Electrochemical Synthesis of Struvite from Wastewater: Implications of Chloride Ion and Initial pH

Abstract

Electrochemical precipitation of magnesium ammonium phosphate (MAP) compounds, specifically as struvite (NH4MgPO4.6H2O) from wastewater is gaining importance as one of the techno-economically feasible pathways of sustainable development [1]. However, systematic exploration of the optimal range of anode potential for maximizing phosphate recovery under varied parametric conditions of pH and ion concentrations, other than NH4 + and PO4 3- has not been attempted so far. Additionally, ion dissolution from sacrificial magnesium anode over stipulated potential range between pitting and secondary passivation needs to be investigated for maximum struvite production with minimal consumption of magnesium. In this study, we have characterized the role of initial pH (4 and 8) and chloride ion concentration (1 M) on phosphate recovery and magnesium dissolution by conducting batch electrochemical experiments with three different anode potentials covering the entire range of feasible potential window. Linear Sweep Voltammetry (LSV) was performed at two different initial pHs to identify the aforementioned potential range. Subsequently, batch experiments at different anode potentials were conducted in an electrochemical cell consisting of pure magnesium sheet as sacrificial anode and stainless steel cathode. 77 mM solution of dihydrogen ammonium phosphate (NH4H2PO4) and 1M sodium chloride was used as electrolyte. Lower value of Pitting potential in presence of chloride ions with respect to that with baseline NH4H2PO4 solution was indicative of the favorable effect of chloride in rupturing the passive layer of magnesium hydroxide that normally forms on anode surface and hinders magnesium dissolution. Higher magnesium dissolution rate and phosphate recovery were recorded above and over respective pitting potentials for both pHs. Moreover, monotonic increasing profiles of precipitate masses with increasing anode potential unambiguously indicates higher dissolution of magnesium with as alternatively reflected in terms of elevated charge transfer as per Faraday’s law. In both cases of different initial pHs, final pH was recorded in alkaline range. Nevertheless, ammonia loss was observed to increase with anode potentials and increasing pH. Precipitates, formed at the surface of anode and in bulk solution, on characterization revealed the presence of several MAP and other magnesium phosphate precipitates including Struvite (MgNH4PO4.6H2O), Dittmarite (MgNH4PO4.H2O), Magnesium phosphate tribasic (Mg3(PO4)2) and Newberyite (MgHPO4.3H2O). Magnesium hydroxide was detected at higher pH. Simulation of saturation indices and product distribution using Visual Minteq 3.1 predicted optimal recovery of Struvite (MgNH4PO4.6H2O) in the pH range 8-10. Reference: [1] Cid, C. A.; Jasper, J. T.; Hoffmann, M. R., Phosphate Recovery from Human Waste via the Formation of Hydroxyapatite during Electrochemical Wastewater Treatment. ACS Sustainable Chem. Eng. 2018, 6 (3), 3135-3142.