Recent advances, influencing factors, and future research prospects using photocatalytic process for produced water treatment Open Access

Advanced oxidation process (AOP), using photocatalytic oxidation, is gaining importance in the area of wastewater treatment (Gogate & Pandit 2004; Raut-Jadhav & Bagal 2020) since this process results in the degradation of pollutants with operation at mild conditions of temperature and most importantly sunlight or ultraviolet radiation for the generation of radicals capable of reacting with a wide range of priority pollutants. The advantages of photocatalytic water treatment over homogeneous-phase AOPs are well documented. For example, photocatalysis has been effectively used to degrade a wide range of organic compounds, including dyes (Dong 2015b; Shoneye et al. 2021) endocrine and mutagenic damaging chemicals (Al-Ghouti et al. 2019) and for degradation of pollutants in petroleum refinery wastewater (Rincon & Pulgarin 2004; Ani et al. 2018). Photocatalysis as a treatment system obviates the necessity for constant demand of precursor chemicals, which is a significant advantage for specific applications, especially those in isolated or resource-limited areas. This promise is reflected in a recent surge of scholarly publications on photocatalytic water treatment: over 5,500 results for papers from 2002 to 2021 (Figure 1).

Despite extensive research during the last few years, photocatalysis applicability in produced water has been quite limited. In light of these facts, how might photocatalysis be efficiently scaled up or approved for oilfield-produced water treatment?

We examine the current state of heterogeneous semiconductor photocatalytic water treatment in this review, highlighting barriers to technology transfer, assessing the feasibility of practical applications, and identifying critical research needs for overcoming obstacles, in order to find answers to this question.

Photocatalysis involves chemical transformations capable of completely decomposing nearly all organic molecules in the presence of a photocatalyst, solar, or Ultraviolet light energy (Bharagava 2020). As shown in Figure 2 and summarised from (Equations (1)–(5)), photocatalysts absorb energy equal to or above their bandgap energy when subjected to a UV light across the catalyst's surface. This exposure leads to the production of electrons and holes pairs in the conduction band and valence band (Equation (1)). The occurrence in photocatalytic processes requires the presence of water and dissolved oxygen, which gives room to the formation of reactive radicals.

As shown in Figure 2, the electrons (e⁻) that are excited by the light (photons) migrate from the valence band to the conduction band, leaving behind an empty unfilled valence band, and thus creating the electron-hole pair (h⁺) (Chong et al. 2010). If the catalyst is suspended in water, the electron-hole pair h⁺ at the valency band reacts with OH⁻ or H₂O to form hydroxyl radical (OH^•), as shown in Equations (2) and (3).

Equation (4) illustrates the reaction at the conduction band. Here, O₂ as an oxidizing agent accepts the photogenerated electron e⁻ to become reduced. The reduction of molecular oxygen O₂ produces superoxide radicals O₂^•−.

Thus photogenerated reactive species O₂^•− and OH^• are mainly responsible for the degradation of the pollutants or mineralized to nontoxic states (Chong et al. 2010; Ala'a & Khraisheh 2015).

In water treatment, the degradation pathway of pollutants to final photocatalytic products involves some specific sequence of steps as depicted in the reaction procedure in Figure 2 (Su et al. 2008; Chong et al. 2010).

• I.

  Mass transfer of reactants from the bulk fluid phase to the external surface of the solid photocatalyst through diffusion

• II.

  Internal mass transfer of the reactants from the external surface of the photocatalyst through the pores to the surface of the photocatalyst and settling on an active site through adsorption

• III.

  Adsorption of the organic contaminant(s) onto the photon activated surface (i.e. surface activation by photon energy occurs simultaneously in this step)

• IV.

  Photocatalysis reaction for the adsorbed phase on the surface of the photocatalyst

• V.

  Desorption of the final products (Product is released from the external surface of the photocatalyst to the fluid media)

• VI.

  Internal counter diffusion of final products

• VII.

  External counter diffusion.

