Chemical Modeling in Aqueous Systems - ACS Publications

3 Redox Equilibria of Iron i n A c i d M i n e Waters D A R R E L L KIRK N O R D S T R O M Department of Environmental Sciences, University of Virginia, Charlottesville, V A 22903

Downloaded by UNIV OF AUCKLAND on May 6, 2015 | http://pubs.acs.org Publication Date: March 19, 1979 | doi: 10.1021/bk-1979-0093.ch003

E V E R E T T A. J E N N E and J A M E S W. B A L L U.S. Geological Survey, Water Resources Division, Menlo Park, C A 94025

As the f o u r t h most abundant element i n the e a r t h ' s c r u s t , no other metal i s as important as i r o n i n geochemistry (1), n a t u r a l water systems (_2), m i c r o b i a l metabolism (_3), the e v o l u t i o n of the e a r t h ' s past and the f o r m a t i o n of ore d e p o s i t s (4^J5). The chemical behavior of i r o n i s known to be a key to the i n t e r p e t a t i o n of p r o c e s s e s i n v o l v i n g t r a c e elements, n u t r i e n t s and o x i d a t i o n - r e d u c t i o n r e a c t i o n s i n n a t u r a l waters, s o i l s , sediments and groundwaters (6-13). An a p p r o p r i a t e s t a r t i n g p o i n t f o r the c y c l i n g of i r o n through the lithosphère i s the weathering of i r o n m i n e r a l s , e s p e c i a l l y p y r i t e because i t commonly occurs i n many rock types and i t p r o v i d e s a major source of s u l f a t e and a c i d i t y as w e l l as i r o n to n a t u r a l water systems. Pyrite weathering a l s o l e a d s to the f o r m a t i o n of a c i d mine d r a i n a g e , a major cause of water p o l l u t i o n i n many f r e s h w a t e r r i v e r s and l a k e s . U n l i k e i r o n t r a n s f o r m a t i o n s i n marine systems or f r e s h w a t e r s of n e u t r a l to a l k a l i n e pH, a c i d mine waters c o n t a i n very high c o n c e n t r a t i o n s of i r o n , which are t h e r e f o r e more e a s i l y determined. There are a l s o minimal problems i n these waters with none q u i l i b r i u m p o l y m e r i z a t i o n and c o l l o i d f o r m a t i o n . Attempts at chemical modeling, however, are c o m p l i c a t e d by the l a r g e q u a n t i t i e s of aqueous complexes t h a t must be c o n s i d e r e d and the inadequacy of i n d i v i d u a l ion a c t i v i t y c o e f f i c i e n t s at moderate i o n i c s t r e n g t h s (up to 0.6 m o l a l ) when the ion a s s o c i a t i o n method i s used. C a l c u l a t i o n s which are based on ion a s s o c i a t i o n and assume mineral-water e q u i l i b r i u m have been a p p l i e d to problems of aqueous geochem i s t r y with encouraging r e s u l t s (14-19). Although we c o n s i d e r our i n i t i a l e f f o r t s as e s t i m a t e s , our c a l c u l a t i o n s can be checked a g a i n s t f i e l d obser0-8412-0479-9/79/47-093-051$07.25/0 This chapter not subject to U.S. copyright Published 1979 American Chemical Society In Chemical Modeling in Aqueous Systems; Jenne, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

52

CHEMICAL MODELING IN AQUEOUS

SYSTEMS


vations. Our r e s u l t s s h o u l d i n d i c a t e where the major weaknesses of t h i s approach remain. We b e l i e v e t h a t any chemical model must be c o n t i n u a l l y t e s t e d a g a i n s t many d i f f e r e n t f i e l d s i t u a t i o n s so t h a t i t can be improved and m o d i f i e d and t h e r e b y enhance our u n d e r s t a n d i n g of n a t u r a l systems. In t h i s paper we s h a l l focus on three important aspects of a c i d mine water e q u i l i b r i a : 1) the measurement and i n t e r p r e t a t i o n of redox r e a c t i o n s , 2) the f o r mation of aqueous complexes and 3) s o l u t i o n - m i n e r a l reactions. Formation of A c i d Mine Waters Numerous i n v e s t i g a t i o n s of p y r i t e o x i d a t i o n and the p r o d u c t i o n of a c i d mine drainage have p a r t i a l l y u n r a v e l l e d the mechanisms of t h i s complex p r o c e s s . The i n i t i a l s t e p i n p y r i t e o x i d a t i o n was demons t r a t e d by Sato (_20) t o be the r e l e a s e of f e r r o u s i o n s and e l e m e n t a l s u l f u r : FeS

Fe

2

2 +

+ S° + 2e-

(1)

L i k e w i s e , the i n i t i a l o x i d a t i o n s t e p of other metals s u l f i d e s ( c h a l c o c i t e , c o v e l l i t e , g a l e n a and s p h a l e r i t e ) appeared t o produce the aqueous d i v a l e n t metal i o n and e l e m e n t a l s u l f u r . In the next s t e p of the o x i d a t i o n , s u l f u r i s o x i d i z e d t o s u l f a t e and protons are produced S° + 8H 0 2

2

+

2 S 0 ~ + 16H 4

+

+ 12e"

(2)

Iron w i l l remain i n the reduced f e r r o u s s t a t e f o r q u i t e some time as long as the s o l u t i o n i s a c i d (pH _< 4 ) . A l t h o u g h the o v e r a l l oxidant which d r i v e s t h i s r e a c t i o n i s oxygen from the atmosphere, s e v e r a l i n v e s t i g a t o r s have demonstrated t h a t d i s s o l v e d f e r r i c i r o n i s the p r i m a r y oxidant which d i r e c t l y a t t a c k s the p y r i t e s u r f a c e ( Z l , 22^). S i n c e the r e a c t i o n r a t e depends upon the a v a i l a b i l i t y of F e ^ , the o x i d a t i o n h a l f - r e a c t i o n +

F e

2+

F e

3+ + e

(3)

has been c a l l e d the r a t e - d e t e r m i n i n g step i n the f o r m a t i o n of a c i d mine drainage (^3). R e a c t i o n 3 proceeds so s l o w l y under a c i d c o n d i t i o n s that a c i d mine waters would not commonly occur were i t not f o r the a c i d o p h i l i c i r o n - o x i d i z i n g b a c t e r i u m Thiobacillus ferrooxidans. T h i s b a c t e r i u m i s such an e f f e c t i v e c a t a l y s t that the o x i d a t i o n r a t e i s

In Chemical Modeling in Aqueous Systems; Jenne, E.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.


3.

NORDSTROM E T A L .

