5 Solution Thermochemistry of H u m i c Substances Acid—Base Equilibria of River Water Humic Substances
1
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Ε. M. PERDUE Environmental Science/Chemistry Department, Portland State University, Portland, OR 97207
Current concern over the p o t e n t i a l e c o l o g i c a l impact o f t r a c e metals i n t h e a q u a t i c environment has l e d t o e x t e n s i v e r e s e a r c h i n two m a i n a r e a s : 1) t h e c o o r d i n a t i o n c h e m i s t r y o f t r a c e m e t a l s i n n a t u r a l waters and 2) t h e r o l e o f t r a c e m e t a l s i n t h e growth o f aquatic organisms. R e c e n t s t u d i e s have shown t h a t n a t u r a l waters p o s s e s s some c a p a c i t y t o c o m p l e x m e t a l s a n d t h a t t h i s "complexation capacity" can strongly influence the biological a c t i v i t y of a m e t a l (]_, 2 ^ 3 ) . Due t o t h e p r e s e n c e o f a d i v e r s e a r r a y o f i n o r g a n i c a n d o r g a n i c l i g a n d s i n n a t u r a l w a t e r s , a v e r y l a r g e num ber o f m e t a l - l i g a n d complexes c o u l d c o n c e i v a b l y c o n t r i b u t e t o the "complexation capacity" of natural waters. The a v a i l a b i l i t y o f s o p h i s t i c a t e d c o m p u t e r p r o g r a m s s u c h a s MINEQL ( 4 J h a s made i t p o s s i b l e to c a l c u l a t e t h e d i s t r i b u t i o n o f f r e e and complexed t r a c e metals i n w e l l - d e f i n e d media ( i . e . , aqueous s o l u t i o n s i n which the a n a l y t i c a l c o n c e n t r a t i o n s o f a l l t r a c e metals andcomp l e x i n g l i g a n d s a r e known a n d w h e r e s t a b i l i t y c o n s t a n t s f o r a l l p o s s i b l e m e t a l - l i g a n d complexes a r e a v a i l a b l e ) . While s t a b i l i t y c o n s t a n t s have been c o m p i l e d f o r a l a r g e number o f t r a c e m e t a l complexes w i t h i n o r g a n i c l i g a n d s and w i t h simple s y n t h e t i c o r g a n i c l i g a n d s s u c h a s NTA a n d EDTA ( 5 J , n o s u c h d a t a a r e a v a i l a b l e f o r i n t e r a c t i o n s between t r a c e m e t a l s and n a t u r a l l y o c c u r r i n g o r g a n i c ligands. U n t i l t h e s e d a t a a r e o b t a i n e d , e q u i l i b r i u m model cal c u l a t i o n s w i l l a t best p r o v i d e an approximate d e s c r i p t i o n o f t h e d i s t r i b u t i o n of trace metals i n natural waters. The c o m p o s i t i o n o f n a t u r a l l y o c c u r r i n g o r g a n i c m a t t e r i n r i v e r w a t e r i s d e t e r m i n e d m a i n l y by t h e i n p u t o f a l l o c h t h o n o u s organic matter (6J which includes biopolymers ( p r o t e i n s , carbo h y d r a t e s , and l i g n i n s ) and geopolymers (humic s u b s t a n c e s ) . Since the chemical c h a r a c t e r of t h e geopolymers r e q u i r e s s p e c i a l i z e d , l e s s common e n z y m e s y s t e m s f o r b r e a k d o w n , m i c r o b i a l a t t a c k intro duces a strong bias f a v o r i n g d i g e s t i o n o f the biopolymers. Con s e q u e n t l y , humic s u b s t a n c e s u s u a l l y c o n s t i t u t e t h e major frac t i o n of organic matter i n r i v e r water (7-13). Humic s u b s t a n c e s a r e g e n e r a l l y c o n s i d e r e d t o be composed o f t h r e e operationally distinct fractions: 1) f u l v i c a c i d , w h i c h i s s o l u b l e i n b o t h 1
Part II of a series.
0-8412-0479-9/79/47-093-099$05.00/0 © 1979 American Chemical Society Jenne; Chemical Modeling in Aqueous Systems ACS Symposium Series; American Chemical Society: Washington, DC, 1979.
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CHEMICAL MODELING IN AQUEOUS SYSTEMS
a c i d i c and b a s i c s o l u t i o n s , 2) humic a c i d , w h i c h i s s o l u b l e i n b a s i c s o l u t i o n b u t i n s o l u b l e i n a c i d i c s o l u t i o n s , a n d 3) h u m i n , which i s i n s o l u b l e i n both a c i d i c and b a s i c s o l u t i o n s . The b u l k o f r i v e r w a t e r humic s u b s t a n c e s g e n e r a l l y r e s e m b l e s t h e more r e a d i l y s o l u b i l i z e d f u l v i c a c i d , w i t h t h e r e l a t i v e amount o f t h e l e s s s o l u b l e h u m i c a c i d p r o b a b l y b e i n g d e p e n d e n t o n t h e pH o f the natural water (10).