The oxidation rates and performance of the photocatalytic system are strongly reliant on some factors such as pH of the solution, light intensity, reaction time, wavelength, catalytic loading and concentration of pollutants (Loeb et al. 2018). Aside from the irradiation source, and the photocatalyst material, the photoreactor design has an impact on photocatalytic performance as measured by pollutant removal efficiency.

pH is one of the most significant operational factors in heterogeneous photocatalytic water systems because it defines the organic compounds' chemistry (e.g. charge, acidity-pKa and partitioning coefficient) and it also affects the charge on the catalyst. Because of the nature of the semiconductor catalyst employed, any change in operational pH is reported to stimulate the isoelectric point or surface charge of the photocatalyst (Gnanaprakasam et al. 2015). Many reports have used the point of zero charge (PZC) to study the pH impact on the photocatalytic oxidation performance (Ochuma et al. 2007; Chong et al. 2009),. The PZC is a condition where the surface charge of photocatalyst is zero or neutral, that lies in the pH range of 4.5–7.0 depending on the catalysts used (Sacco et al. 2012; Chanu et al. 2019).

The amount of catalyst also affects photocatalytic degradation. The rate of photodegradation initially increased as the quantity of catalyst in the photocatalytic process increased. This may be explained by the fact that increasing the quantity of catalyst generally increases the number of active sites on the surface of the photocatalyst, resulting in an increase in the number of OH^• radicals formed, which can participate in the degradation of organic compounds that are present in the solution. Beyond a certain quantity of catalyst, the solution becomes turbid and so prevents UV rays for the reaction to continue, resulting in a decrease in percentage degradation (Coleman et al. 2007). As shown in Figure 3, in the absence of TiO₂, no significant degradation was observed. The degradation rate increased with increasing catalyst loading until 0.7 gL⁻¹, after which a decrease in degradation was observed. The reduction of photocatalytic activity at higher catalyst loading is attributed to the effects of light scattering and shielding by the suspended catalys (Jamali et al. 2013).

Depending on the kind of photocatalysts used, the photochemical impacts of light sources with various wavelength emission ranges will have a significant impact on the photocatalytic process (Molinari et al. 2020). UV light used in the process needs to have sufficient energy to promote electron hole formation (Ameta & Ameta 2018). The corresponding electromagnetic spectrum for UV irradiation is categorised as UV-A, UV-B, and UV-C, based on the wavelength of light that it emits. The UV-A range has its light wavelength range from 315 to 400 nm (3.10–3.94 eV). UV-B has wavelength range of 280–315 nm (3.94–4.43 eV) and the UV-C ranges from 100 to 280 nm (4.43–12.4 eV) (Janssens et al. 2019).

In most of the previous studies, the UV ranges from 254 nm to 315 nm and provides light photons sufficient for photonic activation of titanium dioxide (TiO₂) and zinc oxide (ZnO). TiO₂ has a band gap with an optical absorption in the 310–400 nm-wavelength region, while CeO₂ lies in the corresponding wavelengths of 388 nm (3.2 eV) and 381 nm (3.25 eV) respectively. ZnO with a wide band gap (E_g = 3.37 eV, corresponding to 387 nm) has unique electro optical properties and efficient UV absorptivity (Becheri et al. 2008; Rajendran et al. 2016; Ghamsari et al. 2017) However, some studies have used 253 nm during photocatalysis reaction (Lu et al. 2008; Ali et al. 2010).

A difference in the starting concentration of the water pollutants will result in a varied irradiation period required to accomplish full mineralization. Increased initial organic compound concentration affects photocatalyst degradation efficiency. It might be owing to an increase in the initial concentration of organic chemical, which resulted in more molecules being adsorbed on the surface of the photocatalyst. This results in a lack of available catalyst surface for the formation of hydroxyl radicals, lowering the photocatalytic activity of the catalyst.Additionally, increasing the initial concentration of organic compound lowers the quantity of photons arriving to the photocatalysis surface, hence decreasing the excitation of electrons from the valance band to the conduction band. Consequently, the photocatalytic activity of the produced photocatalyst is decreased.

