New gel-like polymers as selective weak-base anion exchangers.

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.

Introduction

The search for selective polymers or hybrid exchangers or sorbents of anionic species is a fast-developing 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 [16]. 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 [12] or organophosphonate-Zr(IV) adducts for binding of tungstate(VI) and molybdate(VI) [26].

In our previous paper we have shown the preparation and application of novel gel-like polyamine resins for removal of heavy metal cations [27]. 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 [28-31].

Experimental

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 [27]. 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 temperature.

CAUTION

Solutions of perchloric acid in organic solvents may be explosive. Amine perchlorates could detonate upon heating or hitting.

Spectroscopy

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 time—40 ms, cross-polarization contact time—2 ms, sample rotation—5 kHz. The 15N spectra were referred to the signal of external glycine (-347.60 ppm), acquisition time—35 ms, cross-polarization contact time—2 ms, sample rotation—6 kHz. The 29Si spectra were referred to external neat TMS (0.00 ppm), the acquisition parameters were the same as for 15N NMR spectra.

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 1·10–2 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 [32]. Ion-chromatographic analyses were carried out on Methom Vario system.

Isotherms of adsorption, adsorption kinetics, sorption-regeneration experiments

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 -, SO3 2-, SO4 2-, SeO3 2-, SeO4 2-, TeO3 2-, TeO4 2-, PO4 3-, HPO3 2-, H2PO2 -, P2O7 4-, P3O9 3-, AsO2 -, HAsO4 2-, MnO4 -, ReO4 -, CrO4 2-, WO4 2-, MoO4 2-, VO3 - and VO4 3-). 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 (C ) and final, equilibrium (C ) 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 (q ; 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).

Table 1

Methods used for determination of the anion concentration dependent on the counter-ion in polymer.

Anion adsorbed	Analytical method
	ClO4-containing resin	PF6-containing resin
F-	potentiometry	potentiometry
Cl-	argentometry	XRF
Br-, BrO3 -	XRF	XRF
NO3 -, NO2 -	ion chromatography	ion chromatography
ClO3 -	ion chromatography	XRF
ClO4 -	-	XRF
I-, SO3 2-, SO4 2-, SeO3 2-, SeO4 2-, TeO3 2-, TeO4 2-, AsO2 -, HAsO4 2-, MnO4 -, ReO4 -, CrO4 2-, WO4 2-, MoO4 2-, VO3 -, VO4 3-	ICP-OES	ICP-OES
PO4 3-, HPO3 2-, H2PO2 -, P2O7 4-, P3O9 3-	ICP-OES	ion chromatography

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. MoO4 2- or WO4 2-) or anion instability (e.g. SO3 2-). 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 determination.

The adsorption kinetics was studied for 1 × 10–2 M salt concentration, using 20 mg of finely ground adsorbent (150–200 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 (Eq 2): while the second-order rate constants were calculated from the equation (Eq 3): where q and q are the amounts of anion adsorbed after time t and at equilibrium, respectively, (in mmol of adsorbate per 1 g of sorbent); k , k 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 × 10–2 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 × 10–2 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 in brine.

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 R (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 n , n are the amounts of adsorbed ions in the first and n-th adsorption cycle (in mmol), n —the amount of anions released during the n-th regeneration.

Water uptake

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 20–50 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 m and m are the masses of dry and swelled samples respectively.

Thermal analysis

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 studied.

Results and Discussion

Polymer structure

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 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 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.

Fig 1

Synthetic route to studied ion-exchange resins.

Table 2

Elemental analysis data and yields of the resins studied and the parent polymers.

		Elemental composition [%]
Symbol a	Amine	H	C	N	Si	Cl	P
N2	H2NCH2CH2NH2	9.22	46.43	10.14	12.59	-	-
N2HCl	H2NCH2CH2NH2	5.77	26.93	5.92	7.30	14.50	-
N2HP	H2NCH2CH2NH2	4.88	22.64	4.99	6.15	-	10.85
N3	H2N(CH2CH2NH)2H	9.68	47.76	13.43	10.75	-	-
N3HCl	H2N(CH2CH2NH)2H	5.50	24.61	7.00	5.69	17.20	-
N3HP	H2N(CH2CH2NH)2H	5.24	19.99	5.63	4.60	-	12.19
N4	H2N(CH2CH2NH)3H	9.94	48.62	15.68	9.47	-	-
N4HCl	H2N(CH2CH2NH)3H	5.29	23.01	7.47	4.73	18.50	-
N4HP	H2N(CH2CH2NH)3H	5.44	18.49	6.01	3.67	-	13.06
N5	H2N(CH2CH2NH)4H	10.08	49.40	17.70	8.34	-	-
N5HCl	H2N(CH2CH2NH)4H	5.05	21.99	7.82	3.76	19.63	-
N5HP	H2N(CH2CH2NH)4H	5.65	17.42	6.30	3.03	-	13.64
N6	H2N(CH2CH2NH)5H	10.35	50.11	19.22	7.57	-	-
N6HCl	H2N(CH2CH2NH)5H	5.00	21.16	8.14	3.22	20.20	-
N6HP	H2N(CH2CH2NH)5H	5.77	16.77	6.46	2.50	-	14.08

