| Popis: |
The chemical behavior of the trace metals Fe, Al and Mn within the low-salinity (< 1‰) zone of estuaries was examined using laboratory batch-mixing experiments. Two types of experiments were used: (1) one-step mixing; and (2) incremental mixing to a final salinity. Both procedures yielded similar results. During the mixing of unfiltered river water with seawater, Fe concentration (< 0.4 μm) increased considerably within the first 0.5‰ salinity, indicating that mixing with seawater have induced deflocculation of coarse (several μm) Fe-bearing colloidal aggregates. The deflocculation of colloidal aggregates did not cause Al (< 0.4 μm) concentration to change, although the Al was also largely associated with coarse colloids. Mn primarily occurred in dissolved form, or fine particles < 0.03 μm, and the Mn concentration was not affected by the deflocculation. The deflocculation of Fe-bearing colloidal aggregates in the low-salinity zone may be caused by several mechanisms. Upon mixing with seawater, the pH of the acidic river water increases, causing less positive surface charges of Fe colloids. Alternatively, the decrease in the Ca/Na ion ratio with increasing salinity causes Na+ to replace Ca2+ on the particle surfaces in the low-salinity zone, which would diminish Ca2+ binding between colloids. This interpretation is further borne out by an experiment that used NaCl solution that yields results similar to those obtained using seawater. |
