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

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 PO₄³⁻, 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.