Improved Desalination Performance Research Articles

Carbon Black Flow Electrode Enhanced Electrochemical Desalination Using Single-Cycle Operation.

Flow-electrode electrochemical desalination (FEED) processes (e.g., flow-electrode capacitive deionization), which use flowable carbon particles as the electrodes, have attracted increasing attention, holding the promise for continuous desalination and high desalting efficiency. While it is generally believed that carbon particles with abundant microporous and large specific capacitances (e.g., activated carbon, AC) should be ideal candidates for FEED electrodes, we provide evidence to the contrary, showing that highly conductive electrodes with low specific surface area can outperform microporous AC-based electrodes. This study revealed that FEED using solely high surface area AC particles (∼2000 m2 g-1, specific capacitance of ∼44 F g-1, average salt adsorption rate of ∼0.15 μmol cm-2 min-1) was vastly outperformed by electrodes based solely on low-surface area carbon black (CB, ∼70 m2 g-1, ∼0.5 F g-1, ∼0.75 μmol cm-2 min-1). Electrochemical impedance spectroscopy results suggest that the electrode formed by CB particles led to more effective electronic charge percolation, likely contributing to the improved desalination performance. In addition, we propose and demonstrate a novel operation mode, termed single cycle (SC), which greatly simplified the FEED cell configuration and enabled simultaneous charging and discharging. Using SC mode with CB flow electrodes delivered an increased average salt removal rate relative to the more traditional short-circuited closed cycle (SCC) mode, achieving up to 1.13 μmol cm-2 min-1. Further investigations demonstrate that up to 50% of energy input would be avoided when using CB flow electrodes operated under SC mode as compared to that of AC flow electrodes operated under SCC mode. In summary, the FEED process presented in this study provided an innovative and promising approach toward high-efficient and low-cost brackish water desalination.

Carbon electrodes with ionic functional groups for enhanced capacitive deionization performance

Capacitive deionization (CDI) has proved to be a clean and green technology; however, the charge leakage in the electrode pores during the adsorption-desorption cycles lowers the desalination efficiency. In this work, commercially available activated carbon was chemically treated to give the activated carbon surface either immobilized amino groups or sulfonic acid groups. The amino group-containing and sulfonic group-containing carbon materials were used as anode and cathode electrodes, respectively, in assembling a CDI (AC-CDI) cell. The salt adsorption capacity, salt removal efficiency, and charge efficiency of the AC-CDI cell were over two times higher than those of the untreated carbon-based CDI cell and comparable with those of the membrane CDI (MCDI) cell. The greatly improved desalination performance of the AC-CDI cell was attributed to enhanced wettability, faster diffusion of counter ions to each electrode via electrostatic attraction, and the prevention of re-adsorption of ions via repulsion between the charged layers and co-ions during adsorption-desorption cycle. Thus, it can be concluded that chemically modified carbon-based CDI cell is highly promising for practical application.

High-Performance Polyamide Thin-Film Nanocomposite Membranes Containing ZIF-8/CNT Hybrid Nanofillers for Reverse Osmosis Desalination

Thin-film nanocomposite (TFN) membranes incorporating nanofillers in ultrathin and selective polyamide layers have improved desalination performance in conventional reverse osmosis (RO) membranes. ...

Thin film nanocomposite reverse osmosis membrane modified by two dimensional laminar MoS2 with improved desalination performance and fouling-resistant characteristics

