# Nickel (Ni\({}^{2+}\)) Removal from Water Using Gellan Gum-Sand Mixture as a Filter Material
## 3 Results and Analysis  
### Isotherm and Kinetics of Heavy Metal Adsorption Mechanism  
Figure 4 shows the adsorption of nickel solution on each GG-treated sand specimen. The results show that GG and sand had the ability to adsorb nickel. All three conditions (sand, 0.5 HBL, and 1.0 HBL) show a logarithmic trend in the equilibrium state. The equilibrium uptake of nickel adsorbate increased with an increase in the GG content. Figure 4b,c show the Langmuir and Freundlich isotherm models, respectively. All three data were fitted better with the Langmuir isotherm model compared with that of the Freundlich isotherm model, with an \(R^{2}\) value of 0.9881 to 0.9983 for the [PERSON] model  
(Figure 4b) and 0.782 to 0.9197 for the [PERSON] model (Figure 4c). The parameters of the [PERSON] model and the Freundlich model are summarized in Table 3.  
Figure 4: Adsorption isotherm at various initial nickel concentrations (10–1000 mg/L) on gellan gum-treated sand (**a**), the Langmuir adsorption isotherm (**b**), and the Freundlich adsorption isotherm of nickel on sample (**c**).  
The adsorption of Ni\({}^{2+}\) on GG and sand indicated monolayer coverage of the adsorbate on the surface of the adsorbent, which is coherent with that of other studies on heavy metal adsorption by biopolymers [21; 22; 23]. The Langmuir model is the best fitting model for the data set. The slope of Figure 4b was converted into \(q_{max}\) for a better understanding of the adsorption model. The \(q_{max}\) started from 0.19 mg/g for 0 HBL (pure sand) to 0.29 mg/g for 0.5 HBL, 0.40 mg/g for 1.0 HBL, and 0.60 mg/g for 2.0 HBL (Table 3). The results demonstrated that GG had a higher adsorbability compared with that of pure sand. The \(q_{max}\) increased with an increase in GG, which indicated that GG in HBL can improve the nickel adsorption ability of the entire HBL and can be the main adsorbent of the HBL system.  
Kinetic study on Ni\({}^{2+}\) adsorption by GG = 1% HBL with time shows an instantaneous increase in \(q_{e}\) and then steady-state after a specific time (Figure 5) [21; 24]. The adsorption can be illustrated by two stages. In the first stage, there is rapid removal (regardless of the nickel concentration) within the first 1 min, and equilibrium is reached in the later stage in approximately 40 min. The rapid removal during the initial stage might be attributed to ample sites for adsorption on the surface of GG.  
As shown in Figure 6, the SEM EDS spectra show peaks of GG in its pure state and nickel-adsorbed state. The EDS data show a reduction in the potassium peak and appearance of the nickel peak. This can be considered a cation exchange mechanism of GG  
Figure 5: Kinetics of Ni\({}^{2+}\) binding to the gellan gum (GG = 1.0% HBL) at different initial nickel ion concentrations of 25 mg/L, 50 mg/L, and 100 mg/L.  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Parameters** & **0 HBL** & **0.5 HBL** & **1.0 HBL** & **2.0 HBL** \\ \hline \multicolumn{5}{c}{Langmuir isotherm} \\ \hline \(q_{max}\) & 0.19 & 0.29 & 0.40 & 0.60 \\ \(k_{L}\) & 0.20 & 0.21 & 0.30 & 0.30 \\ \(R^{2}\) & 0.9881 & 0.9983 & 0.9866 & 0.9967 \\ \hline \multicolumn{5}{c}{Freundlich isotherm} \\ \hline \(k_{F}\) & 0.03 & 0.05 & 0.06 & 0.09 \\ \(N\) & 2.37 & 2.39 & 2.14 & 2.20 \\ \(R^{2}\) & 0.9197 & 0.9188 & 0.782 & 0.8925 \\ \hline \hline \end{tabular}
\end{table}
Table 3: Parameters of the Langmuir model and the Freundlich model.  
with Ni\({}^{2+}\) ion, similar to that of other adsorbents [21]. The Ni\({}^{2+}\) replacement for potassium in GG indicated that GG is a potassium-type GG [25,26]. Furthermore, the functional groups in GG (carbonyl, methylol, and methyl) were also responsible for chemical interaction with nickel ions [25]. Meanwhile, the sand can adsorb Ni\({}^{2+}\) via the inner-sphere interaction, where Ni\({}^{2+}\) can interact with silanol groups at the sand surfaces [27,28].