Treatment of pretreated coke wastewater by electrocoagulation and electrochemical peroxidation processes

Treatment of pretreated coke wastewater by electrocoagulation and electrochemical peroxidation processes

Accepted Manuscript Treatment of Pretreated Coke Wastewater by Electrocoagulation and Electrochemical Peroxidation Processes Fuat Ozyonar, Bunyamin Ka...

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Accepted Manuscript Treatment of Pretreated Coke Wastewater by Electrocoagulation and Electrochemical Peroxidation Processes Fuat Ozyonar, Bunyamin Karagozoglu PII: DOI: Reference:

S1383-5866(15)30079-4 http://dx.doi.org/10.1016/j.seppur.2015.07.011 SEPPUR 12427

To appear in:

Separation and Purification Technology

Received Date: Revised Date: Accepted Date:

31 October 2014 6 July 2015 7 July 2015

Please cite this article as: F. Ozyonar, B. Karagozoglu, Treatment of Pretreated Coke Wastewater by Electrocoagulation and Electrochemical Peroxidation Processes, Separation and Purification Technology (2015), doi: http://dx.doi.org/10.1016/j.seppur.2015.07.011

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Treatment of Pretreated Coke Wastewater by Electrocoagulation and Electrochemical Peroxidation Processes Fuat Ozyonar1* and Bunyamin Karagozoglu 1 1

Department of Environmental Engineering, Cumhuriyet University, 58140 Sivas, Turkey *Corresponding author: Fuat Ozyonar E-mail address: [email protected], [email protected] Phone: +90 346 219 10 10; fax: +90 346 219 11 77

Abstract In this study, treatment of pretreated real coke wastewater by Electrocoagulation process (EC) and Electrochemical Peroxidation process (ECP) using direct pulse current was investigated. Air stripping process of ammonia was used as a physicochemical process for the pretreatment of wastewater. In the present study, ECP process has been offered to remove chemical oxygen demand (COD), total organic carbon (TOC), phenol, cyanide (CN-) and thiocyanate (SCN-) from coke wastewater. The efficiency of the process and settling characteristic of waste sludge were investigated through changing some operating parameters such as initial pH, initial H2O2 concentration and current density. Direct pulse current (DPC) was used to prevent the passivity or polarization of electrodes and to increase removal efficiency. Under the optimum operation conditions at the EC and ECP process (pH 3, current density 200A/m2, initial H2O2 10 g/L (for ECP), operation time 20 min.), the removal efficiencies of COD, TOC, phenol, CN- and SCN- were observed to be 26%, 20%, 9%, %9.2 and 8.2% (for EC) and to be 92.0%, 90.0%, 97.6%, 90.0% and 93.6% (for ECP), respectively. Operating costs for the EC and ECP process were calculated as 1.46 € /m3 and 5.64€ /m3.

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*Corresponding author: Fuat Ozyonar E-mail address: [email protected], Phone: +90 346 219 10 10; fax: +90 346 219 11 77

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These results asserted that ECP process was more effective than EC process for removal efficiencies of pollutions. But, the ECP process was more expensive than EC process. Keywords: Electrochemical peroxidation, Coke wastewater, Direct pulse current, Iron electrodes, Removal of organic matters.

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1. Introduction Coke wastewater is commonly generated during coal coking, coal gas purification and byproduct recovery processes of a coke factory. The composition of the wastewater is complicated and varies from one factory to another depending on the quality of raw coal, carbonation temperature and methods used for the recovery of by-products [1]. It contains plenty of hazardous organic and inorganic pollutants and most of them are toxic, mutative and carcinogenic substances such as ammonium, sulfate, cyanide, thiocyanate, phenolic compounds, polynuclear aromatic hydrocarbons (PAHs), nitrogen, oxygen and sulfur containing heterocyclic compounds [2-4]. The coke wastewater is a serious pollution problem for human and aquatic life. Conventional treatment of coking wastewater includes solvent extraction of phenolic compounds, stream stripping of ammonia and biological treatment. The activated sludge process is not efficient for coke wastewater. So, new biological treatment processes have been developed. The effluent of common biological treatment includes conventional activated sludge process, anoxic-oxic (A/O) or anarebic-anoxic-oxic (A/A/O), and sequencing batch reactors (SBR) [1,5,6]. These processes are not alone enough to meet legal direct discharge-limits for COD, cyanide, phenol, oil-grease and pH which are respectively set as 150, 0.5, 1.0, 35 mg/L, and 6-9 [7]. Therefore, advanced treatment processes are still necessary for coke wastewater treatment. In recent years, electrochemical treatment methods such as Electro oxidation (EO), Electrocoagulation (EC) and Electroflotation (EF) have attracted increasing attention for the treatment of various types of wastewaters [8]. Electrochemical methods have been reported as a primary technique for treatment of various wastewaters, by virtue of various benefits including environmental compatibility, versatility, energy efficiency, safety, selectivity, amenability to automation and cost effectiveness [9]. Electrochemical processes have some