Redox Equilibria of Iron

i n c r e a s e d by 5 or 6 o r d e r s of magnitude over the a b i o t i c r a t e (2A,2). The b a c t e r i u m can grow i n the absence of l i g h t and i t i s found deep i n s i d e mines as long as some minimal amount of oxygen i s available. The p r o d u c t i o n of a c i d mine drainage i s thus a r a p i d , s e l f - p e r p e t u a t i n g process c a t a l y z e d by b a c t e r i a which c o n t i n u e s as long as a i r , water and p y r i t e are a v a i l a b l e . The geochemical f o r m u l a t i o n of t h i s p r o c e s s i s summarized i n F i g u r e 1. Pyrite i s a t t a c k e d by f e r r i c i r o n , an a c i d f e r r o u s s u l f a t e s o l u t i o n i s produced, and the f e r r o u s i r o n i s c a t a l y t i c a l l y r e o x i d i z e d to f e r r i c by Τ. f e r r o x i d a n s . T h i s c y c l e c o n t i n u e s u n t i l the p y r i t e i s gone or the mine water leaves the s u l f i d e s u r f a c e s ( i s s u e s from the mine) where i t w i l l f u l l y o x i d i z e and hydrolyse t o form amorphous f e r r i c hydroxide (yellow-boy), g o e t h i t e or j a r o s i t e depending upon the a c i d , i r o n , s u l f a t e content of the water and the degree of ageing. Oxygen e n t e r s t h i s r e a c t i o n scheme by a c c e p t i n g the e l e c t r o n donated from the i r o n o x i d a t i o n of r e a c t i o n 3 through the m e t a b o l i c pathways of the b a c t e r i u m . The r e d u c t i o n h a l f - r e a c t i o n i s : h0

2

+ 2H++ 2e"

->

H0 2

(4)

S i n c e these b a c t e r i a are a e r o b i c , t h i s scheme i m p l i e s that an important f u n c t i o n of oxygen i s to p r o v i d e o x i c c o n d i t i o n s f o r adequate r e s p i r a t i o n . These h a l f - r e a c t i o n s (3 and 4) need not be i n e q u i l i b r i u m with each other; o n l y a s m a l l amount of oxygen, enough f o r r e s p i r a t i o n , i s n e c e s s a r y to drive this process. In t h i s study, we are c h i e f l y concerned with the r e a c t i o n s o c c u r r i n g i n the e f f l u e n t water a f t e r i t has l e f t the mines and has e n t e r e d n a t u r a l streams. Field

Site,

Sampling and

A n a l y t i c a l Methods

In Shasta County, C a l i f o r n i a , i n a c t i v e copper mines c o n t a i n i n g s e v e r a l m i l l i o n s of tons of s u l f i des are being weathered to produce h i g h l y a c i d i c mine waters. The watershed s u r r o u n d i n g I r o n Mountain ( F i g u r e 2) was chosen f o r d e t a i l e d i n v e s t i g a t i o n s because i t was the l a r g e s t source of a c i d d r a i n a g e i n the r e g i o n (2_5,2_6). A p p r o x i m a t e l y 14 km of stream waters are a f f e c t e d by a c i d e f f l u e n t i s s u i n g from the mines at Iron Mountain. These streams show a l a r g e g r a d i e n t i n redox s t a t e , pH and t o t a l d i s s o l v e d s o l i d s d u r i n g downstream t r a n s p o r t due to mixing and d i l u t i o n as w e l l as r a p i d o x i d a -



54

C H E M I C A L MODELING IN AQUEOUS

regenerated Thiobacillus

Fe S

2

SYSTEMS

by

ferrooxidans

+ 14Fe + 8 H 0 ^ 15Fe + 2S0£~+ 16H* 3+

hydrolysis Fe(OH) ,

2+

2

and

precipitation

off

amorphous

3

FeO(OH), g o e t h i t e Κ Fe (S0 ) (OH) 3

Figure

4

2

1.

6

,

jarosite

Mechanism

of acid mine water

formation



Figure 2. Location rock Creeks, which

of Spring Creek and its two tributaries, Boulder and Slickcontain acid mine drainage and drain the Iron Mountain watershed,


56

C H E M I C A L M O D E L I N G IN AQUEOUS SYSTEMS

tion rates. The weathering ore bodies are t y p i c a l massive s u l f i d e d e p o s i t s , and they c o n t a i n f i n e g r a i n e d p y r i t e with v a r i a b l e amounts of c h a l c o p y r i t e and s p h a l e r i t e o c c u r r i n g as pods or l e n s e s w i t h i n a r h y o l i t e of Devonian age. Both a i r and groundwaters e a s i l y i n f i l t r a t e the ore bodies and stoped out a r e a s where the a c i d waters d e v e l o p . The a c i d waters e v e n t u a l l y f i n d t h e i r way t o a p o r t a l or a d i t and are f i n a l l y d i s c h a r g e d i n t o e i t h e r Boulder Creek


or

Slickrock

the p r i n c i p a l t r i b u t a r i e s

Creek,

of Spring

Creek ( F i g u r e 2 ) . D u r i n g an 18-month p e r i o d over 1 0 0 water samples were c o l l e c t e d at s e v e r a l d i f f e r e n t l o c a t i o n s on these streams i n order t o i n t e r p r e t the hydrogeochemical p r o c e s s e s c o n t r o l l i n g the heavy metal c h e m i s t r y ( 2 6 ) . Temperature, c o n d u c t i v i t y , pH, Eh and d i s s o l v e d oxygen (D.O.) were measured on s i t e . Water samples c o l l e c t e d f o r a n a l y s i s were f i l t e r e d on s i t e through 0.10 ym membranes i n an a c i d - c l e a n e d p l a s t i c f i l t e r i n g apparatus ( 2 _ 7 ) connected t o a p o r t a b l e p e r i s t a l t i c pump v i a a c i d c l e a n e d s i l i c o n e t u b i n g . Samples were p r e s e r v e d by a c i d i f i c a t i o n t o a pH of about 1 with n i t r i c a c i d f o r heavy metal a n a l y s i s and with h y d r o c h l o r i c a c i d f o r f e r r o u s i r o n , a l u m i num, a l k a l i metals and a l k a l i n e e a r t h m e t a l s . F i l t r a t i o n through 0 . 1 0 ym membranes remove i r o n - o x i d i z i n g b a c t e r i a and p r e v e n t s b i o l o g i c a l o x i d a t i o n while a c i d i f i c a t i o n m i n i m i z e s i n o r g a n i c o x i d a tion. Anion a n a l y s e s were c a r r i e d out on samples which were f i l t e r e d but o t h e r w i s e were not p r e served. Eh measurements were made with a comb i n a t i o n p o l i s h e d p l a t i n u m e l e c t r o d e which was checked at l e a s t once i n a sampling day with ZoBell's s o l u t i o n (_28). The p r e c i s i o n on emf r e a d i n g s was + 5 mv; the p r e c i s i o n on pH r e a d i n g s was + 0 . 0 2 , a l t h o u g h the a c c u r a c y cannot be cons i d e r e d b e t t e r than _± 0 . 0 5 . Measurement of pH i n the most c o n c e n t r a t e d a c i d mine e f f l u e n t (pH - 1 . 0 ) were q u i t e slow t o come t o a s t e a d y value but were always r e p r o d u c i b l e . M e a s u r e m e n t s o f D 0 . were made by a commercial oxygen e l e c t r o d e which was c a l i b r a t e d by a W i n k l e r t i t r a t i o n on a c l e a n oxygens a t u r a t e d stream water. Aluminum, s i l i c o n and z i n c were determined by d.c.-argon plasma j e t e m i s s i o n s p e c t r o p h o t o m e t r y . The remaining c a t i o n s were a n a l y z e d by atomic a b s o r p t i o n s p e c t r o p h o t o m e t r y (AAS) except f e r r o u s i r o n which was done by a m o d i f i c a t i o n of the F e r r o z i n e method ( 2 9 , _ 3 0 , 3 J J . T o t a l i r o n was d e t e r mined by AAS and Fe$ by d i f f e r e n c e * . S u l f a t e was E

+



3.

