Colloidal and Particulate Silver in River and Estuarine Waters of Texas

Abstract

Concentration and phase speciation of Ag in selected Texas rivers and in the Trinity River estuary were measured in order to establish the major factors that control its fate in the aquatic environment from source to sink. Concentrations of Ag in the filter-passing fractions in Texas rivers ranged from <0.01 to 62 ng/L. In the Trinity River estuary (Galveston Bay), they ranged from 0.4 to 6.4 ng/L and showed a non-conservative estuarine mixing behavior. An internal source of filter-passing (≤0.45 μm) and colloidal (1 kDa−0.45 μm) Ag was observed in the upper Trinity Bay. Silver, associated with colloidal macromolecular organic matter, which was isolated using cross-flow ultrafiltration techniques, amounted to 15−70% of the filtered (≤0.45 μm) Ag concentration, decreasing with increasing salinity. Such a trend was similar to that of dissolved and colloidal organic carbon. Estuarine distributions of colloidal Ag were also broadly similar to those of suspended particulate matter. The ratio of colloidal Ag to filter-passing Ag was similar to the ratio of colloidal organic carbon to total dissolved organic carbon, suggesting not only that Ag is complexed by organic macromolecules but also that functional groups with high affinity for Ag were evenly distributed over the different molecular weight fractions. Particulate Ag was found associated mainly with a iron−manganese oxyhydroxide/sulfide phase. The close relation between Ag and Fe in colloidal and particulate phases suggests common surface complexes, probably sulfhydryl groups. In river waters of Texas, 33−89% of the operationally defined dissolved (≤0.45 μm) Ag fraction was present in a large colloidal form (0.1−0.45 μm). The high affinity of Ag for suspended particulates in river and estuarine water was reflected by a high mean particle/water partition coefficient of log Kd = 5.0 ± 0.6 (based on filtration through a 0.45-μm filter) and 5.5 ± 0.5 (based on filtration through a 0.1 μm filter). Particle/water partition coefficients for the surface-adsorbed phase showed particle concentration effects, which, however, disappeared (log Kp1 = 5.0 ± 0.3) when the dissolved Ag data were corrected for the presence of a colloidal fraction.