Air Pollutants: Hydrophobic Or Hydrophilic?

Air pollution is a pressing issue that poses significant risks to human health and the environment. Among the various pollutants, hydrophobic substances stand out as particularly hazardous due to their inherent persistence, bioaccumulation potential, and environmental mobility. These pollutants, which include volatile organic compounds (VOCs), are characterised by their ability to repel water, causing droplets to form. The behaviour of hydrophobic pollutants and their efficient removal from the air have become critical areas of research, with the aim of mitigating their harmful effects on all organisms, including humans. This introduction will discuss the nature of hydrophobic air pollutants, their impact, and the ongoing efforts to address the challenges they present.

Hydrophobic air pollutants are hazardous to all organisms, including humans

The hydrophobic and hydrophilic properties of materials are defined by the geometry of water on a flat surface, specifically the contact angle between a water droplet and the surface. If the contact angle is less than 90 degrees, the surface is hydrophilic, meaning the water spreads out and covers a large area of the surface. On the other hand, if the contact angle is more than 90 degrees, the surface is hydrophobic, causing the water to form droplets that barely touch the surface.

The removal of hydrophobic air pollutants is a critical area of focus to mitigate their harmful impact on the environment and human health. Biological processes, such as biofiltration, biotrickling filtration, and bioscrubbing technologies, are employed to remove these pollutants from the air. For example, a multi-channel capillary bioreactor has been studied for the removal of hydrophobic air pollutants, achieving high removal efficiency for hexane, toluene, and α-pinene vapors.

The challenge in removing hydrophobic compounds from the air lies in the transfer rate of the pollutants from the gas phase to the liquid phase during biofiltration. This process is less efficient for hydrophobic compounds compared to hydrophilic ones. However, advancements in biotechnology are leading to more sustainable processes for air pollution control, including the expansion of biological processes for mitigating greenhouse gases in dilute air streams.

The understanding and manipulation of hydrophobic and hydrophilic properties have significant implications for various technologies. For instance, in power plants, electronics, airplane wings, and desalination plants, the performance and efficiency of these systems can be enhanced by utilizing hydrophobic and hydrophilic surfaces.

The removal of hydrophobic air pollutants can be done through biofiltration, biotrickling filtration, and bioscrubbing technologies

Air pollution is a critical issue faced by industries, and it is essential to control it to protect the environment and living organisms. Air pollutants can be hydrophobic, and these can be removed through biological processes that are more sustainable and economical than conventional physical-chemical processes.

One such biological process is biofiltration, which has gained popularity as a low-cost, straightforward, highly effective, and energy-efficient technique. It utilizes microbial approaches, including bacteria and fungi, to remove a broad range of compounds, odors, and volatile organic compounds (VOCs). Biofilters can be designed as towers or vertical containers, allowing for better process control and exhaust monitoring. The inlet gas flow and humidity must be carefully controlled to prevent clogging. Biofilters are effective for odor control, ammonia control, and VOC emissions control in various industries, including wastewater treatment, food and beverage, and paint manufacturing.

Biotrickling filtration is another technology used to remove hydrophobic air pollutants. Systems like the EcoFilter biotrickling filter can treat very high or low pollutant concentrations. The polluted air is forced into the EcoFilter vessel, where it travels upward through a proprietary synthetic structured media. This media hosts trillions of microorganisms that consume unwanted compounds, eliminating over 99% of offending compounds. Biotrickling filters offer superior performance at a lower cost and reduced environmental impact, as they do not require expensive chemicals or carbon replacement.

Bioscrubbers are also employed for air pollution control, especially when biological degradation products, such as acids produced during H2S and NH3 removal, would harm a biofilter bed. They come in different forms, such as activated-sludge scrubbers and trickling-filter scrubbers, which transfer gaseous contaminants into the liquid phase. Bioscrubbers are effective in removing hydrocarbons and chlorinated organics. One advantage of bioscrubbing is that it produces no residues that require further treatment.

The efficiency of biofiltration for hydrophobic gas pollutants depends on the transfer rate from the gas phase to the liquid phase

The removal of odorous volatile organic compounds (VOCs) from air is becoming increasingly important due to stricter legal regulations and heightened social awareness. These VOCs can be hydrophobic or hydrophilic in nature. Hydrophobic materials repel water, causing the formation of droplets, whereas hydrophilic materials spread water across their surfaces, maximising contact.

Biological processes are being increasingly used for gas purification as a sustainable and economical alternative to conventional physical-chemical processes. Biofiltration, biotrickling filtration, and bioscrubbing technologies are some of the methods used to remove these odorous compounds from the air.

The improvement of biofiltration efficiency for hydrophobic compounds is a current challenge in the design of biological treatment processes. The removal efficiency of hydrophobic air pollutants such as hexane, toluene, and α-pinene vapors can reach values of about 75%, 99%, and 75%, respectively, at a gas contact time of less than 1 second. This is achieved using a multi-channel capillary bioreactor, which is more effective than conventional biological gas purification systems.

The contact angle of a water droplet on a flat surface determines whether a surface is hydrophobic or hydrophilic

The interaction of water with surfaces is often used to define surface properties, and the wettability of a solid surface by a liquid can be quantified using the Young equation. The contact angle is the angle between a liquid surface and a solid surface where they meet. More specifically, it is the angle formed between the liquid–solid interface and the liquid–vapour interface.

Micro-textured or micro-patterned surfaces with hydrophobic asperities can exhibit apparent contact angles greater than 150° and are associated with superhydrophobicity and the “lotus effect”. The roughness of a surface can impact its hydrophobicity, and surfaces can be specially manufactured at the nanometer scale to obtain positive impacts of roughness on both hydrophobicity and hydrophilicity.

The removal of hydrophobic air pollutants has been studied using a multi-channel capillary bioreactor, with removal efficiencies of certain vapours reaching values of up to 99% at a gas contact time of less than 1 second.

Superhydrophobic surfaces are being developed to increase the efficiency of power-plant condensers and desalination plants

The removal of hydrophobic air pollutants is an important aspect of air pollution control and greenhouse gas mitigation. These pollutants are hazardous to all organisms, including humans, due to their persistence, bioaccumulation potential, environmental mobility, and reactivity.

Superhydrophobic surfaces are being developed to enhance the efficiency of power-plant condensers and desalination plants. The key to improving the efficiency of these plants lies in optimizing heat transfer through condensation. Power plants operate by boiling water with fossil fuels to produce steam, which then drives turbines to generate electricity. The steam must then be condensed back into water, which is collected and sent back to the boiler, forming a cycle.

Researchers at MIT have developed an innovative approach to improving heat transfer in these plants. They have found that using a condenser surface coated with a combination of oil and a hydrophobic material with local hydrophilic sites significantly enhances heat transfer. This combination promotes the abundant formation of droplets, which quickly release before growing too large. The quick release of droplets is due to the hydrophobic nature of the surface, while the oil layer attracts the droplets, allowing them to form more easily. This "immersion condensation" technique can be applied to existing condenser structures, making it a practical improvement.

Additionally, a study on a honeycomb-like microporous superhydrophobic surface demonstrated enhanced condensation heat transfer. This surface, created through the electrodeposition method, exhibited stable coalescence-induced condensate droplet jumping under various condensing pressures. The superhydrophobic nature of the surface facilitated the efficient removal of condensed liquid, improving heat transfer by maximizing the surface area in contact with the vapor.

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