Reaction of H2S with Ferric Oxides - American Chemical

11 Reaction of H S with Ferric Oxides 2

Downloaded by UNIV OF PITTSBURGH on August 14, 2015 | http://pubs.acs.org Publication Date: May 5, 1994 | doi: 10.1021/ba-1994-0237.ch011

Some Conceptual Ideas on Its Significance for Sediment-Water Interactions Stefan Peiffer Limnological Research Station, University of Bayreuth, P . O . Box 101251, D-8580 Bayreuth, Germany

Conceptual ideas based on laboratory studies explore the relevance of the interaction of H S with reactive iron in freshwater sediments. The experimental findings suggest a mechanism for sulfate recycling within sediments that involves anoxic oxidation of sulfide by reactive ferric oxides and that may help to explain high sulfate reduction rates. The transport limited supply of the redox equivalents into deeper sediment layers, however, requires very steep microscale horizontal gradients of substances along with the measurable macroscale vertical pore-water gradients. Such a microstructure can be adequately represented by a biofilm model. In addition to its role in sulfate recycling, oxidation of H S by reactive iron also may contribute to pyrite formation. In this process ferric oxides will replace elemental sulfur as the oxidant to form polysulfides, which further react with FeS to form pyrite. 2

2

S U L F U R G E N E R A L L Y I S N O T A L I M I T I N G N U T R I E N T i n e i t h e r aquatic (I) o r terrestrial ecosystems (2), u n l i k e n i t r o g e n o r p h o s p h o r u s . C o n s i d e r i n g t h e various redox states i n w h i c h sulfur can b e f o u n d i n nature (3), its ecological role c o u l d b e d e s c r i b e d as an electron m e d i a t o r . T h i s p r o p e r t y c a n b e o b s e r v e d p a r t i c u l a r l y at interfaces w i t h steep redox gradients, such as t h e b o r d e r l i n e b e t w e e n s e d i m e n t a n d water, w h e r e an intense c y c l i n g o f sulfur c o m p o u n d s occurs (4, 5). C o n s u m p t i o n a n d p r o d u c t i o n o f biomass are freq u e n t l y a c c o m p a n i e d b y sulfate r e d u c t i o n a n d r e o x i d a t i o n o f sulfide v i a several intermediates b y c h e m o a u t o t r o p h i c organisms (6, 7). 0065-2393/94/0237-0371$06.00/0 © 1994 American Chemical Society

In Environmental Chemistry of Lakes and Reservoirs; Baker, L.; Advances in Chemistry; American Chemical Society: Washington, DC, 1994.

372

ENVIRONMENTAL CHEMISTRY O F L A K E S A N D RESERVOIRS

A t t e n t i o n has b e e n focused r e c e n t l y o n s e d i m e n t a r y m i c r o b i a l r e d u c t i o n of sulfate. T h i s process neutralizes atmospheric sulfuric a c i d d e p o s i t e d i n t o soft-water lakes (8-10) t h r o u g h the p r o d u c t i o n of two equivalents of alkalinity p e r m o l e of sulfate r e d u c e d (JI). S 0 ~ +• 2 < C H 0 > 4

2

>H S + 2HC0 -

2

2

(1)

3

w h e r e < C H 0 > denotes organic matter. T h e strong c o u p l i n g b e t w e e n sulfur a n d i r o n c h e m i s t r y b e c o m e s o b v i o u s i n this example. C o n s e r v a t i o n o f alkalinity w i t h i n t h e system is a c h i e v e d o n l y i f the sulfide f o r m e d is p r e v e n t e d f r o m reoxidation, a process that w o u l d restore t h e acidity. P r e v e n t i o n o f reoxidation occurs t h r o u g h t h e u l t i m a t e storage o f sulfide i n sediments, e i t h e r as organic sulfur o r as i r o n sulfides (12, 13). T h e o v e r a l l reaction o f p y r i t e f o r m a t i o n proceeds v i a f o r m a t i o n o f FeS:


2

Fe

2 +

+ H S + 2 H 0 ± = > FeS + 2 H 0 2

2

(2)

+

3

a n d a subsequent reaction w i t h e l e m e n t a l sulfur

H S + i o 2

2

>S° + H 0

(3)

> FeS

(4)

2

S° + F e S

2

w h i c h gives i n total

2S0 ~ + 4 < C H 0 > 4

2

2

+ Fe

2 +

+ £ 0

>

2

2HC0 3

+ FeS

2

+ 2H C0 2

3

+ H 0 2

(5)

T h e ferrous i r o n s u p p l y stems f r o m r e d u c t i v e d i s s o l u t i o n o f ferric oxides, another a l k a l i n i t y - g e n e r a t i n g process: F e O O H + e" + 3 H 0 3

+

>Fe

2 +

+ 5H 0

(6)

2

F o r a d e t a i l e d discussion o f t h e various pathways a n d s t o i c h i o m e t r i c s , see reference 14. H o w e v e r , t h e q u e s t i o n remains o p e n as to w h i c h redox process p r o v i d e s the electrons for reaction 6. W h e n b u r i e d i n sediments, ferric i r o n m a y b e u s e d b y microorganisms as an e l e c t r o n acceptor (15-17). O n t h e other h a n d , it also comes into contact w i t h reductants l i k e H S (18, 19). A l t h o u g h m i c r o b i a l r e d u c t i o n o f ferric oxides u s i n g sulfide as the reductant has not y e t b e e n d o c u m e n t e d (17), various studies support a p u r e l y c h e m i c a l interaction b e t w e e n these t w o c o m p o u n d s (20-22). 2


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Reaction of H S with Ferric Oxides 2


T h e d i s s o l u t i o n rate o f goethite b y sulfide was f o u n d to increase w i t h surface area a n d p r o t o n concentration. P y z i k a n d S o m m e r (21) suggested that H S ~ is the reactive species that reduces surface f e r r i c i r o n after ex c h a n g i n g versus O H " . A subsequent p r o t o n a t i o n o f surface ferrous h y d r o x i d e w o u l d l e a d to d i s s o l u t i o n o f a surface layer. E l e m e n t a l sulfur was t h e p r o m i n e n t oxidation p r o d u c t ; polysulfides a n d thiosulfate w e r e f o u n d to a l o w e r extent. T h e d i s s o l u t i o n rate R (in moles p e r square m e t e r p e r second) o f hematite b y sulfide was d e m o n s t r a t e d to b e p r o p o r t i o n a l to t h e surface concentration o f t h e surface complexes > F e H S a n d > F e S ~ (22). R =

fc{>FeS~}

+

fc'{>FeSH}

(7)

w h e r e { > F e S " } a n d { > F e S H } are surface concentrations (moles p e r square meter) a n d k a n d k denote the c o r r e s p o n d i n g rate constants (per second). f

