Controlled Precipitation of Sparingly Soluble Phosphate Salts Using Enzymes. II. Precipitation of Struvite

Abstract

A novel methodology for the synthesis of magnesium ammonium phosphate (MgNH4PO4·6H2O, MAP or struvite) through precipitation from supersaturated solutions was developed. The development of supersaturation with respect to struvite was catalyzed by the enzymatic decomposition of sodium polyphosphate salts. Solutions containing magnesium and ammonium ions and either sodium tripolyphosphate (Na5P3O10, STP) with alkaline phosphatase or sodium trimetaphosphate (Na3P3O9, STMP) with acid phosphatase were used for the synthesis of MAP. The experiments were done at 25 °C, and the pH was adjusted at 9.80 for the alkaline and at pH 7.50 for the acid phosphatase containing solutions, respectively. The precipitated struvite crystals exhibited a rather irregular morphology. In all cases and as a result of the enzymic activity, the inorganic orthophosphate concentration in solution increased with time reaching the threshold for the spontaneous precipitation of struvite. The supersaturation at the onset of the precipitation process was significantly higher in comparison to the respective values in solutions made by mixing the components of the precipitating solid (magnesium, ammonium, and orthophosphate solutions). Moreover, the rates of precipitation in the latter solutions were significantly higher. Past the supersaturation threshold, struvite crystals formed at rates increasing with the solution supersaturation. The presence of suspended particles affected both nucleation and crystal growth of struvite significantly in the presence of the enzymes tested. More specifically, in the presence of silicate sand the formation of struvite was promoted, yielding higher rates of precipitation. At the same solution conditions, the presence of suspended soil particles resulted in lower rates of precipitation. In both cases, the formation of struvite in the presence of suspended particles formed bridges between neighboring particles. The proposed process is promising for applications in the consolidation of sand containing soils.