aHCl—perchlorate, HP—hexafluorophosphate(V)

Table 3

Results of the chemical analysis of the polymers studied.

Polymer	Parameter
	N1:N2:N3	N:Cl or N:P	N+	WA [%]
N2	0.373:0.621:0.006	-	-	0.30
N2HCl	-	1.035	0.966	0.61
N2HP	-	1.018	0.982	0.55
N3	0.261:0.731:0.008	-	-	0.48
N3HCl	-	1.032	0.969	0.76
N3HP	-	1.023	0.977	0.70
N4	0.193:0.798:0.009	-	-	0.65
N4HCl	-	1.024	0.977	1.03
N4HP	-	1.019	0.981	0.96
N5	0.163:0.830:0.007	-	-	0.77
N5HCl	-	1.010	0.990	1.25
N5HP	-	1.023	0.978	1.20
N6	0.136:0.854:0.010	-	-	0.91
N6HCl	-	1.022	0.979	1.40
N6HP	-	1.016	0.984	1.30

N1:N2:N3—ratio of primary:secondary:tertiary nitrogen atoms in polymer; N:Cl or N:P—moles of counter ion (ClO4 - or PF6 -) per 1 mol of nitrogen atoms; N+—molar ratio of protonated nitrogen atoms to total nitrogen; WA—water uptake

Water uptake by the resins studied was high. The values of this parameter were distinctly higher than that for the parent, non-ionic polymers [27]. 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 conditions—i.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 [27]. The siloxane parts of the molecules gave the characteristic signals at -1.7 ppm (SiH3, cyclic siloxane), -0.1 ppm (SiH3, linear siloxane), 13.6 ppm (SiH2), 23.5 ppm (CH2C), 68.8 ppm (HOH) and 74.0 ppm (H2O). 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 3–7 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 CrO4 2-, 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.

Table 4

NMR data for the polymers studied.

Sample	Chemical shift [ppm]
	13C NMR							29Si NMR	15N NMR
	CH3	CH2Si	CH2 CH2Si	OCH2	CHOH	CHCH2N	CH2N
N2Cl	-1.5; -0.2	13.4	22.1	74.2	68.9	56.7	37.5–53.0	-22.0	-336.2; -337.7
N3Cl	-1.9; -0.1	13.7	23.6	74.3	68.9	54.2	37.1–52.0	-16.5; -22.1	-336.1; -337.5
N4Cl	-2.0; -0.1	13.5	24.1	74.2	69.0	56.0	36.5–52.5	-16.9; -22.0	-325.5; -336.1; -337.9
N5Cl	-1.6; 0.0	13.8	24.0	74.3	69.1	56.6	37.0–52.5	-16.9; -22.1	-336.2; -337.4
N6Cl	-1.5; -0.1	13.8	24.1	74.4	68.7	56.1	37.6–53.0	-16.8; -22.2	-325.7; -336.0; -337.7
N2P	-1.4; -0.1	13.0	22.6	74.6	69.4	56.4	38.0–53.0	-21.9	-336.0; -337.7
N3P	-1.5; -0.1	13.6	23.3	74.6	68.7	53.9	37.0–51.5	-16.4; -22.2	-336.1; -337.8
N4P	-2.1; -0.3	13.6	23.4	74.9	69.1	55.2	36.5–52.0	-17.0; -21.9	-325.4; -336.0; -337.8
N5P	-1.4; -0.3	13.7	23.7	74.1	69.0	55.8	36.0–52.0	-16.8; -22.0	-336.0; -337.5
N6P	-1.5; -0.2	13.9	23.2	74.7	69.3	55.7	38.0–53.0	-16.9; -22.2	-325.4; -336.0; -337.5

IR spectroscopy

All FT-IR spectra of the synthesized resins show the signals characteristic of polysiloxane matrix, i.e. 1255–1265, 1405–1415 (δ (Si-)CH3), 800–810, 760–770 (γ(Si-)CH) and 1000–1100 cm-1 (ν Si-O-Si). These values are very close to those of the parent, neutral polymers [27]. 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 (δ NH2 +, δ NH3 +) and the bands corresponding to δ CH vibration (1430–1470 cm-1). The protonation also changes the shape of the signals in the ν N-H and ν O-H regions. The ν N-H absorption is shifted to smaller wavenumbers (2500–3200 cm-1), while the ν 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.