NORDSTROM E T A L .


a n a l y z e d by the T h o r i n method a f t e r removal of i n t e r f e r i n g c a t i o n s by i o n exchange. F l u o r i d e and c h l o r i d e were e x t r e m e l y d i f f i c u l t t o a n a l y z e because of the high c o n c e n t r a t i o n of i n t e r f e r i n g i o n s , and t h e y were present at very low c o n c e n t r a t i o n s (about 0.1 mg/L or l e s s ) . F o r chemical e q u i l i b r i u m comp u t a t i o n s they were assumed t o be 0.1 mg/L f o r a l l samples. Water samples c o l l e c t e d from the Iron f o u n t a i n watershed p r o v i d e d a l a r g e v a r i a t i o n i n pH (1 t o 7 ) , Eh (350 t o 900 mv), t o t a l d i s s o l v e d i r o n (10 t o 12,000 mg/L), temperature (4° t o 31°C) and the i o n i c s t r e n g t h v a r i e d up t o 0.6 m o l a l . C a l c u l a t i o n s of A c t i v i t i e s

and S a t u r a t i o n

Indices

The water a n a l y s e s were coded and then proc e s s e d with the computer program WATEQ2. This program was m o d i f i e d i n s e v e r a l ways t o handle a c i d mine waters: (a) the Eh c o u l d be c a l c u l a t e d from the F e 2 / F e 3 a c t i v i t y r a t i o or v i c e v e r s a , (b) s e v e r a l s u l f a t e m i n e r a l s were added,(c) metal s u l f a t e and h y d r o x i d e complex c o n s t a n t s were c a r e f u l l y e v a l u a t e d and included, and (d) Mn, Cu, Zn and Cd s p e c i e s were added s i n c e they are major c o n s t i t uents f o r s e v e r a l of the water samples. These m o d i f i c a t i o n s and the e v a l u a t e d thermodynamic data are d e s c r i b e d by B a l l , Jenne and Nordstrom i n t h i s symposium (32). +

+

Redox S t a t u s o f A c i d Mine Waters; Disequilibrium?

E q u i l i b r i u m or

What we mean i n t h i s r e p o r t by e q u i l i b r i u m and d i s e q u i l i b r i u m r e q u i r e s a b r i e f d i s c u s s i o n of d e f i nitions. N a t u r a l p h y s i c o c h e m i c a l systems c o n t a i n g a s e s , l i q u i d s and s o l i d s with i n t e r f a c e s forming the boundary between phases and with some s o l u b i l i t y of the components from one phase i n another depending on the chemical p o t e n t i a l of each component. When e q u i l i b r i u m i s reached by a h e t e r o g e neous system, the r a t e of t r a n s f e r of any component between phases i s equal i n both d i r e c t i o n s across e v e r y i n t e r f a c e . T h i s d e f i n i t i o n demands that a l l s o l u t i o n r e a c t i o n s i n the l i q u i d phase be s i m u l t a n e o u s l y i n e q u i l i b r i u m with both gas and s o l i d phases which make c o n t a c t w i t h t h a t l i q u i d . Homogeneous s o l u t i o n phase r e a c t i o n s , however, are commonly much f a s t e r than gas phase or s o l i d phase r e a c t i o n s and f a s t e r than g a s - l i q u i d , g a s - s o l i d and



58

C H E M I C A L M O D E L I N G IN AQUEOUS

SYSTEMS

l i q u i d - s o l i d transfer reactions. T h e r e f o r e i t seems a p p r o p r i a t e to assume t h a t n a t u r a l waters would be i n e q u i l i b r i u m with r e s p e c t to d i s s o l v e d s p e c i e s i n the aqueous phase but might be i n v a r y i n g stages of d i s e q u i l i b r i u m with r e s p e c t to gaseous and s o l i d phases. Such a system i s i n p a r t i a l or l o c a l e q u i l i b r i u m (3_3). In f a c t , there may be m u l t i p l e and d i v e r s e l o c a l e q u i l i b r i u m s t a t e s i n such a heterogeneous environment. Equilibrium thermodynamics d e f i n e s the e x t e n t to which a n a t u r a l water has reached c h e m i c a l e q u i l i b r i u m even though i t may be undergoing dynamic changes i n f l u i d flow, heat flow, b i o l o g i c a c t i v i t y or mass t r a n s f e r . This assumption of l o c a l e q u i l i b r i u m i s a fundamental p o s t u l a t e of i r r e v e r s i b l e thermodynamics which j u s t i f i e s the c a l c u l a t i o n s of e q u i l i b r i u m p r o p e r t i e s f o r a r e s t r i c t e d s p a t i a l or temporal element i n an i r r e v e r s i b l e process (3_3, 3_4,_^5) · With these concepts i n mind, we can a p p r e c i a t e t h a t a n a t u r a l water may have one f i x e d , r e p r e s e n t a t i v e Eh but i t may not be i n e q u i l i b r i u m with d i s s o l v e d gases or the s o l i d s u r f a c e of a p l a t i n u m e l e c t r o d e . We now c o n s i d e r Eh measurements and c h e m i c a l e q u i l i b r i u m i n n a t u r a l water systems. The Eh of n a t u r a l waters has been c a l c u l a t e d t h e o r e t i c a l l y ( 2 ^ 3 6 . ) , measured with i n e r t metal e l e c t r o d e s , c a l c u l a t e d from a n a l y s e s of i n d i v i d u a l redox s p e c i e s (3_7) measured by e q u i l i b r a t i o n w i t h known redox couples ( _ 3 8 , _ 3 £ ) . Eh measurements have been used q u a l i t a t i v e l y as an o p e r a t i o n a l parameter and q u a n t i t a t i v e l y as an i n d i c a t i o n of a dominant redox c o u p l e . The q u a l i t a t i v e use of Eh, advocated by Z o B e l l ( £ 0 ) , has r e s u l t e d i n a great many measurements (.41 ). As a q u a n t i t a t i v e t o o l , the use of Eh has not enjoyed widespread s u c c e s s . S e v e r a l c r i t e r i a must be met b e f o r e the Eh can be r e l a t e d to a s p e c i f i c redox couple or mechanism: 1) the net exchange c u r r e n t at the e l e c t r o d e - s o l u t i o n i n t e r f a c e , i . e . , the d i f f e r e n c e i n the r a t e s of e l e c t r o n t r a n s f e r to and from the p l a t i n u m s u r f a c e , must be n e g l i g i b l e (an e q u i l i b r i u m c r i t e r i o n r e q u i r e d f o r the a p p l i c a t i o n of the Nernst equat i o n ) , 2 ) the i n d i v i d u a l exchange c u r r e n t , i , s h o u l d be g r e a t e r than about 1 0 " ampere/cm^ ( 4 3 ) , 3 ) a l l aqueous e l e c t r o a c t i v e redox s p e c i e s s h o u l d be i n homogeneous e q u i l i b r i u m , 4 ) the e l e c t r o d e s u r f a c e must be f r e e of e l e c t r o a c t i v e s u r f a c e c o a t i n g s and adsorbed i m p u r i t i e s and 5 ) a c t i v i t i e s of the redox s p e c i e s must be e i t h e r measured or c a l c u l a t e d by c o n s i d e r i n g the e f f e c t s of complexing and i o n i c a

n

d

Q

7



3.

