1. Introduction
he discharge of oil sludge into the environment poses significant risks to human health and the environment [1, 2, 3, 4]. Proper treatment can prevent the relevant contaminants [6, 5]. One of the most critical treatment processes is biodegradation, such as composting [7], which produces contaminated leachate [8, 9, 10]. Koolivand et al. results [11, 12] showed that Hydrogen Peroxide and Fenton have good efficiency in reducing Total Petroleum Hydrocarbons (TPH). One of the advantages of the modified process is the ability to operate in a wide range of pH [13]. Some studies [14, 15] have been conducted to treat municipal waste leachate by Fenton. Other related studies have been done by Hosseini et al. [16], Attarian and Mokhtarani [17], Mahdad et al. [18], Hashemi et al. [19], and Mokhtarani et al. [20]. This study aimed to evaluate the efficiency of the modified Fenton process in leachate treatment from petroleum sludge compost.
2. Materials and Methods 
The leachate used in this study was obtained from the oil sludge composting process by the Windrow method. First, Fenton solution was prepared in the laboratory, and after preparing the desired concentrations, it was added directly to the reactors containing 200 ml of compost leachate. In the Fenton solution, hydrogen peroxide to iron was set to about 10: 1 [21]. Iron was supplied by the chemical substance ferric sulfate Fe2(SO4)3. Independent variables in this study included initial leachate concentration, modified Fenton concentration, reaction time, and pH. The pH of the samples was adjusted at each stage using solutions of sulfuric acid and sodium at 3, 5, and 9. After adding the desired concentrations (20, 50, 100, and 200 mg / l) of modified Fenton solution to the test reactors, sampling was performed at 15, 30, 60, 90, and 120 minutes and COD and TPH of each sample were measured as dependent variables of the study. The initial COD of the models used in the experiments was about 500, 1000, and 1500 mg / l. TNRCC 1005 and TNRCC 1006, provided by the Texas Department of Natural Resources, measured TPH [22].
3. Results
Table 1 shows the effect of pH, initial leachate concentration, and the modified Fenton concentration on the COD reduction efficiency.


Table 2 shows the effect of pH and the modified Fenton concentration on the TPH reduction efficiency of the leachate.


The results showed the lower the pH of the leachate, the higher the COD removal efficiency. Therefore, naturally, the highest COD removal efficiency is obtained at pH=3. At this pH, the highest efficiency (more than 90%) was obtained in 60 minutes with a concentration of 200 mg/l Fenton. According to the results, COD removal efficiency increased from 15 to 60 minutes, and no significant change in removal rate was observed for more than 60 minutes. Therefore, 60 minutes can be considered as the optimum reaction time of the modified Fenton. Also, it was found that the higher the concentration of modified Fenton, the higher the removal efficiency. Therefore, naturally, the highest COD reduction efficiency is related to the 200 mg/l modified Fenton concentration.
The results also showed that the higher the initial concentration of COD, the lower the removal efficiency, so the highest efficiency in COD was 500 mg/l. According to Table 2, which shows the TPH reduction efficiency of the leachate sample in 60 minutes, at modified Fenton concentrations in the range of 20-200 mg/l and pH=3, the TPH removal efficiency is about 31-77%. Be. These cultivars are in the range of 25-72 and 20-65% for pH=5 and pH=9, respectively.
4. Discussion and Conclusion
At a very low pH, the formation of Fe(H2O) 2+, which reacts very slowly with hydrogen peroxide, reduces the number of hydroxyl radicals and thus the process efficiency. The decrease in efficiency in alkaline conditions is due to the conversion of Fe2 + to Fe(OH)3 precipitate, which causes decomposition of H2O2 and prevents the formation of OH radicals [23]. A similar study by Tengrui et al. [23] found an optimum pH of 3 that is consistent with the results of this study. The time required to complete the Fenton reaction depends on several factors, such as the concentration of Fenton used and the nature and concentration of the contaminant [23].
In Fenton’s reaction, the removal efficiency does not change much from time to time. There may even be a slight decrease, which can be due to the production of some metabolites and intermediate products due to the decomposition of the intended contaminant [23]. The results of Farrokhi et al. [14] showed in the Fenton process, the highest amount of COD removal of waste leachate is obtained in the pH range of 3.5-3, and the reaction time is 90 minutes, which is consistent with the results of this study. Among the important factors in the efficiency of the modified Fenton process are the concentration of Fenton used and the initial concentration of the contaminant [23].
The results of the oxidation study of Kerman city waste leachate using the Fenton process carried out by Malakoutian et al. [15] showed the maximum COD removal efficiency is obtained at a contact time of 75 minutes at pH=3 iron concentration of 1400 mg/l. Therefore, based on the results, it can be said that the use of low concentrations of modified Fenton has a slight effect on the removal of TPH, which is consistent with the results of Watts’s study [24]. Overall, it can be concluded that the modified Fenton process can effectively reduce the COD of leachate from petroleum sludge compost.

