Chromium (Cr) in Water Treatment
Quick Answer
Chromium (Cr) is a transition metal with atomic number 24. PubChem lists its standard state as solid and its relative atomic mass as 51.996.
Chromium must be distinguished as Cr(III) or Cr(VI) because toxicity and treatment differ. A common strategy for Cr(VI) is reduction to Cr(III) followed by precipitation and filtration; ion exchange, adsorption or membranes may also be used.
Interesting Facts
- Chromium chemistry changes sharply with oxidation state: Cr(III) and Cr(VI) do not have the same mobility or risk profile.
- Cr(VI) commonly occurs as anionic chromate species, while Cr(III) is cationic or precipitated—one reason the same resin or pH does not treat both alike.
- Reducing Cr(VI) is conversion, not final removal; the resulting Cr(III) must still be separated and the chromium-bearing sludge managed.
Water-treatment Design Implications
An element name alone is not enough to select treatment equipment. Confirm chemical form or speciation, total and dissolved concentration, pH, alkalinity, competing ions and the product-water target. Send the water-analysis results to Watermart to identify additional testing and shortlist processes before equipment is specified.
Summary Data Sources
1. Basic Information
| Atomic Number | 24 |
| Symbol | Cr |
| Atomic Weight | 51.996 g/mol |
| Electron Configuration | [Ar]3d⁵4s¹ |
| Oxidation State | +2, +3, +6 |
2. Physical and Chemical Properties
Chromium is a hard, shiny, brittle metal with a silvery gray color. Some important properties of chromium include:
- Melting point: 1907°C
- Boiling point: 2672°C
- Density: 7.19 g/cm³
- Does not corrode easily in air
- Forms a thin oxide layer that protects the underlying metal
- Can form various compounds with different oxidation states
3. Presence in Water and Health Effects
Water analysis commonly distinguishes total chromium from hexavalent chromium. Cr(III) is generally less mobile, while Cr(VI) often occurs as soluble chromate or dichromate oxyanions. Do not transfer cancer statements from occupational inhalation studies directly to drinking-water exposure; use the applicable drinking-water assessment and laboratory species result.
4. Water Treatment Applications and Removal Methods
Some methods used to remove chromium from water include:
- Ion exchange: Using anion exchange resins to remove Cr⁶⁺ and cation exchange resins for Cr³⁺
- Reduction and precipitation: Reduces Cr⁶⁺ to Cr³⁺ which is then precipitated as hydroxide
- Adsorption: Using activated carbon or other adsorbents
- Membrane filtration: Reverse osmosis or nanofiltration to remove dissolved chromium
- Electrodeposition: Removing chromium from solution through an electrochemical process
5. Industrial Uses in Water Treatment
Historical chromate corrosion inhibitors are a potential source of chromium-bearing wastewater, not a recommendation for modern water treatment. Verify current chemical restrictions and prefer non-chromate programmes where required.
8. Environmental Impacts and Sustainability Considerations
Some environmental and sustainability aspects related to chromium:
- Bioaccumulation: Chromium can accumulate in aquatic organisms, affecting the aquatic food chain
- Persistence: Cr⁶⁺ tends to be more persistent in the environment than Cr³⁺
- Mobility: Cr⁶⁺ is more mobile in soil and groundwater than Cr³⁺
- Ecosystem impacts: High concentrations of chromium can damage fish gills and interfere with the growth of aquatic plants
- Waste management: Disposal of chromium-containing sludge from water treatment processes requires special handling
10. Fun Facts Related to Water Treatment
- Chromium can give glass an emerald green color, which is sometimes used in the design of drinking water bottles
- Some bacteria can use chromium as an electron acceptor in anaerobic respiration, which could potentially be utilized in bioremediation
- Chromium is one of the elements that can be detected in water using inductively coupled plasma spectroscopy (ICP-MS) down to the parts per trillion level
- Some aquatic plants, such as water lilies, have shown the ability to accumulate chromium, which could potentially be used in phytoremediation