A multi-walled carbon nanotube electrode based on porous Graphite-RuO2 in electrochemical filter for pyrrole degradation

Accepted Manuscript A Multi-Walled Carbon Nanotube electrode based on porous Graphite-RuO2 in Electrochemical Filter for Pyrrole Degradation Xiezhen Zhou, Siqi Liu, Anlin Xu, Kajia Wei, Weiqing Han, Jiansheng Li, Xiuyun Sun, Jinyou Shen, Xiaodong Liu, Lianjun Wang PII: DOI: Reference:

S1385-8947(17)31382-7 http://dx.doi.org/10.1016/j.cej.2017.08.042 CEJ 17493

To appear in:

Chemical Engineering Journal

Received Date: Revised Date: Accepted Date:

17 May 2017 7 August 2017 9 August 2017

Please cite this article as: X. Zhou, S. Liu, A. Xu, K. Wei, W. Han, J. Li, X. Sun, J. Shen, X. Liu, L. Wang, A MultiWalled Carbon Nanotube electrode based on porous Graphite-RuO2 in Electrochemical Filter for Pyrrole Degradation, Chemical Engineering Journal (2017), doi: http://dx.doi.org/10.1016/j.cej.2017.08.042

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A Multi-Walled Carbon Nanotube electrode based on porous Graphite-RuO2 in Electrochemical Filter for Pyrrole Degradation

Xiezhen Zhou, Siqi Liu, Anlin Xu, Kajia Wei, Weiqing Han, Jiansheng Li, Xiuyun Sun, Jinyou Shen, Xiaodong Liu*, Lianjun Wang*

Key Laboratory of Jiangsu Province for Chemical Pollution Control and Resources Reuse, School of Environmental and Biological Engineering, Nanjing University of Science and Technology, Nanjing 210094, China

Corresponding author: *Lianjun Wang, Tel/Fax: +86 25 84315941, E-mail address: [email protected]; *

Xiaodong Liu, Tel/Fax: +86 25 84315319, E-mail address: [email protected]

Abstract A novel electrochemical filter has been prepared for the effective adsorption and electrochemical oxidation of aqueous organic pollutants. The adsorptive electrode utilized in the electrochemical filter was a porous Graphite-RuO2 electrode decorated with multi-walled carbon nanotubes (MWCNTs) adsorption layer, which was prepared by sol-gel coating, thermal decomposition, and vacuum filtration synthesis. The morphology and composition of adsorptive electrode characterized by field emission scanning electron microscopy (FESEM), X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) revealed a uniform distribution of RuO2 active coating and MWCNTs adsorption layer. Electrochemical measurements indicated that the adsorptive electrode possessed a favorable oxygen evolution potential (OEP) of 1.42 V and inner voltammetric charge 170.05 mC·cm-2, which were higher than that of non-MWCNTs electrode. Furthermore, the mass transfer of adsorptive electrode was enhanced as high as 4.6-fold in the electrochemical filter. The optimal operation parameters obtained was 3 mA·cm-2 and 7 g·L-1 Na2SO4. Under this condition, the electrochemical filter can remove >97 % of pyrrole and >75 % of total organic carbon. The pyrrole oxidation mechanism of electrochemical filter was also investigated, revealed three dominant reaction stages: adsorption (≤0.3 V), direct oxidation (0.3-1.2 V) and indirect oxidation (>1.2 V). In general, the high electrochemical performance of Graphite-RuO2-MWCNTs filter on pyrrole removal resulted from the enhanced mass transfer, which attributed to the synergistic effect of adsorption from MWCNTs, electrochemical oxidation and convection by filtration. Key words: mass transfer, electrochemical oxidation, carbon nanotube, nitrogen heterocyclic compounds 1. Introduction Nitrogen heterocyclic compounds (NHCs), such as pyrrole, are generated by chemical industry for multiple uses, including pharmaceuticals, pesticides, dyestuffs, and agrochemicals [1, 2]. Due to the existence of the nitrogen atom, NHCs exhibit a high polarity which leads to a better hydrogeological mobility and easily transported through soil [3]. Toxicological studies have indicated that NHCs are teratogenic, mutagenic,

