Low Energy Desalination Using Battery Electrode Deionization

New electrochemical technologies that use capacitive or battery electrodes are being developed to minimize energy requirements for desalinating brackish waters. When a pair of electrodes is charged in capacitive deionization (CDI) systems, cations bind to the cathode and anions bind to the anode, but high applied voltages (>1.2 V) result in parasitic reactions and irreversible electrode oxidation. In the battery electrode deionization (BDI) system developed here, two identical copper hexacyanoferrate (CuHCF) battery electrodes were used that release and bind cations, with anion separation occurring via an anion exchange membrane. The system used an applied voltage of 0.6 V, which avoided parasitic reactions, achieved high electrode desalination capacities (up to 100 mg-NaCl/g-electrode, 50 mM NaCl influent), and consumed less energy than CDI. Simultaneous production of desalinated and concentrated solutions in two channels avoided a two-cycle approach needed for CDI. Stacking additional membranes between CuHCF electrodes (up to three anion and two cation exchange membranes) reduced energy consumption to only 0.02 kWh/m³ (approximately an order of magnitude lower than values reported for CDI), for an influent desalination similar to CDI (25 mM decreased to 17 mM). These results show that BDI could be effective as a very low energy method for brackish water desalination

A Two-Stage Microbial Fuel Cell and Anaerobic Fluidized Bed Membrane Bioreactor (MFC-AFMBR) System for Effective Domestic Wastewater Treatment

Microbial fuel cells (MFCs) are a promising technology for energy-efficient domestic wastewater treatment, but the effluent quality has typically not been sufficient for discharge without further treatment. A two-stage laboratory-scale combined treatment process, consisting of microbial fuel cells and an anaerobic fluidized bed membrane bioreactor (MFC-AFMBR), was examined here to produce high quality effluent with minimal energy demands. The combined system was operated continuously for 50 days at room temperature (∼25 °C) with domestic wastewater having a total chemical oxygen demand (tCOD) of 210 ± 11 mg/L. At a combined hydraulic retention time (HRT) for both processes of 9 h, the effluent tCOD was reduced to 16 ± 3 mg/L (92.5% removal), and there was nearly complete removal of total suspended solids (TSS; from 45 ± 10 mg/L to <1 mg/L). The AFMBR was operated at a constant high permeate flux of 16 L/m²/h over 50 days, without the need or use of any membrane cleaning or backwashing. Total electrical energy required for the operation of the MFC-AFMBR system was 0.0186 kWh/m³, which was slightly less than the electrical energy produced by the MFCs (0.0197 kWh/m³). The energy in the methane produced in the AFMBR was comparatively negligible (0.005 kWh/m³). These results show that a combined MFC-AFMBR system could be used to effectively treat domestic primary effluent at ambient temperatures, producing high effluent quality with low energy requirements

A pH-Gradient Flow Cell for Converting Waste CO₂ into Electricity

The CO₂ concentration difference between ambient air and exhaust gases created by combusting fossil fuels is an untapped energy source for producing electricity. One method of capturing this energy is dissolving CO₂ gas into water and then converting the produced chemical potential energy into electrical power using an electrochemical system. Previous efforts using this method found that electricity can be generated; however, electrical power densities were low, and expensive ion-exchange membranes were needed. Here, we overcame these challenges by developing a new approach to capture electrical power from CO₂ dissolved in water, the pH-gradient flow cell. In this approach, two identical supercapacitive manganese oxide electrodes were separated by a nonselective membrane and exposed to an aqueous buffer solution sparged with either CO₂ gas or air. This pH-gradient flow cell produced an average power density of 0.82 W/m², which was nearly 200 times higher than values reported using previous approaches

Increasing Desalination by Mitigating Anolyte pH Imbalance Using Catholyte Effluent Addition in a Multi-Anode Bench Scale Microbial Desalination Cell

