Chapter 20
Release of Toxic Metals via Oxidation of Authigenic Pyrite in Resuspended Sediments John W. Morse Department of Oceanography, Texas A & M University, College Station, TX 77843
During early diagenesis in anoxic sediments many reactive trace metals are coprecipitated with authigenic pyrite. Metals of potential environmental concern, such as As and Hg, can have in excess of 80% of their reactive fraction incorporated into authigenic pyrite within the top 10 cm of many sediments. Resuspension of sediment into overlying oxic waters can result in the oxidation of pyrite leading to release of the coprecipitated metals. Experimental studies indicate that from about 20% to 50% of the pyrite can oxidize in a day. The loss of trace metals from the pyrite fraction relative to pyrite-Fe is highly variable during the oxidation reaction, but generally is close to or greater than that of pyrite-Fe. Factors controlling the availability of toxic metals to biota in aquatic environments are of major environmental interest (1). Sediments are usually die ultimate sink for these metals. Consequently, chemical reactions that control the burial or remobilization of toxic metals occurring near the sediment-water interface, during early diagenesis, are especially important in determining their fate. Fine-grained terrigenous sediments are commonly found in shallow-water environments, such as lakes,rivers,estuaries and near oceanic coasts. Such aquatic environments are also those that are likely to receive major contaminant toxic metal inputs and be otherwise perturbed by human activities. A common chemical characteristic of these sediments is that there is a sharp transition from oxic to anoxic conditions within centimeters of the sediment-water interface. This transition is dominantly driven by bacterially-mediated oxidation of organic matter within the sediments (2). After the oxygen in pore waters has been exhausted, bacteria utilize other oxidants such as nitrate, iron and manganese oxides, and dissolved sulfate to metabolize organic matter. This sequence of reactions has several important consequences. Sediments become increasingly reducing and pH drops with increasing depth within the sediment. The dominant initial pools for reactive trace metals are most commonly organic matter and metal oxides. These sediment components are significantly decomposed during diagenesis, releasing the associated trace metals. As sulfate reduction becomes a dominant oxidative process, significant
0097-6156/94/0550-0289$06.00/0 © 1994 American Chemical Society
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amounts of hydrogen sulfide are produced which reacts with reduced iron, via a complex pathway, to produce a variety of metastable iron-sulfide minerals and thermodynamically stable pyrite (3). Although it has long been recognized that many different metals can coprecipitate with pyrite (4), until recently it has not been possible to quantify this process for authigenic pyrite in sediments (5). We have recently (6) conducted an extensive investigation of the composition of authigenic pyrite from a wide range of aquatic environments. Our general observations indicate that the coprecipitation of Hg, As, Mo, and Cr with pyrite is a major process in anoxic sediments. The reactive fraction of most class Β and heavy metals appears to generally undergo only minor pyritization. The chemical pathways leading to the coprecipitation of metals with pyrite are currently not known, but may involve initial coprecipitation with metastable precursor iron-sulfide phases such as mackinawite (FeS). Experimental studies of the oxidation kinetics of sedimentary pyrite in seawater indicate that the pyrite can be divided into two majorfractions(7). The first fraction oxidizes rapidly, in a few days or less, and probably consists of submicron individual pyrite grains. The secondfractionoxidizes slowly and can persist in oxic seawater for months to years. It probably consists mostly of pyriteframboidsthat are typically 10 to 30 μπι in diameter. The firstfractionhas been observed to comprise about 15% to 50% of sedimentary pyrite in near-interfacial anoxic sediments from shallow-water environments (2). The observation that veryfine-grained,rapidly oxidizable pyrite is often a significantfractionof the total authigenic pyrite, in near-interfacial anoxic sediments, raises the possibility that if such sediments are resuspended in the water column oxidation of pyrite