## 3 Results and Discussion
### Characterization of Al-Ti Oxide  
SEM photographs of the sorbent with an Al-Ti molar ratio of 2:1, together with Al-oxide (1:0) and Ti-oxide (0:1), are shown in Figure 2a-f. Two magnifications of 1000\(\times\) and 5000\(\times\) are given for each sample. The Al-oxide (a, b) has a rough surface and porous structure. Ti-oxide (c, d) is more likely to agglomerate. The Al-Ti sorbents (e, f) were  
Figure 1: Comparison of phosphorus adsorption using sorbents with different Al and Ti molar ratios, initial pH of 6.8, initial P concentration = 50 mg/L, adsorbent dosage = 0.2 g/L, T = 298 K.  
aggregates of many tiny nanoparticles and formed a rough surface. The dimensions of these nanoparticles were in the range of several tens of nanometers. EDS analysis (Figure 2g) revealed that the molar ratio of Al:Ti was approximately 13:10, which is slightly lower than that in the preparation process (2:1). Furthermore, there were peaks for elemental S on the surface of the sorbent, which is related to residual Al\({}_{2}\)(SO\({}_{4}\))\({}_{3}\) and Ti(SO\({}_{4}\))\({}_{2}\) used in the preparation of Al-Ti oxide. The XRD pattern of the sorbent with an Al-Ti molar ratio of 2:1 is shown in Figure 3. The presence of Al\({}_{2}\)O\({}_{3}\) in the adsorbent corresponds to the diffraction peaks of 20 = 25.6\({}^{\circ}\), 43.4\({}^{\circ}\), and 57.5\({}^{\circ}\) for the (012), (113), and (116) crystal planes, respectively (JCPDS card no. 76-1782). The diffraction peaks at 20.6\({}^{\circ}\), 22.0\({}^{\circ}\), 22.7\({}^{\circ}\), 27.0\({}^{\circ}\), 39.5\({}^{\circ}\), 41.9\({}^{\circ}\), 47.6\({}^{\circ}\), and 76.2\({}^{\circ}\) can be assigned to the (200), (110), (111), (211), (312), (114), (313), and (135) crystal planes of Al(OH)\({}_{3}\), respectively (JCPDS card no. 75-1863). The peaks at approximately 31.6\({}^{\circ}\), 33.3\({}^{\circ}\), and 36.0\({}^{\circ}\) were confirmed as the (026), (022), and (124) crystal planes of Ti\({}_{6}\)O\({}_{11}\), respectively (JCPDS card no. 18-1401). These data indicate that Al-Ti oxide was successfully combined to form a hybrid material through the co-precipitation method. The FT-IR spectra of the composite adsorbent in its original state, after P adsorption and after subsequent regeneration using 0.1 M NaOH solution, are illustrated in Figure 4. For the Al-Ti oxide hybrid material, the broad peak displayed at 3600-3200 cm\({}^{-1}\) can be assigned to the O-H stretching mode of the hydrogen-bond, which implies the presence of water molecules [35]. The peak at approximately 1440 cm\({}^{-1}\) corresponds to O-H deformation vibration [36]. The band at 1073 cm\({}^{-1}\) can be attributed to Al-OH bending vibration [37]. The band from 500 to 700 cm\({}^{-1}\) could be due to metal-oxygen-metal vibrations [38]. The band at 1092 cm\({}^{-1}\) and 696 cm\({}^{-1}\) can be attributed to the asymmetric stretch vibration and bending vibration of S-O in the residual sulfate radical. Figure 4: FT-IR spectra of sorbent (a) as-prepared (b) after adsorption of phosphate, and (c) spent Al-Ti oxide. Figure 3: XRD pattern of the synthesized Al-Ti sorbent, Al-Ti molar ratio of 2:1.  
confirm that a surface complex (M-O-P) between P and Al-Ti oxide may be formed and that the P is adsorbed onto the surface of Al-Ti oxide successfully. The Al-Ti oxide used after P adsorption was regenerated with 0.1 M NaOH solution. For the regenerated sample, the characteristic bands for O-H groups and the Al-OH group were recovered; the peaks for P groups at 1216 cm\({}^{-1}\) and 1150 cm\({}^{-1}\) were greatly weakened. Little to no change in the many peaks of the regenerated sample was observed compared with the as-prepared Al-Ti oxide. This indicates that the adsorption performance of P onto Al-Ti oxide could be recovered by regeneration with the NaOH solution. The zeta potential curve of Al-Ti oxide was presented in Figure 5, which indicates that the point of zero charge (PZC) is about zero at pH 8, and that the charge is in the range from 24 to \(-\)28 mV at pH 3\(-\)11.