## 3 Results and Discussion  
### Conversion of Red Mud into Magnetic Adsorbent  
To determine the crystallography of both red mud and the adsorbents, XRD experiments were conducted. The observed peaks in Figure 1 in the red mud indicate the presence of andradite (JCPDS 87-1971), hematite (JCPDS 33-0664), carnitine (JCPDS 72-2076), clinochlore (79-1270), muscovite (JCPDS 07-0042), and morimotoite (JCPDS 47-1877). After hydrothermal treatment at 120 \({}^{\circ}\)C, no apparent change of the XRD pattern of P-120 was observed. When the temperature increased to 200 \({}^{\circ}\)C, two typical muscovite peaks at 20 = 8.8\({}^{\circ}\) and 26.8\({}^{\circ}\) were not observed and the major peak of andradite at 20 = 32.3\({}^{\circ}\) was significantly reduced in the diffraction pattern of P-200. Instead, the peaks of carnitine, clinochlore, and morimotoite increased. Notably, new peaks at 20 = 30.1\({}^{\circ}\) and 35.4\({}^{\circ}\) appeared, which belonged to an inverse spinel magnetite (JCPDS 19-0629) [19]. Those results indicated that the dissolution of muscovite occurred with the temperature increasing from 120 \({}^{\circ}\)C to 200 \({}^{\circ}\)C, followed by the reductive dissolution of andradite and hematite with ascorbic acid at a higher temperature. Ascorbic acid was a typical reductant, and could react with andradite and hematite at room temperature [20; 21]; however, the process was inhibited by the Si/Al coating (e.g., amorphous aluminosilicate) [22] on the surface of andradite and hematite. With the temperature increasing form 120 \({}^{\circ}\)C to 200 \({}^{\circ}\)C, similar to the dissolution of muscovite in red mud, the Si/Al coating on andradite and hematite was also attacked by hydroxyl with generation of SiO\({}_{3}\)\({}^{2-}\) and Al(OH)\({}_{4}\)\({}^{-}\) into the liquid fraction [23; 24]. Thus, andradite and hematite were exposed to the liquid fraction, and reduced by ascorbic acid. Without adding ascorbic acid, andradite was stable in alkaline conditions, and generated in the presence of carnitine and hematite during the hydrothermal treatment of red mud [25]. In the hydrothermal process, Si, Al and Fe from the dissolution of muscovite, andradite, and hematite were involved in the recrystallization of Fe\({}^{2+}\)-containing minerals, such as morimotoite, clinochlore, and magnetite. With the absence of Fe\({}^{2+}\), Si and Al were recrystallized in the form of carnitine, which was in agreement with the report of [PERSON] et al. [26]. The cages of carcinite's 12-membered ring channels conferred it enough sites for anions coordination [27] and preferentially selected CO\({}_{3}\)\({}^{2-}\) as structure stabilizer over OH\({}^{-}\)[28] when the former was generated from the oxidation of ascorbic acid and present in solution. When the temperature was increased to 270 \({}^{\circ}\)C, the major peaks of andradite and hematite decreased, while the peaks of morimotoite and magnetite increased. By increasing temperature, the dissolution rates of Al and Si were greatly enhanced [29], which accelerated the dissolution of Si/Al coating on the surface of andradite and hematite. Accordingly, the transformation of andradite and hematite into morimotoite and magnetite was promoted. The formation of magnetite in P-200 and P-270 was accompanied by considerable changes in magnetic behavior. Red mud was weakly magnetized (Figure 2A) and not magnetically separated (Figure 2B). Similar to red mud, P-120, prepared at a hydrothermal temperature of 120 \({}^{\circ}\)C, showed a weak magnetic response due to the lack of magnetic species (e.g., magnetite). When the red mud was treated at 200 \({}^{\circ}\)C, the prepared P-200 exhibited magnetization of 4.1 Am\({}^{2}\)/kg, which had a magnetic response and can be magnetically separated from liquid fraction after use (Figure 2B). As the temperature increased to 270 \({}^{\circ}\)C, the saturation magnetization of P-270 further increased to 11.9 Am\({}^{2}\)/kg.