may lead to a release of coprecipitated metals. Resuspension of near-interfacial sediments is a common occurrence in many shallow-water environments. This may be caused by natural processes, such as storms, or by human activities, such as bottom trawling and dredging. This paper reports the results of an experimental investigation of the loss of trace metals from sedimentary pyrite when anoxic sediments are exposed to oxic waters. Study Area Sediments from Galveston Bay, Texas, were chosen for study because of concerns about pollution of this estuary and related studies of toxicant metals in this bay (8). It has a surface area of 1600 km , and is one of the largest embayments on the U.S. coastline. The Bay water is shallow, averaging only about 2 m in depth and is largely cut off from the Gulf of Mexico by the Bolivar Peninsula and Galveston Island. A mean residence time for waters in the Bay of about 40 days has been estimatedfromits average salinity of 15 and an averageriverflow of 12 km y (9). Thirty to fifty percent of the U.S. chemical production and oil refineries are situated around Galveston Bay. The Bay receives more than half of the total permitted waste-water discharges for the state of Texas. The combination of large population, high industrialization, shallow depth and restricted water exchange gives Galveston Bay the potential for serious trace-metal contamination problems. Samples were collected from a soft-bottom shallow-water area where extensive bottom trawling for shrimp resuspends sediments, and from ship channels and a boat basin where maintenance dredging and ship traffic alsofrequentlyresult in resuspension of anoxic sediments. Additional samples were collectedfroma dredgespoil bank on Pelican Island immediately after its formation. 2
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MORSE
Oxidation of Authigenic Pyrite in Resuspended Sediments
Methods and Procedures Sample Collection. Samples were collected from the RV Roman Empire (except for the dredge-spoil bank) either by hand coring with a precleaned plastic core tube or with an epoxy-coated grab sampler in deeper waters. Only the top ~10 cm of sediment was used. This sampling depth was used because it represents "nearsurface" sediments. The sediment was homogenized upon collection in sealed bags from which air was excluded to prevent oxidation. Metal Extraction and Analysis. Metals were extracted from the sediments using leaching techniques. Briefly, the sequential extraction procedure involves digestion of the sediment sample with 1M HC1 (reactive fraction), 10M HF (silicate fraction) and concentrated HNO3 (pyrite fraction). A more complete explanation of the sequential-extraction procedures and the development of the separation method is given elsewhere (5). Trace metals (As, Cr, Cu, Fe, Hg, Mo, Ni, Pb and Zn) were determined by flame atomic absorption (FAA) using a Perkin Elmer model 2380 spectrophotometer. Metals below the detection limit of this instrument were analyzed by direct injection into a Hitachi model 170-70 graphite furnace atomic absorption (GFAA) spectrophotometer with Zeeman background correction. The analytical precision (± relative standard deviation) was normally between 5 and 10% for FAA analyses and between 10 and 15% for GFAA analyses. Salt matrix effects for As determination by GFAA were partially overcome by using a Ni-Pd-ascorbic acid matrix modifier. Determination of Pb by GFAA in samples containing high Fe/Pb ratios (>250) was carried out following the procedure developed by Shao and Winefordner (10). Mercury was determined with a Laboratory Data Control U V monitor equipped with a 30-cm path-length cell, using the cold-vapor technique. For samples suspected of having high dissolved organic matter (DOM) concentrations, Hg was measured after destruction of the DOM with bromine monochloride. All reagents were ACS reagent grade or better. Milli-Q water was used for the preparation of all aqueous solutions. Acidic working standard solutions were always freshly prepared. Materials were carefully cleaned using established acid leaching procedures. The use of the sequential extraction procedures precluded comparisons with standard reference materials for which only total metal concentrations are generally available. Oxidation Kinetics Experiments. Samples that were chosen for the oxidation kinetics experiments were handled exclusively in a glove bag under an Ar atmosphere using deaerated water and fluid reagents to avoid oxidation prior to the experiment. Analyses were made on a subsample to establish the initial portion of reactive metal in the pyrite fraction. A second subsample was then stirred in a covered beaker containing natural seawater with a salinity of 15 (average for Galveston Bay) at room temperature (~23°C) for 1 day. A paddle stirrer was used to avoid grinding of the sample and air was continuously bubbled through the solution. Results and Discussion Pyritization of Metals. Berner (Π) introduced the concept of degree of pyritization (DOP) to describe the extent of reactive-Fe transformation to pyrite. DOP is defined as: DOP =