carcinogenic even in low concentrations [4] and tend to bio-concentrate in food chain after persisting in the environment [5]. Therefore, NHCs pose a potentially serious threat to humans and other animals and have caused extensive concern in environment protection [6]. Conventional processes including biological process and chemical oxidation are reported to be less effectively in degrading toxic and recalcitrant compounds [7, 8], such as NHCs. However, electrochemical oxidation is widely utilized in environmental fields and considered to be versatile, utility and capable of degrading extensive of pollutants, which have drawn growing attention on the destructing toxic and bio-refractory organic contaminants in wastewater in recent years. The mechanism of electrochemical reaction can be classified as four stages: (a) mass transfer to the electrode, (b) adsorption and desorption from the surface of electrode, (c) direct transfer of electron at the electrode surface, and (d) chemical reactions proceed following the transfer of electron [9, 10]. The direct (c) and indirect (d) electron transfer respond the electrochemical transformations immediately. Hence, mass transfer (a) to the surface of electrode is considered as a main limitation factor of overall electrochemical processes [10]. Electrochemical mass transfer limitation is due to the fact that convection becomes negligible near the electrode-solution interface, and the comparatively slow molecular diffusion to the anode cannot be accomplished kinetically with the electron transfer [11]. Therefore, the enhancement mass transfer of the anode would promote the electrochemical transformation as well as lead to reduce of energy consumption. In our previously work, Zhang et al. reported improved electrochemical oxidation of tricyclazole by enhancing mass transfer in a Ti/RuO2 tubular porous electrode electro-catalytic reactor [12]. Recently, it is regarded as an ideal approach to employ an adsorption material as the electrode surface[13], which could promote the mass transfer significantly. Carbon nanotubes (CNTs) have drawn much attention due to their excellent electrochemical property, high tensile strength, favorable chemical stability, and conductivity [14]. As a consequence, CNTs as well as their derivation materials were widely employed in various fields.

Among the CNTs derivation materials, multi-walled nanotubes were composed of several graphitic layers arranged as concentric cylinders, showed great potentials application in electrochemical process due to their unique property of metallic conductivity [15]. Moreover, owing to the large specific surface area, combination of strong π-π interactions and hydrophobic interactions [16], the CNTs have been observed to adsorb various chemical species, especially aromatic compounds [17] and natural organic matter [18]. Liu et al. studied a new type carbon nanotube based electrochemical filter, which was reported to be effective for the adsorption and electrochemical oxidation of common aqueous antibiotic tetracycline [19]. Fan et al. reported a CNTs/Al2O3 membrane to remove natural organic matter, but also achieving high permeability, good selectivity and antifouling ability under electrochemical assistance [20]. The two works above obtained excellent removal efficiency of aqueous pollutant. However, the electrodes they prepared utilized in the electrochemical filter were based on conventional membrane, which were applicable to the condition of relatively low operating voltage (0.5-2.5 V) and pollutants concentration (< 90 mg·L-1). In this study, we prepared a novelty adsorptive electrode consisted of a MWCNTs adsorption layer and a porous Graphite-RuO2 electrode, which took the concept one step further by not only adsorptive removing, but also degrading the target pollutant electrochemically when solution convectively flows through the adsorptive electrode. Furthermore, this new type adsorptive electrode was characterized including surface morphology, crystal structure, electrochemical performance, mass transfer, and adsorption property. Finally, the electrochemical degradation experiment of pyrrole was performed to investigate the practical application of the prepared Graphite-RuO2-MWCNTs electrochemical filter. The removal efficiency of pyrrole and total organic carbon (TOC) removals were two important factors to evaluate the electrochemical oxidation performances. As a result, a fundamental understanding of pyrrole react in this adsorptive electrode system will lead to an improved understanding of the electrochemical filtration mechanism. 2. Materials and methods 2.1 Chemicals and materials

