Stability of Mg-based anode in electrochemical struvite precipitation using pure Mg vs. AZ31 vs. AZ91D

Abstract

Struvite (MgNH4PO4·6H2O) electrochemical precipitation method is a compelling new way to recover phosphorus from wastewater. In this study, a suite of common magnesium materials on the market is tested to understand the effect of anode composition on the electrochemical performance of electrolysis reactors with single and batch modes. The pure magnesium anode demonstrated the fast phosphorus removal efficiency and best Mg2+ releasing among three alloys, but it cannot be sustained and must be replaced frequently. Large amounts of brucite were mixed into precipitates, decreasing their purity during the pure Mg anode electrolysis reaction. The Mg anode materials influenced the elemental composition of struvite produced from the electrolysis reactor, which was not standard 1: 1: 1 (Mg: P: N). Al element is the most significant contaminant released from AZ31 and AZ91D among three impurities (Al, Mn, Zn). The Mg anode, which had higher Al content, released more Al3+ into the electrolyte, which was easy to be adsorbed by struvite precipitates with the cycle. The Al3+ concentration in the effluents also decreased with the experiment prolonged. Results showed that AZ31 could support Mg2+ stable production in batch mode without as large impurity ions releasing as AZ91D in the electrolyte or the precipitated struvite.