Air Pollution: Metal Culprits In Our Cities

Several metals contribute to air pollution in big cities. The primary sources of metal air pollutants are vehicle emissions and industrial emissions. Lead is a significant pollutant released from vehicles, especially those that use leaded petrol. Other sources of lead emissions include industrial sources, waste incinerators, utilities, and lead-acid battery manufacturers. Chromium is another pollutant that enters the air through coal and oil combustion. Copper and cadmium are also present in the air, although copper pollution is more commonly found in water, and cadmium enters the air through the ground.

Lead from vehicle emissions

Lead is a metal that is found naturally in the environment, as well as in manufactured products. It is a major pollutant in the air of big cities, with vehicle emissions being a significant source.

Historically, the fuels used in on-road vehicles, such as cars and trucks, were a primary source of lead emissions. The use of leaded gasoline in these vehicles released lead into the air, causing harm to the environment and human health. Over time, many countries have phased out the use of leaded gasoline, significantly reducing lead emissions from motor vehicles. However, lead is still present in the aviation industry, as leaded aviation gasoline is used in piston-engine aircraft.

In addition to vehicle emissions, industrial sources also contribute to lead pollution in big cities. Lead smelters, waste incinerators, utilities, and lead-acid battery manufacturers are known to release lead into the atmosphere. The concentration of lead in the air is typically highest near these industrial sources, particularly around lead smelters.

The persistence of lead in the environment poses a significant risk to both human health and ecosystems. Lead released into the air can settle onto soil, dust, or other surfaces, leading to incidental ingestion by humans and animals. Once ingested, lead accumulates in the body, primarily in the bones, and can adversely affect multiple systems, including the nervous, reproductive, and cardiovascular systems. The negative impacts of lead exposure are particularly pronounced in children, who may experience behavioural problems, learning deficits, and lowered IQ.

To mitigate the pollution caused by lead from vehicle emissions, proper maintenance of vehicles is essential. Additionally, the use of lead-free petrol or diesel can significantly reduce the amount of lead emitted by automobiles. Regulatory efforts by environmental agencies have played a crucial role in decreasing lead levels in the air, demonstrating the importance of continued enforcement and the development of cleaner technologies.

Copper from industrial wastewater

Copper is a naturally occurring heavy metal found in the environment. It is also a common byproduct of industrial production, and copper ions are often found in industrial wastewater. Copper concentrations in wastewater can range from 2.5 mg/L to 10,000 mg/L.

The presence of copper in industrial wastewater is a significant environmental concern, as it contributes to water pollution. Copper ions can be removed from wastewater through various treatment technologies, including:

• Ion exchange (IX): This process involves passing the wastewater through a resin substrate that captures copper ions. IX is effective for streams with low metal ion concentrations, but it requires regular regeneration and maintenance, and the treatment and disposal of waste products can be challenging.
• Adsorption: This technology uses forces of molecular attraction to separate copper ions from water. The wastewater is passed through adsorbent media such as activated carbon, carbon nanotubes, zeolite, clay, or biomass. Adsorption is technically simple and has low operational costs.
• Chemical precipitation: This method involves adding chemical precipitants to the wastewater stream, causing insoluble precipitates to form, which can then be removed through physical separation. However, the effectiveness of this method depends on the specific chemical precipitants used.
• Membrane separation, electrochemistry, biotechnology, and hydroxide and sulfide precipitation are also used.

While these treatment methods can help remove copper from industrial wastewater, preventing copper pollution at the source is also crucial. This involves implementing proper wastewater management practices in industrial facilities and reducing the use of copper in manufacturing processes where possible.

Chromium from coal and oil combustion

Chromium is one of several metals that contribute to air pollution in big cities. Chromium (Cr) is a highly toxic and carcinogenic metal that is released into the atmosphere through coal and oil combustion. It is a variable-valent metal element and usually occurs as Cr(III)/Cr(VI) in the environment.

