Crystallization of struvite in a hydrothermal solution with and without calcium and carbonate ions

Abstract Hydrothermal experiments with magnesium, ammonium, and phosphate (MAP) solution at a temperature of 120 OC for 24 h and pH (9 and 10), whilst effects of varying Mg2+/Ca2+/HCO-3 ratios on struvite crystallization were examined. The study was performed to investigate their effects on the quality and quantity of crystals using the XRPD Rietveld refinement and SEM method. Obviously, the struvite crystallization was inhibited through the forming of calcite, dolomite, hydroxyapatite, sylvite, and Mg-whitlockite under different pH conditions. In the absence of Ca2+ and HCO-3 ions, struvite and dittmarite were formed at pH solutions (9 and 10). Struvite proportion reduced with pH (9 and 10) under Mg2+/Ca2+/HCO-3 ratios (1:1:1 and 2:1:1), and depleted under the Mg2+/Ca2+/HCO-3 ratio of 1:2:2. An obvious change in morphologies of crystals into nanosized particles was observed. Results of the low proportion of struvite for experiments with Mg2+/Ca2+/HCO-3 molar ratios may be a drawback for phosphate recovery.