Bifunctional Ionic Covalent Organic Networks for Enhanced Simultaneous Removal of Chromium(VI) and Arsenic(V) Oxoanions via Synergetic Ion Exchange and Redox Process

Abstract

Chromium (VI) and arsenic (V) oxoanions are major toxic heavy metal pollutants in water threatening both human health and environmental safety. Herein, the development is reported of a bifunctional ionic covalent organic network (iCON) with integrated guanidinium and phenol units to simultaneously sequester chromate and arsenate in water via a synergistic ion‐exchange‐redox process. The guanidinium groups facilitate the ion‐exchange‐based adsorption of chromate and arsenate at neutral pH with fast kinetics and high uptake capacity, whereas the integrated phenol motifs mediate the Cr(VI)/Cr(III) redox process that immobilizes chromate and promotes the adsorption of arsenate via the formation of Cr(III)‐As(V) cluster/complex. The synergistic ion‐exchange‐redox approach not only pushes high adsorption efficiency for both chromate and arsenate but also upholds a balanced Cr/As uptake ratio regardless of the change in concentration and the presence of interfering oxoanions.

Guanidine-Based Covalent Organic Frameworks: Cooperation between Cores and Linkers for Chromic Sensing and Efficient CO2 Conversion

“…In COFs, the weak chromic effects may be obscured by the strong auxochromic effects of π-conjugating expansion and interlayer interactions, so TG-PH and TG-PY do not show significant visual response to acid or base. Despite the known amphoteric nature of the guanidine unit, acid-/base-responsive chromic properties have not been reported for previous guanidine-based COFs. − …”

Guanidinium-Based Ionic Covalent-Organic Nanosheets for Sequestration of Cr(VI) and As(V) Oxoanions in Water

“…However, the material's response to stimuli (base and acid) allows for a convenient regeneration and reuse of these guanidinium-based nanosheets. For example, we have demonstrated in a recent study 33 that the bound Cr(VI) and As(V) anions in iCON III can be effectively released by treatment with dilute NaOH solution (0.02 M). Afterward, the deprotonated/neutral anion binding sites can be easily restored/reprotonated by treatment with dilute HCl (0.5 M).…”

“…Chromium (Cr) and arsenic (As) are among the major highly toxic and carcinogenic inorganic pollutants present in aquatic systems posing severe threats to human health and environmental safety. − Stringent guidelines have been issued on the limit of chromium (50 ppb) and arsenic (10 ppb) contents in drinking water by The World Health Organization . Under oxidative conditions, waterborne arsenic and chromium species can easily persist as the highly mobile oxoanions at neutral pH, such as (di)hydrogen arsenate (H 2 AsO 4 – and HAsO 4 2– ) and chromate (CrO 4 2– ), presenting a significant challenge in cleanup. , Hence, enormous research interest has been directed toward the effective and selective removal of arsenate and chromate anions in water. − Ion exchange is one of the most common and effective strategies to sequester toxic oxoanions in groundwater and wastewater. − However, conventional inorganic and resin ion exchange systems often have limited efficiency and selectivity toward certain oxoanions of interest due to the lack of well-defined anion recognition/binding elements. − The emerging ionic organic frameworks present a unique venue to effectively incorporate recognition/binding groups into a chemically robust, lightweight, well-structured medium to access novel ion exchange materials. − A number of ionic metal–organic frameworks − and ionic covalent-organic frameworks − with well-defined binding sites have been successfully developed to selectively capture and remove various oxometallate pollutants in water.…”

Super-fast iodine capture by an ionic covalent organic network (iCON) from aqueous and vapor media

“…As shown in figure 1 67 To assess the crystallinity of iCON-4, powder X-ray diffraction experiment was performed. The absence of any strong diffraction peak in PXRD spectra revealed the non-crystalline nature of iCON-4(figure 1c).…”