Simultaneous determination of nucleation and crystal growth kinetics of struvite using a thermodynamic modeling approach

This work concerns the controlled struvite formation (MgNH4PO4·6H2O) by precipitation as an alternative removal of phosphorus and, consequently, of ammonium from wastewater discharges. A new method, based on an integrated methodology, is proposed here for predicting and controlling struvite nucleation and growth rate. Experiments were conducted in an isothermal stirred batch reactor at a temperature of 25 °C from a synthetic aqueous solution at different pH levels (8.5–9.6). The initial concentrations of Mg, PO4 and NH4 are fixed at 3 mmol/L, then at 4 mmol/L, with a molar ratio of Mg/NH4/PO4 equal to 1. Crystal size is determined by laser granulometry and morphometry. A population balance-based model coupled with a thermodynamic model predicts particle size distribution vs. experimental time using a reconstruction model. This approach is particularly numerically stable for the identification of nucleation and particle growth kinetics parameters that are used to predict crystal size distribution. The methodology is based on a thermodynamic model previously developed for which pH control and supersaturation constitute key parameters. The obtained results are of major importance for the design of struvite precipitation reactor, and for the development of crystal growth control methodology.

• This work concerns the controlled struvite formation by precipitation.
• We model nucleation and growth kinetics.
• A thermodynamic model is coupled with a population balance.
• The model predicts particle size distribution vs. experimental time.
• It allows the identification of nucleation and particle growth kinetics parameters.