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Kinetics and mechanism of back-extraction: selected palladium extraction systems
Analytica Chimica Acta, 245 (1991) 225-233 Elsevier Science Publishers B.V., Amsterdam
225
Kinetics and mechanism of back-extraction: selected palladium extraction systems S.J. Al-Bazi and Henry Freiser * Strategic Metals Recovery Research Facility, Department
of Chemistry,
(Received
6th August
University of Arizona,
Tucson, AZ 85721 (U.S.A.)
1990)
Abstract The equilibrium and kinetics of back-extraction (stripping) chloroform extracts obtained with 1-(2-pyridylazo)-2-naphthol 100) or dioctyl sulfide (R2S) were investigated. Replacement prior toKsyk-extraction. The back-extraction equilibria 2SCN -
*
’ Pd(SCN):-
+ 2R,S(o),
with
Kax,
= lO-(‘.‘6
of palladium originally extracted as PdClifrom the (PAN), 7-(4-ethyl-l-methyloctyl)quinolin-8-01 (Kelex of chloride in extracted species by thiocyanate occurs have been described by Pd(SCN),(R,S),($B; * ‘.O’),
and
Pd(SCN)PAN(o)
+ 3SCN-
+ HC
+
*
Pd(SCN)i+ PAN(o), with KBX, = 104.89 * ‘.06. The rate of stripping from Pd-PAN and Pd-Kelex 100 displayed an inverse first-order dependence on the solution pH, a second-order dependence on the thiocyanate concentration and was zero order in both the chloride and the organic phase chelate concentration. More complicated kinetics were observed for palladium stripping in the dioctyl sulfide system. In all systems, the enhancement in stripping rate parallels the size of the “tram effect”. Keywords:
Kinetic
analysis;
Extraction;
Palladium
Separations of ions of the platinum group metals (PGM) would be significantly improved and more effective separation processes could be developed by overcoming the very slow reaction kinetics characteristic of this group. Al-Bazi and Freiser [1,2] have recently taken advantage of phase-transfer catalysis to enhance the slow rates of extraction of these metals. They also reported [3] that the presence of as little as 1.7 X 10e4 M thiocyanate increased the rate of 7-(4-ethyl-lmethyloctyl)quinolin-8-01 (Kelex 100) extraction of palladium by more than 600-fold. Because, in the formation of the extractable species, cleavage of Pd-Cl bonds is required, the catalytic role of thiocyanate is relate to its “truns-effect”, the well known bond-weakening effect that a ligand in a square-planar complex has on the ligand tram to it. These effects greatly facilitate the selectivity of extraction of palladium from PGM mixtures. A 0003-2670/91/$03.50
0 1991 - Elsevier
Science Publishers
complete separation process requires efficient stripping or back-extraction in addition to efficient extraction. In a previous study of mechanisms of stripping involving nickel dithizonates [4], it was found that factors other than simple metal ion-proton exchange on the extracted complexes were significant. This paper is devoted to the examination of the effect of thiocyanate on the back-extraction rate of palladium complexes from acidic solutions.
EXPERIMENTAL
Reagents
Kelex 100 (Sherex Chemicals) was used after purification [3]. 1-(2-Pyridylazo)-2-naphthol (PAN) (Aldrich) and dioctyl sulfide (R2S) (Morton Thiokol) were used as received. The metal B.V.
226
extract solutions were prepared by dissolving an appropriate amount of the corresponding complex in chloroform to give an absorbance of about unity at 675, 309 and 460 nm for the palladium complexes of PAN, R,S and Kelex 100, respectively. All other reagents were of analytical-reagent grade. Chloroform was washed twice with water to remove dissolved ethanol, stored in the dark and used for no longer than 1 week. Distilled, deionized water was used throughout. Preparation of Pd-PAN complex About 0.499 g of PAN was added to a 10% ethanolic solution of 5.0 x lop3 M palladium as PdCl, adjusted to pH 2.50 with hydrochloric acid. The solution was stirred for about 20 h, then left to stand. The Pd-PAN precipitate was filtered, washed several times with a 10% ethanolic solution at pH 2.50 until the washings were PAN-free and then dried at 60°C under reduced pressure. Preparation of Pd- R zS complex A 20-ml portion of 0.10 M R,S in chloroform was equilibrated for 1 day with 100 ml of a 10% ethanolic solution of 5.0 x lop3 M palladium as PdCl, adjusted to pH 2.00 with hydrochloric acid. The organic phase was then separated and shaken several times with hydrochloric acid to remove the dissolved ethanol. The absorption spectrum of the organic phase exhibited an absorption band at 309 nm which corresponds to the PdCl Z(RZS)Z complex. The Pd(SCN),(R,S), complex was prepared by twice equilibrating a 50-ml portion of PdCl,(R,S), in chloroform with an equal volume of 0.010 M thiocyanate solution adjusted to pH 2.60 with sulfuric acid. Complete conversion was confirmed spectrophotometrically by the presence of a corresponding amount of Pd(SCN):in the aqueous phase. Preparation of Pd-Kelex 100 complex A 20-ml aliquot of 0.10 M Kelex lOO(HL) in chloroform was equilibrated for 2 days with 100 ml of 5.0 X lop3 M palladium as PdCl, containing 1.0 x 10e4 M thiocyanate at pH 5.00. The organic phase was then separated and washed twice with distilled water. Its absorption spectrum
S.J. AL-BAZI
showed an corresponds
AND
H. FREISER
absorbance band at 460 nm to the PdL, complex.
which
Apparatus A high-speed stirring apparatus [5] was used to study fast kinetics, and a box-type Eberbach shaker [6] was used for both equilibrium and slow kinetic studies. Varian Car-y 219 and Perkin-Elmer Model 3840 diode-array-based UV-visible spectrophotometers were used for all absorbance measurements. An Orion Research Model 701 A pH meter calibrated with pH 2.00, 4.00 and 7.00 standard buffer solutions was used for pH measurements. Equilibrium procedure A lO.O-ml portion of the aqueous phase containing thiocyanate was shaken in a 50-ml glass vial together with an equal volume of a chloroform solution containing the palladium complex for 48 h to ensure equilibrium. The equilibrium concentration of palladium in the aqueous phase was determined by measuring the absorbance of Pd(SCN)iat 310 nm and that in the organic phase was obtained by difference. Kinetic procedure Each phase was prepared separately in 50-ml volumetric flasks and immersed in a water thermostat at 22” C. After thermal equilibrium, the organic solution was transferred to the reaction flask. Next, the aqueous solution was carefully poured into the flask. The reaction was begun by starting the stirring motor at a rate of 6500 rpm and the rate of decrease in palladium concentration in PdCl(PAN), PdC12(R,S),, Pd(SCN),(R2S)* or Pd(Kelex loo), complexes was followed by measuring the absorbance at 675, 309, 324 and 460 nm, respectively, at l-s intervals. For systems exhibiting slow kinetics, a series of vials containing 10.0 ml each of aqueous and organic phases were shaken with a box-type Eberbach shaker at a shaking speed of 280 oscillaAfter shaking began, vials were retion min-l. moved at predetermined time intervals and the absorbance of Pd(SCN)zin the aqueous phase was measured.
