Petroleum Hydrocarbon Removal from Wastewaters: A Review

: Oil pollutants, due to their toxicity, mutagenicity, and carcinogenicity, are considered a serious threat to human health and the environment. Petroleum hydrocarbons compounds, for instance, benzene, toluene, ethylbenzene, xylene, are among the natural compounds of crude oil and petrol and are often found in surface and underground water as a result of industrial activities, especially the handling of petrochemicals, reservoir leakage or inappropriate waste disposal processes. Methods based on the conventional wastewater treatment processes are not able to effectively eliminate oil compounds, and the high concentrations of these pollutants, as well as active sludge, may affect the activities and normal efficiency of the refinery. The methods of removal should not involve the production of harmful secondary pollutants in addition to wastewater at the level allowed for discharge into the environment. The output of sewage filtration by coagulation and dissolved air flotation (DAF) flocculation can be transferred to a biological reactor for further purification. Advanced coagulation methods such as electrocoagulation and flocculation are more advanced than conventional physical and chemical methods, but the major disadvantages are the production of large quantities of dangerous sludge that is unrecoverable and often repelled. Physical separation methods can be used to isolate large quantities of petroleum compounds, and, in some cases, these compounds can be recycled with a number of processes. The great disadvantage of these methods is the high demand for energy and the high number of blockages and clogging of a number of tools and equipment used in this process. Third-party refinement can further meet the objective of water reuse using methods such as nano-filtration, reverse osmosis, and advanced oxidation. Adsorption is an emergency technology that can be applied using minerals and excellent materials using low-cost materials and adsorbents. By combining the adsorption process with one of the advanced methods, in addition to lower sludge production, the process cost can also be reduced.


Introduction
Oil is a very valuable resource for production of energy and many chemicals. However, negligence during the stages of extraction, transfer, processing, storage, and consumption can cause environmental pollution that is more often severe and irreparable [1]. Multi-ring aromatic hydrocarbons have been identified based on various reports in various sources of groundwater. The United States Environmental Organization has given particular attention to these compounds, due to their toxicity and their carcinogenic potential for humans and various animal species [2]. The connection with industry is also noteworthy (Figure 1) [3]. One of the most important sources of emission of these pollutants to water resources is the leakage of storage tanks, extraction, refineries, and distribution stations [4]. The maximum allowable concentration based on the WHO standard for benzene in drinking water is 10, toluene 700, ethylbenzene 300 µg/L, and for xylene, it is 500 µg/L [5]. The solubility of benzene, toluene, xylene, and ethylbenzene in water is 1780, 535, 175, and 152 mg/L, respectively. The molecular structure of benzene, toluene, ethylbenzene, and xylene, commonly known as BTEX materials, and their physicochemical properties are listed in Table 1. Crude oil is a complex of hundreds of different compounds, often hydrocarbons. Low levels of other elements such as nitrogen (0% to 0.5%), sulfur (0% to 6%), oxygen (0% to 3.5%), and some metals are found to be insignificant in crude oil. The aromatic hydrocarbons (aromatics) are compounds that contain only carbon and hydrogen in their structure, all of which are destructive to the central nervous system. Aromatic hydrocarbons can also cause bone marrow suppression and musculoskeletal malformations [6]. This group of compounds contains benzene and its derivatives, and several condensed benzene rings. Although benzene and most of its compounds are similar, they are not bloated or have a specific aroma. The main source of aromatic hydrocarbons is coal tar. In addition to coal tar, oil sources also extract aromatic compounds such as toluene, benzene, and other compounds with lower boiling points. When hydrogen atoms in the benzene ring are replaced by methyl or ethyl, there are new compounds such as toluene (methylbenzene), xylene (dimethyl benzene), and ethylbenzene. In the case of xylene, there are three types of topical isomers called o-, m-, and p-xylene [7]. The aromatic hydrocarbons have molecular rings, and in general, these compounds are liquid, and they are noticeable in the room temperature of their vapor pressure. This issue, as well as their toxicity, can create significant risks in the workplace. In this group, benzene, toluene, ethylbenzene, styrene, xylene, cumene, and tetralene can be mentioned [8].
Petrochemical hydrocarbons, including BTEX compounds, are known as an indicator for exposure to volatile organic compounds as well as petroleum compounds [9]. Contact with BTEX Table 2. Sources and health effects of petrochemical hydrocarbons pollutants [11,17,18].

