# Sorption of Organic Electrolytes and Surfactants from Natural Waters by Heterogeneous Membranes
## 3 Results and Discussion  
### Exchange Capacity and Sorption of Organic Ions  
Investigation on the exchange capacity (g) of the MK-40 and MA-40 industrial heterogeneous membranes, used in electrodialysis apparatus for desalting water, was carried out in the solutions of inorganic electrolytes NaCl, MgCl2, and Na2 SO4, which are the main components of Black Sea water. The g value was determined by potentiometric titration (the method of separate samples): cationic--in the solutions of NaCl and MgCl2 and anionic--in the solutions of NaCl and Na2 SO4. The results of the study are presented in Figures 11 and 12 in the form of the dependence of g (mg--equiv/g of dry membrane) on the pH of the equilibrium solution. Figure 11: Dependence of membrane MK-40 exchange capacity on pH and composition of solutions. As can be seen from the data presented, the exchange capacity of the MK-40 cation exchange membranes made of the KU-2 strongly acidic cation-exchange resin at pH \(>\) 3 is almost constant and slightly increases with increasing concentration of the test solution. The values of \(g\), obtained for singly and doubly-charged ions Na\({}^{+}\), Mg\({}^{2+}\), are also close to each other in accordance with the theoretical aspects of strongly acidic ion exchangers. The values of g for the MK-40 membranes are 2.6 mg-equiv/g and are in good agreement with the values shown in Figure 12 at pH \(>\) 3. For the MA-40 membrane, the nature of the g-pH dependence is different: there is a significant decrease in the exchange capacity upon transition from the acidic to alkaline region, in accordance with the theoretical aspects of the g dependence for polyfunctional ion exchanger, and in the neutral pH region, it decreases by 3-5 times compared to the maximum measured. According to GOST 17552-72, the exchange capacity of the MA-40 is equal to 3.7 mg-equiv/g and is in good agreement with the data obtained. As for the cation-exchange membrane, there is a slight increase in the exchange capacity with increasing electrolyte concentration, as well as with the transition to doubly-charged anions (SO\({}_{4}\)\({}^{2-}\)) at pH \(<\) 4. From the data obtained by the formulas \(m_{R}=\frac{100-W}{W}\frac{M_{0}}{M}\ g\) and \(X=\rho\ \frac{100-W}{100}\ \frac{M_{0}}{M}\ g\), where \(M_{0}\) is the weight of the ion exchanger in the initial state (in H\({}^{+}\) or Cl\({}^{-}\) form, the ion exchanger is washed and dried), M is the weight of the dry ion exchanger in the test form, \(\rho\) and W are the density and water content of the ion exchanger, the molarity m\({}_{\rm R}\) and the concentration of fixed ions (X) in the membranes were calculated. The m\({}_{\rm R}\) value in the MA-40 membrane was 4.4 for 0.02 N NaCl and 4.3 mg-equiv/g of solvent for 0.03 N Na\({}_{2}\)SO\({}_{4}\), and for the MK-40 membrane in 0.3 N NaCl and 0.06 N MgCl2, it was 4.2 mg-equiv/g. The concentration of fixed ions for the MK-40 in the NaCl and MgCl2 solutions was 1.8: in the BaCl2 solution--1.5 mg-equiv/g of dry membrane and for the MA-40 in the NaCl solutions1.7, in Na2 SO4--1.9 mg-equiv/g. The sorption of organic BS\({}^{-}\) and NS\({}^{-}\) ions by the MA-40 membrane was studied in the concentration range of 10\({}^{-}\)4-10\({}^{-2}\) M and for CP+ by the MK-40 membrane in the concentration range of 10\({}^{-}\)4-10\({}^{-3}\) M. The choice of the upper concentration limit was determined by the possibility of determining sorption for BS\({}^{-}\) and NS\({}^{-}\) by the change in concentration in solution, and for CPCl--by the value of the critical micelle-formation concentration (CMC). The data obtained for individual systems are presented in Figures 13-18. Figure 12: Dependence of membrane MA-40 exchange capacity on pH and composition of solutions. Figure 14: Sorption kinetics of NaBS by membrane MA-40. Figure 13: Sorption isotherms of NaBS by membrane MA-40. Figure 15: Sorption isotherms of NaNS by membrane MA-40.