R e c e n t l y w e p r e s e n t e d (23) t h e results o f an e x p e r i m e n t a l study o n t h e kinetics a n d m e c h a n i s m s o f t h e reaction o f l e p i d o c r o c i t e ( Ύ - F e O O H ) w i t h H S . W i t h respect to the i n t e r a c t i o n b e t w e e n i r o n a n d sulfur, l e p i d o c r o c i t e m e r i t s special attention. It forms b y reoxidation o f ferrous i r o n u n d e r c i r c u m n e u t r a l p H conditions (24), a n d it can therefore b e classified as a reactive i r o n oxide (19). T h e concept o f reactive i r o n was established b y C a n f i e l d (19), w h o differentiated b e t w e e n a r e s i d u a l i r o n fraction a n d a reactive i r o n fraction (operationally d e f i n e d as soluble i n a m m o n i u m oxalate). T h e reactive i r o n fraction is r a p i d l y r e d u c e d b y sulfide o r b y m i c r o o r g a n i s m s . T h i s chapter presents some i m p l i c a t i o n s for s e d i m e n t - w a t e r interactions d e r i v e d f r o m t h e findings o f o u r e x p e r i m e n t a l study. S o m e hypotheses are f o r m u l a t e d c o n c e r n i n g the c o u p l i n g of i r o n a n d sulfur i n s e d i m e n t a r y e n vironments. 2

Reaction of H S with Lepidocrocite 2

I n this study w e p e r f o r m e d i n i t i a l rate e x p e r i m e n t s , reacting H S w i t h l e p idocrocite (23). T h e c o n s u m p t i o n o f H S was m e a s u r e d c o n t i n u o u s l y b y u s i n g a p H S electrode c e l l (25). T o a v o i d interferences of p H buffer solutions w i t h the i r o n oxide surface, the p H was s t a b i l i z e d b y u s i n g a p H - s t a t that a d d e d appropriate amounts o f H C 1 to the s o l u t i o n . T h e a d d e d v o l u m e , w h i c h was also c o n t i n u o u s l y m o n i t o r e d , p r o v i d e d i n f o r m a t i o n about the a m o u n t o f p r o tons c o n s u m e d d u r i n g the reaction. D i s s o l v e d i r o n was m e a s u r e d o n l y i n some runs. 2

2

2

T h e reaction was pseudo-first-order w i t h respect to d i s s o l v e d sulfide. It was surface c o n t r o l l e d ( F i g u r e 1), a n d the reaction rate s h o w e d a strong d e p e n d e n c e o n p H . F i g u r e 2 depicts t h e i n f l u e n c e o f p H o n t h e p s e u d o first-order e x p e r i m e n t a l rate constant fc . T h e i n i t i a l a m o u n t o f d i s s o l v e d sulfide was 10 M i n each e x p e r i m e n t . T h e rate constant increased u p to obs

4



374


Figure 1. The oxidation rate of H S by lepidocrocite is pseudo-first-order with respect to H S. The experimental pseudo-first-order rate constant kobs is plotted as a function of the surface area concentration of y-FeOOH. The reaction rate depends on the surface area (A). 2

2

about p H 7 a n d decreased again at h i g h e r p H . A n e m p i r i c a l rate l a w c a n b e d e r i v e d as follows: F o r 5 < p H < 6, flS(-II)]

=

dt

fc [H ]- [S(-II)]A +

a

(8)

2

F o r 7 < p H < 8.6, flS(-II)]

fc [H P[S(-II)]A b

dt

(9)

+

w h e r e [S(—II)] is t h e concentration o f total d i s s o l v e d sulfide, A is t h e surface concentration i n square meters p e r l i t e r , t is t i m e , a n d k a n d k are rate constants; fc = 1.5 Χ 1 0 M L m m i n , a n d k = 2.1 Χ 1 0 M " m " m i n f o r t h e acidic a n d alkaline ranges, r e s p e c t i v e l y . a

a

1 3

2

2

1

h

h

6

1

2

1

T h e o b s e r v e d rate constants i n the p H range 6 < p H < 7 m a y n o t b e d e t e r m i n e d p r e c i s e l y because o f t h e short t i m e intervals. P r o b a b l y , o u r values reflect o n l y t h e l o w e r l i m i t s o f the fast reaction rates at n e u t r a l p H .


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375


0 1 97

[H*]" ' A

Κ = °'


1

K

-4

4

5

m

10

6.32

[ H + ]

A

9

8

7

6

1.0

pH Figure 2. Experimental pseudo-first-order rate constant kob* (normalized to the surface area concentration A) for the reaction of H S with lepidocrocite plotted as a function of pH. Straight lines a and b correspond to eqs 8 and 9, respectively. k« and k& are the empirical rate constants. 2

B y f o l l o w i n g the reaction scheme p r o p o s e d b y dos Santos A f o n s o a n d S t u m m (22) for the r e d u c t i v e d i s s o l u t i o n of hematite surface sites (Scheme 1), w e w e r e able to explain perfectly the o b s e r v e d p H p a t t e r n of the o x i d a t i o n rate of H S . T h e rate is p r o p o r t i o n a l to the c o n c e n t r a t i o n of i n n e r - s p h e r e surface complexes o f H S ~ f o r m e d w i t h e i t h e r t h e n e u t r a l ( > F e O H ) o r t h e p r o t o n a t e d ( > F e O H ) ferric oxide surface sites. 2

2

+

d[m-} dt

=

fc{>FeS~}

4-

fc'{>FeSH}

(10)

W e w e r e n o t able to measure a n increase of d i s s o l v e d i r o n at a p H h i g h e r than 5.7. E v e n at this p H , t h e r e c o v e r y o f d i s s o l v e d i r o n a c c o u n t e d for o n l y 2 8 % of the d i s s o l v e d sulfide c o n s u m e d . T h i s f i n d i n g agrees w i t h o t h e r studies of the r e d u c t i v e d i s s o l u t i o n o f ferric oxides i n w h i c h d i s s o l u t i o n rates fre q u e n t l y are not detectable at p H 7 (26, 27). A l t h o u g h w e d i d not measure the o x i d i z e d p r o d u c t s o f H S , these p r o d ucts can b e d e d u c e d f r o m the ratio o f c o n s u m e d protons p e r m o l e o f total sulfide c o n s u m e d . I n T a b l e I t h e reaction o f H S w i t h ferric oxide is f o r 2

