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ISSN : 1226-0088(Print)
ISSN : 2288-7253(Online)
Membrane Journal Vol.32 No.1 pp.33-42
DOI : https://doi.org/10.14579/MEMBRANE_JOURNAL.2022.32.1.33

Photocatalytic Membrane for Contaminants Degradation: A Review

Rabea Kahkahni*, Rajkumar Patel**, Jong Hak Kim***†
*Integrated Science and Engineering Division (ISED), Underwood International College, Yonsei University, Incheon 21983, South Korea
**Energy and Environmental Science and Engineering (EESE), Integrated Science and Engineering Division (ISED), Underwood International College, Yonsei University, Incheon 21983, South Korea
***Department of Chemical and Biomolecular Engineering, Yonsei University, Seoul 03722, South Korea
†Corresponding author(e-mail: jonghak@yonsei.ac.kr; http://orcid.org/0000-0002-5858-1747)
February 14, 2022 ; February 22, 2022 ; February 23, 2022

Abstract


Growing industrialization leads to severe water pollution. Organic effluents from pharmaceuticals and textile industries released in wastewater adversely affect the environment and human health. Presence of antibiotics used for antibacterial treatment in wastewater leads to the growth of drug resistance bacteria, which is very harmful for human being. Various small organic molecules are used for the preparation of organic dye molecules in the textile industries. These molecules hardly degrade, which is present in the wastewater effluents from printing and dyeing industries. In order to address these problems, photoactive catalyst is embedded in the membrane and wastewater are passed through it. Through this process, organic molecules are photodegraded and at the same time, the degraded compounds are separated by the membrane. Titanium dioxide (TiO2) is a semiconductor which behave as excellent photocatalyst. Photocatalytic ability is enhanced by the making its composite with other transition metal oxide and incorporated into polymeric membrane. In this review, the degradation of dye and drug molecules by photocatalytic membrane are discussed.


photocatalytic membrane , TiO2 , dye , drugs

오염물질 분해를 위한 광촉매 분리막: 총설

라비아 카갛니*, 라즈쿠마 파텔**, 김 종 학***†
*연세대학교 언더우드학부 융합과학공학부
**연세대학교 언더우드학부 융합과학공학부 에너지환경융합전공
***연세대학교 화공생명공학과

초록


성장하는 산업화는 심각한 수질 오염으로 이어진다. 폐수로 배출되는 약품과 섬유산업에서 나오는 유기배출물은 환경과 생명에게 악영향을 미친다. 항균치료에 사용되는 항생제가 폐수에 존재하면 인체에 매우 해로운 약제 내성균의 성장 을 야기하게 된다. 섬유산업에서 사용되는 유기염료 분자의 제조에는 다양한 유기 저분자가 사용된다. 이러한 분자들은 인쇄 및 염색 산업의 폐수 배출물에 존재하여 분해가 잘 이루지지 않는다. 이러한 문제들을 해결하기 위해 광분해성 촉매를 분리 막에 도입하고 폐수를 처리한다. 이 과정을 통해 유기 분자는 광분해되며 동시에 분해된 화합물들은 분리막을 통과하여 분리 된다. 이산화티타늄(TiO2)은 뛰어난 광촉매 역할을 하는 반도체이다. 다른 전이 금속 산화물과 화합물을 만들고 고분자 막에 도입하여 광촉매 능력을 증가시킨다. 본 총설에서는 광촉매성 분리막에 의한 염료 및 약물 분자의 분해에 대해 논의한다.


    1. Introduction

    Membrane technology is a well-established process for sea water desalination and wastewater treatment. Transition metal oxide is a class of semiconductor that acts as a catalyst under light illuminations. Control of polluted water is a severe problem, which needs to be tackled at the earliest in order to get rid of water borne diseases.

    Effluents from dye, paint, textile, coating and pharmaceutical industries contain various organic molecules present in wastewater. The degradation of these contaminants is possible using transition metal oxides such as TiO2 under illumination by light. In order to remove the degraded product as well as other organic components, these photocatalysts are embedded in the membrane to prepare photocatalytic membranes. Catalytic activity is dependent on the surface area of the exposed surface for which various nanoparticles of metal oxide and nanofiber membranes are used[1-7].

