClO4- or PF6-) per 1 mol of nitrogen atoms;
N+molar ratio of protonated nitrogen atoms to total nitrogen; WAwater uptake
N:Cl or N:P
elemental analysis and N1:N2:N3 ratio in parent polymers. As shown for all studied systems,
the cationized nitrogen atoms constitute over 90% of the total number of nitrogen atoms.
Water uptake by the resins studied was high. The values of this parameter were distinctly
higher than that for the parent, non-ionic polymers . This is a result of higher affinity of
water to cationic N-centres than to amino groups as well as high hydration rates of perchlorate
and hexafluorophosphate(V) ions. For the resins studied, the water uptake increases with
increasing polyamine chain length, which is a result of an increasing number of water binding
groups (N atoms and counter ions) as well as loosening of the polymer structure due to
elongation of the cross-linking chains.
The materials obtained are stable in acidic conditionsi.e. the polymer matrix does not
show any signs of decomposition after stirring for 10 days in 1 M HCl or H2SO4. The IR and
NMR spectra of so treated resins are identical as those of the parent ones (except changes
resulting from ion-exchange). The treatment of the exchangers with strong, concentrated base
solutions (e.g. 1 M NaOH or KOH) leads to fragmentation of poly(siloxane) chains and
decomposition of the polymers studied. They are however stable in less concentrated hydroxide
solutions (< 0.05 M).
CP-MAS spectra of polymers and their complexes
The structures of the polymers obtained are fully confirmed by NMR measurements. The 13C
spectra are very similar to those of parent resins. The signals of non-polyamine part of the
molecules were observed at the same chemical shifts in both protonated and neutral polymers .
The siloxane parts of the molecules gave the characteristic signals at -1.7 ppm (SiCH3, cyclic
siloxane), -0.1 ppm (SiCH3, linear siloxane), 13.6 ppm (SiCH2), 23.5 ppm (CCH2C), 68.8 ppm
(CHOH) and 74.0 ppm (CH2O). The only difference was the greater line widths observed for
the cationic forms. The ionization affects strongly the signals of the carbon atoms bonded to
nitrogen atoms. Protonation resulted in the 37 ppm upfield shift of the signals assigned to the
nitrogen-bonded C atoms. The 29Si NMR spectra of the polymers studied were not affected by
protonation of polyamine part of the molecule, i.e. the signals appeared at -21.8 (linear
polysiloxane) and -16.5 ppm (cyclotetrasiloxane). The 15N NMR spectra of protonated polymers
show the signals at ca. -336, -338 and -325 ppm (traces), corresponding to R2NH2+, RNH3+
and R3NH+ centres. Anion exchange does not affect the 13C, 15N and 29Si NMR spectra, i.e. the
observed shifts are smaller than the measurements accuracy. As the signals of the studied resins
are rather broad, shifts smaller than 2 ppm are not detectable. For the complexes with CrO42-,
a large increase in the NMR line widths was detected, caused by paramagnetic relaxation
induced by Cr3+ ion, formed by reduction of chromate(VI) anions. The NMR data are collected
in Table 4.
All FT-IR spectra of the synthesized resins show the signals characteristic of polysiloxane matrix,
i.e. 12551265, 14051415 ( (Si-)CH3), 800810, 760770 ((Si-)CH) and 10001100 cm-1
( Si-O-Si). These values are very close to those of the parent, neutral polymers . Significant
changes are observed for the absorption bands related to podand moieties. The spectra of
protonated polymers show a signal at ca. 1630 cm-1 ( NH2+, NH3+) and the bands corresponding
to CH vibration (14301470 cm-1). The protonation also changes the shape of the signals in
the N-H and O-H regions. The N-H absorption is shifted to smaller wavenumbers (2500
3200 cm-1), while the O-H signal is sharpened and appears at ca. 3480 cm-1. Counter ions give
the signal at 1115 and 625 cm-1 (perchlorate) or 740 and 505 cm-1 (hexafluorophosphate). The
exemplary spectra are presented in Fig 2.
