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Mechanism and optimization of electrochemical system for simultaneous removal of nitrate and ammonia
Accepted Manuscript Title: Mechanism and Optimization of Electrochemical System for Simultaneous Removal of Nitrate and Ammonia Authors: Qinan Song, Miao Li, Lele Wang, Xuejiao Ma, Fang Liu, Xiang Liu PII: DOI: Reference:
S0304-3894(18)30840-9 https://doi.org/10.1016/j.jhazmat.2018.09.046 HAZMAT 19770
To appear in:
Journal of Hazardous Materials
Received date: Revised date: Accepted date:
31-5-2018 14-9-2018 16-9-2018
Please cite this article as: Song Q, Li M, Wang L, Ma X, Liu F, Liu X, Mechanism and Optimization of Electrochemical System for Simultaneous Removal of Nitrate and Ammonia, Journal of Hazardous Materials (2018), https://doi.org/10.1016/j.jhazmat.2018.09.046 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Mechanism
and
Optimization
of
Electrochemical
System
for
Simultaneous Removal of Nitrate and Ammonia
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Qinan Song, Miao Li*, Lele Wang, Xuejiao Ma, Fang Liu, Xiang Liu
School of Environment, Tsinghua University, Beijing 100084, China
Corresponding author. Tel: +86 10 62772485; E-mail address: [email protected] (M. Li)
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Graphical abstract
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Highlights
A novel electrochemical system for multiple nitrate and ammonia removal is
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established. Simultaneous electrochemical nitrate and ammonia removal was achieved in an
The electrochemical coupling mechanism and concurring evolution of nitrogen were
undivided single cell.
explored by cyclic voltammetry (CV).
Mathematical models were built to optimize the operating parameters by response
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surface methodology.
Abstract In this study, an electrochemical system was established for simultaneous harmless removal of nitrate and ammonia multiple contamination in an undivided single cell. Cyclic
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voltammetry was used to investigate the electrochemical cathode and anode coupling redox mechanism and concurring evolution of nitrate and ammonia. The cyclic voltammograms
showed the cathodic reduction of nitrate to ammonia and nitrite, the chloride ion conversion to hypochlorite and hypochlorous acid, and the oxidation of ammonia to nitrogen gas and nitrate. A circular transformation process was formed in the
electrochemical system and the final product was harmless nitrogen gas. The multiple
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nitrogen pollutants in the original contaminated system were gradually removed with the
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reaction predominantly produced harmless nitrogen gas. Response surface methodology was used to build mathematical models for optimizing the operating conditions. The
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optimum time, NaCl concentration, and current density were 85.38 min, 0.24 g/L, and
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45.13 mA/cm2, respectively. Under the optimum conditions, the nitrate and ammonia
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concentrations in the treated solution were 9.17 and 0.00 mg/L, respectively.
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Keywords Cyclic voltammetry; Optimization; Electrochemical method; Nitrate and Ammonia removal
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1. Introduction
Nitrate contamination is a common environmental problem and is found extensively
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in groundwater. Overuse of nitrogen fertilizer and sewage discharge are major source of nitrate contamination [1,2]. Excessive nitrate accumulation can harm animals and plants
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through toxic effects or oxygen depletion, and pose great threat on human public health. Nitrate can be reduced to nitrite during natural and human activities, which is one of the direct predisposing factors in iron hemoglobin disease and blue baby syndrome [3,4]. The World Health Organization (WHO) limits the maximum acceptable nitrate concentration to 10 mg/L for drinking water [5].
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In general, various remediation methods are employed for nitrate removal and can be mainly divided into physical methods, chemical methods and biological methods [6–11]. Physical methods such as ion exchange and membrane separation only transfer or concentrate nitrate contaminants rather than degrading them, which result in the second contamination [6,7]. Chemical reduction method has issues of long-term reactivity,
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mobility and possible toxicity as it may generate toxic products such as nitrite and ammonia
[8,9]. For biological method, denitrification rely on carbon resources and operating
temperature, and needs a long period for microbial culturing [10,11]. Many studies combined several technologies to solve the above problems, but this increased the investment cost [12,13].
Recently, electrochemical nitrate reduction attract attention due to its no sludge
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production, low investment cost and high treatment efficiency [14–25]. More importantly,
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it can realize the removal of by-products in a single reactor as the reaction proceeds. Nitrate can be reduced with electrolysis and six products are produced, namely NO2−, N2, NH3,
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NH2OH, N2O, and NO [14]. The undesirable by-products can be contained through using
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highly selective electrodes and adding chloride ion into the electrolyte. Cu is one of the most efficient and economic cathodes for electrochemical reduction of nitrate, while
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ammonia and nitrogen gas are the final reduction products [15,16]. Ti/IrO2 anode has been
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reported to be used in electrochemical oxidation and it can give high selectivity for conversion of nitrate to nitrogen gas [17,18]. It has been reported that adding chloride ion into the electrolyte can indirectly oxidize the reductive by-products of ammonia and nitrite
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[19–25]. Pressley et al. used sodium hypochlorite and chlorine to oxidize ammonia and proved that nitrogen gas was the main stable product of the reaction [22]. Li et al. achieved
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complete removal of sole ammonia contamination with a Ti/IrO2 anode by electrochemical method [23]. Li et al. achieved 94.3% nitrate removal, with no detected by-products, in the
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presence of 0.5 g/L NaCl with an Fe cathode [24]. Fan et al. investigated the electrochemical denitrification using a new electrode combination of Ti/IrO2-TiO2-RuO2 and Cu/Zn and studied the kinetics [25]. Ammonia can also enter groundwater with infiltration of litter, humus and manure [26,27]. It is known that active nitrogen cycle occurs in natural environment and is affected by environmental conditions [28]. Nitrate and ammonia often co-exist as coupled multiple 3
contamination rather than as single contamination in most of contaminated water bodies [29]. The current studies of electrochemical removal mainly focus on ammonia oxidation and nitrate reduction separately, but seldom consider the conversion mechanism and concurring evolution of ammonia and nitrate together in an electrochemical system. The simultaneous electrochemical removal of nitrate and ammonia multiple contamination has
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not been reported. This makes the technology far from actual application. It needs to be
determined whether, and under which conditions, multiple nitrate and ammonia can be simultaneously and harmlessly removed to make the technology closer to practical application.
The aim of the present study is to (1) develop an electrochemical system for
simultaneous and harmless removal of nitrate and ammonia multiple contamination in a
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single undivided reactor without a membrane; (2) investigate the operating factors and
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achieve a better removal efficiency; (3) reveal the conversion mechanism and concurring evolution of nitrogen in electrochemical system. A laboratory-scale electrolysis cell was
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used. The Box–Behnken method was used to design experiments for the main operating
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factors, i.e., time, NaCl concentration, and current density, to investigate the interactions among these three factors that affect the responses (nitrate and ammonia removal
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efficiencies). Response surface method (RSM) was used to optimize the operating
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parameters and achieve a better removal efficiency. Cyclic voltammetry (CV) was used to explore the electrochemical coupling mechanism of cathodic reduction and anodic oxidation for multiple nitrogen contamination.
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2. Materials and methods
2.1. Electrochemical system
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In this study, Ti/IrO2 electrode of 24.5 cm2 (9.8 cm×2.5 cm) was used as anode, Cu
electrode of 37.5 cm2 (15 cm×2.5 cm) was used as cathode. An undivided cylindrical
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electrochemical cell was made from polymethyl methacrylate with effective volume of 100 mL. Both electrodes had the same effective reaction area of 20 cm2 in solution. The distance between the anode and cathode was set as 8 mm. A DC regulated power supply was employed with a current range of 0–5 A and a voltage range of 0–50 V. Synthetic solution of multiple contamination with 50.0 mg/L nitrate (NaNO3) and 10.0 mg/L ammonia ((NH4)2SO4) was prepared for electrolysis. 0.50 g/L Na2SO4 was added to 4
improve the conductivity. NaCl at different concentrations (0.0–0.5 g/L) were added as one of the independent variables. Before electrolysis, the cathode was polished well with 100 to 180 mesh sandpaper, then rinsed with deionized water and dried in air. 100 mL synthetic solution prepared as above was poured into the cell and the DC power was connected. The current was switched off after a specified run time, and 0.5 mL of sample was drawn by
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pipette for analysis. All the experiments were carried out at room temperature (25±2℃). 2.2. Cyclic voltammetry measurements
Cyclic voltammetry (CV) was used to study the electrochemical reaction mechanism on the electrode surface. Cyclic voltammograms were recorded by an electrochemical workstation (CHI660D, CH Instruments, ShangHai) and an ALS Software (ALS Limited,
Model 660) on computer. Ti/IrO2 anode or Cu cathode were served as working electrode,
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respectively. Correspondingly, Cu cathode or Ti/IrO2 anode were served as the reference
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electrodes. All potentials in this study were referred to saturated calomel electrode (SCE). The electrode system was cycled in the range of −1.6 V to +0.2 V and −2.0 V to +2.0 V at
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a scan rate of 100 mV/s for three times before test to form a stable state.
