Flow field regulation and sediment particle flocculation mechanisms in ultrasound-assisted electrocoagulation...

Ultrasound-assisted electrocoagulation (US-EC) shows promise in improving flocculation through cavitation and flow field regulation, yet the mechanisms of flow field evolution and particle size control under acoustic-electric coupling remain underexplored. In this study, an acoustic-electric coupling observation platform for suspended sediment flocculation was established. The motion response characteristics of gas-solid and mixed phases under the intervention of acoustic-electric coupling were observed by employing Particle Image Velocimetry (PIV). Then, the effects of different ultrasonic frequencies and current densities on flow field characteristics, particle size distribution, and flocculation behavior were systematically explored, and ultimately, the synergistic mechanism of US-EC in particle fragmentation and aggregation was revealed. According to the experimental results, low-frequency (28 kHz) ultrasound, owing to a strong cavitation effect, can effectively fragment medium-sized particles while enhancing the specific surface area and fluid disturbances. Meanwhile, high current density (40 A/m²) can accelerate electrolytic reactions and promote rapid reduction of Zeta potential, thereby promoting efficient particle aggregation. Under the operating condition of 28 kHz + 40 A/m², the fluid system maintained a moderate flow velocity (11.01 mm/s) with uniform vorticity distribution, leading to a dynamic balance between disturbance intensity and floc structural stability (15 ≤ η/D < 25, 10^-6 ≲ ε ≲ 10^-4 m² s^-3), which significantly enhanced flocculation efficiency. Accordingly, the average particle size increased from 18.89 μm to 60.98 μm, with a sedimentation rate of 87.96 %. Overall, the US-EC process follows a three-phase evolution path: "ultrasound-induced fragmentation, EC-driven aggregation, gravity-induced sedimentation". The flow velocity and vorticity collaboratively regulate the particle migration paths, collision efficiency, and floc stability, while Zeta potential exhibits a control effect on particle aggregation rate. These mechanisms complement each other temporally to enhance the overall treatment performance.