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Mechanistic studies of metal ion binding to water-soluble polymers using potentiometry
Pergamon
0039-9140(94)00231-2
Tahmta, Vol. 42, No. 2, pp. 219-.226, 1995 Copyright ~ 1995 Elsevier Science Ltd Printed in Great Britain. All rights reserved 0039-9140/95 $9.50 + 0.00
M E C H A N I S T I C STUDIES OF M E T A L ION B I N D I N G TO WATER-SOLUBLE POLYMERS USING POTENTIOMETRY NElL V. JARVIS and JUDITH M. WAGENER Department of Process Technology, Atomic Energy Corporation of South Africa Ltd, P.O. Box 582, Pretoria, 0001, South Africa
(Received 15 March 1994. Revised 20 June 1994. Accepted 7 July 1994)
Summary--A method for elucidating metal ion binding mechanismswith water-solublepolymers has been developed in which the polymer is treated as a collection of monomeric units. Data obtained from potentiometric titrations are analysed by the ESTA library of programs and apparent formation constants may be calculated. From this information, predictions may be made as to metal ion separation using complexation-ultrafiltration techniques. The polymer used in this study was Polymin Water-Free and its complexation with Hg(ll), Cd(ll), Pb(ll), Co(lI) and Ni(II) was successfully modelled.
Interest in the use of water-soluble polymers in conjuction with ultrafiltration membranes to separate metal ions from aqueous solutions has steadily grown since the work of Geckeler et al. was published in the early eighties, j The possibility of synthesizing derivatives of commercially available water-soluble polymers such as polyethyleneimine, in order to achieve selective metal ion complexation, was recognized early on. A wide range of applications have been investigated 2 including the nuclear industry 3 and the removal of toxic heavy metal ions such as Pb(II), Cd(II) and Hg(II). 4 As the field ofligand design produces more complicated and selective complexing agents, it has become an attractive option to attach these reagents to polymer backbones in order to minimize losses of expensive compounds. At this time, little effort has gone into the understanding of metal ion complexation mechanisms of water-soluble polymers in aqueous solution. Recent work by Rumeau et al. 2 used potentiometry to calculate stability constants of these polymers with metal ions. We have developed a method to elucidate metal ion bonding mechanisms to chelating resins using two-phase potentiometry. 5 In this paper, we extend this method to water-soluble polymers (although the term "two-phase" may not be entirely applicable here). The repeating unit of the polymer is considered to be the ligand and potentiometric
data are submitted to the ESTA library of programs. 6 Apparent formation constants may be accurately calculated and this allows predictions to be made on metal ion separation. The complexation of Hg(II), Cd(II) and Pb(II) were studied and comparisons could be made with previous ultrafiltration studies. 4 The complexation of C o 0 I ) and Ni(II) were also studied in order to make predictions as to the possible use of complexation-ultrafiltration as a method to separate these two metal ions. EXPERIMENTAL Reagents
Polymin* Water-Free, a polyethyleneimine product in which the ratio of primary, secondary and tertiary amine groups is 1:1:1, was obtained from BASF and used as received. The repeating unit is as follows: 1 - - C H 2 - - C H , - - N - - C H 2--CH 2 - - N H - -
I
CH~
I
CHe
L
NH_,
*Polymin is a registered trademark of the BASF company.
All other reagents used were of analytical grade. Where necessary, metal ion solutions were acidified to prevent hydrolysis and standardized by ICP. Standardized sodium hydroxide solutions were obtained by using Merck Titrisol ampoules.
219
220
N.V. JARVISand J. M. WAGENER
Potentiometry and computing Titrations were performed by a M e t r o h m Titroprocessor 670 using a M e t r o h m 665 dosimat and a M e t r o h m c o m b i n a t i o n glass electrode. The titration solutions were contained in a jacketed vessel through which water at 2 5 + 0 . 1 ° C was circulated from a G r a n t W14 thermostatted bath. The compositions o f the titration solutions are listed in Table I. These were performed beginning at low and ending at high p H by the addition o f 0.050M N a O H in 0.95M N a N O s. All titration solutions were held at a constant ionic strength o f 1.0M N a N O 3 and vigorously stirred. Nitrogen gas was bubbled t h r o u g h the solutions during the titrations. U n d e r these conditions, it appeared as though equilibrium was attained within a n u m b e r o f minutes (stable E M F readings were obtained). The Titroprocessor was p r o g r a m m e d not to add titrant until the electrode drift was less than 0.5 mV/min or a time o f 10 min had elapsed. Most data points were obtained using the former criterion. The remaining points were given low weights by the p r o g r a m E S T A 2 A . D a t a were submitted to E S T A which was loaded on a mainframe computer. The above repeating unit o f Polymin Water-Free was considered to be the ligand. The module E S T A 0 was used to calculate the experimental p r o t o n a t i o n formation function, ZH---the average n u m b e r o f protons b o u n d per ligand; and the d e p r o t o n a t i o n function, 0 = the average n u m b e r o f protons re-
leased on complexation per metal ion. Where these were insensible, the data points were rejected for refinement. All optimization was done using E S T A 2 A with the data weighted. 7 A p p a r ent pK~ values obtained in the titrations without metal ions were fixed during optimization o f metal ion titrations. Hydrolysis constants and pKw were obtained from Smith and Martell s and also held constant. The electrode constant (E0) was calculated regularly using a strong acid-base titration. Once a model had been obtained, E S T A 2 A was allowed to refine E0 resulting in small changes with improved standard deviations for fl values and the H a m i l t o n R-factor.
RESULTS AND DISCUSSION
Equilibrium modelling The results o f modelling are given in Table 2. The low standard deviations o f the fl values and H a m i l t o n R-factors indicate the applicability o f the method. Calculated and experimental formation and d e p r o t o n a t i o n curves were all in g o o d agreement indicating that the chosen models were plausible.