BACK-EXTRACTION
KINETICS
AND
221
MECHANISM
All studies were made under conditions of pseudo-first order with respect to the palladium complex. For reactions that proceeded essentially to completion, pseudo-first-order forward rate constants, kobs, were simply determined from
Pd(SCN),(R,S),(o) Pd(SCN);-
In A/A, = kobst where t is the sampling time, and Ai and corresponds to the absorbance of the complex initial time and time t, respectively. When reaction equilibrium constant was less than rate constants were obtained from ln(Ai-A,)/(A,--%)
ciently close to 2 that the back-extraction described by
(1) A, at the 10,
+ 2SCN-
can be
KBX, G=
+ 2R,S(o)
(3)
where K,, represents the back-extraction constant and he o refers to species in the organic phase. The corresponding equilibrium expression is: K Bx, = D[R,S]f/[SCN-]2
=kobst
= 10-(1.16*0.05)
(4)
(2) where A, is the organic phase absorbance at equilibrium. A, was obtained by allowing the reaction to reach equilibrium on a shaker.
It should be noted that the back-extraction reaction (Pqn. 3) must be preceded by an exchange reaction involving the starting material PdCl,(R,S),, the form in which palladium was extracted:
RESULTS AND DISCUSSION
PdCl,(R,S),(o)
+ 2SCN- 2
Pd(SCN),(R,S),(o)
Palladium
back-extraction equilibrium Plots of log D, vs. log [R,S], and log [SCN-]
+ 2C1-
Otherwise, a fourth-order dependence on thiocyanate and an inverse second-order dependence on dioctyl sulfide concentrations would have been
were linear with slopes of - 1.98 + 0.04 and 1.97 k 0.04, respectively (Table 1). These are suffiTABLE 1 Equilibrium data for back-extraction of palladium from the Pd-R,S Dependence on [R2S],:
[Pd], = 6.4 X 10m5 M, [Cl-] = 0.9 M, [SCN-]
[R2S] x lo2 (M) 0.5 1.0 1.5 2.0 3.0 4.0 5.0
complex = 0.1 M, pH = 2.58:
Log D,
-Log
1.38 0.83 0.41 0.14 -0.14 -0.39 - 0.60
1.22 1.17 1.22 1.26 1.19 1.19 1.20
K,x
Slope of log Dpd vs. log [R2S10 = - 1.98 f 0.04 Dependence on [SCN]: [Pd], = 6.4
X
10m5 M, [R2S10 = 1
[SCN-1(M) 0.02 0.03 0.04 0.06 0.08 0.10 0.15 0.20 Slope of log D,
vs. log [SCN-]
= 1.97 f 0.04
X
10e2
(5)
M, pH = 2.55, ionic strength = 1.0 with NaCI:
ha D,
- Loa KBX
-0.50 -0.12 0.06 0.45 0.69 0.83 1.20 1.15
1.10 1.07 1.15 1.11 1.12 1.17 1.15 1.09
228
S.J. AL-BAZI
observed. The results obtained signify that such an exchange would be achieved at thiocyanate concentrations far lower than those used here for stripping (minimum [SCN-] = 0.020 M). This was confirmed by equilibrating a chloroform solution of PdCl,(R,S), for 1 h with an equal volume of an aqueous phase containing different concentra-
AND
H. FREISER
tions of thiocyanate and chloride salts (Fig. 1). Solutions containing as little as 10e3 M thiocyanate and as much as 0.5 or 0.9 M chloride showed an absorbance band at 304 nm characteristic of PdCl(SCN)(R,S),, while the presence of 0.010 M thiocyanate and 0.90 M chloride or lop3 M thiocyanate in the absence of chloride showed
TABLE 2 Equilibrium data for back-extraction of palladium from the Pd-PAN Dependence on [PAN],: [Pd], = 5.6
X
10F5 M, [Cl-] = 0.9 M, [SCN-]
complex = 0.1 M, [NaOAc] = 0.01 M, pH = 4.10:
[PAN] x 10’ (M)
Log &i
log KBX
10 8 5 3 2 1 Slope of log D,
-0.11 - 0.02 0.16 0.37 0.50 0.75
4.99 4.98 4.96 4.95 4.90 4.85
vs. log [PAN] = -0.84
Dependence on [SCN-1: [Pd], = 5.6
X
f 0.03 lo-’
M, [PAN], = 1
X
10V3 M, NaOAc = 0.01 M, pH = 4.05, ionic strength = 1.0 with NaCl:
[SCN- 1(W
LOaD,
LOa KBX
0.04 0.05 0.06 0.07 0.08 0.09 0.10 Slope of log D,
-0.37 -0.04 0.18 0.37 0.55 0.72 0.83
4.87 4.91 4.90 4.89 4.89 4.91 4.88
vs. log [SCN-]
Dependence on pH: [Pd], = 5.6
= 3.01 * 0.05 X
10m5 M, [SCN-]
= 0.1 M, [Cl-] = 0.9 M, [PAN], = 1 X 10m3 M, NaOAc = 0.01 M:
PH
Log D,
Log KBX
3.75 4.04 4.38 4.68 4.89 5.23 5.37 Slope of log D,,
1.01 0.75 0.50 0.16 - 0.04 - 0.35 - 0.43
4.76 4.79 4.88 4.84 4.85 4.88 4.94
vs. pH -0.91
Dependence on [Cl-]:
f 0.02
[Pd], = 5.6
x
10m5 M, [SCN-]
= 0.05 M, [PAN] = 1 X 10m3 M, NaOAc = 0.01 M, pH = 4.06,
ionic strength = 1.05 with NaClO,:
[Cl - 1(M)
L“g D,dd
L“g KBX
0.1 0.2 0.3 0.5 0.7 1.0 Slope of log D,
0.02 0.00 0.00 - 0.02 - 0.04 - 0.04
4.98 4.96 4.96 4.94 4.92 4.92
vs. log [Cl-] = 0.01 + 0.01
BACK-EXTRACTION
KINETICS
AND
229
MECHANISM
0.01 f 0.01 and -0.91 f 0.02, respectively (Table 2). From these, the following back-extraction stoichiometry and equilibrium for the PAN system were deduced:
0.6
i!i s
0.4
n L
Pd(SCN)L(o)
+ 3SCN-+
H+
KBX, =
: 2
Pd(SCN);-
0.2
+ HL(o) [ Pd(SCN):]
K
(8) [HLlo
BX2= [Pd(SCN)L]0[SCN-]3[H+] 300
350
Wavelength
No.:
[SCN-j(M): [Cl- ] (M):
an absorbance
1
2
3
4
5
6
0.0 0.0
1O-4 0.9
10K3 0.9
lo-* 0.9
1O-3 0.5
1O-3 0.0
band
at 322 nm characteristic
of
Pd(SCN),(R,S),. The exchange reaction (Eqn. 5) constant can be evaluated because it is the composite of the R,S extraction of PdCli(Eqn. 6) the conversion of PdCl$- to Pd(SCN):(Eqn. 7) and the present stripping equilibrium (Eqn. 3): PdC12,- + 2R,S(o)
~~‘PdCIZ(R&(o)
+ 2Cl(6)
PdC12,- + 4SCN-
2 Pd(SCN);-
+ 4c1-
(7)
5) which is seen to by using the values of & = 10’7.5s [7], K,, = 106.06 PI and K,, = lo-‘.‘6 as K, = 10’2.65. Hence, at equilibrium, ‘the presence of as little as 2 x lop4 M thiocyanate in the aqueous solution would be sufficient for the complete conversion of PdCl,Hence,
K, of exchange
(Eqn.
be equal to P4/(KEXzKBX,).can be evaluated
(R,S),
to
(8a)
[SCN-13[H+]
Fig. 1. Absorbance spectra of PdCl,(R,S)2 complex in CHCl3 at different thiocyanate and chloride concentrations. [Pd], = 6.4~ 10V5 M, [R*S], = 1 x 10K2 M, pH = 2.43, extraction time 80 min. Solution
D[HLlo
=
(nm)
WSCN),(R,S),.
