Regular ArticleExploring the electrosorption selectivity and recovery of indium ions with capacitive deionization in acidic solution
Graphical abstract
Introduction
Endangered elements, due to their excessive use or limited resources, have been identified by the ACS Green Chemistry Institute [1]. Indium is one of the eight seriously threatened elements because of its relatively scarcity on earth and its high demand in industry. Indium is an essential industrial ingredient of indium-tin-oxide (ITO), which is mainly used for liquid crystal displays (LCDs) and light-emitting diodes (LEDs) [2]. Due to its good conductivity, low resistivity, and optical transparency, the demand and consumption of indium have increased over the past decade. Indium is mainly produced by China (40%), Korea (32%), Japan (10%), and Canada (8%) [3]. As an important strategic resource, indium is categorized by the European Commission as a critical resource [4]. Efficient production, abundant alternatives, and recovery technologies need to be developed to overcome indium scarcity and maintain a sustainable supply. Making a rapid increase in the supply of indium is difficult because indium is produced from a by-product of zinc. Research on alternatives to indium in LCDs also remains limited. Therefore, it is crucial to recycle indium to stabilize the demand for this rare resource. Conventional methods, including thermal separation [5], [6], adsorption [7], ion exchange [8], [9], chemical precipitation [10], [11], membrane separation [12], [13], and extraction [14], [15], have been widely applied to remove indium from aqueous solutions. However, these methods require either excessive chemical use or large energy consumption [6], [16], [17].
Electrosorption, often referred to as capacitive deionization (CDI), is a promising electrochemical technology for removing ions from water through the sorption of ions on oppositely charged electrodes [18], [19]. CDI exhibits several operational advantages, including energy efficiency, as evidenced in the desalination of low-salinity desalination water sources; direct energy recovery; no required chemical additives; long-term reproducibility; and environmental friendliness [20], [21], [22]. These advantages have broadened the applications of CDI to multiple areas, including water softening, selective ion removal, brackish water desalination, and wastewater treatment [23], [24]. The mechanism of electrosorption is based on the electrical double-layer (EDL) capacitance of porous materials. In the charging stage, ions are electrosorbed on the oppositely charged electrode under an external electric field. At the regeneration stage, the electric field is removed to release the electrosorbed ionic species into the solution [25]. Regarding materials, carbons are employed as a key component in the electrosorption process because of their high surface area, good electrical conductivity and great sorption capacity [26], including activated carbon [27], carbon aerogels [28], carbon nanotubes [29], and graphene [30]. Activated carbons are the most widely used material for electrosorption due to their electrochemical stability, mechanical stability and low manufacturing cost. Many studies have shown that the electrosorption process with activated carbon electrodes has been effective in removing unwanted ions such as Na+, Ca2+, Mg2+, Cl−, NO3−, and SO4− [24], [31] from water.
Currently, research to explore the removal of metal ions from aqueous solutions by electrosorption remains limited. Most studies focus on environmentally harmful ions such as Cr3+, Cd2+, and Pd2+ [32], [33], [34]. In terms of endangered metal ions, Oda and Nakagawa [35] presented the removal characteristics of Cu2+ and Zn2+ using electrosorption with activated carbon electrodes at 1 V of applied voltage (pH = 4.9). Li et al. [36] fabricated α-MnO2/carbon fiber paper composites for the electrosorption of Ni2+ and Mn2+ at an applied voltage of −1.3 V. Tang et al. [37] investigated the electrosorption performance of Zn2+ on activated carbon cloth with an applied voltage of 1.2 V (within a pH range of 4.0–5.8). Liu et al. reported [38] that Co2+ can be electrosorbed on activated carbon cloth at various voltages (0.6–1.2 V) with the coexistence of Sr+ and Cs2+. However, electrosorption experiments with metal ions are often accompanied by simultaneous reactions, including electrodeposition and chemical deposition, thus resulting in the surface area of the electrode decreasing after several charge–discharge cycles. Huang et al. [39] pointed out that electrodeposition of Cu2+ could be suppressed at a voltage less than 0.8 V in CDI. Notably, removing ions under acidic conditions is not included in these reported studies. Since most transition metal ions and poor metal ions dissolve only in acidic solution and then precipitate as salts when the pH value increases, it is essential to study the electrosorption performance under various pH conditions. Huyskens et al. [40] conducted electrosorption with a pH = 2.6 influent by electrosorption and reported that H+ derived from acetic acid competed for electrosorption with Na+ during the charging time. Some studies have pointed out that the pH fluctuation during CDI is caused by Faradaic reactions and the oxidation of electrodes [41], [42], [43]; however, most of these electrosorption processes are conducted in neutral solutions. In this case, the electrosorption of H+ is not significant due to the low molar concentration compared with that of the target ion. It is noted that the study of electrosorption selectivity in acidic solutions during CDI still remains poor. Several researchers have shown that the electrosorption selectivity of ions can be determined by the hydrated radius, ionic charge, initial molar concentration, and electrosorption charging time [31], [44], [45], [46]. To the best of our knowledge, the ion selectivity of In3+ in a carbon-based CDI process at various pH values has not been reported.