Chemical shift [ppm]
Anion exchange does not affect significantly the bands of the organic part of the studied
polymers. The strongest shifts are observed for N-H and O-H, due to different hydrogen
bond interactions between NH and OH protons and bonded anions. Bonding of the anion
results in weakening or even disappearance of the signals assigned to Cl-O (or P-F) bonds and
Fig 2. FT-IR spectra of parent polymer (N6; solid black line), N4 perchlorate (N6HCl; solid blue line) and N4 hexafluorophosphate(V) (N6HP; solid
Bands observed [cm-1]
PO43- 1045 ( P-O), 540 ( P-O)
HPO32- 2316 ( P-H), 1058 ( P-H, P-H), 1025, 1004 ( P-O), 578 ( s P-O), 507 ( a P-O)
H2PO2- 2336 (s P-H), 2304 (as P-H), 1185 (as P-O), 1161 ( P-H), 1091 ( P-H), 1047 (s P-O), 810
( P-H), 473 ( P-O)
P2O74- 1115 ( P-O), 903 (as P-O-P), 551 ( P-O)
P3O93- 1130 ( P-O), 900 (as P-O-P), 540 ( P-O)
SO32- 983 ( S-O),
SO42- 1122 (as S-O), 615 (s S-O), 420 ( S-O)
SeO32- 751 ( Se-O)
SeO42- 881 ( Se-O), 414 ( Se-O)
TeO32- 735 ( Te-O), 710 ( Te-O), 630 ( Te-O)
TeO42- 742 ( Te-O), 670 ( Te-O), 410 ( Te-O)
AsO2- 919 ( As-O), 416 ( As-O)
HAsO42- 844 ( As-O), 806 ( As-O), 742 ( As-O), 404 ( As-O)
ReO4- 909 (s As-O), 891 (as As-O)
MoO42- 933 ( Mo-O; Mo7), 896 ( Mo-O; Mo7), 830 ( Mo-O; Mo1)
WO42- 882 (s W-O), 841 (as W-O)
CrO42- 922 (s Cr-O), 890 (as Cr-O), 765 ( Cr-O-Cr)
VO3- 924 (s VO2), 832 (as VO2), 655 (as VOV)
VO43- 835 ( V-O)
ClO4- 1115 ( Cl-O), 625 ( Cl-O)
ClO3- 973 (s Cl-O), 918 (as Cl-O), 617 ( Cl-O)
BrO3- 795 ( Br-O)
NO3- 1372 (as N-O), 835 ( N-O)
NO2- 1262 (as N-O), 827 ( N-O)
PF6- 740 (a P-F), 505 (as P-F)
appearance of new absorption bands related to the complexed anions. The wavenumbers of the
signals observed for bonded anions are collected in Table 5. The exemplary IR spectra are
presented in Fig 35.
Several important conclusions could be drawn on the basis of the IR measurements for
bonded anions. The first is that the pyrophosphate (P2O74-) and trimetaphosphate (P3O93-)
ions do not undergo hydrolysis upon complexation, since the characteristic vibrations, related
to P-O-P (ca. 900 cm-1) are observed. On the other hand, the sorption of molybdate(VI) ions
(MoO42-) causes its partial polymerisation and formation of heptamolybdate(VI) species
(Mo7O246-). The IR spectra of the Mo(VI) oxoanions adsorbed on the studied resins show the
bands characteristic of MoO42- (830 cm-1) as well as Mo7O246- (896 & 930 cm-1). The signals of
both forms have similar intensities. The band at 765 cm-1 observed for chromate-loaded resins
indicates that the adsorbed ion forms a dimer, i.e. dichromate anion.
