Energy Efficient Carbon Capture through Electrochemical pH Swing Regeneration of Amine Solution

Carbon capture is widely acknowledged as a promising strategy for achieving negative emissions. Electrochemical carbon capture technologies are considered a viable alternative to conventional temperature swing processes. Among these, employing the hydrogen oxidation and hydrogen evolution reactions as a redox couple, along with an ion exchange membrane, offers an effective means of establishing a pH swing for desorbing CO2 and regenerating the alkaline solvent. However, the practical scalability of this approach is impeded by challenges such as high energy demands resulting from a high pH differential between anodic and cathodic environments and operation with solutions with a low conductivity, required to obtain an acceptable current yield. To address these limitations, this study introduces an innovative anion exchange membrane (AEM)-based electrochemical process for solvent regeneration. Our research demonstrates the advantageous utilization of amines as chemical buffers. Selecting an amine solution with a favorable pKa (∼7 to 10) helps in maintaining bicarbonate as the predominant carbon species within the system, thereby ensuring a high current yield (>80%) across various operational conditions (current, load ratio, and solution concentration). Furthermore, our analysis indicates that the use of amine solutions effectively reduces the overpotential of the hydrogen evolution reaction due to a lower local pH. This results in a minimum energy requirement of 63 kJ/mol at a current density of 20 A/m2 to regenerate the solution (MDEA) while maintaining high (>99%) product (CO2) purity.

Hybrid Donnan dialysis-electrodialysis for efficient ammonia recovery from anaerobic digester effluent

Ammonia recovery from wastewater is crucial, yet technology of low carbon emission and high ammonia perm-selectivity against complex stream compositions is urgently needed. Herein, a membrane-based hybrid process of the Donnan dialysis-electrodialysis process (DD-ED) was proposed for sustainable and efficient ammonia recovery. In principle, DD removes the majority of ammonia in wastewater by exploring the concentration gradient of NH₄ ⁺ and driven cation (Na⁺) across the cation exchange membrane, given industrial sodium salt as a driving chemical. An additional ED stage driven by solar energy realizes a further removal of ammonia, recovery of driven cation, and replenishment of OH^- toward ammonia stripping. Our results demonstrated that the hybrid DD-ED process achieved ammonia removal efficiency >95%, driving cation (Na⁺) recovery efficiency >87.1% for synthetic streams, and reduced the OH^- loss by up to 78% compared to a standalone DD case. Ammonia fluxes of 98.2 g_N m^-2 d^-1 with the real anaerobic digestion effluent were observed using only solar energy input at 3.8 kWh kg_N ^-1. With verified mass transfer modeling, reasonably controlled operation, and beneficial recovery performance, the hybrid process can be a promising candidate for future nutrient recovery from wastewater in a rural, remote area.

Direct Air Capture Using Electrochemically Regenerated Anion Exchange Resins

Direct air capture (DAC) aims to remove CO₂ directly from the atmosphere. In this study, we have demonstrated proof-of-concept of a DAC process combining CO₂ adsorption in a packed bed of amine-functionalized anion exchange resins (AERs) with a pH swing regeneration using an electrochemical cell (EC). The resin bed was regenerated using the alkaline solution produced in the cathodic compartment of the EC, while high purity CO₂ (>95%) was desorbed in the acidifying compartment. After regenerating the AERs, some alkaline solution remained on the surface of the resins and provided additional CO₂ capture capacity during adsorption. The highest CO₂ capture capacity measured was 1.76 mmol·g^-1 dry resins. Moreover, as the whole process was operated at room temperature, the resins did not show any apparent degradation after 150 cycles of adsorption-desorption. Furthermore, when the relative humidity of the air source increased from 33 to 84%, the water loss of the process decreased by 63%, while CO₂ capture capacity fell 22%. Finally, although the pressure drop of the adsorption column (5 ± 1 kPa) and the energy consumption of the EC (537 ± 33 kJ·mol^-1 at 20 mA·cm^-2) are high, we have discussed the potential improvements toward a successful upscaling.

Effects of Current on the Membrane and Boundary Layer Selectivity in Electrochemical Systems Designed for Nutrient Recovery

During electrochemical nutrient recovery, current and ion exchange membranes (IEM) are used to extract an ionic species of interest (e.g., ion) from a mixture of multiple ions. The species of interest (ion 1) has an opposing charge to the IEM. When ion 1 is extracted from the solution, the species fractions at the membrane and the adjunct boundary layers are affected. Hence, the species transport through the electrochemical system (ES) can no longer be described as electrodialysis-like. A dynamic state is observed in the compartments, where the ionic species are recovered. When the boundary layer-membrane interface is depleted, the IEM is at maximum current. If the ES is operated at a current higher than the maximum current, the fluxes of both ion 1 and other competing ions, with the same charge (ion 2), occur. This means, for example, ion 1 will be recovered, and the concentration of ion 2 will build up in time. Therefore, a steady state is never reached. Ideally, to prevent the effect of limiting current at the boundary layer-membrane interface, ES for nutrient recovery should be operated at low currents.

