Determination of trace uranyl ions in aquatic mediumby a useful and simple method

Halil Ibrahim Ulusoy

Received: 14 March 2014

� Akademiai Kiado, Budapest, Hungary 2014

Abstract Determination of trace uranyl ions was per-

formed by using mixed micellar system and spectropho-

tometric determination. The method is based on cloud

point extraction of uranyl ions after formation of an ion-

association complex in the presence of Celestine Blue and

sodium dodecyl sulfate. Then, the formed complex was

extracted to non-ionic surfactant phase of Triton X-114 at

pH 8.0. The optimal extraction and reaction conditions

(e.g. concentrations and types of surfactants, concentration

of complex forming agent, incubation conditions) were

studied and analytical characteristics of the method (e.g.

limit of detection, linear range, pre-concentration factor)

were obtained by experimental studies. Linearity was

obeyed in the range of 50–1,500 ng mL-1 for uranium(VI)

ion and the detection limit of is 14.20 ng mL-1. The

interference effects of common ions were also tested and

validation studies were performed by using recovery test.

The method was applied to the determination of ura-

nium(VI) in several real samples.

Keywords Uranyl ions � Cloud point extraction �Celestine blue � Spectrophotometry

Introduction

Uranium is the longest-lived radio nuclides and its pure

form is chemically active. It is one of well-known radio-

active element and the news about uranium related with

radioactive leaking, poising, and carcinogenic effect draw

attention in every country. There is always a potential risk,

although precautions have taken in nuclear reactors and

uranium enrichment plants. Uranium is one of the rarer

elements, but it is actually present in the earth’s crust in

greater amounts than such ‘‘common’’ elements as cad-

mium, bismuth, mercury, and iodine. Most of studies about

uranium are focus on its radioactive and toxicological

properties. It possesses health risks to humans, particularly

relatively high concentrations. The most stable form of

uranium in water is hexavalent uranyl ion (UO2þ2 ) which

can be bonded with chelating agents such as citrate,

bicarbonate anions and plasma proteins in blood. It can be

held on organs such as lung, bone, kidneys and liver and

caused several health problems such as renal damage and

cancer. The tolerable daily intake of uranium is established

as 0.6 lg kg-1 of body weight per day [1, 2]. Therefore,

determination of uranium at trace levels and removal of

uranium from aqueous solutions are always an important

research area in view of environmental and health risks. On

the other hand, uranium alloys are useful in diluting enri-

ched uranium liquid fuel meant for nuclear reactors and

pure uranium coated with silicon and canned in aluminum

tubes are used in production reactors [3, 4].

Large quantities of radioactive wastes in solid or liquid

forms are produced in mining and milling operations. Many

countries have been concern about these problems such as

groundwater contamination, proximity of communication to

populated areas, and emission of radioactive isotopes [5].

Most natural waters (especially sea water) contain

detectable amounts of uranium. The average concentration

in the ocean is about 3 lg L-1 [6]. The uranium concen-

tration of ground and surface waters varies greatly, from a

low of less than 0.1 ng L-1 to several mg per liter. In the

great majority of surface and ground waters the

H. I. Ulusoy (&)

Department of Analytical Chemistry, Faculty of Pharmacy,

Cumhuriyet University, 58140 Sivas, Turkey

e-mail: hiulusoy@yahoo.com

123

J Radioanal Nucl Chem

DOI 10.1007/s10967-014-3229-4

concentration is less than 10 lg L-1. The contamination of

groundwater by uranium increases its contagion rate to

other sources and people.

The determination of uranium in aqueous systems is often

required for environmental control and geochemical pros-

pecting [7–11]. However, conventional spectrometric ana-

lytical techniques such as flame atomic absorption

spectrometry (FAAS), graphite furnace atomic absorption

spectrometry (GFAAS) and inductively coupled plasma

optical emission spectrometry (ICP OES) cannot be used for

determination of trace amounts of uranium. Determination

using FAAS requires nitrous oxide acetylene flame and the

desired sensitivity cannot be obtained by conventional

approaches (about 50 mg L-1) [12]. Application of GFAAS

is also limited for determination of low concentrations of

uranium owing to problems related with the pyrolysis tem-

perature, which must not be higher than 1,000 �C, and ura-

nium forms carbides in a graphite furnace [13]. The

sensitivity of ICP OES has an inadequate detection limit like

over 100 mg L-1 [14]. Considering this, inductively cou-

pled plasma mass spectrometry (ICP-MS) can be evaluated

as a good alternative for the determination of uranium at low

concentrations. However, this technique is very expensive

for a lot of research laboratory. Molecular absorption

(UV–VIS) spectrometry is still a good and accessible option

for most laboratories. Very sensitive determinations can be

directly performed or following a simple pre-concentration

procedure. Most researcher have been developed various

analytical methods for trace uranyl ions by considering this

approaches [15–19].

