Behavior of Ag species in presence of aquatic sediment minerals - In context of aquatic environmental safety

Environmental behavior of silver nanomaterials in aquatic environments: An updated review

The increasing applications of silver nanomaterials (nano-Ag) and their inevitable release posed great potential risks to aquatic organisms and ecosystems. Considerable attention has been attracted on their behaviors and transformations, which were critically important for their subsequent biological toxicities and ecological effects. Therefore, the summary of the recent efforts on the environmental behavior of nano-Ag would be beneficial for understanding the environmental fate and accurate risk assessment. This review summarized the studies on various physical, chemical and biological transformations of nano-Ag, meanwhile, the influencing factors (including the intrinsic properties and environmental conditions) and related mechanisms were highlighted. Surface structure and facets of nano-Ag, abiotic conditions and natural freeze-thaw cycle processes could affect the transformations of nano-Ag under different environmental scenarios (including freshwater, seawater and wastewater). The interactions with co-present components, such as chemicals and other particles, impacted the multiple processes of nano-Ag. Besides, the contradictory effects and mechanisms by several environmental factors were summarized. Lastly, the key knowledge gaps and some aspects that deserve further investigation were also addressed. Therefore, the current review aimed to provide an overall analysis of transformation processes of nano-Ag, which will provide more available information and pave the way for the future research areas.

Transport of silver nanoparticles coated with polyvinylpyrrolidone of various molecular sizes in porous media: Interplay of polymeric coatings and chemically heterogeneous surfaces

Silver nanoparticles (AgNPs) are usually capped with stabilizing agents to protect their activities and improve stability. Polyvinylpyrrolidone (PVP) is one of the most used capping agents of AgNPs, and may affect the transport of AgNPs in porous media. The transport and retention of AgNPs capped with PVPs of different molecular weights (PVP10-AgNP, PVP40-AgNP and PVP360-AgNP) in uncoated, and humic acid (HA)-, kaolinite (KL)- and ferrihydrite (FH)-coated sand porous media were investigated. Among the three AgNPs, PVP360-AgNP exhibited the highest mobility and eluted from all types of porous media. This is because PVPs of higher molecular weight provided stronger steric effect and electrostatic repulsive forces among PVP-AgNPs, inducing stronger blocking and shadow effects. The transport of the PVP-AgNPs increased in the HA-Sand columns, while decreased in the KL- and FH-Sand columns, especially for PVP10-AgNP and PVP40-AgNP. The simulation results using one-site kinetic model indicated that HA-Sand reduced the maximum retention capacity (Smax), while KL- and FH-Sand increased the Smax as well as the first-order attachment rate coefficients (katt), particularly at high ionic strength. The results shed light on the interplay of the capping agents of AgNPs and the surface heterogeneity on the transport of AgNPs in porous media.

Vertical Distribution and Chemical Fractionation of Heavy Metals in Dated Sediment Cores from the Saronikos Gulf, Greece

The Saronikos Gulf is under a lot of anthropogenic pressure, such as the urban expansion of the metropolitan area of Athens, the port of Piraeus and marinas, industrial activities, and tourism. Heavy metal pollution has been a major environmental problem in the area for many decades. Sedimentary cores have proven to be an invaluable indicator of heavy metal pollution, as they can reveal not only the current metal inputs but also the evolution of pollution over time, and with the appropriate geochemical analyses, they can provide information on the potential toxicity of metals. In this study, the temporal evolution and the chemical speciation of eleven elements were examined in sediment cores from Elefsis Bay and the Inner Saronikos Gulf, with an emphasis on the emerging environmental hazards (V and Ag). The results showed extensive pollution of the sediments by Ni, Cr, Cu, Zn, As, Mo, Cd, and Pb from the 1910s and 1960s in Eastern and Western Elefsis Bay, respectively. A significant decrease of the sediment enrichment in V, Ni, Cr, Cu, Zn, As, Cd, Pb, and Ag since 2000 was observed in the part of the Inner Saronikos Gulf that is mainly influenced by the WWTP of Athens. However, a toxicity assessment using the metal contents of the surface sediments showed that most of the trace elements studied still pose a moderate to high risk of toxicity to benthic ecosystems. The present study highlighted the urgent need for focused research and the management of trace element inputs, particularly Ag in the Inner Saronikos Gulf, where severe sediment modification was evident.

The Physical Modeling Analysis of Fate and Transport of Silver Nanoparticles Dispersed by Water Flow

The release of silver nanoparticles (AgNPs) from consumer products into an environment has become a central issue for many countries. Despite that the fate and behaviors of AgNPs incorporated into a wastewater have been investigated by building a model of wastewater treatment process, the transport and retention behaviors of AgNPs influenced by the water flow in a river must be understood. The physical model of simulated river to mimic a natural flow of river was proposed to investigate the behaviors of AgNP transport in the river. The results showed that the large amount of AgNPs deposited on the riverbed as Ag sediment with only 1.26% of AgNPs remained in the water flow. The elemental content of Ag freely dispersed across the riverbed increases from the upstream to downstream area of the simulated river. Verification of the spatial distribution of Ag dispersed along the water flow may contribute to a better understanding of the fate and transport of AgNPs in the aquatic environment.

Adsorption and bonding strength of chromium species by ferrihydrite from acidic aqueous solutions

The adsorption behavior of Cr(III) and Cr(VI) ions onto laboratory-synthesized 2-line ferrihydrite was investigated under a batch method as a function of initial chromium concentration (0.1-1000 mg L-1) and pH (3.0 and 5.0). Moreover, the effect of the type of anion (chloride and sulfate) on Cr(III) adsorption was studied. The affinity of Cr(III) ions for the ferrihydrite surface depended on both the type of anion and pH of the solution and the maximum adsorption capacities decreased as follows: q (SO4 2-, pH 5.0) > q (SO4 2-, pH 3.0) > q (Cl-, pH 5.0) > q (Cl-, pH 3.0), and were found to be 86.06 mg g-1, 83.59 mg g-1, 61.51 mg g-1 and 40.67 mg g-1, respectively. Cr(VI) ions were bound to ferrihydrite in higher amounts then Cr(III) ions and the maximum adsorption capacity increased as the pH of the solution decreased and was 53.14 mg g-1 at pH 5.0 and 83.73 mg g-1 at pH 3.0. The adsorption process of Cr species was pH dependent, and the ions were bound to the surface of ferrihydrite by surface complexation. The Sips isotherm was the best-fit model to the results obtained from among the four isotherm models used, i.e., Freundlich, Langmuir, Dubinin-Radushkevich and Sips, indicating different adsorption centers participate in Cr uptake. In order to assess the bonding strength of the adsorbed chromium ions the modified BCR procedure, dedicated to the samples with a high iron content, was used. The results of the sequential extraction showed that Cr(III) ions were bound mainly in the immobile residual fraction and Cr(VI) ions were bound in the reducible fraction. The presence of Fe (oxyhydr)oxides in soil and sediments increases their adsorption capacity for Cr, in particular for hexavalent Cr in an acid environment due to their properties (high pHPZC).