Environmental Problems/Scenarios/Fate
To better understand the environmental impact of petroleum hydrocarbons, it is necessary to briefly refer to the fate of petroleum on environmental systems ( Figure 2). A variety of processes occur when petroleum products of any kind (e.g., crude oil, gasoline) are released into the environment. These can be physical abiotic processes or biological through interaction with microorganisms and metabolic pathways. The following are abiotic reactions contributing to the breakdown or movement of various compounds and hydrocarbons found in petroleum products. The fastest physical process is the evaporation of light compounds. The speed and efficiency of this can be influenced by environmental factors such as temperature, wind velocity, water turbulence, or surface characteristics [19]. This is often a key process in the initial stages of petroleum release and has been a major factor in the removal of volatile components of petroleum during large scale spills. Soon after initial petroleum release, lighter or shorter chain hydrocarbons can lose large proportions of their mass to evaporation, as much as 99% or more for light alkanes such as propane, butane, and pentane [21]. Photolytic breakdown of hydrocarbons, especially PAHs and BTEX compounds found in petroleum, is another method of petroleum weathering in the environment. By interacting with the aromatic structure pi-bonded system consisting of delocalized electrons, light can be absorbed to promote a higher energy state. This excited state can facilitate the breakdown of the compound, leading to the degradation of light-absorbing compounds within the petroleum mixture. The dissolution of petroleum hydrocarbons can occur based on the solubility of the compounds involved. Solubilities of these hydrocarbons vary based on specific properties, including molecular size, structure, and polarity. Low molecular weight aromatics are typically among the most soluble compounds within a petroleum mixture [19]. Other compounds such as alkanes are non-polar and thus largely hydrophobic and highly unlikely to dissolve in water. Chemical agents are often used in instances of spills to increase the solubility of petroleum compounds. Surfactants and dispersants can create hydrophilic aggregates or emulsions in water, breaking down the petroleum particle sizes and diluting the hazardous compounds and aiding the potential for biodegradation [22]). Many petroleum hydrocarbons have a high affinity for organic matter due to their hydrophobicity. As such, they can easily adsorb to organic matter and will preferentially bind to soil and sediments. Once sorbed, these eight compounds are difficult to remove and can persist for many years before degradation [23]. One underlying factor contributing to the persistence of sorbed petroleum hydrocarbons is the limited surface area, causing a lack of bioavailability for biodegradation and limited access to the physical processes that may otherwise contribute to their breakdown or dilution. In cases where large volumes of petroleum reach shorelines, such as large spills, the presence of petroleum hydrocarbons can increase erosion due to interference with the soil/sediment matrix [24]. Once sorbed, transport of these hydrophobic contaminants can occur through erosion or sediment or soil mixing. Erosion of coastal sediments has been known to redistribute petroleum hydrocarbons into the water column, allowing for transport and potentially re-exposure to plants or marine life.
A special environmental scenario must be done for the case of soil ( Figure 3). Though social and economic development largely depend on petroleum hydrocarbons as a dominant source of energy, it has caused a huge area of contamination and relevant adverse effects [25]. The contamination of petroleum hydrocarbons disseminates from soil and water to human health. Petroleum hydrocarbon contamination on soil petroleum hydrocarbon contamination of soil is a widespread global environmental concern. Oil and fuel spills in soil are among the most extensive and environmentally damaging pollution problems as it is threatening to human health and ecosystems, especially in cold regions. Biochemical and physicochemical properties of soil are deteriorated by refinery products, and it also limits the growth and development of plants. Water and oxygen deficits, as well as to shortage of available forms of nitrogen and phosphorus, are the main changes in soil properties due to contamination with petroleum-derived substances. Petroleum-hydrocarbon-contaminated soil causes organic pollution of underground water, which restricts its use and causes economic loss, environmental problems, and decreases the agricultural productivity of the soil. Microorganisms, plants, animals, and humans are facing vulnerable situations because of the toxicity of petroleum hydrocarbons. Soil enzymes are one of the important biotic components which are responsible for soil biochemical reactions. Petroleum hydrocarbon has adverse effects on soil enzyme activities [25]. Diesel oil has a much stronger inhibitory effect on nitrification than petrol. Diesel oil can cause chronic or acute effects in the plants. Interference in the hydric relations of the plants is caused by diesel oil pollution. Petroleum hydrocarbon released into the sea, normally during transportation, leading to the pollution of several sites, and can eventually reach the coasts. Oil spills ranging from low-level discharges to catastrophic accidents threaten coastal environments; large spills commonly are followed by clean-up efforts, but complete containment is rare. As the solubility of petroleum hydrocarbon in water is generally low, certain fractions of it float in water and form thin surface films, which will facilitate agglomeration of particles and natural organic matter, and impact oxygen transfer. Other heavier fractions will accumulate in the sediment at the bottom of the water, which may affect bottom-feeding fish and organisms [25].

Removal and Purification Methods
The latest methods of removing and eliminating oil pollution from the sea include aeration; adsorption processes with the help of activated carbon and natural and synthetic adsorbents; oxidation by some oxidants; the use of laser technology; the use of dispersants; adsorption and separation of water surface by various types of adsorbents, especially adsorbents prepared using nanotechnology; evaporation of petroleum compounds using vacuum techniques; physical separation by means of foaming; conversion of oil pollution to other compounds by methods such as photo-oxidation as well as biological methods [12].
A high set-up and exploitation cost, the need for skilled personnel to set up and operate, the lack of rapid adaptation of microorganisms to remove these compounds, the presence of electron receptors, and the production of secondary toxic compounds have limited the use of these methods [27]. A summary of the methods used to remove oil compounds is presented in Table 3. Catalytic ozonation process (COP) ozone was generated using a commercial ozone generator marketed as an air purifier, and particle measurements were recorded before, during, and after the release of terpenes from a pine oilbased cleaning product.

Canvas
The use of canvas is now commonly used to capture and seal leakage of oil. These canvases are designed to prevent floating oil from escaping the surface of the water. One type of canvas, called a canopy fence, consists of a PVC barrier that is fitted underneath the polyethylene floats and is provided with apertures for holding the barrier at the bottom. In some of its types, 6-foot-long hinges are used, with a total length of 50-100-feet in length [27].

Chemicals
One of the methods for concentrating oil is the use of chemicals at contaminated water levels that reduces the rate of oil release and causes oil returns at the level of the oil mass. This method is used in real-world oil spills only in calm water [64].

