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In capacitive deionization (CDI), the functionalization or modification of carbon electrodes and the mitigation of positive electrode oxidation are effective methods to improve desalination capacity and stability for practical application. Here, the covalent bond attached three-dimensional (3D) graphene/carbon nanotubes hybrid (G-CNTs) with rich conductive channels was chosen as electrode material, and a periodically alternated (positive and negative) voltage operation (1.2/0/–1.2/0 V) was applied that aims to enhance the efficiency and stability of capacity deionization. The desalination capacity of the G-CNTs electrode increased gradually over the first few cycles in this operation, while the usage cycle of CDI could be pronouncedly extended almost six times, compared with the CDI of regular applied voltage operation (1.2/0 V). Through the characterization of electrode and CDI cell, this enhancement of the desalination performance is attributed to the synergistic benefits of alternating oxidation of negative and positive electrode, which can mitigate the oxidation of the positive electrode as well as achieve negative surface charge enhanced negative electrode. Therefore, a periodically alternated voltage operation is an economical and efficient method to control the oxidation of electrodes for enhancing the desalination performance. [ABSTRACT FROM AUTHOR]

Flow-electrode capacitive deionization (FCDI) is a new desalination technology that delivers sustainable desalination performance. However, higher desalination rate and charge efficiency of FCDI are still desired for practical applications. In this study, voltages greater than 1.23 V – the minimum voltage for water electrolysis – were applied to enhance the driving force for ion migration and adsorption in an FCDI so as to achieve higher desalination rates. When the voltage was raised from 0.6 to 4.8 V, the desalination rate increased by nearly 7 times but the charge efficiency reduced from 92% to 69.5% due to the occurrence of Faradaic reactions such as water electrolysis. The other major factor that influenced the FCDI's charge efficiency was found to be concentration diffusion of ions between the FCDI's middle chamber and the two electrode chambers. To improve the charge efficiency, carbon black (CB) particles with an average diameter of 12 μm were added to the activated carbon flow electrodes at a mixing ratio of 1.5 wt%. With that, the FCDI's charge efficiency increased from 83.5% to 96.5% at 2.4 V. Electrochemical impedance spectrometry (EIS) analysis indicates that the addition of CB particles significantly reduced the FCDI's internal resistance, thereby leading to enhanced desalination performance. [ABSTRACT FROM AUTHOR]

Bioelectrochemical systems (BESs) are integrated water treatment technologies that generate electricity using organic matter in wastewater. In situ use of bioelectricity can direct the migration of ionic substances in a BES, thereby enabling water desalination, resource recovery, and valuable substance production. Recently, much attention has been placed on the microbial desalination cells in BESs to drive water desalination, and various configurations have optimized electricity generation and desalination performance and also coupled hydrogen production, heavy metal reduction, and other reactions. In addition, directional transport of other types of charged ions can remediate polluted groundwater, recover nutrient, and produce valuable substances. To better promote the practical application, the use of BESs as directional drivers of ionic substances requires further optimization to improve energy use efficiency and treatment efficacy. This article reviews existing researches on BES-driven directional ion transport to treat wastewater and identifies a few key factors involved in efficiency optimization. [ABSTRACT FROM AUTHOR]

Abstract: A capacitive deionization (CDI) cell was built with electrodes made of an inexpensive commercial activated carbon fiber (ACF), and then modified by incorporating ion-exchangers into the cell compartment. Three modified CDI designs were tested: MCDI – a CDI with electrodes covered by ion-exchange membranes (IEMs) of the same polarity, FCDI – a CDI with electrodes covered by ion-exchange felts (IEFs), and R-MCDI – an MCDI with cell chamber packed with ion-exchange resin (IER) granules. The cell was operated in the batch reactor mode with an initial salt concentration of 1000 mg/L NaCl, a typical level of domestic wastewater. The desalination tests involved investigations of two consecutive operation stages of CDIs: electrical adsorption (at an applied voltage of 1.2 V) and desorption [including short circuit (SC) desorption and discharge (DC) desorption]. The R-MCDI showed the highest electric adsorption as measured in the present study by desalination rate [670 ± 20 mg/(L h)] and salt removal efficiency (90 ± 1%) at 60 min, followed by the MCDI [440 ± 15 mg/(L h) and 60 ± 2%, respectively]. The superior desalination performance of the R-MCDI over other designs was also affirmed by its highest charge efficiency (110 ± 7%) and fastest desorption rates at both the SC [1960 ± 15 mg/(L·h)] and DC [3000 ± 20 mg/(L·h)] modes. The desalination rate and salt removal efficiency of the R-MCDI increased from ∼270 mg/(L h) and 83% to ∼650 mg/(L h) and 98% respectively when the applied voltage increased from 0.6 V to 1.4 V, while decreased slightly when lowering the salt water flow rate that fed into the cell. The packing of IER granules in the R-MCDI provided additional surface area for ions transfer; meanwhile, according to the results of electrochemical impedance spectroscopy (EIS) analysis, it substantially lower down the R-MCDI''s ohmic resistance, resulting in improved desalination performance. [Copyright &y& Elsevier]

