Comparative study on the production of gluconic acid by electrodialysis and bipolar membrane electrodialysis: Effects of cell configurations

Electro-driven direct lithium extraction from geothermal brines to generate battery-grade lithium hydroxide

As Li-ion batteries are increasingly being deployed in electric vehicles and grid-level energy storage, the demand for Li is growing rapidly. Extracting lithium from alternative aqueous sources such as geothermal brines plays an important role in meeting this demand. Electrochemical intercalation emerges as a promising Li extraction technology due to its ability to offer high selectivity for Li and its avoidance of harsh chemical regenerants. In this work, we design an economically feasible electrochemical process that achieves selective lithium extraction from Salton Sea geothermal brine and purification of lithium chloride using intercalation materials, and conversion to battery grade (>99.5% purity) lithium hydroxide by bipolar membrane electrodialysis. We conduct techno-economic assessments using a parametric model and estimated the levelized cost of LiOH•H2O as 4.6 USD/kg at an electrode lifespan of 0.5 years. The results demonstrate the potential of our technology for electro-driven, chemical-free lithium extraction from alternative sources.

Tactfully introducing amphoteric group into electroactive membrane motivates highly efficient H2O splitting for reversible removal and recovery of nickel(II)

Membrane-based electro-deposition (MED) is an original process promising for reversible removal and recovery of toxic heavy metal ions from wastewater. The removal efficiency of heavy metal ions, however, was limited by the poor membrane surface H2O splitting in the conventional ion exchange membrane (IEM). Inspired by the amphoteric interface-triggered ion exchange resin regeneration phenomenon in electro-deionization, herein we subtly introduced the amphoteric group into IEM as a proof of concept to solve the above bottleneck. By virtue of the "electronic porter" role of the amphoteric -3OS-R-N(CH3)3+, the electron extraction from adsorbed H2O could be accelerated, extending the H2O splitting from the conventional membrane surface to the bulk membrane interior. Such an H2O splitting extension favorably produced an intensified and well-modeled OH- production region at the anodic side of IEM, enhancing the Ni2+ basic deposition accordingly. This special characteristic allowed our MED to realize a super-eminent metal ion removal rate (10.5 mol·h-1·m-2) along with an ultra-low specific energy consumption (0.1 kWh·mol-1) for Ni2+ removal, which considerably surpassed those of state-of-the-art heavy metal ion removal processes reported yet. Further, the deposited Ni2+ could be in situ recovered in conjunction with the facile polarity reversal method. The amphoteric electroactive membrane with high H2O splitting activity is expected to pave the path to engineering MED for efficient heavy metal ion removal and recovery.

Comparison of bipolar membrane electrodialysis, electrodialysis metathesis, and bipolar membrane electrodialysis multifunction for the conversion of waste Na2SO4: Process performance and economic analysis

To convert Na2SO4 into other high-value products (NaOH, H2SO4, and (NH4)2SO4), three types of cell configurations of electrodialysis (ED) were applied (three-compartment bipolar membrane ED (BMED), four-compartment ED metathesis (EDM) and five-compartment bipolar membrane ED multifunction (BMEDM)) and parameters such as average voltage variation, removal ratio of salt, product concentration, conversion rate, ion flux, and energy consumption were calculated and compared. The experimental results and calculations indicated that the overall performance of BMEDM was inferior to that of BMED and EDM. An industrial model was established, which indicated that the net profit from converting Na2SO4 using BMEDM was always higher than that from BMED and EDM. Based on the advantages of low investment (132 $) and energy cost (152 $/t Na2SO4), EDM was applicable to factories with a low output of Na2SO4 (production capacity <45%), whereas BMED (157.3 $/t Na2SO4) and BMED-5 (227.6 $/t Na2SO4) were applicable to factories with a high output of Na2SO4 (production capacity >45%) based on high net profits.

Bipolar Membrane Electrodialysis for Cleaner Production of Diprotic Malic Acid: Separation Mechanism and Performance Evaluation

Bipolar membrane electrodialysis (BMED) is a promising process for the cleaner production of organic acid. In this study, the separation mechanism of BMED with different cell configurations, i.e., BP-A, BP-A-C, and BP-C (BP, bipolar membrane; A, anion exchange membrane; C, cation exchange membrane), to produce diprotic malic acid from sodium malate was compared in consideration of the conversion ratio, current efficiency and energy consumption. Additionally, the current density and feed concentration were investigated to optimize the BMED performance. Results indicate that the conversion ratio follows BP-C > BP-A-C > BP-A, the current efficiency follows BP-A-C > BP-C > BP-A, and the energy consumption follows BP-C < BP-A-C < BP-A. For the optimized BP-C configuration, the current density was optimized as 40 mA/cm² in consideration of low total process cost; high feed concentration (0.5-1.0 mol/L) is more feasible to produce diprotic malic acid due to the high conversion ratio (73.4-76.2%), high current efficiency (88.6-90.7%), low energy consumption (0.66-0.71 kWh/kg) and low process cost (0.58-0.59 USD/kg). Moreover, a high concentration of by-product NaOH (1.3497 mol/L) can be directly recycled to the upstream process. Therefore, BMED is a cleaner, high-efficient, low energy consumption and environmentally friendly process to produce diprotic malic acid.

