Juan Liab,
Zhao-hui Yang*ab,
Hai-yin Xuab,
Pei-pei Songab,
Jing Huangab,
Rui Xuab,
Yi-jie Zhangab and
Yan Zhouab
aCollege of Environmental Science and Engineering, Hunan University, Changsha 410082, PR China. E-mail: yzh@hnu.edu.cn; Fax: +86 0731 88822829; Tel: +86 0731 88822829
bKey Laboratory of Environmental Biology and Pollution Control (Hunan University), Ministry of Education, Changsha 410082, PR China
First published on 9th May 2016
Today, improving the elimination of refractory pollutants in landfill leachate through electrochemical oxidation technology has attracted considerable attention. In this study, a combination of anodic oxidation and cathodic coagulation process using Ti/RuO2–IrO2 and Al electrodes, was adopted to treat the mature landfill leachate with a very low biodegradability ratio (BOD5/COD) of 0.12. The effects of current density, pH, and the chloride ion concentration on the removal of chemical oxygen demand (COD) and ammonia nitrogen (NH3–N) were investigated by response surface methodology (RSM). The optimum condition of 83.7% COD and 100% NH3–N removal was achieved with a current density of 0.1 A cm−2 and a pH of 6.37, the chloride ion concentration 6.5 g L−1, and an electrolytic time of 150 min. In addition, heavy metals were partly removed. A main degradation mechanism of the pollutants, including oxidation, coagulation and precipitation, was elucidated by gas chromatography-mass spectrometry (GC-MS), environmental scanning electron microscopy coupled with energy dispersive spectrometer (ESEM/EDS) and Fourier transform infrared spectroscopy (FT-IR) analysis of organic components in landfill leachate and sludge generated at the cathode. These results indicated that the electrochemical processes could be a convenient and efficient method for the treatment of landfill leachate.
Because of recalcitrant NH3–N and relatively low five-day biological oxygen demand (BOD5)/chemical oxygen demand (COD) ratio, mature landfill leachate (>10 years)9 can not be treated by conventional biological treatment, such as aerobic and anaerobic biological degradation.10 However, the electrochemical oxidation process with high effectiveness, environmental compatibility and easy in operation has been shown as a promising alternative for NH3–N removal.11 In the electrochemical oxidation,12 employing different types of anode materials plays a dominant role, and substantially influences both reaction selectivity and efficiency,13,14 such as Ti, PbO2/Ti, RuO2, Fe, Al, and boron-doped diamond (BDD), etc.15 Among the various anodes used, RuO2 and IrO2 coated Ti anode (Ti/RuO2–IrO2) stands out, which has been utilized widely with well-proven advantages.16 It possesses high stability and catalytic activity, not only for chlorine evolution, but also for oxygen evolution. Several authors have applied Ti/RuO2–IrO2 electrode to the treatment of landfill leachate.3 Usually, cathode is protected against corrosion in the electrooxidation technology. Except for a carrier of the electronic, it does not have substantial effect. On the contrary, taking advantage of the cathode corrosion and investigating the effect in the solution have a certain significance. As the third most abundant element in the earth crust, aluminum and its alloys are recognized to be one of the most suitable metals for future hydrogen production, energy storage and conversion.17,18 Moreover, aluminium as the cathode can produce hydroxide at the expense of sacrificial aluminum, which has a promoting coagulation effect on pollutant removal.17 In consequence, we can construct electrooxidation and coagulation into a system to further improve the efficiency of processing, which has not been studied yet. When anodic oxidation is combined with the cathodic coagulation, structure of reaction tank can be optimized. Compared with the pure electrochemical oxidation, the removal rate of pollutants is improved significantly.