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The a c t u a l s t r u c t u r a l waters remain unresolved.
f e a t u r e s o f humic s u b s t a n c e s i n n a t u r a l I t i s g e n e r a l l y a c c e p t e d t h a t humic
s u b s t a n c e s a r e complex polymers which possess both c a r b o x y l and phenolic hydroxyl a c i d i c functional groups. These f u n c t i o n a l g r o u p s a r e t h o u g h t t o be i n v o l v e d i n m e t a l c o m p l e x a t i o n r e a c t i o n s since protons a r e r e l e a s e d during such r e a c t i o n s ( f o r review, see S c h n i t z e r a n d K h a n , ( 1 4 J ; F l a i g e t a ] _ . , ( 1 5 J . The e x t e n t o f c o m p l e x a t i o n o f a metal w i l l thus depend n o t o n l y on t h e s t a b i l i t y c o n s t a n t o f t h e m e t a l - l i g a n d complex but a l s o on t h e a c i d d i s s o c i a t i o n c o n s t a n t o f t h e a c i d i c f u n c t i o n a l group which serves as t h e c o m p l e x i n g l i g a n d . C o n s e q u e n t l y , many e f f o r t s h a v e been made t o d e t e r m i n e b o t h t h e c o n c e n t r a t i o n s a n d c h e m i c a l c h a r a c t e r i s t i c s of the acidic functional r e v i e w , see Reuter and Perdue,
groups (10)).
of
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In g e n e r a l , w h i l e t h e p o t e n t i o m e t r i c t i t r a t i o n methods u s e d to c h a r a c t e r i z e a c i d i c f u n c t i o n a l groups i n humic s u b s t a n c e s have been u s e f u l i n d e t e r m i n i n g t h e c o n c e n t r a t i o n s o f c a r b o x y l and p h e n o l i c h y d r o x y l g r o u p s , t h e s e methods have n o t been v e r y useful i n determining the pKa values associated with each type of a c i d i c functional group. Some o f t h e m o r e s u c c e s s f u l s t u d i e s have u t i l i z e d v a r i o u s m a t h e m a t i c a l t e c h n i q u e s t o t r y t o t a k e into a c c o u n t t h e e f f e c t o f a c c u m u l a t i n g n e g a t i v e c h a r g e on t h e a c i d d i s s o c i a t i o n c o n s t a n t s o f p r o g r e s s i v e l y weaker a c i d i c f u n c t i o n a l groups (16, 17, ISO. While d i r e c t t i t r a t i o n s u t i l i z i n g f u n c t i o n s f o r e n d p o i n t l o c a t i o n a r e sometimes u s e d (16^ c o n c e n t r a t i o n s o f carboxyl and p h e n o l i c hydroxyl groups substances a r e most o f t e n determined u s i n g t h e c a l c i u m exchange r e a c t i o n and barium hydroxide t o t a l a c i d i t y r e
Gran 1_7), t h e i n humic acetate action,
r e s p e c t i v e l y (1_9). The s p e c i f i c i t y o f t h e s e r e a c t i o n s h a s , how e v e r , b e e n q u e s t i o n e d ( 2 0 , 21_). In summary, t h e need f o r know ledge o f t h e c o n c e n t r a t i o n s and a c i d d i s s o c i a t i o n c o n s t a n t s o f n a t u r a l l y o c c u r r i n g o r g a n i c l i g a n d s has n o t been a d e q u a t e l y met by t h e u s e o f p o t e n t i o m e t r i c t i t r a t i o n m e t h o d s . The
technique
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has been
successfully
used to determine the nature and abundances o f a v a r i e t y o f a c i d i c f u n c t i o n a l groups i n p r o t e i n s (22). S e v e r a l i n v e s t i g a t o r s have made r a t h e r l i m i t e d e f f o r t s t o u s e t i t r a t i o n c a l o r i m e t r y t o s t u d y humic s u b s t a n c e s , u s u a l l y as a method t o d e t e r m i n e t h e c a t i o n exchange c a p a c i t y o r t i t r a t a b l e a c i d i t y o f humic s u b s t a n c e s ( 2 3 , 24). Choppin and K u l l b e r g (25) have r e c e n t l y used t i t r a t i o n calorimetry to determine the enthalpies of n e u t r a l i z a t i o n of a c i d i c f u n c t i o n a l groups i n humic s u b s t a n c e s and have combined t h a t d a t a w i t h pH t i t r a t i o n d a t a t o o b t a i n A G , Δ Η , a n d AS v a l u e s
Jenne; Chemical Modeling in Aqueous Systems ACS Symposium Series; American Chemical Society: Washington, DC, 1979.