The multi-walled carbon nanotubes (MWCNTs) purchased from Beijing Boyu Nanotech Co., Ltd have an average length of 50 µm and a dimeter distribution d=8-15 µm. Thermogravimetric analysis of MWCNTs displayed that it was composed of 0.5-1.0 % amorphous carbon and 3-4 % residual metal catalyst of Fe. The porous graphite plate, with a pore size of 22 µm and a diameter of 30 mm produced by Shanghai Tiankai Co., Ltd, was worked as substrate Regents: Pyrrole (C4 H5N, CAS:109-97-7), Ruthenium Chloride Hydrate (RuCl3·3H2O, CAS:14898-67-0), Dimethyl Sulfoxide (DMSO, C2H6OS, CAS: 67-68-5), Polyacrylonitrile (PAN, (C3H3N)n, CAS: 25014-41-9). All other chemicals were of analytical grade supplied by Aladdin Co., Ltd. All solutions were prepared with deionized water obtained from a Millipore Milli-Q system. 2.2 Preparation of Graphite-RuO2-MWCNTs filtration electrode Before the fabrication, the porous graphite plate was ultrasonically cleaned with acetone and deionized water successively. Then, the RuO2 coating was generated on the porous graphite plate through the method of sol-gel process and thermal decomposition [21]. The precursor solution was prepared with 4 g RuCl3·3H2O in a solvent containing 250 mL isopropyl alcohol and 4 mL HCl. Next, the porous graphite plate was brushed repeatedly to ensure a homogeneous electrode surface at ambient temperature, dried at 105 oC for 25 min and sintered in a muffle furnace at 450 oC for 15 min. For the uniformity of filtration electrode surface, the procedure listed above needed to be repeated at least 10 times [22]. The last annealing process must conduct at 550 oC for an hour. The dispersion solution was prepared by dispersing MWCNTs in dimethyl sulfoxide (1mg·mL-1) with 0.5 wt% polyacrylonitrile (PAN) [13, 20]. Then, the prepared Graphite-RuO2 plate was filtrated by this dispersion solution in vacuum to construct a randomly oriented multi-walled carbon nanotubes layer. Eventually, the adsorptive electrode was finished by heating at 400 oC for 3 h in air. It has been reported that graphene-like carbon from polymer pyrolysis can serve as an adhesive bonding to harden the structure stability of MWCNTs filter layer [23]. Hence, in this work, PAN was employed to improve the adhesion

strength between MWCNTs and Graphite-RuO2 substrate. 2.3 Electrochemical filtration reactor and the experimental procedure All electrochemical experiments were completed in an electrochemical filter reactor as showed in Fig. 1. The Graphite-RuO2-MWCNTs filtration electrode was served as anode with an area of 20 cm2, and an equal size of stainless steel as cathode. The distance between anode and cathode was 5 mm. The current was supplied and controlled by a DC power supply source (HSPY-60-02). The electrochemical experiments were conducted with a continuous flow at a galvanostatic condition. Pyrrole was used to investigate the performance of adsorption and electrochemical oxidation of Graphite-RuO2-MWCNTs filtration electrode. The influent with a specific concentration of 300 mg·L-1 cycled in the reactor with a peristaltic pump. In a certain interval, 5 mL sample was taken and analyzed to obtain the removal efficiency of pyrrole and TOC. 2.4 Analytical method The concentrations of pyrrole were determined by high performance liquid chromatography (HPLC, LC-20, Shimadzu). TOC was measured by Elementar vario TOC cube. Morphological and composition analysis of Graphite-RuO2-MWCNTs adsorptive electrode was performed by field emission scanning electron microscopy (FESEM), energy dispersive X-ray spectroscopy (EDS) using Quantan 250F, X-ray diffraction (XRD) using Bruker D8 Advance with Cu-Kα (λ=1.5418 Å) at the scanning rate of 2° s-1 in 2θ mode from 20° to 80° and X-ray photoelectron spectroscopy (XPS) using PHI Quantera II equipped with a standard monochromatic Al Ka excitation source (hν =1486.71 eV). The electrochemical property and mass transfer behavior was evaluated by linear sweep voltammetry (LSV), cyclic voltammetry (CV) and chronoamperometry (CA), normal pulse voltammetry (NPV) by CHI600E electrochemical workstation respectively. LSV analysis was performed in 0.1 M Na2SO4 solution at a scan rate of 1 mV·s-1 and CV analysis was performed in in 1 M H2SO4 solution at different scan rate ranged from 1 mV·s-1 to 100 mV·s-1. CA and NPV experiments were both conducted in 1000 µM pyrrole solution with 0.01 M Na2SO4 electrolyte.

