Pyritization of trace metals in mangrove sediments - Springer Link
Environ Earth Sci (2012) 67:1757–1762 DOI 10.1007/s12665-012-1620-4
ORIGINAL ARTICLE
Pyritization of trace metals in mangrove sediments R. A. Andrade • C. J. Sanders • G. Boaventura S. R. Patchineelam
•
Received: 9 June 2011 / Accepted: 25 February 2012 / Published online: 7 March 2012 Ó Springer-Verlag 2012
Abstract The present study investigates the levels of Mn, Zn, Ni, and Co pyritization in mangrove sediments along distinct sedimentary zones in Enseada das Grac¸as, 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. Keywords
Mn Zn Ni Co Mangrove Sediment
Introduction The interactions between trace metals and pyrite minerals are directly influenced by specific geochemical conditions (Huerta-Diaz and Morse 1990). The ion type ‘‘Hard Sphere’’ or class A, has tendencies to form strong electrostatic bonds with low polarization (Langmuir 1979). The R. A. Andrade C. J. Sanders (&) S. R. Patchineelam Departamento de Geoquı´mica, Universidade Federal Fluminense, Nitero´i, RJ 24020-007, Brazil e-mail: [email protected] G. Boaventura Departamento de Geoquimica e Recursos Minerais, Instituto de Geocieˆncias Universidade de Brası´lia, Brasilia, Brazil
metal type class B or ‘‘Soft Sphere’’, forms covalent bonds with ligands of high polarization character, such as I, S, and N (Huerta-Diaz and Morse 1990). Transition metals possess an intermediate configuration and are classified as class C. Metallic ions of classes B (Cd, Pb, and Zn) normally form strong soluble complexes with bisulfides and polysulfides. The transition metals do not form bisulfide and polysulfide complexes and preferentially form insoluble metallic sulfides. Increasing H2S concentrations in the interstitial mangrove waters may contribute to the formation of a number of soluble sulfide complexes with metals, belonging to class B (favoring a low DTMP (degree of trace-metal pyritization)) in parallel to the pyritization of As, Cu, Hg, Fe, Mo, Co, and Ni (Huerta-Diaz and Morse 1992). The low DTMP-Zn in the sedimentary phases is likely a consequence of limited interaction of Zn with FeS2. Structural restrictions associated with Zn in pyrite minerals may also justify the low affinity of the ‘‘Class B’’ metals to pyrite in sulfidic-anoxic environments (HuertaDiaz and Morse 1990). The isomorphic substitution of trace metals in the iron sulfide structure could diminish the activity of the solid phase and control the concentrations of metals in anoxic waters (Stumm and Morgan 1981). However, this process may be less effective on class B type metals. According to Huerta-Diaz and Morse (1992), As, Co, Cr, Cu, Hg, Mn, Mo, Ni, Pb, and Zn possess a common characteristic, interacting with metal/pyrite during early diagenesis. This behavior has been observed in anoxic-nonsulfidic and anoxic-sulfidic sediments, in varying levels, depending on the geochemical conditions. This study investigates the levels of Mn, Zn, Ni, and Co pyritization in the sedimentary columns along a distinct mangrove sedimentary system, located on the subtropical coastline of southeastern Brazil.
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The study area is Enseada das Garc¸as, a lagoon-type environment, located on the east side of Sepetiba Bay, Brazil (Fig. 1). Due to high levels of confinement, the salinity may vary between 18 and 40 ppt. The tidal regime plays a significant role in the three distinct sedimentary zones: supratidal flat, mangrove forest, and mud flat, a transect perpendicular to the coastline. Using a geomorphological and a geobotanical basis, sediment cores were sampled in the three distinct stations, as defined in this work.
and Co were 0.7, 0.5, 3.8, and 1.8 lg/l, while the coefficient variations were 8.1, 6.1, 4.4, and 5.6%, respectively. All glasswares used were washed with extram 2.5% and uncontaminated with HCl 10% prior to use. Solutions were prepared with Milli-Q water. This type of sequential analysis permits the description of metal behavior during pyrite formation without being influenced by the total Fe and trace-metal content (Huerta-Diaz and Morse 1992). The DOP and DTMP were obtained by the following calculations.
