Nonequilibrium Systems in Natural Water Chemistry

8 Variations in the Stability of Precipitated

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Ferric Oxyhydroxides D O N A L D L A N G M U I R and D O N A L D O. W H I T T E M O R E Department of Geosciences, and Mineral Conservation Section, The Pennsylvania State University, University Park, Pa. 16802

The apparent

thermodynamic

ide precipitates pQ =

may

waters usually

and goethite, 37.3 to 43.3. 43.5.

Fe

2+

initially

+

-2

and suspended

Variations

leaching

soluble

including 10

in particle and relative

of precipitates

material.

occurs within

Measurable

a few

may take thousands

oxyhydroxproduct

Such precipitates

as mixed amorphous molar

In 24 well waters

trol on absolute H

begin

sometimes

tory solutions

of ferric

by the activity

in solution.

- 3

3+

natural

molar

[OH ]

-log[Fe ]

stability

be described

lepidocrocite. in Fe

containing

to

pQ

was 37.1

the major

of the

crystallization molar

of years in waters

of

Fe

2+

10

-6

10

-5.40

to

con-

oxyhydroxides.

raises pQ by removing -2

was

10

size are probably

hours in 10

laborapQ

3+

-3.33

oxyhydroxides, stabilities

In Fe ,

or

2+

in

material

the

most

precipitates

solutions, molar

in

but Fe . 2+

T t has g e n e r a l l y b e e n a s s u m e d that the t h e r m o d y n a m i c s t a b i l i t y of p r e c i p i t a t e d f e r r i c o x y h y d r o x i d e s increases c o n t i n u o u s l y w i t h a g i n g i n or out of aqueous s o l u t i o n ( J , 2 ) .

L i t t l e t h o u g h t has b e e n g i v e n to the

role of n a t u r a l w a t e r c h e m i s t r y i n changes i n the s t a b i l i t y of the o x y h y d r o x i d e s w i t h t i m e . T h i s r e p o r t first examines the effects of

changes

i n s o l u t i o n c o m p o s i t i o n o n the s t a b i l i t y of f e r r i c o x y h y d r o x i d e s p r e c i p i tated i n f e r r i c a n d ferrous sulfate solutions i n the l a b o r a t o r y . T h e s e s o l u tions are s i m i l a r i n c o m p o s i t i o n to some i r o n - r i c h a c i d m i n e discharges. Results of the l a b o r a t o r y studies a n d of w o r k b y others are t h e n u s e d to e x p l a i n o b s e r v e d v a r i a t i o n s i n the s t a b i l i t y of s u s p e n d e d f e r r i c o x y h y d r o x ides i n some g r o u n d waters of coastal p l a i n N e w Jersey a n d M a r y l a n d . 209 In Nonequilibrium Systems in Natural Water Chemistry; Hem, J.; Advances in Chemistry; American Chemical Society: Washington, DC, 1971.

210

NONEQUILIBRIUM

Naturally-Occurring

Ferric

SYSTEMS

IN

NATURAL

WATERS

Oxyhydroxides

F e r r i c o x y h y d r o x i d e m i n e r a l s or phases w h i c h o c c u r n a t u r a l l y are listed i n T a b l e I , a n d e x c e p t for akaganéite, t h e i r occurrences h a v e b e e n d e s c r i b e d b y P a l a c h e et al. (3).

T h e o x y h y d r o x i d e s f o u n d as p r e c i p i t a t e s

i n n a t u r a l waters are u s u a l l y goethite A m o r p h o u s m a t e r i a l , w h i c h comprises

and x-ray amorphous material. a relatively large proportion

of


most fresh p r e c i p i t a t e s , is f o r m e d u n d e r c o n d i t i o n s of s u b s t a n t i a l s u p e r s a t u r a t i o n w i t h respect to the c r y s t a l l i n e o x y h y d r o x i d e s .

A n amorphous

phase develops b y r a p i d h y d r o l y s i s of d i s s o l v e d f e r r i c species, p a r t i c u l a r l y at p H ' s b e l o w 4 - 5 w h e r e the t o t a l c o n c e n t r a t i o n of s u c h species c a n e x c e e d 0.01 p p m . A m o r p h o u s m a t e r i a l is also p r o d u c e d d u r i n g the r a p i d o x i d a t i o n a n d h y d r o l y s i s of ferrous i r o n - r i c h solutions. Table I.

N a t u r a l l y O c c u r r i n g Ferric Oxyhydroxides Ferric

Oxyhydroxide

Amorphous Goethite Akaganéite Lepidocrocite Hematite Maghemite

Ideal

Formula

indefinite a-FeOOH $-FeOOH γ-FeOOH a-Fe2C>3 y-Fe 0 2

3

G o e t h i t e p r e c i p i t a t e s b y the r e l a t i v e l y s l o w o x i d a t i o n a n d h y d r o l y s i s of aqueous or s o l i d ferrous i r o n species.

U n d e r sterile l a b o r a t o r y c o n d i

tions, goethite forms most r a p i d l y at p H ' s greater t h a n 3.5 b e c a u s e the o x i d a t i o n rate of d i s s o l v e d ferrous i r o n is e x t r e m e l y s l o w u n d e r m o r e a c i d conditions ( 4 ) . phous

G o e t h i t e is also f o r m e d b y the l o n g - t e r m a g i n g of a m o r

m a t e r i a l p r e c i p i t a t e d d u r i n g h y d r o l y s i s of f e r r i c salt solutions.

U n d e r natural conditions, the oxyhydroxides w h i c h precipitate on a n d coat b o t t o m a n d b a n k sediments i n streams p o l l u t e d b y a c i d m i n e d i s charges are u s u a l l y m i x t u r e s of goethite a n d a m o r p h o u s m a t e r i a l . L e p i d o c r o c i t e has most often b e e n p r e c i p i t a t e d i n the l a b o r a t o r y at p H ' s b e t w e e n 4 a n d 7 b y the o x i d a t i o n of aqueous or s o l i d ferrous i r o n species.

T h e m i n e r a l is u s u a l l y a c c o m p a n i e d b y goethite.

the rarest of the o x y h y d r o x i d e s u n d e r n a t u r a l c o n d i t i o n s .

Akaganéite is Perhaps the

o n l y p u b l i s h e d d e s c r i p t i o n s of n a t u r a l occurrences are b y V a n T a s s e l and Chandy (6).

(5)

T h e m i n e r a l has b e e n p r e c i p i t a t e d i n the l a b o r a t o r y

at r o o m t e m p e r a t u r e s b y the s l o w h y d r o l y s i s of 0 . 0 2 - 0 . 0 8 M F e C l

3

solu-

tions ( 7 ) . M a g h e m i t e is r a r e as a d i r e c t p r e c i p i t a t e , a n d u s u a l l y forms b y t h e o x i d a t i o n of

magnetite

(Fe 0 ) 3

4

or the d e h y d r a t i o n of

lepidocrocite.

H e m a t i t e a n d goethite are p r o b a b l y the most stable f e r r i c o x y h y d r o x i d e s u n d e r earth-surface c o n d i t i o n s .

H e m a t i t e is rare as a d i r e c t p r e c i p i t a t e

In Nonequilibrium Systems in Natural Water Chemistry; Hem, J.; Advances in Chemistry; American Chemical Society: Washington, DC, 1971.

8.

211

Stability of Ferric Oxyhydroxides

LANGMUiR A N D WHITTEMORE

b u t s l o w l y c r y s t a l l i z e s f r o m a m o r p h o u s m a t e r i a l b y d e h y d r a t i o n or l o n g t e r m a g i n g out of s o l u t i o n . I n most cases, the first o x y h y d r o x i d e p r e c i p i t a t e is a m o r p h o u s , h a v i n g b e e n f o r m e d u n d e r c o n d i t i o n s of s u b s t a n t i a l s u p e r s a t u r a t i o n . E a r l y i n p r e c i p i t a t i o n , p o o r l y c r y s t a l l i n e goethite a n d p o o r l y c r y s t a l l i n e l e p i d o crocite m a y also f o r m . O t h e r o x y h y d r o x i d e s t h a n these three are r a r e l y precipitated from natural waters. Downloaded by UNIV OF CALIFORNIA SAN DIEGO on November 17, 2014 | http://pubs.acs.org Publication Date: June 1, 1971 | doi: 10.1021/ba-1971-0106.ch008

O x y h y d r o x i d e p r e c i p i t a t e s are g e n e r a l l y m i x t u r e s of different phases. T h e a p p a r e n t t h e r m o d y n a m i c s t a b i l i t y or s o l u b i l i t y of

such mixtures

d e p e n d s i n l a r g e p a r t o n the s o l u b i l i t y of the least stable p h a s e present. T h i s phase is of course

often

amorphous.

u s u a l l y c o n t a i n s u b s t a n t i a l a m o u n t s of

O x y h y d r o x i d e precipitates

collodial-sized material.

Sizes

f r o m m o l e c u l a r F e ( O H ) ° to m i c r o n - s i z e crystals or c r y s t a l aggregates 8

are p o s s i b l e

(8).

B o t h crystalline and amorphous precipitates remain

c o l l o i d a l i n d e f i n i t e l y i n some solutions. major

P a r t i c l e size is p r o b a b l y

c o n t r o l o n the t h e r m o d y n a m i c s t a b i l i t y of

the

the

oxyhydroxides.

O t h e r i m p o r t a n t controls o n s t a b i l i t y are m i n e r a l o g y , c r y s t a l l i n i t y , the d e g r e e of h y d r a t i o n of the p r e c i p i t a t e , a n d the presence of i m p u r i t i e s . S o l u t i o n reactions for a l l the o x y h y d r o x i d e s c a n b e w r i t t e n i n the same f o r m , as s h o w n b y expressions 1, 2, a n d 3. Fe(OH) = Fe + + 3 0 H ~ amorphous α-FeOOH + 1/2 ( a - F e 0 ) 2

3

(1)

3

3

H 0 = Fe + + 3 0 H "

(2)

3

2

+ 3/2 H 0 = F e 2

3 +

+ 30H"

(3)

H e r e a n d h e n c e f o r t h , the c o m p o s i t i o n of a m o r p h o u s m a t e r i a l is g i v e n for s i m p l i c i t y as F e ( O H ) . T h e n e g a t i v e l o g a r i t h m of the t h e r m o d y n a m i c 3

a c t i v i t y p r o d u c t expressions f o r these reactions m a y b e w r i t t e n i n e a c h case as pK

=

-

log [ F e + ] [ O H - ] 3

w h e n the a c t i v i t y of w a t e r is close to u n i t y .