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2.3. Design of experiments
Response surface method (RSM) is an efficient optimization method which can
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effectively evaluate the interaction of multiple complex factors and get an optimal response
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with a minimum number of well-designed experiments [30–32]. Box−Behnken design (BBD) is one of the RSM designs with many advantages, which only requires 15 runs for a three-parameter experimental design [33]. RSM optimization has been used widely in
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pollutant removal [34–36]. In this study, the RSM optimization was achieved by Design Expert 8 Application (StatEase, registered) regression program. The independent variables
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and coded levels were defined by Box–Benken design and are presented in supplementary materials (Table S1).
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For the response surface models, the independent variables were the electrolysis time
(X1), NaCl dosage (X2), and current density (X3), respectively. The low, center and high level of each variable were set to −1, 0, and +1, respectively. The dependent variables were the nitrate concentration (Y1) and ammonia concentration (Y2). The experiment results of dependent variables for 15 runs were given along with independent variables as shown in Table 1. These experiments were carried out in the designed order to minimize the external 5
effect of on the responses. The central point (0, 0, 0) was repeated 3 times to evaluate the error for reliability verification. Significance of coefficient of the model was determined by p-values. If p-values < 0.05, the response variables are significant [34]. No nitrite was detected in the treated water under all conditions. 2.4. Analysis
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All analyses were done according to standard methods [37]. Nitrate, nitrite and ammonia were detected by UV-spectrophotometry, N-1-naphthyl-ethylenediamine
spectrophotometric method and Nessler reagent spectrophotometry by a highly sensitive
spectrophotometer (DR500, HACH, USA), respectively. The LoD for chemical analysis of nitrogen were shown in the supplementary materials. Power density (PD, mW/m2) is
calculated by PD = IU/S, where I and U are current (A) and voltage (mV), respectively, S
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(m2) is the geometrical area of the anode.
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3. Results and discussion 3.1. Fitting and analysis of the response models
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Fitting of empirical models to the experimental data was carried out by RSM to
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quantitatively describe the behavior of the response. The mathematical-statistical relationship between the independent variables (X) and the response functions (Y) can be
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characterized by a quadratic polynomial as follows: (13)
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Y = b0 +b1 X1 +b2 X2 +b3 X3 +b12 X1 X2 +b13 X1 X3 +b23 X2 X3 +b11 X21 +b22 X22 +b33 X23
Equation 14−15 show the response functions with the defined coefficients for nitrate concentration and ammonia concentration.
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Ynitrate = 63.2375 − 0.6719X1 +38.6833X2 − 0.10833X3 − 0.1083X1 X2 +4.4333×10-3 X1 X3 −0.1640X2 X3 +1.7384×10-3 X21 − 29.3667X22 − 4.2533×10-3 X23 (R2 =0.9992) (14)
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Yammonia = 71.6525 − 1.3777X1 − 344.66X2 +1.6654X3 +1.096X1 X2 +4.7467×10-3 X1 X3
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−0.4992X2 X3 +6.2806×10-3 X21 +390.92X22 − 0.0245×10-3 X23 (R2 =0.9676)
(15)
In the equations, X1, X2 and X3 represent time, NaCl concentration and current
density, respectively. Ynitrate and Yammonia indicate the predicted nitrate and ammonia concentration in multiple contamination. The closer the correlation coefficient (R2) is to 1, the polynomial equality is more accurate [35,38]. The calculated R2 (R12=0.9992, R22=0.9666) indicate the response function predictions are in good agreement with experimental data and the predictability of the models is at 95% confidence level, i.e., only 6
0.08% and 3.34% variations are not explained by the regression model in Equation 14 and 15, respectively. The equation coefficients show the impact of independent variables (X) on response functions (Y). The sign ahead shows its improvement for ammonia removal and inhibition for nitrate removal, which is consistent with the previous studies [19,39]. According to
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these, the performances of electrochemical multiple nitrate and ammonia removal are
analyzed. The coefficients of −0.6719 and −1.3777 indicates that time (X1) promotes both nitrate and ammonia removal rate. +38.6833 and −0.34466 of NaCl dosage (X2) for Ynitrate
and Yammonia show that NaCl addition inhibited nitrate removal but greatly promoted ammonia removal. The higher absolute value of coefficients for X2 than other variables
show the nitrate reduction and ammonia oxidation behaviors mainly controlled by NaCl
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dosage. It is indicated that NaCl dosage has a more significant impact on the multiple
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contamination, especially for ammonia (344.66). Similarly, the current density (X3) is negatively correlated with Ynitrate (−0.10833) and positively correlated with Yammonia
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(+1.6654), indicating that nitrate removal is promoted but ammonia removal is inhibited
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as the current density increases. In sum, the three factors with the significance level order of NaCl dosage > current density > time.
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The variance (ANOVA) analyses (Table 2 and Table S2) show the fitting results for
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the response surface model. Significance of model is verified by p-values and F-values. The F-value means the ratio of regression mean square to the estimated parameter standard deviation. The p-value means the probability of the occurrence of F-value [40]. In this
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study, both models are significant (Fnitrate is 655.34, pnitrate less than 0.0001; Fammonia is 16.06, pammonia is 0.0035 less than 0.05). The coefficients of variation (C.V., ratio of the
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standard deviation to the average of the data) for nitrate and ammonia model are 2.29% and 52.5%, respectively. This demonstrates the discrete level of the tested items is low.
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Table 2 shows significance of the coefficients of Equation 15. Statistical analysis for this model shows that the terms of X2 and X22, i.e. the liner and quadratic term of NaCl concentration are significant (p is 0.0002 and 0.00, respectively). Again, the results of the coefficient analysis are proved here. The interactive terms of time and NaCl concentration (X1X2) (p is 0.0797), NaCl concentration and current density (X2X3) (p is 0.6546) and time and current density (X1X3) (p is 0.6944) are all insignificant. The quadratic terms of time 7
(X12) (p is 0.2068), current density (X32) (p is 0.3708) also presents insignificant. All the results prove that the change of NaCl concentration has the most important effect on ammonia removal in this system. The C.V. of 52.5% demonstrates the low discrete level of data of every tested item, suggesting that the regression model is also relevant and credible. The ANOVA test analyses for response function Ynitrate (Table S2) are shown in the
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supplementary materials.
The three-dimensional (3D) response surface plots and relevant contour plots (Figure 1) were built depended on the model functions (Equation 14 and 15), they intuitively illustrated the interaction effect of two independent variables on the response functions,
i.e., nitrate concentration and ammonia concentration [38]. Figure 1 (A) and (B) represents the interaction of NaCl concentration with current density when time is at the center point
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of 60.00 min. Figure 1 (C) and (D) show the interaction of NaCl concentration with time
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when the current density is 37.5 mA/cm2. All the 3D response surface plots show a nonlinear curved surface, and their trends are consistent with those shown in Table 1. The
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other three-dimensional response surfaces are shown in the supplementary materials
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(Figure S2). However, the residual nitrate concentration and the energy consumption need to be further reduced, therefore the operating conditions should be optimized.
