Transport and Retention of Silver Nanoparticles in Water Saturated Porous Media

Silver nanoparticles have unique physic-chemical properties that make them useful for a variety of commercial applications. By contrast, silver nanoparticles can negatively impact natural ecosystems. For the last 20 years, the widespread manufacturing, use, and subsequent disposal of silver nanoparticles has led to the ever-increasing exposure of environmental systems to the risk of silver nanoparticles. It is thus crucial to understand the fate and transport behavior of silver nanoparticles through natural ecosystems.
A well-defined water-saturated Ottawa sand filled column was used first to study the transport behavior of silver nanoparticles. The column transport experiments were performed to investigate the impact of solution ionic strength as well as the sand grain size on the transport behavior of proteinate-capped silver nanoparticles. Results showed that the AgNP retention in the columns increased with the solution ionic strength and reduction in mean sand particle diameter, with the influence from ionic strength having the more significant effect on the retention. The increased nanoparticle-sand interaction instead of aggregation of nanoparticles contributed to the increased retention of nanoparticles. In almost all cases, significant amounts of silver nanoparticles exited the columns, suggesting the silver nanoparticles are relatively mobile in sand porous media.
The transport of silver nanoparticles was subsequently evaluated through a porous ceramic medium which has the same characteristics as a point-of-use ceramic filter for drinking water treatment. Two types of experiments were performed: i) pulse injection of silver nanoparticle suspensions at different ionic strengths; effluent samples were collected over time and analyzed for silver concentration; ii) immobilization of silver nanoparticles on ceramic disk using a paint-on, dipping, or fire-in method; a synthetic, moderately hard water sample with both monovalent and divalent inorganic ions was used as an influent solution, and the effluent was collected and analyzed for silver over time. For the first investigation, the retention of silver nanoparticles ranged from 10% to 13% depending on specific conditions. The mobility decreased with increasing ionic strength and to a less extent with increasing nanoparticle diameter. Citrate-capped particles were found to be slightly less mobile than proteinate-capped particles. In the second part, the fire-in-treated disks had a significant lower release rate of silver nanoparticles compared to ceramic disks treated with either paint-on or dipping method, suggesting that the fire-in method is a superior method with respect to silver retention in the ceramic filter media.
The last porous medium investigated was manufactured geosynthetic clay liners. The transport behavior was evaluated under different ionic strengths and with two types of electrolytes. In general, at least 30% of silver nanoparticles breakthrough the geosynthetic clay liner in all scenarios, except with the high ionic strength water (100 mM), where full retention was achieved. The retention rate was found to increase with the ionic strength. The electrolyte type (monovalent or divalent) had a lesser impact on the total retention of silver nanoparticles relative to ionic strength. The retained silver nanoparticles under high ionic strength can be released by introducing that with low ionic strength. This observation suggested the existence of both primary and secondary energy minimum in nanoparticle-surface interaction profile. Overall, the results indicate that a geosynthetic clay liner does not effectively retain silver nanoparticles in under certain water chemistry conditions.
In the last part of this dissertation, silver-impregnated ceramic water filters were closely evaluated from the perspective of social, economic and environmental sustainability. By defining a functional unit as the total volume of water consumed by a typical household over ten years (37,960 L), a side-by-side comparison was performed between a centralized water system and the ceramic filters. From a case study in South Africa, it showed that ceramic filters are generally 3-6 times more cost-effective than the centralized system in term of the reduction of waterborne diarrheal disease. The ceramic filters also scored higher in the following four categories (out of five categories investigated): energy use, water use, global warming potential, and particulate matter emissions (PM10). This triple-bottom-line-based assessment offers strong evidence that ceramic filters are a more sustainable choice for drinking water treatment in developing countries than the centralized water system.