Manganese Dynamics in Lake Richard B. Russell - Advances in

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Manganese Dynamics in Lake Richard B. Russell Tung-Ming Hsiung and Thomas Tisue Department of Chemistry, Clemson University, Clemson SC 29634-1905

Lake Richard B. Russell is an impoundment of the Savannah River. The lake's waters are soft and mildly acidic. Organic matter is abundant. Seasonal oxygen depletion occurs during stratification, and reduced species, including Mn , accumulate in the hypolimnion. Electron paramagnetic resonance studies indicate that Mn in the water column is present almost entirely as soluble and colloidal Mn species, except near the surface where particulate forms sometimes predominate. Field and laboratory studies were used to estimate the rate of oxidation of Mn in the water column, and to characterize the flux of reduced Mn across the sediment-water interface. Surprisingly, incubating bottom deposits with oxygenated bathylimnetic water released more Mn than did maintaining anoxia. 2+

2+

2+

^ J A N G A N E S E E X H I B I T S C O M P L E X B E H A V I O R i n natural water systems, cy-

c l i n g r e a d i l y a m o n g various oxidation states i n response to c h a n g i n g e n v i r o n m e n t a l conditions (1,2). T h e b e h a v i o r o f manganese i n seasonally anoxic h y p o l i m n e t i c waters generally follows t h e m o d e l d e v e l o p e d b y D e l f i n o a n d L e e (3), w h o traced t h e m i g r a t i o n o f t h e b o u n d a r y b e t w e e n o x i d i z e d a n d r e d u c e d forms from b e l o w t h e s e d i m e n t - w a t e r interface u p i n t o t h e w a t e r c o l u m n as anoxia d e v e l o p e d d u r i n g stratification. M a n g a n e s e thus resembles i r o n i n its response to c h a n g i n g redox c o n d i t i o n s , a n d t h e biogeochemistries of t h e two elements are closely l i n k e d (4). A s a r u l e , h o w e v e r , t h e oxidation o f r e d u c e d M n is slower than that o f r e d u c e d F e . T h e reverse is true f o r r e d u c t i o n ; M n is released first from sediments as anoxia develops (5, 6). T h e n e t result is that t h e p r o p o r t i o n o f particulate F e to total F e is generally larger than t h e p r o p o r t i o n o f particulate

0065-2393/94/0237-0499$07.50/0

© 1994 American Chemical Society

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

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

M n to total M n . T h e s e differences i n kinetics a n d speciation c a n cause fractionation o f t h e t w o elements w h e n redox p o t e n t i a l changes r a p i d l y , o r w h e n particle a n d solute transport are disjunct.


M a n g a n e s e is essential for t h e o x i d a t i o n o f water to o x y g e n i n t h e p h o tosynthetic process, o c c u r r i n g at the active site o f p h o t o s y s t e m I I w h e r e its redox changes are l i n k e d to t h e f o u r - e l e c t r o n oxidation o f water. It is t h e o n l y m e t a l that has b e e n f o u n d to b e associated w i t h t h e w a t e r - s p l i t t i n g apparatus i n a l l the o x y g e n - e v o l v i n g organisms s t u d i e d to date. A l t h o u g h it is an essential e l e m e n t i n plants a n d animals, elevated concentrations o f M n are toxic to a variety of aquatic organisms (7). I n a d d i t i o n , r e d u c e d M n makes water unpalatable a n d causes f o u l i n g a n d c o r r o s i o n i n water systems a n d c o o l i n g towers.

Oxidation Oxidation of M n i n aqueous s o l u t i o n appears to occur i n a stepwise fashion io M n 0 , w i t h M n O O H (8) a n d M n 0 (9) as possible i n t e r m e d i a t e s . T h e reaction exhibits t h e i n d u c t i o n p e r i o d a n d kinetics characteristic o f an a u tocatalytic process (JO). M n ( I I ) is strongly s o r b e d to t h e surfaces of the n e w l y f o r m e d , i n s o l u b l e oxides, w h e r e its oxidation is greatly facilitated. 2 +

