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Application of the Fenton and Fenton-like processes in the landfill leachate tertiary treatment
Journal of Environmental Chemical Engineering 7 (2019) 103352
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Application of the Fenton and Fenton-like processes in the landfill leachate tertiary treatment
T
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Júlia Nercolini Göde , Diego Hoefling Souza, Viviane Trevisan, Everton Skoronski Environmental and Sanitary Engineering Department, Laboratory for Water and Waste Treatment, Santa Catarina State University, 88520-000, Lages, Santa Catarina, Brazil
A R T I C LE I N FO
A B S T R A C T
Keywords: Fenton reaction Fenton-like reaction Advanced oxidative process Landfill leachate
The urban solid waste disposal on landfills is currently a widespread alternative in many countries. After its disposal, the waste undergoes decomposition process, generating leachate as a by-product. This wastewater presents complex composition and contains humic and fulvic substances, which are difficult to degrade. The present study evaluated the performance of Fenton and Fenton-like processes for the tertiary treatment of landfill leachate. In this case, we evaluated the use of ferric ions for the Fenton-Like process. We aimed to investigate color and toxicity reduction, chemical and biochemical oxygen demand, as well as fecal coliforms, ammonia, iron, and phosphorus contents. Leachate color reductions were 98.17% and less than 70% for the Fenton and Fenton-like processes, respectively. The Fenton and Fenton-like processes reduced the toxicity of raw effluent. Observed toxicity of the Fenton process-wastewater was significantly lower. Whereas the Fenton-like process might be considered an alternative for tertiary treatment of landfill leachate, its performance is lower compared to the Fenton one.
1. Introduction Solid waste generation rates are rising in recent decades, due to several factors such as world population growth, technological innovations and changes in habits and lifestyle patterns [1]. In 2025 it is estimated approximately 2.2 × 109 solid waste tonnes are generated per day, worldwide [2]. However, the collection and appropriate waste treatment is still a challenge, mainly in developing countries [3]. Phytoremediation has been extensively performed for wastewater treatment [4–6]; however, this technology generates solid hazardous waste that requires immobilization before landfill [7,8]. Although landfill solid waste destination is considered an economical alternative in many countries, environmental problems are generated, such as the production of hazardous leachate from the decomposition of organic compounds as well as the percolation of rainwater through buried waste [9]. This landfill leachate varies in composition according to factors such as the nature of the waste, the hydrological and meteorological conditions of the site, the age of the landfill and the operational practices [10]. The leachate can be characterized as a complex of different mixtures of organic and recalcitrant inorganic contaminants, including humic and fulvic acids, polycyclic aromatic hydrocarbons, pesticides, trace elements and high levels of ammoniacal nitrogen [11].
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Regarding the environmental control, it is crucial leachate treatment to proper disposal of effluents, per legislation established standards. Traditionally, the biological treatment consists of a reliable and simple method, which has a positive cost/benefit ratio. It is carried out with microorganisms under aerobic and anaerobic conditions [12]; however, it is only applicable to the treatment of biodegradable organic compounds [13]. Also, biological treatment is mostly applied to young sanitary landfills, where the Biochemical Oxygen Demand / Chemical Oxygen Demand (BOD/COD) ratio remains around 0.25. For old landfills, the biological treatment fails to treat toxic and recalcitrant compounds present in the leachate, making it necessary for the application of physicochemical treatments [14]. Physical-chemical treatment usually consists of coagulation-flocculation processes [15], reverse osmosis [16], and activated carbon [17,10], or even a combination of these treatments [18]. Also, the immobilization of organic wastes such as liquid scintillator [19] and textile wastes [20] is also very important before its disposal. Advanced Oxidative Processes (AOPs) are physical-chemical treatment processes for the degradation of the organic medium by the generation of high oxidizing free radicals [21]. The Fenton reaction is one of the most commonly used AOPs and has been widely studied for application in effluent treatment as a complementary step [22]. It is a reaction discovered in 1894 by the British Henry Fenton [23], in which
Corresponding author. E-mail address: [email protected] (J.N. Göde).
https://doi.org/10.1016/j.jece.2019.103352 Received 6 May 2019; Received in revised form 7 August 2019; Accepted 8 August 2019 Available online 09 August 2019 2213-3437/ © 2019 Elsevier Ltd. All rights reserved.
Journal of Environmental Chemical Engineering 7 (2019) 103352
J.N. Göde, et al.
Fe2+ ions act as catalysts, and hydrogen peroxide (H2O2) as the oxidizing agent of the reaction, which generates the hydroxyl radical (•OH) in an acid medium (pH between 2.5 and 3.0) [21]. The hydroxyl radical can degrade recalcitrant and toxic organic compounds employing a series of reactions in solution, summarized by Eq. (1) [1]. Fe2+ + H2O2 → Fe3+ + •OH + OH−
(1)
Fe3+ + H2O2 → Fe2+ + HO2• + H+
(2)
The main objective of Fenton oxidation is to transform the dissolved organic carbon into carbon dioxide (CO2) and dissociate H2O2 into oxygen and water, presenting advantages such as simplicity and lowtoxicity, since it is operated at ambient temperature and atmospheric pressure [24]. Despite this, the high operational cost, the interference of factors such as pH and H2O2 concentration, and the generation of large volumes of iron sludge are the main drawbacks of this process [10]. For this reason, the substitution of the Fe2+ catalyst for Fe3+ species has been investigated, which is known as the Fenton-like reaction, which is briefly described by Eq. 2 [1]. During the Fenton-like reaction, the hydroperoxyl radical (HO2•) is generated. This radical is responsible for oxidizing organic compounds. However, it has a lower oxidation capacity compared to •OH. Consequently, the rate of mineralization occurs faster due to the immediate formation of hydroxyl radicals in the Fenton reaction [25]. In the last 15 years, the Fenton process was extensively investigated for dye discoloration [26,23,27,28] as well in wastewater treatment [1,24]. Also, specific applications such as olive mill wastewater treatment [29–34] (which present high organic load with high toxic features and low biodegradability [35]), landfill leachate [21,36,37] and remediation of oil-contaminated soil [38–40], using process such as Photo-Fenton [14], Electro-Fenton [41,36] and Fenton ultrasound [42] have been reported. Fenton-like oxidation has been also investigated in wastewater decolorisation [24,43–46], olive mill [31,47,48], sawmill [49] as well as pulp and paper [50] wastewater treatment. In landfill leachate treatment, few studies have been conducted such as photoFenton-like systems [51], Fenton-like using Fe3O4 particles decorated Zr pillared bentonite [22] and Fenton-like using zero valent iron [52]. Studies on the applicability of the Fenton and Fenton-like reaction in the tertiary treatment of landfill leachate have not been published to date. No global scale study has reused Fe3+ sludge as the catalyst for new Fenton-like reactions for the treatment of landfill leachate, but for application in the treatment of agro-industries wastewater [1] and discoloration of azo dyes [53]. The reuse of the iron-rich sludge produced by the Fenton’s reaction also was evaluated in the treatment of olive mill wastewater [54] and post-biological treated milk whey wastewater [55] with satisfactory results. There is a limitation since the treatment efficiency decreases with the number of reuses [55]. However, the use of this sludge requires simple steps and can reduce the wastewater treatment costs, prevent negative impacts in ecosystems and save natural resources [54]. The objective of this research is to compare the performance of Fenton and Fenton-like processes in the tertiary treatment of landfill leachate. Additionally, we have assessed the characteristics of the treated effluent as well as its toxicity.
