Rapid and selective adsorption of organic dyes with ultrahigh adsorption capacity using Na and Fe co-doped g-C3N4

Singlet Oxygen Formation Mechanism for the H2O2-Based Fenton-like Reaction Catalyzed by the Carbon Nitride Homojunction

The singlet oxygen (1O2) oxidation process activated by metal-free catalysts has recently attracted considerable attention for organic pollutant degradation; however, the 1O2 formation remains controversial. Simultaneously, the catalytic activity of the metal-free catalyst limits the practical application. In this study, carbon nitride (HCCN) containing an intramolecular homojunction, a kind of metal-free catalyst, exhibits excellent activity compared to g-C3N4 (CN) and crystalline carbon nitride (HCN) for tetracycline hydrochloride degradation through the H2O2-based Fenton-like reaction. The rate constant for HCCN increased about 16.1 and 8.9 times than that of CN and HCN, respectively. The activity of HCCN was enhanced, and the dominant reactive oxygen species (ROS) changed from hydroxyl radicals (•OH) to 1O2 with an increase in pH from 4.5 to 11.5. A novel formation pathway of 1O2 was revealed. This result is different from the normal reference, in which •OH is always the primary ROS in the H2O2-based Fenton-like reaction. This study may provide a possible strategy for the investigation on the nonradical oxidation process in the Fenton-like reaction.

Selective adsorption of organic dyes from aqueous environment using fermented maize extract-enhanced graphene oxide-durian shell derived activated carbon composite

A secure aquatic environment is essential for both aquatic and terrestrial life. However, rising populations and the industrial revolution have had a significant impact on the quality of the water environment. Despite the implementation of strong and adapted environmental policies for water treatment worldwide, the issue of organic dyes in wastewater remains challenging. Thus, this study aimed to develop an efficient, cost-effective, and sustainable material to treat methylene blue (MB) in an aqueous environment. In this research, maize extract solution (MES) was utilized as a green cross-linker to induce precipitation, conjugation, and enhance the adsorption performance of graphene oxide (GO) cross-linked with durian shell activated carbon (DSAC), resulting in the formation of a GO@DSAC composite. The composite was investigated for its adsorptive performance toward MB in aqueous media. The physicochemical characterization demonstrated that the cross-linking method significantly influenced the porous structure and surface chemistry of GO@DSAC. BET analysis revealed that the GO@DSAC exhibited dominant mesopores with a surface area of 803.67 m2/g. EDX and XPS measurements confirmed the successful cross-linking of GO with DSAC. The adsorption experiments were well described by the Harkin-Jura model and they followed pseudo-second order kinetics. The maximum adsorption capacity reached 666.67 mg/g at 318 K. Thermodynamic evaluation indicated a spontaneous, feasible, and endothermic in nature. Regenerability and reusability investigations demonstrated that the GO@DSAC composite could be reused for up to 10 desorption-adsorption cycles with a removal efficiency of 81.78%. The selective adsorptive performance of GO@DSAC was examined in a binary system containing Rhodamine B (RhB) and methylene orange (MO). The results showed a separation efficiency (α) of 98.89% for MB/MO and 93.66% for MB/RhB mixtures, underscoring outstanding separation capabilities of the GO@DSAC composite. Overall, the GO@DSAC composite displayed promising potential for the effective removal of cationic dyes from wastewater.

Acetamide- or Formamide-Assisted In Situ Approach to Carbon-Rich or Nitrogen-Deficient Graphitic Carbon Nitride for Notably Enhanced Visible-Light Photocatalytic Redox Performance