Fig 2

FT-IR spectra of parent polymer (N6; solid black line), N4 perchlorate (N6HCl; solid blue line) and N4 hexafluorophosphate(V) (N6HP; solid red line).

Anion exchange does not affect significantly the bands of the organic part of the studied polymers. The strongest shifts are observed for ν N-H and ν 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 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 3–5.

Table 5

Characteristic IR absorption bands of ions adsorbed on the resins studied.

Anion	Bands observed [cm-1]
PO4 3-	1045 (ν P-O), 540 (δ P-O)
HPO3 2-	2316 (ν P-H), 1058 (δ P-H, P-H), 1025, 1004 (ν P-O), 578 (δ s P-O), 507 (δ a P-O)
H2PO2 -	2336 (νs P-H), 2304 (νas P-H), 1185 (νas P-O), 1161 (δ P-H), 1091 (ω P-H), 1047 (νs P-O), 810 (ρ P-H), 473 (δ P-O)
P2O7 4-	1115 (ν P-O), 903 (νas P-O-P), 551 (δ P-O)
P3O9 3-	1130 (ν P-O), 900 (νas P-O-P), 540 (δ P-O)
SO3 2-	983 (ν S-O),
SO4 2-	1122 (νas S-O), 615 (νs S-O), 420 (δ S-O)
SeO3 2-	751 (ν Se-O)
SeO4 2-	881 (ν Se-O), 414 (δ Se-O)
TeO3 2-	735 (ν Te-O), 710 (ν Te-O), 630 (ν Te-O)
TeO4 2-	742 (ν Te-O), 670 (ν Te-O), 410 (δ Te-O)
AsO2 -	919 (ν As-O), 416 (δ As-O)
HAsO4 2-	844 (ν As-O), 806 (ν As-O), 742 (ν As-O), 404 (δ As-O)
ReO4 -	909 (νs As-O), 891 (νas As-O)
MoO4 2-	933 (ν Mo-O; Mo7), 896 (ν Mo-O; Mo7), 830 (ν Mo-O; Mo1)
WO4 2-	882 (νs W-O), 841 (νas W-O)
CrO4 2-	922 (νs Cr-O), 890 (νas Cr-O), 765 (ν Cr-O-Cr)
VO3 -	924 (νs VO2), 832 (νas VO2), 655 (νas VOV)
VO4 3-	835 (ν V-O)
ClO4 -	1115 (ν Cl-O), 625 (δ Cl-O)
ClO3 -	973 (νs Cl-O), 918 (νas Cl-O), 617 (δ Cl-O)
BrO3 -	795 (ν Br-O)
NO3 -	1372 (νas N-O), 835 (δ N-O)
NO2 -	1262 (νas N-O), 827 (δ N-O)
PF6 -	740 (νa P-F), 505 (νas P-F)

Fig 3

FT-IR spectra of N6HCl (solid black line) and this resin after ion-exchange with: H2PO2 - (dashed black line), HPO3 2- (solid blue line), PO4 3- (dashed blue line) and P2O7 4- (solid red line).

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), MoO4 2- (dashed blue line), WO4 2- (solid red line).

Several important conclusions could be drawn on the basis of the IR measurements for bonded anions. The first is that the pyrophosphate (P2O7 4-) and trimetaphosphate (P3O9 3-) 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 (MoO4 2-) causes its partial polymerisation and formation of heptamolybdate(VI) species (Mo7O24 6-). The IR spectra of the Mo(VI) oxoanions adsorbed on the studied resins show the bands characteristic of MoO4 2- (830 cm-1) as well as Mo7O24 6- (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 molecules co-adsorbed).

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 250°C. Contrary, PF6 - salts decompose less rapidly, however at lower temperatures than that of the parent, neutral polymers which decompose in the temperature range 325–525°C [27]. Decomposition of ammonium polymers studied consists of several steps. Firstly, between 120 and 150°C, an endothermic process accompanied by an insignificant mass loss (ca. 5–10%) is observed. It is assigned to desorption of water adsorbed in the resin. The major decomposition is observed in the temperature range 200–350°C with two peaks on DSC curve, the first at 270°C (mass loss ca. 25–40%) and the second at 310°C (mass loss of 60–65%). Over 350°C the final decomposition step is observed, resulting in the next substantial loss of mass (up to 75%). It ends at ca. 400°C. 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 instability of ammonium salts which undergo elimination reaction, with production of R3NH+ and C = C double bond.

