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Environ Eng Res > Volume 28(6); 2023 > Article
Ritoré, Morillo, Arnaiz, Coquelet, and Usero: Chemical oxidation of hydrocarbon-contaminated soil: oxidant comparison study and soil influencing factors


The objective of this research is to investigate Fenton reaction, permanganate and persulfate oxidation as in-situ remediation technology for the treatment of gasoline-diesel contaminated soil. These oxidants were compared in various soils to study the influence of soil texture and soil organic matter. The different fractions of hydrocarbons, which have been scarcely investigated, were also analyzed and the compounds were clustered into aliphatic and aromatic hydrocarbons. Maximal removal rates were obtained with permanganate (74%), followed by persulfate (60%) and hydrogen peroxide (55%). High levels of clay in the soil (silty clay soil) reduced the efficiency of permanganate and persulfate hydrocarbon oxidation by 18%. On the other hand, 5% soil organic matter decreased the effectiveness of permanganate (18%). The removal rates of hydrocarbons decreased while hydrocarbon size increased, and aromatic hydrocarbons were more oxidized than aliphatic ones. In general, elimination percentages higher than 80% are achieved for chains >C6–C8 and lower than 20% for those in the range >C21–C35. The results observed can be used to increase the efficiency and improve the design of advanced oxidation processes for treating gasoline and diesel contaminated soil.

1. Introduction

Soil and groundwater contamination by petroleum hydrocarbons is a critical environmental and health problem worldwide. The potential contamination during the transport and storage of petroleum products, such as gasoline and diesel, is high and it is a continuous source of contaminants in soils. These fuels are a complex mixture of more than 2000 compounds [1]. Among them, aromatic hydrocarbons such as benzene, toluene, ethylbenzene, xylenes (BTEX) and polycyclic aromatic hydrocarbons (PAHs) are worrisome because of their highly toxic, mutagenic and carcinogenic properties [2, 3]. PAHs with heavy molecular weight are not removed from the soil by technologies such as venting or vapor extraction [4] due to their low volatility and strong absorption to soil components. Diesel fuel is one of the most common petroleum products used in both industries and our daily lives and usually comprises alkanes, cycloalkanes, and aromatics with 9–27 carbon atoms [5]. Most diesel fuel components are harmful to the environment [6].
The large number of places contaminated by gasoline and diesel fuel have drawn attention to the need for research on environmentally friendly technologies to remediate, treat and control the contamination [79]. Among the technologies, advanced oxidation processes (AOPs) stand out. The purpose of oxidant injection is to react with the contaminant and produce compounds that do not endanger the safety of the environment or people. Furthermore, in-situ chemical oxidation (ISCO) could be applied as a pre-treatment for bioremediation [10, 11]. The most common oxidants are Fenton’s reagent, permanganate and persulfate [12].
Fenton processes are based on the decomposition of hydrogen peroxide catalyzed by Fe2+ (Eq. 1). Fe2+ increases the oxidative strength of hydrogen peroxide due to the production of hydroxyl radicals (OH*) [13]. Fe2+ is oxidized to Fe3+, although there are mechanisms for reducing Fe3+ to Fe2+ (Eq. 2) [14]. The form of iron that catalyzes the oxidation reaction is Fe2+ but in soil remediation processes iron is applied as Fe3+ because this prevents an unproductive use of iron in the initial phases of the system (Eq. 3) [15].
Persulfate anion (S2O82−) is a strong oxidant but is kinetically slow to react, at 25 °C, the half-life of S2O82− is approximately 600 days [16]. Persulfate must be activated to form free radicals (SO4*, OH*) with higher oxidative strength [17]. The most common method of activating persulfate is with iron (Eq. 4). Iron sulfates have traditionally been added to persulfate solutions to activate oxidation reactions. Another way to add iron in order to catalyze the reaction is employing zero valent iron (ZVI), because it is necessary to add it in nanoparticles form [18]. In addition, other metals such as manganese, silver or copper can also be used, even if they are not common in remediation processes for contaminated soils. However, persulfate could be activated in other ways: heat activation [19, 20], alkaline activation [2123] and photoactivation [24, 25]. Reactions that involve radicals can be propagation reactions (Eq. 5 and 6) or chain termination reactions (Eq. 7 and 8) [12].
The ion permanganate (MnO4) is a strong oxidant that can react with organic compounds without forming radicals due to the high oxidation state of the manganese atom [26]. Permanganate and persulfate can remain active in the soil for long periods, whereas hydrogen peroxide is quickly consumed within a few hours [27]. Permanganate has been scarcely studied to remediate gasoline and diesel fuel contaminated soil [2830]. Bajagain et al. [28] and Chen et al. [30] studied the effect of permanganate in soil microorganism, while Picard and Chaouki [29] were focused on compare permanganate with other oxidants. None of them investigated the importance of soil properties in permanganate oxidation.
Petroleum hydrocarbons are hard to be separated from soil due to two aspects: 1) they can be strongly sorbed to soil organic matter (SOM) and encapsulated in soil mineral, which makes the remediation process difficult [30]; 2) soil is a heterogeneous system, mass transfer process plays a role in the remediation. Plenty of organic and inorganic colloids, various microorganisms, flora and fauna in soil may affect remediation as well [31]. The quickly contact between oxidant solution and the pollutant compounds is a crucial aspect because the oxidant can react with other components of the soil, resulting in unproductive oxidant use. A “natural oxidant demand” measurement is a direct estimate of the oxidant consumption by organic and inorganic components in the matrix (soil or water). A high consumption of oxidant by the soil makes the oxidants unavailable for removing contaminants [12]. Several characteristics that can restrict the removal of contaminants are soil texture (sand, silt and clay contents) and soil organic matter (SOM) content [14].
No previous studies have been found to use the mixture of gasoline and diesel fuel as a pollutant for chemical oxidation soil remediation. This mixture is a common contaminant in polluted soil service stations and should be investigated. Also, in previous research, the effect of soil organic matter in the chemical oxidation of hydrophobic organic compounds employed various soils with different characteristics that can produce interferences, so the study of SOM in the same soil it is necessary to clarify its influence. Among the interferences that may occur and that have been described in the scientific literature is the influence of soil pH [32, 33] and metal oxide content [34].
In this study, a) Fenton’s reagent, permanganate and persulfate were investigated to treat gasoline and diesel fuel contaminated soils; b) the influence of soil texture and SOM in petroleum hydrocarbon removal rates were evaluated; c) the overall and fractional removal rates of petroleum hydrocarbons were analyzed, both in aromatic and aliphatic compounds.
The main novelties of this research are: (i) the pollutant is a mixture of gasoline and diesel fuel (usual conditions in a contaminated service station). No previous studies have been found that have used this fuel mixture as a pollutant for chemical oxidation soil remediation. This research analyzed the different ranges of aliphatic and aromatic hydrocarbons (>C6–C8, >C8–C10, >C10–C12, >C12–C16, >C16–C21 and >C21–C35) in a mixture of gasoline and diesel fuel. Consequently, fractional removal rates can be obtained instead of overall removal rates, (iii) the method of analyzing the influence of soil organic matter. We used a type of soil, but with different soil organic matter content. By calcining a soil, the organic matter was eliminated and by mixing the original and the calcined soil, a soil with an intermediate organic matter content was prepared. In previous research the effect of soil organic matter in chemical oxidation of hydrophobic organic compounds used different soils with different characteristics that can produce interferences, (iv) permanganate has been scarcely studied as oxidant of/in gasoline and diesel contaminated soil remediation.

