Abstracts
The formation of pyrite and associated trace metals during diagenesis in a tropical pristine environment was investigated. Except for Cr, there was a generic trend towards increasing trace metal piritization with sedimentary depth. The results suggest that Co, Ni e Cu would be removed from the reactive fraction to the pyrite fraction through a sedimentary profile parallel to the diagenetic formation of pyrite. The results obtained in pristine environments may be used as a baseline to study contaminated areas for they represent true natural laboratories.
Pristine Environments; Pyrite; Trace Metals; Diagenesis; Authigenic Sulfide Mineral
A formação de pirita e metais traço associados durante diagênese em um ambiente tropical sem contaminação foi investigada. Exceto para o metal Cr, houve uma tendência genérica de aumento da piritização de metais com a profundidade sedimentar. Os resultados sugerem que Co, Ni e Cu seriam removidos da fração reativa para a fração pirita no perfil sedimentar, paralelo a formação diagenética da pirita. Os resultados obtidos em ambientes sem contaminação podem ser usados como linha de base para estudar áreas contaminadas, uma vez que estas representam verdadeiros laboratórios naturais.
Ambientes sem contaminação; metais-traço; diagênese; minerais autigênicos
Geociências
Uptake of Co, Cr, Ni and Cu by Pyrite in a tropical marine environment
Regina Célia Bastos de Andrade
Doutoranda em geociências, área de concentração em Geoquímica Ambiental
Universidade Federal Fluminense/Niterói - RJ
E-mail: regina@geoq.uff.br
Sambasiva Rao Patchineelam
Dr. Prof. Universidade Federal Fluminense/Niterói - RJ
E-mail: geosam@vm.uff.br
Resumo
A formação de pirita e metais traço associados durante diagênese em um ambiente tropical sem contaminação foi investigada. Exceto para o metal Cr, houve uma tendência genérica de aumento da piritização de metais com a profundidade sedimentar. Os resultados sugerem que Co, Ni e Cu seriam removidos da fração reativa para a fração pirita no perfil sedimentar, paralelo a formação diagenética da pirita. Os resultados obtidos em ambientes sem contaminação podem ser usados como linha de base para estudar áreas contaminadas, uma vez que estas representam verdadeiros laboratórios naturais.
Palavras-chave: Ambientes sem contaminação, metais-traço, diagênese, minerais autigênicos.
Abstract
The formation of pyrite and associated trace metals during diagenesis in a tropical pristine environment was investigated. Except for Cr, there was a generic trend towards increasing trace metal piritization with sedimentary depth. The results suggest that Co, Ni e Cu would be removed from the reactive fraction to the pyrite fraction through a sedimentary profile parallel to the diagenetic formation of pyrite. The results obtained in pristine environments may be used as a baseline to study contaminated areas for they represent true natural laboratories.
Keywords: Pristine Environments, Pyrite, Trace Metals, Diagenesis, Authigenic Sulfide Mineral
1. Introduction
Pyrite is known to be the most thermodynamically stable iron sulfide mineral. Its formation is limited by the availability of metabolizable organic matter, elemental sulfur and reactive iron mineral (Skei,1988). According to previous studies (Luther III,1982), pyrite may be found either as a framboid, result from the oxidation of iron monosulfide with elemental sulfur, which is a very slow process, or as a single crystal rapidly obtained from direct precipitation.. The solubility of the trace element, the composition of pore fluids as well as the formation of complex ions may influence the distribution of trace metals in pyrite (Raiswell and Plant,1980). The interactions between trace metals and authigenic sulfide minerals in anoxic sediments are responsible for the bioavailability of these trace elements. These interactions possibly happen through coprecipitation and/or adsorption of trace metals on pyrite and on acid volatile sulfides (AVS) (Morse,1994).
This research was performed with the aim of studying the distribution of Cr, Co, Ni and Cu into sedimentary pyrite from a pristine mangrove on the northern coast of Amapá - Brazil.
