Mobilization Of Silver Nanoparticles By Three Typical Organic Compounds From Saturated Porous Medium

Silver nanoparticles (AgNPs) are widely applied in medical treatment, food, cosmetics, textile, water purification and other fields due to their favorable properties of electric conduction, catalysis, sensing and antibiosis. The wide use of AgNPs inevitably leads to their release to environment and ultimately to the ground-water system in the form of "the three wastes" during the process of their production, use and discard. The quality of the ground-water has a direct impact on people’s health condition. Research has demonstrated that AgNPs have different levels of side effect on plants and bacteria, and possibly cause toxic reaction in liver, kidney and nervous system of human body. The toxic effect of the AgNPs has a direct relationship with their mobility in the environment. Organic compounds may control the mobility of AgNPs via complex mechanisms, leading to a change in the scale and shape of the pollutant plume and making it a challenging task to predict the environmental impact of AgNPs on groundwater ecolosystem. As different organic compounds may interact with AgNPs by different mechanisms, a systematic evaluation on the effect of several typical environmental organic compounds on AgNPs deposition and release has profound significance.In this thesis work, polyvinylpyrrolidone (PVP) coated silver nanoparticles (PVP-Ag NPs) were used as a model and laboratory experiments combining static mixing experiments and dynamic column experiments were employed to explore the impact of N-acetyl-L-cysteine (NAC), Sodium Dodecylbenzenesulphonate (SDBS) and Ethylenediaminetetraacetic acid (EDTA) on the deposition and release of the nanoparticles in quartz sand (QS) and iron oxide coated sand (IOCS). Our main findings are summarized as follows:(1) The deposition of silver nanoparticles in porous media is jointly controlled by silver nanoparticle core (Ag core) and PVP coating, with the latter being the main factor. The Quartz sand is negatively charged, repelling the negatively charged silver nanoparticle core; nonetheless the silver nanoparticles can also deposit irreversibly through the PVP coating and the Si-O-H bonding on the surface of the Quartz sand. Iron Oxide coated sand is positively charged, attracting the silver nanoparticle core; nonetheless the iron oxide impurities may reduce the available area of the bare quartz surface for the PVP link, lowering the efficiency of the deposition. The particles affiliated with the iron impurities may be mobilized by chemical perturbation, i.e. a change in solution pH.(2) Organic compounds containing sulfur such as SDBS/NAC and sulfur-free complexing agent like EDTA can separate silver nanoparticles and its polymer coating via destruction of the bond between them, and decrease the particle size. But none of them could separate the PVP coating completely and simultaneously, this coating alternation mechanism cannot induce significant release of the silver nanoparticles from the granular surface.(3) SDBS/NAC can mobilize silver nanoparticles deposited reversibly on the iron oxide coated sand. The release mechanism is that SDBS/NAC can be adsorbed onto the iron oxide coated surface, modify and even reverse the medium surface electrical property, enhance the electrical double layer repulsion, and thus cause the instant release of silver nanoparticles.(4) The micelle structure is beneficial to improve the electrical regulating ability of SDBS on the sand surface and enhance the mobilizing efficiency of the organic compound. Meanwhile, the micelle structure allows SDBS to effectively block more deposition sites of silver nanoparticle on the granular surface and thusly reduces the rate of further deposition.