Radiotracer study of synergistic effects of neutral donors on the extraction of gold with N-(Thioacetyl)benzamide in chloroform

Article abstract of DOI:10.1134/S1066362212020105

The extraction of Au(III) from an aqueous acidic medium with N-(thioacetyl)benzamide in chloroform was studied in the presence of organophosphorus donors (tributylphosphine oxide, triphenylphosphine oxide, tributyl phosphate) at pH 3.0 using radiotracer t

Full text of DOI:10.1134/S1066362212020105

ISSN 1066-3622, Radiochemistry, 2012, Vol. 54, No. 2, pp. 153–158. © Pleiades Publishing, Inc., 2012.
Published in Russian in Radiokhimiya, 2012, Vol. 54, No. 2, pp. 143–147.
Radiotracer Study of Synergistic Effects of Neutral Donors on 1
the Extraction of Gold with N-(Thioacetyl)benzamide in Chloroform
a
b
a
P. Dey* , S. Banerjee , and S. Basu**
a
Department of Chemistry, University of Burdwan, Burdwan, 713104 India;
e-mail: * pulakchem@yahoo.co.in, ** basu_sakulyan@yahoo.co.in
b
Department of Chemistry, Raja Rammohon Roy Mahavidyalaya, Radhanagore, Hooghly, 712406 India
Received June 21, 2011
Abstract—The extraction of Au(III) from an aqueous acidic medium with N-(thioacetyl)benzamide in chloro-
form was studied in the presence of organophosphorus donors (tributylphosphine oxide, triphenylphosphine
oxide, tributyl phosphate) at pH 3.0 using radiotracer technique. The influence of various parameters on the
extraction equilibrium was examined. The extent of extraction of Au(III) in the form of the binary complex
increases with temperature, but the reverse trend is observed with the ternary complex. The overall equilibrium
constants for each of the three ternary extraction systems were estimated, and the role of different thermody-
namic functions is discussed. The extraction mechanism is supported by the IR spectra of the ternary adduct.
Keywords: synergistic extraction, N-(thioacetyl)benzamide, gold-198
DOI: 10.1134/S1066362212020105
The phenomenon of synergism, when two extrac- known neutral donors capable of dehydrating the metal
tants used in combination extract a metal ion much ions, facilitating their transfer into an organic medium
more efficiently as compared to the normal additive
[19, 20]. However their effect on the Au(III) extraction
effect of these extractants used separately, has been is less studied [21, 22], and such a study might be of
extensively studied from both theoretical and practical
paramount importance in the field of hydrometallurgy.
points of view. Various combinations of extractants The effects of various parameters (concentration of
causing synergistic effects in metal ion extraction have ligand and organophosphorus donors, temperature,
been described [1–7]. An additional extractant seems etc.) are reported. The role of mixed extractants has
to play an important role, presumably replacing the been elucidated in terms of such effects.
coordinated water molecules to form organophilic ad-
ducts with the metal–ligand complexes.
EXPERIMENTAL
Chemicals and devices. The chelating extractant,
The liquid–liquid extraction of Au(III) from aque-
N-(thioacetyl)benzamide, was synthesized by the reac-
ous acidic medium has long been studied [8–10]. Be-
tion of benzoic acid and thioacetamide, which were
ing a soft cation, it tends to form complexes with soft
obtained from Aldrich, USA. The donor reagents used
bases like thiourea and substituted thioureas, which
in this study (tributyl phosphate, TBP; tributyl-
was used [11–15] for the gold transfer into a nonpolar
phosphine oxide, TBPO; triphenylphosphine oxide
organic medium. Amides and their derivatives are
TPPO) were also purchased from Aldrich. The sol-
promising extractants for many metal ions. Numerous
vents were purified by standard procedures. All the
studies describe the extraction with amides [16–18].