The extraction method using surfactants, termed ‘‘cloud

point (CP) extraction or micelle mediated extraction’’

provides an alternative to the conventional extraction sys-

tems due to its easy steps and lack of requirement for

organic solvents. The concentration at which surfactants

begin to form micelle is known as the critical micelle

concentration (CMC). The CMC of a surfactant depends on

several factors, such as its molecular structure, and

experimental conditions such as ionic strength, counter

ions, temperature, etc. Upon appropriate alteration of the

conditions such as temperature or pressure, addition of salt

or other additives, the solution becomes turbid at a tem-

perature known as CP due to the diminished solubility of

the surfactant in water [20–24]. The determination of

uranyl ions in different samples by means of surfactant

mediums were presented in literature [25–31]. The use of

surfactant molecules not only increases sensitivity of

determination method and also eliminates most of inter-

ference effects with selectivity reactions.

This paper proposes a method for pre-concentration and

determination of uranium by spectrophotometry based on

cloud point extraction (CPE) of the complex of ura-

nium(VI) with Celestine Blue (CB) in mixed surfactant

media. Method variables were optimized in detail. The

validation of developed methods was performed by

recovery tests in spiked samples. Finally, method was

applied to real samples in order to determine uranyl levels.

Experimental

Instrumentation

Spectrophotometric measurements were performed on a

UV–VIS spectrophotometer (Shimadzu, UV–Visible 1800,

Japan) equipped with a 1 cm quartz cell. This spectropho-

tometer has a wavelength accuracy of ±0.2 nm and a

bandwidth of 2 nm in the wavelength range of

190–1,100 nm. A pH meter with a glass–calomel electrode

(Selecta, Spain) was used to measure the pH values. A

thermostatic water bath (Microtest, Turkey) was used to keep

constant the temperature. A centrifuge (Hettich, Universal

120, England) was used for complete phase separation. A

microwave digestion system (CEM Mars X6, USA) was used

to dissolve and prepare the samples to analysis.

Reagents

All reagents used were of analytical grade. Ultra-pure

water with a resistivity of 18.2 MX cm was used in all

experiments provided by ELGA (Flex III, UK) water

purification system. All containers (glassware, PTFE bot-

tles) were treated with diluted HNO3 solution and finally

rinsed with deionized water prior to experiments. Stock

solution of UO22? (1,000 lg mL-1) were prepared by

dissolving appropriate amounts of nitrate salt (Merck) in

water. 3.0 9 10-3 mol L-1 of CB (Sigma, St. Louis, MO,

USA) solution was prepared by dissolving 50 mg reagent

in methanol (Merck, Darmstadt, Germany) and diluting

with water. The buffer solution of pH 8.0 ± 0.1 was pre-

pared with sodium citrate and sodium mono hydrogen

phosphate. The solutions of all surfactant [TritonX-114,

Triton X-100, sodium dodecyl sulfate (SDS), cetyl pyrid-

inium chloride (CPC) and cetyl tri methyl ammonium

bromide (CTAB)] (Sigma, St. Louis, MO, USA) were used

without further purification. Solutions of 5 % (w/v) non-

ionic surfactants (Triton X-114 and PONPE 7.5) were

prepared by dissolving 5.0 g of surfactant in 100 mL of

deionized water.

