Fertilizer derived from alkaline hydrothermal alteration of K-feldspar: A micrometer to nanometer-scale investigation of K in secondary reaction products and the feldspar interface

Global food security concerns have spurred increasing demand for locally sourced and produced K-fertilizers. Various processes been explored more than a century; one promising solution is based on the alkaline aqueous alteration of feldspar-rich rocks at elevated temperatures. However, knowledge overall physico-chemical reactions comprising dissolution feldspar precipitation secondary phases still rudimentary, in particular how structure evolves nm-scale during hydrolysis conditions. Here we report results study aimed converting potassium feldspars to K-rich fertilizer sanidine microcline samples 190 °C pH 12 Ca(OH)2 solutions 24 h. Based X-ray diffraction Rietveld refinement, authigenic minerals that precipitated are primarily composed Ca-carbonate (calcite, vaterite), Ca-(Al)-silicates, such as tobermorite hydrogrossular. Short-term bench top leaching experiments water prove hydrothermal product releases up two orders magnitude K unaltered K-feldspar starting material, pointing its application ready-to-use K-deficient soils. Detailed chemical mapping energy dispersive spectroscopy (FESEM- TEM-EDXS) analyses precipitates μm show distribution associated with very heterogeneous, both spatially terms concentrations. Using various analytical transmission electron microscopy (TEM) techniques, e.g., HRTEM, TEM-EDXS, EFTEM, investigate chemistry interface, find no evidence change or nm-scale, even though continuously decreases volume each grain. Our observations also existence an amorphous surface altered layer (SAL) variable thickness (10–~100 nm) forming interface. Nanometer-scale measurements this SAL rich K, therefore may be important reservoir easily leachable K. We hypothesize it forms situ expense by coupled interfacial dissolution-reprecipitation process (CIDR).