Application of Pulsed Potential Waveform in Electrochemical Phosphate Recovery As Struvite from Wastewater

Abstract

Phosphate production is apprehended to peak as early as 2030, and post-peak, phosphate production, and thus food supply, will be limited by dwindling resources, strained economics, and reduced rock quality, with remaining resources expected to last 50-300 years. The limited supply of phosphate demands P conservation by changing the food production system, from fertilization to wastewater treatment. Stuvite (MgNH4PO4.6H2O) has been recognized as a high-value, slow-release, effective fertilizer that can be obtained from both solid (e.g., food, animal, and human waste) and liquid wastes (e.g., wastewater from industrial, municipal, and agricultural operations). The goal of our research is to recover struvite from wastewater using an alkaline electrochemical reactor with a magnesium alloy anode and a cathode constructed of standard catalysts for reducing water to hydrogen gas and hydroxide. Over time with successive reactor batches, the magnesium electrode used to generate magnesium cations in solution and precipitate struvite becomes severely fouled, which in turn increases energy demand and decrease struvite fertilizer recovery. A dynamic, pulsed voltage waveform, as opposed to a constant voltage, causes a stress-strain-like effect at the magnesium electrode surface, thereby preventing or reducing the amount of struvite precipitate that would attach to the electrode. In an aqueous solution of ammonium dihydrogen phosphate (NH4H2PO4, 10 mM) at neutral pH, a significant 50% increase in magnesium release is observed compared to constant voltage applications, resulting in a 30% increase in phosphate recovery. Studies of the recovered precipitate using X-ray diffraction and scanning electron microscopy (SEM) indicate the presence of struvite.