Chemical Modeling in Aqueous Systems - American Chemical Society

37

Chemical

Modeling

of T r a c e M e t a l E q u i l i b r i a i n

Contaminated Soil Solutions Program

Using

the Computer

GEOCHEM

Downloaded by CORNELL UNIV on September 26, 2016 | http://pubs.acs.org Publication Date: March 19, 1979 | doi: 10.1021/bk-1979-0093.ch037

SHAS V. MATTIGOD and GARRISON SPOSITO Department of Soil and Environmental Sciences, University of California, Riverside, CA 92521 The study of trace metal chemistry i n s o i l s has assumed added importance r e c e n t l y because of the p o s s i b l e d e l e t e r i o u s e f f e c t s on s o i l organisms, p l a n t s , and human beings that could r e s u l t from the accumulation of these metals t o t o x i c l e v e l s i n s o i l s contaminated, e.g., by the land d i s p o s a l o f sewage sludges or by the seepage of geothermal brines l o s t a c c i d e n t l y from storage lagoons. Although the s o i l s o l u t i o n i s an open, dynamic, n a t u r a l water system whose composition r e f l e c t s many r e a c t i o n s o c c u r r i n g simultaneously among i t s aqueous c o n s t i t u e n t s and between those c o n s t i t u e n t s and the assembly of mineral and o r ganic s o l i d phases present, a knowledge of the general features of s o i l trace metal e q u i l i b r i a i s expected t o be a u s e f u l guide to p r e d i c t i n g what w i l l occur i n nature i f contamination takes place. These general features cannot be assessed conveniently by laboratory experiments because of the complexity of s o i l s o l u t i o n s . A more v i a b l e a l t e r n a t i v e i s assessment by a computer model that i s based on thermodynamic a s s o c i a t i o n and s o l u b i l i t y product constants. The computer program GEOCHEM i s adapted and being developed for s o i l s o l u t i o n s from the REDEQL2 program created o r i g i n a l l y by Morel and Morgan and t h e i r coworkers at Caltech (jL) · The p r i n c i p a l ways i n which GEOCHEM d i f f e r s from REDEQL2 are: 1) i t includes thermodynamic data f o r a few hundred a d d i t i o n a l s o l u b l e complexes and s o l i d s that are p a r t i c u l a r l y relevant t o trace metal e q u i l i b r i a i n s o i l , 2) i t contains a subroutine f o r c a t i o n exchange on constant charge surfaces that i s based on a thermo dynamic model, and 3) i t contains a subroutine f o r the e s t i mation of s i n g l e - i o n a c t i v i t y c o e f f i c i e n t s at i o n i c strengths up to 3 M. A number of important t h e o r e t i c a l problems have a r i s e n i n connection w i t h the development of GEOCHEM. These problems may be c l a s s i f i e d broadly i n t o four c a t e g o r i e s : (a) s t a b i l i t y con stants f o r trace metal complexes with inorganic l i g a n d s ; (b) s t a b i l i t y constants f o r trace metal complexes with organic l i g a n d s ; (c) s o l u b i l i t y product constants f o r s o i l c l a y m i n e r a l s , and (d) thermodynamic c a t i o n exchange constants and exchanger 0-8412-0479-9/79/47-093-837$05.00/0

© 1979 American Chemical Society

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

838

CHEMICAL

MODELING IN AQUEOUS

SYSTEMS

phase a c t i v i t y c o e f f i c i e n t s . Any reasonably accurate computa t i o n of trace metal e q u i l i b r i a i n a s o i l s o l u t i o n must be pre ceded by a s u c c e s s f u l attack on these four problem areas.


