# Removal of Fluoride and Arsenate from Aqueous Solutions by Aluminum-Modified Guava Seeds
## 3 Results and Discussion
### Fourier Transform Infrared Spectroscopy  
Figure 3 shows the FTIR results of unmodified guava seeds (GSs) and aluminum-modified guava seeds (Al-GSs). Different functional groups, like C-O-C, -C=O, -COOH, -NH, -CH\({}_{2}\), and -OH are present on GSs' surface (Figure 3a). These groups are characteristic of lignocellulosic materials. Absorption bands from 1700 cm\({}^{-1}\) to 1200 cm\({}^{-1}\) were attributed to carbonyl groups (C=O) of aldehyde and ketone and to C-O group [27]. Comparing GS and Al-GS spectra (Figure 3a), variations on absorption bands can be observed at 3402.3 cm\({}^{-1}\), corresponding to -OH groups which can be attributed to the fact that intermolecular hydrogen bonds can exist because of hydroxyl groups adjacent to glucose units in the cellulose molecule, which is one of the main components of GS biosorbent [26]. The changes in vibrational frequencies around 3030-2385 cm\({}^{-1}\), corresponding to amino and aldehyde groups indicate the attachment of Al metal ions to these functional groups on the GS surface [27]. Moreover, new peaks are observed in SG-Al spectra after modification with Al, particularly those at 1445 cm\({}^{-1}\) and 1546 cm\({}^{-1}\) that have been associated with the presence of surface hydroxide groups (-AlOH) [28] or to Al-O bending. Furthermore, a shift on the absorption band at 1751 cm\({}^{-1}\) corresponding to C=O  
group was observed (Figure 3a), which has been attributed to the formation of coordination bonds between this group (O atoms) and the Al ions [29]. Figure 1: SEM analyses of Al-GS samples: (**a**) Al-GS sample observed at 250\(\times\); (**b**) wrinkled zone observed at 1000\(\times\); (**c**) granules observed at 1500\(\times\) on Al-GS sample; (**d**) micropores observed at 2000\(\times\). Figure 2: SEM-EDS analyses of Al-GS samples: (**a**) Al-GS sample observed at 2000\(\times\); (**b**) SEM-EDS mapping of Al-GS observed at 1000\(\times\); (**c**) Al-GS after fluoride and arsenate sorption observed at 1000\(\times\). Figure 3: FTIR spectra: (**a**) GS and Al-GS samples; (**b**) GS and Al-GS-F samples; (**c**) Al-GS and Al-GS-As samples. According to FTIR spectra observed in Figure 3b, it can be noticed that the functional groups associated with fluoride ions removal by Al-GS are: hydroxyl, carbonyl, and methoxy methyl ether. The OH groups' disappearance was observed at 3402 cm\({}^{-1}\). This can be attributed to the fact that fluoride and hydroxyl ions have similar dimensions and they can exchange each other to form Al-F complexes due to the interaction of F\({}^{-}\) ions with aluminum ions on the surface of the material, without breaking the Al-O-Al bonds. [26; 30]. This could explain the changes in pH values of fluoride solutions before and after adsorption. Moreover, the bands that appeared in the range from 400 to 700 cm\({}^{-1}\) in the Al-GS-F spectrum have been attributed to the vibration of the aluminum-fluoride bonds [26], as well as the band present at 880 cm\({}^{-1}\)[31]. Besides, a band at 761.3 cm\({}^{-1}\) can be observed after fluoride adsorption, corresponding to bending motion of CF\({}_{2}\) and skeletal bending of CF-CH-CF [32]. A FTIR analysis for the biosorption of As(V) onto Al-GS was also carried out. It can be observed (Figure 3c) that the band at 738 cm\({}^{-1}\) corresponds to the symmetric stretch of As-OH bond [33; 34]. Besides, symmetric and asymmetric stretching modes of the As-O bonds in the FTIR spectrum can be observed at 878 and 907 cm\({}^{-1}\), respectively [33]. Moreover, the bands at 844-855 cm\({}^{-1}\) are assigned to the As-O vibration of an inner-sphere Al-O-As complex [33; 35; 36]. These results indicate that the mechanisms involved on As(V) sorption onto Al-GS biosorbent are those related to the interactions between arsenate ion and aluminum on the surface of the guava seeds, along with the active sites on the lignocellulosic structure of GSs.