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注册: 2008-10-25
性别: MM
专业: 配位化学

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Abstract The concentration of thorium in aqueous samples
has been determined by means of alpha-spectroscopy
and UV–Vis photometry after chemical separation and preconcentration
of the actinide by cation exchange and liquidliquid
extraction using Chelex-100 resin and 30%TBP in
dodecan, respectively. Method calibration was performed
using thorium standard solutions and resulted in a high
chemical recovery for cation exchange and liquid extraction.
Regarding, the effect of physicochemical parameters (e.g.,
pH, salinity, competitive cations, and colloidal species) on
the separation recovery of thorium from aqueous solutions
by cation exchange has also been investigated. The investigation
was performed to evaluate the applicability of cation
exchange and liquid extraction as separation and pre-concentration
methods prior to the quantitative analysis of
thorium in water samples, and has shown that the method
could be successfully applied to waters with relatively lowsalinity
and metal ion contamination.
Keywords Thorium determination Cation exchange
Liquid extraction Physico-chemical parameters
Chemical recovery
Introduction
Thorium is a naturally occurring, slightly radioactive metal
found in small amounts in most rocks and soils. Soil
commonly contains an average of around six parts per
million (ppm) of thorium. Thorium compounds in natural
systems including waters are stable in the ?4 oxidation
state. Thorium, like uranium and plutonium, can be used as
fuel in a nuclear reactor, and is attractive because of its
much greater abundance on earth, the superior physical and
nuclear properties as fuel material, better proliferation
resistance, and reduced waste production [1].
Knowledge of the thorium concentration and radioactivity
in natural waters, process liquids, and industrial
wastewaters is of particular interest. Because screening
procedures are connected with an enormous number of
samples to be analyzed, the development and application of
cost-effective techniques is of particular interest. Among
the detection techniques used for thorium analysis, UV–Vis
photometry [2], alpha spectroscopy [3, 4] and ICP-MS and
ICP-OES [5], the two former methods are widely used
analytical techniques because of their simplicity and is the
main reason why these methods have been used in this
study. The advantage in using the two latter techniques is
the good energy resolution, which allows isotope identification.
However, ICP-OES and particularly ICP-MS have
far lower detection limits regarding thorium analysis [5].
Nevertheless, prior analysis of thorium in environmental
matrices, sample pre-treatment is usually a prerequisite. In
this context, the separation and pre-concentration procedure
involved for sample preparation prior analysis is a key
parameter in the design and selection of a cost-effective
technique. Chelex-100 as cation exchanger [6] and tributyl
phosphate (TBP) as complexing agent in liquid extraction
[4] have been extensively investigated in the separation and
pre-concentration of uranium and plutonium from aqueous
solutions. Chelex-100 has been successfully applied in the
separation of uranium from natural waters [7, 8].
In this study, we have investigated the applicability of
Chelex-100 and 30% TBP/dodecan in the separation and
pre-concentration of thorium from aqueous solutions prior
analysis. Results obtained from experimental investigations
regarding chemical recovery and the effect of physicochemical
parameters such us pH, salinity, and the presence
of competitive cations (e.g., Fe3? and Ca2?), and humic
acid and silica colloid concentration, on the separation of
thorium from aqueous solutions by Chelex-100 are presented
and discussed. The investigation was performed to
evaluate the applicability of cation exchange and liquid
extraction as separation and pre-concentration procedures
prior to the determination of thorium in water samples.
Materials and methods
In all experiments analytical grade reagents and de-ionized
water were used. The Th standard solution (992 ± 2 mg/L
Th in HNO3 solution, d = 1.010) was obtained from
Aldrich Co. The Chelex-100 resin (18–50 mesh, Merck)
was used as received. HNO3 solutions of various concentrations
(e.g., 2 and 8 M HNO3) were prepared by diluting
a concentrated solution (68% HNO3, Aldrich Co). 30% v/v
TBP solution was freshly prepared by mixing the appropriate
volumes of tributyl phosphate and dodecan, both
obtained from Aldrich Co. (NH4)2SO4 solutions were
prepared by dissolving ammonium sulfate (99.999%,
Aldrich Co.) in de-ionized water.
Radiometric analysis was performed using a high-resolution
alpha-spectrometer (STC Amplituda, Doza) equipped
with semiconductor detectors, after pre-concentration
and separation of the radionuclides by cation exchange and
liquid extraction, and (finally) their electrodeposition on
stainless steel discs. Additionally, quantitative analysis of
thorium was performed by spectrophotometry using arsenazo-
III. The photometer was calibrated using a series of
thorium standard solutions and from the slope of corresponding
calibration curve the molar extinction coefficient
e of the Th(IV)-arsenazo at 650 nm was determined,
e = (28500 ± 700) (L/mol cm). All measurements were
repeated and the error bars in the graphs correspond to the
standard deviation, which was calculated based on the
repeated measurements. A diagrammatic representation
(flow chart) of the analytical procedure applied is given in
Fig. 1.