It was shown that an increase in photocatalytic reaction temperature (>80 °C) promoted the recombination of electron hole charges (Gaya & Abdullah 2008). Moreover, temperatures below 80 °C actually favour adsorption of contaminants on the TiO₂ surface and operating at this temperature can reduce the heating capacity of water (Herrmann 2005). Li Puma & Yue (2002) reported that temperatures between 20–80 °C seem to be more effective for organic reactions with UV light (Li Puma & Yue 2002).

When increased, light intensity results in much greater energy waste rather than degradation. The increase in light intensity also results in a significant rise in costs (Guozheng et al. 2010). A significant portion of light energy is lost so it is difficult to have an extensive application, when light intensity is not logically controlled, which is one of the most significant photocatalytic components. Light energy losses occur when the photon with less energy than threshold photon energy (h_ν0) is unable to ignite an electron–hole pair, and its energy is lost as heat hv<h_ν0 and when h_v>h_ν0 is absorbed by a photocatalyst, h_ν0 energy is used to excite an electron–hole pair and the remaining energy h_ν − h_ν0 is lost as heat leading to energy waste (Yang & Liu 2007; Guozheng et al. 2010). It has been reported that sometimes the higher light intensity causes much more energy waste instead of much more degradation of organic compounds. It is critical to establish suitable light intensities in order to reduce energy losses caused by electron–hole recombination and to ensure similar values in all the measurements at different times (Dillert et al. 2013).

Photocatalytic reactors for water treatment can be categorised into those reactors using suspended photocatalyst particles and reactors where photocatalysts are immobilised on a continuous inert carrier as shown in Figure 4.

Photocatalytic reactors can run in batch, batch with recirculation, or continuous mode and their geometry should be designed to optimise the collection of light emitted by the selected sources (Cassano & Alfano 2000). According to Pareek et al. (2008), the most critical parameters to consider when designing a photocatalytic reactor are the total irradiation surface area of the catalyst per unit volume and the light dispersion inside the reactor.

Recent studies by Enesca (2021) reported that using the inappropriate photoreactor design may significantly increase energy consumption, improperly distribute photons, or cause the catalyst to deactivate, all of which have an adverse effect on photocatalytic efficiency. Thus the main obstacle in the development of highly efficient photocatalytic reactors is the establishment of effective reactor designs for intermediate and large-scale use, as demanded for the purpose of industrial or commercial applications (Abhang et al. 2011). A scale-up for industrial applications will offer opportunities to increase the performance of photocatalytic water treatment systems through improved reactor design. The importance of additional research in the areas of reactor design have been emphasised by (Chong et al. 2010; Loeb et al. 2018). They concluded that the relatively slow translation of photocatalytic research to industrial practise is most likely due to the difficulty of large-scale system design, which is frequently overlooked at the bench scale. Thus, large-scale photocatalytic treatment process with high efficacy can be achieved through rapid evaluation of different possible pilot plant configurations. According to Hossain (2018) a compact photocatalytic reactor will enable efficient decomposition of organic compounds in a liquid or gas phase, incorporating a flexible and light-dispersive capacity (Hossain 2018).

The photocatalytic process has shown significant promise as a low-cost, ecologically friendly, and sustainable treatment method that is compatible with the water/wastewater industry's ‘zero’ waste policy. This advanced oxidation technology's effectiveness to eliminate persistent organic contaminants and microorganisms in wastewater has been extensively proven. Table 1 gives an overview of the organic compounds treated, the conditions of the catalytic process, as well as the new advances in this technology for each study.

However, the application of photocatalysis systems to water treatment is limited by some technical problems that need further research (Foteinis & Chatzisymeon 2020).

