Hydrodynamic study of bubble characteristics and bubble rise velocities in batch electrocoagulation with vibration-induced electrode plates using the PIV technique

In most of electrocoagulation processes, the efficiency in removing pollutant is not just dependent on anode oxidation but is also determined by the action of gas bubbles produced during the treatment. The bubbles formation and bubbles rise velocities are important especially in electrochemical treatment for which generated gas bubbles improved the ionic transfer and dispersion of particles. This study used the Particle Image Velocimetry (PIV) technique to investigate the hydrodynamics of bubbles in terms of size, distribution and rise velocity in batch electrocoagulation with vibration-induced electrode plates in comparison with stationary plates. An experimental set up consisting of an electrocoagulation reactor and electrode plates equipped with a high-speed camera recorder and a computer embedded with PIVlab software was employed to determine the bubble sizes and the rise velocities in varying conditions, such as current intensity, initial pH, and vibration intensity. The results showed that, for both stationary and vibration-induced plates, smaller bubble sizes were obtained in acidic conditions (pH 5) compared with alkaline conditions (pH 10). The larger bubble sizes were found resulted from the coalescence between smaller and larger bubbles before detachment from the plates. However, due to the vibration of the plates, the bubbles were able to completely detach from the plate surfaces starting at a vibration intensity of 1.8 V. With stationary electrode plates, the bubble rise velocities were found to be higher at initial pH 5 with velocities ranging from 4.7 to 15.3 cm/s, while the velocities measured at initial pH 10 ranged from only 2 to 4.7 cm/s. The flow trajectory of the stationary plates showed that the bubbles flowed only in a single direction parallel to the plate (y-direction). On the other hand, increasing the vibration intensity of the vibration-induced plates from 0.8 to 3.3 V during the treatment resulted in a decrease in the bubble rise velocities at both initial pH 5 and pH 10. The gas bubbles flow was governed by both the x- and y-direction of the electrode plates, with the flow in the x-direction observable at a vibration intensity of 2.8 V or 80 Hz, which is the resonant frequency of the vibrating plates.