This study aims to evaluate the feasibility of electrosorption removal of In3+ from aqueous solutions using activated carbon electrodes. To optimize the indium removal efficiency, the effects of various voltages, initial In3+ concentrations, and pH values on the electrosorption performance were studied. To elucidate the interaction between In3+ and the electrode surface, cyclic voltammetry and surface characterization were performed. The CDI system was operated in single-pass mode for consecutive charging–discharging cycles to investigate the electrosorption selectivity between In3+ and H+. In particular, the displacement of electrosorbed ions was observed to enhance the In3+ removal efficiency. This study demonstrates a promising perspective for using CDI as an energy-efficient means to recover indium ions from acidic aqueous solutions.
Section snippets
Preparation and characterization of the electrode
A carbon electrode for CDI was made by mixing activated carbon powder (ACS20, China Steel Chemical Corporation) with polyvinylidene fluoride (PVDF, MW = 534,000, Sigma-Aldrich) polymer binder at a weight ratio of 9 to 1 in an N-methyl-2-pyrrolidone (NMP, MARCON) solution. The carbon slurry was coated on a titanium plate, dried at 120 °C for 2 h and then dried at 80 °C in a vacuum oven for 2 h to remove any residual organic solvent. The weight of the electrode was approximately 0.7 g, and the
Characteristics of electrodes
The pore structure of the prepared AC electrodes was determined by N2 adsorption–desorption and SEM. The specific surface area and total pore volume of the prepared AC electrode were approximately 1368 m2/g and 0.71 cm3/g, respectively. In Fig. S1a, the curves can be classified as a type-IV isotherm with a hysteresis loop, which is attributed to the presence of meso/macropores in the porous structure. Such pore structure may improve the transport of ions and accessibility to the pore surface
Conclusions
The present work provides fundamental knowledge on the removal characteristics of In3+ from acidic solutions during an electrosorption process. The deionization capacity of AC electrodes strongly depends on the applied voltage, initial indium concentration, and initial pH value. At pH values lower than 4, the occurrence of indium deposition on the electrode is restricted, as verified by SEM and EDS characterization. In3+ ions can be adsorbed onto the AC surface by pure electrostatic
CRediT authorship contribution statement
Yu-Yi Shen: Methodology, Investigation, Formal analysis, Writing - original draft. Shan-Wen Wu: Conceptualization, Methodology, Data curation. Chia-Hung Hou: Conceptualization, Project administration, Supervision, Writing - review & editing, Funding acquisition.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
We are grateful for the financial support from the Ministry of Science and Technology, Taiwan (106-2221-E-002-020-MY3; 107-2628-E-002-001-MY3; 109-2223-E-002-002-MY3) and the National Taiwan University from Excellence Research Program-Core Consortiums (109L891206) within the framework of the Higher Education Sprout Project by the Ministry of Education (MOE) in Taiwan.
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