Vanadium, Niobium, and Tantalum

Abstract

Vanadium (V), niobium (Nb), and tantalum (Ta) are transition metals from group V. They have partly filled d shells, so they are defined as transition elements. Vanadium and niobium are widely distributed in earth's crust, but there are few concentrated deposits of these elements. Tantalum is less abundant in the earth's crust; it occurs in the same minerals as niobium, and their separation is complex. The main commercial sources of both are the columbite–tantalite series of minerals [(Fe/Mn)(Nb/Ta)2O6], with various Fe/Mn and Nb/Ta ratios. Pure or almost pure elements in massive form are gray-colored, ductile metals with high (V, Ta) or moderate (Nb) hardness and very high melting points. Vanadium group elements are resistant to chemicals, and this resistance increases with the atomic number. At room temperature they are not affected by air, water, or alkalies. Vanadium dissolves in oxidizing acids (e.g., nitric acid, concentrated sulfuric acid, aqua regia) and in hydrofluoric acid. Niobium and tantalum can be dissolved by HNO3/HF mixture, and they are slowly attacked by hydrofluoric acid. All these elements dissolve very slowly in fused alkalies, producing salts: vanadates, niobates or tantalates, and hydrogen. Vanadium, niobium, and tantalum pentaoxides are the main products of air oxidation at high temperatures; vanadium can also form trioxide and tetraoxide under these conditions. At elevated temperatures, metals combine with some other nonmetals, for example, with hydrogen, nitrogen, carbon, and silica, giving compounds, many of which are interstitial and non-stoichiometric. All of these elements have five valence electrons; however, electronic configuration of valence orbitals is different. Vanadium compounds are the most toxic among all the three elements; tantalum compounds are practically nontoxic. Reported LC50 values for vanadium pentoxide are between 70 and 200 mg/m3. There is no data on niobium. Vanadium compounds are moderately toxic when given orally; reported LD50 values are in hundreds mg/kg body weight. Niobium and tantalum compounds given orally are practically nontoxic; reported LD50 values are in thousands mg/kg body weight. All elements and their compounds are absorbed from the respiratory tract, and eliminated through the kidney. Their absorbtion from the gastrointestinal (GI) tract is poor. They are distributed to internal organs and there are data indicating that vanadium and tantalum might accumulate in bone. Vanadium and niobium have an irritant effect on mucous membranes and skin. Therefore, irritant effects on the upper respiratory tract and lungs are observed when animals are exposed by inhalation to vanadium and niobium compounds; however, vanadium compounds have stronger effects. Many studies have documented the mitogenic potential of vanadium compounds. The results of mutagenicity studies of vanadium are conflicting, and no firm conclusions can be drawn. There is no such information on niobium and tantalum. However, there are no data indicating that any of these compounds may have carcinogenic potential. Indeed, there are studies suggesting that some vanadium and niobium compounds may have antitumor activity.