ARTICLE pubs.acs.org/IECR
Photo-Fenton Treatment of Actual Agro-Industrial Wastewaters Bedoui Ahmed,† Elalaoui Limem,‡ Ahmed Abdel-Wahab,§ and Bensalah Nasr*,†,§ †
Faculty of Sciences of Gabes, University of Gabes, Erriadh City, 6072 Gabes, Tunisia University of Gafsa, Ennour Ciy, 2100 Gafsa, Tunisia § Texas A&M University at Qatar, Education City, P.O. Box 23874, Doha, Qatar ‡
ABSTRACT: In this work, the treatment of actual agro-industrial wastewaters by photo-Fenton process was investigated. The actual agro-industrial wastewaters (AIW) were received from physicochemical treatment plants of wastewaters coming from the olive oil milling industry. These brown colored aqueous wastes have been characterized by high organic content with chemical oxygen demand (COD) in the range 20007000 mg O2 L1, which makes difficult the degradation of these wastewaters by traditional biological methods. The photo-Fenton process was successfully used to totally decolorize these effluents and to satisfactory remove aromaticity and COD contained in these wastes. The influence of some experimental parameters such as H2O2 and Fe2þ doses, initial COD content, initial pH, and temperature on color, aromaticity, and COD removals has been studied to find out the optimum conditions leading to maximum efficiency of the photo-Fenton process. The best results of photo-Fenton process treatment of AIW (2000 mg O2 L1) have been obtained using 3 g L1 H2O2 and 30 mg L1 Fe2þ at pH 3 and T = 26 °C and after 3 h UV irradiation. The comparison among photo-Fenton, Fenton, and UV/H2O2 processes has shown that the photo-Fenton process is more efficient and cost-effective than the two other processes. During the photo-Fenton process, photodecomposition and catalytic by Fe2þ ions decomposition of H2O2 leads to the production of higher amounts of hydroxyl radicals, proving that the efficiency of an advanced process is generally related to the amount of hydroxyl radicals produced during the treatment. The degradation of organics contained in the actual agro-industrial wastes during photo-Fenton treatment involves many successive oxidation/reduction reactions, including a rapid release of chromophores leading to the total decolorization, then an oxidative opening of benzene rings into aliphatic intermediates without carbon dioxide formation, and finally a slower oxidation of the aliphatic intermediates by hydroxyl radicals to achieve high COD removal.
1. INTRODUCTION Recently, numerous researches have been focused on the study of new technologies for the treatment of industrial wastewaters polluted with organic compounds. The treatment of these actual wastewaters by conventional technologies is complicated.13 The complex mixture of compounds usually disgruntles the recuperation of the organics,4 the low calorific power of the wastes disappoints the use of incineration,5 and the nonbiodegradability fails the biological treatment.6 Under these situations the use of advanced oxidation processes (AOPs) appears as a very promising solution to solve the environmental problem generated by the discharge of these effluents. Advanced oxidation processes are defined as oxidation processes producing free radical oxidants, mainly hydroxyl radicals (HO•). HO• radicals for water and wastewaters treatment are generated in situ by processes including H2O2/UV, O3/UV, Fenton’s reagent, O3/H2O2/UV, TiO2/UV, electron beam irradiation, electrochemical oxidation, and others.7,8 The free hydroxyl radicals react in solution with most organic and inorganic compounds at rates that are close to being diffusion controlled.9 These radicals are very powerful oxidants (E° = 2.80 V vs SHE), which leads to a very effective oxidation process. The Fenton process is a simple and easy-to-manipulate advanced oxidation technology in which a mixture of hydrogen peroxide and iron(II) salts is added directly to the wastewater. The first step involved in the Fenton oxidation can be shown by the following reaction:10,11 Fe2þ þ H2 O2 f Fe3þ þ HO• þ OH r 2011 American Chemical Society
ð1Þ