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3.2. Optimization by desirability functions
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The numerical desirability function was used to find a specific maximum desirable point (Figure S2). The weight was altered to fit the desirable goal. Five options were offered for response in the goal fields, i.e., none, minimum, maximum, within range and
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target. This study aimed at minimizing the nitrate and ammonia concentration to the standard concentration. Therefore, the objectives for the independent variables (time, NaCl
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concentration and current density) were set to “minimum” with corresponding weight of 1. The lower limit value of time, NaCl concentration and current density were set to 30 min,
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0.0 g/L and 25 mA/cm2, respectively. Similarly, the objectives for the dependent variables (nitrate and ammonia concentration after electrolysis) were set to “minimum” with corresponding weight of 1. The target ranges of nitrate and ammonia concentration were set to 9−10 mg/L and 0−0.5 mg/L, respectively. The optimization program was carried out under the above settings and boundaries. As the results, the optimal operating conditions of time, NaCl concentration and current density were 82.08 min, 0.24 g/L and 45.31 8
mA/cm2, respectively. Under the optimum conditions, the nitrate and ammonia concentrations in the treated solution were 9.17 and 0 mg/L, respectively. The predicted result was verified. According to the data obtained from the optimized results, three groups of experiments were carried out under the optimal operating condition (82.08 min, 0.24 g/L NaCl and 45.31 mA/cm2). The result showed that the average concentrations of nitrate
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and ammonia were 8.87 and 0 mg/L, respectively. It was responsible as its deviations from predicted result were both less than 5%. The final concentration met the standards for drinking water quality (nitrate less than 10 mg/L). To sum up, combined application of Box−Behnken design and desirability function can effectively optimize the experiment
design on electrochemical removal of nitrate and ammonia multiple contamination. No
nitrite was detected in the treated water under all conditions. The results indicate that
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simultaneous electrochemical removal of nitrate and ammonia multiple contamination can
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be achieved in a single undivided cell. Furthermore, CV was used to investigate the mechanisms of cathodic reduction and anodic oxidation for the electrochemical
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degradation system.
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3.3. Mechanism of anodic oxidation and cathodic reduction 3.3.1. Reaction process on surface of Cu cathode
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Cyclic voltammograms obtained for the Cu cathode in an electrolyte containing 0.3
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g/L NaCl (line c) and a NaCl-free electrolyte (line b) at a scan rate of 100 mV/s in the potential range −1.8 to +0.2 V are shown in Figure 2. These are used to clarify the reaction process on the cathode surface. In all experiments, the blank electrolyte (line a) contained
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only deionized water and 0.5 g/L Na2SO4 to exclude other potential interference. The four groups of curves in Figure 2 were obtained by scanning the Cu cathode with different
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contaminated matrix types. In all the curves, a reduction peak is observed at −0.3 V (labelled p), which is presumably related to the electrolytic reaction of water under aerobic
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conditions, at the standard oxidation–reduction potential (equation 1; the reference potential of the saturated calomel electrode is +0.2415 V). The peak current increases in the presence of NaCl (curve c), as observed in Figure 2 (A), which shows that chloride ion can promote electrochemical aerobic hydrolysis. O2 + 2H2 O + 2e– → 2OH– (E = –0.401 V)
(1)
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In Figure 2 (B) and (D), the cathodic current caused by the hydrogen evolution reaction on the Cu cathode starts to increase at −1.0 V (equations 2–4) [41]. The reduction peak at −0.8 V (labelled r) in Figure 2 (B) and (D) corresponds to nitrate reduction. The peak is not present in Figure 2 (A) and (C), i.e., in the absence of nitrate. This shows that
consistent with previous studies [42]. NO3− + H2 O + 2e− → 2OH− + NO2− NO3− + 3H2 O + 5e− → 6OH− +
(2)
1 N↑ 2 2
(3)
NO3− + 6H2 O + 8e− → 9OH− + NH3 ↑
(4)
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nitrate was reduced to nitrite, nitrogen gas, and ammonia (equations 2−4), which is
In all the curves, a clear peak is observed near −0.7 V (labelled q). This peak
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corresponds to the oxidation/reduction of Cu2O to Cu [39]. In Figure 2 (B) and (D), peak
q is hidden by the prominent peak r, which arises from active nitrate reduction. Figure 2
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(C) shows a gradual forward shift of the current peak q on addition of ammonia and NaCl.
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This is because the added chloride ion was oxidized to acidic substances, which reduced
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the pH of the system. Acidic electrolyte solution promoted electrochemical corrosion of the Cu electrode [43,44]. To confirm this speculation, the working electrode was changed
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to the anode for three cycles. A floating layer of a green substance formed on the surface of the electrolyte near the electrode. It supposed to be tribasic copper chloride
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[CuCl2·3Cu(OH)2] formed by the reaction between the Cu electrode and chloride ion [44]. Curves b and c in Figure 2 (B) and (D) show a similar cathodic current change, but a
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more negative potential was required to initiate the hydrogen evolution reaction in the case of multiple nitrate and ammonia contamination (D) than in the case of single nitrate
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contamination (B). This is because ammonia decreases the nitrate removal rate, according to the ion-exchange equilibrium. In contrast, the trends and peak locations for the CV
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curves in Figure 2 (A) and (C) showed no significant differences, indicating that ammonia did not participate in the cathodic reduction. To sum up, the basic cathode reaction can be described based on the above CV curves,
which is consistent with the results of independent experiments and previous studies [42– 44]. However, some peaks on curve c in the cathodic CV curves should be discussed in combination with anodic reaction due to the existence of NaCl. Furthermore, the questions
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raised by independent experiments regarding how chlorine affects ammonia removal and why chloride inhibits nitrate removal have not been answered. CV scanning of a Ti/IrO2 anode therefore needs to be performed to explain the reaction mechanism of the entire electrochemical system. 3.3.2. Reaction process on surface of Ti/IrO2 anode
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The cyclic voltammograms obtained for a Ti/IrO2 anode in different electrolytes at a
scan rate of 100 mV/s in the potential range −2.0 V to + 2.0 V are shown in Figure 3. They
were used to investigate the reaction process on the electrode surface. In all experiments,
a blank electrolyte that contained only deionized water and 0.5 g/L Na2SO4 was used to
exclude other potential interference. Cyclic voltammograms for the anode were obtained in an electrolyte containing 0.3 g/L NaCl (line a) and in a NaCl-free electrolyte (line b).
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The peaks on the dotted line in Figure 3 (C) and (D) correspond to the oxidation peak of
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ammonia at a potential of +1.2 V (labelled m), which are not present in Figure 3 (A) and (B). Kapałka et al. suggest that this is because direct electrolytic ammonia oxidation occurs
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around the anode at the above potential (equation 5). This can also explain the decrease in 1
NH3 + 3H2 O → N2 + 3H+ + 3e– 2
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the amount of ammonia in the absence of NaCl (Table 1) [45]. (5)
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The oxidation peak (labelled n) on line a at +1.4 to +1.5 V is not present on line b in
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all four diagrams, indicating that the peak is related to NaCl. The potential at which peak n appears is consistent with the standard redox potential of hypochlorous acid (the
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reference potential of the saturated calomel electrode is +0.2415 V), therefore the peak is attributed to the oxidation of chloride ion to chlorine gas at the anode and then conversion to hypochlorous acid and hypochlorite (equation 6−8) [19,42,46–48]. (6)
Cl2 + H2 O → HClO + H+ + Cl−
(7)
HClO → + H+ + ClO−
(8)
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2Cl− → Cl2 + 2e−
Subsequently, hypochlorite and hypochlorous acid, which are strongly oxidizing,
oxidize ammonia to nitrogen and nitrate (equation 9−12) [22]. The above results for anode manifest that chlorine ions can be transformed to strong oxidizing substances (mainly hypochlorite and hypochloric acid) when the anode relative potential is higher than +1.5 V, then promoting oxidation removal of ammonia. 11
2NH+4 + 3HClO → 3H2 O + 5H+ + 3Cl− + N2 ↑
(9)
2NH+4 + 3ClO− → 3H2 O + 2H+ + 3Cl− + N2 ↑
(10)
NH+4 + 3HClO → H2 O + 6H+ + 4Cl− + NO3−
(11)
NH+4 + 3ClO− → H2 O + 2H+ + 4Cl− + NO− 3
(12)
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Nitrite is supposed to be one of the main by-products according to equation 2. However, no nitrite was detected in the treated water under all conditions. That’s because
nitrite is easy to be transformed to ammonia and nitrate by hydrogen ion and hypochlorite (equation 13−14) according to the previous studies [14,49]. NO2− + HClO → NO3− + H+ + Cl−
(13)
NO2− + 8H+ → 3H2 O + NH+4
(14)
The changes to the CV curves of cathode caused by NaCl addition can be explained
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based on the results of anode. Figure 2 (B) shows a more negative potential on curve c for
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nitrate reduction (peak r) than on curve b. This is because oxidizing hypochlorite and
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hypochlorous acid formed at the anode inhibit the cathodic reduction of nitrate in the
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presence of chloride ion. Curve c in Figure 2 (C), which was obtained in the absence of nitrate, shows an unexpected nitrate reduction peak r. This is because nitrate was generated from ammonia by oxidation with hypochlorous acid and hypochlorite (equation 11−12). A
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comparison of Figure 2 (C) and (D) shows a better coincidence degree for the nitrate
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reduction peak r on the three curves for multiple contamination (D), indicating that NaCl has no effect on the nitrate reduction rate in this case. The possible reasons for this
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phenomenon are as follows. The mechanism of the effect of chloride on nitrate reduction is mainly related to competitive adsorption of chloride ion and interference with the redox
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potential of the system by hypochlorous acid. In the presence of ammonia, hypochlorous acid formed by oxidation of chloride ions will therefore be consumed rapidly during ammonia oxidation, and conversion of chloride ions to hypochlorous acid will be
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accelerated. It is therefore concluded that the presence of ammonia can counteract the effect of NaCl on the nitrate reduction rate. So far, the mechanism of simultaneous electrochemical removal of nitrate and ammonia multiple contamination in an undivided electrochemical system (Figure 4) is clear.
