Niobium [Nb, 41]
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30.5 PLANTS
Niobium is relatively mobile under humid conditions, and therefore may be available to plants. Its biochemical function is unknown. The common Nb amounts in
food plants are very broad, within the range of 0.02–1.1  mg/kg. Cereal grains of
Sweden contain Nb from 0.5 to 1.7 mg/kg (Eriksson 2001a). Its very low concentration (<0.1 μg/L) is reported for wines (Szefer and Grembecka 2007).
In some herbs from the Ural Mountains, the mean contents of Nb are 0.4 and
1.0  mg/kg, in tops and roots, respectively (Grankina et  al. vide Kabata-Pendias
2011). Several native plants have the capability to extract Nb from soils, for example,
Rubus species, grown in the Nb-mineralized area of Komi (Republic in Russian
Federation). These plants are proposed as indicators for the Nb ore deposits.
30.6 HUMANS
According to Emsley (2011), Nb content of human soft tissues is 0.14 mg/kg, with a
total amount of 1.5 mg in the whole body. Niobium contents (mg/kg) are present in
the blood (0.004), the kidney (0.01), the lung (0.02), the muscle (0.03), and the testis
(0.009) (Nielsen 1986).
Metallic Nb has a low order of toxicity due to a poor absorption from stomach
and intestines. Niobium and its compounds may be toxic. Apart from measuring its
concentration, no research on Nb in humans has been undertaken. Once inhaled, it
resides primarily in the lungs and can be absorbed into bones. It interferes with Ca
as an activator of enzyme systems. No side effects have been reported because it does
not react with human tissues. The English diet supplied only 0.020 ± 0.004 mg Nb
daily (Nielsen 1986).
Niobium and some of its alloys are physiologically inert and thus hypoallergenic.
For this reason, Nb is found in many medical devices such as pacemakers (Mallela
et al. 2004). It is frequently used in jewelry and has been tested for use in some medical implants. The Nb nitride coatings show a better cell adhesion than the uncoated
stainless steel (Ramírez et al. 2011).
The Nb-based coatings do not have any toxic effects on cells and have a similar
biocompatibility to Ti. Niobium and its compounds cause skin and eye irritation and
can be toxic, but human poisoning has not been reported. When inhaled, the element
is retained in the bones and lungs, and interferes with Ca.
The inhalation of Nb pentoxide and nitride results in scarring of the lungs in
laboratory animals. In rats, the Nb supplement elevated Cu, Mn, and especially Zn,
in variety of organs, with most marked changes occurring in the heart and liver
(Nielsen 1986).