2

3

4

M o r g a n (11) d e r i v e d a rate l a w that adequately describes t h e o b s e r v e d kinetics a n d was able to extract rate constants for b o t h h o m o g e n e o u s a n d particle-catalyzed reactions. I n laboratory e x p e r i m e n t s w i t h sterile, filtered synthetic solutions, M n oxidation proceeds m u c h m o r e slowly t h a n i n natural waters. It m a y n o t o c c u r at a l l at n e u t r a l o r a c i d i c p H , especially i n the absence of catalytically active surfaces s u c h as p r e f o r m e d o x i d a t i o n p r o d ucts (12, 13). 2 +

T h e c o n v e n t i o n a l i n t e r p r e t a t i o n is that r a p i d oxidation o b s e r v e d u n d e r natural circumstances indicates m e d i a t i o n b y m i c r o o r g a n i s m s (14), a l t h o u g h an i m p o r t a n t caveat was offered b y T i p p i n g et a l . (12, 15). T h e s e authors p o i n t e d o u t that it is difficult to p r o v e c o n c l u s i v e l y w h e t h e r o x i d a t i o n is strictly b i o l o g i c a l l y m e d i a t e d o r is catalyzed b y abiotic particulate matter as w e l l . F i l t r a t i o n removes abiotic catalysts along w i t h m i c r o o r g a n i s m s , a n d poisons u s e d to halt b i o l o g i c a l activity also change c h e m i c a l p r o p e r t i e s s u c h as p H , redox p o t e n t i a l , a n d speciation.

Reduction M n O can b e r e d u c e d to M n ( I I ) i n natural waters b y several means, i n c l u d i n g d i r e c t reaction w i t h Fe(II) species (16, 17). Stone a n d M o r g a n (18-21) and others (22) s h o w e d that r e d u c t i o n o f M n 0 is r a p i d i n t h e presence o f r e a d i l y oxidizable organic c o m p o u n d s s u c h as catechols, h y d r o q u i n o n e s , a n d r e l a t e d c o m p o u n d s . O x a l i c acid is an effective reductant u n d e r acidic conditions (23), c o n v e r t i n g hausmanite ( M n 0 ) to manganite ( M n O O H ) . M a n y M n 0 v

2

3

4


2

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reactive functionalities are present i n natural waters i n a p p r e c i a b l e c o n c e n trations as m i c r o o r g a n i s m a l metabolites o r t h e degradation p r o d u c t s o f n o n l i v i n g organic matter. R e d u c t i o n b y this means thus is a distinct p o s s i b i l i t y , a l t h o u g h its o c c u r r e n c e b y strictly abiotic means appears n o t to have b e e n c o n c l u s i v e l y d e m o n s t r a t e d . C e r t a i n l y , h o w e v e r , t h e presence o f large amounts o f exogenous organic matter, s u c h as tannery effluent (24), w i l l potentiate M n r e d u c t i o n (25). R e d u c e d forms o f sulfur, s u c h as sulfide a n d thiols, also react r a p i d l y w i t h M n 0 (26-28) as w e l l as w i t h F e O . H o w e v e r , sulfur i n fresh w a t e r is often present i n s u b s t o i c h i o m e t r i c amounts w i t h respect to i r o n . T h u s little or no free r e d u c e d S is present e v e n u n d e r strongly anoxic c o n d i t i o n s because of the f o r m a t i o n o f v e r y i n s o l u b l e F e S species. O u r e q u i l i b r i u m calculations (29) indicate that c o m p l e x a t i o n w i t h r e d u c e d sulfur species is n o t a q u a n titatively i m p o r t a n t aspect o f M n speciation i n L a k e R i c h a r d B . R u s s e l l ( R B R ) . H o w e v e r , this result does not r u l e out t h e o c c u r r e n c e of s u c h species as transient intermediates. A t h i r d m e c h a n i s m for M n r e d u c t i o n has b e e n d e m o n s t r a t e d m o r e u n e q u i v o c a l l y . R e d u c t i o n o f manganese oxides b y naturally o c c u r r i n g organic matter is p r o m o t e d b y sunlight. S u n d a et a l . (30) suggested that t h e sur p r i s i n g p r e d o m i n a n c e o f M n ( I I ) i n ocean surface waters is attributable to t h e reaction o f ΜηΟ . w i t h h y d r o g e n p e r o x i d e , p r o d u c e d p h o t o c h e m i c a l l y b y sunlight i n the presence of organic matter (fulvic a c i d , F A ) , perhaps a c c o r d i n g to eqs 1 a n d 2.