The Cottrell equation, eq. 1, describes the time-current functional relationship for diffusion-limited electrochemical systems and can be employed to estimate the molecular diffusion coefficient [24]. I=nFAD1/2 cπ-1/2 t-1/2

(1)

In eq. 1, n means the number of electrons transferred, D is the diffusion coefficient (cm2·s-1), c represents the bulk concentration of the molecule to be electrolyzed (mol·cm-3), A is the geometric area of electrode, and I is the current at time t. In this study, n of the pyrrole electrochemical oxidation is 26. C4 H5 N+11H2 O→4CO2 +NO-3 +26e-+27H+ Furthermore, an estimation of diffusion layer thickness, ∆, can be computed using the equation bellow (eq. 2)[11]. ∆=√2Dt

(2)

3. Results and discussion 3.1 Characterization of Graphite-RuO2-MWCNTs filtration electrode 3.1.1 Surface morphology The morphologies of Graphite-RuO2-MWCNTs filtration electrode were investigated by FESEM analysis (Fig. 2). As can be seen from the Fig. 2a, graphite showed a rough morphology because of a random distribution of graphite flakes. The size of RuO2 crystallites was ranged from 100 nm to 300 nm (Fig. 2b). The surface of RuO2 coating was compact and almost homogeneous, indicating a potential electrochemical activity. Fig. 2c and 2d showed both aerial and cross-sectional FESEM images of the adsorptive electrode surface. The MWCNTs network had an effective filtration area of 20 cm2 and abundant interconnected pores without cracks, the thickness of MWCNTs network was about 150 µm (Fig. S1). This microstructure of porous MWCNTs layer could accelerate adsorption and improve electrochemical oxidation performance, because it can provide a great number of easily accessible and reactive sites compared with conventional Ti/RuO2 plate electrode (Table S1) [12]. As a result, the pyrrole molecular could access to the surface sites in the anodic MWCNTs as the liquid convectively flowing through the network. Therefore, electrochemical

filter with porous MWCNTs adsorption layer may result in an enhancement of mass transfer. 3.1.2 Surface composition The surface chemical composition and the valence states of the anode coating were characterized by using XPS. The survey XPS spectra of Graphite-RuO2 and Graphite-RuO2-MWCNTs were showed in Fig. 3a, many distinct Ru XPS signals such as 3s, 3p, 3d, 4s and 4p appeared in addition to the C1s and O1s (~529-534 eV) signals corresponding to the C–O bonds in graphite, carbon nanotubes and Ru-O bonds in RuO2. However, owing to the MWCNTs filtration layer, some XPS signals were blocked in the spectra of Graphite-RuO2-MWCNTs. The core-level XPS spectrum of C1s was displayed in Fig. 3b, a main peak centered at 284.6 eV was clearly observed which was related to the graphitic sp2 carbon atoms [25], and the weak peak around 286 eV was relate to the carbon atoms connecting with oxygenate groups, including C–O and O–C=O [26], indicating a presence of residual oxygenate groups on the surface of graphite plate and MWCNTs. The presence of residual PAN on the Graphite-RuO2-MWCNTs surface was further conformed by the core-level N1s XPS spectrum shown in Fig. 3c, the N1s core-level spectrum would be ascribed to – N= at 398.4 eV and N+ at 400.5 eV [27]. Fig. 3d displayed the core-level XPS spectrum of Ru2p with Ru2p3/2 and 2p1/2 signals centered at 463.4 and 484.6 eV, suggesting the formation of graphene-RuO2 composite at porous graphite plate surface [28]. The XPS analysis results revealed an excellent binding force between RuO2 coating and porous graphite substrate. Consequently, the electrochemical oxidation property and service life of prepared Graphite-RuO2-MWCNTs adsorptive electrode could also be improved. Fig. 4 displayed the XRD signals of Graphite-RuO2-MWCNTs electrode. The peak located at 26.4° was contributed by graphite. Besides, the intensity peaks observed at 28.3°, 35.5°, 40.5° and 55.0° with (110), (101) and (211) were ascribed to RuO2 (JCPDS Card No: 40-1290) [29]. Elemental profile and EDS mapping were conducted to affirm chemical fluctuations of various elements of Graphite-RuO2-MWCNTs filtration electrode shown in Fig.S2. It noted that carbon (97.96 wt%) was constituted as the most abundant element due to the graphite substrate and MWCNTs adsorptive layer, which leaded to a low detection value

