Self-cleaning properties of L-Histidine doped TiO₂-CdS/PES nanocomposite membrane: Fabrication, characterization and performance

Highlights

• The self-cleaning membrane was successfully fabricated with addition of L-Hisitidine-TiO₂-ZnO nanophotocatalyst.

• The nanocomposite membranes exhibited high performance in treating POME compared to neat PES membrane.

• Elevated permeate flux observed for nanocomposite membranes.

• Improved FRR, Rir and Rt value of blended membrane observed compared to unfilled PES.

• The hydrophilic properties and photocatalytic nature of NPs lead to high antifouling properties.

Abstract

The polyethersulfone (PES) membrane incorporated with L-Histidine doped TiO₂-CdS photocatalytic nanocomposite is fabricated to improve the membrane performance and its antifouling properties. The L-Histidine doped TiO₂-CdS/PES nanofiltration membrane was characterized by scanning electron microscopy (SEM), contact angle and atomic force analysis (AFM) analyses. SEM images and contact angle results are demonstrated that membrane porosity and its hydrophilic properties increase with adding of L-Histidine doped TiO₂-CdS photocatalytic nanocomposite. Moreover, AFM images of the modified membranes indicate favorable changes in surface roughness resulting in the improvement of antifouling properties. The pure water flux, flux recovery ratio (FRR) during filtration of milk powder and activated sludge suspension were also measured in a dead end cell. Increasing membrane permeability and FRR value approved its effective role of the added nanoparticles into the PES matrix. The M₃ (0.5 wt% of NPs) membrane is selected as an optimal nanocomposite membrane which exhibits a much higher pure water flux (42.1 kg/m² h) and FRR value (90 and 97% during filtration of milk powder and activated sludge suspension, respectively). The photocatalytic capability and antifouling properties of the M₃ membrane as an optimum membrane were also investigated in a cross flow cell under continuous visible light irradiation. As a result, the high permeation flux (34.7 kg/m² h), COD removal (100%) and FRR value (99%) was achieved during filtration of biologically treated palm oil mill effluent (POME) with COD concentration of 1000 mg/L at feed flow rate of 150 L/h and pressure of 5 bar. Effect of feed flow rate (50 and 150 L/h) and COD concentration (1000, 3000 and 5000 mg/L) on the membrane performance and its fouling resistance was also evaluated. Increasing feed concentration shows a negative effect on the membrane performance while feed flow rate increases separation performance and self-cleaning properties of the M₃ membrane.

Introduction

Industrial wastewaters containing toxic and xenobiotic organic compounds are the largest challenging for the future reclamation and reuse of water in a worldwide [1]. Different treatment methods such as anaerobic and aerobic digestion processes [2], [3], [4], [5], [6], photocatalytic oxidation [7], [8], coagulation and flocculation processes [9], [10], adsorption [11] and membrane filteration [12] have been applied to treat various wastewaters with different compositions. Other than the treatment methods mentioned, natural treatment systems are cost-effective processes that have been also used for treatment of various industrial wastewaters. Due to their relatively long retention time required for treatment process, mainly suffer from low reactor volume yield [13], [14]. The use of membrane separation technology is widely expanded to treat polluted water due to its advantages such as lack of phase change, low energy utilization, good efficiency, simplicity of operation, etc. [15], [16]. Polymeric materials are widely used in the membrane preparation due to low cost, easy control of pore forming [17]. However, membrane fouling was formed due to concentration polarization or pore blockage which is extensively limited application of membrane process. Membrane fouling decreases permeation flux and separation performance [18], [19]. Also, membrane technology cannot degrade pollutants only transfer them from one phase to another. Therefore, it is necessary to remove the organic pollutants from water and wastewaters [20].

Metal nanoparticles especially TiO₂ nanoparticles (NPs) as an additive agent in the matrix of polymeric membranes has attracted most interest due to their benefits of the composite membrane (inorganic–organic composite) [21], [22]. Addition of the TiO₂ NPs into polymeric membrane structure improves porosity, tensile strength, permeation flux, separation performance, hydrophilicity and antifouling properties [23], [24], [25]. TiO₂ composite membrane as a single unit not only has a physical separation of membrane process but also degrades organic compound_s and improves antibacterial properties by a TiO₂ photocatalysis role [21]. To active TiO₂ photocatalyst, the composite membrane is exposed to UV irradiation, however the polymeric materials are degraded under long time irradiation [26]. The use of visible light or solar light instead of UV light is a suggestion to reduce the risk of polymer degradation under UV light irradiation.

TiO₂ is active under UVA irradiation due to its wide band gap. Also, the photoformed electron and hole pairs during TiO₂ photocatalysis was easily recombined, so quantum efficiency of TiO₂ photocatalyst was reduced [27]. The doping (metal, nonmetal and multi doped-TiO₂), deposition of noble metal, organic surface modification, coupling with narrow semiconductor and dye sanitization was applied to produce visible active TiO₂ [28], [29].

TiO₂ codoped with different elements is one of the favorable methods to promote a considerable use of sunlight and increase its photocatalytic activity [30]. The metal doped TiO₂ suffers from poor thermal stability, low quantum efficiency and high processing costs [28]. As compared atomic p level of nonmetal elements such as N, F, C, B, S and P with nonbonding pπ state of O atom in the TiO₂ structure indicates C and N codoped-TiO₂ resulting in a more effective visible light response of codoped materials [31]. However, dopants are inevitably recombination center for the photogenerated electron and holes [32]. Therefore, a novel doped-TiO₂ coupling with narrow band gap semiconductors is prepared to enhance light harvesting or reduce the recombination rate of photoproduced electron and hole pairs and compensate disadvantages of the individual components due to its synergistic effect [33].

Accordingly, L-Histidine doped TiO₂-CdS/PES composite membranes with different NPs loadings of NPs (0.1, 0.5 and 1 wt%) were fabricated. The influence of the nanoparticle addition on the properties of PES membranes such as cross-sectional morphology, hydrophilicity, permeation flux and fouling was also determined. Finally, the photo-catalytic capability of optimum composite membrane was also studied in a cross flow system during filtration of biologically treated palm oil mill effluent (POME) under continuous visible light irradiation.