2. Area Descriptions
The northern coast of Amapá forms a transition zone of 1600km of a mud belt from the Amazon river mouth to the Orinoco Delta - Guianas. The Amazon River discharges approximately 1.2x108 tons.yr -1 of sediment into the Atlantic ocean, creating extensive fine-grained sediment deposits (e.g., Seasonal Superficial Layer-(SSL), mean grain size F 4µm) (Allison et al., 1995). Around 10% of the suspended sediment carried by the Amazon river is advected to the northwest through the interaction of the North Brazil Current (NBC) with winds and surface gravity waves. The volume of the remaining SSL sediment in northern Amapá is probably responsible for the propagation of the mangrove fringe (Allison et al., 1995). The sampling was made in a Rhizophora mangle L from the mangrove fringe in Cape Cassiporé (03º 54.700N 51º 06.375W), a pristine environment highly influenced by the lateritic sediment carried by the Amazon river (Allison et al.,1995).
3. Material and Methods
This research work is an integral part of the first author's dissertation. Cores were collected on the northern coast of Amapá in 1996. However, only the results from Rhizophora mangle L from the mangrove fringe will be presented. The sediment samples were taken using a gravity core (30-cm in length), subsampled in inert atmosphere (sectioned at the intervals of 0-2, 2-5, 5-10, 10-15, 15-20, 20-25 and 25-30cm) , stored and subsequently freeze-dried. Afterwards, these freeze-dried solid samples were leached to separate sedimentary pyrite from associated trace metals (Huerta-Diaz and Morse,1990). This sequential extraction procedure was used in order to obtain the trace metals associated with the reactive fraction (amorphous and crystalline iron and manganese oxyhydroxides, carbonates and hydrous aluminosilicates), with the silicate fraction (trace metals in clay minerals) and with the pyrite fraction. Trace metals (Fe, Cr, Co, Ni and Cu ) were determined by ICP-MS (Perkim Elmer/ Model Elan 6000). All reagents used were reagent grade and all aqueous solutions were prepared with Milli-Q water.
4. Results and Discussion
Figures 1, 3 Figure 3 - Co-Pyrite (circles) and Fe-Pyrite (squares) in sedimentary depth. , 5 Figure 5 - Ni-Pyrite (circles) and Fe-Pyrite (squares) in sedimentary depth. and 7 Figure 7 - Cu-Pyrite (circles) and Fe-Pyrite (squares) in the sedimentary depth. show the profiles for pyrite-Fe and Cr, Co, Ni and Cu in the pyrite fraction in relation to the sedimentary depth. Figures 2, 4 Figure 4 - Relationship between Co-Pyrite and Fe-Pyrite. , 6 Figure 6 - Relationship between Ni-Pyrite and Fe-Pyrite. and 8 Figure 8 - Relationship between Cu-Pyrite and Fe-Pyrite. According to the results, Co, Ni and Cu were probably removed from the reactive fraction to the pyrite fraction through a sedimentary profile parallel to the diagenetic formation of pyrite (Figures 3, 5 and 7), following the tendency for the association Fe-pyrite to increase along the sedimentary column. This behavior wasn't observed for Cr (Figure 1) due to the instability of chromium sulfides and the rapid formation of insoluble chromium hydroxides (Smillie et al,1981). A correlation coefficient of 0.0034 was found for Cr-pyrite and the pyrite fraction (Figure 2). A significant correlation coefficient - 0.9808 - was obtained for Co-pyrite and the pyrite fraction (Figure 4). A regular correlation of 0.5220 and a low correlation of 0.0017 with pyrite were observed for Cu-pyrite and Ni-pyrite, respectively. These correlations could be justified by the reduction in the degree of pyritization in the lowest layers of the core (Figure 5). show the relationship between pyrite-Fe and Cr, Co, Ni and Cu in the pyrite fraction. The results suggest that except for Cr, in a mangrove covered with Rhizophora, pyritization of trace metals tend to increase with sedimentary depth. Thus, it is reasonable to infer that this metal underwent a simultaneous association with pyrite during sedimentary diagenesis.
