(I) Electrochemical Oxidation (direct/indirect)Investigated parameters: currentdensity and reaction temperature in electrochemical reactor (Körbahti and Artut, 2010),initial pH and cell voltage (Yan et al., 2011).(II) Electro-Fenton In theElectro-Fenton process, Fe(II) is oxidized by H2O2 to form Fe(III).This lead to forming a hydroxyl radical (HO•) and a hydroxide ion (OH?) in the process aswell.
In the next step Fe(III) is then reduced back to Fe(II) by anothermolecule of H2O2, forming a radical of (HOO•)and a proton (H+). The main effect of adding H2O2 isto create two different oxygen-radical species, with water (H+ + OH?)as a byproduct (Ishakand Malakahmad, 2013). Fe2+ + H2O2 ? Fe3+ + HO• + OH? (1) Fe3+ + H2O2 ? Fe2+ + HOO• + H+ (2) In thesecond reaction free radicals of HOO are produced. Hydroxyl radical (HO•)is an authoritative, strong, and non-selective oxidant which can start the newreactions rapidly. Oxidation of anorganic compound by Fenton’s reagent can be done very quickly but it involvedwith exothermic reactions that results in increasing the temperature of thesolutions.
The main purpose of this process is to oxidation of pollutants toprimarily carbon dioxide and water (Kavitha, V., & Palanivelu, K., 2005).
Generally, Fe(II) sulfate (FeSO4)is used as catalyst in the reactions. In case of electro-Fenton process,hydrogen peroxide is produced in situ from the electrochemical reduction ofoxygen. Also, Fenton’s reagent during the radical substitution reaction is usedin organic synthesis for the hydroxylation of aromatic hydrocarbon (Casado et al., 2005).
For instance, classical conversionof benzene (C6H6) into phenol (C6H5OH)can be expressed as: (Casadoet al., 2005) C6H6 + FeSO4 + H2O2 ? C6H5OH (3) Meinero and Zerbinati(2006) investigatedthe oxidative and energetic efficiency of various electrochemical oxidationprocesses. The electro-Fenton process was verified to have the best degradationefficiency in terms of energy consumption: for that case the specific energyconsumption was 0.
3 kWh/g of COD, corresponding to 41.8 kWh/m3.Manyworks classified electro-Fenton or the very Fenton process as advancedoxidation process (AOP). Some of AOPs are, electro-Fenton process, TiO2/H2O2,photocatalysis reactions, etc., that are chemical oxidation processes mainlyused as an attractive pretreatment method to improve the biodegradability ofvarious industrial discharges, that is able to generate and use hydroxyl freeradicals (•OH) as strong oxidant (Klamerthaet al.
, 2010; Sin et al., 2011). The application of AOPs not onlyreduces the COD load and contaminants levels in wastewater, but also generatesfewer toxic effluents. Besides, AOPs augment the biodegradability of wastewaterthrough forming intermediates similar to the metabolic pathway substances (Ollis, 2000). Advanced oxidation process (AOP)which employ strong oxidant agents (ozone, hydrogen peroxide and UV, Fenton,etc.), are able to remove organic and phenolic pollutants of the Olive Mill Wastewater(OMW) (Madani et al.
2015). TheFenton process could be enumerated as one of the promising alternativeoxidation methods because of its cost efficiency, operation simplicity, lack ofresidue, and ability to treat a spectrum of substances. Fenton process, whichis in fact a synthesis of oxidation and coagulation reaction, reduces toxicityand COD concentration using hydrogen peroxide and ferrous sulfate (Madani et al.
2015). To be specific, the oxidationmechanism by the Fenton process is due to the generation of hydroxyl radical inan acidic solution by the catalytic decomposition of hydrogen peroxide and inpresence of ferrous (II) ions (Ledakowiczet al., 2001). Fenton’sreagent (a solution of hydrogen peroxide (H2O2) and an iron catalyst (like FeSO4,iron electrode, FeSO4.7H2O (ferrous sulfateheptahydrate), etc.)) is used to oxidize contaminants or organic compounds inwastewaters such as trichloroethylene (TCE), tetrachloroethylene (perchloroethylene, PCE), andrefinery wastewater to augment biodegradability.
