Degree Of Trace Metal Pyritization Research Articles

Geochemistry of the redox-sensitive trace elements molybdenum, tungsten, and rhenium in the euxinic porewaters and bottom sediments of the Pettaquamscutt River estuary, Rhode Island

Molybdenum (Mo), tungsten (W), and rhenium (Re) concentrations were measured in the porewaters and sediments of the highly stratified Upper Basin of the Pettaquamscutt River estuary (Rhode Island) to investigate the geochemical processes that influence their concentrations in euxinic systems. Total dissolved sulfide (i.e., ΣS-II) concentrations in the porewaters averaged 3.95 mmol kg−1, whereas the porewaters exhibited average Mo, W, and Re concentrations of 830 nmol kg−1, 69 nmol kg−1, and 7.4 nmol kg−1, respectively. The sediments were subjected to chemical extractions to characterize the reactive fraction (i.e., associated with iron (Fe) monosulfides, Fe/manganese (Mn) oxides/oxyhydroxides, and/or Fe/Mn carbonate minerals) and the pyritized fraction of Mo, W, and Re in the sediments. The reactive fraction of each metal decreased with depth from subsurface maxima near the sediment-water interface, whereas the pyritized fractions exhibited a number of subsurface maxima. Rhenium exhibited the greatest degree of trace metal pyritization (DTMP) in the estuary sediments, followed closely by Mo. Specifically, the DTMP for Re ranged from 75 to 88% and exhibited a mean ( ± 1σ) of 80 ± 4% for the entire sediment core, whereas the DTMP for Mo ranged from 34 to 89% with a mean ( ± 1σ) of 73 ± 18%. In contrast, the DTMP for W was lower, ranging from 48 to 73%, but with a mean ( ± 1σ) of 58 ± 8%. Therefore, about 80%, on average, of the Re and 74% of the Mo that enters the northern portion of the Pettaquamscutt River estuary by the combination of stream water inflow (i.e., Gilbert Stuart Stream) and submarine groundwater discharge (SGD) is eventually fixed within the pyrite fraction of the euxinic bottom sediments. However, only about 60% of the W appears to be fixed in the pyrite fraction in these sediments, whereas the other 40% remains in the reactive sediment pool. We suggest that the different behavior of W compared to Re and Mo in the euxinic porewater and sediments of the Pettaquamscutt River estuary's upper basin reflects, in part, differences in their aqueous speciation and particle reactivity that appears to be dictated by differential uptake and fixation of their respective thioanions (i.e., MoS42−, WS42−, ReS4−) with Fe sulfide minerals.

Sulfide influence on metal behavior in a polluted southern Mediterranean lagoon: implications for management.

The degree of pyritization and degree of trace metal pyritization (DTMP) were investigated in sediments from Ghar El Melh Lagoon (northern Tunisia) in order to study metal deposition. A sediment core and 28 samples were thus taken in summer 2008, and metals and sulfate were analyzed in pore water/pyrite. Acid-volatile sulfide and metals were simultaneously extracted from these two fractions and the role of pyrite in the metal cycling studied. To examine pyrite presence and mineralogical form in sediments, X-ray diffraction of the washed and decarbonated sediment was performed along with scanning electron microscopy. Results showed that pyrite is present in fromboidal and euhedral forms. Thermodynamic calculation highlighted the formation of metallic sulfides and the co-precipitation of metals with iron sulfides. The DTMP increases with depth, indicating that these metals are either sequestered as sulfides or that they co-precipitate with pyrite into the deep sediment.

