ABSTRACT
Despite the advancements in alternative fuels and energy sources, there continues to be a significant global dependence on oil production and extraction. A substantial volume of oilfield-produced water (OPW) is generated during the production and extraction processes of oil fields. Recurrent OPW treatments encountered significant challenges in addressing this particular type of wastewater. Advanced oxidation processes (AOPs) are regarded as a promising alternative approach for the degradation of recalcitrant organic compounds in the OPW. This review focuses on the characterization of OPW. The treatment of organic pollutants in wastewater using AOPs, such as ozonation, Fenton oxidation-based processes, heterogeneous photocatalysis, and persulfate oxidation, is comprehensively reviewed in terms of their efficiency for pollutant degradation. The primary challenges in this field and the future directions for development are proposed, with the aim of providing a valuable reference for achieving highly effective treatment of OPW.
HIGHLIGHTS
Characteristics of oilfield-produced water were analyzed.
Treatment of oilfield-produced water using advanced oxidation processes was discussed.
Recommendation and future research are proposed.
INTRODUCTION
The utilization of oil plays a pivotal role in meeting the global demand for power generation (Scafutto & de Souza Filho 2016). The advancement of global industry has spurred an increasing demand for oil, resulting in extensive extraction of crude oil through artificial means (Jia et al. 2019). The majority of oil fields in China are developed through water injection, particularly during the middle and late stages of development. The water content in the produced liquid typically reaches 70–80% at this stage and may even exceed 90%. During the process of oil and gas exploitation, a substantial volume of formation water and injection water is discharged, commonly referred to as oilfield-produced water (OPW). The OPW is a complex and multifaceted mixture. The composition of OPW is typically influenced by geological conditions, formation depth, oil production techniques, crude oil transportation conditions, and dehydration technologies. Direct discharge of untreated produced water (PW) may result in irreversible harm to organism survival (Lu et al. 2021), human health, and the ecological environment (Zhou et al. 2021). Relevant treatment measures should therefore be implemented to ensure the proper management of substance discharge or disposal, thereby effectively mitigating potential environmental impacts.
The petroleum industry has yet to fully explore advanced oxidation processes (AOPs), which are widely recognized as environmentally friendly technologies capable of minimizing the generation of secondary waste (Hasan et al. 2011). The exceptional reactivity of AOP technology enables effective interaction with a diverse range of organic compounds through the generation of hydroxyl radicals (HO‧) and other reactive species during the process. These processes facilitate the mineralization of recalcitrant organic compounds into carbon dioxide and water or at least their degradation into innocuous substances (Ferrarese et al. 2008; Boczkaj & Fernandes 2017). The application of AOPs, such as ozonation, Fenton oxidation-based processes, heterogeneous photocatalysis, and persulfate (PS) oxidation, has demonstrated significant efficacy in the removal of soluble compounds in OPW (Scott et al. 2008; Afzal et al. 2015; Leshuk et al. 2016; Liu et al. 2016c; Jiménez et al. 2019; Fang et al. 2020). Liu et al. (2021) employed the ozonation process for the treatment of OPW. They found that the ozonation system significantly augmented the rates of oxidation and effectively eliminated the polycyclic aromatic hydrocarbons (PAHs) within 30 min. Finally, the sequential oxidation of the OPW matrix by ozonation is highly recommended. Mario et al. (2018) studied the degradation of high hydrophobic organics in saline-PW through photocatalysis using reduced graphene oxide (rGO)-–TiO2 nanocomposites. The photocatalytic degradation rates exhibit the following increasing order: acetic acid < phenols < naphthalene < xylenes < toluene. However, the degradation of acetic acid is almost not degraded under the same experimental conditions. The findings of this study can contribute to the exploration of novel perspectives in designing high-performance photocatalysts based on graphene-like TiO2 for the efficient removal of hydrophobic bio-recalcitrant pollutants from saline water. Dong et al. (2024) investigated the utilization of Cu0 as a catalyst to activate PS for the degradation of hydroxypropyl guar gum (HPG) in oilfield water. They found that the degradation rate of HPG reached up to 81% at a temperature of 45 °C, pH value of 7, oxidant ratio of 2:3 (H2O2:Na2S2O8), total oxidant dosage of 10%, and Cu0 dosage of 0.3 g/L. The results demonstrated that the combined degradation of HPG pollutants by Cu0-activated H2O2/Na2S2O8 exhibits a broad spectrum of applications, offering novel strategies and mechanistic insights for the advanced oxidation of oilfield pollutants.