Two dimensional (2 D) layered materials with special properties provide opportunities to design energy-efficient membranes for water desalination. A negatively charged layered graphene analogue, molybdenum disulfide (MoS2) was synthesized through liquid exfoliation. A novel thin film nanocomposite (TFN) reverse osmosis (RO) membrane was successfully fabricated with prepared laminar MoS2 as fillers in the organic phase through interfacial polymerization of m-phenylenediamine and trimesoyl chloride monomers. The laminar MoS2 nanosheets were observed by transmission electron microscopy (TEM), and the nanosheets were dispersed in the polyamide (PA) matrix as well as on the membrane surface. The properties of the resultant TFN membranes were effectively modified by taking the advantage of two dimensional features, i.e., the negative charge and the hydrophilic sites of layered MoS2. Compared to a classical TFC membrane, the MoS2 incorporated TFN membranes were found to have an increased salt rejection and higher water flux. Moreover, 0.01 wt% MoS2-TFN membrane showed a superior fouling resistance against proteins as foulants. The antifouling behavior of this TFN membrane was observed by field-emission scanning electron microscopy (FE-SEM). Stability tests demonstrated that there was a loss of MoS2 in the initial 2 h filtration, but the process is stable in the long-term.

Improvement of capacitive deionization performance via using a Tiron-grafted TiO2 nanoparticle layer on porous carbon electrode

A novel ion-selective inorganic-carbon composited electrode was designed to improve the performance of a capacitive deionization (CDI) process. Disodium 4,5-dihydroxy-1,3-benzenedisulfonate (Tiron) was grafted on the surface of titania nanoparticles, and a thin titania layer with a thickness of 10-12 μm was formed on porous activated-carbon (AC) electrode and used as the negative electrode in a CDI full cell. The resulting Tiron-grafted titania nanoparticles showed an excellent ion-exchange capacity (1.51 meq/g). As a result, the Tiron-titania/AC composited electrode was found to have improved desalination properties in terms of specific adsorption capacity, specific adsorption rate and current efficiency compared with the pristine CDI electrode. Improved desalination performance was attributed to a reduction in co-ion expulsion effect by ion-exchangeable functional groups in Tiron-grafted titania. In addition, the improved desalination performance through the introduction of a porous layer of Tiron-grafted titania was similar to that of the conventional membrane capacitive deionization (MCDI) using an ion-exchange membrane. From the results obtained, it has been experimentally proven that the use of Tiron-grafted TiO2/AC composite as the negative electrode in the CDI process is a simple and effective way to achieve high desalination performance.

Characterizing the Impacts of Deposition Techniques on the Performance of MnO2 Cathodes for Sodium Electrosorption in Hybrid Capacitive Deionization.

Capacitive deionization (CDI) is currently limited by poor ion-selectivity and low salt adsorption capacity of porous carbon electrodes. To enhance selectivity and capacity via sodium insertion reactions, carbon aerogel electrodes were modified by depositing amorphous manganese dioxide layers via cyclic voltammetry (CV) and electroless deposition (ED). MnO2-coated electrodes were evaluated in a hybrid capacitive deionization system to understand the relationship between oxide coating morphology, electrode capacitance, and sodium removal efficacy. Both deposition techniques increased electrode capacitance, but only ED electrodes improved desalination performance over bare aerogels. SEM imaging revealed ED deposition distributed MnO2 throughout the aerogel, while CV deposition created a discrete crust, indicating that CV electrodes were limited by diffusion. Sodium adsorption capacity of ED electrodes increased with MnO2 mass deposition, reaching a maximum of 0.77 mmol-Na+ per gram of cathode (2.29 mmol-Na+ g-MnO2-1), and peak charge efficiency of 0.95. The presence of MnO2 also positively shifted the electrode potential window of sodium removal, reducing parasitic oxygen reduction and inverting the desalination cycle so that energy discharge coincides with salt removal (1.96 kg-NaCl kWh-1). These results highlight the importance of deposition technique in improving desalination with MnO2-coated electrodes.