    Band gap and crystal plane of transition metal oxide can be tuned by mixing with other metal oxide, so that catalytic activity can undergo even with lower energy such as visible light[8-10]. Materials cost is an important factor for scaling up the process and commercial applications. Based on these requirements, zinc oxides as well as sulfides are excellent materials but their higher band gap is another issue. In order to resolve this problem, zinc oxysulfide are prepared which shows photocatalytic activity under visible light illumination. Zinc oxide nanorod grown on nickel seeded membrane enhance the separation between electron and hole as well as adsorption of photon which in turns enhances the catalytic efficiency of the photocatalytic membrane[11-13].

    This review is divided mainly into two section. In the first section, the degradation of cationic and anionic dye molecule such as rhodamine B, methylene blue and methylene orange in the photocatalytic membrane under light illumination are discussed. In the second section, antibiotics such as amoxicillin, tetracycline and chloramphenicol are described. The schematic presentation and summary of photocatalytic membranes for contaminants degradation used in this review are shown in Fig 1 and Table 1, respectively.

    2. Degradation of Dye

    The photocatalytic efficiency of pristine TiO2 semiconductor can be enhanced by the incorporation of small amounts of tin dioxide[14]. The nanocomposite of mixed metal oxides was embedded in the hydrophobic, chemically stable poly(vinylidene fluoride) (PVDF) matrix. PVDF nanocomposite membrane was fabricated by non-solvent induced phase separation method. Rhodamine B (RhB), a cationic dye was photodegraded by in the photocatalytic membrane bioreactor by the exposure to ultraviolet (UV) light. Among various compositions of SnO2/TiO2, the molar ratio of 1:6 exhibited the highest photocatalytic efficiency. The weight percent of the mixed metal oxide nanocomposite was varied and found out that 7 wt% showed the highest rate of photodegradation of 91.84 %. Light is absorbed by the photocatalyst to emit an electron and results with a hole (h+). This hole interacts with hydroxyl ion (OH-) to generate hydroxyl free radical (OH.). Time of separation between the hole and the electron directly related to the free radical generation which is the key to the degradation mechanism of photocatalytic membrane.

    Martins et al. suggested an alternative recycling solution for the problem of expensive photocatalytic nanoparticles by filtration and separation after water treatment[15]. Immobilized titanium dioxide TiO2 makes the recovery of nanoparticles easier. Hence, researchers prepared different photocatalytic membranes with and without zeolites (NaY) based on immobilization of TiO2 nanoparticles into hydrophobic poly(vinylidenefluoride– trifluoroethylene), P(VDF-TrFE) copolymer matrix. The addition of NaY accounts for the acquisition of the hydrophilic behavior of membranes. The goal was to compare the photocatalytic efficiency on methylene blue (MB) of the membranes with that of dispersed TiO2. The results of the photocatalytic examination of TiO2 in suspension vs. immobilized TiO2 showed that photocatalytic degradation of MB is better when induced by TiO2 suspension. The deficit in the efficiency of the photodegradation performance of immobilized TiO2 was fixed by zeolite addition. Zeolites lead to higher permeability and percolation of MB and reduced efficiency loss of the membrane degradation performance from 13% to 3%. Finally, the degradation rates increased with the increasing concentration of TiO2 in the nanocomposite.

    Shang et al. reported the fabrication of a multifunctional photocatalytic nanofibrous mat that is composed of three main components: TiO2 nanocables, Ag nanoparticles, and graphitic carbon coating[16]. A hydrophilic poly(vinyl pyrrolidone) (PVP) was mixed with titanium tetrabutoxide precursor and silver nitrate in certain ratio and then electronspun to prepare composite nanofiber membranes. It was heated to 280°C un- der air to generate transition metal oxides and further calcined at 550°C under inert atmosphere to convert PVP to graphitic carbon. This multifunctional mat is a good photocatalytic material because its three functional components are operating synergistically and it has excellent charge transport properties. First, the graphitic coating serves as an adsorption material and a charge-transfer material. Secondly, Ag nanoparticles act as light sensitizing agents and charge transfer material. Finally, TiO2 nanocables are UV-sensitive components of the nanofibrous mat and present a flexible surface for the other components of the mat. Furthermore, the porous carbon coating makes the mat flexible and allows robust large-area fabrication. The multifunctional photocatalytic nanofibrous mat has various photocatalysis applications against RhB and phenol.

