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ISSUES IN ASSESSING LOW LEVEL IONIZABLE CONTAMINANT PARTITIONING IN SOILS AND SEDIMENTS
Citation:
Loux, N T. ISSUES IN ASSESSING LOW LEVEL IONIZABLE CONTAMINANT PARTITIONING IN SOILS AND SEDIMENTS. Presented at Society for Risk Analysis Symposium, Atlanta, GA, December 5-8, 1999.
Description:
Solubilization has profound implications for such diverse risk assessment activities as assessing sediment contaminant porewater exposures to benthic fauna, determining half lives of refractory toxicants in natural soils and sediments, and assessing the fate and transport of these compounds in surface and ground waters. The mechanisms governing toxicant partitioning are only partially understood. At high soil/sediment porewater ionizable contaminant concentrations or in the presence of porewater sulfides, dissolved ionic contaminant concentrations can be limited by precipitation reactions with background reactants including hydroxide, bicarbonate, carbonate, sulfate, phosphate and sulfide ligands. In the absence of sulfides and/or at lesser porewater contaminant concentrations, solubility limitations may occur through solid-solution formation by trace toxicants with natural minerals or by adsorption of these compounds on environmental surfaces.
The last topic, adsorptive phenomena, is the focus of the present discussion. Many adsorption reactions with trace metals occur through a reaction of the form: Mez+ + >SOH <==> >SOMe(z-1)+ + H + (where Mez+ represents a metal ion of valence z+ and >SOH designates a bound site on an environmental surface; e.g., >SOH can represent an adsorptive site on particulate organic carbon, aluminosilicates, quartz and hydrous oxides of iron, manganese and aluminum ). Although it is believed that environmental surfaces display these adsorptive phenomena, limitations exist in our ability to reliably quantify these processes in environmental systems. Specific questions include: 1) can laboratory findings be extended to the field scale, 2) what time period is necessary to assume that equilibrium has occurred, 3) does "irreversible" adsorption occur, 4) are environ-mental adsorptive surfaces "additive" with respect to adsorptive phenomena, and 5) do current surface complexation adsorption paradigms contain assumptions that may be inappropriate for environmental application? These and other questions will be examined.