Mobility of Nanogold Particles Through Porous Media.

This work investigates the mobility of nanogold (nAu) particles through porous media as a precursor to understanding the potential transport of nanoparticles in the environment.  For this work, citrate-coated nanogold particles (mean particle diameter (MPD) of approx. 41 nm) were synthesized in batch and purified using ultrafiltration.  A pulse of a 4 ppm (w/v) particle suspension (in 20 mM NaBr background with bromide used as the tracer) was leached at 0.4 ml min^-1 through duplicate columns packed with fine sand.  Effluent was collected using a fraction collector, and analyzed for a variety of solution and particle experimental endpoints.  The experimental data showed that the relative effluent nAu concentration (C/Co) approached unity by approx. 33 pore volumes of leaching.  nAu breakthrough curves showed significant retention (compared to the bromide tracer) yet there was little evidence of particle straining in spite of high accumulation of Au within the column.  DLS data showed that the effluent particles were increasingly polydisperse with continued leaching, containing a MPD of approx. 90 nm.  These results showed that the small particle size did facilitate short-distance movement of nAu through the fine sand, however, significant NP loss from the effluent occurred through particle attachment and/or "exchange" with the sand as well as aggregation, most likely due to the higher particle concentrations in the intrapore spaces.