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Co2+ oxidation as an advanced oxidation process: Comparison with traditional Fenton oxidation for treatment of landfill leachate
water research 43 (2009) 4363–4369
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Oxone/Co2D oxidation as an advanced oxidation process: Comparison with traditional Fenton oxidation for treatment of landfill leachate Jianhui Sun, Xiaoyan Li, Jinglan Feng*, Xiaoke Tian College of Chemistry and Environmental Sciences, Henan Normal University, Henan Key Laboratory for Environmental Pollution Control, Xinxiang, Henan 453007, PR China
article info
abstract
Article history:
In this paper, the application of Fenton and Oxone/Co2þ oxidation processes for landfill
Received 20 February 2009
leachate treatment was investigated. The removal of the chemical oxygen demand (COD),
Received in revised form
suspended substances (SS) and the color of the landfill leachate by Fenton oxidation to that
15 June 2009
by Oxone/Co2þ oxidation were compared under optimal operational conditions. For Fenton
Accepted 20 June 2009
oxidation process, the optimal conditions were determined as: [H2O2] ¼ 80 mmol L1,
Published online 25 June 2009
[H2O2]/[Fe2þ] ¼ 2.0, initial pH ¼ 2.5, reaction temperature ¼ 37.5 1 C, reaction time -
Keywords:
removal efficiency was achieved, but the SS and the color of the treated landfill leachate
Oxone/Co2þ
increased due to the generation of a large quantity of ferric hydroxide sludge. In reference
Fenton
to Oxone/Co2þ oxidation process, the optimal conditions were determined as: [Oxo-
¼ 160 min, number of stepwise addition ¼ 3. Under the given conditions, 56.9% of the COD
Landfill leachate
ne] ¼ 4.5 mmol L1, [Oxone]/[Co2þ] ¼ 104, pH ¼ 6.5, reaction temperature ¼ 30 1 C, reac-
AOPs
tion time ¼ 300 min, number of stepwise addition ¼ 7, the COD, SS and the color removal efficiencies were 57.5, 53.3 and 83.3%, respectively. From this work, it can be concluded that Oxone/Co2þ oxidation process demonstrated higher degradation efficiencies of the COD, SS and color for landfill leachate treatment than that by Fenton oxidation process. It also suggested that Oxone/Co2þ oxidation process could be considered as one of the most promising technologies for practical applicability to treat landfill leachate in large scale. For further improving the efficiency of Oxone/Co2þ oxidation process, we proposed that combination of it with other technologies in future such as ultraviolet, ultrasound and biological methods. ª 2009 Elsevier Ltd. All rights reserved.
1.
Introduction
In China, 160 million tons of municipal solid wastes (MSW) were produced every year, and about 80% of them were buried in the ground (Feng et al., 2003). One of the problems arising from this form of waste disposal was the generation of leachate. Landfill leachate, an aqueous effluent, generated by
the decomposition of landfill wastes and supplemented by rainwater percolating through waste materials. Landfill leachate is generally characterized by a high strength of pollutants, such as organics, ammonium, inorganic salts and heavy metals (Horan et al., 1997). In order to minimize the risks of contamination and insure the safety of receiving media, suitable treatment must be conducted to the leachate.
* Corresponding author. Tel.: þ86 373 332 5971; fax: þ86 373 332 6336. E-mail addresses: [email protected] (J. Sun), [email protected] (X. Li), [email protected] (J. Feng), xktian@ yahoo.com.cn (X. Tian). 0043-1354/$ – see front matter ª 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.watres.2009.06.043
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The biological treatments including aerobic and anaerobic processes have been indicated to be very effective for the young age leachate because of a high value of BOD/COD in the early period, however, the ratio generally decreased with the increasing of the age of landfill (Chian and DeWalle, 1977). As a result, the biological treatments might become ineffective for the treatment of old landfill leachate (Medez et al., 1989). In recent years, many physical/chemical processes have been investigated for the treatment of old or refractory landfill leachate. A group of available treatment technology of these processes widely used is the so-called advanced oxidation processes (AOPs). AOPs have demonstrated to be innovative suitable technologies to reduce organic load or toxicity and enhance biotreatability of landfill leachate, because they are able to oxidize refractory organic compounds into harmless substances and even lead to mineralization end-products such as carbon dioxide and water (Pe´rez et al., 2002; Gogate and Pandit, 2004; Pera-Titus et al., 2003; Chidambara and Quen, 2005). Therefore, several countries in the world had begun to use AOPs to deal with landfill leachate directly or indirectly (Kim et al., 2001; Yoo et al., 1998). Fenton oxidation process is one of such AOPs approaches, which is currently applied to treat landfill leachate. Fenton’s reagent, firstly discovered by Fenton in the 1890s (Fenton, 1894), is an aqueous mixture of Fe2þ and H2O2 that produces OH. Due to its high oxidation power, Fenton oxidation process stands out among the most promising AOPs for the treatment of refractory organic pollutants wastewater. In the last few decades, using Fenton oxidation process for the treatment of landfill leachate has been widely reported (Loizidou et al., 1993; Gau and Chang, 1996; Yoon et al., 1998; Kim et al., 2001; Lopez et al., 2004; Rivas et al., 2004; Zhang et al., 2005; Gotvajn and Tisler, 2008; Zhang et al., 2009). However, Fenton reaction has some limitations in the practical application, such as the requirement for acidic pH, large quantity of chemical reagents, large production of ferric hydroxide sludge, and very slow catalysis of the ferrous ions generation (Walling and Goosen, 1973; Arnold et al., 1995; De Laat and Gallard, 1999; Rivas et al., 2001). Oxone, commercial name of potassium peroxomonosulfate (2KHSO5 $ KHSO4 $ K2SO4), is a good source of generating a strong oxidant peroxymonosulfate (HSO 5 ). Since has a higher oxidation potential than H2O2 HSO 5 (CoOHþ þ HSO5. / CoOþ þ SO. E0 ¼ 1.82 V/SHE 4 þ H2O, versus H2O2 þ 2Hþ þ 2e / 2H2O, E0 ¼ 1.76 V/SHE), Oxone demonstrates to be a suitable oxidant for the decomposition of dyes and organic compounds (Renganathan and Maruthamuthu, 1986; Campaci and Campestrini, 1999; Legros et al., 2001; Yu et al., 2006). For the activation of Oxone, Co2þ has been found as the most efficient transition metal among Agþ, Ce3þ, Co2þ, Fe2þ, Fe3þ, Mn2þ, Ni2þ, Ru3þ and V3þ (Anipsitakis and Dionysiou, 2003). Coupling of Oxone with Co2þ (Oxone/Co2þ) has recently demonstrated greater efficiencies and several operational advantages compared to Fenton reagent for treatment of certain organic wastewater in the homogeneous system (Bandala et al., 2007). Most importantly, it is possible to use the Oxone/Co2þ with no pH adjustment after pre-treatment at biocompatible pH-values (Yu et al., 2006). This is an important advantage considering that water pre-treatment is followed generally by the low cost biological
treatment. Rivas et al. (2005) have reported that landfill leachate could be effectively degraded by Oxone-promoted wet air oxidation. However, the using of Oxone/Co2þ for resistant wastewater treatment was not investigated. A quantitative study of the effect of the operating conditions on the oxidation efficiency of Oxone/Co2þ in treatment of landfill leachate has not yet been reported in the literature. The aims of this study were: (i) to assess the performance of Fenton and Oxone/Co2þ in the treatment of landfill leachate; and (ii) to investigate the influence of some very important operating parameters, such as pH, temperature, dosages of reagents, reaction time, and addition modes of reagents. The efficiency of each experiment was evaluated by measuring the COD value, and the performance of the two different oxidation processes were compared. Then we further established cost-efficient operating conditions for the potential application of Fenton and Oxone/Co2þ oxidation process to treat landfill leachate. The results can provide fundamental knowledge for the treatment of landfill leachate by both treatment processes.
