Influences of changes in pH, temperature, and particle size on surface reaction resistance and diffusion resistance in the struvite dissolution process

Struvite (MgNH 4 PO 4 ·6 H 2 O，MAP), a type of high-quality, slow-release fertilizer, is the main component of urinary stones in animals and humans and causes scale formation in industrial pipes. Therefore, MAP dissolution is an important research topic. Herein, MAP dissolution experiments are performed and a thermodynamic model is developed to evaluate the influences of pH, temperature ( T ), and particle size ( P s ) on surface reaction resistance ( 1/k s ) and diffusion resistance ( 1/k d ). The results show that MAP solubility decreases with an increase in pH and the effects of pH change are more obvious than those of T or P s . The total resistance ( 1/k T ) of MAP dissolution increases with pH increase, P s growth, or T reduction. The comparison of 1/k s and 1/k d reveals surface reaction as the MAP dissolution control step and indicates a linear relation among 1/k s , 1/k d , and the natural logarithm of P s at pH 7.0.