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advantages such as simple equipment, easy operation, shortened retention time, rapid-settling and decreased amount of precipitate or sludge [9]. Electrochemical methods were frequently used for treatment of wastewaters containing heavy metals [10-12], foodstuff [13], for wastewaters of oil industries [14,15], textile industries [1618], dye removal [19, 20], aqueous suspensions of ultrafine particles [21], nitrate [22], removal of iron from drinking water [23], phenolic compounds [24], arsenic and arsenate [25], refractory organic pollutants including lignin and EDTA [26], landfill leachate [27-29] and liquid organic fertilizer plant [30]. Especially, electrocoagulation and electrochemical peroxidation processes are widely used as an effective treatment processes for organic matter content wastewater. Many researches have obtained successful results. But few studies adopted electrocoagulation and electrochemical oxidation process. In these studies, electrochemical processes have been applied using biologically pretreated coke wastewater [4, 31, 32]. So, this paper is highly crucial for the investigation of treatment of real coke wastewater by EC and ECP processes. The aim of the present study was to investigate the pollutant removal efficiencies from pretreated coke wastewater (PCW) by electrocoagulation and electrochemical peroxidation (ECP) processes using iron electrodes and periodic inversion of polarity in DC electric current in a batch mode operation. The effects of operating parameters such as initial pH, current density, initial H2O2 concentration and operation time for EC and ECP processes on the removal efficiencies were studied to determine the optimum operating conditions. The amount and settling of sludge from EC and ECP processes were also investigated and operation costs for EC and ECP were calculated as €/m3.

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Removal mechanisms of electrocoagulation and electrochemical peroxidation processes The EC processes consist of coagulant generation in situ by electrodissolution of soluble anodes such as aluminum or iron upon application of a direct current [30]. The EC processes are characterized by simple equipment, easy operation, brief reactive retention period, negligible equipment for adding chemicals and decreased amount of sludge [31]. When iron electrode is used as anode and cathode, the generated Fe3+ ions will immediately undergo further spontaneous reactions to produce corresponding hydroxides and/or polyhydroxides [15]. For example, ferric ions generated by electrochemical oxidation of iron electrode can form monomeric ions, Fe(OH)3 and hydroxyl complexes namely: Fe(H2O)62+,

Fe(H2O)2+,

Fe(H2O)4(OH)2+,

Fe2(H2O)8(OH)24+

and

Fe2(H2O)6(OH)44+.

Formation of these complexes depends strongly on solution pH [15, 34]. Above pH>9, Al(OH)4- and Fe(OH)4- are the dominant species. Already formed Fe(OH)3 remains in the aqueous streams as a gelatinous suspension that can remove the pollutants from wastewater by coagulation, adsorption, co-precipitation and sweep flocculation [33, 35, 36]. When iron is used as electrode material, two mechanisms have been proposed for the production of Fe(OH)n , Where n= 2 or 3 [15,37]. Fe(s) → Fe2+ (aq) + 2e −

(anode)

(1)

2 H 2 O( I ) + 2e − → H 2 ( g ) + 2OH − ( aq )

(cathode)

(2)

Fe 2+ ( aq ) + 2OH − ( aq ) → Fe(OH ) 2 (s )

(in Solution)

(3)

Fe ( s ) + 2 H 2 O ( I ) → Fe ( OH ) 2 ( s ) + H 2 ( g )

(in Solution)

(4)

2Fe(s) + 5H2O(I) + O2(g) → 2Fe(OH)3(s) + 2H2(g) (in Solution)

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(5)

Recently, the applications of electrochemical method in Fenton process, named as EF process, have been reported [38-40]. In general, there are two different Electrochemical advanced oxidation process (EAOPs) applications. First of them is the EF system where Fenton’s (Fe(II) and H2O2) are added to the reactor from outside and inert electrodes having high catalytic are used as anode material, named as Fered-Fenton process. In the second one, H2O2 is added from outside and Fe(II) is provided from sacrificial cast iron anodes, named as electrochemical peroxidation (ECP) process [40]. This method was used for the treatment of PCW. The effect of initial H2O2 concentration on the efficiency of COD, phenol, cyanide, thiocyanide and TOC removal from the PCW was investigated. The ECP process based on the employment of a sacrificial iron anode delivers Fe2+ ions into the solution (Eq.1). Simultaneously occurs the reduction of water at the cathode (Eq.2); then the hydrogen peroxide is added in order to provide conditions for the Fenton reactions (Eqs. (6) and (8)). Moreover, Fe2+ according to Eq. (9) may be continuously regenerated at the cathode depending on the electrolytic cell setup [30,39].

Fe +2 + H 2 O 2 → Fe 3+ + OH* + OH −

(in the solution)

(6)

Fe+2 + OH* → Fe3+ + OH* + OH−

(in the solution)

(7)

Fe 3+ + H 2 O 2 → Fe 2+ + H + + HO * 2 (in the solution)

Fe3+ + e− → Fe2+

(cathode)

(8)

(9)

Advanced oxidation processes are frequently used to oxidize complex organic constituents found in wastewaters. In these processes, the hydroxyl radical (OH*) is used as a strong oxidant to destroy the complex organic compounds [40]. Huang et al. [41] have discussed the advantages and disadvantages of several advanced oxidation processes and have

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concluded that the methods focus on hydrogen peroxide to promote the formation of hydroxyl radical. These methods do not involve the use of particularly dangerous chemicals and their operation is simple and cost-effective [42]. In these processes, a chain reaction then occurs between the hydroxyl radical and an organic compound R [43, 44].