2.
Materials and methods
2.1.
Landfill leachate
Landfill leachate samples were collected with polyethylene bottles from municipal sanitary landfill, which were located in Xinxiang, Henan Province, China. Samples were stored in a refrigerator at 4 C and used without any previous treatment. The main parameters (average values) of samples were: pH ¼ 8.17, COD ¼ 1116 mg L1, ammonia nitrogen (NH4– N) ¼ 25.4 mg L1, suspended substances (SS) ¼ 17 mg L1, color ¼ 300 times.
2.2.
Chemical reagents
Hydrogen peroxide (30%, w/w), ferrous sulfate (FeSO4 $ 7H2O), cobalt sulfate (CoSO4 $ 7H2O), dipotassium titanium oxide dioxalate (K2TiO(C2O2)2), sulfuric acid, and sodium nitrite (NaNO2) were obtained from Shanghai Chemical Reagents Co. (Shanghai, China). Oxone (95%, Aldrich, manufactured by DuPont) was purchased from Shunyi Development Co. (Guangdong, China). All chemicals used in this study were analytical grade. Deionized water was used throughout this study.
2.3.
Experimental procedures
All these experiments were carried out in a 200 mL double glass cylindrical jacket reactor, allowing cycle water to maintain the temperature of reaction system. Temperature was adjusted by a thermostat and a magnetic stirrer was used to stir reaction solutions (stirring rate was 280 rpm). To start each test, 100 mL landfill leachate was placed in double glass cylindrical jacket reactor where appropriate volumes of stock activator were put into the reactor. The initial oxidant dosage was based on the stoichiometric ratio with respect to the COD value assuming complete oxidation. For Fenton oxidation process, the pH value of each reaction solutions was adjusted
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to the desired level using the prepared 4 mol L1 sulfuric acid or 4 mol L1 sodium hydroxide solutions. After that, predetermined amounts of hydrogen peroxide was added to the reactor and magnetically stirred during the reaction time. As for Oxone/Co2þ oxidation process, since the pH would drop sharply after Oxone addition (Yu et al., 2006), we first added relevant volumes of Oxone solution to the reactor which already have appropriate volumes of landfill leachate and stock activator, then adjusted pH to the desired level rapidly. All samples were taken out from the reactor using a pipette, then adjusting pH value to terminate Fenton oxidation reactions and adding a proper amount of NaNO2 solution to terminate Oxone/Co2þ oxidation reactions within 1 min. Following this, samples were filtered through 0.45 mm filter paper. To eliminate the interference of H2O2 in COD measurement, H2O2 concentration was determined spectrophotometrically using titanium oxalate (Sellers, 1980; Gallard and De Laat, 2000), then deducted from COD calculation. Blank experiment was carried out simultaneously.
2.4.
Fenton oxidation
40
oxone/Co
2+
oxidation
35 30 25 20 15 10 5 0
0
1
2
3
4
5
6
7
8
9
pH Fig. 1 – The effect of pH on COD removal. Experimental conditions: temperature [ 20 ± 1 8C; reaction time [ 60 min; [H2O2] [ 50 mmol LL1 [Fe2D] [ 25 mmol LL1 [Oxone] [ 4 mmol LL1 and [Co2D] [ 0.4 mmol LL1.
Analytical methods
The pH value of solutions was measured by a pHS-3C digital pH meter. Before measurement, pH meter was calibrated with standard buffers (pH 4.0, 6.86 and 9.18) at 25 C. The COD was measured by 5B Intelligent CODCr Fast Measuring Instrument (Lanzhou, China), which was adjusted with potassium hydrogen phthalate standard solution. Ammonium nitrogen, color and SS were respectively determined according to the National Standard Method of Water quality – determination of ammonium Nessler’s reagent colorimetric method (China GB7479-87), water quality – determination of color (China GB11903-89) and water quality – determination of suspended substance – gravimetric method (China GB11901-89).
3.
45
Results and discussion
3.1.
Optimization of system parameters
3.1.1.
Effect of initial pH
The initial pH effect on both processes was tested to determine an experimental condition for further research. In this study, optimum pH that resulted in the highest COD removal was determined by running the reactor under varying pH while the other operating parameters were kept constant. pH was adjusted in the interval from 1 to 8 (note that the pH changed at the end of the experiment by approximately 0.5 unit). With changes of the initial pH, the corresponding COD removal efficiencies are presented in Fig. 1. As shown in Fig. 1, the COD removal strongly depended on the initial pH in both of the processes. The COD removal efficiency was slightly influenced by pH from 2 to 6 in Fenton oxidation process and pH from 5 to 7 in Oxone/Co2þ oxidation process. COD removal efficiency rapidly decreased with increasing pH above 6 in Fenton oxidation and above 7 in Oxone/Co2þ oxidation. Therefore, the best initial pH value was 2.5 in Fenton oxidation and 6.5 in Oxone/Co2þ oxidation, with achieving COD removal efficiencies peak above 36.5 and 41.0%, respectively.
Moreover, Fig. 1 illustrates that Oxone/Co2þ oxidation process gave higher degradation efficiency at elevated pH (Chu et al., 2007) and lower COD removal at pH 4. This is because Oxone is a strong oxidizing agent and sulfate radicals, generated during Oxone decomposition, demonstrate comparable standard reduction potential to hydroxyl radicals at neutral pH. At the same time, low concentrations of HCO 3 and H2PO 4 can produce complex compound (bicarbonate-Co and peroxyphosphate-Co) which may become active sites in Oxone/Co2þ oxidation reaction for pH from 5 to 7 (Anipsitakis when pH et al., 2005). Similarly, the sheer quantity of SO2 4 below 4 can also improve Oxone/Co2þ oxidation.
3.1.2.
