Determination of Uranium and Thorium in the Industrial Phosphoric Acid Using X-Ray Fluorescence Spectrometry with Wavelength Dispersive ( WDXRF )

The method of determination for uranium and thorium in the industrial phosphoric acid by wavelength-dispersive X-ray fluorescence spectrometry (WDXRF) responds positively to all the validation tests and can be adopted for the dosage for these elements. It is clear that the procedure of the proposed method is simple and requires less time to complete the analysis. The concentration for uranium and thorium in the Tunisian phosphoric acid is in the order of 22 ppm for uranium with a coefficient of variation of 0.77% and of about 4 ppm for thorium with a coefficient of variation of 1.37%.


Introduction
Industrial Phosphoric acid is one of the most important products of the chemical industry, it is used in the manufacture of phosphate fertilizers; TSP: Ca(H 2 PO 4 ) 2 and DAP: (NH 4 ) 2 HPO 4 , feed livestock (CaHPO 4 .2H 2 O) and sequestering phosphate (sodium hexametaphosphate).The production of phosphoric acid is based on the chemical treatment of the rock phosphate with sulfuric acid, this process can be described by the following overall reaction of (Slack, 1968;Waggaman, 1969): Ca 10 (PO 4 ) 6 F 2 +10 H 2 SO4 + 20 H 2 O → 10 (CaSO 4 2 H 2 O) + 2 HF + 6 H 3 PO 4 (1) The sulfuric attack dissolved at the same time as apatite, almost all existing impurities in the soil.These consist of heavy metals, radionuclides and other natural elements (Becker, 1989).Some of these impurities can be recycled such as (U, Th, Y, Sc, rare earth ...) and could be recovered, and others that are penalizing for further use of acid such as (Cd , Cr, Pb, Mg ...), they must be eliminated or minimized.
In this work we focus on uranium and thorium and particularly to techniques for determining these elements.The methods currently used are: -Gravimetry which consists in the precipitation of the uranyl ion with 2-hydroxy-1-naphthaldehyde, uranium is determined as U 3 O 8 after calcining the precipitate at 850 ° C. (Lopez et al., 1989) or the precipitation of the UO 2 2+ ion, in the pH range 3-5, by a solution of p-aminobenzoic acid and pyridine in water.An amorphous yellow precipitate forms, which is easily filtered and has the composition UO 2 (H 2 N-C 6 H 4 -COO) 2 .2Py.2H 2 O. which is converted by calcinationto U 3 O 8 (Ripan and Sacelean, 1965).The uranium (VI) is precipitated quantitatively in the range of pH 5.0 to 7.5 with N-benzoyl-o-tolylhydroxylamine in the form of UO 2 (C 14 H 12 O 2 N) 2 (Mrinal and Majumdar, 1970).The thorium, forms an insoluble complex with vanillin in the range of pH 4.0 to 6.2.Thorium (IV) can be determined gravimetrically and separated from cerium, uranium (VI) and trivalent rare earth.The precipitate obtained is converted into oxide ThO 2 by calcination and then weighed (Jain and Singh, 1962).Thorium (IV) make a precipitate with benzenephosphonic acid in the form Th(C 6 H 5 PO 3 ) 2 • 3H 2 O at pH around 0.5.The precipitate containing three molecules of H 2 O and it is stable between 140 ° C and 180 ° C. between 240 ° C and 300 ° C, the precipitate contains only two molecules of H 2 O. Th(HPO 4 ) 2 is obtained anhydrous above 800 ° C (Charles banks and Rodney Davis, 1955).
-Volumetry is used for dosing the uranium (VI) by carrying out a reduction using ferrous sulfate in phosphoric medium.The excess of Fe 2+ is determined by the dichromate (Al Ammar and Basheer, 1993).
-The inductively coupled plasma coupled to mass spectrometry (ICP-MS) (Aydin and Soylak, 2007): the separation and concentration of thorium (IV) and uranium (VI) by solid phase extraction on Duolite XAD761 (resin adsorption).The chelates of thorium (IV) and uranium (VI) with 9-phenyl-3-fluorone formed and adsorbed on the XAD761 Duolite.Thorium (IV) and uranium (VI) are quantitatively eluted with HCl 2 mol L -1 and determined by inductively coupled plasma coupled to mass spectrometry (ICP-MS).
Another method to extract the uranium present in an aqueous phase; we have the extraction of the uranium using the D2EHPA and Synergistic D2EHPA-TOPO Mixture (Ali et al., 2012;Beltrami et al., 2013;Beltrami et al., 2014;Orabi et al., 2015), using a synergistic DOPPA-TOPO mixture (Krea and khalaf, 2013), using of DNPPA and bidentateoctyl(phenyl) CMPO (Mondal et al., 2014) and the adsorption method of the uranium ( Hussein and Morsy, 2011;Morsy and Hussein, 2012;Ahmed et al., 2016) -The spectrophotometric method; ion UO 2 2+ make a chelate with meloxicam by the following reaction: 2 (meloxicam H) + UO 2 2+ → UO 2 (meloxicam) 2 + 2H + , the product is a yellow complex with an absorbance passes through a maximum at 398 nm (Lutfullah et al., 2008).Thorium, extract from a hydrochloric solution using the acid di (2-ethylhexyl) orthophosphoric, where the color is developed in the organic solution by adding an Arsenazo III solution and isopropanol, is determined by spectrophotometry with a maximum absorbance at 660 nm (Cerrai and Ghersini, 1967).
