Environ. Sci. Technol. 2000, 34, 4533-4537
Effect of Short-Term Resuspension Events on the Oxidation of Cadmium, Lead, and Zinc Sulfide Phases in Anoxic Estuarine Sediments STUART L. SIMPSON,* SIMON C. APTE, AND GRAEME E. BATLEY Centre for Advanced Analytical Chemistry, CSIRO Energy Technology, PMB 7, Bangor, New South Wales 2234, Australia
The stability of metal sulfides to oxidation in seawater was investigated in short-term (24 h) resuspension experiments using natural sediments and synthetic metal sulfide phases. Using acid volatile sulfide (AVS) measurements, ZnS, PbS, and CdS phases were shown to be resistant to oxidation. The AVS in resuspended, highly anoxic, contaminated sediments was rapidly oxidized with 98%) was checked by measuring the absence of residual metal present in the dissolved phase and the sulfide content of the mineral phase during the AVS extraction. Sediment Sample Preparation. To minimize sample oxidation artifacts, all sediment manipulations (e.g., metal additions, mixing, and equilibration) were performed in a N2(g)-filled glovebox. The equilibrated sediments were sampled using a plastic corer (8 cm diameter, 20 cm length). The top 5 cm of the core was discarded, and the remainder VOL. 34, NO. 21, 2000 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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TABLE 1. Physical and Chemical Properties of Test Sediments Prior to Metal Additions or Resuspension Experimentsa sediment
Kings Bay
Cooks River
Iron Cove
AVS (µmol/g): 0 h, 24 hb particle size: 95% of the expected SEM concentration (µmol/g) in each sample was used as a check of the quantitative transfer process. To check errors and precision, all experiments were replicated (generally in triplicate). Replicate resuspension experiments were performed for the measurement of dissolved oxygen and pH. Simulated resuspension experiments were performed for periods of 0-24 h on the three unmodified sediments as well as on preparations of the model metal sulfide phases, CdS, FeS, PbS, or ZnS (70 µmol), that were resuspended for 24 h in seawater in both the absence and the presence of each sediment (1 g dry wt). The effect of oxidized sediments on the recovery of the model sulfides (in the AVS extraction) was investigated by the addition of the sulfides, immediately prior to AVS analyses, to samples of each sediment that had been resuspended for 4534
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24 h in seawater. The effect of sediment/water ratios ranging from 0 to 50 g(dry wt)/L were examined for each sediment to determine the effects on PbS and ZnS oxidation. The effect of oxidized iron phases on the oxidation of or recovery of model sulfides was investigated by addition of freshly prepared Fe(OH)3 suspensions either prior to or following resuspension experiments. The effect of added Fe(OH)3 on AVS recovery for ZnS resuspended in seawater with Kings Bay sediment was also investigated. The effect of suspension pH on the oxidation ZnS in the presence of Iron Cove sediment was investigated by buffering suspensions in the range of 6.7-8.1 using the biological buffers (0.01 M) PIPES (pKa ) 6.8) and EPPS (pKa ) 8.0). Resuspension experiments, for periods of 0-24 h, were also performed on the Cooks River sediment spiked with Zn(II) or ZnS and Iron Cove sediments spiked with the different lead and zinc solid materials.
Results and Discussion AVS and SEM analyses are useful tools for interpreting changes occurring to metal sulfide phases as a result of oxidation processes. The AVS extraction procedure used (1 M HCl for 45 min) extracts H2S, amorphous FeS, mackinawite (FeS(1 - x)), some greigite (Fe3S4), but no pyritic sulfides (FeS2) from sediments (16). The sulfide phases (CdS, PbS, and ZnS) are completely extracted by the procedure, while dissolution of CuS and NiS phases is limited under these conditions (14, 17, 18). For our studies, the decrease in the amount of AVS measured after sample resuspension in oxygenated seawater was interpreted in terms of oxidation occurring to discrete monosulfide phases (principally FeS, CdS, PbS, and ZnS). Sediment Properties. The physical and chemical measurements on the collected sediments are summarized in Table 1. The samples varied in organic carbon content and particle size distribution. Not surprisingly for an urban estuary, zinc and lead were the major heavy metal contaminants. It was notable that for these metals SEM was comparable to TPM concentrations, suggesting that the concentrations of pyrite-bound and slowly dissolving phases were low. The AVS concentrations (82, 127, and 212 µmol/g) were of similar magnitude to the simultaneously extractable iron concentrations (109, 136, and 234 µmol/g), indicating that the primary source of AVS was FeS. During the 4-month study, the AVS and SEM concentrations in the test sediments remained the same, within experimental error ((7%), regardless of when subsampling occurred.
FIGURE 1. Changes in AVS concentration during a 24-h resuspension of sediments in seawater prior to metal additions. Kings Bay (b), Cooks River (2), Iron Cove (0). SEM ∑(Cd, Pb, Zn) concentrations are represented by the horizontal lines: KB(‚‚‚), CR (- - -), IC (-), respectively.