2


376

E N V I R O N M E N T A L CHEMISTRY O F L A K E S A N D RESERVOIRS

m u l a t e d for various oxidation p r o d u c t s of H S ~ . T h e w i d e span of ratios ( ~ 1 - 1 5 ) makes it possible to differentiate c l e a r l y a m o n g the p r o d u c t s . F i g u r e 3 shows that the m e a s u r e d ratios range m o s t l y b e t w e e n 0.5 a n d 3.5, a n d it reveals a distinct p H d e p e n d e n c e . Sulfate does not appear to be a major p r o d u c t i n these i n i t i a l rate ex p e r i m e n t s . T h i s c o n c l u s i o n is i n contrast to the findings o f dos Santos A f o n s o and S t u m m (22), w h o u s e d steady-state e x p e r i m e n t s to measure m a i n l y


• r e v e r s i b l e adsorption of H S >FeOH + HS-

>FeS' +

H 0 2

*-i • r e v e r s i b l e e l e c t r o n transfer > F e S " *=>

>Fe»S'

*-et

• r e v e r s i b l e release of the o x i d i z e d p r o d u c t >Fe"S + H 0

>Fe OH n

2

• d e t a c h m e n t of F e

2

+

+ S'~

2 +

>Fe"OH

2

+

H+ — • » n e w surface site +

Fe

2 +

*3

Scheme I. Proposed mechanism for the reaction of H S with ferric hydroxide surface, according to dos Santos Afonso and Stumm (22). 2

Table I. A H : A H S +

2

T O

T Ratio for Products of the Reaction of H S with γ - F e O O H 2

Reaction

àH

6FeOOH 8FeOOH 2FeOOH 8FeOOH 8FeOOH

+ + + + +

+

4HS- + 2 H 0 - * S ^ + 6Fe + 140H" 5 H S - 4- 3 H 0 + 8Fe + 190H" H S - + H 0 -> S° + 2 F e + SOH" 2 H S - + 3 H 0 - » S 0^ + 8Fe + 160H~ H S - + 3 H 0 - » SO^ + 8 F e + 150H" 2

2 +

4

2 +

2

2 +

2

2

2 +

2

2 +

2

Formation of solid-phase bound F e 6 F e O O H + 10HS6FeS + S *- + 8 0 H " + 4 H 0 6 F e O O H + 6 > F e O H + 4HS~ - » 6FeO-Fe + S ~ + 80H- + 4H O

AH S T 2

3.5 3.8 5.0 8.0 15.0

2 +

4

+

4

2

2

a

0.8 2.0

S O U R C E : Reproduced from reference 23. Copyright 1992 American Chemical Society.


TO


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Figure 3. Measured ratio Δ Η :àH S for the reaction of H S with plotted as a function of pH. +

2

2

y-FeOOH

sulfate, thiosulfate, a n d traces o f sulfite as oxidation products of H S . O u r study reflects m o r e the e x p e r i m e n t a l conditions d e s c r i b e d i n P y z i k a n d S u m m e r ' s w o r k (21). T h u s w e may also assume e l e m e n t a l sulfur o r p o l y sulfides (S - a n d S ~) to be the m a i n p r o d u c t s . A black coloration that appeared d u r i n g the e x p e r i m e n t s at p H values > 6 . 5 i n d i c a t e d the f o r m a t i o n of F e S . H o w e v e r , the black d i s a p p e a r e d again t o w a r d the e n d of the experiments. A p p a r e n t l y most of the sulfide stored i n F e S was also o x i d i z e d , a n d o n l y a small p o r t i o n of the sulfide m a y have r e m a i n e d as F e S . T h i s d e v e l o p m e n t is not s u r p r i s i n g because w e w o r k e d w i t h excess ferric oxide, i n contrast to P y z i k a n d S o m m e r ' s study (21) i n w h i c h F e S c o u l d accumulate d u r i n g the e x p e r i m e n t s . W e c o n c l u d e d that the ferrous i r o n released after r e d i s s o l u t i o n of F e S adsorbs to the ferric oxide surface a n d forms a surface c o m p l e x > F e O - F e (reaction 7), to w h i c h polysulfides m a y b o n d . 2

4

2

5

2

+

A c o m b i n a t i o n of the various processes (formation of polysulfides a n d e l e m e n t a l sulfur, p r e c i p i t a t i o n of F e S , a n d a d s o r p t i o n of F e ) leads to the 2 +


378


l o w Δ Η : A H S ratios o b s e r v e d at p H > 6. A t l o w e r p H values F e S does not f o r m a n d a d s o r p t i o n o f F e decreases; these conditions p r o v i d e a n increased A H : â H S ratio. I n s u m m a r y , t h e reaction o f H S w i t h 7 - F e O O H is a fast surfacec o n t r o l l e d process. E q u a t i o n s 8 a n d 9 can b e u s e d to estimate an u p p e r l i m i t of sulfide oxidation rates i n sediments w i t h reactive i r o n (assuming reactive i r o n to b e r e p r e s e n t e d b y lepidocrocite). T h e surface-area c o n c e n t r a t i o n A of reactive i r o n can b e calculated a c c o r d i n g to +

2

2 +

+

2

2


A = F«w(l-)pSe

(11)

where F e is t h e reactive i r o n content (between 0.01 a n d 1 mg/g; cf. réf. 19); φ is porosity (0.8); ρ is d e n s i t y (1.5 k g / L ) ; S is specific surface area ( 1 0 - 3 X 1 0 m / k g ) ; a n d θ is surface coverage r e s u l t i n g f r o m surface precipitates a n d adsorption o f d i s s o l v e d organic c a r b o n ( D O C ) a n d other s o r b i n g substances (0.1-0.01) so that A ranges b e t w e e n 0.3 a n d 360 m / L . T h e o r e t i c a l sulfide oxidation rates range b e t w e e n 9 X 10 " a n d 1.8 M p e r day (assuming S ( - I I ) concentrations r a n g i n g b e t w e e n 10~ a n d 10~ M i n s e d i m e n t p o r e waters a n d a p H o f 7). T h e u p p e r value is certainly a n o v e r estimate. H o w e v e r , aerobic m i c r o b i a l sulfide oxidation rates o n t h e o r d e r of Ι Ο M p e r day range w i t h i n t h e l i m i t s calculated p r e v i o u s l y a n d indicate the e n v i r o n m e n t a l relevance o f this process. T h e values are taken f r o m reference 28, T a b l e IV, a n d c o n v e r t e d f r o m flux densities into rates b y a s s u m i n g t h e oxidation takes place w i t h i n t h e u p p e r c e n t i m e t e r . r e a c

5

5

2

2

7

6

4

3

T h e extent to w h i c h H S contributes to t h e release o f ferrous i r o n into pore-water solution t h r o u g h dissolution o f reactive ferric oxides s u c h as l e p i d o c r o c i t e o r a m o r p h o u s f e r r i h y d r i t e remains unclear. A c c o r d i n g to C a n field (19), l i b e r a t i o n of ferrous i r o n i n sediments stems m a i n l y f r o m m i c r o b i a l dissolution o f ferric oxides. T h e release rates o f F e m e a s u r e d i n his study range b e t w e e n 3 X 10 a n d 4 X 10 M p e r day, at t h e l o w e r l i m i t o f the theoretical i n t e r v a l . 2

2 +

6

5

T h e o b s e r v e d reaction rate m a x i m u m a r o u n d ρ H 7 corresponds to t h e p H usually f o u n d i n anoxic s u l f i d e - b e a r i n g s e d i m e n t p o r e waters (29). I n a d d i t i o n , the f o r m a t i o n of F e S is favored u n d e r these c o n d i t i o n s . P o l y s u l f i d e s m a y b e expected to b e at least a n i n t e r m e d i a t e p r o d u c t o f the reaction (21), w h i c h m a y b e f u r t h e r o x i d i z e d to sulfate o n a l o n g e r t i m e scale (22).

