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Thiourea-based coordinating polymers: synthesis and binding to noble metals
Reactive polymers ELSEVIER
Reactive Polymers 24 (1995) 165-181
Thiourea-based coordinating polymers: synthesis and binding to noble metals Guangju Zuo, Mamoun Muhammed * Department of lnorganic Chemistry, The Royal Institute of Technology, S-100 44, Stockholm, Sweden Received 14 December 1993; accepted in revised form 5 October 1994
Abstract
The synthesis of several coordinating polymers by grafting thiourea functional groups onto commercial macro-
porous polystyrene polymer matrices are described. In this paper, we also report the study on the absorption and elution of precious metal ions (or together with base metal ions, Cu2+ and Fe3+) from acidic chloride solutions. The results showed that these resins have appreciable capacity and good selectivity for the absorption of gold from acidic solutions.The extraction kinetics of the resins is slower than the corresponding monomolecular 'free' extractant analogues. However, some of the resins show acceptable extraction rates. Keywords: Thiourea; Resin; Coordinating polymer; Au; Ag
I. Introduction
Coordinating resins are polymers with covalently bound functional groups containing one or more donor atoms that are capable of forming complexes directly with metal ions or with their complexes. These polymers can also be made very selective to be used for a specific separation of one or more metal ions from solutions with different chemical environment [14]. As selective ion exchangers, the resins may achieve highly selective separation while maintaining appreciable capacity. Selective ion exchange resins can be very useful in treating a variety of solutions for the separation or preconcentration of certain ions. They therefore have great potential for application in environment engineering and hydrometallurgy, especially for * Corresponding author.
the recovery of precious and platinum group metals [5-6]. Precious metals (PM) usually exist together with a much large excess of 'base metals', such as copper, iron, etc. With the exhaustion of highquality PM-containing ores, there are increasing demands for highly selective separation techniques and reagents in order to separate and recover PM from low quality ores and secondary sources. The use of chlorine/chloride solutions as leaching agents has found several industrial applications [6]. Thus there is an increasing interest in preconcentration, recovery and separation of precious metals and platinum group metals (PGM) from hydrochloric acid media. Precious metals, in their normal oxidation states, tend to form their most stable complexes with ligands containing 'soft' donor atoms. Thus, coordinating resins containing donor N- and especially S-atoms in their functional groups are
0923-1137/95/$09.50 © 1995 Elsevier Science B.V. All rights reserved. SSDI 0 9 2 3 - 1 1 3 7 ( 9 4 ) 0 0 0 8 2 - 4
166
G. Zuo, M. Muhammed / Reactive Polymers 24 (1995) 165-181
promising polymers. During the past few years, a number of coordinating and chelating resins have been synthesized and tested for the separation and recovery of precious metals and platinum group metals. Grote and Kettrup [7] have synthesized a number of resins based on Sbonded tetrazolium (P-TD) and S-bonded dithizone (P-D) grafted on polystyrene matrix. P-TD provides fast kinetics for adsorption of noble metals through anion-exchange reactions, while most of the metals were complexed on P-D with tl/2 in the range of 0.5-4 h. Hudson and Thorns [8] have synthesized a polymer containing NCSSH-groups for the extraction of rhodium and iridium. The polymer is uncrosslinked and therefore offers high metal uptake rates. However, the polymer is unstable in sulphuric acid solution, but stable in hydrochloric acid solution. The recovery of the extracted metal is done after the chemical decomposition of the polymer. Myasoedova et al. [9] reported a new chelate forming 'Polyorgs' -resins made of different polymeric matrices and heterocyclic amine and amidoxime groups. The resins were mainly used for the preconcentration and analytical determination of PM in different samples. Siddhanta and Das [10] synthesized a chelating resin containing thiosemicarbazide. The sorption capacity for Pt(IV), Pd(II), Ru(III) and Rh(III) by these resins is at maximum in 1-2 M HC1, while Au(III) and Os(VI) are instantaneously reduced to the metal form when loaded on the resin. Warshawsky et al. [11] reported the synthesis of styrene divinylbenzene copolymer incorporating isothiourea as a functional group. The experimental data showed that the ion pair formation by anion exchange mechanism with the protonated form of the functional group is predominant in high acidity of HCI, and the ion pair binding followed by a slow conversion to coordination type complex via S- or N-atom or both whenever the metal-loaded resin is subjected to low acidity conditions. This resin has been tested in several hydrometallurgical processes [12]. In a previous study, we reported the synthesis and characterization of a family of thiourea-based
reagents. These reagents have proved to be of high selectivity and fast kinetics for the extraction of gold and silver ions through complex formation from HC1 solutions [13]. This earlier investigation led us to prepare resins with a suitable selective behaviour by incorporating thiourea into macroporous polystyrene polymers.
2. Experimental Commercial macroporous polystyrene-based polymers Bonopore 1, Amberlite 2 XAD-2, Amberlite XAD-4, and weak base resin Amberlite IR-45 were selected as the polymeric matrices. All starting polymers (except for Amberlite IR-45) were carefully washed as follows: 1 M NaOH and 1 M HC1 alternatively at 60°C, H20 (25"C), DMF (40°C), 1 M n C l (60°C), H20 (60°C), CH3OH (20°C), 3:2 (v/v) CHaOH: CH2C12, 1 : 3 (v/v) CH3OH: CH2C12, 1 : 9 (v/v) CH3OH: CH2C12 and pure CH2C12. For IR-45, the resin was stirred in 2 M ammonia solution for an hour at first and then washed with H20 (600C), CHaOH (20°C), 3:2 (v/v) CH3OH: CH2CI2, 1 : 3 (v/v) CHaOH: CH2C12, 1 : 9 (v/v) CHaOH: CH2C12 and pure CH2C12 sequentially. The samples were then dried at 600C under vacuum to a constant weight. All chemicals used in this work were of analytical grade. Gold stock solution was prepared by dissolving a known amount of pure gold metal in aqua regia, boiling to dryness and then dissolving the residue in 20 ml 37% HCI solution, and diluting to 1000 ml with deionized water. The stock solution of silver was prepared by dissolving 0.787 g AgNO3 in 1000 ml 1% HNO3. The analysis of different metal ions in aqueous phase was done by Inductively Coupled Plasma Atomic Emission Spectrometry technique (ARL3520 ICP-AES instrument). The instrument was calibrated using standard solutions of different metal ions before the measurement. The solutions were diluted to a proper concentration 1 Bonopore is a trademark of Nobel Industries, Sweden. 2 Amberlite is a trademark of Rohm & Haas Inc., U.S.A.
G. Zuo, M. Muhammed / Reactive Po~mers 24 (1995) 165-181
range. The wavelengths used for the quantitative determination were: Au, 242.793 nm; Ag, 328.071 nm; Cu, 324.758 nm; Fe, 238.200 nm. For batch tests, a known amount of resin beads was stirred with a solution containing single or several metal ions at a known HC1 concentration for a certain period at room temperature. The amount of metal ions adsorbed on the resin was calculated by mass balance from the analysis of the aqueous phase. For column operations, small-scale columns with 0.68 cm inner diameter have been employed. A known amount of resin beads (about 4 ml) was charged into the column, sufficient to obtain a height more than 100 mm. For PTU-1 resin, because most of the beads could float, a small ball of glass-wool was placed on the top of the resin bed. A peristaltic pump (Gilson, minipuls 2) was used to pump the feed solution or the elution solution downflow through the column. The effluent was periodically collected with a fraction sample collector and analyzed with ICP to obtain the loading and elution profiles. Both loading and elution flow rates were adjusted to about 2 ml/min.
167
of the analogous reagents (see Ref. 13). However, the reaction conditions are very different for these heterogeneous reactions and had to be developed in the present study. The reaction steps include chloromethylation, followed (when necessary) by the addition of the spacer, amination, and finally conversion to the thiourea derivative. For the chloromethylation, the traditional reagent, chloromethylmethylether (CMME), bis-chloromethylether (BCME) and their bromo analogues, known to be notorious carcinogens, were not employed. Instead, we used chloromethyloctylether, a safer chloromethylation reagent [14] that was synthesized by ourselves. The procedure can be presented by the following reactions. For the preparation of PTU-1, PTU-2 and BTUO-1 reaction 3 was not adopted. While for the preparation of ITU, after washing the IR-45 beads with 1 M NaOH and deionized water sequentially, only the last step was implemented. (1) Chloromethylation of polymer [14]: II~--H + C H 3 - - ( C H 2 ) 7 - - O - - C H 2 C I - - CH2 C1 +
CH3-- (CH2)7--O H
'
,~
(1)
3. Results
3.1. Synthesis of the coordinating polymers The synthesized coordinating resins and their relevant starting polymers are listed in Table 1. The procedure for the preparation of coordinating polymers is quite similar to the synthesis
(2) Amination of the chloromethylated polymer [15]: ~--CH2C1 (HMT~) ~ - - C H 2 - - N H 2
where HMTA denotes hexamethylenetetramine.
Table 1 Thiourea-based coordinating polymers synthesized and the corresponding matrices used Starting polymer
Resin
Structure
Amberlite XAD-4 Amberlite XAD-2 Amberlite IR-45 Bonopore Bonopore Bonopore
PTU-1 PTU-2 ITU BTUO-1 BTUL-1 BTUL-2
IR--CH2--NH--CS--NH2 IR--CH2--NH--CS--NH2 ~--CH2--NH--CS--NH2 ]R--CH2--NH--CS--NH2 R--CH2--NH--(CH2)2--NH--CS--NH2 R--CH2--NH--(CH2)3--NH--CS--NH2
R denotes the polymer matrix.
(2)
(HCI)
168
G. Zuo, M. Muhammed / Reactive Po~,mers 24 (1995) 165-181
(3) Grafting chain extension (not for PTU-1, PTU-2, ITU or BTUO-1): ]R-- CH2C1 + H2N-- (CH2)n--NH2
~-
]R--CH2--NH-- (CH2)n --NH2 + HCI
(3)
(4) Conversion to the thiourea derivative: ll~'--NI-I 2 -~- NH4SCN
~ ' - - N H - - C S - - N H 2 + NH3 1"
(4)
where R denotes the polymer matrices and ~' presents R--CH2-- or ~--CH2 - - N H - (cn2)n --.
