Biokinetics of yttrium and comparison with its geochemical twin holmium.

The transition metal yttrium (Y, atomic number 39) is chemically similar to elements in the lanthanide family (atomic numbers 57-71) and is found with the lanthanides in rare earth ores. Yttrium and the lanthanide holmium are referred to as geochemical twins because they generally show little fractionation from metamorphic or weathering processes, due to their closely similar chemical properties and nearly identical ionic radii. Extensive measurements on rocks, soils, and meteorites indicate that the Y/Ho mass concentration ratio rarely falls far from the so-called chondritic or solar system ratio of ∼26. This paper presents a new biokinetic model for yttrium in adult humans and examines whether yttrium and holmium may be biological as well as geochemical twins, considering model-based comparisons of their systemic behaviours in adult humans and model-free comparisons of their concentration ratios in human tissues and various types of vegetation. It appears that yttrium and holmium behave similarly in the human body and that their concentration ratios tend to cluster near the chondritic value in human tissues as well as plants, but the comparative information is too limited and imprecise to determine whether they are extremely close biological analogues.