Activated carbon electrodes: Electrochemical

Chemosphere xxx (2015) xxx–xxx

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Activated carbon electrodes: Electrochemical oxidation coupled with desalination for wastewater treatment Feng Duan a,b, Yuping Li a,⇑, Hongbin Cao a,⇑, Yi Wang a, John C. Crittenden c, Yi Zhang a a

Key Laboratory of Green Process and Engineering, Institute of Process Engineering, Chinese Academy of Sciences, Beijing 100190, China University of Chinese Academy of Sciences, Beijing 100049, China c Brook Byers Institute for Sustainable Systems, Georgia Institute of Technology, Atlanta, GA 30332, United States b

h i g h l i g h t s Simultaneous organics degradation and salt removal were realized. Salt and phenol removal efficiency were enhanced by bubbling O2 gas. Phenol degradation was related to the electrogenerated chlorine oxidants.

a r t i c l e

i n f o

Article history: Received 21 October 2014 Received in revised form 17 December 2014 Accepted 18 December 2014 Available online xxxx Handling Editor: E. Brillas Keywords: Electrosorption Desalination Capacitive deionization Electrochemical oxidation Phenol Wastewater treatment

a b s t r a c t The wastewater usually contains low-concentration organic pollutants and some inorganic salts after biological treatment. In the present work, the possibility of simultaneous removal of them by combining electrochemical oxidation and electrosorption was investigated. Phenol and sodium chloride were chosen as representative of organic pollutants and inorganic salts and a pair of activated carbon plate electrodes were used as anode and cathode. Some important working conditions such as oxygen concentration, applied potential and temperature were evaluated to reach both efficient phenol removal and desalination. Under optimized 2.0 V of applied potential, 38 °C of temperature, and 500 mL min1 of oxygen flow, over 90% of phenol, 60% of TOC and 20% of salinity were removed during 300 min of electrolysis time. Phenol was removed by both adsorption and electrochemical oxidation, which may proceed directly or indirectly by chlorine and hypochlorite oxidation. Chlorophenols were detected as degradation intermediates, but they were finally transformed to carboxylic acids. Desalination was possibly attributed to electrosorption of ions in the pores of activated carbon electrodes. The charging/regeneration cycling experiment showed good stability of the electrodes. This provides a new strategy for wastewater treatment and recycling. Ó 2014 Elsevier Ltd. All rights reserved.

1. Introduction Water shortage poses a great threat to the sustainability of human beings, and water recycling has been an effective way to solve the problem. Municipal wastewater, industrial wastewater, or wastewater that comes from household sources can be treated and recycled for non-potable and potable purposes. The treatment methods vary with the wastewater characteristics and end uses (Chen et al., 2013). For example, coking wastewater is common in steel industry, which contains many pollutants such as ammonia and phenolic compounds (Li et al., 2010). Even after the biological ⇑ Corresponding authors at: No. 1 North Second Street, Zhongguancun, Haidian District, Beijing 100190, China. Tel.: +86 10 82544844; fax: +86 10 82544844 816. E-mail addresses: [email protected] (Y. Li), [email protected] (H. Cao).

treatment, inorganic ions (like Cl ion) and refractory organic compounds still exist (Li et al., 2011) which needs to be removed if the wastewater will be reused in the production process. Electrochemical advanced oxidation processes (EAOPs) have been developed as a promising technology for removing persistent organic contaminants, as they use electron as reagent and is environmentally clean. The main drawback is relatively low current efficiency and high energy consumption (Brillas et al., 2009). The organic pollutants can be destroyed by direct oxidation or indirect oxidation (such as chlorine, S2O2 8 ) (Martinez-Huitle and Ferro, 2006). The anode materials are usually DSA-type electrodes (Chatzisymeon et al., 2010; Li et al., 2013), or boron doped diamond (BDD) electrodes (Daghrir et al., 2014). Carbon anodes are not often reported due to their low O2 evolution overpotential and less efficient production of oxidants like hydroxyl radical

http://dx.doi.org/10.1016/j.chemosphere.2014.12.065 0045-6535/Ó 2014 Elsevier Ltd. All rights reserved.

