Nanofiltration: Role of the electrolyte and pH on Desal DK performances

Nanofiltration through Desal DK polyamide membranes is considered. The membrane separation efficiency is determined through permeation experiments with NaCl-water solutions as well as with CaCl2-water solutions. Membrane characterization is completed by introducing a wide study on proton rejection properties. Salt rejections and proton rejections are measured in a wide range of salt concentrations and pH values in the feed side, in the pressure range from 3 to 30 bar, at room temperature. The role of the electrolyte type as well as the influence of the feed pH on membrane separation properties is assessed in a general way: the common aspects and the main differences are put in evidence. In the concentration range investigated, NaCl rejection always decreases as the salt concentration increases; on the contrary, CaCl2 rejection goes through a maximum value as the salt concentration increases. For both solutions, salt rejection is greatly affected by the feed pH, and proton rejection remarkably depends on operative conditions (feed pH and pressure). Salt and proton rejections are dependent upon the membrane charge (value and sign) existing at the corresponding conditions. On the other hand, the membrane charge is greatly affected by the electrolyte type and by the feed pH. All the behaviors observed can be simply explained case by case taking into account of the membrane charge behavior. In the case of CaCl2-water solutions, the electrolyte concentration is the key parameter in determining the amphoteric behavior of the membrane.