Chromium 6+

CHEMICAL SUBSTANCE IDENTIFICATION

Chemical name                 

 Chromium (6+)

Synonyms                           

 Chromium hexavalent ion; Hexavalent chromium; Chromium(6+)ion [2]

IUPAC name

 chromium(6+)

CAS No

 18540-29-9 [2,3]

REACH registration number

EC No

Molecular formula              

 Cr⁺⁶ 

Substance group/chemical family

a monoatomic hexacation and a chromium cation [2]

The most common Cr(VI) forms are chromate (CrO42–), and hydrogen chromate (HCrO4–) also called bichromate. The relative amount of these two species depends on pH. Dichromate (Cr2O7 2–) can also occur. Cr(VI) compounds are anions. [8]

Appearance

Physical state

Odour

Form

    Colour

USES AND HANDLING ISSUES

Relevant identified uses

Hexavalent chromium is used in textile dyes, wood preservation, anti-corrosion products, chromate conversion coatings, and a variety of niche uses. Industrial uses of hexavalent chromium compounds include chromate pigments in dyes, paints, inks, and plastics; chromates added as anticorrosive agents to paints, primers, and other surface coatings; and chromic acid electroplated onto metal parts to provide a decorative or protective coating. Hexavalent chromium can be formed when performing "hot work" such as welding on stainless steel or melting chromium metal. In these situations the chromium is not originally hexavalent, but the high temperatures involved in the process result in oxidation that converts the chromium to a hexavalent state. Hexavalent chromium can also be found in drinking water and public water systems [5]

Handling considerations

PHYSICO-CHEMICAL PROPERTIES

Molecular weight                                  

  51.996 g/mol [2]

Bulk density/Specific gravity

pH

Particle size

EC

Melting point

Boiling point

Flash point

Flammability

Vapour density

Vapour pressure

Solubility in water

Some hexavalent compounds, such as chromium(VI) oxide (or chromic acid), and the ammonium and alkali metal salts (e.g., sodium and potassium) of chromic acid are readily soluble in water. [9]

Solubility in organic solvents

Solubility in inorganic solvents

Hydrolysis

Ionicity in water

Surface tension

Dispersion properties

Specific surface

Stability and reactivity

Chemical stability

Chromium compounds are most stable in the trivalent state under environmental conditions and occur in nature in ores, such as ferrochromite (FeCr2O4). The hexavalent (VI or chromate) is the second most stable state; however, it only occurs naturally in rare minerals such as crocoite (PbCrO4) [9]. Cr(VI) is far more mobile than Cr(III) and more difficult to remove from water. [8]

Reactivity hazards

Corrosivity

Polimerization

Incompatibility with various substances

Special remarks on reactivity

Physical, chemical and biological coefficient

Koc

Kow

pKa

log Kp

Henry-constant

ENVIRONMENTAL FATE AND BEHAVIOUR

Artificial pollution sources

General terrestrial fate

General aquatic fate

Cr (VI) is readily soluble in water. Under high Eh (oxidizing)

and alkaline (pH above 7) conditions, Cr(VI) can be predominant in groundwater. However, in the presence of organic matter, ferrous iron (Fe II) and sulfide, Cr (VI) can be readily reduced to Cr(III) and immobilized. Adsorption of Cr (VI) by clayey soil and natural aquifer materials is low to moderate under near neutral pH ranges commonly encountered in groundwater. [8, 9]

General atmospheric fate

General persistence and degradability

Abiotic degradation and metabolites

Biodegradation and metabolites

Bioconcentration

Although chromium does bioaccumulate, it is not reported to undergo biomagnification in the food chain.

Volatilization

Photolysis

Hydrolysis

Soil adsorption and mobility

ENVIRONMENTAL CONCENTRATIONS

Measured data

Hexavalent chromium (chromium(VI), Cr(VI), chromium 6) is any chemical compound that contains the element chromium in the +6 oxidation state (thus hexavalent). Virtually all chromium ore is processed via hexavalent chromium, specifically the salt sodium dichromate. In rocks and soil, Cr is a common trace mineral, found as amorphous Fe(III), Cr(III) hydroxides, eskolaite (Cr2O3(s)), and chromite (FeCr2O4(s)), all Cr(III) forms. There is a wide natural variation in Cr concentrations in soils and rocks. The concentration of naturally occurring chromium in U.S. soils ranges from 1 to 2 000 parts per million (ppm) [1]. Shales, suspended river material, and soils with fine grain sizes have the highest concentrations, whereas granite, carbonates, and large grained (sandy) sediments have the lowest concentrations [8]. Dissolved concentrations of total Cr in groundwater from natural processes are typically below 10mg/l [8]. In contaminated areas, Cr(VI) concentrations are commonly 300 to 500 mg/l and have been reported to reach 14 g/l [8]. A yellow color is imparted to the water at about 1 mg/l Cr(VI) [8].  In air, the concentrations generally range between 0.01 and 0.03 microgram per cubic meter (µg/m3) [1]. Drinking water levels are generally less than 2 parts per billion (ppb) [1].

