Rejuvenation of reverse osmosis polyamide membranes degraded by chlorine in the presence of ferric chloride

Abstract

Reverse osmosis (RO) polyamide membranes are widely used for water treatment applications. However, certain processes such as wastewater reuse require regular membrane cleaning and disinfection with oxidants, which can lead to early membrane degradation. Furthermore, some metal ions present in the water can act as a catalyst for further accelerating the degradation. This early degradation of RO membranes poses significant challenges, resulting in operational inefficiencies, early disposal of membranes, and elevated operational costs. Fortunately, there is the possibility of recovering some part of this performance loss by means of chemical treatment through rejuvenating agents. This study aims to investigate the effectiveness of a commercially available rejuvenating agent containing tannic acid for restoring salt rejection and permeability parameters on degraded thin-film polyamide membranes. The membranes were first degraded using 250 ​ppm sodium hypochlorite (NaOCl) and 0.05 ​ppm ferric chloride (FeCl₃) at various pH levels (pH ​= ​4, 7 and 9). After applying the rejuvenation treatment to the degraded membranes, the efficiency of the rejuvenating agent was determined based on the improvement achieved for performance testing with respect to salt rejection and permeability. Analytical characterization of the membranes was carried out with Fourier Transform Infrared Spectroscopy-Attenuated Total Reflection (FTIR-ATR). It was found that the chlorine degradation of membranes was accelerated in the presence of FeCl₃ at all studied pH levels but more prominently in the acidic region. This acceleration effect was attributed to the formation of ($·\text{OH}$, $·\text{OCl}$) radicals. Under the conditions studied in this work, rejuvenating agent treatment effectively enhanced the salt rejection capability of the degraded membranes but was unable to restore the permeate flux.