2

t

x

λ

FA + 0 20 -

+ 2 H

2

Ά

2

+ F A

ΟΓ >H 0

+

2

+ 0

2

(1)

+

(2)

2

H y d r o g e n p e r o x i d e c a n f u n c t i o n as a r e d u c i n g agent w i t h respect to M n ( I I I , I V ) , b u t the c h e m i s t r y doesn't necessarily stop w i t h r e d u c t i o n . M n 0 can b r i n g about t h e d i s p r o p o r t i o n a t i o n o f H 0 , as s h o w n i n eqs 3 a n d 4. 2

2

Mn0

2

+ H 0 2

Mn

2 +

2

+ 2 H + H 0 2

+

= Mn

+ 0

2 +

= Mn0

2

2

2

+ 2H 0

2

2

+ 2 H

(3) (4)

+

B o t h o f these reactions are t h e r m o d y n a m i c a l l y feasible i n n e u t r a l s o l u t i o n and c o u l d p l a y a role i n M n d y n a m i c s t h r o u g h o u t t h e w a t e r c o l u m n . H o w ever, w e w e r e unable to detect H 0 i n L a k e R B R e v e n w i t h t h e l u m i n o l c h e m i l u m i n e s c e n t m e t h o d (31). M a n g a n e s e i n seawater acts as a n effective scavenger o f superoxide, whose d i s p r o p o r t i o n a t i o n i t also catalyzes. 2

Mn

2 +

2

+ 0 - + 2 H 2

Mn

3 +

+ ^Η 0 2

2

+

= Mn

3 +

+ H 0

= Mn

2 +

+ H

2

+

(5)

2

+ ^0

2


(6)

502


Adsorption A d s o r p t i o n p h e n o m e n a are an i m p o r t a n t aspect of M n d y n a m i c s i n several ways. Because of t h e i r h i g h surface activity, freshly f o r m e d h y d r o u s m a n ganese oxides can strongly i n f l u e n c e the c y c l i n g of nutrients, heavy metals, a n d organic substances (32, 33). M n O has a greater affinity for H and m u l t i v a l e n t cations t h a n for alkali m e t a l cations. M u r r a y (34) f o u n d the zero p o i n t of charge for a fresh M n 0 suspension at p H 2.25. H i s e l e c t r o n m i croscope studies s h o w e d h i g h l y aggregated particles ( 0 . 2 - 1 . 0 - μ π ι diameter) that b e c a m e less reactive u p o n aging, perhaps because of condensation a n d dehydration. +

r


2

M a n g a n e s e oxides f o r m e d as precipitates b y raising the p H o f M n c o n t a i n i n g solutions are h i g h l y h y d r a t e d a n d o f variable a n d nonstoichiom e t r i c c o m p o s i t i o n . L i k e the c o r r e s p o n d i n g i r o n phases, the h y d r o u s m a n ganese oxides are h i g h l y sorptive of M n ( I I ) . Because adsorption also enhances the rate of Mn(II) oxidation, it is difficult to d i s t i n g u i s h the r e m o v a l of M n ( I I ) f r o m solution b y each o f the two m e c h a n i s m s . M a n y surfaces ( i n c l u d i n g those of the oxides of T i ( I V ) , Si(IV), Sn(IV); calcite; clay m i n e r a l s ; a n d feldspar) m a y accelerate the oxidation o f M n ( I I ) (35). M o r r i s a n d Bale (36) a n d C o u g h l i n a n d M a t s u i (37) n o t e d that the r e m o v a l of M n f r o m s o l u t i o n is associated w i t h the presence of s u s p e n d e d particles. W i l s o n (38) p r e s e n t e d e v i d e n c e that this effect is attributable at least i n part to catalysis of M n ( I I ) o x i d a t i o n . T h e effects of p H o n M n adsorption always m u s t b e taken into account because t h e y can mask or m i m i c o t h e r effects, as H o f f m a n n a n d E i s e n r e i c h (39) d e m o n s t r a t e d . F o r example, l o w e r i n g p H releases soluble M n from adsorption sites o n particles e v e n w h e n no r e d u c t i o n is i n v o l v e d . 2 +

