Changing of mechanisms of self-diffusion of water molecules under nanofiltration of electrolyte solution Original Research Article

It is known, that volume flux of membranes depends on their chemical nature and structural-porous characteristics. There are data on change of a condition and mobility of water molecules on membrane surface. Hence, it is possible to assume, that as a result of the complex processes occuring on a surface of membranes and in volume of a solution during treatment, the state and mechanisms of self-diffusion of water molecules changes in comparison with the same solution, but not taking place through a membrane. To test this assumption, characteristics of mechanisms of self-diffusion of water molecules in a water solution of calcium chloride before and after membrane treatment have been quantitatively obtained. These characteristics have been calculated by the spectra of quasi-elastic slow neutron scattering (QENS) measured experimentally. As characteristics are valued: total coefficient of self-diffusion (D), the contribution to it from collective (Dl) and single-particle motions (Df), and also molecule lifetime at vibrational state (τ0). For usual water according to QENS data: D = 2.23·10−9 m2s−1; Dl = 0.46 × 10−9 m2s−1; Df = 1.77 × 10−9 m2s−1; τ0 = 2.8 × 10−12 s. A nanofiltration membrane OPMN–KM3 has been used for study. Initial concentration of calcium chloride in a solution was 1.311 mmol/l. Concentration of this substance in a filtrate was 0.836 mmol/l. As a result of the complicated processes occuring during filtration of the solution through the membrane it has been established that mechanisms of water molecules self-diffusion changed in comparison with a solution of the same concentration which was not under the membrane treatment.