Natural Clinoptilolite Materials Enriched in Nitrogen and Phosphorous for Agricultural Purposes: A Comprehensive Study on Their Development

Zeolitic materials enriched in phosphorus and nitrogen, with a pronounced content for this last element, were obtained from natural clinoptilolite (CLI) treated with ammonium hydrogenphosphate (DAP) and urea. The interaction of phosphorus and nitrogen species retained on CLI was analyzed using different research methods. FTIR spectra showed the presence of NH₄⁺, PO₄³⁻ and urea on CLI. The retention of ammonium involves both ion-exchange in clinoptilolite, and adsorption. Phosphate is retained by adsorption, and it can be incorporated into an electrical double layer with accompanying cations including NH₄⁺. The FTIR bands of the amino group of urea appear to be shifted relative to their location in the urea spectrum. This is associated with weakening of N–H bond from amino group due to its interaction with the clinoptilolite framework via hydrogen bonding. The urea is arranged on the zeolite surface, which is consistent with SEM observations and elemental (N and P) mapping by STEM. As a result, the surface area and micropore volume of these materials, treated with DAP and urea, decrease with respect to those modified with DAP alone. Accordingly, the release studies showed that the subsequent delivery of phosphorus species occurs in a delayed mode. These materials have great potential to be used as slow-release nutrient fertilizers for crop plants. They have a significant amount of nitrogen available on their surface, allowing plants to easily assimilate it early in their growing cycle. Zeolitic matrix leads to obtain materials at low-cost for massive applications, efficient use of nutrients and environmental benefits.