NORDSTROM E T A L .


s t r e n g t h on a c t i v i t y c o e f f i c i e n t s , s i n c e the a c t i v i t i e s , r a t h e r than the c o n c e n t r a t i o n s of the part i c i p a t i n g s p e c i e s are the q u a n t i t i e s that determine the p o t e n t i a l due to the redox c o u p l e . These c r i t e r i a are d i f f i c u l t to meet and have l e d to the s k e p t i c a l o u t l o o k that most Eh measurements are not amenable to q u a n t i t a t i v e i n t e r p r e t a t i o n (2,42,43). C o n t a m i n a t i o n of p l a t i n u m e l e c t r o d e s u r f a c e s by oxygen i n a e r a t e d waters (4^3,_44), by s u l f u r i n anaerob i c waters (^4) and by i r o n i n s u r f a c e sediments (45) may cause e r r o r s i n the measured v a l u e s . Furthermore, many Eh measurements are thought to be mixed p o t e n t i a l s (j43 ). For these reasons, most Eh measurements have been used o n l y i n a q u a l i t a t i v e sense. In c o n t r a s t to t h i s g e n e r a l pessimism, s e v e r a l i n v e s t i g a t o r s have found good agreement between measured Eh and a dominant redox c o u p l e . Sato (20) found a r e l a t i o n between the measured Eh and pH of s u b s u r f a c e waters and the peroxide-oxygen redox couple. Berner (4_6) demonstrated a N e r n s t i a n r e l a t i o n s h i p between measured Eh and the S ~ / S ° couple i n a n o x i c marine sediments. T h i s e q u i l i b r i u m has a l s o been s u b s t a n t i a t e d by Kryukov, e t a l . (47), S k o p i n t s e v , e t a l . (48) and W h i t f i e l d (49). T h o r s t e n s o n (_37) found good agreement between the redox couples of s u l f u r and n i t r o g e n s p e c i e s f o r f i v e d i f f e r e n t r e d u c i n g environments, i m p l y i n g that mixed p o t e n t i a l s may not be a major problem. In l a b o r a t o r y experiments, N a t a r a j a n and Iwasaki (50) found N e r n s t i a n behavior of p l a t i u m e l e c t r o d e s i n the presence of v a r y i n g d i s s o l v e d oxygen and ferrous-ferric ratios. B r i c k e r (5_1) a c h i e v e d r e v e r s ible e q u i l i b r i u m behavior with a p l a t i n u m e l e c t r o d e i n s o l u b i l i t y s t u d i e s of manganese oxides and hydroxides. Eh measurements of s u l f u r - r i c h waters can d e v i a t e from c a l c u l a t e d s u l f i d e redox values due t o the presence of c o l l o i d a l s u l f u r , but Boulegue (52) has shown that these i n t e r f e r e n c e s can be i d e n t i f i e d by a c i d - b a s e t i t r a t i o n s of the sample. Thus, amid the s k e p t i c i s m , there i s a l r e a d y evidence from s e v e r a l d i f f e r e n t environments i n d i c a t i n g that Eh measurements can be amenable to q u a n t i t a t i v e , t h e r modynamic i n t e r p r e t a t i o n s . A c i d mine waters s h o u l d be p a r t i c u l a r l y w e l l s u i t e d to r e l i a b l e Eh measurements because the high c o n c e n t r a t i o n s of e l e c t r o a c t i v e i r o n s p e c i e s would c l e a r l y s a t i s f y c r i t e r i a 1) and 2) above (42,43). In a d d i t i o n , the a c i d c o n d i t i o n s i n h i b i t s s u r f a c e 2

coatings

of

iron

oxides

on

the e l e c t r o d e . The measured Eh



60

v a l u e s i n t h i s study were compared with c a l c u l a t e d v a l u e s based on f e r r o u s - f e r r i c a n a l y s e s of 60 samples a l l of which had c a t i o n - a n i o n balances b e t t e r than 30 p e r c e n t . C a l c u l a t e d v a l u e s were d e r i v e d from the F e / F e ^ a c t i v i t y r a t i o s a f t e r s p e c i e s d i s t r i b u t i o n and temperature c o r r e c t i o n s to the s t a n d a r d f e r r o u s - f e r r i c couple had been computed withWATEQ2. The N e r n s t e q u a t i o n used f o r the c a l c u lation is 2 +


Eh = Ε*

+

(T. Pe2+/Pe3+, - »J m F

|Iê£| [Fe ] 3 +

where the square b r a c k e t s i n d i c a t e a c t i v i t i e s , R i s the i d e a l gas c o n s t a n t , Τ i s the K e l v i n temperature, F i s the Faraday c o n s t a n t and E° i s the temperaturedependent 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 f o r the f e r r o u s - f e r r i c couple. The r e s u l t s are shown i n F i g u r e 3, where the s o l i d l i n e r e p r e s e n t s p e r f e c t agreement. Seventy-seven p e r c e n t of the values f a l l w i t h i n — 30 mv, which would be an a n t i c i p a t e d u n c e r t a i n t y f o r most Eh measurements. No value i s g r e a t e r than 80 mv from the other and over h a l f of the v a l u e s agree to w i t h i n — 10 mv, which i s twice the p r e c i s i o n of the emf r e a d i n g s . T h i s e x c e l l e n t agreement suggests a simultaneous v a l i d a t i o n of the e q u i l i b r i u m c o n d i t i o n of a c i d mine waters, of the c h e m i c a l model and i t s a b i l i t y to r e p r e s e n t that e q u i l i b r i u m , and of the a c c u r a c y of the Eh measure ment . S e v e r a l f e a t u r e s of t h i s s t u d y s h o u l d be p o i n t e d out. F i r s t , the r e l i a b i l i t y of the Eh measurement depends g r e a t l y on the t e c h n i q u e . We used a c l o s e d l i n e t o pump water from the stream to a f l o w - t h r o u g h c e l l where a continuous s u p p l y of f r e s h sample water made c o n t a c t with the e l e c t r o d e . No streaming p o t e n t i a l e f f e c t s were n o t i c e d because t h e r e was l i t t l e or no change i n p o t e n t i a l when the pumping was stopped. The l a c k of streaming poten t i a l was p r o b a b l y due to the slow flow v e l o c i t i e s and the h i g h i r o n c o n c e n t r a t i o n s . The method of using a c l o s e d flow-through c e l l supplying a large volume of water to the s u r f a c e of the Eh and pH e l e c t r o d e s has been found to be q u i t e s u c c e s s f u l f o r a wide range of n a t u r a l waters ( 5_3 ). We a l s o would commonly remove the p l a t i n u m e l e c t r o d e and g e n t l y b u f f the p o l i s h e d s u r f a c e with a s o f t c l o t h and r e p e a t the measurement i f we n o t i c e d any d r i f t . Second, we f e l t i t was v a l u a b l e t o see the amount of change i n the c a l c u l a t e d Eh when a c t i v i t y



3.