Fig 6

TG curves of N4 (solid black line) parent polymer and its hexafluorophosphate(V) salt (N4HP; solid blue line).

Fig 7

DSC curve of N4HP resin.

Anion exchange

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 C , C are the equilibrium concentrations of the adsorbate (A) and counter ion (B) released to the solution, q —the amount of ions bonded at the concentration C , Q maximum amount of ions A bonded, x, y—valence of the ions A and B, K—equilibrium constant of the exchange reaction.

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 [35]: Calculated K and Q values and are given in Table 6.

Table 6

Parameters of anions adsorption by the selected polymers studied in water at 298 K (pH = 5.0).

Anion	Adsorption parameters: q m (mmol g-1; ± 0.02) and K (± 0.05)
	N2Cl		N2P		N4Cl		N4P		N6Cl		N6P
	q m	K	q m	K	q m	K	q m	K	q m	K	q m	K
PF6 -	3.99	0.14	3.50	1.00	4.99	0.13	4.20	1.00	5.11	0.15	4.54	1.00
ClO4 -	4.25	1.00	3.70	7.21	5.20	1.00	4.36	7.24	5.70	1.00	5.19	6.80
F-	2.15	<0.02	1.93	0.11	2.31	<0.02	1.98	0.10	2.10	<0.04	1.75	0.15
Cl-	4.22	0.11	3.65	0.77	5.13	0.13	4.33	0.90	5.42	0.12	4.83	0.86
Br-	4.20	0.11	3.61	0.75	5.00	0.19	4.21	1.26	5.20	0.15	4.67	1.07
I-	3.80	0.10	3.43	0.75	5.01	0.17	4.27	1.11	5.45	0.15	4.95	0.94
ClO3 -	4.00	0.67	3.55	4.88	5.04	0.73	4.20	5.10	5.49	0.67	4.95	4.40
BrO3 -	3.95	0.58	3.39	4.11	4.98	0.64	4.20	4.52	5.26	0.63	4.69	4.27
NO2 -	2.03	0.11	1.72	0.80	2.80	0.10	2.33	0.27	3.88	0.07	3.42	0.30
NO3 -	4.07	0.72	3.57	5.10	4.89	0.80	4.15	5.80	5.15	0.85	4.50	5.72
H2PO2 -	3.34	0.53	2.96	3.75	4.02	0.59	3.38	4.14	4.44	0.65	4.05	4.33
HPO3 2-	3.20	0.90	2.77	6.41	3.97	1.00	3.35	7.00	3.52	0.99	3.23	6.69
PO4 3-	3.40	2.06	2.94	14.77	4.04	2.18	3.44	15.99	3.95	2.16	3.53	14.05
P2O7 4-	1.89	3.15	1.72	23.50	2.50	3.33	2.07	25.02	2.58	3.80	2.29	25.20
P3O9 3-	1.86	2.98	1.68	21.00	2.27	3.31	1.93	25.13	2.40	3.63	2.13	24.17
AsO2 -	1.02	<0.02	0.81	<0.05	1.75	<0.02	1.50	0.11	2.02	<0.02	1.81	<0.10
HAsO4 2-	2.78	3.40	2.49	24.39	3.68	3.45	3.13	23.54	3.92	3.34	3.47	22.12
SO3 2-	1.46	<0.02	1.23	<0.05	2.40	<0.04	2.00	0.40	2.84	0.09	2.52	0.66
SO4 2-	2.99	3.12	2.56	22.79	3.69	3.05	3.07	21.46	3.95	3.17	3.54	21.49
SeO3 2-	1.32	<0.04	1.12	0.15	2.22	0.05	1.90	0.66	2.90	0.08	2.69	0.50
SeO4 2-	3.03	3.94	2.66	27.99	3.99	4.20	3.35	30.05	4.07	4.13	3.62	27.23
TeO3 2-	1.72	0.11	1.50	0.68	3.08	0.09	2.67	0.87	3.80	0.14	3.37	1.00
TeO4 2-	0.99	<0.02	0.77	<0.05	1.70	<0.02	1.34	0.23	2.07	<0.02	1.89	0.17
VO3 -	2.47	2.31	2.22	16.41	3.51	2.63	2.99	19.03	3.91	2.56	3.60	17.07
VO4 3-	2.86	5.58	2.50	41.60	3.74	5.73	3.21	44.08	3.97	5.67	5.05	37.51
CrO4 2-	2.45	2.99	2.24	22.06	3.49	3.15	2.86	24.23	3.61	3.22	3.27	21.27
MoO4 2-	2.22	3.09	2.00	21.45	3.13	3.30	2.60	25.38	3.48	3.19	3.09	21.24
WO4 2-	2.16	3.41	1.84	21.95	2.97	3.39	2.50	26.08	3.24	3.45	2.97	22.50
MnO4 -	2.97	0.35	2.55	2.06	3.96	0.43	3.36	3.31	4.00	0.39	3.62	2.31
ReO4 -	3.07	5.08	2.69	33.10	4.03	5.17	3.36	39.77	4.31	5.22	3.84	35.00