Fig. 1. Leachate treatment flowchart of Lages (SC) landfill.
6Mo7O24), Ascorbic Acid (C6H8O6 0.1 M), Sodium Sulfate (Na2SO4), Copper Sulfate (CuSO4) and, Antimony and Potassium Tartrate (C8H10K2O15Sb2). Seeds of Lactuca sativa "Buttercrunch" and the enzyme catalase were also used.
2. Materials and methods 2.2. Landfill leachate 2.1. Materials This study was carried out with the landfill leachate produced in the municipality of Lages, located in the southern region of Brazil (27°44′77′'S and 50°09′93′'O). The site is characterized by a temperate climate type (moist mesothermal with mild summers), according to the climatic classification of Köppen-Geiger (1936) [56]. Average annual temperature varies between 15 and 16 °C, being observed negative temperatures during winter. The average annual cumulative rainfall remains between 1500 and 1600 mm [57]. The landfill has been in
Reagents of analytical grade were used in this work such as: Hydrogen Peroxide (30% H2O2), Iron Sulphate (FeSO4.7H2O 53.4 g/L), Iron Chloride (FeCl3 13 g/L), Sodium Hydroxide (NaOH 1 mol/L), Sulfuric Acid (3% H2SO4), Nitroprussiate-Phenol Solution, Sodium Hypochlorite Solution (NaClO), LMX Plus 100 Readycult, Potassium Dichromate (K2Cr2O7), Mercury Sulfate II (HgSO4), Silver Sulfate (Ag2SO4), Phenolphthalein (C20H14O4), Ammonium Molybdate ((NH4) 2
Journal of Environmental Chemical Engineering 7 (2019) 103352
J.N. Göde, et al.
Table 1 Fenton's reaction conditions and color reduction for the first stage (Raw leachate color of 399 Cu). (Id.)
F1' F2' F3' F4' F5' F6' F7' F8' F9' F10' F11'
Concentration (mg/L)
Table 3 Fenton-like reactions conditions and color reduction (Raw leachate color of 398 Cu).
Color reduction (%)
H2O2
FeSO4
1492.5 1492.5 8265.8 8265.8 0.0 9677.4 4918.0 4918.0 4918.0 4918.0 4918.0
1168.9 205.6 6246.9 1220.8 0.0 2338.3 0.0 606.7 606.7 606.7 606.7
(Id.)
PF1 PF2 PF3 PF4 PF5 PF6 PF7 PF8
85.4 25.6 96.6 95.1 0.0 92.5 54.5 76.8 68.9 39.4 34.8
Concentration (mg/L)
Color reduction (%)
H2O2
FeCl3
299.7 1195.2 299.7 1195.2 1195.2 299.7 598.8 1195.2
855.1 2088.5 1064.7 1064.7 2088.5 537.6 1064.7 2088.5
35.3 53.9 44.1 47.0 48.2 34.4 36.3 48.9
reaction. 2.4. Fenton reaction
operation since 2007, which is considered old. Currently, the leachate is subjected to a combination of biological and physicochemical treatments, as observed in Fig. 1. Steps 4A and 4B are related to the physical-chemical processes of coagulation, flocculation, and decantation of the leachate. Samples of the leachate were collected in a zone previously positioned to the chlorination process. The treated leachate is sent to the "Ribeirão da Casa Queimada" Class 2 river, according to CONAMA Resolution N°. 357 of 2005 [58], which deals with the classification of water bodies and environmental guidelines for their classification.
The experiments described below were performed using H2O2 and FeSO4 at concentrations described in Tables 1 and 2. Initially, 50 mL of raw leachate sample was added to each Becker. Carefully, H2O2 and FeSO4 were inserted. After this step, the pH of the solution, which was around 7.03, was adjusted to the value of 3.1 ± 0.04, by applying 980 μL of 3% sulfuric acid solution, since that the ideal pH for the occurrence of the Fenton and Fenton-like reactions is approximately 3.0 [59]. From preliminary experiments, no significant reduction of color was observed during four hours of reaction. In this way, this interval of time was considered for all conditions, which were succeeded at the temperature of 20 °C. After the stabilization of the color reduction, the pH was rised to values ranging from 6.9 to 7.9 in order to enable the decantation of the iron in the hydroxide form. Separation of the solid phase from the liquid was carried out in a centrifuge at 4000 rpm for 5 min. The treated leachate color was measured using a Pharo 300 spectrophotometer (Merck®. Germany) and 50 mm quartz cuvettes. After the last stage of the Fenton reactions, the three conditions that showed the highest levels of leachate color reduction were repeated in triplicate in order to certify the most favorable condition to Fenton treatment and to use its sludge of Fe3+ decanted as the catalyst of the Fenton-like reactions. The procedure to perform the three conditions most favorable to the Fenton reaction was similar to that previously mentioned, with the difference that the amount of raw sample used in the reactions was changed from 50 mL to 250 mL. The amounts of H2O2 and FeSO4 increased proportionally. 56 mL of 3% H2SO4 and 4.0 mL of 1 mol/L NaOH, respectively, was required to adjust the pre and post reaction pH.
2.3. Definition of condition Eleven initial Fenton reaction conditions (Table 1) were determined by means of previous studies carried out with the same leached material. The conditioning factors were the concentrations of H2O2 and FeSO4. As the response variable was considered the leachate color reduction after the reaction, at all stages. The condition that guaranteed the greatest reduction in leachate color at the end of the treatment was used for the establishment of a new set of eleven conditions (Table 2) in order to enhance the interaction of the reactants during the reaction. The Fenton-like process was performed by two different methods: in the first, eight different conditions were defined (Table 3) but in this case, the conditioning factors were H2O2 and FeCl3 concentrations. The level of these conditions was determined from previous experiments; in the second, the iron sludge decanted after the end of the best Fenton reaction condition was used as the iron ion source of the Fenton-like
2.5. Fenton-like reaction In the first method which the Fenton-like process was performed, it was used FeCl3 as a source of Fe3+, using analytical grade reagent. It was added 50 mL of raw leachate in a Becker and, H2O2 and FeCl3 (Table 3) in sequence. The pH of the solution was corrected to 3.0 using 800 μL of H2SO4, and the reaction last 4 h. The solution was stirred with magnetic bars, at a temperature of 20 °C. In the second method, the Fenton-like process was generated by reusing all the Fe3+ sludge produced in the previous Fenton reaction (approximately 0.12 g) with the highest level of color reduction. The same concentration of H2O2 used in this condition was used in a cycle of reuse of the iron sludge decanted of five stages. To obtain the sludge, after the completion of the Fenton reaction and adjustment of the pH, 20 μL of the catalase enzyme solution was added and the solution was kept for 24 h in Falcon tubes without a cap, to allow the release of the residual hydrogen peroxide. After the rest period, the samples were centrifuged for five minutes at 4000 rpm and the decanted sludge of Fe3+ was collected. This sludge was combined with a small volume of
Table 2 Fenton's reaction conditions and color reduction for the second stage (Raw leachate color of 421Cu). (Id.)
