Aggregation behavior of graphitic C3N4 nanosheets in aqueous environment: Kinetics and mechanisms

Effects of Cr(VI) oxyanion, humic acid and solution chemistry on the aggregation and colloidal stability of green synthesized chlorapatite nanoparticles

Aggregation is a crucial process determining the fate, mobility and ecological risks of nanomaterials. Chlorapatite nanoparticles (nClAP) exhibit widely applications in environmental remediation and consequently will inevitably enter aquatic systems. However, the aggregation characteristics of nClAP are still mostly uncovered. This study investigated the aggregation kinetics and colloidal stability of nClAP as a function of pH, humic acid (HA), Cr(VI) oxyanions, monovalent and divalent electrolytes. Results showed that pH values from 5 to 9 had a notable impact on the aqueous behaviors of nClAP. The addition of HA made the zeta potential (ZP) of nClAP more negative and thus enhanced nClAP stability through electrostatic and steric effects. Similarly, the adsorption of Cr(VI) on the surface of nClAP created a physical barrier and negative charge, improving the stability of nClAP by inducing steric force. Lower ZP and hydrodynamic diameter (HDD) reflected that the enhanced stability of nClAP by HA was more significant than Cr(VI). In comparison, the presence of Ca2+ ions were more effective than monovalent Na + ions in promoting the aggregation of nClAP. The classical DLVO theory incorporating the steric repulsion were used to interpret the aggregation and dispersion of nClAP, making it was easier to overcome energy barriers and agglomerate. This study provides new mechanistic insights which could help better understand the effects of Cr(VI) oxyanions and HA on nClAP's colloidal stability.

Photocatalytic degradation of pharmaceuticals by pore-structured graphitic carbon nitride with carbon vacancy in water: Identification of intermediate degradants and effects of active species

Pharmaceuticals are increasingly used in daily life and have been massively discharged to the aquatic environment. The removal of pharmaceuticals from water by various nanomaterials including graphitic carbon nitride (g-C₃N₄) has received extensive attention. Herein, we synthesized a carbon-defective carbon nitride with pore structure through a simple thermal polymerization method for photodegradation of lidocaine, mepivacaine and ropivacaine (typical amide local anesthetics). The results showed that the degradation process conformed to the pseudo-first-order reaction kinetics, and the degradation rate constant of organic pollutants using CCN-600 (i.e., g-C₃N₄ synthesized at 600 °C) reached 5.05 × 10^-2 min^-1, about 2.5 times higher than that of the prototype g-C₃N₄ (2.09 × 10^-2 min^-1). The capture experiment of active species and the electron paramagnetic resonance (EPR) test demonstrated that superoxide radical (O₂^-) played a major role in the degradation process. Based on the possible photodegraded intermediate products identified, the degradation pathways were deduced. This study provides not only a new strategy for fabrication of pore-structured g-C₃N₄ with carbon vacancy, but also a reference method for the treatment of pharmaceuticals in water bodies.

Exfoliation method matters: The microstructure-dependent photoactivity of g-C3N4 nanosheets for water purification

Exfoliation of carbon nitride (g-C₃N₄) into an ultrathin nanostructure significantly improves its photoactivity. However, the effects of the exfoliation method on the microstructure and photocatalytic performance of g-C₃N₄ nanosheets remain largely unknown. Herein, several typical strategies, such as thermal, chemical, ultrasonic and one-step exfoliation, were applied to exfoliate g-C₃N₄ nanosheets for photocatalytic applications. A procedure capable of controlling the morphology, microstructure, light-absorption property, and visible light photoactivity of g-C₃N₄ nanosheets was attempted. We found that nanosheets prepared from one-step exfoliation present superior photocatalytic efficiency under visible light than those fabricated by thermal exfoliation and ultrasonic exfoliation. The kinetic constants for bisphenol A (BPA) photodegradation over these samples were determined to be 6.5, 4.5 and 2.3 times higher than bulk g-C₃N₄, respectively. For chemical exfoliation, excessive oxidation by H₂SO₄ can lead to the structural defects and deactivation of urea-derived g-C₃N₄ nanosheets. Carbon nitride nanosheets synthesized by one-step exfoliation exhibited high specific surface area, optimal band gap energy structure, and high charge separation efficiency, thereby increasing visible-light photoactivity. Enabling cost-effective production of ultrathin and robust g-C₃N₄ nanosheets, one-step exfoliation offers a potential strategy to exploit high-performance g-C₃N₄ for water purification applications.