Reaction 1 is propagated from Fe2þ regeneration that takes place mainly by reduction of Fe3þ. Besides the oxidation carried out by hydroxyl radicals generated by catalytic decomposition of hydrogen peroxide, iron(III) ions generated during the oxidation stage promote the removal of other pollutants by coagulation and sedimentation.1214 However, large amounts of oxidation-refractory compounds are usually obtained at the end of the Fenton process. This behavior is explained in terms of limited amounts of hydroxyl radicals produced during the Fenton process because of the slow regeneration of Fe2þ ions, which play an important role in the Fenton oxidation mechanism by enhancing the continuous catalytic decomposition of H2O2.15,16 The slow regeneration of iron(II) is due to the formation of stable hydroxoiron(III) complexes such as [Fe(OOH)]2þ and [Fe(OH)]2þ by the reaction of iron(III) ions with H2O2 and OH as shown by the following reactions:17 Fe3þ þ H2 O2 a ½FeOOH2þ þ Hþ
ð2Þ
Fe3þ þ OH a ½FeOH2þ
ð3Þ
Furthermore, iron(III) ions can also form stable complexes with aliphatic carboxylic acids18,19 generated during the oxidation of the Received: February 6, 2011 Revised: April 9, 2011 Accepted: April 20, 2011 Published: April 20, 2011 6673
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initial organics given by reaction 4: Fe3þ þ RCO2 a ½FeRCO2 2þ
ð4Þ
The rate of regeneration of iron(II) could be increased when a UV irradiation source is applied to the system.1820 The advantages of UV irradiation are related to the photoreduction of iron(III) to iron(II) ions, a step that produces greater amounts of HO• radicals, accelerates the decarboxylation of [FeRCO2]2þ complexes to rapidly regenerates iron(II) ions that can further react with H2O2 molecules.21,22 The regeneration of iron(II) by UV irradiation of iron(III) complexes can be represented by reactions 57: ½FeOOH2þ þ hυ f Fe2þ þ O2 H•
ð5Þ
½FeOH2þ þ hυ f Fe2þ þ OH•
ð6Þ
½FeðOOCRÞ2þ þ hυ f Fe2þ þ R þ CO2
ð7Þ
Furthermore, it has been established that UV irradiation of Fenton’s reagent, also called the photo-Fenton reaction, enhances the reaction rate of hydroxyl radicals’ production through photodecomposition and catalytic with iron(II) decomposition of H2O2 and the photoreduction of [Fe(OH)]2þcomplexes.23,24 In recent years, although increasing interest has been given to photochemical processes for water treatment, only a few papers have investigated the treatment of actual industrial wastewaters by the photo-Fenton process. These studies have proved that the photo-Fenton process can achieve almost complete removal of organic pollutants from water. Likewise, low chemical amounts and low energy consumption were required.2327 The goal of this work was to study the treatment of actual agroindustrial wastewaters by the photo-Fenton process to explore possible applications of photochemical treatments in industrial wastewaters treatment plants. The actual agro-industrial wastewaters (AIW) were received from physicochemical treatment plants of wastewaters coming from olive oil milling industry. The effects on color, aromaticity, and COD removals of H2O2 and Fe2þ doses, organic content, pH, and temperature were evaluated during the treatment of AIW by photo-Fenton system. The efficiency and costs of photo-Fenton process were compared to two other AOPs, i.e., the Fenton process and UV/H2O2 system to validate the results obtained with UV/H2O2/Fe2þ in treating industrial wastewaters.
2. MATERIALS AND METHODS 2.1. AIW Effluents. AIW effluents were received from physicochemical treatment plants of wastewaters coming from olive oil milling industry involving modern three phases’ extraction. Olive oil mill wastewaters were pretreated by sedimentation and sand filtration to eliminate suspended matter and a part of soluble organics by adsorption. AIW effluents are brown colored, slightly acidic, malodorous, and turbid aqueous wastes. These effluents are characterized by high organic content with a total organic carbon (TOC) in the range 180300 mg C L1 and a chemical oxygen demand (COD) in the range 20007000 mg O2 L1. The organic content is composed of a large variety of aromatic and aliphatic compounds such as polyphenols and long-chain carboxylic acids. All samples were collected in amber glass bottles and kept at 4 °C until use.