Sedimentary trace element sinks in a tropical upwelling system

The accumulation of trace elements in sediments from highly productive continental margins may depend on the affinity of these elements for organic matter and their degrees of further incorporation into pyrite (FeS2). We tested the hypothesis that the relative contributions of these geochemical phases play a substantial role as trace element (As, Cd, Cr, Cu, Mn, Ni, and Zn) sinks in the highly bioturbated sediments from the tropical upwelling system off Cabo Frio, southeastern Brazil. Four sediment cores sampled across the Cabo Frio continental shelf were submitted to a sequential extraction procedure performed to separate three different operationally defined fractions, i.e., the geochemical phases soluble in 1 M HCl (considered as the “reactive” fraction), concentrated H2SO4 (considered as the organic matter-bound phase), and concentrated HNO3 (considered as the pyrite-bound phase). The trace metal incorporation into pyrite was assessed by estimating the degree of trace metal pyritization (DTMP), while the pyrite sulfur stable isotope signatures (δ 34SPyr) were used as proxies for sulfur redox cycling intensity. Relative contributions of trace element fixation by organic matter and pyrite were positively correlated for Mn, Cr, and Ni on one hand, and negative correlated for Cu, on the other hand. The positive correlations imply in synergistic roles of these geochemical phases in determining the trace elements sedimentary sinks, while the negative relationship found for Cu reflects differences in the predominant retention mechanisms along with sediment burial. The δ34SPyr signatures were negatively correlated with DTMP values of As, Cd, and Mn, suggesting a diminishing effect of the sulfur redox cycling on trace elements pyritization. These δ 34SPyr signatures were not correlated with DTMP values of Cr, Cu, and Ni, which were dominantly associated with the high organic matter contents found in this upwelling system. The role of pelagic organic matter scavenging of metals and later fueling of benthic microbial sulfate reduction and pyrite accumulation were evidenced as highly variable across the Cabo Frio shelf sediments. Differences in the organic matter accumulation in response to upwelling-enhanced primary productivity and in the intensity of bioturbation-driven sulfur redox cycling help to explain the spatial variability in the biogeochemical processes affecting the sedimentary trace metal sinks.

Trace metals partitioning among different sedimentary mineral phases and the deposit-feeding polychaete Armandia brevis

Trace metals (Cd, Co, Cu, Fe, Mn, Ni, Pb, Zn) were determined in two operationally defined fractions (HCl and pyrite) in sediments from Ensenada and El Sauzal harbors (Mexico). The HCl fraction had significantly higher metal concentrations relative to the pyrite fraction in both harbors, underlining the weak tendency of most trace metals to associate with pyrite. Exceptionally, Cu was highly pyritized, with degrees of trace metal pyritization (DTMP) >80% in both harbors. Dissolved Fe flux measurements combined with solid phase Fe sulfide data indicated that 98mt of Fe are precipitated as iron sulfides every year in Ensenada Harbor. These Fe sulfides (and associated trace metals) will remain preserved in the sediments, unless they are perturbed by dredging or sediment resuspension. Calculations indicate that dredging activities could export to the open ocean 0.20±0.13 to (0.30±0.56)×103mt of Cd and Cu, respectively, creating a potential threat to marine benthic organisms. Degrees of pyritization (DOP) values in Ensenada and El Sauzal harbors were relatively low (<25%) while degrees of sulfidization (DOS) were high (~50%) because of the contribution of acid volatile sulfide. DOP values correlated with DTMP values (p≤0.001), indicating that metals are gradually incorporated into pyrite as this mineral is formed. Significant correlations were also found between DTMP values and −log(Ksp(MeS)/Ksp(pyr)) for both harbors, indicating that incorporation of trace metals into the pyrite phase is a function of the solubility product of the corresponding metal sulfide. The order in which elements were pyritized in both harbors was Zn≈Mn<Fe<Cd≈Pb<Ni≈Co<<Cu. Lastly, a strong correlation (r2=0.87, p<0.01) was found between average reactive trace metal concentrations and metal concentrations measured in Armandia brevis (a deposit-feeding Opheliid polychaete), suggesting that these labile sedimentary metals are preferentially accumulated by the polychaete, making it a useful biomonitor of sedimentary metal exposure.

