Recent progress in chromium removal from wastewater using covalent organic frameworks - A review

Cellulose/covalent organic framework aerogel for efficient removal of Cr(VI): Performance and mechanism study

Cellulose composites have exceptional qualities, particularly in removing heavy metal ions. Nevertheless, these materials' poor mechanical qualities and the restricted exposure of surface-active sites reduce the effectiveness of their removal. The removal efficiency of adsorbent materials largely depends on their macroscopic structural characteristics. This study successfully developed a novel cellulose composite aerogel adsorbent (TBPM). TBPM aerogel possesses not only numerous active sites but also excellent mechanical strength. It is particularly suitable for the efficient removal of hexavalent chromium (Cr(VI))-containing wastewater. The aerogel exhibited a low density of 0.0238 g/cm3 and high porosity of 98.51 %. Incorporating the covalent organic framework (BT-Dg) increased the active sites in the composite aerogel, enhancing its adsorption performance. Compared with other common heavy metal ion adsorbents, the TBPM aerogel demonstrated superior removal performance, with a maximum adsorption capacity of 411.12 mg/g and a removal efficiency of 95.01 % in Cr(VI) solution at 15 °C, pH = 3. The adsorption of Cr(VI) followed pseudo-second-order kinetics and the Langmuir isotherm model. Even after seven adsorption-desorption cycles, TBPM aerogel maintained over 80 % removal efficiency. In addition, the TBPM aerogel successfully filtered 3439 mL of Cr(VI)-containing wastewater. This composite aerogel expands the application of cellulose composites in purifying wastewater.

Polydopamine-integrated cellulose/graphene oxide monoliths: A versatile platform for efficient continuous-flow iodine capture and photothermal-enhanced reduction of Cr(VI)

The global challenge of wastewater contamination, especially from persistent pollutants like radioactive isotopes and heavy metals, demands innovative purification solutions. Radioactive iodine isotopes (131I and 129I), stemming from nuclear activities, pose serious health risks due to their mobility, bioaccumulation, and ionizing radiation, particularly impacting thyroid health. Similarly, hexavalent chromium, Cr(VI), is highly toxic and persistent in water, linked to cancer and other severe health issues. Developing effective technologies for iodine capture and Cr(VI) reduction is therefore critical for public health and environmental protection. This study presents two distinct cellulose-based composite materials tailored for environmental remediation: cellulose/graphene oxide/polydopamine (cellulose/GO/PDA) monoliths for iodine capture and cellulose/graphene oxide/polydopamine/palladium nano-crystals (cellulose/GO/PDA/Pd) monoliths for the reduction of Cr(VI). PDA substantially enhances the adsorptive, catalytic and photothermal properties of monoliths. The monoliths demonstrated exceptional performance in both batch and continuous-flow reactor studies. Complete iodine removal was achieved within 15 s, while Cr(VI) was entirely reduced within 9 min under dark conditions and 5 min under photothermal conditions. Continuous-flow experiments showed sustained iodine adsorption of 92 % and Cr(VI) reduction of 81 % over 240 min. This research highlights the potential of PDA-enhanced cellulose-based composites as highly efficient and sustainable platforms for practical water remediation and environmental protection.

Surfactant-modified zein nanoparticles adsorbents for ultrafast and efficient removal of Cr(VI)

The adsorption and removal of heavy metal ions Cr(VI) is of great significance for human health and ecological environment. Here, an ultrafast and high efficient adsorbent for Cr(VI) was developed based on cetyltrimethylammonium bromide (CTAB)-modified zein nanoparticles (C-ZNPs). In comparison to pristine zein nanoparticles (ZNPs) (11.199 m2 g-1), the surfactant-modified C-ZNPs exhibited larger specific surface area (17.002 m2 g-1). Moreover, C-ZNPs had superior dispersion and more positive charge distribution, which contributed to the improvement for adsorption performance. The results showed that the saturated adsorption of Cr(VI) was reached up to 192.27 mg/g using the C-ZNPs nanosorbent at T = 298 K, pH = 4, t = 10s, and C0 = 125 mg/L. The removal rate was significantly faster than that reported natural polymer-based adsorbents. The experimental values were followed Freundich isothermal model and pseudo-second-order kinetic model, indicating that the adsorption occurred primarily through a multimolecular layer adsorption process, with a strong emphasis on chemisorption. Mechanistic investigations further revealed that the adsorption of Cr(VI) onto C-ZNPs was mediated by various interactions, including electrostatic attraction, complexation, and ion exchange. These findings provide insights into the efficient removal of Cr(VI) by C-ZNPs and suggest potential applications in water treatment and environmental remediation.

