# Removal of Fluoride and Arsenate from Aqueous Solutions by Aluminum-Modified Guava Seeds
## 3 Results and Discussion
### Fluoride and Arsenic Adsorption Isotherms  
Quantitative analysis of adsorbate-adsorbent interactions is important for the evaluation of the adsorbent. Adsorption isotherms of fluoride and arsenate onto Al-GSs are presented in Figures 10 and 11, respectively. Experimental data were fitted to isotherm models of Langmuir, Freundlich, and Langmuir-Freundlich (L-F) by non-linear regression analyses [26]. Results show that fluoride adsorption isotherm can be well described by the Langmuir and Langmuir-Freundlich (L-F) models (R = 0.9885). However, the L-F model indicates sorption on heterogeneous surfaces implying different sorption energies of the adsorption sites; therefore, this model can be used for the description of fluoride equilibrium data. This is in agreement with the proposed mechanisms for fluoride removal described above where different types of sorption sites are responsible for the F\({}^{-}\) removal. A maximum adsorption capacity of 0.3445 mg of F\({}^{-}\)/g of Al-GS was determined. Supernatants were also analyzed for Al leaching after adsorption, by AAS with a nitrous oxide-acetylene flame, but Al concentrations were below the detection limit (0.4 mg/L).  
Arsenate adsorption onto Al-GS (Figure 11) showed a slightly different behavior compared to fluoride sorption isotherm. In this case, Freundlich and L-F models fitted well arsenate sorption equilibrium data at these experimental conditions. A relatively high sorption capacity was determined at 4 mg of As(V)/g of Al-GS. A similar sorption behavior was reported for the arsenate removal by Al-Based nanoparticle impregnated sawdust [47]. Table 3 shows a comparison of fluoride and arsenate sorption capacities for different adsorbents. Guava seed surface without aluminum modification showed a low fluoride adsorption capacity, but a significant arsenate sorption capacity; thus, the biosorbent was modified to enhance fluoride adsorption capacity and to use this material in cases where simultaneous removal of both ions is required. This aluminum-modified adsorbent has the advantage over aluminum-based coagulants of not producing sludge and also a better recovery of this element. Adsorption capacities around 1 mg/g and 11 mg/g for fluorides and arsenates, respectively, have been reported using activated alumina as adsorbent [48]. By comparing these adsorption capacities with those obtained in this work, Al-GS can be considered as a relatively efficient and low-cost alternative for the adsorption of these ions from water.  
Figure 9: Effect of pH on the removal of fluoride by Al-GSs.  
Figure 11: Adsorption isotherm of arsenate on Al-GS, at 25 \({}^{\circ}\)C and pH 6, fitted to isotherm models.  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Adsoorbent** & **Adsorbate** & **qmax (mg/g)** & **Solution pH** & **Reference** \\ \hline Al-GS & As(V) & 4 & 6 & This work \\ Fe-Al modified zeolite & As(V) & 3.31 & 2 & [49] \\ Fe-Al modified pozzolan & As(V) & 3.35 & 2 & [49] \\ Stem of _Teconella undulata_ & As(V) & 0.159 & 7 & [50] \\ Watermelon rind & As(V) & 3.29 & 5.3 & [51] \\ Aluminum-enriched biochar from Tetra Paks & As(V) & 3.32 & 3 & [52] \\ Al-GS & F & 0.3445 & 6 & This work \\ Fe-Al modified zeolite & F– & 0.866 & 2 & [49] \\ Fe-Al modified pozzolan & F– & 0.834 & 2 & [49] \\ Stem of _Teconella undulata_ & F– & 6.16 & 7 & [50] \\ Active carbon of _Fiscus racemosa_ & F– & 1.65 & 7 & [53] \\ Sweet lemon peel powder & F– & 0.744 & 4 & [54] \\ \hline \hline \end{tabular}
\end{table}
Table 3: Maximum adsorption capacities (qmax) of fluoride and arsenate of Al-GSs and other adsorbents.  
Figure 10: Adsorption isotherm of fluoride on Al-GS, at 25 \({}^{\circ}\)C and pH 6, fitted to isotherm models.