Hidróxidos duplos lamelares como matrizes para interação com fósforo: avaliação físico-química e aplicações

Abstract

LAYERED DOUBLE HYDROXIDE AS MATRICES FOR PHOSPHATE INTERACTION: PHYSICO-CHEMICAL ASSESSMENT AND APPLICATIONS: Phosphorus is an essential element for all living organisms and is indispensable for diverse economical activities, specially agriculture. The development of control release systems is an important and necessary alternative to correct delivery of phosphorus to the necessary mediums, avoiding economic and environmental losses. Layered Double Hydroxides (LDH) due to the singular characteristics are excellent materials to be explored as matrices for phosphate interaction. However, there is no consensus in literature about the structural factors which may favor or harm the interaction process between LDH and the anion. This thesis, was investigated the role of the metallic cation (M2+) of LDH structure has on adsorption and desorption process and on stability of the structure when in contact with phosphate. Hydrotalcite, the most common LDH, was first studied due to the M2+ composition (M2+=Mg), which has the short ionic radius and then the structure slightly open. It was notice that in low and intermediate phosphate concentrations the [Mg-Al]-LDH structure is well preserved, with anion intercalation, while at elevated concentrations, the [Mg-Al]-LDH structure is replaced by new precipitates formed by magnesium and phosphorus. From these results, were evaluated the interaction of LDH with Zn2+ in the structure (intermediate ionic radius). The formation of new phases happens in minor concentrations than that observed for Mg2+ and fewer amount of phosphate was adsorbed. However, these materials were appropriated for phosphate release when incorporated in dental resins, and may contribute to remineralization and maintenance of dental enamel. Besides, the reinforcement properties of this LDH on dental polymeric matrix were evaluated. Lastly, the interaction of phosphate with a more closed structure of LDH, formed by Ca2+ was studied. Similarly to [Zn-Al]-LDH, new phases were formed at intermediated phosphate concentrations. Due to [Ca-Al]-LDH characteristics, these materials were used as phosphate source for nitrifying bacteria, extremely important for legume cultivation. [Ca-AlPO4]-LDH was able to increase the bacterial growth. About [Mg-Al-PO4]-LDH, the capacity of phosphate release from its structure had already been verified in aqueous medium, therefore this material was used to phosphate release in a dynamic system, as fertilizer for wheat crops. [Mg-Al-PO4]-LDH was able to provide the phosphate nutrition than the other sources investigated, in a shorter time, with the advantage of increasing the soil pH, preventing the soil immobilization of phosphate. Soil analysis, after the cultivation, revealed that [Mg-Al-PO4]-LDH has potential to provide phosphate in longer times, around 1.5 fold longer than commercial fertilizers.