In this study, mature landfill leachate was treated by the combination of electrooxidation–coagulation processes using Ti/RuO2–IrO2 anode and Al cathode. The main objectives can be divided into three aspects. Firstly, the effects of various operating variables e.g. electrolytic time, electrode gap, current intensity, pH and initial concentration of chloride ions on COD, NH3–N, colour and heavy metals removal were investigated. In parallel, response surface methodology (RSM) was considered to be an effective means to evaluate their interactions and determine the optimum operational conditions.10 Secondly, some associated mechanisms were presented, regarding oxidation and coagulation that occurred in the electrode/solution boundary. Finally, energy consumption was used to examine its performance in the electrochemical process.
Parameters | Unit | Range | Average |
---|---|---|---|
pH | — | 7.80–8.28 | 8.04 |
Conductivity | mS cm−1 | 12.05–13.08 | 12.62 |
CI− | mg L−1 | 2300–2800 | 2500 |
BOD5 | mg L−1 | 440–520 | 480 |
COD | mg L−1 | 3640–4296 | 3968 |
BOD5/COD | — | 0.10–0.14 | 0.12 |
NH3–N | mg L−1 | 1840–2042 | 2000 |
Sodium | g L−1 | 3.528–3.800 | 3.664 |
Potassium | g L−1 | 1.264–1.386 | 1.325 |
![]() | (1) |
On the basis of the single factor test results, three independent variables (current density (x1), pH (x2) and the chloride ion concentration (x3)) and two responses (COD and NH3–N removal) were investigated in this experiment. The practical design parameters and their levels were presented in Table 2, with the help of the Design Expert software (Version 8.0.6, Stat-Ease Inc, Minneapolis, MN). Then, it was also used for handle of the experimental data to obtain the equations and analysis of variance (ANOVA).10 The test of statistical significance must be based on the total error criteria with a confidence level of 95.0% (p < 0.05). R2, which ranged from 0 to 1, was used to express the fit quality of the polynomial model equation. When R2 value closer to 1, it meaned the model was more accurate. Three dimensional (3D) response surface plots were constructed from the developed models in order to study the individual and interactive effect of the process variables on the responses. And all response surface plots have clear peaks, meaning that the optimum conditions were located to find out maximum values of the responses.
Variables | Range and level | |||||
---|---|---|---|---|---|---|
−1.682 | −1 | 0 | 1 | 1.682 | ||
Current density (A cm−2) | x1 | 0.04 | 0.05 | 0.07 | 0.09 | 0.1 |
pH | x2 | 5.00 | 5.81 | 7.00 | 8.19 | 9.00 |
The chloride ion concentration (g L−1) | x3 | 2.50 | 3.31 | 4.50 | 5.60 | 6.50 |
The percentage removal of pollutant in the aqueous solution was calculated by using eqn (2):
![]() | (2) |
Electric energy per mass, EEM (kW h kg−1), was proposed by Bolton to judge economic feasibility, whether was suitable for large scale application.22 It was defined as the electric energy in kilowatt hour (kW h) required to degrade a kilogram of a specific pollutant in contaminated water, as described by eqn (3):
![]() | (3) |
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Fig. 2 Effects of electrode materials, electrolytic time on COD, NH3–N (a) and color (b) removal, and effect of electrode gap on COD and NH3–N removal (c). |
On the other hand, Fig. 2(a) showed the influence of reaction time on the COD and NH3–N removal rate when it was varied from 0 to 180 min. Electrolytic time had a positive effect on mineralization and decolorization of leachate. It was noted that the maximum COD and NH3–N removal was obtained with an optimal electrolytic time of about 150 min. When the allowed reaction time longer than 150 min, the removal rate were not further improved considerably.
In a parallel-plate monopolar reactor, the electrical field and conductivity could be controlled by varying electrode gap.23 In order to investigate the effect of inter-electrode distance on the efficiency of the process, the reactor was arranged such that electrodes were positioned at 1 cm to 6 cm. Fig. 2(c) showed the COD and NH3–N removal rates obtained from different distances. We could conclude that COD and NH3–N removal rates increased with an increase in electrode gap, until it was 5 cm. This might be related to diffusion limitations at small gap system. Subsequently, the removal rates was decreased. This suggested that the resistivity of the solution increased and it will reduce the mass transfer efficiency. Hence, the recommended gap in our experiment was 5 cm, which was kept constant in all experiments.
