Performance evaluation of an integrated photo-Fenton – Electrocoagulation process applied to pollutant removal from tannery effluent in batch system

Performance evaluation of an integrated photo-Fenton – Electrocoagulation process applied to pollutant removal from tannery effluent in batch system

Chemical Engineering Journal 197 (2012) 1–9 Contents lists available at SciVerse ScienceDirect Chemical Engineering Journal journal homepage: www.el...

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Chemical Engineering Journal 197 (2012) 1–9

Contents lists available at SciVerse ScienceDirect

Chemical Engineering Journal journal homepage:

Performance evaluation of an integrated photo-Fenton – Electrocoagulation process applied to pollutant removal from tannery effluent in batch system A.N. Módenes ⇑, F.R. Espinoza-Quiñones, F.H. Borba, D.R. Manenti Department of Chemical Engineering – Postgraduate Program, West Parana State University, Campus of Toledo, rua da Faculdade 645, Jd. La Salle, 85903-000 Toledo, PR, Brazil

h i g h l i g h t s " Integrated processes were applied to remove pollutants from tannery effluents. " Both photo-Fenton and electrocoagulation processes were optimized and integrated. " A sludge yield reduction was studied searching a minimum environmental impact. " A minimum H2O2 content was investigated to drive an efficient low-cost treatment. " High pollutant removals were attained with values below the allowed environmental limits.

a r t i c l e

i n f o

Article history: Received 1 November 2011 Received in revised form 3 May 2012 Accepted 5 May 2012 Available online 14 May 2012 Keywords: Integrated processes Photo-Fenton Electrocoagulation Tannery effluent

a b s t r a c t The treatment of tannery industrial effluent (TIE) by integrating the widely used photo-Fenton and electrocoagulation (EC) processes was investigated. The optimization of the photo-Fenton process by using solar irradiation was made performing a full 33 factorial experimental design. An irradiation time of 120 min for the photo-Fenton reaction was more favorable to pollutant removal in acidic medium using Fe2+ concentrations ranging from 0.4 to 0.5 g L1 and H2O2 concentrations ranging from 15 to 30 g L1. Nonetheless, a reduction in sludge production and a minimum residual content of hydrogen peroxide were attained within 540 min of irradiation time, with 0.4 g L1 Fe2+ and 15 g L1 H2O2 initial concentrations, with almost the same efficiency of COD, color, and turbidity removal obtained under the optimal experimental conditions. When the pretreated TIE samples were submitted to an EC process, the results of inorganic pollutants removal were better than with the conventional method (use of a combination of filtration, chemical coagulation, and sedimentation processes). In addition, the application of the integrated photo-Fenton and EC process for TIE treatment is cheaper than the conventional one. All the results showed that the integrated photo-Fenton and EC process could be applied as an efficient low-cost alternative treatment for the removal of organic and inorganic pollutants from tannery industrial effluent with a low environmental impact. Ó 2012 Elsevier B.V. All rights reserved.

1. Introduction The increasing standard of drinking water supply and the very severe environmental regulations regarding wastewater discharge have required innovative wastewater treatment technologies. Due to their high toxicity, tannery wastewaters containing a complex mixture of organic and inorganic pollutants are strictly regulated and must be treated before being discharged into water bodies. These substances are derived from the hides and skins themselves and from the addition of reagents during the leather tanning process [1,2]. Classical physicochemical and biological processes are often inadequate to completely remove pollutants from tannery wastewaters [3]. For this reason, more efficient methods such as ⇑ Corresponding author. Tel.: +55 45 3379 7092; fax: +55 45 3379 7002. E-mail address: [email protected] (A.N. Módenes). 1385-8947/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved.

electrochemical or advanced oxidation processes have been proposed. Advanced oxidation processes (AOPs) have the great advantage of degrading pollutants by introducing highly oxidizing species, such as hydroxyl radicals. Among the AOPs, the most widely used techniques are Fenton and photo-Fenton reactions [4,5]. A series of advantages and disadvantages of AOPs have been earlier discussed by Huang et al. [6], suggesting that methods like Fenton process are the most promising technologies for the treatment of wastewaters. The photo-Fenton process has been applied to the treatment of tannery [1,2], food processing [7], wood processing wastewaters [8], and landfill leachate [9], as well as pharmaceutical pollutants [10], organic compounds [11], and dyes [12–15]. Based on the electrocoagulation (EC) advantages, many studies on wastewater treatment have been reported in order to remove a wide range of organic and inorganic pollutants and to ensure good


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quality effluent before its discharge into aquatic environment [16]. The EC technology has been widely applied to the treatment of effluents from tannery [17,18], textile [19,20], and agro-food industries [21], as well as to heavy metal removal [22,23]. In addition, a combination of EC and electro-oxidation techniques with ionizing radiation was proposed as a reliable method for the treatment of highly colored and polluted industrial wastewater [24]. An integrated EC-ozone process has been also applied to improve the wastewater quality [25]. A study of an integrated tannery effluent treatment system in batch mode based on the application of both widely used photoFenton and electrocoagulation techniques was carried out. A full 33 factorial experimental design with a 3D response analysis was applied to the photo-Fenton process data. Effects of irradiation time, initial pH, and Fenton reagent concentration were analyzed. The production of sludge and the amount of residual H2O2 were also analyzed. After the tannery effluent treatment by the photoFenton process, the EC treatment was performed under the optimal EC experimental conditions. An operational cost analysis was performed in order to show the economic viability of treatment systems based on the integration of photo-Fenton and EC processes.