Effect of temperature
At optimum pH, six different temperature values between 20 and 45 C were tested for both processes. Then two neighboring spots of the optimum point were examined in order to find the most suitable temperature. Other experimental conditions apart from temperature were kept the same. Results are depicted in Fig. 2. It was concluded from Fig. 2 that 37.5 and 30 C were found to be the most suitable temperature in Fenton oxidation process and Oxone/Co2þ oxidation process, respectively; and the corresponding COD removal efficiencies achieved 50.6 and 42.2%. As for Fenton oxidation process, the COD removal efficiency slightly increased up to 50.6% when temperature was lower than 37.5 C. But as temperature increased from 37.5 to 45 C, the COD removal efficiency decreased by 4.3% instead of increased. So, the temperature was not the higher the better. One reason was that the rates of side-reactions may be accelerated dramatically at high temperature, another reason was that the high temperature caused a sharp reduction of dissolved oxygen which can rapidly and irreversibly react with carbon-centered radicals (R.) in water. For Oxone/Co2þ oxidation process, the COD removal efficiency increased by 1.6% as temperature increased from 20
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52
Fenton oxidation 2+
oxone/Co
50
oxidation
COD removal (%)
48
45
40
44 40 36 32 28 Fenton oxidation
35
2+
24 20
25
30
35
40
oxone/Co
0
45
Temperature (.C) Fig. 2 – The effect of temperature on COD removal. Experimental conditions: pH [ 2.5 (Fenton), pH [ 6.5 (Oxone/Co2D); reaction time [ 60 min; [H2O2] [ 50 mmol LL1; [Fe2D] [ 25 mmol LL1; [Oxone] [ 4 mmol LL1 and [Co2D] [ 0.4 mmol LL1.
to 30 C but decreased by 2.15% as temperature increased from 30 to 40 C. The influence of temperature on Oxone oxidation process was less pronounced than that observed on Fenton oxidation process. Overall, COD removal efficiency was slightly influenced by the temperature in Oxone/Co2þ oxidation process, and the COD removal efficiency at room temperate was very similar to that at optimum temperature. Therefore, Oxone oxidation is more useful than Fenton oxidation by considering the optimal pH and the most suitable temperature, which is more easily to be achieved in practical application.
3.1.3.
Effect of reagents’ dosage
Like other AOPs, Fenton oxidation process and Oxone/Co2þ oxidation process were affected by [oxidant]/[activator] molar ratio and the concentration of oxidant and activator. Optimum [oxidant]/[activator] molar ratio was determined by running the reactor under six different [oxidant]/[activator] molar ratio at the optimum pH and the most suitable temperature for both of the processes. Then we examined two neighboring points of the optimum point to find the best value. Other experimental conditions except the [oxidant]/ [activator] molar ratio were kept the same. Results are illustrated in Fig. 3, which indicate that COD removal widely varied as the [H2O2]/[Fe2þ] molar ratio increased. When we increased [H2O2]/[Fe2þ] molar ratio above 2, the COD removal of landfill leachate dropped sharply. This may have been caused by the scavenging effect of excessive H2O2 to OH. Therefore, a suitable molar ratio of [H2O2]/[Fe2þ] for treatment of landfill leachate was experimentally determined as 2, with the corresponding COD removal efficiency was 50.6%. As for Oxone/Co2þ oxidation process, the changes of COD removal efficiency was not as obvious as in Fenton process, it only increased from 42.2 to 50.0% as [Oxone]/[Co2þ] increased from 10 to 104. But with an increase of [Oxone]/[Co2þ] molar
1
oxidation
2
3
4
5
6 2+
Molar ratio of [H2O2]/[Fe2+] and lg[oxone]/[Co
]
Fig. 3 – The effect of different [oxidant]/[activator] molar rations on COD removal. Experimental conditions: pH [ 2.5 (Fenton), pH [ 6.5 (Oxone/Co2D); temperature [ 37.5 ± 1 8C (Fenton), temperature [ 30 ± 1 8C (Oxone/Co2D); reaction time [ 60 min; [H2O2] [ 50 mmol LL1and [Oxone] [ 4 mmol LL1.
ratio above 105, the COD removal of landfill leachate dropped. Thus, the most suitable [Oxone]/[Co2þ] was 104 and the COD removal efficiency of landfill leachate was 50.0%. The effect of H2O2 dosage on COD removal of landfill leachate is examined by varying concentration of H2O2 from 10 to 100 mmol, and the COD removal efficiency are shown in Fig. 4. From Fig. 4, it can be concluded that increasing the dosage of H2O2 from 10 to 80 mmol could enhance the COD
60
0
2
4
6
8
10
50
40
30
Fenton oxidation (Bottom X-Axis)
20
Oxone/Co
0
20
40
60
2+
oxidation (Top X-Axis)
80
100
Oxidant dosage (mmol L-1) Fig. 4 – The effect of different dosages of oxidant on COD removal. Experimental conditions: Fenton: pH [ 2.5; temperature [ 37.5 ± 1 8C; [H2O2]/[Fe2D] [ 2.0; reaction time [ 60 min; Oxone/Co2D: pH [ 6.5; temperature [ 30 ± 1 8C; [Oxone]/[Co2D] [ 104; reaction time [ 60 min.
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removal efficiency from 20.3 to 53.0% within 60 min. However, when increasing the dosage of H2O2 from 80 to 100 mmol, the COD removal efficiency was only improved by 1.0%. Therefore, 80 mmol L1 was selected as the suitable H2O2 dosage. Just like the H2O2 plays a very important role as a source of .OH generation in Fenton’s reaction (Sun et al., 2009), so does oxidation reaction the Oxone in Oxone/Co2þ þ þ (CoOH þ HSO5 / CoO þ SO. 4 þ H2O). Oxone dosage can be critical to landfill leachate degradation while overdosing could be a potential problem theoretically to deteriorate the process similar to that of H2O2 in Fenton oxidation process (Chu et al., 2005). In this study, various Oxone concentrations were introduced into the reaction system. As shown in Fig. 4, a higher COD removal efficiency of landfill leachate was observed when more Oxone was introduced. By increasing the dosage of Oxone from 1 to 4.5 mmol, the COD removal efficiency increased from 35.5 to 50.4% within 60 min. However, when further increasing the dosage of Oxone from 4.5 to 8 mmol, the COD removal efficiency was just slightly improved by 1.3%. Thus, 4.5 mmol of Oxone was considered to be a suitable dosage for the treatment of landfill leachate, and the corresponding COD removal efficiency was 50.4%. The optimal molar ratio of [H2O2]/[Fe2þ] and [Oxone]/ [Co2þ] were 2 and 104, respectively, and the COD removal efficiency of landfill leachate achieved 50% for both systems. Suitable dosage of H2O2 and Oxone were selected as 80 and 4.5 mmol, respectively, in each liter landfill leachate. The results showed that the addition of Co2þ at concentration 4.5 104 mmol per liter landfill leachate were sufficient to mediate Oxone decomposition at a concentration of 4.5 mmol. Thus the potential toxicity of Co2þ may be overcome. The dosage of oxidant and catalyst used in Oxone/Co2þ oxidation process was much smaller than that used in Fenton process with a similar COD removal efficiency. Considering the dosage of reagents, pH adjustment, sludge treatment and so on, [Oxone]/[Co2þ] oxidation process was much cheaper than Fenton oxidation process (Anipsitakis and Dionysiou, 2003). Consequently, it is believed that [Oxone]/[Co2þ] reagent might be an alternative technology for treatment of landfill leachate.
3.1.4.
the suitable reaction time for the treatment of landfill leachate by Oxone/Co2þ was 300 min and the COD removal efficiency was 54.5%.
3.1.5.
Effect of addition modes of reagents
In addition to the above factors, the number of dosing steps was also found as an important factor affecting the COD removal of landfill leachate in both of oxidation process. Generally, reagents were added to landfill leachate in a singlestep. However, such addition may cause self-decomposition of oxidant due to high concentrations at the point of injection (Deng and Englehard, 2006). To ensure complete usage of oxidant for oxidation, we tested various numbers of stepwise addition from one to 10 times, and we added even reagent quantity in every step and kept the reagent total quantity invariable. The corresponding COD removal efficiencies are illustrated in Fig. 6. Based on the results, it can be concluded that three and seven times were found to be the best at the given conditions for Fenton oxidation and Oxone/Co2þ oxidation with corresponding COD removal efficiencies of 56.9 and 57.5%, respectively. It seems like the concentrations of H2O2 and Oxone were not the higher.
