Avaliação do papel estabilizante/desestabilizante de polifosfatos em dispersões coloidais de argilas

Abstract

In the development of controlled release fertilizer (CRF) or intelligent fertilizers, one of the challenges is the synthesis of multi-stock materials, such as release of nutrients, pesticides, herbicides and fungicides. One of the vehicles used in CRF's is the hydrogel and, through the use of clays as crosslinkers between polymer chains, this material can be synthesized. Three samples of different clays were studied: a crude natural, from the city of Pedra Lavrada - PB, denominated locally bentonite variety pink; an industrialized natural clay, standard bentonite enogel; a synthetic clay, Laponite RD (LRD). The present work aims to evaluate the stabilizing and / or destabilizing role of polyphosphates in colloidal dispersions of clays in order to obtain new hydrogels that can be used in the future as CRF’s. The clays, pink and enogel, were previously treated. The pink clay was treated with sodium carbonate and the enogel clay was washed with distilled water to remove impurities. Both clays were characterized by X-ray fluorescence assays, X-ray diffraction, Foster swelling, methylene blue index and zero load point. They presented MgO, CaO and K2O contents, possess smectite clay, kaolinite, quartz and other mineral phases. The treated clays showed high swelling, high methylene blue index when compared to untreated clays. Based on the results obtained, the interaction of the clays, pink and enogel, with short chain polyphosphate (PPSC) and long chain polyphosphate (PPLC) were done with the clays after treatment. The study of the stabilizing / destabilizing role of polyphosphates started with visual observations of the clay-polyphosphate system. The concentration of polyphosphate (PP) is important in the colloidal stability of the clay dispersions. At lower concentrations of PP, it exerts stabilizing role, while, in higher concentrations, it destabilizes the dispersions promoting the separation of phases. The results of dynamic light scattering, zeta potential and mobility showed that in 1,0 × 10^-2 mol.L^-1 PP, it acts to stabilize the colloidal dispersions, and, in parallel, promotes bridging interactions between clay particles, and the reflection of these interactions are particles of micrometric size and with highly negative charge. In dispersions where there are bridge interactions, the ionic strength changes the bridge's persistence length, and hence the size of the aggregate formed. Visual observations and rheological measurements of LRD with PP showed that addition of polyphosphate, chain size and clay concentration alter the elastic response of LRD dispersions. By increasing the size of the polyphosphate chain it is possible to modify the viscosity of the dispersions by changing the final state of these dispersions. The effect of chain size was evident in dispersions with 2% (m / v) LRD due to the formation of a trapped state, Wigner glass, whose predominant interactions are repulsive. The system rose pink clay with a concentration of 1,0 × 10^-2 mol.L^-1 PP presented the best conditions to develop a material with potential to act as an intelligent fertilizer.