3.2. Kinetics of metal adsorption The adsorption kinetics was tested under the following conditions. About 0.03 g resin was stirred with 100 ml solution containing 200 ppm Au(III) and 2 M HCI at ambient temperature. At predetermined intervals, aliquots of 5 ml solution were taken out for analysis. The concentration of metal ions in the supernatant solution was determined by ICP spectroscopy and the amount
Table 2 The halftimeof the loadingof Au(Ill) on differentresins
tl/2 (min)
Resin PTU-1 PTU-2 ITU BTUO-1 BTUL-1 BTUL-2
170
36 78 33 24 17
of metal ions loaded on the resin phase was calculated by mass balance (in mmol/g resin). The results, shown in Fig. 1 as percentage of the capacity against contact time, provide numerical data for comparing the kinetic behaviour of all the different resins. The loading half time, tl/2, defined as the time needed to reach 50% of the resin's final loading capacity, was estimated from these curves. The half-time values, listed in Table 2, give a reasonable basis for comparison of the kinetics results. The variation of the adsorption kinetics of the different resins indicates a significant influence of the inner microstructure of the starting polymers on the kinetic behaviour of the resins. The
1°° f
--
2 40 [~/v/ ~7'/ ~ ]~r a_ 2
o [] ~ •
BTUL-2 BTUL-1 BTUO-1 PTU-2
0
5
10 Contact
15 time
20
25
30
(h)
Fig. 1. The kinetics of Au-binding by different resins. Experimentalconditions:batch experiments,0.03 g resin, 100 ml feed solution containing 200 ppm Au(III) and 2.0 M HC1, room temperature.
G. Zuo, M. Muharnmed / Reactive Polymers 24 (1995) 165-181
resins prepared using 'Bonopore' matrix showed improved kinetics. Comparing the sorption profiles for BTUO-1, BTUL-1 and BTUL-2, which were prepared from Bonopore, but with different spacer lengths (1, 3, and 4 carbon chain, respectively), it is observed that the longer the spacer, the faster the adsorption of the metal ions by the resin.
ferent metal ions. In these experiments, an exact amount of resin (0.02 g) was shaken with solution containing 0.3 mM Au(III) and Ag(I), 6 mM Cu(II) and Fe(III) and at various hydrochloric acid concentrations. The separation factor for precious metals over base metals is defined by: SM1/M2 - -
3.3. Capacity for binding of various metals The resin batch capacity (Q) for the extraction of Au(III), Ag(I), Cu(II) or Fe(III) was determined by the following method. About 0.05 g resin beads were stirred with 100 ml solutions containing different metal ions at a given concentration in 2 M HC1, at room temperature, for 24 h to ensure that equilibrium is attained. The initial concentration of gold, copper or iron ions was about 200 ppm, while that of silver was about 30 ppm. The loading capacity for different metal ions on the resins was calculated from the difference between the metal ion concentrations before and after extraction. The data are shown in Table 3.
3.4. Selectivity The data for the extraction of metal ions from solution containing single metal, shown in Table 3, give an indication of potential selectivity of the resins for different metals ions. However, resin selectivity can be more realistically determined under conditions where the extraction is carried out from solutions containing several dif-
169
[M1][M2] --
[M2][M1]
DM, -- - -
DMz
(5)
where M1 denotes precious metal ion, gold or silver, while M2 represents base metals, copper or iron, and DM denotes the distribution coefficient for the corresponding metal. The bar refers to the concentrations in the resin phase. The concentration of metals in the aqueous phase is expressed in molarity while that in the resin phase is given in mmol/g resin. The separation factors for Au(III) over Cu(II) and Fe(III) from solutions with various HCI acidity are shown in Figs. 2 and 3 for the different resins studied. The general trends observed from the extraction experiments given below. (1) The separation factors for all resins employed in this work, except for PTU-1, decrease significantly with increasing the acidity in the aqueous solution. The optimum range of acidity is < 1 M of HC1. For the resin PTU-1, the separation factor is low and independent of the concentration of HCI in the acidity range studied. (2) For resins prepared with the same starting polymer, e.g. Bonopore, the increase in spacer length between the matrix and the functional group results in a better selectivity for Au vs Fe and Cu.
3.5. Resin stability test
Table 3 Metal binding capacity of different coordinating resins Resin
Capacity (mmol/g) QAu
QAg
Qcu
Ql,'e
PTU-1 PTU-2 ITU BTUO-1 BTUL-1 BTUL-2
0.35 0.37 1.63 1.01 1.19 1.26
0.070 0.075 0.43 0.23 0.26 0.32
0.059 0.018 0.21 0.10 0.15 0.014
0.012 0.024 0.27 0.29 0.23 0.071
The resins PTU-1, ITU, BTUO-1, BTUL-1 and BTUL-2 were subjected to several loading and elution operations. The conditions employed in this study are as follows: 0.04-0.05 g resin beads were stirred with 100 ml of a solution containing 211 ppm Au(III) and 2 M HC1 for 24 h at room temperature. The elution of the resins was carried out by shaking the resins with 100
170
G. Zuo, M. Muhammed / Reactive Polymers24 (1995) 165-181 600
5O0
o0400 k0
oo
0 0
0
~-500
BTUL2 BTUL1 ITU BTU01 PTU1
~200 0 Q. O)
U3 IO0 0
0.0
I .0
2.0
3.0 4.0 Cone. of HCI (M)
~
~
5.0
6.0
I
I
7.0
Fig. 2. The influence of HCI on the selective extraction of Au over Cu. Experimental conditions: 0.02 g resin, 0.03 mM Au(III) and Ag(I), 6 mM Cu(II) and Fe(III) at various concentration of HCI, contact time is 24 h.
600
50O u"~400
o a <>
a 300
•
tO
BTUL2 BTUL1 ITU BTU01 PTU1
:-,: 200 0 Q.. co I 0 0 0 0 0.0
I .0
2.0
O
3.0
Conc.
of
--, , 4.0 HCI (M)
O ~ 5.0
~
~ 6.0
,
l 7.0
Fig. 3. The influence of HCI on the selective extraction of Au over Fe. Experimental conditions are the same as in Fig. 2.
ml solution containing 5% thiourea and 1% hydrochloric acid for at least 5 h to ensure that equilibrium was achieved. The operating capacity was calculated from both loading and elution tests. The results from both tests agreed within
5% error. The results, shown in Fig. 4, indicate that after two cycles the operating capacity was much lower than the initial capacity (first cycle), but remained unchanged when used in subsequent cycles.
G. Zuo, M. Muharnmed / Reactive Polymers 24 (1995) 165-181
171
350~ b b
F
%oo.
\\\
O9
\
~250 .< 2OO
8 >.150
_+__ °_ 0
~-100
121 0
.<
A
A
5O
0 o [] ±
ITU BTUL2 BTUL1 BTU01 PTU1
•
o
I
I
I
I
I
I
]
1
2
3
4
5
6
7
Cycle
No
Fig. 4. Evaluation of the stability of the resins for Au extraction in batch operation. Loading tests: 0.04-0.05 g resin, 100 ml feed solution containing 211 ppm Au(lll) and 2.0 N HCI contact for 24 h. Elution: 100 ml solution containing 5% thiourea and 1% HC1, contact >5 h.
3.6. Column tests Loading and elution tests Although the performance of coordinating polymers can be assessed in batch processes, column tests are more realistic when considering application situations. During the loading step, the feed solution containing 60 ppm Au(III) and 2 M HC1 was allowed to pass through the column in downflow. The breakthrough curves, in which the profiles are plotted as a dimensionless concentration factor C/Co (C = the concentration of the metal ions in the solution outcoming the columns and Co is the concentration of the same metal ions in the feed solution) versus the volume of feed solution passed through the column given as bed volume (BV, is also a dimensionless parameter calculated from the total volume of the feed solution passed through the column expressed in the resin bed volume units). Fig. 5 shows an example for the breakthrough curves obtained using ITU and PTU-1 resins. The breakthrough capacity is calculated from the volume of feed solution passed through the column when the
Table 4 Breakthrough capacities for gold using different resins Resin
Capacity (mmol/g)
PTU-I ITU BTUO-1 BTUL-1 BTUL-2
0.01 0.52 0.39 0.50 0.53
Au(III) concentration in the effluent is 10% of its initial concentration [16] (i.e. C/Co = 0.1). The breakthrough capacity (or working capacity) of the different resins under these conditions was calculated and is listed in Table 4. The elution solution used in this test is the same as that used in the batch experiments. The elution profiles shown in Fig. 6 were produced by monitoring the concentration of Au in the effluent. The concentration factor C/Co here indicates how many times the metal component had been concentrated in the effluent fractions after the complete extraction-elution operation. Each curve has a sharp elution peak with a minimum tail; a typical characteristic for good coordinating polymers.
172
G. Zuo, M. Muhammed / Reactive Polymers 24 (1995) 165-181 1.0
0.8
~0.6
G
PTUI ITU
•
0
o 0.4
0.2 f
0.0
v
400
200
800
600
1000
1200
BV Fig. 5. The loading of Au on ITU and PTU-1 resins in column operation. Resin bed: diameter 0.68 cm, ITU 1.48 g, PTU-1 2.93 g. Feed solution: 60 pm Au(III) and 2.0 N HCI, flow rate 2 ml/min, room temperature. 80
60
C 5°
"<50~
<> •
~
ITU PTU1
10 I ~
0
,50
1O0 BV
I
150
~
I ~
'
200
Fig. 6. The elution profiles of Au-loaded ITU and PTU-1 columns. Elution solution: 5% thiourea and 1% HC1. Other experimental conditions are the same as for Fig. 5.
Resin cycling tests T h e stability o f t h e resins was investigated by using P T U - 1 resin. T h e t e c h n i q u e e m p l o y e d in the e x p e r i m e n t s was similar to t h a t u s e d in
t h e b a t c h experiments. A f t e r t h e e l u t i o n is c o m pleted, m o r e t h a n 3 B V d e i o n i z e d w a t e r was p u m p e d d o w n f l o w t h r o u g h t h e resin b e d to ensure t h a t elution solution was c o m p l e t e l y re-
G. Zuo, M. Muhammed I Reactive Polymers 24 (1995) 165-181
173
1.0
0.8
0.6
4~
J
o [] 0
0.2
~~,~ . J ~e k~ , ~ , _ ~ - ' ' ~ " ' - ~ ' ~
0.0~
0
50
_
I00
,
~
150
,
,
,
I
. . . .