Arsenic fractionation in estuarine sediments: Does coastal eutrophication influence As behavior?

The Paranaguá Estuarine Complex (PEC) includes the naturally oligotrophic (NO) Mel Island which is surrounded by sea grasses, a naturally eutrophic (NE) Benito Inlet adjacent to mangrove wetlands and the highly impacted eutrophic (IE) Paranaguá Bay, home of one of Brazil’s largest ports. The results from this study indicate that reactive As and pyrite increase with sediment depth near Paranaguá port in the IE region. At the NE region, near a mangrove fringe, the reactive As, Fe, Mn and pyrite remained relatively high along the sediment column while near the sea grasses at NO the As contents were low. The degree of trace metal pyritization (DTMP) and the degree of pyritization (DOP) was highest at the IE site, slightly increasing with depth. These baseline results indicate that influence of trophic conditions and presence of marine vegetation may be directly related to As behavior in coastal systems.

High variability in geochemical partitioning of iron, manganese and harmful trace metals in sediments of the mining port of Santa Rosalia, Baja California Sur, Mexico

Total trace metal concentrations and the HCl-soluble (MeHCl) and pyrite (Mepyr) fractions were evaluated in two sediment cores (SR1 and SR2) and mine tailings from the copper mining town of Santa Rosalia (SR), Baja California Sur, Mexico. MeHCl were significantly different in the two cores (p≤0.001), in particular Cd and Fe in core SR2 were 29 and 1.0x104 times higher than concentrations found in SR1, respectively. These higher concentrations in core SR2 are likely due to the fact that the core was collected from a navigational channel last dredged between 1931 and 1948. Assuming that the trace metal (Me) reactive fraction (MeRF=MeHCl+Mepyr) is the most labile, then for each ton of dredged harbor sediment, up to 4.5±3.6kg of CuRF (equivalent to almost 0.5% of Cu on a weight basis) could potentially be exported into the ocean. Since an important portion of the reactive trace metals are in the Mepyr fraction, as indicated by their high degree of trace metal pyritization (DTMP=100Mepyr/MeRF) values (>98% for Cu, Fe, Pb, Zn), export outside of the harbor will oxidatively dissolve trace metals and release them into the marine environment. Trace metal enrichments (EFcrust), relative to Al-normalized Earth's crust values (ECV) ranged from 0.94±0.14 to (11.5±5.6)x102 times the ECV for Ni and Cd, respectively. These EFcrust values were differentiated into three groups: EF≈1 (Ni, Fe), 10<EF<100 (Mn, Co, Pb, Zn), and EF>100 (Cu, Cd). Copper enrichments are not surprising considering its high concentrations in the SR mine tailings; however, the higher Cd enrichments (6 to 11 times higher than Cu) were unexpected. Further analyses of mine tailings showed that the Cd enrichments increased considerably through ore processing. Concentrations for a number of trace metals in SR sediments are high even when compared with wastes from mines of other countries, indicating that these sediments essentially should be treated as mine tailings. Finally, our results show that the distribution of the different trace metals is not the result of diagenetic processes, but rather the result of input from ore-derived sources.

Trace metal pyritization variability in response to mangrove soil aerobic and anaerobic oxidation processes

The degree of iron pyritization (DOP) and degree of trace metal pyritization (DTMP) were evaluated in mangrove soil profiles from an estuarine area located in Rio de Janeiro (SE Brazil). The soil pH was negatively correlated with redox potential (Eh) and positively correlated with DOP and DTMP of some elements (Mn, Cu and Pb), suggesting that pyrite oxidation generated acidity and can affect the importance of pyrite as a trace metal-binding phase, mainly in response to spatial variability in tidal flooding. Besides these aerobic oxidation effects, results from a sequential extraction analyses of reactive phases evidenced that Mn oxidized phase consumption in reaction with pyrite can be also important to determine the pyritization of trace elements. Cumulative effects of these aerobic and anaerobic oxidation processes were evidenced as factors affecting the capacity of mangrove soils to act as a sink for trace metals through pyritization processes.