RH + OH * → H 2 O + R *

(10)

R * + O 2 → ROO

(11)

*

ROO * + RH → ROOH + R *

(12)

So, the main advantage of adding H2O2 is to produce this hydroxyl radical, which can react with the organic pollutants in the wastewater [41]. 2. Materials and methods 2.1 Wastewater source and characteristics The raw coke wastewater was collected from a coking plant in Turkey producing approximately 600 m3 of wastewater per day. A physicochemical (air stripping) process was used for the removal of ammonia from the wastewater in the laboratory. The wastewater used in this study was the effluent of this air stripping process. The characteristics of wastewater used in the experiments are listed in Table 1. Table 1. Characteristics of wastewater used in the experiments. 2.2 EC reactor The schematic figure of electrochemical reactor is shown in Figure 1. The EC and ECP reactors were made of plexiglass with dimensions of 130 mm x 100mm x 100mm. Four plate electrodes (two anodes and two cathodes) with dimensions of 50 x 70 x 2 mm (purity ≥ 7

99.5%) were used in the study. The total effective electrode area was 210 cm2 and the spacing between electrodes was 20 mm. The electrodes were connected to a digital DC power supply (Alpha 10A-50V DC power supply) in monopolar-parallel mode. Figure 1. (a) Photograph and (b) Schematic diagram of experimental setup. 2.3 Description of Direct pulse current electrocoagulation An adjustable time relay plugged into the DC power supply in order to obtain alternating pulse current (DPC) was performed. In our study, DPC presents alternating current. According to EC and ECP units with time relay system, turn on and turn off modes switch to positive pole to negative pole or vice versa. For example, cathode and anode are operating every five minutes, then they are replaced or interchanged with each other until EC or ECP are completed. Typical diagram for direct pulse current used in this study is shown in Figure 2. Figure 2. Typical diagram for DPC rectangular pulse current. 2.4 Experimental procedure All experiments were performed at constant temperature (25oC), mixing speed (250 rpm) and with 1000 mL of wastewater solution. Before each run, electrodes were washed with acetone to remove surface grease, then the impurities on electrode surfaces were removed by dipping for 1 min into a solution freshly prepared by mixing 100 mL of HCl solution (35%) and 200 mL of hexamethylenetetramine aqueous solution (2.80%) [9]. After electrodes had been washed and then were dried for the removal of the residuals on their surfaces, the next step of the experiment was started. To avert interference of H2O2 in the electrochemical peroxidation process, samples were taken into the tubes with sodium hydroxide to quench the reaction by increasing pH around 10.0. The samples were filtered and then the filtrate was analyzed. 8

2.5 Analytical technique Chemical oxygen demand (COD), Phenol, Cyanide (CN-), thiyocyanide (SCN-), Sulfur (S2-), and Sulfite (SO3) determinations were carried out by implementing standard analysis methods [44]. Biological oxygen demand (BOD5) was determined by respirometric method (Oxitop IS6, German), Total organic carbon (TOC) levels were determined through combustion of the samples at 680 °C using a non-dispersive IR source (Tekmar Dohrmann, Apollo 9000, USA). The UV-Vis spectra of samples were measured by using a UV-Vis spectrophotometers (Merck spectroquant Pharo 300, German). The pH and conductivities of samples were measured by means a pH meter (C931, Consort, Belgium) and conductivity meter (340I, WTW, USA), respectively. The analysis of H2O2 was done by the permanganometric method (Merck). Sludge settling properties were evaluated by using sludge volume index (SVI) described in standard methods [44]. The percentage removal efficiency of COD, TOC, phenol, CN-, and SCN- was calculated using the following equation, Eq. (15).  (C − C)   x100 Percentage removal efficiency (%) =  o  C o  

(15)

2.6. Operation Cost The operating cost is one of the most important parameters in EC processes because it influences the application of any method of wastewater treatment. The operating cost includes material (mainly electrodes) cost, electrical energy cost, as well as labor, maintenance and other costs. The latter cost items are largely independent of the electrode material. Thus, in this study, the operating cost was calculated together with electrodes, electrical energy and chemical costs. So, energy, electrode and chemical consumption costs are taken into account as major cost items [8]. Calculation of operating cost is expressed as; 9

Operating Cost= AEnergy consumption (EnC) + BElectrode consumption (ElC) + CChemical consumption (ChC)

…….Where energy consumption and electrode consumption are consumption quantities per m3 of wastewater treated. Unit prices, A, B and C, given for the Turkish Market, September 2013, are as follows; electrical energy price 0.072 €/kWh, electrode material price 0.85 €/kg for iron and chemical costs 0.43 €/ kg H2O2, 0.73 €/kg for NaOH, and 0.29 €/kg for H2SO4. The electrode and energy consumptions in the EC process were calculated using the following equations;

Energyconsumption =

(V .I .t ) ν

(16)

…..Where Energyconsumption is energy consumption (kWh/m3), V is voltage (Volt), I is current (Ampere), t is EC time (s) and v is volume of the treated wastewater (m3), respectively. According to Faradays law, electrode material consumption and charge loading are calculated in the following equations; Faraday ( I .t ) = ( F .v ) m3

Electrodeconsumption =

(17) ( I .t.M w ) ( z.F .v)

(18)