Sediment and Fore-Water Data T h e v e r t i c a l profiles o f solid a n d d i s s o l v e d substances i n sediments usually are i n t e r p r e t e d a c c o r d i n g to a v e r t i c a l sequence o f organic matter d e c o m p o s i n g processes (11, 30). T h e reason f o r such a succession is g e n e r a l l y e x p l a i n e d i n terms o f decreasing metabolic free energy gain for successive m i c r o b i o l o g i c a l reactions (e.g., 31, 32). A s l o n g as oxygen c a n diffuse into a certain d e p t h o f the s e d i m e n t , i t w i l l b e the p r e d o m i n a n t t e r m i n a l electron acceptor. O n c e t h e o x y g e n is


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exhausted, other electron acceptors are u s e d . A s a consequence, characteristic reaction p r o d u c t s such as ferrous i r o n , H S , C H , or other f e r m e n t a t i o n products a l l o w the identification of v e r t i c a l zones w i t h respect to the p r e d o m i n a n t redox process. T h e i n d i v i d u a l m i c r o b i a l processes are c o n s i d e r e d exclusive at a certain s e d i m e n t d e p t h (33, 34). I n other w o r d s , c e r t a i n m i croorganisms are able to o u t c o m p e t e other m i c r o o r g a n i s m s i f t h e i r m e t a b o l i s m is l i n k e d to a h i g h e r gain i n free energy.


2

4

O n the basis of this m o d e l , L o v l e y et al. (17) a r g u e d that r e d u c t i v e dissolution of f e r r i c oxides m u s t b e a m i c r o b i o l o g i c a l process because the zone of sulfide generation is distinct f r o m the zone of m a x i m u m ferric oxide r e d u c t i o n . H i g h l y e u t r o p h i c e n v i r o n m e n t s w o u l d be an e x c e p t i o n . I n these systems the zone of d e c o m p o s i t i o n w i t h oxygen as t e r m i n a l e l e c t r o n acceptor d i r e c t l y overlies the zone of sulfate r e d u c t i o n . P o r e - w a t e r profiles are f r e q u e n t l y i n t e r p r e t e d a c c o r d i n g to this concept. F o r e x a m p l e , W h i t e et a l . (35) d e s c r i b e d a c o n c e p t u a l m o d e l of biogeoc h e m i c a l processes o f sediments i n an acidic lake (cf. F i g u r e 4). T h e y discussed the n u m b e r e d points i n F i g u r e 4 as follows: D i f f u s i o n of d i s s o l v e d oxygen across the s e d i m e n t - w a t e r interface leads to oxidation of ferrous i r o n a n d to an e n r i c h m e n t of ferric oxide (point 1). B a c t e r i a l r e d u c t i v e d i s s o l u t i o n of the ferric oxides i n the d e e p e r zones releases ferrous i r o n (point 2). T h e decrease i n sulfate concentration stems f r o m sulfate r e d u c t i o n , w h i c h p r o duces H S to react w i t h ferrous i r o n to f o r m m o s t l y p y r i t e i n the zone b e l o w the ferric oxide a c c u m u l a t i o n (point 3). 2

Interpretation of these data suggests a q u e s t i o n : W h a t is the o x i d a t i o n process l e a d i n g to the f o r m a t i o n of p y r i t e (analytically d e t e r m i n e d as c h r o m i u m - r e d u c i b l e sulfide, C R S ) instead of s i m p l e p r e c i p i t a t i o n of F e S (analytically d e t e r m i n e d as acid-volatile sulfide, A V S ) ? A V S c o n s t i t u t e d less than 10% i n the study of W h i t e et al. (35). A s B e r n e r (36) p o i n t e d out i n his classic w o r k , the f o r m a t i o n o f p y r i t e is c o u p l e d to a process i n w h i c h free sulfide is o x i d i z e d to f o r m p o l y s u l f i d e s , w h i c h again react w i t h F e S to f o r m p y r i t e . I n this study e l e m e n t a l sulfur was the oxidant. H o w e v e r , e l e m e n t a l sulfur was always less than 1 % of the total sulfur content i n the study of W h i t e et a l . (35). T h e findings of the e x p e r i m e n t a l studies discussed o n the interaction b e t w e e n H S a n d ferric oxides (20-23), i n c o m b i n a t i o n w i t h the field observations, suggest a m e c h a n i s m i n w h i c h ferric i r o n oxides are the oxidants to f o r m polysulfides a n d subsequently pyrite. 2

8FeOOH + 5H S 2

4FeS + S " 5

2

+ 2H

+

> 8Fe

+ S "

2 +

> 4FeS

5

2

2

+ 2H 0 + 140H" 2

+ H S

(12a) (12b)

2

In summary, 8FeOOH + 8H S 2

> 4FeS + 4Fe 2

2 +

+ 8 H 0 4- 8 0 H " 2


(13)


380


Concentration

WM Fe -0X1 l l i i CRS SO4

——

Ferjjss

Figure 4. A generalized profile of Fe and S chemistry in sediment pore waters from an acidic lake. Numbered points are discussed in the text. (Reproduced with permission from reference 35. Copyright 1989 American Geochemical Society.) T h e i m p o r t a n c e o f polysulfides i n t h e p y r i t e f o r m a t i o n process was outl i n e d b y several studies (37, 38). S c h o o n e n a n d Barnes (37) s h o w e d that n o p r e c i p i t a t i o n f r o m homogeneous solution c a n b e o b s e r v e d w i t h i n a reasonable t i m e scale, e v e n i n solutions h i g h l y supersaturated w i t h respect to p y r i t e , unless p y r i t e seeds are already existing. T h e r e f o r e f u t u r e studies s h o u l d address t h e role o f ferric oxide surfaces i n p r o m o t i n g t h e n u c l e a t i o n of p y r i t e . Reactions 12a a n d 12b c o n s u m e d i s s o l v e d sulfide. T h i s fact fits n i c e l y w i t h t h e data o f W h i t e et a l . (35), w h o c o u l d n o t detect free sulfide i n t h e i r study. D i s s o l v e d sulfide is f r e q u e n t l y absent i n freshwater sediments (e.g., 39; see U r b a n , C h a p t e r 10, for a discussion). T h i s lack o f sulfide is e x p l a i n e d b y a n excess o f reactive i r o n o v e r t h e total sulfide concentration (19, 40). P r o b a b l y t h e H S p r o d u c e d at a certain d e p t h diffuses b o t h u p a n d d o w n i n t h e s e d i m e n t . A s l o n g as t h e r e q u i r e m e n t s for F e S p r e c i p i t a t i o n are 2


11.

