Professor, Center for Waste Management, Graduate Program in Mechanical Engineering, Diponegoro University, Semarang, Indonesia

Speech Title: Hydrothermally mediated MAP precipitation for resource recovery from wastewater: The effects of calcium and carbonate ions

Abstract: Valuable compounds from waste stream can be recovered through implementing "simple to use" hydrothermal technology. Magnesium, ammonium, and phosphate (MAP) precipitation under hydrothermal process is a potential alternative approach for inexpensive wastewater treatment and providing an environmentally sound and renewable nutrient source to the agricultural industry. In the present study, the influence of Ca²⁺ and CO₃ ions with Mg to Ca ratio (1:1) on the MAP precipitation was investigated through mineralogical phase analysis. The stock solution was prepared by mixing powder of MgCl₂ and NH₄H₂PO₄ (0.025 M) into the distilled water. Similarly, CaCl₂•2H₂O and KHCO₃ (0.025 M) was added to provide calcium and carbonate ions in the solution. Prior to hydrothermal process, the stock solution was initially adjusted to pH 9.0 and 10.0 by the addition of KOH (50/50 wt. %). The hydrothermal experiment was carried out in a batch mode using an autoclave bomb-reactor, which was set at the temperature of 120^OC for 24 h and then cooled down to room temperature. The crystals formed were then examined by X-ray powder diffraction (XRPD) for phase mineralogical phase analysis and scanning electron microscopy (SEM) for morphological crystal. The Rietveld analysis of XRPD pattern verified that many minerals, including calcite (CaCO₃), carnalite [KMgCl₃•6(H₂O)], dolomite [CaMg(CO₃)₂], schertellite [(NH₄)₂Mg(HPO₄)₂.4H₂O)], sylvite [KCl], struvite [NH₄MgPO₄• 6H₂O] and whitlockite [Ca₉(MgFe)(PO₄)6PO₃OH] were formed at pH 9.0. Likewise, the mixed product of calcite, calcium trisphosphate (TCP) hydrate, hydroxyapatite [Ca₁₀(PO₄)₆(OH)₂], dolomite, schertellite, sylvite, struvite was found in the reaction temperature of 120 ^OC and pH 10.0. SEM micrograph showed that the crystal solids have different morphologies with nanosized particles. In the case of phosphate recovery, Ca and carbonate ion concentrations were more sensitive influence than pH to promote struvite formation under hydrothermal reaction. This hydrothermal data would be valuable for understanding nucleation and growth of crystals under influence of various ions present in the wastewater.