A Novel Integrated Flow-Electrode Capacitive Deionization and Flow Cathode System for Nitrate Removal and Ammonia Generation from Simulated Groundwater

Abstract

Electrochemical reduction of nitrate is a promising method for the removal of nitrate from contaminated groundwater. However, the presence of hardness cations (Ca 2+ and Mg 2+ ) in groundwaters hampers the electroreduction of nitrate as a result of the precipitation of carbonate-containing solids of these elements on the cathode surface. Thus, some pretreatment process is required to remove unwanted hardness cations. Herein, we present a proof-of-concept of a novel three-chambered flow electrode unit, constituting a flow electrode capacitive deionization (FCDI) unit and a flow cathode (FC) unit, which achieves cation removal, nitrate capture and reduction, and ammonia generation in a single cell without the need for any additional chemicals/ electrolyte. The addition of the FCDI unit not only achieves removal of hardness cations but also concentrates the nitrate ions and other anions, which facilitates nitrate reduction in the subsequent FC unit. Results show that the FCDI cell voltage influences electrode stability but has a minimal impact on the overall nitrate removal performance. The concentration of coexisting anions influences the nitrate removal due to competitive sorption of anions on the electrode surface. Our results further show that stable electrochemical performance was obtained over 26 h of operation. Overall, this study provides a scalable strategy for continuous nitrate electroreduction and ammonia generation from nitrate contaminated groundwaters containing hardness ions.

Utilizing an Integrated Flow Cathode–Membrane Filtration System for Effective and Continuous Electrochemical Hydrodechlorination

“…Despite the occurrence of competitive HER and electrostatic repulsion between the negatively charged reactant and flow electrode, the inhibitory impact of the applied potential was not as significant as reported in other fixed-electrode hydrodechlorination studies, − ,, possibly as a result of improved mass and electron transfer as a result of the proximity between the Pd@AC particles and 2,4-DCP in the FC system. We would like to highlight that no apparent difference in the cathodic pH was observed at all potentials (≤−1.0 V; Figure S7), which is in agreement with the findings of our previous FC studies. , This observation suggests that the lack of further increase in 2,4-DCP removal by lowering the cathode potential below −1.0 V is not a result of pH change but is, rather, due to mass transfer limitation and/or strong HER at < −1.0 V that hampers H* generation, as has been reported in various other studies. ,,,, Even though the occurrence of HER at lower applied potential causes a decrease in [H + ], the continuous influx of H + from the anode chamber (where H + is generated via water splitting , ) counterbalances the pH change in the cathode chamber, with the cathodic pH stabilizing at ∼11 during the EHDC experiment. The products of 2,4-DCP hydrodechlorination (Figure S7) are the same at all cathode potentials (≤−1.0 V), suggesting that the mechanism of hydrodechlorination of 2,4-DCP does not change with variation in the cathode potential.…”

“…While there have been reports that NO 3 – can be electrochemically reduced via DET and/or indirectly by H* (e.g., on the surface of Co 2+ ), , no change in NO 3 – concentration (Figure S11b) indicates that the synthesized Pd 0.04 @AC (20 mg in total) is unable to facilitate the reduction of NO 3 – , at least on the timescales investigated here. The presence of Ca 2+ has been reported to hamper the cathodic reduction efficiency by precipitating on the cathode ,,, as a result of the high pH in the vicinity of the cathode surface. However, a negligible impact of Ca 2+ on 2,4-DCP reduction in this FC system was observed with this lack of impact, possibly due to the relatively short electrolysis duration of <1 h used here compared to 4 h used in previous studies. , Nonetheless, as highlighted in a number of previous studies of Ca and Mg mineral deposition on cathode surfaces, ,, we recognize that the presence of hardness ions may reduce the effectiveness of the EHDC process described here.…”

Selective Nitrate Electroreduction to Ammonia on CNT Electrodes with Controllable Interfacial Wettability

“…CA and operando FITR spectroscopic measurements are used to determine that TTAB-CNT, which provides the greatest hydrophobicity of the tested surfactants, allows fewer interfacial water molecules to reach the CNT surface, thereby significantly inhibiting the competitive HER and promoting NH 3 selectivity. The produced NH 3 could be further recovered by struvite precipitation, electrochemical stripping, capacitive deionization, and other routes. Overall, this work highlights the importance of controlling the hydrophobicity of the microenvironment of electrocatalysts to maximize the selective generation of NH 3 , which can be applied to a broad range of heterogeneous catalytic systems that involve the control of selectivity.…”

Electrodialysis and Nitrate Reduction to Enable Distributed Ammonia Manufacturing from Wastewaters

“…To address this knowledge gap, we varied the NR background electrolyte anion identity and initial pH in an isolated two-chamber reactor to identify the optimal NR environment (see SI Section S1.3). The background electrolyte concentration was fixed as 1 M (cation concentration) to ensure high NO3RR activity, 34,40 , Fig. 1c).…”

“…8,13,14 Unlike processes with discrete reactant separation and catalysis steps, reactive separations utilize separations to create favorable and stable reaction environments from complex feedstocks, and reactions to produce product mixtures that inform separations. Electrochemical reactive separations have been demonstrated to recover carbon (reactive carbon capture), [15][16][17][18][19] sulfur, [20][21][22][23][24] and lithium, [25][26][27][28][29][30] but have rarely been used to recover NH3 from NH 4 + -containing [31][32][33] and from NO 3 --containing [34][35][36][37] wastewaters, and even more rarely for wastewaters containing both NH 4 + and NO 3 -.…”