The Majority Of Atmospheric Mercury Is Produced By

Atmospheric mercury, a potent neurotoxin, poses a significant threat to human health and the environment. Understanding its sources is crucial for developing effective mitigation strategies. While natural processes contribute to the global mercury cycle, the majority of atmospheric mercury is undeniably produced by human activities, primarily through industrial processes. This article delves into the various anthropogenic sources of atmospheric mercury, exploring the processes involved and their relative contributions to the global mercury budget.

Anthropogenic Sources of Atmospheric Mercury: A Deep Dive

The release of mercury into the atmosphere due to human activities has increased dramatically since the industrial revolution. These activities range from large-scale industrial operations to smaller-scale artisanal mining, each contributing a unique profile of mercury emissions. Let's break down the key players:

1. Coal Combustion: The Dominant Contributor

Coal combustion is by far the largest anthropogenic source of atmospheric mercury globally. Coal naturally contains trace amounts of mercury, which are released during the combustion process in power plants and industrial boilers. The amount of mercury in coal varies depending on its geological origin, with some regions exhibiting significantly higher concentrations than others.

Here's a closer look at the process:

• Mercury in Coal: Coal deposits accumulate mercury over geological timescales, absorbing it from the surrounding environment.
• Combustion Release: When coal is burned, the mercury is volatilized due to the high temperatures, transitioning into gaseous forms.
• Emission Pathways: These gaseous forms of mercury are then emitted into the atmosphere through smokestacks, often in the form of elemental mercury (Hg0) and oxidized mercury (Hg2+).

The impact of coal combustion is substantial:

• Global Scale: Coal-fired power plants are widespread globally, particularly in rapidly developing economies, leading to a significant cumulative effect on atmospheric mercury levels.
• Long-Range Transport: Elemental mercury, being relatively stable in the atmosphere, can be transported long distances, impacting regions far removed from the source of emissions.
• Deposition and Bioaccumulation: Eventually, mercury is deposited onto land and water bodies, where it can be converted to methylmercury, a highly toxic form that bioaccumulates in aquatic organisms and poses a risk to human consumers of seafood.

2. Artisanal and Small-Scale Gold Mining (ASGM): A Significant and Difficult-to-Control Source

Artisanal and small-scale gold mining (ASGM) represents another major anthropogenic source of atmospheric mercury, particularly in developing countries. ASGM typically involves the use of mercury to extract gold from ore through a process called amalgamation.

Here's how it works:

• Mercury Amalgamation: Miners mix mercury with gold-containing ore, forming a mercury-gold amalgam.
• Heating and Vaporization: The amalgam is then heated, vaporizing the mercury and leaving behind the gold.
• Atmospheric Release: The mercury vapor is often released directly into the atmosphere without any pollution controls, resulting in significant emissions.

The challenges associated with ASGM are multifaceted:

• Widespread Practices: ASGM is practiced in numerous countries, often in remote and unregulated areas.
• Lack of Awareness: Many ASGM miners are unaware of the health risks associated with mercury exposure and lack access to safer alternatives.
• Economic Dependence: ASGM often provides a crucial source of income for impoverished communities, making it difficult to implement effective regulations.

The consequences are dire:

• Local Contamination: ASGM activities result in severe local contamination of air, water, and soil.
• Health Impacts: Miners and their families are exposed to high levels of mercury vapor, leading to neurological damage, kidney problems, and other health issues.
• Global Contribution: While ASGM is a localized activity, its widespread nature contributes significantly to the global mercury budget.

3. Non-Ferrous Metal Production: Smelting and Refining Processes

Non-ferrous metal production, particularly the smelting and refining of metals like gold, silver, copper, lead, and zinc, also releases mercury into the atmosphere. Ores of these metals often contain trace amounts of mercury, which are liberated during the high-temperature smelting process.

The processes involved are complex:

• Mercury in Ores: Metal ores can contain mercury as a natural impurity.
• Smelting Release: During smelting, the ore is heated to high temperatures to extract the desired metal. This process volatilizes the mercury, releasing it into the air.
• Refining Processes: Further refining processes can also contribute to mercury emissions.

The impact varies depending on the metal:

• Gold Production: Gold smelting can be a significant source of mercury emissions, especially if mercury amalgamation was used in the initial extraction process.
• Base Metal Production: Copper, lead, and zinc smelting also release mercury, although the concentrations may be lower than in gold production.
• Emission Controls: Modern smelters are often equipped with pollution control technologies to reduce mercury emissions, but older facilities may lack these safeguards.

4. Cement Production: Calcination and Raw Materials

Cement production is another notable source of atmospheric mercury, although often overlooked. The process involves heating limestone and other raw materials to produce clinker, the main component of cement. These raw materials can contain trace amounts of mercury, which are released during the calcination process.

Here's a breakdown:

• Mercury in Raw Materials: Limestone, shale, and other raw materials used in cement production can contain mercury.
• Calcination Release: During calcination, the raw materials are heated to high temperatures (around 1450°C) in a kiln, converting calcium carbonate into calcium oxide and releasing carbon dioxide. This process also volatilizes any mercury present in the raw materials.
• Emission Pathways: The mercury is then emitted into the atmosphere through the kiln's exhaust gases.