(4)

3

V a l u e s of p K r a n g e f r o m

a b o u t 37.1 for f r e s h , r a p i d l y p r e c i p i t a t e d a m o r p h o u s

m a t e r i a l (8)

to

p r o b a b l y 44 or m o r e for c r y s t a l l i n e goethite a n d h e m a t i t e , b a s e d o n other w o r k ( 9 ) a n d s o l u b i l i t y measurements d e s c r i b e d i n this p a p e r . Particle

Size

Effect

It m a y b e s h o w n t h a t the decrease i n p K ( — δ ρ Κ ) of a n o x y h y d r o x i d e r e l a t i v e to the p K of its t h e r m o d y n a m i c a l l y most stable f o r m a n d o w i n g to the p a r t i c l e size effect equals


212

NONEQUILIBRIUM

-Sptf =

SYSTEMS

IN

NATURAL

WATERS

(5)

10.5 S(gfw)/çx

I n this expression, S is the surface e n e r g y of the o x y h y d r o x i d e i n ergs p e r s q u a r e centimeter, gfw

is its g r a m f o r m u l a w e i g h t , ρ its d e n s i t y i n grams

p e r s q u a r e c e n t i m e t e r , a n d a s s u m i n g t h a t p a r t i c l e s of the o x y h y d r o x i d e are cubes, χ is the c u b e edge l e n g t h i n A n g s t r o m s . D e n s i t i e s of h e m a t i t e a n d goethite are 5.26 a n d 4.28 g m / c m , r e s p e c t i v e l y ( 3 ) . 3


of s o l u t i o n measurements at 7 0 ° C , F e r r i e r (10) t h a l p i e s of h e m a t i t e ( as 1 / 2 « - F e 0 2

3

B a s e d o n heat

f o u n d the surface e n

) a n d goethite ( as α - F e O O H ) to be

770 a n d 1250 e r g s / c m , r e s p e c t i v e l y . A s s u m i n g as a n a p p r o x i m a t i o n that 2

surface enthalpies a n d G i b b s free energies are e q u a l , a n d constant f r o m 25° to 70 ° C , equations for the v a r i a t i o n i n δρΚ w i t h p a r t i c l e size of h e m a t i t e a n d goethite are, r e s p e c t i v e l y -hpK

=

123/z

(6)

-$pK

= 272/χ

(7)

and

a n d have b e e n p l o t t e d i n F i g u r e 1. H e m a t i t e a n d goethite particles of 100 Â a n d less i n average d i m e n s i o n are c o m m o n i n l a b o r a t o r y solutions a n d n a t u r a l waters. B a s e d o n F i g u r e 1, s u c h p a r t i c l e s are at least 1.2 a n d 2.7 p K units less stable t h a n w e l l - c r y s t a l l i z e d samples of h e m a t i t e a n d goethite, r e s p e c t i v e l y .

F e r r i e r s studies f u r t h e r s h o w t h a t t w o c o e x i s t i n g

o x y h y d r o x i d e s c a n reverse t h e i r r e l a t i v e stabilities because of p a r t i c l e size effects.

T h u s , for

example, if we

consider

equal-sized hematite

and

goethite p a r t i c l e s , at 70 ° C the e n t h a l p y of the r e a c t i o n 1/2 a - F e 0 2

3

+

1/2 H 0 = a - F e O O H 2

(8)

equals ΔΗ°343

(cal/mole) =

(-

1720 ±

250) +

1.90 X 10*/χ

( 1 0 ) , w h e r e χ is the e d g e l e n g t h of cubes of h e m a t i t e or goethite.

(9) This

expression is r o u g h l y e q u i v a l e n t to AG°

2 9

8

(cal/mole) =

for R e a c t i o n 8 ( 9 , 11).

(-

250 ±

300) +

1.90 X 10*/x

(10)

E x p r e s s i o n 10 is p l o t t e d i n F i g u r e 2 a n d shows

that for e q u a l - s i z e d h e m a t i t e a n d goethite crystals, goethite is m o r e stable t h a n h e m a t i t e w h e n χ exceeds 760 Â, b u t less stable t h a n h e m a t i t e at s m a l l e r p a r t i c l e sizes. V a r i a t i o n s i n A G °

2 9 8

c a n b e c o n s i d e r a b l y greater

t h a n this w h e n u n e q u a l p a r t i c l e sizes of h e m a t i t e a n d goethite are i n volved

(11).


8.

L A N G M U i R AND W H I T T E M O R E

Eh-pH

213


Relations

T h e behavior of precipitated ferric oxyhydroxides i n solution c a n best b e d e s c r i b e d i n terms o f E h a n d p H . F i g u r e 3 is a n E h - p H d i a g r a m showing

fields

Fe-H 0-0 . 2

2

o f solids a n d p r e d o m i n a n t i o n i c species

i n t h e system

D e t a i l e d procedures for the construction of E h - p H

dia-

grams are g i v e n b y H e m a n d C r o p p e r ( 13) a n d G a r r e l s a n d C h r i s t (14). I n F i g u r e 3, i o n - i o n b o u n d a r i e s are d r a w n a t e q u a l i o n a c t i v i t i e s . I o n s o l i d b o u n d a r i e s a r e d r a w n f o r 1 0 " M d i s s o l v e d i r o n species

(5.6 p p m


4

i r o n ) , a t y p i c a l concentration i n the iron-rich ground waters of N e w Jersey a n d M a r y l a n d d e s c r i b e d b e l o w .

T h e f e r r i c a n d ferrous

species-

f e r r i c o x y h y d r o x i d e b o u n d a r i e s h a v e b e e n d r a w n f o r pfC values o f 37.1 a n d 44. T h i s différence i n p K means that for a g i v e n p H a n d E h , a w a t e r can

c o n t a i n r o u g h l y seven orders o f m a g n i t u d e m o r e d i s s o l v e d i r o n i n

e q u i l i b r i u m w i t h fresh, a m o r p h o u s

material than i n equilibrium w i t h

w e l l - c r y s t a l l i z e d goethite or h e m a t i t e .

10

100 1,000 10,000 Particle Size (Angstroms)

Figure 1. Decrease in pK (—SpK) with decreasing particle size of goethite (a-FeOOH) and hematite (l/2a-Fe O ) 2

s

T h e E h a n d p H o f i r o n - r i c h n a t u r a l w a t e r s , most o f w h i c h c o n t a i n s u s p e n d e d f e r r i c o x y h y d r o x i d e s , places these w a t e r s o n t h e f e r r i c o r ferrous

species-ferric

oxyhydroxide

boundary.

Acid-mine

discharges

u s u a l l y h a v e p H ' s b e t w e e n 2 a n d 5 a n d E h s f r o m w i t h i n the F e d o w n t o zero volts.

3 +

field

I r o n - r i c h g r o u n d waters a r e u s u a l l y o n t h e same

b o u n d a r y , w i t h p H ' s b e t w e e n 5 a n d 8 a n d E h s f r o m + 0 . 3 to —0.1 v o l t .


214

NONEQUILIBRIUM SYSTEMS IN N A T U R A L WATERS

Calculation

of Apparent

Stabilities

It is u s e f u l to c o m p u t e w h a t m a y b e c a l l e d t h e a p p a r e n t s t a b i l i t y of a m i x t u r e o f o x y h y d r o x i d e s . T h i s a p p a r e n t s t a b i l i t y m a y b e d e f i n e d i n terms o f pQ w h e r e i n the s o l u t i o n pQ = -

log [ F e ] [ O H - ] 3 +

(11)

3


F o r solutions h a v i n g a constant aqueous c h e m i c a l c o m p o s i t i o n a n d c o n stant c o m p o s i t i o n o f associated f e r r i c o x y h y d r o x i d e s , t h e m e a s u r e d pÇ> w i l l e q u a l the s t a b i l i t y o f a l l the f e r r i c o x y h y d r o x i d e s present. F o r s o l u tions i n w h i c h o n e f e r r i c o x y h y d r o x i d e is a l t e r i n g o r d i s s o l v i n g t o f o r m another, the m e a s u r e d pQ w i l l represent a s t a b i l i t y greater t h a n t h a t of the d i s a p p e a r i n g phase a n d less t h a n t h a t o f the phase w h i c h is f o r m i n g . I n f e r r i c s u l f a t e - r i c h solutions, pQ m a y b e c a l c u l a t e d f r o m a k n o w l e d g e o f p H a n d [ F e ] , the a c t i v i t y o f u n c o m p l e x e d f e r r i c i o n . T h e c o m 3 +

plexes w h i c h must b e c o n s i d e r e d HS0 ", FeS0 4

4

+

, and F e O H . 2 +

t o evaluate i o n i c strength (I) a r e

D i s s o c i a t i o n constants f o r these

complexes

h a v e b e e n m e a s u r e d at 2 5 ° C b y N a i r a n d N a n c o l l a s ( 1 5 ) , W i l l i x ( 1 6 ) , a n d M i l b u r n (12), r e s p e c t i v e l y , a n d are -500

Porticle Size of Cubes (Angstroms)

Figure 2. Gibhs free energy of formation of goethite (a-FeOOH) from hematite (1 /2a-Fe O ) and water as a function of particle size, assuming equal particle sizes of goethite and hematite 2

s


8.

Stability

L A N G M U i R AND WHITTEMORE

of Ferric Oxyhydroxides


τ

I

1

1

1

1

Ο

2

4

6

8

215

Γ

1

1

1

10

12

14

PH

Figure 3. Eh-pH relations between solids and predominant ionic species in the system Fe-H 0-0 . The posi tion of the Fe -ferric oxyhydroxide boundary is shown for pK's of 37.1 and 44. The position of the Fe *FeOH boundary is based on Milburn (12). Other thermochemical data used to construct the figure are from Langmuir (8). 2

2

2+

3

2+

ΚΗΒΟΓ =

[H+][S0

4

2

-]/[HS0 -] =

#Feso + = [ F e + ] [ S 0 - ] / [ F e S 0 + ] 3

4

4

10" · 1

4

2

4

= 10"

4 1 5

#Feo 2+ = [Fe +][OH~]/[FeOH +] = 1 0 ~ 3

H

2

(12)

9 6

U 8 3

(13) (14)

W h e n significant ferrous i r o n is present, this species m u s t also b e c o n sidered.

T r a n s p o s i n g a n d t a k i n g t h e a n t i l o g o f E q u a t i o n 2 5 gives t h e

relationship between

[Fe

2 +

],

[Fe

3 +

] , a n d the measured E h at 2 5 ° C ,

w h i c h is [Fe

2 +

]/[Fe

3 +

] =

10< °E

E H ) / 0

-

0 5 9 6 1

(15)

w h e r e E° is t h e s t a n d a r d e l e c t r o d e p o t e n t i a l o f t h e f e r r i c - f e r r o u s i r o n c o u p l e ( see b e l o w ). K n o w n c o n c e n t r a t i o n s o f t o t a l d i s s o l v e d i r o n , f e r r o us i r o n , a n d s u l fate species y i e l d t h e f o l l o w i n g mass b a l a n c e e q u a t i o n s .


216


raFe(total)

=

raFe + 3

+ mFeOH + + mFeS0 + + wFe + 2

mFe(II) m80

4

2

(16)

2

4

= mFe +

(17)

2

- (total) = m F e S 0 + +

roHSOr

4

+ mS0

4

2

"

(18)

I f N a O H is the base u s e d to p r e c i p i t a t e the o x y h y d r o x i d e s , t h e n


mNa

+

(total) =

raNa+

(19)

T h e o v e r - a l l charge b a l a n c e e q u a t i o n for a c i d solutions is m N a + + raH+ + 3 m F e + +

2mFeOH + +

3

raFeS0 +

2

+

4

2wFe +

=

2

2mS0 " + m H S 0 " 2

4

(20)

4

T h e d e f i n i t i o n of i o n i c s t r e n g t h is I = where

1/2 Στη&*

(21)

a n d Zt are t h e m o l a l i t y a n d valence of i o n i c species i, respec

t i v e l y . T h e f u l l expression for I is t h e n / =

1/2 [mNa+ +

mH+ + 9mFe + + 4 m F e O H + + 3

raFeS0 +

2

4

+

4mFe + + 4 m S 0 " + w H S O r ] 2

4

(22)

2

B e c a u s e significant concentrations of c o m p l e x ions are present, J a n d [ F e ] m u s t b e c a l c u l a t e d b y a m e t h o d of successive 3 +

approximations.

