Study On AC Impedance Characteristics Of Heavy Metal Zinc-Contaminated Sand

In order to explore the application of AC resistivity and impedance characteristics monitored the zinc contaminated sand, zinc, which was chosen as the typical heavy metal, was mixed in pure sand of different physical properties to form different pollution degrees sand. Firstly, the AC impedance and temperatures of the unsaturated zinc contaminated sand were tested, and the data influenced by temperatures were corrected. Then the influences of the test frequency, water content, void ratio and zinc contamination content on the sand AC resistivity and the impedance spectroscopy were systematically analyzed. Finally, the qualitative and quantitative relationships of each influencing factors about the AC resistivity and impedance characteristics were obtained and the empirical formula about the AC resistivity and impedance characteristics of unsaturated zinc contaminated sand were analyzed and tentatively established. The main conclusions show as follows:(1) With the increasing of the current frequency the AC resistivity values of the sand decreased gradually. A small part of sandy particles and its surface solution in the sand formed small capacitors, which was the main reason of the current frequency influencing the AC resistivity. The AC resistivity was significantly influenced by the water content of the sand. When the water content was low, the sand AC resistivity changed gradually, while in high water content, the sand AC resistivity changed slowly. The void ratio weakly influenced on the AC resistivity relative to the water content.(2) The AC resistivity was also significantly influenced by the zinc contamination content of the sand. Especially, when the zinc contamination content in the range of 0 to 250 mg·kg-1, the slightly increasing of the zinc contamination content could cause the sand AC resistivity decreased greatly; with the increasing of the zinc contamination content, the effect gradually reduced and the sand AC resistivity tended to be stable.(3) In the empirical formula, the relevant indexes of the relationships between the water content, void ratio and the sand AC resistivity were fitted. The index values of the water content were very close to 1. The index values of the void ratio distributed in the 0.8～2.2, it had less stability and affected by the water content.(4) The ions in the pore water solution of the zinc contaminated sand had a greater influence on the sand AC resistivity. In the analysis of the zinc contamination content and the sand AC resistivity, the zinc contamination content could be used to express the influence by anionic and cationic on the sand AC resistivity. There was a good power function relationship between the zinc contamination content and the sand AC resistivity, and the index values ranged smaller, relatively stable.(5) The empirical formula using to predict the zinc contamination content was proposed and verified. The zinc contamination content could be quantitatively showed by combining the sand AC resistivity and its physical properties. The index values of the zinc contamination content changed with the water content, it needed to properly select according to the water content and the values table was showed in the appropriate water content range.(6) With the increasing current frequencies, phase angles decreased firstly and then increased in the Bode phase angle diagrams under the different zinc contamination content, water content and void ratio. In each curve, the frequencies of the minimum phase angles were very close. In conclusion, when the current frequency was 50 kHz, the phase angles of each curve were relatively small. In the Nyquist diagrams, with the increasing current frequencies, the values of each curve increased firstly and then decreased. When the current frequency was 50 kHz, Z" got to the peak values and the lines of each curve liked a hill. With the increasing of the zinc contamination content, the range of the hill-like curves gradually decreased.