200
I
,
,
First Second Third Fourth i
250 BV
i
I
i
300
I
r
I
I
Cycle Cycle
Cycle Cycle I
t
550
I
I
I
I
400
i
I
I
I
i
450
~
I
t
J
500
Fig. 7. Evaluation of the stability of PTU-1 resin for Au extraction in column operation. Experimental conditions are the same as for PTU-1 for Fig. 5. Loading Au on PTU-1 from a solution containing about 60 ppm Au(III) in 2 N HCI. Bed: diameter --- 0.68 cm; height = 19 cm; volume = 7 ml. Flow rate: 17 bed V/Hr.
35 30 25
o 0 z~
20 o
First Third Fourth
Cycle Cycle Cycle
(.D 10
0
20
40
60
BV
80
1O0
120
140
Fig. 8. Evaluation of the stability of the resin from elution experiments of the PTU-1 resin. Experimental conditions are the same as for PTU-1 for Fig. 6. Elution of PTU-I: 5% thiourea and 1% HCI. Rate: 17 bed V/Hr. Bed: diameter = 0.68 cm; height = 19 cm; volume = 7 ml. m o v e d . T h e m e t a l i o n c o n c e n t r a t i o n profiles for the loading a n d elution experiments are pres e n t e d i n Figs. 7 a n d 8, respectively. T h e a n o m a l y
t h a t t h e r e s i n capacity o f t h e first l o a d i n g cycle is l o w e r t h a n t h a t o f t h e s e c o n d cycle m a y b e e x p l a i n e d by t h e h y d r o p h o b i c p r o p e r t y o f t h e
G. Zuo, M. Muhammed / Reactive Po~mers 24 (1995) 165-181
174
freshly prepared resin which causes reduced penetration of the solution into the resin. This was normalized after the second cycle after which the resin capacity remained unchanged and consistent with the results of the batch tests.
related complexation of Cu by these resins. In early stage of loading Cu, Ag and Au are all absorbed on the resin. As the loading of the resin bed progresses, Ag and Au replace Cu which is then 'eluted' from the column resulting in C/Co values higher than 1 and then level up to C/Co = 1.0. The further loading of the resin, results in similar pattern for Ag as it is replaced by Au. This is well demonstrated in all resins tested. The loaded column was eluted with solution containing 5% thiourea and 1% HC1 passing downflow through the column. The effluent fractions were collected and analyzed for different metal ions by ICE The results are shown as the curves shown in Figs. 12-14. Each elution profile of Au and Ag for the resins has a sharp peak with a minimum tail. The maximum concentration factor C/Co of Au or Ag for the resins is in the order of BTUL-2 > BTUL-1 > BTUO-1. On the other hand, the concentration factor of Cu and Fe for the resins has the opposite sequence. This suggests that the selectivity of the different resins for Au/Ag over Fe/Cu in column operation is in the order of BTUL-2 > BTUL-1 >
Selectivity tests in columns The batch adsorption tests indicated that some of these resins have good potential for selective extraction of gold. The selectivity of the resins was therefore also tested in a column operation mode using an aqueous solution containing a mixture of 0.3 mM Au(III) and Ag(I), 1 m M Cu(II) and Fe(III) ions in 2 M HC1. The Bonopore-based polymers, i.e. BTUO1 and BTUL-2, showed a better selectivity for extraction of Au(III) against other metal ions in solution, were selected for this experiment. The breakthrough capacity for the individual metal was also evaluated in the same way as for the data given in Figs. 9-11. The profile of Cu(II) extraction by the different resins have a common feature. The concentration factor C/Co steadily increases to values higher than 1 (up to 1.4) before it decreases to 1.0. This is a result of the
1.5
1.0
G
~ v v
-*., z~
oi I
r,_)
/ 1 /~
O-
~, ~
,/
/
/
Y 1O0
200
300
uTOl " 0 x
Ag, BTUO-1 Cu, 8TUO-1 Fe' BTUO-1
400
500
600
BV Fig. 9. The loading of different metals by the resin BTUO-1 in column operation. Resin bed: diameter 0.68 cm, 1.03 g resin. Feed solution: 0.3 mM Au(III), 0.3 mM Ag(1), 1 mM Cu(II) and 1 mM F¢(III) at 2.0 N HCI. Flow rate 2 ml/min, room tempgrature.
175
G. Zuo, M. Muhammed / Reactive PolYmers 24 (1995) 165-181 1.5
1.0
~7~-0 O <> /
--~-II--~
Zf4 ~ /
-
r...)
U, i 0.5 ~ ~<~ IF /
0.0
0
[] • 0 ×
/
~
'
100
Au, Ag, Cu, Fe,
200
BTUL-1 BTUL-1 BTUL- 1 BTUL-1
,300
/~<
400 BV
500
600
700
800
Fig. 10. The loading of different metals by the resin BTUL-1 in column operation. Resin bed: diameter 0.68 cm, 1.00 g resin. The experimental conditions are the same as for Fig. 9.
1.5
/ / i
'X
1.0 ¢
"
/
(_)
0.0
;I
!,
I:
F/
; ~
0
/
./ ," /" //
o •
¢' ×
Au, Ag, Cu , Fe,
BTUL-2 BTUL-2 BTUL-2 BTUL-2
/
/ / '
.... ~("
' O O C, ~ O O' O 0 0 (S ) 100 200 ,300 400 BV
~'
500
'
I
600
700
Fig. 11. The loading of different metals by the resin BTUL-2 in column operation. Resin bed: diameter 0.68 cm, 0.99 g resin. The experimental conditions are the same as for Fig. 9.
B T U O - 1 , t h e s a m e as in b a t c h o p e r a t i o n . T h e s h a r p n e s s o f t h e e l u t i o n curves for t h e resins b e i n g in t h e o r d e r o f B T U L - 2 > B T U L - 1 >
B T U O - 1 i n d i c a t e s that t h e e l u t i o n rate for t h e l o a d e d B T U L - 2 resin is faster t h a n B T U L - 1 , sequentially BTUO-1.
G. Zuo, M. Muhammed / Reactive Polymers24 (1995) 165-181
176 50
30 o • 0 ×
Au, BTUOAg, B T U O Cu, B T U O Fe, B T U O -
1 1 1 1
10
~li
o
lO
20
Ai
^1
~,
I
I
I
I
~
I
i
30
40 50 60 70 80 BV Fig. 12. Elution of loaded BTUO-1 resin. Resin bed is the same as for Fig. 9. Elution solution: 5% thiourea and 1% HC1. Flow rate: 2 ml/min, room temperature.
50
30
20
D • 0
Au, Ag, Cu,
BTUL- 1 BTUL- 1 BTUL-1
10
×
Fe,
BTUL- 1
~.' 30
.~
._-, 40 BY
_-I
50
~
I
60
,
I
70
,
80
Fig. 13. Elution of loaded BTUL-1 resin. Resin bed is the same as for Fig. 10. Experimental conditions are the same as for Fig. 12. 4. D i s c u s s i o n
4.1. Elemental analysis o f the resins Elemental analysis was carried out (by Mikro Kemi
AB,
Uppsala)
on the synthesized
resins in
o r d e r t o d e t e r m i n e H , C, S, a n d N c o n t e n t . T h e r e s u l t s a r e g i v e n in T a b l e 5 f o r t h e I T U resin. T h e t o t a l l i g a n d c o n c e n t r a t i o n in t h e r e s i n w a s calculated from the molecular formula and N a n d S analyses. A s s e e n t h e c o n c e n t r a t i o n o f t h e l i g a n d in t h e r e s i n b a s e d o n S a n a l y s i s is in a v e r y
G. Zuo, M. Muhammed / Reactive Polymers 24 (1995) 165-181
177
100
80
o • 0
01
0
?o
"
O'
so
Au, B T U L - 2 Ag, B T U L - 2 Cu, B T U L - 2 Fe, B T U L - 2
01
,
40
O I
so
,
~
,
60
i
70
..........
80
BV Fig. 14. Elution of loaded BTUL-2 resin. Resin bed is the same as for Fig. 11. Experimental conditions are the same as for Fig. 12.
Table 5 Elemental analysis of the resin ITU Ligand concentration (mmol/g)
Elements (%) C
No. 1 No. 2 Average
63.6 64.0 63.8
H
6.7 6.7 6.7
N
12.8 12.8 12.8
S
11.8 12.1 12.0
good agreement with that determined experimentally from the synthesis by mass difference. This indicates that the structure of the functional groups is as expected by the formula given in Table 1. However, the values of the concentration of ligand obtained from the N analysis are higher than those obtained experimentally, indicating the presence of unreacted NH2 groups (which is expected for this type of reaction). 4.2. Kinetics of metal extraction One obvious limitation for the interpretation of the results of the kinetic experiments is the wide distribution of the resin bead size employed. The h/2 can be taken only as an ap-
Calculated from elemental analysis
Determined from
by N content
by S content
HC1 experiments
4.57 4.57 4.57
3.69 3.77 3.73
3.74
proximate assessment of the kinetic behaviour of the resins. There was, therefore, no attempt to draw conclusions about the kinetic mechanism of the metal adsorption with these resins from these data. For the extraction of Au(III) or Ag(I) from chloride solutions by dodecylthiourea (DTH) reagent (containing the same ligand contained in the resin), the reaction equilibrium can be achieved in 1 min [13]. However, the extraction rate of Au(III) from same solutions with the resins employed in this work was much slower, as seen in Fig. 1. Attempts have been made using PTU-2 resin, in order to improve the kinetic properties of the resins, e.g. crushing the resin beads to in-
G. Zuo, M. Muhammed / Reactive Polymers 24 (1995) 165-181
178
Table 6 The properties of polymeric matrices and tl/z of gold binding to resins Resin
Matrix
Average pore diameter (A)
Surface area (m2/g)
tl/2 (rain)
BTUO-1 PTU-2 PTU-1
Bonopore XAD-2 XAD-4
80 90 50
800 330 750
33 36 170
crease the contact area with aqueous solution, treating the resin with ultrasonic, introducing hydrophilic group via sulphurization of the resin, and treatment of the resin by an non-ionic surfactant, Span 80 (sorbitan monooleate). These treatments did not result in any significant improvement of the resin kinetics. The properties of the polymer matrix used as well as the way by which the functional groups are attached to the support seem to be very important factors determining the kinetics of metal extraction by the resin. From the data presented in Fig. 1 and Table 2, it can be concluded that the resins prepared from different polymers with same length of spacer, i.e. BTUO-1 (from Bonopore), ITU (from Amberlite IR-45), PTU-2 (from Amberlite XAD-2), and PTU-1 (from Amberlite XAD-4) have different kinetics for the binding of gold. BTUO-1 and PTU-2 have faster loading speed (shorter tl/2) than PTU-1 and ITU. The role of the internal microstructure of the starting polymeric can be examined from the data given in Table 6 where the properties of polymer matrices Bonopore, Amberlite XAD-4 and XAD-2 are listed. The average pore diameter seems to be the more important parameter in determining the kinetics of the metal extraction of the resin rather than the surface area of the polymer. The resins BTUO-1 and PTU-2 have identical functional group and a show similar gold extraction rate. However, for these two resins, the matrices used have very different surface areas, but almost the same average pore size. On the other hand, the resin PTU-1, also having the same functional group, shows much slower kinetics of gold extraction. The polymer matrix used in this resin, Amberlite XAD-4, has a comparable surface to the Bono-
pore polymer, but much smaller average pore diameter. The influence of the spacer on the kinetics of metal extraction can be examined from the data presented in Fig. 1. Three resins with the same starting polymer, Bonopore; i.e. BTUL1, BTUL-1 and BTUO-1, but having different lengths of spacer (see Table 1), show significant differences in their kinetic behaviour. Kinetic data indicate that the longer spacer gives more flexibility for the ligand to form a complex with the metal ions, and consequently increases the extraction rate.