According to Lin et al. (2020), the relatively slow transfer of photocatalysis treatment techniques from bench to industrial scale is likely due to the system's design currently used in water treatment; another key challenge identified as a constraint for photocatalysis application is the treatment scope for contaminants.

That being said, Kang et al. (2019) reported a poor quantum yield due to the very effective recombination processes acting in the bulk solid and at the solid/electrolyte interface using UV-irradiated semiconductors (Kang et al. 2019). Also Jiang et al. (2021) reported that obtaining selective oxidation of the target pollutant or pollutants in the presence of other oxidizable organic substrates is relatively complex, which has become a major set back for photoctalysis systems.

Selectivity is the oxidants’ capacity to distinguish between the multiple compounds present in solution (in heterogeneous photocatalytic reactions, h⁺ vb and OH). For example, recent reports had shown that it is challenging to oxidize benzene or other BTEX chemicals selectively in the presence of a high concentration of DOM (dissolved organic matter) (Coha et al. 2021; Giannakis et al. 2021; Marco et al. 2021).

In terms of intermediates produced during treatment not many studies have reported the management or handling of these compounds; in reality these intermediates may be more dangerous to humans and the environment in the short run (Jamjoum et al. 2021).

A photocatalytic treatment process can be more efficient in the removal of organics from oilfield-produced water. The most mature and feasible processes, such as ozonation and Fenton, are also associated with lower efficiency in oilfield-produced water. This assessment was based on the comprehensive report by Coha et al. (2021), who classified the various AOPs based on their current developmental stage.

A search of the Scopus database using the keywords ‘produced water’ and ‘photocatalysis’ indicated that there were fewer than 90 peer-reviewed articles on the subject (Figure 1). Thus applications of photocatalysis techniques to treat oilfield-produced water and, more specifically, aromatic and polyaromatic compounds are still under-studied (Ani et al. 2018).

In environmental remediation, limited reports depict the use of photocatalytic studies for the simultaneous degradation of organic compounds such as BTEX, PAHs, and phenols, contained in OPW and co-existing together, although research in the literature shows that petroleum wastewaters have been studied using a heterogeneous photocatalysis method (Table 1). Nevertheless, most of the studies have primarily focused on the degradation of a single pollutant (Jain et al. 2007; Mu et al. 2017).

However, wastewater on an industrial scale contains a mixture of organic and inorganic compounds in different classifications co-existing, such as produced water during oil and gas extraction. Scaling up from laboratory to industrial-scale applications will require additional scientific research, including the fabrication and modification of semiconducting photocatalysts and the treatment of multiple pollutants, as well as process optimization studies to reduce the cost of treatment, as conceptualized in Figure 5.

Hong et al. (2018) investigated the degradation of biocides, such as Glutaraldehyde (GA), considered one of the most harmful contaminants in produced water, which restricts biological activities, making biological treatment a non-viable alternative for produced water treatment. GA was treated under ultraviolet (UV) irradiation with removal efficiency ranging from 52 to 85% within one hour irradiation (Hong et al. 2018). Also, in the study carried out by Andreozzi et al. (2018), linear plot of ln(C/C_o) versus the treatment time for the organic constituents of saline produced water in presence of reduced graphene oxide/titania rGO(10%)/TiO₂-P25 shows that organic constituents removal was influenced by the rGO/TiO₂ weight ratio. Among rGO/TiO₂-P25 photocatalysts with different weight ratios (1, 5, 10 and 20%), the highest photocatalytic efficiency was achived with a ratio of 10:1, as depicted in Figure 6.

The photocatalytic reaction rates increase as follows: acetic acid < phenols < naphthalene < xylenes < toluene (Andreozzi et al. 2018).

Sheikholeslami et al. (2018) studied the removal efficiency of BTEX compounds using γ-Fe₂O₃ nanoparticles. At the optimal catalyst concentration of 150 mg/L and pH 3, a 97% decrease in COD was achieved after 90 min of irradiation. Thus optimizing the three principal independent parameters, as depicteded in Figure 7, can have significant impact on BTEX removal.