COD removal rate:
y1 = 52.91 + 9.40x1 − 9.46x2 − 2.81x3 + 0.33x1x2 + 1.46x1x3 + 1.23x2x3 + 3.68x12 + 0.84x22 + 0.33x32 | (4) |
NH3–N removal rate:
y2 = 76.74 + 18.33x1 + 11.54x2 + 1.44x3 + 3.62x1x2 + 3.67x1x3 − 2.70x2x3 − 3.95x12 − 5.03x22 − 0.29x32 | (5) |
On the basis of the experimental values, statistical testing was carried out using Fisher's test for ANOVA of regression parameters in quadratic model. Results were listed in Table 3 and indicated the second-order equation fitted well. Because the Prob > F of model was less than 0.05, and total determination coefficient R2 of COD and NH3–N reached 0.9535, 0.9749, respectively.
Source | Sum of squares | Degree of freedom | Mean square | F-Value | Prob > F | ||
---|---|---|---|---|---|---|---|
COD removal (%) | Model | 2186.53 | 9 | 242.95 | 22.78 | <0.0001 | Significant |
x1 | 845.93 | 1 | 845.93 | 79.32 | <0.0001 | ||
x2 | 1074.18 | 1 | 1074.18 | 100.72 | <0.0001 | ||
x3 | 0.49 | 1 | 0.49 | 0.046 | 0.8343 | ||
x1x2 | 22.51 | 1 | 22.51 | 2.11 | 0.1769 | ||
x1x3 | 99.55 | 1 | 99.55 | 9.33 | 0.0121 | ||
x2x3 | 13.42 | 1 | 13.42 | 1.26 | 0.2883 | ||
Residual | 106.65 | 10 | 10.67 | ||||
Lack of fit | 87.06 | 5 | 17.41 | 4.44 | 0.0637 | Not significant | |
Pure error | 19.60 | 5 | 3.92 | ||||
S.D. = 3.27, PRESS = 690.55, R2 = 0.9535, Radj2 = 0.9116, Adeq precision = 16.430. | |||||||
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NH3–N removal (%) | Model | 7246.63 | 9 | 805.18 | 43.19 | <0.0001 | Significant |
x1 | 4586.53 | 1 | 4586.53 | 246.04 | <0.0001 | ||
x2 | 1818.37 | 1 | 1818.37 | 97.55 | <0.0001 | ||
x3 | 28.50 | 1 | 28.50 | 1.53 | 0.2445 | ||
x1x2 | 104.84 | 1 | 104.84 | 5.62 | 0.0392 | ||
x1x3 | 107.75 | 1 | 107.75 | 5.78 | 0.0370 | ||
x2x3 | 58.54 | 1 | 58.54 | 3.14 | 0.1068 | ||
Residual | 186.41 | 10 | 18.64 | ||||
Lack of fit | 180.50 | 5 | 36.10 | 30.55 | 0.0009 | Significant | |
Pure error | 5.91 | 5 | 1.18 | ||||
S.D. = 4.32, PRESS = 1533.46, R2 = 0.9749, Radj2 = 0.9524, Adeq precision = 22.476. |
![]() | ||
Fig. 3 3D surface plots for COD (a–c) and NH3–N (d–f) removal efficiency as a function of two independent variables (other variables were held at their respective center levels). |
Fig. 3(b) showed COD and NH3–N removal with the variation of current density (x1) and the chloride ion concentration (x3), as well as the interaction between them. With current density at low levels, COD removal was higher with the decrease of the chloride ion concentration owning to the decrease of oxidation capacity of anode in high NaCl dosage. On the contrary, with current density at high levels, the higher removal of COD was obtained at high chlorine ion concentration. That was probably because more active free chlorine could be generated by increasing the current density and chloride concentration simultaneously, according to Czarnetzki and Janssen reported.25 It was obviously seen that the NH3–N removal exhibit the same tendency, as shown in the Fig. 3(e).