of Ru (0.16 wt%) and O (2.15 wt%) element. Furthermore, the EDS mapping revealed a uniform distribution of MWCNTs and RuO2 coating. 3.1.3 Liner sweep voltammetry (LSV) and cyclic voltammetry (CV) analysis In practical utilization of anodes, oxygen evolution potential (OEP) is an important parameter of electro-catalytic activities for organic oxidation. The liner sweep voltammetry measurements were performed to compare the OEP between Graphite-RuO2 and Graphite-RuO2-MWCNTs. As shown in Fig. 5a, the OEP of Graphite-RuO2 and Graphite-RuO2-MWCNTs are 1.16 V and 1.42 V (vs Ag/AgCl) respectively. Generally, OEP value of an anode is controlled by two factors: the resistance of electrode material and the resistance to charge transfer at the electrode-solution interface [30, 31]. In this case, the hydrophobic MWCNTs layer leaded to a relatively higher charge transfer resistance at the electrode-solution interface, resulting a higher OEP of Graphite-RuO2-MWCNTs adsorptive electrode [32]. The relatively higher OEP means less side reaction of oxygen evolution occurred during electrochemical oxidation process as well as high current efficiency [33]. Fig.

5b

and

5c

showed

Graphite-RuO2-MWCNTs.

the

cyclic

voltammetry

(CV)

curves

of

Graphite-RuO2

and

Electrochemical surface area, which is an important parameter of

electrochemical oxidation, affects the electrode performance significantly. It has been reported that researchers normally employ CV analysis to evaluate electrochemical surface area [34, 35], because the voltammetric charge (q * ) of an electrode was calculated by a graphical integrator from the cyclic voltammetry cures. Additionally, it can be seen from the FESEM images the RuO2 oxide film and MWCNTs adsorption layer was microporous. These micro-pores on the anode surface was “inner surface” and had less accessibility for proton-donating species. Therefore, the voltammetric charge (q * ) declined with the increase of potential scan rate (v) contributed by the difficulty of electrolyte ions entered the “inner surface” [36, 37]. As the scan rate (v) reaches (∞), the voltammetric charge (q* ) closes to the outer voltammetric charge (q *O) and relates to the most accessible electroactive surface. Oppositely, with the scan rate (v) reaches 0, the

voltammetric charge (q* ) tends to the total voltammetric charge (q*T ) and relates to the entire electroactive surface [38]. Inner voltammetric charge (q *I ) concerns with the “inner surface” is the difference value between q*T and q *O.The values of q *I /q*T is an indicator of electrochemical porosity. From the Fig. 5d, the (q* )-1 was linear correlation to v1/2 . Total charge (q*T ) educed from the extrapolation of these straight lines to v=0. The outer charge (q *O) obtained from the extrapolation to v=∞ (v-1/2 =0) in the plot of  ∗ versus v-1/2 (Fig. 5e). Owing to the uncompensated ohmic drop that decreased the value of  ∗ , the linear relationship deviated at high potential scan rate [36, 39]. Table 1. summarized the total, outer and inner charges of Graphite-RuO2 and Graphite-RuO2-MWCNTs electrode. The total voltammetric charge (q *T) of Graphite-RuO2-MWCNTs (297.61 mC·cm-2) was larger than that of Graphite-RuO2 (168.63 mC·cm-2), suggesting an electrochemical surface area enhancement due to the existence of MWCNTs adsorption layer. Particularly, because the porous microstructure of MWCNTs favored electrochemical performance, the inner voltammetric charge (q *I ) of Graphite-RuO2-MWCNTs (170.05 mC·cm-2) was much larger than that of Graphite-RuO2 (49.37 mC·cm-2). Therefore, it implied a potential electrochemical activity of inner surface of Graphite-RuO2-MWCNTs adsorptive electrode. Because these porous and 3D structures formed by MWCNTs, Graphite-RuO2-MWCNTs provided more real surface area and active sites compared with the non-MWCNTs electrode, resulting the increases in the convective mass transfer of the target molecules to the electrode surface [40]. 3.1.4 Chronoamperometry (CA) and normal pulse voltammetry (NPV) analysis Fig.6a showed the results of chronoamperometry analysis in static and filtration condition compared between Graphite-RuO2 and Graphite-RuO2-MWCNTs. After the test time of 15 s, the current was tended to a horizontal line and approached a steady state, 0.15 A of Graphite-RuO2-MWCNT/Filtration, 0.11 A of Graphite-RuO2/Filtration, 0.05 A of Graphite-RuO2-MWCNT/Static and 0.03 A of Graphite-RuO2/Static respectively. As can be seen from the CA curves, the higher value of plateau means higher mass transfer. Compared with static, the value of steady state in filtration is much greater. It suggested a considerably