-The high resolution gamma spectrometry is used to determine the concentration of uranium and thorium in the surface soils, where soil samples are ground and sieved then stored for four weeks before analysis, it is carried for 18 hours (Tzortzis and Tsertos, 2004).
-The thin layer chromatography (Hodisan at al., 1998) is also used for the determination of uranium and thorium in presence of other metal ions using the iso-acid propyldithiophosphoric (PRDTP-i) in mobile phase as a complexing agent to differentiate between the species studied by modification of their retention time.
-The determination of the uranium isotope and thorium isotope is possible by coupling the ion chromatography and Inductively Coupled Plasma coupled to mass spectrometry IC-ICP-MS (Benkhedda et al., 2005;Zheng and Yamada, 2007;Rathore, 2008;Goodall and Lythgose, 1999;Betti, 1997), but the IC-ICP-MS has limitations due to the structure of the sample (capacity of the column, the acidity ...).
-The determination of thorium is possible by Energy dispersive X-ray fluorescence (EDXRF) (Mirashi et al., 2010), but we have problems of resolution and selectivity with (EDXRF).
-Other techniques such as the fluorimetry (Rathore, 2008;Sanjukta et al., 2008) The neutron activation analysis (Shinotsuka and Ebihara, 1997;Byrne and Benedik, 1988) and the alpha spectrometry (Hou and Roos, 2008;Saidou et al., 2008;Benedik et al., 2009;Jia et al., 2008) can be used for the determination of uranium and thorium, but these techniques require very long preparation of the sample (pre-concentration of the sample separation ...).
Although a variety of methods for the determination of uranium and thorium in different types of samples have been developed, there are several disadvantages for every method; -The Precipitation methods: uranium and thorium have the same chemical properties, so they are difficult to separate, they precipitate together.We must then search the optimal conditions for selectivity.In addition the sensitivity of these gravimetric methods depends on a certain number of factors, the principal one being the interference, it is caused by the structure and concentration of the matrix elements (interference with the rare earth essentially).
-For colorimetric methods there are the stability problems for complexes and the problems of selectivity for the two elements (uranium and thorium).
-The inductively coupled plasma coupled to mass spectrometry (ICP-MS): the disadvantages of this method are the salinity of the matrix of samples processed and the isobaric interferences.
-The neutron activation analysis and the alpha spectrometry require a long process (counting and purification) and the use of a radioactive tracer.
-The gamma spectrometry, despite that it allows direct determination of uranium and thorium through their decay products, requires a long time to establish an equilibrium radiochemical decay products.
These methods require the use of many reagents and require multiple steps in preparations which causes a large error in measurement, in addition these methods require a very long time.Therefore it would be useful to have a simple, fast and accurate method for the determination of uranium and thorium in industrial phosphoric acid.The X-ray fluorescence spectrometry (XRF) is always an acceptable technique due to its acceptable precision and accuracy associated with its lower cost compared to the techniques mentioned above.We propose to perform the analysis of uranium and thorium in the industrial phosphoric acid by the wavelength-dispersive X-ray fluorescence spectrometry (WDXRF) with the use of the intensity of the line Lα (Lα1 and Lα2) for the determination of uranium and thorium.The X-ray fluorescence was used to determine traces of uranium and thorium in zircon, forming a tablet with 100 mg of zircon (Voldet, 1972), the author has determined the values in the range of 100 to 2000 ppm of uranium and 100 to 300 ppm of thorium in rocks zircon of Greenland.XRF is also used for the determination of thorium in the range of 0.1 to 1.2% of the plutonium in (Woltermanet al., 1974).

Spectrometer
An X-ray fluorescence spectrometer (Magix 3kW, PW2403, PANalytical) was used for the determination of the uranium; it is a sequential spectrometer with wavelength-dispersive with a channel of measure based on a single goniometer covering the complete range of measure.It is equipped with an X-ray tube which is the X-ray source, the anode of the X-ray tube is in rhodium.

Scintillation Detector
It is constituted by a crystal of iodide of sodium in which the atoms of thallium are distributed in a homogeneous way (NaI; Tl), by a photocathode and by a tube photomultiplier.
It works by converting the X-ray in light which is then measured with a photomultiplier.

