Abstract
As nanoparticles have been found to cause a range of harmful impacts in biota, understanding processes and transformations which may stabilize and increase their persistence time in the environment are of great importance. As nanoparticles carried in riverine or wastewaters will eventually reach estuaries, understanding their behavior and transport potential in this transition zone from fresh to marine waters is essential, particularly as estuaries are sensitive ecological zones, oftentimes encompassing ornithologically important areas. In this direction, we report on the influence of combined gradients of riverine and marine natural organic matter (NOM) on the temporal stability of biocorona-encapsulated silver nanoparticles in terms of ion release kinetics. In parallel, salinity, pH and oxygen saturation were simultaneously varied to create a model to mimic the complex estuarine environment. While humic acid (HA) and alginate (Alg) disrupted the stabilizing ability of the nanoparticle protein corona to a greater and lesser degree, respectively, they slowed the rate of ion release in freshwater at pH 6.6 and in saltwater at pH 8, respectively, while oxygen saturation was also found to be an important factor. Thus, as the type of NOM changes with pH along a salinity gradient in an estuary, conditions required to increase the persistence time of nanoparticles are serendipitously met, with greater colloidal stability achieved in cases where there is more rapid replacement of HA with Alg. Despite the strong gradients in ionic strength, pH and oxygen saturation, the protein corona was not sufficiently disrupted at the nanoparticle surface to be substituted by NOM indicating the greater adsorption energy of the protein’s hydrophobic domains. Ultimately, it is the specific NOM profile of individual estuaries that may provide the best indicator for predicting the stability and persistence of silver nanoparticles as they transition from fresh to salt water environments.
Highlights
Research in the area of nanoscale materials has continued to grow apace over the past two decades
Sample preparation typically comprised of mixing AgNP with organic matter and adding Artificial seawater (ASW) to give the required salinity; for example, 75 μL AgNP dispersion was vortexed with 125 μL organic matter solution (HA, Alg, bovine serum albumin (BSA) or combinations thereof) in mQ water to which was added 100 μL ASW of appropriate concentration
Silver nanoparticles undergo oxidative dissolution over time in all environmental matrices, e.g., natural waters, sediment and soil, those that have high oxygen or sulphur content [8]. The kinetics of such dissolution may be modulated by natural organic matter, either through formation of a corona around the nanoparticles or by influencing agglomeration and aggregation processes, and the surface area of the nanoparticles in contact with the surrounding medium
Summary
Research in the area of nanoscale materials has continued to grow apace over the past two decades. Reports in the literature on the behavior of silver nanoparticles in aquatic matrices typically focus on variations of individual parameters such as salinity or NOM concentration in isolation, and more realistic data on the fate of silver nanoparticles when multiple parameters are varied simultaneously are lacking In this direction, the present study reports on the physico-chemical behavior of silver nanoparticles in a multi-parameter model simultaneously encompassing gradients of NOM and the primary abiotic factors of estuarine environments with a view to gaining greater understanding of the fate of these nanomaterials in brackish waters. Oxygen saturation of the aqueous matrices in the prior experiment is reduced to determine the role of dissolved oxygen on silver nanoparticle fate as a function of salinity, natural organic matter, and pH This approach provides systematic data on how multiple abiotic parameters and combinations of NOM simultaneously influence silver nanoparticle behavior as they transition from fresh to salt water in estuaries
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