T h e first step is to e s t a b l i s h w h i c h d i s s o l v e d species are the m a j o r ones. A t this p o i n t , differences ignored.

b e t w e e n i o n activities a n d m o l a l i t i e s m a y

T h e p H is u s e d to estimate the r e l a t i v e i m p o r t a n c e of

and m H S C V through K s o - , and H

raFe

3+

4

vs.

raFeOH

2

4

t h r o u g h ΚρβοΗ**·

2+

A s a first a p p r o x i m a t i o n i n most waters w i t h p H > 2.5, m S 0 " — m S 0 4

( t o t a l ) , w h i c h p e r m i t s a n estimate of

raFeSCV

vs. m F e

be

raS0 ~

3 +

2

2

4

"

through K s o . F e

4

+

W i t h a r o u g h estimate of I at values b e l o w 0.1, a p p r o x i m a t e i n d i v i d u a l i o n a c t i v i t y coefficients H u c k e l equation (17).

m a y be c a l c u l a t e d u s i n g the e x t e n d e d

Debye-

T o a g o o d a p p r o x i m a t i o n , the f o l l o w i n g equalities

m a y be a s s u m e d : y F e O H

2 +

— y F e , and y F e S 0 2 +

4

+

— yHS0 " = 4

yHC0 ". 3

S u b s t i t u t i n g values for these coefficients i n t o E x p r e s s i o n s 12 t h r o u g h 15, w e m a y refine the m o l a l r e l a t i o n s h i p s of free a n d c o m p l e x i o n

species.

C o m b i n i n g these r e l a t i o n s h i p s w i t h mass b a l a n c e E q u a t i o n s 16 a n d 18 p e r m i t s c a l c u l a t i o n of a p p r o x i m a t e

m o l a l i t i e s of

a l l the i o n i c

species

present a n d of a m o r e a c c u r a t e i o n i c strength t h r o u g h E x p r e s s i o n 22. T h e c y c l e is r e p e a t e d a b o u t t w o or three times u n t i l n o f u r t h e r c h a n g e i n i o n i c strength c a n be detected.

T h e final v a l u e of [ F e ] is t h e n u s e d 3 +

to c o m p u t e pÇ>.


8.


L A N G M u i R AND WHITTEMORE

217

T h e p r e c e d i n g c a l c u l a t i o n s are possible u s i n g a t o t a l d i s s o l v e d i r o n a n d E h analysis w i t h or w i t h o u t a n analysis for raFe ( I I ) , a l t h o u g h the most accurate results are o b t a i n e d w i t h b o t h t o t a l a n d ferrous i r o n values. Because m F e K

F e

on

2 +

in y F e

3 +

is a s m a l l p e r c e n t a g e of raFe ( I I I ) , errors i n K

are m a g n i f i e d i n the final v a l u e of [ F e ] .

F e

so

4

+

and

A l s o , the u n c e r t a i n t y

3 +

at m o d e r a t e i o n i c strengths is l i k e l y to b e significant. I n v i e w of

3 +

s u c h u n c e r t a i n t i e s , c a l c u l a t e d p Q values b a s e d o n the a b o v e a p p r o a c h Downloaded by UNIV OF CALIFORNIA SAN DIEGO on November 17, 2014 | http://pubs.acs.org Publication Date: June 1, 1971 | doi: 10.1021/ba-1971-0106.ch008

are c o n s i d e r e d a c c u r a t e to ± 0 . 4

u n i t w h e n o n l y a t o t a l i r o n analysis

is a v a i l a b l e , a n d to ± 0 . 2 u n i t w h e n m F e ( I I )

has also b e e n m e a s u r e d .

T h e most r e l i a b l e values of pQ are b a s e d o n the raFe ( I I ) analysis i n solutions of k n o w n i o n i c strength. T h i s a p p r o a c h is a p p l i c a b l e i n m i x e d f e r r i c - f e r r o u s salt solutions as a b o v e , or i n ferrous salt solutions.

An

e q u a t i o n r e l a t i n g p Q to s o l u t i o n c o m p o s i t i o n m a y b e d e r i v e d as f o l l o w s . F o r the r e d u c t i o n r e a c t i o n Fe E h = E° + w h e r e E°

3 +

+ e - = Fe +

(23)

2

1.9842 Χ Ι Ο " Τ X log ( [ F e ] / [ F e ] ) 4

3+

(24)

2+

is the s t a n d a r d e l e c t r o d e p o t e n t i a l of the r e a c t i o n a n d Τ the

t e m p e r a t u r e i n degrees K e l v i n . E x p a n d i n g the l o g t e r m a n d t r a n s p o s i n g gives - l o g [Fe +] = 3

A d d i n g —3 l o g [ O H ]

(E°

-

E h ) / 1 . 9 8 4 2 X 10~*T -

log [ F e ] 2+

(25)

to b o t h sides, the left h a n d side t h e n equals p Q

a n d the final expression m a y b e w r i t t e n E h ) / 1 . 9 8 4 2 X 10r*T -

log [Fe +] -

3(log K

+ pH)

(26)

pQ = (E°

-

where K

is the a c t i v i t y p r o d u c t of w a t e r . T h i s expression w a s u s e d t o

w

2

w

c a l c u l a t e pÇ) i n b o t h f e r r i c - f e r r o u s sulfate a n d ferrous sulfate l a b o r a t o r y solutions a n d i n c o a s t a l - p l a i n g r o u n d w a t e r s of N e w Jersey a n d M a r y l a n d . I o n i c strength was c o m p u t e d for the l a b o r a t o r y solutions f r o m the d e t a i l e d i o n content of the w a t e r t h r o u g h E q u a t i o n 21. T h e N e w Jersey g r o u n d waters s t u d i e d w e r e chiefly of the c a l c i u m b i c a r b o n a t e t y p e w i t h i o n i c strengths less t h a n 4 Χ 10" so that I c o u l d b e c o m p u t e d a c c u r a t e l y w i t h 3

the e m p i r i c a l e q u a t i o n I =

1.5 Χ 1 0 " μ

(27)

5

w h e r e μ is the specific c o n d u c t a n c e i n m i c r o m h o s at 25 ° C

(18).

Ionic

strengths of the M a r y l a n d g r o u n d waters s t u d i e d , w h i c h are a l l less t h a n 5 Χ 10" , w e r e p r o v i d e d b y W . B a c k 3

(19).

B e c a u s e p Q is q u i t e sensitive to £ ° a n d values of £ ° at other t e m peratures t h a n 25 ° C h a v e not b e e n p u b l i s h e d , c a r e f u l l a b o r a t o r y m e a s u r e -


218


ments of this q u a n t i t y w e r e m a d e f r o m 5 ° to 35° C at a p H of 1.5 i n m i x e d f e r r o u s - f e r r i c p e r c h l o r a t e solutions. A d e t a i l e d d e s c r i p t i o n of these m e a surements w i l l be g i v e n elsewhere (20). values of E°

Resultant smoothed and rounded

at 5 ° , 1 0 ° , 1 5 ° , 2 0 ° , 2 5 ° , 3 0 ° , a n d 3 5 ° C are 0.746, 0.752,

0.758, 0.764, 0.770, 0.775, a n d 0.780 volts, r e s p e c t i v e l y , a n d are p r o b a b l y accurate to ± 0 . 0 0 1 volt. T h e e q u a t i o n


E° =

-

1.226 Χ Ι Ο " + 4.147 Χ 10-*!Γ 2

δ.111 X 1 0 - T 6

(28)

2

fits b o t h m e a s u r e d a n d s m o o t h e d values of E° w e l l w i t h i n the u n c e r t a i n t y of the measurements. FeOH

+

is the o n l y c o m p l e x i o n i n the f e r r o u s - r i c h l a b o r a t o r y s o l u

tions or the g r o u n d waters e x a m i n e d . T h e e q u i l i b r i u m constant, K ,

for

eq

the r e a c t i o n Fe + +

H 0 = FeOH+ +

2

2

H+

(29)

equals K

eg

= [FeOH+][H+]/[Fe +] = 2

10" · 8

(30)

30

at 25°C. A t t e m p e r a t u r e s near 25°C K

eq

= K

w

· 10*

(31)

7 0

T h i s expression is p r o b a b l y a c c u r a t e to a b o u t ± 0 . 0 5 l o g u n i t at The F e O H

(18).

+

c o m p l e x is c l e a r l y n e g l i g i b l e r e l a t i v e to F e

2 +

15°C

at p H ' s

b e l o w a b o u t 6.3, b u t a m o u n t s to a m a x i m u m of a b o u t 1 0 % of the t o t a l dissolved F e

2 +

content i n g r o u n d w a t e r s a m p l e 172 at a p H of 7.75

I I I ). V a l u e s of K

w

(Table

as a f u n c t i o n of t e m p e r a t u r e u s e d i n E x p r e s s i o n s 26

a n d 31 are f r o m A c k e r m a n

(21).

I n the l a b o r a t o r y studies, the largest uncertainties i n terms of E q u a t i o n 26 are i n the m e a s u r e d values of E h a n d p H , w h i c h l e a d to a n u n c e r t a i n t y of a b o u t ± 0 . 2 u n i t i n pQ.

Measurement uncertainties i n E h ,

p H , a n d m F e ( I I ) are significant f o r the g r o u n d w a t e r s e x a m i n e d . resultant u n c e r t a i n t y i n pQ about

±0.3

The

for g r o u n d waters h i g h e s t i n m F e ( I I )

u n i t ; for g r o u n d waters lowest i n m F e ( I I ) , a b o u t

is

±0.4

unit. Laboratory

Studies

Methods of Chemical Analysis. D u r i n g o x i d a t i o n a n d h y d r o l y s i s studies of ferrous sulfate solutions, ferrous i r o n concentrations w h i c h r a n g e d f r o m 570 to 157 p p m w e r e a n a l y z e d b y t i t r a t i o n w i t h p o t a s s i u m d i c h r o m a t e s o l u t i o n u s i n g s o d i u m d i p h e n y l a m i n e sulfonate as a n i n d i c a t o r (22). T h e same m e t h o d w a s u s e d to a n a l y z e for ferrous i r o n i n m i x e d f e r r i c - f e r r o u s sulfate solutions. T o t a l d i s s o l v e d i r o n concentrations, w h i c h r a n g e d f r o m 560 to 100 p p m d u r i n g h y d r o l y s i s of m i x e d f e r r i c - f e r r o u s


8.

