Eco-Friendly utilization of kitchen waste ashes through crystallization and hardening methods

The powdery nature of waste ashes constrains eco-friendly utilization besides temporary pH shocks in some crops. This study tested crystallization after acid dissolution firewood and charcoal ashes and hardening methods with cow dung, human urine, and water. Compounds in the crystals were determined by X-ray Diffraction while elemental solubility of hardened ashes was conducted by Atomic Absorption Spectrometry. Compounds formed in the hardened ashes were predominantly Al₂O₃, CaCO₃, amorphous, and crystalline carbon (C). Urine-hardened charcoal ashes had more compounds than dung-hardened ashes, followed by water-hardened ashes. Water-hardened firewood ashes had more compounds than urine-hardened ashes which had the same number of compounds as the dung-hardened. Interestingly, three sulfur minerals, ZnS, CuFeS₂, and FeS₂, were found in the dung- and water-hardened ashes. The ash crystals contained Brucite, calcium sulfate, and calcium phosphate. After leaching, Na and K were more soluble, Ca was sparingly soluble, particularly in charcoal ashes, whereas Mg was insoluble in un-hardened ashes. Hardening increased Mg solubility slightly and reduced Ca solubility in both ashes but substantially enhanced Na and K solubility by two to five-fold. The study shows that ash is a resource whose utilization offers many opportunities in many sectors.