Accepted Manuscript Removal of COD from landfill leachate by advanced Fenton process combined with electrolysis Zhe Wang, Jiangbo Li, Weihua Tan, Xiaogang Wu, Heng Lin, Hui Zhang PII: DOI: Reference:
S1383-5866(18)30252-1 https://doi.org/10.1016/j.seppur.2018.06.048 SEPPUR 14702
To appear in:
Separation and Purification Technology
Received Date: Revised Date: Accepted Date:
1 February 2018 18 June 2018 18 June 2018
Please cite this article as: Z. Wang, J. Li, W. Tan, X. Wu, H. Lin, H. Zhang, Removal of COD from landfill leachate by advanced Fenton process combined with electrolysis, Separation and Purification Technology (2018), doi: https://doi.org/10.1016/j.seppur.2018.06.048
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Removal of COD from landfill leachate by advanced Fenton process combined with electrolysis Zhe Wang a, Jiangbo Li a, Weihua Tana, Xiaogang Wu a,b, Heng Lin a, Hui Zhang a,* a
Department of Environmental Science and Engineering, Hubei Environmental
Remediation Material Engineering Technology Research Center, Wuhan University, Wuhan 430079, China b
School of Urban Construction, Yangtze University, Jingzhou 434023, China
* Corresponding author. Tel: + 86-27-68775837; Fax: + 86-27-68778893. E-mail address:
[email protected] (H. Zhang)
Abstract Zero-valent iron (ZVI) was used as the iron source for the advanced Fenton treatment of landfill leachate in an electrochemical cell. In this process, Fe0 was dissolved from the surface of ZVI powder and then the generated Fe2+ would activate the added H2O2 to produce hydroxyl radical ( •OH), which is a strong oxidant and can lead to the removal of COD from landfill leachate. The effect of initial pH, H2O2 dosage, Fe0 dosage, current density and inter-electrode gap on the COD removal of landfill leachate was investigated. The results indicate that COD removal followed pseudo-second order kinetic model. The rate constant and COD removal efficiency increased with Fe0 dosage being increased from 0.524 to 1.745 g/L, but decreased when initial pH rose from 2 to 4. The optimal conditions of H2O2 dosage, current density and inter-electrode gap were determined to be 0.187 mol/L, 20.6 mA/cm2 and 1.8 cm, respectively. The humic acid-like organics in the leachate were effectively degraded and over 70% of COD could be removed from the leachate, showing high efficiency of the electro-advanced Fenton process.
Keywords: Landfill leachate; Advanced Fenton; Zero-valent iron; Electrochemical; COD removal
1. Introduction Increasingly affluent lifestyles, continuing industrial and commercial growth in many countries around the world in the past decades has been accompanied by rapid increases in both the municipal and industrial solid waste production [1]. The generation of municipal solid waste (MSW) keeps on growing both in per capita and overall terms [2]. The sanitary landfilling for the ultimate MSW disposal continues to be widely accepted and used as a result of its economic advantages [3]. However, the leachate would be derived from the rainfall, run off of surface drainage and decomposition of organic waste with dark grey and foul smelling [4]. Landfill leachate is very difficult to treat by traditional biological methods since it contains a wide variety of refractory contaminants [3,4]. In recent years, advanced oxidation processes (AOPs) have been widely used in wastewater treatment. Fenton process (Fe2+/H2O2) which generates powerful oxidant hydroxyl radicals (•OH) to degrade organic matters has been confirmed to be an effective approach on landfill leachate treatment [5,6]. The oxidation mechanism of Fenton process is as follows [6,7]: Fe2+ + H2O2 → Fe3+ + OH− + •OH
k2 = 53 ‒ 76 M‒1s‒1
(1)
RH + •OH → H2O + R•
k2 = 107 ‒ 109 M‒1s‒1
(2)
H2O2 + Fe3+→ Fe2+ + HO2• + H+
k2 = 0.01 M‒1s‒1
(3)
