Organic Fouling and the Oxidative Cleaning Effect of Sodium Hypochlorite on Graphene Oxide Nanofiltration Membranes

Membrane fouling that inevitably results in the consistent decline of a membrane’s permeability and retention over the course of its operational lifespan is persistently reported to be the main disadvantage of membrane use.^1,2 Meanwhile, Graphene oxide (GO) has emerged as a prominent nanomaterial for both fabrication and integration into composites of membranes ranging from nanofiltration (NF) and reverse osmosis (RO) to ultrafiltration membranes primarily due to its abundant oxygenated functional groups, hydrophilicity, charge properties, and antimicrobial properties.³ These properties have been found to be ideal for improving the antifouling properties of polymeric NF and RO membranes as increased hydrophilicity results in weakened adhesion and increased repulsive forces working between the surface and foulants reduce deposition on the membrane surface.⁴ GO has been incorporated into composite membranes with the aim of improving the inherent properties and fouling resistances of membranes among which are PVDF, polyamide (PA), polysulfone and polyethersulfone. Additionally, the use of available chlorine as a pre-treatment step in water processing and its use as an oxidative cleaning agent effective against fouling composed of proteins, glucans and other organic matter⁵ has led to the development of NF and RO membranes with chlorine resistances up to doses of 1,000 ppm.h⁶ with modifications leading to resistances as high as 50,000 ppm.h⁷. Graphene oxide has also been implemented in polymeric membranes, such as polyamide membranes to protect against amide bond degradation via hydrogen bonding shielding to increase their chlorine resistance. However, little work has been done on describing the fouling mechanisms and chlorine resistance of GO films.

The fouling of GO membranes (GOM) was quantified using a resistance-in-series model that allows for each of the intrinsic membrane resistance and the resistance caused by organic adsorption, concentration polarization or the formation of a gel layer to be determined.⁸ GOM⁹ fouling by Tannic Acid, Bovine Serum Albumin and Methyl blue occurs primarily by absorbance, where hydraulic cleaning has no effect on the removal of the foulant, resulting in an increase in the resistance caused by concentration polarization. Ethanol results in an overall cleaning efficiency (OCE) of 22% for a GOM fouled with methyl blue; however, it exacerbates the compression of the gel layer for a membrane fouled with tannic acid. Cleaning with 100 ppm of sodium hypochlorite results in 98.96% and over 100% OCE by removing any resistances caused by absorbance of TA or MB to the surface of the GO, the resistance of the cleaned membrane is less than that of non-fouled membrane and is indicative of membrane instability after prolonged exposure to sodium hypochlorite. However, the BSA fouled GOM has an OCE of 62% due to the formation of a persistent gel layer. FTIR and XRD measurements of GO films exposed to 100,000 ppm.h of NaOCl show that high levels of chlorine exposure result in an interlayer spacing increase from 7.86 to 9.26Å with a corresponding increase in hydroxyl, carboxyl and epoxy functionalities. In an effort to reduce the chlorination of GO, GO was doped with graphene (G) in a ratio of 10:1 GO:G, an FTIR analysis of which revealed relatively less oxidation with fewer new functionalities resulting in an interlayer spacing increased from 8.12 to 9.24 Å. Similarly, doping with Montmorillonite (MMT) shows a similar improvement in chlorine resistance. Thus, this work aims to discuss the mechanisms by which the membrane is fouled and the effect of sodium hypochlorite on the organic foulants and the graphene oxide membrane. Additionally, doping with graphene and MMT may provide a simple method to improve chlorine resistance of GO membranes.

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