Please cite this article in press as: Duan, F., et al. Activated carbon electrodes: Electrochemical oxidation coupled with desalination for wastewater treatment. Chemosphere (2015), http://dx.doi.org/10.1016/j.chemosphere.2014.12.065

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F. Duan et al. / Chemosphere xxx (2015) xxx–xxx

(Ghernaout et al., 2011). Fan et al. (2008) investigated Amaranth azo dye degradation with activated carbon fiber (ACF) as anode. Electro-oxidation or electro-reduction of the pollutant was achieved by applying positive and negative potential, respectively. In addition to organic pollutants, the salts need to be removed for water reuse. Recently capacitive deionization (CDI) has received great interest as a desalination technology due to advantages such as low energy-consumption and no secondary pollution (Oren, 2008; Porada et al., 2013). The ions are electrosorbed by applied a potential to the porous electrodes and desorbed by shorting the electrodes or reversing the potential. The most frequently used electrodes are porous carbon materials with high surface areas, abundant pore structures and some surface groups (Avraham et al., 2011; Gao et al., 2013, 2014; Wang et al., 2014). In this work, the decontamination of wastewater containing phenol and NaCl was investigated by electrochemical oxidation coupled with desalination using activated carbon (AC) electrodes, which has been rarely reported. The target is to develop a technology that is efficient in removal of organic pollutants and inorganic salts simultaneously and applicable in the advanced treatment of industrial wastewater. We examined the effects of oxygen concentration, applied voltage and temperature on the removal efficiency of phenol and salt. The phenol degradation pathway and intermediates were carefully studied. 2. Materials and methods 2.1. Preparation of carbon electrode

10 mm 10 mm, weight: about 10 mg) was used as the working electrode, with platinum sheet as counter electrode and saturated calomel electrode (SCE) as reference electrode. CV was measured in 17.1 mM NaCl at scan rate of 1–50 mV s1 and potential of 1.0 to 1.5 V (vs. SCE) under N2 or O2 bubbling condition. In some cases, 1.1 mM or 5.5 mM phenol was added to evaluate whether phenol could be oxidized directly. 2.4. Analytical methods The conductivity of the effluent was constantly monitored using a conductivity meter (Orion 3-Star, Thermo Fisher Scientific Inc.). The relationship between the conductivity and concentration was calibrated before the experiments. The concentration of phenol and its oxidation intermediates was analyzed by high performance liquid chromatography (HPLC, 1200 infinity series, Agilent) equipped with Zorbax SB-C18 column. The mobile phase was a mixture of methanol and water containing 10 mM H3PO4 (25%/ 75%, V/V). Total organic carbon (TOC) was measured by a TOC analyzer (TOC-VCPH, Shimadzu). The H2O2 concentration was determined by spectrophotometric analysis with potassium titanium (IV) oxalate reagent at 400 nm (Sellers, 1980), while the total concentration of possible oxidants (such as H2O2 and chlorine) was determined by the iodide method at 350 nm (Kormann et al., 1988). The specific energy consumption (EC) of TOC and NaCl removal was calculated by the following equation (Santos et al., 2010):

Rt 1000 U 0 Idt DTOC V Rt 1000 U 0 Idt 1 ECNaCl ðkW hðkg NaClÞ Þ ¼ DC V 1

ECTOC ðkW hðkg TOCÞ Þ ¼ To prepare the carbon plate electrode, 1.6 g AC was first mixed with 0.67 g Polytetrafluoroethylene (PTFE) emulsion (60 wt%) to obtain weight percentage of 80%:20%. Ethanol was added and the mixture was stirred at 70 °C to form a carbon paste. Then it was rolled, treated at 240 °C for 40 min and 360 °C for 1 h. The thermal treatment may increase the physical stability of electrode (Wang et al., 2013) and surface oxygen-containing groups which affects the electrosorption of ions. The carbon flake (size of about 25 mm 25 mm 0.5 mm) was pressed on graphite paper which was used as current collector. 2.2. Electrochemical setup The electrochemical setup used here is similar to that reported previously (Duan et al., 2014), as shown in Fig. SM-1. The undivided cell consists of two AC electrodes that are placed parallel and separated by a rubber gasket. Batch mode experiments were conducted using the continuously recycling system equipped with an electrochemical cell, a peristaltic pump, a power supply and a conductivity/pH meter. Typically the mixed solution of 100 mL of 17.1 mM NaCl and 1.1 mM phenol was employed as feed solution. The flow rate was kept at 40 mL min1. The applied potential and temperature were optimized in the range of 1.5–3.0 V and 28– 48 °C, respectively. Prior to the experiment, the solution was purged with O2 or N2 at 500 mL min1 for 30 min. The gas purging continued during the polarization process. 2.3. Electrode characterization The morphology of the electrode was characterized by scanning electron microscopy (SEM, Quanta 250, FEI). Nitrogen sorption isotherm was measured at 77 K with an automated gas sorption analyzer (autosorb iQ, Quantachrome). Before measurement, the sample was degassed under vacuum at 120 °C for 12 h. Cyclic voltammetry (CV) was performed in the electrochemical workstation (Autolab PGSTAT302 N, Metrohm). The AC electrode (size:

ð1Þ ð2Þ

where U is the applied potential (V), I is the current (A), t is the polarization time (h), DTOC is the difference in TOC excluding that removed by electrode adsorption (mg L1), DC is the change in salt concentration (mg L1), V is the solution volume (L). 3. Results and discussion 3.1. Characterization of AC electrode The SEM image, nitrogen adsorption–desorption isotherm and pore size distribution (PSD) of the electrode are shown in Fig. SM-2. The SEM image displays that the addition of the PTFE polymer to AC powder yields excellent physical stability. Note that the isotherm exhibits obvious hysteresis at high relative pressure, suggesting the presence of mesopores. The BET specific surface area is 300 m2 g1, while the micropore surface area is 98 m2 g1. The PSD curve displays that most of the pores are above 1 nm, which is beneficial to ion adsorption. The CV curves of AC electrode are presented in Fig. 1. Fig. 1a and b shows the CV curves under O2 or N2 atmosphere. The anodic or cathodic peak is not seen at sweep rate of 10 mV s1 or 50 mV s1, while they are observed at a low sweep rate of 1 mV s1 (the blue1 line). The anodic current increases quickly at a potential higher than 1.2 V, which may due to Cl oxidation. A cathodic peak is also observed at 1.0 V. The CV curves under O2 or N2 atmosphere at 1 mV s1 are compared in Fig. 1c. The main difference is that the cathodic current is higher under O2 atmosphere, as O2 will be reduced at a negative potential. Fig. 1d displays that a new oxidation peak appears at about 0.8 V in the presence of phenol, and it increases with the phenol 1 For interpretation of color in Fig. 1, the reader is referred to the web version of this article.


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concentration. The peak is possibly attributed to phenol oxidation. The direct phenol oxidation is viable by electrochemical method (Skowronski and Krawczyk, 2009). 3.2. Effect of O2 concentration on phenol and ion removal Fig. 2a shows the phenol removal under different O2 concentrations at 28 °C of temperature and 2.0 V or 0 V of applied potential. The phenol removal efficiency by adsorption at open circuit condition is 44%. When 2.0 V is applied and N2 is bubbled, the removal efficiency is similar, indicating removal by adsorption. The phenol conversion is moderately 57% after 300 min of polarization when no gas is bubbled. It is significantly accelerated when bubbled with O2 at 500 mL min1 of flow rate. The possible oxidants in the solution are detected, such as H2O2 (Eq. (3)), or Cl2 and ClO (Eqs. (4) and (5)) (Ozcan et al., 2008). Surprisingly, the oxidants are mostly Cl2 and ClO (as the solution pH is 9.0), and H2O2 is not detected. As seen in Fig. 2b, more chlorinecontaining oxidants are produced when the O2 concentration is increased, which may greatly improve the removal of phenol (Comninellis and Nerini, 1995). The potential at the anode is measured using SCE as reference electrode, and the result is shown in Fig. 2c. The anodic potential (about 1.31 V) is higher at O2 bubbling condition than at no gas or N2 bubbling condition, which contributes to Cl2 production. To further elucidate the mechanism, a divided cell is constructed with the same AC electrode as anode and cathode, separated by a Nafion 117 cationic membrane. It reveals that Cl2 is produced in the anodic chamber, while H2O2 is accumulated in the cathodic chamber when O2 is bubbled instead of N2 (seen in Fig. SM-3). It is assumed that the presence of O2 decreases cathodic potential (or facilitates cathodic reaction) and increases anodic potential. As H2O2 is possibly consumed by reaction with Cl2 (Eq. (6)) or oxidized to H2O in the cathode (Eq. (7)) in the undivided cell, it is not detected in the experiment.

