3.1.1. Catalytic converters for automobiles

Emissions from automobiles are known to be a one of the major contributors of pollutants which have affected environment and biotic health. Many technologies have been working to tackle the problem. The use of catalytic converters is one such method. They have acted as a boon in the field and have helped in reducing environmental pollutants by chemically converting harmful gases to less harmful ones. They catalyse the redox chemical reactions which help in the breaking down of Nitrous oxide (N2O), Carbon monoxide (CO), and other Volatile organic compounds (VOCs) before they are emitted from the vehicles (CK-12 Earth Science For High School, 2019). There are two types of converters, namely a two-way catalysts converter and a three-way catalysts converter. A two-way converter was predominant in 1980s and used metals like platinum and palladium as catalysts. Modern day vehicles have three-way converter which uses specific catalyst formulation of metals like platinum, palladium, gold, and rhodium. Due to issues like high costs and toxicity, there has been a shift for the use of green and sustainable alternative metals like copper, zinc, cobalt, iron, nickel (Dey and Mehta, 2020; Egorova and Ananikov, 2016). They have been observed to have superior activity for the catalytic reduction of oxides of nitrogen (Ribbens and Ribbens, 2017; Samuels et al., 1982). It is made up of a large metallic body fitted beneath the vehicles. It consists of an input pipe which is connected to the engine cylinder to collect the polluted fumes and an output pipe which is connected to the exhaust. The polluted fumes are passed over the catalyst where redox chemical reactions take place, and convert them into other gases that are less harmful (Woodford, 2020). Copper catalytic converters reduce CO exhaust emissions up to 16.67% (Shoffan et al., 2019), while Copper–Zinc catalyst decreases up to 47.71% (Chafidz et al., 2018). Five year study of Hong Kong evaluated significant positive effect of continuous usage of catalytic converters on reduction of air pollutants such as CO, NOX, VOCs (Cui et al., 2021; Yao et al., 2019). Similarly copper-, cobalt-doped titanates reduced NO levels by 100% at 350 °C (Glisenti et al., 2014). The three way catalyst has been seen to reduce major emissions by 90% (Ribbens, 2013). Hence, it can be observed that its application on larger scale would result in positive impact on climate change. Depending upon the material, size, quality, and durability, the cost of catalytic converter varies within a reasonable price of $5000-$10,000 (Catalytic Converter Market, 2019). Groups are further working to bring down the costs by either working on recycling of these costly catalyst metals, or by finding substitutes for these costly catalyst (Morcali, 2020; Thakur, 2019). The leading companies include Faurecia SA of France, Sango Co. Ltd. of Japan, European Exhaust and Catalyst Ltd. of United Kingdom, Yutaka Giken Co. Ltd. of Japan, and few more. These multinational companies launch new products and collaborate with other market leaders to innovate and launch the products so as to meet the increasing needs and requirements of consumers (MarketWatchReports, 2019). In current times, most of the automobile models possess in-built catalytic converters which help in controlling pollutants at individual levels. However, its efficient usage, maintenance, and timely replacement (in case of damage) protocols need awareness so as to enhance its functioning and longevity.

3.1.2. AIR-INK or KAALINK™

Team of experts of Graviky Lab from Singapore have invented and patented an innovation which is capable of capturing the pollutants and converting it into a safe, water resistant, reliable, and high-quality ink (Graviky Labs, 2019a, 2019b). This ink can be effectively used for markers, screen-printing, helmet/device painting, etc. This device is attached to the exhaust pipe of vehicles, generators, and chimneys for some time. During this duration it collects the exhausts or carbon soot. The dark black substance collected is the incompletely burnt organic matter/pollutant. This soot is then gradually and carefully processed to remove the presence of any heavy metals, dust particles, and harmful substances (like carcinogens) to obtain a carbon-rich pigment. This dark carbon-rich pigment is then used for inks, dyes, and paints. It is estimated that 45 min of tailpipe pollution can is sufficiently capable to capture 85–95% of soot emissions from a vehicle and produce 30 mL of ink at an affordable cost between $25 and $30 (MIT Media Labs, 2018). According to preliminary literature, it could be stated that despite being at an initial phase it is a successful way to reduce the emission of air pollutants and utilize the soot in a productive manner. As the innovation seems pocket-friendly, it could be used at individual levels (for automobiles) and also by groups/organizations/industries (for larger generators and chimneys). It is being modified further to extend its application on a larger pollution exhaust systems. Further, it is being planned to utilize the carbon soot to release oil based paints, fabric paints, outdoor paints, dyes, and more (Chaudhuri, 2017; Thakur and Sindhi, 2019).