2. Materials and Method

2.1. Reagents

Hydrogen peroxide (30%), sodium persulfate (>98%) and potassium permanganate (>99%) were supplied by Sigma Aldrich. Hydrogen peroxide was activated by iron (III) sulfate hydrate (>97%) from Sigma Aldrich 12 and sodium persulfate was catalyzed by iron (II) sulfate heptahydrate (>99%) from Labkem. Gasoline and diesel fuel were purchased from a local gas station (Seville, Spain). Gasoline used is unleaded gasoline 95 octane and diesel is a commercial diesel car.

2.2. Soil Sampling, Characterization and Spiking

Uncontaminated soils were collected at a depth of 10–40 cm in Andalusia (Spain): Los Marines soil (37º53′51.41″ N, 6º37′48.84″ O), Almonte soil (37º13′25.38″ N, 6º30′30.16″ O) and La Puebla del Río soil (37º12′24.69″ N, 6º7′33.06″ O). The soil samples covered a wide diversity in sand, silt and clay contents. The soil properties are shown in Table 1.
Uncontaminated soils were homogeneously mixed, air dried and passed through a 2 mm sieve. Then, the soils were spiked with a mixture of gasoline and diesel (60–40%), in a proportion of 16.5 mL/kg soil. The contaminated soils were kept out of light for 2 weeks in closed bottles. Then, the initial concentration of hydrocarbons in soil was analyzed (Table 2).

2.3. Experimental Procedure

The experiments were carried out in 2 L glass bottles with screw Teflon® caps. 200 g of spiked soil were remediated with 1 L of oxidant solutions (5mL g−1). The bottles were shaken by hand for 30 seconds before they were kept in the dark. The soils were analyzed 40 days later. Iron was used as activator in the Fenton and persulfate experiments. The molar ratios were 0.01 between iron (iron (III) sulfate) and hydrogen peroxide [15, 35] and between iron (iron (II) sulfate) and sodium persulfate [27]. Iron solutions were added 10 minutes before the oxidants. The hydrocarbons removed was calculated from the difference between the initial and the final concentrations.

2.3.1. Oxidant concentration

The optimal concentration of oxidant solution was determined. Los Marines soil was used in these experiments. Various oxidant concentrations (mass/volume) were prepared: 1, 2 and 4% hydrogen peroxide, 5, 10 and 20% sodium persulfate and 0.5, 2 and 5% potassium permanganate.
A maximum of 4% hydrogen peroxide was used because at higher concentrations it use is dangerous due to the exothermicity of the chemical reactions and also greater danger can be expected in a full-scale oxidation process. A concentration of persulfate higher than 20% was not used because this oxidant at this concentration or higher can cause corrosion problems in the equipment involved in remediation processes such as pumps and injection or extraction wells. Finally, the highest permanganate concentration was 5% since this compound has a solubility limit close to 6%. The optimum concentrations obtained from these tests were used in the subsequent oxidation experiments.

2.3.2. Soil texture

The most suitable concentration of each oxidant found in section 2.3.1 was applied to each of the three collected soils (La Puebla del Río, Los Marines and Almonte) in order to analyze the influence of soil texture on the oxidation of gasoline and diesel hydrocarbons.

2.3.3. Effect of soil organic matter

Los Marines soil was used to reveal the role of SOM in the oxidation process. Three organic matter contents were prepared. The soil with a lower concentration of organic matter (0% by weight) was obtained after calcination of the collected soil for 24 hours at 550 ºC. The soil with the highest content of organic matter was Los Marines soil without heat treatment (5% by weight). Finally, a soil with 2.5% organic matter was obtained from a mixture of natural untreated soil and calcined soil in a 1:1 ratio. A muffle furnace (Nabertherm, 19/12/S27) was required for the heat treatment of soil.

2.4. Analysis Methods

The organic carbon in the soil was determined by gravimetric analysis [36], pH was measured with a pH meter Thermo 920A [37] and soil texture was analyzed according to the pipette method [38].
After centrifugation at 3,500 rpm for 15 min, the aqueous phase was completely separated to soil. The soil hydrocarbons were then analyzed by gas chromatography. The extraction of soil hydrocarbons was carried out by Soxhlet extraction [39, 40]. The volatile hydrocarbon fractions (>C6–C10) were analyzed by gas chromatography/mass spectrometry (GC/MS) [41, 42] and gas chromatography with a flame ionization detector method (GC/FID) was used for the detection of non-volatile hydrocarbons (>C10–C35) [43]. All analysis were carried out in triplicate.

2.5. Statistical Analysis

The results from the laboratory tests were analysed by SPSS and statistical significance was determined by either the t-test or analysis of variance (ANOVA) and Tukey test.

