Greywater and bacteria removal with synchronized energy production in photocatalytic fuel cell based on anodic TiO2/ZnO/Zn and cathodic CuO/Cu
Graphical abstract
Introduction
Interest on alternative water sources has garnered global attention as they are being challenged by increased water shortage and impacts from climate change throughout the past few years. Greywater is urban indoor wastewaters derived from several different activities, but specifically excludes any sewage inputs. Typical contaminants in greywater include shower foam, soap, oil, grease, microorganism, salt and dirt (Patil and Munavalli, 2016; Wu, 2019). It is noteworthy that these organic wastes in the wastewater comprised rich chemical energy, which is discharged heat as by-products upon the removal process and difficult to be recycled through conventional treatments (Wu et al., 2016; Pan et al., 2017; Zhang et al., 2018a). Thus, it is idea to develop alternative techniques not only undergo waste reduction, but also should provide concomitantly positive energy and environmental impacts through conversion, recycling and waste reclamation approaches.
In recent years, photocatalytic fuel cell (PFC) is regarded as the most sustainable and promising approach, which both combine photocatalysis and fuel cells technologies that can derive electrical power from the decomposition of organic wastes (Ye et al., 2018; He et al., 2019; Li et al., 2019). The whole process is generally operated only by light source as the driving force and semiconductors as the anodic catalyst materials (Khalik et al., 2016; Lee et al., 2017). When the light is irradiated on photoanode, photogenerated electrons and holes are separated, organics are degraded in the photoanode to provide electrons that are transported to cathode through an external circuit to form electrical current. Simultaneously, the electrons migrated to the cathode can also react with target organics and thereby, the waste chemical energy in the organics could be recycled and reused. For this reason, PFC is claimed to be a resilient candidate for a better technology to recuperate of energy from wastewater treatment.
Despite light-responsive photocatalytic degradation of organics and synchronously electrical power generation have been revealed, majority of the PFC devices are constrained to the cage of noble metal or rare earth-bearing photoanode materials. For example, incorporation of surface plasmon resonance and doping of rare earth up conversion materials on semiconductors (Deng et al., 2018; Qiao et al., 2018) may confine the practical applications of such PFC devices as these methods are complex, high cost and required specific instrument for fabrication process. Under these conditions, it is great importance to establish an efficient and cost-effective PFC device.
Moreover, the state-of-the-art advances in the PFC field are still far from being optimal and there are still several breakthroughs to be made before it can be considered as an economically viable process. Myriad research works have been explicated on the effects of operating factors in the PFC system (Khalik et al., 2016; Lee et al., 2017; Zhao et al., 2017; Li et al., 2019). The operating factors were fuel type, catalyst loading and pH value that could affect the cell performance. These earlier works were mostly emphasized on the overall operating performance for the PFC. It should be mentioned that the cell performance was governed by main components, such as photoanode, cathode and electrolyte, which independently vary with the operating factors. In this regard, versatile materials for applications in PFC need more investigations and each component in the cell with superior enhanced properties.
Inspired by the above consideration, we have successfully constructed a greywater-fed PFC with anodic TiO2/ZnO/Zn and cathodic CuO/Cu. The ZnO is the most promising material option owing to its ease of availability, photooxidizing ability, and high photoactivity and TiO2 is utilized to boost up the light absorption and electronic features of ZnO without changing its oxidation-reduction potential (Lee et al., 2016, 2017). Apart from that, CuO is another semiconductor which is copious in nature, and it has been also recognized to be stable and served effective cathode material because of mismatches the Fermi level of photoanode (Masudy-Panah et al., 2016, 2017). Undoubtedly, greywater as a promising sustainable water source was chosen to be treated in this study, the electrical current production and photocatalytic as well as stability of the electrodes tests were measured. The photoelectrochemical, photoluminescence (PL) and the radical trapping tests were also evaluated for photocatalytic mechanism and discussion. Furthermore, the antibacterial performance against Escherichia coli (E. coli) was assessed in the cell.
Section snippets
Material and reagents
Commercially available ZnO (purity 99.5%), TiO2 (purity 98.0%) and terephthalic acid (TA, purity 99%) were both purchased from Acros Organics. Zn and Cu foils was purchased from Stem Chemicals Sdn. Bhd. Isopropanol (IPA, purity 99%) was obtained from HmbG Chemicals, and p-benzoquinone (BQ, purity 98%) and catalase (10,000 units ≥ per mg) was supplied from Sigma-Aldrich. Ethylenediaminetetraacetic acid (EDTA) was purchased from Chemical Solutions. Sodium chloride (NaCl, purity 99%) and nutrient
Morphology and crystallography characteristics
The morphological and crystallographic characteristics of as-prepared electrodes are shown in Fig. 1. Fig. 1a shows an FESEM image of ZnO/Zn foil. The commercial ZnO particles consisted of tetragonal and rod-like shapes approximately 70–710 nm in diameter on the surface of Zn foil. The morphology of CuO/Cu foil is displayed in Fig. 1b. The CuO particles consisted of flake-like shapes approximately 2100–2800 nm in length and 280–360 nm in width. The FESEM image in Fig. 1c illustrated the TiO2
Conclusions
A PFC using anodic TiO2/ZnO/Zn and cathodic CuO/Cu was successfully constructed and utilized for greywater degradation, bacteria disinfection and synchronously electricity production. This cell was able to provide an active catalyst for the degradation of greywater effluent under UV light as well as sunlight irradiation. The amount of TiO2 in the ZnO/Zn was controlled to ensure optimal light activity to the cell in the heterojunction form. When the PFC operated under UV light over 60 min
Author contributions section
Dr. Sin and Dr. Lam worked on the development of the photocatalytic fuel cell using anodic TiO2/ZnO/Zn and cathodic CuO/Cu photocathode. The physico-chemical, optical and electronic properties of the developed photo-electrodes have been also tested. We also evaluated the PFC performance through the removal of greywater as a sustainable water source and its recovery of energy by the investigation of key component in PFC, reusability of the electrode material, photocatalytic mechanism discussion
Acknowledgements
The research was supported by Universiti Tunku Abdul Rahman (UTARRF/2019–C1/L03), Ministry of Higher Education of Malaysia (FRGS/1/2016/TK02/UTAR/02/1 and FRGS/1/2019/TK02/UTAR/02/4), Research Funds of The Guangxi Key Laboratory of Theory and Technology for Environmental Pollution Control, China (1801K012 and 1801K013), and L’Oréal-UNESCO Research Grant.
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