Photocatalysis experiments were also performed to test the influence of the wavelength and intensity. Jimenez et al. (2019) reported a low removal of phenol with 50% degradation; results show that the configuration and geometry of the equipment can affect the profit of the light. Taghizadeh et al. (2020), using a combination of forward osmosis and photocatalyst system for simultaneous salt removal and treatment of produced water, demonstrated that benzene, toluene, ethylbenzene, and xylene (BTEX) removal efficiency in the cellulose triacetate with TiO₂ and TiO₂/GO membrane under UVC radiation was 62 and 78% (Figure 8), respectively. Results show that the use of TiO₂ and TiO₂/GO membranes significantly improved the permeability, water flux, and photocatalytic degradation of pollutants and desalination of produced water.The result for CT (cellulose triacetate) and CTG membranes in comparison with the C membrane showed that the presence of TiO₂ and GO nanoparticles improved the permeability, increased hydrophilicity and enhanced the FO efficiency. Furthermore, the best BTEX removal efficiency obtained was 80% in membrane CTG under visible light.

There is a direct correlation between organic pollutant and surface coverage of photocatalyst (Dong et al. 2015b) as organic pollutants that can attach properly to the photocatalyst surface, are more likely to be oxidized directly. The photo-induced transfer of electrons that take place with adsorbed species over semiconductor photocatalyst depends on the band gap energy of the semiconductor (Kang et al. 2019) and so the activation energy must be enough to transfer electrons from the valence band to the conduction band (Schneider et al. 2014).

Figure 9 displays the bandgap energy (Eb_g) for the photoexcitation of different photocatalysts. However, besides band gap, other factors play an important role in photocatalytic reactions; as the advancement of photocatalytic technology in the water treatment sector will require the use of a photocatalyst exhibiting some ideal features such as high adsorption and surface area, light absorption, efficient charge separation and high mobility of charge carriers.

Different authors (Zhou et al. 2012; Neaţu et al. 2014) agree about the main factors of an ideal photocatalyst: for instance, the size and shape of the catalyst influence its surface structure, including its adsorption capacity, then resulting in varying photocatalytic performance (Rani et al. 2018; Li et al. 2020a). Also, the crystallite size of a catalyst plays an important role in the photocatalytic process, as photo reactivity can be enhanced by improving the crystallinity. Because adsorption of pollutants is a key phase, the surface area of the photocatalysts materials, also has a significant impact on photocatalytic activity. Thus, a large surface area contains more active sites and displays improved photocatalytic activity (Tian et al. 2014).

Silva et al.(2019) used real samples of OPW collected from an oil company located in Rio Grande do Norte, Brazil. The photocatalytic processes were effective for the removal of dissolved organic matter in OPW. The best treatment was at pH 7, this was attributed to the calcination effect on the photocatalyst during treatment. Thus an increase in the kinetic energy of reaction results in a higher oxidation rate of phenol.

Andreozzi et al. (2018) reported that titania coupled with reduced graphene oxide shows higher activity than bare TiO₂ nanoparticles for reducing the TOC of saline produced water (SPW). The higher photoactivity of rGO/TiO₂-HM pristine TiO₂-HM compared to commercial bare P25 can be related to the specific surface area of rGO/TiO₂-HM, which is three times higher than TiO₂-P25. On the other hand, the differences in shape, phase and particle size demonstrated to play a role in the photoactivity of these materials.

Other studies carried out for treating produced water by photocatalytic processes demonstrated that high rate of photogenerated (e⁻ and h⁺) recombination, poor adsorption capacity of pollutants and high concentration of chloride ions in SPW compete with organic substrates and reduce the effect of pristine TiO₂, which is frequently associated with low efficiency and a lack of reduction in the toxicity of treated SPW (Santos et al. 2006; Li et al. 2007; Gouma & Lee 2014). Also a more crystalized photocatalyst is capable of re-positioning light-generated carriers to its originated state thereby preventing recombination (Kim & Kwak 2007) and on some occasions more energy can be absorbed (Marotti et al. 2006). The surface charge properties of the photocatalyst under acidic or alkaline conditions can result in a higher oxidizing activity at lower pH but excess H⁺ at very low pH can decrease reaction rate (Horvath 2003; Sun et al. 2006).