3. Results and Discussion

3.1. Determination of Oxidant Concentration

Fig. 1 shows the TPH removal rates (percentage of compound eliminated relative to the initial concentration) in Los Marines soil obtained with various concentrations of hydrogen peroxide (1, 2 and 4%), persulfate (5, 10 and 20%) and permanganate (0.5, 2 and 5%). The increase in the oxidant concentration enhanced the elimination of petroleum hydrocarbons from the soil. At the highest concentration, permanganate obtained the best results, 55% elimination of TPH, followed by persulfate (43%) and hydrogen peroxide (33%). These results are consistent with Yen et al. [27] who showed the same order for the degradation of diesel fuel: permanganate > persulfate > hydrogen peroxide. In addition, Chen et al. [30] also used hydrogen peroxide persulfate and permanganate to treat diesel-contaminated soil. They observed that increasing the concentration of oxidants favours the degradation of hydrocarbons.
The better removal rate of permanganate could be explained by its high persistence in soil and lower sensitivity to pH conditions. Potassium permanganate showed the greatest differences in removal rates, according to oxidant concentration. At a concentration of 0.5% permanganate, 30% of TPH were oxidized, while 34 and 55% oxidation rates were attained at concentrations of 2 and 5%, respectively. The results in Fig. 1 agreed with Lemaire et al. [44], who found better hydrocarbon removal rates when the permanganate doses were increased. Persulfate achieved 36% of TPH removal at 5% concentration, and 43% at 10 and 20% concentrations solution. The increase in persulfate concentration from 10 to 20%, did not manage to augment the elimination of hydrocarbons. The same result was observed by Lemaire et al. [45] and Ferrarese et al. [46]. However, in those studies the soils were not contaminated by gasoline or diesel but with PAHs. High oxidant concentrations can even reduce hydrocarbon oxidation, because of an increase in chain termination reactions (Equations 7 and 8). Hydrogen peroxide activated by ferric sulfate was the oxidant which provided the worst results, only oxidizing between 22% (1% concentration solution) and 33% (4% concentration solution) of the hydrocarbons in the soil. These results support the work of Santos et al. [47], who obtained a similar hydrocarbon removal rate when oxidizing gasoline with hydrogen peroxide at various concentrations in different soils. Another research study achieved higher removal rates of up to 70%, but they used higher hydrogen peroxide concentrations (35%) [48]. This high concentration is not recommended for full-scale application because it could be dangerous due to the high exothermic capacity of the Fenton process.
The lower efficiencies observed for Fenton and persulfate may be caused by iron precipitation at near neutral pH. At a pH of above 4.0 iron begins to precipitate out of solution [33, 49, 50] forming ferric hydroxide, thus removing the iron from the reaction and reducing the efficiency of process [51]. pH reduction in a full-scale oxidation remediation process is not an option due to the associated costs and potential environmental damage to soil. To avoid these issues, chelating agents could be used to form complexes with iron, thus preventing precipitation. However, the use of chelating agents should be optimized since some studies have shown that chelators did not improve hydrocarbon removal and should be avoided due to their cost and possible toxicity in the soil [44].
Subsequent sets of experiments were carried out with the oxidant concentrations which obtained the highest percentages of TPH removal: 4% hydrogen peroxide, 5% permanganate and 10% persulfate. The concentration of 10% was chosen for persulfate instead of 20% because it was the most efficient in the ratio oxidant concentration/hydrocarbon elimination.

3.2. Influence of Soil Texture on TPH Oxidation

This section analyzes the performance of hydrogen peroxide, persulfate and permanganate in the elimination of hydrocarbons from soil with three different textures (Table 1): loamy sand soil, silt loam soil and silty clay soil.
TPH were removed to a greater extent in the loamy sand soil (Fig. 2). In this soil, permanganate was the oxidant which eliminated more hydrocarbons (71%), followed by persulfate (60%) and hydrogen peroxide (55%). In silt loam soil and silty clay soil, the destruction of contaminants was smaller. The oxidation capacity of hydrocarbons by permanganate decreased substantially as the size of the soil particles was reduced, from 61% in the silty loam soil to 53% in the silty clay soil. Similar results were found for persulfate, which removed 47% of TPH from silt loam soil and 42% from silty clay soil; and for hydrogen peroxide, which reached an oxidation level of 43 and 56% in silt loam and silty clay soils, respectively (Fig. 2). These data show that a decrease in the size of the soil particles leads to a reduction in the percentages of hydrocarbon removal, probably because the capillary forces that hold hydrocarbons to the soil pores are higher in the small pores (especially abundant in soils with high amounts of silts and clays), and for this reason the hydrocarbons are less available to be oxidized. Santos et al. [52] showed a high TPH elimination with the Fenton process in soil with a high content of sand. Sandy soils present a low sorption capacity of contaminants and favor hydrocarbon degradation by oxidants. It should be noted that there may also be an overlapping effect because of the higher organic matter content in the soil that has the smaller particle size (Table 1). This fact is analyzed in detail in the following section.
According to Fig. 2 the Fenton oxidation of TPH increased in silty clay soil. It is possible that the soil minerals produced a better activation of hydrogen peroxide. A soil with high contents of iron and manganese minerals could facilitate the creation of hydroxyl radicals and in addition, the large contact surface provided by small particles such as silt and clay could explain the better oxidation produced by Fenton in silty clay soil [53]. Some studies [34, 48, 54, 55] indicated that metal oxides like magnetite, hematite, goethite and manganese oxide could efficiently catalyze Fenton and persulfate for hydrophobic organic compound oxidation in contaminated soils. The efficiency of the activation of metal oxide depends on the accessibility of iron or manganese from the mineral oxides. The metal oxide content in a soil can differ from several hundred (the average concentration of iron in the lithosphere is 51 g kg−1) to less than one gram per kilogram of soil [52]. For this reason, some soils cannot be activated by natural metal oxides and require iron addition.

