Mechanisms of releasing magnesium ions from a magnesium anode in an electrolysis reactor with struvite precipitation

Abstract

The electrochemical precipitation of struvite (MgNH4PO4) using the sacrificed magnesium anode is a new and effective method to recover phosphorus from wastewater. However, insufficient knowledge about the mechanism of Mg2+ released from the magnesium anode and the variation of the outer anode layer related to the hydrogen evolution and electrolysis reactions limits the development and application of electrochemical precipitation methods. To better understand the role of magnesium anode in electrochemical reactions, an electrolysis reactor with a magnesium anode was used to recover phosphate from synthetic wastewater containing 10 and 100 mg/L-P, respectively. The results showed that the composition of the produced precipitates mainly depended on the phosphorus concentration in the synthetic wastewater, because the predominant precipitates changed from Mg(OH)2 to MgNH4PO4 as the concentration increased. Besides, the proportion of Mg2+ from the hydrogen evolution reaction in total Mg2+ production decreased with increased applied current. Meanwhile, the current application facilitated struvite precipitation and weakened the adsorption of struvite on the Mg anode surface. The results revealed that when the Mg anode was polarized in ammonia and phosphorus solution, an outer mixed film consisting of Mg(OH)2/MgNH4PO4/MgCO3 on top of a MgO-rich inner layer formed on the Mg anode surface. These findings are contributing to the comprehensive understanding and improvement of using magnesium anode for recovering phosphorus from wastewater.