3.1.3. Air-filtering pollution-busting bus

The city of Southampton of United Kingdom, which had been suffering from the problem of air pollution, has come up with a unique idea in the field with their sustainable city models. Go-Ahead Group, UK's largest bus and rail operators, have introduced “Bluestar Buses” or “Air Filtering Buses” which have filters attached at their top. These filters have been designed by Pall Aeroscape. A 100-days trial started during late 2018 has shown that these filters are capable of removing fine particles from the air up to a height of 10 m and nearly 65 g of particulate matter, with an efficiency of nearly 99.5%, thereby effectively cleaning up the polluted air in Southampton and releasing pure air as it goes through the filters (Dogra, 2019; Miller, 2019). The initial trial cost is high and is expected to be around £100,000 (or $126826 USD). Its success has given an opportunity for rolling out more such buses and technologies on a larger scale. This would be effective to control the emissions on daily basis, especially in developed and populated cities (Steffen, 2020; Vyas, 2018). For widespread applications, the filters could be fitted on light-, medium- and heavy-weight automobiles, rail systems, etc. The filters could also be developed to fit to stagnant structures that could be installed at parks, near busy roads, industrial areas, etc. to reduce air pollutants. As the reported efficiency is quite high, its prolonged use at country level would be beneficial to society.

3.2.1. Fuel cells

A new developing technology is the usage of fuel cells. It is a device which uses electro-chemical reactions to convert chemical potential energy present in molecular bonds into electrical energy. The fuel cells are efficient as they utilize hydrogen gas (H2) and oxygen gas (O2) as fuel to produce water (H2O), electricity, and heat (Cummins Inc, 2019; Schumm, 2019). Many research groups are actively working and developing new strategies to widen the usage of fuel cells in vehicles, laptop computers, cellular phones, video recorders, hearing aids (National Geographic, 2019). Utilization of fuel cells have been seen to reduce greenhouse gases (GHGs) by 55%–65% (Cox, 2014). Many leading pioneer companies which are vendors of global fuel cell market are ElectroChem Inc. of Raynham Massachusetts, Ballard Power Systems of Burnaby, Nuvera Fuel Cells, Inc. of USA, Acumentrics, Inc., of Massachusetts. In India, Indian Railways Organisation for Alternate Fuels (IROAF), Indian auto industry lobby Society of Indian Automobile Manufacturers (SIAM) are experimenting with Hydrogen-powered fuel cells for power generation. Direct Methanol Fuel Cell Corporation (DMFCC) of Altadena, California has licensed an extensive portfolio of direct methanol fuel cell patents from Pasadena-based California Institute of Technology (Caltech) as well as from the University of Southern California (USC) (Dutta, 2017; Technavio, 2016). Earlier the fuel cells cost around $1000 per kilowatt of power they generated. Currently, various R&D groups are working to make it further economical and tackle many problems without compromising with the quality of performance. For example, the US Department of Energy is working to bring down the cost of production to $61 per kilowatt per fuel cell. Similarly, Ballard Power Systems is experimenting to enhance the platinum with carbon silk which is expected to bring down 30% reduction in the cost. They are focusing on fuel cell stack designs, improving production capacity, and recycling of catalysts used (Mohapatra and Tripathy, 2018). Recently CSIR and KPIT Technologies, India have developed a 10kWe (Kilowatt-electric) automotive grade low-temperature PEM fuel cell (LT-PEMFC) (Express Drives Desk, 2021).