Ammonia recovery from anaerobic digester centrate using onsite pilot scale bipolar membrane electrodialysis coupled to membrane stripping

Ammonia recovery from centrate of an anaerobic digester was investigated using an onsite bipolar-electrodialysis (BP-ED) pilot scale plant coupled to two liquid/liquid membrane contactor (LLMC) modules. To investigate the process performance and robustness, the pilot plant was operated at varying current densities, load ratio (current to nitrogen loading), and in continuous and intermittent current (Donnan) mode. A higher load ratio led to higher total ammonium nitrogen (TAN, sum of ammonia and ammonium) removal efficiency, whereas the increase in the applied current did not have a significant impact the TAN removal efficiency. Continuous current application resulted in the higher TAN removal compared with the Donnan dialysis mode. The lowest specific energy consumption of 6.3 kWh kg_N^-1 was recorded in the Donnan mode, with the load ratio of 1.4, at 200 L h^-1 flowrate and current density of 75 A m^-2. Lower energy demand observed in the Donnan mode was likely due to the lower scaling and fouling of the ion exchange membranes. Nevertheless, scaling and fouling limited the operation of the BP-ED stack in all operational modes, which had to be interrupted by the daily cleaning procedures. The LLMC module enabled a highly selective recovery of ammonia as ammonium sulfate ((NH₄)₂SO₄), with the concentration of ammonia ranging from 19 to 33 g_N L^-1. However, the analysis of per- and polyfluoroalkyl substances (PFASs) in the obtained (NH₄)₂SO₄ product revealed the presence of 212-253 ng L^-1 of 6:2 fluorotelomer sulfonate (FTS), a common substitute of legacy PFAS. Given the very low concentrations of 6:2 FTS (i.e., < 2 ng L^-1) encountered in the concentrated stream, 6:2 FTS was likely released from the Teflon-based components in the sulfuric acid dosage line. Thus, careful selection of the pilot plant tubing, pumps and other components is required to avoid any risks associated with the PFAS presence and ensure safe use of the final product as fertilizer.

Pilot-scale magnetic recovery of vivianite from digested sewage sludge

Phosphorus (P) is an essential resource for food production and chemical industry. Phosphorus use has to become more sustainable and should include phosphorus recycling from secondary sources. About 20% of P ends up in sewage sludge, making this a substantial secondary P source. There is currently a technological gap to recover P from sludge locally at wastewater treatment plants (WWTP) that remove P by dosing iron. Vivianite (Fe₃(PO₄)₂•8(H₂O)) is the main iron phosphate mineral that forms during anaerobic digestion of sewage sludge, provided that enough iron is present. Vivianite is paramagnetic and can be recovered using a magnetic separator. In this study, we have scaled up vivianite separation from lab-scale to bench- and pilot-scale. Bench-scale tests showed good separation of vivianite from digested sewage sludge and that a pulsation force is crucial for obtaining a concentrate with a high P grade. A pilot-scale magnetic separator (capacity 1.0 m³/h) was used to recover vivianite from digested sewage sludge at a WWTP. Recirculating and reprocessing sludge allows over 80% vivianite recovery within three passes. A concentrated P-product was produced with a vivianite content of up to 800 mg/g and a P content of 98 mg/g. P recovery is limited by the amount of P bound in vivianite and can be increased by increased iron dosing. With sufficient iron dosing, the vivianite content can be increased, and subsequently more P can be recovered. This would allow compliance with existing German legislation, which requires a P recovery larger than 50%.

Electrochemically mediated precipitation of phosphate minerals for phosphorus removal and recovery: Progress and perspective

Phosphorus (P) is an essential element for the growth and reproduction of organisms. Unfortunately, the natural P cycle has been broken by the overexploitation of P ores and the associated discharge of P into water bodies, which may trigger the eutrophication of water bodies in the short term and possible P shortage soon. Consequently, technologies emerged to recover P from wastewater to mitigate pollution and exploit secondary P resources. Electrochemically induced phosphate precipitation has the merit of achieving P recovery without dosing additional chemicals via creating a localized high pH environment near the cathode. We critically reviewed the development of electrochemically induced precipitation systems toward P removal and recovery over the past ten years. We summarized and discussed the effects of pH, current density, electrode configuration, and water matrix on the performance of electrochemical systems. Next to ortho P, we identified the potential and illustrated the mechanism of electrochemical P removal and recovery from non-ortho P compounds by combined anodic or anode-mediated oxidation and cathodic reduction (precipitation). Furthermore, we assessed the economic feasibility of electrochemical methods and concluded that they are more suitable for treating acidic P-rich waste streams. Despite promising potentials and significant progress in recent years, the application of electrochemical systems toward P recovery at a larger scale requires further research and development. Future work should focus on evaluating the system's performance under long-term operation, developing an automatic process for harvesting P deposits, and performing a detailed economic and life-cycle assessment.

