Chromium and the Plant : A dangerous affair?

PRADO FERNANDO EDUARDO; HILAL MIRNA; CHOCOBAR PONCE SILVANA; PAGANO ANTONIO EDUARDO; ROSA MARIANA DANIELA; PRADO CAROLINA DEL VALLE

Plant Metal Interaction: Emerging Remediation Techniques

Elsevier

Año: 2015; p. 149 - 166

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: atomicnumber 24, atomic weight 51.996 atomic mass unit, density 7.19 g/cm3, melting point 1907 C andboiling point 2671 C. Its electronic configuration,1s2,2s2,2p6,3s2,3p6,3d5,4s1,makes it very resistant toatmospheric oxidants, but it will oxidize to form a thin layer that acts as a protective coating to preventits own corrosion and also the corrosion of other metals (Emsley, 2011). Chemically, Cr is a redox-typeelement that is able to exist in a wide range of oxidation states (2 to þ6), which control itsgeochemistry and toxicity (Daulton and Little, 2006). Most stable forms of Cr occurring in theenvironment are Cr(0) (native form), Cr(III) (trivalent form) and Cr(VI) (hexavalent form) (Zayed andTerry, 2003). Chromium(0) rarely occurs in nature, but some sites containing uncombined nativechromium have been reported (Motzer, 2005). Uncombined native Cr was first discovered in theXizang site in the Tibetan region of China (Zhu and Liu, 1981); more reliable samples are from theUdachnaya 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(Gorshkov et al., 1996). Combined native Cr(0) occurs in some mineral alloys such as: ferchromide(Cr3Fe10.6), chromferide (Fe3Cr10.6), 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 ametallurgical ore source. Conversely, in nature, Cr compounds are most stable as trivalent form andoccur 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 (Babula et al., 2008). Minerals containing Cr(VI) are less abundantthan those containing Cr(III). Of the 82 Cr-containing minerals naturally occurring, only 24 are Cr(VI)-bearing minerals (Motzer, 2005). They include anhydrous chromates, compound chromates, compoundphosphates, 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) arehemihedrite [Pb10Zn(CrO4)6(SiO4)2F2], deanesmithite ðHg2þHg32þCr6þO5S2Þ, and edoyleriteðHg32þCr6þO4S2Þ (Barthelmy, 2002). Chromate mineral is also found in nitrate-rich evaporite depositscalled ?caliche? in the Chilean Atacama Desert. Chromate is largely confined to iodine-bearingnitrate ores that consist of caliche-containing sodium nitrate (NaNO3), lautarite [Ca(IO3)2], otheriodates, and dietzeite [Ca2(IO3)2(CrO4)] (Barthelmy, 2002). Chromite is the world?s principal source ofCr metal and its main producers are South Africa, Kazakhstan, India, and Turkey (Brown et al., 2013).The Cr industry consumes approximately 30 million tons of chromite per year, 90% of which isconsumed as ferrochrome dedicated to stainless steel production with China and Western Europe asmain consumers (Roskill, 2014). Interestingly, some precious stones (e.g., ruby, emerald) and semipreciousstones (e.g., spinel, tanzanite, green tourmaline) owe their colors to chromium, either from Crbeing an important part of their chemistry or just as a trace element (Matlins, 2010).