FIGURE 2. Effect of sediment/water ratio (0-50 g (dry weight)/L) on the recovery (as AVS) of 70 µmol of ZnS (filled symbols) or PbS (open symbols) following 24 h resuspension in the presence of the sediments Kings Bay (b, O), Cooks River (2, 4) and Iron Cove (9, 0). Error bars represent one standard deviation.
TABLE 2. Recovery of CdS, PbS, and ZnS Added to Sediments (i) Prior to Resuspension, and (ii) Following Sediment Resuspension but 5 min before AVS Analysisa
values) were 97 ( 6% for ZnS, 91 ( 8% for CdS, 91 ( 11% for PbS, and 98%, Table 1, Figure 1). After 24-h resuspension in seawater, the AVS concentration had decreased to 1.8 (( 0.5), 6.7 (( 0.3), and 4.0 (( 0.4) µmol/g, respectively (Figure 1). The AVS values were well below the SEM concentration of each sediment. This indicated either that 95%, 73%, and 82% of the PbS and ZnS phases in the respective sediments had been oxidized or that the majority of the lead and zinc phases in the sediments were not present as discrete metal sulfide phases. Given that the earlier experiments demonstrated rapid formation of metal sulfides upon addition of dissolved, ionic metals to anoxic sediments and resistance of Pb, Zn, and Cd sulfides to oxidation during resuspension, it was hypothesized that that the majority of the lead and zinc present in the unmodified anoxic sediments were not in the form of discrete metal sulfides. In most cases, the chemical form of metals introduced into sediments is unknown but is likely to include both solid and dissolved phases from point source industrial discharges and diffuse drainage inputs. One possibility is that these metals were input as metallic metals or metal oxides or salts whose surfaces become sulfidized. These coatings may prevent the complete sulfidization of the solids. To investigate 4536
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this, the effects of adding various lead and zinc solid phases to the Iron Cove sediment were investigated. As seen in Table 3, apart from Zn(powder), the added lead and zinc phases dissolved completely during the AVS/SEM extraction procedure (recoveries of 88-107% reported as SEM). Upon resuspension in seawater, the AVS concentration of sediments spiked with ZnCl2(s) and ZnCO3(s) reached a plateau that corresponded to only 75% of the added material forming sulfide phases (Table 3). Sulfide formation was less for the other solid phases (10-31%) and decreased in the following order: PbCl2(s) > ZnO(s) > Zn(powder) > PbO(s). Analyses of AVS/SEM 1 and 3 months after metal additions, respectively, did not show any consistent changes in material reactivity with time. This experiment suggests that a significant portion of metals entering aquatic environments as particulate materials may not become sulfidized for long periods of time (years). These results are consistent with the rapid formation of insoluble sulfide coatings on the outer surface of the added particles that arms them against further reaction. Canfield et al. (20) noted that the sulfidization of different iron minerals in 1 mM sulfide solutions was quite variable with half-lives of 2.8 h (ferrihydrite), 1) and in the absence of other binding phases, porewater metal concentrations may be high and toxicity may occur. Berry et al. (8) provided evidence for this relationship holding for each of the metal ions Cd, Cu, Ni, Pb, and Zn. A fundamental principle of the AVS/SEM theory is that in sulfide-rich sediments the trace metal contaminants are
predominantly present as metal sulfide phases and that nonsulfidic binding phases (e.g., hydroxide/carbonate phases) are insignificant. It is assumed that the sulfide phases extracted by the AVS procedure are predominantly monosulfide phases (e.g., FeS, MnS, and ZnS) and that for every mole of sulfide measured as AVS there will be a corresponding mole of metal measured in the SEM fraction. Our findings are in agreement with previous studies that trace metals added to sediments in an ionic form will react to form discrete sulfide phases, MeS (5, 19, 22-24). Recent studies have suggested that AVS/SEM theory is, at most, applicable to assessing the bioavailability of Cd-, Pb-, and Zn-contaminated sediments. However, even for these metals, assumptions regarding their partitioning to the sulfide phases in sediments may not always be justifiable (14, 17). This is further supported by the findings of this study. Sediments artificially contaminated with metal powders, oxides, or solid metal salts reacted slowly with porewater sulfide. In the anthropogenically metal-contaminated, sulfiderich estuarine sediments investigated, high proportions of the zinc and lead contaminants may not be present as metal sulfides despite high reactive sulfide concentrations (AVS). If sulfide coatings are limiting the reactions of oxidized metal surfaces but these surfaces are soluble under the AVS/SEM extraction procedures, an excess of SEM over AVS will be observed, when in fact such SEM is unavailable. Potential toxicity will therefore be falsely predicted (type II error). Caution is therefore required in extrapolating from laboratory studies where sediments have been artificially contaminated by spiking with metals to studies with naturally contaminated sediments, particularly when considering AVS/ SEM measurements as a predictor of metal bioavailability and toxicity.
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Received for review December 28, 1999. Revised manuscript received July 26, 2000. Accepted July 31, 2000. ES991440X
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