Antony Prince James Selvaraj

There has been a growing interest in water purification by graphene oxide (GO) laminate membranes due to their exceptional hydrophilicity, high throughput, and extraordinary separation performance originating from their two-dimensional and well-defined nanostructure. However, the swelling and stability in an aqueous environment are areas of concern for the GO laminate membranes. Here, a novel methylimidazolium ionic liquid-reduced GO (mimG)-assembled GO laminate membrane (mimG-GO) with remarkable stability was fabricated by a vacuum-assisted strategy for water purification. Methylimidazolium-based ionic liquid-reduced graphene oxide (mimG) was prepared by a facile nucleophilic ring-opening mechanism. Fabricated membranes were thoroughly characterized for stability, structural, permeance, and rejection properties in an aqueous environment. A combination of cationic mimG and GO nanosheets improves membrane stability in the aqueous environment via cation-π interactions and creates nanofluidic channels for facile water transport while yielding significant enhancement in the salt and dye separation performance. The pore size and the number of nanofluidic channels were precisely controlled via material deposition and laminate thickness to remove salts from water. The mimG-GO laminate membrane containing 72.2 mg m^-2 deposition showed a permeance of 14.9 LMH bar^-1, 50% higher than 9.7 LMH bar^-1 of the neat GO laminate membrane, in addition to an increase in Na₂SO₄ salt rejection from 46.6 to 77.4%, overcoming the flux-rejection trade-off. The mimG-GO laminate membrane also rejected various anionic dyes (i.e., 99.9% for direct red 80 (DR 80), 96.8% for reactive black 5 (RB 5), and 91.4% for methyl orange (MO)). The mimG-GO laminate membrane containing 361.0 mg m^-2 deposition showed the highest rejection for Na₂SO₄ (92.1%) and 99.9% rejection for DR 80, 99.0% rejection for RB 5, and 98.1% rejection for MO dyes keeping a flux of 2.6 LMH bar^-1. Partial reduction and covalent grafting of ionic liquid moieties on GO helped to enhance the cation-π interaction between GO laminates, which showed enhanced stability, frictionless water transport, with high salt and dye rejection. Moreover, a simultaneous improvement in water permeance and solute rejection reveals the great potential of ionic liquid-functionalized GO laminate membranes for water-based applications.

In this study, polyelectrolyte assembled functionalized graphene oxide (PE-GO) membranes were fabricated through a one-step charge facilitated deposition method for high performance dye/salt separation. According to the intercalation of polydopamine (PDA) and (ionic liquid) IL functional moieties into the GO membranes, the pore size of the resulted PE-pGO and PE-iGO membrane increased from 2.69 nm to 4.13 nm and 6.54 nm, respectively. Correspondingly, a pure water flux of 13.8 ± 2.2, 36.7 ± 3.4, and 52.1 ± 6.7 L m⁻¹ h⁻¹ bar⁻¹ was achieved for PE-GO, PE-pGO and PE-iGO membrane, respectively. PE-iGO membrane with the largest pore size could be operated with significant water permeability (28.3 to 38.3 L m⁻¹ h⁻¹ bar⁻¹) at a low operating pressure range of 0.5–2 bar (dye concentration = 100 ppm, salt concentration = 5 g/L). More importantly, functionalities introduced to the GO nanosheets are found to impact the dye adsorption to the membrane surface. The IL intercalation promotes the elution of dye molecules from the IL moieties at elevated pH, therefore enhancing the efficiency of alkaline washing of the membrane. By contrast, the intercalation of PDA weakens such efficiency due to its strong adhesion force to the dye molecules even at the alkaline condition.

Charge-gated channels are nature’s solutions for transport of water molecules and ions through aquaporins in biological membranes while excluding undesired substances. The same mechanism has good potentials to be adopted in pressure or electrically driven membrane separation processes. Herein, we report highly charged nanochannels created in polyelectrolyte (PE) intercalated amine reduced graphene oxide membrane (PE@ArGO membrane). The PE@ArGO membrane, with a rejection layer of ∼160 nm in thickness, features a laminate structure and a smooth top surface of a low roughness (typically ∼17.2 nm). Further, a modified PE@ArGO membrane (mPE@ArGO membrane) was developed in situ using free chlorine scavenging post-treatment method, which was designed to alter the charge while keeping alteration to the layered structure minimal. The surface charge of the PE@ArGO and mPE@ArGO membrane was +4.37 and −4.28 mC/m² respectively. In pressure driven processes, the pure water permeability for PE@ArGO and mPE@ArGO was 2.9 and 10.8 L m^–2 h^–1 bar^–1, respectively. Salt rejection is highly dependent on the charge density of the membrane surface, the valence of the co-ions and the size of ions in hydrated form. For example, in the positively charged PE@ArGO membranes, the rejection of the salts follows the order of: R(MgCl₂), 93.0% > R(NaCl), 88.2% ≈ R(MgSO₄), 88.1% > R(Na₂SO₄), 65.1%; while in the negatively charged mPE@ArGO membranes, the rejection of the salts follows the order of: R(Na₂SO₄), 90.3% > R(NaCl), 85.4% > R(MgSO₄), 68.3% > R(MgCl₂), 42.9%. To the best knowledge of the authors, this is the first study to report graphene oxide based membranes (GOBMs) with high density positive/negative charge gated ion transport behavior. What’s more, the high rejection rate along with high water permeability of the PE@ArGO and mPE@ArGO membranes has not been achieved by other types of GOBMs.

Bio-fouling is a serious problem in many membrane-based separation processes for water and wastewater treatment. Current state of the art methods to overcome this are to modify the membranes with either hydrophilic additives or with an antibacterial compound. In this study, we propose and practise a novel concept to prevent bio-fouling by developing a killing and self-cleaning membrane surface incorporating antibacterial silver nanoparticles and highly hydrophilic negatively charged carboxylic and amine functional groups. The innovative surface chemistry helps to reduce the contact angle of the novel membrane by at least a 48% and increase the pure water flux by 39.4% compared to the control membrane. The flux drop for the novel membrane is also lower (16.3% of the initial flux) than the control membrane (55.3% of the initial flux) during the long term experiments with protein solution. Moreover, the novel membrane continues to exhibit inhibition to microbes even after 1320[emsp14]min of protein filtration. Synthesis of self-cleaning ultrafiltration membrane with long lasting properties opens up a viable solution for bio-fouling in ultrafiltration application for wastewater purification.