First, when a current is supplied, the nitrate around the cathode, as an electron
donor, is reduced to nitrite, ammonia, and nitrogen gas. Chloride ion around the anode is 12
oxidized to chlorine gas, which reacts with water to produce hypochlorite and hypochlorous acid. This is the crucial step in the reaction. A small portion of ammonia is converted to nitrogen gas by direct electrolytic oxidation at the anode. Nitrite and most of the ammonia are oxidized by the generated hypochlorite and hypochlorous acid to nitrogen gas and nitrate. A circular transformation process is therefore established. As the reaction
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proceeds, multiple nitrogen pollutants in the original contaminated system are gradually removed as the reaction predominantly produces harmless nitrogen gas. Finally, the goal of harmless removal of multiple nitrate and ammonia contamination is achieved. 4. Conclusions
The following conclusions can be drawn from the present study.
(1) A novel electrochemical system for multiple nitrate and ammonia removal is
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established and studied the conversion mechanism and concurring evolution of nitrogen
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for the first time. Simultaneous and harmless removal of nitrate and ammonia multiple contamination were achieved in a single electrochemical cell without a membrane, which
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makes the technology more feasible in practical application.
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(2) According to the CV, the electrochemical cathodic reduction and anodic oxidation is found to form a coupling action on degradation of the nitrate, ammonia and nitrite. The
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crucial step is the oxidation of chloride ion to hypochlorite and hypochlorous. A circular
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transformation process is established in the electrochemical system. The multiple nitrogen pollutants in the original contaminated system are gradually removed with the reaction predominantly produces harmless nitrogen gas.
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(3) This study applied RSM to investigate the effect of time, NaCl dosage and current density on the nitrate and ammonia removal in electrochemical system. To further explore
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the optimum the interaction of the operating conditions, two response functions were modelled: nitrate and ammonia concentration. The most sensitive parameter for nitrate and
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ammonia removal is NaCl concentration. Operating conditions were all independent for the ammonia removal response surface. Interaction effects between time and current density, time and NaCl concentration were observed for the modelled nitrate removal response surface. The optimum removal rate was achieved when time, NaCl concentration, and current density were 82.08 min, 0.24 g/L, and 45.31 mA/cm2, respectively. Under these
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conditions, the nitrate and ammonia concentrations were degraded to 8.87 and 0 mg/L, respectively.
Acknowledgments
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The authors thank the National Water Pollution Control and Treatment Science and Technology Major Project (2017ZX07202002), Beijing Natural Science Foundation
(J150004) and Key Technology and Project of Jinan Water Environment Control (No.201509002) for the financial support of this work.
J. Zhou, B. Gu, W.H. Schlesinger, X. Ju, Significant accumulation of nitrate in Chinese semi-
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[1]
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doi:10.1016/j.apcatb.2017.02.005.
Environ.
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SC RI PT U N A M Three-dimensional response surface plots for effect of NaCl dosage and current
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Figure 1.
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density on (A) ammonia removal and (B) nitrate removal (mg/L); and for NaCl dosage and
A
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time on (C) ammonia removal and (D) nitrate removal.
19
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Cyclic voltammetry curves for Cu cathode. Line a, b and c imply the blank,
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Figure 2.
blank adding 0.3 g/L NaCl and electrolyte solution adding 0.3 g/L NaCl, respectively. The
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electrolyte solution above contains: (A) 0.5 g/L Na2SO4, (B) 0.5 g/L Na2SO4 +50.0 mg/L nitrate, (C) 0.5 g/L Na2SO4 +10.0 mg/L ammonia, (D) 0.5 g/L Na2SO4 +50.0 mg/L nitrate
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+10.0 mg/L ammonia.
20
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Figure 3. Cyclic voltammetry curves for Ti/IrO2 anode. Electrolyte solution used contains:
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(A) 0.5 g/L Na2SO4, (B) 0.5 g/L Na2SO4 +50.0 mg/L nitrate, (C) 0.5 g/L Na2SO4 +10.0
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mg/L ammonia, (D) 0.5 g/L Na2SO4 +50.0 mg/L nitrate +10.0 mg/L ammonia. Line a and
A
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line b imply whether 0.3 g/L NaCl is added or not, respectively).
21
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Mechanism of electrochemical system for removing nitrate and ammonia
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multiple contamination in the presence of Cl−.
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Figure 4.
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Observed nitrate and ammonia concentrations X1:Time (min)
X2:NaCl concentration (g/L)
X3:Current density (mA/cm2)
Y1:Nitrate concentration (mg/L)
Y2:Ammonia concentration (mg/L)
1
60.00
0.50
25.00
23.60
0.06
2
30.00
0.00
37.50
23.05
77.15
3
60.00
0.00
50.00
10.35
44.80
4
90.00
0.00
37.50
5.50
44.04
5
30.00
0.25
50.00
24.25
0.41
6
60.00
0.50
50.00
14.50
0.00
7
90.00
0.50
37.50
10.05
0.00
8
30.00
0.50
37.50
30.85
0.23
9
60.00
0.25
37.50
17.45
0.28
10
30.00
0.25
25.00
35.20
7.74
11
90.00
0.25
50.00
7.85
0.00
12
60.00
0.25
37.50
17.60
0.26
13
60.00
0.00
25.00
17.40
38.62
14
60.00
0.25
37.50
17.85
0.27
A
N
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Obs StdOrder
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Table 1.
A
CC
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D
M
15 90.00 0.25 25.00 12.15 0.21 Initial nitrate concentration is 50 mg/L; Initial ammonia concentration is 10 mg/L.
