Influence of water, H₂O₂, H₂SO₄, and NaOH filtration on the surface characteristics of a graphene oxide-iron (GO-Fe) membrane

Highlights

• A GO-Fe TFC membrane was prepared by adding divalent Fe for stability enhancement.

• H₂O filtration exhibited a relatively higher flux due to membrane hydration.

• H₂SO₄ filtration exhibited a relatively lower flux by destructing the membrane surface.

• H₂O₂ filtration increased the rate of oxygenation on the membrane surface.

• NaOH filtration increased the rate of protonation on the membrane surface.

Abstract

Graphene oxide (GO)-based membranes formed by stacked nano-scale GO layers can be used to separate contaminants from the water via filtration. The membrane surface and permeation performance are altered by different filtration solutions. This study presented a facile vacuum-assisted approach of preparing a GO-Fe membrane by bonding between multivalent Fe and oxygenated functional groups on the membrane surface. Different solutions including water (pH 7.0), hydrogen peroxide (H₂O₂, pH 4.5), sulfuric acid (H₂SO₄, pH 3.0), and sodium hydroxide (NaOH, pH 9.7) were used. The impact of filtration on the membrane surface characteristics and permeation flux was investigated. In the results, filtration using water and H₂SO₄ exhibited the highest and lowest fluxes, respectively. Water filtration increased the O-H component of the membrane by hydration and improved the permeation flux (109.5 L m⁻²h⁻¹ bar⁻¹). However, this also potentially increased the interlayer distance of the membrane and reduced its molecule rejection rates. H₂SO₄ filtration destructed the membrane surface by enhancing the release of Fe. Filtration with H₂O₂ and NaOH maintained moderated spacing and permeation fluxes of the membrane by increasing the rates of oxygenation and protonation, respectively. The findings provide insights into how different types of solutions affect the stability and surface characteristics of a GO-based membrane modified by a multivalent cation.

Introduction

Water scarcity has grown globally. Water reclamation has become inevitable due to the need of developing alternative water resources. Membrane filtration is a promising technology in the applications of wastewater reclamation and reuse [1], [2]. Hube et al. indicated the great potential of membrane technology in wastewater treatment given its fine treated water quality, resource recovery, and sustainable operation, particularly when biological treatment is infeasible [3]. Besides conventional pollutants, rejection of emerging contaminants including pharmaceuticals and endocrine disruptors by membranes such as nanofiltration was reported [4]. However, the variability in water quality affects the membrane performance. The removal efficiencies of organic micro-pollutants in waters by nanofiltration was known to be influenced by the hydrophobicity of the membrane and those compounds [5]. Increasing hydrophobicity of membrane generally leads to more vulnerability to fouling due to hydrophobic interactions between the membrane surface and the hydrophobic foulants [6]. Hydrophobic acidic and neutral fractions in waters played critical roles in the fouling of the ultrafiltration membrane [7].

Graphene, a two-dimensional carbon allotrope with sp² hybridized atom orbital, exhibits excellent mechanical, thermal, and electrical properties [8], [9]. Owing to the oxygenated groups including hydroxyl, epoxy, carbonyl, and carboxyl groups on the basal planes and edges, graphene oxide (GO) typically has a hydrophilic nature and high specific surface area by retaining the layer structure of graphene with a larger and irregular inter-planar spacing [10], [11]. Studies have shown the highly modifiable nanostructures of GO for the improvement of certain physicochemical properties such as hydrophilicity and catalytic activity [12], [13]. For example, the preparation of GO with iron (II, III) oxide (Fe₃O₄) has been reported to be capable of adjusting the inter-planar spacing of GO layers and enhancing the catalytic activity for adsorption and degradation of organic compounds [14], [15]. The implementation of GO and its derivatives decorated with different organic and inorganic compounds for wastewater treatment have been intensively studied [16], [17], [18].

