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EC number: 231-176-9 | CAS number: 7440-67-7
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Additional information
The present discussion has been mainly elaborated on the basis of the research work of E.Ferrand (2005 ; "Etude de la spéciation en solution, de la rétention dans les sols et du transfert sol-plante du zirconium."- ANDRA thesis). This work extensively considered the speciation of Zirconium in water and soil.
Zirconium metal holds three valences II, III and IV. Nevertheless, only the valence IV of zirconium is relevant under environmental conditions. In aqueous solution, the cation Zr4+is unstable and is subject to complexation with hydroxide ions (OH-) at all pH. This means that Zr4 +as such does not exist in the environment.
In pure water, zirconium tends to form different complex forms [Zr(OH)x(OH2)8-x](4-x)+, where x is a function of the pH of the aqueous solution.It can also exists under [Zr(OH)x](4-x)-, in alcalin medium.Moreover, depending on the pH, the concentration of Zr and the nature of the surrounding minerals, Zr can polymerize to form hydroxo bridges. The predominant species in solution are Zr(OH)4and Zr(OH)5- at environmentally relevant pH. At pH values below 5, the predominant species in solution are Zr(OH)3+, Zr(OH)22 +and ZrOH3+(Blumenthal WR, 1958, The Chemical Behaviour of Zirconium).
The behaviour of Zr in aqueous solution is higly complex and speciation has not been as widely studied as for other more common metals. Speciation models like ECOSAT give the following indications:
1) For high concentration of Zr (> 10 -5 M) precipitation and polymer species prevail [Zr(OH)x(OH2)8-x](4-x)+for pH between 3 and 12
2) In contrast, for very low concentration of Zr (< 10 -6M) monomer species prevail [Zr(OH)x](4-x) and is mainly in solution.
For Zr metal, it should be noted that in contact with moisture/water, Zr is directly oxidised and a relatively stable passivation layer of ZrO2 is formed at the surface. As ZrO2 is poorly soluble (< 0.055 mg/L), the behaviour of Zr in water is driven by the solubilisation and complexation of ZrO2, which is highly dependent on the water chemistry and the hydroxide species presents in solution that could lead to complex formation as described above.
The peer-reviewed database HSDB confirms that the aqueous chemistry of zirconium is characterized by a high degree of formation of polymeric species and various complex ions. It also mentions that Zirconium forms very stable oxides and that Zirconium compounds would not volatilize from aqueous solution due to their ionic character.
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