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 SO3 2- or SeO3 2- are under one-tenth of those of SO4 2- or SeO4 2-. 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 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 (PO4 3-) is less effectively adsorbed than diphosphate(V) (pyrophosphate, P2O7 4-) or trimetaphosphate(V) (P3O9 3-) 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 (H2P2O7 2-) and trimetaphosphoric acid dissociates almost completely (the molar fraction of P3O9 3- 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 -, CrO4 2-) 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.

Fig 8

UV-Vis reflectance spectra of N4HP resin after exchange with MnO4 - (solid black line) and CrO4 2- (solid blue line).

The kinetic parameters of ion exchange were studied for selected ions. The ions undergoing reaction with the resin (MnO4 -, CrO4 2-) or polymerizing during adsorption (MoO4 2-) were excluded from the study. Also some ions which were very weakly bonded by the studied materials (e.g. F-, AsO2 -, SO3 2-) 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. 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 [36]. 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 exchangers.

Fig 9

Pseudo-second order kinetic curves of anion exchange: ○ —N2HCl + SO4 2-, □ —N6HCl + SO4 2-, ▲ —N6HCl + ReO4 -, ■ —N6HCl + TeO3 2-, ● —N6HCl + P3O9 3-.

Table 7

The kinetics of ion exchange by the polymers studied in water (278 K; pH = 5.s0)—pseudo-second order rate constants.

Anion	Kinetic parameters—rate constant (×1000 g (mmol s)-1); ± 0.1
	N2HCl	N2HP	N4HCl	N4P
Br-	6.8	6.8	7.4	7.2
I-	6.0	5.9	6.9	7.0
ClO3 -	5.2	5.3	6.0	6.0
BrO3 -	4.8	4.5	5.6	5.8
NO3 -	6.7	6.6	7.2	7.0
H2PO2 -	5.0	4.9	6.0	6.0
HPO3 -	5.0	5.0	5.9	6.1
PO4 3-	5.1	4.9	6.2	6.0
P2O7 4-	3.5	3.4	4.7	4.6
P3O9 3-	3.0	3.0	4.5	4.5
HAsO4 2-	4.0	4.2	5.3	5.2
SO4 2-	5.2	5.1	6.1	6.1
SeO4 2-	4.1	4.3	5.4	5.4
TeO3 2-	3.3	3.2	5.0	5.2
VO3 -	3.3	3.0	4.6	4.8
VO4 3-	3.0	3.1	4.4	4.7
WO4 2-	3.4	3.1	4.4	4.3
ReO4 -	2.8	2.6	4.2	4.1

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 8–14 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 10

Ion-exchange capacity (IEC) of N2HCl (●) and N6HCl (■).

Fig 11

The pH dependence of maximal amount of ion bonded (Q) for N6HCl and: ▼ —ClO4 -, ○ —SO4 2-, ● —SeO4 2-, Δ —H2PO2 -, ■ —AsO2 -.

Sorption/regeneration experiments

Sorption-regeneration experiments were carried out for N6HCl, N6HP and selected ions: Br-, SO4 2-, SeO4 2-, ReO4 - and HAsO4 2- (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 sorption-regeneration 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. 60–70%. 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 D parameter is over 0.9.

Fig 12

Anion retention (R) and release (D) dependence on the number of sorption-regeneration cycles (base/acid regeneration procedure).

Fig 13

Anion retention (R) and release (D) dependence on the number of sorption-regeneration cycles (brine regeneration procedure).

Conclusions

It has been shown that the studied synthetic weak-base anion exchangers show excellent ion-exchange properties. They have high ion exchange capacity (2–7 mmol g-1), much higher than those of the most commercially used anion-exchange resins (0.5–2 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.