OCCURRENCE AND USES
Chromium, Atomic Number 24, Atomic Weight 51.996 is a metallic element in Group VIb of the Periodic Table of the Elements. The name “chromium” is derived from the Greek for “Color” probably because many salts of chromium are highly colored. Chromium naturally occurs in rocks, animals, plants, soil and in volcanic dust and gases. In its metallic state, chromium is stable and self-passivating, and is used as an ingredient in specialty alloys and steels, imparting superior corrosion resistance to the alloys. This is illustrated by the discovery that the “Terracotta Army” in Xian, China (Qin Dynasty, 3rd century BC) carried chromium-coated bronze lances and swords which, even after all these centuries exhibit no corrosive attack.
Chromium salts come in several different forms including trivalent chromium and hexavalent chromium. Trivalent chromium, often referred to as Chromium (III), or Cr+3, is an essential, non-toxic, poorly absorbed nutrient for the human body. Deficiency of Cr+3 is reported to cause glucose intolerance. The safe and adequate daily intake of Cr+3 is stated to be 0.05 to 0.2 milligrams.
Hexavalent chromium, Chromium (VI), or Cr+6, is generally used or produced in industrial processes. Water sources can be affected by hexavalent chromium naturally, or through contamination plumes from industrial centers, landfills and improper discharge of industrial processing streams.
EXPOSURE AND HEALTH EFFECTS
As stated, trivalent chromium is an essential micronutrient, whereas hexavalent chromium has been shown to be toxic, causing liver and kidney damage, internal hemorrhages and respiratory disorders. If inhaled, Cr+6 is known to cause cancer. On that basis, EPA classified it as a human carcinogen. The health effects of hexavalent chromium through ingestion (the dominant exposure route for drinking water) have seen only limited study which yielded uncertain conclusions. However, a study conducted by the National Toxicology Program (NTP) that was published in 2007 concluded that hexavalent chromium is carcinogenic when ingested in drinking water.
EPA currently regulates chromium (VI), or Cr+6, as part of the total chromium drinking water standard. EPA has an enforceable drinking water standard of 0.1 milligrams per liter (mg / L) or 100 micrograms per liter (µg / L) for total chromium, which includes chromium (VI) and chromium (III).
New health effects information has become available since the original standard was set, and EPA is reviewing this information to determine whether there are new health risks that need to be addressed. Additional information can be found on the chromium in drinking water website.
In 1977 California Department of Health Services (CDHS) adopted what was then a “National Drinking Water Standard” for total chromium of 0.050 milligrams per liter (mg / L) or 50 micrograms per liter (µg / L). The total chromium MCL was established to address exposure to chromium (VI) the more toxic form of chromium. Chromium (III), (trivalent chromium) is a required nutrient.
Effective July 1, 2014 California Department of Public Health (CDPH) set the maximum contaminant level for hexavalent chromium (VI) at 0.010 milligrams per liter (mg / L) or 10 micrograms per liter (µg / L). Note: Refer to Drinking Water-Related Regulations at the Drinking Water Law Book.
HEXAVALANT CHROMIUM REMOVAL PROCESSES
- Reduction / Coagulation / Filtration (RCF)
- Anion Exchange
- Reverse Osmosis
- Electrodialysis (ED) and Electrodialysis Reversal (EDR)
- Coagulation Assisted Membrane Process (CAMP)
- Activated Carbon Filtration
- Lime Softening
- Titanium Dioxide Adsorption Enhanced by UV Light