219


L A N G M u i R AND WHITTEMORE

sulfate solutions, w e r e s i m i l i a r l y t i t r a t e d after a c i d i f i c a t i o n w i t h H C 1 a n d b o i l i n g w i t h r e d u c t i o n b y S n C I (22). Samples for dissolved total iron analysis w e r e w i t h d r a w n f r o m the c l e a r e d s u p e r n a t a n t s o l u t i o n i n the r e a c t i o n vessels after the s o l u t i o n h a d stood u n d i s t u r b e d f o r 48 h o u r s . U p o n w i t h d r a w a l , samples w e r e passed t h r o u g h 0 . 4 5 - m i c r o n filter p a p e r p r i o r to analysis. A l t h o u g h some s u s p e n d e d f e r r i c o x y h y d r o x i d e s c o u l d h a v e b e e n present a n d h a v e p a s s e d t h r o u g h t h e filter p a p e r , the a m o u n t of s u c h m a t e r i a l m u s t h a v e b e e n n e g l i g i b l e for purposes of this s t u d y i n that pQ v a l u e s c a l c u l a t e d u s i n g the m F e ( t o t a l ) analyses w e r e i d e n t i c a l to s u c h values c a l c u l a t e d f r o m m F e ( I I ) concentrations m e a s u r e d i n the same samples. A n a l y z e d ferrous i r o n values are p r o b a b l y a c c u r a t e to ± 0 . 5 % ; t o t a l d i s s o l v e d i r o n values to ± 1 % . A c o m b i n a t i o n glass, s i l v e r - s i l v e r c h l o r i d e reference electrode w a s used for p H measurement, a p l a t i n u m t h i m b l e electrode a n d s i l v e r - s i l v e r c h l o r i d e reference electrode for E h m e a s u r e m e n t . E h a n d p H m e a s u r e ments w e r e m a d e w i t h a C o l e m a n M e d a l l i o n M o d e l 37 b a t t e r y - or l i n e o p e r a t e d p H - m i l l i v o l t m e t e r a n d / o r a C o r n i n g 12 R e s e a r c h p H - m i l l i v o l t meter. T h e p H measurements w e r e c a l i b r a t e d w i t h n o m i n a l p H 4 a n d 7 buffers. D o u b l e buffer checks w e r e w i t h i n ± 0 . 0 2 p H u n i t . E h m e a s u r e ments w e r e c a l i b r a t e d w i t h Z o b e l l s o l u t i o n [ 0 . 0 0 0 3 M K F e ( C N ) , 0 . 0 0 0 3 M K F e ( C N ) · 3 H 0 , a n d 0 . 1 M K C 1 ] . F r o m 0 ° to 2 5 ° C , the E h of Z o b e l l s o l u t i o n obeys the e q u a t i o n


2

3

4

c

6

2

E h ( v o l t s ) = 0.429 + 0.0024(25 -

t)

(32)

w h e r e t is i n degrees C e l s i u s (23). L a b o r a t o r y E h a n d p H measurements are p r o b a b l y a c c u r a t e to ± 0 . 0 0 2 v o l t a n d 0.02 p H u n i t , r e s p e c t i v e l y . P r e c i p i t a t e s w e r e s m e a r - m o u n t e d a n d d r i e d o n a glass s l i d e for x - r a y diffraction analysis u s i n g a G e n e r a l E l e c t r i c x - r a y diffractometer a n d F e K a r a d i a t i o n . A n estimate of the size of c r y s t a l l i n e p a r t i c l e s i n the precipitates w a s m a d e b y m e a s u r e m e n t of the w i d t h of the m a j o r diffract i o n p e a k for the m i n e r a l of interest at h a l f - m a x i m u m i n t e n s i t y (24). P r e c i p i t a t e s f o r e x a m i n a t i o n b y e l e c t r o n m i c r o s c o p y w e r e first d i s a g g r e g a t e d w i t h a n u l t r a s o n i c v i b r a t o r so that single crystals c o u l d be s t u d i e d . A d r o p of the d i s p e r s i o n w a s t h e n p l a c e d o n a c o l l o d i o n - c o v e r e d g r i d i n a Zeiss e l e c t r o n m i c r o s c o p e , M o d e l 9S. E l e c t r o n m i c r o g r a p h s of the p r e c i p i t a t e s s h o w p r e f e r r e d o r i e n t a t i o n of goethite a n d l e p i d o c r o c i t e crystals ( F i g u r e s 5 a n d 7 ) . G e o t h i t e c o m m o n l y occurs as a c c i c u l a r a n d n a r r o w p r i s m a t i c crystals e l o n g a t e d i n the [001] or c-axis d i r e c t i o n . L e p i d o c r o c i t e is t y p i c a l l y present i n b l a d e d p r i s m a t i c o r m i c a c e o u s forms flattened o n {010} ( 3 ) . T h u s , sizes b a s e d o n the x - r a y d i f f r a c t i o n m e t h o d represent the m e a n thickness of goethite crystals i n the [110] d i r e c t i o n , a n d of l e p i d o c r o c i t e crystals i n the &-axis or [020] d i r e c t i o n . T h e a c c u r a c y of x-ray d e t e r m i n e d values for these m e a n d i m e n s i o n s is p r o b a b l y ± 2 0 - 4 0 % f o r the sizes e n c o u n t e r e d i n this s t u d y . Experimental Methods. Studies w e r e m a d e of the p r e c i p i t a t i o n a n d a g i n g of f e r r i c o x y h y d r o x i d e s f o r m e d i n sulfate solutions i n i t i a l l y a b o u t 1 0 " M i n ferrous i r o n . T h r e e representative r u n s are d e s c r i b e d b e l o w . O x i d a t i o n of ferrous i r o n was a c c o m p l i s h e d b y b u b b l i n g the solutions w i t h air. H y d r o l y s i s was w i t h 0 . 1 M N a O H or 0 . 5 M N a H C O a . Runs were m a d e i n a c o n s t a n t - t e m p e r a t u r e b a t h at 25° ± 0.2 ° C w i t h a b o u t 3.5 2



220


Figure 4. Observed changes in Eh, pH, and calculated pQ values during Runs 5 and 8. Bracketed letters A, G, and L indicate that the precipitate is mostly amorphous, goethite, or lepidocrocite, respectively. Bracketed letters g and I indicate the presence of minor amounts of goethite or lepidocrocite, respectively. Runs 5 and 8 were begun at points labeled "acidify" and "start base," respectively. liters of s o l u t i o n i n 4 - l i t e r b o r o s i l i c a t e glass beakers. T h e beakers w e r e fitted w i t h a i r t i g h t L u c i t e covers w h i c h h a d holes to a c c o m m o d a t e stopper-fitted E h a n d p H electrodes, a specific c o n d u c t a n c e p r o b e , gas i n t a k e a n d d i s c h a r g e tubes, a t h e r m o m e t e r , a n d a b u r e t t i p for base a d d i t i o n . Sol u t i o n s w e r e s t i r r e d w i t h a teflon-coated s t i r r i n g b a r p o w e r e d f r o m b e n e a t h the b e a k e r b y a w a t e r - d r i v e n m a g n e t i c rotor. P e r i o d i c a l l y d u r i n g t h e r u n s , w h i c h l a s t e d f r o m 4 to 10 h o u r s , a n d later d u r i n g a g i n g , m e a s u r e ments w e r e m a d e of E h , p H , a n d specific c o n d u c t a n c e , samples of s o l u t i o n w e r e w i t h d r a w n for t o t a l d i s s o l v e d i r o n or ferrous i r o n analysis, a n d p r e c i p i t a t e w a s c o l l e c t e d for x-ray a n d e l e c t r o n m i c r o s c o p e analysis. Hydrolysis of F e ^ S O ^ a Solutions. R u n 8 i n v o l v e d the h y d r o l y s i s w i t h 0 . 1 M N a O H of a s o l u t i o n i n i t i a l l y 1 0 " M i n t o t a l i r o n . T h e s o l u t i o n w a s p r e p a r e d w i t h r e a g e n t - g r a d e f e r r i c sulfate w h i c h c o n t a i n e d a b o u t 1 % of ferrous i r o n . T h e base w a s a d d e d p e r i o d i c a l l y d u r i n g f o u r h o u r s of the r u n so t h a t the final s o l u t i o n c o n t a i n e d about 100 p p m of d i s s o l v e d t o t a l i r o n species. D u r i n g the r u n , o v e r 9 0 % of m F e ( I I I ) w a s present as t h e F e S C V c o m p l e x . E h a n d p H changes d u r i n g the r u n a n d subse2


8.

LANGMuiR

221


AND W H i T T E M O R E

q u e n t a g i n g are s h o w n i n F i g u r e 4. O t h e r results e v a l u a t e d d u r i n g a g i n g are l i s t e d i n T a b l e I I . T h e s o l u t i o n w a s a l l o w e d to evaporate to a b o u t one t h i r d its o r i g i n a l v o l u m e d u r i n g 109 days f o l l o w i n g t h e r u n . T h e goethite w h i c h h a d a p p e a r e d as of 112 d a y s ' a g i n g e v i d e n t l y f o r m e d b y c r y s t a l l i z a t i o n of the a m o r p h o u s m a t e r i a l . E l e c t r o n m i c r o s c o p i c e x a m i n a t i o n of the p r e c i p i t a t e after 198 days s h o w e d r o d - l i k e crystals of goethite s u r r o u n d e d b y a m o r p h o u s - a p p e a r i n g m a t e r i a l . T h e goethite crystals w e r e a b o u t 50 to 100 Â t h i c k a n d 300 to 600 Â l o n g . Oxidation and Hydrolysis of F e S 0 Solutions. I n i t i a l s o l u t i o n of reagent grade F e S 0 i n R u n 5 ( F i g u r e 4 ) g a v e a h y d r o l y s i s p H of 4.12. T h e s m a l l amounts of s u s p e n d e d f e r r i c o x y h y d r o x i d e s present h a d a p Q v a l u e of 38.8. A d d i t i o n of a f e w d r o p s of c o n c e n t r a t e d H S 0 b r o u g h t the p H to 3.08 a n d l e a c h e d a w a y the most s o l u b l e o x y h y d r o x i d e m a t e r i a l present, l e a v i n g b e h i n d r e l a t i v e l y m o r e c r y s t a l l i n e p a r t i c l e s w i t h a pQ of 40.2. L e a c h i n g of the f e r r i c o x y h y d r o x i d e s i n c r e a s e d t h e [ F e ] / [ F e ] r a t i o a n d so r a i s e d the E h . W i t h the b e g i n n i n g of a e r a t i o n a n d base a d d i t i o n , E h d r o p p e d because of p r e f e r e n t i a l h y d r o l y s i s a n d p r e c i p i t a t i o n of F e , whereas the rate of F e o x i d a t i o n at these p H ' s is e x t r e m e l y s l o w ( 4 ) . S o l u t i o n c o m p o s i t i o n thus m o v e d a l o n g t h e ferrous i o n - f e r r i c o x y h y d r o x i d e b o u n d a r y . S u b s t a n t i a l p r e c i p i t a t i o n b e g a n at a p H of 4.34, w h e n pQ = 38.4, i n d i c a t i n g that the first p r e c i p i t a t e w a s chiefly a m o r p h o u s . T h e v a l u e of pQ s u b s e q u e n t l y i n c r e a s e d to 39.1 a n d p r o b a b l y h i g h e r for t w o reasons. F i r s t , i n i t i a l p r e c i p i t a t i o n occurs i n the v i c i n i t y of drops of a d d e d base a n d thus d e v e l o p s u n d e r h i g h l y s u p e r s a t u r a t e d c o n d i t i o n s . W i t h m i x i n g , m u c h of this r e l a t i v e l y u n s t a b l e p r e c i p i t a t e redissolves i n the b u l k of s o l u t i o n , l e a v i n g b e h i n d a m o r e stable (less s o l u b l e ) f r a c t i o n . A l s o as the degree of s u p e r s a t u r a t i o n decreases, p r e c i p i t a t i o n occurs m o r e s l o w l y w i t h the c o n s e q u e n t d e v e l o p m e n t of better c r y s t a l l i z e d m a t e r i a l . A t pQ = 39.1, the s o l u t i o n c o n t a i n e d 335 p p m of d i s s o l v e d ferrous i r o n .