The aim of this review is to explore the treatment and removal of dissolved organic matter in OPW through AOPs (including ozonation, Fenton oxidation-based methods, heterogeneous photocatalysis, and oxidation) as documented in scientific literature. The mechanism of oxidation was discussed. Finally, the practical application of these processes in real-world scenarios and the potential challenges that may arise are taken into consideration.
OPW CHARACTERISTICS
The composition of OPW consists of a combination of particulate matter as well as dissolved organic and inorganic compounds. Its physicochemical properties vary with the production region and the predominant hydrocarbon types present in the crude oil. The variability of OPW is influenced by factors such as geographical location, oil and gas composition, reservoir characteristics, and history of water injection. Table 1 summarizes the main components of OPW. The key components in OPW can be broadly categorized into the following groups:
Characteristics and composition of OPW
Dissolved organic compounds
The natural organic matter in OPW is closely associated with the composition of crude oil, as it originates from the process of crude oil extraction. It contains a rich variety of dissolved organic compounds including PAHs, organic acids, BTEX (benzene, toluene, ethylbenzene, and xylenes), and phenol in the OPW (Binet et al. 2011). These aromatic compounds have been classified as high-priority chemicals (Neff et al. 2011a; WHO 2019). They are defined as polyaromatic hydrocarbons with two or more fused aromatic rings that exhibit resistance to removal from OPW. The presence of BTEX can be detected in OPW water from various sources. Table 1 illustrates PAH concentrations ranging from 0.04 to approximately 3 mg/L, BTEX concentrations ranging from 0.73 to about 24.1 mg/L, and phenol concentrations ranging from 0.4 to around 23 mg/L present in OPW.
Dissolved inorganic compounds
The reservoir's significant burial depth results in OPW being characterized by a high concentration of inorganic ions, typically ranging from 1,000 to 40,000 mg/L. The types and concentrations of these ions vary considerably under different geological conditions. Inorganic ions commonly found in OPW include Fe2+, Na+, Ca2+, Mg2+, K+, Ba2+, Sr+, A13+, Li+, Cl−, CO32−, HCO3−, SO42−, OH−, and GN− (Neff et al. 2011a). Among them, the abundance of inorganic anions follows the order: Cl− > SO42− > Br− > HCO3− > I−. Similarly, the abundance of inorganic cations obeys the order: Na+ > Ca2+ > Mg2+ > K+. These compounds significantly enhance conductivity, salinity, and the scale formation (Jiménez et al. 2018). In addition, OPW may contain a range of heavy metals such as mercury, lead, cadmium, chromium, zinc, and nickel (Igunnu 2014; Dwyer & McDonald 2016). The composition and concentration of toxic metals present can vary across different oilfields due to factors such as well longevity and geological formation (Gregory et al. 2011).
Oil and grease
Oil and grease, whether emulsified, dispersed, or free, are of utmost concern as OPW components in both onshore and offshore fields. They have emerged as a pivotal control parameter for legislators (Islam et al. 2013). The concentration of oil and grease ranges from 2 to 654 mg/L.
Produced solids
The oil production wastewater contains two types of solids: natural and artificial. Natural solids, such as plankton, silt, algae, clay, sediment, and sand, are inherent reservoir materials that exhibit upward mobility toward the surface. Artificial solids introduced into the system through external interventions include waxes, proppants, corrosive debris, gravel pack particles, drilling mud, and cement fine particles. The presence of these solids leads to obstructions in the wellbore, resulting in reduced flow rates and subsequent production decline (Amakiri et al. 2022).
Treatment chemicals
To ensure the stable production of oil fields, it is imperative to incorporate specific chemical additives during the reproduction process. These chemicals, encompassing gas treatment agents, oil displacement agents, biocides, corrosion inhibitors, coagulants and flocculants, scale inhibitors, emulsion breakers, wax removal agents, defoaming agents and demulsifiers, and so on, effectively mitigate operational issues encountered in the oil and gas production such as pipe corrosion, scaling, wax accumulation in production tubing and enhanced oil recovery (Al-Ghouti et al. 2019; Pedro Da Silva Almeida 2019). However, the addition of chemicals may potentially introduce undesirable side effects or potentiate the toxicity of OPW (Beyer et al. 2020).
Dissolved gases
The OPW solution is characterized by a high concentration of dissolved gases, including natural gas components (methane, ethane, propane, and butane), carbon dioxide, nitrogen, oxygen, and hydrogen sulfide. The presence of significant quantities of these gases can pose a potential hazard due to their toxic and corrosive nature. Notably, hydrogen sulfide has garnered significant attention (Amakiri et al. 2022).