The major disadvantage of Fenton process is the production of Fe(OH)3 sludge since a large amount of iron salt is need, which results in the requirement of further separation and disposal. Therefore, elemental iron (Fe0) was proposed as an alternative iron source in Fenton system, which was named as advanced Fenton process (Fe0/H2O2) [8]. The use of zero valent iron (ZVI) could avoid the introduction of counter anions into Fenton system [9,10]. Moreover, a faster regeneration of Fe3+
into Fe2+ occurs at the iron surface [8,11] via Eq. (4). 2Fe3+ + Fe0 → 3Fe2+
(4)
In parallel, to overcome the very slow regeneration of ferrous ion via Eq. (3) in Fenton process, the electrochemical method has been combined with Fenton process [12-16], which is called Fered-Fenton process. In this process, both Fe2+ and H2O2 are simultaneously applied to an electrolytic cell, in which Fe2+ can be regenerated at the cathode as shown in Eq. (5) at the presence of electrical current. Fe3+ + e− → Fe2+
E0 = 0.77 V/SHE
(5)
Although there are quite a few reports on the combination of advanced Fenton with ultrasonic irradiation [17-20] or ultraviolet irradiation [21,22], the coupling of electrochemical method and advanced Fenton (electro-advanced Fenton) was seldom reported. Babuponnusami and Muthukumar [23] observed that the addition of electrochemical effect with Fenton-like reaction using nano-ZVI and hydrogen peroxide improved the phenol removal from 65.7% to 87.4% in 60 min treatment. However, there is no report on the application of electro-advanced Fenton process to the treatment of landfill leachate. Therefore, in this study, the treatment of landfill leachate was performed in an electrochemical cell in which both Fe0 and H2O2 were externally applied. The effects of operating conditions, such as initial pH, H2O2 and Fe0 dosages, current density (j), and inter-electrode gap (d), on chemical oxygen demand (COD) removal were investigated. The evolution of Fe2+ and H2O2 concentrations under various operating conditions was monitored to explore insight into the electro-advanced Fenton process. The kinetics of electro-advanced Fenton process was determined and three-dimensional excitation and emission matrix (3D-EEM) fluorescence spectroscopy of landfill leachate before and after treatment was analyzed.
2. Materials and Methods 2.1 landfill leachate The landfill leachate was taken from a sanitary landfill in Wuhan, China and stored at 4 °C in a refrigerator. The characteristics of the leachate are listed in Table 1. 2.2 Experimental procedure The electrolytic cell was made of polymethyl methacrylate (PMMA) with the size of 12 cm × 10 cm × 20 cm. A Ti/IrO2-RuO2-TiO2 anode and a titanium cathode, both with the size of 10 cm×15 cm, were arranged parallel to each other at a distance of 0.9, 1.8 or 2.7 cm. Before each run, concentrated sulfuric acid was used to adjust the initial pH value of the leachate. When iron powder and hydrogen peroxide were added into the reactor at the same time, a direct current (DC) power supply (Model WYK-305, Yangzhou Jintong Source Co., Ltd., China) and a mechanical agitator (Model DW-3, Yingyu Yuhua Instrument Plant, China) were switched on. At pre-selected time intervals, samples were withdrawn from the reactor and immediately the pH was adjusted to above 8.0. The COD values of the supernatants were analyzed after the samples were settled for two hours to destruct residual Fe2+ and H2O2. Separate aliquots were taken at the same intervals and the concentration of Fe2+ and H2O2 were immediately measured. 2.3 Analytical methods The pH value was measured with a Mettler-Toledo FE20 pH meter . COD was determined using a fast digestion-spectrophotometric method based on the Standard of the People’s Republic of China for Environmental Protection [24]. The concentration of hydrogen peroxide was analyzed by using a spectrophotometric determination with titanium oxalate [25]. Ferrous iron was determined using a 1, 10-phenanthroline colorimetric method [26].