Abstract: A separator coupled circulation stacked microbial desalination cell (c-SMDC-S) was constructed to stabilize the pH imbalances in MDCs without buffer solution and achieved the stable desalination. The long-term operation of c-SMDC-S, regular stacked MDC (SMDC) and no separator coupled circulation SMDC (c-SMDC) were tested. The SMDC and c-SMDC could only stably operate for 1week and 1month owing to dramatic anolyte pH decrease and serious biofilm growth on the air cathode, respectively. The c-SMDC-S gained in anolyte alkalinity and operated stably for about 60days without the thick biofilm growth on cathode. Besides, the chemical oxygen demand removal and coulombic efficiency were 64±6% and 30±2%, higher than that of SMDC and c-SMDC, respectively. It was concluded that the circulation of alkalinity could remove pH imbalance while the separator could expand the operation period and promote the conversion of organic matter to electricity. [Copyright &y& Elsevier]

Microbial desalination cells are considered a low-energy-consumption, clean technology to simultaneously purify wastewater and desalinate saline water by utilizing the in situ energy source contained in wastewater. To enhance desalination performance and achieve an optimal membrane stack configuration, an enlarged stacked microbial desalination cell (SMDC) has been developed and tested with 6–14 desalination cells. The cross-membrane area of the enlarged SMDC is 100 cm 2 . The anode and cathode volumes are both 200 mL. To reduce internal resistance, the width of desalination cells is kept as <0.5 mm. The optimal configuration with 10 desalination cells achieves the highest total desalination rate (TDR) of 423 mg/h and the highest charge transfer efficiency (CTE) of 836% when treating the 20 g/L NaCl solution. During this process, the junction potential across membranes increases from 0 to 374 mV, and occupies up to 74% of the total potential loss inside the SMDC. This shows that the SMDC used in this work achieves the highest TDR and CTE among the reported studies, and the junction potential should be effectively controlled to achieve the desired desalination performance in future practical applications. [ABSTRACT FROM AUTHOR]

A new design of membrane capacitive deionization (MCDI) cell was constructed by packing the cell's flow chamber with granular activated carbon (GAC). The GAC packed-MCDI (GAC-MCDI) delivered higher (1.2–2.5 times) desalination rates than the regular MCDI at all test NaCl concentrations (∼100–1000 mg/L). The greatest performance enhancement by packed GAC was observed when treating saline water with an initial NaCl concentration of 100 mg/L. Several different GAC materials were tested and they all exhibited similar enhancement effects. Comparatively, packing the MCDI's flow chamber with glass beads (GB; non-conductive) and graphite granules (GG; conductive but with lower specific surface area than GAC) resulted in inferior desalination performance. Electrochemical impedance spectroscopy (EIS) analysis showed that the GAC-MCDI had considerably smaller internal resistance than the regular MCDI (∼19.2 ± 1.2 Ω versus ∼1222 ± 15 Ω at 100 mg/L NaCl). The packed GAC also decreased the ionic resistance across the flow chamber (∼1.49 ± 0.05 Ω versus ∼1130 ± 12 Ω at 100 mg/L NaCl). The electric double layer (EDL) formed on the GAC surface was considered to store salt ions during electrosorption, and facilitate the ion transport in the flow chamber because of the higher ion conductivity in the EDLs than in the bulk solution, thereby enhancing the MCDI's desalination rate. [ABSTRACT FROM AUTHOR]