3.2.
Treatment of landfill leachate by different processes
As discussed in Section 3.1, the optimal operational conditions of Fenton’s reagent in treatment of landfill leachate were the following: [H2O2] ¼ 80 mmol L1, [H2O2]/[Fe2þ] ¼ 2.0, initial pH ¼ 2.5, reaction temperature ¼ 37.5 1 C, reaction time ¼ 160 min, number of stepwise addition ¼ 3. And for Oxone/ Co2þ oxidation process, the optimum operational parameters were: [Oxone] ¼ 4.5 mmol L1, [Oxone]/[Co2þ] ¼ 104, pH ¼ 6.5, reaction temperature ¼ 30 1 C, reaction time ¼ 300 min, number of stepwise addition ¼ 7.
0
80
160
240
320
400
480
55
Effect of reaction time
The effect of the reaction time on COD removal of landfill leachate for Fenton oxidation process was examined by varying time from 20 to 200 min, and the COD removal efficiencies are shown in Fig. 5. It can be seen that increasing the reaction time from 20 to 160 min could enhance the COD removal efficiency from 35.9 to 56.7%. However, when increasing the reaction time from 160 to 200 min, the COD removal efficiency was only increased by 0.3%. Therefore, a suitable reaction time for treatment of landfill leachate was experimentally determined as 160 min and the COD removal efficiency was 56.7%. Similarly, the effect of reaction time on COD removal of landfill leachate for Oxone/Co2þ oxidation process was examined by varying the time from 60 to 480 min and COD removal efficiency are also presented in Fig. 5. It can be observed that COD removal efficiency tended to increase as one oblique line within 300 min, and as another trend line between 300 and 480 min. And the inclination of two lines was that the former was far larger than the latter. Therefore,
50
45
40 Fenton oxidation (Bottom X-Axis) 2+
Oxone/Co
oxidation (Top X-Axis)
35 40
80
120
160
200
Fig. 5 – The effect of different reaction time on COD removal. Experimental conditions: Fenton: pH [ 2.5; temperature [ 37.5 ± 1 8C; [H2O2]/[Fe2D] [ 2.0; [H2O2] [ 80 mmol LL1; Oxone/Co2D: pH [ 6.5; temperature [ 30 ± 1 8C; [Oxone]/[Co2D] [ 104; [Oxone] [ 4.5 mmol LL1.
240
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58
4.
COD removal (%)
56
54
52
50 Fenton oxidation 2+
Oxone/Co oxidation 48
0
2
4
6
8
10
Fig. 6 – The effect of different dosing steps on COD removal. Experimental conditions: Fenton: pH [ 2.5; temperature [ 37.5 ± 1 8C; [H2O2]/[Fe2D] [ 2.0; [H2O2] [ 80 mmol LL1; reaction time [ 160 min; Oxone/ Co2D: pH [ 6.5; temperature [ 30 ± 1 8C; [Oxone]/ [Co2D] [ 104; [Oxone] [ 4.5 mmol LL1, reaction time [ 300 min.
Both treatment processes, Oxone/Co2þ and Fenton, were carried out under optimum conditions for the treatment of landfill leachate and the results are summarized in Table 1. It can be seen from Table 1 that the COD removal efficiencies were very similar, 56.9% in Fenton treatment and 57.5% in Oxone/Co2þ treatment. However, the value of SS and color of the landfill leachate increased after treatment by Fenton oxidation process. As for Oxone/Co2þ oxidation process, the removal efficiency of SS and color were 53.3 and 83.3%, respectively. From this work, it was concluded that Oxone/ Co2þ oxidation process demonstrated higher degradation efficiencies than that by Fenton oxidation process. In addition, there were a number of perceived benefits for Oxone/Co2þ oxidation becoming an alternative method to Fenton oxidation process. First, the optimum pH was close to neutral pH in Oxone/Co2þ oxidation process. Second, the most suitable temperature was 30 C, lower than that in Fenton treatment. Third, the suitable concentration of oxidant and catalyst in Oxone/Co2þ oxidation process was much lower than that used in Fenton process. Fourth, there was no sludge production in Oxone/Co2þ treatment. Herein, it also should be noted that Fenton process needs a shorter reaction time and less number of stepwise additions compared to Oxone/Co2þ process.
Table 1 – Comparison of Fenton and Oxone/Co2D processes under the optimum conditions respectively. pH COD removal (%) Fenton Oxone/ Co2þ
2.3 6.9
(þ)56.9 (þ)57.5
SS removal (%)
Color removal (%)
()233.3 (þ)53.3
()66.7 (þ)83.3
Conclusions
This paper provided the first comparative study on the Oxone/ Co2þ oxidation and Fenton oxidation processes for the treatment of landfill leachate, which is very difficult to be treated by common wastewater treatment methods. Under the conditions of neutral pH, low temperature and less dosage of oxidant and catalyst, Oxone/Co2þ oxidation process demonstrated higher degradation efficiencies of COD, SS and color than that by Fenton oxidation process. But a longer reaction time and more number of stepwise additions were the disadvantages of Oxone/Co2þ treatment. However, by considering all of these operating parameters: pH, temperature, dosage of oxidant and catalyst, reaction time and addition modes of reagents, it is suggested that Oxone oxidation has more advantages compared to Fenton process for practical applicability in large scale, which can be considered as one of the most promising technologies to treat landfill leachate. Moreover, for improving the efficiency of Oxone/ Co2þ oxidation process, it might be beneficial to further investigate its integration with other technologies (i.e., biological methods, ultrasound, and UV technologies) for more enhanced treatment of landfill leachate in the future studies.
Acknowledgement The research was supported by Key Scientific and Technological Project in Henan province, with Grant No. 0522030700 and Key Scientific and Technological Project of Henan province, People’s Republic of China (Grant No. 0611020900).
