Improvement of Electrochemical Wastewater Treatment through Mass

Apr 13, 2009 - A seepage carbon nanotube electrode (SCNE) reactor was designed in order to facilitate contaminant mass transfer from bulk solution to ...
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Environ. Sci. Technol. 2009, 43, 3796–3802

Improvement of Electrochemical Wastewater Treatment through Mass Transfer in a Seepage Carbon Nanotube Electrode Reactor JI YANG,* JUN WANG, AND JINPING JIA School of Environmental Science and Engineering, Shanghai Jiao Tong University, Shanghai 200240, P. R. China

Received December 2, 2008. Revised manuscript received March 13, 2009. Accepted March 16, 2009.

A seepage carbon nanotube electrode (SCNE) reactor was designed in order to facilitate contaminant mass transfer from bulk solution to the electrode surface, therefore to break the high cost bottleneck of electrochemical wastewater treatment. The innovative concept behind the reactor design is that the overall mass transfer would be significantly improved via contaminant migration through the porous carbon nanotube electrode. It was found out that the surface diffusivity Ds,i in the external film was the controlling coefficient for electrochemical treatment, and the proposed process could improve the overall mass transfer coefficient by 116-161% compared with conventional electrochemical reactors under the same conditions. The research also showed that the current efficiency of the SCNE reactor was 340-519% higher than that of conventional reactors, and the energy consumption to mineralize the same amount of organics was only 16.5-22.3% of the conventional reactors. Also, the influences of potential, pH, and electrolyte concentration were investigated to optimize the operating parameters for the SCNE reactor. These results show that the SCNT reactor is promising because of its energy efficiency and has potential for application in wastewater treatment.

Introduction The application of electrochemical techniques in organic wastewater treatment has drawn considerable attention in the past few years (1-6), since the electrolytic process is easy to control by potential and current, and such a process can operate at low temperature and pressure without adding other reagents (7). Though the electrochemical technique has many advantages, it is not widely applied in the wastewater treatment because of its limitations, i.e., the high cost and low current efficiency caused mainly by low contaminant mass transfer from water to the electrode surface (8-10). Therefore, development of energy efficient electrochemical reactors is a current and great challenge in science and technology and one that will have important economic consequences. Previous research shows that although the destruction of organic compounds by electrochemical treatment is achieved by oxidation, which can occur directly at the anode and/or indirectly by species (e.g., O•, OH•, and O3) generated by the anode (7), the reaction rates are actually limited by the * Corresponding author phone: +86-21-54742817; fax: +86-2154742817; e-mail: [email protected]. 3796

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contaminant diffusion governed by liquid diffusion coefficient DL,i, pore diffusivity DP,i, and surface diffusivity Ds,i in the external liquid film on the electrode surface and in the internal pores (11), as shown in Figure 1. Therefore, contaminant oxidation is controlled by the rate at which organic molecules are carried from the bulk solution to the electrode surface, rather than by the rate at which electrons are delivered to the anode (8-10). From review of the literature, it is fair to say that so far most of the studies carried out on the electrochemical treatment pertain to the experimental conditions, with the primary objective of these studies being the development of appropriate parameters for contaminant removal, such as selection of electrode materials, electrolytic voltage, or electrolytes (1-6, 12-15). Although mass transfer in electrochemical reactors has been studied by researchers (8-10), the work has been mainly focused on increasing the contaminant movement in bulk solution or reducing the external liquid film thickness by mechanical stirring. There has been little study of the fundamental problem of electrochemical efficiency related to contaminant diffusion in the liquid film. It is in this context that the present idea was proposed with a primary view to cut the energy consumption of the electrochemical process by improving the overall mass transfer. An original reactor with the seepage electrode made of carbon nanotubes (CNTs) has been developed and successfully applied to degrade organics with much higher CE and lower EC. The reason to employ CNTs is that they are very interesting nanomaterials known to exhibit superior and unique electron transfer (ET), thermal, and mechanical properties. The possibility of CNT utilization for organic oxidation has already been demonstrated (16), because CNT electrodes have an extended surface area that promises to provide an additional advantage for chemicals to react. Reactive Brilliant red X-3B(X-3B) solutions were employed as a model system to evaluate the reactor and to compare with the conventional electrochemical processes, because dyes are biorefractory environmental pollutants (17) with strong absorbance in the UV and visible range. It is estimated that more than 15% of the world dye production, about 400 t per day, are released into the environment during synthesis, processing, and use (18). The aim of this study was to provide greater comprehension of the parameters that control the overall mass transfer in the seepage carbon nanotube electrode (SCNE) reactor and to use that information to optimize future reactor design. The significance of this research is to reduce the cost of the electrochemical method by improving the overall mass transfer of the SCNE reactor. The research shows that the SCNE reactor is more energy efficient than conventional electrochemical setups and might broaden the electrochemical application more extensively in wastewater treatment.