3.3. Effect of Soil Organic Matter on TPH Oxidation

Fig. 3 shows the oxidation efficiency of Fenton, persulfate and permanganate in three soils with different SOM contents. SOM exerted a considerable influence on the hydrocarbon removal by permanganate. A higher content of SOM reduced the oxidation of hydrocarbons (Fig. 3). The percentage of hydrocarbon elimination changed from 74% when there was no SOM, to 67 and 61% when the soil samples contained 2.5 and 5% SOM, respectively. It shows that the organic matter of the soil consumes and competes for the oxidant. This fact has generated environmental concerns in the use of this technique such as the negative effect on soil microbial communities and the breakdown of natural organic matter [56, 57]. In this way, an unproductive use of the oxidant took place when the concentration of SOM increased, causing a decrease in the TPH removal. Another negative effect of the presence of SOM in the elimination of hydrocarbons is that soil organic matter could adsorb hydrophobic organic pollutants [58, 59].
The results found with the reagents persulfate and hydrogen peroxide (Fenton) are not conclusive. There is no clear trend deriving from the data obtained in these two cases. With persulfate, the destruction of hydrocarbons rose from 41% when there was 0% organic matter, to 47% when 5% of SOM was present in the soil samples. Hydrogen peroxide oxidized 37% of TPH in the absence of organic matter and 43% in a test with a higher content of it (Fig. 3). These results may seem unexpected, but some studies have reported an increase in the removal of hydrophobic organic compounds when the concentration of SOM rises. Lee and Lee [60] detailed the way in which humic acid increases the production of hydroxyl radicals in the Fenton reaction in order to oxidize trichloroethylene (TCE). Bissey et al. [61] showed that, at neutral pH, some SOM decreases the decomposition of hydrogen peroxide and increases the generation rate of hydroxyl radicals, increasing the effectiveness of the Fenton process in the remediation of contaminated soils in comparison with lower SOM concentrations. Humic acids are part of SOM soil and could act as iron chelators which catalyze the formation reaction of radicals [52, 62]. Although SOM consumes some oxidant radicals, it also serves as a chelator. SOM forms chelates (Fe-SOM) that could keep iron in the solution and promote an increase in the elimination of organic pollutants. This process has been studied in some research works [63, 64], in which the formation of Fe-SOM is related to an increase in hydroxyl radical production from hydrogen peroxide and, therefore, a greater oxidation of hydrophobic organic compounds was achieved. Functional groups such as alcohols and ethers facilitate adsorption and complex formation between iron cations and SOM [64]. Recently, Xu et al. [63] showed that the SOM with numerous humic acids and amino (−NH2) and hydroxyl (−OH) functional groups, combined with iron cations, has a higher catalytic activity, compared to that with only amino groups. Then, not only the content of SOM would have an influence on the oxidation process but also its nature.

3.4. Hydrocarbon Constituents of Gasoline and Diesel Fuel

The hydrocarbons were divided into groups according to the length of the carbon chain (>C6–C8, >C8–C10, >C10–C12, >C12–C16, >C16–C21 and >C21–C35; Ci hydrocarbons containing i C atoms in the molecule) were analyzed in the tests carried out with oxidants in soils with different concentrations of SOM. Moreover, in each fraction, aliphatic and aromatic hydrocarbons were measured.
Fig. 4 shows the elimination of hydrocarbons grouped by size. In general, the increase in the hydrocarbon size reduced their elimination from the soil as can be seen in Fig. 4. This effect is lower in permanganate since it removed between 54 and 95% of petroleum hydrocarbons of all sizes in soil with 0% SOM (Fig. 4a). In contrast, hydrogen peroxide and persulfate showed a greater decrease in effectiveness when oxidizing heavier compounds. Persulfate and Fenton attained more than 70% removal rates in the fractions >C6–C8 and >C8–C10, but they always eliminated below 14% in the fractions >C12–C16, >C16–C21 and >C21–C35. In soils with a SOM content of 2.5 and 5% (Fig. 4b and 4c), a similar downward trend to remove larger hydrocarbons can be observed. In these soils, permanganate removed less hydrocarbons from the >C12–C35 fractions in comparison with soil without SOM. In spite of this, permanganate continued to eliminate percentages greater than 44%. On the other hand, Fenton and persulfate increased their removal rates of the heaviest compounds with respect to the soil without SOM although only slightly. They managed oxidation percentages below 20% in the fraction >C12–C35, always with worse results than permanganate. These results show how hydrocarbons lighter than C12 are efficiently removed by any oxidant regardless of the SOM content. By contrast, the heaviest hydrocarbons are only significantly eliminated by permanganate, which is negatively affected by the SOM increase. In addition, it was revealed how SOM fundamentally affects the elimination of larger gasoline and diesel fuel hydrocarbons. The low long-chain hydrocarbon removal by oxidants has been reported in previous studies. Usman et al. [54] reported a maximum degradation of C14–C29 n-alkanes of 10–15% and almost insignificant elimination of C31–C36 n-alkanes using persulfate catalyzed by Fe (II). Lominchar et al. [65] showed that aliphatic hydrocarbons of diesel-polluted soil greater than C16 were difficult to oxidize with persulfate. Goi et al. [66] reported that n-alkanes C11–C18 were eliminated more readily than heavier n-alkanes C19–C26 and Xu et al. (2017) showed that alkanes C14–C22 were removed better than larger alkanes. The removal of the heavier hydrocarbons may be restricted by the high hydrophobicity of these compounds [65].
Fig. 5 shows the difference between aromatic and aliphatic hydrocarbon oxidation. In addition, all the hydrocarbon fractions analyzed with the three oxidants in soil with different organic matter content are presented. More than 80% of aliphatic and aromatic hydrocarbons in the ranges >C6–C8 and >C8–C10 were removed in all cases. Fenton in <C8–C10 aromatic fractions showed the worst results (between 59–74% elimination). Hydrocarbons heavier than C10 showed notable differences with respect to lighter compounds because significant differences appear between aliphatic and aromatic hydrocarbons. In the aliphatic range >C10–C12, permanganate eliminated 36–46% and Fenton and persulfate between 5–23%, while the aromatic increased up to 87–100%, 71–82% and 43% for permanganate, persulfate and Fenton, respectively. Permanganate also showed large differences in elimination between aliphatics (40%) and aromatics (90%) in the fractions >C12–C16, >C16–C21 and >C21–C35. However, the difference in the removal rates between aromatics and aliphatics were smaller with Fenton and persulfate. Persulfate showed an increase in aromatic hydrocarbon removal as SOM increased, while aliphatics did not. Persulfate always removed less than 20% of >C10 aliphatics compounds whereas in the soil with 5% of SOM persulfate oxidized 82% (>C10–C12), 58% (>C12–C16), 37% (>C16–C21) and 35% (>C21–C35) of aromatic hydrocarbons. Fenton removed very few aromatics and aliphatics >C12, less than 20% in all cases.
The selective degradation of aromatic hydrocarbons compared to aliphatic ones (especially the largest ones) by hydrogen peroxide and persulfate could be explained by the affinity of the electrons of the aromatic ring with the hydroxyl radicals and/or by the low solubility and the high facility of the heaviest aliphatic hydrocarbons to be absorbed by the soil [67]. The substitution reaction in aromatic hydrocarbons is rapid, aiding in a quick hydrocarbon elimination [13]. The results of these tests confirmed the research of Chen et al. [68] and Watts et al. [35], which showed that compounds with aromatic structure present in petroleum hydrocarbons were oxidized by the Fenton process preferentially. Watts et al. [35] attained 95% of BTX removal rates; however, only 37, 7 and 1% of nonane, decane and dodecane, respectively were eliminated. Lominchar et al. [65] studied the efficiency of alkaline activated persulfate to remediate an aged diesel fuel contaminated soil and obtained the same result: the removal of aromatic hydrocarbons was higher than that of aliphatics.
Several studies about the degradation of PAHs with different oxidants agree with our research that permanganate can remove aromatic hydrocarbons more efficiently than other oxidants [26, 44]. Permanganate is highly effective in oxidizing hydrocarbons containing carbon-carbon double bond because it can easily react with the more available electrons [12]. For this reason, permanganate can oxidize aromatic hydrocarbons through the benzene ring double bonds. In addition to the great results showed in this study by permanganate, Bajagain et al. [28] and Chen et al. [30] demonstrated that it can be used as pre-treatment for subsequent bioremediation. Accordingly, permanganate could be an available oxidant for gasoline and diesel fuel contaminated soil remediation.