Sample Preparation
The samples of industrial phosphoric acid are analyzed directly without any prior preparation.Indeed, the industrial phosphoric acid is put in a cup (special sample holder for liquid) whose bottom is a polymer film and it is subjected directly to incident X-rays from the X-ray tube.In our case, it comes to an inverted optical.The X-ray tube is located under the sample.If the film broken during the analysis of the liquid, it can damage these parts.It is therefore necessary to adopt a sufficiently resistant film and make a compromise with the X-ray absorption and the presence of additives in the film (which allow the film to more resistant to some products).This problem does not arise in the case of direct optical.
For this problem, we used a film for liquid analysis by XRF which does not absorb X-rays and which has no effect or interference matrix which may influence our results.This film is X-Ray film MYLAR ® , diameter 63.5mm.We used a mass of 3 g with 0.1 mg of accuracy for industrial phosphoric acid sample.The analysis is made under helium (900 hPa pressure) to prevent boiling under the vacuum.
The WDXRF analysis of uranium and thorium in industrial phosphoric acid is shown in Figure 1.

Linearity, Detection Limit and Quantification Limit
The linearity of the proposed method was investigated by repeating the analysis (n = 6) at six concentration levels of 0, 20, 40, 60, 80 and 100 ppm for uranium and 0, 10, 20, 30, 40 and 50 ppm for thorium.
The intensity obtained at each concentration was plotted against the initial concentration of uranium and thorium and the linear regression equation was evaluated in the statistical treatment of calibration data.The characteristics of the regression were calculated using Microsoft Excel TM .We have the following curves in figure 2 and in figure 3: The measurement collected to perform the linearity can be used to calculate the sensitivity (b 1 ) and its standard deviation S(b 1 ) and the blank value b 0 and its standard deviation S(b 0 ).
These standard deviations (S(b 1 ) and S(b 0 )) define as follows the limits of detection and quantification, based on the two definitions: The limit of detection:LD =  0 +3.S( 0 ) 1 (2) The limit of quantification: LQ =  0 +10.S( 0 )  1 (3) standard deviation of the sensitivityb 1 ( 6) )standard deviation of the blankb 0 (7) 2 (): experimental variance of the regression.(): Sum of squared deviations for the variable .(, ): Sum of the products of deviations for the variable  and y.
The results of the study linearity test are summarized in the following The results of the test of evaluation of the linearity showed that the model of regression is considered acceptable for both for uranium and thorium and that the domain of linearity is validated.

The Specificity
To perform this test, we made 16 (p=16) standard additions on selected samples to cover the scope of the method.The regression line that passes from the points from the following equation is calculated: C 0 the intercept of the regression line.
C 1 the slope of the regression line.v i added content of uranium or thorium in the samples.r i observed content of uranium or thorium = content measured after addition -content measured before addition.We carryon a graph the values of v i additions and observed values r i and the straight recovery calculated and the line of

Interpretation
To check if the slope of the regression line is equal to1, a test is performed by calculating the  which follows aStudent distribution withp -2degrees of freedom. it is a criterion calculated to check the slope as it is described in the validation standard.

𝑆𝐶𝐸(𝑣):
Sum of squared deviations for the variable .( 1 ):standard deviation of the slopeC 1 a) If   is less than or equal to the critical value read from the Student table to the risk1 -α/2 with ν= p-2= 14degrees of freedom, it is possible to conclude that the absence of interference and that the specificity is acceptable.
b) If   is greater than the critical value ,the method is not specific.In general, the risk of error α is taken to 1%.  b) If ′  is greater than the critical value, it is possible to conclude that the intercept is not equal to 0 and that the method is not specific.In general, the risk of error α is taken to 1%.
 We obtain t' obs for uranium = 0.849 and t' obs for thorium = 0.085    <   it is possible to conclude that the absence of interference and that the specificity is acceptable for uranium and for thorium.
 ′  <   it is possible to conclude that the intercept is not different from 0 for uranium and for thorium.
 The slope of the regression line for uranium and for thorium is equal to1 and the intercept of the regression line for these two elements is equal to 0. It is thus possible to conclude that the absence of interference in the defined scope and the specificity for these two elements is acceptable.

3.4
The Fidelity (Repeatability and Reproducibility)

Repeatability
In the case of the study of the repeatability of the method of determination of uranium and thorium, 10 industrial phosphoric acid samples covering the scope were analyzed in triplicate repeatability conditions.
The internal repeatability variance of the alternative method   2 () is equal to the sum   ()divided by the number of degrees of freedom appropriate.We can calculate the coefficient of variation of repeatability: We obtain a value of standard deviation of repeatability intra-laboratory S r (): S r () for uranium = 0.147 and S r () for thorium = 0.042 and an average coefficient of variation of repeatability intra-laboratory   :     = 0.67 % and    ℎ = 1.02 %.