F1” F2” F3” F4” F5” F6” F7” F8” F9” F10” F11”
Concentration (mg/L)
Color reduction (%)
H2O2
FeSO4
2961.5 2961.5 16538.6 16538.6 0.0 19185.3 9909.5 9909.5 9909.5 1021.3 1021.3
2553.2 878.3 2553.2 878.3 1307.3 1307.3 0.0 661.6 1307.3 133.6 133.6
97.2 89.5 97.6 90.2 65.9 77.1 54.0 92.2 92.1 91.3 92.1
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raw leachate, stirred for about ten seconds on the Vortex-type shaker, and transferred to a Becker, ready for reuse. Subsequently, 250 mL of the raw leachate was added to the Becker, and then the H2O2 was added. The initial pH of the solution (7.3 ± 0,0) was adjusted to the value of 3.2 ± 0.09 by the addition of a mean of 4.4 ml of 3% H2SO4. The reaction occurred for four hours. At the end of the reaction, the pH (3.6 ± 0,2) was again corrected to the value of 7.2 ± 0.12 by inserting 1.80 mL of 1 mol/L NaOH in the solution. After that, the solution was centrifuged at 4000 rpm for 5 min. The color of the treated leachate was determined, and the decanted material was used to generate a new Fenton-like reaction under the same conditions. This process was repeated four more times. The conductivity of the treated leachate was not determined since the main goal of this work was to reduce leachate color, avoiding its reuse, being discarded at the end of each treatment. 2.6. Kinetic study The most favorable Fenton and Fenton-like reactions condition were also used to perform a kinetic analysis, monitoring reaction product color over time, in order to verify the possibility of greater efficiency of the treatments that justified the application for a longer period of time. In this manner, 10 mL samples were taken at times 2, 4, 6 and 8 h from the beginning of the reactions. The pH of the samples was adjusted to 7.0 before centrifugation for 5 min, and the color of the leachate at each interval of time was performed as described previously. 2.7. Toxicity assessment The toxicity test was performed with lettuce seeds (Lactuca sativa) and followed the procedures of the United States Environmental Protection Agency [60,61], method "Seed Germination / Root Elongation Toxicity Test". Five 100 cm x 15 cm Petri dishes, each containing a layer of filter paper and previously sterilized, were used for each treatment. The treatments evaluated were raw leachate (before being treated), leachate treated by Fenton reaction, leachate treated by Fenton-like reaction and control treatment, with distilled water in place of the sample. The residual H2O2 concentration in the Fenton and Fenton-like treatments were removed by the addition of catalase, thus, it did not affect the toxicity of the treated leachate. Two mL of sample and five seeds of L. sativa Buttercrunch were added to each plate on the filter paper. The plates were wrapped in a plastic film to prevent loss of moisture to the medium and were kept in the dark for five days at room temperature (17 °C). The variables considered in the analysis of the ¯ ), root length results were the mean number of germinated seeds (GS ¯ ). The Germination Effect (%GE), Root Growth ¯ ) and stem length (SL (RL Inhibition (%RGI) and Germination Index (%GI) were calculated by applying Eqs. 3–5, respectively.
%GE =
%RGI =
Number of germinate seeds × 100 Total number of seeds
Fig. 2. Leachate color reduction over time. A) Fenton process; B) Fenton-like process. Note: Raw leachate color: 462 Cu.
and for this reason, it is important to evaluate them. The analysis of the leachate color reduction over the eight-hour of Fenton and Fenton-like reactions can be seen in the graphs of Fig. 2. Analyzing the graphs of Fig. 2, a trend of efficiency increase of the treatment over time can be observed. In both cases (A and B) it is observed that a plateau is reached after approximately two hours of reaction and no changes in the efficiency are observed. The average efficiency of Fenton treatment was 94.12%, while Fenton-like presented an average of 50.67%. Thus, the Fenton process showed better results than Fenton-like for leachate color reduction and treatment stability. The efficiency of landfill leachate treatment is generally evaluated based on before and after treatment analyzes. The parameters COD, BOD5, and Color are routinely determined in this kind of research [12]. In addition, leachate toxicity analysis is important for a more complete assessment of the quality of treatment. For these reasons, the present study evaluated the raw and treated leachate by the analyzes described in Table 5, and the toxicity by the test with lettuce seeds (Table 6). The three conditions that resulted in the greatest leachate color reduction (F3', F1” and F3”) were redone in triplicate. In this case, the most favorable condition to the Fenton reaction was the F3' (Table 4). Fig. 3 shows the raw leachate next to the treated leachate in the F3' condition in triplicate, after the centrifugation process, stored in Falcon tubes. The decanted sludge in this condition (F3') was reused in the
(3)
Mean control root growth − Mean sample root growth × 100 Mean control root growth (4)
%GI =
Sample root length × Number of germinated seeds in the sample Control root length × Number of germinated seeds in the control (5)
3. Results and discussion
Table 4 Color Reduction of the three best Fenton reaction conditions.
3.1. Treatments efficiency
Id ¯ (%) CR
The time that the solutions are maintained reacting is a parameter that can influence the efficiency of the Fenton and Fenton-like processes
F3' 98.1
Note: Raw leachate color: 469.5 Cu. 4
F1” 93.1
F3” 98.1
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Table 5 Comparative analysis between raw leachate, treated by Fenton, treated by Fenton-like and standards described by Brazilian legislation. Parameter
Raw Leachate
Fenton
FL
430/11*
Iron (mg/L Fe) BOD (mg/L O2) COD (mg/L O2) Ammonia (mg/L N) Coliforms - E. Coli Phosphor (mg/L P)
0.3 43.3 485.1
6.7 16.7 (61%) 414.0
24.1 16.7 (61%) 475.0
Unt. 15.0 Min. 60% rem. NS
< 0.2 5.0 < 0.5
< 0.2 0.0 < 0.5
< 0.2 0.0 < 0.5
Unt. 15.0 NS NS
E. coli given in colony forming units per 100 mL sample; Raw = Leached before treatment; FL = Fenton-like; rem = removal; NS = Not specified. Note:*CONAMA Legislation - National Council of the Ministry of Environment. Table 6 Toxicity analysis for the tested treatments.
Control Raw Leachate Fenton Fenton-like
(%GE)
(%RGI)
(%GI)
100 72 96 50
0.0 95.5 68.2 77.3
100 3.3 30.5 11.4
Fig. 4. Removal of color by A) Fenton-like with FeCl3 (Raw leachate color: 398Cu) and B) reuse of iron sludge (Raw leachate color: 408.5 Cu in the 1 st stage; 391.5Cu in the 2nd stage and 369.5Cu in the 3rd, 4th and 5th stage of the cycle).
Fig. 3. Triplicate of treated leachate in the F3' condition (B, C, D) compared to raw leachate (A).