This study demonstrated that the oxidant that most hydrocarbons removed from soils contaminated with a mixture of diesel-gasoline was permanganate followed by persulfate and Fenton. The best TPH elimination was achieved with high doses of permanganate and Fenton’s reagent.
The characteristics of the soil showed a significant influence on the oxidation of petroleum hydrocarbons. Moreover, a higher content of SOM decreases the TPH oxidation by permanganate. The results found with the reagents persulfate and hydrogen peroxide (Fenton) are not conclusive. There is no clear trend deriving from the data obtained in these two cases.
A low removal rate of aliphatic hydrocarbons was observed in this study, except those with lower molecular weight. Aromatic hydrocarbons were destroyed better than aliphatic ones; this effect is especially relevant when the permanganate oxidant was used. It should be noted that with this oxidant, high efficiencies for aromatic hydrocarbon removal were achieved regardless of the soil texture or the SOM content. The ability of permanganate to remove these most dangerous petroleum hydrocarbon compounds from the soil is remarkable. In an area exclusively contaminated by aromatic hydrocarbons or where the aromatic fraction of TPH is present at a harmful level for the environment or human health, a chemical oxidation design based on permanganate may enhance the relative cost-efficiency significantly in comparison with other oxidants.


This work was supported by INERCO Inspección y Control S. A. (PI 1790/43/2018). The project was the incentive of the Corporación Tecnológica de Andalucía (CTA) and partially funded by IBERDROLA, S. A.


Conflict of Interest

The authors declare that they have no conflict of interest.

Author Contributions

E.R. (Ph.D. Student) bibliographic search, formal analysis, conducted all the experiments and wrote the manuscript. J.M. (Professor) developed the theory and idea, conceptualization, data analysis, wrote and corrected the manuscript. C.A. (Professor) conceptualization, data analysis, correction, wrote and corrected the manuscript. B.C. (Researcher) conceptualization, supervision, project administration, and funding acquisition. J.U. (Professor) research direction, critical review of the manuscript.


1. Salehian E, Khodadadi A, Hosseini B. Remediation of diesel contaminated soils using surfactants: Column study. Am. J. Environ. Sci. 2012;8:352–359. https://doi.org/10.3844/ajessp.2012.352.359

2. ATSDR (Agency for Toxic Substances and Disease Registry). Interaction profile for: benzene, toluene, ethylbenzene, and xylenes (BTEX) [Internet]. Atlanta: ATSDR; c2004. [cited 8 March 2023]. Available from: https://www.atsdr.cdc.gov/interactionprofiles/ip05.html

3. ATSDR (Agency for Toxic Substances and Disease Registry). Toxicological profile for PAHs [Internet]. Atlanta: ATSDR; c1995. [cited 8 March 2023]. Available from https://wwwn.cdc.gov/TSP/ToxProfiles/ToxProfiles.aspx?id=122&tid=25

4. Khan FI, Husain T, Hejazi R. An overview and analysis of site remediation technologies. J. Environ. Manage. 2004;71:95–122. https://doi.org/10.1016/j.jenvman.2004.02.003
crossref pmid

5. Khudur LS, Shahsavari E, Miranda AF, Morrison PD, Nugegoda D, Ball AS. Evaluating the efficacy of bioremediating a diesel-contaminated soil using ecotoxicological and bacterial community indices. Environ. Sci. Pollut. Res. 2015;22:14809–14819. https://doi.org/10.1007/s11356-015-4624-2
crossref pmid

6. Zhao J, Zhang A, Héroux P, Sun Z, Liu Y. Remediation of diesel fuel polluted soil using dielectric barrier discharge plasma. Chem. Eng. J. 2021;17:128–143. https://doi.org/10.1016/j.cej.2020.128143

7. Ritoré E, Coquelet B, Arnaiz C, Morillo J, Usero J. Guidelines for surfactant selection to treat petroleum hydrocarbon-contaminated soils. Environ. Sci. Pollut. Res. 2021;29:7639–7651. https://doi.org/10.1007/s11356-021-15876-1
crossref pmid pmc

8. Befkadu AA, Chen Q. Surfactant-enhanced soil washing for removal of petroleum hydrocarbons from contaminated soils: a review. Pedosphere. 2018;28:383–410. https://doi.org/10.1016/S1002-0160(18)60027-X

9. Hahladakis JN, Lekkas N, Smponias A, Gidarakos E. Sequential application of chelating agents and innovative surfactants for the enhanced electroremediation of real sediments from toxic metals and PAHs. Chemosphere. 2014;105:44–52. https://doi.org/10.1016/j.chemosphere.2013.11.022
crossref pmid

10. Sutton NB, Grotenhuis T, Rijnaarts HHM. Impact of organic carbon and nutrients mobilized during chemical oxidation on subsequent bioremediation of a diesel-contaminated soil. Chemosphere. 2014;97:64–70. https://doi.org/10.1016/j.chemosphere.2013.11.005
crossref pmid