Uniform hydrophobic electrospun nanofibrous layer composed of polysulfone and sodium dodecyl sulfate for improved desalination performance

Abstract Recently, water reclamation and water reuse have received considerable attention because of high water demand. Membrane processes constitute the most convenient technology for water treatment and desalination. In this study, a novel class of enhanced performance membranes consisting of a highly hydrophobic nanofibrous layer and hydrophilic cellulose filter paper (CFP) for membrane distillation (MD) was thoroughly examined. The nanofibrous layer was produced through electrospinning of polysulfone (PSF) doped with a sodium dodecyl sulfate (SDS) surfactant for uniformity and homogeneity in fiber diameter. The SDS surfactant was found to be the ideal additive for increasing the conductivity of the PSF polymeric solution, which helps in lowering the critical voltage required to initiate the electrospinning process, resulting in greater elongation of the nanofibers because of the increase in charge density. In the fabricated membranes, the PSF–SDS nanofibrous layer acts as a top active layer, whereas the CFP acts as the bottom supportive layer. Cellulose paper can absorb water molecules, which enhances vapor flux across the membrane in MD. The surface area, pore size distribution, fiber diameter, mechanical strength, water flux, and rejection percentage of the modified dual-layer PSF–SDS/CFP membranes were studied. The salt rejection of the PSF–SDS/CFP membrane was more than 99%, and a high permeate flux of approximately 9 LMH was maintained for long-term operation of 16 h.

Understanding functionalized silica nanoparticles incorporation in thin film composite membranes: Interactions and desalination performance

Incorporation of functional nanoparticles in polymer membranes is a promising approach for the development of advanced thin film nanocomposite (TFN) membranes with improved desalination performance. This study was aimed to fabricate TFN membranes incorporated with functionalized silica nanoparticles (SNs), which were synthesized with different sizes (~50 and ~100nm) and surface functionalities (hydroxyl, epoxy and amine). Research focused on getting a better understanding of the interfacial interactions between SNs and polymer matrix and structure-property correlation of the TFN membranes. The physicochemical properties of the functionalized SNs and the TFN membranes were characterized using ATR-FTIR, TGA, DLS, SEM, TEM, XPS, tensiometer and streaming potential analysis. The results confirm successful incorporation of the surface functionalized SNs into the polymer membranes. The concentration, size and surface functionality of SNs were found to have strong impact on the surface hydrophilicity, morphology and chemistry of the TFN membranes. The SNs hybridized TFN members showed increased permeate flux and insignificant change in salt rejection. The surface functionalization of SNs with amine and epoxy moieties could facilitate the chemical interaction between SNs and polymer monomers, resulting in stabilizing the TFN membranes. Our research outcomes have provided new insight into the structure-performance correlation of TFN membranes and can be beneficial for fabrication of a wide range of nanoparticle incorporated membranes.

Clay nanosheets as charged filler materials for high-performance and fouling-resistant thin film nanocomposite membranes

Montmorillonite (MMT), a cationic clay, and layered double hydroxide (LDH), an anionic clay were prepared through “top-down” and “bottom-up” methods, respectively, with lateral dimension on hundred-nanometer scale as well as high aspect ratio. These two types of nanosheets, served as additives in interfacial polymerization process, were utilized to fabricate thin film nanocomposite (TFN) reverse osmosis (RO) membranes containing cationic and anionic clay nanosheets. The properties of TFN membranes were effectively modified with the advantage of unique charged properties and hydrophilic nature of these nanoclay filler materials,. Both MMT and LDH filled TFN membranes showed increased hydrophilicity and improved desalination performance. More importantly, the incorporation of MMT and LDH nanoclays resulted in a more and less negatively charged membrane surface, respectively, and thus exhibited different electrostatic repulsion effects and improved antifouling performances towards protein, cationic surfactant, and natural organic matter foulants. The results indicate that clay nanosheets are effective filler materials to tailor membrane surface properties and fouling resistance. Since the modification by nanoclays can be achieved in the membrane preparation process without altering existing procedures, this approach can be considered as a facile way to produce novel high-performance and fouling-resistant TFN RO membrane for desalination.

