Dual Roles of Melamine in the Formation of Calcium Oxalate Stones

Melamine (Mel) is widespread in food and the environment. The effects of Mel on calcium oxalate (CaOx) crystallization and phase transformation were studied in aqueous solution and synthetic urine systems. The mechanism of Mel affecting the crystallization process was analyzed from the perspective of thermodynamics and kinetics. The morphology, structure, and composition of CaOx crystals were characterized; the ion selective electrode method was used to study the crystallization rate. For the first time, it was found that in synthetic urine, Mel could stabilize thermodynamically unstable calcium oxalate trihydrate (COT) and inhibit its conversion to calcium oxalate monohydrate (COM). Because Mel enriched calcium ions and oxalate on its surface through the dual effects of electrostatic adsorption and hydrogen bonding, the local supersaturation increased. High supersaturation is conducive to the stability of COT, thus inhibiting the formation of stones. Kinetic analysis showed that Mel could accelerate the crystallization rate of COM in aqueous solution by reducing the average activation energy, thus increasing the risk of stone formation. This discovery allows us to fully utilize the stabilizing effect of Mel on COT to prevent the formation of stones and to find the treatment of stones by understanding the process of Mel promoting COM crystallization.