Microorganisms
Many studies have shown that microorganisms (fungi, bacteria, and yeasts) are the main factors in the decomposition of pollutant molecules, including hydrocarbons, in aquatic and terrestrial environments. Selective microorganisms should be able to accept hydrocarbons as substrates (primary). These microorganisms must have the ability to synthesize enzymes with the ability to accelerate the digestive process of hydrocarbons, and, in addition, harmful products produced by them are not harmful to the environment. Nevertheless, no microorganism alone is able to completely disperse crude oil [65]. Crude oil distillation involves a series of forward chain reactions in which specific microorganisms initiate an initial attack on petroleum compounds. This process results in the production of intermediate compounds that will result in the use of a different class of microorganisms. Local native microorganisms, or LMOs, in the ground and underground waters of oil regions that have become accustomed to these conditions over time can use carbon from pollutants as sources of energy. These germs convert the pollutants into final products (carbon dioxide and water)

Adsorption Method
Sorbents, materials, or mixtures of insoluble materials are used to recover fluids during adsorption, physical attachment, or both [66][67][68][69][70][71][72][73][74][75][76]. Adsorbents must be absorbent of oil compounds and also waterproof [77]. Although they may be used as the only cleaning method for small leaks, adsorbents are often used to remove the final oil trap. Also, the benefits of using adsorbents are to limit the speed of the playback of the layer and help clean it. Debris can be divided into three basic groups: organic, natural, and combined organic. Natural organic adsorbents include carbon monoxide, nitrogen, dry grass, sawdust, corn wood, feathers, and other products, mainly carbonreadable. Organic adsorbents can absorb between 3 and 15 times the weight of their oil, but their use also has some disadvantages. Some organic adsorbents tend to absorb water as much as absorbing oil. This causes the absorbent to drown.
Many organic adsorbents lose particles such as sawdust, which is difficult to collect after being spread over the surface of the water [77]. Natural mineral adsorbents can absorb oil from 4 to 20 times their weight. Mineral absorbers, like organic adsorbents, are inexpensive and readily available in large amounts. These types of adsorbents are not used on the surface of the water. Combined absorbents include human-made materials that resemble plastics, such as polyurethane, polyethylene, and polypropylene, which are designed to absorb fluids on their surface (such as sponges). Other compound absorbents include shrink-bonded polymers and rubber materials, which absorb fluids on their solid structure, causing the swelling of absorbent materials.
Most composite adsorbents can absorb oil up to 70 times their weight. The properties of both absorbent and oil types should be considered when selecting adsorbents to clean up leakages. Oil adsorption with light petroleum products is faster. Also, the oils with thicker layers are more effective at attaching surfaces. Lightweight oil with less viscosity than the heavier and more viscosity type is easier to handle during retrieval of sealing materials, which results in secondary contamination [77]. Johnson and coworkers suggested the potential of using linen fiber to clean up oil spills (Johnson et al. (1973)). Further studies were performed by Cloud and Choi on milkweed. In this paper, the canopy was investigated, and it was found that in the case of high viscosity oil adsorption, it is comparable to polypropylene. Hemp flax and milk, without requiring any process on them, have hydrothermal characteristics due to the presence of wax on their surface, and these have been used as adsorbents in research on the cleaning of oil spills on water and drought [78].
In a report presented by Schrader, the adsorption capacity of mineral, organic, and synthetic adsorbents was compared. According to the results of this study, inorganic adsorbents had a minimum adsorption capacity, while synthetic adsorbents had the highest adsorption capacity. Adsorbents such as canopy also had medium to high adsorption capacity [79]. In another article, the adsorption capacities of various synthetic and natural absorbents were studied [80]. According to this study, natural absorbents such as plant milk, silk fiber, cotton, linen, and wool showed higher adsorption capacities than synthetic adsorbents such as polyester, polypropylene, nylon 6, and nylon 66. By studying the adsorption mechanism by scanning electron microscopy, it was determined that the milk absorbs oil by capillary action, and the wool uses surface adsorption [80]. Methods based on phase separation or surface adsorption, such as activated carbon, cannot completely eliminate the effects of oil contaminants in water, but their use can be very useful in reducing the contamination of oil in water [81]. These methods do not eliminate the organic matter of the water completely [81].

Soil Vapor Extraction (SVE)
BTEX compounds are volatile and, analogous to the volatile organic compounds (VOCs), can be easily removed by soil vapor extraction (SVE) if the conditions are suitable [82,83]. The SVE does not work well in wet or otherwise impermeable oil [83,84] and is considerably slower in cold than in warm conditions [84,85].

Plasma Treatment Method
Various gas-phase electric discharge methods, also known as plasma treatments, are currently the subject of intense development. Electric discharge plasma is defined as a mixture of free moving electrons and positively charged ions [86]. Non-thermal plasma is a source of gas-phase free radicals (O, OH, and H) and other active species, which are useful for destroying pollutants. Reactive species in non-thermal plasma, such as OH radicals, O3 molecules, and O atoms, can react with odorous and toxic gases and vapors and convert them into non-odorous and non-toxic molecules. Plasma can be generated in a medium between two spatially separated electrodes, where a high AC voltage (10-30 kV) is applied for a very short time (1-10 ms) [87]. Plasma may be generated by electric discharges of various types: dielectric barrier discharge (DBD), arc, gliding arc, spatial, glow, pulsed, or streamer discharge, with or without dielectric barriers [88]. The main advantages of low-temperature plasma technologies in BTEX abatement are their relatively low energy consumption and generally moderate cost [89]. Several studies on BTEX removal, side product analysis, and degradation mechanisms by non-thermal plasma (NTP) have been reported over the past few years. Satoh et al. explored the effect of O2 proportion in carrier gas on the removal of benzene by different manners of discharge. The results show that at low oxygen concentration, the byproducts are primarily C2H2, HCN, NO, and HCOOH, while only HCOOH is found at high oxygen concentrations [90]. Stefan et al. investigated the degradation of cyclohexene and BTEX in an NTP air purifying system, and the degradation efficiency order of benzene (<xylene (3%) ≈ ethylbenzene < toluene (11%) ≈ cyclohexene) was found [91].

