Discussion on preparation of millimeter-size struvite and growth mechanism

Struvite, an insoluble salt, has the problem of slow crystal growth and fine size, which affects sewage treatment and fertilization efficiencies. This study investigated the crystallization process of struvite by changing the precursors and analyzed the growth mechanism. The results showed that the prepared precursor compound with Mg(OH)2 @ MgHPO4 (labeled as P-3) could slowly release Mg2+, PO43-, and OH− to keep the dimensionless ionic product (Asp) in the narrow metastable zone width (MSZW). Moreover, the surface reaction and diffusion resistances were 8977 and 1859 min/mol respectively when P-3 as a precursor, which considerably exceed those of other precursors, avoiding explosive nucleation and enabling a continuous growth of the crystals in the surface reaction zone. Consequently, rod-shaped with a length of 5–6 mm (a maximum of 1 cm for the largest presently known) and the average growth rate with 3.8 × 10−8 m/s could be obtained for struvite crystals.