# Effect of Soil Washing with Ferric Chloride on Cadmium Removal and Soil Structure
## 3 Results and Discussion
### Changes in Soil Clay Minerals during Extraction  
Washing with FeCl\({}_{3}\) or HCl could decrease the soil pH values (Table 2), potentially resulting in a negative disturbance of the soil structure [15,27,28]. However, due to the buffering capacity of soil, the equilibrium pH values were restored to 2.12, 3.94, and 4.88 in the initial pH 2.0, 3.0, and 4.0 FeCl\({}_{3}\) solutions versus 2.77 in the initial pH 2.0 HCl solutions. In the initial pH 2.0 FeCl\({}_{3}\) treatment, the soil solution pH had a slight resilience until equilibrium compared to that in the HCl treatment; however, both washed soils deviated far from the untreated soil in terms of the pH value. At a similar Cd extraction efficiency, the initial pH 3.0 FeCl\({}_{3}\) treatment restored the pH more closely to that of the untreated soil compared to the HCl treatment. The XRD was performed on HCl- and FeCl\({}_{3}\)-treated soils to reveal the effects of these reagents on the soil mineral components. The results (Figure 3) indicate that the untreated soil was mainly composed of quartz (SiO\({}_{2}\)) and kaolinite (Al\({}_{2}\)O\({}_{3}\)Si\({}_{2}\)-2H\({}_{2}\)O); hematite (Fe\({}_{2}\)O\({}_{3}\)), goehite (FeO(OH)), and magnetite (Fe\({}_{3}\)O\({}_{4}\)) were probably also present. The FeCl\({}_{3}\)-treated soils did not differ from the untreated soils in terms of mineral components, indicating that no minerals disappeared under the decrease in pH due to FeCl\({}_{3}\) addition. Moreover, [PERSON] et al. reported a new formation of fine-crystalline Fe phases with ferrihydrite-like morphology in FeCl\({}_{3}\)-treated soil by transmission electron microscopy analysis; iron (oxy)hydroxides can be converted to more stable and crystalline iron oxides over time [24,29]. In contrast, the Fe\({}_{3}\)O\({}_{4}\) peak did not appear in HCl-treated soil. The results of the XRD can, however, not sufficiently prove that FeCl\({}_{3}\) treatment did not damage the soil components, as we performed no quantitative analysis. Kaolinite (Al\({}_{2}\)O\({}_{3}\)Si\({}_{2}\)-2H\({}_{2}\)O) is the most abundant and basic clay mineral of the soil [15], and Al can indicate the extent of kaolinite dissolution into the aqueous phase in HCl- and FeCl\({}_{3}\)-treated soils (Figure 4). The amount of released Al into solution as a proportion of total soil 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. These results are in agreement with the findings of [PERSON] et al., who indicated that washing with FeCl\({}_{3}\) is not likely to cause serious damage to the soil clay mine-rals [10]. The amount of Al in the initial pH 2.0 HCl and FeCl\({}_{3}\) solutions fell within the range of 1.0-5.0%, which was significantly greater than that in the control treatment due to the considerably lower pH. At the same initial pH value, the percentage of Al loss in FeCl\({}_{3}\)-treated soil was significantly higher than that in HCl-treated soil (\(p<0.05\)), most likely because FeCl\({}_{3}\)-treated soil had a lower equilibrium pH value because of the higher amount of hydrogen ions from the chemical equilibrium of hydroxide formation. \begin{table}
\begin{tabular}{c c c c} \hline
**Treatments** & **TOC (g-kg\({}^{-1}\))** & **DOC (mg-L\({}^{-1}\))** & **Fe (II) (mg-L\({}^{-1}\))** \\ \hline Control & 22.3 \(\pm\) 0.24 a & 6.63 \(\pm\) 0.30 c & 0.08 \(\pm\) 0.00 c \\ HCl, pH 2.0 & 19.0 \(\pm\) 0.06 b & 9.01 \(\pm\) 0.19 b & 0.37 \(\pm\) 0.08 b \\ FeCl\({}_{3}\), pH 2.0 & 16.4 \(\pm\) 2.15 c & 19.0 \(\pm\) 1.62 a & 28.9 \(\pm\) 0.75 a \\ FeCl\({}_{3}\), pH 3.0 & 18.5 \(\pm\) 0.08 bc & 7.64 \(\pm\) 0.08 c & 0.12 \(\pm\) 0.04 c \\ FeCl\({}_{3}\), pH 4.0 & 21.9 \(\pm\) 0.45 a & 7.07 \(\pm\) 0.14 c & 0.08 \(\pm\) 0.00 c \\ \hline \end{tabular}  
* Lowercase letters within the same column compare different treatments at the equilibrium stage. Values represent means \(\pm\) SE. Values followed by the same letter do not differ significantly accor-ding to [PERSON]’s test (\(p<0.05\)).  
\end{table}