Organic Fouling and Chemical Cleaning with Sodium Hypochlorite on Graphene Oxide Nanofiltration Membranes

Membrane fouling, a phenomenon that results in the inevitable and consistent deterioration of a membrane’s performance, is reported to be the main disadvantage of the wide membrane use.^1,2 Thus, research that focuses on the antifouling properties of novel nanomaterials has become popular. One such nanomaterial is Graphene Oxide (GO) which has emerged as a material favoured for integration into composite membranes and in nanofiltration membrane fabrication due to its abundant oxygenated functional groups, hydrophilicity, charge properties, and antimicrobial properties.³ These properties have been found to be optimal for the improving the antifouling properties of the polymer matrices it is incorporated into via increased hydrophilicity, which weakens the adhesion between the membrane surface and foulant, and increased repulsive forces, that reduce the deposition of foulants on the membrane surface.⁴ GO has been incorporated into polymeric matrices include PVDF, polyamide (PA), polysulfone and polyethersulfone to improve the fouling resistance of these membranes to great effect. Additionally, the necessity of available chlorine as a pre-treatment in water processing and its oxidising potential that has been proven to be effective treating membrane 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. GO has also been used to increase the chlorine resistances of polymeric membranes by integrating it into polyamide membranes it is used to prevent amide scission by protecting these reactive sites via hydrogen boding. However, little work has been done on describing the fouling mechanisms or the chlorine resistance of GO films. Furthermore, while GO has been used to increase the chlorine resistance of other NF and RO membranes, there is little understanding of the effect that NaOCl will have on GO membranes.

The fouling of graphene oxide membranes was quantified using a resistance-in-series model that allows for an overall fouled membrane resistance to be separated into the contributions of the intrinsic membrane resistance, organic adsorption resistance, or concentration polarization or gel layer resistance .⁸ GO membranes⁹ (GOM) fouled by Tannic Acid, BSA and Methyl blue was determined to predominated by absorbance, where hydraulic cleaning had no effect on foulant removal, 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 an OCE of 98.96% for Tannic Acid and over 100% OCE by removing any resistances caused by absorbance of TA or MB to the surface of the GO, an efficiency greater than 100% indicates damage to the membrane due to the implication of the resistance of the cleaned membrane being less than the non-fouled membrane. BSA fouled GOM has an OCE of 62.69% due to the formation of a persistent gel layer. GO films were exposed to doses of 100,000 ppm.h of NaOCl show that 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 fewer oxygen functionalities with a smaller increase in the interlayer spacing from 8.12 to 9.24 Å. Similar chlorine resistances were obtained by doping GO with Montmorillonite (MMT). Thus, this work aims to discuss the mechanisms by which the GOM is fouled and the oxidative cleaning effect of sodium hypochlorite on organic foulants and graphene oxide membrane. Additionally, doping with graphene and MMT may provide a simple means to improve chlorine resistance of GO membranes.

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