2.2. Chemicals. Hydrogen peroxide was a 30% (w/w) solution (AR grade, Fluka chemical). The other chemicals such as FeSO4 3 7H2O, K2Cr2O7, and Na2SO3 are of analytical grade and purchased from Sigma-Aldrich or Acros. Solutions were prepared with deionized water obtained from a Milli-Q system, with resistivity >18 MΩ cm at 25 °C. The initial solution pH was adjusted to the appropriate value with analytical grade sulfuric acid or sodium hydroxide both purchased from Acros. 2.3. Photo-Fenton Experiments. Photochemical experiments were performed in a batch thermostated Pyrex photoreactor of 2 L capacity equipped with a 40 W Heraeus Noblelight YNN 15/32 low pressure mercury vapor lamp (Hanau, Germany) located in a quartz sleeve at the center of the reactor in an axial position and emitting at 254 nm. Because the light source produces heat, the lamp was surrounded with a water coaling jacket, made of glass, to maintain a constant temperature. All experiments were conducted using 1 L of AIW. The temperature of AIW was kept constant during the experiments using a thermo-regulated water bath. The pH of the solution was adjusted to the desired value by addition of sodium hydroxide or sulfuric acid. After pH adjustment, a given weight of iron(II) salt (FeSO4 3 7H2O) was added. The iron(II) salt was mixed very well with the AIW. After the light of the photoreactor is turned on, a precise amount of hydrogen peroxide (30%) was added to the reactor. A specific H2O2 quantity was adjusted at the beginning of the treatment and no further H2O2 was added during the irradiation of AIW. All the experiments lasted around 6 h and periodically (10 min) samples of 10 mL were collected and quenched with Na2SO3 and then analyzed immediately to determine UVvisible absorbance at two wavelengths (λ = 280 nm and λ = 400 nm), COD, and TOC in some samples. 2.4. Analytical Methods. The carbon concentration was monitored using a Shimadzu TOC-5050 analyzer. Chemical oxygen demand (COD) was determined using a HACH DR200 analyzer and measured according to colorimetric methods. Absorbance measurements were monitored by a HACH DR2500 UVvisible spectrophotometer using a quartz cuve and 1 cm optic path length at two wavelengths 280 and 400 nm. The pH was measured by Micronal pH-meter (model B474). Hydrogen peroxide was measured according to Eisenberg method.28
3. RESULTS AND DISCUSSION The UVvisible spectrum of AIW presents a band at 400 nm corresponding to the brown color of the effluent and another band at 280 nm which can be attributed to π f π* transition in aromatic compounds. Changes in UV400nm and UV280nm will be used to measure color and aromaticity removals, respectively. Series of experiments were conducted to examine the influence of some experimental parameters on the kinetics and efficiency of the treatment of AIW by photo-Fenton process. H2O2 and Fe2þ doses, initial COD content, temperature, and initial pH are considered as the main experimental parameters to optimize to obtain highest color, aromaticity, and COD removals. 3.1. Influence of H2O2 Dose on the Treatment of AIW by Photo-Fenton Process. The efficiency of photo-Fenton process
in treating wastewaters containing organic pollutants rises with the increase of the amount of hydroxyl radicals (HO•) generated from the photocatalyzed decomposition of H2O2.29 Hydrogen peroxide is then the major source of hydroxyl radicals; even little amounts of HO• can be formed by water photolysis and photoreduction of iron(III) complexes. Consequently, the H2O2 dose 6674
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Figure 2. COD removal vs H2O2 dose for the treatment of AIW by the photo-Fenton process. Experimental conditions: COD0 = 2000 mg O2 L1, 30 mg L1 Fe2þ, pH 3, T = 26 °C, UV illumination time = 5 h.
Figure 1. Influence of H2O2 dose on the changes with time of UV400nm removal (a) and UV280nm removal (b) during the treatment of AIW by the photo-Fenton process. Experimental conditions: COD0 = 2000 mg O2 L1, 30 mg L1 Fe2þ, pH 3, T = 26 °C.