Ammonia nitrogen was analyzed by spectrophotometric method [27] using a Three-dimensional excitation and emission matrix (3D-EEM) fluorescence spectroscopy was recorded on a fluorescence spectrophotometer (Spectrofluorometer FS5) with a xenon lamp as the excitation source. The EEM spectroscopy was obtained by measuring the emission spectra from 200 to 550 nm repeatedly, at the excitation wavelengths from 200 to 500 nm, spaced by 5 nm intervals. The scanning speed was set at 6000 nm/min for all measurements. Excitation and emission slit widths were 5 nm. 3. Results and Discussions 3.1 Effect of initial pH on COD removal In advanced Fenton process, pH value is an important factor, which impacts the generation of Fe2+ and decomposition of H2O2 to a great extent [28]. Many researchers have reported that advanced Fenton process was effective in acid pH range [11,28]. In this study, the effect of initial pH (pH0) on COD removal was investigated at pH0 2, 3 and 4 [28]. It can be seen from Fig. 1(a) that COD removal increased with the drop of initial pH value. To understand the kinetics of COD removal, the data as shown in Fig. 1(a) were fitted with pseudo-zero, pseudo-first order or pseudo-second order kinetics. As can be seen from Table S1, the pseudo-second-order kinetic model fitted the experimental data better than either the pseudo-zero or pseudo-first order kinetic model, and the pseudo-second order rate constant (k2) decreased with increasing pH0 (Fig. 1b), which is in agreement with the dependence of COD removal with pH0 as presented in Fig. 1(a). Ghanbari and Moradi [29] also observed that the COD removal followed pseudo-second order kinetic model when the textile wastewater was treated by iron-based electrochemical processes such as electro-Fenton and electrochemical Fenton in which hydrogen peroxide was
externally applied while an iron anode was used. In advanced Fenton process, Fe0 needs to be dissolved from the surface of zero valent iron powder under strongly acidic conditions as given in Eq. (6) [28]. The produced ferrous ion then rapidly reacts with hydrogen peroxide to generate hydroxyl radicals as shown in Eq. (1), just like in Fenton process. However, the production of hydroxyl radicals becomes slower at a higher pH value, due to the formation of the ferric-hydroxo complexes [11,28]. As a result, the highest COD removal was obtained at pH0 2. Fe0 + 2H+ → Fe2+ + H2
(6)
The evolution of residual hydrogen peroxide indicates that H2O2 was nearly completely decomposed after 90 min reaction at pH0 2, which was faster than that at pH0 3 or 4 (Fig. 1c). Thereafter, the COD removal levelled off even though the extension of electrolysis. It is due to the employment of dimensionally stable anode (IrO2-RuO2-TiO2) in this work. Although hydroxyl radicals could be generated by oxidation of water at the DSA anode surface (M) as given in Eq. (7), the M( •OH) is chemisorbed and less available for organic degradation [7, 30]. More importantly, the formed intermediate products such as carboxylic acid which cannot be further oxidized by hydroxyl radicals and consequently were accumulated in the reaction system [31]. M(H2O) → M(•OH) + H+ + e
(7)
As can be seen in Fig. 1(d), the ferrous ion present in the reaction system kept a relatively stable and low concentration before H2O2 was nearly completely decomposed. It indicates that the generated ferrous ion from the dissolution of metallic iron was consumed to decompose hydrogen peroxide. The lowest Fe 2+ concentration was observed at pH0 2, illustrating that the advanced Fenton process
was more efficient at pH0 2 than at pH0 3 or 4. When little H2O2 was remained in the reaction system, the Fenton reaction ceased and Fe 2+ concentration increased sharply. Based on the above results, the subsequent experiments were performed at pH0 2. 3.2 Effect of H2O2 dosage on COD removal Hydrogen peroxide is a source of •OH radicals that are referred as the moieties responsible for the degradation of organic pollutants in the advanced Fenton process [9,11]. As can be seen from Fig. 2a, only 42.4% of COD removal was obtained at the H2O2 dosage of 0.078 mol/L, corresponding to 0.5 of stoichiometric value theoretically required to completely remove COD from the leachate. When the H2O2 dosage was doubled to 0.156 mol/L, the stoichiometric value theoretically needed to totally remove COD from the leachate, COD removal efficiency increased to 54.1%. It indicates the additional dosed H2O2 was not fully used to degrade organic pollutants in the leachate. To quantitatively investigate the efficiency of H2O2 utilization (η),
(8) where the available O2 (mg/L) was the theoretical amount of reactive oxygen in the applied hydrogen peroxide. At 0.078 mol/L of H2O2 dosage, theoretically 50% of COD could be oxidatively removed from the leachate. The 42.4% of COD removal indicates the efficiency of H2O2 utilization was relatively high, specifically, 84.9%. When 0.156 mol/L H2O2 was applied, COD removal increased to 54.1% but the η value dropped to 54.2%. It should be noted that further doubling of H2O2 dosage to 0.312 mol/L gave rise to the decrease of COD removal to 50.9%. It is due to the scavenging effect of overdosed H2O2 on •OH radical, as given in Eq. (9). In this case, the efficiency of H2O2 utilization was as low as 25.5%.
