## 3 Results and Discussion
### Accessible Surface Area Determination  
The surface area of adsorbents can be measured via nitrogen gas adsorption isotherms (via BET) and compared to both the starting materials and a variety of powdered adsorbents. However, gas adsorption provides limited information concerning the porosity and available surface area for cases when the adsorbent undergoes swelling in aqueous media. To evaluate the accessible surface area (SA) for anion adsorption in a liquid medium, PNP adsorption isotherms were obtained at pH 8.5, and analyzed via the Sips model to obtain \(Q_{e}\). At this pH, PNP is deprotonated since the pH lies above the pKa of PNP (ca. 7.2) and approximates an oxyanion species with a single negative charge. A noticeable feature of the PNP isotherms is that Cu-TMC reveals greater SA versus the Al and Fe-TMC materials, according to the dye adsorption method. It is expected that PNP exists in its nonionized phenol form at pH 6.3-6.7, where the role of potential metal cation-\(\pi\) interactions occur, in addition to London dispersion forces between the dye and biopolymer framework. This contrasts with the adsorption of inorganic oxy-anions that involve anion-cation interactions (metal cation-oxyanion). At pH 8.5, the phenolate form of PNP prevails and additional contributions due to phenolate-metal ion interactions may prevail, which is supported by the systematically higher \(Q_{e}\) and SA values at alkaline pH conditions, as revealed in Table 6.  
For comparison, the BET surface area of NaAlg was measured as 1.46 m\({}^{2}\)/g, and that of Chi as 1.06 m\({}^{2}\)/g. While Al-TMC observed a similar SA in its dry state compared to NaAlg or Chi, Cu-TMC showed almost a 30-fold increase in its SA, after Fe-TMC with a SA of 70 m\({}^{2}\)/g. For the adsorption properties, however, it is germane to include solvent swelling and increased access to surface adsorption sites, hence PNP adsorption isotherms were determined. Although the magnitude of the SA values of all composites are similar, the SA estimated for Cu-TMC was the highest, followed by Fe-TMC, and the lowest for Al-TMC. The isotherms of PNP adsorption at pH 8.5 are shown in Figure 5. This corroborates the observations made for SEM-EDX mapping, where a more porous appearance (Cu-TMC and Fe-TMC) is reflected in a drastically increased (dry) SA compared to Al-TMC. In addition, the discrepancy between the N2 gas BET estimate and the PNP dye estimate indicates that the SA in the dry state does not reflect the relative adsorption capacity toward the oxyanions, due to solvent swelling effects because of variable surface chemical and porosity effects. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**adsorbent** & **Al-TMC** & **Fe-TMC** & **Cu-TMC** \\ \hline \(Q_{e}\) (mmol/g) & 1.52 & 2.03 & 2.06 \\ K & 0.11 & 0.14 & 0.52 \\ n & 8.62 & 4.69 & 2.5 \\ SA PNP (m\({}^{2}\)/g) & 271 & 286 & 311 \\ SA BET (m\({}^{2}\)/g) & 1.59 & 70.2 & 30.5 \\ \hline \hline \end{tabular}
\end{table} 
Table 6: Surface area (SA) determination via isotherm parameters fit with the Sips model for PNP adsorption at pH 8.5 (wet state) and comparison with the measured BET surface area (dry state). \begin{table}
\begin{tabular}{c c c c} \hline \hline  & **Al-TMC** & **Fe-TMC** & **Cu-TMC** \\ \hline Orthophosphate (mmol/g) & 0.96 & 0.79 & 0.18 \\ Arsenate (mmol/g) & 0.57 & 0.47 & 0.22 \\ \hline \hline \end{tabular}
\end{table} 
Table 5: Adsorption capacity (\(Q_{e}\)) of arsenate (140 g/mol) versus orthophosphate (97 g/mol) at pH 6.3-6.7. In addition to the dye-based SA determination via PNP in an aqueous environment, the binding mechanism was examined via IR spectral results that consider a potential anion exchange mechanism. As shown in Figure 6, the disappearance of the 1390 cm\({}^{-1}\) signals in Al-TMC and Fe-TMC indicates the displacement of nitrate counter ions, supporting the role of the PNP anion at pH 8.5, as compared to the neutral dye species at slightly acidic conditions (ca. pH 6.3-6.7).