Improving potassium recovery with new solubility product values for K-struvite

Abstract

Crystallisation of ammonium (NH4) struvite is a viable method of recovering valuable nutrients such as nitrogen and phosphorus from waste waters. Little work has been done on potassium (K) recovery since neither is it an environmental pollutant nor is society facing global potassium shortages. However, potassium is an essential plant macronutrient, worldwide imbalances in nutrient and fertiliser use exist and there is a need for a slow-release potassium fertiliser. The goal of this research was to develop a fundamental understanding of K-struvite formation as the first step in recovering potassium to allow for producing a full complement nitrogen–phosphorus–potassium slow-release fertiliser from waste waters. Specific objectives included determination of solubility product values for K-struvite at 10, 25 and 35°C. By using these values, optimal supersaturation conditions for K-struvite precipitation were modelled using aqueous equilibrium modelling software and subsequently validated by experiment. The new solubility product values confirmed that K-struvite was less soluble than previously reported. This research adds to the body of literature in nutrient recovery by furthering the understanding of K-struvite formation.