TREATMENT OF OPW BY AOPS
Schematic of the advantages and disadvantages of AOPs (Gogate & Pandit 2004; Pignatello et al. 2006).
Schematic of the advantages and disadvantages of AOPs (Gogate & Pandit 2004; Pignatello et al. 2006).
Schematic diagram of treatment of oilfield-produced water (Olajire 2020).
Ozonation oxidation
Ozone is a widely used potent oxidizing agent, and its initial application in water treatment primarily focused on disinfection by eliminating bacteria in wastewater and eliminating chroma. Ozonation technology has been extensively employed for the removal of organic matter from wastewater. In comparison to traditional treatment methods, ozonation technology offers several advantages: (1) The AOP does not involve biological reactions, making it a relatively straightforward physicochemical process that could be easily automated and commonly utilized for pretreatment or subsequent advanced treatment (Herndndez-Leal et al. 2011). (2) It exhibits rapid reaction rates with most pollutants within the range of rate constants between 106 and 109 M−1·s−1 (Yang et al. 2021). (3) Ozonation reactions typically require milder operating conditions, resulting in reduced energy consumption. (4) The hydroxyl radicals play a crucial role as intermediate products and reactants, while also serving as initiators for subsequent chain reactions. This makes them highly suitable for the treatment of recalcitrant pollutants that are difficult to degrade using conventional methods. (5) Hydroxyl radicals exhibit nonselective reactivity toward most pollutants and intermediates present in wastewater, leading to their degradation into smaller molecular products such as H2O and CO2. This significantly enhances the degradation rate of COD in wastewater.
The efficiency of contaminant removal through ozonation is marginally inferior compared to other AOPs. Enhancing the efficacy of organic matter elimination becomes imperative. The degradation of pollutants in OPW treatment using ozone as an oxidizing agent generally demonstrates a slightly lower performance compared to other reported AOPs, as indicated by the existing literature (Coha et al. 2021).
Messele et al. (2020) employed carbon-based metal-free materials as catalysts for the ozonation of 1,3-adamantanedicarboxylic acid (ADA). They observed a remarkable removal efficiency of 65% for ADA in the catalytic ozonation process, which was nearly twice as high as the 33% achieved through single ozonation. In addition, the first-order rate constant for the catalytic ozonation reaction (0.0631 min−1) was found to be double that of the ozonation reaction alone (0.0258 min−1). The degradation mechanism of ADA was determined to be governed by hydroxyl radicals. These findings highlighted the exceptional efficiency and promising potential catalytic ozonation as an approach for environmental treatment.
The study conducted by Jiménez et al. (2019) employed ozonation as a treatment method for PW. They found that all the organic components were removed, including a high percentage of acetic acid, in the ozonation combined with the H2O2 system. The recalcitrant nature of acetic acid in PW was well recognized. Under the conditions of an O3 dosage of 4 g/L, H2O2 concentration of 1,500 mg/L, pH value at 10, and a reaction time of 2 h, removal efficiencies for total organic carbon (TOC) and acetic acid were found to be 74 and 78%, respectively. In addition, in terms of TOC, ozonation system had the lowest energy consumption per target compound.
The study conducted by Vaiopoulou et al. (2015) demonstrated that the ozonation of a mixture of naphtenic acids (NAs) followed pseudo-first-order kinetics. In this investigation, ozone exhibited a preferential reactivity toward NAs with high cyclic and molecular weight structures. The reactivity of ozone toward acyclic/unicyclic model NAs was found to be dependent on their molecular structure. Notably, the introduction of branching, tertiary, and quaternary carbons in NAs resulted in reduced ozone reactivity. By combining ozonation with biodegradation, 89% removal of NA and a 15-fold reduction in toxicity were achieved. Therefore, the combined treatment method involving ozonation and biodegradation proved to be highly effective.
Wang et al. (2016a) reported that ozonation emerged as the most efficacious technique for treating oil sands process-affected water. By utilizing a dosage of 2.0 mmol/L O3, ozonation exhibited nearly complete removal of fluorescent aromatics, sulfur-containing naphthenic acids, and classical naphthenic acids. The reaction mechanism of ozonation was found to involve molecular ozone reaction followed by HO· reaction. Qualitative and quantitative confirmation of the removal of aromatics and naphthenic acids was further obtained through the 1H NMR analysis. Furthermore, the ozonation process demonstrated a reduction in acute toxicity toward Vibrio fischeri and goldfish primary kidney macrophages.
