关键词: 纳米银颗粒, 水环境, 氧化溶解, 氯化, 硫化, 影响因素

Abstract:

Nanosilver is the most widely utilized and commercialized nanomaterial in the world. Owing to its increasing application in many technical fields, the mass production and use of nanosilver has led to its inevitable release into the natural aquatic environment during various processes, posing great potential risks to aquatic organisms and ecosystems. Engineered silver nanomaterials are mostly composed of an Ag⁰ core with varying particle sizes and morphologies (e.g., silver nanoparticles, silver nanoplates, silver nanowires, and silver nanocubes) and a shell of capping agents, including carboxylic acids, polymers, polysaccharides, and surfactants; silver nanoparticles are the most widely used. The low redox potential of Ag⁺/Ag⁰ and its large surface area are thermodynamically favorable for the redox reaction, and Ag⁺ dissolved from silver nanoparticles is prone to precipitation with sulfide and chloride ligands because of the low solubility of these Ag-containing species. The composition of the natural water environment is complex; once it enters the water environment, silver nanoparticles are affected by water physiochemistry and aquatic organisms, and their morphology and properties change. Silver nanoparticles transform into aquatic environments and may cause microbial perturbations in aquatic ecosystems, leading to complex toxicological effects. Considerable attention has been paid to their behavior and transformations, which are critically important for their subsequent biological toxicity and ecological effects. Therefore, a summary of the recent efforts on the environmental behavior of silver nanoparticles would be beneficial for understanding the environmental fate and accurate risk assessment. This review summarizes the studies on the sources of silver nanoparticles in an aqueous environment and the possible transformation behavior of silver nanoparticles in an aqueous environment, including physical, chemical, and biological transformations, as well as the influencing factors (including intrinsic properties and environmental conditions) and related mechanisms. This study mainly focuses on the chemical transformation mechanism of silver nanoparticles in aqueous environments and the influence of different environmental factors on different types of chemical transformations of silver nanoparticles, including oxidative dissolution, sulfidation, and chlorination of silver nanoparticles in aqueous environments. The oxidative dissolution process is considered a heterogeneous oxidation process on the surface of metallic Ag. Dissolved oxygen is involved in the formation of an oxide layer, AgO_x(OH)_y, followed by the oxidation of the inner Ag⁰ core. In this case, the oxidative dissolution of silver nanoparticles was divided into two stages: fast and slow. The oxidative dissolution process is accompanied by the loss of surface coating and a decrease in particle size, which destabilizes the silver nanoparticles, and aggregation occurs simultaneously. A higher ionic strength is known to enhance the release of silver nanoparticles, which has been considered during the risk assessment of the use and disposal of silver nanoparticles-based consumer products. The presence of natural organic matter might exert dual and even completely opposite effects on the dissolution of silver nanoparticles due to the diverse composition, N- and S-containing groups, molecular weight, and hydrophobicity. The dissolution process of silver nanoparticles can be affected by other co-present components in the aquatic environment. Silver nanoparticles can be oxidized by denitrifying nitrogen species (e.g., NO₂⁻, NO, and N₂O), even under anoxic conditions, and this process can also be facilitated by a reduced solution pH. Owing to the formation of Ag₂S with extremely low solubility, the presence of sulfides (such as HS⁻ in anoxic water and metal sulfides under aerobic conditions) drives the sulfidation of silver nanoparticles, resulting in a substantial decrease in the free Ag⁺ ions. Biogenic H₂S is also recognized as a sulfide source that reacts with silver nanoparticles and limits the release of dissolved Ag⁺ ions into the physiological environment. After sulfidation, the silver nanoparticles became more negatively charged and less hydrophobic, leading to enhanced particle stability. The presence of cations significantly affected the surface charge of the silver nanoparticles, which determined their attraction affinity for sulfide. The binding of silver nanoparticles with inorganic sulfides and organosulfur dissolved organic carbon promotes the formation of Ag₂S in municipal wastewater and natural aquatic environments, and its association with natural organic matter plays a critical role in the sulfidation process. In addition, silver nanoparticles can undergo sulfidation by solid metal sulfide nanoparticles (such as CuS and ZnS nanoparticles) in water under aerobic conditions via the following reactions: oxygen-dependent dissolution releases silver ions, followed by cation exchange reactions with metal sulfide nanoparticles to form Ag₂S nanoparticles. Chlorination of silver nanoparticles refers to the oxidative dissolution that releases Ag⁺ ions followed by precipitation with chloride ions to form AgCl. Because of the high concentrations of chloride ions in wastewater treatment plants and seawater, which could exceed hundreds of ppm, further formation of soluble AgCl_x^(x−1)− could be the dominant species. The morphologies and states of the silver nanoparticles could be modified by Cl⁻, which is strongly dependent on the concentration of Cl⁻. In addition to the factors mentioned in the dissolution transformations, the wide range of Cl/Ag ratios is another extremely important factor during chlorination. Silver nanoparticles experienced multistep chlorination, which was dependent on the concentration of Cl⁻ in a non-linear manner. This study describes the re-reduction of silver nanoparticles. The chemical transformations of nanosilver in real environments are influenced by various factors, and the mechanism requires further investigation. The surface structure and facets of silver nanoparticles, abiotic conditions, and natural freeze-thaw cycle processes could affect the transformation of silver nanoparticles under different environmental scenarios (including freshwater, seawater, and wastewater). Interactions with co-present components, such as chemicals and other particles, affect the multiple processes of silver nanoparticles. In addition, the contradictory effects and mechanisms of several environmental factors were summarized. Key knowledge gaps and aspects that deserve further investigation are addressed. In the future, more advanced instrumental analysis techniques will be able to better analyze the chemical transformation process of nanosilver in complex water environments, and could provide a more systematic evaluation of the impacts of nanosilver on the environment and human health. Therefore, the current study aimed to provide an overall analysis of the chemical transformation processes of silver nanoparticles, which will provide more information and pave the way for future research.

Key words: silver nanoparticles, aquatic environment, oxidative dissolution, chlorination, sulfidation, influence factors