DUAL TRACER LABORATORY EXPERIMENTS USING CARBON NANOPARTICLES AND BROMIDE IN HETEROGENEOUS AND REACTIVE POROUS MEDIA

Transport of nanoparticles (NPs) through porous media is an important field of investigation. It has been shown that some manufactured NPs in consumer products have deleterious effects on plants and animals once released into the environment. In addition, NPs have been developed for use in remediation of contaminants. The transport of NPs through porous media can be influenced by the potential for the NPs to flocculate, settle, be filtered and react with solid materials. One way to increase our understanding of transport is to use NPs which have been developed to have neutral buoyancy, be hydrophilic, have a neutral charge, not flocculate and have minimal filtration. These NPs can then be part of a suite of tracers used to understand the flow and transport properties of other NPs in natural porous media.

Fluorescent, hydrophilic, non-toxic carbon NPs with a near neutral charge (zeta potential: -1 to -2 mV) and diameter of 5-10 nm were recently developed. Previous laboratory experiments have shown that these carbon NPs are transported identically to a conservative tracer in transport through a uniform silica or calcite porous medium. Additional experiments indicate that the carbon NPs do not diffuse into immobile zones as readily as dissolved Br.

These carbon NPs were used in several laboratory column tracer studies to further test their transport properties in heterogeneous and reactive porous media. One column experiment was designed to emphasize diffusion between mobile and immobile zones. During injection of the tracer, flow was interrupted for six days to allow diffusion to occur. The breakthrough curves (BTCs) show a pronounced drop in both the NP and Br after the flow was resumed, indicating that both diffused into the matrix. Another test was performed using a mixture of silica sand and zeolite which has been surface-modified to remove anions. The BTCs clearly show that Br was significantly retarded and that the carbon NPs had little or no retardation. Modeling of the tracer test provided an estimate that the transport of Br was retarded nearly 9 times greater than the NPs.

The results of these lab experiments further indicate that these carbon NPs are non-reactive in porous media. They have the potential to be an excellent tracer for understanding flow and transport of other NPs and for characterizing natural porous media.