Similarly, when log D values obtained in palladium back-extraction from the Pd-PAN complex were plotted against log [PAN],, log [SCN-1, log [Cl-] and pH, linear relationships were obtained with slopes of -0.84 f 0.03, 3.01 f 0.05,
for which a value of KBX, = 104s9 * ‘.06 was calcuof palladium from PAN lated. The higher K,, compared with that calculated for the R,S system, 10-1.‘6, is strongly affected by the proton affinity of the PAN anion (pK,, = 11.2) [7], a competing factor that does not come into play in the case with R,S. Palladium back-extraction kinetics The effect of thiocyanate in the concentration range 0.04-0.2 M on the rate of palladium stripping from the organic phase containing 2.4 X 10e4 M PAN, when plotted as ln(Ai - A,)/(A, - A,) vs. time, showed straight lines passing through the origin, confirming that the extraction rate is first order with respect to the metal. A logarithmic plot of the observed rate constants against the thiocyanate concentration is linear with a slope of 1.83 f 0.05 (Table 3). Similarly, the values of the observed rate constants were found to have an almost inverse firstorder dependence (- 0.73 + 0.01) on solution pH but were independent of chloride (0.01 f 0.01) and PAN (0.01 f 0.01) concentrations (Table 3). A back-extraction mechanism consistent with these results can be represented as follows: (1) Distribution of the metal complex between the organic and the aqueous phase: PdCl(L)(o)
“3
PdCl( L)
fast
where HL represents PAN. (2) Protonation of the phenolic PdCl(L)
+ H+ ‘5
PdCI(HL)
+
(9)
oxygen of PAN: fast
00)
S.J. AL-BAZI
230
AND
H. FREISER
TABLE 3 Kinetic data for back-extraction of palladium from the Pd-PAN Dependence on [SCN-1: [Pd], = 5.6 ionic strenath = 1.3’M with NaCl:
WN-I
X
10T5 M, [PAN], = 3
X
Complex
10e4 M, [NaOAc] = 0.1 M, pH = 4.24,
M
- Log kobs
0.02 0.04 0.06 0.10 0.14 0.20 Slope of log kobs vs. [SCN-]
3.59 3.28 3.00 2.56 2.21 2.03 in thiocyanate range 0.04-0.20 M = 1.83 * 0.05
Dependence on pH: [Pd], = 5.6
X
10m5 M, [SCN-]
= 0.2 M, [Cl-] = 1.0 M, [PAN], = 1 X lo-’
PH
- Log kobs
3.15 3.60 4.25 4.59 4.82 5.15 Slope of log Kobs vs. pH = - 0.73 f 0.01
1.26 1.62 2.05 2.30 2.50 2.13
Dependence on [Cl-]: [Pd], = 5.6 ionic strength = 1.3 with Na,SO,:
X
lop5 M, [SCN-]
= 0.2 M, [PAN], = 1 X 1O-3 M, [NaOAc] = 0.1 M, pH = 4.25,
ICI-1 04
- L‘S kobs
0.1 0.2 0.3 0.5 0.7 1.0
2.05 2.06 2.04 2.05 2.05 2.04
Slope of log kobs vs. [cl-]
= 0.01 + 0.01
Dependence on [PAN],,: [Pd], = 5.6 [PAN]
M, NaOAc = 0.1 M:
x
X
10F5 M, [Cl-] = 1.0 M, [SCN-]
lo3 (M)
= 0.2 M, [NaOAc] = 0.1 M, pH = 4.61: - Log kot,s
0.1 0.2 0.4 0.6 0.8 1.0
2.40 2.38 2.40 2.40 2.40 2.32
Slope of log kobs vs. log [PAN] = 0.01 f 0.01
This protonation either weakens or breaks the Pd-0 bond, facilitating the formation of a PdSCN bond, i.e.,
gen) bonds because of its “trunk effect”, and therefore enhances the rate of addition of the second thiocyanate ion:
PdCl(HL)+
PdCl( SCN)HL
+ SCN-
2 PdCl( SCN)HL
fast (11)
(3) The thiocyanate ion in PdCl(SCN)HL can weaken one of the Pd-N (pyridine or azo nitro-
+ SCN - k,\
PdCl(SCN),HL(4)
The
remaining
slow Pd-N
(12) bond
in
the
BACK-EXTRACTION
KINETICS
AND
231
MECHANISM
PdCl(SCN)2HL is expected to be very weak and easily replaced with the thiocyanate ion:
which, by suitable substitution, - d[Pd]/dt
PdC1(SCN)2HL-+
= [K&,/K&,]
SCN- 3 x [PdClL],[H]+
PdCl(SCN);-
+ HL
fast
Pd(SCN):-
fast
(14)
The enhancement in back-extraction kinetics due to the truns effect was confirmed by the effects on the extraction rates when other trunslabilizing reagents such as thiourea (TU) and bromide were studied. The enhancement observed, TU > SCN- X- Br-, decreased in the same order as their labilizing “tram effect” [9-111. The rate-determining step can be described by -d[Pd]/dt
= k,[PdCl(SCN)HL][SCN-]
TABLE
4
Kinetic
data for back-extraction
Dependence [SCN -
on [SCN-1:
[Pd],
of palladium = 8.9
X
(15)
from the Pd-Kelex
10m5 M, [HL],
= 4
X
Dependence
100 complex
10m4 M, pH = 2.90, ionic strength
1(W
= 1.0 M with NaCI:
- Log hbs
0.2 0.4 0.5 0.6 0.8 1.0 Slope of Log k,,
vs. [SCN-]
on pH: [Pd],
in the thiocyanate
= 8.9
X
(16)
where K,’ is the acid dissociation constant of PdCl(HL)+, K, the formation constant of PdCl(SCN)(PAN) and K,, the distribution constant of the PdCl(PAN) complex. The effect of thiocyanate on the rate of palladium back-extraction from a chloroform solution of the Pd-Kelex 100 chelate containing excess of Kelex 100 was seen to have a first-order dependence on the metal concentration. From a series of experiments at different pH and [SCN-1, the observed rate constants showed a second-order (1.94 f 0.06) dependence on the thiocyanate concentration and an inverse first-order (- 1.09 f 0.03) dependence on the solution pH (Table 4). These results are consonant with those obtained for palladium back-extraction from the Pd-PAN complex, therefore indicating a rapid pre-equilibrium protonation of the phenolic oxygen in Kelex 100 followed by the addition of two thio-
+ SCN- KI, + Cl-
[SCN-I2
(13)
(5) Both the truns and cis effects of the thiocyanate weaken the Pd-Cl bond and therefore accelerate the addition of the fourth thiocyanate ion: PdCl(SCN):-
becomes:
range 0.4-1.0
10V5 M, [SCN-]
PH 1.91 2.16 2.40 2.62 2.88 3.08 Slope of log kobs vs.pH = - 1.09 rt 0.03
4.01 3.71 3.50 3.33 3.10 2.94 M = 1.94 f 0.06
= 0.2 M, [Cl-]
= 1.0 M, [HL], - Log kobs 2.85 3.13 3.38 3.60 3.95 4.11
= 4
X
10m4 M:
S.J. AL-BAZI
232
ions back-extraction
to the palladium mechanism, i.e.,
cyanate
PdL,(o)
“FPdL,
complex
the
(17) +
fast
PdL( HL) + + SCN - 2 PdL(SCN)HL PdL( SCN)HL
(18) fast
(19)
+ SCN - k,\ IO
PdL( SCN)*HL PdL(SCN),HL-+ Pd(SCN)i-
-
slow 2SCN-+
+ 2HL
fast
20
30
40
Time(min)
(21)
By assuming that the reaction in Eqn. 20 is the rate-determining step and incorporating the effect of the rapid pre-equilibrium in Eqn. 19, then -d[Pd]/dt
H. FREISER
2.5
fast
PdL, + H+ “z’PdL(HL)
in
AND
Fig. 2. Effect of [SCN- ] on rate of palladium back-extraction. [Pd], = 6.4 X 10K5 M, [R2S],, = 4 x 10m4 M, ionic strength = 1.0 M with NaCl, pH = 2.58. Using high-speed stirring: [SCN-] = (1) 1.0, (2) 0.6, (3) 0.4 and (4) 0.3 M. Using Eberbath shaker: [SCN- ] = (a) 0.01, (b) 0.03, (c) 0.05, (d) 0.07 and (e) 0.1 M.