T h e m e t a l oxide surface apparently is not necessarily a passive p a r t i c i p a n t i n photoassisted r e d u c t i o n reactions i n v o l v i n g h u m i c a n d f u l v i c substances. R e d o x processes can b e i n d u c e d or e n h a n c e d b y i n t e r a c t i o n of l i g h t w i t h c h r o m o p h o r e s o n the oxide surface itself. T h e s e processes l e a d to accelerated particle dissolution (40). A n o t h e r factor to consider is that l i g h t a b s o r p t i o n b y organic matter m a y result i n the p r o d u c t i o n of cationic species that are b o u n d m o r e strongly than t h e i r n e u t r a l counterparts to the u s u a l l y n e g a t i v e l y charged surfaces of m e t a l oxides (41). Suwanee R i v e r f u l v i c a c i d a n d M n 0 u n d e r w e n t a redox reaction w h e n i l l u m i n a t e d at a rate significantly greater than i n the dark; 0 was not r e q u i r e d (42). I n soft a c i d i c fresh waters, especially those h i g h i n s u s p e n d e d m i n e r a l matter or h u m i c substances, photoassisted d i s s o l u t i o n of manganese oxides m a y o c c u r b y a different m e c h a n i s m than i n ocean surface waters. 2

2

Microbial Mediation M a n y of the reactions discussed so far are subject to m i c r o b i a l m e d i a t i o n o r i n f l u e n c e (43, 44).

I n fact, a c o m m o n v i e w is that M n b i o g e o c h e m i s t r y i n


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503

fresh w a t e r is d o m i n a t e d b y t h e element's i n v o l v e m e n t i n t h e life processes of various types o f organisms. O r i g i n a l l y , this hypothesis was based l a r g e l y o n e v i d e n c e from field studies, s u c h as t h e o c c u r r e n c e o f t e m p e r a t u r e max i m a for M n o x i d a t i o n rates (45), seasonal redox c y c l i n g i n s y n c h r o n y w i t h m i c r o b i a l p o p u l a t i o n fluctuations, a n d i n h i b i t i o n b y w e l l - k n o w n poisons s u c h as azide (14). T h e i m p o r t a n c e o f m i c r o b i o l o g i c a l i n v o l v e m e n t also has b e e n strongly s u p p o r t e d b y m o r e recent discoveries i n c l u d i n g , for e x a m p l e , bacterial strains that c a n use Μ η Ο as t h e i r sole t e r m i n a l e l e c t r o n acceptor (46). M a n y different types o f m i c r o o r g a n i s m s o x i d i z e manganese, i n c l u d i n g bacteria, yeast, a n d f u n g i (47). R i c h a r d s o n a n d N e a l s o n (48) d i v i d e d m i c r o b i a l l y r e l a t e d M n redox reactions i n t o c o n c e p t u a l categories, i n c l u d i n g d i r e c t o x i d a t i o n i n v o l v i n g specific proteins that are often extracellular; d i r e c t r e d u c t i o n , as w h e n o x i d i z e d M n is u s e d as the t e r m i n a l e l e c t r o n acceptor for anaerobic r e s p i r a t i o n ; i n d i r e c t c h e m i c a l o x i d a t i o n , associated w i t h increases i n the e n v i r o n m e n t a l p H o r redox p o t e n t i a l as a result of m i c r o b i a l activity; a n d i n d i r e c t c h e m i c a l r e d u c t i o n , c a r r i e d out b y reductants released f r o m m i c r o b i a l cells, s u c h as sulfide (27) o r oxalate (23). T h e s e p h e n o m e n a have b e e n o b s e r v e d i n b o t h water a n d sediments. A l e x a n d e r (49) p o i n t e d o u t that m i c r o b i o l o g i c a l i n v o l v e m e n t is l i k e l y to b e least p r o m i n e n t b e l o w p H 5 . 5 , w h e r e exchangeable M n ( I I ) p r e d o m i n a t e s , a n d above p H 8.0, w h e r e o x i dation b y oxygen b e c o m e s r a p i d .