4.3. Acidity dependence of the extraction of metals The profiles shown in Figs. 2 and 3 suggest that the selectivity of these resins is likely to depend on the concentration of hydrochloric acid. In order to check the dependence of the complexation of the resins to metal ions on the H + concentration in solution, an amount of resin BTUO-1 and ITU was titrated with H + in the range pH = 5-2. It was found that all the H + added was kept in solution and did not bind to the resins. This indicates that the resins did not protonate in this pH range. On the other hand, if the resins protonated, the protonation would increase with the increase in concentration of HC1. It could be thus concluded that the adsorption of metal ions would be through anion exchange mechanism and facilitated by increasing HCI concentration. On the other hand, the distribution coefficient of Au(III) adsorbed by the resins decreases with the increase of HC1 concentration, as shown in Table 7, where the distribution coefficient of Au(III) loading on BTUL-1 is given for various concentrations of
G. Zuo, M. Muhammed / Reactive Polymers 24 (1995) 165-181 Table 7 Distribution coefficient of different metals adsorbed on BTUL-1 [HCI]
DAu x 10 -3
DCu x 10 -1
DFe x 10 -1
0.5 1.0 2.0 3.0 4.0 5.0 6.0
5.47 2.54 2.13 2.05 1.79 1.38 0.95
1.80 4.89 6.37 9.58 12.5 9.75 7.60
7.40 6.19 7.59 9.08 11.9 8.66 5.55
HCI. This indicates that the protonation of the resins in the high concentration range of HC1 is also improbable under the experimental conditions used in this study. Thus, the dependence of the adsorption of Au by the resins on HC1 concentration can be reasonably assigned to the variation of C1- concentration. The variation of the distribution coefficients of the adsorption of different metal ions by BTUL-1 at various concentration of HC1 (Table 7) also suggests that the decrease of the selective adsorption of Au(III) over Cu(II) and Fe(III) is mainly attributed to the decrease in the amount of Au bound to the resins. This is consistent with our earlier results of the solvent extraction study with the extractant D T H [13]. When the concentration of CI- in aqueous solutions was <1 M all the resins showed much higher selectivity and capacity for gold extraction, with the exception of PTU-1. 4.4. The metal adsorption behaviour of the resins From the analysis of the distribution data in our previous study [13], it was concluded that the extraction of precious metals by the thioureacontaining reagents takes place through the formation of different metal-ligand complexes. The exclusion of the possibility of the protonation of the functional groups suggests that the adsorption of metal ions by the thiourea-containing resins proceeds through a coordination between the ligand and metal ions rather than anion exchange reaction. This results in a compara-
179
tively slower binding rate, lower capacity and better selectivity than, for example, isothioronium resins, which extract P G M with an anion exchange mechanism in high acidities (2-6 M HCI) [11]. Although Swaminathan and Irving [17] suggested that only sulphur to metal bond exists in the metal-thiourea complexes, the studies of the coordination between noble metal ions and thiourea-like ligands, vinylthiopropioamide and isothiouronium, in macroporous resins by Zhixing S u e t al. [18] and Warshawsky et al. [11] indicate that the possibilities of the complexation of metals via nitrogen or with both nitrogen and sulfur cannot be excluded. The resin loading capacity determined from experiments with solutions containing single metal ions can give us an indication about the selectivity of the resin. However, column tests using solutions containing several metal ions provide more useful measure of the resin selectivity under conditions similar to application conditions. We define, for comparison, a selectivity parameter OtM1/M2 = QflQ2, to describe the separation behaviour of a resin, e.g., for BTUO-1 the O/B.Au/C u = 10, O/B.Au/F e = 4, and OtC.Au/Cu = 21, Otc Au/Fe = 39, where the subscripts B and C refer to the selectivity parameter calculated from batch experiment with single metal ion loading capacity and the column breakthrough capacity, respectively. In general, it is noticed that higher selectivity for precious metals over copper and iron were shown in column operation than those obtained from batch experiments. In comparing the structure of the resins, the role of the spacer was most important for influencing the selectivity of the resin. Comparing BTUO-1, BTUL-1 and BTUL-2 (Figs. 2 and 3) prepared from the same starting polymer Bonopore suggests that longer spacer provides more spacial possibility and ease for the ligands to re-arrange and form a complex with preferred metal ions. In this work, BTUL-2 resin provides the best selectivity of gold extraction from acidic chloride solutions. The selectivity of the resins is comparable with the selectivity of the extraction of metals by
180
G. Zuo, M. Muhammed / Reactive Po~mers 24 (1995) 165-181
thiourea-based reagents dissolved in chloroform, and the data available for the complexation of thiourea to metal ions in aqueous solutions (see Ref. 13). Thiourea and its organic derivatives form the strongest complexes with Au(III), and with other metal ions in the order of Ag(I) > Cu(II) > Fe(III). The adsorption of different metals by the coordinating resins are also in the same order, i.e. Au(III) > Ag(I) > Cu(II) > Fe(III). The separation factors for the 'free' reagent (as an extractant with the same functional group) for Au(III) over Cu(II) and Fe(III), e.g. DTH, under similar conditions are much higher (more than 1000) than those for the corresponding coordinating polymers. This is in agreement with the conclusion resulting from studies on other chelating agents [19,20] where the ligand could lose part of the specificity when incorporated into a polymeric matrix.
Table 8 Metal-binding capacities of ITU resin Metal
Binding capacity (mmol/g)
Au(IlI) Pd(II) Pt(IV) Ag(I) Rh(IlI)
1.63 0.69 0.38 0.43 0.22
lutions at 2 M HCI by resin ITU, are listed in Table 8. These results show that the metals forming only true chloride complexes give comparatively higher loading capacities than the metals forming both true and pseudochloride complexes. It is also indicated that the resins employed in this work show potential for the extraction and separation of PGM from base metals such as Cu and Fe. 5. Conclusion
4.5. Adsorption of Pt(1V), Pd(I1) and Rh(III) In hydrochloric acid media, the PGM and precious metals form complexes with CI-, OHand H20. Au, Ag, Pd and Pt construct the only true complex through the complete range of HCI concentration, namely: AuCI~-, PdCI2-, PtC12-, and AgC12 with AgCI43- in equilibrium. However, Rh, Ir, Ru and Os exist as mixture of true chloride and pseudochloride complexes, e.g. at equilibrium: R h C 1 4 ( H 2 0 ) 2 e,
~ R h C 1 5 ( H 2 0 ) 2 - ,~
~. RhC1 a -
However, this equilibrium exists only at moderate acid concentrations. In highly concentrated HC1, the true chloride complexes are dominant [11]. For these true chloride complexes, AuCI~-, AgC12, PdCI2-, PtCI 2- and RhCI~-, taking the donated electrons by the donor into account, the outer-shell electron configuration of all the metal ions is s2d 1°. Thus, it can be expected that these elements should present the similar extraction behaviour. The loading capacities of Au(III), Ag(I), Pd(II), Pt(IV) and Rh(III) from single metal so-
This study indicates that it is possible to prepare stable coordinating ion exchange resins by incorporating thiourea into macroporous styrene-divinylbenzene polymer matrices. The resins may have appreciable capacity for binding of precious metal ions from hydrochloric acid solutions and good selectivity for gold and silver over copper and iron in both batch and column operations. The adsorption of metal ions by the resins can be explained by the coordination between ligand and metal ions rather than anion exchange mechanism. The metal binding behaviour of the resins depends on the inner microstructure of the matrices and the length of spacer. The results of this investigation indicate the potential use of these resins for the separation of precious metals, especially gold over iron and copper, from chloride acidic solutions. Acknowledgement
The authors thank Dr. Christina Moberg for the fruitful discussions on the synthesis of the coordinating polymers.
G. Zuo, M. Muhammed / Reactive Polymers 24 (1995) 165-181
References [1] C.A. Fleming and G. Cromberge, J. S. Aft:. Inst. Min. Metal., 84 (1984) 125. [2] ED. FaweU, C.E Vernon, C. Klauber and H.G. Linge, React. Polym., 18 (1992) 47. [3] A. Warshawsky. In: M. Streat and D. Naden (Eds.) Ion Exchange and Sorption Processes in Hydrometallurgy, John Wiley & Sons, Chichester, New York, 1984, p. 127. [4] G. Den Boef and A. Hulanicki, Pure Appl. Chem., 55 (1983) 553. [5] L.L. Tavlarides, J.H. Bae and C.K. Le, Sep. Sci. Technol., 22 (1987) 581. [6] A. Warshawsky, Sep. Purif. Methods, 11 (1982/83) 95. [7] M. Grote and A. Kettrup, In: D. Naden and M. Streat (Eds.) Ion Exchange Technology, Ellis Horwood Limited, London, 1984, pp. 618. [8] M.J. Hudson and J.E Thorns, Hydrometallurgy, 11 (1983) 289. [9] G.V. Myasoedova, I.I. Antoko'skaya and S.B. Savvin, Talanta, 32 (1985) 1105.
