Abstract: Soil washing is an effective approach for the remediation of heavy metal contaminated soil. Soil washing agent is a crucial factor which directly affects the efficiency of washing. We examined the performances of four washing agents, inclusive of acetic acid, citric acid, CaCl₂, and FeCl₃, in purifying farmland soil slightly contaminated by Cadmium (Cd) and the influential factors. We found that the effect of FeCl₃ is the optimum, followed by citric acid, acetic acid, and CaCl₂ in sequence.The H⁺ hydrolyzed from the exchange between Fe₃⁺ from FeCl₃ solution and Cd²⁺ from the soil stimulated the dissolution of carbonate bounded form of Cd; Cl^- together with Cd²⁺ could also form a stable chelate. In optimized operation condition (solid-liquid ratio at 1∶5, and washing for 4 hours), the maximum removal rate of Cd was 30.24%, 24.62%,24.82%, and 81.90% by acetic acid, citric acid, CaCl₂, and FeCl₃, respectively. The pH value of soil had large impact on the washing effect. Apart from CaCl₂,the washing effects of the other three washing agents all declined with the rising of pH value within 2.5-5.0, among which the washing effect of FeCl₃ underwent the most severe decline.

Key words: farmland soil, cadmium contamination, soil washing, FeCl₃, organic acids