increase of molecule diffusion to the electrode surface attributed to hydrodynamic flow by peristaltic pump and adsorption by MWCNTs network [41]. For the static situation, the turbulence promoters can improve the mass transfer between the gap of the anode and cathode. However, it is unavoidable that many molecules will flow away with the solution and bypass the electrode surface where the electrochemical oxidation occurred [42]. Differently, in the electrochemical filter the pyrrole molecules could flow through the electrode directly with the solution via the impetus from the peristaltic pump. Thus, the flow direction of molecules in this study was towards to the electrode which increased collision probability [43]. The current versus time-1/2 is plotted in the inset of Fig. 6 and shows a linear relationship over an intermediate at intermediate time range (20-40 s). From the slope of this line, the diffusion coefficient D of Graphite-RuO2 and Graphite-RuO2-MWCNTs in non-filtration situation was calculated (eq. 1) to be 5.84 ⅹ 10-4 cm2·s-1 and 9.56 ⅹ 10-4 cm2·s-1 respectively. Obviously, the mass transfer of Graphite-RuO2-MWCNTs in the non-filtration condition was 1.64 times higher than that of Graphite-RuO2 because the absorbability provided by MWCNTs.

Quantitatively,

the

diffusion layer

thickness of Graphite-RuO2 and

Graphite-RuO2-MWCNTs were estimated (eq. 2) to be 3.42 mm and 4.37 mm respectively. The thickness of diffusion layer in the electrochemical filter is expected to be lesser under the same experimental condition because the hydrodynamic compression of the diffusion layer. Hence, the eq. 1 and eq. 2 are only suitable in static situation. NPV analysis was employed to investigate mass transfer in filtration situation. Fig. 6b displayed the NPV curves of Graphite-RuO2-MWCNTs/Static and Graphite-RuO2-MWCNTs/Filtration. As can be seen from the Figure, the current increased linearly with the increase of potential and then reached a potential independent, mass transfer limited plateau in both curves. The linear increase of current with the potential increasing was attributed to the increasing direct electron transfer kinetics. Because the electrochemical oxidation of pyrrole was kinetically faster than pyrrole diffusion, the concentration of pyrrole on the MWCNTs filtration anode surface decreased, leading to the

formation of surface concentration gradient [44]. Finally, the potential will increase to a value (>0.5 V for static and >0.7 V for filtration), where the pyrrole concentration on the electrode surface was zero and mass transfer to the electrode surface became the limiting factor for electrochemical oxidation of pyrrole. Therefore, the further increase of anode potential will not lead to any current increase and a plateau occur [11]. The following current increase was caused by the oxygen evolution. The limited current of mass transfer in electrochemical filtration condition was 0.14 A, and for comparison the mass transfer limited current of static is 0.03 A, which was 4.6-fold. Thus, the diffusion layer of filtration can be computed to be 1/4.6 of the static value, 0.95 mm. The conclusion that mass transfer in the electrochemical filter was significantly promoted about 4.6-fold than static. 3.2 Electrochemical oxidation of pyrrole The current density plays a very important part in the electrochemical oxidation process as it regulates the capability of hydroxyl radical generation on the electrode surface. Thus, electrochemical experiments was performed under different current density of 1, 3, 5, 7 mA·cm-2. As can be seen from the Fig. 7a, increasing current density had a positive influence on the degradation of pyrrole from 1 to 3 mA·cm-2 at first. The maximum removal efficiency of pyrrole and TOC was 97.7 % and 75.7 % at current density of 3 mA·cm-2. Then the removal efficiency of pyrrole declined with the increasing of the current density. The removal efficiency of pyrrole and TOC at 7 mA·cm-2 was 89.1 % and 61.8 %. It is because that at higer applied current density, the potential of anode exceeded the oxygen evolution potential. The bubbles generated and attached in the porous MWCNTs network and porous Graphite-RuO2 electrode which shield the reactive surface [40], leaded to a decline in the pyrrole and TOC removal efficiency. The electroconductivty of aqueous also plays a significant role in electrochemical oxidation, the influence of supporting electrolyte on the degradation and mineralization of pyrrole were conducted at 3 mA·cm-2 with different concentration of Na2SO4 electrolyte (3, 5, 7, 9 g·L-1). As shown in Fig. 7b, the removal efficiency of pyrrole and TOC was increased with the increasing of Na2SO4 concentration from 3 g·L-1 of

