DERIVATIVE SPECTROPHOTOMETRIC DETERMINATION OF SOME RARE EARTH ELEMENTS IN EGYPTIAN MONAZITE

Abd El-Rehim S.S 1 , Mohammed A.A 2 , El-Wakil A.F 2 and Abd Al AzizR.S 2 . 1. Faculty of Science, Ain Shams University, Cairo, Egypt. 2. Nuclear Materials Authority, P.O. Box 530 El-Maadi, Cairo, Egypt. ...................................................................................................................... Manuscript Info Abstract ......................... ........................................................................ Manuscript History

1634 fluorimetry is based on used the first-derivative and Kalman-filter techniques. The procedure is satisfactory for the simultaneous determination of these four lanthanides (Jianjun L., et al., 1990).
A sensitive simple method for the determination of trace amounts of samarium by spectrophotometry is based on the formation of the samarium-chrome azurol S (CAS)-cetylpyridinium chloride (CPC) complex in micellar medium (Marezenko M., 1986. Beer's law is obeyed from 0.05-2 mg l -1 of samarium at 505 nm as Sm-CAS-CPC complex. Optimal conditions for the samarium determination were studied (Soylak M. and Türkoğlu O., 2000).
The used concentrate rare earth oxide has been extracted from Egyptian monazite upgraded to about 97 % mineralogical grade. It was obtained from black sands deposited at Rosetta area on Mediterranean coast. Monazite is resulted as a byproduct during the recovery of the economic heavy minerals. The mineral monazite is composed of rare earth phosphatesalong with thorium and uranium , SoylakM.,et.al, 1997.
In the present work, the ability to simultaneous determine Samarium, Europium and Gadolinium in the produced concentrate rare earth oxide using chrome azurol S (CAS) dye and cetylpyridinium chloride (CPC) as cationic surfactant was studied. Optimum factors affecting the formation and measuring of the complexes such as maximum wavelength, pH, initial concentration of dye and surfactant, molar ratios and calibration curves were investigated.

Experimental:-Instrumentation:-
A double beam UV-Visible spectrophotometer model Shimadzu (UV-11601) from Japan was used, The spectrophotometer ranges from 190 to 1100 nm, with a resolution of 2 nm and a wavelength accuracy of ±0.5 nm were conducted.
A NEL 980 pH meter was used for recording the pH of the studied solutions. This was calibrated with three successive buffer solutions (4, 7 and 10).

Reagents:-
The chemicals and reagents used through the experiments carried out in the present study were analytical grade. Doubly distilled water was used for preparing all solutions.
Sodium hydroxide, Ammonium hydroxide, Sodium tetraborate, Hydrochloric acid, Sulphoric acid, Nitric acid,Perchloric acid,Cetylpyridinium chloride (Sigma-Aldrich), Dowex50W-X8 (Acros-Organic), EDTA (Merck), Chrome azurol S, Standard solutions 1000 ppm Sm, Eu and Gd (Fluka). A solution of 10 -3 M of dye solution was prepared by dissolving 0.2 g of Chrome azurol S (CAS) dye with double distilled water in a 100 ml volumetric flask and completed to the mark to give a concentration of 0.2%. Cetylpyridinium chloride (CPC) solution 10 -3 M was prepared by dissolving 0.2 g of cetylpyridinium chloride in doubly distilled water in 100ml volumetric flask and completed to the mark to give a concentration of 0.2%. Buffer solution was prepared by dissolving 0.4 g of sodium hydroxide and 1.0061 g of sodium tetraborate in double distilled water in 100ml volumetric flask and completed to the mark to obtain buffer solution of 0.1 M sodium hydroxide and 0.05 M sodium tetraborate.

Results and Discussions:-
Spectrum of chrome azurol S dye and complexes:-Absorption spectrum of chrome azurol S dye could be defined when measured against water as a blank. Figure (1 ) represents the absorption spectrum of chrome azurol S dye as well as the absorption spectra of Sm, Eu and Gd-CAS-CPS complexes against water as a blank in the visible regios between 250 nm-650 nm. Inspection of the data reveal that the maximum absorbance value for the dye is at 450 nm while for the three complexes is at 350 nm. Selection of the type of derivative for the spectrophotometric determination of the complexes:-Derivatisation of spectra is the simplest method for increasing the selectivity, this operation allows to overcome spectral interferences and as a consequence leads to increase the selectivity of assay. Derivatisation of digital data is well known method of separation useful signals from noised data, and by tested first, second, third and fourth derivative it is found that, the selectivity increased by fourth derivative.

Selection of the suitable pH for the spectrophotometric determination of the complexes:-
It is found necessary to study the effect of the pH values on the formation of the complexes. Most colored metallochrome reagents are pH indicators (pH dependence reagents) where they change colour normally in complexation with metals but also due to their indicator properties. Thus the pH value plays an important role in the colour intensity of the dye itself and the formed complex. Firstly the effect of different pH values on the absorption spectrum of free chrome azurol S dye will be studied. For this reason the basicity of the aqueous solution was changed for different pH values ranging from 6.5 to 8 (Soylak M. and Türkoglu O, 2000) and the dye concentration was kept constant in all solutions where the final volume of these solutions was completed to 25 ml. It is clearly evident that the maximum absorption occurs at pH 7.5 which adjusted by borax buffer. Table (1) summarized the effect of changing pH on the absorbance of complexes.