Photocatalytic Oxidation
Among AOPs, heterogeneous photocatalytic oxidation, which involves the acceleration of a photoreaction in the presence of a semiconductor catalyst, has proved to be of utmost interest due to its efficiency in degrading recalcitrant organic compounds [92,93]. Developed in the 1970s, heterogeneous photocatalytic oxidation has been provided considerable attention from the scientific research community. During the past two decades, numerous studies have been carried out on the application of heterogeneous photocatalytic oxidation processes with an aim to decompose and mineralize recalcitrant organic compounds. Photocatalysts are the class of compounds that generate electron-hole pairs on coming in contact with or on the absorption of light quanta, leading to the chemical transformation of substrates that comes in contact with them and regenerate them. These substances are invariably semiconductors. Semiconductor heterogeneous photocatalysis has enormous potential to treat organic contaminants in water and air. Many semiconductors have been studied for the degradation of petrochemical compounds. Several semiconductors (TiO2, ZnO, Fe2O3, CdS, ZnS) can act as photocatalysts [93]. However, TiO2 has been most widely and universally studied because of its high reactivity, reduced toxicity, chemical stability, lower costs, its ability to break down organic pollutants, resulting in complete mineralization.
The available literature revealed that TiO2, along with other semiconductors/nanomaterials, has been widely studied for the degradation processes in visible and UV light irradiation. In addition, most of the degradation studies were carried out at laboratory scale under UV light. The uses of alternative light sources such as LEDs are promising initiatives for the reduction in power consumption. It is imperative to develop more efficient techniques in which solar energy is used for photocatalysis processes. Only 5% of solar radiation is used for catalysis processes. Also, various studies reported that the intermediates are sometimes more toxic than the original compound. Therefore, intermediates or end products of photocatalytic degradation may prove harmful to various organisms in the environment. However, TiO2 is only responsive to the UV region due to its large band gap (3.2 eV). To expand the application of TiO2, the doping of metals were investigated to enhance the photo-response of TiO2 toward the visible region [94]. Some indicative examples are given in following paragraphs.
There are many examples in the literature reporting the doping of TiO2 with various metals for BTEX degradation. Ce-doped TiO2 nanostructures (CeT) with different amounts of Ce (0.5, 0.75, 1.0, 1.5, and 2.0 wt %) were synthesized using a sonochemical processing method by Lee et al. [94]. The prepared CeT exhibited significantly improved photocatalytic performance towards the degradation of toluene and o-xylene, which was much higher than that observed for pure TiO2 and commercial P25 TiO2. Particularly, photocatalytic degradation efficiencies by the prepared CeT catalysts increased remarkably in the case of o-xylene (up to 99.4%) compared to toluene (up to 49.1%).
Another important study by Laokiat et al. investigated the synthesis and evaluation of transition metal (Fe, V, and W)-doped TiO2 via the solvothermal technique and immobilized onto fiberglass cloth (FGC) for uses in photocatalytic decomposition of gaseous volatile organic compoundsbenzene, toluene, ethylbenzene and xylene (BTEX)-under visible light [95]. Results were compared to that of the standard commercial pure TiO2 (P25)-coated FGC. All doped samples exhibit higher visible light catalytic activity than the pure TiO2. The V-doped sample shows the highest photocatalytic activity, followed by the W-and Fe-doped samples. The UV-vis diffuse reflectance spectra reveal that the V-doped sample has the highest visible light absorption, followed by the Wand Fe-doped samples.

Nanostructure materials
Researchers at Rice University in Texas discovered that nanotechnologies could be used to clean the environment from pollutants, particularly the leakage of oil in groundwater. The researchers showed that nanoparticles of thin metal and carbon can trap droplets of water, causing these droplets (over 10 million drops) to be stacked together and form small spherical packages. The researchers also discovered that ultraviolet light and magnetic fields can be used to orient the nanoparticles to form packages and release their cargo. The researchers in the study found multi-faceted nanowires that are similar to nanoscale structures. These nanowires, when incorporated into a mixture of water and oil, can separate oil from water. The researchers are investigating the nanoscale coatings covered with zirconium particles for this purpose. A group of researchers has made a nanomat that can be used as a tool to clean up oil spills and other organic materials. Nanomaterial has a structure like paper and can be used in the near future as an important tool in the oil industry and its related industries. The new technology involves a new membrane that can absorb particles and spots up to 20 times its weight. Additionally, reusing it without damaging its structure is another notable feature of this nanomat. This has been invented while more than 200,000 tons of oil has recently been distributed on the surface of the world, and so far, there has been no effective way of collecting these stains [96].

