A COMPARATIVE STUDY OF THE BIO-EFFECTS OF NANOSILVER PARTICLES ON MODEL ORGANISMS GROWING IN SYNTHETIC AND NATURAL WATERS

AbstractThe fast growth of nanotechnology has stimulated research on the potential impacts of engineered nanomaterials (ENMs) on both human health and the environment. With regard to the latter, it is believed that aquatic systems will likely behave as terminal sinks for the accumulation of these emerging pollutants. Of serious concern are silver nanoparticles (AgNPs), which are widely incorporated into both consumer and medical products, due primarily to their well-established antimicrobial properties. Accordingly, current literature abounds with papers on the potential toxicity of AgNPs. However, the risks to organisms exposed to AgNPs are still poorly understood. One of the challenges has been the limited ability to accurately predict the environmental fate and bio-impacts of AgNPs from data generated under laboratory conditions. Several recent studies have reported on the stability of AgNPs in natural water matrices and synthetic growth media, however, toxicity studies remain primarily limited to traditional dose-exposure experiments conducted in AgNPs spiked synthetic growth media. In fact, there is still a lack of published studies in which natural waters are used as both suspension and growth media in bioassays. As a result, current studies fail to take into account the full complexity of natural waters, making it difficult to easily extrapolate from laboratory generated data to the potential environmental impacts of AgNPs.In this study, an experimental approach in which selected model aquatic organisms (Ceriodaphnia dubia and Raphidocelis subcapitata) were first allowed to adapt and grow in selected natural waters was used. Water samples collected from a spring-fed river (high ionic strength) and an organic-rich aquatic system (high dissolved organic carbon or DOC) were first fully characterized using a combination of field and laboratory techniques (e.g. pH, major ions, DOC, dissolved inorganic carbon, types of prevalent natural organic compounds). Filtered waters (0.45mm) were used to prepare AgNP suspensions which were characterized using SEM-EDS and through measurements of particle size distribution, diameters, zeta potential (?), and electrophoretic mobility. The potential for AgNP dissolution was measured by centrifugal ultrafiltration (Amicon Ultra-15, Millipore). Similar analyses, whenever relevant, were conducted on AgNP suspensions prepared in tested synthetic growth media. Finally, comparative toxicity assays using AgNPs and AgNO3 and the above model organisms were conducted along a gradient of increasing water chemistry complexity as illustrated in the inserted graphical abstract. The major findings can briefly summarized as follows. The toxicity results determined using synthetic growth media may not always be used as good predictor of potential AgNP toxicity in natural waters. If in high ionic strength and low DOC waters an acceptable approximation may be expected, in waters with high DOC content, biological responses in natural waters differed significantly from those obtained on organisms growing in synthetic media. A combination of relatively high ionic strength and low DOC content resulted in high toxicity while the presence of high DOC levels significantly reduced the toxicity of AgNPs to exposed aquatic organisms. The dissolved ionic Ag fraction in tested natural waters was very small as compared to synthetic media, and the coupling of AgNP dissolution rates with toxicity results suggested that the dissolution of Ag from AgNPs is not the only cause of observed toxic biological responses. Therefore, studies assessing the potential contribution of particle related toxicity mechanisms such as the presence and types of reactive oxygen species are necessary. Overall, this research showed that the behavior and toxicity of AgNPs in natural waters cannot always be predicted by simple alterations of synthetic growth media. Therefore, more studies should focus on establishing needed correlations between water matrix dependent AgNP properties and the potential toxicity implications, using experimental approaches which closely mimics natural conditions.

BIOGRAPHYDr. Bonzongo is an Associate Professor in the Department of Environmental Engineering Sciences/Engineering School of Sustainable Infrastructure and Environment at the University of Florida. He has a PhD in Environmental Sciences from the University of Rennes-1, France. His research follows two main tracks in which fundamental knowledge in thermodynamics, geochemistry, microbiology, and toxicology is used to study the fate and impacts of pollutants in natural and engineered systems. The first research track is aimed at developing an "understanding of the potentially complex interplay between emerging pollutants and both living organisms and ecosystem functions". Recent research includes studies on the environmental application and implications of engineered nanomaterials, and the development of a plasma-based technique for treatment poly-fluoro-alkyl substances (PFAS) contaminated waters. The second research track focuses on the "biogeochemistry of trace metals". This research component emphasizes studies on the environmental cycling, regulation, implication, and the biological role of metals. Current research projects include studies on adding value to waste materials for remediation of metal-contaminated soils and sediments, and mitigation of lead exposure through drinking water exposure.