NORDSTROM E T A L .

c o e f f i c i e n t s and complexes were not c o n s i d e r e d , that i s , when measured t o t a l c o n c e n t r a t i o n s of f e r r o u s and f e r r i c i r o n , r a t h e r than t h e i r r e s p e c t i v e c a l c u l a t e d a c t i v i t i e s , were used to compute the Eh. We chose two measured Eh v a l u e s , one which agreed to w i t h i n 1 mv of the c a l c u l a t e d value and another which d i f f e r e d from the c a l c u l a t e d value by 79 mv. W i t h o u t a c t i v i t y and complexing c o r r e c t i o n s , the d i s c r e p a n c i e s i n c r e a s e d to 30 and 180 mv, respect i v e l y , a l a r g e e r r o r i n each case. I t appears that the l a r g e s t source of e r r o r i n t h e s e comparisons i s the a n a l y t i c a l data. The next l a r g e s t source of e r r o r seems to be the adequacy of a c t i v i t y c o e f f i c i e n t s and s t a b i l i t y c o n s t a n t s used i n the model and l a s t i s the r e l i a b i l i t y of the f i e l d Eh measurement. C l o s e i n s p e c t i o n of F i g u r e 3 shows a s l i g h t b i a s of c a l c u l a t e d Eh values towards more o x i d i z i n g p o t e n t i a l s . F e ( I I I ) complexes are q u i t e s t r o n g and i t i s l i k e l y that some important complexes, p o s s i b l y FeHSO| (5^,_55), should be i n c l u d e d i n the chemical model, but the t h e r modynamic data are not r e l i a b l e enough to j u s t i f y i t s use. F i n a l l y , we r e t u r n to a c l a r i f i c a t i o n of what p a r t s of the system are i n e q u i l i b r i u m . The comp a r i s o n of c a l c u l a t e d with measured Eh values g i v e s s t r o n g evidence f o r homogeneous s o l u t i o n e q u i l i b r i u m and f o r e q u i l i b r i u m c o n d i t i o n s at the s u r f a c e of a p l a t i n u m e l e c t r o d e i n these waters. To determine whether the D.O. content of the water r e l a t e d to these e q u i l i b r i a and whether the o x i d a t i o n of i r o n consumed oxygen from the stream water, we made measurements at the mouth of Boulder Creek and downstream i n S p r i n g Creek where most of the i r o n has been o x i d i z e d . We compared our D.O. measurements with the s a t u r a t i o n oxygen s o l u b i l i t y f o r the a p p r o p r i a t e temperature and b a r o m e t r i c pressure and found each p a r t of the stream very c l o s e to s a t u r a tion. The streams should c o n t a i n s a t u r a t e d amounts of D.O. s i n c e they are commonly t u r b u l e n t and w e l l mixed. The r e s u l t s i n Table I, however, show that the Eh c a l c u l a t e d using the O 2 / H 2 O c o u p l e i s c o n s i d e r a b l y higher than the measured v a l u e . The redox s t a t e of the water i s thus determined by the f e r r o u s - f e r r i c r a t i o ; oxygen d i s s o l v e d i n the water, w h i l e i n e q u i l i b r i u m with the atmosphere, i s not i n e q u i l i b r i u m with the f e r r o u s - f e r r i c c o u p l e . This s i t u a t i o n a l s o r e f l e c t s the f a c t that the exchange c u r r e n t f o r the 0 / H 0 c o u p l e i s f a r l e s s than f o r +

9

9



62


Figure

3.

Comparison of measured and calculated Eh values. Errors mV are given for the uncertainty in the Eh measurements.

of

SYSTEMS

±30

Figure 4. Distribution of dissolved iron redox species for varying concentrations of reduced and oxidized iron, (a) Hornet effluent; (b) Boulder Creek; (c) Spring Creek; (d) Spring Creek reservoir. Analyses for these four samples are shown in Table II.



2

B* 25.5 1.10 .622 65000 173 685 92.5 128 9050 2650 1400 130 60000

(Concentrations

1280 188 55 5310

6. 0 2. 00 378 9900 45 10 7 14. 13.8

G 5.5 2.50 .650 2970 17 27.5 6.00 3.75 260 354 47 26 1280

J

i n mg/L except where

noted)

208 47 32 1080 Creek.

5.5 2.90 .770 1810 18 30.8 5.40 2.40

Ν

2.32 11.8 .640 1.197

1.85 11.0 .625 1.227

3.02 10.5 .694 1.156

6.40 10.3 .385 0.948

Table II

(J) Spring Cr.

(G)Boulder Cr.

(C)Boulder Cr.

(H)Spring Cr.

Table I of measured Eh v a l u e s with those c a l c u l a t e d from d i s s o l v e d oxygen and the C^/HoO redox couple

*B - Hornet E f f l u e n t , G - B o u l d e r Creek, J - S p r i n g Creek, Ν - S p r i n g * * U n i t s f o r s p e c i f i c conductance are microSiemens.

9

T°C pH Eh (V) Spec. Cond.** Ca Mg Na Κ Fe ( I I ) Fe ( I I I ) Al Si0 s o /

Site

2

pH D.O.(mg/L) Eh, measured (V) Eh, 0 / H 0 (V)

Comparison


64


2 +

3 +

the F e / F e the measured

c o u p l e (_43) potential.

and

does not

SYSTEMS

affect


S o l u t i o n Complexes of I r o n i n A c i d Mine Waters The wide range of Eh, pH and d i s s o l v e d s o l i d s f o r the a c i d mine waters i n the I r o n Mountain watershed p r o v i d e s an o p p o r t u n i t y to survey the dominant complexes which are c a l c u l a t e d from an ion a s s o c i a t i o n model. Rather than choosing some "average" composition f o r a c i d mine water, we have used n a t u r a l l y - o c c u r r i n g waters which r e p r e s e n t the range of c o n d i t i o n s found i n the study s i t e . The a n a l y s e s shown i n Table II are a s e r i e s of water samples c o n t a i n i n g the most f e r r o u s - r i c h to the most f e r r i c - r i c h in iron content. These a n a l y s e s show p r o g r e s s i v e downstream d i l u t i o n and o x i d a t i o n b e g i n n i n g with the source water coming from the Hornet t u n n e l which d i s c h a r g e s i n t o Boulder Creek (see F i g u r e 2 ) . The two S p r i n g Creek samples were taken at 1) a p o i n t j u s t below Boulder Creek c o n f l u e n c e and 2) a p o i n t 4 km below the Boulder Creek c o n f l u e n c e at the mouth to the S p r i n g Creek Reservoir. The l i s t of i r o n complexes which were used i n the aqueous modeling computations are shown i n T a b l e I I I . Of these complexés, the major ones are expected to be hydroxide and s u l f a t e . The d i s t r i b u t i o n of s p e c i e s i s shown i n F i g u r e 4 as percentages of the t o t a l d i s s o l v e d i r o n . Only species which c o n t r i b u t e d 0.2 p e r c e n t or more of the t o t a l d i s s o l v e d i r o n were i n c l u d e d . From F i g u r e 4 , i t i s c l e a r t h a t o n l y the FeSO£ c o n t r i b u t e s to the complexing of f e r r o u s i r o n but t h a t t h i s complex alone can c o n s t i t u t e up to n e a r l y 50 p e r c e n t of the t o t a l ferrous iron. F e r r i c i r o n i s much more h i g h l y complexed than f e r r o u s and under most c o n d i t i o n s the f r e e f e r r i c i r o n i s never more than about 8 p e r c e n t of the t o t a l i r o n . FeSO+ i s always the dominant f e r r i c complex, f o l l o w e d by F e i S O ^ - > F e ( O H ) > Fe2(OH)4+ > Fe(OH)+ i n order of d e c r e a s i n g abundance. An o b j e c t i o n might be r a i s e d t h a t o r g a n i c complexing p l a y s an important r o l e i n the s p e c i e s d i s t r i b u t i o n of these waters. T h i s p o s s i b i l i t y i s c e r t a i n l y very r e a l , and there i s some evidence that the d i s s o l v e d o r g a n i c carbon i n these waters i s very h i g h , but the good agreement on the Eh c a l c u l a t i o n s and the low pH v a l u e s of these waters suggest that any o r g a n i c s p r e s e n t are p r o b a b l y f u l l y p r o t o n a t e d and have o n l y a minor e f f e c t on complexing. 2 +