Fig. 3(c) presented the interaction between pH (x2) and the chloride ion concentration (x3) and their effects on the COD and NH3–N removal. Increasing the chloride ion concentration (x3) to 4.5 g L−1 at a range from 5 to 7 for the pH (x2) decreased COD removal rate, whereas further increase in the chloride ion concentration (x3) made the removal rate of COD remain unchanged. From 7 to 9 of the pH (x2), the chloride ion concentration increasing was usually accompanied a moderate but significant acceleration of treatment rate in terms of COD removal. Previous studies showed similar results of various electrolytes like NaCl, KCl, NaNO3, NaSO4, etc.26 But, due to low cost and easy availability, NaCl was worthy of being selected as the best electrolyte.27 For NH3–N removal shown in Fig. 3(f), there was the just the opposite with the COD removal results.
As can be seen in Fig. 3, average removal rate of NH3–N were higher than COD during the electrolysis, which was agreement with the reports by Chiang28 and Feki et al.29 During the electrochemical process, both COD and NH3–N could be removed simultaneously and there would be a competition between them yet. According to the report by Deng and Englehardt, the rule of competition between removal of COD and NH3–N seemed to be that the removal of NH3–N was greater than that of COD when indirect oxidation was prevalent, whereas COD removal took priority under direct anodic oxidation.30
Response | Current density (A cm−2) | pH | The chloride ion concentration (g L−1) | Removal rate (%) | Error | Desirability | |
---|---|---|---|---|---|---|---|
Predicted | Observed | ||||||
COD | 0.10 | 6.37 | 6.50 | 84.26 | 83.93 | 0.33 | 87.2% |
NH3–N | 0.10 | 6.37 | 6.50 | 100 | 100 | 0.00 | 87.2% |
Anode | Cathode | Current density (A cm−2) | pH | Reaction time (min) | Initial COD concentration (mg L−1) | COD removal (%) | Initial NH3–N concentration (mg L−1) | NH3–N removal (%) | References |
---|---|---|---|---|---|---|---|---|---|
a NA-not applied; NS-not specified. | |||||||||
Ti/RuO2–IrO2 | Ti | 0.116 | 8.25 | 180 | 1855 | 73 | 1060 | 49 | 31 |
Ti/RuO2–IrO2 | Ti/RuO2–IrO2 | 0.200 | 8.60 | 240 | 3973 | 87.4 | 1726.6 | NS | 32 |
Ti/RuO2–IrO2 | Stainless steel | 0.060 | 8.40 | 180 | 2091 | 20.2 | 2531 | 57.7 | 33 |
Ti/RuO2–IrO2 | Stainless steel | 0.244 | 7.60 | 41.78 | 1375 | 54.99 | 1200 | 71.07 | 34 |
Ti/RuO2–IrO2 | Zr | 0.032 | 3.00 | 240 | 2960 | 65 | 14 | NS | 35 |
Ti/RuO2–IrO2 | Cu/Zn | 0.025 | 7.80 | 360 | NA | NA | 60 | 95.98 | 36 and 37 |
Ti/RuO2–IrO2 | Fe | 0.020 | 7.00 | 180 | NA | NA | 100 | 87 | 37 |
Besides enhanced the treatment efficiency of COD and NH3–N, this procedure also had the potential to eliminate possible heavy metals, like chromium, zinc and part of the aluminum introduced during the cathodic corrosion process. A number of studies demonstrated the natural attenuation of heavy metals within a landfill. However, there were many varieties of heavy metals in landfill leachate, such as Fe, B, Al, Ni, Zn, Cr, As, Pb, Co, Se, and Cu, the concentration of which was relatively low, as shown in Table 6. After 150 min electrolytic time on the optimal conditions, the removal rates of heavy metals comparing with the initial concentrations were 99.60%, 28.57%, 100.00%, 93.33%, 16.67%, 33.33%, 95.00%, 90.00%, 100.00%, 80.00%, and 100.00%, respectively. These results could be explained with respect to cathode corrosion, where sludge provided functional groups (hydroxyl) on the large surface to remove heavy metals through electrostatic absorption or frequent coagulation.38
Species | Initial concentration (mg L−1) | Final concentration (mg L−1) | Removal rate (%) |
---|---|---|---|
Fe | 14.90 | 0.06 | 99.60 |
B | 2.80 | 2.00 | 28.27 |
Al | 0.70 | 0.00 | 100.00 |
Ni | 0.30 | 0.02 | 93.33 |
