Research On Early Aggregation Kinetics And Long-term Fate Mechanism Of AgNPs In Aquatic Environment

Silver nanoparticles(AgNPs)are currently one of the most widely used and fastest growing nanomaterials.While entering into the aquatic system,AgNPs could be toxic to the organisms,and generally enrich in the biological chain,which would have potentiall threat to human health and safety.When in aquatic system,AgNPs would be affected by environmental factors,resulting in aggregation/dissolution,thus changing the nature of AgNPs and directly affects its toxicological.Therefore,more attentions have been paid to AgNPs fate in aquatic system.However,the current studies on the fate of AgNPs were mainly based on fresh synthetic AgNPs,little research has been done on the long-term fate and aged particle proerties of AgNPs in the enironemnt.In this dissertation,stable and spherical monodisperse AgNPs were systhesized by chemical reduction of Tollen's reagent with D-maltose.The AgNPs early aggregation kinetics influences by electrolyte,NOM and coating layer were systemayically studied using DLS,UV-vis,ZetaPlus and TEM.On the basis of summarizing their early aggregation kinetics,several important factors were selected,including coating layer,pH value,electrolyte,NOM and light exposure to investigate the long-term fate of AgNPs.The results are showed as follows:(1)Dissolution-accompanied aggregation kinetics of AgNPs and surface-modified AgNPs observed in NaCl,NaNO3,or CaCl2 electrolytes,but their aggregation behaviors are still consistent with the classical DLVO theory.? During the aggregation process,AgCl deposits formed in the Cl-containing system as the AgNPs dissolving,producing particles and polymers with a smooth continuous surface,resulting in almost no single particles present in the system;Whereas in the NaNO3 system,AgNPs were still disperse in system.? Low concentration of NOM had little effect on the AgNPs' early aggregation kinetics,however the long-term fate of AgNPs differed with/without NOM.? When with surface coating,AgNPs were stabilized with steric repulsive force resulting in respressed dissolution.Whereas the use of low molecular weight molecules(such as trisodium citrate)has no apparent electrostatic stability on AgNPs,which resulting in little difference between Citrate-nAg and Bare-nAg's early aggregation kinetics.The non-ionic space stabilizer Tween 80 could significantly improve the stability of the particles,which keeping the size and shape of AgNPs and significantly reducing the dissolution of the particles.(2)The research results which used fresh AgNPs as object could mostly be used to estimate the actual fate of aged AgNPs in aquatic system,however some results could not,such as the synergistic effect of pH and Cl",and some specific coating layer effects.When facing the case of similar conditions in the actual water system,the properties of fresh synthetic AgNPs could not be directly used to predict their fate,thereby misleading the toxicological and transporting ability in water.? As a result of pH could change the surface charge and participate in AgNPs dissolution,oxidation,and Ag+ reduction reaction,It had the greatest impact on the properties of AgNPs:The AgNPs fate turned out totally different in acidic and basic condition,and they also differed by the different electrolyte and NOM effects:In the Cl-system,the AgNPs size decreases with increasing pH due to the formation of AgCl(s),which is totally different from the resuts in the Cl" free system(NO3-);However,when with NOM,the pH effects would not be so significant;? Due to its lower molecular weight,SRFA which in the same mass concentration conditions as SRHA could have higher molar concentration,the stability of Tween-nAg-10 in SRFA is higher than in SRHA with the same mass concentration;However the enhancement of the stability on Bare-nAg of SRFA and SRHA is almost indistinguishable;increasing the concentration of NOM could reduce the solubility of Bare-nAg and enhance its stability.This results are different from the effects of NFA on AgNPs early aggregation kinetics;? Special electrolytes such as Cl-,which could react with Ag+ in aquatic system and produce complexes or precipitations,could limit the dissolved silver concentration within a low value,and in non-alkaline pH conditions,Cl-could react with the Ag+ which adsorbed to the surface of AgNPs and produce AgCl(s)coating on the surface of AgNPs,thus enhancing the stability of AgNPs;And the electrolyte concentration effects results showed that the AgNPs properties were different in 2 mM and 6 mM electrolyte concentration,however the differences were negligible when comparing with other factors effecting;?Light irradiation could enhance AgNPs dissolving,resulting in higher dissolved silver concentration in light than that in dark,in addition the reduction of Ag+ with NOM to form new AgNPs could be enhanced by irradiation effect,thus resulting in the dissolved silver rate in aged AgNPs in light condition was lower than that in dark condition on the contrary;(3)The release mechanism of AgNPs when without special ion is consistent with the first-order reaction kinetic model:[Ag+]release=[Ag+]total[1-exp(-at)].And when containing special ions,such as Cl-,the dissolved silver concentration will be affected by Cl-,and Ag will exist as complex or sedimentations in the environment.? The dissolution and release rate of AgNPs decreased with increasing pH and SRHA.Due to the complexity of light exposure,it was considered that light irradiation could enhance the dissolution and release of AgNPs in the early stage,and light could enhance the Ag+ reduction rate in the presence of NOM.? The concentrations of dissolved silver detected by AgNPs in 2 mM NaCl system were basically consistent with the theoretically derived dissolved silver concentration.Based on the research results on green synthetic monodisperse AgNPs in various aquatic environment and light conditions,it is helpful to have a fully understanding on the long-term fate of new synthetic AgNPs affected by various factors,which would be beneficial to the further understanding of the residual "aged AgNPs" properties in aquatic system,and laid a solid theoretical foundation for the study of the fate of AgNPs,providing a more effective theoretical basis for the study and prediction of AgNPs toxicity in aquatic system.It would also be meaningful to get knowledge of the hazards of AgNPs pollution and the possible follow-up treatment.