S t a b i l i t y Constants f o r Soluble Inorganic Complexes of Trace Metals Studies i n v o l v i n g t r a c e metal s p e c i a t i o n i n s o i l s o l u t i o n s r e q u i r e the values of s t a b i l i t y or a s s o c i a t i o n constants of complexes of the trace metals with a number of i n o r g a n i c and organic ligands which e x i s t i n these environments. Much of these data can be obtained r e a d i l y from p u b l i s h e d compilations (_2-7.). However, experimental data on a s s o c i a t i o n constants for complexes of the trace metals w i t h ligands such as CO^ , HC0 ", P0 3~ HPO^ ", t^PO^" are not o f t e n a v a i l a b l e . I n view of t h i s lack o f experimental data, modeling s t u d i e s of chemical e q u i l i b r i u m i n n a t u r a l water systems have t y p i c a l l y followed two approaches. One of the approaches, represented by computer programs such as SOLMNEQ and WATEQ (8^, 9 ) , i s t o i n c l u d e only those complexes f o r which the values of experimental a s s o c i a t i o n constants are a v a i l a b l e . The second approach c o n s i s t s of i n c l u d i n g i n the e q u i l i b r i u m c a l c u l a t i o n s estimates of the values of the a s s o c i a t i o n constants of complexes that are b e l i e v e d t o form, but f o r which experimental data do not yet e x i s t , along w i t h experimentally d e r i v e d values of a s s o c i a t i o n constants (10, 11). The l a t t e r approach seems p r e f e r a b l e because i t tends to i n c l u d e i n the c a l c u l a t i o n s a l l known i n t e r a c t i o n s of metals and ligands through complex formation and thus provides r e s u l t s which are expected t o be q u a l i t a t i v e l y more r e l i a b l e than the r e s u l t s obtained through the former approach. The methods a v a i l a b l e f o r c a l c u l a t i n g i o n a s s o c i a t i o n constants c o n s i s t of t h e o r e t i c a l methods based on e l e c t r o s t a t i c s (12, 13) and e m p i r i c a l methods based on c o r r e l a t i o n s of a s s o c i a t i o n constants w i t h p r o p e r t i e s of ions such as e l e c t r o n e g a t i v i t y , charge, r a d i i , c o o r d i n a t i o n number, e t c . (14, 15, 16, 17, 18). Recently, Mattigod and S p o s i t o (19) estimated the a s s o c i a t i o n constants f o r complexes of a number of cations of the f i r s t t r a n s i t i o n metal s e r i e s w i t h many of the i n o r g a n i c ligands considered to be of importance i n the t r a c e metal chemistry of s o i l s o l u t i o n s . The a s s o c i a t i o n constants c a l c u l a t e d by Mattigod and Sposito (19) w i t h the method of Rester and Pytkowicz (13), f o r carbonate and phosphate complexes appear t o be sys t e m a t i c a l l y d i f f e r e n t from those estimated r e c e n t l y w i t h the c o r r e l a t i o n method of Nieboer and McBryde (17). The l a t t e r method has a f i r m e r e m p i r i c a l b a s i s and, t h e r e f o r e , has been p r e f e r r e d . Values of the r e v i s e d estimated and measured assoc i a t i o n constants f o r some t r a c e metal-inorganic l i g a n d complexes are l i s t e d i n Table I . These and other estimated values have been incorporated i n t o GEOCHEM. They w i l l be replaced when r e l i a b l e experimental values become a v a i l a b l e . 2

3

4

s


37.

Computer Program

M A T T I G O D A N D SPOSITO

GEOCHEM

839


There i s some experimental evidence which i n d i c a t e s that mixed-ligand complexes of t r a c e metals may be very s t a b l e and dominant i n s o i l s (20). I n modeling the s p e c i a t i o n of elements i n sea water, Dyrssen and Wedborg (10) found that mixed-ligand complexes of c e r t a i n t r a c e metals were s i g n i f i c a n t . At present, experimental data on mixed-ligand complexes are r a t h e r sparse. However, s t a b i l i t y constants can be estimated by a s t a t i s t i c a l method suggested by Dyrssen, Jagner and Wengelin (21). C u r r e n t l y , GEOCHEM i n c l u d e s estimated s t a b i l i t y constants of hydroxy-chloro complexes of a few trace metals. Lack of data on many mixed-ligand complexes i s c l e a r l y one of the s i g n i f i c a n t shortcomings i n modeling s t u d i e s at present.

