Uso de hidroxiapatita na adsorção do íon Mn2+ e posterior estudo de liberação para aplicação como fertilizante

Abstract

Manganese is an essential element for organisms in low quantities, as it participates in some protein synthesis, enzymatic functioning and as a cofactor of several enzymes. It is also a necessary micronutrient for plants, used as fertilizer. However, in high concentrations, can cause serious human health problems, such as neurological syndromes, newborn size changes, among others. Its bivalent solution-stable and highly soluble Mn2+ form has great pollutant potential. Thus, it is important to investigate effective ways to remove Mn2+ ions from the aqueous medium to reduce the harmful effect of their presence in the medium. Linked to the harmful effect of excess Mn2+ in wastewater, there is the beneficial effect of this ion when used as fertilizer. Thus, the focus of this dissertation study is to use commercial hydroxyapatite and hydroxyapatite synthesized by the coprecipitation method in the adsorption of Mn2+ ions. Selecting hydroxyapatite as adsorbent was based on its adsorptive potential and its use as a fertilizer. Subsequently, hydroxyapatite, with the adsorbed ion, had its potential as a fertilizer evaluated by studying the release of Mn2+ and phosphate ions in a medium that simulates the soil. Thus, the present dissertation studies the removal of Mn2+ from a medium where the presence of the ion produces harmful effects and aims to redirect it positively to agribusiness. The hydroxyapatites characterizations were satisfactory, especially for the coprecipitated hydroxyapatite, which had a larger surface area (60.40 m²/g) than the commercial hydroxyapatite (40.61 m²/g), in addition to greater purity according to the analysis of DRX, FTIR and thermal analysis. The Mn2+ adsorption tests were carried out close to the isoelectric point. Transmission microscopy (MET) revealed smaller average particle sizes for the coprecipitated hydroxyapatite (13.32 nm), with spherical shape, than for the commercial hydroxyapatite (22.60 nm), with rod shape. The results of adsorption of Mn2+ ions are promising, presenting a favorable adsorption profile for both hydroxyapatites, with a maximum adsorption capacity of 14.07 mg/g for the commercial hydroxyapatite and 27.93 mg/g for the coprecipitated hydroxyapatite, adjusted to the Langmuir model for room temperature, pH 7, 30 minutes of contact and 2.5 g/L of hydroxyapatite. The release study in citric acid medium 2% (m/v) at 40°C also obtained positive results, with the release of manganese and phosphorus for both hydroxyapatites, with better results for the coprecipitated hydroxyapatite, since it showed greater release of macronutrient (phosphorus) and less of micronutrient (manganese) in the period studied of five days. In general, both commercial and coprecipitated hydroxyapatites showed favorable results, offering a possible alternative to redirect Mn2+ ions, along with hydroxyapatite, as sources of micronutrient and macronutrient in the area of mineral fertilizers.