Coal combustion, particularly the combustion of bituminous coal, is a major source of chromium emissions. Bituminous coal is the major fuel used in most power plants worldwide, especially in China. During coal combustion, chromium is released as particulates and may be oxidized into toxic hexavalent forms, such as hexavalent chromium (Cr(VI)). The release and capture of chromium from coal and its subsequent capture in fly ash depend on the feed coal properties, chromium speciation in coal, and combustion conditions. The combustion temperature also plays a significant role in the transformation and release of chromium during coal combustion.

At lower combustion temperatures (<800°C), the degree of Cr oxidation increases, with the fraction of Cr(VI) reaching a maximum at 800°C. However, as the combustion temperature rises above 800°C, the Cr(VI) ratios in coal ash decrease, and chromium is retained as more stable Cr(III) compounds. The presence of iron-bearing minerals during high-temperature combustion can enrich Cr in ash and prevent Cr oxidation.

In addition to coal combustion, oil combustion also contributes to chromium emissions. China, being the largest coal producer and consumer, has witnessed significant atmospheric chromium emissions from both coal and oil combustion. The atmospheric emission of chromium from coal-fired power plants in China has been increasing annually, reaching approximately 159 tons in 2019.

The combustion of high-chromium coals can lead to potential health risks, public disease, and environmental deterioration. Chromium can be released into the environment through the combustion process and the disposal of combustion solid wastes, such as bottom ash and fly ash. Chromium's long-term interaction with surface water and groundwater systems has also raised concerns, with reported incidents of chromium leaching from fly ash contaminating surface water.

To mitigate the impact of chromium emissions from coal and oil combustion, it is crucial to develop removal technologies and control emission levels. Proper maintenance of vehicles, the use of lead-free petrol or diesel, and the implementation of air pollution control measures, such as electrostatic precipitators and scrubbers in industries, can help reduce pollutants and improve air quality in big cities.

Cadmium from the ground

Cadmium is a naturally occurring metal that enters the air through the ground. It is mined and released into the environment, primarily during the smelting process. Once in the environment, cadmium moves through the soil and enters the food chain.

Cadmium is a known human carcinogen. It is toxic to humans and animals, and it is harmful to the environment. Cadmium is released into the air through industrial processes such as smelting, soldering, and other high-temperature operations. These industrial areas can have atmospheric cadmium levels of up to 7 µg/m3. When inhaled, cadmium particles deposit in the airways and lungs.

Cadmium enters the food chain through various pathways. Plants absorb cadmium from the soil through their roots and from the air through their leaves. Certain plants, such as tobacco, rice, cereal grains, potatoes, and other vegetables, absorb cadmium more readily than other heavy metals. Cadmium is also present in meat, particularly offal like liver and kidney. In some areas, shellfish, mushrooms, and even drinking water may have elevated cadmium levels.

Cadmium pollution is a significant concern, especially in industrialized areas. People who spend time outdoors in these areas may inhale or ingest cadmium through the air or contaminated food sources. Cadmium residues in plants are typically less than 1 µg/kg, but daily intake can vary greatly depending on location and population group. In non-industrialized rural areas, cadmium intake through food is estimated to be 10-60 µg/d, while in polluted areas of Japan, values as high as 500 µg/d have been reported.

Zinc

Metals are one of the many pollutants that contaminate the air in big cities. While lead, copper, chromium, and cadmium are some of the metals that are commonly found in the air of big cities, zinc is another metal pollutant that has been detected.

The health impacts of zinc air pollution are significant. Studies have shown that exposure to zinc compounds in the air is associated with increased hospital admissions and higher CVD population mortality rates. The impact of zinc air pollution on cardiovascular health is a serious concern, contributing to the already high annual cost of CVD, which is estimated at $315 billion in the United States alone.

While individuals can take steps to reduce their risk of CVD through a healthy diet, regular physical activity, and abstaining from smoking, the environmental factor of zinc pollution is beyond their control. Therefore, it is crucial to recognize the role of zinc as an air pollutant in big cities and to implement measures to mitigate its release into the atmosphere, protecting the health and well-being of those who live and work in these urban areas.

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