Ultrasound-assisted soil washing processes for the remediation of heavy metals contaminated soils: The mechanism of the ultrasonic desorption

Abstract

Ultrasound-assisted soil washing processes were investigated for the removal of heavy metals (Cu, Pb, and Zn) in real contaminated soils using HCl and EDTA. The ultrasound-assisted soil washing (US/Mixing) process was compared with the conventional soil washing (Mixing) process based on the mechanical mixing. High removal efficiency (44.8% for HCl and 43.2% for EDTA) for the metals was obtained for the most extreme conditions (HCl 1.0 M or EDTA 0.1 M and L:S = 10:1) in the Mixing process. With the aide of ultrasound, higher removal efficiency (57.9% for HCl and 50.0% for EDTA) was obtained in the same extreme conditions and similar or higher removal efficiency (e.g., 54.7% for HCl 0.5 M and L:S = 10:1 and 50.5% for EDTA 0.05 M and L:S = 5:1) was achieved even in less extreme conditions (lower HCl or EDTA concentration and L:S ratio). Therefore, it was revealed that the US/Mixing was advantageous over the conventional Mixing processes in terms of metal removal efficiency, consumption of chemicals, amount of generated washing leachate, and volume/size of washing reactor. In addition, the heavy metals removal was enhanced for the smaller soil particles in the US/Mixing process. It was due to more violent movement of smaller particles in slurry phase and more violent sonophysical effects. In order to understand the mechanism of ultrasonic desorption, the desorption test was conducted using the paint-coated beads with three sizes (1, 2, and 4 mm) for the free and attached conditions. It was found that no significant desorption/removal of paint from the beads was observed without the movement of beads in the water including floatation, collision, and scrubbing. Thus, it was suggested that the simultaneous application of the ultrasound and mechanical mixing could enhance the physical movement of the particles significantly and the very high removal/desorption could be attained.

Keywords: Cavitation; EDTA; HCl; Heavy metals; Soil washing; Ultrasonic desorption.

Fig. 1

Schematic of the double-bath-type ultrasonic system used in this study.

Fig. 2

Removal efficiency in the mechanical (Mixing) and combined ultrasonic and mechanical (US/Mixing) washing processes using HCl and EDTA (The number above each of the bar group represents the average removal efficiency of the group of Cu, Pb, and Zn.).

Fig. 3

Heavy metal removal in the mechanical (Mixing) and combined ultrasonic and mechanical (US/Mixing) washing processes using HCl (The number above each of the bar represents the removal efficiency based on the initial concentration.).

Fig. 4

Ultrasonic paint removal from the glass beads for under various conditions (Free beads: the glass beads were not attached to the vessel bottom; Attached beads: the beads were attached to it).

Fig. 5

The paint-coated beads and ultrasound-treated beads for various bead sizes.