The implications of pulsating anode potential on the electrochemical recovery of phosphate as magnesium ammonium phosphate hexahydrate (struvite)

A sustainable approach to address the limited availability of rock phosphate is to electrochemically recover phosphorus (P) as struvite (magnesium ammonium phosphate hexahydrate, MgNH₄PO₄·6H₂O) fertilizer. On Mg surfaces, however, magnesium hydroxide (Mg(OH)₂) and/or struvite form resistive films that limit the release of divalent Mg ions and reduce struvite precipitation. The present work enhances struvite recovery through the application of a dynamic voltage with a sinusoidal waveform. Compared to a constant potential, our results demonstrated that a dynamic potential could enhance the rupture of the passivating film, and facilitate the release of Mg²⁺ from the metal anode, thereby overcoming passivation problems. Ammonium dihydrogen phosphate (NH₄H₂PO₄, 10 mM) solutions at neutral pH were used as test solutions. A 160–192 % increase in magnesium release was observed versus constant voltage applications, resulting in a 1.3x increase in phosphate recovery. The scanning electron microscopy (SEM) and X-ray diffraction (XRD) characterizations indicated that the precipitate recovered contained struvite. An analysis of electrode surfaces using laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) confirms a greater extent of passivating film breakdown under dynamic voltages.