181
[10] S. Siddhanta and H.R. Das, Talanta, 32 (1985) 457. [11] A. Warshawsky, M.M.B. Fieberg, E Mihalik, T.G. Murphy and Y.B. Ras, Sep. Purif. Methods, 9 (1980) 209. [12] A. Warshawsky, In: D. Naden and M. Streat (Eds.) Ion Exchange Technology, Ellis Horwood Limited, London, 1984, p. 604. [13] G. Zuo and M. Muhammed, Sep. Sci. Technol., 25 (1990) 1785. [14] A. Warshawsky, A. Deshe and R. Gutman, Br. Polym. J.., 16 (1984) 234. [15] A. Warshawsky, A. Deshe, G. Rossey and A. Patchornik, React. Polym., 2 (1984) 301. [16] D. Lindsay, D.C. Sherrington, J.A. Greig and R.D. Hancock, React. Polym., 12 (1990) 75. [17] K. Swaminathan and H.M.N.H. Irving, J. Inorg. Nucl. Chem., 26 (1964) 1291. [18] Zhixing Su, Xijun Chang, Keli Xu, Xingyin Luo and Guangyao Zhan, Anal. Chim. Acta, 268 (1992) 323. [19] S.K. Sahni and J. Reedijk, Coord. Chem. Rev., 59 (1984) 1. [20] D. Lindsay, D.C. Sherrington, J. Greig and R. Hancock, React. Polym., 12 (1990) 59.
Reactive Polymers 24 (1995) 165-181
Thiourea-based coordinating polymers: synthesis and binding to noble metals Guangju Zuo, Mamoun Muhammed * Department of lnorganic Chemistry, The Royal Institute of Technology, S-100 44, Stockholm, Sweden Received 14 December 1993; accepted in revised form 5 October 1994
Abstract
The synthesis of several coordinating polymers by grafting thiourea functional groups onto commercial macro-
porous polystyrene polymer matrices are described. In this paper, we also report the study on the absorption and elution of precious metal ions (or together with base metal ions, Cu2+ and Fe3+) from acidic chloride solutions. The results showed that these resins have appreciable capacity and good selectivity for the absorption of gold from acidic solutions.The extraction kinetics of the resins is slower than the corresponding monomolecular 'free' extractant analogues. However, some of the resins show acceptable extraction rates. Keywords: Thiourea; Resin; Coordinating polymer; Au; Ag
I. Introduction
Coordinating resins are polymers with covalently bound functional groups containing one or more donor atoms that are capable of forming complexes directly with metal ions or with their complexes. These polymers can also be made very selective to be used for a specific separation of one or more metal ions from solutions with different chemical environment [14]. As selective ion exchangers, the resins may achieve highly selective separation while maintaining appreciable capacity. Selective ion exchange resins can be very useful in treating a variety of solutions for the separation or preconcentration of certain ions. They therefore have great potential for application in environment engineering and hydrometallurgy, especially for * Corresponding author.
the recovery of precious and platinum group metals [5-6]. Precious metals (PM) usually exist together with a much large excess of 'base metals', such as copper, iron, etc. With the exhaustion of highquality PM-containing ores, there are increasing demands for highly selective separation techniques and reagents in order to separate and recover PM from low quality ores and secondary sources. The use of chlorine/chloride solutions as leaching agents has found several industrial applications [6]. Thus there is an increasing interest in preconcentration, recovery and separation of precious metals and platinum group metals (PGM) from hydrochloric acid media. Precious metals, in their normal oxidation states, tend to form their most stable complexes with ligands containing 'soft' donor atoms. Thus, coordinating resins containing donor N- and especially S-atoms in their functional groups are
0923-1137/95/$09.50 © 1995 Elsevier Science B.V. All rights reserved. SSDI 0 9 2 3 - 1 1 3 7 ( 9 4 ) 0 0 0 8 2 - 4
166
G. Zuo, M. Muhammed / Reactive Polymers 24 (1995) 165-181
promising polymers. During the past few years, a number of coordinating and chelating resins have been synthesized and tested for the separation and recovery of precious metals and platinum group metals. Grote and Kettrup [7] have synthesized a number of resins based on Sbonded tetrazolium (P-TD) and S-bonded dithizone (P-D) grafted on polystyrene matrix. P-TD provides fast kinetics for adsorption of noble metals through anion-exchange reactions, while most of the metals were complexed on P-D with tl/2 in the range of 0.5-4 h. Hudson and Thorns [8] have synthesized a polymer containing NCSSH-groups for the extraction of rhodium and iridium. The polymer is uncrosslinked and therefore offers high metal uptake rates. However, the polymer is unstable in sulphuric acid solution, but stable in hydrochloric acid solution. The recovery of the extracted metal is done after the chemical decomposition of the polymer. Myasoedova et al. [9] reported a new chelate forming 'Polyorgs' -resins made of different polymeric matrices and heterocyclic amine and amidoxime groups. The resins were mainly used for the preconcentration and analytical determination of PM in different samples. Siddhanta and Das [10] synthesized a chelating resin containing thiosemicarbazide. The sorption capacity for Pt(IV), Pd(II), Ru(III) and Rh(III) by these resins is at maximum in 1-2 M HC1, while Au(III) and Os(VI) are instantaneously reduced to the metal form when loaded on the resin. Warshawsky et al. [11] reported the synthesis of styrene divinylbenzene copolymer incorporating isothiourea as a functional group. The experimental data showed that the ion pair formation by anion exchange mechanism with the protonated form of the functional group is predominant in high acidity of HCI, and the ion pair binding followed by a slow conversion to coordination type complex via S- or N-atom or both whenever the metal-loaded resin is subjected to low acidity conditions. This resin has been tested in several hydrometallurgical processes [12]. In a previous study, we reported the synthesis and characterization of a family of thiourea-based
reagents. These reagents have proved to be of high selectivity and fast kinetics for the extraction of gold and silver ions through complex formation from HC1 solutions [13]. This earlier investigation led us to prepare resins with a suitable selective behaviour by incorporating thiourea into macroporous polystyrene polymers.
2. Experimental Commercial macroporous polystyrene-based polymers Bonopore 1, Amberlite 2 XAD-2, Amberlite XAD-4, and weak base resin Amberlite IR-45 were selected as the polymeric matrices. All starting polymers (except for Amberlite IR-45) were carefully washed as follows: 1 M NaOH and 1 M HC1 alternatively at 60°C, H20 (25"C), DMF (40°C), 1 M n C l (60°C), H20 (60°C), CH3OH (20°C), 3:2 (v/v) CHaOH: CH2C12, 1 : 3 (v/v) CH3OH: CH2C12, 1 : 9 (v/v) CH3OH: CH2C12 and pure CH2C12. For IR-45, the resin was stirred in 2 M ammonia solution for an hour at first and then washed with H20 (600C), CHaOH (20°C), 3:2 (v/v) CH3OH: CH2CI2, 1 : 3 (v/v) CHaOH: CH2C12, 1 : 9 (v/v) CHaOH: CH2C12 and pure CH2C12 sequentially. The samples were then dried at 600C under vacuum to a constant weight. All chemicals used in this work were of analytical grade. Gold stock solution was prepared by dissolving a known amount of pure gold metal in aqua regia, boiling to dryness and then dissolving the residue in 20 ml 37% HCI solution, and diluting to 1000 ml with deionized water. The stock solution of silver was prepared by dissolving 0.787 g AgNO3 in 1000 ml 1% HNO3. The analysis of different metal ions in aqueous phase was done by Inductively Coupled Plasma Atomic Emission Spectrometry technique (ARL3520 ICP-AES instrument). The instrument was calibrated using standard solutions of different metal ions before the measurement. The solutions were diluted to a proper concentration 1 Bonopore is a trademark of Nobel Industries, Sweden. 2 Amberlite is a trademark of Rohm & Haas Inc., U.S.A.
G. Zuo, M. Muhammed / Reactive Po~mers 24 (1995) 165-181
range. The wavelengths used for the quantitative determination were: Au, 242.793 nm; Ag, 328.071 nm; Cu, 324.758 nm; Fe, 238.200 nm. For batch tests, a known amount of resin beads was stirred with a solution containing single or several metal ions at a known HC1 concentration for a certain period at room temperature. The amount of metal ions adsorbed on the resin was calculated by mass balance from the analysis of the aqueous phase. For column operations, small-scale columns with 0.68 cm inner diameter have been employed. A known amount of resin beads (about 4 ml) was charged into the column, sufficient to obtain a height more than 100 mm. For PTU-1 resin, because most of the beads could float, a small ball of glass-wool was placed on the top of the resin bed. A peristaltic pump (Gilson, minipuls 2) was used to pump the feed solution or the elution solution downflow through the column. The effluent was periodically collected with a fraction sample collector and analyzed with ICP to obtain the loading and elution profiles. Both loading and elution flow rates were adjusted to about 2 ml/min.
167
of the analogous reagents (see Ref. 13). However, the reaction conditions are very different for these heterogeneous reactions and had to be developed in the present study. The reaction steps include chloromethylation, followed (when necessary) by the addition of the spacer, amination, and finally conversion to the thiourea derivative. For the chloromethylation, the traditional reagent, chloromethylmethylether (CMME), bis-chloromethylether (BCME) and their bromo analogues, known to be notorious carcinogens, were not employed. Instead, we used chloromethyloctylether, a safer chloromethylation reagent [14] that was synthesized by ourselves. The procedure can be presented by the following reactions. For the preparation of PTU-1, PTU-2 and BTUO-1 reaction 3 was not adopted. While for the preparation of ITU, after washing the IR-45 beads with 1 M NaOH and deionized water sequentially, only the last step was implemented. (1) Chloromethylation of polymer [14]: II~--H + C H 3 - - ( C H 2 ) 7 - - O - - C H 2 C I - - CH2 C1 +
CH3-- (CH2)7--O H
'
,~
(1)
3. Results
3.1. Synthesis of the coordinating polymers The synthesized coordinating resins and their relevant starting polymers are listed in Table 1. The procedure for the preparation of coordinating polymers is quite similar to the synthesis
(2) Amination of the chloromethylated polymer [15]: ~--CH2C1 (HMT~) ~ - - C H 2 - - N H 2
where HMTA denotes hexamethylenetetramine.