Materials and methods
DOP ð%Þ ¼ Fe-Pyrite= ðFe-Reactive þ Fe-PyriteÞ and DTMP ð%Þ ¼ Metal-Pyrite= ðMetal-Reactive þ Metal-PyriteÞ:
In February 2001, samples were collected by manually inserting pre-cleaned transparent acrylic tubes, 8 cm in diameter and 30 cm in length, in each of the stations along a transect (supratidal flat with little vegetation; mangrove; forest with grassy mud flat) within the study area (Fig. 1). The sediment cores were sectioned into 2 cm intervals and placed in a nitrogen-purged glove bag and stored at 4°C. An aliquot of the samples were freeze-dried and carefully homogenized, and sieved through a 63-lm screen. Subsamples were then submitted to sequential extractions; a HCl (1 M) cold extraction to analyze metals in association with the reactive sedimentary phases and then extracted using concentrated HNO3 to determine the metals associated to pyrite minerals (Huerta-Diaz and Morse 1990). To quantify metal content a plasma-coupled spectrophotometer (ICP-AES, by Spectro Analytical Instruments, model FVM05) was utilized. The detection limits for Mn, Zn, Ni,
Results The DOP, DTMP-Mn, DTMP-Zn, DTMP-Ni and DTMPCo contents measured in the supratidal flat, mangrove forest, and mud flat are presented in the Tables 1, 2, and 3, respectively. Site-specific contents may be noted along the sediment columns, where supratidal flat presented low levels of DOP and DTMP with little deviation, while the mangrove forest sediments indicated high values of DOP and DTMP, increasing with depth (Tables 1, 2, and 3). The highest levels of Fe pyritization, as well as general metal DTMP were in the mud flat, diminishing to the mangrove forest and supratidal flat, respectively. Individual DOP data related to the respective DTMP results are shown for each station on Fig. 2a–c. Figure 3 shows the results for the
Brazil
-22.90
N
Baía de Sepetiba
Legenda Pontos de amostragem
-23.00
Atlantic Ocean
-23.10
0
-44.00
Fig. 1 Study area, Sepetiba Bay, Brazil
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-43.90
-43.80
-43.70
0.1
0.2
-43.60
Environ Earth Sci (2012) 67:1757–1762
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Table 1 Level of pyritivation at specific depth (cm) intervals along with sediment core average and standard deviation (SD) in the lowvegetated supertidal flat; degree of pyritization (DOP) and degree of trace-metal pyritization (DTMP) of Mn, Zn, Ni, and Co
Table 3 Level of pyritization at specific depth (cm) intervals along with sediment core average and standard deviation (standev) in the grassy mud flat; degree of pyritization (DOP) and degree of tracemetal pyritization (DTMP) of Mn, Zn, Ni and Co
Depth
DOP
Mn
Zn
Depth
DOP
Mn
Zn
Ni
Co
0–2
20.2
1.8
0.4
9.3
5.1
0–2
58.3
18.9
0.9
59.7
48.1
2–4
1.9
1.1
1.4
12.2
4.6
2–4
37.0
3.2
0.4
62.6
35.5
3–6
20.9
2.3
0.8
9.6
9.8
3–6
62.9
19.2
1.2
69.3
55.7
6–8
1.8
0.3
0.1
8.0
4.3
6–8
65.0
14.8
1.3
68.9
51.4
8–10
0.4
0.3
1.1
3.3
7.4
8–10
57.4
16.4
1.0
83.3
54.8
10–12
0.6
0.2
0.0
2.3
3.7
10–12
64.0
19.8
1.8
72.5
56.6
12–14
2.0
0.5
0.8
25.6
14.0
12–14
60.9
17.0
1.5
78.9
56.1
14–16
22.7
0.6
1.7
27.3
16.6
14–16
64.2
33.7
1.1
82.0
56.3
16–18
6.6
0.2
1.0
23.4
5.2
16–18
55.9
29.3
0.7
78.9
59.2
18–20
36.0
0.7
2.7
6.8
5.0
18–20
61.7
47.7
1.6
73.4
59.4
Average
11.3
0.8
1.0
12.8
7.6
20–22
64.4
61.7
1.1
78.1
58.1
SD
12.6
0.7
0.8
9.2
4.5
22–24
61.6
14.6
1.2
78.4
52.3
Average
59.4
24.7
1.2
73.8
53.6
7.7
16.2
0.4
7.5
6.6
Ni
Co
SD Table 2 Level of pyritization at specific depth (cm) intervals along with sediment core average and standard deviation (SD) in the mangrove forest; degree of pyritization (DOP) and degree of tracemetal pyritization (DTMP) of Mn, Zn, Ni, and Co Depth
DOP
Mn
Zn
Ni
Co
0–2
9.3
3.5
0.1
49.0
22.1
2–4
5.8
1.9
0.0
53.6
23.5
3–6
4.1
0.7
0.1
57.5
21.1
6–8
47.4
13.9
2.8
53.5
29.7
8–10
17.0
2.3
0.1
54.1
31.9
10–12
12.6
1.8
0.6
59.9
26.6
12–14
35.1
5.7
1.6
46.3
23.8
14–16
53.5
4.9
2.7
54.7
28.2
16–18
64.3
12.3
2.2
54.1
34.6
18–20
47.3
6.1
1.9
53.7
26.4
20–22
61.6
16.6
1.8
49.8
31.8
22–24
56.6
56.6
1.2
50.6
24.2
Average
34.6
10.5
1.3
53.1
27.0
SD
23.3
15.4
1.1
3.7
4.3
three distinct geobotanical zones. An increasing order of DOP and DTMP was found which indicated: supratidal flat \ mangroves \ mudflat. Indeed, the mud flat contains the highest average levels of DOP and DTMP along the sediment core. Zinc DTMP was relatively low in the three distinct environmental settings. Through a box plot of the analytical data, created with the StatsoftStatistica program, a perspective on the levels of DOP demonstrates three distinct depositional environments (Fig. 4). Indeed, specific levels of metal pyritization may be noted in the three sedimentary zones within the Enseada das Garc¸as, which are clearly identified through
the box plots. From this data, supratidal flat, mangrove forest, and mud flat sediments may be grouped as three specific geochemical depositional settings.
Discussion The data obtained in this study suggest that the formation of pyrite and the incorporation of metals is an important process in (sub)tropical marine sedimentation, in specific, mangrove systems. However, this does not agree with the tendencies described by Huerta-Diaz and Morse (1990, 1992). The DTMP of Ni and Co show an increasing horizontal pyritization gradient, where the lowest values may be noted in the supratidal flat to the highest in the mud flat. This content gradient demonstrates the differing geochemical processes taking place along the sedimentary stations. The percentages of Ni and Co DTMP in the mud flat are similar to As, Hg and Mo in the Gulf of Mexico (HuertaDiaz and Morse 1992). These metals were classified as having elevated affinity with pyrite, a ratio of DTMP/DOP [1 in all sedimentary environments (Huerta-Diaz and Morse 1992). According to Huerta-Diaz and Morse (1992), Ni and Co are strongly associated with pyrite, with a ratio of DTMP/DOP approximately equal to 1. The supratidal flat is an oxidative sedimentary environment, as a result of prolonged exposure to the atmosphere, while the mangrove forest possesses anoxic-sulfidic sediments, similar to the mud flat. Because of the geochemically distinct study sites in this work, it is fundamental to compare the pyritization processes and the trace-metal
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DOP
(a) Station 1 40
70
35
60
30
50
25
40
20
30
15
20
10
10
5
0
DOP DTMP-Mn DTMP-Zn DTMP-Ni DTMP-Co
1
0 0
5
10
15
20
25
30
DTMP Mn
Zn
Ni
2
3
Stations Fig. 3 Bar graphs illustrating the differing degree of DOP and DTMP along the three stations of this work
Co
(b) Station 2 70 60
DOP
50 40 30 20 10 0 0
20
40
60
80
DTMP Mn
Zn
Ni
Co
(c) Station 3 70 65
Fig. 4 Box plot of DOP and DTMP of the three stations studied in the mangrove system of this work: (1) supertidal flat; (2) mangrove forest; (3) mud flat
DOP
60 55 50 45 40 35 30 0
20
40
60
80
100
DTMP Mn
Zn
Ni
Co
Fig. 2 a DTMP depth profile against the DOP values in the lowvegetated supertidal flat, b DTMP depth profile against the DOP values in the mangrove forest, c DTMP depth profile against the DOP values in the grassy mud flat
association in these coastal ecosystems. The diagenetic formation of pyrite mineral is an important sink for metals, which are thermodynamically stable in coastal marine environments. Morse (1995) made evident that metals are associated with pyrite as a co-precipitate, isomorphically substituted with Fe, which is absorbed on the surface of minerals or in the formation of a solid precipitate.
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The formation of pyrite and its parallel-associated metals can be seen in the three sedimentary zones described (Fig. 3). The Enseada das Garc¸as coastline is characterized by distinct tendencies of DOP and DTMP in the specific sedimentary zones, where there is an active transfer of Fe and reactive metals associated to the pyrite fractions. This process occurs during sedimentation and within the sediment column of coastal marine systems, as the conditions favor Fe and Mn oxy-hydroxide reduction to Fe(II) and Mn(II). At this juncture, when dissolved oxygen is exhausted in the sedimentary column, the Fe(III) and Mn(IV) become unstable and are reduced to Fe(II) and Mn(II), reacting with sulfide ions (Strekopytov 2003). The (oxy)hydroxides amorphous ferric iron of goethite type, hematite and magnetite, present a high level of kinetic reactions (Canfield 1989). The dissolution of these mineral phases can be described by following reactions:
Environ Earth Sci (2012) 67:1757–1762 1=4
CH2 O þ FeðOHÞ3 þ 2Hþ ! 1=4 H2 CO3 þ Feþþ þ 5=2 H2 O CH2 O þ 4FeOOH þ 8Hþ ! H2 CO3 þ 4Feþþ þ 6H2 O CH2 O þ 2Fe2 O3 þ 8Hþ ! H2 CO3 þ 4Feþþ þ 4H2 O CH2 O þ 2Fe3 O4 þ 12Hþ ! H2 CO3 þ 6Feþþ þ 6H2 O:
A part of the released Fe(II) and Mn(II) ions in the interstitial water diffuse towards the surface of the sediment column, where they are oxidized as hydroxides and oxides, while the remaining particles diffuse downward resulting as authigenic minerals such as iron sulfide (Berner 1984). The formation of pyrite is an indicator of sedimentary diagenesis, becoming unstable in the presence of oxygen. A remobilization of metals associated with (oxy)hydroxides occurs during the process of organic matter oxidation. When this process is mediated by sulfate-reducing bacteria, the formation of metastable sulfide, as a final product, which may transforms into pyrite (Morse 1994). Morse (1995) summarized the steps for the pyrite formation: I-Organic Material þ SO4 ¼ ðmediated by bacteriaÞ ! H2 S II-iron minerals þ H2 S ! AVS, acid volatile sulfide ðFeS-amorphous; mackinavite and greigiteÞ III-H2 S þ O2 ðmediated by bacteriaÞ ! S0 ðPart of H2 S produced by sulfate reducing is oxidized partially as elemental sulfur:Þ IV-AVS þ S0 ! pyrite ðFeS2 Þ: The lowest values of DOP (%) are located in the supratidal flat, where Fe is pyritized in the range of 6.6–36.0%, while the highest percentage values are present in the mud flat (37.0 of 64.4%; Fig. 4). The increasing gradient of DOP towards the mud flat is an indication of the formation of the distinct sedimentary zones along mangrove systems. The DOP represents the pyritization of Fe, or the degree in which the formation of iron sulfide mineral has undergone during recent diagenesis. Factors, such as the concentration of organic matter, detrital iron mineral, elemental sulfur, and sulfate may be limiting in the formation of pyrite (Berner 1984). The supratidal flat is a region with little vegetation, sandy sediments and characterized as a highly oxidized environment. These characteristics result in the limiting formation and accumulation of FeS2. The process of pyrite formation is restricted in these geochemical conditions, where there are low levels of H2S as a result of major oxygen advection. As the supratidal flat likely contains lower concentrations of H2S, where Zn is not in the form of soluble complexes with polysulfides or bisulfides, but may be associated to oxidative fractions such as (oxy)hydroxides or even organic material (Ye et al. 2010).The prolonged aerial exposure of the supratidal flat sediments influences the
