## 1 Introduction  
Fly ash is a by-product generated by coal-fired power plants generating electricity, which is one of the earth's most significant sources of air pollution worldwide and leads to waste management proposals [1]. Nowadays, fly ash can be recycled and reused in different engineering applications, in the cement industry and in wastewater heavy metal adsorption, reducing environmental footprint [2]. The wastewater from industry and householders has a multi-pollutants composition requiring simultaneous advanced treatment processes, such as adsorption and photocatalysis [3]. These processes can be performed sequentially, with the disadvantage that they increase the technological costs [4, 5]. The integration of adsorption and photocatalysis in a one-step treatment process can enhance the overall removal efficiency [6]. Advanced oxidation processes (AOPs) represent a suitable alternative to the traditional organic removal methods in terms of sustainability [7]. Various photocatalysts were studied in AOP, starting with monocomponent structures such as TiO\({}_{2}\)[8, 9], ZnO [10, 11], SnO\({}_{2}\)[12, 13],and WO\({}_{3}\)[14; 15], which have the disadvantage of limited light absorption range [16; 17], low chemical stability [18; 19], and high charge carriers recombination rate [20; 21]. In order to overcome these issues, complex photocatalysts multi-component structures based on metal oxides [22], metal sulphur [23], graphene [24], and polymers [25] were reported. Coupling semiconductors (oxide and sulfide) with a clear difference between the valence band and conduction band energies (the two difference-shifts being in the same direction) is a common strategy to minimise recombination [26]. These materials have relatively good photocatalytic efficiency, but the competition between adsorption and photocatalytic degradation should be optimised in order to decrease the operation time and to increase the materials lifetime. The heterojunctions are used as photocatalyst due to their carrier-injecting ratio properties, improved light excitation response, and efficient separation of photoexcited electron-hole pairs, inducing a higher photocatalytic reaction rate and overall output [27]. However, there are still issues related to the treatment of highly concentrated organic pollutants, which can not be avoided based only on photocatalytic processes. A range of VIS-active photocatalysts is those based on tandem or diode type structures, displaced by the association of two semiconductors (n-n type or n-p type). These materials are relatively good adsorption substrates, but the competition between the two processes should be optimised so that the adsorption of organic compounds to be followed by photolysis and adsorption of inorganic compounds does not distort the properties of VIS-activated photocatalyst. Titanium dioxide was largely investigated as the photocatalyst for wastewater pollutant decontamination [28], although the process efficiency is low under VIS irradiation [29; 30] due to the bandgap of 3.2 eV and easy recombination of the generated hole and electrons pairs [31]. Several paths are followed: (i) properties that have an impact in photocatalysis, including chemistry defects about electronic structure, semiconductor properties, composition, and morphology, will be optimised; (ii) TiO\({}_{2}\) must be coupled with other semiconductors and designed multi-layered materials [32], such as TiO\({}_{2}\)/Cu\({}_{x}\)S [33], ZnO/TiO\({}_{2}\)[34], WO\({}_{3}\)/TiO\({}_{2}\)[35], TiO\({}_{2}\)/Fe\({}_{2}\)O\({}_{3}\), [36], and TiO\({}_{2}\)/SnO\({}_{2}\)[37]. Coupling semiconductors (oxide and/or sulfide) with a clear difference between the energy of the valence bands as well as between the energy of the conduction bands (the two difference-shifts being to the same direction) is a common strategy to minimise recombination.