Advanced and Chemical Oxidation
The conventional oxidation treatment involves the addition of an oxidizing agent to water or a solution containing contaminants. After such simple chemical oxidation, the pollutants are decomposed and removed. Usually, chemical oxidation in wastewater treatment means adding oxidizing agents such as ozone, hydrogen peroxide, permanganate, chlorine dioxide, chlorine (Cl2, HOCl), and oxygen to the sewage to change the chemical composition of one or more pollutants [97].
Also, some of the most important applications of oxidation in sewage management have been to reduce the amount of bacteria and sewage viruses in order to control the odor and eliminate ammonia and solutes. Chemical oxidation is also effective for the removal of odorous compounds such as sulfides and mercaptans. In addition to the aforementioned uses of chemical oxidation, in order to improve the biological purification ability of organic or non-biodegradable organic compounds, the removal of inhibitory effects of some organic and inorganic materials on microbial growth, reducing or eliminating the toxicity of certain organic and inorganic materials on microbial growth and aquatic plants are used to remove BOD, COD, ammonium, and resistant organic compounds [97]. Types of chemical oxidation treatment methods are purification by conventional oxidation and purification by advanced oxidation methods.
Chemical oxidation with ozone (O3) to remediate the soil has been gaining incremental attention over the past few decades. The strong oxidizing power of ozone should overcome the heavy fraction's residual problem by making them more soluble (thus more bioavailable) and simpler in structure (more biodegradable). Ozone and the hydroxyl radical (OH•), which can be produced form ozone, can react with all types of hydrocarbons and produce hydrophilic compounds, but through different mechanisms. The dominant oxidation pathways for OH• are hydrogen abstraction and OH• addition. In contrast, ozone attacks hydrocarbons via molecule addition, which leads to bond breaking [98]. Chen et al. [99] evaluated how gas-phase O3 interacts with residual petroleum hydrocarbons in soil. Total petroleum hydrocarbons (TPH) were 18 ± 0.6 g/kg soil, and TPH carbon constituted ~40% of the dichloromethane-extractable carbon (DeOC) in the soil. At the benchmark dose of 3.4 kg O3/kg initial TPH, TPH carbon was reduced by nearly 6 g C/kg soil (40%), which was accompanied by an increase of about 4 gC/kg soil in dissolved organic carbon (DOC) and a 4-fold increase in 5-day biochemical oxygen demand (BOD5). Disrupting gas channeling in the soil improved mass transport of O3 to TPH bound to soil and increased TPH removal. Ozonation resulted in two measurable alterations of the composition of the organic carbon. First, part of DeOC was converted to DOC (~4.1 g C/kg soil), 75% of which was not extractable by dichloromethane. Second, the DeOC containing saturates, aromatics, resins, and asphaltenes (SARA) was partially oxidized, resulting in a decline in saturates and aromatics, but an increase in resins and asphaltenes. Ozone attack on resins, asphaltenes, and soil organic matter led to the production of NO3 -, SO4 2-, and PO4 3-. The results illuminate the mechanisms by which ozone gas interacted with the weathered petroleum residuals in soil to generate soluble and biodegradable products. Also, hydrogen peroxide concentration plays an important role in the reduction of BTEX. The excess amount of H2O2 acts as hydroxyl radical's scavenger (HO2), and as a strong oxidizer, hydrogen peroxide breaks down BTEX into by-products (H2O2 + O2• → •OH + OH − + O2) [100].
Permanganates are crystalline solids well known as oxidizing agents. They are available as sodium and potassium permanganates. It is easily handled, readily available, strong and versatile oxidizing agent with a relative oxidizing power of 1.24. Their versatility in reacting with a wide range of organic compounds over a wide range of pH is attributable to the variable oxidation state of manganese in the reactions at different pHs [101]. At very acidic pHs, i.e, at pHs less than 3.5, they have been found to react as MnO4 − + 8H + + 5e − Mn 2+ + 4H2O. In this, manganese has been reduced from manganite (VII) to manganite (II). An important contribution was published by Bajagain et al. [102], who studied the permanganate pretreatment to degrade petroleum hydrocarbons in unsaturated soil in combination with subsequent bioaugmentation. The pretreatment of dieselcontaminated unsaturated soil with 0.5-pore-volume (5%) potassium permanganate (PP) by solution pouring and foam spraying (with a surfactant) achieved the total petroleum hydrocarbon (TPH) removal efficiencies of 37% and 72.1%, respectively. The PP foam, when coupled with bioaugmentation foam, further degraded the TPH to a final concentration of 438 mg/kg (92.1% total reduction). This experiment was conducted without soil mixing or disturbance. The relatively high TPH removal efficiency achieved by the PP-bioaugmentation serial foam application may be attributed to an increase in soil pH caused by the PP and effective infiltration of the remediation agent by foaming. The applied PP foam increased the pH of the acidic soil, thus enhancing microbial activity. Furthermore, 94% of the group of relatively persistent hydrocarbons (C18-C22) was removed by PP-bioaugmentation, as verified by chromatogram peaks. The results revealed that PP can degrade soil TPH and significantly enhance the biodegradation rate in unsaturated dieselcontaminated soil when combined with bioaugmentation foam.
Advanced oxidation processes (ozone, ozone, hydrogen peroxide, ozone/ultraviolet, hydrogen peroxide/ultraviolet) are suitable alternatives for decomposing organic biodegradable organic waste from conventional wastewater treatment [103,104]. The advanced oxidation process has significant advantages (such as the lack of high levels of sludge production) compared to conventional filtration methods. This method usually produces hydroxyl radicals at ambient temperature, which unintentionally attacks all organic and inorganic pollutants in the sewage system. Since advanced oxidation processes are expensive and have high operating costs, in recent years, a new approach to these processes, called catalytic ozonation processes (COP), has been considered [105][106][107]. Recently, various metals such as iron, aluminum, zinc, chromium, and copper have been used as catalysts to enhance the efficiency of advanced oxidation processes [108].
One of the benefits of advanced oxidation is that the processes are generally able to completely destroy specific substances. Therefore, using a biological method as a final treatment will complete the process. Heterogeneous photocatalysts, along with semiconducting processes, extinction, Fenton and sonolysis and similar processes are more efficient than other processes such as wet oxidation and electro dialysis. However, most studies have been conducted with the model of aqueous solutions and surface water (including rivers and lakes), while less attention has been paid to the actual wastewater from wastewater treatment plants or effluent from industrial units [109].
At this point, it is important to mention the Fenton (or the photo-Fenton oxidation process). Fenton's reaction is a non-expensive and environmentally friendly oxidation method, which was discovered by Fenton in 1894 when he strongly improved tartaric acid oxidation with the use of ferrous ion (Fe 2+ ) and H2O2 [110]. This process is widely used in wastewater treatment. Fenton's reagent is a solution of H2O2 and ferrous ions with a complex mechanism, which can be simplified by the following reactions [111,112]: Briefly, the reaction between ferrous ions and H2O2 produces hydroxyl radicals with high oxidative power (a) that attack the organic compounds present in the water (b  parallel reactions occur (d-f), and so the hydroxyl radicals are not only consumed to degrade the organic matter but also to produce other radicals, with less oxidative power, or other species (scavenging effect of HO • ). In addition, this leads to the undesired consumption of H2O2 (d). On the other hand, reaction (g) and (h) indicate generation of Fe 2+ by the reaction between H2O2 and Fe 3+ (Fenton-like process); this way, ferrous ion is restored, acting as a catalyst in the overall process [111,112]. Contrary to the materials mentioned for the function of the advanced oxidation process, if the concentration of carbonate and bicarbonate ions is significant in the environment, it can react with hydroxyl ions and reduce the efficiency of the advanced oxidation process. As the pH increases, carbonate ions and bicarbonate ions increase significantly, reducing the process efficiency [113]. Conventional wastewater treatment methods have been incapable of meeting the new standards and regulations of the environment. Therefore, methods such as oxidation using cavitation and ultrasound [114], and hybrid methods such as sonication, followed by wet-acid oxidation (SONIWO), sonoid-photochemical degradation [115], and removal by sono-electrochemical and others [116] are used.