The CPE procedure

In a typical CPE procedure, 10 mL of sample containing

uranium in the range of 50–1.500 ng mL-1, 2.0 mL of pH

8.0 buffer, 0.5 mL of 3 9 10-3 mol L-1 CB, 0.5 mL of

3 9 10-3 mol L-1 SDS and 0.5 mL of 5 % (w/v) Triton

J Radioanal Nucl Chem

123

X-114 were added to a Falcon tube and dilute to the mark

(50 mL) with ultra-pure water. This solution is allowed to

stand for about 5 min before it placed into a thermostatic

controlled water bath at 45 �C for 15 min. Separation of

the two phases was achieved by centrifugation for 5 min at

3,500 rpm. On cooling in an ice-bath for 15 min, the sur-

factant-rich phase (SRP) became viscous. Then the aque-

ous phase was separated by inverting the tubes. 1 mL of

methanol was later added before measurements in order to

reduce viscosity of SRP and facilitate determination. The

final volume of sample was about 1.2 mL. The analytical

signal (absorbance) of this solution was monitored at

630 nm in a micro quartz cell against pure methanol.

Analysis of water and rock samples

The proposed method was applied to different real samples.

Three different water samples (river, tap and drinking

water) and a rock sample including radioactive elements

were selected for application of new developed method.

River water was collected from Kızılırmak River (Sivas,

Turkey) in cleaned PTFE container and kept in dark until

analysis. Tap water was taken allowing to run for 20 min

and approximately 2.0 L of tap water was collected directly

from laboratory at Cumhuriyet University. Drinking waters

with different origin were bought from a local market. The

ground rock sample including radioactive elements was

obtained from a research group in our university.

Then, the proposed methods were applied to all samples

after a microwave digestion process. The microwave

parameters were given in Table 1. The samples were

neutralized by a few drop of ammonia until its acidity

eliminated and filtrated by 0.45 lm filter paper. Then the

proposed method was applied to samples.

Results and discussion

Figure 1 shows the absorption spectra of U(VI)–CB–SDS

ion-pair complex in SRP against methanol for five different

uranyl levels. As can be seen in Fig. 1, the absorbance of

complex gradually increases by increasing uranyl

concentration at 519 and 630 nm. The formed complex can

be monitored by both wavelengths. 630 nm was used

throughout experiments due to its higher sensitivity. In

addition, the spectrum of blank solution shows that

absorbance of CB without uranyl ions is very low.

After all parameters (pH, concentration of surfactants,

ionic strength, incubation conditions, etc.) were optimized

as explained in following sections, concentration of uranyl

ions in samples can be measured by using calibration

equation given in Fig. 2. As can be seen in Fig. 2, DA

values meaning difference between absorbance of blank

and sample are proportional with uranyl concentration.

Table 1 Microwave parameters for real samples

Amount of

sample

0.5 g of rock sample 20 mL of water

sample

Digestion reagent ?10 mL of 50 %

HNO3

?10 mL of 1 M

HNO3

Time programme 0–5 min 50 �C

5–10 min 100 �C

10–20 min 150 �C

Wavelenght, nm400 450 500 550 600 650 700

Abs

orba

nce

0,000

0,500

1,000

1,500

2,000

2,500

3,000630 nm

519 nm

ab

c

d

e

f

Uranyl Concentrations;a: 0 ng mL-1

b: 100 ng mL-1

c: 300 ng mL-1

d: 500 ng mL-1

e: 900 ng mL-1

f: 1000 ng mL-1

Fig. 1 Absorption spectra of ion-associated complex for five differ-

ent U(VI) concentrations. Absorption spectra of a 3.0 9

10-5 mol L-1 of Celestine Blue with increasing U(VI) concentration

at levels of 100, 300, 500, 900 and 1.000 lg L-1 at pH 8.0 against

methanol after pre-concentration with CPE under the optimized

reaction conditions

Concentration of uranyl, ng mL-10 200 400 600 800 1000 1200 1400 1600

Δ A

0,0

0,2

0,4

0,6

0,8

1,0

1,2

1,4

Abs = 6,7x10-4 [U(VI)] + 0,1918r² = 0,9942

Fig. 2 Calibration curve obtained in optimized conditions

J Radioanal Nucl Chem

123

Effect of pH

The pH effect on CPE depends on the characteristics of

both surfactants and analytes. If target species (uranyl ions)

have different acidic equilibriums changing by pH, even a

little change in pH can cause important effects on CPE

efficiency. For organic molecules, especially including

ionizable species, maximum extraction efficiency is

achieved at pH values where the uncharged form of the

analyte prevails, and therefore, target analyte is favored to

be partitioned into the micellar phase. The ionic form of a

neutral molecule formed upon deprotonating of a weak

acid or protonation of a weak base normally does not

interact with and bind the micellar aggregate as strongly as

its neutral form does. However, changing the pH will

change the ionization form of certain analytes and will

thereby affect their water solubility and extractability [32].