H2O2 + •OH → HO2• + H2O
(9)
By adding a 20% excess of H2O2 theoretical dosage (0.187 mol/L), the highest COD removal was achieved. The similar trend was observed for pseudo-second-order rate constant. When H2O2 dosage increased from 0.078 to 0.156 to 0.187 mol/L, the k2 value increased from 2.2110‒6 to 4.4410‒6 to 6.0510‒6 L/mg/min. Then it decreased to 2.2110‒6 L/mg/min as H2O2 dosage was further raised to 0.312 mol/L. At 120% of H2O2 theoretical dosage, the Fe2+ ion present in the system began to rise up swiftly (Fig. 2d) when H2O2 was almost totally decomposed at 110 min (Fig. 2c), and the similar trend was observed at 50% or 100% of H2O2 theoretical dosage. Interestingly, the Fe2+ concentration kept a relatively low value and this value was even lower during the last stage of the process when the applied H2O2 concentration was 200% of theoretical dosage. It is because the residual H2O2 concentration was as high as 0.0422 mol/L at 150 min despite that H2O2 also kept on decreasing during the whole reaction time. Considering the highest COD removal (65.7%) and relatively high efficiency of H2O2 utilization (54.9%) at 120% of H2O2 theoretical dosage, the following experiments were carried out with 0.187 mol/L H2O2 dosage. 3.3 Effect of Fe0 dosage on COD removal In the electro-advanced Fenton process, ferrous ions originate from metallic iron (Eq. 6) and then catalyze hydrogen peroxide to generate hydroxyl radicals (Eq. 1). Moreover, Fe2+ would be regenerated at the ZVI surface after it is converted to Fe3+ as given in Eq. (4) [8,11]. To investigate the effect of Fe0 dosage, four different iron powder additions between 0.524 and 1.745 g/L were tested. As can be seen, the COD removal rate constant increased from 4.4610‒6 to 9.5710‒6 L/mg/min (Fig. 3b) and COD removal efficiency increased from 61.4% to 70.1% (Fig. 3a), when Fe0 dosage
increased from 0.524 and 1.745 g/L. With the increase of Fe0 addition, more Fe2+ ion would be dissolved into the bulk liquid to decompose H2O2. Fig. 3(c) shows the rate of H2O2 decomposition increased with increasing Fe0 dosage, and correspondingly, hydrogen peroxide needs less time to decompose completely and Fe2+ concentration needs less time to lift sharply (Fig. 3d). For example, hydrogen peroxide was almost thoroughly decomposed at 90 min with Fe0 addition being 1.745 g/L, which was faster than other ones. At that time, the Fe2+ concentration began to rise sharply. As a result, more •OH radicals would be formed via Eq. (1). This would promote COD removal efficiency and COD removal rate. Based on the above result, the subsequent experiments were carried out with 1.745 g/L Fe0 addition. The maximum rate constant of 9.5710‒6 L/mg/min achieved in this study was compared with other systems for the treatment of landfill leachate. As can be seen in Table 2, this value is higher than not only those in chemical AOPs such as ozonation and O3/H2O2 process [33], but also that in electrochemical oxidation [34]. This is because advanced Fenton reagent was coupled with electrochemical process to generate more hydroxyl radicals in this work. 3.4 Effect of current density on COD removal Different current densities of 13.7, 20.6 and 27.4 mA/cm2 were implemented in the experiments to investigate the influence of applied current on COD removal. As can be seen in Fig. 4(a), when the current density rose from 13.7 to 20.6 mA/cm2, the COD removal efficiency increased from 60.4% to 70.1%, and the corresponding rate constant lifted from 6.69 ×10‒6 to 9.57 ×10‒6 L/mg/min (Fig. 4b). An increase in current density results in a quicker regeneration of Fe2+ ion via cathodic reduction reaction as shown in Eq. (5). This would improve H2O2 decomposition to produce •
OH radicals [12,14]. Nevertheless, the cathodic reduction reaction (5) would be
inhibited since the side reaction such as hydrogen evolution (Eq. 10) becomes significant [12,14] when the current density further increased to 27.4 mA/cm2. Therefore, the COD removal efficiency and COD removal rate decreased to 65.7% and 8.84 ×10‒6 L/mg/min, respectively at a higher current density of 27.4 mA/cm2. Fig. 4(c) indicates that H2O2 decomposed more quickly at current density of 20.6 mA/cm2, though after 80 min reaction, little H2O2 was monitored at all three current densities. The absence of sufficient concentration of H2O2 gives rise to the lift of Fe2+ concentration in the reaction system, as shown in Fig. 4(d). As a result, 20.6 mA/cm2 of current density was selected in the next experiments. 2H+ + 2e− → H2
(10)
