Concurrent Desalination and Hydrogen Generation Using Microbial

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Environ. Sci. Technol. 2011, 45, 340–344

Concurrent Desalination and Hydrogen Generation Using Microbial Electrolysis and Desalination Cells HAIPING LUO,† PETER E. JENKINS,‡ AND Z H I Y O N G R E N * ,† Department of Civil Engineering and Department of Mechanical Engineering, University of Colorado Denver, Denver, Colorado 80004, United States

Received July 1, 2010. Revised manuscript received November 10, 2010. Accepted November 17, 2010.

The versatility of bioelectrochemical systems (BESs) makes them promising for various applications, and good combinations could make the system more applicable and economically effective. An integrated BES called microbial electrolysis and desalination cell (MEDC) was developed to concurrently desalinate salt water, produce hydrogen gas, and potentially treat wastewater. The reactor is divided into three chambers by inserting a pair of ion exchange membranes, with each chamber serving one of the three functions. With an added voltage of 0.8 V, lab scale batch study shows the MEDC achieved the highest H2 production rate of 1.5 m3/m3 d (1.6 mL/h) from the cathode chamber, while also removing 98.8% of the 10 g/L NaCl from the middle chamber. The anode recirculation alleviated pH and high salinity inhibition on bacterial activity and further increased system current density from 87.2 to 140 A/m3, leading to an improved desalination rate by 80% and H2 production by 30%. Compared to slight changes in desalination, H2 production was more significantly affected by the applied voltage and cathode buffer capacity, suggesting cathode reactions were likely affected by the external power supply in addition to the anode microbial activity.

oped to simultaneously desalinate salt water and produce electricity (10-12). This system has significant advantages compared to traditional desalination processes, as it does not require intensive energy inputs or high water pressure (13). The versatility of BESs makes them promising for various applications. By using good combinations of complementary functions, the system could be more applicable and economically effective. For example, ion exchange membranes have been used in BESs to separate the anode and the cathode, but the high resistance and costs made such configurations difficult to scale-up. On the other hand, single chamber, membrane-less BESs improved energy production but suffered low energy recovery and contamination. Call and Logan (14) reported that single chamber MECs doubled H2 production rate to a maximum 3.12 m3 H2/m3 d compared to two-chamber systems, but most single chamber MECs experienced H2 loss due to microbial consumption and substantial methane contamination. So far, no effective methods have been found to inhibit methanogenesis (5, 15, 16). The development of MDCs leads to a new challenge: if membranes have to be applied in the system, what would be the most beneficial way to use them. Anion exchange membrane (AEM) and cation exchange membrane (CEM) were used to separate the MDCs into three chambers, with the anode chamber used for organic oxidization, the cathode for current production, and the middle chamber for desalination. The proof-of-concept study has shown a desalination rate approaching 90%, but the current system is restrained by significant pH variation and power output fluctuation (11). Considering the above, we developed an integrated BES system called microbial electrolysis and desalination cell (MEDC) to concurrently desalinate salt water, produce hydrogen gas, and potentially treat wastewater. The advantage of this combination is to generate pure and collectable hydrogen gas without dealing with the contamination or voltage fluctuation and to achieve enhanced desalination facilitated by external power supply. We also characterized the effects of applied voltages, buffer strengths, and operational parameters on H2 production and salt removal. The difference of ion balances across the chambers between this system and the MDC system were also discussed.

Introduction

Materials and Methods

Water and energy are the two most pressing technological issues facing the world. The social and economic developments are driving the search for sustainable supply of both water and energy. Recently developed bioelectrochemical systems (BESs) represent one of the newest approaches for generating clean water and energy. BESs use microorganisms to catalyze the oxidization of organic and inorganic electron donors in the anode chamber and deliver electrons to the anode. The electrons can be captured directly for current generation (microbial fuel cells, MFCs) (1-3) or supplemented by external power input for producing other energy carriers, including hydrogen and methane gas (microbial electrolysis cells, MECs) (4, 5). The electrons can also be used to produce chemicals (microbial chemical cells, MCCs) or remediate contaminants (6-9). Most recently, a new type of BES called microbial desalination cell (MDC) was devel-