Scott et al. (2008) employed ozonation as a treatment method for precipitation-free oil sands process water. A 50-min ozonation process resulted in the production of nontoxic wastewater and led to an approximate 70% reduction in naphthenic acid concentrations. Following 130 min of ozonation, the residual concentration of naphthenic acids in filtered oil sands process water was measured at 2 mg/L. The TOC remained constant throughout the entire 130-min ozonation process; however, there was a significant decrease of approximately 50% observed in chemical oxygen demand (COD) levels. Gas chromatography-mass spectrometry analysis revealed that ozonation caused a reduction in the proportion of high-molecular-weight naphthenic acids.
Fenton oxidation-based processes
The Fenton process is widely recognized as one of the most prominent AOPs due to its utilization of simple reactants and rapid degradation kinetics for various pollutants. Moreover, it exhibits versatile applicability and demonstrates a remarkable capacity to withstand interference (Wang et al. 2016b; Zhang et al. 2019).
(a) Possible Fenton-like degradation mechanism (Yi et al. 2019); (b) photo-Fenton degradation mechanism (Zhang et al. 2022); and (c) electro-Fenton degradation mechanism (Pan et al. 2020).
(a) Possible Fenton-like degradation mechanism (Yi et al. 2019); (b) photo-Fenton degradation mechanism (Zhang et al. 2022); and (c) electro-Fenton degradation mechanism (Pan et al. 2020).
The generation of PW represents the largest wastewater flow in the context of oil and gas exploration. PW is the largest wastewater flow generated during oil and gas exploration. Afzal et al. (2018) investigated the application of Fenton oxidation for the degradation of organic matter in PW. The experiments focused on the removal of COD and involved varying concentrations of hydrogen peroxide (0.12 × 10−−3 mol/L to 3 mol/L), molar ratios of [H2O2]/[Fe2+] (2 to 75), and reaction times ranging from 30 to 200 min. The Fenton reaction was initiated using ferric sulfate heptahydrate (FeSO4‧7H2O) and H2O2. The enhancement in COD removal was significantly amplified with an increase in the molar ratio of [H2O2] to [Fe2+]. The removal of COD was observed to increase with an elevation in the concentration of H2O2 from 0.001 mol/L up to an optimal value of 0.01 mol/L, beyond which the degradation efficiency decreased at higher concentrations of H2O2. The COD removal exceeded 90% under the optimal reaction conditions. The Fenton oxidation process exhibited excellent efficacy in the removal of organic matter from PW.
Jin & Davarpanah (2020) demonstrated that photo-Fenton treatment exhibited significant efficacy in the removal of both solids and chemicals from flow-back water. The implementation of chemical-enhanced oil recovery techniques results in a remarkable 75% reduction in water consumption. Moreover, this translates to an annual conservation of approximately 104 MM m3 of water, which was sufficient to sustain the livelihoods of 1,000 individuals. Furthermore, these methods obviated the necessity for substantial utilization of freshwater resources during hydrocarbon production.
The removal of benzene (B), toluene (T), and p-xylene (X) in aqueous solution was investigated using the EF method under various electrode conditions, as reported by Radwan et al. (2019). The complete degradation of BTX can be achieved within 30 min under optimal conditions. The removal efficiencies of TOC were 95.4, 88.7, and 87.6% when sacrificial stainless steel, carbon, and nickel anodes were applied, respectively. The Fe2+ ions were continuously generated in the stainless steel anode through sacrificial electrode mode in the solution. The cost of EF treatment for one cubic meter of BTX-contaminated water was estimated to be $1.40 under specific conditions.
Heterogeneous photocatalysis
The polyphase photocatalysis technique, pioneered by Fujishima and Honda in the 1970s, represents a highly promising technology for wastewater treatment aimed at effectively eliminating recalcitrant organic compounds such as PAHs, phenolic compounds, sulfides, and other substances originating from OPW. The process is based on the utilization of metal oxides as catalysts and oxygen as electron acceptors to facilitate the generation of oxidizing species, such as HO‧, O2‧−, and HO2‧ (Hasan et al. 2011).
Schematic diagram illustrating the principle of photocatalysis (Ahmed et al. 2011).
Schematic diagram illustrating the principle of photocatalysis (Ahmed et al. 2011).
