# Physicochemical and Microstructural Properties of Red Muds under Acidic and Alkaline Conditions
## 4 Variation in The Main Metal Elements Under the Acidic and Alkaline Conditions  
Table 4 indicates the changes in the main metal elements (i.e., Na, Al, Fe, Ca), and Si and Cl in the weight percentage of the solid particles for the Shanxi silty soil, Shanxi red mud and Shandong red mud immersed in a 4 mol/L HCl or NaOH solution for 120 days (28 days only for the Shandong red mud under alkali action). The measured element contents (EDX method) are actually the contents of some elements of the solid particle sample by the analysis of the dispersive X-ray spectra. In fact, after the acid or alkali treatment, the mineral structure of the solid particles changes obviously and generally exists in the form of compounds or ions due to the addition of acid or alkali solution. Some ions are leached in the filtrate, which causes the difference in the various elements for the same soil to some extent. For clarity, the compositions of the solid particle were expressed in the form of element. Overall, the measured results can explain the difference in the main metal elements of solid particles for the different types of soils (e.g., between the Shanxi silty and Shanxi red mud). Table 4 shows that the Shanxi silty soil initially contained 4.94% Ca (i.e., untreated sample) but almost no Na. The immersion in the NaOH solution results in a 9.42% increase in Na. The Si content in the original Shanxi silty soil accounted for 19.66% of the total weight, which is much higher than that in the Shanxi red mud and Shandong red mud (i.e., 9.09% and 8.24%, respectively). However, immersion \begin{table}
\begin{tabular}{c c c c c c c c c c} \hline \hline \multirow{2}{*}{**Main Elements**} & \multicolumn{3}{c}{**HCl Solution Treatment**} & \multicolumn{3}{c}{**Untreated**} & \multicolumn{3}{c}{**NaOH Solution Treatment**} \\ \cline{2-10}  & **Shanxi** & **Shanxi** & **Shandong** & **Shanxi** & **Shanxi** & **Shandong** & **Shanxi** & **Shanxi** & **Shandong** \\  & **Silty** & **Red** & **Red** & **Silty** & **Red** & **Red** & **Silty** & **Red** & **Red Mud** \\  & **Soil** & **Mud** & **Mud** & **Soil** & **Mud** & **Mud** & **Soil** & **Mud** & **(28 Days)** \\ \hline Na & 0.52 & 1.9 & 0.92 & 0 & 7.83 & 8.87 & 9.42 & 14.23 & 15.88 \\ Al & 5.78 & 9.11 & 5.52 & 6.53 & 10.04 & 9.63 & 5.22 & 8.99 & 9.64 \\ Fe & 4.86 & 2.71 & 12.64 & 7.32 & 3.59 & 19.30 & 6.68 & 4.55 & 16.84 \\ Ca & 4.14 & 11.75 & 1.1 & 4.94 & 11.64 & 0 & 2.06 & 10.34 & 0 \\ Si & 18.38 & 7.01 & 4.06 & 19.66 & 9.09 & 8.24 & 19.72 & 7.67 & 8.15 \\ Cl & 15.96 & 25.39 & 6.71 & 0 & 0 & 0 & 0.82 & 0 & 0.83 \\ \hline \hline \end{tabular}
\end{table} 
Table 4: The main elements of the solid particle for Shanxi silty soil, Shanxi red mud and Shandong red mud after acid or alkali treatment. \begin{table}
\begin{tabular}{c c c c} \hline \hline \multicolumn{1}{c}{**Type of Solution**} & **Shanxi Silty Soil** & **Shanxi Red Mud** & **Shandong Red Mud** \\ \hline HCl solution (4 mol/L) & 18.89 & 13.798 & 60.492 \\ Untreated & 28.432 & 8.906 & 22.931 \\ NaOH solution (4 mol/L) & 1.670 & 5.737 & 8.530 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: The specific surface areas of soil samples immersed in HCl or NaOH solution (m\({}^{2}\)/g). in the HCl solution results in almost no increase in Na but a 15.96% increase in Cl for the Shanxi silty soil. Table 4 also shows that there is a significant difference in the chemical composition between Shanxi and Shandong red mud. For instance, the Shandong red mud contains less Ca (not detected due to its small amount) but more Fe (19.30%) than the Shanxi red mud (11.64% and 3.59%, respectively). This conclusion can also be obtained based on the measurement result after the acid treatment (i.e., 4 mol/L HCl solution) for the Shanxi red mud and Shandong red mud (i.e., Ca: 11.75% and 1.1%; Fe: 2.71% and 12.64%, respectively; see the HCl solution column in Table 4). Hence, the Shandong red mud presents a red-brown color (see Figure 1b) due to the existence of a large number of Fe\({}^{3+}\) ions.