Coagulation and Electrocoagulation
Electrocoagulation is an electrochemical process that has proven to be effective for wastewater treatment and water purification. Despite the high efficiency and environmental-friendly potential of the process, high capital and operating costs have made its application on an industrial scale not so promising. Also, the major disadvantages of electrical and chemical coagulation are the production of large quantities of dangerous sludge that are unrecoverable and often repelled. The elemental composition of petroleum sludge is N, P, K, Fe, Cu, Fe, Ca, Mg, Cd, Cr, Zn, Pb [117,118]. Petroleum sludge is a complex mixture containing different quantities of waste oil, wastewater, sand, and mineral matter. Petroleum industries are responsible for the generation of large quantities of sludge, which is a major source of environmental pollution. Oily sludges are hazardous wastes according to the Environment Protection Act and Hazardous Wastes Handling Rules. These sludges cannot be disposed of as landfill even if they are de-oiled unless they are totally remediated. Sludges generated by petroleum industries accumulate in crude oil tanks, refinery products tanks, desalters, and elsewhere during oil production and processing. The sludges containing recoverable oil less than 40% are considered as low oil content sludges. These sludges have to be treated and made harmless before disposal [117,119].
The efficacy of the electrocoagulation process (ECP) is usually evaluated for the treatment of petroleum-contaminated groundwater and quantified as the total petroleum hydrocarbon (TPH) removed by electrode materials (majorly aluminum, iron, and steel). The maximum TPH removal has been achieved using steel-iron as the anode-cathode electrode arrangement. In most cases, batch and continuous experiments showed that the ECP could be efficient in eliminating TPH from water and thus may be a promising technique for treating petroleum-contaminated groundwater [118]. An indicative example of the above is a published study, based on which electrocoagulation process was used for the removal of turbidity from petrochemical wastewater. The batch experiments were carried out by varying the values of current density, NaCl concentration and electrolysis time and maximum turbidity removal efficiency was 97.43% [120]. In addition, treatment of gas refinery oily wastewater by electrocoagulation using aluminum electrodes was investigated in another study and gas chromatographic analysis of raw and treated wastewater revealed that most normal hydrocarbons (nearly 99%) were removed during the electrocoagulation process [121].
Verma et al. investigated coagulation-flocculation as a pretreatment process for the treatment of purified terephthalic acid (PTA) wastewater [122]. The effect of various inorganic and organic coagulants on the treatment of wastewater collected from a flow equalization tank of an effluent treatment plant was studied. The settling and filtration characteristics of the sludge were also studied. The jar tests revealed that the wastewater was best treated when 3000 mg/L of ferric chloride was dosed at pH 5.6. At optimum conditions, the COD of the wastewater was reduced by 75.5%. The results of the gravity filtration of the treated wastewater showed that the addition of cationic polyacrylamide (175 mg/L) to ferric chloride coagulation improved the filtration characteristics and reduced the specific cake resistance.
Another important contribution is a study published by El-Ashtoukhy et al. [123] who explored the possibility of using electrocoagulation to remove phenolic compounds from oil refinery waste effluent using an electrochemical reactor with a fixed bed anode made of randomly oriented Al raschig rings packed in a perforated plastic basket located above the horizontal cathode. The study revealed that the optimum conditions for the removal of phenolic compounds were achieved at current density = 8.59 mA/cm 2 , pH = 7, NaCl concentration = 1 g/L, and temperature of 25 °C. Remarkable removal of 100% of phenol compound after 2 h was achieved for 3 mg/L phenol concentration of real refinery wastewater. The new anode design of electrocoagulation cell permitted high efficiencies with lower energy consumption in comparison with other cell design used in previous studies.
Additionally, a study of Saeedi and Khalvati-Fahlyani [124] reported the treatment of gas refinery oily wastewater by electrocoagulation using aluminum electrodes. The effects of electrode distance, initial pH, sodium sulfate (Na2SO4) as a supporting electrolyte, polyaluminum chloride dosage as a coagulant aid, and current density on the efficiency of chemical oxygen demand (COD) removal were examined. The results revealed that the COD removal rate increases by applying more current density and polyaluminum chloride and, to a lesser extent, Na2SO4 dosage. The results also showed that 97% of COD can be removed at optimum operational conditions. Specific electrical energy consumption could be reduced from 19.48 kWh (kg COD removal) −1 to 11.057 kWh (kg COD removal) −1 using Na2SO4 as a supporting electrolyte. Gas chromatographic analysis of raw and treated wastewater also revealed that most normal hydrocarbons (nearly 99%) were removed during the electrocoagulation process. Table 4 shows the advantages and disadvantages for the removal of petroleum compounds. Ozonation Yes Effective and fast removal of contaminants Low solubility of ozone in water; ozone scavengers commonly found in environment; incomplete oxidation [44] Plasma No Can simultaneously remove gas pollutants, airborne microbes, and even particles.
May produce O3, NOx, and other harmful byproducts. High voltage and high energy consumption.