Boosting photocatalytic degradation of tetracycline under visible light over hierarchical carbon nitride microrods with carbon vacancies

Graphitic carbon nitride is considered as one of the promising photocatalysts for pollution elimination from wastewater. Manipulating the microstructure of carbon nitride remains a challengeable task, which is essential for improving light absorption, separating photogenerated carrier and creating reactive sites. Herein, a carbon vacancy modified hierarchical carbon nitride microrod (CN1.5) has been prepared templated from a melamine-NH₂OH·HCl complex. The hierarchical microrods are demonstrated to be comprised of interconnected nanosheets with rich carbon vacancies, which endows it with high specific surface area, enhanced light utilization efficiency, available reactive sites, improved charge carrier separation and numerous mass-transport channels. The resultant photocatalyst CN1.5 exhibits an excellent photodegradation efficiency of 87.9% towards tetracycline under visible light irradiation. The remarkable apparent rate constant of 4.91 × 10^-2 min^-1 is 7.3 times higher than that of bulk CN. In addition, the degradation pathways are deduced base on the observation of degradation intermediates generating in the photocatalytic process. Mechanism investigation indicates that the major contribution for photodegradation is attributed to ·O₂^-, ¹O₂ and H₂O₂ species. This work provides new insights into advancing carbon nitride's microstructure to improve photocatalytic degradation performance for highly efficient antibiotic removal and environment remediation.

Aggregation kinetics of fragmental PET nanoplastics in aqueous environment: Complex roles of electrolytes, pH and humic acid

The aggregation kinetics of fragmental polyethylene glycol terephthalate (PET) nanoplastics under various chemistry conditions in aqueous environment were firstly investigated in this work. The aggregation of PET nanoplastics increased with increasing electrolyte concentrations and decreasing solution pH, which became stronger with the presence of divalent cations (e.g. Ca²⁺ and Mg²⁺) than that of monovalent cations (e.g. Na⁺ and K⁺). The effect of cations with the same valence on the aggregation of PET nanoplastics was similar. The measured critical coagulation concentrations (CCC) for PET nanoplastics at pH 6 were 55.0 mM KCl, 54.2 mM NaCl, 2.1 mM CaCl₂ and 2.0 mM MgCl₂, which increased to 110.4 mM NaCl and 5.6 mM CaCl₂ at pH 10. In addition, the aggregation of PET nanoplastics was significantly inhibited with the presence of humic acid (HA), and the CCC values increased to 558.8 mM NaCl and 12.3 mM CaCl₂ (1 mg L^-1 HA). Results from this study showed that the fragmental PET nanoplastics had the quite higher CCC values and stability in aqueous environment. In addition, the aggregation behaviors of PET nanoplastics can be successfully predicted by the Derjguin Landau Verwey Overbeek (DLVO) theory.

Mn2+ effect on manganese oxides (MnOx) nanoparticles aggregation in solution: Chemical adsorption and cation bridging

Manganese oxides (MnO_x) and Mn²⁺ usually co-exist in the natural environment, as well as in water treatments for Mn²⁺ removal. Therefore, it is necessary to investigate the influence of Mn²⁺ on the stability of MnO_x nanoparticles, as it is vital to their fate and reactivity. In this study, we used the time-resolved dynamic light scattering technique to study the influence of Mn²⁺ on the initial aggregation kinetics of MnO_x nanoparticles. The results show that Mn²⁺ was highly efficient in destabilizing MnO_x nanoparticles. The critical coagulation concentration ratio of Mn²⁺ (0.3 mM) to Na⁺ (30 mM) was 2^-6.64, which is beyond the ratio range indicated by the Schulze-Hardy rule. This is due to the coordination bond formed between Mn²⁺ and the surface O of MnO_x, which could efficiently decrease the negative surface charge of MnO_x. As a result, in the co-presence of Mn²⁺ and Na⁺, a small amount of Mn²⁺ (5 μM) could efficiently neutralize the negative charge of MnO_x, thereby decreasing the amount of Na⁺, which mainly destabilized nanoparticles through electric double-layer compression, required to initiate aggregation. Further, Mn²⁺ behaved as a cation bridge linking both the negatively charged MnO_x and humic acid, thereby increasing the stability of the MnO_x nanoparticles as a result of the steric repulsion of the adsorbed humic acid. The results of this study enhance the understanding of the stability of the MnO_x nanoparticles in the natural environment, as well as in water treatments.