As a cationic dye CB is an organic ampholyte which a

proton can attract its pyridine nitrogen atom in acidic

medium while o-hydroxy group can dissociate in basic

medium with acidity constants of pKa1: 0.50 and pKa2:

8.00. It is generally used as a redox reagent in determina-

tion of organic and inorganic species by means of catalytic/

kinetic reactions [33, 34]. In another study, we used a

polymeric material including CB in order to remove uranyl

ions from aquatic mediums [35].

The different buffer systems such as NH3/NH4Cl,

H2PO4-/HPO4

2-, citrate and universal Britton–Robinson

(BR) were used independently in order to determine the

best extraction conditions. The best analytical signal was

obtained by HPO2�4 /citrate buffer system. The selected

buffer system (citrate/HPO2�4 ) only doesn’t keep to con-

stant the pH, also citrate ions in buffer eliminate possible

interfering ions owing to its chelating property.

The optimization of pH was performed by a buffer series

in the range of 4.0–11.0. As can be seen in Fig. 3, the max-

imum signal was obtained at pH 8.0. After pH 8.0, signal is

decreasing because of second dissociation constant of

ligand. So, pH 8.0 was selected as an optimal value.

The effect of buffer concentration on the analytical

signal was also studied in the range of 0–5 mL (in final

volume of 50.0 mL) and the best analytical signal was

obtained by 2.0 mL.

Effect of complexing agent concentration

In this study, CB was selected as a chromogenic and cationic

agent with an oxidizing character and positive charge. The

positive charge of CB is facilitated formation of ion-asso-

ciated complex with charged species. And, its chromogenic

properties are helped to follow the complex formation by

spectrophotometrically. When Fig. 1 was re-examined, it

can be understood better formation of complex. The first

curve in the Figure is spectrum of CB before complex for-

mation. The signals at both wavelength is increased with

uranyl concentration. The increase in absorbance is related

with the increase in uranyl concentration. And, this rela-

tionship is linear in the range of 50–1.500 ng mL-1 for

uranyl ions. After system was optimized for every param-

eter, uranyl concentration can be calculated by monitoring

increase in absorbance and using linear regression

equation.

The effect of CB concentration on analytical response was

shown in Fig. 4. As it can be seen, absorbance of U(VI)–CB–

SDS complex increases in the range of 0.5–73.0 9

10-5 mol L-1, where the reaction is quantitatively com-

pleted. So, a concentration of 3.0 9 10-5 mol L-1 of CB was

chosen as the optimal value.

pH3,00 4,00 5,00 6,00 7,00 8,00 9,00 10,00 11,00

Abs

orba

nce

0,600

0,800

1,000

1,200

1,400

1,600

1,800

2,000

2,200

2,400

Fig. 3 Effect of pH on analytical signal. Conditions: 2 mL of pH

buffer, 250 lg L-1 of U(VI), 0.5 mL of 3.0 9 10-3 M CB, 0.4 mL

of % 5 (w/v) Triton X-114

Concentration of Celestine Blue, mol L-1

0 10-5 2,0x10-5 3,0x10-5 4,0x10-5 5,0x10-5 6,0x10-5

Abs

orba

nce

0,400

0,600

0,800

1,000

1,200

1,400

1,600

1,800

2,000

Fig. 4 Effect of CB concentration on analytical signal. Conditions:

2 mL of pH 8.0 buffer, 250 lg L-1 of U(VI), 0.4 mL of % 5 (w/v)

Triton X-114

J Radioanal Nucl Chem

123

Effect of surfactant concentration

The concentration of surfactant could affect the perfor-

mance of CPE process significantly. A successful CPE

should be able to maximize the extraction efficiency

through increasing phase volume ratio (Vsurfactant-rich phase/

Vaqueous phase), so as to improve the pre-concentration factor

(PF). Depending on the nature of the hydrophilic group,

surfactantsare classified as non-ionic, zwitterionic, cat-

ionic, and anionic. In CPE experiments, non-ionic, zwit-

terionic and anionic surfactants are most widely used for

pre-concentration of inorganic metal ions, drugs, bioma-

terials, and organic compounds. However, ionic surfactants

are needed generally in the presence of charged species

(ligand or analyte) in the solution [36]. It is very important

to select the most appropriate surfactant or mixture of

surfactants for a successful CPE analysis since it can

directly affect success of pre-concentration procedure, and

accuracy of analytical results.