A microbial desalination cell (MDC) uses exoelectrogenic bacteria to oxidize organic matter while desalinating water. Protons produced from the oxidation of organics at the anode result in anolyte acidification and reduce performance. A new method was used here to mitigate anolyte acidification based on adding non-buffered saline catholyte effluent from a previous cycle to the anolyte at the beginning of the next cycle. This method was tested using a larger-scale MDC (267 mL) containing four anode brushes and a three cell pair membrane stack. With an anolyte salt concentration increased by an equivalent of 75 mM NaCl using the catholyte effluent, salinity was reduced by 26.0 ± 0.5% (35 g/L NaCl initial solution) in a 10 h cycle, compared to 18.1 ± 2.0% without catholyte addition. This improvement was primarily due to the increase in buffering capacity of the anolyte, although increased conductivity slightly improved performance as well. There was some substrate loss from the anolyte by diffusion into the membrane stack, but this was decreased from 11% to 2.6% by increasing the anolyte conductivity (7.6 to 14 mS/cm). These results demonstrated that catholyte effluent can be utilized as a useful product for mitigating anolyte acidification and improving MDC performance

Substantial Humic Acid Adsorption to Activated Carbon Air Cathodes Produces a Small Reduction in Catalytic Activity

Long-term operation of microbial fuel cells (MFCs) can result in substantial degradation of activated carbon (AC) air-cathode performance. To examine a possible role in fouling from organic matter in water, cathodes were exposed to high concentrations of humic acids (HA). Cathodes treated with 100 mg L^–1 HA exhibited no significant change in performance. Exposure to 1000 mg L^–1 HA decreased the maximum power density by 14% (from 1310 ± 30 mW m^–2 to 1130 ± 30 mW m^–2). Pore blocking was the main mechanism as the total surface area of the AC decreased by 12%. Minimization of external mass transfer resistances using a rotating disk electrode exhibited only a 5% reduction in current, indicating about half the impact of HA adsorption was associated with external mass transfer resistance and the remainder was due to internal resistances. Rinsing the cathodes with deionized water did not restore cathode performance. These results demonstrated that HA could contribute to cathode fouling, but the extent of power reduction was relatively small in comparison to large mass of humics adsorbed. Other factors, such as biopolymer attachment, or salt precipitation, are therefore likely more important contributors to long-term fouling of MFC cathodes

Regenerable Nickel-Functionalized Activated Carbon Cathodes Enhanced by Metal Adsorption to Improve Hydrogen Production in Microbial Electrolysis Cells

While nickel is a good alternative to platinum as a catalyst for the hydrogen evolution reaction, it is desirable to reduce the amount of nickel needed for cathodes in microbial electrolysis cells (MECs). Activated carbon (AC) was investigated as a cathode base structure for Ni as it is inexpensive and an excellent adsorbent for Ni, and it has a high specific surface area. AC nickel-functionalized electrodes (AC-Ni) were prepared by incorporating Ni salts into AC by adsorption, followed by cathode fabrication using a phase inversion process using a poly­(vinylidene fluoride) (PVDF) binder. The AC-Ni cathodes had significantly higher (∼50%) hydrogen production rates than controls (plain AC) in smaller MECs (static flow conditions) over 30 days of operation, with no performance decrease over time. In larger MECs with catholyte recirculation, the AC-Ni cathode produced a slightly higher hydrogen production rate (1.1 ± 0.1 L-H₂/L_reactor/day) than MECs with Ni foam (1.0 ± 0.1 L-H₂/L_reactor/day). Ni dissolution tests showed that negligible amounts of Ni were lost into the electrolyte at pHs of 7 or 12, and the catalytic activity was restored by simple readsorption using a Ni salt solution when Ni was partially removed by an acid wash

Influence of Chemical and Physical Properties of Activated Carbon Powders on Oxygen Reduction and Microbial Fuel Cell Performance

