Interactions between cerium dioxide nanoparticles and humic acid: Influence of light intensities and molecular weight fractions

Distribution characteristics of TiO2 NPs in Daihai lake and their stability regulated by abiotic factors

Accurate detection and stability analysis of titanium dioxide (TiO2 NPs) in surface water are critical for ecological risk assessment. Quantitative analysis of TiO2 NPs in water and sediment of Daihai Lake was performed and their occurrence forms was characterized by TEM, XRD and FTIR. Further investigation into the impact of pH, ionic strength (IS) and dissolved organic matter (DOM) on the stability of TiO2 NPs was carried out, along with a correlation analysis between abiotic factors and TiO2 NPs concentration. Results showed that TiO2 NPs in water and sediment are 1.01 × 105 and 5.66 × 105 particles/mL in July 2023, respectively. After transformed in water sample from Daihai Lake, the hydrodynamic particle (HDD) of TiO2 NPs reach 600-700 nm, with reduced crystallinity and observable characteristic peaks of NaCl, which is related to the high concentration of NaCl in Daihai Lake. Structural defects and changes in surface properties of TiO2 NPs are introduced due to the adsorption of organic matter, leading to higher aggregation and sedimentation rates. Specifically, Ca2+ promote the sedimentation of TiO2 NPs more than Na+. Conversely, A/A0 increased to 0.80-0.94 from 0.64 after humic acid (HA) and fulvic acid (FA) added. Hydroxyl and carboxyl groups of FA enhanced steric hindrance and inhibited TiO2 NPs sedimentation. While a strong promotion effect on the sedimentation of TiO2 NPs is detected when 10 mmol/L Ca2+ coexisted with 10 mg/L HA, with 70% of the TiO2 NPs settled within 120 min. Additionally, the concentration of TiO2 NPs in Daihai Lake is significantly positively correlated with dissolved organic carbon (DOC) and humification index, which might be relate to the enhancement of TiO2 NPs stability of aromatic organic matter. And the high concentration of DOC will also lead to the stable existence of TiO2 NPs in water and exacerbate aquatic toxicity.

Understanding the Interactions of Nanoparticles and Dissolved Organic Matter at the Molecular Level by 1H 2D Multi-Exponential Transverse Relaxation NMR Spectroscopy

The interaction between humic acid (HA) and engineered nanoparticles (NPs) is critical in environmental sciences, especially for understanding the behavior of NPs in natural waters. This study employs 1H 2D Multi-Exponential Transverse Relaxation (METR) NMR spectroscopy to examine the molecular-level interactions between Pahokee Peat humic acid (HA) and carboxyl-functionalized iron oxide nanoparticles (NPCOs). First, 1H 2D METR NMR spectroscopy allowed not only the identification of HA in terms of its chemical composition but also the separation of molecules with the same chemical shift values but different rates of molecular tumbling. Then, using solutions with varying NPCO concentrations (0, 10, 40, and 100 μM), we observed significant changes in the T2 relaxation times of HA components, indicating interactions between HA and NPCO. Analysis showed the biggest effect on two chemical shift regions, corresponding to lipids and carbohydrates, revealing that smaller molecules within these regions exhibit the most significant changes in T2 values upon the addition of NPCO. This suggests that these molecules are the initial sites of interaction, with the entire HA system being affected at higher NPCO concentrations. These findings highlight the utility of METR NMR spectroscopy in studying complex environmental mixtures and provide insights into the behavior of HA and NPs, essential for understanding the fate of NPs in the environment.

Dissolved organic matter promotes photocatalytic degradation of refractory organic pollutants in water by forming hydrogen bonding with photocatalyst

Removing refractory organic pollutants in real water using photocatalysis is a great challenge because coexisting dissolved organic matter (DOM) can quench photogenerated holes and thus prevent generation of reactive oxygen species (ROS). Herein, for the first time, we develop a hydrogen bonding strategy to avoid the scavenging of photoexcited holes, by which DOM even promotes photocatalytic degradation of refractory organic pollutants. Theoretical calculations combined with experimental studies reveal the formation of hydrogen bonding between DOM and a hydroxylated S-scheme heterojunction photocatalyst (Mo-Se/OHNT) consisting of hydroxylated nitrogen doped TiO2 (OHNT) and molybdenum doped selenium (Mo-Se). The hydrogen bonding is demonstrated to change the interaction between DOM and Mo-Se/OHNT from DOM-Ti (IV) to a hydrogen bonded complexation through the hydroxyl/amine groups of DOM and the OHNT in Mo-Se/OHNT. The formed hydrogen network can stabilize excited-state of DOM and inject its electron to the conduction band rather than the valence band of the OHNT upon light irradiation, realizing the key to preventing hole quenching. The electron-hole separation in Mo-Se/OHNT is consequently improved for generating more ROS to be involved in removing refractory organic pollutants. Moreover, this hydrogen bonding strategy is generalized to nitrogen doped zinc oxide and graphitic carbon nitride and applies to real water. Our findings provide a new insight into handling the DOM problem for photocatalytic technology towards water and wastewater treatment.

