Pangloss Foundation,Berkeley, California
Mary S. Quinby-Hunt
Lawrence Berkeley National Laboratory, University of California, Berkeley
After Fourth International Conference on the Biogeochemistry of Trace Metals, 23-26 June 1997, Berkeley, California, Iskandar, I. K., Hardy, S. E., Chang, A. C., and Pierzynski, G. M. (eds.) Proceedings Volume, p. 339-340
A serious problem during clean-up of various hazardous waste sites is the possibility of introducing various ions to the aquatic environment which are toxic or can chelate to form toxic substances. Removal of soils and sediments from their ambient environment may result in altered chemical state, as may various treatments and disposal scenarios. Typically, the ambient environment for soils and sediments is anoxic. If the sediments are placed in a more oxic environment, various ions may be mobilized as a function of the redox conditions. Several other factors influence the mobilization of potentially toxic ions or chelates, including pH, ionic strength, and salinity of the receiving aqueous environment as well as increase in the porosity and permeability of the material. Because disturbing of sediments and soils results in changing physical conditions and may result in an altered chemical environment, it is important to understand the potential chemical transformations that may occur based on the nature of the contaminants present.
Various scenarios must be considered [see table below]. The processes involved may include several steps each associated with different alterations of Eh and/or pH. For example, removal of bottom sediments from the ocean initially can induce increased oxygenation and therefore higher Eh. If the dredged material is then disposed on land it will be subjected to water of much lower pH than those waters from which it was initially deposited. Under some reburial conditions, the Eh could be expected to drop again, thereby exposing the sediments to low Eh conditions, but at a much lower pH. Conversely, sediments deposited on land, in fresh water, or even in estuarine conditions and subsequently deposited in marine waters, may go from anoxic, low pH conditions to oxic low pH conditions, through oxic conditions, but nearer pH 8, and finally to anoxic conditions with the pH ranging from near 7 to somewhat above 8.
Scenarios resulting in altered Eh/pH conditions TRANSFER PROCESS MEDIUM AFFECTED Land to land Fill Ground Water Land to reclamation Fill Rivers, Lakes Land to estuaries Dumping Brackish Water Land to ocean Dumping Sea Water Bay bottom to land Dredging Ground Water Bay bottom to estuary Dredging Brackish Water Bay bottom to ocean Dredging Sea Water Ocean bottom to land Dredging Ground Water Ocean bottom to ocean Dredging Sea Water
Thermodynamically (not necessarily kinetically) at pH 7 with increasing Eh, the hierarchy of ionic mobilization is V; Pb: Fe, Co, Zn, Cd; Hg, Ni, and Cu at the specific concentrations considered . Measurements in the marine environment show that both Cu and Cd are dramatically mobilized
when introduced to more oxic conditions 12]. Cu may be toxic at the levels released into marine waters. The deleterious effects of Cu on the diatom Thalassiosira pseudonana have been noted . The toxic effects of Cd and Pb are well documented. Fe is released at intermediate Eh over a wide range of pH [4,5]. While Fe is not toxic at the levels observed in natural waters, ions adsorbed or included in iron or manganese oxides or sulfides may be released if exposed to particular Eh/pH conditions [6-9]. The occluded ions may be toxic.
Flocculation conditions are strongly influenced by salinity. Therefore, salinity changes at the disposal site may result in release of potentially toxic substances. For example, clayey soils and sediment introduced into "fresh" water environments (less than 2 parts per thousand salinity) form clay-organic platelets with a low settling velocity. These platelets have a high contact time with more oxic surface waters, thus have a greater potential to reach a new equilibrium with the partial to total dissolution of the constituent metal complexes. In more saline environment found in estuaries and the ocean, due to the higher ionic strength, the platelets form floccules with a higher settling velocity thus having a shorter contact time with the generally more oxic surface waters. However, such sediment may be remobilized in estuaries and the open ocean due to tides and waves again increasing the contact time with oxic waters and potential dissolution.
This paper discusses the thermodynamic hierarchy of stability of various natural metal complexes, which as sulfides, sulfates, oxides, carbonates etc. are generally stable in the anoxic environments on land, but would remobilize in more oxic conditions. Various redox scenarios are examined from fresh water to normal marine receiving waters as a function of typical pH and commonly occurring soil and sediment mineralogy likely to be encountered at potential clean-up sites.
The goal here is to prevent creating a new hazard while suggesting scenarios which can be followed to insure safe and long-term clean-up of hazardous sites.
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