4

4

2

4

3 +

2 +

3 +

2 +

W i t h the e n d of base a d d i t i o n , o x i d a t i o n of F e to f e r r i c o x y h y d r o x ides b y the a i r b e c a m e the i m p o r t a n t r e a c t i o n , c a u s i n g a n increase i n E h a n d decrease i n p H . T w o days after cessation of b u b b l i n g w i t h a i r , pQ = 41.0, a n d x-rays s h o w e d the c r y s t a l l i n e f r a c t i o n of the p r e c i p i t a t e to b e p r e d o m i n a n t l y l e p i d o c r o c i t e ( c r y s t a l thickness 64 ± 15 Â ) w i t h a f e w percent goethite ( c r y s t a l thickness 54 db 1 5 Â ) . A f t e r 93 days (last p o i n t s h o w n i n F i g u r e 4 ) , pQ = 43.3, a n d the goethite to l e p i d o c r o c i t e r a t i o h a d d o u b l e d . T h e thickness of goethite crystals r e m a i n e d u n c h a n g e d , w h i l e that of l e p i d o c r o c i t e h a d i n c r e a s e d to 75 d= 15 Â. X - r a y d i f f r a c t i o n 2 +

Table II. Days After Run 3 21 112 147 192

pH 2.65

-

2.15 2.14 2.11

Some Results D u r i n g A g i n g of R u n 8

Fe(II), ppm

— -

16.1 16.1

Fe(III), ppm 100

-

309

-

X-ray Analysis and Crystal Thickness, A 0

vQ 38.5

-

40.0 40.0

Amorphous Amorphous G o e t h i t e , 67 ± 20 G o e t h i t e , 58 =fc 20 G o e t h i t e , 61 ± 20



222


Figure 5. Electron micrograph taken after 298 days' aging of Run 5, showing lath-like lepidocrocite crystak and smaller needle-like goethite crystals after 293 days s h o w e d the c r y s t a l thickness of goethite f a i r l y constant at 63 ± . 15 Â, w h i l e that of l e p i d o c r o c i t e h a d i n c r e a s e d to 93 ± 20 Â . A t this t i m e , the goethite to l e p i d o c r o c i t e r a t i o h a d a g a i n d o u b l e d . A f t e r 258 a n d 298 days, the e l e c t r o n m i c r o s c o p e s h o w e d l e p i d o c r o c i t e aggregates a n d l a t h - l i k e single crystals, a n d r e l a t i v e l y s m a l l e r goethite needles or rods ( F i g u r e 5 ) . T h e goethite crystals w e r e e l o n g a t e d i n the c-axis d i r e c t i o n , a n d the l e p i d o c r o c i t e laths l a y w i t h their fo-axis v e r t i c a l . T h i s o r i e n tation was c o n f i r m e d b y the r e l a t i v e intensities of reflections i n the x - r a y d i f f r a c t i o n patterns. A s s h o w n i n F i g u r e 5, the w i d t h of the l e p i d o c r o c i t e laths a v e r a g e d about 400 À, their l e n g t h a b o u t 2300 Â . T h e goethite rods w e r e 40 to 60 Â i n w i d t h , a n d a v e r a g e d a b o u t 500 Â i n l e n g t h . R u n 4 ( F i g u r e 6 ) was also b e g u n w i t h a 1 0 " M F e S 0 s o l u t i o n . A c i d l e a c h i n g of s m a l l amounts of s u s p e n d e d o x y h y d r o x i d e s i n i t i a l l y present r a i s e d pQ f r o m 40.1 to 41.8. I n c r e m e n t s of 0 . 5 M N a H C O * w e r e t h e n i n t r o d u c e d w i t h o u t a e r a t i o n . Base w a s a d d e d m o r e r a p i d l y t h a n i n R u n 5 so that the s o l u t i o n b e c a m e m o r e h i g h l y s u p e r s a t u r a t e d w i t h respect to c r y s t a l l i n e o x y h y d r o x i d e s , a n d p r e c i p i t a t i o n of a m o r p h o u s m a t e r i a l b e g a n w i t h pQ = 37.3. A s i n R u n 5, pQ i n c r e a s e d because of r e s o l u t i o n of the most s o l u b l e m a t e r i a l a n d a d e c r e a s i n g rate of p r e c i p i t a t i o n . X - r a y a n a l y sis at pQ = 38.4 s h o w e d the c r y s t a l l i n e f r a c t i o n of the p r e c i p i t a t e to be chiefly l e p i d o c r o c i t e ( c i y s t a l thickness 115 =t 35 Â ) w i t h m i n o r goethite ( c r y s t a l thickness 100 ± 30 Â ) . S u b s e q u e n t x - r a y analyses d u r i n g the r u n at pQ = 39.1 a n d 39.2 s h o w e d a r a p i d l y i n c r e a s i n g p r o p o r t i o n of goethite r e l a t i v e to l e p i d o c r o c i t e w i t h r o u g h l y e q u a l amounts of the t w o phases present at pÇ> = 39.2. T h e m e a n c r y s t a l thickness of goethite r e m a i n e d r o u g h l y constant d u r i n g this t i m e ; h o w e v e r , t h a t of l e p i d o c r o c i t e 2

4


8.

LANGMuiR AND WHITTEMORE


223

i n c r e a s e d to a b o u t 200 db 60 Â, p r o b a b l y b e c a u s e s m a l l e r p a r t i c l e sizes of this m i n e r a l w e r e r e d i s s o l v e d o r c o n v e r t e d to goethite. A e r a t i o n w a s t h e n b e g u n , w i t h the r e s u l t t h a t F e concentrations d r o p p e d to n e g l i g i b l e values ( < 0.005 p p m ) . A f t e r one d a y a n d also after 216 d a y s ' a g i n g ( l a s t p o i n t i n F i g u r e 6 ) , the c r y s t a l l i n e f r a c t i o n of the p r e c i p i t a t e c o n t a i n e d r o u g h l y t w i c e as m u c h goethite as l e p i d o c r o c i t e , w h i l e c r y s t a l thicknesses r e m a i n e d a b o u t 100 a n d 200 Â, r e s p e c t i v e l y . I n 296 d a y s , the g o e t h i t e - t o - l e p i d o c r o c i t e r a t i o h a d i n c r e a s e d to greater t h a n 3, a l t h o u g h c r y s t a l thickness of the t w o m i n e r a l s r e m a i n e d u n c h a n g e d . E l e c t r o n m i c r o g r a p h s t a k e n at 262 a n d 302 d a y s ( F i g u r e 7 ) s h o w c o m p a c t aggregates w i t h single a n d i n t e r g r o w n crystals of goethite a n d l e p i d o c r o c i t e p r o j e c t i n g r a d i a l l y f r o m t h e i r surfaces. I s o l a t e d n e e d l e s h a p e d goethite crystals h a d a n average w i d t h a n d l e n g t h of r o u g h l y 100 a n d 2000 Â, r e s p e c t i v e l y . L e p i d o c r o c i t e o c c u r r e d o n l y i n the aggregates w h e r e its c r y s t a l l i n e d i m e n s i o n s c o u l d n o t b e a d e q u a t e l y m e a s u r e d . X - r a y d i f f r a c t i o n intensities i n d i c a t e d f a i r l y r a n d o m o r i e n t a t i o n of crystals i n the aggregates. D i s c u s s i o n . O b s e r v a t i o n s of other w o r k e r s p r o v i d e f u r t h e r insights


2 +

i n t o the b e h a v i o r of p r e c i p i t a t e d f e r r i c o x y h y d r o x i d e s . L a m b a n d J a c q u e s (25)

f o u n d that i n p u r e f e r r i c salt solutions the p a r t i c l e size a n d s t a b i l i t y

of o x y h y d r o x i d e s increases w i t h the c o n c e n t r a t i o n of d i s s o l v e d f e r r i c i r o n present d u r i n g p r e c i p i t a t i o n . F e i t k n e c h t a n d M i c h a e l i s (26) et al.

(7)

n o t e d that c r y s t a l l i n e o x y h y d r o x i d e s ι

•

1

.5--

and Watson

s u c h as goethite Π

1

Run 4 : O.OI M FeS0 , 4

-.2

J

«

1 4

1

1 6

'

1 8

l

-

J

PH

Figure 6. Observed changes in Eh, pH, and calcu lated pQ values during Run 4; bracketed letters have the same significance as in Figure 4


and

224


akaganéite g r o w r a p i d l y w i t h a g i n g i n f e r r i c salt solutions, b u t m a y f o r m or c r y s t a l l i z e e x t r e m e l y s l o w l y i f at a l l i n solutions w h i c h c o n t a i n o n l y trace amounts of d i s s o l v e d f e r r i c i r o n .

Feitknecht and Michaelis

(26)

o b s e r v e d the c o m p l e t e c r y s t a l l i z a t i o n of a m o r p h o u s m a t e r i a l to goethite w h e n the a m o r p h o u s phase w a s p r e c i p i t a t e d i n the presence of F e

2 +

ions.

T h i s s t u d y s i m i l a r l y shows that the rate of f o r m a t i o n of c r y s t a l l i n e p a r ticles of goethite or l e p i d o c r o c i t e is p r o p o r t i o n a l to the d i s s o l v e d ferrous Downloaded by UNIV OF CALIFORNIA SAN DIEGO on November 17, 2014 | http://pubs.acs.org Publication Date: June 1, 1971 | doi: 10.1021/ba-1971-0106.ch008

or f e r r i c i r o n c o n c e n t r a t i o n a n d is faster i n ferrous t h a n i n f e r r i c i r o n solutions of the same m o l a r i t y .

Figure 7. Electron micrograph taken after 302 days' aging at Run 4, showing compact crystalline intergrowth of goethite and lepidocrocite O t h e r generalizations b a s e d o n results of the s t u d y are

possible.

These are: 1. T h e i n i t i a l p r e c i p i t a t e of f e r r i c o x y h y d r o x i d e is u s u a l l y l o w i n p Q , reflecting the presence of r e l a t i v e l y large amounts of a m o r p h o u s m a t e r i a l . T h e faster the rate of p r e c i p i t a t i o n b y o x i d a t i o n a n d / o r h y d r o l y s i s , the greater the s u p e r s a t u r a t i o n w i t h respect to c r y s t a l l i n e o x y h y d r o x i d e s , a n d the l o w e r the i n i t i a l pÇ>. 2. B e c a u s e of the effect of p a r t i c l e size o n r e l a t i v e s t a b i l i t i e s , m i n e r als s u c h as l e p i d o c r o c i t e a n d goethite ( R u n 4 ) or goethite a n d h e m a t i t e m a y reverse i n s t a b i l i t y . 3. P a r t i a l s o l u t i o n or l e a c h i n g of p r e c i p i t a t e s removes the least stable ( most s o l u b l e ) m a t e r i a l first, so that pÇ> values i n s o l u t i o n increase.