The photocatalytic degradation pathway for treating OPW involves a series of sequential steps: (i) diffusion-driven mass transfer of reactants from the bulk fluid phase to the outer surface of the photocatalyst, (ii) internal mass transport of reactants through pores to reach the active sites on the surface where they could be adsorbed, (iii) adsorption of organic pollutants onto the photoactivated surface of the photocatalyst, (iv) desorption of photodegradation products, and (v) internal and external diffusion of the final photodegradation products.
γ-Fe2O3 nanoparticles were synthesized by Sheikholeslami et al. (2019) using co-precipitation methods for the purpose of photocatalysis. The degradation of BTEX was employed as an indicator for assessing the efficiency of PW treatment. The impact of varying parameters, including pH (3–7), catalyst concentration (0–250 mg/L), UV light intensity (0–100 W), and visible light intensity (0–225 W), was investigated. The results demonstrated that the removal efficiency of BTEX was 95% within a span of 5 days and reached 97% in just 90 min when employing maghemite nanoparticles under both visible and ultraviolet light conditions. In addition, the adsorption (approximately 30%) and photolysis processes (approximately 20%) of maghemite nanoparticles significantly contributed to the removal efficiency observed in the photocatalytic process.
The photocatalytic treatment of naphthalene in real offshore PW was conducted by Liu et al. (2016a, 2016b) using UV irradiation and nanoscale TiO2 powder (aeroxide®P25) as the catalyst. Furthermore, it was observed that the presence of bivalent cations induced catalyst agglomeration. Thus, the reduction of active sites on the catalyst surface leads to a decrease in the photocatalytic rate of the analytes. The rate constants of the photocatalytic reaction in the immobilized systems reached 0.00305 min−1, surpassing that of the homogeneous system (0.00219 min−1). The immobilized catalyst exhibited excellent photo-oxidation performance.
Liu et al. (2016a, 2016b) employed UV and TiO2 to investigate the impact of the OPW matrix on the photodegradation of PAHs. The TiO2 nanocatalyst with an average particle size of approximately 12–17 nm was synthesized using an enhanced sol-gel method. The photocatalytic experiments conducted on degrading PAHs in distilled water samples revealed that a degradation rate is 10–100 times higher than that observed in OPW. The photocatalytic performance was hindered by the complex composition of OPW. The impacts of various operational parameters, namely, salinity, insoluble particulate matter, and organic composition, on the degradation of PAHs were investigated. The removal rates of PAHs with lower molecular weights increased as the ion strength increased. The salinity variation and presence of insoluble matter played a pivotal role in the efficient removal of high-molecular-weight PAHs, primarily due to their limited solubility caused by the ‘salting out’ effect and the competitive adsorption between PAHs, insoluble matter, and TiO2. The attenuation of ultraviolet light primarily stems from organic components. The present study offered an opportunity for further investigation into the influence of these organic substances and halogen ions on the efficacy of photocatalytic techniques in treating OPW.
Hayati et al. (2018) investigated the catalytic efficiency of a ternary nanocomposite heterojunction catalyst consisting of ZnO/TiO2 anchored on rGO for the degradation of phenol in aqueous solutions. The effect of various operational conditions, including pH, catalyst dosage, phenol concentration, and light illumination, on the photocatalytic performance was investigated. The experimental findings demonstrated that a complete degradation rate of phenol was achieved at an initial concentration of 60 mg/L, with a catalyst dosage of 0.6 g/L, light intensity set at 150 W, pH maintained at 4, and a reaction time of 160 min. The introduction of radical scavengers, namely, isopropyl alcohol, p-benzoquinone, and ethylenediaminetetraacetic acid, into the reaction system indicated the presence of hydroxyl radicals for photocatalytic oxidation. The removal ratios of TOCt/TOC0 and CODt/COD0 were observed to decrease to 14 and 8%, respectively, after treatment duration of 440 min for the real petrochemical effluent.
PS oxidation
PS oxidation is a highly efficient advanced oxidation technique that employs thermally activated PS, ultraviolet light, and/or acidic pH conditions to generate free radicals for the degradation of various contaminants. The electron transfer from transition metals, such as iron, manganese, and cobalt, along with the potential mechanisms are illustrated by Equations (15)–(19):
The oil industry generates significant volumes of PW, which is characterized by the presence of organic pollutants such as NA and high concentrations of inorganic salts. Aher et al. (2017) employed membranes functionalized with iron oxide (FexOy nanoparticles) as catalysts and PS as an oxidizing agent for the oxidation of NA from water containing high concentrations of total dissolved solids (TDS). The results indicated that the concentration of NA decreased by up to 8 mg/L (from 34 mg/L) after the second passage through a FexOy functionalized membrane with the addition of PS at 40 °C. They reported that this system had demonstrated promising potential as an alternative to NA treatment.