..…Where F is Faraday’s constant (96485 C/mol), Mw is the molar mass of iron (56 g/mol) and z is the number of electron transfer (zFe:2), respectively. 3. Results and discussion 3.1 Electrocoagulation studies A series of experiments in the treatment studies by EC process of PCW were performed, but remarkable results could not be obtained. EC process was carried out at 10

different initial pH (2-10), current density (50-200A/m2) and 80 minutes operation period. As a results of the experimental studies, removal efficiencies of PCW at optimum conditions (pH :3, current density: 150A/m2 and EC time: 80 min.) were obtained as 32.9% for COD, 27% for TOC, 15.9% for phenol, 18.1% for CN- and 16.2% for SCN- (Table 2). Consequently, satisfactory results at the removal efficiencies of COD, TOC, phenol, CN- and SCN- were not obtained. The same results by other authors the different wastewater treatment by electrocoagulation were obtained [30,36]. Owing to the inefficiency of EC process in the treatment of coke wastewater, the experimental studies were carried out by using electrochemical peroxidation process. The operating costs at 50-200 A/m2 were calculated to be between 1.05-5.85 €/m3 (Table 2). Value of the OCs at the optimum current density and time were 1.46 €/m3. Table 2. Experimental results of coke wastewater at different initial pH and current density in the EC Process 3.2 Electrochemical Peroxidation studies In the ECP process, many useful or interfere reactions can take place successfully in the removal of pollution from wastewater. The removal efficiencies of ECP methods can be explained by oxidation reactions. It is possible to keep these reactions dominant by using proper operating conditions such as initial pH, initial H2O2 concentration, current density and operation time. Therefore, this study was focused on the determination of optimum values of these parameters. In the treatment studies, ECP process was found to be very effective.

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3.2.1 Effect of initial pH on pollutant removal efficiency Initial pH is an important operating factor influencing the performance of the ECP processes. In ECP process, the effects of initial pH together with operating time (0-80min) for treatment of PCW were studied at 100 A/m2 and 10000 mg/L initial H2O2 and initial pH range of 2-6. It is well known that Fenton’s reactions occur at low pH values. According to Zhang et al., [42] and Panizza and Cerisola [45], optimum pH values for EF method were between 2 and 5. A high concentration of the main oxidizing agent, OH*, is expected to be produced from Fenton’s reaction within this range, this range, thus yielded a rapid mineralization of pollutants [46]. Therefore at the 2-6 range, different pH levels were carried out so as to evaluate its effect. The pH values were adjusted by addition of H2SO4. Results obtained from the experiments are presented in Figure 3. It can be seen clearly from Figure 3 that best removal efficiency was the best at pH 3.0 and such finding complies with the findings of earlier studies [28, 36, 47]. The low COD, TOC, phenol, CN- and SCN- removal efficiencies were observed at the initial pH lower and higher than 3.0. The high removal efficiencies of PCW at initial pH 3 for 45 min operation time were obtained as 92.6% for COD, 90.6% for TOC, 98.8% for phenol, 93.8% for CN- and 96.2% for SCN-. The removal efficiencies of the pollutants of PCW at initial pH 2-4 increased. This increment can be explained with the hydroxyl radicals which occur with Fenton’s reactions (6-7) during electrochemical treatment as these radicals react with organic pollutants and destroy them as seen at the reaction (10). The removal efficiencies of COD, TOC, phenol, CN- and SCN- at initial pH 3.0 for the range of operation time 0-45 min. were founded to be increased rapidly respectively from 63.1% to 90.1%, from 65.1% to 84.8%, from 88.8% to 98.0%, from 67.1% to 91.4% and from 78.8% to 93.6%.

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The decrease in the removal efficiencies at low pH is probably caused by the formation of Fe(OH)+, which formed at low pH and can compete with ferrous ion to react with hydrogen peroxide. However, Fe(OH)+ reacts more slowly with hydrogen peroxide, therefore, produces less hydroxyl radicals. This reduces the removal efficiency. In addition, the scavenging at very low pH and also the reaction of Fe3+ with hydrogen peroxide is inhibited [48,49]. On the contrary, ferrous ions are unstable at pH >4.0 and they easily form ferrics ions (Eqs. (6), (7) and (8)), which promotes the tendency to produce ferric hydoroxo complexes [50]. Moreover, the oxidation efficiency of Fenton’s reagent may decrease because ferric ions could form Fe(OH)3. This form has a low activity and will not react with hydrogen peroxide. So, the optimum initial pH for this study was selected to be 3.0. Figure 3. Effect of initial pH on (a) COD removal, (b) TOC removal, (c) Phenol removal, (d) CN- removal and (e) SCN- removal at the ECP process (Operating conditions; Current density: 100 A/m2, H2O2 concentrations: 10 g/L). The energy and electrode consumptions for the ECP process at pH 2-6 and 20 min. were 1.93-3.30 kW h/m3 and 0.98-0.74 kg/m3. OCs varied from 5.32 to 5.25 €/m3 (Table 3). 3.2.2. Effect of current density on pollutant removal efficiency Current density is another important parameter for controlling the reaction rate in most electrochemical processes such as the EC and ECP processes. Current density determines the rate of iron hydroxides dosage and bubble generation rate and size. The bubble size decreases with increasing current density [51], which is profitable to the separation process. Moreover, the OH* formation rate is controlled by the applied current density during the electrolysis. The amount of Fe+2 ions dissolved from the sacrificial iron anodes increases (Eq. 1) when current density applied to the electrochemical reactor increases. This is the most important thing to increase the efficiency of ECP process, since adequate amount of Fe2+ in the reactor propagates to Fenton’s reaction. Thus, this parameter should have a significant impact on 13