3.5 Effect of the inter-electrode gap on COD removal Experiments with different inter-electrode gap at 0.9, 1.8 and 2.7 cm were conducted and the result is illustrated in Fig. 5. As can be seen in Fig. 5(a) and 5(b), the highest COD removal efficiency and COD removal rate were obtained at the optimal inter-electrode gap of 1.8 cm. This is similar to our previous studies when Fered-Fenton process was used for the treatment of leachate in both batch and continuous modes [15,16]. At a shorter inter-electrode gap of 0.9 cm, the electro-regenerated Fe2+ at the cathode would be easily oxidized to Fe3+ at the anode [12,35]. When the inter-electrode gap was set as 2.7 cm, longer distance between the electrodes gives rise to more energy consumption [36], and more importantly, the limiting mass transfer of Fe3+ to the cathode surface which governs Fe2+ regeneration [12,37]. Therefore, decomposition rate was a little lower at 0.9 or 2.7 cm compared with that at 1.8 cm, as indicated in Fig. 5(c), despite the fact that less than 10% H2O2 was remained after 80 min reaction at all three inter-electrode gap values. Accordingly, the Fe2+ concentration was observed to increase rapidly first at 1.8 cm. as illustrated in
Fig. 5(d). To quantitatively evaluate the energy consumption at different inter-electrode gap, the specific electrical energy consumption (SEEC) was calculated by Eq. (11) [29,38]: SEEC (kWh/kg COD) =
(11)
where U is the average cell voltage (V), I is the operating current (A), t is the electrolysis time (h) and V is the volume of treated leachate (L). As can be seen in Fig. 5(b), the lowest SEEC value was achieved at 1.8 cm despite that the cell voltage increased with the increasing inter-electrode gap. Therefore, 1.8 cm tends to be the optimal inter-electrode gap considering the highest COD removal and the lowest SEEC value were obtained at this value. The electrical energy consumption for the removal of COD from leachate was compared with various electrochemical oxidation processes and summarized in Table 2. It can be seen that the present electro/Fe0/H2O2 process is more energy saving than the electrochemical processes [39-41] since advanced Fenton reagent was applied to generate more hydroxyl radicals. Vlyssides et al. [42] observed a lower SEEC value of 4.29 kWh/kg COD, but it was based on the 41.6% of COD removal, which is much lower than the present work (70.1%). It has been reported that when COD removal efficiency reaches a relatively high value, the energy consumption increases sharply with further increase in COD removal [41,43,44]. 3.6 The performance of COD removal in different systems To illustrate the advantage of coupling electrochemical process with advanced Fenton process, the control experiments including electro-oxidation and Fe0/H2O2 treatment of leachate were performed. As indicated in Fig. 6, the electro-oxidation process could remove only 37.7% of COD from the leachate after 150 min treatment. In the electro-oxidation process, the organics in the leachate could be degraded by the
hydroxyl radicals produced from water electrolysis at the surface of DSA anode as shown in Eq. (7). However, the generated hydroxyl radicals are tightly adsorbed to the DSA anode surface and are ineffective for pollutant degradation [7, 30]. When advanced Fenton (Fe0/H2O2) process was employed to treat the leachate, the COD removal increased to 50.9%. It is because more hydroxyl radicals were generated in the bulk solution via Fenton reaction (Eq. 1) after the dissolved Fe2+ was released from the ZVI surface as given in Eq. (6). Moreover, the ferrous ion could be regenerated by Eq. (4) as it was converted into the ferric ion by Fenton reaction (Eq. 1). In the electro-advanced Fenton process, the regeneration of Fe2+ could be realized by the reduction of Fe3+ at the ZVI surface (Eq. 6) and at the cathode (Eq. 5). The Fe3+/Fe2+ cycle would be accelerated and much more hydroxyl radicals would be formed by the Fenton reaction. Fig. 6 shows a 72.4% of COD removal was achieved by coupling electrochemical process with advanced Fenton process. To investigate the contribution of homogeneous Fenton to the COD removal, ZVI was replaced by Fe2+ (in the form of FeSO4) with the same concentration of dissolved total iron detected in the electro-advanced Fenton process. The Fered-Fenton (electro/Fe2+/H2O2) process could also achieve 68.4% COD removal, which was just a little lower than in the electro/Fe0/H2O2 electro-advanced Fenton process. It confirms that the electro-advanced Fenton is still a homogeneous process since hydrogen peroxide is catalyzed by Fe2+ from the dissolution of Fe0. However, the use of ZVI rather than Fe2+ could avoid the introduction of counter anions (SO42‒ in this case) into the electro-advanced Fenton system [9,10]. To explore the transformation of dissolved organic matter (DOM) in the leachate during the electro-advanced Fenton treatment, the EEM spectra of raw leachate and
