Heavy Metal Sorption In Anoxic Sedments
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Date
1992-05
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Abstract
The U.S. Environmental Protection Agency is currently developing Sediment Quality Criteria
(SQC) for heavy metal chemical contaminants. The approach used in their development is based on
Equilibrium Partitioning between sediment and porewater phases. The toxicity of these chemicals is
related to the porewater, free-metal chemical potential. Therefore, anything that reduces this activity
will reduce the toxicity of the sediment.
Sediment studies have identified a reactive pool of solid phase, acid volatile sulfide (AVS)
which controls porewater metal activity. In all sediment studies, no toxicity is observed when the
metal/AVS molar ratio is less than one. In theory, when the metal/AVS molar ratio is greater than
one, sediment porewater activity can potentially reach toxic levels. In certain copper toxicity studies,
however, where sediment acid volatile sulfide concentrations were extremely low and sediment copper
concentrations relatively high, no toxicity was observed. This apparent anomaly does not refute the
acid volatile sulfide theory. It does, however, suggest the presence of one or more additional metalbinding
phases which can reduce free-metal copper porewater activity.
In this thesis, metal binding to freshwater sediments was investigated in the same oxygen-free
and pH buffered conditions found in subsurface, freshwater sediment environments. Achieving these
objectives required the development of a new experimental technique called an Anoxic Sequential Batch Titration (ASBT). The technique is a titration method where sediments are spiked with metal
ions under anoxic and pH buffered conditions. Its ability to maintain these conditions was proven by
experiment. System anaerobicity, the kinetics of copper sorption, the dependence or independence of
sorption on solids concentration, and pH buffering ability is described in detail. After these system
parameters are shown to be controllable, the results of sediment/metal sorption studies are presented.
Correlation of sediment sorption capacities to total sediment organic carbon strongly suggests that this
is the sediment property responsible for the additional metal binding and reduction in sediment
toxicity.
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Keywords
Heavy Metal Sorption, Anoxic Sediments