Pyrite-Fe Pyrite-Fe + Reactive-Fe
( 1 )
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where pyrite-Fe is assumed equal to 0.5 χ (total inorganic reduced sulfur - acid volatile sulfides), based on the 1:2 stoichiometry of Fe:S in pyrite. Subsequently, Huerta-Diaz and Morse (5) expanded this concept to metals (Me) other than Fe defining the degree of trace metal pyritization (DTMP) similarly to DOP for iron. By comparing DTMP with DOP it is possible to relate the pyritization of a given trace metal to that of Fe, which is the dominant metal that is pyritized. DTMP =
Pyrite-Me Pyrite-Me + Reactive-Me
( 2 )
The relationships between the DTMP of trace metals and DOP in near interfacial Galveston Bay sediments are shown in Figure 1. The plots are arranged in approximate order of increasing tendency of metals to become pyritized. All metals have a significant degree (>10%) of pyritization in at least some samples. However, Ni, Zn and Pb are only slightly pyritized in most samples. They have, therefore, not been considered in the experiments to study the release of trace metals during pyrite oxidation. It is important to note that three of the metals that undergo extensive pyritization, Cu, As, and Hg, are ones that have often been of major environmental concern because of their toxicity in aqueous ecosystems (1). Oxidation of Pyrite and Release of Metals. Results of the experiments on the oxidative release of metals from pyrite are summarized in Table I. For the four metals studied (As, Cu, Hg and Mo), 88% of the results indicated at least a 25% oxidation of the pyritized metal. However, the extent of metal oxidation was highly variable for both a given metal between different samples and for different metals within a single sample. The relationship between the initial degree of pyritization of a metal and its percent oxidation is shown in Figure 2. There is a broad scatter in this relationship. However, if the relationship is examined for individual metals it is observed that they can be divided into two groups. Fe and As oxidation are not correlated (r = 0.00) with their initial extent of pyritization. However, Cu, Hg and Mo exhibit a moderately good correlation (r = 0.58, 0.55 and 0.52, respectively) between extent of oxidation and initial extent of pyritization. The correlations are positive for Cu and Mo, but negative for Hg. The extent of loss of As, Cu, Hg and Mo from the pyritefractionis compared with the extent of pyrite oxidation, as represented by loss of Fe from the pyrite fraction, in Figure 3. Again there is a wide scatter in the results. Generally (in 80% of the samples), the oxidation of metals from the pyritefractionis greater than the extent of pyrite oxidation. About a third of the samples exhibit particularly high (>70%) extents of metal oxidation for As and Cu. One possible reason for the large scatter in the data presented in Figure 3 is that "apples and oranges" are perhaps being mixed, in that near surficial (top 10 cm) sediments and deeper sediments from the dredge spoil site are both included. It is reasonable to expect the deeper, and therefore older, sediments from the dredge spoil site may contain a smallerfractionof highly reactive, veryfine-grainedpyrite and the behavior of trace elements might also differ. This hypothesis is supported by the data in Table II. Fe and Cu are about twice as reactive, and Mo is about three times as reactive in the surficial sediments as in the dredged sediments. 2
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Oxidation of Authigenic Pyrite in Resuspended Sediments
0.0
0.2
0.4
0.6
DOP
0.8
1.0
0.0
0.2
0.4
0.6
0.8
1.0
DOP
Figure 1. The relationship between I TMP and DOP for trace metals in Galveston Bay sediments. The line on the plots represents an ideal relationship.
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