    Zhang et al. prepared titanium dioxide graphene carbon nanofiber membranes (TiO2@GO@C) using electrospinning which has higher photocatalytic efficiency against MB than pristine TiO2[17]. Polyacrylonitrile (PAN) was dissolved in dimethylformamide (DMF) and then mixed with graphene oxide (GO). Separately, titanium isopropoxide (TTIP) precursor was dissolved in DMF and small amount of acetic acid was added to enhance the hydrolysis of metal ligand linkage nanofiber. Finally, both the solution was mixed and calcined under two different steps similar to the process described in the earlier reference. The nanofiber membrane has great photocatalyst efficiency and strength. TiO2@GO@C nanocomposite has two distinctive advantages. First, GO encapsulated in TiO2 showed good thermal conductivity and a large number of functional groups on the surface. Secondly, TiO2@GO wrapped by carbon nanofiber through the electrospinning process showed the spherical shape of the TiO2@GO@C, which improves the crystallinity of TiO2 and prevents from transforming from anatase to rutile phase. Because of the carrier transport property of graphene, GO@TiO2 improves the light absorption, reduces the probability of recombination of the hole-electron pair, as well as enhances the transport and photocatalytic degradation performance on MB. Furthermore, the dispersion strengthening effect of TiO2@GO strengthens the carbon nanofiber. Only 0.3 wt % GO showed the best photocatalytic efficiency of 98.5 %.

    In another study, Zr doped SiO2 shell/TiO2 core particle (EC-ZSTs) particles were embedded into PVDF to prepare EC-ZTP photocatalytic membrane[18]. The TEM and BET characterization of EC-ZSTs showed that EC-ZSTs have larger channels in the SiO2 shell compared with ZSTs. It showed that SiO2 shell channels improved mass and light transfer that leads to the better photocatalytic performance of both EC-ZSTP and EC-ZST. The photocatalytic activity investigation of both membranes by photodegradation of methyl orange (MO) solution and oil in wastewater indicates that EC-ZSTP membrane has a lower degradation rate due to better mass transfer. EC-ZSTP is an excellent membrane for immobilizing TiO2, which makes the recycling of nanoparticles easier, and has good photocatalytic performance.

    2.1. Degradation of drug molecules

    Huh et al. prepared membranes with both photocatalytic and separation capability along with self-cleaning property[19]. Yttria-stabilized zirconia/silica nanofiber (YSZ/silica NF) was coated with TiO2 and prepared by electrospinning, followed by calcination in air, as shown in Fig. 2. The YSZ/silica NF membrane acquired degradation/separation ability due to the controlled concentration of TTIP precursor during TiO2 coating. As the concentration of TTIP increased, the TiO2 layer became thicker which led to the reduced pore size of the membrane, thus a better rejection rate of polymeric particles, indicating an improved separation ability. Moreover, raising the layer of TiO2 led to a better photocatalytic performance in degrading pollutants. The results of the study show that the membrane has excellent adsorption and degradation performance on humic acid (HA), MB, and tetracycline (TC). Degradation efficiency of the photocatalytic membrane reactor against HA, MB and TC was about 88.2%, 92.4% and 99.5%, respectively. The reusability tests showed that the adsorption/degradation efficiency dropped by a small percentage, which indicated that the membranes have a good potential for recycling.

    Photocatalysts with photo-Fenton process ability were incorporated into submerged magnetic separation membrane photocatalytic reactor (SMSMPR)[20]. Fig. 3 represents the schematic of photocatalytic reactor. The photocatalyst was synthesized in an autoclave by hydrothermal process. TiO2-Fe3O4 nanocomposites could be recollected from the medium such as wastewater magnetically with the help of an external electric field. The photo-Fenton degradation performance of TiO2-Fe3O4 with amoxicillin trihydrate (AMX) as a pollutant was about 85.2 %. The results showed that the composite had efficient degradation performance. Moreover, due to backwashing treatment and magnetic separation, the SMSMPR still showed a high removal efficiency percentage after four cycles as well.