Zn | 0.30 | 0.25 | 16.67 |
Cr | 0.30 | 0.20 | 33.33 |
As | 0.20 | 0.01 | 95.00 |
Pb | 0.10 | 0.01 | 90.00 |
Co | 0.08 | 0.00 | 100.00 |
Se | 0.05 | 0.01 | 80.00 |
Cu | 0.02 | 0.00 | 100.00 |
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Fig. 5 (a) ESEM image, (b) EDS spectra and (c) FT-IR spectra of the sludge generated in the electrochemical process. |
Fig. 5(c) showed FT-IR spectra of sludge in the 500–4000 cm−1 range, which revealed formation of new species in electrochemical process. From curve, apparition of a peak at 528.5 cm−1, 657.73 cm−1 at 1377.17 cm−1 were ascribed to Al–OH, Al–O and Al–H bending, which were characteristic of Al(OH)3 or Al(OH)4−.41 As a coagulant, hydroxides of aluminum could be considered the responsible constituent of heavy metals removal. Additionally, peaks 1419.61 cm−1 and 1637.56 cm−1 were also observed corresponding to –COOH stretching and H–O–H bending respectively. C–H vibration in aromatic structures was represented by the band at 3051.39 cm−1.42 These indicated the part of organic pollutants in landfill leachate might be adsorbed on coagulant surface. It also had absorbance bands with maxima at 3442.94 cm−1 representing O–H stretching of hydroxyl groups from hydrogen bonding.43 Thus, all of these showed that coagulation process duo to the cathodic corrosion were successfully remove some pollutants.
In anode: Pollutions removal in the presence of electrolyte (NaCl) were carried out in two ways viz.:
(i) Direct oxidation: On Ti/RuO2–IrO2 anode, almost complete mineralization of some organic matter with very high current density was obtained, which occurred through direct electron transfer in the potential region. In addition, hydroxyl radicals or other reactive species were generated from water electrolysis owing to the high overpotential for oxygen production, and participated in the electrochemical oxidation at the anode surface.39 They could promote the oxidation/reduction reactions of the organic pollutants, contained in the electrochemical cell, which improved the removal of large recalcitrant organic molecules or transformed them into more easily biodegradable substances.44 This property led to an excellent COD removal efficiency.
(ii) Indirect oxidation: With the chloride ion concentration, the ability of electric conduction could be improved and the passivation of the electrode could be relieved. Moreover, chloride ions also competed with organic matter to be oxidized at the anode.45 During the electrochemical process, the chloride ion (Cl−) would be discharged at the anode to generate dissolved gas chlorine (Cl2), then the Cl2 could be chemically converted to hypochlorite ion (OCl−). This was the reason for that the chloride ion concentration in the solution had been decreased, until reached a constant value. The possible reactions occurring were listed below:
2Cl− − 2e− → Cl2 | (6) |
Cl2 + H2O → HOCl + H+ + Cl− | (7) |
HOCl → H+ + OCl− | (8) |
The sum of the three species: Cl−, Cl2, and ClO− were termed free chlorine. In the normal pH range of pond water (6–7.5), ClO− was the major component of free chlorine. In turn, as “active chlorine” possessing a high stability and oxidation capacity, OCl− could accelerate the mineralization of organics effectively. In this case, NH3–N in the leachate could be also removed preferentially through the mechanism similar to “breakpoint reactions”:46
HOCl + NH4+ → NH2Cl + H2O + H+ | (9) |
HOCl + NH2Cl → NHCl2 + H2O | (10) |
NHCl2 + H2O → NOH + 2H+ + 2Cl− | (11) |
NHCl2 + NOH → N2 + HOCl + H+ + Cl− | (12) |
On the whole, both direct and indirect oxidations were involved in COD and NH3–N removal. And COD removal by direct oxidation occurred at a higher rate than that of NH3–N, while indirect oxidation preferred removal of NH3–N than that of COD.
