Probing the effect of humic acid on the nucleation and growth kinetics of struvite by constant composition technique

Struvite crystallization has been so far proved to be a potential technology for nitrogen and phosphorus recovery from nutrient-rich wastewater. However, humic acid (HA) among organic matters in nutrient-rich wastewaters would inevitably exert an adverse impact on the crystallization. Therefore, in the present study, we innovatively propose a process monitor chart to accurately differentiate and quantify the nucleation and crystal growth of struvite crystallization by means of constant composition technique, thereby probing the effect of HA on the nucleation and growth kinetics of struvite. Moreover, the mechanism underlying the interaction between HA and struvite crystals was also explored by FTIR and XPS analysis. The results indicated that HA not only significantly prolonged the induction time, but also decreased the growth rate of struvite crystals. This can be explained by the fact that HA is prone to complex with the reactant, and its subsequent adsorption on the surface of struvite crystals performs during the crystallization process. Consequently, the crystal morphology initially varied from rod-like shape to trapezoidal prism, and ultimately to triangular prism, as HA concentration gradually increased from 0 to 100 mg/L. Interestingly, the interaction between HA and struvite crystals is likely to be triggered by the formation of amide, HA-Mg2+ complex and phosphate ester on the crystal surface. These findings herein put forward an innovate method to elucidate the inhibitory effect of organic matter on the nucleation and growth kinetics of struvite, which can provide a theoretical basis for understanding the interference of organic pollutants with struvite crystallization.