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DOP formation and creates less favorable conditions for the formation of sulfide minerals. In this sedimentary region, the predominance of the oxide fractions is the forms in which the majority of the metals are associated. Due to the geochemical processes involved in formation of DOP in mangrove systems, there is general increase with depth. The results in this work indicate a favorable environment in the mangrove mudflat for the formation and accumulation of pyrite in the lower depths. This increasing tendency of DOP is confirmed where marine sediments undergo the pyritization process, with increasing FeS2 content in relation to depth. The extent to which Fe is pyritized varies as a function of the geochemical conditions (Huerta-Diaz and Morse 1992). Pyrite formation is not favorable in the absence of organic matter (Berner 1984). This limiting factor is defined as organic matter that would be metabolized by sulfate-reducing bacteria. The sulfate reduction rate is dependent on the concentration of this biogeochemical parameter. As a result, it is valid to assume that organic material is a limiting factor in the production of FeS2. Hence, organic material should also directly influence the degree of pyritization. The data from Berner (1984) show a linear correlation between DOP and organic matter content. Berner (1984) demonstrated that below a specific depth, there is a decrease in organic matter, acid volatile sulfide (AVS); however, there is an increase in pyrite mineral content and DOP. As a result, the percentage of Fe pyritized tapers off in relation to depth (Berner 1984). In this work, it is not possible to verify a significant decrease in pyritization, as the process involved in the diminishing Fe pyritization likely extend beyond the depths studied. Berner (1984) explains that the organic material could directly influence the transformation of AVS into FeS2. The ‘‘in vitro’’ experiments by Berner (1984) demonstrate that the referred reaction involves the addition of sulfur over the removal of Fe. During the origin of authigenic minerals from metastable iron sulfide minerals, inorganic oxidation agents transform AVS into FeS2. As made evident in laboratory experiments, S and S2O3 act as oxidizing agents in marine environments (Berner 1984). In specific marine systems, such as mangrove forests, the production of H2S (during the anaerobic decomposition of organic matter) is sufficient for the formation of AVS. However, it is insufficient for the transformation of all the AVS into FeS2. The direct influence of the high organic matter flux would generate an ideal environment for an elevated DOP, consequently increasing the rate of sulfate reduction and the formation of detrital Fe and H2S. The high DTMP-Mn content in the mud flat area demonstrates the level of sulfidation in this sedimental zone (Huerta-Diaz and Morse 1990). The fact that Mn is not pyritized in the same level as Fe, suggests differing geochemical behaviors for these two
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metals within redox cycling processes and neo-formation of minerals along the three sedimentary zones. In the presence of H2S concentrations greater than 1.0 M and metabolizable organic matter content higher than 2.0%, 40.0–80.0% of Mn is pyritized (Huerta-Diaz and Morse 1990). The results in this work indicate the importance of H2S and organic material, along with favorable geochemical conditions, for significant Mn pyritization. It is apparent that the concentrations of H2S could influence the higher values of DTPM-Mn in the mud flat, intermediate in the mangrove sediment and the low levels in the supratidal flat (consequences of the oxidation of H2S by percolation or advective oxygen). This percolation of oxygen is inferred by an alternation in the two range percentages of Fe pyritization. The DOP levels in the supratidal flat are likely influenced by the small tidal channel, as indicated by the dark coloration of sandy sediment noted during sampling. This information infers that there is likely some influence from the tidal channel, originating from the intertidal flat, on the perspective sedimentary column. The percolation of the Sepitiba Bay waters (during spring tides) likely plays a significant role by oxidizing a fraction of the pyrite formed. Earlier studies on oxidation have indicated that a decrease in the pyrite concentration correlates with an increase in the ferrous iron concentration and the reprecipitation of iron hydroxides. The majority of the works cited in this study suggest that lmolecular oxygen likely oxidizes pyrite directly through: (1) infiltration of water with dissolved O2 through the sediment column (King 1988; Luther 1991), (2) infiltration of surface water by bioturbation (Luther 1991), and (3) advection of air in the surface sediment during the low tidal regime (Luther 1991). On the other hand, some fraction of Fe-sulfides may dissolve where bio-irrigation supplies dissolved O2. The dissolved Fe may then be available to the mangrove vegetation (Kristensen and Alongi 2006).
Conclusion As highlighted in this study, there is a lack of published work dealing with the kinetics of pyrite formation and metals associated with a variance of DOP and DTMP in mangrove sediments. This study suggests elevated pyritization in mud flat, diminishing towards the mangroves forests, and being the lowest in the supratidal flat. These conclusions are based on the presence, in particular, of a high Ni and Co DTMP. The average DTMP-Ni and
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DTMP-Co values from the supertidal flat to the mud flat are 12.8–73.8 and 7.6–53.6, respectively. The DOP values also increase drastically, average values from 11 to 59, from the supertidal to the mud flat. Given the tendencies shown in this work, further investigations on the kinetics of pyrite formation and the respective interaction with metals are needed to better understand the geochemical processes that cause the formation Mn, Zn, Ni, and Co pyritization in specific regions of the mangrove system. Acknowledgments This work was supported by the Brazilian Government (CNPQ) with CAPES and FAPERJ support, Grant (E-26/101.952/2009), to Christian J. Sanders.