Effect of surfactant concentration on the absorbance of the complexes:-
It was necessary to optimize the concentration of CPC as a cationic surfactant , studying the effect of its concentration on the sensitivity and molar absorbitivity of the produced complexes. For this purpose different volumes of (10 -3 M CPC) solution were added to a series of 25 ml volumetric flasks containing fixed volumes of (10 -3 M dye) and (Sm, Eu or Gd) and adjust to pH 7.5 by borax buffer. The final volumes of these solutions were completed with doubly distilled water. The absorbance of each solution was then measured against blank. Figure (2) shows that, maximum absorbance was attained at 0.5×10 -3 M of (CPC) for the three metals.

Effect of duration time on the stability of the complexes:-
The (M-CAS-CPC) complexes were left for period of time. Their absorbance were measured periodically each 10 minutes for the first half hour, then left to another half hour and then measured again. (Table 2) indicates that, the best absorbance of the complexes were between 10 minutes and 30 minutes. 1637

Composition of the complex:-
In the present work, continuous variation method was used to investigate the ratios between dye and metal. In continuous variation method the molarities of both components (metal-CAS) were changed while keeping the total number of moles of both components constant while surfactant and buffer solutions unvaried. Figures (4, 5 and 6) indicate that, the molar ratio of the complexes Sm, Eu and Gd-CAS is 1:1.

Construction Of Calibration Curves:-
After the detailed study of the relevant factors affecting the chrome azurol S spectrophotometric determination of Samarium, Europium and Gadolinium, it was found necessary to determine the concentration ranges of Samarium, Europium and Gadolinium above and below which no further complexes could be formed and hence determined.
These two limits could be identified through the construction of calibration curves for the complexes. This spectrophotometric determination must be carried out within the concentration range which Beer's law is obeyed.
The fourth derivative spectra provide linear calibration graphs in the range from 0.3 to 12 μg ml -1 for Sm, Eu and Gd respectively as shown in Figures (7, 8 and 9).

Studies the interference Effect:-
It was necessary to study the interference effect of the elements under studying (Sm, Eu and Gd) on each other, and some other rare earth elements associated with them.   were measured at the same optimized conditions. Tables (6, 7, 8 and 9) indicate that the concentration of La, Nd, Pr and Y thatmixed with Sm, Eu and Gd should not exceed than 0.007, 0.06 and 0.008 µg ml -1 respectively. The excess above these concentrations cause systematic error on determination of Sm, Eu and Gd using conventional spectrophotometry.    Accordingly, it was found necessary to apply derivative spectrophotometry technique to overcome the effect of interfering elements on measuring Sm, Eu and Gd in the REE concentrate. It was found that fourth order derivative is the suitable one for the determination of Sm, Eu and Gd-CAS-CPC complex in presence of Pr and Nd and in the presence of each other but it's necessary to eliminate La element firstly. To eliminate La element about 15g of the pure concentrate rare earth oxides was dissolved in the minimum amount of concentrated HCl and the obtained solution was completed up to 500 ml by distilled water after pH adjustment to 2.5 (total REO concentrate about 30 g/l) . About 450 ml of the latter was used to saturate an adsorption resin bed (Dowex 50W-X8,100-200 mesh size) in its hydrogen form packed in a small column (2.2 cm height and 1.1cm diameter). This was followed by washing the resin bed with distilled water until chloride-free. The La load was then selectively eluted using the normal elution procedure, the normal elution procedure (1ml/min) was performed with a known volume of 0.015 M EDTA solution.
The effects of anions (Cl -, NO -3 , SO 4 -2 and ClO -4 ) on measuring of Nd with CAS and CPC at the optimized conditions were studied. For this purpose a series of standard solutions of the studied anions (from 0.1×10 -4 M to 2×10 -2 M) were prepared, then mix 1ml of each standard solution with 1ml of (10 -3 M) of each Sm, Eu and Gd, 1ml of (10 -3 M) CAS and 0.5ml of (10 -3 M) CPC, at pH 7.5, then up to volume in 25ml volumetric flask. The standard solution of each matals in the absence and presence of various concentrations of the anions were measured at the optimized conditions against reagent blank. The critical concentration of each anion used above which interference began to disturb the absorbance of pure metal-CAS-CPC complex was determined. Table (10) reveals that the measuring of metals in presence of Cl -, NO -3 , SO 4 -2 and ClO -4 be possible conventionally up to 2, 1×10 -2 , 0.9×10 -2 and 0.7×10 -2 M respectively, the excess above these concentrations cause systematic error upon determination. From studying the interference effect of these anions, it was shown clearly that conventional spectrophotometry method is inapplicable for determination in presence of other elements due to the interference effect, but use fourth order derivative to overcome the interference.

Application:-
The Egyptian monazite is mostly derived from granite and pegmatite (Hammoud, 1966). Placer monazite is the most important source of rare earth elements, it has been largely abandon because of its high thorium content. (El-Hinnawi, 1964 andDabbour, 1994) revealed that, this monazite contains 5.92 to 6.04 % ThO 2 , 0.45 and 0.48 % U 3 O 8 and 70.7. and 61.31 wt % RE 2 O 3 . Therefore processing of monazite is of great importance as a source of rare earth elements and thoriumuranium concentrates production.
To test the utility of the method, use concentrate rare earth oxide extracted from Egyptian monazite upgraded to about 97 % mineralogical grade. The oxide was obtained from black sands deposited at Rosetta area on Mediterranean coast. Monazite is resulted as a byproduct during the recovery of the economic heavy minerals. The standard deviations are 0.208, 0.208 and 0.305 and errors are 3.5 % , 4.9 % and 5.1 % for Sm, Eu and Gd respectively. Table (11) shows the results from the present method and compared with ICP-OES (Nuclear Material Authority).

Concolusion:-
The proposed method involves the use of fourth derivative spectra for the determination of Samarium, Europium and Gadolinium complex with chrome azurol S dye in the presence of cetylpyridinium chloride. The method is