effluent were analyzed and the results are shown in Fig. 7. Contour lines in the EEM spectra usually represent the fluorescence intensity, and thicker contour lines indicate higher fluorescence intensity. Fluorescence intensity generally increases with the increase of molecular weight and concentration [44]. As can be seen in Fig. 7(a), there is an apparent peak in raw leachate and the maximum fluorescence intensity of this peak located at excitation/emission wavelength of 395/485 nm, indicating the peak was related to humic acid-like organic compounds [45]. The peak blue-shifted to 340/405 nm in the effluent (Fig. 7(b)), corresponding to the fulvic acid-like organics [46], which usually have smaller molecular weight than humic acid-like organics. Moreover, the fluorescence intensity of effluent decreased significantly than that of the raw leachate, indicating the macromolecules with strong fluorescence emission (e.g. humic acid-like organics) in landfill leachate were degraded into substances with less/without fluorescence characteristics. 4. Conclusions This study demonstrates that the electro-advanced Fenton process is efficient to remove COD from landfill leachate. The effect of important parameters, such as initial pH value, H2O2 and Fe0 dosages, current density and inter-electrode gap, on the treatment efficiency was investigated. The Fe2+ concentration present in the reaction system kept relatively stable and low before H2O2 was nearly completely decomposed. When H2O2 was almost consumed up, Fe2+ concentration began to increase swiftly and COD removal levelled off. EEM analysis indicated the humic acid-like organics were converted to fulvic acid-like organics and other small molecules without fluorescence characteristics during electro-advanced Fenton process. Under the optimal conditions of pH0 2, 0.187 mol/L H2O2 dosage, 1.745 g/L Fe0 addition, 20.6
mA/cm2 current density and 1.8 cm inter-electrode gap, over 70% of COD removal efficiency was achieved. The proposed electro-advanced Fenton could be integrated with other post-treatment processes such as biological methods to further remove COD after the oxidized leachate was neutralized by adjusting pH between 7 to 8 and the dissolved iron was removed by precipitation.
Acknowledgements This work was supported by the National High-Tech R&D Program (863 Program) of China (Grant No. 2008AA06Z332), Wuhan Science and Technology Bureau through “The Gongguan Project” Hubei Province
201060723313), Natural Science Foundation of 2014CFB334), the Science and Technology Program of
Department of Education of Hubei Province
Q20141309).
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‒
Fast
Digestion-Spectrophotometric Method, HJ/T 399-2007. [25] R.M.Sellers, Spectrophotometric determination of hydrogen peroxide using potassium titanium(IV)oxalate, Analyst, 105 (1980) 950-954. [26] APHA, AWWA, WPCF, Standard Methods for the Examination of Water and Wastewater, 18th ed., American Public Health Association, American Water Works Association, Water Pollution Control Federation, Washington, DC, USA, 1992. [27] National Standard on Environmental Protection of the People’s Republic of China, Water quality ‒ Determination of ammonia nitrogen ‒ Nessler’s reagent spectrophotometry, HJ 535-2009. [28] N. Ertugay, N. Kocakaplan, E. Malkoç, Investigation of pH effect by Fenton-like oxidation with ZVI in treatment of the landfill leachate, Inter. J. Mining, Reclam. Environ. 31 (2017) 404-411. [29] F. Ghanbari, M. Moradi, A comparative study of electrocoagulation, electrochemical Fenton, electro-Fenton and peroxi-coagulation for decolorization of real textile wastewater: electrical energy consumption and biodegradability improvement, J. Environ. Chem. Eng. 3 (2015) 499-506. [30] N. Oturan, E.D. van Hullebusch, H. Zhang, L. Mazeas, H. Budzinski, K. Le Menach, M.A. Oturan, Occurrence and removal of organic micropollutants in landfill leachates treated by electrochemical advanced oxidation processes, Environ. Sci. Technol. 49 (2015) 12187-12196. [31] M. Kallel, C. Belaid, T. Mechichi, M. Ksibi, B. Elleuch, Removal of organic load
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Highlights
pseudo-
Figure Captions Fig. 1 Effect of initial pH on COD removal (a), rate constant (b), H2O2 (c) and Fe2+ (d) evolution ([H2O2] = 0.156 mol/L, [Fe0] = 0.874 g/L, j = 20.6 mA/cm2, d = 1.8 cm) Fig. 2 Effect of H2O2 dosage on COD removal (a), rate constant and H2O2 utilization (b), H2O2 (c) and Fe2+ (d) evolution ([Fe0] = 0.874 g/L, j = 20.6 mA/cm2, d = 1.8 cm, pH0 2) Fig. 3 Effect of Fe0 dosage on COD removal (a), rate constant (b), H2O2 (c) and Fe2+ (d) evolution ([H2O2] = 0.187 mol/L, j = 20.6 mA/cm2, d = 1.8 cm, pH0 2) Fig. 4 Effect of current density on COD removal (a), rate constant (b), H2O2 (c) and Fe2+ (d) evolution ([H2O2] = 0.187 mol/L, [Fe0] = 1.745 g/L, d = 1.8 cm, pH0 2) Fig. 5 Effect of inter-electrode gap on COD removal (a), rate constant and SEEC (b), H2O2 (c) and Fe2+ (d) evolution ([H2O2] = 0.187 mol/L, [Fe0] = 1.745 g/L, j = 20.6 mA/cm2, pH0 2) Fig. 6 Comparison of COD removal in different systems ([H 2O2] = 0.187 mol/L, [Fe0] = 1.745 g/L , j = 20.6 mA/cm2, d = 1.8 cm, pH0 2) Fig. 7 Typical EEM spectra of raw leachate and effluent: (a) raw leachate; (b) effluent.
