Electrochemically-driven struvite recovery: Prospect and challenges for the application of magnesium sacrificial anode

An excellent approach to recover nutrients from wastewater is by struvite precipitation. The process requires an equal molar ratio of Mg:NH₄:PO₄, which can be acquired through the addition of Mg. In the last decades, electrochemically-driven struvite precipitation has gained much interest due to the effective recovery of high purity struvite. The utilization of Mg sacrificial anode in the electrochemical precipitation of struvite emerges as a new potential Mg source that reduces the need for chemical addition for struvite precipitation or pH adjustment. This review discusses some electrochemical processes that applied Mg sacrificial anode for nutrient recovery. The objective is to fill the knowledge gaps regarding the process mechanisms (i.e., the corrosion behavior of magnesium and their effect on the struvite precipitation), recovery products, and the challenges during the process. The anode material compositions, solution pH, and applied current may influence the Mg corrosion behavior and dissolution rate. The possible formation of Mg(OH)₂ passivation film on the anode surface may inhibit the Mg²⁺ ions transfer and delay struvite formation. However, the increase of applied current slightly above the pitting potential could increase film-free area and speed up the anodic and cathodic chemical reaction, creating higher magnesium dissolution rate and phosphate removal efficiency. Therefore, electrochemical processes utilizing Mg anode can be applied to enhance struvite recovery, i.e., over 90% phosphate removal, with higher purity. Research on the promising utilization of pure-Mg anode and electrodialysis systems requires further developments.