1997 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, 1996 through November 14, 1997
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 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(12):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 Summary: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 the pure phase and in sediment, 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 MS (HTMS); and (2) those synthesized from sodium sulfide and the appropriate metal salt, which are termed low temperature MS (LTMS).
Oxidation Rates: A summary of the oxidation rates measured using oxygen as the oxidant are presented below.
There is approximately an order of magnitude variation among various sediment types. The oxidation rate is always faster than the HTMS for the MS. There also is a trend of increasing oxidation rate CdS < CuS < ZnS < PbS. The results of these experiments are used in the modeling analysis.
Modeling: A numerical one-dimensional transport and reactive sediment model has been developed to predict AVS and SEM profiles for sediments exposed to molecular oxygen through contact with an overlying water column. The flux to the overlying water also is computed. The model divides the sediment into multiple thin vertical layers in which the reactions take place. The model considers the vertical profiles of dissolved oxygen, particulate organic carbon (POC), dissolved sulfide, iron sulfide, iron oxyhydroxide, iron(II), dissolved and sorbed metal (to POC and iron oxyhydroxide), and MS. The sediment mass balance equation takes the following form:
where [c(z)] is the concentration of chemical per unit bulk volume of the sediment as a function of the depth z, fp is the fraction of the chemical that is in the particulate form, fd is the fraction of the chemical in the dissolved form, Dp(z,t) is the diffusion coefficient for particulate phase mixing (bioturbation), Dd is the diffusion coefficient for aqueous phase mixing, and f is the sediment porosity. The term Sx Kx represents the sum of all sources or sinks of the chemical that are first order with respect to the chemical concentration [c(z)]. The term Sy Ky represents the sum of all sources or sinks of the chemical that are zero order with respect to the chemical concentration.
The structure of the model is presented in Figure 1. The features incorporated in the model are: the oxidation of POC in the aerobic layer using oxygen (a) and in the anaerobic layer using sulfate (c); the formation (d) and oxidation (b) of iron sulfide (FeS(s)); the formation (g) and oxidation (e) of MS(s) as a source and sink of dissolved metals, respectively; partitioning of metals (f) to iron oxyhydroxide (FeOOH) and POC; and particulate and diffusive mixing to model bioturbation and pore water diffusion, respectively. Experiments are being carried out to support the model development. The oxidation kinetics of the MS are being determined for pure phase MS and from naturally contaminated sediments. A series of experiments are being
Figure 1. Structure of the sediment flux model.
performed to measure the metal fluxes from spiked and intact field collected cores and to determine the vertical profiles of SEM and AVS.
The sediment metal flux model has been generalized from its predecessor (Di Toro, et al., Environmental Toxicology and Chemistry 1996;15(12):2168-2186) in the following ways: (1) the model computes metal fluxes to the overlying water and the flux computations have been compared to observations; (2) the quantity of iron sulfide is now computed from the kinetics and solubility of iron sulfide rather than the empirical partition coefficient method used previously; and (3) the metals considered have been extended from only cadmium in the previous model to nickel, lead, and zinc.
For the final year of the project, the model will be calibrated to the flux data being generated.