Table 1 Thiourea-based coordinating polymers synthesized and the corresponding matrices used Starting polymer
Resin
Structure
Amberlite XAD-4 Amberlite XAD-2 Amberlite IR-45 Bonopore Bonopore Bonopore
PTU-1 PTU-2 ITU BTUO-1 BTUL-1 BTUL-2
IR--CH2--NH--CS--NH2 IR--CH2--NH--CS--NH2 ~--CH2--NH--CS--NH2 ]R--CH2--NH--CS--NH2 R--CH2--NH--(CH2)2--NH--CS--NH2 R--CH2--NH--(CH2)3--NH--CS--NH2
R denotes the polymer matrix.
(2)
(HCI)
168
G. Zuo, M. Muhammed / Reactive Po~,mers 24 (1995) 165-181
(3) Grafting chain extension (not for PTU-1, PTU-2, ITU or BTUO-1): ]R-- CH2C1 + H2N-- (CH2)n--NH2
~-
]R--CH2--NH-- (CH2)n --NH2 + HCI
(3)
(4) Conversion to the thiourea derivative: ll~'--NI-I 2 -~- NH4SCN
~ ' - - N H - - C S - - N H 2 + NH3 1"
(4)
where R denotes the polymer matrices and ~' presents R--CH2-- or ~--CH2 - - N H - (cn2)n --.
3.2. Kinetics of metal adsorption The adsorption kinetics was tested under the following conditions. About 0.03 g resin was stirred with 100 ml solution containing 200 ppm Au(III) and 2 M HCI at ambient temperature. At predetermined intervals, aliquots of 5 ml solution were taken out for analysis. The concentration of metal ions in the supernatant solution was determined by ICP spectroscopy and the amount
Table 2 The halftimeof the loadingof Au(Ill) on differentresins
tl/2 (min)
Resin PTU-1 PTU-2 ITU BTUO-1 BTUL-1 BTUL-2
170
36 78 33 24 17
of metal ions loaded on the resin phase was calculated by mass balance (in mmol/g resin). The results, shown in Fig. 1 as percentage of the capacity against contact time, provide numerical data for comparing the kinetic behaviour of all the different resins. The loading half time, tl/2, defined as the time needed to reach 50% of the resin's final loading capacity, was estimated from these curves. The half-time values, listed in Table 2, give a reasonable basis for comparison of the kinetics results. The variation of the adsorption kinetics of the different resins indicates a significant influence of the inner microstructure of the starting polymers on the kinetic behaviour of the resins. The
1°° f
--
2 40 [~/v/ ~7'/ ~ ]~r a_ 2
o [] ~ •
BTUL-2 BTUL-1 BTUO-1 PTU-2
0
5
10 Contact
15 time
20
25
30
(h)
Fig. 1. The kinetics of Au-binding by different resins. Experimentalconditions:batch experiments,0.03 g resin, 100 ml feed solution containing 200 ppm Au(III) and 2.0 M HC1, room temperature.
G. Zuo, M. Muharnmed / Reactive Polymers 24 (1995) 165-181
resins prepared using 'Bonopore' matrix showed improved kinetics. Comparing the sorption profiles for BTUO-1, BTUL-1 and BTUL-2, which were prepared from Bonopore, but with different spacer lengths (1, 3, and 4 carbon chain, respectively), it is observed that the longer the spacer, the faster the adsorption of the metal ions by the resin.
ferent metal ions. In these experiments, an exact amount of resin (0.02 g) was shaken with solution containing 0.3 mM Au(III) and Ag(I), 6 mM Cu(II) and Fe(III) and at various hydrochloric acid concentrations. The separation factor for precious metals over base metals is defined by: SM1/M2 - -
3.3. Capacity for binding of various metals The resin batch capacity (Q) for the extraction of Au(III), Ag(I), Cu(II) or Fe(III) was determined by the following method. About 0.05 g resin beads were stirred with 100 ml solutions containing different metal ions at a given concentration in 2 M HC1, at room temperature, for 24 h to ensure that equilibrium is attained. The initial concentration of gold, copper or iron ions was about 200 ppm, while that of silver was about 30 ppm. The loading capacity for different metal ions on the resins was calculated from the difference between the metal ion concentrations before and after extraction. The data are shown in Table 3.
3.4. Selectivity The data for the extraction of metal ions from solution containing single metal, shown in Table 3, give an indication of potential selectivity of the resins for different metals ions. However, resin selectivity can be more realistically determined under conditions where the extraction is carried out from solutions containing several dif-
169
[M1][M2] --
[M2][M1]
DM, -- - -
DMz
(5)
where M1 denotes precious metal ion, gold or silver, while M2 represents base metals, copper or iron, and DM denotes the distribution coefficient for the corresponding metal. The bar refers to the concentrations in the resin phase. The concentration of metals in the aqueous phase is expressed in molarity while that in the resin phase is given in mmol/g resin. The separation factors for Au(III) over Cu(II) and Fe(III) from solutions with various HCI acidity are shown in Figs. 2 and 3 for the different resins studied. The general trends observed from the extraction experiments given below. (1) The separation factors for all resins employed in this work, except for PTU-1, decrease significantly with increasing the acidity in the aqueous solution. The optimum range of acidity is < 1 M of HC1. For the resin PTU-1, the separation factor is low and independent of the concentration of HCI in the acidity range studied. (2) For resins prepared with the same starting polymer, e.g. Bonopore, the increase in spacer length between the matrix and the functional group results in a better selectivity for Au vs Fe and Cu.
3.5. Resin stability test
Table 3 Metal binding capacity of different coordinating resins Resin
Capacity (mmol/g) QAu
QAg
Qcu
Ql,'e
PTU-1 PTU-2 ITU BTUO-1 BTUL-1 BTUL-2
0.35 0.37 1.63 1.01 1.19 1.26
0.070 0.075 0.43 0.23 0.26 0.32
0.059 0.018 0.21 0.10 0.15 0.014
0.012 0.024 0.27 0.29 0.23 0.071
The resins PTU-1, ITU, BTUO-1, BTUL-1 and BTUL-2 were subjected to several loading and elution operations. The conditions employed in this study are as follows: 0.04-0.05 g resin beads were stirred with 100 ml of a solution containing 211 ppm Au(III) and 2 M HC1 for 24 h at room temperature. The elution of the resins was carried out by shaking the resins with 100
170
G. Zuo, M. Muhammed / Reactive Polymers24 (1995) 165-181 600
5O0
o0400 k0
oo
0 0
0
~-500
BTUL2 BTUL1 ITU BTU01 PTU1
~200 0 Q. O)
U3 IO0 0
0.0
I .0
2.0
3.0 4.0 Cone. of HCI (M)
~
~
5.0
6.0
I
I
7.0
Fig. 2. The influence of HCI on the selective extraction of Au over Cu. Experimental conditions: 0.02 g resin, 0.03 mM Au(III) and Ag(I), 6 mM Cu(II) and Fe(III) at various concentration of HCI, contact time is 24 h.
600
50O u"~400
o a <>
a 300
•
tO
BTUL2 BTUL1 ITU BTU01 PTU1
:-,: 200 0 Q.. co I 0 0 0 0 0.0
I .0
2.0
O
3.0
Conc.
of
--, , 4.0 HCI (M)
O ~ 5.0
~
~ 6.0
,
l 7.0
Fig. 3. The influence of HCI on the selective extraction of Au over Fe. Experimental conditions are the same as in Fig. 2.
ml solution containing 5% thiourea and 1% hydrochloric acid for at least 5 h to ensure that equilibrium was achieved. The operating capacity was calculated from both loading and elution tests. The results from both tests agreed within
5% error. The results, shown in Fig. 4, indicate that after two cycles the operating capacity was much lower than the initial capacity (first cycle), but remained unchanged when used in subsequent cycles.
G. Zuo, M. Muharnmed / Reactive Polymers 24 (1995) 165-181
171
350~ b b
F
%oo.
\\\
O9
\
~250 .< 2OO
8 >.150
_+__ °_ 0
~-100
121 0
.<
A
A
5O
0 o [] ±
ITU BTUL2 BTUL1 BTU01 PTU1
•
o
I
I
I
I
I
I
]
1
2
3
4
5
6
7
Cycle
No
Fig. 4. Evaluation of the stability of the resins for Au extraction in batch operation. Loading tests: 0.04-0.05 g resin, 100 ml feed solution containing 211 ppm Au(lll) and 2.0 N HCI contact for 24 h. Elution: 100 ml solution containing 5% thiourea and 1% HC1, contact >5 h.
3.6. Column tests Loading and elution tests Although the performance of coordinating polymers can be assessed in batch processes, column tests are more realistic when considering application situations. During the loading step, the feed solution containing 60 ppm Au(III) and 2 M HC1 was allowed to pass through the column in downflow. The breakthrough curves, in which the profiles are plotted as a dimensionless concentration factor C/Co (C = the concentration of the metal ions in the solution outcoming the columns and Co is the concentration of the same metal ions in the feed solution) versus the volume of feed solution passed through the column given as bed volume (BV, is also a dimensionless parameter calculated from the total volume of the feed solution passed through the column expressed in the resin bed volume units). Fig. 5 shows an example for the breakthrough curves obtained using ITU and PTU-1 resins. The breakthrough capacity is calculated from the volume of feed solution passed through the column when the
Table 4 Breakthrough capacities for gold using different resins Resin
Capacity (mmol/g)
PTU-I ITU BTUO-1 BTUL-1 BTUL-2
0.01 0.52 0.39 0.50 0.53
Au(III) concentration in the effluent is 10% of its initial concentration [16] (i.e. C/Co = 0.1). The breakthrough capacity (or working capacity) of the different resins under these conditions was calculated and is listed in Table 4. The elution solution used in this test is the same as that used in the batch experiments. The elution profiles shown in Fig. 6 were produced by monitoring the concentration of Au in the effluent. The concentration factor C/Co here indicates how many times the metal component had been concentrated in the effluent fractions after the complete extraction-elution operation. Each curve has a sharp elution peak with a minimum tail; a typical characteristic for good coordinating polymers.
172
G. Zuo, M. Muhammed / Reactive Polymers 24 (1995) 165-181 1.0
0.8
~0.6
G
PTUI ITU
•
0
o 0.4
0.2 f
0.0
v
400
200
800
600
1000
1200
BV Fig. 5. The loading of Au on ITU and PTU-1 resins in column operation. Resin bed: diameter 0.68 cm, ITU 1.48 g, PTU-1 2.93 g. Feed solution: 60 pm Au(III) and 2.0 N HCI, flow rate 2 ml/min, room temperature. 80
60
C 5°
"<50~
<> •
~
ITU PTU1
10 I ~
0
,50
1O0 BV
I
150
~
I ~
'
200
Fig. 6. The elution profiles of Au-loaded ITU and PTU-1 columns. Elution solution: 5% thiourea and 1% HC1. Other experimental conditions are the same as for Fig. 5.