23
Sum of Squares df
Mean Square F Value
p-value Prob > F
Model
8114.87
9
901.65
16.06
0.0035
A–Time
213.02
1
213.02
3.79
0.109
B–NaCl
5218.13
1
5218.13
92.95
0.0002
C–Current density
0.25
1
0.25
4.51E-03
0.9491
AB
270.21
1
270.21
4.81
0.0797
AC
12.68
1
12.68
0.23
BC
9.73
1
9.73
0.17
A
118
1
118
2.1
B2
2204.36
1
2204.36
39.26
54.24
1
54.24
0.97
Residual
280.71
5
56.14
Lack of Fit
280.71
3
93.57
Pure Error
9.80E-05
2
4.90E-05
Cor Total
8395.57
14
1.91E+06
N
C
2
A
2
A
CC
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TE
D
M
R2 = 0.9666, Adj R2 = 0.9064, Pred R2 = 0.4650, C.V. = 52.5%
24
significant
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Source
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Table 2. ANOVA test for response function Yammonia
0.6546 0.6944 0.2068 0.0015 0.3708
< 0.0001
significant
S0304-3894(18)30840-9 https://doi.org/10.1016/j.jhazmat.2018.09.046 HAZMAT 19770
To appear in:
Journal of Hazardous Materials
Received date: Revised date: Accepted date:
31-5-2018 14-9-2018 16-9-2018
Please cite this article as: Song Q, Li M, Wang L, Ma X, Liu F, Liu X, Mechanism and Optimization of Electrochemical System for Simultaneous Removal of Nitrate and Ammonia, Journal of Hazardous Materials (2018), https://doi.org/10.1016/j.jhazmat.2018.09.046 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Mechanism
and
Optimization
of
Electrochemical
System
for
Simultaneous Removal of Nitrate and Ammonia
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Qinan Song, Miao Li*, Lele Wang, Xuejiao Ma, Fang Liu, Xiang Liu
School of Environment, Tsinghua University, Beijing 100084, China
Corresponding author. Tel: +86 10 62772485; E-mail address: [email protected] (M. Li)
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Graphical abstract
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Highlights
A novel electrochemical system for multiple nitrate and ammonia removal is
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established. Simultaneous electrochemical nitrate and ammonia removal was achieved in an
The electrochemical coupling mechanism and concurring evolution of nitrogen were
undivided single cell.
explored by cyclic voltammetry (CV).
Mathematical models were built to optimize the operating parameters by response
1
surface methodology.
Abstract In this study, an electrochemical system was established for simultaneous harmless removal of nitrate and ammonia multiple contamination in an undivided single cell. Cyclic
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voltammetry was used to investigate the electrochemical cathode and anode coupling redox mechanism and concurring evolution of nitrate and ammonia. The cyclic voltammograms
showed the cathodic reduction of nitrate to ammonia and nitrite, the chloride ion conversion to hypochlorite and hypochlorous acid, and the oxidation of ammonia to nitrogen gas and nitrate. A circular transformation process was formed in the
electrochemical system and the final product was harmless nitrogen gas. The multiple
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nitrogen pollutants in the original contaminated system were gradually removed with the
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reaction predominantly produced harmless nitrogen gas. Response surface methodology was used to build mathematical models for optimizing the operating conditions. The
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optimum time, NaCl concentration, and current density were 85.38 min, 0.24 g/L, and
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45.13 mA/cm2, respectively. Under the optimum conditions, the nitrate and ammonia
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concentrations in the treated solution were 9.17 and 0.00 mg/L, respectively.
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Keywords Cyclic voltammetry; Optimization; Electrochemical method; Nitrate and Ammonia removal
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1. Introduction
Nitrate contamination is a common environmental problem and is found extensively
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in groundwater. Overuse of nitrogen fertilizer and sewage discharge are major source of nitrate contamination [1,2]. Excessive nitrate accumulation can harm animals and plants
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through toxic effects or oxygen depletion, and pose great threat on human public health. Nitrate can be reduced to nitrite during natural and human activities, which is one of the direct predisposing factors in iron hemoglobin disease and blue baby syndrome [3,4]. The World Health Organization (WHO) limits the maximum acceptable nitrate concentration to 10 mg/L for drinking water [5].
2
In general, various remediation methods are employed for nitrate removal and can be mainly divided into physical methods, chemical methods and biological methods [6–11]. Physical methods such as ion exchange and membrane separation only transfer or concentrate nitrate contaminants rather than degrading them, which result in the second contamination [6,7]. Chemical reduction method has issues of long-term reactivity,
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mobility and possible toxicity as it may generate toxic products such as nitrite and ammonia
[8,9]. For biological method, denitrification rely on carbon resources and operating
temperature, and needs a long period for microbial culturing [10,11]. Many studies combined several technologies to solve the above problems, but this increased the investment cost [12,13].
Recently, electrochemical nitrate reduction attract attention due to its no sludge
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production, low investment cost and high treatment efficiency [14–25]. More importantly,
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it can realize the removal of by-products in a single reactor as the reaction proceeds. Nitrate can be reduced with electrolysis and six products are produced, namely NO2−, N2, NH3,
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NH2OH, N2O, and NO [14]. The undesirable by-products can be contained through using
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highly selective electrodes and adding chloride ion into the electrolyte. Cu is one of the most efficient and economic cathodes for electrochemical reduction of nitrate, while
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ammonia and nitrogen gas are the final reduction products [15,16]. Ti/IrO2 anode has been
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reported to be used in electrochemical oxidation and it can give high selectivity for conversion of nitrate to nitrogen gas [17,18]. It has been reported that adding chloride ion into the electrolyte can indirectly oxidize the reductive by-products of ammonia and nitrite
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[19–25]. Pressley et al. used sodium hypochlorite and chlorine to oxidize ammonia and proved that nitrogen gas was the main stable product of the reaction [22]. Li et al. achieved
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complete removal of sole ammonia contamination with a Ti/IrO2 anode by electrochemical method [23]. Li et al. achieved 94.3% nitrate removal, with no detected by-products, in the
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presence of 0.5 g/L NaCl with an Fe cathode [24]. Fan et al. investigated the electrochemical denitrification using a new electrode combination of Ti/IrO2-TiO2-RuO2 and Cu/Zn and studied the kinetics [25]. Ammonia can also enter groundwater with infiltration of litter, humus and manure [26,27]. It is known that active nitrogen cycle occurs in natural environment and is affected by environmental conditions [28]. Nitrate and ammonia often co-exist as coupled multiple 3
contamination rather than as single contamination in most of contaminated water bodies [29]. The current studies of electrochemical removal mainly focus on ammonia oxidation and nitrate reduction separately, but seldom consider the conversion mechanism and concurring evolution of ammonia and nitrate together in an electrochemical system. The simultaneous electrochemical removal of nitrate and ammonia multiple contamination has
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not been reported. This makes the technology far from actual application. It needs to be
determined whether, and under which conditions, multiple nitrate and ammonia can be simultaneously and harmlessly removed to make the technology closer to practical application.
The aim of the present study is to (1) develop an electrochemical system for
simultaneous and harmless removal of nitrate and ammonia multiple contamination in a
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single undivided reactor without a membrane; (2) investigate the operating factors and
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achieve a better removal efficiency; (3) reveal the conversion mechanism and concurring evolution of nitrogen in electrochemical system. A laboratory-scale electrolysis cell was
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used. The Box–Behnken method was used to design experiments for the main operating
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factors, i.e., time, NaCl concentration, and current density, to investigate the interactions among these three factors that affect the responses (nitrate and ammonia removal
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efficiencies). Response surface method (RSM) was used to optimize the operating
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parameters and achieve a better removal efficiency. Cyclic voltammetry (CV) was used to explore the electrochemical coupling mechanism of cathodic reduction and anodic oxidation for multiple nitrogen contamination.
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2. Materials and methods
2.1. Electrochemical system
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In this study, Ti/IrO2 electrode of 24.5 cm2 (9.8 cm×2.5 cm) was used as anode, Cu
electrode of 37.5 cm2 (15 cm×2.5 cm) was used as cathode. An undivided cylindrical
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electrochemical cell was made from polymethyl methacrylate with effective volume of 100 mL. Both electrodes had the same effective reaction area of 20 cm2 in solution. The distance between the anode and cathode was set as 8 mm. A DC regulated power supply was employed with a current range of 0–5 A and a voltage range of 0–50 V. Synthetic solution of multiple contamination with 50.0 mg/L nitrate (NaNO3) and 10.0 mg/L ammonia ((NH4)2SO4) was prepared for electrolysis. 0.50 g/L Na2SO4 was added to 4
improve the conductivity. NaCl at different concentrations (0.0–0.5 g/L) were added as one of the independent variables. Before electrolysis, the cathode was polished well with 100 to 180 mesh sandpaper, then rinsed with deionized water and dried in air. 100 mL synthetic solution prepared as above was poured into the cell and the DC power was connected. The current was switched off after a specified run time, and 0.5 mL of sample was drawn by
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pipette for analysis. All the experiments were carried out at room temperature (25±2℃). 2.2. Cyclic voltammetry measurements
Cyclic voltammetry (CV) was used to study the electrochemical reaction mechanism on the electrode surface. Cyclic voltammograms were recorded by an electrochemical workstation (CHI660D, CH Instruments, ShangHai) and an ALS Software (ALS Limited,
Model 660) on computer. Ti/IrO2 anode or Cu cathode were served as working electrode,
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respectively. Correspondingly, Cu cathode or Ti/IrO2 anode were served as the reference
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electrodes. All potentials in this study were referred to saturated calomel electrode (SCE). The electrode system was cycled in the range of −1.6 V to +0.2 V and −2.0 V to +2.0 V at
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a scan rate of 100 mV/s for three times before test to form a stable state.
