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Thin, sensitive and selective mixture

Research produces filters through the junction of metallic oxides and cellulose

A new family of chemical filters with potential for use in laboratories and industries has been developed by research workers of the Institute of Chemistry of the State University of Campinas (Unicamp). They are thin and microscopic films, composed of cellulose fibers or membranes with a covering of metallic oxides, in a junction called a composite. These filters have selective characteristics according to each individual formulation. When the filters are used in a chemical analysis, they enable the substance which is of particular interest to researchers to stand out from the other multitude of substances.

A good example is the identification of chromium 6, a potentially highly toxic element for both man and the environment. Chromium 6 is found in industrial waste, mainly in effluents from leather tanning industries and in general is accompanied by chromium 3 (with low toxic levels), thus making its identification difficult. By using a composition of titanium oxide and cellulose as a filter, it is possible to detect minute levels of chromium 6.

The research was coordinated by Professor Yoshitaka Gushikem of the Department of Inorganic Chemistry of the Institute of Chemistry of Unicamp. The project: “Properties of Composites of Metallic Oxides/ Cellulose: Development of New Chemically Modified Membranes and Fibers” was financed by FAPESP via its Research Support program. The work, which was carried out between 1997 and 1999, resulted in a patent request and the publication of the chapter “Preparation of Oxide-Coated Cellulose Fiber”, in the book “Polymer Interfaces and Emulsions”, edited in the USA by Professor Kunio Esumi of the Science University of Tokyo.

Although the cellulose/metallic oxide composites are still restricted to the university, their potential applications to industry are substantial. For example, they are able to remove all the heavy metals present in ethanol fuel. Another application of this filter would be the removal of copper from the Brazilian sugar cane drink, cachaça, thus guaranteeing a purer quality drink. Traces of this metal are found in cachaça because preparation takes place in distilleries which contain copper.

Compounded with oxygen
Substances are used in the manufacturing of composites, which react to what one wants to detect, filter or isolate. The variation of the oxygen compounds occurs according to the choice of the metal. Metals are compounded with oxygen and for this reason are called metallic oxides. According to Gushikem, it would be difficult to use other substances in the place of metallic oxides. As a rule, acids and chemical bases, contain no metal in their composition and do not remain deposited on cellulose. Furthermore, metallic oxides remain insoluble in the liquid environment in which the chemical reactions take place. Made up of microscopic particles, the aspect of a metallic oxide is one of a very dispersed fine powder like a very fine talcum powder.

Cellulose was chosen due to its stability both in mechanical and chemical terms. Research scientists were accustomed to placing very fine coverings of metallic oxides on other surfaces such as silica. Based on years of know-how built up from research, it has become easier to develop filters substituting silica and other substrates for cellulose. When heated, manipulated or inserted into other chemical substances, cellulose does not break, nor crumble and is not easily destroyed. For the filter to be prepared, a coating with an extremely fine layer of film made of metallic oxides of titanium, zirconium, antimony, aluminum or niobium is necessary.

Renewable nature
The cellulose fiber or membrane covered by a film of metallic oxide is given the name of metallic oxide/cellulose composite because there is a new chemical formula after the reaction. Among the advantages of using cellulose in the construction of filters is the ease with which it can be molded into the form of fibers or membranes as well as its renewable nature, given that it is a biopolymer, or in other words, a polymer found in nature. The polymers are aggregates of simple molecules with a three dimensional structure which repeat themselves on countless occasions. The molecules join themselves not only in a horizontal direction, side by side, but also in a vertical direction, one above the other. In the case of cellulose, each molecule is constituted by two units, derivatives of glucose and united by an oxygen atom. This molecule can be repeated one thousand, ten thousand or one hundred thousand times.

The fibers are withdrawn from an organic tissue of a plant in exactly the same form as they exist in their natural habitat by means of chemical processing. They can subsequently be transformed into membranes by mechanical processing such as crushing under pressure. In this case, instead of a fiber similar to a thread, one obtains a membrane with a surface formed by compressed fibers.

Gushikem says that the secret to the whole project was to overcome the relative chemical inertia of cellulose, which in isolation has minimal reactive characteristics. “When thin metallic oxide films are placed on its surface, they function as a linkage interface between the cellulose and other chemical substances, or in other words, the cellulose becomes more reactive”, he explains.

The patent that originated from the project and for which Unicamp has applied for the National Institute of Industrial Property (Instituto Nacional de Propriedade Industrial – INPI), relates to the process for the preparation of a polyelectrolyte which is soluble in water — which was isolated on modified cellulose with aluminum oxide. The inventors are Professor Gushikem and the post-graduate students, Reni Ventura da Silva Alfaya and Antônio Alberto da Silva Alfaya.

Gigantic ion
The polyelectrolyte, the subject of the patent request, is a type of gigantic, positively charged ion, which in contact with the cellulose/aluminum oxide composite material produces thin films on the aluminum oxide surface, leaving it positively charged.

For the moment, the polyelectrolyte is only being used in the laboratory, but its potential applications are ample. Gushikem explains that the polyelectrolytes are anionic switch mechanisms, which means they substitute negative ions for other negative ions. In industry, these switch mechanisms are used in conjunction with cationics (which substitute positive ions) in processes for the desalinization of industrial water. Examples of this are boilers, which require very pure water to operate, and the nuclear reactors installed in constructions containing large pools of desalinized water.

Professor Gushikem says that another important application of polyelectrolyte will be for the substitution of phosphate or nitrate in sea water for a type of chloride. These two substances, even at relatively low levels, cause serious pollution problems for the marine environment. In this case, the switch of ions with polyelectrolyte is an alternative method of clearing up sea pollution.

The academic balance
The whole project cost R$ 39 thousand, employed in equipment and reagents. Professor Sandra de Castro of the Institute of Physics of Unicamp also collaborated in the project, which generated three doctorate theses and another two still in preparation. Twenty articles were also published, one in Brazil and the other in 19 American and European scientific magazines.Among the research scientists involved, of special note are Eduardo Aparecido Toledo, of the State University of Maringá, who completed his doctorate in 1999 and was the co-author of the chapter published in the book “Polymer Interfaces and Emulsions”, and Professor Ubirajara Pereira Rodrigues, of the Institute of Chemistry of USP, São Carlos, who completed his doctorate in 1996 and was the pioneer in the group for this kind of work. All of them contributed to the research as a whole on these new compounds and established a unique technology for the chemical industry.

Yoshitaka Gushikem, 58, graduated from Institute of Chemistry of USP and has a Ph.D. from the same university. He has concluded a post-doctorate course in Japan in the area of chemical-physics. He is professor for the Department of Inorganic Chemistry of the Institute of Chemistry of Unicamp, where he has taught since 1971. Gushikem was twice awarded the Zeferino Vaz prize from Unicamp, for academic recognition in 1997 and 1999.
Project: Properties of Composites of Metallic Oxides/Cellulose: Development of New Chemically Modified Membranes and Fibers.
Investment: R$ 39.000,00