Aquatic Chemistry - American Chemical Society

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Downloaded by MONASH UNIV on December 7, 2014 | http://pubs.acs.org Publication Date: May 5, 1995 | doi: 10.1021/ba-1995-0244.ch005

Reaction Rates and Products of Manganese Oxidation at the Sediment-Water Interface Bernhard Wehrli , Gabriela Friedl , and Alain Manceau 1

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Swiss Federal Institute for Water Resources and Water Pollution Control (EAWAG), CH-6047 Kastanienbaum, Switzerland Environmental Geochemistry Group, L G I T - I R I G M , University Joseph Fourier and Centre National de la Recherche Scientifique (CNRS), BP 53X, F-38041 Grenoble, France

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Manganese(II) oxidation rates in a eutrophic lake were calculated from a 4-year record of sediment-trap data, and the structure of the pre vailing manganese oxides were determined by extended X-ray absorp tion fine structure (EXAFS) spectroscopy. The oxidation rate near the sediment surface showed a distinct seasonal pattern, with maxima of up to 2.8 mmol/m

2

per day during summer. The average half-life of

Mn(II) during stagnation in summer was 1.4 days. A review of pub lished oxidation rates showed that this half-life, which cannot be ex plained with available data of abiotic surface catalysis, is within the typical range of microbiological oxidation. EXAFS

revealed that the

oxidation product consists mainly of vernadite (δ-MnO ), 2

an X-ray

-amorphous Mn(IV) oxide.

I N T E R E S T I N T H E A Q U A T I C R E D O X C H E M I S T R Y of manganese is at least as old as Werner Stumms scientific career. The first P h . D . student in his laboratory at Harvard University worked on the chemistry of aqueous Mn(II) and Mn(IV) ( J ) . Since then aquatic chemists have refined their analytical tools (2), their conceptual (3)

and numerical (4)

models of manganese cycling, and

their

understanding of heterogeneous redox reactions in general (5,6). This chapter examines the biogeochemical and mineralogical aspects of the manganese re-

OO65-2393/95/0244-Olll$O8.72/O © 1995 American Chemical Society

In Aquatic Chemistry; Huang, C., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1995.

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dox cycle, with an emphasis on lakes. Davison (3) showed how the limnological view can be scaled to marine systems.


Aquatic Geochemistry of Manganese The redox cycle of manganese in lakes (Figure 1) and oceans is triggered at the oxic-anoxic boundary, which is often located near the sediment-water interface. In this zone, settling manganese oxide particles undergo reductive dissolution. Burdige and Nealson (7) showed that, under conditions of micro bial sulfate reduction, the rate of reductive dissolution of manganese oxides is limited by bacterial sulfide production. Sulfide and Fe(II) (8) rapidly reduce manganese oxides by direct chemical mechanisms. Ehrlich (9) reviewed the growing literature on microbial reduction of manganese oxides. Bacteria were found that reduce manganese oxides even in the presence of oxygen (10). Transport of dissolved Mn(II) accumulated by reductive dissolution is gov erned by molecular diffusion in pore waters and by eddy diffusion in the stratified hypolimnia of lakes. Transport follows the concentration gradients,

Figure 1. Box model for the calculation of Mn redox cycling near the sediment-water interface. Sedimentation rates are measured with sediment traps. The burial rate S is estimated from dated sediment cores. In situ sampling techniques (flux chambers and peepers) are used to quantify the diffusive flux across the sediment-water interface F . The resuspension rate R is estimated from the increase in the mass flux of settling material between the 81- and 86-m honzons. b

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Μη Oxidation at Sediment-Water Interface

which decrease toward the oxic zone. At the sediment-water interface such diffusive fluxes can be calculated from concentration profiles in pore waters (11 ) or by direct measurements with benthic flux chambers (12, 13). Murray (14) showed that both techniques may yield values comparable with fluxes calculated from hypolimnetic mass balances. Oxidation of dissolved Mn(II) by molecular oxygen is very slow in the absence of microorganisms or mineral particles. Even after 8 years, Diem and Stumm (15) found no precipitated manganese oxides in their experiments with 10 μΜ Mn(II) at p H 8.5 under air-saturated conditions. However, Davies and Morgan (16) reported that Mn(II) is oxidized completely within a few hours under similar conditions in the presence of a 10 m M suspension of goethite (α-FeOOH). Because Mn(II) is only weakly adsorbed at a p H below the zero point of charge (ZPC) of iron hydroxides ( p H ~ 8), the oxygenation rate is rather slow at p H 7.5. This value is typical of the hypolimnia of productive lakes during stagnation. In natural systems microbiological oxidation may offer a faster pathway, particularly at p H < 8 and low concentrations (20%) of hausmannite ( M n 0 ) and manganite ( 7 - M n O O H ) . The former would produce a shift in the M n - O peak in the R D F , and the latter would be easily detected in the R D F by a M n - M n peak of distorted octahedra at a long distance of d _ . Although the formation of these min erals at redox gradients in the hydrosphere was suggested by several authors (17, 23, 25, 63), we found no spectroscopic evidence for their presence in the bottom waters of Lake Sempach. The results of E X A F S spectroscopy have two major implications: 2