Experimental Section Materials and Reagents. The Reactive Brilliant red X-3B was purchased from Shanghai Jiaying Chemical Co., Ltd., China. Viscose-based activated carbon fiber felts (V-ACFs) were purchased from Nantong Senyou Carbon Fiber Co., Ltd., China. The CNTs (external diameter is 60-100nm) were provided by Shenzhen Nano-Technologies Port Co., Ltd., China, and the SEM is shown in Figure 2, which indicates that the CNTs used have an even diameter (Shenzhen NanoTechnologies Port Co., Ltd., China). All other reagents used were of analytical grade. The pore size distribution of the ACF and CNTs could be referred to previous publications (19, 20). 10.1021/es8034285 CCC: $40.75

 2009 American Chemical Society

Published on Web 04/13/2009

FIGURE 1. Mass transfer and reaction on electrode surface.

FIGURE 2. Schematic setup of the SCNTS reactor (1, 2, ACF+CNTs DC power source). Electrolytic Reactor. The experimental setup employed is shown in Figure 2. Two pieces of activated carbon fiber (ACF) felts (4 cm × 7 cm) which had CNTs (0.5 g) packed evenly between them were employed to act as anode and cathode. An insulated porous membrane was inserted at the center of the CNTs to separate the anode and the cathode. A DC voltage-stabilized power supply (Shanghai Liyou Electrification Co., Ltd., China) was connected to both the anode and cathode and was used to control the potential. The simulated dye wastewater was circled by a peristaltic pump (Baoding Longer Precision Pump Co., Ltd., China). For the conventional experiments with CNTs electrodes, the same reactor was employed while the electrodes were emerged vertically at the center of the reactor. And for the conventional experiments with ACF electrodes, only two pieces of ACF with same dimension as SCNT electrodes were employed as anode and cathode and situated 1 cm from each other. Other parameters were kept the same as SCNT reactor. The simulated dye wastewater, which comprised a known concentration of sodium sulfate to simulate the ionic strength in actual situation and to ensure conductivity, was prepared from commercial dye with distilled water to desired concentrations. For each run, 300 mL of simulated dye wastewater was poured into the reactor and continuously circled by a peristaltic pump with a constant flow of 80.2 mL/min. To eliminate the effect of adsorption, the electrodes had been

electrode; 3, insulated porous membrane; 4, peristaltic pump; 5, presaturated with the dye before each experiment. At appropriate time intervals, samples of 10 mL were taken from the reactor and analyzed to determine the intermediate properties of the reaction medium during electrochemical conversion. Analysis. The color removal was evaluated by the absorbencies of the solutions at 538 nm using a UV/vis spectrophotometer (UNICO Instruments Co., Ltd.). Concentrated dye solutions were diluted prior to measurement to allow the absorbance to be within a linear range. Chemical oxygen demand (COD) values of the solutions were determined by standard method which was based on the method of acidic oxidation by dichromate to evaluate the mineralization extent of the dyes. The pH value was measured with a PHS-3C pH meter (Model, Shanghai, China).