= (K,k,/K,“K,,)[PdL,],[H+] x [SCN-I2
(22)
It is interesting to observe that back-extraction rates of Pd from Pd-PAN are faster than from Pd-Kelex 100 (Tables 3 and 4) (an 800-fold higher rate from the former in solutions containing 0.20 M thiocyanate and at pH 3.0). This is probably due to the smaller K,, of Pd-PAN, which has one hydrophobic ligand molecule whereas the Pd-Kelex 100 complex has two. The kinetics of palladium back-extraction from PdC12(R2S)2 were found to be complex. At thiocyanate concentrations < 0.3 M, the stripping rate increases slowly at first but more sharply at longer times (Fig. 2). On the other hand, at higher thiocyanate concentrations, a linear enhancement in the stripping rate was observed. This may indicate that two consecutive reactions are involved in the rate-determining step. Under similar conditions of thiocyanate (0.1 M), chloride (0.9 M) and solution pH (2.58), the rate of back-extraction of palladium from Pd = 15 s) was linear and much (SCN),(R), (h/2 faster from PdCl,(R,S), (t,,, = 500 s). This indicates that substitution of chloride rather than dioctyl sulfide in PdC12(R2S)2 is involved in the rate-determining step. is a square-planar complex Pd(SCN),(R,S),
that exists mainly in the tram form. In the cis form, the Pd-SR, bond is greatly weakened by the “tram effect” of the thiocyanate ion. As cistruns isomerization is considered to be very fast [12], the “tram effect” of thiocyanate can be the result of fast substitution of R,S by thiocyanate ion. In the presence of 0.2 M thiocyanate, changes in solution pH in the range 1.59-2.90 had no effect on the rate of palladium back-extraction from the PdCl,(R,S), complex. In the light of the findings in the previous systems where, in contrast to R,S, the ligands PAN and Kelex 100 had measurable proton affinity, and were loosened in the chelate by protonation, it was expected that an absence of pH dependence would be observed in the dioctyl sulfide system. Therefore, the stripping mechanism can be explained by the schematic diagram in Fig. 3. Although the replacement of chloride in PdC12(R2S), with thiocyanate is expected to be rapid, the slow transformation to PdCl(SCN) (R2S)2 and Pd(SCN)2(R2S)2 results from the low aqueous concentration of PdCl 2(R2S)2 or PdCl (SCN)(R,S), as a result of their high distribution constants. Further, the replacement of chloride with thiocyanate in PdC12(R2S), would be slower than that in PdCl(SCN)(R,S), as a result of the
BACK-EXTRACTION
KINETICS
AND
MECHANISM
233
0
PdCl
2
(R
2
S)
I1 PdC12(R2S;G A
2
PdClKiC,N)(R2S)2
” PdCI(SCN)(R2S)2~
2R2S
SCN-
+ Cl-
Fig. 3. Schematic diagram of stripping mechanism.
Pd(SCN)
2
(R
2
s)
LSCN2zGi-
Pd(SCN)
24
” + 2R2S
+
Cl0 = Organic; A = aqueous.
“trans effect” of thiocyanate. This may result in a sharper increase in the stripping rate at longer times (Fig. 2). The contribution of the “tram effect” in labilizing the leaving chloride group was confirmed by the effect of 0.010 M thiourea, thiocyanate and bromide present in aqueous solutions at pH 2.72 on the rate of stripping of palladium from 6.4 X 10e5 M PdCl,(R,S),, which decreased in the order thiourea x==thiocyanate > bromide. According to the proposed mechanism, the stripping rate can be further improved if the reaction of thiocyanate with PdCl,(R,S), occurs in the organic phase where a high concentration of the complex is present. This was accomplished by shaking 6.4 X 10F5 M of the metal complex in chloroform containing 1 x lop5 M tetraheptylammonium thiocyanate with an equal volume of an aqueous solution containing 0.02 M thiocyanate and 0.01 M chloride at pH 2.61. In the absence of Q+,SCN-, the plot of In &/A, vs. time showed a small enhancement in the extraction rate at the beginning of stripping (log kobs = - 3.24), followed by a sharper increase (log kobs = - 3.05) at longer times. In the presence of Q+,SCN-, however, only one straight line passing through the origin and with a higher stripping rate (log kobs = - 2.21) was observed. These results may also indicate that replacement of the first chloride in PdCl,(R,S), with thiocyanate is the rate-determining step. In conclusion, the presence of trans-labilizing reagents in the aqueous or organic phase can play an important role in catalyzing the stripping rates. This, in combination with the protonation of the oxygen in phenolic chelates, can be considered as
a novel method for stripping metals from organic solutions.
