Homogeneous precipitation of magnesium phosphates from seawater solutions

Homogeneous (unseeded) nucleation of Mg phosphate from modified Ca-free sea water solutions was investigated at 20 degreesC and pH of 8. Precipitated s olid phase was characterized using chemical analysis, X-ray diffraction, sc anning electron microscopy, and IR-spectroscopy. Effect of aqueous phosphat e and ammonia concentrations and the intensity of stirring on the induction period time (tau) of Mg phosphates nucleation were studied. A linear relat ionship between logarithm of bobbierite (Mg-3(PO4)(2) . 8H(2)O) saturation index and log tau was established with a slope close to 2, Aqueous NH4+ (up to 0.002 M) has no effect on the nucleation kinetics and does not incorpor ate in the precipitated solid phase. Stirring of solution has a dramatic ef fect on nucleation kinetics: the induction period decreases by a factor of 100-10000 in unstirred solutions compared to stirred ones. The relative dif fusion/chemical reaction control mechanism of Mg phosphates precipitation f rom supersaturated solutions is discussed. It is shown that spontaneous ino rganic precipitation of Mg-(ammonium) phosphates (struvite and bobbierite) in modern marine environment is impossible because of very sluggish kinetic s. (C) 2001 Elsevier Science B.V. All rights reserved.