Construction of controlled hyper-crosslinked nanofibrous tubes for Cr(VI) removal: Response surface, kinetics, and isotherm

Porous organic polymers (POPs) exhibit significant potential for adsorbing toxic metal ions in wastewater. Developing POPs with controlled morphologies is a pivotal direction in this field. This study synthesized a series of novel hyper-crosslinked nanofibrous tubes designated HCNT-Cn (n = 4, 8, 12, 16) via Friedel-Crafts alkylation and quaternization reactions. These reactions were fine-tuned through a post-synthetic strategy involving varying alkyl chain lengths. These materials were characterized using FT-IR, SEM, N₂ adsorption-desorption isotherms, among others, and they were specifically evaluated for their ability to adsorb Cr(VI). Among the variants, HCNT-C₄ exhibited the highest specific surface area (495.26 m2 g-1), superior hydrophilicity (CA = 48.7°), and optimal adsorption performance. The adsorption kinetics of HCNT-C₄ conformed to a pseudo-second-order model, while its adsorption isotherm aligned with the Langmuir model. An investigation into the impact of Cr(VI) removal was conducted using three independent variables in a Central Composite Design (CCD) response surface model, revealing that under optimal conditions, the Cr(VI) removal efficiency reached 98%. Additionally, a mechanism for Cr(VI) adsorption on HCNT-C₄ was proposed. It was also found that HCNT-C₄ could be reused up to four times, maintaining a removal efficiency of 70%. This study suggests potential applications for removing Cr(VI) from contaminated wastewater.

Dipole moment regulation for enhancing internal electric field in covalent organic frameworks photocatalysts

Covalent organic frameworks (COFs) have recently emerged as a kind of promising photocatalytic platform in addressing the growing threat of trace pollutants in aquatic environments. Along this, we propose a strategy of constructing internal electric field (IEF) in COFs through the dipole moment regulation, which intrinsically facilitates the separation and transfer of photogenerated excitons. Two COFs of BTT-TZ-COF and BTT-TB-COF are developed by linking the electron-donor of benzotrithiophene (BTT) block and the electron-acceptor of triazine (TZ) or tribenzene (TB) block, respectively. DFT calculations demonstrate TZ block with larger dipole moment can achieve more efficient IEF due to the stronger electron-attractive force and hence narrower bandgap. Moreover, featuring the highly-order crystalline structure for accelerating photo-excitons transfer and rich porosity for facilitating the adsorption, BTT-TZ-COF exhibited an excellent universal performance of photocatalytic degradations of various dyes. Specifically, a superior photodegradation efficiency of 99% Rhodamine B (RhB) is achieved within 20 min under the simulated sunlight. Therefore, this convenient construction approach of enhanced IEF in COFs through rational regulation of the dipole moment can be a promising way to realize high photocatalytic activity.

Enhancement of Mass Transfer Process for Photocatalytic Reduction in Cr(VI) by Electric Field Assistance

The removal of Cr(VI), a highly-toxic heavy metal, from industrial wastewater is a critical issue in water treatment research. Photocatalysis, a promising technology to solve the Cr(VI) pollution problem, requires urgent and continuous improvement to enhance its performance. To address this need, an electric field-assisted photocatalytic system (PCS) was proposed to meet the growing demand for industrial wastewater treatment. Firstly, we selected PAF-54, a nitrogen-rich porous organic polymer, as the PCS's catalytic material. PAF-54 exhibits a large adsorption capacity (189 mg/g) for Cr(VI) oxyanions through hydrogen bonding and electrostatic interaction. It was then coated on carbon paper (CP) and used as the photocatalytic electrode. The synergy between capacitive deionization (CDI) and photocatalysis significantly promotes the photoreduction of Cr(VI). The photocatalytic performance was enhanced due to the electric field's influence on the mass transfer process, which could strengthen the enrichment of Cr(VI) oxyanions and the repulsion of Cr(III) cations on the surface of PAF-54/CP electrode. In addition, the PCS system demonstrates excellent recyclability and stability, making it a promising candidate for chromium wastewater treatment.