Reactor Construction. The MEDC reactors were constructed from polycarbonate, and the electrode chambers were produced by drilling a hole with 3-cm diameter in a solid block (Figure S1, Supporting Information) (17). Three chambers were clamped together and divided by placing an AEM (AMI 7001, Membranes international, NJ) between the anode and middle desalination chambers and a CEM (CMI 7000, Membranes international, NJ) between the middle and cathode chambers. After inserting the electrodes, the volumes of the anode, middle, and cathode chamber were 25, 17, and 36 mL, respectively. Heat treated graphite brushes (25 mm diameter × 25 mm length, Golden brush, CA) were used as the anode (17), and a stainless steel mesh (Type 304, McMaster, IL) was selected as the cathode material. The gas produced at the cathode bubbled into the cathode chamber and was collected using a sealed anaerobic tube glued to the top of the reactor (14). The top of the tube was sealed with a butyl rubber stopper and aluminum crimp top. Reactor Start-up and Operation. The reactors were inoculated from a mixed culture by transferring the preacclimated anodes of active acetate-fed MFCs as the actual

* Corresponding author phone: (303)556-5287; fax: (303)5562368; e-mail: [email protected]. † Department of Civil Engineering. ‡ Department of Mechanical Engineering. 340

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10.1021/es1022202

 2011 American Chemical Society

Published on Web 12/01/2010

MEDC anodes. The anodic medium for both MFC and MEDC contained (per liter): CH3COONa 1 g, Na2HPO4 4.58 g, NaH2PO4 · H2O 2.45 g, NH4Cl 0.31 g, KCl 0.13 g, trace metals solution 12.5 mL, and vitamin solution 12.5 mL (18). The cathode chamber was filled with 50 mM phosphate buffer solution (Na2HPO4 4.58 g/L, NaH2PO4 · H2O 2.45 g/L, pH ) 7.0) unless mentioned otherwise. The middle chamber was filled with 10 g/L NaCl solution for desalination. In a fedbatch operation mode, the anolyte and catholyte were replaced every 24 h cycle to avoid significant pH change. In the anode recirculation mode, the anolyte was continuously recirculated from a 400 mL reservoir through the anode chamber at a rate of 1.5 mL/min using a peristaltic pump (Model 3385, Fisher Scientific Co., Fairlawn, NJ). A fixed voltage was added to the circuit of MEDC reactor using a programmable power source (model 3646A, Circuit Specialists Inc., AZ). An external resistor (10 Ω) was connected in series with the power supply negative lead and the cathode for current calculation. A saturated Ag/AgCl reference electrode was inserted into the anode chamber to measure changes of the electrode potential. Analyses and Calculations. The voltage across the external resistor (Re) was recorded using a data acquisition system (model 2700, Keithley Instruments, Inc. OH). The current density (I, A) through the electrical circuit was determined from the measured voltage (E, V) according to I ) E/Re. Salt concentrations were evaluated by conductivity measurements using a conductivity meter (Sension 156, HACH Co., Loveland, CO). The Na+ and Cl- concentrations were determined by using suppressed conductivity detection ion chromatography (model 4500I, Dionex, CA). The volume of H2 produced by the MEDC was measured by the modified biochemical methane potential (BMP) test (19, 20). Specifically, the produced gas in the cathode chamber was intermittently released into a 10 mL glass syringe until the pressure equilibrates with atmospheric pressure. The gas composition and volumetric fraction of H2 was analyzed by using a 100 µL gastight syringe and a gas chromatograph (model 8610C, SRI Instruments, CA) equipped with a thermal conductivity detector with nitrogen as the carrier gas. The accumulated H2 production was calculated by VH2,t ) VH2,t-1 + Vh(xH2,t - xH2,t-1) + xH2,t(Vm,t - Vm,t-1) (1) where VH2,t and VH2,t-1 are cumulative H2 volumes at corresponding times, (Vm,t - Vm,t-1) is the gas production during the time interval, xH2,t and xH2,t-1 are the volumetric factions of H2 in the current and previous intervals, respectively, and Vh is the volume of headspace in the cathode chamber (4). The H2 production rate (QH2, mL H2/h) was calculated from the accumulated H2 production and the operation time, given by QH2 ) (VH2,t - V0)/t. The volumetric H2 production rate (QV,H2, m3 H2/m3 d) was calculated as QV,H2 ) QH2/V, where V is the volume of anode chamber. The cathodic hydrogen recovery (rcat) was calculated by rcat )

nH2 nCE

)

nH2

∫

t

t)0

(2)