can be considered as the main factor that limits the production of hydroxyl radicals during wastewaters treatment by UV/H2O2/ Fe2þ. Figure 1 presents the influence of H2O2 dose on the changes with time of UV400nm and UV280nm removals during photo-Fenton treatment of AIW containing 2000 mg O2 L1 using 30 mg L1 Fe2þ at pH = 3 and T = 26 °C. Also, Figure 2 shows the changes of COD removal vs H2O2 dose at same experimental conditions and after 5 h UV irradiation. The results of Figures 1 and 2 showed that kinetics and efficiency of photo-Fenton process depend largely on H2O2 dose. The increase of H2O2 dose up to 3 g L1 enhances both kinetics and efficiency of photo-Fenton process in terms of UV280nm and COD removals, but no important influence on UV400nm removal was observed with all H2O2 doses used. When the H2O2 dose increases from 0.5 to 3 g L1, UV280nm and COD removals increase from 53 and 38% to 85% and 82%, respectively. However, the increase of H2O2 dose higher than 3 g L1 does not lead to significant changes in UV280nm and COD removals. Accordingly, H2O2 dose of 3 g L1 is optimal to reach highest efficiency of photo-Fenton process in terms of color, aromaticity, and COD removals. At low H2O2 doses the amount of hydroxyl radicals generated by decomposition of hydrogen peroxide is able to remove color from AIW but cannot satisfactory remove COD contained in AIW. Increasing the H2O2 dose up to 3 g L1 leads to generating satisfactory amounts of HO• radicals and as a result the aromaticity and COD removals were highly improved. For H2O2 doses above 3 g L1, although theoretically higher amounts of HO• radicals are supposed to be produced, no significant increase
in aromaticity and COD removals was observed. This can be explained by the limited production of hydroxyl radicals because the lamp power and Fe2þ dose were maintained unchanged in all experimental sets. Also, the competition between the reaction of hydroxyl radicals with pollutants and with H2O2 (eqs 8 and 9) does not provide an increase in the quantity of HO• radicals. Furthermore HO2• radicals formed during the reaction between H2O2 and OH• are less powerful oxidants than HO• that does not affect rates of aromaticity and COD removals. Additionally, the autodecomposition of H2O2 into O2 and H2O (eq 10) consumes non-negligible amount of H2O2 and therefore the quantity of HO• radicals does not augment. HO• þ R / ROH
ð8Þ
OH• þ H2 O2 / OH2 • þ H2 O
ð9Þ
2H2 O2 / O2 þ 2H2 O
ð10Þ
3.2. Influence of Fe2þ Dose on the Treatment of AIW by Photo-Fenton Process. In the second set of experiments the
Fe2þ dose was varied in the range 060 mg L1 during photoFenton treatment of AIW containing 2000 mg O2 L1 using a fixed dose of H2O2 of 3 g L1 at pH 3 and T = 26 °C. The changes of UV400nm and UV280nm removals with time at different Fe2þ doses are presented in Figure 3. These results validated a higher efficiency for photo-Fenton process in comparison to the UV/ H2O2 process by adding ferrous iron to irradiated AIW. The results indicated also that the Fe2þ dose has an important influence on the rates and the percentages of UV280nm and UV400nm abatement. The increase of Fe2þ dose from 0 to 30 mg L1 leads to the augmentation of color and aromaticity removals, but increasing the iron(II) dose more than 30 mg L1 does not yield higher UV280nm and UV400nm removals. The addition of large amounts of iron(II) ions can function as direct scavengers of hydroxyl radicals by increasing the rate of formation of the H2O• radicals.30 Also when high amounts of Fe2þ are added to the wastewater, Fe(OH)3 can be precipitated, which makes difficult the regeneration of Fe2þ catalyst. Thus, the use of catalytic amounts of Fe2þ is highly recommended to avoid the problem of sludge formation and iron contamination, which 6675
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Figure 3. Influence of Fe2þ dose on the changes with time of UV400nm removal (a) and UV280nm removal (b) during the treatment of AIW by the photo-Fenton process. Experimental conditions: COD0 = 2000 mg O2 L1, 3 g L1 H2O2, pH 3, T = 26 °C.
