Ammonia Nitrogen Removal and MAP Crystal Morphology Affected by Reaction Conditions in High-Concentration Wastewater

The MAP (magnesium ammonium phosphate) method is a convenient and efficient approach for the recovery of ammonia nitrogen from high-concentration wastewater, with the resulting product being suitable for use as a slow-release fertilizer. The crystal morphology of MAP is a key indicator of its appropriateness for this application, yet there is a lack of systematic research on this topic. This paper explores the relationship between the efficiency of ammonia nitrogen removal, morphological characteristics of the product, and reaction conditions (i.e., pH, reaction temperature and time, phosphorus–nitrogen (n(P):n(N)) and magnesium–nitrogen mole ratios (n(Mg):n(N)), and stirring speed). The results show that the influence of the reaction parameters on the nitrogen removal efficiency decreases in this order: the pH > n(Mg):n(N) > the stirring speed > n(P):n(N). The highest ammonia nitrogen removal efficiency (97.97%) was achieved under the following optimal conditions: pH 9.5, n(Mg):n(N) = 1.3, n(P):n(N) = 1.0, a stirring speed of 150 rpm, a reaction time of 30 min, and a temperature of 30 °C. The obtained products were MAP crystals with different morphologies, which gradually transitioned from X- to needle-shaped with a decreasing crystal size as the values of the pH, n(Mg):n(N), stirring speed, and reaction time increased. These findings are relevant for both the effective removal of ammonia nitrogen from high-concentration wastewater and the control of MAP crystal morphology.