In cathode: Picard et al.47 showed that there was a chemical attack on the aluminum cathode by hydroxide ions generated during water reduction eqn (13), leading to increase of the pH essentially. It was well established that the dissolution occurred through the intermediate of an oxide/hydroxide film,18 which was formed spontaneously and existed on the surface of aluminium. As expressed by eqn (14) and (15), aluminum cation along with OH− ion formed a hydroxide of a network structure, large surface area and high absorption. As colloid coagulant, mainly at pH values in the range of 6.0–7.0, they promoted the generation of sweep flocs inside the treated wastewater, whose enmeshment made pollutants removed. Once the colloidal matter was destabilized, it could be separated from the wastewater. In addition to COD and NH3–N removal, this mechanism played a key role in removal of heavy metals from landfill leachate. It was found that the corrosion rate of aluminium increased during cathodic polarization, being coupled with the hydrogen evolution arising from the attack by hydroxide ions near the electrode surface. And the amount of hydroxide generated in the process was strongly influenced by the pH and the current density. Aluminum had a very low corrosion rate in neutral solutions due to the formation of an insoluble passive film, but the rapid cathodic aluminum dissolution could be observed in low or high pH electrolytes, which was in a good agreement with the results of Moon and Pyun.17,48 It was also noted that the corrosion rate increases with increasing applied cathodic current density. These could justify the important contribution of the chemical dissolution of aluminum in the cathode to the COD, NH3–N and heavy metals removal.
2H2O + e− = H2 + 2OH− | (13) |
Al + 3OH− = Al(OH)3 | (14) |
Al(OH)3 + OH− = Al(OH)4−. | (15) |
In Fig. 8, it reported the variation of specific energy consumption, as function of COD and NH3–N removal, in the optimum operating condition found previously. For low current density, the specific energy consumption increases almost linearly, while EEM(COD) increased slowly and EEM(NH3–N) increased sharply for high current density. This behaviour could be probably explained by the decrease of organic content or the formation of more refractory product in the solution. Under the optimum conditions, the electrochemical treatment for 1 kg COD and 1 kg NH3–N in landfill leachate required the power consumption of 61.59 kW h and 106.91 kW h respectively, which was close to other studies.2 Additionally, the mass loss of an aluminum electrode for a liter of leachate being treated was 0.46 g.
Observed the effects of variables using RSM, an optimal operating condition were found to be: current density of 0.1 A cm−2, pH of 6.37, the chloride ion concentration of 6.5 g L−1, electrolysis time 150 min and electrode gap 5 cm, respectively. Under these conditions, the removal rates of COD and NH3–N were found to be 83.93% and 100%, respectively, which were consistent with the overlay plot results. Therefore, RSM could be effectively adopted to optimize the operating multifactor in complex electrochemical process. In addition, the behaviors of COD, NH3–N and heavy metals removal were investigated. The predominant mechanisms included oxidation, coagulation and precipitation, confirmed by GC-MS, ESEM/EDS and FTIR analyses.
In most cases, a single technology was insufficient to achieve acceptable levels of pollution decrease. Thus, the further development of integrated different techniques is in demand for taking into account a technically and economically feasible option. The experiment proved that this method was convenient and efficient for primary or deep treatment of wastewater. Coupling with a biological unit will be a promising way, which can obtain an effluent for its reuse or discharge to natural water sources.
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