Thin film nanocomposite reverse osmosis membrane modified by reduced graphene oxide/TiO2 with improved desalination performance

A novel thin film nanocomposite reverse osmosis (TFN-RO) membrane was prepared by interfacial polymerization of m-phenylenediamine (MPD) and trimesoyl chloride (TMC) monomers and embedding reduced graphene oxide (rGO)/TiO2 nanocomposite in its polyamide layer. The rGO/TiO2 nanocomposite was added to the MPD solution with aim of improving RO performance, enhancing antifouling property and increasing chlorine resistance. Surface scanning electron microscopy (SEM) images show that all of the membranes have a rough surface with a layer of “ridge-and-valley” structure that is typical morphology of MPD/TMC polyamide membranes. The contact angle analysis showed that hydrophilicity of the membranes was improved by increasing the concentration of rGO/TiO2 due to the presence of various hydrophilic and negatively charged functional groups on the rGO/TiO2 nanocomposite. The superior performance of the rGO/TiO2/RO membrane was observed in the case of 0.02wt% rGO/TiO2 with water flux of 51.3L/m2h and salt rejection of 99.45%. Antifouling ability of the membranes studied by bovine serum albumin (BSA) solution filtration showed that 0.02wt% rGO/TiO2/RO membrane had the best antifouling property. Also, after the chlorination tests, the salt rejection of the bare RO membrane decreased by 30%, while this value was only 3% for 0.02wt% rGO/TiO2/RO membrane which confirmed its great chlorine resistance.

Improved performance in capacitive deionization of activated carbon electrodes with a tunable mesopore and micropore ratio

To improve the desalination performance, the coconut shell-based activated carbon electrodes were prepared by using a two-stage activation method. Both the specific surface area and the ratio of mesopore to micropore can be successfully manipulated by the activation process of KOH etching plus CO2 gasification. The results showed that the activated carbon electrodes with controlled mesoporosity exhibited higher specific capacitance and better rate capability as compared to the commercial one. The coexistence of mesopores and micropores can provide large surface area for ions to form an electrical double layer, while the enlarged mesoporosity can not only facilitate the ion transport but also improve the accessible surface area, suggesting the improved capacity of capacitive ion storage. From the desalination experiments at 1.0V, the activated carbon electrode, associated with a specific surface area of 2105m2g−1 and a 70.7% ratio of mesopore to total pore volume, presented an electrosorption capacity of 9.72mgg−1 and electrosorption rate constant of 0.060min−1, which were considerably higher than the micropore-dominant carbon electrodes. Therefore, the significantly improved desalination performance can be ascribed to the high surface area and the high ratio of mesoporosity in the activated carbon-based capacitive deionization.

Integrated experimental investigation and mathematical modeling of brackish water desalination and wastewater treatment in microbial desalination cells

Desalination of brackish water can provide freshwater for potable use or non potable applications such as agricultural irrigation. Brackish water desalination is especially attractive to microbial desalination cells (MDCs) because of its low salinity, but this has not been well studied before. Herein, three brackish waters prepared according to the compositions of actual brackish water in three locations in Israel were examined with domestic wastewater as an electron source in a bench-scale MDC. All three brackish waters could be effectively desalinated with simultaneous wastewater treatment. The MDC achieved the highest salt removal rate of 1.2 g L−1 d−1 with an initial salinity of 5.9 g L−1 and a hydraulic retention time (HRT) of 0.8 d. The desalinated brackish water could meet the irrigation standard of both salinity (450 mg L−1 TDS) and the concentrations of major ionic species, given a sufficient HRT. The MDC also accomplished nearly 70% removal of organic compounds in wastewater with Coulombic efficiency varied between 5 and 10%. A previously developed MDC model was improved for brackish water desalination, and could well predict salinity variation and the concentrations of individual ions. The model also simulated a staged operation mode with improved desalination performance. This integrated experiment and mathematical modeling approach provides an effective method to understand the key factors in brackish water desalination by MDCs towards further system development.

Coupling ion-exchangers with inexpensive activated carbon fiber electrodes to enhance the performance of capacitive deionization cells for domestic wastewater desalination

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.