The increasing industrial development, increase in emissions of combustion products, ever-expanding urban areas are some of the causes of one of the most serious problems of modern society: air pollution.
The EEA (European Environment Agency) defines pollution as the alteration, caused directly or indirectly by the man, of biological, physical and chemical properties of the environment, when is created a real or potential risk for man’s health or safety and well-being of living species. Air pollution is the alteration of the natural air conditions due to the emission of polluting compound in the atmosphere.
The sources of pollution can be natural (volcanic eruption releasing SO2, fires where PM10 is released) or anthropogenic (human activity).
Pollutants can be divided in two categories: primary if they are emitted directly into the atmosphere and do not endure changes once emitted, and secondary if are generated in the atmosphere as a result of chemical reactions between various substances, pollutants or not.
The main compounds that cause air pollution are:
- Sulphur dioxide (SO2): generated by the combustion of fuels containing sulfur, in power plants, during metal melting and the process of crude oil. It causes acid rain and respiratory diseases
- Nitrogen oxides (NOX): generated in combustion processes of motor vehicles, in power plants and heating systems. They cause respiratory diseases, photochemical smog and acid rain.
- Volatile Organic Compounds (VOCs): a particular mention should be given to these compounds so discussed nowadays, as they are not only atmospheric pollutants but also the major responsibles for indoor pollution. In fact, many products and materials present in domestic houses contain VOCs which may be released over time such as: paints and associated products, vinyl flooring, wood materials, plywood (MDF), carpets, ceilings, insulating. VOCs (from acronym Volatile Organic Compounds) are many different compounds, approximately more than 300, characterized by a certain volatility, and are easily able to evaporate at room temperature already. As the name implies these organic compounds are a number of all carbon-containing substances, mainly present in the atmosphere in gaseous form. VOCs can be simple hydrocarbons, that is organic compounds based solely of carbon and hydrogen, of aliphatic nature (linear or branched chain of which the main actor is methane – CH4) or aromatic (cyclic chains of which the main actor is benzene – C6H6); or more complex molecules containing atoms of nitrogen, oxygen, chlorine and other elements. Part of the VOCs family are aromatic hydrocarbons (benzene and derivatives, toluene, ethylbenzene, xylenes known as BTEX), aliphatic hydrocarbons (from n-hexane to n-hexadecane and methyl hexane), chlorinated hydrocarbons (chloroform, dichloromethane, chlorobenzenes), the alcohols (ethanol, propanol, butanol and derivatives), esters, ketones and aldehydes (formaldehyde). The main role of atmospheric volatile organic compounds is connected to the formation of secondary pollutants. VOCs have both natural and anthropogenic nature.
VOCs have either natural or anthropic.
Volatiles organic compounds of natural origin come from plants; for example terpenes are a VOCs family almost ubiquitous in superior plants, particularly accumulated in some species (e.g.: rosemary, lavender, mint, citrus, parsley, anise, chamomile). VOCs are produced and accumulated in all organs of the plants (stems, leaves, buds, flowers, seeds, fruits, wood, roots), although they are more present in aerial organs.
Anthropogenic VOCs are man-built molecules, using some derivatives from petroleum refining to enable for example the production of organic solvents widely used in the production of paints and inks, in surface treatment (washing, painting, prints), in leather jacket and shoe production, in rubber conversion, in the extraction of vegetal oils, in pharmaceutical chemistry, in incomplete combustion and evaporation of solvents and fuels. Benzene ,in particular, is present in the air mainly due to vehicle emissions. In general VOCs interfere in respiratory processes and irritate eyes. They cause respiratory diseases, are carcinogenic and contribute to photochemical smog.
- PAHs (Polycyclic Aromatic Hydrocarbons) are other carbon organic compounds consisting of more benzene rings. In the atmosphere, there have been identified over one hundred species of PAHs from naphthalene present in the gas phase with two benzene rings up to compounds with seven or more rings that can be adsorbed on particulate. Historically PAHs are the first chemical species to be identified as carcinogen. The origin of PAHs is associated mainly to human activities such as industrial processes, domestic heating, power generation and traffic, being produced by the incomplete combustion of coal, oil, wood and other organic materials. Exposure to PAHs leads to various damages to blood and lungs. According to the International Agency for Research on Cancer (IARC) among PAHs are probable human carcinogens benzo (a) pyrene, benz (a) anthracene and dibenz(a,h)anthracene; while they are possible carcinogenic benzo (b) fluoranthene, benzo (k) fluoranthene and indeno (1,2,3,c,d) pyrene
- Carbon dioxide (CO2) produced using fossil fuels in all energy, industrials and transport sectors. This causes global warming
- Ozone (secondary pollutant) is generated in the troposphere by reactions between NO2 and VOCs with UV rays. It is responsible for photochemical smog and respiratory diseases. The emission of volatile organic compounds (VOCs) contribute to the formation of so-called photochemical smog, a pollution phenomenon favored by solar ultraviolet radiation which involves, besides VOCs, nitrogen oxides (which, as we said, come from all combustions like traffic, heating, industrial production, storage and distribution of fuels etc.); this phenomenon is precisely highlighted with the formation of ozone in the troposphere, the part of atmosphere where we live.