Bipolar Membranes Containing Iron-Based Catalysts for Efficient Water-Splitting Electrodialysis

Water-splitting electrodialysis (WSED) process using bipolar membranes (BPMs) is attracting attention as an eco-friendly and efficient electro-membrane process that can produce acids and bases from salt solutions. BPMs are a key component of the WSED process and should satisfy the requirements of high water-splitting capability, physicochemical stability, low membrane cost, etc. The water-splitting performance of BPMs can be determined by the catalytic materials introduced at the bipolar junction. Therefore, in this study, several kinds of iron metal compounds (i.e., Fe(OH)₃, Fe(OH)₃@Fe₃O₄, Fe(OH)₂EDTA, and Fe₃O₄@ZIF-8) were prepared and the catalytic activities for water-splitting reactions in BPMs were systematically analyzed. In addition, the pore-filling method was applied to fabricate low-cost/high-performance BPMs, and the 50 μm-thick BPMs prepared on the basis of PE porous support showed several times superior toughness compared to Fumatech FBM membrane. Through various electrochemical analyses, it was proven that Fe(OH)₂EDTA has the highest catalytic activity for water-splitting reactions and the best physical and electrochemical stabilities among the considered metal compounds. This is the result of stable complex formation between Fe and EDTA ligand, increase in hydrophilicity, and catalytic water-splitting reactions by weak acid and base groups included in EDTA as well as iron hydroxide. It was also confirmed that the hydrophilicity of the catalyst materials introduced to the bipolar junction plays a critical role in the water-splitting reactions of BPM.

Electrodialysis Desalination with Simultaneous pH Adjustment Using Bilayer and Bipolar Membranes, Modeling and Experiment

A kinetic model of the bipolar electrodialysis process with a two-chamber unit cell formed by a bilayer (bipolar or asymmetric bipolar) and cation-exchange membrane is proposed. The model allows describing various processes: pH adjustment of strong electrolyte solutions, the conversion of a salt of a weak acid, pH adjustment of a mixture of strong and weak electrolytes. The model considers the non-ideal selectivity of the bilayer membrane, as well as the competitive transfer of cations (hydrogen and sodium ions) through the cation-exchange membrane. Analytical expressions are obtained that describe the kinetic dependences of pH and concentration of ionic components in the desalination (acidification) compartment for various cases. Comparison of experimental data with calculations results show a good qualitative and, in some cases, quantitative agreement between experimental and calculated data. The model can be used to predict the performance of small bipolar membrane electrodialysis modules designed for pH adjustment processes.

Application of bipolar membrane electrodialysis for simultaneous recovery of high-value acid/alkali from saline wastewater: An in-depth review

With the development of comprehensive utilization of high-salinity wastewater, salt resources regeneration has been considered as the fundamental requirement for process sustainability and economic benefits. As one of the potential candidates, bipolar membrane electrodialysis (BMED) was rapidly developed in recent years for the treatment of saline wastewater. Different from other methods directly obtaining salts or condensed wastewater, BMED could utilize and convert the dissolved waste salt into higher-value acid and alkali simultaneously, which has various advantages including outstanding environmental effects and economic benefits. In this review, the recent applications of BMED for waste salt recovery and high-value acid/alkali generation from saline wastewater were systematically outlined. Based on the summary above, the economy analysis of BMED was further reviewed from the roles of desalination and resources recovery. In addition, the BMED-based processes integrated with in-situ utilization of the generated acid/alkali resources were discussed. Furthermore, the influence of operating factors on BMED performance were outlined. Finally, the strategies for improving BMED performance were concluded. Furthermore, the future application and prospects of BMED was presented. This work would provide guidance for the applications of bipolar membrane electrodialysis in saline wastewater treatment and the high-value conversion of salt resources into acids and alkalis.