Fenton-like (FL) reaction cycle, which resulted in the values presented in Fig. 4 as described in item 2.5. The first stage of the cycle showed the best efficiency in reducing the leachate color, probably because in the following stages a continuous loss of iron ions occurs, which can seriously affect the efficiency of the Fenton and Fenton-like processes [53]. According to the work done by Quadrado and Fajardo (2017), the reuse of iron ions can be employed in at least five consecutive processes of the studied dye discoloration, without losing efficiency or catalytic stability [53]. The results obtained by the Fenton-like reaction carried out with the FeCl3 solution as a catalyst can be also observed in Fig. 4. In general, the Fenton-like reaction cycle that used iron sludge as the source of Fe3+ ions proved to be more efficient than the process that used FeCl3 solution, as shown in Fig. 4, where the five conditions that resulted in the greater reduction of the leachate color by the FeCl3 solution process and the cycle of five reactions with the decanted iron sludge are compared. Fig. 5 shows the raw leachate next to the treated leachate in the FL3' (1) condition, in triplicate, after centrifugation. Thus, conditions F3', FL3'(1) and raw leachate were compared to the conditions for effluent discharges into Class 2 rivers, described by CONAMA Resolution 430/2011 [62] in many parameters, which can be seen in Table 5. The Fe3+ sludge generated in the Fenton reaction was used as a
Fig. 5. Triplicate of the treated leachate in the FL3' (1) (B,C, D) condition compared to the raw (A).
catalyst for the Fenton-like reaction, to promote raw material savings and, consequently, reduce the cost of the treatment. However, in evaluating the results obtained, it is noticeable that the Fenton-like process does not achieve the same efficiency of the Fenton. This fact can be explained by the lower oxidative capacity that the radical HO2•, generated in the Fenton-like reaction, presents when compared to the radical •OH, generated in the Fenton reaction [25]. According to the work done by Jung et al. (2017) [38], the Fenton process exhibits a high treatment performance in the optimal conditions because •OH can 5
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Considering both variables (number of germinated seeds and root length of germinated seeds), the raw leachate revealed to be more toxic to L. sativa than the leachate treated by Fenton-like, with a GI almost 3.48 fold lower. It is possible to notice that all the treatments have some level of toxicity since they differed from the control, which proved not to be toxic to the lettuce, according to the analysis of the number of germinated seeds. The highest level of significance was observed between the control group and raw averages, as expected. Fernandes et al. [27] performed the same toxicity test for lettuce with the objective of evaluating the removal of azo dyes after treatment of the effluent by Fenton and Fenton-like and reached the result that, after 24 h of samples exposure, lettuce mortality levels were in the range of 0 to 45%, suggesting a generation of toxic degradation products, considering that in most cases, the degradation of the dye was almost 100%. The reason why Fenton oxidation is more effective in removing toxicity from leachate compared to Fenton-like may be due to two factors: the hydroperoxylic radicals generated, which also attack organic contaminants, but are less sensitive than the hydroxyl radicals. Considering that iron added in small amounts acts as a catalyst, while H2O2 is continuously consumed to produce hydroxyl radicals [59], the second reason may be due to the fact that no further doses of iron were added in the Fenton-like process, only the Fe3+ sludge from the Fenton process was reused, which may have hindered the interaction between the iron ions and the H2O2, thus inhibiting the catalytic action of the reaction.
effectively decompose the dissolved hydrophobic and hydrophilic organic matter [38]. Duarte et al. [41] achieved an 80% removal of COD from the contaminated effluent studied, by the Fenton reaction treatment, which was as efficient as the electrochemical treatment employed, however, faster [41]. The treatment of dyes by Fenton compared to Fenton-like, however, has shown opposite results. In the research conducted by Fernandes et al. [27] the results indicated that the Fenton-like type process is superior to Fenton to degrade Disperse Red 343, a type of azo dye [27]. Hu et al. [28] showed that the degradation rate of malachite green, a toxic chemical used as a dye, was 59.34% in the Fenton process and 92.7% in the Fenton-like process [28]. The high iron ion content present in the treated leachate samples, especially after the Fenton-like process, is shown in Table 5. Although the contents are in accordance with the legislation in the case of the Fenton oxidation, iron can become a difficulty for the application of the treatment, considering that the process itself is based on the use of Fe2+ and Fe3+ ions as catalysts of the main reaction, substantially increasing the concentration of these elements in the treated effluent. The Fenton and Fenton-like reactions are strongly dependent on three main factors: pH of the solution, iron ion concentration, and hydrogen peroxide concentration. The reaction temperature is a parameter that showed no significant levels of influence on the efficiency of the treatments [25]. The optimum pH determined for the Fenton reaction was approximately 3.0, regardless of the target substrate [59], and about 4.5 for Fenton-like reaction [37]. Because of this reason, the pH was carefully adjusted in each of the solutions carried out in this research, resulting in standardized pH solutions. Based on this premise, pH was not a determinant of the efficiency of the Fenton and Fentonlike reaction tests performed. Typically, the rate of organic and inorganic compounds degradation increases as the ferrous ions concentration increase. However, excess ions may lead to an undesired amount of unused iron salts, which can generate a high amount of dissolved solids contained in the effluent flow, affecting the treatment quality [59,25]. The H2O2 concentration also has a crucial role in the overall efficiency of the pollutant degradation process by the Fenton and Fenton-like reactions. It has been observed that degradation increases with the H2O2 dosage increase [25]. However, care should be taken with the added concentration, since the unused portion of H2O2 during the process contributes to the COD increase. Therefore, in excess, it is not recommended [59].
4. Conclusion Concerning the landfill leachate color reduction, the Fenton process presented more satisfactory results (98.1% of color reduction) than the Fenton-like process (less than 70% of color reduction). Except for iron analysis, a reduction in all the parameters analyzed in the raw leachate and treated by Fenton and Fenton-like were observed, obtaining better results in the Fenton reaction. The toxicity analysis showed that, whereas both treatments reduced the toxicity of Landfill Leachate, Fenton has a lower toxic potential than Fenton-like. As an improvement to the research, it is suggested that a larger number of both reactions conditions should be performed, so that even more satisfactory results can be found. It is also suggested that the influence of pH on the efficiency of the reaction is analyzed. It would be interesting if the Fenton and Fenton-like processes were compared in efficiency and cost with other Fenton variations, such as photo-Fenton. It is also suggested that a study be carried out on the operational and economic viability of the application of Fenton and Fenton-like treatments on a real scale.
3.2. Toxicity analysis It was decided to determine the ecological effects of the toxic substances present in the raw and treated leachate through the analysis with Lactuca sativa because this is a plant species used for phytotoxicity tests and its use for this purpose has been recommended by many international organizations [63]. Besides, tests using L. sativa are simple, fast, inexpensive, as they do not require expensive equipment, and are as reliable as the widely known and commonly used Allium cepa [64]. In the toxicity test of the leachate treated by the Fenton and Fentonlike reactions, samples were taken of the conditions in which 847.6 mg/ L of H2O2 and 696.8 mg/L of FeSO4 were added to the Petri dishes. The ¯ ) and root length (RL ¯ ) and average number of germinated seeds (GS ¯ ) of each situation were analyzed. Table 6 shows the arithmetic stalk (SL average of the toxicity test results. The treatments applied were the control, raw (before treatment), Fenton and Fenton-like. The number of lettuce germinated seeds and root lengths are parameters considered sufficient for toxicity analysis of a substance, as was done by Lyu et al. [63]. The leachate treated by the Fenton process showed to be less toxic to L. sativa as compared to raw leachate or the treated by Fenton-like process, with a GI value closer to 1, considered to be ideal (Priac et al., 2017) [1]. Although raw leachate had a higher germination index than that treated by Fenton-like, it exhibited higher levels of inhibition.
