Spectroscopic study of MA-41P and MK-40 membranes in electromembrane purification of process solutions containing cobalt, copper, and cadmium ions

Infrared (IR) spectra were obtained for the surface layer of heterogeneous membranes — cation-exchange MK-40 and anionexchange MA-41P — widely used in electromembrane processes. Spectra were recorded for air-dry, statically water-saturated, and operational (dynamically water-saturated) membrane samples. Dynamic water saturation was achieved during the electrodeionization purification of a solution containing cobalt, copper, and cadmium ions. Water saturation was found to increase the intensity and bandwidth of the absorption band at ν = 3000÷3700 cm –1 , corresponding to the OH stretching vibration region. The appearance of an additional peak at ν ≈ 3287 cm –1 was attributed to the formation of stronger hydrogen bonds in the membrane pore space. The absence of shifts in the absorption bands corresponding to the membrane matrix components under air-dry, statically, and dynamically saturated conditions indicates their chemical stability. In the MA-41P membrane, after use in electrodeionization, changes were observed in both the intensity and position of absorption peaks in the ν  1220÷1000 cm –1 region, associated with the functional groups of the anion exchanger. The observed spectral changes were evaluated by calculating the normalized peak intensities of the absorption bands. It was shown that in the dynamically water-saturated state, both MK-40 and MA-41P membranes exhibit a reduction in the amount of weakly bound (“free”) water and the formation of stronger hydrogen bonds. The results of optical density calculations for characteristic polyethylene absorption bands — the main component of the membrane matrix — are presented. Changes in optical density upon water saturation indicate conformational rearrangements of polyethylene macromolecules. The amounts of chemically unbound solute components retained within the membrane volume were quantified; these species do not affect the membranes’ chemical stabiliy or operational performance.