Effect of extended exposure to alkaline cleaning chemicals on performance of polyamide (PA) nanofiltration membranes

• Exposure to alkaline cleaning induces both slow and fast changes in PA NF membranes.
• Together the slow and fast changes increase permeability and decrease retention.
• The change was found to be influenced by the exposure temperature.
• Alkali induced performance changes were partly recovered after long recovery time.
• The simple rate laws were not sufficient to describe the changes caused at 70 °C.

This study aimed to increase the knowledge on the influence of alkaline cleaning on performance of polyamide based nanofiltration membranes. In the experiments the Desal-5 DL, XN45 and NF270 membranes were exposed to alkaline conditions at temperatures of 40, 50, 60 and 70 °C and the influence of the extended alkaline exposure to the membrane permeability, glucose and MgSO4 retention was measured. The characteristic features of the experimental kinetic data for permeability were a rapid increase in permeability after a short time exposure and a subsequent steady increase when the exposing time was continued while the characteristic features for the retention data were a rapid decrease in retentions of glucose and MgSO4 after a short time exposure and a subsequent steady decrease when the exposing time was continued. The model developed in this study described well the both rapid and slow changes and it can be used to predict the performance of the tested membranes at the temperature range between 40 and 60 °C. The increase in exposure temperature accelerated the changes in membrane performance. The alkali induced changes caused at the 70 °C exposure could not be described with the simple rate law. However, only minor changes in chemical structure of the membranes were noticed in IR spectra after the exposure to alkaline conditions at 70 °C. This indicates that changes observed in permeability and retention could mainly be linked to conformational changes occurring in the polymeric matrix of the membrane skin layers. It was also shown that after a relatively long recovery time (144.5 h) the alkali induced changes in membrane performances were at least partly recovered.