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process through a statistical technique. Chemical Engineering Science 60, 5305–5311. Chu, W., Kwan, C.Y., Chan, K.H., Kam, S.K., 2005. A study of kinetic modeling and reaction pathway of 2, 4-dichlorophenol transformation by photo-Fenton-like oxidation. Hazardous Materials 121, 119–126. Chu, W., Choy, W.K., Kwan, C.Y., 2007. Selection of supported cobalt substrates in the presence of oxone for the oxidation of monuron. Agricultural and Food Chemistry 55, 5708–5713. De Laat, J., Gallard, H., 1999. Catalytic decomposition of hydrogen peroxide by Fe(III) in homogeneous aqueous solution: mechanism and kinetic modeling. Environmental Science and Technology 32, 2726–2732. Deng, Y., Englehard, J.D., 2006. Treatment of landfill leachate by the Fenton process. Water Research 40, 3683–3694. Feng, Z.Y., Na, D., Hong, L.J., Zhong, X.C., 2003. A new pyrolysis technology and equipment for treatment of municipal household garbage and hospital waste. Renewable Energy 28, 2383–2393. Fenton, H.J.H., 1894. Oxidation of tartaric acid in the presence of iron. Chemical Society 65, 899–910. Gallard, H., De Laat, J., 2000. Kinetic modelling of Fe(III)/H2O2 oxidation reaction in dilute aqueous solution using atrazine as a model organic compound. Water Research 34 (12), 3107–3116. Gau, S.H., Chang, F.S., 1996. Improved Fenton method to remove recalcitrant organics in landfill leachate. Water Science and Technology 34, 445–462. Gogate, P.R., Pandit, A.B., 2004. A review of imperative technologies for wastewater treatment II: hybrid methods. Advances in Environmental Research 8, 553–597. Gotvajn, A.Zˇ., Tisˇler, T., Zagorc-Koncˇan, J., 2008. Comparison of different treatment strategies for industrial landfill leachate. Hazardous Materials, doi:10.1016/j.jhazmat.2008.06.037. Horan, N.J., Gohar, H., Hill, B., 1997. Application of a granular activated carbon-biological fluidised bed for the treatment of landfill leachates containing high concentrations of ammonia. Water Science and Technology 36, 369–375. Kim, J.S., Kim, H.Y., Won, C.H., Kim, J.G., 2001. Treatment of leachate produced in stabilized landfills by coagulation and Fenton oxidation process. Chemical Engineering 32, 425–429. Legros, J., Crousse, B., Bourdon, J., Bonnet-Delpon, D., Be´gue´, J.P., 2001. An efficient and robust fluoroketone catalyst epoxidation. Tetrahedron Letters 42, 4463–4466. Loizidou, M., Papadopoulos, A., Kapetanios, E.G., 1993. Application of chemical oxidation for the treatment of refractory substances in leachates. Environment Science and Health A: Toxic/Hazardous Substances and Environmental Engineering 28, 385–394.
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Oxone/Co2D oxidation as an advanced oxidation process: Comparison with traditional Fenton oxidation for treatment of landfill leachate Jianhui Sun, Xiaoyan Li, Jinglan Feng*, Xiaoke Tian College of Chemistry and Environmental Sciences, Henan Normal University, Henan Key Laboratory for Environmental Pollution Control, Xinxiang, Henan 453007, PR China
article info
abstract
Article history:
In this paper, the application of Fenton and Oxone/Co2þ oxidation processes for landfill
Received 20 February 2009
leachate treatment was investigated. The removal of the chemical oxygen demand (COD),
Received in revised form
suspended substances (SS) and the color of the landfill leachate by Fenton oxidation to that
15 June 2009
by Oxone/Co2þ oxidation were compared under optimal operational conditions. For Fenton
Accepted 20 June 2009
oxidation process, the optimal conditions were determined as: [H2O2] ¼ 80 mmol L1,
Published online 25 June 2009
[H2O2]/[Fe2þ] ¼ 2.0, initial pH ¼ 2.5, reaction temperature ¼ 37.5 1 C, reaction time -
Keywords:
removal efficiency was achieved, but the SS and the color of the treated landfill leachate
Oxone/Co2þ
increased due to the generation of a large quantity of ferric hydroxide sludge. In reference
Fenton
to Oxone/Co2þ oxidation process, the optimal conditions were determined as: [Oxo-
¼ 160 min, number of stepwise addition ¼ 3. Under the given conditions, 56.9% of the COD
Landfill leachate
ne] ¼ 4.5 mmol L1, [Oxone]/[Co2þ] ¼ 104, pH ¼ 6.5, reaction temperature ¼ 30 1 C, reac-
AOPs
tion time ¼ 300 min, number of stepwise addition ¼ 7, the COD, SS and the color removal efficiencies were 57.5, 53.3 and 83.3%, respectively. From this work, it can be concluded that Oxone/Co2þ oxidation process demonstrated higher degradation efficiencies of the COD, SS and color for landfill leachate treatment than that by Fenton oxidation process. It also suggested that Oxone/Co2þ oxidation process could be considered as one of the most promising technologies for practical applicability to treat landfill leachate in large scale. For further improving the efficiency of Oxone/Co2þ oxidation process, we proposed that combination of it with other technologies in future such as ultraviolet, ultrasound and biological methods. ª 2009 Elsevier Ltd. All rights reserved.
1.
Introduction
In China, 160 million tons of municipal solid wastes (MSW) were produced every year, and about 80% of them were buried in the ground (Feng et al., 2003). One of the problems arising from this form of waste disposal was the generation of leachate. Landfill leachate, an aqueous effluent, generated by
the decomposition of landfill wastes and supplemented by rainwater percolating through waste materials. Landfill leachate is generally characterized by a high strength of pollutants, such as organics, ammonium, inorganic salts and heavy metals (Horan et al., 1997). In order to minimize the risks of contamination and insure the safety of receiving media, suitable treatment must be conducted to the leachate.
* Corresponding author. Tel.: þ86 373 332 5971; fax: þ86 373 332 6336. E-mail addresses: [email protected] (J. Sun), [email protected] (X. Li), [email protected] (J. Feng), xktian@ yahoo.com.cn (X. Tian). 0043-1354/$ – see front matter ª 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.watres.2009.06.043
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The biological treatments including aerobic and anaerobic processes have been indicated to be very effective for the young age leachate because of a high value of BOD/COD in the early period, however, the ratio generally decreased with the increasing of the age of landfill (Chian and DeWalle, 1977). As a result, the biological treatments might become ineffective for the treatment of old landfill leachate (Medez et al., 1989). In recent years, many physical/chemical processes have been investigated for the treatment of old or refractory landfill leachate. A group of available treatment technology of these processes widely used is the so-called advanced oxidation processes (AOPs). AOPs have demonstrated to be innovative suitable technologies to reduce organic load or toxicity and enhance biotreatability of landfill leachate, because they are able to oxidize refractory organic compounds into harmless substances and even lead to mineralization end-products such as carbon dioxide and water (Pe´rez et al., 2002; Gogate and Pandit, 2004; Pera-Titus et al., 2003; Chidambara and Quen, 2005). Therefore, several countries in the world had begun to use AOPs to deal with landfill leachate directly or indirectly (Kim et al., 2001; Yoo et al., 1998). Fenton oxidation process is one of such AOPs approaches, which is currently applied to treat landfill leachate. Fenton’s reagent, firstly discovered by Fenton in the 1890s (Fenton, 1894), is an aqueous mixture of Fe2þ and H2O2 that produces OH. Due to its high oxidation power, Fenton oxidation process stands out among the most promising AOPs for the treatment of refractory organic pollutants wastewater. In the last few decades, using Fenton oxidation process for the treatment of landfill leachate has been widely reported (Loizidou et al., 1993; Gau and Chang, 1996; Yoon et al., 1998; Kim et al., 2001; Lopez et al., 2004; Rivas et al., 2004; Zhang et al., 2005; Gotvajn and Tisler, 2008; Zhang et al., 2009). However, Fenton reaction has some limitations in the practical application, such as the requirement for acidic pH, large quantity of chemical reagents, large production of ferric hydroxide sludge, and very slow catalysis of the ferrous ions generation (Walling and Goosen, 1973; Arnold et al., 1995; De Laat and Gallard, 1999; Rivas et al., 2001). Oxone, commercial name of potassium peroxomonosulfate (2KHSO5 $ KHSO4 $ K2SO4), is a good source of generating a strong oxidant peroxymonosulfate (HSO 5 ). Since has a higher oxidation potential than H2O2 HSO 5 (CoOHþ þ HSO5. / CoOþ þ SO. E0 ¼ 1.82 V/SHE 4 þ H2O, versus H2O2 þ 2Hþ þ 2e / 2H2O, E0 ¼ 1.76 V/SHE), Oxone demonstrates to be a suitable oxidant for the decomposition of dyes and organic compounds (Renganathan and Maruthamuthu, 1986; Campaci and Campestrini, 1999; Legros et al., 2001; Yu et al., 2006). For the activation of Oxone, Co2þ has been found as the most efficient transition metal among Agþ, Ce3þ, Co2þ, Fe2þ, Fe3þ, Mn2þ, Ni2þ, Ru3þ and V3þ (Anipsitakis and Dionysiou, 2003). Coupling of Oxone with Co2þ (Oxone/Co2þ) has recently demonstrated greater efficiencies and several operational advantages compared to Fenton reagent for treatment of certain organic wastewater in the homogeneous system (Bandala et al., 2007). Most importantly, it is possible to use the Oxone/Co2þ with no pH adjustment after pre-treatment at biocompatible pH-values (Yu et al., 2006). This is an important advantage considering that water pre-treatment is followed generally by the low cost biological
treatment. Rivas et al. (2005) have reported that landfill leachate could be effectively degraded by Oxone-promoted wet air oxidation. However, the using of Oxone/Co2þ for resistant wastewater treatment was not investigated. A quantitative study of the effect of the operating conditions on the oxidation efficiency of Oxone/Co2þ in treatment of landfill leachate has not yet been reported in the literature. The aims of this study were: (i) to assess the performance of Fenton and Oxone/Co2þ in the treatment of landfill leachate; and (ii) to investigate the influence of some very important operating parameters, such as pH, temperature, dosages of reagents, reaction time, and addition modes of reagents. The efficiency of each experiment was evaluated by measuring the COD value, and the performance of the two different oxidation processes were compared. Then we further established cost-efficient operating conditions for the potential application of Fenton and Oxone/Co2þ oxidation process to treat landfill leachate. The results can provide fundamental knowledge for the treatment of landfill leachate by both treatment processes.
