ABSTRACT
The problem of heavy metal contamination of soil at industrial sites in China is serious. The identification of heavy metal contamination is currently focused on chemical analysis. However, this method is limited in engineering practice due to the complexity and cost of operation. Therefore, a physical method is urgently needed to monitor the level of contamination at the site. In the study, the combined acoustic monitoring system has been used to measure acoustic wave velocity of loamy soil under the condition of different concentration of cadmium and moisture content. The results showed that the P-wave velocity in different contaminated soil increased and then stabilized with the addition of cadmium. In the low concentration of cadmium in adsorption range of 5.56 mg·kg−1 to 728.91 mg·kg−1, cadmium was rapidly adsorbed by adsorption sites with high binding energy in the soil colloid. The large number of complexes generated from the adsorption stage were distributed in pores, which led to a rapid increase in P-wave velocity. With saturation of adsorption sites with the high binding energy, the adsorption sites with low binding energy performed adsorption of cadmium. The existence of charge repulsion led to a significant reduction in the adsorption rate of cadmium. The generation rate of complexes in the pores decreased, and therefore wave velocity increased slowly. Based on the adsorption principle, the variation pattern between P-wave velocity and heavy metal concentration can be investigated. This method can be used to evaluate the contamination boundary of a site by applying ultrasonic technique.