Funding This survey did not receive any specific grant awards from public, commercial, or non-profit sectors. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. References [1] V. Leifeld, T.P.M. dos Santos, D.W. Zelinski, L. Igarashi-Mafra, Ferrous ions reused as catalysts in Fenton-like reactions for remediation of agro-food industrial wastewater, J. Environ. Manage. 222 (2018) 284–292, https://doi.org/10.1016/j. jenvman.2018.05.087. [2] D. Hoornweg, P. Bhada, What a waste. A global review of solid waste management, Urban Dev. Ser. Knowl. Pap. 281 (2012) 44 p, https://doi.org/10.1111/febs.13058. [3] F.A.M. Lino, K.A.R. Ismail, Evaluation of the treatment of municipal solid waste as renewable energy resource in Campinas, Brazil, Sustain. Energy Technol. Assessments 29 (2018) 19–25, https://doi.org/10.1016/j.seta.2018.06.011. [4] H.M. Saleh, Water hyacinth for phytoremediation of radioactive waste simulate
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Application of the Fenton and Fenton-like processes in the landfill leachate tertiary treatment
T
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Júlia Nercolini Göde , Diego Hoefling Souza, Viviane Trevisan, Everton Skoronski Environmental and Sanitary Engineering Department, Laboratory for Water and Waste Treatment, Santa Catarina State University, 88520-000, Lages, Santa Catarina, Brazil
A R T I C LE I N FO
A B S T R A C T
Keywords: Fenton reaction Fenton-like reaction Advanced oxidative process Landfill leachate
The urban solid waste disposal on landfills is currently a widespread alternative in many countries. After its disposal, the waste undergoes decomposition process, generating leachate as a by-product. This wastewater presents complex composition and contains humic and fulvic substances, which are difficult to degrade. The present study evaluated the performance of Fenton and Fenton-like processes for the tertiary treatment of landfill leachate. In this case, we evaluated the use of ferric ions for the Fenton-Like process. We aimed to investigate color and toxicity reduction, chemical and biochemical oxygen demand, as well as fecal coliforms, ammonia, iron, and phosphorus contents. Leachate color reductions were 98.17% and less than 70% for the Fenton and Fenton-like processes, respectively. The Fenton and Fenton-like processes reduced the toxicity of raw effluent. Observed toxicity of the Fenton process-wastewater was significantly lower. Whereas the Fenton-like process might be considered an alternative for tertiary treatment of landfill leachate, its performance is lower compared to the Fenton one.
1. Introduction Solid waste generation rates are rising in recent decades, due to several factors such as world population growth, technological innovations and changes in habits and lifestyle patterns [1]. In 2025 it is estimated approximately 2.2 × 109 solid waste tonnes are generated per day, worldwide [2]. However, the collection and appropriate waste treatment is still a challenge, mainly in developing countries [3]. Phytoremediation has been extensively performed for wastewater treatment [4–6]; however, this technology generates solid hazardous waste that requires immobilization before landfill [7,8]. Although landfill solid waste destination is considered an economical alternative in many countries, environmental problems are generated, such as the production of hazardous leachate from the decomposition of organic compounds as well as the percolation of rainwater through buried waste [9]. This landfill leachate varies in composition according to factors such as the nature of the waste, the hydrological and meteorological conditions of the site, the age of the landfill and the operational practices [10]. The leachate can be characterized as a complex of different mixtures of organic and recalcitrant inorganic contaminants, including humic and fulvic acids, polycyclic aromatic hydrocarbons, pesticides, trace elements and high levels of ammoniacal nitrogen [11].
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Regarding the environmental control, it is crucial leachate treatment to proper disposal of effluents, per legislation established standards. Traditionally, the biological treatment consists of a reliable and simple method, which has a positive cost/benefit ratio. It is carried out with microorganisms under aerobic and anaerobic conditions [12]; however, it is only applicable to the treatment of biodegradable organic compounds [13]. Also, biological treatment is mostly applied to young sanitary landfills, where the Biochemical Oxygen Demand / Chemical Oxygen Demand (BOD/COD) ratio remains around 0.25. For old landfills, the biological treatment fails to treat toxic and recalcitrant compounds present in the leachate, making it necessary for the application of physicochemical treatments [14]. Physical-chemical treatment usually consists of coagulation-flocculation processes [15], reverse osmosis [16], and activated carbon [17,10], or even a combination of these treatments [18]. Also, the immobilization of organic wastes such as liquid scintillator [19] and textile wastes [20] is also very important before its disposal. Advanced Oxidative Processes (AOPs) are physical-chemical treatment processes for the degradation of the organic medium by the generation of high oxidizing free radicals [21]. The Fenton reaction is one of the most commonly used AOPs and has been widely studied for application in effluent treatment as a complementary step [22]. It is a reaction discovered in 1894 by the British Henry Fenton [23], in which
Corresponding author. E-mail address: [email protected] (J.N. Göde).
https://doi.org/10.1016/j.jece.2019.103352 Received 6 May 2019; Received in revised form 7 August 2019; Accepted 8 August 2019 Available online 09 August 2019 2213-3437/ © 2019 Elsevier Ltd. All rights reserved.
Journal of Environmental Chemical Engineering 7 (2019) 103352
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Fe2+ ions act as catalysts, and hydrogen peroxide (H2O2) as the oxidizing agent of the reaction, which generates the hydroxyl radical (•OH) in an acid medium (pH between 2.5 and 3.0) [21]. The hydroxyl radical can degrade recalcitrant and toxic organic compounds employing a series of reactions in solution, summarized by Eq. (1) [1]. Fe2+ + H2O2 → Fe3+ + •OH + OH−
(1)
Fe3+ + H2O2 → Fe2+ + HO2• + H+
(2)
The main objective of Fenton oxidation is to transform the dissolved organic carbon into carbon dioxide (CO2) and dissociate H2O2 into oxygen and water, presenting advantages such as simplicity and lowtoxicity, since it is operated at ambient temperature and atmospheric pressure [24]. Despite this, the high operational cost, the interference of factors such as pH and H2O2 concentration, and the generation of large volumes of iron sludge are the main drawbacks of this process [10]. For this reason, the substitution of the Fe2+ catalyst for Fe3+ species has been investigated, which is known as the Fenton-like reaction, which is briefly described by Eq. 2 [1]. During the Fenton-like reaction, the hydroperoxyl radical (HO2•) is generated. This radical is responsible for oxidizing organic compounds. However, it has a lower oxidation capacity compared to •OH. Consequently, the rate of mineralization occurs faster due to the immediate formation of hydroxyl radicals in the Fenton reaction [25]. In the last 15 years, the Fenton process was extensively investigated for dye discoloration [26,23,27,28] as well in wastewater treatment [1,24]. Also, specific applications such as olive mill wastewater treatment [29–34] (which present high organic load with high toxic features and low biodegradability [35]), landfill leachate [21,36,37] and remediation of oil-contaminated soil [38–40], using process such as Photo-Fenton [14], Electro-Fenton [41,36] and Fenton ultrasound [42] have been reported. Fenton-like oxidation has been also investigated in wastewater decolorisation [24,43–46], olive mill [31,47,48], sawmill [49] as well as pulp and paper [50] wastewater treatment. In landfill leachate treatment, few studies have been conducted such as photoFenton-like systems [51], Fenton-like using Fe3O4 particles decorated Zr pillared bentonite [22] and Fenton-like using zero valent iron [52]. Studies on the applicability of the Fenton and Fenton-like reaction in the tertiary treatment of landfill leachate have not been published to date. No global scale study has reused Fe3+ sludge as the catalyst for new Fenton-like reactions for the treatment of landfill leachate, but for application in the treatment of agro-industries wastewater [1] and discoloration of azo dyes [53]. The reuse of the iron-rich sludge produced by the Fenton’s reaction also was evaluated in the treatment of olive mill wastewater [54] and post-biological treated milk whey wastewater [55] with satisfactory results. There is a limitation since the treatment efficiency decreases with the number of reuses [55]. However, the use of this sludge requires simple steps and can reduce the wastewater treatment costs, prevent negative impacts in ecosystems and save natural resources [54]. The objective of this research is to compare the performance of Fenton and Fenton-like processes in the tertiary treatment of landfill leachate. Additionally, we have assessed the characteristics of the treated effluent as well as its toxicity.
