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Precipitation Diagrams of Struvite and Dissolution Kinetics of Different Struvite Morphologies

Vesna Babić-Ivančić ; Laboratorj for Precipitation Processes, Division of Materials Chemistry, Ruđer Bošković Institute, P. O. Box 180, HR-10002 Zagreb, Croatia
Jasminka Kontrec ; Laboratorj for Precipitation Processes, Division of Materials Chemistry, Ruđer Bošković Institute, P. O. Box 180, HR-10002 Zagreb, Croatia
Damir Kralj ; Laboratorj for Precipitation Processes, Division of Materials Chemistry, Ruđer Bošković Institute, P. O. Box 180, HR-10002 Zagreb, Croatia
Ljerka Brečević ; Laboratorj for Precipitation Processes, Division of Materials Chemistry, Ruđer Bošković Institute, P. O. Box 180, HR-10002 Zagreb, Croatia

Sažetak

Spontaneous precipitation in the systems MgCl2-H3P04-NH40H-NaCl (0.15 mol dm-3)-H20, MgS04-NH4H2P04-H20 and MgS04-NH4H2P04-NaCl (0.15 mol dm-3)-H20 was studied at 25 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 the predominant phase formed in a wide range of reactant concentrations. The thermodynamic solubility products of struvite, Ks° = c(Mg2+) · y2 · c(NH4+) · n · c(P043-) · y3, (pKs° = 13.359 ± 0.284 at 25 °C and pKs° = 13.269 ± 0.113 at 37 °C) were calculated from the final, equilibrium, pH values taking ali 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 kdiss. were found to be 0.075, 0.103 and 0.152 dm3 mol-2/3 s-1 for rod-like crystals, and 0.331, 0.286 and 0.596 dm3 mol-2/3 s-1 for dendrites at 25, 30 and 37 °C, respectively. The activation energy for the dissolution of rod-like crystals was found to be (44.8 ± 1.1) kJ mol-1 and that for dendrites (37.6 ± 1.6) kJ mol-1. Analysis of the kinetic data suggests the diffusion of constituting ions away from the surface of the dissolving crystal or the desorption of ions from the crystal adsorption layer as the possible rate determining mechanisms of struvite dissolution.

Ključne riječi

struvite; precipitation; thermodynamic solubility product; dissolution kinetics

Hrčak ID:

127487

URI

https://hrcak.srce.hr/127487

Datum izdavanja:

4.2.2002.

Posjeta: 1.718 *