Gold

Gold (Au)

1. Basic Information

2. Physical and Chemical Properties

Gold is a yellow-colored metal that is highly malleable and ductile. It is an excellent conductor of electricity and heat. Gold is inert and resistant to most chemical reagents. In colloidal form, gold can be red, purple, or black. Pure gold is very soft so it is usually alloyed with other metals to increase its strength.

Gold has a melting point of 1064°C and a boiling point of 2856°C. Its density is 19.3 g/cm3 at room temperature. Gold does not oxidize in air or water at any temperature, making it one of the most corrosion-resistant metals.

3. Presence in Water and Health Effects

Gold is generally present in very low concentrations in natural water, typically less than 0.05 μg/L. Its presence in ground and surface water usually comes from natural mineral deposits or mining activities. Gold in metallic form is considered non-toxic and insoluble in human body fluids. However, some gold compounds such as auric chloride can cause irritation to the skin and mucosa if exposed to large amounts.

Although gold is generally considered safe, long-term exposure to gold dust or vapor can cause a condition known as chrysiasis, in which the skin and body tissues become bluish-gray in color. This is more common in the medical use of gold for the treatment of arthritis than environmental exposure.

4. Water Treatment Applications and Removal Methods

Although gold is rarely a contaminant of concern in drinking water treatment, there are several methods that can be used to remove gold from water:

• Ion exchange: Specialized anion exchange resins can be used to capture the aurocyanide complex formed during the cyanidation process in gold mining.

• Adsorption: Activated carbon can adsorb gold from solution, especially in the form of complexes.

• Precipitation: Gold can be precipitated from solution using reducing agents such as sodium borohydride or hydrazine.

• Membrane filtration: Membrane technologies such as nanofiltration or reverse osmosis can remove colloidal gold particles or dissolved gold complexes.

• Electrolysis: Gold can be recovered from solution through electrolysis deposition on the cathode.

In industrial wastewater treatment, especially from mining or electroplating operations, gold removal and recovery is becoming more important due to its economic value.

5. Industrial Uses in Water Treatment

Although gold itself is rarely used directly in water treatment, some gold-related applications in the water treatment industry include:

• Gold nanoparticles are used in some emerging water purification technologies due to their unique catalytic properties.

• Gold-plated electrodes are sometimes used in water electrolysis systems for hydrogen production or in water quality sensors due to their high corrosion resistance.

• In gold mining wastewater treatment, specialized water treatment technologies are developed to recover gold and remove associated contaminants such as cyanide.

6. Case Studies and Real World Application Examples

Case study: Gold Recovery from Electroplating Effluent

A gold electroplating facility in Japan implemented a gold recovery system for their wastewater. The process involves several stages:

• Initial filtration to remove solid particles.

• Use of a special ion exchange resin to capture gold ions from the solution.

• Elution of gold from the resin using a thiocyanate solution.

• Electrolysis of the gold-rich solution to obtain pure gold metal.

The system is capable of recovering over 99% of gold from wastewater, reducing waste disposal costs and creating an additional revenue stream from the recovered gold.

7. Regulatory Guidelines and Standards

Due to its low toxicity, gold does not have strict regulatory limits in drinking water like other heavy metals. However, some relevant guidelines include:

• WHO does not set guidelines for gold in drinking water as it is considered not risky at the levels typically found.

• The US EPA does not have drinking water standards for gold, but does have guidelines for the disposal of gold-containing waste from mining and processing operations.

• The European Union does not have a specific standard for gold in drinking water, but regulates the use of gold nanoparticles in applications that could potentially affect water supplies.

8. Environmental Impacts and Sustainability Considerations

Although gold itself is considered non-toxic, gold mining and processing can have significant environmental impacts:

• The use of cyanide in gold extraction can contaminate water sources if not properly managed.

• Gold mining is often associated with deforestation and erosion, which can affect surface water quality.

• Mercury, often used in small-scale gold mining, is a highly toxic water contaminant.

Sustainability efforts in the gold industry include:

• Development of more environmentally friendly cyanide-free extraction methods.

• Increased efficiency in water use and recycling at mining operations.

• Implementation of advanced wastewater treatment systems to recover gold and remove contaminants.

9. Future Trends and Research in Water Treatment Involving Gold

Some promising research areas and trends include:

• The use of gold nanoparticles in photocatalysis for water treatment, utilizing their ability to enhance the degradation of organic pollutants under light.

• Development of gold-based sensors for real-time and ultra-sensitive detection of water contaminants.

• Exploration of antimicrobial properties of gold nanoparticles for water disinfection.

• Improved gold recovery technologies from industrial and mining wastewater, including biosorption approaches using microorganisms.

• Research on the potential of gold in advanced membrane technologies for desalination and water purification.

10. Interesting Facts Related to Water Treatment

• Seawater contains small amounts of gold, with estimates totaling about 20 million tons of dissolved gold in the world's oceans.

• Some microorganisms can accumulate gold from their aquatic environment, suggesting potential for bioremediation and gold recovery.

• Gold nanoparticles have been shown to be effective in removing mercury from contaminated water, demonstrating the potential of using one precious metal to remove another in water treatment.

• In some advanced water treatment applications, thin layers of gold are used on membranes to improve performance and lifespan.

• Although expensive, gold is sometimes used in water treatment system components in the microelectronics industry due to its highly pure and corrosion-resistant properties.