Table I . Estimated and measured common logarithms of a s s o c i a t i o n constants f o r some t r a c e m e t a l - i n o r g a n i c l i g a n d i o n p a i r s at 25°C, 1 atm

Co

.2+ Ni

Cu

5.31

5.53

5.78

6.73

2.72

2.89

3.08

4.29

2.79

7.93

8.13

8.37

9.85

8.02

2+ Mn

_ 2+ Fe

co ~

4.52

HCO3-

1.95

Ligand 3

2

3

7.19 HP0

4 4

2

3.58 1.35

H P0 " 2

4

(22),

^ S c h i n d l e r et a l 3

Morgan

(24),

4

2

+

3.60

5

3.04

2.93

2.70

5

1.49

1.53

6

η

Zn

4.76

1

3.20

2+

6

1.76

3.30 1.60

2

7

7

2 , Huston and Stumm (23) Bilinski

.

Smith and A l b e r t y (25),

S i g e l et a l (27_) ,

Stability Metals

r,

Nriagu

Nriagu (26),

(28)

Constants f o r Soluble Organic Complexes of Trace

The s o l u b l e , metal-complexing, organic f r a c t i o n of sewage sludge i s a heterogeneous assembly of molecules that i s charac t e r i z e d by wide ranges of chemical composition, molecular



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CHEMICAL

M O D E L I N G IN

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weight, and f u n c t i o n a l group a c i d i t y (29). This complexity poses a d i f f i c u l t problem f o r the chemical modeling of s o i l s o l u t i o n s contaminated by the i n c o r p o r a t i o n of trace metalbearing sludges. A s u c c e s f u l model w i l l r e q u i r e , i n p a r t i c u l a r , information about the many p o s s i b l e r e a c t i o n s of trace metals w i t h the f u l v i c a c i d f r a c t i o n of sludges. The d i s s o c i a t i o n of protons from f u l v i c a c i d (FA) e x t r a c t e d from a n a e r o b i c a l l y - d i g e s t e d sewage sludge has been i n v e s t i g a t e d by p o t e n t i o m e t r i c t i t r a t i o n (29). I n that study, a number of important f a c t s was brought to l i g h t which suggest that the behavior of f u l v i c m a t e r i a l i n a s o i l s o l u t i o n w i l l indeed be complex. According to Sposito and Holtzclaw (29), there appear to be four separate c l a s s e s of d i s s o c i a b l e f u n c t i o n a l groups that range i n a c i d i t y from very strong ( i o n i z e d at pH < 2) to very weak ( i o n i z e d at pH > 10). (These c l a s s e s are j u s t four out of a continuum of c l a s s e s of f u n c t i o n a l group a c i d i t y i n sludge-derived FA, since no a c i d or a l k a l i n e f i n a l end p o i n t s appear i n the t i t r a t i o n curve between pH 1 and 11.) Based on t h i s evidence, Sposito et a l . (30), suggested that the a c i d i c f u n c t i o n a l group c l a s s e s i n sludge-derived FA be designated I, I I , I I I and IV f o r those groups that t i t r a t e approximately i n the pH ranges < 3, 3-5, 5-8, > 8, r e s p e c t i v e l y . The r e a c t i o n s between sludge-derived f u l v i c m a t e r i a l and t r a c e metals are poorly understood and, at present, no r e l i a b l e thermodynamic s t a b i l i t y constants are a v a i l a b l e . A p r o v i s i o n a l approach to r e s o l v i n g t h i s d i f f i c u l t y , which has been employed o f t e n i n modeling s t u d i e s , i s to i d e n t i f y c a r e f u l l y c e r t a i n c l a s s e s of known organic acids whose proton d i s s o c i a t i o n con s t a n t s f a l l i n t o the ranges observed f o r the s o i l s o l u t i o n organics and which can be expected to be present or to simulate c l o s e l y the organic acids present (whether " n a t u r a l " or from contamination) i n a s o i l s o l u t i o n . Table I I l i s t s a set of aromatic, a l i p h a t i c , and amino acids that are f r e q u e n t l y observed i n s o i l leachates, together w i t h the r e l a t i v e concentrations of each i n a mixture of model water-soluble s o i l organics that provides the t o t a l of 2.2 meq of H found per gram of sludged e r i v e d FA (30). I t has been observed that the f u n c t i o n a l groups i n these acids are s i m i l a r to the f u n c t i o n a l groups present i n sludge f u l v i c m a t e r i a l . The measured s t a b i l i t y constants f o r trace metal complexes w i t h these model organic acids are assumed to be good approximations to the unknown s t a b i l i t y constants f o r the assembly of s o i l s o l u t i o n organics, i n the sense that a mixture of the model organic acids whose proton t i t r a t i o n curve s e m i q u a n t i t a t i v e l y simulates that of sludge-derived FA (Figure 1) w i l l a l s o produce a comparable d i s t r i b u t i o n of various trace metals among organic and i n o r g a n i c species i n a s o i l s o l u t i o n . The s e v e r a l obvious l i m i t a t i o n s and dangers i n t h i s p r o v i s i o n a l approach, p r e s e n t l y used i n GEOCHEM, are not considered to be as serious as completely n e g l e c t i n g the organic s p e c i a t i o n of trace metals i n a s o i l



