# Adsorption of Ni2+ and Cd2+ from Water by Calcium Alginate/Spent Coffee Grounds Composite Beads
## 3 Results and Discussion
### Analysis of Adsorption Isotherms
Likewise, the experimental values of \(q_{e}\) of CA-SCGs beads in the plots of both metals fit better with the expected values of the Freundlich and Sips models (Figures 10 and 11), which describe heterogeneous adsorption. The obtained heterogeneity index (1/\(n\)) of the Sips model of CA-SCGs beads are 0.633 for Cd\({}^{2+}\) and 0.698 for Ni\({}^{2+}\) (more heterogeneous), while the heterogeneity index (1/\(n\)) of CA beads are about 1.0 for Cd\({}^{2+}\) and Ni\({}^{2+}\) (almost homogeneous adsorbent). Thus, for the CA beads, the Sips equation is reduced to the Langmuir model, indicating the preferential binding of metals on all sites of the adsorbent, which act as a homogenous surface. Therefore, the models support the idea that the adsorption sites of CA-SCGs beads are not uniformly distributed through the bead and some adsorption sites are preferred than others. The EDX spectra suggest that at a depth of 1 \(\upmu\)m, the CA-SCGs beads have a greater proportion of adsorption sites in comparison to the CA beads. Figure 10: Plots of the results of the adsorption models for each adsorbent and the experimental values for cadmium: (**a**) CA beads and (**b**) CA–SCG beads. Table 4 shows a comparison of the theoretical maximum sorption capacities of different alginate, algal-based, or spent coffee adsorbents reported in the literature using Langmuir (L) and Sips (S) models. The theoretical adsorption capacity of SCGs for Cd\({}^{2+}\) obtained in this study was higher than the value reported by [PERSON], [PERSON] et al. [35] and similar to the value reported by [PERSON], [PERSON] et al. [18]. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Langmuir (Cd2+)** & **SCGs** & **CA Beads** & **CA/SCGs Beads** \\ \hline \(q_{max}\) (mg/g) & 10.671 & 38.049 & 25.495 \\ \(b_{L}\) (L/mg) & 2.415 & 0.031 & 0.101 \\ \(\chi^{2}\) & 5.669 & 0.307 & 1.533 \\ \hline
**Freundlich (Cd2+)** & **SCGs** & **CA Beads** & **CA/SCGs Beads** \\ \hline Kf (mg/g)(L/mg)(1/\(n\)) & 8.353 & 1.519 & 3.340 \\ \(1/n\) & 0.054 & 0.752 & 0.563 \\ \(\chi^{2}\) & 1.856 & 0.241 & 0.430 \\ \hline
**Sips (Cd2+)** & **SCGs** & **CA Beads** & **CA/SCGs Beads** \\ \hline \(q_{max}\) (mg/g) & 12.752 & 38.650 & 91.184 \\ \(b_{s}\) (L/mg) & 1.290 & 0.030 & 0.005 \\ \(1/n\) & 0.453 & 0.993 & 0.633 \\ \(\chi^{2}\) & 5.622 & 0.301 & 0.587 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Adsorption isotherm constants and chi-square test \(\chi^{2}\) values for each model (Cd2+). Figure 11: Plots of the results of the adsorption models for each adsorbent and the experimental values for cadmium: (**a**) CA beads and (**b**) CA–SCG beads. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Langmuir (Ni2+)** & **SCGs** & **CA Beads** & **CA/SCGs Beads** \\ \hline \(q_{max}\) (mg/g) & 5.608 & 14.668 & 12.320 \\ \(b_{L}\) (L/mg) & 0.803 & 0.019 & 0.056 \\ \(\chi^{2}\) & 0.818 & 0.063 & 0.191 \\ \hline
**Freundlich (Ni2+)** & **SCGs** & **CA Beads** & **CA/SCGs Beads** \\ \hline Kf (mg/g)(L/mg)(1/n) & 2.854 & 0.508 & 1.299 \\
1/\(n\) & 0.177 & 0.676 & 0.508 \\ \(\chi^{2}\) & 0.115 & 0.137 & 0.0461 \\ \hline
**Sips (Ni2+)** & **SCGs** & **CA Beads** & **CA/SCGs Beads** \\ \hline \(q_{max}\) (mg/g) & 7.813 & 14.224 & 20.958 \\ \(b_{s}\) (L/mg) & 0.233 & 0.020 & 0.014 \\ \(1/n\) & 0.391 & 1.015 & 0.698 \\ \(\chi^{2}\) & 0.042 & 0.064 & 0.024 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Adsorption isotherm constants and chi-square test \(\chi^{2}\) values for each model (Ni2+). The theoretical values of adsorption reported in the literature for alginate beads vary. The values for Ni\({}^{2+}\) range from 25.60 mg/g obtained by [PERSON], [PERSON] et al. [36] to 97.93 mg/g obtained by [PERSON], [PERSON] et al. [37]. The values of the current study were below the values for adsorption of Ni\({}^{2+}\) by alginate beads reported in the literature. The values for Cd\({}^{2+}\) range from 10.20 mg/g obtained by [PERSON], [PERSON] et al. [38] to 198.43 mg/g obtained by [PERSON], S.K. (Sips model) [15].