11. Kim I, Lee M. Pilot scale feasibility study for in-situ chemical oxidation using H2O2 solution conjugated with biodegradation to remediate a diesel contaminated site. J. Hazard. Mater. 2012;241–242:173–181. https://doi.org/10.1016/j.jhazmat.2012.09.022
crossref pmid

12. ITRC (The Interstate Technology & Regulatory Council). Technical and regulatory guidance for in situ chemical oxidation of contaminated soil and groundwater [Internet]. Washington, D.C: ITRC; c2005. [cited 8 March 2023]. Available from: http://www.itrcweb.org

13. Watts RJ, Asce M, Teel AL. Chemistry of modified Fenton’s reagent (catalyzed H2O2 propagations – CHP) for in situ soil and groundwater remediation. J. Environ. Eng. 2005;131:612–622. https://doi.org/10.1061/(ASCE)0733-9372(2005)131:4(612)

14. Yap CL, Gan S, Ng HK. Fenton based remediation of polycyclic aromatic hydrocarbons-contaminated soils. Chemosphere. 2011;83:1414–1430. https://doi.org/10.1016/j.chemosphere.2011.01.026
crossref pmid

15. Watts RJ, Dilly SE. Evaluation of iron catalyst for the Fenton-like remediation of diesel-contaminated soil. J. Hazard. Mater. 1996;51:209–224. https://doi.org/10.1016/S0304-3894(96)01827-4

16. Liu H, Bruton TA, Doyle FM, Sedlak DL. In situ chemical oxidation of contaminated groundwater by persulfate: Decomposition by Fe(III)- and Mn(IV)-containing oxides and aquifer materials. Environ. Sci. Technol. 2014;48:10330–10336. https://doi.org/10.1021/es502056d
crossref pmid pmc

17. Matzek LW, Carter KE. Activated persulfate for organic chemical degradation: a review. Chemosphere. 2016;51:178–188. https://doi.org/10.1016/j.chemosphere.2016.02.055
crossref pmid

18. Pardo F, Santos A, Romero A. Fate of iron and polycyclic aromatic hydrocarbons during the remediation of a contaminated soil using iron-activated persulfate: a column study. Sci. Total Environ. 2016;566–567:480–488. https://doi.org/10.1016/j.scitotenv.2016.04.197
crossref pmid

19. Zhao D, Liao X, Yan X, Huling SG, Chai T, Tao H. Effect and mechanism of persulfate activated by different methods for PAHs removal in soil. J. Hazard. Mater. 2013;254–255:228–235. https://doi.org/10.1016/j.jhazmat.2013.03.056
crossref pmid

20. Tsitonaki A, Petri B, Crimi M, Mosbk H, Siegrist RL, Bjerg PL. In situ chemical oxidation of contaminated soil and groundwater using persulfate: a review. Crit. Rev. Environ. Sci. Technol. 2010;40:55–91. https://doi.org/10.1080/10643380802039303

21. Liang C, Guo YY. Remediation of diesel-contaminated soils using persulfate under alkaline condition. Water Air Soil Pollut. 2012;223:4605–4614. https://doi.org/10.1007/s11270-012-1221-6

22. Houtz EF, Sedlak DL. Oxidative conversion as a means of detecting precursors to perfluoroalkyl acids in urban runoff. Environ. Sci. Technol. 2012;46:9342–9349. https://doi.org/10.1021/es302274g
crossref pmid

23. Guan YH, Ma J, Li XC, Fang JY, Chen LW. Influence of pH on the formation of sulfate and hydroxyl radicals in the UV/Peroxymonosulfate system. Environ. Sci. Technol. 2011;45:9308–9314. https://doi.org/10.1021/es2017363
crossref pmid

24. Lau TK, Chu W, Graham NJD. The aqueous degradation of butylated hydroxyanisole by UV/S2O8 2−: Study of reaction mechanisms via dimerization and mineralization. Environ. Sci. Technol. 2007;41:613–619. https://doi.org/10.1021/es061395a
crossref pmid

25. Hori H, Yamamoto A, Hayakawa E, et al. Efficient decomposition of environmentally persistent perfluorocarboxylic acids by use of persulfate as a photochemical oxidant. Environ. Sci. Technol. 2005;39:2383–2388. https://doi.org/10.1021/es0484754
crossref pmid

26. Shih YJ, Binh NT, Chen CW, Chen CF, Dong C. Treatability assessment of polycyclic aromatic hydrocarbons contaminated marine sediments using permanganate, persulfate and Fenton oxidation processes. Chemosphere. 2016;150:294–303. https://doi.org/10.1016/j.chemosphere.2016.01.112
crossref pmid

27. Yen CH, Chen KF, Kao CM, Liang SH, Chen TY. Application of persulfate to remediate petroleum hydrocarbon-contaminated soil: Feasibility and comparison with common oxidants. J. Hazard. Mater. 2011;186:2097–2102. https://doi.org/10.1016/j.jhazmat.2010.12.129
crossref pmid

28. Bajagain R, Gautam P, Jeong SW. Degradation of petroleum hydrocarbons in unsaturated soil and effects on subsequent biodegradation by potassium permanganate. Environ. Geochem. Health. 2020;42:1705–1714. https://doi.org/10.1007/s10653-019-00346-y
crossref pmid

29. Picard F, Chaouki J. NaClO/NaOH soil oxidation for the remediation of two real heavy-metal and petroleum contaminated soils. J. Environ. Chem. Eng. 2017;5:2691–2698. https://doi.org/10.1016/j.jece.2017.05.005

30. Chen K, Chang Y, Chiou W. Remediation of diesel-contaminated soil using in situ chemical oxidation (ISCO) and the effects of common oxidants on the indigenous microbial community: a comparison study. J. Chem. Technol. Biotechnol. 2016;91:1877–1888. https://doi.org/10.1002/jctb.4781

31. Rajabi H, Sharifipour M. Geotechnical properties of hydrocarbon-contaminated soils: a comprehensive review. Bull. Eng. Geol. Environ. 2019;78:3685–3717. https://doi.org/10.1007/s10064-018-1343-1

32. Zhang T, Liu Y, Zhong S, Zhang L. AOPs-based remediation of petroleum hydrocarbons-contaminated soils: efficiency, influencing factors and environmental impacts. Chemosphere. 2020;246:125726 https://doi.org/10.1016/j.chemosphere.2019.125726
crossref pmid