This research the Engineering Foundation.
of platinum
group
was supported by a grant from Division of the National Science
REFERENCES 1 S.J. AI-Bazi and H. Freiser, Solvent Extraction Ion Exchange, 5 (1987) 265. 2 S.J. Al-Bazi and H. Freiser, Solvent Extraction Ion Exchange, 5 (1987) 997. 3 S.J. Al-Bazi and H. Freiser, Solvent Extraction Ion Exchange, 4 (1986) 1121. 4 K. Ohashi and H. Freiser, Anal. Chem., 52 (1980) 2214. 5 H. Watarai, L. Cunningham and H. Freiser, Anal. Chem., 54 (1982) 2390; B.E. McClellan and H. Freiser, Anal. Chem., 36 (1964) 2262. 6 S.J. Al-Bazi and H. Freiser, Solvent Extraction Ion Exchange, 5 (1987) 265. 7 D. Betteridge, Q. Fernando and H. Freiser, Anal. Chem., 35 (1963) 294. 8 L.G. Sillen and A.E. Martell, Stability Constants of Metal Ion Complexes, Special Publication No. 17, Chemical Society, London, 1964. 9 C.H. Langford and H.B. Gray, Ligand Substitution Processes, Benjamin, New York, 1974. 10 A. Peloso, Coord. Chem. Rev., 10 (1973) 123. 11 L.I. Elding, Inorg. Chim. Acta, 28 (1978) 225. 12 F. Basolo and R.G. Pearson, Mechanisms of Inorganic Reactions: a Study of Metal Complexes in Solution, Wiley, New York, 1967.
225
Kinetics and mechanism of back-extraction: selected palladium extraction systems S.J. Al-Bazi and Henry Freiser * Strategic Metals Recovery Research Facility, Department
of Chemistry,
(Received
6th August
University of Arizona,
Tucson, AZ 85721 (U.S.A.)
1990)
Abstract The equilibrium and kinetics of back-extraction (stripping) chloroform extracts obtained with 1-(2-pyridylazo)-2-naphthol 100) or dioctyl sulfide (R2S) were investigated. Replacement prior toKsyk-extraction. The back-extraction equilibria 2SCN -
*
’ Pd(SCN):-
+ 2R,S(o),
with
Kax,
= lO-(‘.‘6
of palladium originally extracted as PdClifrom the (PAN), 7-(4-ethyl-l-methyloctyl)quinolin-8-01 (Kelex of chloride in extracted species by thiocyanate occurs have been described by Pd(SCN),(R,S),($B; * ‘.O’),
and
Pd(SCN)PAN(o)
+ 3SCN-
+ HC
+
*
Pd(SCN)i+ PAN(o), with KBX, = 104.89 * ‘.06. The rate of stripping from Pd-PAN and Pd-Kelex 100 displayed an inverse first-order dependence on the solution pH, a second-order dependence on the thiocyanate concentration and was zero order in both the chloride and the organic phase chelate concentration. More complicated kinetics were observed for palladium stripping in the dioctyl sulfide system. In all systems, the enhancement in stripping rate parallels the size of the “tram effect”. Keywords:
Kinetic
analysis;
Extraction;
Palladium
Separations of ions of the platinum group metals (PGM) would be significantly improved and more effective separation processes could be developed by overcoming the very slow reaction kinetics characteristic of this group. Al-Bazi and Freiser [1,2] have recently taken advantage of phase-transfer catalysis to enhance the slow rates of extraction of these metals. They also reported [3] that the presence of as little as 1.7 X 10e4 M thiocyanate increased the rate of 7-(4-ethyl-lmethyloctyl)quinolin-8-01 (Kelex 100) extraction of palladium by more than 600-fold. Because, in the formation of the extractable species, cleavage of Pd-Cl bonds is required, the catalytic role of thiocyanate is relate to its “truns-effect”, the well known bond-weakening effect that a ligand in a square-planar complex has on the ligand tram to it. These effects greatly facilitate the selectivity of extraction of palladium from PGM mixtures. A 0003-2670/91/$03.50
0 1991 - Elsevier
Science Publishers
complete separation process requires efficient stripping or back-extraction in addition to efficient extraction. In a previous study of mechanisms of stripping involving nickel dithizonates [4], it was found that factors other than simple metal ion-proton exchange on the extracted complexes were significant. This paper is devoted to the examination of the effect of thiocyanate on the back-extraction rate of palladium complexes from acidic solutions.
EXPERIMENTAL
Reagents
Kelex 100 (Sherex Chemicals) was used after purification [3]. 1-(2-Pyridylazo)-2-naphthol (PAN) (Aldrich) and dioctyl sulfide (R2S) (Morton Thiokol) were used as received. The metal B.V.
226
extract solutions were prepared by dissolving an appropriate amount of the corresponding complex in chloroform to give an absorbance of about unity at 675, 309 and 460 nm for the palladium complexes of PAN, R,S and Kelex 100, respectively. All other reagents were of analytical-reagent grade. Chloroform was washed twice with water to remove dissolved ethanol, stored in the dark and used for no longer than 1 week. Distilled, deionized water was used throughout. Preparation of Pd-PAN complex About 0.499 g of PAN was added to a 10% ethanolic solution of 5.0 x lop3 M palladium as PdCl, adjusted to pH 2.50 with hydrochloric acid. The solution was stirred for about 20 h, then left to stand. The Pd-PAN precipitate was filtered, washed several times with a 10% ethanolic solution at pH 2.50 until the washings were PAN-free and then dried at 60°C under reduced pressure. Preparation of Pd- R zS complex A 20-ml portion of 0.10 M R,S in chloroform was equilibrated for 1 day with 100 ml of a 10% ethanolic solution of 5.0 x lop3 M palladium as PdCl, adjusted to pH 2.00 with hydrochloric acid. The organic phase was then separated and shaken several times with hydrochloric acid to remove the dissolved ethanol. The absorption spectrum of the organic phase exhibited an absorption band at 309 nm which corresponds to the PdCl Z(RZS)Z complex. The Pd(SCN),(R,S), complex was prepared by twice equilibrating a 50-ml portion of PdCl,(R,S), in chloroform with an equal volume of 0.010 M thiocyanate solution adjusted to pH 2.60 with sulfuric acid. Complete conversion was confirmed spectrophotometrically by the presence of a corresponding amount of Pd(SCN):in the aqueous phase. Preparation of Pd-Kelex 100 complex A 20-ml aliquot of 0.10 M Kelex lOO(HL) in chloroform was equilibrated for 2 days with 100 ml of 5.0 X lop3 M palladium as PdCl, containing 1.0 x 10e4 M thiocyanate at pH 5.00. The organic phase was then separated and washed twice with distilled water. Its absorption spectrum
S.J. AL-BAZI
showed an corresponds
AND
H. FREISER
absorbance band at 460 nm to the PdL, complex.
which
Apparatus A high-speed stirring apparatus [5] was used to study fast kinetics, and a box-type Eberbach shaker [6] was used for both equilibrium and slow kinetic studies. Varian Car-y 219 and Perkin-Elmer Model 3840 diode-array-based UV-visible spectrophotometers were used for all absorbance measurements. An Orion Research Model 701 A pH meter calibrated with pH 2.00, 4.00 and 7.00 standard buffer solutions was used for pH measurements. Equilibrium procedure A lO.O-ml portion of the aqueous phase containing thiocyanate was shaken in a 50-ml glass vial together with an equal volume of a chloroform solution containing the palladium complex for 48 h to ensure equilibrium. The equilibrium concentration of palladium in the aqueous phase was determined by measuring the absorbance of Pd(SCN)iat 310 nm and that in the organic phase was obtained by difference. Kinetic procedure Each phase was prepared separately in 50-ml volumetric flasks and immersed in a water thermostat at 22” C. After thermal equilibrium, the organic solution was transferred to the reaction flask. Next, the aqueous solution was carefully poured into the flask. The reaction was begun by starting the stirring motor at a rate of 6500 rpm and the rate of decrease in palladium concentration in PdCl(PAN), PdC12(R,S),, Pd(SCN),(R2S)* or Pd(Kelex loo), complexes was followed by measuring the absorbance at 675, 309, 324 and 460 nm, respectively, at l-s intervals. For systems exhibiting slow kinetics, a series of vials containing 10.0 ml each of aqueous and organic phases were shaken with a box-type Eberbach shaker at a shaking speed of 280 oscillaAfter shaking began, vials were retion min-l. moved at predetermined time intervals and the absorbance of Pd(SCN)zin the aqueous phase was measured.