Furthermore, a study conducted by Marzouk et al. (2021) shows that dip-coated commercial TiO₂ membranes with different concentrations of SiO₂ (Figure 10) were tested for oil rejection from raw PW. The TOC content in the SPW filtered through Mu was as low as 28 mg/l giving a removal efficiency of 72%. However, futher enhancement with SiO₂, reported 89, 91, 90 and 87% for membrane (M1, M2, M3 and M4); respectively,which can be attributed to the pore size of the membranes within the NF range.

Research has also been performed to incorporate photocatalysts into inert substrates, such as glass or polymers, most notably to resolve post-treatment catalysts recovery problems. Liu et al.(2016) compared the removal efficiency of naphthalene in real offshore PW achieved by the catalyst suspended or immobilized on glass slides. Even if naphthalene reduction after 12 h was around 85%, immobilized TiO₂ promoted a larger removal rate, mainly due to a much lower increase in system turbidity upon catalyst addition.

Various modification routes exist, such as doping, thermal treatment and optimisation of application process towards augmenting or improving the optical and material response (performance) of photocatalysts.

The justification for using these material engineering methods is to maximize rates of the photocatalytic reaction by introducing electron acceptors or changing the catalyst shape and composition through different mechanisms, as reported by (Chong et al. 2010). Metallic materials such as copper (Cu), tin (Sn), nickel (Ni), platinum (Pt), gold (Au), silver (Ag) and Vanadium (V) may elicit an appropriate bandgap change and stimulate effective degradation.

For instance, Gao et al. (2015) studied the effect of metal ion-doping on the photocatalytic activity of TiO₂ for rhodamine B removal. Ag⁺, Al³⁺, Mn²⁺, Ni²⁺ and Zn²⁺ were investigated. Results show that Zn²⁺ could exhibit an excellent photocatalytic reduction efficiency because of the effects of the weight fractions of the anatase phase, S_BET, and band gap (E_g). The use of Mn²⁺ and Ni²⁺ as dopants decreased photocatalytic activity.

The application of non-metal dopants such as fluorine nitrogen, sulphur, and carbon (F, C,S, N and P) can enhance the photoactivity and feasibility of catalysts for industrial application through the introduction of impurites in the bandgap of photocatalyst and thus, reduce the photonic energy requirements, narrowing the bandgap and promoting the adsorption between photocatalyst and pollutants, increase conductivity within photocatalyst and the mobility of charge carriers (Fujishima et al. 2008; Niu et al. 2018). The choice of phosphorus as a dopant allows the possibility of both cationic (P⁵⁺) and anionic (P³⁻) doping of anatase TiO₂ (Yu et al. 2003) and there are several reports on high activity of P-doped TiO₂ nanoparticles (Gopal et al. 2012). Ganesh (2017) studied the influences of solution pH, band-gap energy, structural, surface and photocatalytic characteristics of different amounts of P-doped TiO₂ nano-powders. Result shows that 1 wt.% P-doped TiO₂ nano-powder calcined at 400 °C exhibits the highest rate of reaction for photocatalytic methylene blue (MB) degradation (Ganesh, 2017). P-doped TiO₂ nano-powder photocatalysts prefer a solution pH of 5.5 for improved activity in the MB degradation reaction. Nitrogen as a dopant has numerous potentials. Nitrogen doping improves optical absorption and photocatalytic degradation capability significantly (Ansari et al. 2016).

According to prior research, the effectiveness of photocatalysts in degrading environmental pollutants is influenced by variables such as the photocatalyst type and the influence of light in photocatalytic reactions depends on the type of photocatalyst (Dong et al. 2015b).

