Cyanex 272 is also used to extract rare earth elements in industrial operations [ 34 35 ]. Xie et al. highlighted that Cyanex 272 and its derivatives have the ability to extract rare earth elements [ 36 ]. Notably, the saponification of Cyanex 272 is necessary to maintain a high extraction efficiency of rare earth elements [ 35 37 ]. When the solution pH is controlled, the use of Cyanex 272 is better than that of Cyanex 301 for the separation of Pr(III) and Nd(III) over La(III) owing to its good extraction capacity and high separation factor. Compared to hydrogen ions, the sodium ions in saponified Cyanex 272 can be better replaced by the rare earth element ions. Therefore, the solution pH can be controlled and thus the extraction efficiency can be maintained. Besides, the appreciable differences in the extraction behavior of Pr(III), Nd(III), and La(III) between Cyanex 301 and Cyanex 272 may be attributed to the acidity of the metal ions which is related to their ionic radii. This is because Pr(III) and Nd(III) ions (hard acids) with smaller ionic radii would prefer to bind to the saponified-Cyanex 272 (hard bases) than Cyanex 301 (soft base). Secondly, the separation factor for Pr(III)/Nd(III) and La(III) was low in the use of Cyanex 301 due to the increase in the co-extraction of La(III) as the Cyanex 301 concentration increased [ 35 ].
A high loading capacity of Cyanex 302 for Fe(III) (29.41 g Fe(III)/100 g Cyanex 302) was obtained from a 1 M HCl solution [ 29 ]. Additionally, Cyanex 301 and Cyanex 302 are considered to have a high affinity for the silver and palladium present in concentrated HCl solutions owing to the strong interactions between soft bases (extractants) and soft acids (metal ions) [ 30 32 ]. By contrast, Cyanex 272 contains oxygen which is considered to be a hard base and thus is less attractive to Pd(II). Au(III) was selectively extracted by Cyanex 272 in the HCl concentration range of 0.5 to 9 M, whereas Pd(II), Pt(IV), Rh(III) and Ir(IV) are not extracted at all [ 3 ]. Cyanex 301 was employed to separate Pd(II) over Pt(IV) from hydrochloric acid solutions [ 33 ]. This result is consistent with a previous study [ 31 ] showing that Cyanex 301 is an effective extractant for Pd(II). Cyanex 301 can completely extract Pd(II) in the HCl concentration range of 1 to 9 M, while Cyanex 272 is not effective.
Adekola et al. discovered that Cyanex 272 is effective for the selective extraction of Co(II) over Ni(II) in chloride solutions at pH 4 [ 27 ]. Moreover, the addition of amine extractants like Alamine 308 into Cyanex 272 enhanced the extraction efficiency and the selectivity of Co(II) over Ni(II) [ 28 ]. This could be ascribed to the fact that amine can extract hydrogen ions which are released during extraction, whereby the equilibrium pH can be controlled and thus Co(II) extraction is improved. Compared to Cyanex 272, Cyanex 301 showed a higher extraction efficiency for Co(II) from the synthetic leach liquor of spent lithium-ion batteries [ 7 ]. The advantage of Cyanex 301 lies in its higher loading capacity for Co(II) from dilute chloride solution with pH 6 than Cyanex 272. This can be related to the structual characteristics of Cyanex 301 which contains only sulfur donor atoms in its functional group compared to Cyanex 302 and Cyanex 272. However, the extraction efficiency depends on the temperature and the polarity of the diluents. In general, the strong interaction between polar diluents and extractants leads to a decrease in the extraction efficiency of Co(II) [ 7 29 ]. Therefore, a diluent with low dielectric constants like kerosene is appropriate for Cyanex 301 and Cyanex 272.
