Abstract
The phase transformation of iron minerals induced by aqueous Fe(II) (Fe(II)aq) is a critical geochemical reaction which greatly affects the geochemical behavior of soil elements. How the geochemical behavior of rare earth elements (REEs) is affected by the Fe(II)aq-induced phase transformation of iron minerals, however, is still unknown. The present study investigated the adsorption and immobilization of REEs during the Fe(II)aq-induced phase transformation of ferrihydrite. The results show that the heavy REEs of Ho(III) were more efficiently adsorbed and stabilized compared with the light REEs of La(III) by ferrihydrite and its transformation products, which was due to the higher adsorptive affinity and smaller atomic radius of Ho(III). Both La(III) and Ho(III) inhibited the Fe atom exchange between Fe(II)aq and ferrihydrite, and sequentially, the Fe(II)aq-induced phase transformation rates of ferrihydrite, because of the competitive adsorption with Fe(II)aq on the surface of iron (hydr)oxides. Owing to the larger amounts of adsorbed and stabilized Ho(III), the inhibition of the Fe(II)aq-induced phase transformation of ferrihydrite affected by Ho(III) was higher than that by La(III). Our findings suggest an important role for the Fe(II)aq-induced phase transformation of iron (hydr)oxides in assessing the mobility and transfer behavior of REEs, as well as for their occurrence in earth surface environments.
Highlights
The aqueous Fe(II) (Fe(II)aq )-induced phase transformation of ironoxides is one of the most important reactions in the soil iron cycle and imposes great effects on the environmental behaviors of metals [1]
We found that divalent metal ions, e.g., Ba(II), Ca(II), Mg(II), Mn(II), Co(II), Ni(II) and Zn(II), inhibited the Fe(II)aq -induced phase transformation of ferrihydrite, while the metal ions were immobilized during this process [5]
In the present study, we focused on the cation effects of two typical rare earth elements (REEs) ions, lanthanum (La) and holmium (Ho), as representatives of light REEs (LREEs) and heavy REEs (HREEs), respectively, on the phase transformation of ferrihydrite, as well as the stabilization of these two REEs metals during the formation of secondary ironoxides
Summary
The aqueous Fe(II) (Fe(II)aq )-induced phase transformation of iron (hydr)oxides is one of the most important reactions in the soil iron cycle and imposes great effects on the environmental behaviors of metals [1]. It was proposed in the 1980s that Fe(II)aq could catalyze the phase transformation of iron (hydr)oxides through electron transfer and atom exchange with structural Fe(III) (Fe(III)oxide ) [2]. Coexisting metallic ions in the system might be involved via adsorption, enwrapping and substitution, and influence the phase transformation process; once the metallic ions were fixed in the iron (hydr)oxides, their environmental mobility and bioavailability were eventually altered [5,6]
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