pollutant removal efficiencies. Effect of the current density on pollutant removal efficiencies of ECP process was studied in the range of 100-400 A/m2 at operation time 0-80 min. and at initial pH of 3.0. The results of studies are shown in Figure 4. Removal of COD, TOC, phenol, CN- and SCN- was directly proportional to the current density and increased with operation time. However, rate of pollutants removal decreases with the application of the current density higher than 200 A/m2. Such a decrease can be attributed to excessive rise of Fe+2 concentrations in the reactor. In recent studies, similar results were observed by Atmaca [28] and Zhang et al, [38]. These studies showed that an increase of the Fe+2 can inhibit the degrading rate of organics due to the competitive reactions (7) and (8) [28, 52, 53]. Another result derived from Figure 4 showed that the required time for the treatment decreased with the increasing applied current density. Especially, the required time was drastically shortened when the applied current density was raised from 100 to 200 A/m2. Figure 4 (a), (b), (c), (d) and (e) indicated that significant improvements in removal efficiencies of COD, TOC, phenol, CN- and SCN- at 10 min were observed at range of 100200 A/m2. The removal efficiencies were increased from 63.1% to 88.5% at 10 min and from 89.2% to 95.5% at 80 min for COD; from 60.1% to 82.7% at 10 min and from 83.4% to 95.6% at 80 min. for TOC; from 89.7% to 94.4% at 10 min and from 97.2 to 99.8% at 80 min. for phenol; from 59.1% to 78.1 at 10 min. and from 95.7 % to 98.1 % at 80 min. for CN-; and finally from 79.8% to 86.9% at 10 min. and from 93.4% to 98.8% at 80 min. for SCN-. The optimum current density for this study was selected as 200 A/m2. OCs at 50-200 A/m2 and 20 min were calculated as 4.89-7.70 €/m3. The OCs increased with increasing current density for the ECP since the cost increased considerably with electrodes consumption. OC the optimum current density was 5.64 €/m3.

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Figure 4. Effect of current density on (a) COD removal, (b) TOC removal, (c) Phenol removal, (d) CN- removal and (e) SCN- removal at the ECP process (Operating conditions; initial pH:3, H2O2 concentrations: 10 g/L). 3.2.3 Effect of initial H2O2 concentration on pollutant removal H2O2 plays a critical role as an oxidizing agent in the Fenton reaction. Generally, it has been observed that the removal efficiency of pollutants increases with increasing H2O2 concentration [36, 48, 49, 54]. It is well known that main source of OH* radical is H2O2 used in the ECP process. So, determination of the optimum H2O2 concentration in the ECP process is very important for removal efficiency. The effect of initial H2O2 concentrations on pollution removal efficiencies was studied in the range of 0-30 g/L at the optimum conditions (200 A/m2 and pH 3). The removal efficiencies of COD, TOC, phenol, CN- and SCN- were found to be directly proportional to H2O2 concentration when the current density applied to the ECP process was sufficient (200 A/m2). In ECP experiments, if high H2O2 concentrations, low current density and uncontrolled pH conditions are studied, H2O2 interferences will very important [36]. Szpykowicz et al., [55] stated under increasing unreacted H2O2 and pH conditions, the following reaction should cause an increase in measured COD of the solutions;

K 2 Cr2 O 7 + 3H 2 O 2 + 4H 2 SO 4 → K 2SO 4 + Cr2 (SO) 4 + 7H 2 O + 3O 2

(17)

As given above reaction (17), COD values are normally higher than the initial COD. This increase in COD values is because of high pH values since pH increases during ECP process. This phenomenon was the reason for sustaining the pH levels within acidic values [36]. COD, TOC, phenol, CN- and SCN- removal efficiencies increased with increasing initial H2O2 concentration. When initial H2O2 concentration was increased from 5-10g/L in 10 15

min, COD, TOC, Phenol, CN- and SCN- removal efficiencies increased. But there were not any remarkable changes in the removal efficiencies of pollutants in the initial H2O2 concentration up to 10 g/L. The similar results were reported by various previous studies [28, 30, 36, 53, 56]. This may be explained with the recombination of hydroxyl radicals and also hydroxyl radical reaction with H2O2, contributing to the OH* scavenging capacity (reaction (19), (20), (21). Oxidation potentials of hydroperoxyl radicals (1.25 ev) were lower than of H2O2 (1.3 ev) [36,56]. H 2 O 2 + ∗ OH → H 2 O + HO 2

HO



∗ 2



(19)

+ ∗ OH → H 2 O + O 2

(20)

OH+∗ OH → H2 O + 1/2O2

(21)

HO* + RH → R * + H 2 O

(22)