ECOTOXICOLOGICAL INFORMATION

General adverse effects on ecosystem

Acute toxicity (LC50, EC50)

Aquatic systems

 LC50: 145 µg/l (Ceriodaphnia dubia)

Results of acute tests of chromium (VI) on C. dubia indicated that the LC50 was in the range of acute values reported earlier for similar species such as C. reticulata, Daphnia magna, and D. pulex [10]

Terrestrial systems

Chronic toxicity (NOEC, LOEC)

Aquatic systems

NOAEC: 111 µg/l (Ceriodaphnia dubia) [10]

Terrestrial systems

HUMAN HEALTH EFFECTS and PROTECTION

Routes of human exposures

inhalation [5, 6], ingestion through drinking water  [6], dermal (through chromate dyed leather goods) [6]

General effects

According to the classification provided by companies to ECHA in CLP notifications this substance may cause cancer, is very toxic to aquatic life with long lasting effects and may cause an allergic skin reaction. [3]

Endocrine disruption

Mutagenicity

Carcinogenicity

Hexavalent chromium compounds are genotoxic carcinogens. Due to its structural similarity to sulfate, chromate (a typical form of chromium(VI) at neutral pH) is transported into cells via sulfate channels [6]. According to the classification provided by companies to ECHA in CLP notifications this substance may cause cancer [3]

According to Toxnet [4] Cr +/ is a human respiratory carcinogen. [5]

Reprotoxicity

Teratogenicity

Skin, eye and respiratory irritations

According to the classification provided by companies to ECHA in CLP notifications this substance may cause an allergic skin reaction. [3]

Metabolism:

absorption, distribution & excretion

When swallowed, it can upset the gastrointestinal tract and damage the liver and kidneys, however evidence suggests hexavalent chromium does not cause cancer when ingested, most likely because it is rapidly converted to the trivalent form after entering the stomach [1].

Exposure limits

NIOSH REL for all Cr(VI) compounds: 0.2 μg Cr(VI)/m3 8-hr TWA.  [7]

Other U.S. occupational exposure limits (OELs) PEL: 5 μg Cr(VI)/m³ 8-hr TWA [7]

TLV  water soluble  Cr(VI) compound(s): 50 μg Cr(VI)/m³ 8-hr TWA [7]

TLV  insoluble  Cr(VI) compound(s): 10 μg Cr(VI)/m³ 8-hr TWA [7]

The IDLH (immediately dangerous to life or health) level  for chromic acid and chromates: 15 mg Cr(VI)/m3 [7].

Drinking water MAC

For drinking water the United States Environmental Protection Agency (EPA) does not have a Maximum Contaminant Level (MCL) for hexavalent chromium. California has finalized a Public Health Goal of 0.02 parts per billion (micrograms per liter) and established a MCL of 10 ppb. [6]

Other information

Within the European Union, the use of hexavalent chromium in electronic equipment is largely prohibited by the Restriction of Hazardous Substances Directive [6].

[1] US EPA, Contaminated site clean-up Information, CLUE-IN https://clu-in.org/contaminantfocus/default.focus/sec/chromium_VI/cat/Toxicology/, Accessed: 2018.12.05

 [2] NIH, US National Library of Medicine, Open Chemistry Database. https://pubchem.ncbi.nlm.nih.gov/compound/Chromium_VI_#section=InChI-Key, Accessed: 2018.12.05

 [3] ECHA, European Chemical Agency https://echa.europa.eu/de/substance-information/-/substanceinfo/100.132.559, Accessed: 2018.12.06

 [4] Toxnet, CHhem IDplus. https://chem.nlm.nih.gov/chemidplus/sid/0018540299, Accessed: 2018.12.05

  [5] Park RM, Bena JF, Stayner LT, Smith RJ, Gibb HJ, Lees PSJ. (2004) Hexavalent chromium and lung cancer in the chromate industry: a quantitative risk assessment. Risk Anal; 24: 1099–1108.

[6] Wikipedia: https://en.wikipedia.org/wiki/Hexavalent_chromium, Salnikow, K.; Zhitkovich, A. (2008). "Genetic and Epigenetic Mechanisms in Metal Carcinogenesis and Cocarcinogenesis: Nickel, Arsenic, and Chromium". Chem. Res. Toxicol. 21 (1): 28 44. doi:10.1021/tx700198a. PMC 2602826. PMID 17970581, Accessed: 2018.12.06

[7] National Institute for Occupational Safety and Health, DEPARTMENT OF HEALTH AND HUMAN SERVICES  (2013) Criteria for a Recommended Standard: Occupational Exposure to Hexavalent Chromium, Publication No. 2013–128,  Accessed: 2018.12.06

[8] Chromium (VI) Handbook . United States of America: CRC Press. 2004. pp. 280-298. ISBN 9781566706087, https://web.archive.org/web/20170110203935, Accessed: 2018.12.06

 [9] Agency for Toxic Substances and Disease Registry (ATSDR) (2012). Toxicological profile for Chromium. Atlanta, GA: U.S. Department of Health and Human Services, Public Health Service. https://www.atsdr.cdc.gov/ToxProfiles/tp.asp?id=62&tid=17, Accessed: 2018.12.06

[10] Baral, A., Engelken, R., Stephens, W. et al. (2006) Evaluation of Aquatic Toxicities of Chromium and Chromium-Containing Effluents in Reference to Chromium Electroplating Industries, Arch Environ Contam Toxicol, Volume 50, Issue 4, pp 496–502, https://doi.org/10.1007/s00244-005-0068-x, Accessed: 2018.12.06