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2.3. Design of experiments
Response surface method (RSM) is an efficient optimization method which can
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effectively evaluate the interaction of multiple complex factors and get an optimal response
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with a minimum number of well-designed experiments [30–32]. Box−Behnken design (BBD) is one of the RSM designs with many advantages, which only requires 15 runs for a three-parameter experimental design [33]. RSM optimization has been used widely in
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pollutant removal [34–36]. In this study, the RSM optimization was achieved by Design Expert 8 Application (StatEase, registered) regression program. The independent variables
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and coded levels were defined by Box–Benken design and are presented in supplementary materials (Table S1).
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For the response surface models, the independent variables were the electrolysis time
(X1), NaCl dosage (X2), and current density (X3), respectively. The low, center and high level of each variable were set to −1, 0, and +1, respectively. The dependent variables were the nitrate concentration (Y1) and ammonia concentration (Y2). The experiment results of dependent variables for 15 runs were given along with independent variables as shown in Table 1. These experiments were carried out in the designed order to minimize the external 5
effect of on the responses. The central point (0, 0, 0) was repeated 3 times to evaluate the error for reliability verification. Significance of coefficient of the model was determined by p-values. If p-values < 0.05, the response variables are significant [34]. No nitrite was detected in the treated water under all conditions. 2.4. Analysis
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All analyses were done according to standard methods [37]. Nitrate, nitrite and ammonia were detected by UV-spectrophotometry, N-1-naphthyl-ethylenediamine
spectrophotometric method and Nessler reagent spectrophotometry by a highly sensitive
spectrophotometer (DR500, HACH, USA), respectively. The LoD for chemical analysis of nitrogen were shown in the supplementary materials. Power density (PD, mW/m2) is
calculated by PD = IU/S, where I and U are current (A) and voltage (mV), respectively, S
U
(m2) is the geometrical area of the anode.
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3. Results and discussion 3.1. Fitting and analysis of the response models
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Fitting of empirical models to the experimental data was carried out by RSM to
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quantitatively describe the behavior of the response. The mathematical-statistical relationship between the independent variables (X) and the response functions (Y) can be
D
characterized by a quadratic polynomial as follows: (13)
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Y = b0 +b1 X1 +b2 X2 +b3 X3 +b12 X1 X2 +b13 X1 X3 +b23 X2 X3 +b11 X21 +b22 X22 +b33 X23
Equation 14−15 show the response functions with the defined coefficients for nitrate concentration and ammonia concentration.
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Ynitrate = 63.2375 − 0.6719X1 +38.6833X2 − 0.10833X3 − 0.1083X1 X2 +4.4333×10-3 X1 X3 −0.1640X2 X3 +1.7384×10-3 X21 − 29.3667X22 − 4.2533×10-3 X23 (R2 =0.9992) (14)
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Yammonia = 71.6525 − 1.3777X1 − 344.66X2 +1.6654X3 +1.096X1 X2 +4.7467×10-3 X1 X3
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−0.4992X2 X3 +6.2806×10-3 X21 +390.92X22 − 0.0245×10-3 X23 (R2 =0.9676)
(15)
In the equations, X1, X2 and X3 represent time, NaCl concentration and current
density, respectively. Ynitrate and Yammonia indicate the predicted nitrate and ammonia concentration in multiple contamination. The closer the correlation coefficient (R2) is to 1, the polynomial equality is more accurate [35,38]. The calculated R2 (R12=0.9992, R22=0.9666) indicate the response function predictions are in good agreement with experimental data and the predictability of the models is at 95% confidence level, i.e., only 6
0.08% and 3.34% variations are not explained by the regression model in Equation 14 and 15, respectively. The equation coefficients show the impact of independent variables (X) on response functions (Y). The sign ahead shows its improvement for ammonia removal and inhibition for nitrate removal, which is consistent with the previous studies [19,39]. According to
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these, the performances of electrochemical multiple nitrate and ammonia removal are
analyzed. The coefficients of −0.6719 and −1.3777 indicates that time (X1) promotes both nitrate and ammonia removal rate. +38.6833 and −0.34466 of NaCl dosage (X2) for Ynitrate
and Yammonia show that NaCl addition inhibited nitrate removal but greatly promoted ammonia removal. The higher absolute value of coefficients for X2 than other variables
show the nitrate reduction and ammonia oxidation behaviors mainly controlled by NaCl
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dosage. It is indicated that NaCl dosage has a more significant impact on the multiple
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contamination, especially for ammonia (344.66). Similarly, the current density (X3) is negatively correlated with Ynitrate (−0.10833) and positively correlated with Yammonia
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(+1.6654), indicating that nitrate removal is promoted but ammonia removal is inhibited
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as the current density increases. In sum, the three factors with the significance level order of NaCl dosage > current density > time.
D
The variance (ANOVA) analyses (Table 2 and Table S2) show the fitting results for
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the response surface model. Significance of model is verified by p-values and F-values. The F-value means the ratio of regression mean square to the estimated parameter standard deviation. The p-value means the probability of the occurrence of F-value [40]. In this
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study, both models are significant (Fnitrate is 655.34, pnitrate less than 0.0001; Fammonia is 16.06, pammonia is 0.0035 less than 0.05). The coefficients of variation (C.V., ratio of the
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standard deviation to the average of the data) for nitrate and ammonia model are 2.29% and 52.5%, respectively. This demonstrates the discrete level of the tested items is low.
A
Table 2 shows significance of the coefficients of Equation 15. Statistical analysis for this model shows that the terms of X2 and X22, i.e. the liner and quadratic term of NaCl concentration are significant (p is 0.0002 and 0.00, respectively). Again, the results of the coefficient analysis are proved here. The interactive terms of time and NaCl concentration (X1X2) (p is 0.0797), NaCl concentration and current density (X2X3) (p is 0.6546) and time and current density (X1X3) (p is 0.6944) are all insignificant. The quadratic terms of time 7
(X12) (p is 0.2068), current density (X32) (p is 0.3708) also presents insignificant. All the results prove that the change of NaCl concentration has the most important effect on ammonia removal in this system. The C.V. of 52.5% demonstrates the low discrete level of data of every tested item, suggesting that the regression model is also relevant and credible. The ANOVA test analyses for response function Ynitrate (Table S2) are shown in the
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supplementary materials.
The three-dimensional (3D) response surface plots and relevant contour plots (Figure 1) were built depended on the model functions (Equation 14 and 15), they intuitively illustrated the interaction effect of two independent variables on the response functions,
i.e., nitrate concentration and ammonia concentration [38]. Figure 1 (A) and (B) represents the interaction of NaCl concentration with current density when time is at the center point
U
of 60.00 min. Figure 1 (C) and (D) show the interaction of NaCl concentration with time
N
when the current density is 37.5 mA/cm2. All the 3D response surface plots show a nonlinear curved surface, and their trends are consistent with those shown in Table 1. The
A
other three-dimensional response surfaces are shown in the supplementary materials
M
(Figure S2). However, the residual nitrate concentration and the energy consumption need to be further reduced, therefore the operating conditions should be optimized.