3

Mn

4

Mn

1. The large specific surface area of vernadite (typically >200 m /g) (64) and its low p H (~2.5) provide an efficient scav enging mechanism for metal cations in the bottom waters of productive lakes such as Lake Sempach. The manganese cycle is therefore likely to interfere with the accumulation of metal ions and radiotracers such as Pb. Anions are less strongly adsorbed to the negatively charged surface of δ-Μη0 . How ever, a study of phosphate adsorption to hydrous manganese oxide (65 ) indicates that in hard-water lakes adsorption of C a may significantly enhance the adsorption of H P 0 ". This pos sible link of Ρ and M n cycles at the sediment-water interface should be studied in more detail. 2

Z P C

2 1 0

2

2 +

4

2


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Mn Oxidation at Sediment-Water Interface

2. Vernadite on the surface of microorganisms may be part of a catalytic system in this oxidative biomineralization process. To clarify this point, a more detailed study of the oxidation kinetics of Mn(II) adsorbed to δ-Μη0 should complement the availa ble data (I ).


2

3. The direct oxidation of Mn(II) to Mn(IV) provides an efficient transfer of oxidizing equivalents to the sediment surface. Re cent peeper results from Lake Sempach showed that large min eralization rates during summer release enough dissolved ions to the overlying water so that a density stabilization occurs (66). The stagnation of water layers near the sediment surface inhibits the transport of dissolved oxidants such as O and N 0 " . However, the flux of particulate M n 0 is at maximum during such periods. a

3

2

Conclusions Analysis of a 4-year time series of M n sediment-trap data yielded Mn(II) oxidation rates with a strongly variable seasonal pattern. The average half-life of Mn(II) (1.4 days) cannot be explained by published abiotic oxidation rates. It is, however, in line with typical microbiological oxidation rates. E X A F S spectroscopy revealed that the products of Mn(II) oxidation con sist chiefly of vernadite, an X-ray-amorphous Mn(IV) oxide. No evidence was found for reduced forms of M n oxides such as hausmannite ( M n 0 ) or man ganite ( 7 - M n O O H ) . In a subsequent study we used E X A F S to elucidate the structure of reduced M n phases that control Mn(II) solubility within the sediment. In situ measurements of M n fluxes with benthic chambers and dialysis samplers confirmed the seasonal variability of the M n redox cycle. They i n dicated that the reduction of particulate M n oxides is a fast process that occurs close to the sediment surface within a time scale similar to that of Mn(II) oxidation. A maximum release rate of 5.5 mmol/m per day was measured in July. This rate indicates a close coupling between the oxidation of Mn(II) in the bottom waters and the reduction of M n 0 at the sediment surface. 3

4

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Acknowledgments The time series of sediment-trap analyses in Lake Sempach was initiated by René Gachter. We thank him for access to these data and Erwin Grieder, Antonin Mares, André Steffen, and Alois Zwyssig for the analytical work. Christian Dinkel performed the sediment lander and peeper experiments, and Judith Hunn analyzed the sediment cores. We thank the staff of the L U R E


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synchrotron facility. This chapter benefited from valuable comments by James J. Morgan, Carola Annette Johnson, Dieter Diem, Noel Urban, and three anonymous reviewers. We acknowledge the support of the Schweizer Nationalfonds ( N F P - 2 4 ) and of the European Environmental Research Organisation ( E E R O ) . C N R S provided a visiting fellowship to B. Wehrli.


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R E C E I V E D for review October 2 3 , 1 9 9 2 . A C C E P T E D revised manuscript April 19, 1 9 9 3 .


Aquatic Chemistry - American Chemical Society

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