other chemicals used were of A.R. grade. A stock solu-
This study deals with the extraction of Au(III) spiked
with 198Au radiotracer into chloroform with a newly
tion of gold was prepared by dissolving AuCl
3
(
Johnson Matthey) in deionized water and standardized
synthesized complexing agent, N-(thioacetyl)benz-
amide, and with the effect of various phosphine oxides
and tributyl phosphate on such extraction. The organo-
phosphorus compounds used in this study are well-
by gravimetry [23]. A working solution (~230 ppm)
was prepared by appropriate dilution. An appropriate
volume of the stock solution of Au(III) was mixed
with a few drops of a tracer solution containing 1 Au
98
1
as AuCl , supplied by BRIT (India), and finally
The text was submitted by the authors in English.
3
1
53

1
54
DEY et al.
aqueous phases, with checking the material balance.
Synthesis and characterization of the chelating
agent. The chelating ligand was synthesized by the
general condensation reaction between benzoyl chlo-
ride and thioacetamide (1 : 1). The starting material,
A.R. grade benzoic acid (2.00 g), was refluxed for 2 h
with successive addition of thionyl chloride. Excess
chlorinating agent was removed by slow evaporation.
In a beaker, a saturated solution of thioacetamide
SOCl
2
Refluxing for 2 h
NH
2
CSCH
3
PhNO , 1 h
2
(
1.24 g) in double-distilled anhydrous nitrobenzene
was prepared. The solvent was purified by drying over
calcium oxide, followed by distillation before use. This
thioacetamide solution was then condensed with ben-
zoyl chloride for 1 h on a boiling water bath.
The brown precipitate formed in the process was fil-
tered off, recrystallized from chloroform, and dried.
The dried compound melts at 94 ± 2°C. The proposed
formula of the ligand (Scheme 1) is in good agreement
Scheme 1. Synthesis of the chelating ligand, N-(thioace-
tyl)benzamide.
brought to the mark. The CHN microanalysis of the
synthesized ligand was made with a Perkin–Elmer
400 CHNS-O elemental analyzer. The IR spectra of
2
1
with the analytical and spectroscopic (IR, H NMR)
the ligand and adducts were recorded with a Perkin–
Elmer FTIR Model RX1 spectrometer. A Systronics
model 335 digital pH-meter equipped with a single
electrode was used for pH measurement. A single-
channel γ-ray spectrometer coupled with a well-type
NaI(Tl) detector (Nucleonix, India) was used for radio-
data. Found, %: C 60.26, H 4.98, N 7.65. Calculated,
%
: C 60.33, H 5.02, N 7.82. The IR spectrum of
–1
the ligand does not contain the band at 3760 cm (ν
OH
in the initial acid) but contains a narrow band at
–1
1
689 cm (carbonyl group) [24], which shows that
only the hydroxy group of the acid is involved in the
1
98
activity measurements. The Au tracer was assayed
by its 412 keV photopeak.
–1
condensation reaction. The ν
band at 1175 cm
C=S
confirms the preservation of the thione group of thio-
1
Extraction. In the general extraction procedure,
ml of an aqueous solution containing ~100 μg of
acetamide in the reaction. H NMR spectrum, δ, ppm:
5
7.6 (5H, aromatic protons), 2.3 (s, 3H, methyl group),
8.1 (s, 1H, NH) [25].
spiked Au(III ) was adjusted to pH 3.00 and extracted
with an equal volume of chloroform solution of the
chelating extractant for 30 min in a mechanical shaker.
For synergistic study, the organic phase was mixed
with appropriate donor reagent of desired concentra-
tion. After the equilibration, the two phases were sepa-
rated and the radioactivities of equal volumes of both
phases were measured. The distribution ratios D were
calculated as the radioactivity ratio of the organic and
RESULTS AND DISCUSSION
Effect of equilibration time. The equilibration
time for the extraction of Au(III) with N-(thioacetyl)-
benzamide in CHCl was within 30 min. An increase
3
in the equilibration time to 1 h did not affect the ex-
traction equilibrium.