Millimeter‐scale resolution of trace metal distributions in microbial mats from a hypersaline environment in Baja California, Mexico

Microbial mats from two ponds with different salinities from the saltern of Guerrero Negro (Mexico) points toward millimeter-scale coherent variations in trace metal (Me) concentrations (Cd, Co, Cu, Fe, Mn, Ni, Pb, Zn). Total, HCl-leachable and pyrite-associated Me showed a trend of increasing concentrations with increasing depth suggesting gradual addition of reactive Me probably as a result of metal sulfide precipitation at depth. The trends in Me profiles can be ascribed to the establishment and maintenance of microzones that promote geochemical processes, bacterial population distributions, and differential mass transport within the mats. Degrees of trace metal pyritization (1 ± 1% for Zn to 24 ± 7% for Cd) as well as metals associated with the pyrite fraction (<1.4-36 ± 18 nmol g(-1) for Zn and Mn, respectively) were low, as expected from a reactive Fe-limited system like Guerrero Negro. Calculated enrichment factors showed that Ni (2.6 ± 2.1), Co (5.5 ± 4.0), Pb (9.4 ± 7.4), and Cd (57 ± 39) were, on average, enriched in the microbial mats of Guerrero Negro. Natural enrichments of Cd, Pb, and Co in sediments along the coast of Baja California and metabolical requirements of Co and Ni by the predominant cyanobacteria in the Guerrero Negro mats may explain these enrichments. Metal characteristics in microbial mats could be advantageously used as biosignatures to identify their presence in the geological record or in other planetary systems.

Pyritization of trace metals in mangrove sediments

The present study investigates the levels of Mn, Zn, Ni, and Co pyritization in mangrove sediments along distinct sedimentary zones in Enseada das Gracas, a lagoon-type estuary located on the southeastern coast of Brazil. The coastal geology is characterized by intense interactions of trace metals, forming pyrite minerals. Specific orders of DOP (degree of pyritization) and DTMP (degree of trace-metal pyritization) are shown: supratidal flat < mangrove forest < mud flat. Distinct changes in content along the sediment profiles are noted, where a supratidal flat presented low levels of DOP and DTMP with little variance along the sedimentary depths. The mangrove forest showed relatively high values of DOP and DTMP in the lower depths, while the mud flat showed the highest levels of DOP and DTMP.

Iron and Trace Metals in Microbial Mats and Underlying Sediments: Results From Guerrero Negro Saltern, Baja California Sur, Mexico

Total trace metals (Cd, Co, Cu, Fe, Mn, Ni, Pb, Zn), Al, and pyrite- and reactive-associated metals were measured for the first time in a microbial mat and its underlying anoxic-sulfidic sediment collected in the saltern of Guerrero Negro (GN), Baja California Sur, Mexico. It is postulated that the formation of acid volatile sulfide (AVS) and pyrite in the area of GN could be limited by the availability of reactive Fe, as suggested by its limited abundance (mat and sediment combined average value of only 19 ± 10 μmol g−1; n = 22) as well as the low pyrite (0.89–7.9 μmol g−1) and AVS (0.19–21 μmol g−1) concentrations (for anoxic-sulfidic sediments), intermediate degrees of pyritization (12–50%), high degrees of sulfidization (14–100%), generally low degrees of trace metal pyritization, and slight impoverishment in total Fe. This is a surprising result considering the large potential reservoir of available Fe in the surrounding desert. Our findings suggest that pyrite formation in the cycling of trace metals in the saltern of GN is not very important and that other sedimentary phases (e.g., organic matter, carbonates) may be more important reservoirs of trace elements. Enrichment factors [EFMe = (Me/Al)sample/(Me/Al)background] of Co, Pb, and Cd were high in the mat (EFMe = 2.2 ± 0.4, 2.8 ± 1.6 and 34.5 ± 9.8, respectively) and even higher in the underlying sediment (EFMe = 4.7 ± 1.5, 14.5 ± 6.2 and 89 ± 27, respectively), but Fe was slightly impoverished (average EFFe of 0.49 ± 0.13 and 0.50 ± 0.27 in both mat and sediment). Organic carbon to pyrite-sulfur (C/S) molar ratios measured in the mat (2.9 × 102–27 × 102) and sediment (0.81 × 102–6.6 × 102) were, on average, approximately 77 times higher than those typically found in marine sediments (7.5 ± 2.1). These results may indicate that ancient evaporation basins or hypersaline sedimentary environments could be identified on the basis of extremely high C/S ratios (e.g., >100) and low reactive Fe.

