# Effect of Soil Washing with Ferric Chloride on Cadmium Removal and Soil Structure
## 3 Results and Discussion  
### Effects of Ferric Chloride on Cd Extraction Efficiency  
To comprehend the extraction process for Cd by ferric chloride, a set of kinetic experiments was conducted at a reaction time from 0.17 to 288 h. According to the results, a two-step process occurred during the reacting period, with a rapid extraction of soil me-tals within the first 12 h, followed by a gradual release up to equilibrium at 144 h. In the control treatment, the proportion of Cd desorption to the total soil Cd was 1.73%, versus values of 48.0%, 40.3%, and 6.4% at an initial pH of 2.0, 3.0, and 4.0, adjusted by FeCl\({}_{3}\) (Figure 1). In contrast, HCl extracted Cd by 39.7% at an initial pH of 2.0, which was close to the value at an initial pH of 3.0 but lower than that at an initial pH of 2.0 of the FeCl\({}_{3}\) solutions. At the same initial pH, ferric chloride had a better extraction efficiency than HCl, most likely because ferric iron could continuously generate hydrogen ions through the equilibrium reaction of hydroxide formation. [PERSON] et al., using FeCl\({}_{3}\) and Fe(NO\({}_{3}\))\({}_{3}\) to remove pH from shooting range soil, obtained similar results, with higher removal efficiencies than HCl and HNO\({}_{3}\) at an initial pH of 2.5 [24].  
In the untreated soil, Cd bound to Fe-Mn oxides was the main fraction (37.9%), followed by exchangeable Cd (28.1%), residual Cd (16.2%), carbonate Cd (16.0%), and Cd bound to organic matter (1.80%; Figure 2). After washing with FeCl\({}_{3}\), the concentrations of exchangeable Cd and carbonate Cd significantly decreased in all three treatments. At the initial pH values of 2.0, 3.0, and 4.0, the concentration of exchangeable Cd decreased by 87.4%, 75.5%, and 21.8% versus 59.1% in the washed soil by HCl, and the concentration of carbonate Cd decreased by 68.8%, 47.7%, and 25.2% versus 64.4% in the HCl treatment. The efficient extraction of soil Cd by FeCl\({}_{3}\) is mainly attributed to the lowering of the solution pH through proton release from the chemical equilibrium of hydroxide formation [24]. Moreover, Fe\({}^{3+}\), as a trivalent cation, might partly promote ion exchange with Cd\({}^{2+}\) on the surface of soil particles compared to HCl [13]. Additionally, Cd bound to Fe-Mn o-xideswas even reduced by 60.2% and 44.2% in the initial pH 2.0 and 3.0 FeCl\({}_{3}\) solutions versus 58.3% in the initial pH 2.0 HCl treatment. [PERSON] and [PERSON] reported that the removal of labile heavy-metal species from the soil would disturb the chemical equilibrium among different species, thus potentially re-establishing such an equilibrium and shifting heavy metals back to more available chemical forms [25]. [PERSON] et al. reported that the easily reducible and oxidizable fractions of Cd were transformed into easily extractable fractions after the soil was leached by the composite reagents with citric acid and FeCl\({}_{3}\)[16]. Thus, Cd bound to Fe-Mn oxides might have undergone this transformation during the washing process and been removed by FeCl\({}_{3}\) or HCl in this study. Similarly, [PERSON] et al. reported that Pb was extracted into solution by EDTA as a result of a fractional change from the strongly bound fractions to soluble, exchangeable, and carbonated fractions [26]. In our study, the Cd bound to the organic matter fraction was also reduced by 42.1%, 36.8%, and 9.41% in the initial pH 2.0, 3.0, and 4.0 FeCl\({}_{3}\)-treated soil versus 35.1% in HCl-treated soil. This may relate to the reduction of organic carbon in soils treated with FeCl\({}_{3}\) and HCl (Table 1).  
Figure 1: Kinetics for the Cd extracted from contaminated paddy soil by FeCl\({}_{3}\).  
Figure 2: Sequential extraction of Cd from contaminated paddy soil with FeCl\({}_{3}\). Note: Exch, Carb, Fe–Mn, Org, and Re refer to exchangeable Cd, Cd bound to carbonates, Cd bound to iron and manganese oxides, Cd bound to organic matter, and residual Cd.