High current density with spatial distribution of Geobacter in anodic biofilm of the microbial electrolysis desalination and chemical-production cell with enlarged volumetric anode

The aim of this study was to establish the relationship between spatial distribution of Geobacter and electric intensity in the microbial electrolysis desalination and chemical-production cell (MEDCC) and to investigate the effect of enlarged volumetric anode on the performance of MEDCC. The MEDCC was constructed with nine carbon brush anodes (length × diameter = 11 cm × 3 cm) as enlarged volumetric anode, and operated by feeding with 1 g/L acetate as substrate and 35 g/L NaCl as artificial seawater under the applied voltages of 1.2-4.5 V. Spatial distribution of Geobacter in the anodic biofilm was determined according to the bacterial community analysis on 27 biofilm samples from the top, middle and bottom layers of anodes (i.e., with distance of 4.5, 10, and 15.5 cm to the cathode, respectively). Results showed that the enlarged volumetric anode significantly improved the performance of MEDCC. The maximum desalination rate and current density reached 338.5 ± 21.8 mg/L∙h and 55.7 ± 3.7 A/m² in the MEDCC, respectively. The electric intensity values decreased with the distance from the anode to the cathode and formed an uneven distribution in the anode chamber. The samples in the top layer of anodes had the highest average 16S rRNA gene copy number of Geobacter of 1.55 × 10⁷ copies/μL, which was 18 times higher than that in the bottom layer of anodes. A linear relation was established between the spatial distribution of Geobacter and electric intensity (R² = 0.994-0.999). The electric intensity gradient created the uneven spatial distribution of Geobacter in the biofilms of volumetric anode. Results from this study could be useful to enrich Geobacter in the anodic biofilm thus to improve the performance of MEDCC.

Factors Influencing the Formation of Salicylic Acid by Bipolar Membranes Electrodialysis

Salicylic acid is an intermediate product in the synthesis of dyes, medications and aspirin. An electrodialysis module has been constructed with commercial cationic, anionic and bipolar membranes for the conversion of sodium salicylate into salicylic acid. The effect of operating conditions such as applied electric potential, salt concentration, initial acid concentration and volumetric flow on bipolar membrane electrodialysis (BMED) yields were investigated using Taguchi analysis. The results obtained in 210 min of work show an average concentration of salicylic acid of 0.0185 M, an average electric current efficiency of 85.3%, and a specific energy consumption of 2.24 kWh/kg of salicylic acid. It was concluded that the proposed bipolar membrane electrodialysis process is an efficient alternative to produce salicylic acid (SAH) from sodium salicylate (SANa) in an environmentally friendly manner. Furthermore, the production of sodium hydroxide was obtained as a by-product of the process carried out.

Membrane applications in the food industry

Current trends in the food industry for the application of membrane techniques are presented. Industrial solutions as well as laboratory research, which can contribute to the improvement of membrane efficiency and performance in this field, are widely discussed. Special attention is given to the main food industries related to dairy, sugar and biotechnology. In addition, the potential of membrane techniques to assist in the treatment of waste sources arising from food production is highlighted.

A comprehensive review on the synthesis and applications of ion exchange membranes

Ion exchange membranes (IEMs) are undergoing prosperous development in recent years. More than 30,000 papers which are indexed by Science Citation Index Expanded (SCIE) have been published on IEMs during the past twenty years (2001-2020). Especially, more than 3000 papers are published in the year of 2020, revealing researchers' great interest in this area. This paper firstly reviews the different types (e.g., cation exchange membrane, anion exchange membrane, proton exchange membrane, bipolar membrane) and electrochemical properties (e.g., permselectivity, electrical resistance/ionic conductivity) of IEMs and the corresponding working principles, followed by membrane synthesis methods, including the common solution casting method. Especially, as a promising future direction, green synthesis is critically discussed. IEMs are extensively applied in various applications, which can be generalized into two big categories, where the water-based category mainly includes electrodialysis, diffusion dialysis and membrane capacitive deionization, while the energy-based category mainly includes reverse electrodialysis, fuel cells, redox flow battery and electrolysis for hydrogen production. These applications are comprehensively discussed in this paper. This review may open new possibilities for the future development of IEMs.

Eco-friendly rice husk pre-treatment for preparing biogenic silica: Gluconic acid and citric acid comparative study

Carboxylic acid leaching has been established eco-friendly pre-treatment method for producing biogenic silica (BSi) from rice husk. The most urgent issue is for carboxylic acid to promote new readily biodegradable acids and enhance carboxylic acid sustainability in BSi preparation. This research investigates gluconic acid (GA) applicability for biogenic silica preparation from rice husk compared with citric acid (CA). The results demonstrated that GA was preferable to CA on BSi recovery with 89.91% efficiency. Although GA leaching promoted slightly higher silica loss, the primary metal alkali impurities, such as K₂O, Na₂O, and Al₂O_3, were effectively removed at 92-93%, 89-93%, 95-97%, respectively. The combination effect of silica loss and high removal impurities resulted in lower rice husk thermal decomposition activation energy. The characteristics of BSi prepared by GA leaching were comparable with CA leaching, mainly mesoporous with 114.06 m²/g of specific surface area and 0.23 cm³/g of the pore volume. In addition, GA leaching was environmentally better than CA leaching, indicated by minor contribution to all environmental impact indices. The findings suggested that GA could be a potential replacement for prevalent carboxylic acids in BSi preparation.