As reported by the Technological Institute of Meteorology of the Brazilian Parana State [27], a mean solar irradiation of approximately 1.13 MJ m2 h1 was measured at the nearest meteorological station from Toledo city. As reported in a previous work [18], the electrocoagulation (EC) experimental condition for TIE treatment using a pair of aluminum electrodes might be set at 68 mA cm2, with initial solution pH of 8.3 and interelectrode distance of 4 cm, for optimal performance. Based on this information, a laboratory-scale EC reactor consisting of a 1.5 L cylindrical glass container, a pair of aluminum plates (7.5 cm  12.12 cm  0.17 cm) with a 4.0 cm gap between them, and a 150 rpm magnetic agitation system was used. The aluminum electrodes were dipped into the beaker containing 800 mL of the TIE effluent. The aluminum plates were partially immersed (approx. 7.8 cm) in the effluent, resulting in an active electrode surface area of 58.8 cm2. The aluminum electrodes were operated in a mono-polar mode and were connected to terminals of direct current power supply (Instrutemp DC Power Supply, FA 1030) that provided the stabilized currents and voltages, ranging from 0 to 10 A, and from 0 to 30 V, respectively. 2.3. Photo-Fenton and electrocoagulation experiments

2. Materials and methods 2.1. Sampling and chemicals An amount of 50 L of raw tannery effluent was weekly collected from an equalization tank of a non-treated tannery industrial effluent (TIE), which is a by-product of the leather finishing process in a factory located in Toledo, Paraná (Brazil). A slight variability on their physico-chemical properties of the real tannery effluent samples was observed during collections from the equalization tank. Besides, these variations are not greater than 20% within two months of sample collection. In order to maintain a representativeness of the raw tannery effluent, all collected samples were mixed in an equalization tank, providing a homogenized non-treated TIE before experiments and stored according to the standard methodologies recommended by the American Public Health Association [26]. All chemicals used were of analytical-reagent grade. A 100-mL standard solution (Combicheck 20, Merck) containing a COD concentration of 750 mg L1, which is traceable to NIST, was used without dilution instead of the sample solution for checking the quality of the photometric measurement system, as well as to identify sample-dependent effects on the COD results. In addition, a 100-mL multielement atomic spectroscopy standard solution, prepared with high purity salts and nitric acid (70002, Fluka), was used in order to obtain the sensibility curves of the SR-TXRF (Synchrotron Radiation Total Reflection X-ray Fluorescence) spectrometer for K and L series X-rays and determine the element concentrations in TIE samples. For SR-TXRF analysis, a 2 mL aliquot of each TIE sample was taken and 20 lL of a standard solution (1.0 g Ga L1) was added as an internal standard. An aliquot of 5 lL was deposited on a pre-cleaned acrylic disk and dried at room temperature. The same procedure was repeated for the multi-elemental standards at five different concentrations. All the disk-samples were prepared in triplicate, except for the standard samples, which were prepared in quintuplicate. 2.2. Photo-Fenton and electrocoagulation reactors In order to perform the pollutant removal from TIE samples in batch system, photo-Fenton reactors consisting of magnetically stirred 500 mL vessels were exposed to the external environment for 5 h of solar irradiation on non-cloudy days, during the summer.

