Article pubs.acs.org/est
Development of the Microbial Electrolysis Desalination and Chemical-Production Cell for Desalination as Well as Acid and Alkali Productions Shanshan Chen,† Guangli Liu,†,‡,* Renduo Zhang,†,‡,* Bangyu Qin,† and Yong Luo† †
School of Environmental Science and Engineering, Sun Yat-sen University, Guangzhou 510275, China Guangdong Provincial Key Laboratory of Environmental Pollution Control and Remediation Technology, Guangzhou 510275, China
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S Supporting Information *
ABSTRACT: By combining the microbial electrolysis cell and the microbial desalination cell, the microbial electrolysis desalination cell (MEDC) becomes a novel device to desalinate salty water. However, several factors, such as sharp pH decrease and Cl− accumulation in the anode chamber, limit the MEDC development. In this study, a microbial electrolysis desalination and chemicalproduction cell (MEDCC) was developed with four chambers using a bipolar membrane. Results showed that the pH in the anode chamber of the MEDCC always remained near 7.0, which greatly enhanced the microbial activities in the cell. With applied voltages of 0.3−1.0 V, 62%−97% of Coulombic efficiencies were achieved from the MEDCC, which were 1.5−2.0 times of those from the MEDC. With 10 mL of 10 g/L NaCl in the desalination chamber, desalination rates of the MEDCC reached 46%−86% within 18 h. Another unique feature of the MEDCC was the simultaneous production of HCl and NaOH in the cell. With 1.0 V applied voltage, the pH values at 18 h in the acid-production chamber and cathode chamber were 0.68 and 12.9, respectively. With the MEDCC, the problem with large pH changes in the anode chamber was resolved, and products of the acid and alkali were obtained.
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INTRODUCTION The microbial desalination cell (MDC) is a device to desalinate salty water using electricity generated by the microbial fuel cell (MFC).1 However, the desalination efficiency of the MDC is limited by the fluctuated voltages produced by the exoelectrogenic bacteria.2 Then the microbial electrolysis desalination cell (MEDC) was developed by combining the microbial electrolysis cell (MEC) and the MDC.3,4 In these studies, the MDC and MEDC were composed of an anode chamber, a desalination chamber, and a cathode chamber. An anion exchange membrane (AEM) was installed between the anode chamber and the desalination chamber, and a cation exchange membrane (CEM) was between the desalination chamber and the cathode chamber. It was reported that 98.8% of 10 g/L NaCl and 68% of 20 g/L NaCl were removed using the MEDC in a single fed-batch cycle.3,4 Stacked MDCs with multiple desalination chambers have also been tested to enhance the desalination rate.5,6 Nevertheless, the MDC or MEDC are still suffering from several barriers. First, the desalination process in the MDC or MEDC results in pH decrease in the anode chamber, because the AEM only allows anions (e.g., Cl−) to go through. Mahanna et al.2 reported a decrease of the anolyte pH from 7.2 to 4.5 with 2 g/L acetate as the fuel in the MDC. In Jacobson et al.,7 the anolyte pH decreased from 6.85 to 5.70 in the MDC. The © 2012 American Chemical Society
pH decrease in the anode chamber is harmful to the microbial activities because most of bacteria in the cell favor a neutral pH condition.8,9 The microbial metabolism in the anode chamber can be inhibited at low pH values, resulting in low current density.10 To avoid sharp pH changes in the chamber, Cao et al.1 replaced the anolyte every 12 h and Chen et al.5 replaced the anolyte twice in one desalination cycle. Second, the Cl− concentration in the anode chamber can dramatically increase because Cl− ions transfer from the middle chamber to the anode chamber to balance the protons produced by the exoelectrogenic bacteria. The Cl− accumulation in the anode chamber may also inhibit the bacterial activities.4 Third, a high phosphate buffer solution (50 mM) used in the MDC or MEDC3,4 may result in phosphate groups transferring through the AEM to the desalination chamber. The phosphate transfer may cause heavy deposit of Mg2+ and Ca2+ in the desalination chamber during the treatment of seawater or brine water. Bipolar membrane (BPM) has been successfully used in many chemical processes, such as separation of ions from the corresponding salts, electroextraction, back-extraction, purificaReceived: Revised: Accepted: Published: 2467
September 22, 2011 January 10, 2012 January 11, 2012 January 11, 2012 dx.doi.org/10.1021/es203332g | Environ. Sci. Technol. 2012, 46, 2467−2472
Environmental Science & Technology
Article
The aim of this study was to demonstrate the advantages of the MEDCC over the MEDC as well as the electrolysis desalination and chemical-production cell (EDCC). Specifically, desalination rates, Coulombic efficiencies, acid- and alkaliproduction performances of these three types of reactors were measured and compared. Efficiencies of acid-production and alkali-production were also discussed.
tion of acid and bases, and production of organic acids and soy protein isolates.11 With an electric field in the BPM, the water dissociation reaction occurs between the two layers, in which H+ migrates through the cation exchange layer and OH− migrates through the anion exchange layer.12,13 To control pH changes in the anode chamber, eliminate the negative effects of Cl− and phosphate, and reclaim Na+ in the cell, we propose a new device using BPM. The device is called the microbial electrolysis desalination and chemical-production cell (MEDCC). The device consists of four chambers: an anode chamber, an acid-production chamber, a desalination chamber, and a cathode chamber (the alkali-production chamber) (Figure 1A,B). A CEM is installed between the cathode
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MATERIALS AND METHODS Reactor Construction. The MEDCC reactor was made of poly(tetrafluoroethylene). Each electrode chamber was constructed by drilling a hole with a diameter of 3 cm in a solid poly block. The available volumes of the cathode chamber, the desalination chamber, the acid-production chamber, and the anode chamber were 30, 10, 10, and 30 mL, respectively. CEM (Ultrex CMI-7000), AEM (Ultrex AMI-7001), and BPM (Fumasep-FBM) were used for the construction. Graphite brush (25 mm diameter ×30 mm length) was used as the anode. The cathode consisted of a 30% wet-proofed carbon cloth with platinum (0.5 mg/cm2) and four diffusion layers on it with the effective surface area of 7 cm2 (3 cm in diameter)15 (Figure 1A,B). For comparison, the MEDC and the EDCC were also constructed. The structure of the MEDC was almost the same as that of the MEDCC, but without the acidproduction chamber and the BPM. The structure of the EDCC was the same as that of the MEDCC, but without adding microbes in the anode chamber during the operation. Medium and Operation. Ten milliliter effluent of matured single-chamber MFC anolyte was inoculated in the anode chambers of the MEDCC and MEDC. The anodic solution for the MFC, MEDCC, MEDC, and EDCC contained (in 1 L deionized water) 1 g of CH3COONa, 4.0896 g of Na2HPO4, 2.544 g of NaH2PO4, 0.31 g of NH4Cl, 0.13 g of KCl, 12.5 mL of trace metal solution, and 12.5 mL of vitamin solution.16 The initial pH of the solution was adjusted to 7.0 using HCl and NaOH. The acid-production chamber, the desalination chamber, and the cathode chamber were filled with 10, 10, and 30 mL of 10 g/L NaCl at the beginning of each batch cycle, respectively. During the start-up operation, the solutions in the chambers were replaced every 24 h. The sampling and data collection operations began after stable voltage outputs were achieved. During the operations, the solutions in all chambers were replaced when the current was
