Effects of organic pollutants on struvite crystallization kinetics and the molecular mechanism of inhibition on crystal growth

The abilities of improving phosphorus (P) resources sustainability and reducing water eutrophication make struvite crystallization technology attract increasing interest in wastewater treatment, but struvite crystallization process may be affected by various impurities in wastewater. In this study, the effects of nine representative ionic surfactants including three types (anionic, cationic and zwitterionic) on crystallization kinetics and product quality of struvite were investigated, and the influencing mechanism was further probed. The results demonstrated that anionic surfactants significantly inhibit crystal growth so as to reduce crystal size especially in a-axis direction, change crystal morphology and decrease P recovery efficiency, and also lead to a slight decline in product purity. In contrast, cationic and zwitterionic surfactants have no obvious influence on the formation of struvite. A series of experimental characterizations and molecular simulations collectively revealed that the inhibition of crystal growth by anionic surfactants is attributed to the adsorption of anionic surfactant molecules on struvite crystal surface and subsequent blockage of active growth sites. The binding ability of surfactant molecules with the Mg2+ exposed on struvite crystal surface was highlighted to be the most essential factor determining the adsorption behavior and adsorption capacity. Anionic surfactants with stronger binding ability with Mg2+ have more intense inhibitory effect, but a large molecular volume of anionic surfactants will weaken the adsorption capacity on crystal surface so as to reduce the inhibitory effect. Contrastively, cationic and zwitterion surfactants without binding ability with Mg2+ have no inhibitory effect. These findings enable us to have a clearer understanding of the impact of organic pollutants on struvite crystallization and make a preliminary judgment on the organic pollutants that may have the ability to inhibit the crystal growth of struvite.