67.2 % and 59.8 % and reached maximum of 97.7 % and 75.7 % at 7 g·L-1, then decreased as the continue increasing of electrolyte concentration at 9 g·L-1 of 71.3 % and 50.7 %. The good electroconductivtity 2-

leaded to a faster electron transfer. However, the overmuch salinity may result in an adsorption of SO4 ions on the electrode surface and minimize the number of active site of electrode, which lead to a decrease of electrochemical oxidation of pyrrole [45]. The effect of initial pyrrole concentration was investigated with different substrate concentration (100, 300, 500 and 700 mg·L-1) at 3 mA·cm-2. As observed in Fig. 7c, the removal efficiency of pyrrole and TOC decreased from 99.1 % and 78.3 % to 72.7 % and 51.3 % with the increasing concentration of pyrrole from 100 to 700 mg·L-1. It is because that when the concentration increased, more pyrrole molecues transferred to the anode surface, however, the ·OH radicals generation amount were limited [46]. In general, the greater concentration of initial pollutants leads to the removal of more amounts of contaminants per unit of time. Electrochemical oxidation performance of pyrrole were evaluated compared between Graphite-RuO2 and Graphite-RuO2-MWCNTs electrode under filtration conditions. As shown in Fig. 7d, the pyrrole removal efficiency was 97.7 % of Graphite-RuO2-MWCNTs and 70.9 % of Graphite-RuO2. The excellent removal efficiency was ascribed to the high specific surface area of MWCNTs and its effective adsorptive of pollutants [47]. Due to the larger reactive sites and better mass transfer, Graphite-RuO2-MWCNTs showed a improved electrochemical oxidation performance. During the electrochemical filtration process, the high aspect ratio MWCNTs are easily formed into free-standing, thin-film and 3D networks of high porosity [44]. This kind of porous sturcures could provide fast adsorption and electrochemical oxidation attributed by the abundant

of

accessible

and

reactive

sites.

Therefore,

the

electrochemical

filter

with

Graphite-RuO2-MWCNTs electrode resulted in a elevation of mass tranfser and electrochemical oxidation ability. The functional relationship of anode potential on the anode current and pyrrole oxidation at an influent concnetration of 100 mg·L-1 was presented in Fig. 8a. The predominant degradation path, i.e., adsorption,

direct electron transfer or indirect electron transfer, can be concluded from the data analysis. The effluent pyrrole concentration decreased with the tenuous increase of anode potential at the initial stage before 0.3 V vs Ag/AgCl. During this period, the effect of direct electron transfer was not obvious and the decrease of pyrrole concentration may be mainly attributed by the adsorption provided by MWCNTs layer. Then the concentration decreased and reached a plateau of 65 mg·L-1 from 0.8 to 1.2 V. The anode current showed a corresponding increase until 0.9 to 1.2 V where the plateaus at mass transfer limited current density was 2 mA·cm-2. At this period, the electrochemical oxidation of pyrrole was faster than its diffusion. The concentration of pyrrole on the anode surface was zero and mass tranfer became the limiting factor of pyrrole electrochemical oxidation, which leaded to a plateau in current density and pyrrole effluent concentration. Therefore, it can be inferred that the decrease of pyrrole concentration up to 1.2 V was due to the increasing rate of direct electron transfer. This result was consistant with the mass transfer limited current plateau presented in NPV curve showed in Fig. 6b. As the anode potential increased to >1.2 V, the effluent pyrrole concentration continued to decrease, which indicated indirect and direct oxidation occurring simultaneously. The activation of indirect oxidation ignored the limitation of mass transfer due to the electro-generated oxidatants diffused to the bulk solution and reacted with pyrrole molecules which are not directly oxidized[44]. According to the previous research [48, 49]: H2 O→4H+ +O2 +4e-

(3)

The E0 of the equation (3) is 1.01 V vs SCE which is similar to the observed anode potential, 1.2 V, at which the indirect oxidation became active. The produced oxygen will immediately react with the radicals generated from direct oxidation of pyrrole and form reactive oxygen species such as peroxy radicals that could indirectly oxidize pyrrole. Furtherly, the current of two curves intersection was about 3 mA·cm-2 suggested an optimum value of electrochemical oxidation, which corresponded with the current density of pyrrole electrochemical oxidation expeiment showed in Fig. 7a. In general, at low anode potentials, adsorption (≤0.3 V) and direct oxidation (0.3-1.2 V) was the main pyrrole degradation path at different react