References Berner RA (1984) Sedimentary pyrite formation: an update. Geochim Cosmochim Acta 48:605–615 Canfield DE (1989) Reactive iron in marine sediments. Geochim Cosmochim Acta 53:619–632 Huerta-Diaz MD, Morse JW (1990) A quantitative method for determination of trace metal concentration in sedimentary pyrite. Mar Chem 29:119–144 Huerta-Diaz MD, Morse JW (1992) Pyritization of trace metals in anoxic marine sediments. Geochim Cosmochim Acta 56:2681–2702 King GM (1988) Patterns of sulfate reduction and the sulfur cycle in a South Carolina salt marsh. Limnol Oceanogr 33:376–390 Kristensen E, Alongi DM (2006) Control by fiddler crabs (Ucavocans) and plant roots (Avicennia marina) on carbon, iron, and sulfur biogeochemistry in mangrove sediment. Limnol Oceanogr 51:1557–1571 Langmuir D (1979) Techniques of estimating thermodynamic properties for some aqueous complexes of geochemical interest. In: Chemical modeling in aqueous systems, american chemical society symposium serious, vol 93, Ch 18, pp 353–387. doi: 10.1021/bk-1979-0093 Luther GW III (1991) Pyrite synthesis via polysulfide compounds. Geochim Cosmochim Acta 55:2839–2849 Morse JW (1994) Interactions of trace metals with authigenic sulfide minerals- implications for their bioavailability. Mar Chem 46:1–6 Morse JW (1995) Dynamics of trace metal interactions with authigenic sulfide minerals in anoxic sediments. In: Dynamics of trace metal in anoxic sediments. Ann Arbor Press Inc., Texas, pp 188–199 Strekopytov SV (2003) Geochemistry of diagenesis of sediments in the Barents Sea: forms of iron and sulfur. Lithology Mineral Res 38(1):1–11 Stumm W, Morgan JJ (1981) Aquatic chemistry: an introduction emphasizing chemical equilibria in natural waters. Wiley, USA, Ch 10: Case studies, phosphorus, iron and manganese pp 534–538 Ye S, Laws EA, Wu QA, Zhong SJ, Ding XG, Zhoa GM, Gong SJ (2010) Pyritization of trace metals in estuarine sediments and the controlling factors: a case in Jiaojiang Estuary of Zhejiang Province, China. Environ Earth Sci 61:973–982
ORIGINAL ARTICLE
Pyritization of trace metals in mangrove sediments R. A. Andrade • C. J. Sanders • G. Boaventura S. R. Patchineelam
•
Received: 9 June 2011 / Accepted: 25 February 2012 / Published online: 7 March 2012 Ó Springer-Verlag 2012
Abstract The present study investigates the levels of Mn, Zn, Ni, and Co pyritization in mangrove sediments along distinct sedimentary zones in Enseada das Grac¸as, 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. Keywords
Mn Zn Ni Co Mangrove Sediment
Introduction The interactions between trace metals and pyrite minerals are directly influenced by specific geochemical conditions (Huerta-Diaz and Morse 1990). The ion type ‘‘Hard Sphere’’ or class A, has tendencies to form strong electrostatic bonds with low polarization (Langmuir 1979). The R. A. Andrade C. J. Sanders (&) S. R. Patchineelam Departamento de Geoquı´mica, Universidade Federal Fluminense, Nitero´i, RJ 24020-007, Brazil e-mail: [email protected] G. Boaventura Departamento de Geoquimica e Recursos Minerais, Instituto de Geocieˆncias Universidade de Brası´lia, Brasilia, Brazil
metal type class B or ‘‘Soft Sphere’’, forms covalent bonds with ligands of high polarization character, such as I, S, and N (Huerta-Diaz and Morse 1990). Transition metals possess an intermediate configuration and are classified as class C. Metallic ions of classes B (Cd, Pb, and Zn) normally form strong soluble complexes with bisulfides and polysulfides. The transition metals do not form bisulfide and polysulfide complexes and preferentially form insoluble metallic sulfides. Increasing H2S concentrations in the interstitial mangrove waters may contribute to the formation of a number of soluble sulfide complexes with metals, belonging to class B (favoring a low DTMP (degree of trace-metal pyritization)) in parallel to the pyritization of As, Cu, Hg, Fe, Mo, Co, and Ni (Huerta-Diaz and Morse 1992). The low DTMP-Zn in the sedimentary phases is likely a consequence of limited interaction of Zn with FeS2. Structural restrictions associated with Zn in pyrite minerals may also justify the low affinity of the ‘‘Class B’’ metals to pyrite in sulfidic-anoxic environments (HuertaDiaz and Morse 1990). The isomorphic substitution of trace metals in the iron sulfide structure could diminish the activity of the solid phase and control the concentrations of metals in anoxic waters (Stumm and Morgan 1981). However, this process may be less effective on class B type metals. According to Huerta-Diaz and Morse (1992), As, Co, Cr, Cu, Hg, Mn, Mo, Ni, Pb, and Zn possess a common characteristic, interacting with metal/pyrite during early diagenesis. This behavior has been observed in anoxic-nonsulfidic and anoxic-sulfidic sediments, in varying levels, depending on the geochemical conditions. This study investigates the levels of Mn, Zn, Ni, and Co pyritization in the sedimentary columns along a distinct mangrove sedimentary system, located on the subtropical coastline of southeastern Brazil.
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The study area is Enseada das Garc¸as, a lagoon-type environment, located on the east side of Sepetiba Bay, Brazil (Fig. 1). Due to high levels of confinement, the salinity may vary between 18 and 40 ppt. The tidal regime plays a significant role in the three distinct sedimentary zones: supratidal flat, mangrove forest, and mud flat, a transect perpendicular to the coastline. Using a geomorphological and a geobotanical basis, sediment cores were sampled in the three distinct stations, as defined in this work.
and Co were 0.7, 0.5, 3.8, and 1.8 lg/l, while the coefficient variations were 8.1, 6.1, 4.4, and 5.6%, respectively. All glasswares used were washed with extram 2.5% and uncontaminated with HCl 10% prior to use. Solutions were prepared with Milli-Q water. This type of sequential analysis permits the description of metal behavior during pyrite formation without being influenced by the total Fe and trace-metal content (Huerta-Diaz and Morse 1992). The DOP and DTMP were obtained by the following calculations.
