Precipitation diagrams of struvite and dissolution kinetics of different struvite morphologies

Babić Ivančić, Vesna; Kontrec, Jasminka; Kralj, Damir; Brečević, Ljerka (2002) Precipitation diagrams of struvite and dissolution kinetics of different struvite morphologies. Croatica Chemica Acta, 75 (1). pp. 89-106. ISSN 0011-1643

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Abstract

Spontaneous precipitation in the systems MgCl2-H3PO4-NH4OH-NaCl (0.15 mol dm(-3))-H2O, MgSO4-NH4H2 PO4-H2O and MgSO4-NH4H2PO4-NaCl (0.15 mol dm(-3))-H2O was studied at 25 and 37 degreesC 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, K-s(o) = c(Mg2+) (.) gamma(2) (.) c(NH4+) (.) gamma(1) (.) c(PO43-) (.) gamma(3), (pK(s)(o) = 13.359 +/- 0.284 at 25 degreesC and pK(s)(o) = 13.269 +/- 0.113 at 37 degreesC) 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 k(diss) were found to be 0.075, 0.103 and 0.152 dm(3) mol(-2/3) s(-1) for rod-like crystals, and 0.331, 0.286 and 0.596 dm(3) mol(-2/3) s(-1) for dendrites at 25, 30 and 37 degreesC, 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.

Item Type:

Article

Uncontrolled Keywords:

struvite; precipitation; thermodynamic solubility product; dissolution kinetics; calcium

Subjects:

NATURAL SCIENCES > Chemistry

Divisions:

Division of Materials Chemistry

Projects: