Electrochemical removal of amphoteric ions

Abstract

Several harmful or valuable ionic species present in seawater, brackish water, and wastewater are amphoteric, weak acids or weak bases, and, thus, their properties depend on local water pH. Effective removal of these species can be challenging for conventional membrane technologies, necessitating chemical dosing of the feedwater to adjust pH. A prominent example is boron, which is considered toxic in high concentrations and often requires additional membrane passes to remove during seawater desalination. Capacitive deionization (CDI) is an emerging membraneless technique for water treatment and desalination, based on electrosorption of salt ions into charging microporous electrodes. CDI cells show strong internally generated pH variations during operation, and, thus, CDI can potentially remove pH-dependent species without chemical dosing. However, development of this technique is inhibited by the complexities inherent to the coupling of pH dynamics and ion properties in a charging CDI cell. Here, we present a theoretical framework predicting the electrosorption of pH-dependent species in flow-through electrode CDI cells. We demonstrate that such a model enables insight into factors affecting species electrosorption and conclude that important design rules for such systems are highly counterintuitive. For example, we show both theoretically and experimentally that for boron removal, the anode should be placed upstream and the cathode downstream, an electrode order that runs counter to the accepted wisdom in the CDI field. Overall, we show that to achieve target separations relying on coupled, complex phenomena, such as in the removal of amphoteric species, a theoretical CDI model is essential.

Electrochemical Methods for Water Purification, Ion Separations, and Energy Conversion

“…Ions constitute a major subset of contaminants found in water. When present even at low concentration, ions like F – , CrO 4 2– , AsO 4 3– , Hg 2+ , and Pb 2+ can pose a threat to the health of humans and animals. − For this reason, researchers have studied and developed platforms for targeted removal of ionic contaminants using CDI. − Selective adsorption by CDI can also be employed to recover valuable elements, such as lithium, − phosphorus, − and nitrogen. ,− In this section, we briefly review several experimental works that focus on selective separation of ions from multicomponent solutions using porous carbon electrodes. In particular, we focus on studies that involve either two monovalent ions or one monovalent and one divalent ion, and we exclude studies that involve mixtures of more than two competing ions because of the complexity of these systems. − We then discuss the quantification of ion selectivity via a separation factor.…”

Boron Removal from Reverse Osmosis Permeate Using an Electrosorption Process: Feasibility, Kinetics, and Mechanism

“…While capacitive ion storage is a predominant electrochemical process in an electrosorption system, inevitable Faradaic reactions (e.g., water electrolysis and oxygen reduction) may, depending on the applied potential, also occur simultaneously. , While these reactions may have a negative influence on the electrosorption performance and system stability, they could also be utilized beneficially for the removal and recovery of amphoteric ions such as ammonium, phosphate, and borate. For instance, significant pH fluctuation occurring during the electrosorption process, if managed appropriately, could be used for selective removal or recovery of specific amphoteric ions. , In our previous studies, a flow-electrode capacitive deionization (FCDI) apparatus was incorporated with a gas-permeable membrane (GPM) contactor to extract ammonium, a typical amphoteric ion, from both low and high-strength wastewaters. Taking advantage of the increase in catholyte pH that occurred as a result of Faradaic reactions, ammonium ions that migrate to the cathode chamber will be deprotonated and transformed into uncharged ammonia molecules, which can selectively pass the GPM and be absorbed by an acid solution on the upstream side of the GPM and fixed as a salt such as ammonium sulfate. , …”

“…Boron can undergo a similar conversion as a result of the occurrence of Faradaic reactions and, potentially, be removed in an electrosorption system without the requirement for chemical additives. While this concept has been examined to some extent in recent studies, challenges still exist in developing a workable approach to boron removal. , For instance, Shocron et al developed a design rule (i.e., that the anode should be placed upstream of the cathode) for boron removal in a flow-through electrosorption system based on the outcomes of theoretical modeling. However, the primary challenge lies in the in situ generation of large internal pH gradients in the system such that complete conversion of the neutral B­(OH) 3 molecule into negatively charged B­(OH) 4 – ions, a prerequisite for boron migration in an electrical field, occurs.…”

“…For instance, significant pH fluctuation occurring during the electrosorption process, if managed appropriately, could be used for selective removal or recovery of specific amphoteric ions. 31,32 In our previous studies, a flow-electrode capacitive deionization (FCDI) apparatus was incorporated with a gaspermeable membrane (GPM) contactor to extract ammonium, a typical amphoteric ion, from both low and high-strength wastewaters. Taking advantage of the increase in catholyte pH that occurred as a result of Faradaic reactions, ammonium ions that migrate to the cathode chamber will be deprotonated and transformed into uncharged ammonia molecules, which can selectively pass the GPM and be absorbed by an acid solution on the upstream side of the GPM and fixed as a salt such as ammonium sulfate.…”

Square-Wave Alternating Voltage for Enhancing Cr(VI) Reduction in a Carbon Fiber-Based Flow-Through Electrode System

“…However, the Cr(VI) anion (HCrO 4 − or CrO 4 2− ) reduction efficiency was strongly suppressed by poor accessibility to the cathode due to electrostatic repulsion. 9,14 As an influent solution was pumped continuously through the FES under a direct voltage power supply, the electrochemical removal of amphoteric ions was significantly influenced by the order of the anode and cathode, 15 which were involved with the sequential reduction−oxidation (Red-Ox) or oxidation−reduction (Ox-Red) systems. Additionally, it was well known that the anodic reaction was related to the adsorption of HCrO 4 − (the major Cr(VI) species at pH values between 2.0 and 4.0), 16 which was also accompanied by generating H + and O 2 species via H 2 O oxidation.…”