Materials and methods
DOP ð%Þ ¼ Fe-Pyrite= ðFe-Reactive þ Fe-PyriteÞ and DTMP ð%Þ ¼ Metal-Pyrite= ðMetal-Reactive þ Metal-PyriteÞ:
In February 2001, samples were collected by manually inserting pre-cleaned transparent acrylic tubes, 8 cm in diameter and 30 cm in length, in each of the stations along a transect (supratidal flat with little vegetation; mangrove; forest with grassy mud flat) within the study area (Fig. 1). The sediment cores were sectioned into 2 cm intervals and placed in a nitrogen-purged glove bag and stored at 4°C. An aliquot of the samples were freeze-dried and carefully homogenized, and sieved through a 63-lm screen. Subsamples were then submitted to sequential extractions; a HCl (1 M) cold extraction to analyze metals in association with the reactive sedimentary phases and then extracted using concentrated HNO3 to determine the metals associated to pyrite minerals (Huerta-Diaz and Morse 1990). To quantify metal content a plasma-coupled spectrophotometer (ICP-AES, by Spectro Analytical Instruments, model FVM05) was utilized. The detection limits for Mn, Zn, Ni,
Results The DOP, DTMP-Mn, DTMP-Zn, DTMP-Ni and DTMPCo contents measured in the supratidal flat, mangrove forest, and mud flat are presented in the Tables 1, 2, and 3, respectively. Site-specific contents may be noted along the sediment columns, where supratidal flat presented low levels of DOP and DTMP with little deviation, while the mangrove forest sediments indicated high values of DOP and DTMP, increasing with depth (Tables 1, 2, and 3). The highest levels of Fe pyritization, as well as general metal DTMP were in the mud flat, diminishing to the mangrove forest and supratidal flat, respectively. Individual DOP data related to the respective DTMP results are shown for each station on Fig. 2a–c. Figure 3 shows the results for the
Brazil
-22.90
N
Baía de Sepetiba
Legenda Pontos de amostragem
-23.00
Atlantic Ocean
-23.10
0
-44.00
Fig. 1 Study area, Sepetiba Bay, Brazil
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-43.90
-43.80
-43.70
0.1
0.2
-43.60
Environ Earth Sci (2012) 67:1757–1762
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Table 1 Level of pyritivation at specific depth (cm) intervals along with sediment core average and standard deviation (SD) in the lowvegetated supertidal flat; degree of pyritization (DOP) and degree of trace-metal pyritization (DTMP) of Mn, Zn, Ni, and Co
Table 3 Level of pyritization at specific depth (cm) intervals along with sediment core average and standard deviation (standev) in the grassy mud flat; degree of pyritization (DOP) and degree of tracemetal pyritization (DTMP) of Mn, Zn, Ni and Co
Depth
DOP
Mn
Zn
Depth
DOP
Mn
Zn
Ni
Co
0–2
20.2
1.8
0.4
9.3
5.1
0–2
58.3
18.9
0.9
59.7
48.1
2–4
1.9
1.1
1.4
12.2
4.6
2–4
37.0
3.2
0.4
62.6
35.5
3–6
20.9
2.3
0.8
9.6
9.8
3–6
62.9
19.2
1.2
69.3
55.7
6–8
1.8
0.3
0.1
8.0
4.3
6–8
65.0
14.8
1.3
68.9
51.4
8–10
0.4
0.3
1.1
3.3
7.4
8–10
57.4
16.4
1.0
83.3
54.8
10–12
0.6
0.2
0.0
2.3
3.7
10–12
64.0
19.8
1.8
72.5
56.6
12–14
2.0
0.5
0.8
25.6
14.0
12–14
60.9
17.0
1.5
78.9
56.1
14–16
22.7
0.6
1.7
27.3
16.6
14–16
64.2
33.7
1.1
82.0
56.3
16–18
6.6
0.2
1.0
23.4
5.2
16–18
55.9
29.3
0.7
78.9
59.2
18–20
36.0
0.7
2.7
6.8
5.0
18–20
61.7
47.7
1.6
73.4
59.4
Average
11.3
0.8
1.0
12.8
7.6
20–22
64.4
61.7
1.1
78.1
58.1
SD
12.6
0.7
0.8
9.2
4.5
22–24
61.6
14.6
1.2
78.4
52.3
Average
59.4
24.7
1.2
73.8
53.6
7.7
16.2
0.4
7.5
6.6
Ni
Co
SD Table 2 Level of pyritization at specific depth (cm) intervals along with sediment core average and standard deviation (SD) in the mangrove forest; degree of pyritization (DOP) and degree of tracemetal pyritization (DTMP) of Mn, Zn, Ni, and Co Depth
DOP
Mn
Zn
Ni
Co
0–2
9.3
3.5
0.1
49.0
22.1
2–4
5.8
1.9
0.0
53.6
23.5
3–6
4.1
0.7
0.1
57.5
21.1
6–8
47.4
13.9
2.8
53.5
29.7
8–10
17.0
2.3
0.1
54.1
31.9
10–12
12.6
1.8
0.6
59.9
26.6
12–14
35.1
5.7
1.6
46.3
23.8
14–16
53.5
4.9
2.7
54.7
28.2
16–18
64.3
12.3
2.2
54.1
34.6
18–20
47.3
6.1
1.9
53.7
26.4
20–22
61.6
16.6
1.8
49.8
31.8
22–24
56.6
56.6
1.2
50.6
24.2
Average
34.6
10.5
1.3
53.1
27.0
SD
23.3
15.4
1.1
3.7
4.3
three distinct geobotanical zones. An increasing order of DOP and DTMP was found which indicated: supratidal flat \ mangroves \ mudflat. Indeed, the mud flat contains the highest average levels of DOP and DTMP along the sediment core. Zinc DTMP was relatively low in the three distinct environmental settings. Through a box plot of the analytical data, created with the StatsoftStatistica program, a perspective on the levels of DOP demonstrates three distinct depositional environments (Fig. 4). Indeed, specific levels of metal pyritization may be noted in the three sedimentary zones within the Enseada das Garc¸as, which are clearly identified through
the box plots. From this data, supratidal flat, mangrove forest, and mud flat sediments may be grouped as three specific geochemical depositional settings.