3.2.2. Microbial fuel cell (MFC)

It is a bio-electrochemical device which utilises microbes (especially bacteria) as the bio-catalysts in an anaerobic anode compartment to oxidize organic and inorganic matter for generating bio-electricity (Locan et al., 2006; Rahimnejad et al., 2011, 2015). They rely on living biocatalysts to facilitate the movement of electrons throughout their systems instead of the traditional chemically catalysed redox reaction which is oxidation of the fuel at the anode and reduction at the cathode (Alternative Energy Solutions for 21st Century, 2019). MFC technology has been improved significantly in the recent decades (Rahimnejad et al., 2015). It has been shown that for every 1,000 tons of wastewater treated, approximately 15,000 tons of carbon dioxide (CO2) is being released in to the atmosphere which causes other problem (Gude, 2015). Preliminary studies revealed MFC removes nearly 90.2% of azo-dye pollutants in wastewater (Hu et al., 2018). Integration of two MFCs, where one can be used for nitrification and the other for autotrophic denitrification, removes 99.9% nitrogen (Peera et al., 2021). Indian Institute of Technology-Madras (IIT-M) & garment dyeing unit of Tripura have worked on a start-up prototype MFC technology wastewater treatment plant which would generate power. The setup will be suitable for small-scale installations with an organic load of atleast 100 kg Biochemical Oxygen Demand (BOD)/day (IANS, 2019). It is being worked upon for many uses like power production, treatment, and sensing (Gajda et al., 2018). However, it has encountered several challenges in scale-up and practical application like high-cost maintenance, system performance, complexity of installation, and improvement in power density (Rahimnejad et al., 2015; Trapero et al., 2017). Use of cathode catalysts manganese dioxide and molybdenum disulfide in combination with graphite is seen to be effective and low cost effort (Peera et al., 2021). Global uses of MFCs are expected to increase to 9% in future. Novel strategies to design MFCs which are capable of handling different substrate-containing wastewaters with ability of resource recovery are required. Some of the leading companies of global MFC market include Pilus Energy LLC of USA, Fluence Corporation Limited of USA, Triqua International BV of Netherlands, MICROrganic Technologies Inc. of USA, Prongineer R&D Ltd. of Canada, Vinpro Technologies of India (Market Research Reports, 2019).

3.2.3. Biofuels

These are renewable sources of energy which are made from organic matter or wastes. They use biocatalysts (microorganisms) for the conversion of chemical energy into electrical energy. Usage of biofuels has risen over past decades as they emit less CO2 compared to conventional fuels. To increase its use for transportation purposes, it is blended with existing fuels such as gasoline and diesel. At present nearly 3% of road transport fuels in use around the world are represented by biofuels (Shell Global, 2019). In India, the Ministry of New & Renewable Energy (MNRE) holds the major responsibility for Research & Development, Policy governance, and overall Coordination concerning biofuels. Ministry of Environment & Forests (MoEF), Ministry of Petroleum & Natural Gas (MoP&NG), Ministry of Rural Development (MoRD), and Ministry of Science & Technology also deal with various aspects of biofuel development as well as its promotion in the country (Ministry of New & Renewable Energy Govt. of India, 2019). In the future, they can be particularly important to help decarbonise the aviation, marine and heavy-duty road transport sectors. A report generated by the National Renewable Energy Laboratory (NREL) of USA has shown that biodiesels produce 78.5% less CO2 emissions, and has reduced greenhouse gas emissions by up to 86% (Office of Energy, 2020; Strickland, 2019). When compared with conventional batteries, biofuels are renewable, cheaper, less pollutant, and non-toxic. Depending upon many factors like land costs, feedstock, oil market, and agricultural subsidies, the current cost of production of biofuels is quite expensive and may vary from $0.4-$1/lge (gasoline equivalent). However with advancement and scaling-up of new processes, the price could still be lowered to a considerable amount (Advantages and Disadvantages of Biofuels, 2019; Walker, 2013).

