Mechanochemical synthesis of layered Zn-Mo compounds as slow-release Mo fertilizers

This study explores a mechanochemical (MC) technique to synthesize a slow-release fertilizer containing zinc (Zn), molybdenum (Mo) and minor amounts of sulfur (S). The reactants ZnO, MoO3, and ZnSO4.7H2O were milled in a ball mill with a two-step operation. The two metal oxide reactants were initially milled for 1 h to activate the oxides followed by milling with ZnSO4.7H2O or minor water for another 2 or 4 h to accelerate the formation of layered zinc molybdenum oxide-sulfate. The products were characterized by X-ray diffraction (XRD), Fourier-transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM), and thermogravimetric analysis (TGA), confirming the successful synthesis of a product with low Mo solubility containing mostly Zn and Mo. The product milled with ZnSO4.7H2O formed a clear layered structure which further slowed the release of Mo. The slow-release character of the product with 1 h + 2 h milling was observed in a column dissolution study; the solubility of Mo was less than half the product made without the second milling step. Column dissolution studies demonstrate the material's biphasic release behaviour, highlighting its potential for sustainable agriculture as a slow-release fertilizer. Furthermore, the products maintained lower solubility across a wider pH range compared to the reactants.