How Particles Behave in the Environment
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It is a matter of fact that once particles come out of their origin, for example, the chimney stack of an incinerator, as already mentioned, they travel and reach distances which can be enormous. As an already mentioned example, the grains of sand in the Sahara Desert, far coarser than the particles we deal with, reach the eastern shores of the United States, where we found them. It is equally clear that the smaller the particle volume, the greater the distance it reaches. It is also this aspect which the various diffusion models should take into particular account Therefore, if the fallout map must be reliable, in addition to considering many other aspects, including those related to weather events, it is essential to know the composition of the particles with respect to their mass.
Another important aspect of the characteristic behaviour of the particles once they have fallen to the ground is the fact of penetrating, especially dragged by the rain, into the aquifers, polluting them. But they also change the composition of the soil, for example, because of what is released as a result of corrosion. This changes the habitat of the microflora and the microfauna with consequences which are often hard to predict. A notable phenomenon is their possible interference with the root system of plants (Fig. 3.16).
Figure 3.16 Tomato roots with nanoparticles. Scanning electron microscopy image of a root of a tomato plant immersed in a solution of water with 500 mg/l of nanoparticles of TiO2 (INESE-IIT-Project 2009-2013). The arrows indicate a cluster of nanoparticles of TiO2 which occlude the pores of the roots, hindering the adsorption of nutrients.
Figure 3.17 (a) Mouth and (b) wings of a contaminated bumblebee. The greenhouse we set up to study the effects of solid, inorganic and non-biodegradable nanoparticles was divided into two specular sectors, each of which contained the same plants and the same insects (bumblebees). Only one of the two halves of the greenhouse was sprayed with particles. In that part of the greenhouse the bumblebees could no longer fly because of the particles which were deposited on their wings. Because of this, they could no longer feed themselves and they all died. The images show the pollution (white dots) around the mouth and on the wings.
In the course of one of our experiments, we also observed that the particles can settle on the wings of insects, making their flight difficult until it becomes utterly impossible, thereby causing their death. This aspect should be studied carefully and, in any case, taken into consideration when studying the progressive rarefaction of bees, a phenomenon which is probably not only due to the increasing presence of chemical pollutants or the use of pesticides (Fig. 3.17).
What Is Being Done to Prevent Particles from Entering the Environment?
To be sure, the situation is somewhat curious: We have known all along that breathing and eating dust is bad for our health. Asbestosis, silicosis, bauxite fibrosis, berylliosis, siderosis and byssinosisa are just a few among the dust-induced pathologies we have known for many years. Pneumoconiosis, that is, the class of occupational lung diseases caused by the inhalation of dust is a word all doctors know. Nevertheless, we continue to produce finer and finer (i.e. more and more aggressive) dust and allow it to enter relentlessly into the environment, the only one we have available, not yet being able to escape from this planet, if we will ever be. And what is even more curious is that there are ‘scientists’, luckily enough nowadays a dwindling minority, who still wonder whether particles are really harmful. This for the relief and the joy of industrialists and politicians who, thus, find a 'scientific' excuse to contin ue undeterred the former to pollute and the latter to allow it.
However, though with some exceptions, the problem is perceived, and the proof is that there are legal and technical systems which, admittedly in a shy and not particularly effective way, try to tackle it
If, at least in many cases, laws look more like the result of negotiations between polluters and legislators than derived from
-’Byssinosis is an occupational pathology affecting the lungs, caused by cotton fibres. Once we had the chance to be present at a surgical operation on a patient diagnosed as affected by pleural mesothelioma. It was only our electron microscopy observation on the surgical samples that showed that it was not asbestos but cotton to be the cause of the illness.
scientific and medical considerations, technical countermeasures do not seem to be particularly effective either.
This is not a technical book addressed to technicians and engineers, who have much better texts available for their information. So, what little follows is no more than few, simple reflections about a couple of preventive systems currently in use.
At first glance, mechanical filtration could seem the most immediate and reasonable way to stop particles issued from any source.
Bag filters are very widely used, particularly, though certainly not only, in waste incinerators. They are composed of many long cylinders made of close-woven fabric capable, in the highest-performing products, of resisting a temperature of around 170°C. Through them, dust-laden gas (very often air) is forced, and particles, as happens in the bronchi, are generally captured by four different mechanisms: inertial collection, interception, Brownian movement and electrostatic forces. The efficiency claimed, in truth without much explanation, is above 99%, but that numeric datum seems at least questionable. The particles taken into account are only the primary, filterable ones, while the primary condensable and the secondary particles, far more important in terms of quantity, are disregarded. The second questionable point is weight. That 99 point something percent is calculated as the weight of the dust captured. Since, as already mentioned, for simple geometrical reasons, weight is proportional to the cube of the particle’s radius; if the filter stops particles whose diameter is 10 pm but can’t stop those whose diameter is 0.1 and whose weight is, therefore, one-millionth of the former particles, efficiency, at least at first glance, looks very high. Unfortunately, the smaller the particle, the more penetrating it is, and bag filters are inefficient as regards nanoparticles. So, that claim does not take real efficiency into account with regards to health, missing condensable and secondary particles and, what is even more important not in terms of weight but in terms of health, smallsized ones. It must also be added that, not to clog the filters, what they capture is made to drop to the bottom of the system together with the ash which accounts for roughly 1/3 of the mass of what is being burned in a waste incinerator.
The obvious conclusion, if scientific data and nothing else are considered, is that incineration is a technique by which waste is, at least in part, made to disappear from view (though hidden in various ways, ash does not disappear) but is transformed into something which is far more aggressive for health and environment. Only to be added marginally: each ton of waste needs as much quantity of other material to be treated (water, methane, ammonia, bicarbonate, active carbon, etc.) and all those materials must somehow be disposed of.
Urban Pollution Filters
A fewyears ago, we had the opportunity to work in New Delhi (India) with an air filtration system similar to what we experimented on the roof of a bus in Rome (Figs. 3.18-3.20). What we tried in India was a device which was not in motion but was placed in a square, much larger in size than the one we used in Rome, where the air entered mechanically sucked and then was emitted purified. Its efficiency was very high, but its range of action was limited, with the aggravating circumstance of having continuously polluted air
Figure 3.18 Electro-filter system located on the top of a bus in Rome.
arriving. Therefore, such a system would have been excellent in a closed place such as, for example, an airport indoor environment, but in fact it proved to be ineffective outdoors. The particles found show an extremely serious pollution situation with particles also composed of uranium and thorium as well as a large variety of other chemical elements.
Figure 3.19 (a) Pollution identified in the electro-filter system in Rome. The particles are those captured by the electro-filter of the filter box placed on the roof of the bus which circulated in Rome for a week. Their shape and composition shows what floats in the air of a big city, (b) The spherical particle shown in the photograph comes from the air of Rome. Its shape demonstrates a high-temperature origin. Many iron-based particles were identified, few of lead.
Figure 3.20 (a) Pollution identified in the static electro-filter in New Delhi, India. The presence of lead-based particulate matter found in New Delhi most likely originates from gasoline which still contains tetraethyl lead as an antiknock. In countries where that substance has been banned for several decades, the lead pollution it has caused still remains in the environment, (b) A particle containing also uranium and thorium which are radioactive, demonstrates how toxic the pollution we inhale every day is.