Protonation of Polymin Water-Free The p r o t o n a t i o n formation curves (Fig. 1) show no marked inflections indicating that the iigand's d e p r o t o n a t i o n reactions overlap in the p H region studied. F r o m the species
Table 1, Compositions of experimental solutions Metal ion 0.010M 0.050M HNO3 IM NaNO~ solution* Polymin 2M NaNO2 in 0.95M NaNO3 Cation H+
Pb(ll) Cd(ll) Hg(II) Co(ll) Ni(ll)
Titration 1 2 3 4 1 2 3 1 2 3 1 2 3 1
2 3 1
2 3
(rnl ) 35.0 33.0 30.0 40.0 30.0 28.0 25.0 30.0 28.0 25.0 36.0 34.0 31.0 18.0 15.0 10.0 18.0 15.0 10.0
(ml )
(ml )
5.0 5.0 5.0 5.0 5.0 5.0 2.0 2.0 2.0 5.0 5.0 5.0 5.0 5.0 5.0
5.0 7.0 10.0 5.0 5.0 7.0 10.0 5.0 7.0 I0.0 5.0 7.0 10.0 12.0 15.0 20.0 12.0 15.0 20.0
(ml )
(rnl )
5.0 5.0 5.0 5.0 5.0 5.0 2.0 2.0 2.0 5.0 5.0 5.0 5.0 5.0 5.0
10.0 10.0 10.0 5.0 5.0 5.0 5.0 5.0 5.0 5.0 5.0 5.0 5.0 10.0 I0.0 10.0 10.0 10.0 10.0
*Solutions made up as follows: 0.00941M Pb(NO 3)2 in 0.050M HNO~, 0.00955M Cd(NO 3),. 4H, O in 0.050M HNO~, 0.00911M Hg(NO02 in 0.050M HNO3, 0.00999M Co(NOO_,.6H_,O and 0.00980M Ni(NO3).,.6HzO.
221
Metal ion binding to water-soluble polymers Table 2. Apparent protonation and formation constants for Polymin Water-Free*
Cation H+
Pb(ll)
Cd(H)
Hg(II)
Co(ll)
Ni(lI)
Equilibriumf
logK
H + L = HL HL + H = H,_L H2 L + H = H 3L 2 L + 3H = H3L ., 2L + 5H = HsL 2
9.71 7.70 2.64 30.87 41.58
M + OH = MOH MOH + OH = M(OH)_, M(OH)2 + OH = M(OH)3 MOH + M = MzOH 3M + 4OH = M~(OH)4 M:~(OH)4 + M = M4(OH) 4 M + 2L = ML 2 ML2 + H = ML.,H 2M + 3L = M2L ~
6.4++ 4.5++ 3.0++ 1.2+ 32.1++ 3.9++ 12.53 + 0.02 8.64 __+0.02 23.06 +_ 0.06
M + OH = MOH MOH + OH = M(OH)., M(OH) 2 + OH = M(OH) 3 M(OH) 3 + OH = M(OH) 4 MOH + M = M2OH 4M + 4OH = M4(OH)4 M + L = ML ML + OH = MLOH M LOH + OH = ML(OH), ML + L = ML 2 ML~ + H = ML2H M L 2 + OH = ML2OH
3.4++ 3.6++. 2.6++ 1.7++ 1.0:~ 24.2++ 8.57 + 0.03 4.91 + 0.04 4.42 __+0.04 7.84 + 0.03 6.66 + 0.04 4.22 + 0.03
M + OH = MOH MOH + OH = M(OH)2 MOH + M = M2OH 3M + 3OH = M3(OH)3 M + L = ML ML + L = ML 2 ML, + H = ML2H ML: + OH = ML2OH
10.0++ I 1.1++ 14.6++ 35.4++ 16.06 + 0.03 13.46 + 0.04 4.35 __+0.04 2.64 + 0.05
MOH MOH + OH = M(OH) 2 M(OH) 2 + OH = M(OH) 3 M + 4OH = M(OH)4 2M + OH = M2OH 4M + 4OH = M4(OH)4 M + 2L = ML 2 M L + H = ML, H
4.0:~ 4.7++ 1.2++ 9.2++ 2.5++ 25.0++ 16.62 _+0.03 6.75 4- 0.03
M + OH = MOH M(OH) + OH = M(OH), M(OH) 2 + OH = M(OH) 3 2M + 20H = M2(OH)2 4M + 4OH = M4(OH)4 M + 2L = ML2 ML 2 + H = ML~H
3.7++ 5.0++ 3.0++ 4.7++ 27.9++ 19.00 _+ 0.02 6.04_+ 0.02
M + OH
=
*All data at 2 5 C in 1.0M N a N O 3. fln these equilibria, charges on metal for simplicity. L = Monomeric H = proton and OH = hydroxide ++Estimated from Ref. 8,
distribution
of Polymin
Water-Free
Number of data points
0.00850 (0.00116)
436
0.0256 (0.00184)
183
0.0230 (0.00163)
227
0.00370 (0.00176)
244
0.0256 (0.00347)
240
0.0130 (0.00281)
299
pKw was fixed at 13.79 under these conditions? ions, the ligand and complexes have been omitted unit of Polymin Water-Free, M = metal ion, ion.
p l o t ( F i g . 2), it is c l e a r t h a t c o m -
plete protonation
__+0.005 + 0.006 + 0.04 +0.01 + 0.02
Hamilton R-factor (R-limit)
does
possible nearest neighbour
i n t e r a c t i o n in w h i c h
intramolecular
bonding
hydrogen
is
taking
n o t o c c u r in t h i s p H r e g i o n . T h i s p h e n o m e n o n
p l a c e , is g i v e n b y t h e p r e s e n c e o f s p e c i e s i n v o l v -
has
ing
been
previously
reported. 9 Evidence
for
two
repeating
units.