O2 þ 2Hþ þ 2e ! H2 O2

ð3Þ

2Cl ! Cl2 ðgÞ þ 2e þ

ð4Þ

Cl2 ðgÞ þ H2 O ! 2H þ Cl þ ClO

ð5Þ

H2 O2 þ Cl2 ! 2HCl þ O2

ð6Þ

H2 O2 þ 2Hþ þ 2e ! 2H2 O

ð7Þ

Fig. 2d shows that the conductivity remains relatively constant at applied potential of 0 V, while it decreases significantly when potential of 2.0 V is applied. This is because ions are electrosorbed on the surface of porous carbon electrodes. The conductivity decrease is also more significant at higher O2 concentration. The phenomenon is contrary to the literature which shows better desalination performance at the nitrogen atmosphere (Bouhadana et al., 2011). However, in their experiment, the low applied potential (0.9 V) and lack of oxygen in the system may prevent the possible Faradaic reactions and the charge consumed results in adsorption of ions. However, in our system when the applied potential is much higher and oxygen is abundant, the reactions such as oxygen reduction and Cl oxidation will happen, as indicated from the pH changes in Fig. SM-4. The enhancement of salt removal may be caused by these reactions. For example, as more Cl2 is generated at higher O2 concentration and escapes from the solution, the conductivity will decrease. 3.3. Effect of applied potential on phenol and ion removal Fig. 1. Cyclic voltammograms in 17.1 mM NaCl under (a) nitrogen atmosphere and (b) oxygen atmosphere; (c) comparison of cyclic voltammograms under different atmospheres at 1 mV s1 of scan rate; (d) comparison of cyclic voltammograms in 17.1 mM NaCl in the presence and absence of phenol at 1 mV s1 of scan rate.

To investigate the effect of applied potential, the experiments were conducted in the range of 1.5–3.0 V. As seen in Fig. 3a and b, increasing the potential notably accelerates the rate of phenol


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and TOC conversion. The phenol removal efficiency is almost 100% at 3.0 V after 300 min of polarization, while only 40% and 83% of phenol is removed at 1.5 V and 2.0 V, respectively. Besides, about 36% of TOC is depleted after 300 min at 1.5 V, and this value increases to 61% at 3.0 V. Nevertheless, 100% mineralization is not reached because the oxidation ability of chlorine is limited and many degradation intermediates still remain in the solution. Fig. 3c shows that the conductivity decreases more sharply with the potential increasing from 1.5 V to 3.0 V, possibly because the electrostatic attraction will become larger (Wang et al., 2012). The chlorine production at various applied potentials is shown in Fig. SM-5a. No chlorine is generated at 1.5 V. The chlorine concentration is lower at 3.0 V than at 2.0 V, probably because more chlorine is consumed or stripped from the solution. The pH change at 3.0 V of potential is more violent than at other potentials, as seen in Fig. SM-5b. The result of specific energy consumption is revealed in Fig. 3d. The value of ECTOC is not shown at 1.5 V as no chlorine is produced and TOC is totally removed by adsorption rather than electrochemical oxidation. Besides, ECTOC is roughly similar at 2.0 V or 3.0 V. However, the value of ECNaCl notably increases with the increasing potential. Since more parasitic reactions will occur at higher applied potential, the energy consumption increases. Not that ECNaCl is much lower than ECTOC.

3.4. Effect of temperature on phenol and ion removal The effect of temperature on the performance of the electrodes was investigated in the range of 28–48 °C. Fig. 4a and b shows that the phenol removal efficiency increases gradually with the temperature, while TOC conversion efficiency increases first and then decreases a little. Increasing the temperature will not only enhance the reaction rate between the oxidants and organics, but also increase the decomposition rate of oxidants. Therefore the temperature should not be too high. The desalination performance is nearly independent of the temperature, as seen from Fig. 4c. The results of chlorine production and pH change at various temperatures are shown in Fig. SM-6. Fig. SM-6(a) displays that less chlorine is detected at higher temperature possibly due to more chlorine consumption in the reaction. Fig. SM-6(b) shows that pH is not much affected by the temperature. The results of EC are shown in Fig. 4d. ECTOC is lowest at 38 °C with a value of 89 kW h (kg TOC)1. The value is similar to other applications of electrochemical processes reported in the literature (Li et al., 2013). However, it is still high and does not contain that of bubbling oxygen or raising solution temperature, which may be attributed to the low electrical conductivity of dilute saline concentration and activated carbon electrodes (Hussain et al., 2013). The value of ECNaCl increases slightly from 28 to 48 °C. The temperature of 38 °C seems more suitable as it ensures both high removal efficiency and low energy consumption.

