engleski

Precipitation Diagrams of Struvite and Dissolution Kinetics of Different Struvite Morphologies

Spontaneous precipitation in the systems MgCl2-H3PO4-NH4OH-0.15 mol dm-3 NaCl-H2O, MgSO4-NH4H2PO4-H2O and MgSO4-NH4H2PO4-0.15 mol dm-3 NaCl-H2O was studied at 25 °C and 37°C and initial pH = 4. Precipitates aged for 24 hours and 30 days were characterized by means of chemical analyses, X-ray diffractometry, FT-IR spectroscopy and thermogravimetry. Struvite was found to be a predominant phase formed in a wide range of reactant concentrations. The thermodynamic solubility products of struvite, Kso = c(Mg2+)×g2×c(NH4+)×g1×c(PO43-)×g3, (pKso = 13.359 ą 0.284 at 25 °C and pKso = 13.269 ą 0.113 at 37 °C) were calculated from the final, equilibrium, pH values taking all the relevant magnesium, phosphate and ammonium species into account. The dissolution kinetics of dendrites and rod-like crystals of struvite in water was followed by recording pH as a function of time and the linear dependence of the dissolution rate on the absolute undersaturation was found. The dissolution rate constants kDIS were found to be equal to 0.075, 0.103 and 0.152 dm3mol-2/3s-1 for rod-like crystals, and 0.331, 0.286 and 0.596 dm3mol-2/3s-1 for dendrites at 25, 30 and 37 °C, respectively. The activation energy for dissolution of rod-like crystals was found to be (44.8 ą 1.1) kJ mol-1 and for dendrites (37.6 ą 1.6) kJ mol-1. Analysis of the kinetic data suggests, the diffusion of constituting ions away from the surface of dissolving crystal or the desorption of ions from the crystal adsorption layer, as the possible rate determining mechanisms of struvite dissolution.

struvite; precipitation; thermodynamic solubility product; dissolution kinetics

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