    Chitosan (CS) is a hydrophilic biopolymer that is eco-friendly, malleable, and cheap and it was used to prepare CS membrane (CSM)[21]. The photocatalytic performance of the membrane was tested with tetracycline hydrochloride (TC) in an aqueous solution. The performance was compared to that of CS powder and commercial TiO2-P25. Consequently, the result shows that CSM has better photocatalytic activity under visible light, which can reach 90% degradation efficiency of TC, then that of TiO2 known commercially as P25 and chitosan powder (CSP). The membrane has good visible light photocatalytic performance due to self-sensitization and excited delocalized pi-electrons of the residual acetyl groups of CS. Moreover, radical scavenging and EPR experiments reveal that O2- and h+ are the major oxidation species participating in the photodegradation of TC. Finally, CSM has good reusability potential. After being washed with NaOH solution the membrane could be reused five times with a degradation efficiency remaining above 80%.

    Recently, a flexible and floatable membrane was prepared by covering Vietnamese traditional paper (VTP) with Cu2O/rGO catalyst[22], as shown in Fig. 4 and 5. Cu2O/rGO was obtained from the green reduction of Cu(OH)2 to Cu2O and reduced graphene oxide. VTP’s fibrous structure and multiple layers give the membrane flexibility and porosity, which makes VTP a good supporting material for the catalyst. Precisely, the microfibrillated cellulose of VTP acts as an adsorbent of ciprofloxacin (CIP), which is the model pollutant of the study. This improves the photocatalytic performance on CIP. The results showed that the photodegradation of CIP reached 80% under solar irradiation. The membrane also exhibited good reusability capacity up to 5 cycles.

    Insoluble poly(ethylene oxide) (PEO) nanofibers were synthesized through electrospinning after the addition of pentaerythritol tetraacrylate (PETA) into precursor solutions, followed by electron beam irradiation [23]. Fig. 6 represents photodegradation mechanism. The nanofibers were subjected to electron beam (EB) irradiation that induces crosslinking in the PEO membrane’s structure and changes its properties. Bi2O2CO3 as a photocatalyst was added onto the PEO membrane. It achieved excellent photocatalytic performance on chloramphenicol (CPL). The membrane itself showed no cytotoxicity; however, the slight changes in cell viability in the water samples may have occurred due to toxic intermediates that come with the photocatalytic process. The PEO/Bi2O2CO3 membranes have great biocompatibility and can be an efficient photocatalyst membrane for wastewater

    2.2. Others

    To address the problem of toxic algae blooms, fluorine- doped titanium F-TiO2 nanocomposites was synthesized by the hydrothermal method which acts as a photocatalytic algaecide[24]. Fluorine doping enhances the anatase crystalline structure of TiO2, which leads to improved photocatalytic activity. The results show that fluorine-doped titanium has excellent photocatalytic degradation performance on algae cells. The pH of 10 accounts for the best photocatalytic effect of F-TiO2, while the pH of 6.25 serves the decline in chlorophyll a of the algae. Moreover, the photocatalytic inactivation process of F-TiO2 involved a simultaneous degradation of the metabolic products and microcystin- LR produced by Microcystis aeruginosa. In addition, it was reported that the reactive oxygen species that participated in the photodegradation were h+ and OH radicals and O2- with different levels of contribution.

    The presence of nitric oxide (NO) in the air is dangerous for human health. Therefore, numerous methods have been developed to remove it[25]. A porous carbon nitride nanosphere with reduced graphene oxide (HCNS/rGO) was synthesized then immobilized on porous carbonized polymer nanofibers CNCF by electrospinning and carbonization. The HCNS/rGO composite improves the light absorption and photocatalytic efficiency of CNCF, due to the hollow porous morphology of HCNS and rGO’s superior electronic properties. The results showed that the membrane had the highest removal percentage of low concentration NO compared to previously synthesized visible-light photocatalysts. Moreover, the HCNS/rGO and CNCF membrane show photochemical stability as well as recyclability potential.