Any

chemical

modeling of a c i d mine waters



b)

a)

complexes

and s o l i d

2

2 +

,

2

2

3

4 } 2

3

(0H)

2

6

Fe(OH)

3

4

F e r r i c hydroxide, amorphous

2

KFe (S0

2

Jarosite

2

F e ( S 0 ) « 9H 0

6

Coquimbite

4

3

Fe F 3+(S0 ) (OH) ·20H 0 e

F e F

Copiapite

2 +

4

2'

FeF

FeS0 « 7H 0

2

3

f o r computation

Fe (OH)5+ FeSO+, F e ( S 0

considered

Fe(OH)°, Fe(OH)-, Fe (OH)4+,

FeCl+, FeCl°, FeF +,

Melanterite

Solids:

FeCl

FeOH +, Fe(OH)+,

FeSO°

Table I I I phases of i r o n

FeOH+, Fe(OH)°, Fe(OH)~,

Complexes:

Aqueous


4

66


r e l i e s h e a v i l y on a c c u r a t e thermodynamic data f o r i r o n s u l f a t e complexes. U n f o r t u n a t e l y , data f o r i o n t r i p l e t s such as FeCSC^)" are not a c c u r a t e l y known and p o s s i b l e complexes such as FeHSO| and Fe(HS04)2 h & been p r o p e r l y i d e n t i f i e d . B e t t e r data are needed f o r these s p e c i e s i n o r d e r to enhance the c a p a b i l i t y of g e n e r a l i z e d c h e m i c a l models. +

v e


Water-Mineral

n

o

t

Equilibrium Relationships

A s u i t e of both o x i d i z e d and reduced i r o n m i n e r a l s has been found as e f f l o r e s c e n c e s and p r e c i p i t a t e s i n or near the a c i d mine water of Iron Mountain. The dominant m i n e r a l s tend to be melant e r i t e (or one of i t s d e h y d r a t i o n p r o d u c t s ) , c o p i a p i t e , j a r o s i t e and i r o n h y d r o x i d e . These m i n e r a l s and t h e i r chemical formulae are l i s t e d i n T a b l e I I I from the most f e r r o u s - r i c h at the top to the most f e r r i c - r i c h at the bottom. These m i n e r a l s were c o l l e c t e d i n a i r - t i g h t c o n t a i n e r s and i d e n t i f i e d by X-ray d i f f r a c t o m e t r y . I t was a l s o p o s s i b l e to check the m i n e r a l s a t u r a t i o n i n d i c e s ( l o g ( A P / K ) , where AP = a c t i v i t y product and Κ = s o l u b i l i t y product c o n s t a n t ) o f the mine waters with the f i e l d o c c u r r e n c e s of the same m i n e r a l s . By c o n t i n u a l checking of the s a t u r a t i o n index (S.I.) w i t h a c t u a l minéralogie o c c u r r e n c e s , i n a c c u r a c i e s i n c h e m i c a l models such as WATEQ2 can be d i s c o v e r e d , e v a l u a t e d and c o r r e c t e d (19), p r o v i d e d t h a t these o c c u r r e n c e s can be assumed to be an approach towards e q u i l i b r i u m . We have chosen an e q u i l i b r i u m zone around the S . I . equal to the e s t i m a t e d u n c e r t a i n t y of the s o l u b i l i t y product c o n s t a n t . O u t s i d e of these l i m i t s , the s o l u t i o n i s e i t h e r s u p e r s a t u r a t e d ( p o s i t i v e d i r e c t i o n ) or u n d e r s a t u r a t e d ( n e g a t i v e d i r e c t i o n ) . The S.I. v a l u e s were then p l o t t e d vs. the l o g cond u c t i v i t y to show approaches to m i n e r a l s a t u r a t i o n as an approximate f u n c t i o n of the t o t a l d i s s o l v e d solids. C o n d u c t i v i t y was chosen as a convenient parameter because 1) i t p o i n t s out d i l u t i o n t r e n d s or u n d e r s a t u r a t i o n and corresponds to a s p a t i a l t r e n d f o r downstream s i t e s , 2) i t changes more cons i s t e n t l y than other parameters, and 3) i t covers a l a r g e r range of v a l u e s than Eh, pH or an i n d i v i d u a l ion. A c t i v i t y diagrams (_53J c o u l d have been used but then c e r t a i n assumptions have to be made about the m i n e r a l s t a b i l i t y boundaries such as constant a c t i v i t y of water or of pH or of s u l f a t e . Melanterite. The most c o n c e n t r a t e d a c i d mine i n



3.

NORDSTROM E T A L .


waters, such as those f l o w i n g from the Hornet tunn e l , commonly p r e c i p i t a t e m e l a n t e r i t e , a pure f e r r o u s s u l f a t e , along t h e i r edges. Water which had i n f i l t r a t e d a massive ore body would a l s o p r e c i t i pate m e l a n t e r i t e along the s u r f a c e of an open cut exposed to dry o u t s i d e a i r . But m e l a n t e r i t e was found o n l y on s u l f i d e s u r f a c e s which were r e g u l a r l y washed by r a i n f a l l or running water. In s h e l t e r e d a r e a s , such as the entrance to the Richmond mine, the dominant m i n e r a l o c c u r r i n g on open cuts i s c o p i a p i t e or a mixture of c o p i a p i t e and coquimbite. In F i g u r e 5 we have p l o t t e d S.I. v a l u e s f o r m e l a n t e r i t e i n d i c a t i n g a t r e n d towards s a t u r a t i o n f o r the Hornet e f f l u e n t ( l a b e l e d B). A l l of the o t h e r waters ( c o l l e c t e d at downstream s i t e s ) have been d i l u t e d and o x i d i z e d and t h e r e f o r e appear undersaturated. The r e s u l t s of these c a l c u l a t i o n s compare q u i t e f a v o r a b l y with f i e l d o b s e r v a t i o n s . U n f o r t u n a t e l y , there i s a l a r g e u n c e r t a i n t y assoc i a t e d with the thermodynamic s o l u b i l i t y constant for melanterite. Although i t s s o l u b i l i t y i s w e l l known, the thermodynamic e q u i l i b r i u m constant i s d i f f i c u l t to o b t a i n because the compound i s h i g h l y s o l u b l e and t h e r e f o r e becomes s a t u r a t e d o n l y at high ionic strenths. More i m p o r t a n t l y , i t should be noted that m e l a n t e r i t e commonly forms on rock s u r f a c e s , espec i a l l y along f r a c t u r e s where water can be p u l l e d up from a w a t e r - s a t u r a t e d zone to the s u r f a c e by c a p i l l a r y f o r c e s . Thus, m e l a n t e r i t e s a t u r a t i o n i s o c c u r r i n g i n a microenvironment i n many i n s t a n c e s which makes sampling and i n t e r p r e t a t i o n d i f f i c u l t . Copiapite. T h e o r e t i c a l l y , m e l a n t e r i t e can o x i d i z e to form c o p i a p i t e by the r e a c t i o n : 5FeS0 -7H 0 + 0 4