2.
Materials and methods
2.1.
Landfill leachate
Landfill leachate samples were collected with polyethylene bottles from municipal sanitary landfill, which were located in Xinxiang, Henan Province, China. Samples were stored in a refrigerator at 4 C and used without any previous treatment. The main parameters (average values) of samples were: pH ¼ 8.17, COD ¼ 1116 mg L1, ammonia nitrogen (NH4– N) ¼ 25.4 mg L1, suspended substances (SS) ¼ 17 mg L1, color ¼ 300 times.
2.2.
Chemical reagents
Hydrogen peroxide (30%, w/w), ferrous sulfate (FeSO4 $ 7H2O), cobalt sulfate (CoSO4 $ 7H2O), dipotassium titanium oxide dioxalate (K2TiO(C2O2)2), sulfuric acid, and sodium nitrite (NaNO2) were obtained from Shanghai Chemical Reagents Co. (Shanghai, China). Oxone (95%, Aldrich, manufactured by DuPont) was purchased from Shunyi Development Co. (Guangdong, China). All chemicals used in this study were analytical grade. Deionized water was used throughout this study.
2.3.
Experimental procedures
All these experiments were carried out in a 200 mL double glass cylindrical jacket reactor, allowing cycle water to maintain the temperature of reaction system. Temperature was adjusted by a thermostat and a magnetic stirrer was used to stir reaction solutions (stirring rate was 280 rpm). To start each test, 100 mL landfill leachate was placed in double glass cylindrical jacket reactor where appropriate volumes of stock activator were put into the reactor. The initial oxidant dosage was based on the stoichiometric ratio with respect to the COD value assuming complete oxidation. For Fenton oxidation process, the pH value of each reaction solutions was adjusted
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to the desired level using the prepared 4 mol L1 sulfuric acid or 4 mol L1 sodium hydroxide solutions. After that, predetermined amounts of hydrogen peroxide was added to the reactor and magnetically stirred during the reaction time. As for Oxone/Co2þ oxidation process, since the pH would drop sharply after Oxone addition (Yu et al., 2006), we first added relevant volumes of Oxone solution to the reactor which already have appropriate volumes of landfill leachate and stock activator, then adjusted pH to the desired level rapidly. All samples were taken out from the reactor using a pipette, then adjusting pH value to terminate Fenton oxidation reactions and adding a proper amount of NaNO2 solution to terminate Oxone/Co2þ oxidation reactions within 1 min. Following this, samples were filtered through 0.45 mm filter paper. To eliminate the interference of H2O2 in COD measurement, H2O2 concentration was determined spectrophotometrically using titanium oxalate (Sellers, 1980; Gallard and De Laat, 2000), then deducted from COD calculation. Blank experiment was carried out simultaneously.
2.4.
Fenton oxidation
40
oxone/Co
2+
oxidation
35 30 25 20 15 10 5 0
0
1
2
3
4
5
6
7
8
9
pH Fig. 1 – The effect of pH on COD removal. Experimental conditions: temperature [ 20 ± 1 8C; reaction time [ 60 min; [H2O2] [ 50 mmol LL1 [Fe2D] [ 25 mmol LL1 [Oxone] [ 4 mmol LL1 and [Co2D] [ 0.4 mmol LL1.
Analytical methods
The pH value of solutions was measured by a pHS-3C digital pH meter. Before measurement, pH meter was calibrated with standard buffers (pH 4.0, 6.86 and 9.18) at 25 C. The COD was measured by 5B Intelligent CODCr Fast Measuring Instrument (Lanzhou, China), which was adjusted with potassium hydrogen phthalate standard solution. Ammonium nitrogen, color and SS were respectively determined according to the National Standard Method of Water quality – determination of ammonium Nessler’s reagent colorimetric method (China GB7479-87), water quality – determination of color (China GB11903-89) and water quality – determination of suspended substance – gravimetric method (China GB11901-89).
3.
45
Results and discussion
3.1.
Optimization of system parameters
3.1.1.
Effect of initial pH
The initial pH effect on both processes was tested to determine an experimental condition for further research. In this study, optimum pH that resulted in the highest COD removal was determined by running the reactor under varying pH while the other operating parameters were kept constant. pH was adjusted in the interval from 1 to 8 (note that the pH changed at the end of the experiment by approximately 0.5 unit). With changes of the initial pH, the corresponding COD removal efficiencies are presented in Fig. 1. As shown in Fig. 1, the COD removal strongly depended on the initial pH in both of the processes. The COD removal efficiency was slightly influenced by pH from 2 to 6 in Fenton oxidation process and pH from 5 to 7 in Oxone/Co2þ oxidation process. COD removal efficiency rapidly decreased with increasing pH above 6 in Fenton oxidation and above 7 in Oxone/Co2þ oxidation. Therefore, the best initial pH value was 2.5 in Fenton oxidation and 6.5 in Oxone/Co2þ oxidation, with achieving COD removal efficiencies peak above 36.5 and 41.0%, respectively.
Moreover, Fig. 1 illustrates that Oxone/Co2þ oxidation process gave higher degradation efficiency at elevated pH (Chu et al., 2007) and lower COD removal at pH 4. This is because Oxone is a strong oxidizing agent and sulfate radicals, generated during Oxone decomposition, demonstrate comparable standard reduction potential to hydroxyl radicals at neutral pH. At the same time, low concentrations of HCO 3 and H2PO 4 can produce complex compound (bicarbonate-Co and peroxyphosphate-Co) which may become active sites in Oxone/Co2þ oxidation reaction for pH from 5 to 7 (Anipsitakis when pH et al., 2005). Similarly, the sheer quantity of SO2 4 below 4 can also improve Oxone/Co2þ oxidation.
