The Passivating Layer Influence on Mg-Based Anode Corrosion and Implications for Electrochemical Struvite Precipitation

Abstract

The removal and recycling of phosphorous from wastewater streams by electrochemical precipitation of struvite is a new and exciting approach; however, previous studies showed low percent yields in single cell batch experiments without pH adjustment. To investigate improvement of the percent yield, the surface-area-to-volume ratio of the electrodes was increased by 1.9-fold; consequently, a 27% increase in struvite production was observed for the pure-Mg anodes, but only a 2% increase for the AZ31 alloy. Potentiodynamic polarization experiments revealed 1.8-fold higher corrosion rates for the AZ31 alloy, which is in contradiction with the 2.8-fold higher magnesium dissolution rates calculated for the pure-Mg during struvite precipitation. This discrepancy between the techniques is attributed to the difference in the electrochemical environment, where the formation of an insulating layer of struvite on the anodes during the batch precipitation experiments is a critical difference. Based on characterization of the morphology and chemical structure of the precipitate, studied by Fourier-transform infrared spectrometry and scanning electron microscopy, pure struvite was obtained with a particle size of ca. 73 μm in length and ca. 13 μm in width for pure-Mg and ca. 44 μm in length and ca. 8 μm in width for the AZ31 alloy anode, respectively.