Ion Exchange Conversion of Na-Birnessite to Mg-Buserite for Enhanced and Preferential Cu²⁺ Removal via Hybrid Capacitive Deionization

Abstract

Layered manganese oxides (LMOs) have recently been demonstrated to be one of the most promising redox-active material platforms for electrochemical removal of heavy metal ions from solution via capacitive deionization (CDI). However, the impact of interlayer spacing of LMOs on the deionization performance of electrodes in a hybrid capacitive deionization (HCDI) system with an LMO cathode and a carbon anode (i.e., LMO/C electrodes), and their phase transformation behaviors, particularly during the desalination operations, have yet to be extensively evaluated. In this study, we thoroughly evaluate Mg-buserite obtained by ion exchange of fresh Na-birnessite and Na- and K-birnessite as HCDI electrodes to remove copper ions (Cu2+) from saline solutions. Among the three LMO/C electrodes, the Mg-buserite/C (MgB/C) electrodes demonstrate the best deionization performance in terms of salt adsorption capacity (SAC), electrosorption rate, and cycling stability, followed by K-birnessite/C (KB/C) and Na-birnessite/C (NaB/C). More importantly, MgB/C exhibits the highest Cu2+ ion adsorption capacity (IAC) of 89.3 mg Cu2+ per gram electrode materials at a cell voltage of 1.2 V in 500 mg L–1 CuCl2 solution, with an IAC retention as high as 96.3% after 60 charge/discharge cycles. Given that electrosorption of Cu2+ ions is often competed by alkali and alkaline earth metal ions, our data reveal that the MgB/C electrodes demonstrate selectivities of 4.7, 7.7, and 8.1 for Cu2+ over Na+, Ca2+, and Mg2+, respectively. Moreover, X-ray diffraction and spectroscopic analyses show that the enhanced deionization performance and preference for Cu2+ are mainly attributed to the expanded interlayer spacing of LMO minerals. This study provides a promising strategy for tailoring LMO minerals for improving their electrosorption capacity and preference for copper ions from a multivalent-ion solution via an HCDI platform.

Structural/Compositional‐Tailoring of Nickel Hexacyanoferrate Electrodes for Highly Efficient Capacitive Deionization

“…[42] Additionally, the NiHCF-NF electrode shows the maximum integrated area under the relevant CV curve, implying the highest ion storage capacity and thereby the superior desalination performance. [37,43] These findings are in good agreement with the GCD results shown in Figure 3b, where the NiHCF-NF electrode exhibits the longest charge/discharge plateau at a specific current of 1 A g −1 and voltage from 0.6 to 0.3 V, demonstrating the greatest specific capacity among these electrodes. [32,44] It is noteworthy that all the GCD curves at varying specific currents (Figure S7a−e, Supporting Information) are featured by an obvious charge/discharge plateau with a high degree of symmetry, indicating high reversibility and Faraday behavior of these electrodes.…”

“…This observation is likely to ascribe to the unique hierarchical structure and the high crystallinity of the NiHCF-NF electrode, allowing for much more amount of sodium ions being intercalated and subsequently stored. [33,37] Specifically, the nanoframe-structured NiHCF with smaller particle size and higher surface area than pristine NiHCF cubes, serving as host for intercalated ions, can provide sufficient electrode/electrolyte contact area and reduced diffusion length for electrons and Na + ions transfer to realize the higher desalination capacity. In addition, the preferential etching of NiHCF yielded the lowest surface Fe/Ni ratio of 1.14 (Table S1, Supporting Information), indicating the highest concentration of surface defects that can act as additional reaction sites for Na + capturing.…”

“…As PBA electrodes often exhibit high ion selectivity when applied to CDI, [16,18,50,51] the mono/divalent ion selectivity of these NiHCF electrodes was also comprehensively explored at 1.2 V in feedwaters with varying binary ions (i.e., F1: Ca 2+ / Na + = 10:10 mm; F2: Ca 2+ /Na + = 30:10 mm; F3: Mg 2+ /Na + = 10:10 mm; F4: Mg 2+ /Na + = 30:10 mm), following the procedure in our previous work. [37] The selectivity, or preference of monovalent ions (e.g., Na + ) over divalent ions (e.g., Ca 2+ ) is quantified by a separation factor, β M/D (defined as the ratio of the percent of monovalent ions removed to the percent of divalent ions removed simultaneously). [18] As shown in Figure 6f, the change in concentration (i.e., Δc) of Na + ions varied in the range of 2.3-2.6 mm, regardless of its initial relative concentration with the divalent ions (i.e., the Na + /D 2+ ratio, D 2+ = Ca 2+ , Mg 2+ ), whereas the corresponding Δc of Ca 2+ and Mg 2+ oscillated over the range of 0.6-0.9 mm, and 0.4-0.6 mm, respectively, yielding an average separation factor (β) of 3.34, 8.87, 5.38, and 13.13 for feedwaters F1, F2, F3, and F4, respectively.…”

Unveiling the role of water in enhancing the performance of zinc-ion batteries using dimethyl sulfoxide electrolyte and the manganese dioxide cathode

“…Thus, it is assumed that the d-MnO 2 (birnessite) cathode was transformed into the water-rich MnO 2 having a layered structure (Zn-buserite) for the 5 wt% sample. 37,38 This characteristic is the same as found in the water-based ZIBs. 35 To observe the real-time evolution of the d-MnO 2 cathode, operando XRD was conducted.…”

Efficient and Superstable Mineralization of Toxic Cd2+ Ions through Defect Engineering in Layered Double Hydroxide Nanosheets

“…Cadmium (Cd) is a heavy metal with high toxicity . Wastewater and agricultural soils polluted with Cd 2+ ions have become a global issue, owing to the fact that cadmium can accumulate in organisms throughout the food chain and a serious threat to ecosystems and human health. − Recently, a wide range of methods have been explored for cadmium removal from water and soils including adsorption, chemical precipitation, membrane filtration, ion exchange, electro-kinetic extraction, solidification, phytoremediation, and soil washing. − However, their large-scale application is limited due to their high cost, stability, and operational difficulties in practical settings. − Accordingly, adsorbent materials such as carbon-based materials, zeolites, and metal–organic frameworks remain the preferred materials for Cd 2+ capture and remediation. − However, the cadmium adsorption capacity and stability of current adsorbents are still not satisfactory, motivating the search for novel adsorbents that can selectively immobilize cadmium ions with very high adsorption capacities …”