Results and Discussion The runs were carried out in duplicate, and average results are presented in the following figures. The data points for duplicate runs at any time were not different from each other more than 4%. The polymeric formation on electrodes has not been detected in any run, which, in fact, was observed in literature studies. Degradation of X-3B in SCNT Reactor. Prior to the comparison experiments, the capability of the SCNT reactor to degrade organics was tested. It can be seen in Figure 3 VOL. 43, NO. 10, 2009 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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overall mass transfer coefficient km (cm/s), which could be determined as follows (25): ln

FIGURE 3. UV/vis absorption spectrum for X-3B degradation by SCNTS reactor (X-3B concentration, 50 mg/L; potential 10 V; pH, 7; Na2SO4 concentration, 3 g/L). that the absorption spectrum of X-3B was characterized by the main band in the visible region with its maximum absorption at 538 nm, and three bands in the ultraviolet region which were at 235, 285 and 330 nm, respectively. The absorbance features of X-3B at 235, 330, and 538 nm were respectively ascribed to the benzene ring, naphthalene ring, and azo linkage. The absorbance feature at 285 nm may be the fine multipeaks of the benzene ring and naphthalene ring (21-23). For 50 ppm initial feed concentration, X-3B concentration at the reactor outlet decreased with increasing treating time as shown in Figure 3. From the experimental results, the absorbance feature at 538 nm in the visible region diminished quickly and nearly disappeared after 90 min, which indicated that the azo linkage was destroyed first. At 15, 30, and 45 min of treatment time, X-3B removal was 25.2%, 42.0%, and 58.4%, respectively. Above 75 min, almost all X-3B was eventually consumed in the reactor. However, the absorbance features in the ultraviolet region did not reduce significantly in the first 45 min and only started decreasing after 45 min. The reason might be that the degradation of X-3B had two stages: The first stage was the decolorization characterized by degradation of azo linkage, and the second stage was mineralization of organic compound by destroying the benzene ring, naphthalene ring, and other functional groups. The experimental results indicated that the electrochemical setup has the ability not only to destroy azo linkage but also to destroy the benzene ring and naphthalene ring, which was manifested by the COD data later. During the reactions, the color of the solution at the outlet stream turned to light red from dark red then to transparent due to increased treatment time, but the color change did not affect the performance of the reactor, mainly cell current density. SCNT vs Conventional Setup. Contaminants Mass Transfer. Conventional electrochemical experiments with CNT’s electrodes and ACF electrodes were also performed to treat X-3B. Comparison results of the color and COD removal are shown in Figure 4. In the investigated concentration, SCNT reactor removed total color and COD by 94.4% and 57.6%, respectively, in 90 min, which is much higher than 32.8-37.4% and 28.0-32.7% removal by conventional electrochemical processes under the same treatment conditions, although CNTs and ACF have been proven to be effective electrode materials at organic mineralization (16, 24). Since all the parameters were kept the same for the SCNT experiments and conventional runs, the excellent performance of the seepage reactor was attributed to the increased 3798

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COD0 kmtAe ) CODt VR

(1)

where Ae is surface area of the electrode (cm2), VR is volume of the reactor (cm3), and COD0 and CODt are chemical oxygen demand at times 0 and t (g O2/L). On the basis of eq 1, the mass transfer coefficient of the SCNT reactor (90 min) was 1.7 × 10-3 cm/s, while it was 7.9 × 10-4 and 6.5 × 10-4 for conventional CNT electrodes and ACF electrodes reactor, respectively, which indicated that the movement of organic molecules from the bulk solution to the electrode surface was faster in the SCNT reactor than that in conventional setups. Since the mass transfer in bulk solution were kept the same for all the electrolytic processes, the significant differences in overall mass transfer could be attributed to the different internal mass transfer as shown in Figure 1, mainly DL,i which is the Lliquid diffusion coefficient, DP,i pore diffusivity, and Ds,i surface diffusivity. On the basis of the correlation presented by Hayduk and Laudie (26) and Crittenden (27), these internal mass transfer coefficients could be determined as follows: DL,i )

13.26 × 10-5 µ1.14V b0.589

(2)

DL,i τP

(3)

DP,i )