For the evaluation of the pre-analytical procedure (e.g.,
the ion-exchange efficiency) the present method was
applied to laboratory solutions of constant thorium concentration
(2 9 10-6 mol/L) and variable composition
(0.1, 0.3, 0.5, 0.7, and 1 M NaCl; 0.1, 0.3, 0.5, 0.7, and 1 M
Ca(NO3)2; 0.05, 0.1, 0.5, and 1 mM FeCl3; 5, 10, 50, 100,
and 150 ppm SiO2; 5, 10, 50, 100, and 150 ppm humic
acid, sodium form). The solutions were prepared by dissolution
of the appropriate amount of the corresponding
Results and discussion
Separation efficiency of cation exchange and liquid
extraction
The efficiency of the pre-analytical procedures is basically
associated with the cation exchange and liquid extraction
chemical recoveries. The chemical recovery of cation
exchange and liquid extraction for thorium is graphically
presented in Fig. 2. The experimental data shown in Fig. 2
correspond to aquatic Th(IV) solutions of 0.1 and 8 M
HNO3 pre-treated with cation exchange and liquid extraction,
respectively. According to the experimental data
(Fig. 2), cation exchange by Chelex-100 and liquid
extraction presents efficiency/chemical recovery for thorium
(80 ± 5) and (65 ± 5)%, respectively.
Comparing these data with previously published data [4]
it becomes evident that the chemical recovery of thorium
by liquid extraction is almost similar (65 ± 5 vs.
70 ± 5%) in both studies. However, the chemical recovery
of cation exchange for thorium found in this study is significantly
higher (almost 60%) than the corresponding
chemical recovery found in the previous study (about 20%)
[4]. This difference in the chemical recovery of thorium is
basically attributed to the thorium concentration used in the
two different studies. In the previous study thorium was
present in ultra-trace amounts (fmol range), because it was
present in solution as daughter-nuclide of the 236Pu tracer
used in the respective study [4], whereas in this study the
thorium concentration in the test solution is several orders
of magnitude higher (lmol range). Hence, the lower
chemical recovery of cation exchange for thorium in the
previous study can be ascribed to thorium losses on the
resin during cation exchange. The small amounts of thorium
lost are reflected to a higher degree in the chemical
recovery determined using test solutions of lower concentration.
This effect is not observed in the case of liquid
extraction because mass transfer processes are significantly
faster in those systems and the binding/complex formation
of thorium with the active moieties quantitatively
reversible.
The combination of the particular chemical recoveries
results in an overall chemical efficiency of the pre-analytical
procedures, which is calculated to be (52 ± 7)%, at
the mBq- or lmol-concentration range. It becomes clear
that the pre-concentration and separation method of actinide
from aqueous solutions, which is described here and
combines cation-exchange and liquid extraction, presents
relatively high chemical recovery for thorium. This effect
is solely attributed to the increased charge of the thorium
cation (Th4?), which forms even at low pH strong
complexes with chelating iminodiacetic moieties of the
Chelex-100 resin and the phosphato moieties of tributyl
phosphate [6, 9].
Because among the pre-analytical procedures applied to
thorium separation from water samples, cation exchange
represents generally the first step resulting in significant
reduction of the sample amount, the effect of composition
of the test solution on the chemical recovery of this procedure
is of particular interest. Hence, the effect of various
physicochemical parameters such as pH, salinity, competitive
cation concentration, and humic acid and silica colloid
concentration on the chemical recovery of thorium by
cation exchange using Chelex-100 is further investigated.
Effect of pH
The interaction between Th4? and the iminodiacetic moiety
is schematically given in Fig. 3 and could be described
as a cation exchange reaction between the protons and the
thorium cations. Because proton concentration is a key
factor governing both, the proton dissociation of the
iminodiacetic moieties and Th(IV) hydrolysis [10], the
binding of Th(IV) by the resin is strongly pH dependent.