The pyritization of trace metals was investigated in different anoxic marine sediments in the Gulf of Mexico (Huerta-Diaz and Morse, 1992). The results showed that pyrite is an important sink for As, Hg and Mo, moderately important for Co, Cu, Mn, and Ni , and unimportant for Cr, Pb, Zn and Cd. The uptake of As by pyrite was studied in near-shore marine sediments. A strong correlation (correlation coefficient of 0.9810) between arsenic and pyrite in the Laurentian Trough, Canada (Belzile and Lebel,1986) was observed. The following trace metals - Co, Ni and Cu - presented the generic behavior of associating with pyrite in a mangrove covered with Rhizophora, sharing the tendency shown in other recent sedimentary environments.
The existing studies have shown the interactions between trace metals and pyrite (e.g., Belzile and Lebel,1986; Morse and Cornwell,1987; Morse,1991; Morse,1994; Andrade and Patchneelam, 1998). However, there is not enough information on pyritization of trace metals in tropical environments. Questions regarding the similarity of the factors controlling trace metals and sulfide minerals interactions in tropical and temperate environments as well as pyritization degrees of trace metals may arise.
The present study was carried out in a pristine environment which constitutes a true natural laboratory to study the interactions between trace metals and sulfide minerals, thus enabling the comprehension of the biogeochemical processes that occur in tropical marine sedimentary environments. The results herein obtained may be used as a baseline to study contaminated areas.
Acknowledgements
This work was supported by CAPES, Brazil and the Department of Geochemistry -UFF. The authors wish to thank Dr. Ricardo Santelli (UFF) for his assistance in this work. and Dra. Teresa Cristina (CENPES-PETROBRAS).
Artigo recebido em 24/05/2001 e aprovado em 28/08/2001 .
- ALLISON, M.A, Nittrouer, C.A, Kineke, G.C. Seasonal sediment storage on mudflats adjacent to the Amazon River. Marine Geology, v.125, p.303-328, 1995.
- ANDRADE, R.C.B., PATCHINEELAM, S.R.Distribution of trace metals associated with diagenetic pyrite in northern coast Amapá Niterói - RJ - Brazil: Department of Geochemistry - UFF, 1998. (Dissertation).
- BELZILE, N., LEBEL, J. Capture of arsenic by pyrite in near-shore marine sediments. Chemical Geology, v.54, p.279-281, 1986.
- HUERTA-DIAZ, M.D., MORSE, J.W. Quantitative method for determination of trace metal concentration in sedimentary pyrite. Marine Chemistry, v.29, p.119-144, 1990.
- HUERTA-DIAZ, M.D., MORSE, J.W. Pyritization of trace metals in anoxic marine sediments. Geochimica et Cosmochimica Acta, v.56, p.2681-2702, 1992.
- LUTHER III, G.W. et alii. Pyrite and oxidized iron mineral fhases formed from pyrite oxidation in salt marsh and estuarine sediments. Geochimica et Cosmochimica Acta, v. 46, p. 2665-2669, 1982.
- MORSE, J., CORNWELL., J.C. Analysis and distribution of iron sulfide minerals in recent anoxic marine sediments. Marine Chemistry, v. 22, p. 55-69, 1987.
- MORSE, J. Oxidation kinetics of sedimentary pyrite in seawater. Geochimica et Cosmochimica Acta, v. 55, p. 3665-3667, 1991.
- MORSE, J.W. Interactions of trace metals with authigenic sulfide minerals: implications for their bioavailability. Marine Chemistry, v. 46, p. 1-4, 1994.
- RAISWELL, R., PLANT, J. The incorporation of trace elements into pyrite during diagenesis of Black Shales, Yorkshire, England. Economic Geology, v.75, p. 684-699, 1980.
- SKEI, J.M. Formation of framboidal iron Sulfide in the water of a permanently anoxic Fjord - Framvaren, South Norway. Marine Chemistry, v.23, p. 345-352, 1988.
- SMILLIE, R.H. et alii. Reduction of chromium(VI) by bacterially produced hydrogen sulfide in a marine environment. Water Res., v.15, p.1351-1354, 1981.
Publication Dates
-
Publication in this collection
28 June 2002 -
Date of issue
Dec 2001
History
-
Accepted
28 Aug 2001 -
Received
24 May 2001