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.