Solvent extraction No
Efficient method and very fast process High cost and environmentally unfit, heavy metals cannot be removed by this process [126] Centrifugation No Easy to process, no need for any solvent and environmentally safe Large among of energy required, economically unfit and lower size molecules difficult to settle down [127] Froth flotation No Easy to apply and less energy required Highly viscous oily wastewater cannot be offered to this process [128] Ultrasonic irradiation Yes Fast and effective, no need any chemicals Heavy equipment cost, unable to treat heavy metals [129] Surfactant EOR Yes Easy to process and limited application in heavy metals

Effect of factors on Efficiency
The efficiency of petroleum hydrocarbons removal was strongly affected by a series of factors (Table 5). In the following subsections, there is a brief overview of them.

Effect of Solution pH
The pH of the solution is one of the most important factors affecting the chemical reactions that affect the structure of the pollutant, the surface properties of adsorbents and the pathway, and the reaction kinetics [154]. Veiga et al. (1999) studied the effect of pH on alkyl benzene degradation (between pH 3.5 and 7.0) and found that alkylbenzene degradation increases with pH increase [155]. Lu et al. (2002) reported the maximum degradation for BTEX between pH values of 7.5 and 8.0 [156]. However, the ideal pH of the biofilter medium depends on the pollutant being treated and the characteristics of the microbial ecosystem [157].
A good reference is the application of the Fenton process for the purification of petroleum compounds. The success of this treatment depends on the generation of OH. Higher concentrations of H2O2/COD in the presence of iron generated more hydroxyl radicals and enhanced process efficiency. However, excessive amounts of hydrogen peroxide and Fe particles had adverse effects on removal efficiency. The undesirable reactions between hydroxyl/hydroperoxyl radicals and excessive amounts of H2O2 and Fe ions may be explained by the following reactions.
Although higher concentrations of Fe ions might have been advantageous in terms of COD reduction by postcoagulation, a high dosage of iron ions scavenged the hydroxyl/hydroperoxyl radicals [158,159]. pH is also a determinant factor in the Fenton process. The highest concentration of Fe 2+ occurred at pH values in the range of 2.0-4.0 and beyond which the efficiency decreased. Moreover, hydrogen peroxide is most stable at pH levels in the range of 3.0-4.0, and its decomposition at higher pH values formed another reason for the inefficiency of the process [158].

Effect of Temperature
Working temperature is an important factor affecting the decomposition of oil compounds. The microorganisms that most effectively degrade BTEX compounds in the biological processes are mesothermic, and their optimum activity temperatures are between 30 and 40 °C. In fact, the proper temperature increases the rate of biofilm development and biomass accumulation [160]. Deeb and Alvarez-Cohen (1999) indicated that in the optimum range of operating temperature, the degradation performance on biofilter can increase 2-fold by 10 °C increase in temperature [161]. This can be explained by the fact that an increase in temperature causes a decrease in BTEX compounds and dissolved oxygen in water and diffusion transfer increases. Due to the exothermic nature of the biodegradation reactions, the variation of temperature in the filter bed is a consequence of microbial activity [162].

Effect of Reaction Time
The reaction time is one of the most important variables affecting the design and operation of each chemical process. In fact, the reaction time is the time it takes to achieve the desired purification targets. In the catalytic ozonation processes, the adsorption, direct oxidation, and indirect oxidation processes are involved in removal, so the catalytic ozonation process will lead to higher yields than conventional ozonation over a shorter period of time [163].