Table 1. Influence of pH on the Results of Photo-Fenton Treatment of AIWa temperature
color removal
pH
(°C)
(%)
aromaticity removal COD removal (%)
(%)
2
26
83
50
26
3 3
26 20
86 84
55 48
39 23
3
30
89
60
48
3
35
94
66
57
4
26
76
45
3
5
26
56
36
0
Experimental conditions: COD0 = 2000 mg O2 L1, 3 g L1 H2O2, 30 mg L1 Fe2þ. UV irradiation time: 60 min. a
requires further treatment stages and then increases costs of photo-Fenton process. Furthermore, the iron level in water was regulated by international organizations to a maximum contaminated level of 30 mg L1. Therefore, a dose of 30 mg L1 Fe2þ is kept as optimal dose for AIW treatment by photo-Fenton process. 3.3. Influence of pH and Temperature on the Treatment of AIW by Photo-Fenton Process. According to literature, it was frequently reported that the catalytic decomposition of H2O2 with ferrous iron (Fenton reaction) can lead to very high efficiency in
Figure 4. Influence of COD0 content on the changes with time of UV400nm removal (a) and UV280nm removal (b) during the treatment of AIW by the photo-Fenton process. Experimental conditions: [H2O2] (mg L1)/TOC0 (mg C L1) = 1.5, [H2O2] (mg L1)/[Fe2þ] (mg C L1) = 100, pH 3, T = 26 °C.
acid conditions.3032 However, some studies reported that pH should be in the range 35 to reach maximum organic removals.31 It was also stated that in high acid conditions (pH < 3) protons can function as scavengers for hydroxyl radicals;33 however, at low acid medium (pH > 5), the regeneration of Fe2þ catalyst becomes difficult because of Fe(OH)3 precipitation. In addition, temperature is also a key variable that needs to be optimized to minimizing costs of wastewater treatment by the photo-Fenton process because it is well-known that the kinetics of homogeneous reactions are highly affected by temperature. However, little research was focused on the influence of pH and temperature on the efficiency of the photo-Fenton process.3436 To examine the influence of pH and temperature on the efficiency of the treatment of AIW contaminated with 2000 mg O2 L1 by the photo-Fenton process, some experiments were conducted at different pH and temperatures. In these experiments the initial pH was changed in the range 25 and the temperature was varied between 20 and 40 °C (because in this range no risks for either UV lamp or the photoreactor can be caused by temperature) and keeping H2O2 and Fe2þ doses fixed to 3 and 30 mg L1, respectively. The results of photo-Fenton treatment at different initial pH and temperatures are illustrated in Table 1. The results obtained confirmed that highest color, aromaticity, and COD removals were achieved at pH 3. Increasing pH from 3 to 4 and then to 5 markedly decreases the efficiency of photo-Fenton in terms of color, aromaticity, and COD removals. The lowest 6676
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Figure 5. Influence of COD0 content on the changes of COD removal with time during the treatment of AIW by the photo-Fenton process. Experimental conditions: [H2O2] (mg L1)/TOC0 (mg C L1) = 1.5, [H2O2] (mg L1)/[Fe2þ] (mg C L1) = 100, pH 3, T = 26 °C.
efficiency of the photo-Fenton process was obtained at pH 5, proving that Fe(OH)3 precipitation limits hydroxyl radicals production by decreasing quantum yield and making harder the regeneration of Fe2þ ions. At pH 2 the efficiency of the process is lower than that at pH 3, which suggests that the scavenging effect of protons toward hydroxyl radicals starts to be more important at high acidic conditions. Temperature also has a non-negligible influence on efficiency of photo-Fenton because the increase of this parameter can yield higher removals of color, aromaticity, and COD, as shown in Table 1. Although high temperature gives greatest efficiency for the photo-Fenton process, an increase of temperature higher than 26 °C (a temperature close to AIW temperature in the source) it is not recommended to not increase costs of photo-Fenton process. 