## 3 Results
### Adsorption of AsO\({}_{4}\)\({}^{3-}\) on Source Materials and Composite Adsorbents  
The parameters of AsO\({}_{4}\)\({}^{3-}\) adsorption on source materials (SK, KB and KF) and on the composites (SK+KB and SK+KF), calculated according to the Langmuir and Freundlich models, are summarised in Table 6. As is evident, the Langmuir and Freundlich fits were applicable just to the adsorption on both FA and SK+KB. Compared to Pb\({}^{2+}\), the adsorption capacities (\(q_{\text{max}}\) and \(Q_{t}\)) reached an order-of-magnitude lower values, which corresponded well to a generally worse adsorption of anions, including AsO\({}_{4}\)\({}^{3-}\), on aluminosilicates and other silicate-rich materials [12; 34; 35; 36]. Indeed, all the source materials belong to this group. The other parameters, such as \(K_{L}\) and \(\Delta G\), indicated a balanced AsO\({}_{4}\)\({}^{3-}\) adsorption on KB  
Figure 5: Efficiency of Pb\({}^{2+}\) adsorption on source materials and composite adsorbents. \begin{table}
\begin{tabular}{c c c c c c c c c c} \hline \hline  & & \multicolumn{3}{c}{**Langmuir Model**} & \multicolumn{6}{c}{**Freundlich Model**} & \\  & \(q_{\text{max}}\) & \(Q_{t}\) & \(K_{L}\) & \(R^{2}\) & \(q_{\text{max}}\) & \(n\) & \(K_{F}\) & \(R^{2}\) & \(\Delta G\) \\  & **(mmol g\({}^{-1}\))** & **(mmol g\({}^{-1}\))** & **(L.mmol\({}^{-1}\))** & **(mmol g\({}^{-1}\))** & **(mmol g\({}^{-1}\))** & **(mmol g\({}^{-1}\))** & **(k[k].mol\({}^{-1}\))** \\ \hline SK & 0.04 & - & - & - & - & 0.04 & - & - & - & - \\ KB & 0.42 & 0.90 & 76.6 & 0.986 & 0.42 & 0.87 & 26.92 & 0.986 & \(-10.7\) \\ KF & 0.43 & 0.23 & 140.6 & 0.667 & 0.43 & 0.66 & 2.59 & 0.736 & \(-12.3\) \\ SK+KB & 0.50 & 0.58 & 512.1 & 0.933 & 0.50 & 0.80 & 50.90 & 0.980 & \(-15.5\) \\ SK+KF & 0.47 & 1.07 & 213.1 & 0.938 & 0.47 & 0.61 & 11.91 & 0.911 & \(-13.3\) \\ \hline \hline \end{tabular}
\end{table} 
Table 5: Parameters of Langmuir and Freundlich models for Pb\({}^{2+}\) adsorption. and KF, and less robust adsorption on SK+KB, most probably thanks to the presence of clay in the adsorption mixture. The efficiency of AsO\({}_{4}\)\({}^{3-}\) adsorption on the tested materials (Figure 6) confirmed the results given by the sorption parameters, i.e., the excellent solvability of AsO\({}_{4}\)\({}^{3-}\) on KB and KF, and almost sufficient AsO\({}_{4}\)\({}^{3-}\) adsorption on composite adsorbents (SK+KB \(>\) SK+KF) at a higher sorbent dosage (25-30 g.L\({}^{-1}\)). The selectivity of KB and KF to AsO\({}_{4}\)\({}^{3-}\) can be explained by the high pH\({}_{\rm ZPC}\) value (Table 1), which, despite a high silicate portion in the FA composition (Table 1), clearly corresponded to anion-active adsorbents [30; 37]. Effect of Preparation Temperature on Sorbability of Pb\({}^{2+}\) and AsO\({}_{4}\)\({}^{3-}\) on Composite Adsorbents  
A possible temperature effect on the structural and surface properties of the composite adsorbents was discussed in Section 3.1. As was found, a higher preparation temperature (65 \({}^{\circ}\)C or 110 \({}^{\circ}\)C in an autoclave) did not affect the structural and surface properties of the composites. In terms of adsorption properties resulting from the Langmuir and Freundlich models (Tables 7 and 8), no significant trend was observed in connection with increasing temperature. In the case of Pb\({}^{2+}\), all adsorption systems were fitted by the Langmuir and Freundlich models, at similar values of parameters for different temperatures. The data of AsO\({}_{4}\)\({}^{3-}\) adsorption on composite adsorbents prepared at different temperatures were not fittable by the adsorption models; however, the experimentally measured maximum adsorption capacities (\(q_{\rm max}\)) even showed a decreasing trend with growing preparation temperatures. The presumed loss of anion-active adsorption sites for heat-treated composites was confirmed by a slight decrease in pH\({}_{\rm ZPC}\) during heating (Table 3), which could be induced by chemical changes in the available compounds (e.g., alkali oxides and hydroxides).