A disadvantage of large-scale application would be the incorrect choice of catalyst for pollutant degradation. That is, utilizing a photocatalyst that is ineffective or incompatible with the contaminants (Hlongwane et al. 2019; Zhang et al. 2019; Quesada-Cabrera & Parkin 2020). Xu et al. (2019), as shown in Figure 11, categorised photocatalyst into nitrides based photocatalysts, metal oxide-based photocatalysts and chalcogenides-based photocatalysts (Xu et al. 2019).

Nitrides based photocatalysts are well suited for catalytic applications as they have good electrical conductivity and Pt-like band structures, thus showing catalytic activity in hydro-denitrogenation and hydro-desulfurization (Rao et al. 2021). Also nitrides nanostructures possess good mechanical toughness, including an outstanding stability in aqueous solution during electrochemical reactions (Qureshi et al. 2017). For example, the application of Gallium nitride (GaN) as a photocatalyst recorded immense success in sensing fields due to high mechanical strength and chemical stability compared to chalcogenides and oxides-based photocatalysts (Yonenaga 2001), and Tantalum nitride (Ta₃N₅) was effectively used for solar water splitting, due to its suitable band edge potential and narrow direct band gap (2.1 eV) (Tabata et al. 2010).

Chalcogenides-based nanomaterials had shown tremendous progress in the photocatalytic hydrogen evolution; hydrogen production from water splitting under visible light (Li et al. 2020b) among the chalcogenides, CdS CuS and ZnS semiconductors are well applied for photocatalytic reactions, such as water decomposition and CO₂ reduction (Zhang et al. 2019). CuS/ZnS composites constructed by Zhang et al. (2011) showed excellent photocatalytic hydrogen evolution activity under visible light, and the best H₂-production rate reached 4,147 μmol h⁻¹ g⁻¹ with quantum efficiency of 20% at 420 nm. Chen et al. (2016) constructed CuS/CdIn₂S₄/ZnIn₂S₄ photocatalysts, which, compared to P25, CuS/CdIn₂S₄/ZnIn₂S₄ present outstanding photocatalytic activity to degrade MO under simulated sunlight and visible light, as a result of the large surface area (239.3 m²/g) and effective visible light response with an absorption edge of 670 nm.

Metal oxides play a significant role in environmental remediation technology. Most metal oxides may be used as photocatalysts due to their promising light absorption capabilities, electronic configuration, and charge transport characteristics. The most important characteristics of the photocatalytic system include high surface property, suitable material properties, such as high crystallinity, and electronic properties for enhanced absorption behaviour (Hernández-Ramírez & Medina-Ramírez 2016). Metal oxides are largely considered effective photocatalysts for degradation of toxic organic compounds (as pollutants) present in wastewater (Khan et al. 2015) and regarded as low cost photocatalysts with good application prospect in photocatalysis. Murthy et al. (2015) reported that TiO₂, ZnO and CeO₂ are suitable photocatalysts for the degradation of mutagenic compounds, which induce heritable change in cells or organisms, and carcinogenic compounds, which induce unregulated growth processes in cells or tissues of multicellular organisms (Akbari-Fakhrabadi et al. 2015; Murthy et al. 2015).

At present, it is difficult to make comparative evaluation not only across photocatalytic systems but also between photocatalysts (Hoque & Guzman 2018). The vast majority of the approaches are based on the use of TiO₂ semiconductors (Dong et al. 2015a), but further research should be carried out. As the upscale of photocatalytic processes may be challenging without adequate studies on more economical and available materials, also the design of more reliable photocatalysts through coating or doping on fixed supports would allow for the simultaneous degradation and separation of contaminants from the effluent stream.

To the best of our knowledge, there has been no report on a systematic comparison of appropriate semiconductor photocatalysts, although the application of photocatalysts such as TiO₂, ZnO, and CeO₂ has been reported due to their exceptional ability to generate charge carriers when stimulated with the required amount of light energy (Chen et al. 2012; Kalathil et al. 2013; Ansari et al. 2014).

