# Removal of Fluoride and Arsenate from Aqueous Solutions by Aluminum-Modified Guava Seeds
## 3 Results and Discussion
### Fourier Transform Infrared Spectroscopy
According to Figure 4, most of the fluoride biosorption was attained in the first 60 min of contact, where it reaches a removal of approximately 40% with an adsorbent dose of 10 g/L. From this time, fluoride biosorption rate was slower, until no significant changes were observed in the concentration of F\({}^{-}\) in the adsorbent. Therefore, the equilibrium point was established at 150 min. This behavior could be attributed to the high concentration gradient between the solution and the Al-GSs, so the biosorption rate was fast at the initial stages of the process, as well as to the large availability of sorption sites on the beginning of biosorption. As the process continues, the available adsorption sites and the concentration gradient began to decrease, and a slower sorption rate was observed [37]. The capacity of Al-GSs for fluoride removal at equilibrium was 0.08 mg/g. Kinetics experiments using unmodified guava seeds as biosorbent showed low fluoride adsorption at these experimental conditions. These results indicate that aluminum modification improved fluoride removal from aqueous solutions in single systems. Concerning the arsenate adsorption kinetics with Al-GSs, a similar behavior was observed since a rapid biosorption was noticed in the first 90 min, reaching approximately 60% removal. From this time, equilibrium of the system was reached and no significant changes in arsenate biosorbent concentration were observed; as a result, this time was chosen for the further biosorption experiments. An equilibrium adsorption capacity of 0.85 mg/g was reached. It was observed that the kinetics of removal of As is slightly faster than the kinetics of removal of F\({}^{-}\). \(K_{2}\) = pseudo-second-order rate constant, (g/mg min). \(q_{t}\) = quantity of ion adsorbed at any given time \(t\), (mg/g). \(q_{e}\) = quantity of ion adsorbed at equilibrium, (mg/g). The pseudo-second-order model is based on the supposition that the rate-limiting step may be chemisorption involving valence forces through sharing or exchange of electrons between adsorbent and adsorbate [39]. The Elovich equation has been usually reported for chemisorption of gases on solids. This equation can be expressed as follows [40]:  
\[q_{t}=\frac{1}{b}\ln(1+abt) \tag{4}\]  
where,  
\(q_{t}\) = adsorbate concentration at any time, \(t\), per weight of adsorbent, (mg/g). \(a\) = Elovich constant related to the initial adsorption rate, (mg/g min). \(b\) = Elovich constant related to the desorption rate, (g/mg). Figures 6 and 7 show the fitted plots to the kinetic models. Tables 1 and 2 show the model's parameters and correlation coefficients for fluoride and arsenate adsorption onto Al-GSs, respectively. The model that best describes the adsorption experimental data is the pseudo-first-order model, or Lagergren equation since the correlation coefficients for fluoride and arsenate removal are higher than for the other two models. However, in the case of fluoride biosorption, pseudo second-order model shows a good fit to experimental data (R\({}^{2}\) = 0.9127); thus it can describe fluoride sorption. For the case of arsenate removal, the Lagergren model describes well biosorption kinetic data, but pseudo-second-order and Elovich equations are not suitable to describe arsenate sorption on Al-GS. \begin{table}
\begin{tabular}{c c c} \hline \hline
**Model** & **Kinetic Parameters** & **Correlation Coefficient (R)** \\ pseudo-first-order (Lagergren) & \(\begin{array}{c}\text{K}_{\text{L}}\text{ (min}^{-1})=0.0231\\ \text{q}_{\text{e}}\text{ (mg/g)}=0.0813\end{array}\) & 0.9719 \\ \hline pseudo-second-order & \(\begin{array}{c}\text{K}_{\text{2}}\text{ (g/mg min)}=0.0993\\ \text{q}_{\text{e}}\text{ (mg/g)}=0.0813\end{array}\) & 0.9554 \\ \hline [PERSON] & \(\begin{array}{c}\text{a (mg/g min)}=0.0003\\ \text{b (g/mg)}=1.0162\end{array}\) & 0.7531 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Kinetic models parameters for fluoride adsorption onto Al-GSs. Figure 7: Adsorption of arsenate by Al-GSs as a function of time fitted to the empirical kinetic models.