Resin cycling tests T h e stability o f t h e resins was investigated by using P T U - 1 resin. T h e t e c h n i q u e e m p l o y e d in the e x p e r i m e n t s was similar to t h a t u s e d in
t h e b a t c h experiments. A f t e r t h e e l u t i o n is c o m pleted, m o r e t h a n 3 B V d e i o n i z e d w a t e r was p u m p e d d o w n f l o w t h r o u g h t h e resin b e d to ensure t h a t elution solution was c o m p l e t e l y re-
G. Zuo, M. Muhammed I Reactive Polymers 24 (1995) 165-181
173
1.0
0.8
0.6
4~
J
o [] 0
0.2
~~,~ . J ~e k~ , ~ , _ ~ - ' ' ~ " ' - ~ ' ~
0.0~
0
50
_
I00
,
~
150
,
,
,
I
. . . .
200
I
,
,
First Second Third Fourth i
250 BV
i
I
i
300
I
r
I
I
Cycle Cycle
Cycle Cycle I
t
550
I
I
I
I
400
i
I
I
I
i
450
~
I
t
J
500
Fig. 7. Evaluation of the stability of PTU-1 resin for Au extraction in column operation. Experimental conditions are the same as for PTU-1 for Fig. 5. Loading Au on PTU-1 from a solution containing about 60 ppm Au(III) in 2 N HCI. Bed: diameter --- 0.68 cm; height = 19 cm; volume = 7 ml. Flow rate: 17 bed V/Hr.
35 30 25
o 0 z~
20 o
First Third Fourth
Cycle Cycle Cycle
(.D 10
0
20
40
60
BV
80
1O0
120
140
Fig. 8. Evaluation of the stability of the resin from elution experiments of the PTU-1 resin. Experimental conditions are the same as for PTU-1 for Fig. 6. Elution of PTU-I: 5% thiourea and 1% HCI. Rate: 17 bed V/Hr. Bed: diameter = 0.68 cm; height = 19 cm; volume = 7 ml. m o v e d . T h e m e t a l i o n c o n c e n t r a t i o n profiles for the loading a n d elution experiments are pres e n t e d i n Figs. 7 a n d 8, respectively. T h e a n o m a l y
t h a t t h e r e s i n capacity o f t h e first l o a d i n g cycle is l o w e r t h a n t h a t o f t h e s e c o n d cycle m a y b e e x p l a i n e d by t h e h y d r o p h o b i c p r o p e r t y o f t h e
G. Zuo, M. Muhammed / Reactive Po~mers 24 (1995) 165-181
174
freshly prepared resin which causes reduced penetration of the solution into the resin. This was normalized after the second cycle after which the resin capacity remained unchanged and consistent with the results of the batch tests.
related complexation of Cu by these resins. In early stage of loading Cu, Ag and Au are all absorbed on the resin. As the loading of the resin bed progresses, Ag and Au replace Cu which is then 'eluted' from the column resulting in C/Co values higher than 1 and then level up to C/Co = 1.0. The further loading of the resin, results in similar pattern for Ag as it is replaced by Au. This is well demonstrated in all resins tested. The loaded column was eluted with solution containing 5% thiourea and 1% HC1 passing downflow through the column. The effluent fractions were collected and analyzed for different metal ions by ICE The results are shown as the curves shown in Figs. 12-14. Each elution profile of Au and Ag for the resins has a sharp peak with a minimum tail. The maximum concentration factor C/Co of Au or Ag for the resins is in the order of BTUL-2 > BTUL-1 > BTUO-1. On the other hand, the concentration factor of Cu and Fe for the resins has the opposite sequence. This suggests that the selectivity of the different resins for Au/Ag over Fe/Cu in column operation is in the order of BTUL-2 > BTUL-1 >
Selectivity tests in columns The batch adsorption tests indicated that some of these resins have good potential for selective extraction of gold. The selectivity of the resins was therefore also tested in a column operation mode using an aqueous solution containing a mixture of 0.3 mM Au(III) and Ag(I), 1 m M Cu(II) and Fe(III) ions in 2 M HC1. The Bonopore-based polymers, i.e. BTUO1 and BTUL-2, showed a better selectivity for extraction of Au(III) against other metal ions in solution, were selected for this experiment. The breakthrough capacity for the individual metal was also evaluated in the same way as for the data given in Figs. 9-11. The profile of Cu(II) extraction by the different resins have a common feature. The concentration factor C/Co steadily increases to values higher than 1 (up to 1.4) before it decreases to 1.0. This is a result of the
1.5
1.0
G
~ v v
-*., z~
oi I
r,_)
/ 1 /~
O-
~, ~
,/
/
/
Y 1O0
200
300
uTOl " 0 x
Ag, BTUO-1 Cu, 8TUO-1 Fe' BTUO-1
400
500
600
BV Fig. 9. The loading of different metals by the resin BTUO-1 in column operation. Resin bed: diameter 0.68 cm, 1.03 g resin. Feed solution: 0.3 mM Au(III), 0.3 mM Ag(1), 1 mM Cu(II) and 1 mM F¢(III) at 2.0 N HCI. Flow rate 2 ml/min, room tempgrature.
175
G. Zuo, M. Muhammed / Reactive PolYmers 24 (1995) 165-181 1.5
1.0
~7~-0 O <> /
--~-II--~
Zf4 ~ /
-
r...)
U, i 0.5 ~ ~<~ IF /
0.0
0
[] • 0 ×
/
~
'
100
Au, Ag, Cu, Fe,
200
BTUL-1 BTUL-1 BTUL- 1 BTUL-1
,300
/~<
400 BV
500
600
700
800
Fig. 10. The loading of different metals by the resin BTUL-1 in column operation. Resin bed: diameter 0.68 cm, 1.00 g resin. The experimental conditions are the same as for Fig. 9.
1.5
/ / i
'X
1.0 ¢
"
/
(_)
0.0
;I
!,
I:
F/
; ~
0
/
./ ," /" //
o •
¢' ×
Au, Ag, Cu , Fe,
BTUL-2 BTUL-2 BTUL-2 BTUL-2
/
/ / '
.... ~("
' O O C, ~ O O' O 0 0 (S ) 100 200 ,300 400 BV
~'
500
'
I
600
700
Fig. 11. The loading of different metals by the resin BTUL-2 in column operation. Resin bed: diameter 0.68 cm, 0.99 g resin. The experimental conditions are the same as for Fig. 9.
B T U O - 1 , t h e s a m e as in b a t c h o p e r a t i o n . T h e s h a r p n e s s o f t h e e l u t i o n curves for t h e resins b e i n g in t h e o r d e r o f B T U L - 2 > B T U L - 1 >
B T U O - 1 i n d i c a t e s that t h e e l u t i o n rate for t h e l o a d e d B T U L - 2 resin is faster t h a n B T U L - 1 , sequentially BTUO-1.
G. Zuo, M. Muhammed / Reactive Polymers24 (1995) 165-181
176 50
30 o • 0 ×
Au, BTUOAg, B T U O Cu, B T U O Fe, B T U O -
1 1 1 1
10
~li
o
lO
20
Ai
^1
~,
I
I
I
I
~
I
i
30
40 50 60 70 80 BV Fig. 12. Elution of loaded BTUO-1 resin. Resin bed is the same as for Fig. 9. Elution solution: 5% thiourea and 1% HC1. Flow rate: 2 ml/min, room temperature.
50
30
20
D • 0
Au, Ag, Cu,
BTUL- 1 BTUL- 1 BTUL-1
10
×
Fe,
BTUL- 1
~.' 30
.~
._-, 40 BY
_-I
50
~
I
60
,
I
70
,
80
Fig. 13. Elution of loaded BTUL-1 resin. Resin bed is the same as for Fig. 10. Experimental conditions are the same as for Fig. 12. 4. D i s c u s s i o n
4.1. Elemental analysis o f the resins Elemental analysis was carried out (by Mikro Kemi
AB,
Uppsala)
on the synthesized
resins in
o r d e r t o d e t e r m i n e H , C, S, a n d N c o n t e n t . T h e r e s u l t s a r e g i v e n in T a b l e 5 f o r t h e I T U resin. T h e t o t a l l i g a n d c o n c e n t r a t i o n in t h e r e s i n w a s calculated from the molecular formula and N a n d S analyses. A s s e e n t h e c o n c e n t r a t i o n o f t h e l i g a n d in t h e r e s i n b a s e d o n S a n a l y s i s is in a v e r y
G. Zuo, M. Muhammed / Reactive Polymers 24 (1995) 165-181
177
100
80
o • 0
01
0
?o
"
O'
so
Au, B T U L - 2 Ag, B T U L - 2 Cu, B T U L - 2 Fe, B T U L - 2
01
,
40
O I
so
,
~
,
60
i
70
..........
80
BV Fig. 14. Elution of loaded BTUL-2 resin. Resin bed is the same as for Fig. 11. Experimental conditions are the same as for Fig. 12.