Computer Program GEOCHEM

841


I.OOp

-11.00-10.00 -9.00 -8.00 -7.00 -6.00 -5.00 -4.00 -3.00 -2.00

log [H ] +

Figure 1. Comparison between the potentiometric titration curve for a sewagesludge-derived fulvic acid ( ) and the simulated titration curve for a model fulvic acid ( )


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M O D E L I N G IN

AQUEOUS

SYSTEMS

s o l u t i o n whose organic c o n s t i t u e n t s may approach 10 M i n t o t a l c o n c e n t r a t i o n (e.g., a s o i l s o l u t i o n a f f e c t e d by sewage sludge application)·

Table I I .


Organic acids used as a model f o r sludge-derived f u l v i c a c i d

Acid

Benzenesulfonic Salicylic Phthalic Citric Maleic Ornithine Lysine Valine Arginine

Concentration (pM)

4.27 4.27 3.97 4.14 3.97 4.36 4.36 4.36 4.49

S o l u b i l i t y Product Constants f o r S o i l Clay M i n e r a l s S o l i d phases comprise the dominant f r a c t i o n of the s o i l volume and, as such, the d i s s o l u t i o n , p r e c i p i t a t i o n and i o n exchange p r o p e r t i e s of the s o i l minerals have a profound i n f l u e n c e on the composition of s o i l s o l u t i o n s . Therefore, i t i s e s s e n t i a l to i n c l u d e the r e a c t i o n s these s o l i d s may undergo i n s o i l s i n s i m u l a t i o n s t u d i e s of chemical e q u i l i b r i u m . R e l i a b l e thermodynamic data on the s t a b i l i t y of many of the important oxides, hydroxides, carbonates, phosphates, and s i l i cates are a v a i l a b l e and can be i n c o r p o r a t e d i n t o a computer program such as GEOCHEM. I n many s o i l s , p h y l l o s i l i c a t e s dominate the clay-s'ize f r a c t i o n . Smectites as a group of the p h y l l o s i l i c a t e s are ubiquitous i n s o i l s and sediments. Because of t h e i r large surface areas and high c a t i o n exchange c a p a c i t i e s , smectites, when present, play a s i g n i f i c a n t part i n i n f l u e n c i n g the composition of s o i l s o l u t i o n s . U n l i k e other c l a y m i n e r a l s , smectites a l s o e x h i b i t a broad range of chemical compositions and, t h e r e f o r e , i t i s u n l i k e l y that the standard f r e e energy of formation of every n a t u r a l l y - o c c u r r i n g smectite w i l l be determined e x p e r i m e n t a l l y . This d i f f i c u l t y has prompted a



37.