• The system performs best at 6 h hydraulic retention time. • It achieved 0.215 g/h salts removal at a wastewater treatment capacity of 10 L/d. • Microbial community structures are consistent with the spatial structure of cathode. • Catholyte circulation should be effective for the stratified microbial structures. In this study, two dual-anode assembled microbial desalination cells (DA-MDCs) composed of two anodes, one cathode and two membrane stacks were constructed, with a hollow fiber membrane (HFM) module coupling into the cathode to filtrate suspended solids and microbes. Enriched domestic wastewater treatment performance of the DA-MDC system under different hydraulic retention time (HRT) was investigated in detail. When HRT sequentially decreased from 10 h, 8 h, 6 h to 4 h, the conductivity in diluent effluent kept stable at first, but increased 519% at 4 h compared with 6 h. Nevertheless, the chemical oxygen demand, total nitrogen and total phosphorus in the effluent met the wastewater discharge standards in China at all HRTs. Additionally, transmembrane pressure of the HFM modules increased sharply when HRT changed from 6 h to 4 h. Therefore, HRT 6 h was selected as the optimal condition. Under such short HRT, the system realized wastewater treatment capacity of 10 L/d, surpassing all bio-cathode microbial desalination cell systems aiming to realize efficient treatment of domestic wastewater. Additionally, the salt removal rate of 0.215 g/h was more than two times compared with most similar systems. Finally, stratified microbial community structures in the cathodes of the system were found. This gave evidence on the efficient catholyte recirculation which might facilitate nitrogen removal from wastewater. [ABSTRACT FROM AUTHOR]

Abstract This study introduced a novel dual-anode assembled microbial desalination cell to enhance the performance of domestic wastewater treatment. Two parallel units were fabricated with two anodes and one cathode, which is separated by two ion exchange membrane stacks. A hollow fiber membrane module was inserted in the cathode to intercept suspended solids and microbes. Based on preliminary experiments where synthetic wastewater was utilized, anode hydraulic retention time of 10 h and cathode aeration rate of 0.16 m3/h were chosen as the operating conditions. By innovatively connecting four membrane stacks in cascades, which multiplied flow rate without adding extra circulation pumps, the desalination rate of the system was improved 214.8% compared with single membrane stack mode. When modified domestic wastewater was applied, the average removal efficiencies of chemical oxygen demand, ammonia nitrogen, total nitrogen and total phosphorous reached 96.9%, 99.0%, 98.0% and 98.3%, respectively. Highlights • A novel duel-anode microbial desalination cell was fabricated. • Connecting four membrane stacks of MDC in cascades improved desalination efficiency. • Practicability of the novel MDC was evaluated with modified domestic wastewater. [ABSTRACT FROM AUTHOR]

In this study, a hollow fiber microfiltration membrane (HFM) coupled biocathode microbial desalination cell (MDC) was fabricated. With real wastewater flowing serially from anaerobic anode to oxic biocathode, filtered by the HFM and finally to the central membrane stack, the HFM-MDC took advantages of conventional anaerobic/oxic process, membrane biofilm reactor, and eletrodeionization technology, realizing advanced removal of organics, suspended solids, and salinity during 105 days of operation. The produced diluate was 86.1 v%, with final conductivity, chemical oxygen demand, total nitrogen, and total phosphorus (TP) reaching 59.2 ± 12.2 μS/cm, 35.5 ± 4.0 mg/L, 1.65 ± 0.39 mg/L, and 0.14 ± 0.05 mg/L respectively. The corresponding concentrate (13.9 v%) concentrated 6.7 times the initial conductivity (6044 ± 168 μS/cm), recovered 98.7% of phosphorous (TP of 40.9 ± 4.9 mg/L) and part of nitrogen. In addition, the HFM was stably operated for 105 days due to coefficient aeration flushing and regular chemical backwashing, with trans-membrane pressure keeping at ≤ 15 kPa for most of time. Moreover, this integrated process only needs wastewater as sole water input, and possesses a relative cheap HFM coupled biocathode compared with conventional Pt cathode, providing a promising solution for practical application of MDC in wastewater purification. [ABSTRACT FROM AUTHOR]

Conventional microbial desalination cells (MDCs) extract organic energy from wastewater for in situ desalination of saline water, but cannot treat industrial wastewater containing high strength organics that may cause blocking in the thin membrane stack. In this study, a multi-stage MDC (M-MDC) was fabricated and operated for simultaneous treatment and desalination of diluted industrial wastewater. With wastewater (COD: 8723 ± 456 mg/L, conductivity: 24,612 ± 772 μS/cm) flowing serially from anode-1 → cathode-1 → anode-2 → cathode-2, the maximum power density of the M-MDC reached 566.1 mW/m 2 , with removal efficiencies of COD, TN, NH 4 + -N, and conductivity reaching 97.8%, 90.6%, 98.4%, and 31.6% respectively with addition of 1 V external voltage. The high strength organics were significantly removed due to alternative anaerobic/oxic conditions, the nitrogen removal was enhanced by simultaneous electrical migration and biological nitrification/denitrification. Moreover, the addition of external voltage enhanced current generation (67.4 ± 3.9 mA), desalination efficiency, coulombic efficiency (14.1%), and organics removal in two anodes (47.3%), producing an effluent (COD < 500 mg/L, TN < 15 mg/L) qualified to be discharged into municipal wastewater pipe systems. These results indicated great potential of M-MDC to work as a pretreatment technology for high strength industrial wastewater. [ABSTRACT FROM AUTHOR]

