## 3 Results and Discussion
### Conversion of Red Mud into Magnetic Adsorbent
This finding was consistent with the presence of magnetite in P-200 and P-270, as shown in Figure 1.  
Figure 1: X-ray powder diffraction (XRD) patterns of the red mud, P-120, P-200 and P-270. In the figure, the peak was numbered: 1: andradite; 2: hematite; 3: magnetite; 4: Morimotoite; 5: canrinite; 6: muscovite; 7: clinochore. The dissolution of muscovite and andradatite in red mud after hydrothermal treatment indicated the difference in the composition of the prepared adsorbents. As shown in Figure 3A, the Si content was 10.7 wt.% for red mud, which slightly decreased to 9.6 wt.% for P-120. Accordingly, the Si concentration in the supernatant of P-120 was 24.1 mg/L. The loss of Si in P-120 was subjected to the dissolution of Si-containing minerals, such as muscovite and andradatite. When the temperature increased from 120 \({}^{\circ}\)C to 200 \({}^{\circ}\)C, the Si content remained unchanged, and the contents of both Ca and Fe increased from 16.1 wt.% and 12.7 wt.% for P-120 to 19.3 wt.% and 16.2 wt.% for P-200. In contrast, the Al content dramatically decreased from 12.1 wt.% for P-120 to 7.5 wt.% for P-200. This finding was attributed to the reductive dissolution of muscovite and andradatite. When the structural Fe in the muscovite and andradatite was reduced to Fe\({}^{2+}\) by ascorbic acid, dissolution of muscovite and andradatite occurred, followed by a release of Al into the liquid solution. The released Al was rapidly converted into Al(OH)\({}_{4}\)\({}^{-}\) under alkaline conditions, and coordinated to the surface sites of residual minerals [16]. However, abundant OH\({}^{-}\) was present in the liquid solution and competed with the coordinated Al(OH)\({}_{4}\)\({}^{-}\) via ion exchange reaction, which led to release of Al(OH)\({}_{4}\)\({}^{-}\) into the liquid solution. Thus, the high-loss of Al in P-200 was observed. Furthermore, the Al concentration in the supernatant was 281.6 mg/L for P-120, and was considerably higher for P-200 (4466 mg/L), which  
Figure 3: (**A**) Composition of red mud, P-120, P-200, and P-270, and (**B**) the major cationic concentrations in the supernatant after hydrothermal treatment. Figure 2: Magnetic hysteresis curves of red mud, P-120, P-200, and P-270. was consistent with the loss of Al in P-200 in comparison to that of P-120. This finding indicates that the dissolution of muscovite and andradradite accelerated when the temperature was above 200 \({}^{\circ}\)C. With the temperature increasing from 200 \({}^{\circ}\)C to 270 \({}^{\circ}\)C, P-270 showed a similar composition to P-200 except that the Fe content was slightly decreased from 16.2 wt.% for P-200 to 15.8 wt.% for P-270. Accordingly, the Fe concentration in the supernatant increased from 33 mg/L for P-200 to 86 mg/L for P-270. Muscovite was completely dissolved with the temperature increasing to 200 \({}^{\circ}\)C. Thus, the loss of Fe in P-270 was subjected to the release of Fe\({}^{2+}\) via the reduction of structural Fe in andradite. The major iron-bearing minerals in red mud were andradite and hematite. During the hydrothermal process, muscovite was dissolved under alkaline conditions by increasing the temperature to 200 \({}^{\circ}\)C. When ascorbic acid was added, it has the ability to provide electrons to the structural Fe in andradite and hematite with the formation of Fe\({}^{2+}\) and dehydroascorbic acid (DHA) [21]. This led to the reductive dissolution of andradite and hematite (Equation (8)). During the dissolution of muscovite, andradite, hematite, Ca, Fe, Si, and Al were released into the liquid fraction (Figure 4 (Step 1)). Simultaneously, ascorbic acid also reacted with dissolved oxygen, which accelerated the consumption of ascorbic acid. When ascorbic acid was exhausted, extra dissolved oxygen was left.