Electrochemical removal and recovery of phosphorus as struvite in an acidic environment using pure magnesium vs. the AZ31 magnesium alloy as the anode

Magnesium (Mg) electrodes were investigated as the only source of magnesium for anodically-driven struvite precipitation in a single-cell electrochemical batch reactor. The cell was operated in an acidic environment with no pH adjustment. The effect of electrode composition on cell efficiency toward struvite production was investigated for pure-Mg versus an AZ31 Mg alloy. 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 and a 2.8-fold higher steady-state current density. The measured Mg 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 Mg alloy anode, respectively. The structure, morphology, and composition of the electrochemically precipitated struvite were analyzed by x-ray diffraction, scanning electron microscopy, x-ray photoelectron spectroscopy, and energy-dispersive x-ray spectroscopy. Results showed a crystalline struvite state, with an elongated needle-shaped morphology and a particle size of ca. 30 µm in length and ca. 6.5 µm in width. The smooth sharp edges are an indication of high-quality pure struvite, with no evidence of other precipitates or interfering cations.