# Adsorption of Ni2+ and Cd2+ from Water by Calcium Alginate/Spent Coffee Grounds Composite Beads
## 3 Results and Discussion
### Kinetic Experiments
In addition to the different pH level, this can be explained by the affinity of the metal toward the lignin and the alginate. The differences in kinetics of adsorption by CA-SCGs beads seen in the previous study with Cu\({}^{2+}\), and in this current study with Cd\({}^{2+}\) and Ni\({}^{2+}\), match the affinities of Cu\({}^{2+}\), Cd\({}^{2+}\) and Ni\({}^{2+}\) toward the alginate and lignin seen in studies reported in the literature [28; 46]. Figure 12: Plots of percent of removal of metal vs. time: (**a**) initial concentration of 10 ppm of metal and (**b**) initial concentration of 100 ppm of metal. Figure 13: Plots of pseudo-second-order linear equations: (**a**) initial concentration of 10 ppm of metal and (**b**) initial concentration of 100 ppm of metal. [PERSON] and [PERSON] deduced the selectivity order for ion exchange of CA for heavy metals with the following result: Pb \(>\) Cu \(>\) Cd \(>\) Ni \(>\) Zn \(>\) Co. The reported estimated equilibrium constants in their study were: K\({}_{\rm Cu}\) = 649 kgL/mol\({}^{2}\), K\({}_{\rm Cd}\) = 172 kgL/mol\({}^{2}\) and K\({}_{\rm Ni}\) = 30.8 kgL/mol\({}^{2}\)[46]. Our previous study with Cu\({}^{2+}\) showed a great improvement in the kinetics of adsorption by the CA-SCGs beads versus the CA beads, despite that the study was at pH of 4, with more protons competing with the metal for adsorption sites [17]. Nevertheless, the current study shows improvement in the kinetics of adsorption of Cd\({}^{2+}\) in CA-SCGs versus the CA beads, but did not show any improvement in the kinetics of adsorption of Ni\({}^{2+}\) in CA-SCGs versus the CA beads. These results of kinetics could be related to a poor affinity of Ni\({}^{2+}\) to the CA. [PERSON], [PERSON] et al. showed in their study that lignin has affinity with metal ions in the following order: Pb(II) \(>\) Cu(II) \(>\) Cd(II) \(>\) Zn(II) \(>\) Ni(II) [28]. Lignin affinity is more important than alginate affinity because one of the major differences between the CA beads and the CA-SCGs beads is the presence of the phenolic groups of lignin in the CA-SCGs beads. Again, the order of affinity with lignin follows the order of the kinetics seen in the previous study with Cu\({}^{2+}\) and the current study with Cd\({}^{2+}\) and Ni\({}^{2+}\). As was discussed earlier, the lignin contains two main types of adsorption sites attributed to carboxylic- and phenolic-type groups, but the phenolic sites have a higher affinity for metal ions than the carboxylic sites [28]. Alginate does not have phenolic groups; therefore, the phenolic groups could follow the same pattern of affinity to the metals than the carboxylic groups. A third explanation for the differences in adsorption kinetics is the structural changes in the bead caused by the metal adsorbed. An unusual phenomenon was seen during the test of adsorbent dose effect at the lowest dose of adsorbent (0.025 g of adsorbent in a solution with a concentration of 20 ppm of metal). This phenomenon was the swelling of the beads after of the adsorption of nickel. The swelling is also related to the affinity of the metal to alginate. In the study of [PERSON] and [PERSON] [46], metals such as Pb, Cu, and Cd make the bead shrink under some experimental conditions. Metals such as Ni, Zn, and Co, which have less affinity to alginate, swelled over all the concentration ranges studied [46]. The swelling of the beads after the adsorption of Ni\({}^{2+}\) allowed a gradual entrance of water inside of the bead. This gradual entrance of water during the process of swelling changed the distribution of SCCGs throughout the bead obtained during the instant precipitation of the bead in the original synthesis. Also, the water inside the bead changed the overall structure of both types of beads, altering the adsorption because the exchange of water between the inside and outside of the bead will not be the same as if the bead maintained a more rigid structure.