BACK-EXTRACTION
KINETICS
AND
221
MECHANISM
All studies were made under conditions of pseudo-first order with respect to the palladium complex. For reactions that proceeded essentially to completion, pseudo-first-order forward rate constants, kobs, were simply determined from
Pd(SCN),(R,S),(o) Pd(SCN);-
In A/A, = kobst where t is the sampling time, and Ai and corresponds to the absorbance of the complex initial time and time t, respectively. When reaction equilibrium constant was less than rate constants were obtained from ln(Ai-A,)/(A,--%)
ciently close to 2 that the back-extraction described by
(1) A, at the 10,
+ 2SCN-
can be
KBX, G=
+ 2R,S(o)
(3)
where K,, represents the back-extraction constant and he o refers to species in the organic phase. The corresponding equilibrium expression is: K Bx, = D[R,S]f/[SCN-]2
=kobst
= 10-(1.16*0.05)
(4)
(2) where A, is the organic phase absorbance at equilibrium. A, was obtained by allowing the reaction to reach equilibrium on a shaker.
It should be noted that the back-extraction reaction (Pqn. 3) must be preceded by an exchange reaction involving the starting material PdCl,(R,S),, the form in which palladium was extracted:
RESULTS AND DISCUSSION
PdCl,(R,S),(o)
+ 2SCN- 2
Pd(SCN),(R,S),(o)
Palladium
back-extraction equilibrium Plots of log D, vs. log [R,S], and log [SCN-]
+ 2C1-
Otherwise, a fourth-order dependence on thiocyanate and an inverse second-order dependence on dioctyl sulfide concentrations would have been
were linear with slopes of - 1.98 + 0.04 and 1.97 k 0.04, respectively (Table 1). These are suffiTABLE 1 Equilibrium data for back-extraction of palladium from the Pd-R,S Dependence on [R2S],:
[Pd], = 6.4 X 10m5 M, [Cl-] = 0.9 M, [SCN-]
[R2S] x lo2 (M) 0.5 1.0 1.5 2.0 3.0 4.0 5.0
complex = 0.1 M, pH = 2.58:
Log D,
-Log
1.38 0.83 0.41 0.14 -0.14 -0.39 - 0.60
1.22 1.17 1.22 1.26 1.19 1.19 1.20
K,x
Slope of log Dpd vs. log [R2S10 = - 1.98 f 0.04 Dependence on [SCN]: [Pd], = 6.4
X
10m5 M, [R2S10 = 1
[SCN-1(M) 0.02 0.03 0.04 0.06 0.08 0.10 0.15 0.20 Slope of log D,
vs. log [SCN-]
= 1.97 f 0.04
X
10e2
(5)
M, pH = 2.55, ionic strength = 1.0 with NaCI:
ha D,
- Loa KBX
-0.50 -0.12 0.06 0.45 0.69 0.83 1.20 1.15
1.10 1.07 1.15 1.11 1.12 1.17 1.15 1.09
228
S.J. AL-BAZI
observed. The results obtained signify that such an exchange would be achieved at thiocyanate concentrations far lower than those used here for stripping (minimum [SCN-] = 0.020 M). This was confirmed by equilibrating a chloroform solution of PdCl,(R,S), for 1 h with an equal volume of an aqueous phase containing different concentra-
AND
H. FREISER
tions of thiocyanate and chloride salts (Fig. 1). Solutions containing as little as 10e3 M thiocyanate and as much as 0.5 or 0.9 M chloride showed an absorbance band at 304 nm characteristic of PdCl(SCN)(R,S),, while the presence of 0.010 M thiocyanate and 0.90 M chloride or lop3 M thiocyanate in the absence of chloride showed
TABLE 2 Equilibrium data for back-extraction of palladium from the Pd-PAN Dependence on [PAN],: [Pd], = 5.6
X
10F5 M, [Cl-] = 0.9 M, [SCN-]
complex = 0.1 M, [NaOAc] = 0.01 M, pH = 4.10:
[PAN] x 10’ (M)
Log &i
log KBX
10 8 5 3 2 1 Slope of log D,
-0.11 - 0.02 0.16 0.37 0.50 0.75
4.99 4.98 4.96 4.95 4.90 4.85
vs. log [PAN] = -0.84
Dependence on [SCN-1: [Pd], = 5.6
X
f 0.03 lo-’
M, [PAN], = 1
X
10V3 M, NaOAc = 0.01 M, pH = 4.05, ionic strength = 1.0 with NaCl:
[SCN- 1(W
LOaD,
LOa KBX
0.04 0.05 0.06 0.07 0.08 0.09 0.10 Slope of log D,
-0.37 -0.04 0.18 0.37 0.55 0.72 0.83
4.87 4.91 4.90 4.89 4.89 4.91 4.88
vs. log [SCN-]
Dependence on pH: [Pd], = 5.6
= 3.01 * 0.05 X
10m5 M, [SCN-]
= 0.1 M, [Cl-] = 0.9 M, [PAN], = 1 X 10m3 M, NaOAc = 0.01 M:
PH
Log D,
Log KBX
3.75 4.04 4.38 4.68 4.89 5.23 5.37 Slope of log D,,
1.01 0.75 0.50 0.16 - 0.04 - 0.35 - 0.43
4.76 4.79 4.88 4.84 4.85 4.88 4.94
vs. pH -0.91
Dependence on [Cl-]:
f 0.02
[Pd], = 5.6
x
10m5 M, [SCN-]
= 0.05 M, [PAN] = 1 X 10m3 M, NaOAc = 0.01 M, pH = 4.06,
ionic strength = 1.05 with NaClO,:
[Cl - 1(M)
L“g D,dd
L“g KBX
0.1 0.2 0.3 0.5 0.7 1.0 Slope of log D,
0.02 0.00 0.00 - 0.02 - 0.04 - 0.04
4.98 4.96 4.96 4.94 4.92 4.92
vs. log [Cl-] = 0.01 + 0.01
BACK-EXTRACTION
KINETICS
AND
229
MECHANISM
0.01 f 0.01 and -0.91 f 0.02, respectively (Table 2). From these, the following back-extraction stoichiometry and equilibrium for the PAN system were deduced:
0.6
i!i s
0.4
n L
Pd(SCN)L(o)
+ 3SCN-+
H+
KBX, =
: 2
Pd(SCN);-
0.2
+ HL(o) [ Pd(SCN):]
K
(8) [HLlo
BX2= [Pd(SCN)L]0[SCN-]3[H+] 300
350
Wavelength
No.:
[SCN-j(M): [Cl- ] (M):
an absorbance
1
2
3
4
5
6
0.0 0.0
1O-4 0.9
10K3 0.9
lo-* 0.9
1O-3 0.5
1O-3 0.0
band
at 322 nm characteristic
of
Pd(SCN),(R,S),. The exchange reaction (Eqn. 5) constant can be evaluated because it is the composite of the R,S extraction of PdCli(Eqn. 6) the conversion of PdCl$- to Pd(SCN):(Eqn. 7) and the present stripping equilibrium (Eqn. 3): PdC12,- + 2R,S(o)
~~‘PdCIZ(R&(o)
+ 2Cl(6)
PdC12,- + 4SCN-
2 Pd(SCN);-
+ 4c1-
(7)
5) which is seen to by using the values of & = 10’7.5s [7], K,, = 106.06 PI and K,, = lo-‘.‘6 as K, = 10’2.65. Hence, at equilibrium, ‘the presence of as little as 2 x lop4 M thiocyanate in the aqueous solution would be sufficient for the complete conversion of PdCl,Hence,
K, of exchange
(Eqn.
