Magnesium source with function of slowly releasing Mg and pH control for impurity-resistance synthesis ultra-large...

Struvite (MgNH₄PO₄·6H₂O, Magnesium ammonium phosphate, MAP), recovered from wastewater, has potential application as a slow-release fertilizer. However, crystal size distribution (CSD) of recovered MAP typically lied in the range of 50-300 μm, due to fast nucleation rate and notably narrow metastable zone width (MSZW) of MAP, with purity levels 40-90 %. In order to control the rate of nucleation, a novel magnesium source with the form of MgHPO₄·3H₂O wrapped with Mg(OH)₂ was prepared, referred to as P-3. This compound gradually released Mg²⁺ and PO₄^3-, regulating solution concentration kept in MSZW to promote crystal growth. The inherent Mg(OH)₂ within P-3 also acted as a pH regulator in wastewater, eliminating the necessity for additional acid or alkali adjustments during crystallization process. The MAP precipitated by P-3 exhibited an impressive CSD of 5000-7000 μm, with a maximum size reaching 10,000 μm. This represented the largest CSD reported in literature for recovered MAP from wastewater. The significance of the ultra-large MAP precipitated by P-3 lied in its enhanced resistance to impurity adsorption, resulting in MAP with a remarkable purity 97 %, under conditions of low heavy metal ion concentration approximately 5 mg/L. Furthermore, the removal efficiency of ammonia nitrogen (NH₄⁺) can reach 92 %. In comparison, two other magnesium sources, soluble salts (MgCl₂ and Na₂HPO₄, P-1) and a combination of insoluble salts (Mg(OH)₂ and MgHPO₄, P-2) were evaluated alongside P-3. The CSD of MAP precipitated from P-1, P-2 was both <100 μm, with purity levels of 90 and 92 % and NH₄⁺ removal efficiency of 92 and 90 %, respectively. Importantly, the strategy of obtaining ultra-large size MAP from wastewater in this study provided novel insights into the crystallization of other insoluble salts with large sizes.