# Sorption of Organic Electrolytes and Surfactants from Natural Waters by Heterogeneous Membranes
## 3 Results and Discussion
### Exchange Capacity and Sorption of Organic Ions
It was found that the sorption isotherms are of the Langmuir nature: sorption increases with increasing concentration of the substance and reaches saturation at high concentrations of the substance (Figures 13, 15 and 17). Figure 16: Sorption kinetics of NaNS by membrane MA-40. Figure 17: Sorption isotherms of CPCl by membrane MK-40. Figure 18: Sorption kinetics of CPCl by membrane MK-40. In Figure 19, data on sorption isotherms are compared, and Figure 20 shows the kinetics of establishing sorption equilibrium in coordinates of sorption in fractions of the maximum for a given ion \(\theta=X/X_{max}\). When comparing the data, it was found that the rate of establishing sorption equilibrium depends on the size of the organic radical: with increasing size of the radical, the rate increases. The sorption limit for NaBS is reached in 2 days, for NaNS the sorption equilibrium is established in 14 days, and for a typical surfactant CPCI it is reached in 35 days. The data obtained indicate a strong influence of the steric factor and specific action on the membrane sorption of organic ions (especially in the MK-CP\({}^{+}\) system). The influence of this factor increases with the elongation of the hydrocarbon chain and the branching of organic molecules. It was found: that the rate of establishing sorption equilibrium also depends on the concentration of the substance in the solution: at high concentrations, it is established faster than at small ones. The linearity of the initial portion of the \(g\)-\(\sqrt{\tau}\) curves for \(10^{-2}\) solutions of NaBS and NaNS indicates that the limiting stage is diffusion in the membrane itself, i.e., helium kinetics occur. By comparing the sorption isotherms (Figure 19), it was found that the sorption value increases with the size of the organic radical: thus, the sorption of NS\({}^{-}\) ions on the MA-40 membrane is greater than for  
Figure 19: Sorption isotherms of organic ions by ion-exchange membranes. Figure 20: Sorption kinetics of organic ions by ion-exchange membranes. BS\({}^{-}\). However, the maximum sorption of CP\({}^{+}\) ion by the MK-40 membrane (2.1 mg-equivv/g) is slightly less than the membrane exchange capacity (2.35 mg-equivv/g against 10\({}^{-2}\) M NaCl), which indicates steric hindrance for the long-chain CP\({}^{+}\) ion. Figure 20 shows dependence of the fraction of filling with an organic ion of the ion exchanger in the membrane on the sorption time. A comparison of the sorption values of BS\({}^{-}\) and NS\({}^{-}\) in the neutral pH range with the values of the exchange capacity of the MA-40 membrane in 10\({}^{-2}\) M NaCl (Figure 21) shows the significant specificity of BS\({}^{-}\) and especially NS\({}^{-}\), compared with Cl\({}^{-}\): \(g\) (Cl\({}^{-}\)) = 0.8 mg-equivv/g, which is significantly less than the sorption of BS\({}^{-}\) and NS\({}^{-}\) (1.4 mg-equivv/g and 2.8 mg-equivv/g, respectively). It can also be seen from Figure 21 that the X values for BS\({}^{-}\) in the neutral pH range obtained by the two methods (potentiometric and by decreasing concentration) coincide, indicating the ion-exchange nature of sorption and the equivalence of the inorganic anion-organic anion exchange. Based on the Langmuir nature of sorption, the values of the limit sorption X\({}_{\rm max}\) and the equilibrium constants of the study processes were estimated: X\({}_{\rm max}\) (BS\({}^{-}\)) = 1.4 mg equiv/g and X\({}_{\rm max}\) (NS\({}^{-}\)) = 2.9 mg-equiv/g. It was found that the calculated and obtained experimental data were almost identical, which indicates the establishment of equilibrium. The greater difference between the calculated maximum sorption value for CP\({}^{+}\) (X\({}_{\rm max}\) (BS\({}^{-}\)) = 2.35 mg-equiv/g) and the experimentally determined value (X\({}_{\rm max}\) (BS\({}^{-}\)) = 2.1 mg-equiv/g) is apparently due to steric hindrance. The equilibrium constants were determined for BS\({}^{-}\)-3, for NS\({}^{-}\)-8 on the MA-40, and for CP\({}^{+}\)-28 on the MK-40.