Systematic comparative studies have not been reported in the treatment of oilfield produced water with photocatalysis processes, according to Scopus data (Figure 1). According to research, there is a rise of scientifically appealing but inappropriate materials that are brittle, chemically undesirable, costly, and potentially hazardous. WO₃ and CdS, for example, are visible light photocatalysts that are relatively effective, but the toxic nature of CdS is incompatible with safe and sustainable water treatment practices (Nyamukamba et al. 2018). Also, the application of nanostructured materials to water treatment, if discharged to the environment, could possibly lead to further environmental pollution (Pietroiusti et al. 2018) and so concerns about the environmental behaviour and toxicity of various nanomaterials cast doubt on the long-term viability of nanotechnology for wastewater treatment and purification (Hlongwane et al. 2019). Thus, a system scale for photocatalytic process should consider the bio-toxicity and environmental hazards associated with nanomaterials.

In this regard, Wang et al. (2021), using a life cycle assessment (LCA), as shown in Figure 12, evaluated the environmental impact of CdS, ZnFe₂O₄, and NiFe₂O₄ under visible light irradiation. After 4 h of photocatalysis treatment, the removal efficiency of MB was greatest for CdS followed by NiFe₂O₄ and ZnFe₂O₄. Moreover, based on the results of this study, ZnFe₂O₄ and NiFe₂O₄ have lower environmental impacts than CdS, both show promise as photocatalysts, as shown in (Figure 12(a)), Cds had a higher environmental impact with high respiratory effect, climate change and carcinogenic effects compared to ZnFe₂O₄, and NiFe₂O₄. Thus, CdS will not be recommended as a treatment catalyst. Additionally, NiFe₂O₄ and ZnFe₂O₄ are recommended for further studies in a pilot treatment system due to their low environmental impacts (Wang et al. 2021).

In another work, the LCA between heterogeneous photocatalysis photo-Fenton treatment process shows that an industrial wastewater treatment facility based on heterogeneous photocatalysis has a greater environmental effect than the photo-Fenton option, which scores 80–90% lower in the majority of impact categories evaluated. Further materials engineering studies were recommended to be carried out as the combination of heterogeneous photocatalysis process to the existing biological wastewater treatment can reduce the eutrophication potential, but require higher site area and electricity consumption (Chong et al. 2010).

In terms of environmental applications, photocatalysis has promising importance in wastewater treatment, including applications in the oil and gas industry, as shown by the recent growth in relevant research. Furthermore, the discharge water quality may be as high as required in light of the aim and allowable discharge concentration under various laws in different parts of the world. The present state of photocatalysis does not take into account intermediates generated during treatment; these intermediates may be more dangerous to humans and the environment in the short term. As a result, future technical progress should put into consideration for the intermediates formed as well as increase material efficiency.

Environmental pollution caused by oilfield-produced water is a threat to the human race and the environment at large due to the inability to eliminate non-biodegradable and persistent pollutants from OPW before discharge due to the conventional treatment methods used in the majority of oil and gas companies. As a result, this review article discussed the most current advancements in the treatment of OPW based photocatalysis systems. The primary benefit of photocatalytic treatment is its capacity to degrade contaminants to mineral end products without conveying contaminants through one phase to another, as is usual for conventional treatment methods. To upscale photocatalysis systems for water treatment, photocatalysis has to be competitive with other AOP technologies. A systematic comparison between photocatalysts in photocatalytic degradation research should be a primary method to identify an appropriate photocatalyst before further modification to achieving a high activity photocatalyst, suitable for industrial applications. The scope of treatment should consider vast contaminants. Also, studies on the life cycle assessment of photocatalysts at the preparatory and application stage should be intensified. Using life cycle assessment (LCA) concepts may aid in developing environmentally friendly and sustainable green synthesis techniques.

All relevant data are included in the paper or its Supplementary Information.