The kinetics of the spontaneous precipitation of struvite was investigated
in aqueous supersaturated solutions containing stoichiometric concentration
s of Mg2+, NH4+ and PO43- ions, ionic strenght 0.15 M NaCl and at 25 degree
s C in a batch, stirred reactor at constant supersaturation. The induction
times preceding the onset of struvite precipitation and the initial rates o
f precipitation were measured directly from the traces of titrants added in
order to maintain the solution supersaturation. From the measurement of th
e induction times as a function of the solution supersaturation, the stabil
ity diagram of the system was constructed. In all cases the only solid-phas
e forming was identified as struvite. Kinetic analysis of the rates, which
depended strongly on the solution supersaturation yielding a second-order d
ependence, suggested a surface diffusion mechanism. The precipitated struvi
te crystals showed a high negative charge which increased as a function of
the solution pH while the presence of magnesium ions affected the microelec
trophoretic mobility of struvite dispersions yielding an isoelectric point
at pMg of 1.75. (C) 2000 Elsevier Science B.V. All rights reserved.