Sequestration of metals through association with pyrite in subtidal sediments of the Nanpaishui Estuary on the Western Bank of the Bohai Sea, China

Measurements were made of the degree of trace metal pyritization (DTMP) and ancillary characteristics of four undisturbed sediment cores collected from the subtidal zone of the Nanpaishui Estuary on the Western Bank of the Bohai Sea, a seriously polluted inland sea in northeastern China. The remarkably low concentrations of organic carbon (<0.72%) in these sediments likely constrained sulfate reduction rates, and the low concentrations of acid volatile sulfide (AVS) (<12.5μmolg−1) limited the sequestration of metals through association with pyrite. The most consistent cause of inter-station differences and depth variations in the degree of pyritization was differences in pyrite metal concentrations rather than reactive metal concentrations. Reactive metal concentrations were in several cases negatively correlated with pore water concentrations, consistent with a dissolution/precipitation mechanism. The relationship between pore water metal concentrations and DTMPs was evidenced by a qualitative similarity of the inter-station variability of these same parameters.

Pyritization of trace metals in estuarine sediments and the controlling factors: a case in Jiaojiang Estuary of Zhejiang Province, China

Three undisturbed sediment samples were collected from the intertidal zone of the Jiaojiang Estuary of Zhejiang Province, China. The sediments were found to contain remarkably low concentrations of organic carbon (<0.6%) and acid volatile sulfide (AVS) (<30 μmol g−1). The availability of these two substrates likely constrained sulfate reduction and pyritization of several trace metals, respectively. This was especially true at one station where AVS concentrations in the upper 20 cm averaged less than 0.05 μmol g−1. Although the depth dependence of the degree of trace metal pyritization was generally consistent with expectations based on redox conditions, depth profiles of reactive-metal and pyrite-metal concentrations in several cases revealed more complex behavior and a positive correlation between reactive-metal concentrations and pore water metal concentrations.

Redox status and heavy metal risk in intertidal sediments in NW Spain as inferred from the degrees of pyritization of iron and trace elements

Mariculture is an important economic activity in shallow marine areas of the Rías Baixas (Galicia, NW Spain). The maintenance of high product quality requires surveillance of environmental quality, including the risk of metal toxicity. In this study the redox status of intertidal sediments in the Bay of San Simón, and the risk of toxicity posed by their As, Cd, Cr, Cu, Mn, Ni, Pb and Zn contents, were evaluated by determination of operationally defined reactive, silicate-bound, organic and pyrite-related fractions of these elements and of Fe. The large silicate-bound fractions of most of these metals indicate their lithogenic origin; the main exception is Pb, which in all respects exhibits singular behaviour associated with its predominantly anthropogenic origin in a ceramics factory. In sediments with larger fine-grained particle contents, which are oxic only in the top few centimetres, greater proportions of the trace elements are present as sulphides or associated with sulphide minerals: the degree of pyritization of Fe (DOP) is 46% overall, and the pyrite fraction of some elements doubtless increases at the expense of the reactive fraction, most overall degrees of trace metal pyritization (DTMPs) lying in the range 10–50%. A decline in pyrite genesis at depths below about 18 cm in these sediments is attributed to the exhaustion of organic matter susceptible to metabolization by sulphide-generating bacteria. In coarse-grained, oxic sediments the oxidation of sulphides makes pyrite-related fractions very small, and reactive and silicate-bound fractions are negatively correlated; reactive fractions associated with Fe–Mn oxyhydroxides are large, and DOP and DTMPs are low (generally much lower than in mud-rich sediments), except for Pb. Most of the elements studied are mainly present in forms that are neither bioavailable nor potentially bioavailable, and so do not constitute a significant environmental threat. However, the high DTMPs of Cu and Pb indicate significant potential bioavailability, which should be taken into account in evaluations of environmental quality and the risk to bivalve cultures.