8.

LANGMuiR



225

A s was e v i d e n t f r o m R u n s 4, 5, a n d 8, after l o n g p e r i o d s of a g i n g , the average thickness of goethite rods or needles m a y r e m a i n i n the 50 to 100 Â r a n g e , w i t h average lengths of f r o m 500 to 2000 Â .

Lepidocrocite

crystals s i m i l a r l y c a n r e m a i n as laths a v e r a g i n g a b o u t 8 0 - 2 0 0 A t h i c k , 400 Â w i d e , a n d 2300 A i n l e n g t h . If the entire surfaces of s u c h crystals w e r e i n contact w i t h the s o l u t i o n , one c o u l d p r e d i c t a l a r g e p a r t i c l e size effect o n t h e r m o d y n a m i c s t a b i l i t y . F o r goethite,

Equation 5 may

be


m o d i f i e d to give - S p i f = 45.3 A/V

(33)

w h e r e A is the surface area of a c r y s t a l i n A n g s t r o m s s q u a r e d , a n d V is its volume i n Angstroms cubed.

A t the close of R u n 8, the m e a n d i a m e t e r

of the r o d l i k e goethite crystals w a s 60 =b 15 Â, t h e i r m e a n l e n g t h 500 =t 100 Â , a n d p Ç =

40.0. B a s e d o n E q u a t i o n 33, δρΚ is 3.2 for the goethite,

a n d the p K of m a c r o s c o p i c

goethite must t h e n e x c e e d 43.2 because the

goethite is f o r m i n g b y c r y s t a l l i z a t i o n of less-stable a m o r p h o u s m a t e r i a l . P r e l i m i n a r y estimates for goethite crystals i n R u n 5 ( F i g u r e 5 ) s i m i l a r l y y i e l d δρΚ =

3.2.

If the l e p i d o c r o c i t e crystals i n this r u n are c o n

s i d e r e d r e c t a n g u l a r p r i s m s w i t h a surface e n e r g y the same as that of goethite, t h e n δρΚ for the l e p i d o c r o c i t e equals 1.3. T h e a b o v e estimates of δρΚ are c a l c u l a t e d f r o m the g e o m e t r y of i s o l a t e d crystals a n d m a y b e i n error for several reasons, i n c l u d i n g n o n u n i f o r m i t y of c r y s t a l shapes a n d sizes for a p a r t i c u l a r m i n e r a l , u n c e r t a i n t y i n the surface energy of l e p i d o crocite, a n d differences i n the state a n d c o m p o s i t i o n of c r y s t a l aggregates. W h e n aggregates or i n t e r g r o w t h s of a m i n e r a l are present, as i n F i g u r e 7 (see

also W a t s o n et al., Réf. 2 7 ) , δρΚ values b a s e d o n the g e o m e t r y

of

i s o l a t e d crystals w i l l be too large.

Suspended

Ferric

Oxyhydroxides

in Some Ground

Waters

Methods of Chemical Analysis. A n a l y s e s of the i r o n content i n g r o u n d waters f r o m the C a m d e n , N e w Jersey area ( T a b l e I I I ) w e r e m a d e i n a trailer m o d i f i e d as a m o b i l e c h e m i c a l l a b o r a t o r y . F e r r o u s i r o n values w e r e m e a s u r e d w i t h i n one h o u r of c o l l e c t i o n u s i n g a m o d i f i e d v e r s i o n of the b a t h o p h e n a n t h r o l i n e s p e c t r o p h o t o m e t r i c m e t h o d of L e e a n d S t u m m (28) w h i c h involves e x t r a c t i o n of the ferrous i r o n - b a t h o p h e n a n t h r o l i n e c o m p l e x f r o m aqueous s o l u t i o n i n t o n - h e x y l a l c o h o l . T h e e x t r a c t i o n is desir able i n that c o l l o i d a l - s i z e d f e r r i c o x y h y d r o x i d e s present i n the s a m p l e are e x c l u d e d f r o m the extractant. A s e c o n d a d v a n t a g e is that l o w s a m p l e i r o n concentrations m a y be d e t e r m i n e d b y e x t r a c t i n g the i r o n f r o m a large s a m p l e v o l u m e i n t o a r e l a t i v e l y s m a l l v o l u m e of n - h e x y l a l c o h o l . L e e a n d S t u m m r e c o m m e n d a c i d i f y i n g a n d b o i l i n g the s a m p l e p r i o r to ferrous i r o n analysis. B o i l i n g a n d a c i d i f i c a t i o n w e r e a v o i d e d i n this s t u d y because S h a p i r o ( 2 9 ) has s h o w n that s u c h t r e a t m e n t reduces f e r r i c to ferrous i r o n , i n amounts d e p e n d e n t o n the p H a n d t i m e of b o i l i n g .


226



Table III.

Description of Wells and Chemical D a t a for and Magothy Formations

Well No.

Altitude above Sea Level Ft

Screen Setting, Interval in Ft

103 104 123 125 127 131 162

19 19 59 10 45 44 65

171 172 187 188 189

100 100 40 45 65

118-148 204-224 298-338 211-272 248-288 309-367 452-473, 541-594 425-445 369-389 325-375 258-293 220-272

a

Date

Temp., °C

12-17-69 12-17-69 12-15-69 12-14-69 12-13-69 12-16-69 12-15-69

13.0 12.9 13.8 13.1 13.0 14.2 15.0

12-14-69 12-14-69 12-15-69 12-13-69 12-15-69

16.2 15.9 13.2 13.8 14.0

Specific conductance (μ) is in micromhos at 25°C.

T o t a l i r o n concentrations g i v e n i n T a b l e I I I w e r e d e t e r m i n e d w i t h the u n m o d i f i e d b a t h o p h e n a n t h r o l i n e m e t h o d of L e e a n d S t u m m (28). A l l s p e c t r o p h o t o m e t r i c measurements w e r e m a d e w i t h a B a u s c h a n d L o m b S p e c t r o n i c 20, u s i n g 1-inch s a m p l e test tubes ( l i g h t p a t h 2.235 c m ) . T h e a c c u r a c y a n d r e p r o d u c i b i l i t y of ferrous a n d t o t a l i r o n analyses w i t h the b a t h o p h e n a n t h r o l i n e m e t h o d was a b o u t ± 0 . 0 0 5 p p m at c o n c e n t r a tions n e a r 0.02 p p m , a n d a b o u t ± 0 . 1 p p m at c o n c e n t r a t i o n s n e a r 10 p p m . T h e same electrodes w e r e u s e d for E h a n d p H m e a s u r e m e n t , b o t h i n the l a b o r a t o r y a n d the field. M e a s u r e m e n t s of p H w e r e m a d e w i t h the C o l e m a n M e d a l l i o n M o d e l 37 m e t e r after c a l i b r a t i o n i n n o m i n a l p H 4 a n d 7 buffers b r o u g h t to w i t h i n 0 . 5 ° C of g r o u n d w a t e r t e m p e r a t u r e . D o u b l e buffer checks w e r e a l w a y s w i t h i n ± 0 . 0 5 p H unit. E h values w e r e r e a d w i t h t h e C o l e m a n m e t e r or a n O r i o n M o d e l 407 b a t t e r y p o w e r e d p H - m i l l i v o l t meter. T h e E h response of the electrodes a n d meters was c h e c k e d d a i l y w i t h Z o b e l l s o l u t i o n . G r o u n d w a t e r for E h analysis was f o r c e d b y w e l l - p u m p pressure t h r o u g h one side a r m of a glass U t u b e a n d out the o p p o s i t e side a r m . T h e p l a t i n u m t h i m b l e a n d reference electrodes w e r e i n s e r t e d t h r o u g h r u b b e r stoppers into the l a r g e r U t u b e openings to g i v e a w a t e r - t i g h t seal. E h was r e c o r d e d w i t h t i m e d u r i n g constant flow t h r o u g h the U t u b e u n t i l v a l u e s c h a n g e d b y less t h a n 0.005 v o l t i n 20 m i n u t e s . A t this p o i n t , flow was s t o p p e d to e l i m i n a t e the flowing p o t e n t i a l ( u s u a l l y —0.010 to —0.030 v o l t ) a n d the final E h r e a d i n g t a k e n . E h measurements t y p i c a l l y r e q u i r e d f r o m one to t w o hours for c o m p l e t i o n because of r e l a t i v e l y l o w r e d o x c a p a c i t y of the g r o u n d w a t e r a n d the n e e d to flush a l l extraneous o x y g e n f r o m p l u m b i n g b e t w e e n the g r o u n d w a t e r a q u i f e r a n d electrodes. A d e t a i l e d e x p l a n a t i o n of t h e m e t h o d s , t h e o r y , a n d l i m i t a t i o n s of E h m e a s u r e m e n t are g i v e n elsewhere (23). F i e l d E h values l i s t e d i n T a b l e I I I are c o n s i d e r e d a c c u rate to ± 0 . 0 2 0 volt.


8.

LANGMuiR AND WHITTEMORE

227



Well-Water Samples from the Potomac G r o u p and Raritan near Camden, N e w Jersey" Feitotal), ppm

Fe(II), ppm

μ

pQ

+0.220 +0.246 -0.092 -0.049 -0.046 -0.088 -0.090

0.74 0.17 9.4 11.2 11.5 2.6 0.67

0.65 0.15 8.6 10.2 10.5 2.5 0.51

64 66 173 128 146 226 186

43.0 41.0 42.5 42.8 42.6 42.3 41.0

-0.034 -0.040 +0.295 -0.053 -0.085

0.22 0.21 0.024 10.3 0.61

253 253 40 170 212

39.9 39.5 42.9 42.7 41.8

pH

Eh, Volts

4.92 5.64 6.52 6.15 6.22 6.71 7.36 7.65 7.75 5.00 6.21 7.25

0.07 0.11 0.022 9.3 0.21

Specific c o n d u c t a n c e was m e a s u r e d w i t h a B e c k m a n c o n d u c t i v i t y b r i d g e , M o d e l R C - 1 6 8 2 , a n d a d i p p i n g glass c o n d u c t i v i t y c e l l w i t h a c e l l constant n e a r 1 c m " . T h i s e q u i p m e n t w a s c a l i b r a t e d p e r i o d i c a l l y i n a s t a n d a r d K C 1 s o l u t i o n . C o n d u c t a n c e values w e r e c o r r e c t e d to 25 ° C a n d are p r o b a b l y accurate to w i t h i n ± 2 % . 1

Results of t h e c h e m i c a l analyses of g r o u n d waters f r o m the t w e l v e w e l l s near C a m d e n , N e w Jersey, are g i v e n i n T a b l e I I I . G r o u n d Waters Near Camden, N e w Jersey. R e s u l t s of the l a b o r a t o r y studies a n d of r e l a t e d w o r k b y others m a y b e u s e d to e x p l a i n the b e h a v i o r of s u s p e n d e d f e r r i c o x y h y d r o x i d e s i n g r o u n d w a t e r . T h e g r o u n d waters to be c o n s i d e r e d are p u m p e d f r o m the P o t o m a c G r o u p a n d R a r i t a n a n d M a g o t h y F o r m a t i o n s of C r e t a c e o u s age as t h e y o c c u r n e a r C a m d e n , N e w Jersey, a n d i n s o u t h e r n M a r y l a n d . A d e t a i l e d d e s c r i p t i o n of the i r o n content of these g r o u n d waters i n N e w Jersey has b e e n p u b l i s h e d b y L a n g m u i r (18,30),

w h i l e the i r o n content of the M a r y l a n d g r o u n d waters

has b e e n e x a m i n e d b y B a c k a n d B a r n e s

(31).

The Potomac G r o u p and Raritan and Magothy Formations, w h i c h m a y be c o n s i d e r e d a single a q u i f e r system, are m a d e u p of sands, silts, a n d gravels w i t h a c o m b i n e d thickness of 2 0 0 - 2 5 0 feet i n the o u t c r o p area near C a m d e n , N e w Jersey ( F i g u r e 8 ) .

T h e formations d i p 40 to

100

f t / m i l e to the southeast w h e r e t h e y are o v e r l a i n b y silts a n d clays of t h e Merchantville and Woodbury

Formations.

G r o u n d w a t e r s present i n

artesian parts of the a q u i f e r system h a v e e n t e r e d as r e c h a r g e i n the o u t c r o p area a n d b y v e r t i c a l m o v e m e n t t h r o u g h o v e r l y i n g sediments w i t h i n a f e w miles southeast of the o u t c r o p area.



228


Figure 8. Location of the New Jersey study area and generalized pH values of ground water in the Potomac Group and Raritan and Magothy Formations as shown by contours, 1966-67; crosshatching denotes the outcrop area of the formations W h e r e f r e s h , the g r o u n d w a t e r is of the c a l c i u m b i c a r b o n a t e t y p e . Its d i s s o l v e d solids content ranges f r o m less t h a n 100 p p m w h e r e u n p o l l u t e d i n the o u t c r o p area to 500 p p m d o w n d i p ten or m o r e m i l e s , w h e r e fresh g r o u n d waters m i x w i t h r e s i d u a l saline g r o u n d waters. I n a n d near the o u t c r o p a r e a , p H ' s are u s u a l l y b e t w e e n 5 a n d 6 ( F i g u r e 8 ) b e c a u s e of H traces of the F e S

2

+

i o n p r o d u c t i o n r e s u l t i n g f r o m the o x i d a t i o n of

m i n e r a l s p y r i t e a n d m a r c a s i t e a n d f r o m s o l u t i o n of

soil-zone C 0 . G r o u n d waters i n this area are p r o b a b l y at most a f e w 2

m o n t h s i n age.

T h e chief H

i o n p r o d u c i n g reactions r e q u i r e o x y g e n .

+

W i t h o u t f r e s h sources of o x y g e n i n artesian parts of the a q u i f e r s y s t e m , p H values rise to a b o u t 8 as H

+

ions are d e p l e t e d w i t h t i m e b y h y d r o l y s i s

reactions w i t h s i l i c a t e m i n e r a l s a n d traces of c a r b o n a t e s h e l l m a t e r i a l s present i n the f o r m a t i o n s . A r g u m e n t s p r e s e n t e d e l s e w h e r e (30)

support

a n age of s e v e r a l t h o u s a n d years or m o r e for g r o u n d waters i n the area h a v i n g a p H of a b o u t 8. T h u s , the p H contours s h o w n i n F i g u r e 8 are a m e a s u r e of the r e l a t i v e age of the g r o u n d w a t e r . A m a p of t o t a l i r o n concentrations b a s e d o n analyses of 180 w e l l waters (30)

is s h o w n i n F i g u r e 9. A l s o i n d i c a t e d i n the figure are l o c a -

tions of 12 w e l l s w h i c h w e r e s a m p l e d a n d t h e i r w a t e r s c h e m i c a l l y a n a l y z e d i n D e c e m b e r 1969 ( T a b l e I I I ) . A c o m p a r i s o n of F i g u r e s 8 a n d 9 shows that t o t a l i r o n contours p a r a l l e l p H contours.

T h e latter

figure

shows that t o t a l i r o n concentrations increase r a p i d l y to a m a x i m u m a n d


8.

LANGMuiR



229

t h e n g r a d u a l l y decrease d o w n d i p . T o t a l i r o n is present c h i e f l y as ferrous i r o n species w h i c h r a n g e d f r o m 0.022 to 10.2 p p m i n the 12 w e l l w a t e r s . T h e differences b e t w e e n t o t a l a n d ferrous i r o n values i n T a b l e I I I are the c o n c e n t r a t i o n of s u s p e n d e d f e r r i c o x y h y d r o x i d e s w h i c h r e p r e s e n t e d f r o m 5 to 7 0 % of the t o t a l i r o n content i n the 12 w e l l w a t e r s . F i l t r a t i o n studies ( 1 8 )

h a v e s h o w n that the s u s p e n d e d m a t e r i a l ranges f r o m less

t h a n 0.01 m i c r o n (100 Â ) to greater t h a n 5 m i c r o n s a n d averages Downloaded by UNIV OF CALIFORNIA SAN DIEGO on November 17, 2014 | http://pubs.acs.org Publication Date: June 1, 1971 | doi: 10.1021/ba-1971-0106.ch008

microns.

1-2

U n f o r t u n a t e l y , i t was not possible to c o l l e c t e n o u g h of t h i s

m a t e r i a l o n a m e m b r a n e filter for x - r a y analysis. H o w e v e r , l a b o r a t o r y studies b y others d e s c r i b e d elsewhere (18)

s u p p o r t a m i x t u r e of a m o r -

p h o u s m a t e r i a l a n d goethite f o r the suspension. T a k i n g p H as a m e a s u r e of r e l a t i v e age, F i g u r e 10 is a p l o t of p H vs. other c a l c u l a t e d a n d m e a s u r e d p a r a m e t e r s i n the g r o u n d w a t e r . W e l l s 103, 104, a n d 187, w h i c h are at or adjacent to the o u t c r o p area, p l o t to the left of the d i a g r a m . T h e other w e l l s tap artesian g r o u n d w a t e r s d o w n -

Figure 9. Generalized total iron concentration in ground waters of the formations in parts per million, 1965 (18) ; total iron values used to construct the map were measured in the laboratory by the spectrophotometric bipyridine method (32)


230


d i p . T h e d i a g r a m shows t h a t specific c o n d u c t a n c e s increase w i t h age of the g r o u n d w a t e r f r o m 50 m i c r o m h o s i n the o u t c r o p area to 250 m i cromhos d o w n d i p .

I n the o u t c r o p of the f o r m a t i o n s , g r o u n d w a t e r r e -

c h a r g e , w h i c h is chiefly f r o m p r e c i p i t a t i o n , is l o w i n d i s s o l v e d solids b u t h i g h i n d i s s o l v e d o x y g e n so that E h is h i g h a n d ferrous i r o n c o n c e n t r a tions are less t h a n 1 p p m ( 1 0 " M ) .

The Fe

4 7

2 +

m a x i m u m just southeast

of the o u t c r o p area reflects the absence of sources of o x y g e n a n d m i x i n g Downloaded by UNIV OF CALIFORNIA SAN DIEGO on November 17, 2014 | http://pubs.acs.org Publication Date: June 1, 1971 | doi: 10.1021/ba-1971-0106.ch008

of the g r o u n d w a t e r w i t h i r o n - r i c h r e c h a r g e w h i c h enters the a q u i f e r system f r o m o v e r l y i n g sediments. F u r t h e r southeast, F e

2 +

concentrations

decrease b y h y d r o l y s i s a n d p r e c i p i t a t i o n as p H rises. T h e r e are n o other ferrous i r o n sources d o w n d i p . V a r i a t i o n s i n pÇ) m a y b e e x p l a i n e d i n l i g h t of the b e h a v i o r of f e r r i c oxyhydroxides

i n l a b o r a t o r y studies.

T h e two ground waters i n a n d

adjacent to the o u t c r o p area w i t h p H s near 5 ( w e l l s 103 a n d 187) supplied with H

+

ions b y reactions a l r e a d y n o t e d .

are

T h e a c i d i t y tends to

l e a c h a w a y the less stable o x y h y d r o x i d e s , r a i s i n g pÇ> values close to 43. I n the w a t e r f r o m w e l l 104 ( p H = relatively l o w F e

2 +

5.64), p r e c i p i t a t i o n is o c c u r r i n g at

concentrations so that p Q is r a t h e r l o w ( 4 1 . 0 ) .

Com-

p a r a t i v e l y h i g h pQ values ( 4 2 . 5 - 4 2 . 9 ) are f o u n d w i t h m a x i m u m ferrous

a

300

-ι

Γ"

5 m 250 » | 200

I

s

§.« 150

«ε

£.£ g

4

100

50 .3-

I

2

Ο

|.

ω

0-

\

Eh \

-.1· 3-

Γ Ν

4-

£

5-

i

β

-I

Η

Η

1^

1-

-LogCFe *3 2

Η

1

Κ

43 Ο

42

α.

PQ 4140-

Figure 10. pH vs. other measured and calculated chemical parameters for the 12 New Jersey well waters


8.

231


L A N G M U i R AND W H i T T E M O R E

i r o n concentrations of a b o u t 10 p p m .

T h u s , as i n t h e l a b o r a t o r y , the

r e l a t i v e l y stable o x y h y d r o x i d e s d e v e l o p i n the presence of h i g h F e

2 +

con-

centrations, u n d e r w h i c h c o n d i t i o n s fresh p r e c i p i t a t e s c a n r e c r y s t a l l i z e t o m o r e stable forms. further d o w n d i p .

P r e c i p i t a t i o n continues as the g r o u n d w a t e r m o v e s H o w e v e r , p Q values g r a d u a l l y decrease b e c a u s e t h e

o x y h y d r o x i d e s r e c r y s t a l l i z e m o r e s l o w l y i n the presence of

decreasing


a m o u n t s of ferrous i r o n .

5.5

5.0

4.5

-logCFe D 2+

Figure 11. p Q vs. — Zog[Fe ] for 12 New Jersey well waters (circles) and 12 Maryland well waters (triangles); open symbols denote waters with pH values of 5.00 or less 2+

Comparison with G r o u n d Waters in Southern Maryland. T h e p r i n ciples d e s c r i b e d a b o v e also a p p l y to the b e h a v i o r of s u s p e n d e d

ferric

o x y h y d r o x i d e s i n g r o u n d waters f r o m the R a r i t a n a n d M a g o t h y f o r m a tions i n s o u t h e r n M a r y l a n d .

F i g u r e 11 is a p l o t of p Q vs. — l o g [ F e ] 2 +

for the 12 N e w Jersey w e l l waters a n d f o r 12 w e l l waters f r o m M a r y l a n d w h i c h w e r e s t u d i e d b y B a c k a n d B a r n e s (31). Fe

2 +

I n the M a r y l a n d w a t e r s ,

concentrations w e r e g e n e r a l l y h i g h e r (1.3 to 26 p p m ) , E h s s l i g h t l y

h i g h e r ( - 0 . 0 2 0 to + 0 . 3 8 4 v o l t ) a n d p H ' s l o w e r (3.66 to 6.87)

than i n

the N e w Jersey waters. T h e s e differences reflect r e l a t i v e l y greater a b u n dances of H

+

and F e - p r o d u c i n g minerals ( F e S , pyrite, a n d marcasite ) 2 +

2

i n l i g n i t e w i t h i n the M a r y l a n d sediments a n d the fact that a l l t h e M a r y l a n d w e l l s tap g r o u n d waters w h i c h are r e l a t i v e l y o x y g e n - r i c h , u n d e r w a t e r t a b l e or at m o s t s e m i c o n f i n e d

conditions.