Effect of Ionic Strength
The desorption rate of petroleum hydrocarbons decreased as the increase of ionic strength decreased, which may arise because the solubility of organic matter in nonpolar or weakly polar water usually decreases as the ionic strength increases. Lower solubility decreased the rate of petroleum hydrocarbon desorption in absorbent. Turner proposed that the increased ionic strength can compress the absorbent surface, similar to an electric double layer, thus changing the structure of organic matter and increasing the desorption of petroleum hydrocarbons. Therefore, the increased ionic strength also compresses the surface of clay and organic matter in absorbent on the surface of the electric double-layer structure, which decreases net negative charge in the absorbent, even when the absorbent is close to its isoelectric point. This is more conducive to the adsorption of hydrophobic organic compounds, resulting in the decreased desorption rate of petroleum hydrocarbons.
Special attention must be given to bilgewater [164]. Bilgewater is a complex shipboard wastewater composed of seawater, various fuels, lubrication oils, cooling water, and other pollutants (e.g., detergents, solvents, solid particulates) from various sources such as machinery washes, condensation, leaks, and other activities. The presence of oil-in-water emulsions makes bilgewater difficult to treat solely by gravity-driven separation processes and often requires the use of secondary  [165] reported the influence of electrolytes on the destabilization of oil-in-water emulsions by breaking them using the inorganic salts calcium chloride and aluminum chloride. As expected, the addition of electrolytes lowered the zeta potential of the droplets, and destabilization of the emulsion was observed [165,166]. In addition, the presence of salts in an emulsion system can influence the solubility of nonionic surfactants [167] and increase the density of the aqueous phase, which can impact the stability of sterically stabilized emulsions. Given the impact of ionic strength on emulsion stabilization, electrical conductivities of bilgewater are often reported in bilgewater research. Electrical conductivity is a simple indirect method for measuring the ionic strength of a water sample. Conductivities of bilgewater have been reported to range from 0.679 to 8.4 mS/cm [168,169].
It is important to mention that, usually, there is a noticeable difference from heavy oils to VOCs treatment. So, Table 6 shows the different types of hydrocarbons presented in petroleum wastewaters, and Table 7 shows the most common approach of them based on this classification. to the boiling point of an n-alkane series, or its retention time on a non-polar b.p. gas chromatographic (GC) column [170,171]. # Aquifer properties: effective porosity 0.25, fOC 0.025 to 0.0025, and bulk density 2500/kg/m −3 . Rf = 1 + (Kd × bulk density)/porosity. Koc is the soil organic carbon-water partitioning coefficient. H is Henry's Law Constant. Different types of oil and petrochemical industry wastewater with different components were studied. The methods described above are mostly used for petrochemical wastewater treatment but are generally described in Table 7 as anaerobic and hybrid methods for the treatment of most heavy oil and petrochemical wastewater.
Moreover, in the adsorption process, in the early stages, a large number of free-surface adsorbents are available for adsorption, and after a while, the remaining surface spaces are not readily available, being occupied by the absorbing material, due to the repulsive forces between the soluble molecules and the solid-phase mass. In the advanced oxidation process, xylene is eliminated more than ethylbenzene, toluene, and benzene, and the amount of xylene and ethylbenzene adsorption is very close, and adsorption is much higher than benzene and toluene. This condition is due to lower benzene and toluene dissolution than ethylbenzene and xylene and higher molecular weight and higher boiling point of xylene and ethylbenzene than benzene [145,172,173]. Lu and coworkers found that in a preliminary concentration of 20060 mg/L of benzene and toluene, the amount of toluene adsorbed per unit mass of mass was absorbed more than benzene [174]. Aivalioti et al. reported that the adsorption capacity of diatoms for toluene removal is greater than that of benzene [174]. In their studies, Daifullah and Grizzis reported that the amount of toluene adsorption per unit mass was greater than that of benzene [175]. Su et al. showed that the adsorption capacity of benzene and toluene by nanotubes modified with sodium hypochlorite at an initial concentration of 200 mg/L, a contact time of 240 min, and a concentration of adsorbent 600 mg/L was 212 and 225 mg/g of absorbent. The toluene adsorption capacity was more than benzene [176]. In a study by Garoma on removing oil compounds using the conventional ozonation process, the removal efficiency of these compounds was reported to be 27% [3]. Table 7. Anaerobic and hybrid (anaerobic-aerobic) bioreactors used for hydrocarbon-containing wastewater treatment [177].

Wastewater Type
Bioreactor Configuration Operational Conditions Treatment Efficiency Ref.
Oilfield produced water ABR Start-up and operational performance (total 212 days) with mixed acclimated oilfield and urban sewage sludges.
COD and oil removals of 65% and 88% [177] Petroleum refinery effluent UASB Mesophilic conditions (38 ± 1 °C) for over120 days. Digested sludge from a dairy industry. Acrylonitrile butadiene styrene resin-manufacturing wastewater Stirred-tank HA with a series of algal photobioreactors The wastewater was treated for 36 h in a batch process and the effluent was applied to the algal microcosm treatment using Chlorella sp.
COD removal of 97.8% [182] Petroleum refinery wastewater Pilot HyVAB Granular sludge from paper and pulp wastewater treatment facility. Continuously operating at varying organic loading rates for 92 d.
86% of the total COD and 91% of the soluble COD removal [183] Metformincontaining wastewater Pilot HyVAB Granular sludge from petrochemical wastewater treatment bioreactor. Co-digest pharmaceutical-containing wastewater with the wastewater rich in easily degradable organics.

Conclusions
Oil compounds are one of the most hazardous pollutants in water and wastewater treatment. The main source of emissions in the environment is petrochemical, transportation, and industrial activities. So, the petrochemical hydrocarbons must be treated. This review focused on major techniques applied to petrochemical wastewater treatment. The major conclusions can be given as  Conventional refining techniques are not able to affect the effective elimination of oil compounds, and the high concentrations of these pollutants may affect the activity and efficiency of the treatment plant due to the high toxicity of these compounds that affects the activity of the active sludge pool and creates layers in the film which can cause blockage of the tubes.  Purification and removal of oil pollutants are necessary, especially in industries; the output of sewage chemical purification (coagulation, DAF flocculation) can be transmitted to the biologic reactor for further purification.  Electromethods are more advanced than conventional physical and chemical methods, such as electrocoagulation and flotation.  Physical methods can separate large amounts of petroleum compounds, and, in some cases, these compounds can be recycled with a number of processes.  Third-party refinement can provide water reuse targets with methods such as nanofiltration, reverse osmosis, and advanced oxidation.  Adsorption is an emergency technology for petrochemical wastewater treatment that can be applied by using minerals and organic materials. By using low-cost adsorbent materials and combining the adsorption process with one of the advanced methods, sludge production may be lowered and can reduce the cost of the process. Funding: This research received no external funding.

Conflicts of Interest:
The authors declare no conflict of interest.