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 (MgNH 4 PO 4 ‚6H 2 O) 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 Mg 2+ , NH 4 + , and PO 4 3ions, 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.
The precipitation of calcium carbonate from aqueous solutions has been studied at supersaturations sufficient for spontaneous precipitation but low enough to enable highly reproducible experiments to be carried out with respect to induction periods and rates of precipitation. The threshold for the onset of calcium carbonate precipitation at pH 8.60 has been established. The induction periods and rates of precipitation are both strongly dependent on supersaturation. Treatment of the kinetics data according to classical nucleation theory yielded an estimated 68 erg cm-2 value for the surface energy of the solid precipitating. From kinetics plots it is also suggested that the number of precipitating calcium carbonate crystals increases rapidly at the start of nucleation. At a later stage the particles stop increasing in number and subsequently grow. The apparent activation energy of 155 kJ rno1-I calculated from experiments carried out at 25, 35 and 45 "C points to a surface-controlled mechanism for the precipitation of calcium carbonate.
Scale formation of calcium carbonate on tube walls is studied in a once‐through flow system under isothermal conditions. The effects of supersaturation ratio, flow velocity and liquid temperature on the deposition rates are investigated. For relatively small supersaturation ratios (in the range 3.5 to 8) the deposition rate tends to increase with increasing supersaturation. For higher fluid supersaturations with respect to calcium carbonate, the deposition rates remain roughly constant but they are influenced strongly by the fluid velocity. This dependence of the deposition rate on fluid flow suggests a mass transfer‐controlled process.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.