3.4. Influence of COD0 Content on the Treatment of Aiw by Photo-Fenton Process. As previously mentioned, AIW organic content can vary in the range 20007000 mg O2 L1; hence it seems necessary to examine the influence of initial COD content on UV/H2O2/Fe2þ. This requires that the ratios of hydrogen peroxide dose to COD0 content ([H2O2] (mg L1))/ (TOC0 (mg C L1)) and Fe2þ dose ([H2O2] (mg L1))/ ([Fe2þ] (mg C L1)) must be maintained constant and respectively equal to 1.5 and 100 because a H2O2 dose of 3 g L1 and Fe2þ dose of 30 mg L1 were determined as the optimal doses of hydrogen peroxide and ferrous iron to treat AIW containing a COD0 of 2000 mg O2 L1. Figures 4 and 5 present changes of UV400nm, UV280nm, and COD removals as a function of time during photo-Fenton treatment of AIW containing different COD0 contents at pH 3 and 26 °C. The influence of COD0 content on UV280nm and COD is much higher than that on UV400nm. Increasing COD0 content from 2000 mg O2 L1 to 8000 mg O2 L1 leads to a significant decrease in the UV280nm and COD removals, but similar removals of UV400nm were achieved. This can be explained by the fact that high COD0 contents absorb higher amounts of UV radiation and then reduce the quantum yield used for photodecomposing hydrogen peroxide and reducing iron(III) complexes to produce hydroxyl radicals. Also, when high COD0 content is present in AIW, a high hydrogen peroxide dose must be added to the wastewaters, which promote the competition of H2O2 decomposition into hydroxyl radicals with secondary reactions such as autodecomposition of H2O2 to O2 and H2O and then reducing
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Figure 6. Changes with time of UV400nm and UV280nm during the treatment of AIW by the photo-Fenton process. Experimental conditions: COD0 = 2000 mg O2 L1, 3 g L1 H2O2, 30 mg L1 Fe2þ, pH 3, T = 26 °C.
Figure 7. Changes with time of COD and TOC during the treatment of AIW by the photo-Fenton process. Experimental conditions: COD0 = 2000 mg O2 L1, 3 g L1 H2O2, 30 mg L1 Fe2þ, pH 3, T = 26 °C.
hydroxyl radicals’ formation. Also, when COD0 content increases, higher amounts of carboxylic acids can be accumulated during photo-Fenton treatment. These carboxylic acids can form stable complexes with ferric iron, making difficult their oxidation by hydroxyl radicals and then the amount of refractory compounds detected at the end of the treatment will be important. 3.5. Kinetic Analysis of Degradation of Organics by the Photo-Fenton Process. It is difficult to determine the different steps and to establish the kinetics of degradation of organics during the treatment of AIW by the photo-Fenton process because of the presence of a complex mixture of compounds in this effluent. However, only an approach based on the changes with time of UV280nm, UV400nm, COD, and TOC was used to characterize kinetics of degradation of organics during photoFenton treatment of AIW under optimal conditions. Figure 6 presents changes with time of UV400nm and UV280nm during the photo-Fenton treatment (3 g L1 H2O2, 30 mg L1 Fe2þ, and pH = 3) of AIW containing 2000 mg O2 L1. As can be seen, similar profiles for both UV400nm and UV280nm were obtained, but the decrease in UV400nm was more rapid and important than 6677
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Table 2. Results of the Treatment of AIW by Fenton, UV/ H2O2, and Photo-Fenton Processes under Optimized Experimental Conditionsa H2O2 dose Fe2þ dose COD removal estimated pH
(g L1)
(mg L1)
(%)
costs ($/m3)
3.5
10.0
240
75
17.50
5.8 UV/H2O2 UV/H2O2/Fe2þ 3.0
6.0 3.0
0 30
83 95
4.00 3.15
H2O2/Fe2þ
Experimental conditions: COD0 = 2000 mg O2 L1, T = 26 °C, 3 h treatment. a
that of UV280nm. These results showed that color removal is more rapid than aromaticity removal. Taking into account that color removal occurs by the release of chromophore groups from aromatic compounds and aromaticity removal happens by oxidative opening of benzene rings into aliphatic carboxylic acids, this means that color removal and aromaticity removal take place in successive steps during organics degradation. Figure 7 presents the changes of COD and TOC with time during the photo-Fenton treatment of AIW in the same conditions as for Figure 6. COD and TOC concentrations are satisfactorily reduced at the end of the treatment (92% and 83% of COD and TOC removal, respectively) but different profiles were observed for COD and TOC. TOC concentration remains constant during the first hour of photo-Fenton treatment. An apparent decrease in TOC is observed only after about 90 min, which indicates that the mineralization of organics takes place (carbon dioxide formation). For this time, the removals of color, aromaticity, and COD are of 86, 70, and 64%, respectively. Also, the changes of COD with time is more rapid than that observed for TOC, suggesting that during initial stages of the photo-Fenton treatment there are processes that engage oxidation of complex organic compounds to more simple organics without carbon dioxide formation. 