Alternatively,
when
222
N . V . JARVIS and J. M. WAGENER 3.0
2.5
2.0
Jill
1.5 1.0
0.5
0.0
'
~
I
2
3
4
,
i
,
5
p
i
6
7
.
.
.
.
.
8
p
9
~
10
i
11
pH
Fig. 1. Experimental (points) and modelled (lines) protonation formation curves for Polymin Water-Free. ZH is the protonation formation function (the average number of protons bound per ligand) and pH is the negative logarithm of the free hydrogen ion concentration. The compositions of the solutions are in Table 1. The titrations are represented by ((3) titration 1, (A) titration 2 and (V-I) titration 3. All solutions were at 2 5 C and 1.0M with respect to NaNO 3.
going from low to high pH, the sequence of deprotonation gives some insight as to what is happening at the molecular level. Species involving two repeating units are found between those involving one. This means that, when going from H3L to H2L or from H2L to HL, every second repeating unit becomes deprotonated in preference to each unit giving rise to the intermediate species HsL 2 and H3L2, respectively. The species H6L2, H4L2 and H2L2, were also submitted to the model but were rejected with ESTA2A preferring to retain species with the same stoichiometric ratios but containing one repeating unit (i.e. H 3L, H2 L and HL). Evidence for the species HL2 could not be found.
Hg(II), Cd(II) and Pb(II) complexation by
Polymin Water-Free The deprotonation curves (Figs 3 5) show good agreement between observed and calculated values. (~ values for Hg(II) rise above zero, indicating complexation, at low pH. This is in contrast with Cd(II) and Pb(II) where (~ becomes sensible above pH 5 and 6, respectively. Thus it may be seen that Hg(II) is more strongly complexed by Polymin Water-Free. Interpretation of the (~ curves is diffficult--as is often the case. However, with the Hg(ll) system, some prediction of complexes for the model is possible. It can be seen (Fig. 5) that at
90 /
80 H5L2
70 A
o~
g
H2L \\
,
// ,
HL \
/
HaL2
/
L \
50
L1
\
40
\ /
O
/
/ \
60
/
30
/
\
,\H3L // \\
'\ \,
/
/
\\
'\,
20
/
//
/×\
10 0 2
3
4
5
6
7
8
9
10
pH
Fig. 2. Species distribution curves for the protonation of Polymin Water-Free at 25 C and 1.0M NaNO~, as calculated from the apparent formation constants in Table 2.
Metal ion binding to water-soluble polymers
223
2.2 2.0
,. ~;,.. ~-~ r, ~...,
.
1.8
-.
/"
: "
1.4
1., lq
Ui 1
,!i U m ' -
~{
- . C ~ ;~
~ ~,
<-#
-,~..
,4~
~L,/~/
~.o
'.
<,,, ~
I~i~,,,
0.8
v
0.6 0.4 0.2 0.0
~" .... " ~ " ~ ' : i ~ : 5 6
-" 7
'
'
8
9
10
11
1
pH Fig. 3. Experimental (points) and modelled (lines) deprotonatlon curves for Pb(II) complexation by
Polymin Water-Free. ~ is the deprotonation function (the average number of protons released on complexation per metal ion). The dashed line is the ri curve, where a is the protonation state of the ligand in the absence of the metal ion. The compositions of the solutions are in Table 1. The titrations are represented by (O) titration I, (A) titration 2 and (I-I) titration 3. All solutions were at 25 'C and 1.0M with respect to NaNO~.
pH 6, the dashed curve which represents ~q, the protonation state of the ligand in the absence of the metal ion, is at a value of 2. At this point, has an inflection at 4. Therefore, the existence of a complex in which a total of five protons have been lost from the ligands in the complex per metal ion is possible. This corresponds to an MLEH species which was indeed found during modelling. The MEL3 species in the Pb(II) system was essential for obtaining a good Q fit. The exist-
ence of this complex may be due to the large ionic radius of Pb(II) with a repeating unit preferring to bridge two Pb(II) ions. Species involving one repeating unit in the Pb(II) system were not accepted into the model by ESTA2A. A possible explanation for this may be advanced from comparing the pH values in the deprotonation curves where Q rises above zero. It may be seen that Pb(II) is not as well complexed by Polymin Water-Free as Cd(II) or Hg(II). Complexation begins for Pb(II) at pH 6
3.0 2.5
,C ?
.~E~ ~ ] ~
,k:
--
2.0 IO
l i, N
1.5
~d c;~
"
~=' r~'
1.0 0.5 0.0
~.t
3
~r~, ~
4
E3",~= 5
"
6
7
8
9
10
,
11
12
OH Fig. 4. Experimental (points) and modelled (lines) deprotonation curves for Cd(lI) complexation by Polymin Water-Free. The compositions of the solutions are in Table I. The titrations are represented by (O) titration l, (A) titration 2 and ( n ) titration 3. All solutions were at 25 C and 1.0M with respect to NaNO 3.
224
N.V. JARVIS and J. M. WAGENER
I0
I
2
3
4
5
6
7
10
8
i
---I
11
pH
Fig. 5. Experimental (points) and modelled (lines) deprotonation curves for Hg(lI) complexation by Polymin Water-Free. The compositions of the solutions are in Table 1. The titrations are represented by (O) titration 1, (A) titration 2 and (IS]) titration 3. All solutions were at 25°C and i.0M with respect to NaNO3.