Effect of microplastics on the binding properties of Pb(ii) onto dissolved organic matter: insights from fluorescence spectra and FTIR combined with two-dimensional correlation spectroscopy

Heavy metal cations are a typical type of inorganic pollutant that has persistent distribution characteristics in aquatic environments and are easily adsorbed on carriers, posing serious threats to ecological safety and human health. Some studies have shown that the coexistence of dissolved organic matter (DOM) and microplastics (MPs) promotes the adsorption of heavy metal cations, but the mechanism of promoting the adsorption process has not been thoroughly studied. In this study, the effect of polystyrene microplastics (PSMPs) on the binding properties of Pb2+ onto humic acid (HA) in aquatic environments was investigated by spectral analysis and two-dimensional correlation (2D-COS) analysis. When PSMPs co-existed with HA, the adsorption capacity of Pb2+ increased. On the one hand, Pb2+ is directly adsorbed on HA through the mechanism of complexation reaction, ion exchange and electrostatic interaction. On the other hand, Pb2+ is first adsorbed on PSMPs by electrostatic action and indirectly adsorbed on HA in the form of PSMPs-Pb2+ owing to the interaction between HA and PSMPs, which increases the adsorption amount of Pb2+ on HA. This study is significant for studying the migration and regression of heavy metal cation contaminants when PSMPs co-exist with DOM in an aqueous environment.

Stability, aggregation, and sedimentation behaviors of typical nano metal oxide particles in aqueous environment

The wide use of nano metal oxide particles (NMOPs) brings about their inevitable release into the water environment, affecting the environment and human health. Therefore, the stability, aggregation, and sedimentation process of four typical NMOPs (ZnO NPs, CeO₂ NPs, TiO₂ NPs, and CuO NPs) were investigated in artificial water and real municipal sewage to reveal their complicated behavior. Results showed that NMOPs aggregated at the pH of zero-charge point, and their hydrodynamic diameters and aggregation rates could reach the maximum values. The hydrodynamic diameters and aggregation rates of ZnO NPs, CeO₂ NPs, TiO₂ NPs, and CuO NPs at the zero-charge point were 617, 1760, 870, 1502 nm, and 31.7, 1158.1, 48.3, 115.7 nm/min, respectively. In addition, the dissolution of NMOPs led to the sedimentation rates under acidic conditions being much lower than those under neutral and alkaline conditions. The aggregation and sedimentation performance of NMOPs were affected by not only pH but also ionic strength (IS) and species. The aggregation rates of NMOPs increased with the increase of IS (0-10 mM), and the maximum aggregation rate of CeO₂ NPs was 470.1 nm/min (pH = 7 and CaCl₂ = 10 mM). According to Coulomb's law, divalent cations (Mg²⁺, Ca²⁺) were more competitively adsorbed on the surface of NMOPs than monovalent cations (K⁺, Na⁺), which increased the zeta potential and aggregation rate of NMOPs. Furthermore, the NMOPs were easier to aggregate in municipal sewage because of the homogeneous aggregation between nanoparticles and heterogeneous aggregation with natural colloids. The total interaction energy between NMOPs was calculated by the classical Derjaguin-Landau-Verwey-Overbeek (DLVO) theoretical formula, which was consistent with the experimental results.

Investigation of the Associations between a Nanomaterial's Microrheology and Toxicology

With the advent of Nanotechnology, the use of nanomaterials in consumer products is increasing on a daily basis, due to which a deep understanding and proper investigation regarding their safety and risk assessment should be a major priority. To date, there is no investigation regarding the microrheological properties of nanomaterials (NMs) in biological media. In our study, we utilized in silico models to select the suitable NMs based on their physicochemical properties such as solubility and lipophilicity. Then, we established a new method based on dynamic light scattering (DLS) microrheology to get the mean square displacement (MSD) and viscoelastic property of two model NMs that are dendrimers and cerium dioxide nanoparticles in Dulbecco's Modified Eagle Medium (DMEM) complete media at three different concentrations for both NMs. Subsequently, we established the cytotoxicological profiling using water-soluble tetrazolium salt-1 (WST-1) and a reactive oxygen species (ROS) assay. To take one step forward, we further looked into the tight junction properties of the cells using immunostaining with Zonula occluden-1 (ZO-1) antibodies and found that the tight junction function or transepithelial resistance (TEER) was affected in response to the microrheology and cytotoxicity. The quantitative polymerase chain reaction (q-PCR) results in the gene expression of ZO-1 after the 24 h treatment with NPs further validates the findings of immunostaining results. This new method that we established will be a reference point for other NM studies which are used in our day-to-day consumer products.

Enhanced SO2 Absorption Capacity of Sodium Citrate Using Sodium Humate

A novel method of improving the SO2 absorption performance of sodium citrate (Ci-Na) using sodium humate (HA–Na) as an additive was put forward. The influence of different Ci-Na concentration, inlet SO2 concentration and gas flow rate on desulfurization performance were studied. The synergistic mechanism of SO2 absorption by HA–Na and Ci-Na was also analyzed. The consequence shows that the efficiency of SO2 absorption by Ci-Na is above 90% and the desulfurization time added with the Ci-Na concentration rising from 0.01 to 0.1 mol/L. Both the desulfurization efficiency and time may increase with the adding of HA–Na quality in Ci-Na solution. Due to adding HA–Na, the desulfurization efficiency of Ci-Na increased from 90% to 99% and the desulfurization time increased from 40 to 55 min. Under the optimum conditions, the desulfurization time of Ci-Na can exceed 70 min because of adding HA–Na, which is nearly doubled. The growth of inlet SO2 concentration has little effect on the desulfurization efficiency. The SO2 adsorption efficiency decreases with the increase of inlet flow gas. The presence of O2 improves the SO2 removal efficiency and prolongs the desulfurization time. Therefore, HA–Na plays a key role during SO2 absorption and can dramatically enhance the SO2 adsorption performance of Ci-Na solution.