The X-O stretching bands of all studied oxoanions bonded to resins are quite broad and
composed of many close lying signals. They are distinctly broader than those reported for
anhydrous inorganic or tetraalkylammonium salts. The observed effect is a result of the
possibility of realisation of different modes of hydrogen bonding between oxoanion adsorbed and the
O-H and N-H donors (hydroxyl and ammonium moieties of the resin as well as water
Fig 3. FT-IR spectra of N6HCl (solid black line) and this resin after ion-exchange with: H2PO2- (dashed black line), HPO32- (solid blue line),
PO43(dashed blue line) and P2O74- (solid red line).
Fig 4. FT-IR spectra of N6HCl (solid black line) and this resin after ion-exchange with: HAsO42- (dashed black line), SeO42- (solid blue line) and
SO42- (dashed blue line).
The thermograms of the selected polymers studied are presented in Fig 6. The perchlorate
forms of the synthesized ionic resins were not studied by TG and DSC because of the violent,
explosive decomposition above 250C. Contrary, PF6- salts decompose less rapidly, however at
lower temperatures than that of the parent, neutral polymers which decompose in the
temperature range 325525C . Decomposition of ammonium polymers studied consists of several
steps. Firstly, between 120 and 150C, an endothermic process accompanied by an insignificant
mass loss (ca. 510%) is observed. It is assigned to desorption of water adsorbed in the resin.
The major decomposition is observed in the temperature range 200350C with two peaks on
DSC curve, the first at 270C (mass loss ca. 2540%) and the second at 310C (mass loss of 60
65%). Over 350C the final decomposition step is observed, resulting in the next substantial
loss of mass (up to 75%). It ends at ca. 400C. As the low-temperature processes should be
assigned to pyrolysis of organic part of the polymer, the high temperature ones corresponds to
evolution of PF5, HF and other products of PF6- decomposition. As for N2-N6 polymers, the
mass loss of ionic resins increases with the number of ethyleneamine units. The residual mass
varies from 45% (N2HP) to 32% (N6HP). The residual material contains mainly Si, P, O and
traces of F and C. All processes observed on heating of the samples are endothermic (Fig 7).
None of the observed processes is reversible, therefore the studied resins do not undergo a
glass-transition in the studied temperature ranges. The observed differences in thermal stability
between the parent neutral polymers and its protonated derivatives are a result of thermal
Fig 5. FT-IR spectra of N6HCl (solid black line) and this resin after ion-exchange with: ReO4- (dashed black line), VO3- (solid blue line),
MoO42(dashed blue line), WO42- (solid red line).
instability of ammonium salts which undergo elimination reaction, with production of R3NH+
and C = C double bond.
All polymers studied are able to exchange anions. The binding capacity of the resins depends
on its structure and the anion character. Although the Misak model (Eq 7) is much more
adequate for describing the ion exchange [33,34], for the anions originating from weak, polyprotic
acids, the determination of the major ionic species adsorbed on a resin is difficult.
where CA, CB are the equilibrium concentrations of the adsorbate (A) and counter ion (B)
released to the solution, qAthe amount of ions bonded at the concentration CA, Q maximum
amount of ions A bonded, x, yvalence of the ions A and B, Kequilibrium constant of the
Therefore, the maximum amount of the ions bonded (Q, mmol of anion per 1 g of polymer)
and the binding equilibrium constant K, were calculated assuming that the adsorption process
was described by the Langmuir model (with Boyd modification; Eq 8), which is widely used in
ion exchange studies :
Calculated K and Q values and are given in Table 6.
For the studied weak base anion exchange resins, the affinity of monovalent halogen anions
to polymers is very low. All of them interact with the resins studied weaker than PF6- and
ClO4-. The interaction of oxoanions depends on their valence and the strength of the parent
acid. The monovalent oxoanions of strong acids (ClO4-, BrO4-, ReO4- etc.) show similar affinity
to the resin, however the larger anions (e.g. ReO4-) bind stronger to the less cross-linked
polymers of a more flexible structure (N5H, N6H). The monovalent anions of very weak acids
(AsO2- or As(OH)2O-) practically do not exchange with ClO4- and only slightly with PF6- ions.
