Chromium (Cr)

1. Basic Information

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

Chromium can be found in water in two main forms:

• Trivalent chromium (Cr³⁺): The more common form and considered an essential nutrient in small amounts
• Hexavalent chromium (Cr⁶⁺): The more toxic form and harmful to human health

Health effects of excess chromium exposure, especially Cr⁶⁺, include:

• Skin irritation and allergic reactions
• Respiratory problems such as asthma and bronchitis
• Liver and kidney damage
• Changes in genetic material
• Increased risk of lung cancer (for Cr⁶⁺)

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

Although chromium is generally removed from water, some chromium compounds are used in industrial water treatment:

• Chromium(III) chloride is used as a coagulant in wastewater treatment
• Chromium compounds are sometimes used in corrosion inhibitors for industrial cooling systems
• Chromium dioxide is used in some catalytic applications for wastewater treatment

6. Case Studies and Real World Applications

Examples of application of chromium treatment in water:

• Hinkley, California case: Groundwater contamination by hexavalent chromium from a Pacific Gas and Electric Company gas compressor facility. The case became famous through the movie "Erin Brockovich".
• Electroplating wastewater treatment: Many electroplating facilities use ion exchange systems to remove chromium from their wastewater before discharge.
• Groundwater remediation at former factory sites: At many old industrial sites, chromium has contaminated groundwater and requires long-term cleanup efforts using various technologies including pump-and-treat and penetrating reactive barriers.

7. Regulatory Guidelines and Standards

Some standards and guidelines for chromium in water:

• WHO: Recommends a maximum limit of 50 μg/L for total chromium in drinking water
• US EPA: Sets a Maximum Contaminant Level (MCL) of 100 μg/L for total chromium in drinking water
• European Union: Set a standard of 50 μg/L for total chromium in drinking water
• Indonesia: Minister of Health Regulation No. 492 Year 2010 sets the maximum limit for total chromium in drinking water at 50 μg/L.

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

9. Future Trends and Research

Some of the research areas and emerging trends include:

• Development of new adsorbents that are more effective and environmentally friendly
• Utilization of nanotechnology to improve chromium removal efficiency
• Exploration of bioremediation methods using microorganisms or plants
• Development of real-time sensors for better chromium monitoring
• Optimization of reduction-precipitation processes to improve efficiency and reduce waste
• Research on long-term impacts of low-dose chromium exposure

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
• Although toxic in high concentrations, trivalent chromium is actually required by the human body in small amounts for glucose metabolism
• Some aquatic plants, such as water lilies, have shown the ability to accumulate chromium, which could potentially be used in phytoremediation