As showed in Figure 5, COD removal efficiency increased from 65.3% to 88.5% with increasing initial H2O2 concentration from 5 g/L to 10 g/L in 10 min. and the removal efficiencies at 15 g/L, 20 g/L and 30 g/L respectively reached to 90.4%, 91.2% and 91.5%. The TOC removal efficiencies in the range of 5-30 g/L initial H2O2 concentration at 10 min. were obtained as 65.9%, 82.7%, 86.4%, 85.5% and 86.4%, respectively. These removal efficiencies at 80 min reached to 85.9%, 95.6%, 95.9%, 96.1% and 96.5%. In the removal of phenol, CN- and SCN, results similar to COD and TOC removal were obtained. The removal efficiencies of phenol, CN- and SCN- in initial 10 g/L H2O2 at 10 min. were found to be 94.4%, 78.1%, and 87.0%, respectively. The removal efficiencies at 80 min reached to 99.8%, 98.1% and 98.8%. As seen in Figure 5, there are significant differences between pollutant removal rates of 10 g/L H2O2 and without H2O2 treatments. Without H2O2, removal efficiencies of COD, TOC, CN- and SCN- were not sufficient. Such a case indicate that H2O2 16

concentration had a very important impact on ECP treatment of PCW. EC process is not effective process for treatment of PCW because of low oxidization potential of organic matter. In EC method, reactions are coupled with coagulation occurring due to existence of ferric/ferrous ions. These metals ions play a significant role as a coagulant [36]. But, ECP process was found to be very effective process for the treatment of PCW. In EC process, three major interaction mechanisms are co-precipitation, adsorption and sweep coagulation [30,35]. These mechanisms occur at different pH ranges. At low pH values, metal species like Fe3+ are generated at the anode. They neutralize their charges and reduce their solubility [30]. This process is called as precipitation. At pH values over (pH>6), second mechanism, adsorption, takes place. It includes adsorption of organic matters on amorphous metal hydroxide precipitates forms amorphous M(OH)3(s) (sweep flocs) [30]. At a pH range of 4-6, the EC process was explained as co-precipitation [35]. Figure 5. Effect of initial H2O2 on (a) COD removal, (b) TOC removal, (c) Phenol removal, (d)CN- removal and (e) SCN- removal at the ECP process (Operating conditions; initial pH:3, current density: 200 A/m2). The optimum initial H2O2 concentration may be selected as 10 g/L. Consequently, it is worth to note that the performance of ECP process also depends heavily on initial H2O2 concentration as well as current density. The removal efficiencies of pollutants in 5 min were lower than the efficiencies obtained in 10 min. COD, TOC, phenol, CN- and SCN- removal efficiencies for pollutants at 5-80 min. were obtained as 77,68%-95,5% for COD, 73%-95,6% for TOC, 90.1%-99.8% CN- and 81.2%-98.8% SCN-, respectively. In the optimum conditions, COD, TOC, phenol, CN- and SCN- removal efficiencies were 92.0% (500mg/L), 90.0% (230mg/L), 97.7% (35mg/L), 90.0% (21mg/L), 93.6% (27 mg/L), respectively. Table 3 showed that OC values calculated by considering H2O2 (0-30 g/L/) concentration varied between 1.42 and 13.85 €/m3. The OC of the treated PCW depends on initial H2O2

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concentration because the OC increased with increasing H2O2 concentration. The OC at optimum initial H2O2 concentration was 2.4 times lower than the OC at 30 g/L. The ECP process seemed to be more expensive but, pollutants were removed with higher removal efficiencies and lower operating time with the ECP process as compared to the EC process. With regard to COD (150 mg/L) and phenol (1 mg/L) concentrations, the levels were below the allowable direct discharge limits. The operational conditions of ECP process were obtained as initial pH of 3, current density of 200 A/m2, initial H2O2 concentration of 15 g/L and operation time of 20 min. However, OC was calculated as 10.63 €/m3. The results of experimental studies are provided in Table 3. ECP process was an effective treatment process to treat PCW in short treatment times. In addition, Figure 6 has shown the actual appearance of the coke wastewater before and after treatment. The effluent of ECP process is cleaner and more transparent than the raw wastewater and effluent water of EC process.

Figure 6. Raw wastewater and effluent of EC and ECP processes (Operating conditions: initial pH: 3 and current density: 150 A/m2 for EC, initial pH: 3, current density: 200 A/m2 for ECP at 80 min). Table 3. Experimental results for coke wastewater treatment at different initial pH, current density and initial H2O2 in the ECP process at 20 min. 3.2.4 Comparison of removal efficiencies of DC and DPC Direct current is widely used in EC process. Usually, passivization is observed by the others authors during the EC process [57-60]. While an impermeable oxide layer is occurs on the cathode materials, corrosion formation may be on the anode material due to the oxidation [60]. This event decreased the effective current transfer between the anode and cathode and at the current efficiency [60]. Generally, to solve this problem have been chosen by alternating pulsed current in EC process [59]. Researchers found that alternating pulse current is effective 18