Ds,i ) SPDFR(Dp,ilpCb,i)/Fpqe,i

(4)

in which µ is fluid viscosity (g cm-1 s-1), Vb is molar volume at NBP (mL/mol), τp is tortuosity (dimensionless), SPDFR is the surface to pore diffusion flux ratio (dimensionless), lp is porosity of the electrode (dimensionless), Fp is electrode density (M L-3), Cb,i is influent bulk liquid-phase concentration (ML-3), qe,i is the solid-phase concentration at equilibrium with the influent concentration in the liquid phase. It can be seen that DL,i and DP,i are the same for both SCNT reactors and conventional CNT electrode reactors. However, the Ds,i of the SCNT reactor is reasonably larger than that of the conventional CNT electrode reactor since the SPDFR for both conventional processes are negligible.This directly explains the better performance of the SCNT reactor and proves that Ds,i is the controlling mass transfer. This also demonstrated that the idea proposed by the authors could improve the mass transfer of the electrochemical process and therefore might break the high cost bottleneck of traditional electrochemical wastewater treatment. Current Efficiency (CE) and Energy Consumption (EC). The CE and EC are two major considerations when it comes to the industrialization of electrochemical wastewater treatment. The CE of the electrochemical processes is defined as the current fraction used for the organic compound oxidation (28). The current efficiency is calculated according to the values of chemical oxygen demand (COD) of the wastewater (29), and the CE and EC could be determined as follows (29-32): CE (%) ) 100FV EC )

∆COD 8I∆t

UIt 3.6∆CODV

(5) (6)

where 4COD is the difference of the COD at time 0 and t (g O2/L), I is the current (A); F is the Faraday constant (96 487 C/mol), and V is the volume of solution (L), t is the treatment time (s), and U is the voltage (V).

FIGURE 4. Comparison of color removal between SCNTS reactor and traditional setups under same conditions (X-3B concentration, 50 mg/L; potential 10V; pH, 7; Na2SO4 concentration, 3 g/L).

FIGURE 5. Comparison of CE and EC during the electrolysis process between SCNTS reactor and traditional setups under same conditions (X-3B concentration, 50 mg/L; potential 10 V; pH, 7; Na2SO4 concentration, 3 g/L). Figure 5 shows the changes of current efficiency and energy consumption with treatment time. Relatively stable CE was established approximately after 45 min of elapsed time, and the corresponding value was 33.1%, 7.5%, and 5.3% for SCNT reactor, CNT reactor, and ACF reactor respectively. It could be seen that the SCNT’s CE was 340.2-519.1% higher than the controlled reactors, which indicates that SCNT is more energy efficient. And the CE of SCNT is even higher than the reported values (10-21%) on Ti/RuO2, Ti/IrO2, Pt, SnO2, PbO2, and Si/BDD electrodes (33-35). The EC values give the same trend. For every kilogram of COD consumed, the EC of SCNT is 101.34 kWh, which is only 16.5-22.3% of the conventional setups, showing that this reactor could decolorize and mineralize the dye solution effectively and economically. Optimal Operating Parameters for SCNTS Reactor. Potential. Potential is always the first to manipulate when it comes to optimizing electrochemical reactor because it