The role of temperature in PS oxidation is of significant importance. The effect of temperature on oxidant consumption was investigated by Ferreira et al. (2022), revealing an inverse relationship between temperature and oxidant consumption. The reasons for this phenomenon could be attributed to two factors. First, the presence of sulfate radicals led to the oxidation of organic compounds in solution. Second, an increase in the levels of sulfate anions, protons, and oxygen was observed as the oxidant decomposition progressed (Domínguez et al. 2021). The reduction of COD and Biochemical oxygen demand (BOD) was achieved through heat-activated PS oxidation; however, the biodegradability of the PW remained unchanged due to the formation of by-products.
Gholami et al. (2021) conducted experiments on the electro-activation of persulfate (EC/PS) for real PW treatment and investigated the influence of operational conditions on COD and ammonia removal. The results obtained indicated that a maximum COD removal efficiency of 71% was observed. The ammonia removal efficiency of 69% was observed at a pH of 10, which can be attributed to the physicochemical adsorption facilitated by air stripping and the thermal activation of PS. The removal of contaminants was predominantly governed by the hydroxyl radical mechanism, accounting for 67% of the overall contribution, while sulfate radicals contributed 33%. Compared to other AOPs, the EC/PS process demonstrated superior cost-effectiveness and efficiency.
Interactions among chemical constituents can induce alterations in the composition of hydraulic fracturing fluids. Manz et al. (Manz & Carter 2017) investigated the application of activated PS oxidation for the degradation and transformation of 3-furfuraldehyde (or furfural). The utilization of two chemicals as additives in hydraulic fracturing was prevalent. A study was conducted to investigate the impact of elevated temperatures, varying PS dosing and Fe (III) concentration, initial pH, and the addition of other chemical additives on furfural degradation. The reaction was found to exhibit pseudo-first-order kinetics with respect to the furfural concentration. The inclusion of radical scavengers, methanol, and tert-butyl alcohol in the reaction supports the involvement of a HO· mechanism. The addition of iron sulfate to a solution with a pH of 5.4 did not exert any influence on the activation energy of furfural oxidation, which remained at 107 kJ/mol. However, when the pH was reduced to 2.54, the activation energy decreases to 75 kJ/mol.
CONCLUSIONS AND PERSPECTIVES
This article provides a comprehensive review of the application of AOPs for pollutant removal in PW treatment. Specifically, ozonation, Fenton oxidation-based processes, heterogeneous photocatalysis, and PS oxidation are critically examined. However, it should be noted that the utilization of these AOPs for treating OPW still presents significant challenges, and further research is warranted to overcome current barriers. Several potential areas for future investigation include:
- (1)
The combined ozonation process is considered as an alternative to address the limitations of conventional ozonation, including low ozone dissolution and slow mass transfer from the gas phase to the liquid phase, which results in high-energy consumption.
- (2)
The utilization of photocatalysis in industrial water treatment facilities is limited due to the inherent constraints of heterogeneous photocatalysis, including challenges associated with the separation of colloidal catalysts from the treated solution and mass transfer limitations imposed by fixed catalyst surfaces on substrates.
- (3)
Given that OPW exhibits a natural pH in close proximity to 8, which holds significant importance for the oil industry, subsequent research endeavors have been directed toward the development of treatment methods under neutral pH conditions.
- (4)
The consideration of the influence exerted by diverse cations and anions present in the PW on the oxidation process is imperative.
- (5)
The investigation of diverse water quality systems at various well sites is necessary to provide robust data support, propose effective technical solutions for practical water quality issues, and offer insights into the scalability and commercial viability of this technology.
- (6)
The treatment of aromatics compounds in micro concentration is not addressed in previous studies, which is a persistent issue.
ACKNOWLEDGEMENT
This work was carried out with the financial supports of the Key Research and Development Project of Weinan City (Grant No. 2022ZDYFJH-80), Shaanxi Provincial Education Department (Grant No. 23JK0443) and Talent project of Weinan Normal University (Grant No. 2022RC14).
DATA AVAILABILITY STATEMENT
All relevant data are included in the paper or its Supplementary Information.
CONFLICT OF INTEREST
The authors declare there is no conflict.