A solubility and thermodynamic study of struvite.

Accumulation of magnesium ammonium phosphate hexahydrate (struvite) on surfaces in contact with wastewater, especially in anaerobic sludge digestion and post-digestion processes, is a widely reported problem in the wastewater treatment industry. The solubility and thermodynamic properties of struvite at different temperatures was studied. Struvite thermodynamic solubility products at temperatures between 10 and 60 'C were determined by variation of solution ionic strength and extrapolation to zero ionic strength, using an appropriate activity coefficient model. The pKsp value of struvite at 25 degrees C was found to be 13.36 (+/-0.07). The pKsp value for a temperature range of 10-60 degrees C varies from 14.36 (+/-0.05) to 14.01 (+/-0.03) with the minimum value of 13.17 (+/-0.05) at 30 degrees C. The effect of ionic strength, pH and temperature on struvite solubility was also studied. The solubility of struvite determined in deionized water was found to be 169.2 (+/-4.3) mg l(-1) at 25 degrees C, with the maximum value of 212.7 (+/-3.8) mg l(-1) at 35 degrees C. Standard enthalpy of reaction, delta Hr degrees calculated from the average Ksp values for the temperature range of 10-30 degrees C, was 23.62 Kcal mol(-1). An analytical expression for the Ksp as a function of temperature has been developed by fitting experimental data.