Co(II) and Ni(II) complexation by Polymin Water -Free
where each repeating unit has already lost a proton hence making species involving two repeating units more possible. For Cd(II) and Hg(II) where complexation begins at pH 5 and below pH 2 respectively, species also involving only one repeating unit are possible. The three metal ions are complexed over a wide pH range. Cd(II) and Pb(II) would be best removed at high pH by ultrafiltration. The pH dependence of Hg(II) removal would not be so critical. These observations are in complete agreement with the complexation-ultrafiltration work performed by Buckley and co-workers. 4
Ni(II) is somewhat better complexed by Polymin Water-Free than Co(II). This may be seen by comparing the deprotonation curves (Figs 6 and 7). This is a source of optimism that separation between these two metal ions may be achieved using complexation-ultrafiltration by carefully fixing the pH of the feed stream. However, a more complete picture is obtained by comparing species distribution curves generated by task SPEC in ESTAI. The Co(II)
3.0 2.5 2.0 I0
1.5 1.0 0.5 /
0.0
t
5
6
7
8
9
10
11
pH
Fig. 6. Experimental (points) and modelled (lines) deprotonation curves for Co(li) complexation by Polymin Water-Free. The compositions of the solutions are in Table 1. The titrations are represented by (O) titration 1, (A) titration 2 and (I-1) titration 3. All solutions were at 25 C and 1,0M with respect to NaNO3.
Metal ion binding to water-soluble polymers
IO
2
,'-)J:~
225
"A
\ 4
5
6
I
i
7
8
,
9
X 10
11
12
pH
Fig. 7. Experimental (points) and modelled (lines) deprotonation curves for Ni(ll) complexation by Polymin Water-Free. The compositions of the solutions are in Table I. The titrations are represented by (O) titration 1, (A) titration 2 and (IS]) titration 3. All solutions were at 25 C and 1.0M with respect to NaNO~.
system is shown as an example in Fig. 8. If only the curves representing the free metal ions are shown, then plots such as Fig. 9 may be obtained. Here it may be seen that these are close together, making separation difficult. It is, however, hoped that such plots may be used to predict the separation of other metal ions from one another using complexation-ultrafiltration. For both Ni(II) and Co(lI), the ML2 and ML2H species retained by ESTA2A during modelling are consistent with the deprotonation curve information. It may be seen that reaches an inflection at roughly a value of three
indicating that three protons have been lost on complexation per metal ion. At pH values around 7-8, ~ is close to 1.5. Thus for two repeating units, three protons are lost in the absence of the metal ions. The total number of protons lost for an ML2 species is six which is consistent with the addition of (~ plus 2 x (total protons on completely protonated repeating unit, ti) which gives a value of six. At lower pH values, where ti equals two, the same argument may be used to arrive at the M L2H species. This confirms the best-fitting model reported in Table 2. Species involving one repeating
1 00 90
ML2 ML2H
80 70 60 50 40 o
30 20 10 0 4
7
8
9
10
pH
Fig. 8. Species distribution curves for the complexation of Co(ll) at 25 C and 1.0M NaNO~ (titration 3), as calculated from the apparent formation constants in Table 2. T A L 42 2
F
N . V . . l a r v l s and J. M. waGeNer
226
1 00 9O 80 .~_
70
~, "~, E
60
× (D el.
40
E
30
~
2o
o
Co(H) Ni(ll)
50
10 0
5
6 pH
Fig. 9. Species distribution curves for the complexation of Co(ll) and Ni(ll) at 25 C and 1.0M NaNO~ (titration 3), as calculated from the apparent formation constants in Table 2. Only the uncomplexed metal ion plots are shown. 40
"~
30 • Hg(ll)
IJ_
._= E
20 *Ni(ll) Cd(ll), eCo(ll)
G_
oc~
Pb(ll)
10
5
10
15
20
25
3"0
Iog82(ethylenediamine) Fig. I0. Plot of log//,(Polymin Water-Free) rs. Iog/] z (ethylenediamine) for the metal ions studied in this paper.
unit were not accepted by ESTA2A into the model.
Linear.D'ee energy correlations A plot of Iog/~_. (Polymin Water-Free) obtained in this study t's. log#_~ (ethylenediamine) '~ shows a linear correlation (Fig. 10) for the metal ions studied. Such a correlation may be expected. The fact that this was indeed obtained is gratifying and lends credibility to the method of treating the polymer as a collection of monomeric units for modelling purposes. .4ckmm'/ed, k,eme,t.s The authors v, ish to lhallk the Atomic Energy Corporation for permission to publish this v.ork and A n n a h Rangoaga for performing some of thc polentiomctric titrations.
REFERENCES
I. K. Geckeler, G. Lange, H. Eberhardt and E. Bayer. Pm'e Appl. C/tent.. 198[), 52, [883. 2. M. Rumeau, F. Persin, V. Sciers, M. Persin and J. Sarrazin, J. Memhr. Sci., [992, 73, 313. 3. A. P. Novikov. V. M. Shkincv. B. Ya. Spivakov, B. F. Myasoedov. K. E. Geckcler and E. Bayer. Radiochim. ,4eta, 1989. 46, 35. 4. L. P. Buckley. V. T. Le. G. J. McConeghy and J. F. Martill. ,{;eh'ctil'e Remora/ ~[ Di.ssoh'ed Toxic .14etals /i'OIll Gl'Olllldwtllcq" /).l Ullrt4/lllrdtion ill Comhhldlion with Chemh'a/ 7)'eatme,I. AECL-[0030, 1989. 5. N. V. Jarvis and J. M. Wagener, T~thl,ta, 1994 (in press). 6. P. M. May, K. Murray and D. R. Williams. Tahmta, 1985. 32, 483. 7. P. M. May and K. Murray. Tuhmtu, 1988. 35, 927. 8. A. E. Marlell and R. M. Smith. ('riticul Stahililr ('o,~ta,t.~', Vols I 6. Plenum, Ne~' York, 1974 1988. 9. O . H o r u , in Po/vmel'ic .~nl/ll('3 and.*~llllllOllilllll StilLs', E. J. Goethals (ed.), p. 335. Pergamon Press, London. 1980.
