A SOLUBILITY AND THERMODYNAMIC STUDY OF STRUVITE

Abstract

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 pK_sp value of struvite at 25°C was found to be 13.36(±0.07). The pKsp value for a temperature range of 10-60 °C varies from 14.36(±0.05) to 14.01(±0.03) with the minimum value of 13.17(±0.05) at 30°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°C, with the maximum value of 212.7 (±3.8) mg l^-1 at 35°C. Standard enthalpy of reaction, ΔH_γ °, calculated from the average K_sp values for the temperature range of 10-30 °C, was 23.62 Kcal mol^-1. An analytical expression for the K_sp as a function of temperature has been developed by fitting experimental data.

Keywords:

• SOLUBILITY
• STRUVITE
• IONIC STRENGTH
• TEMPERATURE
• ENTHALPY