## 3 Results and Discussion
### Effect of Coexisting Ions and HA  
It is well known that environmental water contains not only pollutants, but also high concentrations of salts, which may affect the removal of pollutants [36]. Therefore, the effect of ionic strength on BPA adsorption was investigated by performing adsorption equilibrium experiments at different concentrations of NaCl and CaCl\({}_{2}\). As shown in Figure 6a, the adsorption capacity of BPA improved in the presence of NaCl. This phenomenon could be explained by the negative charge of Mag-PVP and the molecular form of BPA. The Cl\({}^{-}\) was placed between the surface of Mag-PVP and the BPA molecules, which produced a screening effect of the surface charge. It favored adsorbate-adsorbent interactions and enhanced the adsorption of BPA. The presence of NaCl in solution also caused a salting-out effect via decreasing the solubility of BPA, thereby enhancing its adsorption onto Mag-PVP. The advantageous effect of electrolytes on BPA adsorption was previously  
\begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**Temperature (\({}^{\circ}\)C)** & \multicolumn{2}{c}{**Langmuir Model**} & \multicolumn{3}{c}{**Freundlich Model**} \\ \hline Adsorbents & \(K_{L}\) & \(Q_{m}\) & \(R^{2}\) & \(K_{F}\) & \(n\) & \(R^{2}\) \\ Mag-PVP & 0.09 & 115.87 & 0.94 & 16.20 & 2.09 & 0.93 \\ Fe\({}_{3}\)O\({}_{4}\) & 0.22 & 42.05 & 0.94 & 11.27 & 2.67 & 0.74 \\ nanospheres & & & & & & \\ \hline \hline \end{tabular}
\end{table}
Table 1: Parameters of the Langmuir and Freundlich models for the adsorption of BPA on Mag-PVP and Fe\({}_{3}\)O\({}_{4}\) nanospheres at 20 \({}^{\circ}\)C. \(K_{L}\) = [PERSON] binding constant. \(Q_{m}\) = maximum adsorption capacity of the polymers. \(K_{F}\) = Freundlich constant. \(n\) = an indication of the favorableness of the adsorbent/adsorbate interaction.  
Figure 5: (a) Adsorption equilibrium for bisphenol A (BPA) on Fe\({}_{3}\)O\({}_{4}\) nanospheres and Mag-PVP and their model fitting by Langmuir and Freundlich isotherm models (adsorbent dosage = 0.2 g/L; agitation speed = 150 rpm; shaking time = 4 h; temperature = 20 \({}^{\circ}\)C); (b) The effect of contact time on the adsorption of BPA by Mag-PVP (adsorbent dosage = 0.2 g/L; agitation speed = 150 rpm; temperature = 20 \({}^{\circ}\)C; BPA concentration = 10 mg/L). Error bars represent standard deviation values from three replicates. \(Q_{e}\) = equilibrium adsorption capacity. \(C_{e}\) = equilibrium concentration of BPA.  
reported for active carbon and graphene [8; 13]. The amount of BPA adsorbed decreased rapidly in the presence of CaCl\({}_{2}\) (Figure 6b). The observed phenomenon was interpreted in terms of the ability of Ca\({}^{2+}\) to form a calcium-PVP complex, resulting in the adsorption sites of Mag-PVP being occupied by Ca\({}^{2+}\). Similar findings have been reported in previous studies [37]. Humic acid (HA) is naturally occurring macromolecule in organic matter, which is ubiquitous in the natural environment. It has potential effects on the sorption of chemicals to sorbents [38; 39]. The adsorption effect by HA were shown in Figure 6c. When the concentrations of HA were varied from 0-40 mg/L, the adsorption capacity of BPA by Mag-PVP decreased due to the competitive adsorption between HA and BPA on Mag-PVP. The functional groups contained in HA can interact with Mag-PVP, occupying its adsorptive site. When the concentration of HA exceeded 40 mg/L, there was little change in the adsorption capacity, indicating that the adsorptive site of Mag-PVP for adsorbing HA was completely occupied.