This is a result of several cooperative effects. The first is that at pH = 5.0 they exist in a neutral
form, so they do not bind to the protonated resin via electrostatic interaction. Moreover, the
oxygen atoms of arsenate(III) acid are poorer hydrogen bond acceptors than those of
perchlorate anion. Comparing the ions containing the same central atom, it is clearly seen that the
affinity to the resin increases with increasing oxidation state of the central atom, e.g. the K values
for SO32- or SeO32- are under one-tenth of those of SO42- or SeO42-. This is related to increasing
acid strength and, in consequence, the domination of multiple charged species in the solution.
At pH = 5.0 the selenous or sulfurous acids exists mainly as monovalent, HXO3- anions. They
make weaker electrostatic interactions with N+ centres than the multiple charged ions.
Moreover, the increasing number of oxygen atoms in the anions containing central atom of high
Fig 7. DSC curve of N4HP resin.
oxidation state, stabilises adducts due to the formation of higher number of cooperative
hydrogen bonds with NH and OH donors from the resin. The same effects are observed for the series
of phosphates(V). The orthophosphate ion (PO43-) is less effectively adsorbed than
diphosphate(V) (pyrophosphate, P2O74-) or trimetaphosphate(V) (P3O93-) ones. At pH = 5.0 the
major form of phosphoric(V) acid is the monovalent anion (H2PO4-), since in the same
conditions diphosphoric(V) acid forms double charged ion (H2P2O72-) and trimetaphosphoric acid
dissociates almost completely (the molar fraction of P3O93- is ca. 0.999). The effect of the
number of possible hydrogen bonds is clearly seen for metavanadate(V) and orthovanadate(V). At
pH = 5.0, they both exist as monovalent ions (molar fraction of H2VO4- and VO3- is ca. 0.94),
however for metavanadate, K values are distinctly lower. The strong oxidizing anions (MnO4-,
CrO42-) undergo partial reduction after adsorption. Fig 8 presents the diffusion reflectance
UV-Vis spectra of N6HCl loaded with permanganate and (di)chromate ions. Besides the
absorption of Mn(VII) or Cr(VI) (520 nm and 350 nm, respectively), the signals characteristic of
MnO2 (430, 600 nm) and Cr3+ (410, 560 nm) are observed.
The kinetic parameters of ion exchange were studied for selected ions. The ions undergoing
reaction with the resin (MnO4-, CrO42-) or polymerizing during adsorption (MoO42-) were
excluded from the study. Also some ions which were very weakly bonded by the studied materials
(e.g. F-, AsO2-, SO32-) were not investigated. The kinetics of the reaction was measured for four
resins, two perchlorate ones (N2HCl and N6HCl) and their PF6- analogues (N2HP, N6HP). The
exemplary kinetic curves are presented in Fig 9. The kinetic parameters are collected in Table 7.
Adsorption parameters: qm (mmol g-1; 0.02) and K ( 0.05)
The equilibrium of the reaction is reached quite quickly, in less than 60 min., depending on the
anion adsorbed. The adsorption process was analysed using two kinetic models, i.e. pseudo-first
order and pseudo-second order . The experimental data were much better reproduced by the
second one. Although this simplified approach does not take the sorption mechanism into
account, it is widely used for description of the sorption kinetics. Therefore, the presented results
should be used only for the comparison of the sorption kinetics properties of the studied
materials with literature data and have no physical meaning. The exchange was distinctly slower for
N2-derived resins than for N6-derived ones. This is a consequence of the slower diffusion of ions
in the polymer matrix cross-linked with shorter polyamine chains. The differences observed are
more distinct for large ions (e.g. ReO4-) than for those of small radii (Cl-), whose diffusion is less
hindered by the close distances between the polysiloxane chains. There is no significant
difference in the process kinetics between the perchlorate and hexafluorophosphate forms of
The pH dependence of the ion-exchange capacity (IEC) of the studied resins is shown in
Fig 10. As expected, IEC is almost constant below pH = 5.5. Above this value, IEC drops and
reaches 0 at pH > 8. This is a result of deprotonation of NH+ centres of the resin. Above pH = 8,
the resins exist in neutral, amine form. Any binding of anions in the pH range 814 is a
physisorption or a result of NHO and OHO hydrogen bonding between resin heteroatoms and anion
oxygen atoms. The influence of pH of the solution on the maximum amount of ions bonded (Q)
is presented in Fig 11. At low pH values (pH < 3) the anions of strong acids (sulphate, selenate,
perchlorate etc.) are preferably adsorbed. Anions of strong acids exist in ionic form even at low
pH. The uptake of these anions rise insignificantly with increasing pH, these changes are more
distinct for multivalent anions. As the second and subsequent dissociation constants of
polyprotonic acids are smaller than the first one, the increasing pH value results in the rise of the
population of double (and triple) ionized species, which interact stronger with the protonated resin.