0039-9140(94)00231-2
Tahmta, Vol. 42, No. 2, pp. 219-.226, 1995 Copyright ~ 1995 Elsevier Science Ltd Printed in Great Britain. All rights reserved 0039-9140/95 $9.50 + 0.00
M E C H A N I S T I C STUDIES OF M E T A L ION B I N D I N G TO WATER-SOLUBLE POLYMERS USING POTENTIOMETRY NElL V. JARVIS and JUDITH M. WAGENER Department of Process Technology, Atomic Energy Corporation of South Africa Ltd, P.O. Box 582, Pretoria, 0001, South Africa
(Received 15 March 1994. Revised 20 June 1994. Accepted 7 July 1994)
Summary--A method for elucidating metal ion binding mechanismswith water-solublepolymers has been developed in which the polymer is treated as a collection of monomeric units. Data obtained from potentiometric titrations are analysed by the ESTA library of programs and apparent formation constants may be calculated. From this information, predictions may be made as to metal ion separation using complexation-ultrafiltration techniques. The polymer used in this study was Polymin Water-Free and its complexation with Hg(ll), Cd(ll), Pb(ll), Co(lI) and Ni(II) was successfully modelled.
Interest in the use of water-soluble polymers in conjuction with ultrafiltration membranes to separate metal ions from aqueous solutions has steadily grown since the work of Geckeler et al. was published in the early eighties, j The possibility of synthesizing derivatives of commercially available water-soluble polymers such as polyethyleneimine, in order to achieve selective metal ion complexation, was recognized early on. A wide range of applications have been investigated 2 including the nuclear industry 3 and the removal of toxic heavy metal ions such as Pb(II), Cd(II) and Hg(II). 4 As the field ofligand design produces more complicated and selective complexing agents, it has become an attractive option to attach these reagents to polymer backbones in order to minimize losses of expensive compounds. At this time, little effort has gone into the understanding of metal ion complexation mechanisms of water-soluble polymers in aqueous solution. Recent work by Rumeau et al. 2 used potentiometry to calculate stability constants of these polymers with metal ions. We have developed a method to elucidate metal ion bonding mechanisms to chelating resins using two-phase potentiometry. 5 In this paper, we extend this method to water-soluble polymers (although the term "two-phase" may not be entirely applicable here). The repeating unit of the polymer is considered to be the ligand and potentiometric
data are submitted to the ESTA library of programs. 6 Apparent formation constants may be accurately calculated and this allows predictions to be made on metal ion separation. The complexation of Hg(II), Cd(II) and Pb(II) were studied and comparisons could be made with previous ultrafiltration studies. 4 The complexation of C o 0 I ) and Ni(II) were also studied in order to make predictions as to the possible use of complexation-ultrafiltration as a method to separate these two metal ions. EXPERIMENTAL Reagents
Polymin* Water-Free, a polyethyleneimine product in which the ratio of primary, secondary and tertiary amine groups is 1:1:1, was obtained from BASF and used as received. The repeating unit is as follows: 1 - - C H 2 - - C H , - - N - - C H 2--CH 2 - - N H - -
I
CH~
I
CHe
L
NH_,
*Polymin is a registered trademark of the BASF company.
All other reagents used were of analytical grade. Where necessary, metal ion solutions were acidified to prevent hydrolysis and standardized by ICP. Standardized sodium hydroxide solutions were obtained by using Merck Titrisol ampoules.
219
220
N.V. JARVISand J. M. WAGENER
Potentiometry and computing Titrations were performed by a M e t r o h m Titroprocessor 670 using a M e t r o h m 665 dosimat and a M e t r o h m c o m b i n a t i o n glass electrode. The titration solutions were contained in a jacketed vessel through which water at 2 5 + 0 . 1 ° C was circulated from a G r a n t W14 thermostatted bath. The compositions o f the titration solutions are listed in Table I. These were performed beginning at low and ending at high p H by the addition o f 0.050M N a O H in 0.95M N a N O s. All titration solutions were held at a constant ionic strength o f 1.0M N a N O 3 and vigorously stirred. Nitrogen gas was bubbled t h r o u g h the solutions during the titrations. U n d e r these conditions, it appeared as though equilibrium was attained within a n u m b e r o f minutes (stable E M F readings were obtained). The Titroprocessor was p r o g r a m m e d not to add titrant until the electrode drift was less than 0.5 mV/min or a time o f 10 min had elapsed. Most data points were obtained using the former criterion. The remaining points were given low weights by the p r o g r a m E S T A 2 A . D a t a were submitted to E S T A which was loaded on a mainframe computer. The above repeating unit o f Polymin Water-Free was considered to be the ligand. The module E S T A 0 was used to calculate the experimental p r o t o n a t i o n formation function, ZH---the average n u m b e r o f protons b o u n d per ligand; and the d e p r o t o n a t i o n function, 0 = the average n u m b e r o f protons re-
leased on complexation per metal ion. Where these were insensible, the data points were rejected for refinement. All optimization was done using E S T A 2 A with the data weighted. 7 A p p a r ent pK~ values obtained in the titrations without metal ions were fixed during optimization o f metal ion titrations. Hydrolysis constants and pKw were obtained from Smith and Martell s and also held constant. The electrode constant (E0) was calculated regularly using a strong acid-base titration. Once a model had been obtained, E S T A 2 A was allowed to refine E0 resulting in small changes with improved standard deviations for fl values and the H a m i l t o n R-factor.
RESULTS AND DISCUSSION
Equilibrium modelling The results o f modelling are given in Table 2. The low standard deviations o f the fl values and H a m i l t o n R-factors indicate the applicability o f the method. Calculated and experimental formation and d e p r o t o n a t i o n curves were all in g o o d agreement indicating that the chosen models were plausible.