τ

E x t r e m e anoxia is n o t r e q u i r e d for a n d m a y actually i n h i b i t t h e release of r e d u c e d forms o f M n near t h e s e d i m e n t - w a t e r interface i n lakes. Studies i n L a k e C o n s t a n c e , for e x a m p l e , r e v e a l e d that M n c a n b e m o b i l i z e d o u t o f the s e d i m e n t at o x y g e n concentrations as h i g h as 4 m g / L (50). I n E s t h w a i t e W a t e r , r e d u c e d M n i n t h e surficial s e d i m e n t r e a c h e d a m a x i m u m u n d e r well-mixed conditions and Mn(II) accumulated i n the hypolimnion u n d e r oxic c o n d i t i o n s (51). T w o s e d i m e n t - t r a p e x p e r i m e n t s s h o w e d that t h e flux of r e d u c e d M n into t h e w a t e r c o l u m n was actually h i g h e r d u r i n g o v e r t u r n than d u r i n g the seasonal anoxic p e r i o d (52, 53). T h e s e observations suggest that t h e release o f M n a c c o m p a n y i n g m i n e r a l i z a t i o n o f organic matter m a y b e a m o r e i m p o r t a n t source at t i m e s than r e d u c t i v e d i s s o l u t i o n o f M n O . W e w i l l discuss this hypothesis i n greater d e t a i l because i t is consistent w i t h the b e h a v i o r o f M n i n L a k e R B R . x

Lake Richard B. Russell T h e structure i m p o u n d i n g L a k e R B R was c o m p l e t e d i n D e c e m b e r 1983, a n d f u l l p o o l e l e v a t i o n (145 m above sea level) was r e a c h e d i n N o v e m b e r 1984. T h e i m p o u n d m e n t inundates part o f t h e Savannah R i v e r w a t e r s h e d b e t w e e n L a k e H a r t w e l l D a m a n d t h e headwaters o f S t r o m T h u r m o n d L a k e , i n t o w h i c h t h e tailrace discharges (see F i g u r e 1). M a x i m u m d e p t h is about 40 m , a n d t h e surface area is a p p r o x i m a t e l y 105 k m (26,000 acres). 2




504

Figure 1. Locations of Lake Richard B. Russell and of sampling stations 60, 100, and 120.


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H a r v e s t a b l e t i m b e r was r e m o v e d f r o m the lake b e d d u r i n g site p r e p a ration, b u t the r e m a i n i n g forestation was left u n t o u c h e d except for some s h o r e l i n e c l e a r i n g . T h e lake's waters are soft ( c o n d u c t i v i t y a r o u n d 50 μ8/ατι), slightly a c i d i c ( p H 6-7), a n d c o n t a i n a b u n d a n t organic matter; d i s s o l v e d organic c a r b o n ( D O C ) is 2 - 5 m g / L . A t h e r m o c l i n e develops at depths a r o u n d 10 m b e g i n n i n g i n M a r c h , a n d the lake remains stratified f r o m t h e n u n t i l N o v e m b e r . O x y g e n d e p l e t i o n occurs i n the h y p o l i m n i o n d u r i n g stratification, b u t b e c o m e s p r o n o u n c e d (dissolved oxygen < 5 m g / L ) o n l y i n the b o t t o m f e w m e t e r s a n d o n l y d u r i n g the w a r m e s t m o n t h s . A n u n u s u a l feature of the lake is the o x y g e n i n j e c t i o n system i n s t a l l e d b y the U . S . A r m y C o r p s o f E n g i n e e r s , w h i c h is c o n s t r a i n e d b y the states of S o u t h C a r o l i n a a n d G e o r g i a to m a i n t a i n at least 6 m g / L of d i s s o l v e d o x y g e n i n the discharge waters. T h i s c r i t e r i o n often is m e t b y p u l s e i n j e c t i o n of o x y g e n j u s t p r i o r to discharge, w h i c h m i n i m i z e s the i m p a c t o f oxygenation o n the p o w e r p o o l as a w h o l e . T o study the effectiveness o f the o x y g e n i n j e c t i o n s y s t e m , as w e l l as to investigate other w a t e r - q u a l i t y issues, the W a t e r w a y s E x p e r i m e n t Station ( W E S ) , U . S . A r m y C o r p s of E n g i n e e r s , c o n d u c t e d extensive studies i n b o t h L a k e R B R a n d L a k e T h u r m o n d . T h e s e o n g o i n g studies, w h i c h b e g a n p r i o r to creation o f the i m p o u n d m e n t , p r o v i d e a d e t a i l e d p i c t u r e o f the course o f events since i m p o u n d m e n t . R e p o r t s c o n t a i n i n g these data (54) f o r m a v a l uable basis for o t h e r studies. A n i n t e r e s t i n g observation that e m e r g e d early i n the W E S studies was a c c u m u l a t i o n of r e d u c e d soluble M n i n the h y p o l i m n i o n at concentrations u p to several m i l l i g r a m s p e r l i t e r . O u r attention was d r a w n also to a p r e l i m inary r e p o r t b y T u r n e r (55) i n d i c a t i n g r a p i d rates o f r e o x i d a t i o n of M n despite the lake's l o w p H . 2 +

