Abstract: The soil-water characteristic and microscopic mechanism of heavy metals arsenic, cadmium and their composite contaminated soils were studied in this paper. The evolution law of soil-water characteristic curves of contaminated soils was investigated by using centrifuge method, and the microscopic characteristics of contaminated soils was determined by using Zeta potential, NMR and SEM tests. The experimental results show that there is a link between microscopic characteristics and macroscopic soil-water characteristics of the contaminated soils, that is to say the thinner the thickness of the diffused double layer of soil particles, the weaker the water holding capacity of soil, the lower the absolute value of Zeta potential, the larger pores corresponding to large agglomerates, the smaller the air entry value of matrix suction, otherwise the opposite. The type and concentration of heavy metals affect the soil-heavy metal interaction, leading to changes in microscopic characteristics and soil-water characteristics of contaminated soils. When the concentration of arsenic in contaminated soils increases, the dominant reaction changes from electrostatic adsorption of sodium ion to specific adsorption of arsenate, leading to the thickness of the diffused double layer first decrease and then increase. When the concentration of cadmium in contaminated soil increases, the dominant effect changes from crystal precipitation of cadmium carbonate to electrostatic adsorption of cadmium ion, both of which compress the double electric layer but the former is more significant. The dominant role in the composite contaminated soil is the electrostatic effect of cadmium ions and the specialized adsorption of arsenate ions, which have an opposite effect on the thickness of the diffused double layer.

Key words: composite contaminated soil, soil-water characteristic curve, microscopic characteristics, diffused double layer, soil-heavy metal interaction