Optimizing the dimensions of magnesium ammonium phosphate to maximize its ammonia uptake ability

Magnesium ammonium phosphate (MgNH4PO4·6H2O:MAP) releases approximately 70% of its ammonia at a mild temperature of 378 K. The resultant material (magnesium hydrogen phosphate (MgHPO4:MHP)-like material), which appears to be an amorphous phase, to remove ammonia from wastewater was investigated. Because the original size of MAP crystals was found to critically affect the kinetics of ammonia uptake by the corresponding MHP-like material, MAP with different sizes were synthesized by changing concentration, pH and temperature of the synthesis solution. The variation in the synthesis concentration was found to change the size of MAP as well as the aspect ratio of the long-axis to the short-axis. The rate of ammonia uptake depends primarily on the dimension of the short-axis of the corresponding MAP crystals. Furthermore, analysis of ammonia uptake using a method similar to the shrinking-core model shows approximately 0.5 μm from the surface is effectively used for ammonia uptake over a period of a realistic process time. Thus, the results suggest it is crucial to synthesize small MAP crystals with the size of short-axis less than 1 μm. Our results also show that small MAP crystals can be used at least four times repeatedly for ammonia uptake.