Eco-efficient materials for agricultural crops based on a mineral rich in MOR- and HEU-type zeolites.

Abstract

Natural zeolites have great potential as nutrient carriers to develop eco-efficient materials for massive use in agriculture. Zeolitic minerals usually contain only one dominant zeolite type. The use of minerals with mixtures of zeolites in similar proportions can affect the interaction of chemical species with the zeolitic matrix, altering the behaviour of the resulting materials. In this work, a mineral consisting mainly of a mixture of two zeolites, mordenite (MOR) and clinoptilolite-heulandite (HEU) with equivalent fractions, was used to develop materials carrying nutrients (N, P, and K) for agricultural crops. The mineral matrix provides important elements such as K and Si, while N and P were incorporated into the material by treatment with ammonium hydrogenphosphate and urea. The presence of superficially adsorbed PO₄ ^3-, NH₄ ⁺ exchanged in zeolites, and urea arranged on the surface so that it covers the material and interacts with the zeolitic frameworks, was evidenced by Fourier-transform IR spectroscopy, adsorption measurements, scanning electron microscopy, scanning transmission electron microscopy, and other methods, as well as through culture tests. The complexity of the multiphase zeolitic support leads to changes in the position and intensity of FTIR bands compared to other similar materials developed using simpler zeolitic carriers dominated by HEU zeolite. The most intense NH₄ ⁺ band was observed at 1402 cm^-1, while for a HEU zeolite it was at 1540 cm^-1. This difference was associated with a higher NH₄ ⁺ content in MOR compared to HEU. Accordingly, the shift experienced by the urea amino group bands when it interacts with the frameworks of these zeolites is different. The applied treatments did not affect the structures (as evidenced by XRD) and other qualities of these zeolites, highlighting their ion-exchange and adsorption properties for nutrient release and reversible water retention. This is essential for the use of this material as a slow-release fertilizer that efficiently provides nutrients for the agroecological development of plants, as evidenced in the cultivation tests.