The Fenton reaction is shownin Eqs. (4) to (13). At acidic pH it leads to the production of ferric ion andhydroxyl radical (Ishakand Malakahmad, 2013): H2O2 + Fe2+ ? Fe3++ •OH + OH- (4) Fe3+ + H2O2 ? Fe-OOH2+ + H+ ? •H2O + H+ (5) Hydroxyl radicals may be scavengedby reaction with another Fe2+ or with H2O2: •OH + Fe2+ ? OH? + Fe3+ (6) •OH + H2O2 ? HO2 • + H2O (7) Hydroxylradicals may react with organic and starting a chain reaction: •OH + RH ? H2O + R• (RH=organic substrate) (8) R• + O2 ? ROO• ? products of degradation (9) Ferrous ion and radicals are produced during the reactions: H2O2 + Fe3+ ? H+ + FeOOH2+ (10) FeOOH2+ ? HO2• + Fe2+ (11) HO2• + Fe2+ ? HO2? + Fe3+ (12) HO2• + Fe3+ ? O2 + Fe2++ H+ (13) Ishak and Malakahmad(2013) showedthat Fenton process is able to augment the biodegradability of refinerywastewater as a pretreatment for recalcitrant contaminants. Studied operationalparameters were reaction time (20 – 120 min), H2O2/COD (2 – 12) andH2O2/Fe2+ (5 – 30) molar ratios. Theydetermined that BOD5/COD as an index of biodegradability ofwastewater increased from 0.27 to 0.
43 under optimum conditions of operationalparameters, including reaction time (71 min), H2O2/COD (2.8) and H2O2/Fe2+(4) molar ratios: the process was optimized using response surface methodologybased on a five-level central composite design. In addition to low biodegradability of petroleum refinerywastewater, the higher concentration of COD in characterized refinerywastewater is because of presence of some compounds such as phenols andsulfide. So, such wastewater with low BOD and high COD is consider as lowbiodegradability wastewater (Metcalf and Eddy,2003).
Moreover, considering high concentration of some contaminantsincluding oil and grease; Benzene and Toluene as PHCs; Ethylbenzene and Xyleneas aromatic hydrocarbons, it could be implied that the petroleum wastewater orother oily wastewaters containing biorecalcitrant contamination or heavy metalsrequires pretreatment before application of any biological decontamination (Ishak and Malakahmad, 2013).According to Ishak and Malakahmad(2013), although the range of time factor in Fenton process was from 20to 120 min, the results revealed that in the first 20 minutes of the Fentonreaction, more than 90% of COD and BOD removal was achieved. Also, BOD5/CODratio of 0.
40 was attained within 20 minutes. This finding, shows very shortperiod of time required for a significant biodegradability improvement andpollution reduction in a Fenton process which is of special interest in theindustrial application of Fenton’s reagent: hydroxyl free radicalsbear a the short half-life, so the extension of reaction timedoes not improve degradation. Even though by increasing of H2O2concentration better organic degradation will be attained due to moregeneration of more hydroxyl radicals (Kang andHwang, 2000), at a certain limit, the complete organic removal could notbe obtained even with higher than stoichiometric quantity of H2O2/CODand this eventually led to reducing the removal efficiency. Generally, it meansbiodegradability declined after increasing H2O2/COD molarratio to more than 2 (Ishak and Malakahmad, 2013).Regarding the third studied influential factor, i.e. H2O2/Fe2+,it has been verified that both peroxide dose and iron concentration (Fe2+)are influential factors in the Fenton reaction for better degradation efficiency and reaction kinetics, respectively (Kavitha and Palanivelu, 2005; Siedlecka and Stepnowski,2005).
In that experiment, decrease of H2O2/Fe2+molar ratio (i.e. higher concentration of Fe2+) caused morebiodegradability and higher removal of the target compound and formation ofearly intermediates, i.e. generating more hydroxyl radicals for the degradationprocess (Catalkaya and Kargi, 2007; Ishak andMalakahmad, 2013). Excessive amount of Fe2+ competes with theorganic carbon for hydroxyl radicals when high Fe3+ concentration isused.