33. Kakarla PKC, Watts RJ. Depth of Fenton-like oxidation in remediation of surface soils. J. Environ. Eng. 1997;123:11–17. https://doi.org/10.1061/(ASCE)0733-9372(1997)123:1(11)

34. Wang X, Dong W, Brigante M, Mailhot G. Hydroxyl and sulfate radicals activated by Fe(III)-EDDS/UV: comparison of their degradation efficiencies and influence of critical parameters. Appl. Catal. B-Environ. 2019;245:271–278. https://doi.org/10.1016/j.apcatb.2018.12.052

35. Usman M, Hanna K, Faure P. Remediation of oil-contaminated harbor sediments by chemical oxidation. Sci. Total Environ. 2018;634:1100–1107. https://doi.org/10.1016/j.scitotenv.2018.04.092
crossref pmid

36. Watts RJ, Haller DR, Jones AP, Teel AL. A foundation for the risk-based treatment of gasoline-contaminated soils using modified Fenton’s reactions. J. Hazard. Mater. 2000;76:73–89. https://doi.org/10.1016/s0304-3894(00)00173-4
crossref pmid

37. NEN (The Royal Netherlands Standardization Institute). Soil determination of organic matter content in soil as loss-on-ignition [Internet]. Delph: NEN; 5754:c2014[cited 8 March 2023]. Available from: https://www.nen.nl/en/nen-5754-2014-nl-197942

38. Fernández Linares LC, Rojas Avelizapa NG, Roldán Carrillo TG, et al. Manual de técnicas de análisis de suelos aplicadas a la remediación de sitios contaminados. Mexico D.F: Instituto Mexicano del Petróleo; 2006. 19–21. Available from: https://biblioteca.semarnat.gob.mx/janium/Documentos/Ciga/Libros2011/CG008215.pdf

39. NEN (The Royal Netherlands Standardization Institute). Soil determination of clay content and particle size distribution in soil and sediment by sieve and pipet [Internet]. Delph: NEN; 5753:c2018[cited 8 March 2023]. Available from: https://www.nen.nl/en/nen-5753-2018-nl-244166

40. USEPA (United States Environmental Protection Agency). Method 3540C. Soxhlet extraction [Internet]. USEPA; c1996. [cited 8 March 2023]. Available from: https://www.epa.gov/hwsw846/sw-846-test-method-3540c-soxhlet-extraction

41. USEPA (United States Environmental Protection Agency). Method 3541. Automatic soxhlet extraction [Internet]. USEPA; c1994. [cited 8 March 2023]. Available from: https://www.epa.gov/hw-sw846/sw-846-test-method-3541-automated-soxhlet-extraction

42. USEPA (United States Environmental Protection Agency). Method 5021A. Volatile organic compounds (VOCs) in various sample matrices using equilibrium headspace analysis [Internet]. USEPA; c2014. [cited 8 March 2023]. Available from: https://www.epa.gov/hw-sw846/sw-846-test-method-5021a-volatile-organic-compounds-vocs-various-samplematrices-using

43. USEPA (United States Environmental Protection Agency). Method 8260D. Volatile organic compounds by gas chromatography/mass spectrometry (GC/MS) [Internet]. USEPA; c2018. [cited 8 March 2023]. Available from: https://www.epa.gov/hwsw846/sw-846-test-method-8260d-volatile-organic-compounds-gas-chromatographymass-spectrometry

44. USEPA (United States Environmental Protection Agency). Method 8015D. Nonhalogenated organics using gas chromatography/flame Ionization Detection (GC/FID) [Internet]. USEPA; c2003. [cited 8 March 2023]. Available from: https://www.epa.gov/hw-sw846/validated-test-method-8015d-nonhalogenatedorganics-using-gas-chromatographyflame

45. Lemaire J, Buès M, Kabeche T, Hanna K, Simonnot MO. Oxidant selection to treat an aged PAH contaminated soil by in situ chemical oxidation. J. Environ. Chem. Eng. 2013;1:1261–1268. https://doi.org/10.1016/j.jece.2013.09.018

46. Lemaire J, Laurent F, Leyval C, Schwartz C, Buès M, Simonnot M. PAH oxidation in aged and spiked soils investigated by column experiments. Chemosphere. 2013;91:406–414. https://doi.org/10.1016/j.chemosphere.2012.12.003
crossref pmid

47. Ferrarese E, Andreottola G, Oprea IA. Remediation of PAH-contaminated sediments by chemical oxidation. J. Hazard. Mater. 2008;152:128–139. https://doi.org/10.1016/j.jhazmat.2007.06.080
crossref pmid

48. Kong S. Treatment of Petroleum-Contaminated Soils Using Iron Mineral Catalyzed Hydrogen Peroxide. Chemosphere. 1988;37:1473–1482. https://doi.org/10.1016/S0045-6535(98)00137-4

49. Miklos DB, Remy C, Jekel M, Linden KG, Drewes JE, Hübner U. Evaluation of advanced oxidation processes for water and wastewater treatment - A critical review. Water Res. 2018;39:118–131. https://doi.org/10.1016/j.watres.2018.03.042
crossref pmid

50. Paiva VAB, Paniagua CES, Ricardo IA, et al. Simultaneous degradation of pharmaceuticals by classic and modified photo-Fenton process. J. Environ. Chem. Eng. 2018;6:1086–1092. https://doi.org/10.1016/j.jece.2018.01.013

51. Miralles-Cuevas S, Ollera I, Agüera A, Ponce-Robles L, Sánchez Pérez JA, Malato S. Removal of microcontaminants from MWTP effluents by combination of membrane technologies and solar photo-Fenton at neutral pH. Catal. Today. 2015;252:78–83. https://doi.org/10.1016/j.cattod.2014.11.015

52. Santos A, Scheres D, Emmel A, Siedlecki K. Evaluation of the Fenton process effectiveness in the remediation of soils contaminated by gasoline: Effect of soil physicochemical properties. Chemosphere. 2018;207:154–161. https://doi.org/10.1016/j.chemosphere.2018.05.087
crossref pmid

53. Voelker BM. Rates of hydroxyl radical generation and organic compound oxidation in mineral-catalyzed Fenton-like systems. Environ. Sci. Technol. 2003;37:1150–1158. https://doi.org/10.1021/es020874g
crossref pmid