Donnan Dialysis for scaling mitigation during electrochemical ammonium recovery from complex wastewater

Inorganic scaling is often an obstacle for implementing electrodialysis systems in general and for nutrient recovery from wastewater specifically. In this work, Donnan dialysis was explored, to prevent scaling and to prolong operation of an electrochemical system for TAN (total ammonia nitrogen) recovery. An electrochemical system was operated with and without an additional Donnan dialysis cell, while being supplied with synthetic influent and real digested black water. For the same Load Ratio (nitrogen load vs applied current) while treating digested black water, the system operated for a period three times longer when combined with a Donnan cell. Furthermore, the amount of nitrogen recovered was higher. System performance was evaluated in terms of both TAN recovery and energy efficiency, at different Load Ratios. At a Load Ratio 1.3 and current density of 10 A m^-2, a TAN recovery of 83% was achieved while consuming 9.7 kWh kg_N^-1.

Minimal Bipolar Membrane Cell Configuration for Scaling Up Ammonium Recovery

Electrochemical systems for total ammonium nitrogen (TAN) recovery are a promising alternative compared with conventional nitrogen-removal technologies. To make them competitive, we propose a new minimal stackable configuration using cell pairs with only bipolar membranes and cation-exchange membranes. The tested bipolar electrodialysis (BP-ED) stack included six cell pairs of feed and concentrate compartments. Critical operational parameters, such as current density and the ratio between applied current to nitrogen loading (load ratio), were varied to investigate the performance of the system using synthetic wastewater with a high nitrogen content as an influent (NH₄ ⁺ ≈ 1.75 g L^-1). High TAN removal (>70%) was achieved for a load ratio higher than 1. At current densities of 150 A m^-2 and a load ratio of 1.2, a TAN transport rate of 1145.1±14.1 g_N m^-2 d^-1 and a TAN-removal efficiency of 80% were observed. As the TAN removal was almost constant at different current densities, the BP-ED stack performed at a high TAN transport rate (819.1 g_N m^-2 d^-1) while consuming the lowest energy (18.3 kJ g_N ^-1) at a load ratio of 1.2 and 100 A m^-2. The TAN transport rate, TAN removal, and energy input achieved by the minimal BP-ED stack demonstrated a promising new cell configuration for upscaling.

Enhanced Phototrophic Biomass Productivity through Supply of Hydrogen Gas

Industrial production of phototrophic microorganisms is often hindered by low productivity due to limited light availability and therefore requires large land areas. This letter demonstrates that supply of hydrogen gas (H₂) increases in phototrophic biomass productivity compared to a culture growing on light only. Experiments were performed growing Synechocystis sp. in batch bottles, with and without H₂ in the headspace, which were exposed to light intensities of 70 and 100 μmol/m²/s. At 70 μmol/m²/s with H₂, the average increase in biomass was 96 mg DW/L/d, whereas at 100 μmol/m²/s without H₂, the average increase in biomass was 27 mg DW/L/d. Even at lower light intensity, the addition of H₂ tripled the biomass yield compared to growth under light only. Photoreduction and photosynthesis occurred simultaneously, as both H₂ consumption and O₂ production were measured during biomass growth. Photoreduction used 1.85 mmol of H₂ to produce 1.0 mmol of biomass, while photosynthesis produced 1.95 mmol of biomass. After transferring the culture to the dark, growth ceased, also in the presence of H₂, showing that both light and H₂ were needed for growth. A renewable H₂ supply for higher biomass productivity is attractive since the combined efficiency of photovoltaics and electrolysis exceeds the photosynthetic efficiency.

