L1608_C02.fm Page 23 Thursday, July 15, 2004 6:57 PM 2 Chemistry, Geochemistry, and Geology of Chromium and Chromium Compounds William E. Motzer and Todd Engineers CONTENTS 2.1 Chromium Chemistry .................................................................................24 2.1.1 Background ......................................................................................24 2.1.2 Elemental/Metallic Chromium Characteristics .........................25 2.1.3 Ionic Radii ........................................................................................29 2.1.4 Oxidation States...............................................................................30 2.1.5 Stable and Radioactive Isotopes ...................................................31 2.1.6 Characteristics of Chromium Compounds.................................34 2.2 Natural Chromium Concentrations..........................................................34 2.2.1 Mantle ...............................................................................................46 2.2.2 Chromium Minerals........................................................................46 2.2.3 Chromium Ore Deposits................................................................46 2.2.3.1 Stratiform Mafic-Ultramafic Chromite Deposits .........62 2.2.3.2 Podiform- or Alpine-Type Chromite Deposits ............63 2.2.4 Crude Oil, Tars and Pitch, Asphalts, and Coal..........................63 2.2.5 Rock ...................................................................................................64 2.2.6 Soil .....................................................................................................66 2.2.7 Precipitation (Rain Water) and Surface Water ...........................67 2.2.8 Groundwater....................................................................................67 2.2.9 Sea Water ..........................................................................................67 2.2.10 Air ......................................................................................................67 2.2.11 Biogeochemical Cycling .................................................................68 2.3 Chromium Geochemistry...........................................................................70 2.3.1 Cr(III) Geochemistry.......................................................................70 2.3.2 Cr(VI) Geochemistry.......................................................................71 2.3.3 Chromium Reaction Rates (Kinetics)...........................................73 2.4 Chromium Distribution in Primary Environments ...............................74 2.4.1 Possible Sources of Natural Cr(VI) in Rocks..............................74 2.4.2 Known Sources of Natural Cr(VI) in Rocks ...............................77 1-5667-0608-4/01/$0.00+$1.50 23 © 2004 by CRC Press LLC L1608_C02.fm Page 24 Thursday, July 15, 2004 6:57 PM 24 Chromium(VI) Handbook 2.5 Chromium Distribution In Secondary Environments ...........................78 2.5.1 Known Natural Cr(VI) Occurrences in Surface Water and Groundwater ................................................................78 2.6 Forensic Geochemistry................................................................................80 2.6.1 Soil .....................................................................................................80 2.6.2 Groundwater....................................................................................80 2.6.3 Air ......................................................................................................81 2.7 Acknowledgments.......................................................................................81 Bibliography ........................................................................................... 82 2.1 Chromium Chemistry 2.1.1 Background In 1797, the French chemist Nicholas-Louis Vauquelin hypothesized that chromium (Cr) was a separate and distinct element. He had isolated the oxide of this element from a Siberian mineral known as crocoite (PbCrO4). In 1798, Vauquelin successfully isolated metallic chromium by heating (reducing) chromic oxide (Cr2O3) with charcoal. He then named the new element after the Greek word χρωµα (chro^ma), pronounced khrma, for color because it produced chemical compounds with distinct and unique colors. Vauquelin also analyzed a Peruvian emerald, determining that its green color was due to the presence of chromium. About two years after chromium’s discovery, Tassaert, a German chemist, determined that chromium was present 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 industrial metal because of its many applications in ferrous (cast iron and stainless steel) 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, where they enhance corrosion and heat resistance in other metals, and in plated product fabrication where they are used for metal decoration or increased wear resistance. They are also used in nonferrous alloy metal fabrication to impart special qualities to the alloys; in production and processing of insol- uble salts, as chemical intermediates; in the textile industry for dyeing, silk treating, printing, and moth proofing wool; in the leather industry for tan- ning; in the manufacture of green varnishes, inks, paints, and glazes; as catalysts 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 25 Thursday, July 15, 2004 6:57 PM Chemistry, Geochemistry, and Geology of Chromium 25 While chromium in its Cr(III) form is not considered a toxic element and is a required diet nutrient with recommended daily adult dosages ranging from 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). Acute exposure to Cr(VI)-laden dust results in skin rashes, ulcers, sores, and eczema in occupational workers. In humans, Cr(VI) exposure caused marked irritation of the respiratory tract and ulceration and perforation of the nasal septum in workers in the chromate producing and -using indus- tries. Ingestion of 1.0 to 5.0 g of Cr(VI) as chromate results in severe acute gastrointestinal disorders, hemorrhagic diathesis, and convulsions. Death may occur following cardiovascular shock. Doses in animals of Cr(VI) greater than 10 mg/kg mainly affect the gastrointestinal tract, kidneys, and hematopic system (IPCS, 1988). Cr(VI) causes cancerous tumors in mice by inhalation and is considered a possible human carcinogen by this route because workers engaged in the production of chromate salts and chromate pigments experience an increased risk of developing bronchial carcinomas. However, ingestion of Cr(VI) has not been observed to cause cancer because it is believed that Cr(VI) is reduced to Cr(III) in the gastrointestinal tract (IPCS, 1988; WHO, 1988 and 1996; Smith and Huyck, 1999; CDHS, 2003). The understanding of chromium chemistry and geochemistry is therefore important in developing remediation systems that can deal with industrial- caused pollution (see Chapter 8). This chapter is a review of the character- istics of chromium in the natural environment; its concentration within the earth’s crust, atmosphere, and biosphere; and its geochemistry. 2.1.2 Elemental/Metallic Chromium Characteristics Chromium (atomic number 24) is a transition metal occurring in Group VIB of the periodic table. General elemental chromium characteristics are sum- marized in Tables 2.1a to 2.1d. Chromium has a ground state electron con- figuration of 1s22s22p63s23p64s13d5 (Table 2.2). In the periodic table, transition metals (Groups IB to XIIIB) occur between the main group elements (Groups IA to IIA and Groups IIIA to VIIA and the inert gases—Group VIIIA) (Drew, 1972; Timberlake, 2003). The atoms of transition elements have electrons filling d subshells consisting of 5d orbitals. The transition metals are noteworthy because they: 1. Form alloys with one another and the main group metals. 2. Commonly are white lustrous metals with high melting and boiling points. The transition metals vary in abundance in the continental crust from iron, which is common at 5.63% to scandium which is rare at 22 (parts per million) ppm (Ronov and Yaroshevsky, 1972). 3. Have high melting points and densities because the electrons in the d orbitals, bind atoms together in the crystal lattice. L1608_C02.fm Page26Thursday,July15,20046:57PM 26 TABLE 2.1A Elemental Chromium Properties Periodic Table Atomic no. 24 Atomic mass 51.9961 Group no. 6 B Mass Number 52 Group name Transition metals Symbol Cr Period no. 4 Block d Atomic No. (Z) 24 Chemical Registry CAS no. 7440-47-3 (most common isotope: 83.789%) Properties Physical Electrical Thermal Atomic radius (nm) 0.185 e– in shell 1,2,3,4 2,8,13,1 Boiling point 2944 K; 2631 K Atomic volume (cm3/mol) 0.139 e– configuration {Ar}4s13d5 Melting point 2180°C; 1907°C 1 5 Bond length: Cr-Cr (nm) 0.250 /2 Fills of subshell 3d Heat of fusion (kJ/mol) 0.325 Covalent radius (nm) 0.118 Electron binding energies Table 2.1b Heat