be equal to P4/(KEXzKBX,).can be evaluated
(R,S),
to
(8a)
[SCN-13[H+]
Fig. 1. Absorbance spectra of PdCl,(R,S)2 complex in CHCl3 at different thiocyanate and chloride concentrations. [Pd], = 6.4~ 10V5 M, [R*S], = 1 x 10K2 M, pH = 2.43, extraction time 80 min. Solution
D[HLlo
=
(nm)
WSCN),(R,S),.
Similarly, when log D values obtained in palladium back-extraction from the Pd-PAN complex were plotted against log [PAN],, log [SCN-1, log [Cl-] and pH, linear relationships were obtained with slopes of -0.84 f 0.03, 3.01 f 0.05,
for which a value of KBX, = 104s9 * ‘.06 was calcuof palladium from PAN lated. The higher K,, compared with that calculated for the R,S system, 10-1.‘6, is strongly affected by the proton affinity of the PAN anion (pK,, = 11.2) [7], a competing factor that does not come into play in the case with R,S. Palladium back-extraction kinetics The effect of thiocyanate in the concentration range 0.04-0.2 M on the rate of palladium stripping from the organic phase containing 2.4 X 10e4 M PAN, when plotted as ln(Ai - A,)/(A, - A,) vs. time, showed straight lines passing through the origin, confirming that the extraction rate is first order with respect to the metal. A logarithmic plot of the observed rate constants against the thiocyanate concentration is linear with a slope of 1.83 f 0.05 (Table 3). Similarly, the values of the observed rate constants were found to have an almost inverse firstorder dependence (- 0.73 + 0.01) on solution pH but were independent of chloride (0.01 f 0.01) and PAN (0.01 f 0.01) concentrations (Table 3). A back-extraction mechanism consistent with these results can be represented as follows: (1) Distribution of the metal complex between the organic and the aqueous phase: PdCl(L)(o)
“3
PdCl( L)
fast
where HL represents PAN. (2) Protonation of the phenolic PdCl(L)
+ H+ ‘5
PdCI(HL)
+
(9)
oxygen of PAN: fast
00)
S.J. AL-BAZI
230
AND
H. FREISER
TABLE 3 Kinetic data for back-extraction of palladium from the Pd-PAN Dependence on [SCN-1: [Pd], = 5.6 ionic strenath = 1.3’M with NaCl:
WN-I
X
10T5 M, [PAN], = 3
X
Complex
10e4 M, [NaOAc] = 0.1 M, pH = 4.24,
M
- Log kobs
0.02 0.04 0.06 0.10 0.14 0.20 Slope of log kobs vs. [SCN-]
3.59 3.28 3.00 2.56 2.21 2.03 in thiocyanate range 0.04-0.20 M = 1.83 * 0.05
Dependence on pH: [Pd], = 5.6
X
10m5 M, [SCN-]
= 0.2 M, [Cl-] = 1.0 M, [PAN], = 1 X lo-’
PH
- Log kobs
3.15 3.60 4.25 4.59 4.82 5.15 Slope of log Kobs vs. pH = - 0.73 f 0.01
1.26 1.62 2.05 2.30 2.50 2.13
Dependence on [Cl-]: [Pd], = 5.6 ionic strength = 1.3 with Na,SO,:
X
lop5 M, [SCN-]
= 0.2 M, [PAN], = 1 X 1O-3 M, [NaOAc] = 0.1 M, pH = 4.25,
ICI-1 04
- L‘S kobs
0.1 0.2 0.3 0.5 0.7 1.0
2.05 2.06 2.04 2.05 2.05 2.04
Slope of log kobs vs. [cl-]
= 0.01 + 0.01
Dependence on [PAN],,: [Pd], = 5.6 [PAN]
M, NaOAc = 0.1 M:
x
X
10F5 M, [Cl-] = 1.0 M, [SCN-]
lo3 (M)
= 0.2 M, [NaOAc] = 0.1 M, pH = 4.61: - Log kot,s
0.1 0.2 0.4 0.6 0.8 1.0
2.40 2.38 2.40 2.40 2.40 2.32
Slope of log kobs vs. log [PAN] = 0.01 f 0.01
This protonation either weakens or breaks the Pd-0 bond, facilitating the formation of a PdSCN bond, i.e.,
gen) bonds because of its “trunk effect”, and therefore enhances the rate of addition of the second thiocyanate ion:
PdCl(HL)+
PdCl( SCN)HL
+ SCN-
2 PdCl( SCN)HL
fast (11)
(3) The thiocyanate ion in PdCl(SCN)HL can weaken one of the Pd-N (pyridine or azo nitro-
+ SCN - k,\
PdCl(SCN),HL(4)
The
remaining
slow Pd-N
(12) bond
in
the
BACK-EXTRACTION
KINETICS
AND
231
MECHANISM
PdCl(SCN)2HL is expected to be very weak and easily replaced with the thiocyanate ion:
which, by suitable substitution, - d[Pd]/dt
PdC1(SCN)2HL-+
= [K&,/K&,]
SCN- 3 x [PdClL],[H]+
PdCl(SCN);-
+ HL
fast
Pd(SCN):-
fast
(14)
The enhancement in back-extraction kinetics due to the truns effect was confirmed by the effects on the extraction rates when other trunslabilizing reagents such as thiourea (TU) and bromide were studied. The enhancement observed, TU > SCN- X- Br-, decreased in the same order as their labilizing “tram effect” [9-111. The rate-determining step can be described by -d[Pd]/dt
= k,[PdCl(SCN)HL][SCN-]
TABLE
4
Kinetic
data for back-extraction
Dependence [SCN -
on [SCN-1:
[Pd],
of palladium = 8.9
X
(15)
from the Pd-Kelex
10m5 M, [HL],
= 4
X
Dependence
100 complex
10m4 M, pH = 2.90, ionic strength
1(W
= 1.0 M with NaCI:
- Log hbs
0.2 0.4 0.5 0.6 0.8 1.0 Slope of Log k,,
vs. [SCN-]
on pH: [Pd],
in the thiocyanate
= 8.9
X
(16)
where K,’ is the acid dissociation constant of PdCl(HL)+, K, the formation constant of PdCl(SCN)(PAN) and K,, the distribution constant of the PdCl(PAN) complex. The effect of thiocyanate on the rate of palladium back-extraction from a chloroform solution of the Pd-Kelex 100 chelate containing excess of Kelex 100 was seen to have a first-order dependence on the metal concentration. From a series of experiments at different pH and [SCN-1, the observed rate constants showed a second-order (1.94 f 0.06) dependence on the thiocyanate concentration and an inverse first-order (- 1.09 f 0.03) dependence on the solution pH (Table 4). These results are consonant with those obtained for palladium back-extraction from the Pd-PAN complex, therefore indicating a rapid pre-equilibrium protonation of the phenolic oxygen in Kelex 100 followed by the addition of two thio-
+ SCN- KI, + Cl-
[SCN-I2
(13)
(5) Both the truns and cis effects of the thiocyanate weaken the Pd-Cl bond and therefore accelerate the addition of the fourth thiocyanate ion: PdCl(SCN):-
becomes:
range 0.4-1.0
10V5 M, [SCN-]
PH 1.91 2.16 2.40 2.62 2.88 3.08 Slope of log kobs vs.pH = - 1.09 rt 0.03
