## 3 Results and Discussion  
### Sample Characterization  
The X-diffraction patterns of NH\({}_{4}\)H\({}_{2}\)PO\({}_{4}\)/RM composite powders, red mud, and NH\({}_{4}\)H\({}_{2}\)PO\({}_{4}\) are shown in Figure 2, respectively. From Figure 2, we can see that the NH\({}_{4}\)H\({}_{2}\)PO\({}_{4}\)/RM composite powders and red mud powders have the similar X-ray diffraction patterns. The diffraction patterns of the composite powders also present the characteristic diffraction peaks of NH\({}_{4}\)H\({}_{2}\)PO\({}_{4}\) (2\(\theta\) = 19.6\({}^{\circ}\), 27.6\({}^{\circ}\), 36.4\({}^{\circ}\), 42.1\({}^{\circ}\)) after loading the NH\({}_{4}\)H\({}_{2}\)PO\({}_{4}\) particles. The results indicate that the red mud was the main component of the composite powders and the NH\({}_{4}\)H\({}_{2}\)PO\({}_{4}\) particles were loaded on the red mud successfully. The SEM images of NH\({}_{4}\)H\({}_{2}\)PO\({}_{4}\) powders, red mud, and the NH\({}_{4}\)H\({}_{2}\)PO\({}_{4}\)/RM composite powders are shown in Figure 3. As shown in Figure 3A, the sizes of NH\({}_{4}\)H\({}_{2}\)PO\({}_{4}\) particles are large and not uniform, and the diameters are from 5 to 20 \(\upmu\)m. The surface of red mud is rich in pores and the pores are in irregular distribution, as shown in Figure 3B. The surface of the NH\({}_{4}\)H\({}_{2}\)PO\({}_{4}\)/RM is coated with small particles according to Figure 3C, which indicates that NH\({}_{4}\)H\({}_{2}\)PO\({}_{4}\) are successfully loaded on the surface of red mud, forming the uniform composite powders. Figure 3: _Cont._  
Figure 2: X-ray diffraction (XRD) patterns of NH\({}_{4}\)H\({}_{2}\)PO\({}_{4}\)/RM composite powders, red mud (RM), and NH\({}_{4}\)H\({}_{2}\)PO\({}_{4}\). The N\({}_{2}\) adsorption-desorption isotherms of red mud and the NH\({}_{4}\)H\({}_{2}\)PO\({}_{4}\)/RM composite powders are shown in Figure 4. Both of the isotherms are type IV in the IUPAC classification with the H4 hysteresis loop. The results indicate that these two powders possess the mesoporous structure, and capillary condensation occurred during the adsorption-desorption process. After loading the NH\({}_{4}\)H\({}_{2}\)PO\({}_{4}\) particles, the adsorption capacity of the composite is lower than red mud in the P/P\({}_{0}\) range of 0.5-1.0, indicating the decrease of mesopores in the NH\({}_{4}\)H\({}_{2}\)PO\({}_{4}\)/RM composite powders and the formation of a granulation structure during the loading process. The data of this experiment confirmed the changes of the NH\({}_{4}\)H\({}_{2}\)PO\({}_{4}\)/RM morphology that are presented in SEM images (Figure 3). The specific surface area and pore volume of red mud are 123.16 m\({}^{2}\cdot\)g\({}^{1}\) and 0.158 cm\({}^{3}\cdot\)g\({}^{1}\). After loading the NH\({}_{4}\)H\({}_{2}\)PO\({}_{4}\) particles, the data of the composite powders are reduced to 75.83 m\({}^{2}\cdot\)g\({}^{1}\) and 0.116 cm\({}^{3}\cdot\)g\({}^{1}\). It means that the NH\({}_{4}\)H\({}_{2}\)PO\({}_{4}\) particles were loaded on the surface and/or in the pore of red mud. The TG-DSC curves of the NH\({}_{4}\)H\({}_{2}\)PO\({}_{4}\)/RM composite powders are shown in Figure 5. From the TG curve, there are three weight loss stages in the whole process. The first stage from 120 \({}^{\circ}\)C to 220 \({}^{\circ}\)C is attributed to the lost and the thermal decomposition of adsorbed and intercalated moisture of the NH\({}_{4}\)H\({}_{2}\)PO\({}_{4}\)/RM composite powders. The second stage from 220 \({}^{\circ}\)C to 300 \({}^{\circ}\)C could be ascribed to the decomposition of NH\({}_{4}\)H\({}_{2}\)PO\({}_{4}\). The third stage from 300 \({}^{\circ}\)C to 420 \({}^{\circ}\)C could be ascribed to the decomposition of the hydroxides (such as: Al(OH)\({}_{3}\) and Fe(OH)\({}_{3}\)). After 420 \({}^{\circ}\)C, there is no further weight loss, indicating that the composite powder became stable. The total weight loss of the NH\({}_{4}\)H\({}_{2}\)PO\({}_{4}\)/RM composite powders is 93.7%.