to avoid the passivity or polarization of electrodes and lower efficiencies. In this study, DCP was established and proposed to control the passivity [57, 58]. Figure 2 exhibits applied DCP, where either cathode or anode is typically operating for 5 min. and then it is replaced or interchanged until ECP or EC process is completed. The optimum operation conditions (200 A/m2, 10 g/L H2O2 concentration and pH of 3 for ECP process) were carried out for removal of COD, TOC, phenol, CN- and SCN- both applying Direct current mode (DC) and Direct pulse current mode (DPC). In the EC process, the current efficiency is 100% or over 100% without passivization and corrosion formation. Accordingly, decrease of current efficiency during EC or ECP will be observed. So the current efficiency can be used as an index to reflect the extent of passivization [59]. This parameter was observed by authors [59,60]. As seen in Figure 7, the ECP process using DCP mode was found to be more efficient than DC mode with a slower polarization and passivity, better pollutant removal efficiencies. The current efficiency of operation time has shown an important change at DC mode. The current efficiency decreased during EC process. This may be cause a decrease of the removal efficiencies due to passivization and corrosion formation. In optimum conditions, at DC mode, COD, TOC, phenol, CN- and SCN- removal efficiencies were obtained as 50.2%, 51.0 %, 60.1%, 55.2%, 53.3%, respectively. Figure 7. Effect of DC and DPC on COD, TOC, phenol, CN- and SCN- removal by ECP process (operating conditions; pH:3, 200 A/m2, 10 g/L H2O2, operation time 20 min.) and change of current efficiency at different operation time. 3.2.5 Amount and settling property of sludge One of the most important advantages of electrochemical methods is the formation of lower amounts of sludge with better settling properties when compared to conventional chemical treatment methods [15, 38]. In general, settling properties of waste sludge are determined by sludge volume index parameter (SVI). According to Tchobanoglous et al. [61], sludge with a SVI value of lower than 100 is considered to have good settling characteristics. 19

SVI values estimated in different operating conditions for ECP process are shown in Figure 8. As seen in Figure 8, SVI values were determined to be above 100 in the initial pH values of 2, 2.5, 4, 6, 8 and 10. SVI values in the initial pH of 3 and 3.5 were obtained to be below 100. SVI values decreased with increment from 100 to 150 A/m2. This caused the rise in the entrance speed of Fe ions into reactor. In addition, some increases were observed in the values when initial H2O2 concentration was >15 g/L and current density was >200 A/m2. The reason of these may be decomposition of the flock structures due to gasses production at electrode surfaces and increment of initial H2O2, concentration in the previous study [28]. The lowest SVI values were obtained as 90 (pH 3) and 80 (pH 3.5). This may be a result of the pH and/or final iron concentrations in the reactor. It is well known that Fe(OH)3 flocks are large and more stable flocks under slight alkali conditions. In addition, the amount of sludge produced during PCW treatment was an important problem due to the solid waste. The sludge production was proportional to current density and operation time for ECP process. The sludge was collected by vacuum filtration after ECP process and dried at 105 °C for 24 h in the oven. Amounts of the sludge produced in the ECP process at 20min. increased from 0.89 kg/m3 to 4.23 kg/m3 at 100-400 A/m2. This value at current density of 200 A/m2 was obtained as 1.89 kg/m3. Figure 8. SVI values of the sludge for different test conditions. Conclusions In this study, treatment of pretreated coke wastewater with highly toxic substances, was aimed and the effects of operational parameters on the ECP and EC processes were evaluated based on removal efficiencies (COD, TOC, phenol, CN- and SCN-). ECP process was found to be successful for the treatment of PCW whereas EC process was not effective alone. The optimum operation conditions for PCW were determined as 200 A/m2, 10 g/L H2O2 concentration and initial pH of 3 for ECP process at 20 min. Under these conditions, 20

COD, TOC, phenol, CN- and SCN- removal efficiencies were obtained as 92.0%, 90.0%, 97.7%, 90.0%, and 93.6%, respectively. The obtained results indicated clearly that the ECP process was very effective for the removal efficiencies of COD, TOC, phenol, CN- and SCNfrom PCW. Amount of the sludge production for ECP were found to be 1.89 kg/m3. The operating cost for the ECP process was almost 3.8 times higher than that for the EC process because price of H2O2 affected the cost drastically. Overall, ECP process for the treatment of PCW can be a promising technology for applications in wastewater treatment. Acknowledgements This research was supported by the Cumhuriyet University Research Foundation (Project Code: M-427). References [1]

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Figure Captions Figure 1. (a) Photograph and (b) Schematic diagram of experimental setup. Figure 2. Typical diagram for direct rectangular pulse current (DPC). Figure 3. Effect of initial pH on (a) COD removal, (b) TOC removal, (c) Phenol removal, (d) CN- removal and (e) SCN- removal at the ECP process (Operating conditions; Current density: 100 A/m2, H2O2 concentrations: 10 g/L). Figure 4. Effect of current density on (a) COD removal, (b) TOC removal, (c) Phenol removal, (d) CN- removal and (e) SCN- removal at the ECP process (Operating conditions; initial pH:3, H2O2 concentrations: 10 mg/L). Figure 5. Effect of initial H2O2 on (a) COD removal, (b) TOC removal, (c) Phenol removal, (d)CN- removal and (e) SCN- removal at the ECP process (Operating conditions; initial pH:3, current density: 200 A/m2). Figure 6. Raw wastewater and effluent of EC and ECP processes treatment (Operating conditions: initial pH: 3 and current density: 150 A/m2 for EC, initial pH: 3, current density: 200 A/m2 for ECP at 80min). Figure 7. Effect of DC and DPC on COD, TOC, phenol, CN- and SCN- removal by ECP process (operating conditions; pH:3, 200 A/m2, 10 g/L H2O2, operation time 20 min.) and change of current efficiency at different operation time. Figure 8. SVI values of the sludge for different test conditions.

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Figure 1. (a) Photograph and (b) Schematic diagram of experimental setup.