Electrochemical Regeneration of Spent Alkaline Absorbent from Direct Air Capture

CO2 capture from the atmosphere (or direct air capture) is widely recognized as a promising solution to reach negative emissions, and technologies using alkaline solutions as absorbent have already been demonstrated on a full scale. In the conventional temperature swing process, the subsequent regeneration of the alkaline solution is highly energy-demanding. In this study, we experimentally demonstrate simultaneous solvent regeneration and CO2 desorption in a continuous system using a H2-recycling electrochemical cell. A pH gradient is created in the electrochemical cell so that CO2 is desorbed at a low pH, while an alkaline capture solution (NaOH) is regenerated at high pH. By testing the cell under different working conditions, we experimentally achieved CO2 desorption with an energy consumption of 374 kJ·mol-1 CO2 and a CO2 purity higher than 95%. Moreover, our theoretical calculations show that a minimum energy consumption of 164 kJ·mol-1 CO2 could be achieved. Overall, the H2-recycling electrochemical cell allowed us to accomplish the simultaneous desorption of high-purity CO2 stream and regeneration of up to 59% of the CO2 capture capacity of the absorbent. These results are promising toward the upscaling of an energy-effective process for direct air capture.

Competition of electrogens with methanogens for hydrogen in bioanodes

Bioelectrochemical systems (BES) can provide an energy efficient way to recover nutrients from wastewaters. However, the electron donors available in wastewater are often not sufficient to recover the total amount of nutrients. This work investigates hydrogen (H₂) as an additional substrate for bioanodes. This hydrogen can be produced in the fermentation of complex organic waste or could be recycled from the cathode. Understanding how to influence the competition of electroactive microorganisms (EAM) with methanogens for H₂ gas from different sources is key to successful application of H₂ as additional electron donor in bioelectrochemical nutrient recovery. Ethanol (EtOH) was used as model compound for complex wastewaters since it is fermented into both acetate and H₂. EtOH was efficiently converted into electricity (e^-) by a syntrophic biofilm. Total recovered charge from 1 mM EtOH was 20% higher than for the same amount of acetate. This means that H₂ from EtOH fermentation was converted by EAM into electricity. Low EtOH concentrations (1  mM) led to higher conversion efficiencies into electricity than higher concentrations (5 and 10  mM). Thermodynamic calculations show this correlates with a higher energy gain for electrogens compared to methanogens at low H₂ concentrations. Cumulatively adding 1 mM EtOH without medium exchange (14 times in 14 days) resulted in stable conversion of H₂ to e^- (67%-77% e^-) rather than methane. With H₂ gas as electron donor, 68 ± 2% H₂ was converted into e^- with no carbon source added, and still 53 ± 5% to e^- when 50  mM bicarbonate was provided. These results show that under the provided conditions, electrogens can outcompete methanogens for H₂ as additional electron donor in MECs for nutrient recovery.

Energy-Efficient Ammonia Recovery in an Up-Scaled Hydrogen Gas Recycling Electrochemical System

Nutrient and energy recovery is becoming more important for a sustainable future. Recently, we developed a hydrogen gas recycling electrochemical system (HRES) which combines a cation exchange membrane (CEM) and a gas-permeable hydrophobic membrane for ammonia recovery. This allowed for energy-efficient ammonia recovery, since hydrogen gas produced at the cathode was oxidized at the anode. Here, we successfully up-scaled and optimized this HRES for ammonia recovery. The electrode surface area was increased to 0.04 m² to treat up to 11.5 L/day (∼46 g_N/day) of synthetic urine. The system was operated stably for 108 days at current densities of 20, 50, and 100 A/m². Compared to our previous prototype, this new cell design reduced the anode overpotential and ionic losses, while the use of an additional membrane reduced the ion transport losses. Overall, this reduced the required energy input from 56.3 kJ/g_N (15.6 kW h/kg_N) at 50 A/m² (prototype) to 23.4 kJ/g_N (6.5 kW h/kg_N) at 100 A/m² (this work). At 100 A/m², an average recovery of 58% and a TAN (total ammonia nitrogen) removal rate of 598 g_N/(m² day) were obtained across the CEM. The TAN recovery was limited by TAN transport from the feed to concentrate compartment.

Load ratio determines the ammonia recovery and energy input of an electrochemical system

Complete removal and recovery of total ammonia nitrogen (TAN) from wastewaters in (bio)electrochemical systems has proven to be a challenge. The system performance depends on several factors, such as current density, TAN loading rate and pH. The interdependence among these factors is not well understood yet: insight is needed to achieve maximum ammonium recovery at minimal energy input. The aim of this study was to investigate the influence of current density and TAN loading rate on the recovery efficiency and energy input of an electrochemical cell (EC). We therefore defined the load ratio, which is the ratio between the applied current and the TAN loading rate. The system consisted of an EC coupled to a membrane unit for the recovery of ammonia. Synthetic wastewater, with TAN concentration similar to urine, was used to develop a simple model to predict the system performance based on the load ratio, and urine was later used to evaluate TAN transport in a more complex wastewater. High fluxes (up to 433 gN m^-2 d^-1) and recovery efficiencies (up to 100%) were obtained. The simple model presented here is also suited to predict the performance of similar systems for TAN recovery, and can be used to optimize their operation.