Experimental Methods Hydrological data including temperature, dissolved oxygen, p H , and conduc tivity were collected by using a water-quality instrument package (Hydrolab Surveyor II, H y d r o l a b C o r p . , A u s t i n , Texas) on 11 occasions i n 1988. Three sites were sampled: Stations 60, 100, and 120 are shown on the map i n F i g u r e 1. These stations correspond, respectively, to the power pool just b e h i n d the dam, the upstream e n d of the power pool, and a location further upstream at the head of the principal basin. Water samples for chemical analyses were p u m p e d through Tygon tubing by a submersible p u m p , then stored i n linear polyethylene containers that had been cleaned by soaking i n 10% nitric acid, followed by extensive rinsing w i t h deionized (Nanopure system) water. Samples were kept on ice i n the dark until they reached the lakeside field laboratory. Filtration through 0.45-μιη cellulose acetate membranes and acidification took place w i t h i n a few hours following collection, Subsamples for electron spin resonance (ESR) spectrometric deter minations were usually frozen p r o m p t l y i n hematocrit tubes, w h i c h were thawed just prior to analysis and inserted directly into the spectrometer. O t h e r subsam-



506


pies for atomic absorption (AA) speetrophotometrie determinations were pre served at p H 2 or below and stored at room temperature i n precleaned linear polyethylene bottles. Standards treated i n the same way as water samples showed no significant changes i n M n concentration or speciation. A gravity dredge was used to recover samples of the soft, dark, organic-rich muds that have accumulated thinly over the inundated clay soils of the watershed. In some locations, soft deposits such as alluvial silts c o u l d be recovered w i t h a 2.5-inch-diameter gravity corer. A n atomic absorption spectrophotometer ( P e r k i n - E l m e r 4000) was used w i t h standard operating conditions to determine dissolved and total M n , irrespective of oxidation state. Dissolved organic carbon was determined w i t h an organic carbon analyzer (Beckman m o d e l 915B T O C m a s t e r ) . The basis of E S R spectrometry is the measurement of the energy r e q u i r e d to reverse the spin of unpaired electrons i n an external magnetic field. A t a given external magnetic field strength H (in gauss), the energy Δ Ε (in ergs) required for the transition between two quantized electron spin states (parallel and antiparallel to the field) is given as Δ Ε = βμ Η where g is the so-called spec troscopic splitting factor (2.0023 for the free electron) and μ , the B o h r magneton, equals 0.92371 Χ 10" erg/G. In the octahedral aquo complex, M n ( H 0 ) , manganese has a 3 d electron configuration w i t h a g-value close to 2.0. Microwaves are conveniently generated w i t h klystrons i n the X - b a n d region around 9 G H z . F o r g values around 2, these frequencies require magnetic fields of about 3 k G to produce spin transitions. H y p e r f i n e structure i n the spectrum results from interactions of the electron spin (S) w i t h the nuclear spin (/), leading to 21 + 1 transitions. In mobile fluids, anisotropic spin coupling averages to zero, leaving isotropic coupling as the only interaction observed. Because ί = 5/2 for M n (100% abundance), aquo M n gives a six-line absorption spectrum. Spectral line w i d t h varies inversely w i t h the excited-state lifetime according to Heisenberg's principle, Δ Τ Χ άΗ = /ι/2ιτ, where Δ Γ is the lifetime of the excited spin state, h is Planck's constant, and ΔΗ is the effective w i d t h of the absorption signal. Excited-state lifetimes are subject to environmental (including chemical) influences. The