Electrosorption Integrated with Bipolar Membrane Water Dissociation: A Coupled Approach to Chemical-free Boron Removal

Abstract

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.

Surface Engineering of Electrified MXene Filter for Enhanced Phosphate Removal

“…As the voltage was increased from 0 to 2 V, the current had a proportional increase, with a surface charge density at pH 7 of 0.146 mA/cm 2 . Thus, the increased kinetics for the phosphate removal in the presence of an electric field could be due to the increased near-surface electromigration as well as the greater electrostatic interactions with the sorption sites. , Additionally, anodic capacitive charging of the h-Ti 3 C 2 T x (24 h) surface could increase the stability of the adsorbed phosphates and limit desorption, leading to an increase in the kinetics for phosphate sorption. , …”

“…35,36 Additionally, anodic capacitive charging of the h-Ti 3 C 2 T x (24 h) surface could increase the stability of the adsorbed phosphates and limit desorption, leading to an increase in the kinetics for phosphate sorption. 26,37 The physicochemical properties of h-Ti 3 C 2 T x filters and the phosphate species were closely correlated to the pH of the solution. Thus, changes in the removal of phosphate in the h-Ti 3 C 2 T x (24 h) electrofiltration mode as a function of solution pH were monitored, as shown in Figure 3d.…”

Origin of Limiting and Overlimiting Currents in Bipolar Membranes

“…Accordingly, BPMs are nowadays mostly used for the production and recovery of inorganic acids and bases from salt solutions (e.g., from NaCl, NaNO 3 , Na 2 SO 4 , and NH 4 F ,− ). However, various other applications have been proposed, including CO 2 capture, , lithium recovery, production of alkoxides (i.e., by alcohol dissociation at the BPM junction), recovery of ammonia from wastewater, and more recently, BPM assisted electrolyzers, , fuel cells, , electrosorption, and batteries. ,− Notably, the latter three applications involve the use of BPMs not only under reverse but also forward bias (CEL facing the anode), to recombine H + and OH – ions to water (Figure a). An acid–base flow battery (ABFB), for example, operates with alternating bias.…”

Rapid, Selective, and Chemical-Free Removal of Dissolved Silica from Water via Electrosorption: Feasibility and Mechanisms

“…38 Moreover, chemical-free electrosorption of boron, which remains uncharged at circumneutral pH and requires high pH for ionization, has also recently been demonstrated. 37,39,40 In flow-through electrosorption, where the feedwater directly passes through the porous electrodes, the local pH variation at the electrodes is intensified compared to flow-by electrode configurations. 41,42 These characteristics make flow-through electrosorption a promising platform for the chemicalfree removal of pH-dependent ions, like silica.…”

“…For instance, the high pH at the cathode during electrosorption has been used to convert ammonium ions into ammonia, facilitating subsequent ammonia recovery . Moreover, chemical-free electrosorption of boron, which remains uncharged at circumneutral pH and requires high pH for ionization, has also recently been demonstrated. ,, …”