5.
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culties leading
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i n l o c a t i n g p o t e n t i o m e t r i c e n d p o i n t s were e n c o u n t e r e d , t h e s e r e s e a r c h e r s t o s i m p l y assume t h a t t h e i r humic a c i d
c o n t a i n e d equal c o n c e n t r a t i o n s o f c a r b o x y l and p h e n o l i c g r o u p s . None o f t h e s e s t u d i e s h a s t a k e n a d v a n t a g e o f t i t r a t i o n c a l o r i m e t r y
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as a t e c h n i q u e f o r the d e t e r m i n a t i o n o f c o n c e n t r a t i o n s o f a c i d i c f u n c t i o n a l g r o u p s and f o r t h e s i m u l t a n e o u s d e t e r m i n a t i o n o f both AHa and pKa v a l u e s f o r i o n i z a t i o n o f t h e a c i d i c f u n c t i o n a l groups o f humic s u b s t a n c e s . A l t h o u g h pKa v a l u e s are g e n e r a l l y regarded as b e i n g q u i t e c h a r a c t e r i s t i c o f a p a r t i c u l a r f u n c t i o n a l group, i t i s not w i d e l y recognized t h a t AHa values are a l s o q u i t e u s e f u l i n d e t e r m i n i n g t h e n a t u r e o f an a c i d i c f u n c t i o n a l g r o u p . In F i g u r e 1 , t h e r e l a t i o n s h i p between A H a and p K a f o r a l a r g e number o f c a r b o x y l i c a c i d s , p h e n o l s , a n d ammonium i o n s i s p l o t t e d . The l o c a t i o n and s i z e o f t h e d o m a i n o f e a c h f u n c t i o n a l g r o u p was d e t e r m i n e d b y t h e mean v a l u e a n d s t a n d a r d d e v i a t i o n o f A H a a n d pKa v a l u e s computed from a v a i l a b l e d a t a (26J u t i l i z i n g d a t a for a l l t a b u l a t e d compounds c o n t a i n i n g o n l y c a r b o n , h y d r o g e n , oxygen, and r e d u c e d n i t r o g e n . I t i s a p p a r e n t f r o m F i g u r e 1 t h a t an a c i d i c f u n c t i o n a l g r o u p i s i d e n t i f i e d a t l e a s t a s r e a d i l y by i t s A H a v a l u e a s by i t s p K a v a l u e . Of c o u r s e , i f b o t h p K a and A H a a r e known f o r t h e i o n i z a t i o n o f an a c i d i c f u n c t i o n a l g r o u p , t h e t a s k of i d e n t i f i c a t i o n of the f u n c t i o n a l group i s f u r t h e r s i m p l i f i e d . Furthermore, ( e . g . , 25°C) temperatures
o n c e p K a a n d A H a v a l u e s a r e known a t o n e t e m p e r a t u r e i t i s p o s s i b l e to c a l c u l a t e pKa v a l u e s at o t h e r u s i n g standard thermodynamic r e l a t i o n s h i p s .
As p o i n t e d o u t by C h r i s t e n s e n e t aj_. ( 2 7 J , the shape of a thermometric t i t r a t i o n curve (heat evolved v s . moles of titrant) i s d e t e r m i n e d by b o t h t h e e n t h a l p y c h a n g e ( Δ Η ) and e q u i l i b r i u m c o n s t a n t (K) f o r t h e r e a c t i o n t a k i n g p l a c e i n t h e c a l o r i m e t e r . In r e a c t i o n s f o r w h i c h l o g K > 4 ( e . g . , the r e a c t i o n of a carboxy l i c acid with hydroxide ion for which log K-10), the e q u i l i b r i u m f a v o r s t h e f o r m a t i o n o f p r o d u c t s so s t r o n g l y t h a t t h e added i n crement of t i t r a n t i s e s s e n t i a l l y completely r e a c t e d . In t h i s c a s e t h e amount o f h e a t e v o l v e d i s d e t e r m i n e d by t h e Δ Η f o r t h e r e a c t i o n and t h e amount o f added t i t r a n t . The r e s u l t i n g t h e r m o m e t r i c t i t r a t i o n c u r v e c o n s i s t s o f two s t r a i g h t l i n e s e g m e n t s which i n t e r s e c t s h a r p l y at the equivalence p o i n t . In s u c h r e a c t i o n s , t h e K - v a l u e c a n n o t be d e t e r m i n e d f r o m t h e t h e r m o m e t r i c t i t r a t i o n curve. However, t h e e n d p o i n t o f the t i t r a t i o n and t h e ΔΗ o f t h e r e a c t i o n a r e r e a d i l y o b t a i n e d . In r e a c t i o n s f o r w h i c h l o g K