Discussion The data obtained in this study suggest that the formation of pyrite and the incorporation of metals is an important process in (sub)tropical marine sedimentation, in specific, mangrove systems. However, this does not agree with the tendencies described by Huerta-Diaz and Morse (1990, 1992). The DTMP of Ni and Co show an increasing horizontal pyritization gradient, where the lowest values may be noted in the supratidal flat to the highest in the mud flat. This content gradient demonstrates the differing geochemical processes taking place along the sedimentary stations. The percentages of Ni and Co DTMP in the mud flat are similar to As, Hg and Mo in the Gulf of Mexico (HuertaDiaz and Morse 1992). These metals were classified as having elevated affinity with pyrite, a ratio of DTMP/DOP [1 in all sedimentary environments (Huerta-Diaz and Morse 1992). According to Huerta-Diaz and Morse (1992), Ni and Co are strongly associated with pyrite, with a ratio of DTMP/DOP approximately equal to 1. The supratidal flat is an oxidative sedimentary environment, as a result of prolonged exposure to the atmosphere, while the mangrove forest possesses anoxic-sulfidic sediments, similar to the mud flat. Because of the geochemically distinct study sites in this work, it is fundamental to compare the pyritization processes and the trace-metal
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DOP
(a) Station 1 40
70
35
60
30
50
25
40
20
30
15
20
10
10
5
0
DOP DTMP-Mn DTMP-Zn DTMP-Ni DTMP-Co
1
0 0
5
10
15
20
25
30
DTMP Mn
Zn
Ni
2
3
Stations Fig. 3 Bar graphs illustrating the differing degree of DOP and DTMP along the three stations of this work
Co
(b) Station 2 70 60
DOP
50 40 30 20 10 0 0
20
40
60
80
DTMP Mn
Zn
Ni
Co
(c) Station 3 70 65
Fig. 4 Box plot of DOP and DTMP of the three stations studied in the mangrove system of this work: (1) supertidal flat; (2) mangrove forest; (3) mud flat
DOP
60 55 50 45 40 35 30 0
20
40
60
80
100
DTMP Mn
Zn
Ni
Co
Fig. 2 a DTMP depth profile against the DOP values in the lowvegetated supertidal flat, b DTMP depth profile against the DOP values in the mangrove forest, c DTMP depth profile against the DOP values in the grassy mud flat
association in these coastal ecosystems. The diagenetic formation of pyrite mineral is an important sink for metals, which are thermodynamically stable in coastal marine environments. Morse (1995) made evident that metals are associated with pyrite as a co-precipitate, isomorphically substituted with Fe, which is absorbed on the surface of minerals or in the formation of a solid precipitate.
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The formation of pyrite and its parallel-associated metals can be seen in the three sedimentary zones described (Fig. 3). The Enseada das Garc¸as coastline is characterized by distinct tendencies of DOP and DTMP in the specific sedimentary zones, where there is an active transfer of Fe and reactive metals associated to the pyrite fractions. This process occurs during sedimentation and within the sediment column of coastal marine systems, as the conditions favor Fe and Mn oxy-hydroxide reduction to Fe(II) and Mn(II). At this juncture, when dissolved oxygen is exhausted in the sedimentary column, the Fe(III) and Mn(IV) become unstable and are reduced to Fe(II) and Mn(II), reacting with sulfide ions (Strekopytov 2003). The (oxy)hydroxides amorphous ferric iron of goethite type, hematite and magnetite, present a high level of kinetic reactions (Canfield 1989). The dissolution of these mineral phases can be described by following reactions:
Environ Earth Sci (2012) 67:1757–1762 1=4
CH2 O þ FeðOHÞ3 þ 2Hþ ! 1=4 H2 CO3 þ Feþþ þ 5=2 H2 O CH2 O þ 4FeOOH þ 8Hþ ! H2 CO3 þ 4Feþþ þ 6H2 O CH2 O þ 2Fe2 O3 þ 8Hþ ! H2 CO3 þ 4Feþþ þ 4H2 O CH2 O þ 2Fe3 O4 þ 12Hþ ! H2 CO3 þ 6Feþþ þ 6H2 O:
A part of the released Fe(II) and Mn(II) ions in the interstitial water diffuse towards the surface of the sediment column, where they are oxidized as hydroxides and oxides, while the remaining particles diffuse downward resulting as authigenic minerals such as iron sulfide (Berner 1984). The formation of pyrite is an indicator of sedimentary diagenesis, becoming unstable in the presence of oxygen. A remobilization of metals associated with (oxy)hydroxides occurs during the process of organic matter oxidation. When this process is mediated by sulfate-reducing bacteria, the formation of metastable sulfide, as a final product, which may transforms into pyrite (Morse 1994). Morse (1995) summarized the steps for the pyrite formation: I-Organic Material þ SO4 ¼ ðmediated by bacteriaÞ ! H2 S II-iron minerals þ H2 S ! AVS, acid volatile sulfide ðFeS-amorphous; mackinavite and greigiteÞ III-H2 S þ O2 ðmediated by bacteriaÞ ! S0 ðPart of H2 S produced by sulfate reducing is oxidized partially as elemental sulfur:Þ IV-AVS þ S0 ! pyrite ðFeS2 Þ: The lowest values of DOP (%) are located in the supratidal flat, where Fe is pyritized in the range of 6.6–36.0%, while the highest percentage values are present in the mud flat (37.0 of 64.4%; Fig. 4). The increasing gradient of DOP towards the mud flat is an indication of the formation of the distinct sedimentary zones along mangrove systems. The DOP represents the pyritization of Fe, or the degree in which the formation of iron sulfide mineral has undergone during recent diagenesis. Factors, such as the concentration of organic matter, detrital iron mineral, elemental sulfur, and sulfate may be limiting in the formation of pyrite (Berner 1984). The supratidal flat is a region with little vegetation, sandy sediments and characterized as a highly oxidized environment. These characteristics result in the limiting formation and accumulation of FeS2. The process of pyrite formation is restricted in these geochemical conditions, where there are low levels of H2S as a result of major oxygen advection. As the supratidal flat likely contains lower concentrations of H2S, where Zn is not in the form of soluble complexes with polysulfides or bisulfides, but may be associated to oxidative fractions such as (oxy)hydroxides or even organic material (Ye et al. 2010).The prolonged aerial exposure of the supratidal flat sediments influences the
1761
