Photochemical Formation Mechanism Of Silver Nanoparticals Mediated By Plant Litters

Silver nanoparticals（AgNPs）have at least one dimension in the range of 1～100nm,and are widely used because of their unique excellent properties.Besides anthropogenic discharge,AgNPs have also been found in rivers,mineral deposits and other environments,as well as in some bacteria and plants,indicating the possibility of natural synthesis of AgNPs.In this thesis,taking the litters as the sources of organic matter.The effects of tree species,environmental factors and litter decomposition on the formation and stability of AgNPs in litter leachate under simulated sunlight irradiation conditions in the laboratory were investigated.And the mechanism of action of organic matter and Ag⁺was revealed by Ultraviolet-visible spectroscopy（UV-vis）,Excitation-Emission Matrix（EEM）and Fourier Transform Ion Cyclotron Resonance-Mass Spectrometry（FTICR-MS）,X-ray Diffraction（XRD）and Transmission Electron Microscopy（TEM）.The main conclusions are as follows:1.A large amount of DOM in plant litter leachate can reduce silver ions（Ag⁺）to AgNPs,resulting in the color change in DOM-Ag solution and UV absorption peaks around 400 nm.The formation process of AgNPs is light-driven.There are great differences in the amount and type of organic matter contained in litter leachate of different tree species,the amount and physicochemical characteristics of AgNPs generated（particle size,degree of agglomeration,etc.）,and the sensitivity of AgNPs formation to changes in reaction conditions.2.The formation of AgNPs in the natural environment and their physicochemical properties depend on environmental factors such as irradiation,freezing,concentration,pH,and coexisting ions.The formation of AgNPs can be promoted the formation,aggregation and precipitation by long-term light,high concentration DOM/Ag⁺solution,alkaline DOM-Ag solution,Fe²⁺/Fe³⁺and trace Cl^-（mass ratio of Cl^-/Ag⁺<5000）.With the increase of Cl^-/NO₃^-content and the decrease of freezing temperature,the inhibitory effect on the formation of AgNPs gradually increased.3.The decomposition of litter causes changes in the composition and structure of organic matter,resulting in differences in the number and characteristics of AgNPs.In the early stage of litter entering the water,water-soluble organic matter and easily decomposable components are rapidly dissolved,which is conducive to the formation of unstable AgNPs.And then,a large number of easily decomposable components are decomposed and utilized,and AgNPs with stable properties are formed in the solution.In the late stage of decomposition,the refractory components in the litter are dissolved and difficult to be decomposed and utilized by microorganisms,and only minor of AgNPs with unstable properties can be formed in the solution.4.Under the combined interaction of light and organic matter,Ag⁺combines with OH^-to form Ag₂O and then reduced to elemental silver（Ag⁰）.Ag⁰ continues to aggregate to form AgNPs clusters.Under the joint action of O₂ and H⁺,AgNPs are oxidized to release Ag⁺.After the reaction is complete,the DOM-Ag solution reaches equilibrium.At the same time,the reduction of Ag⁺promotes the transformation of saturated compounds to unsaturated compounds and increases the stability of organic matter,which is not conducive to the further decomposition of plant residues.This study revealed the photochemical formation mechanism of AgNPs in leaf litter leachate system,which not only provides a new direction for the green synthesis of AgNPs,but also has great significance for elucidating the migration and transformation of silver in the natural environment.And also provides a reference for scientific evaluation of the environmental behavior and effect of AgNPs in water.