Protonation of Polymin Water-Free The p r o t o n a t i o n formation curves (Fig. 1) show no marked inflections indicating that the iigand's d e p r o t o n a t i o n reactions overlap in the p H region studied. F r o m the species
Table 1, Compositions of experimental solutions Metal ion 0.010M 0.050M HNO3 IM NaNO~ solution* Polymin 2M NaNO2 in 0.95M NaNO3 Cation H+
Pb(ll) Cd(ll) Hg(II) Co(ll) Ni(ll)
Titration 1 2 3 4 1 2 3 1 2 3 1 2 3 1
2 3 1
2 3
(rnl ) 35.0 33.0 30.0 40.0 30.0 28.0 25.0 30.0 28.0 25.0 36.0 34.0 31.0 18.0 15.0 10.0 18.0 15.0 10.0
(ml )
(ml )
5.0 5.0 5.0 5.0 5.0 5.0 2.0 2.0 2.0 5.0 5.0 5.0 5.0 5.0 5.0
5.0 7.0 10.0 5.0 5.0 7.0 10.0 5.0 7.0 I0.0 5.0 7.0 10.0 12.0 15.0 20.0 12.0 15.0 20.0
(ml )
(rnl )
5.0 5.0 5.0 5.0 5.0 5.0 2.0 2.0 2.0 5.0 5.0 5.0 5.0 5.0 5.0
10.0 10.0 10.0 5.0 5.0 5.0 5.0 5.0 5.0 5.0 5.0 5.0 5.0 10.0 I0.0 10.0 10.0 10.0 10.0
*Solutions made up as follows: 0.00941M Pb(NO 3)2 in 0.050M HNO~, 0.00955M Cd(NO 3),. 4H, O in 0.050M HNO~, 0.00911M Hg(NO02 in 0.050M HNO3, 0.00999M Co(NOO_,.6H_,O and 0.00980M Ni(NO3).,.6HzO.
221
Metal ion binding to water-soluble polymers Table 2. Apparent protonation and formation constants for Polymin Water-Free*
Cation H+
Pb(ll)
Cd(H)
Hg(II)
Co(ll)
Ni(lI)
Equilibriumf
logK
H + L = HL HL + H = H,_L H2 L + H = H 3L 2 L + 3H = H3L ., 2L + 5H = HsL 2
9.71 7.70 2.64 30.87 41.58
M + OH = MOH MOH + OH = M(OH)_, M(OH)2 + OH = M(OH)3 MOH + M = MzOH 3M + 4OH = M~(OH)4 M:~(OH)4 + M = M4(OH) 4 M + 2L = ML 2 ML2 + H = ML.,H 2M + 3L = M2L ~
6.4++ 4.5++ 3.0++ 1.2+ 32.1++ 3.9++ 12.53 + 0.02 8.64 __+0.02 23.06 +_ 0.06
M + OH = MOH MOH + OH = M(OH)., M(OH) 2 + OH = M(OH) 3 M(OH) 3 + OH = M(OH) 4 MOH + M = M2OH 4M + 4OH = M4(OH)4 M + L = ML ML + OH = MLOH M LOH + OH = ML(OH), ML + L = ML 2 ML~ + H = ML2H M L 2 + OH = ML2OH
3.4++ 3.6++. 2.6++ 1.7++ 1.0:~ 24.2++ 8.57 + 0.03 4.91 + 0.04 4.42 __+0.04 7.84 + 0.03 6.66 + 0.04 4.22 + 0.03
M + OH = MOH MOH + OH = M(OH)2 MOH + M = M2OH 3M + 3OH = M3(OH)3 M + L = ML ML + L = ML 2 ML, + H = ML2H ML: + OH = ML2OH
10.0++ I 1.1++ 14.6++ 35.4++ 16.06 + 0.03 13.46 + 0.04 4.35 __+0.04 2.64 + 0.05
MOH MOH + OH = M(OH) 2 M(OH) 2 + OH = M(OH) 3 M + 4OH = M(OH)4 2M + OH = M2OH 4M + 4OH = M4(OH)4 M + 2L = ML 2 M L + H = ML, H
4.0:~ 4.7++ 1.2++ 9.2++ 2.5++ 25.0++ 16.62 _+0.03 6.75 4- 0.03
M + OH = MOH M(OH) + OH = M(OH), M(OH) 2 + OH = M(OH) 3 2M + 20H = M2(OH)2 4M + 4OH = M4(OH)4 M + 2L = ML2 ML 2 + H = ML~H
3.7++ 5.0++ 3.0++ 4.7++ 27.9++ 19.00 _+ 0.02 6.04_+ 0.02
M + OH
=
*All data at 2 5 C in 1.0M N a N O 3. fln these equilibria, charges on metal for simplicity. L = Monomeric H = proton and OH = hydroxide ++Estimated from Ref. 8,
distribution
of Polymin
Water-Free
Number of data points
0.00850 (0.00116)
436
0.0256 (0.00184)
183
0.0230 (0.00163)
227
0.00370 (0.00176)
244
0.0256 (0.00347)
240
0.0130 (0.00281)
299
pKw was fixed at 13.79 under these conditions? ions, the ligand and complexes have been omitted unit of Polymin Water-Free, M = metal ion, ion.
p l o t ( F i g . 2), it is c l e a r t h a t c o m -
plete protonation
__+0.005 + 0.006 + 0.04 +0.01 + 0.02
Hamilton R-factor (R-limit)
does
possible nearest neighbour
i n t e r a c t i o n in w h i c h
intramolecular
bonding
hydrogen
is
taking
n o t o c c u r in t h i s p H r e g i o n . T h i s p h e n o m e n o n
p l a c e , is g i v e n b y t h e p r e s e n c e o f s p e c i e s i n v o l v -
has
ing
been
previously
reported. 9 Evidence
for
two
repeating
units.
