Controlled release of fertilizers from Ca(II)-alginate matrix modified by yerba mate (Ilex paraguariensis) waste

Abstract

Yerba mate (Ilex paraguariensis) is an infusion traditionally drunk in many countries of South America. Along its industrial manufacture, a fine yerba mate powder represents a considerable proportion of the total production, consisting of an industrial by-product not safe for human consumption, rich in minerals, that could be useful as organic compost. In line with current bioeconomic trends, here we present the recycling of yerba mate industrial by-products into urea, potassium and phosphate-containing capsules as an environmentally friendly strategy for the design of controlled-release fertilizers. The capsule matrix is based on Ca(II)-alginate with a final yerba mate powder (YMP) content of 83% w/w. The aim of this work was to study the kinetics of mineral fertilizers release both in soil and water, along with an exhaustive macro and microstructural characterization (PIM, DSC, TGA, SEM-EDX, X ray scan tomography and SAXS) to allow the analysis of structure/function relationships. The encapsulation efficiency and release in water of phosphorous were affected, being significantly lower than urea and potassium and releasing under different mechanism. Retention and degradation in soil was mainly related tocapsule structure, as well as their thermal properties. The incorporation of different fertilizer induced structural changes in Ca(II)-alginate network as assessed by SAXS.