# Adsorption of Ni2+ and Cd2+ from Water by Calcium Alginate/Spent Coffee Grounds Composite Beads
## 3 Results and Discussion
### Characterization of CA Beads, CA-SCGs Beads, and SCGs by Raman Spectroscopy  
The photos of Figure 2 reveal a more uniform structure for the CA beads while the CA-SCGs beads have a more heterogeneous structure with differences in bead texture and color. The Raman spectrum of CA beads in Figure 3 shows bands in 817.2, 891.4 and 964.8 cm\({}^{-1}\). These bands can be attributed to skeletal C-C, C-O stretching, and C-C-H, C-C-O bending modes [24]. The spectra of CA beads and CA-SCGs beads show a band at 1085 cm\({}^{-1}\); this band is associated with the glycosific ring breathing mode [24]. The Raman spectrum of CA beads shows bands in 1317.1 cm\({}^{-1}\) (carboxylate stretching vibration: symmetric stretching or C-O single bond stretching vibration) and 1437.6 cm\({}^{-1}\) (symmetric carboxylate stretching vibration) [24]. These bands could be present in the spectrum of  
Figure 1: Determination of pH\({}_{\text{pzc}}\) using pH drift method. CA-SCGs beads, but they could be overlapping with other bands. The CA bead spectrum has a band at 1626.9 cm\({}^{-1}\) (asymmetric carboxylate stretching vibration), this band could also be present in CA-SCGs beads. The presence of carboxylate groups and glycosidic rings in both kinds of beads is confirmed by the IR spectra. The most significant differences can be observed at 1580.1 cm\({}^{-1}\) and 1085 cm\({}^{-1}\). At 1580.1 cm\({}^{-1}\), the CA-SCGs beads and SCGs show strong intensity, whereas the CA beads do not. Regarding the 1085 cm\({}^{-1}\) wavelength, the CA-SCGs beads and CA show strong intensity, whereas the SCGs do not, which is characteristic of the glycosidic ring. Bands around 1600 cm\({}^{-1}\) are related to lignin [25], specifically to C=C bonding in the polyphenols of lignin. Therefore, the presence of this band confirms the presence of polyphenols in CA-SCGs beads and their absence in CA beads. This component could be the main difference in the adsorption by CA-SCGs beads in comparison with CA beads. Lignin is a main component of SCGs, values of 23.90 and 26.51% _w/w_ have been reported [26; 27]. Lignin contains two main types of adsorption sites attributed to carboxylic- and phenolic-type, but the phenolic sites have a higher affinity for metal ions than the carboxylic sites [28]. This could be the reason of why the adsorption sites of CA-SCGs and SCGs have a greater affinity for the adsorption of metals, achieving values of adsorption closer to 100%, especially at the lower range of concentration of metal. Although CA beads have more adsorption sites for metals because they have more carboxylic groups than CA-SCGs beads, the CA-SCGs beads have a combination of carboxylic groups and phenolic groups; this combination resulted in a greater affinity to the adsorption of metals than if only the carboxylic groups were considered to be adsorption sites in the study of [PERSON], [PERSON] et al. [28]  
Figure 3: Comparison of the Raman spectra of (**a**) CA beads, (**b**) SCGs and (**c**) CA–SCGs beads. Figure 2: Photos of the analyzed regions by Raman spectroscopy: (**a**) CA beads, (**b**) CA–SCGs beads (**c**) SCGs. Characterization of CA Beads, CA-SCGs Beads, and SCGs by Fourier-Transform Infrared Spectroscopy (FT-IR)  
The FT-IR spectra (Figure 4) shows a broadband approximately at 3325 cm\({}^{-1}\) in CA beads and CA-SCGs beads, and with less intensity in SCGs. It is attributed to the presence of -OH groups. The bands at 2928 cm\({}^{-1}\) and 2856 cm\({}^{-1}\), present only in CA-SCGs and SCGs, can be attributed to asymmetric and symmetric stretching of C-H bonds in aliphatic chains. The band at 1744 cm\({}^{-1}\), also only present in CA-SCGs beads and SCGs, can be attributed to a carbonyl vibration (C=O) in aliphatic esters or in triglycerides [27]. The bands at 1595 cm\({}^{-1}\) and 1418 cm\({}^{-1}\) can be attributed to -COO\({}^{-}\) (asymmetric stretching) and -CH\({}_{2}\) (bending), they are mostly present in the spectra of CA beads and CA-SCGs beads [29].