Acetamide- or formamide-assisted in situ strategy is designed to synthesize carbon atom self-doped g-C₃ N₄ (AHCN_x ) or nitrogen vacancy-modified g-C₃ N₄ (FHCN_x ). Different from the direct copolymerization route that suffers from the problem of mismatched physical properties of acetamide (or formamide) with urea, the synthesis of AHCN_x (or FHCN_x ) starts from a crucial preorganization step of acetamide (or formamide) with urea via freeze drying-hydrothermal treatment so that the chemical structures as well as C-doping level in AHCN_x and N-vacancy concentration in FHCN_x can be precisely regulated. By using various structural characterization methods, well-defined AHCN_x and FHCN_x structures are proposed. At the optimal C-doping level in AHCN_x or N-vacancy concentration in FHCN_x , both AHCN_x and FHCN_x exhibit remarkably improved visible-light photocatalytic performance in oxidation of emerging organic pollutants (acetaminophen and methylparaben) and reduction of proton to H₂ in comparison of unmodified g-C₃ N₄ . Combination of the experimental results with theoretical calculations, it is confirmed that AHCN_x and FHCN_x show different charge separation and transfer mechanisms, while the enhanced visible-light harvesting capacity and the localized charge distributions on HOMO and LUMO are responsible for this excellent photocatalytic redox performance of AHCN_x and FHCN_x .

Tuning Band Gap in Fe-Doped g-C3N4 by Zn for Enhanced Fenton-Like Catalytic Performance

Multiple oxidation states of first-row transition-metal cations were always doped in g-C₃N₄ to enhance the catalytic activity by the synergistic action between the cations in the Fenton-like reaction. It remains a challenge for the synergistic mechanism when the stable electronic centrifugation (3d¹⁰) of Zn²⁺ was used. In this work, Zn²⁺ was facilely introduced in Fe-doped g-C₃N₄ (named xFe/yZn-CN). Compared with Fe-CN, the rate constant of the tetracycline hydrochloride (TC) degradation increased from 0.0505 to 0.0662 min^-1 for 4Fe/1Zn-CN. The catalytic performance was more outstanding than those of similar catalysts reported. The catalytic mechanism was proposed. With the introduction of Zn²⁺ in 4Fe/1Zn-CN, the atomic percent of Fe (Fe²⁺ and Fe³⁺) and the molar ratio of Fe²⁺ to Fe³⁺ at the catalyst's surface increased, where Fe²⁺ and Fe³⁺ were the active sites for adsorption and degradation. In addition, the band gap of 4Fe/1Zn-CN decreased, leading to enhanced electron transfer and conversion from Fe³⁺ to Fe²⁺. These changes resulted in the excellent catalytic performance of 4Fe/1Zn-CN. Radicals •OH, •O₂^-, and ¹O₂ formed in the reaction and took different actions under various pH values. 4Fe/1Zn-CN exhibited excellent stability after five cycles under the same conditions. These results may give a strategy for synthesizing Fenton-like catalysts.

Peroxidase-like activity of hollow sphere-like FeS2/SC boosted by synergistic action of defects and S-C bonding

Pyrite FeS₂ has been applied as a peroxidase due to its easy preparation and low cost. However, the low peroxidase-like (POD) activity limited its wide application. A hollow sphere-like composite (FeS₂/SC-5.3%) composed of pyrite FeS₂ and sulfur-doped hollow sphere-shaped carbon was synthesized by a facile solvothermal method, where the S-doped carbon was in situ formed during FeS₂ formation. The synergistic action such as the defects at the carbon surface and the formation of S-C bonding improved the nanozyme activity. The S-C bonding was a bridge between the carbon and the Fe atom in FeS₂, which enhanced the electron transfer between the Fe atom and the carbon and accelerated the conversion from Fe³⁺ to Fe²⁺. The optimum experimental conditions were obtained by the response surface methodology (RSM). The POD-like activity of FeS₂/SC-5.3% was significantly improved compared to that of FeS₂. The Michaelis-Menten constant (K_m) of FeS₂/SC-5.3% is 80 times lower than that of horseradish peroxidase (HRP, natural enzyme). FeS₂/SC-5.3% can be used to detect cysteine (Cys) with a limit of detection (LOD) as small as 0.061 μM at room temperature in only 1 min.