## 3 Results and Discussion
### Effect of SO\({}_{4}\)\({}^{2-}\) on Adsorption and Selectivity of PTSA  
The selective adsorption property of an exchanger towards ASAs is of particular significance for its future application because inorganic anions often coexist with ASAs at a high level in the industrial waste streams. Excessive inorganic anions may interfere with the adsorption of ASAs on the resin due to the competitive effect. Thus, Na\({}_{2}\)SO\({}_{4}\) was selected as a typical inorganic salt due to its widespread coexistence in ASA chemical effluents. Figure 4 shows the influence of coexisting Na\({}_{2}\)SO\({}_{4}\) on PTSA adsorption by the five resins from water.  
In the absence of Na\({}_{2}\)SO\({}_{4}\), the PTSA uptake increased in the order of SD1 \(<\) SD2 \(<\) SD3 \(<\) SD4 \(<\) SD5, which agreed with the functional group density of resins. Hence, the higher the functional group density of a resin, the higher the PTSA adsorption capacity. Nevertheless, the adsorption amount of resins significantly decreased with increase in the initial concentration of Na\({}_{2}\)SO\({}_{4}\), suggesting that there was a strong competitive effect between the sulfate radical and the PTSA anion [24]. When the Na\({}_{2}\)SO\({}_{4}\) content in the solution increased to 1%, it was found that the adsorption amount order completely reversed to SD5 \(<\) SD4 \(<\) SD3 \(<\) SD2 \(<\) SD1. In other words, reducing the functional group density of resins could improve the adsorption selectivity of resins toward PTSA in the presence of a competing sulfate anion.  
In general, there are two main mechanisms involved in the adsorption of ASAs on a polymeric anion exchanger, i.e., hydrophobic interaction and electrostatic interaction [35; 36]. The resin with a lower functional group density possessed the higher hydrophobicity, which was conducive to hydrophobic interaction between resin phase and PTSA rather than sulfate. Another possible reason for this observation could be ascribed to the changing distance between functional groups of resin surface. When the two adjacent active sites were far apart resulting from the decrease of functional group density, they would be unlikely to interact with a single divalent anion such as sulfate. PTSA anion is monovalent, a trait which was unaffected by the distance between functional groups on the resin.  
To further quantitatively compare the selectivity of different resins, binary competition tests were conducted. The distribution ratio \(K_{d}\) (mL/g) and the selectivity coefficient \(\alpha\) were calculated by Equation (1) [37] and Equation (2) [38], respectively.  
\[k_{d}=\frac{Q_{e}}{C_{e}} \tag{1}\]  
Figure 4: Effect of Na\({}_{2}\)SO\({}_{4}\) concentration on PTSA adsorption by different resins.  
\[\alpha=\frac{K_{d1}}{K_{d2}} \tag{2}\]  
where \(C_{e}\) is the equilibrium concentration of adsorbate in solution; \(Q_{e}\) is the equilibrium adsorption capacity of adsorbent; \(K_{d1}\) and \(K_{d2}\) represent the distribution ratios of PTSA (target adsorbate) and sulfate (competitive adsorbate) on resins, respectively; and \(\alpha\) represents the relative affinity of the adsorbent for PTSA with respect to sulfate.  
It can be observed that the \(\alpha\) values of five resins sequentially decreased in the order of SD1 \(>\) SD2 \(>\) SD3 \(>\) SD4 \(>\) SD5 under the three different initial concentrations in Table 2, signifying that the salt resistance property of all resins decreased in the same order. In addition, the values of the selectivity coefficient \(\alpha\) of the resins decreased continuously when the concentrations of PTSA and sulfate increased, owing to the intensified competition between PTSA and sulfate.