Spontaneous Precipitation of Struvite from Synthetic Wastewater Solutions

Abstract

Phosphorus recovery from wastewater by precipitation in the form of crystalline struvite is an attractive option contributing toward sustainable development. Struvite or magnesium ammonium phosphate hexahydrate (MgNH4PO4·6H2O) is a crystalline salt, which may be used either as a fertilizer or as raw material for the production of phosphorus. The supersaturation of wastewater with respect to the target mineral is the key parameter determining the extent and the rate of recovery of struvite. In the present work, we investigated the stability domain of struvite in synthetic wastewater (SWW) and report on the kinetics of precipitation of the respective salt forming spontaneously. The investigation was carried out in aqueous solutions with compositions typically encountered in municipal wastewaters. The supersaturation with respect to struvite was varied through the appropriate variation of the concentrations of the Mg2+, NH4+, and PO43- ions, for which the stoichiometric molar ratio of 1:1:1 was observed. All experiments were done at 25 °C and at a constant solution pH 8.50 in a stirred batch reactor closed to the atmosphere. The component salt concentrations were selected so that the respective supersaturated solutions were not stable. Precipitation of struvite from the supersaturated solutions prepared in SWW was initiated spontaneously past the lapse of well-defined induction times. The measurements of the induction times showed that the stability range of the supersaturated solutions in SWW was very narrow. The induction times, preceding the formation of struvite, were inversely proportional to the solution supersaturation and followed the dependence predicted by the classical nucleation theory. It was thus possible to calculate a surface energy of 15 mJ m-2 for the struvite nuclei forming. The rates of the struvite precipitating past the end of the induction period were measured at a constant driving force by the addition of stoichiometric titrant solutions throughout the precipitation process, using solution pH as a master variable monitored by a glass electrode sensor. The rates of precipitation measured from the titrants addition showed a parabolic dependence on the solution supersaturation. The high order (>1) of dependence of the rates on the solution supersaturation suggested a surface diffusion-controlled mechanism.