Ethical Considerations
Compliance with ethical guidelines
This study has been registered in Arak University of Medical Sciences with codes 2645 and 2765.

Funding
Thihs study was supported by the Arak University of Medical Sciences.

Authors' contributions
All authors met the standard writing criteria based on the recommendations of the International Committee of Medical Journal Publishers.

Conflicts of interest
The authors declared no conflict of interest.
 

References

1. Koolivand A, Naddafi K, Nabizadeh R, Saeedi R. Optimization of combined in-vessel composting process and chemical oxidation for remediation of bottom sludge of crude oil storage tanks. Environ Technol. 2018; 39(20):2597-603. [DOI:10.1080/09593330.2017.1362037] [PMID]
2. Varjani SJ. Microbial degradation of petroleum hydrocarbons. Bioresour Technol. 2017; 223:277-86. [DOI:10.1016/j.biortech.2016.10.037] [PMID]
3. Thion C, Cébron A, Beguiristain T, Leyval C. PAH biotransformation and sorption by Fusarium solani and Arthrobacter oxydans isolated from a polluted soil in axenic cultures and mixed co-cultures. Int Biodeterior Biodegradation. 2012; 68:28-35. [DOI:10.1016/j.ibiod.2011.10.012]
4. Zhang C, Qi J, Cao Y. Synergistic effect of yeast-bacterial co-culture on bioremediation of oil-contaminated soil. Bioremediat J. 2014; 18(2):136-46. [DOI:10.1080/10889868.2013.847402]
5. Muangchinda C, Rungsihiranrut A, Prombutara P, Soonglerdsongpha S, Pinyakong O. 16S metagenomic analysis reveals adaptability of a mixed-PAH-degrading consortium isolated from crude oil-contaminated seawater to changing environmental conditions. J Hazard Mater. 2018; 357:119-27. [DOI:10.1016/j.jhazmat.2018.05.062] [PMID]
6. Zhang Y, Zhao Q, Jiang J, Wang K, Wei L, Ding J, et al. Acceleration of organic removal and electricity generation from dewatered oily sludge in a bioelectrochemical system by rhamnolipid addition. Bioresour Technol. 2017; 243:820-27. [DOI:10.1016/j.biortech.2017.07.038] [PMID]
7. Poorsoleiman MS, Hosseini SA, Etminan A, Abtahi H, Koolivand A. [The efficiency of acinetobacter radioresistens strain ka2 isolated from oily sludge for degrading of crude oil‎ (Persian)]. J Arak Univ MedSci. 2019; 22(5):78-89. [DOI:10.32598/JAMS.22.5.78]
8. Koolivand A, Godini K, Saeedi R, Abtahi H, Ghamari F. Oily sludge biodegradation using a new two-phase composting method: Kinetics studies and effect of aeration rate and mode. Process Biochem. 2019; 79:127-34. [DOI:10.1016/j.procbio.2018.12.003]
9. Poorsoleiman MS, Hosseini SA, Etminan A, Abtahi H, Koolivand A. Bioremediation of Petroleum Hydrocarbons by using a two-step inoculation composting process scaled-up from a mineral-based medium: Effect of biostimulation of an indigenous bacterial strain. Waste and Biomass Valorization. 2021; 12(4):2089-96. [DOI:10.1007/s12649-020-01140-z]
10. Mnif I, Mnif S, Sahnoun R, Maktouf S, Ayedi Y, Ellouze-Chaabouni S, et al. Biodegradation of diesel oil by a novel microbial consortium: Comparison between co-inoculation with biosurfactant-producing strain and exogenously added biosurfactants. Environ Sci Pollut Res Int. 2015; 22(19):14852-61. [DOI:10.1007/s11356-015-4488-5] [PMID]