    To tackle the problem of membrane fouling that hinders the filtration process, an ultrafiltration (UF) membrane was fabricated by immobilizing titanium dioxide nanoparticles (TiO2 NPs) on PVDF matrix[26]. Fig. 7 represents the photocatalytic mechanism. The membrane exhibited auto regeneration potential and reduced fouling. The photocatalytic nanoparticles, TiO2, induced the hydrophilicity of the membrane, resulting in increased membrane flux. The investigation on the effect of temperature revealed that the increase in temperature did not affect the photocatalytic process. It rather affected the viscosity of water that lead to increased permeate flux. Furthermore, the photocatalytic performance on HA showed that the deposition rate of HA should remain under 0.55 g min-1 for a fouling-free process with a photodegradation rate constant of 0.55 s-1 during consistent photodegradation. Finally, the membrane performance stayed consistent after 4 cycles, indicates excellent reusable property.

    3. Conclusions

    Chemical industries preparing various kind of materials such as dye, coating and painting release wastewater containing small molecule which is harmful to environment and affects human health severely. Pharmaceutical industries effluents are contaminated with various kinds of drugs, leading to drug resistance bacteria and entering into aquatic animal through which human health can be affected. Before releasing wastewater to any stream, it needs to be purified. Photodegradation of these organic molecules by exposure to light in presence of photocatalyst is a well-established method, but the development of efficient, cheap and reusable catalyst is still challenging. This review discussed these aspects, keeping main focus on photodegradation of dye and drug molecule by photocatalytic membrane reactor.

    Acknowledgments

    This work was supported by the Material & Component Technology Development Program (20010846, Development of nano sized biofilter and module for virus removal) funded by the Ministry of Trade, Industry & Energy (MOTIE, Korea).

    Figures

    MEMBRANE_JOURNAL-32-1-33_F1.gif

    Schematic representation of photocatalytic membrane for contaminants degradation.

    MEMBRANE_JOURNAL-32-1-33_F2.gif

    HR-TEM micrographs of YSZ/silica and TiO2-coated YSZ/silica NF: (A) EDS spectra of pristine NF and 1.0 M TiO2- coated NF and elemental maps of (B) pristine NF and (C) 1.0 M TiO2-coated NF (Reproduced from Huh et al., 19, Nature Research).

    MEMBRANE_JOURNAL-32-1-33_F3.gif

    Schematic illustration of the submerged magnetic separation membrane photocatalytic reactor (SMSMPR) (Reproduced from Li et al., 20, The Royal Society of Chemistry).

    MEMBRANE_JOURNAL-32-1-33_F4.gif

    Fabrication of the Cu2O/rGO/VTP photocatalyst: (a) schematic illustration of the fabrication of the photocatalyst by the roller coating method; (b) photograph of a VTP sheet coated with GO solution; (c-e) SEM images of VTP (c), GO/VTP (d), and Cu2O/rGO/VTP (e) (Reproduced from Nhi et al., 22, The Royal Society of Chemistry).

    MEMBRANE_JOURNAL-32-1-33_F5.gif

    Adsorption of CIP onto Cu2O/rGO/VTP: (a) illustration of the highly flexible and floatable catalyst membrane; (b) the point of zero charge (PZC) of the catalyst; (c) the effects of pH on the removal efficiency of CIP by the catalyst; (d) the effects of temperature on the removal efficiency of CIP by the catalyst, (e) plot of fitting between the experimental data to the pseudo-first-order and the second-order adsorption models; (f) plot of fitting between the experimental data to the Langmuir and Freundlich isotherm models (Reproduced from Nhi et al., 22, The Royal Society of Chemistry).

    MEMBRANE_JOURNAL-32-1-33_F6.gif

    Proposed photodegradation mechanism of the CPL (Reproduced from Xu et al., 23, The Royal Society of Chemistry).

    MEMBRANE_JOURNAL-32-1-33_F7.gif

    Schematic of the photocatalytic UF process (Reproduced from Younas et al., 26, The Royal Society of Chemistry).

    Tables

    Summary of the Photocatalytic Membranes

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