Electrochemical recovery of phosphate from synthetic wastewater with enhanced salinity

Abstract

Electrochemical recovery of phosphorus from synthetic wastewater using a sacrificial Mg anode has been explored as a function of key parameters, including pH, voltage, and salinity. The effect of chloride (NaCl) concentrations in the range of 0–1500 mM on both Mg corrosion and phosphate precipitation was investigated in acidic and alkaline (i.e., pH 4 and 8) solutions of dihydrogen ammonium phosphate. The primary phosphate precipitate was expected to be struvite (MgNH4PO4·6H2O), and collected precipitates were characterized to determine composition and phase. Batch experiments were run at selected potentials identified by linear sweep voltammetry experiments. Higher corrosion current densities (jcorr), which indicated higher Mg dissolution and corrosion rates (υcorr), were achieved in solutions with higher pH and salt concentrations. Both Mg corrosion and phosphate recovery were proportional to [Cl−], so 1500 mM Cl− at pH 8 resulted in increased Mg dissolution and phosphate recovery by ∼5x and 2x, respectively, compared to the solution without chloride. Elemental composition analysis (ion chromatography), crystallography (X-ray diffraction), surface chemical composition analysis (Fourier-transform infrared spectrometry) and characterization (scanning electron microscopy images) of precipitates suggested the presence of struvite. The Mg/P composition of precipitates formed in 1000 mM Cl− solution showed a greater extent of deviation from the theoretical molar ratio in struvite when the electrochemical cell was operated at more positive anodic overpotential and low initial solution pH. The elevated Mg/P ratio could indicate the presence of mixed precipitates. Simulation of product distribution analysis and specific energy requirement results suggest optimum electrochemical struvite production in the pH range of 8–10 in the presence of chloride.