Salt and temperature dependent permeability changes of a NF membrane

Abstract

Nanofiltration at increased temperatures enhances the membrane permeability, which correlates well with an Arrhenius equation up to a certain temperature [1]. Several studies on polyamide nanofiltration membranes have reported both increasing and decreasing flux during nanofiltration. The changes in membrane permeability have been seen after alkaline cleaning, during nanofiltration of salt solutions and during pH changes. A decrease in membrane permeability due to the presence of salt has been related to membrane swelling by AFM measurements [2], where a decrease in permeability is related to decreased swelling of the skin layer of polyamide. Opposite results have been seen in another study [3] where salt increased the water permeability, which was related to an increase in membrane thickness and pore-radius by membrane modelling. It seems that these changes are very hard to predict and that there is no uniform model that can describe these changes. However, such effects can have a large effect on the separation characteristics and it is particularly interesting to study them at elevated temperatures, since temperature changes all the physical properties to some extent [2]. It is also important to consider changes in membrane characteristics during fouling studies, so that flux decline due to fouling can be differentiated from positive or negative flux changes due to membrane-ion interactions. An increased understanding of the influence of salt and temperature will contribute to more accurate and complex models.