MATTIGOD AND

SPOSITO

Computer Program

GEOCHEM

843

number of attempts t o estimate AGf 298 15 for s i l i c a t e s i n general (31, 32, 33) and smectites i n p a r t i c u l a r (34, 35). These methods are based on simple p h y s i c a l models which are r e l a t e d p r i m a r i l y to c l a s s i c a l e l e c t r o s t a t i c s . The success of these methods i s based on the observation that the short-range c o o r d i n a t i o n environment of a c a t i o n that i s found i n s i l i c a t e minerals does not tend to vary much from one mineral t o another. Since covalency plays i t s most important r o l e i n nearest neigh bor i n t e r a c t i o n s , i t f o l l o w s that i t s c o n t r i b u t i o n t o the s t r u c t u r a l energy of a mineral w i l l be about the same among s i l i c a t e s and, t h e r e f o r e , that the r e l a t i v e s t a b i l i t i e s of the minerals l a r g e l y w i l l be determined by longer-ranged i o n i c i n t e r a c t i o n s . Nriagu (34), i n comparing h i s method w i t h the method of Tardy and G a r r e l s (35), concluded that the accuracy of estima t i o n of Gf 298.15 values of p h y l l o s i l i c a t e s i n e i t h e r method was about the same. But, each of these methods contains a c r i t i c a l , ad hoc assumption that i s d i f f i c u l t t o j u s t i f y on geochemical grounds. Tardy and G a r r e l s (35) had t o assume that the hydroxyl ions i n Mg-bearing l a y e r s i l i c a t e s are a s s o c i a t e d only w i t h magnesium i o n s , i n s o f a r as thermochemical c a l c u l a t i o n s are concerned. Nriagu (34) was forced t o decrease the values of AGf 298.15 for solid KOH and NaOH by about 15 percent from the measured values i n order t o o b t a i n agreement between the r e s u l t s of h i s method and experimental data f o r Na- and K-containing c l a y m i n e r a l s . These problems w i t h the methods of Tardy and G a r r e l s (35) and Nriagu (34) prompted the development of a method by Mattigod and Sposito (36) f o r e s t i m a t i n g the AGf 298 15 values of smectites which not only shows a s l i g h t improvement i n p r e d i c t i o n , but a l s o i s free of the a r b i t r a r y assumptions i n herent i n some of the other methods. This method employs the concept of i o n i c bonding as a p p l i e d t o c l a y m i n e r a l s , mentioned e a r l i e r , along w i t h a s t a t i s t i c a l equation t o r e l a t e the charge on a smectite due t o isomorphous s u b s t i t u t i o n and the valence and radius of the i n t e r l a y e r c a t i o n t o the free energy changes brought about by changes i n c o o r d i n a t i o n environment. These changes i n free energy occur i n t r a n s f e r r i n g a c a t i o n from r e l a t i v e l y h i g h - p o t e n t i a l s i t e s i n a hydroxide t o a r e l a t i v e l y l o w - p o t e n t i a l i n t e r l a y e r s i t e i n a smectite. The r e l a t i o n s h i p developed by Mattigod and Sposito (36) i s : AG

and

= Σn. AG

In δ

(n.) - (Σn.Ζ.-12) AG H 0 2

( 1 )

-

δ

= 1.9283 C + 0.3501 R - 0.2819 Ζ + 3.5427

where, AGf = standard free energy of formation of a smectite n i = r e a c t i o n c o e f f i c i e n t of the i hydroxide Zi = charge on the i c a t i o n ( i n c l u d i n g S i ) th

th


(1)

844

CHEMICAL

AG^ C

R Ζ

MODELING IN AQUEOUS

= s t a n d a r d f r e e energy of f o r m a t i o n of the i h y d r o x i d e component = c a t i o n e x c h a n g e c a p a c i t y (due t o i s o m o r p h o u s s u b s t i t u t i o n ) o f the s m e c t i t e per formula u n i t , i n eq/fw ο = P a u l i n g r a d i u s (A) o f t h e e x c h a n g e a b l e (interlayer) cation = v a l e n c e of the exchangeable c a t i o n