Checking the Stability of Fidelity
It is performed by means of the test of Cochran to the risk error α = 1%.This test is to ensure if the fidelity is constant throughout the application domain.
this is a ratio calculated from the equation cited above to verify the stability of fidelity.
value of a variable of Cochran to the risk α=1% with n number of distributionand number of sample p.
We compare   with   which is the critical value of the table of Cochran at the risk of error 1 %.The Statistical results of the verification of the stability of fidelity are presented in Table 2. Table 2. Statistical results of the verification of the stability of fidelity   for uranium and for thorium is lower than the value of the table at the risk of error 1 %.It is possible to conclude that the fidelity in terms of repeatability intra-laboratory is constant in all the field of application for these two elements.
The fidelity in terms of repeatability is acceptable.

Reproducibility
This test is used to calculate the intra-laboratory reproducibility of the method studied, that is to say his fidelity when repetitions are made by several operators or longer time intervals with respect to the method.A sample of industrial phosphoric acid was analyzed for 10 different days (p=10) with 3 replicates (n=3).The results are summarized in Table 3 for uranium and in Table 4 for thorium.To calculate the variance of internal reproducibility, we rely on these formulas:  We obtain a value of coefficient of variation of reproducibility intra-laboratory CV R : = 0.77 % and    ℎ = 1.37 %.
 Fidelity in terms of internal reproducibility is acceptable.

The Accuracy
In the case of the study of the accuracy of the method of determination of uranium and thorium in industrial phosphoric acid, 10 repetitions were performed on a NIST (National Institute of Standards and Technology) standards for uranium and thorium (the content of U is equal to 20 ppm and the content of Th is equal 10 ppm to ).We obtained an average concentration of 19,865 ppm for uranium with a variance of 0.205 and 10.083 ppm for thorium with a variance of 0.095.
To evaluate the accuracy from a reference standard:  Repeat by the same operator determining the analyte n test taken.
 Calculate the differencebetween the averages ofndetermination ( x ̅) and the certifiedconcentration ofuranium and thorium in the reference standard ( SR ).
 Calculatethe standard deviation ofmeasurements obtained S R ().The decision rule is as follows:  If t  > t  : error accuracy is significant, it is possible to conclude that the method is not justified.
 If t  ≤ t  : error accuracy is not significant, it is possible to conclude that the method is justified.
 The value of t  for uranium and for thorium is less than the value of the Student table.It is possible to conclude that the accuracy error is not significant.The method is considered justified.
 After evaluating the criteria of linearity, specificity, limit of detection and quantification, accuracy and fidelity, the method is acceptable in the scope defined.

Conclusion
The present work has developed a method for the determination of uranium and thorium in the industrial phosphoric acid obtained with wet process.This method has been validated on Tunisian phosphoric acid used as a liquid concentrate without dilution.It offers several advantages in particular the speed of measurement, accuracy and low cost of operation.The proposed method has the advantage of not using reagents for the preparation of samples for analysis, the samples were analyzed directly without dilution or preparation which increases the accuracy of the analyzes.Moreover the relatively lower cost of the sensor for the wavelength dispersive X-ray fluorescence (WDXRF) offers a major advantage compared to the energy dispersive analyzer (EDXRF) spectrometry, which has been frequently employed for determination of trace elements in phosphoric acid and the (WDXRF) analyzer has many advantages regarding resolution and selectivity over the (EDXRF).The application of the proposed method for the determination of uranium and thorium in industrial phosphoric acid shows that it is contains more uranium than thorium(of about22ppm of uranium and 4 ppm of thorium).

Figure 2 .Figure 3 .
Figure 2. Calibration of the method for the determination of uranium slope1 (equation y = x).The curves are shown in figure4for uranium and figure 5 for thorium.

Figure 4 .Figure 5 .
Figure 4. Study of the specificity of the method for the determination of uranium

Table 1
crit value of a variable of Student to the risk1 -α/2 withν= p-2degrees of freedom, the risk of error α is taken to 1% ( t crit = 2.977 from the Student table).Weobtain     = 0.788 and    ℎ = 2.046To check if the intercept is equal to 0, a test is performed by calculating the ′  which follows a Student distribution withp -2 degrees of freedom.t' obs it is a criterion calculated to check the intercept as it is described in the validation standard.
( 0 ):standard deviation of the interceptC 0 a) If ′  is less than or equal to the critical value read from the Student table to the risk1 -α/2 withp -2 degrees of freedom, it is possible to conclude that the intercept is not different from 0.

Table 3 .
Results of reproducibility test for uranium ) : repetition number per day (n=3).
this is a ratio calculated from the equation cited above to verify the accuracy.t  =2.083 for uranium and for thorium ist  =2.747  Obtain t  =  ,1− t 2 from the Student table.(t  = 3.355)  Compare t  tot