Alternatively,
when
222
N . V . JARVIS and J. M. WAGENER 3.0
2.5
2.0
Jill
1.5 1.0
0.5
0.0
'
~
I
2
3
4
,
i
,
5
p
i
6
7
.
.
.
.
.
8
p
9
~
10
i
11
pH
Fig. 1. Experimental (points) and modelled (lines) protonation formation curves for Polymin Water-Free. ZH is the protonation formation function (the average number of protons bound per ligand) and pH is the negative logarithm of the free hydrogen ion concentration. The compositions of the solutions are in Table 1. The titrations are represented by ((3) titration 1, (A) titration 2 and (V-I) titration 3. All solutions were at 2 5 C and 1.0M with respect to NaNO 3.
going from low to high pH, the sequence of deprotonation gives some insight as to what is happening at the molecular level. Species involving two repeating units are found between those involving one. This means that, when going from H3L to H2L or from H2L to HL, every second repeating unit becomes deprotonated in preference to each unit giving rise to the intermediate species HsL 2 and H3L2, respectively. The species H6L2, H4L2 and H2L2, were also submitted to the model but were rejected with ESTA2A preferring to retain species with the same stoichiometric ratios but containing one repeating unit (i.e. H 3L, H2 L and HL). Evidence for the species HL2 could not be found.
Hg(II), Cd(II) and Pb(II) complexation by
Polymin Water-Free The deprotonation curves (Figs 3 5) show good agreement between observed and calculated values. (~ values for Hg(II) rise above zero, indicating complexation, at low pH. This is in contrast with Cd(II) and Pb(II) where (~ becomes sensible above pH 5 and 6, respectively. Thus it may be seen that Hg(II) is more strongly complexed by Polymin Water-Free. Interpretation of the (~ curves is diffficult--as is often the case. However, with the Hg(ll) system, some prediction of complexes for the model is possible. It can be seen (Fig. 5) that at
90 /
80 H5L2
70 A
o~
g
H2L \\
,
// ,
HL \
/
HaL2
/
L \
50
L1
\
40
\ /
O
/
/ \
60
/
30
/
\
,\H3L // \\
'\ \,
/
/
\\
'\,
20
/
//
/×\
10 0 2
3
4
5
6
7
8
9
10
pH
Fig. 2. Species distribution curves for the protonation of Polymin Water-Free at 25 C and 1.0M NaNO~, as calculated from the apparent formation constants in Table 2.
Metal ion binding to water-soluble polymers
223
2.2 2.0
,. ~;,.. ~-~ r, ~...,
.
1.8
-.
/"
: "
1.4
1., lq
Ui 1
,!i U m ' -
~{
- . C ~ ;~
~ ~,
<-#
-,~..
,4~
~L,/~/
~.o
'.
<,,, ~
I~i~,,,
0.8
v
0.6 0.4 0.2 0.0
~" .... " ~ " ~ ' : i ~ : 5 6
-" 7
'
'
8
9
10
11
1
pH Fig. 3. Experimental (points) and modelled (lines) deprotonatlon curves for Pb(II) complexation by
Polymin Water-Free. ~ is the deprotonation function (the average number of protons released on complexation per metal ion). The dashed line is the ri curve, where a is the protonation state of the ligand in the absence of the metal ion. The compositions of the solutions are in Table 1. The titrations are represented by (O) titration I, (A) titration 2 and (I-I) titration 3. All solutions were at 25 'C and 1.0M with respect to NaNO~.
pH 6, the dashed curve which represents ~q, the protonation state of the ligand in the absence of the metal ion, is at a value of 2. At this point, has an inflection at 4. Therefore, the existence of a complex in which a total of five protons have been lost from the ligands in the complex per metal ion is possible. This corresponds to an MLEH species which was indeed found during modelling. The MEL3 species in the Pb(II) system was essential for obtaining a good Q fit. The exist-
ence of this complex may be due to the large ionic radius of Pb(II) with a repeating unit preferring to bridge two Pb(II) ions. Species involving one repeating unit in the Pb(II) system were not accepted into the model by ESTA2A. A possible explanation for this may be advanced from comparing the pH values in the deprotonation curves where Q rises above zero. It may be seen that Pb(II) is not as well complexed by Polymin Water-Free as Cd(II) or Hg(II). Complexation begins for Pb(II) at pH 6
3.0 2.5
,C ?
.~E~ ~ ] ~
,k:
--
2.0 IO
l i, N
1.5
~d c;~
"
~=' r~'
1.0 0.5 0.0
~.t
3
~r~, ~
4
E3",~= 5
"
6
7
8
9
10
,
11
12
OH Fig. 4. Experimental (points) and modelled (lines) deprotonation curves for Cd(lI) complexation by Polymin Water-Free. The compositions of the solutions are in Table I. The titrations are represented by (O) titration l, (A) titration 2 and ( n ) titration 3. All solutions were at 25 C and 1.0M with respect to NaNO 3.
224
N.V. JARVIS and J. M. WAGENER
I0
I
2
3
4
5
6
7
10
8
i
---I
11
pH
Fig. 5. Experimental (points) and modelled (lines) deprotonation curves for Hg(lI) complexation by Polymin Water-Free. The compositions of the solutions are in Table 1. The titrations are represented by (O) titration 1, (A) titration 2 and (IS]) titration 3. All solutions were at 25°C and i.0M with respect to NaNO3.
