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Recycling

Vegetable plastic

Mango stone membranes can be used for water treatment

EDUARDO CESARBiodegradable plastic made from mango stones was shown, in laboratory tests, to have potential to be employed in fine membranes used to purify water and to treat effluents, in hemodialysis and in the controlled release of medication. The research that developed this new material was conducted by researchers from the Federal University of Uberlândia (UFU), in Minas Gerais state, and from the University of Caxias do Sul, in Rio Grande do Sul state. The stone is the external hard layer of the mango seed, comprised mainly of the fiber that protects the embryo. Cellulose, hemicellulose and lignin, the components that form plant cell walls, are found in large amounts in this part of the fruit. “In the process that we developed, we extract the cellulose from the mango stone and make acetate with it to manufacture the plastics used in the membranes,” says professor Guimes Rodrigues Filho, the coordinator of the project that is being conducted by the Polymer Recycling Laboratory of the Chemistry Institute of UFU.

The idea of making use of this fruit waste arose when the researchers discovered that at least two thousand tonnes of mango stones from every crop are discarded by the juice industry just in the Triangulo Mineiro region. As Brazil’s mango production amounts to some 1.3 million tonnes a year – used mainly to make juice – the volume of stone waste totals on average about 480 thousand tonnes a year. “The stones account for 30 to 40 percent of the mango’s weight, depending on the variety,” says Rodrigues Filho. This mountain of waste normally ends up burnt or in the garbage. Besides the mango stone, the researchers have already produced cellulose acetate from newspaper and sugarcane bagasse. They are now working with corn straw. The membranes currently used in filtering processes are usually made from wood pulp or cellulose.

Molecular sieve
In an article published in May in the journal Carbohydrate Polymers, the researchers compared the performance of cellulose acetate membranes from newspaper and from the stones of mangoes of the Tommy Atkins variety (which is reddish, with sweet pulp and fairly resistant) in water purification and treatment processes called reverse osmosis. Here, the water is separated from contaminants, such as dissolved solids, suspended solids, bacteria and organic matter, with a semi-permeable membrane. This membrane behaves like a molecular sieve, selectively rejecting almost all the dissolved molecules and only allowing pure water to pass through it.

As there are many applications for this filtering material, before producing the membranes one must know how they are to be used. “Their porosity depends on whether a greater or smaller flow of liquids is to pass through them,” said the project coordinator. The pores are controlled by adding certain salts to the mixture, such as magnesium perchlorate, which works like an inducing agent for the formation of the pores on the membrane’s surface, aiding the  filtering processes. In the structural evaluation conducted with a scanning electron microscope, the membrane made with mango stones and the newspaper one essentially achieved the same standards of performance. However, the porous substructure is dense in the membranes of cellulose acetate made from the fruit stones, because of the material’s higher molecular mass, yielding better performance. “Even when it was put under pressure in separation processes, the membrane didn’t break,” said Guimes.

The researchers tested not only various materials, but several processes for producing cellulose acetate membranes. One of the methods developed by the research group is currently being patented by INPI, the National Institute of Industrial Property. The company Orbita, from the Enterprising Activities Incubation Center at the Minas Gerais university, is to be responsible for partnering with other companies and for transferring the technology.

Biodegradable packaging
The company was created by the researchers to enable them to take part in the development of a project approved in 2008 via an agreement between Finep, the Studies and Projects Finance Agency, and Sebrae, the Brazilian Support Service for Small Enterprises, also based on using mango stones mixed with sugarcane bagasse to produce biodegradable tubelets – packaging in which plant seedlings raised in nurseries are placed.

The researchers are now working on the final formulation for the molding of the tubelets. Furthermore, they are studying the material’s biodegradation at the University of Caxias do Sul. The containers offered by the market, made from petroleum products, such as PVC and polypropylene, take about 150 years to degrade in nature. Based on the results of this project, the researchers decided to use the same vegetable waste to make membranes, a line of research already under way at the laboratory of the Minas Gerais university since 1996. However, instead of using the cellulose mixture from mango stones and sugarcane as a base, they worked on each type of waste separately and added other materials, such as newspapers.

Scientific article
MEIRELES, C.S. et al. Characterization of asymmetric membranes of cellulose acetate from biomass: Newspaper and mango seed. Carbohydrate Polymers. v. 80, n. 3, p. 954-61. mai. 2010.

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