54. Usman M, Faure P, Hanna K, Abdelmoula M, Ruby C. Application of magnetite catalyzed chemical oxidation (Fentonlike and persulfate) for the remediation of oil hydrocarbon contamination. Fuel. 2012;96:270–276. https://doi.org/10.1016/j.fuel.2012.01.017

55. Do SH, Kwon YJ, Kong SH. Effect of metal oxides on the reactivity of persulfate/Fe(II) in the remediation of diesel-contaminated soil and sand. J. Hazard. Mater. 2010;182:933–966. https://doi.org/10.1016/j.jhazmat.2010.06.068
crossref pmid

56. Sirguey C, de Souza e Silva PT, Schwartz C, Simonnot MO. Impact of chemical oxidation on soil quality. Chemosphere. 2008;72:282–289. https://doi.org/10.1016/j.chemosphere.2008.01.027
crossref pmid

57. Schroth MH, Oostrom M, Wietsma TW, Istok JD. In situ oxidation of trichloroethene by permanganate: effects on porous medium hydraulic properties. J. Contam. Hydrol. 2001;50:79–98. https://doi.org/10.1016/S0169-7722(01)00098-5
crossref pmid

58. Pignatello JJ, Xing B. Dual-mode sorption of low-polarity compounds in glassy Poly (Vinyl Chloride) and soil organic matter. Environ. Sci. Technol. 1997;31:792–799. https://doi.org/10.1021/es960481f

59. Weber WJ, Mcginley PM, Katz LE. A Distributed reactivity model for sorption by soils and sediments. 1. Conceptual basis and equilibrium assessments. Environ. Sci. Technol. 1992;26:1955–1962. https://doi.org/10.1021/es00034a012

60. Lee Y, Lee W. Degradation of trichloroethylene by Fe(II) chelated with cross-linked chitosan in a modified Fenton reaction. J. Hazard. Mater. 2010;178:187–193. https://doi.org/10.1016/j.jhazmat.2010.01.062
crossref pmid

61. Bissey LL, Smith JL, Watts RJ. Soil organic matter-hydrogen peroxide dynamics in the treatment of contaminated soils and groundwater using catalyzed H2O2 propagations (modified Fenton’s reagent). Water Res. 2006;40:2477–2484. https://doi.org/10.1016/j.watres.2006.05.009

62. Sherwood MK, Cassidy DP. Modified Fenton oxidation of diesel fuel in arctic soils rich in organic matter and iron. Chemosphere. 2014;113:56–61. https://doi.org/10.1016/j.chemosphere.2014.04.048
crossref pmid

63. Xu J, Cao Q, Zhang M, Yang C. Active iron bound to SOM catalyzes the oxidization of alkanes in soil by H2O2 . Environ. Sci. Process. Impacts. 2018;20:1273–1284. https://doi.org/10.1039/c8em00129d
crossref pmid

64. Xu J, Fan X, Huang F, Li X. Iron bound to soil organic matter catalyzes H2O2 to oxidize crude oil in soil. J. Hazard. Mater. 2017;322:516–524. https://doi.org/10.1016/j.jhazmat.2016.10.018
crossref pmid

65. Lominchar MA, Santos A, de Miguel E, Romero A. Remediation of aged diesel contaminated soil by alkaline activated persulfate. Sci. Total Environ. 2018;622–623:41–48. https://doi.org/10.1016/j.scitotenv.2017.11.263
crossref pmid

66. Goi A, Trapido M, Kulik N, Palmroth MRT, Tuhkanen T. Ozonation and Fenton Treatment for Remediation of Diesel Fuel Contaminated Soil. Ozone Sci. Eng. 2006;28:37–46. https://doi.org/10.1080/01919510500479130

67. Siegrist RL, Crimi M, Simpkin TJ. In Situ Chemical Oxidation for Groundwater Remediation. New York: Springer-Verlag; 2010. 33–88:147–191. https://doi.org/10.1007/978-1-4419-7826-4

68. Chen CT, Tafuri AN, Rahman M, Foerst MB. Chemical oxidation treatment of petroleum contaminated soil using Fenton’s reagent. J. Environ. Sci. Health. 1988;33:987–1008. https://doi.org/10.1080/10934529809376772

Fig. 1
TPH (>C6–C35) Removal Rate (%) with Different Oxidant Concentrations in Los Marines Soil. Hydrogen Peroxide: 1, 2 and 4%. Persulfate: 5, 10 and 20%. Permanganate: 0.5, 2 and 5%.
Fig. 2
Percentage Removal of TPH after Application of Hydrogen Peroxide (Fenton), Persulfate and Permanganate in Three Soils with Different Textures.
Fig. 3
Percentage Removal of TPH after Application of Fenton (agua oxigenada), Persulfate and Permanganate in Soil with Different Contents of Organic Matter (% by weight).
Fig. 4
Percentage Removal of Petroleum Hydrocarbon Fractions after Application of Fenton, Persulfate and Permanganate in Soils with Different Amounts of SOM.
Fig. 5
Percentage Removal of TPH Fractions (aliphatics and aromatics) after Application of Fenton, Persulfate and Permanganate in Soil with Different SOM content. Ci Hydrocarbons Containing i C Atoms in the Molecule.
Table 1
Soil Properties
La Puebla del Río Los Marines Almonte
Sand (%) 3.8 ± 0.7 20.6 ± 0.41 86.2 ± 1.60
Silt (%) 53.5 ± 1.34 58.2 ± 1.45 12.4 ± 0.31
Clay (%) 42.7 ± 1.07 21.2 ± 0.53 1.4 ± 0.03
Texture Silty clay Silt loam Loamy sand
Organic matter (%) 7.2 ± 0.11 5.1 ± 0.08 1.5 ± 0.02
pH 8.3 ± 0.10 5.9 ± 0.20 6.5 ± 0.10
Table 2
Initial Hydrocarbon Concentration (mg Hydrocarbon/kg Soil).
Hydrocarbon size

>C6–C8 >C8–C10 >C10–C12 >C12–C16 >C16–C21 >C21–C35 >C6–C35
Aliphatic 820 ± 70 610 ± 60 580 ± 38 1300 ± 85 1410 ± 92 640 ± 30 5360 ± 322
Aromatic 690 ± 26 870 ± 28 570 ± 29 390 ± 20 510 ± 28 140 ± 8 3170 ± 190
Total 1510 ± 143 1480 ± 148 1105 ± 72 1690 ± 110 1920 ± 125 780 ± 47 8530 ± 512
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