A detailed study was really performed on the integration of both EC and photo-Fenton processes in order to minimize the amount of final sludge yielded. From preliminary tests (data not shown), it was observed that applying in first place the photo-Fenton process a mineralization of organic matter is obtained but with a high residual amount of dissolved iron. Then, applying the EC process a good performance on the removal of suspended and dissolved matters was achieved, resulting in a significant lower amount of final sludge than that obtained in an inversion on the integration of EC and photo-Fenton processes, as first and second stages, respectively. Photo-Fenton experiments were carried out using different hydrogen peroxide and ferrous ion concentrations, and different initial pH values. The iron salt was mixed with the tannery effluent solution before the addition of hydrogen peroxide. The pH value of all samples was adjusted with H2SO4 and NaOH solutions. The reaction mixture inside the cell, consisting of 200 mL of TIE sample and a precise amount of Fenton reagent, was magnetically stirred. From previous photo-Fenton tests, 120 min of solar irradiation was enough to evaluate the influence of each reactor operation parameter (ROP) on pollutant removal. The three ROPs, namely concentrations of Fe2+ and H2O2, and initial pH, were labeled as q1, q2, and q3, respectively. For a statistical analysis and modeling of the optimization procedure, planned photo-Fenton experiments based on the Response Surface Methodology (RSM) were carried out [28,29]. Considering a combination of two-levels and one point at the center of each experimental region, a full 33 factorial experimental design (FED) was applied to optimize the photo-Fenton reactor conditions. From previous photo-Fenton tests, the ranges of 0.25– 0.5 g L1, 15–30 g L1, and 3–7 for Fe2+ and H2O2 concentrations, and initial pH values, respectively, were considered for FED analysis, under 300 rpm controlled stirring and 120 min of solar irradiation in batch mode. Besides, a most-frequently used second-order polynomial model, described by Eq. (1), was applied to model the photo-Fenton experimental data. A set of response variables (R) such as removal of COD and total suspended solids was used in order to optimize the performance of the photo-Fenton reactor. An optimized variable response in 3D graphical representation was obtained by applying the Lagrange criteria [30]. By application of ANOVA with a 95% confidence level (p < 0.05), all modeled variable responses were tested for their statistical significance.


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R ¼ a0 þ



SK ¼ 20:274  4:678 Z þ 0:3494 Z 2  0:0104 Z 3 þ 0:00011 Z 4

3 3 X 3 3 X 3 X 3 X X X ai qj þ bij qi qj þ wijk qi qj qk i¼1 j¼1

3 X 3 X


i¼1 j¼1 k¼1

C¼ 2 2 ij qi qj



ð2Þ ð3Þ

i¼1 j¼1

where R is the experimental response, q is the set of ROP values, a0 is the constant, a is the set of coefficients of the linear terms, and b, w, and v are the set of weight coefficients representing the various types of interactions between the ROP values. For the TIE treatment by the electrocoagulation process, 800 mL TIE samples, which had been previously treated by the photo-Fenton process, were placed into the EC reactor. Before each run, the oxide and/or passive layers on the aluminum electrode surface were mechanically removed underwater by using a fine grade abrasive paper, degreased in acetone, rinsed with distilled water and then dried. Because high conductivities were determined for all samples, addition of chemicals to increase the passage of current was avoided. The previously treated TIE sample was constantly agitated by using a non-conductive impeller. During the experiments the direction of the current was reversed every 30 min in order to limit the formation of passive layers. At the end of each run, the floating and precipitated materials were withdrawn and the clarified effluent sample was collected. In addition, the sample was allowed to settle for a few hours in a polyethylene flask. Finally, the clarified supernatant was collected and stored for subsequent analysis. 2.4. Analytical methods

3. Results and discussion 3.1. Experimental results The characterization of the tannery effluent is presented in Table 1. Based on the full 33 factorial experimental design, the photoFenton experimental data containing the COD and TSS removal values under solar irradiation are summarized in Table 2. Regarding the application of the second-order model to the photo-Fenton experimental data, COD and TSS removal percentages are represented by Eqs. (4) and (5), respectively, with 95% statistical significance for the linear and quadratic coefficients of ROPs. As confirmed by ANOVA results that are shown in Table 3, both COD and TSS removal models were validated by using the StatisticaÒ software.

RCOD ¼ 80 þ 2q1 þ 5  2q2  13q3  0:5q1  q2  0:1q1  q3 þ 0:6q2  q3  0:5q21  0:2q22 þ 1:2q23  0:3q1 q22 þ 0:5q21 q2 þ 0:4q1  q23  0:5q21 q3 þ 2q2 q23 þ 0:6q22  q3  0:3q21 q22 þ 0:2q21 q23 þ 0:3q22 q23


RTSS ¼ 21 þ 2q1 þ 2q2  4q3 þ 1q1  q2  2q1  q3  2q2  q3 þ 0:4q21 þ 1:4q22  0:8q23  0:4q1 q22 þ þ0:06q21 q2

