Effects of Solution Chemistry on the Retention and Dissolution of Silver Nanoparticles in Water-Saturated Porous Media
Abstract
Potential health and environmental effects have been attributed to both silver nanoparticles (nAg) and the silver ion (Ag+), necessitating a thorough understanding of mechanisms governing the fate and transport of nAg in natural systems. Batch and column experiments were conducted to assess nAg transport, retention and dissolution kinetics as a function of pH, electrolyte and dissolved oxygen (DO) content. Batch experiments were performed at pH 4, 5.5 and 7, DO levels of <0.15 mg/L, 2mg/L and 8.9 mg/L, and with 10mM nitrate, acetate or borate as the background electrolyte. Batch solutions containing ca. 2 mg/L nanosilver were monitored regularly for 48 hours and analyzed for mean particle diameter, zeta potential, nanoparticle concentration and silver ion concentration. Silver nanoparticle dissolution increased with decreasing pH and with dissolved oxygen content. Increased aggregation and less negative zeta potential values (tending closer to the point of zero charge) indicate that acetate causes greater instability in nAg suspensions as compared with nitrate at the same ionic strength. Column experiments were performed in glass columns (11 cm length x 2.7 cm diameter) packed with washed 40-50 mesh Ottawa sand and saturated with a background electrolyte solution. Following a non-reactive tracer test, a three pore volume pulse of nAg suspension (ca. 3 mg/L silver) was introduced at a flow rate of 1.0 ml/min (pore water velocity of ca. 7.0 m/d), followed by three pore volumes of nanoparticle-free solution. Column experiments were conducted with 10mM sodium nitrate at pH 4 and 7 and under oxygen rich (DO = 8.9 mg/L) and lean (DO < 0.15 mg/L) conditions. Hyper-exponential retention profiles were observed, with the highest attachment measured at the column inlet. Under oxygen rich conditions, approximately 85% of the input nAg was retained in sand at pH 4, compared with 25% at pH 7. Consistent with batch experimental results, dissolution of retained nanoparticles increased with DO and with decreasing pH. At pH 4 and 7, input suspensions initially contained 0.07 mg/L and 0.03 mg/L silver ion, respectively, while effluent samples contained a maximum of 0.17 mg/L and 0.04 mg/L Ag+, respectively. Batch and column experiments indicate that the retention and dissolution of silver nanoparticles are strongly dependent on solution chemistry. The release of silver ions from retained particles can be substantial under certain environmentally relevant conditions, namely saturated oxygen levels and lower pH values.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2011
- Bibcode:
- 2011AGUFM.H43M..02M
- Keywords:
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- 1832 HYDROLOGY / Groundwater transport