The degree of trace metal pyritization in subtidal sediments of a mariculture area: Application to the assessment of toxic risk

Four 2–3 m sediment cores were taken at the sites on the periphery of mussel raft concentrations in the subtidal zone of the inner Ría de Vigo (Galicia, NW Spain) with a view to evaluate the potential risk to mariculture from sediment-borne trace elements (As, Cd, Cr, Cu, Mn, Ni, Pb and Zn). The distribution of each of these elements in reactive, organic, pyrite and silicate-bound fractions was determined at 64 samples, and these data were used to calculate the degree of trace metal pyritization (DTMP) of each metal/metalloid. In the top 10–20 cm, relatively oxic conditions led to As, Cd, Cu, Pb and Zn having large reactive fractions due to their association with Fe and Mn oxyhydroxides. At lower levels, anoxic conditions favoured by intense diagenesis led to the precipitation of trace metals and metalloids as sulphides, with or without association with pyrite. Particularly large pyrite fractions in the 20–100 cm layer are attributed to the organic matter of this layer being more marine in origin than that of deeper sediments. DTMP was greatest for Cu and As, and least for Pb, Zn and Cr. The risk of trace element toxicity in the event of disturbances instituting oxic conditions in these sediments is discussed.

Trace element diagenesis in pyrite-rich sediments of the Achterwasser lagoon, SW Baltic Sea

The early diagenesis of trace elements (V, Cr, Co, Cu, Zn, As, Cd, Ba, U) in anoxic sediments of the Achterwasser, a shallow lagoon in the non-tidal Oder estuary in the Baltic Sea, was investigated in the context of pyrite formation. The dissolved major redox parameters show a two-tier distribution with transient signals in the occasionally re-suspended fluid mud layer (FM) and a permanently established diagenetic sequence in the sediment below. Intense microbial respiration leads to rapid depletion of O 2 within the uppermost mm of the FM. The reduction zones of Mn, Fe and sulfate overlap in the FM and in the permanently anoxic sediment section which appears to be a typical feature of estuarine sediments, under low-sulfate conditions. Degrees of pyritization (DOP) range from 50% in the FM to remarkably high values > 90% at 50 cm depth. Pyrite formation at the sediment surface is attributed to the reaction of Fe-monosulfides with intermediate sulfur species via the polysulfide pathway. By contrast, intense pyritization in the permanently anoxic sediment below is attributed to mineral growth via adsorption of aqueous Fe-sulfide complexes onto pyrite crystals which had originally formed in the surface layer. The studied trace elements show differential behavior patterns which are closely coupled to the diagenetic processes described above: (i) Zn, Cu and Cd are liberated from organic matter in the thin oxic layer of the sediment and diffuse both upwards across the sediment/water boundary and downwards to be trapped as monosulfides, (ii) V, Cr, Co and As are released during reductive dissolution of Mn- and Fe-oxyhydroxides, (iii) U removal from pore water occurs concomitantly to Fe reduction in the FM and is attributed to reduction of U(VI) to U(IV), (iv) the Ba distribution is controlled by reductive dissolution of authigenic barite in the sulfate reduction zone coupled with upward diffusion and re-precipitation. The incorporation of trace elements into pyrite is most intense for Co, Mn and As, intermediate for Cu and Cr and little to negligible for U, Zn, Cd, V and Ba. The observed trend is largely in agreement with previous studies and may be explained with differing rates for ligand exchange. Slow and fast ligand exchange and thus precipitation kinetics are also displayed by downcore increasing (Mn, Cr, Co and As) or constantly low (Zn, Cu, Cd) pore water concentrations. The downward increasing degrees of trace metal pyritization (DTMP) for Co, Cu, Zn and As are, in analogy to pyrite growth, assigned to adsorption of sulfide complexes or As oxyanions onto preexisting pyrite minerals.