The challenges are similar to other industrial sources:

• Variability in Raw Materials: The mercury content of raw materials can vary significantly depending on their geological origin.
• Scale of Production: Cement production is a large-scale industry, with numerous plants operating worldwide.
• Retrofitting Challenges: Retrofitting existing cement plants with mercury control technologies can be expensive and technically challenging.

5. Waste Incineration: Burning Mercury-Containing Products

Waste incineration can be a significant source of atmospheric mercury, particularly if the waste stream contains mercury-containing products such as fluorescent lamps, batteries, and medical devices. When these products are incinerated, the mercury is released into the atmosphere.

Here's how it happens:

• Mercury in Waste: Many common household and industrial products contain mercury.
• Incineration Release: During incineration, these products are burned, and the mercury is volatilized.
• Emission Controls: Modern incinerators are often equipped with air pollution control devices, such as scrubbers and filters, to remove mercury from the exhaust gases. However, the effectiveness of these controls can vary.

The problem is compounded by:

• Improper Disposal: Improper disposal of mercury-containing products can lead to their incineration along with general waste.
• Informal Waste Management: In some countries, waste management practices are informal, and waste may be burned in open dumps without any pollution controls.
• Global Trade in Waste: The global trade in waste can result in the transfer of mercury-containing products to countries with less stringent environmental regulations.

6. Chlor-Alkali Production: A Legacy and Ongoing Concern

Chlor-alkali production, which uses mercury cells to produce chlorine and sodium hydroxide, has historically been a significant source of mercury emissions. While many countries have phased out mercury cell plants, some still operate, and legacy contamination from past operations remains a concern.

The process and its problems:

• Mercury Cells: Mercury cells use liquid mercury as an electrode to facilitate the electrolysis of brine (sodium chloride solution).
• Mercury Losses: Mercury is lost from the cells through various pathways, including evaporation, leaks, and spills.
• Atmospheric Emissions: A portion of this mercury is released into the atmosphere.

The legacy of chlor-alkali plants:

• Historical Contamination: Sites of former chlor-alkali plants are often heavily contaminated with mercury, posing a risk to human health and the environment.
• Decontamination Challenges: Decontamination of these sites can be a complex and expensive process.
• Ongoing Emissions: Even after plants are decommissioned, mercury can continue to be released from contaminated soil and water.

The Relative Contributions of Different Sources

While each of these sources contributes to atmospheric mercury pollution, their relative contributions vary depending on the region and the time period. Globally, coal combustion is generally considered the largest single anthropogenic source, followed by ASGM. However, in some regions, ASGM may be the dominant source, particularly in areas with extensive artisanal gold mining activities.

Here's a general overview of the relative contributions:

1. Coal Combustion: 30-50%
2. Artisanal and Small-Scale Gold Mining (ASGM): 10-30%
3. Non-Ferrous Metal Production: 5-15%
4. Cement Production: 3-10%
5. Waste Incineration: 2-5%
6. Chlor-Alkali Production: 1-3% (decreasing as plants are phased out)

It's important to note that these figures are estimates and can vary depending on the data sources and methodologies used.

Mitigating Anthropogenic Mercury Emissions

Addressing the problem of anthropogenic mercury emissions requires a multi-faceted approach that includes:

• Transitioning to cleaner energy sources: Reducing reliance on coal-fired power plants by transitioning to renewable energy sources such as solar, wind, and hydro power.
• Promoting cleaner mining practices: Implementing best practices in ASGM to reduce mercury use and emissions. This includes promoting the use of alternative extraction methods, providing training and education to miners, and establishing regulatory frameworks.
• Improving industrial processes: Implementing pollution control technologies in industrial facilities, such as scrubbers, filters, and activated carbon injection systems, to remove mercury from exhaust gases.
• Managing waste effectively: Promoting proper disposal of mercury-containing products and implementing effective waste incineration practices with appropriate pollution controls.
• Phasing out mercury-based technologies: Replacing mercury-based technologies, such as mercury cell chlor-alkali plants, with cleaner alternatives.
• International cooperation: Fostering international cooperation through agreements such as the Minamata Convention on Mercury to reduce mercury emissions and protect human health and the environment.

Conclusion

The majority of atmospheric mercury is undoubtedly produced by human activities, primarily through industrial processes like coal combustion, artisanal gold mining, non-ferrous metal production, and waste incineration. Understanding the specific contributions of each source is crucial for developing effective mitigation strategies. By transitioning to cleaner energy sources, promoting cleaner mining practices, improving industrial processes, managing waste effectively, and fostering international cooperation, we can significantly reduce anthropogenic mercury emissions and protect human health and the environment from the harmful effects of this potent neurotoxin. The challenge is significant, but the potential benefits of reducing mercury pollution are immense, contributing to a healthier planet for current and future generations. We must continue to prioritize research, innovation, and policy initiatives aimed at minimizing mercury releases and ensuring a sustainable future.