Studies have reported the preparation and characterization of GO-based membrane materials [19], [20]. These membranes retain the physicochemical properties of GO and present the flexibility and stiffness by an interlocking-tile arrangement of the nano-scale GO layers. Because pristine GO films readily disintegrate in water due to electrostatic repulsion because the sheets become negatively charged on hydration, multivalent cationic metals are used to crosslink and stable the films [21]. Literature works have coated GO on a suitable substrate to make sure that GO is attached perfectly to the support and not leached out during filtration. Karkooti et al. incorporated different GO derivatives into a polyethersulfone (PES) matrix by a non-solvent induced phase separation method to fabricate nanocomposite membranes [22]. Rastgar et al. fabricated a graphene laminate membrane by depositing a thin layer of graphene onto the PES support [23]. The research group further successfully prepared membranes by laminating polyaniline (PANI)-reduced graphene oxide (rGO) on PES support by pressure-assisted technique [24]. Studies have also indicated that the properties and performance of GO-based membranes are affected by the GO synthesis methods. Sali et al. revealed that the hydrophilicity and porosity of a polysulfone-GO membrane were changed by using different GO preparation methods [25]. The Hummers method is a chemical process commonly used as a rapid and reliable method of producing a large quantity of GO within time limitations [9], [26].

The single-layer thickness and channel size of GO membranes were estimated at 0.34–0.40 and 0.33–0.46 nm, respectively [27]. Varying driving forces in different self-assembly methods including the pressure-, vacuum-, and evaporation-assisted techniques induced slightly different GO assembly structures [28]. Nano-scale pores in a GO lattice and interlayer space between GO sheets provide an opportunity for potential applications such as desalination and pollutant rejection in aqueous environments [29]. Wei et al. discussed several flow enhancement mechanisms through the porous structures of GO membranes considering their promising potentials in filtration and separation applications [30]. The use of GO membranes for selective ion penetration and water purification was demonstrated [31]. Owing to its porous structure and reduced channel length, a GO ultrafiltration membrane that contained nano-channels with a size distribution of 3–5 nm exhibited superior separation of organic and inorganic compounds [32]. Pharmaceuticals represent chemicals of emerging concern to the public because of their occurrences in surface water [33] and groundwater [34] as well as their impacts on water supplies [35]. Li et al. demonstrated that an ultrathin GO membrane fabricated via pressure-assisted filtration exhibited effective separation capabilities of pharmaceuticals by size exclusion and solute-membrane affinity with good stability and high solvent fluxes [36].

Abundant oxygen functional groups on the basal planes and the carboxylic groups on the edges are one critical characteristic of GO sheets. These functional groups can bind to divalent metal ions (e.g., Ca²⁺, Cu²⁺, and Mg²⁺) to significantly enhance mechanical stiffness and fracture strength of a GO membrane consisting of individual GO sheets [37], [38]. In our previous studies, GO-Fe₃O₄ particles that contained both Fe²⁺ and Fe³⁺ were synthesized for adsorption and catalytic oxidation of organic pollutants in water [14], [15]. In this study, we have tried to fabricate a GO-Fe membrane by adding Fe²⁺ with a certain ratio during vacuum-assisted preparation of the pristine GO membrane. The objective of this study was to investigate the impacts of filtration with different solutions on the surface characteristics of this GO-Fe membrane by using the scanning electron microscope (SEM), energy-dispersive X-ray spectroscopy (EDS), X-ray diffraction (XRD), Raman spectroscopy, thermogravimetric analysis (TGA), Fourier-transform infrared spectroscopy (FTIR), and X-ray photoelectron spectroscopy (XPS). The solutions of interest included deionized water (pH 7.0), oxidizing agent (H₂O₂, pH 4.5), acidic H₂SO₄ (pH 3.0), and caustic base (NaOH, pH 9.7). The fluxes of different solutions were measured. The Zeta Potential and water contact angles were analyzed. The results provided insight into how different types of solutions affect the surface structures and stability of a GO membrane modified by divalent metal ions. Additionally, chlorpheniramine, a pharmaceutical frequently used for allergies and respiratory infections and widely identified in municipal wastewater effluents [15], [39], was used to test the separation performance and potential effects on the membrane.