**2. Aqueous chemistry of Ce(IV)**

### **2.1. Ce(IV) in aqueous solution**

Ce is the only lanthanide element that can form stable molecular complexes in the +4 oxidation state. The stability of the +4 state of cerium is attributed to the 4f<sup>0</sup> electron configuration [51]. However, much less is known about the properties of Ce(IV) aqueous species than those of Ce(III). For example, the hydration structure of Ce(II) has been extensively studied by many methods [52–58]. In most cases, it is a part of the systematic study of the trivalent rare earth series. In contrast, even for the simplest aqua species of purely hydrated complex, there is little success in the identification of Ce(IV) aqua complexes in solution. In fact, in the scientific literature, Ce(IV) aqua species are often described only as "Ce(IV)" or "Ce(IV) complex," without specifying their chemical species or composition, simply because of a lack of information [59]. Moreover, because of their high electric charge, Ce(IV) has a strong tendency toward hydrolysis in aqueous solution and undergoes polynucleation or further, leading to colloid formation [60]. Several precedent studies have also implied the formation of soluble polymeric species with oxo- and/or OH-bridging [61]. Based on an extended X-ray absorption fine structure (EXAFS) study and density functional theory (DFT) calculations, Ikeda-Ohno et al. have demonstrated that the Ce(IV) ion in perchloric acid exists in the form of oxo-bridged dimer (**Figure 2**) [59].

20.13 ± 0.37, and 24.14 ± 0.10, respectively [19]. Besides, in sulfuric acid medium, Qiao studied the complexation behavior of fluorine (I) with Ce(IV). The results show that Ce(IV) and F(I) could form

Many extractants have been reported and applied in nitric acid and sulfuric acid for Ce(IV) extraction, for example, acidic organophosphorus extractants [22–27], neutral organophosphorus extractants [28–36], amines [37], and bifunctional ionic liquid extractants (Bif-ILEs) [38–40]. Among them, tributyl phosphate (TBP), di-(2-ethylhexyl) 2-ethylhexyl phosphate (DEHEHP), di-(2-ethylhexyl) phosphate (P204), 2-ethylhexylphosphoric acid mono 2-ethylhexylester (P507), and Cyanex 923 are the most commonly used extractants for Ce(IV) extraction (listed in **Table 2**). Synergistic extraction [26, 35] is also an important method to enhance the extraction efficiency. It was reported that P204 + P507 and P204 + Cyanex 923 had synergistic

Several acidic organophosphorus extractants [22–27] were used to extract cerium(IV); among these extractants, P204 or P507 is a great extractant for Ce(IV) with a high capacity, extraction

with P204 in 1957. Tedesco et al. studied the extraction of cerium in kerosene from sulfuric acid solution by di(2-ethylhexyl) phosphate (P204, HA). The effects of DEHPA concentration and pH on the extraction of cerium were determined. However, the mechanism of extraction of Ce(IV) by P204 is not clear. Tedesco et al. considered that the possible extraction mecha-

When the R-O group in dialkyl phosphoric acid molecule is replaced by R group, such as P507, its pKa value increases and its acidity increases. The distribution ratio of rare earth elements extracted by P507 is lower than that of P204. Li et al. [24] have studied the separation of Ce(IV) with P507 in nitric acid system and sulfuric acid system. The mechanism of extraction

efficiency, and selectivity. Peppard et al. studied the extraction of Ce(IV) from HNO<sup>3</sup>

extraction effects for Ce(IV) extraction from sulfuric acid medium.

2+] and logarithm of the average values of ß was 10.67 [20].

Extraction and Recovery of Cerium from Rare Earth Ore by Solvent Extraction

http://dx.doi.org/10.5772/intechopen.79225

9

<sup>2</sup> = CeH4 A8 + 4H<sup>+</sup> (1)

<sup>2</sup> <sup>=</sup> (CeA4)n <sup>+</sup> 2nH<sup>+</sup> (2)

<sup>2</sup> = CeOH<sup>2</sup> A4 + 2H<sup>+</sup> (3)

<sup>2</sup> <sup>=</sup> (CeOA2)n <sup>+</sup> 2nH<sup>+</sup> (4)

solution

a stable complex in the form of [CeF<sup>2</sup>

**3. Solvent extraction of Ce(IV)**

**3.1. Acidic organophosphorus extractants**

Ce4<sup>+</sup> + 4 (HA)

nCe4<sup>+</sup> + 2 (HA)

