Greywater and bacteria removal with synchronized energy production in photocatalytic fuel cell based on anodic TiO₂/ZnO/Zn and cathodic CuO/Cu

Highlights

• Anodic TiO₂/ZnO/Zn and cathodic CuO/Cu was integrated in a PFC.

• The PFC was assembled to recover energy from greywater and bacteria disinfection.

• Electricity output is promoted by TiO₂ as a charge separator on ZnO/Zn alone.

• Greywater was totally mineralized and ∼99% bacteria disinfection can be achieved.

• The stable cycle operation of the cell has been also obtained.

Abstract

An approach that can recuperate of energy from wastewater treatment process is highly necessitate and would help to surmount the both environmental pollution and energy crisis issues. A photocatalytic fuel cell (PFC) employing an anodic TiO₂/ZnO/Zn and a cathodic CuO/Cu has been applied to degrade the raw greywater, which realized advanced organics destruction, bacteria disinfection, and synchronously electricity production. The improved photocatalytic performance has been observed when the cell was incorporated with anodic TiO₂/ZnO/Zn under UV and sunlight irradiation due to the enhanced electric field conductivity of the catalysts and heterojunction interface of TiO₂. In the constructed UV-activated PFC system, the electricity production capability was observed with the measured voltage and power density of 868 mV and 0.0172 mW cm⁻², respectively. Advanced chemical oxygen demand (COD) removal efficiency of greywater achieved a 100% completion within 60 min of light irradiation. The Escherichia coli (E. coli) colonies decreased significantly and accounted ∼99% disinfection efficiency. Moreover, the photoelectrochemical and photoluminescence (PL) experiments elucidated that the charge carrier separation efficiency were higher when TiO₂ was coupled to ZnO. The organic matter elimination principle was assessed by radical trapping experiment, and the findings indicated that the hydroxyl (OH) radical and hole (h⁺) appeared as major functions in the reaction. The stable cycle operation of the cell has been also obtained owing to the stable and film-type materials of anodic material. This performance was among the highest documented for PFC using real wastewater effluent as the fuel source.

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.