Influence of Cu2 + and Zn2 + as additives on crystallization kinetics and morphology of struvite

Batch crystallization experiments were conducted to investigate the influence of common metal ions in wastewater, i.e. Cu2 + and Zn2 +, on crystallization kinetics and morphology of struvite (MgNHPO4.6H2O). The crystallization process was monitored by measuring the pH of the solution. The pH decreased progressively for the first ten minutes of the run, then started to tail off. The crystallization rates ranged from 3.13/h to 4.18/h, which agree with most published values and the rate was significantly retarded by the presence of either Cu2 + or Zn2 +. The maximum retardation was about 25% which was achieved for crystallization at 25 °C with 5.00 ppm Zn2 +. In the presence of the metal ions tested, cracks and fractures existed on the surface of the struvite crystals but the morphology did not significantly change. The alteration in surface characteristics can be the results of the stresses induced by incorporation of the metal ions into the crystal lattice and/or adsorption of these ions onto the crystal surface. In practical terms, both Cu2 + and Zn2 + could be used as antiscalants for struvite scale control in wastewater treatment.