According to the standard methodologies recommended by the American Public Health Association [26], all measurements of physicochemical parameters were made for each treated and untreated TIE sample. An open reflux/titrimetric method was used to determine the Chemical Oxygen Demand (COD). The pH value of each sample was determined by using a digital pH meter (Tecnal TEC-2). Turbidity (Nephelometric Turbidity Unit, NTU) was determined with a turbidimeter Tecnal, model TB1000. Conductivity was measured by using a conductivity meter (Tecnal R-TEC-04PMP). By using a DR 2010 HACH analyzer, at the 430 nm maximum visible absorbance wavelength, color was determined using the Platinum–Cobalt (Pt–Co) method. The amount of total suspended solids (TSS) containing fixed and volatile solids was determined based on water evaporation at 105 °C. The residual weight that was obtained after the ignition of dried TSS at 550 °C was reported as total fixed solids (TFS), while the weight loss was considered as the total amount of volatile solids (TVS) associated with the organic matter. On the other hand, the total amount of metals dissolved or suspended in the samples was determined by applying the synchrotron radiation total reflection X-ray fluorescence technique (SRTXRF). The SR-TXRF measurements were performed by using a 17.4-keV monochromatic X-ray beam, from the D09-XRF beam line at the Brazilian Light Synchrotron Laboratory. For X-ray detection, a Si(Li) detector with 160 eV FWHM@Mn-Ka line was used, allowing the identification of low-Z chemical elements with atomic numbers ranging from 19 to 40. All X-ray spectra were analyzed using the AXIL program [31]. A polynomial-type function was well fitted (r2 = 0.9921 and v2 = 0.0021) to the relative-to-Gallium sensitivity experimental data for low-Z chemical elements (see Eq. (2)). The elemental concentrations in liquid samples were determined by Eq. (3), where Z is the atomic number ranging from 19 to 40, I represents the fluorescent intensity of the element, C (mg L1) represents the concentration of the element in the liquid phase, and CGa stands for the internal standard concentration (10 mg Ga L1).

 0:3q1  q23  0:5q21 q3  0:6q2 q23  2q22  q3  0:01q21 q22 þ 0:01q21 q23  0:7q22 q23


The models for the removal of COD and TSS are linear functions of [Fe2+] (q1), [H2O2] (q2), and initial pH (q3), and quadratic functions of the interaction effects between these parameters, as represented by Eqs. (4) and (5). According to the positive linear coefficients for both Fe2+ and H2O2 concentrations, the optimal performance of the photo-Fenton process is expected to be enhanced at high concentrations of both Fenton chemical reagents. In addition, when operating the photo-Fenton reactor with lower initial pH values, higher COD and TSS removal are expected. Analyzing the individual effects of ROPs as well as their positive/negative quadratic coefficients and negative interaction coefficient values, an intermediate concentration range for Fe2+ and H2O2 is expected to be adequate to enhance pollutant removal, rather than higher Fenton reagent doses. By the Fe2+/H2O2 association and influence of solar irradiation on the photo-Fenton process, some chemical species are formed, such as hydroxyl radical (OH), which is a powerful oxidizing agent that attacks organic matter as shown in the following equations [32]. Table 1 Physicochemical characteristics and elemental concentrations of TIE. Parameters

Non-treated TIE

pH Conductivity (mS cm1) Turbidity (NTU) COD (mg L1) Color (PtCo) TSS (mg L1) TFS (mg L1) TVS (mg L1) Cr (mg L1) Ca (mg L1) K (mg L1) Fe (mg L1)

8.3 23 2405 ± 85 11,878 ± 132 16,840 ± 61 24,406 ± 284 9822 ± 390 14,587 ± 332 72.1 ± 0.5 3.74 ± 0.36 4.02 ± 0.12 1.68 ± 0.27


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Table 2 Planned photo-Fenton runs under 120 min of solar irradiation and 800 rpm stirring. The photo-Fenton parameters, namely q1–q3, represent the concentrations of Fe2+ and H2O2, and solution pH values, respectively. Run

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27

Fenton parameters

COD and TSS removal (%)

q1 (g L1)

q2 (g L1)








0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.375 0.375 0.375 0.375 0.375 0.375 0.375 0.375 0.375 0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.25

30 30 30 22.5 22.5 22.5 15 15 15 30 30 30 22.5 22.5 22.5 15 15 15 30 30 30 22.5 22.5 22.5 15 15 15

7 5 3 7 5 3 7 5 3 7 5 3 7 5 3 7 5 3 7 5 3 7 5 3 7 5 3

72.3 91.2 97.9 68.3 82.3 96.3 64.3 77.6 89.4 71.8 90.3 96.2 66.4 81.9 89.7 60.3 72.1 89.1 68.3 87.3 95.8 70.4 80.1 88.3 60.3 70.8 89.1

70.4 89.4 98.3 65.4 84.1 94.8 66.4 79.4 88.9 70.4 88.4 97.4 61.3 82.1 91.6 58.3 70.9 90.4 67.1 88.9 95.6 68.3 79.5 87.0 57.2 69.3 88.8

74.0 89.9 98.0 65.9 84.2 95.8 66.1 80.1 90.4 70.9 89.0 94.9 61.2 82.3 90.3 57.1 73.0 91.3 66.2 86.3 95.8 68.0 79.3 87.2 57.9 71.0 90.0

30.3 52.3 65.4 28.7 50.1 68.9 28.4 48 57.9 30.1 47.6 63.1 29.4 47 64.1 27.5 45.8 57.4 25.1 38.7 59.6 30.3 45.2 64.1 22.8 41.8 44.7