D
3.2. Optimization by desirability functions
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The numerical desirability function was used to find a specific maximum desirable point (Figure S2). The weight was altered to fit the desirable goal. Five options were offered for response in the goal fields, i.e., none, minimum, maximum, within range and
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target. This study aimed at minimizing the nitrate and ammonia concentration to the standard concentration. Therefore, the objectives for the independent variables (time, NaCl
CC
concentration and current density) were set to “minimum” with corresponding weight of 1. The lower limit value of time, NaCl concentration and current density were set to 30 min,
A
0.0 g/L and 25 mA/cm2, respectively. Similarly, the objectives for the dependent variables (nitrate and ammonia concentration after electrolysis) were set to “minimum” with corresponding weight of 1. The target ranges of nitrate and ammonia concentration were set to 9−10 mg/L and 0−0.5 mg/L, respectively. The optimization program was carried out under the above settings and boundaries. As the results, the optimal operating conditions of time, NaCl concentration and current density were 82.08 min, 0.24 g/L and 45.31 8
mA/cm2, respectively. Under the optimum conditions, the nitrate and ammonia concentrations in the treated solution were 9.17 and 0 mg/L, respectively. The predicted result was verified. According to the data obtained from the optimized results, three groups of experiments were carried out under the optimal operating condition (82.08 min, 0.24 g/L NaCl and 45.31 mA/cm2). The result showed that the average concentrations of nitrate
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and ammonia were 8.87 and 0 mg/L, respectively. It was responsible as its deviations from predicted result were both less than 5%. The final concentration met the standards for drinking water quality (nitrate less than 10 mg/L). To sum up, combined application of Box−Behnken design and desirability function can effectively optimize the experiment
design on electrochemical removal of nitrate and ammonia multiple contamination. No
nitrite was detected in the treated water under all conditions. The results indicate that
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simultaneous electrochemical removal of nitrate and ammonia multiple contamination can
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be achieved in a single undivided cell. Furthermore, CV was used to investigate the mechanisms of cathodic reduction and anodic oxidation for the electrochemical
A
degradation system.
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3.3. Mechanism of anodic oxidation and cathodic reduction 3.3.1. Reaction process on surface of Cu cathode
D
Cyclic voltammograms obtained for the Cu cathode in an electrolyte containing 0.3
TE
g/L NaCl (line c) and a NaCl-free electrolyte (line b) at a scan rate of 100 mV/s in the potential range −1.8 to +0.2 V are shown in Figure 2. These are used to clarify the reaction process on the cathode surface. In all experiments, the blank electrolyte (line a) contained
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only deionized water and 0.5 g/L Na2SO4 to exclude other potential interference. The four groups of curves in Figure 2 were obtained by scanning the Cu cathode with different
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contaminated matrix types. In all the curves, a reduction peak is observed at −0.3 V (labelled p), which is presumably related to the electrolytic reaction of water under aerobic
A
conditions, at the standard oxidation–reduction potential (equation 1; the reference potential of the saturated calomel electrode is +0.2415 V). The peak current increases in the presence of NaCl (curve c), as observed in Figure 2 (A), which shows that chloride ion can promote electrochemical aerobic hydrolysis. O2 + 2H2 O + 2e– → 2OH– (E = –0.401 V)
(1)
9
In Figure 2 (B) and (D), the cathodic current caused by the hydrogen evolution reaction on the Cu cathode starts to increase at −1.0 V (equations 2–4) [41]. The reduction peak at −0.8 V (labelled r) in Figure 2 (B) and (D) corresponds to nitrate reduction. The peak is not present in Figure 2 (A) and (C), i.e., in the absence of nitrate. This shows that
consistent with previous studies [42]. NO3− + H2 O + 2e− → 2OH− + NO2− NO3− + 3H2 O + 5e− → 6OH− +
(2)
1 N↑ 2 2
(3)
NO3− + 6H2 O + 8e− → 9OH− + NH3 ↑
(4)
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nitrate was reduced to nitrite, nitrogen gas, and ammonia (equations 2−4), which is
In all the curves, a clear peak is observed near −0.7 V (labelled q). This peak
U
corresponds to the oxidation/reduction of Cu2O to Cu [39]. In Figure 2 (B) and (D), peak
q is hidden by the prominent peak r, which arises from active nitrate reduction. Figure 2
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(C) shows a gradual forward shift of the current peak q on addition of ammonia and NaCl.
A
This is because the added chloride ion was oxidized to acidic substances, which reduced
M
the pH of the system. Acidic electrolyte solution promoted electrochemical corrosion of the Cu electrode [43,44]. To confirm this speculation, the working electrode was changed
D
to the anode for three cycles. A floating layer of a green substance formed on the surface of the electrolyte near the electrode. It supposed to be tribasic copper chloride
TE
[CuCl2·3Cu(OH)2] formed by the reaction between the Cu electrode and chloride ion [44]. Curves b and c in Figure 2 (B) and (D) show a similar cathodic current change, but a
EP
more negative potential was required to initiate the hydrogen evolution reaction in the case of multiple nitrate and ammonia contamination (D) than in the case of single nitrate
CC
contamination (B). This is because ammonia decreases the nitrate removal rate, according to the ion-exchange equilibrium. In contrast, the trends and peak locations for the CV
A
curves in Figure 2 (A) and (C) showed no significant differences, indicating that ammonia did not participate in the cathodic reduction. To sum up, the basic cathode reaction can be described based on the above CV curves,
which is consistent with the results of independent experiments and previous studies [42– 44]. However, some peaks on curve c in the cathodic CV curves should be discussed in combination with anodic reaction due to the existence of NaCl. Furthermore, the questions
10
raised by independent experiments regarding how chlorine affects ammonia removal and why chloride inhibits nitrate removal have not been answered. CV scanning of a Ti/IrO2 anode therefore needs to be performed to explain the reaction mechanism of the entire electrochemical system. 3.3.2. Reaction process on surface of Ti/IrO2 anode
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The cyclic voltammograms obtained for a Ti/IrO2 anode in different electrolytes at a
scan rate of 100 mV/s in the potential range −2.0 V to + 2.0 V are shown in Figure 3. They
were used to investigate the reaction process on the electrode surface. In all experiments,
a blank electrolyte that contained only deionized water and 0.5 g/L Na2SO4 was used to
exclude other potential interference. Cyclic voltammograms for the anode were obtained in an electrolyte containing 0.3 g/L NaCl (line a) and in a NaCl-free electrolyte (line b).
U
The peaks on the dotted line in Figure 3 (C) and (D) correspond to the oxidation peak of
N
ammonia at a potential of +1.2 V (labelled m), which are not present in Figure 3 (A) and (B). Kapałka et al. suggest that this is because direct electrolytic ammonia oxidation occurs
A
around the anode at the above potential (equation 5). This can also explain the decrease in 1
NH3 + 3H2 O → N2 + 3H+ + 3e– 2
M
the amount of ammonia in the absence of NaCl (Table 1) [45]. (5)
D
The oxidation peak (labelled n) on line a at +1.4 to +1.5 V is not present on line b in
TE
all four diagrams, indicating that the peak is related to NaCl. The potential at which peak n appears is consistent with the standard redox potential of hypochlorous acid (the
EP
reference potential of the saturated calomel electrode is +0.2415 V), therefore the peak is attributed to the oxidation of chloride ion to chlorine gas at the anode and then conversion to hypochlorous acid and hypochlorite (equation 6−8) [19,42,46–48]. (6)
Cl2 + H2 O → HClO + H+ + Cl−
(7)
HClO → + H+ + ClO−
(8)
A
CC
2Cl− → Cl2 + 2e−
Subsequently, hypochlorite and hypochlorous acid, which are strongly oxidizing,
oxidize ammonia to nitrogen and nitrate (equation 9−12) [22]. The above results for anode manifest that chlorine ions can be transformed to strong oxidizing substances (mainly hypochlorite and hypochloric acid) when the anode relative potential is higher than +1.5 V, then promoting oxidation removal of ammonia. 11
2NH+4 + 3HClO → 3H2 O + 5H+ + 3Cl− + N2 ↑
(9)
2NH+4 + 3ClO− → 3H2 O + 2H+ + 3Cl− + N2 ↑
(10)
NH+4 + 3HClO → H2 O + 6H+ + 4Cl− + NO3−
(11)
NH+4 + 3ClO− → H2 O + 2H+ + 4Cl− + NO− 3
(12)
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Nitrite is supposed to be one of the main by-products according to equation 2. However, no nitrite was detected in the treated water under all conditions. That’s because
nitrite is easy to be transformed to ammonia and nitrate by hydrogen ion and hypochlorite (equation 13−14) according to the previous studies [14,49]. NO2− + HClO → NO3− + H+ + Cl−
(13)
NO2− + 8H+ → 3H2 O + NH+4
(14)
The changes to the CV curves of cathode caused by NaCl addition can be explained
U
based on the results of anode. Figure 2 (B) shows a more negative potential on curve c for
N
nitrate reduction (peak r) than on curve b. This is because oxidizing hypochlorite and
A
hypochlorous acid formed at the anode inhibit the cathodic reduction of nitrate in the
M
presence of chloride ion. Curve c in Figure 2 (C), which was obtained in the absence of nitrate, shows an unexpected nitrate reduction peak r. This is because nitrate was generated from ammonia by oxidation with hypochlorous acid and hypochlorite (equation 11−12). A
D
comparison of Figure 2 (C) and (D) shows a better coincidence degree for the nitrate
TE
reduction peak r on the three curves for multiple contamination (D), indicating that NaCl has no effect on the nitrate reduction rate in this case. The possible reasons for this
EP
phenomenon are as follows. The mechanism of the effect of chloride on nitrate reduction is mainly related to competitive adsorption of chloride ion and interference with the redox
CC
potential of the system by hypochlorous acid. In the presence of ammonia, hypochlorous acid formed by oxidation of chloride ions will therefore be consumed rapidly during ammonia oxidation, and conversion of chloride ions to hypochlorous acid will be
A
accelerated. It is therefore concluded that the presence of ammonia can counteract the effect of NaCl on the nitrate reduction rate. So far, the mechanism of simultaneous electrochemical removal of nitrate and ammonia multiple contamination in an undivided electrochemical system (Figure 4) is clear.