3.1.2.
Effect of temperature
At optimum pH, six different temperature values between 20 and 45 C were tested for both processes. Then two neighboring spots of the optimum point were examined in order to find the most suitable temperature. Other experimental conditions apart from temperature were kept the same. Results are depicted in Fig. 2. It was concluded from Fig. 2 that 37.5 and 30 C were found to be the most suitable temperature in Fenton oxidation process and Oxone/Co2þ oxidation process, respectively; and the corresponding COD removal efficiencies achieved 50.6 and 42.2%. As for Fenton oxidation process, the COD removal efficiency slightly increased up to 50.6% when temperature was lower than 37.5 C. But as temperature increased from 37.5 to 45 C, the COD removal efficiency decreased by 4.3% instead of increased. So, the temperature was not the higher the better. One reason was that the rates of side-reactions may be accelerated dramatically at high temperature, another reason was that the high temperature caused a sharp reduction of dissolved oxygen which can rapidly and irreversibly react with carbon-centered radicals (R.) in water. For Oxone/Co2þ oxidation process, the COD removal efficiency increased by 1.6% as temperature increased from 20
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52
Fenton oxidation 2+
oxone/Co
50
oxidation
COD removal (%)
48
45
40
44 40 36 32 28 Fenton oxidation
35
2+
24 20
25
30
35
40
oxone/Co
0
45
Temperature (.C) Fig. 2 – The effect of temperature on COD removal. Experimental conditions: pH [ 2.5 (Fenton), pH [ 6.5 (Oxone/Co2D); reaction time [ 60 min; [H2O2] [ 50 mmol LL1; [Fe2D] [ 25 mmol LL1; [Oxone] [ 4 mmol LL1 and [Co2D] [ 0.4 mmol LL1.
to 30 C but decreased by 2.15% as temperature increased from 30 to 40 C. The influence of temperature on Oxone oxidation process was less pronounced than that observed on Fenton oxidation process. Overall, COD removal efficiency was slightly influenced by the temperature in Oxone/Co2þ oxidation process, and the COD removal efficiency at room temperate was very similar to that at optimum temperature. Therefore, Oxone oxidation is more useful than Fenton oxidation by considering the optimal pH and the most suitable temperature, which is more easily to be achieved in practical application.
3.1.3.
Effect of reagents’ dosage
Like other AOPs, Fenton oxidation process and Oxone/Co2þ oxidation process were affected by [oxidant]/[activator] molar ratio and the concentration of oxidant and activator. Optimum [oxidant]/[activator] molar ratio was determined by running the reactor under six different [oxidant]/[activator] molar ratio at the optimum pH and the most suitable temperature for both of the processes. Then we examined two neighboring points of the optimum point to find the best value. Other experimental conditions except the [oxidant]/ [activator] molar ratio were kept the same. Results are illustrated in Fig. 3, which indicate that COD removal widely varied as the [H2O2]/[Fe2þ] molar ratio increased. When we increased [H2O2]/[Fe2þ] molar ratio above 2, the COD removal of landfill leachate dropped sharply. This may have been caused by the scavenging effect of excessive H2O2 to OH. Therefore, a suitable molar ratio of [H2O2]/[Fe2þ] for treatment of landfill leachate was experimentally determined as 2, with the corresponding COD removal efficiency was 50.6%. As for Oxone/Co2þ oxidation process, the changes of COD removal efficiency was not as obvious as in Fenton process, it only increased from 42.2 to 50.0% as [Oxone]/[Co2þ] increased from 10 to 104. But with an increase of [Oxone]/[Co2þ] molar
1
oxidation
2
3
4
5
6 2+
Molar ratio of [H2O2]/[Fe2+] and lg[oxone]/[Co
]
Fig. 3 – The effect of different [oxidant]/[activator] molar rations on COD removal. Experimental conditions: pH [ 2.5 (Fenton), pH [ 6.5 (Oxone/Co2D); temperature [ 37.5 ± 1 8C (Fenton), temperature [ 30 ± 1 8C (Oxone/Co2D); reaction time [ 60 min; [H2O2] [ 50 mmol LL1and [Oxone] [ 4 mmol LL1.
ratio above 105, the COD removal of landfill leachate dropped. Thus, the most suitable [Oxone]/[Co2þ] was 104 and the COD removal efficiency of landfill leachate was 50.0%. The effect of H2O2 dosage on COD removal of landfill leachate is examined by varying concentration of H2O2 from 10 to 100 mmol, and the COD removal efficiency are shown in Fig. 4. From Fig. 4, it can be concluded that increasing the dosage of H2O2 from 10 to 80 mmol could enhance the COD
60
0
2
4
6
8
10
50
40
30
Fenton oxidation (Bottom X-Axis)
20
Oxone/Co
0
20
40
60
2+
oxidation (Top X-Axis)
80
100
Oxidant dosage (mmol L-1) Fig. 4 – The effect of different dosages of oxidant on COD removal. Experimental conditions: Fenton: pH [ 2.5; temperature [ 37.5 ± 1 8C; [H2O2]/[Fe2D] [ 2.0; reaction time [ 60 min; Oxone/Co2D: pH [ 6.5; temperature [ 30 ± 1 8C; [Oxone]/[Co2D] [ 104; reaction time [ 60 min.
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removal efficiency from 20.3 to 53.0% within 60 min. However, when increasing the dosage of H2O2 from 80 to 100 mmol, the COD removal efficiency was only improved by 1.0%. Therefore, 80 mmol L1 was selected as the suitable H2O2 dosage. Just like the H2O2 plays a very important role as a source of .OH generation in Fenton’s reaction (Sun et al., 2009), so does oxidation reaction the Oxone in Oxone/Co2þ þ þ (CoOH þ HSO5 / CoO þ SO. 4 þ H2O). Oxone dosage can be critical to landfill leachate degradation while overdosing could be a potential problem theoretically to deteriorate the process similar to that of H2O2 in Fenton oxidation process (Chu et al., 2005). In this study, various Oxone concentrations were introduced into the reaction system. As shown in Fig. 4, a higher COD removal efficiency of landfill leachate was observed when more Oxone was introduced. By increasing the dosage of Oxone from 1 to 4.5 mmol, the COD removal efficiency increased from 35.5 to 50.4% within 60 min. However, when further increasing the dosage of Oxone from 4.5 to 8 mmol, the COD removal efficiency was just slightly improved by 1.3%. Thus, 4.5 mmol of Oxone was considered to be a suitable dosage for the treatment of landfill leachate, and the corresponding COD removal efficiency was 50.4%. The optimal molar ratio of [H2O2]/[Fe2þ] and [Oxone]/ [Co2þ] were 2 and 104, respectively, and the COD removal efficiency of landfill leachate achieved 50% for both systems. Suitable dosage of H2O2 and Oxone were selected as 80 and 4.5 mmol, respectively, in each liter landfill leachate. The results showed that the addition of Co2þ at concentration 4.5 104 mmol per liter landfill leachate were sufficient to mediate Oxone decomposition at a concentration of 4.5 mmol. Thus the potential toxicity of Co2þ may be overcome. The dosage of oxidant and catalyst used in Oxone/Co2þ oxidation process was much smaller than that used in Fenton process with a similar COD removal efficiency. Considering the dosage of reagents, pH adjustment, sludge treatment and so on, [Oxone]/[Co2þ] oxidation process was much cheaper than Fenton oxidation process (Anipsitakis and Dionysiou, 2003). Consequently, it is believed that [Oxone]/[Co2þ] reagent might be an alternative technology for treatment of landfill leachate.
