Electrochemical Precipitation of Struvite in Acidic Environment: Pure Magnesium Vs. AZ31 Alloy Anode

Abstract

Nutrient recovery from phosphorus-rich wastewater streams is essential to reduce environmental impacts and to improve the sustainability of fertilizer production globally. One potential approach is to recover phosphates by electrochemical precipitation of struvite, also known as magnesium ammonium phosphate hexahydrate (MgNH4PO4*6H2O). Struvite is considered a premium grade, slow-release fertilizer, which is a potentially marketable product for the fertilizer industry. The effect of pure-Mg versus AZ31 magnesium alloy as the only source of magnesium on anodically-driven struvite precipitation was investigated in a single-cell electrochemical batch reactor with no pH adjustment, in acidic environment. The results showed an elongated needle-shaped pattern with smooth sharp edges which indicates a high-quality pure struvite and a particle size of ca. 30 µm in length, ca. 6.5 µm in width. In a 6 h batch experiment, the pure-Mg anode out-performed the AZ31 alloy by producing a 4.5-fold greater mass of struvite, next to a 2.8-fold higher current density. The measured magnesium dissolution rates were 1.2 mg cm-2 h-1 for the pure-Mg and 0.8 mg cm-2 h-1 for the AZ31 magnesium alloy anode, respectively. Overall, in this electrochemical reactor setup, the pure-Mg anode showed a better performance towards the electrochemical struvite precipitation.