## 3 Results and Discussion
### Arsenate to Phosphate (As\({}_{\text{i}}\)/P\({}_{\text{i}}\)) Adsorption-Based Selectivity  
To determine the selectivity of an adsorbent toward a target adsorbate species, a variety of methods can be used. The adsorption capacities of both As\({}_{\text{i}}\) and P\({}_{\text{i}}\) in a mixed system are shown in Figure 7.  
Figure 5: PNP adsorption isotherms with various TMCs at pH 8.5. Figure 6: IR spectra for Fe-TMC (**A**), Al-TMC (**B**), and Cu-TMC (**C**) before and after PNP adsorption at pH 6, pH 8.5 low concentration, and pH 8.5 high concentration. An often-used factor is percent removal (R; 9%) due to its wide comparability across different studies in the literature. However, this parameter does not adequately describe the adsorbent efficiency or required dosage. This can be addressed by the incorporation of \(q\) (target equilibrium adsorption capacity). Herein, three different selectivity metrics in the form of the adsorption capacity ratio, the binary separation factor (\(\alpha_{t/c}\)), and the selectivity coefficient \(\beta_{t}\) were used. The adsorption capacity ratio (\(AC\)) can be calculated by employing Equation (3) [9]:  
\[\text{adsorption capacity ratio}=\frac{q_{t}}{q_{(t+c)}} \tag{3}\]  
\(q_{t}\) represents the equilibrium adsorption capacity for the target species, while \(q_{t+c}\) is the equilibrium adsorption capacity of the target species in the presence of a competitor ion species. The calculation of \(\alpha_{t/c}\) is shown in Equation (4), [9]:  
\[\alpha_{(\frac{t}{c})}=\frac{q_{t}C_{e,c}}{q_{c}C_{e,t}} \tag{4}\]  
where \(q_{c}\) is the equilibrium adsorption capacity of the competitor oxyanion species, \(C_{e,c}\) is the equilibrium concentration of the competitor oxyanion species, and \(C_{e,t}\) is the equilibrium concentration of the competitor oxyanion. The selectivity coefficient \(\beta_{t}\) can be calculated from the distribution coefficients \(K_{d,t}\) and \(K_{d,c}\) as shown in Equations (5) and (6), [9]:  
\[K_{d,x}=\frac{C_{0}-C_{e}}{C_{e}}*\left(\frac{v}{m}\right) \tag{5}\]  
\[\beta_{t}=\frac{K_{d,t}}{K_{d,c}} \tag{6}\]  
Figure 7: Selectivity studies on all three TMCs with phosphate (P\({}_{i}\)) alone (**B**), arsenate (As\({}_{i}\)) alone (**A**), and their mixtures (P\({}_{i}\) + As\({}_{i}\)), where the initial concentrations are ca. 20 mg/L. where \(K_{d,x}\) represents either the distribution coefficient \(K_{d,t}\) or \(K_{d,c}\). \(C_{0}\) is the initial concentration of an oxyanion species in a mixture (competitive ion present), while \(C_{e}\) represents the equilibrium concentration within a mixture, \(v\) refers to the volume (mL) and \(m\) is the adsorbent weight (mg). The results for the adsorption capacity ratio, the binary selectivity factor \(\alpha_{t/c}\), and the selectivity coefficient \(\beta_{t}\) are shown in Table 7.  
In a study of the selectivity of arsenate toward the different TMCs, it was shown that while all materials preferentially adsorb arsenate over phosphate, the Al-TMC observed the lowest selectivity among the biocomposites. Both Cu-TMC and Fe-TMC showed considerably higher arsenate selectivity with Fe-TMC surpassing Cu-TMC (cf. Figure 4). Whereas the Sips isotherm parameters suggest heterogeneous binding sites for both Asi and P1, this trend can be explained through hydrogen bonding toward the biopolymer backbone, but mainly due to ligand exchange (especially for Al-TMC and Fe-TMC). In the case of iron-based materials, it has been reported that phosphate can be used to exchange for arsenate, which accounts for the high selectivity of Fe-TMC toward P1 [35; 36]. Further, it has been reported that Al-based materials exhibit competitive P1 adsorption, thus revealing a reduced Asi uptake in the presence of P1 [8; 37]. In the case of copper (i.e., cupric oxides) TMCs, a tendency for As(V) selectivity over P1 has been observed. In this case, inner sphere complexation was suggested, in addition to electrostatic interactions.