2 +

2

+ H S0

2

2

4

+

F e F e 3 ( S 0 ) ( O H ) - 2 0 H 0 + 15H 0„ 4

6

2

2

2

As mentioned e a r l i e r , l a r g e amounts of c o p i a p i t e have been found accumulating on ore s u r f a c e s i n areas p r o t e c t e d f o r d i r e c t r a i n f a l l . The mechanism of i t s f o r m a t i o n i s not at a l l c l e a r but from f i e l d o b s e r v a t i o n s i t appears that a c i d mine water i s b e i n g drawn by c a p i l l a r y f o r c e s to an exposed s u r f a c e where i t q u i c k l y evaporates to m e l a n t e r i t e and/or c o p i a p i t e . Coquimbite i s i n t i m a t e l y assoc i a t e d with c o p i a p i t e and these two m i n e r a l s appear t o be v e r y s t a b l e as long as they are p r o t e c t e d from r a i n f a l l or running water. No thermodynamic data


68

C H E M I C A L M O D E L I N G I N AQUEOUS SYSTEMS


SATURATION

ZONE

JG

J

tf

UNDERSATURATED

N

Ι'ΛΜ

ΪΊΜΟ

ΓΘΟ

ËTëô

?ΙΘ5

^ôô

N

Ν

N

Jâô

Jâô

«Tiëô

TTëô

5.x

LOG CONDUCTIVITY Figure 5. Saturation indices for melanterite plotted as a function of the log conductivity showing an approach to saturation for the most concentrated ferrousrich waters of the Hornet effluent (B)



3.

NORDSTROM E T A L .


are a v a i l a b l e on e i t h e r c o p i a p i t e or coquimbite so t h a t S.I. v a l u e s cannot be c a l c u l a t e d . C o p i a p i t e c o n t a i n s both f e r r o u s and f e r r i c i r o n but a form c a l l e d f e r r i c o p i a p i t e c o n t a i n i n g o n l y i r o n i n the o x i d i z e d f e r r i c s t a t e may a l s o occur. There i s l i t t l e i n f o r m a t i o n on the r e l a t i v e abundance of these m i n e r a l s nor whether c o p i a p i t e i s a n e c e s s a r y p r e c u r s o r to the f o r m a t i o n of f e r r i c o p i a p i t e and coquimbite. A f u r t h e r c o m p l i c a t i o n to the i n t e r p r e t a t i o n of m i n e r a l r e a c t i o n s i s the s o l i d s u b s t i t u t i o n p r o p e r t i e s of c o p i a p i t e . I t not o n l y can have a change i n the f e r r o u s / f e r r i c r a t i o but the d i v a l e n t and t r i v a l e n t s i t e s can accept a l a r g e number of d i v a l e n t and t r i v a l e n t ions such as Mg , C u , Z n , C d + and A l + . Jarosite. The waters which are a s s o c i a t e d with such s o l u b l e s a l t s as m e l a n t e r i t e and c o p i a p i t e com monly have pH v a l u e s of 0.8 t o 1.5. As the pH r i s e s to the range of 1.5 t o 2.5, j a r o s i t e w i l l commonly p r e c i p i t a t e i f the d i s s o l v e d i r o n content i s high enough. A f i n e - g r a i n e d y e l l o w p r e c i p i t a t e forms i n B o u l d e r Creek e v e r y summer and f a l l , which when washed and f i l t e r e d , g i v e s a good X-ray p a t t e r n f o r potassium j a r o s i t e . This mineral i s p r e c i p i t a t i n g d i r e c t l y from the water and i t appears to have good crystallinity. Both c o p i a p i t e and j a r o s i t e are a b r i g h t y e l l o w c o l o r and may e a s i l y be confused with sulfur. In F i g u r e 6,the S.I. v a l u e s f o r potassium j a r o s i t e are g i v e n f o r a l l the samples. The values show s u p e r s a t u r a t i o n f o r n e a r l y every sample and prompts us to ask s e v e r a l q u e s t i o n s : (a) are these waters t r u l y s u p e r s a t u r a t e d and i s t h i s a k i n e t i c r e q u i r e ment f o r p r e c i p i t a t i o n ? (b) are o r g a n i c complexes important f o r these waters? (c) are the t h e r modynamic data r e l i a b l e ? Some of these q u e s t i o n s can be answered. F i r s t , the amount of supers a t u r a t i o n i s more than the ί 1.1 log Κ u n i t u n c e r t a i n t y i n the thermodynamic d a t a . Second, the model does not appear to be inadequate f o r the Eh comparison and o t h e r c a l c u l a t i o n s , and i f o r g a n i c complexing were i n t r o d u c i n g a s i g n i f i c a n t e r r o r i t would have shown up i n the c a t i o n - a n i o n balance or other c a l c u l a t i o n s . Downstream i n S p r i n g Creek where no j a r o s i t e has been found the S.I. i s o f t e n more s u p e r s a t u r a t e d than i n Boulder Creek. This s u p e r s a t u r a t i o n suggests t h a t j a r o s i t e s o l u b i l i t y does not c o n t r o l the water c h e m i s t r y of any of the streams. The remaining p o s s i b i l i t y i s that j a r o s i t e i s p r e c i p i t a t i n g o n l y i n microenvironments where the 2 +

2 +

2 +

2

3



70


Ν

Ν

N

L

h 7

K

j

NL

Ν N N N

e J

SYSTEMS

„

\

J

j

4

'

J

6

L

F

SAT U RATION ZON Ε

1.00

1.40

1.80

ë\2Ô

2\60sTÔÔ

^^40

3^»

TTËÔ

7^60

δ". 00

LOG CONDUCTIVITY Figure

6.

Saturation indices for jarosite plotted as a function of the log tivity showing supersaturation for nearly all values

conduc



3.

NORDSTROM E T AL.


