23 1-5667-0608-4/01/$0.00+$1.50© 2004 by CRC Press LLC 2 Chemistry, Geochemistry, and Geology of Chromium and Chromium Compounds William E. Motzer and Todd EngineersCONTENTS 2.1 Chromium Chemistry.................................................................................242.1.1 Background ......................................................................................242.1.2 Elemental/Metallic Chromium Characteristics .........................252.1.3 Ionic Radii ........................................................................................292.1.4 Oxidation States...............................................................................302.1.5 Stable and Radioactive Isotopes ...................................................312.1.6 Characteristics of Chromium Compounds.................................342.2 Natural Chromium Concentrations..........................................................342.2.1 Mantle ...............................................................................................462.2.2 Chromium Minerals........................................................................462.2.3 Chromium Ore Deposits................................................................462.2.3.1 Stratiform Mafic-Ultramafic Chromite Deposits .........622.2.3.2 Podiform- or Alpine-Type Chromite Deposits ............632.2.4 Crude Oil, Tars and Pitch, Asphalts, and Coal..........................632.2.5 Rock ...................................................................................................642.2.6 Soil .....................................................................................................662.2.7 Precipitation (Rain Water) and Surface Water ...........................672.2.8 Groundwater....................................................................................672.2.9 Sea Water ..........................................................................................672.2.10 Air ......................................................................................................672.2.11 Biogeochemical Cycling .................................................................682.3 Chromium Geochemistry...........................................................................702.3.1 Cr(III) Geochemistry.......................................................................702.3.2 Cr(VI) Geochemistry.......................................................................712.3.3 Chromium Reaction Rates (Kinetics)...........................................732.4 Chromium Distribution in Primary Environments ...............................742.4.1 Possible Sources of Natural Cr(VI) in Rocks..............................742.4.2 Known Sources of Natural Cr(VI) in Rocks ...............................77 L1608_C02.fm Page 23 Thursday, July 15, 2004 6:57 PM24 Chromium(VI) Handbook 2.5 Chromium Distribution In Secondary Environments ...........................782.5.1 Known Natural Cr(VI) Occurrences in Surface Water and Groundwater ................................................................782.6 Forensic Geochemistry................................................................................802.6.1 Soil .....................................................................................................802.6.2 Groundwater....................................................................................802.6.3 Air ......................................................................................................812.7 Acknowledgments.......................................................................................81Bibliography ........................................................................................... 82 2.1 Chromium Chemistry 2.1.1 Background In 1797, the French chemist Nicholas-Louis Vauquelin hypothesized thatchromium (Cr) was a separate and distinct element. He had isolated theoxide of this element from a Siberian mineral known as crocoite (PbCrO 4 ).In 1798, Vauquelin successfully isolated metallic chromium by heating(reducing) chromic oxide (Cr 2 O 3 ) with charcoal. He then named the newelement after the Greek word χρωµα (chro ^ ma), pronounced khrma, for colorbecause it produced chemical compounds with distinct and unique colors.Vauquelin also analyzed a Peruvian emerald, determining that its green colorwas due to the presence of chromium. About two years after chromium’sdiscovery, Tassaert, a German chemist, determined that chromium waspresent in an ore that we now know as chromite (Greenwood and Earnshaw,1998; ChemGlobe, 2000; Papp, 2000; Winter, 2002).Since its discovery, chromium has become a very important industrialmetal because of its many applications in ferrous (cast iron and stainlesssteel) and in nonferrous (aluminum, copper, and nickel) alloy metal fabrica-tion, and in the chemical industry (metal finishing, plating, corrosion control,pigments and tanning compounds, and wood preservatives) (Papp, 2000).Chromium and chromium compounds are used in a wide variety of indus-trial and manufacturing applications including steel alloy fabrication, wherethey enhance corrosion and heat resistance in other metals, and in platedproduct fabrication where they are used for metal decoration or increasedwear resistance. They are also used in nonferrous alloy metal fabrication toimpart special qualities to the alloys; in production and processing of insol-uble salts, as chemical intermediates; in the textile industry for dyeing, silktreating, printing, and moth proofing wool; in the leather industry for tan-ning; in the manufacture of green varnishes, inks, paints, and glazes; ascatalysts for halogenation, alkylation, and catalytic cracking of hydrocar-bons; as fuel and propellant additives; and in ceramics (Spectrum Labora-tories, 1998). L1608_C02.fm Page 24 Thursday, July 15, 2004 6:57 PMChemistry, Geochemistry, and Geology of Chromium 25While chromium in its Cr(III) form is not considered a toxic element andis a required diet nutrient with recommended daily adult dosages rangingfrom 0.5 to 2 mg per day (required for glucose metabolism), in its Cr(VI)form, it does have toxic effects (see Guertin, Section 6, this volume). Acuteexposure to Cr(VI)-laden dust results in skin rashes, ulcers, sores, andeczema in occupational workers. In humans, Cr(VI) exposure causedmarked irritation of the respiratory tract and ulceration and perforation ofthe nasal septum in workers in the