Transient analysis of bipolar membrane assisted electrosorption: Implications for boron removal

Boron removal from aqueous solutions has long persisted as a technological challenge, accounting for a disproportionately large fraction of the chemical and energy usage in seawater desalination and other industrial processes like lithium recovery. Here, we introduce a novel electrosorption-based boron removal technology with the capability to overcome the limitations of current state-of-the-art methods. Specifically, we incorporate a bipolar membrane (BPM) between a pair of porous carbon electrodes, demonstrating a synergized BPM-electrosorption process for the first time. The ion transport and charge transfer mechanisms of the BPM-electrosorption system are thoroughly investigated, confirming that water dissociation in the BPM is highly coupled with electrosorption of anions at the anode. We then demonstrate effective boron removal by the BPM-electrosorption system and verify that the mechanism for boron removal is electrosorption, as opposed to adsorption on the carbon electrodes or in the BPM. The effect of applied voltage on the boron removal performance is then evaluated, revealing that applied potentials above ∼1.0 V result in a decline in process efficiency due to the increased prevalence of detrimental Faradaic reactions at the anode. The BPM-electrosorption system is then directly compared with flow-through electrosorption, highlighting key advantages of the process with regard to boron sorption capacity and energy consumption. Overall, the BPM-electrosorption shows promising boron removal capability, with a sorption capacity >4.5 μmol g-C-1 and a corresponding specific energy consumption of <2.5 kWh g-B-1.

The increasing threat of water scarcity poses a significant challenge to water-intensive industries in Taiwan. There is a pressing requirement to increase freshwater production from seawater and wastewater. In the existing seawater treatment process, two-pass reverse osmosis (RO) and pH adjustment are generally adopted. The first stage RO aims at removing salts within normal pH levels, while the second stage RO focuses on eliminating boron under high pH conditions (>10). Under alkaline conditions, the dominant species, borate ions, are more readily rejected by RO membranes due to their larger hydrated size and negative charge. However, these procedures require substantial chemical dosing and energy consumption, which is uneconomical and environmentally unsustainable. Hence, the urgent need to replace the second stage RO with alternative technologies featuring low operational costs and chemical dosing is evident.Recently, bipolar membrane (BPM) has drawn increasing attention due to the ability to produce acids and bases from the corresponding salt solutions without chemical addition. Under reverse bias, water dissociation reaction occurs, generating protons and hydroxyl ions from water molecules. The H+ and OH− migrate through the cation and anion exange layer of the BPM, leading to the formation of acidic and alkaline pH on opposite sides of the membrane and creating a pH gradient across the BPM.The aim of the study is to achieve boron removal without chemical addition using bipolar membrane electrodialysis (BMED).The characteristics of the commercial BPM, including current-voltage curve and co-ion leakage ratio, are first analyzed. Subsequently, a 4-channel BMED system will be developed to simultaneously remove boron in the alkaline chamber while concentrating boron in the acidic chamber. Increasing the pH value in the alkaline chamber to above the pKa value of 9.24 effectively converts boric acid into brate ions, facilitating efficient boron removal in BMED. In addition, the feasibility of using BMED for boron removal will be further examined under different operational conditions, such as applied voltage (3 V, 4 V, and 5 V), flow rate (5 mL/min and 10 mL/min), and influent boron concentration (0.5 mM, 1 mM, and 10 mM), in sigle-pass mode. This study provide an alternative solution for the treatment of boron in seawater. Figure 1

Experimental and theoretical investigations of steady and transient states in systems of ion exchange bipolar membranes

Simultaneous removal and recovery of boron from waste water by multi-step bipolar membrane electrodialysis

Increasing global water stress motivates a growing interest in seawater desalination by reverse osmosis. Boron is typically weakly removed by reverse osmosis membranes at seawater pH, necessitating expensive post-processes such as caustic agent dosing of the permeate followed by additional filtration steps. It has been previously demonstrated that membraneless capacitive deionization can enable chemical-free boron removal from reverse osmosis permeate, with basic design rules established. However, the level of boron electrosorption per cell charge was limited to ∼0.5 µmol/g, a level too low for practical applications. We here explore, both theoretically and experimentally, methods to enhance boron removal by capacitive deionization. We found that reversing the polarity of the applied voltage during the discharge step resulted in an order of magnitude increase in boron electrosorption to nearly 4 µmol/g with promising energy consumption of 0.2 kW⋅h/m3. The promise of these results is highlighted when compared with recently-developed boron electrosorption cells requiring bipolar membranes, which demonstrate similar boron removal of 4.35 µmol/g but with much higher energy consumption of 18.3 kW⋅h/m3 while incurring significant membrane costs. Overall, we demonstrate for the first time that membrane- and chemical-free electrochemical technologies can remove sufficient boron from RO permeate in a single pass, significantly enhancing its potential to provide energy- and cost-efficient boron removal.

Effects of process conditions on simultaneous removal and recovery of boron from boron-laden wastewater using improved bipolar membrane electrodialysis (BMED)

Graphene oxide modified porous P84 co-polyimide membranes for boron recovery by bipolar membrane electrodialysis process

Comparison of two electrodialysis stacks having different ion exchange and bipolar membranes for simultaneous separation of boron and lithium from aqueous solution

Bipolar membrane electrodialysis (BMED) was used to remove boron from aqueous solution. BMED was compared with conventional electrodialysis (ED) using aqueous solutions of sodium tetraborate containing 100 mg/L of boron. With use of BMED, more than 90% of the boron was removed under both acidic (pH 2.3) and basic (pH 9.1) conditions, whereas only 35−45% of the boron was removed using conventional ED. Over 90% of the boron was removed from the diluate solution over a wide range of initial pH from 2.3 to 12.0. A high initial pH reduces the current efficiency of boron removal because of the high mobility and high concentration of hydroxide ions compared with those of borate ions. The power requirement for boron removal increased as the initial pH and concentration of sodium chloride increased, but decreased as the applied voltage was increased. BMED is a promising option for removal of boron from aqueous solution.

Reverse osmosis (RO) is currently the most cost-efficient method for seawater (SW) desalination; however, producing high-quality water with a low boron concentration typically requires a two-pass process, which increases the required area and chemical consumption. We propose a sustainable and economic pathway for boron removal in a single RO step, thus reducing the area footprint. At the same time, chemicals are produced onsite from the RO brine using bipolar membrane electrodialysis (BMED), thus reducing the chemical footprint. We conducted BMED using natural and synthetic feed solutions and studied the acid and base production kinetics and electricity consumption to assess the feasibility. In terms of energy efficiency, the divalent cationic impurities in the feed are more detrimental than the anionic ones. We found that monoselective cation-exchange membranes are not efficacious in eliminating these, and hence, precipitation/nanofiltration before BMED is essential. As a BMED feed, the nanofiltered SWRO brine was the best option over SW or nanofiltered SW. Economical analysis shows that as compared to purchasing chemicals, BMED integration can reduce the process cost by 45%. In addition, the results point to the flexibility of the proposed design that increases its robustness toward fluctuation in chemicals and electricity prices.