Effect of organic diluents. The extraction with
–
2
Table 1. Effect of diluents on the extraction of Au(III) from
1
.65 × 10 M extractant in benzene, toluene, xylene,
aqueous chloride medium
methyl isobutyl ketone (MIBK), CHCl , CCl , isoamyl
3
4
Diluent
D
0
E, %
alcohol, and n-butanol was studied at pH 3.0. From the
distribution data, CHCl and MIBK appeared to be the
Chloroform
20.21 95.28
19.14 95.04
15.24 92.64
10.45 91.26
5.42 84.42
4.26 80.98
2.11 67.84
1.41 58.51
3
most effective. The distribution ratio D (Table 1) in-
Methyl isobutyl ketone
Isoamyl alcohol
n-Butanol
Toluene
Carbon tetrachloride
Xylene
0
creases in the order benzene < xylene < CCl < tolu-
4
ene < n-butanol < isoamyl alcohol < MIBK < CHCl .
3
Effect of pH in aqueous phase. The extraction of
–2
Au(III) from chloride solution with 2.83 × 10 M ex-
tractant in CHCl was studied as a function of pH over
3
Benzene
the range of pH 1.0–4.0. The extraction of Au(III) was
RADIOCHEMISTRY Vol. 54 No. 2 2012

RADIOTRACER STUDY OF SYNERGISTIC EFFECTS OF NEUTRAL DONORS
155
only 6% at pH ~1.7. With an increase in pH, the ex-
traction regularly increased and became quantitative at
pH 3.0–4.0. The plot of logD vs. pH (in the pH range
0
2
.0–3.0) is a straight line with a slope of ~3 (Fig. 1),
indicating that three protons were liberated on com-
plexation of Au(III) with the extractant to give the ex-
tractable species in the organic phase.
Extraction of Au(III) with N-(thioacetyl)-
benzamide (HA). Extraction of Au(III) from HCl so-
0
Fig. 1. Plot of logD vs. pH of aqueous the phase for the
extraction of Au(III) with a solution of HA in chloroform.
Slope 2.79.
lutions was studied with CHCl as diluent. The extrac-
3
–2
tant concentration was varied from 0.33 × 10 to 3.5 ×
–2
1
0 M (Table 2). The extraction percentage increases
with an increase in the ligand concentration. At higher
concentration of the extractant, it efficiently displaces
water molecules from the coordination sphere of the
hydrated gold ion, which makes the complex much
more organophilic. At pH 3.0, the log–log plot of D
0
vs. extractant concentration is linear with a slope of
~
2.89 (Fig. 2). This fact indicates that three molecules
of bidentate HA ligand are involved in the extraction
of gold:
0
Fig. 2. log–log plot of Au(III) distribution ratio D in the
extraction with a solution of HA in chloroform vs. extrac-
tant molar concentration. Slope 2.89.
k
Au3 +a q + 3HAorg ҙ [AuA
org + 3Н +a q
.
(1)
3
]
The extraction equilibrium constant k for the binary
system is related to the distribution ratio by
logD
0
= logk – 3log[HA] – 3pH.
(2)
Extraction with pure neutral donors (S). Among
the three organophosphorous compounds, tributyl
phosphate (TBP) does not noticeably extract Au(III)
under the examined conditions. Two phosphine oxides,
TBPO and TPPO, however, noticeably extract Au(III)
from aqueous solution at pH 3, with the distribution
m 0
Fig. 3. log–log plot of D – D vs. extractant concentration
in chloroform at fixed donor concentration. Donor, concen-
ratio (D ) increasing with the donor concentration.
S
tration (mM), slope: (1) TBP, 0.38, 2.69; (2) TBPO, 0.25,
Slope analysis shows that only one molecule of any
2
.73; and (3) TPPO, 0.23, 2.76.
donor per AuCl molecule is involved in the extrac-
3
K
tion.