Idt/2F

where nH2 is the moles of H2 actually produced at the cathode, nCE is the moles that theoretically could produced from the current (I, A), dt(s) is the interval over which data are collected, 2 is used to convert moles of electrons to moles of H2, and F is Faraday’s constant (96485 C/mol electrons) (4). The desalination rate (QD, mS/cm h) was calculated by QD ) (Ct - C0)/t, where C0 and Ct are the initial and the final conductivity of saltwater in the middle chamber over a interval time of t. Electrochemical impedance spectroscopy

FIGURE 1. Accumulated H2 production and desalination efficiency in MEDC during batch operation with an applied voltage of 0.8 V. (Arrows indicate electrolytes replacement.) (EIS) was used to determine the internal resistance for the MEDC system by using a potentiostat (G 300, Gamry Instruments Inc. NJ). EIS measurements were conducted at the condition of open circuit voltage (OCV), by using the anode as the working electrode and the cathode as the counter and reference electrode. The internal resistances of the cell were obtained from Nyquist plots, where the intercept of the curve with the Zre axis is defined as the ohmic resistance (21, 22).

Results H2 Production and Desalination Performance in Batch Mode. Concurrent desalination and hydrogen gas production were observed soon after the transfer of the active MFC anodes into the assembled MEDC reactors. To prevent substrate limitations for bacteria and significant changes in pH value, the anolyte and catholyte were replaced every 24 h. Figure 1 shows that a total of 48.7 mL of H2 was produced within 4 cycles (96 h) at an applied voltage of Eap ) 0.8 V, and 98.8% of the salt was removed from the middle chamber during the same period. The maximum current density obtained was 87.2 A/m3, and the cathodic hydrogen recovery was 72%. The values are comparable to the operation of other lab scale MEC systems (4, 16, 23, 24). Instead of the H2 loss due to microbial consumption that was reported by many single chamber MEC studies, the cathodic H2 loss in the MEDCs should be attributed to the intermittent sampling, gas bubble holdup on the cathode surface, and limited H2 diffusion across the membranes (4). In an attempt to characterize the reaction kinetics during the cycles, the H2 production rate and desalination rate showed parallel variations, as both rates increased linearly during the first 8-10 h and then decreased slowly until the end of each batch (Figure 2). The highest rates of H2 production and desalination were both achieved at around 8-10 h of each batch, with the maximum H2 production rate of 1.5 m3/m3 d (1.6 mL/h) and the maximum desalination rate of 0.42 mS/cm h. A further characterization of pH and anode open circuit potential (OCP) variation showed that pH continuously declined during one batch cycle, from 7 to around 5, while the anode OCP first decreased from -442 mV (vs Ag/AgCl) at the beginning to -490 mV at 10 h and then increased to -428 mV at the end of the batch (Figure S2). Figure 2 also shows that the highest reaction rates of different batches declined gradually during the operation. Such decline was presumably due to the increase of the system ohmic resistance as a result of conductivity decrease in the middle chamber during salt removal. The EIS measurement showed the system resistance increased from 70-250 Ω at the beginning of one cycle to 850-1100 Ω at end of the cycle, increasing by a factor of 4-16 (Figure S3). Similar findings were observed in the MDC system as well VOL. 45, NO. 1, 2011 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 2. Variations of H2 production (QH2) and desalination rates (QD) during batch operation with an applied voltage of 0.8 V. (Arrows indicate electrolytes replacement.)

FIGURE 3. Effects of applied voltage on H2 production and desalination in typical 24-h batch cycles. as other MFC reactors, as the low electrolyte condition may also increase the membrane solution interface resistance (11, 25). In conjunction with the conductivity measurements, the desalination performance of the reactor was also examined on the basis of both Cl- and Na+ mass increments in the anolyte and catholyte, respectively. By comparing the concentration change of Cl- (in anolyte) and Na+ (in catholyte) between the beginning and end of each 24-h cycle, it was observed that there were always more Na+ migrated to the cathode than Cl- to the anode chamber. For each batch, the average increase of Na+ in the catholyte was 1.00 ( 0.05 mmol, while the average increase of Cl- in the anolyte was 0.83 ( 0.03 mmol. This imbalance of ion transfer was also confirmed by the molar ratio increase between Cl- and Na+ left in the middle chamber at the end of each batch. Effects of Applied Voltage and Catholyte Buffer Capacity on System Performance. The effects of applied voltage on H2 production and desalination were characterized by varying the applied voltage from 0.4-0.8 V. Similar to many MEC studies, the H2 production rates increased consistently with the increasing voltage (Figure 3). By increasing applied voltage from 0.4 V to 0.8 V, the average H2 production rate increased by 2.6 times, from 0.23 mL/h to 0.6 mL/h. However, little changes were observed in desalination efficiency with the voltage variation. The average desalination rate varied from 0.17 mS/cm h at 0.4 V to 0.23 mS/cm h at 0.8 V, representing only a 35% increase. The consumption of protons for H2 evolution (MEC) or oxygen reduction (MFC and MDC) generally causes the pH to increase in the cathode chamber, especially when membranes are used. Therefore, a buffer solution has been widely used to counter the pH change and stabilize the system performance. Figure 4 shows the different effects of the catholyte buffer concentration on H2 production and de342