resulting line-shape changes y i e l d information about the chemical environment of the M n atoms. B o t h spin-lattice and s p i n - s p i n relaxation mechanisms can contribute to the overall lifetime. Spin-lattice relaxation (time T ) results from dissipation of the excited-state energy among vibrational modes of the matrix. In mobile liquids, vibrational fluctuations are spread over a very wide frequency range. This configuration decreases the probability of spin-lattice coupling. As a result, T is long and thus makes a negligible contribution to the line w i d t h . S p i n - s p i n relaxation (time T ) can result from both intermoleeular homonuclear exchange coupling and d i p o l e - d i p o l e interactions, but only the latter is observable at M n concentrations < 1 0 " M . M n forms both inner- and outersphere complexes. I n symmetrical inner-sphere complexes like M n ( H 0 ) , the s p i n - s p i n coupling is strongly forbidden, T is long, and lines remain narrow. W h e n nonsymmetric inner-sphere complexes form, the resulting anisotropy of the electric field leads to allowed s p i n - s p i n transitions that produce very small values of T and very broad, perhaps even unobservable, lines (56). Fortunately, M n does not form strong (inner-sphere) complexes at the ligand concentrations normally present i n natural waters. F o r example, nitrate, chloride, bicarbonate, and sulfate do not form observable complexes i n fresh water, and p H has no effect i n the range from 2 to 7. W e a k l y interacting species that form only outer-sphere complexes, such as naturally occurring organic mat ter, c o u l d have some influence on line w i d t h , but solvent-ligand exchange i n 0

Β

φ

Β

20

2

6

2

6

2 +

3

5 5

2 +

x

l

2

2 +

2

2 +

2

2

2 +


2 +

16.

HSIUNG & TISUE


the absence of a strong ligand field produces little perturbation of the electronspin system. L i g a n d exchange rates for outer-sphere complexes w o u l d be ex pected to be very close to solvent fluctuation rates. This similarity w o u l d make their line-broadening effect, i f any, difficult to observe (57). E S R offers compelling advantages i n studies of M n biogeochemistry (57, 58). C h i s w e l l and Mokhtar (59) compared various means for studying M n spe ciation. They concluded that E S R offers optimal specificity for M n , sufficient sensitivity, and m i n i m a l alteration of the sample. W i t h its 3 d electron config uration o n M n , M n ( H 0 ) gives a strong, highly characteristic spectrum at a field strength w e l l separated from other commonly occurring paramagnetic spe cies. Several groups have reported (60-62) that the only E S R signal detectable in nonchemically isolated humic material was that of M n . Thus, association of Mn w i t h even relatively large colloids does not interfere w i t h its determination by E S R spectroscopy. A n E S R spectrometer (Varian model E-3) was used to observe and quantify Mn species at a field strength of 3155 ± 50 G and a frequency of 9.5 G H z . A flat fused silica " r i b b o n " cell (Wilmad Glass N o . WG-812) was used at very low concentrations to optimize the signal-to-noise ratio b y m i n i m i z i n g dielectric losses. Microwave power was set routinely to 4 m W , but was occasionally raised to optimize sensitivity at very low concentrations. Quantitation was based o n the height of the lowest-field peak i n the first derivative of the absorption spec trum. A s reported by others (63), this technique is characterized b y precision and accuracy of about 1 % relative standard deviation over a linear range from

Manganese Dynamics in Lake Richard B. Russell - Advances in

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