Remoção de cromo (vi) a partir da utilização de subprodutos de Mangifera indica L.

Abstract

The anthropic action has promoted several problems and damage to the environment, particularly with regard to the dumping of contaminants in water collections. The metal chromium (Cr) stands out because it is an element widely used for various industrial actions such as electroplating, textile processing, metallurgy, among others. This metal can be found in nature, preferably in the hexavalent and trivalent forms - Cr (III) and Cr (VI), respectively. The Cr (VI) is the oxidation state which is capable of causing harm to human health (allergies, mutations, cancer), becoming a social problem as regards their disposal in aquatic environments. Allied to this factor, making use of agricultural wastes (such as peels and seeds of fruits) to remove this toxic metal becomes a viable measure as many of these wastes are rich in phenolic and carbonyl compounds, which may act in removing the chromium. This work aims to study the capacity of Cr (VI) removal, using by-products derived from Mangifera indica L. (mango). Different materials were selected: peels and seeds of four mango cultivars (coité, mallika, rosa and tommy) and mangiferin (substance extracted from the plant) encapsulated with chitosan. There was high total phenolic contents for the peels and seeds of mango, especially the seeds of tommy with 240.76 mg g-1 GA. Chemical characterization by High Performance Liquid Chromatography (HPLC) identified different substances, namely, gallic acid, 3,4 dihydroxybenzoic acid, methyl gallate, brevifolin carboxylic acid, methyl brevifolin carboxylate, mangiferin, isomangiferin, tetra and pentagallate. Besides these substances, there is an emphasis on the hexa, hepta- and octagallate (22,61; 24,57; 26,72; 10,48 mg g-1 extract) compounds found only in seeds of tommy. The characterization of the surface groups of the peels and seeds were evaluated by infrared spectra, pHPZC and characterization of surface functional groups. The adsorption/reduction of Cr (VI) in aqueous solutions was investigated varying the pH, contact time, initial concentration of Cr (VI) and the amount of adsorbent. Removal of Cr (VI) by peels and seeds mango showed the conditions set at pH 1, 3 g L-1 of the adsorbent, except for the cultivar rosa, which was 4 g L-1 (both the peel and seeds); and Contact time of 60 min. The results show a good capacity for absorption/reduction of Cr (VI) and a process of adsorption which follows the kinetics model pseudo-second-order. The most efficient material removal was tommy seeds (100%), with adsorption capacity (q) of 103.94 mg g-1, indicating a correlation with the total and individual content of phenolic compounds and chromium removal. For particles of chitosan and mangiferin (PQM) it was observed maximum adsorption capacity of 57.14 mg g-1, adjusting better to the Langmuir isotherm model and kinetic pseudo-second-order. Thus, the studied mango by-products may be mentioned as materials that remove Cr (VI), giving a highlight for the seeds of the cultivar tommy. In addition, PQM showed better adsorption than the chitosan alone, also highlighting the PQM as a promising material in the removal of hexavalent chromium.