32.4 51.3 68 30 51.1 66.8 30.8 50.1 60.3 29.8 47.5 64.3 30.5 47.5 62.9 29.4 48.3 59.2 25.4 40.6 60.1 29.4 46.7 62.8 22.1 40.8 46.7

31.4 52.0 66.9 29.5 50.5 67.4 29.4 48.9 58.8 30.1 47.8 63.7 29.8 47.3 63.5 28.4 47.2 58.2 25.6 39.5 59.3 29.3 45.8 63.4 22.3 40.9 45.8

 3þ  Fe2þ ðaqÞ þ H2 O2 ! FeðaqÞ þ OH þ OH


2þ þ  Fe3þ ðaqÞ þ H2 O2 ! FeðaqÞ þ O2 H þ HðaqÞ


2þ þ  Fe3þ ðaqÞ þ O2 H ! FeðaqÞ þ O2 þ HðaqÞ


H2 O2 þ h v ! 2 OH


By representing the photo-Fenton response variables by threedimensional curves, with one of the three ROPs fixed at its optimal value, the optimal regions can be observed in Fig. 1. Regarding the minimum and maximum levels of Fe2+ concentration, response surfaces at the end of the photo-Fenton process have shown a broad range of Fe2+ concentration (0.375–0.5 g L1) where maximum COD and TSS removal is achieved (see Fig. 1a and d). Besides, it can be observed that there are small differences between the pollutant removal values (see Fig. 1b and e) for fixed pH 3 and a wide H2O2 concentration range (15–30 g L1). In addition, as can be seen in Fig. 1c and f for a fixed pH of 3 and irradiation time of 120 min, broad concentration ranges of Fe2+ (0.375–0.50 g L1) and H2O2 (25–30 g L1) might be considered in order to remove around 90% of the COD, whereas at least 50% TSS removal was achieved

for broad concentration ranges of Fe2+ (0.375–0.50 g L1) and H2O2 (15–30 g L1). Furthermore, the optimal experimental condition for the photo-Fenton process could be attained when the initial values of the ROPs were set as pH 3 and Fe2+ and H2O2 concentration of 0.5 and 30 g L1, respectively, according to the response variable optimization analysis. Under optimal experimental condition, the H2O2/Fe2+ and H2O2/UV associations have probably yielded higher amounts of hydroxyl radicals (OH), reacting effectively with a variety of organic compounds and removing up to almost 95%, as suggested by the highest TVS removal value. 3.2. Effects of photo-Fenton parameters Regarding the optimal condition for the photo-Fenton reactor to remove pollutants from TIE samples, further photo-Fenton experiments were performed in order to evaluate the effect of irradiation time on the TIE treatment efficiency, considering COD, color, turbidity, and TSS removal as response variables, among other operating parameter effects. All these results are shown in Fig. 2. As can be seen in Fig. 2a, almost 100% removal of COD, color, and turbidity were attained within 120 min of irradiation, while around 90%, 65%, and 30% removal of TVS, TSS, and TFS were achieved, respectively. Furthermore, a photo-Fenton process limited to 120 min of irradiation was adequate to degrade the organic matter and probably mineralize it into harmless species (CO2, H2O, etc.), as suggested by higher removal values of TVS (90%). However, the remaining 70% of TFS was a residual product that should be removed by another coupled process. By considering 120 min of irradiation, the effect of initial pH on the pollutant removal performance was also analyzed in terms of the defined response variables. When H2O2 and Fe2+ concentrations were fixed at their optimal values, the initial pH value significantly affected the degradation of TIE as suggested by the removal profile of COD, color, turbidity, TVS, TSS, and TFS, with the highest removal in a narrow pH range (2.5–3.0), while the pollutant removal performance followed the tendency towards lower efficiency values as solution pH increased, as shown in Fig. 2b. As in Fenton experiments the pollutant degradation is mainly governed by the generation of hydroxyl radicals (OH) that are formed from the Fe2+/H2O2 association according to Eq. (6), the effects of Fe2+ and H2O2 concentrations on the TIE degradation efficiency were also monitored by a set of response variables (COD, color, turbidity, TSS, TFS, and TVS). In this way, a series of further photo-Fenton experiments was performed by varying Fe2+ concentration from 0 to 1.0 g L1 with a fixed H2O2 concentration of 30 g L1 as well as varying the concentration of H2O2 from 0 to 80 g L1 with a fixed Fe2+ concentration of 0.5 g L1. In both cases, all experiments were conducted at pH 3 with 120 min of irradiation. It can be noticed (see Fig. 2c) that low concentrations of Fe2+ (below 0.2 g L1) were added to the TIE solution, the degradation efficiency was less than 80% for the removal of COD, color, and turbidity, whereas it was less than 65%, 40%, and 20% for the removal of TVS, TSS, and TFS, respectively. The TIE degradation efficiency progressively increased as Fe2+ concentration increased,