stages. As the anode potential increased above 1.2 V, the contribution of indirect oxidation became significantly. Therefore, electrochemical oxidaiton of pyrrole in the electrochemical filter could be divided into three stages: (a) mass transfer enhanced by hydrodynamically, (b) adsorption of MWCNTs, and (c) direct oxidation and indirect oxidation at anode surface. As displayed in Fig. 8b, the adsorption experiments of pyrrole were performed with a new Graphite-RuO2-MWCNTs adsorptive electrode and the electrode after 5 times electrochemical-filtration processes, the pyrrole adsorption removal efficiency were 45.1 % and 36.8 % respectively. This experiment suggested that the prepared adsorptive electrode possessed the adsorbability to pyrrole without electrochemical assistance due to the MWCNTs adsorption layer. Then, after the electrochemical oxidation process, most of the pyrrole adsorbed on the MWCNTs can be degraded. Thus, this result indicated that the prepared adsorptive electrode could degrade pyrrole not only by adsorptive removing, but also by electrochemically oxidation. 4. Conclusion In summary, a novel Graphite-RuO2-MWCNTs adsorptive electrode was successfully fabricated by sol-gel coating, thermal decomposition and vacuum filtration synthesis. Due to the MWCNTs adsorption layer and porous Graphite-RuO2 electrode, the adsorptive electrode had lager specific area, better mass transfer and good electrochemical oxidation performance. Optimal parameter of pyrrole electrochemical oxidation was obtained at current density of 3 mA·cm-2 and Na2SO4 concentration of 7 g·L-1. As compared with non-MWCNTs electrode, the adsorptive electrode possessed higher removal efficiency of pyrrole. This study investigated the mechanism of pyrrole oxidation in the electrochemical filter. The electrochemical filtration process was described by a reactive transport mechanism consisting of three primary stages: (a) mass transfer enhanced by hydrodynamically, (b) adsorption of MWCNTs, and (c) direct oxidation and indirect oxidation at anode surface. The key to effective oxidation in electrochemical filter was the 4.6-fold increase in mass transfer due to adsorption of MWCNTs and convection of the pyrrole molecule through the

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Fig.1 Schematic diagram of electrochemical filter system

Fig.2 FESEM images of porous graphite (a); Graphite-RuO2 (b); aerial (c) and cross-sectional (d) of MWCNTs adsorptive layer

Fig.3 The survey XPS spectra of Graphite-RuO2 and Graphite-RuO2-MWCNTs (a). The high resolution XPS spectra of C1s (b), N1s (c) and Ru3p (d)

Fig.4 XRD patterns of Graphite-RuO2 -MWCNTs

Fig.5 LSV analysis compared between Graphite-RuO2 and Graphite-RuO2-MWCNTs (a); CV analysis of Graphite-RuO2 (b) and Graphite-RuO2-MWCNTs (c); Extrapolation of the (d) total voltammetric charge (qT*) and (e) outer voltammetric charge (q O*).

Fig.6 CA analysis compared between Graphite-RuO2 and Graphite-RuO2-MWCNTs (a); NPV analysis compared between Graphite-RuO2 and Graphite-RuO2-MWCNTs (b).

Fig.7 Removal efficiency of pyrrole changes as a function of electrolysis time at different current densities (a); Removal efficiency of pyrrole changes as a function of electrolysis time at different support electrolyte concentrations (b); Removal efficiency of pyrrole changes as a function of electrolysis time at different initial concentrations (c); Removal efficiency of pyrrole compared between Graphite-RuO2 and Graphite-RuO2-MWCNTs (d).

Fig.8 Effect of anode potential on pyrrole oxidation and anodic current density during electrochemical filtration (a); Adsorption experiments of pyrrole without electrochemical assistance (b).

Graphite-RuO2 Graphite-RuO2-MWCNTs

q*T mC·cm-2 

q*O mC·cm-2 

q*I mC·cm-2 

q*I /q*T

168.63 297.61

119.26 127.56

49.37 170.05

0.292 0.571

Table 1 Total, outer and inner charges for Graphite-RuO2 and Graphite-RuO2-MWCNTs electrode

(1) An adsorptive electrode used in electrochemical filter was fabricated and tested; (2) The mass transfer was enhanced 4.6-fold in the electrochemical filter; (3) The high pyrrole degradation efficiency was gained in the electrochemical filter; (4) The pyrrole oxidation was classified as three stages in the electrochemical filter.