Fig. 1. Leachate treatment flowchart of Lages (SC) landfill.
6Mo7O24), Ascorbic Acid (C6H8O6 0.1 M), Sodium Sulfate (Na2SO4), Copper Sulfate (CuSO4) and, Antimony and Potassium Tartrate (C8H10K2O15Sb2). Seeds of Lactuca sativa "Buttercrunch" and the enzyme catalase were also used.
2. Materials and methods 2.2. Landfill leachate 2.1. Materials This study was carried out with the landfill leachate produced in the municipality of Lages, located in the southern region of Brazil (27°44′77′'S and 50°09′93′'O). The site is characterized by a temperate climate type (moist mesothermal with mild summers), according to the climatic classification of Köppen-Geiger (1936) [56]. Average annual temperature varies between 15 and 16 °C, being observed negative temperatures during winter. The average annual cumulative rainfall remains between 1500 and 1600 mm [57]. The landfill has been in
Reagents of analytical grade were used in this work such as: Hydrogen Peroxide (30% H2O2), Iron Sulphate (FeSO4.7H2O 53.4 g/L), Iron Chloride (FeCl3 13 g/L), Sodium Hydroxide (NaOH 1 mol/L), Sulfuric Acid (3% H2SO4), Nitroprussiate-Phenol Solution, Sodium Hypochlorite Solution (NaClO), LMX Plus 100 Readycult, Potassium Dichromate (K2Cr2O7), Mercury Sulfate II (HgSO4), Silver Sulfate (Ag2SO4), Phenolphthalein (C20H14O4), Ammonium Molybdate ((NH4) 2
Journal of Environmental Chemical Engineering 7 (2019) 103352
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Table 1 Fenton's reaction conditions and color reduction for the first stage (Raw leachate color of 399 Cu). (Id.)
F1' F2' F3' F4' F5' F6' F7' F8' F9' F10' F11'
Concentration (mg/L)
Table 3 Fenton-like reactions conditions and color reduction (Raw leachate color of 398 Cu).
Color reduction (%)
H2O2
FeSO4
1492.5 1492.5 8265.8 8265.8 0.0 9677.4 4918.0 4918.0 4918.0 4918.0 4918.0
1168.9 205.6 6246.9 1220.8 0.0 2338.3 0.0 606.7 606.7 606.7 606.7
(Id.)
PF1 PF2 PF3 PF4 PF5 PF6 PF7 PF8
85.4 25.6 96.6 95.1 0.0 92.5 54.5 76.8 68.9 39.4 34.8
Concentration (mg/L)
Color reduction (%)
H2O2
FeCl3
299.7 1195.2 299.7 1195.2 1195.2 299.7 598.8 1195.2
855.1 2088.5 1064.7 1064.7 2088.5 537.6 1064.7 2088.5
35.3 53.9 44.1 47.0 48.2 34.4 36.3 48.9
reaction. 2.4. Fenton reaction
operation since 2007, which is considered old. Currently, the leachate is subjected to a combination of biological and physicochemical treatments, as observed in Fig. 1. Steps 4A and 4B are related to the physical-chemical processes of coagulation, flocculation, and decantation of the leachate. Samples of the leachate were collected in a zone previously positioned to the chlorination process. The treated leachate is sent to the "Ribeirão da Casa Queimada" Class 2 river, according to CONAMA Resolution N°. 357 of 2005 [58], which deals with the classification of water bodies and environmental guidelines for their classification.
The experiments described below were performed using H2O2 and FeSO4 at concentrations described in Tables 1 and 2. Initially, 50 mL of raw leachate sample was added to each Becker. Carefully, H2O2 and FeSO4 were inserted. After this step, the pH of the solution, which was around 7.03, was adjusted to the value of 3.1 ± 0.04, by applying 980 μL of 3% sulfuric acid solution, since that the ideal pH for the occurrence of the Fenton and Fenton-like reactions is approximately 3.0 [59]. From preliminary experiments, no significant reduction of color was observed during four hours of reaction. In this way, this interval of time was considered for all conditions, which were succeeded at the temperature of 20 °C. After the stabilization of the color reduction, the pH was rised to values ranging from 6.9 to 7.9 in order to enable the decantation of the iron in the hydroxide form. Separation of the solid phase from the liquid was carried out in a centrifuge at 4000 rpm for 5 min. The treated leachate color was measured using a Pharo 300 spectrophotometer (Merck®. Germany) and 50 mm quartz cuvettes. After the last stage of the Fenton reactions, the three conditions that showed the highest levels of leachate color reduction were repeated in triplicate in order to certify the most favorable condition to Fenton treatment and to use its sludge of Fe3+ decanted as the catalyst of the Fenton-like reactions. The procedure to perform the three conditions most favorable to the Fenton reaction was similar to that previously mentioned, with the difference that the amount of raw sample used in the reactions was changed from 50 mL to 250 mL. The amounts of H2O2 and FeSO4 increased proportionally. 56 mL of 3% H2SO4 and 4.0 mL of 1 mol/L NaOH, respectively, was required to adjust the pre and post reaction pH.
2.3. Definition of condition Eleven initial Fenton reaction conditions (Table 1) were determined by means of previous studies carried out with the same leached material. The conditioning factors were the concentrations of H2O2 and FeSO4. As the response variable was considered the leachate color reduction after the reaction, at all stages. The condition that guaranteed the greatest reduction in leachate color at the end of the treatment was used for the establishment of a new set of eleven conditions (Table 2) in order to enhance the interaction of the reactants during the reaction. The Fenton-like process was performed by two different methods: in the first, eight different conditions were defined (Table 3) but in this case, the conditioning factors were H2O2 and FeCl3 concentrations. The level of these conditions was determined from previous experiments; in the second, the iron sludge decanted after the end of the best Fenton reaction condition was used as the iron ion source of the Fenton-like
2.5. Fenton-like reaction In the first method which the Fenton-like process was performed, it was used FeCl3 as a source of Fe3+, using analytical grade reagent. It was added 50 mL of raw leachate in a Becker and, H2O2 and FeCl3 (Table 3) in sequence. The pH of the solution was corrected to 3.0 using 800 μL of H2SO4, and the reaction last 4 h. The solution was stirred with magnetic bars, at a temperature of 20 °C. In the second method, the Fenton-like process was generated by reusing all the Fe3+ sludge produced in the previous Fenton reaction (approximately 0.12 g) with the highest level of color reduction. The same concentration of H2O2 used in this condition was used in a cycle of reuse of the iron sludge decanted of five stages. To obtain the sludge, after the completion of the Fenton reaction and adjustment of the pH, 20 μL of the catalase enzyme solution was added and the solution was kept for 24 h in Falcon tubes without a cap, to allow the release of the residual hydrogen peroxide. After the rest period, the samples were centrifuged for five minutes at 4000 rpm and the decanted sludge of Fe3+ was collected. This sludge was combined with a small volume of
Table 2 Fenton's reaction conditions and color reduction for the second stage (Raw leachate color of 421Cu). (Id.)