Table 3 Two-way ANOVA test of the predicted model for COD and TSS removal values from the TIE treatment by the photo-Fenton process. Parameter




Regression Residues Total Regression Residues Total

Sum of squares

11755.92 259.91 12015.83 15795.54 103.09 15898.63

Degrees of freedom

7 73 80 15 65 80

Mean square


Level of significance (%)



1679.42 3.56




1053.04 1.59




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Fig. 1. Experimental response surface results obtained by applying a 33 factorial design for COD (a–c) and TSS (d–f) removal, when one of the ROP’s is maintained fixed at its optimal value for the photo-Fenton experiments.

due to the expected formation of higher amounts of hydroxyl radicals (OH), according to the classical Fenton’s reaction (Eq. (6)), that directs oxidation processes to the mineralization of pollutants or, at least, to their transformation into harmless products. Above Fe2+ concentrations of 0.4 g L1, there was no significant increase in TIE degradation efficiency, which remained constant with almost 100% removal of COD, color, turbidity, and TVS. On the other hand, it was observed that adding low H2O2 amounts (0–5 g L1) led to an increase in TIE degradation efficiency, removing progressively up to 55% of COD, color, and turbidity, whereas low removal values were achieved for TVS (30%), TSS (20%), and TFS (10%). However, almost 95% removal of response variables such as COD and color was attained for Fe2+ concentration above 20 g L1 (see Fig. 2d). Besides, the profile of TVS removal was slightly different from that of TFS removal. This difference could be attributed to

the chemical attack of the produced sludge by an excess of hydrogen peroxide, consequently releasing more inorganic matter into the TIE samples. 3.3. Sludge production and residual H2O2 analyses In spite of the results obtained from the experimental design and the effects of Fenton parameters have been optimal for the organic pollutant removal, a series of further photo-Fenton experiments was performed, to analyze sludge production and residual hydrogen peroxide as a function of irradiation time, considering three initial H2O2 concentrations (30, 20, and 15 g L1) with fixed Fe2+ concentration of 0.4 g L1 and pH 3. In Fig. 3a, it can be noticed that sludge production depends on irradiation time. Beyond the enough time (120 min) to obtain the maximum pollutant removal


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Fig. 2. Removal profiles for COD, color, turbidity, and TSS as a function of irradiation time (a), initial pH (b), H2O2 concentration (c), and Fe2+ concentration (d), obtained from photo-Fenton experiments at optimal reactor operating parameters.

as pointed out by the removal values of COD, turbidity, and color (see Fig. 2), there is a persistent process of mineralization of the sludge due to the presence of the residual amount of H2O2 in longer irradiation times. Similar profile was observed in the reduction of sludge production when different initial H2O2 concentrations were tested. As seen in Fig. 3b, after 540 min of irradiation, only 75% of the H2O2 was consumed when a high initial H2O2 concentration (30 g L1) was used. A lower residual H2O2 concentration was attained when low initial H2O2 concentration was used in the photo-Fenton process. Regarding the minimum amount of sludge produced as well as the minimum residual H2O2 concentration, it can be suggested that the best reactor parameters might be 540 min irradiation time, pH 3, Fe2+ concentration of 0.4 g L1 and H2O2 concentration of 15 g L1. Under this experimental condition, the values of all physicochemical parameters (COD, color, turbidity, and TSS) were similar to those obtained for the reactor operating at Fe2+ and H2O2 concentrations of 0.5 g L1 and 30 g L1, respectively (see Fig. 2). Analyzing the chromium content in the treated tannery effluent and sludge samples, it was observed (see Fig. 4) that chromium was continuously released from the sludge to the aqueous medium, suggesting that mineralization of the sludge was occurring due to the action of the residual hydrogen peroxide in the medium. After 540 min of irradiation, almost all chromium was in the liquid phase, with a very small amount remaining in the sludge. As a high load of inorganic matter is available in the treated TIE samples, another treatment based on the EC process was applied in order to reduce the chromium concentration to within the limits recommended by environmental requirements for discharge of wastewater into water bodies. 3.4. Integrated photo-Fenton and EC processes Performing the integration of both photo-Fenton and EC processes, a 800 mL TIE sample, which was previously treated by the

Fig. 3. Sludge and residual hydrogen peroxide profiles over time.

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Fig. 4. Chromium concentration profiles in treated tannery effluent and generated sludge samples after photo-Fenton process at pH 3, [H2O2] = 30 g L1, and [Fe2+] = 0.5 g L1.