Au3 +a q + 3HAorg + Sorg ҙ [AuA
·S]org + 3H +a q
.
(3)
3
Synergistic extraction of gold complexes in the
presence of donors. The distribution ratio of Au(III)
extracted with HA increases on adding organophos-
phorus compound S. This is attributable to the forma-
tion of a more lipophilic adduct with these donors. In
the ternary extraction system, the log(D – D ) (D is
The equilibrium constant K in the ternary system is
K = [AuA
3
·S]org[H+] 3a q/([Au3+]aq[HA] 3o rg[S]org).
(4)
Table 2. Exaction of Au(III) with a solution of HA in chlo-
roform at pH 3.0 without organophosphorus donors
m
0
m
the distribution ratio in the presence of HA and S) vs.
log[HA] plots have a slope of ~3 (Fig. 3), whereas the
log(D – D ) vs. log[S] plots are straight lines with a
Concentration, mM
D
0
E, %
m
0
3
.3
0.245
0.540
5.50
19.67
35.06
84.61
95.39
slope of ~1 (Fig. 4), suggesting extraction of the spe-
6
.6
cies AuA ·S. The overall reaction of the synergistic
9.9
16.5
3
extraction of the gold ion is given by Eq. (3):
20.7
RADIOCHEMISTRY Vol. 54 No. 2 2012

1
56
DEY et al.
concentrations will correspond to the number of ligand
molecules in the mixed complex. The adduct formation
reaction in the organic phase can be presented as
K
S
(
6)
[
3
AuA ]
org + Sorg ҙ [AuA ·S]org.
3
Correspondingly,
logK
S
= logK – logk,
(7)
m 0
Fig. 4. log–log plot of D – D vs. organophosphorus do-
nor concentration in chloroform at fixed extractant concen-
where K is the adduct formation constant in the or-
S
tration of 6.6 mM. Donor, slope: (1) TBPO, 0.891; (2) TPPO,
ganic phase.
0
.832; and (3) TBP, 0.826.
The experimental data on synergistic effects of each
of the two phosphine oxides and TBP on the extraction
of gold with HA are given in Table 3. As can be seen,
in all the cases the extraction percentage and synergis-
tic coefficient (SC) increase with the donor concentra-
tion, showing enhanced replacement of water mole-
cules from the metal environment. The synergistic co-
efficient and apparent formation constant of the ternary
complex are defined as
It is related to the distribution ratio D by
m
logK = log(D
m
– D
0
) – 3log[HA] – log[S] – 3pH. (5)
+
It is clear from Eq. (5) that, at fixed H and HA
concentrations, the slope of the log(D – D ) vs.
m
0
log[S] dependence will correspond to the number of
donor molecules in the adduct. Similarly, the slope of
+
the log(D − D ) vs. log[HA] at fixed H and donor
m
0
SC = log[D
m
/(D
0
+ D
S
)],
β = (D – D
m
o
)/(D
0
[S]).
Table 3. Synergistic extraction of Au(III) with a mixture of
chelating extractant and organophosphorus donor. Extrac-
= 0.540a
As seen from Table 3, SC and β decrease in the or-
der TBPO > TPPO > TBP, in accordance with the
trend in the electronic effects of substituents. The equi-
librium constants for the binary and ternary systems
are given in Table 4. They show that fairly stable ad-
ducts are formed in chloroform.