F1” F2” F3” F4” F5” F6” F7” F8” F9” F10” F11”
Concentration (mg/L)
Color reduction (%)
H2O2
FeSO4
2961.5 2961.5 16538.6 16538.6 0.0 19185.3 9909.5 9909.5 9909.5 1021.3 1021.3
2553.2 878.3 2553.2 878.3 1307.3 1307.3 0.0 661.6 1307.3 133.6 133.6
97.2 89.5 97.6 90.2 65.9 77.1 54.0 92.2 92.1 91.3 92.1
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raw leachate, stirred for about ten seconds on the Vortex-type shaker, and transferred to a Becker, ready for reuse. Subsequently, 250 mL of the raw leachate was added to the Becker, and then the H2O2 was added. The initial pH of the solution (7.3 ± 0,0) was adjusted to the value of 3.2 ± 0.09 by the addition of a mean of 4.4 ml of 3% H2SO4. The reaction occurred for four hours. At the end of the reaction, the pH (3.6 ± 0,2) was again corrected to the value of 7.2 ± 0.12 by inserting 1.80 mL of 1 mol/L NaOH in the solution. After that, the solution was centrifuged at 4000 rpm for 5 min. The color of the treated leachate was determined, and the decanted material was used to generate a new Fenton-like reaction under the same conditions. This process was repeated four more times. The conductivity of the treated leachate was not determined since the main goal of this work was to reduce leachate color, avoiding its reuse, being discarded at the end of each treatment. 2.6. Kinetic study The most favorable Fenton and Fenton-like reactions condition were also used to perform a kinetic analysis, monitoring reaction product color over time, in order to verify the possibility of greater efficiency of the treatments that justified the application for a longer period of time. In this manner, 10 mL samples were taken at times 2, 4, 6 and 8 h from the beginning of the reactions. The pH of the samples was adjusted to 7.0 before centrifugation for 5 min, and the color of the leachate at each interval of time was performed as described previously. 2.7. Toxicity assessment The toxicity test was performed with lettuce seeds (Lactuca sativa) and followed the procedures of the United States Environmental Protection Agency [60,61], method "Seed Germination / Root Elongation Toxicity Test". Five 100 cm x 15 cm Petri dishes, each containing a layer of filter paper and previously sterilized, were used for each treatment. The treatments evaluated were raw leachate (before being treated), leachate treated by Fenton reaction, leachate treated by Fenton-like reaction and control treatment, with distilled water in place of the sample. The residual H2O2 concentration in the Fenton and Fenton-like treatments were removed by the addition of catalase, thus, it did not affect the toxicity of the treated leachate. Two mL of sample and five seeds of L. sativa Buttercrunch were added to each plate on the filter paper. The plates were wrapped in a plastic film to prevent loss of moisture to the medium and were kept in the dark for five days at room temperature (17 °C). The variables considered in the analysis of the ¯ ), root length results were the mean number of germinated seeds (GS ¯ ). The Germination Effect (%GE), Root Growth ¯ ) and stem length (SL (RL Inhibition (%RGI) and Germination Index (%GI) were calculated by applying Eqs. 3–5, respectively.
%GE =
%RGI =
Number of germinate seeds × 100 Total number of seeds
Fig. 2. Leachate color reduction over time. A) Fenton process; B) Fenton-like process. Note: Raw leachate color: 462 Cu.
and for this reason, it is important to evaluate them. The analysis of the leachate color reduction over the eight-hour of Fenton and Fenton-like reactions can be seen in the graphs of Fig. 2. Analyzing the graphs of Fig. 2, a trend of efficiency increase of the treatment over time can be observed. In both cases (A and B) it is observed that a plateau is reached after approximately two hours of reaction and no changes in the efficiency are observed. The average efficiency of Fenton treatment was 94.12%, while Fenton-like presented an average of 50.67%. Thus, the Fenton process showed better results than Fenton-like for leachate color reduction and treatment stability. The efficiency of landfill leachate treatment is generally evaluated based on before and after treatment analyzes. The parameters COD, BOD5, and Color are routinely determined in this kind of research [12]. In addition, leachate toxicity analysis is important for a more complete assessment of the quality of treatment. For these reasons, the present study evaluated the raw and treated leachate by the analyzes described in Table 5, and the toxicity by the test with lettuce seeds (Table 6). The three conditions that resulted in the greatest leachate color reduction (F3', F1” and F3”) were redone in triplicate. In this case, the most favorable condition to the Fenton reaction was the F3' (Table 4). Fig. 3 shows the raw leachate next to the treated leachate in the F3' condition in triplicate, after the centrifugation process, stored in Falcon tubes. The decanted sludge in this condition (F3') was reused in the
(3)
Mean control root growth − Mean sample root growth × 100 Mean control root growth (4)
%GI =
Sample root length × Number of germinated seeds in the sample Control root length × Number of germinated seeds in the control (5)
3. Results and discussion
Table 4 Color Reduction of the three best Fenton reaction conditions.
3.1. Treatments efficiency
Id ¯ (%) CR
The time that the solutions are maintained reacting is a parameter that can influence the efficiency of the Fenton and Fenton-like processes
F3' 98.1
Note: Raw leachate color: 469.5 Cu. 4
F1” 93.1
F3” 98.1
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Table 5 Comparative analysis between raw leachate, treated by Fenton, treated by Fenton-like and standards described by Brazilian legislation. Parameter
Raw Leachate
Fenton
FL
430/11*
Iron (mg/L Fe) BOD (mg/L O2) COD (mg/L O2) Ammonia (mg/L N) Coliforms - E. Coli Phosphor (mg/L P)
0.3 43.3 485.1
6.7 16.7 (61%) 414.0
24.1 16.7 (61%) 475.0
Unt. 15.0 Min. 60% rem. NS
< 0.2 5.0 < 0.5
< 0.2 0.0 < 0.5
< 0.2 0.0 < 0.5
Unt. 15.0 NS NS
E. coli given in colony forming units per 100 mL sample; Raw = Leached before treatment; FL = Fenton-like; rem = removal; NS = Not specified. Note:*CONAMA Legislation - National Council of the Ministry of Environment. Table 6 Toxicity analysis for the tested treatments.
Control Raw Leachate Fenton Fenton-like
(%GE)
(%RGI)
(%GI)
100 72 96 50
0.0 95.5 68.2 77.3
100 3.3 30.5 11.4
Fig. 4. Removal of color by A) Fenton-like with FeCl3 (Raw leachate color: 398Cu) and B) reuse of iron sludge (Raw leachate color: 408.5 Cu in the 1 st stage; 391.5Cu in the 2nd stage and 369.5Cu in the 3rd, 4th and 5th stage of the cycle).
Fig. 3. Triplicate of treated leachate in the F3' condition (B, C, D) compared to raw leachate (A).