Pyritization of iron and trace metals in anoxic fjord sediments (Nordåsvannet fjord, western Norway)

The accumulation and storage of trace metals in coastal sediments is an environmental concern. It is, therefore, important to understand better how these metals are bound or released under different redox conditions. This study of Fe and trace metal fixation under continuously anoxic conditions in the bottom sediments and the lower water column of the Nordåsvannet fjord in western Norway contributes further to such understanding. It allows investigation of both an end member redox state and one important mechanism of Fe and trace metal accumulation in sediments, the pyritization of Fe and trace metals. Pyrite formation occurs both in the water column and in the sediments of the Nordåsvannet fjord and favours the fixing of Fe and trace metals in the bottom sediments of the fjord. Thus, these sediments act as a continuous sink for Fe and trace metals. The DOP, and the degrees of trace metal pyritization for Mo, Ni and Cr correlate with organic matter content. While it is generally thought that Fe is the factor limiting pyrite formation in anoxic environments, this study found that degrees of pyritization of Fe (DOP) are clearly below 100%, and the availability of metabolizable organic matter is limiting pyrite formation. This is an important finding, because it indicates that increased supply of organic and mineral matter by higher runoff from land would further enhance the fixation of these metals in the fjord sediments, as would higher organic matter availability from increased productivity due to higher nutrient supply. The metals stored in the bottom sediments could be released into the biogeochemical cycle if redox conditions were to change from anoxic to suboxic or oxic. The fjord would then become a source rather than a sink for these metals.

Degree of trace metal pyritization in sediments from the Pacific coast of baja california, mexico

We analyzed sediments from a core collected on the Pacific coast of Baja California, ~45 km off the city of Tijuana and at 1257 m water depth (32º29.5′N, 117º28.3′W), for trace metal content in two operationally-defined fractions, HCl and pyrite. Our results indicate a transference of Cu &gt; Ni &gt; Zn &gt;&gt; Hg and Ag from the HCl to the pyrite fraction. Sediments have degrees of pyritization (DOP) that averaged 7.2 ± 4.9%, with a maximum value of 18.5%. Average degrees of trace metal pyritization (DTMP) ranged from 6.2 ± 2.1% to 83 ± 18% for Mn and Hg, respectively, although maximum values for some metals were close to 100%. This transference is apparently a function of the solubility products of metal sulfides and the relative abundances of metals in the HCl fraction, as suggested by the significant correlation (p &lt; 0.001) observed between these two parameters and the DTMP of a number of trace metals. A similar correlation was obtained from published data of two cores collected in the Gulf of Mexico.