Waters

being from

M a r y l a n d w e l l 18 h a v e a p H of 3.66 because of o x i d a t i o n of F e S m i n e r a l s 2


232


so that o x y h y d r o x i d e p r e c i p i t a t e s h a v e b e e n l e a c h e d , a n d p Q is a h i g h 43.5. F o r the other M a r y l a n d w e l l w a t e r s , the p l o t shows a g o o d c o r r e l a t i o n b e t w e e n pÇ) a n d — l o g [ F e ] , i n d i c a t i n g that the s t a b i l i t y of sus2 +

p e n d e d o x y h y d r o x i d e s increases w i t h the F e

2 +

concentration.

Summary


F e r r i c o x y h y d r o x i d e s p r e c i p i t a t e d i n n a t u r a l waters are u s u a l l y m i x tures of x - r a y a m o r p h o u s m a t e r i a l a n d goethite ( α - F e O O H ) . T h e a p p a r ent t h e r m o d y n a m i c s t a b i l i t y of o x y h y d r o x i d e p r e c i p i t a t e s m a y b e s c r i b e d i n terms of the i o n a c t i v i t y p r o d u c t pQ = solution.

V a l u e s of pQ

solved iron and (or)

de

—log[Fe ] [ O H ' ] 3 +

3

in

are c a l c u l a b l e f r o m measurements of t o t a l d i s ferrous i r o n , p H , E h , a n d a k n o w l e d g e o f i o n i c

strength. F e r r i c o x y h y d r o x i d e s p r e c i p i t a t e d i n the l a b o r a t o r y f r o m s o l u tions i n i t i a l l y 1 0 " M i n i r o n as f e r r i c sulfate or ferrous sulfate h a d 2

values r a n g i n g f r o m 37.3 to 43.3.

pQ

T h e lowest values w e r e f o u n d w i t h

freshly p r e c i p i t a t e d a m o r p h o u s m a t e r i a l , the h i g h e s t values w i t h m i x t u r e s of goethite a n d l e p i d o c r o c i t e f o r m e d i n F e S 0

4

aged

solutions. B a s e d

o n x - r a y d i f f r a c t i o n a n d e l e c t r o n m i c r o s c o p y , goethite o c c u r r e d as r o d or n e e d l e - l i k e crystals e l o n g a t e d p a r a l l e l to the c-axis d i r e c t i o n . T h e crystals t y p i c a l l y r a n g e d f r o m 50 to 100 Â i n d i a m e t e r a n d 500 to 2000 Â i n l e n g t h . L e p i d o c r o c i t e a p p e a r e d as laths r a n g i n g f r o m 60 to 200 Â t h i c k a l o n g the fo-axis, a v e r a g i n g 400 Â w i d e i n the c-axis d i r e c t i o n a n d 2300 Â l o n g i n the α-axis d i r e c t i o n . C h e m i c a l analyses w e r e m a d e of 24 w e l l w a t e r s f r o m coastal p l a i n N e w Jersey a n d M a r y l a n d i n w h i c h the i r o n is present i n s o l u t i o n chiefly as aqueous ferrous species ( 1 0 oxyhydroxides ( 1 0

3 3 3

to 10"

5 4

0

M F e ) and suspended ferric

to 1 0 ~ M F e , or 5 to 7 0 % of the t o t a l i r o n c o n

4 1 3

7 4

t e n t ) . T h e s u s p e n d e d o x y h y d r o x i d e s are p r o b a b l y m i x t u r e s of a m o r p h o u s m a t e r i a l a n d goethite.

B a s e d o n filtration studies, t h e y h a v e a p a r t i c l e

size f r o m < 1 0 0 to > 50,000 À a n d a v e r a g i n g 10,000 to to 20,000 Â . V a l u e s of pÇ> r a n g e d f r o m 37.1 i n a s h a l l o w , o x i d i z e d g r o u n d w a t e r w i t h a c t i v e p r e c i p i t a t i o n to 43.5 i n a s h a l l o w g r o u n d w a t e r w i t h a c t i v e l e a c h i n g of oxyhydroxides b y H

+

ions ( p H =

3.66) f r o m o x i d a t i o n of F e S m i n e r a l s . 2

T h e highest pQ values f o r d e e p e r , artesian g r o u n d w a t e r s r a n g e d f r o m a b o u t 41 to 43 a n d w e r e f o u n d i n r e l a t i v e l y y o u n g w a t e r s at the h i g h e s t m e a s u r e d ferrous i r o n concentrations.

G r o u n d waters at e v e n

greater

depths i n coastal p l a i n N e w Jersey, w h i c h are p r o b a b l y s e v e r a l t h o u s a n d years o l d , h a d pÇ) values as l o w as 39.5, h a v i n g a g e d a l o n g w i t h ferrous i r o n concentrations ( 1 0 "

5

9

0

low

M Fe).

S o m e g e n e r a l conclusions of the s t u d y a r e : 1.

T h e greater the s u p e r s a t u r a t i o n w i t h respect to c r y s t a l l i n e o x y -

h y d r o x i d e s , the faster the p r e c i p i t a t i o n a n d the l o w e r the i n i t i a l


pQ.

8.

LANGMUiR AND WHITTEMORE

2.


233

H i g h pÇ) values c a n result f r o m H i o n l e a c h i n g o f o x y h y d r o x i d e +

p r e c i p i t a t e s w h i c h p r e f e r e n t i a l l y dissolves t h e least stable m a t e r i a l . 3.

T h e rate o f c r y s t a l f o r m a t i o n a n d p Q increase o f a p r e c i p i t a t e

( u s u a l l y goethite ) is p r o p o r t i o n a l to t h e d i s s o l v e d i r o n c o n c e n t r a t i o n a n d is faster i n ferrous t h a n i n f e r r i c i r o n solutions. M e a s u r a b l e c r y s t a l l i z a t i o n of a m o r p h o u s m a t e r i a l occurs w i t h i n m i n u t e s i n 1 0 ~ M ferrous i r o n s o l u 2

tions, b u t m a y take thousands o f years i n waters w h i c h c o n t a i n

about

1 0 " M ferrous i r o n . Downloaded by UNIV OF CALIFORNIA SAN DIEGO on November 17, 2014 | http://pubs.acs.org Publication Date: June 1, 1971 | doi: 10.1021/ba-1971-0106.ch008

6

4.

F e r r i c o x y h y d r o x i d e s w i t h c r y s t a l thicknesses less t h a n 100 Â c a n

r e m a i n i n d e f i n i t e l y i n some waters. 5. T h e r e l a t i v e stabilities o f t w o c r y s t a l l i n e o x y h y d r o x i d e s s u c h as goethite a n d h e m a t i t e o r goethite a n d l e p i d o c r o c i t e c a n reverse w i t h t i m e o w i n g to p a r t i c l e size effects. Acknowledgment F i n a n c i a l s u p p o r t f o r this s t u d y w a s p r o v i d e d b y t h e M i n e r a l C o n s e r v a t i o n S e c t i o n a n d t h e Institute f o r R e s e a r c h o n L a n d a n d W a t e r R e sources, b o t h o f T h e P e n n s y l v a n i a State U n i v e r s i t y , U n i v e r s i t y P a r k , P a . Literature

Cited

(1) Garrels, R. M., in "Researches in Geochemistry," p. 25, Philip H . Abelson, Ed., Wiley, New York, 1959. (2) Welo, L . Α., Baudisch, O., Chem. Rev. (1934) 15, 1-43. (3) Palache, C., Berman, H . , Frondel, C., "The System of Mineralogy," Vol. I, p. 527, 642, 680, 708, Wiley, New York, 1944. (4) Singer, P. C., Stumm, W., Science (1970) 167, 1121-3. (5) Van Tassel, R., Bull. Soc. Belge Geol. (1959) 68, 360-7. (6) Chandy, K. C., Indian J. Phys. (1962) 36, 484-9. (7) Watson, J. H . L . , Heller, W., Poplawski, L . E., Third European Regional Conf. Elect. Microscopy (1964) 315-6. (8) Langmuir, D . , U. S. Geol. Surv. Profess. Paper

(9) (10) (11) (12)

(1969) 650-B, B180-

B184. Langmuir, D., Am. J. Sci. (1971), in press. Ferrier, Α., Rev. Chim. Minerale (1966) 3, 587-615. Langmuir, D., Geol. Soc. Am. Ann. Mtg., Milwaukee, with Abstracts. Milburn, R. M . , J. Am. Chem. Soc. (1957) 79, 537-40.

(13) Hem, J. D . , Cropper, W . H . , U. S. Geol. Surv. Water

(14) (15) (16) (17) (18) (19)

1970, Program Supply

Paper

(1959) 1459A, 4. Garrels, R. M . , Christ, C. L . , "Solutions, Minerals, and Equilibria," p. 172, Harper and Row, New York, 1965. Nair, R. L . S., Nancollas, G . H . , J. Chem. Soc. (1958) 4144-7. Willix, R. L . S., Trans. Faraday Soc. (1963) 59, 1315-24. Klotz, I. M., "Chemical Thermodynamics," revised ed., p. 419, Benjamin, New York, 1964. Langmuir, D., U. S. Geol. Surv. Profess. Paper (1969) 650-C, C224C235. Back, W., U . S. Geological Survey, Washington, D . C., written commu nication, 1970.


234 (20) (21) (22) (23)


(24) (25) (26) (27) (28) (29) (30) (31) (32)

NONEQUILIBRIUM

SYSTEMS IN N A T U R A L

WATERS

Whittemore, D . O., Langmuir, D . , i n preparation. Ackerman, T., Z. Electrochem. (1958) 62, 411-9. Kolthoff, I. M., Sandell, Ε. B . , "Textbook of Quantitative Analysis," 3rd ed., p. 571, 579, M a c M i l l a n , N e w York, 1952. Langmuir, D . , "Procedures i n Sedimentary Petrology," C h . 26, p. 5 9 7 635, Robert E . Carver, Ed., W i l e y , N e w York, 1971. Cullity, B. D . , "Elements of X - r a y Diffraction," p. 97, 261, AddisonWesley, Reading, Mass., 1956. L a m b , A . B., Jacques, A . G., J. A m . Chem. Soc. (1938) 60, 1215-25. Feitknecht, W . , Michaelis, W . , Helv. Chim. Acta (1962) 45, 212-24. Watson, J . H. L., Cardel, R. R., Jr., Heller, W., J. Phys. Chem. (1963) 66, 1757-63. Lee, G . F . , Stumm, W . , J. A m . Water Works Assoc. (1960) 52, 1767-74. Shapiro, J . , Limnol. Oceanog. (1966) 11, 293-8. Langmuir, D . , N. J. Department of Conservation and Economic Develop ment, N. J. Water Resources Circ. (1969) 19, 43 p. Back, W . , Barnes, L., U. S. Geol. Surv. Profess. Papers (1965) 498-C, C1-C16. Rainwater, F . H., Thatcher, L. L., U. S. Geol. Surv. Water Supply Papers (1960) 1454, 184.

R E C E I V E D July 30,

1970.


Nonequilibrium Systems in Natural Water Chemistry

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