4.01 3.71 3.50 3.33 3.10 2.94 M = 1.94 f 0.06
= 0.2 M, [Cl-]
= 1.0 M, [HL], - Log kobs 2.85 3.13 3.38 3.60 3.95 4.11
= 4
X
10m4 M:
S.J. AL-BAZI
232
ions back-extraction
to the palladium mechanism, i.e.,
cyanate
PdL,(o)
“FPdL,
complex
the
(17) +
fast
PdL( HL) + + SCN - 2 PdL(SCN)HL PdL( SCN)HL
(18) fast
(19)
+ SCN - k,\ IO
PdL( SCN)*HL PdL(SCN),HL-+ Pd(SCN)i-
-
slow 2SCN-+
+ 2HL
fast
20
30
40
Time(min)
(21)
By assuming that the reaction in Eqn. 20 is the rate-determining step and incorporating the effect of the rapid pre-equilibrium in Eqn. 19, then -d[Pd]/dt
H. FREISER
2.5
fast
PdL, + H+ “z’PdL(HL)
in
AND
Fig. 2. Effect of [SCN- ] on rate of palladium back-extraction. [Pd], = 6.4 X 10K5 M, [R2S],, = 4 x 10m4 M, ionic strength = 1.0 M with NaCl, pH = 2.58. Using high-speed stirring: [SCN-] = (1) 1.0, (2) 0.6, (3) 0.4 and (4) 0.3 M. Using Eberbath shaker: [SCN- ] = (a) 0.01, (b) 0.03, (c) 0.05, (d) 0.07 and (e) 0.1 M.
= (K,k,/K,“K,,)[PdL,],[H+] x [SCN-I2
(22)
It is interesting to observe that back-extraction rates of Pd from Pd-PAN are faster than from Pd-Kelex 100 (Tables 3 and 4) (an 800-fold higher rate from the former in solutions containing 0.20 M thiocyanate and at pH 3.0). This is probably due to the smaller K,, of Pd-PAN, which has one hydrophobic ligand molecule whereas the Pd-Kelex 100 complex has two. The kinetics of palladium back-extraction from PdC12(R2S)2 were found to be complex. At thiocyanate concentrations < 0.3 M, the stripping rate increases slowly at first but more sharply at longer times (Fig. 2). On the other hand, at higher thiocyanate concentrations, a linear enhancement in the stripping rate was observed. This may indicate that two consecutive reactions are involved in the rate-determining step. Under similar conditions of thiocyanate (0.1 M), chloride (0.9 M) and solution pH (2.58), the rate of back-extraction of palladium from Pd = 15 s) was linear and much (SCN),(R), (h/2 faster from PdCl,(R,S), (t,,, = 500 s). This indicates that substitution of chloride rather than dioctyl sulfide in PdC12(R2S)2 is involved in the rate-determining step. is a square-planar complex Pd(SCN),(R,S),
that exists mainly in the tram form. In the cis form, the Pd-SR, bond is greatly weakened by the “tram effect” of the thiocyanate ion. As cistruns isomerization is considered to be very fast [12], the “tram effect” of thiocyanate can be the result of fast substitution of R,S by thiocyanate ion. In the presence of 0.2 M thiocyanate, changes in solution pH in the range 1.59-2.90 had no effect on the rate of palladium back-extraction from the PdCl,(R,S), complex. In the light of the findings in the previous systems where, in contrast to R,S, the ligands PAN and Kelex 100 had measurable proton affinity, and were loosened in the chelate by protonation, it was expected that an absence of pH dependence would be observed in the dioctyl sulfide system. Therefore, the stripping mechanism can be explained by the schematic diagram in Fig. 3. Although the replacement of chloride in PdC12(R2S), with thiocyanate is expected to be rapid, the slow transformation to PdCl(SCN) (R2S)2 and Pd(SCN)2(R2S)2 results from the low aqueous concentration of PdCl 2(R2S)2 or PdCl (SCN)(R,S), as a result of their high distribution constants. Further, the replacement of chloride with thiocyanate in PdC12(R2S), would be slower than that in PdCl(SCN)(R,S), as a result of the
BACK-EXTRACTION
KINETICS
AND
MECHANISM
233
0
PdCl
2
(R
2
S)
I1 PdC12(R2S;G A
2
PdClKiC,N)(R2S)2
” PdCI(SCN)(R2S)2~
2R2S
SCN-
+ Cl-
Fig. 3. Schematic diagram of stripping mechanism.
Pd(SCN)
2
(R
2
s)
LSCN2zGi-
Pd(SCN)
24
” + 2R2S
+
Cl0 = Organic; A = aqueous.
“trans effect” of thiocyanate. This may result in a sharper increase in the stripping rate at longer times (Fig. 2). The contribution of the “tram effect” in labilizing the leaving chloride group was confirmed by the effect of 0.010 M thiourea, thiocyanate and bromide present in aqueous solutions at pH 2.72 on the rate of stripping of palladium from 6.4 X 10e5 M PdCl,(R,S),, which decreased in the order thiourea x==thiocyanate > bromide. According to the proposed mechanism, the stripping rate can be further improved if the reaction of thiocyanate with PdCl,(R,S), occurs in the organic phase where a high concentration of the complex is present. This was accomplished by shaking 6.4 X 10F5 M of the metal complex in chloroform containing 1 x lop5 M tetraheptylammonium thiocyanate with an equal volume of an aqueous solution containing 0.02 M thiocyanate and 0.01 M chloride at pH 2.61. In the absence of Q+,SCN-, the plot of In &/A, vs. time showed a small enhancement in the extraction rate at the beginning of stripping (log kobs = - 3.24), followed by a sharper increase (log kobs = - 3.05) at longer times. In the presence of Q+,SCN-, however, only one straight line passing through the origin and with a higher stripping rate (log kobs = - 2.21) was observed. These results may also indicate that replacement of the first chloride in PdCl,(R,S), with thiocyanate is the rate-determining step. In conclusion, the presence of trans-labilizing reagents in the aqueous or organic phase can play an important role in catalyzing the stripping rates. This, in combination with the protonation of the oxygen in phenolic chelates, can be considered as
a novel method for stripping metals from organic solutions.
This research the Engineering Foundation.
of platinum
group
was supported by a grant from Division of the National Science
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