Hydrogen Gas Recycling for Energy Efficient Ammonia Recovery in Electrochemical Systems

Recycling of hydrogen gas (H₂) produced at the cathode to the anode in an electrochemical system allows for energy efficient TAN (Total Ammonia Nitrogen) recovery. Using a H₂ recycling electrochemical system (HRES) we achieved high TAN transport rates at low energy input. At a current density of 20 A m^-2, TAN removal rate from the influent was 151 g_N m^-2 d^-1 at an energy demand of 26.1 kJ g_N^-1. The maximum TAN transport rate of 335 g_N m^-2 d^-1 was achieved at a current density of 50 A m^-2 and an energy demand of 56.3 kJ g_N^-1. High TAN removal efficiency (73-82%) and recovery (60-73%) were reached in all experiments. Therefore, our HRES is a promising alternative for electrochemical and bioelectrochemical TAN recovery. Advantages are the lower energy input and lower risk of chloride oxidation compared to electrochemical technologies and high rates and independence of organic matter compared to bioelectrochemical systems.

Clinoptilolite-based mixed matrix membranes for the selective recovery of potassium and ammonium

A clinoptilolite-based mixed matrix membrane (MMM) was developed and studied for the selective recovery of ammonium and potassium. Adsorption of sodium (Na(+)), potassium (K(+)) and ammonium (NH4(+)) was investigated with single salt and equimolar salt solution under static and dynamic conditions. Furthermore, the adsorption capacity of clinoptilolite was investigated when embedded in the MMM and in clay form. Two conditioning methods were compared: HCl and NaCl. Conditioned clinoptilolite with NaCl gave higher static adsorption capacities than with HCl which alters the chemical structure of clinoptilolite. The adsorption of Na(+) was not detected in the static adsorption experiments and results showed that Na(+) adsorbed during the conditioning process it was exchanged by K(+) and NH4(+).The clinoptilolite embedded in MMM reduced the porosity of the MMM so the highest adsorption capacity was reached when the amount of polymer was the lowest: 30 wt% polymer and 70 wt% clinoptilolite. The application of MMM in a dead-end filtration cell (dynamic adsorption) resulted in higher adsorption capacities compared to static conditions and comparable results between synthetic solutions and diluted urine samples. This indicates that MMM is a suitable method for the recovery of K(+) and NH4(+) directly from a diluted urine matrix. The desorption (recovery) of K(+) and NH4(+) from MMM was higher using water at 60 °C than using an acidic treatment.

Ammonium recovery and energy production from urine by a microbial fuel cell

Nitrogen recovery through NH(3) stripping is energy intensive and requires large amounts of chemicals. Therefore, a microbial fuel cell was developed to simultaneously produce energy and recover ammonium. The applied microbial fuel cell used a gas diffusion cathode. The ammonium transport to the cathode occurred due to migration of ammonium and diffusion of ammonia. In the cathode chamber ionic ammonium was converted to volatile ammonia due to the high pH. Ammonia was recovered from the liquid-gas boundary via volatilization and subsequent absorption into an acid solution. An ammonium recovery rate of 3.29 g(N) d(-1) m(-2) (vs. membrane surface area) was achieved at a current density of 0.50 A m(-2) (vs. membrane surface area). The energy balance showed a surplus of energy 3.46 kJ g(N)(-1), which means more energy was produced than needed for the ammonium recovery. Hence, ammonium recovery and simultaneous energy production from urine was proven possible by this novel approach.

Effects of ammonium concentration and charge exchange on ammonium recovery from high strength wastewater using a microbial fuel cell

Ammonium recovery using a two chamber microbial fuel cell (MFC) was investigated at high ammonium concentration. Increasing the ammonium concentration (from 0.07 to 4 g ammonium-nitrogen/L) by addition of ammonium chloride did not affect the performance of the MFC. The obtained current densities by DC-voltammetry were higher than 6A/m(2) for both operated MFCs. Also continuous operation at lower external resistance (250 Ω) showed an increased current density (0.9A/m(2)). Effective ammonium recovery can be achieved by migrational ion flux through the cation exchange membrane to the cathode chamber, driven by the electron production from degradation of organic substrate. The charge transport was proportional to the concentration of ions. Nonetheless, a concentration gradient will influence the charge transport. Furthermore, a charge exchange process can influence the charge transport and therefore the recovery of specific ions.