# Effect of Soil Washing with Ferric Chloride on Cadmium Removal and Soil Structure
###### Abstract  
In China, arable soils contaminated with cadmium (Cd) threaten human health. Ferric chloride (FeCl\({}_{3}\)) is a highly efficient agent that can remove Cd from contaminated soils. However, it is unknown whether FeCl\({}_{3}\) damages the soil structure and consequently affects crop growth. In this study, we investigated the impacts of Cd extraction by FeCl\({}_{3}\) on the structure of a paddy soil on the basis of comparisons of control (without washing agents) and hydrochloric acid (HCI) treatments. According to our results, the removal efficiency increased with the decrease in soil initial pH, as adjusted by FeCl\({}_{3}\). However, the low pH of 2.0 caused a partial loss of soil mineral components, with an Al release of 4.4% in the FeCl\({}_{3}\)-treated soil versus 1.3% in the HCl-treated soil. In contrast, the amount of released Al was less than 0.2% in the control and in the FeCl\({}_{3}\) treatments with initial pH values of 3.0 and 4.0. The washing agents caused soil TOC loss of 27.1%, 17.5%, and 2.76% in the pH 2.0, 3.0, and 4.0 FeCl\({}_{3}\) treatments, compared with 15.5% in the initial pH 2.0 HCl treatment. The use of FeCl\({}_{3}\) represents an optimum tradeoff between removal efficiency and the loss of soil components to restore Cd-polluted soils by adjusting the initial pH to 3.0 with the addition of FeCl\({}_{3}\). Under this condition, the amount of Al loss was less than 0.2%, and the extraction efficiency reached 40.3%, compared to an efficiency of 39.7% with HCl at an initial pH of 2.0. In conclusion, FeCl\({}_{3}\) could effectively remove Cd from contaminated soil.  
cadmium pollution; ferric chloride; soil restoration; soil structure +
Footnote †: journal: Applied Sciences  
portion of metal-EDTA complexes were retained in the soil after washing [10], most likely because metal-EDTA complexes, such as Ni-EDTA, Co(II)-EDTA, and Cd-EDTA, have a quinquedentate configuration; therefore, the carboxylate group not coordinated to the metal center can be adsorbed onto the soil mineral surface [11]. Similarly, the low-molecular-weight organic acids (LMWOAs) such as citric, oxalic, and succinic acids, secreted by plant roots, can form soluble complexes with Cd, thus increasing the release of Cd from the soils into the soil solution. However, in a previous study, the Cd from Cd-LMWOA complexes was easily adsorbed onto the soil particles with the biodegradation of the LMWOAs. Renewal of LMWOAs is necessary to maintain the steady release of Cd from the soils [12].  
Unlike organic chelating reagents, ferric chloride (FeCl\({}_{3}\)) is a stable and highly efficient extracting agent; ferric iron can dissociate water molecules to release hydrogen ions, which can extract heavy metals adsorbed onto soil surfaces [13; 14; 15]. This difference is reflected in the pH of the extraction solution, indicating that the primary mechanism of Cd extraction by FeCl\({}_{3}\) is proton release coupled with hydroxide generation, as iron hydroxides are insoluble. Washing with FeCl\({}_{3}\) led to the formation of Cd-chloride complexes, enhancing Cd extraction from the soils [13]. Moreover, the formation of metal-chloride composites can also enhance the extraction efficiency. Above an extraction pH of 2.4, FeCl\({}_{3}\) could extract soil Cd more effectively than HCl [13]. [PERSON] et al., using leaching experiments, reported that 20 mmol\(\cdot\)L\({}^{-1}\) FeCl\({}_{3}\) removed 66.5% Cd from a contaminated paddy soil in Hunan Province, China [16]. Although FeCl\({}_{3}\) is a promising washing reagent, it is not clear whether it causes damage to the soil structure, an important factor for crop cultivation.  
In this context, the objectives of this study were to reveal the impacts of Cd extraction by FeCl\({}_{3}\) on the soil mineral components and organic matter on the basis of comparisons with hydrochloric acid (HCI) and to confirm the feasibility of this method to restore Cd-contaminated arable soils.