Transforming syenite through alkaline-hydrothermal processes: A sustainable strategy to overcome potassic fertilizer shortages in tropical regions

Abstract

Silicate rocks contribute to soil health and crop productivity, but their nutrient release is often slow. This study evaluates alkaline hydrothermal treatment effect on potassium (K) and multi-nutrient (phosphorus (P), calcium (Ca), magnesium (Mg), sulfur (S), and silicon (Si)) availability from syenite rock. Powdered raw and CaO – KOH materials, hydrothermally treated using a design of experiments (DOE) approach were characterized. Along with petrographic characterization, analytical techniques such as X-ray fluorescence (XRF), X-ray diffraction (XRD), Quantitative Evaluation of Minerals by Scanning Electron Microscopy (QEMSCAN), electron probe microanalysis (EPMA), and Fourier transform infrared spectroscopy (FTIR) were employed. In addition, powdered syenite underwent a sequential leaching (1, 24, 194, and 624 h) in deionized water and the leachate was analyzed by Inductively Coupled Plasma-Atomic Emission Spectrometry (ICP-AES). The results revealed that hydrothermal treatment induced significant mineralogical, chemical, and structural changes, with KOH-treated materials releasing up to 1631 mg/L of K after 26 days. Nevertheless, altered orthoclase remained the dominant K-feldspar (80–90 wt%), while new Ca- and K-bearing phases formed. Regardless of the additive, treated samples released 3 to 69 times more nutrients than raw material (except Mg). However, nutrient dissolution followed a power function with a low rate (0.03–0.9), indicating slow but sustained nutrient availability. The study also revealed antagonistic interaction parameters that may inhibit nutrient release, highlighting key mechanisms of rock weathering. This insight facilitates an 8 % increase in K-release from 7 to 1.5 h of treatment. Therefore, powdered syenite could be a nutrient reservoir and buffer for acidic soils.