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FIGURE 4. Effects of catholyte phosphate buffer concentration on the system performance in terms of (A) accumulated H2 production and (B) desalination efficiency in 24 h of operation. salination in the MEDC at an applied voltage of 0.8 V. Similar to the applied voltage effects, H2 production appeared to be more affected by the availability of PBS buffer than the desalination. When no buffer was provided in the catholyte, very little H2 (3.9 mL) was produced in the first 12 h, and then the gas production stopped, presumably due to the limited supply of proton. In contrast, similar amounts of H2 (16.1-17.2 mL) were produced from the cathode chamber with 50, 100, or 200 mM phosphate buffer, respectively. The gas production rates were also comparable. However, the catholyte buffer capacity showed little effects on desalination, as similar overall salt removal rates were observed in different cycles with various buffer concentrations. Higher gas production and desalination rates were both observed at the 100 mM buffer level in the first 12 h of batch operation, suggesting such concentration may be more compatible to the electron and mass transfers in this MEDC system. The pH change in the catholyte after each cycle was proportional to the buffer concentration, as shown in Figure 5. For example, the catholyte pH increased only slightly, from 7.0 to 7.3 when 200 mM PBS was used, while the pH increased to 12.5 when no buffer was used. The pH of the anolyte decreased after each cycle from 7.0 to between 5 and 6 when 50 mM of PBS was used. Effect of Anolyte Recirculation on System Performance. During batch operation, the anode potential varied from -490 mV at 10 h to -428 mV at the end of each batch cycle, resulting in a decrease of anode as well as system performance (Figure S2). This change is largely due to the decrease of pH and accumulation of Cl- at the anode chamber, which inhibited anode microbial activity. To alleviate such problems, an anolyte recirculation operation was conducted by recirculating the solution through a 400 mL feed reservoir and the anode chamber. The flow rate was set at 1.5 mL/min using

FIGURE 5. pH changes of anolyte and catholyte at the end of each batch cycle with different initial phosphate buffer concentrations. Initial pH was 7.0, and all anolyte buffer concentration was 50 mM to prevent interaction.

FIGURE 6. Accumulated H2 production and desalination efficiency during anolyte-recirculation operation with an applied voltage of 0.8 V. (Arrows indicate electrolytes replacement.) a peristaltic pump, generating a hydraulic retention time of 17 min in the anode chamber. As shown in Figure 6, with the applied voltage of 0.8 V, the continuous operation produced 49.5 mL of H2 within 60 h and also removed 98.2% of the salt from the middle chamber. The corresponding maximum H2 production rate was 2.03 mL of H2/h (1.95 m3/m3 day), and the desalination rate was 0.76 mS/cm h. These improvements are directly related to the significant increase of the system current density, which was calculated as 140 A/m3 (anode chamber), a 61% increase compared to the batch mode.

Discussion The integration of desalination, H2 production, and potentially waste removal in the MEDC reactors provides new applications for the bioelectrochemical systems. A recent life cycle assessment (LCA) study shows that the chemical production in BESs may provide more environmental benefits compared to direct electricity generation and conventional anaerobic treatment (26), and the multiple functions offered by the MEDC are more likely to add further such benefits. The results obtained from this study showed that the MEDC can remove 98.8% and 98.2% of the salt in batch mode and recirculation mode, respectively, as well as achieving high H2 production rates. With an applied voltage of 0.8 V, the maximum H2 production rate from the batch operation was 1.5 m3/m3 day, and the current density approached 87.2 A/m3, comparable to other MEC systems with membranes (23, 27). Anolyte recirculation further increased the current density to 140 A/m3 and the H2 production rate to 1.95 m3/m3 day. Also, compared to the proof-of-concept MDC study, comparable volumes of the anode, cathode, and middle chambers were used, bringing this technology one step closer to