Co(II) and Ni(II) complexation by Polymin Water -Free
where each repeating unit has already lost a proton hence making species involving two repeating units more possible. For Cd(II) and Hg(II) where complexation begins at pH 5 and below pH 2 respectively, species also involving only one repeating unit are possible. The three metal ions are complexed over a wide pH range. Cd(II) and Pb(II) would be best removed at high pH by ultrafiltration. The pH dependence of Hg(II) removal would not be so critical. These observations are in complete agreement with the complexation-ultrafiltration work performed by Buckley and co-workers. 4
Ni(II) is somewhat better complexed by Polymin Water-Free than Co(II). This may be seen by comparing the deprotonation curves (Figs 6 and 7). This is a source of optimism that separation between these two metal ions may be achieved using complexation-ultrafiltration by carefully fixing the pH of the feed stream. However, a more complete picture is obtained by comparing species distribution curves generated by task SPEC in ESTAI. The Co(II)
3.0 2.5 2.0 I0
1.5 1.0 0.5 /
0.0
t
5
6
7
8
9
10
11
pH
Fig. 6. Experimental (points) and modelled (lines) deprotonation curves for Co(li) complexation by Polymin Water-Free. The compositions of the solutions are in Table 1. The titrations are represented by (O) titration 1, (A) titration 2 and (I-1) titration 3. All solutions were at 25 C and 1,0M with respect to NaNO3.
Metal ion binding to water-soluble polymers
IO
2
,'-)J:~
225
"A
\ 4
5
6
I
i
7
8
,
9
X 10
11
12
pH
Fig. 7. Experimental (points) and modelled (lines) deprotonation curves for Ni(ll) complexation by Polymin Water-Free. The compositions of the solutions are in Table I. The titrations are represented by (O) titration 1, (A) titration 2 and (IS]) titration 3. All solutions were at 25 C and 1.0M with respect to NaNO~.
system is shown as an example in Fig. 8. If only the curves representing the free metal ions are shown, then plots such as Fig. 9 may be obtained. Here it may be seen that these are close together, making separation difficult. It is, however, hoped that such plots may be used to predict the separation of other metal ions from one another using complexation-ultrafiltration. For both Ni(II) and Co(lI), the ML2 and ML2H species retained by ESTA2A during modelling are consistent with the deprotonation curve information. It may be seen that reaches an inflection at roughly a value of three
indicating that three protons have been lost on complexation per metal ion. At pH values around 7-8, ~ is close to 1.5. Thus for two repeating units, three protons are lost in the absence of the metal ions. The total number of protons lost for an ML2 species is six which is consistent with the addition of (~ plus 2 x (total protons on completely protonated repeating unit, ti) which gives a value of six. At lower pH values, where ti equals two, the same argument may be used to arrive at the M L2H species. This confirms the best-fitting model reported in Table 2. Species involving one repeating
1 00 90
ML2 ML2H
80 70 60 50 40 o
30 20 10 0 4
7
8
9
10
pH
Fig. 8. Species distribution curves for the complexation of Co(ll) at 25 C and 1.0M NaNO~ (titration 3), as calculated from the apparent formation constants in Table 2. T A L 42 2
F
N . V . . l a r v l s and J. M. waGeNer
226
1 00 9O 80 .~_
70
~, "~, E
60
× (D el.
40
E
30
~
2o
o
Co(H) Ni(ll)
50
10 0
5
6 pH
Fig. 9. Species distribution curves for the complexation of Co(ll) and Ni(ll) at 25 C and 1.0M NaNO~ (titration 3), as calculated from the apparent formation constants in Table 2. Only the uncomplexed metal ion plots are shown. 40
"~
30 • Hg(ll)
IJ_
._= E
20 *Ni(ll) Cd(ll), eCo(ll)
G_
oc~
Pb(ll)
10
5
10
15
20
25
3"0
Iog82(ethylenediamine) Fig. I0. Plot of log//,(Polymin Water-Free) rs. Iog/] z (ethylenediamine) for the metal ions studied in this paper.
unit were not accepted by ESTA2A into the model.
Linear.D'ee energy correlations A plot of Iog/~_. (Polymin Water-Free) obtained in this study t's. log#_~ (ethylenediamine) '~ shows a linear correlation (Fig. 10) for the metal ions studied. Such a correlation may be expected. The fact that this was indeed obtained is gratifying and lends credibility to the method of treating the polymer as a collection of monomeric units for modelling purposes. .4ckmm'/ed, k,eme,t.s The authors v, ish to lhallk the Atomic Energy Corporation for permission to publish this v.ork and A n n a h Rangoaga for performing some of thc polentiomctric titrations.
REFERENCES
I. K. Geckeler, G. Lange, H. Eberhardt and E. Bayer. Pm'e Appl. C/tent.. 198[), 52, [883. 2. M. Rumeau, F. Persin, V. Sciers, M. Persin and J. Sarrazin, J. Memhr. Sci., [992, 73, 313. 3. A. P. Novikov. V. M. Shkincv. B. Ya. Spivakov, B. F. Myasoedov. K. E. Geckcler and E. Bayer. Radiochim. ,4eta, 1989. 46, 35. 4. L. P. Buckley. V. T. Le. G. J. McConeghy and J. F. Martill. ,{;eh'ctil'e Remora/ ~[ Di.ssoh'ed Toxic .14etals /i'OIll Gl'Olllldwtllcq" /).l Ullrt4/lllrdtion ill Comhhldlion with Chemh'a/ 7)'eatme,I. AECL-[0030, 1989. 5. N. V. Jarvis and J. M. Wagener, T~thl,ta, 1994 (in press). 6. P. M. May, K. Murray and D. R. Williams. Tahmta, 1985. 32, 483. 7. P. M. May and K. Murray. Tuhmtu, 1988. 35, 927. 8. A. E. Marlell and R. M. Smith. ('riticul Stahililr ('o,~ta,t.~', Vols I 6. Plenum, Ne~' York, 1974 1988. 9. O . H o r u , in Po/vmel'ic .~nl/ll('3 and.*~llllllOllilllll StilLs', E. J. Goethals (ed.), p. 335. Pergamon Press, London. 1980.