Quantitative recovery and regeneration of acidic ionic liquid 1-butyl-3-methylimidazolium hydrogen sulphate via industrial strategy for sustainable biomass processing

Quantitative recovery is necessary for scale-up application of acidic ionic liquids (AILs). Ultrafiltration and bipolar membrane electrodialysis (BMED) was employed for the recovery and regeneration of acidic ionic liquid 1-butyl-3-methylimidazolium hydrogen sulphate (Bmim[HSO₄]) after biomass pretreatment. Ultrafiltration was designed for the purification of BMED feed solution. During BMED treatment, Bmim⁺ retention with OH^- generation occurred in mixing section and SO₄^2- immigration with H⁺ generation occurred in aciding section. Resulting aqueous Bmim[OH] in mixing section and H₂SO₄ in aciding section could be utilized for quantitative synthesis of Bmim[HSO₄]. Influence of BMED operating mode and major parameters including BMED feed concentration and current density of BMED module were studied in detail. The highest recovery ratio for Bmim⁺ and SO₄^2- reached 96.2% and 96.0%. And the lowest energy consumption of specific Bmim[HSO₄] recovery approached 9.0 kw∙h/kg. Insight gained from this study suggested a sustainable biomass processing methodology using AILs.

Accurately-controlled recovery and regeneration of protic ionic liquid after Ionosolv pretreatment via bipolar membrane electrodialysis with ultrafiltration

Efficient recovery and regeneration of ionic liquid is significant for industrial Ionosolv pretreatment. Complicated electrolyte composition restricts the scale-up recovery and application of protic ionic liquid such as triethylammonium hydrogen sulfate [TEA][HSO₄] in biomass-related research. Recovery of [TEA][HSO₄] after Ionosolv pretreatment for miscanthus powder was studied using bipolar membrane electrodialysis (BMED) assisted with ultrafiltration (UF) by the divisional recovery of TEA⁺ as TEA and recovery of SO₄^2- as H₂SO₄ in different BMED compartments. Hence accurately-controlled regeneration of [TEA][HSO₄] could be realized. Influence of current density and feed concentration of BMED module was studied in detail. In this study, the highest recovery ratio for TEA⁺ and SO₄^2- reached 93.7% and 96.4%. The lowest energy consumption of specific [TEA][HSO₄] recovery was about 6.2 kwh/kg. Insight gained from this study suggests a potentially industrial methodology for complicated protic ionic liquid recovery after biomass processing.

Relationship between surface hydrophobicity and flux for membrane separation

Surface hydrophobicity of anodic aluminum oxide (AAO) membranes was controlled via carbon coating using the CVD method or O₂ plasma treatment with insignificant changes of pore diameter. This study first demonstrated that a larger hydrophobic pore surface and hydrophilic membrane surface are favorable for developing high performance membranes.

This study demonstrated that hydrophobic pore surfaces and hydrophilic membrane surfaces are more favorable in enhancing water flux, providing an important insight into the development of high performance membranes.

Application of Pulsed Electric Field in Antifouling Treatment of Sodium Gluconate Mother Liquor by Electrodialysis

Contamination of ion exchange membranes is one of the major problems in electrodialysis. Among the solutions that have been proposed and tested to alleviate membrane fouling during electrodialysis so far, applying a pulsed electric field (PEF) at a fixed application time (T_on) followed by a pause time (T_off) has been proved to be effective. In this study, the PEF was applied to desalinate sodium gluconate mother liquor by ED. The experimental properties of conventional ED and pulsed ED and their effects on membrane fouling were compared. The results show that compared with conventional ED, pulsed ED can alleviate concentration polarization and enhance the performance of ED. Similarly, in the process of continuous batch treatment of mother liquor under the PEF condition, large organic molecules can be effectively prevented from depositing on the membrane surface. Therefore, an anion exchange membrane (AEM) under the condition of PEF is contaminated mainly by organic molecules with a relatively smaller size. Both the surface and interior of AEM membrane were affected by organic pollutants under conventional electric field (CEF) conditions.