# Adsorption of Ni2+ and Cd2+ from Water by Calcium Alginate/Spent Coffee Grounds Composite Beads
## 3 Results and Discussion
### Characterization of CA Beads and CA-SCGs Beads by SEM-EDX Spectra  
The SEM images (Figures 5 and 6) showed differences in the surface morphology of the beads. Before adsorption, the CA beads had a round shape with a smoother surface without much porosity (Figure 5a). After the adsorption of any of the two metals, the surface texture of the CA beads changed showing marks and scratches (Figure 5b,c). The CA-SCGs beads had a more irregular shape than the CA beads with a rough surface and more porosity (Figure 6a). After the adsorption process, the surface  
Figure 4: Comparison of the FT–IR spectra of (**a**) SCGs, (**b**) CA beads and (**c**) CA–SCGs beads. texture of the CA-CSGs beads did not change (Figure 6b,c). Figure 7 shows an example of the EDX spectra obtained from the beads. The EDX spectra of CA beads and CA-SCGs beads before adsorption showed a significant presence of Cl\({}^{-}\). This could suggest that although the beads were washed several times with deionized water after synthesis to remove the excess of CaCl\({}_{2}\), the removal was not complete. The CA beads and CA-SCGs beads showed the expected peak of Ca\({}^{2+}\). Carbon and oxygen were found in the largest proportion, as was expected. Other elements were detected at level of traces (less than 1 wt. %), these elements such as S and Al have been seen before in spectra of alginate beads and can be products of algae components or impurities associated with the alginate extraction step [16]. Table 1 shows the values of wt. % obtained for Ca\({}^{2+}\), Cd\({}^{2+}\) and Ni\({}^{2+}\) in the EDX spectra for six beads, before and after metal adsorption. The data presents the Ca\({}^{2+}\) content and release, also the adsorption of Cd\({}^{2+}\) and Ni\({}^{2+}\) in the analyzed bead volume. The average (Ave.), standard deviation (S. D.), median (Med.) and median absolute deviation (M.A.D.) are also included in Table 1. The set of data does not have a normal distribution; therefore, the median is more appropriate than the average to  
Figure 5: SEM images of CA beads: (**a**) before adsorption; (**b**) after adsorption at 100 ppm Cd\({}^{2+}\) and (**c**) after adsorption at 100 ppm Ni\({}^{2+}\). Figure 6: SEM images of CA–SCGs beads: (**a**) before adsorption; (**b**) after adsorption at 100 ppm Cd\({}^{2+}\) and (**c**) after adsorption at 100 ppm Ni\({}^{2+}\). Figure 7: EDX spectrum of CA–SCGs bead after adsorption at 100 ppm Ni\({}^{2+}\) (X-ray lines K or L for each element). describe a representative value. Likewise, the M.A.D. is more appropriate than the standard deviation as a measure of how spread out is the set of data. Table 1 shows patterns for each bead type, before and after adsorption. The first pattern is the content of Ca\({}^{2+}\) before adsorption. The EDX spectra of CA beads before adsorption showed a higher median for Ca\({}^{2+}\) content than the CA-SCGs beads (20.03 wt. % versus 8.56 wt. %). The CA-SCGs beads had a higher M.A.D. for the Ca\({}^{2+}\) content (4.49 wt. % versus 2.53 wt. %), while the content of Ca\({}^{2+}\) was more heterogeneous in the CA-SCGs beads. The second pattern seen in the data is the Ca\({}^{2+}\) content after adsorption of Ni\({}^{2+}\) or Cd\({}^{2+}\). The CA-SCGs beads showed a higher median for Ca\({}^{2+}\) content of versus the content of Ca\({}^{2+}\) in the CA beads after adsorption of Ni\({}^{2+}\) (1.04 wt. % versus 4.76 wt. %) or Cd\({}^{2+}\) (2.87 wt. % versus 6.49 wt. %). This could suggest a lesser release of Ca\({}^{2+}\) by the CA-SCGs beads during the adsorption of Ni\({}^{2+}\) and Cd\({}^{2+}\). Finally, the third pattern is related to the content of Ni\({}^{2+}\) and Cd\({}^{2+}\) in the CA beads and the CA-SCGs beads after metal adsorption. The CA-SCGs beads have a higher median of Ni\({}^{2+}\) (2.80 wt. %) versus 2.25 wt. %) and Cd\({}^{2+}\) (0.87 wt. % versus 4.31 wt. %) content. The difference in the Cd\({}^{2+}\) bead content was greater than the difference seen in the adsorption of Cd\({}^{2+}\) from the solution.