CeO<sup>2</sup><sup>+</sup> + 2 (HA)

nCeO<sup>2</sup><sup>+</sup> + n (HA)

of Ce(IV) in sulfuric acid system by P507 is as follows:

nism is as follows [22]:

When pH < 1.0

when pH = 1.7–2.0

**Figure 2.** A proposed structure of the cationic dinuclear Ce(IV) species present in perchloric acid.

Using synchrotron X-ray and Raman spectroscopies and EXAFS, Ellis et al. also found that in strong acidic nitrate solution, ammonium ceric nitrate is a dinuclear Ce(IV) complex with a bridging oxo ligand, formulated as [(H2 O)<sup>x</sup> CeIV-O-CeIV(OH<sup>2</sup> ) x ] 6+ (x = 6 or 7) [62]. On the contrary, the present quantum chemical calculations confirm that the Ce4+ coordination number is 9 and the relative free energies of Ce4+ is the 10- and 8-coordinate isomers in aqueous solutions.

#### **2.2. Ce(IV) complexes with anions in aqueous solution**

Ce(IV) is unstable in perchloric acid aqueous solution because its standard electrode potential in perchloric acid aqueous solution is 1.61 V [19]. Therefore, when water decomposes and releases oxygen, Ce(IV) would be slowly reduced to Ce(III). In addition, Ce(IV) is very easy to hydrolyze and polymerize like other tetravalent cations. It is necessary to maintain high acidity in the medium to avoid it. Due to these difficulties, the data of the stability constants of Ce(IV) complexes are very scarce.

Although the studies on nitrato-cerium complexes started earlier, there is contradictory information about these complexes in the relevant literature [63–66]. It was found by potentiometric method that when [NO<sup>3</sup> − ] < 3.2 mol/L, there was only one complex in the form of Ce(NO<sup>3</sup> ) 3+ in nitric acid aqueous solution [67]. The other two methods, the spectrophotometry and the extraction method, indicate that the ligand number of nitrato-cerium complexes may vary from 1 to 6. The distribution of Ce(IV) species in aqueous media was studied by measuring the total optical absorbance of Ce(IV) species in different nitric acid-perchloric acid mixture solutions. The stability constants of the nitrato-cerium complexes were determined spectrophotometrically [67].

There are considerable evidences of complex formation between ceric ions and sulfate ions in aqueous solution. Jones et al. have measured the migration in high sulfur concentration solutions and found that the color migrated to the anode. Some researchers [68–70] have measured the electromotive force of the cerium sulfuric acid solution; the results show that a complex is certainly formed, but its nature cannot be determined clearly. Besides, evidence for complexing has been found by Moore et al. [71] from kinetic studies on the reaction of Ce(IV) with arsenite ions. The first complex of Ce(IV) and sulfate in perchloric acid solution was studied by spectroscopic method [72]. The results show that the instability constant of the first complex varies with the concentration of total Ce(IV) ion plus sulfate and a higher complex was also found in this system. Hardwick et al. [73] made a spectral study on the association of Ce(IV) with sulfate. The results show that Ce(IV) interacts with one, two, and three sulfate ions in turn to form complexes. As is expected, as the number of sulfate ions in the complexes increases, the trend of association becomes smaller. Nevertheless, there were no higher complexes of more than three sulfate complexes with Ce(IV).

The stability constants of the fluoride complexes of cerium(IV) in 1 M (HCIO<sup>4</sup> , NaClO<sup>4</sup> ) medium have been measured potentiometrically using a fluoride ion-selective electrode. This procedure ensures the stability of the oxidation state and prevents hydrolysis and polymerization of Ce(IV). Logarithms of the average values of ß<sup>1</sup> , ß<sup>2</sup> , ß<sup>3</sup> , and ß<sup>4</sup> were estimated to be 7.57 ± 0.04, 14.50 ± 0.03, 20.13 ± 0.37, and 24.14 ± 0.10, respectively [19]. Besides, in sulfuric acid medium, Qiao studied the complexation behavior of fluorine (I) with Ce(IV). The results show that Ce(IV) and F(I) could form a stable complex in the form of [CeF<sup>2</sup> 2+] and logarithm of the average values of ß was 10.67 [20].
