COLL 394 |
| Zinc-sulfide and other metal-sulfide nanoparticles (or colloids) are known to persist both in oxic and anoxic aquatic environments. These nanoparticles can control the bioavailability and increase the mobility of pollutant metals; however, the processes that facilitate their stability in natural waters are relatively unknown. The objective of this study was to evaluate the role of dissolved natural organic matter (NOM) for stabilizing colloidal ZnS. We assessed the relative growth rates of nucleating and aggregating ZnS particles in the presence of humic acid. Aggregation of ZnS particles was also tested in the presence of low-molecular weight organic acids that are surrogates for naturally-occurring NOM. Particle formation and size was monitored over time by dynamic light scattering. Zn speciation was also measured by filtering the ZnS solutions (<0.2 um) and using anodic stripping voltammetry to confirm that Zn was coordinated to sulfide during the aggregation experiments. Observed growth rates of ZnS aggregates varied by orders of magnitude, depending on the type and concentration of organic ligand in solution. Growth rates were slowest in the presence of thiol-containing ligands: cysteine, glutathione, and mercaptoacetate. In contrast, ZnS particle growth rates were generally not affected by the oxygen- and amine-containing ligands: oxalate, serine and glycolate. The relative ZnS nanoparticle growth rates were observed to decrease as the ZnS(s) saturation index decreased in the presence of thiol ligands. Thus, slow aggregation of ZnS nanoparticles is likely caused by specific attachment of the thiols on surface Zn sites. These studies suggest the importance of thiol-containing organics for stabilizing naturally-occurring and anthropogenic ZnS nanoparticles in the aquatic environment. |
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The Physical Chemistry of Environmental Interfaces
2:00 PM-6:00 PM, Wednesday, April 9, 2008 Morial Convention Center -- Rm. 225, Oral
Division of Colloid & Surface Chemistry |