- Fine particles can be compared to a slow and silent killer; they are so small that can be inhaled and gradually accumulated in the respiratory system. When it comes to fine particles, we usually refer to so-called PM10, but in the last decade scientists have revealed another form of pollution linked to fine particles of a smaller diameter, called PM 2,5. How big are these particles and what do PM10 and PM 2,5 mean? PM10 fine particles have a diameter of 10 microns, as well as PM 2,5 particles, measuring only 2,5 microns. To understand these kind of dimensions, you can just imagine the thickness of a hair: this fine dust has a diameter 30 times smaller than the one of our hair! If PM10 are inhalable and accumulate in the lungs, PM 2,5 are even breathable, meaning that they can get into our lungs and can accumulate in blood and reach various part of our body. That is, if damages connected with PM10 particles are confined to the respiratory system, those related to PM 2,5 may be extended to other tissues. Environmental damages of fine particles are linked to the defacement of monuments, relics and stones; they even are coadjutant of acid rain. The worst damages are those caused to human health and fauna: fine particles are transported everywhere by the wind and are deposited in the agricultural soil as well as road surface: traffic raises this dust from the ground and passers inhale/breath it, damaging themselves unconsciously. Furthermore, fine particles can accumulate in waterways or soil to reach the groundwater making lakes and streams acidic, thereby altering the balance of water and rivers nutrients. We often speak about fine dust, atmospheric dust, total suspended particulates to indicate suspended substances in the air without any reference to the size and nature of these particles (fibers, heavy metals, liquid, gaseous or solid pollutants, carbon particles…). When it comes to fine particles reference is made to particulate or suspended particulate, that are all the particles released into the atmosphere (or accumulated in soil and waterways) which have a diameter ranging from a few nanometers up to 500 microns: for this reason in specific we make a division between PM 10 and PM 2,5. Fine particles with a diameter bigger than 10 microns are defined as “gross particles”, and they don’t arouse great concern because they can overcome the larynx and penetrate into the human respiratory system in a minimal way. Again, the environment is affected by the problem of nano powders also called ultrafine particles, that is inhalable dust that can reach air sacs and falls in the order of nanometers (in order of description PM 1, PM 0,1 and PM 0,001). Since when it comes to fine dust we refer to a large amount of particles, it is obvious that there’s not a single source. In general we can say that smaller fine particles are formed mainly from combustion residues. They can be generated by natural phenomena, such as soil erosion or, more commonly, by vehicles exhaust gases or pollution of industrial plants.
ADVANCED OXIDATION PROCESSES
It’s not possible to treat and purify all the air around us. The only way to fight pollution is prevention: minimize emissions of toxic substances and use alternative and renewable energy sources (solar, wind, etc.). Anyway, there are several traditional methods of air purification.
In industrial plants, for instance, it is necessary to purify the air that is used in the production phase, ad this is done through traditional methods such as:
- Adsorption on activated carbons. The adsorption capacity of these materials is used to remove pollutants from the air stream.
- Thermal incineration. The pollutant is introduced into a burner (1000-1200°C) and heated to trigger the oxidation to carbon oxide and water. It’s a versatile but very expensive technique.
New alternative technologies have joined the conventional ones in recent years.
Among all these technologies much attention was attracted by Advanced Oxidation Processes (AOPs is the acronym).
These are a series of processes that generate chemical species with a high oxidizing power capable of degrading the pollutant molecules present in the atmosphere. These advanced oxidation processes are emerging in recent years and are presented as a viable alternative to conventional depuration systems. They can lead to a complete degradation of recalcitrant pollutants or, possibly, to a transformation of these in biodegradable molecules.
Among the various advanced oxidation processes the one who found the most widely use is heterogenous photocatalysis. Research and technology have shown that pollution and dirt can be fought in a cheap and simple way.