Rural enterprises amount to about 5.8 million in Brazil, 4.3 million of which practice subsistence farming. They put enormous pressure on the water table, due to the existence of septic tanks, pigsties and abattoirs, beside the large plantations that deposit agro-chemical in the soil. In these cases, water can contaminate and pollute rivers and the subsoil. Concerned with this, the physicist specialized in materials engineering Odílio Benedito Garrido de Assis, from Embrapa Instrumentação Agropecuária, located in São Carlos, where he has just finished a project that lays the foundations for cheapening and improving the water filtering processes in the rural environment.
And so the study began with the analysis of data that showed that, where there was pollution, the water can have around 140 million particles of pollutants per cubic meter (m3). It is a problem for which the best conventional purification systems can successfully remove around 90% to 98%, explains Assis, a percentage that is even very acceptable in world-wide terms.
But the remaining part of this venture is tough. The 2% to 10% of pollutants that are left over in the water can mean millions of particles. It is relatively easy to clean water in the range of from 50% to 90%, says Assis. From there on, the gains in efficiency start to be measured in smaller and smaller, and equally more expensive units. With 99.99% efficient filtering, the number of particles drops to 14,000 per m3. An even better level would be reaching 99.9999%, which would leave the water with 140 particles per m3.
A mission to date not fulfilled, because, due to the impossibility of retaining some pollutants in water, the costs with treatment can grow 400% for a minimal increase in efficiency. “When we started to work on the subject, in 1999, we were after a system that would make it possible both to make filtering cheaper and to improve its efficiency, in particular in removing the residue left by agro-chemicals”, Assis explains.
And so the project was born. To begin with, it had touse an easily accessible and low cost material. Recycled and ground glass was chosen to work as the porous medium. “All the other materials, ceramics, metals and polymers, are more expensive”, explains Assis. Collected in public places, bottles were broken and ground down to 10 microns – one micron is a thousandth part of a millimeter -, then compacted and sintered, that is, the particles were regrouped by heating, to form a new material. The resulting membranes had the format of a little circular biscuit full of pores, from 0.6 to 4 millimeters in thickness and 3 to 4 centimeters in diameter. The pores are in a range of between 8 and 16 microns in diameter.
These membranes were the material for the first stage of testing, called mechanical filtering, which means the ability to retain particles just as a physical barrier. “Every particle larger than the pores would be retained”, Assis explains. But bacteria and chemical elements are usually even smaller than the pores in the membranes. The first tests showed that new arrangements had to be made. A new stage started at the meeting between Assis and Professor Sérgio Campana Filho, of the Macromolecule Chemistry and Physical Chemistry Department of the Institute of Chemistry of the University of São Paulo (USP), in São Carlos.
Since 1993, Campana has been researching chemical compounds that show potential for establishing linkages with various elements, among which heavy metals, which are residues left by agro-chemicals and even radioisotopes. “Nothing exactly new as an idea, but little explored in practical terms, as it calls for much study and experimenting”, Campana adds.
Joining these lines of research was what made the work innovative, including a presentation of the results by Assis, as a guest speaker at the Frontiers of Materials Science workshop held in Trieste, Italy, last year. In addition, the work made the pages of international scientific publications. “Our plan was to add a biochemical function to the mechanical filter, covering the glass membrane with chemical elements that would react in a specific way with the pollutants, in an aqueous medium”, Campana recalls.
Among several elements tested was lysozyme, a natural enzyme of animal origin that, in contact with bacteria, destroys the protective layer of these microorganisms. To this glass membrane one can also can add some kinds of polysaccharides – chains of molecules of sugars – like chitosan, a product obtained from chitin, a raw material taken from the shells of crustaceans, which reacts strongly to heavy metals, and carboxymethyl cellulose, which performs the function of solving the components tested.
In a series of experiments carried out, always on a laboratory scale, the researchers made water contaminated with the Escherichia coli bacterium pass through a membrane processed without any covering. Then they repeated the operation, but using a membrane covered with lysozyme. “The results were impressive”, Assis points out. “The slides on which the samples were collected show two extremes. Without lysozyme, the level of survival of the bacteria went from 70% to 92%. Whereas with the enzyme, the results also varied, but in one of the experiments the level of elimination came close to 90%.”
According to Assis, the most promising results were achieved with chitosan,which reacted to agro-chemicals, even at low levels. “This was very important in proving the possibility of interaction with agro-chemicals”, says Campana. Assis says that the biggest problem is that agro-chemicals are very functional. To show economical results, “they are made not to react with the elements that dissolve them, which is why their residual effects are so powerful.” In the tests carried out in the ambit of the project on porous membranes, the level of reaction to the pores, and the consequent elimination of the pollutants, was in the order of 12%, which fell short of the levels expected.
In spite of the fact that this experiment showed a low level of efficiency, the researchers recharged their batteries, both at Embrapa Instrumentação Agropecuária and in USP’s Institute of Chemistry. They now have nine pupils working in research that supplements the original project, three at the doctorate level and six for master’s degrees. “We need to see new reactions, to test on a larger scale”, says Campana. One idea is to set up a filter made only of chitosan divided into compartments. “In the tests here at USP, we saw that chitosan has a very large capacity for absorbing bacteria. When this filter is set up, we need to develop ways of formulating solutions for its format to be maintained, as chitosan dissolves in contact with an acid medium. When we achieve this format, we will be able to use the chitosan filter hundreds of times, since all that we have to do is to get rid of the stock of bacteria that it is capable of stocking.”
For Assis, there now is another way for his studies. According to him, the membranes showed that they also work as sensors, in addition to filtering. This means to say that they will be able to indicate the presence of undesirable chemical elements in various media, such as foods that have a permeable paste or pulp in the form of paste, such as tomatoes and strawberries, by the direct interaction with the active films immobilized on the vitreous surfaces. Amongst these contaminating agents are the organophosphorus compounds, which are carcinogenic.
Development of Permoselective Bioactivated Membranes to be Employed in the Decontamination of Contaminated Waters (nº 97/08178-9); Modality Regular Line of Benefits for Research; Coordinator Odílio Benedito Garrido de Assis – Embrapa; Investment R$ 40,199.76 and US$ 22,652.00