ficienc COD removal ef
y (%)
60
45
30
15 0 150
T im
4 120
e (m
90
3 60
2
30
in )
pH
0
(a)
k2106 (mg-1 L min-1)
5.0 4.0 3.0 2.0 1.0 0.0 2
3 pH0
(b)
4
H2O2 concentration (mol/L)
0.16 pH0 2 pH0 3
0.12
pH0 4
0.08 0.04 0.00 0
20
40
60
80
100 120 140 160
Time (min)
(c)
250
200
pH0 3 pH0 4
150
100
2+
Fe concentration (mg/L)
pH0 2
50
0 0
20
40
60
80
100
120
140
160
Time (min)
(d)
Fig. 1
ficienc COD removal ef
y (%)
75 60 45 30 15 0.312 0.234
0 150
T im
120
e (m
in )
90
0.156 60
0.078
30 0
O2
H2
(M
)
(a)
k2106 (mg-1 L min-1)
8.0
80
6.0
60
4.0
40
2.0
20
0.0
0 0.078
0.156
0.187
H2O2 (mol/L)
(b)
0.312
η (%)
k2 η
[H2O2]0= 0.078 mol/L
H2O2 concentration (mol/L)
0.32
[H2O2]0= 0.156 mol/L [H2O2]0= 0.187 mol/L
0.24
[H2O2]0= 0.312 mol/L
0.16 0.08 0.00 0
20
40
60
80
100 120 140 160
Time (min)
(c)
200
Fe2+ concentration (mg/L)
[H2O2]0=0.078 mol/L [H2O2]0=0.156 mol/L
160
[H2O2]0=0.187 mol/L [H2O2]0=0.312 mol/L
120 80 40 0 0
20
40
60
80
100 120 140 160
Time (min)
(d)
Fig. 2
ficienc COD removal ef
y (%)
75 60 45 30 15 2.0
0 150
T im
120
e (m
in )
1.5 90
1.0 60
0
0.5
30 0
(a)
Fe
(g
/L
)
k2106 (mg-1 L min-1)
8.0 6.0 4.0 2.0 0.0 0.524
0.874
1.309
1.745
0
Fe (g/L)
(b)
H2O2 concentration (mol/L)
0.20
Fe0= 1.745 g/L Fe0= 1.309 g/L Fe0= 0.874 g/L Fe0= 0.524 g/L
0.16 0.12 0.08 0.04 0.00 0
20
40
60
80
100 120 140 160
Time (min)
(c)
Fe2+ concentration (mg/L)
Fe0= 1.745 g/L Fe0= 1.309 g/L Fe0= 0.874 g/L Fe0= 0.524 g/L
600
400
200
0 0
20
40
60
80
100 120 140 160
Time (min)
(d)
Fig. 3
ficienc COD removal ef
y (%)
75 60 45 30 15 0 150
T im
27.4 120
e (m
90
20.6 60
13.7
30
in )
0
j(
cm
/m
(a)
k2106 (mg-1 L min-1)
8.0 6.0 4.0 2.0 0.0 13.7
20.6 j (mA/cm2)
(b)
27.4
2
A
)
H2O2 concentration (mol/L)
0.20
j = 13.7 mA/cm2 j = 20.6 mA/cm2 j = 27.4mA/cm2
0.16 0.12 0.08 0.04 0.00 0
20
40
60 80 Time (min)
(c)
100
120
Fe2+ concentration (mg/L)
400 j = 13.7 mA/cm2 j = 20.6 mA/cm2 j = 27.4 mA/cm2
300
200
100
0 0
20
40
60
80
100
Time (min)
(d)
Fig. 4
60
ficienc COD removal ef
y (%)
75
45 30 15 0
2.7 150
T im
120
e (m
in )
90
60
0.9
30 0
1.8 ) m (c d
(a)
k2 SEEC
k2106 (mg-1 L min-1)
8 6.0 6 4.0 4 2.0
2
0.0
0 0.9
1.8 d (cm)
2.7
(b)
H2O2 concentration (mol/L)
0.20
d = 0.9 cm d = 1.8 cm d = 2.7 cm
0.16 0.12 0.08 0.04 0.00
0
20
40
60 Time (min)
80
100
120
SEEC kWh(kg COD)-1
10
8.0
(c)
d = 0.9 cm d = 1.8 cm d = 2.7 cm
300
200
100
Fe
2+
concentration (mg/L)
400
0 0
20
40
60
Time (min)
(d)
Fig. 5
80
100
COD removal efficiency (%)
80
60
40
20
0 Electro-oxidation
Fe0/H2O2
Fig. 6
Electro/Fe0/H2O2
(a)
(b)
Fig. 7 Table 1 Characteristics of the landfill leachate.