directly determines the contaminant removal and side reactions. As shown in Figure 6, the color removal exhibited a positive correlation with potential, especially in the first 60 min. However, while the potential was higher than 10 V, the color removal increased only slightly. The reason may be that the secondary reaction of oxygen evolution is also accelerated by increasing potential (36). The electrochemical oxidization of organics on anodes has been widely investigated (33, 37). It is believed that the solution is expected to discharge at the anode to produce adsorbed hydroxyl radicals. In particular, at nonactive electrodes, hydroxyl radicals are physisorbed while in active electrodes they are chemisorbed. In the presence of the pure system supporting electrolyte-water, dioxygen is produced involving physisorbed and chemisorbed “active oxygen”, respectively, which can otherwise oxidize any oxidizable organics present in solution. According to the literature (36), an oxidization process is favored with respect to a partial VOL. 43, NO. 10, 2009 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 6. Effect of potential on color removal (X-3B initial concentration, 50 mg/L; Na2SO4 concentration, 2 g/L; pH ) 7). oxidation at nonactive electrodes when physisorbed “active oxygen” is prevalent, and the organic removal is expected to be dependent on the anode potential, similarly to what was observed in our case. Electrolyte. Inorganic anions (Cl-, SO42-, etc.) present in wastewater or added as reagents may have a significant effect on the overall reaction rates in the electrochemical process. Figure 7 presents the effect of electrolyte type and addition of salt in the decoloration of X-3B. As shown in Figure 7, the color removal exhibited a positive correlation with applied electrolyte concentration, since the addition of Na2SO4 could increase conductivity of solution and therefore increase the CE. The influence of the electrolyte type was also investigated. The treatment results can vary depending on the nature of the supporting electrolyte. Generally, electrolysis, with NaCl as a background electrolyte, is reported to generate active chlorine species such as chlorine radical (Cl•), dichloride radical anion (Cl2•-), and hypochlorous acid/hyperchlorite (HOCl/OCl-) via surface-bound hydroxyl radical-mediated pathways. Therefore, it is reported that using NaCl as supporting electrolyte can increase organic removal, which was not observed in this work as shown in Figure 7. It was

FIGURE 8. Effect of initial pH on color removal (X-3B initial concentration, 50 mg/L; Na2SO4 concentration, 2 g/L; potential, 10 V). seen that the difference caused by different electrolytes (Na2SO4, NaCl) is trivial. Initial pH. Since the electrochemical treatment of many organics has been found to be dependent on the pH of the solution, the effect of this parameter was studied in four samples, each containing 50 mg/L X-3B and 2 g/L Na2SO4, with pH values of 2, 5, 7, and 10, respectively. Interestingly, as shown in Figure 8, no significant effect of pH on color removal was observed, thus showing that electrochemical oxidization in the SCNT reactor is a promising process for the treatment of organic wastewater in a wide range of pH. It is generally believed that free radicals are involved in the organic oxidization during the electrochemical process as follows: H2O f OH • + H+ + e

(7)

OH • + organics f CO2 + H2O

(8)

Therefore, a lower pH will shift the equilibrium to the left side of the reaction, which offsets the beneficial increase of conductivity. Apparently the pH does not play a significant role within the range of 2-10.

FIGURE 7. Effect of electrolyte on color removal (X-3B initial concentration, 50 mg/L; pH ) 7; potential, 8 V). (a) Effect of Na2SO4 concentration on color removal. (b) Effect of Na2SO4 and NaCl on color removal. 3800

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(6)

(7) (8) (9)

(10) (11)

FIGURE 9. Effect of initial X-3B concentration on color removal (potential, 10 V; Na2SO4 concentration, 3 g/L; pH ) 7). Concentration. The degradation efficiency with various initial X-3B concentrations was also examined and is presented in Figure 9. The results indicated that the color removal of all samples was almost the same after 100 min of treatment. From the results, it was seen that at the preliminary phase (about 60min), color removal was relatively lower while the initial concentration of X-3B was 100 mg/L or 200 mg/L. The probable reason was that the treatment load of the experimental setup was limited while the dye was degraded if the electrolytic time was prolonged. Application Prospect. An SCNE reactor has been developed and successfully applied in the treatment of X-3B solutions. Excellent performance with much lower energy consumption was achieved by improving the overall mass transfer of contaminants. In future work, we will apply this technique to treat various environmentally important organic pollutants. Although the SCNE reactor is still at laboratory scale stage, it has promise for application considering its energy efficiency, and the commercialization of this technique is a goal worth pursing.

Acknowledgments This research is based upon work supported by the Natural Science Foundation of China (No. 20777050) and Ministry of Education of China (No. NCET-06-0408). Any opinions, findings, conclusions, or recommendations expressed in this publication are those of the authors and do not necessarily reflect the view of the supporting organizations. The authors are grateful to Yalin Wang for advice and troubleshooting the experimental apparatus and sample analysis.

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