The enrichment factor depends on volume of SRP (Vs)

which varies with the surfactant concentration. Researches

show that the smaller the surfactant concentration, the

higher the PF. In addition, when the Vs too small, the

extraction process becomes difficult, and accuracy and

reproducibility will probably suffer [37]. Therefore a bal-

ance between the surfactant concentration required for a

maximum PF and an adequate volume Vs for subsequent

volume manipulation is critical. The efficiency of extrac-

tion/pre-concentration relies on the magnitude of analyte

solubilization into the micelle (non-polar core and polar

micelle–water interface), analyte polarity and solution

composition [38]. Therefore, any experimental approach

should focus on these parameters to ensure maximum

extraction efficiency.

The Triton X-114 and PONPE 7.5 were chosen as

nonionic surfactants because of their commercial avail-

ability in a high-purified homogenous form, low toxico-

logical properties and cost. The variation of absorbance the

formed complex was shown as a function of concentrations

of two nonionic surfactants in Fig. 5. According to this

investigation, Triton X-114 was more suitable than PONPE

7.5 and the optimum signal was obtained at a concentration

of 0.05 % (v/v) Triton X-114. At lower concentrations, the

extraction efficiency of complexes is low probably because

of the inadequacy of the assemblies to entrap the hydro-

phobic complex quantitatively. Above these concentra-

tions, the analytical signal gradually decreases because of

decreasing at PF as mentioned above.

The charge of uranyl ions and used ligand CB are positive

in aquatic medium. The formed complex between uranyl ions

and CB can be entrapped into nonionic surfactant phase, but

ionic surfactants should be improved this transfer by

increasing electrostatically interactions. So, the experiments

were repeated by using three different ionic surfactants like

cetyl pyridinium chloride (CPC), cetyl tri methyl ammonium

bromide (CTAB), and SDS. As can be seen in Fig. 6, cationic

surfactants such as CPC, CTAB increase the analytical signal

a little or not. But the signal increased considerable amount in

the presence of SDS. The maximum signal was obtained by

using 3 9 10-5 mol L-1 of SDS. The addition of SDS as a

negative charged surfactant is increased the extraction effi-

ciency by providing electrostatic balance in the solution. The

formed ion-association complex ðUOþ2 CBþ ððSDSÞ2Þ�Þ can

be easily transferred as a uncharged complex.

Effect of the incubation temperature and time

The incubation time and equilibration temperature are two

important parameters in CPE. It is desired the shortest

Concentration of Nonionic Surfactants, (w/v) %0,00 0,02 0,04 0,06 0,08 0,10 0,12

Abs

orba

nce

0,000

0,200

0,400

0,600

0,800

1,000

1,200

1,400

1,600

1,800

2,000

TRITON X-114PONPE 7.5

Fig. 5 Effect of nonionic surfactant concentration on analytical

signal. Conditions: 2 mL of pH 8.0 buffer, 250 lg L-1 of U(VI),

0.5 mL of 3.0 9 10-3 M CB

Concentrations of Ionic Surfactants, x 10-5 mol L-1

0,00 2,00 4,00 6,00 8,00 10,00

Abs

orba

nce

0,600

0,800

1,000

1,200

1,400

1,600

1,800

2,000

2,200

CPCCTABSDS

Fig. 6 Effect of ionic surfactant concentration on analytical signal.

Conditions: 2 mL of pH 8.0 buffer, 250 lg L-1 of U(VI), 0.5 mL of

3.0 9 10-3 M CB, 0.5 mL of % 5 (w/v) Triton X-114

J Radioanal Nucl Chem

123

equilibration time and the lowest possible equilibration

temperature for efficiently separation of phases. The

dependence of extraction efficiency upon equilibrium

temperature and time was studied in the range of 20–60 �C

and 5–50 min, respectively. The results showed that an

equilibrium temperature of 45 �C is appropriate for CPE

experiments. The formed complex can be decomposed at

high temperatures and it is not formed CP at low temper-

atures. The other important parameter is incubation time

which can affect by complex stability. As a result of

experimental studies, it was understood that 15 min incu-

bation at 45 �C was enough for quantitate extraction.

