Fabrication of Porous Phosphate/Carbonate Composites: Smart Fertilizer with Bimodal Controlled-Release Kinetics and...

A simple method to prepare phosphate/carbonate composites for use as porous sponge-like phosphate fertilizers (ps-PO₄Fs) is presented. The composites ps-PO₄Fs were prepared by ion-exchange implantation of phosphate onto the surface of vaterite-phase calcium carbonate (CaCO₃) microparticles. The ps-PO₄Fs obtained under the optimized conditions were found to contain a nanoscale porous network of calcium phosphate covering the CaCO₃ support. In addition, ps-PO₄Fs exhibited two distinct phosphate release modes having different kinetics: a fast-release step over the initial 24 h period following a parabolic diffusion model, indicating controlled diffusion from external surfaces/edges, and a second slow-release step over the course of a month following the Ritger-Peppas model, indicating the release and diffusion of phosphate adsorbed at specific sites. The ps-PO₄Fs also adsorbed glyphosate well because of their porous structure and large surface area. However, glyphosate adsorption prevented phosphate release at concentrations greater than 10 mg L^-1. The ps-PO₄Fs were tested for their effects on plant growth and showed effects similar to commercial fertilizers. In summary, these smart, eco-friendly, and multifunctional fertilizers having two-stage phosphate release could enable the application of lower amounts of fertilizer and remove excess glyphosate from the environment.