The effect of citrate and phosphocitrate on struvite spontaneous precipitation

The effect of citrate and phosphocitrate ions on the spontaneous precipitation of magnesium ammonium phosphate hexahydrate (MgNH4PO4-6H(2)O, struvite) at pH 8.50 and 25 degrees C was investigated at conditions of constant solution supersaturation. The supersaturated solutions were prepared in synthetic wastewater. The presence of citrate and phosphocitrate in the working solutions showed reduction of the initial rates of struvite precipitation when compared with solutions in their absence. The parabolic dependence of the rates of precipitation on the relative solution supersaturation suggested a surface-diffusion-controlled mechanism. Presence of citrate and phosphocitrate in the supersaturated solutions at concentration levels from I to 10 mu M resulted in a drastic increase in induction times. The presence of citrate and phosphocitrate inhibited struvite precipitation up to 75-80%. The reduction of precipitation rates was related to additive concentration following an expression suggested by the kinetic Langmuir-type adsorption model. The affinity constant calculated for citrate (8.2 x 10(5)) was considerably higher than the respective value for phosphocitrate (0.6 x 10(5)). Finally, it should be noted that the morphology of the prismatic, elongated struvite crystals precipitated in the presence of citrate and in the presence of phosphocitrate showed differences only in size.