Dissolution Characteristics Of Zinc Oxide Nanoparticles And Impact Of Dissolved Organic Matter In Aquatic Environment

With the rapid development of nanotechnology,Zinc oxide nanoparticles(ZnO-NPs)are extensively used in many fields.Therefore,they are inevitably released into aquatic environments and potentially do harm to aquatic organisms.The stability of ZnO-NPs is largely influenced by the propensity to aggregate and form larger particles as well as the propensity to dissolve and release Zn2+.Understanding the impact of these environmental behavior is important in the fate of ZnO-NPs.Previous studies have shown that the presence of ZnO-NPs have posed adverse effects to various aquatic organisms.However,their toxicity mechanism is still controversial due to the lack of studies for environmental behavior of ZnO-NPs in natural waters.In this study,the traditional Anodic Stripping Voltammetry(ASV)was improved and applied for studying the persistence and lifetime of ZnO-NPs in natural waters without the addition of external organic pH buffers.This method was also used to study the effects of different types of ligands and DOM on the lifetime and dissolution kinetics of ZnO-NPs.The results of this study are help to understanding the environmental behavior of ZnO-NPs in complicated environments and evaluating their toxicity to aqueous organisms.The main content and conclusions of this study are as following.1.Both Flame Atom Absorption(AA)and ASV are used to study dissolution kinetics of ZnO-NPs in four commonly used aqueous media.The results showed that AA can be used for any aqueous media,but the requirement of separation is time consuming and hard to be interpreted.While ASV realize the in situ measurement of dissolved Zn2+ released from ZnO-NPs and each measurement takes only a few minutes.However,ASV was not suitable for the study of ZnO-NPs dissolution in waters with a lot of natural organic matter(NOM).Based on the results and water chemistry,the test solutions were bubbled with the N2/CO2 gas mixture,instead of pure N2 gas.The use of an N2/CO2 gas mixture instead of fixing the pH with an additional component foreign in the original samples reproduces chemical conditions that are more germane for studying the dissolution kinetics of ZnO-NPs.2.We have assessed the persistence and lifetime of ZnO nanoparticles(ZnO-NP)by performing dissolution experiments in 3 different aqueous media(KCl solution,wastewater effluent and Lake Michigan Water).These experiments were performed at ZnO-NPs concentration levels close to the solubility of zincite(?8 ?M or 650 ?g/L of ZnO)-a concentration that is orders of magnitude higher than current estimated environmental relevant concentrations.Our results show that,under these conditions,ZnO-NPs readily dissolve with a lifetime expectancy that does not exceed 90 minutes.The addition of anHEPES buffer in solution tends to speed up the dissolution process in contrast to the commonly present chemical species in natural waters.Considering that all three solutions were maintained at about the same pH and ionic strength,the difference in dissolution rates of ZnO-NPs in all three aqueous media was attributed to differences in the quality of DOM present in the 2 WWE samples.3.LMW samples,with the addition of three different types of DOM(L-cysteine,Na2-EDTA and sodium alginate acid),were used for the further study of the effect of DOM on ZnO-NPs dissolution.The results show that,dissolution processes vary with different types of DOM,which are independent of the amounts of DOM.Dissolution rates significantly increase in the presence of strong chelating agents,EDTA and L-cysteine,while the addition of polymeric DOM,such as sodium alginate,has the opposite effect.4.In order to quantitatively determine the dissolution of ZnO-NPs in different aqueous media,we used a kinetic model to calculate the dissolution rate and half-life time of ZnO-NPs.The lifetime of the nanoparticles is short and varies within a few minutes to an hour,depending on the specific water chemistry.This finding suggests that chronic toxicity results cannot be related to the nano-phase material but to the dissolved Zn released.What will then govern the toxicity is the chemical speciation of Zn2+ since it will control its bioavailability.In both waters,organic Zn complexes only account for small fractions(8?16%).Consideringthat free Zn2+ is the most bioavailable and toxic Zn species,these speciation results indicate that the presence of DOM would not significantly affect Zn toxicity due to the dissolution of ZnO-NPs.Overall,by sparging the solution with a N2/CO2 gas mixture one can effectively control the pH of the dissolution experiments while maintaining the chemistry of the aqueous solution close to its original composition.ZnO-NPs dispersed in solution at concentration levels just below the solubility of zincite readily dissolve in aqueous media.The lifetime of the nanoparticles is short and varies within a few minutes to an hour.This finding provides a severe time limit for conducting acute toxicity experiment at environmentally relevant concentrations.It also suggests that chronic toxicity results cannot be related to the nano-phase material but to the dissolved Zn released.Therefore,the adverse effects of the release of ZnO-NPs to natural waters depends primarily on the chemical speciation of dissolved zinc.