Chemistry of metal sulfides in anoxic sediments

Using sequential extraction of solid sulfides, the determination of acid volatile sulfides (AVS) and chromium reducible sulfurs (CRS) in anoxic sediments from the Authie Bay (in northern France) has been undertaken because of the importance of the sediments as sinks for iron, sulfur and trace metals and as possible sources of pollution when reduced sediments are mixed with oxic waters (as a result of a sediment remobilization induced by physical disturbances such as tidal currents and dredgings), and subsequently oxidized. Chemical analysis of solutions recovered after sequential leaching of sediments with 1 M HCl, 1 M HF and concentrated HNO3 has enabled us to obtain profiles, s. sediment depth, of trace metals associated with pyrite. Porewater concentration profiles s. depth have been determined for several cations (Ca2+, Cd2+, Cu2+, Fe2+, Mg2+, Mn2+, Na+, Pb2+, Sr2+ and Zn2+) and anions (CO32−, PO43−, SO42− and S2−). Using the chemical equilibrium modeling program MINEQL+ with these analytical data, thermodynamic calculations have given information about the possibility of precipitation of discrete metal sulfide phases (FeS as greigite and amorphous FeS; ZnS, PbS, CuS and CdS), and coprecipitation with adsorption on solid FeS to produce solid solutions with iron sulfides. The degree of trace metal pyritization, DTMP, has been determined for these metals and compared to the degree of pyritization, DOP. The findings suggest that in Authie-bay sediments Mn is well pyritized; whereas Zn, Cu, Ni and above all Cd are weakly pyritized (MnZn≃Cu>NiCd). These observations seem to be intimately related to the existence of the discrete/separate solid phases CuS, CdS and ZnS, as predicted by thermodynamic calculations. Finally, analysis of crude sediments, heavy minerals and pyrite extracted by a heavy liquid density separation method, has been performed with a Raman microprobe to gain information about the geochemical and mineralogical characteristics of these sediments. The efficiency of sequential leachings of sediments (which were used for sedimentary pyrite recovery/attack and analysis of pyritic Fe and trace metal) has also been evaluated by these techniques.

Selective Extraction Chemistry of Toxic Metal Sulfides from Sediments

Experimental evidence suggests that formation of metal sulfides in anoxic sediments limits the bioavailability of several toxic elements. Our ability to quantify the processes by which these metal sulfides form is dependent upon our ability to determine the speciation of solid phase metals in sediments. Our work indicates that an entire suite of Cu-Fe and Ni-Fe sulfide minerals can form upon the exposure of mackinawite to aqueous Cu and Ni. Furthermore, we have demonstrated that the solubility of pure metal sulfide minerals and their iron-metal derivatives in HCl directly correlates with the observed trends in the ''Degree of Trace Metal Pyritization'' in natural sediments. Current extraction schemes cannot distinguish discrete trace metal sulfides from trace metals associated with pyrite.

Pyritization of trace metals in anoxic marine sediments

The pyritization of reactive trace elements in different anoxic marine sediments was investigated to determine the importance of factors such as ∑H 2S, reactive-Fe, pyrite content and salinity in controlling this process. The areas studied included anoxic-sulfidic sediments (Baffin Bay, a hypersaline coastal lagoon from Texas and Green Canyon, a hemipelagic oil seepage area), anoxic-nonsulfidic sediments with high sedimentation rates (Atchafalaya Bay-Mississippi Delta system), hemipelagic anoxic-nonsulfidic sediments with low sedimentation rates (Gulf of Mexico shelf and slope and Orca Basin, an euxinic hypersaline basin) and organic-rich marsh sediments (Atchafalaya Bay). Results indicate that the degree of trace metal pyritization (DTMP) of all trace metals, except Cd, increased with increasing degree of pyritization (DOP), irrespective of the type of sedimentary environment involved. However, for As, Hg and Mo, the DTMP DOP values were generally above the 1:1 ratio line, whereas the transition metals Co, Cu, Mn and Ni displayed a close to linear increase in DTMP with DOP and moderate incorporation into pyrite. Chromium and the class B metals Pb and Zn were also gradually incorporated into the pyrite phase but without reaching the DTMP levels exhibited by the transition metals. Cadmium was not incorporated to a significant extent. These results are consistent with the chemical attributes of these different classes of trace elements. Availability of dissolved trace metals and organic matter content are apparently important factors controlling the incorporation of Co, Cr, Cu and Ni into pyrite in anoxicsulfidic (Fe-poor and H 2S-rich) environments.