3.6. Comparison among Fenton, UV/H2O2, and PhotoFenton Processes in Treating AIW. A comparative study between the Fenton process, UV/H2O2, and the photo-Fenton process in treating AIW was also performed. The three processes are advanced oxidation technologies based on decomposition of H2O2 to produce hydroxyl radicals. Table 2 and Figure 8 present the results of the treatment of AIW by the three technologies. Cost evaluation for UV processes was based on electrical energy per order (EE/O) using the following formula:37 EE=O ¼
P t 1000 CODinf V 60 log CODef f
where P is rated power (in kW), V is the volume of water treated (in L), t is time (min), and CODinf and CODeff are the initial and final concentrations of contaminant in COD (mg O2 L1). The chemical prices were obtained from the Chemical Market reporter's web site for year 2010. The electrical energy price was obtained from the Tunisian Electricity & Gas Company (STEG) (1 kWh = $0.04). UV irradiation and direct chemical oxidation by H2O2 can remove only about 28 and 35% of UV400nm, but no significant changes in COD were detected in these two cases. However, UV/H2O2, Fenton, and photo-Fenton processes, under optimized conditions, resulted respectively in 75%, 83%, and 95%, of UV400nm removal and 65, 71, and 82% of COD removal after 3 h treatment. These results confirmed that AOPs are efficient
Figure 8. Changes with time of UV400nm removal during the treatment of AIW containing COD0 = 2000 mg O2 L1 by different advanced oxidation processes at T = 26 °C. Fenton experimental conditions: 10 g L1 H2O2, 240 mg L1 Fe2þ, pH 3. UV/H2O2 process experimental conditions: 6 g L1 H2O2, natural pH (5.8). Photo-Fenton experimental conditions: 3 g L1 H2O2, 30 mg L1 Fe2þ, pH 3. Chemical oxidation with H2O2 experimental conditions: 10 g L1 H2O2, natural pH (5.8).
technologies for the removal of organic pollutants from industrial wastewaters.3842 From these results the photo-Fenton process is more efficient than the Fenton and UV/H2O2 processes, which can be explained by the production of higher amount of hydroxyl radicals by photodecomposition and catalytic decomposition of H2O2. Also, the UV/H2O2 process presents a lower efficiency than Fenton, which is due to the slower decomposition of H2O2 by UV irradiation. Estimated costs calculated on the basis of electrical energy requirements and chemicals prices in Tunisia showed that, besides its highest efficiency, the photo-Fenton process is more economic than the two other processes.
4. CONCLUSION The photo-Fenton process was effectively used to treat agroindustrial wastewaters. Almost complete color and aromaticity removals were achieved and at the same time more than 90% of COD and 80% of TOC were reduced during the treatment of AIW polluted with 2000 mg O2 L1 under the following optimized conditions: 30 g L1 H2O2, 30 mg L1 Fe2þ, pH 3, T = 26 °C. Kinetic analysis indicated that numerous successive steps are involved in the degradation of organics by photoFenton oxidation including (i) release of chromophores from aromatics to form colorless aromatic compounds, (ii) oxidative opening of aromatic rings into aliphatic intermediates, and (iii) fragmentation of aliphatic compounds into small carboxylic acids and then to carbon dioxide. Our results have shown also that the photo-Fenton process is more efficient and economic than the Fenton and UV/H2O2 processes, which confirms that higher amounts of hydroxyl radicals are produced by H2O2 decomposition with UV irradiation in the presence of ferrous iron. The photo-Fenton process can be a feasible and economically effective technology for actual agro-industrial wastewaters treatment. ’ AUTHOR INFORMATION Corresponding Author
*Tel. 0021675392600. Fax. 0021675392421. E-mail: nasr.bensalah@ issatgb.rnu.tn;
[email protected]. 6678
dx.doi.org/10.1021/ie200266d |Ind. Eng. Chem. Res. 2011, 50, 6673–6680
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