3.5. Possible degradation mechanism of phenol

Fig. 2. The curves of (a) phenol concentration, (b) chlorine concentration, (c) anodic potential and (d) conductivity at condition of oxygen, nitrogen or no gas purging (temperature: 28 °C, applied potential: 2.0 V).

Various degradation intermediates were detected under O2 atmosphere. Fig. 5a shows that the concentration of 2-chlorophenol (2-CP) and 4-chlorophenol (4-CP) increases at the first 180 min and then decreases. The trend of concentration change of dichlorophenols and trichlorophenols is similar. Besides, the concentration of maleic acid and fumaric acid is increasing with the time, as seen in Fig. 5b. Benzoquinone and catechol are also detected at a low concentration, which indicates that phenol may be oxidized directly.



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Fig. 4. Effect of temperature on the (a) phenol removal, (b) TOC removal, (c) conductivity decrease and (d) specific energy consumption (applied potential: 2.0 V).

Fig. 3. Effect of applied potential on the (a) phenol removal, (b) TOC removal, (c) conductivity decrease and (d) specific energy consumption (temperature: 28 °C).

For better understanding of the mechanism, two experiments were conducted. One is to add 10 mM t-butanol to the reaction and the result is shown in Fig. SM-7. The phenol degradation is


6


Fig. 6. The curves of (a) phenol concentration vs. time and (b) conductivity vs. time in several charging/regeneration cycles.

Fig. 5. Evolution of (a) chlorophenols and (b) other degradation intermediates during electrolysis of 1.1 mM phenol in 17.1 mM NaCl at 28 °C and 2.0 V.

not inhibited in presence of t-butanol but enhanced somehow. The reason is not clear. However, it can be concluded that hydroxyl radical may not be involved in phenol conversion. Another experiment of chemical oxidation of phenol with NaClO was conducted. Fig. SM-8 displays that phenol is first oxidized to 2-CP and 4-CP, and then dichlorophenols and 2,4,6-trichlorophenol are formed. In conclusion, the electrochemical degradation of phenol in our system is possibly performed by two pathways (Fig. SM-9). One is through indirect chlorine mineralization. Chlorophenols are formed as intermediates in this pathway and finally oxidized to carboxylic acids and CO2. Another way is through direct oxidation. Benzoquinone and catechol are generated by hydroxylation through this way (Chatzisymeon et al., 2010; Chen and Kupferle, 2014).

3.6. Stability and reuse of the electrodes The electrodes were used in several consecutive charging/ regeneration cycles. The charging stage lasted for 180 min at 17.1 mM NaCl, 1.1 mM phenol, 500 mL min1 of O2 flow, 2.0 V and 28 °C, then the potential was reversed to 1.0 V for 10 min and kept at 0 V (short circuit) for 110 min to regenerate the electrodes. As seen from Fig. 6a and b, the removal efficiency of phenol and salt decreases gradually after 5 charging/regeneration runs. The performance deterioration may be attributed to organic fouling of the electrodes. Besides, the anode is probably oxidized at 2.0 V and O2-abundant condition (Cohen et al., 2013). To clarify this, a solution of 0.1 M NaOH is circulated to wash the electrodes,

and then they are used for the sixth run. It is found that the phenol and salt removal are still not good. To have a better performance, the polarity of electrodes is reversed. The cathode used in the first to sixth run is used as anode, while the initial anode used in the first to sixth run is used as cathode. Fig. 6a and b shows that the phenol removal is restored in the seventh run, and after some fluctuation in the initial 60 min, the conductivity decreases quickly just like in the first run. The reversal of electrode polarity may be useful to keep the long-term stability of performance. 4. Conclusions In this work, the simultaneous removal of inorganic salt and organic pollutant in NaCl and phenol containing solutions by activated carbon electrodes was investigated. The phenol would be removed by adsorption, direct electrochemical oxidation and chlorine-mediated oxidation, meanwhile the electrosorption of ions was significant. Ongoing researches will focus on development of new electrode materials, testing the applicability in other solutions (for example, sulfate containing solutions) and the electrode stability in long-term runs. Acknowledgments The authors greatly appreciate the financial support from National Natural Science Foundation of China (Grant No. 20607023, No. 21177130 and No. 21377130). We also acknowledge support by the Brook Byers Institute for Sustainable Systems, Hightower Chair, and the Georgia Research Alliance at the Georgia Institute of Technology.



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