DOP formation and creates less favorable conditions for the formation of sulfide minerals. In this sedimentary region, the predominance of the oxide fractions is the forms in which the majority of the metals are associated. Due to the geochemical processes involved in formation of DOP in mangrove systems, there is general increase with depth. The results in this work indicate a favorable environment in the mangrove mudflat for the formation and accumulation of pyrite in the lower depths. This increasing tendency of DOP is confirmed where marine sediments undergo the pyritization process, with increasing FeS2 content in relation to depth. The extent to which Fe is pyritized varies as a function of the geochemical conditions (Huerta-Diaz and Morse 1992). Pyrite formation is not favorable in the absence of organic matter (Berner 1984). This limiting factor is defined as organic matter that would be metabolized by sulfate-reducing bacteria. The sulfate reduction rate is dependent on the concentration of this biogeochemical parameter. As a result, it is valid to assume that organic material is a limiting factor in the production of FeS2. Hence, organic material should also directly influence the degree of pyritization. The data from Berner (1984) show a linear correlation between DOP and organic matter content. Berner (1984) demonstrated that below a specific depth, there is a decrease in organic matter, acid volatile sulfide (AVS); however, there is an increase in pyrite mineral content and DOP. As a result, the percentage of Fe pyritized tapers off in relation to depth (Berner 1984). In this work, it is not possible to verify a significant decrease in pyritization, as the process involved in the diminishing Fe pyritization likely extend beyond the depths studied. Berner (1984) explains that the organic material could directly influence the transformation of AVS into FeS2. The ‘‘in vitro’’ experiments by Berner (1984) demonstrate that the referred reaction involves the addition of sulfur over the removal of Fe. During the origin of authigenic minerals from metastable iron sulfide minerals, inorganic oxidation agents transform AVS into FeS2. As made evident in laboratory experiments, S and S2O3 act as oxidizing agents in marine environments (Berner 1984). In specific marine systems, such as mangrove forests, the production of H2S (during the anaerobic decomposition of organic matter) is sufficient for the formation of AVS. However, it is insufficient for the transformation of all the AVS into FeS2. The direct influence of the high organic matter flux would generate an ideal environment for an elevated DOP, consequently increasing the rate of sulfate reduction and the formation of detrital Fe and H2S. The high DTMP-Mn content in the mud flat area demonstrates the level of sulfidation in this sedimental zone (Huerta-Diaz and Morse 1990). The fact that Mn is not pyritized in the same level as Fe, suggests differing geochemical behaviors for these two
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metals within redox cycling processes and neo-formation of minerals along the three sedimentary zones. In the presence of H2S concentrations greater than 1.0 M and metabolizable organic matter content higher than 2.0%, 40.0–80.0% of Mn is pyritized (Huerta-Diaz and Morse 1990). The results in this work indicate the importance of H2S and organic material, along with favorable geochemical conditions, for significant Mn pyritization. It is apparent that the concentrations of H2S could influence the higher values of DTPM-Mn in the mud flat, intermediate in the mangrove sediment and the low levels in the supratidal flat (consequences of the oxidation of H2S by percolation or advective oxygen). This percolation of oxygen is inferred by an alternation in the two range percentages of Fe pyritization. The DOP levels in the supratidal flat are likely influenced by the small tidal channel, as indicated by the dark coloration of sandy sediment noted during sampling. This information infers that there is likely some influence from the tidal channel, originating from the intertidal flat, on the perspective sedimentary column. The percolation of the Sepitiba Bay waters (during spring tides) likely plays a significant role by oxidizing a fraction of the pyrite formed. Earlier studies on oxidation have indicated that a decrease in the pyrite concentration correlates with an increase in the ferrous iron concentration and the reprecipitation of iron hydroxides. The majority of the works cited in this study suggest that lmolecular oxygen likely oxidizes pyrite directly through: (1) infiltration of water with dissolved O2 through the sediment column (King 1988; Luther 1991), (2) infiltration of surface water by bioturbation (Luther 1991), and (3) advection of air in the surface sediment during the low tidal regime (Luther 1991). On the other hand, some fraction of Fe-sulfides may dissolve where bio-irrigation supplies dissolved O2. The dissolved Fe may then be available to the mangrove vegetation (Kristensen and Alongi 2006).
Conclusion As highlighted in this study, there is a lack of published work dealing with the kinetics of pyrite formation and metals associated with a variance of DOP and DTMP in mangrove sediments. This study suggests elevated pyritization in mud flat, diminishing towards the mangroves forests, and being the lowest in the supratidal flat. These conclusions are based on the presence, in particular, of a high Ni and Co DTMP. The average DTMP-Ni and
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Environ Earth Sci (2012) 67:1757–1762
DTMP-Co values from the supertidal flat to the mud flat are 12.8–73.8 and 7.6–53.6, respectively. The DOP values also increase drastically, average values from 11 to 59, from the supertidal to the mud flat. Given the tendencies shown in this work, further investigations on the kinetics of pyrite formation and the respective interaction with metals are needed to better understand the geochemical processes that cause the formation Mn, Zn, Ni, and Co pyritization in specific regions of the mangrove system. Acknowledgments This work was supported by the Brazilian Government (CNPQ) with CAPES and FAPERJ support, Grant (E-26/101.952/2009), to Christian J. Sanders.
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