Table 5 Elemental analysis of the resin ITU Ligand concentration (mmol/g)
Elements (%) C
No. 1 No. 2 Average
63.6 64.0 63.8
H
6.7 6.7 6.7
N
12.8 12.8 12.8
S
11.8 12.1 12.0
good agreement with that determined experimentally from the synthesis by mass difference. This indicates that the structure of the functional groups is as expected by the formula given in Table 1. However, the values of the concentration of ligand obtained from the N analysis are higher than those obtained experimentally, indicating the presence of unreacted NH2 groups (which is expected for this type of reaction). 4.2. Kinetics of metal extraction One obvious limitation for the interpretation of the results of the kinetic experiments is the wide distribution of the resin bead size employed. The h/2 can be taken only as an ap-
Calculated from elemental analysis
Determined from
by N content
by S content
HC1 experiments
4.57 4.57 4.57
3.69 3.77 3.73
3.74
proximate assessment of the kinetic behaviour of the resins. There was, therefore, no attempt to draw conclusions about the kinetic mechanism of the metal adsorption with these resins from these data. For the extraction of Au(III) or Ag(I) from chloride solutions by dodecylthiourea (DTH) reagent (containing the same ligand contained in the resin), the reaction equilibrium can be achieved in 1 min [13]. However, the extraction rate of Au(III) from same solutions with the resins employed in this work was much slower, as seen in Fig. 1. Attempts have been made using PTU-2 resin, in order to improve the kinetic properties of the resins, e.g. crushing the resin beads to in-
G. Zuo, M. Muhammed / Reactive Polymers 24 (1995) 165-181
178
Table 6 The properties of polymeric matrices and tl/z of gold binding to resins Resin
Matrix
Average pore diameter (A)
Surface area (m2/g)
tl/2 (rain)
BTUO-1 PTU-2 PTU-1
Bonopore XAD-2 XAD-4
80 90 50
800 330 750
33 36 170
crease the contact area with aqueous solution, treating the resin with ultrasonic, introducing hydrophilic group via sulphurization of the resin, and treatment of the resin by an non-ionic surfactant, Span 80 (sorbitan monooleate). These treatments did not result in any significant improvement of the resin kinetics. The properties of the polymer matrix used as well as the way by which the functional groups are attached to the support seem to be very important factors determining the kinetics of metal extraction by the resin. From the data presented in Fig. 1 and Table 2, it can be concluded that the resins prepared from different polymers with same length of spacer, i.e. BTUO-1 (from Bonopore), ITU (from Amberlite IR-45), PTU-2 (from Amberlite XAD-2), and PTU-1 (from Amberlite XAD-4) have different kinetics for the binding of gold. BTUO-1 and PTU-2 have faster loading speed (shorter tl/2) than PTU-1 and ITU. The role of the internal microstructure of the starting polymeric can be examined from the data given in Table 6 where the properties of polymer matrices Bonopore, Amberlite XAD-4 and XAD-2 are listed. The average pore diameter seems to be the more important parameter in determining the kinetics of the metal extraction of the resin rather than the surface area of the polymer. The resins BTUO-1 and PTU-2 have identical functional group and a show similar gold extraction rate. However, for these two resins, the matrices used have very different surface areas, but almost the same average pore size. On the other hand, the resin PTU-1, also having the same functional group, shows much slower kinetics of gold extraction. The polymer matrix used in this resin, Amberlite XAD-4, has a comparable surface to the Bono-
pore polymer, but much smaller average pore diameter. The influence of the spacer on the kinetics of metal extraction can be examined from the data presented in Fig. 1. Three resins with the same starting polymer, Bonopore; i.e. BTUL1, BTUL-1 and BTUO-1, but having different lengths of spacer (see Table 1), show significant differences in their kinetic behaviour. Kinetic data indicate that the longer spacer gives more flexibility for the ligand to form a complex with the metal ions, and consequently increases the extraction rate.
4.3. Acidity dependence of the extraction of metals The profiles shown in Figs. 2 and 3 suggest that the selectivity of these resins is likely to depend on the concentration of hydrochloric acid. In order to check the dependence of the complexation of the resins to metal ions on the H + concentration in solution, an amount of resin BTUO-1 and ITU was titrated with H + in the range pH = 5-2. It was found that all the H + added was kept in solution and did not bind to the resins. This indicates that the resins did not protonate in this pH range. On the other hand, if the resins protonated, the protonation would increase with the increase in concentration of HC1. It could be thus concluded that the adsorption of metal ions would be through anion exchange mechanism and facilitated by increasing HCI concentration. On the other hand, the distribution coefficient of Au(III) adsorbed by the resins decreases with the increase of HC1 concentration, as shown in Table 7, where the distribution coefficient of Au(III) loading on BTUL-1 is given for various concentrations of
G. Zuo, M. Muhammed / Reactive Polymers 24 (1995) 165-181 Table 7 Distribution coefficient of different metals adsorbed on BTUL-1 [HCI]
DAu x 10 -3
DCu x 10 -1
DFe x 10 -1
0.5 1.0 2.0 3.0 4.0 5.0 6.0
5.47 2.54 2.13 2.05 1.79 1.38 0.95
1.80 4.89 6.37 9.58 12.5 9.75 7.60
7.40 6.19 7.59 9.08 11.9 8.66 5.55
HCI. This indicates that the protonation of the resins in the high concentration range of HC1 is also improbable under the experimental conditions used in this study. Thus, the dependence of the adsorption of Au by the resins on HC1 concentration can be reasonably assigned to the variation of C1- concentration. The variation of the distribution coefficients of the adsorption of different metal ions by BTUL-1 at various concentration of HC1 (Table 7) also suggests that the decrease of the selective adsorption of Au(III) over Cu(II) and Fe(III) is mainly attributed to the decrease in the amount of Au bound to the resins. This is consistent with our earlier results of the solvent extraction study with the extractant D T H [13]. When the concentration of CI- in aqueous solutions was <1 M all the resins showed much higher selectivity and capacity for gold extraction, with the exception of PTU-1. 4.4. The metal adsorption behaviour of the resins From the analysis of the distribution data in our previous study [13], it was concluded that the extraction of precious metals by the thioureacontaining reagents takes place through the formation of different metal-ligand complexes. The exclusion of the possibility of the protonation of the functional groups suggests that the adsorption of metal ions by the thiourea-containing resins proceeds through a coordination between the ligand and metal ions rather than anion exchange reaction. This results in a compara-
179
tively slower binding rate, lower capacity and better selectivity than, for example, isothioronium resins, which extract P G M with an anion exchange mechanism in high acidities (2-6 M HCI) [11]. Although Swaminathan and Irving [17] suggested that only sulphur to metal bond exists in the metal-thiourea complexes, the studies of the coordination between noble metal ions and thiourea-like ligands, vinylthiopropioamide and isothiouronium, in macroporous resins by Zhixing S u e t al. [18] and Warshawsky et al. [11] indicate that the possibilities of the complexation of metals via nitrogen or with both nitrogen and sulfur cannot be excluded. The resin loading capacity determined from experiments with solutions containing single metal ions can give us an indication about the selectivity of the resin. However, column tests using solutions containing several metal ions provide more useful measure of the resin selectivity under conditions similar to application conditions. We define, for comparison, a selectivity parameter OtM1/M2 = QflQ2, to describe the separation behaviour of a resin, e.g., for BTUO-1 the O/B.Au/C u = 10, O/B.Au/F e = 4, and OtC.Au/Cu = 21, Otc Au/Fe = 39, where the subscripts B and C refer to the selectivity parameter calculated from batch experiment with single metal ion loading capacity and the column breakthrough capacity, respectively. In general, it is noticed that higher selectivity for precious metals over copper and iron were shown in column operation than those obtained from batch experiments. In comparing the structure of the resins, the role of the spacer was most important for influencing the selectivity of the resin. Comparing BTUO-1, BTUL-1 and BTUL-2 (Figs. 2 and 3) prepared from the same starting polymer Bonopore suggests that longer spacer provides more spacial possibility and ease for the ligands to re-arrange and form a complex with preferred metal ions. In this work, BTUL-2 resin provides the best selectivity of gold extraction from acidic chloride solutions. The selectivity of the resins is comparable with the selectivity of the extraction of metals by
180
G. Zuo, M. Muhammed / Reactive Po~mers 24 (1995) 165-181
thiourea-based reagents dissolved in chloroform, and the data available for the complexation of thiourea to metal ions in aqueous solutions (see Ref. 13). Thiourea and its organic derivatives form the strongest complexes with Au(III), and with other metal ions in the order of Ag(I) > Cu(II) > Fe(III). The adsorption of different metals by the coordinating resins are also in the same order, i.e. Au(III) > Ag(I) > Cu(II) > Fe(III). The separation factors for the 'free' reagent (as an extractant with the same functional group) for Au(III) over Cu(II) and Fe(III), e.g. DTH, under similar conditions are much higher (more than 1000) than those for the corresponding coordinating polymers. This is in agreement with the conclusion resulting from studies on other chelating agents [19,20] where the ligand could lose part of the specificity when incorporated into a polymeric matrix.
Table 8 Metal-binding capacities of ITU resin Metal
Binding capacity (mmol/g)
Au(IlI) Pd(II) Pt(IV) Ag(I) Rh(IlI)
1.63 0.69 0.38 0.43 0.22
lutions at 2 M HCI by resin ITU, are listed in Table 8. These results show that the metals forming only true chloride complexes give comparatively higher loading capacities than the metals forming both true and pseudochloride complexes. It is also indicated that the resins employed in this work show potential for the extraction and separation of PGM from base metals such as Cu and Fe. 5. Conclusion
4.5. Adsorption of Pt(1V), Pd(I1) and Rh(III) In hydrochloric acid media, the PGM and precious metals form complexes with CI-, OHand H20. Au, Ag, Pd and Pt construct the only true complex through the complete range of HCI concentration, namely: AuCI~-, PdCI2-, PtC12-, and AgC12 with AgCI43- in equilibrium. However, Rh, Ir, Ru and Os exist as mixture of true chloride and pseudochloride complexes, e.g. at equilibrium: R h C 1 4 ( H 2 0 ) 2 e,
~ R h C 1 5 ( H 2 0 ) 2 - ,~
~. RhC1 a -
However, this equilibrium exists only at moderate acid concentrations. In highly concentrated HC1, the true chloride complexes are dominant [11]. For these true chloride complexes, AuCI~-, AgC12, PdCI2-, PtCI 2- and RhCI~-, taking the donated electrons by the donor into account, the outer-shell electron configuration of all the metal ions is s2d 1°. Thus, it can be expected that these elements should present the similar extraction behaviour. The loading capacities of Au(III), Ag(I), Pd(II), Pt(IV) and Rh(III) from single metal so-
This study indicates that it is possible to prepare stable coordinating ion exchange resins by incorporating thiourea into macroporous styrene-divinylbenzene polymer matrices. The resins may have appreciable capacity for binding of precious metal ions from hydrochloric acid solutions and good selectivity for gold and silver over copper and iron in both batch and column operations. The adsorption of metal ions by the resins can be explained by the coordination between ligand and metal ions rather than anion exchange mechanism. The metal binding behaviour of the resins depends on the inner microstructure of the matrices and the length of spacer. The results of this investigation indicate the potential use of these resins for the separation of precious metals, especially gold over iron and copper, from chloride acidic solutions. Acknowledgement
The authors thank Dr. Christina Moberg for the fruitful discussions on the synthesis of the coordinating polymers.
G. Zuo, M. Muhammed / Reactive Polymers 24 (1995) 165-181
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