It has been reported that Cyanex 301/302 is better than Cyanex 272 in Cu(II) extractions from hydrochloric acid solutions in terms of loading capacity and solution pH. For instance, Cu(II) was selectively extracted (96.3%) over other metals such as Pb(II), Zn(II), a low trace of Mn(II), Fe(III), Co(II) and Cd(II) by 0.2 M Cyanex 272 at an equilibrium pH of 5.0, while the same performance was obtained by 0.08 M Cyanex 301 at an equilibrium pH of 3.8 [ 25 26 ]. Between HCl and HSOsolutions, the separation of Cu(II) by Cyanex 302 from a HCl solution was more efficient than from HSOsolution because Cu(II) exists as the hydrated copper species ([Cu(OH)) in the sulfate medium which would not be well extracted [ 26 ].
Cyanex 272 can selectively extract Fe(III) from sulfate solutions over Cr(III) and Zn(II) or Co(II), Ni(II), Mn(II) in the solution pH range of 1.8 to 2.5, while Cyanex 302 could only separate Fe(III) over Co(II) and Ni(II) [ 6 45 ]. In contrast, metal ions such as Cu(II), Zn(II), Fe(III), Co(II) and Ni(II) can be extracted from sulfate solutions in the pH range of zero to 2.5 by Cyanex 301 [ 6 ].
However, Cyanex 302 is considered to be more efficient at extracting Co(II) and Ni(II) in the presence of Mg(II) than Cyanex 272 [ 41 42 ]. Indeed, both Cyanex 272 and Cyanex 302 show good performances for Co(II), and Cyanex 302 shows selectivity for Co(II) over Mg(II) from sulfate solutions [ 43 ]. In sulfate solutions containing Co(II), Ni(II) and Mg(II), Co(II) separation can be achieved by two steps: (i) the separation of Co(II) and Mg(II) over Ni(II) by Cyanex 272; (ii) the separation of Co(II) from Mg(II) by Cyanex 302. In particular, the control of the solution pH has a significant influence on the extraction percentage of the metal ions. The extracted complexes by Cyanex 272 and Cyanex 302 are proposed to be [Co(HA] and [Co(HA(H], respectively.
It has been reported that Cyanex 272 is the most suitable extractant for the separation of Co(II) and Ni(II) from sulfate solutions due to the stability it provides conventional oxidants, good physical and chemical properties, and the ability to avoid the crystallization of gypsum in electrochemical circuits [ 38 ]. In practice, the use of Cyanex series extractants for the separation of Co(II) can be achieved under various experimental conditions. However, very high pH conditions should not be taken to avoid the formation of some precipitates. In addition, the replacement of sulfur in functional groups increases the acidity of Cyanex 301/302 and leads to a more efficient extraction of Cu(II), Co(II), and Ni(II). Table 3 lists the equilibrium constants for the extraction reactions of Cu(II), Co(II) and Ni(II) with Cyanex series extractants [ 39 ]. As expected, the extraction order of Cu(II) follows Cyanex 301 >> Cyanex 302 > Cyanex 272 and a complete extraction of Cu(II) by Cyanex 301 and 302 was obtained at a low pH. Cyanex 272 reacts with Cu(II) to form a square planar mononuclear complex such as Cu(HA, while the solvent extraction of Cu(II) by Cyanex 301 and Cyanex 302 is accompanied by a redox reaction in the organic phase: (i) reduction in Cu(II) to Cu(I) and (ii) the oxidation of extractants to disulfides (R-R) [ 14 ]. Namely, CuR(extracted complex of Cu(II) with Cyanex 301/Cyanex 302) is further polymerized in the organic phase to form oligomeric multinuclear copper complexes. The extraction reactions of Cu(II) and sulfur-containing ligands are presented as Equations (8) and (9) [ 40 ].where HR = Cyanex 301/Cyanex 302.
When oxygen atoms of the functional group of organophosphorus acids like Cyanex 272 are replaced by sulfur atoms, the Ag(I) extraction efficiency from nitric media is enhanced. Cyanex 301/302 can extract Ag(I) in a complex form of Ag(HA) (HA = Cyanex 301/302). The increase in the extraction percentage of Ag(I) could be explained by the formation of durable complexes due to the high affinity between soft Lewis acids (Ag(I)) and soft Lewis bases (Cyanex 301/302). In addition, the replacement of sulfur atoms increases the acidity of the extractants, resulting in the shift of the metal extraction to a lower pH in the ligand order: RPO> RPSO> RPS 46 ]. However, the durable complexes of Ag(I) with Cyanex 301 may make stripping difficult and thus a very high concentration of HNO(16 M) was employed for complete stripping [ 46 ].
