A thermodynamic modeling approach for solubility product from struvite-k

Abstract

Crystallization of struvite-k could be an economical and sustainable alternative for potassium and phosphorus recovery from wastewater and ashes of biomass combustion. Knowledge with regards to thermodynamics that are involved in the formation of struvite-k in reactor is key in determining the optimal conditions for an efficient process. However, less research work has been done on struvite-k solubility product. A thermodynamic model for struvite-k precipitation was in this study thus proposed to determine the struvite-k solubility product, as well as conversion rate for struvite-k and its accompanying crystal cattiite over a pH value range of 10.5–12.5. The model was based on numerical equilibrium prediction of involved system Mg-K-PO4-H2O. The model was presented by a set of nonlinear equations that were solved by an optimization strategy with a three-step method. The method consists of a preliminary search by Lagarias Simplex search method for initialization, Successive Quadratic Programming (SQP) for resolution of system, and extrapolating calculated data to zero ionic strength for determination of struvite-k solubility product. A pKstruvite-k value of 10.872 for struvite-k at 25 °C was obtained in the model. This novel model will provide a theoretical reference for recovery of potassium and phosphate, as well as its possible application in magnesium potassium phosphate cement (MKPC).