3.3.1. Photocatalytic hydrophobic coatings

Photocatalytic Coating is one of the modern day air purification methods with self-cleaning effect which works on the principle of photocatalysis. These coatings are based on the usage of Titanium dioxide (TiO2) and reacts with light to degrade particulates (Exterior Coatings Protecting your investments, 2019). During photocatalysis reaction, the light energy (from the sun or an electrical light source) is converted into a chemical energy that is transferred to water vapour to produce active oxygen species at the coated surface (ecotio2, 2019). When ultraviolet (UV) light hits the TiO2 coated surface, electrons get excited from the valence band to the conduction band, leaving behind a positively charged hole. These positively charged holes and electrons act as active species and readily participate in redox reactions with H2O, O2, adsorbed organic and inorganic compounds and leads to the degradation of the pollutants. One of the limitations include the excitation of electrons by UV light (λ < 380 nm). Therefore there is a need to find photo-catalysts, which can get activated and work using light from the visible spectral region utilizing wavelengths between 380 and 500 nm, and giving a greater amount of profit from the solar radiation (Binas et al., 2017). This technique is being used in building materials (such as concretes, plasters, tiles) as it is effective, long-lasting, non-toxic, non-expensive, and has been shown to abate or decomposes NOX, smog, pollution and stain causing substances into harmless by-products (Zouzelka and Rathousky, 2017). TiO2 has three crystalline structures; anatase, rutile, and brookite. Combination of anatase and rutile is seen to be photo-catalytically more active as compared to pure anatase or rutile phases (Agrios et al., 2003; ecotio2, 2019). Studies have shown that photocatalytic coatings are capable of degrading nitric oxide (NO) to nearly 50% (Hot et al., 2017). The reasonable price of commercially available water-based TiO2 and ultra-fine TiO2 is nearly about $20.40 per kg (Shen et al., 2012). Some of the globally acknowledged manufacturers of Photocatalytic coatings are TOTO, Kon Corporation, and Mitsubishi Chemical of Japan; PURETiClean of Cincinnati Ohio; Green Earth Nano Science Inc. (GENS) of Canada (360 Research Reports, 2019). To improve the technique, extensive efforts are being focused to improve existing materials, prepare new materials, improve application of photo-catalytic materials and coatings, substantially increasing the specific surface area, increasing the number of reactive sites available, enhancing its activity by doping TiO2 with non-metal and metal ions (Binas et al., 2017).

3.3.2. Capturing CO2 to make cement and carbon fibers

Blue Planet carbon sequestration plants in California, a well-known organization, have many unique, cost effective, and efficient solutions or technologies that utilize CO2 as a raw material for making carbonate rocks. The carbonate rocks thus produced are used in place of natural limestone rock, which is the principal component of concrete. CO2 from flue gas is treated with some water based solutions to give carbonate. This carbonated solution is used to form a carbonate mineral coating over a substrate to form a synthetic limestone coating of varying sizes. It has been estimated that each ton of CO2-sequestered limestone traps nearly 440 kg of CO2. Blue Planet's product was recently used for constructing a boarding area in San Francisco Airport, and satisfying test results were observed as the material met all necessary specifications (Blue Planet, 2019; Lutteman, 2019).

Another team of researchers at Technological University of Munich have developed an eco-friendly and cost effective process by which algae are used to produce carbon fibers using atmospheric CO2. The process developed is at a preliminary stage in which the algae capture the CO2 and convert it to algae oil. The glycerol in the algae oil is then extracted and processed to form polyacrylonitrile (PAN) which gives rise to carbon fibers of lightweight and high strength quality. These carbon fibers are no different from conventional fibers and thus can be used in all existing processes (Arnold et al., 2018; Francis, 2019).

3.3.3. Air-purifying tower

Air purifying tower or Smog Towers are large scale air purifiers used to reduce air pollutants. China has recently installed a 60 m and a 100 m Air purifying tower, developed by Institute of Earth Environment of the Chinese Academy of Sciences, in Shaanxi Province to tackle the long term severe problem of air pollution (Steffen, 2019). The tower has been designed in such a way that the base has greenhouses which sucks in the polluted air. The air is heated up using solar energy which rises up the tower and gets filtered through many filters. The purified air is then released from the tower. Though the project is in preliminary stages, it has shown a positive effect by cleaning about 75 million cubic metres of air per day, and has shown to be an effective, low-cost method to artificially remove pollutants from the atmosphere. In India, a similar eco-friendly and cost effective 40 feet tall air purifying tower is being planned to be installed in Delhi by Kurin Systems, an organization that has recently got the patent for the "world's largest and strongest air purifier” by the World Intellectual Property Organization (Gandhiok, 2018, 2019). The tower costs $2,30,878-$2,63,861 USD which is comparatively cheaper as compared to ones present in other countries that costs nearly $3,00,000 USD. This tower will utilize solar energy for functioning and thus would be environmentally friendly. It will be containing pre-filters and H14 grade HEPA filters to clear 99.99% of pollutants from air, thereby cleaning nearly 32 million cubic metres of air per day (Online Desk, 2018; Steffen, 2019).

