1998 Progress Report: A Modeling and Experimental Investigation of Metal Release from Contaminated Sediments The Effects of Metal Sulfide Oxidation and ResuspensionEPA Grant Number: R825277
Title: A Modeling and Experimental Investigation of Metal Release from Contaminated Sediments The Effects of Metal Sulfide Oxidation and Resuspension
Investigators: Di Toro, Dominic M. , Mahony, John D.
Institution: Manhattan College
EPA Project Officer: Hahn, Intaek
Project Period: November 15, 1996 through November 14, 1999 (Extended to November 14, 2000)
Project Period Covered by this Report: November 15, 1997 through November 14, 1998
Project Amount: $544,463
RFA: Risk-Based Decisions for Contaminated Sediments (1996) RFA Text | Recipients Lists
Research Category: Hazardous Waste/Remediation , Land and Waste Management
Objective:The objective of this research project is to construct and validate a mechanistically realistic model of the release of potentially toxic metals from contaminated sediments to the overlying water. These are: cadmium, copper, nickel, lead, and zinc, all of which form metal sulfides (MS) more insoluble than iron sulfide. The model is intended to be used in conjunction with water column fate and transport models. The model also is intended to be compatible with sediment quality criteria based on simultaneously extracted metals (SEM) and acid volatile sulfide (AVS) concentrations in sediments as well as interstitial water toxic units (Ankley et al., Environmental Toxicology and Chemistry 1996;15:2056-2066). The previously available model formulations for computing the metal flux from sediments are incomplete because they take no account of the critical and central importance of MS formation, dissolution, and oxidation. They are based on partition coefficients. As a consequence, they provide only a rough approximation to what is actually controlling the release of metals from sediments. The purpose of this project is to remedy this situation by producing a model that calculates the flux of metals from the sediment to the overlying water. The model can be incorporated into presently available water quality models for metals.
Progress has been made on both the modeling and experimental phases of the
project. Oxidation experiments are being conducted for CdS, CuS, PbS, and ZnS.
Our objective is to evaluate the rate constant, k, for the oxidation of heavy
MS, in both pure phases and in sediments, and evaluate factors affecting k. We
examined oxidation of the pure phase by both oxygen and peroxide, as this would
provide an instructive comparison with the body of research that exists for H2S.
Two classes of MS are employed: (1) those synthesized at high temperatures,
which are termed high temperature metal sulfides (HTMS); and (2) those
synthesized from sodium sulfide and the appropriate metal salt, which are termed
low temperature metal sulfides (LTMS).
Data Analysis. In accordance with previous determinations of sulfide oxidation kinetics, it is assumed that the decrease of MS due to oxidation is first order with respect to both the MS and oxidant:
Because less than 10 percent of the initial MS is oxidized in any of the experiments performed, the concentration of solid MS remains virtually constant. Therefore, the kinetics expression may be rewritten as:
where k' is a pseudo-first-order rate constant that describes the decrease of MS(s) on a per-time basis. The decrease of MS is directly proportional to the increase in metal ion concentration:
Therefore, combining these two equations yields:
Plots of dissolved metal concentration versus time are fit with a linear regression. The slope of the regression line is divided by the initial mass of MS(s)0 to obtain k'. An example is presented below. The dissolved concentration is the sum of the measured aqueous concentration and the metal sorbed to the MS particles that is extracted separately. The linear increase confirms the expected result.
Modeling. Experiments designed to produce data for modeling calibration also are being generated. Spiked sediment and naturally contaminated sediments are placed in core tubes with aerated overlying water. The resulting dissolved concentrations are monitored in time; at the end of the experiment, typically 100 days, the sediment core is sliced, and the SEM and AVS are measured. These data for nickel, cadmium, zinc, and lead will form the basis for model calibration.