Table 4 Final concentrations of Cr, Fe, and K, and physico-chemical parameters after EC treatment of TIE previously treated by a photo-Fenton process ([Fe2+] = 0.4 g L1, [H2O2] = 15 g L1, initial pH of 3 and irradiation time of 540 min). A 5–60 min reaction time range was considered, under optimal EC condition (68 mA cm2, initial pH of 8.3 and 4 cm of interelectrode distance). Parameters

Nontreated TIE

After photoFenton treatment

COD (mg L1) Color (PtCo) Turbidity (NTU) TSS (mg L1) TFS (mg L1) TVS (mg L1) Initial pH Cr (mg L1) Fe (mg L1) K (mg L1)

11,878 16,840 2405 24,406 9822 14,587 8.3 72.1 1.68 3.23

522 690 77 8664 7042 1619 3.0 70.6 104.05 7.02

After EC treatment time (min)






214 1010 164 8231 6832 1399 8.3 0.8 4.91 3.06

107 791 162 7723 6509 1214 8.3 0.24 3.16 2.33

119 909 156 7160 6080 1080 8.3 0.23 2.99 2.57

214 842 106 6549 5540 1009 8.5 0.15 1.52 1.37

154 842 137 6432 5332 1100 8.7 0.11 1.49 1.25

photo-Fenton process (540 min irradiation time, pH 3, Fe2+ = 0.4 g L1, and H2O2 = 15 g L1), was placed into the EC reactor, operating under the following optimal condition: 68 mA cm2, initial pH of 8.3, and 4 cm of interelectrode distance for a specific reactor cell geometry (parallel plates) in a lab-scale tannery effluent treatment system. Nonetheless, in the case of increasing the electrodes separation in another reactor cell size with the same geometry, the current density value is expected to be mainly controlled by the applied voltage, conductivity value of the aqueous medium and electrodes separation, due to such parameters might be theoretically correlated by solving an approximation of the Laplace equation and applying another equation that resembles the ´ s Law for a parallel plates geometry [33]. Ohm The results obtained for the physicochemical parameters and for Cr, Fe, and K contents for a 5–60 min EC reaction time range are summarized in Table 4. Small variations on the final pH were observed after the EC treatment, showing a constant value of 8.3 up to 30 min. reaction time and then increasing slightly up to 8.7 at 60 min. reaction time, due to the generation of hydrogen gas and OH- radical. The same pH value of 8.7 was observed for reaction times above 60 min. After 15 min of EC reaction, strong reduc-

Fig. 5. Comparison of physicochemical parameters and chromium removal by the integrated photo-Fenton and EC method and the conventional method.

tions on Cr and Fe concentrations were attained, reaching 0.24 and 3.16 mg L1, respectively, which are below the limits recommended by the Brazilian environmental norm [34] for discharge of wastewater into freshwater bodies. After the photo-Fenton process, the production of sludge from the EC process was 0.04 g L1. In addition, there was around 80% COD removal, while a slight reduction of the physicochemical parameter values was achieved. Furthermore, the integration of both photo-Fenton and Electrocoagulation processes has shown a high efficient removal of organic and inorganic pollutants, as well as a low production of sludge. The best results for the TIE treatment were thus achieved using the photo-Fenton process with 540 min of irradiation time, pH 3, Fe2+ = 0.4 g L1, and H2O2 = 15 g L1, and the EC process with 15 min of reaction, 68 mA cm2, initial pH of 8.3 and 4 cm of interelectrode distance. In comparison with the conventional method, a slightly better result was obtained for the reduction of chromium concentration, as shown in Fig. 5. For turbidity, TSS, TFS, and TVS,


A.N. Módenes et al. / Chemical Engineering Journal 197 (2012) 1–9

Table 5 Total costs of the tannery effluent treatment by the integrated photo-Fenton/EC method and by the conventional method, considering both the operational and sludge disposal costs in each treatment method. Treatment method

Sludge produced (kg m3)

Sludge disposal price (US$ kg1)

Sludge disposal cost (US$ m3)

TIE treatment cost (US$ m3)

Total cost (US$ m3)

Photo-Fenton EC Photo-Fenton and EC Conventional

0.40 0.03 0.43 2.4

24.24 24.24 24.24 24.24

9.70 0.73 10.43 58.56

54.43 1.36 55.79 10.50

64.13 2.09 66.22 69.06

the photo-Fenton & EC process was also a little bit better than the conventional treatment. A significant improvement in COD removal was obtained with the photo-Fenton & EC treatment when compared with the conventional method, with an increasing of 87% in the organic matter removal. 3.5. Cost analysis In an economic analysis for an industrial plant, total operation cost is composed by direct cost items such as electric energy and material (electrodes and chemical reagents) consumption, sludge transportation and disposal costs, as well as by indirect cost items such as labor, maintenance and depreciation of the major equipments, being the whole of it calculated by using the economic data relative to the goods and service prices in each country. Based on the lab-scale optima operating data of an integrated photo-Fenton-Electrocoagulation process, a cost analysis has been carried out considering only the direct cost items. The operational cost of the photo-Fenton reactor in US$ per m3 of treated effluent was estimated considering the amount of Fenton reagents (0.4 g L1 of Fe2+ and 15 g L1 of H2O2) and their commercial prices (US$ 0.44 per kg of ferrous sulfate heptahydrated and US$ 1.19 per liter of 30% v/v H2O2), resulting in a Fenton cost of US$ 54.43 per m3 of treated TIE within a 10% of standard deviation. Regarding the commercial price of NaOH (US$ 15 0 per kg) and its amount (4  104 kg of NaOH per m3) used to adjust the pH value at 8.3, this reagent cost (US$ 0.006 per m3) was negligible as compared to the Fenton cost. In addition, the operational cost of the EC reactor (OCEC) in US$ per m3 of treated effluent was also calculated, taking into account the electrical energy and electrode material consumption quantified by Faraday’s law as summarized in Eq. (10), resulting in an OCEC of US$ 1.36 per m3 of TIE. Furthermore, the total cost of the integrated process was of US$ 55.79 per m3 of TIE (see Table 5) within a 10–15% range of standard deviation.