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Reaction of H S with Ferric Oxides

381

2


f u l f i l l e d , reaction 12b a n d subsequently reaction 13 w i l l p r o c e e d . H o w e v e r , reaction 12b w i l l stop at sulfide activities l o w e r than t h e s o l u b i l i t y p r o d u c t of F e S , whereas reaction 12a m a y c o n t i n u e . U n d e r s u c h circumstances the polysulfides f o r m e d m a y f u r t h e r react w i t h ferric oxides i n a redox process and b e s l o w l y r e o x i d i z e d to sulfate (22). It is therefore not s u r p r i s i n g to f i n d a sulfate peak b e l o w the p y r i t e peak (point 4 i n F i g u r e 4). Subsurface sulfate peaks also w e r e r e p o r t e d i n other lakes (10, 12). In this context, the concept o f sulfate r e c y c l i n g seems to b e h e l p f u l (28, 41). U r b a n ( C h a p t e r 10) p o i n t e d to the fact that a l l m e a s u r e d sulfate r e d u c t i o n rates i n freshwater sediments indicate a m u c h h i g h e r t u r n o v e r o f sulfate than w o u l d b e p r e d i c t e d b y calculation o f diffusive fluxes f r o m the concentration gradients. M o r e than 5 0 % o f sulfate r e d u c t i o n occurs b e l o w a d e p t h o f 2 c m , w h e r e diffusive gradients are n e g l i g i b l e . U r b a n c o n c l u d e d that o n l y sulfate regeneration r e s u l t i n g f r o m reoxidation can e x p l a i n sulfate r e d u c t i o n rates as h i g h as those f o u n d u n d e r m a r i n e c o n d i t i o n s , despite the l o w sulfate concentrations i n freshwater systems. Possible oxidants w i l l b e oxygen, ferric oxides, o r manganese oxides. O x y g e n usually shows v e r y steep gradients at the s e d i m e n t - w a t e r interface i n b o t h m a r i n e (42) a n d freshwater (43) systems. T h e r e f o r e it w i l l serve as an oxidant o n l y i n the u p p e r f e w m i l l i m e t e r s o f a s e d i m e n t (the redox process m e d i a t e d , e . g . , b y Beggiatoa). M a n g a n e s e was s h o w n to o x i d i z e sulfide (44). H o w e v e r , the p o o l o f solid manganese i n freshwater sediments is usually m u c h smaller than that o f solid i r o n (e.g., 35, 45-47). Because d e e p e r layers are f r e q u e n t l y i m p o v e r i s h e d w i t h respect to manganese, it m a y w e l l b e that ferric oxides are the electron acceptors responsible for the p o s t u l a t e d sulfate regeneration, p a r t i c u l a r l y i n d e e p e r layers o f s e d i m e n t . C o n s e q u e n t l y , a n oxic sulfide oxidation rates s h o u l d counterbalance sulfate r e d u c t i o n rates. A s an example, the m a x i m u m o f the sulfate r e d u c t i o n rate m e a s u r e d i n the p o r e waters o f the F O A M site i n the study o f C a n f i e l d (reference 19, 3 Χ 10" M p e r day) corresponds n i c e l y to theoretical oxidation rates calculated b y u s i n g eqs 9 a n d 11 (2.4 Χ 10" to 6.7 Χ Ι Ο M p e r d a y ; F e = 0.15 mg/g, φ = 0.35, p H was estimated to b e 7, a n d d i s s o l v e d sulfide c o n c e n tration [S(—II)] = 10~ M ; for the range o f the other parameters, cf. e q 11). 4

5

4

r e a c

6

In s u m m a r y , it seems m e a n i n g f u l to c o n s i d e r b o t h the f o r m a t i o n of p y r i t e f r o m the reaction o f H S w i t h reactive ferric oxides a n d sulfate r e c y c l i n g as a result o f this process i n any discussion o f the early diagenesis o f sulfur a n d i r o n i n sediments. 2

Formation of Pyrite in Sediments: A Kinetic Approach T h e f o r m a t i o n of p y r i t e i n sediments d e p e n d s o n the availability o f three parameters: i r o n , sulfate, a n d organic matter (48). A l t h o u g h organic matter content controls the f o r m a t i o n rate u n d e r m a r i n e sulfate-rich c o n d i t i o n s , sulfate concentration is usually r e g a r d e d as the l i m i t i n g factor u n d e r fresh-


382



w a t e r c o n d i t i o n s . H o w e v e r , i n t e n s i v e r e c y c l i n g o f sulfate has b e e n f o u n d i n b o t h coastal (41) a n d l i m n e t i c (28) sediments. T h e s e data suggest that the r e c y c l i n g rate of sulfate controls the f o r m a t i o n of p y r i t e m o r e than its c o n centration. I n contrast to sulfate, i r o n generally exists i n sufficient amounts i n sediments (with the exception of calcareous, i r o n - p o o r systems). T h e availability of ferrous i r o n for the f o r m a t i o n of p y r i t e , h o w e v e r , w i l l b e c o n t r o l l e d b y the reactivity of ferric oxides (19). A s S c h o o n e n a n d Barnes (49) p o i n t e d out, the existence of F e S is a necessary p r e r e q u i s i t e for the f o r m a t i o n of p y r i t e . H o w e v e r , h i g h p y r i t e f o r m a t i o n rates w e r e o b s e r v e d , p a r t i c u l a r l y i n salt marshes o r o t h e r systems exposed to t e m p o r a r y oxygen i n t r u s i o n i n t o s u l f i d e - b e a r i n g sediments, a l t h o u g h no F e S c o u l d be d e t e c t e d (50, 51). T h e s e p h e n o m e n a can b e exp l a i n e d b y the g r o w t h of already-existing p y r i t e i n favor of F e S (37). I n o t h e r w o r d s , the rate of formation o f F e a n d H S i n such systems is h i g h e n o u g h to cause p r e c i p i t a t i o n of F e S . It is, h o w e v e r , l o w e r t h a n the c o n s u m p t i o n rate o f b o t h species b y the two processes of reoxidation a n d p r e c i p i t a t i o n as pyrite. 2 +