Fenton-like (FL) reaction cycle, which resulted in the values presented in Fig. 4 as described in item 2.5. The first stage of the cycle showed the best efficiency in reducing the leachate color, probably because in the following stages a continuous loss of iron ions occurs, which can seriously affect the efficiency of the Fenton and Fenton-like processes [53]. According to the work done by Quadrado and Fajardo (2017), the reuse of iron ions can be employed in at least five consecutive processes of the studied dye discoloration, without losing efficiency or catalytic stability [53]. The results obtained by the Fenton-like reaction carried out with the FeCl3 solution as a catalyst can be also observed in Fig. 4. In general, the Fenton-like reaction cycle that used iron sludge as the source of Fe3+ ions proved to be more efficient than the process that used FeCl3 solution, as shown in Fig. 4, where the five conditions that resulted in the greater reduction of the leachate color by the FeCl3 solution process and the cycle of five reactions with the decanted iron sludge are compared. Fig. 5 shows the raw leachate next to the treated leachate in the FL3' (1) condition, in triplicate, after centrifugation. Thus, conditions F3', FL3'(1) and raw leachate were compared to the conditions for effluent discharges into Class 2 rivers, described by CONAMA Resolution 430/2011 [62] in many parameters, which can be seen in Table 5. The Fe3+ sludge generated in the Fenton reaction was used as a
Fig. 5. Triplicate of the treated leachate in the FL3' (1) (B,C, D) condition compared to the raw (A).
catalyst for the Fenton-like reaction, to promote raw material savings and, consequently, reduce the cost of the treatment. However, in evaluating the results obtained, it is noticeable that the Fenton-like process does not achieve the same efficiency of the Fenton. This fact can be explained by the lower oxidative capacity that the radical HO2•, generated in the Fenton-like reaction, presents when compared to the radical •OH, generated in the Fenton reaction [25]. According to the work done by Jung et al. (2017) [38], the Fenton process exhibits a high treatment performance in the optimal conditions because •OH can 5
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Considering both variables (number of germinated seeds and root length of germinated seeds), the raw leachate revealed to be more toxic to L. sativa than the leachate treated by Fenton-like, with a GI almost 3.48 fold lower. It is possible to notice that all the treatments have some level of toxicity since they differed from the control, which proved not to be toxic to the lettuce, according to the analysis of the number of germinated seeds. The highest level of significance was observed between the control group and raw averages, as expected. Fernandes et al. [27] performed the same toxicity test for lettuce with the objective of evaluating the removal of azo dyes after treatment of the effluent by Fenton and Fenton-like and reached the result that, after 24 h of samples exposure, lettuce mortality levels were in the range of 0 to 45%, suggesting a generation of toxic degradation products, considering that in most cases, the degradation of the dye was almost 100%. The reason why Fenton oxidation is more effective in removing toxicity from leachate compared to Fenton-like may be due to two factors: the hydroperoxylic radicals generated, which also attack organic contaminants, but are less sensitive than the hydroxyl radicals. Considering that iron added in small amounts acts as a catalyst, while H2O2 is continuously consumed to produce hydroxyl radicals [59], the second reason may be due to the fact that no further doses of iron were added in the Fenton-like process, only the Fe3+ sludge from the Fenton process was reused, which may have hindered the interaction between the iron ions and the H2O2, thus inhibiting the catalytic action of the reaction.
effectively decompose the dissolved hydrophobic and hydrophilic organic matter [38]. Duarte et al. [41] achieved an 80% removal of COD from the contaminated effluent studied, by the Fenton reaction treatment, which was as efficient as the electrochemical treatment employed, however, faster [41]. The treatment of dyes by Fenton compared to Fenton-like, however, has shown opposite results. In the research conducted by Fernandes et al. [27] the results indicated that the Fenton-like type process is superior to Fenton to degrade Disperse Red 343, a type of azo dye [27]. Hu et al. [28] showed that the degradation rate of malachite green, a toxic chemical used as a dye, was 59.34% in the Fenton process and 92.7% in the Fenton-like process [28]. The high iron ion content present in the treated leachate samples, especially after the Fenton-like process, is shown in Table 5. Although the contents are in accordance with the legislation in the case of the Fenton oxidation, iron can become a difficulty for the application of the treatment, considering that the process itself is based on the use of Fe2+ and Fe3+ ions as catalysts of the main reaction, substantially increasing the concentration of these elements in the treated effluent. The Fenton and Fenton-like reactions are strongly dependent on three main factors: pH of the solution, iron ion concentration, and hydrogen peroxide concentration. The reaction temperature is a parameter that showed no significant levels of influence on the efficiency of the treatments [25]. The optimum pH determined for the Fenton reaction was approximately 3.0, regardless of the target substrate [59], and about 4.5 for Fenton-like reaction [37]. Because of this reason, the pH was carefully adjusted in each of the solutions carried out in this research, resulting in standardized pH solutions. Based on this premise, pH was not a determinant of the efficiency of the Fenton and Fentonlike reaction tests performed. Typically, the rate of organic and inorganic compounds degradation increases as the ferrous ions concentration increase. However, excess ions may lead to an undesired amount of unused iron salts, which can generate a high amount of dissolved solids contained in the effluent flow, affecting the treatment quality [59,25]. The H2O2 concentration also has a crucial role in the overall efficiency of the pollutant degradation process by the Fenton and Fenton-like reactions. It has been observed that degradation increases with the H2O2 dosage increase [25]. However, care should be taken with the added concentration, since the unused portion of H2O2 during the process contributes to the COD increase. Therefore, in excess, it is not recommended [59].
4. Conclusion Concerning the landfill leachate color reduction, the Fenton process presented more satisfactory results (98.1% of color reduction) than the Fenton-like process (less than 70% of color reduction). Except for iron analysis, a reduction in all the parameters analyzed in the raw leachate and treated by Fenton and Fenton-like were observed, obtaining better results in the Fenton reaction. The toxicity analysis showed that, whereas both treatments reduced the toxicity of Landfill Leachate, Fenton has a lower toxic potential than Fenton-like. As an improvement to the research, it is suggested that a larger number of both reactions conditions should be performed, so that even more satisfactory results can be found. It is also suggested that the influence of pH on the efficiency of the reaction is analyzed. It would be interesting if the Fenton and Fenton-like processes were compared in efficiency and cost with other Fenton variations, such as photo-Fenton. It is also suggested that a study be carried out on the operational and economic viability of the application of Fenton and Fenton-like treatments on a real scale.
3.2. Toxicity analysis It was decided to determine the ecological effects of the toxic substances present in the raw and treated leachate through the analysis with Lactuca sativa because this is a plant species used for phytotoxicity tests and its use for this purpose has been recommended by many international organizations [63]. Besides, tests using L. sativa are simple, fast, inexpensive, as they do not require expensive equipment, and are as reliable as the widely known and commonly used Allium cepa [64]. In the toxicity test of the leachate treated by the Fenton and Fentonlike reactions, samples were taken of the conditions in which 847.6 mg/ L of H2O2 and 696.8 mg/L of FeSO4 were added to the Petri dishes. The ¯ ) and root length (RL ¯ ) and average number of germinated seeds (GS ¯ ) of each situation were analyzed. Table 6 shows the arithmetic stalk (SL average of the toxicity test results. The treatments applied were the control, raw (before treatment), Fenton and Fenton-like. The number of lettuce germinated seeds and root lengths are parameters considered sufficient for toxicity analysis of a substance, as was done by Lyu et al. [63]. The leachate treated by the Fenton process showed to be less toxic to L. sativa as compared to raw leachate or the treated by Fenton-like process, with a GI value closer to 1, considered to be ideal (Priac et al., 2017) [1]. Although raw leachate had a higher germination index than that treated by Fenton-like, it exhibited higher levels of inhibition.
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