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 the metals 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 are slowly attacked by hydrofluoric acid. All these elements dissolve very slowly in fused alkalies, producing salts, vanadates, niobates, or tantalates, as well as 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 nonmetals, for example, with hydrogen, nitrogen, carbon, and silica, giving compounds, many of which are interstitial and nonstoichiometric. All 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 (V2O5) are between 70 and 200 mg/m3. There are no data on niobium. Vanadium compounds are moderately toxic when given orally, and their toxicity increases with the oxidation states. Reported LD50 values are in the tens to hundreds of mg/kg body weight. Niobium and tantalum compounds given orally are practically nontoxic; reported LD50 values are in several thousand mg/kg body weight. All elements and their compounds are absorbed from the respiratory tract and eliminated through the kidney. Their absorption 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. Results of mutagenicity studies of vanadium are conflicting. Recent studies indicate that the effect can be related to the ability of vanadium to generate reactive oxygen species. There are a few data on tantalum showing negative potential of mutagenicity. There is no such information on niobium. The results from 2-year NTP inhalation study on F344/N rats and B6C3F1 mice show clear evidence of carcinogenic activity of vanadium pentoxide based on the occurrence of alveolar/bronchiolar neoplasms. No studies were found that specifically examined cancer in animals after oral exposure to vanadium. On the other hand, there are studies suggesting that some vanadium, as well as niobium, compounds may have antitumor activity. On the basis of NTP studies, the International Agency for Research on Cancer concluded that vanadium pentoxide, a pentavalent vanadium compound, is a possible human carcinogen and classified the compound to group 2B. The European Commission has classified vanadium pentoxide as Category 3 for mutagenicity based on positive results in a range of in vivo and in vitro assays for different vanadium compounds and as Category 3 for reproductive toxicity based on a number of studies that vanadium compounds have effects on the developing fetus via oral, intraperitoneal (i.p.), subcutaneous, and intravenous routes. According to new CLP Regulation on classification, labelling and packaging (CLP Regulation), vanadium pentoxide has been classified as mutagen Category 2, reproductive toxicant Category 2, acute toxicant Category 4, and chronic aquatic toxicant Category 2. Keywords: background levels; dust; hematological effects; immunological effects; marine invertebrates; mutagenicity; niobium; niobium compounds; odor; tantalum; tantalum compounds; vanadium; vanadium compounds