First, when a current is supplied, the nitrate around the cathode, as an electron
donor, is reduced to nitrite, ammonia, and nitrogen gas. Chloride ion around the anode is 12
oxidized to chlorine gas, which reacts with water to produce hypochlorite and hypochlorous acid. This is the crucial step in the reaction. A small portion of ammonia is converted to nitrogen gas by direct electrolytic oxidation at the anode. Nitrite and most of the ammonia are oxidized by the generated hypochlorite and hypochlorous acid to nitrogen gas and nitrate. A circular transformation process is therefore established. As the reaction
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proceeds, multiple nitrogen pollutants in the original contaminated system are gradually removed as the reaction predominantly produces harmless nitrogen gas. Finally, the goal of harmless removal of multiple nitrate and ammonia contamination is achieved. 4. Conclusions
The following conclusions can be drawn from the present study.
(1) A novel electrochemical system for multiple nitrate and ammonia removal is
U
established and studied the conversion mechanism and concurring evolution of nitrogen
N
for the first time. Simultaneous and harmless removal of nitrate and ammonia multiple contamination were achieved in a single electrochemical cell without a membrane, which
A
makes the technology more feasible in practical application.
M
(2) According to the CV, the electrochemical cathodic reduction and anodic oxidation is found to form a coupling action on degradation of the nitrate, ammonia and nitrite. The
D
crucial step is the oxidation of chloride ion to hypochlorite and hypochlorous. A circular
TE
transformation process is established in the electrochemical system. The multiple nitrogen pollutants in the original contaminated system are gradually removed with the reaction predominantly produces harmless nitrogen gas.
EP
(3) This study applied RSM to investigate the effect of time, NaCl dosage and current density on the nitrate and ammonia removal in electrochemical system. To further explore
CC
the optimum the interaction of the operating conditions, two response functions were modelled: nitrate and ammonia concentration. The most sensitive parameter for nitrate and
A
ammonia removal is NaCl concentration. Operating conditions were all independent for the ammonia removal response surface. Interaction effects between time and current density, time and NaCl concentration were observed for the modelled nitrate removal response surface. The optimum removal rate was achieved when time, NaCl concentration, and current density were 82.08 min, 0.24 g/L, and 45.31 mA/cm2, respectively. Under these
13
conditions, the nitrate and ammonia concentrations were degraded to 8.87 and 0 mg/L, respectively.
Acknowledgments
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The authors thank the National Water Pollution Control and Treatment Science and Technology Major Project (2017ZX07202002), Beijing Natural Science Foundation
(J150004) and Key Technology and Project of Jinan Water Environment Control (No.201509002) for the financial support of this work.
J. Zhou, B. Gu, W.H. Schlesinger, X. Ju, Significant accumulation of nitrate in Chinese semi-
N
[1]
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doi:10.1016/j.apcatb.2017.02.005.
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SC RI PT U N A M Three-dimensional response surface plots for effect of NaCl dosage and current
D
Figure 1.
TE
density on (A) ammonia removal and (B) nitrate removal (mg/L); and for NaCl dosage and
A
CC
EP
time on (C) ammonia removal and (D) nitrate removal.
19
SC RI PT U N A M
Cyclic voltammetry curves for Cu cathode. Line a, b and c imply the blank,
D
Figure 2.
blank adding 0.3 g/L NaCl and electrolyte solution adding 0.3 g/L NaCl, respectively. The
TE
electrolyte solution above contains: (A) 0.5 g/L Na2SO4, (B) 0.5 g/L Na2SO4 +50.0 mg/L nitrate, (C) 0.5 g/L Na2SO4 +10.0 mg/L ammonia, (D) 0.5 g/L Na2SO4 +50.0 mg/L nitrate
A
CC
EP
+10.0 mg/L ammonia.
20
SC RI PT U N A M
Figure 3. Cyclic voltammetry curves for Ti/IrO2 anode. Electrolyte solution used contains:
D
(A) 0.5 g/L Na2SO4, (B) 0.5 g/L Na2SO4 +50.0 mg/L nitrate, (C) 0.5 g/L Na2SO4 +10.0
TE
mg/L ammonia, (D) 0.5 g/L Na2SO4 +50.0 mg/L nitrate +10.0 mg/L ammonia. Line a and
A
CC
EP
line b imply whether 0.3 g/L NaCl is added or not, respectively).
21
SC RI PT U
Mechanism of electrochemical system for removing nitrate and ammonia
A
CC
EP
TE
D
M
A
multiple contamination in the presence of Cl−.
N
Figure 4.
22
Observed nitrate and ammonia concentrations X1:Time (min)
X2:NaCl concentration (g/L)
X3:Current density (mA/cm2)
Y1:Nitrate concentration (mg/L)
Y2:Ammonia concentration (mg/L)
1
60.00
0.50
25.00
23.60
0.06
2
30.00
0.00
37.50
23.05
77.15
3
60.00
0.00
50.00
10.35
44.80
4
90.00
0.00
37.50
5.50
44.04
5
30.00
0.25
50.00
24.25
0.41
6
60.00
0.50
50.00
14.50
0.00
7
90.00
0.50
37.50
10.05
0.00
8
30.00
0.50
37.50
30.85
0.23
9
60.00
0.25
37.50
17.45
0.28
10
30.00
0.25
25.00
35.20
7.74
11
90.00
0.25
50.00
7.85
0.00
12
60.00
0.25
37.50
17.60
0.26
13
60.00
0.00
25.00
17.40
38.62
14
60.00
0.25
37.50
17.85
0.27
A
N
SC RI PT
Obs StdOrder
U
Table 1.
A
CC
EP
TE
D
M
15 90.00 0.25 25.00 12.15 0.21 Initial nitrate concentration is 50 mg/L; Initial ammonia concentration is 10 mg/L.
23
Sum of Squares df
Mean Square F Value
p-value Prob > F
Model
8114.87
9
901.65
16.06
0.0035
A–Time
213.02
1
213.02
3.79
0.109
B–NaCl
5218.13
1
5218.13
92.95
0.0002
C–Current density
0.25
1
0.25
4.51E-03
0.9491
AB
270.21
1
270.21
4.81
0.0797
AC
12.68
1
12.68
0.23
BC
9.73
1
9.73
0.17
A
118
1
118
2.1
B2
2204.36
1
2204.36
39.26
54.24
1
54.24
0.97
Residual
280.71
5
56.14
Lack of Fit
280.71
3
93.57
Pure Error
9.80E-05
2
4.90E-05
Cor Total
8395.57
14
1.91E+06
N
C
2
A
2
A
CC
EP
TE
D
M
R2 = 0.9666, Adj R2 = 0.9064, Pred R2 = 0.4650, C.V. = 52.5%
24
significant
SC RI PT
Source
U
Table 2. ANOVA test for response function Yammonia
0.6546 0.6944 0.2068 0.0015 0.3708
< 0.0001
significant