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Mobilization of selected trace metals in the aquatic environment (sediment to water column and benthic invertebrates) Bindra, Kuldip Singh


Laboratory experiments were conducted to determine the processes affecting mobilization of trace metals (Cu, Fe, Mn, Pb and Zn) across the sediment-water and sediment-benthic invertebrate interfaces. The release of trace metals from two contaminated well-characterized sediments was studied under quiescent and agitated conditions. Trace metal release was studied under different conditions of salinity (0-29.5 ‰), oxygen (air saturated and nitrogen gas purged) and pH (5, 7, 10). Four groups of benthic invertebrates, namely the opossum shrimp, an amphipod, chironomids, and oligochaetes were exposed to the two sediments for periods up to six weeks. Organisms were analyzed for Cu, Fe, Mn, Pb and Zn to determine whether accumulation occurred. Rapid agitation confirmed many of the observations made under long term (30 day) quiescent experiments. Under oxic (air saturated) freshwater conditions concentrations of the more toxic metals (Cu, Pb and Zn) were less than 10 μg/l. More Zn (27 μg/l) was released under saline conditions. Iron and Mn were released in high concentrations under anoxic (nitrogen gas purged) conditions. Extreme pH (5, 10) resulted in very high concentrations of all metals. Release at pH 10 was attributed to dissolution of humic substances which can bind the metals. Variation in release could not always be related to the sediment trace metal geochemistry. The sediment organic content and particle size were important in determining trace metal release. Microcosm studies indicated that total sediment trace metals are not necessarily indicative of levels in benthic invertebrates. The geochemistry of the trace metals as well as the physico-chemical character of the sediment influenced bioavailability. Contaminated sediments were most toxic to the opossum shrimp. Chironomids showed the greatest uptake of all trace metals. Oligochaetes appeared to have the best capability to mobilize and excrete trace metals from their tissue.

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