3.1.4.
the suitable reaction time for the treatment of landfill leachate by Oxone/Co2þ was 300 min and the COD removal efficiency was 54.5%.
3.1.5.
Effect of addition modes of reagents
In addition to the above factors, the number of dosing steps was also found as an important factor affecting the COD removal of landfill leachate in both of oxidation process. Generally, reagents were added to landfill leachate in a singlestep. However, such addition may cause self-decomposition of oxidant due to high concentrations at the point of injection (Deng and Englehard, 2006). To ensure complete usage of oxidant for oxidation, we tested various numbers of stepwise addition from one to 10 times, and we added even reagent quantity in every step and kept the reagent total quantity invariable. The corresponding COD removal efficiencies are illustrated in Fig. 6. Based on the results, it can be concluded that three and seven times were found to be the best at the given conditions for Fenton oxidation and Oxone/Co2þ oxidation with corresponding COD removal efficiencies of 56.9 and 57.5%, respectively. It seems like the concentrations of H2O2 and Oxone were not the higher.
3.2.
Treatment of landfill leachate by different processes
As discussed in Section 3.1, the optimal operational conditions of Fenton’s reagent in treatment of landfill leachate were the following: [H2O2] ¼ 80 mmol L1, [H2O2]/[Fe2þ] ¼ 2.0, initial pH ¼ 2.5, reaction temperature ¼ 37.5 1 C, reaction time ¼ 160 min, number of stepwise addition ¼ 3. And for Oxone/ Co2þ oxidation process, the optimum operational parameters were: [Oxone] ¼ 4.5 mmol L1, [Oxone]/[Co2þ] ¼ 104, pH ¼ 6.5, reaction temperature ¼ 30 1 C, reaction time ¼ 300 min, number of stepwise addition ¼ 7.
0
80
160
240
320
400
480
55
Effect of reaction time
The effect of the reaction time on COD removal of landfill leachate for Fenton oxidation process was examined by varying time from 20 to 200 min, and the COD removal efficiencies are shown in Fig. 5. It can be seen that increasing the reaction time from 20 to 160 min could enhance the COD removal efficiency from 35.9 to 56.7%. However, when increasing the reaction time from 160 to 200 min, the COD removal efficiency was only increased by 0.3%. Therefore, a suitable reaction time for treatment of landfill leachate was experimentally determined as 160 min and the COD removal efficiency was 56.7%. Similarly, the effect of reaction time on COD removal of landfill leachate for Oxone/Co2þ oxidation process was examined by varying the time from 60 to 480 min and COD removal efficiency are also presented in Fig. 5. It can be observed that COD removal efficiency tended to increase as one oblique line within 300 min, and as another trend line between 300 and 480 min. And the inclination of two lines was that the former was far larger than the latter. Therefore,
50
45
40 Fenton oxidation (Bottom X-Axis) 2+
Oxone/Co
oxidation (Top X-Axis)
35 40
80
120
160
200
Fig. 5 – The effect of different reaction time on COD removal. Experimental conditions: Fenton: pH [ 2.5; temperature [ 37.5 ± 1 8C; [H2O2]/[Fe2D] [ 2.0; [H2O2] [ 80 mmol LL1; Oxone/Co2D: pH [ 6.5; temperature [ 30 ± 1 8C; [Oxone]/[Co2D] [ 104; [Oxone] [ 4.5 mmol LL1.
240
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58
4.
COD removal (%)
56
54
52
50 Fenton oxidation 2+
Oxone/Co oxidation 48
0
2
4
6
8
10
Fig. 6 – The effect of different dosing steps on COD removal. Experimental conditions: Fenton: pH [ 2.5; temperature [ 37.5 ± 1 8C; [H2O2]/[Fe2D] [ 2.0; [H2O2] [ 80 mmol LL1; reaction time [ 160 min; Oxone/ Co2D: pH [ 6.5; temperature [ 30 ± 1 8C; [Oxone]/ [Co2D] [ 104; [Oxone] [ 4.5 mmol LL1, reaction time [ 300 min.
Both treatment processes, Oxone/Co2þ and Fenton, were carried out under optimum conditions for the treatment of landfill leachate and the results are summarized in Table 1. It can be seen from Table 1 that the COD removal efficiencies were very similar, 56.9% in Fenton treatment and 57.5% in Oxone/Co2þ treatment. However, the value of SS and color of the landfill leachate increased after treatment by Fenton oxidation process. As for Oxone/Co2þ oxidation process, the removal efficiency of SS and color were 53.3 and 83.3%, respectively. From this work, it was concluded that Oxone/ Co2þ oxidation process demonstrated higher degradation efficiencies than that by Fenton oxidation process. In addition, there were a number of perceived benefits for Oxone/Co2þ oxidation becoming an alternative method to Fenton oxidation process. First, the optimum pH was close to neutral pH in Oxone/Co2þ oxidation process. Second, the most suitable temperature was 30 C, lower than that in Fenton treatment. Third, the suitable concentration of oxidant and catalyst in Oxone/Co2þ oxidation process was much lower than that used in Fenton process. Fourth, there was no sludge production in Oxone/Co2þ treatment. Herein, it also should be noted that Fenton process needs a shorter reaction time and less number of stepwise additions compared to Oxone/Co2þ process.
Table 1 – Comparison of Fenton and Oxone/Co2D processes under the optimum conditions respectively. pH COD removal (%) Fenton Oxone/ Co2þ
2.3 6.9
(þ)56.9 (þ)57.5
SS removal (%)
Color removal (%)
()233.3 (þ)53.3
()66.7 (þ)83.3
Conclusions
This paper provided the first comparative study on the Oxone/ Co2þ oxidation and Fenton oxidation processes for the treatment of landfill leachate, which is very difficult to be treated by common wastewater treatment methods. Under the conditions of neutral pH, low temperature and less dosage of oxidant and catalyst, Oxone/Co2þ oxidation process demonstrated higher degradation efficiencies of COD, SS and color than that by Fenton oxidation process. But a longer reaction time and more number of stepwise additions were the disadvantages of Oxone/Co2þ treatment. However, by considering all of these operating parameters: pH, temperature, dosage of oxidant and catalyst, reaction time and addition modes of reagents, it is suggested that Oxone oxidation has more advantages compared to Fenton process for practical applicability in large scale, which can be considered as one of the most promising technologies to treat landfill leachate. Moreover, for improving the efficiency of Oxone/ Co2þ oxidation process, it might be beneficial to further investigate its integration with other technologies (i.e., biological methods, ultrasound, and UV technologies) for more enhanced treatment of landfill leachate in the future studies.
Acknowledgement The research was supported by Key Scientific and Technological Project in Henan province, with Grant No. 0522030700 and Key Scientific and Technological Project of Henan province, People’s Republic of China (Grant No. 0611020900).
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