Figure 2. Typical diagram for direct rectangular pulse current (DPC).

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Figure 3. Effect of initial pH on (a) COD removal, (b) TOC removal, (c) Phenol removal, (d) CN- removal and (e) SCN- removal at the ECP process (Operating conditions; Current density: 100 A/m2, H2O2 concentrations: 10 g/L).

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Figure 4. Effect of current density on (a) COD removal, (b) TOC removal, (c) Phenol removal, (d) CN- removal and (e) SCN- removal at the ECP process (Operating conditions; initial pH:3, H2O2 concentrations: 10 g/L).

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Figure 5. Effect of initial H2O2 on (a) COD removal, (b) TOC removal, (c) Phenol removal, (d)CN- removal and (e) SCN- removal at the ECP process (Operating conditions; initial pH:3, current density: 200 A/m2).

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Figure 6. Raw wastewater and effluent of EC and ECP processes treatment (Operating conditions: initial pH: 3 and current density: 150 A/m2 for EC, initial pH: 3, current density: 200 A/m2 for ECP at 80min).

Figure 7. Effect of DC and DPC on COD, TOC, phenol, CN- and SCN- removal by ECP process (operating conditions; pH:3, 200 A/m2, 10 g/L H2O2, operation time 20 min.) and change of current efficiency at different operation time.

33

Figure 8. SVI values of the sludge for different test conditions.

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Table Captions Table 1. PCW used in the

Parameter (unit)

Value

pH

10.5-11.5

Characteristics of experiments.

Table 2. Experimental results of coke wastewater at different initial pH and current density in the EC Process. Table 3. Experimental results of coke wastewater at different initial pH. Current density and initial H2O2 in the ECP Process at 20 min.

Table 1. Characteristics of PCW used in the experiments.

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Conductivity (mS/cm) COD (mg/L) BOD(mg/L)

12.4 ± 2 6150 ± 150 650 ± 30

TOC(mg/L)

2100 ± 100

Phenol (mg/L)

1400 ± 100

-

CN (mg/L) SCN- (mg/L)

210 ±5 410± 10

S2- (mg/L)

1.4

SO3 (mg/L)

40 ± 10

Table 2. Experimental results of coke wastewater at different initial pH and current density in the EC Process at 80 min. pH 2 2.5 3 3.5 4 6 8 10 Current Density (A/m2) 50 100 150 200

COD (%) 28.0 30.0 32.9 31.0 13.0 9.2 22.2 26.0

TOC (%) 22.1 23.0 27.0 25.0 9.2 10.0 13.0 15.0

Phenol (%) 9.3 14.0 16.0 16.0 7.6 6.6 7.0 9.9

CN(%) 10.0 15.2 18.1 14.3 8.1 11.4 10.0 10.0

SCN(%) 15.0 15.0 16.2 14.3 10.0 14.1 13.1 17.1

25.4 26.4 33.0 30.2

24.4 26.0 27.0 27.5

12.0 14.0 16.0 14.0

13.2 14.2 18.1 14.2

11.1 14.1 16.2 16.1

pHfinal 6.7 7.3 7.0 7.1 7.0 8.8 9.4 11.8

6.0 6.7 7.0 7.6

ElC EnC Ws (kg/m3) (kg/m3) (kWh/m3 ) 4.60 2.92 22.40 4.63 2.90 25.73 4.68 2.92 18.40 4.70 2.90 23.60 4.65 2.89 22.40 4.67 2.85 14.53 4.50 2.90 18.40 4.87 2.84 20.67

1.43 3.45 4.68 5.15

0.97 1.94 2.92 3.89

4.13 8.53 18.40 47.07

OC (€/m3) 4.09 4.32 3.81 4.16 4.07 3.47 3.79 3.90

1.05 2.11 3.48 5.85

Table 3. Experimental results of coke wastewater at different initial pH. current density and initial H2O2 in the ECP Process at 20 min. pH

pHfinal

Ws

ElC

36

EnC

OC

(kg/m3) (kg/m3) (kWh/m3) (€/m3) 2 2.5 3 3.5 4 6 Current Density (A/m2) 100 150 200 300 400 Initial H2O2 (g/L) None 5 10 15 20 30

2.7 3.6 4.4 4.5 4.9 8.0

1.20 1.30 1.47 1.50 1.66 1.45

0.98 0.95 0.84 0.81 0.75 0.74

1.93 2.05 2.60 2.91 3.23 3.30

5.32 5.30 5.26 5.27 5.25 5.25

4.0 4.4 4.6 4.8 5.3

0.89 1.47 1.89 3.45 4.23

0.55 0.84 1.08 1.56 2.01

1.31 2.60 4.41 8.51 17.64

4.89 5.26 5.64 6.44 7.70

6.4 5.4 4.6 4.2 4.2 3.8

1.78 2.14 1.89 2.10 2.34 2.24

1.05 1.04 1.08 1.09 1.06 1.04

5.46 4.62 4.41 4.49 5.67 5.88

1.42 3.48 5.64 7.66 10.05 13.85

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Research highlights ►ECP and EC processes for treatment of PCW were investigated. ►Effect of initial pH, Current density, initial H2O2 concentration and operation time were studied in detail. ► ECP was an effective process for removal of COD, TOC, phenol, CN- and SCN- from PCW.

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