E q u a t i o n 1 p r o v i d e d estimates of A G ^ 298 15 ^ smectites w h i c h showed b e t t e r agreement w i t h t h e e x p e r i m e n t a l l y d e r i v e d v a l u e s t h a n t h e o t h e r two methods ( 3 4 , 3 5 ) . A c o m p a r i s o n among t h e methods i s g i v e n i n T a b l e I I I . o


SYSTEMS

C a t i o n Exchange

r

Phenomena

The t a s k o f c a l c u l a t i n g t h e c o m p o s i t i o n o f a s o i l e x c h a n g e r p h a s e i n e q u i l i b r i u m w i t h a s o i l s o l u t i o n h a s two d i s t i n c t p a r t s : t h e t h e r m o d y n a m i c e x c h a n g e e q u i l i b r i u m c o n s t a n t s must be d e t e r m i n e d and t h e a c t i v i t y c o e f f i c i e n t s o f t h e components o f t h e e x c h a n g e r p h a s e must be e s t i m a t e d . The f i r s t p a r t o f t h e p r o b l e m , t h a t o f o b t a i n i n g exchange e q u i l i b r i u m c o n s t a n t s , is n o t p a r t i c u l a r l y d i f f i c u l t s i n c e a number o f measurements o f AG f o r t h e common c l a y m i n e r a l s ( e . g . , s m e c t i t e s , v e r m i c u l i t e s , and k a o l i n i t e s ) h a s b e e n p u b l i s h e d . A l t e r n a t i v e l y , a semie m p i r i c a l a p p r o a c h s u c h as t h e J a m e s - H e a l y m o d e l i n c l u d e d i n REDEQL2 (I) may be e m p l o y e d t o e s t i m a t e a s t a n d a r d f r e e e n e r g y of adsorption for a c a t i o n i c species. e x

S m e c t i t e s a r e one o f t h e most i m p o r t a n t s o i l c l a y m i n e r a l s as r e g a r d s c a t i o n e x c h a n g e . The r e v e r s i b l e e x c h a n g e r e a c t i o n s o f t h e s e m i n e r a l s w i t h m e t a l c a t i o n s may be p i c t u r e d as k i n e t i c a l l y favored ( i . e . , rapid) p r e c i p i t a t i o n - d i s s o l u t i o n reactions (44). From t h i s p o i n t of v i e w , i t i s m e a n i n g f u l to w r i t e the "exchange half-reaction": MX ( s , a q ) m

= M^Uq)

+ mX" (s,aq) 1

(2)

where X r e f e r s t o one e q u i v a l e n t o f t h e a n i o n i c p o r t i o n o f a s m e c t i t e e x c h a n g e r and M*™ i s an e x c h a n g e a b l e c a t i o n . The t y p i c a l e x c h a n g e r e a c t i o n t h e n i s p i c t u r e d as a s e t o f p a i r s o f r e a c t i o n s s u c h as E q . 2, w i t h e a c h r e a c t i o n p a i r i n v o l v i n g two d i f f e r e n t metal c a t i o n s . The a n a l o g y b e t w e e n E q . 2 and t h e dissolution reaction for a solid is evident. H o w e v e r , t h e r e a r e some i m p o r t a n t d i f f e r e n c e s b e t w e e n t h e e q u i l i b r i u m c o n s t a n t f o r E q . 2 and t h e u s u a l K o · First, the compound on t h e l e f t - h a n d s i d e o f E q . 2 i s n o t a d r y s o l i d a t s t a n d a r d t e m p e r a t u r e and p r e s s u r e , b u t i n s t e a d i s a h o m o i o n i c s m e c t i t e i n c o n t a c t w i t h an aqueous s o l u t i o n . The s t a n d a r d s t a t e f o r M X ( s , a q ) a c c o r d i n g l y i s the homoionic c l a y m i n e r a l at s t a n d a r d t e m p e r a t u r e and p r e s s u r e i n e q u i l i b r i u m w i t h an i n f i n S

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Chemical Modeling in Aqueous Systems - American Chemical Society

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