Mechanical properties evolution and microscopic mechanisms of arsenic and cadmium co-contaminated clayey soils

Understanding the interaction mechanism between different heavy metals and soils is a primary and important task for remediation of contaminated soils. In this study, we conducted series experiments to explore the co-contamination mechanism of arsenic (As) and cadmium (Cd) on clayey soils. The results showed that As and Cd exhibited opposite effects on physical and mechanical properties of soils under single contamination, while producing a counteracting effect under co-contamination. Specific adsorption, electrostatic adsorption, and precipitation adsorption co-existed simultaneously in As- and Cd-co-contaminated soils, and precipitates found by XRD is the main cause of the counteraction of As and Cd in co-contaminated soils. The shear strength and permeability coefficient of single As-contaminated soils were decreased by 23.02 kPa and 0.61 x 10(-6) cm/s at a concentration of 5000 mg/kg, respectively, and soil particles were dispersed into small aggregates with more ultramicropores (0.1-5 mu m) and cryptopores (< 0.1 mu m). However, for single Cd contaminated soils, the shear strengths were increased by 27.93% on average, the maximum permeability coefficient increased to 6.63 x 10(-6) cm/s, and soil aggregations were enlarged with more micropores (5-30 mu m). Furthermore, for co-contaminated soils with As and Cd, the shear strength and permeability coefficient were between those of single As- and Cd-contaminated soils at the same concentration, and decreased with an increasing concentration of As. This study provides significant experiment and theory support for remediation and reclamation of heavy metal co-contaminated soils with different charges.