Chromium and the plant: a dangerous affair?

Abstract

Chromium (Cr) is a transition metal belongs to group VI of Periodic Table of Elements. It is a steelygray,bright, hard, and brittle metal. Physicochemical properties of chromium metal are: atomic number 24, atomic weight 51.996 atomic mass unit, density 7.19 g/cm3, melting point 1907 _C and boiling point 2671 _C. Its electronic configuration,1s22s22p63s23p63d54s1, makes it very resistant to atmospheric oxidants, but it will oxidize to form a thin layer that acts as a protective coating to prevent its own corrosion and also the corrosion of other metals. Chemically, Cr is a redox-type element that is able to exist in a wide range of oxidation states (_2 to þ6), which control its geochemistry and toxicity. Most stable forms of Cr occurring in the environment are Cr(0) (native form), Cr(III) (trivalent form) and Cr(VI) (hexavalent form). Chromium(0) rarely occurs in nature, but some sites containing uncombined native chromium have been reported. Uncombined native Cr was first discovered in the Xizang site in the Tibetan region of China; more reliable samples are from the Udachnaya diamonds mine located in the Siberian region of Russia. This mine is a kimberlite pipe, rich in diamonds, and the reducing environment helped produce both elemental Cr and diamonds. Combined native Cr(0) occurs in some mineral alloys such as: ferchromide (Cr3Fe1_0.6), chromferide (Fe3Cr1_0.6), and isovite [(Cr,Fe)23C6] from Russia and tongbaite (Cr3C2) and zhanghengite (Cu,Zn,Fe,Al,Cr) from China. None of these native Cr(0) minerals has been used as a metallurgical ore source. Conversely, in nature, Cr compounds are most stable as trivalent form and occur in many minerals such as chromite (FeCr2O3), ferrochromite (FeCr2O4), magnesiochromite (MgCr2O4), zincochromite (ZnCr2O4), bentorite [Ca6(Cr,Al)2(SO4)3(OH)12$26H2O], tarapacaite (K2CrO4), uvarovite [Ca3Cr2(SiO4)3], stichtite [Mg6Cr2CO3(OH)16$4H2O], and vauquelinite [CuPb2CrO4PO4OH], among others. Minerals containing Cr(VI) are less abundant than those containing Cr(III). Of the 82 Cr-containing minerals naturally occurring, only 24 are Cr(VI)-bearing minerals. They include anhydrous chromates, compound chromates, compound phosphates, compound borates, compound iodates, and multiple oxides, among others. Crocoite (PbCrO4) is the most abundant Cr(VI)-bearing mineral. Less abundant minerals containing Cr(VI) are hemihedrite [Pb10Zn(CrO4)6(SiO4)2F2], deanesmithite ðHg2þHg32þCr6þO5S2Þ, and edoylerite ðHg3 2þCr6þO4S2Þ. Chromate mineral is also found in nitrate-rich evaporite deposits called ?caliche? in the Chilean Atacama Desert. Chromate is largely confined to iodine-bearing nitrate ores that consist of caliche-containing sodium nitrate (NaNO3), lautarite [Ca(IO3)2], other iodates, and dietzeite [Ca2(IO3)2(CrO4)]. Chromite is the world?s principal source ofCr metal and its main producers are South Africa, Kazakhstan, India, and Turkey. The Cr industry consumes approximately 30 million tons of chromite per year, 90% of which is consumed as ferrochrome dedicated to stainless steel production with China and Western Europe as main consumers. Interestingly, some precious stones (e.g., ruby, emerald) and semiprecious stones (e.g., spinel, tanzanite, green tourmaline) owe their colors to chromium, either from Cr being an important part of their chemistry or just as a trace element.