Commercially available activated carbon (AC) powders made from different precursor materials (coal, peat, coconut shell, hardwood, and phenolic resin) were electrochemically evaluated as oxygen reduction catalysts and tested as cathode catalysts in microbial fuel cells (MFCs). AC powders were characterized in terms of surface chemistry and porosity, and their kinetic activities were compared to carbon black and platinum catalysts in rotating disk electrode (RDE) tests. Cathodes using the coal-derived AC had the highest power densities in MFCs (1620 ± 10 mW m^–2). Peat-based AC performed similarly in MFC tests (1610 ± 100 mW m^–2) and had the best catalyst performance, with an onset potential of <i>E</i>_onset = 0.17 V, and <i>n</i> = 3.6 electrons used for oxygen reduction. Hardwood based AC had the highest number of acidic surface functional groups and the poorest performance in MFC and catalysis tests (630 ± 10 mW m^–2, <i>E</i>_onset = −0.01 V, <i>n</i> = 2.1). There was an inverse relationship between onset potential and quantity of strong acid (p<i>K</i>_a < 8) functional groups, and a larger fraction of microporosity was negatively correlated with power production in MFCs. Surface area alone was a poor predictor of catalyst performance, and a high quantity of acidic surface functional groups was determined to be detrimental to oxygen reduction and cathode performance

Using Flow Electrodes in Multiple Reactors in Series for Continuous Energy Generation from Capacitive Mixing

Efficient conversion of “mixing energy” to electricity through capacitive mixing (CapMix) has been limited by low energy recoveries, low power densities, and noncontinuous energy production resulting from intermittent charging and discharging cycles. We show here that a CapMix system based on a four-reactor process with flow electrodes can generate constant and continuous energy, providing a more flexible platform for harvesting mixing energy. The power densities were dependent on the flow-electrode carbon loading, with 5.8 ± 0.2 mW m^–2 continuously produced in the charging reactor and 3.3 ± 0.4 mW m^–2 produced in the discharging reactor (9.2 ± 0.6 mW m^–2 for the whole system) when the flow-electrode carbon loading was 15%. Additionally, when the flow-electrode electrolyte ion concentration increased from 10 to 20 g L^–1, the total power density of the whole system (charging and discharging) increased to 50.9 ± 2.5 mW m^–2

Extracellular Palladium Nanoparticle Production using Geobacter sulfurreducens

Sustainable methods are needed to recycle precious metals and synthesize catalytic nanoparticles. Palladium nanoparticles can be produced via microbial reduction of soluble Pd­(II) to Pd(0), but in previous tests using dissimilatory metal reducing bacteria (DMRB), the nanoparticles were closely associated with the cells, occupying potential reductive sites and eliminating the potential for cell reuse. The DMRB Geobacter sulfurreducens was shown here to reduce soluble Pd­(II) to Pd(0) nanoparticles primarily outside the cell, reducing the toxicity of metal ions, and allowing nanoparticle recovery without cell destruction that has previously been observed using other microorganisms. Cultures reduced 50 ± 3 mg/L Pd­(II) with 1% hydrogen gas (v/v headspace) in 6 h incubation tests [100 mg/L Pd­(II) initially], compared to 8 ± 3 mg/L (10 mM acetate) without H₂. Acetate was ineffective as an electron donor for palladium removal in the presence or absence of fumarate as an electron acceptor. TEM imaging verified that Pd(0) nanoparticles were predominantly in the EPS surrounding cells in H₂-fed cultures, with only a small number of particles visible inside the cell. Separation of the cells and EPS by centrifugation allowed reuse of the cell suspensions and effective nanoparticle recovery. These results demonstrate effective palladium recovery and nanoparticle production using G. sulfurreducens cell suspensions and renewable substrates such as H₂ gas

Energy Recovery from Solutions with Different Salinities Based on Swelling and Shrinking of Hydrogels

Several technologies, including pressure-retarded osmosis (PRO), reverse electrodialysis (RED), and capacitive mixing (CapMix), are being developed to recover energy from salinity gradients. Here, we present a new approach to capture salinity gradient energy based on the expansion and contraction properties of poly­(acrylic acid) hydrogels. These materials swell in fresh water and shrink in salt water, and thus the expansion can be used to capture energy through mechanical processes. In tests with 0.36 g of hydrogel particles 300 to 600 μm in diameter, 124 mJ of energy was recovered in 1 h (salinity ratio of 100, external load of 210 g, water flow rate of 1 mL/min). Although these energy recovery rates were relatively lower than those typically obtained using PRO, RED, or CapMix, the costs of hydrogels are much lower than those of membranes used in PRO and RED. In addition, fouling might be more easily controlled as the particles can be easily removed from the reactor for cleaning. Further development of the technology and testing of a wider range of conditions should lead to improved energy recoveries and performance