2

A parameter i n d i c a t i n g the flux of F e a n d H S w o u l d b e the m e a s u r e d i o n activity p r o d u c t , I A P (52). A l o w p I A P v a l u e , c o r r e s p o n d i n g to a m o r phous F e S , does not necessarily m e a n that other, m o r e stable, s o l i d F e S phases do not exist (the system w o u l d be supersaturated w i t h respect to these phases), b u t it may indicate that the f o r m a t i o n rate of b o t h F e and H S is h i g h . A t l o w net fluxes, other solid phases have t i m e to f o r m . C o n sequently, i n v e r s e gradients can be o b s e r v e d i n systems w h e r e the net fluxes of F e a n d H S are h i g h ( p I A P increases w i t h depth) a n d i n systems w h e r e the net fluxes of F e a n d H S are l o w at the s e d i m e n t - w a t e r interface ( p I A P decreases w i t h depth) (cf. réf. 52). 2 +

2

2 +

2

2 +

2

2 +

2

V a r i a t i o n of p I A P d e p t h profiles also occurs w i t h t i m e . F i g u r e 5 presents p I A P values m e a s u r e d i n L a k e K i n n e r e t sediments (53) after an algal b l o o m ( M a y 30, 1988), d u r i n g the stratification p e r i o d (October 24, 1988), a n d after o v e r t u r n (January 5, 1989). T h e organic matter d e c o m p o s i t i o n after s e d i m e n t a t i o n o f algae caused a b u i l d u p of d i s s o l v e d sulfide (also i n the h y p o l i m n i o n ) a n d therefore an increase i n the rate of f o r m a t i o n of a m o r p h o u s F e S ( p I A P < 3; 54) i n the u p p e r s e d i m e n t layers. D u r i n g the course of the stratification p e r i o d , the f o r m a t i o n rate decreased a n d the p I A P values i n creased to a m o r e or less u n i f o r m v a l u e t h r o u g h o u t the s e d i m e n t , c o r r e s p o n d i n g to a m o r e crystalline F e S phase (mackinawite). A f t e r o v e r t u r n , oxygen penetrates into the s e d i m e n t ; the sulfate r e d u c t i o n rate (and thus the sulfide supply) was decreased ( F i g u r e 5). T h e l o w sulfide c o n c e n t r a t i o n that diffusively a c c u m u l a t e d i n d e e p e r layers was r a p i d l y c o n s u m e d t h r o u g h reaction w i t h ferric oxides. A s this short discussion shows, the kinetics of f o r m a t i o n of the single parameters ( F e a n d H S ) m a y c o n t r o l the extent a n d the pathway of p y r i t e f o r m a t i o n . O x i d a t i o n of sulfide b y e l e m e n t a l sulfur to f o r m p o l y s u l f i d e s (pathw a y 1) s h o u l d p r e d o m i n a t e at the o x y g e n - s u l f i d e interface o f v e r y p r o d u c t i v e 2 +

2



11.

PEIFFER

383


sz

Figure 5. Profiles of ion activity products (pIAP values) of FeS measured in Lake Kinneret sediments after the end of an algae bloom (May 30, 1988), during the stratification period (October 24, 1988), and after overturn (January 5, 1989). Straight lines correspond to solubility products of various FeS phases according to the reaction FeS + H Fe + HS. (Based on data from ref 53.) +

2+

(eutrophic) e n v i r o n m e n t s of h i g h organic matter s u p p l y (e.g., salt marshes; 51, 55). S u l f u r f o r m a t i o n m a y be m e d i a t e d b y bacteria (e.g., Beggiatoa, 56). H i g h rates of m i c r o b i a l r e d u c t i v e d i s s o l u t i o n of ferric oxides together w i t h h i g h sulfate r e d u c t i o n rates cause a v e r y sharp separation of r e d u c i n g a n d o x i d i z i n g m i c r o e n v i r o n m e n t s . I n contrast, oxidation of sulfide b y ferric oxides to polysulfides (pathway 2) m a y o c c u r i n those e n v i r o n m e n t s w h e r e d e c o m position rates are not h i g h e n o u g h for d e v e l o p m e n t of s u c h a redox m i c r o structure (e.g., s e d i m e n t systems, as discussed i n F i g u r e 5). F e S can still b e f o u n d i n s u c h e n v i r o n m e n t s because of a n insufficient s u p p l y of an oxidant to react w i t h sulfide. T h i s insufficiency m a y h a p p e n w h e n either no sulfide-reactive i r o n exists or the rate of r e o x i d a t i o n of ( m i -


384


erobiologieally produced) ferrous i r o n to sulfide-reactive f e r r i c oxide is too l o w (e.g., i n sediments u n d e r l y i n g a n anoxic h y p o l i m n i o n , s u c h as i n L a k e K i n n e r e t ) . I n o t h e r w o r d s , t h e c o n c e n t r a t i o n o f sulfide-reactive f e r r i c oxides s h o u l d l i m i t t h e f o r m a t i o n o f p y r i t e i n e n v i r o n m e n t s w h e r e pathway 2 p r e dominates. T h e rate o f t h e reaction o f ferric oxides w i t h H S controls t h e rate o f p y r i t e f o r m a t i o n f r o m F e S . 2

T h e C R S : A V S ratio reflects t h e t r o p h i c state o f a lake (cf. U r b a n (28), T a b l e V ) . T h i s observation m a y b e e x p l a i n e d b y the p r e c e d i n g k i n e t i c c o n siderations. T h e h i g h organic matter s u p p l y i n e u t r o p h i c lakes leads to a n i n t e n s i v e m i n e r a l i z a t i o n rate b y b o t h i r o n - a n d s u l f a t e - r e d u c i n g bacteria. H o w e v e r , reoxidation o f F e to ferric oxide o r o f sulfide to sulfate does n o t take place because an anoxic h y p o l i m n i o n prevents p e n e t r a t i o n o f o x y g e n . T h e r e f o r e F e S can b u i l d u p , b u t the s e d i m e n t b e c o m e s d e p l e t e d w i t h respect to reactive i r o n .