The nature of the anionic complexes plays an important role in the extraction of metal ions such as Th(IV), Ag(I), Zr(IV) and La(III) from strong mineral acid media [ 46 48 ]. According to the HSAB principle, the hard Th(IV) strongly interacts with a hard base like the oxygen atom in the functional group of Cyanex 272. The replacement of oxygen with sulfur in organophosphorus acids such as Cyanex 301 and Cyanex 302 increases the soft character. The extraction efficiency of Th(IV) from HNOand HCl solutions is in the order Cyanex 272 > Cyanex 302 > Cyanex 301, while the efficiency of the sulfuric acid solution is reversed. This can be attributed to the strong tendency of Th(IV) to form complexes with sulfates, which results in a decrease in its hard character. Thus, the extraction of Th(IV) from a sulfate medium by Cyanex 301 is better than from other mediums. Based on slope analyses, the extracted species are proposed to be Th(NOand ThSO(with R denoting the anion of organophosphorus acids) [ 47 ].
The extraction of metal ions by Cyanex 272, Cyanex 301 and Cyanex 302 is generally carried out in weak acidic media. Therefore, higher acid conditions are favorable to affect stripping [ 13 ]. It was reported that many metal ions in the loaded phases of Cyanex extractants can be stripped by acidic agents such as HCl, HSO, HNO, and HClO 31 ]. For example, Co(II) from the loaded Cyanex 272 can be effectively stripped by a 1.0 M HCl solution [ 27 ]. Depending on the soft&#;hardness of the desired metal ion, suitable stripping agents can be selected. The stripping of Pd(II) from a Cyanex 302 system requires the use of a soft ligand such as thiourea in HCl, HClO, and HNO 31 ]. Along with these inorganic solutions, a mixture of ammonia and ammonium chloride was employed to strip Ni(II) from the loaded Cyanex 301 by utilizing the complex formation tendency between Ni(II) and ammonia [ 49 ]. The addition of ammonium chloride to ammonia could enhance the phase transfer rate and lead to a good separation of the two phases. The optimum composition for the stripping of Ni(II) was reported to be the mixture of 5% NHCl and 75% ammonia [ 49 ].
30,50,3 solution [CoR 2 &#; O 2 CoR 2 nO 2 &#; HR CoR 3 + H 2 O
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(10)
However, some complexes of metal ions like Cu(II), Ag(I), Pd(II) and heavy rare earth elements taken in by Cyanex 301 and Cyanex 302 are difficult to strip [ 13 51 ]. This might be related to the strong interaction between the functional group of Cyanex 301/302 and metal ions according to the HSAB principle. Namely, the stability of the complexes of Ag(I) and Cyanex 301/Cyanex 302 in the organic phase is so high that quantitative stripping is only possible by a 16 M HNOsolution [ 46 ]. Therefore, the nondestructive stripping of the metal ions from these extractants is very complicated [ 46 52 ]. Moreover, the difficulty in stripping Cu(II) from Cyanex 301/302 is also attributed to the reduction in Cu(II) to Cu(I). Therefore, mild acid stripping is ineffective because the re-oxidation of the metal ion to its divalent is required for an effective stripping [ 13 26 ]. In the other words, Cu(II) is more strongly solvated by water than Cu(I) and thus stripping can be easily performed. In particular, Cu(II) was stripped from the loaded organic phase of Cyanex 302 by using a high concentration of nitric acid (8&#;10 M) which acts as an oxidizing agent [ 26 53 ]. When such concentrated acids are used, there is some significant negative effect on the extractants. These might be a discoloration and increased viscosity of the organic phases, as well as a deterioration of phase separation. The decomposition and long-term stabilities of Cyanex 301 and Cyanex 302 in contact with sulfuric and nitric acids have been investigated because of its serious impact on environmental and economic issues [ 14 ]. Besides, the effect of metal oxidation in the loaded Cyanex 301 has been investigated. Some researchers have found metal to oxidize by air during solvent extractions [ 54 55 ]. They suggested that Co(II) is possibly oxidized to Co(III) by air in the organic phase. The resulting Co(III) complex is kinetically very inert, leading to the difficulty in stripping. For example, a cobalt oxidation reaction in the presence of Cyanex 301 is proposed as
Denote. HR: Cyanex 301.