practice. Substrate removal in the anode was not focused in the study, as the anolyte was replaced every 24 h in batch mode. In the MDC system for direct current generation, the transfer of ionic species from the middle chamber is solely driven by the anode microbial activity and is proportional to the electrons delivered by the microbes from the anode chamber (11, 12). However, the added external power supply complicates the reactions and ion transfers in the MEDC. By monitoring concentration changes of the two ion species in individual chambers, we found that, though initially balanced, there was 12-20% more Na+ transferred from the desalination chamber to the catholyte than Cl- to the anolyte within one batch cycle. In the mean time, the pH of the desalination chamber kept stable at 6.6-6.8, indicating no significant proton or hydroxyl ion migration occurred. Such imbalance of Na+ and Cl- transfer can be attributed to several factors, such as more Cl- adsorption on the high-surface carbon brush anode than the Na+ adsorption on the stain steel cathode, other competitive across-membrane ion transfers such as phosphate, protons, and hydroxyl ions, and higher reaction kinetics on the cathode than on the microbial anode. One other affecting factor might come from the external voltage supply, as it provides external potential or driving force for the cathode reactions. This may help explain the results of why H2 production was significantly affected by the applied voltage and cathode buffer capacity, while desalination was only slightly changed with same variations. One possible explanation is that the MEDC cathode H2 production was primarily affected by the external power supply and proton availability in the catholyte, but the desalination was mainly limited by the anode electron transfer. Repeated trends of H2 production and desalination were observed in fed-batch operation, with the highest rates occurred at 8-10 h during each 24-h cycle. Such pattern is likely caused by the changes of anode microbial activity due to the pH variation. As shown in Figure S2, the anode OCP reached a negative peak at the same 8-10 h period, which indicates a maximum microbial substrate utilization rate and leads to the highest rates of H2 production and desalination. However, with the pH kept declining to below 6, the microbial activity was inhibited more significantly until the replacement of new media. The recirculation of anolyte significantly improved the system performance, leading to a 61% increase in current density, 80% increase in desalination rate, and 30% increase in gas production. The differences in improvements support the hypothesis that the desalination is primarily limited by the anode and suggest that the recirculation helped facilitated the mass transfer and lifted the constraints on the anode chamber, such as pH drop, accumulated Cl- concentration, and substrate variation. This is also confirmed by a stable anode open circuit potential of -440 mV as compared to the fluculation of the anode potential in the batch mode. As a new multifunctional process, the MEDC research is facing many challenges that remain to be solved, such as pH variation in the anode and cathode chambers, increased ohmic resistance, and stack system development. Higher flow rate in the anode and cathode chamber compared to the middle chamber, or an anode-cathode flow through system should improve the H2 production and waste treatment efficiency, as well as reduce the imbalance of pH (28). On the other hand, value-added chemicals, such as caustic solutions, could be produced in the cathode chamber, as the catholyte pH could easily reach 13 without a high buffer solution (6). Biofilm was observed on the AEM shortly after the batch operation, indicating membrane biofouling may be another challenge to address. As the key components in the MEDC, high flux, high stability, and low fouling potential ion exchange membranes should also be investigated to reduce VOL. 45, NO. 1, 2011 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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the cost and increase system performance. To reduce the middle chamber salt solution to very low levels, the anode solution was replaced or recirculated multiple times in this study in order to eliminate pH and substrate limitations on bacteria in the anode chamber. In practice, rather than using an MEDC to accomplish completely salt removal, the electrolyte flows can be comparable among the chambers for more efficient partial water desalination. Such operation will leave more salt in the middle chamber and alleviate the pH variation and internal resistance increase problems. The effluent can be either applied for direct beneficial uses such as agricultural irrigation or groundwater recharge, where higher salt limits are allowed (TDS 500-2000 mg/L), or used as a pretreatment for downstream RO processing to reduce the energy consumption and membrane fouling (12, 29).

Acknowledgments This work was supported by the Office of Naval Research (ONR) under Awards N000140910944 and N0001410M0232. We thank Drs. John Regan, Shaoan Cheng, and Hong Liu for the valuable discussions and Dr. Pei Xu for the measurement of ion concentrations.

Supporting Information Available Three additional figures. This material is available free of charge via the Internet at http://pubs.acs.org.

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