water c h e m i s t r y i s d i f f e r e n t than i n the bulk s o l u tion. The evidence f o r t h i s i s that j a r o s i t e p r e c i p itation i s always found on stream bed s u r f a c e s where b a c t e r i a have been growing, and i s a l s o found i n s i d e of a c i d s l i m e streamers produced by these bacteria. Beyond t h i s , there may be important par t i c l e s i z e e f f e c t s (JJ2 ). There c o u l d s t i l l be l o c a l e q u i l i b r i u m c o n d i t i o n s e x i s t i n g here, but they are p r o b a b l y below the r e s o l u t i o n of the sampling t e c h niques. C l e a r l y , more work i s needed before the j a r o s i t e g e o c h e m i s t r y can be a d e q u a t e l y i n t e r p r e t e d . Amorphous f e r r i c h y d r o x i d e . Most of S p r i n g Creek below the Boulder Creek c o n f l u e n c e i s i r o n s t a i n e d and a c t i v e l y p r e c i p i t a t e s i r o n h y d r o x i d e . The pH v a l u e s are commonly i n the range of 2.5 t o 3.5 depending on the season. The i r o n p r e c i p i t a t e s are amorphous to X-rays and appear to be amorphous f e r r i c hydroxide from t h e i r c o l o r and t e x t u r e . S.I. v a l u e s c a l c u l a t e d f o r amorphous f e r r i c hydroxide show a t r e n d towards s a t u r a t i o n with downstream d i l u t i o n and o x i d a t i o n ( F i g u r e 7 ) . We have used the upper s o l u b i l i t y l i m i t of pK = 37.1, where pK i s the apparent s t a b i l i t y c o n s t a n t f o r f e r r i c oxyhydroxides (12) and most of the downstream s i t e s ( l a b e l e d Ν) f a l l n e a t l y i n t o the r e g i o n of pK = 37 t o 39 as would be expected f o r a f r e s h l y p r e c i p i t a t e d material. Very l i t t l e s u p e r s a t u r a t i o n appears i n our computations. A g a i n , the S.I. c a l c u l a t i o n s match v e r y w e l l with the f i e l d o b s e r v a t i o n s and with p r e v i o u s s t u d i e s of f e r r i c hydroxide e q u i l i b r i u m i n n a t u r a l waters ( l ^ l _ 5 f l Z ) . noticeable hiatus o c c u r s at about l o g Κ = 41. Below t h i s value the samples p l o t t e d i n F i g u r e 7 are very high i n f e r r o u s i r o n , low i n pH and show no s i g n of f e r r i c hydroxide precipitation. A

Conclusions We have u t i l i z e d a chemical model i n t h i s i n v e s t i g a t i o n to i n t e r p r e t the e q u i l i b r i u m b e h a v i o r of i r o n i n a c i d mine waters. A successful correla t i o n between c a l c u l a t e d and measured Eh v a l u e s has been found, u s i n g WATEQ2, the computerized i o n a s s o c i a t i o n model. This c o r r e l a t i o n supports the b a s i c assumption of homogeneous s o l u t i o n e q u i l i b r i u m i n these waters and s i m u l t a n e o u s l y c o r r o b o r a t e s both the v a l i d i t y of the aqueous model and the q u a n t i t a t i v e i n t e r p r e t a t i o n of Eh measure ments i n these waters. T h i s i n t e r p r e t a t i o n makes i t p o s s i b l e to c a l c u l a t e the d i s t r i b u t i o n of i r o n


CHEMICAL MODELING IN AQUEOUS

72


SUPERSATURATED

p

SYSTEMS

K-37

c u

κ

J

ι'.8θ

e'.eo

LOG

e'.eo

i.oo

M.eO

4.60

CONDUCTIVITY

Figure 7. Saturation indices for amorphous ferric hydroxide plotted as a function of the log conductivity showing an approach to saturation for the more dilute and more oxidized downstream waters



3.

NORDSTROM E T

AL.


73

s p e c i e s based on Eh measurements when f e r r o u s - f e r r i c a n a l y s e s are not a v a i l a b l e . A l t h o u g h the stream waters show e q u i l i b r i u m s a t u r a t i o n with d i s s o l v e d oxygen, there i s no e q u i l i b r i u m between the d i s s o l v e d oxygen and f e r r o u s - f e r r i c redox c o u p l e . M i n e r a l s a t u r a t i o n i n d i c e s f o r m e l a n t e r i t e and amorphous i r o n h y d r o x i d e agree q u i t e w e l l with f i e l d o c c u r r e n c e s of the same m i n e r a l s . J a r o s i t e , how e v e r , appears to be s u p e r s a t u r a t e d f o r n e a r l y a l l of the samples r e g a r d l e s s of the presence or absence of the m i n e r a l i n these streams. F i e l d observations i n d i c a t e that j a r o s i t e p r e c i p i t a t i o n occurs i n the microenvironment of b a c t e r i a l c o l o n i e s where the c h e m i c a l c o n d i t i o n s may be q u i t e d i f f e r e n t from the bulk s o l u t i o n . These c o n s i d e r a t i o n s l e a d us to suggest that there i s a k i n e t i c b a r r i e r which h i n ders j a r o s i t e p r e c i p i t a t i o n but does not h i n d e r f e r r i c hydroxide p r e c i p i t a t i o n and that t h i s b a r r i e r i s overcome by the s u r f a c e s of b a c t e r i a l c o l o n i e s (both i r o n - o x i d i z e r s and u n i d e n t i f i e d nonoxidizers). The c h e m i c a l e q u i l i b r i a approach used i n t h i s s t u d y have enabled us to i d e n t i f y which p a r t s of an a c i d mine d r a i n a g e system are i n e q u i l i b r i u m and which p a r t s are not. Our r e s u l t s have p r o v i d e d g r e a t e r i n s i g h t i n t o the chemical p r o c e s s e s of a c i d mine waters i n p a r t i c u l a r and the redox r e l a t i o n s of iron in general. Acknowledgements The r e s e a r c h i n t h i s paper was done i n p a r t i a l f u l f i l l m e n t of a Ph.D. d i s s e r t a t i o n completed by the s e n i o r author. Support from the U.S. Geological Survey, e s p e c i a l l y Bob A v e r e t t , Rich F u l l e r , Jo B u r c h a r d , D o r i s York, Dave V i v i t and the s t a f f of the Redding F i e l d O f f i c e , are g r a t e f u l l y acknowl edged. C r i t i c a l comments by G. A. Parks, Κ. B. Krauskopf, J . 0. L e c k i e and B. H a l l e t are g r e a t l y appreciated. We a l s o wish to acknowledge the c a r e f u l review of t h i s manuscript by L. N. Plummer and D. C. T h o r s t e n s o n , both of the U.S. Geological Survey. We a l s o thank Janet B a t t e n f o r t y p i n g the manuscript. Abstract A d e t a i l e d hydrogeochemical i n v e s t i g a t i o n of 14 km of stream waters contaminated by a c i d mine drainage i n Shasta County, California p r o v i d e s i n s i g h t i n t o the


74

CHEMICAL MODELING IN AQUEOUS SYSTEMS

equilibrium transformations of iron during the oxi dative weathering of p y r i t i c ore. Over 60 water analyses covering a range of pH, redox state, total iron concentration, temperature and ionic strength were processed with the computerized chemical model WATEQ2 so that a c t i v i t i e s of free ions and complexes and satutation indices could be determined. The results demonstrate (a) measured Eh values agree quite well with Eh values calculated from the ferrous/ferric a c t i v i t y r a t i o , (b) the dominant complexes in acid mine waters are FeS°4, FeSO+4, Fe(S0 )-2 Fe(OH) , Fe (OH)4+2 and Fe(OH)+2 and (c) acid mine waters low in pH (1 to 2) and high in reduced iron approach saturation with respect to melanterite whereas progressive downstream oxidation and dilution (pH = 2 to 3.5) of these waters forces these waters to become saturated with respect to amorphous iron hydroxide. For these two minerals, the f i e l d observations match the saturation calculations very well. However, a l l of the waters show super -saturation with respect to jarosite regardless of the presence or absence of jarosite in the stream waters. These results suggest a strong kinetic hindrance to the precipitation of jarosite. 2+


4

2

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CHEMICAL MODELING IN AQUEOUS SYSTEMS

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Disclaimer: The reviews expressed and/ or the products mentioned in this article represent the opinions of the author(s) only and do not necessarily represent the opinions of the U.S. Geological Survey. RECEIVED

November 16, 1978.


Chemical Modeling in Aqueous Systems - ACS Publications

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