According to experimental data shown in Fig. 4, the optimum
pH for the separation efficiency of thorium is given at
pH 2 and can be explained by the increased chemical
affinity of Th4? for the iminodiacetic moieties of the
Chelex-100 resin. Below and above pH 2 the affinity is
decreasing because on the one hand the concentration of
the competing protons increases dramatically and on the
other hand the Th(IV) starts, forming very stable hydroxo
complexes and polynuclear species in solution [10]. For
comparison, Fig. 4 shows also the maximum separation
efficiency of Chelex-100 for uranium, which is given at
significantly higher pH (pH = 4.5) [6], indicating that a pH
adjusted selectivity of Chelex-100 for these two naturally
occurring actinides could be possible.
Effect of salinity and competitive cations
Because waters to be treated are complex containing dissolved
species species has been investigated as a function of their concentration
in the test solution [11–13].
The effect of salinity on the chemical recovery of thorium
is summarized graphically in Fig. 5. Increasing
salinity (e.g., [NaCl]) results in lower chemical recovery of
thorium, supporting basically the assumption that the
binding of Th4? by the Chelex-100 resin at pH 2 is based
on electrostatic interactions and an cation exchange
mechanism. Hence, increasing salinity (e.g., [Na?]) in
solution results in decreasing the amount of resin-bound
thorium, indicating that the method could be restricted only
to low-salinity waters.
Further the chemical recovery has been investigated
as a function of Ca2? concentration in solution and the corresponding data are summarized together with the
salinity data in Fig. 5. According to Fig. 5, increasing
[Ca2?] results in reduced separation efficiency of thorium
down to 5% for [Ca2?] = 1 M. The effect of the [Ca2?] on
the chemical recovery of thorium is more effective (at low
concentrations) than the corresponding effect of salinity
most probably because of the duple charge of the Ca2?
cation, which interacts stronger than the Na? cation with
the iminodiacetic moieties of the Chelex-100 resin. However,
calcium concentrations that may affect thorium
recovery by Chelex-100 are not expected in natural waters
and could be found only in industrial wastewaters [11–13].
On the other hand, iron even at low concentration (10-4
mol/L) affects significantly the thorium recovery and leads
to almost no separation recovery at [Fe3?] = 0.01 mol/L
(Fig. 6). The effect is attributed to the higher affinity of the
resin for the Fe3? cations under the given conditions (pH
2). Fe3? cations present high charge as well as significantly
higher tendency for complex formation than Ca2? cations
and hence compete effectively Th4? cations regarding
binding by the resin through its iminodiacetic moieties,
resulting in lower chemical recovery of the radioelement.
The study has been focused on the effect of these three
different metal ions because they are generally the most
abundant metallic species in environmental systems and
have different ionic charges. The latter implies that similarly
charged cations may behave in a similar way.
Effect of colloids
The effect of colloidal species on the chemical recovery
of thorium from aqueous solution by Chelex-100 has
been investigated using silica and humic acid colloids,representing the inorganic and organic colloids, respectively.
Silica colloids were chosen because these species
are omnipresent and present even at low pH negatively
charged surface (pzc * 2), which attracts positively
charged metal ions. On the other hand humic acids are also
naturally occurring colloids affecting strongly the chemical
behavior and migration of pollutants in the geosphere [14].
The effect of colloidal species (e.g., silica and humic
acid colloids) present in solution on the chemical recovery
of thorium by Chelex-100 from aqueous solutions is shown
in Fig. 7. The data in Fig. 7 clearly show that increasing
both silica and humic acid concentration results in dramatic
reduction of the chemical recovery of thorium. The effect
is more pronounced for the organic colloids indicating on
the higher affinity of humic acid for thorium. The interaction
of thorium with the colloids results in the formation
of very stable thorium pseudocolloids [14], which stabilize
the metal ion in solution. The formation of the thorium
pseudocolloids is based on the interaction of the Th4?
cations with the hydroxy groups of the metal oxide and the
carboxylic groups of the humic acid colloids [15, 16].Conclusions
The results obtained from this study lead to following
conclusions:
• The maximum separation efficiency of Chelex-100 for
thorium is given at pH 2 and differs significantly (about
2.5 pH units) from the corresponding separation efficiency
for uranium (pH 4.5), indicating that a pHadjusted
selectivity of Chelex-100 for these two naturally occurring actinides could be possible. The
chemical recovery for thorium under these conditions is
(80 ± 5)%. Taking into account the chemical recovery
for thorium by liquid extraction using TBP, which is
(65 ± 5)%, the overall chemical efficiency, is calculated
to be (52 ± 7)%, at the mBq concentration range.
• Increasing salinity (e.g., [Na?] and [Ca2?]), iron
([Fe3?][10-4 mol/L) and colloid concentration in
solution results generally in decreasing chemical
recovery. Nevertheless, the method could be successfully
applied for thorium determination in waters of
low-salinity and metal ion contamination.

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