Effects of ionic strength

The effect of ionic strength can be discussed in two main

chapters. First, most of the real samples have high ionic

strength. So, the application of newly developed method in

the presence of concentrated electrolytes provides that the

method can be applied to real samples without any negative

effect of common ions. The second approach is related

with salting-out effect. It is known that the presence of

electrolytes decreases the CP temperature and increases

efficiency of separation. The salt concentration is also a

key parameter in CPE. The CP of micellar solutions can be

altered by salt addition, presence of alcohol, other surfac-

tants, polymers, and some organic or inorganic compounds,

which can cause an increase or decrease on the phase

micellar solubility [39].

The addition of an inert salt to the solution can influence

the extraction/pre-concentration process since it can alter

the density of the aqueous phase. In order to study the

effect of electrolyte on CPE of uranyl ions, NaCl solution

was investigated as electrolyte in the range of

0.0–2.0 (w/v) %. The results show that addition of NaCl

does not have an important effect on CPE experiments until

a concentration of 2.0 (w/v) % or 0.34 mol L-1. As can be

seen in Fig. 7, there isn’t an important effect of ionic

strength on developed method. These results shows that the

proposed method can be applied to samples with high ionic

strength without any interference effect.

Interference studies

Commonly encountered matrix components such as alkali

and earth alkali elements generally do not form stable

complexes and are not extracted in the proposed system.

The effects of representative potential interfering species

on the extraction of U(VI) were tested by using syntheti-

cally prepared solutions including uranyl and interfering

ion. The possible interfering ions in different concentra-

tions were added to a solution containing 200 lg L-1 of

U(VI) and the proposed method was applied under

optimum conditions. The tolerance limits were determined

as ratio of interfere ion concentration to analyte ion causing

an error of ±5 %. The results were given in Table 2. These

results demonstrate that the common coexisting ions did

not have significant effect on the determination of the

analyte ions. The proposed method was observed to be

fairly selective for U(VI) ions at pH 8.0. Since commonly

present ions in water samples did not affect significantly

the recovery of U(VI), therefore the methods can be

applied to determination of uranyl ions in environmental

water samples.

Analytical characteristics

In CPE, extraction needs to be carried out under optimal

conditions in order that the PF can be maximized to

achieve 100 % extraction efficiency. The PF is a parameter

that presents an idea how much analyte ions can be pre-

concentrated.

Concentration of NaCl, (w/v) %0,0 0,5 1,0 1,5 2,0 2,5

Abs

orba

nce

0,0

0,5

1,0

1,5

2,0

2,5

Fig. 7 Effect of ionic strength on analytical signal. Conditions: 2 mL

of pH 8.0 buffer, 250 lg L-1 of U(VI), 0.5 mL of 3.0 9 10-3 M CB,

0.5 mL of 3.0 9 10-3 M SDS 0.5 mL of % 5 (w/v) Triton X-114

Table 2 Tolerance limit of possible interfering ions at determination

of 200 ng mL-1 U(VI)

Interfering ion(s) Tolerance ratio Recovery %

Anions

CO32-, NO3

-, Cl-, Br-, SO42-, 1,000 97.3–105.4

S2-, S2O32-, PO4

3- 500 96.2–101.5

Cations

Na?, K?, Ca2?, NH4? 1,000 98.3–100.4

Fe3?, Fe2? 500 98.2–101.5

Co2?, Cr3? 250 96.5–103.2

Cd2?, Zn2?, Hg2? 200 98.5–102.5

Pb2?, Cu2? 50 95.4–103.7

J Radioanal Nucl Chem

123

The calibration graphs of U(VI) ions in Fig. 2 was

obtained by using 50 mL of standard solutions containing

known amounts of the analytes in the presence of CB, SDS,

and Triton X-114. The analytical characteristics of pro-

posed method was summarized in Table 2 such as regres-

sion equation, linear range, and limits of detection and

quantification, reproducibility and PFs. The limits of

detection and quantification were 14.28 and

42.81 ng mL-1, respectively. The linear range of proposed

method is appropriate in order to follow uranyl concen-

trations in water samples according to USEPA standards.