Parameter
Value
COD (mg/L)
2500
pH
8.50
NH4-N (mg/L)
2917
Alkalinity (mg/L)
1547
Conductivity (mS/cm)
12.92
Turbidity (NTU)
1369
UV254
1.590
Fe2+ (mg/L)
2.8
Table 2 Comparison of pseudo-second order rate constant (k2) and specific electrical energy consumption (SEEC) under the different systems for the treatment of the leachate.
Treatment process
k2×10 Operating parameters
6
(L/mg/min)
SEE C
Refe
(kWh/kg
rence
COD)
Influent COD: 1300 mg/L, pH0 11, [O3] = 400 O3/H2O2 process
mg/L/h, mole ratio of O3/H2O2: 1:2, temperature:
2
‒
[33]
3
‒
[33]
4.62
‒
[34]
28 °C, treatment time: 2.5 h, COD removal efficiency: 35%. Influent COD: 1200 mg/L, pH0 11, ozone dose Ozonation
400 mg/L/h, temperature: 28 °C, treatment time: 3 h, COD removal efficiency: 50%. Anode: Anode: Ti/TiO2 (70%)-RuO2 (30%), cathode: titanium, influent COD: 1855 mg/L, pH0 7.0 – 9.5, j
Electroche mical oxidation
= 116.0 mA/cm2, inter-electrode gap: 3 mm, temperature: 40 °C, recirculation rate of leachate: 2000 L/h, treatment time: 3 h, COD removal efficiency:
73%.
Anode: Ti/IrO2-RuO2-TiO2, Cathode: titanium, Influent COD: 2500 mg/L, pH0 2.0, Electro/Fe0/ H2O2
Fe0 loading: 1.745 g/L, H2O2 dosage: 0.187 mol/L, current
this
9.57
8.12
‒
4.29
[42]
‒
35
[39]
work
density: 20.6 mA/cm2, inter-electrode gap: 1.8 cm, treatment time: 2 h, COD removal efficiency: 70.1%. Reactor: 6 L cylindrical vessel, anode: Ti/Pt, cathode: perforated stainless steel, COD: 60000 mg/L, electrolyte: 4% NaCl, Electroche mical oxidation
solution pH 5.5, applied voltage 20 V, temperature: 80 °C, FeSO4∙7H2O: 4 g/L, input rate of leachate: 60 mL/min, treatment time: 5 h, COD removal efficiency: 41.6%. Anode: Ti/IrO2-RuO2, Cathode: zirconium, electrolyte: 1 mol/L HClO4,
Electroche
influent COD: 2960 mg/L,
mical oxidation
pH 3.0, current density: 32 mA/cm2, inter-electrode gap: 1 cm, temperature: 80 °C, treatment time: 4 h, COD 40
removal efficiency: 90%. Anode: Boron-doped diamond (BDD) on silicon, cathode: stainless steel, Electroche mical oxidation
influent COD: 860 mg/L, pH 8.2, flow rate: 300 L/min,
‒
53
[40]
current density: 30 mA/cm2, treatment time: 7 h, effluent COD: < 160 mg/L. Anode: Ti/PbO2, cathode: stainless steel, influent COD: 780 mg/L, Electroche mical oxidation
recirculation rate of leachate: 420 L/h, pH 8.2, current: 2 A, temperature: 50 °C, inter-electrode gap: 0.5 cm, treatment time: 3 h, effluent COD: < 160 mg/L.
41
‒
145. 16
[41]
