Micro-Interfacial Molecular Adsorption Behavior Of AgNPs In Sediment And Its Toxic Effects On Algae

Silver Nanoparticles（AgNPs）are highly toxic metallic nanomaterials,which inevitably pose serious ecological risks during migration in the environment.Due to the scale effects,the intersorption of AgNPs with environmental components is regarded as the most fundamental environmental behavior and therefore profoundly influences the phase distribution,complexation,precipitation,redox and biotransformation.However,the adsorption mechanism of AgNPs in sediment is still unclear and its stress mechanism on algal is relatively scarce.To this end,we explored a method to prepare AgNPs with a stable size.The adsorption behavior and mechanism of AgNPs with silica and humic acid were separately studied by using characterization techniques and Density Functional Theory（DFT）.Besides,the toxic effect of AgNPs on algae was analyzed from a metabolomic perspective.The specific research contents of this study were as follows:（1）The synthesis mechanism of stable size and well-dispersed spherical AgNPs was investigated.The spherical AgNPs were prepared based on the coupling effects of PVP and STPP.During the preparation process,the low-solubility Ag₅P₃O₁₀ formed from STPP and the steric hindrance of PVP jointly limited the reduction rate of Ag⁺,preventing the uneven particle size distribution caused by the excessive reduction rate and stabilizing the particle size.Meanwhile,PVP limited the polynegative effect of P₃O₁₀^5-and prevented the linear induction of P₃O₁₀^5-,stabilizing the spherical structure.（2）The main components of the sediment were identified and the factors affecting the adsorption of AgNPs on sediment were studied.The main components of sediment were silica,heavy metals and humic acid.The optimal temperature and pH value for the adsorption were 25℃ and 6.When the temperature was too high the excessive energy would lead to the desorption of AgNPs,while too low energy would not ensure the occurrence of adsorption.When the acidity was too strong the structure of AgNPs would be destroyed,and when the alkalinity was too strong the electrostatic effect of the adsorption system would be weakened,which was not conducive to adsorption.（3）Based on the atomic interaction relationship,the mechanism of AgNPs adsorption on silica was investigated.Multiple characterization techniques（XPS,SERS,XRD,UV-vis）were combined with DFT calculations to analyse the adsorption process.The binding energy analysis of Ag 3d,O 1s and Si 2p suggested electron transfer between Ag and O after adsorption,and SERS provided evidence of Ag—O bonding.Structural optimization verified that both Ag⁰ and Ag⁺ were chemisorbed on silica surfaces via Ag-O covalent bonds.The average adsorption energy of Ag⁺（-5.445 eV）on silica was higher than that of Ag⁰（-3.61 eV）,indicating that Ag⁺adsorption was thermodynamically preferred.Ag⁰ lost an average of 0.86e in bonding to the O4 of silica.By contrast,few electrons,which bonded with O1,were transferred from silica to Ag⁺.Ultimately,adsorption mechanism depended on 2p-4d orbital hybridization of O and Ag.（4）The molecular characteristics and microscopic mechanism for the adsorption of humic acid with AgNPs were studied.Five aromatic hydrocarbons（AH）containing Ar-OH and-COOH were selected to simulate the adsorption process.The difference of binding energies of Ag 3d,O 1s,C-O and O-C=O indicated the changes of chemical state.FTIR analysis speculated that the process was weak force-based physical adsorption.Structural optimizations and energy calculations indicated that the Ar-OH and-COOH had the greatest influence on the adsorption site,deformation and energy when they are in adjacent positions.The electronic structure calculation showed that the charges in C and H atoms were transferred to O atoms.While there was little charge transfer between AgNPs and AH molecules.The final analyses elucidated that Ar-OH and-COOH were the active functional groups for the adsorption reaction.（5）The toxic effects of AgNPs on algae were explored and the damage mechanism of algal cells was revealed.The levels of ROS,SOD and chlorophyll in algal cells were significantly changed by exposure to AgNPs.Meanwhile,AgNPs also caused physical damage to algal cells.Cell rupture,plasma-wall separation,organelle expansion and material aggregation were the macroscopic manifestation of algal cell damage.Based on the theory of metabolomics,the effects of AgNPs on the metabolites of algal cells were analyzed.The results showed that the glucometabolic and biosynthesis of various amino acids were inhibited,seriously impairing the normal physiological functions of algal cells.