    jVAe t op jA t op M EMP EEP þ eF e nF V eff V eff


where Veff is the volume of the treated wastewater (L), F is the Faraday’s constant (96,487 C mol1), eF is the apparent Faradaic efficiency (1.85), j is the current density (71.1 A m2), V is the applied voltage (10 V), Ae is the effective area (56.25 m2), top is the treatment time (s), EEP is the electrical energy price (US$ 0.14 per kWh), and EMP is the electrode material price (US$ 6.0 per kg of aluminum). For a complementary analysis regarding the direct cost items, it is worth to compare the operational cost of the conventional method by using a combination of filtration, chemical coagulation, and sedimentation processes and the proposed integrated method, in regard with some important aspects such as sludge transportation and disposal costs. As reported by the Brazilian tannery industries, the mean chemical reagent cost is estimated to be US$ 10.50 per m3, varying within a 10–20% range. In spite of showing lower chemical reagent cost (see Table 5), the conventional method produced a larger amount

of sludge than the integrated photo-Fenton and EC process, indicating that the conventional method needs to discard huge amounts of sludge in landfills and thus becomes more expensive (US$ 58.56 per m3 of sludge) in this regard. Considering the sludge disposal cost, a total cost of US$ 69.06 per m3 of treated TIE would be expected for the conventional method, while the total cost for the integrated photo-Fenton and EC method would be of US$ 66.22 per m3 of treated TIE obtained. Compared with the conventional method, the integrated photoFenton and EC process brings a series of benefits, considering its greater performance on the removal of organic and inorganic pollutants from the TIE, its huge reduction in sludge production after the treatment (82%), and its reduction in the total cost (4%). Based on these advantages, it can be pointed out that the integrated photo-Fenton and EC method could be applied to the treatment of tannery effluents with optimal results from the point of view of preservation of the environment and cost.

4. Conclusion The integration of the widely used photo-Fenton and Electrocoagulation processes was evaluated as a tannery effluent treatment system. From the application of a full 33 factorial experimental design, the optimal performance of the photo-Fenton process was obtained using 120 min of solar irradiation in acidic media but within broad ranges of Fe2+ (0.375–0.50 g L1) and H2O2 (20–30 g L1) concentrations, with the best response achieved at initial pH of 3 and Fe2+ and H2O2 concentrations of 0.5 and 30 g L1, respectively. Nonetheless, the pollutant removal by the photo-Fenton process was strongly improved when the photo-Fenton reactor was operated above 120 min of irradiation time, with 0.4–0.5 g L1 of Fe2+ and 15–30 g L1 of H2O2. Within the 15–30 g L1 H2O2 concentration range, sludge production decreased with increasing irradiation time. Minimum amount of sludge was observed within 540 min of irradiation time, with minimum residual hydrogen peroxide and most of the chromium available in the aqueous medium, when the photo Fenton reactor was operated with initial concentrations of 15 g L1 H2O2 and 0.4 g L1 Fe2+, maintaining the same efficiency of COD, color, and turbidity removal. When the pretreated TIE samples were submitted to an electrocoagulation process under the following condition: 15 min reaction time, 68 mA cm2, 8.3 initial pH and 4 cm interelectrode distance, the best results regarding the removal of pollutants were attained for the final TIE treatment. In comparison with the conventional method for TIE treatment, an appreciable improvement in the results of COD removal and smaller amounts of sludge were obtained for the integrated photo-Fenton and EC treatment, while slightly better results were obtained for chromium removal, as well as for the removal of turbidity, TSS, TFS, and TVS. In addition, the application of the integrated photo-Fenton and EC process for the TIE treatment is cheaper than the conventional one. Based on all environmental and economical advantages, the integrated photo-Fenton and EC process could be applied as an efficient alternative to treat tannery industrial effluent.

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Acknowledgments Authors thank to CNPq, CAPES and Araucaria Foundation for the financial support and the Brazilian Light Synchrotron Laboratory (LNLS) for the partial financing (Project # XRF-8133) of this study.

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