tant concentration 6.6 mM, pH 3.0, chloroform, D
0
[
Donor],
mM
Donor
TBPO
D
S
D
m
E, %
SC
logβ
0
.25
0.019 1.22 54.95 0.338 3.704
0.051 1.76 63.76 0.473 3.663
0.064 2.19 68.53 0.561 3.615
0.031 0.99 49.75 0.239 3.561
0.061 1.19 54.34 0.289 3.523
0.089 1.33 57.08 0.325 3.501
0.93 48.19 0.234 3.274
0.49
0
0
.74
.23
Effect of temperature. The extraction of Au(III)
from aqueous HCl with a mixture of N-(thioacetyl)-
benzamide and phosphine oxides or TBP was also
studied in relation to temperature (25–45°C, Fig. 5),
and thermodynamic functions (enthalpy of formation
TPPO
TBP
0.35
0
0
.46
.38
0.62
–b
1.14 53.28 0.326 3.255
1.26 55.75 0.369 3.252
0
0
0
.75
ΔH , entropy of formation ∆S , free energy of forma-
0
a
(
D
m
) Distribution ratio in presence of mixed extractant; SC,
synergistic coefficient, SC = log[D /(D + D )]; β = (D
)/(D [S]), apparent formation constant.
Pure TBP extracts Au(III) under these conditions negligibly.
tion ∆G ) were calculated using the van’t Hoff equa-
m
0
S
m
–
tion [26, 27]. The results are given in Table 5.
D
0
0
b
The experimental data were processed by linear
regression analysis. The data clearly show that the ex-
0
traction of gold with HA only is endothermic (∆H >
0
). When a cation is transferred from an aqueous me-
Table 4. Equilibrium constants for binary and ternary sys-
tems
dium into an organic phase through an ion exchange–
complexation mechanism, the net enthalpy and entropy
associated with the extraction will be mainly due to
two opposing processes: (1) dehydration of the ex-
tracted cation and hydration of the exchanged protons;
(2) metal coordination by the organic ligand and de-
protonation of the ligand [28]. The positive entropy
Ternary systems
Au(III)–ligand system, logk
donor
TBPO
TPPO
TBP
logK
0.975
0.825
0.545
logK
S
3.245
3.095
2.815
–
2.27
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RADIOTRACER STUDY OF SYNERGISTIC EFFECTS OF NEUTRAL DONORS
157
TPPO–TBPO. Thus, the adduct formation reaction
occurs spontaneously, exothermally, with the entropy
0
0
loss. The negative ΔH and ∆S values are characteris-
tics of outer-sphere complexation [26]. Thus, the syn-
ergistic effect of the donor is due to expansion of the
coordination sphere of an Au(III) atom, which accom-
modates an additional electron pair donated by organo-
phosphorus compounds.
IR spectra. The IR data [24] clearly support the
formation of [AuA ·S] in the organic phase. The ligand
3
Fig. 5. Temperature dependence of the equilibrium con-
stants for the binary and ternary systems. Donor: (1) none,
is bound to the metal ion through the C–S bond, as
–1
indicated by the shift of ν
from 1152 cm in the
C=S
(2) TBPO, (3) TPPO, and (4) TBP.
–
1
free ligand spectrum to 1141 cm in the spectrum of
–1
the complex. A new band appears at 1596 cm , absent
0
change ∆S in the binary extraction system is appar-
ently due to dehydration of the gold ion before its ex-
traction into the organic phase with the release of wa-
ter molecules. The dehydration of the metal cation also
in the free ligand spectrum and assignable to ν ; its
C=N
appearance is due to deprotonation of the N atom upon
complex formation in the organic phase. Similarly,
–1
ν
has shifted from 1144 to 1120 cm , suggesting
P=O
0
involves a positive enthalpy change (∆H > 0) as a re-
bonding of phosphine oxide molecules with the metal
chelate.
sult of the cleavage of the ion–water bonds. The posi-
0
tive entropy change (∆S > 0) can be associated with
complete dehydration of the metal cation whose effect
is not compensated by the simultaneous hydration of
ACKNOWLEDGMENTS
+
One of the authors (P. Dey) thanks CSIR (India) for
providing him a fellowship.
the equivalent amount of H ions. Positive entropy
change generally contributes to the enhanced stability
of chelates [27]. Thus, the binary extraction proceeds
nonspontaneously in the forward direction, endother-
mally with entropy gain.
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