Plants: Our Natural Air Purifiers

Air pollution is a serious issue that has plagued humanity since the earliest days of burning wood and biomass for fuel. Over time, the problem has only worsened with the introduction of fossil fuels, leading to millions of premature deaths annually. However, there is hope on the horizon as researchers have been working tirelessly to find solutions. From the Clean Air Act, which has successfully reduced pollution in the US since 1970, to the development of new materials that capture pollutants, the world is making strides towards cleaner air. In this discussion, we will delve into the various methods and materials being employed to absorb air pollution, exploring their effectiveness and potential impact on mitigating this global health crisis.

Plants and grasses

Plants with dense branching and twig structures are effective at trapping pollutants. Evergreen trees, for example, can remove more pollutants, but many conifer species are sensitive to common pollutants. Plants with hairy, coarse, and resinous leaves capture more particles than those with smooth leaves, and smaller leaves are generally more efficient collectors. Additionally, herbaceous species may adsorb more gaseous pollutants.

The Royal Horticultural Society (RHS) has identified certain "super plants" that are particularly effective at absorbing air pollution. These include bushy, hairy-leafed plants like cotoneaster, which can soak up pollution on busy roads. In a study, cotoneaster was found to be at least 20% more effective at absorbing pollution compared to other shrubs on roads with heavy traffic. Other "super plants" include ivy, which is excellent at cooling buildings, and hawthorn and privet, which help mitigate intense summer rainfalls and reduce localised flooding.

When selecting plants for phytoremediation, it is important to consider the specific pollutants present in the environment and the plant's tolerance to those pollutants. Additionally, choosing plants that require minimal maintenance, are long-lived, and have pest and disease resistance can ensure the long-term viability of the phytoremediation efforts.

In addition to plants, grasses can also play a role in absorbing air pollution. Vetiver grass, for example, has been found to significantly lower the half-life of certain pollutants in the soil. By using a combination of plants and grasses with different pollutant removal mechanisms, it is possible to create effective air quality buffers that can trap and remove pollutants from the air.

Sodium alginate and silica fume

Sodium alginate is a polysaccharide that can be extracted from naturally abundant seaweed and algae. Silica fume, on the other hand, is a high-volume industrial by-product of ferrosilicon or silicon metal alloy processing. Together, they form a porous hybrid material that can be used to absorb air pollution.

This hybrid material is created by combining the gelling properties of alginate with the decomposition of food-grade sodium bicarbonate. The result is a compound that can absorb and remove pollutants with 94% efficiency, as demonstrated by tests using methylene blue dye as a model pollutant.

The sodium alginate and silica fume compound has several advantages over other absorbents, such as activated carbon. Firstly, it is cheaper and cleaner to produce, and can be easily scaled up. Secondly, it can be shaped into a porous solid, making it ideal for use in environmental applications such as nanoparticle sequestration. Finally, it offers a four-fold advantage in addressing environmental pollution:

• Reduction of the quantity of by-products
• Conversion of potentially toxic waste into safe materials
• Development of low-cost, efficient adsorbents
• Pollution management at reasonable costs

Metal organic frameworks (MOFs)

Metal-organic frameworks (MOFs) are three-dimensional networks created by the coordination bonding of organic ligands and metals. They combine the properties of organic polymers with those of inorganic compounds. MOFs have been used to address environmental pollution, disease, and toxicity issues, and their use has led to the rapid development of nanotechnology.

MOFs are particularly useful for air quality management (AQM) due to their intrinsically tunable chemical structure and multifunctional properties. They have been found to significantly enhance adsorption capacities, catalytic degradation, and the removal of diverse airborne pollutants and other vapors. Their high surface area, high porosity, and tunable features make them attractive materials for the expulsion of harmful pollutants.

MOFs have been studied as adsorbents for major air pollutants such as nitrogen dioxide (NO2) and sulfur dioxide (SO2). Han et al. studied the isothermal adsorption of MOFs and confirmed that MFM-300 (Al) can interact with highly reactive NO2, making it a potential practical solid absorbent. Dinca et al. investigated the ammonia capacities of various microporous triazolate-based MOFs, with Co2Cl2BBTA and Co2Cl2BTDD exhibiting high ammonia breakthrough capacities.

While MOFs have shown great potential in AQM, they also have some drawbacks, including poor selectivity, high energy and fiscal costs, low capacity, and difficulties in regeneration. However, with ongoing research and development, MOFs may provide a great material as a catalyst for air pollution remediation in the near future.

Rain

The scavenging effect of rain washes away PM (particulate matter), soot, and other gaseous pollutants from the atmosphere. However, the effectiveness of rain in reducing air pollution depends on various factors, such as the altitude of clouds, the size of their droplets, and the diameter and concentration of aerosols. Atmospheric chemists can use these factors to predict the likelihood of a raindrop sweeping a particle out of the atmosphere.

Research has shown that even the heaviest rains only reduce small pollutants by about 8.7%. Light to medium rain has a negligible effect on PM2.5 (fine particulate matter with a diameter of 2.5 micrometres or less). It is easier for rain to wash away larger particles, with moderate rain having an effect of 10% or less on larger particles. The most intense rainfall events can have an impact of up to 30% on reducing particulate air pollution.

It is worth noting that the correlation between rain and PM2.5 pollution reduction is relatively weak compared to other factors like wind, which has a more substantial influence on decreasing pollution levels. Additionally, the effect of rain on air pollution can vary across different cities, as the composition of air pollutants (large vs. small particles) may differ between locations.

Moving away from fossil fuels

Fossil fuels have been the primary energy source for centuries, and transitioning away from them will be challenging. However, it is essential to mitigate the severe environmental damage they cause, including harmful air pollution that kills 4.2 million people annually.

The COVID-19 pandemic provided a glimpse of what a world less reliant on fossil fuels might look like. With much of the world under lockdown, demand for oil plummeted, and air pollution levels dropped significantly.

To effectively move away from fossil fuels, several strategies must be implemented. Firstly, governments must take aggressive policy action to support the transition to clean energy. This includes subsidy reform, carbon pricing, and investments in renewable energy sources. For example, China has delayed or halted work on 151 coal power plants and created a fund to support retraining and reallocating workers in the coal and steel sectors.

Secondly, diversifying economies, especially those rich in fossil fuels, is crucial. While this transition may impact certain regions and communities, renewable energy companies are the fastest-growing source of jobs globally, and government-led initiatives can help ensure a just transition. For instance, Italy's ENEL utility is transitioning away from coal while generating employment in affected communities.

Thirdly, standards for building and appliance efficiency, along with innovative financing and public-private partnerships, can drive the shift towards clean energy. India's Energy Efficiency Services Limited, a government-backed company, has achieved significant energy and cost savings by promoting high-efficiency lighting and appliances.

Finally, continued advancements in carbon capture and storage technologies can help reduce emissions from heavy industry. While some question the effectiveness of capturing CO2 to produce more fossil fuels, studies show that captured CO2 can be permanently stored underground or used in industrial processes, resulting in negative emissions.

In conclusion, moving away from fossil fuels is essential to address the harmful impacts of air pollution on human health and the environment. While this transition will not be easy, a combination of policy action, economic diversification, efficiency improvements, and technological advancements can help achieve a more sustainable and clean energy future.

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