The volume of the solution was 50 mL before CPE and its

final volume was 1.2 mL before determination step. So, the

PF can be calculated about 42 by using ratios of phases.

Determination of uranyl in real samples

The accuracy and validity of the proposed method were

checked by applying the determination of uranyl ions in

various samples. The collected samples were prepared to

analysis according to procedure the mentioned in ‘‘Ana-

lysis of water and rock samples’’ Section and determined

uranyl concentration by using proposed method.

The results were shown in Table 3. Recovery studies

were also carried out after it was spiked to samples known

concentrations of uranyl ions at levels of 100 and

300 ng mL-1. The recoveries are close to 100 % and

indicate that the proposed method was helpful for the

determination of uranium in the real samples. The accuracy

of the method was statistically (Table 4) tested by evalu-

ating the obtained results with the proposed method based

on pre-concentration with CPE.

Conclusions

In this work, the usage of new micellar system was pre-

sented as an alternative method for pre-concentration and

determination of trace uranyl ions before detection by

UV–VIS spectrophotometry. The proposed method offers

several important advantages including inexpensive, rapid,

safe, lower-toxicity, high sensitivity, high recovery, low

LOD and good precision.

The mixed surfactant medium (Triton X-114 and SDS)

have been used for pre-concentration of uranium in several

water samples and rock sample. The limit of detection of

the presented method seems to be satisfactory in contrast to

some familiar pre-concentration techniques. Furthermore,

in contrast to some familiar pre-concentration techniques

like solvent extraction methods, it is much safer, because

only a small amount of the surfactant, which has a low

toxicity, is used. In addition, the linear range of proposed

method is highly suitable for determination of trace uranyl

ions in real samples. The proposed method is a combina-

tion of CPE and UV–VIS spectrophotometry as a detection

tool for uranyl ions. The method is very versatile and

economic because it exclusively used a conventional

spectrophotometry which is available in almost every lab-

oratory. It may be a useful analytical approach as an

Table 3 Analytical characteristics of the proposed method

Parameters The obtained values

Linear range 50–1.500 ng mL-1

Slope 0.00067

Intercept 0.0192

Correlation coefficient (r2) 0.9912

Recovery % (n 5) 98.7–103.5

RSD (%) (25 and 250 lg L-1, n 5) 4.16 and 3.25

LOD (lg L-1)a 14.28

LOQ (lg L-1)b 42.81

Preconcentration factorc 42

a Based on statistical 3Sblank/m-criterion for ten replicate blank

absorbance measurementsb Based on statistical 10Sblank/m-criterion for ten replicate blank

absorbance measurementsc Preconcentration factor is defined as the ratio of the initial solution

volume to the volume of surfactant rich phase

Table 4 Determination of uranyl ions in several samples

Sample Added

U(VI)

ng mL-1

Founda

U(VI)

ng mL-1Recovery

%

RSD %

River water – 32.18 ± 1.35 – 4.19

100 129.24 ± 3.58 97.8 2.77

300 325.17 ± 8.52 97.9 2.62

Tap water – Non detected – –

100 103.57 ± 2.11 104.4 2.04

300 308.25 ± 8.48 102.8 2.75

Drinking waterb – Non detected – –

100 96.51 ± 3.25 96.5 3.37

300 303.24 ± 7.24 101.1 2.39

Drinking waterb – Non detected – –

100 104.64 ± 3.54 104.6 3.38

300 315.04 ± 9.01 105.0 2.86

Drinking waterb – Non detected – –

100 95.38 ± 2.72 95.4 2.86

300 287.25 ± 7.87 95.8 2.74

Rock including

radioactive

elements

– 88.44 ± 3.84 – 4.34

100 194.12 ± 8.07 100.9 4.16

300 400.61 ± 9.95 103.1 2.48

a Average of five replicate determinations ±sb Three different drinking water samples were bought from a local

market

J Radioanal Nucl Chem

123

alternative to expensive and time consuming techniques

such as ICP-MS, ICP-OES.

Acknowledgments The present study was performed with partly

contributions obtained from other projects supported by Cumhuriyet

University Scientific Research Projects Commission. The author also

wishes to express his gratitude to Asst. Prof. Dr. Selcuk Simsek for all

expert discussions; his suggestions contributed enormously to the

preparation of the manuscript.

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