3.4.1. Algae-filled curtains

Increasing CO2 emission has been a serious concern worldwide. A team of researchers from Ireland have developed a new concept of ‘Algae-filled urban curtains’. This curtain is made up of long (>20 feet) and large bio-plastic sheets containing built-in channels or maze-like network of tubes which are filled up with microscopic algae. The curtain is designed in such a way that it captures the polluted air from the bottom which eventually rises up through the tubes. The CO2 is utilised by the algae for photosynthesis and in-turn liberates O2. According to a London-based firm EcoLogicStudio, these curtains might costs around $350 per square meter and can suck more than two pounds of CO2 from the air in a day (Chow, 2018; Peters, 2018). The method is considered to be at experimental level. Much work is required to be done to tackle the drawbacks. The bio-plastic material used for the construction is not completely biodegradable as it contains little amount of plastic. Furthermore, since the algae keeps on growing, regular harvesting and maintenance is required thereby adding to its costs. The curtains have been installed at small scale levels in some cities, but its large scale implication and cost management still remains under process (Shaw, 2019).

3.4.2. Plastic eating bacteria

Plastic waste is one of the leading pervasive contributors of pollution. It has added onto the global burden on diseases, bioaccumulated up the food chain, deposited into the ocean and polar ice caps, forms debris and contaminates land, water and air. The Center for International Environmental Law has reported that the current rate of plastic consumption is believed to contribute 1.34 gigatons of CO2 emissions per year by 2030 (Irfan, 2019). Plastic incineration contributes to release of many dangerous substances like heavy metals, Persistent Organic Pollutants (POPs), and other toxics into the environment (Gaia, 2019). Hence alternative methods are required which could degrade the plastic. One such method has been published in Proceedings of the National Academy of Sciences USA, where it has shown that researchers have discovered a known natural enzyme which feeds on plastics. Plastic bottles contain patented plastic polyethylene terephthalate (PET). The enzyme, nomenclature as PETase, is secreted from a newly discovered Ideonella sakaiensis 201-F6, and is able to degrade this plastic PET, polyethylene furandicarboxylate (PEF), and its other derivatives (Austin et al., 2018; SCIEU Team, 2018). The wild-type of I. sakaiensis, typically works to mineralize about 75% of degraded plastic into CO2 (Hehe, 2019). PETase converts PET into mono (2-hydroxyethyl) terephthalic acid (MHET), terephthalic acid (TPA), and bis(2-hydroxyethyl)-TPA as products. Another enzyme called MHETase (MHET-digesting enzyme), further converts the generated MHET into the two monomers, which are TPA and ethylene glycol (EG). Both the enzymes (PETase and MHETase) are secreted by I. sakaiensis and likely act synergistically to depolymerize PET. Currently, many research are focusing on increasing the activity of PETase enzyme by modifying the active site (Austin et al., 2018). A team of researchers at Carbios, France aims to optimize an enzyme that could depolymerize 97% of PET into monomers in 24 h. This would enable treatment of up to 200 kg of PET waste in 24 h (Drahl, 2018). Similarly, Oberbeckmann et al. have identified that incubation of members of Erythrobacter genus with microplastics lead to formation of a layer of biofilm to utilize polycyclic aromatic hydrocarbons (PAH) (Oberbeckmann et al., 2018). Other microorganisms like Shewanella, Moritella sp., Psychrobacter sp., Pseudomonas sp., Zalerionmaritimum, Phormidium sp., have been also been studied to interact with microplastics in cold marine conditions; while studies conducted on fresh water systems have revealed dominance of the Burkholderiales, Acidovorax, Undibacterium, and Chitinimonasin most biofilms which showed the ability to degrade Poly (3-hydroxybutyrate-co-3-hydroxyhexanoate) (PHBH) (Morohoshi et al., 2018; Paço et al., 2017; Sekiguchi et al., 2011; Urbanek et al., 2018). However, proper insight into the degradation of these microplastics by the microbes and its large scale applications are still under investigation.

3.4.3. Genetically modified plants

Since past decades, researchers have been working upon genetically modified or transgenic plants which could help in tackling the pollution. Both the root and shoots are being worked upon so as to uptake the pollutants and break down the impurities. A team of researchers of University of Washington have created a genetically engineered poplar trees in the laboratory that can remove nearly 91% of contaminant trichloroethylene and benzene, a pollutant that is associated with petroleum. Currently much work is being focused on to make genetically modified plants like Arabidopsis sp., which are tolerant to ozone stress (Ainsworth, 2017; Choi, 2007). University of Washington has worked on pothos ivy or devil's ivy to modify a mammalian protein, called 2E1 which helps them to transfer compounds like chloroform and benzene from the air around it and utilize it for plant growth. The result have shown that the chloroform level dropped by 82% in 3 days, while the level of benzene decrease by 75% in 8 days (Miley, 2018). However further enquiry is required for its large scale application.