Contrary, the anions of weak acids show high increase in the affinity to the resin with decreasing
solution acidity. At low pH, these acids exist as non-ionized molecules or the molar ratios of
ionic forms are very low. The increase in pH results in ionisation of weak acids and formation of
anionic species, which show affinity to the resin. The anion of the weakest acid studied (AsO2-)
adsorbs at pH > 8 and the formed adduct is stabilised by hydrogen bonds.
Fig 9. Pseudo-second order kinetic curves of anion exchange:
N6HCl + P3O93-.
N2HCl + SO42-, N6HCl + SO42-, N6HCl + ReO4-, N6HCl + TeO32-,
Fig 10. Ion-exchange capacity (IEC) of N2HCl () and N6HCl ().
Sorption-regeneration experiments were carried out for N6HCl, N6HP and selected ions: Br-,
SO42-, SeO42-, ReO4- and HAsO42- (these formulas correspond to the composition of salts used
as an anion sources, not to the form of the anions existing in solution or bonded to the polymer
after sorption, which may be different due to protolytic equilibria). The results of these
experiments are shown in Fig 12 and 13. Fig 12 presents the efficiency of two-step regeneration, i.e.
desorption of bonded ion in NaOH solution followed by re-protonation of the polymer
material. Synthesized material shows excellent reversibility of the exchange properties and high
stability. Small (ca. 5%) decrease in the sorption efficiency (R) is shown in the first
sorptionregeneration cycle, but the further amounts of exchanged ions are the same up to the tenth
cycle. Over 97% of the ions adsorbed are released during treatment with NaOH. The
regeneration with concentrated salt (NaClO4 or NaPF6) solutions gives somewhat different results
(Fig 13). Since the regeneration with NaClO4 almost restores the starting sorption ability of the
materials studied, the regeneration with NaPF6 is much less effective. The recovery of
oxoanions of strong acids, showing high affinity to the resins studied, in this regeneration method is
ca. 6070%. This is a result of much stronger bonding of these ions than that of a PF6- anion,
which does not form hydrogen bonds with the polymers studied. For simple anions (e.g. Br-),
having lower affinity to the resin than PF6-, the Dn parameter is over 0.9.
It has been shown that the studied synthetic weak-base anion exchangers show excellent
ionexchange properties. They have high ion exchange capacity (27 mmol g-1), much higher than
those of the most commercially used anion-exchange resins (0.52 mmol g-1) and comparable
to those reported for modern, recently reported adsorbents [28,37]. The materials studied are
thermally stable and could be regenerated many times without significant loss of activity. They
show high affinity to strong acids-related anions, so they could be used for separation of ions
containing the same atom at different oxidation states (e.g. arsenate(III) from arsenate(V)).
Their excellent adsorption properties to metal oxoions make them interesting materials for
separation and pre-concentration of valuable metals (e.g. vanadium or rhenium) in
hydrometallurgical processes or for removal of toxic metalloids (selenium, arsenic) from drinking water.
Conceived and designed the experiments: BG MZ. Performed the experiments: BG MC.
Analyzed the data: BG MC MZ. Contributed reagents/materials/analysis tools: BG. Wrote the
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