2 +

Sulfate Reduction in Lake Sediments: A Biofilm Model T h e o b s e r v e d sulfate r e d u c t i o n rates i n freshwater sediments cannot b e e x p l a i n e d b y diffusion o f sulfate f r o m the lake water into t h e s e d i m e n t , because m u c h steeper sulfate c o n c e n t r a t i o n gradients s h o u l d t h e n b e o b s e r v e d . A s s u m i n g diffusive s u p p l y alone, U r b a n (28) calculated that t h e change o f sulfate concentration w i t h d e p t h s h o u l d take place w i t h i n 1 m m instead o f several centimeters, w h i c h are usually m e a s u r e d . T h i s a s s u m p t i o n , h o w e v e r , also means that t h e sulfate r e c y c l i n g process a n d t h e sulfate r e d u c t i o n rate s h o u l d n o t b e l i m i t e d b y t h e v e r t i c a l transport o f sulfide to (frequently solid) oxidants o r to t h e oxic b o u n d a r y layer. Instead o f a s s u m i n g that diffusive fluxes o c c u r o n l y v e r t i c a l l y , f o l l o w i n g the macroscopic, measurable, large-scale c o n c e n t r a t i o n gradients, i t seems reasonable to c o n s i d e r also lateral microscale concentration gradients w i t h i n m i c r o n i c h e s (57-59). A b i o f i l m p r o v i d e s a v e r y h e l p f u l a p p r o a c h to e x p l a i n i n g the p h e n o m e n o n o f m i c r o n i c h e s . T h e b i o f i l m concept, m o s t l y a p p l i e d i n t e c h n i c a l systems, describes t h e activity o f m i c r o o r g a n i s m s a d h e r i n g to surfaces a n d thus separated from t h e b u l k s o l u t i o n (60-63). M i c r o o r g a n i s m s i n s e d i m e n t p o r e waters benefit above a l l f r o m t h e e n h a n c e d n u t r i t i o n a l status at m i n e r a l surfaces (64). M o r e generally, they exert b e t t e r r e g u l a t i o n o r c o n t r o l o f t h e i r m i c r o e n v i r o n m e n t (65). O f particular interest is the f o r m a t i o n o f m i c r o b i a l consortia, the d e v e l o p m e n t o f a s y n t r o p h i c c o m m u n i t y o f t w o o r m o r e bacterial species (66). S u c h consortia are often s t r i k i n g l y c o m p l e x , s h o w i n g t e m p o r a l a n d s t r u c t u r a l heterogeneity, b u t e x h i b i t i n g f u n c t i o n a l h o m o g e n e i t y based o n i n t e r c e l l u l a r fluxes o f organic c a r b o n c o m p o u n d s , inorganic e l e c t r o n acceptors, a n d r e d u c i n g equivalents (65, 67). M o d e l i n g o f interactions b e t w e e n organisms attached to p o r e walls a n d the b u l k s o l u t i o n o f a porous m e d i u m s u c h as a s e d i m e n t has b e e n d o n e for


11.

PEIFFER

385



g r o u n d w a t e r systems (68, 69), b u t there is still a great lack of t h e o r e t i c a l u n d e r s t a n d i n g of these interactions (W. Schâfer, U n i v e r s i t y of H e i d e l b e r g , personal communication). In theory, m i c r o b i a l activities w i t h i n b i o f i l m s are r e g a r d e d as d i f f u s i o n c o n t r o l l e d (70). H o w e v e r , the diffusion l e n g t h is shorter t h a n the macroscale concentration gradient thickness, w h i c h is u s u a l l y several c e n t i m e t e r s . T h e separation of m i c r o e n v i r o n m e n t s w i t h i n a b i o f i l m , w h e r e organisms benefit f r o m the metabolites of other organisms, leads to c o n c e n t r a t i o n gradients on a v e r y s m a l l scale. T h e m o r e intense the m e t a b o l i c activity of a certain species is, the steeper the concentration gradients a n d the greater the fluxes of substances. T h e intensity of d e c o m p o s i t i o n d e p e n d s o n the availability of b o t h organic matter a n d an adequate e l e c t r o n acceptor. T h e latter w i l l b e s t i m u l a t e d if the p r o d u c t o f a certain o r g a n i s m g r o u p (e. g . , sulfide) is r a p i d l y r e c o n v e r t e d to a reactant (e.g., sulfate) i n close p r o x i m i t y to this o r g a n i s m g r o u p . Steep, b u t i n v e r s e , concentration gradients of b o t h sulfide a n d sulfate w i l l therefore enhance the d e c o m p o s i t i o n rates of sulfate-reducing bacteria i n close p r o x i m i t y to a sulfide-regenerating ferric oxide surface. A sufficient organic matter s u p p l y m u s t be p r e s u m e d i n this m o d e l . In lake sediments the b u l k solution consists of lake water e n c l o s e d b y the s e d i m e n t e d material. A t the m o m e n t of b u r i a l the b u l k s o l u t i o n has the same c h e m i c a l c o m p o s i t i o n as the lake water. It is, h o w e v e r , t h e n exposed to early diagenetic processes, i n c l u d i n g g r o w t h of a b i o f i l m o n the p o r e w a l l s . T h e right side of F i g u r e 6 depicts a b i o f i l m - c o v e r e d , v e r t i c a l l y d i r e c t e d s e d i m e n t pore that is separated into five boxes, each r e p r e s e n t i n g a c e r t a i n s e d i m e n t d e p t h . T h e change of concentration o f the r e d o x - d e p e n d e n t substances ( D O , S O / ~ , F e , a n d H S ) w i t h d e p t h reflects the macroscale v e r tical concentration gradients w i t h i n the b u l k s o l u t i o n . M n was o m i t t e d for simplicity. 2 +

2

2 +

T h e p o r e w a l l consists of m e t a l oxides. Its reactive part is c o n s u m e d i n p r o p o r t i o n to d e p t h because of m e t a b o l i c activity w i t h i n a b i o f i l m . T h e b i o f i l m separates the b u l k solution f r o m the p o r e w a l l . T h e h o r i z o n t a l b i o f i l m concentration profiles o n the left side of F i g u r e 6 c o r r e s p o n d to the c e n t e r of each of the five boxes. ( C o n c e n t r a t i o n is o n the v e r t i c a l axis a n d b i o f i l m thickness is o n the h o r i z o n t a l axis.) Excess organic matter is a s s u m e d w i t h i n the u p p e r few centimeters. T h e arrows indicate net flux densities of various substances i n a n d out of the b i o f i l m . Substances i n the b u l k solution diffuse into the b i o f i l m , w h e r e they are c o n s u m e d (such as oxygen, p o i n t 1 i n F i g u r e 6) or r e c y c l e d (such as sulfate t h r o u g h stepwise reoxidation of H S f r o m sulfate r e d u c t i o n , p o i n t 5). W i t h i n the b i o f i l m , v e r y steep gradients exist for o x y g e n or h y d r o g e n sulfide a n d also for ferrous i r o n f r o m r e d u c t i v e d i s s o l u t i o n o f f e r r i c oxides. T h e s e gradients result f r o m the coexistence of anaerobic a n d aerobic metabolisms s u c h as aerobic r e s p i r a t i o n (point 1), r e d u c t i o n of f e r r i c oxides (point 3), a n d sulfate 2


386


E N V I R O N M E N T A L CHEMISTRY O F L A K E S A N D RESERVOIRS

Figure 6. An idealized scheme for a sedimentary porous medium with pore walls covered by a biofilm. High sulfate reduction rates are maintained even in depths to which sulfate cannot diffuse because of recycling of sulfate within the biofilm. Numbered points (in black circles) denote the following processes: 1, Respiration consumes oxygen. 2, Microbial reduction of reactive metal oxides. Reduction of reactive ferric oxides is in equilibrium with reoxidation of ferrous iron by 0 . Thus, no net loss of reactive iron takes place in these layers. 3, Microbial reduction of ferric oxides. 4, Sulfate reduction rate (denoted as SRR). 5, Sulfide oxidation, either microbiologically or chemically. 6, Sulfide builds up within the biofilm, sulfate consumption increases, reactive iron pool decreases. 7, Formation of iron sulfides. 2


11.