Nitrogen exists in municipal wastewater in the form of ammonia/ammonium which is a recoverable resource. Recovering nitrogen to improve nitrogen-use efficiency should be carefully considered to promote global environmental sustainability. However, ammonia/ammonium is always not well recovered but removed as one of the main pathways for nitrogen release into the environment, which is due to ineffective ammonia capture. In this study, a new method of ion-exchange resin ammonium recovery combined with membrane capacitive deionization (MCDI + IE process) pretreatment is proposed. Selective ion removal by MCDI is demonstrated and ammonium ratio to the total prior ion (K + , Ca 2+ and Mg 2+ ) in outflow of MCDI cell is respectively increased from ∼15% to over 30% and 45% after 3 and 5 MCDI cycles. The MCDI + IE process could deal with 2.48 and 3.89 times of inflow, respectively, compared with the single IE process after 3 and 5 MCDI charge–discharge cycles and recover more than 65% of ammonium in a resin regeneration cycle after 3 MCDI cycles. It ascribes to MCDI desalination eliminating the negative effects of divalent cations, such as Ca 2+ and Mg 2+ , on the IE absorb–desorb process. [ABSTRACT FROM AUTHOR]

Highlights: [•] Mixed resin packed MDC removed multiple anions better than multiple cations. [•] Both molar conductivity and exchange selectivity affected migration of multiple ions. [•] Scaling was observed on the surface of CEM when desalinating multiple cations. [•] Conductivity of resin decreased when desalinating multiple cations for long time. [•] Resin packed MDC is more suitable for desalination of water with lower hardness. [Copyright &y& Elsevier]

Abstract: A novel configuration of microbial desalination cell (MDC) packed with ion-exchange resin (R-MDC) was proposed to enhance water desalination rate. Compared with classic MDC (C-MDC), an obvious increase in desalination rate (DR) was obtained by R-MDC. With relatively low concentration (10–2g/L NaCl) influents, the DR values of R-MDC were about 1.5–8 times those of C-MDC. Ion-exchange resins packed in the desalination chamber worked as conductor and thus counteracted the increase in ohmic resistance during treatment of low concentration salt water. Ohmic resistances of R-MDC stabilized at 3.0–4.7Ω. By contrast, the ohmic resistances of C-MDC ranged from 5.5 to 12.7Ω, which were 55–272% higher than those of R-MDC. Remarkable improvement in desalination rate helped improve charge efficiency for desalination in R-MDC. The results first showed the potential of R-MDC in the desalination of water with low salinity. [Copyright &y& Elsevier]

l Why FCDI can be part electrodialysis and part electrosorption? l Resistance to charge transport is essential towards the performance of FCDI. l Distinguish the electronic and ionic resistance within a flow-electrode. l Enhancing electronic conduction greatly contributes to improve FCDI performance. Flow-electrode capacitive deionization (FCDI), as a novel electro-driven desalination technology, has attracted growing exploration towards brackish water treatment, hypersaline water treatment, and selective resource recovery in recent years. As a flow-electrode-based electrochemical technology, FCDI has similarities with several other electrochemical technologies such as electrochemical flow capacitors and semi-solid fuel cells, whose performance are closely coupled with the characteristics of the flow-electrodes. In this review, we sort out the potentially parallel mechanisms of electrosorption and electrodialysis in the FCDI desalination process, and make clear the importance of the flowable capacitive electrodes. We then adopt an equivalent circuit model to distinguish the resistances to ion transport and electron transport within the electrodes, and clarify the importance of electronic conductivity on the system performance based on a series of electrochemical tests. Furthermore, we discuss the effects of electrode selection and flow circulation patterns on system performance (energy consumption, salt removal rate), review the current treatment targets and system performance, and then provide an outlook on the research directions in the field to support further applications of FCDI. [ABSTRACT FROM AUTHOR]