226
C. De Stefano . C. Foti . A. Gianguzza . D. Piazzese . S. Sammartano
Using results previously obtained from equilibrium analysis studies performed on
various acid base-systems in artificial sea water (SSWE) both as six components and
as single salt, we can now show a comprehensive picture of the binding capacity of
SSWE towards 0- and N-ligands and organometallic cations, with the aim (a) of drawing up a rigorous speciation picture of the acid-base systems under investigation, and
(b) of defining general relationships useful in estimating the behaviour of a whole class
of ligands.
The interactions of carboxylate, amine, amino acid and phosphate classes ofligands
and of organotin(IV) cations with the components of SSWE as a single salt BA will be
discussed. The binding of some divalent cations by the anion of BA was also considered using predictive relationships.
9.3.1
Organic Ligands
9.3.1.1
Carboxylic Ligands
In speciation studies of natural waters, different classes of ligands have to be considered. Among these, carboxylates are the most common and ubiquitous naturally occurring organic complexants, present in all the fractions of natural organic matter,
particularly in fulvic compounds (Buffle 1988). It can be estimated that the main
binding sites of aquagenic refractory organic matter and fulvic acids are -COO(2-10 mmol g-l), often of aliphatic nature, and phenolic -OR (1-5 mmol g-l). A number (about 25 fig of C atoms rl) of carboxylic groups are also present in sea waters as
both free and combined hydrolysable amino acids (Daumas 1976; Lee and Wakeham
1989; Stumm and Brauner 1975; Williams 1971) and as fatty acids (about 40 Ilg C r 1 ,
free and combined) (Stumm and Brauner 1975). Some important di- and tri-carboxylic acids such as pyruvic, succinic and citric acids are always present in natural waters
and derive from the biochemical processes of living organisms. Therefore, the importance of carboxylic and polycarboxylic ligands in the general picture of organic complexation in natural waters is evident. A number of data concerning the binding capacity of carboxylic ligands (Martell and Smith 1997; SilIen and Martell 1964, 1971; Pettit
and Powell 1997) are reported in literature, but no data are reported for the complex
formation of these ligands in sea water. The binding capacity of carboxylic ligands towards alkali and alkaline earth metals has been extensively studied at different ionic
strengths (Daniele et al. 1985, 1994 and references reported therein). The stability of
different complexes of various polycarboxylic anions with Na +, Ca 2 + and Mg2+ follows
a regular trend with respect to the charges involved in the formation reaction, as shown
in Fig. 9.1, where the formation constants of ML species are plotted vs. [(ZaZc)2/3 -1].
Linearity is quite good and ensures good predictive power. Moreover, this homogeneous trend indicates that sodium, magnesium and calcium cations have very similar
binding capacities towards carboxylic anions.
With the aim of giving a comprehensive picture of the binding capacity of low
molecular weight carboxylic ligands in sea water, apparent protonation constants and
complex formation constants with the cationic macro components of sea water were
determined for the following carboxylic ligands (see formulas and abbreviations in
C. De Stefano . C. Foti . A. Gianguzza . D. Piazzese . S. Sammartano
Using results previously obtained from equilibrium analysis studies performed on
various acid base-systems in artificial sea water (SSWE) both as six components and
as single salt, we can now show a comprehensive picture of the binding capacity of
SSWE towards 0- and N-ligands and organometallic cations, with the aim (a) of drawing up a rigorous speciation picture of the acid-base systems under investigation, and
(b) of defining general relationships useful in estimating the behaviour of a whole class
of ligands.
The interactions of carboxylate, amine, amino acid and phosphate classes ofligands
and of organotin(IV) cations with the components of SSWE as a single salt BA will be
discussed. The binding of some divalent cations by the anion of BA was also considered using predictive relationships.
9.3.1
Organic Ligands
9.3.1.1
Carboxylic Ligands
In speciation studies of natural waters, different classes of ligands have to be considered. Among these, carboxylates are the most common and ubiquitous naturally occurring organic complexants, present in all the fractions of natural organic matter,
particularly in fulvic compounds (Buffle 1988). It can be estimated that the main
binding sites of aquagenic refractory organic matter and fulvic acids are -COO(2-10 mmol g-l), often of aliphatic nature, and phenolic -OR (1-5 mmol g-l). A number (about 25 fig of C atoms rl) of carboxylic groups are also present in sea waters as
both free and combined hydrolysable amino acids (Daumas 1976; Lee and Wakeham
1989; Stumm and Brauner 1975; Williams 1971) and as fatty acids (about 40 Ilg C r 1 ,
free and combined) (Stumm and Brauner 1975). Some important di- and tri-carboxylic acids such as pyruvic, succinic and citric acids are always present in natural waters
and derive from the biochemical processes of living organisms. Therefore, the importance of carboxylic and polycarboxylic ligands in the general picture of organic complexation in natural waters is evident. A number of data concerning the binding capacity of carboxylic ligands (Martell and Smith 1997; SilIen and Martell 1964, 1971; Pettit
and Powell 1997) are reported in literature, but no data are reported for the complex
formation of these ligands in sea water. The binding capacity of carboxylic ligands towards alkali and alkaline earth metals has been extensively studied at different ionic
strengths (Daniele et al. 1985, 1994 and references reported therein). The stability of
different complexes of various polycarboxylic anions with Na +, Ca 2 + and Mg2+ follows
a regular trend with respect to the charges involved in the formation reaction, as shown
in Fig. 9.1, where the formation constants of ML species are plotted vs. [(ZaZc)2/3 -1].
Linearity is quite good and ensures good predictive power. Moreover, this homogeneous trend indicates that sodium, magnesium and calcium cations have very similar
binding capacities towards carboxylic anions.
With the aim of giving a comprehensive picture of the binding capacity of low
molecular weight carboxylic ligands in sea water, apparent protonation constants and
complex formation constants with the cationic macro components of sea water were
determined for the following carboxylic ligands (see formulas and abbreviations in