Promote heterogeneous nucleation of struvite in livestock wastewater by regulating the surface properties of suspended particles with ultrasound-assisted pretreatment.

Excessive nutrients in livestock wastewater pose significant environmental threats, yet also offer opportunities for resource recovery through struvite crystallization. However, high content of suspended solids (SS) may hinder phosphorus recovery via struvite crystallization by unpredictably inhibiting or promoting nucleation. This study proposes a novel ultrasound-assisted pretreatment to regulate the physicochemical properties of SS and enhance heterogeneous struvite nucleation. The results showed that the optimized conditions of pH 9.0, Mg: N: P = 1.2:1:1.2 were achieved by response surface methodology (RSM), and the removal rate of NH₃-N exceeded 90 % as well as the struvite purity of 84.9 % with the ultrasound-assisted pretreatment of 50 W and 600 s. Surface characterization using FTIR, XPS, SEM-EDS, and AFM revealed that ultrasound significantly modified SS morphology, surface energy, and functional groups to promote crystal formation. Classical nucleation theory confirmed that ultrasound pretreatment reduced the nucleation energy barrier by 55 % and shortened induction time by 70 %. Meanwhile, it could balance the crystallization performance and energy input. These findings offer mechanistic insights into ultrasound-enhanced crystallization and demonstrate a scalable, green strategy for nutrient recovery from high-solid livestock wastewater.