PEIFFER

387


r e d u c t i o n (point 4). W i t h i n the b i o f i l m , r e c y c l i n g of ferrous i r o n to f e r r i c i r o n w i l l take place as l o n g as oxygen is available (point 2). I n a d d i t i o n , H S is r e c y c l e d to sulfate because o f the reaction of H S w i t h f e r r i c oxides (point 5). T h e b u l k s o l u t i o n b e c o m e s i m p o v e r i s h e d w i t h respect to oxygen. H o w ever, the sulfate c o n c e n t r a t i o n remains constant as l o n g as the r e c y c l i n g rate of r e d u c e d sulfur to sulfate is h i g h e r than the sulfate r e d u c t i o n rate (denoted S R R i n F i g u r e 6). I n the opposite case the sulfate concentration of the b u l k solution also decreases, a n d H S is s l o w l y e n r i c h e d (point 6). T h e r e q u i r e ments for F e S p r e c i p i t a t i o n a n d subsequent p y r i t e f o r m a t i o n are t h e n f u l filled (point 7). 2

2


2

In spite of its l o w concentration c o m p a r e d to ferric oxides, s o l i d m a n ganese also m a y p l a y a role i n r e c y c l i n g sulfide i n the u p p e r layers (44). T h e kinetics o f homogeneous oxidation of M n to manganese oxide b y d i s s o l v e d oxygen are rather s l o w (32). N e v e r t h e l e s s , some r e c y c l i n g of M n because of heterogeneous catalysis o f the oxidation process w i l l o c c u r at oxide surfaces (71). F i e l d data i m p l y a r e l a t i v e l y fast d e p l e t i o n of solid manganese w i t h d e p t h (e.g., 47), so the r e c y c l i n g process s h o u l d be r e s t r i c t e d to the u p p e r c e n t i m e t e r of a sediment. A c c o r d i n g to the m o d e l p r e s e n t e d i n F i g u r e 6, reactive ferric oxides p l a y a k e y role i n the r e c y c l i n g of sulfate a n d therefore w i l l c o n t r o l the gross sulfate r e d u c t i o n rate. If the reactive i r o n p o o l i n boxes 2 a n d 3 (1.0 a n d 1.5 cm) d i d not exist, ferrous i r o n c o n c e n t r a t i o n w o u l d decrease i n the b u l k solution a n d sulfate w o u l d b e c o n s u m e d r a p i d l y i n the u p p e r layers. T h i s m o d e l places special emphasis o n the r e c o v e r y of reactive m e t a l oxides i n the u p p e r layers o f the s e d i m e n t b y d i s s o l v e d oxygen. I n o t h e r w o r d s , the oxidation capacity of d i s s o l v e d oxygen ( D O ) is transferred onto m e t a l oxides, w h i c h are t h e n b u r i e d b y f u r t h e r s e d i m e n t a t i o n . T h e o x i d a t i o n capacity is thus s h u t t l e d into d e e p e r layers, w h e r e it w i l l enhance t h e a n aerobic t u r n o v e r of organic c a r b o n i n s e d i m e n t layers that c o u l d not b e m a i n t a i n e d b y diffusive s u p p l y of sulfate alone. A shuttle of o x y g e n e q u i v alents m a y also influence the pathways of organic matter d e c o m p o s i t i o n . 2 +

2 +

O n the other h a n d , a p e r m a n e n t s u p p l y of ferric oxides to the sediments is p r o v i d e d b y s e d i m e n t a t i o n of allochthonous m a t e r i a l . It is u n k n o w n to what extent these oxides are reactive w i t h respect to sulfide or w h e t h e r a p r e d i g e s t i o n of ferric oxides b y bacteria is n e e d e d . V a r i o u s studies i n d i c a t e that ~ 5 0 % of freshwater s e d i m e n t i r o n exists as i r o n oxide a n d ~ 2 0 % o f the i r o n is reactive (72). F u t u r e studies s h o u l d b e d i r e c t e d to a b e t t e r u n d e r standing o f the existence o f reactive i r o n .

A Personal View M a n y of the hypotheses p r e s e n t e d i n this chapter are based o n e x i s t i n g f i e l d and laboratory studies. A s t h e y are often not yet p r o v e n , t h e y m i g h t stimulate




388

controversial discussion. I hope they will provide a contribution to the development of future research objects in a field of great environmental relevance. The fate of substances of environmental concern may be closely linked to the processes discussed. O f particular relevance are substances that interact with the surface of ferric oxides, such as trace metals (73-76) or phosphate (77, 78). In addition to a better understanding of the reaction of sulfide with ferric oxides and its role in pyrite formation, a more exact definition of the term "reactive iron" is critical. Does reactive iron mean a different iron oxide fraction for bacterial dissolution (e.g., weathering products such as goethite or hematite) than for reaction with sulfide (e.g., reoxidized lepidocrocite)? In other words, is there a predigestion of ferric oxides by bacteria that allows a subsequent rapid interaction of sulfide with ferric oxides? The biofilm concept, applied to sediment-water interactions, breaks with classical strategies to model early diagenesis (i.e., the vertical redox zonation). Although far from completely developed, this concept may overcome modeling problems, such as an adequate description of recycling of substances.

Acknowledgments This chapter benefited from stimulating discussions with Noel Urban and Bernhard Wehrli during a postdoctoral year at the Lake Research Institute of E A W A G , Kastanienbaum, Switzerland. I am grateful to Jeff White for the discussion of his data during the A C S meeting. I am also indebted to Wolfgang Durner for critically commenting on the biofilm concept and to two reviewers for their helpful comments. I further wish to thank Barbara Staudinger for making data available and Elisabeth Schill for preparing the drawings.

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for review September 26, 1991.

ACCEPTED

revised manuscript July 24,


Reaction of H2S with Ferric Oxides - American Chemical

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