11. Koolivand A, Naddafi K, Nabizadeh R, Jafari AJ, Nasseri S, Yunesian M, et al. Application of hydrogen peroxide and fenton as pre-and post-treatment steps for composting of bottom sludge from crude oil storage tanks. Pet Scie Technol. 2014; 32(13):1562-8. [DOI:10.1080/10916466.2012.697961]
12. Koolivand A, Naddafi K, Nabizadeh R, Nasseri S, Jafari AJ, Yunesian M, et al. Degradation of petroleum hydrocarbons from bottom sludge of crude oil storage tanks using in-vessel composting followed by oxidation with hydrogen peroxide and Fenton. J Mater Cycles Waste Manag. 2013; 15(3):321-7. [DOI:10.1007/s10163-013-0121-1]
13. Nam K, Rodriguez W, Kukor JJ. Enhanced degradation of polycyclic aromatic hydrocarbons by biodegradation combined with a modified Fenton reaction. Chemosphere. 2001; 45(1):11-20. [DOI:10.1016/S0045-6535(01)00051-0]
14. Farrokhi M, Kouti M, Mousavi G R, Takdastan A. [The study on biodegradability enhancement of landfill leachate by fenton oxidation (Persian)]. Iran J Health Environ. 2009; 2(2):114-23. https://www.sid.ir/en/journal/ViewPaper.aspx?id=160390
15. Malakootian M, Ahmadian M, Loloei M. [Influence of fenton process on treatability of Kerman city solid waste leachate (Persian)]. Iran J Health Environ. 2010; 3(2):123-34. http://ijhe.tums.ac.ir/article-1-118-en.html
16. Hassani A, Mokhtarani N, Bayatfard A. [Post treatment of composting leachate using combination of aerobic completely mixed and plugs flow reactors (Persian)]. J EnvironSci Technol. 2012; 14(1):4-9. https://jest.srbiau.ac.ir/?_action=articleInfo&article=2209&lang=en
17. Attarian P, Mokhtarani N. [Post-treatment of composting leachate by Sequencing Batch Reactor (SBR) (Persian)]. Modares Civil Eng J. 2018; 18(1):171-82. https://mcej.modares.ac.ir/article-16-15839-en.html
18. Mahdad F, Younesi H, Bahramifar N, Hadavifar M. [Optimization of compost leachate treatment using advanced oxidation process h₂o₂/uv (Persian)]. Modares Civil Eng J. 2017; 17(2):247-56. https://mcej.modares.ac.ir/article-16-305-en.html
19. Hashemi H, Alipor Samani E, Amin M M, Bina B. [Urvey on electrocoagulation process efficiency on Isfahan composting plant leachate treatment (Persian)]. J Health System Res. 2013; 9(9):969-78. http://hsr.mui.ac.ir/article-1-669-en.html
20. Mokhtarani N, Khodabakhshi S, Ayati B. [UV-TiO2 Photocatalytic degradation of compost leachate (Persian)]. Modares Civil Eng J. 2014; 14(20):137-46. https://mcej.modares.ac.ir/article-16-951-en.html
21. Goi A, Trapido M. Degradation of polycyclic aromatic hydrocarbons in soil: The Fenton reagent versus ozonation. Environ Technol. 2004; 25(2):155-64. [DOI:10.1080/09593330409355448] [PMID]
22. Wu C, Chen W, Gu Z, Li Q. A review of the characteristics of Fenton and ozonation systems in landfill leachate treatment. Sci Total Environ. 2021; 762:143131. [DOI:10.1016/j.scitotenv.2020.143131] [PMID]
23. Tengrui L, Al-Harbawi A, Jun Z, Bo LM. The effect and its influence factors of the Fenton process on the old landfill leachate. J Appl Sci. 2007; 7(5):724-7. [DOI:10.3923/jas.2007.724.727]
24. Watts RJ. Hydrogen peroxide for physicochemically degrading petroleum-contaminated soils. Remediat J. 1992; 2(4):413-25. [DOI:10.1002/rem.3440020407]