2 CoR 2 + R &#; R = 2 CoR 3
(11)
The reaction can also occur by an oxidizer, i.e., the disulfide of the dithiophosphinic acid (R &#; R) present in the Cyanex 301, as follows:
The back reaction, namely the reduction in Co(III) to Co(II) under normal conditions, is very slow due to the inertness of the cobalt(III) complexes [ 55 ]. In order to prevent the oxidation of the extracted Co(II) to Co(III), as well as the oxidation of Cyanex 301 to a disulfide by oxygen, the extraction was performed in an inert gas stream (nitrogen or carbon dioxide) [ 40 ]. This route can result in an efficient stripping of Co(II) and prevent the oxidation of Cyanex 301. However, the use of concentrated hydrochloric acid for the stripping is a disadvantage of this process [ 40 ].
33,4, and HNO3, but not H2SO4 because of the protonation of thiourea [3 solution [Various stripping agents have been tested to overcome the above-mentioned problems [ 31 56 ]. The stripping of Pd(II) from Cyanex 302 and some mixtures of Cyanex 301 require the use of a soft ligand like thiourea in HCl, HClO, and HNO, but not HSObecause of the protonation of thiourea [ 31 33 ]. In this case, thiourea is not a stripping agent and helps to form complexes which makes metal stripping easy with acid. The stripping rate increases with increasing thiourea concentrations owing to the formation of the complexes. The utilization of ligands containing sulfur atoms was also effective in the stripping of Pd(II) with a HCl solution containing LiSCN [ 31 ]. In the case of Cu(II), stripping from the loaded phase of Cyanex 301, an aqueous solution of thiourea, hydrazine, and sodium hydroxide was used, resulting in a quantitative stripping as well as the regeneration of the extractant. In this mixture, thiourea acted as the stripping agent, while hydrazine lessened the formation of disulfide. However, the high cost of thiourea and the toxicity of hydrazine limits the commercial application of this method [ 40 ]. The stripping performance was enhanced when Y(III) was loaded into an organic mixture of 8-hydroxyquinoline and Cyanex 301 and a mixture of 8-hydroxyquinoline and Cyanex 302. Y(III) in the organic loaded phase was quantitatively stripped by a 0.06 M HNOsolution [ 56 ]. Since stripping depends on the nature of the metal complexes in the loaded organic phase, the nature and the concentration of acidic stripping solutions affect the stripping efficiency. An important objective is to find the most suitable stripping agent to recover the metal with the highest possible purity. The stripping efficiency of some metal complexes from the loaded phase of Cyanex 272/301/302 is also summarized in Table 4
The other name of hypo phosphorous acid is phosphinic acid. Hypo phosphorous acid is a strong reducing agent. Hypo phosphorous acid has a property to get soluble in solvents like dioxane, water and alcohol.- In the question it is asked about the number of &#;OH groups and number of hydrogen atoms attached to hypo phosphorous acid.- To know about the &#;OH groups and number of hydrogen atoms we should know the structure of the hypo phosphorous acid.- The chemical formula of hypo phosphorous acid is ${{H}_{3}}P{{O}_{2}}$ .- The structure of hypo phosphorous acid is as follows.- In the above structure we can see clearly that there is one &#;OH group and two hydrogen atoms attached directly to the phosphorus atom.- Therefore in the structure of hypo phosphorous acid One OH group and 2-H atoms are directly attached to P.Hypo phosphorous acid is going to donate only one hydrogen atom, which is attached to oxygen. Hypophosphorous acid is going to be used in fischer esterification as an additive. In chemical reactions hypophosphorous acid is going to act as a reducing agent.
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